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Applications of NIRS

The use of NIRS in both research and clinical environments is becoming more accepted. It is utilised in wide variety of departments e.g. neonatology, geriatrics, surgery, psychology, urology, neurology, and (sport) physiology. NIRS is most commonly used to measure oxygenation changes in muscle and brain tissue. With NIRS it is also possible to measure absolute values. The application of NIRS to these fields is explained in more detail below.

 Tissue Saturation Index

Our NIRS devices can use multiple distances between the receiver and a number of transmitters. Wit this it is possible to calculate absolute values of oxygenation by means of spatially resolved spectroscopy. This technique is used in real time and is of special interest e.g. in monitoring brain oxygenation during surgery or muscle oxygenation during exercise.

 

Muscle applications of NIRS

Our NIRS devices can use multiple distances between the receiver and a number of transmitters. Wit this it is possible to calculate absolute values of oxygenation by means of spatially resolved spectroscopy. This technique is used in real time and is of special interest e.g. in monitoring brain oxygenation during surgery or muscle oxygenation during exercise.

Venous occlusion technique

When venous occlusion is applied to the upper arm or leg by inflating a blood pressure cuff to a pressure of approximately 50 mmHg, there is results in (arterial) inflow of blood but no outflow. The observed increase in blood volume equals the blood flow into the limb, and can be measured with NIRS by monitoring the increase in the tHb signal following occlusion. Figure 1 is an example of a NIRS trace during venous occlusion of the arm.

Arterial occlusion technique

An example of a NIRS tracing. In this case an arterial occlusion (A.O.) is applied to the upper arm. The NIRS optodes were attached to the brachio-radialis muscle of the forearm. During A.O. there is no arterial inflow or venous outflow. This results in an increase of deoxyhaemoglobin and decrease in oxyhaemoglobin. From these signals oxygen consumption and blood flow into the limb can be calculated

The blood flow into a limb can be stopped completely by inflating a blood pressure cuff to a pressure above 250 mmHg. It is then possible to calculate the local oxygen consumption in the muscle tissue from the gradient of the subsequent decrease in the O2 Hb signal.

Once the cuff pressure is released, the tissue will exhibit a hyperemic reaction. The re-saturation recovery time observed with NIRS can be used as a measure of, e.g. the leg vascularization in patients with peripheral vascular disease.

By combining the NIRS data with arterial saturation (SaO2), measured for example by pulse oximetry, it is possible to quantify the absolute blood volume of the examined tissue.

The effect of a small, gradual, and transient change in SaO2 on O2Hb concentration is monitored. A decrease in SaO2 of around 10%, induced by lowering the inspired oxygen concentration, is sufficient to calculate the blood volume. Provided that blood flow, volume, and oxygen consumption remain constant during the procedure, the tissue blood volume (TBV) can be calculated. When using this method, an absolute change in arterial saturation is compared to a relative change in O2Hb concentration, which can then be quantified.

Organ blood flow measurements using NIRS are based on the Fick principle, which states that the accumulation of a tracer in an organ equals the difference between the inflow (arterial concentration x flow) and outflow (venous concentration x flow). If we measure within the tracer's transit time through the organ, the venous concentration will be zero. In NIRS, the tracer used is an O2Hb bolus, which can be induced by suddenly increasing the inspired oxygen concentration. The concentration of the bolus can be measured by attaching a pulse oximetry probe to the organ. The O2Hb increase as measured by NIRS represents the accumulation of the bolus in the organ.

Brain applications of NIRS

NIRS measurements of the visual cortex during visual stimulation show the activation of the visual cortex by changes in the oxy– and deoxy-hemoglobin ([O2Hb] and [HHb]) concentration.

The figures below show an example of NIRS measurement during visual cortex stimulation.

The curves trace the left (upper panels) and right (lower panels) hemisphere oxygenation changes in response to right and left hemi-field visual stimuli. The stimulus (10 s) was a green and white reversing (8 Hz) hemi-field checkerboard. Both [O2Hb] and [HHb] (in µM) are tim-averaged over six right and left hemi-field stimuli. Left and right panels refer to the left and right visual cortex areas, respectively. The vertical lines indicate the stimulus duration. Circles: [O2Hb], triangles: [HHb].

Visual stimulation provoked an increase in [O2Hb] accompanied by a smaller decrease in [HHb]. The reported decrease in [HHb] shows the ability of fNIRS to detect the localized changes: left hemi-field stimuli induce a decrease in [HHb] in the right visual area and no decrease in the left visual area and vice versa.

Functional NIRS (fNIRS) can be done using single channels or multiple-channel NIRS devices. Multiple channels allow brain-mapping to be performed.

Oxygenation monitoring during surgery

NIRS monitoring of the brain during surgery provides information about cerebral perfusion and oxygenation. Two examples are given below.

