Acoustic Shadowing Due to Refractive and Reflective Effects

Transcription

1 Acoustic Shadowing Due to Refractive and Reflective Effects F. GRAHAM SOMMER, 2 R. A. FILLY, AND MICHAEL J. MINTON Acoustic shadowing may be seen distal to the margins of rounded structures having different acoustic velocities from the surrounding tissue. Refractive and reflective mechanisms for such shadowing are reviewed and in vitro demonstrations of such shadowing are presented. Clinical examples of refractive and reflective shadows are given, and their Importance is discussed. It is well known that acoustic shadowing in sonograms may be seen distal either to a strongly reflective interface, such as a bony structure, or to a region of high sound attenuation, such as a collection of bowel gas. This paper shows that acoustic shadows are commonly observed on clinical sonograms secondary to refractive and reflective effects occurring at interfaces between media of differing acoustic velocity. Recognition of such acoustic shadows and an understanding of their origins will prevent confusion with those from pathologic processes. Recognition of the refractive origins of an acoustic shadow may also provide important evidence of the presence of a fluid-filled structure. Physical Principles Acoustic Shadowing due to refraction may occur distal to the margins of a rounded structure containing a substance of lower acoustic velocity than the surrounding tissue. The source of this acoustic shadowing can be explained by applying the law of refraction (fig. 1) to this imaging situation [1]. Paths of the ultrasonic beam tangent to the sides of a rounded region of low acoustic velocity surrounded by higher velocity tissue are considered (fig. 2). Application of the refraction law to the ultrasonic beams arriving tangentially at the acoustic interfaces yields: - SinO2 SinO2 _. _V2 SinO1 Sin(90#{176}) SInO2 where V2/V, is a quantity less than 1, indicating that a beam at this point is refracted toward the lower velocity structure. The beam undergoes a second refraction when exiting the region of lower velocity (fig. 2). The low velocity region therefore acts as a weakly focused acoustic lens resulting in acoustic shadows distal to its lateral margins. A second mechanism for acoustic shadowing may occur distal to the margins of a rounded region of relatively high acoustic velocity surrounded by a medium of lower velocity (fig. 3). Since the ultrasonic beam passes from a medium of lower to a medium of higher velocity, that part of the beam having an angle of mcidence at the interface equal to or greater than a critical angle, O(., will be deflected. This is expressed by the relation: 0,. = Sin where V3 and V2 are the velocities of the lower and higher velocity media, respectively. The critical angle will be exceeded at the lateral margins of the high velocity structure. The result, again, is zones of acoustic shadowing. In Vitro Demonstration of Acoustic Shadowing Due to Refractive and Reflective Effects In vitro investigations of acoustic shadowing due to refraction were performed using a phantom containing a tissue equivalent gel (developed at SRI International, under contract to Picker Corp.) (fig. 4). The gel was formulated to have acoustic properties and a B-scan appearance similar to that of liver tissue (velocity of sound in the gel is m/sec; attenuation is 1.7 db/ cm at 2.25 mhz). Six regularly spaced condoms provide ports for liquids to be examined. These ports were filled with five solutions of albumin having concentrations of 5%-25% albumin by weight, with the sixth port containing distilled water. Using mineral oil as a coupling agent, the phantom was scanned with a focused 2.25 mhz transducer in direct contact with the gel surface. The resulting ultrasonogram (fig. 5) demonstrates acoustic shadowing distal to the margins of those ports containing water and albumin solutions of concentrations up to 15%. A steplike discontinuous appearance of the far wall of the phantom indicates that the velocity of sound in water and in albumin solutions of 5%-15% is lower than that in the gel. To verify this conclusion, the velocity of sound in water and in each of the albumin solutions was determined using an apparatus consisting of a Lucite tube having a repetitively pulsed 2.25 mhz emitter and a receiving transducer separated by a known distance. With the liquid to be tested in the Lucite tube, the apparatus was immersed in a constant temperature water bath and time-of-flight measurements between the two transducers were determined using an oscilloscope. Acoustic velocities for the albumin solutions were then calculated (table 1). The results of these measurements confirm the impression that acoustic velocities in water and in albumm solutions of concentrations up to 15% are lower than that in the gel (1,540 m/sec). These differences in Received September 19, 1978; accepted after revision February 6, Department of Radiology, University of california, San Francisco, California Address reprint requests to R. A. Filly. a Present address: Department of Radiology, Yale University, New Haven, Connecticut AJR 132: , June 1979 C 1979 American Roentgen Ray Society x/79/1326-o973 $0.00

