Lithotripters can use a number of methods of generating shock
waves. Our research is an electrohydraulic lithotripter which
uses a spark source placed at the first focus (F1) of an ellipsoidal
reflector, shown in Fig. 1. The ellipsoid reflects the energy
and focuses it at the second focus (F2) of the ellipse where the
kidney stone should be positioned. Figure 1 shows the ray paths
emanating from the spark plug and focusing on F2. Our research
machine is patterned after the Dornier HM3 lithotripter which
was the original clinical lithotripter. The first HM3 was installed
in the United States in 1984 and many HM3s are still in wide use.
Figure 2 shows
a typical waveform recorded at the focus of a lithotripter using
a PVDF membrane hydrophone. The waveform consists of a compressive
spike with peak amplitude of about 40 MPa and duration 1 µs.
The front of the waveform is shocked and the measured rise time
of 30 ns is limited by the hydrophone. Theoretically it should
be less than 1 ns. The spike is followed by a tensile tail that
lasts more than 3 µs and has a peak negative pressure of
about 10 MPa. It is the tensile tail that is responsible for the
cavitation generated by a lithotripter.
Figure 3 shows the distribution of the peak positive pressure
in the field of an HM3 based on calculations using the KZK equation.
Contours for 10, 20, and 30 MPa are shown. The geometrical focus
is at 12.8 cm. Note that the maximum in the peak positive pressure
actually occurs just beyond the focus because of self-refraction
associated with nonlinear propagation. Figure 4 shows the peak
negative pressure which achieves a maximum in front of the geometrical
focus. 
Click on the image to view a short animation of the calculated
pressure field of the HM3. The radius has been normalised to the
77.5 mm, the axial distance by 128 mm and the pressure by 6 MPa.
This page is maintained by robinc@bu.edu
Last Updated July 1999