Effect of lithotripter focal width on stone comminution in shock wave lithotripsy

Using a reflector insert, the original HM-3 lithotripter field at 20 kV was altered significantly with the peak positive pressure ( p + ) in the focal plane increased from 49 to 87 MPa while the −6 dB focal width decreased concomitantly from 11 to 4 mm. Using the original reflector, p + of 33 MPa with a −6 dB focal width of 18 mm were measured in a pre-focal plane 15-mm proximal to the lithotripter focus. However, the acoustic pulse energy delivered to a 28-mm diameter area around the lithotripter axis was comparable ( ∼ 120 mJ ) . For all three exposure conditions, similar stone comminution ( ∼ 70 % ) was produced in a mesh holder of 15 mm after 250 shocks. In contrast, stone comminution produced by the modified reflector either in a 15-mm finger cot (45%) or in a 30-mm membrane holder (14%) was significantly reduced from the corresponding values (56% and 26%) produced by the original reflector (no statistically significant differences were observed between the focal and pre-focal planes). These observations suggest that a low-pressure/broad focal width lithotripter field will produce better stone comminution than its counterpart with high-pressure/narrow focal width under clinically relevant in vitro comminution conditions.