Brain imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) can see the structure of brain tissue, but cannot monitor what the brain is doing dynamically. This gap can be addressed by a class of functional brain imaging techniques that detect brain activation and associated changes in brain blood flow and metabolism, such as functional near-infrared spectroscopy (fNIRS) and functional MRI (fMRI). fNIRS was first introduced in the early 1990s and has since progressed as a promising noninvasive brain imaging tool for monitoring brain activity continuously and under natural conditions.

A critical comparison between fNIRS and fMRI is described in the perspective by Fantini et al., covering the technique’s intrinsic limitations, key challenges, and potential for brain functional connectivity studies and for noninvasive brain studies with near-infrared light.

The fNIRS approach employs specific light frequencies to measure where oxygenated and deoxygenated hemoglobin changes occur in the brain, indicating active areas of the brain and functional connectivity between regions. The technique has to overcome challenges, such as having limited or no sensitivity to brain structures deeper than the brain cortex, as well as a strong sensitivity to physiological noise from scalp and skull tissues. To tackle the latter, the authors outlined some approaches based on collecting data at short source-detector separations that are almost exclusively sensitive to physiological noise.

Given the broad applicability of fNIRS, its safety, its portability, its wearability and the capability of real-time monitoring, this near-infrared technique could extend its usage in real-life settings, field applications and bedside monitoring, opening new research and clinical opportunities in the future.

Source: “Perspective: Prospects of non-invasive sensing of the human brain with diffuse optical imaging,” by Sergio Fantini, Blaise Frederick, and Angelo Sassaroli, APL Photonics (2018). The article can be accessed at