Nonlinear effects in HIFU propagation and attenuation in biological tissue

Abstract

For therapeutic applications of medical ultrasound, such as hyperthermia, acoustic surgery, and hemostasis, it is desirable, for precise targeting, to induce effective local absorption of the acoustic energy at a small focal area of the acoustic beam. A source with high-focusing gain can be used to achieve the most effective regime of focusing, in which nonlinear effects increase dramatically very close to the focal point. Sharp shocks are developed and the corresponding absorption rate increases at the focus without extra nonlinear attenuation in the prefocal region. In an earlier presentation [Khokhlova et al., J. Acoust. Soc. Am. 98, 2944 (1995)], a modified spectral approach was used that enables us to model propagation of nonlinear plane waves, including shocks, in biological tissue. Here, this method is extended for the description of high-intensity focused sound beams. The acoustic pressure field is calculated numerically for specific focusing gains and tissue absorption over a wide range of the source amplitudes. The effect of nonlinearity, focusing gain, absorption on the waveforms, amplification and spatial localization of the intensity, heating rate, and positive and negative peak pressure at the focus is discussed. [Work supported by ONR and CRDF.]