© 2009, Elsevier Limited. All rights reserved. The right of Juriy Wladimiroff and Sturla Eik-Nes to be identified as editors of this work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Publishers. Permissions may be sought directly from Elsevier's Health Sciences Rights Department, 1600 John F. Kennedy Boulevard, Suite 1800, Philadelphia, PA 19103-2899, USA: phone: (+1) 215 239 3804; fax: (+1) 215 239 3805; or email: h
[email protected]. You may also complete your request online via the Elsevier homepage (www.elsevier.com), by selecting ‘Support and contact’ and then ‘Copyright and Permission’. ISBN-13: 978-0-444-51829-3 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress Notice Neither the Publisher nor the Editors assume any responsibility for any loss or injury and/or damage to persons or property arising out of or related to any use of the material contained in this book. It is the responsibility of the treating practitioner, relying on independent expertise and knowledge of the patient, to determine the best treatment and method of application for the patient. The Publisher
The publisher's policy is to use paper manufactured from sustainable forests
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Contributors
Domenico Arduini MD Associate Professor Cattedra Medicina dell’Eta Prenatale Universita di Tor Vergata Rome Italy Fetal biometry, estimation of gestational
Bruno Cacciatore MD, PhD Professor of Obstetrics and Gynaecology Department of Obstetrics and Gynaecology Helsinki University Hospital Helsinki, Finland
age, assessment of fetal growth
Doppler ultrasonography in gynaecology
Bernard Benoit Hôpital Princesse Grace Monaco
José M. Carrera PhD Professor of Obstetrics and Gynaecology Fetal Medicine Unit Department of Obstetrics and Gynaecology Institute University Dexeus Barcelona Spain
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Harm-Gerd K.Blaas MD, PhD National Centre for Fetal Medicine St Olav’s Hospital University Hospital Trondheim Trondheim Norway
Investigation of early pregnancy
Investigation of early pregnancy
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Contributors
✩ ✩✩✩✩✩✩✩✩✩✩✩ Rabih Chaoui Centre for Prenatal Diagnosis and Human Genetics Berlin Germany Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Frank A. Chervenak MD Given Foundation Professor and Chairman Department of Obstetrics and Gynecology Weill Medical College of Cornell University New York USA Ethics and patient information
Werner Diehl Department of Prenatal Diagnosis and Therapy AK Barmbek Hamburg Germany Multiple pregnancies
Francis A. Duck PhD, DSc Medical Physicist Department of Medical Physics and Bioengineering Royal United Hospital Bath UK Biological effects and safety aspects
Sturla H. Eik-Nes Professor of Obstetrics and Gynaecology Department of Obstetrics and Gynaecology National Centre for Fetal Medicine University Hospital of Trondheim Trondheim Norway Physics and instrumentation
viii
Amniotic fluid and placental localization Examining the cervix by transvaginal ultrasound
Annegret Geipel MD Priv. Doz. Dr. Med Leitende Oberärztin Pränatalmedizin Abteilung für Geburtshilfe und Pränatale Medizin Universitätsklinikum Bonn Sigmand-Freud-Str. 25 53105 Bonn Germany Evaluation of fetal and uteroplacental blood flow
Ulrich Gembruch Professor of Obstetrics and Gynaecology Abteilung fur Praenatale Medizin und Geburtshilfe Zentrum fuer Geburtshilfe und Frauenheilkunde Rheinische Friedrich-WilhelmsUniversitaet Bonn, Germany Evaluation of fetal and uteroplacental blood flow
Francesco Giacomello MD Professor Department of Surgery University Tor Vergata Rome Italy Fetal biometry, estimation of gestational age, assessment of fetal growth
Kurt Hecher Professor of Obstetrics and Gynaecology Department of Prenatal Diagnosis and Therapy AK Barmbek Hamburg, Germany Multiple pregnancies
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Assessment of the placenta and umbilical cord
Davor Jurkovic MD, PhD Consultant Early Pregnancy and Gynaecology Ultrasound Unit Department of Obstetrics and Gynaecology King’s College Hospital London UK Gynaecological pathology: the uterus
Laurence B. McCullough PhD Dalton Tomlin Chair in Medical Ethics and Health Policy Professor of Medicine and Medical Ethics Associate Director for Education Center for Medical Ethics and Health Policy Baylor College of Medicine Houston Texas USA Ethics and patient information
Hylton B. Meire Consultant Radiologist (Ultrasound) Bromley UK Medico-legal implications of ultrasound imaging in obstetrics and gynaecology
Israel Meizner md Professor Ultrasound Unit Department of Obstetrics and Gynecology Rabin Medical Center – Beilinson Campus Petah-Tikun and Sackler Faculty of Medicine Tel Aviv University Tel Aviv, Israel
Contributors
Eric Jauniaux MD, PhD, MRCOG Professor in Obstetrics and Fetal Medicine Academic Department of Obstetrics and Gynaecology UCL EGA Institute for Women’s Health Royal Free and University College London London UK
Prenatal diagnosis of fetal anomalies
Ana Monteagudo MD Professor of Obstetrics and Gynecology Department of Obstetrics and Gynecology New York University School of Medicine New York USA Scanning techniques in obstetrics and gynaecology
Eduard J.H. Mulder Msc, PhD Professor Department of Perinatology and Gynaecology University Medical Center Utrecht The Netherlands Fetal movement patterns and behavioural states
Kypros H. Nicolaides MD Professor of Fetal Medicine Department of Obstetrics & Gynaecology Kings’s College Hospital London UK Prenatal diagnosis of fetal anomalies
David A. Nyberg MD Seattle Ultrasound Associates Seattle USA Normal fetal anatomy at 18–22 weeks
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Contributors
✩ ✩✩✩✩✩✩✩✩✩✩✩ Rüdiger Osmers MD, PhD Professor of Obstetrics and Gynaecology Department of Obstetrics and Gynaecology University Hospital Gottingen Gottingen Germany Gynaecological pathology: tubes and ovaries Doppler ultrasonography in gynaecology
Gianluigi Pilu MD Associate Professor of Obstetrics and Gynaecology Department of Obsetrics and Gynaecology University of Bologna Bologna Italy Prenatal diagnosis of fetal anomalies
Roberto Romero MD Professor of Obstetrics and Gynecology Wayne State University and Chief of the Perinatology Research Branch of the National Institute of Child Health and Human Development National Institutes of Health Bethsedu, MD USA Prenatal diagnosis of fetal anomalies
Rehan Salim MRCOG Early Pregnancy and Gynaecology Ultrasound Unit Department of Obstetrics and Gynaecology King’s College Hospital London UK Gynaecological pathology: the uterus
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Kjell Å. Salvesen Dr Med, PhD National Centre for Fetal Medicine St Olav’s Hospital University Hospital Trondheim Trondheim Norway Examining the cervix by transvaginal ultrasound
Waldo Sepulveda Professor of Obstetrics and Fetal Medicine University of Santiago de Chile San Jose Hospital and Director Fetal Medicine Center Clinica Las Cordes Santiago, Chile Prenatal diagnosis of fetal anomalies
Povilas Sladkevicius MD, PhD Department of Obstetrics and Gynaecology Malmö University Hospital Lund University Malmö Sweden Normal gynaecological anatomy (uterus, tubes, ovaries)
Vivienne L. Souter MD, MRCOG Seattle Ultrasound Associates Seattle USA Normal fetal anatomy at 18–22 weeks
Ilan E. Timor-Tritsch MD Professor of Obstetrics and Gynecology Department of Obstetrics and Gynecology New York University School of Medicine New York USA Scanning techniques in obstetrics and gynaecology
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Normal gynaecological anatomy (uterus, tubes, ovaries)
Yves Ville MD Professor of Obstetrics and Gynaecology Service Gyneco/Obstetrique Centre Hospitalier Inter Communal Poissy France
Kim Wherey MD Seattle Ultrasound Associates Seattle USA Normal fetal anatomy at 18–22 weeks
J. W. Wladimiroff Emeritus Professor of Obstetrics & Gynaecology Department of Obstetrics & Gynaecology Erasmus University Medical Centre Dr Molewater plein 40 3015 GD Rotterdam The Netherlands
Contributors
Lil Valentin MD, PhD Professor in Obstetrics and Gynaecology Department of Obstetrics and Gynaecology Malmö University Hospital Lund University Malmö Sweden
Amniotic fluid and placental localization
Invasive procedures in obstetrics
Gerard H.A. Visser MD, PhD Professor of Obstetrics and Gynaecology Department of Perinatology and Gynaecology University Medical Center Utrecht The Netherlands Fetal movement patterns and behavioural states
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Preface
This textbook is the result of a joint venture between the International Society for Ultrasound in Obstetrics and Gynaecology (ISUOG) and the European Board and College of Obstetrics and Gynaecology (EBCOG). Both organizations play an important role in training. The book aims to provide the reader with the information necessary for everyday ultrasonography in obstetrics and gynaecology, rather than a summary of the latest developments in the field. The book follows the traditional pattern of starting with the physical and biological aspects of diagnostic ultrasound, followed by a wide range of clinical applications in obstetrics and gynaecology. Each chapter has been written by one or more experts actively involved in ultrasound teaching. Ultrasound images are presented either in the text or separately on a CD at the end of the book. Multiple choice questions are presented at the end to allow the reader to test his or her knowledge. We hope that this textbook will serve all those who are active in day-to-day ultrasound scanning. Juriy W. Wladimiroff, Sturla H. Eik-Nes
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Physics and instrumentation Sturla H Eik-Nes
Abstract This chapter provides an overview of the fundamental physical principles that make it possible to produce images of human tissue using sound. The physical laws are explained without the use of complicated formulas. Sound is a mechanical vibration in a medium such as air or human tissue. The upper frequency limit for sound to be heard by humans is 20 kHz. Frequencies above 20 kHz are called ultrasound. Medical images are made with a frequency above 3 MHz. The basic principle for making images of human tissue is to send a pulse into the tissue with a transducer and detect the echoes emerging from structures in the tissue. Imaging may be done in real time by electronic scanning. A variety of sizes and shapes of transducers have been produced for the various applications of ultrasound in medical diagnosis. A proper transducer must be used for a specific task. The ultrasound beam is the essential tool to make images. It must be focused by the user and the image must be properly adjusted with respect to the gain. Measurements can be made and a basic understanding of the resolution in the three planes is necessary for measurements and interpretation of the images. The main artifacts such as edge shadows, attenuation shadows, enhancements and reverberation must be understood. Basic principles of ultrasound scanning must be followed to extract the maximum information from the scan.
Keywords A-mode, artifacts, B-mode, focus, M-mode, real-time scanning, technical principles of ultrasound in obstetrics and gynaecology, time gain compensation.
Introduction In the practice of clinical ultrasound in obstetrics and gynaecology, it is essential that the examiner has a basic understanding of the physics that makes it possible
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ to produce images of human tissue using sound. In addition, the examiner must be able to handle artifacts properly, know about the basic performance of the instrument and be aware of artifacts, safety and risk factors. This chapter provides an overview of the fundamental physical principles without the use of complicated formulas to explain the physical laws. The focus is to give the reader an overall understanding of how an ultrasound machine works and the skill to operate the machine and to manage the necessary adjustments in order to produce images of high quality for diagnostic use. For indepth knowledge of the physics of ultrasound, the reader is referred to excellent textbooks. (See selected list at the end of this chapter.)
Sound Sound is mechanical vibrations travelling in a physical medium such as air, water, metal or even human tissue. Whether the airborne vibrations come directly from the source or are reflected, they produce impressions on the eardrums of our vestibular organs. We interpret these vibrations as sound. Sound may be categorized according to various frequency levels:
• infrasound (0–20 Hz) • audible sound (20–20 kHz) • ultrasound (>20 kHz) • diagnostic ultrasound (1–20 MHz). Humans do not hear the infrasound but other species such as whales, dolphins, elephants, hippopotamuses and rhinoceros do; they use infrasound to communicate with other members of their species over long distances. The upper frequency limit for humans is 20 kHz. Frequencies above 20 kHz are called ultrasound. Some species may hear sound frequencies which for humans are categorized as ultrasound, for example mice (10–70 kHz), dogs (40–60 kHz) and bats (20–200 kHz). There is even some evidence that bats utilize the change in pitch of the echo to determine the relative movement of the object that reflects sound – the Doppler effect. Marine mammals may produce very complex signals ranging from low frequencies for long-range use to high frequencies for local chatting!
Short History of the Development of Ultrasound in Medicine
2
In 1912, the passenger ship Titanic hit an iceberg on its maiden trip crossing the Atlantic from Southampton to New York. In the time that followed, physicists took an interest in using sound to detect large objects submerged in water. Initially their research for that purpose was unsuccessful. During World War I, the French physicist Paul Langevin was responsible for developing the hydrophones needed to detect submarines; this underwater sonar technology resulted in the first sinking of a German submarine in 1916. In 1917, Langevin invented the quartz sandwich transducer which served as the basis for the modern ultrasonic era. Between
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Physics and instrumentation
World War I and World War II, the development of sonar (Sound Navigation and Ranging System) and radar (Radio Detection and Ranging) took place. The latter technique used electromagnetic waves rather than ultrasound. The next important step was the use of ultrasound to detect flaws in metal using high-frequency ultrasound. The metal flaw detectors became increasingly important as World War II was approaching, but were reported after the war.2,4 After World War II, Howry and Bliss, in Denver, started to experiment with sonar equipment and amplifiers from the navy.7 They developed a pulse-echo technique in 1948–49, and later produced cross-sectional images of a human partly submerged in water. At the same time, Wild in Minneapolis developed a breast scanner and actually made a diagnosis of breast lesions with his device.12 The Swedish physician Inge Edler and physicist Helmut Hertz, at the University of Lund, borrowed a metal flaw detector from Kockum's Shipyard in Malmö, Sweden. In 1953, they managed to trace the movements of the human cardiac valves by means of the sound waves emitted and received by their modified instrument.5 This was the start of a new era in cardiology relying on sound technology.6 The next breakthrough was by the Scottish physician Ian Donald, in Glasgow, who conducted the basic research for the development of a machine for clinical use employing ultrasound to make two-dimensional images of human tissue. Donald had served in the Air Force during World War II and his past experience influenced his prototype machine, which consisted of two metal flaw detectors. His Lancet paper of 1958, ‘Investigation of abdominal masses by pulsed ultrasound’, is considered to be one of the most important for the development of clinical ultrasound.3 Since the late 1950s, the development of ultrasound in medicine in general and in the field of obstetrics and gynaecology in particular has continued in an exponential way. Breakthrough advances have been repeatedly made in spite of claims that the development of ultrasound in medicine has reached its physical limits.
Sound, Waves and Propagation Sound is a mechanical vibration in a medium. The medium may be, for example, air, water or human soft tissue. The sound wave propagates through the medium as a longitudinal compression wave. When we think of waves we may picture a stone being thrown into a quiet lake and observe the concentric rings that propagate from the centre, or we may think of the waves in the ocean as seen from the shore or from a boat. These waves are transversal waves. Sound waves, however, are longitudinal waves and the medium that they travel through is subject to cyclic variations in pressure as the medium is being compressed or rarefied (Fig. 1.1). Make a small experiment by putting your index finger on the top of your larynx, then make the sound of a z-z-z. With your finger you will feel the vibrations caused by your vocal cords that are your own sound system, that cause the z-z-z to be heard in the room. You have now produced longitudinal sound waves that travel
3
Distance
λ
Compression
Decompression
Pressure
Ultrasound in obstetrics and gynaecology
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Distance
Moves with wave velocity, c Fig. 1.1 (Upper panel) A schematic illustration of a sound wave as it travels in a medium causing periodic compressions and rarefaction of the medium. (Lower panel) The dislocation of the particles.
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through the room and cause compression and rarefaction of the air in their path. When the sound waves hit the eardrums of someone in the room, the process is reversed and causes the eardrums to vibrate and the person will hear your z-z-z. The sound wave is a longitudinal wave caused by compression and rarefaction of a physical medium in the direction of the movement of the wave. This sound wave may further be described by intensity and frequency. If you have a piano, you can carry out a small experiment in your living room by hitting A above middle C. You will hear a chamber tone with a frequency of 440 Hz. If you move up one octave on your piano and hit A, you will hear it at a frequency of 880 Hz. If you move up one more octave to the next A, you will hear an A note with the frequency of 1760 Hz. The frequency tells us about the degree of highness or lowness of a tone. The frequency is the number of vibrations per second that produce the sound. Hit the A on your piano very lightly and you will barely hear the chamber tone of 440 Hz; hit the key with force and you will hear the same chamber tone with the frequency of 440 Hz, but much louder. This tells us that the same tone may differ in intensity or loudness. The intensity tells us something about the loudness or strength of the sound signal. A sound wave travelling in a medium produces compression and rarefaction of the medium as shown in Figure 1.1. The velocity of propagation of the sound wave is dependent on the medium and is 330 m/s in air, 1480 m/s in water, 1589 m/s in
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λ=
v f
(1)
A chamber tone (440 Hz) has a wavelength of 0.75 m, propagating in air at the velocity of 330 m/s. It is obvious from equation 1 that the wavelength will vary with the frequency and velocity of sound in the tissue. The higher the frequency, the shorter the wavelength; the higher the velocity of sound, the longer the wavelength. Because the speed of sound in human tissue has been standardized at 1540 m/s in the equation, the wavelength will vary with the frequency (Table 1.1). The higher the frequency of ultrasound in human tissue, the shorter the wavelength. An ultrasound wave with a frequency of 5 MHz (M is the Greek abbreviation for mega which means big, but used in acoustics it means million) has a wavelength of 0.31 mm. It is important to understand what really happens when a sound wave moves through the medium. A scene we all are familiar with will demonstrate the principle (Fig. 1.2). When a sound wave propagates through a medium, the wave moves while the medium remains in place. Thus, when ultrasound propagates through human tissue, it is the wave that moves, not the tissue. Let's go back to the sound waves. Low-frequency sound (a human voice, music) will spread all over a room. You can easily hear the voice of a person talking with his back turned to you. Very high-frequency sound behaves like light – it moves like a beam along a straight line. High-frequency ultrasound propagates through tissue in a relatively narrow beam and may be focused by acoustic lenses. In order to make a simple ultrasound machine, we need to be able to produce high-frequency sound. In the 1880s the Curie brothers discovered the piezoelectric effect which implies that a crystal, for example a quartz crystal, will produce an electrical current if subject to mechanical pressure. Conversely, an electrical current that is applied to a quartz crystal will cause the crystal to change its shape. The change in shape will have an impact on the surrounding medium. If alternating
Physics and instrumentation
muscle and 3500 m/s in bone. The hardness or stiffness of the medium is the main factor determining the propagation velocity of sound. Ultrasound machines are now standardized and calibrated to use 1540 m/s as the speed of sound in human tissue. Based on the propagation of the sound wave in a particular medium (v) with a particular frequency (f), we arrive at the first important equation for the wavelength λ:
Table 1.1 Various ultrasound frequencies and the corresponding wavelength Frequency (MHz)
Wavelength (mm)
3.5 5 8 10
0.44 0.31 0.19 0.15
5
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Ultrasound in obstetrics and gynaecology
Wave motion
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Fig. 1.2 The person on the shore throws a stone into the water. The stone creates waves in the form of concentric rings that approach the cork. Instead of being ‘pushed away’ the cork moves up and down as the wave passes by.
current is applied to the crystal, the crystal will repeatedly change its shape and the movements of the crystal will produce a wave transmitted through the medium. By using a piezoelectric material (quartz crystal) it is possible to produce highfrequency sound waves that emerge from the crystal into human tissue. The same crystal can be made to pick up the echoes emerging from the depth of the tissue. Such echoes will have an impact on the crystal that produces an electric pulse that we may detect and process further. If you have been at an outdoor rock concert in front of a full-blast subwoofer, you will have experienced the impact that sound can have on your body, in particular on your air-filled chest cavity. Imagine the sound level scaled down to an impact you cannot feel and then a very sensitive instrument introduced to detect the sound waves; then you have a demonstration of the basic principle of receiving low-impact echoes. Making images with sound is about sending and receiving sound waves in the form of a pulse (Fig. 1.3). We now have enough knowledge to make a one-dimensional ultrasound image of the fetal skull the way it was done in the late 1950s and early 1960s. It was called A-mode (A stands for amplitude) (Fig. 1.4). In the early days of the clinical use of ultrasound, A-mode technology made it possible to measure the fetal biparietal diameter and the conjugata vera, to locate the placenta, including placenta praevia, and to diagnose polyhydramnion, detect the fetal heart activity, diagnose a molar pregnancy and a variety of other diagnoses. The interpretation of such images was difficult and required extensive training and imagination of the examiner. Still, sophisticated diagnoses were made by dedicated pioneers.9
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Physics and instrumentation Fig. 1.3 (Bottom) An electric current is applied to the transducer and a pulse is sent out. (Top) A pulse is received and generates an electric current that can be displayed by the instrument. The stronger the returned pulse (echo), the higher the amplitude of the electric current.
The natural step forward was to make two-dimensional images. The strength of the echoes was then displayed as a white dot instead of as an amplitude; the higher the intensity of the returned echo, the larger the dot. This was called B-mode (B stands for brightness). In a one-dimensional system, these signals were impossible to interpret but moving the transducer in a plane across the area to be examined (scanning) during sending and receiving made it possible to display all the echoes emerging from structures in that plane. Together, these were converted into a relatively easy-to-read two-dimensional image (Fig. 1.5). This manual scanning made it much easier to produce and interpret two-dimensional images produced with ultrasound. The image quality was further improved by the development of the analogue scan converter, so that grey scaling could be applied as well as scaling of the image and calliper movements on the screen. The next technical step was to produce real-time two-dimensional images. This was achieved mechanically in the 1960s by Krause and Soldner in Erlangen, Germany.10 A more sophisticated way was to align a set of crystals to make a linear transducer, described by Nicolaas Bom in Rotterdam, in 1971.1 The principle was further developed by Martin Wilcox who produced a clinically most successful real-time scanner in 1972 (Fig. 1.6). The principle of displaying the returned signals appropriately is simple: the speed of sound is known and the time from when a pulse is emitted until it comes back can be calculated. It is obvious that each submitted pulse will hit many structures in the path of the beam, thus many echoes will be returned separated by a short time interval. Electronic real-time scanning implies that the transducer sends a pulse, and then it switches to the listening mode. A linear transducer may typically have 196 or more crystals aligned in a row. Typically crystals number 1–50 are fired, and then number 2–52, etc. The examiner is presented with an image frame rate of approximately 30 per second, which for the human eye will make the on-screen image appear flicker free with movements in real time.
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Ultrasound in obstetrics and gynaecology
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8
Fig. 1.4 A-mode. A single ultrasound beam is sent through the fetal skull and, in sequence reflected from the parietal bone closest to the transducer, the falx cerebri, the skull bone distal to the transducer and, finally, the posterior uterine wall. Depending on the strength of the returned pulses (echoes), the quartz crystal will generate a high- or low-amplitude current.
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Physics and instrumentation
Fig. 1.5 Twin pregnancy. B-mode image obtained in 1964 by Diasonograph, Nuclear Enterprises Ltd, Edinburgh, UK. The image is made by ‘compound scanning’, i.e. by rocking the probe back and forth during the process of moving the scanning arm slowly across the pregnant abdomen. Reproduced by permission from Bertil Sundén.11
Finally, we need to understand the physical principle of M-mode (M stands for motion). M-mode is used to trace the movement of a structure. For example, tracing the movement of a heart valve or the movement of the atrial wall and the ventricular wall of the fetal heart simultaneously on the very same image makes it possible to discriminate a dissociation of the rhythm, i.e. supraventricular tachycardia, various kinds of AV block, etc. During an M-mode recording, we register the movements of the echoes along one single line in our image (the y-axis) while time runs along the x-axis. The principle is easy to understand if we imagine that we put a long paper strip on our desk, hold a pen against the paper and move the pen up and down while pulling the paper strip in a direction perpendicular to the movement of the pen. In our example, the pen represents the moving echoes and the up-and-down movement of the pen will result in a curved line on the paper reflecting the movements of the pen. An M-mode scan is shown in Figure 1.7.
One Transducer for each Purpose A variety of sizes and shapes of transducers have been produced for the various applications of ultrasound in medical diagnosis. Transducers have various sizes of ‘footprints’, i.e. the part of the transducer that touches the skin or other tissue. Transducers with a small footprint are necessary in, for example, cardiology, for sending a beam between the ribs to reach the heart as a target organ. To reach the heart and thoracic aorta, even an oesophageal transducer may be used; in urology and proctology, the prostate or lower part of the intestines is reached by inserting a transducer into the rectum. The gastroenterologist may examine the liver from the surface of the abdomen or insert a slim transducer through the gastric scope to reach the surrounding organs including the ductus pancreaticus and the pancreas.
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Ultrasound in obstetrics and gynaecology
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Fig. 1.6 (Upper panel) The ADR linear scanner (in Europe manufactured under the name of ADR-Kranzbühler, image by courtesy of the company, 1980). (Lower panel) The basic principle of scanning in real time. The crystals fire the sound beams, which travel into the human tissue, hit structures and are reflected. The reflected echoes are picked up and displayed accordingly on a screen.
10
In vascular surgery, imaging through a catheter has been developed for target organs such as the neck vessels and coronary arteries. In the field of obstetrics and gynaecology, curvilinear transducers are extensively used for transabdominal examination (Fig. 1.8). The shape of the transducer fits well to the pregnant and non-pregnant abdomen, the footprint is small, while the view deep in the tissue is wide due to the sector-shaped image. The use of a convex transducer also reduces the effect of reverberations and wave front aberrations (see later). Transducers designed for transvaginal scanning make the early pregnancy and the non-pregnant uterus accessible at a close range; thus, they are widely used in gynaecology and obstetrics. Transducer technology has become complex. The essential unit, the sound-emitting crystal, was made of natural materials such as quartz. Nowadays most of the crystals are made of artificial ceramics mixed with plastic materials with various
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Movement with time
Depth
M-mode
Physics and instrumentation
A-mode
Movement with time Grey-scale amplitude along beam at fixed time
Time
Fig. 1.7 The principle of M-mode. The basic principle is described in the text.
Fig. 1.8 Sector, curvilinear, linear and transvaginal transducers.
forms of damping material to produce a clean pulse and a pulse of short duration. The electrical excitement is made through thin silver electrodes connected to the ceramic material. The basic principle for producing a pulse wave and receiving an echo, which generates a current, remains the same, as illustrated in Figure 1.8.
The Ultrasound Beam Near Field and Far Field Ideally, an ultrasound beam would emerge from a crystal, be narrow and circular and shoot into the tissue along a straight line. Then it would return along the same line from structures it may hit, to the very same crystal, which would be excited by the echoes and produce an electrical current. In real life, the beam is not ‘narrow and circular’ but advanced engineering has, over time, worked to
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ modify the beam towards the ideal form. Put simply, the beam has a near field and a far field. In the near field we may influence the shape of the beam by focusing. In the far field we cannot do that. When we make our images we are operating in the near field. So from an imaging point of view, we would like the beam to have a long near field (Fig. 1.9). The depth (d) at which the transition of the beam from the near field to the far field takes place is given by equation 2. r is the diameter of the circular transducer: (2) r2 d= λ This equation tells us that the near field is relatively long if the diameter of the circular transducer is large and/or the wavelength (λ) is short, i.e. the frequency is high. It follows that the near field is relatively short if the transducer has a small diameter and/or the wavelength is long, i.e. the frequency is low.
Focusing This brings us to the next important feature, which is the focusing of the beam. The required effect of focusing the beam is to reduce the width of the beam. Focusing may be achieved by employing lenses in various forms. F ×λ (3) BW = 2r Figure 1.10 shows the trade-off of having a narrow beam width as an effect of focusing: an increased divergence of the beam distal to the focal distance. Considering equations 1–3, we may conclude that a focused transducer with a large diameter (aperture) and a high frequency (short wavelength) will provide a narrow beam in our region of interest (at the focal distance). So why do we not settle for transducers with a large aperture and a high frequency? The quick answer is that a large aperture may not be acceptable for a particular application and high-frequency ultrasound is absorbed to a greater extent than low-frequency ultrasound. The range of a relatively low-frequency transducer is longer than for a relatively high-frequency transducer. Near field
Far field
2r Transducer d
12
Fig. 1.9 Schematic illustration of an ultrasound beam emerging from a transducer with a circular surface with a diameter 2r. The beam has a near field reaching into the depth of d and a far field.
✩✩✩✩✩✩✩✩✩✩✩ ✩ Near field
Far field
2r Transducer
Lens
BW
Fig. 1.10 Schematic illustration of an ultrasound beam emitted from a transducer with a circular surface with a diameter 2r. The focal distance is set at F and the beam width (BW) is the effect of the focusing.
Physics and instrumentation
F
The solution is to use high frequency if we are looking at structures close to the transducer and low frequency if we are looking at structures further away. Let us go back to the ultrasound beam. Ideally, one would like the ultrasound beam to be thin and round, and shoot into the tissue along a straight line, hitting structures which cause echoes that return to the transducer along the same line. Then only structures in the thin path of the beam would cause echoes. We have learned that this is not so. The beam has a near field where we may manipulate the beam and a far field where the beam diverges due to diffraction, where it is not possible to manipulate the beam. The beam has a main lobe and side lobes (Fig. 1.11). The side lobes may be considered ‘skirts’ around the main lobe body. When such a complex beam is shot into the tissue, all the structures hit by the
Side lobe Main lobe
Fig. 1.11 Schematic sketch of an ultrasound beam, demonstrating the main lobe and the side lobes.
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ main lobe and structures which in reality are located on the side of the main lobe, but within the side lobes, will cause echoes to be returned to the transducer and be displayed along the centre of the imaginary line through the centre of the main lobe. This will cause a ‘smear-out’ effect of the image. The presence of side lobes in addition to the main lobe reduces the quality of our image. Structures outside the main lobe will be picked up by the side lobes and on the final image they will be displayed along the centre line through the main lobe. Improving the overall beam quality is accomplished through the focusing process which may be achieved in various complex ways. One technique, dynamic focusing, may help us understand the principle of focusing. One submitted pulse may cause many returned echoes which hit the transducer surface over a time period, depending on how far the echoes have travelled on their way down to the various reflectors and then back. Focusing is a process that may be done on the way out and on the return of echoes. Focusing on the returned echoes is always done. Since we know when a pulse has been transmitted, we may focus the returned echoes by changing the focus level in the tissue at certain time intervals following the transmission of the pulse. This will cause echoes, which originate from a depth of, for example, 2, 4, 6, 8 and 10 cm, to be focused separately on return. Thus, the focusing process will affect the area between 2 and 10 cm, in the example above. Additionally, we may focus our area of interest especially on the way out to obtain the highest image quality possible in the specific area where we are looking. Arrows along the side of the image indicate the manually set foci. Optimum quality is usually achieved employing two to three foci in the area of interest. The process of directing the focus of our beam to the area we are looking is one of the most important manual adjustments we make during ultrasound scanning. Unfortunately, focusing is one of those manual adjustments which are most often forgotten, a practice that exemplifies a lack of technical understanding of the person performing the scanning.
Resolution
14
To be able to interpret our image, define discrete structures and make precise measurements on an ultrasound image, we need to understand the basic principles of resolution. Resolution is defined as the smallest distance we can have between two structures and still be able to distinguish them as two separate structures. On a two-dimensional ultrasound image, we have an axial plane, a lateral plane and an elevation plane (Fig. 1.12). The resolution in these three different planes is determined by various physical laws that we have to understand to optimize the adjustment of our machine settings, select the best transducer for our purpose and make measurements as precise as possible. The axial resolution may be called the range resolution or the radial resolution. The resolution in the axial plane is the best of the three. The axial resolution is mainly determined by the length of the transmitted pulse. A ‘pulse’ always consists of a few oscillations in spite of effective damping factors. The absolute length
✩✩✩✩✩✩✩✩✩✩✩ ✩ Transducer
Physics and instrumentation
Axial plane
Azimuth plane
Elevation plane Fig. 1.12 The three planes on an ultrasound image: the axial, the azimuth and the elevation plane.
5 MHz 2.5 MHz
5 MHz 2.5 MHz 5 MHz 2.5 MHz Fig. 1.13 In the upper part, a 5 MHz pulse is shown travelling towards a target, which may be a blood vessel. The pulse is short enough to be able to hit the anterior and posterior walls separately, thus two separate echoes will be reflected and make two separate dots on the screen when they hit the transducer. Below, the 2.5 MHz pulse is longer and the echoes from the anterior and posterior walls of the vessel will overlap, so only one large dot will be displayed on our screen. The 2.5 MHz pulse was not able to resolve the two vessel walls as two separate structures.
of a pulse may therefore be reduced by increasing the ultrasound frequency. The principle of the axial resolution is demonstrated in Figure 1.13. The total pulse length of a 5 MHz pulse is typically shorter than that of the 2.5 MHz pulse. A good axial resolution requires a short pulse. Several factors may contribute to a short pulse: one of them is the wavelength. A high frequency (i.e. short wavelength) will make the pulse relatively short and improve the axial resolution.
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ The lateral resolution affects measurements across the azimuth plane, which is perpendicular to the direction of the beam. The lateral resolution is governed by different physical laws from the axial resolution and is poorer than the axial resolution. Among the factors that affect the lateral resolution are the quality of the beam and the size of the side lobes (see Fig. 1.11). In the process of optimizing the beam quality, the aim is to have a thin main lobe and small side lobes. The lateral resolution perpendicular to the direction of the beam is poorer than the axial resolution. Measurements made in the axial direction are more precise than those made across the image perpendicular to the beam.
Measurement Generally, when we measure a distance in the axial direction, we put one electronic calliper on an echo and move the next calliper to another echo to assess the distance between the two. However, we are not actually measuring the distance between the two, but rather the time it takes for a pulse to travel from the transducer to the structure closest to the transducer and to the structure further away. So when we measure a distance, our calculations are based on time rather than on a physical distance. We have to take into account that of the two, axial resolution is better than lateral resolution. If we measure in the plane perpendicular to the beam, the beam quality will influence our measurement. A relatively thick beam will make the endpoint of a structure appear blurred and make the distance between two points appear slightly larger than in reality. This phenomenon has a consequence for the measurements across the screen, for example the occipitofrontal diameter of the skull and even the femur length.8
Time Gain Compensation
16
When a pulse propagates through the tissue, it will gradually lose its energy. This loss is caused mainly by power absorption and to a smaller extent by reflection, scattering and geometric spread. This process takes place as the pulse travels away from the transducer and as the echo is on its way back to the transducer. The absorption of ultrasound energy increases with increasing frequency. The attenuation causes the reflected echoes from structures deep in the tissue to be weaker than those emerging from nearby structures. If we do not compensate for this phenomenon, our image will appear imbalanced (Fig. 1.14). The speed of sound in the human tissue is constant; the echoes emerging from the deeper areas arrive later than those from the upper structures. Thus, we may compensate for the loss of power from the late-arriving echoes by inserting a time variable gain in the receiver amplifier. This is called time gain compensation (TGC). The basic TGC is preset in modern machines, but we may have to adjust manually to fine-tune our image. Usually it is possible to make an overall adjustment of the gain as well as adjustments affecting the local area ranging from the near to the far field of the image. The setting of the TGC also affects our measurements and it is an important part of the training to learn how to set it correctly. A TGC adjusted too high will produce blurry edges and measurement of distance between structures will be longer than in real life.
✩✩✩✩✩✩✩✩✩✩✩ ✩
Physics and instrumentation
Fig. 1.14 A section through the planum biparietale. The area close to the transducer is correctly adjusted while the distal area has hardly visible low-energy echoes as a consequence of the insufficient compensation for the attenuation of sound emerging from the deeper sections of the tissue.
The fine-tuning of our image using the TGC is one of the most important adjustments we make. The grey-scale level of the image ought to appear well balanced from the upper to the lower part of the image. The adjustment must aim at achieving the full register of grey tones between the black areas and the white highlights. The setting of the TGC has an influence on our measurements.
Artifacts Artifacts in ultrasound imaging may be distortions or any form of incorrect appearance affecting an image and giving misleading information as we try to interpret from the image. The imaging process using ultrasound technology may cause numerous artifacts that we have to be aware of. Some of the main artifacts are as follows:
• Edge shadows • Attenuation shadows • Enhancement • Reverberations. Edge Shadows In obstetrics, edge shadows are mainly observed during scanning of the fetal head. When the sound enters a round structure such as the fetal head it emerges from tissue with a velocity of 1540 m/s through the bone of the fetal skull that has a sound velocity of ≈3000 m/s. The sound will then be refracted and leave a shadow-like impression on both sides of the fetal skull (Fig. 1.15).
Attenuation Shadows Bone absorbs ultrasound and the echo amplitude will then be reduced behind ossified structures. This is frequently observed during fetal heart scanning when the image of the heart may be in the shadow of the ribs or the vertebrae. In gynaecology, dense structures such as myomas may to a lesser degree reduce the
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩
Fig. 1.15 Edge shadows. On both sides of the fetal skull, the ultrasound beam is refracted and then leaves a shadow below.
amplitude of the sound. Such shadows may give us information about the structure that is causing the shadow.
Enhancement Enhancement is the opposite of attenuation shadow. The phenomenon may be seen behind cysts (Fig. 1.16). This artifact may also be used to characterize the structure causing the enhanced area.
18
Fig. 1.16 Simple cyst demonstrating the enhancement artifact. The sound that is passing through the cyst is not attenuated in the same degree as the sound passing through the tissue on the right and left side of the cyst. Therefore, the amplitude of the sound immediately below the cyst is higher than on the sides and consequently it looks as if the area below the cyst has been enhanced by selectively turning up the time gain compensation.
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ReverberationS Physics and instrumentation
The artifact referred to as reverberation or multiple reflections is common and may distort the image in several ways. The basic principle of making an image using sound is to send a pulse, wait for the pulse to return as an echo and then a dot is put on the screen corresponding to the time the pulse has taken to travel on its way down to the reflecting structure and back again. A pulse may also be reflected back and forth between interfaces before returning to the transducer. The extra travel time this process takes will cause the false echoes to arrive later than echoes emerging directly from the original structure so that then several lines on the image may present themselves as copies of the original (Fig. 1.17). Such reverberations may easily be recognized. Layers of fat may also cause reflections and reverberations in the image that presents itself as a diffuse cloud of noise and is thus not so easy to recognize as artifacts. The lower mechanical impedance of sound in fat (sound velocity ≈1420 m/s) and muscle tissue (sound velocity ≈1560 m/s) may cause reverberations. Reverberations may be complex in their appearance and not always easy to detect. Using a curved array transducer may reduce the effect of reverberations. The echoes are scattered out of the field, which causes the curved array to have a good near-field view.
Main echo
Reverberation
Object
Image
A
Main echo Reverberation
Object
Image
B Fig. 1.17 Two examples of reverberations. In the upper panel (A) the main echo schematically is represented by a blood vessel. A ‘copy’ of these echoes may be found as reverberations at a lower level. Fatty tissue may also cause reverberations which may show up as a diffuse haze (B).
