Nonlinear FDTD-Modeling of Ultrasound Thermotherapy Including Temperature Dependent Tissue Parameters
M.Liebler, S.Ginter, T.Dreyer, R.E.Riedlinger (Institut fuer Hoechstfrequenztechnik und Elektronik / Akustik, University of Karlsruhe, Germany)
Ultrasound thermotherapy (USTT) is used to induce thermal coagulation in human tissue by high intensity focused ultrasound. In thermotherapy the absorbed ultrasound energy mainly determines the temperature achieved. It strongly depends on the counteracting processes of nonlinear ultrasound propagation and tissue attenuation. The heating process leads to a change of tissue properties and therefore influences the therapy itself. Here an USTT-model, implemented with a FDTD-algorithm, is presented, combining nonlinear ultrasound propagation with a broadband frequency-power-law attenuation. Special importance is attached to an efficient numerical implementation of the frequency dependent attenuation in the time domain method. Calculation of temperature distributions is done using Pennes Bio-Heat-Transfer-Equation. The temperature dependence of tissue parameters is taken into account for the complete model. Simulation results show the influences of nonlinear effects on therapy. Furthermore it is demonstrated that the temperature dependence of tissue parameters can not be neglected for accurate predictions of the required heating time.
Section : 5