2 974 SOMMER ET AL. AJR:132, June 1979 velocity cause refraction of the ultrasonic beam at the of sound in the 20% and 25% albumin solutions, on the liquid-tissue interface and result in acoustic shadowing other hand, were comparable to those in the gel. No distal to the margins of the fluid collections by the acoustic shadows were seen distal to the margins of mechanism previously discussed (fig. 2). The velocities ports containing these solutions. In a second study, bile from T-tube drainage was concentrated in a distillation apparatus. Successive frac- Medium 1 Medium 2 Velocity of Sound = LAW OF REFRACTION Sin O V1 Sin 02 V2 Im:. S.- Lower Acoustic : Velocity V1 RefIected Beam - Fig. 1.-Law of refraction (Snell s law) applied to ultrasound. Ratio of sine of angle of incidence to sine of angle of refraction equals ratio of velocities of sound in two media. Acoustic Shadowing Acoustic Shadowing Fig. 2.-Mechanism of acoustic shadowing caused by refraction of ultrasound. Ultrasonic beams tangential to interface between two media of different acoustic velocity are refracted, resulting in zones of shadowing distally. Fig. 3.-Mechanism of acoustic shadowing distal to periphery of rounded region of higher acoustic velocity surrounded by medium of lower velocity. At lateral parts of interface, critical angle of incidence, O, is exceeded, leading to total reflection of incident ultrasound and causing acoustic shadowing distally. Fig. 4.- Phantom for in vitro investigation of acoustic shadowing. Lucite box contains tissue-equivalent gel in which six condoms are embedded to contain fluids being imaged. Fig. 5.-Phantom with ports contaming distilled water and albumin in water solutions of 5%, 10%, 1 5%, 20%, and 25% by weight. Two experiments were done accounting for the cornbined image. Shadowing (angled arrows) seen distal to peripheries of ports containing water and albumin solutions up to 15%. Shadows appear as discontinuities in far wall of phantom. Backward displacement of far wall of phantom distal to these ports (straight arrows) indicates velocity of sound lower in these liquids than in surrounding gel.

3 AJR:132, June 1979 ACOUSTIC SHADOWING 975 TABLE 1 Velocity of Sound at 22#{176}C Medium velocity (rn/sec ± 4 m/ sec) H20 1,490 Albumin (%): 5 1, , , , ,548 GEL..... *,. : I -..,_p #{149}..:it:. -,#{176}. --.,.1... :. i1: =..#{176}.. #{176} & : ME Fig. 6.-Phantom containing four bile fractions of increasing concentration (a-d). Appearance of far wall (straight arrows) of phantom mdicates acoustic velocities in bile fractions are (left to right): lower than gel for first two specimens, about equal gel in the third, and higher than gel in the fourth. Acoustic shadows (curved arrows), distal to margins of two ports at left, produced by refractive effects. Acoustic shadows distal to periphery of right port (curved open arrow) secondary to total reflection of sound at margins of fluid collection. Again, acoustic shadows more easily seen as discontinuities of far wall of phantom. No acoustic shadows seen distal to margins at third bile fraction since velocity of sound in gel and in bile fraction are about equal. Internal echoes appear in highest concentrations of bile. tions of increasingly concentrated bile were subsequently scanned in the phantom (fig. 6). If the velocity of sound in the bile was either higher or lower than that of the gel, as indicated by the appearance of the far wall of the phantom, acoustic shadowing was demonstrated. Acoustic shadowing due to refraction is noted distal to the periphery of the two bile collections having acoustic velocities lower than that of the surrounding gel (ports a and b, fig. 6). For the bile fraction having a higher acoustic velocity than the gel (port d, fig. 6), acoustic shadows are present due to the mechanism of total reflection of ultrasound (fig. 3). The bile fraction having an acoustic velocity similar to that of the gel (port c, fig. 6) demonstrated neither refractive nor reflective shadows. Clinical Examples of Refractive and Reflective Shadowing Acoustic shadows have been noted distal to the margins of cysts occurring in the breast. These shadows Fig. 7.-Parasagittal ultrasonogram. Septated cyst (C) of multilocular dysplastic type in right kidney (K). Well defined refractive shadow (arrow). D = diaphragm. Fig. 8.-Right hepatic lobe abscess (A) presents confusing ultrasonic appearance. Numerous internal echoes suggest lesion is solid. Well defined refractive shadow (arrowheads) leads to correct conclusion of echoproducing fluid rather than solid tissue. D = diaphragm; L = liver. have been interpreted as an important indication of the cystic nature of a breast lesion [4, 5]. While the cause of these shadows has been attributed to either refraction [4] or the effect of total reflection [5], it seems that the refractive mechanism outlined earlier and documented in vitro provides the best explanation of their origins. By the same mechanism, acoustic shadowing may be seen distal to the margins of other fluid containing structures within the body. Cysts in other organs may show margin shadows, providing additional important information that a fluid filled structure is present (fig. 7). More significant is that atypical fluid collections, such as those that contain numerous internal echoes, may well display refractive shadows (fig. 8). This finding leads to a correct interpretation of the lesion as fluid containing.