19
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Ultrasound in obstetrics and gynaecology
References 1. Bom N, Lancée CT, v Zwieten G, Kloster FE, Roland J. Multiscan echocardiography I. Technical description. Circulation 1973;48(5):1066–1074 2. Desch CH, Sproule DO, Dawson WJ. The detection of crack in steel by means of supersonic waves. J Iron Steel Inst 1946; 153:319 3. Donald I, Wicar WA, Brown TG. Investigation of abdominal masses by pulsed ultrasound. Lancet 1958;1:1188 4. Firestone FA.The supersonic reflectoscope, an instrument for inspecting the interior of the solid parts by means of sound waves. J Acoustic Soc America 1946;17:314 5. Edler H, Hertz CH. The use of ultrasonic reflectoscope for the continuous recording of movements of heart walls. Kgl Fysiograph Saellskap Lund Förh 1954;40:23 6. Edler I. Ultrasound cardiography. The diagnostic use of ultrasound in heart disease. Acta Med Scand 1955;308(suppl):32
7. Howry, DH, Bliss WR. Ultrasonic visualisation of soft tissue structures of the body. J Lab Clin Med 1952;40:579 8. Jago JR, Whittingham TA, Heslop R. The influence of ultrasound scanner beam width on femur length measurements. Ultrasound Med Biol 1994;20(8):699–703 9. Kratochwil A. Ultraschalldiagnostik in Geburtshilfe und Gynäkologie. Georg Thieme Verlag, Stuttgart, 1968 10. Krause W, Soldner R. Ultraschallbildverfahren (B-Scan) mit hoher Bildfrequenz für medizinische Diagnostik. Elektromedica 1967;4:1 11. Sundén B. On the diagnostic value of ultrasound in obstetrics and gynæcology. Acta Obstet Gynaecol Scand 6(suppl):114 12. Wild JJ, Reid JM. Application of echoranging techniques to the determination of structure of biological tissues. Science 1952;28:226–230
Further reading Angelsen B. Ultrasound Imaging. Waves, Signals, and Signal Processing. Basic Principles, Wave Generation, Propagation, and Beam forming in Homogenous Tissue. Vol I. Emantec, Trondheim, Norway, 2000. www.ultrasoundbook.com Angelsen B. Ultrasound Imaging. Waves, Signals, and Signal Processing. Propagation and Scattering in Homogenous, Nonlinear Tissue with Contrast Agent. Imaging and Doppler Measurement. Vol II. Emantec, Trondheim, Norway, 2000. www.ultrasoundbook.com Hatle L, Angelsen B (eds). Doppler ultrasound in cardiology. Physical principles and clinical applications. Lea and Febiger, Philadelphia, 1986 Kremkau FW. Diagnostic ultrasound. Principles and Instruments, 7th edn. WB Saunders, Philadelphia, 2006 Maulik D (ed). Doppler ultrasound in obstetrics and gynecology. Springer, New York, 1997 Woo J A short history of the development of ultrasound in obstetrics and gynecology. www.ob-ultrasound.net/history1.html
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Biological effects and safety aspects Francis A Duck
Abstract Full exploitation of diagnostic ultrasound requires careful consideration of potential risks. Ultrasound causes small increases in tissue temperature. Whilst commonly of only fractions of a degree, some conditions can give temperature increases which could approach 10°C, particularly at exposed bone during Doppler modes. Safety thresholds are derived from studies into thermal teratology. Tissues can also be damaged mechanically by gas body activation, although this mechanism appears to be of very minor concern for most obstetric applications. Another bioeffects mechanism is radiation pressure, whose presence is demonstrated by acoustic streaming. Epidemiological studies have yet to demonstrate unequivocally any causal relationship between exposure to ultrasound in utero and developmental changes, although all published studies relate to earlier, low-intensity exposure regimens. There is yet insufficient understanding of the interaction between ultrasound and the developing embryo and fetus at all stages in pregnancy, and this lack of detailed knowledge still advises care and prudence in the use of ultrasound in obstetrics. On-screen safety indices may assist clinical users to make improved safety judgements.
Keywords Epidemiology, exposure, gas body activation, mechanical index, non-thermal effects, regulations, thermal effects, thermal index, ultrasound safety.
Introduction Diagnostic ultrasound has an enviable reputation for safety, and the lack of evidence of significant hazard and consequent risk has been one of the key factors which has established it as the pre-eminent imaging method in obstetrics. Whilst the severe biological effects associated with x-radiation became abundantly clear
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ very early, ultrasound gives no obvious evidence of tissue damage until very high intensities are used. However, it is now appreciated that even diagnostic levels of ultrasound can cause small but potentially significant tissue responses. Therefore, both the design and clinical use of equipment which uses ultrasound for diagnosis must be subject to the general rule that the diagnostic benefit must be sufficient to outweigh the potential for harm – a risk/benefit judgement has to be made. Until about a decade ago, manufacturers designed ultrasound scanners for particular applications – for example for cardiology, ophthalmology, obstetric or vascular scanning. Regulation in the USA restricted output intensity from obstetric scanners to be about eight times lower than the highest available. These limits served also to constrain output from equipment available in other countries. In the early 1990s regulations in the USA were relaxed, in part to allow Doppler modes to be used in obstetrics, allowing the highest output to be used for all applications. Manufacturers now sell equipment for obstetric use that can operate at levels previously reserved only for peripheral vascular applications. They are required also to display values of safety indices, which reflect the changing output of the machine as it is used clinically for different applications and patients. These values of the mechanical index (MI) and thermal index (TI) are intended to allow users to make a risk/benefit judgement. In order to do this, clinicians and other users of the equipment must know of the potential hazards inherent in using ultrasound, and be advised about the interpretation of the safety indices. This chapter is intended to introduce the reader to these issues. More detailed information may be found in other publications2,10,11 and in a series of safety tutorial articles which are available on the web page of the European Federation of Societies for Ultrasound in Medicine and Biology (www.efsumb.org/ecmus. htm) and the International Society for Ultrasound in Obstetrics and Gynecology (www.isuog.org).
Acoustic Output of Diagnostic Ultrasound Scanners
22
Exposure to ultrasound at sufficiently high levels is capable of causing lethal damage to tissues. Knowledge of acoustic output serves to ensure that diagnostic exposures are limited to levels that may be used safely. Broadly, two aspects of the ultrasound beam are measured, which guide answers to two questions: how much energy is in the beam and how big are the pulses of ultrasound? The energy may be described in terms of total acoustic power (energy per second) or acoustic intensity (power through a specific area). Both of these are related to the temperature rise in tissue. Commonly the spatial-peak temporalaverage intensity is quoted rather than the acoustic power (Ispta, in milliwatts per square centimetre). The size of the pulse is usually measured by its peak rarefactional pressure, pr, in megapascals (MPa). One megapascal is approximately equal to 10 atmospheres. This quantity is related to the potential for gas body activation or acoustic cavitation. Normally, tables giving Ispta and pr present the highest values reached anywhere, and these are found typically near to the focus of the ultrasound beam.
✩✩✩✩✩✩✩✩✩✩✩ ✩ Table 2.1 Summary of median and maximum values of spatial peak, temporal average intensity, Ispta, from a 1998 survey22 Maximum value, mW cm−2
A- or M-mode
81
604
Real-time B-mode
94
1330
Colour Doppler
328
2030
Spectral Doppler
1420
7500
There have been a number of published surveys of output, and these have been summarized by Whittingham.22 He shows that the peak rarefaction pressure used for all modes is about the same, with median about 2.5 MPa and maximum about 5 MPa, whether operating in imaging mode, M-mode, spectral Doppler or Doppler imaging. Thus gas body effects are equally likely to occur whatever mode is in use. The situation is different when considering Ispta. This is shown in Table 2.1, which summarizes the median and maximum values reported by Whittingham for a 1998 survey. Two remarks may be made. First, on average, intensities become higher as the mode is changed from M-mode, through B-mode and colour Doppler, to become highest in spectral Doppler mode. This trend occurs in both the maximum and median values. Therefore, on average, the highest intensities and hence probably the greatest heating are associated with Doppler modes, particularly spectral Doppler. However, the second remark is perhaps of greater general importance. The overlap between peak intensities in each mode is very large. It is possible to find B-mode intensities on one scanner which exceed the highest Doppler intensities on another. Moreover, for any selected transducer it is often true that the intensity used for Doppler imaging exceeds that used for spectral Doppler. For this reason it is now becoming common only to give general advice on safety rather than to give specific advice for the use of pulsed Doppler. Surveys have also demonstrated a trend towards increased output during the past 20 years or so. Increases have occurred in output from ultrasound scanners used for obstetrics, partly due to the changed regulations in the USA, and partly because of a general trend to design scanners that operate towards the top end of the available performance range.
Biological effects and safety aspects
Median value, mW cm−2
Tissue Warming by Diagnostic Ultrasound The fundamental biochemical processes controlling the behaviour and function of living cells depend strongly on temperature. Mammalian tissues can survive and operate effectively within quite a small range of temperatures, and elevated temperatures sustained for extended times may alter cell function and can result in cell death. Temperature elevation is a potent teratogen, and thus it is appropriate to establish the extent by which ultrasound scanners are capable of increasing temperature within tissue.
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Ultrasound in obstetrics and gynaecology
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24
Ultrasound pulses lose energy as they penetrate tissue, a fact ultimately limiting the ability to scan to great depths. Almost all the energy lost from the ultrasound wave is deposited as heat in the tissue and this causes small rises in temperature in this tissue.3 The temperature rise is affected by a number of factors. The first is the energy in the beam: the higher the intensity, the greater the heating. The energy distribution is also important, for example whether the beam is narrowly focused, and whether it is scanned. The thermal and acoustic properties of tissue also determine the temperature elevation. Amongst these properties, the two most important are the acoustic absorption coefficient of the tissue and its blood perfusion rate. Bone is the tissue which absorbs ultrasound energy to the greatest extent and so, wherever the ultrasound scan plane intercepts bone, it will be here that the temperature rise will be most rapid and of greatest elevation. In obstetric scanning, the developing fetal skeleton warms first and to the highest temperature. As the fetal bones mature throughout gestation, the absorption of ultrasound increases, and so does the temperature they may attain (see Fig. 2.1). Blood perfusion controls temperature elevation, returning local temperature towards the core temperature. This effect is seen most strongly near large blood vessels. Fetal tissue is adequately, though not strongly, perfused, and so this may not be a significant factor in controlling ultrasound-induced warming. Tissues may also be warmed as a secondary effect from an elevated temperature in a nearby structure. This is important when considering heating of fetal central nervous tissue, which is known to be particularly sensitive to thermal damage.3 Whilst fetal brain itself has a relatively low ultrasound absorption coefficient, the brain tissue which lies alongside the skull heats as a secondary effect of skull heating. It is therefore the bone temperature that is critical for safety judgements. The second situation when secondary heating may be important is transducer self-heating. Ultrasound transducers heat because the electrical power is converted rather inefficiently into ultrasound power, the remaining power being dissipated as heat in the transducer. Tissues close to the transducer can have their temperature raised by several degrees, by contact heating. Whilst this probably is not important for a skin-coupled transducer, a transducer for transvaginal scanning could, in principle, pose a problem. International standards for transducer design now limit the contact temperature rise to 6°C, and the contact temperature to 43°C.9 Currently available clinical scanners are capable of causing temperature elevations in bone which approach 10°C, and in soft tissues of about 3°C, when operating in pulsed Doppler mode. Whilst these results relate to rather extreme experimental conditions, which omit the protection given by any overlaying tissue layers, they emphasize that present clinical scanners are easily able to cause significant heating within tissues when operated at the extreme upper limits of output. One example of bone heating is shown in Figure 2.1, which shows measured temperature rises in samples of human fetal vertebrae, exposed to ultrasound in vitro.5 In this case the frequency was 3 MHz, and the acoustic power, 50 mW, can be easily achieved in vivo with modern scanners.
✩✩✩✩✩✩✩✩✩✩✩ ✩ 2.0
1.6
39 weeks
Temperature rise, C
1.4 1.2 1.0 0.8 0.6 0.4
14 weeks
0.2 0.0
0
50
100
150 200 250 Time, seconds Fig. 2.1 Measured surface heating curves for two human fetal vertebrae, exposed in vitro to 3 MHz focused ultrasound at a diagnostic power (50 mW); 14 weeks and 39 weeks gestation. Redrawn from reference 5 with permission.
From the scientific evidence of the effects of hyperthermia, it is generally accepted that tissues containing a large component of actively dividing cells are particularly sensitive to heat. Abnormalities in cell pathology and biochemical processes can occur following an increase in temperature above normal basal levels. There are critical periods during gestation when the embryo and fetus are particularly sensitive to thermal effects. During formation of the neural plate and closure of the neural tube, animal studies have demonstrated that elevated temperature can result in neural defects, retarded brain development, exencephaly and microphthalmia. Exposure at preorganogenesis stages can result in cardiovascular abnormalities, whilst later heating can affect skeletal and visceral systems. There is now a substantial literature on thermal teratology7 which suggests that an elevated temperature of 2–2.5°C, if sustained for an extended period, is sufficient to cause major developmental abnormalities, at least in small mammals. Recognizing the difficulty of transferring animal data to humans, these data still serve as a reminder that remarkably small changes in fetal temperature are capable of causing developmental changes of major significance. It is not possible to interpret thermal bioeffects studies without considering the time over which the temperature elevation is generated and the time for which it is sustained. Review of the thermal teratology literature has led the World Federation for Ultrasound in Medicine and Biology to recommend that ‘a diagnostic exposure that elevates embryonic and fetal in-situ temperature above 41°C (4°C above normal temperature) for 5 minutes or more should be considered potentially hazardous’.11,12 Of course, clinical scanning commonly takes place in a manner such that regions are not examined continuously for more than a few seconds at a time. Exceptions to this are most probably in cardiovascular studies, when the
Biological effects and safety aspects
1.8
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Ultrasound in obstetrics and gynaecology
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26
time variation of a particular region is of interest. Exposed bone can approach a steady-state temperature within about 30 seconds (see Fig. 2.1), soft tissue somewhat longer. Assuming that bone may be exposed anywhere within the examined volume, it is prudent to take particular care to limit output if the examination requires the probe to be stationary for more than 30 seconds.
Non-Thermal Mechanisms and their Safety Implications Ultrasound pulses can alter cells in other ways than by heating the tissue. Broadly, heating changes rates of biochemical reactions, whereas the damage from mechanical effects is primarily to the cellular and tissue structures. Non-thermal mechanisms fall into two classes: those which involve ‘gas bodies’ and those which do not.
Gas Body Effects of Diagnostic Ultrasound It is now accepted that diagnostic ultrasound does not cavitate soft tissues. That is, microscopic gas bubbles are not generated within tissue by diagnostic ultrasound pulses under normal conditions. However, cells and tissue can be damaged when exposed to diagnostic ultrasound pulses if they lie close to a region of gas already contained within tissue. The shear forces generated at the tissue/gas interface may be sufficient to cause damage. Known examples include the rupture of capillaries at the lung surface, resulting in extravasation of blood components into the extracellular space, and the formation of petechiae in the intestine. Gas bubble contrast agents are being introduced into the practice of clinical ultrasound and similar shear forces are created at the surface of these agents when exposed to ultrasound. A process known as ‘sonoporation’ can occur, which is the transient opening of ‘pores’ or gaps in cell membranes, allowing the passage of larger biomolecules into the intracellular space. At sufficiently high acoustic pressures, haemolysis occurs. The response of gas-filled structures to an ultrasound field has been termed ‘gas body activation’ because it differs in many respects from acoustic cavitation. One common factor, however, is that all effects are related to thresholds in acoustic pressure. Judgements about safety therefore depend on an estimate of these thresholds, and a comparison with estimates of acoustic pressure in vivo. The displayed MI is intended to inform these judgements. In the context of obstetric ultrasound, much of the safety discussion about gas bodies has little relevance. Cavitation is not initiated in soft tissues. There are no pre-existing gas bubbles within the uterus so no gas body activation can occur. It is appropriate to use caution when using gas bubble contrast agents for hysterocontrast salpingography, using the displayed MI to limit the possibility of inertial cavitation of free bubbles released when the contrast agent is destroyed. Present advice is to avoid the use of intravenous contrast agents during pregnancy, because it is yet to be determined whether fragments may pass the placental barrier and enter the fetal circulation.
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Other Mechanical Bioeffects Mechanisms
Evidence from Epidemiology This section summarizes briefly the outcome of the more important epidemiological studies into ultrasound exposure in utero. Fuller reviews may be found elsewhere.10 There have been three well-managed case–control studies into ultrasound and childhood malignancies, all of which were of sufficient size to have statistical validity. No association between childhood malignancy was found in any study. Some early studies suggested an association between exposure and birthweight or subsequent growth, but subsequent studies have been unable to demonstrate such an association. In view of the conflicting evidence presented by these studies, the present consensus is that there is no association between exposure to ultrasound and birthweight. A range of neurological functions has been examined and no association between ultrasound exposure in utero and subsequent hearing, visual acuity, cognitive function or behaviour has been found. An association with dyslexia reported earlier14 was not found in later larger studies.15,16 Studies have suggested a possible association between ultrasound exposure and handedness,17 with a gender-biased tendency towards left-handedness.18 At present, there is no explanation of this association and no firm conclusions can be drawn.19 A controlled randomized study from Australia indicated the relationship between repeated Doppler examinations and growth restriction in the fetus20 but the same research group could not find any effect on postnatal follow-up of the children.21 In summary, there is no independently verified evidence to suggest that ultrasound exposure in utero may cause an alteration in the development and growth of the fetus. All studies have either proved to be negative, or, when positive findings have appeared, they have not been verified or have been shown to result from poorly designed studies. New studies will be difficult to structure, because of the difficulty of finding an unexposed control group, resulting from the widespread use of ultrasound during pregnancy throughout the world. It is necessary to sound
Biological effects and safety aspects
A brief mention should be made of a further means of interaction between ultrasound and tissue – radiation pressure. Ultrasound waves push the material through which they pass. If the medium is a liquid, such as amniotic fluid or blood, the result is movement of the liquid. This is called acoustic streaming and may sometimes be observed with modern scanners. Whilst streaming itself is not apparently a hazard, the radiation pressure causing it is also exerted on all tissues within the beam. The forces are small but it is important to recognize that little is known of their effects. Radiation pressure can induce neurological and auditory effects at sufficiently high levels, and some cells can respond to the effects of external shear forces. Caution is needed here as elsewhere as diagnostic techniques are being developed.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ a note of caution, however. There are no studies which have explored outcomes following exposure to pulsed Doppler or Doppler imaging, where intensities and powers are known to be higher than in pulse-echo imaging. While the results of epidemiological studies so far are comforting, they cannot be used to support an argument that it is safe to extend exposure in utero to higher levels. Further epidemiological studies focused specifically on Doppler exposure would be needed before such confidence can be claimed.
The Management of Safety The successful management of safety in medical ultrasound practice operates at several levels. It involves manufacturers, users and international and national professional and regulatory bodies. Manufacturers must comply with standards and regulations intended to make the equipment safe. Users must make sure that they use the equipment in an appropriate and safe manner. Basic scientists provide the evidence from which safety judgements are made, and which informs the recommendations of national and international bodies.4
The Users' Responsibility Clinicians using ultrasound equipment should have specific training in safety aspects of its use. From this training they are expected to be able to use the real-time safety indices to manage the machine settings with appropriate attention to safety. A summary of the meaning and function of these safety indices follows.
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Thermal indices Since it is impossible for the user to know the temperature increase in the body, thermal indices (or TI) have been developed to provide guidance. A TI is a rough estimate of the increase in temperature that occurs in the region of the ultrasound scan. A TI of 2.0 suggests that a temperature rise may reach 2°C, if the transducer is held stationary for long enough. There are three thermal indices – one for soft tissue (TIS), one for bone at depth (TIB) and one for bone at the surface (TIC). These TI values are more helpful than any other information available to the user, because they are informative about the state of the machine output as it is being used. However, the methods for calculating TI include some important simplifications and as a result the true temperature rise may be somewhat higher or lower than the value indicated, perhaps by as much as a factor of 2. Whilst the displayed TI values are the best information currently available, they should be used only as rough, rather than absolute, indicators of the thermal hazard. They may be useful, however, to identify which machine settings are more likely to generate significant temperature increases in tissue, so that particular care may be made to avoid their use for critical examinations. On current equipment, TI values can usually be found around the edge of the scanner screen, often in the top right corner, indicated by the letters TIS, TIB or TIC
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Mechanical index At high enough pressure amplitudes, cavitation becomes ‘inertial’ and its potential for damage increases considerably. An analysis of inertial cavitation has resulted in the formulation of a mechanical index (MI). The MI was developed to quantify the likelihood of onset of inertial cavitation, for which a threshold of MI 0.2 if bubbles pre-exist has been suggested. The MI is proportional to the peak rarefactional pressure, and has a weak frequency dependency. It has since been related also to thresholds for lung damage, and contrast behaviour. The MI is displayed on the scanner screen together with, or instead of, the TI. For applications in obstetrics and gynaecology, the MI is of use primarily when contrast agents are to be used.
Biological effects and safety aspects
followed by a number which changes when the scanner controls are altered. The most cautious approach is to display TIB most of the time. TIS should be displayed only if there is no bone, developing bone or cartilage anywhere in the region being scanned.
The Manufacturers' Obligations The Medical Device Directive in Europe and the Food and Drug Administration (FDA) regulations in the USA both make demands of manufacturers regarding the safe design and performance of their scanners and provision of output information to users. Europe sets no upper limit to the allowed output from ultrasound equipment; the USA, through the FDA, has such limits in place. Intensity (Ispta) must not exceed 720 mW cm–2 and the MI must not exceed 1.9. In order to use these output levels, manufacturers must provide a real-time display of safety information by means of the TI and MI.1,9 Manufacturers must also comply with international standards as set by the International Electrotechnical Commission for electrical and thermal safety.9
Safety Practice Keeping up to date with current thinking on ultrasound safety and risk minimization allows clinicians to make the best decisions on how to maximize the benefit to the patient whilst reducing the risk. Present estimates of risk encourage clinicians primarily to use equipment in such a way as to maximize the opportunity to make a good diagnosis. There is more chance of causing harm by misdiagnosis than through heating or cavitation. With this in mind, the following sections summarize the particular safety considerations relating to obstetric scanning early and late in pregnancy, and to the scanning of patients with fever.
Diagnostic Ultrasound During the First Trimester Probably the most critical question concerns the exposure of the embryo during the early stages of pregnancy.6 This is a period of rapid development and complex biochemical change, which includes organ creation and cell migration.
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ There is widespread evidence that during this period the developing embryo is particularly sensitive to external agents, whose effect on subsequent development may range from fatal developmental malformation to minor and subtle biochemical disturbance. It is because of this sensitivity that the ISUOG13 and EFSUMB 8 have recommended caution with the use of Doppler in early pregnancy. The EFSUMB have advised that ‘until further scientific evidence is available, investigations using pulsed or colour Doppler should be carried out with careful control of output levels and exposure times’.8 This statement recognizes both that there are gaps in our knowledge and understanding of the way in which ultrasound may interact with embryonic tissue, and that any adverse effect may result in developmental problems because of the particular sensitivity of the tissue at this time. Moreover, this sensitivity may be cyclic, with some tissues being sensitive only during particular time-bands of rapid cell development and differentiation. Heat is a teratogen and any temperature increase from the absorption of ultrasound can disturb subsequent development, if of sufficient magnitude and maintained for sufficiently long. Fortunately, the tissue with the greatest tendency to heat, bone, only starts to condense at the end of the first trimester. In the absence of bone, current evidence suggests that temperature elevations greater than 1.5°C are unlikely to occur within embryonic tissue at present diagnostic exposures. This suggests that significant developmental changes probably do not occur. The kinetics of biochemical processes are known to be temperature sensitive, however, and little research has investigated the influence of small temperature changes induced locally on membranes and signal transduction pathways. There is no evidence for cavitation, as there are no gas bubbles to activate within the uterus. The effects of radiation pressure on the developing embryo and fetus are unknown. Thus, although our current understanding suggests that present practice is safe, there is sufficient uncertainty about the detailed interaction processes to advise caution.
Scanning During the Second and Third Trimesters
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Bone ossification is the main developmental change during the second and third trimesters of pregnancy that is of significance to ultrasound safety. As bone condenses, it forms local regions of high ultrasound absorption. Ultrasound energy is absorbed more by the fetal skeleton than by fetal soft tissues, and so it is preferentially heated. This is important in part because soft tissues alongside this bone will also be warmed by thermal conduction, reaching a higher temperature than expected from ultrasound absorption alone. Neurological tissues are known to be particularly sensitive to temperature rise, and the development of brain tissue, and of the spinal cord, could be affected if adjacent skull or vertebral bone were heated too much. Within the fetal haematopoietic system, the bone marrow is the main site of blood formation in the third trimester of pregnancy. Neither cavitation nor gas body activation will occur because of the absence of nucleation sites and pre-existing bubbles.
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Obstetric Scanning on PatIents with Fever
Conclusion Ultrasound has an enviable record for safety. Indeed, it is partly its lack of toxicity which has allowed it to grow to the point where ‘more than one out of every four imaging studies in the world is an ultrasound study’. All the evidence points to the conclusion that past and current practice presents no actual risk to the patient, and may be considered as safe. Nevertheless, there is ample evidence that modern scanners, designed in accordance with national and international standards and regulations, can warm tissues by several degrees under some circumstances. If gas bubbles or other pockets of gas lie in the ultrasound field, the tissues may be damaged from stresses caused by cavitation-like oscillations. Current scanning equipment displays safety indices, allowing users greater feedback for safety judgements to be made. Safety in diagnostic ultrasound depends both on manufacturers to produce equipment that is safe to use, and on the users of ultrasound in managing their scanning practice.
Biological effects and safety aspects
It is noted in the WFUMB recommendations12 that ‘care should be taken to avoid unnecessary additional embryonic and fetal risk from (heating due to) ultrasound examinations of febrile patients’. If a mother has a temperature, her unborn child is already at risk of maldevelopment as a result of the elevated temperature. This being so, it is sensible not to increase this risk unnecessarily. This does not mean withholding obstetric scanning from patients if they have a temperature. The methods of limiting exposure, including minimizing the TI, limiting the duration of the scan and avoiding casual use of Doppler techniques, should be employed with particular vigilance in these cases.
References 1. American Institute for Ultrasound in Medicine/National Electrical Manufacturers' Association. UD 3-1992: standard for real-time display of thermal and mechanical acoustic output indices on diagnostic ultrasound equipment. American Institute for Ultrasound in Medicine/National Electrical Manufacturers' Association, Rockville, MD, 1992 2. Barnett SB, Kossoff, G (eds). Safety of diagnostic ultrasound: progress in obstetric and gynaecological sonography series. Parthenon, London, 1998 3. Barnett SB, Rott H-D, ter Haar GR, Ziskin MC, Maeda K. The sensitivity of biological tissue to ultrasound. Ultrasound Med Biol 1997;23:805–812
4. Barnett SB, ter Haar GR, Ziskin MC, Rott H-D, Duck FA, Maeda K. International recommendations and guidelines for the safe use of diagnostic ultrasound in medicine. Ultrasound Med Biol 2000;26:355–366 5. Doody C, Porter H, Duck FA, Humphrey VF. In vitro heating of human fetal vertebra by pulsed diagnostic ultrasound. Ultrasound Med Biol 1999;25:1289–1294 6. Duck FA. Is it safe to use diagnostic ultrasound during the first trimester? Ultrasound Obstet Gynecol 1999;13: 385–388 7. Edwards MJ. Hyperthermia as a teratogen: a review of experimental studies and their clinical significance. Teratogen Carcinogen Mutagen 1986;6:563–582
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8. European Federation of Societies for Ultrasound in Medicine and Biology. Clinical safety statement for diagnostic ultrasound. 2008: www.efsumb.org 9. International Electrotechnical Commission 2002 IEC Standard 60601-2-37: medical electrical equipment – particular requirements for the safety of ultrasound medical diagnostic and monitoring equipment. International Electrotechnical Commission, Geneva 10. Salvesen KJ, Eik-Nes SH. Ultrasound during pregnancy and birthweight, childhood malignancies and neurological development. Ultrasound Med Biol 1999;25:1025–1031 11. Ter Haar G, Duck FA (eds). The safe use of ultrasound in medical diagnosis. British Medical Ultrasound Society/British Institute of Radiology, London, 2000 12. World Federation for Ultrasound in Medicine and Biology Symposium on Safety of Ultrasound in Medicine. Conclusions and recommendations on thermal and non-thermal mechanisms for biological effects of ultrasound. Ultrasound Med Biol 1998;24(suppl 1):1–55 13. Abramowicz JS, Kossoff G, Marsal K et al. Safety statement, 2000 (reconfirmed 2003). International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). Ultrasound Obstet Gynecol 2003;221:100 14. Stark CR, Orleans M, Haverkamp AD et al. Short- and long-term risks after exposure to diagnostic ultrasound in utero. Obstet Gynecol 1984;63:194–200
15. Salvesen KA, Bakketeig LS, Eik-Nes SH et al. Routine ultrasonography in utero and school performance at the age 8–9 years. Lancet 1992;339:85–89 16. Salvesen KA, Vatten LJ, Jacobsen G et al. Routine ultrasonography in utero and subsequent vision and hearing in primary school age. Ultrasound Obstet Gynecol 1992;2:243–247 17. Salvesen KA, Vatten LJ, Eik-Nes SH. Routine ultrasonography in utero and subsequent handedness and neurological development. BMJ 1993;307:159–164 18. Kieler H, Axelsson O, Haglund B et al. Routine ultrasound screening in pregnancy and the children's subsequent handedness. Early Hum Dev 1998;2:233–245 19. Salvesen KA, Eik-Nes SH. Is ultrasound unsound? A review of epidemiological studies of human exposure to ultrasound. Obstet Gynecol 1995;4:293–298 20. Newnham JP, MacDonald J, Hall C. Characterisation of the possible effect on birthweight following frequent ultrasound examinations. Early Hum Dev 1996;45:203–214 21. Newnham JP, Doherty DA, Kendall GE et al. Effects on repeated ultrasound examinations on childhood outcome up to 8 years of age: follow-up of a randomized controlled trial. Lancet 2004;364: 2038–2044 22. Whittingham TA. Acoustic outputs of diagnostic machines. In: Ter Haar G, Duck FA (eds) Safety of medical diagnostic ultrasound. British Institute of Radiology, London, 2000, pp 16–91
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Scanning techniques in obstetrics and gynaecology Ilan E Timor-Tritsch Ana Monteagudo
Abstract In the last 25 years ultrasonography became the ‘right hand’ of modern obstetricians and gynaecologists. This was possible due to the advances of ultrasound transducer technology and the ‘explosion’ of computer science. In order to realize the power of ultrasound diagnostics and the ultrasound-guided procedures, the astute provider of women's health has to understand how and when to apply the different scanning techniques. This chapter enumerates the different ways in which ultrasound is used in obstetrics and gynaecology. However, it starts with the very basic concepts of how to set up a simple but efficient ultrasound examining room and the ways to approach scanning the everyday patient in a simple office setting. The text leads the reader through the scanning of specific organs as well as some of the mandatory protocols of obstetric and gynaecological examination. Certain more prevalent clinical entities are mentioned in greater detail than others. The interested reader should be clear that ultrasound technology like any other technology based upon electronics, is developing rapidly and without any doubt, by the time these pages are read, there will be new and exciting ways to help us provide better, faster and more accurate diagnoses for our patients. Constant reading and updating our knowledge in ultrasonography is mandatory.
Keywords Colour Doppler, gynaecology, obstetrics, transabdominal ultrasound, transvaginal ultrasound, ultrasound.
Introduction The subject of scanning techniques in obstetrics and gynaecology can be presented from various angles. One way is to start with the purely physics aspects of
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ scanning. This, however, may deter some clinicians from reading through this part of the book. The other angle for introducing the ways we scan in the office and in the dedicated ultrasound laboratories is to describe and discuss the practical aspects of the daily use of ultrasound in its clinical set-up. We decided to take this route, thinking that it has more practical value. An additional decision had to be made: to avoid discussion of older scanning techniques which were ‘cutting edge’ in their time, but which have no practical use at the time of this writing.
General Aspects We will deal with the basic requirements as far as the ultrasound equipment, orientation and aspects of the technicalities involved in conducting the gynaecological ultrasound examination are concerned. If additional information is needed, the reader is referred to some of the more detailed resources in the literature.6,8,14,16,17,20,28,33,35,40,57
Empty or Full Bladder It is now known almost to everyone engaging in obstetric and gynaecological scanning that transabdominal ultrasound is performed most of the time with a full bladder. The full bladder serves as an acoustic window and pushes the bowel out of the sound path (Fig. 3.1). However, transvaginal sonography is best performed with an empty bladder, which enables the pelvic organs to reach a closer proximity to the tip of the high-frequency transvaginal probe. The difference between the two approaches lies in the different physical properties of the transabdominal and transvaginal probes.10,22,25,27,48,50 The transabdominal probe produces a more or less panoramic view of the pelvis, showing the interrelationship of the major anatomical structures within the pelvis and their possible pathology. The transvaginal probe, however, is able to furnish a more targeted image of the organ of interest. The transvaginal probe,
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Fig. 3.1 Orientation using transabdominal scanning. (A) Directions in the sagittal plane. (B) Directions in the transverse plane.
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Scanning techniques in obstetrics and gynaecology
therefore, will permit an effective imaging usually to not more than 7–10 cm in depth. Lately more advanced probe technologies as well as signal processing have enabled deeper penetration with minimal loss of resolution. The bowel interferes with transvaginal sonography mainly in cases in which the uterus has been removed and the available space is taken up by the gas/fluid of the bowel. Depending on the goal of the scan, the sonographer can select the transabdominal or the transvaginal scanning method. It is important to stress the fact that it is easier and faster to empty the urinary bladder than to fill it up, and this should always be considered before the patient is sent to empty her bladder. The protocol of most laboratories is to perform first a general transabdominal scan of the pelvis, which requires a full bladder to get an overview of the anatomy. However, if a patient presents with an empty bladder, the scan should not be postponed, and transvaginal sonography can be performed as a first-line imaging technique. If transabdominal examination of the pelvis is still required, it should be performed after filling the bladder. Benacerraf et al questioned whether a full bladder is still necessary for pelvic sonography.3 After scanning 206 consecutive patients prospectively, they concluded that transvaginal sonography (TVS) with an adjunctive transabdominal sonography (TAS) with an empty bladder approach can replace the full bladder technique for routine pelvic sonography. Filling the bladder may help in imaging a low-lying placenta, diagnosing placenta accreta overlying the bladder or outlining the cervix in the median plane, and at the time of chorionic villus sampling, it may help to ‘straighten’ the uterine body to match the axis of the cervix and the vagina for better approach by the sampling catheter. If the patient is suspected of having an ectopic pregnancy, extreme caution should be exercised to prevent the patient drinking water, because an emergency surgery can be required if a bleeding ectopic pregnancy is diagnosed. In this case, if it is really necessary to obtain abdominal views, the bladder should be filled using an indwelling catheter or by infusing about 1 L of IV fluids.
Patient Information Patient information is important regardless of the gynaecological or obstetric procedure plan. Informing the patient about the transabdominal or transvaginal sonography is essential. Patients usually have a general idea about ultrasound but we should not take this for granted. There are still countries and communities in which, due to mostly religious views, transvaginal scanning is not accepted and therefore not used. The kind of examination she is about to undergo should be explained to the patient in several short sentences. This information can be conveyed in three ways. The first, which is probably the best way, is at the time of the bimanual pelvic examination in the office of the gynaecologist or in the emergency room. At this time, the words ‘transabdominal’ and ‘transvaginal’ should be mentioned and explained. A second way to inform the patient is by the way of brochures available in several languages and placed in the waiting area, or given to the patient by the receptionist. The third and probably the worst way to explain the scanning to the patient is at the time of the scan itself, while she is seated on the examination table. At this time,
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ the patient does not have the opportunity to ask many questions or she is under stress and does not really remember the right questions to be asked. The similarity between the familiar vaginal speculum, or the Papanicolau test, and the vaginal transducer could be used as an effective comparison. The patient also should be reassured that the dimensions of the probe are small, and it is helpful to show her the probe. Some laboratories allow the patient to insert the probe into her own vagina. However, in general, this is not regulated in any way. Over the years, patient information regarding the use of the transvaginal probe will probably become redundant. Remember that several years ago, when transabdominal sonography was introduced, emphasis was placed on patient information, and today patients know enough about transabdominal sonography that can be used almost without any in-depth explanation. At times transvaginal scanning is contraindicated. Almost similar quality images can be obtained by using the transrectal approach. The vaginal probe is inserted in the rectum55 and after its insertion the same protocol as for a vaginal probe can be followed. The biggest ‘hurdle’ is to properly explain to the patient the harmless nature of such a scan.
The Examination Table It should be explained at the outset that any ultrasound examination can, and if necessary should, be carried out on any available examination table or even in the patient's bed. Certain additions, such as elevating the pelvis for a better transvaginal ultrasound examination, may be necessary. More and more gynaecological and obstetric pelvic scans, however, are performed with the patient on a gynaecological examination table. This is equipped with a footrest, which allows the patient to assume the lithotomy position for convenient transvaginal scanning, and a retractable leg support, which can be extracted for a better transabdominal sonographic evaluation of the patient. It is rather cumbersome and almost impossible to perform transvaginal ultrasound scanning on a flat table, unless an elevation below the pelvis is provided. Such an elevation will enable the backward tilt of the probe handle. The newer probes that have the ability to electronically or mechanically steer the scanning plane may be used even with a flat examination table. Some gynaecological examination tables are fitted with a swinging arm and a small platform that can support a small portable ultrasound box, similar to the swinging arm on the other side of the table supporting the colposcope (Fig. 3.2). Remember that, for specific reasons, such as a detailed examination of the cervix, the patient can be scanned, using the transvaginal probe, in a standing position.
Bimanual Pelvic Examination Preceding the Scan
36
Patients undergoing transvaginal or even transabdominal sonography usually have a pelvic exam that precedes the imaging procedure. If transvaginal ultrasonography is to be performed in the gynaecologist's office, a bimanual exam should definitely be
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performed before the scanning. If the patient is referred to the imaging laboratory, the gynaecologist should explain the palpatory finding for which the patient is referred for further imaging studies. The imaging laboratories rely on the information provided by the obstetrician-gynaecologist and are guided by these findings. These specialized ultrasound laboratories usually do not subject the patient to a routine pelvic examination. It is, however, a good idea to take the time to examine the patient before or even after the ultrasound examination to correlate the findings with the image obtained. Such a pelvic palpatory examination is even more important if a discrepancy between the image obtained and the image expected occurs. A basic distinction between the transvaginal ultrasound examination in the gynaecologist's office and that performed in the specialized laboratory must be made. In the gynaecologist's office, a bimanual examination is performed after taking the history of the patient. This examination usually guides the clinician as to the necessity of performing other laboratory tests. Among these laboratory tests, ultrasound may be considered. If ultrasound equipment is available in the office, the gynaecologist may proceed to complement and enhance his or her bimanual palpatory examination with this simple imaging technique. In this case, the bimanual pelvic examination and the transvaginal ultrasound complement each other to arrive at the clinical decision. If referral of the patient to a more sophisticated and usually remotely situated imaging laboratory is selected, the targeted pelvic ultrasound exam is performed by imaging specialists who must rely on the pelvic examination previously performed by the referring physician.
Scanning techniques in obstetrics and gynaecology
Fig. 3.2 A gynaecological table fitted with a swinging arm on which the small, portable ultrasound machine is placed. This can be used during examinations performed with the patient on the table.
Equipment The practical aspects of using the equipment are discussed below. The first step in scanning a patient is to prepare the equipment before the patient is examined. This preparation is even more crucial when a transvaginal scan is planned.
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38
The patient's demographic data, her last menstrual period and important pertinent observations should be entered. We found it useful to have the last menstrual period appear on the screen of the ultrasound machine next to the patient's name; thus it will be visible on each of the printed pictures, and it will therefore be easy to calculate the gestational age of a pregnancy or, in a gynaecological scan, the day of the patient's cycle. Recording devices should be switched to a stand-by position that enables their instantaneous operation. Ultrasound machines now enable recording onto CDs and even electronic data transmission over networks and the Internet. It is helpful to have a separate foot-switch which freezes the screen and even a second one to take hard copies of the desired images. This becomes important when transvaginal sonography is performed, because the operator needs both hands free, one to manipulate the transvaginal probe and the second to place on the abdominal wall and facilitate the location and mobilization of the pelvic structures. All operators should use gloves when scanning. They should at least put a glove on the hand handling the probe during the examination. This protects the users from a possible transmittable infection and reassures the patient as to a procedure performed under clean circumstances. Transabdominal probes are usually not covered with a probe cover (unless a sterile procedure is contemplated). However, they should be cleaned between patients. Probe cleaning is of the utmost importance. Both the transabdominal and transvaginal probe should be wiped, with gel first and then some means of probe cleanser can be applied. An alcohol spray or sponge or some other disinfectant (usually a quaternary alcohol compound) may be used to clean the probe. Ordinary bleach also can be used for this. However, it is important to ask the probe manufacturers about their preferred method of disinfecting the probe. Odwin et al provided guidelines to the different solutions and their efficacy in cleaning the probes.36 Local disinfection protocols should be followed carefully to minimize the spread of infection and the liability of the operators. A clean condom or the digit of a surgical rubber glove should cover the transvaginal probe. These must be clean but need not be sterile. Ultrasound coupling gel should be placed inside the protective cover to enable smooth passage of the sound waves from the transducer to the pelvic organs. A ready-to-use, prelubricated, individually prepackaged, thin plastic transvaginal probe cover is available for use. This cover is particularly useful if the patient is allergic to latex. Latex allergy is more widespread than believed, therefore it is a good routine to ask patients about such allergies before using a latex product at scanning. It is also a good idea to post a sign in the waiting area which asks the patients to report any previously known latex allergy before the actual scanning. Coupling gel should be applied to the tip of the probe before its insertion into the vagina. Coupling gels seem to be important to generate a clear and clinically meaningful ultrasound picture on the screen. K-Y gel (Johnson & Johnson, Skillman, NJ) or the ultrasound coupling gel, which is basically clean to begin with, can be used for this purpose. Mineral oil is a good inexpensive coupling agent if the more expensive gels are not available. However, infertility patients
✩✩✩✩✩✩✩✩✩✩✩ ✩ approaching their midcycle should be scanned using normal saline because coupling gels may be detrimental for sperm motility and viability.
