Cavitation-induced surface damage of a model stone in a high-intensity focused ultrasound field

Within the context of the clinical treatment of human body calculi, this paper addresses the role of ultrasonically induced cavitation bubble clouds in the stone comminution. The fragmentation of calculi is widely assumed to be due to the high acoustic wave energy, while the exact contribution of cavitation is currently unknown. Based on in-situ experimental observations, post-mortem microtomography and acoustic simulations, the present work provides direct evidence on the role of cavitation bubbles in the stone surface fragmentation by correlating the detected damages to the observed bubble activity. Our results show that not all clouds erode the stone, but only those located in preferential nucleation sites whose locations are herein examined. In addition, quantitative characterizations of the bubble clouds and their trajectories within the ultrasonic field are discussed. These include experiments with and without the presence of a model stone in the acoustic path length. Finally, the optimal stone-to-source distance maximizing the cavitation-induced surface damage area has been determined. Assuming the pressure magnitude within the focal region to exceed the pressure cavitation threshold, this location does not correspond to the acoustic focus, where the pressure is maximal, but rather to the region where the acoustic beam and thereby the acoustic cavitation activity near the stone surface is the widest.