Estimating ‘‘true’’ HIFU-induced temperature changes using thermocouples

Abstract

An easy and economical way to measure temperature rises in tissue and tissue-mimicking phantoms is with imbedded thermocouples. However, a thermocouple undergoing HIFU exposure is subject to the formation of a viscous boundary layer, leading to an additional local temperature rise not experienced by the rest of the focal region. The resulting temperature measurement is then not indicative of the average undisturbed temperature in the region. This is the so-called thermocouple artifact (we will argue against the use of the word artifact). Motivated by our bubble-enhanced heating experiments, we present a simple method for estimating HIFU-induced temperature changes. The influence of the thermocouple is modeled by an effective increase in the local sound absorption coefficient. The effective absorption coefficient is estimated by measuring the increased rate of heating brought on by the thermocouple. The temperature rise in the medium adjacent to the thermocouple is then predicted by incorporating the effective local absorption into the acoustic source term in the 3D heat conduction equation. Comparisons between simulation and experiment and between the reported method and other methods for thermocouple-artifact correction are presented. [Work supported by the U.S. Army.]