Abstract
In recent years, it has been demonstrated that using functional near-infrared spectroscopy (fNIRS) channels with short separations to explicitly sample extra-cerebral tissues can provide a significant improvement in the accuracy and reliability of fNIRS measurements. The aim of these short-separation channels is to measure the same superficial hemodynamics observed by standard fNIRS channels while also being insensitive to the brain. We use Monte Carlo simulations of photon transport in anatomically informed multilayer models to determine the optimum source-detector distance for short-separation channels in adult and newborn populations. We present a look-up plot that provides (for an acceptable value of short-separation channel brain sensitivity relative to standard channel brain sensitivity) the optimum short-separation distance. Though values vary across the scalp, when the acceptable ratio of the short-separation channel brain sensitivity to standard channel brain sensitivity is set at 5%, the optimum short-separation distance is 8.4mm in the typical adult and 2.15mm in the term-age infant.
Highlights
Functional near-infrared spectroscopy is an optical technique that uses near-infrared light to monitor cortical functional activation
SS channels are essential for accurate Functional near-infrared spectroscopy (fNIRS) measurements because they enable the extra-cerebral signal contribution to be regressed from standard separation channels
This reduces the chance that extra-cerebral hemodynamics will be falsely interpreted as functional brain activation
Summary
Functional near-infrared spectroscopy (fNIRS) is an optical technique that uses near-infrared light to monitor cortical functional activation. The fluence distribution produced by a source transmitting light into such highly scattering media can be determined using a range of numerical methods, including the finite-element method, (which discretizes the diffusion approximation to the radiative transfer equation into mesh elements3) and more explicit Monte-Carlo simulation approaches.[4,5] By taking the element-wise product of the fluence distribution of the source with the adjoint fluence distribution of the detector modeled as a source, the photon measurement density function (PMDF) can be calculated This distribution gives the probability of the detected near-infrared light traveling through a given region of tissue, which can be thought of as how sensitive the fNIRS measurement will be to changes in chromophore concentration in that region of tissue
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