Spectral response of optical fiber probe with closely spaced fibers

Background: Optical fiber probe spectroscopy can characterize the blood content, hemoglobin oxygen saturation, water content, and scattering properties of a tissue. A narrow probe using closely spaced fibers can access and characterize a local tissue site, but analysis requires the proper light transport theory. Methods: Monte Carlo simulations of photon transport specified the response of a two-fiber probe as a function of optical properties in a homogeneous tissue. The simulations used the dimensions of a commercial fiber probe (400-micron-diameter fibers separated by 80-microns of cladding) to calculate the response to a range of 20 absorption and 20 reduced scattering values. The 400 simulations yielded an analysis grid (lookup table) to interpolate the probe response to any given pair of absorption and scattering properties. Results: The probe in contact with tissue is not sensitive to low absorption but sensitive to scattering, as occurs for red to near-infrared spectra. The probe is sensitive to both absorption and scattering for shorter visible spectra (purple-orange). The non-contact probe held above the tissue delivers light to/from a spot on the tissue and fails to collect light that spreads laterally to escape outside the collection spot. Such partial collection can distort the spectra. Conclusions: Optical fiber spectroscopy using closely spaced fibers requires proper calibration. An analysis subroutine is provided for analysis of a two-fiber probe with the dimensions of a commercial probe (Ocean Insight), but the method can be applied to any probe design. A closely spaced fiber probe can document blood in the shorter visible wavelengths, but has difficulty detecting red and near-infra-red absorption. Hence detection of hydration is difficult. The strength of the closely spaced fiber probe is detecting scattering that depends on tissue structure at the micron to sub-micron scale.