Investigation on cavitation bubble dynamics induced by clinically available Ho:YAG lasers

Abstract

Abstract Background and objective: Endoscopic laser lithotripsy is the preferred technique for minimally invasive destruction of ureteral and kidney stones, and is mostly performed by pulsed holmium:yttrium-aluminum-garnet (Ho:YAG) laser irradiation. The absorbed laser energy heats the water creating a vapor bubble which collapses after the laser pulse, thus producing a shock wave. Part of the laser energy strikes the stone through the vapor bubble and induces thermomechanical material removal. Aim of the present study was to visualize the behavior and the dynamics of the cavitation bubble using a specially developed ultra-short-time illumination system and then to determine important characteristics related to clinically used laser and application parameters for a more detailed investigation in the future. Materials and methods: In accordance with Toepler’s Schlieren technique, in the ultra-short-time-illumination set-up the cavitation bubble which had been induced by Ho:YAG laser irradiation at the fiber end, was illuminated by two Q-switched lasers and the process was imaged in high contrast on a video camera. Cavitation bubbles were induced using different pulse energies (500 mJ/pulse and 2000 mJ/pulse) and fiber core diameters (230 μm and 600 μm) and the bubble dynamics were recorded at different times relative to the Ho:YAG laser pulse. The time-dependent development of the bubble formation was determined from the recordings by measuring the bubble diameter in horizontal and vertical directions, together with the volume and localization of the center of the bubble collapse. Results: The results show that the bubble dynamics can be visualized and studied with both high contrast and high temporal resolution. The bubble volume increases with pulse energy and with fiber diameter. The bubble shape is almost round when a larger fiber core diameter is used, and elliptical when using a fiber of smaller core diameter. Moreover, the center of the resulting bubble is slightly further away from the fiber end and the center of the bubble collapse for a smaller fiber core diameter. Conclusion: The experimental set-up developed gives a better understanding of the bubble dynamics. The experiments indicate that the distance between fiber tip and target surface, as well as the laser parameters used have considerable impact on the cavitation bubble dynamics. Both the bubble dynamics and their influence on the stone fragmentation process require further investigation.