Ultrasonic mapping of temperature in hyperthermia: the thermal lens effect

Recently, a method was proposed to noninvasively estimate temperature during hyperthermia treatments (E.S. Ebbini et al., IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 43, no. 6, p. 1063-73, 1996). Temperature in rubber phantoms and bovine muscle slabs has been estimated by this method, and exhibits strange artefacts. In order to understand these effects, experimental evolutions of an acoustical Green's function with temperature are presented. The results clearly show that the technique appears to be strongly sensitive to some refraction effects. Indeed, the heated area changes its sound velocity and acts as a thermal lens for the ultrasonic beam. A set of finite difference simulations shows an example of the deformations that a focused ultrasonic beam undergoes, while propagating through an inhomogeneous temperature distribution. This thermal lens effect not only acts on the diagnostic beam, but also on the heating beam. These combined effects have to be understood, in order to know the limits of a precise temperature estimation.