Multi-wavelength and photon time-of-flight for quantitative constituent measurement in scattering media and tissue

The investigation of the interaction of light with tissue has many applications in the field of diagnostic medicine. The objective of the project was to obtain quantitative measurements of constituents in a medium in which the absorption and scattering optical properties vary. Initially the project was divided into two separate experiments, namely multiwavelength steady state and time resolved measurements. Both methods are applicable to complex media, providing information related to absorption and scattering.The first phase was the study of Near Infrared diffuse reflectance for the determination of pulmonary edema. Non-contact pulmonary edema measurements were made on in vitro isolated perfused animal lungs. Multispectral analysis with Partial Least Squares (PLS) depicted three distinct changes associated with fluid increase: an increase in the O-H absorption, a decrease in the C-H absorption, and a decrease in the spectral baseline or scattering contribution. PLS estimates of relative water content had an R2 of 0.973 with a standard error 10.6 +/- 2.9% relative to the weight of lung.The behavior of light in a highly scattering medium is a complex phenomenon. A method is described using analytical descriptors for the separate determination of the absorption and scattering properties from time resolved photon distributions. Estimates with the analytical descriptors were tested using simulated data generated from the time dependent diffusion equation. Objective selections of the descriptors were made using a Stepwise Multi-Linear Regression technique to independently estimate the absorption and scattering coefficients. In a medium where scattering and absorption vary simultaneously multiple parameters are required for quantitative estimates. Relative standard errors of less than 0.5% were obtained with the analytical descriptors estimates.Estimates using the analytical descriptors were tested experimentally with the use of a photon time-of-flight instrument. Different light collection geometries and their relation to the descriptors were investigated in both transmittance and reflectance configuration. Absorption and scattering estimations of less than 9% error were attained for both collection geometries.Information on both the absorption and scattering properties in highly variable media can assist in quantitative multispectral measurement. A method was developed whereby a multiwavelength scatter correction was made using optical coefficients determined from time resolved distributions. Optical coefficient corrected estimates produced a 70% improvement in the error over non-corrected spectra. The study suggests a practical method to correct and quantitate constituent measurements in variable media