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Discrimination between scattering and absorption inhomogeneities using time-resolved transmittance imaging

An effective projection imaging technique is proposed that allows the detection of realistic optical inhomogeneities in highly diffusive media and the discrimination between scattering and absorption contributions. The method is based on information derived from time-resolved transmittance measurements. Scattering inhomogeneities are discriminated using maps of the transport scattering coefficient, as provided by best fitting of the diffusion theory to experimental data. Absorption variations are effectively classified by time-gating on the tail of the transmittance pulse. Images were constructed from matrices of time-resolved transmittance measurements performed with a mode-locked dye laser and an electronic chain for time-correlated single photon counting. Data were collected from realistic tissue phantoms containing cylindrical inhomogeneities (1-cm height and 1-cm diameter) embedded in a 5 cm thick turbid slab. The optical coefficients of the inclusions were varied separately ((mu) s' equals 5 divided by 20 cm-1 and (mu) a equals 0.025 divided by 0.4 cm-1) with respect to the background values ((mu) s ' equals 10 cm-1 and (mu) a equals 0.1 cm-1). The overall acquisition time was less than 15 min. In all the experimental conditions considered, the method discriminated efficiently the scattering from the absorption contribution when either of the coefficients or both of them were inhomogeneous.