The measurement of tissue optical properties allows quantitative assessment of the blood perfusion, mixed arterio-venous oxygen saturation, bilirubin content and scattering properties of a tissue. An optical fiber spectrometer offers a convenient rapid measurement of in vivo tissue sites.
Various methods can specify the absorption coefficient (μa [cm-1]) and reduced scattering coefficient (μs' [cm-1]). Such methods require the measurement of 2 experimental parameters which are sufficiently different that they uniquely specify the two unknowns (μa and μs'). For example, measurement of the escaping flux from a tissue at different distances from the source, often called R(r) for reflectance as a function of radial position, is sufficient to specify the optical properties at one wavelength of light. Measurements are done at many wavelengths, and the optical properties are specified at each wavelength measured. ADVANTAGE: This method is able to characterize materials with unknown constituents. DISADVANTAGE: The probe may need to be large to accomodate more than one source-collector separation.
A very convenient approach is to measure the transport of light from a source fiber to a collection fiber, both contacting the tissue, then analyze the entire spectrum in terms of biological constituents, such as blood, oxygen saturation of that blood, porphyrins (bilirubin, PPIX) and tissue scattering. In the case of skin, melanin can be added to the analysis. ADVANTAGE: This method allows measurements of the common constituents of tissue, and the scattering properties, using a library of absorption spectra for water and hemoglobin and a simple description of scattering. The analysis has advantage in overcoming noise since it fits the data with known spectra, essentially using all the information over bands of wavelengths. ...and the probe can be as small as possible, although too close a source-collector spacing decreases the sensitivity to the tissue absorption coefficient. DISADVANTAGE: The method assumes known spectra for constituents. Unknown absorbers will frustrate the method. However, it is often possible to fit the known parameters and identify an unknown constituent by the wavelength dependence of the error between data and the fit.
For example, in our work on reflectance spectra from horse skin during photodynamic therapy (PDT) using injected delta-aminolevulinic acid (ALA) to induce production of protoporphyrin IX (PPIX), we discovered a discernible discrepancy in our spectra which turned out to be the absorption due to elevated PPIX in the skin. Now, we include the PPIX absorption in our fitting.
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