Derating of nonlinear high intensity focused ultrasound fields to predict millisecond boiling in tissue.

Abstract

The most commonly used method for derating high intensity focused ultrasound (HIFU) fields from water to tissue is based on multiplying the acoustic intensity measured in water by an exponential factor to compensate for attenuation in the tissue path assuming linear wave propagation. Yet, in nonlinear HIFU fields, the intensity provides little information about either heating or negative and positive pressure amplitudes, which are important in predicting bioeffects. In this work, a new derating method is presented and tested for a 2 MHz high gain focused ultrasound source. Focal waveforms are experimentally measured and modeled after propagation through both water and tissue paths at output intensities of up to 24 000 W/cm2. The focal waveforms measured after propagation through tissues were made equivalent to those obtained in water by increasing the pressure amplitude at the source. From the change in source amplitude pressure, the attenuation of the tissue was determined. The focus was then shifted to within the tissue sample, and the measured attenuation was used to calculate the time to reach 100°C. Calculations were in excellent agreement with the time measured to attain boiling in the tissue, which was only several milliseconds. [Work supported by NIH DK43881 and NSBRI SMS00402.]