NIRS and transcranial Doppler on the middle cerebral artery are recorded simultaneously in a patient during an elective cardiac arrest in order to test a newly placed pacemaker (figure on the left). The pacemaker was placed because of cardiac arrhythmia. After the cardiac arrest the pacemaker stimulates the heart to start again. Optodes were placed on the frontal side of the head, 5.5 cm apart. The NIRS measurement shows a decrease in the hemoglobin concentrations and the total blood volume during the cardiac arrest. After the cardiac arrest, the hemoglobin concentrations recover. The transcranial Doppler measurement shows a similar decrease during the cardiac arrest, and then recovery of hemoglobin concentrations.

NIRS measurement from a pig's head taken with clamped arteries, shows decreased oxygenated hemoglobin concentrations and similar increases in deoxygenated hemoglobin concentration. First the left artery, then the right and then both arteries were clamped (figure on the right). Post clamping recovery and overshoot are visible.

Motor task— finger tapping

 Brain mapping measurement with NIRS during finger tapping[Colier WNJM, Quaresima V, Baratelli G, Cavallari P, van der Sluijs MC, Ferrari M.; SPIE Proc. 1998: 390-396].  

NIRS can be used to study human motor-cortex oxygenation changes in response to motor tasks. An example is shown at the right with a 12-channel measurement. A map of [O2Hb] and [HHb] concentrations over an area of 7 x 7 cm measured over the left motor cortex is shown. It is recorded while the subject performed a finger tapping task with his right hand for 20 seconds at 2 Hz.

With our (Oxysoft) software, the data can be presented topographically (brain mapping) as shown in the example below. The data can also be viewed seperately for each channel and be filtered (low pass, high pass, band pass, moving average, etc). Within a repetitive measurement, the data can be averaged and de-trended over the cycles as shown below. In addition, the standard deviation over the cycles can be added to the graph.

Brain-computer interface

The yellow block should be hit by the black snake. The snake is controlled by the oxyhemoglobin concentration changes measured with NIRS in the motor cortex of the subject caused by finger tapping.

NIRS can be used to build a real-time NIRS-based brain-computer interface. Our device can be connected to a buffer from the Matlab toolbox ‘FieldTrip’, which makes it possible to do online data analysis and manipulation in Matlab. As an example, it is possible to play Brain Snake. The hemodynamic response in the motor cortex to finger tapping was used to play the computer game ‘Snake’. Two NIRS channels with transmitter-receiver distances of 4cm and 1.5 cm were placed over the motor cortex. A few seconds after starting the finger tapping, a response was visible at the 4 cm channel. The signal of the superficial 1.5 cm channel was subtracted from the 4 cm channel in order to cancel out the Mayer waves. The Mayer waves mask the task related changes in the cortex. The difference signal was used to control the computer game. A few seconds after starting finger tapping the oxyhemoglobin concentration increased. When the signal amplitude increased above a certain threshold, the snake would make a right turn. This simple setup demonstrates that a computer can be controlled using NIRS-detected signals from the brain.

Recently a movie has been made by our users at the Sint Maartenskliniek using their BCI. Please have a look at the results on YouTube!

fNIRS and EEG

Electroencephalography (EEG) and NIRS both offer information about brain function, complementing each other in their ability to resolve information about the spatial and temporal characteristics of neural activity. The electrical potentials in brain tissue can be measured by EEG with high temperal resolution. NIRS measures the changes in oxygenation and blood volume, which also reflect neural activity and provide spatial information.

The Oxymon does not interfere with EEG signals. It is possible to combine head caps for EEG and the Oxymon. The analog outputs of the Oxymon offer the option of synchronized EEG and NIRS measurements. In addition, a special analog box is available which outputs the concentrations measured by the Oxymon (up to 8-channel NIRS). This box can be used to couple the Oxymon to EEG (or other) equipment for a synchronized NIRS-EEG measurements.

fMRI and fNIRS

 Figure 9: Combined measurement with NIRS and fMRI

NIRS can be used in conjunction with to fMRI to study hemodynamics in the brain. The advantages of NIRS over MRI are:

• High temporal resolution.
• Excellent intrinsic sensitivity to hemoglobin - fMRI can only monitor [HHb] changes (the BOLD effect).
• Measurements can be applied to a variety of tasks.
• Easily adapted to incorporate other techniques (EEG, fMRI).
• Economical, portable, scalable.
• No contra-indication to ferrous implants.
• No patient claustrophobia.
• Fewer motion artifacts.

The figure on the left shows the time curve of a finger-tapping activity represented by vertical peaks, with periods of rest represented by horizontal lines (top); time curve of changes in [HHb], [O2Hb] and [tHb] concentrations as measured by NIRS (middle), and the time curve of changes in the maximum BOLD-fMRI signal (bottom) over the left motor cortex in a subject during seven cycles of contralateral finger-tapping for 20 sec and rest for 40 sec.

A statistical parametric mapping (SPM) tool is available for combining NIRS signals from Oxymon with MRI data. [Ye JC, Tak S, Jan KE, Jung J, Jang J. NIRS-SPM: statistical parametric mapping for near-infrared spectroscopy Neuroimage 2009; 44(2): 428-47]

With special NMR-compatible fibers the Oxymon can be used in the MRI. The fiber length can be up to 10 m.

OxyMon PortaMon PortaLite Oxysoft Applications of NIRS Accessories