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General principles of endoscopic ultrasonographic imaging
General Principles of Endoscopic Ultrasonographic Imaging Steven M. Shaw, MD, and Michael B. Kimmey, MD
Endoscopic ultrasonography (EUS) uses high-frequency acoustic waves within the gastrointestinal lumen to obtain detailed images of the gut wall and surrounding structures. Knowledge of basic ultrasound (US) physics is important for both acquisition of highquality images and accurate interpretation. US waves are produced by transducers, which also act as receivers for reflected waves, or echoes. Images are formed by processing of the electrical signals resulting from reception of echoes by the transducer. Tiny pulses of US energy are emitted into tissue along a single plane by either a mechanically rotated transducer or a fixed array of transducers. A structure's depth is determined by the time it takes for the US pulse to return to the transducer. The intensity of a returning echo depends on the reflectivity of the target structure and the amount of energy loss, or attenuation, in tissue. Real time imaging is possible because of rapid renewal of images several times per second. Axial (or depth) resolution is determined by US pulse length; it is optimal at high US frequencies. However, tissue penetration decreases as frequency is increased, limiting the depth of imaging. Lateral resolution is a function of the width of each US wave or beam. Transducer size and frequency determine the shape and dimensions of the beam, which typically converges at a distance of 2 to 3 cm from the transducer and diverges thereafter. Lateral resolution is best in the narrowest portion, termed the focal zone. In general, optimal resolution occurs with use of the highest possible frequency that allows adequate penetration to the structures of interest. Two types of echoes are produced in tissue: scattering of US waves by tiny particles or irregular surfaces, and reflection at interfaces between layers of tissue with different acoustic properties. EUS imaging of the gut wall is used to illustrate several of the fundamental US principles detailed in this article. An overview of the basic types of EUS instruments and certain applications, such as Doppler or duplex scanning, is also provided. Copyright 9 2000 by W.B. Saunders Company
ver the past decade, endoscopic ultrasonography (EUS) has become an important tool in the diagnosis of gastrointestinal (GI) disease. Imaging of the gut wall and surrounding structures by conventional transcutaneous ultrasound is of limited value, for 2 main reasons: First, intervening structures such as fat, bone, and gas within the GI tract (or lungs) impair transmission of ultrasound (US) waves. Second, resolution is limited by the lower frequencies needed to penetrate deeply through tissue. These problems have been circumvented by the incorporation of a US transducer into the tip of a flexible
O
From the University of Washington Medical Center, Seattle, WA. Address reprint requests to Michael B. Kimmey, MD, Professor of Medicine, Director of Gastrointestinal Endoscopy, University of Washington Medical Center, Box 356424, Seattle, WA 98195-6424. Copyright 9 2000 by W.B. Saunders Company 1096-2883/00/0202-0002510.00/0 doi:10.1053/TG.2000.5430 50
endoscope, allowing closer apposition of the transducer to structures of interest. This permits use of higher frequencies, which improves resolution. Although the technical performance of EUS is not exceedingly difficult in most cases, correct interpretation of the images obtained can be quite challenging. Knowledge of the salient anatomy m different imaging planes and a basic understanding of US physics are therefore indispensable. This article reviews the fundamental principles of US waves, their interaction with different types of tissue and media, and the formation of endosonographic images. Details pertaining to the interpretation of images, including artifacts, are discussed by other authors.
Basic Ultrasound Physics Acoustic Waves Acoustic waves with frequencies higher than the range of audible sound (20 to 20,000 Hz) are termed ultrasound. Relatively high frequencies, generally between 2 million and 20 million Hertz (Hz), or 2 to 20 megahertz (MHz), are required for US imaging in medicine. Longitudinal waves are produced and emitted into tissue by a transducer, which also acts as a receiver for reflected waves, or echoes. US transducers are composed of piezoelectric materials, which convert electrical signals into acoustic waves. This is accomplished by realignment of the crystalline elements (dipoles) in response to a voltage spike, causing vibration. The frequency of the resulting sound waves is determined by inherent properties of the crystal, or its resonant frequency. Similarly, electrical signals are produced when the piezoelectric material is struck by returning echoes. These signals are then processed so that information can be displayed in an interpretable fashion. The velocity of an acoustic wave is related inversely to the density and compressibility of the media through which it travels. Because these properties are similar for most soft tissues, the velocity of ultrasound in vivo is nearly constant ( - 1 , 5 4 0 m/s). Thus, the interval between wave production and echo reception defines the depth of a given structure (distance from the transducer) on the resulting image. The wavelength (the length that one wave occupies in space) is equal to the velocity divided by the frequency.
