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Sufficiently high intensity ultrasound can create hyperechoic regions in an ultrasound image due to local bubble generation. We explore the link between the temporal extent of these hyperechoic regions and tissue damage caused by ultrasound therapy. The decay rate of increased echogenicity from the focal zone in insonated live exteriorized canine kidney was quantified and correlated to the spatial extent of tissue damage. The decay half-time, t/sub half/, defined as the time for echogenicity enhancement to decay by a factor of 2, was observed in all cases to be greater than 41 s in spatial zones in which extensive histological damage was observed. In cases in which the measured t/sub half/ was less than 11 s, the damage was limited to minor hemorrhage, or it was not detected. These t/sub half/ discrimination boundaries of 41 and 11 s were not statistically different for cases in which contrast agent was used to enhance therapeutic efficiency. This was true even though contrast agent infusion significantly reduced the therapy pulse duration threshold for damage production.