Empirical model description of photon path length for differential path length spectroscopy: Combined effect of scattering and absorption

Differential path length spectroscopy (DPS) is a method of reflectance spectroscopy that utilizes a specialized fiber geometry to make the photon path length (τ) insensitive to variations in tissue optical properties over a wide range of absorption (μ a) and total scattering (μ s) coefficients, which are common within the ultraviolet/visible (UV/VIS) wavelength region. This study extends the description of τ to larger μ a and smaller μ s values, optical properties that are representative of the near-infrared region (NIR), a region where the DPS path length may be dependent on both coefficients. This study presents a novel empirical relationship between τ and the combined effect of both μ a (range: 0.1-12 mm -1) and μ s (range: 1.5-42 mm -1), anisotropy of 0.8, and is applicable to DPS probes containing a wide range of fiber diameters (range: 100-1000 μm). The results indicate that the simple empirical formula, including only one fitted parameter, is capable of accurately predicting τover a wide range (r= 0.985; range: 80-940 μm) and predictions are not biased versus μ a or μ a. This novel relationship is applicable to analysis of DPS measurements of tissue in both the UV/VIS and NIR wavelength regions and may provide information about the wavelength-specific tissue volume optically sampled during measurement.

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