Comparison of stone comminution produced by two different reflector configurations in an HM-3 lithotripter
Clinical experience has demonstrated that the performance of the 2nd and 3rd generation shock wave lithotripters in terms of stone comminution has become inferior to the 1st generation Dornier HM-3 lithotripter. A primary change in the design of the newer generation lithotripters is the enlarged aperture of the shock source, leading to a higher peak pressure with a smaller focal area. To evaluate the effect of beam size on stone comminution in the same lithotripter, we developed a reflector insert for the HM-3 that can substantially increase the peak pressure while concomitantly tightening the beam size of the lithotripter field. Using a light spot hydrophone, we have characterized the acoustic fields produced by the two reflector configurations at 20 kV. The peak positive pressure and the -6dB beam size were found to be 87 MPa and 4 mm for the HM-3 with the reflector insert, which are significantly different than the corresponding values (51 MPa and 13 mm) for the original reflector. In contrast, the peak tensile pressure and the total acoustic energy of the lithotripter pulse were found to be similar (-12 MPa and 59 mJ with the reflector insert, -11 MPa and 52 mJ for the original reflector). Stone comminution was evaluated in four different holders in order to determine the effect of lateral spreading of residual fragments on lithotripsy outcome. After 250 shocks, the results of stone comminution in a mesh holder of D = 15 mm (63% vs. 61%) were similar. However, the corresponding values produced by the HM-3 with reflector insert in a finger cot of D = 15 mm (45%) and in a membrane holder of D = 30 mm (14%) were significantly lower than those produced by the original reflector (56% and 26%). Further, the efficiencies of stone comminution in both the mesh holder and the finger cot were much higher than that in the membrane holder. These results suggest that a wide beam size in the original HM-3 may increase stone comminution efficiency when the fragments are spread out to a large area during lithotripsy. In contrast, when the fragments are confined during lithotripsy the beam size may not influence significantly the treatment outcome.
