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abdominal aortic aneurysm American Academy ofOtolaryngology-Head Neck Surgery American Association for the Surgery ofTrauma allergic bronchopulmonary aspergillosis N-acetylcysteine acetylcholinesterase acquired cystic kidney disease anterior cervical space adenocorticotropic hormone aortocoronary venous bypass graft
AFLP AFP ACS AHA AlP AICC ALL AML APKD APUD ARDS ASD ASH ASNR ASSR ATAI ATS AUA AUSPV AV
acute fatty liver of pregnancy alpha-fetoprotein adrenogenital syndrome American Heart Association acute interstitial pneumonia American Joint Committee on Cancer acute lymphocytic leukemia acute myeloic leukemia acquired polycystic kidney disease amine precursor uptake and decarboxylation (cell line) adult respiratory distress syndrome atrial septal defect asymmetric septal hypertrophy American Society ofNeuroradiology American Society of Spine Radiology acute traumatic aortic injury American Thoracic Society American Urological Association anomalou~ unilateral single pulmonary vein atrioventricular
AVM BAL BALT BlP BMT BO BOOP BPH bpm BS CAD CBV CCC CCD CHF CLL CM CML CMN CMP CMV COLD COP COPD CPR CRP CS CSF CT CTA CTAP CTC CTm CTEPH CTHA CTP
arteriovenous malformation bronchoalveolar lavage bronchus-associated lymphoid tissue bronchiolitis obliterans interstitial pneumonia bone marrow transplant bronchiolitis obliterans bronchiolitis obliterans with organizingpneumonia benign prostatic hyperplasia beats per minute buccal space coronary artery disease cerebral blood volume cholangiocellular carcinoma charge couplet device chronic heart failure chronic lymphatic leukemia contrast medium chronic myeloid leukemia contrast medi urn nephropathy cardiomyopathy cytomegalovirus chronic obstructive lung disease [~COPDl ~togenic organizing pneumorna chronic obstructive pulmonary disease curved-planar reformation C-reactive protein carotid space cerebrospinal fluid computed tomography CT angiography CT during arterial portography CT cholangi ography CT dose index chronic thromboembolic pulmonary hypertension CT hepatic arteriography cr perfusion imaging
7 Principles ofCT, Spiral CT, and Multislice CT M. Prokop
1 Principles ojCT Spiral CT, and Multislice CT Computed tomography (CT) has been one of the biggest breakthroughs in diagnostic radiology. The first clinical CT scanner was developed by Godfrey N. Hounsfield for examinations of the head and was installed in 1971 at Atkinson-Morley's Hospital in Wimbledon, England. The first body CT scanner was installed in 1974, and before the end of the 1970s the basic technical evolution of CT was complete (Table 1.1). Technical details were refined during the 1980s, and CT technology remained on a plateau until the early 1990s, when the advent of spiral (helical) CT scanning sparked a further, rapid evolution leading to improved diagnostic capabilities, 3D imaging techniques, and CT angiography. The latest innovation is the introduction of multislice CT in 1998. This new technology is vastly expanding the performance of CT scanners: it truly transforms CT from a trans axial imaging modality to a 3D technique that yields high quality images in arbitrary planes and forms the basis for an expanding variety of 3D visualization techniques, including virtual endoscopy. In addition, these scanners have the potential to revolutionize cardiac imaging with CT.
Table 1.1 phy
Historical milestones in computed tomogra-
1929
Radon: basic mathematical principles
1963
Cormack: image reconstruction
1971
Hounsfield (EMI Laboratories): technology
1971
Head scanner (EMI Mark I)
1974
Whole-body scanner (ACTA)
1974
Third-generation scanner (Artronix)
1977
Fourth-generation scanner (AS&E scanner)
1979
Nobel Prize awarded to Hounsfield and Cormack
1980s
Technical refinements
1983
Dynamic spatial reconstructor
1983
Electron beam CT scanning
1987
Scanners with a continuously rotating tube
1989
Spiral CT (helical CT)
1991
Dual-slice spiral CT (Elscint)
1991
CTangiography
1995
Real-time reconstruction (CT fluoroscopy)
1998
Multislice CT (4 detector rows _~~~
1999
Multislice cardiac imaging
2001 /2
Multislice CT (8/16 detec~to"l'!u r--ow-~"tr
Future
Cone beam CT (a 128 detectorrows)
Computet Tomograpfy Scanning Principle
Computed tomography is an x-ray tomographic technique in which an x-ray beam passes through a thin axial section of the patient from various directions (Fig. 1.1).Parallel collimation is used to shape the x-ray beam to a thin fan, which defines the thickness of the scan plane. Detectors measure the intensity of the attenuated radiation as it emerges from the body. A mathematical image reconstruction (inverse Radon transformation) calculates the local attenuation at each point within the CT section. These local attenuation coefficients are translated into "CT numbers" and are finally converted into shades of gray that are displayed as an image. With conventional CT scanners the volume of interest is scanned in a sequential fashion, usually proceeding one section at a time. The first two generations of CT scanners (Table 1.2) were superseded in the late 1970s by third- and fourth-generation scanners, which are still in use today. In third -generation scan-
Fig. 1.1
Principle of CT scanning.
Image Reconstruction Table 1.2
Comparison of CT scanner generations
Type
First generation
and types
Second generation
Third generation
Translation-rotation
Principle
Single
Series
Arc (30°_60°)
Active detector rows
2
1-2
1-16
3-52
256-1000 0.4-10 s
Scan time
135-300 s
5-150 s
ners, tube and detector array rotate synchronously around the patient. The detector array covers the full width of the fan beam (Fig.1.2). In fourth-generation scanners, the detector elements cover a full circle around the scanner opening and remain stationary during the scan, while only the x-ray tube rotates around the patient (Fig. 1.2). Third-generation scanners, however, offer better scatter suppression and require less detector elements, which is the reason why all multislice CT scanners use third-generation technology. Attempts to speed up the imaging process led to the development of a multi-tube CT scanner called the dynamic spatial reconstructor (the "Mayo monster" equipped with 28 tubes, able to scan up to 240 sections, each of 1 mm thickness, in one 360° rotation), electron beam CTscanning, spiral CT, and, recently, multislice CT. Of these procedures, only spiral and multislice CT have achieved large-scale clinical impact.
Image Reconstruction The detector signals registered during a scan are preprocessed to compensate for inhomogeneities in the detector system and to correct for beam-hardening effects within the patient. After various correction steps and transformation from signal intensities into x-ray attenuation values these data are called the CT raw data (Fig.1.3).The raw data sets for third- and fourthgeneration scanners each consist of the attenuation profiles of some 500 to 1500 projections for each 360° rotation of the x-ray tube. Each projection in turn is composed of some 500 to 1500 single attenuation values. Image reconstruction from the raw data sets finally yields the image data set.
Electron beam scanning
Rotation
Detectors
Detector elements/row
Fourth generation
Electron beam deflection
Ring (360°)
Semicircular
(210°)
4 600-4000
432/864
1-5 s
~50 ms
Scanning principle
Image reconstruction
3rd Generation
Fig.1.2 Comparison of scanning principle and image reconstruction in third-generation (a, b) and fourthgeneration CT scanners (c, d). The fourth generation uses a stationary detector ring, and the data acquired by one detector are assembled into a projection for the various tube positions.
Image reconstruction starts with the selection ofthe desired field of view. Each ray from the tube to the detector that passes through this field of view is used for reconstruction. The attenuation coefficient for each image point is determined by averaging the attenuation values for all rays that cross this point (back projection). But this type of unfiltered back projection yields a very unsharp image with blurred edges, so multi-
3
I
1 Principles oICT, Spiral CT, and Multislice CT
Scan Data
-+
Fig. 1.3
Id
Prepro-
Convolutionj -+ filtering
cessing
~
,...~~ , •. ~J
Imaae data Back projection
~~\~J.j
Processes involved in CT image reconstruction.
pie rays are assembled into a projection, and the resulting attenuation profile is subjected to an edge-enhancing mathematical filtering (convolution) process. The so-called "convolution kernel" determines the type offiltering. Back projection of the filtered attenuation profiles then yields a sharp image. The convolution kernel (reconstruction algorithm) used for the/itered back projection determines the properties of the reconstructed CT sections in terms of spatial reso-
a
Filtered raw data -+
b
Fig. 1.4 Effect of the convolution kernel on spatial resolution and image noise. In the lung parenchyma (high intrinsic contrast), the use of a sharp kernel (b) instead of a standard kernel (a) increases image sharpness. In
lution and image noise, and can vary from soft or smooth to sharp or edge-enhancing (Fig. 1.4). Third and fourth-generation scanners differ in the way that the attenuation values are assembled into a fan-shaped projection. The third generation uses the fan from a single tube position to the detector, while the fourth generation takes the data acquired by a single detector for various tube positions and assembles them into a projection (Fig. 1.2).
c
d
the liver (low intrinsic contrast), a soft kernel (c) is preferable to a sharp kernel (d) because the increased noise could otherwise obscure low-contrast structures.
Image Display and Documentation Fig. 1.5 The pixels in the CT matrix actually represent volume elements (voxels) in the scanned body region. Note that for a standard section thickness of 10 mm, each voxel has a matchstick shape.
I 0.5mm:10mm
I
FOV=25 em
Matrix= 5122
Image Display and Documentation •
Image Matrix and Field of View
ACT image is composed ofa square image matrix that ranges in size from 256 x 256 to 1024 x 1024 picture elements, or pixels. Since a CT section has a finite thickness, each pixel actually represents a small volume element. or voxel. The size of this voxel depends on the matrix size, the selected field of view (FOV) , and the section thickness (Fig. 1.5).In most CT examinations the voxel has a matchstick shape, i.e., the pixel size measured in the plane of the section, the x-y plane, is 10 to 20 times smaller than the section thickness, measured along the z-axis. This anisotropy (nonuniform shape) of the voxels can be decreased only by greatly reducing the section thickness. Only with multislice CT is it possible to obtain nearly isotropic (cube-shaped) voxels for larger body areas. (See also p. 46 for a more complete discussion of the issue.) There may be a difference between the actual image matrix that is reconstructed from the raw data and the display matrix that is shown on the viewing monitor or is printed out to film. Although the display matrix is usually identical to the image matrix, a larger display matrix may be selected (e.g., 1024 x 1024 instead of 512 x 512) to improve the image quality. It is not usually necessary to use data from the entire body cross section for image reconstruction, and afield of view (FOV) of more restricted size can be reconstructed from the raw data. This field of view is characterized either by its size in mm or by the zoomfactorrelative to the maximum field of view possible on that scanner. Depending on the scanner it can either be round or square. Some manufacturers call this the reconstruction field of view (RFOV) to distinguish it from the display field of view (DFOV) that can be
selected from that field and magnified for display on the monitor. Usually such a magnified image is less sharp than one directly reconstructed from the raw data because it uses only a portion of the image data rather than all the information contained in the raw data set. The reconstruction and display field of view have to be distinguished from the scan field of view (SFOV) available on some scanners. The SFOV is a reduced, centrally located area from which data are acquired, which can increase the sampling rate and thus improve spatial resolution. Usually this technique is applied in examinations of the extremities (calcaneus). spine or in the head and neck region. Since a reduced SFOV uses a fan beam with a narrower angle, the technique also reduces radiation exposure to those portions of the patient that are outside the SFOV. For this reason, it is used on some scanners to reduce radiation exposure for cardiac multislice CT imaging. H
CT Numbers
During image reconstruction, a numerical value (CT number) is assigned to each voxel according to the degree of x-ray attenuation in that voxel. To reduce dependence on the energy of the radiation and to obtain numerical values of convenient size, the CT number is defined as follows: CT=
1000
X
(u-U.water)/U-water
The unit for CT attenuation is called the Hounsfield unit (HU). The numbers are set on a scale in which -1000 represents the attenuation of air and 0 is the attenuation of water. Note that there is no upper limit to the scale. The available range of CT numbers varies between scanners and available bits per pixel (e.g., from -1024 to 3071 HU with 12 bits or up to 64,500 HU for 16 bits) .
5
pending on viewing conditions). Consequently, there is no point in assigning the complete diagnostic range of CT numbers (some 4000 HUl to the available range of gray levels (from white to black) because discrimination between structures with small differences in CT numbers would no longer be possible. It is therefore better to display just a portion of the CT scale. This so-called window is defined by its width, which affects image contrast. and by its level (center). which determines image brightness. Reducing the window width increases image contrast: lowering the window level brightens the image, and raising the window level darkens the image (Fig. 1.7). Examples of window settings shown in Table 1.3.
3000
2000 Cortical bone 1000 Cancellous bone Soft tissues Water
'ot ~Lung .1000~Air
•
Fig.lo6 The scale of CT attenuation numbers is defined by the values for air (-1000HU) and water (0 HU). Soft tissues occupy a narrow range around 50 HU.
The scale of IT attenuation numbers is shown graphically in Fig. 1.6. Except for fat, lowprotein fluids, and fresh blood, there are no "typical" values that would permit the specific characterization of soft [issues by their CT numbers (see also Fig. 7.5). •
Window
Settings
The human eye can distinguish only a limited number of gray levels (from about 40 to 100, de-
Image
Processing
and Analysis
The software used in CT scanners provides various options for processing and manipulating the cr sections. The features of greatest practical importance are the measurement of lengths and angles and the analysis of CT numbers within a selected region of interest (ROI). This ROJ may be selected interactively, allowing the operator to choose between predefined shapes (circle, ellipse, rectangle) or freely outlined regions of arbitrary shape. A computer program can then calculate mean CT numbers and standard deviations within the ROJ and can generate histograms. The scanned volume (an also be manipulated to produce reformatted images in any secondary plane of section ("multi planar reforms-
:::~:-I~j,
-1000 10
0
",~ •••••••••••• 1500/-650
-1000 . 1300 Fig. 1.7 Window settings. For optimum contrast, the gray levels available for vlewinq or filming are assigned to a designated portion of the CT scale. The window is
lung wIndow
.
Bone window
defined by its width, which determines contrast. and by itslevel,whlchdetermlnesbrightness.
Gantry Angulation The gantry, or frame that houses the scanning equipment, can be tilted about the x-axis to perform oblique scans. The range of gantry angulation may be ::: 30°, depending on the scanner. Gantry angulation is used mainly for scanning the head, neck. or spine. It is unnecessary for most other applications and is increasingly being abandoned with multislice scanning and reconstruction of tilted sections from a volume dataset.
Section Thickness • tion." MPR) and various types of three-dimensional reconstruction (see Chapter 2, Image Processing). •
CT
Documentation
images from conventional scanners are usually printed on film for image review. It is importent to document all the window settings that are relevant for the examined body region. For example, chest scans should be documented in both the lung window and soft-tissue window settings. A bone window setting may be added to screen for skeletal metastases. Attempts to display two non-overlapping window settings simultaneously (e.g., mediastinum and lung) have been abandoned, because structures whose CT numbers lie between the two windows (e.g.. pleural lesions) are too easily missed. This dual window technique is no longer used. With modern spiral CT or multislice scanners, or with the introduction of picture archiving and communication systems (PACS), printing of CT images is increasingly abandoned in favor of direct viewing on CRT or flat screen monitors and storing image data in a digital archive.
Section
Profile
The beam collimation setting determines the thickness of a CT section. But as in conventional radiography. the x-ray tube emits a conically divergent beam rather than parallel rays. To obtain sections of reasonably uniform thickness, it is necessary to use collimators located directly behind the tube. Some scanners use additional post-patient collimators in front of the detectors for further optimization of the section profile. Despite these measures, a truly planeparallel section is never obtained in practice, and adjacent portions of the scanned object are always included. In addition, the finite size of the focus of the x-ray tube causes an area outside the primary beam to receive radiation of less intensity. This region is called the penumbra (Fig.l.8a). The effect on the acquired section can be described in terms of the section sensitiviry profile (SSP) or slice profile {Fig. 1.8), which shows how much a point in the object contributes to the image as a function of its distance from the center of the section. The ideal section profile is a rectangle whose width equals the desired section thickness, so that all points outside the section contribute nothing to the measured attenuation while all points within the section contribute uniformly to the CT number. Real section profiles have rounded "edges". meaning that adjacent regions also contribute slightly to the image. While the profile of thick IT sections (7-lOmm) closely approximates to an ideal rectangular shape, thin sections exhibit a more bell-shaped profile (Fig. 18b).
7
•
8
1 Principles
of CT, Spirol CT, and Multislice
IT
Sensitivity profiles Focal spot
lOmm 100%
50 %
~~
5mm
3mm
slice
2mm slice
t mm slice
'~'
I-+--+-+------f--H-H-\---I
---++--tt-
O%4.~~~-4~~~~~--~~~~ b
FWHM
FWTA
-4.0
4.0 8.0 Table position (mrn)
Hq. U:I Because of the x-ray beam geometry, regions located outside the selected section thickness are ineluded in the scan (a). This results in a rounded section sensitivity profile, which approximates to an ideal rectilinear shape only in thicker sections (b). The effective
section thickness (* section width SWj can be measured by the width of the section profile at half of its peak value, called the full width at half maximum (FWHM), or the width that encompasses 90% of the area under the curve, called the full width at tenth area (FWTA)(c).
In clinical practice, the increasingly large "tails" of thinner section profiles do not substantially affect image quality because the decreased width of the profile is more important and leads to improved resolution along the z-axis.
A more rigorous measure of the width of the section profile is the width that encompasses 90% of the area under the curve, called the full width at tenth area (FWTA). The FWTA indicates the width at which object elements located outside the section contribute just 10% to the cr number. FWHM and FWTA yield similar values in conventional cr when thick sections are chosen, but they differ markedly in spiral cr and when thin collimation is used in conventional cr. The slice profile quality index (SPQI) is another, more recent measure. It describes the percentage of the area under the section sensitivity profile that is bounded within an ideal rectangular profile of the same section width.
•
Effective
Section
Thickness
The width of the section sensitivity profile is usually quantified by measuring the width of the curve at 50% of its peak value, called thefull width at haJfmaximum (FWHM) (Fig.1Sc). This value is also known as the effective section thickness or section width (SW). It is by definition equal to the section collimation (SC) or nominal section thickness in conventional CT. It should be noted, however, that the effective section thickness is not equivalent to the section collimation in spiral cr. It is the commonest measure for describing the spatial resolution along the patient axis (a-axis),
Scanning Parameters 9 Partial Volume
Effect
The CT number of a pixel is determined by the x-ray attenuation that occurs in the corresponding voxel. If tissues with different attenuation properties occupy the same voxel (e.g., blood vessels and lung), the resulting CT number willwith good approximation-represent the sum of the different attenuation values (partial volume
effect or partial volume averaging):
tal.
C T =viXC Ti + V2 XC T 2 + ... , where the partial volume elements vi add
Given the much greater extent of a voxel along the z-axis than in the x-y plane, the section collimation contributes a great deal more to this partial volume effect than the field of view or pixel size (Fig. 1.9). • Section Collimation A number of anatomic structures (portions of the aorta, chest wall, or liver borders) are oriented parallel to the long axis of the body. The usual transaxial section orientation in CT means that the scan will cut the corresponding tissue boundaries at a perpendicular angle, which tends to minimize partial volume effects. For this reason, a section collimation of 7-10 mm is most commonly used in conventional body CT examinations. Partial volume effects are particularly troublesome in scans that are oblique or parallel in relation to tissue boundaries (diaphragm, apex
of lung, pole of kidney) and in the evaluation of small structures (small vessels, bronchi, adrenals). A collimation of3-5 mm can be used to evaluate structures that are parallel to the scan plane (e.g., the pancreas) or for evaluating small organs such as the adrenals. Thin collimation of 1 -2 mm is preferred for the lung, where a detailed structural analysis is required for the diagnosis of interstitial lung disease. With multislice CT scanning thin collimations have become standard (see below). Table Feed
In conventional CT, a tissue volume is scanned section-by-section. This is accomplished by moving the patient table by a designated amount, called the table feed (table increment), between consecutive scans. Contiguous imaging is generally performed, meaning that the section thickness and table feed are equal. Overlapping Scans
Reducing the table feed produces an overlapping scan pattern that increases the radiation dose to the patient. While overlapping scans have been recommended to improve the 3D imaging of skeletal structures with conventional CT, the introduction of spiral scanning has made them obsolete. Discontinuous
Respiratory
Fig. 1.9 Partial volume effect. Because of the matchstick-like configuration of the voxels, the CT number of a voxel is affected not only by the object of interest (e.g., a round lesion) but also by adjacent structures (e.g., lung parenchyma). This creates an averaging effect that distorts the CT number of the voxel.
Scanning
When the table feed is increased, intersection gaps are produced. This may be useful in selected cases where it is necessary only to search for gross pathologic changes that extend over larger ranges. Misregistration
Respiratory misregistration in CT occurs when structures that move with respiratory excursions are missed due to variations in the depth of respiration between sections. However large the efforts to reproduce a consistent depth of breath holding for 5 to 20 respiratory cycles, some gaps along the z-axis will generally occur. The thinner the collimation and the smaller the lesion, the greater the risk of respiratory misregistration will be (Fig. 1.10).This has only a moderate effect on the detection of hepatic lesions or the evaluation of the kidneys and adrenals, but it can seriously hamper the search for pul-
1 Principles ofCT, Spiral CT, and Multislice CT
.~ 15l· Fig. 1.10 Respiratory misregistration: different depths of breath holding can cause a small lesion (e.g., a pulmonary nodule) to be missed in sequential scans.
monary metastases. To obtain seamless coverage, a section collimation of less than 5mm should be avoided in areas that move with respiratory excursions. This creates conflicting demands in conventional Cl, where it is not possible both to minimize partial volume effects and avoid respiratory misregistration in the same scan. Reconstruction Algorithm (Convolution Kernel) The convolution kernel used in reconstructing an image from the raw data determines the relationship between spatial resolution and image noise. Noise limits contrast resolution, and thus the ability to differentiate objects that show very little attenuation difference from their surroundings. High contrast resolution is important for the detection of lesions in parenchymal organs like the liver and pancreas. High spatial resolution is necessary for the detection of very fine morphologic changes in lung or bone. Highresolution convolution kernels [HR kernels, sharp kernels) improve spatial resolution but disproportionately increase noise as well. Conversely, soft or smooth kernels lead to a concomi-
Image noise Fig. 1.11 The use of hiqher-resolution convolution kernels can improve spatial resolution but leads to a disproportionate increase in image noise.
tant reduction in noise and spatial resolution (Fig. 1.11). Standard kernels are designed as a compromise for good spatial resolution and reasonably low image noise for most body applications. Partial Scan Because attenuation of an x-ray beam is identical in both directions (from the tube to the detector and from the detector to the tube), enough data for reconstructing a cr image can be obtained from less than a full 360° rotation. Instead, data from a partial rotation of 180° plus the fan angle of the x-ray beam (usually some 60°) is sufficient for this purpose. This partial scan or half scan reconstruction can be used to reduce the scan time per CT section and thus reduce motion artifacts. In particular, it is employed for cardiac imaging using electron beam CT or multislice Cl, when temporal resolution is of utmost importance.
VIII
Abbreviations CTSI CUP CVS CVS CWP DAD DCF DEQCT DES DIP DLDCF DLP DORV DPB DRE DSA D-TGA DVT EAA EAP EBCT EBT ECA ECG ECST EHE ELCAP EMLA ERC ERCP ESR FBI FBSS FDG FIGO FLC FNAC FNH FOV FSH FWHM FWTA Gd-MRA GFR
CT severily index carcinoma of unknown primary calcium volume score continuous volume scanning (ECBn coal worker's pneumoconiosis diffuse alveolar damage deep cervical fascia dual-energy QCT diethylstilbestrol desquamative interstitial pneumania deep layer of the deep cervical fascia dose - length product double outlet right ventricle diffuse panbronchiolitis digital rectal examination digital subtraction angiography complete transposition of the great arteries deep venous thrombosis extrinsic allergic alveolitis early arterial phase electron beam CT electron beam tomography external carotid artery electrocardiogram European Carotid Surgery Trial epithelioid hemangioendothelioma Early Lung Cancer Action Project eutectic mixture oflocal anesthesia endoscopic retrograde cholangiography endoscopic retrograde cholangiopancreatography erythrocyte sedimentation rate fat - blood interface failed back surgery syndrome f1uorodeoxyglucose Federation Internationale de Gynecologie et d"Obstetrique fibrolamellar carcinoma fine needle aspiration cytology focal nodular hyperplasia field of view follicle-stimulating hormone full width at half maximum full width at tenth area Gadolinium-enhanced MRA glomerular filtration rate
GI GIP GIST GSD GTD HAE HAP HBV HCA HCC HD HDL HIV HL HMG-CoA HNP Ho:YAG HOCM HRCT HSV HVP ICA ICFT ICU IGCCCG IHE IHSS
gastrointestinal giant ceU interstitial pneumonia gastrointestinal stromal tumor glycogen storage diseases ~estational trophoblastic disease epatic alveolar echinococcosis hepatic arterial phase hepatitis B virus hepatocellular adenoma hepatocellular carcinoma Hodgkin's disease high density lipoproteins human immunodeficiency virus Hodgkins lymphoma hydroxymethylglutaryl coenzyme A
International Germ Cell Cancer Collaborative Croup infantile hemangioendothelioma idiopathic hypertrophic subaortic stenosis
IIAC ILP IMA INR IPF IPH
idiopathic infantile arterial calcification interstitial laser photocoagulation inferior mesenteric artery international normal ratio idiopathic pulmonary fibrosis idiopathic pulmonary hernosiderosis
IPMT IV IVC IVF IVP KUB LA LAA LAD
intraductal papillary mucinous tumor
intravenous inferior vena cava
in vitro fertilization intravenous pyelogram kidney abdomen pelvis (plain abdominal radiograph) left atrium left atrial appendage left anterior descending coronary artery
Scanning Principle Spiral (Helical) CT
During the past decade, spiral (= helical) CT has become the standard technology for the maj ority of clinical indications for CT scanning.
(see Fig. 1.15). The higher the pitch, the lower the radiation dose to the patient and the greater the available range of scan coverage. Advantages
Scanning Principle
Spiral CT requires a scanner with a continuously rotating x-ray tube. A tube with a large heat capacity is needed that can operate continuously for the duration of the scan. More than 100 seconds scan duration are available with current scanners. In contrast to conventional CT, the patient is not scanned section-by-section but is translated through the scan plane at a uniform table speed during acquisition of the raw data (Fig. 1.12). The technique is named spiral or helical CT after the spiral or helical pattern that is traced by the scan. A CT image can be generated from any segment within the scanned volume, so the table feed is unrelated to the site of image reconstruction. Sectional images can be produced at arbitrary levels, and individual images can be overlapped as desired without increasing radiation exposure. The spacing between the reconstructed sections is called the reconstruction interval, increment or index. The section collimation (nominal section thickness) and table feed can be varied independently of each other in spiral CT. The pitch defines the ratio of the table feed (table increment) per gantry rotation to the section collimation
Position of
The advantages of spiral CT arise from continuous data acquisition and short total scanning time. With conventional CT, small lesions such as pulmonary or hepatic metastases may be missed due to respiratory misregistration (see Fig. 1.10). But spiral CT can acquire data in a seamless volume during one breath hold, eliminating respiratory motion and interscan gaps. The use of overlapping reconstruction intervals allows for optimum visualization of small lesions while eliminating partial volume effects. When thin collimation is used, 2 0 reformatted images can be generated in arbitrary planes of section, and 3D reconstructions of good quality can be produced. Owing to the short scan time, most spiral examinations can be performed during a single breath hold. Intravascular contrast medium can be used more effectively, providing either higher contrast or a reduction in the volume of contrast material. Arterial phase imaging has become available only with spiral scanning. These advantages can Significantly improve the detection of hepatic and pancreatic lesions compared with conventional scanning. High vascular contrast is the foundation for CT angiography (CTA), which is a technique that is not available with conventional CT. Volume acquisition and short scan times are utilized in this technique to capture the arterial enhancement and generate angiogram-like vascular images (see Chapter 24).
tube focus Disadvantages
Fig. 1,12
Principle of spiral CT.
Most disadvantages of spiral CT result from the use of older scanners. In these scanners the continuous tube operation necessitates a lower radiation dose per rotation and may lead to a marked increase in image noise. Modern scanners employ an improved tube technology, so noise is no longer a limiting factor. While the scan duration is shorter with spiral CT, there are more sections to be processed. This may increase the time required for image reconstruction, especially when overlapping
11
Abbreviations LAM LAO LAP LCA LCX LDH LDL LH LHA LHV LI LIMA LIP LITI LOCM LPV L-TGA LV MAC MALT MEN MFH MHV MIBC MinIP MIP mIP MLDCF MLI mLV Mn-DPDP MPNST MPR MRA MRCP MRI mRV MS MSAD MTF MTT NASCET NASH NASS Nd:YAG
lymphangioleiomyomatosis left anterior oblique late arterial phase left coronary artery left circumflex [coronaryartery[ lactate dehydrogenase low density lipoproteins luteinizinghormone left hepatic artery left hepatic vein linear interpolation left internal mammary artery lymphocytic interstitial pneumania
laser-induced thermotherapy low-osmolar contrast media left portal vein corrected transposition of the great arteries left ventricle Mycobacterium avium complex mucosa-associated lymphoid tissue
multiple endocrine neoplasia mal~nant fibrous histiocytoma mid e hepatic vein meta-iodobenzylguanidine minimum intensity projection maximum intensity projection minimum intensity projection middle layer of the deep cervical fascia multislice linear interpolation morphologic left ventricle mangafodipir-trisodium malignant peripheral nerve sheath tumor multiplanar reformation magnetic resonance angiography magnetic resonance cholangiopancreatography magnetic resonance imaging morphologic right ventricle masticator space multiple slice average dose modulation transfer function mean transit time
North American Symptomatic Endarterectomy Trial nonalcoholic steatohepatitis North American Spine Society neodymium: yttrium-aluminumgarnet
NHL NIPF n.p.o. nr-MIP NSCLC NSIP OLT OM 00 OP OR PA PACS PAI-l PAN PAP PAPVR PCP PCS PDA PDA PDV PE PEEP PEl PET pi PID PIE PLC
non-Hodgkin"s lymphoma nonspecific interstitial pneumania and fibrosis nothing by mouth (nil per as) noise-reduced MIP non-small cell lung cancer nonspecific interstitial pneumania
progressive massive fibrosis polymethylmethacrylate pharyngeal mucosal space paroxysmal nocturnal hemoglobinuria primary pulmonary hypertension parapharyngeal space parotid space prostate specific antigen prothrombin time percutaenous transluminal angioplasty percutaneous transhepatic cholangiography
IX
1 Principles ofCT, Spiral CT, and Multislice CT sections are used. While this was a time-consuming process with older scanners. new scanners provide fast reconstruction and often require less than one second per image. An increased number of images needs to be viewed. documented and archived. which again may increase time and costs. There is a basic tradeoff in spiral CT between a large scan volume and a high spatial resolution in the z-direction. Short scan ranges. such as the inner ear, can be covered with thin collimation but long ranges, such as in thoracoabdominal CT examinations, require a thicker collimation. The short scan time also complicates the administration of contrast medium, and new types of artifact may occur. A faulty contrast technique can lead to suboptimum examinations or misleading findings (see Figs. 7.39-7.41).
Image •
Reconstruction
Interpolation
Algorithm
and
Section Profile
The table movement during the scan will produce motion artifacts if the raw data acquired during a 360° rotation are used directly for image reconstruction. This is because the first and last projections in the 360° rotation sample different data (due to the table motion during tube rotation). To avoid these artifacts, interpolation of the raw data before image reconstruction is required. The goal of the interpolation is to obtain a complete set (360°) of projections at the desired z-axis position in the scanned volume. The simplest linear interpolation of the projection data is called 360° LJ (Fig. 1.13a). At every angular position of a 360° rotation it interpolates between the two projections in the spiral data set that are closest to the chosen position along the z-axis. This interpolation from 720° of data results in a complete (360°) set of projections for the chosen z-position. The 360° Ll interpolation provides the least image noise but substantially broadens the section profile (Fig.1.l3b). More advanced interpolation algorithms exploit the fact that x-ray attenuation is independent of direction, i.e., the attenuation along a ray between the tube and detector is equal in both directions. This makes it possible to com-
pute a virtual second spiral (conjugated data) for the attenuation values along a ray from the detector to the tube, and to interpolate the projections at corresponding angles between the real and virtual spiral. This algorithm is called 180° LI (Fig. 1.13a) but actually uses data from 360° plus the fan angle of the x-ray beam. The resulting section profile is substantially narrower (Fig.U3b) because the distances between corresponding projections in the real and virtual spirals are less than between corresponding projections in the real spiral alone. Differences between the 360° and 180° interpolations are best appreciated in multiplanar reformations and are most pronounced when pitch factors> 1 are used (Fig. 1.14). The 180° Ll algorithm, however, results in a larger image noise since only half the data are used for interpolation as compared to 360° LI. In fact, the noise with 180° Ll is as high as it would be with a 360° Ll and half the exposure dose. Higher-order interpolation algorithms use not only two points from adjacent (real or virtual) spirals but instead, apply a more complex weighting function (longitudinal filtration, z-filtering) to the spiral projection raw data. This zfilter function defines how much each projection contributes to the final image depending on its distance to the reconstructed section. Such algorithms can be optimized to obtain more rectangular section profiles at the cost of more image noise (e.g., 180° HI). or they can reduce noise (and thus dose requirements) at the expense of a slightly broadened section profile (e.g., HRLF -10, SmartHelical, GE). Effective Section Thickness (Section Width)
While the width of the section profile in conventional CT is equal to the section collimation (nominal section thickness), the bell-shaped section profile in spiral scanning must be described in terms of the effective section thickness or section width (SW). The effective section thickness depends on the section collimation and a number of other factors that include the table feed and interpolation algorithm. The most frequently used measure for the effective section thickness is the full width at half maximum (FWHM), i.e., the width of the profile at one-half ofits peak value. For a pitch of 1 the section width, given as FWHM, is identical to the section collimation if
X Abbreviations PTCA PTLD FIT PV PVOD PVP PVP PVS QCT RA RAO RAS RBF RB-ILD RBV RCA RCC REAL RF RHA RHV RIMA RLD RN ROI RPS RPV RSV RV RVOT SAPHO SBO SC SCCa SCF SCLC SEQCT SFOV SI SLAP SLDCF SLE SLS SMA SMS SMV
percutaenous transluminal coronaryangioplasty post-transplantation lymphoproliferative disorder partial thromboplastin time portal vein puhnonary veno-occlusive disease portal venous phase percutaneous vertebroplasty prevertebral space quantitativeCT right atrium right anterior oblique renal artery stenosis regional blood flow respiratory bronchiolitis associated interstitial lung disease regional blood volume righ t coronary artery renal cell carcinoma Revised European-American Lymphoma (classification) radiofrequency right hepatic artery right hepatic vein right internal mammary artery right lateral decubitus regenerative nodule region of interest retropharyngeal space right portal vein respiratory syncytial virus right ventricle right ventricular outflow tract synovis tis-acn e-pustulosis- hyperostosis-osteitis (syndrome) small bowel obstruction slice collimation squamous cell carcinoma superficial cervical fascia small cell lung cancer single-energy QCT scan field of view sacroiliac
superior labral anterior - posterior (tear) superficial layer of the deep cervical fascia systemic lupus erythematosus sublingual space superior mesenteric
artery
superior
vein
submandibular space mesenteric
SPECT SPIO SSD SSP SIT SVC SVS TACE T-ALL TBC TEE TF TGA THAD THPE TIA TIPS TNB TNM TNMS
single photon emission computed tomography superparamagnatic iron oxide (particles) shaded surface displays section sensitivity profile scaphoid - trapezium - trapezoideum superior vena cava
(staging classification) tubo-ovarian abscess TOA TRUS transrectal ultrasound thyroid stimulating hormone TSH time to peak TIP transurethral resection TUR ultrafastCT UFCT Union Internationale Centre Ie UICC Cancer DIP usual interstitial pneumonia UP) ureteropelvic junction US ultrasound ultrasmall superparamagnatic USPIO iron oxide (particles) V/Q, scanning ventilation-perfusion scintigraphy VATS video-assisted thoracic surgery VOl volume of interest VRT volume rendering technique VS visceral space VSD ventricular septal defect WHO World Health Organization ZEEP zero end-expiratory pressure 13-hCG P-human chorionic gonadotropin
Image Reconstruction
13
Image plane
.r ,
"
,, ,, ,
\'
' '
a
o
4
2 3 Z-position/collimation
a
360' L1:Linear interpolation between projectional data atzundz:» TF 180' L1:Linear interpolation between conjugated data at z" and projectional data atz' + TF
b Fig. 1.14 Quality comparison of coronal reformatted images using 360' LI (a) and 180' LI (b). The examination was performed with 3mm collimation, 6mm table feed, and a 2mm reconstruction increment.
measured spiral data set calculated conjugated data set Image plane
a 180 LI interpolation is used (see Fig.1.l3b). With a 360 LI interpolation, the section width is 28% larger. The same 28% larger section width is obtained if a 180 LI interpolation with a pitch of 2 is used (see Fig. 1.15b). 0
0
270
0
Image Noise
Image interpolation always includes a projection whose position corresponds precisely to the center of the section (Fig. 1.13a). This projection is not interpolated in the algorithms described, whereas its 180 counterpart undergoes maximum interpolation. This leads to a position-dependent discrepancy of spatial resolution and noise in the scan plane, as image areas near the tube show slightly better sharpness due to the lack of interpolation while opposite image areas show a reduction in noise (see Fig. 7.48). Newer interpolation algorithms (e.g., 180 adaptive interpolation) can correct for these discrepancies.
90
0
z"
3
4
Z-position/collimation
b % 100
"
- -,
, , ,
80 60
1800 LI 3600 LI
r'
u:
" "" ",
"
;, "
,.
_ 1__
"
" " " ,r , ,'' , , , ,
40 20
Pitch
p";l
0
, , , "
r
-5.0
, "" "
" ", , , ,
, , 0 -10
Collimation SC=Smm
,, ,'' , ,'
Standard CT ~ ,
.A
0
~5.010.0 Table position (mm)
Fig. 1.13 Principle of raw data interpolation, (a) Conventional diagram, (b) angluar diagram, (c) comparison of section profiles for 360' and 180' linear interpolation (L1).
XI Contents
Principles of cr, Spiral cr, and Multislice cr Image Processing and Display Techniques
p.45
Patient Preparation
p.83
Optimization
and Contrast Media
of Scanning Technique
p. 131
cr-Guided
p. 161
Interventional
Procedures
Image Analysis
p.187
Neck
p.223
Mediastinum,
system
Pleura, and Chest Wall
p.279 p.373
The Liver
p.405
The Biliary Tract
p.477
The Spleen
p.497
The Pancreas
p.513
The Gastrointestinal
Tract
p.541
Peritoneal Cavity and Retroperitoneum
p.595
Adrenal Glands
p.625
The Kidneys
p.639
Urinary Bladder
p.683
Female Pelvis
p.699
Male Pelvis
p. 725
Lymphatic System
p. 741
Heart
p. 759
Vascular System
p.825
Musculosceletal References Index
'.
p.109
Radiation Dose and Image Quality
Lungs and Tracheobronchial
4'"
p.1
System
p.929 p.998 p. 1052
1 Principles ofCT, Spiral CT, and Muitislice CT As mentioned above, a comparison of 360 LI and 180 LI interpolation shows that 360 LI yields a 28% larger section width (less spatial resolution along the z-axis) and a 29% reduced image noise, Variants such as SmartHelical (GE) lead to a 10% wider section profile than 180 LI but only suffer from 8-16% less noise (depending on the pitch), 0
Acquisition
Section Collimation
0
0
Scanning Parameters
The variable scan parameters in spiral CT examinations are reviewed in Table L4. Three basic parameters for spiral CT scanning are userselectable, In most scanners these are section collimation (SC), table feed per rotation (TF), and reconstruction interval (RI), In some scanners the table feed TF is substituted by the pitch factor P. All the other parameters are varied only in exceptional cases, For this reason, the basic scanning parameters provide an excellent idea of how a spiral CT scan was performed, In this book, we use a triplet of numbers {SCITFlRlj to describe these basic parameters, Section collimation, table feed and pitch are the most important acquisition parameters, while the reconstruction increment is the most important parameter for image reconstruction.
Table
1.4
Scan
parameters
Acquisition
parameters
SC
=
collimation
TF
=·Table
P P*
81.
=
Slice
Scan
RI
=
Reconstruction
width
Table Feed and Pitch
nominal
section
thickness
=N X PX SC=P' X SC = TF /[N x SC] = beam pitch = TF /S C = N X P slicepitch 'TI XTS= TI x N X PX SCIRT e
(mm) increment
(mm)
=
effective
=
reconstruction
section
thickness interval
parameters
=
Table
=
Scan time
"Equipment
•
The table feed per tube rotation (TF), also called the table increment, can be selected independently of the section collimation in spiral scanning, Pitch (P) is defined as the ratio ofthe table feed per tube rotation to the collimation (Fig, US), The following rules should be noted, Scanning at a pitch less than 1 produces an overlapping scan pattern that increases the radiation dose to the patient Overlapping scans may offer slight advantages for the 3D reconstruction of contours that are roughly parallel to the scan plane (e.g, the calvaria), but there are
parameters
Section
Derivative
The section collimation (SC) determines the spatial resolution that is achieved along the zaxis (the direction of table travel), SC can be varied in fixed increments, depending on the scanner. Usually the manufacturer predefines these increments, but with some scanner units they can be modified at installation, The following settings can be recommended as a good compromise between clinical requirements and number of collimation settings: SC=O,Smm, 1 mm, 2mm, 3mm, Smm, 7 mm, or 10mm,
CT
o
(mm)
(em)
=
=
rotation
pitch
length
Reconstruction
RT
and multislice
Pitch
-Volume =
i1 spiral
(mm)
feedltube
SW
N
Parameters
0
speed
(mm/s)
=
(s)
IF/RI
=
scan ciJration
=
duration
=
L1TS
=
(L X RD/(N
X P x S
parameters
Rotation
"Number
time
(s)
of detector
rows
Single
slice:
of one tube N
=
rotation
1; dual slice:
N
=
2; multislice:
Scanning Parameters 15
Pitch=2
Pitch=l
-SC
"
,I r
Pitch= 1 TF=Smm
j
\
/
Pitch = 2 TF= 10 mm
,
\
I I
80
I'
"
"
40
1\
1\ 1\ \
,, ,' I
\
, ,
20
,, ,,
:
o
-S.O
S.O
"
" ,_.• - -, .!I-
_..•.
.
,
I
1\
"
Table feed TF=Smm
'
" "
. - ....•
I,
"
TF
Pitch = 1 ----"--' : " SC=Smm ::
60 1180 LlI----
o~~~~~-,--~~~~-,--10
,
, ,
\'
-l-·-···_-\,
% 00
"\'
f
"
" I
se
Collimation SC=Smm
I
',
rf
60
20
I
••..1 I...•• 14-~
TF
SC
I
40
Pitch=2
--
TF
% 100 80
Pitch-'l
Pitch =2 SC= 2.Smm
J
:
"
"" ,, '' "
, ',
O~-r~/r'~,-,,-,~,"~,--,-10
-10
-S.O
o
S.O
10
Fig. 1.15 Increasing the pitch while keeping the section collimation SC constant has the effect of stretching outthe spiral (a). This widens the section profile (b). The resulting profile with 180 LI at pitch 2 is identical to the section profile with 360 LI at pitch 1 (compare Fig. 1.13 c). Thus, using a pitch of2 instead of 1 will yield
an only 30% wider section width but cover twice the scan length. Increasing the pitch by reducing the section collimation SC covers the same scan length (c). With 180 LI, this leads to a 35% narrower section width than with a pitch of 1 (d).
very few indications in which the minimal improvement of image quality justifies the increased exposure. Moreover, there is little if any effect on image noise, and the higher radiation dose essentially goes unutilized. The only exceptions are scanners with dual detectors (see below, p. 20), which have special z- filtering algorithms that can utilize the higher applied dose to improve image quality (useful in examinations of the intervertebral disks, for example). If the pitch is greater than 2, the volume of interest will be under-sampled, leading inevitably to artifacts. This may be acceptable in some instances, such as trauma cases where priority is placed on the rapid assessment of large volumes rather than on high image quality. But as a general rule, it is better to avoid scanning at pitch values greater than 2. One exception is scanning with very thin collimation, where even conventional CT sections have a bell-shaped profile. Due to broadening of the section profile,
a pitch as high as 3 can be used without causing serious artifacts if the collimation is 1 mm or less (Fig. 1.16).
0
0
0
•
Effective Section Thickness and Pitch
The most Widely used interpolation algorithms at present (180 U) do not increase the section width (effective section thickness) over the section collimation when a pitch of 1 is used (Fig. 1.15 b). Increasing the pitch to 2 reduces the radiation dose by half and doubles the scan length but does not increase the effective section thickness to a proportionate degree. The section profile obtained with 180 LI and a pitch of 2 equals the profile obtained with 360 LI and a pitch of 1, i.e., the scan length is increased and radiation exposure reduced, each by a factor of 2, with identical resolution along the z-axis (Fig.USb). 0
0
0
16
1 Principles oICT, Spiral CT, and Multislice CT reduces the radiation dose but also keeps the effective section thickness as small as possible. A pitch of 2 would be most favorable on theoretical grounds but is more vulnerable to motion and spiral artifacts. A pitch between 1.5 and 2 is the most reasonable tradeoff for most clinical investigations. Rotation Time Fig. 1.16 For narrow collimation, the pitch can be increased to 3 without markedly compromising image quality. Note the excellent quality of multiplanar reformations with a SCITF /RI = 1/3/1 protocol for visualization of subsegmental pulmonary emboli (arrow). Note that the vertebral end plates are well delineated but suffer from minor horizontal interpolation artifacts (arrow head).
Increasing the pitch may produce an increase or decrease in the section width. If the section collimation is kept constant while the table feed is increased, the section width increases (Figs. 1.15a, b). Conversely, the section width is reduced if the section collimation is decreased while the table feed remains unchanged (Figs. LISc, d). In clinical practice, the length L of the scan range and the available scan duration Tl (usually a maximum of 30 seconds for breath-hold scanning) determine the selection of the table feed (see formula in Table 1.4). In this situation it is generally best to use a pitch greater than 1 to maximize the spatial resolution along the z-axis for a given scan length (Table 1.5). This not only
Table 1.5 Effect of pitch on z-axis resolution (effective slice thickness) for a constant table feed
Scan parameters Pitch SC TF
1800 LI
3600 LI
3mm 4mm Smm 6mm
3.9mm 4.6mm 5.3mm 6.0mm
6.9mm
sc=
6mm 6mm 6mm 6mm
2.0 1.5 1.2 1.0
Section width (FWHlvlp
7.2mm 7.5mm 7.8mm
slice collimation, TF = table feed, FWTA full , width of section profile at half maximum Eo
The rotation time (RT) of the x-ray tube (duration of one revolution) is 1 second in most scanners but can vary from 0.42 to 2 seconds, depending on the scanner type. The table speed (TS), or scan length per unit time, can be calculated by dividing the table feed TF by the rotation time (see formula in Table 1.4). The IS is one-third higher in 0.75 second scanners and one-third lower in 1.5second scanners than in standard 1 second scanners. With a shorter rotation time, the z-axis coverage within a given scan time can be increased. All scanning protocols (basic parameters) for spiral CT presented in this book are based on scanners with a rotation period of 1 second. To achieve the same coverage with a slower scanner, the scan time must be increased by the factor RT compared with a 1 second scanner. This is feasible only in regions that do not show respiratory motion (usually skeletal examinations). In all other cases, slower scanners require wider section collimation. Conversely, faster scanners can apply narrower collimation. Because of the detector properties in some scanners, the number of projections available for image reconstruction must be reduced at the shortest rotation time available with these scanners. This leads to a degradation of image quality (loss of sharpness) in the scan plane. If the rotation time is user-selectable (not available in all scanners), it is advantageous to select a longer rotation time for studies that require higher in-plane spatial resolution (e.g., high resolution lung or skeletal examinations). Scan Time
The maximum scan time (scan duration) available in a spiral scanner depends on the x-ray
Image Review, Display and Documentation tube. The higher the dose requirement. the shorter the available scan time will be. Old scanners were limited to a scan time of 24 seconds. but curren t units can achieve scan times up to lOOseconds. In practice, scan times longer than 30 seconds are feasible only for body regions that can be examined without breath holding-the neck, the musculoskeletal system, and in some cases the pelvis. Thoracic and abdominal examinations generally require breath holding to ensure optimum image quality. In some regions (e.g., the lower abdomen), scanning can proceed while the patient slowly exhales after holding his or her breath and continues breathing shallowly. This will cause no substantial loss of quality and will enable scan times longer than 30 seconds.
Reconstruction •
Parameters
Raw Data Interpolation
Most modern scanners use the 180 LI raw data interpolation algorithm, but there are variants available among different units (e.g., SmartHelical on GE scanners, slim2 on Siemens scanners). Fourth-generation scanners (Marconi/Philips) use an interpolation (z-filtering) in which the effective section thickness is largely independent of the pitch (for a constant collimation), but image noise increases with the pitch. This differs from standard interpolations (360 LI. 180 LI), in which image noise is independent of pitch for a given collimation. 0
0
0
Reconstruction Algorithm (Convolution KerneO
The convolution kernels in spiral CT are -no different from those in conventional CT (see above).
The reconstruction increment has nothing to do with section collimation or section width. Rather, it defines the degree of overlap between the axial sections. For the detection of small structures (e.g., pulmonary nodules) it is important to choose Rl so that at least a 30% section overlap is generated. For optimum multiplanar reformations or 3D reconstructions, the reconstruction increment should not exceed half the section width (effective section thickness). This provides a 50% degree of section overlap and generates more than twice as many CT images as conventional scanning. Non-overlapping reconstruction wastes an important advantage of spiral CT, and small lesions located at the boundary between two sections may be missed due to partial volume effects. Overlapping reconstruction ensures optimum lesion display. The theoretic optimum for RI is even smaller than half the section width (see Chapter 4, p. 124), but the added value in clinical practice is generally low.
Image
Review,
cr
Axial
Increment
One of the main advantages of spiral scanning is the continuous acquisition of data, which makes itpossible to generate axial CTsections retrospectivelyat arbitrary z-positions within the scanned volume. The reconstruction increment (RI) defines the spacing of the reconstructed images.
Images
Spiral CT images differ very little in appearance from conventional CT sections obtained with a similar radiation exposure, an identical convolution kernel, and an identical number ofprojections. This is because the raw data interpolation eliminates image artifacts, and the spatial resolution in the axial scan plane corresponds to the spatial resolution of conventional CT images. Compared with conventional CT, however, the image noise for the same scan dose may be 18% lower (360 LJ) or 15% higher (180 LJ), depending on the type of interpolation algorithm used. Spiral CT with a small reconstruction increment generates a large number of images. For example, spiral CTofthe abdomen with 1 s rotation time, a table feed of8 mm, a scan time of 30 seconds, and image reconstruction at 2 mm intervals (as in CT angiography) will generate (30s x 8mm/s/2mm=) 120 images. This large number of images cannot be conveniently documented and reviewed with traditional methods. 0
Reconstruction
Display and
Documentation
0
Principles «ct. Spiral CT, and Muitislice CT •
Cine Mode and Image Documentation
As a rule, not all images can be documented on film One solution is to document every second to fourth image on film, depending on the type of examination, Abnormalities must be adequately documented, however, and a suspicious or abnormal finding must be reproducible, For this reason, selected overlapping sections or multiplanar reformations (MPRs) should also be documented if required, Image quality (noise) can be improved by retrospectively increasing the section width (approximately 7-10mm) for documentation instead of using the original sections, In some scanners, this can be done directly from the raw data by fusing projections from multiple tube rotations (similarly to multislice CT), These thicker sections reduce noise, which is particularly beneficial in scans taken with a low dose or a thin collimation, If this option is not available, thicker sections can be produced by averaging the data from several overlapping images (e.g, using thick axial reformations;' This is usually too time-consuming
for routine
examinations
but may be appropriate in selected cases where image noise is excessive, It is, however, the preferred technique for image documentation in almost all multislice CT applications,
If not all the reconstructed images are printed, all available images should be viewed in an interactive cine display so that an accurate evaluation can be made, This mode displays the individual images in the scanned volume on the monitor in a movie-like sequence, providing a three-dimensional impression of complex, nonlinear structures that have multiple intersections with the image plane, The radiologist can control the speed and direction of the image display sequence interactively by manipulating a computer mouse or trackball, Image Processing
A data set composed of overlapping axial spiral CT images is well suited for generating multiplanar reformatted images (MPRs) in arbitrary sectional planes through the scanned volume (see pp,48), In cases with equivocal findings, MPRs can assist the diagnosis by providing information in an extra viewing plane, When thin sections have been used, reformatted images of good quality can be produced, Three-dimensional images (see pp.Gtl) of the skeleton or vascular system improve anatomic orientation and can enhance the presentation of findings to referring colleagues,
Multislice CT
Multislice CT, or multidetector-row CT, multidetector CT, or volume CT, as it is also called, represents the next breakthrough in CT technology, Multislice CT scanners provide a huge gain in performance that can be used to reduce scan time, reduce section collimation, or to increase scan length substantially, Multislice CT transforms CT from a trans axial into a truly three-dimensional imaging technique, Multislice CT has gained a rapid acceptance by the radiological community, There is an almost exponential growth of the number of scanners: in 1998 there were 10 scanners installed, in 1999it was 100by the middle of the year, and by the end of 2000, over 1000 scanners were installed worldwide,
Scanning Principle
Unlike standard systems that use a single detector arc or detector ring, multislice CT systems are equipped with two or more parallel detector arrays and always utilize a third-generation technology with synchronously rotating tube and detector array as well as solid state detectors (Fig, 1.17),Although the very first CT scanners in the 1970s also employed a split detector, the first dual or split detector systems with continuously rotating tubes becarue available only in the early 1990s, Systems with four active detector arrays were introduced in 1998, and systems with 8, 10 or 16 active detector arrays are now available, The performance of many of these systems is further improved by a faster rotation time, As a result, a four-detector-row scanner with 0,5 s
Scanning Principle 19 Table 1.6 Advantages of higher scanner performance with multislice CT or subsecond spiral CT Shorter scan duration Reduced motion artifacts: • Children • Trauma patients • Acutely ill patients Improved scanning of parenchyma! organs • Well-defined phase of contrast enhancement Reduced volume of contrast medium Perfusion imaging Longer scan ranges Trauma
II»
Thoraco-abdominal
spine/blunt trauma
CT angiography: • Aorta and peripheral run-off • Thoraco-abdominal aorta • Carotids from arch to intracerebral circulation Thinner sections Fig. 1.17 Principle of multislice CT scanning. More than two detector rows are exposed simultaneously.
rotation has an about 8 times higher performance than a conventional 1 s single-detectorrow scanner. Multislice CT does not always involve spiral data acquisition, although spiral acquisition is implied in this book unless stated otherwise. A sequential mode (step-by-step scanning) as in conventional CT with possibilities of image fusion is also available, although it will be used only in a minority of applications, such as HRCT of the lungs or interventional procedures. Advantages
The performance of multislice CT is at least 4 times, with modern scanners 8-20 times higher than that of a conventional spiral CT scanner. This enormous increase in performance can be used for shorter scan duration, longer scan ranges, and thinner sections (Table 1.6). In general, the performance gain will be used to improve all of the above to a substantial amount. In consequence, multislice CT has overcome one of the most severe limitations of spiral CT, namely the inverse relation between scanning range and section collimation.
IMear-isotropic imaging: • • • • •
Temporal bone imaging Musculoskeletal imaging Arbitrary imaging planes Multiplanar reformats 3D rendering
Shorter scan duration will reduce the danger of motion artifacts, especially in children or critically ill patients. Marked improvements can be seen for trauma patients or in dyspneic individuals with suspected pulmonary embolism. Shorter scan duration also will allow for scanning of the liver or other parenchymal organs in a more well-defined phase of contrast enhancement, which again improves lesion detection and characterization. Shorter scans will also allow for substantial reduction of the volume of contrast material to be administered as long as only an arterial phase of contrast enhancement is required. Longer scan ranges are especially important for CT angiography. CTA-examinations become feasible that include the abdominal aorta and the peripheral run-off vessels down to the feet. Alternatively, the whole aorta can be scanned with high spatial resolution, or the carotids can be examined form the aortic arch to the intracranial circulation. Long scan ranges such as in thoraco-abdominal studies no longer pose a
1 Principles «ct. Spiral CT, and Multislice cr problem, even for indications that require a high spatial resolution. Finally, thinner sections and even isotropic imaging will become feasible yielding almost identical spatial resolution in all directions. This makes near-isotropic multiplanar imaging possible with a spatial resolution in any arbitrary plane that equals, or often exceeds, the resolutionofMRI. Disadvantages
The downside is a markedly increased data load, especially if near-isotropic imaging is performed. A scan of the chest and abdomen (60cm) can be performed with a 4 x 1 mm collimation in 50s and produces up to 500-800 images, depending on the degree of overlap. A chest scan with an identical collimation will produce as many images if reconstructed in a smoothing convolution kernel for the mediastinum and an edge-enhancing kernel for the lungs. ACTA of the aorta and peripheral arteries may produce 1000 images and more. The only way to avoid this enormous amount of data is to (acquire and) reconstruct thicker sections. This, however, will forgo many of the advantages of multi slice CT. Current scanning protocols frequently are modified versions of standard spiral CT protocols with a somewhat thinner section collimation. Most present scanners and workstations can easily handle the data created by such protocols. However, if near-isotropic imaging is performed, image reconstruction speed and data handling become limiting factors for some of the present scanners and workstations and make this technique cumbersome for clinical routine use. There is not yet an in-depth experience with near-isotropic imaging, nor do standardized protocols exist. This book suggests protocols optimized for clinical routine work as well as specialized protocols for more advanced applications and multiplanar imaging. Image processing adds to the increased time requirements for data analysis if a full-resolution scan is performed. No standardized protocols are available yet but manufacturers are working on techniques to include 3D processing in the standard workflow. Image noise grows as section collimation is reduced. For this reason, it is important to reconstruct thicker sections (MPR or axial sections) to keep image noise low. With very thin
collimation the geometric efficiency of the detector deteriorates (see Fig. 5.5). It varies between manufacturers and depends on scanner geometry, the implementation of beam collimation and image interpolation algorithms. With 16-slice scanners, the geometric efficiency approaches that of single slice CT. An increase in patient dose is only necessary if thin-section images of high quality are required. In all other cases, multislice CT requires less dose than conventional CT or a similar dose as spiral CTwith a pitch of2. Detector Types Dual Detector Systems
Dual or split detector systems are based on a detector array that is twice as wide as a conventional CT detector and is split in half (Fig. 1.18). Elscint was the first company to use this concept on a spiral CTsystem (ElscintTwin). Now, most vendors offer such dual-slice systems. Multidetector-row
Systems
True multidetector-row or multislice systems are able to acquire at least four simultaneous sections. To be able to choose between various section collimations, the detector arrays have to be subdivided into multiple detector rows. Data from each of the scanned sections is recorded by a data acquisition system (DAS), which consists of one detector row or a combination of detector rows, depending on the chosen section collimation (Figs. 1.19-1.21).The basic types of detectors available are described below. The width of each detector row is not given as its real width but instead as the width of the corresponding xray beam at the center of the scan field. Matrix Detector
Matrix detectors consist of multiple detector rows of identical width (Fig.1.l9a). The GE matrix detector is a typical example. It uses 16 parallel detector arrays with a width of 1.25mm each. One has to note that the detector width mentioned in this text (as well as most publications) is not the "real" width of the detector elements but the width of the detector-row beam
Detector Types
~~'.~.x-ray tube ..~.~~ Beam collimators
. X-ray beam ... Central trajectory
I I I I I Single-slice
Fig. 1.18 Comparison
of single and dual (split) detector systems.
collimation, measured in the center of the scan field. The real width of the elements of the x-ray detector is about twice as large, depending on the scanner geometry and the distance from the center of the scan field to the actual detectors. By exposing only the innermost four detector rows, a 4 x 1.25 mm collimation can be produced. For wider collimator settings the signals of groups of two or more adjacent detector rows have to be added to obtain 4 x 2.5 mm, 4x3.75 mm and 4x5 mm collimations (Fig. 1.19b). By partially covering the innermost two detector rows, also 2 x 0.63 mm sections can be generated. The same principle of grouping adjacent arrays holds true with 8-detector-row scanners, which will allow for 8 x 1.25 mm and 8x2.5 mm section collimation (Fig.1.l9c). The total collimation (e.g., 8 x 1.25 mm = 10 mm, or 8x2.5 rnrn Zfl mm) is limited by the total width of the detector array. e
•
Adaptive
Multislice
Dual-slice
Array Detector
Adaptive array detectors consist of detector rows that grow in width from the center of the section to the periphery (Fig.1.20a). Philips (Asymmetrix detector) and Siemens (AAD) have adopted this concept and use the same detector in their scanners. By collimating half of the two innermost 1 mm-wide detector arrays of 4-detector -row scanners, a section collimation of 2 x 0.5 mm can be obtained. By partially collimating the
next arrays, 4 x 1 mm sections can be gained. Adding the innermost two detector rows will yield 4 x 2.5 mm collimation (Fig. 1.20b). The rationale behind this approach is based on the fact that the x-ray beam hits the more peripheral detectors at an oblique angle. Any perpendicular septa in the more peripheral portions of the detector will block some of these oblique rays and will thus decrease the detection efficiency. Reducing the number of septa in the peripheral regions of the detector array should thus increase geometric dose efficiency. Indeed, the detector has a dose efficiency that is only slightly less than that of a single-slice scanner, if a section collimation s2.5mm is used (for more detailed discussion on geometric efficiency see Chapter 5 and Fig. 5.5). The detector geometry is optimized only for a particular number of active detector rows. A new detector configuration is needed as the number of detector rows progresses from 4 to 16 (Fig. 1.20 c). With more rows, also more septa have to be added. Hybrid
Detectors
Hybrid detectors are similar to matrix detectors with the exception that the innermost detector rows are thinner than the outer ones (Fig. 1.21a). This concept is embraced by Toshiba (selectable slice-thickness multirow detector, SSMD). These hybrid detectors are based on a detector array of a total width of 32 mm that con-
21
22
1 Principles of CT, Spiral CT, and Mu/tislice CT _16x1.25mm
__ 5mm
1.5mm 1 2.5mm :1 ':l.m
mm 2.5mm
.
Srnm
Center of scan plane ___
20mm_
20mm
2xO.63mm 4xl.25mm 4x2.5mm 4x3.75mm 4x5mm
2xlOmm
8xl.25mm 8x2.5mm 16xO.63mm 16xl.25mm Fig. 1.19 Matrix detectors consist of detector rows of identical widths. For the GE HiLight detector this width is chosen so that each detector row covers a 1.25 mm-thick section in the center of the scan field (a). Collimating the x-ray beam, or combining the signal from adjacent detector rows, or both, will yield various section collimations (b). An 8-row scanner provides 8 x 1.25 mm and 8 x 2.5 mm collimation. The 16-row scanner uses a hybrid detector with 16 x 0.63 mm and 16 x 1.25 mm collimation (c).
sists of 1 mm-wide detector rows, With 4-detector-row scanners the innermost 2 mm of this array are further subdivided into four 0.5 mm wide detector rows. With this detector configuration there is a multitude of possible collimations, ranging from 4 x 05 mm, 4 x 1 mm to
2xO.5mm
4x1 mm
4x2.5mm 4x5mm
2x8mm
•
Mll
II
i ••.
iiWIiI
1111t.tIIIIII!l%~
16xO.75mm 16x1.5mm
8x3mm
11111,1,llllllh
11111111111111111
Fig. 1.20 Adaptive array detectors have outer detector rows that are wider than the innermost detector rows (a). The detector used by Philips and Siemens covers a total section width of20 mm and allows for section collimations ranging from 2 x 0.5 mm to 4 x 5 mm (b). A new detector configuration is chosen for the new 16row scanner, which will be ableto provide 16 X 0.75 mm to 16 x 1.5 mm collimation (c). Philips will additionally offer a 8 x 3 mm collimation.
4 x 8 mm (Fig. 1.21b; Table 1.7). With 8- and 16detector-row scanners the innermost 8 or 16 rows of the 32 mm-wide detector array have a width of 05 mm (Fig. 1.21c). The Toshiba detector uses dividers between the various detector rows that are opaque to the
Detector Types
~-
4xO.5mm
15xl mm
15xl mm--
Fig. 1.21 Hybrid detectors are matrix detectors with thinner innermost detector rows (a). Toshiba uses a detector that covers 32 mm and allows for 4 x 0.5 mm to 4 x 8 mm collimation (b). The new detector configuration for a 16-row scanner will be able to provide 16 x 0.5 mm to 16 X 2 mm collimation (c).
32mm
scintillation light produced by each detector element (thus avoiding cross-talk) but which are transparent to the incident radiation. This effect
Cone Beam Systems (Volumetric
increases dose efficiency but also makes the de-
on cone beam technology in which a wide x-ray beam is used to cover a large range along the zaxis. Prototypes by various vendors include a
tector slightly more vulnerable radiation. The 16-detector-row
to scattered
CT)
Future generations of CT scanners will be based
scanners of all other
2S6-row detector and a 1024' flat panel detec-
vendors (Figs. 1.19c and 1.20c) also use thinner
tor. Such scanners may provide enormous spatial resolution and high-speed data acquisition.
innermost detector rows and therefore
be considered hybrid detectors.
should
Cone beam systems that are already on the market are reserved for rotational
angiography.
23
•
1 Principles ofCT, Spiral CT, and Multislice CT They use image intensifiers or (CCD) flat panel detectors as the radiation receptor and require a num ber of seconds for acquiring one (single-rotation) data set. The present systems are only able to depict high-contrast structures such as vessels during intra-arterial contrast injection. The reason is the large amount of scattered radiation that is not blocked from hitting the detector-as opposed to multislice CT where the relatively narrow beam and the various septa between the detector rows achieve adequate scatter suppression. This scattered radiation destroys low-contrast resolution if no adequate scatter-reduction techniques are developed. Unless this major obstacle is solved, true cone beam CTwill not become clinically feasible. Another major problem with cone beam detectors is the bandwidth of the signal transmission system that has to transfer the various detector signals out of the gantry to the image reconstruction computer. The data throughput in these systems will become enormous and will grow by a factor of 64 relative to a 4-detector row scanner if 256-detector rows are employed. At present there is no system that can transmit and process the acquired data with sufficient speed. The third and biggest problem is image noise, which will increase substantially. Image noise for isotropic resolution with voxels of a size of x3 is theoretically proportional to IIx'. This means that image noise will grow by a factor of 2*~ 16 if the size of an isotropic voxel is reduced from (Irnm}' to (0.5mm)3, and by 10,000 if the voxel size is decreased to (0.1 mm)3 As a consequence, sophisticated noise suppression and data reconstruction techniques will be necessary to create clinically useful images. For the reasons mentioned above, it will be several years until cone beam scanners become available for clinical use.
1a'~
System Performance The performance of multislice CT scanners increases with the number of active detector rows, the speed of x-ray tube rotation, and with the available pitch factor. This increased performance can be used to reduce scan time, reduce section collimation, or to increase scan length (Table 1.6). At the same time, increase in
performance has to be counterbalanced by the amount of artifact induced and by increased image noise with thinner sections. Rotation Speed
The rotation speed of the x-ray tube is commonly described by the time required for one tube revolution, the so-called rotation time RT. All multislice scanners have a rotation time of 0.8 seconds or less. Most high-end scanners even allow for a 0.5 s rotation. With the newest generation of scanners, rotation times of 0.42 s are available. Pitch
There are two definitions ofpirdi factor available with multislice CT scanners, depending on whether a single section collimationSC or the total cOiKmatwnofthedetectorarray(N x SC)ischosen as the reference. To distinguish between them, an asterisk is used to indicate the definition used by most manufactures for 4-slice scanners (volume pitch P*, also called sh'ce pitch or detector pitch), while P denotes the definition agreed upon internationally (lEC) and preferred by most physicists (pitch P), also called beam pitch:
P=TFI(NxSCj
P*~TF/SC
Pitch Volume pitch
While P is independent of the number of detector rows, P* increases as the number of detector rows grows. As with single-slice spiral CT, the pitch P can be increased up to 2, independent of the number of detector rows N. In multislice scanners with four active detector rows this corresponds to P*=8. Some manufacturers have limited the maximum pitch P to 1.5 (corresponds to P* = 6 for four-slice scanners). Relative System
Performance
System performance is proportional to the number of detector rows N and improves with a shorter rotation time RT of the x-ray tube. This concept can be applied to standard spiral CT as well as to dual detector scanners or multislice CT. Table 1.7 gives an overview of the relative performance of various scanner types and scanning protocols. It has to be noted that the quality of axial images may deteriorate when the highest possible pitch factors are chosen.
Image Reconstruction Table1.7 Examples of system performance relative to a 1 sec single-slice spiral CT scanner with pitch System
1s 1s 0.8s 0.5 s 1s 0.5s 0.8s 0.8s 0.5 s 0.5 s 0.5 s 0.42s
Image Reconstruction
Image reconstruction in 2- and 4-detector-row (T systems does not have to use sophisticated interpolation if a section-by-section mode is used, but even then, the cone beam geometry of the system will lead to artifacts for regions outside the center of the gantry. With spiral acquisition, the situation is becoming more complex because the trajectories of the various detector rows may overlap and yield "redundant" data, depending on the pitch factor employed. For this reason, more sophisticated raw data interpolation schemes have to be used. •
The Cone Beam Problem
Due to the nature of the x-ray beam, which arises from a small focal spot, all (T scanning relies on cone beam geometry. With single-slice CTthese effects are hardly visible but they play a major role in multislice scanning because the same structure may be depicted on different detector rows during one revolution of the x-ray tube (Fig.1.22a). Only in the center of rotation are structures always captured by identical detector rows. The effect becomes more prominent the further a structure is from the isocenter (rotational axis), and the more detector rows are used (Fig. 122c). Simple raw data interpolation algorithms make the assumption that all beams are parallel (Fig.1.22 b), which yields reasonable results up to four active detector rows but fails with a higher number of rows. While adaptation for
Detector rows (N)
Pitch P
Pitch p.
=
2
Rei. performance
0.5 2 2 2 2 2 0.75 1.5 1.5 2 1.35
1 2 2 4 4 4 4 8 16
2 2 2 4 4 3 6 6 8 10.8 32
2
1.25 2 2 4 1.875 3.75 6 8 10.8 38
thin section and 8-slice scanners may still be possible, 16-slice scanners require more sophisticated cone beam algorithms for image reconstruction. •
Multislice Linear Interpolation
Algorithms for 4-slice systems that are analogous to 180 Ll and 360 LI from spiral (T are called 180 MLI (multislice linear interpolation) and 360 MLi (Fig. 1.23). For each projection angle, they use the projection data from the two detectors that are closest to the scanning pane (3600 MLI only real trajectories, 1800 MLI also virtual trajectories from the detector to the x-ray tube). Section profiles with these algorithms vary between those from conventional 180 Ll and 360 LI spiral (T interpolation, but the dependence on the pitch factor is more complex because of the sampling patterns described below. It is difficult to represent these effects using the standard diagrams for single-slice spiral (T. A diagram that displays the rotational angle of the x-ray tube for varying z-positions of each DASchannel (active detector row) is more suitable for this purpose (Fig. 123 b, compare also Fig. 113a). 0
0
0
0
0
•
0
The Sampling Problem
Depending on the pitch factor, the spiral trajectory of the first detector row of a 4-slice scanner may overlap with that of the second, third or fourth row for P* = 1, 2, and 3, respectively (Fig. 1.24). For a pitch of 0.5 (P* = 2), even the
25
•
7 Principles aICT, Spiral CT, and Multislice CT
Assumption for reconstruction: parallel rays
a
FOV z-axis
c
4 detector rows
8 detector rows
Fig. 1.22 Cone beam geometry. Peripheral structures will be seen by different detector rows during one revolution of the x-ray tube (a). Simple raw data interpolation algorithms make the assumption that all beams are parallel like in convention spiral CT scanning (b). Such algorithms will fail with more than four detector rows be-
cause the cone beam effect will become more prominent when more detector rows are used (c). The distance ASC traveled by the outermost rays within the field-of-view (FOV) becomes larger than the section collimation SC and will cause artifacts in standard reconstructions.
conjugate data from the virtual spiral overlap the real spiral at the isocenter (Fig. 1.24b). This makes data sampling particularly inefficient and only allows for an interpolation between data samples that are one section collimation apart. As a consequence, the section width is widened Similar to a 360 LI interpolation. Because of
varying degrees of overlap with changing pitch factors, the sampling density and thus the section width varies (Fig. 1.25). In principle this sampling problem applies also to 8- and 16-slice scanners but, because of the cone beam effect, the projection rays for the overlapping detector rows are not completely
0
Image Reconstruction Fig. 1.23 Multislice linear interpolation for a 4-slice scanner and a pitch P = 1.5 (volume pitch P* =6). The conventional representation (a) and the angular diagram (b) are equivalent but the angular diagram provides a better overview. Note that the trajectories of the original spiral (detector row 4) and the conjugated data (from detector row 1 to x-ray source) overlap.
lrnaqeplane
3
2
a
5
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Z-position/collimation Image plane 360
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Fig. 1.24 Overlapping data sampling occurs when trajectory of one detector row superimposes another, e.g. for a pitch of 0.75 (P* = 3) (a). In the center of rotation, even the conjugated data from
b the on isothe
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=4
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6
virtual spiral overlap the real spiral trajectory for a pitch of 0.5 (P* =2) (b). Note, however, that the cone beam geometry ameliorates this effect, especially for image points farther away from the isocenter.
27
28
1 Principles ofCT, Spiral CT, and Multislice CT
SW/SC 300%
200%
128% 100 % I-----~::::::-.------
---Pitch
f-----_.__---t-------,-------\
1.5
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2
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redundant outside the isocenter. Still, there are constellations in which the sampling is particularily advantageous or disadvantageous. While Philips and Siemens have made their interpolation algorithms independent of pitch, Toshiba and GE provide preferred pitch factors to ensure particularly advantageous data sampling for their 4-, 8- and 16-slice systems. Siemens has chosen another approach, in which a z- filtering for 4-slice systems and cone-beam reconstruction for 16-slice systems is chosen in a way that makes section width, image noise and radiation dose independent of the sampling pattern with varying pitch (see Figs. 1.25c and 1.28c). •
Fig. 1.25 Effective section thickness (section width) as a function of pitch for 180 LI and 360 LI with single-detector-row CT (a) compared to 180 MLI and 360 MLI with 4-slice scanners (b). Adaptive interpolation schemes (c) keep section width independent of the pitch. HQ = high quality mode, MS = high speed mode on GE 0
0
0
scanners.
0
Z-Filter Interpolation
Z-filter interpolation is reserved for 4- and 8slice scanners uses a similar concept as higherorder interpolation algorithms for conventional spiral CT. Not only the two projections from the detectors that are closest to the scan plane are used but also adjacent projections [multipoint interpolation). These projections are weighted according to their distance from the scan plane (Fig. 1.26).This filter function may even contain negative portions that result in "edge enhancement" along the z-axis and thus can ensure that the section width is equal to the collimation even at unfavorable pitch factors. This, however, will come at an increased image noise. By using a wide filter function, noise is reduced and the section width becomes larger. Z- filtering controls the width of the slice profile (section width SW) of the reconstructed images. While most manufacturers (Philips, Siemens, Toshiba) always display the correct section width SW on their user interface, GE has chosen to display only multiples of SC as potential section widths SW on their scanners. This is correct for the vast majority of parameter combinations. For the thinnest section widths at pitch P> 1, however, the true width of the section profile at half maximum (FWHM) is approximately 30% larger than SC (for 4- and 8slice scanners but also for 16-slice units) and than the displayed value for SW. This widening of the section profile is also present with other manufactures but is correctly represented in the displayed section width SW. Depending on the manufacturer, various combinations between chosen section collimation and reconstructed section width are available (Tables 1.8-1.10). In principle, the only re-
Image Reconstruction
360
Fig. 1.26 Z-filtering is a technique that weights projection data according to distance from the reconstructed imaging plane.
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straint is that the section width must be larger than or equal to the collimation. The main difference between manufacturers is how they treat the minimum available section width. If standard 180 MLI or 360 MLI algorithms (or variants) are used, the section width varies between 100% and" 128% ofthe section collimation, depending on the pitch factor. The minimum section width with the GE high speed mode (P *= 6) on 4-slice scanners is some 30% wider than the collimation SC (see Table 1.8). It is only in this mode and in the UF (P*=10.8, P=1.35) and US (P*=13.4, P= 1.675) modes in 8-slice scanners that the "reconstruction slice thickness' indicated on the user interface is not approximately equal to the section width. Philips provides the real section width on theiruserinterface. The available minimumnumbers vary with pitch (between SC and SC+30%). 0
0
Siemens varies their z- filter (adaptive array interpolation) with the pitch in such a way that section width, image noise, and dose requirements remain constant independent of pitch (Fig.1.25c). As a standard, they offer a section width that is some 30% wider than the section collimation. An edge-enhancing z- filter is also available for the 4 xl mm detector configuration that provides 1 mm section width independent of the pitch. The resulting images, however, suffer from a substantially higher noise Oust like the difference between 180 LI and 360 LI in spiral CT). Toshiba uses a z-filter of varying width (MUSCOT algorithm) that provides a given section width independent of the chosen pitch factor. The minimum section width is identical to the collimation and can be increased in steps of 0.5 mm. If SW is chosen equal to SC, the noise is substantially increased (like with all other vendors). 0
0
29
1 Principles ofCT, Spiral CT, and Multislice CT
•
Cone Beam Interpolation
Z-filter interpolation still relies on the assumption that rays are parallel (Fig. 1.22c). Real cone beam corrections require more complex calculations. Such algorithms are mandatory with 16 or more detector rows because cone beam artifacts (see Fig. 7.47) increase substantially with wider detector arrays. Most of these algorithms are still being refined, and use various types of compensation techniques for the cone beam geometry. Variants of 3D back projection (e.g., COBRA, Philips or ConeView, Toshiba) theoretically should yield the least artifacts. They rely on reb inning of projection data and back projection along the correct cone angles. For each voxel, the projectional rays that are next to it are actually used for backprojection. The Adaptive Multiple Plane Reconstruction (AMPR, Siemens) shifts the plane of interpolation from an axial orientation to an oblique position with a maximum angulation determined by the cone angle (Fig. 1.27). This yields a set of oblique planes (as an intermediate step) that rotate with different z-positions. Interpolation between these oblique planes then creates axial, coronal, or arbitrarily oriented sections of any desired section width without necessarily having to go through a real reconstruction of an orthogonal 3D data set. GE uses a combination of approaches (Crossbeam correction, Hyperplane and Conjugate Ray reconstruction) to address the various problems of cone-beam scanning, i.e. cone-beam artifact reduction and thin-slice, high-pitch scanning. At present, artifact behavior of these various algorithms has not yet been studied.
The cone beam interpolation used in the 16-slice scanners of Philips, Siemens and Toshiba allow for arbitrary pitch factors up to 1.5 to 2. However, Toshiba suggests preferred pitch values (at present P*=ll, 15 and 23; P= 0.6875, 0.9375, and 1.4375, respectively). Similar to their 4-slice and 8-slice systems, GE supports fixed pitch factors (P*=9, 15, 22 and 28; P= 0.5625, 0.9375, 1.375 and 1.75, respectively), for which their cone beam algorithms have been specifically optimized. •
Image Noise
Image noise in multislice CT depends on the zfilter (or cone beam algorithm) and the chosen section width. To understand how, we have to look back at conventional spiral CT. There, the noise was independent of the pitch but varied with the chosen interpolation algorithm. In fact, it was much smaller with 360 LI as compared to 180 LI; for identical noise, the dose with 180 LI would have to be doubled. The 180 LI algorithm had an identical performance to the 360 LI only at a pitch of 2 and twice the mAs setting (same section profile, identical noise at identical dose). This should have made the 360 LI algorithm the interpolation of choice for low pitch factors, and is another argument for using 180 LI only in combination with high pitch factors. The effects becomes clear when the noise data are normalized to identical patient exposure (Fig. 1.28 a), which can be done by increasing the mAs setting proportional to the pitch so that the effective mAs (=mAs/Pitch) remain constant. Multislice CT overcomes the limitation of single-slice spiral CT because of the higher number of detector rows and, thus, faster coverage. With 4-slice scanners, the 360 MLI algorithm has similar advantages in terms of image noise and dose requirements over 180 MLI as the corresponding algorithms from single-slice CT. When considering the pitch-dependence of the section width (Fig. 1.25) and normalizing the noise behavior to identical patient exposure (Fig. 1.28b), it becomes clear that a 360 MLI algorithm should be preferred for pitches up to 1 (slice pitch up to 4), and the 180 MLI algorithms should be used for pitches up to 2 (P" up to 8). Since a variant of 180 MLI is used for both the HO. (P*=3) and HS (P*=6) modes ofGE scanners, the noise per dose is higher for the HQ. mode than with the HS mode. 0
0
0
0
0
0
0
0
o
0
0
0
0
Fig. 1.27 Cone beam interpolation by interpolating raw data to obtain multiple angulated imaging planes that are then interpolated to form a 3D volume.
Image Reconstruction ReI. dose requirements for constant noise
0
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100 % i-------''-soo >-900 >-SOO >-SOO < 1S0
1200/300 300/700 1200/300 1200/300 200/1 SO
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Curve Ramp Ramp Ramp Ramp Inv. Ramp
Alternative
Visualization
Techniques
Fig. 2.48 Virtual endoscopy yields perspective views by using a central projection instead of the parallel projection used in other techniques. Internal surfaces can be displayed using SSD (a) or volume rendering (b).
a
b
a
b
Fig. 2.49 Virtual endoscopy relies on a high contrast between lumen and surrounding tissues. Most applications such as virtual cystoscopy (a) rely on air as a negative contrast material. For virtual angioscopy ( b) or cholangioscopy, the positive contrast in the structure of interest requires an inverted opacity curve. Note the small entry tear in this type B.
differentiate a colonic fold from a real polyp in CT colonography. Alternative Visualization Techniques
Casts ofa luminal organ are known from CTA and there represent the standard display technique with SSD or VRT. Similar techniques are possible with the tracheobronchial system, and yield a tree-like structure. Distal branches are displayed best if the data was acquired with thin-section multisliceCT. Forthecolon, these casts resemble a monocontrast barium enema, but they are superior because they provide a three-dimensional display of the colonic surface (Fig.2.50a). Casts, however, have an intrinsic drawback: they are less sensitive for displaying small endoluminal protrusions such as polyps orwall-adherent tumors. These lesions appear as small indentations on the surface of the cast and may easily be overlooked.
Tissue transition projections render the walls of the structure of interest in an opaque or translucent fashion. They can be created either by using SSD or VRT with a narrow range ofCT numbers that correspond to the transition zone between luminal contents (air or contrast medium) and the surrounding tissues. This display technique, for example, produces images that simulate a double-contrast barium enema (Fig.2.50b). Alternatively, a tissue transition MPR (see also Fig. 2.36) can be chosen that provides similar results (Fig. 2.50c). ThickMPRis useful for displaying small vessels (thickness 3-5 mm) or the central tracheobronchial system (thickness 7-20 mm). They provide an overview of anatomy and pathology (see Fig. 2.36) and make it easier to detect focal lesions because these will alter the attenuation of the vessel or bronchus. Sliding thin-slab VRT is a method similar to sliding thin-slab MIP but uses volume rendering
79
80
2 Image Processing and Display Techniques
a
b
d
e
9 Fig. 2.50 Alternative visualization techniques may provide a helpful adjunct to virtual endoscopy, as demonstrated in this patient with a colonic lipoma and small polyps in the proximal transverse colon, (a) Intraluminal cast. (b) Tissue transition projection using VRT. (c)
Tissue transition MPR. (d) Thin-slab VRT. (e) Coneddown VRT. (f) Virtual colonic dissection ("virtual gross pathology"), (g) Virtual colonoscopyusing a panoramic view ("unfolded cube"), (h) MPR demonstrating fat attenuation in the lesion, proving it to be a colonic lipoma.
Virtual Endoscopy-Applkations
instead of maximum intensity projections. This technique can be employed in CTA. tracheobronchial imaging as well as CT colonography (Fig.2.50d). It provides a good overview of the anatomy even in complex and spatially contorted cases. Coned-down VRT allows for in-detail analysis of a subvolume of interest and is helpful for vascular structures as well as CT colongraphy (Fig.2.50e). Virtual dissection (virtual gross pathology) is a new technique for CT colonography that finds the centerline of the colon. stretches it out and performs a virtual longitudinal cut in order to obtain a fiattened view of the inner colonic surface (Fig.2.50f). In order to avoid spatial distortion. only slim stripes of the colon wall are displayed. A panoramic view (Fig.2.50g) yields endoscopic projections in and against the direction of movement as well as views of the sides of the colon. This allows for almost complete endoscopic evaluation of the colonic surface and improves detection of polyps hidden behind larger folds. Other techniques have been tried as an alternative to virtual colonoscopy and include Mercator's projection, bull's eye views and others, but they have not yet galned clinical acceptance. Finally, multiplanar reformations (Fig. 2.50 h) must not be neglected because they are able to yield information about CT attenuation, contrast enhancement as well as transmural extent of a lesion.
Artifacts and Pitfalls Threshold-based techniques (SSD) are inherently more susceptible to artifacts than volume rendering techniques. Even with optimum thresholding, SSDs are degraded by pseudo-occlusions and pseudo-stenoses in small vessels (virtual angioscopy) and by pseudo-defects in the bronchial or colonic wall (virtual bronchoscopy and colonoscopy, respectively). These effects are less pronounced in volume rendering when suitable parameters are selected. With VRT even smaller vessels and airways can be evaluated. In virtual colonoscopy, it is often difficult to distinguish between intraluminal contaminants and polyps. In virtual colonoscopy and gastros-
copy, residual intraluminal fluid can obscure the surface of the stomach or bowel and prevent their evaluation. Optimum preparation is critically important. Thin bowel walls, especially when two bowel loops are adjacent to each other, or haustra may appear translucent and require readjustment of threshold or opacity values. In virtual bronchoscopy, breathing and pulsation may lead to ring-like distortion of the bronchial walls. Mucous can mimic polypoid lesions, although it often appears elongated and is therefore easily distinguished from real pathology. Measurements of luminal diameters and degrees of stenosis are strongly dependent on the viewer's perspective and, especially, on the selected threshold value.
Applications
Virtual colonoscopy is the most widely used virtual endoscopic technique (see also Figs. 15.1618). It is part of a CT colonography study and has been proposed for colon cancer screening as well as for detection of concomitant lesions in stenosing colon cancers. However, very few people use it to examine the whole colon. Instead it is used mainly as a tool to differentiate colonic folds that mimic polyps from real polypoid lesions. With further refinement of the technique, however, virtual colonoscopy may also become a method for screening the whole colon because it should have a higher sensitivity for detecting small polyps that are easily missed on cross-sectional images alone. Virtual bronchoscopy at present has very few clinical indications because most diagnostic decisions can also be made based on alternative display techniques such as thick MPR. Because of the strong enlargement of structures, however, it might gain a role with multislice scanning of the tracheobronchial system. Virtual bronchoscopy can more readily detect small polypoid protrusions into the bronchial lumen and is able to depict lymph nodes that bulge the bronchial wall at the level of a bronchial bifurcation, a sign considered indicative of malignant disease by bronchoscopists. What remains is a gutding tool for fiberoptic bronchoscopy that
81
2 Image Processing and Display Techniques helps in biopsies of more peripheral lesions, or for determining the safest point for transbronchial biopsies in a central location close to a main vessel. Virtual cystoscopy has the potential to detect very small polypoid bladder lesions, even in regions that may be hard to inspect by real cystoscopy, For best results, the technique requires filling of the bladder with contrast material or air. Virtual angioscopy has provided almost no additional information over other visualization
techniques, The only indication may be the display of the relative location of the struts of aortic stent grafts relative to the ostium of branching vessels, A major disadvantage of this techniques is the overestimation of stenoses due to calcified plaques (blooming effect),
Virtual cholangioscopy, virtual gastroscopy, virtual enteroscopy, or endoscopy of other organ
systems have been suggested as well, but most techniques fail to provide helpful information over other, less complex visualization tools,
3 Patient Preparation M. Prokop, A. J.
Preparation
and Contrast Media Application
van der Molen
for CT Examinations
.
Preparation of Patients with Impaired Renal Function (p. 84) Preparation of Patients with Allergy to Contrast Media (p. 86) Preparation of Patients with Hyperthyroidism (p. 87) Bow •• Preparation (p. 87) Positioning (p, 88) Vascular Access (p. 88) Planning the Scan (p. 89) Gastrointestinal Contrast Me30 ml of ionic highosmolarCM • Extravasated volume >100 ml of nonionic lowosmolarCM • Skin blistering • Altered tissue perfusion (capillary refill) or change in sensation in hands
93
3 Patient Preparation and Contrast Media Application Table media
3.12
Symptoms
K Early reactions
of adverse
reactions
to contrast
(up to 60 min postinjection)
Nausea or vomiting Urticaria Diffuse erythema, angioedema Bronchospasm
may have an anaphylactoid etiology but can also result from osmolar effects (e.g .. direct irritation of the venous wall) or the number of carboxylic and hydroxylic side chains (chemotoxicity). The incidence of adverse reactions depends on whether the medium is ionic or nonionic and on the specific product that is used. It is also influenced by the concentration and volume of the contrast medium. the rate of contrast injection. and a number of patient-related factors (Table 3.13). Published incidence data vary dramatically depending on how adverse reactions are defined. A possible grading is as follows: • Minor reactions: nausea, vomiting, heat sensation. flushing. mild urticaria. sneezing • Intermediate reactions: mild hypotension. generalized urticaria. mild bronchospasm. diffuse erythema. angioedema. vasqvagal reaction • Severe reactions: hypotension (systolic pressure 350mg iodine/ml) have a substantially increased viscosity and require pre-warming of the contrast material. Without the use of a saline flush, the duration of contrast enhancement drops early because the (more viscous) contrast material remains in the injection" veins.
Intravenous ContrastAdministration dium by IV bolus injection (preferablyusingmechanical injection at the same rate as the diagnostic scan) and performing several trial scans in a reference region using the lowest possible tube output. The scans may be taken at 1-2 second intervals, depending on the dose and type of scanner. Even for these trial scans, it is advantageous to have an 8-12 second delay from the start of the injection to the initial scan. The delay time depends on the patient's presumed circulatory status but rarely exceeds 20 seconds (except in patients with heart failure or valvular disease). The test is evaluated by identifying the image that shows maximal enhancement in the target region (e.g., the aorta). The time to peak At, measured from the start of the injection to the time index ofthe selected image, equals the optim um scan delay time. Experience shows, however, that the best results are achieved by adding 2 to 5 seconds to the calculated delay time (Table 3.17).
Scan Delay Ind ivid ual d iff erences in circulation tim e are difficult to assess since they are based on heart rate, patient age, and previous illnesses. Because ofthese differences, the scan delay time should be individualized so that imaging can be performed in the optimum phase of enhancement. This is the more important the faster the data acquisition becomes. Modern multislice scanners require just 6 s for a liver scan with 4x 2.5 mm collimation, and the next generation of 16-row scanners requires even less time (2-4 s. depending on collimation). The risk of mis-timing the injection increases with such equipment. The arterial phase of perfusion is the most difficult one to capture. Individual adaptation is mandatory for fast scanners. The parenchymal or portal phase of perfusion is less critical, thus standard delay times can be used for most routine examinations. Even in these cases, better results can be achieved by individualizing the scan delay (Table 3.17). •
Bolus Triggering
A test bolus is not the preferred method for determining the scan delay. A more efficient method is bolus triggering, in which the contrast bolus itself is used to initiate the scan. A growing number of manufacturers are offering this technique, in which "monitor scans" are
Test Bolus
The injection ofa test bolus is a robust method available for determining the individual circulation time and the optimum scan delay. It consists of administering 10-20 ml of contrast meTable
3.17
Tailoring
the scan delay to the patient's
circulation
time
Test bolus Reference
Contrast Scan
region:
parameters:
parameters:
Scan delay 0
Ascending
aorta Start of scanning
(Thoracic
(CTA
region
CTA)
of neck or abdomen)
10/5/8-12 5 mm section collimation, ning sequence: 20-30 s Calculated D = At + 2
minimal
from time to peak
dose,
scans
at 2-4
s
intervals;
duration
of scan-
At.
s (At < 20 s) D=At+5s(At>20s)
Bolus triggering Target
region:
Spleen
(5) or portal
Contrast parameters:
According
Scan
Section
parameters:
(Thorax, eTA)
Ascending aorta (A) Abdominal aorta (A) to protocol,
collimation
Scan delay 0
Depending
Notation:
V/FIDX, aorta
(Arterial phase in liver or pancreas) (Portal venous phase)
veins
no test injection
as in spiral scan,
on target
region:
e.g.: 120 + 60/4/5
A
minimal
dose,
delay starts when =
scan
initiated 5
scans at 1 to 4s intervals
enhancement
s
in
after appearance
ROI
exceeds
of contrast
50
HU
in the
101
3 Patient Preparation and Contrast Media Application
taken at a suitable table position using a greatly and the contrast material is injected rapidly C'mlls). On the other hand, a delay of SsA'aAiAsAA
High-contrast artitacts can be prevented by adequate dilution of the contrast medium (1:3 to 1:5, i.e., < 100 mg iodine/ml). If necessary they can be somewhat reduced by using the highest possible tube current (usually 140kV).
4 Optimization of Scanning Technique M. Prokop
Scanning
Strategy
.
Scanogram (p. 110) Settnq up the Scan Sequence (p. 110) Scan Parameters
.
General Acquisition Parameters (p. 112) Gantry Angulation (p. 112) Scan Field of View (SFOV) (p. 113) Scan Time and Rotation Period (p. 113) Scan Duration and Phase of Respiration (p. 113) Scan Length (p. 114) Scanning Direction (p. 114) Tube Voltage (kVp) (p. 115) Tube Load (mAs) (p. 115) Tube Output Characteristics (p. 115) Single-slice Spiral Acquisition Parameters (p. 116) Section Collimation (p. 116) Table Feed and Ptch (p. 117) Multislice Acquisition Parameters (p. 118) Section Collimation (p. 119) Table Feed and Pitch (p. 120) Vendor-specific Approaches (Four-slice Scanners) (p. 121) Eight- and 16-row Multislice Scanners (p. 122) Reconstruction and Display Parameters (p. 123) Section Width and Raw Data Interpolation (p. 123) Reconstruction Interval (p. 124) Convolution Kernel (Reconstruction Field of View (p. 127) Window Selection (p. 127) Image Processing (p. 128) 2D Processing (p. 128) 3D Processing (p. 129) Special Scanning Techniques (p. 130) Ouantitative CT (OCT) (p. 130) Dual-Energy CT (p. 130)
Algorithm)
(p. 124)
4 Optimization of Scanning Technique
Scanning Strategy Scanogram
The scanogram, known alternatively as a scan projection radiograph, topogram, scout view, or pilot scan, is used to locate and define the regions to be examined by CT. A scanogram is obtained by keeping the x-ray tube stationary while the patient is moved through the scan plane, producing a projection radiograph that resembles a conventional plain film. In the scanogram, however, the contiguous slit scans and collimated detectors greatly reduce scattered radiation compared with plain films. This has numerous advantages, such as a marked improvement in the delineation of the abdominal organs (important in determining the scan length). Spatial resolution is relatively poor, however, depending on the selected slice thickness (0.5-2 mm). Especially when low-dose scanning is performed, the scanogram may contribute substantially to overall patient dose. For this reason, the lowest possible mAs and kVp settings should be used in such situations (Fig. 4.1). Setting up the Scan Sequence
The following guidelines are helpful in devising a strategy for CT examinations:
additional, secondary target regions. For example, the pancreas is the primary target organ for thoracoabdominal staging in a patient with known pancreatic carcinoma while the thorax, liver, and the rest of the abdomen represent secondary target regions. (Intravenous) contrast administration should be tailored to the primary target organ. If timing is not critical for the primary target region, use the secondary target organ for which contrast timing is most critical. Determine the enhancement phases in which imaging should be performed (see chapters dealing with specific body regions). Next decide how many scans are required and determine the sequence in which the primary and secondary target regions will be scanned. The choice of scan parameters strongly depends on the available technology. With multislice CT scanning, either a volumetric imaging protocol with near-isotropic resolution or a fast spiral scanning mode with rapid data acquisition will be chosen. With single slice spiral CT the length of the individual scanning regions and the available time window determines the choice of scan parameters.
• The clinical problem will dictate the primary target organ for scanning as well as
Fig. 4.1 Scanogram using 120 kVp and 100 mAs (manufacturer's standard) (a) and 80 kVp and 50 mAs (approximately one fifth ofthe dose)(b). a
b
Setting up the Scan Sequence
Increasing attention is being given to radiation safety issues in CT examinations. The level of radiation exposure in CT is multiple times higher than that incurred in conventional radiography. This mandates ajudicious selection of imaging parameters, for even slight changes in the parameters can lead to marked absolute differences in radiation exposure. An effort should be made to reduce radiation exposure in accordance with the patient's size and the clinicalpro blem. The diagnostic accuracy of CT will generally be enhanced with a more specific clinical request and a more limited target region • Non-contrast scanning followed by a contrast -enhanced examination are usually indicated in patients with suspected hemorrhage, renal lesions, or suspected hypervascular hepatic tumors. • For most contrast-enhanced studies, the upper abdominal organs should be scanned first during the contrast injection, because organ perfusion and enhancement in that region will critically influence the quality of the examination. • Thoracic scanning should precede examination of the lower abdomen, because
111
vascular opacification in the pulmonary hilar region is more critical. The neck should be scanned last (interstitial enhancement), preceded if necessary by the injection of additional contrast medium. Repositioning of the arms should be considered when neck and chest have to be examined. Keeping the arms in the scan filed will cause major image degradation (Fig.4.2) The scan parameters should be tailored to the scanned region according to the anticipated lesion size, bearing in mind the limitations imposed by the available scanner technology. For CTA and virtual endoscopy, the smallest possible FOV (for better resolution) and a soft convolution kernel (for less noise and better 3D rendering) should be used. Sample protocols for CT examinations of varying complexity are shown in Table 4.1.
b Fig. 4.2 Raising the arms above the head should be tried whenever possible for scanning of the chest and abdomen: There are substantial streak artifacts with the
arms placed next to the body (a) while raising at least one arm above the head leads to a substantial reduction in artifacts and decreases dose requirements (b).
112
4 Optimization of Scanning Technique Table 4.1 Single-slice
Sample protocols of varying complexity D
for a single-slice scanner (left) and 4-slice scanner (right)
MultisliceD SC/TF/RI
NxSC/TF
V/F/DX
SW/RI'
MPRb
V+N/F/DX
7{12{6
1. Chest, inspiration
1{10 1{30
t t
5{1O{5 2. Tumor + hila:
t
2{4{2
1 i
noncontrast
4x2.5{15
1.Abdomen
noncontrast
8{8
HR'
1{1
HR'
1{1
noncontrast
1. Adrenals:
4xl16
j
3{2
90{3{30
2. Chest: tumor region
4x1{6
i
3{3 1.5{1.5
non contrast cor
90 + 50{3{30
cor+saq''
lymph node staging token over 1~hr p'eriod
- Preparation: 7 L oral eM 1. Liver + chest:
7{12{6
i
120{2{40
1. Liver to neck:
2. Rest of abdomen:
7{12 6
t
105 ISO
2. Rest of abdomen:
- Reposition arms next to body 3. Neck:
3{5{3
t
jl t
Suspected retroperitoneal hemorrhage 1. Abdomen
d
50{2{20
4x2.5{15
t
5{4
~=~~~:=====:;:i::;:;i===1:::::50{4{25 lOs ISO
non contrast
CPR
150+50{5{5A'
cor
lOs ISO
105 ISO
lOs ISO
SC = slice collimation (mm), TF = table feed (mm{rotation), RI = recon. increment (mm), i t = scanning direction, SW = effective section width (mm), MPR = multiplanar reformats. axial = axial sections. cor= coronal, sag = sagittal eM = contrast material, Contrast concentration = 300 mg/ml iodine V=volume of CM (rnl). N = volume of saline flush (ml) F=flow rate (ml{s), 0= start delay, X=trigger region a axial section for diagnostic review, an additional reconstruction to create a "secondary raw data set" may be necessary b MPR are reconstructed from a .secondary raw data set: with SWIRl = 1-1.5/0.7 Pancreas: CPR along pancreatic duct; CTA: MIP I VRT for routine display mode, CPR for stenoses C sequential CT d reconstruction from the same raw data set e read: 5 A = start delay 5 s after reaching 50 HU enhancement in the infrarenal aorta
Scan Parameters User-selectable scan parameters are tailored to the specific clinical request so that adequate spatial and contrast resolution will be achieved. Reasonable trade-offs should be made that will balance the requirements of an acceptable radiation dose with acceptable image quality. The selection of scan parameters is significantly influenced by the available scanner technology. For the notation of scan parameters, we recommend the abbreviations listed in Table3.7.
The following section describes the various trade-offs for the choice of data acquisition, reconstruction, and display.
General Acquisition •
Parameters
Gantry Angulation
Most IT examinations do not require gantry angulation, i.e., the scan plane remains perpen-
General Acquisition Parameters dicular to the table plane. In intervertebral disk studies, however, the gantry angle should be adjusted on the lateral-projection scanogram so that the scan plane is parallel to the center of the intervertebral disk space for each level that is examined. Gantry angulation can also be used in neck scanning to minimize dental artifacts and should always be used in head examinations to exclude the eyes from the scan field. With thin-section multislice scanning, gantry angulation is almost never necessary for body imaging because any desired imaging plane can be reconstructed after the data set has been acquired (see Fig. 25.45). In some scanners, gantry angulation will cause distortion in 3D reconstructions or will not be available for spiral scanning. •
Scan Field of View (SFOV)
On some scanners, the size of the scanned field can be reduced to increase the sam pling rate and thus improve spatial resolution. This is feasible mainly for relatively small object diameters (neck, extremities). If the object diameter exceeds the SFOV, "out-of-field artifacts" will appear at the periphery ofthe object (see Fig. 7.45). For this reason the object should be precisely centered within the gantry aperture, and, if in doubt, a trial scan may be advisable. Out-of- field artifacts influence only a relatively narrow band in the periphery of the (round) SFOV (see Fig. 7.45b). This can be used for indications with a focused interest on only a portion of the body cross-section (suchas in multislice cardiac imaging, see Chapter 23). When positioning the structure ofinterest exactly in the center ofthe gantry aperture, and reconstructing a slightly smaller fieldofview than the actually exposed field, the image quality will be high, and radiation exposure to the more peripheral body portions (e.g., skin,breast) will be reduced. Information outside
the SFOV, however, will be lost. This type of scan is often called a "target scan". For multislice scanning, cone beam artifacts are least severe for structures close to the rotational axis. For this reason, the examined body part should be placed as close as possible to the center of the gantry (com pare Fig. 1.22). •
Scan Time and Rotation Period
Shorter scan times per slice are associated with fewer motion artifacts. The scan time can be shortened by limiting the scan arc to an angle equal to 180 plus the fan angle (partial scan) rather than performing a full 360 rotation. Motion artifacts can be reduced by assembling multiple rotations into one image. Short rotation periods (rotation time, RT) in spiral and multislice CT increase the available scan length and shorten the time required to scan a particular volume. A short rotation time will also allow for using thinner sections while covering a given scan volume. In some scanners, very short rotation periods will cause some degradation of spatial resolution because the number of available projections decreases (e.g., for the "flying focal spot" technology used by Siemens). For high-resolution application, such ultra-short rotation periods should be avoided on these scanners (Fig. 4.3). For cardiac scanning, adapting the rotation time to the heart rate may improve temporal resolution. This will be the case if a two- or threesector reconstruction instead of a one-sector reconstruction can be used (see Chapter 23, Heart). 0
0
•
Scan Duration and Phase of Respiration
When selecting the scan duration, the radiologist should make certain that the patient has adequate breath-holding capacity and that imaging can be performed during the desired enhancement phase. When brief hyperventilation precedes the scan, most patients can tolerate a 30 second breath hold in inspiration.
Fig.4.3 Image quality of HRCT is improved with maximum number of projections. Some scanners using flying focal spot technology require a reduced rotation time (0.75s) (a) for optimum spatial resolution. Resolution with 0.5 s rotation time and the same exposure dose is slightly reduced (b).
a
b
4 Optimization of Scanning Technique
Most body spiral CT examinations (singleslice or multislice) are performed in inspiration. Scans during forced expiration are only required for the chest (high-resolution CT) to detect air trapping or emphysematous lung areas. Multislice scanning during expiration may be used to quantify the amount of air trapping in the whole lungs. There are a few cases in which scanning may be performed during shallow respiration, e.g., for patients in respiratory failure or sedated children. Spiral scanning will su bstantially reduce artifacts on the axial scans if only mild movement is present. If a breath-hold is attempted, the amount of artifacts will commonly increase because such a patient will usually have to continue breathing during the scan with deep inspirations to compensate for his dyspnea (Fig. 4.4). Breath-hold scanning should not be attempted in dyspneic patients, unless the scan duration is very short. Breath holding is unnecessary in examinations of the head, axial skeleton, and extremities. •
Scan Length
The scan range can be determined from the scanogram in most instances. For reasons of radiation protection, pre contrast scans to precisely define the scan range should be avoided.
Fig. 4.4 In dyspneic patients, trying to have the patient hold the breath may cause severe artifacts when the patient has to start breathing again. This can be avoided if the patient is instructed to breathe shallowly. Note the lack of artifact in the upper portion of this craniocaudal scan (arrows) and massive artifact in the lungs and bronchi further distally (arrowheads).
The radiation exposure to the patient (dose-length product, DLP) increases with the scan length. Therefore, the scan length should be kept as short as possible without omitting any portions of relevant organs. Proper patient instruction, training of the technicians and the use of standardized scan protocols will help to reduce the "safety margins" for examinations of the chest and abdomen that are commonly used to make sure all organs are covered, even in a patient with breathing problems. Generally the extension of the scan range will make a prolonged breath-hold and a wider section collimation necessary. This trade-off is less relevant with multislice CT but remains important when deciding on the total amount of contrast material or the scan duration that can be tolerated by the patient. •
Scanning Direction
The direction in which scanning proceeds can substantially influence image quality, depending on the contrast protocol and anticipated respiratory artifacts. A caudocranial direction is best for spiral scanning ofthe thorax, because it reduces motion artifacts toward the end of the scan in the lung apex. For optimum results of breath-holding enough time should be calculated for patients to breathe in and hold their breaths before the scan is started. If the scan is initiated a few seconds after the breath-holding instructions the results
a Fig. 4.5 Starting a caudocranial scan too early causes breathing artifacts (a) that are most prominent on MPR
(b).
General Acquisition
will be substantially improved (Fig. 4.5). This can be accomplished by a start delay of 4-5 s. Caudocranial scanning generally reduces contrast -induced streak artifacts in the cranial mediastinum particularly, if the IV contrast injection is followed immediately by a saline flush at the same inj ection speed: the brachiocephalic veins are then captured at a point in time when they already contain saline solution, the highly concentrated contrast material being flushed to therightatrium. When the liver is examined early during the portal contrast perfusion phase, caudocranial scanning will improve visualization of the hepatic veins because they will be covered during the late part of the scan when they are more readily opacifed. This effect is of course less pronounced with multislice CT because scan duration is much shorter. •
Tube Voltage (kVp)
If the tube voltage is reduced'from 140kV to 80 kV, a greater part of the radiation is absorbed in the prefilter and in the outer fatty tissue layers of the patient. As a result, the CTDIvoi for identical mAs settings is reduced by a factor of 3-4. This is accompanied, however, by a strong increase in noise but also an increase in CT attenuation (and therefore an enhanced radiation contrast) of substances containing high atomic numbers, such as bone, iodinated contrast agents, or metals. Hence, the relationship between image quality and kVp settings is complex (see more detailed discussion Chapter 5). Our own results suggest that a high kVP setting (140kVp) is advantageous for obese patients, while a low kVp setting (80-100kVp) is advantageous in children or in low-absorption areas such as the chest, especially when contrast
Fig. 4.6 Low-dose scan of a patient with chronic thromboembolic pulmonary hypertension performed with 80 kVp and a CTDlvoi=2.3 mGy. There are multiple peripheral pulmonary artery stenoses (arrows). Note the excellent contrast (window settings W/L= 1500/300) on the thin-slab MIP (a) and the good quality ofthe volume rendered display (b).
Parameters
material is injected (Fig. 4.6, see also Figs. 5.16, 5.19, 24.18, 24.19 a, 24.20 a, 24.64). •
Tube Load (mAs)
For constant tube voltage and beam filtration, the tube current is proportional to both, the patient dose and the detector dose. The mAs per tube rotation provides a useful measure of image noise (detector dose), while the mAs per tube rotation divided by the pitch (effective mAs = mAs per rotation/pitch) provides a measure of the patient dose. A high mAs setting results in low noise, permits the use of high-resolution kernels, and improves image quality (see Chapter 5) but also increases radiation exposure to the patient. On most newer scanners, the volume CT dose index (CTDIvoi) is displayed on the user interface (as CTDI) and provides a better information about radiation dose than the mAs settings ever could. Radiologists and technicians therefore should get adapted to using CTDIvoi instead of mAs as a measure of exposure settings to compare scanning protocols. •
Tube Output Characteristics
The x-ray tube output, heat capacity, and heat output are the tube performance characteristics that limit the mAs settings available for the duration of the spiral CT scan. This has a direct effect on image quality. The longer the scan duration or the greater the number of successive spiral scans that are acquired at short intervals, the higher the necessary tube output and the lower the available mAs per tube rotation. In older scanners, trade-offs have to be made in terms of image quality or available enhancement phases. With multislice CT, two, four, or more sections can be acquired simultaneously. Although
115
116
4
Optimization of Scanning Technique
this feature can be used to reduce the requirements on the x-ray tube by a similar factor, most users choose thinner sections instead, which increase the scan duration and keep mAs requirements high. In addition, subsecond rotation times (e.g., 0.5 s) and high pitch factors further increase the rnA settings that have to be used for obtaining a good signal-to-noise ratio. With a pitch of 2 (P * =8) and 0.5s rotation time, the rnA settings have to be four times higher than for a conventional incremental CT (1 s rotation time) at identical patient dose and signal-to-noise ratios.
ters are discussed below. Table 4.2 gives an overview of suggested scanning parameters depending on the clinical indication. Table 4.3 describes the trade-offs for the choice of various acquisition parameters. •
Section Collimation
A narrow section collimation (SC) has the advantage of reduced partial volume effects, better spatial resolution along the z-axis, and a con- f: sequent improvement in the image quality of • multiplanar reformations and 3D reconstrucReduced mAs, soft convolution kernels, tions. The resulting increased image noise can shorter scan lengths, and thicker slice collimation be reduced subsequently by increasing the seccan be used in an attempt to minimize image tion width in axial reformations, which will ennoise without overloading the tube. These tech- hance the signal/noise ratio without corn- ji y niques also reduce the radiation dose to the promising z-axis resolution, The main advantage of a wide section collipatient and, therefore, should be considered in mation is that less radiation dose (lower mAs the formulation of scan protocols. Maximum dose per section is required for setting) is required at a constant level of image very obese patients. This can be achieved by noise, whereas at a constant dose, there is less decreasing the rotation time and increasing the noise resulting in a better contrast resolution, section collimation and mAs. Image noise can be The most practical advantage of a wide collimareduced by reconstructing thicker sections (sec- tion is the ability to scan a given volume more , tion thickness SW= 10mm) and by using soft rapidly because thicker sections can also be re- ( convolution kernels. In single-slice CT, a pitch of constructed from thinner ones using MPR func1 and a 360 LI interpolation algorithm may be tions (or in the case of multislice scanning, the & used (twice as dose-efficient as 180 LI). With raw data of the scan). multislice scanning, low pitch factors are reThe choice of a collimation setting depends F on the clinical request and the nature of the required to allow for high mAs settings. gion that is to be examined. Since most anatomic structures are oriented along the longitudinal axis ofthe body, a section collimation of Single-slice Spiral Acquisition Parameters 5-8 mm can often be used without introducing Spiral CT has become the standard technique for detrimental partial volume effects. Large most applications in the body. The most impor- volumes are imaged with a large section thickness in order to reduce the scan duration. The tant trade-offs in the choice of the scan parame0
0
Table 4.2
Suggested scan parameters for single-slice spiral CT ofthe body
For optimum 3 D results (MPR MIP, SSD, VRT), choose RI = SW/2 incremental scanning technique SC = section collimation (rnrn). TF = table feed (mrn). RI = recon. increment (rnrn). SW =section width (mm)
b
Single-slice Spiral Acquisition Table 4.3
Trade-offs
Acquisition
that influence
Parameter
Section collimation
SC
Table feed TF
the choice
of scanning
Parameters
parameters
Trade-off large small
--
large
->
less nose, better low-contrast resolution, more partial volume effects less partial volume effects, better z-axis resolution, more noise faster coverage,
longer scan range per time, less motion
artifacts
on
MPR smalls
FItch P
smaller
large
->
small-> T uberotationtime
RT
resolution
large
-> more projections coverage _"' less motion
small Scan length L
better z-axis
or faster coverage,
increased
patient
+ better in-plane
artifacts,
better z-axis
less patient
resolution
(some scanners),
resolution
or faster
-> long scan range, thinner collimation possible, more contrast material required -> shorter scan range, less artifacts, less contrast better
anatomic
dose
dose
large
-» large coverage,
small
longer scan duration TI required -> less patient dose, thinner collimation
overview,
more motion material
thicker
possible,
less
coverage artifacts,
required
collimation
shorter
SC or
scan time
mAs settings
high low
-> less noise, better contrast resolution, _" less patient dose, more image noise, low-contrast details
kVp
high
-> better penetration,
low
-r
large small
-> less noise, better low-contrast resolution, more partial volume effects -> less partial volume effects, better z-axis resolution, more noise
large small
->
settings
Reconstruction
more patient dose decreased detection
less dose for given signal-to-noise
patients, especially in abdomen higher contrast (especially with iodinated contrast minirrum dose, less dose for given signal-to-noise patients, chest, and neck
Recon. increment
materia), lower for children, slim
RI
less images bu lesions may be missed more images, better MPR/3D representations,
more lesions
Z-filter
wide -> less image noise and dose requirements, lower z-axis narrow _"' increased noise, higher z-axis resolution
Recon. filter kernel
smoothing
less image noise better 3D images
high resolution Field of view (FOV)
large small
-> substantially
-> better overview, -c-» higher maximum
section collimation should be reduced, however, if it is necessary to depict fine anatomic details, With low-contrast lesions such as hepatic metastases, the signal-to-noise ratio, and thus detectability decreases for lesions that are smaller than the section width, Table
In spiral used for improves phase, or
of small
in obese
Parameter
Section width SW
•
SC possible
less spiral artifacts,
small large
Scan duration TI
collimation
Feed and Pitch
scanning, a pitch of 1.5 to 2 should be the vast majority of indications, This scan coverage during a breath-hold it can be used to improve spatial reso-
and dose requirements,
lower spatial
increased
spatial
noise,
higher
detected
resolution resolution
resolution
smoothing filter kernels are less disturbing spatial resolution (requires HR filter kemels)
lution by reducing section collimation (see Fig, US), At the same time, less dose is required than with conventional slice-by-slice scanning (see Fig, 5.12), A pitch in this range provides an optimum z-axis resolution for a given scan length and simultaneously reduces the radiation exposure, If the collimation is very thin (SC < 1 mm): even a pitch as high as 3 can be used without causing serious additional artifacts (Fig, 4.7),
118
,
4 Optimization of Scanning Technique Fig. 4.7 A pitch of 3 may be possible with thin collimation (SCI TF /RI = 1 /3/1) for CTA of the pulmonary vessels (a) and even the lung parenchyma (b).
b
a
MultisliceAcquisition Parameters
For the vast majority of applications there are two essentially different ways to use a multi-
Table 4.4
Comparison
of multislice CT scanning Volumetric
slice CT scanner: as a fast spiral CT scanner or as a volumetric imaging tool (Table 4.4). Fast spiral scanning takes advantage of the increased acquisition speed but still uses the
protocols for fast spiral scanning
and volumetric
imaging
Fast spiral scanning
imaging
16-slice
Protocol
4-slice NXSClTF
16-slice NXSClTF
4-slice NXSCITF
NXSC/TF
GE
4x1.25/7.5
16XO.625/13.75
4X2.5/15
16X1.25/27.5
Philips/Siemens
4X1/6
16XO. 7 5/16-18
4X2.5/15
16X1.5/32-36
Toshiba
4X1/5.5
16XO.5/11.5
4X3/16.5
16X1-2/23-46
Performances
TT TT TT TT
T
Coverage
T TTT TTT
Speed Z-resolution
lrnaqenolse" Pulsation
artifacts
Breathing
artifacts
TT TT
U U
A
1
Applications Standard,
Tumor
Chest
Tumor staging I nterstitial disease
Metastases Mediastinum
Abdorne?
Pancreas Liver, biliary system Bowel Preoperative evaluation
Standard abdomen Liver (noncontrast phase) Kidneys
Carotids Pulmonary vessels Abdomen
Aorta Oysponeic Veins
Small joints Cervical + thoracic spine Small bones
Pelvis Lumbar spine Long bones
Skeleton
3
c
staging
lymph node staging
Neck
relative to 1 s-single-slice 7/12/6 use 16x1-1.5 mm for volumetric
protocol imaging
b
in obese patients
patients
at identical patient dose (CTDlvoi)
Multislice Acquisition Parameters scanner primarily as an axial imaging tool. It has the advantage that a thinner collimation than with single-slice CT can be used, although the acquisition time is substantially reduced in comparison with single-slice spiral CT Images are reconstructed with a section width that is equal to or slightly less than section width in conventional spiral CT These images serve as the primary evaluation tool. Thinner sections are reconstructed retrospectively only if questions arise, and may be used to create good quality multiplanar reformations, This technique has the advantage of a known workflow, high patient throughput, limited numbers of images on a routine basis, and the option for problem solving with thin sections and multiplanar imaging, However, it does not take full advantage of the capabilities of multi slice CT, and fine detail may be missed if thick axial sections are to be used as the primary tool for making a diagnosis, Volumetric imaging takes full advantage of the near isotropic spatial resolution that becomesavailablewithmultisliceCT Ittransforms CT into a true multiplanar imaging tool that is, in this respect, superior to MRL It is the preferred, if not only, mode used with 16-slice scanners, Volumetric imaging requires the reconstruction of thin, overlapping axial images that serve as a secondary raw data set, These images are used as a 3D data volume from which further images are generated for diagnostic purposes, In most instances, these thin images will suffer from increased noise and require the reconstruction of thicker multiplanar reformations with a section width that is tailored to the clinical question, Such MPR are reconstructed as part of the standard data evaluation and can be "pretailored" to the anatomical site (such as coronal MPRofthe tracheobronchial tree, sagittal MPR of pulmonary masses, curved planar reformations of the pancreatic duct) or they may be created interactively at the CT workstation to probe the data volume interactively and obtain problem-adapted sections which is, in the interactive 3D data evaluation, similar to ultrasound, This approach requires very fast workstations and networks with proper software to be timeand cost-efficient Volume rendering may become the primary tool for evaluating such data because it combines the advantages of MPR with additional depth information,
Volumetric imaging takes full advantage of the capabilities of multislice CT scanning, However, there are marked differences in workflow, and the patient throughput is lower due to the longer processing times, Increased data volume has to be reconstructed, processed, analyzed, and stored, which requires a proper network and archiving system as well as fast, intuitive CT workstations and trained personnel. At present, such constraints are major and lead to markedly increased effort with such a technique, Therefore, volumetric imaging is used most often only in selective cases, such as planning of surgery, CT angiography, or complex cases that cannot be resolved with conventional techniques, Section
Collimation
Fast spiral scanning can rely on a 4x2,5 mm (4 x 2-3 mm) detector configuration for most indications, With a volume pitch (P*) around 6, a table feed of 15 mm per rotation can be obtained, which translates into a table speed of 18.75 mmls (0,8s rotation time) to 30 mmls (0,5s rotation time), This allows for covering the chest (24cm) in 8-12,8 s. which is a very comfortable breath hold period for most patients, A 40 em long abdomen can be covered in 13,3-21 s. which also can be met by most patients, Even combined thoracoabdominal scans become possible: a 60cm range can be covered in 20-32 s. Using a larger collimation but a lower pitch will reduce multislice artifacts but a similar if not better result is obtained if such thicker sections are reconstructed from the 4 x 2,5 mm data, Thicker collimation may be useful for very rapid scanning or in very obese patients as the image noise can substantially be reduced, Volumetric imaging relies on the highest possible spatial resolution at a reasonable patient dose, With current four-slice scanners, this can be achieved with a 4x 1 mrn or 4 x 1.25 mm detector configuration and the reconstruction of thicker sections for image analysis from the raw data or from the secondary raw data set that should consist of LCl-L5 mm thick sections reconstructed every 0.5-1 mm With current four-slice scanners, dose efficiency is reduced by some 20-30% with this detector configuration, In obese patients, a 4x2 mm or 4x2,5 mrn acquisition may be used instead, which requires reconstruction of 2.5-3 mm wide sections every 1-1.5 mm
4 Optimization of Scanning Technique
Similar to single-slice spiral CT a high pitch factor allows thinner collimation for a given scan coverage per time to be used. It has been shown that it is better to use thin collimation with a high pitch factor and then reconstruct thicker sections than to use thicker collimation with a low pitch factor primarily. For an identical reconstructed section width, artifacts are less with
thinner sections, and there is still the opportunity to reconstruct thinner sections if multiplanar imaging has to be performed. Thus, a volume pitch P* of 5.5 to 6 with thinner sections should be preferred over a volume pitch of3 for most applications (Fig. 4.8). Exceptions from this are situations in which the thinnest possible sections have to be reconstructed. In such cases (e.g., for the cervical spine or the extremities, Fig. 4.9), the amount of artifact can be reduced by using lower pitch factors (P *=3, P < 1). Another exception are obese patients, in whom image noise is a limiting factor. If too thin a collimation is employed electronic noise may dominate the images (see Fig. 7.33). Image quality then can no longer be recovered by reconstructing thicker sections, For such patients, thicker sections and maximum mA settings should be used with low pitch factors (p*= 3, P< 1). Adjusting the pitch P even higher, P < 1 to a maximum of 2 (P*=8 for 4-slice units-if allowed by the scanner) will result in an increase of artifacts. However, this may be advisable in cases in which faster scanning or longer coverage is required. Such artifacts can be decreased efficiently by either reconstructing thicker sections or reformatting thicker MPRs. In addition, such high pitch factors may be advisable for low-dose scanning because the mA settings at an identical patient dose is higher for a larger
a
c
Ultra-thin collimation (0.5-0.625 mm) is possible with most multislice CT scanners but there is a substantial increase in patient dose with current systems (see Fig.5.5b), and an increased susceptibility towards electronic noise which further decreases the signal/noise ratio. Such protocols are useful only for very selective applications, e.g., for the lung parenchyma or for small bones, where it can provide exquisite spatial detail. With 16-slice scanners, volumetric imaging is used in almost all cases. Thin (SC= 1-1.5mm) to very thin (SC = 0.5-0.75 mm) sections become available. Because of noise characteristics, thin sections will be the standard mode for the abdomen, while very thin sections can be recommended for the neck, chest and extremities. Only in slim patients and specific diagnostic questions (CT of the pancreas and biliary system, CTA of the mesenteric arteries) very thin sections may be used in the abdomen as well. •
Table Feed and Pitch
b
Fig. 4.8 For identical volume coverage per time (identical table feed), using a higher pitch factor and thinner sections may improve the quality both ofMPRaswell as thick axial sections, at identical dose to the patient. Comparison of a 4x2.5 mm collimation with volume pitch P* of 3 (a, c) to a 4 x 1.25 mm collimation with a
d
volume pitch of 6 (b. d). A CTDlyoi= 12 mGy was usedfor both examinations. Axial sections of 3 mm thickness (a, b) yield comparable image quality but coronal sections of 3 mm thickness (c. d) are substantially better with thinner collimation.
Multislice Acquisition Parameters
a
thickness) as multiples of a single section collimation (1.25 mm, 2.5 mm, 3.75 mm, etc., see Table 1.8) and then decide which detector configuration to use. This approach favors the fast spiral scanning technique and is very intuitive and time-efficient. GE provides only two volume pitches, which are named high-quality (HQ.) mode (P" = 3) and high-speed (HS) mode (P" = 6). As discussed above, the high-speed mode should be employed for optimum system performance (see also Table 1.7). For similar image quality the high-quality mode will require about 50% increase in dose. There are only limited combinations of section width and section collimation possible with the software on current four-detector row scanners (see Table 1.8): it is, for example, not possible to choose a 4 x 2.5 mm collimation for a 7.5 mm section width (reconstruction slice thickness), or a 4 x 1.25mm collimation for a 5mm section width, respectively. To take advantage of direct axial image reconstruction from the raw data, users will have to use the next thicker collimation. Note that the minimum section width is some 28% larger that the numbers displayed on the user interface if a high speed mode is chosen (see Table 1.8). To obtain thick axial sections even when thin-section scanning (volumetric imaging) was employed, the data have to be reconstructed with the minimum possible section width and an overlap of some 50% (e.g., with 1.25mm sections, every 0.7 mm for 4 x 1.25mm configura-
b
Fig. 4.9 For the skeletal system, a lower pitch factor yields fewer artifacts. Comparison of coronal MPRofthe cervicalspine obtained with a volume pitch P* = 3 (a) and a volume pitch P* = 6 (b) from a 4 X 1 mm detector configuration.
pitch, which leads to a higher detector signal and thus less contribution of electronic noise (Fig.4.10). •
Vendor-specific Approaches (Four-slice Scanners)
Table 4.5 compares the parameters settings for the volumetric imaging and fast spiral modes that are available with the various vendors. General Electric (GE) lets the user first choose the section width (reconstruction slice
a
b
Fig.4.10 For low-dose applications, noise decreases substantially with larger pitch factors despite an identical patient dose. Comparison of images obtained with a volume pitch P*=6 (a, c), and P*=8 (b. d) from a 4 x 1 mm detector configuration with 80 kVp and
c
d 40 mAs (CTDlyoi = 1.2 mGy) each. Note that there is a reduction of image noise not only in the 1.25mm thick secondary raw data set (a, b) but also on the 5 mm thick sections reconstructed for image review (c. d).
121
4 Optimization of Scanning Technique tion). After transfer to a CT workstation (e.g.. Advantage Windows) thick axial sections are then reformatted using the MPR mode, and additional MPR can be reformatted in any desired plane. Philips provides the real section width (effective slice thickness) and the pitch factor, not the volume pitch, on its user interface. Fast spiral scanning and volumetric imaging can be applied as described above. Good results are found for a pitch of 1.5 (corresponding to P*=6). Thick sections can be reconstructed from the raw data in a similar fashion as with all other vendors. Siemens has implemented an adaptive interpolation that assures that the section width is independent of the chosen pitch factor. The noise is also kept constant due to the increase of mA with increasing pitch (constant mASeff and thus, patient dose). Artifacts are somewhat lower at lower pitches but the advantage of increased coverage or higher spatial resolution should prompt the user to employ a volume pitch of 6-8, unless bone imaging is performed. Axial images of almost arbitrary width can be reconstructed from any detector configuration as long as the section width is larger or equal to the collimation (see Table 1.9). A special z-filter is available that allows for reconstructing sections of 1 mm width from a 4 x 1 mm collimation. This will increase image noise substantially and therefore cannot be recommended on a routine basis. Fast spiral scanning and volumetric imaging can be applied as described above. Toshiba uses a z- filtering algorithm (MUSCOT) that also assures that the chosen section width differs only minimally from the section width provided on the user interface. Axial images of almost arbitrary width can be reconstructed as long as the section width is larger or equal to the collimation. If the reconstructed section width is identical to the single section collimation, noise will be substantially increased (see Table 5.5). lt is therefore advisable to use a slightly (by 0.5mm) wider section width for most indications of volumetric imaging. The Toshiba detector gives the largest choice of detector configurations. For fast spiral scanning the user has the option of choosing between a 4x2 mm and a 4 x 3 mm collimation. In practice, the thicker collimation can be recommended for obese patients and in all cases where faster data
acquisition is required. The Toshiba hybrid detector is the only one that has four rows of 0.5 mm collimation, making ultrahigh resolution scanning more feasible and less dose-intense. Although the pitch can be varied freely, Toshibasuggests preferred pitch factors (see Table 1.10). Again, higher volume pitches of 5.5 or 6 yield the highest performance. Otherwise, fast spiral scanning and volumetric imaging can be applied as described above. Eight- and 16-slice Scanners
With the advent of more detector rows, there is a shift from fast spiral acquisitions to volumetric scanning with thin sections. These scanners have the advantage of better geometric dose efficiency compared to four-slice scanners, and most will also use more sophisticated detector technology that reduces electronic noise. Nevertheless, ultrathin collimation (0.5mm) will cause a substantial increase in image noise and will require reconstruction of thicker axial and multiplanar sections as described for volumetric imaging. This provides excellent image quality for the lungs as well as for the peripheral skeleton and the cervical spine. It is doubtful whether such thin sections can be used in the abdomen, especially in more obese patients. They prove most useful for the chest, neck and peripheral skeleton, where radiation absorption is not too high. The main difference between 8- and 16slice scanner types and four-slice scanners will be the substantially increased scanning speed, which will all but eliminate movement artifacts, Pulsation effects are spread out over longer distances, thus improving the display of the thoracic aorta and the origin of the supraaortic vessels. Optimization of contrast material administration requires special attention because of the short scanning time. Precise separation of scan phases is possible but timing is a crucial issue and frequently requires longer short delays. Only a limited number of applications allow for savings in contrast material (mainly CTA and chest imaging), most other applications (especially in the abdomen) require similar amounts of intravenous contrast material because organ enhancement is more dependent on the volume than the flow rate during contrast injection. I The application that profits most is cardiac imaging because eight-to 16-row scanners
Reconstruction an d Display Parameters
overcome many of the limitations of spatial and temporal resolution with present four-slice scanners, especially because some new scanners allow for 0.42 s or less tube rotation time. In addition. new techniques for dose reduction (ECGgated mAs modulation) and reduced scan field of view substantially reduce dose requirements with cardiac cr. Since the coverage is substantially increased. lower pitch factors can be chosen to keep artifacts to a minimum without compromising scan speed. However. all manufacturers use more elaborate reconstruction schemes that sufficiently compensate for cone beam gecmetry and allow for a maximum pitch Ps 1.5. Apart from these issues, the same general principles described above for volumetric scanning and fast spiral scanning hold true, and acquisition parameters only have to be adapted to the collimation settings avallable with a specific scanner (Table 4.6). GE supplies an eight-detector row scanner with 0.5 s rotation speed. Similar to their fourslice scanner, multiples of 1.25mm can be chosen as section collimation and reconstructed section width. The manufacturer allows for a volume pitch of 5 (UQ, P= 0.625), 7 (UM, P=0.875), 10.8 (UF, P=1.35), and 13.4 (US, P= 1.675). A volume pitch of 10.8 is suggested for most applications because it provides the best compromise between coverage, dose requirements, and image quality. A detector configuration of 8 x 1.25 mm at a table feed of 13.5 mm/rot, can be suggested for volumetric imaging, and a 8x2.5 mm configuration at 27 mm/rot, yields best results for fast spiral imaging. A special one-slice mode is available for HRCT of the chest. GE is now also supplying a 16-slice scanner that gives the choice between 16 x 0.625 mm and 16 x 1.25 mm collimation. The pitch can be varied in four steps. A volume pitch P*=9 (P =0.5625) and P*= 15 (P= 0.9375) yields a minimum SW that is similar to SC, while P*=22 (P= 1.375) and P*=28 (P=1.75) are more doseefficient but provide a 30% wider SW (0.8mm and 1.6mm, respectively). Apart from high-resolution applications, a pitch P*=22 (28 for CTA) provides the best compromise. Philips and Siemens have introduced a 16-detector row scanner that allows for 16 x 0.75 mm and 16 x 1.5 mm collimation. Like the present scanners, standard section width will be some
30% larger than the collimation (SW= 1 mm for SC=0.75mm, and SW=2mm for SC=1.5mm). This ensures improved signal-to-noise ratios. In addition, thin sections (identical to the collimation) can also be reconstructed but at the cost of a higher image noise. Such thin sections are only useful for the lung parenchyma and the peripheral skeleton. Pitch factors can be increased up to 1.5 (volume pitch P* up to 24) for Siemens and up to 2 (P* = 32) for Philips scanners. A standard pitch around 1.5 (P'" = 24) will suffice for most applications. Both vendors allow for continuous selection of pitch factors. For volumetric imaging ofthe chest, as well as for the abdomen in slim patients, a 16x0.75 mm detector configuration is best suited. In obese patients, a 16 x 1.5 mm configuration should be employed in the abdomen. Fast spiral scanning relies on a 16 x 1.5 mm collimation. Toshiba offers an 8-slice as well as a 16-slice scanner. On the latter, Toshiba provides a 16 x 0.5 mm, 16x 1 mm and 16x 2 mm collimation. Preferred pitch values are P*=ll (P=0.6875), P*=15 (P=0.9375), and 23 (p= 1.4375). When reconstructing sections SW that are identical to the collimation SC, image noise will be increased. Thus, this should only be done for high-resolution applications but not for standard techniques in the chest or abdomen. In case of doubt, it is better to use the next smaller section collimation instead and reconstruct (at least 30%) wider sections. Only for the peripheral skeleton, the neck, and the lungs can a 16xO.5 mm ultrahigh resolution protocol be recommended. In most standard situations in the neck, chest and abdomen, a 16 x 1 mm collimation yields less noise and thus a better image quality for volumetric imaging. A 16x2 mm collimation should be used for volumetric imaging in obese patients as well as for fast spiral scanning in standard patients. Fast image reconstruction is possible if the reconstruction increment RU is chosen 0.2, 0.5,1.0 or 2.0times SC.
Reconstruction and Display Parameters •
Section Width and Raw Data Interpolation
In single-slice spiral CT, the chosen collimation, the pitch factor, and the raw data interpolation algorithm determine the section width (see Figs. 1.13, 1.15). The section width SW grows continuously from 100% to 130% of the section collimation SC when the pitch is increased from 1 to 2 at a 180 LI algorithm. 0
123
Optimization of Scanning Technique A 360' LI algorithm is available only on a few scanners and requires the use of a pitch = 1 (SW=1.3xSC). lts main advantage is a better utilization of the radiation dose. In fact this algorithm reduces the noise as much as an increase in mAs by a factor of 2. Because scan coverage is low. this algorithm has been used mainly for dual slice systems in which this disadvantage is compensated by simultaneous acquisition of data from two detector rows. Reconstruction of thicker section width is possible with both single and multislice spiral CT but there are hardly any implementations in single-slice scanners available yet. Thicker sections for single slice CT have been employed for partial volume artifact reduction in the posterior fossa but have seen few if any use outside this application. They may. however. be used in a similar fashion to multislice CT, and then can serve as a tool to reduce image noise when thinsection spiral scanning had been employed. In multislice CT, the collimation, pitch factor, and reconstruction algorithm determine the minimum available section width SW (see Fig. 1.25). SW varies in a complex fashion between 100% and 130% of the section collimation SC if pitch is increased from 1 to 2 and a 180' MLI interpolation is used. A similar behavior is found for 360' MLI up to a pitch of 1. Adaptive interpolation schemes fix the section width at some 130% of the collimation, independent of the pitch (see Fig. 1.29). For fast spiral scanning, thick sections are reconstructed from the raw data set, even if thin collimation was used to obtain a scan. The section width depends on the size of the patient and the clinical indication (Table 4.5, see also Table 2.2). Thin sections and MPR are only reconstructed as a problem-solving technique if thick axial sections alone cannot resolve the clinical question. For volumetric scanning, overlapping thin sections of 0.5-1.5 mm width are reconstructed every 0.4-1 mm. This "secondary raw data set" serves as the basis for arbitrary cut planes using thick MPR. lt can be employed as a standard for indications such as staging of bronchogenic cancer or CT colonography (see organ chapters for more details), or as an adjunct if suspicious lesions are found (see Fig. 7.29). For indications, in which standard axial, coronal, and sagittal sections are desired for image interpretation, optimum results are gained if the axial plane-
as well as all other required planes-are tioned orthogonally relative to the patient and not relative to the scan table. Hence, positioning of the patient can be easily rected by imageprocessing. •
Reconstruction
posiaxes malcor1
Interval
The reconstruction interval Rl (synonyms: reconstruction increment, reconstruction index, or spacing) in spiral or multislice CT can be chosen independently from the collimation or section width. The advantage of a large reconstruction interval is that it reduces the number of images that must be reconstructed and interpreted. The main disadvantage of non-overlapping sections is that they do not provide optimum lesion detection, resulting in a less confident evaluation and a lower detection rate (up to 30% lower in the liver and lung). Overlapping reconstruction results in a greater number of images that have to be interpreted, processed, and stored. The quality of multiplanar and 3D reconstructions will generally improve with a higher degree of image overlap. Theoretical considerations suggest that a reconstruction increment that is some 30% less than half the section width is an optimum balance between practicality, accuracy, and image quality. In thin-section multislice CT, however, it makes little sense to choose a reconstruction interval that is smaller than the pixel size in the scan plane. For practical reasons, a reconstruction increment that is identical to half the section width suffices for most indications. Usually, in fact, conventional spiral CT as well as multislice CT require an overlap of only 30%, which is sufficient for most routine applications, e.g., lesion detection (Tables 4.2 and 4.5). Convolution Algorithm)
Kernel (Reconstruction
The selection of the convolution kernel always involves a trade-off between spatial and contrast resolution (see Fig. 5.11). High-resolution kernels always result in a disproportionate increase in image noise and may even introduce edge-enhancement artifacts. They are suitable only for regions in which image contrast is high, such as lung parenchyma, and bone. Soft kernels are suitable for cases in which high contrast resolution is required or image noise is excessive due to a low detector signal. This applies particularly to abdominal scanning in obese patients
Reconstruction Table 4.5
Suggested
scan parameters
for multislice
CT scanning
and Display Parameters
of the body (4-slice scanners)
Acquisition
Pitch factor"
N-SC/TF
P
P*
SW/Rlb
SWIRl'
2XO.625/1.S75
1.5
3
0.65/0.5
0.65/0.5
Volumetric resolution)
imaging
(ultrahigh
4X1.25/7.5
1.5
6
1.6/0.Sd
5/3"
Volumetric
imaging
(standard)
4X1.25/3.75
0.75
3
1.25/0.5
1.25-2.5/2
Volumetric
imaging
(skeleton)
4x2.5/7.5
0.75
3
2.5/1.3
7.5/5"
Volumetric patients)
imaging
(obese
4X2.5/15
1.5
6
3/1.5d
5/5
Fast sprial scanning
(standard)
4x3.75/22.5
1.5
6
5.0/2.5
7.5/5
Fast sprial scanning patients)
(obese
0.S-1.5
1.6-3
0.5/0.5
0.7/0.5
Volumetric resolution)
(ultrahigh
Image reconstruction
Application
General Electric
Philips. Siemens 2XO.5/0.S-1.5
imaging
4X1/6-S
1.5-2
6-S
1.25/0.7
3-5/3
Volumetric
imaging
(standard)
4X1/3
0.75
3
1.0-1.5/0.5
1-2/2
Volumetric
imaging
(skeleton)
4X1/S
2
S
2/1
6-S/5
Volumetric patients)
imaging
(obese
4x2.5/15
1.5
6
3/1.5
6/5
Fast spiral scanning
(standard)
4x2.5/20
2
S
4/2
S/5
Fast spiral scanning patients)
(obese
4XO.5/1.5-2.75
0.75-1.375
3-5.5
0.5/0.5
0.5/0.5
Volumetric resolution)
imaging
(ultrahigh
4x 1/5.5
1.375
5.5
1.5/0.7
3-5/3
Volumetric
imaging
(standard)
4X1/3
0.75
3
1.0-1.5/0.5
1-2/2
Volumetric
imaging
(skeleton)
4x2/11
1.375-1.75
5.5-7
2.5/1.3
S/5
Volumetric patients)
imaging
(obese
4X2/11
1.375
5.5
2.5/1.3
6/5
Fast spiral scanning
(standard)
4X3/16.5
1.375
5.5
4.0/2
S/5
Fast spiral scanning patients)
(obese
N = number of active detector rows; SC = section (mm), RI = recon. Increment (mm)
collimation
(mm); TF = table feed (mm), SW = section
width
a Pitch factor
P=TF(NxSC); Volume pitch P* =TF/SC parameters for reconstruction of an overlapping 3D volume (secondary raw data set) from which MPR and 3D displays can be generated; not necessary for fast spiral scanning unless MPR or 3D display is required c parameters for image review and reporting (may vary depending on clinical indication), may be reconstructed by thick MPR from 3D volume (secondary raw data set) d user interface displays SW = 1.25 and 2.5, respectively e needs to be reconstructed by thick MPR from 3D volume (secondary raw data set)
b
as well as to situations that require the use of a greatly restricted dose (e.g., pediatrics and pregnancy). When switching to a more smoothing kernel,images at first appear "blurred" but radiologistsquickly get used to the new appearance.
Apart from classic HRCTapplications, most soft tissue imaging will not require the extra spatial resolution provided by a sharper kernel (see Fig. 1.4). Hence, they can be used to decrease image noise at a lower exposure dose. In obese
N = number of active detector rows; SC = section collimation (mm); TF = Table feed (mm); SW = section width (mm); RI = reconstruction increment (mm) "Pitch P = TF / (N x SC); Volume Pitch P* = TF/SC b parameters for reconstruction of an overlapping 3 D volume (secondary raw data set) from which MPR and 3 D displays can be generated; not necessary for fast spiral scanning unless a second plane of reconstruction of 3 D display is required C parameters for image review and reporting (vary depending on clinical indication, and may be reconstructed by thick MPR from secondary raw data set) d pitch factors may have to be varied according to newest developments by the manufacturers e optional: 8-slice mode (8x3 mm) may be used in very obese patients on a Philips unit
patients with a large body cross-section, the pixel size is large (0.8-1 mm) and limits spatial resolution. In such a situation, the smoothing kernels not only reduce noise, but also do not markedly influence spatial resolution.
Judicious selection of the convolution kernel and mAs will help reduce the radiation dose. A smoothing kernel should be used in obese patients.
Reconstruction
•
Field
of View
The field of view (FOy) selection limits image reconstruction to a particular body region of interest to obtain the best possible spatial resolution (Table 4.7). For example. portions of the subcutaneous fat can usually be omitted from the scanned field. A constant FOV should be used throughout the scanned body region to allow for multiplanar reformatting and 3D reconstructions. For this reason. the FOV should be based on the widest portion of the scanned region. For high-resolution thoracic CT, some authors recommend a separate reconstruction of each lung to obtain the smallest possible FOV. However, the gain is minimal compared with a tightly limited field confined to both lungs. In addition, some modern scanners allow for reconstruction of a 10242 matrix, which will provide a large field of view at optimum spatial resolution.
Table 4.7 a Effect of reconstruction filter kernel spatial resolution (example of single-slice scanner) Kemel
3.2 Lp/cm
5 Lplcm
Smooth
4.0 Lp/cm
6.2 Lp/cm
Standard
4.8 Lp/cm
High resolution
8.5 Lp/cm
7.8 Lplcm 12.6 Lplcm
Very smooth
Exact
numbers
resolution hgher
a small objects b
vary
between
for multi slice
manufacturers,
4J
scanners
that size are displayed
original contrast boundary resdutim
at 4
to two times with 20% of
%MTF
Table 4.7 b Effect of FOV selection attainable spatial resolution
FOV
on the
maximum
Boundary
Pixel size
resolution
S'l Zmm
1mm
5 Lplcm
256mm
0.5 mm
10 Lp/cm
200rrrn
O.39mm
12.0 Lp/cm
150mm ~xel size limits
O.29mm spatial
Choice of reconstruction small FOV
on
Boundary resomon"
Resolution at 20%MTFa
17.6 Lp/cm
resolution filter
for large
kernel
FOV
is limiting
for
and Display Parameters
A limited field (FOV of approximately 25cm) can be useful in studies of the adrenals and pancreas. Reconstructions including the whole cross-sectional body area should additionally be obtained if abnormalities outside this field of view are suspected (e.g., pancreatic cancer). For CT angiography, it is generally unnecessary to reconstruct a 'whole-body cross-section". A FOV of 20-25 em can substantially improve the delineation of small vessels and intraluminal pathology (pixel size: 0.4-0.5 rnrn). Excessive magnification (FOV too small) only amplifies the apparent noise without, improving spatial resolution (FOV < 15 cm 5 is rarely advised). 2. With a large FOV, high-resolution kernels give poor results and are therefore not recommended (Table 4.7). Window
Selection
The window level (or center) should be chosen close to the CT number of the primary structure of interest (e.g., 50-70 HU for most soft tissue imaging). The window should be wide enough to capture all relevant attenuation numbers (e.g., fat should not be completely black and enhanced vessels not completely white) but as narrow as possible if low attenuation differences shall be detected (e.g., liver imaging). In general, no structure of interest should be completely white or completely black, unless more than one window setting is available (e.g., for the chest). The window width should also be increased for imaging high-contrast structures. This particularly applies to the lung, where a window setting that is too narrow (800-1000 HU) can cause pleural lesions to be missed or can exaggerate the apparent size of small lesions and bronchial wall thickness. Narrow windows are useful only in the detection of emphysematous changes. A large window width decreases the visibility of image noise and may be used for low-dose examinations in which local contrast is not critical. In studies that are strictly confined to high-contrast structures (e.g., dedicated skeletal studies), a wide window setting can be used along with a substantially reduced radiation dose.
127
Optimization
of Scanning
Technique
sponse of the human observer. This may improve local contrast for wide window settings but makes it difficult to compare window settings to other scanners.
Some manufacturers (e.g., Philips) use a gradation curve that assigns gray levels nonlinearly to the CT numbers within the chosen CT window to compensate for the gray level re-
Image Processing
cr.
