Measurements of the pressure field and in vitro stone fragmentation of a Storz Modulith SLX lithotripter

The Storz Modulith SLX electromagnetic lithotripter can produce peak pressures higher than most other clinical lithotripters. The pressure field of a clinical SLX was measured with a PVDF membrane hydrophone: the peak-positive pressure varied from 10–115 MPa and the peak-negative pressure from −5 to −15 MPa. The focal spot, based on peak positive pressure, was a cigar shaped volume 25 mm long by 5 mm in diameter. The fragmentation performance was determined by treating artificial kidney stones (cylinders, 9.5 mm long and 6.5 mm diameter, made from Ultracal-30 gypsum) with 400 SWs. After treatment, stone remnants greater than 2 mm in diameter were dried and weighed. We found that fragmentation was very sensitive to correct positioning of the stone. Displacement from the focus by 4 mm or more in the horizontal plane led to a statistically significant decrease in fragmentation (p<0.03). In the vertical direction a displacement of more than 10 mm was necessary to produce a statistically significant reduction in fragmentation. We also found that a slower firing rate of 1 Hz was more effective than 2 Hz (p<0.033) indicating that shock waves can be shielded by cavitation bubbles. [Work partially supported by NIH through P01-DK 43881.]