This paper examines non-invasive methods for absolute determination of the hemoglobin content of arterial blood and the water content of skin. Both methods are based on diffuse reflectance spectrophotometry in the near-infrared band (800 - 1600 nm). Separation of blood and background tissue spectra is accomplished by a technique similar to pulse oximetry, with the added feature that the set of measurement wavelengths is chosen to be sensitive to both hemoglobin and water concentration in the blood. Regressions performed on a simulated tissue spectra suggest that {1060, 1160, 1200 and 1320 nm} is an optimal set of wavelengths for measurement of tissue hydration and {1040, 1120, 1140 and 1200 nm} is an optimal set of wavelengths for measurement of hemoglobin content under typical measurement conditions. A simple in vitro tissue phantom whose optical properties can be altered in a controlled manner was developed to test the feasibility of the methods. Measurements were made with a custom-designed NIR spectrophotometer.
The aim of this study was to develop quantitative methods for relating the microstructure of a tissue to the magnitude and wavelength dependence of its scattering coefficient. Two methods, cell counting and spatial frequency analysis, were used to estimate the distribution of sizes of structures imaged by light and electron microscopy. We found that scatterers in the epidermal layer of the skin exhibit a log-normal size distribution, whereas the spatial fluctuations in the index of refraction of dense fibrous tissues, such as the dermis, follow a power law. The correlations in the refractive indices of a variety of tissues exhibit characteristics of a random fractal with a Hurst coefficient between 0.3 and 0.5. Calculated from the measured distributions and volume fractions, the magnitudes of the scattering coefficient and anisotropy parameters of the tissue were found to be within the range 10 less than (mu) s less than 35 mm-1 and 0.7 less than g less than 0.97, depending on wavelength and tissue structure. Our results suggest that analysis of histological images of tissues is a viable method for estimating the optical parameters of tissues and their wavelength dependence.
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