Comparison of fNIRS with other neuroimaging modalities – fNIRS vs. fMRI
Since its implementation in the 90s, functional magnetic resonance imaging (fMRI) is seen as the gold standard for in-vivo brain imaging. However, fMRI comes with certain technological restrictions which limit its application in some experiments and subjects. Functional Near-Infrared Spectroscopy (fNIRS), which measures the same response as fMRI but using a different technique, is increasingly used as an alternative to fMRI. Read this blog post to learn more about advantages that fNIRS can provide over fMRI, considerations when using fNIRS in a study, as well as literature comparing or using both techniques simultaneously.
Measurement technique of fMRI and fNIRS
Both fNIRS and fMRI measure the so-called hemodynamic response, which conceives local changes in cerebral blood flow during brain activity. When a certain brain region becomes active, the metabolic demand and hence blood flow to this brain region increases, which usually shows after 2 to 4 seconds. These changes in blood flow are captured by fNIRS and fMRI using two different techniques:
fMRI relies on different magnetic properties of oxygenated hemoglobin (diametric, so low interaction with the magnetic field) and deoxygenated hemoglobin (parametric, generating distortion of magnetic field). Using magnetic resonance imaging and radio frequency pulses, high-resolution images that give information about regional brain activity are created. With fMRI, the blood-oxygen-level-dependent (BOLD) response is measured, which reflects changes in deoxygenated hemoglobin due to increase in increase in blood flow, when a brain region is active.
fNIRS relies on the different absorption characteristics of oxygenated and deoxygenated hemoglobin to near-infrared light (NIR). Using NIR light in different wavelengths (650 nm – 1000 nm), the relative concentration changes in oxygenated and deoxygenated hemoglobin can be assessed.
Both fNIRS and fMRI provide relative measurements, thus a baseline needs to be included in the measurements.
Although both techniques measure the same response, the differences in measurement principal lead to certain advantages and disadvantages. In the following, the advantages of fNIRS over fMRI, as well as considerations of using fNIRS will be discussed.
Advantages of using fNIRS
Mobility
fNIRS can be completely portable, enabling the subject to freely move during measurements. Hence, fNIRS allows for measuring brain activity in any settings, even outside of the lab, and during studies that include movement, or even exercise. In comparison, fMRI measurements need to be performed in an MRI scan, which leads to limited application options, i.e., stationary tasks. Due to its portability and comfort, using fNIRS hence provides increased application possibilities to experiments in naturalistic settings and during free movement.
Ease of use
fNIRS is relatively easy and straight-forward to use, and provides a quick setup. Depending on the measurement location and channel amount, fNIRS can be set up within a few minutes. fMRI however requires more preparation time, as well as more training and expertise to achieve good results and perform proper analysis.
No limitations in participants
Compared to other neuroimaging techniques, fNIRS is relatively robust to motion artifacts and, as mentioned before, can be completely wireless and mobile, enabling free movements during measurement. fMRI is highly sensitive to movement artifacts and requires subjects to stay completely still during measurements. This makes acquiring brain activity with fMRI, especially with sensitive or less compliant subjects, such as infants, children, or clinical populations, extremely difficult. Due to its robustness and portability, fNIRS is a perfect alternative for brain imaging in infants, children and neurological or psychiatric patients.
As fMRI uses a magnetic field to detect brain activity, metallic objects should not be in or close to the scanner to avoid severe injuries. Hence, subjects with metal parts implanted in their body (i.e. cochlear implant) cannot be measured in an fMRI scanner. As fNIRS is not sensitive to metal, it does not provide limitations of application in subjects with implants or metal plates.
Affordability
fMRI is a relatively expensive neuroimaging technique, which comes with high costs per performed measurements. In comparison, fNIRS is relatively affordable and in most cases does not include ongoing costs, but is more often a one-time investment. Hence, fNIRS can be especially beneficial in terms of affordability when performing multiple measurements, i.e., in case of a huge sampling, or in case of multiple measurements per subject.
