Heating of bone and soft tissue by ultrasound

Abstract

The absorption of ultrasound in human tissue always causes some local increase in temperature. In the case of ultrasound imaging, the power is usually much less than that used therapeutically and is unlikely to produce any significant physiological effect. However, a reliable means of calculating the maximum temperature rise to be expected in any given case will assist in both the development and the safe use of new ultrasound devices. To validate earlier work on ultrasonic tissue heating, including both experiment and finite element modelling (FEM), an analytical method is described for calculating the steady-state temperature rise along the axis of an axially symmetrical beam of ultrasound incident through water on a two-layer phantom consisting of agar gel and a bone mimic, the practical beam profile being modelled by a pair of coaxial Gaussian functions. It is shown that, in the absence of perfusion, the steady-state temperature distribution for the extended heat source generated by the ultrasound absorption can be obtained by integrating the point-source solution to the Bioheat transfer equation (BHTE). The boundary conditions associated with the difference in thermal properties of the mimic materials are satisfied by introducing images of the extended heat source in the gel/bone–mimic interface.