Comparison of electrohydraulic lithotripters with rigid and pressure-release ellipsoidal reflectors. II. Cavitation fields

Dramatically different cavitation was produced by two separate acoustic pul ses that had different shapes but similar duration, frequency content, and peak positive and negative pressure. Both pulses were produced by a Dornier HM-3 style lithotripter: one pulse when the ellipsoidal reflector was rigi d, the other when the reflector was pressure release. The cavitation, or bu bble action, generated by the conventional rigid-reflector pulse was nearly 50 times longer lived and 3-13 times stronger than that produced by the pr essure-release-reflector pulse. Cavitation durations measured by passive ac oustic detection and high-speed video agreed with calculations based on the Gilmore equation. Cavitation intensity, or destructive potential, was judg ed (1) experimentally by the size of pits in aluminum foil detectors and (2 ) numerically by the calculated amplitude of the shock wave emitted by a co llapsing bubble. The results indicate that the trailing positive spike in t he pressure-release-reflector waveform stifles bubble growth and mitigates the collapse, whereas the trough after the positive spike in the rigid-refl ector waveform triggers inertially driven growth and collapse. The two refl ectors therefore provide a tool to compare effects in weakly and strongly c avitating fields and thereby help assess cavitation's role in lithotripsy. (C) 1999 Acoustical Society, of America. [S0001-4966(49)03708-X].