Ultrasound Interaction with Tissue For a structure to be imaged, it must reflect echoes back to the transducer. This can occur by 2 means: First, there is reflection at interfaces between tissues or structures of different acoustic impedance, which is a measure of resistance to wave propagation. Acoustic impedance is defined as the product of velocity and density. The fraction of a beam that is reflected is directly proportional to the difference in acoustic impedance. These Techniques in Gastrointestinal Endoscopy, Vol 2, No 2 (April), 2000: pp 50-55
echoes are represent a type of specular reflection and are termed interface (or boundary) echoes. Interface echoes appear on US images at the level of the interface, and their thickness is related to the pulse length of the US wave. 1 If the deeper tissue produces its own echoes (see below), the interface echo will blend with the deeper layer. The second type of echo is produced by structures that are smaller than the US wavelength or have highly irregular surfaces, which cause scattering of the US beam. These are termed nonspecular reflectors or scatterers; they produce the basic echotexture of tissues. For example, fat cells and collagen molecules are nonspecular reflectors, probably accounting for the highly echogenic appearance of fat and fibrous tissue, respectively.2 Lean muscle, conversely, contains relatively few nonspecular reflectors and therefore appears hypoechoic. Small differences in the quality and quantity of these tissue reflectors give different visceral organs, such as liver, spleen, and pancreas, distinctive sonographic appearances. Higher US frequencies enhance nonspecular echoes and may improve endosonographic differentiation between tissues, such as small pancreatic neoplasms from surrounding normal parenchyma. 3 The intensity, or amplitude, of an echo depends on the fraction of the US beam that is reflected and the amount of attenuation of the beam by the tissue. Attenuation generally occurs by 2 mechanisms: loss of US energy by absorption (conversion to heat), and redirection of waves by reflection, refraction, and scattering, such that the intensity of echoes returning to the transducer is diminished. Absorption is directly related to frequency and viscosity. It can contribute significantly to the attenuation of US in tissue, but the amount of heat produced in conventional US and EUS is extremely low, owing to the very low energy levels used for imaging. Water produces very little US attenuation, because it causes minimal absorption or reflection. For this reason, water is used as the primary acoustic coupling agent for EUS. Conversely, air and other gases, because of their compressibility, are poor conductors of US waves and are therefore highly reflective. Bone and other high-density structures, such as gallstones, also produce high-amplitude echoes at their interfaces with soft tissues. This reflectivity forms the basis of acoustic shadowing, or the paucity of echoes seen deep to these structures on US images.
Ultrasound Image Production Modes of US Imaging EUS images are acquired by use of a moving transducer or multiple fixed transducers configured in a linear or curvilinear arrangement. They are generally displayed as 2-dimensional B-mode (Brightness) images, representing a "slice" of tissue within a single plane. One dimension depicts the depth of structures (distance from the transducer) and the other their horizontal position. The image comprises individual dots or pixels whose brightness is determined by the intensity of the individual echoes (Fig 1). The simplest type of US display is the A-mode (Amplitude), which depicts echoes in 1 dimension as spikes, whose position represents depth, and whose height indicates echo intensity. This display mode is not used in EUS. The M-mode (Motion) and Doppler US provide information about motion. M-mode imaging displays echoes in a single dimension over time; it is used primarily in echocardiography, PRINCIPLES OF ENDOSCOPICULTRASONOGRAPHICIMAGING
Fmage (dots)
Object
A
Array of transducers
B
Fig 1. Two-dimensional (B-mode) images are formed by combination of echoes received along individual vertical scan lines. Each echo is depicted as a dot (or pixel) (A). Clearer, more detailed images are obtained with larger numbers of scan lines per image; this is unrelated to resolution, which is a property of the transducer itself, individual acoustic waves corresponding to each scan line are emitted and received by either an array of fixed transducers (B) or a single transducer that is mechanically rotated (C).
Doppler imaging, used mainly to detect blood flow, is based on the change in frequency (Doppler shift) that occurs when a US wave encounters a moving object. The Doppler shift produced by movement of blood within vessels is typically about 15,000 Hz, which falls in the range of human hearing. Thus, the Doppler signal can be heard, after amplification, as audible sound. Alternatively, the signal can be displayed graphically, superimposed onto B-mode images, creating a duplex scan. Color duplex scanning conventionally depicts flow toward the transducer as red and flow directed away from the transducer as blue. It is commonly used in EUS (available with certain 51
imaging systems) to facilitate differentiation of cystic from vascular structures and to assist with the detection of vascular invasion by tumors.
Instruments Two different types of transducers are used in EUS instruments: mechanical and electronic array. Radial scanning echoendoscopes use a mechanical transducer, which is housed at the tip of the instrument and is rotated within its plastic casing by a motor located in the handle of the endoscope. Three hundred sixty-degree images are generated in a plane perpendicular to the long axis of the instrument. For the Olympus EU-M20 system, the transducer rotates at a rate of 6.67 revolutions per second, with updated images generated at the same rate. This results in a "real time" video display that simulates motion. However, the human eye can detect individual images when they appear at a rate of up to 16 frames per second, so the picture may appear to flicker. 4 Electronic array echoendoscopes acquire 2-dimensional images by using multiple tiny transducers configured in a line or arc at the tip of the instrument, rather than by rotation of a single transducer. Images are formed by electronic mixing of signals from combinations of these transducers. This allows the images to be updated more rapidly than with mechanical transducers (up to 30 per second, depending on the depth of imaging), producing smoother video pictures without perceptible flickering. The image plane is parallel to the long axis of the endoscope, resulting in a completely different orientation than that produced by radial scanning instruments. Complete circumferential examination requires manual 360 ~ rotation of the instrument within the lumen. The major advantage of this system is the capability of imaging a needle passed through the instrument's accessory channel to direct fine-needle aspiration (FNA) biopsy. Doppler (or duplex) imaging is also available only with the electronic array instruments. US transducers have also been incorporated into catheter probes that can be passed through the working channel of a standard diagnostic endoscope. 5-7 These devices are discussed in greater detail in subsequent articles.
