Numerical investigation on cavitation induced tissue injury during ultrasound treatments.

Cavitation in medical applications is increasingly used for kidney stone fracture in shock wave lithotripsy (SWL), drag and gene delivery through encapsulated bubbles, blood brain barrier (BBB) permeability and in high intensity focused ultrasound (HIFU) among others. Despite the beneficial use of cavitation in these procedures, the exact mechanism of the adverse effects that are being reported, such as hemorrhage, still is not fully understood. In this work we present numerical simulations of cavitation induced soft tissue interaction/injury during ultrasound and shock wave treatments, to elucidate the bubble interaction with soft tissue and rigid bio materials such as kindey stones. To this end a novel numerical solver was developed (ForestFV), that employs a Diffuse Interface Method (DIM) able to capture multi-phase, fluid-solid interactions (FSI), complex shock wave interactions and bubble dynamics, in various spatial and temporal scales using an adaptive mesh refinement (AMR) framework for unstructured grids. The presented numerical framework has been validated against other numerical and experimental test cases. Three different configurations will be presented. In the first cases, a novel tension driven tissue injury mechanism is highlighted for the inertial collapse of attached and detached bubbles on soft tissue. In the next configuration, we demonstrate that the resulting liquid jet from a collapsing bubble can penetrate a blood vessel wall and cause hemorrhage, a typical side effect during high intensity ultrasound. Finally, in the last cases, we present large scale oscillations of bubbles within a capillary the stress development inside the soft tissue wall, as well as the injury potential of stable cavitation to various blood vessels.