Orientation using transabdominal probes is simple. On the monitor or any hardcopy picture, it is sufficient to annotate which is the patient's right or left side. This is much like the conventional orientation used in radiology. On a longitudinal, sagittal transabdominal scan the direction toward the patient's head (cephalad or superior) is displayed usually on the left side and the direction toward the patient's feet (caudad or inferior) is displayed on the right side of the monitor or picture. Anterior (or ventral) points upward and posterior (or dorsal) points downward on the monitor or the picture (see Fig. 3.1). In transvaginal sonography, the orientation is entirely different. The images created are oriented perpendicularly (rotated 90 ° counterclockwise), compared to those generated by the transabdominal probe. Sonologists from around the world display sonographic pictures according to different rules.4 Displaying of the sonographic image requires some explanation, because it becomes important to understand the onscreen image orientation. The sonographic picture generated by transvaginal sonography can be displayed with the apex of the ‘pie’ pointing upward (Fig. 3.3A) or downward (Fig. 3.3B). Some countries, as well as individuals, believe that displaying images with the apex pointing down seems more logical. However, at this time, a uniform worldwide display is highly unlikely to occur. Some sonologists think that displaying a transvaginal picture with the apex of the ‘pie’ pointing to the bottom and a transabdominal picture with the apex pointing to the top would enable a distinction regarding the scanning routine. For example, in Germany (and in a number of other countries), one could distinguish between a transvaginal and a transabdominal picture by looking at the picture orientation. It is important and also useful to introduce some standardization regarding this issue.58 In the United States and many other countries, the images are displayed as follows.
Scanning techniques in obstetrics and gynaecology
Orientation
• If a fetus is scanned, the left and right sides will be determined according to
the position of the fetal stomach and the fetal heart. • In the case of a gynaecological scan, on the longitudinal plane, the bladder appears on the upper left side of the screen with the external cervical os pointing toward the right. If the uterus is anteverted, the fundus appears on the same side as the urinary bladder (Fig. 3.3A). If the fundus in a retroverted uterus points toward the opposite side of the bladder, the retroversion can be ascertained (Fig. 3.3C). • In the cross-section of the pelvis in the transverse (coronal) plane, the patient's right side will be seen on the left side of the monitor (or picture) and the left side of the patient on the right side of the monitor (or picture), as on any radiographic image performed using the coronal plane (Fig. 3.3A).
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40
Fig. 3.3 Orientation in the pelvis using the transvaginal probe. (A) The uterus is displayed with the apex of the ‘pie’ pointing upward. The general directions within the body are marked on the pictures. The left image is the picture of the uterus in the sagittal plane, whereas that on the right side depicts the transverse section. (B) The uterus is displayed with the apex of the ‘pie’ pointing downward (the European approach). (C) A retroverted uterus and its relationship with the bladder. The major directions in the pelvis are identical with the sagittal section of the uterus in Fig 3.3A.
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• If the cervix is scanned usually in the sagittal plane, it is customary that in
– On the coronal planes, the fetal right side should be kept on the left side of the picture and the fetal left side on the right side of the image. This is similar to any coronal x-ray picture of the head (Fig. 3.5A). – On the sagittal planes it is customary to orient the forehead (anterior or frontal) to the left and the occiput (posterior or occipital) to the right side of the picture (Fig. 3.5B). – Using TVS, a series of coronal and sagittal brain sections can be obtained for better definition of eventual pathology.30,56 Some authors recommend the use of transverse and anteroposterior planes only if transvaginal sonography is used.12 A somewhat better approach is to use the relative position of the target organ within the pelvis, which does not necessarily match the cardinal and classic anatomical planes of the pelvis. This scanning method is called ‘organ-oriented scanning’ (Fig. 3.6).38,61 It is practical to refer to the longitudinal axis of the scanned organ, such as the fallopian tube or the ovary (e.g. ‘the longitudinal image of the tube’, etc.), instead of using the conventional orientation of the scanning planes with the known pelvic co-ordinates. Lately the issue of orientation using three-dimensional (3D) ultrasound has surfaced. Interestingly, the major textbooks on the subject of 3D ultrasound in obstetrics and gynaecology have not discussed adequately, or at all, orientation in the acquired volume. It should be stressed that this orientation, mainly in the reconstructed volumes, is a real but surmountable problem. Some machines have a convenient
Fig. 3.4 Sagittal sections of the cervix with a cervical suture. Note that by convention the external os of the cervix with an anteverted uterus is oriented towards the right of the picture.
Scanning techniques in obstetrics and gynaecology
an anteverted pregnant uterus the external os is pointing towards the right side of the picture, while the internal os is on the left side of the image (Fig. 3.4). Of course, the bladder is kept constantly on the upper left side of the picture. In the case of a retroverted uterus the bladder is kept on the upper left side of the picture while the external and internal os are oriented respectively to the left and right side of the picture. • If the fetal brain is scanned, a careful assessment of the left and right side is necessary regardless of TAS or TVS.
41
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Fig. 3.5 Orientation in scanning the fetal brain regardless of the scanning route. (A) The coronal plane. (B) The sagittal plane. Note that the face is on the left side.
labelling protocol which enables the user to label the major directions (right, left, cranial, caudal, anterior, posterior) at the onset of scanning and incorporate it automatically in the displayed images (Medison, Kretz and General Electric systems). These, of course, make orientation in such 3D volumes easier. If, however, such orientation display is not available, the operator should use extreme care to correctly label the image generated. This may avoid later confusion and medical liability. The most important message regarding orientation is the fact that it is mandatory to label all images at all times to be able to correctly indicate left and right, cranial and caudal, sagittal, coronal and axial (horizontal) sections (Fig. 3.7).
Scanning Routine Regardless of the route selected to perform the scan, a methodical and systematic scanning routine should be followed. The following order was found to be helpful.
42
Fig. 3.6 The concept of ‘organ-oriented’ scanning in the female pelvis. (A) The tube (chronic hydrosalpinx) is imaged to display its longest diameter. This is not necessarily in any fixed plane of the pelvis. It is obtained by trial and error. This is correct also if the ovary is scanned. (B) Ninety degrees to any plane that detects the longest measurement of the pelvic organ (in this case an acute salpingitis with the ‘cogwheel sign’) will display the cross-section of that organ.
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Obstetric Scanning Different countries use different protocols for the structures to be included in obstetric scans. These requirements should be kept in mind when performing ultrasound examinations of the fetus. It is customary to divide the scanning routine in obstetrics as follows: firsttrimester scan, basic exam and comprehensive fetal exam. When performing the first-trimester scan (usually between 11 and 14 postmenstrual weeks), a transabdominal or transvaginal probe may be used.1,9,13,29,34,42-44,51,60 The following information should be obtained.
Scanning techniques in obstetrics and gynaecology
Fig. 3.7 Three-dimensional image of the uterus. The upper left image is in the sagittal plane, the upper right in the axial plane, the lower left in the coronal plane. The lower right image is the rendering of the uterine cavity in the coronal plane which is almost never achieved using two-dimensional transvaginal scanning.
• Presence or absence of an intrauterine gestational sac • Identification of embryo or fetus • Yolk sac • Fetal number • Presence or absence of fetal cardiac activity • Crown–rump length (CRL) • Evaluation of uterus and adnexal structures • Evaluation and measurement of the nuchal translucency. I f any obvious anomaly is seen, which may be the case if high-resolution equipment is used, this should trigger a more intensive scan and obviously a follow-up scan. The basic fetal exam should provide the following information.
• Fetal number • Fetal presentation • Documentation of fetal life • Placental location • Assessment of amniotic fluid volume • Assessment of gestational age • Survey of fetal anatomy for gross malformations • Evaluation of the ovaries and possible maternal pelvic masses.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ This is primarily a biometric examination. Nonetheless, a brief survey of fetal anatomy and maternal pelvic organs should be performed. Some major structural malformations of the fetus may be identified during basic examinations, and some basic examinations may suggest the need for a more comprehensive survey.13,29,34,60 In certain circumstances, a ‘limited’ ultrasound examination may be appropriate and desirable. Such circumstances commonly relate to the specific nature of the information required or the urgent nature of the clinical situation. A limited examination may be useful to collect information such as the following.
• Assessment of amniotic fluid volume – amniotic fluid index (AFI) • Fetal biophysical profile (BPP) testing • Ultrasonography-guided amniocentesis, chorionic villus sampling (CVS) • Nuchal translucency measurement9,34,44 • Perumbilical blood sampling (PUBS) • External cephalic version • Confirmation of fetal life or death • Localization of placenta in antepartum haemorrhage • Confirmation of fetal presentation. A comprehensive ultrasound examination may be indicated for a patient who is suspected of carrying a physiologically or anatomically defective fetus by history, clinical evaluation or prior ultrasound examination. A limited examination, as defined above, may be performed by ultrasonographers or specially trained personnel. The basic examination, however, should be performed or reviewed by an appropriately trained operator. An operator with experience and expertise in such scanning should perform the comprehensive examination. In some situations, it may not be possible to perform a full fetal survey. These include:
• oligohydramnios • hyperflexed position of the fetus • engagement of the head • compression of some fetal parts • maternal obesity. Biophysical profile Biophysical profile testing consists of a non-stress test with the addition of four observations made by real-time ultrasound, each receiving a score of two. The five components are as follows.
• Reactive non-stress test. • Fetal breathing movements (one or more episodes of rhythmic fetal breathing movements of 30 seconds or more within 30 minutes).
• Fetal movement (three or more discrete body or limb movements within 44
30 minutes). • Fetal tone (one or more episodes of extension of a fetal extremity with return to flexion).
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• Quantitation of amniotic fluid volume. There is no universal agreement
With this method, a score of 2 (normal) or 0 (abnormal) is assigned to each of the five observations. A score of 8–10 is normal; a score of 6 is considered equivocal (a fetus should be retested in 12–24 hours) and a score of 5 or less is abnormal. In the presence of oligohydramnios, further evaluation may be warranted.26,39 See also Chapter 7.
Gynaecological Scanning If transabdominal sonography is performed, the sonographer may select other target areas for the scanning, such as looking for free fluid in the abdominal cavity, in Morrison's pouch, along the right axial line or below the liver, or scanning the patient's kidneys. It should be stressed that adequate training should preclude scanning non-gynaecological structures. It is important to use the largest possible magnification, which enables orientation as well as recognition of organs and their pathologies. Magnification usually does not alter the resolution of high-frequency probes. The following routine has proven to be effective.2
Scanning techniques in obstetrics and gynaecology
as to the optimal method of assessing amniotic fluid volume. Some investigators consider the detection of a single pocket of amniotic fluid exceeding 2 cm in two perpendicular planes to be adequate. A semiquantitative, four-quadrant assessment of amniotic fluid depth (AFI) is widely used, and cross-sectional nomograms have been developed.32,39 Ideal cut-off levels for intervention using the AFI have yet to be established.
The uterus When evaluating a suspected uterine mass, the practitioner should identify the appropriate anatomical structures. The initial step is to identify the bladder anteriorly and the rectosigmoid posteriorly. The position of the uterus depends on the distension of the bladder and rectosigmoid, masses that may be present extrinsic to the uterus, and intrinsic uterine masses. The normal uterus appears sonographically as a uniform structure. By resting a hand on the abdomen and using the intermittent pressure of a transvaginal probe, the practitioner can determine the mobility of the uterus, the ovaries or any pelvic structure. This sliding movement of the organs can be related to each other or the stationary pelvic floor (‘sliding organs sign’).57 The origin of structures (e.g. ovary versus a pedunculated fibroid) or adhesions can be diagnosed using this manoeuvre. Testing for pain is also possible, with the vaginal probe identifying the touched structure in question on the screen. Lately 3D ultrasound became the most informative and powerful technique to image the uterus. Its main strength is that along with the sagittal and transverse planes, the coronal plane can be displayed. The cervix Scanning the uterine cervix is an integral part of the gynaecological as well as the obstetric ultrasound examination. A wide variety of pathologies ranging from benign or prevalent Nabothian (inclusion) cysts to cervical fibroids or the rare
45
Ultrasound in obstetrics and gynaecology
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46
cervical pregnancy can be identified. The importance of transvaginal ultrasound scanning of the cervix has increased in recent years as it has been found to be predictive of preterm deliveries. Usually the closed cervical canal length is measured. If funnelling is seen the funnel length and width can be measured. Cervical sutures can and should also be evaluated periodically. The myometrium The sonographic appearance of the myometrium and the arcuate vessels within the myometrium should be noted. Leiomyomata tend to be discrete, multiple, spherical masses of varying size. They can be found almost entirely within the endometrial cavity (submucosal) (Fig. 3.8A–C), within the myometrium (intramural) or on the surface of the uterus (subserosal). Ultrasonographically, a leiomyoma often appears hypoechogenic. However, its appearance may vary depending on its location and whether it has undergone internal changes, such as hyaline degeneration, fatty degeneration, calcification or haemorrhagic necrosis. These changes will alter the sonographic appearance of the leiomyoma; for example, the presence of calcium will result in an increase in echogenicity, whereas degeneration will produce a cyst-like structure. Submucosal leiomyomata may give the appearance of a bulge in the endometrial lining. A more detailed investigation of this sign is warranted. This can be accomplished by instilling normal saline via a thin catheter placed in the uterine cavity. The saline will serve as a contrast medium and will outline the mass (Fig. 3.8D,E). Serial ultrasonography can be used to determine whether the leiomyomata are growing or shrinking. This can be especially useful in patients entering menopause. When the uterus of a reproductive-age woman with leiomyomata is evaluated,
Fig. 3.8 Examples of submucous fibroids enhanced by saline infusion sonohysterography. (A,B) Almost entirely intracavitary submucous myoma. (C) It is possible to study the Doppler signal of the feeding vessel to the fibroid. (D,E) Partially submucous myoma bulging into the cavity with approximately 30–40% of its volume.
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The endometrium Sonographically, the interface of the two endometrial surfaces appears as a thin, echogenic line that can be evaluated throughout the menstrual cycle. The endometrium varies in thickness and appearance depending on the stage of the menstrual cycle or the use of exogenous hormones. Measurement of the endometrial thickness should be done on the long axis, with a combined anterior–posterior wall measurement. If fluid is found in the uterine cavity, the measurement should exclude that fluid interface (Fig. 3.9). In postmenopausal women with bleeding, studies indicate that when there is a thin distinct endometrial echo less than 4–5 mm maximum anteroposterior thickness read from a long axis view, this finding is consistently associated with lack of significant tissue on sampling. Thus, such patients may be able to avoid invasive sampling and its risks, expense and discomfort. Presence of an endometrial echo greater than 5 mm is not compatible with atrophy and thus, depending on hormonal status, may indicate the need for sampling. Saline infusion sonohysterography can be used to distinguish symmetrically thickened endometrium in
Fig. 3.9 The technique of measuring endometrial thickness in the presence of intracavitary fluid.
Scanning techniques in obstetrics and gynaecology
the practitioner should be alert to the possibility of a small (4–6 weeks of gestation size) chorionic sac. These small gestations may be difficult to detect and may be found in odd locations. Adenomyosis is diagnosed by noting the presence of endometrial tissue in the stroma of the myometrium. Although this condition may be suspected by the presence of small sonolucent areas and linear shadowing within the myometrium, it cannot be confirmed only on that basis. Usually the anterior or posterior wall containing the adenomyosis is thicker than the other wall. The diagnosis rests on clinical parameters and histological confirmation.
47
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48
which the process is global from endometrial changes that may be focal. In the former, blind sampling is appropriate, whereas the latter requires hysteroscopically directed evaluation. It is to be hoped that current research in 3D techniques, particularly in volume rendering, may prove to be an added source of information distinguishing benign from malignant pathology. Finally 3D techniques are proven to be useful in diagnosing the different degrees of uterine malformations by displaying the contour of the fundus and the cavity at the same time (Fig. 3.10).31 It is also important to examine carefully the entire length of the myometrial– endometrial interface. If the endometrium is irregular or if there is an enlarged area of echogenicity, endometrial pathology should be suspected. An endometrial biopsy or dilation and curettage should be performed to determine the histological status of the endometrium. The myometrial–endometrial interface should be evaluated by continuously shifting the transducer through its long axis and corresponding coronal planes. Increasing attention is being given to the presence of heterogeneous central uterine changes in women who receive tamoxifen for breast cancer. In some such cases, changes originally interpreted as endometrial are actually in the proximal myometrium. Sonohysterography may be used to determine the location (endometrial vs proximal myometrial) of such heterogeneous echoes. During the follicular phase, the endometrium is thin, with a ‘pencil-line’ echo of the cavity and hypoechoic functional endometrium on both sides of the cavity line (three-line sign). This phase is followed by gradual thickening, which reaches its peak immediately prior to ovulation. Following ovulation, coincidental with the rise in progesterone, the echogenicity of the endometrium on both sides of the cavity line increases and equals that of the cavity line, which gradually disappears within the hyperechoic endometrium. This hyperechoic endometrium is then ‘broken down’ at the time of the menstrual flow. The endometrium can be
Fig. 3.10 Three-dimensional rendering of the uterus in the coronal plane. Note the clear contours of the fundus (arrows) and the septated uterus.
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Scanning techniques in obstetrics and gynaecology
measured throughout the first part of the cycle. The endometrium can serve as a natural contrast material in the uterus, leading to better definition of the endometrial–myometrial interface and detection of polyps or submucous leiomyomata, or both. Patients with irregular uterine bleeding may have an endometrial polyp, submucosal myoma or adenomatous hyperplasia. Polyps can occur in patients of any age but tend to be more common in perimenopausal women. Polyps are usually seen as a prominent endometrial echo complex; rarely, discrete masses occupying the endometrial cavity are found. Sonohysterographic fluid enhancement through a thin intrauterine catheter may improve diagnostic capability.18 The branching appearance of the feeding blood vessel can be detected by turning on the colour Doppler feature. Although neither transvaginal nor transabdominal ultrasound evaluation can confirm the presence or absence of cancer of the endometrium, ultrasonography can provide information to aid in diagnosis. In early stages, endometrial carcinoma can appear as a change in the thickness of the endometrial lining and in the endometrial echogenicity. Advanced endometrial or cervical carcinoma may appear as hydrometra, pyometra or haematometra. These conditions will appear sonographically as fluid collection within the uterine cavity. The endometrial– myometrial interface should be defined and monitored to detect pathology at that level. Other conditions that may be detected by ultrasound examination of the endometrium are Asherman syndrome and retained products of conception following spontaneous abortion, therapeutic abortion or delivery. A diagnosis of Asherman syndrome can be strengthened by the presence of an irregular echogenic picture and, occasionally, by the finding of calcification. On ultrasonography, calcification is intensely echogenic and causes acoustic shadowing. The diagnosis can be firmly established by hysteroscopy. Retained products usually can be detected if an irregularly shaped, dilated endometrial cavity containing echogenic material is noted. Asherman syndrome can best be distinguished from retained products of conception by evaluating the patient's history. Uterine anomalies including septae, bicornuate uteri and didelphys may be identified especially when using 3D ultrasonography using ‘thick-slice’ or inversion rendering. Sonohysterography may be useful to measure a fundal septum prior to hysteroscopic resection when habitual abortion is present.
Adnexal Masses Ultrasonographic examination of the adnexa encompasses evaluation of the ovaries, fallopian tubes and parametrial areas. It is important that the examiner be familiar with other anatomical structures in this area, such as the external and internal iliac artery and vein, ureter and bowel. The ovaries usually lie in the ovarian fossa found along the lateral pelvic wall. The ovary can be located by identifying a pulsating linear echo; superior to this is the external iliac artery and posterior and inferior to this the ureter.
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ The normal ovary of teenagers and young adults measures approximately 2 × 2 × 3 cm. The size of the ovary should be measured according to the largest diameter in the three planes. Some investigators have recommended determining ovarian volume, using the formula (length × width × height)/2. The ovarian volume in teenagers and young adults can reach 14 cm3. In postmenopausal women, the average ovarian volume is 2.5 cm3 or less. The following aspects of an adnexal mass should be evaluated. – the mass should be moved by the vaginal probe or by the hand • Mobility
of the operator that is resting on the abdomen (‘sliding organs’ sign).57 – its location should be established by watching the on-screen picture • Pain when touching different organs with the tip of the transvaginal probe. structure – features of an ovarian mass, such as thickness and outer • Wall and inner surface irregularities and papillae, should be described and measured. • Septations – the thickness of the septations should be reported. of the mass – the mass can be completely sonolucent and may • Echogenicity have low-level echogenic contents, may be with or without an echogenic core, may have mixed echogenicity containing all of these components or may be completely echogenic. The presence of the following conditions may make it more difficult to detect ovarian or adnexal masses with ultrasonography.
• Fluid-filled loop of bowel • Faeces in loop of bowel • Closed-loop bowel obstruction • Artifact of multipath reflection of sound waves (stratified echo pattern
resulting from echoes bouncing back and forth) from fluid-filled structure (e.g. bladder) • Mesenteric cysts • Peritoneal inclusion cysts (postoperative or after infections) • Nabothian cysts • Hydrosalpinges (acute and chronic) • Large fibroids.
50
The clinical findings of acute salpingitis may be strengthened by ultrasonographic findings of tubo-ovarian complexes of a fluid-containing structure with thickened walls sensitive to the touch of the probe, adnexa adherent to loop of bowels, or collection of fluid in the cul-de-sac. Chronic salpingitis can be diagnosed on the basis of a painless (to the touch of the probe), thin-walled, pear-shaped, fluidfilled adnexal structure. Abscesses can be detected by ultrasonography, which can also be used to characterize the abscess as unilocular or multilocular and determine the thickness of the abscess wall and anatomical location. This information should be integrated into clinical findings (e.g. pain, fever) and is helpful in determining whether the abscess may be drained percutaneously or transvaginally or whether surgical intervention is required.54
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Peritoneal Fluid Scanning techniques in obstetrics and gynaecology
Small amounts of fluid in the lower pelvis can be visualized with ultrasonography. The fluid should be examined for the presence or absence of floating debris, which will appear as low-level echoes. The tip of the probe can be used to rock the fluid slightly, thus aiding in the observation of floating particles. If the nature of the fluid must be determined, culdocentesis can be accomplished with the aid of transvaginal sonography, which offers the best guidance for needle placement through the needle guide mated to the shaft of the probe. Attempts to assess the quantity of pelvic fluid have been reported in the literature. The smallest amount of fluid that can be detected is about 20–30 mL if a 5 MHz transvaginal probe is used. Although experienced sonographers can estimate the approximate amount, this should be done with extreme caution. Figure 3.11 demonstrates how the approximate amount of free or loculated pelvic fluid collection can be estimated using perpendicular scanning planes. If a larger amount of abdominal or pelvic fluid is suspected, the space between the liver and the right kidney (the Morrison pouch) should be examined. This can be achieved by placing an abdominal transducer parallel to the sagittal plane and overlying the right upper abdomen. One should distinguish between free fluid in the pelvis and loculated fluid. The loculated fluid is found usually as a consequence of pelvic surgery or an inflammatory process. It is characterized by flimsy or denser adhesions creating the pseudoseptations in the fluid. The wall of the pseudocyst is the pelvic wall itself.
Urinary Bladder Ultrasonography can be used to examine the bladder for the presence of extrinsic or intrinsic pathological masses. The urethra and the bladder can be viewed on the sagittal and on an extremely anteriorly directed coronal plane. A transverse scan through the superior portion of the bladder reveals the bladder to be rounded. If the bladder is scanned inferiorly, it will appear square, whereas a longitudinal scan will make it appear triangular. The thickness of the bladder and the
Fig. 3.11 The technique of estimating the almost free (or loculated) pelvic fluid. The formula of the ovoid is used (a × b × c) 0.523 = mL.
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Fig. 3.12 Using transvaginal colour Doppler, the ureteral jets of the left (A) and right (B) ureter can be studied.
presence of polyps or bladder stones will be outlined by the sonolucent urine. A scan performed at the base of the bladder, just proximal to the urethrovesical junction, will permit visualization of the urethral orifices. Ultrasonography may be used to estimate the volume of postvoid residual and, in incontinent women, the mobility of the urethrovesical junction. Observing the urinary jets arising from the two ostia by using grey-scale or colour Doppler, it is possible to determine ureteral patency (Fig. 3.12).52,53
Other Findings Other pathologic processes that can affect organs in the lower pelvis can also be detected. The most prevalent bowel diseases that can be observed are diverticulosis and various degrees of dilation of the small bowel. Dilation of the bowel that can be mistaken for cystic structures can often be differentiated by the presence of peristalsis. Ectopic or low-lying horseshoe kidneys can also be detected by sonography. Transabdominal sonography can be used to identify appendicitis; however, considerable experience is required to do so.
Colour Doppler Studies An increasing number of laboratories are now offering colour flow-directed measurements such as pulsatility and resistance indices as well as flow velocities.2,13,26,29,32,39,57,60 Some colour flow studies are still considered investigational in the USA. The pulsatility index is calculated by the following formula: Systolic Velocity − Diastolic Velocity Mean Velocity The resistance index is calculated by the following formula: 52
Systolic Velocity − Diastolic Velocity Systolic Velocity
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Screening for Ovarian Masses
Scanning techniques in obstetrics and gynaecology
Subsequent chapters will discuss the technique and clinical use of colour Doppler and power Doppler measurements. Preliminary results raise the possibility of detecting increased vascularity and new vessel formation in cases of malignant ovarian masses. In general, vessels in malignant tumours lack the muscle layer and have lower impedance. However, increased flow may also be seen with pelvic inflammatory processes. The presence of a corpus luteum not only may be misleading in the structural evaluation of an adnexal mass since the flow measurement values overlap with those found in ovarian cancer. The corpus luteum is known to have new vessel formation, which lowers the resistance to flow while present. Recent studies have suggested a possible role for colour Doppler in the diagnosis of adnexal torsion.15,23 These studies suggest that, at the site of the torsion, the diameter of the vessels proximal to the occlusion is increased; the disruption of flow is identified on colour Doppler. Within the twisted adnexa, there is significantly diminished flow or no flow at all. The cost–benefit ratio of colour flow studies is still under investigation, and the value of such studies is as yet not fully determined. Moreover, the technique requires a great deal of training, and measurement remains a subjective process.
Transvaginal sonography, with or without colour flow-directed measurements of resistance to flow and flow velocities, has been suggested as a means of screening for ovarian cancer.5,7,24 Although transvaginal sonography is probably the best means of determining the morphological structure of adnexal masses, its efficacy in screening for ovarian cancer has not been adequately established. Both modalities – transvaginal sonography and colour flow-directed measurements – are experimental for these uses.11,49,59 Several studies are under way to determine whether transvaginal sonography and colour Doppler in conjunction with biological markers (proteins) can be used to screen a selected population at high risk for ovarian cancer. Other studies are being done to examine the feasibility of using transvaginal sonography as a first-line modality for screening. Doubts about the value of colour Doppler in the diagnosis of ovarian masses have also been expressed.45-47
Transperineal and Transrectal Scanning This chapter would not be complete without mentioning other scanning routes. The transperineal route is also called translabial scanning. It is mainly used if transvaginal scanning is not possible (no transvaginal transducer is available or a contraindication prevents its use).21,41 A linear or curvilinear transducer is inserted in a glove and applied to the vulvar area in a sagittal fashion. The authors' experience is that there are very few contradictions to the use of transvaginal probes in favour of a transperineal scan. Even in the case of premature
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Ultrasound in obstetrics and gynaecology
✩ ✩✩✩✩✩✩✩✩✩✩✩ rupture of the membranes, it was found that one transvaginal scan can be more useful than one digital examination to predict premature delivery.19,37 In cases where transvaginal scanning is not feasible or is contradicted, transrectal scanning can be used.55
Ultrasound-Guided Puncture Procedures There are two kinds of ultrasound-guided puncture procedures: those guided by a transabdominal transducer and performed transabdominally and those guided by a transvaginal transducer and performed transvaginally. Transabdominal puncture procedures can be done using the ‘free hand’ method or a fixed needle guide. The former requires some degree of experience and good eye–hand co-ordination. Transvaginal puncture procedures should always be performed using a fixed needle guide which is ‘mated’ to the shaft of the transvaginal probe.
Conclusion The technique and clinical aspects of transabdominal and transvaginal ultrasound have been discussed. Those who intend to perform hands-on scanning of obstetric and gynaecological patients should familiarize themselves with the described techniques. In addition, the more specific and detailed texts and published articles should be read. Based upon our experience, the evolution of understanding in this imaging specialty is closely related to advances in the fields of electronics, acoustics and computer sciences as well as to the ability to miniaturize most components of the ultrasound equipment. References
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1. American College of Obstetricians and Gynecologists 1993 ACOG technical bulletin no. 187. American College of Obstetricians and Gynecologists, Washington, DC 2. American College of Obstetricians and Gynecologists 1995 ACOG technical bulletin no. 215. American College of Obstetricians and Gynecologists, Washington, DC 3. Benacerraf BR, Ship TD, Bromley B. Is a full bladder still necessary for pelvic sonography? J Ultrasound Med 2000;19:237–241 4. Bernaschek G, Deutinger J. Current status of vaginosonography – a world-wide inquiry. Ultrasound Obstet Gynecol 1992;2:352–356 5. Bourne TH, Hampson J, Reynolds K, Collins WP, Campbell S. Screening for early ovarian cancer. Br J Hosp Med 1992;48:454–459
6. Callen PW. Ultrasonography in obstetrics and gynecology, 4th edn. WB Saunders, Philadelphia, 2000 7. Campbell S, Bourne T, Bradley E. Screening for ovarian cancer by transvaginal sonography and colour Doppler. Eur J Obstet Gynecol Reprod Biol 1993;49:33–34 8. Chervenak FA, Isaacson GC, Campbell S. Ultrasound in obstetrics and gynecology. Little, Brown, Boston, 1993 9. Cicero S, Sacchini C, Rembouskos G, Nicolaides KH. Sonographic markers of fetal aneuploidy – a review. Placenta 2003;24 (suppl B):S88–98 10. Coleman BG, Arger PH, Grumbach K et al. TVS and TAS sonography: prospective comparison. Radiology 1988;168:639–643 11. Daskalakis G, Kalmantis K, Skartados N et al. Assessment of ovarian tumors using
✩✩✩✩✩✩✩✩✩✩✩ ✩ 25. Lavery MJ, Benson CB. Transvaginal versus transabdominal ultrasound. In: TimorTritsch IE, Rottem S (eds) Transvaginal sonography, 2nd edn. Chapman and Hall, New York, 1991 26. Manning FA, Harman CR, Morrison I, Menticoglou SM, Lange IR, Johnson JM. Fetal assessment based on fetal biophysical profile scoring. IV. An analysis of perinatal morbidity and mortality. Am J Obstet Gynecol 1990;162:703–709 27. Mendelson EB, Bohm-Velez M, Joseph N, Neiman HL. Gynecologic imaging: comparison of TAS and TVS sonography. Radiology 1988;166:321–324 28. Merz E. Three-dimensional ultrasound in obstetrics and gynecology. Lippincott Williams and Wilkins, Philadelphia, 1998 29. Michailidis GD, Papageorgiou P, Economides DL. Assessment of fetal anatomy in the first trimester using twoand three-dimensional ultrasound. Br J Radiol 2002;75:215–219 30. Monteagudo A, Reuss ML, Timor-Tritsch IE. Imaging the fetal brain in the second and third trimester using transvaginal sonography. Obstet Gynecol 1991;77:27–32 31. Monteagudo A, Timor-Tritsch IE. First trimester anatomy scan: pushing the limits. What can we see now? Current Opin Obstet Gynecol 2003;15:131–141 32. Moore TR. Superiority of the four-quadrant sum over the single-deepest-pocket technique in ultrasonographic identification of abnormal amniotic fluid volumes. Am J Obstet Gynecol 1990;163:762–767 33. Nelson TR, Downey DB, Pretorius DH, Feuster A. Three-dimensional ultrasound. Lippincott Williams and Wilkins, Philadelphia, 1999 34. Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. Am J Obstet Gynecol 2004;191:45–67 35. Nyberg DA, Hill LM, Bohm-Velez M, Mendelson EB. Transvaginal ultrasound. Mosby-Yearbook, St Louis, MO, 1992 36. Odwin CS, Fleischer AC, Kepple DM. Probe covers and disinfectants for transvaginal transducers. J Diagn Med Sonogr 1990;6:130–135 37. Rizzo G, Capponi A, Angelini E, Vlachopoulou A, Grassi C, Romanini C. The value of transvaginal ultrasonographic examination of the uterine cervix in predicting preterm delivery in patients with preterm premature rupture of membranes. Ultrasound Obstet Gynecol 1998;11:23–29
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transvaginal color Doppler ultrasonography. Eur J Gynaecol Oncol 2004;25:594–596 12. Dodson MG, Deter RL. Definition of anatomical planes for use in transvaginal sonography. J Clin Ultrasound 1990;18: 239–242 13. Economides DL, Whitlow BJ, Braithwaite JM. Ultrasonography in the detection of fetal anomalies in early pregnancy. Br J Obstet Gynaecol 1999;106:516–523 14. Fleischer AC, Romero R, Manning FA et al. The principle and the practice of ultrasonography in obstetrics and gynecology, 5th edn. Appleton and Lange, Stamford, CT, 1996 15. Fleischer AC, Stein SM, Cullinan JA, Warner MA. Color Doppler sonography of adnexal torsion. J Ultrasound Med 1995;14:523–528 16. Goldstein SR, Timor-Tritsch IE (eds). Ultrasound in gynecology. Churchill Livingstone, New York, 1995 17. Goldstein SR. Endovaginal sonography, 2nd edn. Wiley-Liss, New York, 1991 18. Goldstein SR. Use of ultrasonohysterography for triage of perimenopausal patients with unexplained uterine bleed. Am J Obstet Gynecol 1994;170:565–570 19. Gomez R, Galasso M, Romero R et al. Ultrasonographic examination of the uterine cervix is better than cervical digital examination as a predictor of the likelihood of premature delivery in patients with preterm labor and intact membranes. Am J Obstet Gynecol 1994;171(4):956–964 20. Hendrick WR, Hykes DL, Starchman DE. Ultrasound physics and instrumentation, 3rd edn. Mosby, St Louis, MO, 1995 21. Hertzberg BS, Bowie JD, Weber TM, Carroll BA, Kliewer MA, Jordan SG. Sonography of the cervix during the third trimester of pregnancy: value of the transperineal approach. Am J Roentgenol 1991;157: 73–76 22. Kossoff G, Griffith KA, Dixon CE. Is the quality of transvaginal images superior to transabdominal ones under matched conditions? Ultrasound Obstet Gynecol 1991;1:29–35 23. Kupesic S, Aksamija A, Vucic N, Tripalo A, Kurjak A. Ultrasonography in acute pelvic pain. Acta Med Croatica 2002;56:171–180 24. Kurjak A, Shalan H, Matijevic R, Predanic M, Kupesic-Urek S. Stage I ovarian cancer by transvaginal color Doppler sonography: a report of 18 cases. Ultrasound Obstet Gynecol 1993;3:195–198
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38. Rottem S, Thaler I, Goldstein SR, TimorTritsch IE, Brandes JM. Transvaginal sonographic technique: targeted organ scanning without resorting to ‘planes.’ J Clin Ultrasound 1999;18:243–247 39. Rutherford SE, Phelan JP, Smith CV, Jacobs N. The four-quadrant assessment of amniotic fluid volume: an adjunct to antepartum fetal heart rate testing. Obstet Gynecol 1987;70:353–356 40. Sabbagha RE. Diagnostic ultrasound applied to obstetrics and gynecology, 3rd edn. Lippincott, Philadelphia, 1994 41. Scanlan KA, Pozniak MA, Fagerholm M, Shapiro S. Value of transperineal sonography in the assessment of vaginal atresia. Am J Roentgenol 1990;154:545–548 42. Souka AP, Nicolaides KH. Diagnosis of fetal abnormalities at the 10–14-week scan. Ultrasound Obstet Gynecol 1997;10: 429–442 43. Souka AP, Pilalis A, Kavalakis I et al. Assessment of fetal anatomy at the 11–14week ultrasound examination. Ultrasound Obstet Gynecol 2004;24:730–734 44. Spencer K, Nicolaides KH. Screening for trisomy 21 in twins using first trimester ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years experience. Br J Obstet Gynaecol 2003;110:276–280 45. Tekay A, Jouppila P. Blood flow in benign ovarian tumors and normal ovaries during the follicular phase. Obstet Gynecol 1995;86:55–59 46. Tekay A, Jouppila P. Controversies in assessment of ovarian tumors with transvaginal color Doppler ultrasound. Acta Obstet Gynecol Scand 1996;75:316–329 47. Tekay A, Jouppila P. Intraobserver variation in transvaginal Doppler blood flow measurements in benign ovarian tumors. Ultrasound Obstet Gynecol 1997;9: 120–124 48. Tessler F, Schiller VL, Perrella RR et al. TAS versus endovaginal pelvic sonography: prospective study. Radiology 1980;170: 553–556 49. Timmerman D, Valentin L, Bourne TH et al. Terms, definitions and measurements to describe the sonographic features of adnexal tumors: a consensus opinion from the International Ovarian Tumor Analysis (IOTA) Group. Ultrasound Obstet Gynecol 2000;16:500–505 50. Timor-Tritsch IE, Bar-Yam Y, Elgali S, Rottem S. The technique of TVS
sonography with the use of 6.5 MHz probe. Am J Obstet Gynecol 1988;158:1019–1024 51. Timor-Tritsch IE, Bashiri A, Monteagudo A, Arslan AA. Qualified and trained sonographers in the US can perform early fetal anatomy scans between 11 and 14 weeks. Am J Obstet Gynecol 2004;191:1247–1252 52. Timor-Tritsch IE, Haratz-Rubinstein N, Monteagudo A, Lerner JP, Murphy K. Transvaginal color Doppler sonography of the ureteral jets: a potential method to detect ureteral obstruction. Obstet Gynecol 1997;89:113–117 53. Timor-Tritsch IE, Haratz-Rubinstein N, Murphy K, Monteagudo A. Transvaginal ultrasound in the detection of ureteral jets. Contemporary Reviews in Obstetrics and Gynecology 1997;143–148 54. Timor-Tritsch IE, Lerner JP, Monteagudo A, Murphy KE, Heller DS. Sonographic markers of inflammatory tubal disease. Ultrasound Obstet Gynecol 1998;12:56–66 55. Timor-Tritsch IE, Monteagudo A, Rebarber A et al. Transrectal scanning: an alternative when transvaginal scanning is not feasible. Ultrasound Obstet Gynecol 2003;21: 473–479 56. Timor-Tritsch IE, Monteagudo A. Transvaginal fetal neurosonography: standardization of the planes and sections used by anatomic landmarks. Ultrasound Obstet Gynecol 1996;8:42–47 57. Timor-Tritsch IE, Rottem S (eds) Transvaginal sonography, 2nd edn. Elsevier, New York, 1991 58. Timor-Tritsch IE. Standardization of ultrasonographic images: let's all talk the same language! Ultrasound Obstet Gynecol 1992;2:311–312 59. Ueland FR, DePriest PD, Pavlik EJ, Kryscio RJ, van Nagell JR Jr. Preoperative differentiation of malignant from benign ovarian tumors: the efficacy of morphology indexing and Doppler flow sonography. Gynecol Oncol 2003;91:46–50 60. Whitlow BJ, Chatzipapas IK, Lazanakis ML, Kadir RA, Economides DL. The value of sonography in early pregnancy for the detection of fetal abnormalities in an unselected population. Br J Obstet Gynaecol 1999;106:929–936 61. Zimmer EZ, Timor-Tritsch IE, Rottem S. The technique of transvaginal sonography. In: Timor-Tritsch IE, Rottem S (eds) Transvaginal sonography, 2nd edn. Elsevier, New York, 1991
4 ✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩✩ ✩
Investigation of early pregnancy Harm-Gerd K Blaas José M Carrera
ABSTRACT The preferred approach for the first-trimester examination is transvaginal sonography (TVS) although transabdominal sonography (TAS) could be and sometimes should be used to get a better overview. The presentation of images made by TVS and TAS should be standardized. In early pregnancy, use established measurement methods and measure several parameters: the crown–rump length (CRL), the head width, the heart rate, the diameter of the amniotic cavity and the diameter of the yolk sac; if possible, describe the anatomy. Looking at the heart activity alone is an incomplete examination. A detailed description of the embryonic development starting at week 4 and ending at week 10 is presented.
Keywords Early pregnancy loss, ectopic pregnancy, first trimester ultrasound, miscarriage, sonoembryology.
