Ultrasonic Detection of Calcification in Gallstones:

Transcription

1 J Ultrasound Med 3: Mardl 1984 Ultrasonic Detection of Calcification in Gallstones: "The Reverberation Shadow" Suhas G. Parulekar, MD Observation of reverberations within a gallstone shadow (the "reverberation shadow") resulted in this study. Reverberation shadow was seen in sonograms of 27 patients with gallstones and three patients with milk of calcium bile. Calcification within gallstones was demonstrated by radiography or computed tomography in 22 of the 27 patients with gallstones. associated with the reverberation shadow. Whether the remaining five patients had calcified gallstones remains unknown. This study suggests that the reverberation shadow produced by gallstones indicates the presence of calcification within the gallstones. (Key words: acoustic shadow; reverberations; calcified gallstones; milk of calcium bile) Distal acoustic shadowing is an important sanagraphic feature of gallstones. The shadow produced by gallstones has been described as a "clean" shadow, characteried by distinct margins and a lack of reverberation echoes. 1 Observation of re verberations within a gallstone shadow resulted in this study. The purpose of this study is to determine the cause and the significance of reverberations within a gallstone shadow. MATERIALS AND METHODS All clinical ultrasound examinations were performed in an 18-month period, using a Diasonics,. high-resolution mechanical-sector real-time scanner and a 3.5-MH transducer. The clinical material included ultrasound examinations of27 patients with gallstones and three patients who had milk of calcium bile within the gallbladder. Ultrasound examinations of all 3 patients demonstrated reverberations within the shadows produced by gallstones and milk of calcium bile. All patients had abdominal radiographs following the ultrasound examinations. In-vitro ultrasound examinations of the gallstone specimens from two patients were performed. For in-vitro studies, gallstones were placed on the surface of a tissue-equivalent phantom immersed in a water bath. The ultrasound scanning Aece ved May 23, 1983, from the Department of Radiology, The Mt. Sinai Medical Center. Univers"ty Circle, Cleveland, Ohio 4416, Accepted lor publication October 25, Address correspondence and reprint requests to Dr. Parulekar. of these stones was performed using the same 3.5- MH mechanical-sector real-time transducer used in the clinical studies. For in vitro ultrasound examination, the distance between the transducer and the gallstone was similar to the distance found during the clinical gallbladder examination. The real-time ultrasound probe (Diasonics GP mechanical-sector) used in this study houses a transducer within a chamber filled with a liquid medium. Within this chamber, the transducer itself is located approximately 1 mm from the membrane covering the chamber. Therefore, the distance between the reverberation echo and the stone is equal to the distance between the stone and the patient's skin surface, plus 1 mm. RESULTS Reverberations were observed within the acoustical shadows produced by gallstones in a total of 27 patients. In 18 of these 27 patients, plain radiography of the abdomen after ultrasonography revealed calcified gallstones. Two patients had no evidence for calcification on the plain radiographs; however, computed tomography revealed calcification within the gallstones. In two patients, radiography of the abdomen revealed no evidence of calcification; however, plain radiographs of the gallbladder specimen after cholecystectomy revealed calcification in the gallstones in one patient, and computed tomography of the gallstone specimen of the other patient revealed calcification. Thus, calcification within the gallstones was confirmed in

2 124 PARULEKAR- JOURNAL OF UL TAASOUND IN MEDICINE Figure 1. A (above). sagittal section. Funda menial reverberation echo (arrow) within the acoustic shadow is produced by a calcihed gallstone. Low-level echoes between the gall stone and the fundamental reverberation represent multiple internal reverberations within the calcified gallstone: the "comet tail'' artifact. 8 (right. above), sagittal sonogram and C (right), abdominal radiograph of two ca ci1ied gallstones. The sonogram (8) reveals a reverberation echo (solid arrows) associated with each stone (curved open arrows). of 27 patients with reverberations within the acoustic shadows produced hy gallstones. The remaining five patients had no calcifications on the plain mdiographs of the abdomen and had no further studies such as computed tomography or specimen radiography. Therefore, whether these five patients had calcifications within the gallstones remains unknown. The sonographic observations in this study are confined to the ultrasound examinations of 22 patients with proven calcified gallstones and three patients with milk of calcium bile. In 21 of 22 patients with gallstones, a single strong {high-amplitude) reverberation echo was seen within the shadow produced by gallstones (fig. 1). This strong reverberation echo is recorded at twice the transducer-stone distance and is referred to as fundamental reverberation echo. 2 In only one of 22 patients with gallstones, a second fundamental reverberation echo was seen, at thrice the transducer-stone distance (fig. 2). This patient with two fundamental reverberations had heavily calcified large gallstones, one of which was almost square, with large flat surfaces. With faintly calcified gallstones, the second fundamental reverberation probably is too weak to be recorded. Also, in 14 of 22 patients with gallstones, the transducerstone distance was such that the second fundamental reverberation, if present, would have been outside the field of the image sie, and therefore could not have been seen. In 11 of 22 patients with gallstones, several lowlevel reverberation echoes were observed between the gallstone and the strong fundamental reverberation (figs. la and 2A). These low-level echoes are secondary to multiple internal reverberations within the calcified gallstone. the so-called "comet tail artifact. " 2 The comet tail artifact within the gallstone shadows appeared as a discrete echo band