Image processing gives the best results with data sets that were acquired with thin collimation. For spiral and multislice CT, a thin section width and a reconstruction increment with at least 50% overlap should be chosen for a "secondary raw data set" that serves as the basis for optimum quality of reformations and 3D reconstruction with the acquired data set. In most cases, the section width should be chosen some 20-30% wider than the collimation in order to reduce noise and optimize image quality for a given patient exposure (see above). Image processing, apart from thick axial reformations, is required whenever further evaluation of a volumetric data set is intended. Table 4.8 gives an overview of the types of processing that we suggest for various clinical indications.
single-slice spiral The technique of thick axial reformations is the more effective the thinner the primary sections and the higher the overlap. If such axial sections are reformatted, the operator can also correct for positioning errors, e.g., at the skull base, the neck, the shoulders, or the pelvis. In general. axial. coronal or sagittal reformations should be aligned not to the CT table but to the patient anatomy. The workstation should have a simple tool to perform this realignment of data to patient anatomy. The commonest further processing techniques are coronal or sagittal multiplanner reformations. They are routinely indicated for all situations that require a second imaging plane for optimum evaluation. This is the case for many musculoskeletal applications, such as for the cervical, thoracic, and lumbar spine. For joint trauma in the shoulders, elbows, hands, hip, knee, and foot, biplanar reformations are mandatory. In the chest and abdomen, most diagnostic questions can be resolved by axial sections alone. Coronal, and sometimes sagittal reformations are mainly required for planning surgery, e.g., for staging of bronchial cancer, complex liver lesions, pancreatic tumors, kidney
2D Processing Axial images with a larger section width can be reformatted from the secondary raw data set, even if the scanner does not allow for direct reconstruction of such thick axial sections. The technique can be used for multislice CT as well as for
Table 4.8
Suggested
Processing
image
processing
Technique
Thick axial sections Correct
Standard
for positioning
Coronal
or sagittal
techniques
depending
on clinical
indication
Application
errors
reformations
Petrous Tumor
viewing
technique,
bone,
shoulders,
staging,
bronchi,
noise reduction
pelvis,
intervertebral
bile ducts,
stomach,
disks bowel,
urinary
bladder,
kidneys Interactive
reformation
Curved
planar
Shaded
surface
Maximum
displays
intensity
Minimrm Volume
reformations
intensity rendering
projection projections techniques
Complex
pathology,
Vessels,
pancreatic
Complex
anatomy
CT angiography Bronchi,
pathology aorta,
(standard),
rendering
Virtual endoscopy
CT angiography Preoperative Colon,
interfaces
bile duct
skeletal
lung nodules,
structures interstitial
lung disease
biliary system (standard),
skeleton,
planning Perspective
near tissue
duct, common ofthe
planning
rarely bronchi
or arteries
complex
pathology,
preoperative
3D
tumors, and gastrointestinal malignancies, The evaluation of bowel diseases is improved when coronal reformations are available, Interactive reformation is required in more complex cases that cannot be resolved adequately on axial sections alone, Interactive reformations would be an ideal primary diagnostic interpretation mode but hardly any present workstation is fully capable of the (relatively simple) prerequisites for time-efficient work with this viewing modality (Table 4,9), Basic prerequisites include real-time interaction even for a large data set, full-screen display of the reformatted plane, and easy and intuitive tools for interaction, Interactive choice of the width of the reformatted plane is especially desirable with thin-section multislice imaging because image noise becomes the single most important limiting factor for image quality, In addition, a simple way of aligning the three imaging planes to the actual morphology makes image interpretation more easy, For more complex pathology, interactive variation of the imaging plane is helpful but thick sections should be available for the same reasons as described above, With most present workstations interactive scrolling speed is still insufficient when thick (oblique) sections are reformatted, although appropriate PC-based hardware has been available since the late 1990s, Curved planar reformations (CPR) are used mainly for vessel tracking in CT angiography and allow for optimum display of wall changes and eccentric plaques, and are the prerequisite for accurate measurement of curved vessel segments (e,g" prior to interventions), CPR are mandatory to evaluate the vessel lumen in case of vessel calcification, e,g" of peripheral runoff vessels, but semiautomated programs are required to make this procedure time-efficient Coronal CPR can be Table
4.9 CT workstations: prerequisites use of multi planar capacities of multi slice Real-time
Tools for aligning the three orthogonal planes to the actual anatomy • Interactive
for optimum
CT
adjustment
(with thick sections)
of the imaging
imaging plane
Processing
used as a standard display tool for longitudinal evaluation of the sternum, the spinal column, and the larynx and trachea, Coronal CPR through the pancreatic or common bile duct are used for CT cholangiography and CT pancreatography, Coronal CPR through the ureters are excellent for demonstrating the position of ureter calculi and for differentiating between calculi and ph leboliths. Such CPR, however, are quite time-consuming and difficult, and are made easier if the curve can be interactively corrected,
3D Processing
Shaded surface displays (SSD) have been all but replaced by high-quality (>5122 matrix) volume rendered displays, They are helpful for complex skeletal fractures (especially for the acetabulum and the shoulders) and can be used to display complex vascular anatomy (especially of the thoracic aorta), Maximum intensity projection (M I P) is an excellent display modality for CT angiography, especially where small vessels are concerned, For larger vessels and the display of three-dimensional relationships, volume rendering is superior. With the advent of (semi)automated bone removal, MIP has become a time-efficient imaging tooL Still, time can be saved if the images are prepared by the CT technologists, Sliding thin-slab MIP, however, is an excellent tool for evaluating complex anatomy of small vessels, MIP holds promise for the evaluation of pulmonary parenchymal disease but is still not routinely used for this application, Minimum intensity projections (mIP or MinIP) have only fringe indications in the chest (display of air-filled tracheobronchial structures and emphysematous bullae in the lungs) but other display techniques such as volume rendering are often superior. Volume rendering techniques (VRT) have replaced many other 3D display modalities with the advent of fast workstations and dedicated accelerator boards, VRT have become the standard technique for rapid display and evaluation of vascular disease in CTA Often, only minimum editing is needed for good results, VRT is replacing SSD for skeletal trauma, VRT displays should become standard for initial evaluation of trauma cases, At some point in the future, VRT will probably replace conventional axial sections and even
129
130
4 Optimization of Scanning Technique multiplanar reformations, especially when thinslab techniques and opacity settings that simulate conventional CTwindowing are used, Perspective rendering may make interactive movies look more realistic but it rarely yields additional information over other rendering tools, especially when such tools are combined with sectional planes that may reveal otherwise hidden structures, Using it depends very much on individual user preferences,
Virtual endoscopyis an endoluminal visualization technique which has the advantage that the evaluated structures are substantially enlarged, thus makingit possible to detect even subtle abnormalities, It has a proven value for the differentiation of colonic polyps and folds while the practical value for other organ systems (such as the larynx or bronchi) is very limited,
Special Scanning Techniques Quantitative CT (QCT) Quantitative CT is used primarily for the assessment of bone mineral density, It is most commonly used to assess the bone mineral content of the first three lumbar vertebrae, but in principle QCT can be used in other spinal segments and in the neck of the femur. It requires special analytical software and a suitable calibration phantom, The technique is described fully in Chapter 25 (Musculoskeletal System),
Dual-Energy CT Dual-energy CT is used for the detection and quantification of substances with a high atomic number, such as calcium, iodine or iron, The technique is based on the principle that the xray attenuation and CT numbers of these substances are strongly dependent on the tube voltage that is used, The method has found clinical application in bone densitometry (DEQCT) and for assessing liver iron in patients with hemosiderosis or hemochromatosis, The technique can reduce measurement errors due to variations in the fat content of the bone marrow or liver. Dual-energy CT is used only sporadically at present The base material decomposition technique, although more accurate, is no longer available with current scanners, The postprocessing dual-energy technique is still available and can be performed on any scanner. It involves taking two successive CT scans at the level of interest using two different tube voltages (e.g. , 80kVp and -140 kVp) , Though less precise than the base-material decomposition technique, the postprocessing technique is useful for assessing liver iron and bone mineral
content, which are estimated from the mean CT numbers in the region of interest (ROl) measured at the two energies, The scans can be performed at a low dose, provided a sufficiently large ROI (> 1 cm) is selected, I The attenuation difference (ACT) between' the two measurements is roughly proportional to the iron or bone mineral content of the tissue, Generally, this technique is satisfactory for the assessment of hepatic iron, Calibration phantoms are used in quantitative CT to improve accuracy and further reduce measurement errors due to fat Phantoms may consist of solid calibration standards composed of calcium hydroxyapatite or aqueous solutions ofk2HP04, To correct for fat-related errors, the j absorption difference in fatty tissue must be ! estimated by direct measurement in the subcutaneous or abdominal fatty tissue or in a standard ethanol solution, The ethanol solution is F less accurate, because its atomic composition is not identical to that of fat The following equation is used to calculate the mineral concentration Cm in the ROI from the concentration c of the calibration material and the attenuation i differences ACT for the ROl, ACTM for the calibration material, and ACTp for the fatty tissue: em
=
c x fACT-ACTFXACTM-ACT
A
This type of analysis requires an accurate determinationofACTp,otherwisethemeasurement errors would negate the advantage of the technique, Generally, the results are slightly less reproducible than conventional (single-energy) CT measurements, because the negative expressions in the equation tend to amplify the effect ofstatistical variations, For this reason, the results of the method are seldom worth the increased effort
Image Noise (p. 143) mAs Settings (p. 144) Tube Voltage (p. 144) Reconstruction Filter Kernel (p. 145) z-Filter (p. 146) Section Collimation (p. 146) Section Width and Pitch (p. 147) Body Diameter and Body Region (p. 149) Field-of-View(p.150) Window Setting and Perceptible Image Noise (p. 151) Dose Reduction
Negligible Reduction in Image Quality (p. 152) Trade-offs in Image Quality (p. 152) low-Dose
Scanning
Prerequisites for Low-dose Scanning (p. 153) HRCT(p.154) Screening CT (p. 154) CT Urography and Urolithiasis (p. 155) Children (p. 155)
_
5 Radiation Dose and Image Quality
Radiation Protection
156
Patient Selection (p. 156) Dose n Children (p. 156) Selection of Parameters (p. 157) Radiation Shielding (p. 158) Exposure of the Examiner to Radiation (p. 160)
The absolute values for radiation exposure in CT depend strongly on the scan parameters, scanner characteristics, and the patient. They may be 5 to 100 times higher than with conventional radiographs of the same anatomic region. This emphasizes the importance of radiation dose and tailoring the scan parameters to the individual patient.
Important
Dose
There are numerous parameters that de- ( scribe the radiation dose to the patient but the I following three are most important and have gained international acceptance to describe the local dose (volume CT dose index, CTDlvoi),Ihe total scan dose (dose-length product, DLP) and the radiation risk (effective dose, E).
Parameters
Volume CT Dose Index (CTDlvoi) The volume CT dose index (CTDlvoi) is a tool to indicate the average local dose to a patient within the scan volume. Its unit is the mGy (milli Gray) and there are separate data provided for body applications (measured in a 32cm PMMA phantom) or head scans (l6cm PMMA phantom). The CTDlvoi is a measure of the average local dose delivered by CT (conventional slice-by slice, single-slice or multislice spiral scanning) to a cross-section of such a phantom. The CIDlvoi is the most accessible dose indicator because it can be directly displayed on the user interface of modern CT scanners. It proivides immediate feedback about the dose delivered to the patient. Such a display is required by EU regulations, but outside the EU some manufacturers only provide display of the CTDlvoi when requested by the customer. The CTDlvoi allows for direct comparison of the radiation dose from different scan parameter settings, even for scanners of different manufacturers. It already corrects for the influence of pitch, mAs, kVp, filtering and other scanner-specific parameters. It does not, however, indicate the precise dose for any in-
dividual patient, but is rather an index of dose for a particular scanner and examination. The CTDlvoi is derived from the primary direct measurement of dose in CT, the CT dose index (CTDl) , which is defined as the integral under the dose profile D (z) of a CT section: 1 D(z)dz N.SC CTDl measurements are made at various positions within a body (32-cm diameter) or head (16-cm diameter) PMMA phantom. The phantom provides scattering media similar (though of course not identical) to a patient. The dose profile is markedly wider than the section collimation SC or section profile because of beam divergence and scattered radiation (Fig. 5.1 a). As a result, contributions from adjacent sections increase the local radiation dose during scanningofwhole body regions (Fig. 5.1 b). In theory, CTDl measurements must include all dose contributions from the tails of the dose profile. As this is often not practical, finite measurement limits are used. The subscript 100 in the term CTDhoo denotes that the measurement was made over lOOmm. The term CTDlw (weighted CTDl) represents the average radiation dose across the diameter of the phantom (in body scanning, the periphery and center measureCIDI
rs
133
Volume CT Dose Index (CTDlyoi)
1.0 0.8
2.0 Dose profile
CTDlvo\
Slice profile
1.5 (])
Vl
0.6
0
U (])
>
~rc
0.4
1.0
(])
e:::
0.5
0.2 0.0 -5 a
-4
-3
-2
-1
0
2
em
4
5
0.0 -10
o
-5
em
5
10
b
Fig. 5.1 Dose profile of a single CT section (a). The area under the curve describes the CT dose index (CTDI). Acquisition of multiple contiguous sections increases the local dose due to contributions from adjacent sections
(b). The maximum dose in the center of the scan volume corresponds to the volume CT dose index (CTDlvoi) if the scanned volume is large enough and a relative plateau is reached.
ment can vary by a factor of 2). The CTDIwis factor P [= TF / (NxSC) 1 and is then termed measured using a 100mm ionization chamber volume CTDI (CTDIvoi): that is placed at various positions within a CTDIvoi= CTDIw/ P 32cm (body) phantom (Fig.5.2a). The CTDIw is then calculated from the measurements in the 0::::::::--:: 1 em center and the average of the four measurements in the periphery, 1 cm below the phantom surface: CTDlw =- CTDIc+ -j- CTDlp The CTDIwis a very good measure of the average radiation dose within the scan volume for contiguous CT scans. To extend this to noncontiguous axial scans or spiral scanning with a pitch + 1, the CTDIw has to be corrected by the pitch
32em
o
16 em
o
o o
a
250% 200 % ..-
Fig. 5.2 Measurements of the CT dose index (CTDI) are performed in the center (CTDlc) and 1 cm below the surface (CTDlp) of a cylindrical Plexiglas phantom of 32 cm diameter for the body and 16cm for the head (a). The resulting CTDlw decreases with larger phantom diameters (b), which implies that the real average dose is underestimated for children and slim patients.
_
.
o b
(])
>
~
~
e WOO (e.g., in the lung or skeleton). Conversely, the use of a soft kernel should be considered in cases where a narrow window width is required (e.g., in the liver); otherwise noise can be reduced only by increasing the patient dose.
c
d
with a normal window width (b. 400 HU). The use of a high-resolution (MR) kernel provides excellent definition of the lung parenchyma (c. window width of 1500 HU), but the mediastinum suffers from increased noise (d. 400 HU).
Increasing the window width setting can greatly reduce perceived image noise. Doubling the window width reduces perceived noise by 50%. Doubling the window width also reduces image contrast by 50%. Consequently, structures with a high contrast require a significantly lower radiation dose (doubling the window width reduces the dose requirement by 75%).
151
5 Radiation Dose and Image Quality Dose Reduction
All dose reduction relies on the reduction of CTDIvoi or scan length. Negligible Reduction in Image Quality
On-line adaptive dose modulation will yield excellent results for most body regions where there is a marked difference in attenuation between AP and lateral diameters. This holds true especially for the shoulders but also for the chest, abdomen, and pelvis in slim patients. Dose reduction by some 10-30% will become possible, depending on the shape of the examined body area. Image quality is retained and even improved for the shoulders or the pelvis. Current implementations, however, suffer from a number of problems (see above). Longitudinal dose modulation adapts the dose to the local requirements in the body (e.g., less for the neck and chest, higher for the abdomen). This will become available soon (SmartrnA, GE; DoseRight, Philips; AEC, Siemens; Real EC, Toshiba) and will be an important step towards more constant image quality ("phototiming") in CT. New three-dimensional multislice CT interpolation schemes (noise filtering) substantially reduce noise without causing much smoothing of the data. This technique is not yet commercially available.
a
b
Fig. 5.15 The dose recommended by the manufacturer (a, 300 mAs, standard kernel) can be significantly reduced by the use of a soft kernel (b. 100 mAs). Note that the noise level is identical but the noise appears coarser.
Trade-ofts in Image Quality
The most well known trade-off is the increase in image noise if mAs settings are reduced and all other parameters are kept constant. Image noise can also be reduced by using more smoothing filter kernels for image reconstruction ("soft" instead of "standard", or "standard" instead of "high resolution" or "bone"). Thus there is a trade-off between dose and inplane spatial resolution if a smoothing kernel is employed together with appropriately reduced mAs (Fig. 5.15). Choosing a smoothing kernel for the soft tissues and a higher resolution kernel for the lungs or bones yields excellent results and allows for substantial dose savings (Fig. 5.16).
a
b
Fig. 5.16 The radiation dose in thoracic examinations can be significantly reduced (here: I = 40 mA, RT = 1.0s, pitch =1.6 => 25mASeff; CTDlvoi = 2.3 mGy) by using different kernels (HR and soft) to display the lung parenchyma (a) and mediastinum (b).
Image noise can also be reduced by choosing a thicker collimation or by reconstructing thicker sections (effective section width, SW) from thin axial data. This will reduce z-axis resolution but may be employed to reduce mAs settings. The multiplanar capabilities of multislice CT are best utilized by thin-collimation scan-
Low-Dose Scanning ning. However, patient dose (CTDIvoi) increases with such thinner sections, and image noise also grows substantially. Still, low-dose scanning is feasible with the following approach. An overlapping data set of thin sections (secondary raw data) is reconstructed first. This data set consists of noisy images (see Figs. 2.5, 2.14, and 5.18-19) but has high resolution in all three spatial directions. Thick multiplanar reformations are then performed from these data in any desired direction (axial, sagittal, coronal, oblique). These MPR will have good in-plane resolution as well as sufficient signal-to-noise (see also Fig. 5.13). The only drawback is the reduced through-plane resolution (resolution in the direction perpendicular to the MPR plane) due to the increase in MPRthickness. This is very similar to multiplanar imaging in MRI, where in-plane resolution is high, and a section thickness of 3-8 mm is employed. Image quality with this approach is excellent but-with four-slice scanners-the patient dose will be somewhat increased as compared to scanning with thicker sections because of the higher CTD] at thinner collimation (Fig.5.5a). With 16-slice scanners, geometric efficiency is substantially improved, and this negative effect is almost completely eliminated.
a
b
Fig. 5.17 Low-dose examination for colon cancer screening. Acquisition parameters: 4x 118 at 80kVp and CTDlvoi = 1.88 mGy, resulting in an effective dose of 1.3 mSv. Image noise is excessive with a soft tissue window setting (a) but is acceptable with a wide window
Low-Dose Scanning Prerequisites for Low-dose Scanning
Under the following four conditions, low-dose scanning becomes feasible: •
Reduced image quality requirements
Image quality is a critical feature in radiographic examinations. Reductions in quality are possible if there is pre-existing information about the disease (e.g., follow-up scans), if there is only one specific finding that is of importance in a certain clinical setting (e.g., presence of a polypoid lesion in the colon in colonic cancer screening) , or if only one organ system with high contrast has to be evaluated (e.g., lungs in lung cancer screening). Thick sections have been used as a standard in the past. If thick sections suffice (such as for the search for abdominal abscesses or hematoma) , dose may be reduced compared to protocols with a thinner section and similar signalto-noise ratio (e.g., by a factor of 2 if 10 mm instead of 5 mm sections may be employed). In multislice CT such thick sections may be reconstructed retrospectively as a means to improve in-plane image quality. Thin sections reduce partial volume effects and therefore increase the contrast of small structures. For this reason, an increase in image noise is acceptable on thin sections used for diagnostic re-
c setting ofW/L= 200010 (b) and is sufficient for virtual colonoscopy (c). In more obese patients, higher kVp settings should be used, and dose has to be increased to maintain image quality.
5 Radiation Dose and Image Quality view. The dose therefore needs not be proportionally increased as compared to thicker sections. •
Discontinuous
evaluation
of target organ
If the structure of interest can be evaluated in a discontinuous fashion. such as in HRCT for diffuse lung disease, effective radiation dose is reduced by a factor that is roughly identical to the relation between the width of the section and the table feed (e.g., factor 10 for 1 mm collimation and l Omm table feed in HRCT with a single-section technique). In multislice CT four or more sections are acquired simultaneously. Thus, the advantage of discontinuous scanning is reduced by that factor if no single-slice mode is available on such scanners. As a result, exposure dose for HRCT using such multislice scanners is still lower than with continuous CT but substantially higher than with single-slice CT. In addition, thin sections usually increase the CTDl somewhat, and will cause an increase in image noise that has to be compensated for by an increase in rnA settings. •
High contrast between the structures interest and their surroundings
of
High contrast makes it possible to employ wide window settings. Such wide window settings efficiently suppress noise (which is immediately evident when comparing lung and soft tissue window settings from HRCT of the lung, see Figs.5.14c, d). Thus, major dose reductions become feasible only if a wide window setting is required. This holds true for the lungs and the skeletal system. Improved contrast in CTA (by appropriate contrast injection protocols or scanning protocols) may be used to reduce patient dose while maintaining a reasonable signal-tonoise ratio between vessels and surroundings. •
Examined body part has little x-ray attenuation
Slim patients or children have lower x-ray attenuation than obese patients. This translates into lower dose requirements. The same holds true for organ regions with intrinsically low attenuation such as the lungs, the neck, and the extremities. For roughly every additional 4cm of soft tissue diameter, the required radiation dose for identical image noise doubles in phantom experiments. In clinical practice this increase is probably less (first results suggest some 8 cm for
doubling the dose) because obese patients contain more fat as an intrinsic contrast agent and thus are less vulnerable to noise, while children require somewhat sharper kernels for image reconstruction and thus require more dose than such a calculation would suggest. In practice, however, every 4-8 cm more soft tissue diameter requires double the dose, while for slim patients, the dose can be halved if the diameter is smaller by 4-8 em.
HRCT
High resolution CT (HRCT) of the chest is performed in a discontinuous fashion: 1-2 mm thick sections are acquired every 10-20 mm. This substantially reduces radiation exposure compared to spiral scanning. In standard size patients, the DLP should remain below 50mGyxcm, and the effective dose E should remain well below 1 mSv. Follow-up examinations or expiratory scans may be performed with further reduced dose.
Screening CT CT is excellent for screening of bronchogenic cancer patients since primary thin-
Multislice
section scanning allows for reconstruction of suspicious areas with a high spatial resolution and thus avoids additional scans in patients in whom a nodule was detected. In addition, it becomes much simpler to follow up patients with multiple nodules, especially when automated detection algorithms are used (Computer Aided Diagnosis or CAD). The mAs settings have to be adjusted to the size of the patient so that the CTDIvoi varies between 0.6 and 2mGy. Note, however, that image quality may deteriorate substantially if the patient is too obese and too Iowan exposure setting has been used (see Fig. 7.32, electronic noise). With single-slice spiral CT, a 5 mm collimation with a pitch of 2 is employed for lung cancer screening. Because of less noise on such thick sections than with thin-section multislice CT. an even lower CTDlvoi can be chosen, in the range of 0.3-1 mGy. However, additional thinsection scans are required if a nodule is detected.
Children Fig.5.18 CT Urographywith reduced dose. Acquisition parameters: 4x-1/8at120kVpand CTDlvoi= 1.88 mGy, resulting in an effective dose of 1.6 mSv. I mage noise on the thin-section images from the secondary raw data set (SWIRl = 1.25/0.7) is excessive (a) but can be substantially reduced when 20 mm thick curved planar reformations a re used to demonstrate the course of the ureter and the renal pelvis (b). Note the decrease contrast excretion by the tumor -bearing left kidney. a
In colon cancer screening there is a high contrast between the air-filled lumen and the bowel. Dependent on patient size, a CTDIvoi between 2 and 5 mGy can be used (Fig. 5.16). Smoothing reconstruction is mandatory and a section with of some 2mm is sufficient (and suffers from less noise) even for the secondary raw data set of overlapping axial images. CT Urography and Urolithiasis
CT urography after injection of IV contrast has an intrinsic high contrast. Thus, low-dose scans may be sufficient but mAs settings must be adapted to patient size (CTDIvoi =2-5 mGy). Thin-slab MIP or thick curved planar reformations (10-20 mm) parallel to the course of the ureters are used to evaluate the images (Fig. 5.1S) For the detection of urolithiasis, low-dose scanning is feasible. We found that even with CTDlvoi =2 mGy all therapeutically relevant stones > 2.5 mm could be seen in a 20x30 cm water phantom when thin-slab MIP were employed. However, noise increases quickly with more obese patients. A substantial increase in dose (5 mGy) may be necessary for this patient group.
b
Children
For the newborn and small infants it is advisable to reduce the tube voltage to SO-100 kVp because the CTDI falls with lower kVp. This allows for further dose reduction below the minimum mAs settings available with a standard of 120 kVp. Often as little as 0.3-1.5 mGy at SOkVp is sufficient (Fig. 5.19). The chest requires less dose than the abdomen. Contrast administration improves tissue differentiation and signal-to-noise, and therefore should be considered for examinations of the neck, mediastinum, and abdomen. For children between 4 and IOyears of age, 120 kVp with CTDIvoi= 2-5 mGy can be recommended (see Table 5.6). For all indications, a thin secondary raw data set should be obtained from which thicker sections can be reconstructed in arbitrary planes. The width of the MPR should remain as narrow as possible but as wide as necessary for a good signal-to-noise ratio (chest: 2-5 mm, abdomen 3-S mm).
155
156
5 Radiation Dose and Image Quality
a Fig. 5.19 Low dose examination in a 5-year old boy with a right-sided thoracic mass. Acquisition parameters: 4 x 1 18 at 80 kVp and CTDlvoi = 0.88 mGy, resulting in an effective dose of 0.35 mSv. Excessive noise on a 1 pixel thick coronal reformation from a secondary raw
data set that was reconstructed with a high-resolution kernel (a). The 5-mm thick coronal section from the same data set yields excellent image quality and demonstrates a right-sided interlobar effusion (b).
Radiation Protection
CT accounts for a large percentage (>30%, depending on the country) of the collective effective radiation from medical sources to which the general population is exposed. Patient selection, proper choice of scanning parameters, and radiation shielding can contribute to lowering overall radiation exposure. Patient Selection
The most important step is patient selection based on rigorous criteria. The question that is to be addressed by CT should be defined as clearly as possible, and the imaging modality itself must be capable of answering the question. The radiologist should also consider whether alternative modalities such as MRI or ultrasound could provide comparable information with less invasiveness. MRI should be considered as a primary imaging study mainly in young patients or in patients with non-neoplastic diseases that require frequent follow-up. Dose in Children
Radiation-induced risk in children can be substantially higher than the risk in the adult population. For this reason, the radiologist should
double-check the indication for CT, and should adapt the radiation dose to the body cross-section. Longitudinal and adaptive dose modulation would be ideal techniques, but presently only rough suggestions based on the body weight are available (Table 5.6). One has to keep in mind, however, that the body weight may underestimate the dose requirements in obese children because their body cross-section can be larger than that of a taller child of identical weight. Table 5.6 Suggested exposure settings depending patient size (CTDlvoiderived from data by Donnely, 2001 for aGE CT Ii). The pitch factor should always be 1.5 or larger (120 kVp) Weight(kg)
Selection of Scanning Parameters Selection of Scanning Parameters
Table 5.7 gives recommendations for selecting the convolution kernel, slice collimation, kVp, and CTDIvoi for various clinical indications. The numbers indicated are based on a "standard patient" of 170cm and 70kg body weight (20x30 cm body cross-section), and have to be increased for obese patients. The following considerations are important in the selection of imaging parameters.
For objects with high intrinsic contrast (lung parenchyma and bone), spatial resolution is almost independent of image noise. Thus, the wide window setting used for skeletal imaging and HRCT of the lung permits the use of narrow collimation, a high-resolution kernel, and low mAs values (Fig. 5.16). Contrasts are lower in CT angiography, so a soft kernel must be used to compensate for the narrow collimation that is required (Table 5.7).
Table 5.7 Suggested exposure parameters for various clinical indications for standard patients (70 kg/170 ern). These exposure levels should be adjusted to individual requirements because dose efficiency and filter kernels vary between scannertypes and manufacturers
CTAchest CTAabdomen Obese patients (100kg/170cm) Neck (benign disease) I Neck (tumor staging) I Chest (HRCT) jfefChest (benign disease) Chest (pulmonary embolism) Chest (tumor staging) Liver (pre-contrast) Liver (post contrast) Pancreas (tumor staging) Urolithiasis Abdomen (benign disease) CTAchest CTAabdomen a b
C
smoothing kernel for the mediastinum, high resolution kernel (or display filter) for the lungs 5 mm sections for the mediastinum and lungs; additional 1.5 mm sections for peripheral tumors (multislice CT) thinner sections and higher dose are chosen for preoperative evaluation of tumor patients
157
Radiation Dose and Image Quality
For structures with low intrinsic contrast, detection of small objects (as described by a contrast-detail curve) depends on image noise, and small structures may be completely obscured by noise. As a general rule, one should use the convolution kernel that provides the best trade-off between low noise (smoothing effect) and adequate spatial resolution (acceptable unsharpness). While this trade-off is always based on a subjective appraisal, in most cases it can substantially reduce the dose requirement below manufacturers' recommendations. Contrast resolution in liver studies is critical, because there is often very little attenuation difference between a hepatic lesion and the surrounding parenchyma. Lesions smaller than 3 mm can almost never be detected. A narrow window setting is required for noncontrast examinations, and therefore soft kernels should be used to avoid an excessive increase in dose (Fig. 5.15). Intravenous contrast medium generally increases the attenuation difference between tumor and liver, so the dose can be reduced when a soft kernel is used (Table 5.7). In abdominal examinations, similar principles to contrast-enhanced liver studies apply. In CT examinations of the lungs and neck, relatively little attenuation occurs within the tissues. Since many neck examinations require a detailed structural analysis, a somewhat higherresolution kernel should be used than for mediastinal studies. The dose in thoracic examinations can be reduced by using different display kernels for the lung parenchyma and the mediastinum (Fig. 5.16). Intervertebral disk studies have the highest dose requirement: the attenuation difference between the disk and its surroundings is small, thin collimation is used, and abdominal attenuation is frequently high. Again, the dose requirement can be reduced by using a soft convolutionkerne1. The following parameters are critical: • Number of scans: are precontrast scans, biphasic scans, or delayed scans really necessary? • Scan length (preferably limited to the region of interest). • CTDlvoi (choose mAs judiciously, using a lower setting for thin patients and children) .
• Pitch (should be "1.5 or more whenever possible). Slice width (thicker sections, whether axial or multiplanar. reduce noise and allow for CTDl reduction). Convolution kernel (soft kernels reduce noise and allow for CTDl reduction).
Radiation Shielding Outside the scan volume lead shielding can be used to reduce scattered radiation. It has to be noted, however, that most of the scattered radiation arises from within the patient and therefore cannot be shielded. Lead shields must not be included in the scan range because they cause substantial streak artifacts that warrant repeat scanning of the affected region. Shielding of direct radiation is only in its infancy. Bismuth-containing shields have been suggested that are radio-transparent, but still reduce surface exposure substantially. Such shields are especially helpful for reducing dose to superficial organs such as the breasts, the testes, and the eye lenses. The more superficial the organs, the higher the radiation doses to which they are exposed. This particularly applies to organs that are directly exposed in the scanning field, but it also pertains to those receiving scattered radiation. Testes that are unprotected in the scanning field may receive organ doses higher than 20m Gy per examination. In women, the breasts are exposed to particularly high dose levels (up to 25mSv) if they are within the scanning field. The dose is several times that received in grid mammography, so particularly stringent criteria should be used in selecting young women for CT examinations. It should be noted that in women with large breasts, CT scanning of the liver will incur some radiation exposure to the mammary gland. This can increase the organ dose to more than 50mGy, especially in biphasic examinations. Consequently, the breasts should be positioned to minimize their exposure in liver examinations.
Radiation Shielding Table 5.8
159
Oose and image quality
Examination parameters
t i
mAs-settings Tube voltage
Remarks
radiation exposure radiation exposure
i
Section collimation Pitch facto r
Effect
i
radiation
image noise image noise
1800-lnterpolation
image noise
z-Filtering: SW = SC
image noise
z-Filtering: SW> SC
image noise
t
image noise image noise
High resolution
filter kernel
image noise
Window width
t
noise perception
t
Body cross-section
image noise
t
t = increase; t
i tt
image noise
it;
(t)
t
preferentially
t
between 1.5 and 2
adapt to indication
(see Table 2.2)
for P = 1 for P > 1 increased dose requirements decreased dose requirements
=
standard with multislice
CT
for mediastinum/abdomen
xy-resolution t t t ; contrast t
t ; dose t
as low as possible couple with mAs
only in obese patients
t ; z-resolution t t ; z-resolution t t ; z-resolution t t ; z-resolution = t ; z-resolution ( t ) t ; in-plane-resolution t t ; xy-resolution t
Smoothing filter kernel
Size of lungs
noise
t ; z-resolution t exposure t ; z-resolution
i
MPR-thickness
noise
image noise
3600 -Interpolation
Section width
t ; image i ;image
for lungs/bone for lungs/bone/CTA
requirements
t
reduce mAs chest only; predictable gram
from scano-
= decrease; = = constant
Table 5.9 Relation of mAs settings (at 120 kV) to CTOlvol (single slice data calculated from data from Nagel 2000. multislice data from www.impacscan.org). The multislice data hold true only for 5 mm section collimation. For smaller sections. the CTOI increases (see Fig. 5.11)
Single slice scanner GE9800/HiLight/HiSpeed
series
Philips SR series/AV series/Secura PhilipS/Picker PQ series Philips/Elscint/Picker
Twin Flash
Siemens AR series/Emotion/Balance Siemens Plus 4 series Toshiba Xpeed/Xpress/XVision
Numbers for 120 kV. For 140 kV the CTOI is larger by a factor of approximately 1.36. The mAs settings have to be reduced by 26% to keep the CTOI constant. a Siemens and Philips multislice scanners display the effective mAs-settings. Thus. there is no pitchdependence.
•
160
5 Radiation Dose and Image Quality Exposure of the Examiner to Radiation Only individuals who remain in the scanning room are subject to radiation exposure (interventional procedures, ICU patients, persons accompanying the patient) as structural shielding can easily be designed and installed that essentially eliminates exposure outside of the scan roo 111. Every scanner has its own isodose map with contour lines indicating the spatial distribution of scattered radiation (Fig. 5.20). As always, scattered radiation depends on the applied dose (mAs), tube voltage, and patient cross section. Dose to persons within the scan room can be minimized using the radiation protection principles of shielding (use of a lead apron), time (limit time within the scan room), and distance (move as far away from the scan plane and patient as is possible). Since the radiation emanates from the gantry aperture, a bystander will receive a minimum dose by standing at an angle next to the gantry or close to the narrow side of the gantry. Persons involved in interventional procedures should not leave their hands in the scan plane during imaging (especially during CT fluoroscopy). If necessary, this can be avoided by using suitable holding devices for the interventional instruments.
4
3 2
o -1
-2-
-2
-1
o
2
3
m--
Fig. 5.20 Typical isodose map showing radiation emissions from a modern CT scanner. The dose figures are stated per 100 mAs for 120 kV and a 10 mm slice thickness (CTD1w~ 8 mGy).
6 CT-Guided Interventional Procedures M. Prokop, A.J. van der Molen
CT-Guided Biopsies
....
General Principles (p. 162) Biopsies in the Chest (p. 167) Biopsies in the Abdomen (p. 169) Musculoskeletal Biopsies (p. 171) CT-Guided
Drainage Procedures
General Principles (p. 174) Specific Techniques in the Chest and Abdomen Specialized Interventions
70000/mm3,PPT 50%
These values should be checked against the reference values of the reader's own laboratory. If possible, there should be timely termination of anticoagulation therapy before biopsy to allow a safety window for the procedure. Warfarin is terminated and IV heparin instituted instead. Patients with deranged clotting parameters may require substitution of their blood coagulation factors (e.g., fresh frozen plasma). Patients who are under direct threat of pulmonary embolism from deep venous thrombosis may require implantation of a temporary inferior vena cava filter before anticoagulation can be interrupted. If coagulopathy is suspected, extended clotting tests are obtained before planning the biopsy and special clearance by the hematology department may be required in particular cases.
Planning the Approach
Planning of the biopsy requires review of relevant previous cross-sectional and other imaging. A recent (contrast-enhanced) study of the biopsy region should be available. Alternatively, a diagnostic examination can precede the biopsy. However, in many cases, a limited non-enhanced study directly before the biopsy will suffice for safe evaluation of the access path. Additionally, this study serves as a test to assess if the position is comfortable for the patient. For abdominal lesions it is advantageous to have oral or rectal contrast medium on board to avoid bowel puncture. In doubtful cases an additional quick reference can be obtained with abdominal ultrasound to complement the assessment of bowel peristalsis. When selecting a representative area for tissue sampling, the operator should consider that the central part of a lesion often contains necrotic areas devoid of active tumor cells. In the case of pulmonary lesions, perifocal inflammatory reactions may occur at the periphery of the lesion (near the pleura), so the sample should be obtained as close to the hilum as possible. The route of approach should be as safe as possible (and the patient forewarned of any potential risks when consent is sought). Hence, it is essential to avoid nerves and vessels, the pleura and uninvolved organs if at all possible. In some cases the approach can be substantially facilitated by changing the depth of respiration, moving the patient's arm, or repositioning the patient. Approaches through the liver or lung parenchyma, or small bowel are acceptable, provided a sufficiently thin needle (18 G or smaller) is used and there is no significant amount of pleural or intraperitoneal fluid.
163
164
6 CT-Guided Interventlonal Procedures
b
a
c
Fig. 6.2 Needle biopsy of a small lesion in a lung transplantat patient. Planning (a), local anesthesia (b), fine core biopsy with a 20 G needle (c).
It is a sound general principle to take the simplest route of approach, bearing in mind that changing from a vertical or horizontal path to an oblique path, or even angling the gantry to place the needle path within the scan plane, increases the complexity of the approach. Only experienced examiners should select an approach that cannot be imaged in one plane of section. Multiplanar reformations of spiral or multislice CT data are useful in planning this type of approach. By viewing a suitable reference image, the operator can monitor the depth of infiltration anesthesia, the minimum distance between the needle insertion site and the lesion, and the maximum safe depth of needle penetration on the CT console. The cutaneous puncture site is marked on the z-axis with the laser-positioning device of the scanner (table position of the reference image). The location within the scan plane can be determined from anatomic landmarks (e.g., a vertebral spinous process), or from radiopaque markers attached to the skin (Fig. 6.2). •
Single scans will usually suffice for needle localization, but short spirals may be required for more complex approaches. With multislice CT, multiple sections are Simultaneously available, and the various manufacturers offer different solutions to take advantage of these sections. A good approach is to fuse the innermost sections to obtain a thicker target plane. The sections above and below this target plane serve as a control that easily demonstrates whether the needle deviates in a caudal or cranial direction and requires repositioning (Fig. 6.3).
Needle Localization Scans
For needle localization, it is generally sufficient to use the lowest scanning dose available (the smallest mAs and, if desired, the lowest kV setting). A foot switch and a display monitor in the examination room can vastly speed up the procedure, because the needle position can be quickly checked by the radiologist without having to leave the examination table.
Fig. 6.3 Multislice CT provides simultaneous sections. The inner ones can be used to define the target plane. Note that the needle is seen in the cranial image, indicating in which direction the needle deviates.
General Principles Guiding Systems
Various guiding systems are available to choose the correct needle path in a quicker and more reproducible way. Since less control scans are required. such guiding systems may also allow for a substantial reduction in radiation dose applied to the patient. The less experienced a user. and the more complex the access route. the more will the procedure profit from a guiding system. Simple guidtng devices help to define the correct angulation by providing an angular measure that can be directly attached to the needle or to the patient. Laser guides may either be part of a CT unit or may be provided by a different manufacturer and can be added to an existing unit. After planning the approach the correct angulation is transferred to the laser guiding system that indicates it as a laser beam either outside the gantry or inside the gantry opening. For systems that remain outside the gantry opening. no direct control of the needle position is possible as the needle tip is advanced. This has to be kept in mind when organs that move with respiration are biopsied. Even more sophisticated guiding systems are based on electromagnetic tracking of the tip and angulation of the needle. This information may then be overlaid on a reference CT data set that was acquired prior to the procedure. ifnecessary using contrast-enhancement. Such systems allow for complex angulation and reduced radiation exposure and promise not to compromise biopsy accuracy. Vital organ structures can be avoided more easily. especially since such systems (e.g .. the UltraGuide system) are aiso designed to compensate for respiratory movements. CT Fluoroscopy
CT fluoroscopy is a procedure that allows for online tracking of the needle as it passes through the tissue (see Fig. 1.33). The technique was developed for complex approaches or for regions with substantial respiratory motion (e.g .. lung bases). in which guided biopsies by conventional CT may be cumbersome. CT fluoroscopy should use low-dose settings because radiation dose can quickly become excessive with longer fluoroscopy times. However. such low-dose scans may suffer from
reduced image quality. which is particularly disturbing in difficult needle approaches. For practical purposes. fiuoroscopy should only be used intermittently during critical phases of the procedure. Having the patient change the depth of inspiration during fiuoroscopy may help bring the target lesion into the scanning plane. The needle is then advanced under fiuoroscopic control. Fluoroscopy with multislice scanners can demonstrate in which direction the needle leaves the target plane and thus make repositioning simpler. It has to be noted. however. that there is a potential dose increase if four sections are exposed instead of one. Speciai needle holders are available that make it possible to direct the needle without getting direct radiation exposure to the hand of the radiologist. Biopsy Needles
Percutaneous biopsies may be performed with fine needles (20-22 G). small core needles (1618 G). or large-core needles (14G and up). depending on the type of sample required. the risk of the approach. the anticipated pathology. and the experience of the evaluating pathologist. The smaller the lesion and the riskier the approach. the thinner the needle should be. Generally the presence of a trained cytologist and on-site analysis of the salvaged material help to significantly reduce the number of biopsy passes and to decrease the gauge of the required aspiration and guidtng needles. In many cases the biopsy needle can be substituted by aspiration cytology and unnecessary risk of hemorrhage and pneumothorax is avoided. This is especially important as the choice of the needle type (Fig. 6.4) or biopsy mechanism seems to have less effect on the diagnostic yield than the experience of examiner and cytologist. Aspiration needles can be of several types: Chiba needles (flat bevel tip). Franseen needles (serrated tip) or Westcott needles (with specimen side notch). Core biopsy needles generaily employ the Tru-Cut principle. There is a current trend toward semiautomated or automated high-speed biopsy instruments in which the trocar and core needle are individually or successively fired with a spring-loaded mechanism (Blopty, Temno, Gallini, etc.). There are severai coaxial biopsy systems in which multiple samples can be retrieved
165
6 CT-Cuided Interventional Procedures
Chiba
Greene
Westcott
Rotex
~
~
22,23
G
20,22
G
•
((E
18,20,22
18,19,5,21
~
G
G
22G
~
Fig. 6.4 Various types core biopsy needles.
G
22 G
~
Tru-Cut
G
16,18,20,22
~
Franseen
Otto
22,23
~
Turner
Madayag
making certain that the pleura, peritoneum, or periosteum are adequately infiltrated. For lung biopsies, the pleura should be anesthetized without piercing the pleural tissue with the needle. Striking a nerve will elicit a sharp, shooting pain, and the needle should be repositioned. If desired, localization of the inserted anesthesia needle can be performed to recheck the direction of the approach. Injections in children may be easier if a topical anesthetic cream (e.g., EMLA) is applied to the skin \hour before the procedure.
Size
Type
: ~
and sizes
14-22G.
of fine needles
and
through an indwelling outer cannula. Needles with graduated markings simplify the biopsy procedure, as do sleeve-type depth indicators that are mounted on the needle shaft. Some questions cannot be adequately resolved by fine-needle aspiration biopsy and cytology, and core biopsy is needed to furnish material for histological evaluation (Table 6.2). •
Local Anesthesia
The usual technique is to inject 5 to 20ml of local anesthetic (e.g., lidocaine, mepivacaine), Table 6.2 Techniques opsy indications
For some biopsies as well as for more complex interventions, conscious sedation may be needed for optimal results. In these cases adequate monitoring of pulse oximetry, ECG, heart rate and blood pressure is needed. A number of medications are frequently used, often in combination. Benzodiazepines give anxiolysis and sedation but do not provide analgesia. In older patients, and patients with cardiovascular disease, hypotonia and depression of breathing may be seen. Patients often like the short -term amnesia that may be associated with these drugs. They are contraindicated in patients with allergy, muscular diseases, and in pregnancy. Diazepam is the best known with good anxiolytic properties, but has a long elimination-time. For this reason, midazolam is more often used. Benzodiazepines can be antagonized byflumazenil. Propofoi in a selected dose can be used effectively for sedation in patients who breathe spontaneously. It is short acting and elimination is fast, but it has no analgesic effects. Dosing should be done carefully as overdosing may induce depression of breathing and there is no antidote available. Also, patients should be adequately monitored. In children, ketamine can be used for sedation in short procedures. It has both sedative and anesthetic effects. It is usually combined with atropine sulfate to counteract hypersalivation that might compromise the airway. Emergent reactions preclude the use of this agent in adults, but in children these are rare. For longer procedures it can be combined with benzodiazepines like midazolam. Opiates are mainly used for analgesia, but they may have some sedative effects in higher doses. Side effects include nausea, vomitting,
Biopsies in the Chest
breathing depression and hypotension. Frequently used examples are the strong opiates piritramid and the short-acting alfentanil and remifentanil. These opiates can be combined with benzodiazepines or propofol (dose adaptations'). Naloxone can be used as an antagonist for opiates. Non-steroid anti-inflammatory drugs can also be used for analgesia and also have antipyretic and anti-inflammatory profiles. They can be used to supplement opiates for pain reduction. They should be used carefully in patients with ulcer disease. asthma, and reduced renal function. Parenteral administration is possible with diclofenac or metamizole, but the latter can have serious side effects. Needless to say, adequate training should be sought before these drugs are administered by radiologists and their nursing staff. Hence, conscious sedation is often supervised by the referring clinician or anesthesiologist. Puncture Technique
A small stab incision is made in the skin, and the biopsy needle is introduced and advanced along the pre-selected path. It is easier to control the needle alignment when an assistant stands at the head or foot of the table. The laser guide of the scanner or external biopsy aids can be used to improve orientation. Depending on the risk of the approach, the needle may be inserted directly into the lesion or it may be advanced in stages while each stage is checked with a localization scan. The needle tip appears as a hypodense streak (hardening effect) in continuity with the axis of the needle shaft. In structures that move with respirations, the needle should not be fixed but should be allowed to move with the excursions, as otherwise it might lacerate the liver, lung, or other organs during respiration. The safest technique is to advance the needle while respiration is suspended. Sudden shooting pains signal that a nerve has been struck, and the needle should be repositioned. For a fine-needle aspiration cytology (FNAC), the needle is advanced to the surface of the lesion orjust inside it. The stylet is removed, a syringe is attached (20ml Luer-Lock), and the needle is moved in a fan-shaped pattern ("needling"). hence piercing the lesion several times, while suction is applied. The suction is released
before the needle is withdrawn to avoid seeding potentially malignant cells into the needle tract. When a core biopsy is performed with a core biopsy needle (or its automated or semiautomated variants), the biopsy needle is again advanced up to orjust inside the lesion. It should be noted that the core biopsy needle is surmounted by about a 5 mm-long trocar point that does not collect tissue. Advancing the trocar into the lesion loads tissue into the 5-20 mm-long sample notch of the needle, then the sharp-edged outer sleeve is rapidly advanced over the trocar to collect the tissue core. A spring-loaded mechanism may be used to accomplish one or both of these steps, usually providing a less hemorrhagic sample that is easier to evaluate. With larger core biopsy specimens simultaneous cytology "roll" smears can be made, or when coaxial technique is performed, the core biopsy can be preceded by fine needle aspiration cytology (FNAC). Final Check
After the sample has been obtained, a control CT scan should be taken to exclude a pneumothorax or acute hemorrhage. If a small pneumothorax is detected with CT, a conventional chest radiograph should be taken at 1 -4 hours after the biopsy and the clinicians informed. Depending on the needle size and route of approach, several hours' bed rest may be advised to reduce the risk of post biopsy hemorrhage.
Biopsies in the Chest Lung The location of the lesion is an important factor in deciding whether to perform a trans bronchial biopsy (central bronchial lesion, "positive bronchus sign" of a bronchus directly entering the lesion on CT) or a percutaneous transthoracic fine needle aspiration cytology (FNAC) or small core needle biopsy (peripheral lesion not related to the bronchi). Transthoracic biopsy is contraindicated in patients with a contralateral pneumonectomy, pulmonary hypertension, or pulmonary arteriovenous malformation and severe chronic obstructive pulmonary disease (COPD). FNAC of the lung is most often performed for the diagnosis of carcinoma, while core biopsies are more important for benign lesions,
167
6
CT-Guided Interventional Procedures
sclerotic lesions, lymphomas, and other noncarcinomatous malignancies, For diagnosing malignancy, transthoracic biopsy has reported sensitivities of 70-100% and high specificities, For benign processes, the yield is 85-95% but the specificity is much lower and varies between 15% and 65%, Prerequisites for a higher yield are repeated sampling from different parts of the lesion and expert on-site cytopathological advice with histological confirmation ff needed. Pulmonary lesions should be biopsied with the patient prone (if access is short enough) in shallow breathing and suspended respiration during needle movements, It is sound practice, especially with small lesions, to advance the needle into the lesion before the stiletto is withdrawn, In biopsies of extensive lesions, material should be sampled from perihilar portions of the lesion and not from its necrotic center. The risk of pneumothorax in lung biopsies may be as high as 40%, and patients must have adequate pulmonary function (Oz partial pressure > 60 mm Hg) and should not have serious emphysema or advanced pulmonary hypertension, The risk of pneumothorax is increased in patients with obstructive airway disease, emphysema, multiple needle passes, and in deep-seated or small lesions, Between 10 and 30% of all pneumothoraces have been reported to need chest drainage, The number of air leaks may be reduced by sealing the needle tract with Gelfoam or throm bin at the end of the procedure, Another method that has been successful for avoiding pneumothorax is a coaxial technique using a guiding canula with a sharp and blunt trocar. The sharp trocar is only used to advance the needle close to the pleura, which is then penetrated slowly using the blunt trocar. A sharp biopsy needle is only inserted close to the target lesion, This technique is less prone to causing pleural tears as the needle is advanced into the lung, The risk of a delayed pneumothorax can be reduced by positioning the patient with the biopsied side down, Small intrapulmonary hemorrhages along the needle track (see Fig,9,30a) are common, However, hemoptysis occurs only in 2% to 5% of cases, and more often at biopsy of vascularized
lesions, It is generally self-limiting, Rare complications like air embolism or needle tract seeding occur in 0.1 % and fatal complications have been reported in 0,2% of cases, Mediastinum
Percutaneous biopsy is appropriate for lesions that are inaccessible by the transbronchial route or by mediastinoscopy, It is also used in cases where a previous transbronchial biopsy has been attempted without success, Hilar lesions should be biopsied at a central site to avoid false-negative results from areas of posts ten otic pneumonia or atelectatic lung, The safe biopsy of mediastinal lesions requires that the location of vascular structures be known precisely, and contrast medium must be administered for vascular opacification, The risk of pneumothorax can be reduced By performing the biopsy at full expiration (increases contact area on anterior chest wall) and by injecting 20-50 ml of physiologic saline solution to displace the lungs laterally (Fig, 6,5), The chosen approach will depend on the location of the lesion, and is either anterior parasternal or posterior paravertebral. A direct mediastinal route is preferred to a trans pulmonary one to avoid the risk of pneumothorax, However, if a trans pulmonary approach cannot be avoided, FNAC should be preferred, Another option is the transsternal approach (Fig, 6,6), in which a hollow needle (e.g. a 2mm Ostycut needle (Bard) for biopsy with an 18 gauge cutting needle) is passed just through the sternum to establish access for the aspiration cytology or core biopsy, While FNAC has a high sensitivity and specificity for metastatic carcinoma, core biopsies are needed for non carcinomatous malignancies, lymphoma, and in benign lesions, For the diagnosis of thymoma, a surgical technique is generally suggested and may directly precede tumor resection, The success rate of mediastinal biopsies is Significantly higher with core biopsies (approximately 90%) than with FNAC, Pneumothorax is reported in some 10% of cases and hemorrhage in 3%,
Biopsies in the Abdomen
Fig.6.5 The route of approach can be expanded by the injection of physiologic salinesolution.
• Pleura CT is a good imaging modality in the identification of and guidance for aspiration and tap of loculated collections of the pleura, and biopsy of associated pleural thickening. A supra costal approach is mandatory to avoid damage to the intercostal neurovascular bundle. In most cases, the biopsy path is tangential to the thoracic wall. 18-20 G core biopsies give the best results in pleural masses, because the lesions are often inhomogeneous and immunohistochemical methods are needed for differentiation. For malignant disease, a sensitivity of >80% and specificity near 100% has been reported. In benign disease, the main role is identification of tuberculous pleuritis if CT morphology is equivocal. As with most biopsies, multiple passes are required for optimal accuracy. In experienced hands, major complications occur only in less than 1 % and include pneumothorax or hematothorax. There is a risk of tumor seeding, especially in mesothelioma. For mesothelioma and lymphoma, however, thoracoscopic (VATS) biopsy remain the reference standard. Biopsies in the Abdomen •
a
b
Fig. 6.6 Transsternai approach using a 2 mm Ostycut needle (a) followed by 18 gauge core biopsy (b) of a suspected recurrence ofT -cell non-Hodg kin's lymphoma.
Liver
In selecting patients for a liver biopsy, it should be decided whether the diagnosis could be adequately established by imaging procedures (typical findings in two separate procedures) rather than percutaneous biopsy. If hepatocellular carcinoma (HCC) is suspected, the risk of peritoneal tumor spillage would contraindicate liver biopsy in patients who require surgical treatment regardless of the biopsy result. An ultrasound-guided biopsy is preferred in many cases, because the biopsy can be monitored in real time and can often be completed during one inspiration. Care should be taken in patients with free intraperitoneal fluid as any liver biopsy can induce severe hemorrhage in these patients. Therefore, in patients with diffuse liver disease transjugular biopsy via a liver vein should be preferred, and in patients with focal liver pathology pre- interventional ascites tap may be helpful to reduce the risk of hemorrhage.
169
CT-Guided
imervenuone;
Procedures
A liver core biopsy is preferred over FNAC because of its greater accuracy, especially in the differentiation of benign hepatic lesions, The accuracy rate of core biopsy is greater than 90%, and its complication rate (bleeding, pneumothorax, puncture of other organs) is low, The individual route of approach to a hepatic lesion depends on its location and size, The shortest transhepatic route is generally preferable, Hemangiomas and other hypervascular tumors, should be biopsied through a longer, self-tamponading approach through normal liver tissue to reduce the risk of hemorrhage, If a subphrenic lesion cannot be biopsied in full inspiration via the extrapleural route, the biopsy should be performed in expiration or in the right lateral decubitus position to reduce the volume of lung tissue in the pulmonary recess, Biopsy employing gantry angulation can be beneficial and even trans pleural approaches have been used successfully with low complication rates in selected cases, Gallbladder,
Spleen
Percutaneous biopsy of the gallbladder is rarely indicated, because generally there is no difficulty in diagnosing a lesion that has expanded past the gallbladder to involve other tissues, Splenic masses are usually associated with systemic lymphomatous diseases that are more easily biopsied at other sites, Another reason to avoid gallbladder and splenic biopsies is their high rate of complications (biliary peritonitis and hemorrhage), FNAC should be performed only in highly selected cases, Pancreas
CT (fluoroscopy) and ultrasound are prerequisites for guidance of pancreatic biopsies, as they allow for real-time monitoring of the procedure, Using routine CT (non-fluoroscopy) guidance it is difficult to obtain a representative sample from small lesions, and negative cytology is generally unreliable, The sensitivity of FNAC is variable (70-100%) but is comparable to the accuracy of an intraoperative biopsy, The complication rate of pancreatic biopsy is low when a small-gauge needle is used, Many pancreatic lesions are difficult to detect without contrast medium, hence the planning scan should routinely include contrast CT Pancreatic biopsies can be performed through an anterior or posterior approach, A transgastric
or trans intestinal approach requires 6 hours' fasting to reduce the risk of contamination and infection, For safety reasons FNAC is generally preferred, The biopsy should be performed with a breath-hold in an intermediate position because respiratory excursions cause movement of the liver and other intraperitoneal organs relative to the retroperitoneal pancreas and deep abdominal respiration could withdraw an unfixed anterior needle from the pancreas, Kidney
Renal biopsy is generally not indicated in patients with renal cell carcinoma (RCC), because the biopsy sample cannot often differentiate the lesion from adenoma, the rich vascular supply increases the risk of hemorrhage, and tumors tend to disseminate into the needle tract or the lung, In doubtful cases the diagnosis should be established by surgical enucleation, which can be combined with intraoperative frozen section analysis, Renal biopsy for focal lesions is indicated only in patients with a suspected isolated renal lymphoma or metastasis, Exceptions are patients with tuberous sclerosis and equivocal findings in cross-sectional imaging, in whom repeated laparotomies cannot be performed, More frequent indications are core biopsies of native kidneys for diffuse disease or transplant kidneys for rejection, For best results, "14G core biopsies are taken from the lower-pole cortex, If ultrasound guidance is not preferred, the biopsy can be performed under CT-guidance by a posterior approach, and FNAC will generally furnish an adequate sample, Complications in the form of retroperitoneal or intraperitoneal hemorrhage or gross hematuria are rare, Adrenal Gland
Adrenal biopsy is appropriate in cases where a suspected metastatic lesion or indeterminate adrenal mass cannot be adequately diagnosed with CT MRI or other imaging procedures, In patients with a known primary tumor, FNAC is satisfactory for the detection of adrenal metastasis, In patients with a suspected primary adrenal tumor, an effort should be made to establish a histological diagnosis, The sensitivity and specificity are close to 100% with core biopsy and 90-100% with FNAC, If a pheochromocytoma cannot be excluded, appropriate pharmacological prepara-
Musculoskeletal
tion with' 0,1 and 0.2 blockade is indicated because of the risk of a hypertensive crisis. For this purpose the expertise of the local endocrinology department should be enlisted. Adrenal biopsy is best performed through a posterior approach with the patient positioned prone or with the affected side down. The adrenals can also be biopsied through a lateral approach or through an anterior approach (transhepatic, if necessary). Transrenal and transsplenic approaches should be avoided. Saline injection can expand the posterior paravertebral space and facilitate a posterior extrapleural approach (see Fig. 6.5). Retroperitoneal
and Mesenteric
Masses
A posterior paravertebral approach is most suitable for the biopsy of retroperitoneal lymph nodes and indeterminate retroperitoneal masses. Core biopsies are preferred, although injury to the lumbar arteries can produce extensive retroperitoneal hemorrhage. For lesions located between the vena cava and aorta, FNAC should be preferred using an anterior (trans intestinal) approach avoiding the colon. Nodular mesenteric masses in patients with an unknown primary tumor should be investigated by core biopsy from an anterior approach. Pelvic Region
The difficulty of differentiating scar tissue from recurrent tumor on cross-sectional irnagingespecially in rectal carcinoma-is perhaps the most typical indication for biopsy in this region. FNAC is usually adequate, especially since core biopsies can be very painful and, when scarring is present, often do not yield a diagnostic specimen. The complication rate of FNAC is extremely low, and its accuracy rate is higher than 90%. Structures cranial to the linea terminalis are biopsied via a trans peritoneal approach. Structures caudal to the linea terminalis are accessible through the abdominal wall or sciatic foramen. In selected cases specialized routes may be needed which can give good results (see also section on CT-guided drainage).
Biopsies
Musculoskeletal Biopsies Biopsy of Bone Lesions
In recent years, percutaneous biopsy of bone lesions has become a routine procedure in centers specializing in their treatment. For guidance, CT as well as fluoroscopy or ultrasound can be used. CT-guidance is especially valuable in deep-seated lesions and lesions that lie adjacent to vital structures. Biopsy is planned in close communication with the referring specialist (especially for malignant lesions). Bone scintigraphy is used to select the most accessible site for biopsy. Osteolytic regions are preferable for biopsy and thinslice CT is indispensable before biopsy to guide the appropriate access, as it can better differentiate osteolytic from sclerotic regions within the lesion. It is also used for selecting the proper needle type. Additional MRI may have a supplementary advantage in differentiating vital tumor regions from cysts or necrosis as well as gutding the needle pass in highly vascular lesions. All bone biopsies must be performed under strict aseptic conditions due to the high risk of infection. The periosteum should be thoroughly anesthetized. Sedation and general anesthesia should not be used for biopsies in proximity to neural structures. Bone biopsy can be performed as aspiration or trephine (core) biopsy. In general, trephine biopsy is more efficient than the alternative, both for neoplasms and infections, especially tuberculosis. However, aspiration can be useful in lytic lesions. A multitude of special needles are available for bone biopsy. Short trephine needles (e.g., Yamshidi, Tanzer) have large calibers of 8-11 G and are useful for superficial lesions or the penetration of cortical shells. They have the disadvantage that reinsertion for multiple biopsies is required. Longer trocar systems (e.g., Ackerman, Craig, Laredo-Bard) are placed coaxially and allow for multiple passes in deeper-seated lesions. Another coaxial system is the Bonopty needle which has an eccentric drill and is useful for penetrating intact cortex in peripheral bones. It can be used in combination with softtissue needles for lytic lesions. Pneumatic motor-drill systems (e.g., Ostycut) may be useful in locations where the cortex is thick or in sclerotic lesions. Recommended needle types are summarized in Table 6.3.
171
6 CT-Cuided Interventional Procedures
a
b
Fig. 6.7
c
Sample approaches for vertebral biopsies in the cervical spine (a), thoracic spine (b), and lumbar spine (c).
Table 6.3
Recommended
needle types (after Laredo,
1999) Bone tumors
Superficial bone lessions
Yamshidi, Tanzer
Deep bone lesions
Ackerman, Craig, Laredo-Bard
Sclerotic
Ostycut. Bonopty
Lytic
Tru-Cut core biopsy needle
iytic
+
bony shell
Yamshidi
+
Tru-Cut
Lesions in the thoracolumbar spine are approached via a posterolateral route with the patient in a lateral or prone position (Fig. 6.7). Lesions of the posterolateral body or pedicles are best reached via a transpedicular route, while lesions in the more anterior part require a steeper lateromedial angle (40°-60°) directly to the body. Needle tilt can be important to avoid damage to exiting nerve structures, and may require tilting the gantry. In the cervical spine an anterolateral or lateral approach should be chosen for lesions in the vertebral bodies. Neural arch lesions can be reached via posterior routes with tangential approaches. In peripheral bones the approach may be variable but especially in primary tumors should be chosen in close consultation with the referring surgeon, because the needle path should be resected at surgery. The path should be perpendicular to the cortex (Fig. 6.8) and aspiration of the bone marrow can be performed simultaneously. Penetration of the marrow may be painful, requiring general anesthesia. For some malignant lesions, such as osteosarcomas, open surgical biopsy may be
a
b
Fig. 6.8 Percutaneous biopsy of a lytic lesion in the talus that proved histologically to be a giant cell tumor.
preferable for better control of bleeding complications. In flat bones a tangential approach is better for sampling the maximum amount of pathologic bone and to avoid injuries to underlying structures. It is advisable to sample different sites of the lesion. Both the sampled bone tissue and any blood aspirated from the bone should be submitted to the pathologist. Close communication with the pathologist is needed for proper fixation, e.g., when electron microscopy is contemplated. If an inflammatory process is suspected, the bulk of the biopsy material should also be prepared for microbiological testing. In experienced hands complications are few and can be less than 1 %. Complications may adversely affect surgical treatment and outcome, especially in malignant bone tumors, in a significant number of patients. Therefore, proper
Musculoskeletal technique is important and these biopsies are best performed in specialized centers. In general, percutaneous biopsies are less invasive and associated with a lower morbidity and cost than surgical biopsies. The diagnostic accuracy depends on many factors, like the nature of the lesion, quality of preoperative work-up, selection of biopsy site, biopsy material, and technique and experience of the pathologist. The accuracy cited in the literature ranges from 80% to 95% for bone metastases and round cell tumors (Ewing' sarcoma, lymphoma) and 60-80% for primary bone tumors. In infection, success rates of 90% for skeletal tuberculosis and 60% for pyogenic infections have been reported. •
Biopsy of Malignant Soft Tissue Tumors
For many years, biopsy of malignant soft-tissue tumors has been the terrain of the oncologic surgeon because of the risk of compartmental contamination as well as the histopathological heterogeneity of most soft-tissue tumors. Recently, percutaneous biopsies have been more frequently used for the primary diagnosis with good results. Close interaction with the surgical oncologist is necessary and therefore these biopsies are best done in centers specializing in surgery of the tumors. CT is most commonly used for deep-seated lesions, while ultrasound is a strong competitor for large and more superficial lesions. CT has the added benefit that it can document the needle track accurately. Before biopsy, close communication with the operating surgeon is important as the needle track should be resected during subsequent surgery. General pre-biopsy principles should be followed as outlined above, and generally a preoperative gadolinium-enhanced MRI should be available to select the viable tumor regions. Biopsy can be performed as needle-core biopsy or fine needle aspiration biopsy (FNAC).Core biopsy is usually preferred in larger tumors and has the advantage of allowing specific subtyping of sarcomas. FNACis more difficult in mesenchymal tumors and is often reserved for small lesions.The preference of the surgeon or radiologist as well as the experience of the cytologist and pathologist will also influence the choice of the type of biopsy in the individual patient. Core biopsy is usually performed with 14-18 G side-cutting (e.g., Tru-Cut, Quick-Core, Gallini) or tip-cutting (Surecut, IBI) needles. To
Biopsies
prevent seeding of the biopsy track a coaxial approach may be taken through an outer sheath or Yamshidi needle, which is placed into the periphery of the tumor. Often the side-cutting needles are connected to a biopsy gun for ease of the procedure. Fine-needle biopsy employs 20-23 G side-cutting (Westcott, Burney) or tipcutting (Chiba) needles connected to a syringe. It is best to have a cytopathologic technician onsite for direct evaluation of the biopsy material. A suggested algorithm from the literature is presented in Table 6.4. Complications from percutaneous biopsies are similar to complications for other CT-guided biopsies. They occur in less than 1-4% of biopsies, and are less frequent than with open surgical biopsy. This is important since any complication may alter the timing and the type of subsequent surgical procedures, and thus may considerably influence patient outcome. It has been demonstrated that the number of clinically relevant complications in specialist centers is significantly smaller. Few studies have evaluated the accuracy of this technique. Core biopsies have an accuracy of 84-90%, while the accuracy of fine needle aspiration lies in the 90-96% range. Major reasons for inaccurate results are failures to obtain diagnostic samples and to sample parts of the tumor that are not representative for the whole lesion. The latter is more important in lowgrade malignancies and malignancies with necrotic and cystic areas.
Table 6.4 al.,1996)
Recommended
biopsy algorithm
(Logan et
Soft tissue tumors < 3 cm
20-22 G FNAC
;;. 3 cm
18 G core biopsy
Bone tumors Lytic + soft tissue mass 50°C), Vessels around the lesion may cause rapid cooling of tissue resulting in less optimum coagulation (heat sink effect), It has been demonstrated that overlapping treatment areas can increase the efficacy of thermo ablation in larger lesions, Ideally 6-14 overlapping ablations of 3cm in a lesion of 2-3 cm diameter should be used but for reasons of clinical practicality the number of overlapping areas is usually reduced to below six, Ablation of a cuff of 5-10 mm of normal liver tissue should always be attempted, Ablation needles with a variety of features are available, These include retractable prongs and internal cooling of the tip, Newer devices optimize the amount of tissue that is coagulated in a Single session, Simultaneous intratumoral saline or doxorubicin injections and an occlusion of the local (parenchymal) blood flow (Pringle maneuver) have been used to enhance the effect oftherrnoablation for lesions larger than 3-5 ern. The procedure can be performed intraoperatively as well as percutaneously under guidance ofCT or ultrasound or both, Before the procedure is scheduled, recent CT and MRI scans should be available or additional imaging may have to be obtained prior to intervention. Large-bore venous access is made available and supportive therapy with IV fluids and peri-interventional antibiosis is initiated, One or two ground pads are positioned on the
Liver Tumor Ablation patient's back or thigh and the patient is anesthetized (general anesthesia or local anesthesia including the liver capsule under conscious sedation). The vital parameters are closely monitored at all stages of the procedure. The needle is then advanced into the target area and the electrode is connected to a 100200 W RF generator. After deployment of the electrode prongs, the lesion is ablated with continuous or pulsed protocols at the desired temperature for 8-25 minutes, depending on the equipment and individual lesion size. Temperature sensors in the electrode tip control the temperature delivered to the tissue. Depending on its design, the electrode generates a sphere of tissue coagulation 3-7 cm in diameter (Fig. 6.11). If multiple overlapping ablations are required, the needle can be repositioned and the procedure repeated. Individualized strategies are employed for complex situations with subcapsular tumors and central lesions in close proximity to large vessels. Multiple ablations with only partial deployment of the prongs and the use of different designs of needle may be required to safely enhance efficacy. During and after the procedure, most patients experience pain and nausea that subsides within a week. Therefore good practice should include the best available management by the local pain team including the use of
a
b
Fig. 6.11 Radiofrequency (RF) ablation of a liver metastasis from colorectal cancer. The hypovascular lesion (a) is treated via an anterior approach using a needle with extendable prongs (b). After ablation (c),
epidural on-demand analgesia with morphine analogs. Major complications can include capsular necrosis, intraperitoneal hemorrhage, pleural (hemorrhagic) effusion, perforation of bowel or bile ducts, and cholangitis. A baseline CT to document therapy success is performed within a week after the procedure. Multiphasic technique is employed, preferably with portal venous and early delayed phases for hypovascular (metastases) and arterial and portal phases for hypervascular lesions (HCC). Follow-up with CT, MRI, or contrast -enhanced ultrasound every 3-4 months is recommended, ideally within a standardized surveillance program. As thermal ablation is a new, rapidly changing therapy, no long-term studies or studies with surgical correlation are yet available. Initial studies in HCC reported complete necrosis rates of 48-95% and a tumor -free interval of 12-24 months in 64-71% of patients. For small tumors, survival rates of approximately 60% after 3 years and about one-third after 5 years can be expected. For metastases, complete necrosis was achieved in 52-93% of lesions and tumor-free rate was 11-52% at 9-18 months follow-up. Long-term data indicate a 46% survival after 3 years. Local tumor control appears significantly better for small metastases with patient survival rates of78% for lesions below 2.5 cm diameter and 32% for lesions larger than 4.0cm.
c there is coagulation necrosis with lack of contrast uptake but a small remnant of tumor tissue in the periphery (arrow).
179
6 CT-Cuided
Interventionaf
Procedures
Laser Ablation
A second type of energy that can induce tissue coagulation in the liver is the laser. Laser ablation is commonly referred to as interstitial laser photocoagulation (ILP) or laser-induced thernnotherapy (UTI), and is most frequently used for the treatment of metastasis, Thermal ablation with laser uses neodymium: yttrium-aluminum-garnet (Nd: YAG) or solid-state laser types delivering their energy through 400 urn fibers, Different fiber geometries result in different areas of necrosis, Bare tip fibers will create spherical lesions and cooled-tip diffuser fibers will result in elliptical lesions 10l8mgiml) from transudative fluids ( 2 mm in diameter absence of clinical signs of infection active protein =CRP, leukocytosis) • lack of response to antibiotics
(C-re-
Areas of liquefaction may occur in tumors as well as infectious processes. Generally tumors are distinguished by a larger soft-tissue component, a thicker wall, and less regular margins. In doubtful cases, an effort should be made to establish a tissue diagnosis by fine-needle aspiration or core biopsy.
Infiltration versus Retraction
The margins of a lesion can provide clues to its etiology. Smooth margins are typical of expansile lesions and most metastases. Spiculated margins may indicate an infiltrative lesion or may signify retraction effects due to scarring (Fig. 7.22). Infiltration may have a tumorous or nontumorous etiology. Tumorous infiltration is most commonly seen with primary organ tumors (bronchogenic carcinoma, hepatocellular carcinoma, pancreatic carcinoma) but is occasionally caused by heavily vascularized metastases (Kaposi sarcoma, angiosarcoma). N ontumorous infiltration may be caused by infectious processes with a cellulitic component.
• alveolar cell carcinoma from pneumonia (compare Fig. 9.31 to 9.39 and 9.40) • recurrent tumor from chronic infection following surgery (see Fig. 15.19) • chronic pancreatitis from pancreatitis in pancreatic-head carcinoma • inflammatory bowel disease from intestinal lymphoma • gastric cancer (Tl) or early lymphoma from gastritis • diverticulitis versus colon cancer (compare Fig.15. 21 toI5.15c). While there are no generally valid rules, the following clinical and radiologic features are suggestive of a tumor:
Fig. 7.22 Spicules radiating from a lesion may be the result of an infiltrative process or scarring: bronchial carcinoma (a), scar tissue in silicosis (b).
203
Image
Analysis
Retraction effects are typically caused by scarring as a result of trauma, surgery, radiotherapy, infection, or successful tumor therapy, lt can be difficult to differentiate the various causes of spiculated margins in any given lesion, but the following guidelines should be helpful, Radiating strands of scar tissue generally have well-defined margins and are less tightly clustered than the spicules of an infiltrative lesion, Perifocal edema is rare, Scars may distort the parenchymal architecture, which seldom occurs with infiltrative processes,
Within soft tissues (neck, mediastinum, abdomen), ill-defined spiculated margins on CT scans suggest an infiltrative process (inflammation or high-grade malignancy, especially nonHodgkin lymphoma), Again, well-defined spicules are more consistent with retractile processes due to scarring, Perifocal edema can obscure fat planes and is often indistinguishable from infiltration; its presence signifies an active process (tumor, infection, recent surgery, lymph stasis, venous stasis),
Enhancement Characteristics Normal Enhancement Characteristics Pulmonary Vessels
Opacified blood and nonopacified blood are generally well mixed within the right ventricle, As a result, the maximum enhancement of the pulmonary vessels is identical to peak aortic enhancement Maximum opacification depends strongly on the cardiac output (dilution effect) and the injection parameters (contrast concentration, flow rate), Enhancement of the pulmonary arteries is some 5 s earlier than the enhancement of the pulmonary veins, If there is a temporary enhancement of the aorta before contrast arrives in the pulmonary vessels, a persistent oval foramen can be suspected, Such a foramen opens temporarily as there is a rise in right atrial pressure due to contrast material injection and allows blood to enter the systemic circulation first Systemic Vessels
The aoJta and systemic etteties are defined with exquisite clarity during the arterial phase of enhancement The enhancement of the aorta (and its branching arteries) reaches a relative plateau rapidly after contrast material arrives at a particular level in the aorta, As contrast material is injected the level of this "plateau" increases further (see Fig, 24.3) until shortly after the injection (contrast material and saline flush) stops, The coverage of a large imaging volume with high spatial resolution during this phase requires the use of multi slice scanning, About 20 to 30 seconds after the onset of arterial enhancement, contrast material is
delivered to the portal system, with inflow of contrast occurring first through the splenic vein and later through the mesenteric veins, This delay can lead to artifacts in the portal vein, where non opacified blood can mimic intraluminal thrombosis (see Figs, H.53 and 15,34), The time at which systemic veins enhance after intravenous contrast injection depends on their location, The renal veins enhance very rapidly because of the high renal blood flow, accompanied by enhancement of the intrahepatic segment of the inferior vena cava, A similar pattern is seen in the jugular veins, Venous drainage of the lower extremity and azygos system proceeds more slowly, but even in these regions nonopacified blood at confluence sites can mimic thrombosis, If the contrast material is injected at a very slow rate, the arterial and venous vessels will show similar patterns of enhancement determined solely by the total volume of contrast medium and the patient's circulating blood volume, Various enhancement phases are best separated if the contrast material is injected as a short bolus (limited volume, high flow rates), Arterial Phase
The onset of aortic enhancement after contrast injection depends largely on the circulation time and can range from less than 10 seconds to more than 60 seconds, It correlates with the heart rate only in patients who have a healthy circulatory system. For this reason, tailoring the examination to the individual patient by bolus triggering or test bolus injection is superior to a standardized examination technique,
Normal
The arterial phase denotes the period in which the enhancement of an organ is caused predominantly by contrast medium delivered to the organ by arterial perfusion. This phase continues as long as a high concentration of contrast material is delivered through the aorta. But the purely arterial phase lasts only a short time (15 to 30 seconds) before other effects. such as contrast diffusion into the interstitium. contrast delivery through the portal vessels. and renal parenchymal enhancement. are superimposed. Typical patterns of arterial-phase enhancement are observed in the spleen (serpentine hypo- and hyperattenuating regions due to the differential perfusion of the red and white pulp). the kidney ("corticomedullary phase" with intense cortical enhancement and faint medullary enhancement). the liver (minimal enhancement of normal parenchyma. since 75% of the hepatic blood supply is from the portal vein). the pancreas (strongest enhancement toward the end of the arterial phase). and the thyroid gland (intense enhancement of the entire gland). Mucosal enhancement in the bowel is strongest at the end of the arterial phase and beginning of the parenchymal phase.
Enhancement
Characteristics
portal phase approximately 60 to 90 seconds p.i. In the kidney, the renal cortex shows intense initial enhancement, and shortly thereafter contrast is delivered to the collecting ducts in the renal medulla. Thus the renal cortex shows earlier and more intense peak enhancement than the medulla. The true parenchymal phase starts when the cortex and medulla show approximately equal levels of attenuation (at about 80 to WO seconds p.i.). The normal intestinal mucosa shows the strongest enhancement at the beginning of the parenchymal phase and then loses enhancement over time. The muscles do not show significant enhancement in either the arterial or parenchymal phase. Interstitial
Phase
The interstitial phase is determined by the diffusion of contrast medium into and out of the interstitium; perfusion effects are of minor importance. Little organ enhancement persists during this phase, and the vessels in most organs appear isodense to the parenchyma. Contrast
Excretion
Contrast material is normally excreted almost entirely by the kidneys. At 3 to 5 minutes after Parenchymal Phase the injection, highly concentrated contrast meThe parenchymal phase denotes the period in dium appears in the excretory system of the uriwhich the parenchyma of an organ shows its nary tract. Because contrast medium is denser than urine, it forms a dependent layer in the urihighest level of enhancement. The time required to reach the parenchymal phase depends on the nary bladder. It is very rare for intravenously inorgan (Table 7.3). and this phase shows varying jected contrast to fill the bladder completely; degrees of overlap with the arterial phase or in- generally this requires retrograde contrast administration. terstitial phase of enhancement. The liver excretes approximately 2% of the In the pancreas. for example. maximum parenchymal enhancement is produced by arte- contrast medium into the bile. This percentage rial perfusion and occurs only about 40 to may be substantially higher in patients with im60 seconds after the start of the injection (p.t.). paired renal function. Opacification of the gallIn the liver. however. enhancement results pre- bladder occurs approximately 6 to 24 hours dominantly from the portal delivery of contrast after intravenous contrast administration. medium, so peak attenuation occurs during the
Table 7.3 , Organ
Phases
of enhancement Arterial
phase
Liver
Minimal
Spleen
Tiger-striping
Homogenous
enhancement
Homogenous
Pancreas
Slowly increasing
enhancement
Maximum
Kidneys
Corticomedullary
differentiation
Homogenous
Bowel
Increasing
mucosal
enhancement
Decreasing
after after
50-
120s
after musocal
p.i.
60 s p.i.
enhancement 80-100
at 40-60 s p.i.
enhancement
s p.
7 Image Analysis
Focal Lesions
•
Peripherally Enhancing Tumors
Focal lesions in parenchymal organs have approximately the same CT attenuation as their surroundings, but their vascular supply (perfusion) can differ markedly from the surrounding parenchyma. As a result, contrast administration can improve lesion detection and narrow the differential diagnosis. The best phase of organ perfusion for lesion detection is the phase that provides a maximum attenuation difference between the lesion and its surroundings. The pattern of contrast delivery and clearance relative to the surrounding parenchyma can be used for lesion characterization.
Tumors with a rich peripheral vascular supply are most common in the liver. Most such tumors are metastases (usually from adenocarcinoma), but cholangiocarcinoma (Fig. 7.23 b) may also occur. Rim enhancement is seen during the arterial phase, and may persist through later phases. In some lesions, however, scans during the portal phase may show only the central hypovascular portion of the tumor, resulting in underestimation of tumor size. A similar enhancement pattern may be seen outside the liver (e.g., in neck tumors) but does not have particular diagnostic significance.
•
U
Hypervascular Tumors
Hypervascular tumors show greater enhancement than the surrounding parenchyma during the arterial phase, generally appearing as a focus with transient hyperattenuation. If the scan is performed too late, many hypervascular tumors are no longer detectable. As a result, small lesions can often be detected only with (biphasic) spiral or multislice CT scanning. Examples of hypervascular tumors are renal cell carcinoma (Fig.7.23a), hepatocellular carcinoma, sarcomas, and most malignant tumors with endocrine activity. Benign hypervascular tumors include renal adenomas, hepatic adenomas, focal nodular hyperplasia, pheochromocytoma, and benign endocrine pancreatic tumors.
a
Hypovascular tumors are less perfused than the surrounding parenchyma, so they are most conspicuous in the phase of maximum organ enhancement. Some hypervascular tumors are also hypo attenuating to their surroundings in this phase (e.g., all renal tumors), provided the attenuation produced by arterial perfusion is below the level of parenchymal enhancement. Almost any tumor histology may present as a hypovascular lesion. A large percentage of all hepatic metastases are hypovascular, and hypovascular lesions are typical of pancreatic carcinoma (Fig. 7.23c). The maximum enhancement of an organ during the parenchymal phase is strongly dependent on the contrast injection parameters.
c
b
Fig. 7.23 Lesions are optimally displayed times, depending on the vascularity of the organ: pancreatic metastasis from renal cell in the arterial phase (a), cholangiocarcinoma
Hypovascular Tumors
at different tumor and carcinoma in the early
portal phase (b), pancreatic carcinoma in the parenchymal phase (c). Note the primary tumor and the periarterial invasion (arrows) as well as the atrophic tail of the pancreas with a dilated pancreatic duct (arrowheads).
Focal Lesions
Maximum attenuation in the pancreas occurs about 20 to 30 seconds earlier than in the liver, while maximum attenuation in the renal nephrogram occurs 20 to 30 seconds later than maximum liver enhancement. As a result, the timing of optimum lesion detection varies from organ to organ, and contrast administration always involves a trade-off with regard to imaging goals. •
Tumors Outside Parenchymal Organs
Tumors outside parenchymal organs, such as softtissue tumors and head and neck tumors, must be delineated from muscle tissue. Except for sarcomas, they are frequently hypovascular or slightly hypervascular. Tumor enhancement is based on arterial perfusion and increased contrast diffusion into the interstitium due to altered vascular permeability. Depending on the vascularization, tumors can best be appreciated towards the end of the arterial phase (arterial vascularization) or later, up until the interstitial phase (diffusion effects). Optimum detection of these lesions requires scanning with a long delay (60 s or more) and a narrow window setting. •
Cysts
Cysts do not enhance after contrast administration. A diagnostic criterion for cysts is the absence of a significant attenuation increase (> 10 HU) during any perfusion phase. Attenuation changes less than 10 HU are still within normal accuracy limits or may result from random fluctuations. Scans during a late hepatic phase or during the excretion phase in the kidneys often improve differentiation between tumors and cysts because even small tumors tend to
8-mm Sections
pre-contrast
Fig. 7.24
Pseudoenhancement
post contrast
of small cysts.
have taken up contrast during these phases and become hard to visualize while cysts remain distinctly hypo attenuating. Pseudoenhancement is a problem in areas where there is marked contrast uptake in normal parenchyma outside a cyst. It is most common in the kidneys and is the more pronounced the smaller the lesion. Pseudo enhancement is a consequence of partial volume effects, which change in magnitude as the extracystic parenchyma enhances (Fig. 7.24). For small cysts, a less stringent criterion of > 15 HU enhancement may be used, but it is better to reduce the section width for such an examination and even perform a late scan with thin sections to keep partial volume effects low. Accurate measurements can be expected if the cyst is larger than twice the section width. There are no enhancing (solid) elements found in the periphery of a true cyst. The presence of such elements, even when very small, should raise suspicion of a cystic tumor, or, when membrane-like, of infection. •
Inflammation and Abscesses
Inflammatory diseases of the mucosae (e.g., in the gastrointestinal tract) are characterized by intense mucosal enhancement during the arterial phase and early parenchymal phase. The inflammatory involvement of deeper wall layers leads to enhancement of the entire wall, in some cases obscuring the adjacent fatty tissue. A focal inflammation often produces no detectable changes within a parenchymal organ, but unenhanced scans may occasionally show a hypo dense area (edema), and a slightly hyperdense area may be seen after contrast adininis-
2-mm Sections
pre-contrast
post contrast
207
7 Image Analysis
(ration (arterial hyperperfusion or impaired interstitial back-diffusion). The predominant finding in fatty tissue is a diffuse or streak-like attenuation increase caused by edema and an inflammatory reaction. Abscesses may develop from a region of focal inflammation or from the infection of an existing fluid collection or hematoma. Liquefaction and abscess formation are marked by a central, nonenhancing cystic region with an attenuation of approximately 30HL] (pus) surrounded by an enhancing rim of relatively uniform thickness (Fig.7 .25a) . Usually this granulomatous rim is best appreciated during the interstitial phase of enhancement. Hepatic abscesses may be rimless or display a hypodense rim due to the intense enhancement of the surrounding parenchyma (Fig.7.25b). Because of this often invisible ring of granulation tissue, hepatic abscesses usually appear to have indistinct margins. Superiniected fluids (seroma, biloma, hematoma) have nonspecific CT densities that can range from 0 to 80 H. They do not show central enhancement, and it takes several days for a typical granulomatous rim to develop at the periphery (Fig.7 .25c). Generally this rim is narrow and is most conspicuous during the interstitial phase. A superinfected fluid collection is indistinguishable from a noninfected collection in its early stage. Diagnostic needle aspiration should be considered if laboratory findings are consistent with infection. An enhancing rim may also form in association with resorptive processes such as hem a-
a
b
toma breakdown, so this sign is highly suggestive of, though not specific for, an infectious process. •
Hemangiomas
Cavernous hemangiomas of the liver are loculated, blood-filled cavities with a delayed, centripetal pel-fusion pattern and very thin walls. This accounts for their enhancement characteristics on CT scans: On noncontrast images, hemangiomas are isodense to blood (even in anemic patients). Following contrast administration, the enhancing areas in the lesion show somewhat delayed but approximately equal attenuation to the aorta. Contrast -filled lacunae appear first at the periphery of the lesion and subsequently spread toward its center. By the portal phase at the latest, all the lacunae are isoattenuating to the blood vessels (blood pool effect). The time required for the complete opacification of a hemangioma can range from seconds to more than 30 minutes, the lesion displaying a characteristics pattern of peripheral-to-central ("centripetal") enhancement. If the blood and liver parenchyma are isoattenuating on delayed scans, the hemangioma can no longer be discerned. Note that there are a few atypical hemangiomas that may appear as a homogeneous hypervascular tumor (small lesions up to 2cm), as a heterogeneous tumor (large lesions greater than 3cm), or as a hypovascular lesion (thrombosed hemangiomas).
c
Fig. 7.25 Loculated hepatic abscess with an enhancing rim (a), hepatic abscess with a hypodense rim and air inclusion (b), superinfected biloma (c).
Artifacts and Pitfalls Anatomy
Previous Surgery
Anatomic variants or normal structures scanned on an unusual plane should not be misinterpreted as mass lesions (Fig. 7.26). Differentiation is accomplished by comparing the adjacent slices and by taking enhancement characteristics into consideration (Fig. 7.27). Table 7.4 lists some of the anatomic structures that are most commonly misidentified on CT scans.
Scars can be quite difficult to distinguish from tumors if the scar tissue forms a mass lesion. This is particularly true after proctectomy. Pulmonary metastases after therapy may resemble bronchial carcinoma. Dynamic contrast studies can sometimes differentiate tumors from scar tissue, since tumors enhance during the arterial or parenchymal phase of perfusion, while scar
b
a
Fig. 7.26 An accessory left superior vena cava mimics a mediastinal lymph node (a). The cisterna chyli mimics a retrocrural node (b). Bowel simulates lymph nodes (c),
a
b
c
d
and the fatty infiltration of a lymph femoral vein thrombosis (d).
7 Image Analysis Table 7.4 some anatomic identified on CT scans
structures
that may be mis-
Structure
May be mistaken for
Geniohyoid muscle
Lymph node
Scalene muscles
Lymph nodes
Sartorius muscle
Lymph node
Supra-aortic
Lymph nodes
arteries
Left superior vena cava
Lymph node
Aberrant vein
Lymph node
pulmonary
Pericardiac recess
Lymph node
Right atrial appendage
Tumor, lymph node
Atypical renal veins
Lymph nodes
Paravertebral veins
Lymph nodes
Pelvic venous plexus
Infiltrative growth
Fatty infiltration lymph node
Venous thrombosis
of
tumor
Cisterna chyli
Azygos vein, lymph node
Crus of diaphragm
Lymph node, esophagus
Accessory spleen
Lymph node, renal or adrenal tumor, peritoneal tumor
Bowel
Intra-abdominal tumor, lymph node, varices
Splanchnic varices
bowel loops
tissue does not enhance until the interstitial phase. Bowel loops after reconstructive surgery of the biliary system often collapse and may easily
be mistaken for a local mass. Oral cholegraphic contrast agent may be used to opacify such loops and allow for reliable differentiation from a real tumor (see Chapters 12 and 15). M ul tislice scanning may improve anatomic delineation as long as there is sufficient local abdominal fat between various soft tissue structures. Bladder wall edema following a transurethral resection (TUR) can mimic a residual tumor (see Fig. 19.7). "Cysts" can be seen followingTUR of the prostate. Partial Volume Effects
The influence of partial volume effects depends on the size of the structure relative to the section width and the position of the structure in relation to the scan plane. Thus, structures that are approximately parallel to the scan plane are far more subject to partial volume effects than structures perpendicular to the scan plane, i.e., structures oriented along the z-axis. In studies using relatively thick sections and nonoverlapping reconstructions, partial volume effects can reduce the contrast of small pulmonary or hepatic lesions located in an unfavorable position (between two adjacent slices), causing them to be missed. Scans that cut a portion of an adjacent structure may simulate lesions where none exist. For example, a section traversing an ossified, posteriorly protuberant attachment of the first rib to the sternum can mimic a solitary pulmonary nodule (Fig.7.28a). Similarly, a sec-
c Fig. 7.28 The sternal attachment of the first rib mimics an intrapulmonary hepatic lesion (b).
d nodule (a). The air-filled
colon mimics a
Window Setting Window Setting
a
b
Fig. 7.29 Suspected low attenuation mass in the left adrenal gland on a 5 mm section (a) that can be attributed to partial volume effects due an almost horizontal course of a normal size gland (b) if coronal MPR are reconstructed from an overlapping secondary raw data set in multislice CT (4 x 1/6).
tion cutting the upper pole of the right kidney, the gallbladder, or adjacent air-filled bowel segments can mimic an intrahepatic lesion (Fig.7.28b). The lower part of the caudate lobe may be mistaken for a lymph node at the porta hepatis. The portal venous confluence can simulate a lesion of the pancreas. Thick sections through an obliquely positioned adrenal gland may mimic a mass (Fig. 7.29). Unsharp tumor interfaces in oblique scans may prompt a spurious diagnosis of infiltration.
a
b
Fig. 7.30 Dependence of CT measurements on window width. When a narrow window is used (a), the bronchial wall thickness is overestimated compared with a wide window setting (b). The degree of calcified
CT window settings affect image contrast and noise. If the window is set too wide, image contrast is reduced, with the result that low-contrast lesions in the liver or pancreas may be missed or the enhancement of an abscess membrane, for example, may go undetected. Conversely, too narrow a window setting can significantly increase image noise and abolish grayscale differentiation in fatty tissue areas. This can hamper the evaluation of fine structural details and cause pathologic changes in fatty tissues (edema, infiltration) or free air to escape detection (see Fig. 7.9). The accurate estimation of lengths and diameters on CT scans requires a sufficiently large window width. Both the structure of interest and its surroundings should still contain gray levels and should not appear totally black or white. A typical example is the evaluation of bronchial wall thickness. This requires a window width > 1OOOHU (preferably 1500 HU). Other window settings lead to the overestimation of bronchial wall thickness (Fig.7.30a, b). CTA of calcified stenoses or stents requires wide window settings to correctly assess the degree of narrowing (Fig. 7.30c, d).
c
d
stenoses on CTA of the carotids varies with window settings. While a setting ofW/L=500/150 overestimates the stenosis (c), a setting of 1500/300 displays the lumen correctly (d).
211
7 Image Analysis
Beam-Hardening Artifacts
The radiation emitted from the x-ray tube of a CT scanner represents a spectrum of high and low energies. As the thickness of the scanned object increases, the low-energy spectral components are absorbed more than the higherenergy components. This phenomenon is called beam hardening. In defining CT numbers, a certain "intermediate" energy must be assumed for the absorption of x-rays by water. Since absorption is reduced at higher energies, increased beam hardening would cause a decline in CT numbers. The technical solution to this problem is to estimate the thickness of the object based on the x-ray absorption for each individual projection and mathematically correct for beam hardening based on the estimated thickness. One assumption in this computer correction is that the entire object is composed of just one substance (usually water). If the local atomic composition of the object differs markedly from that of water (bone, metal, concentrated iodinated contrast medium), beam-hardening artifact will continue to occur. Newer techniques are available for further reducing beam-hardening artifacts. This can be done either by using complex computer algorithms to determine beam hardening from the density distribution in the CT image (e.g., RASP, Toshiba) or by acquiring thinner slices that are then assembled to make a larger slice thickness (this can reduce beam hardening by structures that pass obliquely through the slice).
a
b
•
Ribs
Low-attenuation beam-hardening artifacts that occur behind ribs can mimic intrahepatic lesions, but they can usually be correctly identified by their characteristic location (Fig. 1.31 a). •
Extremities
If both arms are left in the scanning field in an adducted position at the patient's sides disturbing horizontal streak artifacts will appear between the bony structures due to increased xray absorption and beam hardening. For this reason, at least one arm should be removed from the scanning field and extended above the patient's head. Similar beam hardening effects can be seen between the shoulders (Fig. 7.31 b) and between the femurs. The denser the skeletal structures, the more pronounced the artifacts. •
Metal Implants
Metallic objects cause significant beam hardening ranging and may also cause almost complete x-ray absorption (photon starvation at the detector), resulting in the appearance of hyperdense and hypodense streak artifacts (Fig. 7.31c). These artifacts generally do not occur with small objects (staples, vascular clips) because of partial volume averaging. Titanium implants absorb less radiation than other metals, so they cause fewer artifacts. Beamhardening artifacts caused by bilateral total hip implants cause decreased attenuation in the lesser pelvis (bladder, obturator compart-
c
d
Fig. 7.31 Beam-hardening artifacts may occur behind ribs (a) and may be caused by the shoulder girdle (b), metal implants (c), and highly concentrated contrast medium in the urinary bladder (d).
Image Noise
merit), making it difficult to evaluate this region. CT is not useful for evaluating metal objects (e.g., checking for implant fracture) unless the range of measurable CT densities is expanded upward (> 3,000 HLJ). This is possible if the scanner already employs 16bits per pixel (maximum CT number> 65,000) or if special software is available that extends the CT number scale. Algorithms for metal artifact suppression are available from some manufacturers. Such algorithms often use iterative correction. In a first step they determine the location of dense objects based on a reconstructed CT image. They then use this information for predicting beam hardening effects and photon starvation effects, and correcting for them. This procedure can be repeated in an iterative manner, thus further improving image quality. Such algorithms, however, require substantially longer for reconstructing the final image. •
Contrast Medium in the Bladder
A contrast-filled urinary bladder typically causes areas of very low attenuation to appear between the lateral bladder wall and the bony pelvis (Fig. 7.31d). •
Gastrointestinal Contrast Media
Barium residues in the gastrointestinal tract can give rise to very intense local streak artifacts. Undiluted iodinated contrast material can produce a similar effect. For this reason, CT examination of the abdomen should be postponed if gastrointestinal contrast residues can be seen on the scanogram. Contrast residues in diverticula may persist for days or weeks, but usually are responsible only for minor artifacts.
Image noise is particularly disturbing shaded surface displays because it will "flying pixels" and irregularities of the contour (Fig. 7.32). Similar effects can countered on volume rendered images. •
on 3D cause object be en-
Electronic Noise and Photon Starvation
If radiation dose to the detector is reduced substantially, then the electronic noise of the detector system (amplifier noise) becomes a major issue. Electronic noise adds random fluctuations of the detector signal and is negligible for standard radiation exposure. The absolute amount of noise is independent of the incident radiation. As the detector signal decreases, the relative effect of electronic noise becomes more apparent. Such a decrease in detector signal can be due to high absorption in the patient (e.g., obese patients, metallic objects, large cross-section such as in the shoulders) or due to low-dose scanning. As there is a continuing trend towards lower exposure dose, electronic noise will become more important. If the number of photons that hit the detector becomes too low (photon starvation), electronic noise becomes the predominant feature (Fig. 7.33). Reconstructing thicker sections or using smoothing kernels to reconstruct the raw data set can no longer compensate for this sudden increase in image noise. For this reason, a photon starvation effect should be avoided by all means because there is no way of compensating for the reduced image quality.
Image Noise
Image noise is a normal effect on CT sections that were not obtained with an excessive amount of radiation exposure. It represents random fluctuations of the measured CT number and is mainly caused by quantum noise (see also Chapter 5). Image noise may reduce the conspicuity of low-contrast detail but can be reduced by increasing the section thickness (axial images or MPR) or using a smoothing filter kernel (see Fig. 2.5).
Fig. 7.32 Image noise will lead to flying pixels on 3D shaded surface displays (a). Using a smoothing kernel for image reconstruction can reduce this effect (b).
Fig. 7.33 As exposure dose is decreased, image noise exceeds the numbers predicted from quantum noise alone. The reason is the growing contribution of electronic noise. which becomes the predominant feature as the detector signal drops below a certain threshold
and photon starvation occurs (a). Reconstructing thicker sections from multislice CT data sets can no longer sufficiently compensate for the reduction in image quality (b, c).
Motion Artifacts
•
Motion in the scanned section during one rotation of the x-ray tube will result in non-consistent projection data because of different configurations of the scanned objects in the various x-ray projections. As a result, varying degrees of motion artifact will appear throughout the reconstructed image, being most pronounced in the region of the moving structure.
Pulsation of the heart, aorta, and pulmonary vessels may cause double or multiple contours to appear on vascular and organ boundaries. When this effect occurs in the ascending aorta, it can simulate an intimal nap (see Fig.23.13). Pulsation of the heart may cause streak artifacts in adjacent organs and may simulate dissection in the descending aorta (Fig.7.34a). Vascular pulsation may lead to displacement of the vessel as such and may cause double contours
Fig.7.34 Pulsation of the heart may cause streak artifacts in the descending aorta simulating dissection (a). Pulsation results in displacement of the renal arteries
leading to serrated contours in this patient after renal transplantation (b). Pulsation artifacts can produce kymographic effects on MPRs (c).
Pulsation
Motion Artifacts
(Fig.7.34b) or serration of the vessel wall (see Fig. 23.13). Pulsating structures in multiplanar reformations cause serration artifacts to appealalong the z-axis (kymographic effect, Fig. 7.34c). Most pulsation effects decrease with faster scanning (multislice CT, e.g., with 4x2. 5 mm detector configuration) because one heart cycle spreads out over a larger range along the z-axis. Pulsation of the heart and great vessels causes the trachea to appear serrated on 3D or multiplanar displays (Fig.7.35a). Pulmonary vascular pulsations or transmitted lung pulsations most commonly occur behind the heart in the left lower lobe, causing herringbone-like distortion of vascular bifurcations in the lung periphery. Hypodense areas appear near the bifurcation sites and can mimic focal emphysema (Fig.7.35b). Pulsation may cause local areas of increased CT density that present as "pseudocalcifications" in the lung hilum, the heart, or the liver (Fig. 7.35 c). Fast ECG gating or prospective triggering can almost completely suppress pulsation effects in the aorta or pulmonary vessels. However, pulsation is non-uniform because pulsation waves travel from the heart to the periphery. Thus, there are different points in time that are optimum for the aorta and the pulmonary vessels. •
Respiratory Artifacts
Insufficient breath holding can cause double contours to appear on organ boundaries that
move with respiration. These artifacts can obscure pathology and are rarely mistaken for true pathology, although double contours within the lung can mimic bronchiectasis or pleural disease (Fig. 7.36a). These effects are caused by vigorous respiratory motion, such as a sudden deep inhalation or a coughing fit (Fig. 1.36 b). At the upper or lower border of large ovalshaped structures such as the kidneys, spiral artifacts may occur if the patient breathes during the scan (Fig.7.36c). Such artifacts do not occur if the patient holds his or her breath properly. With spiral and multislice scanning, acceptable results can be achieved even during shallow respiration. This applies only to axial scans, however. Multiplanar reformations are highly sensitive to any kind of patient movement, and even small respiratory excursions will cause undulating surface contours (Fig. 7.37a). Organs, vessels, or tumors may appear elongated or compressed, depending whether the patient breathes in or out during the scan (Fig. 7.37b). Deeper breathing leads to discontinuities in the moving structures (Fig. 7.37 c). Patient instruction is therefore critical if any type of 3D processing or 3D measurement is to be performed, such as in CT angiography or CT volumetry. •
Other Motion-Related
Effects
Motion artifacts also result from gross movements such as those that occur in patients with acute fractures or unrest due to pain (Fig. 7.38 a). While spiral CT has greatly reduced the motion problem on axial images, conspicuous motion
a Fig. 7.35 Pulsation causes the trachea to appear serrated on multiplanar reformations (a). Pulsation artifacts produce emphysema-like hypodensities at the bi-
furcations of pulmonary calcifications (c).
vessels
(b) and can mimic hilar
215
216
7 Image Analysis Fig. 7.36 Gross respiratory excursions during the scan (e.g., due to coughing) can mimic areas of bronchiectasis (a) or produce a double margin on organ boundaries (b). At the upper or lower border of the kidney, breathing may cause spiral artifacts.
a
b
a
b
Fig. 7.37 On multi planar reformations or volume-rendered displays motion artifacts can be appreciated as undulating contours of the body surface (a) or elonga-
tion or shortening of an organ or lesion (b). Marked breathing may cause discontinuities in vascular structures (c).
Fig. 7.38 Gross movements (here due to swallowing) lead to structural discontinuities that even can simulate fractures (a). A sagittal reformation of the dens in Parkinson's disease shows pronounced motion artifacts (b) that were not visible in the axial sections.
a
b
Artifacts Due to Contrast Media
artifacts can still appear in m ul tiplanar reformations and shaded surface displays (Fig. 7.38 b).
sequent blending of opacified and nonopacified blood from the two vessels can create a spurious intraluminal filling defect, especially when there is laminar flow. Pseudothrombi can occur at numerous sites of venous confluence (Table 7.5). Generally this artifact is easy to recognize, but occasionally it can be mistaken for throm bosis (Fig. 7.39, compare also Figs. H.53c and 15.34c). In the case of a pseudothrombus, scans acquired 30 to 60 seconds later will show complete, uniform opacification of the region in question.
Artifacts Due to Contrast Media
The short scan duration with spiral and multislice CT can increase artifacts caused by differential enhancement of various vascular territories. Venous structures are most commonly affected. •
Pseudothrombus
Artifact
A pseudothrombus artifact occurs most commonly at the confluence of veins that carry blood from anatomic regions with different circulation times, such as the infrarenal vena cava (slow return from pelvic and leg veins) and the renal veins (fast return though the kidneys). The artifact occurs if scanning is performed during a phase that is too early for complete opacification of both venous systems. The con-
Table 7.5
Sites of venous confluence
Affected vein
pseudothrombus
Inferior vena cava
Renal veins
Portal vein
Superior vein
Pseudothrombi
in Pulmonary
Vessels
During early scanning (particularly with multislice CT), there may be differential enhancement between various portions of the pulmonary arteries that rarely may simulate thrombus (Fig. 7.40 a). The differential enhancement of pulmonary arteries and veins during the early phase of
Causative vein,
Superior mesenteric
artifacts
may occur (listed in order of frequency) Mechanism Inflow of opacified
mesenteric
vein
blood
Inflow of nonopacified Reflux of opacified
Splenic vein
blood
blood
Mesenteric vein
Mesenteric
Inflow of nonopacified
blood
Superior vena cava
Azygos vein
Inflow of nonopacified
blood
Common
Internal iliac vein
Inflow of nonopacified
blood
Profunda femoris vein
Inflow of opacified
Brachiocephalic
Inflow of nonopacified
iliac vein
Common femoral
vein
Superior vena cava
a
where
•
veins
b
Fig. 7.39 Pseudothrombus artifacts due to differential portal vein (c), common femoral vein (d).
vein
c venous opacification:
blood blood
d azygos vein (a), inferior vena cava (b),
217
218
7 Image Analysis
a
Fig. 7.40 Pseudothrombus in the pulmonary arteries is a rare finding and is due to differential opacification of various pulmonary artery branches (a). Non-opacified pulmonary veins may simulate thrombi but are easily
scanning is more common, which may lead to misinterpretation of a non-opacified vein as an arterial em bol us (Fig. 7.40b). Because of the shorter scan duration, this effect is seen more frequently with multislice CT. Differentiation of arteries and veins is simple using a cine display or by checking for the bronchus that is adjacent to pulmonary arteries, not veins. •
c
b
Pseudolesions due to Hepatic Veins
Nonopacified hepatic veins appear hypoattenuating to the enhanced liver parenchyma and can appear as pseudolesions in some sections (Fig.7 .40c). This is most apparent during the early portal phase (about 50-60 seconds p.i.), when the portal vein is brightly enhanced but the hepatic veins are not yet opacified. It is better to delay scanning of the liver or to choose a caudocranial scanning direction during this phase, as this will delay imaging of the major hepatic veins until they are more adequately opacified. Pseudolesions can be correctly identified by following a suspected lesion through adjacent sections and comparing it to pre contrast or later contrast -enhanced scans. An identical effect can occur in arterial-portal CT (CTAP), in which contrast medium is injected into the superior mesenteric artery and the liver parenchyma is enhanced purely by contrast that is delivered to it through the portal vein. The hepatic veins are not opacified during the early phase of CTAP (about 20-40 seconds p.i.) and may appear as pseudolesions.
distinguished based on anatomical considerations (b). Non-opacified liver veins may simulate hypovascular lesions, especially during the early portal venous phase (c).
•
High-Contrast
Artifacts
If the concentration of contrast material is too high, the signal at the detector may be so weak (photon starvation) that typical streak artifacts are produced. This effect is accompanied by beam hardening, which also produces low-attenuation artifacts. After the administration of intravenous contrast, artifacts may appear in areas where the venous inflow carries a high concentration of contrast medium, most commonly the brachiocephalic vein or superior vena cava. These artifacts can completely obscure underlying signals, mimicking the CT appearance of an aortic intimal flap or pneumothorax (Fig. 7.41; see also Fig. 24.5). This problem can be reduced by lowering the contrast concentration, or by scanning in a caudo-cranial direction
Fig. 7.41 High-contrast artifact caused by contrast medium in the subclavian vein mimics a pneumothorax.
Scanner-Related
Artifacts
while using a saline flush to wash out the injection veins (see Chapter 3, p. 99). Scanner-Related Artifacts
CT scanners can cause a variety of equipmentrelated artifacts, most of which are due to errors of application, adjustment problems, or scanner defects. •
a
Inaccurate CT Numbers
Improper calibration of the scanner can lead to significant CT number errors (Fig. 7.42a). On multislice CT, improper calibration of the detectors will cause hypo attenuating rings that rotate around the patient and cause a pattern of undulating densities (zebra stripe artifact) on MPR (Fig. 7.42b). The scanner should be calibrated at least once a day, depending on the manufacturer's recommendations. If inaccuracies are noted, the scanner will require recalibration or service. In some scanners, a low radiation dose or certain convolution kernels can lead to inaccuracies in the measured CT numbers. In particular, high-resolution kernels should not be used for CT number measurements (Fig. 7.43). The effect of kilovoltage on CT numbers should also be considered. The more the effective atomic number of the scanned region differs from that of water, the greater the kilovoltage effect. Lowering the kVp will increase the CT numbers of structures with high atomic number (e.g., calcium- or iodine-containing compounds, see also Fig. 7.15), while it will reduce the CT number of fat.
b
Fig. 7.43 A high-resolution kernel for image reconstruction will simulate the presence of calcifications in small nodules (a). Correct assessment requires the use of a standard kernel (b).
•
Tube Malfunction
Breakdown of the x-ray beam during a spiral acquisition yields specific artifacts (Fig. 7.44) that are due to a complete lack of data from some projection angles. For these angles, only electronic noise contributes to the image. In some scanners such raw data cannot be reconstructed at all, others allow for image reconstruction and thus may avoid re-exposing the patient. •
Detector Malfunction
If a single detector malfunctions during one single projection, filtered back-projection along that ray will cause a single black line to appear on the CT image (Fig. 7.45a). The malfunction of several detectors in one projection will be represented as multiple lines centered on the corresponding tube positions. These artifacts
Fig. 7.42 Incorrect scanner calibration: the calibration processes was interrupted and led to a circular band of a low attenuation that simulates perihepatic fluid (a). On multislice CT, alternating bands of low and normal attenuation may occur that present as "zebra stripe artifacts" on MPR (b).
a
b
220
7
Image Analysis
Fig.7.44 Tube malfunction with complete interruption of the x-ray beam leads to purely electronic noise at the detector. Resulting artifacts with partial detector signal (a) and no detector signal (electronic noise only) (b).
b
a
Fig.7.45 Failure ofa single detector during one projection (a). Ring artifacts caused by faulty calibration of the detectors relative to one another (b). Out-of-field arti-
occur sporadically and reflect problems with the detector electronics. If a single detector malfunctions for all the projections in one tube rotation, all the CT numbers will be underestimated, and the image will show a hypodense ring the position of which correlates with the angular position of the faulty detector. This type of artifact may occur sporadically or may signify a permanent detector defect. •
c
b
a
Ring Artifacts
Multiple ring artifacts appear when several detectors malfunction in one tube rotation or, more commonly, when detector sensitivities are poorly calibrated relative to one another (Fig. 7.45 b; see also Fig. 7.41 b). The latter problem can usually be corrected by recalibrating the scanner.
facts in an obese patient, caused by tissues located outside the scanned field of view (c).
•
Out-of-Field
Artifacts
Out -of- field artifacts can occur in very obese patients or if the selected scanned field of view is too small. Areas of the body cross section located outside the scanned field appear hyperattenuating (Fig. 7.45 c, see also 25.14c). Spiral and Multislice Artifacts •
Spiral Artifacts
Spiral artifacts may occur in spiral or multislice CT at interfaces that are slightly angulated relative to the scan plane. They are due to data inconsistencies during one rotation that cannot be corrected by interpolation. They increase with pitch factor and present as an object distortion (Fig. 7.46) or spiral indentation of the object sur-
Splrol and Multislice Artifacts
Fig. 7.46 Spiral artifacts in the dome of the acetabulum from a multislice CT scan (4 x 2.5/15) with 3 mm section width.
face on 3D renderings. They are exaggerated if additional motion occurs (see Fig. 7.35c). •
Cone Beam Artifacts
Cone beam artifacts are due to the geometry of the x-ray beam and are more pronounced the more detector rows present. Only objects in the rotational axis (center of the gantry) are "seen" by the same detector row. Objects localized in the periphery of the scan field will be detected by different detector rows as the tube rotates around the patient (see Fig. 1.22). This effect leads to marked undulations and serrations on MPR (see Fig. 2.9) because the apparent position of the object cannot be precisely localized. New multislice cone-beam interpolation techniques promise to reduce this effect.
The artifacts can be seen in almost all objects with high contrast, such as bone, vessels, and intraluminal air in the bowel. Artifacts are more pronounced with a higher pitch factor and may even cause slight streak artifacts at object interfaces (Fig. 7.47). When comparing the image quality of thin sections with a high pitch factor to thick sections with a low pitch factor, such cone beam artifacts are less with the lowpitch images. However, if images of similar thickness (section width) are reconstructed from a thin-collimation data set, artifacts are further reduced. Thus, scanning with thin collimation yields the best image quality as long as image noise does not become a limiting factor. •
Noise and Spatial Resolution
Noise and spatial resolution are not uniformly distributed throughout a set of spiral CT images. Each table position along the z-axis corresponds to a particular angular position of the x-ray tube. At this position, little raw data interpolation is required, which will yield a locally improved spatial resolution. At an opposite position in the image, maximum interpolation is necessary, which yields less image noise but a somewhat lower spatial resolution (Fig. 7.48). Image noise is increased in thin-section multislice CT images because of the narrower collimation and the reduced detector dose. This makes such a technique more vulnerable to electronic noise as well.
Fig. 7.47 Cone-beam artifacts at high-contrast interfaces in the cervical spine (arrows) are hard to appreciate on axial sections (a) but are obvious on coronal reformations (b) from a data set acquired with N x SCITF=4 x 115 and reconstructed with SWIRl =1.25/0.7.
a
b
221
222
7 Image Analysis Fig. 7.48 Image noise and spatial resolution rotate in conjunction with the angular position of the x-ray tube. Close to the position of the tube, image noise is highest and spatial resolution is best (arrow) while opposite, spatial resolution as well as noise are reduced (arrow heads) (a). Image from a 5/10/5 data set. (b) Image 5 mm further distally. Note that now the areas with high and low noise are shifted by 180°.
Fasciae (p. 232) Approach to Radiographic Evaluation of Masses of the Deep Face and Neck (p. 232) Spaces of the Suprahyoid Neck (p. 232) Spaces of the Infrahyoid Neck (p. 240) Masses Involving Multiple Cervical Spaces (p. 241) Examination Technique Cystic Lesions Thyroglossal
Pharynx CT is an established imaging modality for examination of the head and neck (Table 8.1). but is increasingly being superseded by magnetic resonance imaging, especially in cooperative patients. A simple rule of thumb is that CT is preferable for inflammatory pathology. while MRI is preferred for tumor imaging. For tumor imaging, MRI is currently more important in the higher part of the neck while helical cr is still the preferred modality in the lower part, where more moving structures are present. This rule is, of course, prone to change with new developments in technology, cost and availability, and radiologist experience. The development of short bore magnets, fast imaging sequences, phased array neck coils, and dedicated protocols have led to an increased use of MRI, while with the advent of multislice helical CT some of the indications may shift back to cr. CT has advantages and should be used as the primary modality in several circumstances: • intensive care patients: shorter examination time, better monitoring; • severely debilitated patients: shorter examination time, less motion artifact; • patients with suspected destructive bone lesions or bone erosion.
Table 8.1 neck
Major indications for CT examination of the Detection, characterization, TNM-staging, bone erosion
I \lnfeGtion
Extent of infectious processes, abscess formation, differentiation of infection from tumor
{Ongenita""diseases
Evaluation of extent, characterization
lraUJT1a
Laryngotracheal injury, vascular injury
All in all, in many patients CT and MRI are complementary imaging modalities rather than competitive ones, with MRI the primary modality for tumor imaging in the upper neck. Modern ultrasonography also has a major role in imaging the neck. This is a primary modality for evaluating the superficial parotid and tile thyroid glands, Combined with ultrasoundguided fine needle aspiration it is probably the most reliable technique for evaluation of lymph node staging, although the retropharyngeal nodes are still difficult to assess with this technique.
Anatomy Most cervical structures are arranged symmetrically with respect to the midline. This facilitates side-to-side comparison, and soft-tissue asymmetry may provide the only radiological clue to the presence of a pathologic neck mass.
The anterior tongue consists of three intrinsic muscles (genioglossus. styloglossus. hyoglossus) that are supported by the floor of the mouth. The floor of the mouth is U-shaped and lined with mucosa and is formed by the mylohyoid and the anterior belly of the digastric muscles.
Oral Cavity The oral cavity is the anterior part of the oral aerodigestive tract and is separated from the oropharynx by a ring formed by the junction of the hard and soft palates, circumvallate papillae, and anterior tonsillar pillars. It contains the anterior two-thirds of the tongue, lips, hard palate, buccal mucosa, gingiva, mandible, maxilla, and the floor of the mouth. The major types of soft tissue found here are muscle, fat, and lymphoid tissue.
225
Pharynx The air-containing structures of the pharynx are traditionally divided into three levels, which are still routinely used in the description of squamous cell carcinomas and other tumors (Fig.8.1).
•
8 Neck
middle constrictor muscles. The oropharynx contains the base of the tongue and lingual tonsils in the anterior part and the faucial tonsils laterally, which are part of the Waldeyer's ring (see Fig. 8.3 b,c; Fig.8.4a,b). Hypopharynx
Fig. 8.1 Sagittal anatomy of the pharynx. The neck is commonly subdivided in suprahyoid (SHN) and infrahyoid parts (IHN). NC= nasal cavity; NP= nasopharynx; OC=oral cavity; OP=oropharynx; H = hyoid bone; HP = hypopharynx; L= larynx.
The hypopharynx stretches from the valleculae superiorly to the lower edge of the cricopharyngeal muscle inferiorly. There are three important subsites of the hypopharynx. Piriform sinus. This is formed by invaginations between the aryepiglottic folds medially and the thyroid cartilage laterally and anteriorly (see Fig. 8.3d). Its inferior tip reaches to the level of the cricoarytenoidjoints (true vocal cords). Postcricoid area or pharyngoesophageal junction. This is the anterior wall of the hypopharynx at the level of the cricoid. This site is difficul t to evaluate with cross-sectional imaging techniques. Posterior hypopharyngeal wall. This is composed of mucosa and the constrictor muscle.
Nasopharynx
The nasopharynx is the upper part of the pharynx. It is bordered superiorly by the sphenoid sinus, inferiorly by a horizontal line through the soft palate (see Fig.8.4c), anteriorly by the nasal choanae and posteriorly by the posterior pharyngeal wall. A landmark on the lateral wall is the cartilaginous end of the eustachian tube (torus tubarius). The pharyngeal recess (fossa of Rosenmuller) is located posterior and superior to the torus, which is difficult to evaluate clinically and may harbor malignancy (see Fig.8.3a). The adenoids are situated on the posterosuperior wall (roof) and are the most superior part of the Wakleyer's ring of lymphoid tissue. Oropharynx
The oropharynx stretches from the soft palate superiorly to the valleculae inferiorly. Anteriorly, it is separated from the oral cavity by a ring formed of the junction of the hard and soft palate, circumvallate papillae and anterior tonsillar pillars, and posteriorly by the superior and
Larynx
The soft tissue structures of the larynx are arranged around a cartilaginous framework of the cricoid, thyroid, and arytenoids. The epiglottic cartilage serves as "lid" for the laryngeal "box" (Fig.8.2b) . The thyroid cartilage has protective laminae that meet at an angle anteriorly (Fig. 8.3 e). From the posterior part of the laminae, inferior horns extend downward to articulate with the cricoid and superior horns stretch toward the hyoid bone providing attachment for the thyrohyoid ligaments. The cricoid cartilage is a complete ring. Its inferior border represents the junction of the larynx with the trachea. The greatest part or lamina is located posteriorly and its superior part lies at the level of the vocal cords. The paired, pyramidal arytenoid cartilages sit on top of the cricoid lamina (Fig.8.3e) and serve to move the vocal cords during phonation. All are made of hyaline or elastic cartilage and start to OSSifyaround the age of 20. The true vocal cords span the airway from anterior to posterior and are the inferior part of the vocal cord-ventricle complex. Anteriorly,
Lymphoid Tissue
,----
Epiglottis +---
Fig. 8.2 Corona] (a) and sagittal (b) anatomy of the larynx.
Hyoid bone
Supraglottis
.-+-f,Z-If---
Cricoid cartilage
Subglottis
a
o
Vocal cords and ventricle
Trachea
--tlt-----
b
they converge into the anterior commissure. The false cords are slightly more superior and are the inferior continuation of the aryepiglottic folds. The ventricles are clefts that also run anteroposterior and that curve upwards to a level above the false vocal cords (Fig. 8.2, Fig. 8.4b). The leaf-shaped epiglottis is the flexible anterosuperior part of the larynx and is separated from the hypo pharyngeal piriform sinuses by the aryepiglottic folds (Fig.8.3d). It is made of fibrocartilage and seldom shows any significant calcification. Its function is to protect the airway during swallowing. It is connected to the oropharynx by the anterosuperior pharyngoepiglottic folds and is also attached to the tongue by the glossoepiglottic fold in the midline. Anterior to this "lid" is the preepiglottic fat behind the anterior thyrohyoid membrane. The larynx is routinely subdivided into three major subsites. • The supraglottis extends from the tip of the epiglottis to the laryngeal ventricles. It contains the epiglottis, pre-epiglottic fat, aryepiglottic folds, false cords, arytenoids, and ventricles. • The glottis contains only the true vocal cords and anterior-posterior commissure. • The subglottis extends from the undersurface of the true vocal cords to the inferior edge of the cricoid. It contains only the mucosa lining the cricoid ring. Lymph Nodes
Approximately 300 of the 800 lymph nodes of the body are located in the neck. Although there
are many classifications, the most widely used are the ones of the Union Internationale Contre Ie Cancer/American Joint Committee on Cancer (LJICC/ AJCC) and the American Academy of Otolaryngology, Head and Neck Surgery. Very recently, an imaging-based variant based on these has been introduced-see Chapter 18. In the present chapter, the original regional classification of the American Academy of Otolaryngology, Head Neck Surgery (1991) is used. In this classification nodal groups, like the facial, OCCipital, retropharyngeal, and parotid nodes, are not included and should always be mentioned separately. The major nodal groups are classified into six levels: Level I: Level Level Level Level Level
Submental and submandibular groups II: Upper jugular group-skull base to hyoid II I: Middlej ugular group-hyoid to omohyoid muscle IV: Lower jugular group-omohyoid muscle to clavicle V: Posterior triangle group VI: Parapharyngeal and paratracheal groups
Lymphoid Tissue
Lymphoid tissue is primarily located in Waldeyer's ring, which is composed of the palatine and lingual tonsils, faucial tonsils, and the adenoid. The adenoid lies high in the nasopharynx and may also extend into the lateral fossae (Rosenmuller). The adenoid normally involutes
---------------;;~» 60%). The infantile form is much more benign and has a low incidence of
metastasis. CTshows a homogeneous soft-tissue mass that may erode bone. Calcifications. necrosis. and hemorrhage can be present.
Chondrosarcoma In the neck. chondrosarcoma most often originates in the chondroid larynx skeleton. The thyroid and cricoid are most commonly involved. Other locations are the masticator space and skull base. It is usually well defined with submucosal growth. but shows irregular. ring-like calcifications on CT. The airway may be compromised in laryngeal tumors.
Rhabdomyosarcoma This is the commonest sarcoma in children. usually occurring before the 10th year. The nasopharynx. parapharyngeal space. or infratemporal fossa are involved in 30% of cases. The tumor frequently extends through the skull base into the cavernous sinus. Lymph node. lung. and bone metastases are seen in up to 50% of patients. At CT. the tumors present as homogeneous. infiltrative bulky masses with soft tissue density and variable enhancement. There is often bony erosion or remodeling of the skull base or maxillary sinus.
Synovial Sarcoma
in the retropharyngeal space and patients present with hoarseness and dysphagia. The tumors show slow, infiltrative growth and frequently metastasize to the lung. CT shows homogeneous or heterogeneous lesions that often have welldefined margins. Fluid - fluid levels, hemorrhage, and calcifications are common. Malignant Peripheral Nerve Sheath Tumor (MPNST) MPNST is the unified name for malignant neurogenic tumors and describes a highly malignant sarcoma. These sarcomas may arise de novo or from malignant transformation of a neurofibroma or schwannoma, usually a plexiform neurofibroma. Some 65% show distant metastases. They are commonest in patients with neurofibromatosis I and following radiation (latency period 10-20years), but are uncommon in the head and neck. CT is nonspecific and shows a large, fusiform, invasive tumor with necrosis and hemorrhage. Enhancement is heterogeneous.
Malignant Fibrous Histiocytoma The neck is a rare site for this tumor. CT findings are a circumscribed mass of homogeneous softtissue density, often within muscle or along fasciae. Small foci of calcification, necrosis, or hemorrhage may be present.
The neck is the most usual site of this tumor outside the extremities. Lesions are commonest
Salivary Gland
Pathology
Cysts
Retention cysts develop secondarily to an incomplete or intermittent obstruction caused by inflammatory stricture, calculus, surgery, or trauma to the salivary ducts. They have an epithelial lining and most often occur in the submandibular gland. A ranula is a mucous retention cyst of the sublingual gland. The simple form is a true cyst limited to the sublingual space. The plunging or
diving ranula develops from a rupture of a simple ranula and is in fact a mucocele or pseudocyst without an epithelial lining. It extends over the posterior margin of the mylohyoid muscle into the submandibular space. Lymphoepithelial cysts are associated with AIDS and often multicentric and bilateral. They are usually located in the parotid glands and associated with cervical adenopathy.
8 Neck
and have and lack more illtail into
rhage, or calcification, When left untreated, 25% of the tumors will develop malignant transformation (usually adenocarcinoma), When incompletely resected, the tumor can recur in up to 50% of cases within lOyears after surgery, MRI is the preferred modality in these cases,
Between 70% and 80% of parotid, 40-60% of submandibular, 15-30% of sublingual and 20-40% of minor gland tumors are benign, Imaging is unreliable in giving a definite diagnosis but should be used to select therapy and serve as a roadmap for surgery, Table 8,26 gives an overview of solid lesions in the parotid gland,
Small tumors are homogeneously hyperattenuating and well defined on non enhanced studies, With increasing size the masses are more inhomogeneous with hyperattenuating areas of hemorrhage or calcification andlor more hypodense regions of necrosis and cystic degeneration, Enhancement after contrast injection increases (Fig, 8,26) and becomes more homogeneous over time, and is best evaluated on delayed scans,
CT Morphology
These cysts are smooth, well defined fluid attenuation, The walls are thin calcifications, A diving ranula can be defined and often has a characteristic the sublingual space,
CT Morphology
Benign Tumors
Pleomorphic Adenoma Pleomorphic adenoma or benign mixed tumor is the commonest and represents 70-80% of all benign solid masses in the salivary glands, The majority occurs in the parotid, usually lateral to the plane of the facial nerve (superficial lobe), They are usually solitary, well-defined and ovoidshaped, Larger tumors may have a lobulated contour and may show areas of necrosis, hemor-
These are small, ovoid, well-defined masses often arising in the tail of the superficial lobe, Small tumors are homogeneous, well-delineated masses that can be multifocal or bilateral. In larger tumors, cyst formation is seen frequently, These cysts may have focal tumor nodules in their walls, Contrast enhancement is inhomogeneous, The differential diagnosis for noncystic lesions includes lymphoma, adenopathy, and granulomatous disease, and, for cystic lesions, cavitating metastases, and lymphoepithelial cysts,
carcinoma
Adenoid cystic carcinoma Non-Hodgkin
Warth in's Tumor Warthin's tumor or cystadenoma lymphomatosum is a monomorphic adenoma of the parotid gland accounting for 2-10% of benign tumors, It is the commonest lesion to occur as a multifocal or bilateral (10%) disease, The lesion is rare in non-whites,
be multifocal
cysts
of skin' (In AIDS)';
Oncocytomas
n1g~~del'n
Oncocyto~~~gl:FedU~~~~ymln~ patients over 55 years old, They occur preferentially in the parotid gland, are slow growing, and are well circumscribed,
Malignant Tumors
a
c
b
Fig. 8.26 Pleomorphic adenoma of the deep lobe of the parotid gland ("iceberg tumor") (a). Hypervascularized mass medial to the right mandible displacing the origin of the pterygoid muscles and the soft palate. Bilateral fatty infiltration of the parotid glands (arrows) is
a common finding. A pleomorphic adenoma at the lower pole of the right parotid gland (b, c) simulates a lymph node. There was a typical echo-free appearance on ultrasound with posterior echo enhancement. Note the dental artifacts on the coronal view (c, arrowheads).
•
Table 8.27 1997)
CT Morphology
The imaging characteristics are nonspecific and mimic both pleomorphic adenomas and Warthin's tumor. Malignant Tumors
Malignant tumors are more frequent in smaller salivary glands and occur more often in Inuit Eskimos and in children. Pain, nodal metastases, and facial nerve paralysis are associated with poor prognosis. Staging is based upon the TNMclassification (TableS.27). Mucoepidermoid Carcinomas
Mucoepidermoid carcinomas are the commonest salivary malignancies in adults and children. They represent 30% of parotid malignancies and occur most often in the parotid or minor salivary glands. They can be oflow, intermediate, or high grade. •
CT Morphology
Low-grade tumors are often well described while the high-grade tumors are diffusely infiltrative. Cyst formation is frequent in lower grade tumors, and hemorrhage or necrosis may be present. At imaging the morphology varies with the histological grade and as such may mimic pleomorphic adenoma or squamous cell carcinoma.
T-staging of salivary
gland tumors
(UICC,
T1
Tumor 2cm or less (greatest dimension) withoutextraparenchymal extension
T2
Tumor greater than 2 cm but less than 4 cm without extra parenchymal extension
T3
Tumor greater than 4 cm but less than 6 cm ortumorwith extra parenchymal extension without facial nerve involvement
T4
Tumor greater than 6 cm or tumor invades facial nerve or skull base
Adenoid Cystic Carcinomas
This tumor occurs most often in the minor salivary glands of the palate, in the parotid or submandibular gland of patients in the fifth or sixth decade. It is the commonest malignancy in the submandibular gland. These tumors grow slowly and have a tendency to show perineural invasion that can lead to pain as well as spread along the facial or mandibular nerve. Cd-enhanced MRI is preferable to CT for evaluating cell ularity and perineural extension. •
CT Morphology
Tumors show a more aggressive growth in the minor salivary glands than in other glands where their aspect is more benign, depending on the subtype and cellularity. Tumors in the minor salivary glands arise in the mucosa and
265
8 Neck
therefore cannot be morphologically differentiated from pharyngeal carcinomas. The imaging characteristics are compatible with their histological grade.
Carcinoma ex Pleomorphic Adenoma Carcinoma ex pleomorphic adenoma is a malignant transformation of a (resected) pleomorphic adenoma.
cr
Morphology
Small tumors resemble pleomorphic adenoma with benign characteristics. but larger tumors are more inhomogeneous and infiltrative with areas of necrosis. The CT morphology directly reflects this variation in macroscopy.
Acinic Cell Carcinoma Acinic cell carcinoma occurs almost exclusively in the parotid gland and can be bilateral in 5% of cases. They usually occur after the 40th year but also are the second commonest salivary malignancy in children. CT Morphology
The tumors can either be solid or cystic. The imaging characteristics are nonspecific. The tumors generally have a benign appearance.
Metastases Metastases occur almost exclusively in (the intraglandular nodes of) the parotid gland. The commonest primary is melanoma of the temporal skin or other parts of the neck. Less frequent are head and neck. renal, lung, breast, and Gl carcinomas.
cr
Morphology
Unilateral or bilateral, enlarged intraglandular lymph nodes with a homogeneous density. After contrast administration homogeneous enhancement is seen. Necrosis can be seen occasionally, and is suggestive for malignancy. Differentiation from nodal lymphoma is often impossible.
Inflammatory and Autoimmune Lesions Sial adenitis Acute bacterial and viral sialadenitis commonest pathologies of the salivary Bacterial infections usually ascend from cavity when saliva flow is decreased and pathogens like Staphylococcus aureus,
are the glands. the oral involve
Streptococcus pyogenes. Streptococcus pneumoniae and Haemophilus influenzae. Viral infections can be
caused by mumps, Coxsackie or (Para)influenza viruses and preferentially involve the parotid glands. Intra- and periglandular lymph nodes may be involved in the process. The main role for imaging is to differentiate adenitis from abscess formation. Abscesses are often formed when acute adenitis is undiagnosed or incompletely treated. These abscesses can spread quickly in the para pharyngeal space or other parts of the neck. CT Morphology
The involved gland is dense and somewhat enlarged. It enhances slightly. Abscesses are demarcated regions of low attenuation.
Sialolithiasis Stones can be located in the salivary gland ducts or the extraglandular ductal system. From 80-90% occur in the submandibular gland, W-20% in the parotid and
Trauma
and
Postoperative
Chest
Trauma
Adult
Respiratory
Pneumonectomy lung
Chronic Airway
Distress and
Syndrome
Lobectomy
Transplantation
Reimplantation Acute
367
Changes
(p.367)
Rejection
(p. 368)
(p. 370)
371
.
Response
(p. 371)
(p. 371)
Rejection Complications
(p. 371) (p. 371)
Plain chest radiographs are an indispensable part of the diagnostic work-up of suspected thoracic disease. However. projection effects and limited density resolution limit the value of plain films as a diagnostic tool. Computed tomography. with its excellent contrast resolution and nonsuperimposed views. is the most sensitive modality for evaluating the lung parenchyma. CT is the method of choice for tumor staging. the diagnosis and follow-up of pulmonary and mediastinal lesions. and the detection of occult thoracic lesions that cannot be diagnosed with conventional x-ray films (Table 9.1). High-resolution thin-slice CT scanning (HRCT) improves our ability to define and evaluate fine structural details in the lung parenchyma. HRCT is used for the localization. characterization. and quantification offocal and diffuse parenchyma! lung diseases. The pathomorphologic response pattern of the lung parenchyma is fairly uniform for different etiologies. however. so clinical information is essential for the interpretation offindings. CT is an important adjunct to bronchoscopy for evaluating the airways. By defining the precise location of an abnormality, CT can accurately direct bronchoalveolar lavage (BAL) or bronchoscopic biopsy. CT can visualize surrounding structures as well as bronchial segments that are inaccessible to bronchoscopy.
of parenchymal changes for open lung biopsy or bronlavage
Anatomy •
Trachea
The trachea starts at the inferior border of the cricoid cartilage and is 9 to 12 cm long. In 50% of patients the membranous posterior wall forms a convex bulge projecting into the tracheal lumen. The average transverse diameter of the trachea is 15mm in women and 18mm in men. The upper limits for coronal and sagittal diameters are 26mm in men and 22mm in women; the lower limit of normal is 13 mm in men and 10 mm in women. The extrathoracic tracheal diameter varies markedly during forced inspiration and expiration (by up to 35%), and this should not be misinterpreted as wall instability. The intrathoracic part of the trachea remains fairly constant during the normal respiratory cycle. •
Central Bronchial System
The right main bronchus is shorter than main bronchus and appears adjacent upper lobe bronchus in longitudinal (Figs. 9.1, 9.2). The bronchus intermedius to 4cm distally and presents an elliptical
the left to the section runs 3 section
on the CT scan. The apical segmental bronchus of the lower lobe (B6) arises at approximately the same level as the middle lobe bronchus. The latter then bifurcates into a lateral and medial segment. The left upper lobe bronchus divides into only two segmental bronchi (apicoposterior, B1I2, and anterior, B3). Further distally, the lingular bronchus branches anteriorly from the inferior aspect of the upper lobe bronchus. Its division into a superior (B4) and inferior segmental bronchus (B5) is poorly visualized on axial CT scans because of its oblique course. It originates 1 to 2 cm cephalad to the right middle lobe bronchus. Past the origin of the B6 segmental bronchus, the right lower lobe bronchus divides into four segmental bronchi while the left lower lobe bronchus usually divides into three segmental bronchi (there is no B7). The branching pattern is subject to a considerable range of normal variation.
Fig. 9.1 Anterior view of the central tracheobronchial system. Upper Lobe 1 apical (R) 112 apicoposterior 2 posterior (R) 3 anterior (R/L)
Fig. 9.2 Axial anatomy of the pulmonary segments and central tracheobronchial system. The major and minor fissures can be clearly identified (as fine lines or
avascular zones). The intralobar segmental boundaries (broken lines) can only be estimated from the course of veins and segmental bronchi.
283
9 Lungs and Tracheobronchial System
Thickening of the posterior wall of the intermediate bronchus that borders directly on the lung parenchyma is a sensitive sign of a mass lesion (except in the approximately 2 % of individuals with an aberrant superior pulmonary vein). Pulmonary
Vessels
The main pulmonary trunk bifurcates into a short left and longer right pulmonary artery. The right pulmonary artery runs between the superior vena cava and the right main stem bronchus and divides into an upper branch (truncus anterior) and the descending interlobar branch. The left pulmonary artery bridges over the left main bronchus. In some cases. the left upper lobe artery directly originates from the main artery. More frequently. the main pulmonary artery continues into a vertical interlobar or descending pulmonary artery running posteriorly to the lower lobe bronchus. from which the segmental arteries to the upper and lower lobe originate. The upper limit for the vascular diameter is 29mm for the pulmonary trunk. 28 mm for the left pulmonary artery and 24 mm for the right pulmonary artery. The right inferior pulmonary artery presents a circular cross section in the angle between the middle lobe bronchus and the origin of the 86 segmental bronchus (Fig.9.2d). Increased soft-tissue structures or scalloped borders in this area are a sign ofhilar lymphadenopathy. On the right, there are three pulmonary veins: the descending superior pulmonary vein and the middle lobe vein, which both run into the superior venous confluence at the posterior aspect of the left atrium, and the inferior pulmonary vein running more horizontally into the inferior pulmonary venous confluence. On the left there are two veins: the superior pulmonary vein draining the upper lobe and the lingula and the inferior pulmonary vein draining the lower lobe. They run into the superior and inferior venous confluences, respectively. Pulmonary
of each lung is easily traced on axial CT scans in its
posterosuperior-to-anteroinferior
Table 9.2 Azygos
Accessory fissures fissure
caused by downward invagination of azygos vein
Inferior accessory fissure
between medial segment (7) and the remaining lower lobe
Superior fissure
between upper segment and the basal segments t..pper lobe
accessory
Segments
There are ten pulmonary segments in the right lung and eight in the left lung. The major fissure
course
(Fig. 9.2). The minor fissure between the upper lobe and middle lobe of the right lung runs approximately parallel to the scan plane and is identifiable as a nonvascular zone on one to three axial slices. The anatomic relationship between the segmental bronchus and the concomitant artery is helpful to differentiate the middle from the upper lobe: in the upper lobe the artery runs medially to the segmental bronchus while in the middle lobe the artery is located laterally to the segmental bronchus. Thin slices and multiplanar reformations based on multislice CT data sets better demonstrate the course of the lobar fissures than axial slices alone. Additional fissures can be identified more frequently than with any other technique. The commonest accessory fissures include the azygos fissure, the inferior and superior accessory fissure, and the left minor fissure (Table 9.2). Incomplete fissures are frequent findings that allow for collateral air drift and spread of disease. The bilateral inferior pulmonary ligaments represent parietal pleural reflections extending from the inferior aspect of the hilum to the posterior diaphragm. On axial CT images they appear as broad bands between the posterior mediastinum (region of esophagus) to the diaphragm. CT cannot define the segmental boundaries in normally aerated lung, but the segments can be identified indirectly by tracing the central segmental bronchi and the interlobar veins. Substantial variations may be encountered in segmental pulmonary anatomy. A slice collimation of 5 mm or less is necessary for the positive identification of segmental bronchi.
Left minor fissue
between lirwla upper lobe
and left
(6) ofthe
Anatomy •
Peripheral Bronchi
The airways undergo approximately 23 divisions between the trachea and the respiratory bronchioles (Fig. 9.3). Standard CT can define fourth-order bronchi (3 mrn), while HRCT can resolve eighth-order branches (1-2 mrn). The CT detection of normal bronchi relies on visualization of the bronchial wall. The thickness of the bronchial wall is approximately one-tenth the diameter of the bronchus. Normal intralobular bronchi are not visible, as their wall thickness is less than 0.15mm. The presence of visible bronchial structures in the lung periphery (within 2 to 3cm subpleural space) signifies pathologic bronchial wall thickening or ectasia of the small airways. The small airways of the lung « 2m m in diameter) have a total cross-sectional area greatly exceeding that of the larger airways but account for only about 25% of the total airway
resistance. The small airways must undergo severe destructive changes before pulmonary function is measurably affected. •
Lung Parenchyma
The smallest structural unit visible on CT lung scans is the acinus, which is about 6-8 mm in diameter and consists of alveolar ducts, alveolar sacs and approximately 4000 alveoli distal to the terminal bronchiole (Figs. 9.3, 9.4). The secondary lobule is the smallest discrete portion of the lung that is surrounded by connective tissue. It is the smallest anatomical unit that can be clearly identified on HRCT, and assessment of the distribution of abnormalities within it can be helpful in the differential diagnosis of parenchymal disease. It consists of a maximum of 12 acini and has a characteristic polygonal shape with an edge length of 1-2.5 ern, a septal boundary, and a central core consisting of a bronchiolus and arteriole (Fig. 9.4).
Fig. 9.3 Schematic representation of the bronchial divisions, showing the limits of spatial resolution of CT and HRCT.
Interstiti u m: a) central b) septal c) peripheral
Viszeral pleura
c~ _""f-'--
b
\
)7
~
----
Secondary
Acinus lobule
*
Centrilobular artery -
Interlobular
septum
Terminal ::;', ~\'T-----
bronchiole
ILobular artery
Terminal bronchiole
Respiratory I bronchiole· Central acinar artery *----''
*
Lymph vessels" Pulmonary vein
* Visible only when diseased
• Never visible
Fig. 9.4 Schematic representation ing veins, and interstitium.
of two adjacent
secondary
The arterioles lie 5-10 mm deep to the pleura. The veins take an interlobular course and are visible 1-2 cm below the pleura. Normally the bronchioles cannot be seen on CT. This means that normal Iung contains an immediate subpleural zone 5-10 mm wide that is devoid of visible vascular and bronchial structures. The interstitium is the connective-tissue framework of the lung. Its fiber tracts support the bronchi, the pulmonary vessels, and the membranes for alveolar gas exchange. The interstiti urn consists of the central or axial interstitium, which surrounds the central airways and vascular structures, the septal interstitium, which supports the alveolar walls, and the peripheral interstitium, which forms the basic
lobules with afferent arteries
and bronchioles,
drain-
framework of the lobar and lobular septa. The interlobular septa normally are not visible on CT or may appear only as faint linear structures in the basal lung zones. In the presence of thickened lymphatic vessels or interstitial disease, however, they may become very prominent and define the architecture of the lung parenchyma. The CT density of the lung parenchyma normally shows an anteroposterior and an inspiratory-expiratory gradient. The anteroposterior density gradient is 20 ± IOHU in inspiration and 150±20HU in expiration. The average pulmonary density in inspiration ranges from -900 to -850 HLJ.
Examination Technique
The choice of examination technique for the Iung depends on whether the whole lung is evaluated (usually in cases offocallung disease) or whether a disseminated process can be examined by discontinuous scans with high spatial resol utian (HRCT) . Only multislice scanning allows for a combination of these two different requirements.
Oral contrast is usually not indicated for CT of the lungs unless tumor involvement of the esophagus is suspected (see Chapter 15, p. 548). Intravenous contrast material should always be used for the investigation of mediastinal and pleural abnormalities. Contrast administration is not only mandatory for the evaluation of focal intrapulmonary lesions (tumor, rounded atelec-
Routine Chest CT 287 tasis). Enhancement characteristics are not important for the characterization of diffuse parenchymal changes. Contrast administration is essential for all vascular investigations (pulmonary embolism. aortic studies) and for staging examinations.
Table 9.3
Suggested
protocols
for CT scanning
Routine Chest CT Spiral CT is the standard for examinations of the lung (Table 9.3). Most patients will be scanned in inspiration. Patients with marked dyspnea who are not be able to hold their breath during the scan should breathe shallowly instead.
of the thorax
General Preparation
not necessary
Patient
Supine with arms raised (standard) Prone position with elevated arms: • Differentiating hypostasis from subpleural • Differentiating edema from infiltration
position
Scan range
Respiratory
From thoracic From thoracic phase
inlet to posterior pleural recess (standard) inlet to below the renal veins (suspected bronchogenic
carcinoma)
End-inspiratory scans (standard) End-espiratory scans: • Air trapping: early detection of peripheral airway obstruction • Differentiation of air trapping from ground-glass opacity • Compliance testing (fibrosis versus edema) • Bronchogenic carcinoma: chest wall fixation
SC= slice collimation (mm), TF = table feed (mm/rotation), RI = recon. increment (mm), i t = scanning direction, SW = effective section width (mm), MPR = multi planar reformats, axial = axial sections. cor= coronal, sag = sagittal CM= contrast material, Contrast concentration = 300 mg/ml iodine V = volume of CM (ml), N = volume of saline flush (ml), F = flow rate (rnlts), D = start delay. X = trigger region a Pitch P = TF/(NxSC) should be chosen close to 1.5 with 4-slice CT scanners, 1.3-1.5 with 16-slice scanners b MPRare reconstructed from a "secondary raw data set" with SW/RI = 1-1.5/0.7 or 0.5-0.8/0.6 c read: single section mode HRCTwith 0.5-1.5 mm collimation and 10-20 mm increment between sections dread: 25A (5P) = start delay 25 s (5 s) after reaching 50 HU enhancement in the desc. aorta (pulmonary artery) e choose lower dose and kVp setting (CTDI", = 3-6 mGy; 80-100 kVp, depending on patient size and CT scanner)
II
9 Lungs and Tracheobronchial
System
When such a patient is scanned during inspiration, marked artifacts will occur as soon as the patient begins breathing again, Breathing artifacts decrease with caudocranial scanning, The technician, however, has to take care that there is a sufficiently long waiting period (2-4 seconds suffice) between the inspiration command and the beginning of the scan, Otherwise, the lung bases will suffer from marked artifacts due to the final inspiration of the patient. Patient compliance can be improved if they are instructed prior to the scan and asked to hyperventilate (2-5 deep breaths) immediately prior to the spiral examination, With spiral CT, a 5 mm collimation and a l Ornm table feed (pitch = 2) will give a good compromise between spatial resolution along the z-axts, scan duration (usually some 30 seconds), and patient exposure, Reducing the pitch to 1.5-1.8 will slightly improve image quality at the base of the lung because lower pitches are somewhat less susceptible to pulsation artifacts, For multislice CT with 4-slice scanners in a routine setting, a 4 x 2.5 mm (4 x 2-4 x 3 mm) collimation usually suffices unless there is a high likelihood that coronal or sagittal reformations will have to be reconstructed from the data set Under such conditions, volumetric imaging using 4xl-l.25 mm is advocated, With 8- and 16-slice scanners, 1-1.5 mm section collimation can be used on a routine basis, Reconstructing axial sections of 5-7.5 mm section width is sufficient for rou tine interpretation, Reconstruction of an overlapping secondary raw data set with SWIRl = 3/1.5 (for 4 x 2.5 mm collimation) or SWIRl =l.25-2/0,7 (for 1-1.5 mm collimation on 4- to 16-slice scanners) are required only if multiplanar reformations were initially planned or become necessary to solve a particular question on the axial sections, For optimum image quality, the images of the mediastinum should be reconstructed using a soft kernel, while images of the lung require a high resolution kernel. Instead of directly reconstructing two sets of images from the raw data set (larger amount of images to be processed and archived), display filters may be used to smooth images from a single data set reconstructed with a high resolution kernel. For opacification of the mediastinal vessels, relatively small amounts of contrast material are required for spiral CT or multislice CT acquisitions: the injection of some 50-70ml of contrast
material followed by 50ml of a saline flush (2 mlls) is usually sufficient. Because of the substantially reduced absorption in the lungs the radiation exposure (mAs settings) for the chest can be approximately halved as compared to abdominal scanning, Usually a CTDlvol = 5 mCy (more for obese patients, less for slim tndtviduals) yields good results, One has to keep in mind, however, that image noise in the cranial portions of the liver as well as in the shoulders may then be substantially increased, This usually does not affect diagnostic accuracy because liver imaging using a chest protocol is far from optimum (and should be optimized separately anyway), and the spectrum of neck pathology in chest examinations is very limited (lymph nodes, masses extending into the neck), Image quality in these regions may be improved even at reduced exposure settings with the use of longitudinal dose modula-
tion techniques.
Tumor Staging The highest possible image quality should be employed for tumor staging during the initial evaluation of primary tumors or prior to surgery. Since high space resolution is possible only for a short scan range with spiral CT, a composite approach with thin and thick sections has to be taken. We suggest focusing primarily on the pulmonary lesion including the pulmonary hila first, and then scanning the rest of the lung or repeating a scan of the whole lung using the protocol described above. For the initial high resolution scan, a (SWI TFIRI =) 214/2 protocol can be recommended if the scan range can be covered within one breath-hold phase. It is advisable to test the breath-holding capacity prior to the scan because breathing artifacts substantially impair the quality of reformatted images. For larger ranges, a 3/5/2 protocol can be recommended. When only the primary lesion (peripheral nodule) has to be scanned (for in-detail analysis of morphologic features or for nodule volumetry), a 11211 protocol can be employed. With a 11311 protocol, a 50% longer scan range can be achieved at only slightly reduced image quality (if the scanner supports a pitch of 3). With 1 mm sections, the mAs settings have to be increased to ensure sufficient image quality.
High resolution CT (HRCT)
From this thin-section scan multiplanar reformation should be reconstructed as described below for multislice CTscans. For multislice CT, volumetric imaging with a 0.5-1.25 mm collimation (P ':1.5) yields excellent results for 4- to 16-slice scanners. Only in very dyspneic or obese patients should thicker sections be used. The acquired data are reconstructed as a "secondary raw data set" with SW IRl= 0.7 -1.5/0.7. From this secondary raw data set, axial sections of 4-5 mm width are reconstructed, followed by coronal and sagittal thin sections across an intrapulmonary tumor (SW = 1. 5mm). If lymph node, mediastinal, or chest wall involvement is suspected, additional 3-4 mm coronal and 4-5 mm sagittal reformations are reconstructed across the central portions of the mediastinum (including the hila), or across the affected portions of the chest wall. Additional expiratory scans can determine whether a tumor adheres to the parietal pleura (no movement on expiration). Scans after inducing a pneumothorax have been suggested for the same purpose but are substantially more invasive and have the same problems with differentiation between mere adhesion and chest wall infiltration. For best results, a high resolution kernel should be used for the lungs and a soft kernel for the mediastinum and chest wall. However, this will create up to WOO images per patient (two secondary raw data sets). As a compromise, a Single secondary raw data set can be reconstructed with a high resolution kernel. and only the multiplanar images that are reconstructed from this data set are subsequently subjected to softening display filters. For tumor staging, a volume of 70-100 ml of contrast material should be injected at 2-3mlls, followed by a saline flush of 50ml using the same injection rate. A scan delay of 30 seconds yields good results in most patients. Larger volumes and higher flow rates improve contrast enhancement but add little to the diagnostic yield of the scan. Radiation exposure is no limiting factor for tumor staging. Instead, exposure setting should be chosen so that image quality is optimized. If thick axial (and coronal or sagittal) sections are sufftctent, a CT01voi of 5-10 mGy is recommended, but thin-section imaging requires a CTDlYoiof 10-25 mCy, depending on the size of the patient.
High Resolution CT (HRCT) High resolution CT is indicated for diffuse lung disease. If focal lesions are suspected, a sole examination by HRCT is not indicated. In such cases, an (additional) spiral CT should be performed. When volumetric imaging using multislice CT is performed, thick conventional as well as thin HRCT sections can be reconstructed form the same data set. HRCT examination of the lung parenchyma is usually performed with a slice thickness of 1 -2 mm, a high-resolution kernel and discontinuous acquisition with a 10 to 20mm gap between sections. Scanning is generally performed at end inspiration with the patient in a supine position. Expiratory scans are necessary to detect air trapping and thus early (peripheral) airwayobstruction, and to differentiate air trapping from ground-glass opacities. They have also been suggested for testing lung compliance (fibrosis versus edema) or bronchial collapse. It is possible to use a wider gap of > 20 mm for this purpose. Scans in prone position help differentiate hypostatic edema from subpleural fibrosis, and edema from infiltration. Only the areas that were suspicious on the supine scans need to be examined again (usually the lung bases). Scanning primarily in a prone position may be useful for patient with asbestos exposure (HRCT screening examination) because the initial findings with this disease predominantly affect the posterior portions of the lung. For screening purposes, the number of sections may be reduced to keep radiation exposure low. The mAs settings for HRCT of the lungs should be about 50-100% higher than for routine chest CT because of the higher noise on thin sections. Despite the fact that the CTDlw (dose per section, displayed on the user interface for incremental CT) may be quite high (1020 mGy), the dose to the patient remains low because of the large interslice gap (CTDlvol =1-4 mGy) Multislice CT scanners should be used in a Single slice mode. If available, only a Single section should be acquired at each table position, because a set of adjacent sections rarely adds to the diagnostic information but substantially increases patient dose. Acquiring Single sections is not yet possible with most 4-slice scanners but
9 Lungs and Tracheobronchial System can be clone with most 8- and 16-slice scanners. If no single-section mode is available, dedicated HRCT examinations should preferably be performed on single-slice systems. When 4-slice scanners have to be used, discontinuous sections of 4 x 1 mm to 4 x 1.25 mm width (or 4xO.5 mm width on Toshiba units) should be acquired every 10-15 mm, preferably every 20mm. In order to take full advantage of the increased number of sections per scan, one should not only view the axial sections but create thinslab MIP and mIP across the (four) simultaneously acquired sections. This may improve the detection of initial disease in some patients. Multislice HRCT of the whole lung is based on volumetric imaging with 1-"1.25mm section collimation for 4-to 8-slice scanners and 0.5-0.75 mm collimation for 16-slice scanners. In general. a pitch P s 1.5 (P" '6 for 4-slice scanners) can be recommended. A sufficiently high dose (CTOIvoi 'IOmCy) is required for good results. Image noise is reduced when 1.5 mm thick axial as well as coronal or sagittal reformations are reconstructed from a secondary raw data set. Multislice HRCT of the whole lung may add findings unsuspected from conventional discontinuous HRCT. Because very few of these findings appear to hold therapeutic consequences, HRCT scanning of the whole lung cannot yet be recommended on a routine basis (radiation exposure is increased by a factor of 5-10). Because scanning of the whole lung improves side-by-side comparison of findings, it may be indicated for patients in whom even minor changes during follow-up warrant a change in therapy. In such a follow-up situation, scans with reduced dose may be feasible.
