Near-infrared absorption and scattering spectra of tissues in vivo

Tissue inhomogeneity is often neglected in the analysis of the spectral data collected in tissues. We have measured the absorption and reduced scattering spectra of the human forearm in the wavelength range 633-841 nm, and in the range of source-detector separations 0.7-3.3 cm. We found that the layered tissue structure due to the skin and adipose layers may have a significant effect not only on the data collected at different distances, but also on the data at 633 nm, where the optical absorbance is significantly higher than at the longer wavelengths considered. We conclude that the dependence of the optical penetration depth on the absorption and reduced scattering coefficients should be carefully considered in the quantitative analysis of the optical spectra of tissues.