Nonlinear enhancement of heating due to shock formation in high intensity focused ultrasound fields

High intensity focused ultrasound (HIFU) devices operate at intensity levels of up to 30,000 W/cm2 and are used to perform noninvasive thermal therapy. At these output levels, nonlinear acoustic propagation can be of importance and result in enhanced heating of tissue, especially if shocks form in the focal region. Although HIFU is in clinical use, the role of nonlinear propagation and shocks on treatments has not been studied in detail. The work contained herein investigates the effect of acoustic nonlinearity characterization of HIFU devices, in enhancement of HIFU-induced heating in tissue, and in derating of HIFU fields.;A method to characterize nonlinear HIFU fields in water using a combination of measurement and numerical modeling was developed. Modeling was performed using a Khokhlov-Zabolotskaya-Kuznetsov type equation. Measurements were performed using a fiber optic probe hydrophone. Shock waves with amplitudes of up to 80 MPa were both modeled and measured in the field of a 2 MHz HIFU source at clinically relevant output levels of up to 24,000 W/cm2. Results obtained using measurements and modeling were in good agreement; however, when shocks were present, modeling provided more accurate results for determination of the peak positive pressure than measurements because of the limited bandwidth of the hydrophone.;Nonlinear enhancement of heating rates induced by shock formation was quantified in experiments in tissue-mimicking phantoms and excised tissue samples and compared to the results of full numerical modeling and analytic predictions obtained using weak shock theory. In both media, shock waves developed in the focal region and led to a rapid temperature rise and the initiation of boiling after only milliseconds of HIFU heating. Methods for detecting and distinguishing bubble activity due to either cavitation or boiling during HIFU heating were also developed. Furthermore, to determine nonlinear HIFU parameters in tissue, a method to derate acoustic fields from characterization performed in water was proposed and validated.;The results of this work demonstrate that nonlinear acoustic propagation effects are significant in HIFU therapy as enhanced heating and rapid initiation of boiling can alter treatments.