Low-dose
CT
Because of the low absorption of the lungs, radiation exposure can be substantially reduced ifC T scanning can focus on the lungs only. With spiral CT. a pitch of 2 should be employed to get the best image at a given (low) exposure dose (see Fig. 5.11). For multislice CT, the same holds true: high pitches yield better quality (in terms of noise) at a given low exposure dose, especially when thin sections are employed (less effect of electronic noise, see pp.120, 139, and 213: also compare Fig. 4.10). The CTDIvoi can be reduced to 2 mGy for most patients.
U sing a less edge-enhancing standard filter substantially improves image quality, especially for ultra-low dose applications, such as bronchogenic cancer screening (see below). For HRCT, a reduced dose (CTDIw= 2-6 mGy) is feasible for follow-up examinations (depending on the therapeutic implications and the subtlety of disease). Increasing the gap between sections to 2cm or higher also reduces patient dose but is only feasible for relatively evenly distributed disease processes. If in doubt, additional sections may be added in the gap between the original sections. With multislice CT and thin-section lowdose scanning, the noise on the original axial sections may be unacceptably high, especially in the mediastinum. Reconstructing thicker sections either primarily from the raw data or from a secondary raw data set will substantially improve image quality (see Fig. 5.18). Since low-dose scanning is at the lower edge of acceptable image quality, any further decrease in quality may reduce diagnostic yield. Obese patients require more dose, otherwise image quality may deteriorate substantially, especially in the lung apex (shoulder regions). Also, a higher kVp setting (usually 140 kVp) should be employed to improve radiation penetration. If in doubt, a single section test scan (which is often performed anyway because of the selection of the reconstruction field-ofview) with the same mAs and kVp settings (not CTDIvoi) as compared to the planned spiral or multislice scan can be used to judge if image quality is sufficient. For this test scan, the CTOIvoi provided on the user interface will be 1.5-2 times (= pitch) higher than the CTDIvoi of the actual scan because (per definition) a single section has a pitch of 1. If available, adaptive noise filtering (see Fig.5.7) substantially improves image quality in critical regions (lung apices). For children, the dose should be adapted to their size (see Table 5.6). In small children, 80 kVp and minimum mAs settings may be employed. The test scan used for determining the reconstruction field-of-view can be used to decide whether image quality is too high (lower dose possible) or too low (higher dose or thicker sections necessary). Low kVp settings can also be recommended for imaging of the pulmonary vessels (see also p.837)
kernel for image reconstruction
vtnuet Low-Dose
Screening
of Bronchogenic
Virtual
Bronchoscopy
Bronchoscopy
and CT Bronchography
and
CT
Cancer
Bronchography
For low-dose screening of bronchogenic cancer, even lower dose may be employed, With conventional spiral CT, a 5110/5 protocol at 120 kVp with a CTDlvoi < 0,8 mGy may be feasible in normal size patients (depending on the minimum mAs setting of the scanner), This corresponds to an effective dose of approximately OAmSv for females and 0,3 mSv for males, It has to be compared to conventional chest radiography, which yields some 0,05-0.1 mSv for a single PA projection, and 0.12-0,2 mSv for PA and lateral radiographs, Again, the dose has to be increased in obese patients, A standard filter kernel for image reconstruction substantially improves image quality, If a suspicious nodule is detected on a screening scan, the patient is brought back and a thin-section CT with moderately increased dose (CIDlYol= 2-4 mGy) is performed with a 112/0,5 protocol and a field-of-view (FOV) that is targeted on the nodule of interest This scan is used for in-detail morphologic analysis (presence of calcifications) and for nodule volumetry. Depending on the size of the nodule (usually < 1 cm) and the screening algorithm, the scan is repeated after 6 weeks to 6 months, Changes in nodule volume by 20%-100%, depending on its size, indicate Significant growth and thus malignantpotentiaL With multislice CT, scanning can be performed immediately with thin sections (0,751,25mm collimation for 4- to 16-slice scanners), Thick sections of some 5 mm width are usually used for image review unless an automated detection program is available, If a nodule is detected, thin sections can be reconstructed around this nodule using a small FOV, and morphologtc and volumetric analysis can be performed, Automated programs substantially simplify nodule analysis because local multiplanar reformations centered around the nodule, volume rendered displays, and volumetric analysis become almost instantly available (see Fig, 9.18), No second scan is required, and the detection of nodule growth may become feasible at an earlier point in time (6-12 weeks follow-up),
Virtual bronchoscopy is a three-dimensional (3D) visualization technique that requires perspective rendering and various interactive tools to obtain the visual impression that the observer is moving within the data volume, The technique can be used to assess anomalies of the central tracheobronchial system noninvasively but cannot substitute for fiberoptic bronchoscopy in patients in whom biopsy has to be performed, lis most useful indications are for the planning of complex transbronchial biopsies (peripheral bronchi or central masses next to major vessels), lt is potentially more sensitive for detecting small irregularities of the tracheobronchial wall but such irregularities are often caused by mucous retention, especially in smokers, Other rendering techniques (compare Fig, 2.16, 2.18, and 2,36) are often superior to virtual bronchoscopy for the display of the tracheobronchial system (CT bronchography). Such techniques present an "intraluminal cast" (using SSD or VRT, see Fig.2.36a) or display the tracheobronchial anatomy as three-dimensional structure with semitransparent walls (tissue transition projection, see Fig.2.36b). Virtual bronchoscopy and CT bronchography require a highly resolved data set. Thus a protocol should be used as described above for tumor staging. M ultislice CT has the advantage that there are fewer pulsation or breathing artifacts because of the more rapid scanning procedure. In addition, it not only provides good results for the central tracheobronchial system but also for more distal bronchial segments. Due to the perspective distortion, measurements should not be performed on the endoscopic views but on the axial or reformatted images.
9 Lungs and Tracheobronchial System Congenital Disorders
Congenital disorders of the lungs and airways are very rare and may cause recurrent infections. CT is indicated if chest radiographs and bronchoscopic findings are equivocal. Tracheal Bronchus, Cardiac Bronchus
Tracheal bronchus refers to a bronchus (upper lobar, apical segmental, or middle lobar) arising from the trachea at an atypical site or a supernumerary bronchus arising directly from the trachea. In most cases the anomalous airway arises from the right lateral wall of the distal third of the trachea. Cardiac bronchus is a supernumerary bronchus, usually rudimentary, that most commonly arises from the medial wall of the intermediate bronchus and passes downward and medially toward the heart (Fig. 9.5). These anomalies are clinically significant in that they predispose to recurrent infections and bronchiectasis, frequently isolated to the bronchopulmonary territory of this bronchus. After endobronchial intubation, the balloon may inadvertently obstruct an atypical bronchus causing right upper lobe atelectasis. Bronchial Atresia
The apicoposterior segment of the left upper lobe is most commonly involved. The lesion may be associated with a bronchogenic cyst.
•
CT Morphology
While there is atresia of the lobar or segmental part of the bronchus, the more distal structures are well preserved. Mucus secreted within the airways distal to the obliterated lumen cannot pass the bronchial stenosis and subsequently causes a mucous plug and a mucocele. Collateral air drifts keep the involved segment inflated. It becomes hyperinflated due to expiratory air trapping. CT reveals the mucocele as perihilar branching structure and hyperinflation of the involved segment. Tracheobronchomegaly
Tracheomegaly can have various causes (Table 9.4), the commonest being destruction of the cricoid cartilage. •
CT Morphology
Tracheobronchomegaly is present when the tracheal diameter exceeds 2.5 cm and the diameter of the main bronchus exceeds 2.0 and 2.3 ern, respectively. Absence or hypotrophy of elastic fibers and hypotrophy of the muscles result in pathologic airway dynamics with dilatation in inspiration and collapse in expiration. Intrapulmonary bronchiectasis is a frequent associated finding.
Fig.9.5 VRT(a)and mIP(b)of the central tracheobronchial system in a newborn: two bronchial buds arise from the intermediate bronchus. Images were derived from a reduced dose CT scan (4 x 1/6, CTDlvoi = 1.8 mGy). Note the small size of the bronchial system with a tracheal diameter of 4mm.
upper lobar or diffuse pulmonary sarcoidosis, idiopathic pulmonary
Cyst
Bronchogenic cysts are the commonest bronchopulmonary anomaly and result from abnormal bronchial arborization during embryologic development. They are classified by location as intrapulmonary (70%) or mediastinal (30%). •
area. They communicate only very rarely with the tracheobronchial system. Infection is extremely rare. The mediastinal bronchogenic cyst is usually larger as compared to the pulmonary site (c- 20 em).
CT Morphology
The intrapulmonary type occurs most frequently in the lower lobe. The cyst may reach a diameter of several centimeters (central type) or may be a multilocular mass composed of multiplesmallcysts (peripheral type). Cysts with very proteinaceous contents may have a CT density as high as 50 HU. causing the "cystic" mass to exhibit soft-tissue density. The uninfected lesion does not enhance after intravenous contrast administration. Characteristically the cysts do not communicate with the tracheobronchial tree until they become infected. which occurs in about 75% of cases. When communication is established. the cysts contain air or show an air-fluid level. With infection of the cyst. the usually sharp definition of the cyst border becomes obscured by consolidation in the adjacent lung parenchyma. the cyst wall becomes thickened and contrast-enhancing, in that way simulating the appearance of a lung abscess. Differentiation between a bronchogenic cyst and an acquired cyst (e.g., after adult respiratory distress syndrome (ARDS) or a healed abscess) may be difficult. Mediastinal cysts are most commonly seen in the right subcarinal area but can also be located paratracheally or in the paraesophageal
A pulmonary sequestration is a nonfunctioning mass of aberrant lung tissue that lacks a normal communication with the bronchial system and has a systemic arterial blood supply. Sequestrations are found in the posterior left lower lobe segment in approximately 65% of cases and in the right lower lobe segment in the remaining cases. Two forms are distinguished according to their relationship to the pleura: • intralobar sequestration (75-85%) which is located within the normal pleural covering of the lung and drains into the pulmonary veins, it typically manifes ts in ad ults; • the extralobar form (15-25%) which has its own separate pleural covering and drains into the vena cava or azygos vein, and is diagnosed in the newborn or early infant. The diagnosis should be considered in any patient with persistent or recurrent lower lobe pneumonia. CT Morphology
An intralobar sequestration may be aerated (by collateral air drift through the pores of Kohn) and frequently shows areas of hyperinflation due to air trapping phenomena. In this case it can be identified as a sequestration only by its atypical arterial supply from the descending aorta. Sites of mucoid impaction in the rudimentary bronchi are a typical finding. If the sequestration has become infected or has perforated into the bronchial tree, CT may show uni- or multilocular cysts containing variable amounts of air and fluid. Extralobar sequestrations generally do not communicate with the bronchi and are of softtissue density throughout (Fig. 9.6). Homogeneous enhancement of the mass coinciding with aortic enhancement suggests the correct diagnosis on postcontrast scans. Some patients show emphysematous changes in the adjacent lung parenchyma and cystic areas within the sequestration.
293
294
9 Lungs and Tracheobronchial
System
Fig. 9.6 Extralobar sequestration in a newborn child. Left lower lobe consolidation (a) with an systemic arterial blood supplyfrom the infradiaphragmatic descending aorta (arrows) seen on a 3 mm thick MIP (b). The arrowheads on (b) indicate a pneumothorax treated by a pleural drain (arrowhead in a). The black arrowheads point to a gastric tube. Ultralow dose scan with 0.6 mGy CTDlvoi after intravenous injection of 2 ml contrast material (4 x 1/6).
a
b
The anomaly is frequently associated with other congenital abnormalities. In rare cases the sequestered segment may fistulate to the esophagus or stomach. Venolobar Syndrome, Scimitar Syndrome, Congenital Hypogenetic Lung Syndrome
Venolobar syndrome is a collective term for various congenital anomalies of the thorax, which frequently coexist in the same patient (Table 9.5). •
CT Morphology
CT demonstrates a small, hypoplastic lung with anomalies in the course and branching of the airways. Bronchial diverticula and bronchiectatic areas are common associated findings. The examiner should be alert to anomalous pulmonary venous return to the vena cava, right atrium, or atrial appendage. Most commonly there is a single vein draining the entire right lung, that runs inferiorly parallel to the right Table 9.5
Venolobar
cardiac border into the inferior vena cava below the diaphragm. (On a frontal radiographs this vein has the appearance of a Turkish sword or scimitar.) Atypical pulmonary veins may be found even in the absence of detectable anomalies of the lung parenchyma. The lung usually receives a systemic blood supply from the descending or abdominal aorta, and sequestrations may be present. Anomalous
Pulmonary Venous Drainage
Anomalous venous drainage is characterized by venous drainage directly into the right heart or the systemic veins (Table 9.6). It may be partial or complete, involving some or all veins, and in any case producing an extracardiac left -to-right shunt. The anatomy is highly variable with approximately 30 different patterns. It is frequently associated with other cardiovascular abnormalities (e.g., atrial septal defect). One diagnostic clue is the identical oxygen saturation in all four heart chambers because pulmonary venous blood mixes with systemic venous blood. The long-term effect of the increased
syndrome
Hypoplastic right lung, lobar agenesis, aplasia or hypoplasia
69%
Partial or complete anomalous veins
pulmonary
31 %
of the right puimo-
14%
Absence of hypoplasia nary artery
Table 9.6
Pulmonary sequestration
24%
Partial or complete systemic arterial supply to the lung without sequestration
10%
Absence or disruption of the inferior vena cava Duplication of the diaphragm right hemithorax
separating
7% the
Patterns of anomalous venous drainage
Supracardiac (50%) • Venous return into superior vena cava, persistent left vena cava or the left innominate vein Cardiac (30%) • Venous return into right atrium or right coronary sinus Infradiaphragmatic (15%) • Venous return into the portal vein or its branches
7% Mixed (5%)
Nonneoplastic Tracheal Stenosis
a
b
c
Fig. 9.7 Partially anomalous pulmonary venous return (PAPVR). Axial sections demonstrate a large superior vena cava (SVC) and a symmetrical vein on the left side (a). There is direct drainage of the right upper lobe vein into the SVC (b). The volume-rendered image demon-
strates that the left-sided mediastinal vein drains the almost the whole left lung into the left braehioeephalie vein (c). The right upper lobe vein drains into the superior vena cava (4 x 1 16).
blood flow in the involved lung is pulmonary hypertension.
left superior vena cava lateral of the aortic arch (Fig. 9.7), and a lack of veins that drain into the left atrium. The anomalies, however, are best described and detected on volume-rendered displays of the pulmonary vasculature because such a display gives the best overview over the -often complex-anatomy. In the presence of arterial septal defects there may be a jet of highly concentrated (undiluted) contrast material entering the left atrium which even can be seen on volume-rendered images (see Fig.23.19b).
•
CT Morphology
CT angiography using spiral or multislice CT allows for complete description of such anomalies, including hypoplastic native pulmonary veins. Without prior clinical suspicion, the anomalies can be missed easily on axial sections. Typical features involve a pulmonary vein that directly enters into the right atri urn or the superior vena cava (Fig.9.7a), an aberrant left pulmonary vein in the position of an accessory
Acquired Disorders of the Tracheobronchial Nonneoplastic
Tracheal Stenosis
Tracheal narrowing may affect the entire trachea or portions of it and may result from pulmonary disease, systemic disease, or a process confined to the trachea itself. CT can be used to distinguish an intrinsic lesion from an extrinsic process (compression) and exclude malignant disease. A "saber-sheath" trachea occurs in patients with COPD and is related to an abnormally high intrathoracic transmural pressure.
•
System
CT Morphology
Tracheal stenosis mayor may not be associated with wall thickening (Table 9.7). CT morphology is often nonspecific, so histological evaluation is frequently necessary when wall thickening is present. CT is performed to document the affected tracheal segment, define the degree of stenosis, and visualize involved bronchial segments. A "saber-sheath" trachea is present when the tracheal area is reduced by 70% and the transverse (coronal) tracheal diameter is reduced to less than one-half of its sagittal diame-
295
9 Lungs and Tracheobronchial Table 9.7 Nonneoplastic Tracheal Intrinsic
stenosis
tracheal
System stenosis:
etiology and CT features
without wall thickening
Rare, 2-4 cm in length, smooth or irregular
Idiopathic Saber-sheath
Intrathoracic trachea, long segment involved, elliptical to triangular cross-section, coronal diameter < half of sagittal diameter
trachea
After tracheostomy
Region of stoma or lower edge of tube, hourglass configuration
After intubation
At cuff site or tube tip, hourglass
Post-ischemic
after lung transplantation
Smooth,
Post-traumatic
configuration
Region of anastomosis hourglass
configuration
Extrinsic Vascular
Detection of vessel (aberrant right subclavain duplicated aortic arch, pulmonary sling)
Goiter
Relation to thyroid gland, intense contrast ment, cysts, calcium
Tracheal
stenosis
Osteochondroplastic
with wall thickening t rac he ob ron ch op ath ia
Calcified tracheal
Polychondritis
Submucosal
Scleroderma
Diffuse thickening,
Wegener's
granulomatosis
cartilage,
irregular tracheal
nodularity of anterolateral
central and peripheral stenoses
Rare, smooth or irregular wall thickening, calcified cartilage, focal or diffuse wall thickening, laryngeal involvement Submucosal
Sarcoidosis
Mural granulomas, smooth or irregular, laryngeal involvement (tuberculosis,
fungal, scleroma)
nodules, diffuse>
local
Rare, smooth or irregular, tuberculous
cavities
Smooth or irregular
Postirradiation
ter. Typically ber beginning ing extending
contour
walls
Amyloidosis
Postinfectious
artery,
enhance-
there is abrupt change of the caliat the thoracic inlet, the narrowfor the entire intrathoracic length.
Bronchiectasis Bronchiectasis is a localized dilatation of the bronchial tree. The changes may be very subtle and may be regional or widespread. Today, bronchiectasis is commonest in patients with cystic fibrosis, mucociliary dysfunction, or an immunodeficiency disease (Table 9.S). Reversible bronchiectasis may occur in the setting of pneumonia and may resolve completely in four to six months. HRCT is superior to standard CT
in the detection of bronchiectasis, providing sensitivity and specificity greater than 90%.
a
CT Morphology
One criterion for bronchiectasis is an internal bronchial diameter that is enlarged in relation to the corresponding artery (normally the artery has a slightly larger caliber than the bronchus). But since the ratio of bronchial to arterial diameter shows a relatively large range of physiologic variation (values found in a normal population range from 0.5 to 1.4), two additional CT criteria for bronchiectasis have been introduced:
• failure of the bronchi to taper toward the periphery (a bronchus has the same diameter as its parent branch for a distance of more than 2 cm); and • visualization of bronchi within "1 cm of the costal pleura and visualization of bronchi abutting the mediastinal pleura. A bronchoarterial diameter ratio can be considered a reliable sign of bronchiectasis by itself only when it is at least 1.5. When it is between 1 and 1.5, it must be present in several airways or be associated with other findings such as bronchial wall thickening and lack oftapering, or both. Care should be taken to assess the smallest crosssectional airway diameter, to avoid comparisons near bifurcations of bronchi and vessels and not to overcall bronchiectasis in areas with decreased vessel size as a result oflocal vasoconstriction. On 10 mm collimated scans normal bronchi are seen only in the inner third of the lung, on HRCTimages Table 9.9
normal bronchi can be seen in the inner twothirds of the lung. Optimal HRCT assessment is obtained using a window level of -700 HU and a width of /\lOOOHU; smaller window width leads to artificial bronchial wall thickening. An extended window width (1500 HU) may be used routinely. However, it reduces the contrast between lung parenchymal structures and makes slight attenuation differences less conspicuous. Vessels may have double contours due to pulsations or respiratory motion, and this can mImIC bronchiectasis (see Fig.7.36a). Bronchiectasis may be missed in areas of dense consolidation or atelectasis. Three morphologic types of bronchiectasis are identified in the Reid classification: cylindrical (tubular, fusiform), varicose, and cystic (saccular) (Table 9.9). Depending on the relation of the bronchi to the plane of section, CT scans of cylindrical bronchiectasis may show signet -ring configurations in cross sections or "tram lines" when the bronchus is scanned in-plane. Tram lines often can be traced far into the periphery (Fig.9.8a, d). The rare varicose form is recognized on CT by marked variations in the calibers of airways that are imaged in longitudinal section (Fig. 9.8 b). Areas of cystic bronchiectasis often show retained secretions with fl ui d levels (Fig.9.8c). Air-filled cystic lesions in atelectatic lung areas have a cluster-of-grapes appearance. Dilated bronchi that are filled with secretions appear as Y- or V-shaped structures of soft-tissue density [mucoid impaction, Fig.9.8c). Thickening of the bronchial walls indicates a bronchitic component or mucosal edema (Fig.9.8b). Mucoid impaction in bronchiectasis is particularly common in patients with cystic fibrosis and allergic bronchopulmonary aspergillosis (ABPA).
Bronchiectasis
Cylindrical
Varicose
Cystic
Mildest form Tram lines (horizontal) Signet ring sign (vertical)
Rare (Swyer -james) String-of-beads contours
Most severe form Transverse diameter> 1 em Frequent wall instability Clustered cysts Air-fluid levels
297
298
9 Lungs and Tracheobronchial System
b
a
c
d
Fig. 9.8 Bronchiectasis. (a) Cylindrical bronchiectases (signet ring sign, arrow), (b) Extensive mucous plugging in right lower lobe bronchiectases (arrowheads). The bronchi are larger than the accompanying arteries. (c) Varicose and cystic bronchiectasis with fluid levels. (d) Semicoronal reformat from a low dose multislice CT data set (4x1/6, 1.2mGy) demonstrates cylindrical
and varicose bronchiectasis in a patient with cystic fibrosis. There is bronchial wall thickening (sign of chronic inflammation) and retained secretions (mucoid impaction). The secretion-filled bronchi produce typical Y- or V-shaped structures. The subpleural "tree in bud" pattern is also evident.
Bronchiolectasis is detectable only if the dilated bronchioles are filled with mucus. CT shows thin, branching structures of soft-tissue density in the subpleural lung zone (5-10 mm) that have small bulbous terminations ("tree in bud" pattern, Fig. 9.8d).
Table 9.10
The isolated circumscribed dilatation of a mucus-filled bronchus may be an indirect sign of a small intraluminal tumor (e.g., carcinoid, bronchialcarcinoma). Differentiation of mucoid impaction from arteriovenous malformations is easily achieved after intravenous administration of contrast material.
Bronchiolitis
A variety of pulmonary diseases are characterized predominantly by inflammation of membranous and respiratory bronchioles. Various classifications schemes are found in the literature that differ with respect to whether they are based on etiologic, histologic, clinical or radiologic features (Table 9.10). From a clinical point of view it is most useful to differentiate infectious bronchiolitis from non-infectious, the latter being related to immunologic reactions.
Different forms of bronchiolitis
Etiology
Morphology
Acute infectious
Tree-in-bud, nodules
Chronic inflammatory (asthma, chronic bronchitis)
Bronchial wall thickening, tree-in-bud
Panbronchiolitis
Marked tree in bud, air trapping, bronchial wall thickening
Respiratory
Centrilobular nodules (predominantly in lower lobes)
bronchiolitis
Respiratory bronchiolitis with interstitial disease
Acute infectious bronchiolitis is most commonly the result of infection by viruses (respiratory syncytial virus, adenovirus), Mycoplasma pneumoniae and Chlamydia species. Immunocompromised patients develop acute Aspergillus bronchiolitis or endobronchial spread of tuber-
Bronchiolitis
culosis. In principle, each bacterial infection can cause acute bronchiolitis if confined to the small airways. •
CT Morphology
The hallmarks of infection-related bronchiolitis in HRCT are small centrilobular nodules with branching lines (tree- in-bud) that represent thickening of the bronchiolar wall or filling of dilated bronchioles with granulation tissue, mucus, or pus (Fig. 9.9 a, b). The tree-in-bud sign is most strikingly appreciated in the peripheral one-third of the lung parenchyma. In some cases it is accompanied by scattered areas of ground-glass attenuation or consolidation. Acute infectious bronchiolitis has to be differentiated from chronic inflammatory conditions (asthma, chronic bronchitis), diffuse panbronchiolitis and respiratory bronchiolitis, which present with centrilobular small nodules (see Fig. 9.9 c): The centrilobular nodules in infectious bronchiolitis are usually well-defined and have sharp margins whereas those seen in patients with respiratory bronchiolitis typically have poorly defined margins. In infectious bronchiolitis nodular opacities are usually combined with branching linear opacities (tree- in-bud). In infectious bronchiolitis centrilobular nodules may enlarge then resembling acinar nodules that can come together to form rosettes.
a
b
Fig.9.9 Bronchiolitis. (a) Infectious bronchiolitis with tree-in-bud-pattern and air-trapping (expiration) indicating disease of the small airways, (b) Infectious bronchiolitis with small nodules and tree-in-bud. (c) Respira-
Respiratory Bronchiolitis and Related Disease
Respiratory bronchiolitis and respiratory bronchiolitis associated interstitial lung disease (RBILD) have recently been grouped together with desquamative interstitial pneumonia (D IP) as part of a single spectrum of diffuse interstitial infiltrative pulmonary disease (see Table 9.34) since they have clinical similarities and significant overlapping of histological and HRCT features. Both diseases are strongly associated with cigarette smoking, underlining the concept that all three diseases represent different degrees of severity of small airway and parenchyma! reaction to cigarette smoking. Radiological as well as histological features of respiratory bronchiolitis and extrinsic allergic alveolitis are also in a continuum, since bronchiolitis is a prominent component of extrinsic allergic alveolitis. Inhalational injury caused by S02, oxides of nitrogen, or a variety of other gases or fumes is characterized by identical CT morphology. •
CT Morphology
Respiratory bronchiolitis is characterized by illdefined centrilobular nodular opacities (Fig.9.9c) producing an image that may be virtually indistinguishable from acute extrinsic alveolitis (see Fig.9 .56a). In most patients, the upper lobes are involved predominantly or even exclusively (as opposed to extrinsic allergic alveolitis) .
c
d
tory bronchiolitis with centrilobular nodules and bronchial wall thickening, (d) Follicular bronchiolitis with multiple ill-defined centrilobular nodules, predominantly in subpleural location.
299
9 Lungs and Tracheobronchial System In addition to small centrilobular nodules, patients with RB-ILD show more prominent bilateral areas of ground-glass attenuation mostly in a patchy distribution and subtle findings offibros is (honeycombing, intralobular lines) predominantly in the lower lung zones, An upper lobe centrilobular emphysema is frequently present; bronchial wall thickening may be an associated finding, In contrast to DIP, lung volumes are usually normal, Similar findings as seen in RB and RB-ILD are also seen in asymptomatic smokers, but the findings are usually less extensive, Clinical and radiological findings improve promptly when the patient stops smoking and is treated with corticosteroids,
Diffuse Panbronchiolitis (DPB) Diffuse pan bronchiolitis is a disease ofunknown etiology, It is characterized by luminal obliteration through accumulation of macro phages, The disease has been recognized almost exclusively in East Asia, The patients present with a chronic productive cough and progressive dyspnea, The long term prognosis is poOL CT Morphology
The hallmarks are extensive diffuse centrilobular nodules with branching lines (tree-in-bud), marked bronchial wall thickening, bronchiectasis, and localized areas of decreased attenuation (air trapping) and perfusion,
gates (Fig,9,9d), The nodules are usually small (1-3 mm). and only occasionally larger (1-2 em). Tree-in-bud, bronchial wall thickening, and patchy areas of low attenuation are associated findings,
Bronchiolitis Obliterans (Constrictive Bronchiolitis) Bronchiolitis obliterans (BO) -also termed constrictive bronchiolitis-is characterized by proliferation of submucosal and peri bronchiolar fibrous tissue resulting in a more or less concentric narrowing of the airway lumen (without intraluminal granulation) through to complete obliteration in extreme cases, The idiopathic form is rare, and the condition can have various causes (Table 9.11), Patients present clinically with progressive obstructive lung disease that frequently shows no abnormalities on plain chest radiographs, Bronchiolitis obliterans is a separate entity from bronchiolitis obliterans with organizing pneumonia (BOOP, see P 321), Table
9.11
Etiology
of obliterative
Inhalation of toxic gases • (e.g., nitrogen dioxide, chorine) Infectious
bronchiolitis
nitric oxide,
ammonia,
agents
• Mycoplasma (in children) • Viruses (in adults) Drug reactions • Penicillamine • Lomustine
Collagen diseases
Follicular Bronchiolitis Follicular bronchiolitis is characterized by the presence of abundant lymphoid tissue within the walls of bronchioles and to some extent of bronchi. The finding is not specific and can be found in association with connective tissue disease (Sjogren's disease and rheumatoid arthritis), imm unodeficiency syndrome, and systemic hypersensitivity reactions, CT Morphology
CT shows nodular opacities mainly in a peribronchovascular or subpleural distribution, consistent with the presence of lymphoid aggre-
• Rheumatoid • Scleroderma
arthritis
-SLE Transplantation
(lung, heart,
• Graft-versus-host • Chronic rejection In setting
The hallmark of bronchiolitis obliterans is air ~rapping: the density of the affected lung areas IS normal or reduced on inspiratory scans, and on expiratory scans the normal parenchyma shows increased density while affected areas show little or no increase in attenuation (due to check-valve obstruction, Fig.9.10a, b). Regions of air trapping may show decreased vascularity with redistribution and consequent vasodilatation in other lung areas, giving rise to a "mosaic" pattern (Fig. 9.10c). . Peribro?chiol.ar fibrosis is rarely detected directly but ISmamfested by centrilobular punctate or branching structures that are most conspicuous in the peripheral third of the lung. Bronchiectasis (mainly central), bronchial wall thickening, and signs of interstitial fibrosis are common associated findings but are not primaryfeatures ofbronchiolitis obliterans. It can be difficult to distinguish the mosaic pattern of bronchiolitis obliterans from a geographic pattern of ground-glass opacities. Ground-glass areas of increased density are pathologic and are seen in the setting of acute alveolar diseases and interstitial inflammatory proc~s:es leading to fibrosis. Ground-glass opacities show a very slight, homogeneous density increase that does not obscure pulmonary vessels or parenchyma! structures.
Mosaic pattern is seen in bronchiolitis obliterans (1), an acute inflammatory process (2) or in chronic thromboembolic disease (3), which can be differentiated from each other by expiratory CT scanning (Table 9.12): 1. In the case of air trapping, the areas of increased attenuation are normal while the are~s of decreased attenuation are patholOgIC. They do not show an increased attenuation on expiratory scans but become even more pronounced in stead. 2. In infiltrative disease, ground-glass opacities represent areas of pathologically increased density. In expiratory views, they show an attenuation increase like that observed in normal lung parenchyma. 3. Pulmonary vessels are decreased in size in the low attenuation areas when air trapping or vascular disease are present, but are normal in acute infiltrative disease. Expiratory scans help to exclude air trapping in hypo attenuating areas.
r
a
b
Fig. 9.10. Bronchiolitis obliterans is characterized by air trapping due to check-valve obstruction, (a) Inspiratory CT shows sharply demarcated areas of different lung density with vascu lar rarification in the hyperlucent areas (mosaic pattern), (b) Expiratory CT shows marked increase of lung density in the normal parenchyma
c while lung areas with air trapping show no change in volume or density, (c) Obliterative bronchiolitis secondary to a drug reaction. CT reveals a patchwork of sharply marginated areas of increased lucency (air trapping) adjacent to areas of normal or apparently decreased lucency with dilated blood vessels ("mosaic pattern").
9
Lungs and Tracheobronchial System
Table 9.12
Differential
diagnosis
of mosaic pattern
Bronchiolitisobliterans
Infiltrativedisease
Chronicpulmonary embolism
Small air ways disease
Ground-glass
Vascular
Airtrapping
Density of normal and opacified areas increases with expiration
No air trapping
Normal vessel
Vessel diameter is decreased areas of hypoattenuation
in expiration
Vessel diameter is (slightly) decreased in areas of hypoattenuation
opacification
diameters
obliteration
in
--
Allergic Bronchopulmonary Aspergillosis (ABPA)
Allergic bronchopulmonary aspergillosis is caused by a pathologic immune response to colonization of the bronchi by Aspergillus species (not to be confused with invasive aspergillosis). Patients with asthma or cystic fibrosis are predisposed. Other rarer forms of hypersensitivity reaction to Aspergillus are extrinsic allergic alveolitis and a Loffler-like syndrome. 1
•
CT Morphology
The initial finding is that of migrating alveolar infil trates in a setting of pneumonitis. The infiltrates are usually bilateral and show a predilection for the upper lobes. The hallmarks of ABPA are bronchiectasis and mucoid impaction involving predominantly the segmental and subsegmental airways of the upper lobes (Fig. 9.11). Retained secretions in segmental bronchi lead to V- or Y-shaped opacities that may persist for months. Focal areas of mucoid impaction and bronchiectasis may mimic a nodule or mass lesion on CT. CT typically shows branching tubular opacities that emanate from the hilum (finger-in-glove appearance). Bronchiectasis may also be widespread and involve more peripheral bronchi; involvement of main and lobar bronchi, however, is very uncommon. The combination of bronchiectasis on CT and blood eosinophilia is typical of allergic bronchopulmonary aspergillosis. Segmental atelectasis and peripheral subsegmental atelectasis give rise to patchy areas of consolidation. Postobstructive cavitation may occur. Late changes consist of hyperinflation and fibrosis. Involvement of small bronchioles leads to a "tree- in-bud" pattern and air trapping on expiratory scans.
Bronchiectasis is seen more frequently in asthmatic patients with ABPA than in asthmatic patients without ABPA. However, bronchiectasis may also occur in uncomplicated asthma, and may not be present in all patients with ABPA.
Benign neoplasms of the trachea and bronchi are very rare and are always intraluminal. Commonest are mesenchymal tumors (hemangioma, hamartoma), but occasional paragangliomas, neurofibromas, and papillomas can occur. The detection of extraluminal tumor growth on CT excludes a benign lesion. Tracheobronchial papillomatosis is caused by a viral infection (papilloma virus). It is usually confined to the larynx, rarely spread of papillomas to the trachea, bronchi and lung occurs. Papillomas may be solitary or multiple. Histologically they consist of central fibrovascular bundles covered by layers of well-differentiated squamous cells. Transformation oflesions into invasive squamous cell carcinoma is known. •
a
Fig. 9.12 Tracheal lesions, (a) Shaded surface display of a benign intraluminal tracheal papilloma (2/4/2). (b) Coronal reformatted image of a malignant esophageal tumor invading the trachea (3/5/2). Note the intraluminal and extraluminal components of the tumor.
CT Morphology
They have a pathognomonic CT morphology with nodules that frequently cavitate (Fig. 9.13).
Benign neoplasms usually have smooth, welldefined margins and are less than 2cm in diameter. They do not display specific morphologic CT features, however, and biopsy is necessary to establish a diagnosis (Fig. 9.12). Polypoid and sessile masses protrude into the lumen and may cause subtotal obstruction. Tracheobroncbial papillomatosis may present as polypoid lesions on the surface of the tracheobronchial system, but frequently intrapulmonary papillomas are the only findings on CT.
a
b
b
Fig.9.13 Intrapulmonary papillomatosis. (a) Intrapulmonary cavitating, irregular nodules (HRCT). (b. c) Lowdose multislice CT (4x1/6, CTDlvoi=1.8mGy) of a
Particularly in elderly patients with an obtunded cough reflex, secretions may accumulate in the central airways and create a pseudomass appearance. Usually these can be identified by their teardrop-shaped configuration, and coughing tends to displace the collection or cause it to disappear.
c
~ d
young girl with multiple small cavitating nodules due to papillomatosis. (d) Compare to cavitating intrapulmonary metastases.
303
9 Lungs and Tracheobronchial
System
Malignant Tracheal Tumors
Carcinoid Tumor
Primary malignant tumors of the trachea are extremely rare (180 times rarer than bronchial tumors). Clinical symptoms are a late finding, so generally these tumors are in an advanced stage when diagnosed (significant luminal narrowing in more than 75%, mediastinal invasion in 40%). The three commonest tracheal malignancies are squamous cell carcinoma, adenoid cystic carcinoma, and mucoepidermoid carcinoma. Secondary malignant tumors involving the trachea include the thyroid and the esophagus.
Carcinoids are classified as neuroendocrine tumors. They account for less than 4% of all bronchial neoplasms. They are locally invasive and metastasize to regional lymph nodes, bone, and liver. Approximately 90% are endobronchial tumors that involve the central airways and may become obstructive. Only 10% involve the lung parenchyma. According to their histology and their biological behavior, typical, carcinoids are differentiated from the more aggressive atypical carcinoid (metastasizes into the lymph nodes) and the small-cell carcinomas (mostly associated with bulky mediastinal disease). Radion uclidecoupled somatostatin analogs (octreotide scan) are used to visualize somatostatin-receptorbearing tumors.
•
CT Morphology
Primary tracheal tumors grow exophytically and by infiltration. They may spread over a distance of several centimeters and sometimes up to 10cm. The full extent of intramural (submucosal) spread tends to be underestimated with CT. While squamous cell carcinoma causes irregular sometimes nodular wall thickening, adenoid cystic carcinoma leads to smooth and long-standing wall thickening and consecutive lumen narrowing. For surgical planning, it is crucial to assess the involvement of the infraglottic larynx (indication for laryngectomy) and of the principal bronchi.
a
b
Fig. 9.14 Axial section (a) and shaded surface display (b) of an intraluminal carcinoid at the level of the carina (3/5/2). It is typical to find the smooth-bordered mass located in proximity to bronchial divisions. A carcinoid
•
CT Morphology
Endobronchial carcinoids appear on CT as wellcircumscribed nodular masses that occur predominantly at bronchial divisions (Fig. 9.14). They frequently cause airway obstruction and may be associated with air trapping, bronchiectasis, mucoid impaction, or pneumonia. Calcifications are seen in 30% and due to their good vascularization tumors show a marked enhancement after application of contrast.
c
d tumor (arrowheads) in another patient (c) has caused atelectasis of the anterior segment of the right upper lobe (d). Carcinoid tumors are hypervascular and thus may be hard to distinguish from adjacent atelectasis.
Bronchial Carcinoma 305 sis, idiopathic pulmonary fibrosis, scleroderma), The therapeutic approach depends on the local tumor extent, the invasion of mediastinal and hilar lymph nodes, and the presence or absence of distant metastases, Bronchial carcinoma is characterized by early lymphogenous and hematogenous spread, Lymph node metastases are commonest with small-cell carcinomas, The highest rates of metastasis are to the liver (up to 40%), adrenals (up to 40%), brain (up to 43%), bone (up to 33%), kidneys (up to 25%), and abdominal lymph nodes (up to 30%), The incidence of metastases depends on histology and primary tumor stage,
Bronchial Carcinoma Bronchial carcinoma is the most frequent tumor in the statistics of neoplasm-related death rates and the commonest tumor of the central tracheobronchial system. The five-year survival rate is only 8 to 13% lower as compared to that of other neoplasms (colorectal carcinoma=62%, breast neoplasm=85%), Active cigarette smoking increases the risk for developing a bronchial carcinoma by factor 10, passive smoking by factor 2, The histological classification was revised in 1999 and differentiates the small-cell carcinoma (SCLC=20%), the adenocarcinoma (30%), the squamous cell carcinoma (25%) and the large cell carcinoma (10%) as most frequent tumor types (Table 9.13), Certain occupational agents (asbestos, coal, arsenic, chromium, chloromethyl ether, mustard gas) are known to lead to increased risk of lung cancer, the most important of these being asbestos, A combination of asbestos exposure and Cigarette smoking is multiplicative, Other diseases that have an increased prevalence of lung cancer are characterized by diffuse or focal pulmonary fibrosis (e,g" tuberculosis, asbesto-
Table 9.13
Staging The staging of bronchial carcinoma (Tables 9.149.16) is useful in selecting patients for surgical resection, planning radiotherapy, and monitoring the response to chemotherapy, Stages 1 and 11 are amenable to resection, Stage III was divided into two classes: lllA includes tumors with limited mediastinal or chest wall invasion and are considered potentially re-
Bronchiogenic carcinoma Frequency
Histology
Etiology and
Region of
Squamous
cell carci-
30-35%
noma (subtype:superiorsulcus tumor)
Special features
predilection
Prognosis
Best prognosis; local
2/3 central
Central: atelectasis
metastases nodes
endobronchial 1/3 peripheral
strucnve pneumonitis Peripheral: thick walled
to lymph
or postobcavi-
tation (30%), solitary nodule Small-cell
carcinoma
20-25%
(sci c)
Strong association with smoking; most aggres-
Central (85%)
sive with early lymph node Large-cell
(undifferen-
3 cm, or tumor involving the intermediate or main bronchus but at least 2 cm distal to the carina, or tumor involving the visceral pleura; associated atelectasis or postobstructlve pneumonia must involve less than an entire lung Tumor of any size involving the main bronchus within 2 cm of the carina, or tumor directly invading the parietal or mediastinal pleura (including superior sulcus tumors) diaphragm, chest wall, or pericardium, associated atelectasis or postobstructive pneumonitis involving the entire lung Tumor of any size invading the heart, great vessels, trachea, carina, esophagus, or spinal column, or the presence of malignant pleural effusion, satellite lesions within the some lobe as primary tumor
NO
No metastasis to regional lymph nodes
N1
Metastasis to ipsilateral bronchial or hilar lymph nodes
N2
Metastasis to lpsilateral mediastinal and subcarinallymph nodes
N3
Metastasis to scalene or supraclavicular lymph nodes, or metastasis to contralateral mediastinal or hilar lymph nodes
M1
Distant metastases, satellite lesions in different lobe as primary tumor (ipsi- or contralateral)
sectable, while IIIB is considered non-resectable. T4 means infiltration of vital mediastinal structures (e.g .. heart, trachea, esophagus, great vessels). N1 alters the prognosis but not the surgical approach. N2 diseases are considered resectable but usually patients undergo neoadjuvant chemotherapy sometimes combined with radiation therapy before operation. N3 disease precludes resection. It is therapeutically important to differentiate between stage II and iliA tumors (lobectomy versus pneumonectomy) and between stage lilA andlllB disease (resectable versus unresectable) (Fig. 9.15 b). Transfissural tumor spread, pulmonary vascular invasion, the invasion of a main bronchus, or the involvement
Table 9.15 Size criteria for mediastinal lymph nodes (Glazer 1985 adopted to AJCC/UICC classification 1996) Station
Description
Normal size
2R
Right upper para tracheal nodes
:s7mm
2L
Left upper paratracheal nodes
:s7mm
4 R
Right lower paratracheal nodes
:s 10 mm
4 L
Left lower paratracheal nodes
:s 10 mm
5
Subaortic aortopulmonary nodes
:59mm
6
Para-aortic (anterior mediastinal) nodes
::=;8mm
7
Subcarinal nodes
8R
Right para-esophageal nodes
:s 11 mm :s 10 mm
8L
Left para-esophageal nodes
:s;7mm
10 R
Right hilar nodes
:s 10mm
10 L
Left hilar nodes
:s7mm
Table 9.16 Stages of bronchial from Mountain, 1998)
T1 T2 T3 T4 M1
carcinoma
(adapted
NO
N1
N2
N3
M1
IA IB liB IIIB IV
IIA liB lilA 1116 IV
IliA lilA IliA IIiB IV
IIIB IIiB 1116 IIIB IV
IV IV IV IV IV
of upper and lower lobe bronchi, preclude a lobectomy and necessitate pneumonectomy (iliA). Invasion of a main bronchus less than 2 ern from the carina signifies a T3 tumor (Fig.9.15c). Involvement of the carina or involvement of contralateral lymph nodes indicate advancement beyond stage iliA and preclude surgical intervention. The non-small-cell neoplasms are staged according to the TNM classification (last modified in 1997). A small-cell carcinoma is classified as "limited disease" (stages I through iliA). Stage IIIB, denoting "local extensive disease," is still considered to be localized from a radiotherapeutic standpoint. Stage IV denotes "extensive disease" with detectable distant (ex-
Bronchial Carcinoma
a
b
Fig. 9.15 Central bronchial carcinoma, (a) Tumor has encased the intermediate bronchus (stage I) without invading the pulmonary artery. Small lymph nodes, not considered significant, are visible on the contralateral hilum (arrow), (b) Stage IliA bronchial carcinoma has obliterated the right lower lobe bronchus (arrow) and invaded the pulmonary artery, necessitating a pneumonectomy rather than lobectomy. (c) Tumor has en-
trathoracic) metastases (cervical or axillary lymph nodes or other distant metastases). •
CT Morphology-
T Staging
CT has an accuracy of only 50% to 70% in evaluating the local invasiveness (T staging) of bronchial carcinoma. CT and MRI are equally limited in their ability to assess involvement of a main bronchus, the carina, or the trachea, due to a lack of precision in the delineation of submucosal tumor spread. Intrabronchial structures of soft-tissue density, bronchial wall thickening, and an irregular reduction in bronchial caliber are indicators of bronchial tumor involvement (Fig.9.15). Contrast-enhanced CT is essential, though not always successful, in differentiating bronchogeniC tumor from areas of poststenotic atelectasis and pneumonia. Atelectasis shows more intense and more uniform enhancement than tumor-involved lung tissue on postcontrast scans. Pneumonia shows similar characteristics, though to a considerably lesser degree. The following findings are more suggestive of an occult central tumor with postobstructive consolidation rather than of simple pulmonary consolidation due to pneumonia: • S-shaped distortion of the lung fissure due to tumor-induced lobar atelectasis;
c
d
cased a long segment of the anterior upper lobe bronchus and the right main bronchus to within 2 cm of the carina (stage IliA disease), (d) Inoperable tumor (stage IV) has invaded the mediastinum and encased the aorta. It has also invaded the pulmonary artery and partially obstructed the left main bronchus. The atelectasis shows greater postcontrast enhancement than the tumor itself (arrowheads).
• Loss of volume due to lobar atelectasis, or the presence of an air bronchogram (in patients over age 35); • 'Irregular stenosis in the central bronchial tree; • Hilar lymphadenopathy; • Insufficient regression of pneumonia in response to antibiotics. Reconstruction of thin axial sections and MPR are particularly worthwhile in patients with focal lung lesions that are in the vicinity of pleural interfaces, because they frequently provide additional diagnostic information with respect to segment localization, pleural dissemination, and transfissural tumor growth (Fig. 9.16). It was demonstrated that the sensitivity of CT scanning for the presence of extension through the major fissure was 57% for thick 10mm sections, but could be increased to 87% with thin 2 mm sections. Additional multiplanar reformations (MPR) could further improve sensitivity to 100%. For the minor fissure, 6/51 cases were inconclusive on axial section but only one remained so on MPRs. Three-dimensional images based on shaded surface displays were found to facilitate the differentiation of simple pleural tags (i.e., bands extending from the lesion to the visceral pleura; T-stage =s T1) from pleural puckering associated
308
9 Lungs and Tracheobronchial System
a
b
c
d
Fig. 9.16 Multiplanar reformations improve staging of bronchial carcinoma (4x1/6, 7.2 mGy). (a) Transfissural tumor growth and broad tumor adherence along the superior vena cava and the right atrium, (b) Incomplete lobar fissures support translobar spread of carcinomatous lymphangitis, (c) Large tumor infiltrating the pulmonary artery (arrow) with pathologic sub-
carinal lymph nodes (T4N2, stage IIIB disease) and a contralateral acute pulmonary embolus (arrowhead). (d) Bilateral metastatic lymph nodes in the right perihilar region, pretracheal, and in the aortopulmonary window. These nodes are round but smallerthan 10 mm in short axis diameter.
with an indrawn locally thickened pleura seen with visceral pleural invasion (> T2). While the sensitivity of pleural puckering was high, specificity was only 76% because it was not only seen with tumor invasion but also with reactive fibrotic changes.
sidered a reliable indicator for intrapericardial tumor extension, while infiltration of the inferior pulmonary vein does not necessarily mean that the pericardium is infiltrated. Although chest wall and limited mediastinal invasion of lung cancer are no longer contraindications to surgical treatment, the mortality rate associated with en bloc resection of the tumor and the contiguous structures is still substantial. Therefore, information about mediastinal and chest wall invasion is still one of the important factors in the clinical decision to perform surgery. Gross extrapulmonary chest wall invasion does not impose problems. However, accurate distinction between tumor contiguity with adjacent extrapulmonary structures and actual invasion is often difficult on static images, especially when there is no obvious soft tissue mass in the mediastinum or chest wall. Contiguity of the tumor with parietal pleura was found not to be equivalent to definite invasion of the chest wall, even when associated with a pleural thickening contiguous to the tumor. Apart from gross invasion, focal rib destruction is the only sign that is "100% sensitive for chest wall invasion. All other findings such as pleural thickening, an obliterated epipleural fat plane, an obtuse angle between the tumor and chest wall, more than 3 cm of contact between the tumor and chest wall
Presence of a pleural effusion requires the exclusion of pleural involvement (pleural tap, if negative PET). Pleural involvement corresponds to stage IV and makes the tumor irresectable. Mediastinal invasion represents stage T4 and in almost all cases makes the tumor irresectable. It should be diagnosed if CT shows tumor encasement or constriction of vital mediastinal structures, e.g., of the large mediastinal vessels, the heart, the esophagus, spine, or the trachea, or if the tumor is transgressing the midline (Fig. 9.15d). Local infiltration of the mediastinal fat after transgression of the mediastinal pleura is not considered a criterion for irresectability. MRI is superior to CT in differentiating tumor from fat and will reveal even minimal changes, but it is no better than CT for distinguishing tumor infiltration from an inflammatory reaction of the epipleural or mediastinal fat. Infiltration of the superior pulmonary vein is con-
Bronchial Carcinoma and a ratio between tumor-pleura contact and tumor diameter of > 0.9 showed sensitivities between approximately 60% and more than 90%. Obliteration ofthe epipleural fat plane may be due to tumor invasion or inflammatory reaction and is equally unreliably differentiated with MRI and CT (sensitivity/specificity of 85%/87%withCTand90%/86%withMRI) . In general, there is a superiority of \ mm slices as compared to 10mm slices for the delineation of anatomic chest wall structures and thus for evaluating the presence of chest wall involvement. For the delineation of the extrapleural fat plane and the assessment of soft tissue structures a smoothing standard reconstruction algorithm was superior to the high resolution algorithm due to its lower noise level. Both, CT and MRI have been employed to use dynamic imaging during different phases of respiration to assess the respiratory shift that is defined as a change in the relative location between the peripheral lung tumor and the chest wall with deep inspiration and expiration. Expiratory dynamic CT was found to provide accurate information but only for tumors located in the middle and lower lobes. It has also to be noted that only absence of tumor invasion can be confidentially diagnosed in those patients in which free movement of the tumor directly proves lack of attachment between the visceral and parietal pleura. Benign fibrous adhesion may cause false positive results by simulating tumor infiltration. Such limitation holds true for
a
both CT and MR imaging as well as alternative approaches using ultrasound or pneumothorax CT. The latter is based on the idea that tumor infiltration can be differentiated from the parietal pleura on the basis of air in the absence of tumor infiltration. However, this technique is invasive, it may cause symptomatic pneumothorax (described in 4/43 patients) and therefore has never found broad application. Owing to its superior contrast resolution, MRI used to be the method of choice for imaging of superior sulcus tumors. The use of surface coils provides high resolution images and chest wall invasion is best depicted as a disruption of the extrapleural fat line (bright signal of fat in Tl-weighted images). MRI was also thought to be more accurate than CT in depicting chest wall involvement from superior sulcus tumors with respect to involvement of the subclavian vessels, the brachial plexus and the vertebra. However, high quality spiral CT (thin sections, bolus injection of contrast agents, and sagittal and coronal reformats) provide the same anatomic information of the anatomical environment of the plexus, and MRI may be reserved for patients with extension into the neural foramina and the epidural space (Fig. 9.17). •
Staging
The evaluation of mediastinal lymph nodes is an important aspect of staging in patients with non-smaIl-cell lung cancer. The nodal status is an integral part of the TNM staging system and
b
Fig. 9.17 Superior sulcus (Pancoast) tumor (4 x 1/6). On 5 mm axial sections (a) there is no clear demonstration of chest wall invasion, which is clearly demonstrated on 3 mm coronal reformations from the same
CT Morphology-N
c multislice CT data set (b). Coronal MPR in another patient with small cell lung cancer extending into the cervical soft tissues and invading the superior vena cava (c).
309
9 Lungs and Tracheobronchial
System
provides critical information for planning treatment and determining prognosis. Traditionally imaging evaluation of mediastinal lymph node involvement has relied on anatomic features of the nodes. most notably nodal size. CT has been shown to be the best predictor of nodule volume. With regard to the determination of the nodal status. however. its limitation with relatively low sensitivity (4167%) and low specificity (79-86%) are well documented. based on the fact that enlarged lymph nodes may be hyperplastic rather than neoplastic. especially in patients with post-obstructive pneumonia. and normal-sized nodes may contain neoplastic cells. Microscopic tumor involvement was seen in S-64% of normal-sized lymph nodes. while IS-30% of enlarged lymph nodes (IO-IS mm) were free of tumor. Patients with COPD. recurrent cardiac failure. or previous granulomatous disease also tend to have enlarged lymph nodes. Various approaches have been taken to refine the size criterion by modulating the threshold size for various nodal locations within the mediastinum. or taking into account not only the smallest diameter as measured on the axial scan but also the shape of the lymph node (round versus oval). An increased diagnostic accuracy was described for morphologic evaluation of thin section imaging ofhilar lymph nodes. In 9S% oflocations with normal lymph nodes there was a straight or concave pleural interface between hilar structures and lung parenchyma, while in 9S% of locations with malignant lymph nodes there was a convex interface. Application of this criterion led to an improvement in sensitivity from SO% for the 10 mm size criterion to 87% for the morphologic criterion, and of specificity from 80% to 83 %. The approach of considering size and shape of lymph nodes takes advantage of the availability of thin and high quality multiplanar images provided by multislice CT. No quantitative data of diagnostic accuracy are available up to now. However, even though it is likely that refinement of morphological criteria in evaluating mediastinallymph nodes will improve the diagnostic accuracy, it will most likely never be able to achieve the same diagnostic accuracy as functional imaging using PET or (still investigational) lymph node-specific MR contrast agents (USPlO). Lymph node metastases may arise through a continuous mode of spread (intrapulmonary
=> bronchopulmonary or hilar => ipsilateral mediastinal => contralateral mediastinal). or a group of nodes may be skipped (in up to 30% of cases). Tumors of the right lung tend to metastasize to ipsilateral nodes, with lower lobe tumors usually causing more extensive metastasis. Contralateral nodal metastasis is more common with tumors of the left lung {> 30%). CT serves not only as basis for selecting those patients who have to undergo further invasive procedures (mediastinoscopy, mediastinotomy, thoracoscopy, trans bronchial biopsy) for lymph node staging but also to select the appropriate procedure depending on the location of enlarged lymph nodes to be sampled. Though the inability of CT to detect microscopic metastases within normal sized lymph nodes is known, it is generally agreed that preclusion of enlarged lymph nodes is sufficient to obviate further evaluation and patients directly undergo thoracotomy. However, to avoid over staging it is common understanding that enlarged lymph nodes seen in CT must be sampled by biopsy. The paratracheal, subcarinal, and proximal bronchopulmonary lymph nodes are accessible to mediastinoscopic biopsy, but not the commonly affected aortopulmonary nodes.
Therapeutically significant lymph nodes that appear enlarged on CT (Tables 9.1S, 9.16) should be biopsied due to the limited specificity ofCT. A mediastinoscopy is recommended in all patients with T3 tumors, with centrally located adenocarcinoma because of their high incidence of lymph node metastases, and in patients with chest wall infiltration or with Pancoast tumors, because of their poor prognosis with advanced lymph node staging.
Screening with Low Dose CT Previous studies using chest radiography and sputum cytology had not been able to statistically prove reduction of mortality rates, most likely due to the too low sensitivity of these tests for early tumor stages. Spiral CT is acknowledged as the most sensitive diagnostic method for the detection of small intrapulmonary nodules. The option for Significant reduction of acquisition dose
Bronchial Carcinoma
without comprormsmg the conspicuity of intrapulmonary nodules makes this technique feasible for screening (Fig. 9.18). Such CT examinations have a sensitivity >90% for nodules larger than 5 mm in diameter and an acquisition dose that is only about 2-3 time larger than chest radiographs in two projections. If a nodule is detected, the patient has to be rescanned with thinner sections unless thin-section multislice CT has been employed (see p.291). Multislice CT, in addition, facilitates automated detection, morphologic, and volumetric analysis of nodules. Depending on the geographic area and the frequency of histoplasmosis or tuberculosis, there may be a high rate of nodules with benign histology (granuloma, hamartoma, focal infection, lymph nodes). Thus, the screening protocols may vary locally. Biopsy has been suggested for all nodules> A cm that are not definitively benign (e.g., because of benign calcification, see p.314, or a known long-term course). Nodules between 5 and W mm in size are either biopsied or followed, nodules below 5 mm are followed with a larger time interval for rescanning. •
CT Morphology
Nodule detection is performed on axial sections with a lung window setting and a section width of some 5 mm. Overlapping reconstruction is mandatory, and viewing in a cine display speed up the evaluation and improves the detection rate. Automated detection algorithms work best on thin-section multislice CT data sets. It is not yet clear whether these algorithms are good
a
b
Fig. 9.18 Lung cancer screening Low-dose scans (CTDlvoi=2.3 mGy) demonstrate a small solid nodule (adenocarcinoma, a) and a ground-glass nodule (alveolar cell carcinoma, b). The volumetric assessment of
enough to be used exclusively or whether visual assessment of the whole data set is still necessary. Detection failures in a Japanese trial were due to the misinterpretation of lesions in close vicinity to pulmonary vessels or associated with pre-existing disease due to old tuberculosis. Three types of nodules have been identified: ground-glass nodules (focal areas of ground-glass opacities), solid nodules (no ground-glass associated), and mixed nodules (both features present). For nodules below "I em, patients with ground-glass and mixed nodules may be put on a course of antibiotics and are rescanned after 3-6 weeks. Solid nodules are biopsied or removed if thin-section scanning reveals morphologic criteria that are suggestive of malignancy (see Table 9.18). All other nodules are followed for 6 weeks to 6 months, depending on the local protocols and the available evaluation software. Computer-based evaluation algorithms were found to be able to reliably assess volumetric changes already after 4 weeks. Morphologic analysis is improved if volume rendered displays of the nodule surface are available and if multiplanar reformations centered on the nodules can be performed. •
Whom to Screen and When?
All studies published so far have in common that CT was found to be highly superior to radiography in terms of sensitivity. Bronchial carcinoma was detected in 1.1-2.7% of screened patients. Most (62-93%) tumors were determined to be stage 1. There is no general consen-
c
d
another nodule at the lung base (c) demonstrates no increase in tumor volume in follow-up scans after up to 6 months (d).
311
9 Lungs and Tracheobronchial
System
sus yet, however, as to whether screening can be generally recommended, and which risk groups are eligible. There are numerous potential sources of bias in the present trials (e.g., ELCAP, various Japanese and European studies). A lead time bias mainly affects the initial (prevalence) screening and is caused by tumors that are detected earlier but without an effect on overall life expectancy. A length time bias is caused by tumors, such as adenocarcinomas, that are slow growing and are thus more likely to be detected during follow-up (incidence screening) while more aggressive tumor types, such as small-cell cancers, may be diagnosed based on clinical symptoms in the time intervals between two screening procedures. Peripheral tumors are more likely to be detected early than central tumors {location bias}. In addition, there is the danger of overdiagnosis in those individuals who normally would die from other causes: small tumors detected in patients with large other risk factors (such as coronary artery disease, previous heart attacks, severe emphysema, or other smokingrelated diseases) may induce unnecessary treatment. Most present evidence, however, suggest that lung cancer screening programs in a selected risk population may be even more effective than mammography screening. There are no general rules yet but patients should be over 50 or 60 years old and have a major history of smoking (20-40 pack years). Obviously, the older the patient and the longer the smoking history, the higher is the likelihood that screening would be effective in detecting tumors and reducing overall mortality. Control after Treatment/Tumor Recurrence
CT remains the procedure of choice for routine follow-up or in suspected tumor recurrence in the post-pneumonectomy patient. The superiority of MRI for the detection of soft tissue recurrence is shown by the combination of mass effect and altered MR signal intensity. In fact any inhomogeneous signal in the pneumonectomy space should be suspicious for tumor recurrence (Fig. 9.19). A problematic topic is the reevaluation of the residual tumor and tumor activity after
a
b
Fig. 9.19 Tumor recurrence after lobectomy (4 x 1/6). Axial section (a) and coronal reformation (b) demonstrate a large recurrent tumor that invades the chest wall, the pulmonary arterial trunk, and thejugular vein and superior vena cava, leading to extensive venous collaterals (c).
neoadjuvant chemotherapy to be followed by surgery. CT criteria tend to underestimate the therapeutic effect demonstrated by pathologic examination. PET is likely to be the superior diagnostic method for evaluation of residual tumor or tumor recurrence. •
CT Morphology
There is a considerable variation in the appearance of the post-pneumonectomy space ranging from complete obliteration (in 27%) to a persistent fluid collection (in 73%). Filling in of previously patent ectatic bronchi is suggestive for recurrent disease. A 50% size reduction, a change in tumor morphology (round versus irregular), and the disappearance of density enhancement after application of intravenous contrast have been suggested as criteria for complete tumor response. Virtual Bronchoscopy and CT Bronchography
Virtual bronchoscopy yields excellent displays of the central tracheobronchial system and has various advantages over fiberoptic bronchoscopy. It is less invasive and therefore well tolerated by the patients. It can display the bronchial system distal to a narrow stenosis. Inversion of the viewing direction of the virtual
Secondary Tracheobronchial
bronchoscope is possible and one is able to look towards the proximal portions of the tracheobronchial system. Most important, however, is that all the additional information from the surrounding structures is available from the scans. Thus, there is information of the transmural extent of a tumor, the presence and location of lymph nodes, and the location of suspicious areas wi thin the 1ung parenchyma. In a clinical trial it was found that virtual bronchography improves the recognition of mild changes in airway caliber that otherwise are difficult to appreciate. It also helps in the understanding of complex tracheobronchial anomalies. The technique can be used to noninvasively assess diseases of the central tracheobronchial system (Fig. 9.20) but cannot substitute for fiberoptic bronchoscopy in patients in whom biopsy has to be performed or in whom visual assessment of the mucosal surface (e.g., color changes) is necessary. For pretherapeutic staging, virtual bronchoscopy serves as adjunct to bronchoscopy. Virtual bronchoscopy with transparent walls or colored extramural pathology may be used for guiding and planning trans bronchial biopsy procedures. In post-therapeutic imaging, non- invasive virtual bronchoscopy may be used alternately with fiberoptic bronchoscopy, e.g., in patients after bronchoplastic surgery with complex anatomic situations and a susceptibility for developing bronchial strictures. However, other visualization techniques based on MPR or volume rendering often provide equivalent or superior information (Fig.9.20).
Secondary Tracheobronchial Neoplasms
Neoplasms of the larynx, thyroid gland, or esophagus can involve the trachea and central bronchi by direct invasion or distant metastasis. Breast cancers, renal carcinomas, colorectal carcinomas, and melanoma have a propensity for endobronchial metastasis, which may be palliable by laser ablation and stent insertion, depending on the extent of the extrabronchial lesion and the severity of the obstruction. •
CT Morphology
CT has proven more sensitive than bronchoscopy for evaluation of the subglottic larynx. The subglottic spread of a laryngeal tumor into the trachea or, conversely, the spread of a trachea! tumor to the undersurface of the vocal cords will dictate the preferred therapeutic approach (laryngectomy versus partial tracheotomy). When a thyroid tumor is present, the ill-defined boundary that is often found between the thyroid gland and tracheal wall should not be misinterpreted as a sign of tumor invasion. Tracheoesophageal fistulas are most likely to develop in patients with advanced esophageal tumors, and CT can often locate the fistula more easily than endoscopy. Endobronchial metastases display nonspecific CT features and should be biopsied if doubt exists. Hodgkin's disease can lead to the formation of obstructive endobronchial lesions with atelectasis. In very rare cases, non-Hodgkin's lymphoma (NHL) may infiltrate the central airways and cause diffuse submucosal nodules.
Fig. 9.20 Virtual bronchoscopy (a) and CT bronchography (b) in a patient with a stenosis following sleeve resection of bronchogenic cancer(2/4/1 ).
a
Neoplasms
b
313
9 Lungs and Tracheobronchial System Pulmonary
Nodules
Pulmonary nodules are among the commonest focal pulmonary lesions detected at imaging (Table 9.17). Vascular processes or atelectatic lung areas can have features that resemble pulmonary nodules. For many diseases. however. the presence of a nodule is simply one variant in a range of possible radiologic presentations of the disease.
Table dules
9.17
Differential
Benign
neoplasms
• Hamartoma • Chondroma
Approximately 50% of surgically removed nodules turn out to be benign and noninvasive. Evaluation of the solitary pulmonary nodule remains a substantial challenge in modern medicine. Table9.18 reviews morphologic findings that may be helpful in differentiating benign from malignant lesions. In selected cases. ancillary findings such as pleural thickening. vascular relationships. cavitation. or enhancement characteristics may also aid in making the correctdiagnosis. Clusters of small intrapulmonary nodules with an internodular distance of less than Wmm and without associated infiltrates most frequently represent postinfectious granulomaFeatures
Central Dumbbell-shaped Peripheral lamination Calcification >10% of volume
Eccentric Stellate Disseminated Calcification
CTdensity
>150HU
50%) and spiculated margins (> 70%). Most lesions exhibit a mixed density pattern. It is common to find intact air-filled bronchi within the tumor (pseudocavitation), and the embedded bronchi may appear stretched. Atelectasis generally does not occur. The multifocal or djffuse form resembles air space consolidation due to pneumonia. The spectrum ranges from patchy ground-glass opacities to areas of consolidation (Fig. 9.31 b, c). Involvement of both lungs may occur.
Whenever CT demonstrates multiple lesions that show the morphologic features of bronchial carcinoma or focal pneumonia, alveolar cell carcinoma should be considered in the differential diagnosis. Pleural effusion is seen in approximately onethird of cases, and mediastinal lymphadenopathy is seen in one-fifth. Cavitation is rare. The "CT angiogram" sign appears when pulmonary vessels coursing through the tumor show greater enhancement on postcontrast scans than the surrounding tumor. While a "CT angiogram" is commonly seen with alveolar cell carcinoma, it is not specific and may also occur in consolidation due to lymphoma, pneumococcal infection, or tuberculous infection (Fig. 9.31 d). Lymphoma
Pulmonary involvement by malignant lymphoma occurs in up to 40% of cases and is commoner in Hodgkin's lymphoma than in the nonHodgkin's type (6%) (Table 9.23). In Hodgkin's disease, more frequently, the lung is secondarily involved by widespread mediastinal disease, primarily involvement of the lung parenchyma is extremely uncommon. Recurrent Hodgkin's disease, however, is often
c
d
caused by greater vascular enhancement relative to the surrounding tumor. This sign is common but not specific for alveolar cell carcinoma. Note also the calcified pleural plaque secondary to asbestos exposure.
Lymphoma Table 9.23 Hodgkin's
Types of Lymphoma and lymphoid lymphoma
Lymphoma (WHO classification, listing only the most frequent subtypes)
within the thorax along the radiation treatment port margins. Less than 50% of nonHodgkin's lymphoma initially develop a thoracic manifestation. The most frequent manifestation of thoracic involvement in non-Hodgkin's lymphoma (NHL) is mediastinal or hilar lymph node enlargement. If lymphoma is limited to the lungs (with or without mediastinal lymphadenopathy) it is classified as primary pulmonary lymphoma (no extrathoracic involvement for at least three months), while cases with previous or concomitant extrathoracic lymphoma are classified as secondary pulmonary lymphoma. The most recent working formulation subdivides B-cell non- Hodgkin's lymphoma into low, intermediate, and high grade, and predominantly T-cell angioimmunoproliferative lesions (lymphoid granulomatosis) .
Mucosa-associated lymphoid tissue (MALT) represent one type of intrapulmonary low grade B-cell lymphoma in the lung, also described as bronchus-associated lymplzoid tissue (BALT). Also most high grade NHL are B-cell lymphomas. Patients may suffer from underlying disease such as AIDS, or have undergone organ transplantation. In contrast to patients with low grade lymphoma, they suffer from respiratory or systemic symptoms. High grade non-Hodgkin lymphomas tend to recur at the initial sites of disease. Angiocentric immunoproliferative lesions represent a histologic spectrum and are therefore subdivided into three grades, dependent on the severity of cytologic atypia and polymorphism. While grade 1 describes a benign lymphocytic angiitis, grade 2 is defined as lymphoid granulomatosis, and grade 3 represents true lymphoma. From the literature it appears that there is also some overlap between pulmonary lymphoid hyperplasia and low grade lymphoma, and it remains unclear whether the first represents a truly benign (possibly reactive) process, a premalignant form, or a malignancy difficult to diagnose. Focal
lymphoid hyperplasia
(synonyms:
nodular lymphoid hyperplasia, pseudolymphoma, well-differentiated lymphocytic proliferation, monoclonal B-cell proliferation) can be differentiated from diffuse pulmonary lymphoid hyperplasia. For the latter, two varieties have been described: lymphoid interstitial pneumonia (LIP), predominantly affecting the parenchymal interstitium, and follicular bronchiolitis, predominantly affecting the peribronchiolar interstitium (see also p. 300). Secondary pleuropulmonary involvement with lymphoma in patients with extrathoracic disease is much commoner than the primary condition. Spread occurs either by direct involvement from mediastinal or hilar lymph nodes or by hematogenous dissemination. Post-transplant lymplioproliferative disorder (PTLD) is seen in about 3% of organ transplant recipients and is related to immunosuppression and the Epstein- Barr virus. CT
Morphology
Lymphomas have a broad, nonspecific spectrum of CT findings that range from miliary nodulation and discrete nodules to ground-glass opacities and confluent, pneumonia-like patches of
325
326
9
Lungs and Tracheobronchial System
c Fig. 9.32 CT appearances of pulmonary lymphoma. (a) Well-defined nodule, (b) Patchy nodules with nodular pleural thickening and pleural effusion, (c) Consolidation with perifocal ground-glass opacification in
consolidation with or without an air bronchogram (Fig. 9.32). The pattern of intrapulmonary Hodgkin and non-Hodgkin's manifestations can be identical. In Hodgkin's disease the commonest findings are ill-defined nodules or masses spreading out from the hila along the central bronchovascular bundle. The appearance may be similar to that of sarcoidosis or Kaposi sarcoma. Air bronchogram and cavitations are frequent findings. Recurrent disease also demonstrates intrapulmonary or perihilar nodules and mass lesions, sometimes along the radiation treatment port margins. Care should be paid to the diaphragmatic and pericardia! lymph nodes that are not included in the mantle radiation treatment. In non-Hodgkin's disease chest wall involvement is rather common and results from contiguous spread from the mediastinum. The pleura may be involved by intraparenchymal disease as well as by chest wall involvement. While plaque-like pleural thickening represents lymphomatous spread, pleural effusions are mostly due to lymphatic obstruction and do not represent direct pleural involvement. The lung parenchyma is a common site of involvement by extranodal NHL. Though there is morphologic overlap, some characteristic features or the different grades of NHL can be described: CT findings in low grade NHL (MALT or BALT) comprise solitary nodules or ill-defined focal opacities between 2 and Scm in size with air bronchograms in about 50% (Fig. 9.33). Bronchi
Fig. 9.33 MALT lymphoma. (a) III-defined focal opacity. (b) Diffuse micronodular opacities with tree-in-budpattern and bronchial wall thickening simulating acute endobronchial spread of infection.
within affected lung parenchyma may appear stretched and slightly narrowed. Less commonly there are multiple nodules or infiltrates. They slowly grow over months or years. Lymphadenopathy is primarily seen in less than 5%. CT findings of high grade NHL are nonspecific and range from focal to diffuse consolidations with or without air bronchogram and occasionally with a diffuse reticular pattern. Patients may show rapid progression of consolidations with respiratory failure. The commonest manifestations of englecentric immunoproliferative lesions are multiple
Kaposi Sarcoma
nodules or masses between 0.5 and 8 cm in size. They may start as ill-defined opacities, and develop to confluent nodules or masses. In subpleural location they mimic pulmonary infarcts. Cavitation is seen in up to 40%; air bronchograms are extremely rare. Since medium to small-sized vessels are involved it mimics the appearance ofWegener's disease in the lung but never involves the kidneys (see also p. 328). A focal lymphoid hyperplasia presents with a solitary nodule or a focal area of consolidation usually limited to one lobe, 2 to 5 cm in diameter and rarely exceeding to masses and infiltrates with W cm diameter. Virtually all lesions contain air bronchograms. Typically there is no associated lymphadenopathy; if there is, lymphoma should be suspected. UP is frequently associated with diseases with abnormal immune reaction (Table 9.24, see Fig. 9.65 b). Follicular bronchiolitis has been seen in up to 20% of patients with rheumatoid arthritis. The most frequent abnormalities consist of bilateral areas of ground- glass opacification, small nodules, and mostly centrally located cysts. In follicular bronchiolitis CT shows small nodular opacities in a peribronchiolar and centrilobular location (1-3 mm in diameter, rarely up to 10 mm). In contrast to primary pulmonary lymphoma, secondary involvement tends to affect the central airways resulting in atelectasis and poststenotic pneumonitis. Lymphatic congestion presents with thickened interlobular septa. The commonest intrapulmonary manifestations are solitary or multiple ill-defined nodules or masses, most frequently in the lower lobes, with linear stranding extending into the surrounding parenchyma. Cavitation is rare. Post-transplant lymplioproliferative disease presents with solitary or multiple nodules and occasionally hilar adenopathy. The lesions usually regress with decrease ofimmunosuppression. Table 9.24
Kaposi sarcoma in HIV -infected patients may involve the lung in addition to the skin and gastrointestinal tract. The overall incidence is on the decline (from 60% to the current 15-20%). •
CT Morphology
CT typically demonstrates cloudy nodular masses or patchy opacities that show a predominantly perihilar distribution (bilateral, often asymmetrical) and are oriented along the bronchovascular structures. This may be accompanied by nodular thickening of the interlobular septa, creating a pattern that resembles carcinomatous Iym phangitis. The lesions have ill-defined margins due to infiltration of the surrounding interstitium (local lymphangiosis), leading to spiculated, lanceolate, or flameshaped densities. Similar findings are seen with angiosarcoma (Fig. 9.34). Endobronchial tumor spread can lead to atelectasis. Being highly vascular, Kaposi sarcoma enhances intensely after intravenous contrast administration. Extensive pleural effusions and mediastinal lymphadenopathy are commonly found.
a
b
Fig. 9.34 Kaposi sarcoma (a) appearing as multiple perivascular focal opacities surrounded by halos caused by lesional hemorrhage. Perilesional hemorrhage is also seen with pulmonary angiosarcoma (b).
327
328
9 Lungs and Tracheobronchial System
Pulmonary Blastoma
Pulmonary blastoma is a rare primary lung tumor, histologically consisting of immature epithelial and mesenchymal components. There is a biphasic age distribution with peaks in the 15t and 7th decade. The prognosis is poor and appears to be dependent on the size of the primary tumor. A histologic variant resembling pulmonary blastoma without its sarcomatous stroma is named "well differentiated fetal adenocarcinoma" or "pulmonary endodermal tumor resembling fetal lung" . •
CT Morphology
CT demonstrates a large well-defined intrapulmonary nodule or mass with diameters ranging from 2.5 to 25cm. Occasionally metastases and pleural invasion are seen. Rarely tumors are cavitated, calcified or multiple. Well differentiated fetal adenocarcinoma tend to be smaller than the classic blastoma. Radiologic findings are nonspecific consisting of a peripheral nodule or mass. Pulmonary Infarction
The thromboembolic occlusion of a pulmonary artery restricts blood flow to the more distal lung segments, leading in 10-15% of cases to pulmonary infarction. The hypoperfused tissue is prone to secondary infection (postinfarction pneumonia) or abscess formation (postinfarction abscess) and may cavitate when a connection with the bronchial system is established (postinfarction cavitation). •
CT Morphology
Pulmonary infarction typically appears as a wedge-shaped density that is broadly based on the pleura and points to the hilum (Fig. 9.35). The lower lobes are more commonly affected than the upper lobes. Contrast-enhanced scans show a central hypodense area (necrosis) and peripheral rim enhancement (inflammatory reaction). An accompanying pleural or pericardial effusion may be found.
a Fig. 9.35 Pulmonary infarction presents as a pleuralbased opacification with reduced contrast enhancement that is typically wedge-shaped (a), but may be more rounded (b).
Wegener's Disease
Wegener's disease represents a systemic disease with necrotizing granulomatous vasculitis involving several organs. Approximately 90% of patients develop pulmonary manifestations (renal changes occur in 85%, joint changes in 50%), thus the lung has to be examined obligatorily once the diagnosis of Wegener's disease has been established. Approximately 90% of patients have an increased level of C-ANCA antibodies, which is also seen in patients with microscopic polyangiitis, progressive glomerulonephritis, Churg-Strauss, and some infectious diseases (aspergillosis, amebiasis, bacterial endocarditis, chromomycosis). Wegener's disease is characterized by a necrotizing granulomatous vasculitis of the upper respiratory tract, a disseminated vasculitis (small and medium-sized vessels) and a focal necrotizing glomerulonephritis. •
CT Morphology
Typical CTfindings are multiple intrapulmonary nodules or focal densities (0.5 to 10cm) predominantly in the lower zones and frequently subpleurally located. The nodules may show spiculae or feeding vessels that are also described for metastases or infarcts. Cavitation is seen in up to 50% of cases, resulting in thickwalled cavities with irregular inner margins (see Fig. 9.36 b) that are susceptible to secondary infection (e.g., with Aspergillus], an air broncho-
Churg-Strauss Syndrome
, a
b
c
d
Fig.9.36 Wegener's Disease, (a) Diffuse confluent acinar opacities due to pulmonary hemorrhage, (b) Chronic changes during resorption of pulmonary
hemorrhage, (c) Thin-walled nodule with cavitation an air-fluid level, (d) Thick-walled cavity.
gram in 25% and pleural effusion also in about 25%. Women frequently show a smooth subglottic tracheal stenosis typically over 3 to 4cm length. CT shows inflammatory reactions around the thickened tracheal ring. Strictures can also be located in the main or lobar bronchi. A pneumothorax or pneumoserothorax are the result of subpleural cavitations and the development of bronchopulmonary fistulae. Diffuse bilateral ground-glass opacifications or consolidations are caused by alveolar hemorrhage (Fig.9.36a). Recurrent hemorrhage results in fibrotic changes. Nodules and hemorrhage are seen in active disease, while fibrotic parenchymal bands and septal thickening indicate an inactive phase. Follow-up studies may show marked resolution of parenchymal changes. The lymphomatoid granulomatosis shows identical CT features to Wegener's disease (it does not affect the kidneys) but is considered a neoplasticlymphoproliferativedisease.
phase characterized by pulmonary or gastrointestinal eosinophilic infiltrates and peripheral eosinophilia, and the vasculitis phase with hemoptysis, purpura erythema urticare, glomerulonephritis and gastrointestinal ulcers, depending on the affected organs.
Churg-Strauss
•
and
CT Morphology
The most frequent HRCT findings are patchy ground-glass opacifications and consolidations predominantly in subpleural locations. Rarely, there are bronchiectases, bronchial wall thickening, centrilobular nodules, and septal thickening, the latter caused by cardiac insufficiency and interstitial edema (Fig. 9.37).
Syndrome
Churg-Strauss syndrome (allergic angiitis or allergic granulomatosis) is defined as a combination of systemic necrotizing vasculitis affecting the small and medium sized vessels in at least two organs (e.g., heart, lung, skin, nervous system, or kidneys), blood eosinophilia, and lung infiltrates. There are three clinically distinctive phases: the prodromal phase characterized by asthma and allergic rhinitis, the eosinophilic
a
b
Fig. 9.37 Churg-Strauss syndrome demonstrates areas of consolidation and ground glass opacification indicative of pulmonary hemorrhage. Thickening of interlobular (a) or intralobular septa (b) may be present.
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9 Lungs and Tracheobronchial
System
The following diseases are associated with a peripheral eosinophilia: idiopathic pulmonary fibrosis, allergic bronchopulmonary aspergillosis, chronic eosinophilic pneumonia and Churg-Strauss syndrome.
Vascular
Malformation (AVM)
More often congenital than acquired (post-traumatic) , pulmonary vascular malformations are characterized by the abnormal shunting of blood between the pulmonary arteries and veins or, less commonly, between the bronchial and pulmonary arteries or between the bronchial arteries and pulmonary veins. Because there is no capillary network between the arterial and venous vessel, blood is shunted to the left atri urn without being oxygenated. From 40 to 60% of patients with pulmonary arteriovenous malformation (pA VM) suffer from Osler-Weber-Rendu disease with cutaneous and mucosal teleangiectasias and AVM also in other organs (e.g., liver). •
CT Morphology
Arteriovenous malformations appear as rounded opacities that measure several millimeters to several centimeters in diameter and typically have a feeding arterial vessel and a draining venous vessel (Fig. 9.38). Multiple feeding and draining vessels are seen in complex pAVMs. Most pAVMs are solitary (60-70%) and
a
b
Fig. 9.38 Arteriovenous malformation appearing as a group of peripheral nodules (a). The feeding and draining vessel in another AVM (4 x 1 /6) is demonstrated on a MIP display (b).
are located in the central part of the lung. Calcifications have been described. With spiral CT, an pAVM can be identified on unenhanced scans by its vascular anatomy, which is most clearly demonstrated by shaded surface displays and MIP images (Fig. 9.38). Delineation of the angioarchitecture is important for planning of interventional therapy (embolization). A sign, pathognomonic for pAVM, is its intense contrast enhancement synchronous with the arterial vascular opacification. Only rarely does throm bosis lead to a lack of enhancement.
Infectious Diseases
As a rule, pulmonary infections in themselves are not an indication for CT scanning. CT is used mainly to detect complications (cavitation, abscess formation); detect or exclude predisposing diseases (bronchiectasis, tumor); differentiate among infiltration, atelectasis, and effusion; monitor progression and therapeutic response; and localize lesions prior to bronchoscopy or lavage. Clinical features are important for differentiating various infectious agents. Communityacquired pneumonias occurring in previously healthy individuals have another spectrum of infectious agents as compared to nosocomial
pneumonias acquired in the hospital by patients who were already ill (Table 9.25). Immunocompromised patients are susceptible to a greater diversity of infections that also include atypical forms of pneumonia [Pneumocystis ceruiu, cytomegalovirus, mycobacterial infections, and fungal infections; Table 9.26). Experience has shown that chest radiographs are negative in approximately 10 of patients with pulmonary infection. This percentage is substantially higher in immunocompromised patients. Today, the clinical suspicion of pulmonary infection in a patient with a nega%
Infectious
Table 9.25
Common
Community-acqu
infections
in the normal
Diseases
host
ired pneumonia
Bacterial • Streptococcus
lower lobes,
• Staphylococcus
postviral,
• Haemophilus
lower lobes,
consolidation,
children>
rapid spread,
adults,
cavitation,
bronchopneumonia
round
(children)
or sublobar,
effusion
and cavitation
rarely lobar
empyema
(COPD),
rarely
Atypical diffuse
• Mycoplasma
tions,
centrilobular mosaic
nodules,
pattern
tree in bud, patchy
with air trapping,
• Legionella
upper lobes, segmental
• Chlamydia
similar to Mycoplasma
consolidation,
(lobular)
lymphadenopathy rapid spread
ground
glass
or consolida-
in up to 40%
to diffuse
disease
Vims diffuse acinar shadows, pathyor diffuse ground solidations due to bacterial superinfection
patchy or diffuse ground glass, consolidation post transplantation (first 4 months), under chemotherapy, HIV«20 c ells/mm") associ ated with mucocutaneous ground glass
consolidation
disease ->
fibrosis.
cysts
HIV Atypical Fungal
mycobacterial infections
acinar
or nodular
patchy opacities,
opacities,
consolidation
consolidations,
halo sign
• Candida • Aspergillus
HIV, under chemotherapy,
post transplantation
• Cryptococcus
tive chest radiograph is considered an indication for CT or HRCT. Given the nonspecific response pattern of the lung. it is only possible to reduce the number of causative organisms based on morphologic criteria. Clinical features (e.g .. CD4
cell count) can help to further reduce the differential diagnosis of etiologic agents (Tables 9.279.29).
A lobar pneumonia is characterized by diffuse alveolar wall injury accompanied by hemorrhagic edema. Bronchopneumonia (lobular pneumonia) results from infectious organisms in the epithelium of distal airways causing peribronchiolar exudates. Chronic (carnifying) pneumonia is considered to be present if the infiltrated alveoli do not clear within 15 to 30 days. This condition involves a proliferative fibroblastic inflammation with scarring and fibrous change. Depending on their etiology. bacterial infections are divided into community-acquired and nosocomial infections. The radiographic pattern is quite variable depending on the organisms. One organism may produce different patterns depending on the progression of disease. the patient's immune status, or preexisting or concomitant disease.
Prominence of arteriole -> Visualization of bronchiole b Branched pattern with small bulbous terminations ("tree-in-bud" pattern) -> Dilated mucus-filled bronchioles or peri bronchiolar fibrosis 12 Interstitial (perilymphatic)
nodules
• 1-2 mm in diameter • Usually well-circumscribed • Perilymphatic distribution: peribronchovascular, periseptal, and subpleural; also interlobar, interlobular, and centrilobular 3 Disseminated (perivascular) nodules
2
Fig. 9.51 Schematic representation of nodular and reticulonodular lung changes (see Tables 9.36, 9.37).
• Several mm in diameter • Usually well-circumscribed • Disseminated distribution with no sites of predilection (miliary pattern; perivascular, periseptal, and subpleural but not predominantly interstitial)
• • • •
Several mm to 1 cm in diameter Margins usually ill-defined Ground-glass opacity to soft-tissue density Disseminated distribution, but never subpleural or peri pleural
HRCT features of PLC are smooth or beaded thickening of the central peribronchovascular interstitium and of the interlobular septa producing a characteristic reticular pattern peripheral arcades) and thickened septa outlining distinct pulmonary lobules (polygonal arcades). Thickening of the intralobular axial interstitium results in prominent vascular and bronchiolar structures (centrilobular core structures). Polygonal septal thickening in combination with prominent centrilobular core structures is one of the most distinctive features of PLC (Fig. 9.52). The centrilobular interstitial thickening predominates in only few patients. In approximately 50% of patients the abnormalities appear focal, unilateral, or asymmetric rather than diffuse. Axial, peripheral subpleural and central perihilar bronchovascular thickening may occur together, or one of these features predominates or occurs alone. Associated findings are intrapulmonary nodules, hilar or mediastinallymphadenopathy (38-54%), or pleural effusion. A preservation of normal lung architecture at the lobular level despite the presence of reticulonodular and linear opacities represents an important feature for differential diagnosis.
plastic cells. Tumors that spread in this way include carcinomas of the breast, lung, stomach, pancreas, cervix, prostate, thyroid, and metastases of adenocarcinomas of unknown primary site. PLC usually results from metastatic spread to the lung with subsequent interstitial and lymphatic invasion, but can also occur because of direct lymphatic spread of the tumor or secondary to distension of the vascular and lymphatic channels distal to tumor emboli or tumor obstruction.
Fig. 9.52 Carcinomatous lymphangitis, (a) Irregular beaded thickening of the interlobular interstitium (arrowhead) with accentuation of centrilobular core structures (arrow). (b) Thickened interlobular septa with marked thickening of the bronchovascular bundles (arrow) and pleural effusion.
350
9 Lungs and Tracheobronchial System clues for differentiation between PLC and interstitial edema, in that both are
Diagnostic
characterized by perihilar, peribronchovascular, and reticular septal thickening, are the following: • In PLC, the thickened interstitium is sharply marginated from the adjacent aerated lung, there is no filling-in of the alveoli, which remain well aerated. Interlobular thickening due to PLC is more nonuniform as compared to interstitial edema: different degrees of thickening may occur within one septum as well as alterations between smooth or beaded contours. PLC may occur in a focal and asymmetric distribution. The pulmonary arterial branches adjacent to the bronchi also appear larger than normal, that way maintaining the size relationship of the thick-walled bronchi and adjacent vessels. Interstitial edema is most frequently bilateral and symmetric. The interstitial edema is dependently distributed, various degrees of alveolar opacification represent air space edema. The left ventricle and atrium may be enlarged, and pleural effusions are frequently seen. Administration of diuretics will result in a decrease of interstitial edema within hours. clues for differentiation of PLCfrom sarcoid, pneumoconiosis and pulmonary fibrosis, which may all show nodular or beaded
Diagnostic
peribronchovascular
thickening,
are:
• Septal thickening is less extensive in sarcoidosis and pneumoconiosis than in patients with PLC, and reticular opacities are not the predominant feature. In sarcoidosis, pneumoconiosis, and pulmonary fibrosis cicatricial distortion of the lung architecture and secondary lobule anatomy is common, especially when septal thickening is present. In PLC however, lung architecture remains normal and the lobules preserve their size and shape.
Sarcoidosis Sarcoidosis is a systemic, non caseating epithelioid-cell granulomatosis that can affect all organs and is most commonly manifested in the
lung (90%). The granulomas typically show a perilymphatic distribution in the peribronchovascular (perihilar and centrilobular), interlobular, and subpleural interstitium. The pulmonary changes are accompanied in early (stages 1 and 11) by bilateral hilar and mediastinallymphadenopathy. CT Morphology
The CT appearance of pulmonary sarcoidosis varies greatly and is known to masquerade many other diffuse infiltrative lung diseases. The commonest pattern consists of small (2-10 mm): well-defined nodules in a characteristic "perilymphatic distribution" in relation to the subpleural surface, adjacent to the major fissures, along thickened interlobular septa and adjacent to vessels in the lobular core. As a result, pulmonary vessels may be irregularly enlarged. The nodules may be evenly distributed throughout both lungs though predominantly in the upper and middle lung zones; however, in many cases (50%) they are clustered in the perihilar and peribronchovascular region with relative sparing of the lung periphery, or they may be grouped in small areas uni- or bilaterally. A typical finding is irregular polynodular thickening of the bronchovascular bundle and interlobular septa (Fig. 9.53). Pseudoplaques formed by the coalescence of multiple subpleural granulomas are a characteristic feature. Nodular densities measuring between 1 and 4cm in diameter are seen in 15-25% of patients (so called nodular or nummular sarcoidosis). Confluence of granulomas results in large, mostly ill-defined, opacities or consolidations that are often located centrally around bronchovascular structures. There are patchy areas of ground-glass opacities, which may be superimposed over interstitial nodules or signs of fibrosis. They are rarely seen on the radiograph (0.6%) but are commonly present on HRCT (20-60%) mostly in association with small nodules. Pathological studies have shown that they represent interstitial granulomatous inflammation and occasionally microscopic foci of parenchyma! fibrosis. Approximately 20% of patients develop a pulmonary fibrosis with septal thickening, traction bronchiectasis and honeycombing (Fig. 9.53). Irreversible fibrosis is commonest in stage 3 (diffuse pulmonary disease un associated
Diffuse Nodular Lung Changes
a
b
c
d
Fig. 9.53 Sarcoidosis. (a) Very small diffuse nodular densities that partially coalesce to ground-glass opacification. (b) Grouped nodular opacities partially coalescent to patchy consolidations (arrow) with tiny subpleural nodules ("pseudoplaques", arrowheads), (c) Irregular beaded thickening of the central bronchovascular bundle with nodules in interstitial distribution. (d) Perihilar fibrotic mass with parenchymal distortion and interstitial nodules, (e) Coronal 5mm wide MIP demonstrating the apical predominance of nodular densities (4 x 1/6).
with lymph node enlargement). Conglomerate masses mostly in a perihilar location represent areas of fibrosis causing characteristic traction bronchiectasis. The posterior displacement of the upper lobe bronchi (and later also of the main bronchi) is considered to be an early sign oflung distortion in sarcoidosis, indicating loss of volume in the posterior segments of the upperlobes. Differential diagnosis: conditions that most closely mimic the HRCT appearance of sarcoidosis are pulmonary lymphangitis carcinomatosa (PLC), silicosis. and coal worker's pneumoconiosis (CWP). All of these show small perilymphatic nodules. However, differences of the predominant distribution and the combination with signs of fibrosis represent the diagnostic key factors.
In sarcoidosis, the nodules are predominantly located along the central bronchovascular bundle and in the subpleural area; in PLC nodules are mostly located septally and along the central bronchovascular bundle. Septal thickening in sarcoidosis is a much less dominant feature as compared to PLC and if present, it is usually combined with findings offibrosis and lung distortion, precluding the diagnosis of PLC. There are, however, descriptions of cases in which the parenchymal involvement of sarcoidosis and PLC are quite similar and indistinguishable. In silicosis and CWP, nodules are more evenly distributed throughout the whole lung with a predominantly centrilobular and subpleural location, a finding that is much less typical for sarcoidosis. Differen-
351
Lungs and Tracheobronchial System tiation of conglomerate masses of fibrosis in sarcoiclosis from those in stltcosis can be made by the more common presence of air bronchograms in sarcoidosis. As fibrosis develops over time, sarcoidosis and IPF increasingly share a number of findings. Both show irregular or nodular septal thickening, irregular interfaces and traction bronchiectasis. In sarcoidosis, honeycombing is a less frequent feature of irreversible fibrosis as compared to IPF. However, there appear to be two types of fibrotic progression in sarcoidosis. Most patients develop fibrotic changes with loss of volume, fibrotic bands and cysts predominately in the perihilar region and the upper lobes, that way suggesting the diagnosis of sarcoidosis. There are, however, descriptions of a few cases that developed a diffuse fibrosis with predominant basal and subpleural honeycombing very similar in appearance to UlP. HRCT is the method of choice for showing early it is important to know that HRCT cannot be used to rule out involvement of the lung parenchyma. Several authors report that intrapulmonary granulomas could be histologically shown in patients with normal HRCT appearance of the lung parenchyma. A lot has been written about the meaning of ground-glass opacities in patients with sarcoid. One study indicated that areas of ground-glass opacities are associated with disease activity as assessed by 'Ca-scintigraphy. Correlation with the pathologic specimen, however, could prove that ground-glass opacities reflect the presence of confluence of extensive interstitial sarcoid granulomas rather than active alveolitis. Ground-glass, nodular and irregular linear opacities and interlobular septal thickening were described as representing potentially reversible disease, while cystic air spaces and architectural distortion are irreversible findings. The extent of consolidations and nodular densities showed higher correlations with other indicators of disease activity (serum-angiotensin converting enzyme assay, Ga-scanning and bronchoalveolar lavage) than reticular opacities and ground-glass attenuation. Signs of fibrosis (such as lung distortion and reticular densities),
fibrosis in sarcoidosis. However
however, correlate better with impairment of lung function (obstruction) than extent and distribution of nodules and consolidations. Silicosis and Coal Worker's Pneumoconiosis (CWP) The WHO defines pneumoconioses as irreversible lung changes caused by the inhalation of mineral dust and its deposition in the lungs. Silicosis and CWP are pathologically distinct entities with differing histology, resulting from the inhalation of different inorganic dusts (dust containing crystallized silicon dioxide and coal dust, respectively). However, the radiographic and HRCT appearances are similar and cannot be reliably distinguished. The primary imaging modality is plain chest radiography, but HRCT is more sensitive in the detection of subtle changes. Pneumoconiosis may take an uncomplicated nodular course or a complicated course that is associated with progressive massive fibrosis (PMF). The complicated forms present clinically with severe impairment of pulmonary function and frequently with cor pulmonale. Silicoproteinosis occurs after brief, intense exposure to fine silica dust and is characterized by fulminant respiratory impairment. Silicosis can also predispose to the reactivation of tuberculosis, which may take an accelerated, fulminating course (silicotu berculosis). CT Morphology The nodular form is characterized by multiple 1 to 5 mm nodules, usually with sharp margins, that are scattered throughout the lungs (Fig. 9.54) but which show a predilection for the upper and middle zones especially posteriorly. Small nodules may coalesce to form macronodules (8-20 mm in diameter, with subpleural lesions forming pseudoplaques). In contrast to sarcoidosis, the nodules are typically uniformly distributed in centrilobular (peribronchiolar) and subpleural locations and rarely clustered or distributed along central bronchovascular structures. Calcification may occur within the nodules, and hilar lymph nodes may develop typical eggshell-like calcifications. Sometimes the centrilobular densities show tiny short branches instead of being round, corresponding to irregular fibrosis around the respiratory
Diffuse Nodular Lung Changes
haled stimulating an alveolar and subsequently extensive fibrous tissue (silicoproteinosis). olar ground-glass opacifications crete silica nodules, resembling alveolar proteinosis.
a
b
Fig. 9.54 Silicosis. (a) Diffuse rnicronodular densities (type q' in ILO classification), (b) Conglomerate mass (PMF) with parenchymal distortion, perifocal emphysema, flbrosls. reactive pleural thickening, and diffusely scattered nodules.
bronchioles surrounded by a small zone of focal emphysema. As simple silicosis and CWP progress, the number of nodules increase and increasingly coalesce resulting in distortion of the adjacent lung structure. Coalescence of small nodules to ~arger nodules (> 1 em) and eventually to large, Irregularly defined conglomerate masses refl,e~ts t?e transition from simple to complicated silicosis/pneumoconiosis (Fig. 9.54). The masses of progressive massive fibrosis (PMF) tend to develop in the mid portion or periphery of the upper lung zones and migrate towards the hila. They may cavitate due to ischemic necrosis although this seems to be more common with CWP. Calcifications may befound. Regardless of age and smoking habits, most pati,ent,s with silicosis or coal worker's pneumocomOSIS develop focal areas of centrilobular emphysema. Unlike classic centrilobular emphysema, these areas contain central micronodules and their extent correlates with the impairment of pulmonary function. The extent of emphysema correlates more closely with functional imp,airment than does the degree of nodular profusion. Functional impairment and p:~gn~sis are usually poorer for patients with sIlIcoSIS than for CWP. Accompanying paracicatri~ial emp?ys~ma reflect the process of progressive massive fibrosis, An acute form of silicosis has been described in which a large dose of silica was in-
lipoproteinosis production of CT shows alveinstead of disthe image of
The differential diagnosis of silicosis/CWP includes all diseases that may develop numerous small well-defined nodular opacities such as sarc~idosis, pulmonaly lymphangitic carcinomatosis, Langerhans cell histiocytosis but also infectious diseases such as miliary tuberculosis, fungus infection, and liematogenous metastases. • Dependent on the profusion of the nodules in a seemingly random distributi~n, it may be impossible to distinguish miliary tuberculosis (perivascular nodules) from silicosis (peri bronchiolar nodules). Also the differentiation between sarcoidosis and silicosis can be difficult when the sarcoid nodules are numerous a~d ~o n,ot show the typical perilymphatic distribution but a diffuse centrilobular location. The differentiation between silicosis and Langerhans cell histiocytosis is difficult if the latter is characterized solely by t~e presence of nodules while cystic leSIOns are completely missing. • Si,licosis/CWP and PLC can usually be differentiated quite easily by the different distribution of nodules. In silicosis and CW\, the nodules appear bilaterally, symmetnc and more uniformly distributed. Beaded septa or reticular densities are usually absent. • Perihilar mass-like fibrosis or upper lobe consolidations in combination with p~~en~hymal distortion may occur in both slhcoSlS/CWP and end-stage sarcoidosis. Both entities are characterized by an upp~~ lo~e predominance and by potential calcifications of the nodules. Differentiation of conglomerate masses of fibrosis in sarcoidosis from those in silicosis can be ~ade b~ the, presence of air bronchograms In sarcoidosis and by associated findings such as the distribution of nodules. • Conglomerate masses in PMF are occasionally indistinguishable from tuberculous cavities or bronchial carcinoma.
354
9
Lungs and Tracheobronchial
System
Diffuse Lung Changes with Increased Parenchymal Density
Parenchyma! changes that cause increased radiographic density are evaluated according to the degree of the attenuation increase and the shape and extent of the area of increased attenuation (Tables 9.38, 9.39; Fig. 9.55). The areas may be described as nodular, patchy, geographic, or lobular.
Table 9.38 Differential diagnosis enchymal density in the lung Ground-glass
Fig. 9.55 Patterns of increased (see Tables 9.38, 9.39).
Table 9.39
1 Ground-glass
Hemorrhage Contusion masses
Silicosis, pneumoconiosis Sarcoidosis (end stage)
BOOP
Carnifying pneumonia Wegener's disease
density
opacity
2 Alveolar opacities (PMF)
parenchymal
density
Patchy increase in parenchymal density Vascular structures not obscured Air bronchogram may be present (dark bronchus sign) Thickened alveolar walls, intraalveolar material Occurs with interstitial and alveolarprocesses-> active, treatable, potentially reversible process
BOOP
Conglomerate
Increased
parenchymal
• • • •
(airspace shadowing)
Profuse focal or patchy opacity Obliteration of vascular structures Positive air bronchogram Alveolar airspaces filled with cells or fluid
3 Conglomerate
masses
• Bunched, confluent strands • Traction bronchiectasis • Volume loss
Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis is an allergic response to inhaled organic antigens (farmer's lung, birdbreeder's lung). Known causal antigens include microorganisms (fungi, bacteria, actinomycetes) and animal and plant proteins. The radiographic and pathologic abnormalities are quite similar, independent of the causing antigen The clinical and CT features ofEAA vary with the duration of the exposure. The changes are classified as acute, subacute, or chronic, and combinations of these forms can be found in individuals who are repeatedly exposed. Mostly HRCT is performed in the subacute and chronic stage. Chronic changes are also referred to as chronic allergic pneumonitis. A careful clinical history, typical HRCT findings and concordant serologic findings confirm the diagnosis of EAA thus precluding the need for lung biopsy. In patients with discrepant or atypical HRCT findings diagnosis is mostly based on trans bronchial biopsy and abnormal Tlymphocytes in the bronchoalveolar lavage. •
CT Morphology
The acute form is generally diagnosed clinically, so little information is available on CT morphology.
Chronic Allergic Pneumonitis
In the subacute stage, HRCT typically (in 40-70%) shows small (2-3 mm, maximum 5 mm) centrilo bular acinar densities, usuall y ill- defined and showing low attenuation (Fig.9.56a). The acinar densities may coalesce to form diffuse ground-glass opacities (Fig. 9.56b). All lung zones are affected, but there is a slight middle and lower zone predominance that often spares the subpleural space (unlike the alveolitis in idiopathic pulmonary fibrosis). The changes tend to regress in 7-10 days after exposure to the antigen is discontinued. Bronchial walls may be slightly prominent. Recurrent EAA episodes and the chronic stage of allergic pneumonitis are characterized by signs of irreversible interstitial fibrosis and parenchyma! distortion (irregular reticular opacities, intralobular interstitial and interlobular septal thickening, visible intralobular bronchioles, traction bronchiectasis, and honeycombing) (Fig.9.56c). Frequently there is a mixture of chronic (fibrotic) and subacute (inflammatory) changes in patchy distribution and combined with signs of air trapping. The distribution of the fibrosis may be variable, sometimes predominantly subpleural, in others patchy or peribronchovascular. Honeycombing is usually localized subpleurally. Divergent findings are reported about the lobar predominance of chronic fibrotic changes. While one study reported a lower lobe predomi-
r
Fig.9.56 Extrinsic allergic alveolitis. (a) Diffuse, iii-defined acinar (alveolar) opacities in a centrilobular distribution. (b) The alveolar opacities may coalesce to form ground-glass opacities. Differentiation is required from viral pneumonia, acute Pneumocystis pneumonia, and acute exacerbation of an early form of interstitial fi-
brosis or sarcoidosis. (c) Chronic allergic pneumonitis with an mixed pattern of acute alveolitis (ground-glass opacities) and chronic fibrosis (thickened septa, thickened interstitium, bronchiectasis, subpleural opacities) associated with regional air trapping indicating a process involving the airways.
356
9
Lungs and Tracheobronchial System
nance in about one-third of patients, others described a mid zone predominance or even distribution for the majority of patients. In contrast to idiopathic pulmonary fibrosis, however, there is no subpleural or lower zone predominance. In rare cases chronic EAA may be indistinguishable from a late form of sarcoidosis. • In patients with bilateral nodular, patchy or diffuse ground-glass opacities, the differential diagnosis includes desquamative interstitial pneumonia (DIP) and pulmonary alveolar proteinosis (PAP). DIP, however, is very rare and usually shows a subpleural predominance of the groundglass opacification and is not associated with centrilobular nodules. • Faint acinar nodules, which may coalesce to a diffuse ground-glass opacity, are seen in various disorders including allergic alveolitis, viral infections, and vasculitis. Classic alveolar proteinosis shows a crazy paving appearance and is readily distinguished by bronchoalveolar lavage. The latter is also suited to rule out infectious diseases such as CMV or Pneumocystis cetinii as etiologies for diffuse groundglass opacities. The differential between an alveolar sarcoidosis and subacute EAA requires trans bronchial biopsy. • Patients with chronic EAA and UIP may show identical HRCT findings, and differential diagnosis may be possible only by the clinical history and laboratory findings. Only a predominant location of fibrotic changes in the upper and/or middle lung zones allow for a distinction between these entities.
tings (in children: lymphopenia, thymic aplasia, immunoglobulin deficiency; in adults: lymphoma and leukemia) and infectious diseases (CMV, mycobacteria, Pneumocystis, Histoplasma, Candida species, etc). Pulmonary proteinosis is very rare. In addition to the suggestive HRCT pattern, diagnosis is usually based on characteristic features of BAL fluid. •
CT Morphology
The classic CT finding is a reticulated parenchymal pattern formed by the juxtaposition of normal areas and pathologic areas of increased density (crazy paving). Opacifications range from ground-glass to consolidation, and may be patchy or geographic with sharp demarcation from the surrounding normal parenchyma. While some borders follow anatomic structures of lobar or lobular septa, others are independent of anatomic boundaries. The smoothly thickened inter- and intralobular septa often create a very fine polygonal pattern that is usually seen only in areas with groundglass opacification (Fig. 9.57). Resolution of air space consolidation can be monitored radiographically or by HRCT. The overall extent of disease and the degree of Opacifications correlate well with the impairment of pulmonary function and severity of hypoxemia.
Pulmonary Alveolar Proteinosis (PAP)
Pulmonary alveolar proteinosis is characterized pathologically by filling of the alveoli with a PAS-positive proteinaceous material rich in lipid. It is caused by a dysfunction of pneumocytes that desquamate in the alveolar spaces secondary to a defect of lipid metabolism at the intracellular level. An abnormality in surfactant production, metabolism, or clearance also has been strongly implicated, as well as an association with a variety of immunocompromised set-
a
b
Fig. 9.57 Pulmonary alveolar proteinosis. (a) Acute: ground glass opacification superimposed by smooth intralobular thickening (crazy paving) in a geographic sharply demarcated distribution, (b) Chronic: after treatment, irregularly thickened intralobular septa may persist.
Lipoid Pneumonia
Although the crazy paving appearance is highly suggestive for alveolar proteinosis with a 100% prevalence of this pattern, it is not specific. A recent publication listed 14 airspace and interstitial lung diseases that showed crazy paving with varying but mostly much lower prevalence of crazy paving. The main differential diagnoses include diffuse alveolar damage superimposed on UIP (prevalence of crazy paving in 67%), acute interstitial pneumonia (31 %) and ARDS (21 %). Other diseases with lower prevalence of crazy paving were drug induced pneumonitis (12%), pneumonias (bacterial 6%, tuberculosis 1%, mycoplasma 6%, Pneumocystis cetinii 7%), BOOP (8%), chronic eosinophilic pneumonia (8 %) , radiation pneumonitis ( 4 %) , and cardiogenic pulmonary edema (14 %) . The wide differential diagnosis is understandable considering that any kind of fluid or cellular filling of the air spaces and the interstitial spaces causes crazy paving. In alveolar proteinosis, the crazy paving appearance was found to be due to an accumulation of PAS-positive material in the air spaces adjacent to the interlobular septa rather than to thickening of the septa themselves. Most disease entities with similar HRCT features can be distinguished from alveolar proteinosis by clinical symptoms and associated HRCT findings. It has to be noted that superimposed infection- frequently by Nocardia asteroides-is a common complication of alveolar proteinosis,
and CT is rather limited in differentiating air space consolidation due to infection from underlying disease. Lipoid Pneumonia
Lipoid pneumonia is caused by the aspiration of oily substances (e.g., foods, laxatives, nasal drops) and is commonest in elderly patients with neuromuscular disorders or swallowing difficulties. A large amount of oily material must be aspirated before symptoms develop. It differs from alveolar proteinosis in that the alveoli are filled with a lipid-containing material rather than proteinaceous material. •
CT Morphology
Lipoid pneumonia presents with a homogeneous low attenuation (-35 to -75 HU) consolidation of the affected area (predominately in the middle or lower zones). It may cause alveolar opacities resembling those in alveolar proteinosis. There is also ground-glass opacity with a superimposed reticular interstitial pattern caused by thickened interlobular septa (Fig. 9.58). Fibrosis and inflammation accompany the presence of intra-alveolar lipid material (HU value lower than chest wall musculature but higher than subcutaneous fat). Sometimes cavitation and necrosis are seen. In rare cases, fat -equivalent CT densities are found in the affected bronchi.
Fig.9.58 Lipoid pneumonia. (a) Diffuse changes with dense ground glass opacification and accentuated intralobular septa, (b. c) Focal changes with a tumor-like presentation (b) but fat-attenuation on soft tissue windows (c) in an actor that uses oily nose drops.
a
b
c
357
358
9 Lungs and Tracheobronch'ial System
Eosinophilic Pneumonia
Drug-Induced Lung Disease
Eosinophilic pneumonia is typically associated with blood eosinophilia and responds promptly to steroid therapy (regression within days with complete recovery within months). An acute type (Loffler) is differentiated from the chronic type (disease duration longer than one month). Some 50% of patients have an atopic disposition, 40% have a history of asthma. It may be idiopathic or represent secondary immunologic reaction to drugs, parasites, or other agents.
Pulmonary drug toxicity is increasingly being diagnosed as a cause of acute or chronic lung disease and has to be considered in the differential diagnosis, especially in selected patient groups (e.g., those under chemotherapy or cardiogenic antiarrhythmic therapy). Affected patients typically present with progressive dyspnea, dry cough, and, occasionally, fever. The radiologic findings consist of a broad spectrum of morphology reflecting the underlying histopathologic process. Pulmonary pathology may start to decrease after cessation of drug therapy (e.g., BOOP, eosinophilic pneu-
•
CT Morphology
Dominating features in the acute eosinophilic lung disease (Loffler's syndrome) are groundglass opacities (" 100%) and consolidation (in 92%) in diffuse distribution without a preferred zone (Fig. 9.59). Infiltrates are transient and "migratory". Ill-defined nodules (in 54%), septal thickening (in 70%), and a thickening of the central bronchovascular bundle (in 60%) are other frequent findings. Bronchiectasis and lymph node enlargement are rare. Two-thirds have a pleural effusion. Dominating features in the chronic type are also ground-glass opacities (88%) and consolidations (100%) but now in a predominantly subpleural distribution in the cranial and middle lung areas. Nodules (38%), septal thickening (18 %), thickening of the bronchovascular bundle (38%), and pleural effusions (10%) are rarer than in the acute form.
Fig. 9.59 Eosinophilic pneumonia with subpleural filtrates of varying density.
in-
• relationship between lung injury and dose and duration of therapy
Emphysema monia). Some patients require the administration of corticosteroids. •
CT Morphology
Pulmonary drug reactions include findings caused by diffuse alveolar damage (DAD), nonspecific interstitial pneumonia (NSZP), obliterative bronchiolitis, bronchiolitis obliterans with organizing pneumonia (BOOP), eosinophilic lung disease, pulmonary hemorrhage, edema, hypertension or veno-occlusive disease (Table 9.40). DAD is divided into an acute exudative phase (first week after injury) and a late reparative or proliferative phase (second week), which is characterized by a varying degree of interstitial fibrosis, architectural distortion, and honeycombing. NSf? in its early phase shows scattered or diffuse ground-glass opacities, followed by findings of fibrosis (traction bronchiectasis, honeycombing) predominantly in the basal lung areas (Fig. 9.60). Patients with BOOP show nodular or patchy consolidations, diffuse tree- in- bud opacities and bronchiectasis.
Fig. 9.60 Drug-induced lung disease, (a) Nonspecific interstitial pneumonitis (NSIP) due to Glivec therapy. (b) Bronchiolitis with organizing pneumonia (BOOP) as a consequence ofAmiodarone therapy.
Eosinophilic pneumonia presents with peripherally located subsegmental consolidations (reverse pulmonary edema pattern), while pulmonary hemorrhage leads to bilateral patchy or diffuse ground-glass opacities or consolidations.
Parenchymal Diseases with Decreased Density
A regional or diffuse decrease in the attenuation of the lung parenchyma can have numerous
potential causes, several of which are distinguished by their typical morphologic features on HRCT (Tables 9.41, 9.42; Fig. 9.61).
Emphysema is an irreversible dilatation of the peripheral airways distal to the terminal bronchioles accompanied by destruction of the alveolar walls. Emphysema is classified into three types-centrilobular, panlobular, or paraseptal-according to the arrangement and distribution of the emphysematous areas (Fig. 9.62). Clinical impairment of pulmonary function is related to the extent of parenchymal destruction rather than to the type. The pathogenic mechanisms of peripheral airway enlargement may be based on hypoplasia, atrophy, hyperinflation, or destruction. Hypoplasia of the peripheral airways can occur as a sequel to bronchopulmonary diseases in in-
359
9 Lungs and Tracheobronchial System Table 9.42
Decreased parenchyma! density
1 Emphysema a Centrilobular, b panlobular, c paraseptal Air trapping - Hyperlucent areas with air trapping alternating with areas of normal density • Well-defined margins • On expiratory scans, little or no density increase in hyperlucent areas (air trapping) with a normal density increase in the rest of the lung parenchyma
- Hyperlucent areas with narrowed vessels alternating with areas of slightly increased density with expanded vessels • Margins frequently ill-defined • On expiratory scans, uniform density increase in all areas
5-Table 9.43 Cystic structures
in the lung (see Fig. 9.61)
3 Pulmonary cysts - NHL) or extrinsic bronchial compression (more common in children). Recurrences of Hodgkin's disease tend to occur at the original site after chemotherapy and outside the therapy port after irradiation. The absence of hilar adenopathy virtually excludes pulmonary involvement by Hodgkin's disease.
Non-Hodgkin's
Lymphoma
Non- Hodgkin lymphoma refers to a heterogeneous group of tumors with diverse histologi-
cal features. It is the third most common childhood malignancy after leukemia and eNS tumors butis unusual in children under 5 years of age. The adult form is most prevalent in the sixth and seventh decades. There is a 40 to 100 times greater risk for NHL in patients with immunodeficiency syndromes, after organ transplantation, and with collagen vascular diseases. Treatment depends on the histological classification. Frequently the disease is widespread at the time of diagnosis. •
CT Morphology
Non-Hodgkin lymphomas show a noncontiguous mode of spread with involvement of unusual lymph node groups. Involvement of the mammary and paracardiac nodes and isolated lymphomas in the posterior mediastinum are suggestive of non-Hodgkin lymphoma (Fig.lO. 7a). Extranodal disease is commoner than with Hodgkin's lymphoma (Figs.lO.7b, 10.8). Pulmonary lesions may appear as discrete
Fig. 10.7 (a) Extensive precardiac mass caused by non-Hodgkin lymphoma. (b) T-ALL with massive enlargement of the thymus and encasement of the superior vena cava (arrow).
a
b
Mesenchymal
a
c
b
Fig. 10.8 Non-Hodgkin's lymphoma in a 6-year-old child. Note the enlargement of the thymus (a), the axillary and supraclavicular lymph nodes (arrowheads). the
nodules, parenchymal infiltrates, glass pattern (see Chapter 9).
Tumors
or ground-
In up to 50% of cases, residual masses are seen after the successful treatment of malignant lymphomas. These masses are inactive and consist of sterile fibrous tissue. For distinguishing residual fibrotic masses from active tumor, CT relies on follow-ups showing lack of change in the size of the residual masses. An increase in size signifies an active lymphoma. MRI offers its own criteria of response to therapy: signal decreases on T2-weighted images with treatment, and residual fibrous masses showing low signals in Tl- and T2weighted images. However, so far MRI cannot reliably exclude focal vital lymphoma tissue within a residual mass.
invasion of the lower neck (b) and the stenosis of the trachea seen on a sagittal reformation of a low-dose (4 x 1 /6, CTDlvoi = 2.4 mGy) multislice CT data set (c).
•
CT shows a cystic mass, mostly unilocular, sometimes multilocular or septated, that molds to adjacent mediastinal vessels. The protein-rich content causes attenuation values of up to 40 HU. Unilateral or bilateral effusions may occur and may be chylous. Mesenchymal Tumors
Mesenchymal tumors may occur in all three mediastinal compartments. The benign forms most commonly occur in the anterior mediastinum, and the malignant forms (liposarcoma, fibrosarcoma) are commonest in the posterior mediastinum. •
Lymphangioma
Lymphangioma (cystic hygroma) is a rare congenital malformation of the lymphatic system consisting of lymph channels or cystic lymph spaces. In infants they are most commonly located in the anterior-superior mediastinum and frequently extend into the neck. In adults, lymphangiomas tend to occur in the lower anterior mediastinum and do not communicate with the neck.
CT Morphology
CT Morphology
Lipomas show characteristic fat attenuation on CT scans. They are soft tumors that model themselves to the mediastinal contours rather than displace adjacent organs. Other mesenchymal tumors such as fibroma, fibrosarcoma, and hemangiopericytoma can cause significant displacement. Hemangiomas are hypervascular masses that enhance intensely with intravenous contrast and often contain phleboliths.
385
10 Mediastinum, Pleura, and Chest Wall
Masses Predominantly
Involving the Middle Mediastinum
Lymphoma
Both Hodgkin's lymphoma and NHL may present as a mass in the middle mediastinum, although NHL is commoner than Hodgkin's disease at this location (see above). Tumors oftheTracheobronchial
System
See Chapter 9 (Lungs and Tracheobronchial tems, p. 303).
Sys-
Bronchogenic Cyst, Pericardial Cyst
Bronchogenic cysts are rare congenital anomalies of the tracheobronchial tree (see Chapter 9) that occur more frequently in the mediastinum than in the lungs. They have a fibrous capsule, frequently contain cartilage, are lined by respiratory epithelium, and contain mucoid material. Unlike intraparenchymal cysts they do not usually communicate with the bronchial tree. Pericardia! cysts are mesothelial anomalies (oLitpouching of the parietal pericardium without communication to the pericardial sac). MRI is often superior to CT in differentiating these cysts from mediastinal tumors. •
CT Morphology
Pericardia} cysts are most commonly found in the right cardiophrenic angle but may occur at any pericardial or pleuropericardial site (posterior cardiophrenic angle, superior pericardial recess). Most broncbogenic cysts are located adjacent to the carina on the right side (>50%). They may also occur adjacent to the trachea or esophageal wall or in the retrocardiac region (Fig.W.9). Both types of lesion typically range from 3 to Scm in size and display the features of a bland cyst with a round to oval shape, sharply defined margins, and homogeneous contents. The CT density is dose to that of water or may be higher, signifying mucus formation (pericardia! cyst: 20-50 HLJ; bronchogenic cyst: 0-50 HLJ). The lesions do not enhance after contrast administration.
Fig. 10.9 Bronchogenic cyst. The cyst. the upper border of which is below the carina, extends downward and to the right. terminating behind the left atrium.
Paraganglioma
Paragangliomas are neurogenic catecholamineproducing tumors that are malignant in 10% of cases. N on- functioning paragangliomas are called chemodectomas, functioning paragangliomas correspond to pheochromocytomas. Almost all chemodectomas are aortic body tumors. •
CT Morphology
Aortic body tumors are most often located at the angle of the ductus arteriosus, but can also occur lateral to the brachiocephalic artery, anterolateral to the aortic arch, or above and to the right of the right pulmonary artery. Pheochromocytomas are mostly found in the posterior mediastinum, in or adjacent to the heart or the pericardium, particularly in the wall of the left atrium or the interatrial septum. Paragangliomas are hypervascular and enhance intensely following intravenous contrast administration. Multicentric tumors are possible. Local invasion or metastases indicate malignancy (Fig.10 .10).
Neurogenic Tumors
a
b
c
Fig. 10.10 Malignant paraganglioma with encasement of the left coronary artery (4 x 1/6). There are two overlapping stents placed in the left main and left anterior descending coronary arteries (a). The tumor invades and displaces the pulmonary artery superiorly (b.
Masses
Predominantly
Involving
the Posterior
Neurogenic Tumors
•
With the exception of paragangliomas neurogenic tumors are found exclusively in the posterior mediastinum. Approximately 30% are malignant. Various histologic types are known (see Table 10.5). Neurogenic tumors can be divided into nerve sheath tumors (schwannoma, neurofibroma), ganglion cell tumors (malignant neuroblastoma and benign ganglioneuroma), and paraganglionic cell tumors.
Table 10.5
volume-rendered image). The extent of the tumor is excellently displayed on volume-rendered images of the heart (c).There is residual peripheral enhancement after radiation therapy indicating residual vital tumor tissue (arrowheads) .
Mediastinum
CT Morphology
CT typically shows a paravertebral mass (Fig.10.11) that is ellipsoidal or dumbbellshaped and may cause expansion of an intervertebral foramen. It is common to find sites of pressure erosion and deformation, usually involving the ribs. Foci of bone destruction without sclerotic borders indicate malignancy. Most neurogenic tumors have an oval shape and are of homogeneous soft -tissue attenuation.
Neurogenic tumors
Neuromas
Common
in neurofibromatosis,
show moderate
Neurofibroma
Frequently
expands
Schwannoma
Frequently
ellipsoidal
Paragangliomas
Very rare, show intense enhancement
the intervertebral
Chemodectoma
Frequently
near the aortic arch
Pheochromocytoma
Frequently
near the posterior
Sympathicoblastomas
Heterogeneous
Neuroblastoma
Malignant, age 30HLJ) is often present. Advanced cases may present with pericardial effusion, rib destruction, contralateral involvement, thickening of the interlobar septa, and intrapulmonary metastases. Mesothelioma is distinguished from fibrothorax by the more irregular pleural thickening, which often shows a more intense contrast enhancement. In early cases the differentiation may be difficult (Fig. 10.22a). Pleural Lymphoma
Both Hodgkin' disease and non-Hodgkin lymphoma may involve the pleura. Pleural involvement is rarely the only initial manifestation of the disease. It occurs more frequently in conjunction with mediastinal nodes, thymic involvement, or parenchyma! pulmonary disease, or is due to recurrence of the lymphoma. A pleural effusion (30% patients with Hodgkin's disease) caused by lymphatic obstruction of mediastinal lymph nodes has to be differentiated from lymphomatous involvement of the pleural itself. Pleural involvement in patients with lymphoma alters radiation therapy and unrecognized disease increases the risk for treatment failure. Leukemia may also cause pleural thickening.
395
10 Mediastinum, Pleura, and Chest Wall
a
c
b
Fig. 10.22 Malignant mesothelioma. (a) CT initially demonstrated irregular posterior and mediastinal pleural thickening with calcified plaques that do not show significant enhancement after contrast administration. This resembles the findings seen in chronic effusion with incipient pleural thickening, (b) Scan two
months later shows marked progression of the neoplastic pleural thickening, which now is polynodular. Note the involvement of the mediastinal pleura, (c) Involvement of the major (arrowheads) and minor fissure (arrow) in another patient with extensive pleural mesothelioma (4 x 1/6, sagittal MPR).
Superior Vena Cava Syndrome
Superior vena cava syndrome may result from intravascular or extravascular obstruction of the superior vena cava. Approximately 80% of extrinsic obstructions are caused by malignant disease (bronchial carcinoma, primary mediastinal tumors, lymphoma), and only 20% have a benign cause (fibrosing mediastinitis, radiationinduced fibrosis, catheter-induced thrombosis). CT is useful for establishing the cause of the obstruction and, in acute cases, planning radiotherapy or vascular intervention.
•
CT Morphology
Postcontrast scans can differentiate extravascular causes (compression) from intravascular causes (thrombosis). The development of a collateral supply is manifested by dilatation of the azygos vein, superior intercostal veins, and small cutaneous veins that drain into the azygos-hemiazygos venous system, into pericardiophrenic veins, or rarely into the portal veins (Fig. 10.23).
Fig. 10.23 Superior vena cava syndrome with collateralization. Downhill varices in the esophagus (a). Collaterals via intercostal, pericardiophrenic, azygos and hemiazygos veins (b) seen on a lateral MIP(4X1/6).
a
b
Fibrosing Mediastinitis
Diffuse Mediastinal
Diseases
Acute Mediastinitis
In more than 90% of cases, acute mediastinitis is caused by an esophageal perforation that may be post-traumatic, spontaneous (Boerhaave syndrome), iatrogenic (after surgery or endoscopy), or neoplastic. The remaining 10% of cases are due to complications following sternotomy or an inflammatory process that has spread from the neck along fascial planes (visceral and posterior cervical space). Mediastinitis has a high mortality if untreated. Management include antibiotics, drainage or surgical debridement. •
CT Morphology
In mediastinitis there is a streaky or diffuse increase in CT attenuation in the mediastinal fat (Fig.lO .24a). Complete obliteration of normal mediastinal fat by material of water to softtissue density indicates a severe mediastinitis with edema and inflammatory infiltrates. Abscesses present as well-defined fluid collections with CT attenuation up to 30HL] that mayor may not have an hyperattenuating rim and may contain gas collections. Aspiration may be needed to determine the infection. Pleural effusion is common. Sternal osteomyelitis is characterized by demineralization and permeative destruction of the bone (see Fig. 23.58). It is not necessarily accompanied by increased attenuation or abscess formation in the surrounding fat and soft tissues. Following a median sternotomy, these findings are almost always associated with a
sternal dehiscence of more than 2 mm. Destruction of the sternum by malignant neoplasms is often accompanied by soft -tissue masses (e.g., in multiple myeloma or metastases from lung, breast, prostate, or renal cell carcinoma). Air and fluid collections may be seen for up to 20 days postoperatively, and rarely for up to 50 days, without signifying an inflammatory process. If follow-up examinations show new or enlarging fluid or air collections, the presence of an inflammatory process is highly probable (Fig.lO.24b). Normal CT findings exclude mediastinitis. False-positive CT diagnoses may occur.
Fibrosing Mediastinitis
A chronic granulomatous or fibrosing (sclerosing) mediastinitis may result from an infectious disease (tuberculosis, histoplasmosis, fungal infections, syphilis), a mediastinal hematoma, radiation therapy, or drug therapy (methysergide). Fibrosing mediastinitis is often associated with other fibrotic processes such as pseudotumors of the orbit, Riedel's struma, or retroperitoneal fibrosis (Ormond's disease). •
CT Morphology
The clinical and radiological features of fibrosing mediastinitis vary according to the location and the extent of the fibrosis. Postinfectious forms are marked by a self-perpetuating medi-
Fig. 10.24 Acute mediastinitis with diffuse increase in attenuation of the mediastinal fat in a patient after neck surgery (a). Mediastinal air and haziness of the mediastinal fat in a patient after liver surgery
(b). a
b
10 Mediastinum, Pleura, and Chest Wall
astinal fibrosis that may cause very little stranding or increase in attenuation of the perivascular fatty tissues, and may be difficult to identify. In other cases, however, the fibrosis forms solid tumor-like masses or ring-like zones of increased attenuation around the mediastinal structures. Approximately 10% of cases show obstruction of the vena cava (Fig. 10.25) or the central tracheobronchial system. Extensive calcifications may occur. Mediastinal Lipomatosis
The excessive deposition of mediastinal fat may occur as a nonpathologic process in obese individuals and in patients on cortisone therapy. CT can easily establish the cause of the mediastinal widening that is detected on chest radiographs. •
Fig. 10.25 CT in fibrosing mediastinitis (Ormond's disease) shows an indistinct peripheral increase in paratracheal and paracaval density and narrowing of the superior vena cava (arrow) by fibrotic encasement, leading to a superior vena caval syndrome. Note the intense opacification of the azygos vein due to the collateral circulation (arrowheads).
CT Morphology
CT shows a diffuse increase in mediastinal fat that is most pronounced in the upper mediastinum. Inhomogeneities, as opposed to uniform fat density, indicate a different etiology
such as fibrosis, hemorrhage, acute or chronic inflammation, tumor invasion, or postoperative or postirradiation changes.
Pleural effusion may consist oftransudate or exudate, depending on the protein content of the fluid. Chylothorax refers to an effusion of lymphatic fluid, hemothorax to an effusion that contains blood. Pneumonic infiltrates that border the pleura incite an parapneumonic effusion that progresses from an exudative to a fibrinopurulent stage as it becomes organized. This type of effusion is on a continuum with pleural empyema. A tuberculous pleural effusion results from the perforation of a subpleural pulmonary focus into the pleural space and usually develops 3 to 6 months after the primary infection. Malignant effusions are also exudative and are not" always associated with pleural thickening. Thus needle aspiration is often indicated. Hemorrhagic effusions usually have a neoplastic or traumatic etiology. The most frequent causes
of Chylothorax are lymphomas (> 50%) and trauma (25%). Effusions are a common associated finding in thoracic CT examinations but are not a primary indication for CT. •
CT Morphology
Pleural effusions with a volume of 15 ml or more can be detected with CT. Pleuritis sicca is not visible on CT scans. Free pleural effusions occur in the posterobasal region and form a crescentshaped collection that abuts the chest wall. Effusions that are loculated or encapsulated by fibrous septa may occur anywhere within the pleural cavity (Fig. 10.26). Fibrous septa or fibrin threads cannot be visualized with CT. Interlobar effusions have a typical biconvex shape and can present as a pseudotumor on chest radiographs. An air-fluid level can result from a bronchopleural fistula or prior therapeutic measures (surgery, aspiration biopsy, drainage).
Pleural Empyema
A cbylothorax has a CT density of approximately 0 HU and is usually quite extensive. Transudates are homogeneous and have nearwater attenuation « 15HLJ). Neither the pleura nor the effusion enhance with intravenous contrast. Attenuation values higher than 20 HU indicate an exudative, hemorrhagic, or purulent effusion. The effusion may enhance on late postcontrast scans due to diffusion of contrast medium into the effusion. There are borderline cases in which transudate and exudate are indistinguishable byCT. Withafi brinopurulenteffusion, the thickened pleura shows contrast enhancement in the absence of empyema (reactive pleura! thickening). It is common at this stage to find septations and local encapsulated collections (Fig. 10.26). Increased density and widening of the subpleural fat stripe signifies a long-standing process. In tuberculosis, pleural calcifications and extensive areas of pleural fibrosis are a common finding (d. Fig.10.28a). High CT numbers (>30HU) and heterogeneous attenuation are consistent with hemotborax. Sedimentation effects are seen if the blood in the pleural space does not coagulate. Pleural effusions are very commonly associated with compression atelectasis of adjacent lung areas. Compressed lung tissue floating in a free effusion appears on CT as a tag-like structure (see Fig. 9.13), a small portion of which may be fixed to the chest wall by adhesions. Multiple chest wall adhesions lead to corresponding taglike deformities of the atelectatic lung due to fibrous bands and indicate a septated effusion. Repositioning the patient (lateral decubitus) can be hel pful in distinguishing between free and encapsulated effusions, although ultrasound is the technique of choice for this indication. With a free effusion, atelectatic portions of the lower lobe in the posterior diaphragmatic recess may be mistaken for the diaphragm because of their curved shape, and anterior portions of the effusion may be misinterpreted as subphrenic fluid. A pleural effusion lies posterior (superior) to the diaphragm and extends medially adjacent to the spine. Ascites lies anterior (inferior) to the diaphragm and does not extend medially.
Fig. 10.26 borders.
Loculated
pleural effusion
with scalloped
Pleural Empyema
Pleural empyema is on a continuum with parapneumonic exudate (fibrinopurulent effusion). Most empyemas are accompanied by bronchopneumonic infiltrates (mostly Pneumococcus, and Staphylococcus pyogenes). Tuberculous pleural effusions progress to pleural empyema in approximately 20% of cases. CT can detect areas suspicious for empyema and can distinguish pleural empyema from a peripheral lung abscess. •
CT Morphology
The transition from exudative effusions to pleural empyema can be appreciated on CT scans. Empyema is associated with more intense contrast enhancement and greater thickening of the visceral and parietal pleural layers than exudates (the "split pleura sign", Fig.10.27a). With increasing duration of the empyema, there is increased thickening of the extrapleural fat stripe. Diffuse pleural thickening and calcification may be the end result after healing of a pleural empyema. Air inclusions (bubbles) signify infection by gas- forming bacteria or rarely an esophagopleural fistula, while an air-fluid level results from a bronchopleural fistula or previous needle procedure. Criteria for differentiating between pleural empyema and a subpleural lung abscess (Fig. 10.27 b) are listed in Table 10.8.
399
10 Mediastinum, Pleura, and Chest Wall Fig. 10.27 (a) Pleural empyema with thickened, markedly enhancing visceral (arrow) and parietal pleura ("split pleura sign"), (b) Peripheral lung abscess with an airfluid level. Note the acute pleural angles (arrows) compared with the pleural process in (a), confirming the intrapulmonary origin of the lesion.
a
b
Table 10.8 Features that distinguish pleural empyema from lung abscess Empyema
Peripheral lung abscess
Oval or crescent shape
Round or oval shape
Obtuse angle with chest wall
Acute angle with chest wall
Consistently associated with pleural thickening
Not consistently associated with pleural thickening
No air in lesion wall
Air in lesion wall
Bronchopleural fistula
Communicates with bronchus
Compresses contiguous lung
Distorts contiguous lung
Changes shape with position changes
Does not change shape with position changes
Common features: central air inclusions (depending on pathogen), peripheral contrast enhancement
Pleural Fibrosis, Asbestos-Associated Changes
Asbestos-associated pleural changes (pleural plaques) may not appear until several decades after the initial asbestos exposure. Approximately 3% of asbestos-exposed individuals develop an exudative pleural effusion during the first 10 years after exposure. The effusion may remain asymptomatic or it may cause pain and persist for more than a year. Many patients subsequently develop rounded atelectasis and diffuse pleural fibrosis with restrictive impairment of pulmonary function. Pleural mesothelioma, bronchial carcinoma, and tuberculosis show an increased incidence in asbestosexposed individuals. •
CT Morphology
Pleural plaques most commonly occur on the subcostal pleura and may be 1 to 15mm thick. Thickening of the pleura may be diffuse al-
though more commonly there is focal plaque formation. Plaques typically appear as sharpedged elevations ("mesa" sign) and tend to form calcifications that start at the center of the plaque and are therefore separated from the ribs by an non calcified zone (Fig.10.28b). The plaques spare the lung apices and costophrenic angles. Visceral pleural involvement, particularly within the fissures, and mediastinal pleural involvement are uncommon, and should raise the suspicion of a mesothelioma. Plaques may be associated with fibrosis of the subpleural fat, subpleural pulmonary fibrosis, and the formation of parenchymal bands and adjacent nodular opacities compatible with areas of early rounded atelectasis ("crows' feet" or "rounded atelectasis equivalents").
Pneumothorax Fig. 10.28 Pleural calcifications. (a) Extensive flocculent calcifications in pleuritis calcarea following tuberculosis, (b) Typical asbestosrelated pleural plaque, appearing as an isolated elevation ("mesa" sign) with central calcification (arrow). Note also the posterior pleural thickening.
a
The attachments of the serratus and external oblique muscles, epipleural fat, and intercostal vessels should not be mistaken for pleural plaques.
b
The term asbestosis is reserved for the pulmonary parenchyma! fibrosis caused by asbestos exposure and should not be used for asbestos-related pleural abnormalities.
Trauma and Postoperative Changes
CT is indicated in chest trauma for the evaluation of suspected injury to mediastinal structures, particularly the great vessels. CT angiography is a sensitive, noninvasive procedure that can replace arterial angiography for the diagnosis of aortic rupture (see p.915). In a postoperative setting CT is a problem-solving tool.
•
CT Morphology
CT can detect pneumothorax with high sensitivity by delineating the visceral pleura from the air- filled pleural space. In patients with recurrent pneumothorax, CT can be used to search for causative subpleural bullae, which may be congenital or may form in the setting of obstructive or fibrosing lung diseases (Fig. 10.29).
Pneumothorax
The presence of a pneumothorax is rarely a primary indication for CT. CT is used to evaluate recurrent spontaneous pneumothorax or insufficient pleural drainage (faulty tube placement, frequently in major fissures or in the lung parenchyma). It is also useful for differentiating a pneumatocele or a large bulla from a pneumothorax. Identification of even a small pneumothorax can be important in trauma patients about to undergo mechanical ventilation or general anesthesia for surgery. Therefore it appears useful always to evaluate the most cephalad scans of an abdominal CT examination in a trauma patient in lung window settings to identify otherwise occult pneumothorax.
Fig. 10.29 Patient with spontaneous pneumothorax. A coronal volume-rendered image (4x 1/6) demonstrates small apical bullae due to paraseptal emphy-
401
10 Mediastinum, Pleura, and Chest Wall
Mediastinal Hematoma
Mediastinal hematomas may result from central venous puncture, chest trauma, the spontaneous rupture of an aortic aneurysm, aortic dissection, or a coagulation disorder. If aortic injury is suspected, CT angiography should be performed as an adjunct to non contrast CT (see Chapter 24). •
CT Morphology
A fresh mediastinal
hemorrhage initially produces streak-like densities in the mediastinal fat. Only larger hematomas contain areas of increased attenuation. The approximate location of the bleeding site can be inferred from the site of the hematoma. Contrast extravasation into a fresh hemorrhage is rare and is usually an indication for immediate surgery. Bleeding into the pleural space signifies injury to the lung (Fig. 10.30) or intercostal arteries or an open or confined rupture of the aorta (ascending aorta on the right side, aortic arch or descending aorta on the left side). A (hemorrhagic) pericardia! effusion poses a risk of pericardial tamponade, especially if CT reveals a concomitant dissection of the ascending aorta. Rupture of the brachiocephalic veins due to blunt chest trauma leads to hematoma formation in the upper anterior mediastinum. Traumatic aortic aneurysms occur in up to 95% at the level of the ligamentum arteriosum and are associated with left-sided hematomas in the posterior mediastinum. Older hematomas show decreasing attenuation on CT scans and may have soft-tissue or water attenuation, making them difficult to distinguish from a solid mediastinal tumor (hematomas enhance by less than WHU after intravenous contrast administration).
Fig. 10.30 Massive chest trauma with injury of the right lung and acute intrapleural hemorrhage demonstrated after contrast injection.
surgery, intubation, or endoscopy). Mediastinal emphysema may be an indirect sign that a neoplasm has eroded the esophagus or tracheobronchial tree. Retroperitoneal and intraperitoneal free air can also track into the mediastinum. Mediastinal emphysema is rarely a primary indication for CT scanning. •
CT Morphology
CT can sensitively detect even the smallest mediastinal air collections, but the cause can be determined only when the CT scans are interpreted within the context of clinical findings (Fig. 10.31).
Mediastinal Emphysema
Mediastinal emphysema, known also as pneumomediastinum, usually signifies a tracheobronchial injury or subpleural alveolar rupture with air tracking along the pulmonary interstitium and lung root into the mediastinum. Possible causes are a sudden intra-alveolar pressure rise due to strenuous physical effort or mechanical ventilation (Macklin effect), bronchial rupture, esophageal rupture (Boerhaave syndrome), or therapeutic procedures (prior
A Fig. 10.31 Pneumomediastinum due to iatrogenic injury of the left main bronchus during difficult intubation. Soft tissue emphysema after insertion of a pleural drain.
Diaphragmatic
Rupture
~ a
b
Fig. 10.32 Traumatic diaphragmatic rupture on the left side with herniation of stomach and colon into the thoracic cavity, accompanied by extensive right-sided
pulmonary contusions. Note the exposed stomach wall and pneumothorax on the left side (5/8/4).
Air within the pulmonary interstitium associated with a pneumomediastinum is a sign of alveolar rupture. A traumatic rupture of the trachea or large bronchi occurs within 2.5cm of the carina in 85% of patients and leads to mediastinal emphysema. With rupture of a bronchus distal to the insertion of the pulmonary ligament a pneumothorax will form as well. A rupture is presumed to exist if a large pneumothorax persists despite adequate drainage, if the lung is completely collapsed, or if the lung collapses toward the chest wall rather than toward the hilum ("fallen lung" sign). Direct CT visualization of the rupture is rarely achieved and usually requires spiral or multislice scanning with thin slices (see Chapter 9).
angle. Tears of the central portion of the diaphragm are most difficult to evaluate. Multislice CT strongly improves evaluation of the diaphragm. Reformatted images in the coronal and sagittal plane from spiral and mul tislice CT data sets are extremely helpful to establish the diagnosis (Fig. 10.32). Indirect signs of a diaphragmatic tear are an apparent elevation of the diaphragm with pleural effusion, deformation of the hepatic or splenic contour by herniating viscera, and an unusually thin border separating the stomach, small intestine, or colon from the adjacent lung (lack of diaphragmatic coverage). In addition, thickening of the posterior portions of the diaphragm on the side of the rupture has been reported. This sign, however, must be distinguished from a relative hypertrophy of the diaphragm on the contralateral side, which may be due to phrenic nerve palsy.
Diaphragmatic Rupture
Diaphragmatic rupture is commoner on the left side than on the right. Visceral herniation into the chest is rarely seen in acute cases. The radiological diagnosis of diaphragmatic rupture is a difficult challenge, regardless of the modality used. Even with multiple imaging modalities, a definitive diagnosis must await operation in approximately 50% of cases. •
CT Morphology
A diaphragmatic tear can be directly visualized only on thin CT slices that cut the posterior part of the diaphragm almost at a perpendicular
Cardiac Surgery
CT can be used in postoperative cardiac patients to investigate a new occurrence of mediastinal widening, confirm or exclude suspected postoperative hemorrhage, or search for a focus of infection. •
CT Morphology
During the initial weeks after surgery, it may be difficult to distinguish infection-related changes from changes due to surgical manipulation.
403
10 Mediastinum, Pleura, and Chest Wall
However, a hypoattenuating mass that shows marked rim enhancement after contrast administration is suspicious for an abscess. Presternal or retrosternal edema during this period has no pathologic significance. It is manifested by stranding and increased density of the fat in the middle and anterior mediastinum. Fluid collections may be serous (lD20 HU). bloody (40-80 HU), or of mixed composition. Mediastinal and pericardial air inclusions may occur initially along with a small pericardial effusion. Enlarged mediastinal lymph nodes are a common finding. The abeve changes gradually resolve during the first three or four postoperative weeks and rarely persist for up to 50 days. In patients who have undergone a sternotomy, the osteotomy site should be fully reunioned, although small discontinuities are normal. A gap up to 2 mm wide may still be considered normal, but the risk of sternal instability increases with the width of the residual gap (see Fig. 23.58). Aortic pseudoaneuIJsms are a rare late complication « 1 % at 1 to 2 years after surgery) resulting from dehiscence of the suture line due to an acute or chronic infection. Typical sites of occurrence are the site of an aortotomy (for connection to the heart-lung machine) or an anastomosis between the aorta and a bypass (see Fig. 24.48). Constrictive pericarditis is another rare late complication (0.2% at months to years after surgery) manifested on CT by 3 to 20mm of pericardial thickening with manifestations of right heart failure: caval dilatation, flattening of the right ventricle with curvature of the septum to the left, pleural effusion, and ascites. Pericardial calcifications are rare in postoperative cases but occur in 50% of cases with other etiology (see Fig.23.52b).
Heart Transplantation Acute or chronic rejection after heart transplantation requires biopsy confirmation and is not an indication for CT. In cases of infection, which are responsible for up to 40% of postoperative mortality in heart transplant recipients, CT is a sensitive modality for the detection and localization of the focus (see Chapter 9). The findings are nonspecific, however, and cannot identify
the causative organism (most frequent problem organisms: gram-negative bacteria, viruses, fungi, protozoans). Long-term complications include lymphoma, carcinoma, and coronary sclerosis. CT Morphology Scanning of anastomotic sites in the ascending aorta or right atrium may reveal a constriction or an abrupt change in caliber. The lumen of the inferior vena cava is frequently enlarged. Unusual separation may be noted between the superior vena cava of the recipient and the ascending aorta of the donor or between the recipient pulmonary artery and the donor aorta. Pericardial effusion is frequently encapsulated and may persist for a period of weeks or months. Tricuspid insufficiency is a post-transplant complication that can be suspected from CT if an extreme dilatation of the right atrium including the valvular ring is present (see Fig. 23.57). Steroid therapy can lead to mediastinal lipomatosis and spinal osteoporosis, which is frequently associated with vertebral body compression.
The use of scintigraphy. such as " "Tc-sulfur colloid or IDA, is rapidly declining as these have been replaced by multiphasic spiral CT or MRI. Sulfur colloid scans can be used for focal fatty infiltration, regenerative nodules, and in characterizing primary tumors like focal nodular hyperplasia. Tc-red blood cell scan with SPECT is still a good technique for hemangioma. New techniques like 'In-octreotide imaging may have a niche in the work-up of neuroendocrine tumors, especially gastrinomas, or carcinoid tumors. The role for PET in clinical oncology is also increasing steadily, but the applications in the liver are still scarce. Image fusion of SPECT and PET data with CT data sets is still quite cumbersome but there are combined CTPET or CT-SPECT scanners-some even with multislice technology-becoming available that elegantly solve the problem of co-registration of data sets. While these scanners can provide excellent results, but still suffer from their substantial costs.
Computed tomography is a standard technique for imaging of the liver. It is mainly used for the evaluation of tumors but offers useful information also for a vast range of other diseases (Table 11.1). Ultrasound offers an economical. noninvasive alternative formany indications. but further evaluation by CT or MRI is frequently required in patients prior to surgery. Contrast-enhanced ultrasound may become a competitor in the diagnosis of focal lesions. but this technique is still difficult to perform for coverage of the entire liver. As compared to MRI. CT offers a consistently good image quality and requires only minimal patient cooperation. Complex invasive procedures such as CT hepatic arteriography (CTHA) or CT during arterial portography (CTAP) had been justified only if they had essential therapeutic implications (assessment of resect ability). At present. they are substituted for by biphasic multislice CT and dynamic Cd-enhanced MRI or MRI with liver-specific contrast media.
Hemosiderosis (iron content) Liver and tumor volume
benign-malignant,
characterization
Detection,
Malperfusion Vascular:
The Liver
Anatomy
With a volume of 1400 to 1700ml, the liver is the largest parenchymal organ in the body. On average, it measures 13.5cm in its largest craniocauclal dimension. The size of a particular liver varies with the height and gender of the patient. Hepatomegaly is present in >80% of patients when the liver exceeds 15.5cm in its craniocaudal extent, or the volume of the liver is 2000 ml or more. The normal size ratio between the right and left hepatic lobes is 3 :2. •
Segmental
Anatomy
Knowledge of hepatic segmental anatomy is helpful to communicate CT findings to the surgeon. However, numerous approaches use the same names for different structures. The numbering system for the segmental anatomy is identical for most of these approaches. All approaches are only approximations of reality because there are marked variations in the individual anatomy of almost every patient. The liver is divided into four segments by the planes defined by the inferior vena cava (lVC) and the right, middle, and left liver veins. These define the anterior and posterior segment of the right hepatic lobe, and the medial and lateral segment of the left hepatic lobe. The right and left lobe are separated by the plane defined by the IVC, the middle liver vein, and the gallbladder. These vertically oriented segments are divided into subsegments by the horizontally oriented portal vein. In the most usual surgical classification (Bismuth) in North America, the portal venous Table 11.2
Segmental classification
Anatomic subsegment
Caudate lobe Left lateral superior subsegment Left lateral inferior subsegment Left medial subsegment Right Right Right Right
anterior inferior subsegment anterior superior subsegment posterior inferior subsegment posterior superior subsegment
Couninaud's
supply of the liver defines eight subsegments of the liver. The boundaries of these segments are grossly defined by the hepatic veins (in the craniocaudal direction) and by the plane of the portal vein (horizontal on the right, oblique on the left) (Fig. 11.1). Other numbering systems, such as the one by Couinaud, are more common in Europe. Table 11.2 gives an overview of the most important terminologies. Segment 1is the caudate lobe. Subsegments 11 and III are the portions of the left lobe that are 10-
Fig. 11.1 Bismuth classification of hepatic segments. The left, middle, and right hepatic veins divide the liver into four segments (Couinaud: sectors), which are further subdivided into cranial and caudal subsegments (Couinaud: segments) by the plane of the portal vein. Viewed from the front, the numbering of the (sub)segments proceeds in a clockwise direction (except for segment I and subsegment IVa).
of the liver (Soyer, 1993) Nomenclature Couinaud,
Bismuth,
Goldsmith and Woodburne,
1957
1982
1957
IV V VIII
III IV alb V VIII
Caudate lobe Left lateral segment Left lateral segment Left medial segment Right anterior segment Right anterior segment
VI VII
VI VII
Right posterior segment Right posterior segment
hemiliver, sector, and segment correspond to Bismuth's lobe, segment, and subsegment
Anatomy
cated to the left of the falciform ligament. Segment IV is the medial portion of the left hepatic lobe (the" quadrate lobe ") that is located between the falciform ligament and the vena cava-gallbladder line/middle hepatic vein. In the Bismuth classification segment IV is subdivided into subsegments IVa cranially and IVb caudally. The right hepatic lobe is divided into subsegments V through VlI 1, with a clockwise numbering system starting from the subsegment V inferiomedially next to the gallbladder (Figs. "11.1, 11.2). Common anatomic variants include a hypoplasia of segment IV b and a large left hepatic lobe that fills the left dome of the diaphragm. Complex congenital heart diseases may be associated with a symmetrical, centralized liver
(Fig. 11.3). A left-sided liver is part ofa complete or partial situs inversus. •
Vascular Anatomy and Variants
The common hepatic artery normally originates from the celiac trunk. After giving rise to the gastroduodenal artery behind the pylorus, it becomes the proper hepatic artery that passes through the hepatoduodenal ligament anteromedial to the portal vein and anterior to the common bile duct. In the liver hilum it bifurcates into the right (RHA) and left hepatic arteries (LHA). The middle hepatic artery that supplies segment IV originates with equal frequency from the right or left hepatic artery.