Introduction Approximately 12–15% of all pregnancies end in recognizable miscarriages.1 The most common indication for emergency referral in early pregnancy is vaginal bleeding. However, there are many other reasons for a pregnant woman to visit her doctor, such as abdominal pain, poor obstetric history, recurrent miscarriages, previous pregnancy with anomalous embryonic/fetal development, check-up following assisted fertilization, possible teratogenic exposure, uncertain gestational age or general anxiety. Today an ultrasound assessment of the pregnancy is a natural part of a first-trimester clinical examination.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ An ultrasound examination in the first trimester is expected to provide answers to important questions. Is the embryo/young fetus alive? Is the pregnancy properly located in the cavity of the uterus? Is it a single or multiple pregnancy, and in cases of multiple pregnancy, what is the chorionicity and amnionicity? What is the age of the conceptus? The examiner must recognize the signs of early pregnancy failure such as embryonic demise, spontaneous abortion, ectopic pregnancy, hydatidiform mole, and be able to identify normal anatomy and/or anomalies in very early viable pregnancies.2 The characteristics of the early conceptus are its small size, its constantly changing anatomical appearance, and its uniform development and constant growth. Therefore, the prerequisite for any early scan, in addition to adequate ultrasound equipment, is a thorough knowledge of the normal sonographic appearance of the developing embryo and its associated structures.2 The transvaginal approach is preferred. In this chapter, fetal age is always given in completed weeks and completed days based on the last menstrual period, i.e. the standard in obstetrics.
Description of the Sonoanatomic Development During the last two decades, systematic ultrasound studies have provided important and extensive knowledge about the development of the living embryo up to 10 weeks and the young fetus from 10 weeks on with detailed anatomic descriptions of embryonic organs and extraembryonic structures.3–9 4.5 weeks After approximately 4.5 weeks (LMP-based), a tiny gestational sac (diameter 2 mm) becomes visible within the decidua surrounded by the echogenic trophoblastic ring. 5 weeks 0–6 days, CRL ª0–3 mm At 5 weeks the thin-walled yolk sac usually appears (Fig. 4.1). After ≈5.5 weeks the yolk sac is always visible, which indicates that the pregnancy is properly located in the uterine cavity, even if the embryo is not yet identified. The embryonic pole appears adjacent to the yolk sac. Since the connecting stalk is short, the embryonic pole is located near the wall. The heart rate is about 80–100 beats per minute (bpm) at the end of this week. 6 weeks 0–6 days, CRL ª4–8 mm The embryonic pole, yolk sac and the heart activity are always present. The heart rate increases to 130 bpm (Fig. 4.2).
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7 weeks 0–6 days, CRL ª9–14 mm In sagittal section, the embryonic body appears as a triangle. The sides consist of the back and the roof of the rhombencephalon, and the frontal part includes the head, the basis of the umbilical cord, and the embryonic tail (Fig. 4.3). The embryonic body is slender in the coronal plane. The limbs appear as short hypoechogenic
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Investigation of early pregnancy
Fig. 4.1 5 weeks 1 day old pregnancy: retroverted uterus, trophoblastic ring in fundus; (arrow) small secondary yolk sac.
Chor cavity Yolk sac
Heart
Embryo
Fig. 4.2 6 weeks 1 day old pregnancy: CRL 5.2 mm. The embryo and the yolk sac lie close to the wall (future placenta). The beating heart can easily be identified by real-time ultrasound.
outgrowths. The hypoechogenic brain cavities can be seen. The shallow rhombencephalic cavity is also visible from 7 weeks on. It has a well-defined rhombic shape in the cranial pole of the embryo. The heart can easily be recognized by real-time ultrasound as a relatively large beating structure below the embryonic head. It is large and echogenic, the frequency has increased from 130 to 160 bpm. The thin amniotic membrane surrounding the embryo becomes visible. The mean diameter of the amniotic cavity is approximately identical with the CRL.
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B
A
Mesencephalon Rhombencephalon
Diencephalon
Fig. 4.3 7 weeks, CRL 13 mm. (A) Sagittal section through body; dotted line = section of B. (B) Horizontal section through the head showing measurement of head width and OFD (occipitofrontal diameter).
8 weeks 0–6 days, CRL ª15–22 mm The brain cavities are easily seen as large ‘holes’ in the embryonic head (Fig. 4.4). Choroid plexuses become visible as echogenic areas in the enlarged lateral ventricles and in the roof of the fourth ventricle. The third ventricle is still rather wide, as is the mesencephalic cavity. The mesencephalon is on top of the head. The spine is seen as two echogenic parallel lines. It is possible to recognize the fluidfilled stomach as a small hypoechogenic area on the left side of the upper abdomen below the heart. The physiological herniation of the gut can be identified as an echogenic area in the umbilical cord at the abdominal insertion. Within a few days, this echogenic structure becomes more distinct. At the end of the week, the fingers may be distinguishable.
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9 weeks 0–6 days, CRL ª23–31 mm (Fig. 4.5) At week 9 it is possible to obtain acceptable images of the embryonic profile. The lateral ventricles are always visible. They are best seen in the parasagittal plane, where the C-shape becomes apparent. The bright choroid plexuses of the lateral ventricles are regularly detectable at 9 weeks. The width of the diencephalic cavity narrows gradually while the mesencephalon remains wide.
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Mes Chorionic cavity 3. ventr Rhomb Mes
Lat Lower ventr limb
Lower limb Umb
Amn cavity
Amn cavity 3. ventr
Lower limb
Fig. 4.4 8/1 weeks, CRL 15 mm, sagittal section through embryo lying in amniotic cavity; the dotted line indicates section of image on the right side, horizontal section through the head. Mes, mesencephalic cavity; Rhomb, rhombencephalic cavity.
The choroid plexuses of the fourth ventricle are echogenic landmarks which divide the fourth ventricle into a rostral and a caudal compartment. The cerebellar hemispheres are easily detectable. The spine is still characterized by two echogenic parallel lines. During week 9 the heart rate reaches a maximum of mean 175 bpm. The midgut herniation is now a large hyperechogenic mass in the umbilical cord (Fig. 4.6). 10 weeks 0–6 days, CRL ª32–42 mm, and 11 weeks 0–6 days, CRL ª43–54 mm The fetus has developed a human appearance. The head is relatively large with a marked chin, a prominent forehead and a flat occiput. Ossification starts at about 11 weeks with the occipital bone,10 then the ossification of the spine becomes apparent. The lateral ventricles fill the anterior part of the head and conceal the diencephalic cavity. The cerebellar hemispheres seem to meet in the midline during weeks 11 and 12. The heart rate slows down to 165 bpm at the end of week 11. Anatomical details of the heart become obvious. The midgut herniation has its maximal extension at the beginning of week 10; it returns into the abdominal cavity during weeks 10–11. Fetuses that are older than 12 weeks do not demonstrate any sign of the midgut herniation. The stomach is always visible at 11 weeks. During weeks 9–11 the shape of the yolk sac alters and its wall becomes thinner. The yolk sac enlarges in some cases, while in other cases it shrinks.9
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Chorionic cavity 3rd ventricle Mesencephalic cavity Choroid plexus of 4th ventricle
Amniotic cavity
Spine
Fig. 4.5 Approximately 9-week-old embryo, CRL 22 mm, sagittal section through the embryo in the amniotic cavity. A
Echogenic midgut herniation in umbilical cord
B
Echogenic midgut herniation in umbilical cord
Cord cyst (normal phenomenon in early first trimester)
Body
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Fig. 4.6 Sagittal (A) and horizontal (B) section through an embryo (CRL 28 mm) at the end of week 9. The midgut herniation of the bowel is identified as an echogenic area in the umbilical cord. In (B) horizontal section through the embryonic abdomen; the arrows point at the abdominal insertion of the umbilical cord, containing echogenic bowel.
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Measurements of the Embryo/Early Fetus Investigation of early pregnancy
The crown–rump length (CRL) is measured as the greatest length in a straight line from the cranial to the caudal end of the body in the straightest possible position of the embryo/fetus (Fig. 4.7). The CRL diagram presented by Robinson in 1975 is still widely used for the evaluation and dating of the early pregnancy.11 The width of the head, also designated as the biparietal diameter (BPD), is measured in the horizontal section perpendicular to the body axis. Due to the development of the brain, the largest width alters its position in relation to cerebral landmarks during the embryonic and early fetal period (Fig. 4.8). At 7 weeks, BPD is measured at the height of the rhombencephalon. In the early fetal period the future cranium becomes more distinguished such that the BPD can be obtained by placing the calipers at the outer border of the not yet ossified
10 weeks Correct measurement of the crown−rump length (CRL)
C
9 weeks B 7 weeks A
70
mm
50
30
n = 29
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7
8 9 10 11 12 LMP-based gestational age (weeks)
13
Fig. 4.7 The CRL is measured as the greatest length in a straight line from the cranial to the caudal end of the body in the straightest possible position of the embryo/fetus. At 7 weeks, the rhombencephalic cavity lies at the top of the head; later, the midbrain is at the top. Below: Growth curve of CRL in 29 healthy embryos (reproduced from reference 9 with permission).
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A Head “BPD”
7 weeks B
9 weeks C
13 weeks Fig. 4.8 Measurement of the head at 7, 9 and 13 weeks. The reference plane and landmarks change during the first trimester. At 13 weeks, the third ventricle is visible. The cavum septi pellucidi is not yet developed.
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cranium in a horizontal section at the level of the thalamus. The anteroposterior diameter of the embryonic head, designated as the occipitofrontal diameter (OFD), is measured in the same section perpendicular to the BPD. The embryonic head circumference (HC) measurement is usually calculated from the BPD and the OFD, using the formula for an ellipse. Measurements of the embryonic trunk (abdominal circumference, AC) have been introduced as a possible parameter for the estimation of embryonic age
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Investigation of early pregnancy
during first-trimester biometry.12 It is advantageous to use comparable parameters when describing embryonic and fetal biometry. Instead of measuring the mean diameter of the abdomen and multiplying it with the constant 2π to obtain the AC, one may use a simpler parameter such as mean abdominal diameter (MAD)9 alone. This parameter is derived from two perpendicular measurements taken in the horizontal plane through the upper embryonic abdomen below the heart and above the umbilicus/midgut herniation (Fig. 4.9). Longitudinal examinations of the BPD and MAD in 29 normal pregnancies showed that the growth of the healthy embryo is constant (Figs. 4.7, 4.10).9 In 1973, Robinson showed that the heart rate reached a maximum at 9 weeks in the first trimester.13 The heart can easily be recognized by real-time ultrasound
Spine
Stomach 11 weeks Fig. 4.9 Measurement of the abdomen at 11 weeks. The reference plane is in the height of the embryonic/fetal stomach (arrow). The MAD is calculated from two perpendicular measurements. The abdominal circumference is also calculated from these diameters.
25
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mm
20 15 10 5 0
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7
8
9
10
11
12
13
MAD
mm
15 10 5 n = 29 0
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7 8 9 10 11 12 LMP-based gestational age (weeks)
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Fig. 4.10 Growth curves of the head width (BPD) and MAD in 29 pregnancies showing that the growth of the healthy embryo is constant (reproduced from reference 9 with permission).
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✩ ✩✩✩✩✩✩✩✩✩✩✩ as a relatively large beating structure below the embryonic head. The heart rate should be analysed electronically using the M-mode facility (Fig. 4.11). ‘Manual’ counting results in lower maximum heart rates, which again may result in incorrect counselling and poorer management of the patient. In normal pregnancies, the heart rate develops in a specific pattern, increasing from approximately 100 bpm at the end of 5 weeks to a peak mean of 175 bpm at 9 weeks, and slowly decreasing to 150 bpm in the second trimester (Fig. 4.12). The physiological midgut herniation is recorded by measuring the length of the protruded bowel into the cordal coelom. It is usually detectable at 8 weeks, has its maximal extension at the beginning of week 10, and can be seen until the end of week 11.8,14,15 Significant ossification of the long bones is not seen before 10 LMP weeks and later.16 This was confirmed in a study on the development of the skeleton comparing longitudinal ultrasound imaging from living embryos/fetuses with radiographs
Fig. 4.11 M-mode registration of embryonic heart activity (upper arrows) and maternal pulse (lower arrows). The embryonic heart rate is measured as beats per minute, here 168 bpm.
Heart rate 200
bpm
150
100
50 n = 448 0
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5
6 7 8 9 10 11 12 13 14 CRL-based gestational age (weeks)
Fig. 4.12 Embryonic and fetal heart rate in 448 examinations; the age of the embryos/ fetuses is based on CRL measurements.
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Extraembryonic Structures: the Three Sacs The gestational sac corresponds to the chorionic cavity. Its size has been used to evaluate normal progress of the early pregnancy. The amniotic membrane is easily depicted by transvaginal ultrasound at 7 weeks. For measurements, the calipers are placed on the thin membranes of the chorionic and amniotic cavities. As with the chorionic sac, the amniotic sac is measured by three perpendicular diameters and the arithmetical mean of these diameters is calculated. The yolk sac appears as a small ring with rather bright walls lying within the chorionic cavity (extraembryonic coelom), and lying outside the amniotic cavity after 7 weeks. Due to the loss of its physiological function, the yolk sac alters its shape during weeks 9–11.9 The wall of the yolk sac is thinner than 0.3 mm, but because of the transducer-dependent point spread function and the gain setting, the echogenic wall of the yolk sac appears significantly thicker in the ultraound image. Therefore, the calipers should be placed outside–inside or just on the middle of the yolk sac wall to avoid possible measurement bias. Thus, the measurements that most likely represent the true diameters are obtained by ‘outer–inner’ or ‘middle–middle’ placement of the calipers on the wall of the yolk sac. The growth of the amniotic cavity and the yolk sac is uniform and constant in healthy pregnancies9 (Figs. 4.13, 4.14).
Fig. 4.13 The three sacs: chorionic cavity and measurement of the amniotic cavity and the yolk sac.
Investigation of early pregnancy
obtained from aborted silver nitrate-impregnated embryos and fetuses.10 At 10.5 weeks the ossified part of the femur was just measurable to 2.1 mm by ultrasound. In a 10-week-old silver nitrate-impregnated embryo, the femur length was even shorter. Therefore, measurement of limbs does not have clinical significance in the first trimester.
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mm
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n = 29 6
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n = 29 6
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Fig. 4.14 Growth curves of the mean amniotic cavity diameter and mean yolk sac diameter in 29 healthy pregnancies showing that the growth of extraembryonic structures is constant. (Arrow) At approximately 9 weeks the physiological function of the yolk sac ceases (reproduced from reference 9 with permission).
Multiple Pregnancy: Determination of Chorionicity and Amnionicity The chorionicity and amnionicity can be explained quite easily, considering the developmental stage at which the twinning event occurs.16 Late twinning will be incomplete and will result in conjoined twins. The ‘cleavage’ or twinning event does not take place after 5 completed weeks. The spectrum varies from dichorionic (DC) diamniotic (DA) to monochorionic (MC) monoamniotic (MA) twins, where conjoined twins represent the extreme form of monoamniotic twins. MCDA twins have two yolk sacs; MCMA twins usually have only one. The chorionicity and amnionicity can be diagnosed at the end of week 5, when both embryo and yolk sac are detectable. MCDA twins always have thin dividing amniotic membranes that become visible at 7 weeks, while MCMA twins have a common amniotic cavity without dividing membranes (Fig. 4.15). DC pregnancies always have two thick trophoblastic tissue layers and two amniotic membranes between the twins; on the ultrasound image these layers and membranes appear as one thick wall.17 At the end of the first trimester and beginning of the second, trophoblastic tissue in the angle between two placentas and chorionic cavities constitutes the ‘lambda’ sign, which is characteristic for DC pregnancies.18 68
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Investigation of early pregnancy
B
Fig. 4.15 Twin pregnancies. (A) Dichorionic diamniotic twins at 7 weeks; notice the thick trophoblastic tissue between the gestational sacs. (B) Monochorionic diamniotic twin pregnancy at 8 weeks; the amniotic membrane (arrow) may be difficult to identify.
Evaluation of Early Pregnancy Failure Early Pregnancy Loss According to the literature on embryology and sonoembryology, the size and morphology of both embryonic and extraembryonic structures show little variation in pregnancies of the same age.6–9,16 Of extraembryonic structures, especially the yolk sac and the amniotic sac show a growth pattern that is closely related to embryonic development. Measurement of the gestational sac has been used for pregnancy evaluation, but one must be aware of the rather large variation of its size in normal pregnancies. This knowledge can be used as the basis for the evaluation of the early pregnancy, when significant departures from normal development and measurements are found. A threatened abortion is defined as a painless vaginal bleeding occurring before 24 weeks of pregnancy. A spontaneous abortion may be incomplete or complete. In first-trimester bleeding, neither statistical prediction models based on signs and symptoms nor clinical judgement are valid replacements for ultrasonographic assessment in establishing a diagnosis.19 No single ultrasound measurement of different anatomical features in the first trimester has been shown to have a high predictive value for determining early pregnancy outcome.20 Therefore a systematic evaluation of the early conceptus using combined biometric parameters is recommended.2 69
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✩ ✩✩✩✩✩✩✩✩✩✩✩ Gestational sac (chorionic cavity) and amniotic cavity The size of the gestational sac (chorionic cavity) has been used to evaluate normal progress of the early pregnancy. An abnormal size of the chorionic cavity, compared with the size of the embryo, has traditionally been associated with impending early pregnancy loss21,22 but its significance has not been extensively documented.23 This is probably due to the large variability of its size in normal pregnancies.9 Abnormality is probable when the mean size of the chorionic cavity is >10 mm without a yolk sac or >20 mm without an embryo. Another method of evaluating development is to compare the size of the amniotic cavity with the CRL.23 Embryologists have shown the close relationship between the amniotic cavity volume and fetal size. This has been confirmed by ultrasound studies showing a remarkable similarity in the absolute values of CRL and the mean diameter of the amniotic cavity in normal pregnancies between 7 and 11 weeks.9 A significant discrepancy between these two parameters is a possible sign for abnormality. A mean amniotic cavity diameter that is significantly less or larger than the actual CRL or an amniotic sac that is smaller than the yolk sac, or even absent after 7 weeks, are suspicious signs of abnormal development. If the size of the gestational sac or the embryo is smaller than the expected age, the possibility of incorrect age should always be considered and a repeat scan should be performed after 1 week. Normal growth and appearance of additional anatomical details may then rule out an abnormal early development. Yolk sac The yolk sac plays an important role in the early nutrition of the embryo, and is the source of early haematopoiesis.16 Thus, abnormal embryonic development may be reflected in an abnormal appearance of the yolk sac. However, many pregnancies that end in abortion show normal appearance of the yolk sac at an initial early scan; conversely, changes of shape and echogenicity have been found in uncomplicated pregnancies.23 In general, the finding of a yolk sac which is 7 mm before 9 weeks, absent or clearly irregular in shape indicates a possible abnormal early pregnancy.
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Haematoma Intrauterine haematomas are blood accumulations that are subchorionic, retroplacental or both (Fig. 4.16). The results from numerous studies of the intrauterine haematoma are not unequivocal. Today, the importance of intrauterine haematoma for early pregnancy loss is played down.23 A study from 2001 even concludes that intrauterine haematomas do not have a deleterious effect on pregnancy outcome in a population with recurrent miscarriage.24 But it seems reasonable to assume that if the haematoma lies under the placenta and cord insertion, it has the potential to lead to placental separation and abortion; and that also very large subchorionic haematomas may cause uterine contractions with subsequent pregnancy loss.
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Umbilical cord Uterus
Haematoma Embryo
Trophoblastic tissue
Investigation of early pregnancy
Chorionic cavity
Fig. 4.16 8/4 weeks pregnancy showing an intact normal embryo and its umbilical cord. There is a large haematoma on the outer side of the gestational sac. The amniotic membrane is not visible on this image.
Heart rate There is a good correlation between the heart rate and embryonic size and age. Alterations of the embryonic heart rate such as arrhythmia and/or bradycardia may be associated with maldevelopment.22,25 Embryonic heart rate measurements in early pregnancy may be useful in the prediction of first-trimester spontaneous abortion after ultrasound-proven viability, but a heart rate below the 95% confidence interval of normal does not necessarily indicate a poor outcome.26 A general rule is that if the embryo has a CRL of 6 mm or more, the lack of heart activity is highly suspicious for intrauterine embryonic/fetal death. A significant relationship to abortion has been found when the heart rate is less than 1.2 SD from the mean.21
Trophoblastic Disease Gestational trophoblastic diseases are complete, partial and invasive moles, placental site trophoblastic tumours and choriocarcinomas. An invasive hydatidiform mole is defined by penetration of molar villi into the myometrium or vasculature of the uterus. Both complete and partial moles can become invasive. Ultrasound has replaced all other techniques for early diagnosis and management of these conditions.27 However, a routine pre-evacuation ultrasound examination identifies less than 50% of hydatidiform moles, the majority sonographically appearing as missed or incomplete miscarriage.28 Testing β-hCG levels of maternal serum is still of major importance for the evaluation and follow-up of treatment.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ Complete hydatidiform mole Complete hydatidiform mole develops when the diploid chromosomal set of the conceptus is entirely derived from paternal chromosomes. Patients with complete hydatidiform mole present a large uterus, vaginal bleeding and abnormally high β-hCG levels. The latter causes hyperstimulation of the ovaries, resulting in enlargement through theca lutein cysts in 50% of cases. The ultrasound examination reveals a uterine cavity filled with multiple cysts and echogenic areas of variable size and shape (‘snow-storm’ appearance) in the absence of an embryo or fetus (Fig. 4.17). One must be aware that certain rare uterine tumours may resemble moles. Using ultrasound, approximately 79% of complete hydatidiform moles are detected.28 Partial hydatidiform mole In partial hydatidiform mole, a fetus is found in association with molar degeneration of the placenta. Partial moles are usually of triploid or diandric origin, having two sets of chromosomes of paternal origin and one of maternal origin (69,XXX or 69,XXY).27 Partial mole presents on ultrasound examination as an enlarged placenta; it is thicker than 4 cm at the level of the cord insertion at the second-trimester routine scan and contains many cystic areas (‘Swiss cheese’ appearance). The diagnosis of partial mole is more difficult than that of a complete mole; only 29% were detected in a large study by Fowler et al.28 Doppler investigation plays a limited role in diagnosis or management.27 The fetus is usually growth retarded and shows variable congenital anomalies. Invasive hydatidiform mole An invasive mole usually appears clinically with bleeding after surgical evacuation of a molar pregnancy. Sonographically, nodular areas of increased echogenicity are found in the uterine wall. The lesions may contain fluid-filled cavities.27 Doppler may be used to evaluate the effectiveness of medical therapy. Choriocarcinoma Choriocarcinoma is highly malignant, developing from trophoblastic tissue and metastasizing into lungs, liver or brain. Women with metastases may present
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Fig. 4.17 Complete hydatidiform mole: uterine cavity filled with multiple cysts and echogenic areas of variable size and shape.
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Ectopic Pregnancy A common cause for early pregnancy failure in the western world is ectopic pregnancy. The prevalence of this condition is nearly 2%, accounting for 9% of pregnancy-related deaths of reproductive-aged women in the first trimester in the USA.29 There are many risk factors, the highest risk being found in patients who have had tubal surgery including sterilization, previous ectopic pregnancy, in utero exposure to diethylstilboestrol, IUD and documented tubal pathology.30 Early diagnosis of an ectopic pregnancy is important because it contributes to a decline in morbidity, maternal deaths and treatment costs. As the identification of an ectopic pregnancy can be difficult, the first step in ruling out ectopic pregnancy should be to identify intrauterine pregnancy which can virtually always be identified after 5.5 weeks by transvaginal ultrasound. Quantitative hCG serum analysis is an important additional test, when an intrauterine pregnancy cannot be seen. Ectopic pregnancy should be assumed when the hCG serum test is above the discriminatory zone in which a pregnancy should always be detected by transvaginal sonography (TVS) (β-hCG concentrations =1500 IU/L).30 Sonographically, an extrauterine gestational sac surrounded by an echogenic ring consisting of the trophoblast at the implantation is usually seen (Fig. 4.18). Other ultrasound signs for ectopic pregnancy are any non-cystic extraovarian adnexal mass, complex cystic or solid masses and, of course, a living ectopic pregnancy, which is found in approximately 5–15% of cases. Viability of ectopic pregnancies can be evaluated by transvaginal Doppler ultrasound because of the good vascularization of the trophoblastic ring. The scan for ectopic pregnancies must be performed thoroughly and systematically. One must be aware of special ectopic locations such as interstitial pregnancy, which occurs in 1–6% of all ectopic pregnancies,31 or cervical pregnancy, which accounts for only 0.15%.32 The ultrasound diagnosis of an interstitial pregnancy is made when products of conception are visible in the upper lateral aspect of the uterus, outside the uterine cavity and at least partially surrounded by myometrium.31 In cervical pregnancies, the gestational sac is found below the internal os of the uterus. To miss the diagnosis of these two variants of ectopic pregnancy imposes extraordinary risks to affected women, because these conditions may lead to acute life-threatening bleeding, which may be difficult to treat. One must also be aware of the possibility of concomitant intrauterine and ectopic pregnancies. The occurrence of heterotopic pregnancies is increased in pregnancies achieved by assisted fertilization.
Investigation of early pregnancy
with dyspnoea, abdominal pain and neurological symptoms. The primary symptom is vaginal bleeding. Choriocarcinoma may be found after a molar pregnancy, a miscarriage or after an apparently normal pregnancy. The sonographic appearance of a choriocarcinoma resembles that of an invasive mole. The primary tumour of a choriocarcinoma in an apparently normal placenta is usually small, less than 8 mm. So far, such a primary tumour has not been described sonographically.27
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A
Uterus Ovary
right
left
B
Ectopic pregnancy
Ovary
Fig. 4.18 Ectopic pregnancy. (A) Transverse section through the female pelvis; ovary on the right side of the uterus. (B) Laterally from the right ovary an ectopic pregnancy with a living embryo can be seen. See also standardization, Fig. 4.21.
A review of six different diagnostic algorithms for ectopic pregnancy concluded that a combination of ultrasound and hCG resulted in the best outcomes. Ultrasound as the first step was the most efficient and accurate method of diagnosing ectopic pregnancy.29
Early Anomalies
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High-frequency transvaginal transducers have made it possible to disclose structural developmental disorders of the embryo (before 10 weeks) and the young fetus. An increasing number of studies, reviews and case reports describe ultrasound detection of early anomalies.33–37 Nuchal translucency (NT), which may be found at the early 11–13-week-scan, is a well-known transient marker for fetal disorders.38 Both the transabdominal and the transvaginal approach can be used. Though NT is seen in normal fetuses, it is not only highly associated with chromosomal aberrations, but it may also be found in fetuses with skeletal anomalies, neuromuscular disorders, rare genetic disorders, heart defects or infections.38 The likelihood for associated anomalies increases with the thickness of the oedema. Nicolaides and colleagues have described the criteria of NT measurements: CRL 45–83 mm, 11 weeks 0 days to 13 weeks 6 days, preferably but not necessarily TVS, and good sagittal section of the fetus with appropiate magnification. The maximum thickness of subcutaneous translucency is measured by placing the calipers on the inner lines.38
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Standardization of Transvaginal and Transabdominal Imaging in Gynaecology In 1992, Timor-Tritsch discussed an inquiry performed by Bernaschek and Deutinger 40 that revealed a world-wide discord about the way TVS images were displayed.41 Timor-Tritsch's recommendation was: ‘Let's all talk the same language!’. To help us talk the same language, we should review some of the basic rules taught at medical school, rules that probably represent a world-wide standard. When we examine a patient, we principally position ourselves on the patient's right side and face the patient. This is the starting point of every examination. When we look at the patient we see the patient's left shoulder on the right. We can imagine we are viewing a ‘screen’ with our own eyes. When gynaecologists perform an examination of the uterus and adnex, they will find the patient's left ovary/adnexa on the right side (of the screen/picture), and the patient's right ovary/adnexa on the left side.
Investigation of early pregnancy
The absence of nasal bone ossification at the end of the first trimester is another marker for possible abnormal development, namely trisomy 21. Evidence based on radiological, histomorphological and sonographic studies has shown that nasal bone abnormalities are significantly more common in trisomy 21 fetuses than in euploid fetuses.39
Imaging in Medicine Pictures of organs or parts of the body should present the normal anatomical relations as exactly as possible. The argument for displaying the TVS pictures on the monitor with the ‘footprint’ of the vaginal probe at the bottom of the screen close to the cervix, while the fundus of the uterus points towards the top of the screen, seems correct and self-explanatory.41 In essence, this would be the ‘natural’ way to display the uterus on the screen, at least through the eyes of a practising gynaecologist who performs a bimanual examination on a patient in the supine position. The cervix would be at the tip of the examining fingers and the fundus of the uterus further away, i.e. cephalad. An additional advantage of displaying the picture with the apex of the ‘pie’ pointing downward or upward would be the ability to tell instantly the difference between an image obtained by the transvaginal or transabdominal route: the apex of the ‘pie’ pointing to the bottom of the screen on the transvaginal picture and to the top of the screen on the transabdominal scan. There are more arguments for this form of imaging. If we move the transabdominal transducer from the midsagittal plane to the left or to the right, we will identify the ovaries lying close to and ‘below’ the iliac vessels. This is the normal relation: the ovary is in a mediodorsal position to the iliac vessels. In the TVS image display, this anatomical relation should be maintained. For the transverse plane, the transducer is rotated 90 ° to the left, imaging the right adnexa on the left side of the picture and vice versa for the left adnexa.
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Ventral
Cranial
Caudal
Dorsal Fig. 4.19 Transabdominal sonography (TAS) and transvaginal sonography (TVS) of the female pelvis; sagittal insonation angle.
Ventral
Cranial
Caudal
Dorsal Fig. 4.20 Direction of the sagittal imaging sectors of TAS and TVS through the female pelvis. Note that the cranial part of the pelvis points to the left side, and the caudal part points to the right side. Imagine that the observer's position is on the right side of the patient.
Ventral/cranial
Right
Left
90 Dorsal/caudal
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Fig. 4.21 By turning the transducers 90˚, a transverse section of the pelvis is shown. Note that the right part of the pelvis is shown on the left side of the image, and the left part on the right side. Imagine that the observer looks at the patient's body from below.
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References 1. Regan L, Rai R. Epidemiology and the medical causes of miscarriage. Baillière's Clin Obstet Gynecol 2000;14(5):839–854 2. Blaas H-GK. Editorial: the examination of the embryo and early fetus: how and by whom? Ultrasound Obstet Gynecol 1999;14(3):153–158 3. Takeuchi H. Transvaginal ultrasound in the first trimester of pregnancy. Early Hum Dev 1992;29:381–384 4. Timor-Tritsch IE, Farine D, Rosen MG. A close look at the embryonic development with the high frequency transvaginal transducer. Am J Obstet Gynecol 1988;159:678–681 5. Timor-Tritsch IE, Peisner DB, Raju S. Sonoembryology: an organ-oriented approach using a high-frequency vaginal probe. J Clin Ultrasound 1990;18:286–298 6. Blaas H-G, Eik-Nes SH, Kiserud T, Hellevik LR. Early development of the forebrain and midbrain: a longitudinal ultrasound study from 7 to 12 weeks of gestation. Ultrasound Obstet Gynecol 1994;4:183–192 7. Blaas H-G, Eik-Nes SH, Kiserud T, Hellevik LR. Early development of the hindbrain: a longitudinal ultrasound study from 7 to 12 weeks of gestation. Ultrasound Obstet Gynecol 1995;5:151–160 8. Blaas H-G, Eik-Nes SH, Kiserud T, Hellevik LR. Early development of the abdominal wall, stomach and heart from 7 to 12 weeks of gestation: a longitudinal ultrasound study. Ultrasound Obstet Gynecol 1995;6:240–249 9. Blaas H-G, Eik-Nes SH, Bremnes JB. Embryonic growth. A longitudinal biometric ultrasound study. Ultrasound Obstet Gynecol 1998;12(5):346–354 10. Zalen-Sprock R, Brons JTJ, Vugt J, Harten H, Geijn H. Ultrasonographic and radiologic visualization of the developing embryonic skeleton. Ultrasound Obstet Gynecol 1997;9:392–397
11. Robinson HP, Fleming JEE. A critical evaluation of sonar ‘crown-rump length’ measurements. Br J Obstet Gynaecol 1975;82:702–710 12. Reece EA, Scioscia AL, Green J, O'Connor TZ, Hobbins J. Embryonic trunk circumference: a new biometric parameter for estimation of gestational age. Am J Obstet Gynecol 1987;156:713–715 13. Robinson HP, Shaw-Dunn J. Fetal heart rates as determined by sonar in early pregnancy. J Obstet Gynaecol Br Cwlth 1973;80:805–809 14. Cyr DR, Mack LA, Schoenecker SA et al. Bowel migration in the normal fetus: ultrasound detection. Radiology 1986;161:119–121 15. Timor-Tritsch IE, Warren WB, Peisner DB, Pirrone E. First trimester midgut herniation: a high frequency transvaginal sonographic study. Am J Obstet Gynecol 1989;161:466–476 16. O'Rahilly R, Müller F. Developmental stages in human embryos. Carnegie Institute Publications, Washington, DC 17. Monteagudo A, Timor-Tritsch IE. Early and simple determination of chorionic and amniotic type in multifetal gestations in the first fourteen weeks by high-frequency transvaginal sonography. Am J Obstet Gynecol 1994;170:824–829 18. Sepulveda W, Sebire NJ, Hughes K, Odibo A, Nicolaides KH. The lambda sign at 10–14 weeks of gestation as a predictor of chorionicity in twin pregnancies. Ultrasound Obstet Gynecol 1996;7(6):421–423 19. Waard MW, Bonsel GJ, Ankum WM, Vos J, Bindels PJE. Threatened miscarriage in general practice: diagnostic value of history taking and physical examination. Br J Gen Pract 2002;52(483):825–829 20. Jauniaux E, Johns J, Burton G. The role of ultrasound imaging in diagnosing and
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By simply looking at the image, it should be easy to distinguish whether the scan was done by TVS or TAS. Further, it must be possible to correlate TVS and TAS images from the same patient. When TVS and TAS images are standardized as indicated above, it is easy to recognize a TAS (‘pie apex up’) from a TVS (‘pie apex down’). In the transverse plane we will expect to find the patient's left ovary on the right side of the image and in the sagittal plane we will expect to find the bladder on the right side. If the uterus is pointing towards the right in a sagittal plane it is anteflexion; if it is pointing to the left, it is retroversion – no further explanation needed.
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investigating early pregnancy failure. Ultrasound Obstet Gynecol 2005;25:613–624 21. Falco P, Milano V, Pilu G et al. Sonography of pregnancies with first-trimester bleeding and a viable embryo: a study of prognostic indicators by logistic regression analysis. Ultrasound Obstet Gynecol 1995;7:165–169 22. Makrydimas G, Sebire NJ, Lolis D, Vlassis N, Nicolaides KH. Fetal loss following ultrasound diagnosis of a live fetus at 6-10 weeks of gestation. Ultrasound Obstet Gynecol 2003;22:368–372 23. Jauniaux E, Kaminopetros P, El-Rafaey H. Early pregnancy loss. In: Rodeck J, Whittle M (eds) Fetal medicine: basic science and clinical practice. Harcourt Brace, London, 1999: 835–847 24. Tower C, Regan L. Intrauterine haematomas in a recurrent miscarriage population. Hum Reprod 2001;16(9):2005–2007 25. Schats R, Jansen CAM, Wladimiroff JW. Abnormal embryonic heart rate pattern in early pregnancy associated with Down's syndrome. Hum Reprod 1990;5(7):877–879 26. Achiron R, Tadmor O, Mashiach S. Heart rate as a predictor of first-trimester spontaneous abortion after ultrasoundproven viability. Obstet Gynecol 1991;78:330–334 27. Jauniaux E. Ultrasound diagnosis and follow-up of gestational trophoblastic disease. Ultrasound Obstet Gynecol 1998;11:367–377 28. Fowler DJ, Lindsay I, Seckl MJ, Sebire NJ. Routine pre-evacuation ultrasound diagnosis of hydatidiform mole: experience of more than 1000 cases from a regional referral center. Ultrasound Obstet Gynecol 2006;27:56–60 29. Gracia C, Barnhart K. Diagnosing ectopic pregnancy: decision analysis comparing six strategies. Obstet Gynecol 2001;97:464–470 30. Pisarska M, Carson S, Buster J. Ectopic pregnancy. Lancet 1998;351(9109):1115–1120 31. Hafner T, Aslam N, Ross J, Zosmer N, Jurkovic D. The effectiveness of non-surgical
management of early interstitial pregnancy: a case report of ten cases and review of the literature. Ultrasound Obstet Gynecol 1999;13:131–136 32. Jurkovic D, Hacket E, Campbell S. Diagnosis and treatment of early cervical pregnancy: a review and a report of two cases treated conservatively. Ultrasound Obstet Gynecol 1996;8:373–380 33. Blaas H-G, Eik-Nes SH. First-trimester diagnosis of fetal malformations. In: Rodeck J, Whittle M (eds) Fetal medicine: basic science and clinical practice. Harcourt Brace, London, 1999: 581–597 34. Blaas H-GK, Eik-Nes SH, Isaksen CV. The detection of spina bifida before 10 gestational weeks using 2D- and 3D ultrasound. Ultrasound Obstet Gynecol 2000;16:25–29 35. Blaas H-GK, Eik-Nes SH, Vainio T, Isaksen CV. Alobar holoprosencephaly at 9 weeks gestational age visualized by two- and threedimensional ultrasound. Ultrasound Obstet Gynecol 2000;15:62–65 36. Rottem S, Bronshtein M. Transvaginal sonographic diagnosis of congenital anomalies between 9 weeks and 16 weeks menstrual age. J Clin Ultrasound 1990;18:307–314 37. Souka AP, Nikolaides KH. Diagnosis of fetal abnormalities at the 10–14-week scan. Ultrasound Obstet Gynecol 1997;10:429–442 38. Nicolaides KH, Sebire NJ, Snijders R. The 11–14-week scan. The diagnosis of fetal abnormalities. Parthenon, Carnforth, 1999 39. Sonek J, Cicero S, Neiger R, Nicolaides K. Nasal bone development in prenatal screening for trisomy 21. Am J Obstet Gynecol 2006;195(5):1219–1230 40. Bernaschek G, Deutinger J. Current status of vaginosonography: a world-wide inquiry. Ultrasound Obstet Gynecol 1992;2:352–356 41. Timor-Tritsch I. Opinion: standardization of ultrasonographic images: let's talk the same language. Ultrasound Obstet Gynecol 1992;2(5):311–312
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Normal fetal anatomy at 18–22 weeks David A Nyberg Vivienne L Souter
Abstract A screening obstetric ultrasound during the second trimester has been widely adopted around the world and can provide important information regarding the fetus and pregnancy. The fetus can be examined quite literally from head to toe, which has led to the concept of the fetal ‘anatomical survey’. A fetal anatomical survey requires a systematic approach that should be performed in all secondtrimester fetuses, regardless of the indication for the ultrasound. Familiarity with normal anatomy is essential to recognize deviations from normal or fetal anomalies. Centres should attempt to exceed basic guidelines.
Keywords Fetal abnormalities, fetus, normal, normal anatomy, prenatal sonography.
Introduction A screening obstetric ultrasound during the second trimester has been widely adopted around the world. This can provide important information regarding the pregnancy, including evaluation for placenta praevia, evaluation of the cervix and cervical incompetence and, most importantly, evaluation of the fetus. The fetus can be examined quite literally from head to toe, which has led to the concept of the fetal ‘anatomical survey’. Parents might even consider this their baby's first physical examination. In the vast majority of cases the fetus appears normal and parents can be reassured regarding the health of their baby. At the same time, a systematic fetal survey can now detect the majority of fetal malformations.1 A second-trimester scan is also desired by most prospective parents and has been found to be cost-effective, at least at centres that have reasonable accuracy for detection of fetal anomalies.2
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✩ ✩✩✩✩✩✩✩✩✩✩✩ The timing of the second trimester ultrasound varies between centres. While later scans permit improved anatomical detail and greater sensitivity for many structural defects, earlier scans can both provide useful information earlier and also about the risk of fetal chromosome abnormality. For this reason, fetal surveys may be performed earlier at 15–18 weeks, coinciding with the time of genetic amniocentesis or second-trimester maternal serum screen. At our own centre, patients obtain a scan at 15–18 weeks if they are considering genetic amniocentesis or at 18–22 weeks if they are considered low risk. This approach supports other studies which suggest that, at least among low-risk women, a later scan will provide more information and is less likely to result in a repeat scan.3 Centres that perform a first-trimester (10– 14 weeks) ultrasound that includes nuchal translucency measurements and early fetal evaluation will also usually obtain a later scan at 18–22 weeks.
