Adam Calabrese
Dept. of Phys., Univ. of Mississippi, Oxford, MS 38677
John Allen
Univ. of Washington, Seattle, WA 98105
Ronald A. Roy
Univ. of Washington, Seattle, WA 98105
Recent studies have demonstrated that cavitation can be produced by short pulses of megahertz-frequency ultrasound. These studies employed a detection scheme that keyed only on the onset of detectable cavitation. Cavitation thresholds measured using this approach do not accurately reflect nuclei statistics, for one is concerned only with the pressure required to activate the ``weakest'' nuclei. Moreover, this technique offers little information regarding cavitation production rates, which are likely to be better correlated to a mechanical bioeffect than the threshold alone. In the present study, a passive cavitation detector [Roy et al., J. Acoust. Soc. Am. 87, 2451--2458 (1990)] us used to estimate the rate of cavitation production as a function of acoustic pressure amplitude. This measurement is repeated for a variety of insonating pulse lengths and duty cycles and at frequencies of 1 and 2.25 MHz. Results suggest that the threshold pressure for cavitation is weakly dependent on pulse length, and decreases with increasing duty cycle. In addition, cavitation production rates appear better correlated to duty cycle than pulse length. [Work supported by NIH.]