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Ultrasound: The Requisites PART II Obstetrics and Gynecology Ultrasound: The Requisites Third Edition Barbara S. Hertzberg, MD, FACR Professor of Radiology Associate Professor of Obstetrics and Gynecology Duke University School of Medicine Duke University Health System Durham, North Carolina William D. Middleton, MD, FACR Professor of Radiology Director of Ultrasonography Mallinckrodt Institute of Radiology Washington University School of Medicine Saint Louis, Missouri 1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899 ULTRASOUND: THE REQUISITES, Third Edition ISBN: 978-0-323-08618-9 Copyright © 2016 by Elsevier, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. 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International Standard Book Number: 978-0-323-08618-9 Executive Content Strategist: Robin Carter Content Development Specialists: Margaret Nelson and Gabriela Benner Publishing Services Manager: Julie Eddy Design Direction: Xiao Pei Chen Printed in China Last digit is the print number: 9 8 7 6 5 4 3 2 1 PART I General and Vascular CHAPTER 1 Practical Physics ACOUSTICS DOPPLER OPTIMIZATION Transducer Frequency INSTRUMENTATION Gain Piezoelectric Crystals Power Static B-Mode Systems Pulse Repetition Frequency (Doppler Scale) Real-Time Transducers Wall Filter HARMONIC IMAGING Color Priority Beam Steering REAL-TIME COMPOUNDING Ensemble Length EXTENDED FIELD-OF-VIEW (PANORAMIC) ARTIFACTS IMAGING Shadowing M-MODE IMAGING Posterior Enhancement Mirror Images THREE-DIMENSIONAL ULTRASOUND IMAGING Refraction Speed Propagation FUSION IMAGING Reverberation ELASTOGRAPHY Ring Down Comet Tail GRAY-SCALE IMAGE OPTIMIZATION Side Lobe Transducer Slice Thickness Power Output Anisotropy Gain Electronic Interference Focal Zone Focal Zone Banding Field of View Transducer Crystal Malfunction Line Density Loss of Contact Gray-Scale Curves/Maps Acoustic Streaming Dynamic Range Aliasing Persistence Tissue Vibration DOPPLER SONOGRAPHY Blooming Pulsed Doppler Twinkling Color Doppler CONTRAST AGENTS Power Doppler B Flow FOR KEY FEATURES SUMMARY SEE P. 30 Additional videos for this topic are available online at qualitative and quantitative evaluation of blood flow. expertconsult.com. Development and continuing refinement and experience with ultrasound microbubble contrast agents now allow sonogra- Ultrasonography has long been a valuable method of imaging phy to rival and/or exceed CT and MRI in detecting and the body with several distinct advantages over other modali- characterizing soft-tissue and vascular lesions. Finally, in ties. One of the most important advantages is its lack of ion- the era of medical cost containment, ultrasonography is an izing radiation. Sonography can provide clinically useful attractive imaging study for many clinical problems, espe- information without clinically significant biologic effects on cially in situations in which multiple sequential examinations the patient. This is critical in obstetrics, very important in are necessary or when screening of large patient populations the pediatric patient population, and becoming increasingly is desired. All of these factors make ultrasonography an important in adults due to radiation dose concerns associated extremely valuable tool in the investigation of a vast array of with computed tomography (CT). A second advantage of disorders. sonography is the real-time nature of the examination. This Any individual who performs diagnostic ultrasonography makes it possible to evaluate rapidly moving structures such must have an understanding of the physical principles of this as the heart and easier to examine the moving fetus and technique and the instrumentation available for detecting and structures in patients unable to suspend respiration or coop- displaying the acoustic information. This chapter will be erate with the examination. Its multiplanar imaging, real-time limited to the practical physical principles that are most rel- equipment, and three-dimensional (3D) capabilities enable evant to the practice of diagnostic ultrasound. flexibility in the selection of imaging planes and the ease of altering these planes, allowing rapid determination of the ACOUSTICS origin of pathologic masses and analysis of spatial relation- ships of various structures. The portable nature of the equip- Sound is the result of mechanical energy producing alternat- ment is an advantage over other cross-sectional modalities ing compression and rarefaction of the conducting medium such as CT and magnetic resonance imaging (MRI). Another as it travels in the form of a wave. Humans can generally hear advantage of sonography is its excellent resolution of super- from 20 Hz to 20 kHz. Ultrasound differs from audible sound ficial structures. Doppler techniques add the advantage of only in its higher frequency, and hence the name ultrasound 3 PART I General and Vascular 4 PART I General and Vascular M L Fast R Slow FIGURE 1-1. Interactions of sound with anatomic structures. Trans- verse view of the liver demonstrates the right (R), middle (M), and left (L) hepatic veins. They appear anechoic (black) because the intraluminal blood contains very weak reflectors. The walls of the hepatic veins are specular reflectors and their appearance will depend on their orientation to the sound