Improvement of stone fragmentation during shock-wave lithotripsy using a combined EH/PEAA shock-wave generator-in vitro experiments.

To control the collapse of cavitation bubbles induced during shock-wave lithotripsy (SWL), a piezoelectric annular array (PEAA) shock-wave generator was fabricated and combined with an experimental electrohydraulic (EH) shock-wave lithotripter with a truncated HM-3 reflector. The PEAA generator consists of eight individual transducers of 200-kHz resonant frequency. At a discharge voltage of 15 kV, the PEAA generator produces a shock wave with a peak positive pressure of 8.2 MPa, a positive half cycle duration of 2.9 micros, and a -6-dB beam width of 5 mm. The trigger of the PEAA generator was controlled via fiberoptic link with reference to the spark discharge of the EH generator. Hence, the PEAA-generated shock wave could be used to interact with cavitation bubbles induced by the EH source at various stages of their oscillation. The duration of bubble oscillation during SWL was monitored by a 2.25-MHz focused hydrophone, and this information was used to control the release timing of the PEAA generator. Stone fragmentation tests in vitro were carried out, and demonstrated that stone comminution could be significantly enhanced when the shock wave-bubble interaction occurred during the collapsing phase of the bubbles. A maximum increment of 60% to 80% in stone fragmentation was achieved when the PEAA-generated shock wave arrives near the collapse of the bubbles. Under these conditions, much intensified collapse of the bubbles near the surface of the stone, with strong secondary shock-wave emission and increased stress concentration at the impact site of the solid boundary, was observed using high-speed shadowgraph and photoelastic imaging.