Performance evaluation and design improvement of electromagnetic shock wave lithotripters

This work aims to improve the design and performance of electromagnetic (EM) shock wave lithotripters. To achieve this goal, we have taken a progressive approach to evaluate the effect of beam size on stone comminution in shock wave lithotripsy (SWL), to better understand the difference between electrohydraulic (EH) and EM lithotripters, and to modify the design of EM sources for improved stone comminution in SWL. First, the effect of beam size on stone comminution was investigated by using the original and modified reflectors in an HM-3 lithotripter. The physical characteristics of the acoustic fields, cavitation activities, and stone fragmentation in vitro produced by the two reflector configurations of significantly different beam size were compared. The results suggest that a broad beam size can increase stone comminution efficiency when fragments are spread out or moving due to respiratory motion in a large area during SWL. Second, the original Dormer HM-3 lithotripter (EH) and a Siemens Modularis lithotripter (EM) were compared, including the characteristics of acoustic fields and stone fragmentation both in vitro and in vivo. The results show that at representative output settings (20 kV for the HM-3 and E4 for the Modularis) stone fragmentation in vitro produced by both lithotripters was similar in a finger cot holder with ideal stone localization. However, the HM-3 was found to produce better stone comminution than the Modularis both in vivo and in a membrane holder, which allows the residual fragments to spread laterally. These results indicate that several critical physical properties of the acoustic field may dominate the performance of stone comminution in SWL. Third, a pilot study was carried out, in which the acoustic lens was modified based on an in situ pulse superposition technique to improve the pressure waveform profile while enlarging simultaneously the lithotripter beam size. Using this modified lens design, the 2nd compressive component in the pressure waveform of a typical EM lithotripter can be significantly reduced, while the beam size significantly enlarged under comparable total derived acoustic pulse energy. Finally, stone fragmentation was evaluated both in the finger cot and membrane holders, and the modified lens was found to produce significantly higher comminution efficiency than the original lens. Altogether, these findings provide preliminary encouraging data to support the notion that appropriate modification in the design of the acoustic lens can improve the performance of an EM shock wave lithotripter.