fNIRS (functional near-Infrared spectroscopy) can measure brain activity non-invasively and has advantages such as low cost and portability. While the conventional fNIRS has used laser light, LED light fNIRS is recently becoming common in use. Using LED for fNIRS, equipment can be more inexpensive and more portable. LED light, however, has a wider illumination spectrum than laser light, which may change crosstalk between the calculated concentration change of oxygenated and deoxygenated hemoglobins. The crosstalk is caused by difference in light path length in the head tissues depending on wavelengths used. We conducted Monte Carlo simulations of photon propagation in the tissue layers of head (scalp, skull, CSF, gray matter, and white matter) to estimate the light path length in each layers. Based on the estimated path lengths, the crosstalk in fNIRS using LED light was calculated. Our results showed that LED light more increases the crosstalk than laser light does when certain combinations of wavelengths were adopted. Even in such cases, the crosstalk increased by using LED light can be effectively suppressed by replacing the value of extinction coefficients used in the hemoglobin calculation to their weighted average over illumination spectrum.
Similar to blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI), functional nearinfrared
spectroscopy (fNIRS) observes regional hemodynamic responses associated with neuronal activation. However,
the conventional criteria for detecting true positive fNIRS and fMRI signals appear to be based on different
understandings of cerebral hemodynamics. Considerable numbers of fNIRS studies have ascribed the increase in
oxygenated hemoglobin to a typical sign of functional activation, whereas the corresponding BOLD signal in fMRI
directly correlates with a decrease in deoxygenated hemoglobin. This inconsistency requires solution through the
simultaneous measurements of fNIRS and fMRI. In practice, however, there remain several technical problems
associated with conducting simultaneous measurements with high reproducibility. One issue is the precise spatial
registration of NIRS optodes in MR images. We prepared marker containers of an annular shape that can be coaxially
fixed to the optode. Liquid paraffin with α-tocopheryl acetate, which exhibits a bright contrast in T1-weighted MR
images of human heads, was solidified in each container by adding higher fatty acid. A subject wearing the marker-fixed
optodes at parietal area participated in preliminary fNIRS and fMRI experiments; the subject was instructed to execute
single-sided hand finger tapping. The positions showed that deoxygenated hemoglobin decreases in fNIRS coincided
with the BOLD-positive region in fMRI. The prepared marker is chemically stable and repetitively usable. We believe
that this simple method contributes precision to the co-registration of fNIRS and fMRI.