beam. Because the right hepatic FIGURE 1-2. Sound refraction. When sound travels obliquely through vein is oriented perpendicular to the direction of sound, its walls an interface between substances that transmit sound at different appear echogenic. The left and middle hepatic veins are not oriented speeds, the wavelength changes as shown in the illustration perpendicular to the sound beam and so their walls are hypoechoic. (top). The result is a redirection or bending of the sound, called The liver parenchyma appears intermediate in echogenicity because refraction. it contains multiple small-tissue interfaces that scatter the sound. (i.e., >20 kHz). Diagnostic sonography generally operates at (Fig. 1-2). Refraction is important because it is one of the frequencies of 1 to 20 MHz. causes of mislocalization of a structure on an ultrasound Ultrasound uses short sound pulses that are transmitted image. Refraction is discussed in more detail in the “Artifacts” into the body. The velocity of propagation is constant for a section. given tissue and is not affected by the frequency or wave- Absorption refers to the loss of sound energy secondary length of the pulse. The more closely packed the molecules, to its conversion to thermal energy. Absorption is greater the faster the speed of sound. Therefore in biological tissues, in soft tissues than in fluid, and it is greater in bone than in the speed of sound is lowest in gases, faster in fluid, faster yet soft tissues. Sound absorption is a major cause of acoustic in soft tissue, and fastest in bones. In soft tissues, the assumed shadowing. The combined effects of reflection, scattering, average propagation velocity is 1540 m/s. and absorption result in attenuation in the intensity of the Sound pulses transmitted into the body can be reflected, sound pulse as it travels through matter. Attenuation limits scattered, refracted, or absorbed. Reflection or backscatter the depth of imaging and is greater at higher transmit occurs whenever the pulse encounters an interface between frequencies. tissues that have different acoustic impedances. Acoustic impedance is the product of the speed of sound and the tissue density. The strength of reflection depends on the difference INSTRUMENTATION in acoustic impedance between the tissues as well as the size, Piezoelectric Crystals surface characteristics, and orientation of the interface with respect to the transmitted sound pulse. The greater the acous- Ceramic crystals that deform and vibrate when they are tic impedance mismatch, the greater the backscatter or electronically stimulated generate the sound pulses used for reflection. Large interfaces that are smooth produce strong diagnostic sonography. Each pulse consists of a band of fre- reflections and are referred to as specular reflectors. If spec- quencies referred to as the bandwidth. The center frequency ular reflectors are oriented perpendicular to the direction of produced by a transducer is the resonant frequency of the transmitted pulse, they will reflect the sound directly back the crystal element and is dependent on the thickness of the to the active crystal elements in the transducer and produce crystal. Echoes that return to the transducer distort the a strong signal. Specular reflectors not oriented perpendicular crystal elements and generate an electric pulse that is pro- to the sound will produce a strong reflection, but because the cessed into an image. High-amplitude echoes produce greater reflection will not travel back to the active crystal elements crystal deformation and generate a larger electronic voltage. in the transducer, the signal will be weaker. Scattering refers They are then displayed on the image as brighter pixels to the redirection of sound in multiple directions. Scattering than low-amplitude echoes. Because of this, standard two- produces a weak signal and occurs when the pulse encounters dimensional (2D) gray-scale images are often referred to as an acoustic interface that is smaller than the wavelength of B-mode (brightness mode) images. sound, or a large interface that is rough. The results of these The size and configuration of the transmitted sound pulse interactions are illustrated in Fig. 1-1. determine the resolution of the image. Resolution must be Refraction refers to a change in the direction of the sound considered in 3D, as illustrated in Fig. 1-3. Axial resolution and occurs when sound encounters an interface between two refers to the ability to resolve objects within the imaging tissues that transmit sound at different speeds. Because the plane that are located at different depths along the direction sound frequency remains constant, the wavelength changes of the sound pulse. This is dependent on the pulse length of to accommodate the difference in the speed of sound in the the generated sound pulse, which in turn is dependent on the two tissues. The result of this change in wavelength is a redi- wavelength. Because wavelength is inversely proportional rection of the sound pulse as it passes through the interface to the frequency, higher frequency probes produce shorter ChAPTeR 1 Practical Physics 5 Probe Focal zone Lateral Axial Elevational FIGURE 1-3. Ultrasound resolution. This schematic shows a sound gFeIGnUicR pEi n1s- 4vi.e wVeiedw i no fc raons su sletrcatsioonu.n Td hpeh painntso min