Development of a Self-Focusing Multi-Spark Shock Wave Generator for Lithotripsy

In this thesis, a self-focusing multi-spark (SFMS) shock wave generator is developed to provide flexibility in controlling the beam size and shape in an electrohydraulic shock wave lithotripter. Such a device will allow us to better distribute the shock wave energy to match the anatomic features in the urinary collecting system or respiration movement of the stone to improve stone fragmentation efficiency while reducing tissue injury. In this study, we present the design, fabrication and evaluation of the multi-pin titanium electrodes by 3D printing, integration of the SFMS shock wave generator, acoustic field characterization based on hydrophone measurements, and stone fragmentation tests using stone phantoms confined within a polyurethane rubber holder of elliptical shape. The effects of pin number on pressure output and electrode degradation are evaluated in order to produce a consistent pressure waveform with increased electrode lifespan.

Experiments were conducted using two transducer configurations: case 1 (axisymmetric) – with all transducers connected, and case 2 (non-axisymmetric) - with transducers on the two side sections disconnected. A fiber optic probe hydrophone was mounted on a 3D computer controlled translational stage to perform acoustic field characterization. Stone fragmentation test was conducted with stone phantoms placed inside a stone holder made of soft tissue mimicking material polyurethane rubber to evaluate the stone comminution efficiency. To assess the effect of multi-pin design on electrode damage and output pressure variations, transducer lifespan experiments were performed. Specifically, individual transducers used in the SFMS, but with different pin numbers (1, 10, and 45) were fired up to 2000 shocks.

The FOPH measurement results show that the SFMS can generate an axisymmetric focal zone with the -6 dB focal width of 16 mm, or a non-axisymmetric focal zone with the -6 dB focal width elongated to 28 mm in the side-section direction, while in the perpendicular direction the -6 dB focal width is 16 mm, accompanying a peak positive pressure of 39.0±1.8 MPa at an input electric pulse energy of 600 J. Such a focal zone has stone comminution efficiency of 37.68±5.11% and 69.58±8.29% for stone fragment smaller than 2.0 mm and 2.8 mm, respectively, after 150 pulses. After more than 2000 pulses, the pressure output drops only by 2%, and lifespan of the transducer (defined by a peak output pressure drop less than 10%) is expected to exceed 6000 pulses. Altogether, we have demonstrated that the SFMS can generate an elongated non-axisymmetric focal zone with higher stone comminution efficiency, and has significantly increased electrode lifespan. The SFMS shock wave generator may provide a flexible and versatile design to achieve accurate, stable, and safe lithotripsy for kidney stone treatment.