Exposure levels for ultrasonic cavitation in the mouse neonate

The levels for hind limb paralysis from 1 MHz, continuous wave, unfocused ultrasound in the neonatal mouse were determined at 1 and 16 bars hydrostatic pressure and at 10 and 37 degrees C. Above a specific intensity level at each temperature, the exposure duration for paralysis of 50% of specimens exposed (t50) was found to be greater at 16 bars than at 1 bar suggesting a threshold for cavitational involvement. Using these results, the intensity thresholds for cavitation were found to be in the ranges of 120-150 W/cm2 and 53-74 W/cm2 at 10 degrees C and 37 degrees C, respectively. This temperature dependence is consistent with a cavitation mechanism. In addition, the t50 at 289 W/cm2 and 10 degrees C was measured as a function of hydrostatic pressure and showed that cavitation was suppressed at hydrostatic pressures above approximately 10 bars. This result and the intensity threshold for cavitation at 1 bar and 10 degrees C yielded similar values for the threshold negative total pressure for cavitation in the neonatal mouse.