Shock wave propagation in renal calculi in extracorporeal shock wave lithotripsy.

摘要

Extracorporeal shock wave lithotripsy (ESWL) has become the primary treatment modality for urinary stone removal. Impact of cavitation microjets formed from the collapsing bubbles near stone surfaces has been suggested to be an important mechanism for stone fragmentation. A better understanding of the transient jet-stone interaction, and the propagation of resulting shock waves in the stones, however, is needed.;Using geometrical acoustics, a model for the impingement of cavitation microjet on elastic boundaries and the propagation of the resulting shock waves in the solids was developed. Compared with previous studies, this model provides a complete and general solution for the jet impact problem.;Longitudinal and shear wave speeds for four major types of renal calculi were measured using an ultrasound transmission technique. The densities of the stones were also measured by a pycnometer based on Archimedes' principle. From these, the wave impedances, Young's and shear moduli, as well as Poisson's ratio of the stones were determined.;With the material properties of the stone determined, the model was used to calculate the jet impact pressure at the boundary of a renal calculus and the stress and strain at the propagating shock fronts. Compared with stone material strengths, the model predictions show that damages are most likely to occur at (1) the jet impacting surface of the stone due to the repeated impingements of cavitation microjets and (2) at the back surface of the stone due to reflected tensile waves. The model predictions also agree with experimental results using stone phantoms and clinical experiences of the stone fragility.;Results from this study should help us better understand the mechanism of stone fragmentation in ESWL and improve the design of lithotripters as well as the treatment strategy for non-invasive stone removal.