Progress in Modeling and Simulation of Shock Wave Lithotripsy (swl)

Past research in shock wave lithotripsy (SWL) has shown that cavitation plays an essential role in the comminution of kidney stones. To provide a better understanding of the role of cloud cavitation dynamics in SWL, the flow in the focal region of a lithotripter was modeled using an ensemble averaged two-phase flow model for the bubbly mixture combined with a high-order accurate shock capturing technique. The domain and initial conditions used in the numerical model reflect the appropriate dimensions and intensity of a Dornier HM3 electrohydraulic lithotripter. The impact of factors such as the size and number of bubble nuclei in the liquid, the intensity of the shock wave and the pulse rate frequency (PRF) on the cavitating flow field is analyzed. Conclusions regarding the impact of these parameters on the potential for stone comminution are also presented. INTRODUCTION Several aspects of the treatment of kidney stones using SWL are not fully understood. Although the importance of of cavitation in stone comminution has been firmly established, see [1-2] as well as 3 presented elsewhere in these proceedings, it has not been possible in the past to obtained detailed evolution for the generated cavitation field. Because of short and long term side effects of SWL, there is an extensive research effort to find optimized treatment parameters such as intensity and pulse rate frequency. In order to achieve this goal, it is crucial to understand the specific characteristics of the cavitation cloud which promote stone comminution. At the present stage, the role of numerical modeling has been constrained into predicting the pressure in the field of a lithotripter and the response of a single bubble to the pressure field [4-5]. However, these approaches are valid only in the limit of vanishing void fraction and cannot represent coupled interactions between the pressure and cavitation field. As presented in our earlier work [6-7], the two-phase continuum model is able to represent some of the complex interactions occurring in the focal region of a lithotripter. The present study is a continuation of this work and focuses on the interactions within the collapsing bubble cloud and their importance with regards to stone comminution. By post-processing the numerical results using a more complex bubble model which includes gas diffusion, we were able to calculate an approximate pulse firing rate which would correspond to the conditions implemented.