Session: 1H

Our recent work with cavitation enhanced heating has shown that cavitation used with ablation can improve heating efficiency yielding larger lesions. In this study, we hope to optimize the ultrasound parameters for cavitation enhanced (CE) heating, understand the mechanisms of lesion formation, examine the potential side effects, and develop methods for monitoring and controlling this therapy. First, cavitation thresholds in muscle, fat, and interface tissues of 14 rabbits were measured at 1.1 and 1.7 MHz, for 100 and 500 millisecond exposures. All experiments were performed under MRI guidance with a spherically curved therapy transducer and a cavitation detector ring in a manual positioner/ultrasound coupling system. The threshold results were used to design 4 CE heating exposures which could be tested in vivo and compared to conventional heating sonications. All of the 20-second CE exposures begin with a half-second, suprathreshold burst to induce bubbles at the focus and are followed by constant power CW, pulsed, or a combination of CW with pulsed ultrasound. Such exposure types at 3-4 different energy levels were studied in 12 rabbits. During treatment, MRI thermal images and the passive cavitation signal were acquired at triggered time points and used to access the lesion formation process. Following treatment, MRI was used to measure the lesion size and shape, therapy endpoints. Side effects, near and far field heating, irregular lesion shapes, and skin burn, were also quantified and compared for all exposure types. Exposures with high intensity pulse followed by CW ultrasound produced the best results: largest and most predictable lesion sizes and shapes with minimal side effects. Results indicate that specific bands of the cavitation spectrum could be used to monitor and control cavitation effects while MRI thermometry can be used to control thermal effects. The study characterizes the mechanisms for optimal CE heating and highlights parameters useful in controlling this new more efficient therapy technique.