Reflections and emissions from stones during electrohydraulic lithotripsy.

Abstract

Lithotripsy is a noninvasive procedure to destroy biliary and renal concretions using shock waves generated externally to the patient and focused at the stone site. The effectiveness and safety of lithotripsy procedures are partially determined by the accuracy of stone targeting. The presentation will cover analysis of pressure waves reflected from the stone to determine stone location, degree of pressure wave focusing, and stone geometry (with changes in stone geometry providing a measure of stone destruction). In the experiments, a Wolf model 2137.50 electrohydraulic lithotripter is used in conjunction with an ellipsoidal focusing system. Piezoelectric pressure transducers within the focusing system provide a means to measure pressure waves both before and after reflection from a variety of targets. In addition, a computer simulation for axisymmetric shockwave propagation in an ellipsoid is developed to predict reflections from stones of arbitrary geometry and position relative to the far focus of the ellipsoid. This ray method solution of the high-frequency approximation to the wave equation accounts for the formation of caustics, while other nonlinear effects are neglected. The simulation allows the visualization of time varying pressure fields within the lithotripter focusing system and around the targeted stone. This research could provide a starting point for a method of dose estimation in lithotripsy procedures. [Work supported by NSF, BCS-8957206.]