Modeling optical and thermal distributions in tissue during laser irradiation

✍ Scribed by Steven L. Jacques; Scott A. Prahl

• Publisher: John Wiley and Sons
• Year: 1987
• Tongue: English
• Weight: 805 KB
• Volume: 6
• Category: Article
• ISSN: 0196-8092
• DOI: 10.1002/lsm.1900060604

✦ Synopsis

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. T,, total reflectance, R,, and the on-axis transmission, T,, for incident collimated light that propagated through the dermis of albino mouse skin, over the visible spectrum. The diffusion approximation solution to the onedimensional (l-D) optical transport equation computed the expected & and R, for different combinations of absorbance, k, scattering, s, and anisotropy, g, and by iterative comparison of the measured and computed & and R, values converged to the intrinsic tissue parameters. For example, mouse dermis presented optical parameters of 2.8 cm-', 239 cm-', 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 l-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" C. and the kinetics were first order, with a temperature-dependent rate constant that obeyed an Arrhenius relation (molar entropy 276 callmol-OK, molar enthalpy 102 kcal/mol). The model simulation agreed with the corresponding in vivo experiment that a 2-s pulse at 55 W/cm2 itradiance will achieve coagulation of the skin.