Partial vaporization model for pulsed mid-infrared laser ablation of water

Previous studies have shown that during pulsed infrared laser ablation of tissue, rapidly expanding and collapsing water vapor bubbles are formed. These bubbles are held responsible for inducing collateral mechanical tissue damage. In this study, a theoretical analysis predicting the volume of vaporized water is correlated with measured bubble volumes in water. Laser induced bubble formation was documented using a time-resolved flash photography setup. Evidence for a novel ablation model (partial vaporization) is presented in the form of a picture of small bubblets that form at the very beginning of the vaporization process. The dimensions of the rapidly expanding and collapsing bubble (diameter up to 2.5 mm) induced by Q-switched (500 ns) holmium (2.12 μm) laser pulses, delivered in water via 200, 320, or 400 μm fibers have been measured. The volume of the bubble increases with increasing pulse energy. The partial vaporization theory agreed well with the experimental data. It is concluded that the partial vaporization model may be useful in developing laser delivery strategies that lead to a reduction in bubble volume and hence may lead to reduction in laser-induced mechanical damage to adjacent tissue.