VII~ VII./.l
a
b
(
d
Fig. 11.2 Location of the hepatic segments on CT sections. In scans through the cranial portion of the liver, the segmental boundaries are defined by the vena cava and the main branches of the hepatic veins. At more caudal levels the cava-gallbladder line and the falciform
ligament define the central and left segmental planes. Often the plane of the portal vein can only be estimated. In case of doubt coronal reformations are helpful.
409
11 The Liver
patic ligament and reaches the left lobe via the fissure of the ligamentum venosum (>20% of all patients). Axial CT sections demonstrate such a vessel above the liver hil urn and posterior to the left lobe (Fig. T1.4a). Accessory and replaced arteries can be distinguished by searching for a normal left hepatic artery originating from the proper hepatic artery at the liver hil urn. Variations of the right hepatic artery are almost as common. They all present as an artery in the hepatoduodenal ligament posterior to the portal vein (Fig.l1.4b). By tracing this artery back to its origin, one can distinguish between a RHA arising from the aorta, the superior mesenFig. 11.3 Symmetric liver as an associated anomaly in a teric artery, or one directly from the celiac patient with Kartagener syndrome (immotile cilia synartery. By following it in both directions, it is drome) and incomplete situs inversus. Also note the just as easy to distinguish between an accessory right-sided location of the stomach and an aplasia of the or replaced artery, and to determine whether it intrahepatic segment of the inferior vena cava with azygos continuation. is just an aberrant RHA, or a common hepatic artery, or even the celiac artery that is displayed. Apart from the hepatic artery, there can be Variations in the arterial blood supply of other sources of arterial supply to the liver, which of periphthe liver occur in some 45% of individuals and are important in the vascularization are important for planning surgical or radiologieral hypervascular tumors (parasitic supply) and cal interventions. Various classifications have when the hepatic artery is occluded. The most been proposed. The one by Michels is most important is the right inferior phrenic artery, popular but is not very precise. Therefore, it is but also the left inferior phrenic, internal mammary, left gastric, or intercostal arteries may more advisable to properly describe variations give offbranches to the liver surface (Fig. "11.5). than to try to classify them. Aberrant arteries The main portal vein is formed by the concan either be a replaced (variation in origin) or accessory (additional vessel). Variants that in- fluence of the superior mesenteric vein and the splenic vein and runs posteriorly in the hepavolve more than one artery are common. The commonest variations involve an aber- toduodenal ligament. At the porta hepatis it branches in the right (RPV) and left portal veins rant left hepatic artery that runs in the gastrohe-
Fig. 11.4 The following patterns suggest an aberrant hepatic vascular supply, (a) An aberrant left hepatic artery (almost always from the left gastric artery) is present if there is artery entering the liver above the hilum and anterior to the caudate lobe. (b) An aberrant
right hepatic artery (most commonly from the superior mesenteric artery) is present ifthere is an artery supplying the liver posterior to the portal vein. Further analysis is required to determine the origin of the vessels and to distinguish accessory from displaced arteries.
Anatomy Fig. 11.5 Aberrant vascular supply to the liver in a patient with occlusion of the hepatic artery after embolization of a metastatic gastrinoma. Note the arterial revascularization (arrows) of the thrombosed of the portal vein (a) and collateralization (arrowheads) via phrenic artery branches (b).
a
(LPV). Most commonly, the RPV further divides in anterior and posterior segmental branches, while the LPV divides in superior and inferior segmental branches. Anatomical variants of the MPV (trifurcation, posterior from MPV, anterior from LPV) occur in up to 20% of patients, and variants of segmental branches are present in almost every individual. The anterior branch of the RPV is more constant than the posterior branch. The course of the main LPV almost always remains constant. There are usually three hepatic veins that course in the vertical fissures that separate the liver segments. The right hepatic vein (RHV) drains segments VI -VIII into the IVC. The middle hepatic vein (MHV) drains segments V and IV B, while the left hepatic vein (LHV) drains segments II -IV A. The MHV and LHV often form a common trunk before draining into the IVC. The caudate lobe (segment I) drains separately into the IVC via small veins at a more inferior level. This is the reason why it often hypertrophies in Budd-Chiari syndrome (occlusion of the large hepatic veins). There are numerous variations in the drainage pattern of the several segments to the hepatic veins. Other common variants include accessory right, middle, or left hepatic veins, and an accessory inferior RHV that may be important for atypical liver surgery because it separately drains segments V and VI. There are m ul tiple branching patterns ofthe LHV. •
b
liver attenuation decreases approximately 15 HU for each 10% increase in the proportion of fat. When the fat content reaches 10-15%, the intrahepatic vessels become isoattenuating to the parenchyma and cannot be seen on unenhanced scans (Fig.1l.6b). A further increase in fat content reverses the display of vascular structures, and the intrahepatic vessels appear hyperattenuating to the liver tissue on noncontrast scans (Fig. 11.6a) . In patients with hemochromatosis or hemosiderosis, liver attenuation increases in proportion to the iron load (Fig.11.6d). However, this attenuation increase depends strongly on the effective energy of the x-ray beam and varies with the tube voltage, scanner type, and patient diameter. A concomitant fatty infiltration of the liver may yield a falsely low CT attenuation. Table 11.3 lists other, rarer causes of increased liver attenuation. Table 11.3
Thorium-containing contrast medium (diffuse reticular distribution pattern)
Thallium:
Accidental
Liver Attenuation
Normal liver parenchyma has a CT attenuation of 55 to 65HU (Fig.H.Gc). This slightly higher attenuation as compared to other soft tissues is thought to be due to the glycogen content of the liver. When diffuse fatty infiltration is present,
Causes
(>70HU)
Arsenic: Acute massive
(antiarrhythmic
drug)
use in rheumatoid
or suicidal ingestion
Chronic poisoning protein storage
Glycogen storage diseases
ar-
411
77 The Liver
c
b
a
Fig. 11.6 CT densityofthe hepatic parenchyma: normal (c), hemochromatosis (d).
Examination
d
fatty liver (a, with hepatic metastases),
mild fatty infiltration (b),
Technique
CT scanning of the liver is usually a part of an upper abdominal or complete abdominal examination. With few exceptions (exclusion of hemorrhage, hemochromatosis), every CT of the liver should use intravenous contrast material (Table 11.4). A power injector is required. Table I1.S lists the scanning techniques that we recommend for specific clinical problems. Biphasic multislice CT and various MRI techniques increasingly substitute for procedures that require intra-arterial contrast application such as CT hepatic arteriography (CTHA) or CT during arterial portography (CTAP). Patient Preparation
Except in dire emergencies or CTAP studies, oral contrast medium should be administered. For upper abdominal studies, SOO-600ml of oral contrast medium given 30 minutes prior to the scan is sufficient, while whole abdominal examinations require administration of 1000IS00ml of oral contrast medium over 60-90 minutes before the examination. In trauma cases, SOOml given via nasogastric tube is beneficial, and extra 2S0ml portions may be added if the waiting time for CT is prolonged. Positive contrast material is still standard, but negative contrast material such as water or methylcellulose preparations are becoming in-
creasingly important for arterial phase liver examinations as part of a biphasic or multiphasic upper abdominal study, especially when multislice scanning is employed. Such negative contrast has the advantage that there is no superimposition of bowel and vessels for CTA, and there is a better display of the bowel walls. Acquisition Technique
Spiral scanning is the technique of choice for liver examinations, independent of the mode of contrast material application (Tables 11.4, 11.S). Spiral CT is the prerequisite for scanning the whole liver within the arterial phase of contrast enhancement, a phase that was inaccessible with older techniques such as dynamic incremental CT. Trade-offs and Parameter Optimization
Lesion detection in the liver depends on signalto-noise ratios and thus on the size of the lesion, its contrast, the amount of partial volume averaging, and the image noise. Large lesions usually do not pose a problem as long as there is an attenuation difference to normal liver parenchyma. Small lesions, however, suffer from partial volume effects that may further reduce
Acquisition Technique Table 11.4
Suggested parameters for examination
413
of the liver
General Oral contrast
Liver only: 0,51, administered 30 min prior to examination Abdomen: 1-1.51, administered 60 min prior to examination
Patient position
Supine with arms elevated
Scan range
Liver only: from diaphragm to inferior hepatic border Abdomen: from diaphragm to symphysis
Breath hold phase
Inspiration (Spiral-CT)
Windowing
Noncontrast or delayed CT: After I. v. contrast: CTHA, CTAP: Lipiodol-CT:
W/L=200/40 W/L=350/60 W/L= 500/100 (adjust as needed) W/L=300/40
Scan parameters
1-slice SCfTF/RI
Standard, precontrast
7/12/
Volumetric imaging
5/8/4
Perfusion CT
7-10/0
€ontrast injection
l-slice V/F/D
4-slice V/Ii/Ii)
16-slice 'if./F/f!)d
Abdominal screening
120/2/60
120 + 50/3/60
120 + 5012/205
Portal venous CT
150/4/70
150 + 50/5/60
150 + 50/5/205
Biphasic CT
150/4/25 + 70'
150 + 50/5/25 + 70'
150 + 50/5/10Aa 80 c.
Early delayed phase
3-5 min
3-5min
3-5 min
Late delayed phase
10-15 min
10-15 min
10-15min
Perfusion CT
50/5/10
50 + 50/5/10
50+ 50/5/10
CTHA
50/2/5 La.9
50/2/5 La.9
CTAP
120/2/30 La. 9
120/2/30 + 60
Lipiodol CT
5 ml Lipiodol i.a. h
5 ml Lipiodol i.a. h
Portal venous CT
150/4/70
150 + 50/5/60
4-slice' SC
t i
2-3
16-slice' SC
i
1-1.25
1-1.5
t
i
5-7.5/4-5
0.75-1.25
t
3/3 cor'
4/3
16 x 1.25/2/0'
4 x 5/0'
10-20/-
II f
50/2/15 i.a. 9 120/2/30 + 60 9
9
5 ml Lipiodol i.a. h 150 + 50/5/205
sc = slice collimation (mm), TF= table feed (rnrnjrotanon). RI = recon. increment (mm), T ~ scanning direction, width (mm), MPR = multiplanar reformats. axial = axial sections, cor coronal, sag = sagittal eM = contrast material, Contrast concentration = 300 mg/ml iodine (optional 400 mg/ml for 16-slice scanning) V= volume of eM (ml), N =volume of saline flush (ml), F" flow rate (mils). D = start delay, X = trigger region a Pitch P = $Cf(NxTF) should be chosen close to 1.5 with a-slice scanners. 1.3- 1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SW/RI"1-1.5/0.7 { single section (perfusion) mode with no table increment, scans every 2-5 s dread: 205 (lOA) '" start delay 20 s (lOs) after reaching 50 HU enhancement in the spleen (desc. aorta) e read: biphasic scan (arterial + portal phase) after 25 s (lOA) for the pi for the scan, 70-80s for the 2"d scan =::
SW= effective section
=::
f 9
h
when 400 rnqfml is used for 16-slice scanning, the contrast volume can be reduced to 120 ml injection into proper heaptic artery (CTHA) or superior mesenteric/splenic artery (crAP); prior injection of vasodilator (optional); reduced iodine concentration of 150 mqjml (optional) examination performed approximately 14 days after Upiodol injection inot the proper heaptic artery
their contrast to surrounding low contrast will make such a ceptible to image noise: lesions noise and no longer be visible.
parenchyma. A lesion more susmay "drown" in For this reason,
there are three ways of improving lesion detection (and possibly also characterization): reduce partial volume effects, improve contrast. and limit image noise.
414
11 The Liver Table 11.5
Which CTtechnique
Screening for metastases hypovascular hypervascular Prior to resection Lesion of unknown
dignity
for which clinical indication? Phase after i. v. contrast
injection
i.a. contrast
application
Precontrast
arterial
portal
delayed
GAP
lipiodol
(+)
++
++ ++ ++
(+)
++
++
+
(+)
++
++
+
Suspected
HCC
(+)
++
++
+
Suspected
CCC
(+)
(+)
++
+
Post transplantation
(+)
++
++
Trauma
+
(+)
++
The recommended techniques are printed in blue. The arterial phase is always combined with the portal venous phase (biphasic well suited + ~ may provide additional (+) ~ may provide additional - ~ not recommended
G
++
spiral CT).
++ ~
information information
in selected
cases
Reduction oj partial volwlle averaging relies on the choice of a narrow section width and overlapping image reconstruction. Overlapping reconstruction increases the number of images to be viewed. documented. networked. and archived. There are two other important tradeoffs: thin-section scanning reduces the coverage per time. and it increases image noise substantially. The longer time that is required to cover the liver may become problematic for arterial phase imaging. Because there is only a narrow window for purely arterial phase imaging (see below). there will be varying degrees of overlap between the arterial and the portal phase of contrast enhancement during a single spiral acquisition. Timing. therefore. becomes crucial. In addition. the longer scan duration with thinsection imaging will increase the likelihood of breathing artifacts during the scan. Thinner sections also increase image noise. A reduction of section width by a factor of four from Srnrn to 2 mill will lead to a two-fold ( 4) increase in image noise. Lesion detection therefore becomes dose-dependent. To take full advantage of the increased lesion detection with thin sections. one has to substantially increase patient dose or use other means of noise reduction. This is underlined by a recent study that shows that evaluation of 1 mm mulrislice CT images no longer improves lesion detection but actually reduces it as compared to 2-4mm sections at a dose of 15mGy (CTDlvol). At a dose of
20 mGy. another study shows that the number of detected lesions can be substantially increased with 2.5 mrn section width as compared to 5 rnm or more. Characterization becomes a problem with small lesions. Here. the thinner section width is better because it improves morphologic evaluation. Delayed scans help differentiate between small cysts and tumors. Still. a substantial number of lesions of less than 8 mm diameter will remain undiscovered. It has been shown that the vast majority of such small lesions are benign. even in tumor patients. At the same time. the larger lesions seen in a patient with metastatic disease are only the tip of the iceberg. Depending on the primary. 1.6 to 4 times more metastases smaller than 1 ern have been shown to be present for every lesion> 1 ern. There is no consensus yet as to which section width and radiation dose is appropriate for which indication. Given the results mentioned above. however. we suggest reconstructing a moderate section width (S-7.5mm) at a reduced patient dose (CTDlvol= 6-10 mGy) for benign diseases or indeterminate lesions. a moderate section width (S-7.5mm) and moderate patient dose (8-12mGy) for known malignancies. and a narrow section width (2-4mm) and high dose (12-20 mGy) only for those patients in whom major surgery is planned. Dependent on body diameter in more obese patients. the emlvol has to be increased by a fac-
Acquisition
tor of 2-4 (see Chapter 5). In the near future. automated exposure control will become available. which will make it much easier to individualize the required patient dose. In addition. contrast material application should be optimized so that high liver-to-lesion contrast is guaranteed.
Single-Slice
CT
The main limiting factor for single-slice CT is coverage. To optimize coverage and section width. a pitch factor> 1.5 should be used. For most situations. 5 mm collirnation and a table feed of 8 mm (5/8/4 protocol) is a good solution. For improved scanning speed in the arterial phase. the table feed can be increased to 10 mm. If the spatial resolution has to be optimized, a 3/512 scanning protocol should be considered if a subsecond scanner is available. A table feed of 6mm is also possible but image quality of 3D reconstructions is visibly lower. Such a protocol can be important for planning surgery, but the protocol suffers from increased noise and therefore cannot be recommended for obese patients. In very obese individuals, the section collirnation may even be increased to 7 mm to obtain a s ufficie nt signal-to-noise ratio. Overlapping image reconstruction is necessary to take full advantage ofthe spiral CTacquisition. The number oflesions increases substantially when images are reconstructed in an overlapping fashion. Ideally, there should be 50-60% overlap with respect to the effective section thickness (section width) but in clinical practice, a somewhat lower overlap may suffice.
Multislice
CT
Multislice CT has similar trade-offs as singleslice spiral CT. It can be optimized either to near-isotropic resolution or to more homogeneous liver enhancement. Unless extremely rapid scanning of the liver is necessary « 5 s), a 5mm collirnation should not be used, even with 4-slice scanners This gives the user the choice between a 0.75-1.5 mm collirnation for nearisotropic resolution and a 2.5-4 mm collirnation for rapid scanning. For optimum speed, a pitch P > 1.35 should be used with either technique. A test bolus injection or bolus triggering becomes
Technique
mandatory with contrast injection to avoid missing the optimum perfusion phase. H
With
4-slice Scanners
a narrow collirnation of 1-1.25 mm {volumetric imaging}, multislice CT provides excellent multi planar reformations that are especially helpful before hepatic surgery (e.g., Fig. 11.19). This technique allows for excellent evaluation of the arterial and portal venous anatomy and the relationship of tumors to the liver hilum, the hepatic vessels, and the bile ducts. lt also yields the bestCTangiographic results for the arterial as well as the portal venous supply. However, contrast enhancement of the liver and vascular structures changes over time during arterial phase scanning; the most cranial sections will represent a relatively early arterial phase while the caudal sections will represent a late arterial/early portal venous phase. This problem is less pronounced in portal venous imaging because the portal phase lasts for a longer period. With a collirnation of 4x2.5 mm or even 4x5 mm (fast spiral scanning), the liver can be scanned in some 8-12 s or 4-6 s, respectively. This makes it possible to perform scans in an early and late hepatic arterial phase (see below). Contrast enhancement of the liver and vessels is more homogenous but spatial resolution of multiplanar reformations is lower. In patients with hepatomegaly, higher pitch factors (P" up to 8) or thicker collirnation (e.g., 4x3.75 mm) may be used to cover the liver in a sufficiently short time. As mentioned above, we suggest reconstructing a moderate section width (5-7.5 mm) for benign diseases, indeterminate lesions, or follow-up of known malignancies, and a narrow section width (2-4 mm) for patients before major surgery. CE scanners at present are limited as to which section widths can be reconstructed for a specific collirnation setting. This may require the use of 4x2.5 mm collirnation for the first group of indications if one wants to reconstruct axial 5mm sections (maximum section width with 4xl.25 mm collirnation is 2.5mm). Alternatively, a 4x1.25 mm protocol can be used and 5mm thick MPR can be reformatted from the resulting secondary raw data set of overlapping thin axial sections. Other scanners can use the thin-section protocol and reconstruct images of arbitrary section width (even 7 mm or above).
11 The Liver
In obese patients, thin-section imaging will lead to suboptimum results because of excessive noise. Such patients should be scanned using at least a 4x2.5mm protocol (4x3 mm or 4 x 3.75 mm, depending on the scanner type, will be better) and a section width of7-7.5 mm. In very obese patients, the collimation and section width may be increased even more. 8- and 16-slice Scanners
Newer multislice scanners with 8 or 16 detector rows allow for covering the whole upper abdomen in less than lOs, even with 1-1.5 mm section collimation. This makes it possible to use such a thin collimation and a pitch P >-1.35 as the standard for liver imaging. Thinner collimation (0.5-0.75 mm) is rarely necessary for the liver unless a detailed analysis of the vasculature or biliary system is required. With such thin collimation, higher pitch factors (P , 1.5) should be used. Like with 4-slice scanners, the reconstructed section width (either from the original raw data or an secondary raw data set of overlapping thin sections) depends on the clinical indication, the exposure dose and the size of the patient. Thick coronal and sagittal MPR (3-5 mm Width). especially when aligned parallel and perpendicular to the portal vein, are helpful for the evaluation of the porta hepatis and for the planning of partial liver resection.
Intravenous Contrast Injection While non-contrast CT is easily performed even on non-spiral CT scanners, contrast-enhanced CT of the whole liver was limited to scanning in the portal venous or later phases because the arterial phase of enhancement was too transient for capturing by conventional scanners. Spiral scanning allowed for rapid acquisition of the data volume within one breath hold, but arterial phase imaging has become a standard technique for the liver only with the possibility of quick repeat scanning of the same volume (biphasic CT). With multislice scanners, even more rapid data acquisition becomes possible and an early and late arterial phase of enhancement can be captured separately. Timing of contrast application is a crucial factor for optimum results.
Noncontrast
CT
Noncontrast CT has limited indications in modern practice. It is generally indispensable for evaluating suspected hemorrhage in trauma and is the best phase for documenting confluent fibrosis in cirrhosis, calcifications, and hemochromatosis. Most often however, it is used in the evaluation of hyper vascular liver tumors as part ofa multiphasic approach. Some of these tumors can best be detected on non contrast CT. We especially recommend them for patients with carcinoid or other endocrine tumors because lesion size can be assessed to better advantage. For hypovascular lesions, such as metastases of colorectal cancer, noncontrast CT is not required. Because of the low contrast of many liver lesions, noncontrast CT requires sufficiently high mAs settings depending on the size of the patient (CTOIvol 'IOmGy) and narrow ("liver") window settings for image documentation (150250/40). Blood vessels and lesions generally appear hypoattenuating compared to normal liver parenchyma. If differentiation between blood vessels and liver parenchyma is not possible because of slight fatty infiltration, noncontrast CT is not helpful for identifying lesions. Portal
Phase CT
For most indications in which the liver is just part ofa general work-up, scanning in the portal venous phase is considered sufficient. This is also the case in patients with known malignancies that cause hypovasculcir metastases, such as colorectal cancer. In a pre-surgical work-up, however, biphasic spiral scans should be preferred. For spiral CT in the portal venous phase, 100 to 200ml of contrast medium is administered by power injector (at 3-5 mlls). For optimum lesion-to-liver contrast, liver enhancement should be as high as possible. Peak enhancement increases with larger volumes of contrast material. Best results require injection of 150 ml or more, but there is a strong dependence on patient weight. At present, there is a trend to individualize contrast material injection, with 1.5-2 ml contrast material per kg body weight being a good compromise. There is some contradictory evidence in the literature as to whether flow rates influence he-
Intravenous Contrast Injection
patic parenchymal enhancement. Work by Chambers et a1. (1994) suggests that using injection rates of 3 mlls lead to an earlier and higher hepatic enhancement than rates of 2 mlls. Model calculations by Bae et a1. (1998) could show that peak enhancement is almost independent of flow rates and only depends on the total amount of iodine injected. As a consequence, the volume of contrast material probably cannot be reduced, even for 16-row multislice CT scanning. The duration of the phase of optimum liver enhancement increases with a biphasic injection protocol. The optimum parameters for a biphasic injection vary from patient to patient (because of different cardiac output and variations in splanchnic perfusion) but a combination of 50 ml contrast at 5 mlls, followed by 100mI at 3 mlls yields good results. With rapid scanning using either spiral or multislice technique, however, there is little advantage to such a protocol. As a rule of the thumb, a relative plateau of parenchymal enhancement is reached some 20 seconds after the end of contrast injection (Fig. 11.7), with peak enhancement occurring about lOs later. This enhancement plateau lasts for more than 30s although parenchymal enhancement may vary by up to 15HLJ during this phase. Thus, scanning is initiated about 50 to 100 seconds after the start of the injection depending on the volume and flow rate of the contrast medium. Because there may be substantial variations between patients, results are superior
Enhancement(H
E)
3001-
250
Aorta
200 150
Portal vein ~
100
~Live~veins_
50 a
------~ _ Liver parenchyma
30
40
50
60
70
80
90 Time (5)
Fig. 11.7 Schematic representation of vascular and hepatic enhancement after injection of 150 ml of contrast material at 4 mils.
when the scan delay is tailored to the individual circulation time (bolus triggering). Bolus triggering in the portal venous phase may either use a region of interest in the liver, spleen, or aorta, or rely on enhancement of the liver veins as a visual clue. Portal venous triggering is not always easy because the maximum enhancement of the liver parenchyma may vary strongly between patients, and there are variations in the time delay between start of aortic and optimum portal venous enhancement. Some authors have suggested using an ROI in the liver (enhancement of 50 HU as a trigger level) for starting the scan but more than 30% of patients may never reach such a level. We also suggest a trigger level of 50 HU enhancement, but use either the spleen (15-20 s delay for the portal venous phase, 30-40 s for the hepatic venous phase, see below) or the aorta (40-45 s for the portal venous phase, 55-65 s for the hepatic venous phase). For longer scan duration (e.g., with singleslice CT), we recommend scanning in the caudocranial direction because it delays imaging of the hepatic veins until they are more fully opacified. •
Biphasic Spiral CT
Biphasic spiral CT includes scanning in the arterial phase of contrast enhancement in addition to the portal venous phase. A biphasic examination is the standard technique for planning partial liver resections or whenever hypervascular lesions are suspected (see Tables 11.5, Table 11.6). Some authors advocate additional noncontrast scans in patients with hypervascular metastases but the number of additionally detected lesions is generally small « 5%). Biphasic CT is often sufficient to characterize focal liver lesions (see Figs. 11.12, 11.14-11.17),but a third phase after contrast injection increases the confidence oflesion characterization. Biphasic spiral CT uses similar scanning parameters to monophasic examination, except that scans are performed during both the arterial phase (start delay about 25 seconds) and the portal phase of enhancement (start delay about 60-90 seconds), in each case using a short scan duration. High flow rates improve the conspicuity of hypervascular lesions and good results are obtained with uniphasic flow rates of 4-5 mlls, although flow rates as high as 8 mlls have been advocated and yield excellent
11 The Liver
lesion-to-liver contrast (see Fig. 11.30). A secure venous access route is necessary for such high flow rates to avoid extravasation. Biphasic contrast injection protocols with a flow rate of 5mlls over 10-15 s and a lower flow rate of 3mlls over 20-25 s improve both arterial enhancement and the duration and homogeneity of the portal venous phase. High dose techniques using 150-200ml of a 300-370 mg IIml contrast agent injected at 5mlls have shown very promising results. comparable with spiral CTAP in detection of liver lesions. but with improved characterization. Bolus triggering further optimizes contrast enhancement of hepatic lesions. The trigger level chosen should not be at the top of the scan range because diaphragmatic movements may cause considerable artifacts in the aorta. The aorta at the level of the liver hilum is more suitable for triggering the scans. Arterial phase scans should start 5-10 s (5s for 20s scan duration. lOs for < lOs scan duration) after the enhancement in the trigger ROl exceeds 50HLJ above baseline. while portal phase scans are initiated a fixed time after the arterial phase has finished. Ideally. the inters can delay should be adapted to the duration of the arterial phase scan. We suggest aortic triggering with a 40-45 s delay for the portal venous phase and a 55-65 s delay for the hepatic venous phase (relative to the aortic trigger level). Triphasic Spiral CT
The term "triphasic spiral CT' is used inconsistently in the literature. It may indicate a noncontrast scan followed by a biphasic CT, or a true triphasic CT with three distinct enhancement phases. In this book. only the latter definition is applied. With Single-slice spiral CT, a triphasic examination consists of a scan in the hepatic arterial phase. portal venous phase. and a scan in the interstitial phase after contrast administration. Such a triphasic examination is performed identically to a biphasic CT but adds a third scan afterward with a start delay of either 3-5 minutes (early delayed. vascular equilibrium phase) or 10-15 minutes (late delayed. parenchymal equilibrium phase). A triphasic CT is helpful for the characterization of known hepatic lesions (hemangiomas. HCC. cholangiocarcinomas) and can improve the detection rate of hepatocellular carcinomas, especially in cir-
rhotic patients. Hemangiomas tend to demonstrate a blood-pool effect with an enhancement of the perfused portions of the tumor that is similar to the vascular space (better seen on early delayed scans). HCC may demonstrate a capsule, or may appear as a hypoattenuating lesion (better on late delayed scans). Peripheral cholangiocarcinomas may only be seen on delayed scans as a hyperattenuating lesion spreading parallel to the portal branches. Cysts remain hypoattenuating in all scan phases. An early delayed scan 3-5 minutes after contrast injection helps to identify hemangiomas and small cysts. • A late delayed scan after 10-15 minutes helps to detect small cysts and tumors with a large fibrotic component (e.g., cholangiocarcinoma, HCC with capsule, some nodular NHL). With multislice CT, early and late arterial phase scanning became possible, and a triphasic protocol has been suggested that includes these two phases followed by a later portal (= hepatic venous) phase. Rapid scanning is necessary to capture distinct phases. With 4-slice scanners, 4 x 2.5 mm to 4 x 3.75 mm collimation should be used. A high pitch factor is to be preferred over a wide collimation (GE scanners: HS instead of HQ. mode). Aortic bolus triggering is beneficial: the early arterial phase is started with a 3-5 s delay, the late arterial phase is started 5 s after the first phase ends. The hepatic venous phase is initiated some 20 s after the late arterial phase has ended. Such a protocol yields the best separation of arterial and venous anatomy (early arterial and hepatic venous phases). Lesion detection, however, does not profit from the early arterial phase because most hypervascular lesions are better (or only) detected during the late arterial phase. The protocol may therefore be recommended for a pre-surgical work-up but, because of the improved differentiation of detected lesions, we usually prefer a triphasic examination with an arterial phase scan initiated some lOs after contrast arrival in the aorta, an hepatic venous phase, and a (usually 3-5 min) delayed phase.
Intra-arterial Contrast Injection Techniques An early arterial phase is helpful for pre-surgical 3D reconstruction of the arterial supply but rarely improves tumor detection. Due to the increased radiation exposure. one should consider MRI with dynamic sequences after gadolinium or with liverspecific contrast agents as the first choice technique in younger patients with suspected benign liver tumors.
Intra-arterial
Contrast
Injection
Techniques
Intra-arterial contrast injection techniques have been developed to improve detection of liver lesions. but such techniques at present only hold fringe indications. CT hepatic arteriography (CTHA) may be used during arterial chemoembolization or before surgery of hepatocellular carcinomas (H CC). and CT during arterial portography (CTAP) may be used in critical preoperative situations if more modern CT or MR techniques are not available. •
CT Hepatic Arteriography
(CTHA)
CT hepatic arteriography is an interventional procedure in which a catheter is placed under fluoroscopic guidance into the proper hepatic artery. The patient is then transferred to the CT room and a scan (spiral or multislice CT) is performed during direct contrast injection into the hepatic artery. This procedure yields excellent and purely arterial contrast enhancement of hypervascular hepatic tumors. It is mostly performed in surgical candidates with liver cirrhosis and suspected hepatocellular carcinoma. nowadays often in conjunction with spiral CTAP. With a catheter in the hepatic artery. 7S ml of 1: 1 diluted contrast medium (l.e .. ISOmgll ml) is injected at a rate of 2-3 mlls. Scanning is initiated after 3-10 seconds using a technique that covers the entire liver in one breath hold. Replaced or aberrant hepatic arteries and perfusion abnormalities (cirrhosis) may give problems in the interpretation of the scan. Addition of a delayed phase (biphasic CTHA) is used for improved differentiation of tumor versus perfusion abnormalities. Injection of vasodilators like papaverine. Priscoline or prostaglandin Et has been reported to increase portal flow and to enable more confident differentiation of true le-
sions from pseudolesions due to systemic venous inflow tracts. The advent of multislice scanning probably makes the technique obsolete, because proper timing of the multislice scan and use of increased flow rates can reach high and almost selective arterial enhancement of the liver. CT during Arterial
Portography
(CTAP)
CTAP utilizes the principle of indirect splenic and mesenteric portography. It can detect hepatic lesions with high sensitivity by the selective delivery of contrast medium to the portal venous system (see Fig. 11.14). CTAP is used in the planning of partial hepatic resections (to exclude tumors in residual liver segments), but cirrhosis and large tumors can limit its effectiveness by altering portal venous hemodynamics. Spiral CT during arterial portography offers a detection rate of well above 90% for lesions ss 1 cm in diameter. For lesions smaller than \ cm, results are poorer. Specificity of CTAP is very limited, however, because all types of lesions appear hypoattenuating on these scans. Apart from biphasic multislice CT, MRI with liver-specific contrast agents (SPIO) has been shown to be equally accurate and more specific, and may replace CTAP in the future. The technique involves injection of ISO200ml of normal or 1: 1 diluted (I.e., ISO300 mg lIml) contrast medium at 3-S mlls through catheters selectively placed in the superior mesenteric artery, splenic artery, or both. Some authors claim better results with splenic artery injections. After a delay of 20-60 s spiral or multislice CT of the entire liver is performed during a breath hold. For improved characterization of lesions, biphasic studies during the true portal phase after 20s and late (recirculation) phase after 60s can be performed. Its specificity is still limited, however. The late scan is useful for distinguishing perfusion-related artifacts from lesions. Small rounded lesions that are conspicuous during the portal phase but not in the late phase are interpreted as neoplasms. Combination with CTHA is often performed for improved characterization, but there is controversy about which technique should be used first. As in CTHA, injection of va so dilating agents can enhance portal venous flow, leading to better consptcutty of small lesions and differentiation from pseudolesions.
11 The Liver
CTAP is a completely nonspecific method of lesion detection (except for cysts). Low-attenuation artifacts further reduce its specificity (Fig. 11.8).These artifacts may be caused by laminar flow in the portal vein (poor mixing of splenic and mesenteric blood) or by the compression of portal venous branches. Aberrant portal vessels that drain directly into the liver (e.g., the left gastric vein, the gallbladder veins) can mimic malignant lesions in segment IV or in the gallbladder bed. Compression of the liver parenchyma by ribs may cause hypoperfusion artifacts as well (see Perfusion Abnormalities, below). •
Lipiodol CT
Lipiodol CT is a technique that offers high specificity but only moderate sensitivity in the detection of HCC. Best results are achieved with hypervascular HCC tumors, especially those that are larger than 2 cm. In other cases, sensitivity is poorer. Prior to the CT examination, approximately 5 to 10ml of Lipiodol, an oil-like iodinated substance, is injected into the proper hepatic artery through a catheter under angiographic guidance (taking care to include accessory or aberrant hepatic arteries). Some 2-4 weeks later, a CT examination of the liver is performed without further administration of intravenous contrast material. By this time the marked uptake of Lipiodol by HCC nodules will contrast sharply
a
b
Fig. 11.8 Perfusion artifacts in CTAP may have a typical wedge-shaped appearance distal to a tumor with compression of portal venous branches (a), or they may be
with the very low Lipiodollevels in normal liver parenchyma. Hyperattenuating nodules that show essentially homogeneous high density are considered pathognomonic for HCC (see Fig. 11.34). A homogeneous high density pattern is not seen with all HCCs. Other hypervascular tumors may show nonspecific Lipiodol uptake. Spotty uptake of Lipiodol in the periphery of a lesion does not positively identify the lesionasHCC. Note that Lipiodol is a contrast agent for lymphography that is not approved for intraarterial injection in many countries [off-label use}.
Perfusion CT
Perfusion CT or Single-slice dynamic CT has (re) gained increased popularity for studying liver perfusion. Single-level CT scanning is performed while a bolus of 40-80 ml of contrast medium is administered by fast intravenous bolus injection (5-10 mlls) followed by a saline chaser-bolus (e.g., 40ml). Scans are started at 10-15 s after the begin of the injection and are acquired every 1 to 4 seconds for 60-120 seconds, then at larger intervals for up to 5 minutes after the start of the injection. Perfusion quantification relies on using slope measurements or
c caused by veins that drain directly into the liver, e.g., gastric veins anteriorto the portal vein in segment IV (b) or gallbladder veins into the gallbladder bed (c).
Perfusion CJ compartmental modeling. Several indices (transit time. hepatic perfusion index. peak perfusion values. distribution volume) can be measured and may be displayed in parametric images. To minimize radiation exposure. an 80120%). pancreas (20%). breast (approximately 10%). and lung (approximately 10%). Metastasis can occur anywhere in the liver. but for unknown reasons the right lobe is more frequently affected. The great majority of hepatic metastases are multifocal (>90%). Although the neovascularity in malignancies is derived primarily from hepatic arterial branches. from an imaging standpoint liver metastases are commonly described as either hypovascular or hypervascular, relative to the surrounding parenchyma (Table 11.9). This will guide the most optimal CT technique to be used. Table
3. Tumor Show all lesions
• Segmental •
Size
Ascites factors
Central tumor location ; •
Compression,
thrombosis
or infiltration
hepatic veins, portal vein, hepatic • Bile duct obstruction •
Growth
6.
Prognostic criteria Distant metastases
•
Regional lymph node metastases Number of intrahepatic lesions complicating factors (see Point 5)
4. Functional status of the rest parenchyma Volume of rest parenchyma Signs of cirrhosis •
11.9
of IVC
hyperhyper-
Sarcoma
arteries
j
Hypovascular
lesions
Adenocarcinoma (Gltract. lungs) Breast carcinoma Squamous cell carcinoma (head and neck, anus) Lymphoma
lungs,
437
11 The Liver Most liver metastases are hypovasciilar and for these metastases it is generally accepted that scanning during the portal venous phase is optimal for tumor detection. Routine use ofbiphasic CT is not indicated, but addition of the arterial phase may be beneficial in selected cases such as the depiction of the hypervascular rim for characterization in colorectal metastases. It must. also be noted that independent of the hypo- or hypervascular nature of the primary tumor, in small lesions ( 1cm) the primary blood flow is supplied by the hepatic artery and arterial phase CT may have benefit in improving detection. Thus, it may be beneficial to use biphasic CT in the initial examination or before surgery. Also, liver perfusion studies showed higher risk for patients with increased arterial flow patterns, so that early perfusion imaging may be integrated with portal venous morphologic imaging. Delayed imaging is beneficial for differentiation against benign lesions. Metastases from hypervascular tumors, such as renal cell carcinoma, neuroendocrine tumors (islet cell, carcinoid), thyroid carcinoma, melanoma, sarcoma and breast carcinomas are hypervcisciilar (Table 11.10). However, hypervascular primaries also produce metastases that are hypovascular in comparison with the liver. Multiple studies have shown that biphasic CT is the recommended imaging technique for optimal detection and to assess tumor recurrence in hypervascular lesions treated with chemother-
apy. Although breast carcinoma metastases can be hypervascular, studies did not show added benefit of routine use of biphasic CT over portal phase CT. Routine use of the noncontrast phase is controversial. Some studies in melanoma or hypervascular metastases showed its use to be equivalent or better than arterial phase imaging when added to portal phase imaging and it has also been advocated as a third phase in carcinoid tumors. Other groups have advocated the addition of delayed imaging for improved characterization of lesions with rapid wash-out. For most optimal characterization, unenhanced plus biphasic (or even triphasic) CT is probably the best option, especially for the initial CT examination. Dynamic MRI is beneficial in (inhomogeneous) fatty livers and for the discrimination of small hemangiomas versus hypervascular metastasis. In presurgical evaluation, optimized detection can be achieved with angiographically assisted techniques at the expense of specificity. For hypovascular metastases, CTAP is used (Fig. 11.24), while in hypervascular metastases a combination of CTAP and CTHA gives the best results. One has to keep in mind, however, that pathologic studies have shown that for every metastasis > 1 cm there are on average 1.4 metastases •.; 1 cm in patients with colorectal cancer and up to 4 metastases - 1 cm in patients with other tumor entities. Thus, imaging modalities will only detect part of the total.
Table 11.10 Detection rates of multiphasic he Study
Metastases Fig. 11.24 Compared with portal phase conventional CT (a), CTAP (b) demonstrates significantly more small metastases from a colorectal carcinoma. Note the lack of enhancement of the inferior vena cava in CTAP (arrow).
a
•
CT Morphology
On iinenhcinced CT, a substantial numbers of metastases are indistinguishable from normal liver. Most. however, are hypoattenuating to the liver parenchyma. Lesions may become isoattenuating and may be missed if there is slight fatty infiltration of the liver. Greater fatty infiltration can reverse the usual density pattern, causing metastases to show higher attenuation than the surrounding liver (see Fig. 11.6a). Small lesions are nodular and homogeneous, while larger lesions are more irregular, heterogenous and with ill-defined margins. Calcification is common in mucinous CI tract carcinomas and in chemotherapeutically treated hypervascular lesions (e.g., carcinoid, islet cell tumors). After contrast administration, metastases from hypovasciilar tumors may have an enhancing rim that can be seen during the arterial phase and occasionally during the portal phase.
b
This 'target' sign is considered pathognomonic (Fig. 11.25). Lesions that do not have an enhancing rim show little if any enhancement during the arterial phase and become hypo attenuating during the portal phase. Hypervascular metastases show moderate to intense enhancement during the arterial phase (Figs. 11.26. 11.27) that may persist in the portal phase. However, most show a rapid wash-out and become isodense or hypodense to surrounding liver. Lesions with central necrosis also show the typical "target" appearance. The quicker the examination is performed during the desired phase of enhancement, the better the resul t in terms of lesion detection and discrimination. If scanning is performed too late, the detection rate will be lower because of the diffusion of contrast medium into the lesion ("vanishing" lesion).
Fig. 11.25 Hypovascular metastasis from gastric carcinoma, (a) A faintly enhancing rim is visible during the arterial phase, (b) The lesion is most conspicuous during the portal phase, and a second lesion is also seen (arrow).
a
b
439
11 The Liver Fig. 11.26 Recurrent hypervascular metastases from medullary thyroid carcinoma in a patient after a right hemihepatectomy. Most of the lesions are visible only during the arterial phase (a), but some lesions are not visualized until the portal phase (b).
a
b
a
b
c
Fig. 11.27 Metastases from a carcinoid tumor appear iso-, hypo- and hyperattenuating on the noncontrast scan (a). During the arterial phase (b) and portal phase
(c) a typical variegated pattern is seen, consisting of cystic areas, hypervascular lesions (arrows), and hypovascular lesions.
Detection rates (per lesion basis) of recent studies that specify numbers are summarized in Table 11.10. It must be noted that many studies include focal liver lesions in general, the accuracy in small lesions 1 cm is often considerably lower, and results on a per patient basis are generally better. MRI with liver-specific agents (SP10) often shows better results than helical CT and has a sensitivity equivalent to CTAP but an improved specificity. Thin-section multislice CT or the combination of SPIO with Gel-enhanced MRI may improve techniques even further.
retain their hyperenhancement during the portal venous and delayed phase. Differentiation from FNH may be difficult when the FNH does not show a central scar. Hypovascular metastases can be distinguished from focal fatty infiltration by their different enhancement pattern: metastases tend to become isoattenuating to surrounding liver over time. Necrotic lesions with rim enhancement require differentiation from abscesses, which have a center with lower attenuation, and from cholangiocarcinoma. In all problematic cases, delayed imaging often helps for differentiation.
•
Differential Diagnosis
Hypervascular metastases are to be differentiated from small hemangiomas which usually
Hepatocellular Carcinoma (HOC) 441 Metastases from very hypervascular tumors such as carcinoid, renal cell carcinoma, thyroid carcinoma, and insulinoma may be most conspicuous on noncontrast CT scans, The tumor mass may be slightly underestimated on contrast-enhanced scans, Postcontrast scans of carcinoid metastases often show a heterogeneous mix of cystic components with hypovascular and hypervascular regions (Fig, 11.27),
Hepatocellular Carcinoma (H CC) Hepatocellular carcinoma (HCC) is one of the world's commonest malignancies and the most common primary hepatic tumor, accounting for 80-90% of cases, In children, HCC is the second commonest tumor after hepatoblastoma. The incidence shows considerable geographic variation, mirroring that of chronic hepatitis Band C The annual incidence is low in most of Western Europe, the USA and Australia (1-3/100000), intermediate in Spain, Italy and Argentina (S-IOI 100000), high in Japan and Greece (10-301 100000) and very high in Southeast Asia, China, and sub-Saharan Africa (30-lS0/100000), Male to female ratio is 4-8: 1 and peak occurrence is in the fifth to seventh decades, In patients under 30, the tumor is SOO-fold more common in areas like Mozambique as compared to USA and Europe, Hepatic cirrhosis is the greatest risk factor and underlies HCC in 60-90% of cases, The annual incidence in cirrhotic patients is about 3-S%, While in the USA and Europe the most common under lying cause is alcohol, in Asia and Africa it is hepatitis B or aflatoxlns. Hepatitis B, endemic in Southeast Asia, China, and subSaharan Africa, is a well established risk factor, both in patients with a history of HBV and in chronic carrier states, Hepatitis C, endemic in Southern Europe and Japan, increases risk by promotion of cirrhosis but carriers are not at increased risk, Synergistic effects with hepatitis B have been described, The risk is also increased in persons exposed to chemical carcinogens (aflatoxin, cycasin, Thorotrast), and in patients with metabolic disorders (hemochromatosis, Wilson's disease, tyrosinosis, alpha-l-antitrypsin deficiency, etc.).
HCC may arise de novo in a noncirrhotic liver, but the majority of HCC in cirrhotic livers are believed to arise in a stepwise fashion: regenerating nodule-low-grade dysplastic nodule-high-grade dysplastic nodule-dysplastic nodule with focal HCC-small HCC and (large) HCC (Table ILl-I), Histologically, HCC are graded from well-differentiated to poorly differentiated according to Edmondson, Along with progression of the tumor, nodular lesions show a change in blood supply, Regenerative nodules and dysplastic nodules with or without foci of HCC have a predominant portal blood supply, while (small) HCC has a preferential arterial supply, There is, however, a substantial percentage of small HCCs in which portal supply predominates, Small HCC are usually nodular and can be divided in four types: Single nodular, nodular with extranodal growth, contiguous multinodular, and poorly demarcated nodular types, Large HCC are classified as expansive nodular, infiltrative, and diffuse types, Multinodular forms may consist of multiple small nodules or present as a dominant mass with small, daughter nodules, Well-differentiated tumors are frequently expansive, nodular and enclosed in a fibrous
Table
11.11 Terminology (lWP, 1995)
of hepatocellular
nodules
cirrhosis
New tenninology
Old terminologies
Regenerative nodule
Regenerative
Dysplastic
nodule,
low-grade
nodule
Macroregenerative Adenomatous
Dysplastic
nodule,
type I nodule,
high-grade
hyperplasia
Macroregenerative
nodule,
type II Adenomatous hyperplasia with atypia Borderline hepatocellular
lesion
Dysplastic nodule with focus ofHCC
Macroregenerative
nodule
with microscopicHCC Adenomatous hyperplasia with microscopicHCC
Early HCC Small
HCC «2
HCC(>2cm)
em)
Adenomatous hyperplasia with macroscopic HCC Early advanced HCC Advanced Hepatoma
HCC
in
77 The Liver
capsule. These encapsulated forms are found much more freq uently in Asian than in nonAsian populations and have a better prognosis. Large tumors tend to necrose or bleed centrally but no central fibrosis is present. HCC has a tendency for vascular invasion. more frequently in the portal vein than in the hepatic veins. Biliary invasion is seen less frequently. Lymphogenous spread occurs in the following order of frequency: porta hepatis > hepatoduodenal ligament> pancreatic bed > mesenteric root> mammary lymph nodes. A large tumor may directly invade the diaphragm. abdominal wall. or pancreas. Hematogenous metastasis can occur to the lung « 10%). peritoneum. adrenal glands. and bone. HCC is often diagnosed in advanced stages. with patients complaining of right upper quadrant pain. mass. fever. and weight loss. Asymptomatic cases can be diagnosed by screening risk populations with ultrasound and alphafetoprotein (AFP) measurements. AFP is elevated in 60-70% of HCC in USA and Europe and in 80-90% of HCC in Asia. False positives may be due to chronic hepatitis. cirrhosis. and pregnancy. Clinical staging is done by multiphasic CT or MRI using the TNM-classification (Table 11.12). Prognosis depends on the clinical stage. tumor histology. grade of cirrhosis (Child-Pugh classification) and patient's general condition (Karnofsky scale). Curative therapy is reserved for selected patients with small tumors and good hepatic function. The five-year survival rate is in the order of 30% after curative surgical therapy. Multiphasic CT has a role in the detection and staging of tumors, the exclusion of multifocal disease, and to select candidates for curative surgery. For planning surgical or embolotherapy, precise description of arterial variants helps facilitate these interventions. For confirmation of the diagnosis, MRI or angiographically assisted techniques like CTHA, CTAP or Lipiodol CT may be used. The Asian variant of HCC appears to differ from the Caucasian, tending to show typical patterns of Lipiodol storage more often than the variant seen in Caucasians. Nowadays, with improvements in CT and MRI technology, these invasive techniques are used less frequently and may be reserved for cases amenable to curative therapy. In surgical candidates, needle biopsy of HCC is controversial due
2 em in greatest dimension, without vascular invasion, or multiple tumors limited to one lobe, none >2 em in greatest vascular invasion T3
dimension,
without
Solitary tumor >2 em in greatest dimension, with vascular invasion, or multiple tumors limited to one lobe, none >2 em in greatest dimension with vascular mvasron. or multiple tumors limited to one lobe, any more than 2 cm in greatest dimension with or without vascular invasion
T4
Multiple tumors in more than one lobe, or tumor(s) involve a major branch of the portal or hepatic vein(s), or tumor(s) with direct invasion of adjacent organs otherthan gallbladder, or tumor(s) with perforation ofthe visceral peritoneum
N1
Regional
lymph
duodenal M1
Distant
node metastasis
(hepato-
nodes)
metastasis
at locations ligament)
outside
(including
lymph
nodes
the hepatoduodenal
to the risk of bleeding, tumor rupture, or peritoneal metastasis. Candidates for palliative therapy may undergo fine needle aspiration cytology. CT is also the primary diagnostic modality in the follow-up of patients after surgery or ablative therapies. •
CT
Morphology
The CT features of hepatocellular carcinoma are highly variable, depending on the size, vascularity, histologic composition, and growth pattern of the tumor. It is generally accepted that CT with images in the arterial and portal venous phases are the most useful for detection. Few tumors may only be seen on unenhanced scans. Delayed imaging may be helpful for characterization in hypervascular HCC. On unenhanced CT scans, most HCC present as solitary or multiple hypo- to isoattenuating masses. Calcification is seen in 5-10% of cases. Areas of necrosis or fatty metamorphosis appear as hypoattenuating foci, but fatty metamorpho-
Hepatocellular Carcinoma (HOC)
a
b
Fig. 11.28 Typical CT features of HCC. A heterogeneous hypervascular lesion (a) that may contain fat (arrow) and may show delayed enhancement in a tumor
sis is rare in non -Asian populations. In the presence of diffuse steatosis, the tumor may be undetectable or even slightly hyperattenuating. In small HCC, the nod ular type is often sharply demarcated from the liver and has a tumor capsule in 50% of lesions. This is often thin and may be difficult to see at CT. The other types show nodularity with ill-defined and irregular margins. In large HCC, the expansive nodular type is sharply demarcated. It is encapsulated in 70-80% and shows typical internal mosaic architecture. Infiltrative types are often segmental, heterogeneous, and with ill-defined margins. Strands of tumor frequently invade portal vein branches. The diffuse type is the least common and shows numerous small nodules scattered throughout the liver. All large HCC have a tendency for developing satellite nodules that represent intrahepatic metastases. In hypervasciilar tumors, arterial phase CT demonstrates feeding vessels and intense, enhancement throughout the tumor (Fig. 11.28). Larger tumors often are heterogeneous due to necrosis and hemorrhage. Especially with multislice CT, tiny hypervascular nodules may be detected (Fig. 11.29). If present, the capsule remains hypoattenuating. During the portal venous phase, HCCs wash out rapidly and become isodense to hypodense. relative to normal liver. Larger tumors are generally heterogeneous (mosaic pattern). On delayed phase images, the capsule and fibrous septa are relatively hyperattenuating and show prolonged enhancement
c capsule (b) ortumors associated with portal vein thrombosis (c) are suggestive of HCC. Note the contrast uptake in the tumor thrombus (arrows).
Fig. 11.29 Small hypervascular lesions in a cirrhotic liver (arrows) are characteristic of HCC. Such lesions are frequently multifocal and can be seen only in the arterial phase (here: 4 x 1/6. VRT).
while the tumors are iso- to hypoattenuating to the liver (Fig. 11.28). The diffuse type may not be detectable by imaging studies (including CTAP). and many HCCs are very poorly visualized wi thin a cirrhotic liver. Hypovascular tumors are not uncommon and usually represent early stages like dysplastic nodules with focal HCC orwell-differentiated small HCCs (Fig. 11.30). Often they are best seen on noncontrast scans. They show l ittle if any enhancement during the arterial phase. They may also be poorly visualized on later scans, with isodense to hypodense appearance depending on their portal blood supply.
443
11 The Liver
a
b
Fig. 11.301 HCC nodules are usually hypervascular (arrows) and ofteru only visible during I thearteriall phase (a) .. Hypovascular lesions" which are seen more' clearly during the portal phase, are less common.
Thrombosis of portal venous branches occurs in up to 40% of HCCs and is frequently caused by direct tumor invasion (Fig. 11.31). These tumor thrombi are only moderately hypoattenuating and enhance irregularly after contrast administration, especially in the arterial phase. Tumorous arterioportal shunts, which are characteristic of HCC, show early enhancement of intrahepatic portal branches on arterial phase images (Fig. 11.32). They may be associated with transient perilesional segmental or wedge-shaped areas of hyperenhancement (transient hepatic attenuation differences, THAD, Fig. 11.32). Large, proximal shunts can
a
b
a
b
Fig. 11.31 Tumor thrombus in the portal vein may demonstrate marked arterial enhancement (a) and become hypoattenuating in the portal venous phase (b).
Fig. 11.32 Hypervascular HCC withiai centrall hypodensitymimicking FNH. HCCis.highly likely becauseofarterio-porta] shunting, .which can Ibediagnosedl from uransient hyperattenuation of the perilesional liver parenchyma (arrowheads) (a) or from locally increased enhancement of peripheral portal branches despite substantially less enhancement of the central portal branches (arrow) best demonstrated on a axial thin-slab MIP(4x1/6)(b).
significantly alter perfusion and may lead to imaging features of portal hypertension with esophageal varices. The invasion of hepatic veins (approximately 15% of cases) is less common and may cause occlusion leading to a Budd-Chiari syndrome (see Fig.ll.55c). This occlusion leads to increased perfusion of liver parenchyma and lower attenuation of HCC in arterial dominant phase. Spontaneous rupture ofHCC «10%) is
c Note the hypotrophy of the right and hypertrophy of the left liver lobe. Liver vein infiltration (arrow) can progress to tumor thrombus in the vena cava (c).
Hepatocellular Carcinoma (HOC) 445 creased arterial flow and occurrence of pseudolesions. Detection rates (per lesion basis) of recent studies summarized in Table 11.13.The Peterson study is the only one with explantation results for both positive and negative imaging examinations and thus reflects the true sensitivity in screening situations. It must be noted that many studies deal with focal lesions in general; the accuracy in small lesions < 1 cm is considerably lower and results on a per patient basis are generally better. •
Differential
Diagnosis
In cirrhotic livers, typical hypervascular tumors with capsule and mosaic architecture represent Fig. 11.33 Spontaneous rupture of an HCC (arrow) HCC. If atypical findings are found, differential causes intraperitoenal hemorrhage (a). Fresh extravasadiagnosis includes hemangioma and metastasis, tion can be appreciated if bleeding is active (b). Adenoma, large FNH, or hemangiomas may cause similar but hypervascular metastases in this clinical setfindings. ting are rare. Tumors in non cirrhotic livers are more advanced and differential diagnosis is more difficult. HCC should be distinguished life-threatening and requires emergent treat- from metastases and other rare primary liver ment. It is manifested by hematoma (Fig. "11.33), tumors but also from benign lesions like FNH, lesion surface discontinuity, or separation of HCA or hemangioma. Even needle biopsy is tumor content from a peripheral enhancing rim often unrewarding, since the cytologic and histologic features may vary within the lesion and that may rupture intraperitoneally ("enucleaare very similar for all four diagnoses. Lipiodol tion sign"). Imaging accuracy is often more limited in CT may be helpful in larger tumors (Fig. 11.34). the cirrhotic liver because of architectural distortion with numerous regenerating nodules as well as altered portal hemodynamics with ina
b
Table 11.13
Detection rates of multiphasic
H E3i~/""'j"""j"""j"f
!
helical CT and CTAP/CTHA in the diagnosis
1'1'1'111IU11~~,~~,~:ij~'jIlH'JI!~M1.J11:Jpliasi<MUltiphasicCT AngiographicallyassistedCT'" ANI
The role of CT is tumor staging and the presurgical evaluation of resect ability. •
Fig. 11.34 Lipioclol CT shows homogeneous retention of the multifocal recurrences of HCC the metal clips following the nodule.
pathognomonic, largely oily contrast material in in an Asian patient. Note resection of a solitary
Fibrolamellar Carcinoma (FLC)
Fibrolamellar carcinoma is a rare malignant neoplasm that is now seen as a separate entity. It accounts for 2% of all malignant liver tumors and there are no known risk factors. It is seen in young patients (5-35 years) without cirrhosis and there is no male preponderance. AFP levels are normal, but other markers like vitamin 812 or copper-binding proteins may be elevated. Histologically, FLC is a large mass made up of lamellar fibrous strands that separate markedly eosinophilic hepatocytes in cords. Centrally, a stellate fibrous scar is present. Hemorrhage and necrosis are infrequent features. FLC has a more favorable prognosis and is resectable in 50-75% of patients. Five-year survival is in the order of 60%.
CT Morphology
On noncontrast CT, FLC usually presents as a large, solitary, well-defined mass measuring 4-17 ern, but satellite nodules may be present. The tumor has lobulated contours and is usually hypo attenuating relative to liver. Centrally, a more hypo attenuating stellate central scar can be visualized that shows nodular or stellate calcifications in 55% of cases. After contrast administration there is moderate to marked enhancement of the peripheral part of the tumor, becoming iso to hyperdense compared to liver (Fig. 11.35). There is no enhancement of the scar, but contrast may slowly diffuse into the scar giving hyperattenuation on delayed phase images. Intrahepatic ductal dilatation and portal vein invasion may be seen, but the latter is much less frequent than in HCC. Cholangiocellular Carcinoma (CCC)
The rare intrahepatic (peripheral) cholangiocarcinoma accounts for 10% of primary hepatic tumors and is the second commonest tumor after HCC. Cholangiocarcinomas should not be confused with bile duct carcinomas (extrahepatic Cholangiocarcinomas), and comprise only 10% of all malignant bile duct cancers. Risk factors include a history of Thorotrast administration, primary sclerosing cholangitis, congenital fibrosis, and infection with Clonorchis sinensis (Chinese liver fluke). Histologically, the tumor has a large central core of fibrous tissue with adenocarcinoma cells
Fig. 11.35 Fibrolamellar carcinoma (FLC) is hypervasrular, similar to HCC (a). It may contain fat and is characterized by a central scar that not always contains calcifications (b).
a
b
Cholangiocellular Carcinoma (CCC)
a
c
b
Fig. 11.36 Varying CT appearances of cholangiocarcinoma: (a) diffuse type with disseminated nodules (may have the same appearance as HCC or metastases); (b) nodular tumor with peripheral hypervascularity (differ-
ential diagnosis: metastases); and (c) nodular hypovascular tumor (differential diagnosis: metastases). Fibrous retraction of the hepatic capsule (arrowhead) and small calcifications (arrows) may be observed.
at the periphery. The tumor frequently produces mucin. A nodular type (95%) is distinguished from a diffuse type (5%). As these tumors are usually clinically silent, most are diagnosed at an advanced stage. CT is used for preoperative staging of the nodular type; the diffuse type is difficult or impossible to detect with imaging procedures. •
CT Morphology
Cholangiocarcinoma appears on noncontrast CT scans as a large round or oval mass of low attenuation with irregular margins. Satellite lesions are common (65%) but often are poorly visualized. Centrally, stippled areas of hyperattenuation represent small mucinous substances. Irregular or punctate calcifications may be seen (in 20% of cases). Capsular retraction is typical but is not often visible. Mild, segmental bile duct dilatation is a common finding. Most tumors are hypovascular and show mild rim-like enhancement at the periphery (Fig. H.36) in both arterial and portal venous phases. Progressive tumoral contrast enhancement in the late delayed phase (W-15 minutes) is typical and can aid in detection and characterization (Fig. 11.37). Portal venous invasion is frequent. Some 50% of tumors show hyperenhancing THADs during the arterial phase because of an increased arterial flow to the liver segment.
Fig. 11.37 On interstitial phase images, some cholangiocellular carcinomas (CCC) demonstrate delayed enhancement.
•
Differential
Diagnosis
CCC have to be differentiated from metastases, most commonly of adenocarcinomas. This is not possible by imaging techniques but rather requires histologic proof. Capsular retraction and coarse calcifications may also be found in epithelioid hemangioendothelioma or in metastases from colorectal cancer.
11 The Liver
Hepatoblastoma
Hepatoblastoma is the commonest malignant hepatic tumor in children (>50%). It is most prevalent in children under 3 years of age but may affect children up to age 15. It is more common in patients with Beckwith-Wiedemann syndrome, Wilms' tumor, glycogen storage diseases, and Meckel's diverticulum. The tumor is composed of tissue that resembles fetal hepatocytes and mesenchymal cells. Pure epithelial types are homogeneous, while the more common mixed type may have areas of hemorrhage, osteoid, fibrosis, or cartilage. Alpha-fetoprotein levels are markedly elevated in two-thirds of patients. •
CT Morphology
Unenhanced scans show a solid, hypoattenuating mass that may occupy large portions of the liver. Around 20% are multifocal. The mixed type frequently contains necrotic areas, fibrotic bands, and coarse calcifications (30-50%). Arterial and portal phase images show a hypervascular mass that is often heterogeneous (Fig. 11.38). After chemotherapy, nodules may demonstrate hemorrhagic necrosis (Fig. 11.39). •
Fig. 11.38 Hepatoblastoma, appearing as a hypervascular mass containing areas of necrosis, in a 5-yearold boy.
Differential
Diagnosis
The differential diagnosis includes infantile hemangioendothelioma (fine granular calcifications, multiple hemangioma-like vascularized foci), metastatic neuroblastoma, mesenchymal
a Fig. 11.39 Hepatoblastoma in a 15-year-old girl after chemotherapy. There are multiple necrotic and hemorrhagic tumor nodules (arrows) in segments II to IV (a). A coronal reformation of the portal phase scan demon-
hamartoma (multiple cystic spaces of varying size and density, moderate contrast enhancement), and fibrolamellar HCC (age> 5 years). Undifferentiated Embryonal Sarcoma
Undifferentiated embryonal cell sarcoma is a rare malignant mesenchymal tumor found in older children and young adults (90% of patients are under age 15, with rare cases occurring up to age 30). It is a large solitary mass composed of spindle-shaped sarcomatous cells packed in whirls around a myxoid ground substance, and may contain hematopoietic foci. It is believed to
b strates an additional nodule (arrowhead) in segment VII that was treated by wedge resection (b). The enlarged hypervascular lymph node (wide arrow) proved to be hyperplastic at histology (c).
Epithelioid Heinangioendothefjoma
be the malignant variant of mesenchymal martoma. •
ha-
CT Morphology
CT scans usually show a very large mass (1025cm) located predominantly in the right lobe of the liver (75%). A near-water attenuation is typical (Fig. 11.40) and can be found not only in regions of necrosis and intratumoral hemorrhage but also in areas of solid tumor. The mass is sharply demarcated from surrounding liver by a fibrous pseudocapsule. The tumor parenchyma in the periphery of the lesion may show intense enhancement on arterial phase images, and there typically are hyperattenuating areas within the hypo attenuating central tumor portions (on CT very similar to cystic renal cell carcinoma) . •
(EHE)
Differential Diagnosis
Mesenchymal hamartoma may be distinguished from undifferentiated embryonal sarcoma by its enhancing soft -tissue component and lack of a pseudocapsule. Rhabdomyosarcoma may show extensive tumor necrosis. Epithelioid Hemangioendothelioma (EHE)
EHE is a rare solid vascular tumor which nowadays is considered to be a separate entity with low-grade malignant potential. Most affected patients are women between 20-40 years of age in whom the tumor is usually discovered incidentally. The tumor begins as multiplenodules ofepi thelioid -appearing endothelial cells that coalesce into large masses that are located in the periphery of the liver. Due to slow
Fig. 11.40 Non-d ifferentiated embryona I sarcoma is hard to distinguish from a cystic mass. However, even the cystic appearing portions are usually solid.
growth, compensatory enlargement can be seen in the non-affected liver parenchyma. •
CT Morphology
Noncontrast CT shows multiple well-defined, hypo attenuating lesions. Calcifications are frequently present. The lesions may have coalesced into larger tumors. In a subcapsular location retraction phenomena can be seen. After contrast administration, the hypervascular peripheral portion may enhance to the same amount as surrounding liver tissue. If an outer avascular zone is present, a characteristic target appearance can be seen (Fig. 11.41). When absent, contrast -enhanced CT may underestimate the tumor size. For this reason, noncontrast scans provide better tumor conspicuity and evaluation of tumor extent and should always be included in the work-up.
Fig. 11.41 Epithelioid hemangioendotheliomas present as a rnultifocal. mostly subcapsular hypovascular lesions.
a
b
77 The Liver
Angiosarcoma
Angiosarcoma is an extremely rare primary vascular hepatic tumor derived from endothelial cells. It is more common in adults that received the contrast agent Thorotrast in the years 19281950 (latent period > 15 years), and among patients with an exposure to inorganic arsenic or polyvinylchloride (latent period >4 years). An association with hemochromatosis and Recklingshausen's disease is also known. The tumor is highly aggressive and metastasizes early to the lung, spleen, and regional and peritoneal lymph nodes. Hemorrhagic ascites may occur. Median survival is only 6 months. •
CT Morphology
Unenhanced CT shows multiple or occasionally solitary hypoattenuating lesions with cystic and hyperattenuating components caused by necrosis and hemorrhage. After Thorotrast, strongly hyperattenuating nodular deposits may still be seen in the liver or spleen. Angiosarcomas tend to be very hypervascular and may be scattered throughout the liver (Fig. 11.42). Marked and prolonged peripheral enhancement is usually seen, without complete fill- in towards the center. It is very common to find invasion and tumor thrombosis of the portal vein and hepatic veins.
Angiosarcoma is the only tumor in adults besides cavernous hemangioma that may show peripheral lacunae after intravenous contrast administration. This possibility should be considered in lesions that show a marked increase in size over time. Necrotic and hemorrhagic areas and vascular involvement are helpful in suggesting the correct diagnosis.
Other Mesenchymal Tumors
Other sarcomas are even rarer, and include leiomyosarcoma or malignant fibrous histiocytoma. CT is nonspecific and shows large hypodense masses without calcifications. There is heterogeneous peripheral contrast enhancement. Lymphoma
Primary hepatic lymphoma is very rare and hepatic lymphoma is usually seen in association with systemic disease, both Hodgkin's disease and NHL. In advanced stages these are commonly associated with hepatic involvement (up to 60% in stage IV). There is however a rise in incidence due to growing numbers of AIDS patients and liver transplant recipients who are also at increased risk. Diffuse infiltration of the liver is more common in secondary forms, while large focal nodular disease is the most common presentation of the primary form. •
Fig. 11.42 Angiosarcoma with typical hypervascular portions that may simulate giant hemangioma. Aggressive behavior with infiltration of vascular structures is typical (arrows).
CT Morphology
Diffuse involvement is not detectable on CT scans. Focal lesions are isoattenuating and occasionally hypo attenuating on unenhanced scans, and most are poorly visualized. Portal phase images occasionally demonstrate multiple small hypo attenuating lesions, which may have ill-defined margins (Fig. 11.43). Large lesions frequently display an irregular contour. Primary lymphomas often appear hyperattenuating on delayed scans after contrast administration (due to the diffusion of contrast into the tumor interstitium). Concomitant abdominal lymph node enlargement, if present, will help to suggest the correct diagnosis.
CCC, large fibrous tumor parts or large extracellular
» angiosarcoma,
large
space
metastases
locations
Liver hilum
Biliary cystadenoma/cystadenocarcinoma,
Subcapsular
Hemangioma endothelioma
Gallbladder Segm.IV,
well differentiated
small lesions. hypervascularized
bed
prepartal
Segm. IVb, lig. teres
Gallbladder (THAD)
> metastases, carcinoma,
focal
fatty
hematoma
fatty infiltration/sparing,
Focal fatty infiltration/sparing, Focal
bile duct carcinoma
subcapsular,
CT AP: perfusion
CTAP;
defect
(Klatskin)
» hemangioperfusion
defect
(THAD)
infiltration
Age Up to 3 years
Hepatoblastoma,
Up to 15 years
Hepatotxastoma.
FLC, hamartoma,
years
Adenoma,
FLC, FNH
Up to 40 years
Adenoma,
FNH, FLC, metastases
Up
to
infanWe hemangioendothelioma,
30
Up to 50 years 60 60
years years
FNH, metastases,
adenoma,
Metastases, CCC, HCC Metastases,
HCC, angiosarcoma
undifferentiated
epitheloid
mesenchymal embryonal
hemangioendothelioma
hamartoma sarcoma
451
11 The Liver
and the imaging features resemble lymphoproliferative disorders in other immunocompromised symptoms like AIDS-lymphoma. In PTLD however, the liver is the most common extranodal site. Apart from nodular or infiltrative disease, the liver may also show periportal lymphomatous infiltration which is unique to transplant patients. Other abdominal sites are the spleen, kidneys and small bowel. •
CT Morphology
In the liver three patterns can be seen: Fig. 11.43 Nodular hepatic involvement by NHL. Portal phase scan shows ill-defined hypoattenuating lesions that are indistinguishable from metastases. Concomitant extensive nodular involvement of the spleen suggests the correct diagnosis.
PTLD occurs at a frequency of 2-5% and is the most common malignancy in patients following solid organ transplantation. It encompasses a spectrum of disorders ranging from lymphoid hyperplasia to (non- Hodgkin's) lymphoma. It represents an unregulated B-cell proliferation which is associated with Epstein- Barr virus infection. Although lymph node involvement is common, extranodal disease is much more frequent
• single or multiple low-attenuation nodules 1-4 em in diameter • diffuse infiltration with geographic or illdefined low-attenuation, and • a porta hepatis mass with extension into the biliary tree and associated hilar adenopathy. The diagnosis can be supported if lesions at other sites are found. The spleen can be enlarged, but nodular involvement of the spleen is less frequent. In the GI tract, the distal j ej un um or proximal ileum is most commonly affected, showing circumferential wall thickening, dilatation, ulceration, or a polypoid mass. In the kidney, unilateral solitary nodular lesions are seen most commonly. Enlarged lymph nodes, which do not show central necrosis, can be found in an intraperitoneal or retroperitoneal location.
Diffuse liver Disease
Except for hemochromatosis, diffuse liver diseases are not an indication for CT scanning. However, CT performed for other indications will commonly show findings that are referable to diffuse hepatic disease. Many of these diseases are associated with an increased incidence of HCC, and (multiphasic) CT should be performed in patients with elevated alpha-fetoprotein levels or equivocal sonographic findings. Fatty Infiltration
Fatty infiltration (steatosis) of the liver can be seen in 7% of abdominal CT examinations. It may result from metabolic causes (diabetes
mellitus, obesity, congested liver) or hepatic exposure to noxious agents (alcohol consumption, steroids, chemotherapy, etc.). Rapid changes in the degree of fatty infiltration (days to months) may be observed. Nonalcoholic steatohepatitis (NASH) and acute fatty liver of pregnancy (AFLP) are special types of hepatic steatosis. NASH is frequently seen in obese women with associated diabetes or hyperlipidemia, but has also been described in other patients. The cause is unclear but accumulation of fat may lead to focal inflammation and necrosis. It has a benign course. AFLP is a rare complication of first pregnancies and may progress to liver failure with encephalopathy and coagulopathy.
Cirrhosis Fig. 11.44 Focal fatty infiltration of the liver may lead to focal tumorlike regions of hypoattenuation (a). Non-steatotic areas in a fatty liver may mimic tumor (arrows) (b). Their site coincides with artifact-prone regions in CTAP.
a
b
•
CT Morphology
Hepatomegaly is common (> 7 5%) and is associated with rounding of the liver contours that is most apparent in the left liver lobe. In normal livers, the hepatic vessels are hypo attenuating relative to the liver parenchyma. As fatty infiltration of the liver commences (10% to 15% fat, see Fig. 11.6b), the hepatic vessels become isoattenuating to the surrounding liver. As the proportion of fat increases further, the normal pattern is reversed and the hepatic vessels become hyperattenuating to the liver parenchyma (see Fig.l1.6a) . If only post -contrast scans are available, fatty infiltration can be diagnosed if the attenuation of the (contrast -enhanced) liver parenchyma remains lower than that of the paravertebral muscles. Another indicator is the enhancement difference between liver and spleen on portal phase scans. If CT numbers of the liver are more than 20 HU lower than those of the spleen, fatty infiltration of the liver is very likely (Fig. 11.44). Incipient fatty infiltration can obscure hepatic lesions on unenhanced CT scans.
Cirrhosis
Hepatic cirrhosis is a generalized response of the liver to chronic hepatocellular injury. It is characterized by an irreversible destruction of the liver parenchyma with fibrosis, steatosis, and regenerative nodules, leading to an abnor-
mal reconstitution of the lobar architecture. Among many causes, chronic viral hepatitis and alcohol are most important. Less frequent causes are metabolic (e.g., hemochromatosis, Wilson's disease, glycogen storage disease IV, aI-antitrypsin deficiency, tyrosinemia), prolonged cholestasis (primary biliary cirrhosis, primary sclerosing cholangitis), toxins, malnutrition, and hepatic venous outflow blockage. Three important morphologic types of cirrhosis are distinguished according to the type of regeneration that occurs: • micronodular cirrhosis (nodules < 3mm): usually alcohol-related; • macronodular cirrhosis (nodules > 3 mm to several centimeters): usually related to chronic viral hepatitis; • mixed type: usually the result of chronic biliary obstruction. Sometimes, a fourth type, septal or posthepatitic cirrhosis is distinguished, but most of the time this is grouped under the macronodular type. Macronodular cirrhosis has a particularly high association with HCC (incidence rate> 40%). Multiphasic CT is used in evaluating patients for possible HCC and can help describe the amount and location of collaterals in patients with portal hypertension. For the follow-up of cirrhosis per se MRI is probably the preferred modality as it can perform these same tasks, and evaluate morphology and iron deposition. •
CT Morphology
Cirrhosis slowly progresses from compensated states to decompensated cirrhosis with compli-
453
11 The Liver
cations. Early cirrhosis shows a normal liver or enlargement of the hilar periportal space anterior to the right portal vein. This is associated with atrophy of the medial segment of the left hepatic lobe (segment IV). There may also be hepatomegaly, which is frequently combined with heterogeneous fatty infiltration. With progression, morphologic changes are common and include atrophy of the right hepatic lobe and left medial segment and enlargement of the caudate lobe and left lateral segment (see Fig. 11.31).The lateral segment is preferentially enlarged in viral causes, while the caudate lobe enlargement is more often seen in alcohol cirrhosis. Regenerating nodules produce a nodular liver contour, but usually the nodules themselves cannot be seen until later stages (Fig. H.45), when they may produce mass effects and show an attenuation difference relative to the surrounding parenchyma. With atrophy, the gallbladder fossa enlarges and fills with fatty tissue. Lymphadenopathy is also commonly seen in advanced-stage cirrhosis. In advanced disease, masses of confluent fibrosis can be seen in up to 15% of patients (Fig. 11.45), especially in cirrhosis from sclerosing cholangitis. These are usually wedge-shaped hypoattenuating areas radiating from the porta hepatis, although peripheral bands or total lobar involvement may also be seen. The preferential location is the medial segment of the left lobe and the anterior right lobe. There is often capsular retraction at the liver surface. Secondary extrahepatic changes of cirrhosis include splenomegaly, portal hypertension, portosystemic collateral vessels, varices, and ascites. Postcontrast scans in later stages of the
disease show inhomogeneous arterial enhancement and decreased enhancement of the portal vein and the liver parenchyma. This can hamper the search for HCC nodules and will lead to substantial perfusi on artifacts on CTAP scans. Areas of massive hepatic fibrosis are hypoattenuating on noncontrast scans as well as in the arterial and portal venous phases. The scars only enhance in the interstitial phase due to delayed diffusion of contrast medium back into the bloodstream (Fig. 11.45).
Regenerative Nodule (RN)
Regenerative nodules are normal structural features in hepatic cirrhosis and may also be seen after partial hepatic resections, regional chemotherapy, or liver trauma. They lack a true capsule, but they do contain venous vessels and bile ducts. RNs can be small (micronodular) enlarge (macronodular), but combinations occur as well. The blood supply is invariably portal, but due to increased arterial flow they can be hyperattenuating during CTHA. 25% contain iron (siderotic nodules) and are at an increased risk for developing HCC. •
CT Morphology
Most regenerative nodules are poorly defined on unenhanced CT scans, appearing isoattenuating or slightly hyperattenuating depending on the fat content of the parenchyma. Siderotic nodules may be relatively hyperattenuating. A mass effect may be seen. The nodules may be
Fig. 11.45 Hepatic cirrhosis leads to nodular changes in the liver parenchyma (arrows) that demonstrate similar contrast enhancement to liver parenchyma. Areas of massive fibrosis are hypoattenuating on pre-contrast (a), arterial or portal phase images (b) but become hyperattenuating on delayed phase images (c).
a
b
Hemochromatosis isodense or slightly hyperdense on contrast-enhanced scans, depending on their perfusion,
Dysplastic
Nodule
(ON)
Dysplastic nodules are found in 15-25% of cirrhotic livers and are considered premalignant. They are usually somewhat larger than RNs (1020mm), can have a bulging surface and are histologically divided in low-grade or high-grade types, depending on the cellular atypia present. Blood supply is usually from portal venous branches, but a minority have an arterial supply (better seen on CTHA) , Some may harbor small foci ofHCC and these lesions may be better visible on MRI showing a typical nodule-withinnoclule phenomenon, Siderotic DN that accumulate copper and iron can be seen with MRL •
CT Morphology
On nonenhanced scans, dysplastic nodules are often poorly defined, Following contrast administration, most DN are isodense to hypodense during all phases, Portal and hepatic veins may be visible in large nodules, The minority with arterial blood supply usually do not become hyperattenuating when intravenous contrast is used, but may become hyperattenuating when angiographically assisted CT techniques are used, Additional imaging is often required to distinguish these from well-differentia ted HCC, Dysplastic gutshable HCC.
Primary
nodules are frequently from well-differentiated
Biliary
indistin(small)
Cirrhosis
Primary biliary cirrhosis (PBe) is an infrequently occurring chronic cholestatic liver disease that leads to destruction of small intrahepatic bile ducts, portal inflammation, progressive scarring and eventually to liver failure, lt occurs most commonly in middle-aged women and is associated with disorders of the humoral and cellular immune system. In 95% of patients antimitochondrial antibodies are present. Associated disorders include connective tissue diseases (scleroderma, Sjogren syndrome, CREST syndrome) and hypothyroidism, Muliple stages
can be recognized and as in other types of cirrhosis, portal hypertension and formation of esophageal varices and ascites may be found, Development of HCC is uncommon, but the risk may be increased when coexistent viral hepatitis B or C is present. Advanced disease is usually treated by transplantation within 10 years from diagnosis, and PBC represents the third most common indication for orthotopic liver transplantation in adults, CT Morphology
In early stages of the disease, the liver is normal or increased in size and has a smooth contour. Segmental lace-like fibrosis and regenerative nodules can be seen in a third of patients, Varices and ascites are frequently seen, There is associated lymphadenopathy in 80-90%, With advanced disease, this condition can not be differentiated from other causes of cirrhosis, Typically a small irregular liver, splenomegaly and features of portal hypertension are present. The role of CT is to evaluate complications of cirrhosis and to monitor progression of disease, CT is usually done with noncontrast (fibrosis better seen) and portal venous phases, For improved results in the evaluation of vascular changes, disease progression or HCC detection, an additional arterial phase (triphasic C1) can be beneficiaL The arterial phase is mandatory during a pre-transplantation work-up for evaluation of vascular anatomy,
Hemochromatosis Primary hemochromatosis is an iron-storage disorder that is inherited as an autosomal recessive trait and is characterized by abnormally increased iron absorption from the gut and abnormal reticuloendothelial cells, lt leads to the deposition of iron in hepatocytes and other parenchymal cells (liver, pancreas, heart), leading to cellular toxicity and injury, Hepatomegaly is present in 90% of patients, diabetes mellitus in 50%, The peak incidence is in the fourth and fifth decades, with a 10:1 preponderance of males, In the early stage, the iron deposition is limited to the liver and the pancreas is spared, With advanced disease, also the pancreas shows iron accumultatlon. Characteristically, the spleen is spared because it is comprised of dys-
455
11 The Liver
functional reticuloendothelial cells. The primary form may be complicated by periportal fibrosis. which may progress to hepatic cirrhosis and carries an increased risk ofHCC (up to a 30% incidence rate). Hemosiderosis or secondary (transfusional) hemochromatosis is based on an increased iron intake due to multiple transfusions (chronic anemias), excessive iron ingestion, or increased iron absorption (alcoholic liver disease). This form involves deposition in Kupffer cells and also involves the spleen and bone marrow. In later stages this cannot be differentiated from the primary form. Hemosiderosis is reserved for increased iron deposition without organ damage. CT can be used for the noninvasive (semiquantitative) monitoring of hepatic iron content and is also useful for the detection ofHCC. More and more CT is being replaced by MRI as the primary imaging modality for this indication. CT Morphology
CT shows a generalized increase. or less commonly a focal increase. in liver attenuation (CT numbers> 70HU. sometimes 100-140 HU). Hepatic vessels. especially veins. are plainly visible on unenhanced scans (see Fig.ll.6d). The iron concentration in the liver is proportional to the difference between the measured attenuation of the liver and its normal attenuation (60HLJ). This is true. however. only if the liver is free of fatty infiltration; otherwise dual-energy CT measurements (e.g .. at 80 and 140 kVp) are required. Scans in more advanced cases demonstrate cirrhotic changes. Contrast administration is necessary only in patients evaluated for HCC.
Radiation
Injury
Radiation injury (hepatitis) to the liver may occur after fractionated doses of 35 Cy and more. Onset is typically 2-6 weeks after completion of therapy and patients present with RUQ. discomfort and abnormal liver function tests. It is believed that this injury is a form offibrotic veno-occlusive disease. Most patients show a complete recovery. but some will progress to a chronic stage with marked atrophy in the radiation portal.
CT Morphology
Noncontrast CT shows a sharply delineated band of hypoattenuation due to edema or fatty infiltration. After contrast administration. there is often hypo attenuation in the arterial or portal venous phases with hyperenhancement at delayed imaging. When conformal therapy is used this region may be oval and adjacent to the treated lesion. In chronic cases the irradiated part may become atrophic. with compensatory hypertrophy of the unaffected parenchyma. When patients develop diffuse steatosis. regions of radiation injury are typically spared.
Miscellaneous
Diffuse
Liver Diseases
Sarcoidosis
In 25-75% of patients with sarcoidosis. hepatic and splenic involvement can be seen at pathologic examination. but this rarely leads to hepatic dysfunction. Most commonly the disease produces hepatosplenomegaly. Nodules. ranging from 2-20 mm in size. can be identified in 5-30% of patients. Nodules are hypodense at noncontrast CT and show no substantial enhancement. Lymphadenopathy is often present in the porta hepatis or around the celiac axis. Differentiation of nodular sarcoidosis from lymphoma can be difficult.
Amyloidosis
Amyloid can be deposited in the spaces ofDisse between the endothelial gaps of the hepatic sinusoids. Primary forms are associated with immunoglobulin disorders like multiple myeloma and secondary forms are seen in chronic inflammatory processes such as rheumatoid arthritis. Liver involvement is usually uniform. but can be regional. Hepatomegaly is commonly seen. CT findings are nonspecific. with decreased attenuation of the involved areas on plain CT that show no mass effect. Decreased enhancement with persistent hypodense areas of involved areas is rule. The diagnosis may be suggested when there is an associated lack of splenic enhancement.
Abscesses
Metabolic Storage Diseases Rare
metabolic diseases may involve liver. Wilson's disease leads to an deposition of excess copper with hepatomegaly, chronic hepatitis, and slow progression to cirrhosis, In glycogen
Infectious
storage diseases (GSD) , enzyme-defiCiencies lead to glycogen accumulation with hepatomegaly, steatosis, and eventually cirrhosis, Type I is associated with an increased frequency of hepatocellular adenomas and HCC
Disorders
Hepatitis
Pyogenic Abscess
Hepatitis can be divided in acute or chronic forms, In acute hepatitis changes are present in the intralobular portions with hepatocyte swelling, Chronic hepatitis shows a portal inflammation with additional fibrous changes around the lobules, Causes include alcohol, medications, and viral infections, Neither acute nor chronic hepatitis is an indication for CT examination, The CT features are nonspecific and often unremarkable, In active viral hepatitis CT may show hepatomegaly and periportal low attenuation, while in chronic active forms lymph node enlargement may be noted at the porta hepatis also' In some regions of the world, CT is used in screening high risk populations for HCC
A pyogenic abscess is most often seen in older patients with debilitating diseases like cancer, bacteremia, or after prior surgery, It can result from infection by several routes: biliary (cholangitis) , portal venous (intra-a bdominal sepsis), hepatic arterial (sepsis), local extension or traumatic Patients usually present with fever, pain and malaise, Escherichia coli, Staphylococcus aureus, Enterococcus sp, and anaerobes like Klebsiella or Bacteroides are frequent causative microorganisms, and 50% of abscesses are polymicrobial.
Abscesses Hepatic abscesses may have a pyogenic cause (USA, 85%) or may result from amebiasis (6%) or fungal disease (9%), Regardless of etiology, abscesses progress through three stages of evolution, Within the first 10 days there is necrosis with small areas of liquefaction, At 10-15 days necrotic debris is resorbed and liquefaction will predominate, After 15 days, only little debris is left and is surrounded by a thick fibrous wall, For this reason, the radiological appearance may vary with the stage of the abscess, A mass that is indeterminate by ultrasound can be identified as an abscess based on its CT features, Suspicious lesions can be aspirationbiopsied and, if necessary, drained under CT guidance with good results, If biliary communication is present, drainage should be extended to longer periods, Even with appropriate therapy, large abscesses may take months to resolve,
CT Morphology CT is often nonspecific, but usually shows a round or irregularly shaped hypodense mass, Pus measures some 30 HU, but CT attenuation of superinfected cystic lesions may be much less, Abscesses do not show central enhancement, and peripheral rim enhancement occurs only in lesions walled off by granulation tissue « 50%), With strong enhancement of the liver parenchyma in the portal venous phase, the granulation tissue may appear hypodense relative to the liver. Ill-defined margins before and after contrast administration (caution: partial volume effects) are considered characteristic (Fig, 11,46), Sometimes a double-target appearance is seen due to hypodense edema around the enhancing abscess wall, Abscesses can be unilocular or multilocular with incomplete septa, When abscesses are multiple, they may be clustered C' cluster sign"), Gas formation is common in Klebsiella infection and although gas is a specific sign, it is only present in a minority of cases, Nonspecific accompanying reactions are hepatomegaly, elevation of the right hemidiaphragm, pleural effusion. and basal atelectasis or basal lung infiltrates,
457
11 The Liver
c
b
a
Fig. 11.46 Hepatic abscesses, (a) Large abscess (superinfected biloma) with a moderately enhancing rim. (b) Typical abscesses with indistinct margins, (c) Multiple
abscesses with CT features patic metastases.
Amebic Abscess
sions may have densities comparable with softtissue masses. The margins can be smooth or nodular (60%) and an incomplete ring of edema is often present. Septations are visible in 30%. Associated findings in amebiasis include pleural fluid, perihepatic fluid, and retroperitoneal extension of the disease.
Worldwide, amebiasis is the most frequent cause of a hepatic abscess. It is caused by the parasite Entamoeba bistolytica, which is endemic in tropic and subtropic climates, including the southwestern USA. Abscess formation is the commonest extra-intestinal complication that occurs in less than 10% of cases, 75% of which are found in the right lobe. It is primarily treated by medical therapy (metronidazole). •
CT Morphology
The CT appearance is nonspecific. It shows a solitary, well-defined, round to oval hypo attenuating mass with an enhancing wall (Fig. 11.47).Early le-
that closely
resemble he-
Fungal Abscess
These micro abscesses occur usually in immunocompromised patients, most commonly with hematologic malignancies. Causative organisms, in order of descending frequency, include Candida albicans, Aspergillus and Cryptococcus. Biopsy is usually required for definitive diagnosis as the CT appearance is aspecific and can mimic metastasis, lymphoma, sarcoidosis, or mycobacterial infection. •
CT Morphology
CT typically shows multiple, small hypoattenuating lesions scattered throughout the liver (Fig.11.48a). Some lesions may show rim enhancement. A hyperdense center is characteristic in larger lesions and represents clustered hyphae (Fig.11.48b). Concomitant involvement of the spleen and kidneys is frequent. Sterilized lesions may undergo calcification. Fig. 11.47 Large septated amebic abscess with a small rim of contrast enhancement.
Liver Tubercu/osis
Fig. 11.48 Fungal abscesses. (a) Multiple tiny hypoattenuating lesions are typical for candid sepsis. Aspergillus abscess may present as a lesion with a target enhancement in the portal venous phase (b).
a
b
Hydatid Disease
The dog tapeworm Echinococciis granulosus is endemic in the Mediterranean countries, Russia, the Middle East, Japan, Australia, parts of Africa, and South America, where sheep-farming is common. Its eggs hatch in the duodenum, releasing larvae that enter venous vessels in the wall of the small intestine and become lodged in various capillary filters (liver> lung> peritoneum > kidneys> spleen, etc.). The liver is affected in 55-75% of cases. The larvae develop into slow-growing hyclatid cysts, which are made up of three layers: the outer pericyst of infiltrated host tissue, which forms a fibrous protective zone, the middle laminated membrane, and the inner germinal layer. These latter two form the true wall or endocyst. As the cyst matures and degenerates, small daughter cysts form in the periphery. With further degeneration the pericyst eventually ruptures and the lesion may calcify completely. Cysts are commonly classified in five types, which is useful for treatment selection. Hydatid disease may be complicated by secondary infection and cyst rupture. This rupture can involve either the biliary system, peritoneal or retroperitoneal cavities or the gastrointestinal tract, and the spill of allergenic fluid can incite an anaphylactic response. Hydatid disease does not always produce a positive serologic test (upto 85 % sensitivity). CTis valuable in the detection ofthe cysts and can accurately define the extent of disease prior to surgery orintervention. It is also used in complications or to monitor therapy. Treatment is initially medical
(mebendazole). More recently, it was shown that percutaneous treatment is safe in selected cyst types and CT may be used for guidance. •
CT Morphology
Imaging findings depend on the stage of cyst growth and may show an unilocular cyst or a complex with daughter cysts. On noncontrast CT, a hydatid cyst is a well-defined round to oval cystic mass. Daughter cysts are seen as septate structures within the mother cyst or give it a multilocular appearance. The cyst usually has a fluid-density content, but the attenuation of the cyst contents may be as high as 45 HU (hydatid sand). Daughter cysts can have a lower attenuation than the mother cyst. Frequently the cyst is permeated by thin septa, and it has a high-attenuation wall. Cyst calcification occurs in the outer wall, and dense calcifications indicate non-viability of the lesion. Enhancement of the cyst wall and septa on post -contrast scans is a feature that distinguishes hydatid cysts from hepatic cysts (Fig. n.4g). Liver Tuberculosis
Hepatic TE is commonly associated with the miliary form of the disease. It can have a more common micronodular form or a rarer macronodular form. The latter spreads to the liver via para-aortic or hepatoduodenal ligament nodes. •
CT Morphology
The micronodular form shows moderate hepatomegaly. The liver parenchyma is homo-
459
11 The Liver
Oriental Cholang iohepatitis
a
b
Fig. 11.49 Ecchinococcus cysticus. Hydatid cyst with arterial enhancement of the pericyst (a) and excellent distinction between pericyst (arrow), cyst membrane (arrowhead) and germinative layer (wide arrow) of the endocyst in the portal phase (b). Ruptured hydatid cyst (c. d) shows a crumpled subcapsular membrane and an enhancing cyst wall and septa, accompanied by free intra-abdominal fluid.
geneous or heterogeneous. In the macronodular form, single or multiple hypo attenuating (3545 HU) nodules are scattered through the liver. The nodular lesions may calcify. Associated findings like adenopathy, ascites, and peritoneal disease suggest disseminated TE. Rare Infectious Diseases
Also termed recurrent pyogenic cholangitis, this condition is endemic in SE Asia and due to immigration is now more frequently seen in the West. It is a recurrent cholangitis that may result in progressive cholangiopathy and eventually liver failure. Patients have coliform bacteria in bile or previous Clonorchis infection. Major features include intra- and extrahepatic bile duct dilatation and stones. CT shows intrahepatic bile duct strictures and fusiform or varicose dilatation that is most common in the lateral segment of the left lobe. Other findings include high-attenuatingcalculi, pneumobilia, abscess formation, and segmental atrophy. Calculi are often multiple and in acute forms contrast enhancement of the bile duct wall may be seen. Clonorchiasis
The parasite Clonorchis sinensis (Chinese liver fluke) can infect humans after the ingestion of raw freshwater fish. It is endemic in South East Asia. Cercaria migrate to the small to medium bile ducts in the liver where the mature worms are formed. The number of infected persons increases with age and men are more commonly affected. CT shows diffuse, mild intrahepatic dilatation, usually uniform from porta to the periphery. Extrahepatic duct involvement is rare. In endemic areas, this infection is a common risk factor for HCC.
Schistosomiasis
Infection with Schistosoma japonicum is common in South East Asia. Liver involvement shows a characteristic pattern of capsular and parenchymal calcification perpendicular to the liver capsule. Linear and parenchymal band enhancement is seen. The liver surface can be nodular and fibrosis may show increased periportal fat. There is an increased risk for HCC.
Glanders Disease
This is a very rare infection with Burkholderia mallei causing oropharyngeal mucositis and abscess formation in lung, liver, and spleen. It is endemic in the Middle and Far East, infecting mainly horses and donkeys. Sporadic cases in the West include veterinarians or horse lovers. In the liver it causes multiple, small nodules that may coalesce into larger necrotic abscesses. The infection in humans is lethal when untreated.
Perfusion Abnormalities Vascular
Disorders
and Perfusion
Abnormalities
Vascular disorders used not to be a primary indication for CT and were originally evaluated by (color) duplex sonography. With CT angiography. and especially with the advent of multislice scanning. CT can be used as a primary imaging modality that not only evaluates the liver parenchyma but also vascular changes. A biphasic spiral or multislice CT examination can replace angiography in most instances and allows for preoperative investigations of the hepatic blood supply. Perfusion abnormalities have been known for a long time from experiences with angiographically assisted CTtechniques such as CTHA and CTAP. With the increasing use of multiphasic helical CT using higher injection flow rates. these perfusion abnormalities are frequently encountered in daily practice.
Third hepatic inflow tracts by systemic veins that drain non opacified venous blood into the liver are more frequent and are associated with THADs during multiphasic studies or as flow-related defects during CT during arterial portography. An aberrant right gastric vein may directly drain into the liver giving a perfusion defect in the posterior edge of segment IV. just anterior to the right PV (8% of patients. see Fig. 11.8). An aberrant left gastric vein can give perfusion defects in the posterior edge of segment IV (although less frequently) or in the posterior part of segment II. The cystic vein of the gallbladder can drain into intrahepatic portal vein branches around the gallbladder fossa and create a local portal perfusion defect (see Fig. 11.8). Paraumbilical veins that run into the falciform ligament drain into the left lobe portal branches and give portal perfusion defects anteriorly on both sides of the falciform ligament (Fig. 11.50).
Perfusion Abnormalities Any process (tumor. cirrhosis. etc.) that causes a local or generalized rise in intrahepatic pressure will cause a reduction of portal venous flow in the affected region. This is associated with a compensatory increase in arterial perfusion. Locally altered hemodynamics with increased arterial flow are known as transient hepatic attenuation differences (THAD). also called transient hepatic parenchymal enhancement (THPE). lt results in a regional areas of hyper attenuation in the hepatic arterial phase (HAP) that return to normal in the portal venous phase (PVP). These can be caused by a number of vascular lesions (described in the next section). but can also occur independently. Knowledge of the pattern of these abnormalities and their causes is important for correct interpretation (Table 11.14).
Arterioportal Shunts Arterioportal shunts can occur via several routes: through an iatrogenic fistula (post biopsy). trans-sinusoidal (blockage of venules), transvasal (via tumor thrombus), transtumoral
Table
11.15
Common
causes
and
locations
of perfu-
sion defects
'location IMan-occlusive Aberrant
R gastric
Aberrant
L gastric
Gallbladder
vein
Paraumbilical
Anatomic Variants
APshunt
Systemic arteries may aid in the arterial supply of the liver. especially in tumors or in situations with diminished hepatic arterial flow. The most common are the inferior phrenic. internal mammary. left gastric and intercostal arteries. These variants may give perfusion defects at CTHA.
Occlusive
edge
segment
IV
Posterior
edge
segment
IV
Posterior
edge
segment
II
along gallbladder Anterior Liver
Right hepatic Middle
veins
Posterior
hepatic
vein vein
ligament
periphery
Dorsal in anterior segment Ventral in posterior segment Ventral Medial
Left hepaticvein
falciform
fossa
in anterior
segment
segment
Throughoutleft lobe
11 The Liver
a
b
c
d
Fig. 11.50 Transient hepatic attenuation differences (THAD). (a) Hyperperfusion in the arterial phase due to collaterals from superior vena cava occlusion directly entering the left liver lobe. (b) Transient hyperperfusion in hepatic cirrhosis (negative biopsy), (c) Wedge-shaped arterial hyperperfusion due to a capillary hemangioma
with arterio-portal shunting in the arterial phase, (d) Segmental arterial hyperperfusion due to obstruction of a portal venous branch by a hypovascular metastasis (arrow). Note the concomitant biliary obstruction (wide arrow).
(via draining vein of a hypervascular tumor) or transplexal (peri biliary) . Tumoral shunts (HCC, cholangiocarcinoma or hemarigioma) are usually caused by vascular infiltration of portal vein branches. Nontumoral shunts due to liver cirrhosis, trauma, biopsies, and ethanol injection are less common. These shunts also reflect local blockage of portal venous venules and retrograde filling via trans-sinusoidal AP shunting. Regardless of their etiology, these shunts typically give wedge-shaped THADs in the liver periphery and there may be a dot -like or branching vessel visible resulting from early venous filling. These are best seen in the arterial dominant phase or during CTHA.
Portal Hypertension
Other Causes
Siphoning by hypervascular tumors that steal arterial blood from the surrounding parenchyma will result in hypo attenuation during the HAP and isoattenuation in the PVP. Infiammatoi-y changes caused by acute cholecystitis or hepatic abscesses lead to arterial hyperemia and venous flow stoppage that show typical THAD.
Normal portal flow rate is about llOOml/min and normal pressure is 5- WmmHg, which is 3-6mmHg above the IVC pressure. Portal hypertension is defined as a venous pressure gradient greater than 5 mm Hg and commonly results from increased portal venous resistance, the cause of which may be prehepatic, intrahepatic, or posthepatic. Rarely, portal hypertension may be due to a hyperkinetic condition caused by an arterial-portal fistula (traumatic, congenital, neoplastic). Clinically significant symptomatology is usually present with pressure gradients above 12 mm Hg, when complications will be seen. Prehepatic causes reduce portal blood flow (PV thrombosis, AP fistula), intrahepatic causes are associated with chronic liver disease (diffuse liver disease, cirrhosis) and posthepatic causes reduce outflow into hepatic veins or IVC (venoocclusive disease). Intrahepatic causes may be further divided in presinusoidal, sinusoidal and postsinusoidal. The pressure increase induces the formation of portosystemic collaterals, which are usually dilated pre-existing vessels but active angiogenesis is also seen. Common collateral pathways include the coronary (left gastric), posterior gastric, and short gastric veins to the esophageal and paraesophageal veins;
Portal Venous Obstruction gastrosplenic, gastrorenal and splenorenal shunts to the left renal vein; and several pathways including the omental, paraumbilical, hemorrhoidal, and retroperitoneal (Retzius) veins. Other complications include splenomegaly, encephalopathy and ascites. Banti's syndrome is an idiopathic, noncirrhotic form of portal hypertension that is secondary to obliterative venopathy of the intrahepatic portal venous branches. CT is not the firstline technique for imaging portal hypertension but imaging in the early portal venous phase is excellent for demonstration of complications and collateral pathways. •
Segmental portal hypertension results from the occlusion of splenic or mesenteric veins.
CT Morphology
An increased diameter of the portal vein (> 13 mm) and the superior mesenteric and splenic veins (> 1Omrn) with development of paraumbilical collaterals (>3mm) are highly specific but only moderately sensitive signs of portal hypertension. CT is able to demonstrate the various collaterals (Fig.'11.51). Large (spontaneous) splenorenal shunts may almost normalize the portal venous pressure and reduce the size of other collateral pathways. Paraesophageal varices have to be distinguished from esophageal varices because they cannot be detected with endoscopy. Collateral veins may be difficult to distinguish from collapsed bowel or lymph nodes on
a
unenhanced CT scans but are easily distinguishable after contrast administration. Collateral pathways are well demonstrated on axial sections but targeted (thin-slab) MIP or VRT yield a better overview of the anatomic situation (Fig. 11.52). Splenomegaly is frequently but not always present. Siderotic nodules (Gamna-Gandi bodies) in the spleen may appear as tiny disseminated calcifications.
b
Fig. 11.51 Portal hypertension in a patient with severe liver cirrhosis: enlargement of the portal vein diameter (» 13 mm) and development of prominent venous collaterals. This CTAP with selective contrast enhancement of the portal vessels demonstrates a spontaneous splenorenal shunt with contrast run-off into the renal
Portal Venous Obstruction
Causes of obstruction of the portal vein can be intrahepatic or extrahepatic. Extrahepatic obstruction can be the result of neoplastic involvement, PV thrombosis, extrinsic compression, iatrogenic or due to pressure on the parenchyma. Intrahepatic (presinusoidal) obstruction due to narrowing of portal venules by inflammation or fibrosis. Portal vein thrombosis is the major cause of prehepatic portal hypertension (and variceal bleeding) in the western countries. In young patients it is usually idiopathic or may result
c vein (arrow). After intravenous contrast administration such shunts are characterized by a dilatation of the left renal vein. Multiple hypoattenuating HCC nodules (arrowheads) are visible against a background of micronodular cirrhosis.
463
11 The Liver
to 20% of cases show cavernous transformation of the portal vein (local collateral veins). In main portal vein obstruction, reduced portal blood flow leads to dilatation of the hepatic artery and increased arterial inflow. This can take two patterns: • compensatory increase of regional flow in the area where portal flow is reduced (because of tumor thrombus, or compression or stricture of the portal vein or a branch); and • heterogeneous peripheral increase of arterial flow (in cavernous transformation of thrombosed portal vein), while collateral circulation supplies the central periportal parenchyma. Associated AP shunts may also be present. A chronic pel-fusion deficit (caused by arterial occlusion, portal thrombosis, arterioportal shunting, etc.) can lead to regional fatty infiltration. "
Fig. 11.52 The multitude of abdominal varices in a patient with portal hypertension is best demonstrated on volume-rendered images (4x 1/6, thin-slab VRT anterior to the aorta and SMA). Note the large perigastric varices.
from venous webs (mainly Asian countries). Secondary forms can result from inflammatory or infectious diseases such as pancreatitis, sepsis, myeloproliferative diseases, tumor compression or invasion, hepatic cirrhosis with portal hypertension, and hypercoagulable states (protein CIS or AT3 deficiency, antiphospholipid syndrome). The involvement may be partial or complete and can involve any portion of the portal system. The portal thrombosis induces the development of portosystemic, splenohepatic or mesentericohepatic collaterals, and up
a
b
Fig. 11.53 Non-tumorous portal venous thrombus must not enhance in the portal venous phase (a). Chronic thrombi in portal hypertension may not lead to occlusion but to thrombotic material adherent to the walls, often associated with calcifications (b). Pseudo-
•
CT Morphology
A portal vein thrombosis can only be identified on unenhanced scans if it is fresh with a hyperattenuating thrombus present (Fig. 11.53). The affected venous segment is enlarged (> 15mm in up to 40% of cases) and in long-standing thrombosis shows decreased attenuation. Calcifications may develop in a chronic thrombus or in the wall of the portal vein. They are especially frequent in patients with Banti's syndrome.
c thrombi are a frequent finding on arterial phase images and are due to lack of mixing of enhanced blood from the spleen and non-enhanced blood from the mesenteric circulation (c).
Hepatic Venous Outflow Obstruction Signs of portal hypertension (esophageal, fundal, and paraspinal varices) vary in their conspicuity, depending on the duration of the thrombosis. Ascites is a common secondary phenomenon of decompensation in the acute phase or late stage. Scans after contrast administration show nonopacification of the thrombosed segment or an intraluminal filling defect. Tumor thrombus shows moderate to marked and often irregular enhancement, best appreciated in the arterial phase. If cavernous transformation is present, a network of bypass veins can be seen in the area of the occluded segment (usually the porta hepatis, Fig. n.54). Parenchymal enhancement is often inhomogeneous with relative hypo density of the affected liver areas. The described arterial hyperperfusion patterns both produce THAD. The first pattern leads to a lobar or (sub)segmental hyperattenuation in the HAP (no collaterals), and the second to multiple inhomogeneous peripheral hyperattenuating areas (collaterals or cavernous transform). The peribiliary plexus takes over for periportal liver, and in chronic cases periportal hypertrophy can be seen with atrophy in the affected liver periphery. During the late arterial or early portal phase, contrast -enhanced blood from the splenic vein enters the portal vein while the mesenteric blood is still relatively nonopacified. In-
complete mIxmg of opacified and nonopacified blood can mimic a filling defect, creating a "pseudothrombus" in the portal vein or proximal superior mesenteric vein.
Hepatic Venous Outflow Obstruction
Blockage of the hepatic venous outflow can be due to Budd-Chiari syndrome, congestive rightsided cardiac failure, pericardia! disease, mediastinal fibrosis, or extrinsic compression on the portal vein or IVe. Obstruction of venous outflow will increase postsinusoidal pressure (hypertension), resulting in dilatation of the central veins with delayed portal venous inflow, with or without hepatomegaly. Eventually the flow in the portal vein may be reversed, turning it into the main hepatic draining system. The arterial inflow will be relatively increased. Suprahepatic thoracic inlet obstruction causes blood flow through collateral vessels like the azygos-hemiazygos and lateral thoracic veins to the IVe. The latter communicates via the paraumbilical veins with the portal circulation. Generally this diagnosis of hepatic outflow obstruction is established by ultrasound (color duplex sonography). CT may be used to confirm the diagnosis in selected cases. Budd-Chiari Syndrome
Fig. 11.54 With cavernous transformation of the portal vein, a network of collateral channels occupies the area of the former portal vein.
Budd-Chiari syndrome is a rare disorder that refers to a segmental or global obstruction of the hepatic veins. Acute cases (1/3) present with sudden abdominal pain due to hepatic congestion and ascites that is refractory to treatment (>95%). In the chronic stage, ascites is accompanied byjaundice and signs of portal hypertension with bleeding varices. The syndrome may be caused by hepatic venous thrombosis or nonthrombotic obstruction. It is more frequent in the Middle East, India, and Japan, where it is often caused by membranous webs or septa. In the USA and Europe, most cases are acquired and can be caused by hypercoagulable states (polycythemia vera, PNH, sickle cell disease, oral contraceptives, pregnancy and postpartum states), injuries to the vessel walls (trauma, phlebitis), or tumorous obstruction (HCC, renal
465
77 The Liver
cell or adrenal carcinoma). Concomitant portal vein thrombosis is present in 20% of patients. Based on the location of the obstruction, several types of the syndrome exist. Type I is defined as an occlusion of the IVC and type II patients have occluded major hepatic veins. In hepatic vena-occlusive disease, which has also been termed type III Budd-Chiari, resistance is at the level of the postsinusoidal venules following a progressive inflammation. This is often associated with chemotherapy and radiation ther-
apy. •
CT Morphology
In acute Budd-Chiari syndrome, noncontrast scans show hepatomegaly with decreased liver attenuation due to edema and rapid fatty infiltration. Ascites is present and hyperattenuating thrombi may be visible in the IVC or hepatic veins. The caudate lobe is rarely affected initially. After contrast administration, heterogeneous patchy enhancement (mosaic pattern) is seen in the arterial phase and there may be draining portal vein branches visible. The occluded area may be hyperenhanced during this phase due to arterial hyperperfusion (THAD). In the portal venous phase this patchy enhancement often persists, especially when there is hepatofugal portal vein flow (Fig.TI.55a). On 5-10 minute delayed scans, the patchy poly-
a
b
Fig. 11.55 Budd-Chiari syndrome. Occlusion of the hepatic veins (arrows) in chronic Budd-Chiari syndrome. Patchy areas of enhancement are seen around the larger portal branches, with intervening reticular areas of low attenuation. Parenchymal swelling has compressed the vena cava to a slit (arrowhead, a). Acute Budd-Chiari syndrome in another patient with intrahepatic venous
gonal enhancement of the affected areas disappears. The hepatic veins continue to show low or no luminal attenuation after contrast administration. Due to the edematous swelling of the liver, the IVC may be highly compressed in its hepatic portion. In the subacute stage, multiple hypervascular nodules may be found that correspond to regenerative nodules (Fig.TI.55b). Such nodules are amost impossible to differentiate form hypervascular tumors (Fig. T1.55c). In the chronic stage, the caudate lobe and unaffected liver areas are enlarged in 80-90% of cases, as it has its own veins that drain into the vena cava. The affected liver parenchyma shows the same patchy enhancement pattern with normal opacification of the portal vein and patchy enhancement around the terminal portal branches (Fig.TI.55b). Some patients may actually have a relative homogenous hepatic enhancement. Infarcted areas, especially at the periphery of the liver, become shrunken and fibrotic. They show low attenuation and very little enhancement. The hepatic veins are not visualized or are greatly reduced in caliber.
c collaterals (arrowhead) due to a patent left liver vein (wide arrow). Note the multiple hypervascular nodules (b). Budd-Chiari syndrome in a patient with a large central HCC and hypodense thrombosed hepatic veins (arrowheads) (c). In the periphery of the tumor it is no longer possible to distinguish between hypervascular benign nodules and HCC.
In passive hepatic congestion from heart failure or pericarditis, elevated venous pressure leads to decreased hepatic flow and arterial hypoxemia. This hypertension results in sinusoidal congestion and perisinusoidal edema. Patients can have hepatomegaly and elevated liver function tests. If uncorrected, this may progress to cardiac cirrhosis. •
CT Morphology
CT shows hepatomegaly with normal parenchymal density in the noncontrast phase. The caudate lobe is usually normal. Ancillary findings are ascites, pleural effusions, and cardiomegaly. After contrast administration, a diffusely mottled enhancement of the liver is seen and contrast -enhanced blood may reflux from the heart into the IVC or hepatic veins. There may be intrahepatic perivascular lucency and the IVC and hepatic veins are usually dilated (Fig. 11.56). This distinguishes this disorder from Budd-Chiarisyndrome. Hepatic Arterial Thrombosis and Hepatic Infarction
With obstruction of hepatic arterial flow, there is no increase in portal venous flow. Blockage of large arteries will directly induce new routes of flow from extrahepatic arteries (see Pel-fusion Abnormalities), e.g., through the bare area of the liver and via the suspensory ligaments. A delay in arterial flow may lead to a hypo dense region during arterial phase imaging.
Because of the dual blood supply of the liver, hepatic infarction is uncommon. It can result from coexisting hepatic arterial and portal venous obstruction. It is most commonly seen after liver transplantation. Other less frequent causes of hepatic artery occlusion include hepatic artery atherosclerosis, shock, sepsis, eclampsia, and oral contraceptive use. •
CT Morphology
Hepatic infarcts can take on three configurations. Peripheral wedge-shaped lesions, central or peripheral rounded lesions, or irregular shaped lesions paralleling bile ducts. With time, margins become less distinct. Bile duct necrosis may lead to gas collections or cystic bile collections. In long-standing cases the involved liver segment atrophies. Arterial obstruction will show increased caliber of extrahepatic systemic arteries with peripheral THAD during the arterial phase. Rendu-Osler-Weber Disease
Rendu-Osler- Weber disease (hereditary hemorrhagic telangiectasia) is a hereditary disorder in which multiple pathologic vascular formations occur throughout the body. Liver involvement is characterized by hepatic telangiectasia and usually arteriovenous, less commonly arterioportal, shunts. These shunts can cause a significant increase in cardiac output resulting in a hyperdynamic circulatory status.
Fig. 11.56 Severe hepatic congestion due to right heart failure, with patchy polygonal areas of periporta I enhancement in the liver (a). Note the poor opacification (a) or dilatation (b) of the hepatic veins (arrows).
a
b
468
11 The Liver
a
c
a
Fig. 11.57 Osler's disease is characterized by enlarged hepatic arteries and early enhancement of the hepatic veins already during the arterial phase (a). The liver parenchyma contains very small, hyperdense foci representing shunts (b). Individual portal veins are throm-
bosed (arrows). A volume-rendered display of an arterial phase scan in another patient (c) demonstrates marked enlargement of the hepatic artery and early enhancement of draining veins (arrowheads).
•
diminished on portal phase scans due to the high cardiac output, and little parenchymal enhancement is seen. The best examination techniques are spiral or multislice scanning performed during the arterial phase of enhancement.
CT Morphology
Noncontrast scans demonstrate enlarged vessels (especially the hepatic veins) and frequently also hepatic enlargement. Signs of (hyperkinetic) portal hypertension may be present. Arterial phase images demonstrate enlarged arteries • Differential Diagnosis with early opacification of enlarged hepatic veins due to arteriovenous shunting (Fig. 11.57). Hepatic hemangiomatosis appears as multiple The shunts usually involve the peripheral vessels circumscribed lesions that show a characteristic diffusely, and therefore they are difficult to pattern of peripheral to central enhancement (Fig. 11.58). define. Vascular opacification is markedly
a
b
Fig. 11.58 Liver hemangiomatosis in Gorham's disease with multiple lesions with patchy hypervascular areas in the periphery on arterial phase scans (a) that show progressive enhancement on portal phase images
c (b). There is marked enlargement of the hepatic arteries (arrowheads) (c) and early enhancement of liver veins (arrow) similar to Osler's disease (4 x 1/6, VRT).
Blunt Liver Trauma Fig. 11.59 Peliosis hepatis with multiple focal lesions that demonstrate modest enhancement on arterial (a) and more pronounced enhancement on portal venous phase scans (b).
b
a
Peliosis Hepatis
Peliosis hepatis is a rare disorder in which multiple blood -filled cystic spaces form within the liver parenchyma. These lacunar spaces do not have an endothelial lining and are prone to rupture. They may occur as a complication of aplastic anemia or androgen therapy, chemotherapy, diabetes mellitus and chronic renal failure.
•
CT Morphology
The lacunar spaces seen in peliosis hepatis appear almost isodense to the aorta, and their enhancement follows that of the aorta (Fig. 11.59). They are strongly hyperattenuating in the arterial phase and remain moderately hyperattenuating in the portal venous phase (bloodpool effect). Rupture with formation of intrahepatic and perihepatic hematomas is a dangerous complication.
Trauma Blunt Liver Trauma
The liver is the second most frequently injured solid organ and injuries occur in 15-20% of patients with blunt abdominal trauma. The right lobe is more often injured than the left lobe, and the posterior segments especially are prone to injury by compression or shearing forces. There may be associated rib fractures. In 80% there is associated hematoperitoneum and there may be retroperitoneal bleeding from rupture of the liver surface in the bare area. Injuries can be classified as vascular lesions, lacerations and fractures, intraparenchymal or subcapsular hematomas, contusions and periportal low -attenuation. Grading is often done using the organ injury scale of the AAST (Table 11.16),whereas imaging based classifications (e.g., Mirvis et ai, 1989) may also be used. CT can accurately define the extent of hepatic injury, quantitate hemoperitoneum, reveal associated injuries, and follow healing. This enables the surgeon to select the most appropriate
therapy. Standardization of reporting and grading enhances accuracy and can help in this triage. Based on clinical and imaging findings, 20-40% of patients are currently managed conservatively. Postcontrast scans usually suffice because they allow for the detection of hemorrhage and are more sensitive for parenchymal contusions and the extent of lacerations. If acute bleeding is suspected, a biphasic examination should be performed. CT is also indicated in the diagnosis and management of complications, such as biloma or abscess formation. •
CT Morphology
Vascular injuries are the most severe injuries but are fortunately rare. They most frequently include lesions of the (retrohepatic) inferior vena cava and hepatic veins, and management often requires a combination of surgery and catheter embolization. Lacerations are the commonest type ofinjury and frequently travel along the vessels and fissures. Perihilar lacerations are more frequently
469
11 The Liver Table 11.16 Grade
AAST organ injury scale for liver injuries (1994 resivion) Injury
Hematoma Laceration
Subcapsular, 50%), duodenum (> 10%), and colon (approximately 10%). Regional .lymph node metastasis (hepatoduodenal ligament, peripancreatic nodes, periportal nodes) occurs in more than 50% of cases. Hematogenous or intraductal spread is less frequent, but perineural spread is commonly seen with aggressive tumors. CT is used for tumor staging and is particularly useful for detecting spread beyond the gallbladder and lymphatic metastasis. •
CT Morphology
CT usually shows a hypo attenuating mass in the gallbladder bed infiltrating the adjacent fatty tissue and liver (Fig. 12.11). The tumor is rarely
c heads). There is abrupt termination of the dilated common bile duct (arrow) and contrast-enhancing neoplastic wall thickening best appreciated on semicoronal reformations (4 x 1/6).
Cholelithiasis Table 12.4 Tis
TNM Staging of gallbladder carcinoma
carcinoma in situ
T1 a
tumor infiltrates lamina propria
T1 b
tumor infiltrates gallbladder muscle layer
* T2
Table 12.5 Differential diagnosis noma
tumor infiltrates peri muscular connective tissue
T3
tumor perforates serosa or invasion of liver 2 cm or into 2 or more neighbouring organs (stomach, duodenum, colon, pancreas, omentum, extrahepatic bile ducts)
N1
metastasis in nodes around cystic duct around CBD or in liver hilum (= in hepatoduodenal ligament)
N2
metastasis in peripancreatic head, in periduodenal, celiac and/or superior mesenterial nodes
Hyperplasia of the gallbladder wall, in some cases with a polypoid projection into the lumen Melanoma, lymphoma, or in leukemia Liver, pancreas, duodenum
b
Fig. 12.11 Gallbladder carcinoma with irregular wall thickening and hepatic invasion.
Gallbladder
Thin-section spiral or multislice CT may be used in a preoperative stetting before laparoscopic cholecystectomy (especially in combination with biliary contrast agents, as a CT cholangiography), in patients with acute abdomen and to help differentiate benign form malignant disease. MRI with MRCP is the most important competitor with the advantage that no biliary contrast material is necessary.
carci-
Acute or chronic cholecystitis
Metastatic ment
diagnosed in its early stages, which may be marked by asymmetric generalized or focal thickening of the gallbladder wall or a polypoid intraluminal mass. Tumor demarcation from the lumen is seen most clearly during the late phase after contrast administration. An increase in the CT density of an intraluminal feature by more than WHU is highly suspicious for a tumor. With a mucinous adenocarcinoma, fine granular or punctate calcifications may be seen. Enlarged regional lymph nodes are common. The tumors usually appear hypo attenuating after contrast administration, but some may show intense peripheral arterial enhancement. Table 12.5 provides the differential diagnoses for diffuse or focal thickening or tumors and tumor-like lesions of the gall bladder.
of gallbladder
Diseases
Cholelithiasis
Cholelithiasis is a common disease that predominantly affects middle-aged women. Gallbladder "sludge" is composed of gritty cholesterol crystals and calcium bilirubinate granules. Ultrasound is the imaging modality of choice, and cholelithiasis is usually an incidental finding at CT. For diagnosing choledocholithiasis,
489
72 The Biliary Tract
a risk stratification based on clinical, laboratory and ultrasound features can be used. For patients with low to intermediate risk MRCP is the method of choice. High risk patients and those with a positive US/MRCP should proceed to ERC with stone extraction. It has been suggested that CT cholangiography be used for the preoperative exclusion of cholangiolithiasis prior to laparoscopic cholecystectomy. This type of study requires use of a cholangiographic contrast medium and yields best results with thinsection mul tislice CT. •
CT Morphology
Stones in the biliary tract are visible on unenhanced scans only if their CT attenuation significantly differs from than that of the surrounding bile (0-20 HU). Thus, approximately 15-25% of biliary calculi are not detectable with CT. Cholesterol stones range from slightly hypo-
abc
attenuating (pure cholesterol, Fig.12.12a) to hyperattenuating (containing CaC03 and calcium bilirubinate, Figs. 12.12b, c). Pigment stones (calcium bilirubinate) are hyperattenuating (Fig.12.12d). Gas-containing stones result from dehydration and cavitation (Mercedes Benzsign, Fig.12.12b). In patients with cho]angiolithiasis, pre-contrast CT can demonstrate an obstructing stone in 50-90% of cases. Thin-section spiral and multislice CT yield the best results. If the radiation dose chosen is high enough to keep image noise low and if a narrow window setting (e.g., W/L= 150/10) is used, hypoattenuating noncalcified stones in the extrahepatic bile ducts can be detected to better advantage. However, isoattenuating and intrahepatic stones may be missed. The typical finding is a target or bull'seye pattern (Fig. 12.13). Nonenhanced scans are required to avoid confusing the calcified rim
d
Fig, 12.12 Gallstones: (a) hypoattenuatingr cholesteroll stone fl-oating in the gallbladder, (b) Mercedes Benz sign associatedl with gas-containing stones, (c) ring calcification around stones, arid (d) multiple pigment stones.
Fig. 12.13 Cholangiolithiasis in the common bile duct with a typical target pattern caused by an impacted, calcified stone. Note the decreased conspicuity after IV contrast enhancement.
a
b
Cholecystitis
Fig. 12.14 Hemobilia in the common bile duct causes a target pattern similar to the one seen in Fig. 12.13. (a) Semicoronal reformations demonstrate that the filling defect is not rounded and circumscribed but extends along wider portions of the duct (b) indicating sludge or hemobilia as the cause.
b
a
Table 12.6 Causes of increased CT density of the gallbladder contents Sludge Prolonged fasting (e.g., intensive care patients) Hepatobiliary excretion of intravenous contrast material Clotted blood in hemobilia Parasites (ascariasis) Gallbladder tumors
with a contrast-enhancing bile duct wall. However, sludge and hemobilia may also present as slightly hyperattenuating material in the bile ducts but may be differentiated from stones on multiplanar reformations using multislice CT (Fig. 12.14). Sludge has a moderately higher attenuation than the bile and shows layering in the dependent portion of the gallbladder. Prolonged fasting (intensive care patients) can cause the gallbladder contents to become isoattenuating to the liver tissue (differential diagnosis see Table 12.6). Cholecystitis
Acute cholecystitis is most prevalent in the fifth and sixth decades and affects women with a g3: 1 predominance. It usually results from transient obstruction of the cystic duct by an impacted gallstone. The diagnostic methods of choice are ultrasound and clinical examination (Murphy's sign). CT may be indicated in patients with an acute abdomen and atypical clinical findings. Acute cholecystitis may be found incidentally in CT follow-ups after intra-arterial
chemoembolization of the liver. As consequence of gallbladder hyperemia, there is an increased incidence of transient arterial hyperperfusion of adjacent liver parenchyma on biphasic liver scans. Chronic cholecystitis is the most common inflammatory gallbladder disease. It is an incidental finding in CT and requires differentiation from gallbladder carcinoma. Xanthogranolomatous cholecystitis is a rare chronic inflammatory disease characterized by foreign body giant cells and foamy histiocytes. Emphysematous cholecystitis affects men with a 5 :1 preponderance and shows a high mortality rate (15%). Gangrene and gallbladder perforation are the most frequent complications. The diagnosis is established by ultrasound and/or plain abdominal radiography. CT can detect emphysematous cholecystitis with high sensitivity during the evaluation of an acute abdomen. •
CT Morphology
Acute cholecystitis is characterized by thickening of the gallbladder wall > 3 mm (usually by 9mm). The wall enhances after contrast administration (Fig.l2.15a), occasionally showing a three-layered structure. Gallbladder hydrops (gallbladder width > 4ern) and gallstones are common. Pericholecystic fluid may represent an inflammatory exudate (Fig.l2.15a) or an abscess. Intraluminal air may signify a gallbladder empyema or emphysematous cholecystitis (Fig.l2.15c). With acute cholecystitis following chemoembolization of the liver, some of the highly hyperattenuating Lipiodol-containing embolic agent may occasionally be demonstrated in the gallbladder wall.
492
12 The Biliary Tract
a
b
Fig. 12.15 (a) Acute cholecystitis with wall enhancement and pericystitic fluid, (b) Edematous cholecystitis during chemotherapy for ovarian cancer, (c) Emphy-
c sematous cholecystitis with intraluminal air and airoutside the gall bladder lumen indicating perforation.
Xanthogranulomatous cholecystitis: There is Chronic cholecystitis: Gallstones are invariairregular gallbladder wall thickening, which bly present. Usually the gallbladder is increased in volume and shows smooth or irregular wall even may appear as a lobulated mass, rarely with calcifications. Stones are commonly presthickening (by an average of 5 mm). This thickening often affects the entire circumference of the ent. Pericholecystitic extension can simulate gallbladder, but may be asymmetric (Fig. 12.16). gallbladder carcinoma. Emphysematous cholecystitis: Starting about Postcontrast CT shows moderate wall enhancement, which may be seen only on delayed scans 24 hours after the acute event, air can be dem(Fig. 12.16a). In patients with calcifications ofthe onstrated in the gallbladder lumen (Fig.12.15c) gall bladder wall (Fig.12.16b, c) there is an in- and wall and rarely in the intrahepatic bile creased risk of developing gall bladder carci- ducts. Signs of perforation with a pericholecystic abscess are common (20%). noma. A distinction from malignant wall thickening may not always be possible based on CT criteria alone. Doubtful cases require sonographic follow-up or cholecystectomy.
a
b
Fig. 12.16 (a) Chronic cholecystitis with asymmetric thickening of the gallbladder wall (delayed phase), (b) Chronic cholecystitis with calcified gall bladder wall and
c multiple gall stones. CT cannot rule out the presence of aT1 gall bladder cancer (4 x 1/6, coronal MPR). (c) Porcelain gallbladder.
Postoperative Complications
Pericholecystic Abscess Gangrene that develops in the setting of acute cholecystitis can lead to a subacute perforation of the gallbladder wall. Ultrasound is the modality of choice. but CT may be indicated in cases where technical factors hamper ultrasound imaging (obesity),
Laparoscopic
Cholecystectomy
Preoperative Evaluation Laparoscopy has become the method of choice for the majority of patients scheduled for cholecystectomy, There are, however, a number of conditions that increase the risk of the procedure and may require prior intervention by ERe. The risk of bile duct injuries increases with anatomic variants, most importantly aberrant right hepatic ducts, and short cystic ducts, Chronic inflammatory changes with marked adhesions may complicate the procedure and may require open surgery, Bile duct stones are hard to extract during laparoscopic cholecystectomy and therefore should be removed prior to surgery, Asymptomatic patients with normal liver function tests and normal common bile ducts at ultrasound have a very low rate of bile duct stones, A positive detection of a stone at ultrasound mandates ERC with stone extraction, and a normal ultrasound rarely requires further work-up, Unless clinical, laboratory and US findings mandate direct ERC (e.g. cholangitis), MRCP is the technique of choice for diagnosis of anatomic variants and the detection of bile duct stones before laparoscopic cholecystectomy, CT cholangiography can be expected to yield similarly good results but requires the use of biliary contrast agents, •
CT Morphology
A zone oflow attenuation is usually found in the gallbladder bed and less commonly in the gallbladder wall (intramural abscess) or peritoneal cavity surrounding the gallbladder. Peripheral contrast enhancement may be visible but may be absent in early stages, Most intramural abscesses are detectable only after contrast administration.
CT Morphology
CT cholangiography using thin-section spiral or multislice CT can demonstrate filling defects in the extra- and intrahepatic bile ducts, As opposed to MRCP, CT can easily distinguish between intraductal air and stones due to their characteris tic CT numbers,
Lack of filling of the gallbladder suggests marked inflammatory reactions associated with substantial adhesions around the gallbladder. This finding has been suggested as a contraindication to laparoscopic approach and as an indication for open surgery, The detection of anatomic variants requires close scrutiny of axial images, best using an interactive cine display on a monitor. Volume-rendered images are superior to MIP to demonstrate the spatial relationship between the various portions of the biliary system. Typical variants of the cystic duct include a spiral cystic duct, a low junction of the cystic duct, a short cystic duct, and a cystic duct entering a right hepatic duct Variants of the right hepatic duct include aberrant ducts that drain directly into the common hepatic duct, common bile duct, the cystic duct, or the gallbladder (see Fig, 12,2),
Postoperative Complications Complications after laparoscopic cholecystectomy are strongly correlated with the experience of the surgeon, Laparoscopy itself may lead to injury of almost any abdominal organ, but liver, bowel. ureter, and vessels of the abdominal wall are most frequently involved, During laparoscopic cholecystectomy the common bile duct or a hepatic duct may be mistaken for the cystic duct and may be ligated, strictures may result from laser or cautery injuries, but also bile duct leaks may occur, Retention of stones in the common bile duct is possible, as is incomplete resection of the gallbladder. Clinically, leaks most frequently become symptomatic 1-2 weeks after surgery
493
12 The Biliary Tract
while structures take 2 weeks to several months to cause clinical symptoms. Patients may present with serologic signs of blood loss due to hemorrhage, or abdominal distension, mild pain and low-grade fever due to infected bilomas or bile peritonitis, or mildly abnormal liver function tests or minimal jaundice due to biliary obstruction. In the postoperative setting, CT is often superior over MRI because of less interference by perihepaticfl uid. •
CT Morphology
Normal postoperative findings that should resolve within the first week include pneumoperitoneum, subcutaneous emphysema, small amounts of fluid in gallbladder bed, adynamic ileus, pleural effusion, and lower lobe atelectasis. Postoperative ascites should resolve within two weeks. As a long-term effect after cholecystectomy there is compensatory dilatation of the common bile duct up to 1Omm (Fig. 12.17). Laparoscopy-related complications may present as peritoneal hemorrhage, liver lacerations, hematoma in the abdominal wall or in the rectus sheath, persistent pneumoperitoneum, or ascites. Suspected ureter leaks have to be
proven by extravasation of contrast -enhanced urine on delayed excretory phase scans after > 10 minutes. Bile leaks present as focal fluid collections, especially in the gallbladder fossa and the liver hilum, or as free fluid in the abdomen. Bile peritonitis will after some days cause peritoneal thickening and enhancement. The detection of fluid, however, is nonspecific because it can also be caused by seroma, lymphocele, hematoma, or abscess. A suspected bile leak requires confirmation by fine needle aspiration, CT cholangiography, or ERe. Ligation or stricture of a normal bile duct (most commonly an aberrant right duct or the common bile duct) leads to marked cholestasis in the affected segment. Prolonged cholestasis due to complete obstruction may lead to atrophy and cirrhosis of the drained liver segment. Bile duct ligation can be associated with injuries to the right hepatic artery (hemorrhage, stenosis, occlusion). Stones that have dropped into peritoneal space or that remained in the common bile duct can rarely be detected. (Optimum visualization would require CT cholangiography.)
Fig. 12.17 Unusually pronounced compensatory dilation of the common bile duct in a patient following cholecystectomy.
a
b
Trauma
and Postoperative
Changes
Traumatic Injuries
Postoperative Changes
Blunt abdominal trauma or penetrating injuries may cause hemobilia or biliary leaks. Liver lacerations usually involve the bile ducts. CT is an excellent tool to detect hematoma or biloma, but it is less well suited to prove a bile leak or hemobilia.
General postoperative complications after open cholecystectomy, bile duct surgery, or liver transplantation are similar to those after laparoscopic cholecystectomy, but they occur with different frequency. The most typical complication after bilodigestive anastomosis (usually choledochojejunostomy) is a stricture at the site of the anastomosis sometimes complicated by secondary cholangitis. After liver transplantation, structures at the site of the bile duct anastomosis may develop and are most often caused by ischemia due to a stenosis or thrombotic occlusion of the hepatic artery. In an occluded remnant of the cystic duct, mucoceles may form.
•
CT Morphology
Hemobilia may be hard to detect. It may present as an increase in bile attenuation or as hyperattenuating clots in the biliary system (see Fig. 12.14) or the gallbladder. Blood contents pool in the dependent portion of the gallbladder or of a dilated duct. Such hyperattenuating material. however. may also be caused by sludge, milk of calcium bile, small gallstones, or contrastmaterial. Traumatic avulsion of the gallbladder can best be diagnosed on reformations from thinsection spiral or helical CT that are positioned parallel or perpendicular to the gallbladder axis. There is an increased distance between liver surface and gallbladder and free fluid in the gallbladder fossa. Perihepatic fluid collections may be due to hematoma, biloma, abscess or loculated ascites.
Chemoembolization, Chemotherapy Chemoembolization or intra-arterial chemotherapy may lead to benign bile duct strictures due to focal ischemia. As a consequence offocal or generalized bile duct obstruction, secondary cholangitis and even chologenetic abscesses may develop. Chemoembolization may lead to cholecystitis if the cystic artery received some of the embolic agent. These agents are commonly mixed with radio-opaque contrast material and can be identified as small high-attenuation foci in the hepatic arteries, the liver or the gall bladderwall. CT is not the primary imaging tool but can help detect inf e c ti ous c om plica tions.
CT Morphology
Normal postoperative findings are the same as those described for laparoscopic cholecystectomy. After open surgery, they may persist for a longer period of time, depending on the complexity of the surgical procedure. Air in the biliary system is a normal finding, even years after bilodigestive anastomoses. Periportal hypoattenuation after liver transplantation is a normal finding that may persist for up to four months and is due to lymphatic congestion in the transplanted liver. This finding should not be confused with dilated bile ducts which are more tubular and usually extend more into the periphery. Mucoceles of the cystic duct remnant appear as rounded fluid collections in the liver hilum and often cannot be differentiated from loculated ascites, lymphocele, biloma, abscess, or a fluid-filled Roux-en-Yjejunalloop. Strictures of the bile duct anastomosis lead to a dilation of the intrahepatic bile ducts. Presence of gas in the bile ducts indicates patency but does not rule out strictures. Dilatation without gas implies a tight stenosis. Other findings (biloma, bile leak, free fluid, peritonitis) correspond to the ones described for laparoscopic cholecystectomy.
The spleen is rarely a primary target organ for a CT examination but it is regularly included in an upper abdominal examination (Table 13.1). CT has a second-line role in locating an infectious focus (splenic abscess) or detecting splenic rupture after bl unt abdominal trauma and should be used whenever ultrasound findings are equivocal. CT is not sensitive enough to detect lymphomatous involvement of the spleen, but it is excellent for detecting splenic metastases from other tumors. CT is not helpful for the etiologic assessment of splenomegaly. The major competing modality is ultrasound. CT is superior to ultrasound in cases where sonographic imaging is difficult. MRI can also be used for splenic imaging but has not yet achieved much practical importance. MRI may gain increasing importance for detecting splenic
Table 13.1 spleen
Indications
for CT examination
of the
CT indicated Tumor staging
As part of upper abdominal examination
Search for splenic abscess
If ultrasound findings are equivocal
Blunt abdominal trauma (splenic rupture)
If ultrasound findings are equivocal
CT not indicated Splenic involvement Investigation
by malignant lymphoma
of splenomegaly
involvement in lymphoma using contrast agents specific for reticuloendothelial tissue.
Anatomy
The size and shape of the spleen are highly variable, and the diagnosis of splenomegaly is subjective, at least in the borderline range. The following biometric determinations can be made. The craniocaudal length of the spleen (L) has a normal range of 10-15 cm and is determined by the craniocaudal extent on CT. The maximum diameter (D) on any transverse image is 4-8 cm. The maximum thickness (T) is 3-5 cm (Fig. 13.1) measured at the level of the splenic hilum. If the splenic shape is markedly irregular, a number of measurements may be averaged to yield the thickness T. The splenic index LxDxT should be less than 480. A recent estimate of the splenic volume from spiral CT data has been provided by Prassopoulos et al. (1997): V-30+0.58xLxDxT
The authors found the splenic volume in adults to be largely independent of age and gender, height, weight, and body mass index. The mean volume was 215ml (range, 110340ml). The splenic weight can be estimated by multiplying the volume V by 1.05, the density of soft tissue. The splenic weight ;n vfvo is probably greater than the published weight from post mortem studies due to the large amount of blood contained within the spleen in vi'vo.
Splenic index= Lx W x T «480 is normal) D=maxJmum diameter T =thickness at splenic hilum L=craniocaudal length
Fig. 13.1 Cross-sectional diagram showing the relation of the spleen to the tail of the pancreas, the adrenal, stomach, and left colic flexure.
Examination
A list of causes of a "small" spleen is given in Table 13.2. while causes of splenomegaly are listed in Table 13.6. Normal splenic parenchyma has a CT attenuation of 40-50 HU (lOHU less than the liver). Changes in the relationship of splenic to hepatic attenuation are generally due to hepatic disease rather than a change in the spleen. Increased attenuation values are seen in primary hemochromatosis. sickle-cell anemia. and thalassemia. The spleen is an intraperitoneal organ that relates closely to the gastric fundus, the tail of the pancreas, and the left kidney and adrenal gland (Fig. 13.1). The spleen is sufficiently soft and pliable in texture that left upper quadrant abdominal masses or organ enlargement can cause considerable displacement and deformity in shape. Thus, tumors of these organs commonly displace the spleen but rarely invade it. Perisplenic abscesses or hematomas lead to considerable deformity that does not necessarily indicate splenic infiltration or rupture. The shape of the spleen may vary as a result of increased lobulation or impressions on the
spleen from neighboring organs. Fissures in a lobulated spleen should not be mistaken for splenic rupture. Splenic indentation can mimic a pararenal or pancreatic mass at ultrasound but is easily recognized on CT scans. The splenic artery is often tortuous, and a long segment of the vessel may show calcifications, which should not be mistaken for pancreatic calcification. The splenic vein runs just posterior to the pancreas. Its confluence with the superior mesenteric vein can be identified on axial scans by a slight dilatation of the vessel. The inferior mesenteric vein enters the splenic vein several centimeters to the left of the venous confluence.
Technique
In most cases the spleen is scanned as part of an upper abdominal CTexamination. The examination technique and contrast protocol are geared toward the primary organ of interest (Table 13.3). A non contrast examination of the spleen is useful only for the detection offresh hemorrhage in trauma patients. Most other splenic lesions are difficult to detect on unenhanced scans. Table
Table
Technique
Protocol for CT scanning
of the
The parenchymal phase of enhancement is the most rewarding for diagnostic evaluation of the spleen. During the arterial phase, the splenic enhancement is markedly heterogeneous and may hamper an accurate evaluation ofthe splenic parenchyma (see below). Since scan delays of 40 to 60 seconds are used in examinations of the pancreas and liver, such artifacts should always be considered in evaluations of the spleen.
spleen
General Oral contrast
not mandatory
Patient
Supine
position
Upper
Scan range
with elevated abdomen
arms
from diaphragm
to lower pole of liver, in splenomegaly
includ-
ing pelvis Respiratory Windowing
phase
Inspiration
Noncontrast CT: Contrast-enhanced
CT:
W/L - 350/40 W/L =400/60 Continue
A_
13 The Spleen Table 13.3
(Continue)
I Scan parameters Standard, trauma Focal lesion
1-1.2St
Contrast injection Standard
IS-slice3 SC
axial SWIRl
1-1.5 I
5/4
0.75-1.251
4/3
3/3 cor 16-slice V+IM/F/D'
4-slice V+N/F/D 12012/60
MPR" SWIRl
120 + 5013160
120+50/3/205
SC = slice collimation (rnrn). TF = table feed (mm/rotation), RI = recon. increment (mrn), f [ = scanning direction, SW= effective section width (rnrn). MPR= multiplanar reformats, axial = axial sections, cor = coronal, sag = sagittal CM = contrast material. Contrast concentration = 300 mglml iodine V '"=volume of CM (rnl). N = volume of saline flush (ml), F = flow rate (mils), D = start delay, X '"=trigger region a
Pitch P=T FI (NxSC) should be chosen close to 1.5with 4-slice scanners, 1.3-1.5with 16-slice scanners
b
MPR are reconstructed from a "secondary raw data set" with SWIRl =1-1.510.7
c
read: 20S = start delay 1Os after reaching 50 HU enhancement in the spleen
Enhancement Characteristics
During the arterial phase of enhancement (e.g., in a biphasic liver study), the splenic parenchyma shows a characteristic, heterogeneous enhancement pattern (Fig. 13.2). This effect has not been fully explained yet but has been attributed to a differential perfusion of the
red and white pulp that produces an arciform and wave-like pattern of sharply defined enhancing and non enhancing areas. Because these heterogeneities can mimic focal lesions, the arterial phase should not be used for lesion detection in the spleen.
a b Fig. 13.2 Atypical striped enhancement pattern ofthe spleen is seen on arterial phase images and is caused by differential enhancement of the red and white pulp (a).
In rare cases (b) this pattern can mimic a lesion (arrow). Adjacentscans (c) suggest the correct interpretation by resolving the lesion into band-like structures.
c
Accessory Spleen, Polysplenia, Splenosis
During the parenchymal phase, which begins 60 to 90 seconds after the start of the injection, the normal splenic parenchyma has a
Accessory Spleen, Polysplenia,
CT Morphology
Accessory spleens are generally located in close proximity to the spleen (hilum, gastrosplenic ligament). Rarely they may occur in the pancreas or lesser pelvis. Most are less than 1 cm in
Fig. 13.3 Polysplenia in a patient with Karthagener syndrome and situs inversus. The multiple accessory spleens show an enhancement pattern that is characteristic of splenic tissue. L = liver, P = pancreas.
diameter, but accessory spleens as large as lOcm have been described. They are identified by their rounded shape, smooth margins, and enhancement characteristics that parallel that of the spleen (Fig. 13.3). Splenosis presents with multiple round or oval masses in a patient with a history of splenic trauma (Fig. 13.4). In most cases, no spleen is
Fig. 13.4 Splenosis after splenectomy for splenic trauma (4 x 1/6). There are multiple nodular peritoneal masses (arrows) that enhance similarly to splenic tissue (a) displayed on an oblique coronal section, as well as aVRTimage(b).
a
enti-
Splenosis
The developing spleen fails to fuse normally in 10-30% of the population, and multiple spleens develop in approximately 10-20% of these individuals. More than two accessory spleens occur in less than 5%. Polysplenia denotes a condition in which the spleen is subdivided into multiple parenchymal segments that usually have a rounded shape. It is mostly associated with various other complex abnormalities of the abdominal vasculature and gastrointestinal tract. Splenosis develops when splenic tissue is seeding within the abdomen or chest following trauma. This autotranplanted splenic tissue will undergo hypertrophy, leading to the formation of masses in the peritoneum, along the diaphragmatic surface, the mesentery and omentum, the liver, or even the pleura. Accessory spleens may undergo hypertrophy after splenectomy and may cause recurrence of symptoms in patients with hematologic disorders associated with hypersplenism. •
homogeneous appearance. Consequently, ties during this phase signify pathology.
b
501
13 The Spleen
present (resection after injury). All masses have a homogenous enhancement pattern but the degree of enhancement may vary depending on their blood supply.
Accessory spleens can mimic abnormal lymph nodes or pancreatic, adrenal, or intraperitoneal lesions on unenhanced CT scans.
Cystic Splenic Lesions (see Table 13.4) Table 13.4
Cystic splenic lesions
Congenital Splenic Cysts
Congenital cyst
Hypoattenuating «20 HU), smooth margins, homogeneous, possible wall calcifications
Post-traumatic cyst
Hypo- to hyperattenuating, smooth margins, homogeneous, possible wall calcifications
Pancreatic pseudocyst
Hypoattenuating, often very large, signs of previous pancreatitis
Echinococcal cyst
Calcifications (usually in the wall, occasionally in septa), daughter cysts, contrast enhancement of wall and septa
Abscess
Hypoattenuating, occasionally with indistinct margins; usually does not show rim enhancement
Candida sepsis
Numerous small focal hypodensities, possible wedge-shaped perfusion defects
Lymphangioma
Multiple cysts, relatively small and confluent with well-defined margins; a rare lesion
a Fig. 13.5
(b).
Splenic cysts frequently represent incidental findings at CT. Congenital splenic cysts (also called epidermoid, mesothelial, or primary cysts) are rare. They are epithelialized and are often associated with cystic kidney disease. •
CT Morphology
Cysts appear as well-circumscribed, homogeneous lesions that usually have attenuation values less than 20 HU. They do not enhance and do not show increased peripheral vascularity. Wall calcifications may occur in less than 15% and septations in less than 20% (Fig. 13.5). In 80% of cases, congenital splenic cysts are unilocular and solitary. Infrequently, the cysts show wall trabeculation or peripheral septation. Very rarely, congenital epidermoid cysts may contain cholesterol crystals (negative attenuation values) or blood.
b Splenic cyst (4 X 1/6) with tiny septation
and calcification
on axial image (a) and volume-rendered
image
Splenic Abscess Post-traumatic
Pseudocysts
Pseudocysts after trauma or infarction are not epithelialized and result from the cystic degeneration of hematomas. They are substantially more common than congenital cysts.
• cr
Morphology
Higher attenuation values may be seen after trauma or infarction. Post-traumatic cysts have been reported to become as large as 13 em. Wall calcifications (50%) and peripheral septations (85%) are more frequent in post-traumatic cysts than in congenital ones.
Postpancreatitic
Pseudocysts
Intrasplenic pseudocysts are very rare.
• cr
after acute pancreatitis
Morphology
Pseudocysts secondary to pancreatitis that arise in the tail of the pancreas adjacent to the splenic hilum occasionally extend beneath the splenic capsule or even into the splenic parenchyma. Such intrasplenic pseudocysts can reach considerable size. They are usually surrounded by a well demarcated wall that enhances after intravenous administration of contrast material. Signs of previous pancreatitis will suggest the correct diagnosis. Further splenic complications of pancreatitis include subcapsular hematoma. rupture. infarction. splenic artery pseudoaneurysm, and splenic vein thrombosis.
Echinococcal
Disease
involvement by Ecbinococcus granulosus (cysticus) is present in less than 5% of patients. It is rare, except in endemic areas (e.g., Argentina, Greece, Spain).
Splenic
•
echinococcosis. The cyst wall enhances after contrast adminis tra tion. Percutaneous aspiration should be avoided in an untreated patient because of the risk of allergic reaction to cyst content and spread of infection.
CT Morphology
Echinococcal (hydatid) cysts are well circumscribed low density lesions that enlarge the spleen. Wall calcifications are extremely common (» 50%), while intracystic calcifications are rare. The mother cyst (10-45 HLJ) may contain daughter cysts of slightly lower attenuation at its periphery, confirming the diagnosis of
Splenic Abscess
Splenic abscesses are more often multiple than solitary. Most develop in the setting of septicemia. Microabscesses are commonest in imrnunocompromised patients and usually result from a fungal infection (Candida. Aspergillus, Cryptococcus). Splenic abscesses in AIDS patients are additionally caused by mycobacteria and Pneumocystis carinii. Pyogenic abscesses may develop secondary to superinfection of an infarcted area or post-traumatic hematoma. Predisposing primary infections are endocarditis, urinary tract infection, surgical wound infection, pneumonia, and appendicitis. CT can detect abscesses only a few millimeters in size. The clinical presentation must be considered in order to make an accurate differential diagnosis. CT Morphology
Abscesses appear on CT scans as focal areas of low attenuation (Fig. 13.6). Usually there is no inflammatory rim. Many abscesses remain uniformly hypoattenuating after contrast administration and do not show peripheral enhancement (Fig.13.6a). The margins of the abscess may be ill-defined. Gas collections are very rarely seen (Fig.13.6b). Splenic abscesses are commonly associated with similar lesions in the liver (15-20%). Multilocular abscesses have a fungal etiology in more than 60%, while unilocular abscesses have a bacterial etiology in more than 90%. In Candida sepsis the spleen is permeated by numerous small lesions, and the occlusion of small arteries can lead to wedge-shaped infarcts (Fig. 13.6c). Calcifications are seen in treated Candidamicroabscesses and in lesions caused by other fungi, by mycobacteria, and Pneumocystis carinii. Focal hypoattenuating areas composed of granulomatous tissue may persist after therapy.
503
13 The Spleen
c
b
a
Fig. 13.6 (a) Staphylococcal sepsis with large subcapsular splenic abscesses, (b) Superinfected splenic infarctwith small gas inclusions, (c) Multiple small splenic
A CT scan without focal abnormality does not exclude the possibility of early infection, particularly in hematogenously disseminated fungal disease.
Lymphangiomas
Lymphangiomas are benign tumors that are categorized as capillary, cavernous, or cystic, depending on the size of the abnormal lymphatic channels. In the spleen the cystic type is com-
and hepatic abscesses are typical of Candida immunosuppressed patients (here: AM L).
sepsis in
monest. Lymphangiomas are more frequent in the neck and axilla than in the spleen. •
CT Morphology
Cystic lymphangiomas are composed of multiple, relatively small, sharply circumscribed confluent cysts, often in a subcapsular location, which are filled with lymphatic fluid and do not enhance after contrast administration. CT attenuation varies between 15 and 35 HU. Curvilinear calcifications have been reported. There is generalized enlargement of the spleen.
Solid Splenic Lesions Benign Splenic Tumors
The spleen is rarely affected by benign primary tumors (hemangioma, lymphangioma, hamartoma, fibroma, myxoma, chondroma, osteoma, desmoid tumors) (Fig. 13.7). Hemangioma is the commonest of these tumors (0.01-0.14 % in autopsy series). Splenic hemangiomas are freq uently associated with hemangiomas in other organs. In the spleen, cavernous hemangiomas are more common than capillary hemangiomas. A new entity specific for the spleen is the littoral cell angioma. It is characterized by multiple nodules composed of vascular channels of red pul p, and is more often diffuse than focal. A malignant angiosarcomatous variant has been described.