Quality of EUS Images Image quality and resolution depend on multiple factors, many of which can be modified by the endosonographer using controls on the image processing unit. Typically, very little of the interrogating US beam is refected back to the transducer in
a given echo, so amplification of these returning signals is required. The degree of amplification is determined by setting the overall gain control, which provides nonselective amplification of all echoes. It is usually set at a level that optimizes visualization of structures of interest, while keeping background noise to a minimum. Contrast and brightness are also adjustable. Attenuation of US energy in tissue is cumulative; therefore, echoes arising from deeper structures are generally of lower intensity and require additional amplification. This feature is called time gain compensation (TGC). It is conventionally set so that the amount of gain or amplification is proportionate to depth, so that echoes from the same tissue type appear of equal intensity regardless of distance from the transducer. Certain artifacts, such as enhancement of echoes deep to low attenuation structures (eg, cysts or vascular structures), are produced as a result of TGC. Spatial resolution, the ability to discriminate 2 distinct points in space, is determined by characteristics of the US wave (Fig 2). It can be expressed as the shortest distance between the points at which they still appear separate. Transducers emit US energy in short pulses ( - 1 , 0 0 0 per second) and receive echoes between these pulses. The duration of each pulse is determined primarily by frequency.8 In general, shorter pulses can be produced with higher US frequency, resulting in improved axial resolution, or the ability to distinguish 2 points that lie along the direction of the US beam (Fig 3). Lateral resolution, or point discrimination in a plane perpendicular to the path of the beam, is a function of beam width. The dimensions of the beam vary with distance traveled; they are determined primarily by the size of the transducer and the frequency used (Fig 4). Optimal lateral resolution is achieved in the beam's narrowest portion, termed the focal zone.
Technical Considerations As noted above, water is used as an acoustic coupling agent between the transducer and the gastrointestinal mucosa. This commonly involves instillation of water into a balloon positioned around the transducer. Because of the high reflectivity of air, care is taken to remove all air from the balloon, as well as from the lumen of the gut around the transducer. Controlling the volume of water in the balloon also aids in placing the transducer at the desired distance from structures of interest. As noted above, lateral resolution is best in the focal zone of the
Circular transducer
t3
t4
Lateral resolution Focal zone ~ial resolution Fig 2. The resolution for a US beam produced by a single circular transducer is depicted at 3 different points, or depths from the transducer (t1.3). tl is located in the near field, t2 in the focal zone, and t3 in the far field. Axial resolution, which is determined by pulse length, is the same at all 3 locations. Lateral resolution, however, is determined by the diameter of the beam; it is optimal in the focal zone, or near-far field transition zone, depicted here at t~. Dispersion of the beam in the far field impairs lateral resolution with increasing depth of propagation beyond the focal zone.
52
SHAW AND KIMMEY
tI
Endosonographic Imaging of the Gastrointestinal Wall
t2
Transducer - -
OS u,se
Point targets ~
9
Fig 3. The effect of frequency on axial resolution is illustrated. At higher frequency (f2), the shorter pulse length improves axial resolution, allowing discrimination between 2 points that could not be resolved at a lower frequency (f0-
US beam, which is usually located 2 to 3 cm from the transducer. Overdistension of the balloon is avoided, particularly in the relatively small-caliber lumen of the esophagus or duodenum, because the superficial layers of the gut wall can be compressed by pressure from the balloon. 9 The gut wall and its individual layers can also be artifactually thickened on EUS by tangential imaging (Fig 5); thus, the thickness of these structures should be assessed when the US beam is directed perpendicularly. This is in contradistinction to Doppler US, where the signal is enhanced by a tangentially directed beam.
The gut wall is classically represented as a 5-layer structure when imaged with currently available US endoscopes, which use frequencies of 5 to 12 MHz. These layers do not correspond precisely to histology ~ (Fig 6); therefore, a basic understanding of the US principles described in the preceding sections is important for correct image interpretation. The first sonographic layer is produced by the interface between luminal fluid and the mucosal surface. Because the presence of mucosal glands or pits makes this surface irregular, the thickness of the echoic first layer can vary significantly. The second layer is hypoechoic. It includes the remainder of the mucosal epithelium (deep to the surface echo) and the lamina propria (LP). The interface echo formed by the junction between the LP and muscularis mucosae (MM) is usually thicker than the MM, completely obscuring this layer and forming a continuum with the echoic submucosal layer. ~~ This concept is clinically important because penetration of a lesion through the MM into the submucosa (SM) defines certain disease entities, such as invasive carcinoma and ulcers (as opposed to erosions, which lie superficial to the SM). Thus, endosonographic involvement of the superficial portion of the third layer may not allow such diagnoses to be made with certainty. Recall that the thickness of an interface echo is determined by the axial resolution of the transducer, which is in the range of 0.2 to 0.3 mm for most US endoscopes. The normal MM is --<0.1 mm in thickness. If it is thickened, or if the axial resolution of the transducer is <0.1 mm, the MM may appear as 2 distinct sonographic layers: the specular echo from its interface with the overlying LP, and an echo-poor layer between the interface echo and the deeper SM, representing the remaining thickness of the (hypoechoic) MM not obscured by the interface echo.i~ The interface between the MM and the thicker SM produces an echo, which is obscured amid the nonspecular echoes within the submucosal layer. The third sonographic layer also includes the echo arising from the interface between the SM and the muscularis propria (MP), the remainder of which is
Transducer
Focal zone
"- T2
f2
A
B
Fig 4. (A) The effect of US frequency on lateral resolution is illustrated. Increasing frequency (f2 > fl) causes the location of the focal zone to shift further away from the transducer. Dispersion of the beam in the far field is also reduced at higher frequencies. (B) The effect of transducer size on lateral resolution is illustrated. The smaller transducer (TO produces a narrower beam, resulting in better lateral resolution in the near field and focal zone. However, at greater depths, lateral resolution may be worse than that achieved by the larger transducer ('1"2)because of shortening of the near field and wider dispersion of the beam in the far field.
