The fNIRS Signal
Accompanying neuroactivation, is a coupled hemodynamic response that is sensitive to a host of features of coordinated brain function. Relating these measures to the seemingly endless breadth of human behavior is a principal aim of many scientific investigations.
Learning, language acquisition, sensory and motor functions, emotion, social interactions, and the influence of a host of disease processes all can be explored from measures of the fNIRS signal. Most important, the resources needed to accomplish such measures are easily made wearable, resistant to motion artifacts, economical and can be performed in the natural environment. Further, employing harmless light sources, NIRS is safe for studies with infants and adults as well.
Left: A simple demonstration that red light is penetrating in tissue. Wavelengths in the near infrared region are even more penetrating.
Right: Oxy- and deoxy-hemoglobin are the dominant tissue absorbers within the near infrared range.
In contrast to information derived from the fMRI BOLD signal that can identify portions of the hemodynamic response, the fNIRS signal offers added information regarding the coupling between tissue metabolic activity and its blood supply. Supporting direct measures of both oxy- and deoxyhemoglobin, with deep tissue penetration, the NIRS signal supports real-time evaluation of related biometrics that are known to influence brain function.
New Technologies Require New Understandings:
A Collision of Ideas Born in Brooklyn
As often happens in science, new insights are drawn from the most unlikely of interactions. In our case it involved common understandings from the fields of Life Sciences and Nuclear Physics; the problem of scattering. Whether the form of energy is light or a neutron, the physics of scattering is the same. Armed with this understanding, in 1988 Drs. Randall Barbour from the SUNY Downstate Medical Center and Raphael Aronson from the now NYU Polytechnic University, were the first to show that whereas individual scattered photons have an unpredictable path, bulk movements can be defined .
The characteristic ‘photon banana’ of bulk photon movement.
Using mathematical transforms that leveraged this understanding, these investigators produced the first 3D images of otherwise obscured features from an opaque medium. The result was the birth of a field of imaging that today is opening new vistas in our understanding of tissue function: optical tomography.
Left: Multi-distance source-detector configuration (tomography) to identify cortical activation.
Right: Tomographic brain imaging with fNIRS system by NIRx.
Basics of fNIRS and Optical Tomography
fNIRS Tomography makes use of the fact that light penetrates up to several centimeters of biological tissue. This is demonstrated by an experiment such as depicted in the image to the right. Here, a bright white light source is directed toward a hand, causing a reddish glow in places where light is transmitted through the tissue. The red color is caused by the pigment hemoglobin in the red blood cells, which primarily absorbs wavelengths outside the red and near infrared spectrum. It is seen that no inner structures such as bones or veins are readily revealed by this simple transillumination technique.
fNIRS employs sensitive instrumentation to precisely measure the amount of light that is transmitted through a body site of interest. Many transmission measurements (hundreds or thousands) are performed in different locations on the target surface creating large data sets which are then used to compute an image of the underlying tissue composition.
The following sections are meant to provide more detailed information about the scientific and technological background of biomedical optical imaging.
fNIRS Tomography Characteristics
The following main properties characterize fNIRS tomography and distinguish it from other biomedical imaging methods such as x-ray, CT, MRI, ultrasound, or nuclear imaging:
No Harmful Radiation
fNIRS employs non-ionizing near-infrared light, so no cancerogeneous or genetic effects are possible. The light intensity is kept well below safety limits to avoid the risk of thermal damages. No cumulative harmful effects are known so that measurements can be applied repeatedly or in a continuous manner for monitoring purposes.
No Extrinsic Contrast Agents Required
The principal absorber in the near-infrared region in tissue is the pigment hemoglobin, which due to its confinement in the red cells, is highly concentrated in blood and therefore provides a strong optical contrast. This allows the sensitive imaging of local blood supply without additional contrast agents.
NIRS Functional Imaging (fNIRS)
Using spectroscopic techniques, fNIRS provides both images of blood concentration and oxygen saturation in tissue, which makes it uniquely suited to assess a host of functional parameters including tissue perfusion, local oxygen supply/demand balance, and autoregulatory response. In addition, OT can be expanded to image other tissue constituents (e.g., water, fat, proteins), which essentially makes it a molecular probing technique. Besides these absorption features, the technology can also explore differences in light scattering properties of different tissue components, providing yet another contrast mechanism.
Furthermore, extrinsic contrast agents can be imaged, whose accumulation or activity is influenced by local physiologic factors, such as such tissue type, metabolism, or chemical environment.
Compact, Low-Cost Technology
The development of fNIRS has strongly gained from the advances in microelectronics, computer technology, and optical engineering that were made over the last decades. Typically, semiconductor lasers are employed as light sources because of their efficiency, beam quality, and ease of operation. Fiber optics can be used to transmit light to and from the tissue, thereby allowing for great flexibility in instrument design. Advances in electronics allow precision analog and digital signal processing at ever-decreasing footprints and power consumption. Combined, these technologies allow for compact and cost-effective fNIRS instrumentation. With the advent of further miniaturization and integration such as integrated optics and MEMS devices, wearable and even disposable fNIRS technology can be envisioned.
Temporal & Spatial Resolution
Although optical measurements can be exceedingly fast-on the order of 10-9 seconds and faster-fNIRS image capture rates are much slower than that in order to maximize signal quality. For dynamic applications, typical image frame rates are on the order of several Hz, enough to capture fast physiologic signals, such as the cardiac frequency or perfusion changes due to neuroactivation.
Because of the physics of light propagation in turbid media, the spatial resolution of fNIRS images is relatively low. Depending on the composition and size of the target tissue, the resolution is on the order of 5 mm.
Light intensity is heavily attenuated in tissue and falls off exponentially from the illumination point. The maximum achievable range over which a measurement can be performed is limited by the source strength - determined by the thermal damaging threshold - and the detection sensitivity, given by the detector noise. Imaging depth strongly depends on tissue composition and measurement geometry. For a brain measurement in backreflection mode, the probing depth is about 3 cm, for a cross-sectional limb image, it is about 8 cm, and for a full volumetric breast scan, up to 12 cm.
NIRScout is an ultra-compact fNIRS neuroimaging solution that offers the versatility and scalability to fit a broad range of research applications. For example, investigating motor sensory functions, traumatic brain injury, learning/attention disorders and brain computer interfaces.
NIRSport is the first freely configurable, wearable, multichannel NIRS imaging system to be used on any part of the head. This pocket-book sized device combines LED illumination and innovative active detection technology into a truly wearable brain imaging solution.