EXCIMER-LASER ABLATION OF SOFT-TISSUE - A STUDY OF THE CONTENT OF RAPIDLY EXPANDING AND COLLAPSING BUBBLES

Both holmium (lambda = 2.09 mum) and excimer (lambda = 308 nm) lasers are used for ablation of tissue. In a previous study, excimer laser ab lation of aorta produced rapidly expanding and collapsing vapor bubble s. To investigate whether the excimer-induced bubble is caused by vapo rization of (tissue) water, the threshold fluence for bubble formation at a bare fiber tip in water and on tissue was compared between the e xcimer laser (pulse length 115 ns) and the Q-switched and free-running holmium lasers (pulse length 1 mus and 250 mus, respectively). To ind uce bubble formation by excimer laser light in water, the absorber Oxy buprocaine-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-degrees-C) at ambient pressure and as a f unction of the boiling temperature of water (100-184-degrees-C) at aug mented environmental pressures (1-11 bar). For both the holmium and ex cimer laser, the threshold fluence for bubble formation decreased prop ortionally with increasing water temperature (correlation coefficient r = 0.95-0.99). Correspondingly, the threshold fluence for bubble form ation increased with increasing boiling temperature of water. A simila r correlation was found for the excimer laser-induced bubble formation on tissue. Furthermore, for both the holmium and the excimer laser, t he threshold fluence for water vaporization was much lower than theore tically predicted values. We conclude that the principal content of th e large (up to 3 mm), short lived (<300 mus) bubble induced by a 308 n m excimer laser pulse is tissue water vapor, rather than a mixture of compounds generated by pyrolysis of tissue proteins. Consequently, exp losive vaporization of tissue water may contribute to the excimer lase r ablation of aortic tissue.