The role of bubbles and cavitation in the production of thermal lesions from high-intensity focused ultrasound

Abstract

Rapid hyperthermia resulting in tissue necrosis is a key physical mechanism for focused ultrasound surgery (FUS). At therapeutic intensities, tissue heating is often accompanied by cavitation activity. Although it is well known that bubbles promote mechanical damage, in vitro and in vivo experiments have shown that under certain conditions bubble activity can double the heating rate. With a view towards harnessing bubbles and cavitation for useful clinical work, we report the results of in vitro experiments and modeling for the dynamic and thermal behavior of bubbles subjected to 1-megahertz ultrasound at mega-pascal pressures. The dominant bubble-related heating mechanism depends critically on the bubble size distribution which, in turn, depends on insonation control parameters (acoustic pressure, pulse duration), medium properties (notably dissolved gas concentration), and bubble-destroying shape instabilities. The evidence points to a range of control parameters for which bubble-enhanced FUS can be assured. [Work supported by DARPA and the U.S. Army.]