![]() 1 Light launched into tissue rapidly diffuses. In the visible to near-infrared spectrum, the reduced scattering coefficient of brain, scalp and skull is on the order of 1 inverse mm. In this modality, the time resolution of fNIRS for sensing brain activity is therefore intrinsically limited by the hemodynamic response function (HRF) to be hundreds of milliseconds or longer. By measuring the differential transmittance among the spectral components, fNIRS directly senses changes of the blood oxygenation in the brain tissue, a proxy for metabolism and therefore an indication of neuronal firing within the sensing volume. ![]() ![]() For the application of functional neural imaging, fNIRS systems typically monitor the transmission of light in two or more spectral bins (for example, 690 nm and 852 nm), which straddle the isosbestic point of oxygenated- vs-deoxygenated hemoglobin near 808 nm. Functional near-infrared spectroscopy (fNIRS) is a non-invasive tool to measure the time-varying absorption and scattering of light propagating through tissue. ![]()
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