The relationship between probe positions of near-infrared spectroscopy instruments and functional areas in the brain is very important for the brain function measurement. Light propagation in a standard brain was calculated to consider the broadening of the probing region caused by the tissue scattering in the NIRS measurements to determine the relationship between the probe positions and the functional areas. The NIRS signal tends to reflect the brain activation in different functional areas and the primary functional area is possibly different from that indicated by the simple projection of the measurement point.
Scalp hemodynamics contaminates the signals from functional near-infrared spectroscopy (fNIRS). Numerous methods have been proposed to reduce this contamination, but no golden standard has yet been established. Here we constructed a multi-layered solid phantom to experimentally validate such methods. This phantom comprises four layers corresponding to epidermides, dermis/skull (upper dynamic layer), cerebrospinal fluid and brain (lower dynamic layer) and the thicknesses of these layers were 0.3, 10, 1, and 50 mm, respectively. The epidermides and cerebrospinal fluid layers were made of polystyrene and an acrylic board, respectively. Both of these dynamic layers were made of epoxy resin. An infrared dye and titanium dioxide were mixed to match their absorption and reduced scattering coefficients (μa and μs’, respectively) with those of biological tissues. The bases of both upper and lower dynamic layers have a slot for laterally sliding a bar that holds an absorber piece. This bar was laterally moved using a programmable stepping motor. The optical properties of dynamic layers were estimated based on the transmittance and reflectance using the Monte Carlo look-up table method. The estimated coefficients for lower and upper dynamic layers approximately coincided with those for biological tissues. We confirmed that the preliminary fNIRS measurement using the fabricated phantom showed that the signals from the brain layer were recovered if those from the dermis layer were completely removed from their mixture, indicating that the phantom is useful for evaluating methods for reducing the contamination of the signals from the scalp.
Functional near-infrared spectroscopy (fNIRS) is suitable for measuring brain functions during neurorehabilitation
because of its portability and less motion restriction. However, it is not known whether neural reconstruction can be
observed through changes in cerebral hemodynamics. In this study, we modified an fNIRS system for measuring the
motor function of awake monkeys to study cerebral hemodynamics during neurorehabilitation. Computer simulation was
performed to determine the optimal fNIRS source–detector interval for monkey motor cortex. Accurate digital phantoms
were constructed based on anatomical magnetic resonance images. Light propagation based on the diffusion equation
was numerically calculated using the finite element method. The source–detector pair was placed on the scalp above the
primary motor cortex. Four different interval values (10, 15, 20, 25 mm) were examined. The results showed that the
detected intensity decreased and the partial optical path length in gray matter increased with an increase in the source-detector
interval. We found that 15 mm is the optimal interval for the fNIRS measurement of monkey motor cortex. The
preliminary measurement was performed on a healthy female macaque monkey using fNIRS equipment and custom-made
optodes and optode holder. The optodes were attached above bilateral primary motor cortices. Under the awaking
condition, 10 to 20 trials of alternated single-sided hand movements for several seconds with intervals of 10 to 30 s were
performed. Increases and decreases in oxy- and deoxyhemoglobin concentration were observed in a localized area in the
hemisphere contralateral to the moved forelimb.