Abstract
Similar to functional magnetic resonance imaging (fMRI), functional near-infrared spectroscopy (fNIRS) detects the changes of hemoglobin species inside the brain, but via differences in optical absorption. Within the near-infrared spectrum, light can penetrate biological tissues and be absorbed by chromophores, such as oxyhemoglobin and deoxyhemoglobin. What makes fNIRS more advantageous is its portability and potential for long-term monitoring. This paper reviews the basic mechanisms of fNIRS and its current clinical applications, the limitations toward more widespread clinical usage of fNIRS, and current efforts to improve the temporal and spatial resolution of fNIRS toward robust clinical usage within subjects. Oligochannel fNIRS is adequate for estimating global cerebral function and it has become an important tool in the critical care setting for evaluating cerebral oxygenation and autoregulation in patients with stroke and traumatic brain injury. When it comes to a more sophisticated utilization, spatial and temporal resolution becomes critical. Multichannel NIRS has improved the spatial resolution of fNIRS for brain mapping in certain task modalities, such as language mapping. However, averaging and group analysis are currently required, limiting its clinical use for monitoring and real-time event detection in individual subjects. Advances in signal processing have moved fNIRS toward individual clinical use for detecting certain types of seizures, assessing autonomic function and cortical spreading depression. However, its lack of accuracy and precision has been the major obstacle toward more sophisticated clinical use of fNIRS. The use of high-density whole head optode arrays, precise sensor locations relative to the head, anatomical co-registration, short-distance channels, and multi-dimensional signal processing can be combined to improve the sensitivity of fNIRS and increase its use as a wide-spread clinical tool for the robust assessment of brain function.
Highlights
Functional near-infrared spectroscopy is a wellestablished non-invasive tool to continuously assess regional tissue oxygenation at bed-side
Near-infrared spectroscopy in the brain is made possible by the relative transparency of biological tissues to light for infrared wavelengths ranging from 650 to 925 nm
Like functional magnetic resonance imaging (fMRI), the response of the functional near-infrared spectroscopy (fNIRS) signals to changes in neuronal activity are convolved with a hemodynamic response function which describes the latencies, overshoots and undershoots of the arteriole sphincters that control blood flow into the capillary beds in responses to changes in metabolic demand of the neuronal tissue (Osharina et al, 2010)
Summary
Functional near-infrared spectroscopy (fNIRS) is a wellestablished non-invasive tool to continuously assess regional tissue oxygenation at bed-side It was first described by Jöbsis 40 years ago (Jobsis, 1977) and has been utilized in different clinical settings, especially in the field of neuroscience (Obrig, 2014; Hong and Yaqub, 2019). The brain is a high energy-demand organ and neuronal activation correlates with increases in cerebral blood flow and volume This so-called “neurovascular” coupling is the fundament of many functional neuroimaging techniques, including fNIRS, functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and single-photon emission computerized tomography (SPECT). Group analysis and signal processing can increase the SNR, but it is still unable to analyze individual events, such as seizures, in real-time Techniques such as vector diagram analysis to detect the initial dip in the hemodynamic response have helped to tackle this problem (Zafar and Hong, 2018). Coupled with the ability to simultaneously measure fNIRS together with other neuroimaging analysis techniques such as EEG and neural network learning (Yang et al, 2019) fNIRS has the potential for widespread clinical applications in neuroscience
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