Pressure and temperature fields from high-intensity focused ultrasound: Modeling the impact of bubbles and cavitation

Abstract

The propagation of high-intensity focused ultrasound (HIFU) in tissue-mimicking phantoms is modeled via a finite difference time-domain simulation. Above a threshold pressure, cavitation activity results and the HIFU focal zone becomes a bubbly medium. We assume an effective medium and account for the impact of bubbles by computing modified effective sound-speed and attenuation coefficients, where the latter includes cavitation-related dissipation mechanisms. Stability criteria establish the bubble equilibrium sizes, and comparison with experiments provides an estimate of the bubble number density. Nonlinear bubble responses are computed numerically and the resulting cycle-averaged void fraction is used to estimate the effective sound speed using a Woods approximation. Computed absorption cross sections related to viscous dissipation and the absorption of reradiated sound yield the effective attenuation coefficient. Using the updated sound-speed and attenuation coefficients, the pressure field is recomputed in an iterative process. Heat deposition is estimated using the averaged acoustic intensity as the heat source along with the evolving attenuation coefficient. The space-time-dependent temperature field and thermal dose is then calculated. Results indicate enhanced heating rates as well as a tadpole-shaped lesion that grows towards the HIFU transducer. [Work supported by the US Army.]