Effect of acoustic nonlinearity on heating of biological tissue by high-intensity focused ultrasound

Effect of strong acoustic nonlinearity on the efficiency of heating of a bi ological tissue by high-intensity focused ultrasound in the modes of operat ion used in real clinical setups is studied. The spatial distributions of t hermal sources and the corresponding temperature increments caused by ultra sonic absorption are analyzed. Numerical algorithms are developed for simul ating the nonlinear focusing of ultrasound in the calculations of both the heat sources on the basis of the Khokhlov-Zabolotskaya-Kuznetsov-type equat ions and the temperature field in a tissue on the basis of an inhomogeneous thermal conduction equation with a relaxation term. It is demonstrated tha t in the mode of operation typical of acoustic surgery, the nonlinearity im proves the locality of heating and leads to an increase in the power of the rmal sources in the focus by approximately an order of magnitude. The diffu sion phenomena in the tissue lead to a smoothing of the spatial temperature distributions, as compared to the distributions of thermal sources. In the case of one-second exposure in the nonlinear mode of focusing, the maximal temperature in the focus exceeds the values obtained in the approximation of linear wave propagation by a factor of three. (C) 2001 MAIK "Nauka/Inter periodica".