Temperature estimation and protocol assessment for monitoring high intensity focused ultrasound therapy with diagnostic ultrasound

Characterizing the performance of diagnostic ultrasound (DU) for monitoring high intensity focused ultrasound (HIFU) therapy is essential for identifying effective treatment protocols. Such protocols ensure accurate therapy monitoring in media with known thermal and perfusive properties. The protocols are specified by treatment duration, transducer motion, and the DU acquisition characteristics. The HIFU transducer is constrained to move in the plane perpendicular to its axis, with the DU positioned to capture the focal plane of the HIFU transducer. Differences in RF backscattering observed between two frames, one captured before and one after treatment, are known to result from a temperature distribution generated by the heat deposited during therapy. A functional expansion for the heating is used which provides minimum variance coefficient estimates for stationary backscattering by averaging the Fisher information matrix. We show through approximation, simulation, and experiment that these functions attain the Cramer-Rao bound independent of the particular heating applied or the RF backscattering encountered. Estimator accuracy is thus determined by the material properties, including the spatial correlation, and the protocol employed. Temperature estimates accurate to well within one degree Celsius are possible, and the estimation algorithm is implemented efficiently to enable near real-time monitoring. [Work supported by NIH Grant 5R01CA109557.]