The propagation of light energy in tissues is an important problem in phototherapy, especially with the increased use of lasers as light sources. Often a slight difference in delivered energy separates a useless, efficacious, or disastrous treatment. Methods are presented for experimental characterization of the optical properties of a tissue and computational prediction of the distribution of light energy within a tissue. A standard integrating sphere spectrophotometer measured the total transmission, Tt , total reflectance, Rt , and the on-axis transmission, Ta , for incident collimated light that propagated through the dermis of albino mouse skin, over the visible spectrum. The diffusion approximation solution to the one-dimensional (1-D) optical transport equation computed the expected Tt and Rt for different combinations of absorbance, k , scattering, s , and anisotropy, g , and by iterative comparison of the measured and computed Tt and Rt values converged to the intrinsic tissue parameters. For example, mouse dermis presented optical parameters of 2.8 cm - 1 , 239 cm -1 , and 0.74 for k , s , and g , respectively, at 488 nm wavelength. These values were used in the model to simulate the optical propagation of the 488 nm line of an argon laser through mouse skin in vivo. A 1-D Green's function thermal diffusion model computed the temperature distribution within the tissue at different times during laser irradiation. In vitro experiments showed that the threshold temperature range for coagulation was 60-70°, and the kinetics were first order, with a temperature-dependent rate constant that obeyed an Arrhenius relation (molar entropy 276 cal/mol°K, molar enthalpy 102 kcal/mol). The model simulation agreed with the corresponding in vivo experiment that a 2 s pulse at 55 W/cm 2 irradiance will achieve coagulation of the skin.
S. L. Jacques, S. A. Prahl, "Modeling optical and thermal distributions in tissue during laser irradiation," Lasers Surg. Med.,6, 494-503 (1987).