Separating nonlinear propagation and cavitation effects in HIFU

Abstract

HIFU waves can effectively destroy tumors or stop internal bleeding. The primary physical mechanism in HIFU is the conversion of acoustic energy into heat, which is often enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. The goal of this work is to study and separate the effects of nonlinear propagation and cavitation on HIFU heating of tissue. Transparent polyacrylamide gel was used as a tissue-mimicking phantom. The phantom contained protein that turned opaque at high temperature. The lesions were produced for the same time-averaged intensities, but with variation of acoustic pressure amplitude combined with different duty cycles. A computer program was written to quantify the size and shape of lesions captured on CCD. Substantial increase of the lesion size was observed with high-amplitude waves. In order to separate the two effects, the experiments were repeated under static pressure greater than the pressure amplitude of the sound waves, which suppressed cavitation. It was found that with the same time-averaged power, a shorter, higher amplitude pulse creates a lesion faster and grows the lesion larger, than a longer, lower amplitude pulse. Cavitation was found to play a subtle role in the initial formation of the lesion. [Work supported by NIH, NSF, NSBRI.]