A survey of the acoustic output of commercial extracorporeal shock wave lithotripters

Abstract

A survey of the pressures and intensities generated by different commercial extracorporeal shock wave (ESWL) lithotripters is reported. The lithotripters included in the survey are the Dornier HM3, Wolf Piezolith 2200 and 2300, Siemens Lithostar, Technomed Sonolith 2000 and 3000, and EDAP LT-01. Measurements were made using a polyvinylidene difluoride (PVdF) membrane hydrophone in water. The zero crossing frequency of one complete cycle of the focused pulse from ESWL equipment is in the range 0.1 to 1 MHz. Spatial-peak temporal-peak positive and negative pressures up to 114 MPa and 10 MPa, respectively, have been measured and the rise times of the positive pressure half cycle at maximum output settings are 30 ns or less. The mean spatial-peak temporal-average intensity of the lithotripters is 5.0 × 10 2 W m −2 when operated at a pulse repetition frequency of 1 Hz. The spatial-peak pulse-average intensity ranges from 6.6 × 10 7 to 1.24 × 10 9 W m −2. The estimated acoustic energy in a single pulse (at the focus) at the maximum output setting of the lithotripters varies from 2.0 × 10 −3 J to about 9.0 × 10 −2 J. The beam area in the focal plane varies by a factor of 100 on different lithotripters and the temporal-peak pressure at the position of the skin at the entry point of the beam by a factor of 30. Measurement problems associated with hydrophone damage and the uncertainties in the hydrophone calibration at high pressures are discussed and an estimate of the total uncertainty in the absolute measurements of the spatial-peak temporal-peak positive pressure is given as ±36%.