Why stones break better at slow shockwave rates than at fast rates: in vitro study with a research electrohydraulic lithotripter

Abstract

Background and purpose: Stones break better when the rate of shockwave (SW) delivery is slowed. It has been hypothesized that the greater cavitation accompanying a fast rate shields pulse propagation, thus interfering with the delivery of SW energy to the stone. We tested this idea by correlating waveforms measured at the SW focus with cavitation viewed using high-speed imaging.

Materials and methods: A series of U30 gypsum stones held in a 2-mm mesh basket were exposed to 200 SWs at 30 or 120 SW/min from a research electrohydraulic lithotripter (HM3 clone). Waveforms were collected using a fiberoptic probe hydrophone. High-speed imaging was used to observe cavitation bubbles in the water and at the stone surface.

Results: Stone breakage was significantly better at 30 SW/min than at 120 SW/min. The rate had little effect on SW parameters in the water free field. In the presence of particulates released from stones, the positive pressure of the SW remained unaffected, but the trailing tensile phase of the pulse was significantly reduced at 120 SW/min.

Conclusions: Cavitation bubbles do not persist between SWs. Thus, mature bubbles from one pulse do not interfere with the next pulse, even at 120 SW/min. However, cavitation nuclei carried by fine particles released from stones can persist between pulses. These nuclei have little effect on the compressive wave but seed cavitation under the influence of the tensile wave. Bubble growth draws energy from the negative-pressure phase of the SW, reducing its amplitude. This likely affects the dynamics of cavitation bubble clusters at the stone surface, reducing the effectiveness of bubble action in stone comminution.

Figure 1

Experimental setup. The tip of the fiber optic probe hydrophone was positioned at the focus of the lithotripter in front of a 2 mm mesh basket holding a model stone. Low density polyethylene (LDPE) membrane was used to prevent stone debris (visible in enlargement) from falling into the ellipsoidal reflector.

Figure 2

Stone breakage at 0.5 Hz (30 SW/min) was better than at 2 Hz (120 SW/min).

Figure 3

Cavitation bubbles in the water free field at different firing rates. By inspection, it is clear that the number of bubbles increases as the firing rate of the lithotripter is increased.

Figure 4

Representative waveforms (shot #32 of 200) recorded at 0.5 Hz (30 SW/min) and 2 Hz (120 SW/min) in front of the basket with stone. The leading positive-pressure phase was virtually identical at both rates, but the trailing negative pressure phase was reduced at 2 Hz compared to 0.5Hz.

Figure 5

Cavitation bubbles generated at the surface of a model stone treated at 120 SW/min. These sequential images captured at 120-microsecond steps show that the cavitation cycle lasts only about 600 μs. The first frame (0μs) was recorded when the shock wave had just arrived at the stone, and shows fine particles that were dislodged from the stone by the previous shot. The frame at 600 μs shows implosion of the cavitation cloud at the stone surface, and bubbles are no longer visible in the surrounding water.