Simulation of gallstone fragments by cavitation bubbles during extracorporeal shock wave lithotripsy: physical basis and in vitro demonstration.

During extracorporeal shock wave lithotripsy of gallstones, sonography often shows a swirling pattern of echogenic foci shortly after the application of shock waves. This effect has been thought to represent gallstone fragments in suspension. However, evidence suggests that this finding is in part due to cavitation, a physical phenomenon associated with the formation or movement of gas bubbles in the fluid-filled gallbladder. Condoms filled with degassed water and five human bile specimens were positioned in the focus of an MPL 9000 lithotriptor (Dornier Medical Systems, Munich). A solitary nonradiopaque gallstone was then added to a bile-filled condom, and the sonographic pattern was observed before and after fragmentation. The mean clearance time of the cavitation bubbles was 4 seconds for degassed water and 22 seconds for human bile. Gallstone fragments were distinguished from cavitation bubbles by their prolonged settling time (up to 30 minutes) along the dependent gallbladder wall.