tshheo wdas smheudl twiphleit ee cohvoa-l beam being produced by an ultrasound probe. The ultrasound beam are used to test linear measurement accuracy. The pins in the solid is narrowest at the level of the focal zone, resulting in the best lateral white ovals are used to test lateral resolution. The pins in the black resolution at this level. The focal zone can be adjusted up and down oval are used to test axial resolution. Variably sized cylinders filled by the operator. The elevational resolution, which is equivalent to the with fluid (arrows) and solid material (arrowheads) are used to test slice thickness, is dependent on the shape of the transducer’s crystal contrast resolution. elements and is not variable, except with matrix transducers. In this diagram the elevational focal zone and the lateral focal zone are at the same level, but that is not always the case. The axial resolution is dependent on the transmit frequency and improves with higher frequency transducers. pulses and better axial resolution. As mentioned earlier, high- frequency sound does not penetrate deeply into tissues, so high-frequency probes are only useful for superficial struc- L tures. Lateral resolution refers to the ability to resolve objects within the imaging plane that are located side by side at the same depth from the transducer. This is dependent on the in-plane diameter of the pulse and can be varied within limits by adjusting the focal zone. Elevation resolution (azimuth resolution) refers to the ability to resolve objects that are at the same distance from the transducer but are located perpendicular to the plane of imaging. This is dependent on the out-of-plane diameter of the pulse, which is equivalent to the thickness of the tomographic slice. When crystal elements are arranged in a single row, slice thickness is determined by the FIGURE 1-5. Articulated-arm static B-mode scan of the abdomen taken in 1976. Unlike modern real-time scans, this technique required shape of the crystal elements or the characteristics of fixed several seconds to generate a single image. However, complete cross acoustic lenses and is not adjustable by the user. When crystal sections of the body could be obtained. In this case the image is black elements are arranged in multiple rows and columns, the slice on white, which is opposite from modern scans. In this patient a thickness is variable and can be adjusted by the user. The fluid-filled lymphocele (L) is shown. resolution of different transducers can be tested using phan- toms, as shown in Fig. 1-4. disadvantage of static B-mode imaging was its lack of real- time capabilities. Because of this limitation, static articulated- Static B-Mode Systems arm B-mode devices have been replaced by real-time units. The early 2D units attached a B-mode transducer with a single large piezoelectric crystal element to an articulated arm that Real-Time Transducers was capable of determining the exact location and orientation Mechanical Transducers of the transducer in space. The distance of the reflector from the transducer was obtained by converting the time taken for Real-time images can be generated with a variety of transduc- the echo to return to the transducer based on the speed of ers. The simplest design is the mechanical sector transducer, sound in soft tissues (1540 m/s). This allowed the origin of which uses a single large piezoelectric element to generate the returning echoes to be localized in 2D. Then, by moving and receive the ultrasound pulses. Beam steering is accom- the transducer across the patient’s body, a series of B-mode plished by oscillating or rotating the crystal element itself, or lines of information could be added together to produce a 2D by reflecting the sound pulse off an oscillating acoustic mirror. image. With static B-mode imaging it was possible to view Beam focusing is done using different-shaped crystal elements cross sections of large organs, such as the liver, or of the entire or by attaching an acoustic lens to the transducer. Although body on a single image, much like an axial CT scan (Fig. 1-5). the mechanical movement is fast enough to produce gray- This was an advantage over modern real-time transducers, scale images in real time, it is not fast enough to produce which have a much more limited field of view. The major real-time color Doppler images. Other disadvantages of the 6 PART I General and Vascular Linear-Array Transducers Unlike phased arrays in which all crystal elements are used to generate each transmitted sound pulse, linear arrays acti- vate a limited group of adjacent elements to generate each pulse. Adjacent groups of elements are sequentially excited from one edge of the transducer to the other. If each sound pulse travels in the same direction (parallel) and is oriented perpendicular to the transducer surface, then the image is rectangular. It is also possible to steer the pulses so that the image is sector or trapezoidal in shape. The major advantages of linear-array transducers are high resolution in the near field and a larger superficial field of view. Focusing is uniform in the center and periphery of the image when there is no beam steering. Some loss of focusing and resolution occurs when the beam is steered into a sector or trapezoidal format. FIGURE 1-6. Electronic linear-array function. Groups