3 CALCIFIED GALLSTONES-VOLUME 3. MARCH Figure 2. A (left), sagittal section. Two fundamental reverberations (arrows) produced by a heavily calcified, laminated gallstone (arrow in fig. 28) with a large, flat surface. Notice the prominent internal reverberations (comet tail artifact) between the stone and the two fundamental reverberations. B (right)., heavlly calcified laminated gallstones can be seen on the abdom nal radiograph. pattern between the fundamental reverberations (fig. 2A), rather than as a continuous homogeneous dense echo trail.$ This type of reverberation artifact is dependent upon a number of factors, including the sie, shape and composition of the gallstone; its orientation to the sound wave; and its distance from the transducer. 2 These low-level multiple internal reverberations (comet tail artifact) were most prominent in the patient who had the large square gallstone with a heavily calcified laminated periphery (fig. 2, A and B), and were less prominent in stones with less calcification (fig. l, A and B) and in smaller stones. The sies of the gallstones in this investigation ranged from 3 to 5 mm. Of 18 patients who had radiographic evidence of calcified gallstones, five had faintly calcified stones, nine had moderately calcified stones, and four had heavily calcified stones. In ten of 18 patients, only the peripheries of the stones were calcified, and in eight, the entire stones were calcified. In-vitro ultrasound examinations of the gallstone specimens of two patients were performed. One of these had stones ranging in diameter from 7 to 9 mm, and reverberations could be produced within the shadow (fig. 3). A plain radiograph of the abdomen of this patient revealed no evidence of calcification; however, a radiograph of the gallbladder specimen following surgery revealed faintly calcified gallstones (fig. 3B). The other patient had a large solitary stone 5 em long (fig. 4). On clinical ultrasound examination, a reverberation could be seen corresponding to certain portions of the length of this stone (fig. 4, A and B). Computed tomography of the stone specimen (fig. 4C) revealed calcification involving two thirds of its length. In-vitro ultrasonography of this stone revealed reverbera tions in the shadow produced by calcified portions of the stone (fig. 4D} but no reverberations within the shadow produced by non-calcified portions of the stone (fig. 4E). In many patients with multiple calcified gallstones, multiple reverberations could be seen within the acoustic shadows (fig. 5A). In three patients who had milk of calcium bile within the gall bladder, a layer of echogenic material was seen within the gallbladder and a corresponding layer of reverberation echoes could be demonstrated within the acoustic shadows produced by milk of calcium bile (fig. 5, B- D). In two of these three patients, milk of calcium bile was visible on plain radiographs of the abdomen. In the remaining patient, the milk of calcium bile was not seen on the plain radiographs of the abdomen but could be demonstrated on computed tomography of the abdomen. Sonography of the patient with faintly calcified milk of calcium bile revealed faint reverber ations within the acoustic shadow (fig. 5, B and C ). The patient who had heavily calcified milk of calcium bile and small calcified stones within the milk of calcium bile had stronger reverberations within the acoustic shadow (fig. 5, D and E). DISCUSSION In abdominal ultrasonography reverberations are most frequently produced by air within the loops of bowel or pathologic air collections. Reverberations within the acoustic shadows can be seen in patients with acute emphysematous cholecystitis. 3 None of the patients in this study showed evidence of air collection within the gallbladder on abdom-

4 126 PARULEKAR- JOURNAL OF ULTRASOUND IN MEDICINE Figure 3.. A (above), transverse section. Multiple stones, two of which demonstrate reverberations (arrows) w ~ th i n the shadow. B (right, above), a radiograph of the gallbladder specimen demonstrates peripheral calci'lication in the gallstones. The abdominal radiograph of this patient revealed no evidence for calcifications. C (nghl'), i n ~ v i tro sonogram of one of the stones demonstrates a reverberation (arrow) similar to that seen during the c6nical ultrasound exam nation (hg. 3A). inal radiography. Also,. no patient had any evidence for fissured or airwcontaining gallstones on abdominal radiogmphy. The in-vivo and in-vitro obser.. vations in this study suggest that reverberations produced within the gallstone shadows are related to the calcification within the gallstones. The majority of gallstones produce "clean" shadows, be ~ cause only 2-3 per cent of the incident ultra ~ sound beam is reflected hy majority of gallstones. ' The most likely explanation for the reverberation shadows produced by calcified gallstones is that, because of calcification. the stones reflect a much larger percentage of the incident ultrasound beam, thus producing reverberation echoes within the shadows. The heavier the calcification in the stone, the stronger are the reverberations within the shadow. Analysis of the echogenicity of partially calcified stones producing reverberation shadows also revealed that the calcified portions of the stones were considerably more hyperechoic com- pared with non-calcified portions (fig. 5). The reverberations within the gallstone shadow may not be seen or recognied on routine ultrasound examination of the gallbjadder containing calcified gallstones. Scanning of the stones from different angles is frequently necessary to demonstrate the reverberations. A lack of reverberations within the shadow does not completely excjude calcification within the gallstone, sjnce production of reverberations is dependent upon sie and shape of the gallstone and its orientation to the sound wave, in addition to the degree of calcification. Nine of 27 patients who were found to have gallstone reverberation shadows in this study showed no evidence of calcified gallstones on plain radiog raphy of the abdomen. In two of these nine patients, calcified galjstones were seen on computed tomography, and in two patients. gallstone spec ~

