Dynamics of temperature dependent optical properties of tissue: dependence on thermally induced alteration

Thermal damage in heated bovine myocardial tissue is assessed from measured changes in total reflection and transmission of light. Mathematical expressions, based on random walk analysis of light propagation within tissue slabs, are used to relate the diffuse reflection and transmittance to the absorption coefficient, μ _a, and effective scattering coefficient, μ´ _s for samples of myocardial tissue which were subjected to rapid step changes in temperature. Time-dependent changes in μ´ _s indicate two processes, one with a fast and temperature-dependent rate the other with a slow and apparently temperature-independent rate. For final temperatures above 56.8°C and for the first 500 s after the temperature change, the optical parameters are well fit by exponential forms that exhibit temperature-dependent time constants as predicted by Arrhenius reaction rate theory of thermal damage. The scattering changes are associated with an apparent activation energy, ΔE, of 162 kJ/mole and a frequency constant, A, of 3×10 ²³ s ^-1. This method provides a means for estimating optical coefficients which are needed to assess laser tissue dosimetry.