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Author Topic: Functional near-infrared spectroscopy  (Read 4854 times)

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Functional near-infrared spectroscopy
« on: July 04, 2019, 10:29:12 PM »

Functional near-infrared spectroscopy

Functional Near-Infrared Spectroscopy (fNIRS), is the use of near-infrared spectroscopy (NIRS) for the purpose of functional neuroimaging. Using fNIRS, brain activity is measured through hemodynamic responses associated with neuron behaviour.

Brain activity is measured through hemodynamic responses.  fNIRSWiki has been successfully implemented as a control signal for BCI systems.

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.

Development of NIRS systems proceeded rapidly and by 1985, the first studies on cerebral oxygenation were conducted by M. Ferrari.

NIRS techniques were expanded on by the work of Randall Barbour,

Spectroscopic techniques   

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

There are four current methods of fNIR Spectroscopy.

1. Continuous wave   

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 .

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 the 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 tomographyWiki systems, and more.

Wireless CW systems have been developed, allowing monitoring of individuals in ambulatory, clinical and sports environments.

2. Frequency domain   

In frequency domain (FD) systems, NIR laser sources provide an amplitude modulated sinusoid at frequencies near one hundred megahertz (100 MHz).

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.

3. Time-resolved   

In time-resolved 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.

4. Spatially-resolved spectroscopy   

Spatially-resolved spectroscopy (SRS) systems use localized gradients in light attenuation to determine absolute ratios of oxy-Hb and deoxy-Hb.

Using a spatial measurement, SRS 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).
« Last Edit: July 05, 2019, 05:56:33 AM by Chip »
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