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Physical mechanisms of pulsed infrared laser ablation of biological tissues

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Time-resolved measurement of the stress transients generated by pulsed infrared irradiation and ablation of tissue has demonstrated that these processes are governed primarily by photothermal processes. For ablation of porcine dermis at 2.79 micrometer (Q-sw Er:YSGG) and 10.6 micrometer (CO 2), the onset of material removal has been shown to be delayed with respect to irradiation and the magnitude of the generated stress transients are consistent with a model for explosive material removal. Upon consideration of the threshold radiant exposure for ablation, it appears that the mechanism and dynamics of these processes are controlled by explosive boiling as the tissue water is likely to be significantly superheated. To examine this issue further, we employed time- resolved optical interferometry to measure the surface displacement generated by Q-sw Er:YSGG laser irradiation of pure water for radiant exposures below the ablation threshold. This was done to directly measure the dynamic thermal expansion and interphase mass transfer generated by pulsed laser heating. These results are compared to a model which computes the dynamic thermal field within a semi-infinite pool of water undergoing pulsed irradiation while subject to a surface heat flux condition given by kinetic theory. We find that the measured mass fluxes exceed that predicted by simple kinetic theory arguments. The implications of the experimental and model results to pulsed laser ablation of tissue are discussed. ©2003 Copyright SPIE - The International Society for Optical Engineering.

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