•
CT Morphology
Hemangiomas may be solitary or multiple and range in size from a few millimeters to 15cm. Capillary splenic hemangiomas present as homogeneously hyper- to isoattenuating nodules. Their dynamic enhancement pattern almost parallels that of the aorta but with a markedly lower peak enhancement. Cavernous hemangiomas may present with similar imaging characteristics as hepatic hemangiomas. On unenhanced scans they may be isoattenuating to the aortic blood and slightly hypo attenuating to the splenic parenchyma. Cystic components are frequent, and may be the dominant feature in some tumors. Multiple calcifications are com-
Malignant Primary Tumors of the Spleen Fig. 13.7 Hamartomaofthe spleen (4 x 1/6). Multiple hyperenhancing masses (arrows) seen only on arterial phase images (a) appear isoattenuating in the portal phase (b). There is a small cystic component.
a
mon, and are either punctate and curvilinear, or radially oriented from the center. Rupture and hemorrhage are reported in up to 25% of cases. Littoral cell angioma shows multiple small hypo attenuating nodules in the portal phase, ranging in size from 5-Wmm. The nodules become isodense to splenic tissue on delayed postcontrast images. Table 13.5
b
The CT features of these and other benign lesions are summarized in Table 13.5. Malignant Primary Tumors of the Spleen
Malignant tumors that arise in the spleen include (littoral cell) angiosarcoma, fibrosarcoma,
Solid splenic lesions
Tumor-like lesions Perfusion artifact
Early perfusion phase, ring-like or band-like; compare with adjacent slice
Splenic infarct
Band-like or wedge-shaped defect broadly based on the splenic capsule; coronal or sagittal sections may be required
Sarcoidosis
Rounded hypovascular lesions
Amyloidosis
Rounded hypovascular lesions
Hematopoiesis
Rounded hypovascular lesions
Gaucher's disease
Rounded hypovascular lesions
Peliosis
Isoattenuating to blood, enhancement characteristics similar to splenic artery
Benign tumors Hemangioma
Solitary or multiple, isoattenuating to blood on noncontrast and delayed scans, multiple calcifications, some may show same enhancement pattern as liver hemangioma. Capillary hemangiomas are homogenous, cavernous types often contain cystic portions
Hemang iopericytoma
Multiple small confluent nodules; solid and cystic; intense enhancement (solid portion)
Littoral cell angioma
Multiple small hypodense nodules; isoattenuating on delayed phase; diffuse> focal
Hamartoma
May contain fatty components and amorphous hancement
Fibroma, myxoma
Rounded hypovascular lesions after contrast administration; rare
Desmoid tumors
Hypovascu lar lesions; rare
Chondroma
Rounded hypovascular lesions with amorphous calcifications;
Osteoma
Rounded, homogeneous plaquelike calcifications;
calcifications;
heterogenous en-
rare
rare Continue
>
505
13 The Spleen Table 13.5
Continue
Malignant tumors Lymphomas
Rounded hypovascular lesions, rarely showing rim enhancement or heterogeneities (detected injust 20% all lymphomas involving the spleen)
Metastases
Heterogeneoushypo- or hypervascular lesions, usually multiple. Central necrosis possible. Most common primaries: melanoma > breast carcinoma > bronchial carcinoma > colon carcinoma > renal cell carcinoma >others
Angiosarcoma
Heterogeneous,very hypervascular, history ofThorotrast administration; rare
Heterogeneous hypervascular lesions after contrast administration; rare
and leiomyosarcoma, malignant teratoma, and malignant fibrous histiocytomas. The development of angiosarcomas has been linked to the prior administration of Thorotrast, a thoriumcontaining contrast agent used in the 1930s to 1950s. Some authors have used the term (littorial cell) hemangioendotheliomas to refer to vascular tumors of borderline malignant potential. •
CT Morphology
The CT appearance of these tumors is nonspecific. Primary splenic malignancies appear as relatively large, heterogeneous masses that may contain cystic and solid components. Hypervascular tumors (especially angiosarcomas) show marked, heterogeneous enhancement following contrast administration and often contain areas of necrosis. Attention should be paid to obvious criteria of malignancy such as distant metastases and signs of infiltration (Fig. 13.8). The tumors may be multifocal, involving both the liver and the spleen. Splenic rupture has been reported to occur in one- third of patients. Metastases
Splenic metastases are most commonly seen in patients with advanced primary tumors. The most frequent primaries are malignant melanoma (approximately one-third of melanoma patients), breast carcinoma, and bronchial carcinoma (see Table 13.5). •
CT Morphology
Metastases may appear as ill-defined areas of low attenuation on unenhanced scans. They are more clearly delineated after contrast adminis-
Fig. 13.8 Malignant hemangioendothelioma of the spleen with nodular hypervascular components and necrotic areas. Note the different enhancement characteristics of the hepatic metastases.
tration. Cystic areas of necrosis may occur. Multifocal lesions are the rule, but solitary splenic metastases have been described even in the absence of other metastatic disease. Diffuse infiltration is reported in up to 10% of affected patients. Cystic lesions may occur with metastasis from melanoma, and cancer of the ovary, breast, and endometrium. Calcifications may occur in patients with mucinous adenocarcinoma as a primary tumor. Lymphoma
Lymphoma is the commonest primary malignancy of the spleen. Primary splenic lymphoma is rare (1-2% of all lymphomas) and is usually a non-Hodgkin Lymphoma (NHL) of the small cell type.
Inflammatory Diseases, Parainfectious Splenomegaly Secondary splenic involvement is frequent in both Hodgkin's disease (HD) and NHL. Approximately 25-33% of all patients with HD and NHL are affected. Reported numbers for sensitivity/specificity of CT to diagnose splenic involvement by lymphoma vary substantially, ranging from 30% and 7"!% to 90% and 90%, respectively. Diffuse lymphomatous involvement need not be associated with splenomegaly. Nodular lesions are seen in fewer than 20% of patients with splenic involvement. However, this is the only type of involvement that can be reliably detected by CT.Therefore CTis used primarily for lymph node staging. •
CT Morphology
Splenic involvement in lymphoma can have four forms: homogeneous enlargement, miliary nodules, multifocal lesions of A to 10cm, or a solitary mass. CT can detect focal or multifocal disease more reliably than diffuse infiltration. Commonly, the spleen appears homogeneous on unenhanced scans. Larger focal lesions may cause bulging of the splenic contour. During the parenchymal phase of enhancement, focal lesions are clearly demarcated as hypodense areas (Fig. 13.9). Occasionally their margins are ill-defined. Rim enhancement is rarely observed.
Fig. 13.9 Nodular involvement of the spleen by NHL. The CT morphology is indistinguishable from hypovascular metastases or nodular involvement by sarcoidosis. The slight enhancement of the lesions distinguishes them from abscesses.
Necrosis of large lesions has been reported and can cause an irregular cystic appearance that may mimic an abscess in patients with lymphoma and associated fever. Calcifications are rare before, and more frequent after therapy. In NHL, splenic involvement is associated with para-aortic lymph node involvement in approximately 70% of patients.
Diffuse Splenic Diseases, Splenomegaly (see Table 13.6) Table 13.6
Reactive splenomegaly can develop as a parainfectious process in response to mononucleosis, typhus, bacterial endocarditis, or other infectious diseases. •
CT Morphology
Parainfectious splenomegaly generally has a homogeneous appearance. Abscesses are not present. Intrasplenic hemorrhage may occur as a rare complication. Infiltration of the spleen by Pneumocystis cetinii (mostly in AIDS patients) may show a fine speckled pattern of hypo attenuating lesions after contrast administration (differential diagnosis: fungal abscesses) or may present as splenomegaly with or without multiple punctate calcifications.
13 The Spleen
Calcifications also occur after other granulomatous inflammations such as tuberculosis or histoplasmosis (Table 13.7). Other organs, such as the liver, kidneys, or adrenals, may also be affected.
phomatous involvement, even in a patient with confirmed lymphoma. Diffuse infiltration may not alter the enhancement characteristics of the spleen, but a few cases show a pattern of fine, nodular hypo attenuating lesions.
Lymphoma
Portal Hypertension
Diffuse lymphomatous infiltration of the spleen is more common in NHL than Hodgkin's disease. It is mostly due to low grade lymphomas with associated blood involvement.
Portal hypertension leads to splenomegaly and stimulates the development of portosystemic collaterals.
•
In most cases the enlarged spleen presents a homogeneous pattern on unenhanced scans and during the parenchymal phase after contrast administration. The splenic vein is often dilated (> 1Omm) , and collateral vessels are found in the hilum of the spleen. Portosystemic collaterals involving fundic, gastroepiploic. and gastroesophageal veins are recognized by their increased tortuosity and their sometimes grotesquely enlarged diameters (differential diagnosis: poorly opacified bowel loops) . Enlargement of the left renal vein should prompt a search for a spontaneous splenorenal shunt (see Fig. 11.51).Ascites maybe present. Frequent causes are hepatic cirrhosis (e.g., small liver with a nodular surface, hypertrophy of the left and caudate lobes) or a thrombosis in the splenic or portal vein.
CT Morphology
CT is able to detect splenomegaly (splenic index >480) but is often not useful for the direct detection of diffuse splenic involvement. Diffuse infiltration may or may not lead to splenomegaly. Conversely, the presence of splenomegaly is not necessarily caused by lymTable 13.7
Between 20% and 60% of patients with sarcoidosis develop splenomegaly. CT cannot detect diffuse involvement of the spleen and is useful only for detecting the less common focal involvement. •
CT Morphology
Noncontrast CT shows a uniformly enlarged spleen. On postcontrast scans heterogeneous enhancement may be noted. The spleen may contain sharply circumscribed hypo attenuating lesions corresponding to aggregated granulomas. Coexistent abdominal lymphadenopathy is frequently seen in patients with splenomegaly. The chest radiograph may be normal in up to 25%. The presentation may be morphologically indistinguishable from focal lymphomatous involvement (Fig. 13.9).
Trauma
The spleen is the organ that is most frequently injured by blunt abdominal trauma. The extent of the injury can range from subcapsular hemorrhage and parenchymal contusion, to a parenchymal laceration (with or without capsular injury), or a complete splenic rupture. The detection of a subcapsular hemorrhage is important because there is the potential of a lifethreatening hemorrhage in the event of a capsular rupture. Injury to the vascular pedicle leads to splenic infarcts of variable extent. CT has greater than 95% sensitivity in the detection of splenic injuries and is indicated whenever ultrasound findings are inconclusive. •
CT Morphology
A hematoma may appear hyperattenuating (after fresh hemorrhage, often with clot formation), isoattenuating (older hemorrhage or hyperacute extravasation without separation of red cells and serum), or even hypo attenuating to the splenic parenchyma, depending on the age ofthe collection. Contrast -enhanced scanning is required in addition to precontrast scans for adequate evaluation of splenic trauma, as areas of hematoma or laceration may be isoattenuating to the splenic parenchyma on noncontrastenhanced CT and the splenic injury is more clearly demarcated after contrast administration. On the other hand, fresh hematomas may
a
b
Fig. 13.10 Splenic trauma, (a) Traumatic splenic laceration with a concomitant rupture of the diaphragm. The splenic capsule is disrupted, but the intrasplenic hemorrhage is poorly visualized, (b) Splenic rup-
appear isoattenuating to the splenic parenchyma on enhanced scans. A subcapsular hemorrhages appears as a peripheral crescent-shaped collection that indents and displaces the splenic parenchyma. It may reach a considerable size. An intra parenchymal hematoma following splenic contusion appears as a patchy, ill-defined area of low attenuation with hyperattenuating components (Fig. IS.10a). A laceration or rupture produces a linear, wedgeshaped, or rounded parenchymal defect of low density that shows absent or reduced enhancement (Fig. 13.10b) and covers part or all of the organ cross section. Hemoperitoneum usually accompanies this type of injury. Contrast extravasation is a sign of active hemorrhage that nearly always needs immediate surgical treatment. Traumatic splenic false aneurysms show a CT attenuation similar to blood vessels after contrast administration (Fig. 13.10c) and predispose for a delayed splenic rupture. In rare cases, initial CT scans are normal despite the presence of a splenic injury. Splenic hemorrhage may be absent initially due to hypotension from severe blood loss, but it may commence later when volume replacement is instituted. A delayed splenic rupture is marked by intraperitoneal blood collections that are often denser (fresher) than perisplenic or intrasplenic blood collections.
c ture with a band-like laceration of the parenchyma. (c) Post-traumatic pseudoaneurysm of the spleen with a fine parenchymal Jaceration. Delayed splenic rupture occurred one week later.
13 The Spleen count the integrity of the capsule, the size of the hematoma, the number and length of lacerations, vascular involvement, and the extent of parenchymal devascularization, The later schemes have improved upon older ones, and significant correlation with patient outcome is noted, However, these correlations are not strict for grade III lesions and conflicting results from nonsurgical management have kept the discussion alive, The results of nonsurgical management may be improved by more liberal use of endovascular therapies like embolization, CT can play an important role in patient selection, Lack of splenic parenchymal enhancement without infarction may occur in the setting ofprofound hypotension, It has to be differentiated from arterial disruption after abdominal trauma, Splenic injuries may take several months to fully resolve on follow-up CT examinations, Peritoneal blood and perisplenic hematoma typically resolve in 1 to 3 weeks, Infection may complicate hematoma resolution and produce a splenic abscess, Pseudocysts may develop as a late sequel to parenchymal hematomas,
Even if a parenchyma! defect cannot be directly visualized, the detection of perisplenic blood clots (so called sentinel clots, patchy hyperattenuating structures with CT numbers >60HU as compared to 35-45 HU of a hemoperitoneum) provides an indirect sign of parenchyma! injury, Faint focal hyperenhancing regions in an otherwise homogenous splenic parenchyma may be an indicator of laceration and are associated with an increased risk of delayed rupture, Strongly hyperenhancing regions suggest a traumatic false aneurysm, Splenic laceration may be mimicked by beam-hardening or streak artifacts due to ribs, an air-contrast interface in the stomach, nasogastric tubes, or monitor material. The streaks usually extent beyond the splenic parenchyma, Fissures in a lobulated spleen should not be mistaken for a splenic laceration or rupture, An over-distended stomach compressing the medial border of the spleen may obscure a small laceration, Several CT grading systems (e,g" by Mirvis, 1989 or Federle, 1998) have been proposed to predict which patients can be successfully managed conservatively and which need immediate surgical intervention (Table l3.8), They take into ac-
Table 13.8
AAST
organ
injury scale for splenic
injuries-modified
Hematoma
Subcapsular,
nonexpanding,
Laceration
Capsulartear,
nonbleeding,
Hematoma
Subcapsular,
non expanding,
Intra parenchymal, Laceration
1111
Hematoma
I Laceration
IV
V
Hematoma
«zcm
active
2 em diameter or expanding >3 em parenchymal Ruptured Involving
segmental
Laceration
Shattered
spleen
Hilarvascular
one grade for multiple
depth
intraparenchymal
Laceration
Vascular Advance
Capsulartear,
CT version
injuries
hematoma
or hilar vessels
with
injury that devascularizes to same organ
active
producing
up to grade
spleen III.
bleeding
devascularization
>25%
Splenic Artery Aneurysm
Vascular Disorders Splenic Infarct
Splenic infarcts may be caused by thromboemboli ejected from the heart or an aortic aneurysm. Thrombotic occlusions of splenic arteries can result from atherosclerosis, subendothelial
infiltration
in myeloid
leukemia,
in-
flammatory or neoplastic pancreatic diseases, vascular encasement by gastric carcinoma, hemagglutination in sickle cell anemia, or traumatic intimal •
injury.
CT Morphology
Unenhanced CT demonstrates an acute splenic infarct as a subcapsular area of low attenuation, that classically is well demarcated and wedgeshaped with the base at the splenic capsule. Not infrequently, however, infarcts may appear as more rounded or linear poorly marginated lesions that are hard to distinguish from other splenic lesions. Extension to the capsule suggests the correct diagnosis (Fig. 13.11a). Splenic infarcts show little or no enhancement after contrast administration. Occasionally an enhancing strip of parenchyma is seen just beneath the capsule. This rim sign is caused by opacification via capsular vessels (Fig.l3.11 b). Secondary hemorrhage may occur, causing the lesion to appear hyperattenuating. Over time, the lesion becomes isoattenuating to the surrounding parenchyma on noncontrast scans. A pseudocyst may develop from the infarcted area, or the infarct may undergo secondary calcification.
CT in patients with homozygous sickle cell anemia and chronic recurrent splenic infarcts shows a shrunken, heavily calcified spleen. Patients with the heterozygous form have an enlarged spleen showing multiple subcapsular infarcts with calcifications. Splenic Vein Thrombosis
Thrombosis of the splenic vein may occur in the setting of an inflammatory or neoplastic pancreatic disease, peritonitis, or after trauma. •
CT Morphology
With an acute thrombosis, the splenic vein usually appears dilated and hyperattenuating. Later the vein shrinks and acquires soft-tissue density. It does not enhance after contrast administration. Collaterals develop via short gastric, gastroepiploic, gastroesophageal, and splenorenal venous channels. Splenomegaly is invariably present. Splenic Artery Aneurysm
Aneurysms of the splenic artery are the commonest abdominal artery aneurysms. An incidence of 0.01-2% is reported in autopsy series, rising up to 10% if aneurysms between 0.5 and 1 cm are included. Predisposing conditions include multiparity, portal hypertension, and arteriosclerotic disease. Congenital splenic artery
Fig. 13.11 Splenic infarct. (a) MUltiple linear and wedge-shaped infarctions in a patient with a hematopoietic disorder and splenomegaly (4 x 2.5/15, VRT). (b) Splenic infarct appears as a hypodense wedge-shaped region bounded by an enhancing subcapsular strip of parenchyma. This rim sign is only seen occasionally and can be difficult to distinguish from an abscess membrane.
a
b
511
512
73
The Spleen
Fig 13.12 Patient with chronic renal failure, kidney transplantation, and a stenosis of both native renal arteries (arrowheads) who also has a large partially calcified aneurysm of the splenic artery (arrows). Volume-rendering of a multislice eTA data set (4 x 1/6) in an AP direction (a) and caudocranial direction (b).
a
b
aneurysms are frequently accompanied by aneurysms of the hepatic or celiac artery. A false aneurysm of the splenic artery can develop as the result of a shearing injury (seat-belt trauma) . Splenic artery aneurysms are a common incidental finding at CT. Embolization or resection is recommended if the aneurysm is symptomatic, larger than 2 em, increasing in size, seen in pregnant women or in women of childbearing age. Preoperative visualization can be accomplished with CTA.
•
CT Morphology
CT demonstrates a circumscribed mass in the course of the splenic artery, mostly in the distal third of the artery (Fig. 13.12). The mass enhances strongly during the arterial phase after contrast injection, and its diameter exceeds that of the adjacent vascular segments. It can range from a few millimeters to several centimeters in size. Larger aneurysms are associated with wall calcifications and peripheral thrombosis.
14 The Pancreas The current mainstays in the radiological evaluation of the pancreas are ultrasonography, computed tomography and magnetic resonance imaging, Diagnostic endoscopic retrograde pancreatography (ERCP) is presently being substituted by MR cholangiopancreatography (MRCP), Endoscopic ultrasound is important for small
Table
14.1
Tumor
Indications
diagnosis
Oncologic
follow-up
for CT examination
tumors or ambiguous findings at CT or MR At present, CT offers the broadest spectrum of information in the diagnosis of pancreatic diseases (Table 14.l), Thin-section spiral and multislice CT have increased the sensitivity of CT for the detection of small tumors and have improved the accuracy of tumor staging,
of the pancreas
Pancreatic
carcinoma:
Endocrine
pancreatic
Therapeutic
response,
Trauma
Pancreatic
Pancreatitis
Differentiation
rupture,
tumor
detection,
tumors:
potential
for curative
resection
localization
suspected
recurrence
pancreatitis
of exudative
and necrotizing
forms;
Pretherapeutic CT to determine extent of inflammatory Detection and identification of pancreatic pseudocysts; Evaluate for suspected abscess
spread;
Anatomy The pancreas is located in the anterior retroperitoneal space (Fig, 14.1), which is bounded in front by the parietal peritoneum and behind by Cerota's fascia, The tail of the pancreas usually sweeps upward, and ends intra peritoneally within the splenorenalligament The normal pancreatic parenchyma has CT attenuation values in the range of 30-60 HU, Pancreatic attenuation decreases due to fatty infiltration (which occurs normally with aging) and is increased in hemochromatosis, The size and shape of the pancreas are highly variable and depend on age and fat content (normal upper values for the transverse diameter: 3cm in the head, 2,5cm in the body, 2cm in the tail), A more important criterion of normal pancreatic morphology is an even tapering of the pancreas from the head to the tall. The tail of the pancreas usually sweeps upward, but in 25% of cases it turns downward and can mimic a renal or adrenal mass, or a recurrent tumor following nephrectomy, Atrophy or fatty infiltration may give the pancreas a lobulated appearance, The uncinate process is embedded in the duodenal loop (Fig, 14.1, see also Fig, 12.1), While the mesenteric artery is usually surrounded by a thin layer of fat, the mesenteric
vein is in direct contact with the parenchyma of the uncinate process, The venous confluence lies just posterior to the pancreatic head, and the splenic vein runs along the posterior border ofthe body of the pancreas, The splenic artery is very tortuous, so on axial sections only segments of the vessel can be imaged posterosuperior to the pancreas, The main pancreatic duct (ofWirsung) extends along the central axis of the pancreas, its distal portion having a diameter of 3-5 mm near the papilla, The duct can be visualized in most patients up to the tail only with thin-section spiral or multislice CT (3 mm thickness or less), The main pancreatic duct should not be confused with the fine fatty layer that runs parallel to the splenic vein, A diameter greater than 4mm is considered pathologic. The accessory duct (of Santor in i) is detectable only with thinsection CT The distal portion of the common bile duct runs through the pancreatic head, where its diameter is less than 5 mm, and opens into the duodenum at the papilla of Vater. The common bile duct and the main pancreatic duct have either one orifice in common or two separate orifices lying next to each other (see Fig, 12.1),
Anatomy Fig. 14.1 Diagrams illustrating the sectional anatomy of the pancreas. Pancreatic duct -______. Common hepatic artery Common bi Ie -duct
.__ . /
Splenic a Splenicvein
Celiac (arterial) trunk
Gastroduoden.al ~ artery Superior --mesentericvein Superior mesenteric artery Pancreatic head
Inferior mesenteric
•
Anatomic Variants
Pancreas divisum (5-10%) is the commonest anatomic variant. It results from a failure of fusion of the anterior and posterior halves of the developing pancreas and is defined as a completely separate pancreatic ductal system in a grossly undivided gland. The main portion of the pancreas, including the anterosuperior part of the head, the body and the tail, is drained by the dorsal pancreatic duct through an accessory papilla. The posteroinferior part of the head and uncinate process is drained by a short, narrow ventral pancreatic duct that joins the common bile duct in the ampulla. Pancreas divisum is associated with an increased rate of pancreatitis. Although the overall size of the pancreas may be normal, CT demonstrates broadening of the pancreatic head in the craniocaudal and/or an-
-------" _
vein
teroposterior extent. In rare cases the ventral and dorsal moieties may be distinctly visible, separated by a thin fat plane. Thin-section spiral or multislice CT can detect the separate ductal systems. Annular pancreas is a developmental anomaly in which the main pancreatic duct encircles the duodenum and produces a constriction. An annular pancreas is visible with CT only if there is sufficient parenchyma! involvement in the anomaly. It should not be mistaken for duodenal wall thickening. An annulus composed entirely of connective tissue cannot be demonstrated with CT. An accessory pancreas is subject to many variations. Ectopic pancreatic tissue in the stomach or small bowel usually cannot be detected with CT.
515
516
74 The Pancreas
Examination Technique The examination technique varies strongly between evaluation for pancreatic tumors or pancreatitis (Tables 14.2, 14.3).
Table 14.2
Suggested protocols for CT scanning of the pancreas
General Preparation
Tumor diagnosis:
patient should fast for 6 hours
Oral contrast
Pancreatitis:
depent on clinical situation either none or 500 - 1000 ml contrast medium or water, 30 min prior to examination 1000-1500 ml water, 30 min prior to examination additional 250 ml of or water in RLD posltlon on CT table N-butyscopolamine iv (to relax the duodenum)
Tumor diagnosis:
Patient position
Supine with elevated arms Tumor diagnosis: Right lateral decubitus (optional)
Scan range
Tumor:
pancreas only (detection, T-staging), pancreas + liver (M-staging) Upper abdomen to small pelvis
SC~ slice collimation (mm), TF ~ table feed (rnmjrotatlon), RI ~ recon. increment (mm), i L ~ scanning direction, SW ~ effective section width (mm), MPR ~ multiplanar reformats, axial ~ axial sections, cor= coronal, sag ~ sagittal CM ~ contrast material, Contrast concentration ~ 300 mg/ml iodine V ~ volume of CM (ml), N ~ volume of saline flush (ml), F~ flow rate (ml/s), 0 ~ start delay, X ~ trigger region " Pitch P~ TF/(Nx5C) should be chosen close to l.5 with 4-slice scanners, 1.3-1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SW/RI ~ 1-1.5/0.7 or 0.5-0.8/0.5; oblique coronal MPR through the pancreas, sagittal MPR between the common bile duct and the inferior meso vein; CPR parallel to the pancreatic duct c read: 5-30A (205) ~ start delay 5-30 s (20 s) after reaching 50 HU enhancement in the desc. aorta (spleen)
Patient Preparation The duodenum is difficult to opacity because of its brisk peristalsis, but adequate duodenal distension is essential for tumor diagnosis, A right lateral decubitus position improves filling of the duodenum, so the patient should ingest the last cup of oral contrast while lying in this position on the CT table, A spasmolytic drug (e,g" 20-40 mg N-butylscopolamine IV) should be administered to dilate the duodenum, Afterward the patient is moved to the supine position, and scanning is initiated, If N-butylscopolamine is contraindicated, the examination may be performed while the patient remains in the right lateral decubitus position, There is a sound rationale for using negative contrast media (water, milk, or gas) in the duodenum instead of positive contrast media, as this leaves open the option for CT angiography of the pancreatic vessels, does not mask bile duct stones and may be advantageous for evaluation of gastric or duodenal wall infiltration, lfthe clinical situation allows, oral contrast medium should be used for bowel opacification even in patients with acute pancreatitis, In doubtful cases (patients on n.p.o.) the referring physician has to be consulted, Positive oral contrast material aids in differentiating bowel from abscesses, peripancreatic fluid collections, and pseudocysts, Some institutions, however, prefer water as oral contrast agent as it facilitates the evaluation of the pancreatic head, the periampullary region, the distal common bile duct, and
the pancreatic duct Such a procedure is mainly recommended when CT is performed to rule out biliary pancreatitis but it is inferior for identification of complications (e,g" differentiation between water-filled bowel loops and pseudocysts),
Acquisition Precontrast
Technique Scan
Generally, there is no diagnostic need for precontrast scans of the upper abdomen in patients with suspected pancreatic tumors unless there is coexistent chronic pancreatitis, U nenhanced scans may be used, however, to tailor the subsequent contrast-enhanced scan optimally to the tumor size, U nenhanced CT may be used to verify bile duct or pancreatic duct stones as etiologic agents for acute pancreatitis and to detect calcifications in the chronic or chronic relapsing forms of the disease, However, non-calcified stones may not be visualized with this technique, and calcified stones as well as parenchymal calcification usually can also be detected on contrast-enhanced CT Tumor
Diagnosis
Staging of malignant tumors and detection of islet cell tumors requires thin sections to ensure optimum pancreatic imaging, Because an optimum attenuation difference between tumor and parenchyma can be achieved for only a brief
14 The Pancreas
time after contrast injection, trade-offs must be made between scan length and section thickness in single-slice CT For malignant tumors, the liver has to be examined as well because of the high frequency of hepatic metastases, Patients with potentially resectable pancreatic carcinomas are challenging, Most authors suggest biphasic scanning, The first scan is focused only on the pancreas and uses a thin collimation of 2-3 mm with single-slice spiral CT 1-1.25 mm with 4-slice scanners and 0,5-1.25 mm with 16-slice scanners depending on patient size, It is performed during the parenchyma! phase of pancreatic enhancement with a scan delay of 30-40 seconds, depending on the contrast injection rate (A-4mlls is advised for multiphasic studies), In this phase the contrast between parenchyma and lesions is highest, and there is still sufficient enhancement of the arteries to evaluate vascular tumor involvement Often portal enhancement is also already beginning, To increase spatial resolution, images may be reconstructed using a small fieldof-view that is focused on the pancreas, or with a 10242 matrix and a full field-of-view (ifavailable) , The second scan covers the liver and pancreas during the portal venous phase (60-100 second scan delay) and may use a greater section thickness (5mm with single-slice CT 2-2,5 mm with 4-slice CT 1-1.5 mm with 16-slice C1), In patients with large pancreatic carcinomas that are known to be unresectable, a portal phase scan of the upper abdomen suffices, CT angiograpby of the peripancreatic vasculature has become an integral part of the preoperative staging procedure and can be derived from data acquired during biphasic scanning of the pancreas, A scan in the pancreatic parenchyma! phase is used to create CTA images of the peripancreatic arteries but arterial contrast is improved if the scan delay is reduced, For practical purposes a scan delay of about 30s (or better, 10-15 s after bolus triggering in the aorta) usually provides the best compromise between parenchyma! and arterial enhancement CTA in the portal venous phase is best if the scan delay is around 60s, Thin-section scanning with multislice CT has intrinsic advantages for CTA Imaging of the pancreatic duct has been termed CT pancreatography It relies on a thinsection spiral or multislice CT data set acquired in the parenchyma! phase, Curved reformations (CPR), volume rendering (VRT) or minimum in-
tensity projections (MinlP) may be used to visualize the duct (see Figs, 14,6 and 14,7), Virtualendoscopic views (virtual pancreatoscopy) have been employed to demonstrate ductal irregularities or intradu ctal masses bu trarelyyield su perior information over other display techniques, Endocrine pancreatic tumors are generally hypervascular and often can be visualized only in the arterial phase of enhancement If a benign lesion is to be localized, scanning can focus on the pancreas and should employ thin sections (2-3 mm with single-slice spiral CT 1-1.25 mm with 4-slice scanners and 0.5-0,75 mm with 16slice scanners) in order not to miss a small lesion, Scanning may start shortly after the contrast arrives in the aorta (3-4 s after bolus triggering in the aorta) and should be completed shortly after peak aortic enhancement This requires short data acquisition times of20s or less for optimum contrast between tumor and parenchyma, If a malignancy is suspected, the liver has to be covered in the arterial phase as well, and the section thickness may have to be increased to 3-5 mm (Single-slice spiral C1) or 2-2.5 mm (4-slice scanners), depending on the performance of the scanner. Most authors suggest biphasic scanning with a portal phase scan similar to that for pancreatic carcinoma, In most cases, however, this scan only contributes to the staging of larger tumors with venous involvement If arterial phase scanning of the liver is not possible, a noncontrast examination of the liver is recommended, Pancreatitis
For pancreatitis, most institutions perform only a contrast-enhanced scan in the parenchyma! or portal phase of enhancement A contrast dose of 70-100 ml is sufficient to differentiate between exudative and necrotizing pancreatitis (scan delay 40-60 s). Thicker sections (7-8 mm) can be used for evaluation of the entire abdomen to detect spread of exudates or necrosis, Because it is a benign disease and pathology is often gross, dose reduction may be considered, Patients with chronic pancreatitis in whom a tumor shall be ruled out have to be scanned according to the tumor protocol described above, B
Pancreatic Trauma
In patients who have sustained blunt abdominal trauma, noncontrast CT scanning should be performed on the entire upper abdomen for the de-
Enhancement Characteristics
tection cluded to that nation
of hemorrhage. Parenchymal injury is exwitha contrast-enhanced protocol similar used in pancreatitis. and a survey examiof the pelvis should also be performed.
Image Processing Image processing is rarely required for pancreatitis or pancreatic trauma. For the evaluation of tumor resectability. however. various types of image processing can be recommended. As a standard. axial images should be complemented by oblique coronal reformations parallel to the pancreatic head or tail. depending on the tumor location. An MPR thickness of 3-4 mm reduces image noise. It is usually sufficient to include the para-aortic nodes posteriorly and the superior mesenteric vessels anteriorly in the reconstructed range of MPR. Such coronal sections display the cranial and caudal extent ofa tumor and are optimum for assessing fatty tissue invasion. paravascular spread. and the relation of the abnormality to the common bileduct(cf. Fig. 14.19). Sagittal MPR covering a range that includes the common bile duct and the inferior mesenteric vein are well suited to evaluate proximal vascular spread around the superior mesenteric artery (SMA) and celiac artery as well as the commonbileduct(CBD) .
Enhancement
Characteristics
The blood supply to the pancreas is entirely arterial. Consequently. the attenuation of the pancreatic parenchyma rises more rapidly after contrast administration than in the liver. and the enhancement persists for a shorter time due to the absence of a portal phase. Pancreatic enhancement begins shortly after the start of the aortic enhancement. reaches a peak approximately 5 s after the end of the aortic plateau. and rapidly declines to a level that depends on the remaining intravascular contrast concentration. •
Coronal curved planar reformations (CPR) through the pancreatic duct are excellent for displaying the level of ductal involvement (see Fig. 14.7c). CPR through the superior mesenteric artery. hepatic artery or the superior mesenteric vein and the portal or splenic vein are well suited to demonstrate the relation of the tumor to these critical structures. Thin-slab maximum intensity projections (MIP) are excellent for displaying vessel involvement (irregularities. stenoses). They should be as narrow as possible but still include the whole vessel of interest. The plane of the slab should be tilted appropriately. Thin-slab minimum intensity projections (mIP) may be used to display the pancreatic duct. Slabs should be chosen parallel to the pancreatic body and tail. and to course of the duct in the head and uncinate process. Best results are gained with a relatively narrow slab thickness (some 5 mm). Volume rendering with appropriate cut planes allows for excellent 3D display and evaluation of venous as well as arterial involvement. Opacity settings should be chosen to make the fat transparent but keep the enhancing pancreas opaque. By cutting into the volume. the relation of tumor relative to the arteries and veins can be directly displayed (see Fig. 14.8).
Anerial Phase
The arterial phase starts as contrast material arrives in the abdominal arteries (about 15-25 seconds after the start of the injection). During
the course of the arterial phase there is a steady increase in enhancement of the pancreatic parenchyma with a gradual transition to the parenchyma! phase. In general. the arterial phase lasts from about 20 to 40 seconds after the start of the injection of 150 ml of contrast at 4mlls. It is the only phase in which many hypervascular neuroendocrine tumors can be detected. because at later phases the tumors very quickly become isoattenuating to the pancreatic parenchyma and are no longer detectable. The intense opacification of the peripancreatic arteries during this phase is useful for detecting arterial invasion by tumor. but there is still insufficient opacification to detect the invasion of portal venous vessels.
519
14 The Pancreas
•
Parenchymal Phase
artifacts in the mesenteric and portal venous trunks that can mimic thrombosis or tumor invasion (pseudothrombus artifact).
The parenchyma! phase is characterized by strong enhancement of the pancreas • Portal Phase parenchyma. It overlaps with the arterial phase and lasts from about 30-70 seconds after the The portal phase of hepatic enhancement (peak start of the injection (peak at about 40s), de- at about 60-90 s) overlaps with the parenchypending on contrast volume and flow rates. The mal phase of pancreatic enhancement but permore contrast material is injected, the longer sists for a longer time. this phase persists. At constant volume, higher At 50-70 seconds after the start of the inflow rates increase the level but reduce the du- jection opacification of the portal and superior ration of pancreatic enhancement. mesenteric veins is optimal for detection of Pancreatic carcinomas, necrotic areas, pseu- tumor infiltration and thrombosis. Hypovascudocysts, and abscesses are most clearly demarlar hepatic metastases are well delineated. For cated during this phase, which also gives op- this reason, a scan is performed in tumor tim um delineation of the pancreatic duct and patients during the portal phase after arterial or common bile duct. Arterial opacification is still parenchymal phase scanning has been sufficient to define the vessels, but irregularities completed. in vascular contours may not be depicted as When the contrast material bolus is inclearly as during the arterial phase. The splenic creased to 200ml at 4mlls and scanning is carvein opacities approximately 40 seconds after ried out with a 60s delay, a hybrid phase often the start of the injection, and the mesenteric provides high arterial and portal venous opacifivein opacities about 5-10 seconds later. This cation and a good pancreatic and sufficient hedisparity in opacification times produces flow patic parenchymal enhancement.
Cystic Pancreatic Lesions (see Table 14.4) Dysontogenic Pancreatic Cysts
"True" epithelium-lined pancreatic cysts are found in 10% ofpostmortem examinations. They are clinically silent and are commonly (in 25-50%) associated with cysts in other organs (liver, kidneys, spleen, thyroid, ovaries and testis). Berry aneurysms of the cerebral arteries are present in approximately 10% of patients. Polycystic disease of the kidneys and HippelLindau disease is frequently associated with multiple pancreatic cysts. •
CT Morphology
CT demonstrates sharply circumscribed round or oval masses of low attenuation that do not enhance after contrast administration. In most cases the cyst wall is not detectable or appears only as a thin, faint line (Fig. 14.2). Pancreas pseudocysts generally have thicker walls.
Fig. 14.2 True pancreatic cyst. confirmed at operation. CT demonstrates a faintly visible cyst wall.
Pancreatic Pseudocysts
Pancreatic pseudocysts are loculated collections of necrotic material, old blood, and enzymatic fluid that develop as a sequel to acute or chronic pancreatitis. Pseudocysts form when areas of exudation and necrosis become walled off by
Pancreatic Pseudocysts Table 14.4
Differential diagnosis of cystic lesions of the pancreas
Dysontogenic cyst
Sharply circumscribed with a faint or invisible wall; septations does not enhance after contrast administration
Pancreatic
Thick granulating wall; rarely contains septations; history or direct evidence of pancreatitis
pseudocyst
frequently extrahepatic;
Abscess
Gas collection
Cystic fibrosis
Small cysts may occur; no pancreatitis; fatty involution of pancreas
Lymphangioma
Malformation with cluster of small cysts; calcifications tain lymphatic fluid
Microcystic adenoma
Multiple (>6) cysts 1 -20 mm in diameter with hypervascular septa and central calcifications (40%); central scar; unilocular variant very rare
Macrocystic adenoma
Unilocular or multilocular large cystic structure with irregularly hypervascular septa; calcifications «15%); no central scar
Cystadenocarcinoma
Same as macrocystic adenoma (often indistinguishable); foci of infiltration; hypoattenuating soft-tissue components; lymph node enlargement; metastases
rim history consistent with cystic fibrosis may occur; cysts con-
thickened,
Multilocular cystic lesion, usually in uncinate process, segmental pancreatic duct dilatation; communication of cystic lesion with main duct; bulging papilla mucinous tumor
non-epithelialized granulation tissue. There is a continuous transition from a peripancreatic fluid collection to a pseudocyst. A pseudocyst that persists longer than six weeks is very unlikely to resolve spontaneously. Interventional treatment (percutaneous or transgastric drainage) or surgery (cystogastrostomy, cystoduodenostomy, or cystojejunostomy) is then required depending on cyst size and complications (hemorrhage, superinfection). CT is used for treatment planning and for follow-up (see below p. 534).
or infection. CT morphology alone, however, is unreliable for the diagnosis of superinfection. The presence of gas bub bles within a pseudocyst may be caused by gas-forming bacteria but in rare cases can also be caused by a fistula between the pseudocyst and the gastrointestinal tract without concomitant infection.
IS CT Morphology
Pancreatic pseudocysts are highly variable in their size and location. They may be completely intrapancreatic, but it is more common to find extrapancreatic lesions located in the omental bursa or along Cerota's fascia. Rarely, pseudocysts may occur in the mediastinum or retroperitoneum or may erode into the liver or spleen. The cyst wall is often several millimeters thick and is composed of surrounding tissue and reactive granulation tissue (Fig. 14.3). Calcifications may be visible in the cyst wall. Septations are rare. CT numbers of the fluid of more than 25 HU are indicative of coexisting fat necrosis, hemorrhage,
Fig. 14.3 Pancreatic pseudocyst occurring at a typical site in the omental bursa and displacing the stomach. The thick granulating rim is a common finding. Note the substantially reduced attenuation of the liver due to fatty infiltration in this alcoholic patient.
521
74 The Pancreas
Cystadenoma,
Cystadenocarcinoma
•
CT Morphology
Benign micro cystic (serous) adenoma in more than 80% of cases occurs in patients > 60 years of age, with a female predilection of3 :2-9:2. It is distinguished from potentially malignant macrocystic (mucinous) adenoma and from cystadenocarcinoma, which mainly affects women 40 to 60 years of age. Cystadenomas account for 10-15% of all cystic pancreatic lesions, while cystadenocarcinomas comprise only a few percent of all pancreatic malignancies. Approximately 60-80% of subtypes of cystic neoplasms can be correctly categorized by CT. Recently, the intraductal papillary mucinous tumor (IPMT) was recognized as a separate new subtype of cystic pancreatic neoplasms. In the past, this rare tumor has also been named mucinous ductal ectasia or ductectatic mucinous cystic tumor. It originates from proliferating mucin-secreting cells in the ductal wall that form papillary folds. There is an abundant secretion of viscous mucin leading to obstructive dilatation of the ducts. Unlike other mucinous tumors, the tumor is more frequent in men and presents in the seventh decade. The tumor can be either benign or (low grade) malignant. Its location affects the macroscopic features and several subtypes are recognized: the main duct type, branch duct type, and combined type. Correct identification is important as this tumor has a more favorable prognosis than adenocarcinoma or mucinous cystadenocarcinoma and can be treated by curative segmental or total surgical excision.
The benign microcystic serous adenoma is characterized by mul ti pIe (> 6) cysts that are 1-20 mm in diameter with hypervascular septa (Fig. 14.4). The cysts form a lesion that may exceed 5 cm in size, has a multilobulated contour and a honeycomb-like configuration. The tumor may appear solid on noncontrast scans (especially when consisting of only small cysts < 2 mm), but enhancing septations are usually visible after contrast administration. Scans typically show stellate or "sunburst" calcifications (15-40%) within a central fibrotic scar. This scar may show prolonged enhancement on delayed images. Central calcifications are seen more frequently in serous cystadenoma than in any other pancreatic tumor. The tumor shows no signs of local invasiveness. Very rarely, serous cystadenoma may present as a macrocystic variant. The macrocystic mucinous adenoma consists ofa uni- or multilocular cystic mass (Fig. 14.5) that may reach considerable size (always >2cm, many over 10cm). Sometimes multiple small daughter cysts are found on the inner surface of a large cyst. The contents of the cyst may show water or soft-tissue density. The septa are hyperattenuating on postcontrast scans, but generally they are thicker and more irregular than in the microcystic form (important for differential diagnosis) Macrocystic adenomas most commonly involve the body or tail of the pancreas and peripheral calcifications are specific for this entity.
Fig. 14.4 Macrocystic mucinous adenoma (4x 1/6) presenting as a cystic mass in the pancreatic head with a polypoid intracystic lesion (a, arrow) and a dilatation of
the pancreatic duct (b). No intraductal found at histology.
a lesions
were
Cystadenoma, Cystadenocarcinoma
a
b
c
Fig. 14.5 (a) Macrocystic adenoma appears as a sharply circumscribed, unilocular cystic lesion with an enhancing rim. (b) Small cystadenocarcinoma in the pancreatic tail with peripheral enhancement, (c) Cyst-
adenocarcinoma of the pancreatic tail (arrowheads) with infiltration of the peri pancreatic tissue posteriorto the SMA (arrow). Note the massive infiltration of the spleen.
Macrocystic adenomas may undergo malignant change [cystadenocarcinoma') and are therefore classified as premalignant lesions. The malignant process may involve only a small area of the cystic mass. The thickness of the septa does not correlate with the propensity for malignant transformation. The septa exhibit cacifications in less than IS70 of cases, but there is no central fibrotic scar. A solid tumor component, if present, typically shows less contrast enhancement than healthy parenchyma (Fig.14.4c). While the tumor has a smooth external surface similarly to the microcystic form, the presence of solid papillary excrescences and thick septations enhancing after contrast material application practically exclude the diagnosis of a benign microcystic adenoma. Mucinous cystadenoma cannot be confidently distinguished from cystadenocarcinoma on CT scans (Fig.14.4b). They are also not readily distinguished by cytological evaluation of fine needle aspiration biopsy sampling, since undersampling of portions of the tumor containing carcinoma is likely. Thus surgical resection is indicated. For intraductal papillary mucinous tumors (lPMT) thin section helical or multislice CT is essential. The main duct type will show segmental or diffuse dilatation of the pancreatic duct with or without distal parenchymal atrophy. Intraductal papillae show solid attenuation in the duct wall and sometimes intraductal amorphous calcifications may be seen. As a result of
obstructing mucus, a bulging papilla may be identified (25% of cases). The branch duct type is usually located in the uncinate process, and much less freq uentl y in the body or tail. At CT it shows a lobulated mass of clustered small cysts with central septations (Fig. 14.6). Less frequently a unilocular cyst is demonstrated. Associated dilatation of the main pancreatic duct is segmental. There also may be a bulging papilla. Solid papillary excrescences of the tumor are often difficult to visualize. In the combined type features of both main and branch duct types are present. Differentiating malignant versus benign tumor is difficult (sensitivity 65-70%), but the risk of malignancy is increased when the following features are present: solid mass, dilatation of the pancreatic duct > 1Omm, bulging papilla, diffuse or multifocal involvement, or attenuating intral uminal content. Differential diagnosis includes macrocystic mucinous adenoma, microcystic serous adenoma, obstructive pancreatitis with pseudocyst, and necrotic adenocarcinoma. For correct diagnosis of this tumor, the communication of the cysts with the main pancreatic duct should be demonstrated. If thin-slice CT fails, MRIIMRCP or ERCP are the modalities of choice.
14 The Pancreas
Fig. 14.6 Intraductal papillary mudnous tumor (IPMT) (4 x 1/6). The tiny septa are hard to appreciate (a). A curved reformation (CPR) best demonstrates the location within the pancreatic head (b).
a
b
Solid Pancreatic lesions Pancreatic Carcinoma
Up to 90% of pancreatic carcinomas are adenocarcinomas, and most (80%) occur in the pancreatic head. Tumors in the body (15 %) or tail of the pancreas (5 %) do not cause typical symptoms and usually are not diagnosed until they have reached an inoperable stage. The only effective treatment of pancreatic carcinoma is surgical resection. Because of early lymphogenous and hematogenous spread and a propensity for invasive growth, only a small percentage of pancreatic cancers are resectable at the time of diagnosis (10-30%). Vascular invasion is a relative criterion for the lack of resectability, depending on the extent of vascular involvement and reconstructive options. Metastatic spread makes a tumor irresectable and is most often caused by liver metastases and peritoneal metastatic implants. Because peritoneal and omental metastases may measure only "1-2 mm in size and then can only be detected by direct visualization, some institutions perform a preoperative laparoscopy before the patient is subjected to laparotomy. The sensitivity of CT for the detection of small tumors and accurate tumor staging (Table 14.5) can be significantly improved by the use ofthin-section spiral or multislice CT techniques. •
CT Morphology
Pancreatic carcinomas are almost always isoattenuating to the parenchyma on unenhanced CT scans. The presence of a tumor is suggested by
Table 14.5
TNM Staging
of pancreatic
tumors
Tis
carcinoma
in situ
T1
tumor limited to pancreas,
< 2 cm
T2
tumor limited to pancreas,
>2 cm
T3
invasion of duodenum, or peri pancreatic fat
T4
infiltration of stomach, large vessels
N1a
singular
N1 b
multiple regional
regional
common spleen,
bile duct, colon, or
lymph node metastasis lymph node metastases*
* superior/i
nferior/ anterior (pa ncreaticod uadena I, proximal mesenteric/posterior (pancreaticoduadenal, common bile duct, proximal mesenteric)/splenic/cel iac
focal enlargement of the pancreas (usually the head or uncinate process) or by a circumscribed bulge in the gland contour. The uncinate process may loose its wedge-shaped configuration and become round. Neither of these changes is an early sign, however. In patients with pancreatic lipomatosis, tumors appear as homogeneous areas of increased parenchyma! density or lead to a focal obliteration of the lobular pattern. Fibrofatty septa within the normal parenchyma help to differentiate hypertrophic (benign) parenchyma! lobulation from a malignant tumor. On contrast-enhanced scans during the parenchymal phase adenocarcinoma demarcates as a hypoattenuating mass from the normal parenchyma. Small tumors may become
Pancreatic Carcinoma
isoattenuating just 30 seconds later, so only a brief time window is available for optimum tumor detection. Dilatation of the pancreatic duct is suggestive of a tumor, but beaded dilatation of the duct may be also seen in chronic pancreatitis in the absence of a tumor. Signs strongly suggestive of a tumor are an abrupt cutoff of the duct or dilatation of the duct in the pancreatic body or tail with a normal duct caliber in the pancreatic head. Long-standing ductal dilatation is associated with atrophy of the surrounding pancreatic parenchyma. A concomitant obstruction of the intrapancreatic portion of the common bile duct with no evidence of a stone may be caused by a carcinoma in the pancreatic head (double duct sign, Fig.14.7a). Even in the absence of a demonstrable tumor, the possibility of a periampullary or (intraductal) papillary carcinoma should be considered. Because small tumors that lead to early obstruction of the common bile duct make up the majority of the surgically resectable pancreatic tumors, identification of these lesions is particularly important. Foci of intratumoral necrosis can simulate a pseudocyst [pseudo-pseudocyslj, but their neoplastic origin is suggested by the higher density of the tumor necrosis, a relatively thick and shaggy cyst wall, and the absence of detectable calcifications. Vascular involvement is not an absolute contraindication forresection, but makes the surgical
intervention much more difficult, and careful preoperative planning is necessary. Assessment of the celiac and hepatic arteries, the superior mesenteric artery and vein (SMA, SMV) and the portal vein (PV) are crucial to successful resection. Identification of tumor spread to the retroperitoneum behind the SMA is important (d. Fig.14.5c), since direct intraoperative visualization ofthis area is diffi cuI tun til to the final stage of the operation. Arterial invasion (celiac trunk, superior mesenteric artery or hepatic artery) is less common and for most surgeons an absolute contraindication for tumor resection (Fig. 14.8a, b). Invasion of the superior mesenteric vein (Fig. 14.8c), venous confluence, or portal vein, if it is locally limited, does not preclude surgery. Various grading systems for vascular involvement have been described that assess the resectability of tumors adjacent to a vessel (Table 14.6), and positive predictive values of> 95% can be achieved. Direct contact between the tumor and a vessel with loss of the intervening fat plane does not automatically mean vascular invasion. Vascular involvement is confirmed, however, by a tumor thrombus, by vascular occlusion (nonvisualization or vessel cutoff) or by abrupt changes in vessel caliber. Indistinct vessel contours and a visible vessel "wall" does not necessarily indicate tumor infiltration but may be caused by peritumorous inflammatory reaction. The pancreaticoduodenal vein (PDV) produces an arcade of collateral connections be-
a
c
b
Fig. 14.7 Carcinoma of the pancreatic head. (a) Periampullary tumor (arrow) with associated dilatation of the common bile duct and pancreatic duct (double duct sign, arrowheads), (b) Tumor of the pancreatic head (arrow) with local lymphangitis in the mesenteric fat (broad arrow) and a small lymph node (arrowhead) in a
patient who previously underwent endoscopic stent insertion in the common bile duct. (c) Large tumor of the pancreatic head without contact to the SMA and SMV but causing a double duct sign (CPR, 4 x 1/6). Note the large metastatic node (arrowhead).
525
74 The Pancreas
a
c
b
Fig. 14.8 Volume-rendering (VRT) and MIP displays of vascular involvement in pancreatic adenocarcinoma (4 x 1/6). (a) Carcinoma of the pancreatic body infiltrating the splenic artery (arrow) and causing occlusion of the splenic vein with extensive collaterals (VRT). (b) Carcinoma of the pancreatic head (arrowheads) with en-
casement and narrowing of the SMA (arrow) indicating infiltration and irresectability (VRT). (c) Cystadenocarcinoma of the head and uncinate process causing occlusion (arrow) and stenosis (arrowheads) of the SMV. MIP demonstrates the course of the mesentericosplenic collaterals.
Table 14.6
carcinoma
Criteria for vascular
involvement of pancreatic
Grade
Vascular involvement (Raptopoulos et aI., 1997)
Circumferential vessel Involvement (Lu et al., 1997)
Surgery
Grade 0
normal vessels
no contiguity
Grade 1
loss of fat plane with or without smooth displacement of the vessel
:£25% circumference
resectable resectable
Grade 2
flattening or slight irregularity on one side of any vessel
> 25-50%
circumference
questionably
Grade 3
tumor around two sides of artery rowing of venous lumen
> 50-75%
circumference
unresectable
Grade 4
vessel occlusion
nar-
tween the portal vein and the SMV. Asymmetric enlargement of one vein may be interpreted as a secondary sign of encasement of another major vein and may be the only sign ofunresectability. Dilatation of the posterior superior pancreaticoduodenal vein (> 5m m) is an indirect sign of extrapancreatic extension with compression or occlusion of the pancreaticoduodenal venous arcade. However, the sensitivity of this sign is rather low and detection of abnormalities in these vessels is uncommon even in patients with unresectable tumors. Further secondary signs of encasement of major veins are an increased enhancement of the wall of small bowel loops and a decreased enhancement of the portal vein. A tethered, teardrop-shaped SMV was found to be a reliable sign of the unresectability of tumors.
> 75%
circumference
resectable
unresectable
Indistinct margins and stranding in the peripancreatic fat may be caused by pancreatic tumor spread due to focal carcinomatous lympitangiitis or by a perifocal inflammatory reaction (in 10% of cases) (Fig.14.7b). Tumor invasion into adjacent organs signifies unresectability, with the exception of the duodenum which is resected anyway and therefore poses no contraindication for surgery. Ascites is usually seen in advanced stages and signifies peritoneal carcinomatosis or invasion of the lesser sac. Irregular increase in density of the omental fat is highly suggestive of peritoneal carcinomatosis. It is common to find (hypovascular) hepatic metastases, but splenic metastases are very rare. However, even with spiral CT, up to 20% of patients that were considered operable by CT were found to have un-
Islet Cell Tumors detected liver lesions and peritoneal implants. Preoperative laparoscopy with ultrasound is therefore recommended in some institutions. In 40-65% of cases. lymph node metastases are already present in the area of the mesenteric root. celiac trunk. porta hepaus, or in para-aortic and paracaval or retrocrural sites at the time of tumor detection. CT has important limitations in the detection of nodal metastases because metastatic lymph nodes in this disease are frequently not enlarged. Regional lymph node metastases need to be considered when peripancreatic lymph nodes are at the upper limit orjust beyond the normal size (1-1.5 cm) or when there is a cluster of still normal sized lymph nodes (Fig.l4.7b). Rarely lymph node metastases may be quite extensive and present as a peripancreatic mass. Regional lymph node metastases (N 1) do not necessarily render the tumor unresectable. However. the 5-year survival of patients with positive nodes found at surgery is significantly worse than the prognosis of other patients. Microscopic involvement oflymph nodes is fairly common even in small pancreatic tumors and is not considered as contraindication to resection.
endocrine neoplasia) is associated with hyperplasia or adenoma of the parathyroid in 90%. adrenal cortical tumors in 40%. thyroid tumors in 20%. pituitary gland adenomas in 40% and endocrine pancreatic tumors in 80%. Islet cell tumors may also be seen in association with von Hippel-Lindau disease (17%). Functional tumors produce early symptoms resulting from their hormonal activity. and usually they are only about 1-2 cm in diameter when diagnosed. Most nonfunctional tumors are considerably larger at diagnosis. because they do not produce early symptoms. Less than 10% ofinsulinomas undergo malignant transformation. compared with 60% of gastrinomas. Most malignant tumors are nonfunctional. In functional tumors malignancy may be difficult to determine histologically'. it is indicated by local invasion of adjacent structures as well as distant metastases. CT has a role in the preoperative localization of functional tumors and in the staging of nonfunctional tumors. The results of the examination depend critically on the use of an optimal scanning technique.
Islet Cell T umars
Functional islet cell tumors are usually isoattenuating and rarely hypoattenuating to the surrounding pancreas on unenhanced CT scans. Most islet cell tumors are hypervascular (80% of all insulinomas) and show a marked attenuation increase on arterial phase scans (for differential diagnosis. see Table 14.9). Small tumors « 1 em) can be identified by their enhancement dynamics rather than by a contour irregularity in the gland (Fig. 14.9). Some tumors remain visible in the parenchyma! or portal venous phase of en-
CT Morphology
Islet cell tumor is a collective term applied to tumors arising from endocrine cells of the islet cell apparatus of the pancreas. Approximately 90% of these tumors are benign. and 75% are functional (hormonally active). The principal types are reviewed in Table 14.7. Functional pancreatic tumors are frequently associated with tumors involving cells originating from the APUD system. MEN syndrome type 1 (multiple Table
14.7
Islet cell tumors
Insulinoma
((3 cells)
Gastrinoma
(a cells)
Vipoma
(6 cells)
Glucagonoma Somatostatinoma
(a cells)
Frequency
Malignancy
Site of occurrence
Remarks
60-75%
5-10%
Entire pancreas
10% multiple
20%
60%
Pancreatic head (50%), penpancreatrc or duodenal (35%)
20% multiple; calcifications may occur; thickened bowel folds in CT; hepatic metastases
1Oem). It contains areas of cystic degeneration, necrosis, and solid material in varying proportions, and its CT morphology resembles that of macrocystic adenoma. It shows little or no contrast enhancement, and cystic lesions do not contain septations. The absence of metastasis, even with large tumors, is typical. Pleomorphic
Carcinoma
Pleomorphic carcinoma is a rare tumor (2-7% of all pancreatic tumors) that is histologically similar to sarcoma. It is characterized clinically by a downhill course, and death usually occurs within three months after diagnosis. •
CT Morphology
Pleomorphic carcinoma is commoner in the body or tail of the pancreas than in the head, and most tumors have reached considerable size when diagnosed. The tumor may contain cystic necrotic areas and shows a corresponding inhomogeneoUs pattern of enhancement. Typically there is widespread metastasis by the time of diagnosis, including spread to unusual sites such as the mediastinal lymph nodes, kidneys, and adrenals. Other distant metastases may be found in the liver, lung, and bone. Abdominal lymph node metastases are so extensive that the tumor may be misinterpreted as a lymphoma. Lymphoma
Primary lymphomas of the pancreas are rare « 1 %ofall pancreaticneoplasms). Themajority are non- Hodgkin lymphomas. Secondary lymphomatous involvement of the pancreas is also rare and is generally associated with extensive abdominal lymphomas.
Fig. 14.11 Pancreatic lymphoma with diffuse organ enlargement and extensive stranding of the surrounding mesenteric fat.
•
CT Morphology
Pancreatic lymphomas may appear as large homogeneous masses, but it is more common to find central tumor necrosis. Approximately 60% are larger than 4cm when diagnosed. They can be difficult to distinguish from pancreatic carcinoma. Extensive, diffuse infiltration of the peripancreatic fat and disproportionately mild pain symptoms are suggestive of malignant lymphoma (Fig. 14.11). Lymphomas displace the peripancreatic vessels without invading them, and the pancreas may be displaced anteriorly by enlarged peri pancreatic nodes. With secondary pancreatic involvement by malignant lymphoma, images will typically show extensive manifestations of extra pancreatic disease. Metastases
Carcinomas of the lung, breast, thyroid, kidneys, ovaries, testes, liver (HCC), and melanomas can produce intrapancreatic metastases. With its proximity to other organs, the pancreas may also be invaded by direct extension of tumors from the stomach, colon, gallbladder, liver, or kidney. •
CT Morphology
Pancreatic metastases may be hypovascular or hypervascular, depending on the primary tumor (Table 14.9). There is an even distribution throughout the pancreas. They may be morphologically indistinguishable from primary pan-
529
74 The Pancreas Table 14.9 Differential pancreatic tumors Primary
diagnosis
of
cr
creatic neoplasms on scans, but there is less propensity for infiltration of the peripancreatic fat and vascular involvement.
Pancreatitis Pancreatitis is classified into various forms according to the course and severity of the inflammatory processes. This classification has significant prognostic and therapeutic implications. has two major roles. staging the severity of the inflammatory process and detecting complications. especially the identification and quantification of parenchyma! and peripancreatic necrosis. should be used for diagnosis only in patients with equivocal clinical findings. (See recommendations for use of CT. below.)
cr
cr
Table 10.10 Acute Pancreatitis (clinical classification according to the Atlanta conference of 1992) Mild acute pancreatitis
minimal
Severe acute pancreatitis
associated with organ failure or local complications such as necrosis, abscess, or pseudocyst
Acute fluid collections
lack of a defined wall
Pancreatic
nonenhanced pancreatic parenchyma >3 cm or 4 weeks after onset of pancreatitis
Pancreatic abscess
defined wall. fungi or pus
Acute Pancreatitis On the basis of the severity of the pathologic changes. clinical manifestations and laboratory values. acute pancreatitis is classified as mild or severe (Table 14.10). The predominant macroscopic feature of mild acute pancreatitis is interstitial edema. While parenchymal necrosis is macroscopically not visible. small foci of acinar cell necrosis occasionally may be found. Necrosis of peripancreatic adipose tissue is common. It represents the more common form (70%) and generally runs an uncom plicated course. The diagnosis is based on clinical and laboratory findings and does not require imaging studies. Severe acute pancreatitis is characterized by extensive changes of the peripancreatic fat and hemorrhagic fluid within the retroperitoneal space. There are patchy and confluent zones of parenchymal necrosis with associated necrosis of
organ
dysfunction
little or no necrosis,
vessel walls, foci of hemorrhage and necrosis, and disruption of the pancreatic duct. Clinically, severe pancreatitis is indicated with three or more Ranson criteria or eight or more APACHE 11 criteria any time during the course of disease. Because the pancreas lacks a true capsule, enzymerich pancreatic secretions that escape from the inflamed pancreas can easily reach the peripancreatic fat, producing areas of fat necrosis and leading to more extensive inflammatory spread, manifested by tracks of necrotic material. While the pancreatic parenchyma is still intact in serous exudative pancreatitis, areas of (often hemor-
Acute Pancreatitis rhagic) pancreatic necrosis develop in the hemorrhagic necrotizing form. The superinfection of these areas leads to a suppurative form of pancreatitis with abscess formation. The latter has a poor prognosis with a high mortality rate. While the mild form of pancreatitis shows no or only mild organ dysfunction and usually responds rapidly to conservative treatment, the severe form is associated with much more serious physical and laboratory symptoms (shock, pulmonaryinsufficiency, renal failure, CI bleeding, metabolic disturbances, etc.). The frequency of complications increases with the severity of the pancreatitis. Progression from mild to severe can occur, but is a rare phenomenon. A failure of clinical response within 48 to 72 hours should result in additional diagnostic tests to re-evaluate the initial diagnosis and depict any complication. •
CT Morphology
The mild acute pancreatitis (edematous or interstitial pancreatitis) is characterized by swelling of the affected portions or the entire gland (Fig. 14.12). The edematous areas appear slightly enlarged and show decreased attenuation. The margins of the gland are blurred, and there may be thickening of Cerota's fascia. Postcontrast scans usually show slightly inhomogeneous enhancement of the pancreatic parenchyma with no perfusion defects (necrotic areas). In 30% of patients with definite clinical and laboratory signs of pancreatitis, the pancreas appears completely normal on CT scans. The detection of pancreatic enlargement in itself is inconclusive, given the normal range ofvariation in the size of the gland. Hypoattenuating, non-enhancing acute fluid collections (exudates, < 15 HU) that lack a defined wall are found in serous exudative pancreatitis. Fat necrosis may be hard to differentiate from such exudates but they are less well defined, and have a higher than fluid attenuation (> 25 HU). Inhomogeneities are highly suggestive of fat necrosis. The pathways of such extrapancreatic inflammatory spread initially conform to pre-existing fascial spaces (Fig. "14.13). Inflammatory exudate can spread anteriorly and laterally in the anterior pararenal space along Cerota's fascia, the mesenteric root, and the
Fig. 14.12 Mild acute pancreatitis with interstitial edema, characterized by swelling of the pancreas and blurring of its margins (Stage B, CTSI = 1).
gastrohepatic, gastrosplenic, and gastrocolic ligaments (Fig. 14.14). Infiltration of the pararenal space is usually greater on the left side than on the right. Rarely, inflammation may track around the renal fasciae to involve the posterior pararenal space. Downward extension into the lesser pelvis may also occur. Increased density (infiltration) of the pararenal space with sparing of the fat-filled perirenal space is called the renal halo sign. Enzymatically active exudates must penetrate the fascial planes in order to enter the omental bursa, the perirenal spaces, and the mediastinum. Even early stages of inflammatory spread may incite a small accompanying ascites and a reactive pleural or pericardial effusion. In severe acute pancreatitis (extensive exudative pancreatitis or liemorrliagic necrotizing pancreatitis] the gland is markedly enlarged, nonhomogeneous, and poorly demarcated from surrounding tissues. Necrotic areas in the pancreas appear hypoattenuating, parenchyma! sequesters are isoattenuating, and hemorrhagic areas are hyperattenuating. Intravenous contrast should be used to determine the extent of pancreatic necrosis: viable parenchyma shows contrast enhancement while necrotic parenchyma does not (Fig. 14.15). Extrapancreatic exudates and fat necrosis spread like in the mild form, but the inflammatory and necrotic changes are much more extensive and severe. Absorption of the exudates occurs over a period of weeks to months, with serous elements undergoing more rapid absorption than
531
14 The Pancreas Omental
bursa
Fig. 14.13 Pathways of inflammatory spread in acute pancreatitis. CT demonstrates focal or generalized enlargement of the gland. Inflammatory collections may spread along the anterior pararenal spaces, into the mesenteric root, or into the omental bursa.
Lateroconal
1111111111111
Anterior
pararenal space
Perirenal space
I07m/;7A
a
Posterior Gerota's-fascia
Posterior pararenal space
c
b
Fig. 14.14 Severe acute pancreatitis with moderate serous exudates (4x1 /6). Note the small focus of parenchymal necrosis (arrow) seen on axial (a) as well as coronal sections (b). Parenchyma! perfusion is otherwise unchanged. The caudal spread of the exudates is
hemorrhagiccomponents.Asabsorptionoccurs, the periphery of the inflammatory mass becomes walled off by granulation tissue, first producing an encapsulated fluid collection that subsequently develops into a pseudocyst (Fig. 14.3). These acute pseudocysts evolve at least four weeks or more after onset of extrapancreatic fluid.
best seen on a sagittal volume-rendered image (c). Exudates are located anterior and posterior to the pancreas, in the omental bursa/lesser sac (8). and along Gerota's fascia (arrowheads) extending into the pelvis (Stage E, CTSI=6) .
CT Staging of the Severity of Acute Pancreatitis
Several studies have shown that initial clinical evaluation may miss approximately 60% of patients with severe acute pancreatitis who are likely to develop complications with a high morbidity and mortality. This is because clinical evaluation alone measures only the physiologic or systemic response of the patient. CT can depict
Acute Pancreatitis
533
b Fig. 14.15 (a) Necrotizing pancreatitis after ERCP. Note the stent in the common bile duct, a postinterventional hematoma in the duodenal wall (") and a small infected fluid collection in the porta hepatis (Stage E, CTSI= 6). (b) Hemorrhagic necrotizing pancreatitis with
a perfusion defect in the body of the pancreas. Large inflammatory exudates are present in the left anterior pararenal space (= paracolic gutter). There is free fluid in the peritoneum (Stage E, CTSI = 8).
the extent of glandular damage and identify complications prior to clinical manifestation. Such complications include extensive peripancreatic fluid collections, abscess formation, and vascular, biliary, or gastrointestinal involvement. CT can differentiate the mild edematous from the more severe exudative and necrotizing forms with 95% accuracy, but it is only 60% accurate in distinguishing hemorrhagic necrotizing pancreatitis from the suppurative form. The false negative rate of CT for detecting parenchymal necrosis was reported to be some 20% in patients with minor necrosis and less than 10% in patients with major necrosis. The specificity approaches 100%.
Balthazar and colleagues (1990) defined a composite CT Severity Index (CTSI, Table 14.11) to assess the severity of parenchymal gland necrosis (scores 0 to 6) and the extraglandular inflammatory process (stages A to E, scores 0 to 4). The CTSI ranges from 0 to 10 and shows excellent correlation with the clinical severity, morbidity and mortality rates. Patients with CTS1 of 0 or 1 had no mortality and no morbidity, while patients with an index of 2 had no mortality and only 4% morbidity. In contrast, a CTSI of 7-10 resulted in a 17% mortality and a 92% complication rate.
Table 14.11 CT severity index (Balthazar et al., 1990) Assessment
of acute inflammatory process
StageA:
score 0
Stage B:
score 1
intrinsic pancreatic changes: focal or diffuse gland enlargement, mild heterogeneity of parenchyma, small intrapancreatic fluid collections «3 ern)
StageC:
score 2
like stage B but with mild inflammatory changes of the peri pancreatic tissue
Stage D:
score 3
like C but more prominent peripancreatic inflammatory changes, but not more than one fluid collection
StageE:
score 4
multiple or extensive extrapancreatic fluid collections or abscess
normal pancreas
Assessment of pancreatic necrosis
Normal:
score 0
no necrosis, regular enhancement
Mildly necrotic:
score 2
50% of pancreatic parenchyma
Recommendations for Use of CT in Patients with Acute Pancreatitis
An initial CTscan is indicated: • in patients with clinical diagnosis in doubt (best after at least 48 hours after onset of symptoms); • in patients with clinical signs of severe pancreatitis (Ranson score >3 or APACHE 11 score >8); in patients who do not show adequate improvement within 72 hours of initiation of conservative medical treatment; in patients with suspected necrotizing pancreatitis: after onset of clinical symptoms glandular necrosis develops often within the first 24 hours and virtually always within the first 72 hours; therefore the initial CT scan should be obtained after these 72 hours if the clinical conditions allows; • in patients who develop a sudden change in clinical status (fever, pain, decreasing hematocrit, hypotension, etc) after initial improvement suggesting complications,
Follow-up eTscans
should be obtained:
• after 7 to "10 days andlor before hospital discharge in all patients with a grade D or E pancreatitis (CTSI score of 3-10) to exclude clinically silent complications', • in patients with no or inadequate clinical improvement or deterioration under treatment; • in patients with grade A-C pancreatitis (CTSI 0-2) only when complications are suspected,
Complications of Acute Pancreatitis The complications of acute pancreatitis include pseudocyst formation, infection, hemorrhage, formation of pseudoaneurysms, and venous thrombosis,
CT Morphology
CT occasionally shows a circumscribed peri pancreatic fluid collection of high attenuation (>60HLJ), but it is difficult or impossible to differentiate hemorrhage into the peripancreatic tissue from hemorrhage into a pseudocyst based on CT findings,
Pseudocyst Pseudocysts (see also p, 520) do not develop until 4 to 6 weeks after initialization of the pancreatitis, Mostly they are related to acute fluid collections seen at the onset of pancreatitis, In about 50% of patients these collections resolve; in the remainder pseudocyst formation occurs, For preoperative differentiation of a pseudocyst from other cystic lesions (cystadenoma), cytological studies and tests evaluating enzymatic activity of the peripancreatic fluid can be advocated, CT Morphology
Pseudocysts present as rounded, well encapsulated fluid collections of varying attenuation (0-25 HU), There is contrast enhancement of the granulation tissue in the cyst wall, A reliable correlation between the degree of wall thickness or wall enhancement and the maturity of a pseudocyst (consistency of its capsule) cannot be established, Septations are uncommon, Most frequently pseudocysts are located close to the pancreas but may also be found in the liver, spleen, or mediastinum, A suspected infection (pancreatic abscess) requires needle aspiration because it cannot be ruled out on the basis of CT morphology, Gas bubbles are suggestive of abscess formation, but they are quite infrequent and may also be caused by enteric Itstulatton, or following percutaneous puncture.
Infectious Complications Hemorrhagic Pancreatitis The diagnosis of hemorrhagic pancreatitis is based on histopathologic criteria and not on the demonstration of acute hemorrhage by CT, which is seen in only 2-5% of patients with acute pancreatitis,
Even today, infectious complications of pancreatitis are associated with a high mortality rate, A pancreatic abscess must be differentiated from an infected necrosis because the prognosis is substantially different and the treatment is different: an abscess can be treated effectively
Complications of Acute Pancreatitis with percutaneous catheter drainage while infected necrosis requires surgical debridement. Infected necrosis is an early complication of acute pancreatitis involving a disseminated bacterial inflammation of necrotic pancreatic and peri pancreatic tissue. Affected patients are septic, and the mortality rate ranges between 40% and 70% depending on the extent of infected necrotic tissue (involvement of 50% of glandular tissue). The frequency of bacterial contamination in patients with necrotizing pancreatitis increases with time (24% within 1st week, 36% within 2 weeks and 71 % within 3 weeks). It is very important to confirm bacterial contamination (usually by fine needle aspiration) because well over 50% of patients with severe pancreatitis have clinical symptoms of sepsis but only 50% of them actually suffer from superinfection. Pancreatic abscesses are circumscribed intraabdominal collections of pus located in proximity to the pancreas gland. They contain little or no necrosis and usually develop from infected pseudocysts. Pancreatic abscess is a late complication (> 5 weeks), occurring at a stage when the acute manifestations of pancreatitis are already subsiding. The mortality rates described in the literature vary between 20% and 50%. •
CT Morphology
Infected necrosis shows the same CT features as necrotic pancreatitis, including non-enhancing portions of the gland and peripancreatic fluid collections. Infection may be present in solid or liquefied regions of the gland, or peripancreatic fluid collections, or fat necrosis. Regions of fat necrosis present as heterogeneous hypoattenuating areas that are often ill-defined and have near soft-tissue attenuation (> 20 HU). Gas bubbles in areas of pancreatic gland necrosis or dispersed in necrotic peripancreatic tissue (emphysematous pancreatitis) are highly suggestive of infection. Pancreatic abscesses show CT features similar to pancreatic pseudocysts. They are wellcircumscribed regions of fluid attenuation (1030 HU) with a contrast -enhancing "wall". Pancreatic abscesses are multiple in 30% of cases, and many are located some distance from the pancreatic bed. The detection of gas inclusions in the pancreas and peripancreatic tissue is the only morphologic sign suggestive of bacterial superinfec-
Fig. 14.16 Pancreatic abscess with small gas inclusions (arrow). There is inflammatory spread along Gerota's fascia (Stage E, CTSI=6). Rim enhancement indicates the formation of granulation tissue (arrowheads), but whether this region is also superinfected cannot be determined based on CT criteria alone.
tion, but gas is detected in only 30-50% of cases (Fig. 14.16) and can be also caused by a gastrointestinal fistula in the absence of infection, or by previous percutaneous intervention. If gas collection is not present, it is not possible to distinguish infected from noninfected necrosis, or a pseudocyst from an abscess, based on CT findings alone. Both the abscess membrane and the inflammatory rim surrounding a pancreatic fluid collection show the same pattern of peripheral enhancement. Clinical parameters such as persistent leukocytosis and fever suggest the diagnosis. Equivocal cases should be resolved by fine-needle aspiration biopsy of the suspicious area for microbiologic analysis. A positive Gram stain or culture indicates the need for intervention. Vascular Complications
Hemorrhage results from the enzymatic erosion of small blood vessels. Larger vessels such as the hepatic, splenic and gastroduodenal arteries may develop pseudoaneurysms in up to 10% of cases. Pseudocysts and chronic pancreatitis predispose to thrombotic occlusions of the portal venous system. The splenic vein is most commonly affected, which may lead to splenic infarction.
535
14 The Pancreas •
CT Morphology
Pseudoaneurysms are most clearly depicted during the arterial phase of enhancement (Fig. 14.17) but can also be detected during the parenchymal phase. Venous occlusions lead to non opacification of the affected vessel and incite the development of peripancreatic collaterals. Intraluminal filling defects due to thrombotic material can occasionally be detected in the affected vessels. Chronic Pancreatitis
Chronic pancreatitis is most commonly the result of alcohol abuse or biliary disease. There may be recurring episodes of acute exacerbation (chronic relapsing pancreatitis), or the disease may take a slowly progressive course. Chronic pancreatitis is characterized by irreversible damage to the organ with progressive exocrine andend ocrin eins uffi ci en cy. Patients with pancreatic calcifications [chronic calcifying pancreatitis) show an increased incidence of pancreatic carcinoma. Chronic pancreatitis and a pancreatic tumor coexist in up to 5% of cases. The goals of CT scanning are to detect intraductal lithiasis as the cause of the disease, exclude a pancreatic tumor, stage the severity of disease, detect complications such as pseudocysts, ductal stenosis, and fistula formation,
a Fig. 14.17 Vascular complications of necrotizing pancreatitis. (a) Pseudoaneurysm of the gastroduodenal artery with partial thrombosis that has led to cholestasis with hydrops of the gallbladder, (b, c) Large infected
and assist in the selection of treatment alternatives. •
CT Morphology
CT findings are normal in 10% of patients with clinically diagnosed chronic pancreatitis. The inflammation may lead to focal or diffuse enlargement of the pancreas (hypertrophic form), but more frequently the gland is atrophic. This atrophy is characterized by reduced gland size, decreased structural density of the parenchyma, and ductal dilatation. The detection of ductal dilatation or calcifications signifies the pancreatitis- induced form of atrophy as opposed to the "senile atrophy" that occurs after age 60. Calcifications are more common in the alcoholrelated form (50%) than in the biliary form (approximately 20%, see Table 14.13). Most calcifications are located adjacent to or within the ductal system. Pseudocysts are found in approximately 30% of cases and may have an intra- or extrapancreatic location (Fig. 14.18). Pancreatic ductal calcifications occur in up to 90% of patients with severe, and in up to 55% of patients with only mild to moderate, pancreatitis; thus calcifications per se are a poor predictor of disease severity. Widening of the pancreatic duct to more than 4mm is found in more than 50% of patients. The duct is more often irregular (beaded) than smooth due to distortion by scarring (Fig. 14.19). This contrasts with tumor -related obstruction, which is more likely to cause
b
c
pseudoaneurysm of the splenic artery with gas inclusions and a layered thrombus, which has caused erosion of the duodenum (arrow, c).
Chronic Pancreatitis
Fig. 14.18 (a) Moderate chronic calcifying small cysts, and dilatation of the pancreatic
pancreatitis with enlargement of the gland, multiple calcifications duct. (b) Severe chronic calcifying pancreatitis (VRT, 4 x 1/6).
c
b
a
Fig. 14.19 Pseudotumor of the pancreatic head due to severe chronic pancreatitis (4 x 1 /6). Note the intraductal calcifications on the axial sections (a). The pancreatic duct is markedly dilated and shows irregularities towards the pancreatic tail (b, coronal oblique section).
Table 14.12 1996)
Cambridge
classification
of pancreatic
The pancreatic head is markedly dilated and inhomogeneous but no focal tumor can be discerned (c. coronal section). Note that it may be impossible to differentiate pseudotumor from carcinoma in patients with chronic pancreatitis.
morphology
in chronic pancreatitis
(adapted
from
Freeny,
Changes
ERCP
None
no abnormal
Equivocal
MPDnormal
One of the following: 3 abnormal LSB, MPD 2-4 mm, slight gland enlargement. heterogeneous parenchyma
Moderate
Severe
CT and US LSB
and
normal gland size and shape, homogenous
parenchyma
MPD changes
small cysts 1Omm, intraductal filling defects, calculi, MPD obstruction irregularity, or contiguous organ invasion
LSB = lateral side branch ducts, MPD = main pancreatic duct
or stricture,
enhancement!
severe
MPD
537
538
14 The Pancreas
smooth dilatation and abrupt termination of the duct. However, the shape and extent of the dilatation are not reliable indicators for differentiating benign from malignant disease. An acute exacerbation of chronic pancreatitis can cause focal or diffuse enlargement of the gland with decreased contrast enhancement and occasional infiltration of the peripancreatic tissue. These changes may be morphologically indistinguishable from pancreatic carcinoma at this stage, necessitating further diagnostic studies such as ERCP and biopsy. The International Cambridge Classification is the most widely used system for the morphologic staging of the severity of chronic pancreatitis (Table 14.12). Correlation between morphologic findings and function tests, and between morphologic changes and clinical stages, is high in advanced disease but only moderate in mild or early chronic pancreatitis. Complications of chronic pancreatitis are similar to those of acute pancreatitis and include formation of fluid collections and inflammatory involvement of the vascular, gastrointestinal and bile duct system. Imaging studies playa crucial role in deciding whether conservative medical, surgical, or interventional treatment is more appropriate. Indications for surgical intervention are suspected tumors, persistent pain, and complications such as biliary or duodenal obstruction, or pseudocysts.
Cystic fibrosis may induce degeneration of the pancreas. •
fibrosis
or fatty
CT Morphology
Usually the pancreatic tissue is nonhomogeneous clue to fibrosis and fatty degeneration. Cystic structures (ranging from millimeters to centimeters in size) and calcifications have been described. Atrophy occurs in the late stage. Complete fatty involution is possible (Fig. 14.20). Fig. 14.20 Complete fatty involution of the pancreas is typical for patients with cystic fibrosis.
Pancreatic Trauma Hemochromatosis
Primary terized organs docrine sis (e.g. mia) is •
CT
idiopathic hemochromatosis is characby iron storage in the pancreas and other (liver, spleen, heart, lymph nodes, enorgans, skin), Secondary hemochromatocaused by repeated transfusions for anenot associated with pancreatic changes, Morphology
The CT attenuation of the pancreas is homogeneously increased in hemochromatosis, reaching values of70-130 HU
Lipomatosis
senile atrophy, the end stage of chronic pancreatitis, cystic fibrosis, or following occlusion of the pancreatic duct Lipomatous pancreatic atrophy in children is a rare disorder caused by a vtraltnfectton. CT
Morphology
The fatty tissue deposits cause decreased structural density of the pancreatic parenchyma, accompanied by the displacement of individual gland lobules, The mesenteric vessels and pancreatic duct appear very prominent, but the duct is of normal caliber «5mm), The gland may be small with normal contours, In extensive cases, the parenchyma in total may show fat attenuation (Fig, 14,20),
Increased fat deposition in the pancreatic parenchyma may occur in the setting of obesity,
Trauma and Postoperative Changes Pancreatic
Trauma
Pancreatic injury is infrequent in blunt trauma and is usually associated with injuries to other organs such as duodenum and liver. It is seen more frequently in children or young adults, Usually, the pancreas is compressed from the anterior against the spinal column, e,g" by a seat-belt or steering wheel in car accidents, This may produce a contusion, hematoma, laceration, or fracture that can be graded with the MST organ injury scale (Table 14.14), Lacerations and fractures that rupture the pancreatic duct require emergency surgery in which part of the organ may have to be resected, Overlooking these injuries may lead to recurrent pancreatitis with pseudocyst or abscess formation, Other possible late sequelae include ductal strictures, and fistulization, •
CT
Morphology
In laceration or fracture, CT shows interruption of the pancreatic body with associated hemorrhage in the anterior pararenal space, Less severe injuries may present as edematous swelling, a focal mass, or contour deformities, which may be associated with peripancreatic fluid collections (Fig, 14,21), Indirect signs to look for are fluid collections around the superior mesenteric
Table 14.14 juries Grade
AAST organ
injury scale
for pancreatic
Injury
Description
Hematoma
Minor contusion duct injury
Laceration
Superficial laceration without
Hematoma Laceration
Laceration
duct
in-
without
injury
Major contusion without duct injury or tissue loss Major laceration without duct injury ortissue loss Distal transection or parenchymal injury with
duct
injury
IV
Laceration
Proximal transection or parenchymal injury involving papilla
Laceration
Massive disruption creatic head
Advance one grade for multiple organ up to grade III. Proximal pancreas mesenteric vein.
injuries
is to the right ofthe
of pan-
to same
superior
artery, transverse mesocolon, lesser sac and between the pancreas and splenic vein, In severe cases with ductal laceration, necrotizing pancreatitis may develop,
539
74 The Pancreas
•
Fig. 14.21 Poly traumatized patient with pancreatic contusion (arrow), signs of exudative pancreatitis, free air (arrowhead) in the gall bladder bed due to small bowel perforation, and acute perisplenic contrast extravasation (wide arrow) due to splenic rupture.
Traumatic injuries to the surrounding organs should be carefully sought for, which may require additional oral contrast. Evaluation of duct integrity is the terrain ofMRCP or ERCP, depending on the stability of the patient. Occasionally, the pancreas may be deceptively normal at CT due to isodense lacerations or lack of contour irregularities. Thickening of Cerota's fascia is a very nonspecific sign of an acute or old pancreatic injury. Pancreatic Resection
Various operations such as the Whipple procedure or pancreatectomy involve partial or complete resection of the pancreas. It may also be necessary to remove the spleen and portions of the stomach, omentum, or bowel. In partial pancreatic resections, anastomoses may be constructed to provide for drainage of pancreatic secretions to the stomach or bowel.
CT Morphology
Following resection ofthe pancreatic tail, the rest of the gland retains an essentially normal shape and position. With a resection of the pancreatic head, the tail is anastomosed to the stomach or to a transposed loop of small bowel. Generally the anastomotic site is not clearly visible on CTscans. When the pancreatic head has been resected, the rest of the gland may atrophy. Peri operative fluid collections may signal complications such as seroma, hematoma, abscess, biloma, or pseudocyst. A leaky pancreatic anastomosis can lead to postoperative pancreatitis. In patients who have undergone a pancreaticoduodenotomy for carcinoma, lesions of soft-tissue density in the pancreatic bed that are not identified as prolapsed loops of small bowel should raise suspicion of tumor recurrence. Pancreatic Transplantation
Aside from general postoperative complications, there are no indications for CT scanning after pancreatic transplantation. •
CT Morphology
The pancreatic allograft lies within the lesser pelvis in contact with the iliac vessels and bowel loops. Its enhancement characteristics are typical of pancreatic tissue. Adjacent bowel loops should be adequately filled with oral contrast medium so that they can be positively identified. The morphologic features of acute rejection range from a normal-appearing CT scan to edematous enlargement with regional parenchymal hypoattenuation. Chronic rejection is manifested by atrophy of the pancreatic allograft.
15 The Gastrointestinal Tract C. Schaefer-Prokop,
M.Jorgensen
Anatomy Esophagus(p .543) Stomach and Duodenum Small Intestine (p. 546) Colon (p. 543)
Endoscopy is the commonest initial diagnostic procedure for evaluating diseases of the upper and lower gastrointestinal tract. Laparotomy is used for the staging of tumors of the stomach and colon. Only small bowel studies still rely primarily on imaging procedures. CT in this setting can image the bowel wall and lumen in addition to surrounding structures and therefore is often superior to conventional barium studies, As a rule, conventional CT is not used for the staging of primary gastrointestinal tumors (except esophageal carcinoma). CT is used to investigate suspected hepatic metastases, however, and it is applied in advanced tumor cases to direct the planning of adjuvant pre-
593
operative therapy and to monitor therapeutic response. Presently, new techniques in computed tomography such as hydro-CT, CT enteroclysis, and CT colonography, especially when cornbined with multislice scanning, have increased the sensitivity of CT in detecting and staging gastrointestinal tumors or in evaluating inflammatory diseases, although the relative merits of these techniques have not yet been clearly established. Virtual colonoscopy, in particular, holds great potential as a screening tool for colorectal cancer but has not yet been shown to be cost-effective. CT has gained new indications, such as the evaluation of suspected appendici-
Anatomy tis, acute abdomen, suspected bowel ischemia, diverticulitis, and acute bleeding (Table 15.1). Ultrasound is a potent competitor in pediatric patients, suspected appendicitis, or in patients with an acute abdomen. Endosonography has been widely accepted for evaluating the submucosal extent of tumors or cancers in the esophagus, stomach, and colon. MRI holds great Table 15.1 Tumor
Indications
for CT examination
diagnosis
potential for the evaluation of benign disease such as inflammatory bowel disease, localization of acute gastrointestinal bleeding using blood pool contrast agents, and for the evaluation of rectal cancer with endorectal coils, but has so far failed to be widely accepted for indications concerning the alimentary tract.
of the gastrointestinal
Esophageal,
tract
gastric, and colorectal cancer: therapy planning in advanced M staging (liver)
tumor stages, Detection of small
bowel
tumors
Lymphomas Re-staging after therapy of primarily unresectable tumors Onocologic follow-up in gastric and rectal carcinoma Colon cancer screening Inflammatory
bowel
disease
Mural
involvement,
Acute abdomen
Appendicitis,
General
Postinflammatory,
Other
follow-up indications
ileus,
skip lesion,
fistulae,
conglomerate
obstruction,
search
for hemorrhage
postoperative
masses,
obstruction
post-traumatic
Indeterminate abdominal complaints Equivocal radiographic, sonographic, Retrocardiac mass
or endoscopic
findings
Anatomy Esophagus
The cervical portion of the esophagus lies directly posterior to the trachea. Often it is not clearly demarcated from neighboring structures due to a paucity of intervening fat. The intrathoracic portion of the esophagus is intimately related to the trachea, the left main bronchus, and the left atrium of the heart. Portions of the intrathoracic esophagus may be poorly delineated by CT due to transmitted pulsations. On passing through the diaphragm, a short segment of the esophagus acquires a peritoneal covering as it turns left and enters the gastrtc fundus. For correlation with endoscopic measurements it is helpful to note that the upper esophageal sphincter is located 15cm from the incisor teeth (15cm aboral). From a surgical standpoint, the esophagus is divided into three segments:
• proximal third (extending from the upper , esophageal sphincter to the tracheal bifurcation; 15-25 cm aboral); • middle third (extending from the fourth to seventh thoracic vertebra; 25-32 cm aboral); • distal third (extending from the seventh thoracic vertebra to the gastric cardia; 32-42 em aboral). The wall of the air-filled esophagus should not exceed 3 mm in thickness, although areas of wall thickening that can been seen on CT may be nonspecific. The esophageal mucosa is rarely visible even on contrast-enhanced scans.
Stomach and Duodenum The gastric cardia starts approximately 3 cm past the site where the esophagus passes through the diaphragm. Even when the stomach
544
75
The Gastrointestinal
Tract
is well-distended with contrast medium, the cardia has a thicker wall than the other gastric segments and may produce a focal thickening that mimics carcinoma. Scanning in the left lateral decubitus position can aid in evaluating the region of the cardia. The gastric fundus lies directly infradiaphragmatically and is in contact with the spleen, while the body of the stomach extends anteriorly and establishes contact with the anterior abdominal wall (Fig.15.1 a). The junction of the body and antrum of the stomach
usually has a constriction that should not be mistaken for the pylorus. The pylorus and duodenal bulb turn posteriorly and are intimately related to the left hepatic lobe and the head of the pancreas (Fig. 15.Ib). CT visualization ofthese structures requires the use of oral contrast medium and may require scanning in the right lateral decubitus position. The duodenum may extend far downward and may protrude between the inferior vena cava and the aorta, sometimes simuFig. 15.1 8, b Crosssectional anatomy of the gastrointestinal tract. Left colic flexure
Pylorus DuodenalBulb
Anatomy Fig. 15.1 c-e Crosssectional anatomy of the gastrointestinal tract.
+onzontai oart of :luodenum
. Descending colon mesentericvein ~ight colic
Superior mesenteric
Iexure Inferior epigastric
artery
Small intestine Cecum-~ Appendix~
"----- Ureter
-
Perirectal fascial sheath
-
artery
545
75 The Gastrointestinal Tract lating the appearance of lymphatic tissue. The mesenteric vessels cross the horizontal portion of the duodenum (Fig.l5.1c). Further distally. thejunction of the duodenum with the jejunum is marked by the duodenojejunal flexure. The normal gastric wall ranges from 4-10 mm in thickness, depending on the degree of gastric distention. There may be various degrees of fatty changes in the submucosa of the gastric wall that appear as a hypoattenuating layer on CT sections. Pathologic wall thickening (>4mm) should be diagnosed only when the stomach is optimally distended. Contrast administration improves the delineation of the gastric mucosa.
Small Intestine The small intestine (small bowel) is approximately 3 m long. Loops of small bowel in the left mid-abdomen generally represent jejunum, while bowel loops in the lesser pelvis and right lower abdomen are part of the ileum. The ileocecal region is identified by an abrupt change in the caliber of the bowel loops in the right lower quadrant and by the distinctive gas patterns: intraluminal gas in the small bowel occupies the anterior part of the lumen in the supine patient while the intraluminal gas is present in small bubbles in the non-distended colon. The ileocecal valve appears as a structure of soft-tissue density within the cecal lumen and should not be mistaken for a mass. It may contain fat in the submucosal layer. Small bowel that is not filled with contrast medium is generally in a collapsed state. When the bowel is well distended, the Kerckring folds (valvulae conniventes) can be seen. Adequately distended small bowel segments have a normal wall thickness of 1-3 mm. When the bowel has been filled with water-equivalent contrast material, CT following intravenous contrast administration demonstrates the intestinal mucosa as a thin, moderately hyperattenuating line.
Colon Loops of large intestine (colon) are distingutshed by their larger caliber and the presence of haustral markings and gas-containing feces.
The gas inclusions appear as multiple small bubbles retained within the feces in the bowel lumen. The ascending and descending colon have a retroperitoneal location (Fig.l5.1d). When not distended by contrast material. the descending colon is often in a collapsed state and often can only be distinguished from small bowel loops by its position (far posterior, just lateral to Cerota's fascia). Frequently the sigmoid colon can be identified on CT scans only by carefully tracing the descending colon or the rectum to the sigmoid on sequential scans. Portions of an elongated sigmoid colon may even be found on the right side. The rectum is approximately 12 em long, occupies a presacral extraperitoneal position, and is surrounded by perirectal fat. The perirectal fascial sheath appears as a thin line on CT images (Fig.l5.1e). The ischiorectal fossa is the largest of the anorectal spaces: it is bounded medially by the levator ani muscle and the external sphincter muscles, laterally by the internal obturator muscle and inferiorly by the glutens maximus muscle and the sacrotUberous ligament. The levator ani muscle separates the ischiorectal fossa from the extra peritoneal space of the pelvis (supralevator space) and constitutes the most important anatomic and surgical landmark of the region. Colon that is not distended with contrast medium generally contains feces and has a wall less than 2mm thick. Optimum evaluation of the lumen requires prior colonic cleansing followed by the instillation of positive or negative contrast medium. When the colon has been filled with water-equivalent contrast material, CT scans after intravenous contrast injection will display the colonic mucosa as a thin line of moderately high attenuation.
Examination Technique
547
Examination Technique Examination technique and scan interpretation are particularly challenging in cr examinations of the stomach and bowel. Optimum distension (for hydro Cl, cr enteroclysis, cr colonography, or virtual colonoscopy) combined with the seamless volume coverage of spiral or multislice CT markedly improve diagnostic accuracy compared with conventional cr. Interactive tracing of the stomach and bowel loops on the worksta-
Table lS.2
Protocol
for CT scanning
tion monitor is an essential part of scan interpretation. Injection of intravenous contrast material requires the use of negative intestinal contrast material, and the timing of contrast injection should be either optimized for the liver (if metastatic disease has to be ruled out) or for the enhancement of the intestinal mucosa (3060 s p.i., cr. Table 15.2).
of the gastrointestinal
tract
General Preparation Oral/rectal
see Table lS.3 contrast
see Table 1S.3
Patient position
Supine with elevated
Scan range
Esophagus: Stomach
arms From T1 to below posterior lung recesses; if necessary coverage may be extended to CS or below liver From diaphragm to below kidneys; coverage may include lesser pelvis Form left hemidiaphragm to ischial tuberosities
and duodenum:
Small bowel and colon: Respiratory
phase
Windowing
Inspiration Noncontrast CT: Contrast-enhanced
W/L=300/40 W/L = 400/60
CT:
16-slice"
Scan parameters
l-slice SC/TF/RI
4-slice'
sc
sc
Standard
S/10/S1
2-31
1-1.S1
Volumetric imaging
3/6/31
1-1.25
t
t
MPRb SW/RI
axial SW/RI 5/4
t
0.5-1.25
1-1.251
0.5-01.251
1-1.251
0.5-01.25
3/2
3/3 cor
3/2
3/3 cor
3/2
3/3 cor
CT enteroclysis
3/6/3
CT colonography
5/10/5
Contrast injection
l-slice V/F/D
4-slice V+N/F/D
16-slice V+N/F/DX'
Esophageal tumors
90/3/30
90 + 50/3/30
90 + 50/3/25A
Standard (portal phase)
120/2/60
120 + 50/3/60
120 + 50/3/205
Tumor detecting/ staging Biphasic CT
150/4/50
120 + 50/4/40
120 + 50/5/20A
t
150/4/25
+ 70 d
120 + 50/4/25
t
+ 70
d
120 + 50/5/20A
+ 80 d
SC= slice collimation (mm), TF =table feed (rnmjrotation], RI= recon. increment (mm), it = scanning direction, SW = effective section width (mm), MPR = multiplanar reformats, axial = axial sections, cor=coronal, sag = sagittal CM= contrast material, Contrast concentration = 300 mg/ml iodine V=volume of CM (ml), N = volume of saline flush (ml), F =flow rate (rnlrs), 0 = start delay, X= trigger region a Pitch P = TF/(Nx5C) should be chosen close to 1.5 with 4-slice scanners, 1.3-1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SW/RI = 1-1.5/0.7 or 0.5-0.8/0.5 c read: 20-25A (205) = start delay 20-25 s (205) after reaching 50 HU enhancement in the desc. aorta (spleen) dread: biphasic scan (arterial + portal phase) after 25 s (20A) for the 1" scan, 70-80 s for the 2nd scan
II
15 The Gastrointestinal
Tract
Esophagus Patient
direction and the use of a saline flush injected immediately after the contrast material. Preparation
The use of oral contrast medium is controversial. Positive oral contrast material may improve delineation of the esophageal lumen. Barium pastes (e.g .. Esophocat®) have a high viscosity and therefore may improve the coating of the esophageal mucosa but they are contraindicated if there is a risk of aspiration. However. evaluation of uptake of IV contrast material in the esophageal mucosa may be hampered. and often the oral contrast material passes the esophagus as a bolus. which leaves large portions of the esophagus without intraluminal contrast. For these reasons, we prefer not to use any positive oral contrast material. If CT angiography is planned (e.g., dysphagia lusoria), oral contrast material should always be avoided. Acquisition
Stomach Patient
Technique
A CT examination of the esophagus is part' of a thoracic CT. The scan length may have to be extended in the cranial or caudal direction, depending on the site of a suspected esophageal tumor. Spiral and multislice CT provide the best results when based on a thin-section protocol (Table IS.2). Curved planar reformations along the esophagus best delineate the longitudinal extent of an abnormality. Contrast
Injection
Intravenous contrast material is usually only injected to differentiate vascular structures from other soft tissues. The same protocols as for general mediastinal examinations are employed. Injecting larger amount of contrast material at higher flow rates may improve the detection of hypervascularization of the esophageal mucosa or tumorous structures. Because of streak artifacts in the brachiocephalic veins, we recommend a caudocranial scanning Table
15.3
Patient
preparation
and enteric
Esophagus:
No oral contrast
Stomach
500-1000
Small
and duodenum:
bowel/colon:
Sellinklsmall
bowel
contrast
enema:
medium:
administration
medium
necessary
ml oral contrast
medium
1000-2000
ml oral contrast
up to 2000
ml of dilute barium
methylcellulose Rectal
contrast
Rectum: Colon:
Preparation
An empty and distended stomach is an essential prerequisite for a good examination. Thus, the patient should not eat for 12hours nor drink for 6 hours prior to the CT examination. Hydro-CT is based on optimum distention of the stomach with negative oral contrast medium (water, juice, or a methylcellulose preparation) in conjunction with intravenous contrast material. The patient drinks SOO-IOOOml of contrast medium shortly before the examination. While on the CT table, the patient should be asked to drink another 2 SOml of contrast medium (Table IS.3). A prone position has been recommended for lesions of the gastric body and antrum of the stomach. Scanning in the right lateral decubitus position may be advantageous for the pylorus and duodenal bulb, and scanning in the left lateral decubitus position for the gastric cardia. An antispasmodic agent (N-butylscopolamine, Buscopan®) may be administered for optimum distension of the stomach and relaxation of the gastric muscles. For virtual gastroscopy, gas is required as a negative contrast agent. The patient has to be examined early in the morning to reduce the amount of gastric secretions in the stomach. The same principles of patient preparation hold true as for conventional radiographic barium studies of the stomach. The stomach is distended by giving the patient a C02- producing powder and a little water. The patient then has to wait for some 2-3 minutes to make sure the powder is completely dissolved. CT scanning in both the prone and supine positions improves evaluation ofthe anterior and posterior portion of the gastric wall.
administered
or water;
antispasmodic
medium or by duodenal
intubation
500 III contrast medium or water 1000-2000 ml contrast medium or water
(Buscopan)
Examination Technique •
Acquisition Technique
CT examinations of the stomach and duodenum usually are performed as part of an upper abdominal survey that includes the liver, spleen, and kidneys, For staging examinations, the scan length is extended into the lesser pelvis to detect drop metastases, As a rule, scanning protocols are used that also permit optimum evaluation of the liver (see Chapter 11), Thin-section scanning with single-slice or multislice spiral CT improves the evaluation of horizontally oriented portions of the stomach (TableIfi.Z). With 4-slice scanners, a 4xlL25mm collimation is to be preferred if the patient is not too obese, In obese patients, the increased image noise requires use of a wider collimation (e,g" 4x2, 5 mm): With 16-slice scanners, even thinner sections may be acquired (SC= 0,5-0,75 mm). For image review, thicker sections (SW=4-6mm) should be reconstructed from the raw data or an overlapping secondary raw data set Multiplanar reformations (3-4 mm thick) that are aligned to the position of the stomach (semicoronal and sagittal) are highly recommended for optimum display of abnormalities, •
Contrast Injection
Intravenous contrast injection usually is performed to optimize evaluation of the liver because detection of liver metastases is a major issue for gastric malignancies, Since most gastric carcinomas cause hypovascularized metastases, the upper abdomen is scanned during the portal venous phase, During this phase, there is enhancement of the gastric wall as well as the perigastric vessels, which makes it easier to differentiate them from lymph nodes, During an earlier phase that corresponds to the late arterial phase in liver CT, however, there is superior differentiation of the various layers of thegastricwalL Small •
Intestine
Patient Preparation
Standard examinations require that a sufficient volume of oral contrast medium (10002000ml) is given to the patient over a period of at least 60 to 90 minutes before the examination, The patient should be handed portions of
500ml contrast medium every 15-20 minutes to more evenly distribute the contrast agent within the bowel. CT enteroclysis is a superior technique for indications that focus on the small bowel. The patient should take in ample fluid but no solid meal for 12 hours prior to the examination to cleanse the small bowel. Prior to CT a duodenal tube is placed distal to the duodenojejunal flexure, under fluoroscopic guidance, Then the patient is brought to the CT suite and a total volume of up to 2000 ml contrast medium is instilled at a rapid rate (80-120 mllmin) by a roller pump, The contrast material may consist ofa highly diluted barium suspension, a methylcellulose preparation, or a stearin-containing preparation (Table 15,3), We prefer negative contrast for most indications because it allows for excellent visualization of the bowel wall following intravenous contrast administration, Positive contrast material is better suited to demonstrate stenoses or fistulous tracts, The instillation of contrast material should be timed so that it just ends during or just after the CT scan. Acquisition Technique
Scanning of the small bowel is performed during the course of upper and lower abdominal examinations that include the lesser pelvis, Spiral and multislice CT with a thin-section protocol (Table 15,2) provide the best results and the same technique as for evaluation of the stomach can be used, With 8- and 16-slice scanners, however, 1-1.5 mm collimation generally suffices, Image review is based on 4-6 mm thick axial sections and 3-4 mm thick coronal reformations, Problem-adapted sections may further optimize the display of abnormalities, Contrast Injection
Intravenous contrast material injection relies on similar principles as for gastric examinations, If a hypovascular tumor is suspected, scanning in the portal venous phase is usually sufficient Hypervascular tumors, such as carcinoid tumors, require biphasic examinations, The best delineation of small bowel mucosa also occurs during the late arterial phase and early portal phase, which should preferably be used for the evaluation of inflammatory bowel disease,
75 The Castrointestinal
Tract
Colon Patient Preparation
For a standard abdominal examination that includes the colon, the administration of a large volume of positive oral contrast material (10002000ml) should be given to the patient over a period of at least 60 to 90 minutes prior to the examination, In patients with distal colon lesions, the rectal instillation of contrast medium (SOO-1000 ml) improves bowel distension and facilitates evaluation of the bowel wall, As in all enemas, care has to be taken to avoid perforation of the rectum, CT colonography is a focused examination of the colon, which requires the same patient preparation (bowel cleansing) as a barium enema (Table IS.3), The patient should eat no solid foods for two days, but drink copious amounts of liquids, Laxative medication using a "dry" preparation (e.g. Fleet) yields superior results over a "wet" preparation (e,g" Colite or Golytely), which is commonly used for endoscopy, Fecal tagging is possible if the patient drinks SO-80 ml of a diluted barium suspension with each meal (total of 200ml) on the day prior to the examination, This technique will help opacity stool and distinguish it from real polyps, Colon distension is achieved by rectally insufflated air or COs as a negative contrast material. Colonic distension is better with C02, and there is less colonic spasm and discomfort for the patient during and after the examination because C02 is absorbed the blood and eliminated via respiration, pumps that provide constant disention are now available, Due to the propensity of the colon to collapse, scanning should be performed in a prone and supine position, The filling state of the colon is checked on a low-dose scanogram before starting each spiral or multislice CT scan, If distention is not complete, antispasmodic agents and turning the patient (usually right lateral decubitus position) may improve colonic filling, Commonly used antispasmodic agents are O,S-LO ing glucagon or 20-40 mg N-butylscopolamine (e.g. Buscopan), which are injected subcUtaneously or intravenously, It is not yet clear, however, if they provide an advantage for every patient or whether they are better reserved for patients that experience colonic spasm during the exam.
Water or positive contrast material can also be used as an alternative for colonic distention but handling is more complex and time-consuming, Acquisition Technique
The colon is usually examined as part of an upper and lower abdominal examination, Scan coverage should include the region from the left diaphragm to the ischial tuberosities, It should not be limited to the pelvic region in rectal tumor examinations, since distant metastases to the liver could be missed, CT colonography yields best results using a thin-section protocol (Table IS,2), A 4x 1-1.2S mm collimation should be used for 4slice scanners whenever possible, Obese patients require use of a wider collimation (4 x 2-3.7S mm): With 8- and 16-slice scanners, 1-1.S mm collimation is sufficient even for CT colonography. Images of 4-6 mm thickness can be reconstructed for general colonic examinations but CT colonography requires thinner sections of 2-2.5 mm section width for optimum detection of small lesions and differentiation between polyps and stool, Coronal reformations (2-3 mm thick) should always be performed for dedicated examinations of the colon, Sagittal reformations are useful for the rectum, and further improve the detection rate for virtual colonoscopy, A low-dose technique (CTDlvoi=3-6mGy) with two scans in a prone and supine position without injection of intravenous contrast material is used for screening purposes, A wide window setting decreases the influence of image noise, For clinically indicated colon examinations (e,g" incomplete colonoscopy, cancer staging, or detection of secondary lesions in stenoses that cannot be passes with an endoscope), one scan (e.g. in prone position) can use a low-dose technique, while the second scan (e,g" in supine position) is performed with regular dose (6-12 mGy) and intravenous contrast administration. Contrast Injection
Intravenous contrast administration should be performed in a way that both the bowel and the liver can be adequately evaluated, For this reason, some 120-lS0 ml of contrast material are injected at 4mlls and scanning is initiated some IS-20 s after contrast arrival in the aorta just above the bifurcation, The scan is then per-
Examination Technique formed in a caudocranial direction in order to yield good bowel opacification and good portal phase images in the liver. Image Processing
Evaluation relies on axial sections and coronal multiplanar reformations, preferably using interactive display on a 3D workstation, In case of doubt, additional virtual endoscopic views may help differentiate polyps from thickened folds, There is data that suggests that virtual colonoscopy of the whole colon improves detection of small polyps but also holds the danger of an increased false positive rate, Other display modes (see Fig, 2,50) are also available but there is no consensus yet about which technique is best suited in clinical practice, Automated detection algorithms are being developed that in the future may direct the radiologist's attention to potential polyps,
Acute Abdomen Spiral CT is increasingly used for the diagnostic work-up of patients with acute abdominal pain, Tailoring the examination parameters to the Table 15.4
CT protocols
Suspected
in patients
diagnosis
with
acute
abdomen
suspected markedly
clinical diagnosis improves the diagnostic
Patient
Preparation
Oral contrast material is not usually necessary, especially with multislice scanning, Oral contrast material may improve the differentiation between inflammatory lesions (suspected appendicitis, diverticulitis, abscess), tumors, and collapsed small bowel loops, If possible, it should be administered over a period of at least 60 minutes prior to the scan to ensure adequate bowel opacification, thus making this procedure difficult to use in an emergency situation, In patients with suspected gastric or duodenal disease, pancreatitis, or intraductal gallstones, negative oral contrast (water) results in better distention and demarcation of the stomach and upper small boweL Oral contrast material is contraindicated in patients with suspected high-grade small bowel obstruction or ureter obstruction; it may obscure findings in patients with suspected gastric disease or gastrointestinal bleeding, In patients undergoing 3D CT angiography, oral contrast interferes with the volume-rendering procedure,
tailored
to the
Rectal contrast
Oral contrast
clinical
working
iv contrast'
Urolithiasis
obstruction
PVP
-/water -/water -/water 1000ml -/water
Aortic disease
Abdominal ischemia Venous
thrombosis
Abscess Pancreatitis Bowel
-/500ml
Volumetric
AP+PVP AP+PVP PVP PVP
-/water
Pyelonephritis/abscess Pelvic inflammatory
Standard Volumetric
-/500ml
-/water
Trauma
3
AP
phase,
b
phases in brackets see Table 15.2
Volumetric Volumetric
PVP
=
portal venous
may add information
NP(+DP)
-/500ml
phase,
bu.
NP
=
nephrographic
are not mandatory
Standard Volumetric
PVP phase,
DP
Standard Standard
PVP
lOO0ml
Enteric tumor
Standard
(PVP)
(PVP) NP (+ DP)
-/1000ml
disease
Volumetric
PVP (AP+)NP
-/500ml -/500ml
Diverticulitis
Volumetric
Standard Standard
PVP
Renal infarct Appendicitis
arterial
Standard Volumetric
AP
perforation
Biliary obstruction
=
Scan parameters"
Standard
Hematoma bowel
diagnosis
Volumetric
AP+PVP
Acute abdominal hemorrhage Small
(Table 15.4) accuracy,
=
delayed
phase
75 The Gastrointestinal
Tract
Positive rectal contrast (diluted iodinated contrast agents) can improve the evaluation of suspected diverticulitis and appendicitis. Acquisition Technique
Scanning parameters depend on the size of the suspected abnormalities. For the detection of hematoma. abscess. pancreatitis, renal infarction, or pyelonephritis, thicker sections (SW= S-8 mm) suffice. Thus, spiral scanning can rely on a SI10/S or 7/12/6 protocol. Multislice scanning with a 4x2.S-4x3.7Smm collimation has the advantage that thinner sections can be reconstructed if necessary. For the evaluation of small bowel disease, appendicitis, diverticulitis, urolithiasis, acute bleeding, or suspected perforation, thinner sections should be preferred if possible. With 1 s spiral CT scanners, however, the whole abdomen with a length of some 40cm can only be covered with a S mm collimation. For this reason, the scan range should be reduced wherever possible, and the reconstruction increment should be narrowed to improve z-axis resolution. With subsecond spiral CT or dual-slice scanners 2.S-3 mm collimation can be used for this purpose. For the evaluation of biliary and vascular disease, an even smaller collimation should be chosen, usually in the 3 mm range for
spiral CT. The 4-slice scanners can either employ 4 x 2-3 mm collimation for fast volume coverage or 4x 1-1.2S mm collimation for optimum spatial resolution. With 8- and 16-slice scanners, 1.2S-2 mm collimation suffice. Evaluation relies on interactive viewing of axial sections in a cine display.lfa perforation is suspected or intraperitoneal free air is to be excluded, the images should be viewed using a lung window setting as well, to identify air collections that are not associated with bowel loops. Interactive multiplanar may be used as a problem-solving tool. Volume-rendered displays are favored for vascular disease, and curved planar reformations help to better demonstrate findings in the bile ducts and the ureters. Contrast Injection
No contrast injection should be used for the evaluation of suspected urolithiasis and for suspected abdominal hemorrhage. Even for other indications, intravenous contrast injection is not always necessary although it can substantially improve evaluation if no oral or rectal contrast could be given. Early portal phase scanning provides a good compromise for most indications, even for patients with suspected active hemorrhage.
Esophagus Duplication Cysts and Diverticula
Duplication cysts of the esophagus are rare congenital anomalies (0.S-2.S% of all tumor-like lesions of the esophagus) that may be noted incidentally on a conventional chest radiograph as an indeterminate mediastinal mass and require further investigation by CTor MRI. Esopbageal diverticula may be of the pulsion or traction type. Whereas pulsion diverticula occur predominantly in the pre sphincteric region (cervical, epiphrenic), most traction diverticula occur at parabronchial sites. They are detected incidentally on CTscans. CT Morphology
Duplication cysts are smoothly
marginated, homogeneous masses with water-equivalent attenuation that most commonly occur in the
lower esophagus (60%). They are intimately related to the esophagus on multiple axial scans but rarely communicate with it. The cysts may have a paraesophageal (Fig. IS.2a) or intramural location. Infected cysts may incite a perifocal reaction in the mediastinal fat. The differential diagnosis of esophageal duplication cysts includes bronchogenic and pericardial cysts. Depending on the density of the cyst content they may simulate hematoma, neurofibroma, leiomyoma, or lipoma. Diverticula appear as air-, water- or contrast-filled masses that most commonly occur posteroinferior to the cricoid cartilage as Zenker diverticula (70%, Fig.lS. 2b). Parabronchial diverticula (20%) and epiphrenic diverticula (10%) are not usually visualized on axial CT scans but may be noted on coronal or sagittal reformations of spiral or multislice CT data sets.
Esophageal Carcinoma
Fig. 15.2 (a) Zenker's diverticulum at a typical location in the lower necklposterior superior mediastinum. There is an air-fluid level and the trachea is displaced anteriorly and to the opposite side. (b) Esophageal duplication cyst.
b
a
Benign Tumors
Benign tumors of the esophagus are rare « 1 %), and most are asymptomatic. In rare cases the tumors are manifested clinically by upper intestinal hemorrhage. More than 50% of benign esophageal masses are leiomyomas. Benign esophageal tumors in asymptomatic patients are an incidental finding at CT examination. •
CT Morphology
Scans demonstrate a smooth, well-defined thickening of the esophageal wall. Benign tumors usually appear as a nodular or eccentric mass that differs from the typical circumferential growth pattern of esophageal carcinoma (Fig. 15.3). Even so, benign tumors cannot be confidently distinguished from carcinoma by their CT features alone.
Fig. 15.3 Esophageal leiomyoma presenting as an eccentric mass in the esophageal wall, which can also be appreciated on virtual endoscopy.
Leiomyomas are most commonly found in the distal esophagus and may reach 2-8 cm in size. The eccentric masses, which are sometimes misdiagnosed as extrinsic lesions, tend to distort the esophageal lumen and obstruct the passage of food. Leiomyomas show diffuse contrast enhancement and are the only tumors that may contain calcifications. The less common adenomas are smoothly marginated intraluminal masses that show moderate contrast enhancement. Hemangiomas show intense enhancement. Lipomas are readily identified by their fat attenuation (HU < 0). Esophageal
Carcinoma
By far the most common malignant tumor of the esophagus is squamous cell carcinoma (95 %), followed by adenocarcinoma (4 %). The peak incidence is between 50 and 60 years of age, and men are predominantly affected. Sites of predilection are the three physiologic constrictions of the esophagus, with "15% of lesions occurring in the upper third, 50% in the middle third, and 35% in the lower third. Adenocarcinoma predominates in the distal esophagus and shows a high association with endobrachyesophagus (Barrett's esophagus). A continuum exists between this tumor and carcinoma of the gastric cardia. Most tumors spread by longitudinal extension along the submucosal plane and metastasize early to paraesophageal, mediastinal, cervical, perigastric, and celiac lymph nodes, aided by the lack of a serosa on the cervical and thoracic segments of the esophagus. The re-
553
75 The Castrointestinal
Tract
gional lymph nodes for the cervical esophagus are the cervical nodes, and those for the in- • CT Morphology trathoracic esophagus are the mediastinal and Esophageal carcinomas appear as eccentric or areas of wall thickening perigastric nodes. Affected celiac nodes are circumferential (> 5 mm) that usually involve a shorter segment classified as distant metastases. Hematogenous of the esophagus than inflammatory processes spread occurs to the liver (rare with squamous (Fig. 15.4). Postcontrast CT shows enhancement cell carcinoma, common with adenocarcinoma), of viable tumor tissue with improved delinealungs, adrenals, and bone. The role of CT in staging esophageal neo- tion of tumor boundaries. The gastroesophageal plasms has been controversial. Currently it is junction is a problem area in terms of diagnostic used more in treatment selection (curative ver- evaluation because focal wall thickening at the gastroesophageal junction can be normal, desus palliative) rather than to place the patient into a distinct staging level. In a neoadjuvant sit- pending on the degree of gastric distension, but may mimic neoplasm. uation with chemotherapy or radiation therapy An important criterion for transmural prior to surgery, the role of CT is the initial tumor spread is the obliteration of the paraassessment of the tumor bulk for (radiation) therapy planning and monitoring tumor re- esophageal fat plane, which may be difficult to appreciate in emaciated patients. Even in norsponse to tumor-reducing therapy. Distant mal individuals the fat plane between the metastases and tumor spread in contiguous structures (stage T3, Table 15.5) preclude cura- middle third of the esophagus and the aorta is frequently absent. A small triangle of fat, tive surgery. Thus, the primary task of preoperative CT (chest and upper abdomen) is to detect however, is usually preserved between the esophagus, aorta, and spine in nearly all intransmural tumor growth and distant dividuals. Encasement of 90° or more of the aormetastases (liver) because of the substantial surgical mortality that is associated with tic contour makes invasion highly probable, while less than 45° of contact implies that invaesophageal resection. Invasion of the tracheal bifurcation, bronchi, pericardium, and aorta are sion has probably not yet occurred. The range of critical importance from a surgical stand- between 45° and 90° is considered indetermipoint. nate. Tumor extension into surrounding tissues is definitely indicated by displacement and
a
b
Fig. 15.4 Esophageal carcinoma, (a) Esophageal dilatation with retained secretions and eccentric wall thickening caused by carcinoma. Note the impression on the left main bronchus, (b) Asymmetric, circumferential carcinoma in the distal third of the esophagus. The tumor is still separated from the pericardium by a thin fat plane but is in broad contact with a vertebral
c
d
body. (c) Coronal reformation curved to follow the course of the esophagus demonstrates the longitudinal extent of a middle-third esophageal carcinoma, (d) Recurrence of esophageal carcinoma following resection and gastric transposition. The aorta is partially encased by tumor.
Esophagitis Table 15.5 TNM
Staging
criteria for esophageal
carcinoma
Bt\~~"I3BHmB""8 Ci RBffi BEN mBBBlhEW
staging
•
w
T1
Tumor 5 cm in longitudinal extent with circumferential wall involvement or luminal obstruction
compression of the airways with intraluminal convexity, fistulation to the tracheobronchial tree, and vertebral body erosion, The presence of a fistulous tract is indicated by the passage of orally administered air or contrast medium (do not use barium-containing agents) into the airways, mediastinum, or pleura, Periesophageal lymph nodes larger than 10mm, and infradiaphragmatic nodes of 8mm or larger are considered to indicate metastatic involvement (Table 15,5),
Esophageal Fistula Most esophageal fistulae in children are congenital, while fistulae in adults may be secondary to esophageal carcinoma, esophageal trauma, infections, or radtochemotherapy. Generally CT scanning is not indicated in children, but CT may be necessary in adults for localization and surgical planning, CT can also be used to detect pleuropulmonary or mediastinal inflammatory reactions to esophageal fistulae, • CT Morphology CT can demonstrate fistulous connections between the esophagus and the tracheobronchial system, pleura, pericardium, or mediastinal fat if the fistulous tract is of sufficient size and contains air or oral contrast medium, CT can also detect perifocal reactions in the form of pleuropneumonia, mediastinitis, or mediastinal abscess formation (see also Chapter 10),
Perforation of the Esophagus Perforation of the esophagus is an acute, lifethreatening event that requires prompt diagnosis, More than 50% of esophageal perforations result from iatrogenic manipulations (e,g" endoscopy, dilatation esophageal stenoses, or attempted intubation), Other causes are swallowed foreign bodies, blunt or penetrating chest trauma, caustic ingestion, and spontaneous perforation due to a sudden rise in intraluminal pressure, Esophageal diseases such as strictures, achalasia, and tumors predispose to perforation, Boerhaave syndrome is a special type of esophageal perforation involving a transmural injury of the distal esophageal wall approximately 2-3 cm above the esophagogastric junction, It is precipitated by extreme overeating and severe vomiting, CT Morphology
Esophageal injuries are manifested by wall thickening associated with periesophageal fluid, extra luminal air (pneumomediastinum, subcutaneous emphysema), pleural effusions, and possible contrast extravasation, CT also allows the visualization of very small collections of mediastinal air or contrast material in cases with small tears, It may also demonstrate penetrating foreign bodies that are only faintly opaque and therefore invisible on CI studies, Often the location of the esophageal perforation cannot be accurately defined by CT While distal perforations are usually associated with the development of a left-sided pleural effusion or a hydropneumothorax, a mid-esophageal perforation is more often characterized by a right-sided pleural effusion or pneumothorax, Thus, the side of the effusion or seropneumothorax can be critical in determining the route of the surgical approach,
Esophagitis Inflammatory thickening of the esophageal wall may be noted incidentally during the course of a CT staging examination or postoperative CT follow-up, It is not a primary indication for CT evaluation,
555
15 The Castro intestinal
Tract
CT Morphology
The inflamed esophageal mucosa shows uniform, circumferential wall thickening that usually involves a relatively long esophageal segment Eclematous wall segments appear hypoattenuating after intravenous contrast administration, while inflammatory infiltrates show substantial enhancement The transmural spread of an inflammatory process is uncommon, Intramural nodular densities, deep ulcerations, and fistulae, accompanied by mediastinal lymphaclenopathy, are seen in association with tuberculosis, Inflammatory and neoplastic wall changes cannot be reliably distinguished based on CT findings, Short segments of ulcerative wall thickening are more suggestive of a malignant lesion while longer segments are more consistent with an inflammatory process, Biopsy is usually necessary to establish the diagnosis,
Achalasia Achalasia is a motility disorder in which deglutition evokes incomplete relaxation of the lower esophageal sphincter, leading to dilatation of the esophagus, It is commonest from the third to sixth decades of life, The cardinal symptom is dysphagia, The primary diagnosis of achalasia is based on contrast esophagography or endoscopy, In rare cases (2- 7%), long-standing achalasia is associated with esophageal carcinoma, Cases that present unusual findings at esophagography or endoscopy should be further evaluated by CT CT Morphology
CT shows uniform dilatation affecting a long segment of the esophagus (4,Scm average diameter at the level of the carina) with no wall thickening and with normal-appearing boundary surfaces and mediastinal fat The esophagus narrows abruptly at the esophagogastric junction with no evidence of an intramural or extrinsic obstructive lesion, In contrast to a stricture, the esophageal wall is not thinned at the site of narrowing, and the wall is not thickened as it is with an esophageal tumor or esophagitis,
Strictures Stricture formation with obstructive narrowing of the esophagus may result from caustic ingestion, irradiation, trauma, surgery, or inflammation, The only role of CT is to exclude esophageal compression by an extrinsic mass. CT Morphology
Fibrosis and stricture formation are marked by luminal narrowing with thinning of the esophageal wall, In most cases the outer diameter of the esophagus is reduced over the site of the stricture, and the wall does not show abnormal enhancement It is common to find postinflammatory changes in the adjacent fat or at intrapulmonarysues.
Hiatal Hernia Hiatal hernia is the commonest form of diaphragmatic hernia, accounting for 90% of cases, The hernial opening is the esophageal hiatus, A distinction is drawn between an axial hernia (99%), in which the cardia is displaced into the thoracic cavity, and a paraesophageal hernia (1 %), in which portions of the stomach herniate alongside the esophagus, Mixed forms also occur. Hernias are an incidental finding at CT CT Morphology
With an axial hernia, portions of the stomach that emerge above the diaphragm may be misinterpreted as a retrocardiac mass (Ffg.lfi.Sa) but the demonstration of gastric folds and air in this mass is frequent and pathognomonic. Adequate gastrointestinal opacification is required in indeterminate cases, The diaphragmatic crura show more than IS mm of separation, The fat layer around the distal esophagus may be augmented by herniated portions of omentum, A paraesophageal hernia is associated with fixation of the gastric cardia, As a result, portions of the stomach herniate anterior to the esophagus and present as a contrast-filled intrathoracic mass, Upside-down stomach is an extreme form in which all of the stomach has herniated into the thoracic cavity (Fig, IS,Sb, c), and no portions of the stomach can be detected below the diaphragm, As the hernia enlarges the risk of life threatening complications such as incarceration, volvulus, strangulation, and infarction increases.
Dysp hagia/usoria
a
b
c
Fig. 15.5 Hiatal hernia, (a) Axial herniation of the stomach has resulted in a large retrocardiac mass. (b) Massive paraesophageal hernia (upside down stomach) with focal thickening of the wall due to esophageal
cancer. (4x1 /6). (c) Paraxial hernia with a gastric cancer in the fundus (4 x 1/6, coronal MPR). Note the retained contrast material in the proximal esophagus.
Esophageal Varices
structures that show marked pooling of intravenous contrast medium. Paraesophageal varices are often larger, and have a more serpiginous structure (see Fig. 10.13). Venous aneurysms are relatively frequent. While esophageal varices can be appreciated easily by endoscopy, paraesophageal varices are only seen on CT. In patients with portal hypertension, convoluted venous vessels can be seen in other areas of the upper abdomen that appear on the scan, but hepatic changes may be absent (depending on the etiology of the portal hypertension) . CT following uncomplicated sclerotherapy may show areas of esophageal wall thickening, scant mediastinal fluid, thickening of the diaphragmatic crura, pleural effusions, and small areas of pulmonary atelectasis.
Esophageal and paraesophageal varices are most commonly the result of portal hypertension. These" uphill" varices establish a collateral circulation from the portal vein through the azygos venous system to the superior vena cava. They occur predominantly in the lower half of the esophagus. "Downhill" varices are rare and may develop in the upper half of the esophagus when obstruction or occlusion of the superior vena cava causes blood to flow through the azygos system into the inferior vena cava and portal venous system. Esophageal varices may present as a mediastinal mass on chest radiographs and require CT for further evaluation. Often, however, they are an incidental finding at a thoracic or upper abdominal CT examination. •
CT Morphology
Esophageal varices have a variable CT appearance. Noncontrast scans show only sites of nonspecific esophageal wall thickening and nodular paraesophageal or retrocardiac masses. The enhancement of the varices after contrast injection requires a sufficiently long scan delay (at least portal venous phase), otherwise they may remain isoattenuating to the esophagus. Esophageal varices present as intraluminal (intramural, submucous) tubular, often dot-like
Dysphagia lusoria
Aberrations in the course of the aortic arch or supra-aortic branches can displace or compress the proximal esophagus, leading to dysphagia. The most frequent cause is an aberrant right subclavian artery that arises from the descending aorta and passes behind the esophagus. Other causes are a duplicated aortic arch or an aortic aneurysm.
557
15 The Gastrointestinal Tract
CT angiography (CTA) has already replaced intravenous and intra-arterial DSA in the evaluation of thoracic vascular anomalies and has become the diagnostic procedure of first choice (see Chapter 24, p, 871), Contrast-enhanced MR angiography has recently emerged as a competitive modality especially in young patients in whom, radiation exposure is an issue.
Postoperative Changes Gastric transposition and colon interposition are two reconstructive techniques that are commonly used to restore alimentary continuity after a partial or complete resection of the esophagus, The esophageal substitute may be placed within the esophageal bed or in a presternal or retrosternal position, CT has an essential role for the detection of recurrent esophageal carcinoma in the post esophagectomy patient The strength of CT is to identify an extraluminal mass in the mediastinum and to detect distant metastases, In patients with dysphagia, endoscopy and upper
Gl series may be more helpful in identifying a mucosal lesion or an anastomotic obstruction, but CT is superior in patients with general symptoms such as weight loss and abdominal pain, CT Morphology
Depending on its choice and placement, the esophageal substitute will generally show slight dilatation and a greater outer diameter than the original esophagus, When gastric transposition has been performed, part of the infradiaphragmatic stomach will be absent An interposed colon segment is recognized by its haustrations, Recurrent tumor presents as wall thickening, or as an intra- or extraluminal mass, usually at the site of anastomosis, Extraluminal mediastinal masses or encasement of the esophagus or esophageal substitute are most often present at the site of maximum tumor size before therapy (see Fig.l5.4d), Lymph node enlargement may be found in a paraesophageal or upper abdominal location but also in other site in the mediastinum, Pulmonary or hepatic metastases may be present
Stomach Cysts and Diverticula
Duplication cysts are congenital,
intramural masses of the stomach, Gastric lesions are lined by secretory epithelium and may enlarge over time, Most of these masses are detected at an early age, Diverticula of the stomach are very rare and arise in two locations, True gastric diverticula are more common and occur just below the gastroesophageal junction at the posterior portion of the lesser curvature, They contain all gastric layers and may become quite large (I-IOcm), An antral diverticulum is rarer and represents a focal invagination of the mucosa into the muscularis propria of the gastric wall, Antral diverticula are intramural and rarely larger than a few millimeters, They are almost never detected on CT
CT is indicated in patients with indeterminate obstructive thickening of the gastric or duodenal wall on endoscopy, CT Morphology
Duplication cysts appear
as intramural cystic areas of gastric or duodenal wall thickening with an associated mass effect Cysts on the greater curvature of the stomach may reach more than !Oem in size, The masses are oriented parallel to the gastrointestinal lumen, Generally the cysts are not opacified by oral contrast medium, because they rarely communicate with the lumen of the organ, Diverticula appear as air-, water- or contrast-filled masses located on the posterior wall of the gastric fundus,
Gastric Carcinoma
Benign Gastric Tumors
Benign tumors of the stomach may originate from any layer in the gastric wall. The commonest are adenomas (polypous adenomas), leiomyomas, lipomas, leiomyoblastoma, neurofibromas, and angiomas. Polypoid adenomas have a high risk of malignant transformation. Hyperplastic gastric polyps have no malignant potential; however patients are at increased risk for harboring separate, coexisting gastric carcinomas (between 8% and 28%). Leiomyoblastomas are uncommon smooth muscle tumors usually arising from the antrum, that can become quite large. Dependent on their size they have a tendency for malignant transformation with metastases to the liver or other structures occurring in 10% of patients. Metastases seldom occur with lesions smaller than 6cm. Approximately two-thirds of all leiomyomas occur in the stomach. A large subset of the leiomyomas and leiomyoblastomas that are positive for KIT (CD 117) are currently believed to belong to the group of gastrointestinal stromal tumors (GIST, pS66) There are only few cases (lipomas) in which CT can differentiate benign from malignant wall lesions, so a definitive diagnosis must rely on endoscopic biopsy. •
CT Morphology
Adenomas of sufficient size appear as intral uminal masses on CT scans. Leiomyomas are sharply circumscribed, uniformly enhancing masses with an average size of some Scm. They may contain irregular streaks or clumps of mottled calcifications and
are the most commonly calcified benign gastric tumors. They usually project into the gastric lumen, but occasionally may have an extragastric appearance. Contrast-enhancing normal mucosa usually covers the tumors (Fig.1S.6a) although ulcerations may occur. Neurofibromas are sharply circumscribed oval lesions within the gastric wall that are covered by normal contrast -enhancing mucosa. The tumors usually do not calcify and enhance less strongly than most leiomyomas (Fig. IS.6b). Lipomas range from 1-3 cm in size and are easily distinguished from other masses by their negative attenuation values «-80HU). Up to 7S% of lipomas are located in the antrum. They are endogastric lesions that arise from the submucosal fat in more than 90% of cases and grow towards the lumen. Large lesions develop superficial ulcerations that may bleed. Angiomas show intense enhancement after intravenous contrast administration. Gastric
Carcinoma
Adenocarcinoma (9S%) is one of the commonest tumors in humans with a high incidence in China, Japan, and East Asia, and the lowest incidence in North America and North Africa. There is a declining incidence of distal tumors but increasing number of tumors in the cardia and esophagogastric junction. It is most prevalent after SO years of age and predominantly affects men (2: 1). Most carcinomas infiltrate the gastric wall in the regions of the antrum or pylorus, along the lesser curvature, or in the gastric fundus
Fig. 15.6 Benign gastric tumors. (a) Leiomyoma of the anterior gastric wall, demonstrated by hydroCT in the prone position. The tumor appears as a well-circumscribed, intensely enhancing mass. Absence of infiltration of the gastric mucosa confirms the submucous location of the tumor. (b) Neurofibroma of the gastric wall with a similar morphology but less contrast enhancement.
a
b
15 The Gastrointestinal Tract near the carclia. Early lymphatic metastases occur to the lymph nodes of the lesser and greater curvatures and along the celiac trunk, the gastrocolic and gastrosplenic ligaments, and the greater omentum. Retroperitoneal (paraaortic) spread may occur along the thoracic duct to the left sLipraclavicular region (sentinel node). Affected retropancreatic, mesenteric, and para-aortic lymph nodes and affected nodes along the hepatoduodenal ligament are classified as distant metastases. Advanced disease is characterized by transm ural tumor growth with invasion of the colon, pancreas, or liver, and by peritoneal carcinomatosis with ascites (Table 15.6). Mucinous gastric carcinoma in women tends to seed drop metastases to the ovaries (Krukenberg tumors) or into the cul-de-sac. Hematogenous metastasis occurs to the liver, adrenals, lung, ovaries, bone, and brain. Tumor staging is performed histologically in a surgical specimen. CT is used to assess the presence and extent of transmural or extragastric spread of tumor, thereby assisting in the selection of surgical or conservative therapy. In patients with transm ural tumor extension with peritoneal spread and distant metastases (liver, lung), chemotherapy or palliative surgery are performed. If CT indicates a localized tumor, staging laparotomy and curative surgery will be attempted. CT is also used in "downstaging" to assess therapeutic response and evaluate the Table 15.6
Staging criteria for gastric carcinoma
T1
Tumor confined to the mucosa or submucosal
T2
Infiltration of the muscularis or serosa
T3
Penetration of the serosa
T4 a
Invasion of surrounding tissue
T4b
Invasion of other organs, diaphragm, or abdominal wall
N1
Lymph node involvement on greater or lesser curvature (up to 3 cm from primary tumor)
N2
Involvement of perigastric or celiac lymph nodes (>3 cm from primary tumor)
M1
Distant metastases (liver, lung, ovaries, adrenals, bone, brain), involvement of paraaortic, hepatoduodenal, mesenteric, or retropancreatic lymph nodes
secondary resectability of locally advanced tumors. With thin-section spiral and multislice CT, the accuracy of tumor staging has substantially improved, but its diagnostic role has not yet been established. •
CT Morphology
CT of the adequately fluid-distended stomach reveals focal areas of wall thickening (>4mm) that may be irregular or nodular, or intral uminal masses with asymmetric thickening of the gastric folds. Scirrhous carcinomas (linitis plastica) tend to cause concentric wall thickening with lack of distension of the affected segment (Fig.15.7a). The loss of a normal fold pattern is best detected when the stomach is distended with negative contrast medium (water). Differentiation of nodular tumor from hyperplastic gastric folds is required and improves with interactive cine display or multiplanar reformations. Larger tumors may obstruct the passage of contrast medium and cause prestenotic dilatation. Mucin- producing adenocarcinomas contain hypoattenuating central areas and may appear completely hypodense. They may contain stippled calcifications. Variable enhancementof gastric carcinomas is seen after intravenous contrast injection, with scirrhous carcinomas showing pronounced enhancement. Polypoid carcinomas present as soft tissue masses protruding into the lumen. Mural thickening due to gastric cancer ranges from 6mm to 4cm, the likelihood of transmural tumor-extension is directly correlated to the wall thickness and is quite common when the wall thickness exceeds 2cm. It can be difficult to assess transm ural tumor extension in emaciated patients due to the lack of conspicuous fat planes. Irregularities of the outer gastric margin, perigastric stranding, areas of increased structural density, and ill-defined boundary surfaces, especially in the perigastric fat, are suspicious for transmural tumor extension (stage T3, serosal involvement), although most of the stranding represents desmoplastic reaction. Irregular areas of increased attenuation in the mesenteric fat and omentum, sites of peritoneal thickening, and ascites with solid components are suggestive of peritoneal carcinomatosis in patients who have not had previous abdominal surgery. Proximal gastric cancer may cause direct invasion into the
Gastric Lymphoma
a
c
b
Fig.15.7 Gastric carcinomas (4x1/6, semicoronal MPR). (a) Scirrhous carcinoma with concentric gastric wall thickening and effacement of folds despite the slight degree of gastric distention. The inner layers of the gastric wall show appreciable enhancement, (b) T2
tumor of the gastric fundus with small, round and contrast-enhancing lymph node metastasis (arrow), (c) T4 tumorwith local infiltration of the transverse colon (wall thickening) and the pancreatic tail (arrows).
adjacent esophagus via the gastrophrenic ligament, the left lobe of the liver via the gastrohepatic ligament or the spleen via the gastrosplenic ligament. Distal gastric tumors can invade the neighboring duodenum, the pancreas, or the periaortic nodes, via the inferior aspect of the lesser omentum and the hepatoduodenal ligament, and into the transverse colon via the gastrocolic ligament. Metastases occur most frequently in lymph nodes along the gastrohepatic ligament and along the attachments of the omental folds to the great curvature. For the diagnosis of lymph node metastases, CT shows a sensitivity and a specificity of60-66%. Newer criteria for multislice CT (rounded shape with> S mm; contrast enhancement, no fatty center LIT ratio < 2 and short -axis diameter, see also Chapter 22) promise to improve the staging accuracy (Fig. IS.7b, c).
present. Low-grade MALT lymphoma has a favorable clinical course. Thoracic and abdominal CT is used for tumor staging.
Gastric Lymphoma
The stomach is the commonest site (SO%) of involvement by primary lymphoma or generalized lymphomatous disease in the gastrointestinal tract, with lymphomas accounting for just 3-S% of all gastric malignancies. Most of the tumors are histiocytic or lymphocytic non-Hodgkin lymphomas (90-9S%). MALT lymphomas arise from acquired mucosa-associated lymphatic tissue, with Helicobacter pylori invariably being
•
CT Morphology
Gastric lymphoma may infiltrate the entire stomach diffusely, or it may produce sharply circumscribed areas of pronounced polypoid or nodularwall thickening (Fig.18a, b). Ulcerations may be present (Fig. IS.8c). These areas do not contain necrotic foci and rarely show fistulation. Lymphomas tend to infiltrate the stomach homogeneously and show little contrast enhancement (Fig. IS.8). The mean thickness of the affected gastric wall is 4 to S cm despite extensive lymphomatous infiltration the stomach usually remains pliable and distensible without significant luminal narrowing (Fig.1S.8a). Advanced lesions may become very large. Only very rarely gastric lymphoma presents as cavitated lesion caused by tumor necrosis and excavation. Unlike gastric carcinoma, gastric lymphoma most commonly affects the greater curvature of the antrum and the body. Direct invasion of the pancreas, spleen, colon, and liver may occur. Transpyloric spread of lymphoma into the duodenum is quite common and represents a differentiating feature to adenocarcinoma that invades the duodenum in less than 10%. While splenic involvement is suggestive of lymphoma, hepatic metastases are characteristic of adenocarcinoma (Table IS.7).
561
562
75
The Gastrointestinal
Tract
Fig. 15.8 Gastric lymphoma. (a) Large MALT lymphoma with thickening of the gastric wall along the major curvature and in the antrum. Note the extra gastric tumor extent with encasement of the splenic artery, (b) Lymphoma of the lesser curvature with nodular submucosal tumor growth.
a
b
Table 15.7 Differentiating features between gastric adenocarcinoma CTfeatures
Gastric Lymphoma
Gastric carcinomc
Mural thickness
4cm (1-7 cm)
1.8cm (1-3cm)
Gastric wall
Thickened folds with smooth outer contours
Nodular outer contours + perigastric stranding
Inhomogeneous wall enhancement
uncommon
common
Extent
often diffuse
often focal
Spread into adjacent organs
40%
>70%
Lymphadenopathy below renal hilum without perigastric adenopathy
>40%
Other Malignant Gastric Tumors Gastric Sarcoma
Up to 3% of all gastric malignancies are leiomyosarcomas, which may occur at a young age. Patients present clinically with gastrointestinal obstruction and hemorrhage. Sarcomas tend to metastasize hematogenously to the liver, lung, peritoneum, omentum, and retroperitoneum. More than 90% involve the fundus and the body. Leiomyosarcomas may be difficult to distinguish from leiomyomas; malignancy correlates with size, and tumors larger than 6cm are almost always malignant. A specific subset of these sarcomas that express the c-kit oncogene (CD 117) are currently categorized as gastrointestinal stromal tumors (CIST, see p. 566). •
and lymphoma
CT Morphology
In patients with leiomyosarcoma, CT demonstrates lobulated, irregular areas of wall thickening or focal masses that show intense, nonho-
mogeneous enhancement and may contain areas of hypo attenuating necrosis and deep ulceration. Cavitation with air-fluid levels may be seen. Calcifications are occasionally present. Preferred sites are the anterior and posterior walls of the body of the stomach. The tumors may reach more than 12cm in size, resulting in a large extragastric component. Kaposi Sarcoma
Kaposi sarcoma develops in about 35% of patients with AIDS. Gastrointestinal lesions are seen in about 50% of them and are commoner in homosexuals than in drug abusers. Cutaneous involvement is common. •
CT Morphology
Kaposi lesions represent as submucosal masses of 0.5-3 cm diameter. They are markedly hypervascular. As these masses enlarge, they frequently undergo central necrosis or ulceration (target appearance).
Postoperative
Also polypoid masses or diffuse infiltrations are seen and may be indistinguishable from adenocarcinomas. Since the gastric findings alone are nonspecific. CT is helpful in assessing retroperitoneal lymphoma. splenomegaly or other evidence of Kaposi sarcoma suggesting the diagnosis.
Gastric
•
CT Morphology
Gastric metastases may appear as solitary submucosal masses or as diffuse infiltration. the latter being indistinguishable from a primary scirrhoLis carcinoma. Unenhanced CT attenuation values and enhancement characteristics depend on the primary tumor.
Gastritis,
Ulcer Disease
Endoscopy is used for the diagnosis and followup of gastritis and gastroduodenal ulcer disease. Generally CT is not indicated. but it does have selected indications such as the assessment of complicated cases prior to surgical intervention. •
Varices
Portal hypertension can lead to the portocaval shunting of blood through enlarged venous channels in the gastric wall. or varicose short gastric veins and the coronary vein of the stomach. Like esophageal varices. they are incidental findings at CT. CT Morphology
Metastasis
The stomach may be involved by metastases from malignant melanoma. carcinomas of the breast. lung. colon. or prostate. leukemia. or secondary lymphoma.
CT Morphology
Scans in severe gastritis demonstrate areas of gastric wall thickening that enhance after intravenous contrast administration. Nodular intramural densities suggest a tuberculous process. but CT cannot differentiate benign from malignant wall changes. Intramural air collections are found in complicated forms of bacterial gastritis {phlegmonous gastritis}. Scans in gastroduodenal ulcer disease may reveal local wall thickening that enhances with intravenous contrast. Due to the retroperitoneal position of the duodenal loop. its perforations are usually well contained and do not always present as a pneumoperitoneum (see Fig. 15.33a). Pockets of extraluminal gas may be seen in one or both anterior pararenal spaces or in the soft tissues of the upper abdomen.
Changes
Gastric varices appear as areas of gastric wall thickening (fundus. lesser curvature) with enhancing varicose veins. The varices can be traced as tubular wall segments on contiguous sections. They are best appreciated on MIP or volume rendered images if a negative contrast agent has been used.
Postoperative
Changes
Surgical procedures on the stomach and duodenum (gastrojejunostomy or esophagoje]unostomy) can radically alter the anatomy and physiology of the upper intestinal tract and can significantly affect the transit of food and contrast medium. Some reconstructive procedures (e.g .. a Billroth II reconstruction) may preclude the possibility of endoscopy. A complicated postoperative course or unexplained luminal compression requires evaluation by CT. •
,CT Morphology
A leaky anastomosis is manifested byextraluminal fluid collections with possible contrast extravasation or abscess formation. Contrast extravasation may be hard to distinguish from blind portions of small intestine that are frequently found after these procedures. In these cases. conventional radiological examinations or multislice CTwith multi planar reformations have to be performed. Postoperative luminal narrowing at the anastomotic site may have inflammatory or neoplastic causes (anastomotic recurrence). which are indistinguishable by CTif there is only a moderate mass effect. Given the frequent absence of oral contrast opacification in the afferent loops (e.g .. at the porta hepatis). these loops are easily mistaken for a mass lesion. lymph nodes. or an abscess with air inclusions. Such loops can be opacified by oral administration of biliary contrast agents (see Chapter 12).
563
75 The Gastrointestinal Tract Intestine Cysts and Diverticula
Duplication cysts are rarely seen in the duodenum. Diverticula in the duodenum are more common than in the jejunum and ileum. Diverticula of the small bowel are often located on the mesenteric side of the proximaljejunum. The majority of duodenal diverticula are acquired, asymptomatic diverticula but symptomaticinverted (in tral uminal) duodenaldiverticula in young adults (congenital lesions) may be responsible for unexplained pancreatic or biliary stasis. Asymptomatic colonic diverticula are much more frequent and become increasingly common with aging. Approximately two-thirds of all colonic diverticula involve the sigmoid colon. CT is indicated in cases of unexplained pancreatic or biliary stasis to exclude a tumor of the pancreatic head, or in patients with suspected diverticulitis (see p. 576). Duodenal or colonic diverticula often present as an incidental finding. •
CT Morphology
Duplication cysts appear as cystic areas within the duodenal wall and have an associated mass effect. Small bowel diverticula appear as air-, water- or contrast-filled masses located on the inner aspect of the duodenal C loop (Fig. 15.9) or less commonly on the mesenteric side of the proximaljejunum. Withjuxtapapillary diverticula, dilatation of the pancreatic duct and bile ducts may occur as an accompanying reaction. A
diverticulum containing retained food may simulate an abscess, although the wall of the diverticulum is thin and enhances like other bowel structures. Inverted diverticula present as filling defects in the duodenum and require good luminal opacification by oral contrast material for diagnosis. The differential diagnosis of indeterminate round or oval masses in the gastrointestinal tract should always include small bowel diverticula in addition to tumors. Retained stool in small bowel diverticula can be a cause for the "small bowel feces sign" that is due of abnormal bacterial overgrowth. Colon diverticula present as air-; water- or contrast -filled outpouchings of the colon wall and usually measure between 3 and 10mm in diameter. Chronic diverticular disease causes wall thickening (3-5 mm) in the affected colon segment. Previous inflammatory processes are characterized by residual densities in the pericolic or mesenteric fat. Benign Tumors
Tumors of the small bowel are very rare (4 % of all intestinal tumors), and approximately threefourths of all small bowel tumors are benign. Lipoma of the duodenum is quite common. The commonest benign tumor of the more distal small bowel is leiomyoma, which involves the
Fig. 15.9 Duodenal diverticulum with food remnants and air in a juxtapapillary location in a patient with NHLand retroperitoneal lymph node involvement (a). The normal duodenal lumen is marked by an asterisk. A volume rendered coronal image (4 x 1/6) in another patient demonstrates the typical location of such diverticula (b).
a
b
Malignant Small Bowel Tumors jejunum in 50% of cases. Adenomas most commonly occur in the ileocecal region, and hemangiomas are commoner in the jejunum. Neurofibromatous tumors occur in the setting of Recklingshausen disease. The commonest benign colonic tumors are adenoma's (polyps) and lipomas. The risk of malignant transformation in adenomas depends on the size and histology of the tumor (see Table 15.9). Polyps of more than lcm diameter and those with villous histological features are associated with a higher risk for malignant transformation. Lipomas are found predominantly in the right -sided colonic segments (cecum and ascending colon). Another common benign tumor is endometriosis, with 70% of intestinal deposits showing a perirectal or perisigmoidal location. Leiomyomas, fibromas, neurofibromas, hemangiomas, and lymphangiomas are rare benign colonic tumors. •
CT Morphology
Lipomas usually appear as well-defined round or oval intramural masses of fat density measuring 1 to 3 em in diameter. Lipomas of the horizontal portion of the duodenum (Fig. 15.10a, b) and of the ileocecal valve are relatively common. Lipomas may simulate extramural masses on endoscopy but can be readily identified by CT (see Fig.15.17b). Leiomyomas are moderately hypervascular masses or areas of bowel wall thickening that are prone to necrosis if their size is large. Hemangiomas appear as hypervascular, nodular areas in the bowel wall and may be
a
b
Fig. 15.10 Submucosal lipoma in the duodenum (a). A close relation to the papilla of Vater is seen on a coronal image (4 x 1/6) in another patient (b). Multiple in-
pedunculated. Endometriosis appears as an extramucosal tumor-like infiltrate. Adenomas of sufficient size may be demonstrated as pedunculated or broad-based intral uminal masses of soft-tissue density (Fig.15.1 Oc). In the small bowel, they are best detected by CT enteroclysis, and in the colon by CT colonography or virtual colonoscopy techniques (see p. 571). Malignant Small Bowel Tumors
Carcinomas of the small bowel are rare lesions that occur predominantly in the jejunum, while sarcomas are commonest in the ileum. Malignant tumors of the duodenum [adenocarcinomas, leiomyosarcomas, carcinoid tumors, lymphomas) are very rare. •
CT Morphology
Carcinomas of the small bowel most often involve the jejunum but may also be seen in the duodenum, or least frequently, the ileum. In the duodenum, they predominantly affect the posterior wall. Carcinomas present as areas of wall thickening or intraluminal masses (Fig. 15.11) that enhance after intravenous contrast administration. As opposed to small bowel lymphomas, carcinomas do not tend to cavitate with a communication to the bowel lumen but instead lead to focal small bowel stenoses or obstruction. As a result, signs of bowel dilatation or ileus may be present. The tumors grow
c traluminal pedunculated polyps in the small bowel in a patient with Cowden disease (multiple hamartoma syndrome) (c).
