Studies of the cavitational effects of clinical ultrasound by sonoluminescence: 3. Cavitation from pulses a few microseconds in length

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Abstract
For pt.2 see ibid., vol.33, p.1249-60 (1988). Sololuminescence can readily be seen when aerated is insonated with continuous wave therapeutic ultrasound at room temperature but is not easily observed when short pulses of diagnostic ultrasound are used. In this work on ultrasound generator, operating in the region of 1 MHz and capable of producing pulses of different length and repetition rate, was used for insonation. The pulse repetition rate of the ultrasound was fixed at 1 kHz since this is characteristic of diagnostic machines, and a series of thresholds for sonoluminescence was obtained for two transducers, one therapeutic and one diagnostic, as the number of cycles in each pulse was varied. Sonoluminescence was observed for pulses of a few cycles, but the ultrasound intensity threshold for onset increased sharply with decreasing pulse length. Under all conditions tested, sonoluminescence was more readily sustained than initiated. At about 20 cycles per pulse, peak negative pressures of about 400 kPa initiated sonoluminescence. These conditions are well within the range of some regimens for Doppler ultrasound and not far removed from the diagnostic situation.