A Method to Estimate the Ratio of Absorption Coefficients of Two Wavelengths Using Phase Modulated Near Infrared Light Spectroscopy
Near infrared (MR) spectroscopy provides a very useful tool to monitor the oxygen saturation of living biological tissue non-invasively (1–3). Since the biological tissue has a very high effective scattering factor (µs’), the light travels a relatively long path length making it difficult to compute the absorption coefficient (µa) by usual methods. Phase Modulation Spectroscopy (PMS) is one method to measure the optical path length, and estimate the µ a from a highly scattering media. By measuring the ratio of the µa’s of two different light wavelengths, we can determine the hemoglobin (Hb) oxygen saturation within the tissue (4). A technique using two modulated frequencies at two wavelength has been reported by Sevick et al. (4). This method requires a more complicated system for determining the ratio of µ a . In this report, we present an improved method for obtaining the ratio of the absorption coefficients using a single frequency PMS system.
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