Bubble dynamics in therapeutic ultrasound

Rapid hyperthermia resulting in tissue necrosis has proven to be a useful therapeutic modality for clinical application of high-intensity focused ultrasound. At therapeutic intensities, the hyperthermia is often accompanied by bubble activity. In vitro and in vivo experiments alike have shown that under certain conditions bubble activity can give rise to a doubling of the heating rate. With a view towards harnessing the energy-concentrating effects of bubbles to do useful clinical work, we report the results of experiments and modeling for the dynamic and thermal behavior of bubbles subjected to 1-megahertz ultrasound at megapascal pressures. The dominant heating mechanism depends on bubble size, medium shear viscosity number and frequency-dependent acoustic attenuation. The bubble size distribution, in turn, depends on insonation control parameters (acoustic pressure, pulse duration), medium properties (notably dissolved gas concentration) and bubble-destroying shape instabilities. The evidence obtained so far points to a range of control parameters for which bubble-enhanced heating can be assured, while mitigating the effects on the primary acoustic field.