Compact self focusing piezoelectric generator using electrically pre-stressed transducer for strong sound pulses in therapy

New clinical concepts in therapy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of the shell can be increased so that, the total pressure at the focus remains very high. We propose to electrically pre-stress the piezoelectric material by applying a high electric field in the opposite direction of the polarization. To avoid depolarization, this field has to be applied only during a short time just before the emitting pulse which generates the compressive wave. Using this method, it was possible to reach, on a plane piston 20 mm diameter, a 5 MPa pressure value. According to this idea a very compact shock wave generator was designed. The generator made of a 1-3 piezo composite material has a diameter of 120 mm and a focal length of 120 mm too. The maximum pressure and the width of the compressive wave at the focus were respectively 60 MPa and 1.5 μs. The focal zone measured at - 3dB is an ellipsoid 6 mm high in the propagating axis and 3 mm width in the perpendicular direction. The efficacy of this generator was measured as the number of shocks necessary to totally disintegrate plaster balls 15 mm in diameter mimicking the kidney stones. At full power the number of shocks was only 150 which is rather the same number as the one obtained using electrohydraulic machine generally considered as the gold standard. This results show that piezoelectric material may be advantageously used for the manufacturing of shock wave generators.