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Diffuse optical imaging (DOI) with near-infrared light can reconstruct the spatial distribution of changes in absorption associated with local functional activation of the brain. Most existing studies utilize optical parameters obtained from ex-vivo measurements for the reconstruction. In consideration of the discrepancy in optical properties between individuals and especially in patients with abnormal conditions in the brain, experimental procedures were designed to quantify the optical parameters of each layer in a four-layered human head model: the scalp, skull, cerebrospinal fluid, and cerebral cortex. A near-infrared spectroscopy system combined four probes that covered 12 different source-to-detector separations (SDS) in the range of 0.215-32.4 mm for multiple depth resolved measurements. Shallower layers were measured with smaller SDS and extracted optical properties were used as initial estimates for later quantification of all four layers using larger SDS. Calibration was performed by measuring spectra of phantoms of known optical parameters and comparing to corresponding simulated spectra using the Monte Carlo method. Feasibility was validated on a three-layered solid phantom with errors in extracted optical coefficients below 15%. Finally, the prefrontal area of the human head was measured and scattering and absorption coefficients of each layer were extracted by iterative spectral fitting to Monte Carlo simulation results. Optical properties extracted from in-vivo measurements fell within reasonable ranges of reported values. In contrast to ex-vivo measurements, the presented procedure enables the reconstruction of distributions of absorption changes in the cortex using subject’s own optical properties, widening the application of functional imaging for example to post-stroke patients.
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