Cavitation-induced saturation of the negative-pressure phase of lithotripter shock pulses

Abstract

The tensile stress imposed by the negative-pressure phase of lithotripter shock pulses can cause cavitation. Bubbles continue to grow after the passage of the acoustic pulse; thus, some of the pulse energy is transformed to the kinetic and potential energy of the liquid surrounding the cavitation bubbles and, therefore, no longer belongs to the acoustic field. One might predict that this energy loss should be more pronounced for strong pulses that produce more cavitation. To investigate this, acoustic pulses were measured at the geometric focus of a Dornier HM-3 electrohydraulic lithotripter (water 39°C, dissolved gas ∼8% saturation) using a fiber-optic probe hydrophone FOPH-500. Measurements showed that, while the amplitude and duration of the leading positive-pressure phase increased dramatically as charging potential was increased from 12 to 24 kV, the trailing negative-pressure phase of the pulse remained unchanged. This stabilization of the negative-pressure phase could be due to cavitation restricting the amplitude of the negative pressure that can be transmitted through the liquid, such that further increase of the amplitude at the source would not increase the negative amplitude at the target but would only result in stronger cavitation along the acoustic path. [Work supported by NIH-DK43881.]