Tissue coagulation imaging based on bi-plane RF cross-correlation during high intensity focused ultrasound therapy

Therapy monitoring based on echo-time-shift imaging during high intensity focused ultrasound (HIFU) treatment was described. The echo shift estimated by radio frequency (RF) correlation between adjacent frames is potentially useful for mapping coagulation and tissue temperature. B-mode images are also useful for real-time monitoring, but cannot show the denatured region formed below the boiling point. Echo-shift images are, however, can. To identify each contribution of reversible and irreversible change, we used a bi-plane ultrasound imaging and a laser displacement meter to detect tissue expansion and velocity change independently. Tissue expansion was mapped using ultrasound echo shift and laser measurement with a split HIFU beam in an in vitro experiment. Expansion ratio was about 5%, and this value was larger than estimated value on the basis of thermal effects. Since the expansion remained after cooling, it was suggested that echo-shift imaging could detect region in which coagulation occurred below the boiling temperature.