PROBABILITY OF CAVITATION FOR SINGLE ULTRASOUND PULSES APPLIED TO TISSUES AND TISSUE-MIMICKING MATERIALS

In this study, the negative pressure values at which inertial cavitation consistently occurs in response to a single, two-cycle, focused ultrasound pulse were measured in several media relevant to cavitation-based ultrasound therapy. The pulse was focused into a chamber containing one of the media, which included liquids, tissue-mimicking materials, and ex vivo canine tissue. Focal waveforms were measured by two separate techniques using a fiber-optic hydrophone. Inertial cavitation was identified by high-speed photography in optically transparent media and an acoustic passive cavitation detector. The probability of cavitation (P-cav) for a single pulse as a function of peak negative pressure (p_) followed a sigmoid curve, with the probability approaching one when the pressure amplitude was sufficient. The statistical threshold (defined as P-cav = 0.5) was between p_ = 26 and 30 MPa in all samples with high water content but varied between p_ = 13.7 and >36 MPa in other media. A model for radial cavitation bubble dynamics was employed to evaluate the behavior of cavitation nuclei at these pressure levels. A single bubble nucleus with an inertial cavitation threshold of p_ = 28.2 megapascals was estimated to have a 2.5 nm radius in distilled water. These data may be valuable for cavitation-based ultrasound therapy to predict the likelihood of cavitation at various pressure levels and dimensions of cavitation-induced lesions in tissue. (E-mail: adamdm@umich.edu) (C) 2013 World Federation for Ultrasound in Medicine & Biology.