[Focusing water shock waves for lithotripsy by various ellipsoid reflectors]

A spherical shock wave generated in the first focus of an ellipsoid is focused in the second focus after reflection. Shadowgraphs provide an impression of the converging process, while the pressure field ist measured with speciality designed pressure probes having a high temporal and spatial resolution. Two different types of reflectors have been investigated. The first is like a spherical mirror with a short focal length, the second is a deep reflector "surrounding" the generation point. Variation of the parameters, size of the reflector, and incident shock energy reveal their influence on the focusing process and the point of maximal pressure, which in the case of deep ellipsoids can deviate from the geometrical focus. Here, non-linear propagation effects on the long distances to the focus lead to an increase in the size of the focal region. Although these reflectors utilize much more of the primary shock front, their peak pressure of 400 bar is low compared with the "shallow" reflectors with 1200 bar and their small focus. The effect of the deep type on kidney stones is however greater. The desintegration process and the reaction of the generated cavitation bubbles have been studied with a high speed camera. It can be shown that stone splinters do not injure the kidney tissue, but liquid jets generated by oscillating cavitation bubbles lead to tissue damage.