PRINCIPLES OF ENDOSCOPIC ULTRASONOGRAPHICIMAGING
53
20 mHz EUS
7.5 mHz EUS
Histology
1 2
n mn
3
St]
4
mp
5
B
C
Fig 5. The effect of tangential scanning on gastrointestinal wall thickness is illustrated. Tangential images are produced when the beam encounters the gut wall at a nonperpendicular angle (A). A simplified representation of the artifactual wall thickening is shown in (B), where x is a measure of actual wall thickness and y is the apparent thickness on the tangential US image. Further distortion of the thickness and echo intensity of the gut wall layers on EUS images is related to the lateral resolution of the beam, which plays a larger role as the angle deviates further from 90 ~ The axial resolution for most EUS transducers is generally superior to their lateral resolution, even in the beam's focal zone, where lateral resolution is optimal.
depicted as the hypoechoic fourth layer. Thus, the sonographic third layer is slightly thicker than the corresponding SM, and the fourth layer appears thinner than the histological thickness of the MP.1 The fifth (echoic) layer results from the interface between the MP and the surrounding tissue. The serosal layer, which lies deep to the muscularis propria in all parts of the GI tract except for the esophagus and rectum, is normally too thin to be resolved by EUS. When higher frequencies are used (->15 MHz: currently available with US catheter probe units), the GI wall can be seen as a 7-layer or 9-layer structure, r,ll The MP is depicted as 3 distinct sonographic layers: the hypoechoic inner circular and outer longitudinal muscle layers, and the intervening echoic connective tissue layer. It should be noted, however, that the 3-layer representation of the MP can occasionally be appreci-
ated at the usual frequencies used with current US endoscopes, probably because of nonspecular reflectors within the connective tissue layer, rather than to a true interface echo. The MM also may be resolved at higher frequencies, appearing as 2 distinct sonographic layers as described above, if its thickness exceeds the thickness of the LP-MM interface echo (Fig 6c). As EUS technology continues to improve and become more widely available, basic skills in the performance and interpretation of endosonography will be valuable tools to the practicing gastroenterologist. The principles reviewed in this article are intended to provide a framework for understanding how US interacts with tissue and how images are formed. Clinical applications of these principles, including examination technique, image interpretation, and the recognition and avoidance of artifacts, will be discussed further in subsequent articles.
Image
G u t wall
Gut
lential
I---x
A
[
U~ beam
B
Fig 6. Imaging of the gut wall by EUS. (A) Histological layers are illustrated: mucosa (m), muscularis mucosae (mm), submucosa (sm), muscularis propria (mp), and serosa plus subserosa (s). The dotted line in the mp represents the thin connective tissue layer between the inner circular and outer longitudinal muscle layers. (B) At frequencies used with most US endoscopes, the wall is represented as a 5-layer structure. Specular echoes formed by interfaces between adjacent layers extend into the deeper of the 2 adjacent layers (or beyond, in the case of the muscularis mucosae, which is normally thinner thanmand therefore obscured by--the interface echo). (C) With higher frequencies, the improved axial resolution results in thinner interface echoes, and the gut wall may be observed as a 9-layer structure. 54
SHAW AND KIMMEY
References 1. Kimmey MB, Martin RW, Haggitt RC, et al: Histologic correlates of gastrointestinal ultrasound images. Gastroenterology 96:433-441, 1989 2. Kimmey MB, Silverstein FE, Martin RW: Ultrasound interaction with the intestinal wall: Esophagus, stomach, and colon, in Kawai K (ed): Endoscopic UItrasonography in Gastroenterology (ed 1). Tokyo, Japan, Igaku-Shoin, 1988, pp 35-43 3. Kimmey MB, Martin RW: Fundamentals of endosonography. Gastrointest Endosc Clin North Am 2:557-573, 1992 4. Herbener TE: Fundamentals of ultrasonography, in Van Dam J, Sivak MV (eds): Gastrointestinal Endosonography (ed 1). Philadelphia, PA, Saunders, 1999, pp 3-18 5. Nesje LB, Qdegaard S, Kimmey MB: Transendoscopic ultrasonography during conventional upper gastrointestinal endoscopy. Scand J Gastroentero132:500-508, 1997 6. Yanai H, Fujimura H, Suzumi M, et ah Delineation of the gastric muscularis mucosae and assessment of depth of invasion of early
PRINCIPLES OF ENDOSCOPIC ULTRASONOGRAPHIC IMAGING
gastric cancer using a 20-megahertz endoscopic ultrasound probe. Gastrointest Endosc 39:505-512, 1993 7. Waxman I: Clinical impact of high-frequency ultrasound probe sonography during diagnostic endoscopy: A prospective study. Endoscopy 30:A166-A168, 1998 (suppl 1) 8. Curry TS, Dowdey JE, Murry RC Jr: Ultrasound, in Christensen's Introduction to the Physics of Diagnostic Radiology (ed 4). Philadelphia, PA, Lea & Febiger, 1990, pp 323-371 9. Qdegaard S, Kimmey MB, Martin RW, et al: The effects of applied pressure on the thickness, layers, and echogenicity of gastrointestinal images. Gastrointest Endosc 38:351-356, 1992 10. ~ldegaard S, Kimmey MB: Location of the muscularis mucosae on high frequency gastrointestinal ultrasound images. Eur J Ultrasound 1:39-50, 1994 11. Wiersema MJ, Wiersema LM: High resolution 25-megahertz ultrasonography of the gastrointestinal wall: histologic correlates. Gastrointest Endosc 39:499-504, 1993