565
75 The Gastrointestinal Tract
a
b
Fig. 15.11 (a) Carcinoma of the small bowel. CT reveals wall thickening and subtotal stenosis of thejejunum by a nonhomogeneous mass, accompanied by borderline enlargement of mesenteric lymph nodes (arrow heads). (b) Hypervascular duodenal metastasis of a renal cell carcinoma.
locally invasive with stranding of the peritumoral fat as a sign of infiltration. Advanced duodenal tumors may invade the pancreatic head, obstructing the pancreatic duct and causing biliary stasis. These lesions may be indistinguishable from primary tumors of the pancreatic head. Leiomyosarcomas reach more than 6cm in size and appear as nodular masses with necrotic components that show heterogeneous enhancement and may contain calcifications. In most cases (>60%), the nodular mass is mainly located extrinsically. The mucosa may be stretched and ulcerated (50%), which may result in a fistula communicating between the bowel lumen and the necrotic tumor center. Metastases to the bowel are rare and are almost impossible to differentiate from primary tumors (Fig.15.11 b). Gastrointestinal Stromal Tumor
Castrointestinal stromal tumors (GISTs) are a heterogeneous group of tumors that represent the most common mesenchymal tumors in the gastointestinal tract. They are defined as KIT (CD n7)-positive spindle cell or epitheloid neoplasms and include most tumors previously designated as leiomyoma, leiomyoblastoma, and leiomyosarcoma. However, CISTs differ clinically and pathogenetically from true leiomyosar-
comas (very rare in the GI tract), from leiomyomas (common in esophagus and colon) and from neurogenic tumors like schwannoma. Although not fully understood, they are believed to originate in the plexus myentericus or in a pI uri potential stem-cell that can differentiate into Cajal and smooth muscle cells. Based on the immunohistochemical findings, four categories can be differentiated: tumorwith differentiation towards smooth muscle cells, towards neural elements (gastrointestinal autonomic nerve tumors, CANT), without a specific differentiation (uncommitted type), and with differentiation towards smooth muscle andneural elements. The histologic differentiation can be very difficul tor even impossible even with use of modern immunohistochemical methods. GISTs typically present in older (>50 years) individ uals and are most common in the stomach (60-70 %), followed by the small intestine (2025%), colon and rectum (5%), esophagus «5%) and mesentery and omentum « 5%). Theycanbe benign or malignant (in up to 30%) or have a borderline to low malignant potential. Malignant tumors frequently show metastases in the liver, peritoneum, lung, bone or lymph nodes. Therapy is primarily surgical and for nonresectable tumors very promising results with imatinib (ST1571, Glivec) chemotherapy has recently been reported. The most important prognostic factors are tumor size (or preferably tumor volume), tumor grade, tumor necrosis, and infiltration and metastasis to other sides. •
CT Morphology
GISTs may be submucosal, subserosal or intral uminal. They occur as solitary or m ul ti pIe lesions. Submucosal GISTs appear as round, smoothly defined filling defects while subserosal GISTs appear as extrinsic masses (Fig.15.12a), displacing adjacent bowel loops. Hypervascular intraluminal GIST may cause ulceration and secondary hemorrhage. Calcifications are rare. Benign GISTs are homogeneous with soft tissue density similar to muscle. Malignant GISTs appear as irregular lobulated masses with central liquefactive or cystic necrosis (Fig.15.12b), with or without a fluid-air level. They typically show a tumor diameter >5cm, intralesional necrosis, and infiltration ofthe surrounding structures. Vascular encasement may occur. A smooth contour, however, is also seen
Malignant Small Bowel Tumors Fig. 15.12 Gastrointestinal stroma cell tumor (GIST), (a) Subserosa I GIST of the stomach presenting as an extrinsic mass in the left upper quadrant. (b) Malignant GIST of the ileum with central necrosis.
b
a
in malignant lesions, and in some cases it may be impossible to distinguish benign from malignant types. The enhancement pattern in benign GIST is usually homogeneous, while in malignant types inhomogeneous peripheral enhancement is seen. Although liver metastases are most frequently hypodense, hypervascular types are not infrequent. Therefore, as with other sarcomas, metastatic disease detection in the liver is optimal when Diphasic CT is used. In HIV-positive patients, multiple hypervascular GISTs may simulate multiple Kaposi's sarcoma. Lymphoma, metastases, carcinoid, carcinoma or other mesenchymal tumors may show indistinguishable morphologic findings. The morphology following Clivec therapy has not yet been elucidated, but personal experience suggests that dimishing enhancement and increase in necrosis may precede actual tumor size reduction. Liver metastases may become purely cystic. Carcinoid
Carcinoid tumors are the commonest primary neoplasms of the small bowel (ileocecal region) and appendix. Only about 5% of intestinal carcinoids occur in the colon, and 17% involve the rectum. Carcinoids are neuroendocrine in origin (APUDoma=amino precursor uptake and decarboxylation, representing a cell line arising from islet of Langerhans). Men are affected twice as often as women. Patients are asymptomatic (60%) or suffer from pain (20%), obstruction, or weight loss (20%). Patients with liver metastases
may develop a carcinoid syndrome caused by excess serotonin levels bypassing the liver (diarrhea in 70 %, right heart failure clue to rightsided endocardial fibroelastosis and tricuspid regurgitation (35 %), asthma (15 %), skin lesions (5 %), cutaneous flushing « 5%). While small bowel and colonic carcinoids metastasize relatively early, regardless of their size, the occurrence of metastasis of rectal carcinoids is size-dependent. CT is used for nodal staging and for detecting or excluding distant metastases (lung, liver, bone). •
CT Morphology
The primary tumor can be very small and escape imaging, but the associated desmoplastic reaction and mostly bulky lymphadenopathy is readily seen. Calcifications occur in the majority of cases. In addition to hypervascular areas of bowel wall thickening or mass lesions, CT reveals retraction of the mesentery due to a desmoplastic reaction (Fig. 15.13). Adjacent bowel loops may be displaced, unusually separated, or sharply bent. Encasement of mesenteric vessels leads to chronic ischemia with segmental bowel thickening. Fibrotic retraction of mesenteric vessels may simulate hypervascularity. Venous occlusion may cause mesenteric varices. Affected mesenteric and retroperitoneal lymph nodes contain necrotic components of low attenuation. Advanced tumor stages may be associated with hepatic metastases (that may be near cystic, hypo-, iso-, or hyperattenuating after contrast injection, see Fig. 11.27), pulmonary nodules, and mostly osteoblastic skeletal metastases.
15 The Gastrointestinal
Tract Fig. 15.13 Carci noid. (a) Carcinoid tumor of the ileum with hypervascular nodular wall thickening. (b) Desmoplastic reaction leads to retraction of the mesentery.
a
b
Lymphoma
Lymphomas are the commonest primary small bowel tumors but are rare in the colon (approximately 1.5% of all abdominal lymphomas). The incidence of small bowel involvement by lymphoma is substantially increased in immunocompromised patients. Anatomically, the ileum is the most usual site of small bowel involvement. Multifocal involvement is occasionally seen. The most frequent site of occurrence in the large bowel is the cecum. N on- Hodgkin lymphoma can involve the gastrointestinal tract due either to secondary spread from nodal disease elsewhere or as a primary disease. Primary gastrointestinal lymphoma occur more frequently in the setting of inflammatory bowel disease, immunosuppression, sprue, and Helicobacter gastritis. CT is used in tumor staging to evaluate local tumor manifestations and lymphadenopathy. •
CT Morphology
Intestinal involvement by lymphoma may consist of a discrete mass or diffuse infiltration, resulting in pronounced eccentric or circumferential bowel wall thickening or a focal soft-tissue mass (average size Scm) that shows only slight enhancement (Fig. 15.14). The intestinal lumen may be narrowed as consequence of wall thickening. However, it can also show dilated caliber making it difficul t to differentiate from a small bowel obstruction. Large ulcerations and necrotic areas are also found that may be in continuity with the bowel lumen and thus contain contrast material. Polypoid lesions may act as the leading edge of intussusception, especially
Fig. 15.14 Intestinal lymphoma. CT shows segmental involvement of the small bowel by NHL with pronounced, asymmetric wall thickening and bizarre cavitation caused by complete destruction of the intestinal mucosa.
in children. Fistulae between adjacent loops may simulate Crohn's disease. Scans may also show pronounced regional lymph node enlargement along with mesenteric and retroperitoneal lymphadenopathy. The "sandwich" sign seen with involvement of mesenteric lymph nodes is caused by masses that encroach upon the mesenteric vessels from both sides. Malignant lymphomatous polyposis is a special condition with multiple (> 100) small polypoid lesions in the colon; frequently a large polyp or cecal mass is seen. Typically it is a low grade lymphoma most frequently occurring in men beyond 50 years. As a rule, lymphoma should be considered in the differential diagnosis ofvirtually all gastrointestinal tumors or inflammatory processes.
Colorectal Carcinoma Colorectal Carcinoma Colorectal carcinoma is the commonest malignant tumor of the gastrointestinal tract and the second commonest malignant tumor in men and women. The peak incidence is between 50 and 70 years of age. The predominant histological type is adenocarcinoma (70%). followed by mucfnous carcinoma (20%. known also as signetring or colloid carcinoma). and anaplastic carcinoma. Squamous cell carcinoma and adenoacanthoma are rare. Approximately 90% of all colorectal carcinomas arise from benign adenomatous polyps. Most carcinomas show an exophytic. polypous type of growth with frequent central degeneration. and they tend to infiltrate the bowel wall circumferentially. More than 50% of colorectal carcinomas occur in the rectum and sigmoid colon. Regional lymph node metastases of the colorectal carcinoma involve the pericolic and perirectal nodes and the nodal chains along the mesenteric vessels and internal iliac arteries on both sides. More advanced stages may also metastasize to the para-aortocaval nodes and mesenteric fat nodes. The most common sites of distant metastasis are the liver. lung. andadrenals. Potential complications of colorectal carcinoma include bowel obstruction. perforation. hemorrhage. fistula formation. and unilateral or bilateral hydronephrosis due to ureteral encasement. CT colonography is only indicated to detect additional lesions proximal to a stenotic primary that cannot be passed by an endoscope. From a surgical point of view. CT has no role yet for T-staging of patients with known colon cancer because most primary tumors are resected. even if metastases are present. Crosssectional imaging is indicated for the detection or exclusion of distant metastases (lung, liver) and to direct planning of radiotherapy in patients with locally advanced disease. With thin-section multislice CT, tumor staging becomes more accurate, and CT may regain a role in the pretherapeutic work-up, especially in the setting of neoadjuvant radio-chemotherapy. •
CT Morphology
CT demonstrates sites of focal or circular wall thickening or intramural masses that show enhancement with intravenous contrast. Markedly
hypoattenuating masses with hypoattenuating lymph nodes are characteristic of mucinous adenocarcinoma. Mucinous carcinoma tend to develop calcifications, both in the primary tumor as well as in distant metastases. Wall thickening distal to a large, fungating type of colonic carcinoma in the ascending colon may be due to edema or colitis. Tumor staging (Table 15.8) is problematic with CT but improves substantially with thinTable 15.8 Staging criteria forcolorectal
carcinoma
Dukes classification Dukes A: Dukes
B:
Dukes
C:
Dukes
Tumor
confined
Invasion Lymph
to the bowel wall
of the serosa
or mesenteric
fat
node metastases
Plus tumor bowel wall
growth
C1: Dukes
Plus tumor
extension
confined
to the
into fatty tissue
C2: Dukes D:
Distant metastases
TlIIIM staging
(UICC. 1991)
Tis
Carcinoma
T1
Tumor confined to mucosa
in situ confined to mucosa
T2
Tumor
T3
Transmural penetration into the subserosa or into non-peritonealized pericolic or perirectal structures
T4
Extension
confined
and submucosa
to muscularis
into retroperitoneum,
or adjacent organs (including distant bowel segments) N1
propria
-S3 positive
perirectal
mesocolon, infiltration
or pericolic
of
lymph
nodes N2
> positive nodes
M1
Distant
Tumor
perirectal
or pericolic
lymph
metastases
Stages
(UICC.
1997)
Stage 0
TIS,
NO,
MO
Stage I
rtz.
NO,
MO
Dukes A
Stage II
U4,
NO,
MO
Dukes B
MO
III
anyT,
Nl-2,
Stage IV
anyT,
any N,
Stage
Residual
tumor
classification
Dukes C
Ml after surgery
(R-classification) RO
no evidence
for residual
R1
microscopic
evidence
R2
macroscopic
evidence
tumor
for residual for residual
tumor tumor
569
75 The Castrointestinal
Tract
section multislice scanning. Even multislice CT is not able to differentiate between TI and T2 tumors (Fig.15 .15a). Tumor extension beyond the colon wall (T3) produces stranding (due to tumor and desmoplastic reaction), an indistinct boundary, and nodular protrusions in the pericolic fat (Figs.15.15b, 15.16). Absence of intervening fat planes, broad-based contact, and indistinct boundaries are suggestive of tumor extension to adjacent organs (T4). CT yields best results for staging of tumors that occur on the mesenteric or retroperitoneal wall of the colon. At the antimesenteric side of the colon, only a
a
b
Fig. 15.15 Colon carcinoma, (a) Small T2 rectal tumor (arrow) that is almost impossible to detect in standard CT despite rectal contrast administration. The small, round, contrast-enhancing lymph node (arrow) contained tumor at histology, (b) T3 rectal carcinoma
a
b
Fig. 15.16 -Recurrent colon carcinoma after right hemicolectomy. (a~ T3 tumor (arrow) withl hiqlu grade stenosis that could not.be.passed by an endoscope. (b) Virtual endoscopic view demonstrating the exophytic
very narrow rim of fat (1-2 mm) separates the colon from the peritoneal surface. Therefore, there is very little difference in infiltration depth from a T2 to a T4 tumor located at this side, and an ill-defined outer margin of such a tumor is already suggestive ofT 4 disease. There is a predictable pattern of lymph node drainage from the various sites of the colon. The mesocolic and the inferior mesenteric arterial nodes drain the left-sided colon, rectum and anus, while the right-sided colon and the cecum metastasize to the peripancreatic lymph nodes, simulating primary pancreatic
c with initial infiltration of the perirectal tissue (arrow heads) and round, contrast-enhancing pathologic lymph nodes (arrows), (c) Perforated colonic carcinoma with local abscess formation (arrow).
c nature or tne. lesion, (C)I Tfssue transition projection I resembling eft double-contrast barium enema. The tumor shows: a typicall apple' core configuration, ,andJthere 'is 31 second stenosing tumor at the ileo-colic anastomosis.
Colorectal Carcinoma 571 Table 15.9
Risk of malignancy in colonic polyps (Ferrucci 200_1)
_
Poly size
Likelihood
likelihood adenoma
If adenoma, likelihood cancer i1 IOyrs.
Overall likelihood
<Smm
10 mm) may occur.
of the
Small Bowel Obstruction (SBO)
Fig. 15.27 Whipple's disease, characterized by enlarged, hypoattenuating lymph nodes in the region of the mesenteric root and stranding in the mesenteric and retroperitoneal fat. The bowel loops show no dilatation or wall thickening.
Early complications include necrosis, ulceration, bleeding, and perforation with abscess'and fistula formation. Late complications may manifest clinically only after up to 15 years following radiotherapy, but permanent radiologic changes become apparent within one or two years. Women with gynecologic tumors and patients with bladder tumors are predisposed. The ileum is predominantly affected, and there are accompanying changes in the colon and rectum. Complications include bowel obstruction and fistula formation. Clinical hallmarks are cramping abdominal pain with persistent diarrhea and intestinal bleeding. CT is used to exclude extramural causes and in tumor patients for exclusion of recurrent disease. •
CT Morphology
In the early phase, the affected bowel segments show homogeneous wall thickening with consecutive luminal narrowing and irregular outer contours. The circumferentially thickened hypoattenuating submucosal layer may produce a target pattern in the bowel wall. Extramural changes consist of fibrotic strands in the perirectal or mesenteric fat along with thickened fascial planes and peritoneal adhesions. In the late phase, fibrosis lead to single or multiple minimally enhancing narrowed bowel segments with varying degrees of proximal dilatation. Fistulae, perforation, and abscess formation are rather rare complications. Onlyoccasionally, a shoulder formation at the strictures
Intestinal obstruction is a frequent problem affecting both, in-patients as well as out -patients and may account for up to 20% of hospital admissions. About 80% of obstructions are of small bowel origin. Amechanicalsmall bowel obstruction (SEO) or ileus is indicated by hyperactive high pitched bowel sounds. Prolonged obstruction, however, reduces peristalsis and may lead to atonic bowel and absence of bowel sounds. A history of previous surgery (5 days to many years earlier) favors the diagnosis of adhesions as the cause of obstruction. Other etiologies for a mechanical small bowel obstruction include inflammatory disease (Crohn's disease, post irradiation), tumors (metastases, carcinoid, primary bowel tumors), intussusception, or internal hernias (Table 15.12). The presence of continuous abdominal pain, signs of peritonitis, and a heart rate over 1l0beats/min are suggestive for strangulation of bowel segments. Strangulation implies interference with the blood supply associated with an obstruction that has not necessarily to be complete. There is a growing importance of CT for the diagnosis of SEO for a variety of reasons: 1) it does not rely on peristaltic propulsion and can
be used in cases of complete obstruction or paralytic ileus, 2) it provides global assessment of the abdomen, 3) it does not require oral administration of contrast media, and 4) it provides ready assessment of strangulation. CT has become the diagnostic method of choice to demonstrate or exclude strangulation, which would require immediate surgical intervention. When CT fails to provide a definite diagnosis, enteroclysis may be performed for a more detailed analysis. In patients with low grade or intermittent obstruction, (CT) enteroclysis remains the primary diagnostic modality. Mechanical Bowel Obstruction
A mechanical bowel obstruction is classified as acute, subacute, chronic, or chronic recurring, depending on the time course of the obstructive process. It is further classified by severity, as partial or complete obstruction, and by the anatomic level, as high or low small bowel obstruction or as colonic obstruction. CT is only indicated in patients with mechanical obstruction. •
CT Morphology
In mechanical bowel obstruction, dilated bowel loops with gas- fluid levels are seen proximal to the obstruction. The site of obstruction is characterized by an abrupt caliber change. CT usually provides more detailed information on the cause and location ofthe obstruction. Ifperforation occurs, extraluminal air is seen in the abdominal cavity and ascites is sometimes observed. As opposed to strangulated obstruction with interference of the blood supply (see below), a simple obstruction usually shows no mesenteric changes such as vascular engorgement, there is none, or only minimal haziness and none, or only minimal ascites. Localized peritoneal fluid accumulations should not be misinterpreted as mesenteric haziness. A non-obstructive paralytic ileus is characterized by uniformly dilated loops of small and large bowel with gas-fluid levels and no demonstrable lesion causing the obstruction (Fig. 15.28). The key finding of a mechanical obstruction is the so-called transition zone of obstruction. A transitions zone has three components. There is a proximal dilated segment, that abruptly terminates with a straight or slightly curved demarca-
Fig. 15.28 Paralytic ileus in a patient with retroperitoneal rupture of an abdominal aortic aneurysm after stent graft placement. Note the dilated fluid-filled small bowel loops with an air-fluid interface (arrowheads) but normal mesenteric vessels and lack of bowel wall thickening.
tion. The distal bowel segment has a normal caliber, is collapsed, or of reduced diameter. Between these two components, there is a short, sharply marginated segment that has a compressed lumen (Fig.15.29a). Depending on the underlying disease (adhesions, postinflammatory stricture, tumors), there is a normal wall, a thickened wall, or a mass in or close to the narrowed segment. The "small bowel feces sign" is characterized by gas-inclusions in small bowel loops that resemble colonic contents. The sign has. a sensitivity of around 80% but should be used with caution if it is the only sign present. Obstruction by an extraluminal tumor (Figs. 15.29b), or by a broad adhesive band, produces a less abrupt beak-shaped transition zone. Metastases may cause a more high grade obstruction because of associated desmoplasia, while primary tumors are characterized by lower grade of obstruction without desmoplastic change. The diagnosis of metastases as opposed to a primary tumor is further supported by evidence of additional metastases elsewhere in the abdomen. Carcinoid tumors cause intraluminal filling defects and typically rather obvious desmoplastic changes in the mesenteric fat that retract and displace bowel loops. Adhesions can be produced by extrinsic inflammatory or postoperative changes or are due to intrinsic inflammatory processes (e.g., Crohn's disease). The latter can be differentiated by the wall thickness itself.
584
75 The Gastrointestinal Tract
Fig. 15.29 Mechanical obstruction, (a) Simple obstruction with dilatation of bowel loops but regular wall enhancement. Note the transition point from enlarged
to normal-sized bowel loops (arrow), (b) Obstructing cecal carcinoma with proximal bowel dilatation.
Strangulation/Ischemic Bowel Disease
Interference with the blood supply may occur either from twisting of the bowel on the mesentery or from distention of a closed loop. Obstruction and strangulation are not necessarily combined, but the more severe the obstruction, the higher is the incidence of strangulation. The longer the involved bowel segment, the more severe are the clinical complications, ranging from perforation and peritonitis to shock and death. Pathophysiologically there are different stages of strangulation that can also be differentiated in CT. Initially, the venous return is compromised but arterial infl ux into the closed loop
Strangulation implies an interference of the blood supply associated with an obstruction of the bowel lumen that is not necessarily complete. CT was found to have a high sensitivity (above 90%), but rather low specificity (about 45%) for demonstrating intestinal ischemia. Strangulation occurs as a complication of intussusception, torsion, volvulus or any type of closed loop obstruction. Mesenteric vascular occlusion is also included in the strangulation category.
Table 15.13
Differentiation
of simple and strangulated
CT features
bowel obstruction
Strangulation
(modified from Ha et al., 2000) Simple Obstruction
continues, leading to increasing intravascular pressure and vessel engorgement. Hemorrhage into the bowel lumen and the bowel wall occurs. Eventually perfusion ceases in the bowel wall as result of arterial spasm due to increasing pressure from interstitial hemorrhage, anoxia, edema and capillary stasis. •
CT Morphology
CT scans demonstrate a moderately dilated, fluid filled bowel segment that has a radial or Y-shaped configuration (Table 15.13). Converging and engorged edematous vascular bundles represent congested mesenteric vessels (Fig. 15.30a). Venous strangulation of the bowel wall is suggested when CT shows circumferential thickening of the bowel wall with increased attenuation, hazy mesenteric vessels, and increased enhancement after intravenous contrast administration (target sign). Arterial strangulation may be assumed when there is delayed, reduced, or no contrast opacification of the bowel wall within the closed loop. The latter is frequently associated with the presence of intramural or extraluminal air. The poorly perfused strangulated segments show a stronger delayed enhancement in the interstitial phase 10-15 minutes after completion of spiral CT. The strangulated segments may thus appear hyperor hypo attenuating relative to the adjacent bowel loops, depending on the scan phase. Volume rendering of the superior mesenteric
a
b
Fig. 15.30 Volvulus, (a) A whorled pattern is created by the bowel loops twisted around the fixed mesentery. The torsion has led to strangulation (absence of bowel enhancement), mesenteric edema (arrow), and small amounts of free fluids around the affected bowel segment. (b, c) In another patient, the cecum had rotated
vascular course can be helpful to diagnose the spiral course of twisted branches of the SMA or SMV (Figs.l5.30b, C). Coexisting vascular engorgement can be appreciated as thickened vessels distal to the twisting point. A serrated beak sign due to thickened mucosal folds is suggestive of strangulated obstruction as opposed to a smooth beak seen in patients with simple obstruction. Volvulus, Closed Bowel Loop
A volvulus is a malrotation or axial twisting of portions of the small or large bowel associated with a long mesentery. It usually occurs in the elderly patient. The commonest form is a colonic volvulus, which can lead to bowel obstruction. A closed loop obstruction refers to entrapment of a bowel loop by a contracting adhesive band. A similar situation is seen in patients with an incarcerated internal hernia. All three situations may lead to symptoms of strangulation, dependent on the interference of the vascular supply. •
CT Morphology
The colonic volvulus can affect the cecum or the sigmoid. CT demonstrates a whirled pattern caused by the torsion of the bowel segments (afferent and efferent loops) around the fixed and twisted mesentery. In patients with a rotated cecum, the distended cecum is usually located
c around the terminal ileum and is displaced upwardly, resulting in a typical whirl signs (b). The volume rendered display of the vessels (c ) demonstrates venous stenosis (wide arrow) and engorgement of the right colic veins (ReV).
15 The Castrointestinal Tract in the left upper quadrant. A twisted sigmoid leads to greatly distended paralyzed loops affecting both the small and large bowel. The whirl sign, convergence of mesenteric vessels toward the twisted site or a reversed position of the mesenteric artery and vein, are CT signs indicative of a volvulus or a closed bowel loop. However, the whirl sign is also seen in asymptomatic subjects (approximately 7% of asymptomatic patients), especially at the anatomic level just below the inferior pole of the right kidney. This sign can also be seen in patients after previous abdominal surgical (e.g., gastrectomy). Therefore it is necessary to combine the presence of the whirl sign with a suggestive abdominal symptoms or other morphological findings such as dilated bowel loops. In closed loop obstruction demonstrates a markedly dilated segment of (usually small) bowel with two transition zones. Vascular engorgement and other signs of strangulation may be present (Fig.15 .30a). Intussusception
Intussusception is defined as invagination or prolapse of an intestinal segment (intussusceptum) into the lumen of the adjacent intestine (intussuscipiens). It is much more common in pediatric patients (94%) than in adults (6%). Intussusception is the commonest abdominal emergency of early childhood (75% in patients 50%). A right-sided duodenal hernia (25% of duodenal hernias) is associated with an absent ligament of Treitz. Bowel loops lie lateral and inferior the descending portion of the duodenum. The superior mesenteric artery and branches run along the anterior wall of the right-sided paracluodenal hernia. In a more frequent left-sided paracluodenal hernia the jejunum passes within a sac-like structure through the foramen of Landzert behind the inferior mesenteric vein, and extends into the nonfused mesentery of the descending colon. Small bowel loops and occasionally terminal ileum, cecum, ascending colon, transverse colon, gall bladder, and omentum may protrude
CT is by far more sensitive than plain film radiography for the detection of a small pneumoperitoneum and unsuspected gastroduodenal perforation. Perforations can be the result of an injury (seat belt, steering wheel, or iatrogenic) or an intrinsic lesion (postbulbar ulcer, Crohn'sdisease. diverticulitis). CT Morphology
Perforation of the stomach or duodenal bulb cause extra luminal gas in the peritoneal cavity while perforation of the duodenal loop may lead to air bubbles in the porta hepatis (Fig.l5.33a) or the retroperitoneum e.g., anterior pararenal space. Small pockets of extra luminal air may be trapped beneath the mesenteric leaves or the greater omentum and may be better detected with lung window settings. Larger collections of air are usually localized anterior to the liver in the upper abdomen if the patient is examined in supine position. Perforation of the jejunum, ileum, or colon may cause substantial amounts ofinfra-abdominal free air. Perforations of the ascending, descending, and sigmoid colon as well as the rectum may cause retroperitoneal air that may even extend into the mediastinum. Contained perforations are especially frequent in Crohn's disease or tumors. They present only small air bubbles close to the site of perforation. Usually there are inflammatory changes that seal off part of the peritoneal cavity (Fig.l5.33b, see also Figs. 15.22 c, 16.11 b). Perforations may incite inflammatory reactions in adjacent organs (pancreas, colon, liver).
Mesenteric Ischemia and Infarction
jtig. 15.33 Free abdominal air. (a) D~odenal perforation with free air in the porta hepatis (arrow) and a small fluid collection ("). (b) Bowel ischemia due to strangulation with pneumatosis and a perforation (arrow) leading to a local abscess formation (arrowheads) with free air.
(c) Free air (arrows) in the right infrahepatic space in this elderly patient was not associated with perforation at surgery. Note the streak artifacts caused by placing the arms next to the body (4 x 1/6, coronal MPR).
Fluid extravasation (containing hydrochloric acid and digestive enzymes) from gastroduodenal perforations into the peritoneal cavity gives rise to peritonitis and abscess formation. Postinflammatory strictures may be responsible for bowel obstruction. Free abdominal air is most commonly caused by perforation (Table "15.14) but in rare cases, no perforation can be found at surgery (Fig.15.33c) .
caused by arterial occlusion, non-occlusive ischemia, or venous thrombosis. Embolization of the SMA with emboli originating from the left heart account for more than 50% of cases. Thrombosis of pre-existing atherosclerotic lesions accounts for another 25% of cases with acute mesenteric ischemia. Nonocc1usive mesenteric ischemia accounts for 20-30% of cases with acute mesenteric ischemia, and is related to low mesenteric flow states such as systemic hypotension, cardiac failure, sepsis, or systemic administration of alpha-adrenergic drugs. Chronic intestinal ischemia or intestinal angina is usually caused by atherosclerotic stenosis and less commonly by arterial thromboembolism or compression. Venous thrombosis of the mesenteric vessels may result from extrinsic compression (by a tumor) or be related to risk factors such as thrombophilia, portal hypertension, visceral infections, previous abdominal surgery, or abdominal trauma. The clinical consequence of mesenteric ischemia varies with the sites of involvement. Infarction ofthe small bowel has a mortality of more than 80% and successful treatment requires immediate surgical intervention. By contrast, ischemic colitis is usually self-limiting and responds to supportive care and administration of fluids. One of the major protectors against ischemia of the gut is the presence of extensive collaterals. The small bowel has alternative pathways of blood supply provided by the primary,
Mesenteric Ischemia and Infarction
Mesenteric ischemia can be classified into acute and chronic forms. Acute ischemia can be Table 15.14
Reasons for free abdominal air
Free air in the retroperitoneum Perforation:
rectum cecum
Free air in the peritoneum Perforation:
stomach
duodenum small bowel colon
Free air in the peritoneum Perforation:
sigmoid
and retroperitoneum
colon
589
15 The Gastrointestinal
Tract
secondary, and tertiary arcades with multiple connections between the superior and inferior mesenteric artery and the celiac trunk. In the colon, the marginal artery runs parallel to the mesenteric border. The end arteries-called vasa recta-arise from this vessel. Thesplenicflexureis one of the commonest sites for ischemic colitis because no primary or secondary arcades serve as back-up in this segment. In addition, the vasa recta in the right ascending colon are fewer in number and more prone to vasospasm. Acute mesenteric ischemia is associated with high morbidity and mortality. It is a diagnostic problem because of its nonspecific clinical symptoms and its high mortality if not diagnosed and treated properly and quickly. Arterial angiography remains the gold standard for patients in whom acute mesenteric infarction is suspected clinically. CT angiography, especially with multislice technique, is excellently suited to demonstrate emboli in the SMA and its first segmental branches, and to detect manifest signs of bowel ischemia. •
CT Morphology
The majority of emboli lodge approximately 3-10 cm from the ostium of the SMA,just distal to the origin of the middle colic artery. Approximately 15% of emboli lodge at the origin of the SMA. The arterial embolus may be identified as contrast filling defect or abrupt cutoff of vascular opacification (Fig. 15.34a). Frequently the arterial occlusion is followed by secondary venous thrombosis clue to stasis. In these cases, there is a
a
b
Fig. 15.34 Acute mesenteric ischemia, (a) Direct demonstration of an embolus in the superior mesenteric artery (4 x 1/6). (b) Mesenteric venous thrombosis, (c)
lack ofopacification of the mesenteric veins after injection of contrast media (at> 70 seconds p.i.). CT reveals moderate « 15mm) bowel wall thickening. Submucosal hemorrhage may lead to asymmetric high attenuation areas. Involved bowel segments may be dilated and are often completely atonic with air- fluid levels (paralytic ileus). Scans may also show intramural air inclusions and air collections in the portal and mesenteric veins indicating bowel gangrene (Figs.15.35a, b: see also Fig.15.33b). While in the early evolution of ischemic injury there is intense mucosal enhancement after administration of intravenous contrast (due to slower transit of blood through capillaries), in later phases hypoperfusion of affected bowel segments may be seen (Fig. 15.35c). Gas in intrahepatic portal venous branches accumulates ventrally and extends much closer to the liver capsule than gas in the biliary tree (Fig. 15.35d). In cases of mesenteric venous thrombosis, a prominent intraluminal filling defect in the mesenteric vein can be identified (Fig.15.34b). Care has to be taken not to misinterpret inhomogeneous contrast filling of the mesenteric vein as a thrombotic filling defect if the scan is performed too early. Spiral CT scans acquired during the late arterial or early portal phase after intravenous contrast administration may cause pseudothrombus artifacts that mimic mesenteric vein thrombosis because of differences in transit time for various mesenteric branches and the splenic artery (Fig. 15.34c).
c Pseudothrombus due to laminar flow artifacts contrast enhanced blood from a non-affected segment in Crohn's disease.
of less bowel
Postoperative
Changes
Fig. 15.35 Acute mesenteric infarction. Mural necrosis is evidenced by distended bowel loops and by gas in the bowel wall (a) and mesenteric veins (b), and by lack of
enhancement of the affected segments (c). Air in intrahepatic portal branches extents more close to the liver capsule than aerobilia (d).
There are alternative interventional nonsurgical therapeutic options for treatment of acute arterial or venous occlusion. Preconditions for non-surgical treatment, however, are the lack of bowel wall necrosis (complete lack of enhancement, pneumatosis) and absence of signs of peritonitis (thickening of peritoneal leaves, ascites). Some institutions, therefore, request an exploratory laparotomy for visual inspection of the bowel to exclude manifest intestinal infarction.
tected incidentally at CT and lack specific clinical manifestations. Therefore, the finding of pneumatosis should always be interpreted within the clinical context.
Pneumatosis cystoides intestinalis
This condition involves the appearance of gas bubbles in the subserosa and mucosa of the small and large bowel (mainly the distal colonic segments) in young adults. Primary pneumatosis (15 %) is distinguished from the commoner secondary form (85%) (Table 15.15). Most cases of pneumatosis cystoides intestinalis are de-
•
CT Morphology
CT demonstrates multiple, thin-walled, noncommunicating, air-filled cysts of varying size distributed along the subserosal or submucosal layer of the bowel wall that may mimic pneumatosis due to bowel infarction. The perforation of a cyst can lead to an asymptomatic pneumoperitoneum that may persist for a prolonged period of time. Postoperative Changes
There is a large variety of surgical procedures involving the small bowel and colon. Apart from classical tumor resections, segmental resections, ileocolonostomy, palliative bypass operations in
591
15 The Gastrointestinal Table 15.15 Differential testinal pneumatosis
tumor patients, enteroplasties to increase the lumen of stenotic segments can be performed, and various techniques are available for permanent or temporary percutaneous stomata. CT is indicated only to rule out complications. •
CT Morphology
Anastomotic leaks manifest as extraluminal fluid collections, postoperatively persisting or increasing intraperitoneal air, oral contrast extravasation (bari urn preparations are contraindicated in such patients), or abscess formation. Postoperative fluid collections are quite common but require further work-up (aspiration biopsy, drainage) only if there are concomitant signs of infection. Hematoma or hemorrhagic ascites may be present (see Fig. 16.9). At the site of an enterostomy, a hyperattenuating suture line may be found. Postoperative 1uminal narrowing at the anastomotic site may have an inflammatory or neoplastic cause
Fig. 15.36 Postoperative dilatation of multiple small bowel loops in a patient with Crohn's disease and multiple corrective surgical interventions for intestinal stenoses (4 x 1/6, coronal MPR).
(anastomotic recurrence), which are indistinguishable by CT if there is only a moderate mass effect. Stenosis of an anastomosis is only considered clinically significantifthere is dilatation of the pre-stenotic bowel loop. Multislice CT provides superior delineation of complex bowel changes after surgical interventions (Fig. 15.36).
Trauma Blunt or penetrating abdominal trauma can involve the small or large bowel and can lead to hemorrhage or perforation. Injury from blunt trauma to the gastrointestinal tract is uncommon, and occurs most frequently in rear-seat passengers in motor vehicle accidents. It can lead to contusion. intramural hematoma. or perforation of the bowel. Contusion or hematoma formation are usually treated conservatively. although the latter may cause obstruction when large. Perforation from laceration or transection requires urgent surgery. Associated vascular lesions can lead to ischemia or infarction with risk of perforation or stricture formation. Peritoneal lavage has been substituted for by sequential abdominal ultrasound but CT is becoming a first-line technique in poly traumatized patients (see also Chapter 25. p. 943) for demonstrating direct and indirect signs of bowel injury. Injuries to the stomach usually occur in children and are most frequently located on the ventral wall. In the duodenum the second and third segments are most frequently injured and there may be associated injuries to surrounding organs (liver. pancreas and spleen). Small bowel injuries involve both mobile parts and parts where the bowel is fixed. Colon injuries are very rare. The transverse part is most often involved. but diagnosis is difficult in a collapsed bowel. When suspicion is high. the patient should be rescanned following the administration of 7501000 ml rectal contrast medium. Mesenteric injuries usually occur in combination with injuries to the bowel. Fresh hematomas should prompt a thorough search for associated bowel injuries. Lacerations of major vessels may lead to life-threatening bleeding. CT requires adequate contrast in the bowel for optimal results. Usually only 500-750 ml is administered via a nasogastric tube directly before the scan and consequently only the stomach. duodenum and part of the jejunum are opacified when there is adequate peristalsis (often trauma will lead to reflex paralysis and gastric dilatation). Nevertheless. CT sensitivity has been reported to be 88-93%. with better results obtained with spiral and multislice CT.
CT Morphology
Penetrating bowel injury with perforation
almost always causes free intraperitoneal. sometimes retroperitoneal air. A wide window setting (lung window) should be used in order not to miss small air bubbles. (Hemorrhagic) ascites may bepresent. Active hemorrhage can be demonstrated by scanning preferable in the arterial phase of enhancement. Hemorrhage may involve the peritoneal cavity but may also be present in the mesentery. If hemorrhage is suspected no positive oral contrast should be administered in order not to mask the findings. Blunt trauma is more difficult to diagnose. CT signs of bowel injury are usually subtle. and diagnosis requires a careful search of all bowel segments in cine mode. Contusion or hematomas will show focal bowel wall thickening with mesenteric infiltration. Free fluid is nonspecific but highly sensitive. Interloop fluid presenting as triangular fluid accumulation between bowel loops can be a sign of bowel perforation. A near-water attenuation suggests s perforation. while attenuation> 30 HU suggests hemorrhage. A "misty mesentery" with increased density of the fat. engorgement of vessels. bowel edema. and may be seen. Hemorrhagic areas within the bowel wall are rare. Sbocl-: bowel is characterized by marked wall thickening an increased contrast enhancement that is thought to be due to reperfusion injury after initial vasoconstriction. In shock. the spleen may become small and hypoattenuating due to internal blood redistribution. and vasoconstriction may be detected in mesenteric arteries. A "caved in" (flattened) cava is caused by hypovolemia. Peritoneal or retroperitoneal air. pneumatosis. loculated fluid between bowel loops. and active contrast extravasation are signs that often prompt direct laparotomy.
16 Peritoneal Cavity and Retroperitoneum M. Jorgensen,M.
CT has become the diagnostic method of choice for the evaluation of the peritoneal cavity and the retroperitoneum, except for pediatric patients and young, slim patients, in whom ultrasonography is preferable. CT is superior for patients who are hard to examine by ultrasound due to obesity or superimposing bowel gas. Ultrasound is superior in patients in whom there Table 16.1
617
(p. 623)
is little abdominal fat that serves as a natural negative contrast agent in CT The reproducibility of sectional planes makes CT particularly useful for follow-up examinations. CT is also used to guide diagnostic needle aspirations and biopsies and to direct interventional procedures such as percutaneous drainage of abscesses (Table 16.1).
Indications for CT examination of the peritoneal cavity and retroperitoneum
Tumor diagnosis
Primary evaluation of peritoneal or retroperitoneal masses Nand M staging of other tumors Oncologic follow-up
Suspected abscess
Search for focus
Hematomalhemorrhage
Detection, localization, active bleeding?
Preoperative,
Priorto open abscess drainage or drain insertion
interventional
Diagnostic needle aspiration or biopsy
Indeterminate masses cr fluid collections
Other
Lesions indeterminate by ultrasound Trauma
Anatomy The abdominal cavity is traditionally divided into the peritoneal cavity and the retroperitoneum. The peritoneum, mesenteries, and various fascia and ligaments form the boundaries of the peritoneal and retroperitoneal spaces. A thorough knowledge of the various spaces is helpful for understanding the pathways of spread of infectious and neoplastic disease. It is particularity helpful for successful percutaneous drainage of abdominal fluid collections.
Peritoneal Cavity The peritoneal cavity refers to the portion of the abdominal cavity that is lined by the peritoneum. It consists of the space between the two layers of serosa and the enclosed organs. A ligament is formed by two folds of peritoneum that support a structure within the peritoneal cavity. It may contain lymph nodes, vascular structures, and ducts. It is usually named according to the two structures it connects.
Peritoneal Cavity Table 16.2
Compartments
of the peritoneal
Upper abdomen = supramesocolic Subphrenic
cavity
Separated
space
--H .1
compartment into right and left halves by the falciform
Right subhepatic space
Between
liver and right kidney
Left suphepatic space
Between
liver and stomach
Omental bursa (lesser sac)
Between pancreas hilum
Lower abdomen = inframesocolic
compartment
Right paracol ic gutter
Right of cecum and ascending
Left paracolic
Left of descending
gutter
and stomach, extends to caudate
lobe and splenic
colon
colon
Supra mesenteric space
Above
Inframesenteric
Below root of mesentery
space
ligament
root of mesentery
Pelvic cavity Vesic-outerine
Between
bladder and uterus
Cul-de-sac (in females)
pouch (in females)
Between
uterus and rectum
Rectovesical pouch (in males)
Between
bladder and rectum
An amentum is a large reflection of the peritoneum containing also fat, lymph nodes, and vascular structures. Both the greater and the lesser omentum are connected to the stomach. Ligaments form parts of the omentum. A mesentery connects a portion of bowel to the retroperitoneum. It contains vascular and lymphatic structures and is covered on each side by a peritoneal lining. The space between the parietal and visceral peritoneum is subdivided into various compartments by the organs enclosed by peritoneum and by the corresponding ligaments (Table 16.2). These compartments are not separate but communicate broadly with one another anterior to the transverse colon. This area of communication is continued into the pelvis (Figs. 16.1,16.2).
space, the perisplenic space, and the left subhepatic space. The left triangular hepatic ligament that runs between the superior aspect of the left hepatic lobe and the diaphragm does not compartmentalize the left subphrenic space. The immediate subphrenic space is located between the diaphragm and the gastric fundus.
Right ~ subphrenic Subhepatic
Supramesocolic Compartment
colic
The transverse mesocolon separates the supramesocolic from the inframesocolic compartment. The supramesocolic space consists of the right and left subphrenic spaces, right and left subhepatic spaces, the perisplenic space, and the lesser sac (Fig. 16.1). The falciform ligament separates the right and left subphrenic spaces.
Supr-amesenteric
•
Left Subphrenic
Left subphr-enic
\
\
Hepatoduodenal ligament
mesenteric
Mesenteric root
Space
The left -sided subphrenic space is large and can be divided into the immediate subphrenic
Fig. 16.1
Compartments
of the peritoneal
cavity.
597
16 Peritoneal Cavity and Retroperitoneum
Epiploic foramen
omentum Transverse mesocolon
Transverse colon
Omental ---bursa (lesser sac) "" Greater ----~/"
~H-"'?\--\7"'---'~-\----
Lo 0 P of small bowe - Sigmoid colon
omentum Vesicouterine pouch
Rectum Cul-de-sac
__
Fig. 16.2 Sagittal ity and pelvis.
---f
section through the abdominal
cav-
It is crossed by the gastrophrenic ligament and represents a common location for fluid, abscesses, hemoperitoneum, and gastric processes. The perisplenic space is separated from the left paracolic gutter by the phrenicocolic ligament that runs between the proximal descending colon and the diaphragm. This ligament inhi bits a free flow of ascites between these spaces. The perisplenic space is a common location for splenic processes and processes involving the tail of the pancreas. The left subhepatic space (= gastrohepatic recess) is located between the left hepatic lobe and the stomach and may be involved with processes of the gallbladder, duodenal bulb, left hepatic lobe and lesser curvature of the stomach. •
Right Subphrenic Space and Right Subhepatic Space
The right triangular ligament runs posterolaterally from the dome of the right liver lobe to the diaphragm. It divides the right subphrenic and the right subhepatic space.
The latter is further divided into anterior and posterior portions. The anterior right subhepatic space communicates with the lesser sac via the epiploic foramen (foramen of Winslow) and is involved with processes of the gallbladder, pancreas, stomach, and liver. The posterior right subhepatic space is also described as Morisons pouch or the hepatorenal fossa; it is the most dependent part of the intraperitoneal space of a patient in a supine position and therefore a frequent location of fluid or peritoneal metastases. •
Lesser Sac
The lesser sac [omental bursa} is the largest recess of the peritoneal cavity (Figs. 16.1-16.3). It communicates with the greater peritoneal cavity through the epiploic foramen at the inferior border of the hepatoduodenal ligament, which contains the common bile duct anteriorly, the proper hepatic artery centrally, and the portal vein posteriorly. The lesser sac has three recesses: the superior, splenic, and inferior recess. The superior recess surrounds the medial aspect of the caudate lobe, the splenic recess extends across the midline to the splenic hilum, and the inferior recess extends inferiorly between the stomach, pancreas, and transverse mesocolon. Encapsulated fluid produces a cyst-like bulging ofthe lesser sac. An isolated fluid collection in the lesser sac is suggestive for a pancreatitis or a perforated gastric or duodenal ulcer. A disproportionate amount of fluid in the lesser sac may be evidence of malignant disease. The anterior border of the lesser sac is the lesser omentum, which consists of the hepatogastric ligament and-to a lesser degree-the hepatoduodenal ligament and the gastrocolic ligament. The hepatogastric ligament connects the liver with the lesser curvature of the stomach. It contains the left gastric artery, the coronary vein, lymph nodes, and may contain coronary varices in patients with portal hypertension. It is the primary pathway for the spread of a pancreatic phlegmon or for metastatic disease from the esophagus, stomach, or biliary tumors. The hepatoduodenal ligament forms the inferior part of the hepatogastric ligament and runs from the proximal duodenum to the porta hepatis. It contains the common hepatic duct, common bile duct, hepatic artery, and portal
Peritoneal Cavity Fig. 16.3 T opogra phy of the upper abdominal cavity at the level of the lesser sac (omental bursa).
Left subhepatic Perihepatic space
- Lesser sac space ~
vein, and is an important primary pathway for the spread of biliary neoplasms. The gastrocolic ligament connects the greater curvature of the stomach with the superior aspect of the transverse colon. Gastric neoplasms may spread via this structure and infiltrate the transverse colon or the splenic flexure. It represents the superior aspect of the greater omentum and contains the gastroepiploic vessels. Posteriorly and posterolaterally the lesser sac is bordered by the gastrosplenic and splenorenal ligaments. The gastrosplenic ligament runs between the greater curvature of the stomach and the splenic hilum and contains the short gastric vessels. It may be involved by gastric or pancreatic tail processes. The splenorenal ligament connects the posterior aspect of the spleen and the anterior pararenal space and may be involved with pancreatic processes. The mesocolon borders the lesser sac posteroinferiorly. Inframesocolic
Compartment
Mesocolon and mesentery are identified in CTby their fatty consistency, their anatomic location, and by the vascular structures they contain. The transverse mesocolon suspends the transverse colon from the retroperitoneum lying
anteriorly and inferiorly of the pancreas in the upper abdomen. It contains the middle colic vessels. Processes in the pancreas spread via the mesocolon to the inferior border of the transverse colon and vice versa. Most of the omentum is formed by a peritoneal duplication. The greater omentum is suspended from the transverse colon and hangs over the lower abdominal viscera like an apron (Fig. 16.2). The gastrocolic ligament forms the upper portion of the greater omentum that can be located anterior to the small bowel on axial CT sections. It is formed by four layers of peritoneum and therefore predisposed for becoming involved in neoplastic as well as inflammatory processes. Confluent omental masses are referred to as omental cake. The lesser omentum represents a peritoneal duplication between the lesser curvature of the stomach, the superior border of the duodenum and the porta hepatis. The root of the small bowel mesentery suspends the small bowel from the retroperitoneum and extends from the left upper quadrant (ligament ofTreitz) to the right lower quadrant passing in front of the horizontal part of the duodenum. It contains the superior mesenteric vessels and more than 100 lymph nodes which are normally smaller than 1 cm in size. The lymph nodes can be divided into three groups: adjacent to the bowel wall, adjacent to the
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16 Peritoneal Cavity and Retroperitoneum upper trunk of the superior and inferior mesenteric vessels, and parallel to the major branches of the mesenteric vessels, The mesentery subdivides the inframesocolic compartment into a supramesenteric space and an inframesenteric space, Both spaces communicate with each other above the duodenojejunal flexure (Fig, 16.1), Laterally they extend to the ascending and descending colon, Superiorly they are bounded by the right and left halves of the transverse colon, Inferiorly they communicate with the bilateral paracolic gutters and the pelvic cavity, The superior and inferior ileocecal recesses are located above and below the terminal ileum, The cecum normally lies completely retroperitoneal; however, a retrocecal space may be created when there is an additional peritoneal reflection dorsal to the cecum. The right and left paracolic gutters are located lateral to the ascending and descending colon, The sigmoid mesocolon suspends the sigmoid from the posterior wall of the pelvis and contains the sigmoid vessels, It is the target area of diverticulitis, perforated colon carcinoma, or Crohn's disease, The intersigmoid recess is located in the left lower quadrant along the undersurface of the sigmoid mesocolon,
Pelvic Cavity Lateral to the urinary bladder, the peritoneum is deepened on each side to form the perivesical fossae, The deepest part of the peritoneal cavity lies posterior to the bladder. In females, the uterus subdivides this deep recess into the vesico-uterine pouch anteriorly and the recto-uterine pouch (culde-sac orpouc/i o[Douglas) posteriorly (Fig, 16,2), This cul-de-sac is bounded anteriorly by the broad ligaments and the uterus, and is the most dependent portion of the pelvis in women, In males both peritoneal recesses are combined to form a single space between the rectum and bladder called the recto-vesical pouch. Non-encapsulated fluid collections in the supine patient tend to collect in these deep spaces, where they are detectable by imaging procedures, Larger pelvic fluid collections can extend up the paracolic gutters into the subhepatic and subphrenic spaces (Fig, 16.1),
The broad ligaments envelop the parametria, fallopian tubes, ovaries, uterine vessels and ureters, and run from the uterus to the pelvic sidewall. They are involved in neoplastic as well as inflammatory disease, The round ligaments serve as the anterior suspensory ligaments of the uterus and contain lymphatic channels, They pass lateral to the deep epigastric vessels and continue into the ingumal canal, finally inserting on the major labia, The median umbilical ligament lies in the midline and forms a peritoneal reflection over the obliterated urachus, The lateral umbilical ligaments consists of two parts: the medial part runs along the obliterated umbilical arteries to the falciform ligament The lateral part forms a peritoneal reflection over the deep inferior epigastric vessels,
Retroperitoneum The retroperitoneum is bounded superiorly by the diaphragm, inferiorly by the linea terminalis of the lesser pelvis, anteriorly by the posterior layer of the peritoneum, and posteriorly by the muscles of the back and by the bony pelvis, It is continuous inferiorly with the subperitoneal space of the pelvis, and anteriorly with the preperitoneal space, It has connections with the intra-abdominal organs through the mesenteric root and the sites of attachment of the pancreas, liver, duodenum, ascending colon, and descending colon (Fig, 16.4), The layers of Cerota's fascia (anterior and posterior renal fasciae) subdivide the retroperitoneum into three compartments at the level of the kidneys (Figs, 16.4, 16,5): the perirenal space, formed by the anterior and posterior layers of Cerota's fascia; the anterior para renal space between the posterior layer of the peritoneum and the anterior layer of Cerota's fascia; and the posterior para renal space between the posterior layer of Cerota's fascia and the transverse fascia, Viewed in sagittal section, the perirenal space is sharply tapered inferiorly, The anterior and posterior para renal spaces communicate with the anterior pre peritoneal space approximately at the level of the iliac crest A connection also exists between the right and left perirenal spaces, The retroperitoneal fasciae have a normal thickness of 1 -2 mm, CT can define the fascial
Retroperitoneum Peritoneum
~
Lateroconal fascia
Perirenal space Posterior pararenal space Fig. 16.5 Retroperitoneum (fascial spaces) at the level of the kidneys.
space Perirenal space
Fig. 16.4 Parasagittal section through the peritoneal cavity and retroperitoneum.
Examination
Technique
Patient Preparation
Sufficient oral contrast medium should be administered to improve the delineation of pathologic processes. This is especially important when looking for abscesses. In postoperative patients some bowel loops may not fill after oral contrast administration (e.g., roux-en- Y loops). Here oral administration of a biliary contrast agent 12 hours prior to the CT examination leads to opacification of bile secretions and consequently of the excluded bowel loop if there is a biliodigestive anastomosis (see also CT cholangiography, Chapter 12). Acquisition
planes in approximately 50% of patients. The fasciae can be seen if they are perpendicular to the CT scan plane and if the retroperitoneum contains a sufficient amount of fat. Thin-section imaging improves fascial delineation. Fluid collections and sites of fascial thickening improve the delineation of the fascial planes.
Parameters
CT scanning of the peritoneal cavity and retroperitoneum is performed in the supine patient as part of an upper and lower abdominal survey that includes all the abdominal compartments
and the pelvis (Table 16.3). Longitudinal coverage should extend from the diaphragm to the ischial tuberosities. Standard protocols use 5-7 mm section thickness but the evaluation of more complex intra- and retroperitoneal disease is improved with thinner sections. Multislice CT allows for creating excellent coronal or sagittal reformations that are particularly helpful for demonstrating the involvement of the various retroperitoneal spaces and for display of the mesenteric and retroperitoneal vasculature. However, most indications do not require such elaborated scanning techniques. Best results are obtained with 0.75-1.5 mm high-resolution protocols on 4- to 16-slice scanners. On 4- or 8-slice scanners, thicker collimation (e.g., 4-8 x 2.5 mm) is useful in more obese patients to decrease image noise. Similar results, however, can be obtained with thin section collimation if 5-7 mm wide sections are reconstructed in the axial plane and 3-5 mm wide sections are reformatted in the coronal plane.
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602
16 Peritoneal Cavity ond Retroperitoneum Table 16.3
Suggested
protocol
for CT scanning
of the peritoneal
cavity and retroperitoneum
General Preparation
Tumor diagnosis:
Oral contrast
1000-1500 ml contrast medium, given over 60-90 min prior to examination if necessary administered by gastric intubation. Negative contrast agents (water) may be advantagenous for multislice CT
Patient
position
Scan range
patient
Supine with elevated From diaphragm
Respiratory
phase
Windowing
should fast for 6 hours
arms
to ischial tuberosities
Inspiration Noncontrast CT: Contrast-enhanced
CT:
W/L=300/40 W/L=400/60
Scan parameters
1-slice SC/TF/RI
4-slice' SC
16-slice' SC
Standard
7/12/6
2-4 !
1-2!
1-1.25!
0.75-1.25
Volumetric Contrast
imaging injection
!
5/8/4 ! 1-slice
Standard
CT
5-7.5/5 !
16-slice V+N/F/DXc
100 + 50/2/70
120/2/70 + 70
120 + 50/4/25
3/4 cor 4/4 sag
5/4
4-slice V+N/F/D
V/F/D
MPRb SWIRl
axial SWIRl
100 + 50/2/70 + 70
120 + 50/4j20A
Biphasic CT
150/4/25
Interstitial (delayed) phase
5min
5min
5min
CT urography
2:15 min
2:15 min
2:15 min
d
d
+ 80
d
SC = slice collimation (mm), TF = table feed (mmjrotation), RI = recon. increment (mm), T! = scanning direction, SW = effective section width (mm), MPR = multiplanar reformats, axial = axial sections, cor = coronal, sag = sagittal CM = contrast material, Contrast concentration = 300 mg/ml iodine V = volume of CM (ml), N = volume of saline flush (ml), F = flow rate (mljs), 0 = start delay, X = trigger region a Pitch P=TFj(NxSC) should be chosen close to 1.5 with 4-slice scanners, 1.3-1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SW/RI = 1-1.5/0.7 c read: lOA = start delay 10 s after reaching 50 HU enhancement in the desc. aorta dread: biphasic scan (arterial + portal phase) after 25 s (lOA) for the 1" scan, 70-80 s for the 20d scan
Contrast Injection Many peritoneal and retroperitoneal abnormalities originate from abdominal organs, and intravenous contrast administration should be geared toward the primary lesion. In patients with neoplastic disorders, the IV contrast administration should be tailored to allow optimum evaluation of the liver (hepatic metastases). I!' hemorrhage is suspected, the initial scans should be obtained without intravenous contrast. Scans in the interstitial
phase of contrast enhancement are helpful in fibrous processes such as retroperitoneal fibrosis, while late scans some 15 min after contrast application are required to detect leaks of the urinary tract. With multislice CT, late scans (thin-sections, reduced dose) can provide excellent visualization of the ureter (CT urography). As an alternative, 20 ml of contrast material may be given up to 20 minutes before the examination to provide opacification of the collecting system during arterial or portal phase scanning. An overview is given in Table 16.4.
+ suggested, (+) may yield additional information,
(+)
+
- not recommended
• Preopacifying the pyelocaliceal system with 20 ml of contrast medium 5-20 min before scan replaces excretory phase scanning (not recommended in suspected urolithiasis)
Peritoneal Fluid Collections without
Solid Components
The detection of peritoneal fluid is not a primary indication for CT scanning. CT is the modality of choice, however, for the differential diagnosis of abdominal fluid collections and especially for the exclusion of abscesses and hemorrhage. Table 16.5 provides the most important differential diagnoses for intra-abdominal fluid collections.
with epithelium and a layer of smooth muscles. They occur anywhere in the gastrointestinal tract, but are most common in the esophagus (10-15%) and the small bowel (see also Chapter 15, p. 541). •
IT Morphology
Mesenteric cysts are located between
Mesenteric,
Enteric,
and Omental
Cysts
Mesenteric cysts are asymptomatic lymphatic hamartomas that may contain variable fluid components and are usually detected incidentally. Approximately 80% of the cysts occur in the mesentery of the small bowel. Alimentary tract (enteric) duplication cysts result from developmental errors and are lined
the leaves of the mesentery, mostly of the small bowel (Fig. 16.6). CT demonstrates a single cyst or multilocular cysts that may reach several centimeters in size. The cyst may exhibit water or softtissue CT attenuation, depending on its contents. A fluid-fluid level may be present if the cyst contains fat. Peripheral calcifications may be found. Omental cysts are located in the greater omentum or lesser sac. They are usually pedunculated. Twisting of the pedicle can only be detected in rare cases. Bowel obstruction is marked by dilatation of the proximal bowel segments. Enteric duplication cysts are usually unilocular and have a well-defined enhancing wall, which contains a muscular and mucosal layer. They contain serous or occasionally hemorrhagic fluid. Gas inclusions indicate a communication with the bowel. Most often they are located at the mesenteric surface of the ileum, more rarely at the ileocecal junction, the duodenum or at the greater curvature of the stomach. CT shows elongated tubular or spheri-
II
604
16 Peritoneal Cavity and Retroperitoneum Mesenteric Pseudocyst
Mesenteric pseudo cysts often contain blood and evolve from prior hematomas or abscesses. •
CT Morphology
Pseudocysts can be uni- or multilocular. Differentiation from pancreatic pseudocysts is based on the lack of other signs of inflammation in the retroperitoneum, and thus may be difficult. The wall is usually thicker than in simple cysts and has a stronger contrast enhancement. Fig. 16.6 Mesenteric cyst with wall calcification small bowel mesentery.
in the
cal cystic masses that are intram ural in location or run parallel to the normal bowel lumen. The differential diagnosis includes abscess and old hematoma (mesenteric pseudocyst, see below). Lymphangioma
Lymphangiomas are congenital disorders. They represent the most common cystic tumors of the mesentery. •
CT Morphology
Lymphangiomas may be multilocular with very thin cystic walls that merge with the bowel wall. Imperceptible cystic walls lead to an illdefined opacification of the mesenteric fat that obscures the cystic nature of the tumor (Fig. 16.7a). The fluid within a lymphangioma is variable and may be serous, chylous, or hemorrhagic. The presence of streaky low densities within a multi-cystic lesion are very suggestive for the diagnosis (Fig.l6.7b). A fat-fluid level within cystic spaces is seen only rarely.
Ascites Numerous diseases, including cardiovascular, inflammatory, and neoplastic entities, may be associated with the accumulation of ascitic fluid in the peritoneal cavity. The collection may be transudative or exudative in nature. Special forms are hemorrhagic ascites (secondary to tuberculosis or peritoneal carcinomatosis) and chylous ascites. •
CT Morphology
Ascites often presents as a hypoattenuating rim that surrounds intraperitoneal organs. Fluid volumes of approximately 50 ml or more can be detected on smooth organ surfaces such as the liver. Adequate bowel opacification helps to detect small amounts of fluid in the infracolic spaces. The distribution of ascites is determined by the anatomy of the intraperitoneal compartments, by gravity, and by the position of the patient. In supine patients, nonencapsulated fluid quickly gravitates to the more dependent spaces (the cul-de-sac and posterior subhepatic space). Larger fluid collections usually do not enter the lesser sac. Large amounts of fluid can
Fig. 16.7 Lymphangioma. Tiny cysts are imperceptible on CT but present as an ill-defined opacification of the mesenteric fat (a). Large cystic components are very suggestive of the diagnosis (b).
a
b
Bile Leak
a
c
b
Fig. 16.8 Ascites. (a) A large perihepatic fluid collection has caused medial displacement of the liver. The subphrenic space is subdivided by the falciform ligament. There is a small perisplenic fluid collection and pleural effusions. Atelectasis of the lung bases (arrows) should not be mistaken for the diaphragm (arrow heads), (b) Ascitic fluid in the subhepatic space, the
cause separation of small bowel loops and the fatty layers of the mesentery (Fig. 16.8). Adhesions (postoperative, inflammatory, neoplastic) give rise to cystic fluid collections that may be difficult to distinguish from abscesses. Fibrin septa usually cannot be directly detected by CT (as opposed to ultrasound with which they are excellently displayed). Indirect signs of septations incl ude scalloping of the liver contour. The CT attenuation of ascites ranges from 0-30 H, depending on the protein content of the fluid. Higher attenuation values are characteristic of ascites due to tuberculosis, ovarian or appendiceal tumors, and intraperitoneal hemorrhage. In c/ironfc ascites there may be slight thickening of the parietal peritoneum. Enlarged peritoneal veins should not be mistaken for peritoneal metastatic deposits. Intraperitoneal fluid with solid components, thickening of the peritoneum, and a large amount of fluid in the lesser sac should raise suspicion of peri ron eo I carcinomatosis. Hemorrhage
Infra-abdominal hemorrhage most commonly results from sharp or blunt abdominal trauma (organ or vascular injury), bowel perforation, tumor rupture, ectopic pregnancy, or anticoagulant medication. In hemodynamically stable patients, CT is the modality of choice for detecting or excluding an intra-abdominal hematoma.
supra- and inframesenteric space, and in the right and left paracolic gutters, causing marked displacement of the bowel loops and mesentery. Note the retroperitoneal location of the duodenum (arrows) as well as the ascending and descending colon (arrow heads). (c) Ascites in the supine patient guickly gravitates into the cul-de-sac between the rectum and uterus.
•
CT Morphology
Fresh hemorrhagic collections show soft-tissue attenuation values because there is not yet a separation between high-attenuation erythrocytes and serum. Fresh hemorrhage may be better detected on contrast -enhanced images as a hypo attenuating rim around the liver or spleen. Clots have high CT attenuation, but scans often show only streak-like or rounded, ill-defined areas of increased attenuation within the collection. The CT attenuation of the intraperitoneal hematoma fades within days, reaching a value of 0-20 HU by 1 to 2 weeks. Layering effects (ventrodorsal attenuation gradient, fluidfluid level) are most commonly seen in hemorrhagic ascites, especially in the lesser pelvis (Fig. 16.9). Bile Leak
Intraperitoneal bile leakage may be caused by iatrogenic manipulations, trauma, or spontaneous rupture of the bile ducts. The leak incites a local inflammatory response that usually leads to rapid encapsulation of the intraperitoneal fluid. CT is indicated prior to surgical or interventional procedures and during follow-up. •
CT Morphology
CT generally shows a localized fluid collection (biloma) in the region of the porta hepatis or at
605
606
76 Peritoneal Cavity and Retroperitoneum Fig. 16.9 Hematoma. (a) Fresh hematoma anterior to the left liver lobe, which is hyperattenuating to muscle but isoattenuating to the liver and thus almost imperceptible on the non-contrast scan. (b) On the post-contrast scan, the hematoma is clearly demarcated from the liver parenchyma.
a
b
the hepatic margin. It usually appears encapsulated and has a CT attenuation of about 10-20 HU (Fig. 16.10). Loculation due to fibrin strands is rarely seen on CT. Fibrin strands are much better detected by ultrasound. Peritonitis
Peritonitis usually results from an infection of the abdominal cavity due to peritoneal injury (e.g., surgery) or bacterial seepage through the peritoneum. It may involve the entire peritoneum (diffuse peritonitis) or may be a localized process (local peritonitis). Fibrinous,
purulent exudates cause adhesions to form between the peritoneal layers, leading to localized abscess formation. Adynamic (paralytic) ileus may develop through reflex mechanisms. Clinically overt peritonitis is a serious disorder and an absolute indication for surgical treatment. It does not require further radiological assessment. But if clinical presentation and ultrasound findings are equivocal, CT examination may be required. S
CT Morphology
Acute peritonitis is characterized by ascites with dilatation of mesenteric vascular structures and a hazy increase in attenuation of the mesenteric fat. The peritoneal layers are smoothly thickened (Fig.l6.11a) and enhance after intravenous contrast administration. Chronic forms may be associated with focal peritoneal thickening and calcifications (Fig.l6.11b). Abscess
Fig. 16.10 Biloma. CTfollowing a left-sided partial hepatic resection. This demonstrates a fluid collection at the resection margin and a second, large parahepatic fluid collection on the right side. The latter has lead to substantial liver compression with transient hepatic attenuation difference of the compressed parenchyma (arrow heads).
An isolated inflammatory process secondary to a hollow viscus perforation or surgery may incite a local, circumscribed peritonitis that becomes encapsulated and forms an abscess. The adjacent abdominal compartments may be involved. An encapsulated abscess may also persist after diffuse peritonitis. CT is used in patients with unexplained fever to exclude an abscess or to localize an abscess prior to intervention.
-Abscess
607
b Fig. 16.11 Peritonitis, (a) A postoperative colon perforation led to fecal peritonitis with large amounts of free fluid. The pneumoperitoneum (broad arrows) indicates perforation. The extraluminal gas within the fluid collection (arrows) indicates a high viscosity of the fluid, in this case due to fecal material. Note that there is only
a thickening and enhancement of the peritoneum on the right side (arrow heads) where the perforation was found at surgery, (b) A confined perforation in this patient incited a local peritoneal inflammatory reaction with small amounts offree fluid and extraluminal gas.
•
subphrenic space. Despite the direct communication between Morison's pouch and the lesser sac, a generalized peritonitis usually does not affect the lesser sac because the slit -like connection is often sealed off by adhesions. Inflammatory processes within the lesser sac are highly suggestive for being primarily caused by adjacent organs such as a perforated ulcer from the stomach or duodenum. Left-sided subphrenic abscesses mostly result from complications after gastric, colonic, or splenic surgery (Fig. 16.12). Rarely they may result from a perforated ulcer in the stomach or duodenum.
CT Morphology
An abscess typically appears as a localized fluid collection of "1O-40HU attenuation. Thickened, enhancing peritoneum may be seen as a manifestation of the peritoneal inflammatory response. Inflammatory spread to adjacent structures is manifested by ill-defined boundary surfaces and the obliteration of fat planes. Extral uminal gas is pathognomonic, but postoperative air collections may be detectable for up to 14 days. Secondary signs include scoliosis, ileus, or changes at the lung bases. Developing abscesses may resemble encapsulated ascitic fluid. Thus, any isolated fluid collection is suspicious for an abscess when corresponding clinical signs are present. It requires fine needle aspiration for differentiation. Sites of predilection for abscess formation are the right subphrenic space, the Morison pouch (subhepatic space), the cul-de-sac (see Fig. 16.23), and the rectovesical pouch. The frequency of subphrenic and subhepatic abscesses is 2-3 times greater on the right than on the left. The commonest site is the Morison pouch. Abscesses of Morison's pouch and the right subphrenic space often coexist. This is explained by the fact that negative subdiaphragmatic pressure causes flow from the pelvis up the right paracolic gutter. The right subphrenic space is affected only after Morison's pouch has been contaminated. The falciform ligament avoids direct passage from the right to the left
Fig. 16.12 Large peritoneal patic space.
abscess
in the left subhe-
16 Peritoneal Cavity and Retroperitoneum
As effect of gravity, fluid in the inframesocolic compartment invariably seeks the pelvic cavity (pouch of Douglas, lateral paravesical
fossa) and makes the pelvis the most frequent location of residual abscess formation following generalized peritonitis.
Peritoneal Fluid Collections with Solid Components Pseudo myxoma peritonei
Pseudomyxoma peritonei is a rare condition caused by the rupture of a cystic lesion with the release of mucinous, gelatinous material and mucus- forming epithelium into the free abdominal cavity. The source of the material may be a mucinous cystadenocarcinoma of the ovaries or the gastrointestinal tract. A similar pattern is produced by an appendiceal mucinous adenoma (degenerative mucocele). Once the material has been seeded into the abdominal cavity, it continues to form mucus, leading to adhesions, bowel obstruction, and occasionally to abscess formation.
masses do not enhance with intravenous contrast, but slight enhancement is observed in the septations and solid components of the lesions (Fig. 16.13). The peritoneum itself may also be thickened. The differential diagnosis includes pancreatitis with pseudocysts, pyogenic peritonitis, and widespread echinococcal disease. Mesothelioma
CT demonstrates extensive, hypoattenuating intraperitoneal masses of varying size that progressively displace the bowel loops and mesentery posteriorly. The masses may cause a characteristic scalloping of the liver contour. The thin-walled cystic masses (approximately 15-30 HU) appear as bulky, septated areas of pseudo-ascites. Ring-like or curvilinear calcifications rarely develop in chronic forms and are highly suggestive of pseudomyxoma. The
Primary neoplasms of the peritoneum are very rare, the commonest being peritoneal mesothelioma. The peak incidence is between 55 and 65 years of age. Men are predominantly affected. The disease is associated with asbestos exposure. The peritoneum is the second commonest site of involvement (30%) after the pleura (60%). Benign forms are localized; malignant forms show early spread along the peritoneal surfaces. A cystic type of mesothelioma occurs in young to middle aged women with a prior history of abdominal surgery, pelvic inflammatory disease or endometriosis. This type has no association with asbestos exposure.
Fig. 16.13 Pseudomyxoma peritonei with extensive, hypoattenuating intraperitoneal masses impressing on the liver surface and displacing bowel loops and
mesentery. The solid components and septa show slight contrast enhancement, and peritoneal thickening is observed.
•
CT Morphology
Peritoneal Carcinomatosis and Metastases
deposition and growth of seeded neoplasms in the abdomen depends on the flow of ascites within the peritoneal spaces and is similar to the spread of peritoneal abscesses. Peritoneal Carcinomatosis may be detected as an incidental finding or during CT tumor staging. Because implants may be as small as 2 mm they pose significant problems for CT diagnosis. While CT clearly depicts diffuse nodular peritoneal thickening, it is rather inaccurate for the detection of small lesions especially in the pelvis. • Fig.16.14 Mesothelioma with nodular peritoneal thickening (arrows) on the bowel surface and parallel to the mesenteric vessels (arrow heads).
•
CT Morphology
In malignant" mesot-heh'oma, CTreveals foci ofirregular, nodular peritoneal thickening as well as circumscribed masses that occur predominantly in the anterior parietal peritoneum. There is frequently thickening of the mesentery, omentum, and bowel walls (Fig. 16.14). Focal calcifications may be seen. The masses and sites of nodular peritoneal thickening are more clearly demarcated after IV contrast administration. The disproportionately small amount of clear ascites is suggestive for the diagnosis. A singular nodular mass of the peritoneum is highly suggestive of benign mesothelioma or mesorhe !iaffibroma. In patients with cystic mesothelioma CT shows multiple tiny cysts with thin fibrous septa that are thought to represent inclusion cysts secondary to entrapment of mesothelial cells after laparotomy. Differentiation from a lymphangioma may not be possible based on morphological criteria. Peritoneal Carcinomatosis and Metastases
Secondary peritoneal tumors are far commoner than primary peritoneal neoplasms and most commonly originate from carcinomas of the stomach, colon, ovaries, gallbladder, pancreas, and uterus. Initial tumor extension is a local subperitoneal growth, followed by spread to the peritoneum and intraperitoneal seeding. The
CT Morphology
Peritoneal neoplasms are generally associated with pronounced sero-fibrinous or hemorrhagic ascites with solid components. Peritoneal thickening may be generalized without major nodular components (Fig.16.15a). Small nodular densities are best appreciated on smooth organ surfaces such as the liver or at posteriorly located peritoneal surfaces. They are best detected with thin-section techniques such as multislice CT. Even if solid components are not found, the presence of ascites in tumor patients should always be taken as a potential sign of peritoneal Carcinomatosis. A large fluid collection in the lesser sac reinforces the suspicion of carcinomatosis in a patient with no apparent gastric or pancreatic abnormalities. Sites of predilection for peritoneal Carcinomatosis are the pouch of Douglas (50%), the ileocecal junction (40%), the superior aspect of the mesosigmoid (20%), and the right paracolic gutter (20%). Along the ruffled small bowel mesentery there are a number of peritoneal recesses that run obliquely towards the right lower quadrant. Malignant cells in the ascites will follow these recesses and will finally arrive in the lower recess of the small bowel mesentery in the right infracolic space. Metastases typically displace the distal ileal loops and infiltrate the medial contour of the cecum. In the left lower quadrant the superior border of the sigma is affected in a similarly way. Coalescence of multiple metastatic deposits is typically associated with a dense fibrotic (desmoplastic) reaction. Metastatic implants in the right subphrenic space along the liver surface and along the falciform ligament are most characteristic for ovarian cancer (see Fig. 16.13). They may be more the result of lymphatic drainage to the right-
7 6 Peritoneal Cavity and Retroperitoneum
a Fig. 16.15 Peritoneal carcinomatosis. (a) Broad, plaque-like thickening of the greater omentum (omental cake), which shows marked contrast enhancement.
(b) Peritoneal seeding of cholangiocellular carcinoma with nodules at the liver surface, on the bowel surface and in the mesentery (4 x 1 /6, coronal MPR).
sided diaphragm rather than the result of direct intraperitoneal spread. More rarely metastatic implants can occur in the right paracolic gutter and the Morison pouch. Occasionally peritoneal metastases from mucinous ovarian carcinoma may calcify. Gastric or more rarely colon mucinous adenocarcinomas tend to seed bilateral metastases [Kmkenberg tumors) to the ovaries, which appear as cystic lesions or masses of soft-tissue density, which are usually associated with massive ascites. Metastatic spread into the umbilical region is described as Sister Mary Joseph nodule. This may be caused by carcinomas of the stomach, ovary, colon, or pancreas.
Invasion of the mesenteric fat is shown by a reticular or nodular pattern of increased density. Fluid deposition causes an apparent thickening of the mesenteric vessels. A stellate pattern is produced by stretching and increased density of the mesenteric root. Affected portions of the omentum have a nodular or streak-like increase in attenuation of the fatty tissue, resulting in sheet-like or plaque-like masses called omental cake (Fig.l6.15b). CT following IV contrast administration shows enhancement of the peritoneum and of solid components. Paired vascular structures in the mesenteric fat (arteries and veins) are more easily distinguished from lymph nodes on contrast -enhanced scans.
Solid Processes of Peritoneum and Mesenterium The "Misty" Mesentery
Infiltration of the mesenteric fat by cells, fluid (edema, lymph, blood), tumor or fibrosis has been termed the "misty" mesentery. CT Morphology
The CT attenuation, which is normally around -100 HU, increases to values of -10 HLJ to -60 HU. As a consequence, vascular structures within the opacified mesenteric fat may be obscured and can only be delineated after application of contrast medium. The visceral peri-
toneum-normally not visible-may become recognizable due to fluid deposition. A diffuse mesenteric edema has multiple etiologies (Table 16.6). It extends from the surface of the bowel to the root of the mesenteric vessels. Due to edema, the bowel wall becomes illdefined. If associated with a systemic process the mesenteric edema is mostly combined with a generalized subcutaneous edema or with ascites (Fig. 16.16). Mesenteric edema due to arterial or venous mesenteric thrombosis tends to remain localized (see Fig. 15.34).
Retractile Mesenteritis
Table 16.6 mesentery" Diffuse
Differential
diagnosis
of
the
"misty
meseirlfferic edtenria
As cites Portal hypertension Severe cardiac congestion
Inflammatory processes such as pancreatitis, appendicitis, diverticulitis, or inflammatory bowel disease result in mesenteric inflammation that is more or less locally defined. In abdominal tuberculosis, infiltration of the mesentery is usually patchy. There is an increased number of enlarged lymph nodes that show a strong peripheral enhancement with low-density centers. Concomitant findings in tuberculous infection include high-density ascites, bowel wall thickening, and peritoneal thickening and enhancement. Fresh blood in the mesentery shows typical high CT attenuation (50-70 HU). It may originate from the mesenteric vessels, the bowel wall, the retroperitoneum, or the pelvis. In trauma-related bowel lacerations the alterations in the adjacent mesenteric fat may be more conspicuous than the injury itself. Lymphedema of the mesenteric fat occurs after obstruction of the lymphatics due to congenital anomalies, inflammation, neoplasms,
Fig. 16.16 Mesenteric edema and anasarca with increased (fluid-like) density of the mesenteric, retroperitoneal, and subcutaneous fatas well as ascites and pleural effusion (notshown) in a patientwith hypoalbuminemia. Also note the marked edema of the transverse colon with mucosal enhancement (accordion sign).
surgery, or radiation therapy. In patients with non- Hodgkin's lymphoma it was found that the infiltration of the mesenteric fat becomes more obvious and may persist over years, while the lymphadenopathy may decrease under radiation or chemotherapy. Metastatic spread of tumors of the pancreas, colon, breast, and of carcinoid, leukemia, ovarian cancer, or mesothelioma may develop a similar appearance. Retractile Mesenteritis
Retractile mesenteritis (also called mesenteric lipodystrophy, chronic fibrosing mesenteritis, sclerosing mesenteritis, mesenteric panniculitis) is a very rare disease of unknown etiology characterized by fibrolipomatous thickening of the small bowel mesentery, chronic inflammation with fat necrosis and calcifications, and finally mesenteric fibrosis. The peak incidence is in the sixth decade, and men are predominantly affected. The disease has a nonspecific clinical presentation and is usually discovered incidentally. •
CT Morphology
CT shows a mesenteric mass {mesenteric panniculitis; with a CT attenuation slightly above that of fat that is permeated by soft tissue (connective tissue) and may contain calcifications. The lesion may be sharply demarcated and then has a rim of higher density due to connective
612
16 Peritoneal Cavity and Retroperitoneum Fig. 16.17 Mesenteric panniculitis with increased attenuation of the mesenteric fat (arrows), which is clearly demarcated against normal retroperitoneal fat. The coronal MPR (4 x 1/6) demonstrates multiple small nodules (arrowheads) within the mesentery (b).
a
b
tissue (Fig. 16.17). Mesenteric vessels traverse the lesion in an undisturbed fashion and may be surrounded by a hypo attenuating halo of normal fat. Lesions are well delineated even on unenhanced scans. In more advanced cases, a softtissue dense fibrous mass may develop in the thickened mesentery (fibrosing mesenteritis) is recognized by its stellate configuration. Small bowel loops may be retracted or narrowed, but without signs of obstruction (proximal bowel dilatation). A single nodule or multiple nodular masses (fibromas) may be present in the mesentery. The findings are characteristic but have to differentiated from liposarcomas and carcinoid tumors. Lipoma, Liposarcoma
See Solid Retroperitoneal Processes (p. 617). Carcinoid Tumor
Up to 95% of carcinoid tumors are located in the gastrointestinal tract, mostly in the ileum. The tumors may be multiple, and either metastases or the primary tumor may be located in the mesentery. •
CT Morphology
Carcinoid tumors within the mesentery present as an ill-defined mass with a stellate radiating pattern (see Fig. 15.13). More than 70% show coarse and dense calcifications. Extension of the tumor to the small bowel causes focal wall thickening. There may be multiple nodular tumor metastases in the adjacent mesentery.
The tumor-whether lying within the small bowel or within the mesentery-is typically associated with an excessive peritumoral desmoplastic and fibrotic reaction leading to retraction of the mesentery, beading of the mesenteric vascular bundle, and displacement of adjacent bowel loops (see Fig. 15.13). Encasement of mesenteric vessels may lead to chronic ischemia. Liver metastases occur in 90% of patients with carcinoid syndrome and in >85% of patients with tumors larger than 2 cm. Desmoid Tumor
Desmoid (aggressilivefibrom a tosis) is a rare infiltrative tumor that arises from the aponeurosis or the fascia of muscle groups, or directly form the mesentery. Abdominal desmoid predominantly affects women of childbearing age, and its biologic behavior (non-metastasizing, high recurrence rate) places it in the category oflowgrade sarcomas despite its benign histology. CT is performed as a staging examination. Desmoids can be divided into those that involve the abdominal wall or mesentery, and those that affect other regions (extra-abdominal desmoids). They are associated with Gardner syndrome, multiple pregnancies, and prior trauma. •
CT Morphology
Scans demonstrate large masses measuring 5-20 cm in size, located in the mesentery or in the aponeurosis of the rectus abdominis muscle or internal oblique muscle (Fig.16.18a). The masses may be well-circumscribed at initial
Inflammations and Abscesses
a
c
b
Fig. 16.18 Desmoid tumor, (a) Large mass arising from the fascia of the rectus abdominis muscle. The tumor has invaded the subcutaneous fat anteriorly, and posteriorly it has expanded the muscle and penetrated the peritoneum. Desmoid tumor of the mesentery that is
hypoattenuating during the portal phase (b) after contrast injection but displays marked late enhancement in the interstitial phase (c) due to a large fibrous component.
presentation and even have a pseudocapsule. Recurrent tumors, however, often are less welldefined with respect to the fatty tissue and adjacent organs. Twisting and distortion of bowel loops indicates infiltration of the mesentery. Due to their fibrous origin lesions are slightly hyperattenuating to muscle tissue. They show little or no enhancement in the portal phase after IV contrast administration but en-
hance substantially during the interstitial phase due to their large fibrous component (Fig. -16.18b). Differentiation from lymphoma or other more malignant lesions is based on the presence of a single mass in the mesentery without retroperitoneal adenopathy. Late enhancement is characteristic.
Retroperitoneal Inflammations
Fluid Collections and Infection
and Abscesses
Local reactions or abscesses may develop in the retroperitoneum as a complication of retroperitoneal inflammatory disease (Crohn's disease, retrocecal appendicitis, pancreatitis, pyelonephritis, osteomyelitis, spondylodiscitis), trauma (pancreas, kidneys, vessels), extraperitoneal hollow viscus perforation (gastrointestinal tract), or the superinfection of hematomas and urinomas. Apart from tuberculosis (developing countries), the majority of psoas abscesses are of pyogenic origin (Staphylococcus aureus and mixed gram-negative organisms). Acute abscesses or inflammations lead to serious illness. Chronic forms present with nonspecific clinical signs and symptoms.
CT is indicated in patients with equivocal ultrasound findings, and it is used in the planning of open or percutaneous abscess drainage. H
CT Morphology
Inflammatory reactions in the retroperitoneum may be localized by encapsulation, may be confined to fascia-defined spaces, or may involve the entire retroperitoneum. Extensive processes may invade the fascial space of the psoas muscle and spread within the muscle compartment to the groin. Concomitant involvement of the iliopsoas muscle leads to generalized or focal expansion of the muscle that may be associated with intramuscular abscess formation (Fig. 16.19a). The density increase of an inflammatory collection in the retroperitoneum depends on the protein content and age of the process, resulting
7 6 Peritoneal Cavity and Retroperitoneum
a
b
c
Fig. 16.19 Retroperitoneal abscess, (a) Expansion of the psoas muscle with intramuscular abscess formation. Air inclusions and rim enhancement are pathognomonic. (b, c) Tuberculous abscesses arising from a spondylodiscitis LS/S 1 (c). Fluid collections are adjacent
to the psoas muscles. The collections (arrowheads) between the iliac vessels were located anterior to the sacrum (b). Small calcification are frequently seen in tuberculous abscess formations (arrow).
in attenuation values that usually range from 10-30 HLJ. The fascial planes are most often thickened and may show enhancement with intravenous contrast. Early stages of abscess formation may not yet show the typical rim enhancement seen in more mature abscesses. Exudative processes (e.g., phlegmons, pancreatitis) permeate and obscure the retroperitoneal fat, causing a general increase in CT density. Fat necrosis presents as regions of low attenuation within the fatty tissue and may not be distinguished from abscess formation. The fluid collections that may form in pancreatitis should not be misinterpreted as abscesses, even though these collections may be thick-walled due to granulation tissue. CT does not provide the means of making a definitive diagnosis of abscess in a patient with pancreatitis unless there are gas inclusions, and the patient had not received any type of prior aspiration biopsy. Aspiration biopsy is often the only way to confirm the suspected diagnosis. Tuberculosis used to be the commonest cause of abscess formation in the spine, paraspinal region, and the iliopsoas compartment.
Today there has been resurgence in tuberculous paraspinal infections in HIV -positive patients. Bone destruction, involvement of the disk space with spread along the anterior longitudinalligament, enlargement of the psoas muscle by fluid collections, and associated calcifications within the abscess are typical findings (Fig.16.19b). As opposed to pyogenic infections, tuberculous infections are characterized by a relatively thick and potentially calcified abscess rim, multiple abscess cavitations and only minimal new bone formation. CT-guided aspiration and drainage can be used for confirmation of diagnosis and for treatment. Hematoma
Retroperitoneal hematomas can result from vertebral or pelvic fractures, injuries of the pancreas or urogenital tract, or vascular injuries. Among these causes, trauma of the kidneys is most frequent (see Fig. 18.46). Spontaneous retroperitoneal or pelvic hemorrhage in adults requires exclusion of an aortic rupture (see Chapter
24, p. 878). It may also be due to neoplastic diseases, hemorrhagic diathesis, or anticoagulant medication, which is the most frequent cause of spontaneous psoas hematoma. It is important to look for causative lesions such as tumors (Table 16.7) in patients with spontaneous retroperitoneal bleeding under anticoagulationtherapy. Retroperitoneal hematomas may be localized (e.g., to the perirenal space) or may involve
a
the entire retroperitoneum, depending on the primary lesion and the extent of the collection. Clinical suspicion of retroperitoneal hematoma (e.g., trauma, vascular disease, or a falling hematocrit in anticoagulated patients) or indeterminate retroperitoneal fluid seen at ultrasound requires further evaluation by CT. CT will also influence treatment planning (percutaneous drainage vs. open surgery) and is useful for evaluating response to conservative therapy. Contrast -enhanced CT in the arterial or portal phase can be used to demonstrate active bleeding, which in many cases will mandate immediate therapy, such as surgery or interventional radiological treatment by embolization therapy.
b
Fig. 16.20 (a) Retroperitoneal hematoma with internallayering due to separation between blood serum and cellular blood components. A clotted portion is seen laterally (wide arrow). Note that there is contrast extravasation signifying an active hemorrhage. It arises posteriorly (arrowheads), seeps through and layers on
CT Morphology
Hematomas appear as masses of varying size whose CT attenuation depends largely on the age and size of the collection. Hemorrhage in acutely bleeding patients may be is 0 attenuating to soft tissue (see Fig. 7.12d) or to be more precise to the blood in the aorta. This is the case if fresh blood has not yet separated into its solid and fluid components. Very rapidly, however, sedimentation effects or clot formation occur. Sedimentation causes layering of blood components with a gradient towards the dependent portions of the hemorrhagic material (Fig.16.20a; see also Fig.7.12c). Fluid-fluid levels may occur. In general, sedimentation indi-
c top of the denser portions of the hemorrhage (arrow). There is marked displacement of bowel loops, (b. c) Old retroperitoneal hematoma with typical hyperattenuating center on the noncontrast scan (b) and rim enhancement due to granulation tissue on the postcontrast scan (c). There were no signs of infection.
16 Peritoneal Cavity and Retroperitoneum cates lack of coagulation within the blood collection. Clot formation leads to typical hyperattenuating regions within the hemorrhage (up to 70 HU), which often are ill-defined and rounded in appearance (see Fig. 7.12a). Clot formation indicates that simple drainage will not suffice for removing the hemorrhagic collection. Serous components in acute hemorrhage are hypoattenuating and coexist with hyperattenuating regions. With increasing duration of the hemorrhage serous components are absorbed, and the hemorrhagic area may appear homogeneously or inhomogeneously hyperattenuating. Contrast extravasation in to a hemorrhagic area indicates active bleeding (Fig.l6.20a). It can best be seen if the CT examination has been performed similar to a CTA. A second portal phase scan may be used to confirm increase in size of the actively bleeding focus. The site of extravasation may be used to guide surgical or interventional radiological treatment. Recurrent hemorrhage can be suspected if there is marked heterogeneity in the hemorrhagic area. Large hemorrhages tend to form separate collections due to fibrin strands. As the collection matures, its CT attenuation decreases to near-water density. Hematomas do not enhance after intravenous contrast administration. With increasing resorption of the hemorrhage there will be a contrast -enhancing rim (Fig.l6.20b) that should not be mistaken for superinfection. The only secure sign of superinfection is the presence of gas bubbles in patients who have not had any type of percutaneous puncture. A primary retroperitoneal hematoma may become an intraperitoneal hemorrhage if it ruptures the posterior parietal peritoneum. Hematomas of sufficient size can displace and compress adjacent organs. Unilateral hematomas of the iliopsoas muscle are initially manifested by expansion of the muscle, often without a significant increase in CT density. A chronic hematoma may be confused with an abscess or necrotic mass, and percutaneous needle aspiration may be necessary for definite diagnosis. Urinoma
Injuries of the renal collecting system and lower urinary tract (traumatic, iatrogenic) may allow urine to extravasate into the perirenal fat along
the ureter or collect at perivascular sites. Weeks or years may pass between the injury and the appearance of clinical symptoms. Infection of the urinoma leads to retroperitoneal abscess formation. CT is a very sensitive tool for detection of urinoma but requires late scans after the injection of contrast material for detection of persistent urinary leaks. •
CT Morphology
Any indeterminate postoperative fluid collection detected in a perirenal, periureteral, or perivascular location should be imaged on delayed scans (> 15 min) after contrast administration. Contrast extravasation into the fluid confirms the diagnosis of persistent urinary leak (Fig. 16.2-1). Chronic urinoma, however, may no longer be accompanied by a contrast extravasation and may require needle aspiration for diagnostic confirmation. Old, liquefied hematomas may be indistinguishable from chronic urinomas by CT morphology alone. CT attenuation around 30 HU on pre-contrast scans, and rim enhancement following IV contrast administration should raise suspicion of a superinfection. The absence of an enhancing rim does not exclude infection in the acute stage.
Fig. 16.21 Urinoma resulting from injury to the renal pelvis during the removal of a postoperative drain. Opacified fluid is visible around the kidney 30 minutes after contrast administration.
Lymphoceie
Fig. 16.22 Lymphoceles. (a) Typical bilateral Iymphoceles of water density following lymphadenectomy (metal clips), (b) Atypicallymphocele that appears asawell-encapsulated, nonhomogeneous, nonenhancing mass containing components between -20 HU and 35 HU. The mass is compressing the ureter.
a
b
Cysts
Lymphocele
Cysts in the retroperitoneum are rare. Congenital cysts may arise from the bowel or urogenital tract but are less common than acquired posttraumatic or postinflammatory cysts, which are usually in direct contact with the site of the primary lesion (e.g., the pancreas). Differentiation is required from old hematomas and urinomas and must rely on the clinical presentation in addition to imaging findings.
Chylous fluid collections most commonly occur after lymphadenectomy for prostatic or testicular cancer and occasionally for upper abdominal tumors. They are also seen in patients after kidney transplantation, where they may cause secondary renal obstruction due to compression effects on the ureter. Lymphoceles are usually an incidental CT finding during follow-up of tumor patients.
•
•
CT Morphology
Cysts appear as masses of varying size that have smooth margins, are typically well encapsulated, and are usually of water attenuation, Cysts do not enhance on postcontrast scans and do not show rim enhancement. Teratogenic cysts and dermoid cysts include various embryonic elements and may contain calcifications oreven teeth.
Solid Retroperitoneal Retroperitoneal
CT Morphology
Lymphoceles appear as round or oval masses of water density along abdominal lymphatic pathways (Fig. 16.22a). Heterogeneous areas of nearfat attenuation may also be present (Fig.l6.22b). The mass usually has a very thin wall that cannot be distinguished on CT images and does not enhance with contrast medium. Surgical clips are often found adjacent to the mass.
Processes
Fibrosis
Retroperitoneal fibrosis may occur in a primary form (Ormond's disease), which probably has an autoimmune pathogenesis and is responsive to corticosteroids, or in a secondary symptomatic form (Ormond's syndrome), which can result from inflammation, neoplasms, aortic aneurysms, radiotherapy, drug ingestion, or trauma. Both forms lead to fibrotic induration of
the retroperitoneum. The fibrotic process may involve blood vessels, the ureters, the kidneys, and retroperitoneal portions of the bowel and biliary tract. An acute inflammatory stage is distinguished from a chronic fibrotic stage. Retroperitoneal fibrosis is most prevalent in middle age and predominantly affects males. The malignant form of retroperitoneal fibrosis « 10%) is caused by infiltration of the retroperitoneum by malignant cells. This kind of
76
Peritoneal Cavity and Retroperitoneum
desmoplastic malignant reaction of the retroperitoneal fat is associated with breast, stomach, colon, and lung cancer, and Hodgkin's disease, lymphoma, carcinoid, and sarcoma. CT is used to investigate unexplained hydronephrosis, exclude retroperitoneal masses of other origin, and evaluate response to steroid therapy. However, the differentiation of inflammatory tissue and mature fibrosis is superior with MRI. •
CT Morphology
Retroperitoneal fibrosis presents as masses of soft-tissue density enveloping the aorta and vena cava that may be nodular or plaque-like. It is frequently accompanied by proximal ureteral dilatation. The margins of the mass may be sharply defined or indistinct (Fig. 16.23). Aside from medial deviation of the ureters, no displacement or invasion of retroperitoneal structures will be found in most patients. Retroperitoneal fibrosis usually covers the anterior part of the great vessels and extends to the lateral wall of the vena cava and aorta. In the primary form, the posterior wall of the aorta is typically preserved. The soft-tissue process tends to progress in a caudocranial direction along the midline. The process commonly begins in the area of the bifurcations of the aorta and inferior vena cava. In the commoner limited form of disease the fibrous mass is centered over the fourth or fifth lumbar vertebra. In the rarer extensive form, the fibrotic process may extend downward, spreading along the ureters and ileum into the lesser pelvis (Fig.16.23b), laterally beyond the outer
edge of the psoas muscles and upward through the diaphragm and into the mediastinum. Atypical manifestations of retroperitoneal fibrosis occur mainly in extensive disease. Lesions may appear as a huge mass and may mimic retroperitoneal lymphoma or other malignancies. The renal hilum may be involved. Encasement of retroperitoneal vascular structures can lead to stenosis and thrombosis. Differentiation of the malignant form of retroperitoneal fibrosis is often impossible based on morphological CT signs. An anterior displacement of the aorta due to extension around the posterior wall is only seen in malignant disease. The absence of multifocal, usually asymmetric lymph node enlargement distinguishes retroperitoneal fibrosis from lymphoma. Primary retroperitoneal tumors usually occur at eccentric sites and show a propensity for invasion and metastasis. The detection of cystic areas is more consistent with a malignant tumor. On non contrast CT images, the attenuation values are similar to those of muscle tissue and the mass is very difficult to distinguish from vessels. Therefore IV contrast administration is required. Scans in the late arterial or portal phase best demonstrate contrast uptake in acute inflammation while scans in the interstitial phase of contrast enhancement show contrast uptake in areas of fibrosis. In the secondary form of retroperitoneal fibrosis, it may be possible to demonstrate the causative lesion (tumor, aneurysm) after contrast injection. Caution is required for application of contrast media in patients with renal failure.
Fig. 16.23 Retroperitoneal fibrosis. (a) The soft tissue mass surrounding the aorta is relatively sparing of its posterior aspect. a sign that is more often seen in primary retroperitoneal fibrosis. The mass has streaky, iII-defined margins and has caused medial deviation of the ureters. Subtle contrast enhancement can be seen in the interstitial phase, indicating a chronic fibrotic stage, (b) The symmetrical, plaque-like presacral mass that has encased the veins, arteries, and ureters extends from the small pelvis to the thoracoabdomina Ijunction, indicating an extensive disease.
Pelvic fibrolipomatosis is a disease of unknown etiology in which increased fatty tissue deposition predominates over fibrous and inflammatory components. Men from 25 to 60 years of age are affected much more often than women. The clinical features are nonspecific, and the disease is usually discovered incidentally at CT. •
CT Morphology
The small fibrous component causes the pelvic mass to display predominantly fat density on CT scans. The mass causes displacement, elevation, and narrowing of pelvic soft-tissue structures (rectum, sigmoid colon, bladder, ureters) but no effective compression. There might be mild dilatation of the upper urinary tract, however. The rectum appears elongated, straightened and fusiformly narrowed. The ureters are displaced medially. The lipomatous mass may completely fill the pelvis and can expand the presacral space to more than 10 mm width (Fig. 16.24). Pelvic lipomatosis can be associated with cystitis glandularis or cystitis cystica. Primary Retroperitoneal Tumors
Retroperitoneal neoplasms may arise from any of the three germ layers (Table 16.8). The origins Table 16.8
Classification
of primary retroperitoneal
Mesenchymal (40-80%)
Fig. 16.24 Fibrolipomatosis in a patients with Crohn's disease causes anterior displacement. elongation, and straitening of the rectum (arrowhead). and anterior displacement of the bladder. The presacral space is markedly expanded and filled with a mass offat density with a small fibrous component. The ureters are displaced medially (arrows).
of such tumors are muscles, fasciae, loose connective and fatty tissue, blood and lymphatic vessels, nerve trunks and sheaths, heterotopic tissue and embryonic remnants. Mesenchymal tumors are the commonest, followed by neurogenic, dysontogenic, and epithelial tumors. At least 80% of all primary retroperitoneal neoplasms are malignant, and benign lesions have a strong propensity for malignant transformation.
Neuroblastomas and embryonic rhabdomyosarcomas are most prevalent in children, while neurogenic and teratogenic tumors are commonest in young adults, An increased incidence of neurogenic tumors has been documented in patients with von Hippel-Lindau disease, tuberous sclerosis, multiple endocrine neoplasia (MEN syndrome), and familial neurofibromatosis. Sarcomas have a peak incidence between 50 and 60 years of age. The CT features of these neoplasms depend on the size and origin of the tumor, and the clinical manifestations are largely nonspecific. N onfunctioning tumors are usually diagnosed at a late stage. Good tumor delineation in the absence of metastases does not exclude a malignant process, and therefore the diagnosis must be confirmed by biopsy and histological evaluation. Invasive growth, bone destruction, and distant metastases (lung, liver) are unmistakable signs of malignancy. CT is used to establish the identity of indeterminate retroperitoneal masses and to stage known tumors. The differentiation of retroperitoneal masses is based on the age of the patient, the location of the mass, laboratory findings, and CT morphology. Precontrast CT images are helpful for demonstration of fatty components and calcifications. Contrast media application is necessary for evaluation of the vascular architecture. MRI can be used in place ofC T owing to its good tissue discrimination. MRI is particularly useful in children and young adults. M ultislice CT offers improved tumor delineation by its multiplanar imaging capabilities. The commonest benign and malignant retroperitoneal tumors are described below.
Neuroblastoma Neuroblastomas have approximately the same incidence as Wilms' tumor (nephroblastoma), accounting for 7-10% of all pediatric malignancies. Fifty percent of children are affected before 1 year of age and 90% before age 7. As derivatives of primitive sympathetic ganglion cells, neuroblastomas develop in the region of the sympathetic trunk and adrenal medulla. The commonest site of occurrence is the retroperitoneum (75%), followed by the mediastinum ("15%) and the neck and pelvis (5% each). Neuro-
blastomas metastasize early to the lymph nodes, bone marrow, skeleton, liver, lung, brain, and skin. The tumors almost always secrete catecholamines, which can be detected in the urine.
cr
Morphology
CT reveals a large, nonhomogeneous mass in the adrenal bed or paravertebral area in the region of the sympathetic trunk. Up to 50% of the lesions contain areas of hemorrhage and necrosis, and 30-70% contain calcifications. Mass effect is apparent on adjacent organs. Unlike nephroblastoma, the tumor deforms the kidneys without causing renal destruction. Advanced disease is manifested by retroperitoneal lymphadenopathy and distant metastases to the liver and bones. Invasion of the spinal canal with erosion of the pedicles may be present. Involvement of major retroperitoneal vascular structures may occur (see Fig. 17.8).
Rhabdomyosarcoma Rhabdomyosarcoma is the commonest softtissue sarcoma in children, accounting for approximately 5% of all pediatric malignancies and showing a peak incidence between 2 and 6 years of age. The principal sites of occurrence are the head, neck, urogenital tract, trunk, and extremities. CT Morphology
Masses of muscle attenuation involving the urogenital tract or the muscles of the trunk should raise suspicion of rhabdomyosarcoma. The masses may appear nonhomogeneous due to intralesional hemorrhage and necrosis. The diffuse tumor vascularity causes the masses to enhance intensely after IV contrast administration. Retroperitoneal lymphadenopathy is usually present as an associated finding.
Teratoma Teratomas occur mainly in the gonads, but a few lesions (approximately 4%) may have an intracranial, mediastinal, or retroperitoneallocation. A distinction is drawn between mature and immature teratomas.
Primary Retroperitoneal Tumors •
CT Morphology
The diverse tissue components of teratomas may give them a nonhomogeneous CT appearance. The coexistence of fat, soft-tissue elements, bone, or teeth within a mass is considered pathognomonic. Malignant teratomas may invade the inferior vena cava and may remain relatively homogeneous, and thus have a nonspecific appearance on CT (Fig. 16.25). Lipoma
Lipoma is the commonest benign mesenchymal tumor, showing a peak incidence in the fifth and sixth decades. The tumor may have a superficial (subcutaneous) or deep site of occurrence (retroperitoneum, chest wall, hands, feet). Fewer than 10% oflipomas are multifocal. •
CT Morphology
Lipoma appears as a well-circumscribed, homogeneous mass of fat density (approximately -100 HU). Unlike liposarcoma, the mass does not contain soft-tissue components and does not enhance with intravenous contrast. Intramuscular lipoma very rarely shows locally invasive growth with separation of the surrounding muscle planes [invasive lipoma).
621
Liposarcoma
Liposarcoma is the second commonest malignant retroperitoneal soft -tissue tumor in adults. Like lipoma, it has a peak incidence in the fifth and sixth decades of life. Four histological types are recognized, the myxoid type (40- 50%) being the most common. The tumor presents as a painful mass in only about 10-15% of cases. Ten percent of the myxoid lesions show multicentric involvement. Low-grade liposarcomas have a high recurrence rate but rarely metastasize, while high-grade liposarcomas frequently metastasize to the liver, lung, and bone. •
CT Morphology
The mass may consist predominantly of fat or soft-tissue components, depending on its grade and histology. A lipomatous mass is very likely to be a liposarcoma if it contains streaky densities or solid areas of soft -tissue density in addition to areas of pure fat density (Fig. 16.26). Areas with negative attenuation values that exceed fat density (as compared to other region of abdominal fat) are also suspicious for liposarcoma. Well-differentiated tumors contain large fatty components and tend to have expansile rather than infiltrative margins, whereas poorly differentiated myxomatous, round-cell or pleomorphic liposarcomas have large soft-tissue components and are frequently invasive. Only a few liposarcomas contain no grossly detectable fatty components, and all but a very few show
b Fig. 16.25 (a) Benign teratoma in a 8-year-old girl. There is fat, soft tissue, and ossified portions (arrows) interspersed in this large mass. The small bowel is displaced to the left. (b) Malignant teratoma that presents
as a slightly heterogeneous left-sided pre- and paravertebral mass causing erosion of the anterior vertebral margin.
16 Peritoneal Cavity and Retroperitoneum
a Fig. 16.26 Liposarcoma. (a) CT shows a nonhomogeneous, lipomatous mass that contains streaky components of soft-tissue density in addition to areas of fat density. The mass is expansile and infiltrative. (b) This
liposarcoma appears as an encapsulated mass in the left lower quadrant containing a combination of fat and streaky soft-tissue components.
negative attenuation values. The nonfatty portions of liposarcomas may show contrast enhancement. Calcifications are found in up to "12% of the tumors. It may be difficult to differentiate between an extrarenal angiomyolipoma and a liposarcoma of the retroperitoneum, especially when the tumor is large. In the early stage liposarcomas are most often confined outside the perirenal fascia, while angiomyolipomas grow in the perirenal fat. A fat line between the mass and the kidney, compression of the renal surface and preservation of the renal cortex are suggestive for a liposarcoma, while discontinuity of the renal cortex and a spur sign are suggestive for angiomyolipoma.
•
CT Morphology
Soft -tissue involvement is characterized by masses of muscle density that contain hypodense necrotic areas and frequently measure at least 5 cm in diameter. The masses generally show intense contrast enhancement. The lesions may infiltrate the muscles of the back or abdominal wall, especially the psoas muscle (Fig. 16.27). Scans occasionally demonstrate osseous involvement with periosteal reaction and cortical erosion. Invasion of the inferior vena cava or renal vein usually is not found. This distinguishes malignant fibrous histiocytoma from advanced renal cell carcinoma.
Malignant Fibrous Histiocytoma
Malignant fibrous histiocytoma is the commonest primary malignant soft-tissue tumor of the extremities and retroperitoneum in adults. The average age at diagnosis is 50 years, but all age groups are affected. Primary soft -tissue involvement by the tumor is distinguished from skeletal involvement and from pulmonary involvement, which is very rare. Many patients clinically present with a painless mass that has been evident for several months. The tumors have a high recurrence rate after resection, and 50% have already metastasized (lung, liver, bone, lymph nodes) by the time of diagnosis.
Fig. 16.27 Malignant fibrous histiocytoma that has infiltrated the left psoas muscle (arrow heads) and has led to an anterior displacement of the kidney. The tumor is heterogeneous with hypervascular areas and areas of necrosis.
Secondary
Leiomyoma,
Leiomyosarcoma
These tumors arise from blood vessels, the spermatic cord, or embryonic rests and are seldom benign. They most frequently metastasize to the liver, lung, mediastinum, and soft tissues. •
CT Morphology
The tumors appear on CT as a nonhomogeneous, peripherally enhancing mass. Because of their rapid growth, 75% of the tumors contain areas of necrosis, cystic degeneration, and hemorrhage (Fig. 16.28). Calcifications are rare. Secondary Retroperitoneal Masses and Lymphadenopathy
Secondary retroperitoneal masses result from tumor invasion or tumor seeding into the retroperitoneum. The retroperitoneal lymph nodes may be involved by systemic lymphatic diseases (Hodgkin's disease, non-Hodgkin's lymphoma) or by tumors of the pancreas, kidneys, stomach, or lesser pelvis (testes, ovaries, uterus, bladder, and prostate). Inflammatory processes can also cause changes in the retroperitoneal lymph nodes. CT is the method of choice for the detection of retroperitoneal lymph nodes but its accuracy to detect abnormality in normal-sized lymph nodes is limited.
•
Retroperitoneal
Masses and Lymphadenopathy
CT Morphology
Tumor invasion into the retroperitoneum is seen in large tumors, especially of the kidneys, adrenals, pancreas, and retroperitoneal bowel structures. Usually the primary tumor site is well-delineated and differentiation from primary retroperitoneal tumors poses no problems. Normal para-aortic or paracaval lymph nodes are seen as small, round or oval structures in the fat adjacent to the great vessels. The diagnosis of lymphadenopathy at CT is mainly based on the size criterion, i.e., the minim um diameter on axial sections. Thresholds between benign and suspected malignant involvement vary depending on the anatomic location (Table 16.9; see also Tables 22.6, 22.7). Additional criteria are the presence of a fatty center (fibrolipomatous degeneration) that indicates a benign node. Inhomogeneities or strong enhancement after contrast administration suggests necrotic or hypervascularized nodes either due to tumor or infection. With multislice CT, the longitudinal extent and threedimensional shape of lymph nodes can be evaluated as well. Round nodes (maximum diameter < 1.5 x minim um diameter) suggest malignancy even though they may be smaller than the threshold size.
Fig. 16.28 Leiomyosarcoma presenting as a septated, peripherally enhancing mass that shows foci of cystic degeneration. The mass cannot be differentiated from the inferior vena cava and has encased the aorta.
The only primary indication for a selective CT examination of the adrenal glands is an abnormality of adrenal function (Table 17.1).In those cases, diagnostic evaluation usually starts with the clinical examination and laboratory tests since their findings will determine the need for imaging procedures and direct their selection. Because of its superiority for estimating the fat content of the lesion (in phase and opposed phase GRE images), MRI offers great potential, especially for the differentiation of benign and malignant enlargement of the adrenals. MRI used to have the further advantage of providing coronal imaging, a feature thatis also available with spiral CT. Multislice CT now provides even better quality. CT is the primary method for excluding metastatic disease in the adrenals. Frequently,
however, the radiologist is confronted with incidentally detected adrenal lesions and is challenged to develop diagnostic strategies for differentiating benign from malignant lesions.
Anatomy
Because the adrenal glands are embedded in the retroperitoneal fat, they can be clearly visualized with CT in almost every patient. Occasionally fusion with the liver or with the renal capsule is encountered. Both adrenal glands are located in the upper part of the pararenal space, enclosed in the pararenal fascia. The right adrenal gland lies anteromedial and superior to the upper pole of the kidney, in the angle between the right lobe of the liver and the crus of the right diaphragm, just posterior to the inferior vena cava. The left adrenal gland lies posterolateral to the aorta and is more anteromedial than superior to the upper renal pole (Fig.17.1 a). Occasionally the lower pole of the gland extends very close to the renal vascular pedicle. Shape and Size
An axial CT scan through the adrenal glands shows each gland to consist of a body and two limbs. The right adrenal gland usually displays a linear, comma, or V shape, while the left adrenal is more often triangular, or Y-shaped. The contours of the glands are slightly concave and sharply marginated (Fig.17.1 b). Corticomedullary differentiation is sometimes possible with contrast-enhanced multislice CT. The adrenal glands measure 4-6 cm in their vertical dimension and 2-3 cm in width. The
~
right adrenal
left adrenal
Fig. 17.1 Anatomy of the adrenal glands, (a) Relationship to adjacent organs, (b) Variations in the shape of the adrenal glands.
limbs are 6-8 mm thick. Measurement of the adrenal glands has no significance because of the large range of variation. The qualitative assessment of shape, size, and contours is of greater diagnostic importance.
Contrast Injection Examination
Technique
No patient preparation is necessary for evaluation of the adrenal glands unless pheochromocytoma is suspected. In the presence of a known secreting pheochromocytoma alpha- and beta-adrenergic blockade prior to contrast material injection can be administered to prevent an adrenergic hypertensive crisis. Recently. it has been shown that specific blockade may not be required before contrast medium enhanced scanning with nonionic contrast media.
Thin-section scanning (3 mm or less) is required for detecting small endocrine tumors and for correct measurements of CT attenuation. Section thickness may be increased to 5 mm for malignant diseases or if the scan length has to be extended. Multislice scanning should employ 0.5-1.25 mm collimation on 4-16 slice scanners. Axial section of 2-3 mm thickness usually suffice for image review but coronal (and sometimes sagittal) reformations of 2-3 mm thickness may help to better detect multifocal lesions and improve delineation oflarge adrenal tumors in their relationship to adjacent organs.
Acquisition Technique
Contrast Injection
The technique for CT examination of the adrenal glands is guided strictly by the indication and the anticipated findings. This particularly applies to examinations that focus primarily on the adrenal glands. such as the evaluation of abnormal adrenal function (Table 17.2). With single-slice spiral CT, a low-dose (localizer) scan of the upper abdomen (7-10 mm section thickness) can be performed to provide a clue to the pathology to be expected. and to optimally define the scan range. This range only needs to include the adrenals in case of benign Cushing's or Conn's disease but should be expanded in malignant tumors or suspicious findings on the localizer scan. For localization of a suspected pheochromocytoma the abdomen should be imaged, at least down to the aortic bifurcation.
Imaging in the arterial phase (25-35 sec) is helpful in distinguishing the adrenals from adjacent structures. However, it is generally accepted that early contrast enhancement (arterial phase) does not contribute to the differential diagnosis of benign versus malignant lesions. The mean attenuation difference between nonadenomas and adenomas was found to be highest on scans acqulred 60-90 sec after injection of contrast material due to the faster wash-out of the contrast material in nonadenomas. Delayed scanning 15 minutes after injection of 150ml of contrast material (300 mg iodine! ml) was reported to have a sensitivity and specificity each over 95% in differentiating adenomas from nonadenomas.
Patient Preparation
Table
17.2
Protocol for CT seaming
ofthe
adrenal
glands
General Oral contrast
Unnecessary (adrenal SOOml, administered
Patient
Supine
position
with elevated
Adrenal
Scan range
region) 30min priorto
region:
Retroperitoneum: Respiratory
phase
(retroperitoneum)
chosen
according
from diaphragmatic
to a thick-section crura to aortic
localizer
scan
bifurcation
Inspiration (spiral C'T) Expiration
Windowing
examination
arms
(conventional
NoncontrastCT: Contrast -enhanced CT:
C'T) W/L=300/40 WiL = 400/60 Continue
>
627
628
17 Adrenal Glands Table 17.2
Continue
Scan parameters
l-slice SC/TF/RI
Adrenals (endocrine
2/3/2 disease)
L
Adrenal region (tumor staging)
3/5/3 t
Retroperitoneum (pheochromocytoma)
5/10/5
Contrast
injection
t
4-slice" SC
16-slice" SC
1-1.25t
0.5-1.25
'/6 t
0.75-1.25
2-3 t
1-2
t
L
MPRb SWIRl
3/2
3/3 cor
5/4
3/3 cor 4/4 sag
5/4
3/4 cor
4-slice
16-slice
N/F/D
V+N/F/D
V+N/F/Dc
100/3/30
100 + 50/3/30 15min
80 + 50/3/15A 15min
l-slice
Parenchymal phase Delayed phase (wash-out)
t
axial SWIRl
15min
SC = slice collimation (mm), TF = table feed (rnmjrctation), Rl = recon. increment (mm), i t = scanning direction, SW = effective section width (mm), MPR = multi planar reformats, axial = axial sections, cor = coronal, sag = sagittal CM = contrast material, Contrast concentration = 300 mg/ml iodine V = volume of CM (ml), N =volume of saline flush (ml), F= flow rate (mils), D = start delay, X = trigger region a Pitch P = TF/{NxSC) should be chosen close to 1.5 with 4-slice scanners, 1.3-1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SWIRl = 1-1.5/0.7 or 0.5-0.8/0.5 C read: 15A = start delay 15 s after reaching 50 HU enhancement in the desc. aorta
Endocrine Diseases Fig. 17.2 reviews (he morphologic changes in the adrenal glands that may be observed in endocrine diseases.
Cushing's Syndrome Endogenous Cushing's syndrome perproduction of corticosteroids)
(adrenal hyis due to a
pituitary overproduction of ACTH with a resulting adrenal hyperplasia in 70% of cases, to an autonomous production of cortisol by a neoplasm of the adrenal cortex in 15%, and to an ectopic ACTH production in a further 15%. In adrenal Cushing's syndrome, it is important to differentiate between adenoma and carcinoma. •
CT Morphology
are easy to detect with CT because they usually measure 2-5 em in diameter, and the obesity in Cushing's syndrome creates very favorable imaging conditions. The lesions appear as well-circumscribed round or oval masses of homogeneous density that show little enhancement after IV contrast administration (Fig. 17.3a). Portions of the gland unaffected by tumor and the contralateral gland appear atrophic. Areas of hemorrhage, necrosis, or calcification are rare and are seen only in large tumors. In Cushing's syndrome due to central hypothalamic-pituitary dysfunction (note that this condition does not represent a primary indication for cross-sectional imaging), the ACTH stirnulaCushing adenomas
Diffuse hyperplasia
Nodular hyperplasia
OO~A
Macroadenoma
Microadenoma
Pheochromocytoma
Fig. 17.2 Changes in adrenal morphology that may be found in endocrine diseases. Pheochromocytomas enhance inhomogeniously after contrast injection.
Conn's Syndrome
Fig. 17.3 (a) Adenoma in Cushing's syndrome appears as a homogeneous, sharply circumscribed, slightly hypoattenuating nodule. The "atrophy" affecting the remaining portions of both adrenal glands suggests an ACTH-independent autonomous adenoma rather than macronodular hyperplasia in central Cushing's syndrome, which also may produce adenoma-like nodules.
(b) Adrenal cortical hyperplasia in central Cushing's syndrome. The adrenal glands are enlarged and show no contour irregularities. Given the highly variable configuration of the adrenals, the diagnosis of a central Gushing syndrome requires corroborative evidence of an endocrine disorder.
tion of the adrenal glands induces a sometimes subtle bilateral adrenal hyperplasia (Fig.l7.3b). The shape of the adrenals remains in proportion. This secondary adrenal hyperplasia can be difficult to differentiate from endogenous tnecronodular adrenal hyperplasia, a rare condition that is also characterized by the development of adenoma-like nodules. Both with ACTH-dependent adrenal hyperplasia and with macronodular adrenal hyperplasia the whole gland is hyperplastic or irregularly thickened, but in patients with an autonomous adrenal adenoma the non tumor-bearing portions of the gland are atrophic. An adrenal Cushing syndrome may be caused by an adrenal cortical carcinoma, but in most cases the CT features of these tumors can be clearly differentiated from benign conditions.
scanning is presently the technique of choice and should rely on thin-section spiral or multislice techniques.
Conn's Syndrome
Conn's syndrome (hyperaldosteronism with the clinical hallmark of hypokalemic hypertension) is caused by a unilateral aldosterone-producing adenoma in 70% of cases and by bilateral idiopathic adrenal cortical hyperplasia in 30% of cases. This differentiation has key therapeutic implications, since adenomas are treated surgically while hyperplasia is managed with drug therapy. Because Conn adenomas are small, CT
•
CT Morphology
Adenoma detection is the key to diagnosis, because idiopathic hyperplasia (aside from rare macronodular forms) cannot be identified by its gross morphologic features on CT scans. Aldosterone-producing adenomas are generally less than 2cm in diameter, and some measure only a few millimeters. CT scanning with a thin slice collimation can consistently detect lesions as small as 10mm, especially if they are favorably located (i.e., in the peripheral part of the gland). They appear as small, round or oval, homogeneous masses with slightly hypodense CT attenuation values (Fig. 17.4). The detection of idiopathic hyperplasia is a more difficult task than the localization of adenomas. Only the rare macronodular forms allow a positive diagnosis of hyperplasia, but these lesions are apt to be misinterpreted as aldosterone-producing adenomas. Determination of the affected gland in Conn's syndrome relies on adenoma detection. Therefore the disease cannot be classified when scans show normal-appearing adrenals, even if there is strong clinical suspicion of idiopathic hyperplasia. The detection of bilateral adenomas also precludes lateralization. The evaluation of
77 Adrenal Glands
Fig. 17.4 Aldosterone-producing adenoma in Conn's syndrome. CT typically shows a microadenoma less than 2cm in diameter that appears as a well-defined, homogeneous, hypoattenuating nodule « 10 HU).
these cases requires bilateral adrenal venous sampling for differential hormone assay, or NP59 scintigraphy. Adrenogenital Syndrome
Primary adrenogenital syndrome (ACS) is based on a congenital enzyme defect in steroid synthesis resulting in absent or deficient cortisol and/or aldosterone production and excessive androgen secretion. The cortisol deficiency leads to constant ACTH stimulation of the adrenal cortex, resulting in adrenal hyperplasia. ACS perse is not an indication for CT examination. •
CT Morphology
CT demonstrates marked adrenal cortical hyperplasia in which the adrenal glands show coarse tumorous enlargement with preservation of normal shape. This type of change is not seen in any other adrenocortical endocrine disease. Adrenal cortical hyperplasia may be absent or mild in ACS patients who receive early replacement therapy. Adrenocortical Insufficiency (Addison's Disease)
Primary adrenocortical insufficiency (hypoadrenalism, Addison's disease) results from destruction of the adrenal cortex, while secondary
adrenocortical insufficiency reflects a deficiency of pituitary stimulation. The most frequent cause of pituitary insufficiency is a neoplastic disease. Primary adrenocortical insufficiency is most commonly due to autoimmune processes (idiopathic adrenocortical atrophy). Less frequent causes are specific or nonspeclftc granulomatous processes (tuberculosis, mycosis), hemorrhage, metastases, amyloidosis, hemochromatosis, lymphoma, etc. The condition is differentiated into chronic (more than 2 years), subacute (less than 2 years) and acute forms of insufficiency. At least 90% of the adrenal cortex must be destroyed before clinical manifestations of adrenal insufficiency appear. Imaging studies are used only in the etiologic investigation of primary adrenocortical insufficiency. •
CT Morphology
CT has no practical role in the diagnosis of idiopathic adrenocortical insufficiency, as the only CT change consists of bilateral atrophy in adrenals that otherwise appear normal. Autoimmune-based atrophy is the commonest cause of chronic insufficiency and is characterized by atrophy of the adrenals without calcifications. Active infectious-inflammatory and granulomatous processes (e.g., in AIDS patients with Mycobacterium evium, Mycobacterium intracellulare or CMV) cause subacute adrenal insufficiency with mild to moderate adrenal enlargement, which is usually bilateral and often symmetrical. The basic shape of the glands is unchanged, but frequently their contours are ill-defined. Heterogeneities may result from necrosis or abscess formation. Calcifications suggest a granulomatous infection (tuberculosis, histoplasmosis). Acute adrenal insufficiency is most frequently caused by adrenal hemorrhage seen after severe hypotension, shock, or septicemia. On CT the adrenals are enlarged and hyperattenuating owing to the presence of blood. Pheochromocytoma
Ninety percent of pheochromocytomas arise from the adrenal medulla and 10% from extraadrenal sympathetic tissues (paraganghomas). Ten percent of pheochromocytomas are bilateral, and 10% are malignant.
Pheochromocytoma Extra-adrenal pheochromocytomas occur predominantly in the retroperitoneal space or pelvis (Organ of Zuckerkandl around the aortic bifurcation). Only 3% are supradiaphragmatic, where they are more often mediastinal than cervical. Intracardiac and paracardiac lesions have been described (see Fig. 10.10). The diagnosis is established by laboratory tests (urinary catecholamine excretion, serum levels of epinephrine and norepinephrine). Imaging procedures are used only for preoperative localization. Contrast media administration is therefore often unnecessary, especially since it may trigger an adrenergic hypertensive crisis in some patients. •
CT Morphology
Because almost all pheochromocytomas are larger than 2 cm when diagnosed, CT can easily detect tumors located at a typical adrenal or juxta-adrenal site in the retroperitoneum. CT demonstrates a well-circumscribed round or irregularly shaped mass. Small tumors are relatively homogeneous, but larger tumors are more likely to contain areas of hemorrhage, necrosis, cystic changes, and calcifications (3-10%). The intense enhancement of pheochromocytomas serves to distinguish them from adrenal adenomas (Fig.17.5). MRI and specific radionuclide studies (iodine-131 meta- iodo benzy lguanldtne (MI BG) scintigraphy) are superior to CT in screening for extra-adrenal tumors. • Generally differentiation between benign and malignant tumors cannot be made, although metastasis is a reliable criterion for malignancy. • Pheochromocytomas can mimic some of the symptoms of a neurocutaneous syn-
Fig. 17.5 Pheochromocytoma containing areas of regressive change (necrosis and calcification). Only the peripheral parts of the tumor show typical intense enhancement. Note: intravenous contrast is not essential in patients with biochemically confirmed pheochromocytoma. If required, it should be used with premedication (receptor blockers).
drome or multiple endocrine neoplasia (MEN) syndrome. Bilateral and ectopic pheochromocytomas are frequently associated with MEN. Localization is particularly difficult in MEN syndrome type II (medullary thyroid carcinoma, parathyroid adenoma, pheochromocytoma) and type III (medullary thyroid carcinoma, pheochromocytoma, ganglioneuroma, marfanoid body habitus), because the tumors are often small and bilateral and develop from medullary hyperplasia, which cannot be detected with imaging studies. The incidence of pheochromocytoma is increased in patients with von Hippel- lindau syndrome (5-15%) and neurofibromatosis(5%).
Nonfunctioning Adrenal Lesions
This group consists of lesions that do not cause hormonal abnormalities or other symptoms and therefore are often discovered incidentally during examinations performed for other indications. They include nonhyperfunctioning adenomas, cysts, hematomas, granulomatous changes, myelolipoma, and rare instances of
metastases and primary adrenal carcinoma (Table 17.3). Only some of these lesions have CT features specific enough to suggest a spot diagnosis or presumptive diagnosis. These include myelolipomas, cysts, hemorrhage, and coarse calcifications without a mass effect.
631
77 Adrenal elands Table 17.3 findings Benign
Differential diagnosis of incidental adrenal
lesions
Hypodense(O-10HU).no enhancement
Cysts
Hematomas
Hyperdense (>60HU), no enhancement
Calcifications
Very hyperdense
Silent
tumor (irregular wall thickening), The presence of irregular wall thickening or contents that show a density greater than that of water do not support the diagnosis of a simple cyst Adenomas, adrenal carcinomas, and pheochromocytomas (determination of urinary catecholamine levels) may also represent as cystic tumors,
No definite criteria «10 HU, 10H)
Adrenal Cysts Adrenal cysts are very rare, They may be classified by their pathogenesis as endothelial (45%), epithelial (9%), parasitic (Echinococcus, in 7%), or pseudocystic (40%), The last represents the end stage of a remote hemorrhage, CT Morphology
The typical CT appearance is that of a cystic mass with smooth, uniformly thin walls and contents of fluid attenuation, Many cysts display a calcified rim, Wall thickness seldom exceeds 3 mm, Rim enhancement but no intralesional enhancement can be seen. On unenhanced CT scans it may be distinguish between a benign cyst attenuation adenoma, Ultrasound adjunct for confirming that the liquid,
difficult to and a low is a useful content is
The differential diagnosis of cystic lesions should include the possibility of regressive changes in a
CT Morphology
On CT the adrenals appear round or oval having a similar shape to those with adenomas or metastases, Adrenal hemorrhage can be diagnosed with high confidence when it is fresh and displays high attenuation values, but with increasing delay between the bleeding and the CT examination attenuation values decrease. As in cysts but unlike solid tumors, contrast enhancement is not seen, Older hematomas often appear cystic, in which case they are indistinguishable from pseudocysts of other etiology,
Adrenal Calcifications Pronounced calcifications in the adrenal gland most commonly result from adrenal hemorrhage in early childhood, They may also be residual to adrenal tuberculosis or a different granulomatosis, Tumor-associated calcifications are easily recognized by demonstrating the accompanying mass, Calcifications are present in up to 80% of neuroblastomas, up to 30% ofcarcinomas, and up to 10% of pheochromocytomas,
Nonhyperfunctioning Adenoma Myelolipoma
Adrenal myelolipoma is a benign developmental tumor (hamartoma) that contains varying proportions of fat and hematopoietic tissue elements. In 10% of patients this is associated with endocrine disorders like Cushing's or Conn'ssyndrome. •
CT Morphology
Characteristic feature of these lesions is the presence of fat, which is seen in almost all myelolipomas (Fig. 17.6). The proportion of fat to bone marrow cells varies, and tumors may be completely fatty or retain only a few foci of fatty elements. The myeloid elements show higher precontrast attenuation and significant enhancement, while the enhancement of the fatty structures is minimal. The size of the well-circumscribed tumors varies between some millimeters and 30cm, with Scm being the average. Retroperitoneal and intralesional hemorrhage has been described. In "10% of patients bilateral lesions are seen, and up to 30% of lesions have calcifications that are often punctate. Myelolipomas with a high fat content are easily diagnosed, and only lipoma (fat, no other soft tissue) and adenoma (CT attenuation usually >-10HU) need be considered in the differential diagnosis. With more inhomogeneous appearance, the further differential diagnosis includes a liposarcoma or an upper pole renal angiomyolipoma. The larger the myeloid component, the more difficult it is to identify the tumor. Tumors
Fig. 17.6 Typical near-fat adrenal myelolipoma.
CT attenuation
values
of
that contain very little fat cannot be identified by their CT features, and biopsy is required. Nonhyperfunctioning
Adenoma
At least 30% of all adrenal masses are incidental findings. Nonhyperfunctioning adrenal cortical adenomas have no pathologic significance. If such a lesion is discovered incidentally, e.g., on a CT scan of the abdomen, it is called "incidentalorna". For differentiation between benign and malignant disease see incidentally detected adrenal masses (p. 636). •
CT Morphology
These adenomas can vary greatly in size (O.S-Wcm), but most are smaller than 3cm. They appear as round or oval masses with welldefined margins. The shape of the adrenals is preserved. Adenomas are usually homogeneous and show CT attenuation lower than or equal to that of muscle. CT attenuation below 10 HU on non-contrast scans is considered highly predictable of adenoma, while attenuation above this cutoff level is considered indeterminate (10-40% of lesions, lipid -poor adenomas). Inhomogeneities and calcifications are rare. Contrast enhancement may vary but is usually high initially (more than nonadenomas) and rapidly washes out. Most adenomas have a density below 30 HU (> 40% de-enhancement within 15 min) on delayed scans 1 h after contrast administration (Table 17.3;Fig. 17.7).
Fig. 17.7 Silent adenoma (incidentaloma) has exactly the same CT features as functioning adrenal cortical adenomas. Incidentalomas are elliptical, sharply circumscribed, usually measure below 10 HU.
633
77 Adrenal Glands Ganglioneuroma,
Ganglioneuroblastoma
Canglioneuroblastoinas are generally located in the retroperitoneum (65%), otherwise in the neck, mediastinum, or in the adrenals. Ganglioneuroblastoma is a complex tumor containing both mature ganglion cells and primitive neuroblastic cells. They are seen in children under 10 years of age. Ganglioneuromas are benign tumors that arise from the sympathetic chain or rarely from the uterus, ovary, skin, or GI tract. Adrenal ganglioneuromas occur predominantly between the third and fifth decade. •
CT Morphology
Canglioneuroblastoinas show similar imaging features to neuroblastomas, and require histological proof. Adrenal ganglioneuromas are indistinguishable on CT from adrenal carcinomas, but no signs of local or vascular extension are seen. Specific diagnosis requires biopsy or surgical removal.
infiltrate the bone marrow in such a Way as to be confused with leukemia. The goal of cross-sectional imaging is to stage local extent and metastasis, for which MRI is the current imaging modality of choice. Multislice CT probably will yield similarly good results. •
CT Morphology
Most neuroblastomas display a heterogeneous tumor matrix with areas of necrosis and hemorrhage. CT shows calcifications in up to 80% of tumors (Fig. 17.8), which are finely stippled and linear or ring shaped. Solid tumor portions show moderate contrast enhancement. Local staging is based on tumor size, location, extent, spread across the midline, and supradiaphragmatic involvement. Invasion of the kidneys by neuroblastoma may make it difficult to distinguish this from nephro blastoma (Wilms" tumor). The tumors frequently metastasize to the liver, bone, and lymph nodes. Adrenal Carcinoma
Neuroblastoma
Neuro blastoma is the commonest pediatric abdominal malignancy. The peak incidence is in the second year of life, with 75% of cases occurring before age 2; the remainder are found at all ages. They originate from the adrenal medulla and from the sympathetic ganglia. The origin of large tumors cannot always be determined (see Chapter 12). The tumor may be quite highly differentiated and contain ganglion cells (ganglioneuroblastoma) or be undifferentiated and
Primary adrenal carcinomas are very rare. They remain clinically silent for some time before they are detected by their mass effect or endocrine symptoms (Cushing'ssyndrome, virilization). By that time, many of the tumors have already metastasized. There are two age peaks: before the age of 5 years and between 40 and 50 years. •
CT Morphology
Adrenal carcinomas are large (90% > 6 cm; up to 25cm) at the time of clinical presentation. Thus, while the tumor is easily detected by CT, it can
Fig. 17.8 Neuroblastoma of the right adrenal gland. Calcifications are visible on the precontrast scan (a). On the coronal MPR (3/5/2) after contrast administration, the tumor displaces the kidney downward but is clearly demarcated from it (b).
a
b
Lymphoma be difficult to assign the lesion to a specific organ and delineate it from adjacent organs. Differential diagnosis is aided by the fact that adrenal carcinomas displace and deform adjacent organs for a long time before infiltrating them. Large carcinomas have irregular margins and are heterogeneous due to areas of regressive change (necrosis, hemorrhage). Solid tumor components show marked contrast enhancement (Fig. 17.9). Calcifications are observed in 25-30% of adrenal carcinomas. The distinction between benign and malignantcortical neoplasms is virtually impossible if no signs of infiltration or metastases are present. Large size of tumors, necrosis, and intratumoral hemorrhage are suggestive for malignancy. It can also be difficult to differentiate an adrenal cortical neoplasm from a renal cell carcinoma, since both show similar enhancement characteristics, and patterns of metastatic spread. Here, multiplanar reformations, especially from thin-section helical or multislice CT are particularly helpful.
635
geneous, while larger metastases are heterogeneous due to regressive changes caused by hemorrhage or necrosis (Fig. 17.10). Generally the lesions enhance after IV contrast administration (Table 17.4). Calcifications are rare but do occur in mucin-producing tumors. Approximately 50% of adrenal metastases are bilateral. Adrenal glands that appear grossly normal or hyperplastic (with their basic shape unchanged) do not rule out metastases (see Fig.9.10b). A differentiation between benign and malignant lesions cannot always be made (see discussions below). Lymphoma
Primary adrenal lymphoma is extremely rare. Secondary involvement of the adrenals is more frequent (in approximately 25%). The most common cause is NHL.
Table 17.4 Incidentally detected adrenal masses
Adrenal
Metastases
The tumors that most frequently metastasize to the adrenal gland are breast carcinoma (55 %) , bronchial carcinoma (35%), renal cell carcinoma (25-40%), and melanoma (60%). •
CT Morphology
Adrenal metastases have a highly variable CT appearance. Small metastases are usually homo-
b Fig. 17.9 Adrenal carcinoma (3/5/2) presenting as a large mass with lymph node metastasis (arrowhead). On the contralateral side there is a small silent adenoma (arrow).
17 Adrenal Clands
a Fig. 17.10 Adrenal metastases. (a) CT reveals large, focal adrenal metastases in a patient with bronchial carcinoma. The size and bilateral occurence of the lesions are suspicious for metastases. (b) Metastases that
mimic adrenal hyperplasia are rare (here: bronchial carcinoma). Although the gland is enlarged, it retains an approximately normal configuration.
are a challenge to the radiologist, who must determine their significance. • CT Morphology Choosing an appropriate diagnostic Involvement is bilateral in about 50% of strategy for differentiating between a benign lepatients. CT attenuation values range between sion and a metastasis becomes an important 40 HU and 60 HU; there is little contrast en- issue when an adrenal mass is detected in a hancement (Fig.17. Tl). Necrosis can be seen in patient with a known primary malignancy. The rapidly growing lesions. detection of an adrenal lesion is suspicious for metastasis in such a patient, but it is not conclusive. Published reports indicate that the majorIncidentally Detected Adrenal Masses ity of adrenal masses found in oncologic patients are nonmalignant. Therefore it is cliniIncidentaloma is the term applied to an adrenal cally important to be able to differentiate bemass larger than 1 em, discovered incidentally tween benign and malignant disease in the main a patient with no evidence of endocrine dys- jority of patients. function. These masses are detected in approxi• CT Morphology mately "1-4% of abdominal CT examinations. Thus, incidentalomas are by no means rare and Morphologic differentiating criteria have been established for the etiologic classification and differentiation between benign and malignant adrenal masses discovered by CT (Table 17.4).The most reliable criteria are, in order of importance, attenuation, size, and enhancement characteristics, although these criteria overlap. A useful rule of thumb is that small « 3cm) lesions of uniformly low attenuation « 10 HU) or that show rapid wash-out of the contrast enhancement on delayed scans taken 15-60 minutes after contrast injection are probably benign. Larger heterogeneous masses (> 5 ern) that show marked contrast enhancement are strongly suspicious for malignancy or metastasis, especially in a patient with a known primary tumor. Fig. 17.11 Primary adrenal large cell lymphoma, diffuse enlargement of both adrenal glands.
with
Incidentally Detected Adrenal Masses Diagnostic
indicated, Asymptomatic masses dcm that have a CT attenuation < 10 HU may be classified as benign with high confidence, If the lesion has an attenuation> 10 HU, then thin-section contrast-enhanced CT with early and 15 minute delayed images is done, When the lesion has a relative contrast wash-out >40% or when the density at the delayed scan measures dOHU, benignity is assumed, Further investigations are unnecessary in non-oncologic patients, Oncologic patients can be managed with a wait-and-see approach that includes a single follow-up, provided the mass is not a new lesion. A new lesion is presumed to be metastatic 3. For asymptomatic lesions that are detected incidentally during routine contrast-enhanced abdominal CT. 15 minutes delayed imaging is performed, The lesion is considered benign if it shows a relative contrast wash-out> 60% at 15 minutes, 4, Large (>5cm), heterogeneous, enhancing lesions are suspicious for malignancy and require a definitive diagnosis (fine needle aspiration biopsy), 5, If the lesions do not fulfill the above criteria they are considered indeterminate, Spectflc gutdelines cannot be offered for such lesions, The procedure in a given case depends on the totality of clinical findings, If therapeutic implications compel a decision, fine needle aspiration biopsy should be performed,
Strategy
A recent analysis showed that up to 40% of adenomas are relatively lipid-poor and do not fulfill the densitometry criteria on noncontrast CT. Applying densitometry criteria on delayed contrast CT may be difficult due to differences in CTscanners or contrast injection protocols, leading to significant numbers of indeterminate lesions, Although the density of lipid-poor adenomas resembles that of nonadenomas, their contrast kinetics are similar to lipid-rich adenomas, Using a cutoff value of 40% relative wash-out (("I HUeariy/HUdeiay)x 100%) at 15 minutes, these adenomas can be identified with a high specificity and acceptable sensitivity, For lipid-rich and lipidpoor combined a more appropriate value is 60 %, A practical implication of this technique is the fact that delayed scanning can easily be combined with routine abdominal CT without having to reschedule the patient for a second examination, The following considerations should direct the diagnostic strategy with CT (in those patients where there is an apriori knowledge of a lesion, chemical shift MRI is a good alternative)-see also Table 17,5: L lf an adrenal mass shows features typical of a specific benign lesion (myelolipoma, cyst, hemorrhage) that is consistent with the clinical presentation, further diagnostic studies are not required, 2, When an adrenal lesion has been diagnosed previously, thin-section noncontrast CT is Table 17.5
Differential diagnosis of adrenal masses
Noncontrast
CT
Fa' Cystic areas Hemorrhage Calcification
Myelolipoma
Cyst, old hematoma, Trauma,
neuroblastoma,
Adrenal hemorrhage
echinococcosis, neuroblastoma adrenal carcinoma, metastases in early childhood, adrenal tuberculosis,
The kidney can be excellently analyzed with computed tomography. This is due to its typical enhancement characteristics, which allow for an optimal visualization of the renal parenchyma, renal lesions, and the renal vessels. Helical CT provides all necessary information for surgical management, especially for nephron-sparing resection techniques. Helical CT has improved the detection and characterization of small renal neoplasms although the differentiation between small benign and malignant tumors (ilC!>!i»WilaSII\lll\t13:::W1
..
651
18 The Kidneys
pyonephrosis. Visible wall thickening is suggestive of an inflammatory process or a tumor. At the onset of urinary obstruction, the protruding portion of the renal pelvis is indistinguishable on noncontrast scans from an ampullary type of pyelocaliceal system. On postcontrast scans, however, the layering of contrast medi urn in the renal pelvis points to urinary obstruction rather than an ampullary pelvis. Further progression of the urinary obstruction causes dilatation of the major and minor calices (Fig. 18.9). Late stages are marked by papillary atrophy, and loss of the medullary pyramids due to the high intral uminal pressure. Finally the kidney becomes a hydronephrotic sac surrounded by a thin rim of parenchyma. Renal function at this stage is usually impaired, causing a delay of corticomedullary enhancement
a
b
Fig. 18.9 Acute right-sided urinary obstruction with differential contrast enhancement between the two sides (a), subacute bilateral urinary stasis (b) with pyelocaliceal dilatation and loss of corticomedullary differen-
and delayed contrast excretion into the renal pelvis. Hydronephrosis should be distinguished from ureteropelvic junction (L1Pl) obstruction (Fig. 18.10), which is a congenital anomaly that does not cause dilatation of the ureter. The ureter is dilated proximal to an obstruction (diameter proximal to the obstruction > 5 mm, further proximally> 3 mm) and shows delayed opacification. The ureter distal to the obstruction is collapsed and poorly defined. If dilatation of individual calices is noted (hydro calices) , several possible causes should be considered: crossing vessels, calculi, tumors, or inflammatory changes (common in tuberculosis). CT can demonstrate the cause only if thinslice helical or multislice scanning is performed.
c tiation (scan acquired at 100 seconds), and a hydronephrotic sac representing the end stage of chronic hydronephrosis (c).
Fig. 18.10 Ureteropelvic junction (UPJ) obstruction (4 x 1/6, paracoronal MPR) with marked dilatation of the renal pelvis and normal size of the ureter. Normal size kidney (a). marked thinning of the cortex in untreated massive obstruction (4 x 1/6, sagittal MPR) (b).
a
b
Urof'ithiasis
a
c
b
Fig. 18.11 Urolithiasis. Small calculus in the ureter with soft-tissue rim sign and stranding of the periureteric fat (a). A curved MPR (4 X 1/6) demonstrates the location relative to the ureter (b). A prevesical calculus
d
(arrow) has to be distinguished from phleboliths (arrowhead) (c). Note the comet tail sign due to a small vein (4x 1/6, coronal MPR) (d).
Fig. 18.12 Unilateral perinephric stranding in acute obstruction (a) as compared to bilateral perinephric stranding in elderly patients with chronic renal disease (b). The perinephric fluid in (a) is extensive, suggesting rupture of the pyelocalyceal system.
a
b
Urolithiasis
Noncontrast CT plays an increasingly important role in patients suspected of suffering from a renal colic. Spiral CT is more effective than intravenous urography in identifying ureteral stones and as effective as intravenous urography in determining whether there is renal obstruction. CT is considered the best method of determining the stone volume (with a mean error of < 5 %) and thus providing important information for treatment planning (lithotripsy for stones> 2.5 cm has considerably higher complications). Residual calculi after shock wave lithotripsy are readily assessed. Low-dose scanning usually suffices for making the diagnosis. Best results are obtained with multislice CT.
•
CT Morphology
CT is more sensitive than other imaging modalities for detecting urinary calculi. It can even detect small noncalcified stones that are radiolucent on abdominal radiographs, as their attenuation values are always greater than 100 H (Fig. 18.11).Calcium-containing stones (90% of all stones) and cystine stones have attenuation values of 450-1500 HLJ, while xanthine stones are in the range of 100-600 HLJ. Generally, however, urinary stones cannot be differentiated by their chemical composition on the basis of CT attenuation measurements. Acute obstruction may lead to swelling of the kidney, increased thickness of the renal cortex due to edema, and delayed or reduced contrast enhancement of the affected kidney. Ureteral stones in combination with hydronephrosis, hydroureter, or perinephric fluid
18 The Kidneys
establish the diagnosis of an acute renal obstruction (Fig. 18.12). Dilatation of the collecting system without detectable stone occurs if the stone has already passed into the bladder or was extracted. In acute ureteral 0 bstruction the ureteral wall will initially appear normal but will eventually thicken with the onset of edema or inflammatory reactions. Acute rupture of the renal pelvis presents as substantial amounts of fluid around the affected kidney in patients with complete ureteral obstruction. Perinephric fluid alone is a good predictor of acute obstruction if it occurs unilaterally, and the patient has no history of acute trauma, previous or chronic renal disease (Table 18.5). Papillary
calcifications
and arterial
calcifi-
cations are usually distinguished by their location from stones in the collecting system (Fig. 18.13). The following features on noncontrast CT that are helpful to distinguish ureteral stones from extra-ureteral calcifications such as phleboliths in the small pelvis: • A soft tissue rim around an impacted stone (tissue rim sign) is thought to represent the ureteral wall thickened by edema or inflammation. • Stranding around the calcification represents an inflammatory reaction in the periureteral fat. • Phleboliths may be characterized by a central region of low attenuation (magnified images with bone window settings), a bifid peak on the line histogram and the comet
a
b
Table 18.5
Urolithiasis
/\
Signs of acute obstruction due to urolithiasis
Hyperattenuating focus (>120 H) in the collecting system Dilation of the intrarenal collecting system Hydroureter (ureteral obstruction) Stranding of the perinephric fat (unilateral) Perinephric fluid (markely more pronounced on one side) Unilateral increase in renal cortical thickness Unilaterally reduced cortical enhancement DD phleobolith/ureter
stone
Plebolith:
Comet sign Central low attenuation
Ureter stone:
Tissue rim sign Periureteral stranding Continuity with ureter on curved reformations
sign. The latter is caused by an eccentric, tapering, soft tissue mass adjacent to the round calcification, which represents the noncalcified portion of the pelvic vein containing the phlebolith. With multislice CT longitudinal reformats can directly demonstrate the relation of the ureter to the suspected stone (Fig. 18.11), even in situations where conventional spiral CT remains inconclusive.
c
Fig. 18.13 (a) Papillary calcifications located at thejunction of the medulla and pyelocaliceal system, (b)Severe nephrocalcinosis in oxalosis. (c) Medullary sponge kidney.
Cystic Renal Masses
Differential diagnostic features are provided in Table 18.6. Renal Cysts
Renal cysts are a frequent incidental finding that has no pathologic significance; more than 50% of patients over age 50 have renal cysts. Multiple cysts are common. Cysts may be intraparenchyTable 18.6
mal or parapelvic, and they can reach considerable size. Complicated cysts result from superinfection or hemorrhage into the cyst. While the majority of uncomplicated renal cysts are easily diagnosed, complicated cysts can be very difficult to distinguish from tumors. •
CT Morphology
Uncomplicated cysts appear on noncontrast scans as masses of uniformly low attenuation
Differential diagnosis of cystic renal lesions
Solitary lesion
Uncomplicated cyst
No calcification, no septa, no contrast enhancement «10 HU attenuation difference)
Abscess
III-defined margins, air bubbles, rim enhancement, renal parenchyma
Hematoma, traumatic cyst
Hyperattenuating, nonenhancing
Echinococcalcyst
Thin mural calcification, enhancing capsule and septa
Renal cystadenoma
Multiple cysts enclosed in a capsule, enhancing capsule and septa
Cystic hypernephroma
Enhancement in nodular portion of wall
inflammatory reaction of
Multiple lesions
a
Uncomplicated cysts
No septa, no contrast enhancement
Adult polycystic disease
Numerous cysts of varying density scattered throughout the kidney, accompanied by cysts in the liver and pancreas
Echinococcalcysts
See above
Multicystic and dysplasia
Cluster-of-grapes parenchyma
cysts with wall calcifications, no demonstrable renal
b
Fig. 18.14 Renal cysts, (a) Large, uncomplicated renal cyst with a marginal beak sign caused by the stretched renal capsule, (b) This sign should not be mistaken for a solid tumor component, (c) Parapelvic cysts can be dis-
c
d
tinguished from a dilated renal pelvis by their lack of enhancement on late scans, (d) Smm-thick coronal MPR of parapelvic cysts (4 x 1/6).
18 The Kidneys
(0-20 HU) with well-defined margins (beware of partial volume effects). The cyst wall is not visible with CT, and the attenuation of the mass does not increase after IV contrast administration « 10 HU). Renal cysts can be classified by their location as subcapsular, cortical, medullary, or parapelvic (Fig. 18.14). Parapelvic cysts cause impressions on the pyelocaliceal system (Fig. 18.14c, d), but segmental obstruction ofthe collecting structures is rare. They can best be differentiated from an obstructed renal pelvis on late scans. A beak sign occurs with cysts that stretch the renal capsule (Fig. 18.14a, b) and can mimic wall thickening in cysts that are located at the renal pole.
a
c
b
Fig. 18.15 Complicated cysts (a). Septated benign cyst (4 x 1/6, coronal MPR). ( b) Multiple cysts in von-HippelLindau disease, associated with a renal angioblastoma (arrow). Note the distorted renal parenchyma, (c) Table 18.7
Cysts are called complicated if they have an increased CT attenuation, thickened septa, calcifications, wall thickening, or contain gas (Fig. 18.15). A high-density renal cyst (> 20 HU, up to 90 HU) may be caused by intracystic hemorrhage, infection, a high protein content, or milk of calcium. Calcifications most commonly result from previous infection or intracystic hemorrhage. Infection can lead to uniform wall thickening with rim enhancement or the presence of intracystic gas (see below). The Bosniak classification has been established to differentiate between benign and potentially malignant cystic lesion (Table 18.7).
d
Thickened wall and extensive mural calcification in renal tubercolosis. (d) A hyperdense cyst in the nephrographic phase may simulate tumor (4x 1/6, coronal MPR).
