Excimer laser ablation of soft tissue: a study of the content of rapidly expanding and collapsing bubbles

Both holmium (λ=2.09 μm) and excimer (λ=308 nm) lasers are used for ablation of tissue. In a previous study, excimer laser ablation of aorta produced rapidly expanding and collapsing vapor bubbles. To investigate whether the excimer-induced bubble is caused by vaporization of (tissue) water, the threshold fluence for bubble formation at a bare fiber tip in water and on tissue was compared between the excimer laser (pulse length 115 ns) and the Q-switched and free-running holmium lasers (pulse length 1 μs and 250 μs, respectively). To induce bubble formation by excimer laser light in water, the absorber Oxybuprocaine-hydrochloride (OBP-HCl) was added to the water. Fast flash photography was used to measure the threshold fluence as a function of the water temperature (6-90°C) at ambient pressure and as a function of the boiling temperature of water (100-184°C) at augmented environmental pressures (1-11 bar). For both the holmium and excimer laser, the threshold fluence for bubble formation decreased proportionally with increasing water temperature (correlation coefficient r=0.95-0.99). Correspondingly, the threshold fluence for bubble formation increased with increasing boiling temperature of water. A similar correlation was found for the excimer laser-induced bubble formation on tissue. Furthermore, for both the holmium and the excimer laser, the threshold fluence for water vaporization was much lower than theoretically predicted values. It is concluded that the principal content of the large (up to 3 mm), short lived (<300 μs) bubble induced by a 308 nm excimer laser pulse is tissue water vapor, rather than a mixture of compounds generated by pyrolysis of tissue proteins. Consequently, explosive vaporization of tissue water may contribute to the excimer laser ablation of aortic tissue