Glossary

NIRS Near Infrared Spectroscopy for applications to tissues uses excitation wavelengths in the range from 670 nm through 900 nm; at these wavelengths, the absorption properties of tissue are such that a measurable amount of light can pass through large volumes of tissue. Below 650 nm the absorption of hemoglobin increases to the point that no measurable light can travel through the tissue; above 900 nm the absorption of water makes detection of light passing through tissue difficult. Thus, between 670 and 900 nm there is a unique window within which tissues can be probed by near infrared light; the main absorbers of the tissues in the region are the oxygenated and deoxygenated hemoglobin, and to a lesser extent, water, fat and cytochrome oxidase.
FD–NIRS

Frequency Domain Near Infrared Spectroscopy allows to measure and determine the absorption and scattering coefficients of the tissue (rather than making assumptions on their statistical values or using the differential path length factor).

In frequency domain systems, the NIR laser sources are (a.) either an amplitude modulated sinusoidally at frequencies near one hundred megahertz (100 MHz); or (b.) a train of pulses with a repetition rate of the order of 10-50 MHz. The instrumentation for FDNIRS can be implemented following two paths: (a.) using one single modulation frequency for the excitation source and collecting the signal at three or more locations from the injection point (multi-distance approach); or (b.) use multiple modulation frequencies for the excitation source and collect the signal at one location.

In both implementations, three distinct quantities are measured and recorded: the intensity of the detected signal, its modulation ratio with respect to source modulation and the time the signal takes to traverse the tissue (phase shift). From these measurements the absorption and scattering coefficients of the tissue are determined and, hence, the oxy- and deoxy-hemoglobin concentration of the tissue. The main and unique advantages of FD-NIRS is the capability to provide an absolute baseline of the oxygenation level without making any assumptions and to monitor changes in the oxygenation of the tissues with sampling rates up to 50 Hertz.

In ISS instrumentation, the light source is modulated at high frequency (110 MHz) and delivered to the subject through the sensor at four different distances from the location of the collector fiber (multi-distance technique); the distances vary from 1.5 cm to 4.0 cm. Three quantities are measured and recorded: the intensity of the detected signal, its modulation ratio with respect to the modulation of the source and the time the signal takes to traverse the tissue (phase shift). From these parameters the absorption and scattering coefficients of the tissue are determined and, hence, the oxy- and deoxy-hemoglobin concentrations. In some instruments the role of the emitters' fibers and collector is reversed: light is injected at one location and it is collected at four different locations. The main and unique advantage of FDNIRS is the capability to provide an absolute baseline of the oxygenation level and to monitor changes in the oxygenation in real time.

fNIRS

Functional Near Infrared Spectroscopy is a technique used to obtain information on brain activity following a stimulus (optical, visual, acoustic, etc.). The activity is monitored through the detection of temporal changes in the local concentration of oxy- and deoxy-hemoglobin due to neuron activation. The localization of the signal is confined to a volume of about 5 mm3; the temporal resolution is of the order of 200 ms.

Imagent uses wavelengths at 690 nm and 830 nm; the fibers are paired so that at each contact location of the emitter fibers photons of both wavelengths are emitted. The headgear allows for the user to probe the subject's head with different montages of sources-detectors patterns.

DOT Diffuse Optical Tomography uses the fNIRS technique to reconstruct a 3D image of the region affected by changes of the hemodynamics of the tissue under examination. The 2D image reconstruction is sometimes named "Diffuse Optical Topography".
EROS

Event Related Optical Signal is an fNIRS technique that, instead of using changes in absorption due to the hemodynamics to infer the cognitive response to the stimulus, processes the information carried by the scattering component of the optical signal that probes the cerebral cortex. It is presumed that the changes in the signal are due to the changes in the shape of glia and neurons that are associated with neuron firing (which may be due to the movements of water and ions through the membrane) or to changes in the optical parameters of the membrane itself through the activation. As EROS does not use the changes in absorption due to the hemodynamics, it is a more direct measurement of the cellular activity; it is capable of localizing the brain activity within millimeters with a time scale of a few milliseconds.

The Imagent configuration used for EROS detection usually features one excitation wavelength only (830nm is preferred as it has a better efficiency penetration in the tissue than the 690 nm) and the fibers are not paired. Two parameters are measured: (1.) the amount of light emitted by the source that reaches the detector; (2.) the phase delay (or time delay) of the photons that reach the detector. The event-related measures are recorded by synchronizing the recording to the stimulus presentation. The EROS signal elicited by a given stimulus is analyzed with respect to a pre-stimulus baseline, recorded right before the stimulus presentation.