Management of cystic renal lesions (according to Bosniak, 1994)
Homogeneous attenuation before contrast administration (narrow window setting) and no enhancement with contrast medium (narrow window setting, 15-20 HU) is seen in the renal parenchyma during a contrast-enhanced examination. In these cases, late scans more than 15 minutes following intravenous contrast administration demonstrate a reduction in CT attenuation if the lesion is a vascularized tumor, while CT attenuation remains unchanged « 10HU difference) if it is a cyst (Fig. 18.17). •
Differential
a
Fig. 18.17 Even without a precontrast scan, a suspicious lesion measuring 45 HU on the nephrographic phase (a) can be identified as a tumor by demonstrating de-enhancement to 30 HU in the excretory phase (b).
the renal sin us. In isolated cases it may be difficult to distinguish from parapelvic cysts or transitional cell carcinoma. The slight, delayed enhancement of fibrolipomatosis differentiates it from cysts, however. Metastatic melanoma may show very low attenuation on pre contrast scans, but it differs from cysts in its marked enhancement after contrast administration. Some difficulties may be encountered in distinguishing complicated cysts from renal cell carcinoma. Some helpful differentiating features are listed in Tables 18.6 and 18.7.
Diagnosis
Parapelvic cysts are best distinguished from an ampullary renal pelvis or hydronephrosis during the excretory phase. They show no enhancement, and they cause smooth impressions on the collecting system. Pelvic fibrolipomatosis has a negative to above water-equivalent CT attenuation, depending on the connective tissue content of the fat in
Fig. 18.16 Pseudoenhancementwith increase of enhancement from 20 HU before contrast injection (a) to 35 HU in the nephrographic phase may be seen with small cysts if wide sections (here;SW= 10 mm) are used (b). This partial volume effect is reduced with thin-section imaging: the cysts measures only 23 HU on 3-mm sections reconstructed form the same multislice CT data set (c).
a
b
Echinococcal Cysts
After the liver and lung, the kidney is the third commonest organ infected by Ec1iinococcus cysticus, with renal involvement occurring in 2-7% of
b
c
657
18 The Kidneys
patients with systemic hyclatid disease. Renal involvement by Echinococcus alveolaris is unknown. •
CT Morphology
Renal echinococcosis is marked by the development of sharply circumscribed cysts. Calcification of the cyst wall is seen in approximately one-third of cases. Septations and daughter cysts within the mother cyst are characteristic, and the attenuation difference between the mother and daughter cysts is pathognomonic. The cyst wall is visible with CT, and both the wall and septa enhance with intravenous contrast. Rarely the cysts may contain solid components, which also show intense enhancement. These lesions are generally not recommended for biopsy. Cysts that show septa or wall thickening may be difficult to distinguish from cystic hypernephroma. Hydatid cysts in other organs and serologic test results will suggest the correct diagnosis, however. POlycystic Kidney Disease
The infantile form of polycystic disease (Potter type 1) is inherited as an autosomal recessive trait and is often fatal during the first months of life. It is commonly associated with anomalies of the hepatobiliary system. CT scanning is not usually indicated. The adultform (Potter type I I I) is transmitted as an autosomal dominant trait and causes progressive renal impairment starting at about 30
a
b
Fig. 18.18 Polycystic kidney disease, (a) Multiple bilateral cysts in a 42-year-old-patient. (b) Massive renal enlargement with multiple hyperdense (chocolate)
years of age. It is associated with hepatic cysts (30-60%), pancreatic cysts (10%), and an increased incidence of arteriovenous malformations and aneurysms. There is, however, no increased incidence of renal cell carcinoma in these patients. CT is used to diagnose complications such as hemorrhage and infection and to detect or exclude a tumor in patients with hematuria. •
CT Morphology
In the infantile form, the kidneys are markedly enlarged but retain a normal shape. The numerous cysts are 1-2 mm in size and commonly cannot be defined by CT. Contrast-enhanced scans reveal faint radial striations in the parenchyma, which does not show corticomedullary differentiation. Urinary stasis in the dilated collecting ducts leads to stone formation with medullary nephrocalcinosis that is visible even on plain scans. In the adult form of polycystic disease, the kidneys are enlarged during adolescence and young adulthood and are diffusely permeated by cysts of varying size (Swiss cheese pattern). The number and size of the cysts increase with further progression of the disease, until finally the grotesquely deformed kidneys fill the entire abdomen. Recurrent intracystic hemorrhage is very common (70%) and produces areas ofvariable high attenuation with homogeneous contents (Fig. 18.-18).Calcifications (in> 50%) result from small calculi, hemorrhage, or infection. Intravenous contrast administration improves the
c cysts in a 55-year-old patient (4 x 1/6, coronal MPR). (b). Fresh intracystic hemorrhage (arrow) is less homogeneous and has a fuzzy border (c).
Renal Cystadenoma (Perlmann demarcation of the residual parenchyma from the collecting structures. Areas of fresh intracystic hemorrhage present as heterogeneous regional hyperattenuating structures within a renal cyst (Fig.18.18c) and may warrant surgical intervention in symptomatic patients. Acute infection can lead to thickening and enhancement of the affected cyst wall and may also be manifested by intracystic gas. Malignanttumors (renal cell carcinoma, lymphoma) are easily missed in polycystic kidneys. Multicystic Dysplastic Kidney
Multicystic dysplastic kidney (Potter type II) is a condition marked by severe structural disorganization of the kidney. It is a unilateral anomaly, since bilateral multicystic kidneys are incompatible with life. •
CT Morphology
The kidney is composed of approximately 10 to 20 cysts, each smaller than 4cm, that show wall calcifications and a c1uster-of-grapes appearance. The complete absence of functioning (enhancing) renal parenchyma is pathognomonic (silent kidney). The contralateral kidney is hypertrophic and in 40% of cases show associated abnormalities such as malrotation or ureteropelvicjunction stenosis. Acquired Cystic Kidney Disease
Acquired renal cysts are found in 40-60% of patients who have been on hemodialysis for 3-5 years, and in up to 80% of patients on dialysis
for 5-10 years. The number and size of cysts increase with duration of hemodialysis. Complications and suspected tumors are the only indications for CT. There is an increased incidence of renal carcinoma of about 5%. Ultrasound follow up is indicated once per year for patients who are on dialysis for longer than 3 years. •
CT Morphology
Scans reveal multiple small cortical cysts less than 2 cm in diameter (acquired cystic renal disease). The renal parenchyma appears shrunken (Fig. 18.19), and fatty atrophy is apparent in the renal sinus. Hemorrhage into cysts causing increased attenuation may occur. Diffuse cortical calcifications result from the deposition of oxalate crystals in the cortical tubules. Renal Cystadenoma (Perl mann's Tumor)
Renal Cystadenoma (multilocular cystic nephroma) is a benign encapsulated tumor composed of multiple cysts filled with myxomatous material. It is most prevalent in small boys « 4 years) and older women (40-70 years). •
CT Morphology
The tumor ranges from 3-30 cm in size and consists of numerous cysts, up to lOcm in diameter, which are sharply demarcated from surrounding structures (Fig. 18.20). There are multiple, well-defined septa permeating the cysts. Calcifications are common (10-50%), but areas of hemorrhage and necrosis are rare. Cysts with mucinous contents show soft-tissue attenuation and are indistinguishable from solid tumors on precontrast scans. The cysts do not communi-
Fig. 18.19 Renal changes associated with chronic hemodialysis (aquired polycystic kidney disease, APKD). (a) Multiple small, bilateral renal cysts are a typical finding, (b) Uncomplicated cysts on the left side and a small, hypervascular renal cell carcinoma on the right side in a patient with APKD.
a
s Tumor)
b
659
18 The Kidneys
phroma in adults and cystic Wilms' tumor in children, surgical resection is always indicated, especially since the tumor may undergo sarcomatous transformation in adults. These lesions are in the Bosniak type 3 category (see Table 18.7). If CT shows thickened or nodular enhancing septa the mass must be considered renal carcinoma until proven otherwise. Cystic Renal Cell Carcinoma
Fig. 18.20 Renal cystadenoma (Perl mann's tumor) appears as a focal polycystic lesion with fine enhancing septa.
cate with the collecting system but, not uncommonly, a portion of the tumor herniates into the distorted collecting system. Only the septa and capsule enhance after IV contrast administration. Solid nodular components are not found. Because the CT features of Perlmann's tumor are similar to those of cystic hyperne-
In approximately 20% of all renal cell carcinomas, cystic characteristics are observed in portions of the tumor. •
CT Morphology
The hypernephroma may display low attenuation values (approximately 10-30 HU) because of cystic components, fat, or necrotic areas. An enhancing cyst "wall" of irregular thickness is highly suspicious for a tumor (Fig. 18.21). If even a small portion of a cystic mass displays a feature that is not consistent with a cyst, a malignant tumor should be suspected (see criteria in Table 18.7). Fig. 18.21 (a) Cystic renal cell carcinoma with peripheral, septa-like internal structures that show contrast enhancement, (b) Large cystic hypernephroma with an irregular, enhancing soft-tissue component and irregular calcifications.
a
b
Benign Solid Tumors
CT is excellent for tumor detection but, except for angiomyolipomas, cannot positively identify a renal tumor as being benign. For this reason, tumors that appear benign (Table 18.8) are generally managed by surgical resection (enucleation). Biopsy is not indicated because of its low specificity.
Pseudotumors
Hypertrophic renal columns or persistent fetal lobulation can mimic renal tumors (see congenital anomalies, p.649). Secondary pseudotumors result from the nodular hypertrophy of healthy areas of parenchyma following the atro-
Renal Adenoma Table 18.8
Differential diagnosis of solid renal lesions
I Benign renal lesions:
Noninvasive, no extracapsular
extension
Pseudotumor
Same enhancement pattern as renal cortex (renal columns, fetal lobulation, parenchymal scars)
Angiomyolipoma
Fatty areas «-20 HU) accompanied by areas of soft-tissue attenuation
80% of cases shows a typical spoked wheel pattern of enhancement with a central stellate scar of low CT attenuation (Fig. 18.24). Rare findings are cystic changes or more than one scar. Rarely, oncocytomas occur as multiple lesions (oncocytomatosis) or become very large (giant oncocytoma) . A confident differentiation from renal cell carcinomas, however, cannot be made: lesions smaller than 5 cm may show no scar and on the other hand, a central scar in a renal cell carcinoma can have a very similar appearance Angiomyolipoma
Angiomyolipoma is a relatively common benign hamartosis composed of varying proportions of
Fig. 18.24 Oncocytoma with a faint central scar in the right kidney (a). Large tumor in the right kidney with a central scar (b) that proved to be a renal cell carcinoma at histoloqy.
a
b
Angiomyolipoma fat, atypical muscle fibers, and ectatic vessels. Angiomyolipomas predominantly affect young women, are usually unilateral, and present as lesions of 2-5 cm in size. They may reach 15cm, however, and these large tumors are often symptomatic (flank pain, hematuria). Lymphangioleiomyomatosis (LAM) of the lung and tuberous sclerosis are associated with an increased incidence of angiomyolipoma (4080%). The lesions in tuberous sclerosis are small, multiple, bilateral, asymptomatic, and can be detected in adolescence. In young patients, however, a macroscopic fatty tumor component may be absent. Malignant transformation is unknown. Local lymph node enlargement and even foci in the liver, spleen, or uterus are a reflection ofm ul ticentric disease rather than metastases. Complications include potentially severe retroperitoneal hemorrhage due to rupture, hypertension, and impaired renal function. The frequency ofcomplications increases with a tumor size of> 4 cm. Because angiomyolipomas can enlarge over time, they may warrant therapy at some point. Tumor growth appears to be greatest in patients with multiple angiomyolipomas, regardless of whether the stigmata of tuberous sclerosis are present. It is not clear yet to which extent followup examinations are required. •
CT Morphology
Fatty components are almost always present, and their CT detection suggests the correct diag-
a
b
Fig. 18.25 Angiomyolipoma. (a) CT demonstrates a large angiomyolipoma with characteristic fatty components and angiomatous soft-tissue components, (b) Multiple, often tiny angiomyolipomas offat density are seen in tuberous sclerosis (caution: these should not be
nosis. It is sufficient to identify even small areas with attenuation values less than -20 HU (Fig. 18.25). In doubtful cases, the accuracy of attenuation measurements can be improved by acquiring thin slices with spiral or multislice CT. The tumor grows by expansion and not by infiltration, but it may rupture its capsule or extend into the renal sinus, causing pyelocaliceal compression. Predominantly extrarenal extension occurs in approximately 25% of cases. The presence of an angiomatous component leads to heterogeneous enhancement. Hemorrhage into the tumor or perirenal space appears as a hyperattenuating area. Problems of differential diagnosis may arise if the diagnostically important fatty component of the tumor is very small or in rare cases absent (Fig.18.25c). This can make the angiomyolipoma indistinguishable from renal cell carcinoma. Very rarely, a renal cell carcinoma or nephroblastoma may contain a small amount of fat. Renal lipomas and liposarcomas are extremely rare. Care should be taken not to mistake adipose tissue of the renal sinus engulfed in a renal carcinoma as the fatty component of a tumor suggesting an angiomyolipoma. The combined presence of fat and calcifications is suspicious for carcinoma since angiomyolipomas very rarely contain calcifications. Angiomyolipomas never show an extension into the renal vein.
c confused with small cysts), (c) Large angiomatous component showing intense, nonhomogeneous enhancement in an adolescent patient with tuberous sclerosis (mimics renal cell carcinoma).
663
664
18 The Kidneys
Other Mesenchymal Tumors Fibromas, lipomas, hemangiomas, hemangioendotheliomas, leiomyomas, and histiocytomas are very rare « 1 %) and are generally small,
CT Morphology Only lipoma can be identified by its negative attenuation values and the absence of a softtissue component, although it may be mistaken for a much more frequent renal angiomyolipoma. All other tumors display nonspecific morphologic features on CT scans,
Malignant Solid Tumors Renal Cell Carcinoma (Hypernephroma, RCC) The clinical hallmarks of renal cell carcinoma (RCC) are hematuria (60%) and Hank pain (50%), Approximately one-third are discovered incidentally, Males are affected with a 3: 1 predominance, and the peak incidence is after 50 years of age, The risk of RCC is increased in Hippel-Lindau syndrome (35%), in tuberous sclerosis, and after years of dialysis (6%), The tumors in Hippel-Lindau syndrome are bilateral in 75% of cases and multicentric in more than 85%, Tumor histology is a more important prognostic indicator than tumor size, Hypovascular tumors (5-15%) have a better prognosis than the commoner hypervascular forms, The majority of renal cell carcinomas are of the clear cell subtype (70%), that are mostly (80%) hypervascular; 10-20% of them are cystic. Papillary cell type tumors (10-15%) or chromophilic cell carcinoma « 5 %) are hypovascular and resemble cortical cysts (papillary type 1), lymphoma or metastases (pa pillary type 11), CT is more than 90% accurate in the detection and staging ofRCC CT Morphology RCC typically causes distortion of the renal contours, It is approximately isoattenuating to the renal parenchyma on non contrast scans, but it may also contain hyper- or hypoattenuating areas (due to hemorrhage or necrosis), Fatty components are very rare and may make the tumor difficult to distinguish from angiomyoltpoma: Less than 20% of the tumors contain central or peripheral curvilinear calcifications, The likelihood of calcifications increases with size.
The more common hypervascular tumors show intense enhancement during the early arterial phase after contrast administration (Fig, 18,26), During the parenchymal (nephrographic) phase, RCC is always hypoattenuating to the renal parenchyma regardless of its vascularity (Figs, 18,26, 18,27), Larger tumors with necrotic components show heterogeneous enhancement An indistinct margin between the mass and the renal parenchyma is suggestive of malignancy, This, however, is only common in large tumors, Over 80% of small renal tumors are well demarcated, some of them even showing a pseudocapsule consisting of a thin low-attenuation border. Demonstration of the exact localization of the tumor within the renal parenchyma, its size and relation to the renal vessels is essential prior to nephron-sparing surgery (partial nephrectomy), and can be best accomplished using volume-rendering techniques (Fig, 18,28), The preoperative staging ofRCC (Table 18,9) is based on the invasion of adjacent structures, lymph node status, and the involvement of venous structures, Tumor thrombi are often in contact with appositional thrombi that may extend into the vena cava and right atrium, Table 18.9 T1 T2 T3
TN Staging
to the kidney
> 7 em, confined
to the kidney
Perirenal
invasion
inferior vena T4
Extension
N1
Solitary
node
N2
Solitary
node
<Scm N3
criteria for renal cell carcinoma
7 em, confined
1\
>5cm
and/or
involvement
of the
cava or renal vein
through
Smm) and asymmetric contraction of the anterior wall. Calcifications are typical and suggest the correct diagnosis. The incidence of neoplastic disease is increased following schistosomiasis. Tuberculosis also causes irregular bladder shrinkage. and calcifications are a common finding. Tuberculous bladder abnormalities are always accompanied by changes in the kidneys and ureters. Radiogenic cystilis is caused by irradiation to the bladder. and chemically induced cystitis may follow the local instillation of chemotherapeutic drugs or may follow systemic cyclophosphamide therapy due to breakdown products that damage the bladder mucosa. These forms of cystitis are characterized by irregular thickening of the bladder wall and uniform bladder shrinkage. Cystitis cystica (cystitis glandularis. or follicularis) is a rare proliferative form that develops in response to chronic irritation. It is character-
ized by thin-walled cysts of water density of 1-10 em diameter. Cystitis cystica is considered a premalignant lesion and is associated with an increased incidence of adenocarcinoma. A rare inflammatory disorder is necrotizing granulomatous cystitis, which occurs in the setting of Wegener's disease and causes nonspecific thickening of the bladder wall.
Malacoplakia Malacoplakia is a rare, postinfectious, matory granulomatous wall thickening responsive to antibiotic therapy.
inflamthat is
CT Morphology
There is a circumscribed nodular or plaque-like area of wall thickening that may contain central necrosis and calcifications. It is indistinguishable from a bladder malignancy on CT scans.
Enterovesical Fistulae Enterovesical fistulae can occur in the setting of Crohn's disease, sigmoid diverticulitis, or colonic malignancy. o
CT Morphology
Enterovesical fistulae are manifested by gas in the bladder lumen. !ffocal bladder wall thickening is present, the chronic inflammatory cause is morphologically indistinguishable from an invasive tumor.
Trauma
Extraperitoneal bladder ruptures are more Approximately 10% of patients with pelvic trauma sustain a bladder injury. The risk of in- frequent (60-75% of major bladder injury) and jury is greater in a distended bladder than an associated with penetrating trauma or fractures empty bladder. CT cystography (usually done as of the pelvis, usually the pubic rami or symphysis. part of a general abdominal CT examination) is It is caused by a penetrating bone fragment or by superior to conventional cystography for detect- other mechanical effects relating to trauma-like ing small bladder extravasations, including ret- shearing forces or stress on the puboprostatic ligaments. The site of rupture can be far removed rovesical collections. It also yields information from the site of the bony fracture. In 5% of major on trauma-associated changes in the extravesibladder injuries a combination of both intra- and cal soft tissues. Symptoms of gross hematuria, pelvic fluid and specific pelvic fractures are extraperitoneal ruptures can be found. highly correlated with bladder rupture and may • CT Morphology guide patient selection. The most important traumatic lesions are Optimum CT evaluation of the bladder requires bladder contusion, intraperitoneal bladder rup- contrast instillation through a Foley catheter. Inture and extraperitoneal bladder rupture. A con- juries to the upper urinary tract should be excluded by conventional urography or CT tusion of the bladder wall is an incomplete tear of the mucosa that produces a localized (delayed scans in the excretory phase acquired > 10 min after IV contrast administration, best intramural hematoma. It is the commonest results with m ul tislice CT). form of bladder injury and is treated conservatively. Bladder contusions show a focal area of hyperattenuation in the bladder wall (Fig.19.11 b) An intraperitoneal bladder rupture accounts for 20-35% of major bladder injuries. It may be and are best visualized on non-contrast or early scans with a full bladder. Intravesical hemorcaused by a sudden pressure rise within the full bladder caused by a forceful external blow rhage presents as a hyperattenuating mass on (often after a long automobile ride). The site of early scans and a filling defect on delayed scans (Fig. 19.11a). In intraperitoneal ruptures contrast predilection is the bladder roof. It is a typical medium is seen in the paracolic gutters and outseat-belt or steering wheel injury, that requires lining small bowel loops or other abdominal orprompt surgical intervention.
a
b
Fig. 19.11 (a) Traumatic bladder tamponade with sharply circumscribed filling defects (blood clots). Small air collections surround the inflated tip of the Foley catheter, (b) Sagittal reformation (4 x 1/6) demonstrating a traumatic bladder injury with intramural (arrow-
c heads) and supravesicular hematoma (arrow) that was hard to detect on axial sections, (c) Impression of the contrast-filled bladder ba large abscesses secondary to pelvic trauma.
19 Urinary Bladder
gans. Extraperitoneai ruptures can be simple or complex. In simple ruptures. extravasation is limited to the pelvic perivesical spaces. In complex ruptures the pelvic fascial boundaries have ruptured and contrast extends into the thigh. scrotum. penis. and perineum. Extravasated
Postoperative
Changes
Cystectomy and Urinary Diversion CT can be performed after radical cystectomy with lymphadenectomy (usually combined with prostatectomy/hysterectomy or with resection of the seminal vesicles/ovaries) to detect or exclude postoperative complications such as hematomas. abscesses, and urinomas (Table 19.5). In a classic urinary conduit procedure, the detached ureters are implanted into a suitable isolated bowel segment (ileum, transverse or descending colon) and the urine is diverted through a percutaneous stoma into an external receptacle. A more modern alternative is the continent urinary diversion, in which part of the bowel is used to construct a reservoir pouch for urine. The various procedures differ in the selecTable 19.5 Diagnostic procedures various surgical complications Complications
procedures
recommended
for
Diagnostic
Postoperative Anastomotic Infection,
leak (urinary
leak)
Cystography,
Hemorrhage
CT CT
formation
Ureteral
CT
CT
abscess
Ischemia Fistula
Cystography,CT
obstruction
Cystography,
complications
Reflux Diurnal
Cystography or nocturnal
Infection Ureteral
in conti-
Cystography Laboratory tests, CT
stricture
Calculi
IVP,CT Cystography,
IVP, CT Metabolic
acidosis
Laboratory
tion of the bowel segment used to construct the bladder substitute, the nature of the anti-reflux and continence-preserving mechanisms, and the mode of urine elimination. In all these procedures, small bowel and/or colon is used to construct a "neobladder" into which the ureters are reimplanted. The procedures are subject to numerous modifications that influence the CT appearance of the reconstruction and should be interpreted in consultation with the urologic surgeon. The presence of a functioning external urethra! sphincter permits the construction of an "orthotopic neobladder" that occupies a normal intrapelvic position. Otherwise the reservoir may be placed in the right lower abdomen and fitted with a cutaneous stoma (usually an umbilical stoma) to allow intermittent catheterization. Immediate postoperative complications are distinguished from late complications. (Voiding) Cystography after contrast instillation, and urodynamic studies are the methods of choice for diagnosing ureteral reflux, incontinence, or poor pouch filling due to excessive pressure within the reservoir. Calculi may develop on metal clips that are exposed to urine. CT is used to detect extravesical changes such as abscesses, fluid collections, and enterovesical orvesicocutaneous fistulae (Table 19.6). CT Morphology
IVP, CT Late
urine in the prevesical space may extend cephalad in the abdominal wall or surround the peritoneal cavity, thereby mimicking intraperitoneal ruptures. Extravesical hemorrhage appears as a hyperattenuating mass that may produce bladder impression or displacement.
tests
The reservoir pouch appears as a fluid-filled structure that may be located in the right lower abdomen or may occupy an orthotopic position in the pelvis, depending on the reconstructive technique used (Fig. 19.12). The bowel should be adequately opacified with oral contrast so that the pouch can be distinguished from functioning bowel. A precontrast examination should be performed to detect hyperattenuating fluid collections and calculi. Delayed scans (» IOmin after
Orthotopic bladder replacement (urinary diversion through the urethra) Carney bladder (historical technique) U-shaped ileal loop with each ureter implanted into one end of the loop; used only in males. Ileal neobladder (Hautmann procedure) Reservoir constructed ofdetubularized* terminal ileum Studer bladder Reservoir constructed ofdetubularized* ileum with the (shortened) ureters implanted into a cranially transposed ileal loop
a
Hemi-Kock pouch Reservoir constructed ofdetubularized* ileum, using intussusception (ileal nipple) as an anti-reflux mechanism Orthotopic Mainz I pouch Reservoir constructed of cecum and terminal ileum (ileocecal pouch) Heterotopic bladder replacement (urinary diversion through a stoma) Mainz I pouch Ileocecal pouch, usually with an appendiceal umbilical stoma Kock pouch Ileal pouch with a continence and anti-reflux nipple, often combined with an umbilical stoma Indiana pouch Ileocecal pouch
b Fig. 19.12 Kock pouch in the right lower abdomen with an umbilical stoma (a). Ileum conduit (4x1/6 VRT). The arrows mark the anastomoses ofthe ureters to the conduit. Note the contrast remnants in colonic diverticula (arrowheads) following biliary contrast excretion after an CT urogram (b).
Ureteral implantation into the bowel (urinary diversion through the rectum) (historical technique) Implantation of the ureters into the sigmoid colon
Coffey operation
Mainz II pouch Implantation of the ureters into a reservoir constructed of sigmoid colon and rectum * Detubularization = incising the bowel longitudinally and suturing it to create a reservoir
IV contrast administration) should also be obtained to evaluate the supravesical urinary tract, including the sites of ureteral reimplantation in the pouch. If a fistula or urinary leak is suspected, the pouch should be scanned after percutaneous instillation of contrast material. Intravesical gas is a normal finding after percutaneous catheterization and should not be
mistaken for an abscess. However, extravesical gas collections, like fluid collections, are always abnormal (unless scanning is performed in the immediate postoperative period) and may signify hemorrhage (> 2OHU), urinary leak «20HU, contrast extravasation), or infection with abscess formation (20-40 HU). Recurrent Tumor
MRI is superior to CT for differentiating recurrent tumor and postoperative fibrosis, but it is less effective in differentiating acute radiationinduced changes, tumor, inflammation, and infection. A CT examination to detect tumor recurrence should cover the abdomen and pelvis including the perineum.
19 Urinary Bladder
CT Morphology
Adequate bowel opacification must be obtained in the pelvis to ensure that coils of small intestine are not mistaken for recurrent tumor. A new (enhancing) soft-tissue mass that is detected in the bladder bed more than 3 months after surgery is suspicious for a recurrent tumor (Fig. 19.13). Some 70% of cases with local recurrences also have enlarged pelvic lymph nodes. On the other hand, more than 50% of postoperative patients are found to have abnormally enlarged pelvic lymph nodes that are not associated with a local recurrence. Consistent with the lymphatic pathway that drains the bladder, enlarged para-aortic retroperitoneal nodes are generally associated with enlarged pelvic nodes. There have been reports of local tumor recurrence in the pelvic floor involving only the perineum or vagina and appearing on CT as an isolated thickening of the anterior rec-
Fig. 19.13 carcinoma.
Urethral recurrence
(arrows)
of a urothelial
tal wall or anterior vaginal wall. As with a recurrence of rectal carcinoma, a recurrent bladder carcinoma may directly infiltrate the bony pelvic ring.
20 Female Pelvis K. J. Lehmann, A.
J. van der Molen
Anatomy Examination Technique Anomalies ofthe Uterus and Vagina Predominantly
organs and should be used for assessment of pathologic structures. The first imaging modality that is used after the physical examination of the pelvis is ultrasound, preferably using an transvaginal probe. In diseases of the female pelvic organs, such as the staging of uterine tumors, MRI is increasingly being used as the first-line modality, although multislice CT techniques may yield similarly good results in patients with more advanced diseases. CT scanning of the female pelvis is usually performed for reasons other than primary gynecologic indications (Table 20.1).
CT is not among the first-line procedures that are used in the diagnosis of female pelvic diseases. Unlike ultrasound and MRl, CTcannot define the structures of the uterine wall, particularly the endometrium and its changes during the menstrual cycle. On the other hand, the pelvis is often included as part of an abdominal CT study, and therefore knowledge of normal and pathologic appearances of the pelvic structures is essential. Similar to MRl, the multiplanar imaging capabilities of spiral or multislice CT (sagittal and coronal views) are most helpful for delineation of the interfaces between the pelvic
Anatomy The uterus is covered by peritoneum, which anteriorly continues into the peritoneal covering of the bladder roof. Posteriorly the peritoneum descends to the dorsal vaginal fornix and then along the anterior wall of the rectum, forming the posterior cul-de-sac (pouch of Douglas) . The vagina appears on CT scans as a transverse oval structure of soft-tissue density located anterior to the rectum and medial to the levator sling (Fig. 20.1). Unless a vaginal tampon has been inserted, the vaginal lumen, and the anterior and posterior fornices, usually cannot be visualized with CT, nor can it be demarcated from the cervix. However, after rapid injection of contrast material the vaginal mucosa often enhances and can be better appreciated if no tampon is present.
The cervix displays uniform soft-tissue density, is sharply demarcated from the surrounding fat, and superiorly continues into the uterine body (Fig. 20.1). The shape, size, and position of the uterus are variable and are influenced by many factors including age, hormonal status, pregnancy, the presence ofleiomyomas, and the degree of bladder distension. Viewed in axial section, the cervix of a woman of childbearing age normally measures no more than 3 em in diameter, and the uterine body measures no more than 5 cm. The uterine cavity is delineated as an area of low attenuation only if it contains fluid (e.g., in the perimenstrual period). Although the cervix can be hard to distinguish from adjacent portions of the uterus and vagina (even if a tampon is inserted), they nevertheless can be sepa-
Anatomy Fig. 20.1 Cross-sectional anatomy of the uterus, ovaries, parametria, and vagina.
rated from one another by their configuration: the uterine body usually is somewhat triangular in shape, whereas the cervix has a more rounded appearance. The broad ligament is formed by two layers of peritoneum, which drape the uterus and extend laterally from the uterus to the pelvic sidewall. The superior free edge is formed by the fallopian tube medially and the suspensory ligament of the ovary laterally. The lower edge of the broad ligament ends at the cardinal ligament (Fig. 20.2).
The parametrium is a broad sheet of loose extraperitoneal connective tissue, smooth muscle, and fat that extends laterally from the uterus to the pelvic wall between the layers of the broad ligament. The parametrium contains the fallopian tubes, and the ovarian and cardinal ligaments. In addition, it contains uterine and ovarian vessels, nerves, and lymphatic vessels and a portion of the ureter that runs 2 cm lateral to the cervix through the parametrium. Although the broad ligament is rarely seen unless
701
20 Female Pelvis
~ ~
Uterine round ligament Suspensory of ovary
ligament
Fig. 20.2 Schematic diagram of the parametria, viewed from the posterior aspect.
Uterine tube Ovary Broad ligament Ureter Cardinal
ligament
Uterosacral
ascites is present, its position can be determined by structures that it contains or abuts. The only structures that can be consistently visualized with CT in this region are the principal structures of the uterine ligaments. The uterine round ligament is a band of fibromuscular tissue that passes anterolaterally forward and downward from the lateral angle of the uterus to the internal inguinal ring. It appears on CT as a band of soft-tissue density that gradually tapers from its broad base at the uterus (Fig. 20.1). The cardinal ligament (lateral or transverse cervical ligament, Mackenrodt ligament) passes laterally from the cervix and upper vagina to the fascia overlying the obturator internus muscle (Fig. 20.2). It forms the base of the broad ligament but cannot always be defined over its entire length in CT. The uterine artery runs along its superior aspect. The cardinal ligaments represent the border between the parametrium and the paravaginal tissue [paracolpos). There is a broad variation in the anatomic configuration of the cardinal ligament with respect to thickness, shape and contour. The uterosacral ligament extends laterally and posteriorly from the cervix to the sacral bone and encloses the rectum. The vesicouteral ligament extends laterally and anteriorly and encloses the dorsal wall of the bladder. It is
ligament
common for the uterine ligaments to appear thickened following a course of radiotherapy. The ovaries cannot always be identified as discrete structures on conventional CT scans but they are almost always seen with multislice scanning. They are located anterior to the ureters and posterolateral to the uterus in the ovarian fossa. The origin of the Fallopian tube and the ovarian ligament (between the uterus and the ovary) is usually visible at the lateral angle of the uterus and provides an additional anatomic landmark for identifying the ovary. The ovaries may be of uniform soft tissue density. More commonly, however, the ovaries contain soft tissue stroma admixed with small cystic areas representing normal follicles. The size of the follicles varies with the menstrual cycle. Just before ovulation they may be as large as 2.5 cm. The uterine vessels pass from the internal iliac artery and vein to the inferior border of the broad ligament and cross in front of the ureter before forming the uterovaginal vascular plexus, which can be identified after IV contrast administration. The paired ovarian arteries (which arise directly from the aorta) can be visualized only on arterial phase multislice CT scans, but the ovarian vein can be identified with most techniques on the psoas muscle lateral to the ureter, especially during the postpartum period or if there is varicose dilatation.
Examination
Technique
Patient Preparation
Acquisition
Adequate oral contrast opacification is essential to avoid mistaking nonopacified bowel loops for pelvic masses or fluid collections. The fluid absorbed from oral administration of 10001500ml of an iodine- or barium-containing solution also distends the bladder. displacing the coiled loops of small intestine upward and shifting the uterus to a more upright position. It is important to have the patient drink the oral contrast agent continuously. spread over a period of at least 45 minutes. If scanned too early the distal small bowel loops will often not yet be opacified. while drinking the contrast agent all at once may result in non-opacification of proximal loops. Full bowel distension usually is not necessary because a small amount ofintraluminal contrast already identifies bowel structure. With multislice CT scanning. negative contrast agents such as water or methyl cellulose preparations may be advantageous for better display of the vasculature but this risks failing to distend distal loops. which makes it harder to distinguish them from pelvic masses. With thinsection multislice scanning. however. bowel loops can be followed and thus be distinguished from masses, even if distension is suboptimum. For optimum display of the pelvic structures, a moderate distension of the bladder is ideal. Patients should therefore be asked to urinate about 20-30 minutes before the scan. A moderate amount of rectal contrast (approx. 100ml of 5-10% diluted CM) is advantageous. However, an overdistended rectum leads to compression of the pararectal fat and can make the scans difficult to interpret. Peritoneal instillation of 1000-2000 ml of diluted (1: 20) sterile nonionic contrast agents is not a standard procedure but may be used to optimize staging and diagnosis of recurrence of ovarian cancer. Prior to intraperitoneal chemotherapy such an approach may help determine which portions of the peritoneal cavity are reached by the chemotherapeutic agent when applied via a intraperitoneal catheter.
Thin slices should be used for selective helical CT examinations of the uterus and adnexa (Table20.2). although a 4x2.5 mm collimation will usually suffice for 4-slice scanners. For optimum evaluation of local tumor invasion, however, multislice scanning with 0.75-1.25 mm sections (4- to 16-slice scanners) is advisable to obtain a secondary raw data set from which thicker axial sections (3-7 mm, to improve signal-to-noise ratio) and coronal sections of 2-4 mm thickness as well as sagittal sections of 3-6 mm can be reformatted. The variations in section thickness are required because of the special noise characteristics in the pelvis. Image noise in the pelvis is anisotropic because absorption varies with the rotational angle of the x-ray tube; it is higher for lateral projections than for sagittal projections. The appearance of noise on the scans, therefore, will not be punctuate but instead slightly needle-shaped, oriented in a horizontal direction. Software filters that average raw data sets from high-absorption views (either in-plane or along the z-axis also) can improve image quality substantially and may even allow reduction in dose. They are available for spiral CT and are currently being developed for multislice CT. Another technique is adaptive dose modulation in which the mA settings of the x-ray tube are continuously varied and adapted to the higher requirements in the lateral projections and the lower requirements in AP directions. This technique hold the potential for up to 30% dose reduction at comparable image quality. With primary staging of known malignancies, however, dose is only a minor issue. For this indication, radiation exposure should be chosen high enough for optimum image quality. Especially with obese patients this requires choosing 140 kVp for better penetration, thicker sections (e.g., 4 x 2.5-4 x 3.75 mm for 4-slice scanners), and the highest possible mAs setting. Low pitch factors for 4-slice scanning (P" = 3) make it possible to use higher mAs settings in very obese patients. For spiral CT or dual-detector CT. a 360' LI interpolation with a pitch of 1 (P*=2 for dual detectors) substantially improves signal-to-noise ratio (equivalent to increasing mAs settings by a factor of two) but at the expense of a 30% wide section profile.
Technique
704
20 Female Pelvis Table 20.2
Suggested protocols for CT scanning of the female pelvis
General Oral contrast Rectal contrast
1000-1500 ml positive contrast, administered
Vaginal contrast
100 ml (optional) Insertion of vaginal tampon (optional)
Patient position
Supine with elevated arms Pelvis from iliac crest to below ischiac bone
Scan range Respiratory
60-90 min before examination
phase
Windowing Scan parameters
Inspiration (shallow respirations is also possible) Contrast-enhanced
SC
t
2-3
5/10/5
Small pelvis
3/5/21
Contrast injection
l-slice
Small pelvis Infectous disease Tumor staging d
W/L~400/70
4-slicea
l-slice SC/TF/RI
Routine (whole abdomen)
Routine (whole abdomen)
CT:
i
1-1.251
16-slice" SC 1-1.5
t
0.75-1.251
axial SW/RI
MPRb SW/RI
5/4
4/3
3/3 cor 4/4 sag
V/F/D
4-slice V+N/F/D
16-slice V+N/F/DX'
120/2/60
120 + 50/3/25
120 + 50/3/155
120/2/80 120/4/40
120 + 50/2/80 120 + 50/5/40
120 + 50/2/90 120 + 50/5/20A
SC= slice collimation (mm), TF
2cm) or one containing areas oflow attenuation is suspicious of malignant transformation.
Benign Lesions Benign lesions of the testes and prostate are not an indication for CT, but they are frequently encountered as incidental findings. MRI and ultrasound are clearly superior to CT in differentiating intratesticular from extratesticular lesions and in discriminating solid from fluid structures.
Calcifications Calcifications are a common incidental in the male reproductive tract.
Table
resection
finding
CT Morphology
Calcifications in the spermatic cord, seminal vesicles, or prostate may have a degenerative or inflammatory etiology (Table 2 1.3). Small calcifications in the prostate (up to the size of a grape seed) usually result from nonspecific lithiasis in prostatic secretions and are an innocuous incidental finding (Fig.21.2a). Coarse prostatic calcifications may have an inflammatory etiology, occur in focal benign prostatic hyperplasia (BPH), or may develop following a transurethral resection (TUR) of the prostate
(Fig.21.2b). Calcifications in the seminal vesicles are seen in diabetics or following tuberculosis. Stones may develop rarely in the seminal vesicles in patients with urolithiasis and may be of variable size. Small calcifications lateral to the prostate usually represent phleboliths in the periprostatic venous plexus (Fig.21.2b). Small calcifications may be seen in the testis (microlithiasis) and this is believed to be a risk factor for developing malignant disease. Epididymal calcifications are found in 3% of males.
Cystic Prostatic Lesions
Fig. 21.2
Prostatic calcifications.
(a) Calcifications in the ejaculatory ducts in a patient with BPH. (b) Follow-up scan in a patient who underwent previous TU R of the prostate, which yielded malignant cells. CT demonstrates coarse, nonmalignant-type prostatic calcifications in addition to phleboliths.
a
Benign Prostatic Hyperplasia
Benign prostatic hyperplasia is a very common incidental finding in older males and leads to enlargement of the central part of the gland. BPH represents a proliferation of glandular or interstitial elements in the transitional zone and is frequently accompanied by benign prostatic calcifications ("prostate stones"). Focal changes may lead to nodules or adenomyomata. The stage of the disease is determined by clinical symptoms and not by the size of the prostate or the degree of bladder floor elevation. CT is not used in staging BPH, but prostatic enlargement is commonly seen in pelvic CT studies. •
CT Morphology
BPH is manifested on CT by prostatic enlargement (transverse diameter> 5 ern) that is indistinguishable from prostatic adenoma. The peripheral glandular tissue typically not hyperplastic is demarcated as a hypodense zone after
b
contrast administration (Fig. 21.3a). With faster contrast material injection, hypoattenuating and hyperattenuating nodules may be found in the prostate (Fig.21.3b). There are no data yet that correlate these findings with histology. A sufficiently large prostate may form a curved impression on the bladder floor or may lead to subvesical obstruction. The prostate is still sharply delineated with respect to surrounding fat and the levator sling. Cystic Prostatic Lesions
Cystic prostatic lesions are an occasional incidental finding on CT scans. They are often congenital and are evaluated by ultrasonography and if necessary by percutaneous aspiration. Utricular cysts arise from a dilatation of the utricle and are frequently associated with other anomalies of the genitalia. They are 8-10 mm long and communicate with the posterior
Fig. 21.3 Prostatic hyperplasia. (a) The transitional zone of the prostate appears hyperattenuating after contrast administration. The lateral, peripheral part of the gland is defined as a hypoattenuating area on either side. (b) Hypervascular focus in a patient with normal size gland and normal PSA levels. The CT morphology is highly suspicious for an early prostate cancer. Biopsy should be considered. a
b
27 Male Pelvis
urethra. Mullerian duct cysts are remnants of the muellerian duct system and not associated with other anomalies. Patients may present with urinary retention, infection, or stone formation. There is a stalk to the veromontanum and an increased incidence of carcinomas. Ejaculatory duct cysts are rare. They can be congenital or postinflannmatory and result from obstruction. A vas deferens cyst is rare and involves the ampulla (Table 21.4). •
CT Morphology
Utricular cysts lie in the midline, are small and usually teardrop-shaped. Mi111erianduct cysts
are midline, retrovesical, and larger than utricular cysts. Calcification or stones may be seen. Ejaculatory duct and vas deferens cysts are paramedian. CT is not accurate in the differentiation of these cystic prostatic lesions (Fig. 21.4). Testicular cyst
Intratesticular cysts are rare lesions that are usually solitary. They have idiopathic, postinflammatory, or post -traumatic etiologies. They customarily measure 5-7 mm, are often located adjacent to the rete testis, and are often accompanied by cysts in the epididymis. Tunica albuginea cyst make up 10-20% and are peripherTable21.4
Fig. 21.4 CT following open enucleation adenoma demonstrates a hypoattenuating site of the former adenoma.
of prostatic area at the
ally located. Tubular ectasia of the rete testis a normal variant seen in older men, and often bilateral. It is usually associated with spermatocele or epididymal abnormalities. Epidermoids are rare solitary cystic lesions in the testes that can be neoplastic in 1%. Differentiation from other germ-cell tumors is not possible and therefore epidermoids are resected. •
CT Morphology
These are seen as simple well-defined cysts with fluid -like contents in the testicular parenchyma
Cystic prostatic lesions
Congenital Utricular cyst
Usually 50000ml
AFP
ng/ml;
Intrapelvic and inguinal lymph nodes are considered regional after scrotal or inguinal surgery
anyT anyT
anyT
anyN
anyT
anyN
anyT
anyN
IGCCCG (1997)
MO MO MO MO MO MO M1 M1 M1
prognostic
classification
SO SO
anyS SO-I SO-! SO-I SO-I 52 53 for germ-
T esticularlretroperitoneal primary Pulmonary metastases AF P 20% of the nodal diameter) helps to distinguish them from lymph node metastases due to squamous cell carcinoma. Chronic nodal changes consist of flocculent or homogeneous calcifications.
Other Inflammatory Diseases Enlarged lymph nodes may be seen at the port of entry of the infectious organism or in nodal groups that drain the affected organ. The goal of CT is to differentiate these lymph nodes, which are usually small, from tuberculous and metastatic nodes. CT Morphology
There are no typical morphologic criteria that can differentiate among the various bacterial and viral infections and the various fungal and parasitic diseases. Bacterial pneumonia is usually not associated with mediastinal lymphadenopathy, whereas pneumonias caused by viruses, mycoplasma, or fungi, often present with bihilar adenopathy. Infections with mycoplasma and Pneumocystis cetinii often lead to conspicuous calcifications in the enlarged mediastinal lymph nodes. Other, noninfectious inflammatory changes (e.g., Crobn's disease) and surgical procedures (e.g., thoracotomy) can also lead to mediastinal lymph node enlargement. There is a tendency for lymph node enlargement in patients with
congestive heart disease.
Histoplasmosis Histoplasmosis is a fungal disease that is endemic in central North America (Ohio and Mississippi river valleys) and which has a high prevalence in Middle and South America. The acute or subacute pulmonary form may be associated with generalized mediastinal and hilar lymphadenopathy, while adenopathy is also seen in the disseminated form in patients who are immunocompromised, such as in AIDS. Delayed manifestations include mediastinal granulomas, histoplasmoma, or granulomatous (fibrosing) mediastinitis. CT Morphology
Nonspecific hilar or mediastinal lymph node enlargement or a paratracheallsubcarinal mass with necrosis can be seen. Large nodes may contain popcorn calcifications. Target lesions with central calcifications are pathognomonic for the acute form. Contrast enhancement is often irregular. The mediastinal granuloma manifests as a inhomogeneous, lobulated mass with a thick capsule and irregular septa.
Lymph nodes with inflammatory changes usually appear homogeneous following intravenous contrast administration. However, central liquefaction in lymph nodes, which is common in staphylococcal infections, can lead to central low attenuation with peripheral enhancement.
Cystic Fibrosis Extensive, mediastinal or hilar lymph node enlargement is noted in up to 50% of patients with cystic fibrosis. It should not be mistaken for lymphoma. In nearly all cases, this adenopathy is chronically progressive.
Sarcoidosis The goal detecting mine the tinguish nopathy.
of CT scanning in sarcoidosis, besides intrapulmonary changes, is to deterextent of lymph node changes and disthem from other types of lymphade-
Malignant
•
CT Morphology
Scans usually demonstrate hilar and mediastinal lymphadenopathy chiefly involving the paratracheal and preaortic nodes. Coarse flecks of calcifications develop in the enlarged nodes during the healing stage. Symmetrical hilar involvement is typical and distinguishes sarcoidosis from tuberculosis, which usually shows a unilateral or at least asymmetric pattern of involvement. In cases where the changes are predominantly mediastinal and do not involve the lungs, the differential diagnosis should include malignant lymphoma. This is a likely diagnosis in cases where conglomerate masses have formed. In approximately 75% of cases that have progressed to a chronic stage, enlarged intra-abdominal lymph nodes are also found. Contrast administration does not reveal any changes characteristic of sarcoidosis.
Lymphoma
sis is made from the history and from typical CT findings in the lung. The differential diagnosis should include other forms of pneumoconioses. Asbestosis, for example, is also associated with mediastinal lymphadenopathy but does not lead to eggshell calcifications.
Amyloidosis Amyloidosis is a rare systemic disease characterized by the deposition of amyloid in extracellular tissues. The average age at diagnosis is 55 to 60 years, and males are predominantly affected. A distinction is drawn between primary and secondary forms. The latter is most often secondary to plasmacytoma, rheumatoid arthritis, tuberculosis, or familial mediterranean fever. CT demonstrates a broad spectrum of changes in all organ systems. CT Morphology
Silicosis The goal ofCT scanning in silicosis is to differentiate the affected lymph nodes from other forms of lymphadenopathy. •
CT Morphology
CT demonstrates enlarged hilar and mediastinal lymph nodes with typical fiocculent (eggshelllike) calcifications. Contrast administration does not produce characteristic changes. The diagno-
Malignant
Lymphadenopathy, manifested by nodal enlargement and occasional mottled calcifications, is found in 20% of patients. The enlargement predominantly affects the mediastinal and abdominal para-aortic groups of nodes. The differential diagnosis includes sarcoidosis, silicosis, post-therapeutic lymphoma, and granulomatous diseases. The lymph nodes usually show a nonhomogeneous pattern of contrast enhancement.
Lymphoma
CT has become the imaging modality of choice for the detection, staging, and follow-up of Hodgkin's disease and non-Hodgkin's lymphoma (NHL). Although Hodgkin's disease and NHL have identical CT morphologies, their distribution patterns are dissimilar. A detailed outline of the features that differentiate Hodgkin's disease from NHL is presented in Table 22.11. NHL tends to have a generalized distribution while Hodgkin's lymphoma is often localized. The staging of Hodgkin's disease is based on the number of nodal groups affected, their location in relation to the diaphragm, and the involvement of the spleen and extralymphatic organs (Table 22.12). NHL is staged in similar fash-
ion, except that a distinction is drawn between primary nodal and primary extranodal patterns of involvement. Today, NHL is classified according to the WHO classification published in 1999, which except for minor differences is in agreement with the REAL classification defined in 1994 to unify different European and American classification schemes. Accordingly an attempt was made to standardize the CT morphologic criteria for assessment of therapeutic effects (Table 22.13). This approach, however, uses the long-axis instead ofthe short-axis diameter of the nodes.
756
22 Lymphatic System Table 22.11 Differences between Hodgkin's and non-Hodgkin's lymphoma (NHL)
disease
Table 22.13 Criteria for response to therapy of malignant lymphoma (Cheson 1999)
Involvement
Hodgkin's disease
NHl
Complete remission (CR)
Mediastinum
85%
350 mg/ ml iodine) to improve enhancement at a given fiow rate and contrast volume. More highly concentrated contrast material, however, will be more viscous and always requires the use of a saline chaser in order not to lose contrast material in the injection veins towards the end of the scan. In addition, there will be major high-contrast streak artifacts in the right ventricle (see Fig.23.16b) which may seriously impair evaluation of the right coronary artery. For this reason, we suggest using a standard concentration of 300mg/ml iodine and adjusting the contrast volume and flow rates as needed. Scan duration with current EBCT and 4-row multislice scanners is in the range 30-45 s. It is substantially reduced with 8- and 16-slice scanners. A scan duration of 30-45 s requires an injection time that is in the same range. For this
reason, 150-200 ml of contrast material will have to be injected if a flow rate of 4mlls is to be maintained. Some authors therefore suggest lower flow rates in the range 2-3 mlls, which require contrast volumes of only 90-140 ml but also yield lower vascular contrast (see Table 23.7). Bolus timing can use either bolus triggering techniques or a test bolus injection. Bolus triggering should use the ascending aorta for a trigger region with a threshold of 50 HU and a trigger delay of 5 s. Bolus triggering has the advantage of being fast and quite reliable. Test bolus injection using 10-15 ml of contrast material followed by a 2Il ml saline flush at 4 mlls has the advantage that the actual scan delay can be tailored more precisely. We suggest extending the time to peak by 20% and using this figure as a start delay to compensate for the slower rise of contrast enhancement with reduced cardiac output, which would result in a prolonged timeto-peak of the enhancement curve. In addition, the maximum enhancement during this bolus test gives an indication of cardiac output (higher enhancement is due to less dilution and therefore a reduced cardiac output) and may help decide about the flow rate and contrast volume (lower flow rates with higher test bolus enhancement) . Our suggestion is to limit the total amount of contrast given to 2.5 mllkg body weight (0.75 g iodine/kg BW) if renal function is not impaired. Proper hydration of patients (e.g., oral fluids on the day of the examination) is advisable. The duration of contrast material injection should match the scan duration. We suggest a flow rate of 4 mlls, which should be altered depending on the scan duration (lower with longer duration), the size of the patient (higher rates with larger patients), and-if availablemaximum enhancement during test bolus injection (lower rates and lower contrast volumes with higher enhancement). Cardiac
Morphology
Compared to coronary CTA, a lower but more constant enhancement level through the heart is required for the evaluation of cardiac morphology. Monophasic contrast material injection is not ideally suitable for evaluating cardiac morphology because it leads to a gradual increase in intravascular contrast over time, until a maximum is reached and enhancement
777
23 Heart
rapidly decreases thereafter. This effect will occur at different points in time for the right and left side of the heart and will therefore make homogeneous enhancement of the heart chambers difficult, if not impossible, In addition, there is a rapid contrast uptake in the myocardium, which will decrease contrast between lumen and wall as the scan progresses, Biphasic injection protocols are more suitable for overcoming this problem, To reduce the high-contrast streak artifacts in the right atrium, contrast material concentration may be reduced while maintaining the flow rate during the second phase of contrast injection, This is cumbersome with single barrel injectors but is easily possible with double-barrel injectors by simultaneously injecting contrast material and saline at suitable flow rates, To maintain enhancement in the right side of the heart the injection time has to be prolonged by the right heart transit time of some 5-15 s. depending on cardiac output Because cardiac output is hard to estimate, we suggest determining the required injection time by increasing it over the scan duration by 10-15 s. The first phase of the biphasic injection should last some 10-15 s. A flow rate of 2,5-3 mlls is generally sufficient The duration of the second phase is calculated from the injection time, and a flow rate of 1.5-2 mlls is used, A saline chaser bolus of some 30ml is advisable but not mandatory because of the relatively low flow rates (Table 23,7), Imaging Tasks Cardiac Morphology
For the assessment of cardiac morphologic, ECG synchronization is necessary as soon as the ventricles have to be evaluated, There are not yet definitive recommendations as to which triggering to use but setting the trigger at 50% of the RR interval appears to be well suited for multislice CT and 70-80% for EBCT, given the heart rate is below 70 bpm. For the aorta, pulmonary artery, and the pulmonary veins ECG synchronization is usually not necessary, which means that single slice and multislice spiral CT provide good results, They are even preferable to ECG-synchronized techniques because radiation exposure is lower. Suggested protocols are given in Table 23.7, ECG triggering using EBCT or multislice CT is the preferred technique for evaluating the
morphology of the heart chambers, including the assessment of ventricular aneurysms, cardiac thrombi, cardiac tumors, and congenital heart disease, Suggested parameters are also provided in Table 23.7, Coronary Calcium Scoring
Since CT is very sensitive at detecting high-density structures, numerous techniques for the detection and quantification of coronary calcium have been suggested that range from non ECCsynchronized single slice scanning to ECG-gated multislice scanning and ECG-triggered EECT scanning. EBCTis the classical technique for coronary calcium scoring, The single slice mode with 100 ms scan time and 3 mm collimator setting is chosen, Data are acquired during every sequential heartbeat with an ECG trigger set at 40-80% of the RR interval. The standard was considered to be 80% but recently the 40% setting was shown to provide the best reproducibility, Images are reconstructed with a 512' matrix and a field-of-view (FOV) of220-260mm, Single slice spiral CT scanning without ECG gating can demonstrate coronary calcifications but the reproducibility is less than with ECG synchronization, In addition, it may be hard to rule out minor calcifications as a sign of developing atherosclerosis and potentially unstable plaques, Non-ECG synchronized dual slice spiral CT is a technique that has been widely studied for coronary calcium scoring, In principle, the same problems as with single slice spiral CT apply, Suggested parameter settings are also provided in Table 23.7, ECG-triggered multislice CT scanning should be the standard for multislice CT evaluation of coronary calcium, For this indication, a trigger level is set at 50% RR interval (may vary with higher heart rate), Most reproducible results are obtained if the heart rate is below 70 bpm. Beta-blockers are generally not applied but their use may improve reproducibility of measurements. Retrospective ECG-gating can further improve reproducibility with multislice CT scanning, Low dose settings are available, but the dose is still higher than with ECG-triggering techniques (by a factor of2 to 3), ECG-gating has also been suggested for single spiral CT scanning, The coronary calcium scores correlate well with those from EBCT for a threshold of 130 HU
Table 23.7
Suggested protocols for CTscanning of the heart
General Patient selection
No coronary CTAin patients with severe arrhythmias or frequent extrasystolic beats No coronary CTAin severely dyspnoic patients Consider MRAin young individuals or children (radiation exposure)
Preparation
Avoid caffeine, atropine, nitroglycerine Proper hydration (orally or intravenously) if coronary CTAis planned Beta-blockers for coronary CTAif heart rate> 70 b[)m
Oral contrast
None
Patient position
Supine with elevated arms
Scan range
Always as short as possible, tailored to the clinical problem (see Table 23.3)
Field of View
Standardized, e.g. 26 cm
Respiratory phase
Inspiration
Window setting
Calcium scoring: CTangiography:
Scan parameters
EBG SCfTF
W/L=400/40 W/L= 500/150 (adjust as needed) 4-sliceG SCfTFa
SW/RI
16-sliceG SCfTF'
SW/RI
Calcium scoring. cardiac morphology No ECGsynchronization Prospective ECGtriggering Retrospective ECGgating C
b
3/3 t
2-3/11-16 t 2-3/8-12 t 2-3/2-4 t
3/3 3/2 3/2
1-2/23-46 1-2/16-32 1-2/4-8/1-2
t t
b
3/3 t
1-2/6-12 t 2-3/8-12 t 2-3/2-4 t
1/0.7 3/2 3/2
0.5-1/8-16 t 1-2/16-32 t 1-2/4-8/1-21
1-2/0.7 2/1 1-2/0.7
b
3/2 t
1-2/4-8 t 1-2/1.3-2.5
2/1 1-2/0.7
0.5-1/8-16 1 0.5-1/2-5 1
1-2/0.5-1 0.5-1/0.3-0.7
t
2/1 2/1 1-2/0.7
Bypass grafts No ECGsynchronization Prospective ECGtriggering Retrospective ECGgating C CoronaryGA Prospective ECGtriggering Retrospective ECGgating C Contrast injection
A A A A 50/5 + 30/3/8 A 50/5 + 60/3/5 A 50/5 + 90/3/5 A 45/4.5 + 105/2.7/5 A
SC= slice collimation (mm), TF= table feed (rnmjrotatlon), RI = recon. increment (mm), L = scanning direction, SW::::effective section width (mm). MPR= multiplanar reformats. axial = axial sections. cor= coronal. sag = sagittal CM = contrast material, Contrast concentration = 300 mglml iodine (optional 400 mglml iodine for 16-slice) V=volume of CM (ml), N =volume of saline flush (ml) F=flow rate (mils), D=start delay. X=trigger region • Parameters are given for 0.5 s rotation time RT. For faster I slower RT, the numbers for P' have to be multiplied by b sequential scanning r for retrospective gating the table feed (pitch) has to be adjusted to the heart rate (see Table 23.5) dread: 5-10 A= start delay 5-10s after reaching SO HU enhancement in the aorta e biphasic injection, e.g. 5015 + 30/3/8 A => SOml CM at 5 mils flow followed an additional SOml saline flush injected at 3 mils may be beneficial.
d.e
by 30 ml CM at 3 mils, delay 8 A d:
RT/0.5 s.
II
23 Heart
but, for low calcium scores, differences in assessment of patient risks may occur, The data are evaluated either using the Agatston score or by measuring total coronary calcium mass, The Agatston score is the traditional measure for coronary calcium, which uses a threshold of 121)HLJ to define calcified portions of the coronary arteries, The total area of each calcified region (larger than 1 mm' or 1-3 pixels) is weighted with a multiplication factor depending on the peak density in this region and summed to yield a score that is a representation of the total amount of coronary calcium (see also Fig, 23,34), Risk stratification based on the Agatston score are provided below in Table 23.11 (p, 803), Newer techniques involve the determination of the total volume of the calcified plaques or the total coronary calcium mass (derived from the plaque volume and the mean plaque density), Total calcium mass can be individually calibrated by using a calcium hydroxyapatite phantom that is placed in the scanning mat underneath the patient Total calcium mass appears to be most reproducible because it is less susceptible to partial volume effects than other measures of coronary calcium, In clinical practice, various programs are available that automatically calculate scores, The user has to identify potential plaques, assign them to the various portions ofthe coronary artery tree, and place ROls around them for further evaluation by the automatic program, In general a threshold of 121)HU is used, but a threshold of 90 HLJ has been suggested for non-EBCT evaluation, Coronary
CT Angiography
Coronary CT angiography is the most daunting imaging task in CT Coronary CT angiography using EBCTscanningwith ECG-triggering at 40-80% RR interval (depending on the heart rate of the patient) can cover the whole heart if overlapping scans with 3 mm collimation and 2 mm table feed are performed, The lOOms single slice mode is used, Proper training of patients is necessary, to ensure that they can hold their breath for the scanning duration of some 40-50 heartbeats, If the proximal portions of the coronary arteries only are to be evaluated at higher spatial resolution, a 1.5 mm collimation with 1.5 mm table feed has been suggested (trigger at 80% RR interval), This technique covers two-thirds of the heart in 40-50 sections and is feasible for patients not overly obese,
Three-phase electron beam angiography was developed to optimize imaging of the coronaries in patients with arrhythmia, With this technique, three sections are acquired during each heartbeat (minimum time difference 58 ms between phases), With multislice CT scanning, retrospective gating is mandatory, Multisector reconstruction yields the best results but requires markedly increased patient exposure, A collimation of 1-2 mm (depending on the scanner) can be applied with a pitch factor P between 0,2 and 0,375 (P*=0,8- 1.5 for 4-slice scanners, see Table 23,5), The optimum setting for ECG-gating depends on the heart rate of the patient For heart rates below 65, a gating at 60% of the RR interval usually yields sufficient results with single sector reconstruction (also less dose), In case of movement artifacts, further images at other intervals have to be reconstructed (start with 50% and 70 %, add more if necessary). For heart rates above 65 bpm, dual or multiple sector reconstruction has to be chosen, and the pitch has to be reduced accordingly (higher dose), Images should be reconstructed with 50% overlap relative to the effective section width and a FOV A 250 mm to ensure optimum 3D-reformations, It has been shown that the right coronary artery and the left coronary artery have different time intervals at which they are best demonstrated (40-50% for RCA, 50-60% for LAD, 60-70% for LCX at 250ms duration of data acquisition and 4-slice scanning), With higher heart rates, reconstructing more than one data set rarely can be avoided, A percentage reconstruction setting may not always be optimum, Some researchers suggest an absolute reverse setting with 550ms for the RCA and 450ms for the proximal LAD and LCX, Newer techniques involve dual parameter settings that take into account that the optimum phase at which images should be reconstructed shifts with heart rate (e.g. HeartBeat-RT, Philips), This technique improves visualization of the heart even in patients with (moderate) cardiac arrhythmia, Image Postprocessing Detection of stenoses on axial images is often problematic, Various postprocessing techniques have been suggested that simplify this task. The first step, however is to select the data set with the
least motion artifacts for the vessel segment of interest. Ifheart rate is low enough, one data set only may suffice for these postprocessing efforts. Good results are obtained with curved planar reformations (CPR) along each vessel segment (Fig. 23.12), preferably in at least two perpendicular planes to be able to detect and quantify stenoses. Vessel tracking can be used successfully if little motion artifact is present. Similar results are possible with manual tracking although this is a more time-consuming procedure. Standardized 5 mm wide thin-slab MIP reconstructed in an overlapping fashion every 2.5mm provide an alternative evaluation tool that can be more easily correlated with coro-
nary angiography if three MIP data sets are reconstructed that correspond to the standard angiographicviews: 30° RAO, 45°LAO, and45° LAO with 30° craniocaudad angulation (Fig. 23.13). Table 23.8 gives an indication which vessel segment can best be evaluated on which of these projections. One has to keep in mind, however, that these standard views in coronary angiography are frequently adapted to the individual anatomy, so that one-on-one correlation may only be possible if the precise angiographic angulations are known. Volume rendering techniques (VRT) are helpful for demonstrating of the outer shape of the heart chambers and great vessels and for displaying the anatomy of the coronary arteries.
b
c
a Fig. 23.12 CPR of the coronary arteries improves stenosis quantification. Comparison of a longitudinal section of the LAD parallel to the heart surface (a), perpendicular to the heart surface (b), and three transverse sections perpendicular to the vessel axis (c). Note that
a
b
Fig. 23.13 Optimum projection angles for thin-slab MIP of the coronary arteries for correlation with coronary angiography. Sections are prescribed on an axial image at the level of the mitral valve (see Fig. 23.2).
the eccentric lesion is well seen on (a) but is almost completely missed on (b) due to its eccentricity, which can best be appreciated on the transverse sections. The location of these sections is indicated on (a) and (b).
c Compare 30° RAO(a),45° LAO (b), and 45° LAO with 30° craniocaudad angulation (c). The numbering of vessel segments is in accordance to Fig. 23.4.
23 Heart
Identification of the various coronary branches is frequently easier on VRT images than on cross-sectional images or MIP displays (see Fig. 23.3). Some manufactures provide presets that allow for selective display of the heart (excluding the chest wall and pulmonary vasculature) , and even selective displays of the coronary artery tree have been proposed. Virtual angioscopy of the coronaries is a technique that can be successfully used to impress clinicians, but practical applications are few (e.g. evaluating complex branching patterns or coronary aneurysms). In addition, virtual angioscopic images suffer from blooming effects that will make calcified plaques look much larger than they really are. Assessment of Cardiac Function
Assessment of cardiac function requires at least two data sets (systole and diastole). While the diastolic image is reconstructed at 60-80% of the RR interval (depending on the temporal resolution of the chosen imaging technique), the end systolic image is obtained at 20-30% ofthe RR interval. In-detail analysis of heart movement requires reconstruction of multiple image sets during the whole RR interval in 5-10% steps. For analysis of cardiac functional parameters there are a number of commercial software packages becoming available for CT use (e.g. CTMASS, GE and CT CFA, Toshiba). The most important basic functional indices are summarized in Table 23.9. It should be noted that most norTable 23.8
mal ranges are derived from MRI data. Because the temporal resolution of cine- MRI is better than multislice CT, variations may be higher since endocardial contours are more difficult to draw on CT data and tend to blur more during phases of rapid movement. This can be expected to improve with 16-slice scanners and better multisector reconstruction algorithms. If a dedicated CT software package is not available, a simpler approach could be considered (arealength method ) that uses a short axis view at the base of the papillary muscle (roughly half the distance between the apex and the mitral valve annulus) to evaluate the cross-section of the ventricular lumen and determine its short and long axis diameters a and b (Fig. 23.14). A long axis view is used to measure the length of the ventricle c from the apex to the mitral valve annulus. A simple estimate of the ventricular volume based on the simplified assumption of an elliptical shape of the ventricles is: V=(a.b.c)/2
Data from systolic images (10-20% of the RR interval) and diastolic images (60-80% of the RRinterval) can then be used to estimate functional parameters. Dose
Dose has become an issue of substantial controversy between EBCT and multislice CT manufacturers. The discussion is complex because the users can vary dose arbitrarily and the recommendations of the manufacturers have been
Suggested projection angle for the various coronary artery segments (Becker et al., 2001) 30 RAO
45° LAO
---------45
LAO-30
interventricular groove
atrioventricular groove
long axis
Orientation
along LAD
between RCA and LCX
long axis
Segments through plane
RCA(1,3) LCX(11)
PDA (4) LCA(5) LAD (6-8)
RCA(2) LCX(13)
Segments in plane
RCA (2) PDA (4) LCA(5) LAD (6-8) LCX(13)
RCA (1-3) LCX(11) OM(12)
RCA(1,3) PDA (4) LCA(5) LAD (6-10) LCX(11) OM(12)
CC
LAD = left anterior descending coronary artery (6-10), LCA = left coronary artery (5), LCX = left circumflex coronary artery (11. 13), OM = obtuse marginal branch (12), PDA= posterior descending coronary artery (4), RCA = right coronary artery (1-3)
b
a
c
Fig.23.14 Simple functional data can be derived from the short axis view in diastole (a) and systole (b) by measuring the diameters of an ellipse centered on the
ventricular lumen. The longitudinal diameters are estimated from the corresponding long axis or four-chamber views.
Table 23.9
in cardiac function
Overview of basic quantitative
measurements
Definition
Ventricular Volume V (ml)
L (A.ndo- Apap) x SW
Myocardial Volume MV (ml)
L (A.pi - A.ndo + Apap) x SW
Myocardial mass (g)
1.055
Stroke Volume SV (ml) Ejection fraction
EF (%)
analysis based on short-axis views.
Normal Ranges a
Parameter
RVES54::':: 21 RVED138::':: 40
LVES40::':: 14 LVED121 ::'::34
RV 44::':: 10
LV 150 ::'::37
c
RV 46::':: 11
LV 158 ::'::39
c
VEO- YES
RV 84::':: 24
LV 82::':: 23
(VEO- YES)I VED
RV 61 ::'::7
LV 67::':: 5
x
b
MV
Regional wall motion
CLEO- CLES
Regional wall thickening
(WTEs- WTED) I WTED
Regurgitant volume
SVLV-SVRV
b
RV= right ventricle; LV= left ventricle; ES= end systolic measurement; ED = end diastolic measurement SW = section width of adjacent short axis views covering the whole ventricle A.ndo= area within endocardial contour; A.pi = area within epicardial contour; Apap= area of papillary muscles CL= centerline drawn between endocardial and epicardial contours WT = regional wall thickness measured as the distance between epi- and endocardial contours perpendicular to CL a Based on MRI data from Lorenz, J Cardiovasc Magn Reson, 1999 b Approximation based on Simpson's rule; contours preferably drawn on workstation (quantitative analysis software) C Includes ventricular septum
continuously reduced over the past years, both for EBCTas well as multislice CT.When comparing dose requirements, one has to consider image quality as well. Many of the studies published so far are flawed and have to be viewed with caution. The following aspects, however, are well established (Table 23.10): EBCTuses only a 210° sweep of the x-ray beam from below the patient (see Fig.23.9), which should eliminate direct exposure of the
breasts. Exposure of the thyroid is much less of an issue because the gland is usually kept outside the scan range and therefore is not directly exposed. At an identical CT dose index (CTDI), EBCT will have a slightly lower radiation exposure to the breasts than multislice CT. However, calculations of the effective dose in females did not show a smaller increase over males with EBCT as compared to multislice CT. Techniques that lower the exposure during anterior projec-
•
23
Heart
Radiation exposure in cardiac CT (12 em scan length). Note that the numbers for EBCT are fixed Table 23.10 63 mAs per sweep possible) but numbers for spiral and multi slice CT depend on the actual choice of parameters may vary substantially. Technique Coronary Calcium
SC= section collimation (mm); TF= table feed (mm); mASeff=mAs/P; E = effective dose (mSv) for males; dose is 20% higher for females The mAs and kVp settings refer to Siemens scanners (CTDI values Higher dose is needed for obese patients. Dose reduction by ECG-controlled tube current modulation With low kVp settings, further dose reci.Jction by 20-50% Ca-mass
has to be used instead
ofAgatston
= =45%. is possible
score for Ca scoring
tion angles of a tube rotation have been discussed for (non-cardiac) multislice CT but have hot yet been implemented. Older EBCT scanners with Xe detectors had a quantum efficiency that was some 30% less than the solid-state detectors used in multislice CT scanners. However, solid-state detectors are now also available for EBCT. EBCT scanners use post-patient collimation for 1.5mm sections and then increase dose by a factor of 2 as compared to the 3 mm collimation. Non-gated single slice, dual slice, and multislice spiral CTyield less noise at a given radiation exposure (CTDl) than sequential scans, if high enough pitch factors are chosen (see Fig. 5.12). This holds true in comparison to EBCT as well as to sequential single or multislice CT Thus, non-gated spiral scans (such as in CT A) in general require less dose for a given image quality than EBCT. ECC-triggered scans using EBCT or sequential multislice or dual slice CT use all acquired projection data for CT image reconstruction and
CTDlvoi=
volume
CTdose
6.1 5.0 ·1.54
I
I
index (mGy)
from www.impactscan.org).
in slim patients.
because
Lklder such conditions,
of higher attenuation
in calcified
total
plaques.
therefore should be similar in terms of dose (CTDl) requirements. Differences in image noise result from differences in detector efficiency, geometric efficiency, and from the choice offilter kernel for image reconstruction. Higher resolution kernels will cause more image noise (see Fig. 5.11). Thinner sections will increase image noise at a given exposure dose. This holds true for multislice CT as well as for EBCT Since most EBCT protocols use 3 mm sections while m ultislice protocols use 4 x 2.5 mm or even 4 x 1 mm collimation, noise with multislice CT may be similar (4 x 2.5 mm) or substantially higher (4 x 1 mm). At constant mAs settings, 1 mm sections in 4-slice scanners suffer from an approximately 30% higher dose because of decreased geometric efficiency as compared to 2.5mm sections (see Fig. 5.5 b). In 16-slice scanners, geometric efficiency is much higher; thus dose is not substantially increased for thin sections. As mentioned above, 1.5mm EBCT sections result in a 100% dose increase over 3 mm sections.
Fig. 23.15 Dose reduction in cardiac CT: ECG-adapted dose modulation reduces the mA settings during the systolic phase of the heart cycle. The time interval A TM at which the nominal tube current remains at 100% is chosen larger than the reconstruction time window in order to be able to retrospectively adjust the position of the reconstruction window. Since ATN is independent of the heart rate, the amount of dose reduction decreases with higher heart rates.
Tdel. mA , , Vl
100%
fl.T N
Reconstructed image data
,-, , ,
01 C
B « Q.) Vl
E Q.)
'-
20%
0%
, 'T del, Recon
Electronic noise from the detector is another limiting factor for low-dose applications and thin sections. Substantial differences between manufactures and scanner models appear to be present. In general, 16-slice scanners suffer less from electronic noise because of improved detector electronics. ECC-gated multislice CT uses continuous data acquisition and retrospectively selects the data used for image reconstruction. The higher the temporal resolution of this technique relative to the RR interval derived from the heart rate, the more projectional data have to be abandoned, and the less dose-efficient is the scan. If a 250ms reconstruction window is chosen at a heart rate of60bpm (1000ms RR interval), only one fourth of the data will be used. Hence, the dose at identical image quality is some four times higher than with an ECG-triggered multislice or EBCT technique. With multisector reconstruction of a 80 ms image at a heart rate of 90 bpm (667ms RR interval), the dose at
identical image noise has to be over eight times higher than with ECG-triggered techniques. The advantages of such techniques are a higher spatial and temporal resolution and the ability to reconstruct multiple images during the heart cycle, which would make more efficient use of the dose. Dose modulation techniques (Fig. 23.15) have been suggested that still use retrospective gating but reduce the rnA settings during systole, so that dose is reduced but the ability to retrospectively adjust the reconstruction window is retained with the possibility to reconstruct systolic images (oflower quality) for evaluating cardiac function. While these techniques are still experimental, some manufacturers have already implemented a reduction of the scan field of view to the center of the gantry (e.g., Toshiba). Together with proper positioning of the patient this technique can be used to reduce dose to the chest wall and breasts.
Artifacts •
Pulsation
Artifacts
Pulsation is responsible for incongruence during data acquisition that leads to blurring of structures, if motion during the phase of acquisition is minor, and to marked distortion, double contours, or complete lack of fine detail, if motion is major. While large structures such as the heart, aorta, and pulmonary artery can usually still be evaluated despite pulsation effects, more
detailed analysis of internal structures in the heart chambers (such as the endocardium, papillary muscles, and valves) and the coronary vessels becomes impossible. Strong pulsations can seriously hamper the evaluation of the aortic root and aortic arch (especially the region of a suspected ductus arteriosus). Such aortic pulsations are especially large in younger individuals because of higher elastic-
23 Heart
ity of the aorta. In such patients, alternative techniques such as MRA or ECG-triggered CT acquisitions should be considered. Pulsation artifacts can be used to estimate the motion of a cardiac structure or great vessel (see Fig. 23.10). For the coronaries, pulsation is the main reason for inconclusive examinations. With retrospective gating, image quality may be recovered by reconstructing data sets at various time points during the cardiac cycle but there is a considerable number of patients in whom evaluation of one or more coronary arteries (most frequently the RCA) is impossible with 4slice scanners. The number of conclusive examinations is considerably improved at low heart rates and with 8- and 16-slice scanners. •
Arrhythmia
Arrhythmia is a major cause for suboptimum results with prospectively triggered EBCT or multislice CT scans. Retrospective gating may compensate for changes in heart rate but extrasystolic beats can lead to incomplete or abnormal contraction that will cause images that are not in synchrony with the reset of the data set. Arrhythmia may be responsible for pseudostenoses of the coronary arteries or may obscure stenoses that otherwise would remain within the scan volume (Fig.23.16a). Patients with severe arrhythmias or frequent extrasystolic beats are not good candidates for ECG synchronization. While CT may
a
b
Fig. 23.16 Artifacts. Arrhythmia (a) may simulate stenoses or vessel interruption (arrow). Note the loss of the distal portion ofthe RCA due to breathing (arrow head). High contrast streak artifacts from the right atrium
adequately evaluate cardiac morphology, coronary CTA frequently does not provide diagnostic results. Coronary CTA should therefore not be attempted in these patients. •
Respiratory Artifacts
Artifacts due to respiratory motion during data acquisition are the second commonest problem with ECG-synchronized CT (in EBCT as well as multislice CT). Because of a total scan time that varies between 30s up to 50s with current scanners, proper patient training is extremely important so that breathing at critical moments is avoided. For larger ranges, such as bypass evaluation, caudocranial scanning may be advantageous, so that the heart is imaged first and regions like the upper mediastinum that are less sensitive to breathing are scanned last. Breathing artifacts can simulate vascular discontinuities, stenoses, or aneurysms depending on the direction and amplitude of the motion. Respiratory artifacts are easily recognized by the appearance of steps or undulations in the contours of the skin surface, the chest wall, or the heart. Artifacts appear at precisely the same table position. •
High-Contrast Artifacts
Highly concentrated contrast material in the superior vena cava can cause substantial streak artifacts that obscure the RCA and may make evaluation of the atria, the right ventricle, and the RCA difficult (Fig.23.16b). Streak atifacts caused
c simulate irregularities of this ectasia of the RCA (b). Contrast enhancement of the myocardium as well as loss of enhancement of the right heart may occur towards the end of the scan (c).
by a high concentration of contrast material in the superior vena cava (SVC) can mimic intimal flaps in the ascending aorta (see Fig. 24.5). Mixing of non-enhanced blood (from the IVC) and highly concentrated contrast material (from the SVC) occurs in the right atrium and causes the display of the atrial septum and tricuspid valve to deteriorate. It may be hard or even impossible to detect a right atrial thrombus under these conditions. High-contrast artifacts may be used as a diagnostic clue to detect a R-L shunt. e.g .. in patients with sinus venosus atrial septal defect (ASD; see Fig. 23.21). Using diluted contrast material reduces the effects somewhat. while highly concentrated contrast material (> 350 mg/ml iodine) and high flow rates (> 3 mlls) increases the artifacts. Inhomogeneous
Contrast
Because of sequential enhancement and de-enhancement of the right and left heart chambers, scanning at an early phase will yield less enhancement of the left heart, while later during the scan, contrast enhancement in the right heart starts to drop. If the timing of contrast material injection is not adjusted specifically to take account of these effects, evaluation of portions of the heart chambers may be difficult or even impossible. In addition, there is continuous contrast material uptake in the myocardium that will reduce the contrast between lumen and wall as the; scan progresses (Fig.23.16c).; As ;a consequence, 3D evaluation of the heart chambers or ventricular volumetry may be hampered. Partial Volume Effects
Partial volume effects are particularly troublesome for small details such as the more peripheral coronary artery branches. While an effective section thickness of some 3 mm (such as in EBCT or ECG-triggered multislice CT) is able to display the proximal coronary arteries, smaller and more peripheral vessels will be lost. Partial volume effects also affect the detection and apparent density of coronary calcifications. Because the Agatston score was developed for 3 mm thick EBCT sections, a similar section thickness should be chosen with other techniques as well, if present reference values are to be used for comparison.
Blooming of coronary calcifications is more severe with thicker sections, and thick sections also hamper the evaluation of soft plaques. Evaluation of coronary artery stents is only possible for stents that are markedly larger than the effective section width, which makes it difficult to use coronary CTA for more distally placed coronary stents. Image
Noise
Image noise poses a substantial problem in obese patients if low-dose protocols are used. This is independent of the scanning technique and affects EBCT as well as multislice CT scanning. Since EBCT does not allow for increasing the mAs settings per sweep (63mAs/lOOms), however, there is little opportunity to compensate for image noise, while multislice CT can simply apply a higher mAs setting. Image noise will lead to more substantial variations in pixel values (CT numbers). As a consequence for coronary calcium scoring, more pixels will exceed the thresholds of 130 HU and may be erroneously counted as a coronary calcification (compare Fig. 23.35). Using filters that require more than 2-4 of these pixels to be connected reduces the number of false positive classifications but also reduces the sensitivity for small calcified plaques. For coronary CT A, image noise can substantially hamper the evaluation of smaller arteries and may simulate wall irregularities due to random variations in pixel numbers. Superimposed
Structures
Cardiac veins will be less opacified than the coronary arteries if craniocaudal scanning is performed and contrast material timing was successful. Coronary veins, however, may be hard to distinguish from coronary arteries if both structures are opacified and the continuity of the coronaries is not adequately displayed due to pulsation effects. This mainly concerns the left circumflex branch that runs parallel to the left coronary vein, and diagonal and marginal branches that have a similar course as oblique ventricular veins. The left atrial appendage aimost invariably superimposes over the origin of the left coronary artery and the proximal portions of the LAD and LCX. The RCA runs in the right atrioventricular groove and may be superimposed by these structures. The diaphragm obscures the
23
Heart
undersurface of the heartland the sternum superimposes over the anterior surface of the heart. Proper editing with removal of these structures is necessary prior to 3D rendering. Automated techniques that separate the heart from the chest wall have been suggested and
even techniques that remove the ventricular lumen and isolate the coronaries are being developed. Thin-slab MIP are an alternative for evaluation of the coronaries (see Table 23.8, Fig. 23.13).
lenital Heart Disease
Standard spiral CT scanning is sufficient to evaluate those types of congenital heart disease that mainly involve the great vessels. However, it is only able to detect large abnormalities in the heart and lacks the ability to provide accurate quantitative information on these anomalies. ECG-synchronized scanning using EBCT or multislice CT can successfully evaluate cardiac morphology' but radiation exposure is a limiting factor in younger patients and makes CT unsuitable as a primary diagnostic study. Echocardiography and MRI are the techniques of choice for such indications but CT can be useful in patients with cardiac pacemakers (or other contraindications to MRI) and in adult patients with congenital heart disease. Ventricular Septal Defects
Defects of the ventricular septum (VSD) are the commonest congenital heart disease and account for some 30% of cases. Most are isolated but in approximately 20%, a VSD is associated with other anomalies. Depending on the size and location of the defect, most VSD close spontaneously in the first years of life. Functional problems of VSD are related to the size of the L-R shunt, which may even progress to Eisenmenger syndrome with severe pulmonary hypertension and shunt inversion. CT is not the technique of choice to detect or quantify VSD, but VSD may be a finding that is associated with other congenital heart defects that are evaluated with CT. •
Fig. 23.17 AV channel type ventricular septal defects (VSD) in a one-year-old child with D-TGA (low-dose scan: 80 kVp, CTDlvoi = 0.6 mGy).
Conal {supracristalj VSD (5- W%) are located
just underneath the pulmonary valve and the commissure between the right and left aortic valve cusp. The right aortic valve cusp may herniate into this VSD and lead to aortic valve insufficiency. A muscular VSD is located entirely in the sepal myocardium. Often multiple defects are present that close spontaneously in the first years of life. These defects are rarely examined by CT. The atrioventricular canal type [AV canal, endocardial cushion type, posterior) VSD is located close to the mitral valve and is best seen on axial sections. It is rarely an isolated defect. For this reason, it is frequently encountered on CT scans for other congenital heart disease (Fig. 23.17). ,
CT Morphology
Membranous VSD are most frequent (> 75%) and involve the membranous portion of the septum close to the right and posterior coronary valve cusp. They can best be appreciated on short -axis views as a defect in the posteriocranial portion of the septum.
Atrial Septal Defects
_
Atrial septal defect (ASD) is the commonest congenital cardiac anomaly in adults, and is often diagnosed late in adulthood because of its benign course. Females are four times more
During normal embryonic development, a thin fibrous septum primum forms as the left side of the later atrial septum with a physiologic opening (ostium secundum) close to its superior portion, followed by a muscular septum secundum on the right side that has a large physiologic defect in its mid-portion the foramen ovale (Fig. 23.18). The resulting valve-like opening present during fetal life closes after birth as the left atrial pressure rises and presses the fibrous portion of the septum primum over the foramen ovale. In most individuals the foramen ovale becomes permanently obliterated but in some 25% of cases it remains patent and may allow R-L shunting if there is a rise in right atrial pressure. It may be responsible for paradoxical emboli after acute pulmonary embolism. An ostium secundum ASD is most frequent (» 60 %) and represents a fenestration or absence of the foramen ovale flap of the septum primum. It may be associated with pulmonary valve stenosis, tricuspid atresia, a hypoplastic left heart, and an interrupted aortic arch. An ostium primum ASD (30% of cases) is a defect of the AV endocardial cushion that did not close during embryonic development. It is located inferior and ventral to the foramen ovale. It is associated with endocardial cushion defects (AV canal), cleft mitral valve and anterior fascicular block. A sinus venosus ASD is a defect of the atrial septum close to the entrance of the superior vena cava. It is associated in over 90% with an
aberrant return of the right upper lobe pulmonary vein. A Common atrium is an extreme form of ASD with very large defect of the septum primum and secundum. Only a small remnant of the septum is seen as a ridge along the roof of the atrium near the junction of the SVc. Lutembacher syndrome is the rare combination of ASD and mitral stenosis. The magnitude of the L-R shunt through an ASD depends on the size of the defect, the relative compliance of the ventricles and the relative resistances in the pulmonary and systemic circulations. The L-R shunt causes diastolic overloading of the right ventricle, up to three to six times increased pulmonary blood flow, and may finally result in shunt reversal {Eisenmenger syndrome}. As in VSD, CT is not the technique of choice for detecting or quantifying ASD, but ASD may be an incidental finding or associated with other congenital heart defects that are evaluated with CT. CT is well suited to demonstrate the aberrant pulmonary venous return in sinus venosus ASD. •
CT Morphology
A patent foramen ovale or an ASD may be suspected if the aorta enhances simultaneously or even earlier than the pulmonary artery on test bolus injections or on CTA of the pulmonary arteries. In patients with a patent foramen, the aortic enhancement drops again after an initial peak (Fig. 23.19). This peak is caused by the sudden rise in right atrial pressure and temporary
SVC-----
Pulmonary
Septum ---JJ primum Septum -r-rrr-rw; secundum
~~Abel
/'
pulmor
~
IVC Normal
Fig. 23.18 Embryonic development of the atrial septum. A thin fibrous septum primum forms as the left side of the later atrial septum with a physiologic opening (ostium secundum) close to its superior portion, followed by a muscular septum secundum, on the right side, that has a large physiologic defect in its mid-portion, the foramen ovale (modified from Dahnert, 2000).
Ostium primum defect
~ Sinus venosus defect
Ostium secundum defect
790
23
Heart Fig. 23.19 Paradoxical enhancement of the aorta is indicative of a patent foramen ovale and can be seen on CTA for pulmonary embolism. Early enhancement of the aorta (a) is followed by a drop in enhancement a few seconds later (b). Note the lack of pulmonary venous enhancement on the first scan (arrow), indicating that the contrast passed to the aorta via a R-L shunt. (c) Excellent display of the membranous portion of the atrial septum in a patient with known patent foramen ovale. No opening is seen as long as the left atrial pressure exceeds the right atrial pressure.
a
b
c
R-L shunting at the beginning of contrast material injection. It is followed by the later regular aortic enhancement. CT cannot directly visualize a patent foramen ovale (Fig.23.19c). An ostium secundum ASD is located in the central portion of the atrial septum and is best appreciated on axial sections (Fig. 23.20). An ostium primum ASD is located in the inferior portion of the septum and is also well visualized on axial sections. Because of the L-R shunt, the injected (highly concentrated) contrast medium only enters the left atrium if there is a large defect in the septum. Due to the increased blood flow there is marked dilatation of the right ventricle and the pulmonary arteries which not only involves the main branches but also the periphery. Paradoxical interventricular septal motion may be seen on (ECG-gated or consecutive non-ECG synchronized) sections of the interventricular septum. Sinus venous ASD occurs high in the atrial septum, near the entry of the superior vena cava and is frequently associated with anomalous connection of the right upper lobe pulmonary vein to the junction of the superior vena cava and right atrium (Fig.23.21a). In this type of ASD, contrast material from the IVC may enter the left atrium in ajet that can be demonstrated on axial sections as well as volume rendered images (Fig.23.21 b, c). Right ventricular overload may only be minor. Patent Ductus Arteriosus
The ductus arteriosus is a vessel leading from the bifurcation of the pulmonary artery to the aorta lust distal to the left subclavian arte .
Normal closure of the ductus immediately after birth is due to the sudden increase in oxygen tension that accompanies ventilation. It may present as isolated finding or be associated with other malformations (e.g., coarctation, VSD, pulmonary stenosis, or aortic stenosis). CT may demonstrate a patent ductus arteriosus (PDA) but is not the technique of choice to define shunt volume and pressure differences. PDA may be visualized in patients examined for aortic coarctation. •
CT Morphology
CT demonstrates a connection between the proximal descending aorta and the pulmonary artery bifurcation or proximal left pulmonary artery. This connection may be hard to visualize on single-slice CT because of marked pulsation
Fig. 23.20 Large ostium secundum atrial septum defect (ASD) in a patient with suspected chronic pulmonary embolism. Note the increased noise level and ring artifacts in this low dose multislice study (CTDlvoi= 2.4 mGy).
a
b
c
Fig. 23.21 Sinus venosus ASD with an associated aberrant pulmonary venous return from the right upper lobe (a) that enters the right atrium at the site of the small
defect (arrowheads). Note that there is ajet phenomenon (arrow) as highly concentrated contrast material enters the left atrium (b. c).
effects in the affected regions. With multislice CT, scanning is faster and artifacts are more spread out and thus allow for better detection of even small PDA (Fig. 23.22). Especially in patients examined for the presence of aortic coarctation, one should also search for a (small) PDA. Large PDA are obvious and are associated with enlarged pulmonary arteries and a dilated ascending aorta and aortic arch. In older patients, the PDA may calcify.
Transposition of the Great Arteries (TGA)
Coarctation of the Aorta
Coarctation of the aorta is discussed in detail in Chapter 24 (p. 872). Concurrent cardiac anomalies are rare with localized coarctation but are common with tubular hypoplasia.
Fig. 23.22 Small patent ductus arteriosus in a patient with an aortic coarctation (arrow).
The term transposition identifies a group of malformations that are characterized by abnormal relationships between the cardiac chambers and the great vessels. With complete transposition (D-TGA) , the aorta arises from the right ventricle and the pulmonary trunk emerges from the left ventricle. The position of the atria, ventricles, and valves is normal. This results in two separate circulations with some communication that is needed to sustain life. Patients present with a patent foramen ovale and PDA, or VSD, or another more complex cardiac anomaly. The creation or enlargement of an intraatrial communication provides better mixing of systemic and pulmonary venous blood. It may be created surgically by a Blalock-Hanlon procedure or by balloon septostomy during cardiac catheterization (Rashkind procedure). Corrective repair is accomplished by an intra-atrial baffle that directs the systemic venous return to the left ventricle and the pulmonary venous return to the right ventricle (Mustard and Senning operation). In patients with a VSD and a pulmonary stenosis, a corrective switch operation (Rastelli procedure) is performed to transpose the aorta back to its correct anatomic position on top of the aortic valve. The pulmonary valve is resected and substituted by an extracardiac prosthetic conduit to replace the pulmonary artery and valve. A corrected transposition of the great arteries (L-TGA) is characterized by an inversion of
23 Heart
the morphologic left and right ventricle with the AV valves following their respective ventricles. It may be associated with a membranous VSD, pulmonic stenosis and dextrocardia. In a double outlet right ventricle (DORV), most of the aorta and the pulmonary artery arise from the right ventricle. The aorta straddles a VSD. CT plays a role in adult patients with cardiac pacemakers, particularly for non-invasive detection of complications of corrective surgery (baffle stenosis). •
CT Morphology
L-TGA
A P
=
aorta
= pulmonary artery
DORV
double outlet right ventricle TGA = transposition of great arteries =
Typical patterns of the relationship of the ascending aorta and pulmonary artery are provided in Fig. 23.23. Fig. 23.23 Schematic representation ofthe position of In D- TCA, the aorta is located anterior (and the aorta and pulmonary artery in various forms of frequently to the right) of the pulmonary artery transposition of the great arteries (modified from Dahnert, 2000). (Fig. 23.24). The right ventricle has a normal trabecularization but is markedly hypertrophied with a thickened ventricular wall. After a Mustard or Senning procedure, there may occur a The commonest form ofsub aortic stenosis is stenosis of the systemic venous inflow that is the idiopathic hypertrophic form, also known as directed towards the mitral valve (see hypertrophic cardiomyopathy. The lesion conFig. 23.57). To avoid high-contrast artifacts, it is sists of a membranous diaphragm or fibrous advisable to use diluted contrast material for ring encircling the left ventricular outflow tract just beneath the base of the aortic valve. answering this specific diagnostic question. In L-TCA, the aorta holds the position norSupra valvular aortic stenosis is a localized or mally occupied by the pulmonary artery and diffuse narrowing of the ascending aorta, origivice versa. The aorta is connected to the mor- natingj ust above the level of the coronary arterphologic right ventricle that is located posteri- ies at the superior margin of the sinus of Valorly, in the position normally occupied by the salva. In contrast to other forms of aortic stenoleft ventricle. It is characterized by a marked sis, the coronary arteries are subjected to the wall hypertrophy and trabecularization. The elevated pressure and are often dilated and tortuous. Adherence of the free edges of the aormorphologic left ventricle is located anteriorly and is connected to the pulmonary artery. tic cusps to the sites of supravalvular stenosis In DORV, various types are distinguished, may interfere with coronary arterial inflow. Valvular pulmonic stenosis usually presents depending on the relative position of the aorta and pulmonary artery (Fig. 23.23). There is a in childhood and causes a dilated main and left concomitant VSD that can be easily detected on pulmonary artery. Sub valvular pulmonic stenosis may be infunaxial sections. dibular (such as in tetralogy of Fallot) or sub infundibular caused by hypertrophied anomalous muscle bundles (usually associated with VSD). Valvular Anomalies Supra valvular pulmonic stenosis is caused by A Valvular aortic stenosis occurs in 4 % of narrowing of the pulmonary trunk, pulmonary patients with congenital cardiovascular defects. bifurcation, or more distal arteries. It may be asMost children are asymptomatic, but the lack of sociated with various congenital heart defects as symptoms does not preclude the presence of well as with Williams- Beuren syndrome, Ehlersmoderately severe obstruction. Left atrial en- Danlos syndrome, and post rubella syndrome. CT is not the technique of choice for the largement, concentric left ventricular hypertroevaluation of congenital valvular disease unless phy and poststenotic dilatation of the ascending peripheral pulmonary stenoses are suspected. aorta are common findings.
a
Fig.23.24 Transposition of the great arteries (TGA). In D-TGA the aorta arises from the morphologic right ventricle (mRV) and the pulmonary artery from the mor-
•
c
b
CT Morphology
See below, p. 807.
phologic left ventricle (mLV) (a, b). Typical 3D representation obtained from a single-slice spiral CT data set (3/5/2) (c).
lar to the ventricular septum. The infundibular pulmonic stenosis is also well visualized on these sections. ECG-triggering is suggested for optimum demonstration of the anatomy.
Tetralogy of Fallot
Tetralogy ofFallot is characterized by an obstruction ofthe right ventricular outflow tract (usually an infundibular pulmonic stenosis), right ventricular hypertrophy, a VSD, and an aorta overriding the interventricular septum. It may be accompanied by a bicuspid pulmonic valve, a stenosis of the left pulmonary artery, a right aortic arch, and anomalies of the coronaries and the chest skeleton. If combined with an ASD it is termed pentalogy ofF allot. A trilogy ofF allot consists of a pulmonary stenosis, right ventricular hypertrophy, and a patent foramen ovale. Palliative treatment consists of a BlalockTaussig shunt between a subclavian artery and a pulmonary artery, a Pott operation with anastomosis of the left pulmonary artery to the descending aorta, or a Waterston-Cooley procedure with anastomosis of the right pulmonary artery to the ascending aorta. Definitive treatment consists of correction of the VSD and resection of the obstructing tissue from the RVOT. CT may be used in patients with pacemakers to define morphology but it is not suited to provide functional information. •
CT Morphology
A VSD with the typical overriding aorta is best seen on semicoronal reformations perpendicu-
Ebstein Anomaly
In this rare anomaly there is abnormal morphogenesis of the tricuspid valve. Portions of the septal and posterior leaflets of the tricuspid valve are attached lower than normal to the ventricular wall rather than to the atrioventricu1ar ring (atrialization of the RV inlet). The valve is incompetent, the foramen ovalc is patent, and the right ventricle variably hypoplastic. •
CT Morphology
Ebstein anomaly presents with an extremely large right atrium that extends partially into the superior portions of the right ventricle. Because of tricuspid insufficiency the SVC and IVC are enlarged and contrast material injected via an arm vein may retrogradely enter the IVC and liver veins. The functional right chamber is small and displaced inferiorly (Fig. 23.25). Complex Anomalies
Complex anomalies usually represent combinations of the defects described above and may be associated with various other congenital diseases.
23 Heart
Fig. 23.25 Ebstein anomaly with typical atrialization of the right ventricle due to distal insertion of the tricuspid valve and consecutive massive dilatation of the right atrium and bulging of the atrial septum (a). Note the narrow pulmonary arteries (b). There was reflux of contrast material into the dilated hepatic veins (c), a sign that is indicative of impaired right ventricular function.
a
•
b
CT Morphology
Analysis of complex anomalies is based on the segmental anatomy of the heart (see p. 761). Morphologic and functional heart chambers need not be identical. Levocardia and dextrocardia (see Fig. 23.41) are defined by the direction of the heart axis. The morphologic right chamber is characterized by its increased trabeculation and longer outflow tract. The right atrium is defined by its connection to the inferior vena cava. The bronchial anatomy can be helpful in defining left or right isomerism because the bronchial situs is usually concordant with the atrial situs. Complex anomalies of the great vessels include totally anomalous pulmonary venous re-
turn, truricusarteriosus, pulmonary atresia, pulmonary vein atresia, hypoplastic left heart syndrome, and interruption of the aortic arch. Atrioventricular anomalies are manifold. They include concordant and discordant anomalies, unilateral AV atresia, double-inlet single ventricle, and unusual alignment with crisscrossing or overriding great vessels. Ventriculoarterial anomalies include transpositions, atresia and great vessel straddling over a VSD. Atresia of the pulmonary artery is associated with dilated systemic (usually bronchial) supply. Concomitant valvular incompetence is important but very hard to evaluate by CT.
Acquired Diseases oftheMyoca Valvular disease, cardiomyopathies, and coronary heart disease are associated with known morphologic changes in the cardiac chambers. CT is not indicated as a primary imaging study but may assume increasing importance in assessment of these disease entities. Cardiomyopathy
Congestive cardiomyopathy can be caused by ischemic coronary artery disease, myocarditis, endocardial fibroelastosis, metabolic diseases (glycogenosis, mucolipidosis, mucopolysaccharidosis), and muscular dystrophies, and is seen in children of diabetic mothers. Hypertrophic
obstructive
cardiomyopathy
(HOCM) can be due to autosomal dominant
transmission causing idiopathic hypertrophic subaortic stenosis (IHSS) and asymmetric septal hypertrophy (ASH). Symmetric concentric hypertrophy is a rarer entity that may be diffuse, apical or midventricular. Restrictive cardiomyopathy can be caused by amyloid, glycogen deposits, or as a consequence of hemochromatosis. It may also be due to constrictive pericarditis. Echocardiography is the technique of choice for evaluation of these entities. •
CT Morphology
Congestive cardiomyopathy is characterized by
cardiomegaly with global heart enlargement the (mild) left atrial enlargement is not sociated with enlargement of the left atrial pendage. There is global hypokinesis with
but asapre-
Ischemic Heart Disease
duced pulsation artifacts on axial sections or MPR of non-ECG-gated scans (Fig.23.26a). The ventricular wall is not thickened. In hypertrophic obstructive cardiomyopathy (HOCM) size ofthe ventricular cavities is normal to small with a marked increase in wall thickness (Fig.23.26b). IHSS and ASH are characterized by a markedly asymmetric thickening of the ventricular septum as compared to the free left ventricular wall. Restrictive cardiomyopathy caused by amyloid or glycogen deposits is characterized by left ventricular wall thickening and reduced LV systolic and diastolic function (Fig.23.26c). In amyloidosis, pulmonary congestion and pulmonary deposits of amyloid may be present (patchy infiltrates). Constrictive pericarditis is characterized by a thickened (> 2 mm) and often extensively calcified pericardium. Ischemic Heart Disease
Cardiovascular diseases are the most frequent cause of death in the western world. Classical risk factors include age, (male) gender, total cholesterol levels, increased LDL, decreased HDL-cholesterollevels, high systolic blood pressure, treatment for hypertension, cigarette smoking, and diabetes. Newer independent risk factors include increased fibrinogen, increased factor VIlc, elevated homocysteine, CRP, and increased PAI-1. These factors can be used for risk stratification. The classical risk factors have
a
b
Fig. 23.26 Cardiomyopathy. Congestive cardiomyopathy with massive dilatation ofthe heart chambers and thinning of the LV wall (a). Hypertrophic cardiomyopathy with substantial thickening of the left ventricular
been established as a tool for deciding between various management options ranging from dietary changes to statin therapy or the need for further testing. Acute ischemic heart disease includes new onset of cardiogenic chest pain, myocardial infarction, and sudden cardiac death. Myocardial infarction and sudden death are the most frequent initial presentations of cardiovascular disease. It has been shown that the majority of these patients do not have a significant coronary stenosis (» 50%) prior to their event, which underlines the need for early detection by risk assessment and screening of appropriate patient groups. . .• , , : . /Chronic ischemic heart disease leads to changes in myocardial perfusion, myocardial kinetics, and ultimately myocardial morphology. As a consequence of acute infarctions, wall thinning, aneurysm, and scar formation, and various forms of dyskinesia of the myocardium may result. Ischemic cardiomyopathy is the consequence of repeated acute or chronic ischemic injury to the heart. ECG, stress tests, and (dobutamine) thallium scintigraphy or MRI are the techniques of choice for evaluating patients with increased risk factors or symptomatic ischemic heart disease. CT at present has no role in acute coronary syndromes but may be used to rule out extracardiac diseases in patients with atypical angina. Coronary calcium scoring by CT (see below, p. 800) allows for risk stratification but it is not yet clear whether it provides estimates
c wall even in diastole (b). Restrictive cardiomyopathy due to amyloid deposition (c). Note the hypoattenuating subendocardial region (arrowheads) and an increased thickness of the myocardium.
795
23 Heart
that are independent from classical risk factors. In patients with chronic ischemia, CT holds the potential to demonstrate scars, wall thinning, dyskinetic wall segments, and ventricular aneurysms. These findings may also be accidentally detected by conventional (non ECG-synchronized) CT but are commonly underreported. Although perfusion studies for myocardial viability have been performed with CT, scintigraphic techniques and MR currently are more suitable for this imaging task. •
CT Morphology
Acute infarction will lead to ischemia, myo-
cardial damage, and ultimately scar formation. On late scans (10-40 minutes) after contrast material application, the damaged tissue has been shown to become hyperdense because of increased interstitial trapping of the contrast agent 0, " ,. _ , A, .• , Formation of subendoeardial scars can be appreciated as a hypo attenuating rim in the endocardial region of the ventricles (Figs. 23.27). Subendoeardial, but more frequently transmural infarction leads to focal thinning of the ventricles (Fig.23.26b) with regional disturbance of wall movement. Ultimately, aneurysms or thrombi may form (see Figs. 23.29). The various portions of the myocardium can be assigned to the different coronary vascular territories (see Fig. 23.5) but there is some insecurity at the base of the heart depending on whether the right, left, or both coronary arteries I
a
•••
'-
••
,,_
•••
,,;
_
b
Fig. 23.27 Ischemic scars lead to thinning of the ventricular wall following transmural infarction or to subendoeardial hypoattenuating regions after contrast application, (a) Massive thinning of the ventricular septum and apex (transmural scar) due to infarction of the
supply the posterior descending coronary artery (PDA). In general, however, the LAD supplies the anterior left ventricular wall including the superior portions of the septum, the roof of the left ventricle, the apex, and the anterior papillary muscle. The left circumflex artery supplies the lateral wall and the posterior papillary muscle. The right coronary artery supplies the right heart, and in the majority of patients (via the PDA) also the inferior left ventricular wall and the inferior portions of the septum. Disturbed wall motion may be appreciated by functional analysis of ECG-gated data sets, reconstructed m various -cardiac phases; akinetic wall segments show no movement, hypo kinetic segments have a reduced wall motion, dyskinetic segments show a paradoxical systolic expansion, and asynchrony describes a disturbed temporal sequence of contraction. On VRT images, thin portions of the wall appear more translucent and bright on both systolic and diastolic images (Fig. 23.28). Cardiac Aneurysms
Congenital left ventricular aneurysms are rare and occur in young black adults. The sub mitral type leads to a bulge at the left middle or upper cardiac border while the subaortic type may be small and hard to visualize or lead to massive cardiac enlargement due to aortic insufficiency. The risk of rupture is usually low.
c LAD. Note the subendoeardial extent of the scar in the common territory of LAD and LeX (arrows), (b) Transmural scar in the territory of the LAD with persisting subendoeardial hypodensity. (c) Subendoeardial scar at the anterior wall of the right ventricle.
Card'iacAneurysms 797 Fig. 23.28 Hypokinesiaofthe anterior wall of the left ventricle demonstrated on volume rendered images in systole (a) and diastole (b). The thin portions of the myocardium reveal the bright intraventricular contrast on both, systolic and diastolic images. Images were acquired at 20% and 1O%ofthe RR-interval.
b
a
Acquired left ventricular aneurysms are a complication of transmural myocardial infarction or, rarely, Chagas disease. True aneurysms locally involve the ventricular wall, which becomes noncontractile and dyskinetic. False aneurysms (pseudoaneurysms) are due to a left ventricular rupture, which is contained by fused layers of visceral and parietal pericardium (after pericarditis) as well as extra cardiac tissue. It may be caused by transmural myocardial infarction or trauma and has a high risk of delayed rupture. Echocardiography and cardangiography are the primary imaging modalities but ECG-gated multislice CT may offer similar information.
a
b
Fig. 23.29 Cardiac aneurysms. (a) Aneurysm of the cranial portion of the left ventricle. Concomitant pericardial effusion, (b) Ischemic cardiomyopathy leading to formation of an aneurysm of the apex of the heart, (c)
•
CT Morphology
A true cardiac wall aneurysm often causes only a slight bulge in the heart contour, but there is always marked, localized thinning of the myocardium compared with adjacent areas (Fig. 23.29). On ECG-gated scans, reconstruction in systole demonstrates a characteristic paradoxical expansion. Rarely, a rim of calcium is seen in the fibrotic wall. There is always a wide communication with the heart chamber (no neck). A cardiac pseudoaneurysm may have a globular appearance with a mouth that is smaller than the actual diameter of the aneurysm. It is typically located at the posterolateral/diaphragmatic wall of the left ventricle. Due to delayed filling the contrast enhancement may not be identical to that of the ventricular lumen.
c Extremely calcified aneurysm ofthe left ventricle (4 x II 6). Note the internal mammary graft (arrow) and the reduced pulsation of the aneurysmatic portions of the ventricle.
23 Heart
Cardiac Thrombi
Cardiac thrombi can form at the site of myocardial injury due to infarction (or, rarely, tumors), and in regions of reduced or turbulent blood flow. The latter may be the consequence of tricuspid or mitral stenosis (or other valvular disease) and then most commonly affects the atria, including the atrial appendages. Thrombi may also form in regions with reduced wall motion, and thus may coat the internal surface of ventricular aneurysms. Cardiac thrombi hold an increased risk for peripheral embolization, either into the systemic circulation (from the left heart) or into the pulmonary arteries (right heart). In fact, the presence of peripheral emboli should prompt the search for a source within the heart or the
Fig. 23.30 Intraventricular thrombi, (a) Wall-adherent thrombi frequently form in the region of a myocardial infarction. (b) Round thrombi in the left ventricle may be a source of small peripheral emboli. (c) This large thrombus presenting as a mass in the right ventricle was found incidentally on conventional spiral CT of the ab-
a
b
Fig. 23.31 Intra-atrial thrombi, (a) Thrombus in the left atrial appendage (LAA) seen on a low-dose multislice CT (CTDlvoi= 2.2 mGy) of the pulmonaryvessels, (b) The patient also had an embolic occlusion ofthe left iliac artery seen on the corresponding CT venogram. (c. d)
aorta, or for an open foramen ovale (paradoxical emboli) . Echocardiography is the present technique of choice. CT is superior for detecting thrombi in regions that are hard to assess by transthoracic echocardiography. •
CT Morphology
Cardiac thrombi frequently form broadly walladherent hypoattenuating structures that are best detected after contrast administration. This is the case in the ventricles, in particular in areas of (old) infarction or ventricular aneurysms. Older thrombi may partially calcify (Fig.23.30a). Calcifications may also be seen in the regions of scar tissue, in particular in an endocardiallocation.
domen. Note that the myocardium on this portal phase scan is nearly isoattenuating to the heart chambers, (d) Large embolus in the right atrium that extended through the right ventricle and ended in the main pulmonaryartery.
c
d
Thrombus in the right atrial appendage (arrow) that is hard to see during the arterial phase (c) but better appreciated on a delayed phase scan (d) in this patient with an anastomotic leak (arrow head) after aortic surgery.
Congenital Anomalies
Some thrombi detach and then may appear as a mass-like intraventricular filling defect that can simulate a tumor (Fig.23.30b). As opposed to tumors (compare Fig.23.44b), thrombi do not take up contrast even on delayed images after 2-5 minutes. /\ ' Wall-adherent thrombi in the atria are less frequent. Thrombotic material may accumulate in the atrial appendages, especially in the left atrium in patients with marked atrial dilatation secondary to mitral stenosis (Fig. 23.31). Concomitant infarctions of brain or internal organs as well as occlusion of peripheral arteries may be seen (Fig.23.31b). Arrhythmogenic Dysplasia
Right Ventricular
Arrhythmogenic right ventricular cardiomyopathy or dysplasia may lead to life-threatening ventricular arrhythmias. There is a congenital disposition toward fatty infiltration of the myocardium of the right ventricle leading to changes in conduction (left bundle branch block) and development of arrhythmogenic zones. Typical locations include the ventricular apex, the RV outflow tract and the inferior wall below the tricuspid valves. The dysfunction may lead to variable ventricular dilatation and dysfunction with reduction of ejection fraction.
Fig. 23.32 Arrhythmogenic right ventricular dysplasia (ARVO) with fatty infiltration of the right ventricular wall (arrowheads) and a pericardia I effusion.
•
CT Morphology
CT can show abundant epicardial adipose tissue, RV dilatation, prominent trabeculation of the right ventricle with low attenuation, and a scalloped appearance of the free wall (Fig. 23.32). Intramyocardial fat deposits may be seen in advanced cases when image quality is good. Cine imaging and functional studies may reveal akinetic or dyskinetic areas with bulging in diastole.
Coronary Arteries, Congenital Anomalies
An anomalous left coronary artery from the pulmonary trunk may lead to postnatal infarction or, if sufficient collaterals to the right coronary artery exist, to a flow reversal in the LCA. With massive collaterals, there will be a L-R shunt with volume overload of the heart. Anomalous coronary arteries with a common trunk may take a course between the aorta and the right atrium or between the aorta and the pulmonary outflow tract. Such arteries may be at an increased risk for injury during median sternotomy. ECG-synchronized CT (EBCT or multislice CT) plays a role in those cases in which the precise location is hard to determine on coronary angiograms.
A coronary artery fistula is defined as connection between the coronary arteries and other heart structures. Most commonly (>90%) the fistula drains into right heart structures such as the right ventricle, atrium, pulmonary trunk, coronary sinus, or superior vena cava. As a result, a L-R shunt develops with concomitant cardiomegaly only in large shunts. CT may define the precise anatomic location although cardangiography remains the technique of choice. Muscular bridges are most frequent in the proximal portion of the LAD but may be seen almost anywhere in the coronary artery tree. They lead to a characteristic systolic "milking" on coronary arteriograms with compression and stenosis of the affected vessel.
799
23
Heart
•
CT Morphology
CTA demonstrates a dilated main coronary artery with an anomalous connection. An anomalous left coronary artery arising from the right coronary artery may either take a path to the left between the pulmonary outflow tract and the aorta to the LAD and then the LCX (Fig. 23.33), or it will run between the right atrial appendage and the aorta to the LCX and then the LAD. Similar anomalies are possible if the right coronary artery arises from the left ostium. Two separate ostia for the LCX and the LAD are possible. Coronary artery fistulae may be demonstrated by ECG-synchronized CT. They present as tortuous dilated vessels that are easily seen on axial sections. It is important to determine into which structures the fistula drains. Volumerendered images provide a good anatomic overview. Muscular bridges present as a (short) portion of the coronary artery that is surrounded by soft-tissue dense material (myocardial muscle). The intramuscular course can best be proven by MPR perpendicular to the vessel course. The proximal LAD is most frequently affected. If systolic images of sufficient quality are available as well, thinning of the affected portion of the vessel may be noted.
Calcium Scoring
Calcification in the arterial wall is part of the development of atherosclerosis and is absent in normal vessels. Coronary calcium as an indicator of atherosclerotic coronary artery disease (CAD) has been known for along time. EBCThas substantially increased the sensitivity of radiological methods for the early detection of coronary calcium and therefore has been used both as a screening tool in asymptomatic patient groups as well as in symptomatic patients, to establish the presence and estimate the risk of coronary artery disease, and thus to estimate the risk of acute cardiovascular events. There is an ongoing discussion as to the clinical value ofEBCT and its use in screening and following patients with CAD. Recently, single, dual, andmultislice (spiral) CThave also been suggested for this purpose. Atherosclerotic (cholesterol) deposits have been demonstrated in a small portion of adolescents and young adults with an increasing prevalence with age. In some individuals, coronary calcium may be seen already in the second and third decades of life but it is not an inevitable aspect of aging. Calcium hydroxyapatite is thought to precipitate in atherosclerotic arteries via an active mechanism that is similar to bone formation and breakdown. The amount of coronary calcium is closely related to total plaque burden, although calcifications represent only the tip of the iceberg. The volume of calcified plaques has been estimated to make up 20% of total plaque burden. There may even be individual patients with no calcifications but vulnerable plaques that are prone to rupture. •
Fig. 23.33 Common right coronary artery trunk with left coronary artery running between the aorta and the right ventricular outflow tract (arrowheads).
CT Morphology and Technical Aspects
TheAgatsron score is traditionally used to express the burden of calcification. It is based on a sliceby-slice analysis of EBCT sections. An arbitrary value of 130 HU was chosen to separate calcifications from high-density pixel noise. This threshold was based on late 1980s EBCTtechnologyto be two standard deviations above the CT number of blood. A (vendor-specific) pixel filter can further eliminate singular noise-related pixels by determining the minimum number of connected pixels (usually 1 -4 pixels, or 2 mm') that are included in the evaluation. With current evaluation software, candidate plaques are highlighted on the images (Fig. 23.34). On each section, the user places an ROI around a group of plaques that is located in
Calcium Scoring Fig. 23.34 Agatston score. Each plaque is encircled by a region of interest (ROI). The calcium score of each plaque is calculated from the area of all pixels above 130 HU multiplied by a weighting factor that depends on the maximum density of the plaque. The Agatston score is the sum of all individual plaque scores.
801
Agatston Score: Ag
= L: Area. x C ROI,
I
I
Cofactors Cj: c, = 0 if CTj,max < 130 HU Cj = 1 if CTj,max = 130 199 HU c, = 2 if CTj,max = 200 299 HU Cj = 3 if CTj,max = 300 399 HU Cj = 4 if CTj,max ;:: 400 H U
the course of a coronary artery, and the program calculates the area on each section and multiplies it by a weighting factor that depends on the maximum pixel number in the ROI (Fig. 23.34). The sum of individual scores yields the total calcium score (Agatston score).
is not yet commonly employed and does not work well for obese patients (Fig. 23.35 b). While there is an excellent correlation between single (and dual) slice CT for a threshold of 130 HU. values for 90 HU differ slightly more. A calcium volume score (CVS) has been developed by Callister (1998) to improve repro• Artifacts ducibility over the Agatston score. It uses interCare has to be taken not to include calcifications polation between sections and determines the volume of calcified plaques. The CVS was used of the aortic bulb at the orifice of the right or left coronary artery (Fig. 23.35 a). Calcifications successfully to demonstrate regression of calof the mitral valve annulus should not be miscified plaques in patients treated with HMGtaken for LCXcalcifications (Fig. 23.35 c). CoA reductase inhibitors. Motion artifacts should be included in the The total coronmy calcium mass uses volumetric and density information and a calievaluation ROI to improve reproducibility (Fig.23.35 c). bration phantom of hydroxyapatite (as in bone Image noise is a limiting factor for EBCT. mineral density measurements) to calculate the With spiral CT scanners, a lower threshold of actual mass of the calcified plaques. It should be least influenced by technical factors such as sec90HU yields a higher accuracy (increased sensitivity for the detection of calcium at the cost of a tion thickness and partial volume effects and slightly reduced specificity), but this technique has a high reproducibility.
a
b
Fig. 23.35 Artifacts for calcium scoring: mural calcium in the aortic wall at the orifice of the coronary arteries must not be included (a). Image noise in obese patients is especially cumbersome if the threshold for calcium detection is reduced from 130 HU to 90 HU (b). Pulsa-
c tion artifacts can blur a plaque (c) but should nevertheless be included in the evaluation ROI to improve interobserver variation. Note the calcification in the region of the mitral valve (arrow).
•
23
Heart
New CT workstations provide the traditional Agatston score as well as newer volumetric measurements (CVS, coronary calcium mass) independent of the CT technology used for data acquisition. They also correct for differences in section thickness and examination technique and therefore hold the potential to further improve reproducibility of measurements. Comparison
of CT Techniques
With the advent of spiral and multislice CT scanning there is now a heated argument about the most suitable CT technique. There is, however, an excellent correlation between the various CT techniques, independent of whether ECG synchronization was used or not. The most important differences arise in patients with a low calcium score, and regarding the reproducibility for follow-up examinations. Motion artifacts may be accepted to a certain degree for quantification of coronary calcium (Fig.23.35c) but are disturbing if high reproducibility is required for follow-up examinations. EBCT has the highest temporal resolution of lOOms. The temporal resolution with single or dual slice spiral CT is above 500ms. With 0.5 s multislice CTscanners, temporal resolution is 250-320 ms with prospective triggering and can be further reduced with retrospective ECG gating. Image noise reduces the accuracy for detecting and quantifying small amounts of coronary calcium (Fig.23.35b) and also reduces reproducibility of such measurements. EBCT uses a fixed setting of 63 mAs that assures low radiation exposure but may be insufficient for more obese patients. The various other CT techniques can adapt mAs settings to the clinical requirements and thus ensure reasonably low noise levels at similar radiation exposure for slim patients. This individual adaptation, however, is not always used in present practice. In EBCT the noise problem has been tackled with repeat examinations and averaging the results, which increase radiation exposure as well. EBCT uses sequential 3mm sections. This may reduce reproducibility of measurements if calcified plaques are sectioned differently at follow-up and the Agatston score is used. Plaque volume (CVS) and mass are more robust measures for this purpose. Spiral techniques can use overlapping reconstruction, which5 also can improve reproducibility.
Radiation exposure with EBCT is low but can be kept in the same range with other CT techniques if scan parameters are adjusted accordingly. Only retrospectively gated (single or multislice) techniques inherently require higher radiation dose (see also p. 784). Availability of spiral and multislice CT is more widespread and there are less hardware costs involved with EBCT. To ensure maximum reproducibility, the same techniq ue (scanner, acq uisition techniq ue, kVpsettings,reconstructionkemel,FOV,filtered back projection methods, and scarmer calibration) should be used for follow-up. Data Interpretation
Prevalence of calcifications increases with age. Interpretation of calcium scores may be based either on absolute calcium scores (Table 23.11) or on scores relative to a standard male and female population group (Fig. 23.36). Despite various controversies, the following consensus statements from the 1996 report of the writing group of the American Heart Association (AHA) have been further substantiated over the years: • Negative EBCT makes presence of atherosclerotic plaque very unlikely. • Negative tests occur in a majority of patients with normal cardiac angiograms. • Negative tests are unlikely in the presence of significant luminal obstructive disease. • Negative tests (Ca score ~O) may be consistent with a low risk of cardiovascular events in the next 2-5 years. • High Ca scores may be consistent with moderate-high risk of cardiovascular events in next 2-5 years. • Positive EBCT confirms presence of a coronary atherosclerotic plaque. • The greater the amount of Ca, the greater the likelihood of occlusive CAD, but there is no one-to-one relationship and findings may not be site-specific. • Total amount of Ca correlates best with total amount of atherosclerotic plaque, but true plaque burden is underestimated.
1 400
extensive plaque
high likelihood of significant stenosis
high
aggressive risk factor modification exercise test or pharmacological stress test
CAD = coronary artery disease; CV = cardiovascular Total coronary calcium mass '== Ca score x 0.83
Age Fig. 23.36 Normal values for coronary calciumscores in asymptomatic men (Rumberger 1996).
The following issues are controversial and remain under discussion: There is conflicting evidence as to whether extensive calcification is associated with more mature stable plaques (with less risk for acute events) or whether the risk of acute cardiovascular events is actually further increased. While some studies report that more extensive calcification is found in patients with stable angina than in those with unstable angina or acute infarction, a recent report found annual event rates (for myocardial infarction or coronary death) of 25% in patients with extremely high calcium scores (> WOO). Calcium scores > 100 have been shown to be highly predictive for separating patients who experience cardiovascular events (angina, infarction, PTCA/
surgery) from those who do not, during 2-5 year follow-up. While a calcium score of zero is highly predictive of angiographically normal or near-normal arteries, it does not rule out the presence of vulnerable plaques, especially in young highrisk patient groups. Up to 7% of patients with acute events have been found to have zero calcium scores. This is in accordance with angiographic findings in patients who by chance underwent angiography prior to a cardiovascular event; in this group over 60% of individuals had no significant (» 50%) coronary artery stenosis. •
Indications
There is little consensus about firm indications for coronary calcium scoring. General screening of asymptomatic individuals is currently not recommended. For asymptomatic subjects with risk factors (see ischemic heart disease, p. 795), however, calcium scoring may help identify individuals who benefit from aggressive risk management or further testing. In a large metaanalysis the risk ratio for coronary calcium in regard to infarction or death was found to be only slightly above conventional risk factor assessment (e.g., Framingham). At present, no hard evidence exists that coronary calcification is an independent predictor for cardiac events. Calcium scoring is not recommended for routine use, but can be justified in selected asymptomatic patients (intermediate risk, elderly) when standard risk assessment is insufficient to direct therapy plans. Various large studies (CARDIA, MESA) are under way to determine which com-
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Heart
parative measures are additive to traditional risk factor models. For diagnosis of CAD in symptomatic patients, a large meta-analysis (ACC/AHA consensus) showed a pooled sensitivity of 91-92% and specificity of 49-51 % for the detection of significant stenoses, with a predictive accuracy of around 70% for EBCT when compared to angiography. Although a different cut point may alter these figures, it was concluded that calcium scoring is not superior to other well-validated tests like exercise ECG, stress echo card iography, or perfusion scintigraphy. The low specificity, in particular, may lead to unnecessary work-up and patient anxiety. In patients with atypical chest pain and a low pretest probability, calcium scoring may identify those who are highly unlikely to suffer from CAD. Calcium scoring has been successfully used to assess progression or regression of CAD. High reproducibility of results is important (see above, p. 802). Volume (CVS) and mass scores appear superior to the classical Agatston score. Coronary calcium has been shown to be reliable for distinguishing ischemic from nonischemic cardiomyopathy. Cost Effectiveness
Conducting effective cost-benefit trials is difficult. In model calculations by Rumberger et aI. (1999), EBCT was shown to be cost effective, compared to angiography, treadmill testing, stress echocardiography, and stress thallium scans, in specific subsets of the general population. A testing pathway utilizing quantification of coronary calcium by EBCT as an initial noninvasive approach minimized direct costs, and maximized cost effectiveness in ambulatory patients with low to moderate prevalence for obstructive coronary artery disease (70%). Coronary angiography as the first and only test proved most cost-effective in patients with a high prevalence (> 70%) of disease. The data, however, were deemed insufficient, as yet, by the ACC/AHA expert consensus document to warrant widespread screening efforts. More cost-benefit analyses are considered necessary.
Coronary Artery Stenosis Coronary artery stenosis is associated with coronary ischemia but there are substantial numbers of individuals in whom no significant stenoses could be demonstrated on a coronary angiogram prior to an acute cardiac event. Absence of high-grade stenosis, therefore, does not rule out a high plaque burden, particularly instable plaque prone to rupture. Correlation of high-grade stenosis and acute cardiac events is poor. High-grade stenosis, however, is frequently associated with symptoms of chronic coronary ischemia and usually requires treatment by PTCA, stent graft placement, or surgical revascularization. EBCTand multislice CTA have been found to provide a sensitivity of between 60% and over 90% for the presence of significant stenoses, depending on the size of the vessels. Stenosis detection is more accurate for proximal than for distal stenoses or for small caliber vessels. If these numbers can be improved, CTA may be used as a non-invasive tool to rule out coronary artery disease (CAD) in populations with a low to moderate prevalence, such as asymptomatic patients with high cardiovascular risk factors, unspecific exercise test findings, or patients with atypical chest pain. Coronary CTA is not yet suited to following patients with known CAD, typical angina, or obvious myocardial ischemia at exercise testing, because of a high incidence of marked calcifications that hamper CT evaluation of stenoses. CT Morphology
CT angiography is not sensitive enough to grade coronary stenoses precisely because of the small size of these vessels and considerable problems with pulsation effects. For this reason, a grading scheme has been suggested that uses five categories: Normal o 0% 1 1-49% Nonobstructive disease 11 50-74% Significant stenosis 1lI 75-99% High-grade stenosis Occlusion IV 100% Suspicious areas should be analyzed in at least two perpendicular planes (Fig. 23.37). The suspected stenosis then has to be classified and attributed to a particular segment of the coronary tree (see Fig. 23.4). In addition, the nature of the
Coronary Artery Stenosis
a
c
b
Fig. 23.37 Long, moderately stenotic segment of an intermediate, coronary artery, a third branch arising at the bifurcation of the LCA into the LAD and LCX (8X1.25/2.5), seen on CPR and transverse sections
through the stenosis (a), oblique thin-slab MIP (b) and a volume-rendered display (c). This stenosis was due to a soft plaque and lead to a vascular occlusion 3 month later with massive left ventricular infarction.
stenosis should be stated with respect to plaque composition (see below). Blooming of calcified plaques is a problem that can be reduced by using wide window settings for image analysis. Extensive calcified plaques are often present in areas of remodeling and cause no significant stenoses until the very late and severe stages of atherosclerosis. These areas may be impossible to evaluate with current CTA techniques. Regions with motion artifacts should be excluded from the evaluation (see Fig. 23.16). With retrospective gating, additional data sets can be reconstructed and may be less affected by cardiac motion (see above, p. 774).
•
Table 23.12
Plaque
Morphology
The normal arterial wall measures 0.1 mm and is not visible on CTA. An overview of the various plaque types is given in Table 23.12. Calcified or hard plaques have a density well above fibrous tissue (90 H). Calcified nodules present as small dots of calcium surrounded by dense soft tissue (Fig.23.38a, b). They are rarely associated with acute events in coronary thrombosis and are considered to represent atheromas that did not rupture but directly calcified instead. Extensive coronary artery calcifications without any non-calcified plaque are rarely the locations for significant stenosis.
Plaque morphology (Stary, 1995; Becker 2001)
Plaque
AHA
calcification
classification
noncalcified portions
shape
remodeling
'50HU
well-defined
positive
V
no may be'present
'70HU
well-defined
positive/negati
trhrombus
VI
may be present
'40HU
irregular
high-grade ste or occlusion
oFibrocalcified
VII
large calcification, across vessel
"100HU or absent
irregular
stenosis
.Calcified nodule
VII-VIII
small round nodule or calcification along vessel wall
"100HU or absent
irregular
positive
JPibrotic lesion
VIII
no
irregular
positive/negative
I&Atheroma Thin fibrous cap atheroma
IV
plaque
AHA I - III cannot be visualized by CT
100 HU
805
23 Heart
a
b
c
Fig.23.38 Calcified coronary plaques (4x1/1.S). Small round deposits are due to calcified nodules (a, b), which rarely cause stenosis (arrows). Larger, often circumferential, and longer calcifications and soft tissue
material are caused by fibrocalcified plaques (arrowhead) that are frequently associated with stenoses (b. c). Wide window settings are required for evaluation.
Fibrocalcified plaques are characterized
well below 1 mm. They are rarely detected by CTA. Atheromas contain a large lipid (cholesterol) core and have been described as displaying a CT attenuation of 50 HU or below. CT cannot differentiate a fibrous cap from a lipid core. Atheromas are wall-adherent and may be focal or present as a long region of hypoattenuating, irregular wall thickening (see Fig. 23.37). Remodeling may make the lumen look unaffected, despite considerable amounts of plaque burden, but significant stenoses due to atheromatous lesions may be present, even in patients with a coronary calcium score of O. Fibrous plaques are denser and have been associated with a CT attenuation around 90 HU. They are commonly associated with stenoses.
by longer segments of irregular calcifications associated with dense fibrous tissue (some 90 H) on coronary CTA. They are considered to represent a consequence of plaque rupture with negative remodeling, causing scar tissue and stenosis. These plaques and the associated stenoses are particularly hard to evaluate by CTA (Fig.23.38c) because of "blooming" effects that let the calcifications appear larger than they really are (caused by limitations of spatial resolution) . Soft plaques are non-calcified and may consist of preatheroma, atheroma, fibroatheroma, or fibrous plaques. Preatheromas are thin walladherent layers of soft plaque with a thickness
a
b
Fig. 23.39 Coronary thrombi are hypodense intraluminal filling defects and are always associated with highgrade stenoses or occlusion. The vessel wall is seen as a
c hyperdense rim (a). Occlusion of a segment of the LAD demonstrated on longitudinal (b) and horizontal CPR (c).
Valvular Disease Fig. 23.40 Coronary aneurysm of the LAD in a patient with EhlersDanlos syndrome. Location and calcifications are best demonstrated on VRT (b) while the thrombosed portions are best seen on CPR (a).
a
Fibroatheromas have intermediate CT attenuation, between 50 HU and 90 HU. Coronary thrombi can be suspected if an artery has a dark center and a bright rim (Fig. 23.39). They are always associated with high-grade stenoses or occlusion. The density of the thrombus may vary depending on its age and overlaps with the various types'of soft plaques. Coronary Artery Aneurysm and Dissection
Coronary artery aneurysms are associated with a rast number of systemic diseases including Ehlers-Danlos or Marfan syndrome, SLE, panar-
b
teritis nodosa, Takayasu arterities, Kawasaki disease, or generalized fibromuscular dysplasia. They may be mycotic or posttraumatic but can occur also in atherosclerosis. They represent a focal dilatation of a coronary artery and may be associated with wall calcification, thrombi, stenoses or occlusion. CTA is superior to other imaging modalities (with the exception of intravascular ultrasound) for displaying not only the lumen but also calcification and thrombi in the aneurysm (Fig. 23.40). Coronary artery dissection is a rare complication of type A aortic dissection and is due to a retrograde extension into the coronary orifices. Focal dissections may be due to cardangiographic interventions or surgery.
Valvular Diseasi
Aortic regurgitation may be caused by intrinsic
aortic valvular disease (congenital bicuspid valve, rheumatic endocarditis, bacterial endocarditis, a myxomatous valve in Marfan syndrome, breaks or leaks in a prosthetic valve), by dilatation of the aortic annulus (aortic aneurysm, dissection, syphilitic aortitis, rheumatic diseases, and Marfan syndrome), or by laceration of the aortic valve. Aortic stenosis can be caused by a congenital supravalvular, subvalvular, or valvular stenosis (most frequently the bicuspid valve), by rheumatic valvulitis (almost always combined with mitral valve disease), or senile calcific valve degeneration.
Mitral stenosis is more frequent in females
(8: 1) and is mainly due to rheumatic heart disease. Rarely, an obstructing mass may cause mitral stenosis. Atrial fibrillation is common in these patients. Thrombi forming in the left atrial appendage may cause systemic emboli. Mitral regurgitation in the adult is most frequently caused by mitral valve prolapse, rheumatic heart disease, or dysfunction or rupture of a papillary muscle. Rarely it may be caused by a left atrial myxoma, a split mitral valve leaflet in endocardial cushion defects, or a strongly calcified annulus (especially in older women).
807
23 Heart
Pulmonary stenosis is frequently associated
with various congenital heart defects but may also occur as an isolated finding. Acquired stenoses or pulmonary valve regurgitation are rare. Tricuspid insufficiency is usually a consequence of right ventricular dilatation together with increased RV pressure due to a right ventricular obstruction or severe pulmonary hypertension. Rarely may it be caused by bacterial endocarditis in drug addicts, or by rupture of a papillary muscle. Tricuspid stenosis is almost always due to rheumatic disease, and is then invariably associated with mitral valve stenosis. Rarely it may be caused by a right atrial tumor (myxoma, carcinoid tumor) or by SLE. Valvular disease is evaluated primarily by echocardiography, and by angiography for pressure measurements. MRI is becoming an increasingly important modality. Even standard CT is able to demonstrate indirect signs of valvular disease. ECG-synchronized techniques hold some promise in directly visualizing the valvular cusps and leaflets. •
CT Morphology
Indirect signs of valvular disease are summarized in Table 23.13. Calcifications of the valves have to be differentiated from calcifications of the valvular ring. Aortic regurgitation causes enlargement without thickening of the left ventricle, a large amplitude of LV motion, an increased ejection fraction, and an increased pulsation of entire aorta (best seen on MPR from non- ECG synchronized data). The aorta is normal in intrinsic
Table 23.13 I
•••
; •• ',{
Indirect signs of (compensated)
valvular disease 1\
/If, ~ J\J\J\J\J\J\
val vular disease. In patients with dilatation of the aortic annulus (e.g., Marfan syndrome), aortic regurgitation can be indirectly diagnosed if the aortic root is >5cm wide (>80% probability), and >6cm (always). The aorta in these patients is enlarged, often calcified, and elongated resulting in a tortuous descending aorta. In aortic stenosis, there is frequently a poststenotic dilatation of the ascending aorta and a thick-walled left ventricle (concentric LV hypertrophy) of either normal or enlarged size. In the compensated state there is a hyperdynamic contraction of the LV. Calcifications of the aortic valve are a good indicator of aortic valve stenosis in adults >30 years indicating a gradient above 50mm Hg (Fig. 23.41). Calcification of the aortic valve annulus is frequently present. Only on ECG-gated images may a decreased aortic valve area be visible « 0.8 ern'). Subvalvular aortic stenosis may present as a "1-2mm thin membranous stenosis (type I), a thick collar-like stenosis (type II), an irregular fibromuscular stenosis (type III) , or an excessive thickening of the upper portion of the ventricular septum causing a tunnel-like stenosis of the left ventricular outflow tract (tunnel sub aortic stenosis, type IV). For CT evaluation, ECG-triggering or gating is important. Subvalvular aortic stenosis may be due to these fixed anatomic stenoses, or due to a functional narrowing during systole in asymmetric septal hypertrophy (ASH) , idiopathic hypertrophic subaortic stenosis (IHSS) or hypertrophic obstructive cardiomyopathy (HOCM). These entities are characterized by normal to small, right and left ventricular cavities, and an asymmetrically thickened
1\
PA Aortic
"
regurgitation
LA
LV
Ao
I
II
-II I
-H
Aortic stenosis Pulmonary
regurgitation
Pulmonary
stenosis
f'['
/I
ii
H
T
ii
Mitral regurgitation Mitral stenosis Tricuspid
regurgitation
Tricuspid
stenosis
f
i
/\
/I
/\
'[
I
Ii
""""-
TT
RA= right atrium, RV" right ventricle, PA = pulmonary artery, f = dilatation, /I f = marked dilatation, H = hypertrophy
Acute myocardial infarction Rupture of the ascending aorta (dissection. aneurysm) Rupture of the pulmonary trunk Trauma (blunt/penetrating) Coagulopathy. anticoagulants Cardiac surgery. catheterization Tumors Chemotherapy * No sharp transitions between given HUlimits.
fluid can give suggestions as to the potential etiology of an effusion (Fig. 23.47, Table 23.14). One has to be careful to avoid measurement errors due to pulsation-induced inhomogeneities. Indirect signs of cardiac tamponade include decreased pulsation artifacts in the ascending aorta (indicating low cardiac output) and a widened superior vena cava (indicating decreased right heart filling). Heart size may enlarge rapidly. On ECG-gated acquisitions there is evidence of a diastolic collapse of the RV and cyclic collapse of either atrium.
Pericarditis
Acute pericarditis
can be caused by tuberculosis or viral (Coxsackie B) infections. by chronic renal failure. rheumatoid arthritis. and radiotherapy to the mediastinum. Constrictive pericarditis may develop as the consequence of acute pericarditis and is characterized by fibrous thickening of the pericardium. which leads to restriction of heart motion and decreased filling of the ventricular chambers. It affects males more often than females (3: 1). Concentric neoplastic involvement of the pericardium leads to similar symptoms.
after esophagoscopy (c). Note the contrast material and air in the pericardial sac after oral administration of iodinated contrast material.
Pericardia! Tumors
CT is excellently suited to visualize pericardial morphology in constrictive pericarditis and can aid in pre-surgical evaluation prior to pericardial fenestration or pericardectomy. •
CT Morphology
A thickened or enhancing pericardium together with a pericardial effusion suggests active pericarditis (Fig. 23.48). Adhesions between parietal and visceral pericardium cannot be directly demonstrated by CT but can be suspected if there is an atypical distribution of pericardia I fluid. In constrictive pericarditis the pericardium is thickened to 2 mm or more, often containing extensive linear or plaque-like calcifications. The calcifications are most commonly seen over the right ventricle, the posterior surface of the left ventricle, and in the atrioventricular groove. 3D rendering visualizes the site of the most extensive calcifications prior to surgery (Fig. 23.49a, b). Secondary signs involve dilation of the SVC and IVC, and reflux of contrast into the liver veins and coronary sinus. There is flattening of the right ventricle and bulging of the intraventricular septum towards the left. Pleural effusion and ascites are signs of decompensation. Pericardial Tumors
Benign pericardial neoplasms include teratoma, leiomyoma, hemangioma, and lipoma. Malignant neoplasms include mesothelioma, sarcoma, metastases from lung and breast cancers, lymphoma, leukemia, and melanoma. Metastases or continuous tumor extension into
Fig. 23.48 Infectious pericarditis with pericardial empyema and enhancement of the pericardium.
the pericardium are much commoner than primary tumors. CT is excellent for defining the extent of benign tumors and for demonstrating mediastinal involvement in malignant disease, but echocardiography and MRI often are better suited for demonstration of myocardial infiltration. •
CT Morphology
CT demonstrates a mass adjacent to or within the pericardial sac (Fig. 23.50). A pericardial effusion is more often seen in malignant disease, in which case it is often hemorrhagic (>30HU). Thin-section imaging and ECG-triggering improve delineation of the mass relative to the heart chambers. An intact epicardiac fat line makes myocardial involvement highly unlikely. Malignant pericardial tumors may lead to encasement and stenosis of coronary arteries (see Fig. 10.10).
Fig. 23.49 Constrictive pericarditis after surgical fenestration of the pericardium anterior to the right ventricle. Note the residual calcifications of the pericardium posterior to the left ventricle (a) and at the base of the heart (b). There is substantial dilatation of the atria and increased pulsation of the pulmonary outflow tract (arrows).
a
b
815
23
Heart
Fig. 23.50 Pericardial sarcoma infiltrating the region ofthe left atrial appendage.
Pericardial Cyst
lies (VSD, PDA, mitral stenosis). The smaller foraminal defects are distinguished from complete absence (bilateral, left -sided, or diaphragmatic pericardial aplasia) oflarge portions of the pericardium. Acquired defects are usually caused by surgical procedures to the heart or pericardium (pericardectomy). Pericardial defects are commonly asymptomatic but may be associated with nonspecific intermittent chest pain and cardiac symptoms, as well as positional discomfort. A large foraminal defect requires surgery because herniation of the left atrial appendage, left atrium, or left ventricle may occur. CT is excellently suited to directly demonstrate the pericardium as well as secondary effects due to herniation of cardiac structures. •
CT Morphology
Small foraminal defects will show no abnormalPericardial cysts present as a paracardiac mass, ity other than a short interruption of the periusually in the right or left costophrenic angle. cardial line on axial sections. With moderate Pericardial cysts in the mediastinum are rare. foraminal defects there may be a focal bulge in -the area of the right ventricular outflow tract, See also Chapter 10, Mediastinum (p. 373). main pulmonary artery, or the left atrial ap• GT Morphology pendage. Large defects may lead to herniation of CT demonstrates a smooth, round or ovoid mass cardiac structures or lung, e.g., insinuation of in typical location with a diameter of up to 8 cm lung between the aortic arch and the pulmonary that has attenuation values of 20-40 HU, and artery. A diaphragmatic pericardial aplasia may occasionally higher. lead to interposition of lung between the heart and the left hemidiaphragm. Absence of the anterior portions of the pericardium will cause an increased distance between sternum and the Pericardial Defect heart in the supine patient (lack of the sterCongenital pericardial defects are caused by a nopericardic ligament). Complete absence may failure of pericardial development that may be lead to levoposition of the heart. Patients with associated with bronchogenic cysts, diaphragpneumothorax and pericardial defects may matic hernia, sequestration or cardiac anoma- develop pneumopericardium.
Cardiac
Trauma
Penetrating Trauma
Penetrating injuries to the heart have a high mortality and less than 20% reach the hospital alive. The clinical presentation can be quite deceptive and can range from hemodynamic stability to profound shock with cardiac arrest. The presence of pericardial effusion should raise suspicion of penetration but the classic signs of pericardiac tamponade like Beck's triad (muffled heart tones,
hypotension, and distended neck veins) or Kussmaul's sign (jugular venous distension on inspiration) are usually absent. The commonest causes are stab or gunshot wounds. The anterior located ventricles are at greatest risk, whereas the left atrium is only rarely involved. Stab injuries result in perforation of the pericardium or myocardium that may quickly seal but later the effects of pericardial tamponade seriously compromise car-
Postoperative and Postinterventional Changes diac function. Survival rates may be as high as 60-65% with appropriate therapy. Gunshot injuries usually result in large myocardial and pericardial injury. more often with hemorrhage in the left hemithorax. Survival rates of only 10-15% are reported. Diagnosis is usually made with echocardiography as part of a focused trauma ultrasound protocol. CT is agood alternative in stable patients and has the advantage that it can also evaluate the other vulnerable structures of the thorax and abdomen and that it can show the knife or bullet tract. Unstable patients should proceed directly to emergency thoracotomy with intensive fluid resuscitation and cardiorrhapy. •
CT Morphology
CT can show as little as 25-50 ml pericardial fluid. usually located anteriorly to the right ventricle. Signs of pericardial tamponade (see above. p. 814) have to be excluded. There may be hemothorax. usually on the left side. An abnormal contrast leakage into the pericardium is a direct sign of myocardial injury.
phragmatic pericardium. Pericardial ruptures can be isolated or be associated with cardiac herniation. Dislocation of the heart harbors the risk of incarceration of myocardium or strangulation of a coronary artery. CT performed for blunt chest trauma may demonstrate the lesion. More than 80% of patients with myocardial lacerations after blunt chest trauma die acutely. Myocardial lacerations are present in 0.2-2.0% of patients that reach the hospital. Early diagnosis and emergency surgery can result in survival rates of 50-80%. Although the rupture may involve any part of the myocardium including the atrial appendages. the most frequent location is the right atrium. Patients usually present with a widened mediastinum or with left hemothorax on chest x-ray. Diagnosis is usually made by transesophageal echo cardiography. CT of the thorax in such patients is commonly performed to rule out vascular mediastinal injury and may be life-saving if myocardial rupture is detected. However. the findings are often subtle and easily missed if the radiologist is not actively searching for such a lesion. CT Morphology
Blunt Trauma
Cardiac contusions or infarcts rarely show abnormalities on CT. In pericardial ruptures, the thin
Blunt cardiac injury is usually caused by a direct blow or a deceleration injury in a motor vehicle accident. Relatively more cases are seen following sternal injuries. Cardiac contusions are frequent but rarely manifest clinically. Infarction of myocardial tissue is rare. It is commonly caused by injury to the coronary artery (dissection. thrombosis. or embolism of plaque material). Presently. there is no indication for CT for suspected cardiac contusion. Pericardial ruptures are rare. They most frequently involve the pleuropericardial or dia-
line of the pericardium may be interrupted, and pneumopericardium, cardiac dislocation, or an empty pericardial sac may be seen. In myocardial lacerations, CT will show hemopericardium with or without associated cardiac tamponade. There is mediastinal hemorrhage. Focal areas of abnormal enhancement of the myocardium or (subtle) abnormal contrast leakage into the pericardium are direct signs. A (usually left-sided) hemothorax indicates concomitant injury to pericardium and pleura.
Postoperative
and Postinterventional
CT is the imaging modality of choice for the evaluation of local complications in the mediastinum and after sternotomy. For the heart, the preferred modalities for evaluating postoperative and postinterventional complications are echocardiography, cardiac angiography, and MRl, depending on the clinical question.
Changes CT can provide a minimally invasive procedure for assessing the patency of an aortocoronary venous bypass but still has trouble evaluating bypass stenosis. CT is not used to evaluate cardiac valves or septal defects, but it can be used in the follow-up of complex surgically corrected heart defects in patients with cardiac pacemakers and in cases difficult to evaluate with ultrasound.
817
23 Heart
Normal Postoperative Findings
Retrosternal fluid collections, small hematomas, and air inclusions are considered normal findings during the first three weeks after thoracic operations on the heart and great vessels, and these changes regress with passage of time. Often it is difficult to distinguish normal from pathologic findings in a single examination. Pleural effusions, deficient aeration of the posterobasal lung, and compression atelectasis are consistently observed. Pericardial effusions are not a normal finding. In cases where a median sternotomy was performed, the osteotomy line should not be visible, although it is normal to find areas ofimpaction or slight displacement.
The role of CT is to demonstrate graft patency. This task does not require ECG synchronization. Evaluation of graft stenoses may be possible for ACVB but is very difficult to achieve even with EBCT and gated multislice CT techniques. •
CT Morphology
Aortocoronary venous bypass grafts (ACVB) are usually performed in the middle third of the ascending aorta, although higher or lower anastomoses (e.g., the brachiocephalic trunk) are feasible. Bypasses to the left coronary artery are routed over the pulmonary trunk. Internal mammary arterial bypass grafts using the left (LIMA) or right internal mammary artery (RIMA) are more difficult to evaluate due to the multiple clips that are used to occlude the small side branches.
Since the diameter of a venous bypass is 3-7 mm, ACVBs can generally be visualized with CT (Fig. 23.51). A bypass that does not opacity after intravenous contrast administration is occluded, whereas a stenosed bypass will still show complete opacification. Stenoses most frequently occur at the site of anastomosis. Stenoses at the aortic anastomosis often can be evaluated even without ECG synchronization. Stenoses of the distal anastomosis, however, require triggering or gating techniques. Because of the long scan range of up to 20 em, evaluation with EBCT is difficult, and multislice CT with 4-slice scanners either requires fast gating techniques (P * = 3) or thicker sections (4 x 2.5-3 mrn), which limit spatial resolution. Mural calcifications may develop in the bypass over time without causing occlusion or stenosis. A narrow, nonenhancing band of softtissue density 1-5 mm wide along the course of an expected ACVB signifies a chronic occlusion (Fig.23.51). With LIMA and RIMA bypass grafts, patency is usually detectable, but the lumen is often very small (1-4 mrn). Detection of stenoses is not yet
Fig. 23.51 Aortocoronary venous (ACVB). Volume rendered display of grafts from the ascending aorta to the segment and the distal LCX (a). Chronic
bypass grafts normal venous distal LAD, D1 occlusion of an
c
ACVB to the LCX (arrow), and patent LIMA bypass to the first diagonal branch (arrow head) and ACVB to the LAD (wide arrow) (b. c).
Catheter Ablation
ofArrhythmogenic
Foci
a
b
Fig. 23.52 Volume-rendered display of the interior of the chest wall with RIMA and LIMA (a). LIMA and RIMA
bypass and radial artery transplant in a patient with multiple coronary stenoses (16 x 0.75/3.6) (b).
reliably possible. Preoperative imaging can demonstrate the anatomy of the internal mammary arteries (Fig. 23.52).
ing is mandatory for morphological evaluation of stent patency but even then, blooming effects will make evaluation difficult. Wide window settings are helpful in such situations. Morphological analysis relies on longitudinal and transverse sections using MPR and CPR capabilities (Fig.23.53). Multisection flow studies with EBCT have been shown to indirectly detect stent obstruction by demonstrating malperfusion of the dependent regions.
Coronary Stents
Coronary stents are frequently used to treat coronary stenosis primarily or to salvage acute complications of PTCA. Nevertheless, the risk of restenosis or occlusion is still high with most stents. CTA has a limited spatial resolution for the evaluation of stents in smaller more distal vessels but may be sufficient for the evaluation of proximal stent grafts in larger vessels. Perfusion studies with EBCT have shown promising results. At present, however, there is no hard indication for CT. •
CT Morphology
Any type of ECG-synchronized CT study is able to precisely localize a stent. Thin-section imag-
Catheter Ablation ofArrhythmogenic
Transcatheter ablation of arrhythmogenic foci in the left atrium may be performed in patients with severe atrial fibrillation. For this purpose a loop-shaped electrocoagulation wire is placed in the orifices of the pulmonary veins, thus interrupting the electrophysiological connection between the pulmonary veins and the atria. The anatomy of the pulmonary venous orifices is
Fig. 23.53 Coronary artery stent near the ostium of the RCA in the region of a stenosis due to a calcified plaque. Axial (a), longitudinal (b) and perpendicular MPR (c). Only larger stents in the more proximal portions of the coronary arteries can be evaluated (16XO. 75/ 3.4).
a
Foci
b
c
820
23
a
Heart
c
b
Fig. 23.54 Ablation of arrythmogenic tissue at the pulmonary venous ostia. (a) Display of pulmonary venous anatomy is possible from low-dose spiral scans (4 x 1 /6,
2.2 mGy). (b) Follow-up with occlusion ofthe left upper lobe vein. (c) Note the pulmo-pleural venous anastomosis in this patient (arrow).
highly variable, and cardangiography only provides projection images that provide little information about the sagittal diameter of the veins, thus making the procedure time-consuming and risky because overcoagulation may lead to stenosis or even occlusion of a pulmonary vein. Low-dose multislice CT is sufficient to provide the necessary anatomical information about the left atrium and the pulmonary veins. Gating should not be performed because it requires an unnecessarily high dose for this purpose. Even triggering is not necessary because of only minor movement artifacts in the vast ma-
jority of cases. Contrast material injection is helpful but not mandatory, and can be combined with a Iow-kvp scanning technique.
a
b
Fig. 23.55 Perforation of a right ventricular pacemaker into the epicardial fat (a.) Malposition of a pacemakerthatwas no noted onthe PAradiograph. The
•
CT Morphology
Volume rendering after removal of the aorta is best suited to provide an overview of the venous anatomy (Fig.23.54a), and multiplanar crosssections though the orifices of the pulmonary veins can be used to provide the measurements necessary for the choice of loop size. Follow-up may demonstrate stenoses or even occlusion of one or more pulmonary veins.
c pacemaker entered the coronary sinus (b) and ended in the posterior vein of the left ventricle (c).
