Functional near-infrared spectroscopy
Functional near-infrared spectroscopy, sometimes referred to as NIRS or Optical Topography, is an optical brain monitoring technique which uses near-infrared spectroscopy for the purpose of functional neuroimaging. Using fNIRS, brain activity is measured by using near-infrared light to estimate cortical hemodynamic activity that occurs in response to neural activity. The use of fNIRS has led to advances in different fields such as cognitive neuroscience, clinical applications, developmental science and sport and exercise science. The signal is often compared with the BOLD signal measured by fMRI and is capable of measuring changes both in oxy- and deoxyhemoglobin concentration, but can only measure from regions near the cortical surface.
How it Works
fNIRS estimates the concentration of hemoglobin from changes in absorption of near infrared light. As light moves or propagates through the head, it is alternately scattered or absorbed by the tissue through which it travels. Because hemoglobin is a significant absorber of near-infrared light, changes in absorbed light can be used to reliably measure changes in hemoglobin concentration. Different fNIRS techniques can also use the way in which light propagates to estimate blood volume and oxygenation. The technique is safe, non-invasive, and can be used with other imaging modalities.fNIRS is a non-invasive imaging method involving the quantification of chromophore concentration resolved from the measurement of near infrared light attenuation or temporal or phasic changes. The technique takes advantage of the optical window in which skin, tissue, and bone are mostly transparent to NIR light and hemoglobin and deoxygenated-hemoglobin are strong absorbers of light.There are different ways for infrared light to interact with the brain tissue. fNIRS focuses primarily on absorption: differences in the absorption spectra of deoxy-Hb and oxy-Hb allow the measurement of relative changes in hemoglobin concentration through the use of light attenuation at multiple wavelengths. Two or more wavelengths are selected, with one wavelength above and one below the isosbestic point of 810 nm—at which deoxy-Hb and oxy-Hb have identical absorption coefficients. Using the modified Beer-Lambert law, relative changes in concentration can be calculated as a function of total photon path length.
Typically, the light emitter and detector are placed ipsilaterally on the subject's skull so recorded measurements are due to back-scattered light following elliptical pathways. fNIRS is most sensitive to hemodynamic changes which occur nearest to the scalp and these superficial artifacts are often addressed using additional light detectors located closer to the light source.
Modified Beer–Lambert law
Changes in light intensity can be related to changes in relative concentrations of hemoglobin through the modified Beer–Lambert law. The Beer Lambert-law has to deal with concentration of hemoglobin. This technique also measures relative changes in light attenuation as well as using mBLL to quantify hemoglobin concentration changes.Equipment and Software
fNIRS cap
fNIRS electrode locations can be defined using a variety of layouts, including names and locations that are specified by the International 10–20 system as well as other layouts that are specifically optimized to maintain a consistent 30mm distance between each location. 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.Software
HOMER3
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.NIRS toolbox
This toolbox is a set of Matlab-based tools for the analysis of functional near-infrared spectroscopy. This toolbox defines the +nirs namespace and includes a series of tools for signal processing, display, and statistics of fNIRS data. This toolbox is built around an object-oriented framework of Matlab classes and namespaces.AtlasViewer
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.History
The conceptual origins of functional near-infrared spectroscopy date to 1977, when Frans Jöbsis demonstrated that near-infrared light could penetrate biological tissue and report on oxygenation changes in vivo.Throughout the 1980s, this observation informed early cerebral oximetry instruments used in neonatal medicine and physiology research.
In the early 1990s, several research groups independently reported changes in hemoglobin concentrations during cognitive tasks, establishing fNIRS as a viable functional neuroimaging modality.
Progress occurred internationally: researchers in Japan, Europe, and North America contributed to theory, hardware, and methodological refinement.
The term “optical topography” was introduced in 1995 when Hitachi released one of the first practical multi-channel commercial systems.
With rising sensor density, signal analysis advances, and portable configurations, fNIRS expanded rapidly into cognitive neuroscience, clinical medicine, developmental psychology, and applied research.
Diffuse Optical Spectroscopy/Imaging (DOI/DOS)
Spectroscopic techniques
Continuous wave
Continuous wave system uses light sources with constant frequency and amplitude. In fact, to measure absolute changes in HbO concentration with the mBLL, we need to know photon path-length. However, CW-fNIRS does not provide any knowledge of photon path-length, so changes in HbO 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 approximate absolute quantification of hemoglobin concentrations.Where is the optical density or attenuation, is emitted light intensity, is measured light intensity, is the attenuation coefficient, is the chromophore concentration, is the distance between source and detector and is the differential path length factor, and is a geometric factor associated with scattering.
When the attenuation coefficients are known, constant scattering loss is assumed, and the measurements are treated differentially in time, the equation reduces to:
Where is the total corrected photon path-length.
Using a dual wavelength system, measurements for HbO2 and Hb can be solved from the matrix equation:
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. However, it does not distinguish between absorption and scattering changes, and cannot measure absolute absorption values: which means that it is only sensitive to relative change in HbO concentration.
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. Moreover, thanks to its portability, wireless CW systems have been developed—allowing individuals to be monitored in ambulatory, clinical and sports environments.
Frequency domain
system comprises NIR laser sources which provide an amplitude-modulated sinusoid at frequencies near 100 MHz. FD-fNIRS measures attenuation, phase shift and the average path length of light through the tissue.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; and from the coefficients we determine the changes in the concentration of hemodynamic parameters.
Because of the need for modulated lasers as well as phasic measurements, FD system-based devices are more technically complex than CW-based ones. However, the system is capable of providing absolute concentrations of HbO and HbR.
Time domain
Time domain system introduces a short NIR pulse with a pulse length usually in the order of picoseconds—around 70 ps. Through time-of-flight measurements, photon path-length may be directly observed by dividing resolved time by the speed of light. Information about hemodynamic changes can be found in the attenuation, decay, and time profile of the back-scattered signal. For this photon-counting technology is introduced, which counts 1 photon for every 100 pulses to maintain linearity. TD-fNIRS does have a slow sampling rate as well as a limited number of wavelengths. Because of the need for a photon-counting device, high-speed detection, and high-speed emitters, time-resolved methods are the most expensive and technically complicated.TD-based devices have the highest depth sensitivity and are capable of presenting most accurate values of baseline hemoglobin concentration and oxygenation.