Comparison between an open-cage electrode lithotripter and an encapsulated electrode lithotripter

Abstract

We compare the acoustic and cavitation field of two electrohydraulic lithotripters: one employed an open-cage electrode (OCE) and the other an encapsulated electrode (ECE). Acoustic pressure was measured with a fiberoptic probe hydrophone and cavitation using a passive detector consisting of two confocal transducers. The focused shock waves (FSW) of the two lithotripters were similar in shape and at 20 kV, peak positive pressure (P+) for the OCE was 23.2±4.4 MPa and peak negative pressure (P−) was 9.0±1.5 MPa. For the ECE the peak pressures were 30.8±9.4 and −8.2±1.3 MPa, respectively. However, the direct waves (DW) were different. The OCE-DW was triangular shaped and almost entirely positive pressure, whereas the ECE-DW consisted of numerous cycles and had a peak negative pressure of 1.3 MPa. For both electrodes the passive cavitation measurements exhibited two acoustic emissions characteristic of inertial collapses. For the OCE the first emission was weak (in comparison to the second emission) and was attributed to scattering of the FSW. For the ECE both emissions were of similar amplitude, indicating that the DW generated cavitation bubbles that were forced to a violent collapse by the FSW. The ECE produces two violent cavitation events for every shock wave. [Work supported by HMT and NIH.]