Dynamic cholecystosonography of the contracted gallbladder: the double-arc-shadow sign.

In 41 of 55 patients in whom the gallbladder lumen was not identified by static sonography, a specific image was observed with dynamic scanning. This consisted of two parallel arcuate echogenic lines separated by a thin anechoic space with distal acoustic shadowing: the "double-arc-shadow" sign. The proximal arc represents the near wall of the gallbladder, the anechoic space is bile in the gallbladder lumen, and the distal arc represents the gallstone(s) responsible for the acoustic shadowing. Of the two additional sonographic patterns observed, the "'echo-shadow" pattern, which was seen with static scanning in 44 and with dynamic scanning in eight patients was an accurate, indirect indicator for cholelithiasis. On the other hand, "nonvisualization" was an unreliable sign. Of 11 such patients scanned with static and six with dynamic scanning, four proved to have a normal gallbladder. Comparing static with dynamic cholecystosonography in patients with nonphysiologically contracted gallbladders, the accuracy rate for cholelithiasis increased from 86.3% (44 of 51 patients) to 96% (49 of 51). In 41 of these patients (80%) the double-arc-shadow pattern positively identified the gallbladder lumen, increasing the confidence of the examiner. Furthermore, these images can be produced in a fraction of the time required for complete evaluation with static scanning.