We attempted to selectively determine the absorption coefficient (μa) of bottom regions in two- and four-layered models
with time-domain near infrared measurement. The difference curve in the time-resolved reflectance between a target and
a reference medium was divided into segments, and a slope of each segment was calculated to determine
depth-dependent μa (μaseg). The deviation of μaseg in later time segments from the real μa of the bottom layer was
μaseg in an earlier time segment to that in a later one. Using this function, we could
determine μa in the bottom layer for various target media with different conditions.
We try a new approach with near-IR time-resolved spectroscopy, to separate optical signals originated in the upper layer from those in the lower layer and to selectively determine the absorption coefficient (µa) of each layer in a two-layered turbid medium. The difference curve in the temporal profiles of light attenuation between a target and a reference medium is divided into segments along the time axis, and a slope of each segment is calculated to determine the depth-dependent µa. The depth-dependent µa values are estimated under various conditions in which µa and the reduced scattering coefficient (µ) of each layer are changed with a Monte Carlo simulation and in phantom experiments. Temporal variation of them represents the difference in µa between two layers when µ of a reference is the same as that of the upper layer of the target. The discrepancies between calculated µa and the real µa depend on the ratio of the real µa of the upper layer to that of the lower layer, and our approach enables us to estimate the ratio of µa between the two layers. These results suggest the potential that µa of the lower layer can be determined by our procedure.
Using both experimental and theoretical methods, we examine the contribution of different parts of the head to near-IR (NIR) signal. Time-resolved spectroscopy is employed to measure the mean optical path length (PL), and the absorption (µa) and reduced scattering (µ) coefficients in multiple positions of the human head. Monte Carlo simulations are performed on four-layered head models based on an individual magnetic resonance imaging (MRI) scan to determine µa and µ in each layer of the head by solving inverse problems, and to estimate the partial path length in the brain (p-PL) and the spatial sensitivity to regions in the brain at the source-detector separation of 30 mm. The PL is closely related to the thickness of the scalp, but not to that of other layers of the head. The p-PL is negatively related to the PL and its contribution ratio to the PL is 5 to 22% when the differential path length factor is 6. Most of the signal attributed to the brain comes from the upper 1 to 2 mm of the cortical surface. These results indicate that the NIR signal is very sensitive to hemodynamic changes associated with functional brain activation in the case that changes in the extracerebral tissue are ignorable.
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