Enhancement of ultrasonic heating by ultrasonically localized cavitation for high intensity focused ultrasound treatment

In our previous study, the new method was suggested to treat the tissue efficiently in high intensity focused ultrasound (HIFU) treatment. We have called the method "Triggerd HIFU" treatment, in which a high intensity burst (named "a triggering pulse") was irradiated just before the CW ultrasound (named "heating waves"), at an intensity level and with duration, typical for conventional HIFU ablation. In this experiment, the second-harmonic was superimposed to the triggering pulse, and the cavitation threshold was investigated by comparing with the case of single frequency exposure through detecting the acoustic emission from the bubbles. Cavitating bubbles generated by the triggering pulse were also observed by high speed imaging during both exposure experiments. The experimental results show that cavitation bubbles can be generated more efficiently by the second-harmonic superimposition. The high speed imaging found that the quantity of the triggered cavitation bubbles correlated to the 1/2 subharmonic signal amplitude in the acoustic emission from the bubbles.