There are reports that ultrasonically induced cavitation bubbles locally enhance tissue heating in high intensity focused ultrasound (HIFU) treatment. In this study, a high-intensity burst (named "a triggering pulse") above the cavitation threshold was used to trigger cavitation. Immediately after that, CW ultrasound (named "heating waves"), at an intensity level and duration typical for conventional HIFU ablation was irradiated. Before the ablating sonication, it is necessary to determine the optimum intensity and duration of the triggering pulse. Immediately after a test triggering pulse, low-intensity CW ultrasound (named "a sustaining pulse") was irradiated for a short duration of time, and we detected the cavitation noises (subharmonics and higher harmonics) generated in the focus spot. The relationship between the 1/2 subharmonic signal amplitude and the coagulation volume after the ablating sonication was investigated. The result of this experiment suggests that we can optimize the triggering pulse by detecting the 1/2 subharmonic signal amplitude.
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