High-intensity, focused ultrasonic fields

Abstract

The use of extracorporeal shock wave lithotripsy (ESWL) for disintegration of body stones has increased considerably during recent years. A worldwide activity in this field is reflected in a growing number of international publications and in the development and manufacturing of several ESWL machines marketed by companies in Germany and France, in particular. Two main types of ESWL systems are prevailing, the spark gap-based and the piezoelectric disk-based systems. This paper is introduced by a brief reconsideration of the features of pressure waves in water produced by an electrical discharge across a spark gap emphasizing the parameters of significance to pressure wave amplitude and time course. Moreover, a comparison is presented of results obtained using various theoretical models—including the KZK equation—for the calculation of pressure waveform parameters, spatial pressure distribution, etc. involving nonlinearity, diffraction, and absorption in the high-intensity focused ultrasonic fields produced by an ellipsoid as well as a spherical cap focusing geometry. Data from the development of an ESWL of the piezoelectric disk type are reported including demands to transducers and to the electrical discharge circuit. Measured pressure waves in various regions of propagation will be presented. Calibration demands to transducers used for finite-amplitude pressure wave detection including linearity, dynamic range, etc. are emphasized. Finally, a critical assessment is given of the most essential pressure wave parameters, their relation to the focal system data, and their significance to stone disintegration efficiency