A theoretical model of light propagation in tissue, which accounts for scattering and absorption of light, predicts the spatial profile of the surface intensity of re-emitted light. The model allows for both re-emission of the introduced light and fluorescent light generated by an embedded chromophore either inherent to or introduced in the tissue. Measurement of a series of surface intensity profiles allows reconstruction of the three-dimensional structure of the tissue using inverse analytical techniques. Prototype instrumentation has been developed to capture surface images for analysis. A laser scanning system introduces light into the tissue at a series of sites in the region of interest and the emitted light optically filtered through a dichroic filter and imaged on to a cooled charge coupled detector. Measurements using this instrumentation of fluorescent markers embedded in a highly scattering turbid medium yielded excellent agreement between the reconstructed theoretical prediction and the experimental measurement of these known sites. The techniques are being developed as non-invasive methods to diagnose diseased salivary glands in Sjorgen's syndrome and to identify sentinel nodes for biopsy in breast cancer. Identification of deeper structures within the tissue is possible by extending the instrumentation to capture images in the near infra-red region of the spectrum and by the use of novel infra-red compounds to act as probes localized at desired sites within the tissue.
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