Modeling weak shocks produced by high-intensity focused ultrasound.

Abstract

Shock waves of up to 100 MPa may form at the focus of high-intensity focused ultrasound (HIFU) transducers at clinically reported in situ intensities of up to 30,000 W/cm2. The heating due to shocks is sufficient to boil tissue in milliseconds, which dramatically alters the treatment. Quantification of enhanced heating from shocks is therefore critical to treatment planning. In this work, several approaches and temporal grids of different resolutions were used to simulate HIFU fields. Peak positive pressure, which determines the shock amplitude, and thus the heating rate were found to be the most sensitive to the parameters of the numerical scheme. Heating rates calculated in modeling and estimated using weak shock theory from the measured and modeled waveforms were compared. Time to boil measured in tissue phantoms and tissue was used as a metric of the heating efficiency of shocks. It is shown that the bandwidth limitations in the waveform measurements result in underestimation of the heat rates, although boiling onset predicted in modeling agreed well with the experimental data. An experimentally validated numerical model thus can be an effective tool in both laboratory and clinical HIFU setting. [Work is supported by NIH EB007643 and NSBRI SMST01601.]