4 976 SOMMER ET AL. AJR:132, June 1979 Fig. 9.-Shadowing (arrows) observed from interface of body of gallbladder (GB) and liver. Convoluted nature of cystic duct could not Fig. 10.-Refractive shadow (small arrows) evident distal to lateral explain such a shadow. D = diaphragm; RPV = right portal vein. interface of inferior vena cava (IVC) and liver (L). LPV = left portal vein. A Shadows occurring at the junction of the gallbladder and liver parenchyma are commonly seen and are often particularly evident at the gallbladder neck where they may raise the suspicion of cholelithiasis. It has been suggested that such shadows may be due to the convoluted course of the cystic duct, but the refractive mechanism seems a more likely explanation. This is documented by the demonstration of similar shadows at margins of the gallbladder where no such convolutions may be invoked as an explanation (fig. 9). Similarly, shadowing due to refractive effects may be seen distal to the margins of blood vessels surrounded by tissue of higher acoustic velocity than blood (fig. 10). Acoustic shadows secondary to the mechanism of critical angle of reflection are also evident. The most common examples are seen in obstetric ultrasonography B Fig A, Acoustic shadow (atrows) seen distal to interface of antenor fetal abdominal wall (FB) with amniotic fluid (AF). B, Broad acoustic shadows (arrows) evident distal to margins of fetal head (FH). Two successive interfaces of higher acoustic velocity encountered in this situation: amniotic fluid-scalp interface and scalp-skull interface. P = placenta; Sp = spine. where a solid structure of higher acoustic velocity (fetus) is immersed in a medium or lower acoustic velocity (amniotic fluid). Thus, as the beam path crosses the margin of soft tissue and fluid, shadows are generated at points where the critical angle is met or exceeded (fig. ha). The most dramatic example of such shadowing is noted at the margins of the fetal head. In this instance, total reflection may be expected to occur not only at the amniotic fluid-scalp interface, but additionally at the scalp-skull interface. The result is a broad acoustic shadow (fig. 11B). The obvious importance of understanding the refractive and reflective mechanisms that produce acoustic shadows lies in the realm of clinical interpretation of ultrasonograms. The artifact produced by these physical effects (shadows) is of interest since its recognition may

5 AJR:132, June 1979 ACOUSTIC SHADOWING 977 avoid a misdiagnosis or provide essential information for arriving at the correct interpretation of the scan (fig. 8). Undoubtedly, many more examples of refractive and reflective shadows will be evident on ultrasonograms closely inspected with an eye to these mechanisms. REFERENCES 1. Wells PNT: Physical Principles of Ultrasonic Diagnosis. New York, Academic Press, Goldman DE, Hueter TF: Tabular data of the velocity and absorption of high-frequency sound in mammalian tissues. J Acoust Soc Am 28:35-37, Sommer FG, Filly RA: A phantom for characterization of biological fluids by ultrasound and CT scanning. Paper presented at the annual meeting of the American Institute of Ultrasound in Medicine, San Diego, October Jellins J, Kossoff G, Reeve TS: Detection and classification of liquid-filled masses in the breast by gray scale echography. Radiology 1 25 : , Kobayashi T: Gray scale echography for breast cancer. Radiology 122: , January 1977