Functional near-infrared spectroscopy
fNIRS is a non-invasive imaging method involving the quantification of chromophore concentration resolved from the measurement of near infrared (NIR) light attenuation or temporal or phasic changes.
Using the modified Beer-Lambert law (mBLL), relative changes in concentration can be calculated as a function of total photon path length.
Transillumination (forward-scattering) was of limited utility in adults because of light attenuation and was quickly replaced by reflectance-mode based techniques - resulting in development of NIRS systems proceeding rapidly.
Later, in 1989, following work with David Delpy at University College London, Hamamatsu developed the first commercial NIRS system: NIR-1000 cerebral oxygenation monitor.
NIRS techniques were further expanded on by the work of Randall Barbour, Britton Chance, Arno Villringer, M. Cope, D. T. Delpy, Enrico Gratton, and others.
This effort came into light when the team, along with their leading expert, Dr Hideaki Koizumi (小泉 英明), held an open symposium to announce the principle of "Optical Topography" in January 1995.
The idea had been successfully implemented in launching their first fNIRS (or Optical Topography, as they call it) device based on Frequency Domain in 2001: Hitachi ETG-100.
Later, Harumi Oishi (大石 晴美), a PhD-to-be at Nagoya University, published her doctoral dissertation in 2003 with the subject of "language learners' cortical activation patterns measured by ETG-100" under the supervision of Professor Toru Kinoshita (木下 微)—presenting a new prospect on the use of fNIRS.
Many CW-fNIRS commercial systems use estimations of photon path-length derived from computerized Monte-Carlo simulations and physical models, to approximate absolute quantification of hemoglobin concentrations.
Due to their simplicity and cost-effectiveness, CW-fNIRS is by far the most common form of functional NIRS since it is the cheapest to make, applicable with more channels, and ensures a high temporal resolution.
Still, the simplicity and cost-effectiveness of CW-based devices prove themselves to be the most favorable for a number of clinical applications: neonatal care, patient monitoring systems, diffuse optical tomography, and so forth.
Because of the need for modulated lasers as well as phasic measurements, FD system-based devices are more technically complex (therefore more expensive and much less portable) than CW-based ones.
Through time-of-flight measurements, photon path-length may be directly observed by dividing resolved time by the speed of light.
TD-based devices have the highest depth sensitivity and are capable of presenting most accurate values of baseline hemoglobin concentration and oxygenation.
Diffuse correlation spectroscopy (DCS) is a non-invasive optical imaging technique that utilizes coherent near-infrared light to measure local microvascular cerebral blood flow by quantifying the temporal light intensity fluctuations generated by dynamic scattering of moving red blood cells.
The decay of the autocorrelation curve is fitted with the solution of the correlation diffusion equation to obtain an index of cerebral blood flow.
[13][14][15][16] At least two open-source fNIRS models are available online: HOMER3 allows users to obtain estimates and maps of brain activation.
This toolbox defines the +nirs namespace and includes a series of tools for signal processing, display, and statistics of fNIRS data.
Moreover, understanding the physiological mechanism underlying the bodily response to oxygen deprivation is of major importance and NIRS devices have shown to be a great tool in this field of research.
Multi-channel fNIRS measurements create a topographical map of neural activation, whereby temporal correlation between spatially separated events can be analyzed.
Functional connectivity is typically assessed in terms correlations between the hemodynamic responses of spatially distinct regions of interest (ROIs).
[27] It is an effective aid in Cardiopulmonary bypass, is strongly considered to improve patient outcomes and reduce costs and extended stays.
There are inconclusive results for use of NIRS with patients with traumatic brain injury, so it has been concluded that it should remain a research tool.
Through neuro-vascular coupling, neuronal activity is linked to related changes in localized cerebral blood flow.
Studies relating fMRI and fNIRS show highly correlated results in cognitive tasks.
Multiplexing fNIRS channels can allow 2D topographic functional maps of brain activity (e.g. with Hitachi ETG-4000, Artinis Oxymon, NIRx NIRScout, etc.)
[30] Modern fNIRS systems are combined with virtual or augmented reality in studies on brain-computer interfaces,[31] neurorehabilitation[32] or social perception.
Importantly, the signal is sensitive to hair and skin pigment differences, making it difficult to do between-subject designs.
Dense or extremely curly hair may prohibit placement of electrodes close to the scalp, limiting the ability to use the technique with all individuals.
Among all other facts, what makes fNIRS a special point of interest is that it is compatible with some of these modalities, including: MRI, EEG, and MEG.
