Estimates for the acoustical stimulation and heating of multiphase biotissue

Low-intensity, unfocused, ultrasound-induced diathermy can produce undesired temperature increases at the interface of adjacent tissues within the body; particularly, at the interface of soft tissue and bone. This study provides a computational framework for predicting an upper bound on the temperature profile within a multiphase system composed of gel pad (water), tissue and bone from an input of acoustic energy, at frequencies and power levels consistent with applications of therapeutic hyperthermia. The model consists of solving a (one-dimensional) spatially discretized bioheat transfer equation via finite-difference method and updating the solution in time with a forward-Euler scheme. Simulations are then compared to experimental data to determine the energy-to-heat conversion factors within each constituent material using thermocouple-embedded, tissue-mimicking phantom material, with and without bone. Viscous heating artifacts from the presence of the thermocouples in the experimental phantom tissue are accounted for via additional experimental methods similar to those described by Morris et al. (Phys Med Biol 53:4759, 2008). Finally, an example application of the model is presented via prediction of the maximum temperature at the tissue-bone interface, as well as the peak temperatures in the composite structure at the end of a prescribed 2-min sonication, of blood-perfused, human soft-tissue at 1, 2 and 3 MHz frequencies and a spatial peak temporally averaged intensity of [Formula: see text]. The results of this simulation are then related to comparable experimental studies in the literature.