FEM Model for Non-Invasive Temperature Changes Estimation using Ultrasonic Echo Shifts Based on Mean Scattering Space Study

Ultrasound can be used as a feasible method for estimating temperature changes during hyperthermia therapies due to its high capability of tissue penetration and relative ease of signal processing. During hyperthermia therapies, the proportionality constant between echo-shift and temperature is determined by the local change in the speed of sound due to temperature and the linear coefficient of thermal expansion of the material. A model of the interaction of an ultrasonic pulse with the displacement of a muscle tissue scatterer is presented in this work using the Finite Element Method to estimate temperature changes in a non-invasive way. The method is based on quantifying the time-delays of an ultrasonic pulse passing through a section of muscle tissue with ideal characteristics that has a 1 mm diameter glass scatterer inside; this latter is displaced due to the tissue thermal expansion phenomenon that appears as a result of heat application (37- $61^{\circ}C)$ during hyperthermia therapies. A time-delay of 0.913 ns/°C is quantified in this paper by comparing seven ultrasonic signals that represent the scatterer displacement.