Quantitative Reflectance Spectrophotometry For The Noninvasive Measurement Of Photosensitizer Concentration In Tissue During Photodynamic Therapy

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Abstract
When light is diffusely reflected from tissue containing a dye, such as a photosensitizer, with a known absorption spectrum, the changes in the reflectance spectrum caused by the presence of the dye can be identified and correlated with the dye concentration. A feasibility study' of this method for the noninvasive determination of the concentration of photosensitizers in tissue found, however, that the changes in reflectance also depended on the optical properties of the tissue. In this study we propose a simple model of light propagation which allows quantitative prediction of the sensitivity of the method. The optical absorption and scattering coefficients required as input for the model are obtained from two ancillary noninvasive measurements: the total diffuse reflectance and the spatial variation of the local diffuse reflectance. Experiments performed using tissue-simulating phantoms suggest that the simple model, when combined with the ancillary measurements, allows absolute calibration of the method to within 20%.

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