Threshold ultrasonic dosages for structural changes in the mammalian brain.

The relationship between the acoustic intensity and the time duration of exposure, for a single pulse, necessary to produce a threshold lesion in the cat brain was studied. Focused ultrasound of 1, 3, and 4 MHz was employed with intensities ranging from 10 • to 2X10 • W/cm a with the corresponding pulse durations from 7 to 2XI0 • sec, respectively. Three types of lesions were observed attending three regions. At the lower intensities and long time durations of exposure, the lesion is produced by a thermal mechanism. At the highest intensities and shortest ime durations, cavitation is believed to be the mechanism responsible for the sometimes randomly appearing lesions. At intermediate dosages, the lesions are formed by a mechanical mechanism which is thus far not well understood. These results exhibit good agreement with that of other investigators on both the cat and the rat brain.