Phase propagation in ultrasonic backscatter monitoring of high intensity focused ultrasound therapy

Phase propagation using the Rytov method has recently been proposed as a means for modeling the time-of-flight changes induced by thermal therapy [Speyer et al., J. Acoust. Am. 127]. These results are extended to measurements from a linear array, under which the general problem of imaging material changes is cast. The linear array offers several design components, which can be exploited for therapy monitoring, including the apodization and probing frequency. Phase propagation models are shown to be consistent with many aspects of conventional modeling, linearizing material changes around the same operating points as have been proposed by other researchers, and providing time-of-flight changes linearly related to the temperature distribution under these conditions. Beyond expanding on model properties, experimental evidence is presented, which indicates that phase propagation modeling is significantly more consistent with backscattered ultrasound data than conventional ray approaches. [Work supported by NIH Grant No. 5R01CA109557.]