The absorption spectra of a chromophore in highly scattering media

Frequency domain spectroscopy provides a quantitative measure of the optical properties, namely µa and µs' coefficient spectra, of multiply scattering, macroscopically homogeneous media or tissue. The diffusion model for photon transport provides the theoretical framework for the analytical expressions of the optical properties. Experimentally, we intensity modulated (60 MHz) a light emitting diode, which emits between 620-700 nm. From data sets of relative phase shifts and average intensity at different source-detector separations, we calculated on the basis of the analytical expressions a wavelength resolved absorption µa (?) and scattering µs' (?) coefficient spectrum. The test material was methylene blue, whose absorption spectrum (maximum 656 nm) closely matches the wavelength profile of the diode source. The multiply scattering, macroscopically homogeneous medium for dissolving the methylene blue was provided by a diluted fat emulsion, Liposyn III. The concentrations ofboth the absorbing and scattering materials were adjusted to correspond to ranges typical of µa and µs tissues. We obtained quantitative agreement between the measured µa (?) the scattering medium and a control solution measured in a spectrophotometer under non-scattering conditions