THERMALLY INDUCED OPTICAL PROPERTY CHANGES IN MYOCARDIUM AT 1.06 MU-M

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 μm). 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. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company