Appearance of gas-containing gallstones on sonography and computed tomography.

The diagnostic features of gas-containing gallstones on sonography and CT in 6 patients and the in vitro findings in 30 gas-containing gallstones are analyzed. On plain abdominal radiography, the stellate appearance of gas collections, though infrequent, is quite characteristic. On real-time sonography, gas-containing calculi can be observed to float within bile. Furthermore, larger gas collections within gallstones can be identified since they produce high-level echoes in the acoustic shadow of the stone, probably due to sound reverberation. These phenomena, although interesting, do not interfere with the high accuracy of sonography in the detection of gallstones. CT detects gallstones when their density differs from that of bile. Due to high-density resolution, even minute gas collections are displayed and can reveal gallstones with an isodense matrix. However, under routine abdominal scanning conditions (8 mm collimation), the gas collections often appear round or ovoid, because spatial resolution is inferior to that on plain radiography. This fact should be considered in the differential diagnosis of gas collections in the gallbladder region found on CT.