Toward a better understanding of high intensity focused ultrasound therapy using the Khokhlov–Zabolotskaya–Kuznetsov equation.

Abstract

High intensity focused ultrasound (HIFU) therapy is an emerging medical technology in which acoustic pressure amplitudes of up to 100 MPa are used to induce tissue ablation, often in combination with real-time imaging. The ultrasound energy is typically focused into a millimeter-size volume and used to thermally coagulate the tissue of interest while ideally sparing surrounding tissue. Nonlinear effects are important in HIFU as in situ intensities for clinical applications of up to 30 000 W/cm2 have been reported. Since controlled experiments are often difficult to perform, especially in vivo, modeling can aid in understanding the physical phenomena involved in HIFU-induced tissue ablation. The Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation is applicable to HIFU because it includes all of the basic physical phenomena that are relevant to HIFU including acoustic beams, diffraction, focusing, nonlinear propagation, shock formation, and dissipation. In this paper, an overview of several recent advances in KZK modeling for HIFU applications are described. It is shown that shock-induced heating in tissue can cause localized boiling in milliseconds; furthermore, the bubbles associated with boiling can significantly alter HIFU treatments. [Work supported in part by NSBRI SMST01601, NIH EB007643, and RFBR 09-02-01530.]