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.
Qin, Jun, Simmons, W. N., Sankin, Georgy and Zhong, Pei. "Effect of lithotripter focal width on stone comminution in shock wave lithotripsy." Journal of Acoustical Society of America 127, No. 4 (Spring 2010): 2635-2645. doi:10.1121/1.3308409.