Investigation of stone damage patterns and mechanisms in nano pulse lithotripsy

Abstract

Nano Pulse Lithotripter (NPL) utilizes spark discharges of ~30-ns duration, released at the tip of a flexible probe under endoscopic guidance, to break up kidney stones. Different damage patterns have been observed using BegoStone samples, including crater formation underneath the probe tip, crack development from the distal wall, and radial and ring-shape crack initiation in the proximal surface of the stone. Multiple mechanisms have been proposed for stone disintegration in NPL: dielectric breakdown near the probe tip, shockwave induced by the spark discharge, and asymmetric collapse of bubbles. Experiments have been performed to correlate the proposed mechanisms with the damage patterns observed. Comparison between micro-CT images of the damage initiation sites and COMSOL simulation of the stress field in the stone indicates that the observed cracks are most likely to be produced by the locally intensified tensile stresses produced by surface acoustic waves (SAW) generated by the incidence of the spark-generated, spherically divergent shockwave on the proximal surface of the stone, and their interactions with bulk acoustic waves (P or S) upon reflection at stone boundaries. Dielectric breakdown may contribute to crater formation. However, the contribution of cavitation to stone fragmentation in NPL appears to be minimal.