Effect of diffraction wave at the reflector aperture on shock wave lithotripsy

To study the effect of diffraction wave evolved in the shock wave (SW) propagation on the performance of extracorporeal lithotripter, an edge blocker was designed to fit at the outside surface of the ellipsoidal reflector of a Dornier HM-3 lithotripter. With this modification, the contribution of the diffraction wave to the tensile component of SW at the focal region could be reduced significantly (3.2±0.54 μs vs. 5.83±0.56 μs). However, the amplitude and duration of the compressive wave as well as the -6 dB beam width and the amplitude of the tensile wave were almost unchanged. The suppression on bubble cavitation was confirmed using the passive cavitation detection technique. At the lithotripter focus, while 30 SWs can cause rupture of a blood vessel phantom using the HM-3 lithotripter at 20 kV; no rupture could be found after 300 SWs with the edge blocker. On the other hand, after 200 SWs the HM-3 lithotripter at 20 kV can achieve a stone fragmentation of 50.4±2.0% on plaster-of-Paris stone phantom, which is comparable to that of using the edge blocker (46.8±4.1%, p=0.005). Altogether, the diffraction wave is a major reason for the bubble expansion at the lithotripter focus and the modification on it at the lithotripter aperture can produce satisfactory stone comminution while significantly reducing the potential for vessel rupture in shock wave lithotripsy.