Ultrasonic Refraction by the Rectus Abdominis Muscles:

Transcription

1 J Ultrasound Med 3: , November 1984 Ultrasonic Refraction by the Rectus Abdominis Muscles: The Double Image Artifact Nestor Muller, MD, Peter L. Cooperberg, MD, V. A. Rowley, MD, John Mayo, MD,t Brian Ho, MD,t David K. B. Li, MOt Using a real-time mechanical sector scanner in a transverse plane of section, the authors noticed that a double image of an intrauterine gestational sac or an intrauterine contraceptive device can be obtained when the transducer is passed over the midline of the abdomen. It was suggested that this is caused by refraction of the sound passing through the medial edges of both rectus abdominis muscles. The authors were able to reproduce this artifact in vitro using isolated human cadaver rectus muscles and imaging a single object in a water bath. (Key words: ultrasound artifacts; refraction artifacts) There have been several reports on the artifacts produced by refraction at acoustic interfaces with different tissue velocities. 1...a These have not, how ever, dealt with the double-image artifact often observed when midline structures are scanned with real, time sector scanners (figs. 1 and 2). These are particularly common in the pelvis and are most noticeable when transverse scans are obtained with the transducer in the midline. We were able to obtain similar double images in vitro using isolated rectus abdominis muscles and scanning objects in a water bath. We believe that the artifact is caused by refraction of the ultrasound beam at the fat-rectus abdominis muscle interface. The artifact is unique to the rectus muscles because these represent two apposed curved interfaces which behave like lenses, causing medial deviation of the refracted beam. Thus, a double image can be produced from a single object if it is at the right depth in the body. MATERIAL AND METHODS In Vivo Studies By moving the real-time sector scanner across the lower abdomen in a transverse plane of section, the uterus and contents appear to move perceptibly Received January 25, 1984, from the Department of Radiology, University of British Columbia, and Vancouver General Hospital; and the Health Sciences Centre Hospital.t Vancouver, B. C., Canada. Accepted for publication May 7, Address correspondence and reprint requests to Dr. Cooper berg: Department of Radiology, Vancouver General Hospital, 855 West 12th Ave. Vancouver, B.C., Canada V5Z 1M9. as the transducer crosses the midline. If the transducer is held in a very specific position in the midline, a picture can be obtained showing a double image of a single gestational sac or intrauterine con traceptive device. In Vitro Studies Preparations of anterior abdominal wall muscles were removed from three cadavers. These consisted of 10-cm-long sections of both rectus muscles and the linea alba with the skin and fat superficially and the appropriate fat in the deep aspect. Two specimens were studied fresh and after fixation in formalin. Formalin had several advantages and did not cause any appreciable change in the results. It maintained the biconvex surfaces of the rectus muscles more efficiently. The rectus muscles were applied to the surface of a water bath over a sheet of clear plastic wrap (Saran*). A single cylindrical ob-. ject 2 mm in diameter and 10 em long was suspended in the center of the water bath and fixed to its side using waterproof adhesive tape. The distances between the rectus muscles and the object ranged from 5 to 20 em. Scans through the rectus muscles were obtained using and S.M Hz transducers, on a commercially available real-time mechanical sector scanner (Diasonics RA1 ). RESULTS In Vivo Studies Examples of double images of different structures observed in vivo are shown in Figures 1 to 3. The artifact is usually seen as a "shadow" which 515

2 516 MULLER ET AL- JOURNAL OF ULTRASOUND IN MEDICINE Figure 1. (above, left). Transverse scan showing a duplication of the echo from 1he Copper+? IUD- hence. the Copper-14 artifact. Figure 2. (above, right). A duplicate echo of a single six-week gestational sac simulating twins. Also notice the apparent enlargement of the uterus due to the artifact. can be manipulated so that any object in its path can be affected. At the proper depth. a doubling nrtitact will result. However, at other depths, only partial separation will occur and the object can be made to increase in size. This is illustrated in figures 2 and 4. In Vitro Studies One sp cim ~n consisted of rectus musdes 3 mm thick. In this specimen. we were unable to obtain a double-image artif:tct hd(>re or after fi xation in formalin. The other two cadavers had rectus muscles approximutely 10 mm thick. Both had prominent subcutaneous and extraperitoneal fitt. One was studied fresh from the cadaver and later preserved in formalin. The other was studied fol lowing formalin fixation. In these two specimens, a doubk--image artifact could he produced only with transverse scans, with the transducer held over the linea alba and with the object at least 15 em deep to the rectus muscles (fig. 5). When the object was closer. there was only partial or no separation of the images. There were no apparent dif:. ferences using the 3.5- or 5.0-MH:t. trunsducers. DISCUSSION As sound passes from one tissue plane into another, some of it is reflected and some is refracted. The proportion of sound reflection is directly proportional to the difference in acoustic impedance between the two tissues. The portion of the sound beam that is not reflected (and not absorbed) is refracted. The degree of change in direction is dictated by Snell's law. 5 The speed of sound in fat js approximately 1,450 m/sec.and that in muscle, 1,585 m/sec. Therefore, when the ultrasound beam passes from fat to muscle, the beam will he bent away from the perpendicular; Conversely, on the way out of the rectus muscle, the beam goes from a high- to a )ow-velocity tissue and is refracted to ~ ward the perpendicular. Since the rectus muscle is shaped in cross-section like a lens. the sound beam is deviated medially. In the case of two apposed rectus muscles, when the transducer is pointed loa ward one rectus muscle, the beam is deviated me dially (toward the midline). As the transducer ro ~ tales toward the other rectus muscle, the sound beam is then again directed medially (back toward the midline). If the IUD or gestational sac is at the appropriate depth, it will he detected twice, whe n the sound beam is directed to the right and to the left rectus muscles. Although the sound beam is refracted, the machine does not appreciate that the sound is not traveling in the direction in which the transducer is pointing. It therefore displuys the object at the appropriate distance determined by the time taken for the echo to return to the transducer in the direction that the transducer was pointing

