Bioheat Transfer Model (BHTE) based temperature estimation technique for high intensity focused ultrasound therapy monitoring

Abstract

The spatial distribution of tissue temperature is an essential indicator of thermal therapy progress, treatment safety and efficacy. Here, it is shown through in vitro experiments that temperature rise can be accurately measured during therapy delivery and post-treatment cool down phases using RF backscatter data collected with a modified ultrasound scanner. RF data is acquired prior to, during, and after HIFU exposures, in tissue mimicking phantoms and excised animal tissue. Through two calibration experiments, initial estimates for key BHTE parameters (local thermal diffusivity, and magnitude of the HIFU heat source) and the temperature dependence of ultrasonic travel time are obtained prior to therapy. Tissue heterogeneity over a larger targeted region of interest is modeled as a change in the magnitude of the focal heat source. During therapy this magnitude is updated using an iterative optimization technique that minimizes the difference between predicted and measured travel time values. Temperature and thermal dose maps are generated throughout therapy delivery and post-treatment cooling periods. The ultrasound derived estimates are validated against independent thermocouple measurements close to but not at the HIFU focus. This model-based technique permits noninvasive temperature estimation throughout the entire therapeutic range, and is thus a departure from previously reported techniques.