QUANTITATIVE CONSTITUENT MEASUREMENTS IN SCATTERING MEDIA FROM STATISTICAL-ANALYSIS OF PHOTON TIME-OF-FLIGHT DISTRIBUTIONS

The long term goal of this project is to develop practical methods for quantitative measurements of constituents in scattering media. Pulsed femtosecond illumination and time-gated detection provide information about the photon pathlength. The photon time distribution is non-line arly dependent on three major parameters: the absorption cross-section , the scattering cross-section, and the phase function describing the degree of forward scattered light at each scattering event. To direct the construction of simplified instrumentation, we have investigated t he use of analytic descriptors of the time distributions, such as deri vatives and moments. Objective selection of the processing techniques were made using Stepwise Multi-Linear Regression to independently esti mate the absorption and scattering coefficients. The method was tested with simulated time distributions. Results demonstrate that mean time of the trailing edge of the time profile can provide a quantitative e stimation of the absorption coefficient when scattering is held consta nt. The second moment of the time profile provides the best estimator of the scattering coefficient with absorption held constant. When scat tering and absorption vary simultaneously multiple parameters are requ ired for quantitative estimates. The absorption coefficient estimates require the mean fall time, mean rise time, and the first moment of th e distribution with variable scattering. The second moment, the mean r ise time, the mean fall time, and the peak area provide the best estim ates for the scattering coefficient with variable absorption. These re sults suggest that practical instrumentation may be developed which wo uld allow for robust quantification in highly scattering media.