Cavitation methods in therapeutic ultrasound : techniques, mechanisms, and system design

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, February 2004.Includes bibliographical references (leaves 134-151).Focused ultrasound is currently being developed as a non-invasive 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, the formation and collapse of gas bubbles, can occur. These bubbles can be unpredictable in their timing and location and often interfere with thermal therapies. Therefore, focused ultrasound techniques have tried to avoid bubbles and their effects. However, gas bubbles in vivo have some potential useful features for therapy. They greatly enhance local ultrasound absorption, and can on their own induce mechanical damage to the tissue. In addition, bubble clouds can block ultrasound wave propagation, providing a means to protect vital tissues during ablation of nearby pathology. If induced and controlled properly, cavitation in focused ultrasound therapy could potentially be very beneficial. The first aim of this research is to design and test in vivo ultrasound exposures that induce cavitation at appropriate times and take advantage of their absorption enhancing properties. In addition, methods to monitor and control cavitation induction and the associated therapy will be investigated. Second, a theoretical bubble model and acoustic field simulations will be used to design optimal pressure fields which very tightly control the cavitation location. These models will also be used to investigate methods for reducing the acoustic powers needed to induce cavitation while preventing off focus cavitation. For the final phase of the research a multi-channel, multi-frequency ultrasound amplifier system capable of delivering optimal exposures via large scale phased array systems will be developed and tested. In(cont.) total, the thesis research will justify applications for cavitation in ultrasound therapy, and develop the technology and methodology to optimally use cavitation and monitor its effects in vivo.by Shunmugavelu D. Sokka.Ph.D