THERMODYNAMIC RESPONSE OF SOFT BIOLOGICAL TISSUES TO PULSED INFRARED-LASER IRRADIATION

The physical mechanisms that achieve tissue removal through the delive ry of short pulses of high-intensity infrared laser radiation, in a pr ocess known as laser ablation, remain obscure. The thermodynamic respo nse of biological tissue to pulsed infrared laser irradiation was inve stigated by measuring and analyzing the stress transients generated by Q-sw Er:YSGG (lambda = 2.79 mu m) and TEA CO2 (lambda = 10.6 mu m) la ser irradiation of porcine dermis using thin-film piezoelectric transd ucers. For radiant exposures that do not produce material removal, the stress transients are consistent with thermal expansion of the tissue samples. The temporal structure of the stress transients generated at the threshold radiant exposure for ablation indicates that the onset of material removal is delayed with respect to irradiation. Once mater ial removal is achieved, the magnitude of the peak compressive stress and its variation with radiant exposure are consistent with a model th at considers this process as an explosive event occurring after the la ser pulse. This mechanism is different from ArF- and KrF-excimer laser ablation where absorption of ultraviolet radiation by the collagenous tissue matrix leads to tissue decomposition during irradiation and re sults in material removal via rapid surface vaporization. It appears t hat under the conditions examined in this study, explosive boiling of tissue water is the process that mediates the ablation event. This stu dy provides evidence that the dynamics and mechanism of tissue ablatio n processes can be altered by targeting tissue water rather than the t issue structural matrix.