55
Endoscopic ultrasonography (EUS) uses high-frequency acoustic waves within the gastrointestinal lumen to obtain detailed images of the gut wall and surrounding structures. Knowledge of basic ultrasound (US) physics is important for both acquisition of highquality images and accurate interpretation. US waves are produced by transducers, which also act as receivers for reflected waves, or echoes. Images are formed by processing of the electrical signals resulting from reception of echoes by the transducer. Tiny pulses of US energy are emitted into tissue along a single plane by either a mechanically rotated transducer or a fixed array of transducers. A structure's depth is determined by the time it takes for the US pulse to return to the transducer. The intensity of a returning echo depends on the reflectivity of the target structure and the amount of energy loss, or attenuation, in tissue. Real time imaging is possible because of rapid renewal of images several times per second. Axial (or depth) resolution is determined by US pulse length; it is optimal at high US frequencies. However, tissue penetration decreases as frequency is increased, limiting the depth of imaging. Lateral resolution is a function of the width of each US wave or beam. Transducer size and frequency determine the shape and dimensions of the beam, which typically converges at a distance of 2 to 3 cm from the transducer and diverges thereafter. Lateral resolution is best in the narrowest portion, termed the focal zone. In general, optimal resolution occurs with use of the highest possible frequency that allows adequate penetration to the structures of interest. Two types of echoes are produced in tissue: scattering of US waves by tiny particles or irregular surfaces, and reflection at interfaces between layers of tissue with different acoustic properties. EUS imaging of the gut wall is used to illustrate several of the fundamental US principles detailed in this article. An overview of the basic types of EUS instruments and certain applications, such as Doppler or duplex scanning, is also provided. Copyright 9 2000 by W.B. Saunders Company
ver the past decade, endoscopic ultrasonography (EUS) has become an important tool in the diagnosis of gastrointestinal (GI) disease. Imaging of the gut wall and surrounding structures by conventional transcutaneous ultrasound is of limited value, for 2 main reasons: First, intervening structures such as fat, bone, and gas within the GI tract (or lungs) impair transmission of ultrasound (US) waves. Second, resolution is limited by the lower frequencies needed to penetrate deeply through tissue. These problems have been circumvented by the incorporation of a US transducer into the tip of a flexible
O
From the University of Washington Medical Center, Seattle, WA. Address reprint requests to Michael B. Kimmey, MD, Professor of Medicine, Director of Gastrointestinal Endoscopy, University of Washington Medical Center, Box 356424, Seattle, WA 98195-6424. Copyright 9 2000 by W.B. Saunders Company 1096-2883/00/0202-0002510.00/0 doi:10.1053/TG.2000.5430 50
endoscope, allowing closer apposition of the transducer to structures of interest. This permits use of higher frequencies, which improves resolution. Although the technical performance of EUS is not exceedingly difficult in most cases, correct interpretation of the images obtained can be quite challenging. Knowledge of the salient anatomy m different imaging planes and a basic understanding of US physics are therefore indispensable. This article reviews the fundamental principles of US waves, their interaction with different types of tissue and media, and the formation of endosonographic images. Details pertaining to the interpretation of images, including artifacts, are discussed by other authors.
Basic Ultrasound Physics Acoustic Waves Acoustic waves with frequencies higher than the range of audible sound (20 to 20,000 Hz) are termed ultrasound. Relatively high frequencies, generally between 2 million and 20 million Hertz (Hz), or 2 to 20 megahertz (MHz), are required for US imaging in medicine. Longitudinal waves are produced and emitted into tissue by a transducer, which also acts as a receiver for reflected waves, or echoes. US transducers are composed of piezoelectric materials, which convert electrical signals into acoustic waves. This is accomplished by realignment of the crystalline elements (dipoles) in response to a voltage spike, causing vibration. The frequency of the resulting sound waves is determined by inherent properties of the crystal, or its resonant frequency. Similarly, electrical signals are produced when the piezoelectric material is struck by returning echoes. These signals are then processed so that information can be displayed in an interpretable fashion. The velocity of an acoustic wave is related inversely to the density and compressibility of the media through which it travels. Because these properties are similar for most soft tissues, the velocity of ultrasound in vivo is nearly constant ( - 1 , 5 4 0 m/s). Thus, the interval between wave production and echo reception defines the depth of a given structure (distance from the transducer) on the resulting image. The wavelength (the length that one wave occupies in space) is equal to the velocity divided by the frequency.
