Functional near-infrared spectroscopy

fNIRS cap on a subject

Functional near-infrared spectroscopy (fNIRS), or Optical Topography as it is called in Japan exclusively, is the use of near-infrared spectroscopy (NIRS) for functional neuroimaging. Using fNIRS, cerebral hemodynamic responses are measured by near-infrared light, which go in line with cerebral activation or deactivation. In particular, this technology is capable of visualizing changes both in oxy- and deoxyhemoglobin concentration.

Description

Oxygenated and Deoxygenated Hemoglobin

fNIRS is the absorption of near infrared light by hemoglobin. The light moves, or propagates, through the head and lends information about blood volume, flow and oxygenation. This technique is safe, non-invasive, and can be used with other imaging modalities.

To specify, 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. fNIRS spectrum light takes advantage of the optical window in which (a) skin, tissue, and bone are mostly transparent to NIR light (700–900 nm spectral interval) and (b) hemoglobin (Hb) and deoxygenated-hemoglobin (deoxy-Hb) are strong absorbers of light. These are the same principals adapted from pulse oximeters.

Absorption spectra for oxy-Hb and deoxy-Hb for near-infrared wavelengths

There are four different ways that infrared light can interact with the brain tissue: transmission, reflection, scattering, and 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 (mBLL), relative concentration can be calculated as a function of total photon path length.

Typically, the light emitter and detector are placed ipsilaterally (each emitter/detector pair on the same side) on the subject's skull so recorded measurements are due to back-scattered (reflected) light following elliptical pathways. fNIRS is most sensitive to scalp and skull, so in order to have an increased sensitivity to the superficial cortex, there needs to be a larger source-detector ratio.

Basic functional near infrared spectroscopy (fNIRS) abbreviations BFi = blood flow index CBF = cerebral blood flow CBV = cerebral blood volume CMRO2= metabolic rate of oxygen CW= continuous wave DCS = diffuse correlation spectroscopy FD = frequency-domain Hb, HbR= deoxygenated hemoglobin HbO, HbO2= oxygenated hemoglobin HbT= total hemoglobin concentration HGB = blood hemoglobin SaO2= arterial saturation SO2= hemoglobin saturation SvO2= venous saturation TD=time-domain

History

US & UK

In 1977, Jöbsis reported that brain tissue transparency to NIR light allowed a non-invasive and continuous method of tissue oxygen saturation using transillumination in neonates. 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. Then, by 1985, the first studies on cerebral oxygenation were conducted by M. Ferrari. 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 methods were initially used for cerebral oximetry in the 1990’s. In 1993, four publications by Chance et al PNAS, Hoshi & Tamura J Appl Physiol,  Kato et al JCBFM, Villringer et al Neuros. Lett. demonstrated the feasibility of fNIRS in adult humans. 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. Currently, wearable fNIRS are being developed.

Hitachi ETG-4000

Japan

Meanwhile, in the mid-80's, Japanese researchers at the central research laboratory of Hitachi Ltd set out to build a NIRS-based brain monitoring system using a pulse of 70-picosecond rays. 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. In fact, the term "Optical Topography" derives from the concept of using light on "2-Dimensional mapping combined with 1-Dimensional information", or topography. 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. The company has been advancing the ETG series ever since.

Modified Beer-Lambert Law

Changes in light intensity can be related to changes in relative concentrations of hemoglobin through the modified Beer–Lambert law (mBLL). 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.

Spectroscopic techniques

Currently, there are three modalities of fNIR Spectroscopy:

1. Continuous Wave

2. Frequency Domain

3. Time-Domain

Continuous wave

Continuous wave (CW) fNIRS uses light sources which emit light at a constant frequency and amplitude. 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.

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

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

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

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

Where $d$ is the total corrected photon path-length.

Using a dual wavelength system, measurements for HbO₂ and Hb can be solved from the matrix equation:

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

Due to their simplicity and cost-effectiveness, CW-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 allow CW-based devices to be the most favorable for clinical applications such as neonatal care, patient monitoring systems, optical tomography systems, 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

In frequency domain (FD) systems, NIR laser sources provide an amplitude modulated sinusoid at frequencies near one hundred megahertz (100 MHz). FD-NIRS measures attenuation, phase shift and the average path length of light through the tissue. Multi Distance, which is a part of the FD-NIRS, is insensitive to differences in skin color - therefore 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; 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, frequency domain systems are more technically complex (therefore more expensive and much less portable) than CW-based devices. However, these systems are capable of providing absolute concentrations of oxy-Hb and deoxy-Hb.

Time-Domain

In time-domain spectroscopy, a short NIR pulse is introduced 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 are costly, huge, heavy, immobile, space-consuming and the most difficult to make; even so, they have the highest depth sensitivity and are capable of presenting most accurate values of baseline hemoglobin concentration and oxygenation.

Diffuse Correlation Spectroscopy

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).

System Design

A few open-source fNIRS models are available online:

• http://www.opennirs.org
• https://openfnirs.org/

Data Analysis 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

It is the most recent one. This toolbox is a set of Matlab based tools for the analysis of functional near-infrared spectroscopy (fNIRS). 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.

Application

Cerebral Oximetry

File:Allen Brain Atlas.jpg

Brain Atlas

NIRS monitoring is helpful in a number of ways. Preterm infants can be monitored reducing cerebral hypoxia and hyperoxia with different patterns of activities. 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.

Functional imaging

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 Cap

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.

Brain–computer interface

fNIRS has been successfully implemented as a control signal for Brain-Computer Interface systems.

Pros and Cons

The advantages of fNIRS are, among other things: noninvasiveness, low-cost modalities, perfect safety, high temporal resolution, full compatibility with other imaging modalities, and multiple hemodynamic biomarkers.

However, any system can't be without its limitations. For fNIRS those include: low brain sensitivity, low spatial resolution, and shallow penetration depth.

Future Direction

Despite a few limitations, fNIRS devices are relatively small, lightweight, portable and wearable. Thanks to these features, applications for the devices are astounding—which makes them easily accessible in many different scenarios. For example, they have the potential to be used in clinics, a global health situation, a natural environment, and it could be used as a health tracker.

Finally, in the future, at-risk individuals in hospitals could benefit from neuromonitoring and neurorehabilitation that fNIRS can offer.

fNIRS Compared with other neuroimaging techniques

Comparing and contrasting other neuroimaging devices is an important thing to take into consideration. When comparing and contrasting these devices it is important to look at the temporal resolution, spatial resolution, and the degree of immobility. EEG (electroencephalograph) and MEG (magnetoencephalography) have high temporal resolution, but a low spatial resolution. EEG also has a higher degree of mobility than MEG has. When looking at fNIRS, they are similar to an EEG. They have a high degree of mobility as well as temporal resolution, and they have low spatial resolution. PET scans and fMRIs are grouped together, however they are distinctly different from the other neuroimaging scans. They have a high degree of immobility, medium/high spatial resolution, and a low temporal resolution. All of these neuroimaging scans have important characteristics and are valuable, however they have distinct characteristics.

Among all other facts, what gives fNIRS a special point of interest is that it is compatible with some of these modalities, including: MRI, EEG, and MEG.

See also

• Near-infrared spectroscopy
• Diffuse optical tomography
• Functional neuroimaging
• Cognitive neuroscience
• The Society for Functional Near Infrared Society (external link)