3 RECTUS ABDOMINIS ARTIFACT- VOLUME 3, NOVEMBER Figure 3. (top, left). Although the duplication artifact is more commonly observed in the pelvis, it may also be seen in the upper abdomen. Duplication of the echoes from the aorta and superior mesenteric artery is illustrated. Figure 4. Series of transverse scans of the pelvis, with the transducer being moved from just to the right of the midline, to the midline, and immediately to the left of the midline. A (top,. right). Scan of a gestati.onal sac obtained just to the right of the midline shows partial separation, resulting in an apparent increased transverse diameter of the uterus. A partial doubre image of the gestational sac is obtained. 8 (bottom. left), In the midline, a double lmage artifact is obtained. The appearance simulates the presence of two separate gestauonal sacs. C (bottom, right). Moving the transducer slightly to the left of the midline results in only partial duplication art1fact. at the time. Thus, one object gives two images (flg. 6). Although the object must be an appropriate distance from the refracting muscles, it turns out that the uterus, deep to a filled urinary bladder, is frequently at the appropriate distance from the rectus abdominis muscle and therefore this artifact can be produced with great frequency. Furthermore, only small objects can give a completely separate double image, so that this artifact is seen most commonly

4 518 MOLLER ET AL- JOURNAL OF ULTRASOUND IN MEDICINE , Figure 5. A (left), Scan o ~ the wooden rod (arrow) obtained away from the midline. 1 ne ecnoes 1rom tne rectus muscles are lost In the near field. B (right), Scan through the midfine showing a double image of the rod. with an early gestational sac or intrauterine contraceptive device. (Because we first noticed this duplication artifact with a Copper-7 IUD. it is locally referred to as the "Copper-14'' artifi1ct.) Although no one should be fooled into thinking that there actually are two I 'Ds in the uterus, or twins when there is actually singleton early gestation, it is interesting to understand why this artifnct can arise. Perhaps of greatel' practical importance is the nh ~ servatinn that the doubling artifad can produce nn increase in size of an object not situated at a sufhcient depth to give a completely separate double image. A fibroid, lor example. could be made to appear larger than it actually is so far as its side-to side measurement is concerned because of the artifact. The crown- rump length in the fetus appropriately aligned could also be possibly increased. After completion of our manuscript. similar in vivo observations were reported by Buttery and Davison. 6 Their conclusions are similar to ours. However, they did not perform in vitro studies and their explanation for the phenomenon differs slightly from ours. Whereas they suggest that the refraction occurs at the linea alba and rectus muscle interf.tce. we believe that it is greater in the latter and thus would produce more refra~tion. We also \ Ultrasound beam 1--bladder ,uterus + IUD Figure 6, Exaggerated diagram of the refraction of the sound beam passing through the med ~ al edges of both rectus muscles to converge at the leve1 of the IUD. The Image will appear on the screen at the appropriate depth aiong the vector determined by the direction in which the transducer was pointing at the lime. The solid lines indicate the actual path of the beam. The interrupted lines indicate the Initial direction of the ultrasound beam. As the computer calculations are based on the latter, a double image is obtained.

5 RECTUS ABDOMINIS ARTIFACT- VOLUME 3, NOVEMBER take exception to labeling it a "ghost artifact." Clearly if we give it a name, it should be a "doubleimage artifact." This not only says what it is but also avoids introducing confusing terms. REFERENCES 1. Sommer FG. Filly RA. Minton MJ: Acoustic shadowing due to refractive and reflective effects. AJR , Filly RA, Sommer FG, Minton MJ : Characterization of b'ological f uids by ultrasound and computed tomography_ Radiology 134:167, P'erce G. Golding RH, Cooperberg PL: The effects of tissue velocity changes on acoustical interfaces. J U'trasound Med 1:185, Robinson DE, Wilson LS, Kossoff G: Shadowing and enhance ment in ultrasonic echograms by reflection and refraction. J Clin Ultrasound 9.181, Christensen EE, Curry TS, Dowdey JE; An lntroduct'on to the Physics of Diagnostic Radiology.. Philadelphia. Lea & Febiger, 1978, pp BuHery B. Davison G The ghost artifact J Ultrasound Med 3:49, 1984