A Study On Determination Of Multi-layered Tissue Optical Properties Based On Time-resolved Measurement

The determination of biological tissue optical properties is a question of concern in many laser medicine applications, both diagnosis and therapy. They are believed to be related to important physiologic functions and activities However, the joint influences of absorption and scattering on the photon migration make it difficult to interpret the physical information embedded within measurement. Therefore, for extracting biological physiology and structure, it is fundamental to quantitatively separate the absorption and scattering coefficients. In this paper, a model based on distribution of times of flight(DOTF) of photons, which is measured by a time-correlated single photon counting system, is introduced to retrieve the light absorption and scattering coefficients of multi-layered tissue.The extracting methods for a half-infinite model based on diffusion equation were studied. It's found that the method of featured parameters is fast but instable and the method of curve fitting is perplexed by the zero correction. An improved curve fitting method was applied, which fits the chosen points that most featured, and the precision of extracted optical properties is increased.Monte Carlo simulation is used for the determination of multi-layered tissue optical properties, and it's the non-experimental standard for various approximation of photon propagation in tissue. First, a Monte Carlo model of multi-layered tissue was developed as the forward part of the extracting program to obtain the DOTF. And it was used in iteration of the inverse part to fit measured DOTF. The inverse part results when the mean-squared error of fitting is minimized. The whole program is an unrestraint nonlinear optimization. To accelerate forward part, a condensed Monte Carlo simulation is employed. The database demanded by condensed Monte Carlo was built at a non-absorption tissue. In theory, the results for all absorption and scattering coefficients (if anisotropy factor and refractive index are constant) could be derived by interpolation to the distance variable and curve fitting to the time variable. Also, condensed Monte Carlo simulation avoids the variance between DOTF from two forward Monte Carlo random sampling. Tests with the DOTF from a two-layered solid model at a series of showed that the absorption and scattering coefficients could be retrieved rapidly.