Shock wave emission from a cloud of bubbles

Shock wave emission from a hemispherical cloud of bubbles, generated by short pulses of high-intensity focused ultrasound, was investigated by acoustic measurements and ultra-high-speed photography with framing rates of up to 200 million frames s(-1) and exposure time of 5 ns. The maximum amplitude and the duration of the shock wave emitted during bubble cloud rebound increase with increasing the maximum radius of the cloud and, at equal maximum radius, are larger than the corresponding values obtained in the case of individual spherical bubbles. At a maximum radius of about 1300 mu m, the shock pressure for a bubble cloud is a factor of three larger than the value measured in the case of individual spherical bubbles. In both cases, however, the shock pressure decays proportionally to r(-1) with increasing distance r from the emission centre. The main damage mechanism of bubble clouds collapsing near a boundary material is the high pressure generated inside the cloud at the minimum volume. For a bubble cloud with a maximum radius of 760 mu m, the maximum pressure at the bubble cloud, and thus the pressure acting on a nearby boundary material, is about 1.5 GPa. On the other hand, shock wave-induced mechanical effects on biological tissue are restricted to very small dimensions on a cellular or sub-cellular level. We discuss the implications of the results for the therapeutic applications of high-intensity focused ultrasound such as lithotripsy and histotripsy.