IN-VIVO PRESSURE MEASUREMENTS OF LITHOTRIPSY SHOCK-WAVES IN PIGS

Stone comminution and tissue damage in lithotripsy are sensitive to th e acoustic field within the kidney, yet knowledge of shock waves in vi vo is limited. We have made measurements of lithotripsy shock waves in side pigs with small hydrophones constructed of a 25-mu m PVDF membran e stretched over a 21-mm diameter ring. A thin layer of silicone rubbe r was used to isolate the membrane electrically from pig fluid. A hydr ophone was positioned around the pig kidney following a flank incision . Hydrophones were placed on either the anterior (shock wave entrance) or the posterior (shock wave exit) surface of the left kidney. Fluoro scopic imaging was used to orient the hydrophone perpendicular to the shock wave. For each pig, the voltage settings (12-24 kV) and the posi tion of the shock wave focus within the kidney were varied. Waveforms measured within the pig had a shape very similar to those measured in water, but the peak pressure was about 70% of that in water. The focal region in vivo was 82 mm x 20 mm, larger than that measured in vitro (57 mm x 12 mm). It appeared that a combination of nonlinear effects a nd inhomogeneities in the tissue broadened the focus of the lithotript er. The shock rise time was on the order of 100 ns, substantially more than the rise time measured in water, and was attributed to higher ab sorption in tissue. (C) 1998 World Federation for Ultrasound in Medici ne & Biology.