Optimization of pressure waveform, distribution and sequence in shock wave lithotripsy

This work aims to improve shock wave lithotripsy (SWL) technology by increasing stone comminution efficiency while reducing simultaneously the propensity of tissue injury.{09}First, the mechanism of vascular injury in SWL was investigated. Based on in vitro vessel phantom experiment and theoretical calculation, it was found that SWL-induced large intraluminal bubble expansion may constitute a primary mechanism for the rupture of capillaries and small blood vessels. However, when the large intraluminal bubble expansion is suppressed by inversion of the pressure waveform of the lithotripter shock wave (LSW), rupture of a 200-μm cellulose hollow fiber vessel phantom can be avoided. Based on these experimental observations and theoretical assessment of bubble dynamics using the Gilmore model an in situ pulse superposition technique was developed to reduce tissue injury without compromising stone comminution in SWL. A thin shell ellipsoidal reflector insert was fabricated to fit snugly with the original HM-3 reflector. Using the Hamilton model, the effects of reflector geometry on the pulse profile and sequence of the shock waves were evaluated qualitatively. Guided by this analysis, the design of the reflector insert had been refined to suppress the intraluminal bubble expansion, which was confirmed by high-speed imaging of bubble dynamics both in free field and inside a vessel phantom. The pulse pressure, beam size and stone comminution efficiency of the upgraded reflector were all found to be comparable to those of the original reflector.{09}However, the greatest difference lies in the propensity for tissue injury. At the lithotripter focus, about 30 shocks are needed to cause a rupture of the vessel phantom using the original reflector, but no rupture can be produced after 200 shocks by the upgraded reflector. Overall, the upgraded reflector could significantly reduce the propensity of vessel rupture while maintaining satisfactory stone comminution. Second, to improve stone comminution in SWL a new piezoelectric annular array (PEAA) generator made of 1–3 piezocomposite material was fabricated and retrofitted on a clinical HM-3 lithotripter. The operation of the integrated lithotripter system can be controlled by an automatic program. The shock wave produced by the PEAA generator was used to intensify the collapse of LSW-induced bubbles near the target stone. In vitro experiments have shown that combining the upgraded reflector with the PEAA generator could produce better stone comminution efficiency after 1,500 shock (95.25%) than that produced by the original reflector (81.58%). In animal experiments, a BegoStone phantom was implanted into the renal pelvis of right porcine kidney from the urinary tract and exposed up to 2,000 shocks produced by different lithotripter configurations. Better stone comminution efficiencies can be achieved by using the upgraded reflector and the combined system (91.6% and 93.2%, respectively) than the original HM-3 reflector (87.6%). Meanwhile, the volume percentages of gross injury produced by the upgraded reflector and the combined system (0.92% and 0.71%, respectively) are found to be less than that of the original reflector (1.69%). All together, it has been shown both in vitro and in vivo that optimization of lithotripter pressure waveform, distribution, and sequence can improve stone comminution efficiency and reduce simultaneously the propensity of tissue injury.