Non-invasive measurement of in situ thermal diffusivity and local heat source using backscattered ultrasound for thermal therapy planning and monitoring

Abstract

Bioheat transfer equation (BHTE) estimates of applied dose during HIFU therapy typically use prior knowledge or assume standard values for tissue properties that determine thermal diffusivity (K) and heat source (Q). We have developed a novel signal-processing based technique to noninvasively estimate these parameters in situ based on analysis of raw backscattered RF data from two localized HIFU exposures, one at sub-ablative intensities to determine K, and another at therapeutic intensities to obtain Q. Both exposures are performed prior to therapy. To estimate K, a short HIFU exposure is applied resulting in a temperature rise of less than 15° C. The Gaussian radius of the temperature induced apparent strain profile during cool down is estimated and fit to an approximate analytical heat diffusion expression to obtain K. Independent estimates of K derived using the transient hotwire technique validated the ultrasonic measurements. To estimate Q, an audio range hydrophone was acoustically coupled to the sample to detect the onset of boiling. The time required to bring the sample to boiling was used to estimate Q by iteration of a numerical BHTE model. These results are validated against in situ measured values using thermocouples and linear acoustic calculations.