Performance of an electrohydraulic shock wave lithotripter measured with thin film PVDF transducers.

Abstract

Measurements of the operation and performance of a research lithotripter have been carried out using a new thin film polyvinylidene fluoride (PVDF) pressure transducer. The research lithotripter is modeled after a commercial electrohydraulic machine. Experimental techniques to match nanosecond-response transducers to digital sampling oscilloscopes are described. Shock rise times at F2 of about 45 ns are measured. Techniques have been developed for characterizing lithotripter focal properties by a combination of remote measurement and calculation to minimize damage to transducers. The results of numerical calculations of focusing shock waves in water by the method of shock dynamics are presented. The measurements show that the strength of shock waves in electrohydraulic lithotripters varies from shot to shot by as much as 40%, even though the discharge current of the shock generator is repeatable. Thus every shot in experiments carried out with these machines should be monitored with a reference transducer. It is suggested that the variability is due to the large sensitivity of ellipsoid optics to small changes of the symmetry of the spark discharge. The effects of tissue phantoms on focusing shock waves are reported.