of crystal ele- Linear arrays have largely replaced the traditional phased- ments are sequentially excited to move the scan line (large arrows) array probes, although narrow linear arrays that steer the from one side of the probe to the other. Steering the beam is accom- beam into a sector format closely simulate the traditional plished by adjusting the timing of crystal excitation (small arrows). phased arrays and are often still referred to as phased-array or sector transducers. Images obtained with linear arrays always have a flat superficial surface and are designated on mechanical sector transducer is the fixed focal zone and fixed the image with the letter L followed by the transmit fre- transmit frequency. This forces the operator to switch to a quency. The various image formats of linear arrays are shown completely different transducer to vary the focus distance and in Fig. 1-7. transmit frequency. Curved-Array Transducers Multielement Array Transducers If the surface of a linear array is reformed into a curved Because of their lack of flexibility, mechanical sector trans- convex shape, it is called a curved array, curvilinear array, ducers have been replaced by multiple-element transducers, or a convex array. Curved arrays can be formed in different commonly called arrays. The array transducers contain sizes and shapes. Probes with a short radius of curvature can groups of small piezoelectric crystal elements arranged in a be used for endoluminal scanning and probes with a larger sequential fashion. Transmitted sound pulses are created by radius of curvatures can be used for general abdomen and the summation of multiple pulses from many different ele- obstetrical scanning. Images obtained with curved arrays ments. By altering the timing and sequence of activation of always have a curved superficial surface and are designated the different elements, the transmitted pulse can be steered on the image with the letter C followed by the transmit fre- in different directions and focused at different depths (Fig. quency (see Fig. 1-7). 1-6). In fact, the multielement arrays can scan in real time In addition to transmit frequency, the size of linear and while focusing at multiple levels. curvilinear probes also determines where they can be used. The image created by array transducers consists of multiple For example, large probes are acceptable for abdominal and scan lines arranged side by side. The length of the scan line obstetrical applications, but would not be feasible for scan- (image depth) multiplied by the speed of sound determines ning a finger. how much time it takes to generate each line. This time must Two-Dimensional (Matrix) Arrays be doubled because sound travels the length of the scan line and then back to the transducer. This travel time can then be Standard array transducers can produce variable focusing in multiplied by the total number of lines in the image to deter- the plane of imaging but cannot focus the beam perpendicular mine the time required to generate an entire frame of the to the plane of imaging (i.e., the elevational plane). In other real-time image. Because the speed of sound is essentially words, the slice thickness is fixed and cannot be adjusted. constant, the frame rate of the image can be adjusted by One solution to variable focusing in the elevation plane is changing the depth of the image (length of the scan line), the the 2D array. These probes have crystal elements that are width of the image (number of scan lines), or the line density arranged in columns as well as rows (Fig. 1-8). The 2D arrays (number of lines per degree or lines per cm). are sometimes referred to as matrix arrays. They allow for variable slice thickness while maintaining the other advan- Phased-Array Transducer tages of electronically controlled arrays, such as color Doppler. With the phased-array transducer, every element in the array They also allow for simultaneous scanning in more than one participates in the formation of each transmitted pulse. plane as well as real-time 3D imaging. Because the sound beams are steered at varying angles Intraluminal Probes from one side of the transducer to the other, a sector image format is produced. Compared with the other electronic array Small transducers that can be placed within various body transducers (discussed in the following sections), the phased- lumens were developed in the 1980s and are now very array probe is smaller and therefore capable of scanning common. Because these transducers can be positioned close in areas where acoustic access is limited, such as between to the organ of interest, higher frequencies can be used and ribs. However, phased arrays have a small superficial field of higher resolution images obtained. In addition, the ability view and poor near-field focusing capabilities. The focusing to image organs without having to transmit the sound beam capabilities in the periphery of the image are also limited. through the abdominal wall helps to minimize the image- Phased arrays are good for performance of deep Doppler but degrading properties of adipose tissue and the shadowing pro- poor for superficial Doppler. They have been largely replaced duced by bowel gas. The overall result is that the images are by linear-array probes. of much higher quality than those obtained with a standard