Effect of the dynamic optical properties of water on midinfrared laser ablation.

Water is a primary tissue chromophore in the midinfrared. Absorption of midinfrared radiation by water is a function of temperature and pressure, both of which rise rapidly during an ablative laser pulse. Data show that the absorption coefficient of water changes, by as much as two orders of magnitude, during a high-irradiance laser pulse. We present an ablation model that is fundamentally based upon Beer's Law but considers changes in the absorption coefficient. The model predicts that Er:YAG laser cuts will be deeper than Er:YSGG laser cuts; an opposite prediction would be made based upon the static, low-intensity absorption coefficient of water. The results of in vitro ablation of skin confirm that the dynamic optical properties of tissue need to be considered in the understanding of laser ablation as well as the design, manufacture, use, and regulation of clinical laser systems.