Although both gas and heavy metals create multiple reverberations, they can be distinguished both in vivo and in vitro by the differences in origin (focus) and the duration (persistence) of the artifact. A phantom experiment showed that surgical clip artifacts emanate from short echogenic foci, and are persistent, whereas gas foci are either long or short, with evanescent artifacts. In 78 consecutive post-cholecystectomy patients, 96 per cent (24/26) of the patients with clips demonstrated multiple reverberation artifacts, whereas 6 per cent (2/31) of the patients without clips demonstrated several reverberation artifacts. Analyzing these characteristics as well as the positional relationships of reverberation artifacts in the porta hepatis and gallbladder fossa should enable one to suspect the post-cholecystectomy state and differentiate from an abnormal gas collection.