Thermally induced optical property changes in myocardium at 1.06 microns.

Light in the visible and near-infrared region is diffusely scattered in tissues by macromolecules. It was therefore hypothesized that tissue coagulation caused by high-power continuous wave laser irradiation might significantly alter tissue optical properties, resulting in a redistribution of laser energy during the laser ablation process. Infrared transmittance studies confirmed the hypothesis by demonstrating an irreversible decrease in light transmittance (45%) during heating of a 0.75 mm thick slice of tissue. Absorption and scattering coefficients were then determined from transmittance and reflectance measurements on thin slices of raw and coagulated myocardium irradiated with a Nd:YAG laser (1.06 microns). The scattering coefficient was found to increase fourfold (0.427 mm-1----1.74 mm-1) during tissue coagulation, while the absorption coefficient remained relatively unchanged (0.044 mm-1----0.051 mm-1). Calculations indicate that the coagulation-induced changes in tissue optical properties substantially increase surface back-scattering and reduce tissue penetration.