What is boiling during high-intensity focused ultrasound

Abstract

For treatments that use high-intensity focused ultrasound (HIFU), it is important to understand the behavior of bubbles in the context of both large acoustic pressures and elevated temperatures in the surrounding medium. Based upon clinical and experimental observations, any preexisting cavitation nuclei in tissue or blood are likely to be less than 1 micron. For HIFU conditions characterized by megahertz frequencies and pressures on the order of megaPascals, gas bubbles less than a micron in radius can grow explosively. Calculations for a single, spherical bubble imply that the resulting bubble motions are significantly influenced by evaporation and condensation processes. Consequently, at both high and low ambient temperatures, HIFU-driven bubbles may best be described as gas-vapor bubbles that can exhibit rectified transfer of both heat and noncondensable gases. Moreover, increased vapor pressures associated with ambient temperatures at or above ‘‘boiling’’ may not lead to unbounded bubble growth as expected for a quasistatic bubble in a superheated medium. Instead, calculations suggest that growth of boiling bubbles can be confined. [Work supported by NIH T32 EB001650, NIH DK43881, and NSBRI SMS00402.]