Heart Transplantation 821 Due to segmental venous congestion, an increased lung density and various grades of interstitial edema may be seen. In chronic occlusion, calcifications may occur (Fig.23.54b), and collateral vessels to other segments or even the chest wall may form (Fig. 23.54 c). Pacemakers
CT can be used to define venous cardiac anatomy prior to complex pacemaker placement. It can prove penetration of a pacemaker wire through the myocardium, even in difficult cases that cannot be resolved by fluoroscopy (Fig.23.55a). CT can also depict malposition of pacemaker wires (Fig.23.55b), but this is usually an incidental or concomitant finding. Dislodged catheter or wire fragments in the pulmonary circulation can be precisely located.
Congenital Heart Disease
Patients with surgically repaired heart disease require long-term follow-up, which generally is done with echocardiography. Many cases additionally require MRI for the follow-up of complex defects (transposition of the great vessels, atretic valves, single ventricle, tetralogy of Fallot, etc.) and aortic anomalies (coarctation, duplicated arch, etc.). In most cases CT can also resolve common postoperative questions on the anatomy and morphology of the vessels about the heart (Table 23.15, Figs. 23.57 and 23.58), but image quality may be degraded by pulsation artifacts and there may be radiation concerns if follow-ups are maintained over a period of years. As a result, CT is reserved mainly for use in patients with cardiac pacemakers and in emergency situations. Heart Transplantation
Valve Replacement
Cardiac valves that contain metal are a source of CT artifacts. It is rarely possible to detect perivalvular leaks (Fig.23.56a) or endocarditic valve lesions. Usually these are incidental or concomitant findings on CT examinations performed for other indications. The modalities of choice are echocardiography and cardiac angiography. CTA is well suited, however for detection and mapping of anastomotic leaks after valve replacement (Fig.23.56b).
Cardiac transplantation is an accepted treatment for end-stage cardiomyopathy or coronary artery disease. Improvement in immunosuppression and early diagnosis of rejection has resulted in improved survival rates in the order of 60-80%. In orthotopic transplantation the native heart is removed and the donor heart is joined to the recipient atria, aorta, and pulmonary artery. Heterotopic transplantation is much less often performed and in this procedure the donor heart is placed in the right hemithorax and connected to the recipient in such a way that the donor heart provides the bulk of left-
Fig.23.56 Complications after aortic valve replacement. Paravalvular leak (a). Anastomotic leak with pseudoaneurysm (b). a
Mustard and Senning procedures for atrial inversion
Reverse blood flow at the atrial level to palliate complete transposition of the great arteries (TGA)
Intra-atrial baffle is inserted to connect the SVC and IVC to the subpulmonary ventricle (mLV) and to channel LV blood to the systemic ventricle (mRV)
Stenosis at thejuncti the SVC or IVCwith systemic venous atrium or at thejunction ofthe pulmonary veins with the pulmonary venous atrium
Fontan procedure
Segregate the pulmonary and systemic circulations in patients with anatomic or functional univentricular heart (e.g., single ventricle, tricuspid atresia, etc.)
An anastomosis is created between the SVC, IVC or right atrium and the pulmonary arteries
Anastomotic stenosis, intraatrial thrombi
Blalock-Taussig shunt
Increase pulmonary blood flow (e.g., in tetralogy of Fallot or pulmonary atresia)
An anastomosis is established between the subclavian artery and the ipsilateral PA ora plastic shunt is interposed between the subclavian artery and the PA
Shunt patency, peripheral pulmonary arterial stenoses (area of anastomosis)
Coarctation repair
Bypass or remove the aortic constriction
Multiple options include patch grafting, resection with end-to-end anastomosis, and extracardiac bypass
Exclude aneurysms in the ascending aorta or at the surgical site, exclude recurrentstenosis, confirm by-", pass patency
Rastelli procedure
Bypass or remove pulmonarystenosis, e.g., in TGA with VSDand (sub)pulmonic stenosis
Includes insertion of an extracardiac conduit (e.g., a homograft) between the subpulmonary ventricle and PA
Conduit stenosis
IVC = inferior vena cava, SVC =superiorvena cava, mRV = morphologic right ventricle, mLV = morphologic left ventricle, PA = pulmonary artery, TGA = transposition of great arteries
a
b
Fig. 23.57 Correction of D-TGA by a Mustard/Senning procedure. A baffle in the atria (arrowheads, c) directs the blood from the systemic circulation to the morphologic left (subpulmonic) ventricle, and the blood from
c the pulmonary veins to the morphologic right (subaortic) ventricle. A stenosis of the baffle at the region indicated by the arrows (a, b) is a typical complication.
Sternal Debiscence
a
Fig. 23.58 An arterial switch procedure for correction of transposition of the great vessels results in an abnormal anterior postion of the pulmonary artery but successful separation of the right and left circulation. Low-dose scan (80kVp, CTDlvoi= 0.6 mGy) in a 1-year-
side cardiac output, while the native heart provides the right -side output. Complications are common and often related to the surgical procedure. Non-surgical complications include infection with or without empyema and mediastinits, acute rejection, accelerated graft atherosclerosis, Ieukencephalopathy, and £ost-transplant lymphoproliferative disorder (PTLD). CT is an excellent technique for detection of these complications (see also Chapter 10, Mediastinum). •
c
b
CT Morphology
The normal postoperative morphology in orthotopic transplantation may show a high main pulmonary artery and a large space between the recipient SVC and the donor ascending aorta. There may be a change in caliber between donor and recipient aorta and a waist in the atria at the corresponding anastomosis (Fig. 23.59). The remnant of the donor SVC is medial to the recipient SVC and the donor aorta. In heterotopic transplantation there is obvious right -sided cardiomegaly with anastomoses between both atria, aortae and pulmonary arteries.
old infant (a). In a Fontan operation, the superior vena cava is anastomosed to the right pulmonary artery, bypassing the right ventricle (b. c). Note the lack of a SVC in (b) and the highly concentrated contrast material entering the right pulmonary circulation in (c).
CT is the imaging technique of choice for
this'entity. •
CT Morphology
Sternal dehiscence is seen most clearly on curved coronal reformations. CT demonstrates intrasternal gas bubbles or liquid pre- and retrosternal masses that show peripheral enhancement (abscesses). If a sternal fistula is suspected, the fistulous tract is injected with dilute (" 1:10) contrast material to detect osseous involvement and contrast extravasation into the retrosternal space. These findings are best appreciated on sagittal reformations.
Sternal Dehiscence
Sternal dehiscence following a median sternotomy is defined as a gap of at least 2 mm persisting at the osteotomy site. Sternal dehiscence leads to sternal instability and predisposes to sternal infection, which is diagnosed clinically.
Fig. 23.59 Typical indentation of the left and right atrial anastomosis after heart transplantation. Note the dilatation of the right atrium with secondary tricuspid insufficiency.
824
23
Heart
Mechanical Support of Circulation. Artificial Heart
Various technique have been recently developed for internal or external mechanical support of end-stage heart failure. They include intraaortic ballon pumps, mechanical tube systems that
a
b
Fig. 23.60 Artificial heart implanted in the same patient as in Fig.23.29c. The connections to the atria and ventricle are best demonstrated on MPR (a) while VRT provides a good overview ofthe position ofthe Tef-
connect the heart to an external pump or completely implantable systems (artificial hearts) . CT is a valuable tool to detect complications such as pseudoaneurysms, perforation, hematoma or infection (Fig. 23.60). ECG gating is rarely required.
c Ion sheets placed around the cardiac tubes (b). Note the mediastinal hematoma demonstrated by the axial sections (c).
24 Vascular System M. Prokop, C. Engeike
Anatomy
.
Pulmonary Vessels (p. 827) Aorta (p. 828) Aortic Side Branches (p. 829) Principal Systemic Veins (p. 830) Portal Venous System (p. 831) Examination
Postoperative Findings (p. 918) Malpositioned Catheter Systems (p. 918) Anastomotic Complications (p. 919) Arterial Dissection (p. 919) Stenoses and Occlusions (p. 920) Infected Prosthesis (p. 920) Normal
Angiographic
Interventions:
Patient Selection
Planning of Angiographic Interventional Procedures (p. Follow-up after Angiographic Procedures (p. 921) Planning of Endovascular Aortic Repair (p. 922) Follow-up after Endovascular Aortic Repair (p. 926)
cr angiography (CTA) has revolutionized vascular imaging. Vessels smaller than I mm in diameter can be visualized with single slice CTA, and vessels 2 mm or more in diameter can be diagnostically evaluated. The acquisition of 3D data volumes makes it possible to generate "angiographic" views that facilitate anatomic orientation, With the advent of multislice CT A, near isotropic resolution becomes available for most applications and allows for evaluation of even smaller vessels. Multislice scanning makes CTA less technically challenging and more robust. It reduces scan time and the vulnerability to mo-
921)
tion artifacts, and may be used to substantially decrease the required contrast material volume to as little as 50 ml in selected cases. The advantages ofCTA over arterial angiography (DSA) include substantially lower invasiveness, less cost, less radiation exposure, and better patient tolerance. Diagnostic advantages include the simultaneous visualization of vessel wall and lumen, and the ability to studyvascular anatomy from arbitrary viewing angles using only one data acquisition. CTA even allows for projections (such as caudocranial) that cannot be obtalned with conventional angiographic techniques. Although CTA can replace diagnostic
Anatomy intra-arterial DSA for many indications, the poorer spatial resolution in comparison with intra-arterial DSA is a limiting factor in patients in whom diagnostic evaluation: of small vessels is required (Table 24.1). Ultrasound with its various Doppler techniques may be preferred over CTA in regions that are easily accessible by sonography or when flow information is required. It also holds advantages whenever bedside imaging is to be preferred. Contrast-enhanced magnetic resonance angiography (MRA) offers competitive results to CTA, although spatial resolution is often lower than with multislice CT scanning. The lack of ionizing radiation of MRA is important in young patients, for vascular screening, and for patients requiring repeated follow-up examinations. In patients with impaired renal function, contrast-enhanced MRA requires less (and therefore less nephrotoxic) contrast material. Dynamic MRA examinations offer therapeutically relevant additional information in only a relatively small number of cases. Flow quantification with MR may gain increasing importance for therapeutic decision-making. In acute lifethreatening diseases such as aortic aneurysm, aortic rupture, or pulmonary embolism, CTA is the procedure of choice owing to the short examination time, easier patient monitoring, and less reliance on patient compliance.
The anatomy of the Vascular system is well known from angiographic studies. Crosssectional anatomy, however, provides the opportunity to trace blood vessels to and from their target organs and appreciate their relationship to surrounding structures. The normal ranges of vascular calibers of the major vessels are reviewed in Table 24.2. Pulmonary Vessels
The pulmonary trunk divides at a Y-shaped bifurcation into the left and right pulmonary arteries. The diameter of the pulmonary trunk should not exceed that of the ascending aorta, and the diameters of the pulmonary arteries should be less than the diameter of the de-
Table
24.1
Indications
CT angiography Aorta
for vascular
Hepatic
with CT
All lesions affecting the aorta or wall: arch anomaly, coarctation, aneurysm, occlusion,
Renal arteries
imaging
(eTA) its /\
dissection, stenosis, arteritis, trauma
Suspected stenosis, evaluation living renal donor work-up
for
PTA,
arteries
Preoperative vascular anatomy; detection of stenosis or occlusion after
Mesenteric
Chronic
liver transplantation
I
ischemia,
eTA),
acute
ischemia
arteries
(multislice
Carotid arteries
Suspected dissection
Runoff vessels
Peripheral arterial in non-dabetics
Pulmonary artery
Pulmonary embolism, A V mal formations, arteritis, chronic thromboembolic pulmonary hypertension,
Vena cava, portal vein
Suspicion of thrombosis, tumor invasian (equivocal ultrasound findings)
scending aorta. The pulmonary arteries taper uniformly towards the periphery. All artery segments branch dichotomously into two subsegmental arteries. The only exception is segment 6 which gives rise to three subsegmental arteries. There are, however, numerous variants of this branching pattern with up to four main branches and occasional smaller branches coming from a segmental artery. Two pulmonary veins pass from each lung to the left atrium of the heart. In their central portions the upper lobe veins run directly anterior to the pulmonary arteries, while the lower lobe veins course medial to the arteries (Fig. 24.1). Variants of the pulmonary veins are frequent and become important for planning
827
24 Vascular System Table 24.2 vessels
electroablation in atrial fibrillation because the size of the pulmonary venous orifice may vary substantially between patients, and aberrant vessels may be hard to detect on retrograde angiography. If the vein draining segment 6 (posterioapical lower lobe) runs posterior to the upper lobe bronchus and intermediate bronchus it may simulate a pathologic bronchopulmonary lymph node on non-enhanced scans. This vein enters the left atrium directly from above. Within the lung parenchyma, the pulmonary veins and arteries can be distinguished by the fact that the arteries run parallel to the bronchi.
Normal average diameters of major blood
Pulmonary trunk?
22-35 mm"
Right pulmonary artery
20-23 mm
Left pulmonary artery
18-21 mm
Pulmonary veins
14-16mm
Aortic root
30-38 mm
Distal ascending aorta
28-36 mm
Proximal descending aorta
24-28 mm
Distal descending aorta
22-26 mm
Suprarenal abdominal aorta
20-25 mm
Infrarenal abdominal aorta
18-23 mm
Celiac artery
5-10 mm
Superior mesenteric artery
5-10mm
Renal arteries
5- 9mm
Inferior vena cavl
•
Aorta
The aortic root appears as an elliptical expansion (bulb of the aorta) that contains three smaller dilatations called the aortic sinuses. Pulsations
22-28 mm
_~R.BCV ~_~BCA T
Es
VCS •
L. BCV
L. SeA
AZV
Pericardium
ILAD
Fig. 24.1 Cross-sectional anatomy of the mediastinal vessels. BCAlBCV= brachiocephalic artery and vein SCA=subclavian artery Ao = aorta RA/LA= right/left atrium RVOT = right ventricular outflow tract RV/LV = right/left ventricle RCA/LCA= right/left coronary artery LCX= leftcircumflex branch LAD = Left anterior descending artery Es= esophagus T = trachea AZV = azygos vein PV= pulmonaryvein
Anatomy
in this area produce characteristic crescentshaped double contours or density variations in the ascending aorta that should not be mistaken for an aortic dissection (see Fig. 24.5). The aortic arch is most commonly defined as the portion of the aorta extending from the origin of the brachiocephalic trunk to just behind the origin of the left subclavian artery. An elongation in elderly individuals or in patients with aneurysms of the descending aorta may push the aortic arch to an anterior position. As a result, the proximal portion of the descending aorta may form the most apical point of the aortic convexity. This so-called "aortic hump" (see Fig. 24.46) should not be confused with the aortic arch. The descending thoracic aorta has a markedly smaller diameter than the ascending aorta, and its diameter should decrease very little as the vessel descends to the diaphragm. The abdominal aorta begins distal to the aortic hiatus and tapers slightly at the origin of the infradiaphragmatic side branches to the iliac bifurcation. •
Aortic Side Branches
The brachiocephalic trunk is the first supra-aortic branch. It is followed by the left common carotid and the left subclavian arteries (Figs. 24.1, 24.2a). There are numerous variants, which are described later on p. 870 and p. 892. The left subclavian artery commonly arises from the apex of the aortic arch. The subclavian arteries ascend through the thoracic inlet and then turn downward toward the axilla. As axillary arteries they descend posterior to the accompanying veins, their positions varying somewhat with the position of the arms. The vertebral arteries originate from the apex of the subclavian arteries. A left subclavian artery that arises some distance posterior to the aortic apex may indicate a coarctation or other aortic arch anomaly. The subclavian arteries vary in their origin; the most important variation is an aberrant right subclavian artery that arises from the back of the aortic arch below its apex and passes to the right side behind the trachea and the esophagus causing dysphagia lusoria (see Fig. 24.41). The carotid arteries bifurcate at the level of C2-C4. Tortuous courses of the supra-aortic arteries are quite common in elderly individuals.
Fig. 24.2 Schematic diagrams of the aorta (a) and the venous system of the chest and abdomen (b).
The proximal portions of the bronchial arteries' intercostal arteries, lumbar arteries, and other small aortic branches can be demonstrated even with single-slice CTA technique, which may help in planning interventional procedures. With multislice CT scanning, such arteries can' be followed further into the periphery. Bronchial arteries are rarely identified past the region of the main bronchi. Dilated bronchial arteries (> 2 mm) are common in pulmonary hypertension or other causes of abnormal collateral circulation of the lung. Only with excellent vascular opacification can the anterior spinal artery be identified in some patients when thin-section scanning is employed, but DSA remains the standard for the evaluation of the spinal cord arteries. Identification of the phrenic or suprarenal arteries is usually possible with multislice scanning. The testicular and ovarian arteries can be identified, given sufficient contrast enhancement and thin-section scanning, as
829
24
Vascu far System
structures that run parallel to the ureter. The gonadal arteries commonly arise from the aorta but gonadal arteries that originate from the left, and rarely from the right, renal artery may assume importance for living renal donors. On single- or multislice CTA the celiac and superior mesenteric arteries can easily be identified as separate structures. The division of the celiac trunk into the splenic and hepatic arteries displays a typical swallowtail configuration. The left gastric artery appears as a smaller vessel that commonly runs upward from this division. In cases where the celiac trunk takes a descending preaortic course, portions of the diaphragm may abut the trunk anteriorly, causing a stenosis with a transverse oval shape on axial views (see Fig. 24.80). Anatomic variants of the upper abdominal arteries are found in approximately 50% of patients (see below, p. 898). With thin-section multislice CT, the various branches of the celiac and superior mesenteries artery can be analyzed in more detail (Fig. 24.3). The renal arteries generally arise just caudal to the origin of the superior mesenteric artery at the level between LI and L2. Multiple renal arteries are present in approximately 20% of cases. The inferior mesenteric artery arises from the frontal or left anterior aortic wall between the third and
Celiac artery Common Proper hepatic hepatic artery artery
Left gastric artery
Short gastric arteries
fourth lumbar vertebral bodies. The aortic bifurcation lies approximately at the level ofthe L4-L5 intervertebral space. Tortuous iliac arteries may appear as multiple segments on axial CT sections. The various internal iliac side branches can best be demonstrated on 3D-rendered images but small branches such as the uterine or pudendal arteries are too narrow to detect pathologic changes. The arteries of the arms and legs are easily displayed on volume rendered images but maximum intensity projections (MIP) after bone removal are often more useful for diagnostic evaluation because they follow the anatomic display provided by arterial DSA. •
Principal Systemic Veins
The right jugular vein, right brachiocephalic vein, and superior vena cava (SVC) lie on a vertical axis that is perpendicular to the scan plane (Fig. 24. 2b). The left brachiocephalic vein passes over the front of the aortic arch and terminates in the vena cava. A retro-aortic variant may also occur and should not be mistaken for lymph nodes. The azygos-hemiazygos venous system establishes a collateral pathway in response to occlusion of a portion of the vena cava. The veins, which flank the aorta, are highly variable in
Gastroduodenal artery
Cystic artery
Inferior mesenteric artery
3astrofuodenal artery Superior supra duodena arteries
Gastroepiploic artery
;"
Appendicular artery
Fig. 24.3
Superior mesenteric artery
Normal anatomy of the celiac and mesenteric arteries.
Sigmoid arteries
Anatomy their diameters but normally do not show abrupt caliber changes on adjacent scans (differential diagnosis: paraesophageal lymph nodes). There are extensive connections to the spinal venous plexus. and small connections also to the (left) renal vein. The azygos vein opens into the SVC above the right main bronchus. If an azygos lobe is present. the vein will pursue a characteristic course through the upper lobe of the right lung. The intercostal veins Thl-Th4 drain into a right and left superior intercostal vein. which enters the azygos vein on the right and may either enter the accessory hemiazygos vein on the left or pass across the aortic arch into the left brachiocephalic vein giving rise to an aortic "nipple" on PA chest radiographs. In the abdomen. the azygos and hemiazygos veins continue into a network oflumbar and spinal veins. The inferior vena cava (IVC) is situated slightly to the right of the midline. passing through the right hemidiaphragm. Its intrahepatic segment is compressed into a triangular or lanceolate shape by the right lobe and the caudate lobe of the liver. The three principal hepatic veins (right. middle. and left) are accompanied by a number of accessory veins. One of these veins opens directly into the IVC between hepatic segments 5 and 6 and can be important for planning atypical hepatic resections. The veins of the caudate lobe are small and enter directly into the IVC (this accounts for the hypertrophy of the caudate lobe seen in Budd-Chiari syndrome). The renal veins enter the IVC at about the level of the first lumbar vertebral body, the left renal vein crossing in front of the aorta on its way to the vena cava. A retroaortic left renal vein usually passes obliquely downward. It is single in up to 2.5% of the population. In up to 9% of the population there is a duplicate renal vein, with one vein passing behind and one in front of the aorta. The early opacification of the renal veins
after contrast infusion
can cause
pseudothrombus artifacts within the IVC to appear at the level of their terminations. Early images show a marked decrease in CT attenuation caudal to the renal veins. There are small collaterals from the left renal vein to the lumbar venous system as well as via the adrenal veins to the splanchnic venous system. Such collaterals attain significance in IVC obstruction or portal hypertension.
The pelvic venous confluence lies just below the aortic bifurcation. The left common iliac vein passes behind the iliac arteries, where it presents an oblong cross section. Generally the main pelvic veins run posterior to the arteries. Familiarity with the vascular anatomy of the bifurcation region is important in nodal staging, because veins may not have enhanced if scanning was started too. early, and pathologic lymph nodes may enhance on delayed scans. Scans in the portal venous perfusion phase, which is commonly used for abdominal examinations, may display pseudothrombus artifacts at venous bifurcations. This is caused by differential enhancement of veins draining proximal and distal portions of the pelvis or lower extremities; the internal iliac veins and profunda femoris veins enhance earlier than the external iliac or superficial femoral veins (see Fig. 7.39). Portal Venous System
The portal vein arises from the junction of the superior mesenteric and splenic vein. On reaching the liver hilum, the portal vein divides into a right and left main branch (see Fig. lU). The left portal vein turns a short distance forward in its terminal portion, defining the boundary between the lateral segment of the left hepatic lobe (subsegments II and III) and subsegment IV. The right portal vein divides into an anterior branch, which supplies hepatic subsegments V and VIII, and a posterior branch, which supplies segments VI and VII. Numerous variants may occur, especially in the mode of division of the right portal venous branches. Some of these variations, such as a trifurcation of the portal vein or a left portal vein arising from the anterior branch of the right portal vein, may provide important information for planning partial liver resection.
The inferior mesenteric vein runs parallel to the inferior mesenteric artery in the small pelvis and lower abdomen, then turns upward in front of the left renal vein and drains into the splenic vein several centimeters proximal to the venous confluence. Large numbers of collateral pathways to the systemic circulation exist, via gastroesophageal and anorectal veins, paraumbilical veins, and retroperitoneal veins (liver capsule, common bile duct, splenorenal ligament, omentum and abdominal wall, mesenterico-ovarian). The size of the portal vein should not exceed "13mm, the diameter of the splenic
831
832
24 Vascular System vein and the superior mesenteric vein should remain below 10 mm unless portal hypertension is present.
Examination Tedhnique CfA relies on thin-section spiral imaging and optimum timing of contrast material injection to ensure proper opacification of the target territory. To obtain the best possible spatial resolution along the z-axis, all the spiral scan parameters should be optimized to produce the smallest possible effective slice thickness in singleslice CT. While z-axis resolution is a limiting factor with such single-slice techniques, near isotropic imaging is possible with multislice techniques. Signal-to-noise ratios become a more important limiting factor with multislice scanning because of low detector signals at thin sections. A section width between 1.25 mm and Table 24.3
3 mm suffices, depending on the scan length and the size of the patients. Smaller sections may be technically feasible but should be avoided for most body applications because of excessive noise or dose. In general, the scanning technique with multislice CfA becomes more simple and robust. The contrast protocol must be individually adapted to achieve a sufficiently high and constant level of vascular opacification throughout the examination (see Figs.24.6 and 7). This is done by following the steps outlined below (recommendations for scanning protocols are summarized in Table 24.3):
Suggested protocols for vascular imaging with CT
General Preparation
Not necessary
Oral contrast
None or negative contrast agents (water)
Patient position
Supine with elevated arms (standard) [Carotids: arms parallel to the body
Scan range
As short as possible: tailored to the clinical problem
Respiratory phase
Inspiration (standard) Shallow resplration (patients that are unable to hold their breath)
Window setting
Precontrast scan: a angiography:
Scan parameters
l-slice SqTf/RI
W/l-400/40 W/l= 500/150 (adjust as needed) 4-sliltea SC
SC= slice collimation (mm). TF= table feed (mm/rotation), RI= recon. increment (mm), t t = scanning direction, SW = effective section width (mm), MPR= multi planar reformats, axial = axial sections, cor = coronal, sag = sagittal CM = contrast material, Contrast concentration = 300 mglml iodine (optional 400 mglml for 16-slice scanning) V=volume of CM (rnl), N =volume of saline flush (ml), F=flow rate (mils), D = start delay, X = trigger region a Pitch P = TF/(NxSC) should be chosen close to 1.5 with 4-slice scanners, 1.3-1.5 with 16-slice scanners b
MPRare reconstructed from a "secondary raw data set" with SWIRl = 1-1.5/0.7
C
MIP/VRT routine display mode; CPRfor stenoses suspected on axial sections, MIP or VRT sequential CT
d e
or 0.5-0.8/0.5
localized examinations only (region of suspected pathology) read: 5-30A (5P) = start delay 5-30 s (5 s) after reaching 50 HU enhancement in the aorta (pulmonary artery)'
r basic injection, e.g. 50/5 + 30/3/5A-?50 ml CM at 5 mils flow followed by 30 ml CM at 3 mils, delay 5A' 50 ml saline flush at 2.7-3 mils optional 9 h
choose lower dose and kVp setting (CTDI""r = 80-100 kVp, depending on patient size and CT scanner) dilution 60 ml CM: 90 ml normal saline
r scan in combination with CTA of the pulmonary vessels, total start delay 3 min
• Perform (discontinuous) pre-contrast scans in cases with suspected hemorrhage, mural hematoma. • Determine the scan length L by means of a scanogram, which may be supplemented if necessary by low-dose noncontrast scans. • Administer a test bolus to determine the scan delay time D. The test bolus may be omitted if bolus triggering is available, i.e., the scan is automatically initiated when intravascular contrast appears.
• Determine the scan parameters as a function of the scan length and scan duration TI. • Define the contrast protocol, taking into account the patient's body weight « 2.5 mg iodine/kg body weight) and any impairment of renal function. • Evaluate the images interactively on the viewing monitor, and construct maximum intensity projections (MIP), shaded surface displays (55D), or volume-rendered images.
I
24 Vascular System Patient Preparation
The patients should refrain from eating a few hours before the examination but otherwise no special preparation is needed unless there is known deterioration in renal function or an otherwise increased risk for contrast material application (see also Chapter 3, p. 85). No positive oral contrast material should be employed. This is especially important to remember ifCTA is performed as part of another examination, for example a biphasic liver CT. Acquisition Technique Pre-contrast Scans
Pre-contrast scans are only required in patients with suspected hemorrhage, and mural hematoma in suspected aortic dissection. Rarely, it may also be helpful in suspected aortitis. In most cases, however, such precontrast scans offer only little additional information over those that can be obtained from the contrastenhanced CT angiograms. Because the abnormalities extend over larger regions, discontinuous scanning (e.g., 5mm sections every 20mm) with single-slice CT or low-dose, thick-section scanning with multislice CT (e.g., 4 x 5 mm detector configuration, 7-10 mm section width) usually suffice. Pre-contrast scans are rarely necessary for determining the scan range. In single-slice CT, however, long scan ranges require the use of thicker collimation and thus reduce spatial resolution, or require increased scan time and thus need more contrast material and make motion
Table 24.4
artifacts more probable. For -this reason, precontrast scans can help decide whether an aneurysmatic dilatation of the aorta involves both chest and abdomen, or whether CTA of one of these regions suffices. Scan Length
Scan length is no limiting factor in multislice CT, but is critical for spatial resolution in singleslice CTA. However, keeping the range to a minimum reduces scan time, allowing for thinner collimation and thus better spatial resolution, with reduced motion-related artifacts, contrast material requirements, and patient dose. The cranial and caudal extent of the acquisition can usually be determined on the scanogram for most vascular structures (Table 24.4). Scan Duration and Breath Holding
CT angiography is performed during an inspirational breath hold. As a rule, the duration of breath holding should not exceed 30 seconds. It is helpful to have the patient hyperventilate briefly (i.e., perform several forced inhalations and exhalations) before each scan. Informing the patient about the procedure and the importance of the breath hold is an important part of a successful CTA examination. In critical patients, the technologists should try the breath holding maneuver before the actual scan and watch the patient's abdomen. If it moves during the breath hold phase, the patient has to be instructed again. In particular, movement artifacts may occur if the patient relaxes the diaphragm
Whole chest Diaphragm to aortic arc (dyspneic patients,L_
Thoracic aorta H
"
From below aortic root to above aortic arch (including brachiocephalic trunk)
Whole chest
From above celiac artery to iliac bifurcation (internal/external) or inguinal region
From above celiac artery (ThIO/IT) to inquinal reqion
_
BT'
Abdominal aorta
"1
'
co,
.
, .
Acquisition
Scanning Parameters
during the breath hold or if there are involuntary diaphragmatic movements towards the end of the scan phase. The images obtained during the test bolus series are a good indicator of patient compliance. If the patient is not able to hold the breath for sufficient time. the scan duration should be reduced. This is feasible for most emergency examinations but reduces diagnostic evaluation of smaller vessels because of the need to employ a thicker collimation. If this is not possible. no attempt should be made to force a breath hold examination. In such cases, the patient will continue breathing anyway, resulting in major breathing artifacts. lt is better to have the patient hyperventilate before the scan and then breathe shallowly. This is especially important in patients with suspected pulmonary embolism. In single-slice CTA, the scan duration can be increased to 40-60 seconds by first scanning the part of the imaging volume that is susceptible to respiratory artifacts (e.g., the upper abdomen). After 30 seconds have elapsed, the patient is instructed to resume breathing while the less motion-sensitive part of the volume is imaged. This technique can be used in examinations of the abdominal aorta and renal arteries as a means of reducing the slice thickness. In multislice CTA, scan duration can be substantially reduced for many indications. This should not make the technicians less careful in instructing the patients. A sudden breath hold command in an unprepared patient will often result in substantial breathing artifacts either during the beginning or the end of the scan.
Table 24.5 RT ls ls ls 0.75s 15 0.55 0.4s RT «tube
Single
16 rotation
time, TI
TI
Interpolation
255 245 305 305
360' LI 180'LI 180'LI 180'LI
305 205 85
cone beam
=
scan time,
180'LI 180'MLI
SC
=
Slice
Parameters
Standard parameters can be used in most CT vascular examinations. The parameters listed in Table 24.3 are for single-slice scanners with 1 second tube rotation that permit at least a 50 second scan duration. These parameters may have to be adjusted for the available scanner technology, since different technologies substantially affect the z-axis resolution (Table 24.5) and the available scan length. In very obese patients in whom the maximum mAs settings do not suffice, the use of a thicker collimation may be necessary to improve the signal-tonoise ratio. As described in Chapter 4 (p. 117) it is always desirable to keep the slice collimation SC as small as possible and use a pitch P close to 2. A pitch of 3 may be used for selected indications such as scanning of the carotid arteries or the pulmonary arteries with 1 mm collimation. A 1800 interpolation algorithm and a soft convolution kernel improve the z-axis resolution and reduce image noise. For optimum results of multiplanar reformations and 3D reconstructions, the images should be reconstructed with a high degree of overlap (reconstruction increment RI ss section widthSW/2; Fig. 24.4). Dual
Slice
Parameters
With dual slice scanners, the z-axis resolution can be substantially improved as compared to single slice scanners. In principle, the collimation could be split in half without altering the scan duration, but often it more advisable to re-
CT of the thoracic aorta: effect of scanner technology Detectors
Technique
(scan length 24 em) on z-axis resolution
SC
TF
10mm 4mm 3mm
lOmm lOmm 8mm 6mm
2mm Imm O.5mm
6mm 12mm
5mm
slice collimation,
Pitch
P
p"
12.8mm 6.4mm 5.1 mm 3.8mm
8mm
TF «table
feed, SW
SW
1.5 1.5 =
section
24 width
2.6mm 1.3mm O.5mm (FWHM)
835
24 Vascular System
b
a
Fig. 24.4 Influence of overlapping image reconstruction on the image quality of CTA of the renal arteries: SCITF/RI= 2/2/2 (a) and 2/2/1 (b). Note the display of the small, partially stenosed renal arteries on the left side.
duce the scan duration as well and chose a slightly wider collimation (Tables 24.3, 24.5). This reduces breathing artifacts and makes it possible to use less contrast material. A pitch P of 2 or slightly less (P" < 4) should be used for most indications together with a 180 interpolation algorithm. Only in very obese patients in whom the maximum mAs settings have been reached may it be necessary to use a pitch P of 1 (P*=2) together with a 360 interpolation to decrease image noise. Under these conditions, the same collimation as with single slice CT has to be used. The coverage and z-axis resolution are still equal to that of single slice CTA with pitch 2 but signal-to-noise ratio is better at identical mAs settings. 0
0
•
Multislice Parameters
With 4-slice CT scanning, the number of scanning protocols is substantially reduced. Only in very obese patients may it be necessary to use a section collimation above 2.5mm. Most frequently, volumetric imaging can be applied in the neck, chest, or abdomen (Table 24.3). With current 4-slice scanners, detector configurations of 4 x I mm or 4 x 1.25 mm collimation will be used. Volumetric imaging is important if small vessels, such as aortic side branches or peripheral pulmonary arteries, have to be evaluated. For the aorta (chest, abdomen, or thoracoabdominal), fast spiral scanning with 4 x 2.5 mm collimation (4x2 mm on Toshiba units)
usually yields sufficient results and allows for substantial reduction in scan duration and contrast material volumes. In patients with aortic dissection and abdominal symptoms, however, volumetric imaging (4x1-1.25 mm) may improve evaluation of abdominal side branches but requires 40-50 s scan duration, even with 0.5 s 4-slice scanners. Volumetric imaging with 8- and 16-slice scanners is possible with a collimation of 8 x 1-1.25 mm or 16 x 0.5-1.25 mm which substantially reduces scan duration by up to a factor of 4 (see more details for 8- to 16-slice scanners in Chapter 4 and Table 4.6). Fast spiral scanning using an 8 x 2.5mm collimation with 26mm table feed allows for covering the chest or abdomen in 4-8 s. A 16 x 1.5 mm collimation with a 36 mm table feed will achieve the same in 3-6 s, and a 16 x 2mm collimation with 44mm table feed will require 2-5 s. Such extremely fast scan durations substantially improve evaluation of dyspneic patients and allow for reduction of contrast material volumes. Timing of contrast material, however, will have to be adapted accordingly.
Dyspneic patients with suspected pulmonary
embolism can be examined within some 5s
using fast spiral scanning with 4-slice scanners and 4 x 2.5 mm collimation if only the central portion of the pulmonary arterial system is covered from the top of the lower half of the diaphragm to the top of the aortic arch. In general, however, near-isotropic imaging is preferred for the pulmonary vessels. Proper breath-hold instructions, are mandatory. If breath holding is impossible, shallow breathing should be attempted. Current 8- and 16-slice scanners require no modifications of scan protocols relative to normal patients. For the peripheral vessels, fast spiral scanning with 4-slice units is able to cover the abdominal aorta and the legs (some 120cm) in 40-64 s, depending on the rotation speed of the x-ray tube. With 0.5s rotation, near isotropic imaging becomes feasible within 75-80 s scan duration if (N x SC/TF =) 4 x 1/8 or 4 x 1.25/7.5 protocols are used. Contrast opacification of the arteries usually poses no problem, despite long scan times, because the bolus geometry widens on its way to the feet. There is increasing venous overlay, however, as scan duration increases. Using 8- and 16-slice scanners the scan duration can be reduced by up to a factor of 4, even with
Acquisition
high spatial resolution. If scanning is too fast, however, the bolus may be passed by the scan and distal vessels may be inadequately opacified. For this reason, a reduction of pitch may be advisable to keep scan durations in the range of20-30 s.
CT Perfusion Imaging (CTP) CT perfusion is firmly established only for the brain (see below, p. 895), although applications elsewhere in the body have been suggested for a long time and commercial software is now available. Multiple scans in rapid succession (or even during continuous radiation) are obtained during the injection of a contrast material bolus. The various sections must be registered to obtain the enhancement curve (time-density curve) of each voxel within the organ of interest. For this reason, the organ of interest must be free of (respiratory) motion during data acquisition. The enhancement curve is then used to calculate various perfusion parameters (see below). Perfusion depends on the velocity and volume of the blood flow through large capacity vessels as well as through the capillary vascular bed. CTperfusion usually applies first-pass evaluation using kinetic models that assume that the contrast material is non-diffusible, and neither metabolized nor absorbed by the tissue bed studied. A simple model, which uses the slope of the enhancement curve to determine regional blood flow (RBF) , is most robust. It relies on the Fick principle: change in enhancement (C) in a ROI is proportional to the blood flow x difference in concentration of contrast material between feeding artery (Co) and draining vein (Cv):
dCWdt=RBFx (Ca(VCv(V) This model requires a delta impulse to work best, which means that a sufficient amount of contrast material (e.g., 50ml) has to be injected within as short a time as possible (i.e., using large bore 14 G venous cannulae and a flow rate of8-1 Omlis) . Since the absolute concentration of contrast material in the artery is not known, only relative numbers can be obtained for RBF. The time to peak (TIP) is easily established. Deconvolution models provide both qualitative and quantitative data and allow for slower
Technique
injection rates (50ml at4-5mlls). They use the time-density curve in a feeding artery (which has to be contained in the imaging volume) as the input function that is used to deconvolute the tissue time-density curve to determine RBF, regional blood volume (RBV) , and mean transit time (MTT). Scanning at 80kVp is suggested for both techniques because it provides superior contrast at a lower dose. Scan duration is usually 45-50 s for the brain.
Dose Issues With optimum contrast opacification of the target vessels, wide window settings can be used and a higher image noise can be accepted as compared to standard CT indications. At the same time, reconstruction of thick multiplanar reformations may reduce the contrast of small vessels because of increased partial-volume effects. Thus, there is a delicate balance between dose reduction, acceptable image noise, and vessel contrast. In general, higher doses are necessary for near-isotropic imaging, while less dose can be applied for fast volume protocols. In standard-sized patients, a CTDIvoi of 5-6 mGy will suffice for the chest, while 8-15 mGy will be required for the abdomen. In more obese patients with larger body cross-sections, substantially higher dose may be necessary. In the chest and in slim patients, lowering the kVp settings is a technique to increase vascular contrast because of the higher attenuation of iodine at lower x-ray energies. For CTA of the pulmonary arteries, the use of 100 kVp instead of 140 kVp will, on average, increase vascular opacification by some 80-100 HU for identical injection parameters, and thus allow for scanning the chest with a eTDIvoi of 3-4 mGy. The resulting effective dose will be in the range of 1.5-2 mSv. A tube voltage of 80kVp holds an even higher potential for dose savings in slim individuals and children (effective dose less than 1 mSv) but it requires substantially higher mAs settings in more obese patients because of less penetrating radiation at low kVp settings. Such low settings therefore should be avoided in obese individuals. In the abdomen, x-ray attenuation is also higher, and reducing the kVp settings will only be advantageous in slim patients and allow only for minor dose reduction.
24 Vascular System Adaptive tube current modulation and zaxis modulation is an excellent technique for further dose reduction by some 10-30%, depending on the body region, It should be used wherever possible, For similar image quality 16slice scanners require some 20-30% less dose than 4-slice scanners,
Contrast Injection Contrast injection is critical for CTA procedures, Optimization becomes more important for multislice scanning, especially when decreased volumes of contrast material have to be used, It 'is important to obtain a sufficiently high degree of vascular opacification throughout the imaging volume and to display the vessel of interest with as few artifacts as possible, As a general rule of thumb, small vascular calibers, in-plane vessels, and greater slice thickness require a higher degree of vascular opacification to ensure that the vessels can be evaluated despite partial- volume effects, Only a moderate level of enhancement (» 100 HU) is needed in studies that are limited to large vessels (such as the thoracic aorta or the deep veins of the trunk) or vessels that run perpendicular to the scan plane, such as the carotids and the vessels of the pelvis and lower limb, Contrast volume and flow rates are less critical in such examinations, Good vessel opacification, however, will allow for using less patient dose during data acquisition, For evaluation of suspected pulmonary embolism and for the arteries of the upper abdomen, spatial resolution is critical, In addition, vessels may run parallel to the scan plane and are therefore more susceptible to partial volume effects, For these indications, optimum vascular opacification is mandatory, A similar requirement exists in regions where 3D reconstructions are desired, Good results have been .achieved with the protocols shown in Table 24.3,
Positioning and Venous Access For cubital venous access in CTA, a sufficiently large-bore intravenous catheter « 18 gauge) should be used to enable a high contrast flow rate, The injected arm should be stationary and well extended to prevent catheter kinking and
reduce venous compression effects, Right cubital venous access is preferred for thoracic CTA, because this will avoid streak artifacts caused by the approximately horizontal course of the left brachiocephalic vein, Good results are also achieved by positioning the left arm above the head and the right arm parallel to the body and using right cubital venous access for the contrast injection, This technique reduces compression effects in the area where the brachiocephalic vein crosses the ribs, Central venous lines should be avoided if flow rates above 2mlls are required, The only exceptions are large bore catheters such as for dialysis or hemoftltratton. It is always advisable to test catheters or check with manufacturers to determine the maximum flow rate and pressure that is still safe,
Contrast Volume and Flow Rate The goal of contrast injection in CTA is to cause a constant and sufficiently long phase of vascular enhancement during which the actual data acquisition can take place, Vascular enhancement for CTA of the arterial system is an off-equilibrium effect that is driven by the steady inflow of contrast-enhanced blood from the injection veins via the pulmonary circulation into the aorta, Soon after the injection stops, the vascular enhancement decreases as well. Vascular enhancement depends on dilution effects and is therefore strongly influenced by cardiac output Patients with a high cardiac output, such as young adults, suffer from more dilution, and thus less enhancement, than patients with a reduced cardiac output, such as an acutely ill patient in shock. Recirculation effects will cause a continuous increase of contrast enhancement during the plateau phase, leading to maximum enhancement close to the end of the scan, For most standard indications a monophasic contrast injection is sufficient The user has to choose the injected volume V and the flow rate F as well as the scan delay D, These three parameters are adapted to the clinical imaging task and the resulting scan duration (Fig, 23,5; Table 24,3), The results can be further improved, however, if a saline flush is used to better utilize the injected contrast material. More homogeneous enhancement is gained by a
Contrast Injection biphasic injection technique or even an exponential decrease of flow over time. Using mathematic modeling or test bolus injections. a predefined level of contrast enhancement independent of the individual circulatory state can be achieved with good approximation (see below. Individualization of Contrast Material Injection). For most standard applications. however. the following rules have to be considered.
Saline Flush
The injected contrast material can be used to better advantage if the injection veins are flushed by a bolus of normal saline, injected immediately after the contrast material using similar flow rates. Such a saline flush pushes the contrast material forward and thus increases the length of the contrast plateau. It can also be used to save contrast material without jeopardizing vascular contrast, and to increase vascular opacification if combined with higher flow • A larger contrast volume will prolong the rates. It is particularly important when small plateau phase of enhancement. amounts of contrast material are injected, and • Enhancement increases over the duration therefore becomes essential for short multislice of the plateau phase due to recirculation CT acquisitions. In addition, a saline flush will effects (monophasic injection). A reduce the amount of artifacts in the superior • Hence. larger volumes will increase maxi-A 1 vena cava and brachiocephalic injection veins mum enhancement. when chest CTA acquisitions are obtained in a • Maximum enhancement will occur close caudo-cranial scanning direction (see Fig. 24.13). to the end of the plateau phase. In a single barrel CT injector, layering is • Vascular enhancement increases as more necessary so that contrast material is injected iodine is injected per second. first, followed by normal saline (see procedure • This can be achieved either by higher flow in Chapter 3, p. 99). Because it speeds this prorates or higher contrast concentration. cedure up, we suggest pointing the syringe • Higher flow rates decrease the duration of downwards and pulling up saline first when the plateau phase. loading the injector, followed slowly by contrast • The flow rate F should be at least 3 ml/s. material. Such a technique will not provide a better 4-5 ml/s. for all indications in sharp fluid-fluid level within the syringe but a which high intravascular contrast is resmall mixing zone at the interface between the quired. fluids. This procedure will have to be repeated • Higher contrast concentration increases! for each injection. viscosity and requires pre-warming of the" Double-barrel injectors are presently encontrast material (incubator). tering the market and make a saline flush tech• The duration of the contrast plateau may nique much easier to perform; no refill is necesalso decrease with higher contrast concensary between patients, and preparation of contration unless a saline flush is used to push trast injection is much faster. In addition, such the (high-viscosity) contrast material forinjectors allow for multiphasic injection protoward. cols. The contrast injection can be further op• As a rule of thumb. the volume of contrast timized if normal saline is injected before the material should be equal to the scan duramain contrast bolus as well, because it will open tion (s) x flow rate (mlls). up the injection veins and will fill small'col • For short scan duration (AlOs; multislicel lateral veins with saline, thus creating a more CT). higher volumes, higher flow rates or sharply defined enhancement curve. higher contrast concentrations, and a saIt is not yet clear how much saline has to be line flush are needed. injected for optimum results. Based on our clinical experience, we suggest 40-60 ml. When determining the proper scan delay, any saline that The risk of contrast extravasation with higher is left in the injection lines has to be taken into flow rates can be reduced with pressure-limited account. This is no problem when bolus triggerinjectors, which stop contrast injection when a ing techniques are used but may be problematic sudden pressure increase occurs, or with 30when scan delay is derived from a test bolus incalled extravasation detection accessories. jection.
839
24 Vascular System has shown, however, that the best results are achieved by increasing this delay time by some 10-20% when determining the definitive scan delay D because a larger amount is injected during the actual CTA data acquisition (Table 24.6).
Scan Delay Although correct timing of the contrast material bolus is critical for optimum results of CTA, individual timing is not always mandatory but may avoid occasional failures to obtain adequate vascular opacification. It is particularly important in emergency patients with decreased cardiac output and prolonged circulation time. Individual timing is based on a test bolus injection or bolus triggering.
Bolus Triggering
Bolus triggering (or bolus tracking) is a more elegant technique than a test bolus, because the contrast bolus itself is used to initiate the scan. Most manufacturers equip their scanners for bolus triggering. This involves performing monitor scans with a substantially reduced dose at a suitable table position (trigger region) and defining a region of interest (ROl) in which vascular enhancement is to be measured. Once a sufficient level of enhancement has been reached (determined visually or by a threshold attenuation value), the table moves to the beginning of the scan range, and the actual spiral acquisition is initiated.
Test Bolus Injection
A test bolus of 10 (to 20) ml of contrast material is injected intravenously by rapid manual injection or power injection at 4-5 ml/s. and several low-dose scans are performed at a reference level that is chosen close to the beginning of the scan range. Using 80-100 kVp further decreases dose and improves contrast enhancement of these test scans. Scans are acquired at 1-2 second intervals. depending on the expected scan delay. Even these test scans should be initiated after a certain delay time of usually 8-12 seconds following the start of the test bolus injection. The test scans are evaluated by identifying the image that displays maximum opacification in the region of interest (e.g.. the aorta). The time delay At of this image from the start of the injection determines the scan delay. Experience Table 24.6
Some scanners offer a continuous fluoros-
copy mode for monitor scans. This mode should be avoided in favor of scans every 1-2 s because it may increase patient dose substantially. There is an additional delay between the time at which the threshold level is reached and the time the actual data acquisition is started. This delay varies from scanner to scanner and may be as high as 7 s. This makes it necessary to use
Test bolus and bolus triggering
Test bolus Reference
region:
Level of pulmonary Cranial
Contrast
protocol:
Scanning
protocol:
Scan delay D:
border
V/F!D=
bifurcation
(thorax)
of scan volume
(abdomen)
10/5/8-12
5 mm slice thickness, sequence: 20-30 s Calculated opacification D=AtX".2a
minimal
from the time ofthe target
dose, scans acquired
at 1 to 4s intervals,
interval At from the start ofthe structure
injection
duration
of
to maximum
Bolus triggering Reference Contrast
region: protocol:
Scanning Scan 3
delay
protocol: D:
Ascending
aorta
or left ventricle
No test injection Slice thickness intervals Scanning
add 5-10 s for short scan duration
same as for spiral scanning,
initiated
5-7
S3
after attenuation
and large contrast
volumes
minimal
dose, scans acquired
has increased (16-slice
scanners)
by 50 HU3
at 1 to 2 s
Contrast Injection a low threshold level to determine contrast arrival in the target region, and may even make it advantageous to place the trigger region upstream. For example, a trigger level in the ascending aorta may be used for CTA of the chest, or a level in the descending aorta for CTA of the abdomen. Too short a delay between the initiation of the scan and the actual start of the spiral acquisition is also not advisable because patients have to breathe in and hold their breaths during this short additional period of time. Proper patient instruction and a scan delay of 4-5 s after reaching the trigger level is advisable. With fast multislice acquisitions using 8and 16-slice scanners, however, a higher trigger level should be set or the start delay after reach-
ing this level should be increased. The rationale behind this approach for very short acquisition times is to delay scanning until a relative peak enhancement is reached (Fig. 24.5). Some systems allow for optimization ofROI placement during the monitor scans to be able to correct for malpositioning. If placement of the trigger ROI is difficult it can be advisable to manually start the scan as soon as contrast material arrives in the target region. •
Venous Imaging
Examinations of the venous system may be performed during the phase of direct vascular enhancement or during the recirculation phase. In either case it is unnecessary to individually determine the scan delay.
,0,.HU
,0,.HU
400
400 300 -
Enhancement "plateau" grows
200
200 Optimum scan window 120 ml eM 4mljs
100
100
O~~--~~~--~--~~------~ o
20
40
60
80
20
400
400
300
300 120 ml CM 4ml/s
80 Time
80m1CM+ 50 ml NaCI 4ml/s
200 100
100
o~~--~------~----~------~ 40 20 60 80 o
Time
Fig. 24.5 Effect of contrast material (eM) injection protocol on arterial enhancement (simulated data, compare also Fig. 3.3). With monophasic injection protocols the enhancement increases over time because of recirculation effects (a). The optimum time period for scanning (scan window) is reached when the enhancement exceeds a predefined target value. Increasing the flow rates increases the maximum
60
,0,.HU
,0,.HU
200
40
Time
enhancement
but
O~-e~-L
o
20
L-
40
~
60
~
80 Time
shortens the scan window (b). If the contrast volume is reduced (e.g., because of short scan duration with 16slice scanners), only a short period of peak enhancement is attained and the maximum enhancement is less than with larger volumes (c). A saline chaser bolus (saline flush) has a similar effect as increasing the contrast volume (d).
841
24
Vascular System
during the phase of direct material should be well diluted (1 :5) to avoid high-contrast streak artifacts that would hamper vascular assessment. A scan delay of lOs is sufficient during this phase (see Fig. 24.89). For imaging during the recirculation phase, there is usually an equilibrium of venous and arterial enhancement, and the optimum scan delay is 40-60 s for the superior vena cava, some 100 s for the inferior vena cava, and some 200 s for the legs. The latter is important when an examination of the peripheral veins is added on top of aCTA of the pulmonary arteries (CT venography). For this particular indication, and additional 2-3 min delay period should be added after the pulmonary CTA has ended (see Fig.24.87d), and scanning from the periphery up into the abdomen yields best results. For examinations
enhancement, the contrast
Individualized Contrast Material Injection Ideally, the contrast material injection should provide a predefined and constant enhancement level that is guaranteed over the whole duration of the scan. At present, there are two approaches that come close to this goal (see also Chapter 3, p. 103). Instead of using a monophasic contrast injection, both vary injection rates over time to achieve the desired contrast enhancement curve ("bolus shaping" techniques). Mathematical
Modeling
Mathematic modeling uses a complex pharmacological flow model to calculate the required volume and flow rate from individual patient parameters (sex, weight, and height) and various assumptions regarding cardiac output, organ perfusion, and other physiologic parameters. These parameters are used to determine the enhancement over time for any arbitrary volume and flow rate of contrast material. In addition the technique allows estimation of the amount of contrast material that has to be injected to obtain a predefined enhancement plateau. U sing this technique it can be demonstrated that an exponential reduction of the injection speed yields a constant enhancement plateau if the flow rate is adjusted to the individual patient (Fig.24.6b). In addition, the model predicts that the maximum enhance-
ment does not increase linearly with flow rate but that the gain in enhancement becomes less with higher flow rates because saturation effects due to the limited total blood volume. The technique has not yet been integrated into a commercially available product and relies on a number of assumptions, e.g., regarding cardiac output, which may vary between patients. The technique works for the many patients and allows for an individual increase or decrease of the injected contrast material volume. Test Bolus Technique
This technique assumes that the body responds in a linear fashion toward the injected contrast material. This response can then be described as an individual patient function that describes how the injected contrast is transformed into a particular enhancement curve. The patient function is derived from the test bolus injection and includes information about first pass as well as recirculation effects. With this patient function the individual response to a given contrast material injection can be calculated. Conversely, the patient function can be used to determine the flow rates required to obtain a predefined enhancement curve (Fig. 24.6 c). With the current prototype system, a 16 ml test bolus is injected, followed by 40 ml of normal saline. The resulting time-density curve is then calculated for the target vessel or the aorta at that anatomic level. The time-density curve is then used as the basis of determining the patient function. Then the required enhancement level (e.g., 250 HU) and the duration of the plateau phase is entered into a computer program, and the computer uses the patient function to calculate a biphasic injection protocol that best fits the desired enhancement. Such a protocol is based on a smaller volume and a fast flow rate first, followed by a larger volume at a lower flow rate. It could be shown that such a technique yields more homogeneous contrast enhancement over the desired scan phase than a standard monophasic injection. However, there is still some variation as to the actual enhancement level that is reached (up to 50HU). This may be due to the fact that the assumption of a linear response is not strictly accurate because of the limited total blood volume and resulting saturation effects, or that some of the test bolus is pooled in small veins.
Contrast Injection ~HU
~HU
400
400
300 -
300 _ 105 ml eM
200
200
100
exponentially
100
120 ml eM 4 m1/s
o~----------~--~--~----~ o
20
decreasing flow
40
60
o~-=----------------------~ 40 o 20 60 80
80
Time
Time
b Fig. 24.6 Effect of bolus shaping techniques on contrast enhancement, (a) Monophasic injection with a constant flow rate leads to an increase in arterial enhancement over time. (b) A patient-specific exponential decrease in enhancement can achieve a real enhancement plateau, (c) Individualized biphasic injection techniques substantially improve homogeneity of enhancement as compared to monophasic injection and can be implemented on most CT injectors.
~HU 400 300 200 40 ml eM 100
6.7ml/s + 76 ml eM 2.8 mils
O~~--------~------~----~ 80 o
20
40
60
Time
c
Even where such individualization techniques are not available, biphasic injection is still advantageous. As a rule of thumb, some 50 ml can be injected first at 5 mlls, followed by an amount of contrast material that is chosen so that the total injection time (including the lOs of the first phase) is identical to the scan duration and is injected at a flow rate of 3 mlls (see Table 24.3). Contrast Material in Patients with Reduced Renal Function
With single slice CTA, the volume of contrast material is rarely below 100ml, and is frequently about 150ml or more. Iodinated contrast media are known to be nephrotoxic, and require proper hydration of patients (see Chapter 3, Patient Preparation and Contrast Media Application). Patients with compensated renal
failure should not be examined by CTA unless other imaging modalities, such as contrast -enhanced MRA or duplex ultrasound, do not provide sufficient information. In such cases, proper patient preparation is required to keep the risk to a minimum. Multislice scanning using a fast spiral acquisition with 4 x 2 mm, 4 x 2.5 mm, or 4 x 3 mm collimation reduces the scan duration for the chest or abdomen to 10-12 s, and to 4-6 s for the central pulmonary vessels or the renal arteries. As a consequence, the volume of contrast material may be reduced substantially and often may be kept below 50ml (see Table 24.3). In properly hydrated patients, such low volumes of iodinated contrast material are usually well tolerated. Gadolinium compounds (MR contrast agents) are an alternative for patients with severe allergies to iodinated contrast material who cannot be examined by ultrasound or MRA.
843
24 Vascular System However, they give less enhancement at the centricity of some lesions such as stenoses, same injection rates (comparable to 100- aneurysms, and injuries. Interactive multiplanar 150mg/ml iodine). The total amount of these reformations (MPR) are best suited for this task but again scrolling speed is an important issue. agents required for CTA can be quite high, which In addition, no loss in spatial resolution should may also lead to nephrotoxic effects in patients be accepted when the sectional plane is with severely impaired renal function. changed interactively. Longitudinal sections help identify the length and shape of a lesion Evaluation and Documentation while sections perpendicular to the vessel's Depending on the clinical imaging task, evalua- course help to define the vessel diameter pretion of CTA is primarily based on axial sections cisely and thus the degree of stenosis or the size (aortic dissection, pulmonary embolism) or on of an aneurysm. Curved planar reformations (CPR) along the various 3D displays (aortic aneurysm, pulmonary vascular anomalies). The various 2D and vessel's course are much more comfortable to 3D processing techniques are also discussed in evaluate than standard multiplanar reformations (Fig. 24.7). To use this feature effectively, Chapter 2. the workstation must provide a means of scroll• Axial Sections ing through the data set when defining the Evaluation of axial sections requires an interac- curve on the reference sections. In addition, intive cine mode because of the large number of teractive corrections of the points that define sections, which vary between 80 and 400 for this curve should be possible because positionsingle slice CTA, and between 150 and 1500 for ing errors may frequently occur in more commultislice CTA. The reviewing workstation must plex vascular situations. An immediate update be capable of scrolling through these large data of the resulting CPR further improves handling. sets rapidly. Scrolling speed should be intui- Still, such a process can be quite time-consumtively and interactively adjustable so that fine ing. For this reason, technicians should be detail can be evaluated more slowly while large trained to perform standard tasks such as CPR through the renal arteries, so that the radioloranges can be covered rapidly. Not all current systems are equally suited for this task, es- gist can focus on reporting these images instead pecially when large multislice data sets are in- of having to struggle with technology. One has volved. Interactive viewing is important for the to keep in mind, however, that off-center longievaluation of small vessels that cross the scan tudinal sections can simulate vascular stenosis. plane several times, and for evaluating the ves- CPR is mandatory for evaluation of stents or arteries with heavy wall calcifications such as pesel wall and vessel lumen. ripheral arteries or the carotids, especially in • Multiplanar and Curved Planar diabetics and patients with (secondary) hyperReformations parathyroidism (see Fig. 24.15). Vessel tracking software, which semiautoIf abnormalities are suspected, evaluation in a matically finds the centerline of a vessel second plane is mandatory because of the ec-
c
a
b
Fig. 24.7 Curved planar reformation along both renal arteries. Anteroposterior view (a), caudocranial view
(b), and section perpendicular to the vessel axis in the region of an eccentric stenosis (c).
Evaluation and Documentation without much user interaction, is now commercially available (see Fig. 2.44). Most programs require the user to place a proximal. distal. and possibly some intermediate points on the vessel. and then automatically track the vessel between these points. Such vessel tracking software makes it feasible to evaluate the vessel of interest interactively with respect to wall changes (soft versus hard plaques. aneurysms). measurements of the true length of an abnormality. or relationship of the vessel to surrounding structures (e.g .. pancreatic carcinoma). A feature that allows the imaging plane to rotate around the centerline of the vessel. thus making it possible to evaluate eccentric abnormalities. is particularly helpful (Fig.24.7 a. b). Stretching the vessel out can simplify length measurements but causes the anatomic overview to deteriorate. Vessel cross-sections perpendicular to the centerline substantially improve the grading of stenosis or evaluation of other abnormalities (Fig.24.7c). In addition. software tools are available that automatically calculate the local vascular diameter and display the location and degree of maxim um vessel stenosis or dilatation. Such tools are most reliable for the aorta but may fail for smaller vessels. especially in cases with increased image noise (obese patients). crossing or branching vessels (errors in determining the vessel wall). or limited spatial resolution (too thick a collimation). Thick CPR may be helpful in obese patients and for eccentric stenoses. By increasing the thickness of the CPRjust above the diameter of a vessel. the same effects as in conventional angiography or DSA occur: the local density of a vessel decreases as it gets thinner. This will happen close to the vessel wall but it will also affect stenotic segments, which will appear less dense even in vessels with eccentric stenoses and thus can be more readily appreciated (see Fig. 2.15). In addition, this technique will more readily appreciate irregularities of the vascular lumen, even when not cut perpendicularly. Another beneficial effect is the reduction of image noise. •
Maximum Intensity Projections
Maximum intensity projections (MIP) are an excellent tool for providing angiography-like images of the vascular system (see Chapter 2). They optimize contrast between a vessel and the background, and are therefore excellently suited for displaying small and peripheral ves-
sels that are otherwise hard to demonstrate by 3D techniques. For most applications, prior editing of the data set is necessary to remove all high-attenuating vascular or skeletal structures, or bowel loops containing positive contrast material that obscure the vessels of interest (Fig. 24.9). MIP are well suited to display simple vascular anatomy in which only few vessels overlap (e.g., see Fig. 2.12). For such applications the main disadvantage of MIP, namely the inability to differentiate between foreground and background, does not playa role. A 3D impression can be gained by reconstructing images from various projection angles that can then be displayed in a cine mode. This is the case for the retroperitoneal vessels (aorta and renal arteries). the mesenteric vessels (excluding the aorta by editing procedures, see Fig.24.74a), the thoracic aorta (excluding the pulmonary vessels), the carotid arteries (separate displays for each side, see Fig.24.69a), and the peripheral arteries (unless there are massive calcifications). They are not particularly well suited for displaying the whole abdominal vasculature, the pulmonaryarteries (unless targeted MIP are used), complex anomalies of the aortic arch, or vascular malformations. MIP suffer from an increase in background density when the projected volume is top wide. Overlying contrast-enhancing organs may superimpose over smaller vessels, and even image noise may hamper evaluation of small arteries. For this reason, thin-slab MIP (targeted MIP) of the vessel of interest often provide superior results (Fig. 24.8). In many cases, no prior editing of the skeletal structures will be necessary with this technique. Curved thin-slab MIP provide an excellent overview of branching vessels, for example in the renal hilum, and are thus superior to CPR for this purpose (Fig. 24.9). Calcifications are clearly delineated in MIPs, and this can be helpful in planning vascular interventions or anastomotic surgery in patients with severe atherosclerosis. On the other hand, calcifications superimposed on the arterial lumen can make it impossible to quantify the degree of stenosis. The original axial sections or CPR should be used for evaluating these cases (seeFig.24.82b) . Editing used to be a complex and time-consuming process that hampered the use of (nontargeted) MIP in clinical practice. Software is
845
846
24 Vascular System
a
b
Fig. 24.8 Oblique thin-slab MIPs parallel to the course of the renal arteries require no prior bone removal. The thickness in AP direction can be somewhat smaller
(10mm, a) than in the caudocranial direction (20mm, b).
a
b
Fig. 24.9 After editing the skeletal structures, there are still mesenteric vessels superprojecting over the kidneys on AP projections (a). Best display of renal vessels is possible with curved thin-slab MIP (b). The location of this thin slab is indicated on a caudocranial MIP that was tilted in a way to separate aorta and vertebral column (c)
Eva/uatf'on and Documentation
now available that does this semiautomatically. Many such programs require a seed point to be placed on the structures to be removed and sometimes also the structures of interest. The program then automatically removes unwanted structures with no or very little further interaction (see Fig. 2.43). Such programs are particularly helpful in the chest, abdomen, and for peripheral run-off vessels. •
Shaded Surface Displays
Shaded surface displays are best suited to demonstrate complex 3D-relationships, for example at the aortic arch or abdominal vasculature. They are helpful for preoperative planning, and may be used routinely in the evaluation of thoracic aneurysms (without a large thrombus), coarctations and other aortic arch anomalies, and in the follow-up of extra-anatomic vascular bypass surgery. SSD require a high level of vascular enhancement for good results. Generally there is no need for editing of the image data set prior to reconstruction of a SSD. Most processing tools, from simple cutting function to semiautomated techniques for bone removal, can be applied to the reconstructed 3D object. A useful rule of thumb for the optimum 3D imaging of in- plane and through-plane vessels is that the vascular diameter should be at least 1.5 times the slice collimation. The optimum threshold level depends on numerous factors but should be set at approximately 40-50% of the contrast between a vessel and its surroundings. Common artifacts mimic stenosis or vascular occlusion on SSD. This is due chiefly to partial-volume effects, which are especially pronounced in small vessels that course in plane. As the threshold for rendering is increased, the image depicts fewer vessels but displays an increasing number of pseudo-stenoses and occlusions. On the other hand, setting the threshold too low leads to the underestimation of stenoses (see Fig. 2.21) and produces band-like distortions of small vessels in the direction of the scan (Fig. 24.10). Calcifications can mask stenoses, because they appear to enlarge the vessel. Multiple thresholds can be used together with different colors to distinguish calcifications from true vessel lumen. However, such a technique is not very accurate in depicting the real vessel diameter because of blooming effects that make a calcification look larger and exag-
Fig. 24.10 Effect of threshold level on the shaded surface display (SSD) of a contrast-filled infusion line.
gerate stenoses. Image noise will cause "flying pixels" and is the main reason why SSD does not provide optimum results with thin-section multislice imaging. Volume rendering is the superior technique for this purpose. •
Volume Rendering
Volume rendering has become the standard rendering technique for many CTA indications, especially when multislice scanning is used. Compared to surface shaded displays it is less vulnerable to threshold effects and is easily able to display calcified plaques and lumen with different colors. It has the advantage ofM1P that it retains 3D information as well as density information but requires less editing. However, MIP may be superior when evaluation of more peripheral vessels is required. An appropriate color scale can be used to better differentiate between vessels of varying density (such as arteries and veins, see Fig. 2.31). A color scale may also be used to color code density ranges, which is particularly helpful for the pulmonary parenchyma or the kidneys (see Fig.2.38a). Editing with bone removal and removal of overlying structures may further improve the visualization of small detail (see Fig. 2.43) but is often not essential. Thin-slab rendering is an easy tool to free the data set
24 Vascular System from overlying structures editing. •
Virtual
without
much further
Angioscopy
Virtual angioscopy relies on the same principle as other virtual endoscopic techniques with the exception that the high-density areas (contrastenhanced vessel lumen) have to be rendered transparent. This is possible most easily with volume rendering and properly placed trapezoids for soft tissue (100% opacity below 100150 HU. 0% opacity above 150-200 HU). and for calcifications or metal stents (0% opacity up to maximum CT number in enhanced vessel. 100% opacity for CT numbers some 200-400 HU larger). Virtual angioscopy frequently suffers from noise-related vascular irregularities (see Fig. 24.60). and from difficulties in finding the correct opacity settings. especially in small vascular structures. Vessels that are in direct contact to the vessel of interest may blend and leave a connecting hole in the vascular wall. Virtual angioscopy rarely adds diagnostic information over other 2D or 3D-rendering techniques. In a few cases it may improve the assessment of
stent graft localization relative to branching arteries. Volume rendering with the opacity settings used for virtual angioscopy may often be superior for this purpose. especially when the vessel is cut open perpendicular or longitudinal to the course of the lumen. Documentation
Given the large number of sections generated by spiral scanning. it is impractical to document all images on film. With the increasing use of PACS systems. image documentation on hard copy loses importance. In such situations. only the pathologic findings may be recorded on film or even on paper prints. In a non-PACS situation. however. documentation on hard copy remains important. For single slice CT. it is advisable to document every second to fourth image (spaced 4-8 mm apart) on hard copy. For multislice CT, reconstruction of thick sections (5-10 mm) can be advocated. In any case. all relevant 3D images (MIP, SSDor VRT) together with CPRofthevessels of interest have to be printed because they provide the information pertinent to the further treatment of the patient.
Artifacts Artifacts may be caused by movement (pulsations. respiration). the selected scan parameters. or the rendering technique (MIP. SSD). An awareness of artifacts is important because they can simulate pathology. Pulsation
Artifacts
Artifacts occur when pulsations lead to changes in the diameter. configuration. or position of an opacified structure. The results are double contours. streak artifacts. jagged margins along the z-axis, and a spurious increase or decrease in CT attenuation. Pulsation artifacts can alter the diameter or the configuration of the heart. ascending aorta. and aortic arch. Double contours of the aorta can create the appearance of multiple lumina with different degrees of opacification (Fig.24.11a. b). Strong pulsations can seriously hamper the evaluation of the aortic root and aortic arch. Indeed. one should be cautious in
selecting young patients for CTA of the thoracic aorta, as the elasticity of that vessel can lead to extreme pulsation artifacts. Pulsation- induced position changes may be seen in the abdominal aorta, the renal arteries, and especially in renal graft arteries (Fig.24.11c). This artifact leads to saw-tooth irregularities in vessel margins. Pulsations in the craniocaudal direction can mimic stenosis in inplane vessels and when a high table feed is used. ECG-gating techniques may be used with multislice scanning to reduce pulsation effects. In addition, the faster table speed with multislice scanning, in particular 16-row scanners, will spread out artifacts over wider ranges along the z-axis. They will therefore appear less disturbing on 3D-rendered images (Fig. 24.12).
Artifacts
a
b
Fig. 24.11 Pulsations of the ascending aorta lead to characteristic double contours ofthe vessel wall (a) that can mimic a dissection (b). Positional changes during pulsation causes typical serrations, even on multislice
c CT data ofthe abdomen (c). Note the undulations ofthe abdominal wall due to shallow breathing. They have no additional influence on the display of the mesenteric vasculature.
The key factors in eliminating respiratory artifacts are optimum patient instruction and practice in effective breath holding. •
High-Contrast
Artifacts
Highly concentrated contrast material in the left brachiocephalic vein can cause significant streak artifacts that obscure adjacent structures. Streak artifacts caused by a high concentration of contrast material in the superior vena cava can mimic intimal flaps in the ascending aorta (Fig. 24.13), and they degrade the quality of 3Da
b
Fig. 24.12 Pulsation is spread out along the Z-axis with fast scanning using 16-row scanners (16 x 1.5/36) (a) and is thus less disturbing than with a 4-row scanner (4Xi/6)(b) .
•
Respiratory Artifacts
Artifacts due to respiratory motion during the spiral scan can simulate vascular discontinuities, stenoses, or aneurysms depending on the direction and amplitude of the motion. Respiratory artifacts are easily recognized by the appearance of steps or undulations in the contours of the skin surface or organs that move with respirations (see Fig.24.11c). Spurious vascular changes appear at precisely the same table position.
a
b
Fig. 24.13 High contrast artifacts in the superior vena cava can simulate dissection in the aorta (a). Such artifacts can be substantially reduced when a saline flash is used together with caudocranial scanning (b).
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24 Vascular System rendered views. In examinations of the carotid arteries or thoracic vessels, an effort should be made to avoid these artifacts by using a saline flush and a scanning direction that captures the inj ected veins at the end of the scan. Artifacts due to dental implants will cause similar effects in the neck, particularly close to the region of the carotid bifurcation (see Fig. 24.69). Since the artifacts arise further ventral to the vessel they are frequently less disturbing than contrast-material induced artifacts. They can be reduced by placing the patient's head in such a way that the line of the teeth runs as closely parallel to the scan plane as possible. •
Flow-related
Artifacts
Differential opacification of vascular territories is due to differences in blood velocity, length of a vessel, vascular resistance (arterial phase) or variations in tissue transit times (venous phase). They may cause mixing of well-enhanced and less-enhanced blood that may simulate thrombi. This effect is most pronounced in regions of venous confluences (see Fig. 7.39) but can also be found in the pulmonary arteries (see Fig. 24.24 a, b). Layering of contrast material can occur in a vessels with a low blood velocity because contrast material is denser than blood (Fig. 24.14). •
Partial Volume
Effects
Small vessels that course in-plane are subject to marked partial-volume effects (see Fig. 2.21), especially in cases where a wide slice collimation has to be used. These effects cause a marked
decrease in the attenuation of small vessels on axial sections. As a consequence, small vessels that are poorly opacified may not be visualized on MIP or SSD. Small vessels that course through the imaging plane are clearly defined and show markedly higher contrast than inplane vessels. It should be noted, however, that partial-volume effects can cause segmental stenoses of these vessels to be missed. Partialvolume artifacts frequently affect imaging ofthe pulmonary, renal, and splanchnic arteries in single slice CTA examinations. With multislice CTA such artifacts are substantially reduced and occur only with fast scanning techniques that require thicker sections. •
Spiral Artifacts
The raw data interpolation process with spiral scanning has been shown to cause undulations of the vessel contour even in stationary vessels. In clinical practice such effects rarely affect image interpretation but they may become important with automated analysis of vessel diameters and stenosis quantification. Artifacts increase with larger pitch factors for both single and multislice scanning. •
Calcifications
and Vascular Stents
Metallic vascular stents and calcifications of the vessel wall have a substantially higher attenuation than the opacified vascular lumen. Due to
I
I
a a
b
Fig. 24.14 Contrast material is denser than blood and may for a fluid-fluid level in vessels with slow flow. (a) superficial femoral arteries; (b) abdominal aortic aneurysm.
b
Fig. 24.15 Metal stents will lead to "blooming" effects, which 'lead to underestimation of the true vessel lumen at standard CTA window settings (550/150) (a), wide wide window settings (1500/500) provide a more realistic view (b). Similar effects are seen with calcifications that are directly adjacent to the perfused vessel lumen.
Developmental
the limited spatial resolution of the CT system, both in-plane as well as along the z-axis, stents and calcifications will appear blurred and thus cause a halo of intermediate density CT numbers around them. This "blooming" effect will lead to an overestimation of stenoses if the calcification or stent borders directly to the vessel lumen and standard CTA window settings are used (Fig.24.15a). Due to partial volume effects on the vessel, this depends not only on the density of the stent material/calcified plaque but also on the spatial orientation of the vessel (parallel or perpendicular to the scan plane). Using a specifically adjusted narrow window setting is advocated for this purpose but this technique is cumbersome and requires readjustment for each calcified vessel segment. A simpler technique is visual analysis of the vessel using a very wide window setting that makes sure that the stent or plaque does not appear completely white (Fig. 24.15b, see also Fig. 7.30c. d). •
3D Rendering
Arterial Abnormalities
Table 24.7 Artifacts seen in eTA with various 2D and 3D imaging techniques
• Simulated stenoses with off-center cut-lines
A
MIP • • • •
Increasing background density with thick VOl Calcification superimpose vessel lumen No foreground-background differentiation Eccentric stenoses may appear normal
SSD • No differentiation between lumen and calcified plaque • Degree of stenosis threshold-dependent VRT • Noise may cloud pathology
As discussed above the various rendering techniques suffer from specific artifacts that are summarized in Table 24.7 and are discussed more extensively in Chapters 2 and 7.
Pulmonary Vessels
The main indication for pulmonary CTA is suspected acute pulmonary embolism (PE), for which it is superior to ventilation perfusion scintigraphy. With multislice scanning, even peripheral vessels (up to the sixth to eighth order) can be adequately evaluated. CTA is also an excellent tool for diagnosing developmental abnormalities, arteriovenous malformations, and for ruling out chronic thromboembolism as a cause of pulmonary hypertension. CTA has thus become the primary tool for the diagnostic work-up of the pulmonary vasculature. Developmental Arterial Abnormalities
Developmental pulmonary artery abnormalities in children should be evaluated with MRA whenever possible. If CTA is necessary, low dose techniques with reduced kVp (effective dose 50 years of age and is frequently associated with polymyalgia rheumatica of the shoulders and hips. Unlike Takayasu arteritis. the acute stage lasts only for 1-3 weeks and is associated with flu-like symptoms and often a unilateral headache. The hallmark is a rapid erythrocyte sedimentation rate (ESR >40mm/hr).
Table /I
24.14
Classification
ofTakayasu
Type I
Aortic arch and supraaortic
Type
Descending thoracic abdominal branches
g
Type III
arteritis arteries
+ abdominal
aorta +
Combination oftype I and type II
Concomitant aneurysmal disease of the pulmonary artery and dilatation of the aortic root with aortic valve incompetence has been reported. Syphilitic aortitis occurs in stage III of the disease after an incubation period of more than 10-15 years. It is characterized by periaortitis and mesaortitis. which predispose for aneurysm formation. mainly in the ascending aorta and the aortic arch. There is secondary injury of the intima. leading to accelerated calcific atherosclerosis. Polyarteritis nodosa is a rare vasculitis involving systemic small and medium sized arteries and arterioles. All organs may be involved but kidney, heart, and liver are most frequently affected. CT Morphology
Takayasu arteritis leads to thickening
and enhancement of the vessel wall. The vessel wall may be seen to be hyperattenuating already on precontrast scans. In the acute stage, it takes up contrast substantially, which can already be seen in the arterial phase of CTA but is more prominent in later phases (which also allow for a better differentiation from atherosclerotic wall thickening). Inflammatory wall changes may be detected even if the DSA is normal (Fig.24.66a). Takayasu aortitis commonly involves the aorta, the pulmonary arteries, the left subclavian (Fig.24.66b), left common carotid, and brachiocephalic artery in the chest, and the celiac, superior mesenteric, and renal arteries in the abdomen. Rarely, smaller arteries are involved. Proximal stenoses of the supra-aortic branches are typical findings. The stenoses may be long and smooth but can also be short and irregular. Skip lesions are frequent. There is abundant collateralization in the chronic stage. Fusiform aortic ectasia and aneurysms may be found. Granulomatous arteritis cannot be distinguished morphologically from Takayasu arteritis
Neop/asia
a
c
b
Fig. 24.66 Takayasu aortitis with significant wall thickening in a patient with no angiographic evidence of supra-aortic stenosis (a). Takayasu arteritis with highgrade stenosis and wall-thickening of the subclavian
artery (4 x 1/6) (b). Granulomatous arteritis of the superior mesenteric artery and its side branches (4x 1/6)
but it can be suspected if arteries are involved that are rarely affected by Takayasu disease (Fig.24.66c). It is commonly bilateral and symmetric, and there is a lack of atherosclerotic changes to account for the vascular narrowing. The common carotid artery is usually not involved. , , Syphilitic aortitis also causes aortic wall thickening with increased contrast enhancement due to inflammation. A thin pencil-like calcification of the intima of the ascending aorta is characteristically seen late in the disease, but it can be superimposed by thicker atherosclerotic calcifications. Aneurysms may form that are most commonly saccular and involve the ascending aorta and the aortic arch, and also, more rarely, the sinus of Valsalva or the pulmonary artery. Polyarteritis nodosa is characterized by small saccular aneurysms of 1-5 mm size, which can sometimes be detected by thin-section CTA. Aneurysms can be found in the intrarenal or intrahepatic branches on early arterial phase scans (only possible with multislice scanning) before there is substantial parenchymal enhancement. Aneurysms in mesenteric side branches are more easily detected in (moderately) obese patients. The typical luminal irregularities and stenoses of small arteries are
very hard to detect on CTA. In the kidneys multiple cortical infarcts;may be present.
(c).
Neoplasia
Aortic tumors are rare. The majority are mesenchymal tumors, most notably leiomyosarcoma, angiosarcoma, or malignant fibrous histiocytoma. CT Morphology
Aortic tumors may present initially as proximal occlusions of the aortic branches in cases where tumor growth is confined to the aortic wall. Nodular lesions of soft-tissue density in the aortic wall extending into the para-aortic tissue can suggest the correct diagnosis. The differential diagnosis includes retroperitoneal fibrosis (Ormond disease), inflammatory aneurysms, and aortitis. These conditions are much more common than sarcomas.
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24 Vascular System
The scan range with single slice CTA is limited but evaluation of focal pathology such as stenosis of the carotid bifurcation or evaluation of thoracic outlet syndrome is nevertheless possible. Multislice CTA offers advantages whenever large ranges have to be covered, such as for the evaluation of the carotid arteries from the aortic arch to the intracranial circulation, which now also allows for the diagnosis of tandem stenoses. Anatomic Variants
Anatomic variants of the supra-aortic branching pattern have also been described above (p. 870). A retroesophageal course of the left or right subclavian artery can cause dysphagia lusoria. Knowledge about a direct origin of the vertebral artery from the aortic arch may be helpful for selective catheterization. An abnormally high or low carotid bifurcation as well as the presence of massive carotid kinking or coiling may be important for preinterventional planning. •
CT Morphology
An aberrant subclavian artery and other variants can be identified by tracing the vessels from the aortic origin into the neck or axillary region. In many cases the vessels display a typical arrangement that can be recognized even on noncontrast scans (see Fig. 24.41). The common carotid artery bifurcates between the level of C2 and C4. Abnormally caudal
or cranial branching patterns are well appreciated on curved reformations or thin-slab MIP and are important to detect prior to surgery. Kinking is due to elongation ofthe carotids leading to sharp bends within the vessels (Fig.24.67a), while coiling is an even more massive elongation that causes a full rotation of the vessel, usually in the distal extracranial portion of the vessel. Kinking of the vertebral artery may lead to bony erosion (Fig.24.67b) due to pulsation effects. Such findings are easily overlooked on axial images. Patients are at increased risk of vascular damage during chiropractic manipulations. Carotid Stenosis
Carotid artery stenosis is a common complication of atherosclerosis. Higher grades of carotid stenosis are associated with an increased risk of stroke. Large studies (NASCET, ECST) could show that symptomatic patients with a stenosis benefit from surgical endarterectomy. Even asymptomatic patients with carotid stenosis have been shown to benefit from surgery: the ACAS trial (1995) found a reduced stroke rate of 1% with surgery as compared to 2% with medical treatment but had less influence on clinical practice than the NASCETtrial. The area of the carotid bifurcation is most commonly affected. The stroke risk is increased in patients with ulcerated plaques that facilitate formation of thromboemboli. The size of infarc-
Fig. 24.67 Bilateral carotid artery coiling (4 x 1/6, VRT) (a). Bone erosion by vertebral artery kinking (4X1/6,VRT) (b). a
b
Carotid Stenosis 893 tions depends on intracranial collateral circulation, which may be impaired if portions of the circle of Willis are hypoplastic or stenosed. Single and multislice CTA are excellent for detecting carotid stenosis in the region of the carotid bifurcation and assessing the need for surgical treatment, but the proximal and intracranial portions of the carotid arteries can only be adequately evaluated with multislice CTA. CTA is in direct competition with color duplex ultrasound and MR angiography as a minimally invasive alternative to diagnostic angiography. •
CT Morphology
Because the carotid arteries run perpendicular to the scan plane, maximum opacification is unnecessary for their evaluation. An interactive review of the axial images is still important, however, because MIPs and SSDs may be seriously degraded by superimposed calcified plaques. Thin-slab MIPs and curved planar reformations (CPR) with a slice thickness of 3-10 mm are best for documentation purposes (see Fig. 24.69). Quantification of stenoses is most precise if transverse sections or curved planar reformations are generated perpendicular to the longitudinal axial of the artery. Such CPR are especially important in the bony segment of the carotids, where most other visualization techniques fail. Precise positioning of the centerline of the CPR within the vessel is essential for stenosis quantification because an eccentric cut may simulate stenosis. Vessel tracking may help to automatically track the vessel course but it usually fails for the petrous segment of the carotid arteries. Stenosis quantification in CTA is usually based on the (angiographic) NASCET criteria, which use the diameter of a normal distal segment of the internal carotid artery as a reference for calculating the degree of stenosis (Fig. 24.68). A 70% cutoff is employed for distinction between significant and non-significant stenosis but newer results (Cochrane Reviews 2000) suggest that even 50% stenoses benefit from endarterectomy. The ECST criteria for stenosis quantification are based on sonography and use the whole diameter of the carotid lumen (including any plaques) at the site of stenosis as a reference for calculating the degree of stenosis (Fig. 24.68). According to new results, a 70% cutoff is employed for distinction between significant and non-significant stenosis.
Carotid stenosis (%) NASCET:
100-(a/dx1 00)
ECST:
100-(a/bx1 00)
Fig. 24.68 Quantification of carotid using NASCET and ECST criteria.
artery
stenosis
Fibromuscular dysplasia of the carotid artery is a very rare entity and can be suspected in a younger individual if typical beaded stenoses or formation of small aneurysmatic dilatations of the vessel can be observed. CT attenuation is a good predictor of plaque composition (Fig.24.69a); lipid plaques have the lowest density (below 50 HU), fibrous plaques have densities between 50 HU and 150 HU, and calcified plaques have the highest densities (over 150 HU, often more than 1000 HU). Small plaques, however, are susceptible to partial volume effects. Ulcerated plaques present as marked irregularities on the surface of a soft plaque (Fig.24.69b). Calcified plaques make evaluation of stenoses difficult unless transverse sections or curved planar reformations are generated and a wide window setting is used. Concomitant aneurysmatic dilatation of the carotid (Fig.24.69d) may increase the risk of stroke if there is irregular thrombus formation within the aneurysm sack. It is important to evaluate not only the region of the carotid bifurcation but also more distal segments, as well as the petrous segment of the carotid artery, because such distal segments are hard to evaluate by ultrasound (Fig. 24.70). The same holds true for stenoses close to the origin of the common carotid artery. Tandem stenoses are present if there are stenoses outside the carotid bifurcation as well, usually located either at the origin of the common carotid artery or in the distal or intracranial segments of the carotids (Fig. 24.70).
894
24 Vascular System
a
b
c
fibrous plaque, (b) High-grade carotid artery stenosis with an ulcerated plaque (4 x 1/6, CPR). (c) long subtotal stenosis of the ICA involving also the petrous portion of the vessel (d) Thin-slab MIP of an aneurysm and stenosis of the internal carotid artery.
Fig. 24.69 Internal carotid artery (ICA) stenoses, (a) The plaque density may provide a clue about plaque composition. In this patient there are hypodense «SOH U) regions considered to be lipid plaques and denser regions (50-90 HU) considered to represent
a
b
c
Fig. 24.70 Findings outside the region ofthe carotid bifurction (4 x 1/6, CPR). (a) Tandem stenosis ofthe bifuraction (arrowheads) and the petrous segment of the ICA(arrows). (b)Corresponding axial sections, (c)Stenosis at the origin ofthe left carotid artery, (d) Occlusion of
d
d
e the proximal right common carotid artery, (e) Incidental finding of an intracranial aneurysm and hypoplasia of the A1 segment of the left anterior cerebral artery (VRT).
Stroke Imaging
Associated intracranial abnormalities include intracranial stenoses of the main cerebral arteries as well as aneurysms or hypoplasia of these vessels (Fig.24.70e). Y
present as multiple areas of focal narrowing or aneurysmal dilatation. Similar findings can be seen in vertebral artery dissection, but spatial resolution may become a limiting factor for this indication.
Spontaneous Carotid Dissection
Internal carotid artery dissections occur in young or middle-aged patients and are a major cause for stroke (5-20%) in the young population. Some 70% involve the cervical and petrous portions of the vessel. while 20% affect the cervical part only. In spontaneous (non traumatic) dissections. there is either no precipitating factor or they can occur after trivial trauma such as rapid head turning or during normal sporting activity. The pathogenesis is usually unclear. but underlying arteriopathy like hypertension. fibromuscular dysplasia ("15-20% of cases). cystic medial necrosis. Marfan syndrome. or Ehlers-Danlos syndrome may predispose to dissection. Patients present with symptoms of headache. neck pain, vascular bruits, Homer's syndrome, or focal neurological deficits. Stroke is often delayed for days to weeks when the dissection remains extracranial. The most common extracranial site is the internal carotid, a few cm beyond the bifurcation. Anticoagulation therapy is usually effective, and 85% of patients with minor symptoms do well and only 5% suffer severe strokes. Refractive cases may require surgery, such as an extracranial- intracranial bypass. Vertebral artery dissections are half as common as those involving the internal carotid artery. They are less common in the mid cervical region where the artery is protected by the transverse processes, and usually occur around the CI-C2 region (two-thirds of cases). In up to 20% there may be asymptomatic dissections in other arteries as well. •
CT Morphology
Carotid dissection may present as a smooth, long stenotic segment that may extend intracranially ("string" sign, see Fig. 24.91). Usually the false channel is thrombosed and may appear hyperdense with a crescent shape. A thin rim of enhancing vasa vasorum may be seen around the intramural hematoma. If the false channel is perfused, diagnosis is easy with CTA. Fibromuscular dysplasia as a causative factor may
Stroke Imaging Stroke is the third leading cause of death in most industrial countries. Acute cerebral ischemia accounts for more than 75% of cases. Hemorrhagic brain infarction and acute intracerebral hemorrhage are responsible for about equal numbers of the remaining patients. New therapeutic concepts based on early fibrinolytic therapy of patients with ischemic stroke require a short time to definite diagnosis for optimum therapeutic results. This can be achieved by SPECT, PET, xenon CT, or MRI but probably as well (or even quicker) with the following combination ofCT techniques: 1. Precontrast CT; 2. CT perfusion imaging (CTP) of the brain; 3. CTA of the carotid arteries including the brain. The precontrast scan is used to rule out hemorrhage or signs of manifest infarction. The perfusion scan allows for distinction between infarcted regions and hypo perfused areas (penumbra) that may recover after fibrinolytic therapy. The CTA can detect potential causes for the acute ischemia, such as a high-grade stenosis, an ulcerated plaque, or a (partially thrombosed) aneurysm. In addition, demonstration of an occluded large intracranial vessel suggests that the patient may benefit from intra-arterial thrombolysis. By demonstrating normal perfusion CTP can help to exclude stroke mimics like TIA. migraine, or seizures. For this reason, CT may become an important part of the work-up of hyperacute stroke (3-6 h after onset). CT Morphology
Hyperattenuating areas on the precontrast CT indicate hemorrhage or hemorrhagic brain infarction. No further examinations need to be performed because fibrinolytic therapy is not an option for these patients. The classic signs of ischemic infarction, such as a well-demarcated hypoattenuating region with a CT attenuation slightly below that of normal brain substance, as
895
24 Vascular System
well as loss of gray-white differentiation or locally decreased gyration also suggest areas that are infarcted and also do not profit from lytic therapy. Normal or near-normal findings on the precontrast scan warrant a pel-fusion scan. The dynamics of contrast enhancement are acquired at a representative level within the brain. The level usually just cranial to orbital roof usually includes parts of the anterior, medial, and posterior cerebral arteries, as well as the basal ganglia. The thickest possible sections are used (10 mm for single slice CT, and 2 x 10 mm or 4 x 8mm for multislice CT). Automated programs . for CT perfusion imaging can calculate regional cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), and timeto-peak (TTP). The normal CBF is 50-60 mlilOOg tissue/ min. CBF is reduced in neonates, but increases during early childhood (peaks age 7) and then lowers again until adult values are reached during adolescence. Dysfunction is present if CBF falls below 20mlll00g/min. Neuronal death has been reported to occur if CBF is lower than 10-12mlilO 0 g/ min. SPECT data would show that there is an increased risk for hemorrhage following thrombolysis if the CBF in the ischemic core falls below 35% of normal cerebellar flow. Lack of blood volume and blood flow indicates an area of manifest infarction. Reduced blood volume and blood flow suggest lack of sufficient collateral supply, which very likely
a
b
Fig. 24.71 Perfusion imaging in a stroke patient with a near normal precontrast scan 2 hours after the incident (a). The map of mean transit time (MTT) best shows the affected area (b). The cerebral blood flow map (CBF)
will progress to manifest infarction, while regions with near-normal blood volume but reduced blood flow can be found in areas with good collateral supply (penumbra) that will most probably benefit from fibrinolytic therapy (Fig. 24.71). A similar pattern, however, may be found in patients with chronic cerebral ischemia, although reduction in blood flow is not common in these patients although the arrival time is prolonged. CBV images are a better predictor of the final minimum infarct size, while CBF and MTT images overestimate the size of the infarction. Demonstration of the occlusion of a large intracerebral vessel (either on axial source images or thin-slab MIP) suggests that the patient may profit from intra-arterial thrombolysis if the acute event occurred less than 6 hours earlier (Fig. 24.71). Evaluation of the extracranial circulation has been described above. Thoracic Outlet Syndrome
Thoracic outlet syndrome describes a compression of nerves, veins, and arteries along the course from the chest to the arm. The most frequent congenital causes are a widened or abnormal insertion of the anterior scalene muscle (scalene syndrome), followed by a cervical rib (only 5-10% of patients have symptoms), an unusually straight course of the first rib with narrowing of the costoclavicular space, or a scalenus minimus muscle extending from the trans-
c
d
demonstrates a corresponding hypoperfused zone (arrowheads, c) that is larger than the region with reduced cerebral blood volume (d), which is at increased risk for permanent ischemia.
Thoracic Outlet Syndrome verse process of the seventh cervical vertebra to the first rib, inserting between the brachial plexus and the subclavian artery. Acquired causes are a supraclavicular mass or lymphadenopathy, fracture of the clavicle with exuberant callus formation, or muscular compression of the artery in the pectoralis minor tunnel. Patients present with pain in hand or forearm that increases with elevation of the arm. Paresthesias of the hand are very common. A hyperabduction maneuver will frequently obliterate the radial pulse. A Raynaud phenomenon may be present in up to 40 % of patients. CTA may be performed instead of angiography to demonstrate the site of compression. To be most effective, however, scans in a neutral position and after appropriate postural maneuvers are required. The maneuver that induces most symptoms orchanges in pulse wave or arterial pressure should be used. The Adson maneuver is usually most successful as it tests the compression by the anterior scalene muscle; the patient holds a deep inspiration while the neck is fully extended and the neck is turned to the ipsilateral and contralateral side. The costoclavicular maneuver tests the compression between the clavicle and the first rib and uses an exaggerated military position with the arms at 90° and the shoulders pressed back. The hyperabduction maneuver tests the compression by the humeral head and pectoralis minor muscle while the arm is moved from a neutral position to 180° over the head.
•
CT Morphology
CT angiography demonstrates a focal stenosis or occlusion of the subclavian artery during the proper postural maneuver (Fig. 24.72). One should watch out for mural thrombus (which increases the risk of distal embolization), poststenotic dilatation, or even aneurysms of the subclavian artery, and associated venous obstruction or even thrombosis. Obstruction of the subclavian vein is a frequent incidental finding during intravenous injection of the contrast material and is characterized by substantial flow of highly contrasted blood from the injection vein into cervical and chest wall collaterals while there is marked narrowing or even obstruction of the venous lumen as it crosses between the clavicle and the rib.
Fig. 24.72 Thoracic outlet syndrome. Compression of the subclavian artery during postural maneuver (4 X II 6).
JVbdominal and Peripheral Arteries
Thin-section CTA is necessary for the adequate evaluation of the abdominal aortic side branches. This limits the examination volume with single slice scanning. Multislice CTA allows for excellent displays with near-isotropic resolution while being able to cover the whole chest
or abdomen in a single breath hold. CTA is able to substitute almost completely for diagnostic arterial angiography for the evaluation of aortic side branches but color duplex ultrasound and (Gd-enhanced) MRA are potent competitors.
897
24 VascularSystem Anatomic Variants
Anatomic variants are incidental findings in CT examinations of the abdomen. They assume importance when they cause symptoms or have a bearing on planning of surgery or interventional radiological procedures. 1 , Anatomic variants of the arterial supply to the liver are found in up to 50% of patients. They are typical incidental findings in arterial phase CT examinations of the abdomen. They become diagnostically relevant for planning partial liver resection or interventional radiological procedures. Variants of the superior mesenteric artery, other than a common origin with the celiac artery, are exceedingly rare. Superior mesenteric artery syndrome is caused by compression of the duodenum by the SMA in thin individuals. The nutcracker syndrome is due to compression of the left renal vein and may present as painless hematuria in slim patients. •
CT Morphology
Variants of the upper abdominal arteries can usually be detected on standard arterial phase scans that are part of a biphasic or multiphasic study of the upper abdomen. The radiologist should be alert for the following variants of the arterial supply to the liver: • Branches of the celiac trunk arising separately from the aorta (Fig. 24.73 a).
a
b
Fig. 24.73 Anatomic variants of the hepatic blood supply, (a) All three branches of the celiac trunk arise from the aorta, (b) Accessory right hepatic artery (here
• A vessel running posterior to the portal vein (Fig.24.73b) may be an accessory right hepatic artery, a replaced right hepatic artery, or a replaced proper hepatic artery. Accurate identification requires tracing the vessel into the liver and toward its origin (from the superior mesenteric artery, the aorta, or directly from the celiac trunk) . • A vessel coursing between the caudate and left lobes of the liver superior to the porta hepatis may be an accessory or replaced left hepatic artery arising directly from the left gastric artery (Fig. 24.73 c). • An accessory right hepatic artery that arises from the gastroduodenal artery presents as a second vessel just below the right hepatic artery. Such vessels are easily missed and require thin-section imaging. They are best seen on thin-slab M1P (1-2 ern thickness) parallel to the course of the proper hepatic artery. A good overview of major findings is gained by coronal thin-slab M1P of some 5-6 cm thickness that are placed immediately anterior to the aorta and tilted slightly so that there is a selective display of the celiac and mesenteric branching pattern (Fig. 24.74). Volume-rendered displays are a good alternative that even allows for semitransparent display of the surrounding tissues. Atypical branching patterns of the hepatic arteries are important for evaluation of living
c from the superior mesenteric artery) showing a typical retroportal course, (c) Accessory left hepatic artery arising from the left gastric artery.
Living Renal Donors
Living Renal Donors
Fig. 24.74 Thin-slab MIP of a displaced right hepatic artery from the SMA and an accessory left hepatic artery from the left gastric artery (a).
hepatic donors. In particular, the presence of multiple arteries supplying the right or left lateral segments and segment IV are of importance in deciding about a suitable donor. Segment IV can receive branches from the right or left hepatic arteries and directly from the hepatic arterial bifurcation, which thus becomes a trifurcation. High flow rates and thin-section imaging are required because this imaging task pushes the limit of spatial resolution with CTA. Accessory renal arteries occur in up to 25% of individuals. Usually one vessel is dominant on either side. However, multiple small vessels may be present, and accessory arteries may originate even above the superior mesenteric artery (exceedingly rare) or the iliac vessels (some 1% of cases). Main renal arteries enter the kidney via the hilum, while polar arteries enter the renal parenchyma directly. Such variants are important for interventional radiological dilatation of renal artery stenosis and for the work -up of living renal donors. The gonadal arteries usually arise from the aorta just below the level of the kidneys but rarely may originate from the renal artery (more common on the right) . Early branching of a normal sized artery or multiple or abnormal arteries or veins may make harvesting a kidney from a living renal donor difficul t. / In slim (young) patients, the duodenum or the left renal vein may be compressed between the aorta and the superior mesenteric artery leading to superior mesenteric artery syndrome or nutcracker syndrome, respectively.
The demand for renal transplantation is growing rapidly and has led to an increasing number of living-related donors. Advances in surgery now allow for minimally invasive explantation techniques with low morbidity and shortened hospital stays for the donor. Precise preoperative planning is mandatory, however, because the overview of the intraoperative situs is limited and unexpected findings, such as accessory vessels, may cause major complications. CT has become the imaging technique of choice for evaluating potential renal donors but MR may assume an increasing role in the future. CT must rule out pre-existing kidney disease and should precisely map the preoperative vascular anatomy. It is used to decide which kidney to explant and to plan the surgical procedure. •
CT Morphology
The examination consists of a pre-contrast CT scan to rule out calcifications and calculi, aCTA to demonstrate the arterial (and venous) anatomy, an nephrographic phase scan to exclude parenchymal disease and tumors, and a KUB radiograph immediately afterward to demonstrate the morphology of the renal pelvis and the course of the ureter. The pre contrast and nephrographic phase scans can use less dose and thicker sections than the CTA, which should have a high spatial resolution to demonstrate even small accessory vessels. The main criteria for evaluation are listed in Table 24.15. It is important to rule out concomitant parenchymal disease, tumors, cysts, and renal calculi. CTA is used to evaluate whether a normal arterial and venous anatomy is present, and to describe all anatomic variants that may lead to surgical complications. The extrarenal branching pattern of arteries and veins (length of common segment) should be noted, and the distance and length of accessory vessels relative to the main arteries or veins should be measured. It is important to distinguish between main renal arteries that enter the renal hilum and polar arteries that directly enter the parenchyma (Fig.24. 75a). A high spatial resolution is needed to demonstrate gonadal or suprarenal arteries that may arise from the renal arteries. Diseases of the renal arteries, such as stenosis, fibromuscular dysplasia, or aneurysms have to be ruled out.
899
900
24 Vascular System Table 24.15
Checklist for living renal donors
Renal arteries
Accessory arteries Renal artery stenosis or aneurysm Early branching Atypical course Gonadal arteries
tonal veins Accessory veins • Early branching • Retroaortic/circumaortic renal veins Gonadal and lumbar veins Renal parenchyma
On the venous side, the presence and number of lumbar veins that drain into the renal vein (mainly present on the left) as well as adrenal veins should be documented (Fig.24.75b). Noting the location, size, and potential duplication of gonadal veins helps to simplify the surgical procedure and reduces the risk of postoperative hemorrhage.
Renal Artery Stenosis
Renal artery stenosis (RAS) is an uncommon cause of arterial hypertension (approximately 5%). Because it is a potentially curable condition, early detection is important. Clinical symptoms may appear at a young age and consist of severe hypertension, hypertensive crises, disturbance in diurnal rhythm, a rapid rise in blood pressure within a few months, progression of hypertension following successful medical therapy, and an increase of retention parameters in response to ACE inhibitors. The commonest cause of RAS is atherosclerotic disease (65-70%). Angioplasty has been considered the treatment of choice for these hypertensive patients but newer longerterm studies from the Netherlands suggest that optimum medical treatment is equally effective. There is also an ongoing discussion about the positive effects of angioplasty or stenting on the preservation of renal function in these patients. Thus, the final role of noninvasive techniques (CTA, MRA, ultrasound) for diagnosis of renal artery stenosis will depend on whether interventional therapy is considered or not. Fibromuscular dysplasia is the next commonest cause of renal artery stenosis (some 25%). It affects younger patients and is more prevalent in women (3: 1). Angioplasty is usually a very effective treatment. Six separate forms have been distinguished (Table 24.16), of which medial fibroplasia is commonest (601~%).
. .....
The sensitivity and specificity of CTA are well above 90% for the detection of significant
Fig. 24.75 Important findings during work-up of living renal donors, (a) Small polar artery entering the parenchyma directly. (b) Retroartic left renal vein with branching prior to entering the IVC. a
b
Renal Artery Stenosis Table 24.16 Classification of the renal arteries
of fibromuscular
dysplasia
1. Intimal fibroplasia (1-2%) Circumferential band, poststenotic dilatation main + segmental arteries, often bilateral, children
+ young
adults
CT Morphology
2. Medial fibroplasia (60-700/0) String of beads and aneurysms, branches
=
alternating areas of stenoses' mid + distal renal arteries +
3. Medial hyperplasia (5-15%) Long smooth tubular arteries and branches
narrowing,
main
rena!"
4. Perimedial fibroplasia (20%) Long irregular stenosis, beading, no aneurysms (width" unaffected segments), distal main renal artery
5. Medial dissection (5-10%) False channel in media, aneurysm main renal artery + branches
formation
6. Adventitial fibroplasia «1 %)
~
and patient-related factors (obesity, bowel gas). Its diagnostic yield is substantially reduced under unfavorable conditions. Scintigraphy with ACE inhibitors may yield additional functional information but is inferior in bilateral disease.
I
Long segmental stenosis due to periarterial proliferation offibrofatty tissue, main renal artery large branches
Indirect signs can already be appreciated on standard CT examinations. Differences in size (right kidney 2 cm smaller than left kidney, or left kidney 1.5cm smaller than right kidney). cortical thickness (on corticomedullary phase images), level of enhancement, and time to excretion between the two kidneys suggest unilateral significant renal artery stenosis. Effective interpretation of the actual CTA examination is based primarily on 3D views such as volume-rendered displays (VRT) or curved thin-slab MIP (Figs. 24.76, 24.77; see also Fig. 24.11). While VRT are superior for the main renal arteries, MIP yield better results for small intrarenal branches. In order not to miss eccentric plaques, anteroposterior views as well as caudocranial views should be used. For optimum display, multiple 3D images should be generated that are rotated in 10 steps around the longitudinal and a horizontal axis. Whenever a suspicious finding is encountered, it should be confirmed by cross-sectional images. This can either be done by interactive analysis of axial sections in a cine display, or by generating curved planar reformations (CPR) through the vessel segment of interest (see Fig. 24.7). Cross-sectional images are mandatory whenever calcified plaques are present. In such situations, a wide window setting is necessary not to overestimate a stenosis. If the crosssectional images are disregarded and only the MIP are considered, the diagnostic accuracy of CTA declines to 80-90%. The accurate quantification of renal artery stenoses is very hard due to partial volume effects, but CTA has proven accurate in the grading of stenoses on a five-point scale; no stenosis (grade 0, 5%) group and selects patient for primary cervical spinal CT
tures of the spine or pelvis. Under such conditions, the patients can be on and off the table in less than 10 minutes. To speed up patient positioning and reduce the risk to the patient, a radiolucent support should be used in the emergency room that can be moved on to the CT table together with the patient. With multislice CT scanning, thinner sections can be used even for scanning the chest and abdomen. Depending on the size of the patient, a 4xl to 4x3.75 mm (16xl-2mm) collimation may be used. Suspended respiration is usually not required. Longitudinal dose modulation is highly advantageous in these patients but a high enough dose setting for the shoulders and pelvis has to be chosen. If the scanner allows doing so, even the cervical spine can be included when a 4 x 1 mm collimation with high enough dose is available for a scan duration of some 60 seconds. It is important, however, to first reconstruct thick sections from the raw data (5-7.5 mm) with identical increments to have a quick feedback about potential life-threatening complications. If thin-section scanning is performed, the patient can leave the table immediately after the scan is finished because all information can be gained from the raw data set. If thicker collimation is chosen for data acquisition (see Table 1.8), additional thin-section scans may become necessary in selected patients. After the patient is off the table, thinner overlapping sections (secondary raw data set) should be reconstructed that serve as the basis for sagittal reformations of the thoracic and lumbosacral spine (see Figs. 25.16 and 25.25). These images are excellently suited for detection and display of spinal injuries, in particular fractures that involve the middle column with the posterior surface of the vertebrae. Additional MPR are reconstructed wherever required. Volume-rendered images of the ribs and sternum can improve fracture detection (see Fig. 25.24). Volume-rendered images of the pelvis give an excellent overview of major fragments and the type of pelvic injury (see Fig. 25.30). They can probably substitute for conventional radiographs of the pelvis in these patients.
Torsion IAssessment
TorsionAssessment
CTArthrography
CT provides a standardized and reliable means With the increasing role of MR arthrography, the role of CT arthrography is diminishing. It is for measuring torsion deformity in long bones (Fig. 25.5). With the increasing use of inmost often used in the shoulder for evaluating the glenoid labrum and j oint capsule in patients tramedullary fixation in long tubular bones with shoulder instability. The examination (e.g., medullary nailing), CT has assumed a starts with double-contrast arthrography of the greater role in torsion assessment after fracture shoulder, in which contrast material (approxi- treatment and prior to corrective osteotomy. CT assessment of the torsion of a long bone mately 2-3 ml of a 300 mg IIml compound, more if diluted) and air (12-15 ml) is injected requires scanning of the proximal and distal into the inferior third of the joint capsule from joint regions. Several contiguous 8-10 mm sections or short spiral acquisitions through the an anterior or posterior approach. Iso-osmolar nonionic contrast material is to be preferred be- proximal and distal bone ends are sufficient. cause there is less dilution due to osmotic ef- Only a very low dose (CTDlvoi 15° discrepancy
> 15° discrepancy
7 mm is a definitive sign of transverse ligament rupture (Fig. 25.9). Sagit-
Upper cervical spine fractures
I C1 fractures
Type I
fracture of the anterior arch
frequently associated with a dens fracture
Type II
fracture of the posterior arch
most common fracture
Type III
Jefferson fracture
unstable fracture symmetrical bilateral fracture of the anterior and posterior arches, always associated with a disruption ofthe transverse ligament no displacement offragments anterior displacement and angulation of the anterior arch, additional anterior and cranial displacement of the posterior arch
type I type II type III Type IV
fracture of the lateral mass
Type V
fracture of the transverse process
C2 Dens fractures-Anderson-D'Alonzo
Type I
Fracture of the tip of the dens (usually stable)
Type II
Fracture through the base of the dens (unstable)
Type III
Fracture through the base of the dens and body of C2 (stable)
C2 Hangman's
fractures-Effendi
Type I
Solitary fracture of the axis arch with dislocation 4 mm and/or angulation >110 (unstable)
Type 111
Arch fracture with disc rupture and luxation ofthe C2-C3 facet joints (unstable)
25
Musculoskeletal
System Fig. 25.9 C1 fractures, (a) Isolated fracture of the atlas arch on the right side. (b) Jefferson fracture type I with concomitant fracture of the articular facet.
a
b
tal reformats provide important information as to whether only the anterior arch is anteriorly displaced and angulated (Type II) or whether the anterior and posterior arch are displaced leading to a potentially devastating narrowing of the spinal canal. C2 Fractures
Dens fractures occur in 10-15% of cervical spine
fractures and are caused either by a hyperflexion trauma (anterior displacement of the dens and anterior subluxation of C1 and C2) or, less frequently, a hyperextension trauma (posterior displacement of the dens and posterior subluxation of C1 and C2). Fracture types can be classified according to Anderson and D 'Alonzo (Table 25.9) but because of the rarity of type I fractures other authors simply divide these fractures in high versus low dens fractures. While type I and III fractures are stable, type II is unstable (Fig. 25.10). It is a transverse fracture through the base of the odontoid and is the commonest C2 fracture. Due to dislocation ofC1
with respect to C2 and compression' of the medulla, these fractures may be lethal in 25-40%.
The fusion of the os terminale of the dens with the body of the axis can be delayed or may completely fail and will result in the presence of an odontoid bone. This should not be misinterpreted as type I or II fracture. A rounded and smooth contour of the os odontoideum and a compensatory hypertrophy of the anterior arch of C1 is suggestive of a developmental disorder rather than a fracture. A large os odontoideum can also be the consequence of a pseudarthrosis after previous C2 trauma.
Hangman's fracture or traumatic spondylolisthe-
sis (5-10%) is caused by hyperextension and distraction and describes bilateral fractures through the pedicles (or the arches) of the axis (C2) with anterior displacement of the body and angulation. These fractures can be divided according to Effendi in three subtypes, based on
Fig. 25.10
Anterior displacement
of the dens axis in an Anderson type II fracture (4x1/6). Sagittal view (a), volume-rendered display (b). Note a fracture component that involves the vertebral body as well.
a
b
Lower Cervical Spine (C3-Cl)
Fig. 25.11 Hangman's fracture type II with slight rotation and ventrocaudal dislocation of the body of C2 (unstable) that is better appreciated on the sagittal reformats (b, c) than on axial sections (a) Note the involvement of the intervertebral foramen on the right side (c).
a
subluxation patterns (Table 25.9). The extent of C2-C3 subluxation is best appreciated on sagittal reformats (Fig. 25.11).
b
CT adequately demonstrates the frequent displacement of fragments into the spinal canal. Articular Mass Injuries
Lower Cervical Spine (C3-C7)
Traumatic lesions of a cervical articular mass are infrequent, yet potentially unstable and require C3-C7 Fractures internal fixation. CT is the diagnostic method of Teardrop fractures that are caused by flexion choice but MRI should be preferred if accomtrauma are located in the lower cervical spine panying disc or ligament injuries are suspected. Dislocation of the cervical spinal facet (C5-C6) in more than 70% of cases. Flexion teardrop fractures are extremely unstable and may joints (locked facets) is the result of an extreme flexion of the head and neck. It frequently oclead to a dorsal subluxation of the vertebral body due to the rupture of the anterior longi- curs bilaterally and is an unstable condition due to disruption of the posterior ligament complex. tudinal ligament. There is often a significant swelling of the prevertebral tissues due to In general, the ligamentum flavum, the interhemorrhage. Teardrop fractures must not be un- spinous, and the supraspinous ligaments are inderestimated, despite the minor osseous lesion volved. There is anterior displacement of the with avulsion of a small triangular fragment of upper vertebra leading to an anterior dislocathe anteroinferior surface of the body, as there tion of the inferior articular facet of the upper are often midsagittal fracture lines through the vertebra to the front of the superior articular corpus. If the height of this fragment is smaller facet of the lower vertebra. Unilaterally locked than its sagittal length, this is suggestive for the facets are combined with a malrotation of the presence of an unstable flexion teardrop frac- upper vertebra into a slight oblique location (flexion and rotation trauma). ture as opposed to the stable extension teardrop The locked facet joints, and the anterior and fracture. The clay shoveler's fracture is the result of a rotational displacement of the upper vertebra flexion trauma that leads to an avulsion fracture are both best appreciated on sagittal reformats of the spinous process of C6 or C7 without in- (Fig. 25.12). Sagittal reformats also show the obvolvement of ligaments. It is thus a stable frac- literation of the intervertebral foramen by the dislocated articular mass. Axial scans demonture and is usually not examined with CT. Cervical burst fractures are a result from ex- strate the inverted order of the facets with the plosion of the vertebral body after the inter- triangular articular mass of the lower vertebra in the posterior location instead of the normal vertebral disc herniates through the fractured end plate following axial loading injury. There is anterior location (reversed hamburger bun sign). Note that axial sections of normal facet joints alusually a sagittal fracture line and comminution of fragments. The stability is dependent on the ways show a maximum of two pieces of bone. involvement of the posterior vertebral margin. Any additional bone fragment indicates a frac-
952
25
Musculoskeletal System
a
b
c
d
Fig. 25.12 Unilateral locked facets (0.5/1) as a consequence of hyperflexion/rotational injury. Note the empty facets on axial sections (a, b) and compare the
sagittal MPR of the riding facet joints tralateral side (d).
ture (triple image sign). Four fragments indicate a unilateral biarticular fracture (quadruple image sign). Axial and sagittal planes are complementary for outlining fractures of the articular masses and dislocations of fragments.
stabilizing rib cage, and the change from kyphosis to lordosis in this region. It is a common site of fractures associated with calcaneal fractures after falls orjumps from height. Hyperflexion and compression mechanisms are the main causative factors in this segment, with vertebral body compression fractures the most frequent entity (Figs. 25.13-25.15).
Thoracolumbar Spine Thoracic Spine
Because of the stability provided by the thoracic cage, thoracic spine fractures are relatively uncommon. They can occur in severe trauma, and are frequently associated with neurologic complications because of the large size of the spinal cord relative to the small spinal canal. Because of the natural kyphosis, hyperflexion type injuries are commonest, often resulting in fracturedislocations. Because of the high energy needed, noncontiguous fractures elsewhere in the spine or in the sternum are not uncommon and should be looked for. Normal variants can cause confusion in this region. There is a physiologic wedging in the lower thoracic spine (wedge ratio 0.80-0.90) and thin pedicles can mimic an increased interpedicular distance. Thoracolumbar Junction and Lumbar Spine
The thoracolumbar junction (ThI2- L2) is a vulnerable site for fractures because of the increased mobility of this segment, the lack of a
-
(c) to the con-
Fracture Classification
Based on CT findings, Denis developed his three-column system (see Table 25.3) in the early 1980s. Thoracolumbar fractures were classified in four types: compression fractures, burst fractures, seat -belt injuries and fracturedislocations. Each of these types was further subdivided into three or four SUbtypes. There has been criticism about this model because his three column definition does not match anatomically or biomechanically and the importance of soft -tissue lesions was underestimated. This led to the AO classification in the mid 1990s (Table 25.10), which used a two-column model and formed three categories: -A-vertebral body compression - B-anterior and posterior element injury with distraction, and - C-anterior and posterior element injury with rotation. Each category is further subdivided into nine SUbtypes using the classic AO 3-3-3 scheme.
Tnoracofumbar Spine Table 25.10 AD classification fractures (Magerl et al.. 1994)
• More than a ser/o decrease in vertebral body height • More than 10° angulation of the vertebral body in the cranial or caudal direction compared with adjacent segments • Tearing of the anterior longitudinal ligament (teardrop sign)
• Contour irregularities in the posterior margin of the vertebral body (caution: basivertebral vein) • Decreased height of the posterior marginofthe vertebral body • Displacement of the posterior margin of the vertebral body • Asymmetry or separation of the pedicles
with rotation
C1 : Type A with rotation C2: Type B wrh rotation C3: Rotational shear injuries
Posterior column (sagittal and coronal reformations, also reformations on the plane of the facet joints) Fracture
Stability
In his original model, Denis defined three types of instability, which for CT was simplified to the rule that if any two or all three pillars are damaged (as seen on CT), it is assumed that the spine is unstable, For spinal surgeons however, stability of the spine means addressing several kinds of stability: mechanical, neurological and long-term stability, The AO classification defines instability for each of the classification patterns, including bony and disco ligamentous components, For this reason, MRI has been advocated as the most appropriate technique for use with this classification, For CT, the more simplified stability definitions are still commonly used and the criteria for the anterior, central, and posterior columns are summarized in Table 25.1L Multiplanar reformations are essential for a complete evaluation of spinal trauma (Figs, 25.13-25.15), Involvement of the middle pillar is never an isolated finding and always causes an unstable fracture, Unstable fractures frequently require surgery, while stable fractures have not necessarily to be operated, In the lower thoracolumbar spine a vertebral fracture is considered unstable with • involvement of the dorsal border of the vertebral body (middle column) • a post-traumatic kyphosis of more than 30 0
• Separation and divergence of the spinous processes • Fractures involving the pedicles, laminae, or facet joints • Lateral displacement ofthe articular processes • Subluxation of the facet joints with !,'"
,
In tervertebral Disk Lesions
a
a
b
Fig. 25.50 Spinal stenosis caused by bilateral short pedicles, a sagittal orientation of the facet joints, spondylarthrosis, hypertrophy of the ligamenta flava, and intervertebral disc protrusion (a). Stenosis of the cervical neuroforamina is mainly caused by hypertrophied (osteoarthritic) facet joints (b).
tory narrowing of the intervertebral foramina with true spondylolisthesis; while the body, the pedicles, and the superior articular process slip anteriorly, the spinous process, laminae, and the inferior articular process remain in their normal position due to the defect in the interarticular part of the vertebral arch. The step-off is located at the interspace above the level of the slip. Spinal stenosis may be caused by anterior vertebral body displacement in spondylolisthesis, by the osteoarthritic changes in pseudospondylolisthesis, or by bony hypertrophy at the site of the defect in spondylolysis (Fig. 25.52). The grading of spondylolisthesis, as proposed by Myerding, is based on the amount of forward displacement and is quantified in 25% steps of the sagittal diameter of the vertebral body (1=0-25%, 11=25-50%, 111=50-75% and IV =75-100% displacement) .
b
Fig. 25.51 Synovial cyst of the L4-L5 facet joint on the left side (a). Cyst of the L3 nerve root sheath on the left side (b).
Pitfalls:
• The osseous defects and hypertrophy in spondylolysis should not be mistaken for the facet joints located below the defects. • The detection of disc tissue between the vertebral body margins in spondylolisthesis should not be misinterpreted as a herniated disc. In doubtful cases sagittal MPR clarifies the anatomic situation.
Fig. 25.52 Spondylolysis with bilateral defects of the pars interarticularis (4 X 1/6).
a
b
981
Infection and Inflammatory Processes
MRI is generally superior to CT for evaluating in- • CT Morphology flammatory diseases because it can assess the Septic arthritis leads to destruction of the subextent and activity of the process while also de- chondral bone plate, followed by ill-defined tecting associated edema. This applies to ar- j oint contours and erosive changes that can thritis (including sacroiliitis) , osteomyelitis, culminate in bone destruction (Figs. 25.53; spondylitis, and spondylodiscitis. Exceptions are 25.54a). joint effusion and surrounding soft diagnostic or therapeutic CT-guided interventissue swelling are accompanying features. Airtional procedures involving the axial skeleton. bubbles may suggest gas-forming microor-
ganisms (Klebsiella, Serratia).
Infectious Arthritis
Infectious arthritis can be pyogenic (bacterial) or non-pyogenic (TB, viruses, fungi). The commonest organisms cultured in septic (pyogenic) arthritis are Staphylococcus aureus, Haemophilus influenzas, and Klebsiella and Serratia. Infectious agents may invade the joint directly through a penetrating wound, via a surrounding soft-tissue infection, or hematogenously. Tuberculosis and other mycobacteria (like Mycobacterium avium-intracellulare) may lead to granulomatous synovitis. Septic arthritis needs rapid therapeutic action to avoid severe joint damage, especially in children. CT is not commonly performed forjoint infection. After conventional radiography, scintigraphy (bone or leukocyte scans) or contrast-enhanced MRI are the methods of choice.
a
b
Fig. 25.54 (a) Acute osteomyelitis of the talus with septic arthritis in the surroundingjoints. Note the bone erosions (arrowheads) and subchondral osteoporosis (wide arrows) as well as the faint bone sclerosis due to
Fig. 25.53 Sepfic'arth'ritis of the sacroiliac Joint in a IV drug user. Irregular joint margins with complete destruction of the subchrondral bone. Note the abscess (arrows), the beginning osteomyelitis (arrowhead) and the signs of periostitis.
c incipient talar necrosis (arrows), (b) Tuberculous osteomyelitis ofC1 with bone destruction and abscess formation. (c) Chronic osteomyelitis with sclerosis ofthe right pubic bone.
Infectious Spondylitis and Spondylodiscitis Osteomyelitis Infection of the bone and medullary cavity is regularly seen in daily practice. Infectious agents may directly invade the joint in the setting of trauma or surgery. Other routes are via a surrounding soft-tissue infection or hematogenous spread. In children and in the adult spine the hematogenous route is the most usual way for Staphylococcus aureus infection to spread. When present. the physis acts as a natural barrier to spread. Although nowadays frequently replaced by MRI. CTcan be a useful adjunct to radiography as it can detect cortical destruction and periosteal reaction. as well as the formation of an involucrum (shell of living new bone over dead bone). the tract of a cloaca (opening for discharge of sequester). and sequestrated dead bone. Spiral or multislice CTenables high-quality MPR or 3D reconstructions for optimal visualization. •
CT Morphology
In acute osteomyelitis, initial findings include soft-tissue swelling, osteoporosis, and medullary resorption with indistinct borders (Figs. 25.53; 25.54). Later this progresses to cortical lucencies and periosteal reaction, with subperiosteal abscess formation and creation of an involucrum. This sequence is particularly seen in children. In adults cortical erosion and fracture can occur. Reparative changes are prominent in chronic osteomyelitis (Fig.25.54c). Irregular sclerosis around osteolytic areas, solid periosteal reactions, and the possible formation of a Brodie's abscess or cortical sequester, may be seen, with or without sinus tracts originating in cloacae. Chronic Recurrent Multifocal Osteomyelitis This is a rare chronic form of aseptic osteomyelitis from unknown cause that primarily affects children and adolescents. The disease is seen as a distinct entity but there are known associations with bony lesions from psoriasis, palmoplantar pustulosis, and acne conglobata as well as overlap with features of Carre's sclerosing osteomyelitis and the SAPHO syndrome. The
disease has a chronic'intermittent can take more than 15 years.
course
that
CT Morphology
Chronic recurrent multifocal osteomyelitis is manifested by single or multiple metaphyseal lesions that occur predominantly in the tibia, femur, clavicle, and fibula. The lesions are osteolytic and eccentric and are surrounded by rims of bone sclerosis. Periosteal new bone formation is variable. Infectious Spondylitis and Spondylodiscitis Spondylitis is a hematogenous infection that starts in the anterior medullary cavity of a vertebral body, eventually penetrates the cortex, and spreads along the paravertebral ligaments, or it may penetrate the vertebral end plates and involve the disc space (Spondylodiscitis). It is most frequent in the lumbar spine. Most frequent causative microorganisms are S. aureus, Pseudomonas aeruginosa (drug addicts). Streptococcus (endocarditis). H influenza (meningitis), and Mycobacterium tuberculosis. CT Morphology
In the early stage, the disc space shows a decreased height and decreased density. Within 1-3 weeks paras pinal soft-tissue swelling and osteolytic areas appear in the adjacent vertebral end plates (Fig. 25.55). Rarely, the infectious process destroys a single vertebra or part of the vertebra without invasion of the disc. In cases that respond to antibiotic therapy, serial CT examinations show increasing sclerosis of the cancellous bone. The process may culminate in the fusion of adjacent vertebral bodies (block vertebrae). In other cases the vertebral body may be destroyed, resulting in vertebral wedging and gibbus deformity (most typical for tuberculous Spondylitis). Paravertebral abscesses may gravitate downward along the psoas sheath and spread beneath the inguinal" ligament (gravitation abscess, cold abscess). Tuberculous Spondylitis cannot be differentiated definitively by imaging. Clues to a tuberculous origin are involvement of more than two vertebrae, sparing of disc spaces (Spondylitis), thoracic spine localization, primary spread via the longitudinal ligament with a resulting large
25 Musculoskeletal System Fig. 25.55 Spondylodiscitis with complete destruction of the end plates ofT6 and Th7 (a). Postoperative Spondylodiscitis with asymmetric erosion and bone sclerosis (b). Note the drill holes after explanation of the osteosynthetic material.
a
b
paras pinal abscess that may also, extend Into the epidural space, gibbus deformation, and eburnation (diffuse sclerosis of the vertebral body surrounding osteolytic areas). The most usual complication of tuberculous spondylitis is compression of the thecal sac and the spinal cord with resulting paraplegia. Sacroiliitis
MRI has become the most important modality for diagnosing sacroiliitis, as it furnishes information on inflammatory joint changes and can distinguish active inflammation (joint effusion, synovitis, medullary edema) from inactive chronic inflammation (fatty involution of the medullary cavity in a setting of fibrosis or sclerosis). The role of CT is limited to the detection of bony abnormalities. Conventional tomography is considered obsolete. •
Fig. 25.56 Sacroiliitis due to ankylosing spondylitis. Irregular sclerotic bands and erosive defects are present in both sacroiliac joints.
scan). A more circumscribed sclerosis is seen in osteitis condensans ilii, common in middle-aged active females. The dominant feature of septic sacroiliitis (confirmed by serology) is joint destruction (see Fig. 25.53).
CT Morphology
The early stage of sacroiliitis is characterized by j oint surface irregularities (intra -articular new bone) and microfractures in the cancellous bone. Further progression leads to erosive changes, irregular joint space narrowing, and subchondral sclerosis (Fig. 25.56). Partial or complete ankylosis of the sacroiliac joint occurs as an end stage or late sequel. Differentiation is required from osteophytes and ligamentous calcification due to degenerative changes involving the anterior and posterior joint margins (axial scans) or the superior and inferior joint margins (semicoronal
Heterotopic Ossification (Myositis Ossificans)
Heterotopic ossification can have various causes but presents similar morphologic features. The primary form (generalized progressive myositis ossificans, Munchmeyer's disease) is very rare. Secondary forms are most often caused by trauma, surgical procedures (e.g., total hip replacement), or prolonged immobility due to head trauma, spinal cord trauma, or-long-term ventilation. As a rule, surgical treatment is indicated only if there is severe restriction of move-
Soft-tissue Infections ment or significant deformity. The goal of surgical treatment is complete resection of the heterotopic ossification. Surgery should be deferred until the lesion has "ripened" (approximately 6 months after trauma or 12-18 months following head trauma or long-term ventilation). The differential diagnosis should include paraosseous osteosarcoma, which usually shows central rather than peripheral ossification. •
CT Morphology
Preoperative CT should let the surgeon know how far the lesions have "ripened", i.e., the degree of calcification of the lesions, the periand/ or para -articular location of the heterotopic foci, and their relation to muscles, nerves, and blood vessels. Early lesions are often hypoattenuating to surrounding muscle. Later they calcify from the periphery toward the center while also increasing in size. The lesion is assumed to be "ripe" when it has become completely ossified. Long-term ventilation and head injuries are typically associated with para -articular ossification, while direct trauma tends to induce periarticular ossification, which is usually localized to the area of the trauma (Table 25.19). The most commonly affected joints are the shoulder, elbow, hip, and knee. Three-dimensional reconstructions are helpful in defining the extent and relative location of the ossified areas (Fig.25.57).
Table 25.19
Common sites of heterotopic ossification
• May occur anywhere about the elbow after trauma, usually on the medial or lateral side with frequent involvement ofthe ligaments; on the ulnar side, the ulnar artery and nerve are frequently encased by new bone • Posterior ossification is most common following* head injury; the posterior aspect of the elbow is the most common site in general for heterotopi] ossification in head-injured patients A
Shoulder
• No site of predilection after trauma; frequent involvement of coracoacromial ligament • A common site after head injury is the inferomedial area below the shoulderjoint; pseudojoint without ankylosis Hip
• Ossification in the abductors may occur after total hip replacement or trauma; the foci may enlarge and involve the femoral neck • In head-injuries patients: anteromedial (anterior superior iliac spine -> teres minor) or anterolateral (anterior superior spine -> teres major) periarticular ossification or inferomedial paraarticular ossification adjacent to teres minor Knee
No specific location; the quadriceps tendon is common site of post-traumatic ossification, an medial ossification (similar to Stieda-Pellegrini sign) is common after head trauma • Surgical intervention is usually not required
Soft-tissue Infections
Fig. 25.57 SSD demonstrates extensive heterotopic ossification along the left sacrotuberal ligaments and about the right hip joint; following; long-term ventilation.
Soft-tissue infections can be primary or are associated with j oint infection, osteomyelitis, or diabetes. Primary infections include abscesses, pyomyositis, cellulitis, or fasciitis. Abscesses can be caused by direct inoculation from penetrating wounds or by hematogenous spread from distant sites. Infection is preceded by a phlegmonous phase before it is walled-off and liquefaction occurs. Pyomyositis is uncommon in temperate climates, but is more often seen in the tropics or in immunocompromised patients. Usually the lower extremity is involved with formation of multiple small abscesses. Cellulitis is a phlegmonous infection of the superficial soft tissues with thickening of the skin, subcutaneous fat and fasciae. Necrotizing fasciitis is a
985
25
Musculoskeletal
System
rare, rapidly progressive infection of the deep fasciae and surrounding muscle with massive necrosis rapidly leading to septic shock. In the extremities, ultrasound and MRI are the methods of choice. There is no primary role for CT, but soft -tissue changes may be seen frequently in studies done for bony abnormalities. CT is used in examinations of the trunk (mediastinum, retroperitoneum, pelvis; see the respective chapters). Bursitis
Bursitis is not an indication for CT scanning but may be an incidental finding that can be mistaken for a cystic mass. •
CT Morphology
Bursitis manifests as a fluid-filled mass in a periarticular location that is characteristic for eachjoint. A iliopsoas bursitis may mimic a cystic mass but is seen in a characteristic location medial and cranial to the hip joint within the iliopsoas muscle (Fig. 25.58). Differentiation
b
a
Fig. 25.58 Iliopsoas bursitis presenting as a fluiddense zone within the iliopsoas muscle in a patient with subchrondral cysts in severe osteoarthritis of the hip joint.
from joint recesses and synovial cysts is only possible by their para-articular location and the lack of demonstration of a communication with the joint.
iibne Tumors" Primary bone tumors are relatively uncommon lesions. Diagnosis is still primarily based on patient characteristics and plain radiographs. Additional imaging is mainly done with radionuclide scanning and MRI. CT has a limited role but may be used to assist in the diagnosis of selected bone-forming tumors, to evaluate bone destruction, to assess skeletal stability, and to guide percutaneous ablative therapies. Skeletal metastases in adults are far commoner than primary bone tumors and occur predominantly in the axial skeleton (skull, spinal column, ribs, pelvis) due to the extensive paravertebral plexus of valve-less veins. The bones are imaged in every CT examination, and bone-window scans should be obtained whenever there is clinical suspicion of an osseous infection or neoplasm. Equivocal findings are resolved by CT-guided percutaneous procedures for cytological, histolpgical,. or microbiological sampling.
Primary Bone Tumors
Plain radiographs continue to be the mainstay of the differential diagnosis of bone lesions. CT is indicated if there is uncertainty as to the presence of matrix calcification or ossification, or if it is necessary to detect incipient erosion or define the extent of gross destruction. MRI is the modality of choice for defining the soft -tissue component and detecting invasion of the medullary cavity. The staging of bone tumors (according to Enneking) is based on the histological type and grading, the definition of tumor extent, and the diagnosis of metastases (lung or bone). A differentiation is made between intra compartmental and extracompartmental localization of tumors. Intracompartmental tumors are: • soft tissue lesions confined to the subcutaneous fat; • parosseous lesions that neither infiltrate the bone nor the muscles;
Primary Bone Tumors • intraosseous lesions that have not penetrated the cortical bone; and • soft-tissue tumors that lie within one muscle compartment and have not crossed the surrounding fascia Extracompartmental tumors are: • all tumors that have penetrated the predefined anatomic borders of fascia or cortical bone, or lie within different muscle compartments; • all tumors that infiltrate nerves or vascular structures; and • all tumors that have invaded joints. In addition there are some anatomic areas that are primarily extracompartmental because they lack predefined anatomic borders. These areas are the middle and back-foot, middle hand, popliteal fossa, femoral fossa, and the axilla. MRI is the method of choice for staging of benign and malignant bone lesions. While MRI is frequently not able to define the type of lesions (which is based on radiographic criteria and histology), it plays an important role in differentiating between tumors and infarcts or osteomyelitis. Dynamic contrast -enhanced studies may help in differentiating benign from malignant cartilaginous tumors or in assessing response of bone tumors to chemotherapy. CT has similar potential for the differentiation of intra- versus extracompartmental lesions. Matrix calcifications in a lesion are best illustrated by CT and are useful in differentiating a chondrogenic tumor from an osteogenic tumor. CT can delineate the thin bony wall of an aneurysmal bone cyst, and it can also demonstrate the diffuse permeative destruction and extraosseous soft-tissue component associated with Ewing sarcoma. In these cases CT provides typical morphologic imaging features and in addition makes an essential contribution to the assessment of bone stability. Enostosis
Enostoses or bone islands are common incidental findings that have no pathologic significance. They may occur anywhere and are observed at all ages.
b
a
Fig.25.59 Enostoma in the calcaneus (a). Bone island in L2 (b: 4x1/6, curved coronal MPR). The very dense, homogeneous sclerosis is a typical finding.
•
CT Morphology
Bone islands (Fig. 25.59) appear as sharply demarcated sclerotic foci that are usually located in cancellous bone (ubiquitous in osteopoikilosis). They can be round or ovoid and are usually aligned with the long axis of the trabeculae. Some lesions can be difficult to distinguish from osteoblastic metastases on CT scans but are nonenhancing on MR images and do not show tracer uptake on radionuclide scans. Osteoid Osteoma
Osteoid osteoma most commonly presents in the second and third decade with pain that is typically worse at night and relieved by aspirin. Most instances occur in the shafts of the femur and tibia, but lesions are also common in the hands or feet. Spinal occurrence (approximately 5%) is manifested by a painful scoliosis in which the tumor is located in the posterior elements on the concave side of the curve. Although surgery is the therapy of choice, a newer application for CT is the guidance of locally ablative therapies. Good results have been described with thermocoagulation, interstitial laser photocoagulation, and radiofrequency ablation (see Chapter 6, p. 181).
987
25 Musculoskeletal System
•
CT Morphology
Osteoid osteoma presents as a small nidus of vascular osteoid tissue approximately 0.5-1.5 cm in diameter (the actual tumor), which may have a sclerotic center. It is often, but not always, surrounded by a dense, homogeneous zone of reactive sclerosis. The nidus shows intense contrast enhancement, especially during the early phase, although there is no need for contrast administration in most cases. The CT diagnosis of osteoid osteoma requires thin sections (1-3 mm). For lesions in long tubular bones, a wide window setting (4000 HU) is needed to detect the hypoattenuating nidus within the high-density cortical bone (Fig.25.60a). . ... The cortical type of osteoid osteoma is the most frequent and exhibits intense reactive sclerosis around a nidus located in the cortex (Fig.25.60a). More than one nidus may be present (multifocal osteoid osteoma, very rare). The medullary type is characterized by a densely calcified nidus within the medullary cavity (Fig.25.60b), which is surrounded by a halo of hypervascularized osteoid tissue. In the subperiosteal type, the nidus is calcified and there is little periosteal response and no reactive sclerosis. The periarticular type is located close to a joint and shows little if any evidence of reactive sclerosis (Fig.25.60c; see also Fig. 6.13). Differentiation is required from osteomyelitis with a sequestrum and from Brodie's abscess, in which the central hypodensities are often
a
b
Fig. 25.60 Osteoid osteoma in the femoral shaft (cortical type), displaying a central nidus surrounded by a dense sclerotic zone (a). Medullary osteoid osteoma in the articular process of L2 with a calcified nidus (b). Sub-
larger and irregular. In addition, Brodie's abscess is located close to the growth plate and often demonstrates a serpiginous tract. Both entities often display heterogeneous sclerosis. In the spine the osteoid osteoma may simulate a sclerotic metastasis but demonstration of the (calcified) nidus is pathognomonic. Enchondroma, Osteochondroma, and Chondrosarcoma
Enchondroma, osteochondroma, and chondrosarcoma are cartilaginous tumors. Chondromas may arise within a bone (enchondromas) or on the surface [periosteal chondromas}. They occur in the meta- or diaphysis of tubular bones, most commonly in the small bones of the hand (phalanges and metacarpals). Osteochondromas (osteocartilaginous exostoses) occur on the outer bone surface and usually involve the metaphysis of long bones particularly in the vicinity of the knee and shoulder. With both tumors, pain is a sign of malignant transformation with the development of a secondary chondrosarcoma. Enchondromatosis is characterized by multiple enchondromas {Ollier's disease}. The disease has a strong preference for a single side of the body and leads to knobby swellings of the hand, forearm, or legs. The risk of malignant transformation is much higher. The association of enchondroma to sis with soft tissue hemangiomatosis is called Maffucci's syndrome.
c periosteal osteoid osteoma of the sixth thoracic vertebra with moderate calcification of the nidus and almost no reactive scleroses (c).
Primary Bone Tumors
Chondrosarcomas most commonly occur in
the pelvis and femur (together accounting for approximately 40% of cases). They can be divided in conventional, juxtacortical', or mesenchymal types. Low-grade sarcomas may be difficult to differentiate from enchondromas in contrast to clearly malignant high-grade tumors. Dynamic contrast -enhanced MRI is the diagnostic method of choice for this differentiation. •
CT Morphology
All three tumors are characterized by flocculent, curvilinear, or stippled areas of matrix calcification. Most enchondromas are associated with popcorn-like matrix calcifications. The softtissue component may occupy the entire diameter of the medullary canal. While the lesions are frequently completely radiolucent in short bones, they sometimes demonstrate extensive calcifications in long bones. A typical finding is the scalloping of the inner cortical margin, since the cartilage grows in a lobular pattern (Fig.25.61a). Periosteal chondromas, mostly occurring at the proximal humerus, may lead to a painful swelling and may erode the cortex in a saucer-like fashion, producing a solid buttress of periosteal new bone. Chondromas in long tubular bones show a markedly higher incidence of malignant transformation than those in short tubular bones.
a
b
Fig. 25.61 Enchondroma with typical intramedullary location, popcorn-like calcifications, and sharp margins (a). Osteochondroma of the s" rib with a,wide connec-
Osteochondroma
has a cancellous bone structure at its base that is similar to that of the adjacent host bone. Typically, there is an uninterrupted merging of the cortex of the host bone with the cortex of the osteochondroma. In addition, the medullary portion of the lesion and the adjacent bone communicate (Fig.25.61b). The cartilaginous cap of the lesion displays typical foci of matrix ossification. Enlargement of the cartilaginous cap in adults or a cap that is more than 2cm thick on CT scans and has irregular margins should raise suspicion of malignant change (in less than 1 %). More frequent complications may arise from the fact that the lesion compresses adjacent nerves, vessels, or bones. Multiple osteochondromas are present in hered-
itary multiple exostosis syndrome. Low-grade chondrosarcomas typically show
an eccentric expansile type of growth and display marked matrix calcification, thickening of the cortical bone, and endosteal scalloping. The higher-grade tumors are distinguished by a concentric growth pattern, little matrix calcification, necrotic areas «40HU), cortical destruction, and an extensive soft tissue mass (dedifferentiated chondrosarcoma, mesenchymal chondrosarcoma, Fig.25.61c). Periosteal c1wndrosarcoma has the same morphologic and pathologic features as central chondrosarcoma except for its localization on the surface of the bone. It may be indistinguishable from a periosteal osteosarcoma.
c tion to the bone and chest deformity (b). Chondrosarcoma with destructive growth and a large soft tissue component (c). Note the intratumoral calcifications.
989
25 Musculoskeletal System Giant Cell Tumor Giant cell tumor, also known as osteociastoma, is approximately twice as common as aneurysmal bone cyst and is the second commonest benign spinal tumor after hemangioma, Around 60% of the lesions are localized in the long bones, almost always at the articular end of the bone (proximal tibia, distal femur, distal radius, and proximal humerus), Giant cell tumor may undergo malignant transformation iin 5-10% of cases. CT Morphology
The typically expansile lesions are purely lytic, without a sclerotic rim or any periosteal reaction (compare Fig, 6,8), The cortical bone is eroded, giving the tumor a malignant appearance, although its histology is benign, Giant cell tumors are characterized by highly vascularized tissue containing giant cells of osteoclast type, The tumor has a CT density of 20-70 HU and shows marked enhancement after contrast administration (40-60 HU), Internal septation is visible in rare cases, The precise extent of the soft-tissue component is sometimes difficult to distinguish from reactive edema; MRI is superior for this purpose, Treatment usually consists of wide resection with implantation of an allograft or endoprosthesis, CT is also used for monitoring the incorporation of the graft into the normal bone, and for follow-up because of the high incidence of recurrences, Recurrent lesions exhibit malignant transformation, especially after radiation therapy,
Aneurysmal Bone Cyst Aneurysmal bone cysts occur in children and young adults (90% of patients are younger than 20 years) and most commonly arise in the metaphyseal area of the femur and tibia or in the posterior elements of the vertebrae (transverse and spinous processes, neural arches), There is no risk of malignant transformation, They can develop as a primary lesion or as a result of cystic changes in a pre-existing lesion (chondroblastoma. osteoblastoma, giant cell tumor, fibrous dysplasia),
CT Morphology
An aneurysmal bone cyst causes marked bone expansion (blowout) and cortical thinning, Internal septa are frequently observed, CT may demonstrate fluid levels within the loculi in bedridden patients, The soft-tissue component has a CT density of 60 -70 HU and is well vascularized, showing intense enhancement after contrast administration, A typical and important feature, well demonstrated by CT, is the integrity of the cortex, Recurrence after treatment (surgical removal and bone grafting) is frequent
Hemangioma Hemangioma is the second commonest benign vertebral lesion and is usually an incidental finding, The incidence increases with age, The most usual sites are the spine, especially the thoracic spine, and the skull, CT Morphology
The typical pattern of coarse, predominantly vertical, linear striations produced by the rarefied trabeculae creates a stippled pattern on axial scans through the affected vertebra, Generally the shape and size of the vertebral body are unchanged, The hemangioma may completely fill the vertebra, or involvement may be limited to small areas (Fig, 25,62), Fatty involution is typical, thus differentiation from an isolated focus of osteoporosis may be difficult If clinical symptoms are present, the lesions often show marked contrast enhancement An intraspinal component should be excluded prior to operative treatment or radiological intervention (embolization), In the tubular bones, hemangiomas show a lacelike pattern, and honeycombing as they do in the spine,
FibrousDysplasia Fibrous dysplasia is the most commonly found benign tumor-like lesion of bone, It is usually confined to a Single bone (monostotic) but occasionally is more Widely distributed (polyostotic), Asymptomatic lesions are usually detected incidentally during middle age, while symptomatic forms are usually diagnosed during childhood, The monostotic form most frequently in-
Primary Bone Tumors 991 Fig. 25.62 Vertebral hemangiomas. (a) Typical incidental finding of a thoracic lesion with a coarsened, rarefied, vertically oriented trabeculae that produce a stippled pattern on axial sections. There is extensive fatty infiltration of the medullary cavity, (b) Smaller focal lesion in the lumbar spine.
b
a
volves the femur, especially the femoral neck, the tibia, and the ribs. The polyostotic form has a striking predilection for one side of the body. The pelvis is frequently involved followed by the long bones, the skull ribs, and proximal femur. Sarcomatous transformation is extremely rare.
Pseudotumors
There are numerous entities that may simulate a bone tumor. In cases in which plain films are inconclusive, CT can help identify intraosseous ganglia, brown tumors of hyperparathyreoid-
A
•
CT Morphology
The spectrum of CT appearances is dependent on the lesion's proportion of osseous-to- fibrous content. Lesions with greater osseous content are more sclerotic while those with greater fibrous content are more radiolucent with a characteristic milky or smoky ground-glass appearance. Thus the morphology ranges from pure lytic areas and ground-glass densities to irregular bands of sclerosis. The ground-glass densities have attenuation values of 70-150HU. The cortex may be expanded and thinned or may show compensatory thickening (Fig.25.63a). Involvement of the skull base usually produces a homogeneous, well-circumscribed area of sclerosis and expansion with resultant narrowing of involved foramina. The differential diagnosis includes meningioma, which typically occurs in older patients and has a markedly enhancing soft-tissue component. The most frequent complication of the polyostotic form is the recurrent pathological fracture with resulting deformity.
a
b Fig. 25.63 (a) Typical ground-glass lesion of fibrous dysplasia involving the iliac wing. (b) Hemophilic pseudotumor due to recurrent bleeding with bone destruction.
25 Musculoskeletal System
ism, encystified bone infarcts, posttraumatic myositis ossificans, various infectious lesions and hemophilic pseudotumors (Fig.25.63b).
types can be better circumscribed. There are periosteal reactions, typically in the form of sunburst pattern or a Codman's triangle. Parosteal types typically show a lobulated tumor that grows outward from the cortex.
Osteosarcoma
Osteosarcoma is the second commonest primary tumor of bone. There are many different types, such as conventional, juxtacortical, and telangiectatic Osteosarcoma. The tumors most often present in the second or third decade, but lower-grade types can also be seen in the fourth to fifth decade. Most types are relatively more frequent in men. The most usual locations are the femur, tibia, and humerus, with most osteosarcomas occurring in the vicinity of the knee. Microscopically, they can be primarily osteoblastic, chondroblastic, or fibroblastic. Telangiectatic variants contain large cystic spaces filled with blood. ]uxtacortical tumors proliferate outward from the cortex. The parosteal type is typically of lower grade than other types. MRI is generally superior to CT in diagnosis and follow-up, but CT may be used to assist in diagnosis and staging of the tumor.
Ewing's Sarcoma
Ewing's sarcoma is a highly malignant neoplasm in children and adolescents. Conventional radiographs and MR are the techniques of choice for evaluation of diaphyseal tumors, while CT is well suited for describing the extent of bony destruction in flat bones such as the ribs, pelvis, and scapula. •
CT Morphology
CT demonstrates an ill-defined diffuse permeative or moth-eaten bone destruction with a malignant (aggressive) type of periosteal response and a large soft-tissue component. Skeletal Metastases
Most tumors show a mixture of osteosclerosis and osteolysis, although the telangiectatic type is primarily lytic (Fig. 25.64). The tumors are usually poorly defined, but the lower grade
Metastases are the most frequent malignant skeletal lesions. Approximately 25% of all primary malignant tumors are associated with skeletal metastases. Lesions in cortical bone are more conspicuous than in cancellous bone and can be detected at an earlier stage. While CT is better for delineating lesions in cortical bone,
a
b
Fig. 25.64 Osteosarcoma. (a) Telangiectatic lesion of the proximal tibia (3/5/2) with bone destruction and a
large soft tissue component, sacrum (4 x 1/6).
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CT Morphology
(b) Osteosarcoma
oftt/e
Skeletal Metastases
a
c
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Fig. 25.65 Metastases. (a) Metastatic breast cancer with destruction and pathologic fracture of L2, compression of the spinal canal and a substantial anterior soft tissue component, (b) Diffuse osteoblastic metastasis from prostatic carcinoma. There is extensive
concomitant osteolysis of the left arch of the atlas with incipient displacement of the dural sac. (c) Enhancing intrapelvic metastasis from thyroid carcinoma. The bone destruction is more extensive than the visible area of contrast enhancement.
MRI is superior for diagnosing metastases in cancellous bone that have not destroyed the trabecular architecture. While the determination of the type of a skeletal metastasis, whether it is predominantly sclerotic, lytic, or of a mixed type (Fig. 25.65) cannot furnish a specific diagnosis, it can help to narrow the differential diagnosis in patients with an unknown primary tumor (Table 25.20). CTis superior to MRI in assessing the stability of the affected bone. As rule of thumb , long bones become unstable when more than 50% of the cortex is destroyed. In the spine, the vertebra is considered unstable when the dorsal border of the body is destroyed, when the height of the body is reduced, and when MR imaging shows bone marrow infiltration within the entire vertebral body even if the cortical bone is still intact.
After radiation, hormone treatment, or chemotherapy primary lytic lesions may become sclerotic.
Table 25.20
Incidence of skeletal
Unlike inflammatory processes, spinal metastases spare the intervertebral disc space. If the posterior elements of a vertebral body have been destroyed, attention should be given to the extent of an accompanying intraspinal soft-tissue component (Fig.25.66). Less than IQ% of skeletal metastases are solitary lesions.
