Coupled thermal-acoustic simulation results with temperature-dependent tissue parameters for therapeutic ultrasound

Abstract

Recently the authors used direct simulations of the transient acoustic pressure field to calculate heat energy deposition and, thus, temperature fields in two-dimensional tissue like media [J. Acoust. Soc. Am. 102, 3172(A) (1997)]. It is known that acoustic-induced temperature rises will alter the properties of the medium in hyperthermia situations. In this presentation the simulations are extended to simultaneously solve the wave propagation and tissue heating problems so that the effect of temperature-dependent tissue parameters can be accounted for directly. In other words, there is a continuous feedback of temperature on the sound propagation parameters, which in turn affects the thermal energy deposition. The simulations are second order accurate in time, fourth order in space full wave calculations using the finite-difference time-domain technique, and allow for finite-amplitude wave propagation in an inhomogeneous, thermoviscous fluid. The model allows for spatially and temporally varying sound speed, density, attenuation coefficient, and nonlinearity parameter, as well as variable thermal properties and perfusion. Acoustic pressure, thermal dose, and tissue temperature are calculated, and conclusions are made regarding qualitative and quantitative aspects of focusing behavior contrasted to the case where the propagation and thermal parameters are kept constant. [Work sponsored by ONR and DARPA.]