Continuous Wave fNIRS
Functional Near Infrared Spectroscopy (fNIRS) makes use of the specific interaction of light with biological molecules to noninvasively measure relative concentration changes of hemoglobin in cortical regions of the brain. While fNIRS is a relatively newly used neuro-imaging technique, it has first been demonstrated by Jobsis in the late 1970’s [Jobsis, 1977]. Even with it’s nearly half of century in existence, it hasn’t been until the last decade or so that fNIRS has taken to its own.
The technology has since undergone dramatic technological advancements, and different techniques have been devised to allow the extraction of a host of spectroscopic information with high signal quality. fNIRS is most commonly implemented in one of three spectroscopic methods which will be explained in following sections.
Common to all techniques is the underlying principle of trans-illuminating the tissue with a light source of defined temporal and spectral properties, and to detect the transmitted intensity level and its variation. For neuro-imaging applications it is by far most common to illuminate with two discrete wavelength, which is the minimum requirement to assess relative variations of both oxygenation states of the hemoglobin molecule independently.
The Continuous Wave (CW) method relies on the steady illumination of tissue and the detection of the transmitted light intensity. This conceptually and technically simplest form of tissue spectroscopy assesses the overall light attenuation inside the tissue and cannot differentiate effects of scattering and absorption.
This still allows the extraction of valuable information if one seeks to obtain relative changes of blood volume and oxygenation, as is the case in functional neuroimaging. The hemoglobin molecules are by far the the strongest absorbers present and only their spectroscopic signature shows significant temporal changes on the time scales of interest (~ seconds). Their relative concentration change can therefore be extracted from the background with high reliability and contrast.
This is the most widely used fNIRS method currently.
In this technique the light source is intensity-modulated in the radio (typically, 100 MHz) frequency range. At this rate, the duration between successive intensity peaks becomes comparable to the time constants to be considered in the optical diffusion processes of optical properties and spatial dimensions encountered in tissue spectroscopy. Specifically, the multiple scattering serves to effectively decrease the propagation velocity of photons over a given distance to a fraction of the speed of light (see also TD, below) so that propagating regions of alternating high and low photon are created by the incoming light modulation.This can be described as the propagation of ‘Photon Density Waves’ through the medium. These transmitted waves are then picked up by a detector of sufficiently quick response time and then compared to the impinging intensity waveform. Besides the intensity attenuation, as with CW, this allows the extraction of two more independent quantities; a phase shift (φ) and the decay of modulation depth (ratio of AC to DC component). These quantities are affected differently by absorption and scattering of the tissue so that, in principle, it is possible to distinguish these parameters (i.e., μa, μs).The explicit sensitivity to scattering changes has led to the claim of FD being capable of picking up a fast optical signal (aka, EROS) caused by the temporary nerve swelling and associated change in index of refraction/scattering coefficient.
Time Domain measurements utilize extremely short duration pulses of photons (typ., 100 ps or less) to irradiate the tissue and fast responding detectors to register the shape of the light pulse as it exits the tissue. The statistical encounter of each photon with a different number of scattering events, and their varying travel paths of randomly distributed lengths lead to a smearing-out of the initially tightly concentrated impulse over time. The properties of the received photon distribution such as area under the curve, the time of maximum, and width allow assessment of the tissues absorption and scattering properties.
In principle both fast modulation techniques, FD and TD, allow the obtaining of absolute chromophore concentration quantities.
Comparison of Techniques
When considering the different techniques for fNIRS applications, one needs to weigh off their respective strengths and weaknesses.
The main argument for considering FD or TD techniques are their ability to provide - at least in principle - a greater signal information content. However, this comes at the price of greatly increased complexity of the instrumentation and the analysis. In practice, this means that TD/FD instruments are inferior to CW setups in one or more aspects when comparing cost, size/compactness, channel count, robustness, and usability. As a consequence, the number of commercial systems using fast light modulation available is extremely low.
On the other hand, one should consider whether the prospect of obtaining absolute tissue values is really warranted. While this may be the case for measuring tissue oxygenation levels in critical care situations, most functional neuroimaging applications do not rely on absolute quantification of concentration changes. However, the strengths of the CW method such as robustness, compactness, high channel count/integration/head coverage, and a relative cost benefit are strong arguments for this method. This is why almost all commercial fNIRS systems currently on the market rely on CW measures.
More relevant to fNIRS measures than determination of absolute coefficients is dealing with other detrimental effects and limitations, such as motion artefacts, systemic/superficial hemodynamics, and basic instrumentation sensitivity/stability/dynamic range. We at NIRx have focused our efforts on providing innovative solutions for these concerns, from a user-centric approach.
The modalities differ in many ways. The most obvious of which is cost. Being the most simple measurement, CW instruments tend to be the most affordable of each of the above types. In addition, the different system types also differ in measurement reliability, consistency, and user friendliness.
In considering the most commercially viable system NIRx took the following into account. What would provide users with the most accurate and publishable data? What would NIRx be able to reasonably support? What would be the most widely usable and affordable system type?
Why Continuous Wave?
In answering these questions, NIRx settled on CW as the prominent option. As the measurements of the other two modalities tend to be less reliable and incorporate a much more highly complex analysis, CW fNIRS was chosen as the basis for our fNIRS systems.
By implementing CW fNIRS in our systems, NIRx is able to provide end-users with:
High quality and reliable data collection
A large number of references in terms of CW fNIRS publications
High quality support in terms of application and analysis
It was decided that both FD and TD fNIRS are fantastic mechanisms allowing for very interesting data, but the core mechanisms are more theoretical, and therefore more difficult to support, and the systems are far more costly than our choice of CW fNIRS.
Science. 1977 Dec 23;198(4323):1264-7
Images taken from Scholkmann et al. (2014). A review on continuous wave functional near-infrared spectroscopy and imaging instrumentation and methodology. NeuroImage, 85 (1), 6-27
Abbreviations and Symbols used:
fNIRS functional Near-Infrared Spectroscopy
CW Continuous Wave
FD Frequency Domain
TD Time Domain
μa Absorption coefficient
μs Scattering coefficient
Io, I(t), I Illumination intensity, transmitted intensity (function of time), transmitted
d Tissue thickness or source-detector distance