Evaluation of ultrasound tissue damage based on changes in image echogenicity

The rate of decay of increased echogenicity of the focal zone insonified in vivo canine kidney was quantified and parameterized to evaluate tissue damage. High intensity focused sonification was conducted at high mechanical index in order to assure onset of cavitation. A 2D local correlation method was applied to trace temporal change at each location and for motion compensation. The decay half time of increased echo, t_half, defined as the time for echo enhancement to decay by a factor of 2, was measured. The average t_half determined from 23 occurrences of tissue damage, characterized by coagulation necrosis, pitting, and/or blanching, was approximately 45 seconds. On the other hand, when increased echogenicity decayed significantly faster, the observed damage, if present at all, was limited to minor hemorrhage. In 18 cases where increased echogenicity was observed and associated tissue damage was not found, the average was approximately 13 seconds. The correlation between the decay half time and the observed tissue damage is presented and may provide a useful method for both pre-treatment localization and post-treatment evaluation for non-invasive ultrasound surgery.