Estimating the thermal dose from backscattered RF echoes

Over the years many different investigators have attempted to estimate the temperature rise resulting from ultrasound exposure in therapy applications. The developed methods typically rely on thermal expansion or temperature related sound speed variations both of which are dependent on tissue type and cannot be known a priori on a patient-specific basis. We have developed a method to estimate the thermal dose (temperature over time) from the backscattered RF echoes. RF echoes from within the same tissue type can be used to estimate the in vivo local attenuation (assumed to be the same as tissue absorption) as has been shown by other investigators. Similarly, the RF echoes can be compared to reference echoes while assuming a model for the scattering structures to estimate the total attenuation along the propagation path (i.e., the in vivo power spectrum). Hence, the temperature over time can be estimated from the measured in vivo power spectrum and the measured tissue absorption by solving the bioheat equation directly. The estimated thermal dose can then be used to monitor or plan ultrasound therapy on a patient-specific basis.