Cavitation-enhanced ultrasound heating in vivo: Therapy protocols, mechanisms, and acoustic and MRI monitoring

Focused ultrasound is currently being developed as a noninvasive thermal ablation technique for benign and cancerous tumors in several organ systems. Although these therapies are designed to ablate tissue purely by thermal means, cavitation can occur. These bubbles can be unpredictable in their timing and location and often interfere with thermal therapies. However, gas bubbles in vivo have some potential useful features for therapy. In this research, we design and test in vivo ultrasound exposures that induce cavitation at appropriate times and take advantage of their absorption enhancing properties while yielding reliable lesion sizes and shapes. In addition, MRI and acoustic methods to monitor and potentially control cavitation induction and the associated therapy are investigated. Finally, histology of the resulting cavitation-enhanced heating lesions is performed to assess the type of tissue damage. We demonstrate that cavitation-enhanced heating can be reliable and useful with the appropriate therapy protocol. Properly timed cavitation during thermal therapy yields larger lesions at lower powers taking advantage of bubbles’ higher absorption. In addition, we show that bubble clouds that result from cavitation can block ultrasound wave propagation, providing a means to protect vital tissues during ablation of nearby pathology.