Various shape bubbles generated by Ho:YAG laser irradiation for vascular treatments

Various shape bubbles were generated by changing holmium-yttrium-aluminum- garnet (Ho:YAG) laser irradiation parameters. Intensive pressure waves induced by their bubble collapse were measured. The Ho:YAG laser-induced bubble in water-containing liquid had been reported by many authors regarding its shape and generated collapse pressure. However, controllability of the bubble shape and generated collapse pressure with various irradiation parameters has been still unclear. In our experiments, we changed the core diameter of optical liber (400μm or 600μm), laser pulsewidth (FWHM 100-300μs or 50-120μs, depends on laser output energy), and positions of the optical fiber tip in a sheath. The bubble shapes were observed with the time resolved flashlamp photography. The expansion and contraction rates of the bubble volume were determined by the obtained bubble shapes. The collapse pressure was measured with a small diameter (0,5mm) calibrated hydrophone. The long Ho:YAG laser pulse irradiation made long shape bubble so-called "pear shaped" bubble. This pear shaped bubble generated low collapse pressure comparing to the spherical shape bubble which was generated by the short pulsewidth. Using the constant laser pulse energy, we obtained large volume bubbles with high collapse pressure by the optical fiber of 600μm core diameter. When the optical fiber tip was located in the sheath, the bubble expanded to the lateral direction, and then the high collapse pressure was observed along the lateral direction. Therefore, we could arrange the bubble shape by changing the irradiation parameters. We discussed the proper bubble shape for various intra-vascular applications.