Ultrasound Interaction with Tissue For a structure to be imaged, it must reflect echoes back to the transducer. This can occur by 2 means: First, there is reflection at interfaces between tissues or structures of different acoustic impedance, which is a measure of resistance to wave propagation. Acoustic impedance is defined as the product of velocity and density. The fraction of a beam that is reflected is directly proportional to the difference in acoustic impedance. These Techniques in Gastrointestinal Endoscopy, Vol 2, No 2 (April), 2000: pp 50-55
echoes are represent a type of specular reflection and are termed interface (or boundary) echoes. Interface echoes appear on US images at the level of the interface, and their thickness is related to the pulse length of the US wave. 1 If the deeper tissue produces its own echoes (see below), the interface echo will blend with the deeper layer. The second type of echo is produced by structures that are smaller than the US wavelength or have highly irregular surfaces, which cause scattering of the US beam. These are termed nonspecular reflectors or scatterers; they produce the basic echotexture of tissues. For example, fat cells and collagen molecules are nonspecular reflectors, probably accounting for the highly echogenic appearance of fat and fibrous tissue, respectively.2 Lean muscle, conversely, contains relatively few nonspecular reflectors and therefore appears hypoechoic. Small differences in the quality and quantity of these tissue reflectors give different visceral organs, such as liver, spleen, and pancreas, distinctive sonographic appearances. Higher US frequencies enhance nonspecular echoes and may improve endosonographic differentiation between tissues, such as small pancreatic neoplasms from surrounding normal parenchyma. 3 The intensity, or amplitude, of an echo depends on the fraction of the US beam that is reflected and the amount of attenuation of the beam by the tissue. Attenuation generally occurs by 2 mechanisms: loss of US energy by absorption (conversion to heat), and redirection of waves by reflection, refraction, and scattering, such that the intensity of echoes returning to the transducer is diminished. Absorption is directly related to frequency and viscosity. It can contribute significantly to the attenuation of US in tissue, but the amount of heat produced in conventional US and EUS is extremely low, owing to the very low energy levels used for imaging. Water produces very little US attenuation, because it causes minimal absorption or reflection. For this reason, water is used as the primary acoustic coupling agent for EUS. Conversely, air and other gases, because of their compressibility, are poor conductors of US waves and are therefore highly reflective. Bone and other high-density structures, such as gallstones, also produce high-amplitude echoes at their interfaces with soft tissues. This reflectivity forms the basis of acoustic shadowing, or the paucity of echoes seen deep to these structures on US images.
Ultrasound Image Production Modes of US Imaging EUS images are acquired by use of a moving transducer or multiple fixed transducers configured in a linear or curvilinear arrangement. They are generally displayed as 2-dimensional B-mode (Brightness) images, representing a "slice" of tissue within a single plane. One dimension depicts the depth of structures (distance from the transducer) and the other their horizontal position. The image comprises individual dots or pixels whose brightness is determined by the intensity of the individual echoes (Fig 1). The simplest type of US display is the A-mode (Amplitude), which depicts echoes in 1 dimension as spikes, whose position represents depth, and whose height indicates echo intensity. This display mode is not used in EUS. The M-mode (Motion) and Doppler US provide information about motion. M-mode imaging displays echoes in a single dimension over time; it is used primarily in echocardiography, PRINCIPLES OF ENDOSCOPICULTRASONOGRAPHICIMAGING
Fmage (dots)
Object
A
Array of transducers
B
Fig 1. Two-dimensional (B-mode) images are formed by combination of echoes received along individual vertical scan lines. Each echo is depicted as a dot (or pixel) (A). Clearer, more detailed images are obtained with larger numbers of scan lines per image; this is unrelated to resolution, which is a property of the transducer itself, individual acoustic waves corresponding to each scan line are emitted and received by either an array of fixed transducers (B) or a single transducer that is mechanically rotated (C).
Doppler imaging, used mainly to detect blood flow, is based on the change in frequency (Doppler shift) that occurs when a US wave encounters a moving object. The Doppler shift produced by movement of blood within vessels is typically about 15,000 Hz, which falls in the range of human hearing. Thus, the Doppler signal can be heard, after amplification, as audible sound. Alternatively, the signal can be displayed graphically, superimposed onto B-mode images, creating a duplex scan. Color duplex scanning conventionally depicts flow toward the transducer as red and flow directed away from the transducer as blue. It is commonly used in EUS (available with certain 51
imaging systems) to facilitate differentiation of cystic from vascular structures and to assist with the detection of vascular invasion by tumors.
Instruments Two different types of transducers are used in EUS instruments: mechanical and electronic array. Radial scanning echoendoscopes use a mechanical transducer, which is housed at the tip of the instrument and is rotated within its plastic casing by a motor located in the handle of the endoscope. Three hundred sixty-degree images are generated in a plane perpendicular to the long axis of the instrument. For the Olympus EU-M20 system, the transducer rotates at a rate of 6.67 revolutions per second, with updated images generated at the same rate. This results in a "real time" video display that simulates motion. However, the human eye can detect individual images when they appear at a rate of up to 16 frames per second, so the picture may appear to flicker. 4 Electronic array echoendoscopes acquire 2-dimensional images by using multiple tiny transducers configured in a line or arc at the tip of the instrument, rather than by rotation of a single transducer. Images are formed by electronic mixing of signals from combinations of these transducers. This allows the images to be updated more rapidly than with mechanical transducers (up to 30 per second, depending on the depth of imaging), producing smoother video pictures without perceptible flickering. The image plane is parallel to the long axis of the endoscope, resulting in a completely different orientation than that produced by radial scanning instruments. Complete circumferential examination requires manual 360 ~ rotation of the instrument within the lumen. The major advantage of this system is the capability of imaging a needle passed through the instrument's accessory channel to direct fine-needle aspiration (FNA) biopsy. Doppler (or duplex) imaging is also available only with the electronic array instruments. US transducers have also been incorporated into catheter probes that can be passed through the working channel of a standard diagnostic endoscope. 5-7 These devices are discussed in greater detail in subsequent articles.