Scan Guidelines Guidelines for a normal anatomical survey have been published by various institutions.4,5 However, most centres now routinely include documentation of other anatomical structures beyond the basic set suggested by society guidelines. We have further modified the guidelines to reflect the completeness of a fetal survey performed at most obstetric centres (Box 5.1). Procedures that adhere to these guidelines should result in detection of the majority of major detectable anomalies. Because most anomalies are sporadic and occur in otherwise low-risk women, it is important that all scans performed during the second trimester include a fetal survey as an essential part, regardless of the reason for performing the scan. Detection of anomalies does not require a detailed understanding of pathology; it only requires thorough familiarity with normal anatomy. Deviations from normal or suspected anomalies can then be referred to a high-risk centre for a more detailed fetal ultrasound examination and clinical consultation.
Normal Fetal Anatomy Brain/Calvarium Views of the brain should include three standard axial views: transthalamic, transventricular and transcerebellar (Figs 5.1–5.4). These three views permit a reliable prenatal diagnosis of nearly all significant intracranial anomalies as well as providing important clues for the vast majority of spinal dysraphic defects before the time of viability.6–8,84
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Transthalamic view The transthalamic view (see Fig. 5.1) is the standard plane used for obtaining biometric cranial measurements (biparietal diameter (BPD) and head circumference). At this level, one can also visualize the frontal horns of the lateral ventricles and the cavum septum pellucidum between the frontal horns. The cavum septum pellucidum (CSP), and its posterior extension the cavum vergae, is a fluid-filled midline structure located between the lateral ventricles. Sonographically, it is
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Box 5.1 Elements of fetal anatomical survey at 18–22 weeks
Face/Neck Face (lips, mouth, nose, orbits and ears) Neck (nuchal fold) Spine Longitudinal and transverse views Thorax Heart – documentation of venous–atrial, atrial–ventricular and ventricular–arterial connections which may include the following views: •• Four chamber •• Right ventricular outflow •• Left ventricular outflow •• Aortic arch •• Ductal arch Lungs Bony thorax
Normal fetal anatomy at 18–22 weeks
Head and Brain Calvarium Brain – documentation of thalami, hemispheres, lateral ventricles, cerebellum and vermis which includes the following views: •• Transthalamic •• Transventricular •• Transcerebellar
Abdomen Major organs (stomach, liver, spleen, gallbladder) Gut Anterior abdominal wall Genitourinary tract Kidneys Urinary bladder Genitalia Extremities, bony skeleton Upper extremities including both hands Lower extremities including both feet Data from reference 4. Evaluation should also include estimation of dates (or evaluation of growth). This should include, as a minimum, measurements of biparietal diameter, head circumference, abdominal circumference and femur length (humerus length). Obstetric ultrasound should also include assessment of the placenta, cervix, amniotic fluid and possible adnexa. Adapted from Yoo et al.44
usually identified as a fluid-filled structure anterior to the thalami on axial images. It should not be mistaken for the third ventricle which is smaller and located more posteriorly between the thalami. Presence of the CSP suggests proper formation of the midline cerebral structures.85 When specifically sought, the CSP can be identified in most cases. However, it may be difficult to visualize on standard views, especially before 20 weeks. Both the CSP and corpus callosum can be better seen on transvaginal scans (see Fig. 5.2).9 The corpus callosum shows gradual enlargement during pregnancy, from nearly
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Fig. 5.1 Transthalamic view. Axial view through the mid head shows normal thalami (Th). The cavum septum pellucidum (CSP) is a small midline fluid space anterior the thalami.
17 mm in length at 18 weeks' gestation to 44 mm at term. The ratio of the length of the corpus callosum to the anteroposterior diameter of the brain remains relatively constant from 20–21 weeks' gestation to term. Transventricular view The transventricular view is obtained at a plane just superior to the transthalamic view (see Fig. 5.3). Demonstration of the lateral cerebral ventricles in this view is essential for the early detection of hydrocephalus. Within the ventricular system lies the echogenic choroid plexus, best seen filling the body of the lateral ventricle from medial to lateral wall, and extending into the atrium (or trigone). The choroid plexus does not extend into the frontal horns, and they are identified as A
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Fig. 5.2 Transvaginal scans show normal-appearing corpus callosum (CC) in (A) coronal and (B) sagittal planes. FH, frontal horns; CSP, cavum septum pellucidum.
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Normal fetal anatomy at 18–22 weeks Fig. 5.3 Axial sequence images from superior to inferior as seen with tomographic ultrasound imaging (TUI). The images are set at standard 3 mm intervals in this case. Note that the choroid (CH) fills the lateral cerebral ventricles and this is located at a plane superior to the transthalamic view. TH, thalamus; C, cerebellar hemispheres; CM, cisterna magna.
prominent anechoic anterior components of the lateral ventricles. In contrast, the surrounding cerebral cortex is hypoechoic with scarcely more echogenicity than the CSF. After about 18 weeks, the posterior aspect of the lateral ventricles (atria and occipital horns) is drawn laterally with development of the temporal horns. Simultaneously, the ventricles and choroid appear less pronounced with growth of the cerebral hemispheres.
Fig. 5.4 Transcerebellar view. Slightly oblique scan through the posterior fossa shows cerebellar hemispheres (C) which show a normal biconvex shape, outlined by the cisterna magna (CM). This single normal view excludes nearly all open spinal defects.
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The standard measurement of the cerebral ventricle is obtained in an axial plane through the atrium. Most studies have found the mean size of the ventricular atrium to be in the range of 5.4–6.5 mm, with 10 mm considered to be a cut-off for abnormal.10,11 However, even lesser degrees of ventricular dilation may prove to be significant during the second trimester.12 Although 10 mm is a commonly accepted cut-off for normal size, others have found that ventricle size over 8 mm is unusual before 25 weeks.13 Mild or borderline degrees of cerebral ventricular dilation pose a difficult dilemma since most fetuses are perfectly normal but some have underlying abnormalities or experience adverse outcome.14,15 Idiopathic lateral ventricular dilation is more common among male fetuses16 and in our experience, this is more common in those who are large for gestational age. Detection of borderline or mildly dilated cerebral ventricles is controversial. Careful techniques should be used to evaluate the ventricles. Off-axis or angled planes can overestimate the size of the lateral ventricles.17 Reverberation artifact from bone normally obscures the proximal hemisphere. A technique employing an oblique scan plane angled superiorly through the temporal bone affords markedly improved visualization of the proximal hemisphere.18,19 A common ‘normal variant’ on transventricular views is a choroid plexus cyst. These are identified as often as 3–4% at 15–18 weeks20 and approximately 1% at 18–22 weeks. Choroid plexus cysts have been the subject of much study and debate.21–24 There is now generalized consensus that they are of no direct consequence themselves. They appear to slightly increase the risk of fetal chromosome abnormality, especially trisomy 18. However, as an isolated finding in a low-risk patient, this risk is considered to be low and amniocentesis is not recommended.25 Transcerebellar view The transcerebellar or posterior fossa view is obtained by slightly angling the scan plane down posteriorly from the axial image for BPD determination until the cerebellum and cisterna magna are delineated (see Fig. 5.4).26,27,86 This view is important for identification of the Dandy–Walker malformation and cerebellar agenesis and can provide diagnostic information regarding the presence or absence of the Arnold–Chiari malformation, seen with nearly all cases of open spina bifida. This scan plane is also useful for evaluating the nuchal soft tissue thickness. The cerebellar hemispheres can easily be seen on each side of the echogenic midline vermis, anterior to the cisterna magna. The cerebellum appears slightly more echogenic than the cerebral hemispheres and is separated from supratentorial structures by the tentorium. The cerebellar hemispheres are biconvex in shape. The normal fourth ventricle can be occasionally visualized within the midbrain by high-resolution scans. It appears as a triangular-shaped small fluid space between the cerebellar hemispheres. The cisterna magna appears as a sonolucent space just posterior to the cerebral hemispheres and vermis near the base of the brain. The presence of a normal cisterna magna (CM) excludes nearly all open spinal defects. Standardized measurements of the cisterna magna, with a normal range of roughly 3–10 cm, are taken in the midline from the vermis to the occipital bone.28 Measured values
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Face and Neck
Normal fetal anatomy at 18–22 weeks
at the lowest end of this range are expected only early in gestation, while conversely a cisterna magna of large dimension is generally seen in the third trimester. Occasionally a measurement exceeds these standards, which is usually still normal if the vermis is well seen and intact and the transverse cerebellar diameter is normal for gestational age. An inappropriate scan plane can also lead to an abnormally large measurement of the CM or even the appearance of a Dandy–Walker variant.29 Caution should be exercised in diagnosis of incomplete closure of the vermis in the early second trimester;30 however, the normal vermis should be closed by 18 weeks. In most fetuses, 1–3 linear echoes can be seen traversing the CM posteriorly from the cerebellum. Although originally mistaken for the straight sinus, these lines are now attributed to ‘subarachnoid septa’31 or ‘dural folds’.32 Occasionally they may appear ‘cyst-like’ and simulate a posterior fossa cyst.
Examination of the face is not included in basic examination guidelines; however, it is generally easy to perform and may provide important information. Parents also desire and easily recognize views of the face. Views of the face are particularly important when other anomalies are suspected. For all these reasons, we believe views of the face should be included in any fetal survey at 18–22 weeks. Imaging of the fetal face can be accomplished in coronal, sagittal and axial planes as well as three-dimensional (3D) multiplanar ultrasound (Figs 5.5–5.8). Each scan plane has advantages and disadvantages, and a combination of scan planes is often desired, when positioning is favourable, to adequately delineate all anatomy. A midline sagittal scan, or profile view, is one of the most recognizable images of any fetus (see Fig. 5.5). It is useful for evaluation of size and position of the mandible for exclusion of micrognathia, the nose and nasal bridge, and the tongue. Coronal imaging of the soft tissues of the nose and upper lip can be easily shown at 18–22 weeks (see Fig. 5.6). This view is most useful for exclusion of cleft lip, with or without cleft palate. Although mild forms of cleft lip/cleft palate can be missed earlier, the majority of clefts should be visualized by 18–22 weeks.
Fig. 5.5 Midline sagittal view of the face shows normal structures including a normal nasal bone (NB).
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Fig. 5.6 Coronal view through the superficial soft tissues shows a normal upper lip (L) and nose (N).
On the other hand, cleft palate without associated cleft lip (aetiologically distinct from cleft lip with or without cleft palate) remains undetected throughout pregnancy, with rare exceptions. Although the soft tissue view is most useful, coronal views more posteriorly within the face may show the oral cavity and deep nasal structures. This can be useful for showing the involvement of cleft palate when cleft lip is suspected. Transverse or axial views are also useful when cleft lip/palate is suspected. The upper lip and anterior maxilla can be imaged simultaneously in this plane, and the integrity of the alveolar ridge can be demonstrated.33 The oral cavity may also be nicely imaged in the axial plane. When the upper lip is evaluated on coronal images, the nose also comes into plane. The nose should be normal in size and shape; two ala should be demonstrated. Some familial and racial differences are probably evident in the appearance of the nose. The orbits are best imaged in the axial or coronal plane (see Fig. 5.7). Measurement of the outer orbital diameter (OOD), which correlates with gestational age,34 allows for detection of hypo/hypertelorism. Intraorbital anatomy that can be identified includes the globe, lens and hyaloid artery. The fetal ears are not frequently targeted for imaging, but when necessary can be easily identified as complex soft tissue protrusions external to the skull.35
Fig. 5.7 Axial image through the orbits shows normal measurements of the orbital diameter (OD), binocular diameter (BOD), and interocular diameter (IOD).
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Because of their complex shape, they are best imaged using 3D multiplanar ultrasound (see Fig. 5.8). Normal ear size has been documented in an effort to detect small ears as a sign of fetal chromosome abnormality.36 Fetal hair may be seen during the third trimester37 but is not seen at this time. Evaluation of the soft tissue of the back of the neck (nuchal fold or nuchal thickness) has proven to be one of the best sonographic ‘markers’ for trisomy 21 during the second trimester.38-40 Although detection of sonographic markers is generally performed earlier, with nuchal translucency assessed as early as 10–14 weeks, nuchal thickness can still be evaluated before 20 weeks, and probably as late as 22 weeks. It should be included on any routine fetal anatomical survey during the second trimester. Because nuchal thickness increases with gestational age, absolute cut-offs will be useful throughout the pregnancy. Other neck structures are not routinely sought during the anatomical survey. Normal thyroid size has been documented since 20 weeks.41 However, cystic hygromata or other neck masses can be detected.
Normal fetal anatomy at 18–22 weeks
Fig. 5.8 Face, 3D multiplanar ultrasound. A variety of normal structures on a single image, including normal mouth, jaw, lips, orbits and ear.
Spine The fetal spine is well developed at 18–22 weeks. Each vertebral segment is composed of three ossification centres which appear echogenic sonographically.42 The anterior centre is the developing vertebral body while the posterior centres are formed at the junction of the lamina and the pedicle on each side.43 The ossification centres lie in a symmetrical triangular configuration, with the posterior centres oriented towards the midline. Imaging of the fetal spine can be accomplished in three planes: parasagittal, coronal and transverse. While the parasagittal and coronal images give the best overall views of the spine (Fig. 5.9), transverse images permit simultaneous evaluation of both the ossification centres and the overlying soft tissues (Fig. 5.10). Although transverse images look only at a single level, the entire spine can be quickly evaluated with transverse images with real-time ultrasound. Threedimensional multiplanar ultrasound can also be used to evaluate the spine and other bony structures (Fig. 5.11).
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Fig. 5.9 Longitudinal view shows normal spine (Sp) extending to the sacral spine.
Parasagittal imaging of the entire spine demonstrates two rows of roughly parallel ossification centers, one being the vertebral bodies and the other one a row of posterior centres. While this plane of section nicely demonstrates the overall appearance of the spine and delineates the posterior soft tissues adjacent to the midline, it theoretically might be less sensitive for subtle widening between the paired posterior ossification centres. Coronal images may be oriented to show both posterior ossification centres. However, this plane does not show visualization of the posterior soft tissues.
Heart The 18–22-week scan is an ideal time to evaluate the fetal heart.87 Indeed, the heart can usually be evaluated in great detail at this time. In additional to the requisite four-chamber view, most centres have now adopted the policy of additional views of the outflow tracts and great vessels.44,87,88 A systematic approach using specific views will result in optimal detection of cardiac defects. However, specific views are not a substitute for understanding normal anatomical relationships. Also, it should be stressed that static images are not sufficient for evaluating complex anatomical structures. Nowhere is this more true than when evaluating the dynamic fetal heart.
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Fig. 5.10 Spine. Transverse view shows normal anterior (Ant) and posterior (Post) ossification centres and overlying skin posteriorly.
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Normal fetal anatomy at 18–22 weeks
Fig. 5.11 3D multiplanar view shows normal spine (Sp), ribs (R) and scapula (Sc).
Obtaining a good four-chamber view is usually not difficult at 18–22 weeks although it requires good ultrasound technique (Fig. 5.12).45-47 This view is best obtained from an anterior or left lateral approach to avoid the spine and ribs. The scan plane is transverse through the lower thorax; however, the transducer is actually tilted slightly cephalad toward the spine to include the atria which are located posterior and superior to the ventricles. This can be accomplished by beginning the plane inferior to the heart and angling up slightly to the thorax. Using the four-chamber view, evaluation of the heart begins by overall assessment of the position, axis and size of the heart.48,49 The heart lies anteriorly within the thorax, slightly to the left of midline. The cardiac apex, as determined by a line through the ventricular septum, is directed to the left side of the hemithorax at about a 45 ° angle from the midline or roughly equidistant from the anterior and lateral aspects of the chest.50 The heart should occupy about one-third of the thorax; an abnormal heart to thoracic ratio can help identify a variety of cardiac defects.51 Cardiac rate and rhythm should be noted, with a normal range of 120–160 beats per minute from the second trimester to term.
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Fig. 5.12 Normal four-chamber axial view of the heart. LV, left ventricle; RV, right ventricle; LA, left atrium; RA, right atrium; PV, pulmonary veins; Ao, aorta.
A checklist of normal anatomical relationships can be confirmed on the fourchamber view, including the following.
• There are two atria of approximately equal size. The left atrium is the most posterior chamber.
• The two atrioventricular valves (mitral and tricuspid) show normal mobility.
• There are two ventricles of approximately equal size and thickness. Both
show normal contractility. The right ventricle is anterior in the midline, just behind the sternum, and the left ventricle is positioned left and posterior to the right ventricle. • The atrial and ventricular septa meet the two atrioventricular valves (mitral and tricuspid) to form the crux of the heart. This crux has a slightly offset cross appearance since the septal leaflet of the tricuspid valve inserts slightly lower in the ventricular septum than the mitral valve. The left atrium is the most posterior chamber and is similar in size to the right atrium. The atrial septum is often difficult to image, but appears as a continuous thin structure with the exception of a physiological opening, the foramen ovale, through which blood flows in the fetus from right to left atrium. • The interventricular septum should appear intact.
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The ventricular septum appears as a continuous thick muscular structure separating the ventricles, except for its short, thinner membranous portion near the atrioventricular (AV) valves. In some scan planes the ventricular walls or interventricular septum may have a component which is quite hypoechoic relative to the remaining muscle. This is a normal variant secondary to the complex orientation of the cardiac muscle fibres interacting variably as the scan plane changes.52 Fetal rotation or probe position changes may also alter the appearance
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Fig. 5.13 Four-chamber view without (left) and with (right) colour flow. Colour flow helps to confirm an intact ventricular septum.
Normal fetal anatomy at 18–22 weeks
and identification of some intracardiac structures, such as the ventricular septum which can vary from appearing relatively thin to rather thick when imaging in orthogonal planes. Colour flow Doppler can help confirm an intact ventricular septum (Fig. 5.13). The moderator band may be seen in the right ventricle. A commonly seen normal variant, usually within the left ventricle, is an echogenic focus caused by a specular reflection from the papillary muscles and chordae tendinae.53–55 This has been referred to as an echogenic intracardiac focus or echogenic chorda tendinae. It is observed in approximately 3–4% of the normal population before 20 weeks, and appears to be even more common among Asian populations. It typically resolves later in gestation and has no direct functional significance. However, an echogenic intracardiac focus does appear to increase the risk of fetal chromosome abnormality, especially trisomy 21, at least in non-Asian populations. In addition to the four-chamber view, other views of the heart are required to show normal relationships (Figs 5.14–5.18).42,44,56,57 However, it is important to understand the normal anatomy and circulatory pattern while obtaining these views. This sequential segmental approach is most useful in the evaluation of the heart and especially in the diagnosis of congenital heart disease. In brief, blood enters the right atrium through the superior and inferior vena cavae. Oxygenated blood from the inferior vena cava is preferentially directed across the foramen ovale to the left atrium while deoxygenated blood from the superior vena cava is directed to the right ventricle through the tricuspid valve. Deoxygenated blood is pumped through the pulmonary artery and much of this continues through the ductus arteriosus to the aorta, returning to the placenta and lower fetus. A portion of pulmonary arterial flow goes to the lungs, returning via the pulmonary veins. This blood enters the left atrium where it mixes with oxygenated blood crossing the foramen ovale and then enters the left ventricle through the mitral valve. It is then pumped through the left ventricle into the aorta.
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Fig. 5.14 Left ventricular outflow tract. Plane angled toward the right shoulder from the standard four-chamber view shows normal ascending aorta (Ao). Note the wall of the ascending aorta is continuous with the ventricular septum. RV, right ventricle; LV, left ventricle; LA, left atrium.
Other views help show these normal relationships including the venous–atrial, atrial–ventricular and ventricular–arterial junctions of both the left and right side of the heart. In addition to the four-chamber view described above, these include a view of the upper abdomen to show normal solitus, and views of the great vessels. Left and and right ventricular outflow views (see Figs 5.14, 5.15) should be obtained, if possible. A ‘three-vessel’ view (see Fig. 5.16) should also be attempted by continuing to angle the transducer superiorly from the four-chamber view. A more complete study would include the venous–atrial connections and longitudinal views of the ductal arch (see Fig. 5.17) and aortic arch (see Fig. 5.18). The venous–atrial connections of both the left and right heart can be seen on transverse views sweeping from the abdomen to the four-chamber view. As an optional view, the relationship of the inferior and superior vena cavae with the
Fig. 5.15 Right ventricular outflow tract. Plane angled toward the left shoulder from the standard four-chamber view shows the main pulmonary artery (MPA), and its continuation by way of the ductus arteriosus (DA) which joins the aorta. RPA, right pulmonary artery; RV, right ventricle; Ao, ascending aorta seen in cross-section.
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right atrium can also be seen on a longitudinal view through the right atrium. This plane shows the inferior vena cava entering the right atrium and the tricuspid valve between the right atrium and ventricle. The four-chamber view shows the atrial–ventricular connections. Demonstration of the ventricular–arterial connections and great vessels requires familiarity with normal anatomy. The pulmonary artery arises anteriorly, close to the chest wall, and is directed straight back towards the spine while the aorta arises centrally within the heart from the left ventricle and ascends to the right, posterior to the pulmonary artery (see Figs 5.15, 5.17). The right ventricular outflow tract and branching of the pulmonary artery can be identified on an axial plane just above the level for a four-chamber view with angling slightly toward the left shoulder (see Fig. 5.15) while the left ventricular outflow tract can be visualized by angling toward the right shoulder from the four-chamber view (see Fig. 5.14).
Fig. 5.17 Ductal arch. Midsagittal scan shows the ductal arch. Note the arch is relatively flattened and has no systemic vessels arising from it. Also note that the ductus arteriosus (DA) is a direct extension of the main pulmonary artery (MPA) and so receives most of the blood pumped through the right ventricle (RV). RPA, right pulmonary artery; DA, ductus arteriosus; Ao, ascending aorta in cross-section; Ao(D), descending aorta; PV, pulmonic valve.
Normal fetal anatomy at 18–22 weeks
Fig. 5.16 Three-vessel view. Axial view superiorly shows three vessels: the pulmonary artery (P) and its continuation via the ductus arteriosus, the aorta (Ao) at the level of the aortic arch, and the superior vena cava (SVC). (P)Ao, proximal or ascending aorta; (D)Ao, descending aorta; D, ductus arteriosus; LT, left; RT, right.
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Fig. 5.18 Aortic arch. The aorta (Ao) arises from the left ventricle. It is rounder in shape than the ductus arteriosus and also gives rise to the neck vessels (arrows). Ao(D), descending aorta.
Each of these views should also show the other great artery in cross-section. The left ventricular outflow view should also show continuity of the aortic wall and the ventricular septum. The ductal arch view is obtained on longitudinal midline plane which shows continuation of the ductus arteriosus from the pulmonary artery and its connection with the aorta just below the aortic arch (see Fig. 5.17). The aortic arch can be seen on transverse views above the heart as the aorta curves from right to left (see Fig. 5.16). However, the entire aortic arch and origin of the neck vessels can best be seen on an aortic arch view (see Fig. 5.18). This is an oblique longitudinal plane oriented from the right anterior chest to the left posterior chest. The descending aorta can be seen on longitudinal views, as well as transverse views where it is seen in cross-section just to the left and anterior to the spine. In summary, a checklist of the great vessels should make note of the following.
• The aorta arises from the centre of the heart and ascends as the aortic arch which can be confirmed by showing the origin of head and neck vessels.
• The pulmonary artery arises from the right ventricle and gives rise to the
pulmonary arteries and the ductus arteriosus. • The great arteries are similar in size but the pulmonary artery at the valve ring may be slightly bigger than the aorta. • The great arteries cross each other at their origin. • The ventricular septum is continuous with the aortic wall. The recent introduction of 3D and 4D fetal echocardiography has opened new possibilities of studying normal and abnormal fetal cardiac anatomy.89
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In addition to views of the heart, images directed to the heart will also demonstrate surrounding structures, including the lungs, great vessels, bony thorax and extrathoracic structures. The lungs are observed as homogeneously echogenic structures surrounding the heart. They should be roughly equal in size. They should surround the pulmonary arterial branches and pulmonary veins. Lung size has been evaluated by various ratios, lung length, and more recently by volume using 3D ultrasound.58–60
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Abdomen The basic ultrasound examination of the fetal abdomen and pelvis includes identification of the stomach, kidneys, bladder, umbilical cord insertion and adjacent anterior abdominal wall, and measurement of an abdominal circumference for dating/growth. Other organs that can easily be documented, and in some cases measured, include the liver,61 spleen,62 adrenals,63 large and small bowel and gallbladder,64 as well as major vascular structures. Both axial (Fig. 5.19) and longitudinal (Fig. 5.20) views are useful for evaluating the abdomen. The abdominal circumference (AC) measurement, while useful as an adjunctive parameter for fetal dating, finds its greatest value in the evaluation of fetal growth in the latter part of pregnancy. Fetal growth disturbances are generally detected by a change in the size of the fetal liver. This is reflected sonographically on the AC measurement, obtained on an axial image through the fetal liver where the midline umbilical vein joins the portal venous system. Only a short portion of the umbilical vein deep within the liver should be imaged, since visualization of the vein more anteriorly is only possible with oblique scans as it passes inferiorly towards the umbilicus (see Fig. 5.19). The stomach is a variably sized fluid-filled structure in the left upper quadrant. For confirmation of normal solitus, the stomach should be confirmed to be on the same side as the apex of the heart. Situs inversus seen prenatally usually reflects one of the cardiosplenic syndromes (asplenia or polysplenia). Absence of a visible
Fig. 5.19 Abdomen, axial view, at the level where the abdominal circumference measurement is obtained. St, stomach; L, liver; UV, umbilical vein; IVC, inferior vena cava; Ao, aorta; Sp, spine.
Normal fetal anatomy at 18–22 weeks
The osseous components of the thorax are readily identified due to their inherent subject contrast with the adjacent soft tissue structures. In this location the rib cage is composed of cartilage, which is sonographically hypoechoic and allows good sound transmission. The scapulae, clavicles, ribs and dorsal spine can all be easily identified on directed scanning. Since shadowing caused by the overlying bones makes examination of the intrathoracic contents difficult, especially with advancing gestational age, scan planes that avoid the bony thorax are employed when feasible.
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Fig. 5.20 Longitudinal view in the right abdomen shows the normal liver, diaphragm and more echogenic lung.
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stomach after 14 weeks is unusual. This may indicate underlying fetal abnormality or transient decrease in fetal swallowing. In this situation, a short-interval follow-up scan is indicated for confirmation.65,66 A more common presentation is a small or prominent stomach. However, it is often difficult to know what is too small or too prominent. To help in this situation, Pekindil et al67 proposed a ratio of stomach circumference to abdominal circumference expressed as a percent (SC/AC ratio). They found this was normally distributed from 15 to 39 weeks at a mean of 20.4% and ranged between 14.8% and 27.03% throughout pregnancy. Although the fetal stomach is a dynamically changing organ, the SC/AC ratio can be considered as a potentially useful parameter in assessing fetal stomach size. The fetal duodenum can be identified but dilated duodenum suggests underlying obstruction68 (double bubble sign). The gallbladder is often seen in the right abdomen near the inferior edge of the liver (Fig. 5.21). Care should be taken not to mistake the gallbladder for the umbilical vein. While both structures extend to the region of the porta hepatis, the umbilical vein is of uniform calibre, midline in position, and courses inferiorly to penetrate the abdominal wall; the gallbladder is clearly not in the midline, usually is somewhat teardrop shaped, and does not penetrate the abdominal wall. Persistent intrahepatic right umbilical vein is a relatively common normal variant in which the right umbilical vein persists rather than the left.69 Blazer et al69 observed this finding in 69 of 30,240 consecutive pregnancies at 14–26 weeks.
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Normal fetal anatomy at 18–22 weeks Fig. 5.21 Oblique scan shows a normal gallbladder in the right abdomen. This can be easily confused with the umbilical vein, which is more central in location. St, stomach.
In this situation, the persistent right umbilical vein enters the right lobe of the liver, lateral to the gallbladder rather than medial to it. In the absence of other abnormalities, this variation is associated with a favourable outcome.69 The liver occupies the majority of the upper abdomen, with a prominent left lobe extending well into the left upper quadrant in the fetus. The smaller spleen is identified as a solid organ posterior to the stomach. Much of the remaining abdominal cavity is filled with bowel. Early in the second trimester bowel appears as an area of midlevel to increased echogenicity filling the abdomen from the liver to the bladder. The large bowel progressively enlarges with meconium throughout pregnancy, measuring 3–5 mm at 20 weeks.70,71 Normal small bowel is less distinct since it is smaller, circuitous in course, and changes with peristalsis. Small bowel segments can be transiently identified with small quantities of fluid, particularly with higher-resolution scanners. Echogenic bowel may be seen as a normal variant.72 However, moderate to markedly echogenic bowel has been associated with adverse outcome including chromosome abnormality, in utero infection, growth retardation and fetal demise.73
Anterior Abdominal Wall The site of the umbilical cord insertion into the abdominal wall must be evaluated to confirm a normal-sized cord penetrating into the abdomen. The adjacent abdominal wall must also be examined to confirm its integrity. The musculature of the fetal abdominal wall appears hypoechoic, and may be confused with fetal ascites.74 Knowledge of the hypoechoic nature of fetal musculature and close attention to anatomical detail should easily differentiate the normal from abnormal.
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Confirmation of a normal three-vessel cord may be made by direct imaging of the cord to delineate the two smaller umbilical arteries and the larger umbilical vein. Alternatively, the paired umbilical arteries can be imaged within the fetus, extending along the anterior abdominal wall from the umbilicus to a position lateral to the fetal bladder (Fig. 5.22). This can easily be confirmed with colour flow imaging (Fig. 5.23), a valuable technique early in gestation when direct visualization of the arteries within the cord is suboptimal. The umbilical cord insertion site should be visualized on all routine fetal surveys. Detection of a normal cord insertion excludes the vast majority of anterior abdominal wall defects.
Urinary Tract By 18–22 weeks, the kidneys can be clearly seen as oval masses lateral to the psoas muscles and inferior to the adrenal glands (Figs 5.24, 5.25). Before this time, the kidneys may be difficult to identify with certainty so that guidelines refer to scanning through the kidney regions. Use of colour flow Doppler can be helpful for confirming the presence of two kidneys when they are difficult to visualize on standard grey-scale imaging (Fig. 5.26).
Fig. 5.22 Cord insertion site. Axial view shows the umbilical cord inserting into the umbilicus. The paired umbilical arteries are seen on this view; the umbilical vein deviates from the arteries immediately on entering the abdomen and courses cephalad to the liver. Because the cord insertion is inferior on the abdominal wall, the urinary bladder (B) can often be seen on this view.
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Normal fetal anatomy at 18–22 weeks
Fig. 5.23 Urinary bladder. A slightly inferior plane with colour flow Doppler shows the paired umbilical arteries coursing around the urinary bladder. LUA, left umbilical artery; RUA, right umbilical artery.
The kidneys grow throughout gestation and standard measurements for renal circumference, volume, thickness, width and length have been reported as a function of menstrual age.75 The ratio of kidney circumference to abdominal circumference remains constant at 0.27 to 0.30 throughout pregnancy.76 In general, the normal kidney length spans approximately 4–5 vertebral bodies.
Fig. 5.24 Kidneys. Axial view with the spine anterior in position shows the normal paraspinal kidneys (K), outlined by arrows. A tiny amount of fluid is seen within the central renal pelvis of each kidney.
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Fig. 5.25 Kidneys, coronal view. Coronal view of the abdomen shows both kidneys (K). Again note the small amount of fluid within the central renal pelvis of each kidney.
High-resolution scans can identify normal renal architecture. The medullae are arranged in anterior and posterior rows around the pelvic sinus. The medullae appear hypoechoic, probably because the tubules are thin-walled and fluid-filled, compared to the more peripheral renal cortex. Recognition of this normal renal architecture is important in distinguishing normal kidneys from those with cystic dysplasia.
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Fig. 5.26 Normal renal arteries. Colour Doppler, using the power mode, with the fetus in coronal plane shows both renal arteries arising from the aorta. This can be helpful when the kidneys are difficult to visualize on standard grey-scale imaging.
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Normal fetal anatomy at 18–22 weeks
The central renal pelvis commonly contains small amounts of fluid (urine). Obvious dilation of the renal pelvis may reflect an underlying obstructive process. Therefore, it is important to attempt to distinguish normal physiological amounts of fluid from a true abnormality. An objective method of assessment is measurements of the renal pelvis, best obtained in the anterior–posterior plane with the fetus either spine anterior or posterior relative to the transducer. ‘Cutoffs’ for normal vary with gestational age and will also vary between centres. By 18–22 weeks, we use a cut-off of 5 mm or more for suggesting a possible renal abnormality.77 As a normal variant, mild degrees of renal pyelectasis occur more commonly among fetuses that are large for gestational age and males are affected more often than females. Kent et al78 found that 13 of 37 (35%) fetuses with renal dilation of 4–8 mm at 16–21 weeks went on to require medical or surgical intervention for significant urinary tract anomalies. These anomalies included pelviureteric junction obstruction, dysplastic kidney, vesicoureteric reflux and posterior urethral valves. Follow-up evaluation is suggested at 28 weeks when renal pelvic dilation is suggested. Although not part of the genitourinary tract, the adrenal glands are usually assessed at the same time as the kidneys due to their proximity. The adrenal glands characteristically appear as triangular hypoechoic shadows which outline the upper poles of the kidneys. The right gland is positioned immediately posterior to the inferior vena cava, while the left lies lateral to the aorta. Sonographically the adrenals are hypoechoic peripherally, with a central echogenic layer. The urinary bladder is a fluid-filled structure located low within the pelvis, in the midline. Changes in bladder volume over time are obvious and help differentiate the urinary bladder from other cystic pelvic structures. If in doubt, the umbilical arteries course along the lateral walls of the bladder and confirm it as the urinary bladder. Therefore, this view can confirm the presence of both the urinary bladder and both umbilical arteries.
Genitalia Evaluation of the genitalia is often desired by the prospective parents in order to determine gender, and is also sometimes medically indicated. Certainly fetal genitalia are well visualized by 18–22 weeks (Fig. 5.27), and fetal gender can probably be accurately determined by the late first trimester.79 The first finding with a male fetus is delineation of the penis, a solid structure in contrast to the fluid-filled umbilical cord which may lie between the thighs. The scrotum is a bulbous soft tissue structure increasingly apparent at the base of the penis as gestation progresses. Although the scrotum is visualized, testicular descent is not seen before 26 weeks. Longitudinal scans of the scrotum and penis may produce a ‘turtle’ appearance. Female genitalia are confirmed early in gestation via identification of several parallel linear echos representing the margins of the labia. In the third trimester the prominent soft tissues of the labia majora border the linear echoes of the labia minora.
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Fig. 5.27 Normal genitalia. Axial images of the perineum show normal female (left) and male (right) external genitalia.
Skeleton and Extremities The bones of the extremities are readily identifiable due to their inherent subject contrast with the surrounding soft tissues, from the late first trimester to term. The femur is the only long bone which is routinely measured, however, being a primary parameter for fetal dating as well as a screen for the skeletal dysplasias. Humerus length is also commonly measured during the second trimester, especially as a potential marker for fetal Down syndrome (Fig. 5.28). Mild contour variation in the normal femoral shaft is often apparent, with a straighter appearance on the lateral aspect and a mild ‘bowed’ appearance medially.80 Only the ossified portions of the bone are measured, excluding the hypoechoic cartilaginous epiphyses of the femoral head and condyles distally.81 It is recommended that a survey of all extremities be performed to confirm a grossly normal appearance of the bones and soft tissues to the level of the feet and hands (Figs 5.29, 5.30). Use of 3D multiplanar ultrasound can also help to confirm normal extremities, including the hands and feet (Fig. 5.31). Imaging of specific bones is accomplished by careful progression from one known structure
Fig. 5.28 Longitudinal views of the femur (F, left) and humerus (H, right). These are similar in size during the second trimester.
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to the next, adjusting probe position as needed to obtain the desired images.82 Scanning beyond the femur will outline the hypoechoic cartilages of the distal femur and proximal tibia. Subsequently the tibia and fibula and orientation of
Fig. 5.30 View of a normal hand including the thumb. Note three bones (proximal phalanx, middle phalanx and distal phalanx) of each finger.
Normal fetal anatomy at 18–22 weeks
Fig. 5.29 Feet. Axial view shows normal paired feet (F).
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Fig. 5.31 Hand, 3D view. Three-dimensional ultrasound with surface rendering better shows the complex shape of the hand.
