Frequency dependence of shock wave foci and stone disintegration

Focused high amplitude ultrasonic shock pulses are used in lithotripsy for the disintegration of concrements. In the focal zone of a shock wave source like that of the Siemens Lithostar with a fundamental center frequency of .16 MHz, higher harmonics were created due to the nonlinear behavior of the propagation medium. The spectral components of the pulses were measured in the focus for different shock parameters σ at therapeutic levels. With a broadband, long-term stable PVDF hydrophone, the lateral and axial pressure distributions are measured up to the 50th harmonic. For focal pressure amplitudes of 49 MPa (σ=2.5) the energy contents above the 10th (20th) harmonic contribute only up to 5% (approx. 1%) to the focal energy and thus to the disintegration efficiency