High temporal resolution
Due to technical advances, fNIRS can provide higher temporal resolution and sampling rate than fMRI. Although the hemodynamic response is relatively slow, high temporal resolution can be beneficial to distinguish physiological or motion artifacts, and investigate the hemodynamic onsets.
No distraction
The gradient coils that are used during fMRI vibrate at a certain frequency, which is amplified by the MRI scan itself. This leads to loud noise, which can be experienced as uncomfortable or disturbing by subjects. Further, this noise can influence the brain responses, for instance, in case of studies investigating the perception of sound. fNIRS devices are usually relatively comfortable and light weight, and hence might cause less distraction than the fMRI scanner.
Considerations using fNIRS
Although fNIRS offers certain advantages compared to fMRI, there might also be considerations, that should be kept in mind when considering using fNIRS for a study or research project. These mainly arise from the technology behind fNIRS.
Spatial resolution
When near-infrared light enters the human tissue, it diffuses quickly, resulting in low spatial resolution compared to other modalities, such as fMRI. Thus, the penetration depth is limited and fNIRS can only detect brain activity from the surface, so from cortical brain regions. Measuring deep structures, for instance the amygdala, is not possible with fNIRS.
Lack of anatomical information
Another consideration when using fNIRS is that fNIRS does not initially provide anatomical information on the brain region it is applied on. However, there are multiple solutions to resolve this problem and guarantee correct placement of the optodes, for instance performance of 3D digitalization / usage of brain mapping software solutions based on anatomical information from standard brains (i.e. AtlasViewer).
Studies comparing fNIRS and fMRI
As fMRI is the gold standard technique in brain activity measurements, various studies comparing fNIRS with fMRI have been performed, to compare both techniques and validate usage of fNIRS, or to complementary use information of both modalities. In the following, example studies of using fNIRS and fMRI are presented.
In a recent study, Jalavandi et al. [1] combined fNIRS and fMRI to gain insights into complex brain activity patterns during motor tasks including wrist movement. The results showed strong correlation of fNIRS and fMRI and high effectiveness of fNIRS to detect neural activity during motor tasks. This indicates that fMRI can be used as a reliable alternative method in subjects that are not able to undergo fMRI scans.
In a study by Klein et al. [2] fMRI-based validation of fNIRS to measure brain activity in the supplementary motor area (SMA) during execution and imagination of movement was performed. General results suggest that fNIRS can be reliably used to detect SMA activation when looking at spatial specificity and task sensitivity.
Huppert et al. [3] performed simultaneous measurements of fNIRS and fMRI and fNIRS and magnetoencephalography (MEG) to compare and validate source-localized fNIRS on a group level during parametric median never stimulation. Good correspondence between fNIRS, fMRI and MEG was found, allocating validation of source-localized fNIRS in multimodal measurements to assess brain activity.
Literature
[1] Jalalvandi M, Sharini H, Shafaghi L, Alam NR. Deciphering brain activation during wrist movements: comparative fMRI and fNIRS analysis of active, passive, and imagery states. Exp Brain Res. 2024 Dec 31;243(1):36.
[2] Klein F, Debener S, Witt K, Kranczioch C. fMRI-based validation of continuous-wave fNIRS of supplementary motor area activation during motor execution and motor imagery. Sci Rep. 2022 Mar 4;12(1):3570.
[3] Huppert T, Barker J, Schmidt B, Walls S, Ghuman A. Comparison of group-level, source localized activity for simultaneous functional near-infrared spectroscopy-magnetoencephalography and simultaneous fNIRS-fMRI during parametric median nerve stimulation. Neurophotonics. 2017 Jan;4(1):015001.
In this blogpost, we discussed the technical background of fMRI and fNIRS, potential use cases where fNIRS has advantages over fMRI, considerations of using fNIRS, as well as literature using both techniques. If you would like to discuss, if fNIRS is a suitable fit for your research, please feel free to reach out to us at askforinfo@artinis.com.