Optical & ultrasound (in vivo) · reads
Wearable fNIRS is near-infrared spectroscopy worn on the head, usually without fibre-optic bundles. Small light sources shine near-infrared light into the scalp and nearby detectors catch what scatters back; the change in attenuation is converted into changes in oxygenated and deoxygenated haemoglobin. These blood measures are derived from the light signal, an indirect measure of brain activity that also contains scalp blood flow. A time-domain variant times the arrival of single photons, which in one whole-head headset gave better depth sensitivity than continuous-wave systems.
Wearable fNIRS is functional near-infrared spectroscopy carried on the head, usually without fibre-optic bundles. Sources held against the scalp send near-infrared light (650 to 950 nm) into the tissue and nearby detectors collect the light that scatters back. Attenuation at two or more wavelengths changes with the concentrations of oxygenated and deoxygenated haemoglobin, so the measured quantity is a hemodynamic signal estimated from light attenuation, an indirect measure of brain activity; the recorded signal mixes cortical hemodynamics with substantial scalp blood-flow contributions. Wearable instruments are typically continuous-wave devices: battery powered, with light-emitting diodes coupled directly to the head and data stored on the unit or sent wirelessly; the absence of fibre bundles makes them lighter and less sensitive to movement. A time-domain variant records the arrival time of single photons and builds a timing histogram for each source-detector pair and wavelength. In one whole-head time-domain headset, higher moments and later time gates of that histogram kept high contrast at depths beyond 20 mm, an advantage in depth sensitivity over continuous-wave systems; changes in the absorption coefficient at each wavelength were converted to oxy- and deoxy-haemoglobin concentration changes with the modified Beer-Lambert law. That headset carries its optical modules on plates over the frontal, parietal, temporal and occipital cortices and connects to a computer by a single cable that supplies power and carries data.
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