Spectroscopy and characterization of turbid media within the diffusion and P(3) approximations

Noninvasive measurement of tissue optical properties has become an area of increasing interest in recent years. Because such measurements have the potential to provide a wide range of clinically useful information, techniques for determining tissue optical properties from straightforward measurements are of considerable importance.; In this thesis, the diffusion approximation to the radiative transport equation is applied to radially-resolved measurements of the reflectance emitted from highly scattering media interrogated by a normally-incident pencil beam. Solutions to the photo-diffusion equation are considered which express the reflectance as a function of the tissue absorption and transport scattering coefficients. The expressions are then fitted to measured reflectance data in order to extract these optical properties as fitting Parameters. The expressions am evaluated through a series of Monte Carlo experiments.; The design and testing of a steady-state reflectance spectrometer is described, and it is demonstrated that the diffusion-theory solutions can be used to determine absorption and scattering coefficients of turbid media from experimental reflectance data to accuracies of 10% or better when medium optical properties are within the regime of validity of the diffusion approximation. These techniques are applied to studies of tissue-simulating phantoms containing human red blood cells and to measurements of the hemoglobin oxygenation status of subcutaneous rodent mammary tumors.; Optical diffusion theory is only valid when the tissue transport scattering coefficient is much greater than the absorption coefficient. In cases where absorption and scattering are comparable in magnitude, new theoretical models are needed for accurate quantitation of optical properties. The P₃ approximation to the radiative transport equation is used to develop such a model. Approximate P₃ boundary conditions for semi-infinite media are derived, and a simple model of optical beams suitable for small source-detector separations is proposed. It is demonstrated that the resulting reflectance expressions can be used to accurately extract optical properties from experimental measurements of media with transport albedos as small as 0.59, a value which is typical of tissue in the visible region of the spectrum. The potential for reflectance spectroscopy in the visible to provide information about mitochondrial cytochrome oxidation status is assessed experimentally.; Finally, a steady-state technique for localizing buried sources of limunesence is developed, and it is shown that the method determines the depth of a buried spherical fluorescent bulb to ±1 mm accuracy for source depths as large as 40 mm. A sensitivity analysis is performed to investigate the stability of the technique with respect to errors in initial estimates of the medium optical properties. The corrupting effects of nonspecifically localized lumiphore are considered theoretically, and limits are placed on the effective optical contrast required for practical implementation of the technique.