Abstract:

In an electrohydraulic lithotripter (device to treat kidney stones), an underwater spark at the near focus of a rigid ellipsoidal reflector produces acoustic cavitation at the far focus. A reflector made of pressure release foam, however, dramatically reduces cavitation with little change in acoustic pressure amplitude (~ +35 MPa, ~-10 MPa). The pressure waveform at the focus of a rigid reflector is a positive spike followed by a negative tail. The negative tail sets bubbles into a growth-and-collapse cycle hundreds of ms long. With a pressure release reflector, a negative phase precedes a positive spike. Bubble growth induced by the negative phase is quickly (<10(mu)s) snuffed by the spike, and collapse is weak. Cavitation was measured by (1) listening for bubble collapse and (2) recording bubble-induced pits on aluminum foil placed along the reflector axis. Pit size was used as an experimental measure of cavitation intensity. Intensity was reduced 3--10 times by the pressure release reflector. Results are in agreement with calculations from the Gilmore equation. Comparing the bioeffects of the two reflectors provides a way to assess cavitation's role in lithotripsy. [Work supported by ONR, ARL:UT IR & D, and NIH.]