A theoretical study of the detection of activated position in cerebral cortex from NIR signal

Near infrared (NIR) spectroscopy and imaging have been applied to measure oxygenation change in the brain. Multi-channel NIR system can obtain topographic image of brain function. However poor spatial resolution due to tissue scattering is serious problem of NIR imaging. In this study, we propose a simple method to estimate the centre of the absorption change in the brain from the signal detected with multichannel NIR system. The theoretical analysis of photon measurement density functions for the adjoining source-detector pairs in multi-channel NIR system suggests that the difference in intensity between the adjoining source-detector pairs almost linearly relates with the position of the absorption perturbation. The proposed method to estimate the centre of the absorption change in the brain is evaluated by using three layered head model. The estimated position is almost consistent with the centre of the absorption change. In case where the broad absorption change occurred close to the edge of the measuring region, the estimated position tends to be shifted inward of the measuring region.