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the leg to the foot can be shown. Measurement of foot length and documentation of digits are possible with appropriate fetal positioning and careful scanning. Similar scanning through the upper extremities can detail the humerus, radius and ulna, and hand. The clavicle and scapula define the shoulder girdle. The scapula imaged in long axis coronally has a characteristic shape resembling a ‘Y’ with the supraspinatus, subscapularis and infraspinatus muscles in their respective fossae. The scapula has a triangular shape when imaged posteriorly. The clavicles can be seen if not
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Conclusion A second-trimester scan has now become widely accepted throughout much of the developed world. We believe it should be considered an essential part of obstetric care, but only when performed by experienced sonographers or sonologists. Armed with a systematic approach, a good understanding of normal fetal anatomy and familiarity with normal sonographic appearances and common variants, the sonographer/sonologist can offer accurate reassurance in the vast majority of normal pregnancies, and at the same time detect the majority of major defects in anomalous fetuses. Finally, it will be of interest to see to what extent new techniques like echo-planar magnetic resonance will complement or even replace certain areas of fetal anatomic assessment.90
Normal fetal anatomy at 18–22 weeks
obscured by flexion of the fetal head. They grow at a linear rate of approximately 1 mm per week, reaching a length of 20 mm at 20 weeks and 40 mm at 40 weeks. The humeral head epiphyseal cartilage lies between the ossified distal clavicle, scapula and proximal humeral diaphysis. At the elbow, the non-ossified coronoid fossa delineates the medial and lateral humeral epicondyles. The more proximal extent of the ulna at the elbow distinguishes it from the radius. Demonstration of the ulna and radius ending at the same level at the wrist effectively excludes many radial ray defects. The non-ossified carpals produce a conglomerate zone of grey echoes antenatally, but the ossified metacarpal and phalanges are readily visualized if the fetus extends the hand. The foot length is similar to the ossified femoral shaft throughout much of pregnancy.83
References 1. Grandjean H, Larroque D, Levi S. The performance of routine ultrasonographic screening of pregnancies in the Eurofetus Study. Am J Obstet Gynecol 1999;181(2):446–454 2. Vintzileos AM, Ananth CV, Smulian JC, Beazoglou T, Knuppel RA. Routine secondtrimester ultrasonography in the United States: a cost-benefit analysis. Am J Obstet Gynecol 2000;182(3):655–660 3. Schwarzler P, Senat MV, Holden D, Bernard JP, Masroor T, Ville Y. Feasibility of the second-trimester fetal ultrasound examination in an unselected population at 18, 20 or 22 weeks of pregnancy: a randomized trial. Ultrasound Obstet Gynecol 1999;14(2):92–97 4. American Institute of Ultrasound in Medicine. Guidelines for performance of the antepartum obstetrical ultrasound examination. American Institute of Ultrasound in Medicine, Laurel, MD, 1994
5. American College of Obstetrics and Gynecology. Technical bulletin no. 187. Ultrasound in pregnancy. American College of Obstetrics and Gynecology, Washington, DC, 1993 6. Filly RA, Cardoza JD, Goldstein RB, Barkovich AJ. Detection of fetal central nervous system anomalies: a practical level of effort for a routine sonogram [see comments]. Radiology 1989;172(2): 403–408 7. Reece EB, Goldstein I. Three-level view of fetal brain imaging in the prenatal diagnosis of congenital anomalies. J Matern Fetal Med 1999;8(6):249–252 8. Pilu G, Perolo A, Falco P, Visentin A, Gabrielli S, Bovicelli L. Ultrasound of the fetal central nervous system. Curr Opin Obstet Gynecol 2000;12(2):93–103 9. Malinger G, Zakut H. The corpus callosum: normal fetal development as shown by transvaginal sonography. Am J Roentgenol 1993;161:1041–1043
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10. Cardoza JD, Goldstein RB, Filly RA. Exclusion of fetal ventriculomegaly with a single measurement: the width of the lateral ventricular atrium. Radiology 1988;169:711–714 11. Alagappan R, Browning PD, Laorr A et al. Distal lateral ventricular atrium: reevaluation of normal range. Radiology 1994;193:405–408 12. Hertzberg BS, Lile R, Foosaner DE et al. Choroid plexus–ventricular wall separation in fetuses with normal-sized cerebral ventricles at sonography: postnatal outcome. Am J Roentgenol 1994;163:405–410 13. Farrell TA, Hertzberg BS, Kliewer MA, Harris L, Paine SS. Fetal lateral ventricles: reassessment of normal values for atrial diameter at US [see comments]. Radiology1994;193:409–411. Comment in: Radiology 1994;193(2):315–317 14. McGahan JP The fetal head: borderlines. Semin Ultrasound CT MR 1998;19: 318–328 15. Pilu G, Falco P, Gabrielli S, Perolo A, Sandri F, Bovicelli L. The clinical significance of fetal isolated cerebral borderline ventriculomegaly: report of 31 cases and review of the literature. Ultrasound Obstet Gynecol 1999;14(5):320–326 16. Patel MD, Goldstein RB, Tung S, Filly RA. Fetal cerebral ventricular atrium: difference in size according to sex. Radiology 1995;194:713–715 17. Heiserman J, Filly RA, Goldstein RB. Effect of measurement errors on sonographic evaluation of ventriculomegaly. J Ultrasound Med 1991;10(3):121–124 18. Browning PD, Laorr A, McGahan JP et al. Proximal fetal cerebral ventricle: description of US technique and initial results. Radiology 1994;192:337–341 19. Cronan MS, McGahan JP. A new ultrasound technique to visualize the proximal fetal cerebral ventricle. J Diagn Med Sonography 1991;6:333–335 20. Kraus I, Jirasek JE. Some observations of the structure of the choroid plexus and its cysts. Prenat Diagn 2002;22:1223–1228 21. Nyberg DA, Crane JP. Chromosome abnormalities. In: Nyberg DA, Mahony BS, Pretorius D (eds) Diagnostic ultrasound of fetal anomalies. Yearbook Publishers, Chicago, 1990: 676–724 22. Walkinshaw S, Pilling D, Spriggs A. Isolated choroid plexus cysts: the need for routine offer of karyotyping. Prenat Diagn 1994;14(8):663–667
23. Gross SJ, Shulman LP, Tolley EA et al. Isolated fetal choroid plexus cysts and trisomy 18: a review and meta-analysis. Am J Obstet Gynecol 1995;172:83–87 24. Snijders RJ, Shawa L, Nicolaides KH. Fetal choroid plexus cysts and trisomy 18: assessment of risk based on ultrasound findings and maternal age. Prenat Diagn 1994;14(12):1119–1127 25. Sullivan A, Giudice T, Vavelidis F, Thiagarajah S. Choroid plexus cysts: is biochemical testing a valuable adjunct to targeted ultrasonography? Am J Obstet Gynecol 1999;181(2):260–265 26. Filly RA, Cardoza JD, Goldstein RB et al. Detection of fetal central nervous system anomalies: a practical level of effort for a routine sonogram. Radiology 1989;172: 403–408 27. Nyberg DA. Recommendations for obstetric sonography in the evaluation of the fetal cranium. Radiology 1989;172:309–311 28. Mahony BS, Callen PW, Filly RA et al. The fetal cisterna magna. Radiology 1984;153:173–176 29. Laing FC, Frates MC, Brown DL et al. Sonography of the fetal posterior fossa: false appearance of mega-cisterna magna and Dandy–Walker variant. Radiology 1994;192:247–251 30. Bromley B, Nadel AS, Pauker S, Estroff JA, Benacerraf BR. Closure of the cerebellar vermis: evaluation with second trimester US. Radiology 1994;193:761–763 31. Knutzon RK, McGahan JP, Salamat MS, Brant WE. Fetal cisterna magna septa: a normal anatomic finding. Radiology 1991;180(3):799–801 32. Pretorius DH, Kallman CE, Grafe MR, Budorick NE, Stamm ER. Linear echoes in the fetal cisterna magna. J Ultrasound Med 1992;11:125–128 33. Goldstein I, Jakobi P, Tamir A, Goldstick O. Nomogram of the fetal alveolar ridge: a possible screening tool for the detection of primary cleft palate. Ultrasound Obstet Gynecol 1999;14(5):333–337 34. Jeanty P, Cantraine F, Cousaert E et al. The binocular distance: a new way to estimate fetal age. J Ultrasound Med 1984;3: 241–243 35. Birnholz JC. The fetal external ear. Radiology 1983;147:819–821 36. Lettieri L, Rodis JF, Vintzileos AM, Feeney L, Ciarleglio L, Craffey A. Ear length in second-trimester aneuploid fetuses. Obstet Gynecol 1993;81(1):57–60
✩✩✩✩✩✩✩✩✩✩✩ ✩ 52. Brown DL, Cartier MS, Emerson DS et al. The peripheral hypoechoic rim of the fetal heart. J Ultrasound Med 1989;8: 603–608 53. Schechter AG, Fakhry J, Shapiro LR, Gewitz MH. In utero thickening of the chordae tendinae. A cause of intracardiac echogenic foci. J Ultrasound Med 1987;6(12):691–695 54. Bromley B, Lieberman E, Laboda L, Benacerraf BR. Echogenic intracardiac focus: a sonographic sign for fetal Down syndrome. Obstet Gynecol 1995;86(6):998–1001 55. Manning JE, Ragavendra N, Sayre J et al. Significance of fetal intracardiac echogenic foci in relation to trisomy 21: a prospective sonographic study of highrisk pregnant women. Am J Roentgenol 1998;170(4):1083–1084 56. DeVore GR. The aortic and pulmonary outflow tract screening examination in the human fetus. J Ultrasound Med 1992;11:345–348 57. DeVore GR. Color Doppler examination of the outflow tracts of the fetal heart: a technique for identification of cardiovascular malformations. Ultrasound Obstet Gynecol 1994;4:463–471 58. Yoshimura S, Masuzaki H, Gotoh H, Fukuda H, Ishimaru T. Ultrasonographic prediction of lethal pulmonary hypoplasia: comparison of eight different ultrasonographic parameters. Am J Obstet Gynecol 1996;175(2):477–483 59. D'Arcy TJ, Hughes SW, Chiu WS et al. Estimation of fetal lung volume using enhanced 3-dimensional ultrasound: a new method and first result. Br J Obstet Gynaecol 1996;103:1015–1020 60. Laudy JA, Janssen MM, Struyk PC, Stijnen T, Wladimiroff JW. Three-dimensional ultrasonography of normal fetal lung volume: a preliminary study. Ultrasound Obstet Gynecol 1998;11(1):13–16 61. Vintzileos AM, Neckles S, Campbell WA et al. Fetal liver ultrasound measurements during normal pregnancy. Obstet Gynecol 1985;66:477–480 62. Schmidt W, Yarkoni S, Jeanty P et al. Sonographic measurements of the fetal spleen: clinical implications. J Ultrasound Med 1985;4:667–672 63. Rosenberg ER, Bowie JD, Andreotti RF et al. Sonographic evaluation of the fetal adrenal glands. Am J Roentgenol 1982;139:1145–1147
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37. Petrikovsky BM, Vintzileos AM, Rodis JF. Sonographic appearance of occipital fetal hair. J Clin Ultrasound 1989;17:425–427 38. Benacerraf B, Frigoletto F, Laboda L. Sonographic diagnosis of Down syndrome in the second trimester. Am J Obstet Gynecol 1985;153:49–52 39. Gray DL, Crane JP. Optimal nuchal skinfold thresholds based on gestational age for prenatal detection of Down syndrome. Am J Obstet Gynecol 1994;171:1282–1286 40. Bahado-Singh RO, Oz UA, Kovanci E et al. Gestational age standardized nuchal thickness values for estimating midtrimester Down's syndrome risk. J Matern Fetal Med 1999;8(2):37–43 41. Ho SS, Metreweli C. Normal fetal thyroid volume. Ultrasound Obstet Gynecol 1998;11:118–122 42. Filly RA, Simpson GF, Linkowski G. Fetal spine morphology and maturation during the second trimester. J Ultrasound Med 1987;6:631–636 43. Gray DL, Crane JP, Rudloff MA. Prenatal diagnosis of neural tube defects: origin of midtrimester vertebral ossification centers as determined by sonographic water-bath studies. J Ultrasound Med 1988;7: 421–427 44. Yoo SJ, Lee YH, Cho KS, Kim DY. Sequential segmental approach to fetal congenital heart disease. Cardiol Young 1999;9(4):430–444 45. McGahan JP. Sonography of the fetal heart: findings on the four-chamber view. Am J Roentgenol 1991;156:547–553 46. Copel JA, Gianluigi P, Green J et al. Fetal echocardiographic screening for congenital heart disease: the importance of the fourchamber view. Am J Obstet Gynecol 1987;157:648–655 47. Cook AC, Yates RW, Andersson RH. Normal and abnormal cardiac anatomy. Prenat Diagn 2004;24(1):32–48 48. Brown DL, DiSalvo DN, Frates MC et al. Sonography of the fetal heart: normal variants and pitfalls. Am J Radiol 1993;160:1251–1255 49. Frates MC. Sonography of the normal fetal heart: a practical approach. Am J Roentgenol 1999;173:1363–1370 50. Comstock CH. Normal fetal heart axis and position. Obstet Gynecol 1987;70:255–259 51. Paladini D, Chita SK, Allan LD. Prenatal measurement of cardiothoracic ratio in evaluation of heart disease. Arch Dis Child 1990;65(1 Spec No):20–23
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64. Hata K, Aoki S, Hata T et al. Ultrasonographic identification of the human fetal gall bladder in utero. Gynecol Obstet Invest 1987;23:79–83 65. Millener PB, Anderson NG, Chisholm RJ. Prognostic significance of non-visualization of the fetal stomach by sonography. Am J Roentgenol 1993;160:827–830 66. Pretorius DH, Gosink BB, Clautice-Engle T et al. Sonographic evaluation of the fetal stomach: significance of nonvisualization. Am J Roentgenol 1988;151:987–989 67. Pekindil G, Varol F, Yuce MA, Yardim T. The fetal stomach circumference/abdominal circumference ratio: a possible parameter in assessing fetal stomach size. Yonsei Med J 1998;39(3):222–228 68. Levine D, Goldstein RB, Cadrin C. Distention of the fetal duodenum: abnormal finding? J Ultrasound Med 1998;17(4):213–215 69. Blazer S, Zimmer EZ, Bronshtein M. Persistent intrahepatic right umbilical vein in the fetus: a benign anatomic variant. Obstet Gynecol 2000;95(3):433–436 70. Nyberg DA, Mack LA, Pattern RM et al. Fetal bowel: normal sonographic findings. J Ultrasound Med 1987;6:3–6 71. Parulekar SG. Sonography of normal fetal bowel. J Ultrasound Med 1991;10: 211–220 72. Perez CG, Goldstein RB. Sonographic borderlands in the fetal abdomen. Semin Ultrasound CT MR 1998;19:336–346 73. Nyberg DA, Dubinsky, Mahony BS, Luthy DA, Hickok DE, Sorenson T. Echogenic fetal bowel: clinical importance. Radiology 1993;188:527–531 74. Hashimoto BE, Filly RA, Callen PW. Fetal pseudoascites: further observations. J Ultrasound Med 1986;5:151–152 75. Cohen HL, Cooper J, Eisenberg P et al. Normal length of fetal kidneys: sonographic study in 397 obstetric patients. Am J Roentgenol 1991;157:545–548 76. Grannum P, Bracken M, Silverman R et al. Assessment of fetal kidney size in normal gestation by comparison of ratio of kidney circumference to abdominal circumference. Am J Obstet Gynecol 1980;136:249–254 77. Anderson N, Clautice-Engle T, Allan R et al. Detection of obstructive uropathy in the
fetus: predictive value of sonographic measurements of renal pelvic diameter at various gestational ages. Am J Roentgenol 1995;164:719–723 78. Kent A, Cox D, Downey P, James SL. A study of mild fetal pyelectasia – outcome and proposed strategy of management. Prenat Diagn 2000;20(3):206–209 79. Shapiro E. The sonographic appearance of normal and abnormal fetal genitalia. J Urol 1999;162:530–533 80. Goldstein RB, Filly RA, Simpson G. Pitfalls in femur length measurements. J Ultrasound Med 1987;6:203–207 81. Lessoway VA, Schulzer M, Wittmann BK. Sonographic measurement of the fetal femur: factors affecting accuracy. J Clin Ultrasound 1990;18:471–476 82. Mahony BS, Filly RA. High resolution sonographic assessment of the fetal extremities. J Ultrasound Med 1984;3: 489–498 83. Mercer BM, Sklar S, Shariatmadar A. Fetal foot length as a predictor of gestational age. Am J Obstet Gynecol 1987;156:350–355 84. Barnewolt CE, Estroff JA. Sonography of the fetal central nervous system. Neuroimaging Clin North Am 2004;14:255–271 85. Garel C. Fetal cerebral biometry: normal parenchymal findings and ventricular size. Eur Radiol 2005;15:809–813 86. Hashimoto K, Shimizu T, Shimoya K, Kanzaki T, Clapp YF, Muzeta Y. Fetal cerebellum: US appearances with advancing age. Radiology 2001;221:70–74 87. Caoni R, McEwing R. Three crosssectional planes for fetal color Doppler echocardiography. Ultrasound Obstet Gynecol 2003;21:81–93 88. Allan L. Technique of fetal echocardiography. Paediatric Cardiol 2004;25:223–233 89. De Vore GR. Three-dimensional and fourdimensional fetal echocardiography: a new frontier. Curr Opin Pediatr 2005;17: 592–604 90. Duncan KR, Issa B, Moore R, Baker PN, Johnson IR, Gowland PA. A comparison of fetal organ measurements by echo-planar magnetic resonance imaging and ultrasound. Br J Obstet Gynaecol 2005;112:43–49
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Amniotic fluid and placental localization Juriy W Wladimiroff Sturla H Eik-Nes
Abstract Various pathways determine amniotic fluid production and absorption, which include fetal swallowing, lung fluid and urine production and flow across the chorionic plate. Measurements of the amount of amniotic fluid include the 1 or 2 cm pocket rule and the amniotic fluid index. Oligohydramnios is associated with fetal renal pathology and fetal growth restriction, whereas polyhydramnios is often associated with a wide range of fetal congenital anomalies, maternal diabetes mellitus, multiple pregnancy and nonimmune hydrops. Ultrasound is the method of choice to locate the placenta. The most common indications for locating the placenta are in connection with first-trimester invasive procedures, bleeding in the second and third trimesters, as part of the routine second-trimester exam and prior to external version of the fetus in late pregnancy. The placenta is located in the fundal area, on the left or right lateral side, the posterior or the anterior side or a combination thereof. Clinically it is most useful to distinguish the relation between the inner os of the cervical canal and the edge of the placenta.
Keywords Amniotic fluid absorption, amniotic fluid production, amniotic fluid volume, management of abnormal placental location, oligohydramnios, placenta praevia, placental embryology, placental functional anatomy, placental location, polyhydramnios.
Amniotic Fluid The amount of amniotic fluid surrounding the fetus provides us with information about the fetal condition. To appreciate abnormal changes in amniotic fluid
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Amniotic Fluid Physiology It is during the period of embryonic development that the amnion is formed and surrounds the embryo. In the beginning the amnion itself is surrounded by coelomic fluid which will disappear from 9–10 weeks of gestation and a rapid expansion of amniotic fluid volume will occur thereafter. Amniotic fluid is 98–99% water and its chemical composition varies with gestational age.10 There are five pathways playing a major role in the exchange of water and solutes between fetus and amniotic fluid.14 Excretion from the fetus into the amniotic cavity consists of fetal urine flow and lung flow. The onset of fetal micturition is associated with a reduction of amniotic osmolarity which will continue with advancing gestational age.10 Reabsorption of amniotic fluid takes place through fetal swallowing and absorption into the fetal circulation across the fetal surface of the placenta1 and exchange across the fetal skin before completion of the keratinization process at approximately 22 weeks of gestation. Finally, there appears to be excretion from the fetal salivary glands into the amniotic fluid. A short resumé of a few of the most important pathways will now follow. Fetal urinary production Fetal urine flow constitutes an important source of amniotic fluid, hence the development of severe oligohydramnios in bilateral renal agenesis or urethral obstruction. Diagnostic ultrasound has provided data on hourly fetal urinary production rates, with values from 2–3 mL/h at 20 weeks to 30–35 mL/h at term.17 This would result in a urinary production rate of 700–800 mL/24 h, which is approximately 25% of fetal body weight/day. Over the years different urine production rates have been reported as a result of different measuring techniques. It seems that the above figures are probably more or less correct. Fetal urine contributes not only to amniotic fluid volume but also to its composition, since osmolarity is about onehalf and chloride and sodium concentration are about one-third of plasma values. Lung fluid The production of lung fluid is 250–300 mL/24 h at term, which is approximately 10% of fetal body weight.9 A very small percentage of this fluid remains in the lungs for expansion with growth. Nearly 99% of the fluid leaves the lungs through the trachea. Beyond the trachea about 50% is swallowed and the remaining 50% will appear in the amniotic fluid.
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Flow across the chorionic plate It is likely that the exchange from the amniotic cavity to the fetal blood compartment of water and solutes is quite considerable, with figures of 200–250 mL/day of water in normal fetal development near term.
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Amniotic Fluid Volume
Methods of assessment Several methods of amniotic fluid assessment have been employed to detect adverse fetal conditions. Determination of total intrauterine volume reflecting the sum of the volume of all intrauterine contents (fetus, placenta, amniotic fluid) is achieved from longitudinal, transverse and anteroposterior uterine dimensions. This method was particularly applied for the early detection of fetal growth restriction, but has a low accuracy. A quantitative approach of assessing amniotic fluid volume is the 1 cm or, even better, the 2 cm pocket rule. Using ultrasound, the largest cord-free pocket of amniotic fluid is detected and the vertical and transverse diameter of this pocket is measured with the transducer always at right angles to the maternal abdominal wall. Amniotic fluid volume was considered normal if the pocket measured 1 cm or more in its largest vertical diameter and reduced if the diameter was less than 1 cm.8 However, this approach leads to a pick-up rate of fetal growth restriction of only 4%. Later it was demonstrated that amniotic fluid pockets greater than 1 cm but less than 2 cm should also undergo further investigation for fetal underdevelopment.2 Here, the single deepest pocket was identified. A semi-quantitative method of assessing amniotic fluid volume is the amniotic fluid index technique.13 Using the umbilicus as a reference point, the uterus is divided into an upper and a lower half. The linea nigra is subsequently used to divide the uterus into a right and a left half, resulting in four uterine quadrants. The ultrasound transducer is placed in each quadrant with the transducer head always at right angles to the floor. A more oblique positioning of the transducer head will result in an inaccurate amniotic fluid volume measurement. Moreover, the investigation should extend to the lateral margins of the uterus since often substantial amniotic fluid may be situated in the flanks of the pregnant woman when in the supine position. In each quadrant the largest pocket of fluid is sought according to the above technique. The vertical diameter of each of the pockets is then measured (Fig. 6.1). The numbers obtained from each quadrant are added up. The resulting figure in centimetres represents the amniotic fluid index for that particular pregnant woman. A normal amniotic fluid index ranges between 5 and 25 cm. The amniotic fluid index and single deepest pocket measurement appear to perform best for the identification of normal amniotic fluid volumes, whereas the identification of oligo- and polyhydramnios is of limited value.7 In another study, the amniotic fluid index was not significantly correlated with perinatal outcome.11 The amniotic fluid index seems to offer no advantage in detecting adverse outcomes compared with the single deepest pocket when performed with the biophysical profile.7 Three-dimensional ultrasound has been used in measuring amniotic fluid or gestational sac volume at 11–14 weeks of gestation,4 whereas later in pregnancy magnetic resonance imaging was found to be comparable with ultrasound evaluation for the prediction of oligohydramnios.18
Amniotic fluid and placental localization
Both amniotic fluid volume and its composition reflect the status of mother and fetus.
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4.7 cm
5.8 cm
4.9 cm
6.2 cm
Fig. 6.1 Measurement of the vertical diameter in each of the four quadrants of the uterus. The numbers are added up to calculate the amniotic fluid index.
Normal amniotic fluid volume values Amniotic fluid volume increases from approximately 70 mL at 11 weeks of gestation to 800 mL at 28 weeks followed by a slower increase to about 1000 mL at 34 weeks. A decline in volume takes place during the last 6 weeks of gestation to about 800 mL at 40 weeks. Gestational sac volume appears to be a poor predictor of major chromosomal defects.4 Abnormal amniotic fluid volumes Abnormalities in amniotic fluid volume are associated with increased perinatal mortality and morbidity.
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Oligohydramnios Oligohydramnios is defined as a deepest fluid pocket of less than 2 cm or an amniotic fluid index of 5 cm or less. It develops in 0.5–4.0% of all pregnancies and can be associated with fetal growth restriction as a result of reduced renal perfusion and urinary output. Severe oligohydramnios (deepest fluid pocket smaller than 1 cm) or even anhydramnios may develop in the presence of bilateral renal agenesis or urethral obstruction/stenosis. Pregnancies beyond 40 weeks may be complicated by reduced amounts of amniotic fluid with volumes down
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Polyhydramnios Polyhydramnios is defined as a deepest fluid pocket of more than 8 cm or an amniotic fluid index of at least 25 cm or more. Its incidence has been reported to range from 0.4% to 3.3%. Chronic polyhydramnios which develops gradually over weeks or months is more common than acute polyhydramnios. The aetiology of polyhydramnios is diverse and includes fetal congenital anomalies, notably neural tube defects and neuromuscular defects preventing adequate swallowing, on the one hand, and gastrointestinal obstruction resulting in fluid congestion on the other. Nearly one in five pregnancies with chronic polyhydramnios has been associated with fetal anomalies. Other abnormal maternal and fetal conditions associated with polyhydramnios are maternal diabetes mellitus, macrosomia, multiple pregnancy and non-immune fetal hydrops. Also, lesions of the umbilical cord and placenta have been associated with polyhydramnios. However, in approximately two-thirds of pregnancies with polyhydramnios, no specific cause can be established. Idiopathic polyhydramnios12 does not seem to be less associated with adverse perinatal outcomes than polyhydramnios in which one of the above fetal or maternal conditions has been identified. Polyhydramnios in itself may create obstetric problems such as premature labour, postpartum haemorrhage and PROM resulting in prolapsed cord.
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to about 350 mL at 42 weeks of gestation. Oligohydramnios may also develop in association with drug therapies such as indometacin treatment in premature labour. Decreased renal perfusion may be the underlying mechanism for this. Another cause of oligohydramnios is premature rupture of the membranes (PROM), which occurs in approximately 10% of all pregnancies and is associated with increased perinatal mortality and morbidity due to premature labour, chronic fetal distress or infection. When severe oligohydramnios develops before 20–25 weeks of gestation, there is a high association with fetal pulmonary hypo plasia, fetal facial compression and abnormal position/contractures of hands/feet (oligohydramnion sequence).
Conclusions Various pathways determine production and absorption of amniotic fluid, amongst which are fetal swallowing, fetal urinary production, fetal lung fluid excretion, fluid transfer through the chorionic plate and to a minor extent fluid excretion by the fetal salivary glands. Normal amniotic fluid volumes display a wide distribution with a marked increase up to about 34 weeks and a gradual reduction thereafter. Determination of amniotic fluid volume includes single deepest pocket and amniotic fluid index measurement. Abnormal amniotic fluid volumes include both oligohydramnios and polyhydramnios. Whereas oligohydramnios is mostly associated with fetal pathology, polyhydramnios may be due to abnormal fetal or maternal conditions or may be idiopathic. The predictive value of amniotic fluid index and single deepest amniotic fluid pocket for oligo- and polyhydramnios appears to be limited.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ Measurement of the vertical diameter in each of the four quadrants of the uterus is undertaken. The numbers are added up to calculate the amniotic fluid index.
Placenta Localization The introduction of ultrasound technology represented a unique step forward in our ability to make a diagnosis regarding the human placenta. Rapidly, it became clear that the new technology had far more advantages than x-ray, thermography and scintigraphy in the ability to locate the placenta, and it left the old techniques obsolete. Even the particularly difficult problem of defining the exact delineation of the border in cases where placenta praevia was suspected is now of historical interest only as a consequence of high-resolution ultrasound and the transvaginal scanning approach.
Embryology The placenta is regarded as an organ of fetal origin as it develops solely from the outer cell layer of the blastocyst, the trophoblast. The contact with the uterine endometrium and the trophoblast induces a proliferation of the trophoblast. Some of the trophoblast cells lose their cell membrane and form a syncytium, the so-called syncytioblast. This process stimulates a decidual reaction in the endometrium that causes the stroma to become thicker and highly vascularized, then called decidua. A thin capsule of the decidua called decidua capsularis covers the part of the embryo which is protruding into the endometrial cavity. The decidua at the embryonic pole develops into the decidua basalis, which takes part in the formation of the future placenta. During early development, the vessels supporting the decidua capsularis regress and the smooth chorion or chorion laeve develops.6 The vessels supporting the decidua basalis are retained and the leafy chorion or chorion frondosum is developed and the growth process of the placenta, which takes most of the remaining time of the pregnancy, then commences (Fig. 6.2).
Chorion laeve Decidua capsularis
Chorion frondosum Decidua basalis
Decidua parietalis
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Fig. 6.2 Status of the chorion and the decidua at approximately 8 weeks' gestational age.
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Functional anatomy The human placenta is the interface between the circulations of the mother and fetus for the exchange of nutrients, respiratory gases and waste products.3 The physiological mechanism of the transfer of specific substances is complicated and remains a field for advanced research. This is in contrast to other organ systems where such a mechanism is mostly understood. The human placenta has approximately 120 cotyledons that together comprise the functional unit of the organ. Each of these cotyledons has a primary villus stem which arises from the chorionic plate and is supplied by the primary branches of the fetal vessels. Further down the vascular tree, these branches form the secondary and tertiary stem where the vascular exchange takes place. From the maternal side, the pulsative blood flow from the spiral arteries enters the intercotyledonary space and flushes the maternal side of the vascular space all the way up to the chorionic plate. Between the cotyledons, the blood filters into venous channels and returns to the decidual plate. There is complete separation between the fetal and the maternal blood and all exchange of nutrients and blood gases takes place through the vasculosyncytial membranes separating the two circulatory systems. Development of the placenta as evaluated by ultrasound technology During weeks 8–12 the development of the placenta may be followed and the chorion frondosum (placenta) may be easily differentiated from the chorion laeve (chorion) (Fig. 6.3). From week 12 onwards it becomes possible to differentiate between the placenta, the basal plate facing the maternal side of the placenta and the chorionic plate facing the fetus. The placenta grows as pregnancy progresses, allowing easy identification.
Indications for the Location of the Placenta
• First-trimester invasive procedures such as abdominal or transvaginal chorion villus biopsy
• Transabdominal amniocentesis in the second trimester and other invasive procedures performed any time later in the pregnancy
• Bleeding in the second and third trimesters • Evaluation of the placenta and its location and relation to the uterine wall in cases of suspected placental abruption • Routine fetal examination performed at 18–20 weeks • Prior to external version of the fetus in late pregnancy.
The most common indication for location of the placenta is during systematic evaluation of the uterus and the intrauterine contents during the second-trimester fetal examination or ‘routine fetal examination’ as it is frequently called. The fetal examination is best initiated by the inverted U-movement of the transducer starting at the symphysis, slowly moving the transducer in a transverse plane on one side of the uterus to the top of the uterus and then down to the symphysis
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Fig. 6.3 Pregnancy at 10 weeks' gestational age. The placenta and the chorion are clearly imaged, as is the amniotic sac surrounding the fetus. The amnion has not yet fused with the chorion.
on the contralateral side. During this procedure the placenta may be located and other important features such as the viability of the fetus, the position and the number of fetuses may be established. In early pregnancy, i.e. before the 18–20-week routine fetal examination, there is no reason to register the location of the placenta except for those indicated above. Various locations of the placenta In the second trimester, the chorionic plate or the fetal surface of the placenta is usually seen as a white line. The placenta is usually relatively echogenic, with equally distributed, fine-grained echoes through the full extent of the organ. The basal plate is not always easy to distinguish but the uterine tissue, which is about 1.5 cm thick, appears slightly darker in its fine-grained echo setting compared to the placenta, making the delineation between the placenta and the uterine tissue possible (Fig. 6.4). Normally, the placenta is located in the fundal area on the left or right lateral side, the posterior or the anterior side (Fig. 6.5) or a combination thereof. The most important clinically useful distinction of the location is
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Fig. 6.4 Placenta located on the anterior wall. The uterine wall, the basal plate of the placenta facing the uterus and the chorionic plate facing the fetus are clearly seen.
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Fig. 6.5 Placenta located on the anterior wall including a velamentous insertion of the cord.
the relation between the lower portions of the placenta and the internal os of the uterus (Fig. 6.6). Attempts should be made to demonstrate the lower portion of the placenta and the internal os on the same image. Care must be taken to distinguish between Braxton Hicks contractions and the placenta. The contraction appears darker and less echogenic (Fig. 6.7). The final location of the placenta may require additional sagittal and parasagittal scans. It is not difficult to locate the placenta except when it is on the lower posterior wall. The overview might be difficult due to extremities or a larger presenting part of the fetus casting a shadow on the deeper portion of the image. Scanning from the right or left side of the uterus or scanning transvaginally can help overcome the problem.
Fig. 6.6 The placenta is located on the posterior wall, covering the internal os of the cervical canal. The placental edge is marked with +.
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Fig. 6.7 The placenta is located on the posterior wall. A local contraction of the uterus behind the placenta can clearly be distinguished, making the placenta seem to be protruding into the amniotic fluid.
Placenta praevia The placenta may cover the internal uterine os (see Fig. 6.6). When this is the case, an exact delineation of the location of the placenta and a specific management protocol are required. If more than 2.5 cm of the placenta covers the internal os, it is characterized as placenta praevia. A transvaginal scan may assist in providing a more detailed location of the lower portion of the placenta. In addition to the detailed relation to the internal os, it is important to describe the main location of the placenta. Particular attention is required when the placenta covers the internal os and a major part of the placenta is located in the lower anterior portion of the uterus, which may interfere with a surgical approach to deliver the fetus. Suggested management protocol for suspected placenta praevia Management when the placental edge related to the internal os at 18–20 weeks is:
• =1 cm from internal os. No further scans. Placenta praevia is unlikely. • 12 mm/week), a difference between abdominal diameter and BPD greater than 26 mm and between thoracic diameter and BPD above 14 mm.20 Subcutaneous tissue thickness has been studied as a predictor of macrosomia. The following parameters have been considered: cheek-to-cheek diameter, humeral, shoulder, femoral and abdominal subcutaneous thickness, calculating the most appropriate cut-off value with the ROC curve (from 11 to 13 mm). The application of such measurements in clinical practice is still premature.3
Fetal Biometry, Anomalies and Syndromes The progressive alteration of single biometric values may indicate the presence of fetal malformations as in cases of microcephaly or short-limbed dwarfism. In many instances the diagnosis may not be apparent before the third trimester with progressive alteration of biometric ratios below the first or above the 99th percentile. In such cases the number of SDs below or above the mean value (±3 SD) is more significant to indicate the degree of change in a particular biometric value and the likelihood of a malformation than the percentile. The measurements of some fetal parameters (nuchal translucency, short femur, short humerus) have also been studied in the assessment of the risk of fetal aneuploidy.
Conclusion The careful measurement of selected fetal parameters throughout the pregnancy is the basis for obtaining some of the most important information of the pregnancy, such as the gestational age and expected day of delivery, the size and growth of the fetus and important information for the safe management of the fetus before and after term as well as the delivery and birth. References
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1. Benacerraf BR, Gelman R, Frigoletto FD. Sonographically estimated fetal weights: accuracy and limitation. Am J Obstet Gynecol 1988;159:1118–1121 2. Bettelheim D, Deutinger J, Bernascheck G. Fetal sonographic biometry. Parthenon, Carnforth, 1997 3. Chauhan SP, West DJ, Scardo JA, Boyd JM, Joyner Y, Hendrix NV. Antepartum detection of macrosomic fetus: clinical versus sonographic, including softtissue measurements. Obstet Gynecol 2000;95:639–642 4. Crang-Svalenius E, Jorgensen C. Normal ultrasonic fetal growth ratios evaluated in cases of fetal disproportion. J Ultrasound Med 1991;10:89–92
5. Deter RL, Harrist RB. Growth standards for anatomic measurements and growth rates derived from longitudinal studies of normal foetal growth. J Clin Ultrasound 1992;20:381–388 6. Exacoustos C, Rosati P, Rizzo G, Arduini D. Ultrasound measurements of fetal limb bones. Ultrasound Obstet Gynecol 1991;1:325–330 7. Ferrazzi E, Nicolini U, Kustermann A, Pardi G. Routine obstetric ultrasound: effectiveness of cross-sectional screening for fetal growth retardation. J Clin Ultrasound 1986;14:17–22 8. Gardosi J, Chang A, Kalyan B, Sahota D, Simmonds EM. Customized antenatal growth charts. Lancet 1992;339:283–287
✩✩✩✩✩✩✩✩✩✩✩ ✩ 18. Mongelli M, Wilcox M, Gardosi J. Estimating the day of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol 1996;174:278–281 19. O'Keeffe DF, Garite TJ, Elliott JP, Burns PE. The accuracy of estimated gestational age based on ultrasound measurement of biparietal diameter in preterm premature rupture of the membranes. Am J Obstet Gynecol 1985;151:309–312 20. O' Reilly-Green, Divon M. Sonographic and clinical methods in the diagnosis of macrosomia. Clin Obstet Gynecol 2000;43:309–320 21. Sabbagha RE, Hughey M, Depp R. The assignment of growth-adjusted sonographic age (GASA): a simplified method. Obstet Gynecol 1978;51:383–386 22. Stebbins B, Jaffe R. Fetal biometry and gestational age estimation. In: Jaffe R, Bui TH (eds) Textbook of fetal ultrasound. Parthenon, Carnforth, 1999: 47–57 23. Thompson TE, Manning FA, Morrison I. Determination of fetal volume in utero by an ultrasound method: correlation with neonatal birth weight. J Ultrasound Med 1983;2:113 24. Weiner CP, Robinson D. The sonographic diagnosis of intrauterine growth retrdation using the postnatal ponderal index and the crown–heel length as standards of diagnosis. Am J Perinatol 1989;6:380–383 25. Zelop CM. Prediction of fetal weight with the use of three-dimensional ultrasonography. Clin Obstet Gynecol 2000;43:321–325 26. Tunon K, Eik-Nes SH, Grottum P. A comparison between ultrasound and a reliable menstrual period as predictors of the day of delivery in 15000 examinations. Ultrasound Obstet Gynecol 1996;8: 178–185
Fetal biometry, estimation of gestational age, assessment of fetal growth
9. Hadlock FP, Deter RL, Harrist RB, Park SK. Computer assisted analysis of fetal age in the third trimester using multiple fetal growth parameters. J Clin Ultrasound 1983;11:313–316 10. Hata T, Deter RL. A review of fetal organ measurements obtained with ultrasound: normal growth. J Clin Ultrasound 1992;20:155–174 11. Hill LM, Guzik D, Boyles D, Merolillo C, Ballone A, Ghiter P. Subcutaneous tissue thickness cannot be used to distinguish abnormalities of foetal growth. Obstet Gynecol 1992;80:268–271 12. Jeanty P, Beck GJ, Chevernak FA, Kremkau FW, Hobbins JC. A comparison of sector and linear array scanners for the measurement of the fetal femur. J Ultrasound Med 1985;4:525 13. Jeanty P. A simple reporting system for obstetrical ultrasound examination. J Ultrasound Med 1985;4:591–593 14. Jeanty P. Fetal biometry. In: Fleisher AC, Manning FA, Jeanty P, Romero R (eds) Sonography in obstetrics and gynecology. Principles and practice. Prentice-Hall International, New York, 1996: 131–149 15. Kurniawan YS, Deter RL, Visser GH, Simon NV, van der Weele LT. Prediction of neonatal crown–heel length from femur diaphysis length measurements. J Clin Ultrasound 1994;22:245–252 16. Manning FA. General principles and appli cations of ultrasonography. In: Creasy RK, Resnik R (eds) Maternal-fetal medicine, 4th edn. WB Saunders, Philadelphia, 1999: 169–206 17. Miller JM, Kissling GA, Brown HL, Gabert HA. Estimated fetal weight: applicability to small- and large-forgestational-age fetus. J Clin Ultrasound 1988;16:95–97
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Prenatal diagnosis of fetal anomalies Gianluigi Pilu Kypros H Nicolaides Israel Meizner Roberto Romero Waldo Sepulveda
Abstract Congenital anomalies occur in about 2.5% of all births and are the leading cause of infant mortality and probably of long-term handicap. A well-performed ultrasound examination carried out around midgestation allows identification of about 50% of all major anomalies. Ultrasound may also help in identifying aneuploidies at midgestation, although the specific approach remains controversial. The distinctive features of the sonographic diagnosis of anomalies, as well as the clinical implications, are discussed.
Keywords Chromosomal aberrations, congenital anomalies, fetus, prenatal diagnosis, ultrasound.
An Introduction to Congenital Anomalies Detection of fetal anomalies is one of the major reasons motivating the use of ultrasound in pregnancy. Congenital anomalies are the leading cause of infant mortality and probably one of the leading causes of long-term morbidity. Diagnosis of fetal anomalies is far from simple. It demands expertise in obstetrical ultrasound as well as knowledge in many fields including anatomy, embryology, teratology, genetics, paediatrics and cardiology. There is, however, consensus that a wellperformed basic ultrasound scan, including the evaluation of a well-defined set of quantitative and qualitative parameters, can detect the presence of a substantial number of anomalies, thus allowing the patient to undergo a targeted examination in a centre. The elements of the basic evaluation of fetal anatomy have been discussed in a previous chapter. The proportion of anomalies that will be detected by a basic ultrasound survey of fetal anatomy is controversial, as various studies
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✩ ✩✩✩✩✩✩✩✩✩✩✩ have reported very different results, and we will summarize the available experience at the end of this chapter. Our aim is to review the frequency and impact of congenital anomalies and to provide basic concepts useful for ultrasound identification. Diagnosing fetal anomalies is difficult and we refer the interested readers to the many detailed textbooks available on the subject.1–4 A congenital anomaly consists of a departure from the normal anatomical architecture of an organ or system. Anomalies may result from an intrinsically abnormal primordium (malformation) or from a normal primordium that is affected during development by extrinsic forces, such as vascular accidents (disruptions) or mechanical compression (deformations). Although there are several systems used to classify congenital anomalies, a common method is to divide them into major and minor. A major anomaly is one with medical, surgical or cosmetic importance and with impact on morbidity and mortality. A minor anomaly is one that does not have a serious surgical, medical or cosmetic significance, and does not affect normal life expectancy or lifestyle. Obviously, this classification is subjective and arbitrary. There is an overlap between minor anomalies and normal anatomical or phenotypical variants. A phenotypical variant occurs with a frequency of more than 4% in the general population, whereas minor anomalies occur with a rate of less than 4%. Clearly, this is also an arbitrary definition. The precise incidence of congenital anomalies is difficult to determine. Accurate documentation depends on many factors including:
• age at examination (prenatal period, newborn period, infancy or later in life)
• the experience of the observer (e.g. general paediatrician versus dysmorphologist)
• the definition of an anomaly (major, minor, normal phenotypical variation) • the type of examination (body surface examination, extensive examination including evaluation of internal organs)
• ethnic, geographical and social variations in the incidence of individual malformations.
