Numerical Simulation of Volumetric Ultrasound Heating of Biological Tissue with Surface Cooling

One of the undesirable effects of using ultrasound for extracorporeal therapy is skin overheating, caused by both ultrasound absorption and contact with the heated surface of the acoustic transducer. To suppress this effect, a forcibly cooled contact medium can be placed between the skin and the irradiating surface. A novel ultrasound applicator implementing this approach has recently been proposed and developed at Southern Federal University. It uses a rectangular piezoelectric transducer bonded to an aluminum plate for volumetric heating of subcutaneous biological tissue. The plate is cooled by circulating cold water through laterally drilled channels. This article presents a numerical algorithm for calculating the three-dimensional temperature field in tissue during the operation of this applicator. The simulation was based on the inhomogeneous heat equation. Experimental acoustic holography data obtained for the developed transducer were used to calculate the heat sources in tissue. An example of heating bovine liver tissue ex vivo is considered, with irradiation times ranging from several seconds to several min. The simulation results were compared with experimental data on tissue thermal ablation at an acoustic power of 12 W and an ultrasound frequency of 6.96 MHz. It is shown that the combination of thermal tissue exposure and contact boundary cooling allows for volumetric tissue heating with a temperature maximum at a depth of 8 to 15 mm, while maintaining a negligible temperature change at depths up to 2–3 mm.