In vivo measurements of lithotripsy shock waves in pigs

Abstract

Mechanisms of stone comminution and tissue damage in lithotripsy are sensitive to the acoustic field within the kidney, yet prediction of shock wavesin vivo is difficult. Measurements of lithotripsy shock waves have been made inside pigs with small hydrophones constructed of a 50-μm PVdF membrane stretched over a 21-mm-diam ring. A thin layer of silicone rubber was used to electrically isolate the membrane from pig fluid. An incision was made in the pig flank and a hydrophone inserted either on the anterior (shock wave entrance) or posterior (shock wave exit) surface of the left kidney. The incision was flooded with saline during closure to minimize entrapped air. Four pigs were treated in a Dornier HM-3 lithotripter. Fluoroscopic imaging was used to orient the hydrophone perpendicular to the shock wave. For each pig, various power settings and movement of the shock wave focus throughout the kidney were used. Waveforms measured within the pig had a shape very similar to those measured in water. However, pressure amplitude in the pig was about 60% of that measured in water. The shock rise times were on the order of 100 μs and were hydrophone limited. [Work supported by NIH and the ASA Hunt Fellowship.]