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There is a general consensus that the prevalence of anomalies detected at birth is in the region of 2.5%, while long-term follow-up studies demonstrate much higher figures, in the range of 14–15%. It is important to remember that even severe anomalies may not be detected at birth; for example, some cardiac abnormalities will only be manifest afterwards. The neurological examination of a newborn infant has many limitations, and severe central nervous system anomalies may be undetected up to 1–3 years of age. Causative factors for congenital malformations may be identified in approximately 40% of cases and are usually divided into four major groups: single gene disorders, chromosome abnormalities, multifactorial conditions (involving both environmental and genetic components), and environmental factors. About 7.5% of all congenital malformations are caused by a single gene
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mutation.5,6 Autosomal mutations occur when the gene is located in a non-sex chromosome and may be either dominant (e.g. adult polycystic kidney disease, achondroplasia) or recessive (e.g. infantile polycystic kidney disease, achondrogenesis, short-rib polydactyly syndrome). Autosomal dominant conditions have a recurrence risk of 50% for the subsequent offspring, whereas autosomal recessive conditions have a recurrence risk of 25%. The term X-linked disorder is reserved for single gene mutations in the X chromosome (e.g. fragile X syndrome, fetal akinesia syndrome, oto-palato-digital syndrome). In this case, women are asymptomatic carriers and the disease is usually expressed only in males. Chromosomal anomalies are responsible for about 6% of all serious congenital malformations among live-born infants. They may be numerical or structural in nature. Multifactorial conditions are responsible for 20% of malformations in live-born fetuses. Examples of malformations with a multifactorial inheritance include spina bifida, cleft lip/palate and congenital dislocation of the hip. These anomalies are the result of interactions between a relatively large number of genes with similar effects and non-genetic, usually undefined factors. Currently, 2–3% of the spectrum of congenital malformations is attributed to teratogens, with most malformations resulting from exposures during days 18–40 post conception, except for the palate, central nervous system and genital structures that can be affected at later stages of development. Finally, a significant proportion of congenital malformations of unknown aetiology are likely to be polygenic or at least have an important genetic component.5,6 Congenital anomalies are an important determinant of perinatal and infantile death and long-term morbidity. A substantial fall in maternal and infant mortality rates was achieved during the 20th century. Environmental interventions, improvements in nutrition, advances in clinical medicine, wider access to healthcare, increased surveillance and monitoring of disease, better education and higher living standards contributed to this accomplishment. In Scotland, the overall perinatal mortality declined by 75% between the periods 1939– 1941 and 1974–1976, but over the same 37-year time span, the contribution of congenital anomalies to perinatal mortality increased from 10% to 25%.7 From 1915 to 1997, while the United States experienced a 93% drop in infant mortality (from approximately 100/1000 to 7.2/1000 live births),8 the relative contribution of congenital anomalies to the perinatal death rate increased. In 1995, according to the Centers for Disease Control and Prevention, birth defects were the leading cause of infant mortality in the USA.9,10 From 1968 to 1995, the proportion of infant deaths attributable to birth defects increased from 15% to 22%.11,12 Alongside the impact caused by congenital anomalies in perinatal mortality, there is an increased awareness regarding the role of congenital disease in determining morbidity. It has been estimated that at least 1% of all hospital admissions have a genetic basis or genetic contribution to their disease; as many as one of every four hospitalized children is affected by a disease that is at least partially genetically determined and approximately one of every 20 children is affected by a disorder that is completely genetic in origin. Infants with anomalies detected
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✩ ✩✩✩✩✩✩✩✩✩✩✩ within the first year have a significant increase in the risk of death and in all parameters of evaluated postnatal morbidity. Infants with congenital anomalies also impose an economic burden on society and contribute stress to the family nucleus. For example, the incidence of divorce and sibling social maladjustment is greater in families of children with spina bifida than in families of infants without congenital anomalies.
Central Nervous System Anomalies Most cerebral anomalies diagnosable in utero by ultrasound are easily demonstrated by the use of two transverse sections of the fetal head, one obtained at the level of the lateral ventricles (transventricular plane) (Fig. 10.1) and the other at the level of basal ganglia and cerebellum (Fig. 10.2). Recently, magnetic resonance imaging has become a valuable tool in the diagnosis of suspected brain and spine abnormalities.31,32
Mild Ventriculomegaly
Severe
Anterior midline defects
Alobar holoprosencephaly 160
Lobar holoprosencephaly
Agenesis of corpus callosum
Fig. 10.1 Fetal cerebral anomalies detectable with the transventricular plane.
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Absent cisterna magna banana sign (spina bifida)
Large cisterna magna cerebellar defect (Dandy–Walker complex)
Prenatal diagnosis of fetal anomalies
Normal cisterna magna
Fig. 10.2 Fetal cerebral anomalies detectable with the transcerebellar view.
Neural Tube Defects These include anencephaly, spina bifida and encephalocele. In anencephaly there is absence of the cranial vault (acrania) with secondary degeneration of the brain. Encephaloceles are cranial defects, usually occipital, with herniated fluid-filled or brain-filled cysts. In spina bifida the neural arch, usually in the lumbosacral region, is incomplete with secondary damage to the exposed nerves. The incidence of neural tube defects is subject to large geographical and temporal variations; in Europe the prevalence is about 1–2 per 1000 births with a peak of 5 per 1000 births. Anencephaly and spina bifida, with an approximately equal prevalence, account for 95% of cases and encephalocele for the remaining 5%. Neural tube defects are multifactorial disorders. Chromosomal abnormalities, single mutant genes and maternal diabetes mellitus or ingestion of teratogens, such as antiepileptic drugs, are implicated in about 10% of cases. When a parent or previous sibling has had a neural tube defect, the risk of recurrence is 5–10%. Periconception supplementation of the maternal diet with folate reduces by about half the risk of developing these defects. The sonographic diagnosis of anencephaly during the second trimester of pregnancy is based on the demonstration of absent cranial vault and cerebral hemispheres. The diagnosis can be made after 11 weeks, when ossification of the skull
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normally occurs. Ultrasound reports have demonstrated that there is progression from acrania to exencephaly and finally anencephaly. In the first trimester the pathognomonic feature is acrania, the brain being either entirely normal or with varying degrees of distortion and disruption. Diagnosis of spina bifida requires the systematic examination of each neural arch from the cervical to the sacral region both transversely and longitudinally. In the transverse scan the normal neural arch appears as a closed circle with an intact skin covering, whereas in spina bifida the arch is U-shaped and there is an associated bulging meningocele (thin-walled cyst) or myelomeningocele. The extent of the defect and any associated kyphoscoliosis are best assessed in the longitudinal scan (Fig. 10.3). The diagnosis of spina bifida has been greatly enhanced by the recognition of associated abnormalities in the skull and brain. These abnormalities include frontal bone scalloping (lemon sign) and obliteration of the cisterna magna with either an ‘absent’ cerebellum or abnormal anterior curvature of the cerebellar hemispheres (banana sign). These easily recognizable alterations in skull and brain morphology are often more readily attainable than detailed spinal views.13 A variable degree of ventricular enlargement is present in virtually all cases of open spina bifida at birth, but in only about 70% of cases in the midtrimester. Closed spina bifida may be associated with neurological compromise and the prenatal diagnosis is difficult, because α-fetoprotein is usually within normal limits in both amniotic fluid and maternal serum, there are no cranial signs and the spinal defect may be small and difficult or impossible to identify sonographically. Encephaloceles are recognized as cranial defects with herniated fluid-filled or brain-filled cysts. They are most commonly found in an occipital location (75% of cases) but alternative sites include the frontoethmoidal and parietal regions. Anencephaly is fatal at or within hours of birth. In encephalocele the prognosis is inversely related to the amount of herniated cerebral tissue; overall the
Fig. 10.3 Lumbosacral myelomeningocele in the sagittal (left) and axial (right) view.
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Ventriculomegaly The term ventriculomegaly is commonly used to indicate enlargement of the lateral cerebral ventricles. The incidence of this finding is unclear. Severe ventriculomegaly or hydrocephalus is found in less than 1 per 1000 births. Ventriculomegaly may be the consequence of cerebral malformations, chromosomal abnormalities or congenital infection. Genetic factors play an important role. About 25% of severe ventriculomegaly occurring in males is due to X-linked transmission. Fetal ventriculomegaly is diagnosed sonographically, by the demonstration of abnormally dilated lateral cerebral ventricles. A transverse scan of the fetal head at the level of the cavum septum pellucidum will demonstrate the dilated lateral ventricles, defined by an internal diameter of the posterior horn (or atrium) of 10 mm or more.13 The choroid plexuses, which normally fill the lateral ventricles, are surrounded by fluid. A diameter of 10–15 mm indicates mild ventriculomegaly. A diameter greater than 15 mm indicates moderate to severe ventriculomegaly.33 Certainly before 24 weeks and particularly in cases of associated spina bifida, the head circumference may be small rather than large for gestation. Fetal or perinatal death and neurodevelopment in survivors are strongly related to the presence of other malformations and chromosomal defects.34 Isolated severe ventriculomegaly is associated with an increased risk of perinatal death and a 50% chance of neurological sequelae in survivors. Although isolated mild ventriculomegaly (atrial width of 10–15 mm) is generally associated with a good prognosis, it is also the group with the highest incidence of chromosomal abnormalities (often trisomy 21). In addition, in a few cases with apparently isolated mild ventriculomegaly there may be an underlying cerebral maldevelopment (such as lissencephaly) or destructive lesion (such as periventricular leukomalasia). It has been suggested that ventricles of 10–12 mm, which represent the bulk of these cases, tend to have a good prognosis, with neurological compromise in the range of 4%, while those cases in which the measurement is 13–15 mm are associated with a greater probability of handicap, in the range of 12%.
Prenatal diagnosis of fetal anomalies
neonatal mortality is about 40% and more than 80% of survivors are intellectually and neurologically handicapped. In spina bifida, surviving infants are often severely handicapped, with paralysis in the lower limbs and double incontinence; despite the associated hydrocephalus requiring surgery, intelligence may be normal.
Holoprosencephaly This is a spectrum of cerebral abnormalities resulting from incomplete cleavage of the forebrain. There are three types according to the degree of forebrain cleavage. The alobar type, which is the most severe, is characterized by a monoventricular cavity and fusion of the thalami. In the semilobar type there is partial segmentation of the ventricles and cerebral hemispheres posteriorly with incomplete fusion of the thalami. In lobar holoprosencephaly there is normal separation
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✩ ✩✩✩✩✩✩✩✩✩✩✩ of the ventricles and thalami but absence of the septum pellucidum. The first two types are often accompanied by microcephaly and facial abnormalities. Holoprosencephaly is found in about 1 per 10,000 births. Although in many cases the cause is a chromosomal abnormality (usually trisomy 13) or a genetic disorder with an autosomal dominant or recessive mode of transmission, in many cases the aetiology is unknown. For sporadic, non-chromosomal holoprosencephaly, the empirical recurrence risk is 6%. In the standard transverse view of the fetal head for measurement of the biparietal diameter there is a single dilated midline ventricle replacing the two lateral ventricles or partial segmentation of the ventricles. The alobar and semilobar types are often associated with facial defects, such as hypotelorism or cyclopia, facial cleft and nasal hypoplasia or proboscis. Alobar and semilobar holoprosencephaly are lethal. Lobar holoprosencephaly is associated with mental retardation.
Agenesis of the Corpus Callosum The corpus callosum is a bundle of fibres that connects the two cerebral hemispheres. It develops at 12–18 weeks of gestation. Agenesis of the corpus callosum may be either complete or partial (usually affecting the posterior part). Agenesis of the corpus callosum is found in about 5 per 1000 births and may be due to maldevelopment or secondary to a destructive lesion. It is commonly associated with chromosomal abnormalities (usually trisomies 18, 13 and 8) and more than 100 genetic syndromes. The corpus callosum is not visible in the standard transverse views of the brain but agenesis of the corpus callosum may be suspected by the absence of the cavum septum pellucidum. The lateral ventricles usually are mildly enlarged and have a typical ‘teardrop’ configuration. Agenesis of the corpus callosum is demonstrated in the midcoronal and midsagittal views, which may require vaginal sonography. Partial agenesis of the corpus callosum is extremely difficult to diagnose antenatally because the cavum septum pellucidum is present and only a midsagittal view allows demonstration of the condition. The outcome is dependent mostly upon the association with other anomalies. In about 85% of those with apparently isolated agenesis of the corpus callosum, development is normal.
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The Dandy–Walker complex refers to a spectrum of abnormalities of the cerebellar vermis, cystic dilation of the fourth ventricle and enlargement of the cisterna magna. The condition is classified into (a) Dandy–Walker malformation (complete or partial agenesis of the cerebellar vermis and enlarged posterior fossa), (b) Dandy–Walker variant (partial agenesis of the cerebellar vermis without enlargement of the posterior fossa), and (c) megacisterna magna (normal vermis and fourth ventricle). The Dandy–Walker complex is a non-specific endpoint of chromosomal abnormalities (usually trisomies 18 or 13 and triploidy), more than 50 genetic syndromes, congenital infection or teratogens such as warfarin, but it can also be an isolated finding.
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Prenatal diagnosis of fetal anomalies
Ultrasonographically, the contents of the posterior fossa are visualized through a transverse suboccipito-bregmatic section of the fetal head. In the Dandy–Walker malformation there is cystic dilation of the cisterna magna with partial or complete agenesis of the vermis; in more than 50% of cases there is associated hydrocephalus and other extracranial defects. Enlarged cisterna magna is diagnosed if the vertical distance from the vermis to the inner border of the skull is more than 10 mm. Prenatal diagnosis of isolated partial agenesis of the vermis is difficult and a false diagnosis can be made if the angle of insonation is too steep. Classic Dandy–Walker malformation (that is, a large posterior fossa cyst associated with severe ventriculomegaly) is associated with a high postnatal mortality (about 20%) and a high incidence (more than 50%) of impaired intellectual and neurological development. Experience with apparently isolated partial agenesis of the vermis or enlarged cisterna magna is limited and the prognosis for these conditions is uncertain.
Microcephaly Microcephaly means small head and brain. This may result from chromosomal and genetic abnormalities, fetal hypoxia, congenital infection and exposure to radiation or other teratogens, such as maternal anticoagulation with warfarin. It is commonly found in the presence of other brain abnormalities, such as encephalocele or holoprosencephaly. The antenatal diagnosis of microcephaly is limited for many reasons. There is not an absolute cut-off that distinguishes normal fetuses with constitutionally small heads from microcephalics. Furthermore, microcephaly has a variable natural history. In many cases, the head is of normal size until late gestation and even at birth. Fetal microcephaly should be suspected when the head is smaller than −2 standard deviations below the mean. The diagnosis is rapidly established when the head is extremely small (20 weeks), sufentanyl followed by KCl can be injected in the umbilical vein using the same technique as that described for intrauterine transfusion. This avoids the potentially painful and often difficult intracardiac injection at this gestation. Abortions later than weeks 22 or 24 are accepted in only a few countries. The same goes for fetocide. In monochorionic multiple gestations, KCl injection cannot be used for selective fetocide. Indeed, this would precipitate an acute hypotensive episode in the surviving twin through bleeding into the dead co-twin through the placental anastomoses still present on the placental surface. This would occur irrespective of the histological nature of the vessels. The alternative is to coagulate the umbilical cord. This can be achieved using Nd:YAG laser technology when the cord is still small in diameter; however, the technique may not be used after 20 weeks of gestation. Alternatively, a bipolar forceps of 2–3 mm has been developed that can be passed down a cannula under ultrasound guidance and grasp the cord to coagulate. This is done under continuous ultrasound/colour Doppler control. This efficient technique is still under evaluation. The subsequent risk of preterm premature rupture of the membranes is not precisely known, but could be as high as 20–30%.
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Ultrasound-guided invasive procedures represent the foundation for fetal medicine. They can be learned as variations of the same technical approach, implying the use of both hands of one operator which would shorten the learning curve and improve the safety of the most frequently performed procedures, such as amniocentesis and CVS. There is rarely such a thing as a difficult procedure when done by a well-trained operator who performs a high number of invasive procedures. In addition, it is important to plan the procedure well by following the simple rules mentioned in this chapter. Ultrasound examination prior to performing the procedure is therefore a key element. Visualizing the target and the entry point on the maternal abdomen ensures a straightforward path without interposition of any fetal structures in the path of the needle.
✩✩✩✩✩✩✩✩✩✩✩ ✩ References pregnancies. In: Santoyala-Forgas J, Lemery D (eds) Interventional ultrasound in obstetrics, gynecology and the breast. Blackwell, Oxford, 1998: 146–150 13. Daffos F, Capella-Pavlowsky M, Forestier F. A new procedure for fetal blood sampling in utero: preliminary results of 53 cases. Am J Obstet Gynecol 1983;146:985–998 14. Ghidini A, Sepulveda W, Lockwood C, Romero R. Complications of fetal blood sampling. Am J Obstet Gynecol 1993;168:1339–1344 15. Perry KG, Hess LW, Roberts WE et al. Cordocentesis by maternal fetal fellows: the learning curve. Fetal Diagn Ther 1991;157:858–859 16. Moise KJ. Intrauterine transfusion with red cells and platelets. West J Med 1993;159:318–324 17. Evans MI, Sacks AJ, Johnson MP, Robichaux AG, May M, Moghissi KS. Sequential invasive assessment of fetal renal function and intrauterine treatment of fetal obstructive uropathies. Obstet Gynecol 1991;77:54–55 18. Rodeck CH, Nicolaides KH. Ultrasound guided invasive procedures in obstetrics. Clin Obstet Gynecol 1983;10:515–540 19. Yamamoto M, El Murr L, Robyr l, Leleu F, Takahashi Y, Ville Y. Incidence and impact of perioperative complications in 175 fetoscopy-guided laser coagulations of chorionic plate anastomoses in fetofetal transfusion syndrome before 26 weeks of gestation. Am J Obstet Gynecol 2005;193:1110–1116 20. Firth H. Chorion villus sampling and limb deficiency – cause or coincidence? Prenat Diagn 1997;17:1313–1330 21. Gonce A, Borrel A, Fortuny A et al. First-trimester screening for trisomy 21 in twin pregnancy: does the addition of biochemistry make an improvement? Prenat Diagn 2005;25:1156–1161 22. Stewart KS, Johnson MP, Quintero RA, Evans MI. Congenital abnormalities in twins: selective termination. Cur Opin Obstet Gynecol 1997;9:136–139 23. Westgren M, Selbing A, Stangenberg M. Fetal intracardiac transfusions in patients with rhesus isoimmuniation. BMJ 1988;296:885–886 24. Evans MI, Ciorca D, Britt DW, Fletcher JC. Update on selective reduction. Prenat Diagn 2005;25:807–813
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1. Ville Y, Cooper M, Revel A, Frydman R, Nicolaides KH. Development of a training model for ultrasound-guided invasive procedures in fetal medicine. Ultrasound Obstet Gynecol 1995;5:180–183 2. Timor-Tritsch IE, Yeh MN. In vitro training model for diagnostic and therapeutic fetal intravascular needle puncture. Am J Obstet Gynecol 1987;157:858–859 3. Wapner R. Chorionic villous sampling. In: Santoyala-Forgas J, Lemery D (eds) Interventional ultrasound in obstetrics, gynecology and the breast. Blackwell, Oxford, 1998:45–59 4. Jackson L, Wapner R, Barr-Jackson M. Chorionic villus sampling (CVS) is not associated with an increased incidence of limb reduction defects. Abstract for the American Society of Human Genetics 43rd Meeting, New Orleans, LA, October 1993 5. Brambati B, Oldrini A, Lanzani A. Transabdominal chorionic villus sampling: a freehand ultrasound guided technique. Am J Obstet Gynecol 1987;157:134–142 6. MRC Working Party on the Evaluation of Chorionic Villus Sampling. MRC European trial of chorionic villus sampling. Lancet 1991;337:726–741 7. Canadian Collaborative CVSAmniocentesis. Clinical trial of chorionic villous sampling and amniocentesis. Lancet 1991;337:1491–1509 8. Tabor A, Madsen M, Obel EB, Philip J, Bang J, Noorgard Pedersen B. Randomised controlled trial of genetic amniocentesis in 4606 low-risk women. Lancet 1986;i:1287 9. Nicolaides KH, Brizet ML, Patel F, Snijders R. Comparison of chorion villus sampling and early amniocentesis for karyotyping in 1,492 singleton pregnancies. Fetal Diagn Ther 1996;11:9–15 10. Kappel B, Nielsen J, Brogaard Hansen K, Mikkelsen M, Therkelsen AAJ. Spontaneous abortion following midtrimester amniocentesis. Clinical significance of placental perforation and blood-stained amniotic fluid. Br J Obstet Gynaecol 1987;94:50 11. Andreasen E, Kristoffersen T. Incidence of spontaneous abortion after amniocentesis: influence of placental localisation and past obstetric and gynecologic history. Am J Perinatol 1989;6:268 12. Ville Y, Nicolaides KH. Prenatal diagnosis and therapeutic techniques in twin
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Multiple pregnancies Kurt Hecher Werner Diehl
ABSTRACT Detection of the number of fetuses, chorionicity and amnionicity should be achieved during the first-trimester scan. Invasive diagnostic techniques such as chorion villous sampling and amniocentesis may be used to obtain karyotypes of all fetuses and this should be discussed individually with the couple, taking into account the risk to benefit ratio, i.e. the procedure-related risk of a miscarriage and the individual risk for chromosomal abnormalities. Fetal growth impairment in dichorionic twins will more often reflect uteroplacental insufficiency as compared to singleton pregnancies and fetal surveillance including Doppler ultrasound should be intensified. In monochorionic twins, one should be aware of the risk for the development of twin–twin transfusion syndrome, and amniotic fluid volumes and their relation to bladder filling of both twins should be monitored from the early stages of gestation onwards. Monoamniotic twins, occurring in 5% of monochorionic gestations, show the highest risk for structural anomalities and poor outcome. The assessment of monoamniotic pregnancies implies close fetal monitoring and detection of cord implications. Conjoined twins represent the most severe form of splitting disorders in monozygotic twins. They occur in 1% of monochorionic pregnancies, and their outcome depends mainly on the site of conjoining and the organs involved.
Keywords Amnionicity, chorionicity, early risk assessment, fetal surveillance, multiple pregnancy, zygocity.
Introduction Perinatal mortality and morbidity rates are increased three to seven times in twin pregnancies,8 as compared to singleton gestations. Although twin pregnancies
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✩ ✩✩✩✩✩✩✩✩✩✩✩ account only for 2.5% of the population, they are responsible for up to 12.6% of the overall perinatal mortality rate.28 Additionally, assisted reproduction techniques have led to an increase in the incidence of twinning with almost 50% of the twins resulting from infertility treatment.22 The application of such techniques has also contributed to an increase in the incidence of multiple pregnancies of a higher grade (e.g. triplets, quadruplets). The fact that this population also shows a higher number with women at an advanced age, with an increased age-related risk for chromosomal abnormalities and for impairment of the uteroplacental perfusion, also contributes to the high risk in this collective. Approximately two-thirds of twin pregnancies are dizygotic and therefore dichorionic and diamniotic. One-third is monozygotic; of these, one-third is dichorionic (splitting occurring at less than 4 days after conception) and the other twothirds are monochorionic and diamniotic (splitting occurring from days 4 to 8 after conception). A later splitting (9–13 days) leads to the occurrence of monoamniotic twins, and a division beyond the 14th day to conjoined twins. It is known that mortality and morbidity rates are higher in monochorionic twins.16,17,32 Conditions unique to them, such as twin–twin transfusion syndrome (TTTS), reverse twin arterial perfusion sequence and monoamniotic pregnancies, are responsible for an increased risk of adverse perinatal outcome. Therefore, early assessment of chorionicity and amnionicity plays an important role in the risk stratification of multiple pregnancies and has practical consequences for the management of those pregnancies. Due to the high-risk nature of multiple pregnancies fetal surveillance should be undertaken in appropriate intervals.
First-Trimester Ultrasound Pregnancy Dating The crown–rump length (CRL) of the fetuses is the most important ultrasound parameter for dating of the pregnancy and, if necessary, to correct the gestational age in cases with a non-reliable menstrual history. The onset of early growth retardation in one of the fetuses may indicate a higher risk for chromosomal abnormalities. Normally, the CRLs correlate between co-twins, although some degree of variability has been observed in multifetal pregnancies.19,34 Measurement of the CRL can easily be done at the time of the first-trimester scan (11–14 weeks of gestation). Later in pregnancy, correction of gestational age should be avoided, since growth curves in multiple pregnancies differ from those in singleton pregnancies beyond the second trimester.
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The currently widespread availability of transvaginal ultrasound enables early detection of multiple pregnancies and their localization. Additionally, it allows precise assessment of chorionicity and amnionicity. The diagnosis of twins with
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Fig. 13.1 Assessment of chorionicity. (A) Transverse view of the uterus at 12 weeks of gestation, showing two separate amniotic cavities with two placentae (dichorionicity) and two fetuses. (B) Intertwin membranes in trichorionic triplets at 20 weeks of gestation showing the lambda signs at the placental base. (C) The confluence of the intertwin membranes in a trichorionic triplets pregnancy at 12 weeks of gestation. (D) Pentachorionic quintuplets pregnancy at 10 weeks of gestation.
Multiple pregnancies
the observation of two embryos may be confusing for the parents, if there is subsequent disappearance of one of them during further examinations (vanishing twin phenomenon). The spontaneous incidence of this phenomenon in multiple pregnancies has been reported to be between 21%21 and 50% in triplets23 and occurs most frequently during the first 7 weeks of pregnancy and never beyond 14 weeks. Some authors consider monochorionic twinning as a risk factor for neurological abnormalities in the surviving twin after disappearance of one embryo, since this form of placentation may predispose to vascular events in early fetal life.6 After 10 weeks of gestation a reliable identification of the number of fetuses and their chorionicity may be expected even with transabdominal ultrasound (Fig. 13.1).
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Chorionicity and Amnionicity
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For reasons of risk stratification in multiple pregnancies, one of the most important goals of early ultrasound in this population is the determination of chorionicity and amnionicity.35–37 Early in gestation (6–9 weeks), the presence of two separate gestational sacs indicates dichorionicity. From 10–14 weeks onwards, a thick septum and a triangular tissue projection at the placental base of the separating membrane (lambda sign) predict dichorionicity27 (see Fig. 13.1; Fig. 13.2). This is due to four layers of the intertwin membrane: amniotic and chorionic layers of fetus 1 and chorionic and amniotic layers of fetus 2. Monochorionic twins show a very thin intertwin membrane (only two amniotic layers) and no lambda sign, since there are no chorionic layers between the two amniotic layers of the membrane (see Fig. 13.2). Misdiagnosis of monoamniotic twins due to visualization of a single gestational sac may occur, if identification of the thin separating membrane is difficult. Later during pregnancy, identification of fetal
Fig. 13.2 The lambda sign. (A,C) The lambda sign (arrows) at the placental base of the intertwin membrane in dichorionic pregnancies at 15 weeks (A) and 10 weeks (C) of gestation. (B,D) Absence of the lambda sign (arrows) in monochorionic diamniotic twin pregnancies at 16 weeks (B) and 20 weeks (D) of gestation. Note the thin intertwin membrane (M) and the common anterior placenta (PL).
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Nuchal Translucency
Multiple pregnancies
gender may also be helpful in the assessment of chorionicity, although overall two-thirds of twins are of the same sex: one-third consists of all monochorionic twins and the other one of 50% of all dichorionic twins. Discordant sex indicates dichorionicity. Regarding amnionicity, the lack of an intertwin membrane, despite careful scanning of the whole amniotic cavity, leads to the diagnosis of monoamniotic twinning.25 The presence of a unique yolk sac also indicates monoamnionicity.7
Between 10 and 14 weeks of gestation, it is possible to assess the woman's individual risk for chromosomal abnormalities combining the measurement of the nuchal translucency (NT) and maternal age.29 However, in multiple pregnancies, this risk calculation has to take into account several aspects.38,39 In dizygotic twins, the risk for a chromosomal abnormality is calculated individually for each twin in the same fashion as for singleton fetuses. However, the risk that at least one fetus of this pregnancy is affected is the summation of the two individual risks, which is twice as high as in a singleton pregnancy if the individual risks are almost the same. Squaring the singleton risk derives from the risk that both fetuses are affected. In monozygotic twins, the risk for a chromosomal abnormality is the same as that of a singleton pregnancy, but in cases of an abnormal karyotype both fetuses are affected. A higher false-positive rate for risk calculations of chromosomal defects in monochorionic twins can be explained due to an early manifestation of a twin–twin transfusion syndrome (TTTS), where an increased NT in at least one fetus has been shown as a marker for prediction of TTTS.26 Increased NT in the recipient fetus as a consequence of hypervolaemia is considered as an early sign for TTTS and the risk for the development of the syndrome is increased almost fourfold. The nature of the estimation of a likelihood, the options of invasive diagnostic procedures for the assessment of the fetal karyotype and the possible consequences of an abnormal result have to be explained in detail during the counselling. The knowledge of chorionicity is paramount for risk estimation, the decision regarding the technique of invasive testing and its consequences.
Invasive Diagnostic Procedures Chorionic villous sampling (CVS) can be done as early as 10–12 weeks of gestation and has then a risk for a procedure-related pregnancy loss of about 1%. In another 1% of cases the results may be unclear, for instance due to the presence of mosaicism in the chorionic tissue. However, studies have established comparable risks to those of second-trimester amniocentesis, if performed by experienced operators.20 In dichorionic twins these risks may increase, if double sampling has to be performed to assure obtaining a result for both fetuses. Thus, sampling has optimally
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✩ ✩✩✩✩✩✩✩✩✩✩✩ to be performed below the umbilical cord insertions of the respective twins. The possibility of cross-contamination of the sample, when single puncture is performed, is about 0.6%. Maternal contamination is another problem of CVS, as well as sampling the same fetus twice, but these problems occur in less than 1% of procedures, as reported in recent studies.1,31 The advantage of this procedure, which can be performed earlier than amniocentesis, is that in case of a chromosomal defect of one of the fetuses, an earlier selective fetocide with a lower procedure-related risk for fetal loss (5% versus 16% at 15 weeks or later) may be performed.9 If the individual risk for chromosomal abnormalities, calculated by maternal age and NT, in at least one of the fetuses is greater than 1 in 50, it may be preferable to perform a CVS for fetal karyotyping. For pregnancies with a lower combined risk calculation, an amniocentesis after 15 weeks of gestation may be more appropriate.24 Also, with amniocentesis in dichorionic twins, obtaining a result for both fetuses has to be guaranteed. This can be achieved by either puncturing each amniotic sac separately with two needle insertions or with only one uterine needle insertion by crossing the intertwin membrane and sampling amniotic fluid separately from each sac under ultrasound guidance. In structurally normal monochorionic twins, single sampling may be sufficient, since monozygotic fetuses can be expected to be genetically identical.
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The longitudinal assessment of fetal growth in both twins gives valuable information about their intrauterine well-being. In dichorionic twins biometry should be performed at monthly intervals,28 keeping in mind the higher risk for intrauterine growth retardation, as compared to singleton pregnancies. Beyond the 20th week of gestation even normal twin fetuses may show smaller biometric measurements than singletons and, therefore, specially adapted growth curves should be used.15 If the growth curve of one of the fetuses shows the tendency to approach the 5th percentile for gestational age, control intervals should be shortened to every second week. In small-for-gestational age fetuses, the benefits of Doppler ultrasound should be used. Serial ultrasound examinations from the second trimester onwards, including Doppler velocimetry if necessary, represent the most reasonable antenatal assessment of twin pregnancies.11 However, in monochorionic twins, due to the existence of placental vascular anastomoses which continuously allow interfetal blood flow, ultrasound examinations should be performed at shorter intervals, every 2–3 weeks. Attention should be drawn to the amounts of amniotic fluid in each amniotic cavity and the bladder filling of each fetus. An early TTTS can be recognized following these criteria. The development of growth restriction of one fetus may result as a consequence of TTTS, but also as a consequence of placental insufficiency.16 Doppler assessment of the fetal circulation may help to distinguish between these two conditions.33
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Malformations and Fetal Demise Multiple pregnancies
In general, the risk for malformations in twin pregnancies is elevated. It is increased in monochorionic twins and in monoamniotic twins it is reported to be as high as 38%,4,25,40 where the inherent mechanism for the malformation may be related to the process of splitting itself. The risk for fetal loss in dichorionic twins after selective fetocide, due to the presence of a chromosomal abnormality in one of the fetuses, is about 10%. Spontaneous intrauterine death of one twin usually does not affect the co-twin, because there are no vascular anastomoses in dichorionic placentae. However, the situation is completely different in monochorionic twins. After a single intrauterine death there is a high risk for damage of the co-twin, due to the presence of placental vascular anastomoses.3 As a consequence of an acute loss of blood towards the dying fetus or immediately after its death, a hypotensive and anaemic episode may occur in the co-twin and subsequently lead to its death or to neurological damage (in 20–30%). This has also to be taken into account if one of the fetuses of a monochorionic twin pregnancy shows structural anomalies and selective termination is considered. More recently, invasive techniques have been developed to occlude completely the umbilical vessels by laser or bipolar coagulation of the umbilical cord, to avoid acute haemodynamic imbalance in the co-twin.10
Twin–Twin Transfusion Syndrome In 10–15% of monochorionic pregnancies, severe midtrimester TTTS develops, which is associated with a mortality rate of 80–90% if left untreated.41–43 The underlying cause for the development of the syndrome is the presence of vascular anastomoses in all monochorionic placentae. As a consequence of the different types of anastomoses (arteriovenous, arterioarterial and venovenous) and the blood flow direction in the arteriovenous anastomoses, a net imbalance in intertwin blood flow may ensue. The recipient fetus becomes hypervolaemic and polyuric, leading to polyhydramnios, and may develop congestive heart failure due to cardiac overload. The donor fetus becomes hypovolaemic and anuric, leading to severe oligo- and anhydramnios. Premature rupture of membranes, owing to the extreme polyhydramnios, miscarriage and extremely premature delivery, as well as intrauterine death, are the main complications contributing to the high perinatal mortality. The diagnostic ultrasound criteria for TTTS are the observation of a single monochorionic placenta, the presence of polyhydramnios in the amniotic cavity of the recipient fetus, who shows also a distended bladder (Fig. 13.3), and severe oligo- or anhydramnios in the amniotic cavity of the donor fetus, who shows only a weak or no bladder filling at all. Due to the absence of amniotic fluid in the donor's amniotic sac, the intertwin membrane may not be visible as it is adherent to the fetus who is pressed against the uterine wall or the placenta (stuck twin) (Fig. 13.4). The absence of a visible intertwin membrane may lead to the misdiagnosis of monoamniotic twins. TTTS may develop in the early second trimester (at 16 or 17 weeks of gestation) and within a short period of time (1 or 2 weeks).
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Fig. 13.3 Twin–twin transfusion syndrome. Note the distended (polyuric) bladder of the recipient twin and the massive polyhydramnios (gestational age 21+5 weeks; deepest vertical pool was 14 cm).
Fig. 13.4 Twin–twin transfusion syndrome. The ‘stuck twin’ phenomenon: due to anhydramnios the donor twin (circle) is stuck to the uterine wall.
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Doppler assessment of blood flow in the umbilical arteries and the ductus venosus of both twins provides valuable information about the fetal cardiovascular condition. In the recipient fetus, signs of congestive heart failure, such as abnormal ductus venosus flow, tricuspid and mitral regurgitation, fetal hydrops (ascites, pleural effusions, skin oedema), reduce the probability of survival. In the donor fetus, an increased placental resistance with absent or reversed enddiastolic flow in the umbilical artery is associated with a lower survival rate.33 As fetal viability is not yet achieved during the second trimester of pregnancy, delivery is not a realistic option for the management of these cases. There are two options for therapy: serial amniodrainages and percutaneous fetoscopic laser coagulation of the placental vascular anastomoses. The latter offers a causal therapeutic approach and an overall survival rate of 68% and 81% of pregnancies with
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at least one survivor can be achieved.18 During laser surgery a mean of 5–6 anastomoses can be identified and coagulated. In all cases arteriovenous anastomoses from donor to recipient are present, with the majority of cases also showing anastomoses shunting in the opposite direction, and arterioarterial anastomoses are present in about one-third of pregnancies with TTTS.12 After laser therapy, follow-up scans are performed at weekly intervals first and then every second week, as normally done for fetal surveillance in monochorionic twins, drawing attention to the amounts of amniotic fluid, bladder fillings, growth patterns and Doppler flow velocity waveforms of both fetuses.
Twin Reversed Arterial Perfusion The prevalence of twin reversed arterial perfusion (TRAP) or acardiac twins is about 1 in 35,000 pregnancies. The presence of an arterioarterial and a venovenous anastomosis between both cord insertions in monochorionic twins may lead to a reversed perfusion of one fetus, if one pulse wave predominates over the other early in gestation. In the reversely perfused fetus there is no cardiac development at all or only a rudimentary heart tube can be detected. The development of the upper part of the body is also severely impaired and most of the acardiac fetuses also show acrania and severe hydrops. This condition represents a high risk for heart failure and intrauterine demise or preterm delivery of the pumping twin. The typical ultrasound appearance of the acardiac twin is a hydropic mass without a heartbeat or only with a rudimentary pulsatile cardiac structure. Colour Doppler sonography reveals the reversed perfusion via the single umbilical artery (Fig. 13.5). These ultrasound features are unique to this disorder and may be detected in the first trimester of pregnancy. Treatment strategies range from vessel obliteration by intracardiac application of alcohol to fetoscopic ligation and bipolar or laser coagulation of the umbilical cord of the acardiac twin.
Fig. 13.5 Twin reversed arterial perfusion (TRAP sequence). (A) The hydropic acardiac twin with multiple malformations at 20 weeks of gestation. (B) Colour Doppler depicts the reversed arterial perfusion (blue) to the acardiac twin and the returning blood flow via the umbilical vein (red) to the pumping twin.