Quality of EUS Images Image quality and resolution depend on multiple factors, many of which can be modified by the endosonographer using controls on the image processing unit. Typically, very little of the interrogating US beam is refected back to the transducer in
a given echo, so amplification of these returning signals is required. The degree of amplification is determined by setting the overall gain control, which provides nonselective amplification of all echoes. It is usually set at a level that optimizes visualization of structures of interest, while keeping background noise to a minimum. Contrast and brightness are also adjustable. Attenuation of US energy in tissue is cumulative; therefore, echoes arising from deeper structures are generally of lower intensity and require additional amplification. This feature is called time gain compensation (TGC). It is conventionally set so that the amount of gain or amplification is proportionate to depth, so that echoes from the same tissue type appear of equal intensity regardless of distance from the transducer. Certain artifacts, such as enhancement of echoes deep to low attenuation structures (eg, cysts or vascular structures), are produced as a result of TGC. Spatial resolution, the ability to discriminate 2 distinct points in space, is determined by characteristics of the US wave (Fig 2). It can be expressed as the shortest distance between the points at which they still appear separate. Transducers emit US energy in short pulses ( - 1 , 0 0 0 per second) and receive echoes between these pulses. The duration of each pulse is determined primarily by frequency.8 In general, shorter pulses can be produced with higher US frequency, resulting in improved axial resolution, or the ability to distinguish 2 points that lie along the direction of the US beam (Fig 3). Lateral resolution, or point discrimination in a plane perpendicular to the path of the beam, is a function of beam width. The dimensions of the beam vary with distance traveled; they are determined primarily by the size of the transducer and the frequency used (Fig 4). Optimal lateral resolution is achieved in the beam's narrowest portion, termed the focal zone.
Technical Considerations As noted above, water is used as an acoustic coupling agent between the transducer and the gastrointestinal mucosa. This commonly involves instillation of water into a balloon positioned around the transducer. Because of the high reflectivity of air, care is taken to remove all air from the balloon, as well as from the lumen of the gut around the transducer. Controlling the volume of water in the balloon also aids in placing the transducer at the desired distance from structures of interest. As noted above, lateral resolution is best in the focal zone of the
Circular transducer
t3
t4
Lateral resolution Focal zone ~ial resolution Fig 2. The resolution for a US beam produced by a single circular transducer is depicted at 3 different points, or depths from the transducer (t1.3). tl is located in the near field, t2 in the focal zone, and t3 in the far field. Axial resolution, which is determined by pulse length, is the same at all 3 locations. Lateral resolution, however, is determined by the diameter of the beam; it is optimal in the focal zone, or near-far field transition zone, depicted here at t~. Dispersion of the beam in the far field impairs lateral resolution with increasing depth of propagation beyond the focal zone.
52
SHAW AND KIMMEY
tI
Endosonographic Imaging of the Gastrointestinal Wall
t2
Transducer - -
OS u,se
Point targets ~
9
Fig 3. The effect of frequency on axial resolution is illustrated. At higher frequency (f2), the shorter pulse length improves axial resolution, allowing discrimination between 2 points that could not be resolved at a lower frequency (f0-
US beam, which is usually located 2 to 3 cm from the transducer. Overdistension of the balloon is avoided, particularly in the relatively small-caliber lumen of the esophagus or duodenum, because the superficial layers of the gut wall can be compressed by pressure from the balloon. 9 The gut wall and its individual layers can also be artifactually thickened on EUS by tangential imaging (Fig 5); thus, the thickness of these structures should be assessed when the US beam is directed perpendicularly. This is in contradistinction to Doppler US, where the signal is enhanced by a tangentially directed beam.
The gut wall is classically represented as a 5-layer structure when imaged with currently available US endoscopes, which use frequencies of 5 to 12 MHz. These layers do not correspond precisely to histology ~ (Fig 6); therefore, a basic understanding of the US principles described in the preceding sections is important for correct image interpretation. The first sonographic layer is produced by the interface between luminal fluid and the mucosal surface. Because the presence of mucosal glands or pits makes this surface irregular, the thickness of the echoic first layer can vary significantly. The second layer is hypoechoic. It includes the remainder of the mucosal epithelium (deep to the surface echo) and the lamina propria (LP). The interface echo formed by the junction between the LP and muscularis mucosae (MM) is usually thicker than the MM, completely obscuring this layer and forming a continuum with the echoic submucosal layer. ~~ This concept is clinically important because penetration of a lesion through the MM into the submucosa (SM) defines certain disease entities, such as invasive carcinoma and ulcers (as opposed to erosions, which lie superficial to the SM). Thus, endosonographic involvement of the superficial portion of the third layer may not allow such diagnoses to be made with certainty. Recall that the thickness of an interface echo is determined by the axial resolution of the transducer, which is in the range of 0.2 to 0.3 mm for most US endoscopes. The normal MM is --<0.1 mm in thickness. If it is thickened, or if the axial resolution of the transducer is <0.1 mm, the MM may appear as 2 distinct sonographic layers: the specular echo from its interface with the overlying LP, and an echo-poor layer between the interface echo and the deeper SM, representing the remaining thickness of the (hypoechoic) MM not obscured by the interface echo.i~ The interface between the MM and the thicker SM produces an echo, which is obscured amid the nonspecular echoes within the submucosal layer. The third sonographic layer also includes the echo arising from the interface between the SM and the muscularis propria (MP), the remainder of which is
Transducer
Focal zone
"- T2
f2
A
B
Fig 4. (A) The effect of US frequency on lateral resolution is illustrated. Increasing frequency (f2 > fl) causes the location of the focal zone to shift further away from the transducer. Dispersion of the beam in the far field is also reduced at higher frequencies. (B) The effect of transducer size on lateral resolution is illustrated. The smaller transducer (TO produces a narrower beam, resulting in better lateral resolution in the near field and focal zone. However, at greater depths, lateral resolution may be worse than that achieved by the larger transducer ('1"2)because of shortening of the near field and wider dispersion of the beam in the far field.
