Determination of the shock wave intensity from a laser lithotriptor using a bi-laminar hydrophone

Kidney stones have been treated for some years with extracorporeal shock wave lithotripsy (ESWL) with marked success but this therapy is contraindicated under certain circumstances. In addition, some types of kidney stone do not respond to ESWL and the reason for this is not fully understood although the materials from which the stone are formed is thought to be a crucial factor. An alternative technique uses a laser, which discharges into the urine surrounding the stone and thereby initiates a shock wave. The interaction of this shock wave with the stone is only poorly understood. In order to examine this situation further, artificial stones were created whose material and elastic properties were calculated by ultrasonic methods. The shock wave induced pressures within the stone could then be calculated once the shock wave intensity had been measured. Measuring the shock wave intensity was performed by the use of a bi-laminar hydrophone. The variation in the shock intensity as a function of the laser discharge-hydrophone separation was measured which permitted the intensity at the point of discharge to be estimated. Pressures up to 500 MPa have been measured which is well in excess of the fracture strength of the stones