Thermally induced optical property changes in myocardium at 1.06 μm

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Abstract
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.