PRINCIPLES OF ENDOSCOPIC ULTRASONOGRAPHICIMAGING
53
20 mHz EUS
7.5 mHz EUS
Histology
1 2
n mn
3
St]
4
mp
5
B
C
Fig 5. The effect of tangential scanning on gastrointestinal wall thickness is illustrated. Tangential images are produced when the beam encounters the gut wall at a nonperpendicular angle (A). A simplified representation of the artifactual wall thickening is shown in (B), where x is a measure of actual wall thickness and y is the apparent thickness on the tangential US image. Further distortion of the thickness and echo intensity of the gut wall layers on EUS images is related to the lateral resolution of the beam, which plays a larger role as the angle deviates further from 90 ~ The axial resolution for most EUS transducers is generally superior to their lateral resolution, even in the beam's focal zone, where lateral resolution is optimal.
depicted as the hypoechoic fourth layer. Thus, the sonographic third layer is slightly thicker than the corresponding SM, and the fourth layer appears thinner than the histological thickness of the MP.1 The fifth (echoic) layer results from the interface between the MP and the surrounding tissue. The serosal layer, which lies deep to the muscularis propria in all parts of the GI tract except for the esophagus and rectum, is normally too thin to be resolved by EUS. When higher frequencies are used (->15 MHz: currently available with US catheter probe units), the GI wall can be seen as a 7-layer or 9-layer structure, r,ll The MP is depicted as 3 distinct sonographic layers: the hypoechoic inner circular and outer longitudinal muscle layers, and the intervening echoic connective tissue layer. It should be noted, however, that the 3-layer representation of the MP can occasionally be appreci-
ated at the usual frequencies used with current US endoscopes, probably because of nonspecular reflectors within the connective tissue layer, rather than to a true interface echo. The MM also may be resolved at higher frequencies, appearing as 2 distinct sonographic layers as described above, if its thickness exceeds the thickness of the LP-MM interface echo (Fig 6c). As EUS technology continues to improve and become more widely available, basic skills in the performance and interpretation of endosonography will be valuable tools to the practicing gastroenterologist. The principles reviewed in this article are intended to provide a framework for understanding how US interacts with tissue and how images are formed. Clinical applications of these principles, including examination technique, image interpretation, and the recognition and avoidance of artifacts, will be discussed further in subsequent articles.
Image
G u t wall
Gut
lential
I---x
A
[
U~ beam
B
Fig 6. Imaging of the gut wall by EUS. (A) Histological layers are illustrated: mucosa (m), muscularis mucosae (mm), submucosa (sm), muscularis propria (mp), and serosa plus subserosa (s). The dotted line in the mp represents the thin connective tissue layer between the inner circular and outer longitudinal muscle layers. (B) At frequencies used with most US endoscopes, the wall is represented as a 5-layer structure. Specular echoes formed by interfaces between adjacent layers extend into the deeper of the 2 adjacent layers (or beyond, in the case of the muscularis mucosae, which is normally thinner thanmand therefore obscured by--the interface echo). (C) With higher frequencies, the improved axial resolution results in thinner interface echoes, and the gut wall may be observed as a 9-layer structure. 54
SHAW AND KIMMEY
References 1. Kimmey MB, Martin RW, Haggitt RC, et al: Histologic correlates of gastrointestinal ultrasound images. Gastroenterology 96:433-441, 1989 2. Kimmey MB, Silverstein FE, Martin RW: Ultrasound interaction with the intestinal wall: Esophagus, stomach, and colon, in Kawai K (ed): Endoscopic UItrasonography in Gastroenterology (ed 1). Tokyo, Japan, Igaku-Shoin, 1988, pp 35-43 3. Kimmey MB, Martin RW: Fundamentals of endosonography. Gastrointest Endosc Clin North Am 2:557-573, 1992 4. Herbener TE: Fundamentals of ultrasonography, in Van Dam J, Sivak MV (eds): Gastrointestinal Endosonography (ed 1). Philadelphia, PA, Saunders, 1999, pp 3-18 5. Nesje LB, Qdegaard S, Kimmey MB: Transendoscopic ultrasonography during conventional upper gastrointestinal endoscopy. Scand J Gastroentero132:500-508, 1997 6. Yanai H, Fujimura H, Suzumi M, et ah Delineation of the gastric muscularis mucosae and assessment of depth of invasion of early
PRINCIPLES OF ENDOSCOPIC ULTRASONOGRAPHIC IMAGING
gastric cancer using a 20-megahertz endoscopic ultrasound probe. Gastrointest Endosc 39:505-512, 1993 7. Waxman I: Clinical impact of high-frequency ultrasound probe sonography during diagnostic endoscopy: A prospective study. Endoscopy 30:A166-A168, 1998 (suppl 1) 8. Curry TS, Dowdey JE, Murry RC Jr: Ultrasound, in Christensen's Introduction to the Physics of Diagnostic Radiology (ed 4). Philadelphia, PA, Lea & Febiger, 1990, pp 323-371 9. Qdegaard S, Kimmey MB, Martin RW, et al: The effects of applied pressure on the thickness, layers, and echogenicity of gastrointestinal images. Gastrointest Endosc 38:351-356, 1992 10. ~ldegaard S, Kimmey MB: Location of the muscularis mucosae on high frequency gastrointestinal ultrasound images. Eur J Ultrasound 1:39-50, 1994 11. Wiersema MJ, Wiersema LM: High resolution 25-megahertz ultrasonography of the gastrointestinal wall: histologic correlates. Gastrointest Endosc 39:499-504, 1993
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