Experienced Engineers Using the Best Tools Produce Superior Measurement Results
Our designers use the latest CAD/CAM tools to create our products. Full 3d modeling ensures our mechanical designs fit the first time, while full circuit modeling allows us to define and meet strict electronic noise requirements. These methodologies support our goals of producing the highest quality measurement instruments.
Brush Optodes Solve Problems With Hair Obscuration
Our brush optodes increase signal levels by penetrating through the hair, directly to the scalp. In addition, the system speeds optode application since there is no need to move hair from under each optode. The figure shows typical results obtained with brushed vs flat optodes. For a link to our full paper on the method, see the link at the bottom of this page.
Overview of an fNIRS Signal and Experiment
This is an overview of fNIRS (functional near-infrared spectroscopy), and is intended to give a very basic understanding of the workflow for an fNIRS experiment.
fNIRS measures functional activations of the cerebral cortex using infrared light. The basis of fNIRS is that the optical absorption spectrum is different for oxygenated and de-oxygenated hemoglobin, thus allowing separation of the two absorption signals if measured using multiple wavelengths. Since hemoglobin carries oxygen to tissues, and since tissues utilize oxygen when performing work, measurement of the relative concentrations of oxy and de-oxy hemoglobin gives a measure of the amount of work being carried out. For the cerebral cortex, this means it is possible to use light to measure which areas are active – a functional measurement. There are of course many subtleties in the fNIRS measurement, but measuring the changes in absorption for IR light is the basic enabling idea.
While fNIRS allows measurements of functional activity in a naturalistic environment, there are some limitations to the method. Only shallow areas of the cortex can be detected. Deep brain regions or areas deep in sulci are inaccessible to fNIRS because light does not penetrate the tissues to a great depth. Also, since fNIRS yields only functional information, the structure must be inferred from external landmarks or from a separate structural scan using another technology such as MRI.
Some practical aspects of an fNIRS experiment include:
For the fNIRS system to measure cortex activations, it is necessary for the light to reach the subject's scalp. The main problem is light absorption by the subject's hair. MRRA Inc. developed optodes that penetrate the hair, similar to a brush, and reliably deliver the light to and from the scalp.
It is important that the optodes do not move during the experiment, as any change in optical contact will result in noise obscuring the desired signal. Brushed optodes somewhat compensate for small motions due to the compliance of the small fibers acting as springs, but subject movement should generally be minimized as much as possible.
The light from fNIRS measurements has a very low intensity. For good results, it is necessary that light from external sources be blocked during the experiment. While built-in signal processing allows ignoring some external light signals, for best results a dark cloth should be placed over the headgear to block external light, and the experiment should be done in low-light conditions as much as is practical.
The light from the transmit optodes must not be allowed to pass directly to the detector optodes. If the light does not pass through the head, but passes directly from transmit to receive optode, then it cannot be imprinted with the absorbance signal. Usually this direct light path is blocked by the subject's hair, but for subjects with little or no hair, or for frontal lobe measurements on the forehead, the experimenter must block the direct light path. Typically the direct light path is blocked using black felt or another such dark, soft material.
Experimenters typically use averages of a block experimental design to measure the functional activations. The protocol uses rest and activity blocks as well as a starting block, which allows for a baseline measurement. Activity blocks should be long enough duration to allow the hemodynamic function to fully develop (usually 15 to 30 seconds), while the rest blocks must be long enough to allow the hemodynamics to stabilize (typically 1 minute). Experimental durations range from 10 to 30 minutes. While block design is the most common protocol, it is also possible to use fNIRS technology to establish functional connectivity between brain regions.
It is necessary to have a process for checking the fNIRS scanner for proper operation. Instruments are commonly checked using a phantom or test protocol. Some typical approaches are:
1. Solid or Liquid Phantom: A solid or liquid phantom target consisting of light scattering and absorbing materials is used to check the scanner. These phantoms range from simple to complex, and examples can be found in the literature. MRRA Inc. offers a solid-phase phantom as an optional accessory.
2. Testing a subject's arm during constriction using a blood pressure cuff: The optodes should be placed at a 'meaty' portion of the subject's forearm for this test. As the blood pressure cuff constricts the blood flow in the upper arm, the oxygen in the blood remaining in the forearm is expended, thus producing clear signals for the fNIRS measurement. Care should of course be taken so as not to restrict the blood flow for an extended period of time.
3. Breath Holding: In this method, the subject exhales normally, then stops breathing for a period of 20 to 30 seconds. This causes the body to send all available oxygen to the brain, and so generates a clear signal for the fNIRS device. This measurement is commonly done on the forehead using optodes arranged to measure each frontal lobe independently and also across the central sulcus for comparison purposes.
4. An experiment with a simple protocol yielding unambiguous signals is often used for a test. An example is finger tapping with one hand while performing a bilateral fNIRS measurement of the primary motor cortex. Activations should be observed on the contralateral side.
Anatomy of an fNIRS signal
Fiber Optics Technology
Hair-Penetrating Brush Optode Paper links:
Biomed Opt Express. May 1, 2012; 3(5): 878–898.
Brain Tutor is a free simplified brain atlas for the iPad and iPhone.
Wikipedia article on fNIRS .
Scholarpedia article on fNIRS .
Wikipedia article on the Beer-Lambert absorption law.
fNIRS.org, a link to the fNIRS community.
Wikipedia article on spread spectrum signal processing.
Wikipedia article on lock-in-amplifiers.