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Monoamniotic Twins Monoamniotic twinning occurs in 5% of monochorionic twins (1% of all twins), and the most severe form of splitting disorders in monozygotic twins is conjoined twins. At the 10–14-week scan, the criteria for monoamnionicity are the absence of the intertwin amniotic membrane and the presence of only one yolk sac7 and it should be suspected if the placental cord insertions are close to each other and an unusual intrauterine position of both fetuses, in close proximity to each other, is seen.25 Monoamniotic twins show a significantly increased risk for structural anomalies and for poor perinatal outcome, even in the absence of an intertwin discordance. In both structurally normal and abnormal twins, cord entanglement, which has been reported to be present as early as the first trimester,2 has been responsible for the demise of one or both fetuses in the majority of cases. The aim of fetal surveillance should be to reach at least 32 weeks of gestation and delivery by elective caesarean section should be performed to avoid acute cord complications during delivery. Cord entanglement may be detected by colour Doppler ultrasound. However, its consequences remain controversial, because the incidence of loose entanglement seems to be high, but fetal jeopardy occurs only if this leads to compression of the cords, which may be an acute event. The primary ultrasound feature of conjoined twins is the fact that they are always close to each other with common movement patterns and without separation from each other if observed over a certain period of time. The chance for survival depends on the site of conjoining and the organs involved, and overall about 50% are stillborn. One third of the live-born twins have defects, which are impossible to correct surgically, and in those cases where surgery is attempted, a survival rate of about 60% of babies is achieved.30
Higher-Order Multiple Pregnancies Recently the incidence for multiples of a higher order has increased due to the use of different techniques in assisted reproduction. The use of transvaginal and transabdominal ultrasound to assess the number of fetuses and their chorioni city is fundamental in the early diagnosis and management of these pregnancies. With the measurement of the nuchal translucency, the individual fetal risk for chromosomal abnormalities and other maldevelopments can be calculated and a selective reduction can be performed under ultrasound guidance to reduce the perinatal risk associated with multiples of higher order than twins or triplets. Multifetal reduction has been reported to reduce triplets to twins5 and the risk for fetal loss after the procedure has been continuously diminishing during recent years, as more experience in this technique has been gained. In general, the higher the starting number of fetuses, the poorer is the outcome after reduction, with fetal loss rates reported to range from 15.4% to 4.5% according to starting numbers ranging from six to three fetuses, respectively.13,14 256
✩✩✩✩✩✩✩✩✩✩✩ ✩ References 13. Evans MI, Goldberg JD, Horenstein J et al. Selective termination for structural, chromosomal, and mendelian anomalies: international experience. Am J Obstet Gynecol 1999;181:893–897 14. Evans MI, Berkowitz RL, Wapner RJ et al. Improvement in outcomes of multifetal pregnancy reduction with increased experience. Am J Obstet Gynecol 2001;184:97–103 15. Farina A, Vesce F, Garutti P, Jorizzo G, Bianciotto A. Evaluation of intrauterine growth pattern of twins by linear discriminant analysis of the values of biparietal diameter, femur length and abdominal circumference. Gynecol Obstet Invest 1999;48:14–17 16. Gaziano EP, De Lia JE, Kuhlmann RS. Diamniotic monochorionic twin gestations: an overview. J Matern Fetal Med 2000;9:89–96 17. Hatkar PA, Bhide AG. Perinatal outcome of twins in relation to chorionicity. J Postgrad Med 1999;45:33–37 18. Hecher K, Diehl W, Zikulnig L, Vetter M, Hackelöer BJ. Endoscopic laser coagulation of placental vascular anastomoses in 200 pregnancies with severe mid-trimester twinto-twin transfusion syndrome. Eur J Obstet Gynecol Reprod Biol 2000;92:135–139 19. Isada NB, Sorokin Y, Drugan A, Johnson MP, Zador I, Evans MI. First trimester interfetal size variation in well-dated multifetal pregnancies. Fetal Diagn Ther 1992;7:82–86 20. Jenkins TM, Wapner RJ. First trimester prenatal diagnosis: chorionic villous sampling. Semin Perinatol 1999;23:403–413 21. Landy HJ, Weiner S, Corson SL, Batzer FR, Bolognese RJ. The ‘vanishing twin’: ultrasonographic assessment of fetal disappearance in the first trimester. Am J Obstet Gynecol 1986;155:14–19 22. Loos R, Demron C, Vlietinick R, Demron R. The East Flanders prospective twin survey (Belgium): a population-based register. Twin Res 1998;1:167–178 23. Manzur A, Goldsman MP, Stone SC, Frederick JL, Balmaceda JP, Asch RH. Outcome of triplet pregnancies after assisted reproductive techniques: how frequent are the vanishing embryos? Fertil Steril 1995;63:252–257 24. Pandya P. Ultrasound and multiple pregnancies. Front Fetal Health 2001;3: 89–91
Multiple pregnancies
1. Appleman Z, Vinkler C, Caspi B. Chorionic villous sampling in multiple pregnancies. Eur J Obstet Gynecol Reprod Biol 1999;85:979 2. Arabin B, Laurini RN, van Eyck J. Early prenatal diagnosis of cord entanglement in monoamniotic multiple pregnancies. Ultrasound Obstet Gynecol 1999;13: 181–186 3. Bajoria R, Wee LY, Anwar S, Ward S. Outcome of twin pregnancies complicated by single intrauterine death in relation to vascular anatomy of the monochorionic placenta. Hum Reprod 1999;14:2124–2130 4. Baldwin VJ. The pathology of monochorionic monozygocity. In: Baldwin VJ (ed) Pathology of multiple pregnancy. Springer Verlag, New York, 1994: 199–214 5. Boulot P, Vignal J, Vergnes C, Dechaud H, Faure JM, Hedon B. Multifetal reduction of triplets to twins: a prospective comparison of pregnancy outcome. Hum Reprod 2000;15:1619–1623 6. Brodtkorb E, Myhr G, Gimse R. Is monochorionic twinning a risk factor for focal cortical dysgenesis? Acta Neurol Scand 2000;102:53–59 7. Bromley B, Benacerraf B. Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins. J Ultrasound Med 1995;14:415–419 8. Chitrit Y, Filidori M, Pons JC, Duyme M, Papiernik E. Perinatal mortality in twin pregnancies: a 3-year analysis in Seine Saint-Denis (France). Eur J Obstet Gynecol Reprod Biol 1999;86:23–28 9. De Catte L, Liebaers I, Foulon W. Outcome of twin gestations after first trimester chorionic villous sampling. Obstet Gynecol 2000;96:714–720 10. Deprest JA, Audibert F, Van Schoubroeck D, Hecher K, Mahieu-Caputo D. Bipolar coagulation of the umbilical cord in complicated monochorionic twin pregnancy. Am J Obstet Gynecol 2000;182:340–345 11. Devoe LD, Ware DJ. Antenatal assessment of twin gestation. Semin Perinatol 1995;19:413–423 12. Diehl W, Hecher K, Zikulnig L, Vetter M, Hackelöer BJ. Placental vascular anastomoses visualised during fetoscopic laser surgery in severe mid-trimester twin–twin transfusion syndrome. Placenta 2001;22:876–881
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25. Sebire NJ, Souka A, Skentou H, Geerts L, Nicolaides KH. First trimester diagnosis of monoamniotic twin pregnancies. Ultrasound Obstet Gynecol 2000;16:223–225 26. Sebire NJ, Souka A, Skentou H, Geerts L, Nicolaides KH. Early prediction of severe twin-to-twin transfusion syndrome. Hum Reprod 2000;15:2008–2010 27. Sepulveda W, Sebire NJ, Hughes K, Odibo A, Nicolaides KH. The lambda sign at 10-14 weeks of gestation as a predictor of chorionicity in twin pregnancies. Ultrasound Obstet Gynecol 1996;7:421–423 28. Sherer DM. Is less intensive fetal surveillance of dichorionic twin gestations justified? Editorial. Ultrasound Obstet Gynecol 2000;15:167–173 29. Snijders RJM, Noble P, Sebire NJ, Souka AP, Nicolaides KH. UK multicentre project on assessment of risk for trisomy 21 by maternal age and fetal nuchal translucency at 10–14 weeks of gestation. Fetal Medicine Foundation First Trimester Screening Group. Lancet 1998;352: 343–346 30. Spitz L. Conjoined twins. Br J Surg 1996;83:1028–1030 31. van den Berg C, Braat AP, Van Opstal D et al. Amniocentesis or chorionic villous sampling in multiple gestations? Experience with 500 cases. Prenat Diagn 1999;19: 234–244 32. Victoria A, Mora G, Arias F. Perinatal outcome, placental pathology, and severity of discordance in monochorionic and dichorionic twins. Obstet Gynecol 2001;97:310–355 33. Zikulnig L, Hecher K, Bregenzer T, Bäz E, Hackelöer BJ. Prognostic factors in severe twin–twin transfusion syndrome treated by endoscopic laser surgery. Ultrasound Obstet Gynecol 1999;14:380–387
34. Kalish RB, Gupta M, Perni SC, Berman S, Chasen ST. Clinical significance of first trimester crown–rump length disparity in dichorionic twin gestation. Am J Obstet Gynecol 2004;191:1437–1440 35. Machin GA. Why is it important to diagnose chorionicity and how do we do it? Best Pract Res Clin Obstet Gynecol 2004;18:515–530 36. Menon DK. A retrospective study of the accuracy of sonographic chorionicity determination in twin pregnancies. Twin Res Hum Genet 2005;8:259–261 37. Geipel A, Berg C, Katalinic A et al. Prenatal diagnosis and obstetric outcomes in triplet pregnancies in relation to chorionicity. Br J Obstet Gynaecol 2005;112:554–558 38. Van der Cruys, Faiola S, Auer M, Sebire N, Nicolaides KH. Screening for trisomy 21 in monochorionic twins by measurement of fetal nuchal translucency thickness. Ultrasound Obstet Gynecol 2005;25: 551–553 39. Wald NJ, Rish S, Hackshaw AK. Combining nuchal translucency and serum markers in prenatal sceening for Down syndrome in twin pregnancies. Prenat Diagn 2003;23:588–592 40. Garne E, Andersen HJ. The impact of multiple pregnancies and malformations on perinatal mortality. J Perinat Med 2004;32:215–219 41. Huber A, Hecher K. How can we diagnose and manage twin-twin transfusion syndrome? Best Pract Res Clin Obstet Gynaecol 2004;18:543–556 42. Fisk NM, Tan TY, Taylor MJ. Stagesaved treatment of twin-twin transfusion syndrome. Am J Obstet Gynecol 2004;190:1491–1492 43. Robyz R, Quarello E, Ville Y. Management of fetofetal transfusion syndrome. Prenat Diagn 2005;25:786–795
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Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis Rabih Chaoui Bernard Benoit
Abstract Three-dimensional ultrasound has been the most rapidly evolving technique in fetal imaging in recent years and is mainly known for the demonstration of the face or other fetal surface structures. The potential of this technique in prenatal diagnosis is, however, greater, based on the possibility of acquiring a volume data set of a region of interest which can then be displayed in different ways. This chapter will emphasize these display modes as the demonstrations of reconstructed two-dimensional images either as orthogonal or parallel crosssection planes (tomographic mode) or the rendering of the three-dimensional/ four-dimensional information. Rendering includes the surface mode, the maximum, minimum and inversion modes, as well as glass body mode when combined with colour or power Doppler acquisition. Spatial and temporal image correlation technology enables the acquisition of fetal heart data and the display of one single cardiac cycle in different modes. The chapter supports the idea that we are now moving from the era of ‘sonography in two-dimensional planes’ to ‘volume ultrasound’.
Keywords Inversion mode, maximum mode, minimum mode, prenatal diagnosis, spatial and temporal image correlation, three-dimensional ultrasound, tomographic imaging, volume ultrasound.
Introduction Three-dimensional (3D) ultrasound has become the most rapidly evolving technique in fetal imaging, but some examiners are still using 3D and fourdimensional (4D) techniques only to demonstrate the fetal face to the parents,
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✩ ✩✩✩✩✩✩✩✩✩✩✩ which has made this new technique very popular with them as well. However, the concept of ‘volume ultrasound’ introduced a few years ago enabled a more comprehensive medical and clinical application of 3D technology to prenatal diagnosis.1 A volume data set of the region of interest is acquired digitally, and the information stored can be displayed in different ways to highlight the spatial arrangement of a specific structure in the region of interest. Many colleagues may still be unfamiliar with all of these features, which are now well established in targeted prenatal diagnosis for ruling out or clearly demonstrating fetal malformations. In this chapter we will review the potential of volume ultrasound and the application of some display modes in clinical work.
Volume Acquisition A volume data set can be acquired in different ways. The acquisition can be achieved as a:
• static 3D • real-time 3D or 4D • spatial and temporal image correlation for heart and vessels. Static 3D This is the 3D used in most fetal studies (face, hands, etc.) and consists of a single volume data set. The volume quality is defined by the choice of the acquisition time. Static 3D can also be combined with colour Doppler, power Doppler, high-definition (HD) flow, and B-flow, depending on the question of interest.
Real-Time 3D or 4D Ultrasound Real-time 3D or 4D is achieved today mainly by a mechanical 3D transducer with a rapid acquisition from 1.5 to 40 volumes/sec. A few matrix transducers provide electronic 3D information but their use in obstetric ultrasound is still limited. The advantage of a 4D examination is its ease of use. The direct result on the screen enables online manipulation to acquire the best image by changing the gain and the contrast depending on the mode used. Furthermore, it allows the transducer to be moved depending on the insonation angle. The technique is ideal for studying fetal movements and behaviour throughout pregnancy (smiling, yawning, grimacing, etc.). It can also be used for fetal echocardiography but this requires a great deal of experience.
Spatial and Temporal Image Correlation
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Spatial and temporal image correlation (STIC) is a software application providing an acquisition of a fetal heart volume data set over a period of few seconds (i.e. 7.5–15 sec). It allows the acquisition of numerous planes including
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Volume Data Display Single Plane of Choice, Multiplanar Orthogonal Planes or Multiple Tomographic Parallel Slices From a digitally stored 3D/4D data set, cross-sectional views can be obtained at any desired orientation (a so-called ‘anyplane’), direction and depth. It must be borne in mind that the acquisition (A-) plane provides the best information, whereas the reconstructed planes (B-, C- or others) are of lesser quality. This should be considered during the volume acquisition. The 2D image analysis from a volume can be achieved from a 3D, 4D or STIC data set. The display format is either a single-plane view or a multiplanar view showing three planes which are perpendicular to each other (Fig. 14.1). In the lateral view the intersection of the three planes is a dot and by moving the position of this dot, the examiner can navigate through the volume (see Fig. 14.1). The recent introduction of multislice analysis known as tomographic ultrasound imaging (TUI) is similar to the tomographic assessment known from computed tomography (CT) and magnetic resonance (MR) workstations (Figs 14.2, 14.3). The examiner can define the slice thickness and the number of planes demonstrated. The multiplanar mode can be used to acquire a plane not directly seen on cross-section 2D during live examination, mainly in cases with non-optimal fetal position, so as to demonstrate the corpus callosum, a fetal profile, a limb or the aortic arch. It can be used to visualize exact midline planes after making adjustments in the two other orthogonal planes (for nasal bone assessment). One of the major advantages of a 3D data set is the potential for offline examination of a few volumes at a remote station.4 One of the future potential uses of this mode could be the transfer of data via the Internet to a remote site to get a second opinion or a complete offline evaluation without examining the patient.5 However, since the quality of reconstructed images depends mainly on the original acquisition, the examiner should consider this aspect when acquiring volumes for future studies.
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
additional information from an entire cardiac cycle. The software calculates the mean heart rate acquired and the images in the volume are rearranged according to their temporal event within the heart cycle. The displayed volume then includes a single ‘hypothetical’ heart cycle, which is reconstructed from single selected images of the A-plane (acquisition plane) in the different phases of the heart cycle, whereas the B- and C-planes are reconstructed digitally.2 STIC can be used with grey-scale fetal echocardiography but can also be combined with colour Doppler, power Doppler, HD flow, B-flow, etc.3 Once the acquisition of a volume is achieved, the information can be visualized as either single or multiple 2D images regenerated from the volume and selected by the examiner or as a volume spatial information called 3D rendering, allowing the application of different modes. Some of the actual display modes will be emphasized and illustrated in this chapter.
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Fig. 14.1 Volume data set of a fetal face shown with the ‘multiplanar mode’ with three orthogonal planes. The dot is the intersection of all three planes and can be used to achieve the best position for assessing the profile. In the C-plane (lower panel) the dot is on the nasal bone. In the lower panel in another fetus the volume information of the face was used to achieve a plane of the soft palate after offline processing.
Fig. 14.3 In this fetal thorax, the cross-section volume acquisition in anterior–posterior tomographic mode is used to demonstrate the slices of the heart, lungs, stomach, diaphragm, etc. In the lower middle panel the bifurcation of the trachea is well seen.
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Fig. 14.2 Tomographic ultrasound imaging (TUI) of the brain demonstrating all important brain structures including the lateral ventricles, the cerebellum, the cavum septum pellucidum and insula.
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Fig. 14.4 TUI can be used for the heart combined with STIC. Within one image mainly moving structures can be seen.
Once a volume data set of a fetal region is stored, the examiner can scroll through the volume to get the plane of interest independent from the insonation angle. In a STIC volume the reconstructed cardiac volume can be displayed in the multiplanar or tomographic modes (Fig. 14.4), and played in slow motion or stopped at any time for detailed analysis of specific phases of the cardiac cycle. When combined with colour Doppler, events within the cardiac cycle during systole and diastole can be very well demonstrated.
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The image rendering of the fetal surface is the best known and most commonly used display modality in 3D and 4D. From the volume acquired, the skin is primarily demonstrated (surface) and not the organs inside the body. It is used to visualize the surface of a structure which is best achieved in the interlay between fluid and surface, such as the face of a fetus in amniotic fluid or the valves within the heart. The main advantage of the technique is its ease of use and its impact on patients and doctors due to the lifelike image (Fig. 14.5), and comparison with the postnatal appearance. Clinical applications include demonstration of the whole fetus in the first trimester up to 12 weeks' gestation (see Fig. 14.5), and demonstration of the face (Fig. 14.6), limbs, etc. in order to rule out or confirm anomalies involving the skin as well as facial anomalies, spina bifida, limb anomalies and others. It is best demonstrated using 3D as well as 4D, whereas the latter
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Fig. 14.6 With surface mode, maturation of the face and changes occurring during pregnancy are well recognized. Three fetuses at 14 weeks (left), 26 weeks (middle) and 31 weeks (right).
can be used to analyse behaviour such as fetal movements, grimacing, yawning or eye opening. Surface rendering can be applied to the fetal heart to visualize cardiac cavities and valves (Fig. 14.7). It can be used in the brain to demonstrate cavities such as the lateral ventricles, especially in the presence of brain anomalies.
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Fig. 14.5 Surface mode demonstrating two fetuses at 10 weeks (left) and at 13 weeks.
Maximum Mode Rendering This mode is used to highlight the maximal echo information of a volume data set and is an ideal tool for the 3D reconstruction of bony structures (Fig. 14.8).6 In general, cranial bones, the ribs and other curvilinear bones cannot be clearly seen in a single 2D plane and are therefore better assessed in a maximum mode projection. This technique has been applied in the demonstration of spine and limb abnormalities but was recently used in the assessment of the nasal bones, the cranial bones and corresponding sutures in normal and abnormal conditions.7–11 This technique delivers a picture similar to an x-ray of the bony skeleton in the fetus.
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Fig. 14.7 Acquisition with STIC and surface mode rendering can be used at the level of the heart to visualize the spatial appearance of the heart cavities.
Fig. 14.8 Maximum mode rendering is used to visualize the fetal skeleton; here the bony face with the nasal bones and the metopic suture (left), the skull from the side with skull sutures (middle), and the fetal spine with the scapulae, long bones and pelvis (right).
Minimum Mode Rendering
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This mode is used to highlight the hypoechoic structures in the volume of interest and to demonstrate a 3D projection of vessels, cysts, bladders and others that appear black against a surrounding of more echogenic tissue (Fig. 14.9). It is preferable to make the rendering box narrow in order to focus on the region of interest. Within the box, the presence of amniotic fluid should be avoided as it casts a large black shadow. Images produced with this technique are similar to x-ray projection. Regions of interest are mainly the stomach (see Fig.14.9 right), the bladder, the brain ventricles and the heart with the corresponding vessels.
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Inversion Mode Rendering This display mode inverts the colour of the anechoic information (similar to negative/positive film), thus presenting the hypoechoic structures as echogenic solids.12,13 It blackens most of the surrounding tissue information (Fig. 14.10). By changing certain preset parameters, the image can be improved. This technique was also called negative surface display and it was discovered that the images produced were similar to postmortem casting. Artifacts may result from rib shadowing or from amniotic fluid, etc., but can be eliminated using the electronic scalpel during offline volume manipulation.
Fig. 14.10 Inversion mode in different fetal conditions. (Left top) A dilated ventricle on 2D in a longitudinal view in a fetus with spina bifida. (Left bottom) Inversion mode demonstrating the shape of the dilated ventricles in the same fetus. The black areas are the lack of information of the choroid plexus. (Middle) A fetal bladder with dilated ureter and hydronephrosis. (Right) Inversion mode demonstrating the heart and the crossing of the great vessels.
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Fig. 14.9 Minimum mode rendering demonstrating in the left and middle images the abdomen with bladder (BL), gallbladder (GB), umbilical vein (UV), stomach (ST), inferior vena cava (VCI) and aorta (AO). On the right, as comparison, a double bubble sign observed at 24 weeks in duodenal atresia.
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✩ ✩✩✩✩✩✩✩✩✩✩✩ The role of this technique has been analysed in visualizing cardiac and extra cardiac fluid-filled structures in the fetus. Application fields are not only the heart and vessels (see Fig. 14.10 right) but also kidneys in hydronephrosis (see Fig. 14.10 middle), brain ventricles (see Fig. 14.10 left) and other hypoechoic cystic structures. Regions of interest in the fetus could be the fluid-filled structures as the stomach, the urinary bladder or gallbladder. The shape of the stomach and duodenum in the presence of a double bubble in duodenal atresia could be a clinically important application. The kidneys can mainly be demonstrated in anteroposterior longitudinal projection and clinical benefit can be found in multicystic kidneys and hydronephrosis. Intracranial brain structures, especially the lateral ventricles in early pregnancy, can be clearly demonstrated and malformations with disturbed anatomy of the lateral ventricles can be seen with inversion mode. One of the major fields of interest with inversion mode is the cardiovascular system. The examiner can visualize the heart and vessels in a manner similar to 3D power Doppler ultrasound at a better resolution and with a more rapid acquisition rate. Particularly easily demonstrated is the crossing of the vessels of the heart or the relationship of the ventricles and their size. The main advantage of this technique is that the image is similar to the one acquired by power Doppler but without the difficulties encountered in adjusting the image. The volume can be acquired in grey scale as 3D static or as a STIC, at a high frame rate and resolution, whereas volumes with power Doppler information are at low frame rates and subject to movement artifacts. Thus, the image quality with inversion mode is superior to the quality obtained by power Doppler; however, it lacks the information of neighbouring tissue demonstrated in the glass body mode. Since inversion mode can also be used for volume calculation, it could be more easily used to calculate volumes of structures with irregular shape than with the VOCAL technique.
Glass Body Mode Rendering This mode is used to demonstrate a volume with grey scale and colour or power Doppler information simultaneously. The acquisition can either be achieved as static 3D or as a STIC. Volume data can be displayed in three ways: the colour information alone, the grey-scale information alone or a combination of both as a so-called ‘glass body’ mode (Fig. 14.11). A prerequisite for a good volume is the optimal presetting of the colour during 2D scan before acquiring a volume. One of the main application fields of this mode is the demonstration of the cardiac chambers and the great vessels14 (see Fig. 14.11). Peripheral vessels such as the umbilical cord (see Fig. 14.11), intra-abdominal, thoracic and brain vessels can be well demonstrated.
Volume Calculation
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Biometry is an integral part of the antenatal ultrasound examination and has been achieved for years by measuring distances, circumferences and areas. The acquisition of a 3D volume data set allows easy reconstruction of a selected 2D plane to
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perform well-known measurements such as nuchal translucency, biparietal diameter or femur length, but also offers the potential to accurately calculate the volume of a selected region of interest. Volume measurements can be achieved using either the multiplanar mode or the VOCAL™ software (VOlume CALculation). Recently, another possibility has been developed for liquid-filled structures involving the threshold principle in combination with the inversion mode. Volume measurements are still time-consuming and thus limited to research purposes. Volume measurements and charts were reported for the placenta, the amniotic cavity, the first trimester fetus, the fetal brain, liver and arm, but there was a special interest in measuring fetal lung volume.15,16 Fields of interest in these measurements focused chiefly on the detection of difference in volume in pregnancies complicated by chromosomal anomalies, diabetes, intrauterine growth restriction and congenital diaphragmatic hernia.
Conclusion Three-dimensional ultrasound application in prenatal diagnosis should not be limited to the demonstration of the fetal face to please the parents. The concept of volume ultrasound demonstrated in this chapter enables the acquisition of a digital volume data set and the display of the information in different ways. The different display modes available can be used for the demonstration of the spatial appearance of surface structures as well as the projection of bony structures for a better understanding of skeletal and other findings. The multiplanar mode and tomographic imaging allow the reconstruction of planes not directly seen on the screen and offer new insight into fetal anatomy similar to images now demonstrated by MRI for brain structures. The numerous enthusiastic articles on 3D written in recent years confirm that we are rapidly moving from the era of ‘sonography in 2D planes’ to ‘volume ultrasound’. New rendering modes and clinical features will appear in the near future and the development of faster processors in computer technology will enable the advent of matrix transducers with the possible instant application of these techniques.
Three-dimensional and four-dimensional ultrasound application in prenatal diagnosis
Fig. 14.11 Glass body mode. On the left, a longitudinal view of the brain with vessels (pericallosal artery and ramifications). In the middle, the heart and the great vessels from a STIC volume. On the right, the posterior placenta demonstrating the central insertion of the umbilical vessels.
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References
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1. Chaoui R, Heling KS. Three-dimensional ultrasound in prenatal diagnosis. Curr Opin Obstet Gynecol 2006;18:292–302 2. Devore GR, Falkensammer P, Sklansky MS, Platt LD. Spatio-temporal image correlation (STIC): new technology for evaluation of the fetal heart. Ultrasound Obstet Gynecol 2003;22:480–487 3. Chaoui R, Heling KS. New developments in fetal heart scanning: three- and fourdimensional fetal echocardiography. Semin Fetal Neonatal Med 2005;10:567–577 4. Benacerraf BR, Shipp TD, Bromley B. How sonographic tomography will change the face of obstetric sonography: a pilot study. J Ultrasound Med 2005;24:371–378 5. Vinals F, Mandujano L, Vargas G, Giuliano A. Prenatal diagnosis of congenital heart disease using four-dimensional spatio-temporal image correlation (STIC) telemedicine via an Internet link: a pilot study. Ultrasound Obstet Gynecol 2005;25:15–31 6. Benoit B. The value of three-dimensional ultrasonography in the screening of the fetal skeleton. Child's Nerv Syst 2003;19 (7–8):403–409 7. Dikkeboom CM, Roelfsema NM, Van Adrichem LN, Wladimiroff JW. The role of three-dimensional ultrasound in visualizing the fetal cranial sutures and fontanels during the second half of pregnancy. Ultrasound Obstet Gynecol 2004;24:412–416 8. Benoit B, Chaoui R. Three-dimensional ultrasound with maximal mode rendering: a novel technique for the diagnosis of bilateral or unilateral absence or hypoplasia of nasal bones in second-trimester screening for Down syndrome. Ultrasound Obstet Gynecol 2005;25:19–24
9. Chaoui R, Levaillant JM, Benoit B, Faro C, Wegrzyn P, Nicolaides KH. Threedimensional sonographic description of abnormal metopic suture in second- and third-trimester fetuses. Ultrasound Obstet Gynecol 2005;26:761–764 10. Faro C, Chaoui R, Wegrzyn P, Levaillant JM, Benoit B, Nicolaides KH. Metopic suture in fetuses with Apert syndrome at 22–27 weeks of gestation. Ultrasound Obstet Gynecol 2006;27:18–33 11. Faro C, Wegrzyn P, Benoit B, Chaoui R, Nicolaides KH. Metopic suture in fetuses with holoprosencephaly at 11 + 0 to 13 + 6 weeks of gestation. Ultrasound Obstet Gynecol 2006;27(2):162–166 12. Lee W, Goncalves LF, Espinoza J, Romero R. Inversion mode: a new volume analysis tool for 3-dimensional ultrasonography. J Ultrasound Med 2005;24:201–207 13. Benacerraf BR. Inversion mode display of 3D sonography: applications in obstetric and gynecologic imaging. Am J Roentgenol 2006;187(9):965–971 14. Chaoui R, Schneider MBE, Kalache KD. Right aortic arch with vascular ring and aberrant left subclavian artery: prenatal diagnosis assisted by three-dimensional power Doppler ultrasound. Ultrasound Obstet Gynecol 2003;22:661–663 15. Moeglin D, Talmant C, Duyme M, Lopez AC. Fetal lung volumetry using two- and threedimensional ultrasound. Ultrasound Obstet Gynecol 2005;25:219–227 16. Peralta CF, Cavoretto P, Csapo B, Falcon O, Nicolaides KH. Lung and heart volumes by three-dimensional ultrasound in normal fetuses at 12–32 weeks' gestation. Ultrasound Obstet Gynecol 2006;27(2):128–133
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Fetal movement patterns and behavioural states Gerard H A Visser Eduard J H Mulder
Abstract Fetal movements appear early in pregnancy, are from their inception specific and closely resemble movements after birth. This makes them candidates for diagnostic purposes. In this chapter the normal development of fetal motor patterns and of behavioural states is discussed and clinical implications of altered behaviour are emphasized.
Keywords Fetal behaviour, fetal monitoring, fetal movements, maternal diseases, medication.
Introduction Ultrasound in obstetrics focuses on morphology and Doppler waveform patterns of fetal and maternal vessels. Fetal motility usually gets less attention. However, some knowledge regarding incidence, quality and periodicity of fetal movement patterns is necessary in order to:
• obtain insight into normal developmental aspects of nervous system
functioning (and related phenomena such as fetal heart rate patterns, Doppler flow profiles and fetal micturition) • identify situations with a negative impact on nervous system development and • identify individual fetuses with abnormal brain or neuromuscular functioning. In this chapter the normal development of fetal motor patterns and of fetal sleep or behavioural states is discussed and clinical implications of altered behaviour are emphasized.
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Methodology
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Observation of fetal movements can best be done by using a real-time linear array transducer with a long probe (>9 cm) or a curved array transducer, with a high frame rate (>30 pictures per second). One transducer is sufficient until about 20 weeks of gestation, but thereafter two transducers are necessary if it is intended to observe all movement patterns. Also, for the recording of fetal behavioural states, two transducers are necessary: one for the observation of body movements and one for eye movements. Due to the episodic occurrence of the different movement patterns and the development of fetal behavioural states, it is necessary to make relatively long observations (0.5–2 h). With a recording of 1 hour's duration and a spatial peak temporal average of the equipment of 0.4 mw/cm2, the product of intensity and exposure time is 1.4 J/cm2, which is far below the accepted 50 J/cm2.
The Emergence of Fetal Movement Patterns Endogenously generated (i.e. spontaneous) fetal movements can first be observed after 7 weeks postmenstrual age (i.e. 5 weeks after conception).8 At this early age these movements are difficult to classify because of the small size of the embryo (1–2 cm) and the limited resolution of the ultrasound equipment. All types of movements emerging after 8 weeks are, however, specific and easily recognizable. Surprisingly, all these early emerging movements closely resemble those observed in preterm and full-term newborn infants, which makes it possible to classify them accordingly.8,32 There is an early emergence of different movement patterns and at 15 weeks of gestation 12 distinct patterns can already be distinguished (startle, general movements, hiccup, breathing, isolated arm or leg movement, isolated retroflexion/rotation and anteflexion of the head, jaw movements, sucking and swallowing, hand–face contact, stretch, yawn, body rotation). The developmental profile of these movements plotted according to their first appearance in a group of 12 normal fetuses is shown in Figure 15.1. In addition, slow eye movements can be observed from 18 weeks onwards, while rapid eye movements emerge somewhat later.5,15 These movements, once observed, remain present during the course of pregnancy and their appearance hardly changes. All these data were obtained in the early 1980s with the equipment available at that time, but are still unchallenged. This early emergence of highly organized, specific movement patterns, long before birth, seems surprising, even more so when the minimal development of the nervous system at that age is taken into account. Studies on the ultrastructure of the nervous system of the young fetus are still scarce. The available data suggest, however, that movements commence as soon as the first connective structures are formed.8 The reason why the different movement patterns emerge so early is still unclear. Certain movement patterns have an adaptive effect on the survival or development of the fetus. Frequent and active changes of the intrauterine
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Fig. 15.1 Timetable of emergence of specific movement patterns in a longitudinal study of 12 fetuses. Each dot indicates the first observation of a particular pattern in an individual from the weekly observation. Postmenstrual age is given in weeks and days (reproduced from reference 1, with permission).
position may prevent adhesions and local stasis of the circulation of the skin. Individual movements may prevent the occurrence of contractures, as can be found after prolonged oligohydramnios following leakage of amniotic fluid. Sucking and swallowing movements are necessary for the regulation of the amount of amniotic fluid. Another reason for the early emergence of fetal movements is anticipation of postnatal functions. Some motor patterns emerge during early prenatal development and are regularly performed spontaneously long before they fulfil a meaningful task as part of a complex adaptive function. For example, fetal breathing movements are already present at 10 weeks. These movements might also have a profound influence on lung growth, as in animal experiments spinal cord transection results in fetal lung hypoplasia;18 however, this link is still unclear in the human. The frequent occurrence of many specific movements during the first trimester of pregnancy can be depicted in a complex actogram, as is shown in an example of a 1-hour recording at 13 weeks (Fig. 15.2). At all ages there are large interindividual differences in the incidence of the various types of movements.9 There are, however, specific developmental trends in the quantity (incidence) of the various types of movements. For example, the incidence of general movements increases rapidly until a plateau is reached at 10 weeks (about 12% of recording time), with a slight fall towards term age. The incidence of startles and hiccups declines
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13 weeks Startle General mov. Hiccup Breathing mov. Isol. arm mov. Isol. leg mov. Head retroflexion Head rotation Head anteflexion Jaw opening Sucking+swallow. Hand–face contact Stretch Yawn Minutes 0 30 60 Fig. 15.2 Compiled actogram of 1 h observation of a fetus at 13 weeks of gestation. Note the periodicities and the multitude of specific movement patterns (data extracted from references 1 and 6, with permission).
after 12 weeks of gestation, while breathing movements gradually increase until 30 weeks; at the latter age breathing movements are on average present during 30% of recording time.9,36
Body Movements in Normal Pregnancy
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Several authors have reported on the incidence of fetal body movements. However, because of the absence of a uniform definition and differences in study design and data analysis, the reported mean/median values and ranges of normality differ greatly among the various studies. Figure 15.3 shows nomograms of four incidence parameters of fetal body movements from 24 weeks till term. These data are from a longitudinal study in 29 normal fetuses. Fetal movements were recorded serially for 60 minutes at fortnightly intervals between 24 weeks and 36 weeks of gestation and for 120 minutes weekly from 36 weeks until delivery. Body movements which
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Fig. 15.3 Nomograms of the four incidence parameters of fetal body movements. Presented are the median (solid line), 2.5th and 97.5th centiles (dashed lines), and individually measured values in relation to gestational age for (A) the percentage of time spent making body movements, (B) the number of body movements per hour, (C) their mean duration, and (D) the median onset–onset interval (reproduced from reference 8, with permission).
occurred within 1 second apart were considered as a single burst of movement. The duration of individual fetal body movements remained stable with gestation whereas the onset–onset interval increased, resulting in a gradual decline in the number of movements per hour. The median percentage incidence of fetal body movements decreased from 17% at 24 weeks to about 7% near term. This overall decline in incidence appears to be a developmental phenomenon, rather than the result of developing sleep states, since the declining trends are similar during ‘active’ and ‘quiet’ sleep.40 There is some degree of intrafetal consistency in the incidence of body movements, but intra- (and inter-) fetal variances are generally high, for instance much higher than those for fetal heart rate and its variation. The incidence of fetal body movements is the same for boys and girls.35 Maternal meals do not affect the incidence of body movements. During the second half of gestation there is a diurnal variation in the incidence of general movements, with peak values occurring around midnight.34 This diurnal rhythm as well as those in fetal heart rate variation is related to maternal adrenal activity (cortisol).44
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Fetal Breathing in Normal Pregnancy
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Fetal breathing movements are characterized by a fluent downward movement of the diaphragm, outward displacement of the abdomen and inward displacement of the thorax. Apnoea is defined as an interval between two consecutive breaths of more than 6 seconds. The incidence of fetal breathing increases up to about 30 weeks of gestation. At 20 weeks, breathing is on average present during 5% of recording time and at 30 weeks during 30%. Fetal breathing movements are affected by maternal meals and plasma glucose concentrations, especially during the third trimester of pregnancy. The highest incidence occurs 1.5–2 hours after a meal and 1 hour after the highest blood glucose value. There is also an increase in breathing incidence at night, but this is related to a circadian rhythm and not to glucose concentrations.30
Normal Development of Fetal Behavioural States During early gestation movements are scattered over time, but in the course of pregnancy a progressive clustering occurs in rest/activity cycles and later in behavioural states. These behavioural states develop during the third trimester of pregnancy. They are distinct and discontinuous modes of neural activity and, although defined by a different set of variables, are homologous to the states in the newborn.28 Each state is defined by a specific combination of the parameters of three selected variables: fetal heart rate pattern, body and eye movements. Such a combination is relatively stable, i.e. it is maintained uninterrupted over longer periods and transitions from one state to another are characterized by the almost simultaneous change of state variables. In the healthy fetus behavioural states are fully developed from about 36 weeks onwards, an age at which behavioural states are also present in low-risk preterm newborn infants. In the near-term human fetus four behavioural states have been identified: 1F–4F (F stands for fetal).28 State 1F is characterized by a stable heart rate with a small oscillation bandwidth (FHR pattern A) and absence of eye and generalized body movements. In state 2F, eye movements and periodic body movements are present; fetal heart rate has a wide oscillation bandwidth between frequent accelerations (FHR pattern B). In state 3F body movements are absent, eye movements are present and fetal heart rate has a wide oscillation bandwidth without accelerations (FHR pattern C). In state 4F there are prolonged accelerations (FHR pattern D), numerous body movements and presence of eye movements. If a stable association of the three state parameters exists for at least 3 minutes and if transitions from one state to another do not last more than 3 minutes, the presence of fetal behavioural states is accepted. The fetal states correspond to state 1 to state 4 in the full-term newborn infant, and in the neonate they may also be classified as quiet sleep, REM sleep, quiet awake and active awake, respectively. An example of a fetal behavioural state recording is shown in Figure 15.4. Fetal behavioural states develop gradually and from 28 weeks onwards there is a significant association between the state variables.48 This development results
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Fig. 15.4 Example of a 3-h recording of a healthy fetus at 38 weeks of gestation. It shows from above downwards: (1) the fetal heart rate tracing (FHR) and the occurrence of general movements (GM) and eye movements (EM). Three periods of high heart rate variation (pattern B) were interrupted by two periods of low variation (pattern A); during the former both general movements and eye movements were present (coincidence 2F), but absent during pattern A (coincidence 1F); (2) profiles of the three state variables and the resulting episodes of coincidence 1F and 2F; (3) presence of behavioural states 1F and 2F (transitions 3 mm. 355
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c. The embryo has a CRL of 25 mm; the heart rate is as high as 185 bpm. d. The embryo has a CRL of 10 mm; the yolk sac diameter is 7 mm. 5. One of the following findings is probably consistent with a normal early pregnancy. a. CRL 4 mm, diameter of yolk sac 5 mm, diameter of amniotic cavity 7 mm, heart rate 110 bpm b. CRL 23 mm, diameter of yolk sac 5 mm, diameter of amniotic cavity 24 mm, heart rate 170 bpm c. CRL 29 mm, diameter of yolk sac 5 mm, diameter of amniotic cavity 34 mm, heart rate 150 bpm d. CRL 2 mm, diameter of yolk sac 3 mm, diameter of amniotic cavity 4 mm, heart rate 90 bpm e. CRL 13 mm, diameter of yolk sac 9 mm, diameter of amniotic cavity 12 mm, heart rate 130 bpm
Chapter 5 Normal Fetal Anatomy at 18–22 Weeks 1. Basic guidelines by the American Institute of Ultrasound in Medicine include views of: a. the great vessels of the heart b. hands and feet c. anterior abdominal wall d. face e. all of the above 2. A sequential segmental approach to the heart would include specifice views of all of the following except: a. view of the upper abdomen to show normal solitus b. left and right ventricular outflow views c. four-chamber view d. ductus venosus e. pulmonary artery and ductal arch 3. Markedly echogenic bowel has been associated with what types of outcome? a. normal b. aneuploidy c. fetal infection d. fetal demise e. all of the above 4. All of the following is true about the fetal anatomic survey except: a. it is usually performed during the second trimester b. it is the ideal time for nuchal translucency evaluation c. it requires a systematic approach d. it gives the opportunity to provide important information about the fetus
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Chapter 6 Amniotic Fluid and Placental Localization Test yourself – questions and answers
1. The water content of amniotic fluid is: a. 50–60% b. 70–80% c. 98–99% 2. The normal amniotic fluid index ranges between: a. 5 and 25 cm b. 10 and 30 cm c. 20 and 40 cm 3. There is a high association with pulmonary hypoplasia when severe oligohydramnios develops: a. before 20–25 weeks of gestation b. at 30–35 weeks of gestation c. at 35–40 weeks of gestation 4. The association between chronic polyhydramnios and fetal anomalies is approximately: a. 1 in 25 b. 1 in 15 c. 1 in 5
Chapter 7 Assessment of the Placenta and Umbilical Cord 1. All the following statements concerning placenta accreta are correct except one. a. This anomaly is characterized by myometrial invasion by placental villous tissue. b. It occurs when the decidua basalis is partially or completely absent. c. It is more common in primigravidae than multigravidae. d. Placentas accreta have an overall maternal and fetal mortality of around 10%. e. Caesarean hysterectomy is often needed. 2. The only statement about chorioangioma that is correct is: a. Chorioangiomas are malignant tumours characterized by a proliferation of villous capillaries and trophoblastic tissue. b. Chorioangiomas are often diagnosed during the first trimester of pregnancy. c. Most chorioangiomas are large, single, round, encapsulated, near the cord insertion. d. All chorioangiomas can be complicated by fetal hydrops, due to the chronic shunting, and by polyhydramnios. e. The fetal risk depends more on the proportion of angiomatous versus myxoid tissue inside the tumour than on its exact size. 3. The only ultrasound feature mentioned below that is specific to a triploid partial mole is: a. fetal bilateral cerebral ventriculomegaly b. severe fetal growth restriction
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Chapter 8 Examining the Cervix by Transvaginal Ultrasound
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1. The risk of spontaneous early preterm delivery increases with decreasing cervical length. At a cervical length of 25 mm, the risk is approximately: a. 25% b. 15% c. 10% d. 1% 2. Cervical funnelling is: a. the early onset of labour b. protrusion of membranes into the endocervical canal c. shortening of the cervix 3. A short cervical length is synonymous with cervical incompetence: a. correct b. incorrect 4. Ultrasound measurement of cervical length allows differentiation between true and false labour: a. correct b. incorrect
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Chapter 9 Fetal Biometry, Estimation of Gestational Age, Assessment of Fetal Growth 1. What is the aim of CRL measurement? a. to determine fetal weight b. to determine true gestational age c. both d. other aims 2. The accuracy of pregnancy dating: a. increases with gestational age b. decreases with gestational age c. is independent from gestational age 3. The measurement of biparietal diameter: a. is used for dating pregnancy in the second trimester b. is very accurate for dating pregnancy in the third trimester c. is taken at the same level as the measurement of the cerebellar diameter 4. The abdominal circumference: a. is used for fetal dating in the third trimester b. is used for the evaluation of fetal growth disturbances in the second trimester c. is the main factor in fetal weight determination in most of the mathematical equations 5. For a confident diagnosis of IUGR: a. the calculated fetal length should be below normal values b. the calculated fetal weight should be below normal values c. both AC and fetal weight estimation, made at least 2–3 weeks apart, should be below normal values
Test yourself – questions and answers
5. True cervical incompetence is responsible for which percentage of preterm labour: a. 40% b. 30% c.