Functional near-infrared spectroscopy

The use of fNIRS as a functional imaging method relies on the principle of neuro-vascular coupling also known as the haemodynamic response or blood-oxygen-level dependent (BOLD) response. This principle also forms the core of fMRI techniques. Through neuro-vascular coupling, neuronal activity is linked to related changes in localized cerebral blood flow. fNIRS and fMRI are sensitive to similar physiologic changes and are often comparative methods. Studies relating fMRI and fNIRS show highly correlated results in cognitive tasks. fNIRS has several advantages in cost and portability over fMRI, but cannot be used to measure cortical activity more than 4 cm deep due to limitations in light emitter power and has more limited spatial resolution. fNIRS includes the use of diffuse optical tomography (DOT/NIRDOT) for functional purposes. Multiplexing fNIRS channels can allow 2D topographic functional maps of brain activity (e.g. with Hitachi ETG-4000, Artinis Oxymon, NIRx NIRScout, etc.) while using multiple emitter spacings may be used to build 3D tomographic maps.

10-20 system

fNIRS electrode locations and names are specified by the International 10–20 system. In addition to the standard positions of electrodes, short separation channels can be added. Short separation channels allow the measurement of scalp signals. Since the short separation channels measure the signal coming from the scalp, they allow the removal of the signal of superficial layers. This leaves behind the actual brain response. Short separation channel detectors are usually placed 8mm away from a source. They do not need to be in a specific direction or in the same direction as a detector.[6]

Continuous wave (CW) fNIRS uses light sources which emit light at a constant frequency and amplitude. Changes in light intensity can be related to changes in relative concentrations of hemoglobin through the modified Beer–Lambert law (mBLL).[8] The Beer lambert-law has to deal with concentration of hemoglobin. CWNIRS stands for Continuous-Wave Near infrared spectroscopy. It is the least expensive and has more channels with a high temporal resolution. This technique also measures relative changes in light attenuation as well as using MBLL to quantify hemoglobin concentration changes. As far as things it cannot do, it does not distinguish between absorption and scattering changes and cannot measure absolute absorption values. It means that it is only sensitive to hemoglobin changes. However, the red light emitted from the fNIRS can travel tissues further without absorption.

${\displaystyle {\text{OD}}=\operatorname {log} _{10}(I_{0}/I)=\epsilon \cdot [X]\cdot l\cdot {\text{DPF}}+G}$

Where ${\displaystyle {\text{OD}}}$ is the optical density or attenuation, ${\displaystyle I_{0}}$ is emitted light intensity, ${\displaystyle I}$ is measured light intensity, ${\displaystyle \epsilon }$ is the attenuation coefficient, ${\displaystyle [X]}$ is the chromophomore concentration, ${\displaystyle l}$ is the distance between source and detector and ${\displaystyle {\text{DPF}}}$ is the differential path length factor, and ${\displaystyle G}$ is a geometric factor associated with scattering.

When the attenuation coefficients ${\displaystyle \epsilon }$ are known, constant scattering loss is assumed, and the measurements are treated differentially in time, the equation reduces to:

${\displaystyle \Delta [X]=\Delta {\frac {\text{OD}}{\epsilon d}}}$

Where ${\displaystyle d}$ is the total corrected photon path-length.

Using a dual wavelength system, measurements for oxy-Hb (HbO₂) and Deoxy-Hb(Hb) can be solved from the matrix equation:[9]

${\displaystyle {\begin{pmatrix}\Delta {\text{OD}}_{\lambda _{1}}\\\Delta {\text{OD}}_{\lambda _{2}}\end{pmatrix}}={\begin{pmatrix}\epsilon _{\lambda _{1}}^{\text{Hb}}d&\epsilon _{\lambda _{1}}^{{\text{HbO}}_{2}}d\\\epsilon _{\lambda _{2}}^{\text{Hb}}d&\epsilon _{\lambda _{2}}^{{\text{HbO}}_{2}}d\end{pmatrix}}{\begin{pmatrix}\Delta [X]^{\text{Hb}}\\\Delta [X]^{{\text{HbO}}_{2}}\end{pmatrix}}}$

Due to their simplicity and cost-effectiveness, CW technologies are by far the most common form of functional NIRS. Measurement of absolute changes in concentration with the mBLL requires knowledge of photon path-length. Continuous wave methods do not have any knowledge of photon path-length and so changes in concentration are relative to an unknown path-length. Many CW-fNIRS commercial systems use estimations of photon path-length derived from computerized Monte-Carlo simulations and physical models to provide absolute quantification of hemoglobin concentrations.

Simplicity of principle allows CW devices to be rapidly developed for different applications such as neonatal care, patient monitoring systems, optical tomography systems, and more. Wireless CW systems have been developed, allowing monitoring of individuals in ambulatory, clinical and sports environments.[10][11][12]

In frequency domain (FD) systems, NIR laser sources provide an amplitude modulated sinusoid at frequencies near one hundred megahertz (100 MHz). FDNIRS measures attenuation, phase shift and the average path length of light through the tissue.Multi Distance, which is a part of the frequency domain NIRS,  is insensitive to differences in skin color, the results are the same.

Changes in the back-scattered signal's amplitude and phase provide a direct measurement of absorption and scattering coefficients of the tissue, thus obviating the need for information about photon path-length; from the scattering and absorption coefficients the changes in the concentration of hemodynamic parameters are determined. Because of the need for modulated lasers as well as phasic measurements, frequency domain systems are more technically complex than continuous wave systems. However, these systems are capable of providing absolute concentrations of oxy-Hb and deoxy-Hb.

In time-domain spectroscopy, a short NIR pulse is introduced with a pulse length usually on the order of picoseconds. Through time-of-flight measurements, photon path-length may be directly observed by dividing resolved time by the speed of light. Because of the need for high-speed detection and high-speed emitters, time-resolved methods are the most expensive and technically complicated method. Information about hemodynamic changes can be found in the attenuation, decay, and time profile of the back-scattered signal.Time-Domain NIRS is capable of photon counting technology, but is very expensive. It does have a slow sampling rate as well as a limited number of wavelengths. This device can count 1 photon for every 100 pulses to maintain linearity. The properties of this device are that it has higher depth sensitivity and baseline hemoglobin concentration and oxygenation.

Diffuse correlation spectroscopy (DCS) systems use localized gradients in light attenuation to determine absolute ratios of oxy-Hb and deoxy-Hb. Using a spatial measurement, DCS systems do not require knowledge of photon path-length to make this calculation, however measured concentrations of oxy-Hb and deoxy-Hb are relative to the unknown coefficient of scattering in the media. This technique is most commonly used in cerebral oxymetry systems that report a Tissue Oxygenation Index (TOI) or Tissue Saturation Index (TSI).[13]

HOMER3 allows users to obtain estimates and maps of brain activation. It is a set of matlab scripts used for analyzing fNIRS data. This set of scripts has evolved since the early 1990s first as the Photon Migration Imaging toolbox, then HOMER1 and HOMER2, and now HOMER3.[14]

AtlasViewer allows fNIRS data to be visualized on a model of the brain. In addition, it also allows the user to design probes which can eventually be placed onto a subject.

Brain Atlas

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