5 --- - ~ - X. y (") Figure 4. A (top row, left), a transverse sonogram ~ reveals a reverberation (arrow) within the acoustic ::n shadow. 8 (top row, middle), a sagittal sonogram ~ reveals a reverberation (arrow) from the calcified G> portion of the large 5-cm stone. C (top row, right), f! in-vitro computed tomography of the 5-cm stone ~ reveals calcification in two thirds of the stone. Ab- o domina! radiography of this patient did not dem- ffi onstrate calcification. D (bottom row, left), an in- f vitro sonogram (transverse section) of the calcified < portion of the stone (level X in C) demonstrates l2 reverberation (arrow) within the shadow. E (bot- ~ tom row, right), an in-vitro sonogram (transverse m section) of the non-calcified portion of the stone!-j (level Y in C) demonstrates no reverberation within ~ the acoustic shadow. ~ :I: ~... (I;)...

6 ~ ~ - --b._ ~ Figure 5. A (top row, left), sagittal section dem ~ onstrates four fundamental reverberations (arrows) corresponding to four of the many faintly calcified small stones layering within the gallbladder. B and C (top row), a patient with faint\y calcified milk of calcium bile within the gallbladder. A sagittal sonogram (B, top row, middle) reveals a layer of echo genic material (open arrow) in the fundus of the gallbladder, and a corresponding layer of low-amplitude fundamental reverberations (solid arrows) within the acoustic shadow. An erect abdominal radiograph (C. top row, right) reveals milk of c al ~ cium blle (arrow). D and E (bottom row), this patient had heavily calcified mi'k of calcium bile mixed with a few calcdied stones. The sagittal sonogram (D. left) demonstrates a high-amplitude fundamental reverberation (arrow) corresponding to the highly reflective layer of milk of calcium bile in the gallbladder.. The erect abdominal radiograph (E, right) reveals milk of calcium bile (arrows) containing a few floating stones. ll ~ ~- - 1"1" ~ ~~--~., ~ ~' -~... ~---~ "''J :: c r m " f I <- :: c ' r. ~ I :: 1 ~. ~ " ~.- 4(.: ~ m ;I... -~ I r "TI c Cj (/) c n m

7 CALCIFIED GALLSTONES-VOLUME 3, MARCH imen radiography or computed tomography of the specimens revealed calcifications within the gallstones. A previous in-vitro investigation"' showed that 5 per cent of the patients with calcified gallstones, as demonstrated on in-vitro specimen 1.1- diography, had no evidence of calcification on preoperative abdominal radiography. Therefore, demonstration of reverberations within the acoustic shadows produced by gallstones may be a more sensitive method for detection of calcifications within the gallstones, compared with abdominal radiography. The in-vivo ultrasonogmphic compositional analysis of gallstones for detection of calcification might influence therapeutic decisions regarding selection of patients for chenodeoxycholic acid therapy for gallstone dissolution. Chenodoxycholic acid is ineffective for dissolution of gallstones in the presence of gallstone calcification, as demonstrated on abdominal radiography. 5 6 However, as many as 5 per cent of calcified gallstones may not be visible on plain radiograph of the abdomen. 2 Computed tomography is perhaps the most sensitive imaging modality currently available for detection of calci fication within the gallstones. However, since ultrasonography is currently the primary imaging modality of choice for diagnosis of cholecystolithiasis, the ability to detect calcifications hy dem onstrating the reverberation shadow could be helpful in selection of patients for chenotherapy of gallstones. Whether gallstones with calcification detectable only by ultrasonography or computed tomography and not visible on abdominal radiography will fail to respond to chenotherapy remains to be evaluated. CONCLUSION This study suggests that the reverberation shadows produced by gallstones indicate the presence of calcification within the gallstones. Further ultrasonographic, computed tomographic, and radiographic correlative studies are necessary to determine the sensitivity and specificity of this observation. The author thanks Miss Jeannetta Daniels for assistance with In vitro examinations of gallstones and ga!bladder specimens. REFERENCES 1. Sommer FG, Taylor KJW: Oillerentiat1on of acoust1c shadowing due to calculi and gas collections Radioogy 135:399, Ziskin MC, Thickman 1, Goldenberg NJ, et al The comet tail artifact. J Ultrasound Med 1: Parulekar SG. Sonographic findings in acute emphysematous cholecystitis. Radiology 145: Carroll BA: Gallstones in vitro comparison of physical, radio graphic and ultrasonic characteristics. Am J Aoenlgenol 131 :223, Thistle JL, Hofmann AF. Ott BJ. et al: Chemotherapy for garl. stone dissolution JAMA 239:141, Way LW The National Cooperative Gallstone Sludy and che nodiol. Gastroenterology