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. They are affected by temperature-dependant changes in the speed of sound, by thermal expansion of tissues, and by tissue expansion caused by irreversibly denatured protein and the radiation force generated by HIFU. To separate the effect of radiation force from other thermal changes, we used a split-focus technique with which the peak of ultrasonic intensity can be shifted from the peak tissue temperature. Tissue expansion was mapped with a split HIFU beam with large separation in an in vitro experiment. Since the time course of tissue expansion did not follow that of the temperature change and expansion remained after cooling, it was suggested that echo-shift imaging could detect region in which coagulation occurred below the boiling temperature, which regions are could not be detected by B-mode imaging.