Influence of thermophysical and acoustic properties on temperature elevation at muscle/ bone interface induced by ultrasound hyperthermia

Therapeutic ultrasound heating is a very common technique in the treatment of muscular and joints injuries. However, insufficient or excessive heating delivered to living tissues can be either innocuous or hazardous to the patient. The present work deals with the heat transfer process in living tissues, with particular concern on the behavior at the muscle-bone interface, when ultrasound irradiation is applied. Based on the Generalized Integral Transform Technique (GITT), a hybrid numerical-analytical solution of the Pennes' equation using the open-source UNIT code ("UNified Integral Transforms") is proposed, considering different boundary conditions. The influence of physical properties and parameters, such as beam incidence angle, acoustic attenuation coefficients for refracted longitudinal and shear waves in the bone, and blood perfusion in the muscle, were more closely analyzed. Numerical results of temperature profiles showing variations of up to 2 C-degrees for different situations are presented, illustrating a marked dependence of heating patterns on the perfusion rates as well as on the effect of shear waves. The conclusions are that such type of more detailed analysis is essential for the accurate identification of physical properties in living tissues and phantoms and can contribute to the treatment planning for therapeutic ultrasound.