Abstract
Portable neuroimaging technologies can be employed for long-term monitoring of neurophysiological and neuropathological states. Functional Near-Infrared Spectroscopy (fNIRS) and Electroencephalography (EEG) are highly suited for such a purpose. Their multimodal integration allows the evaluation of hemodynamic and electrical brain activity together with neurovascular coupling. An innovative fNIRS-EEG system is here presented. The system integrated a novel continuous-wave fNIRS component and a modified commercial EEG device. fNIRS probing relied on fiberless technology based on light emitting diodes and silicon photomultipliers (SiPMs). SiPMs are sensitive semiconductor detectors, whose large detection area maximizes photon harvesting from the scalp and overcomes limitations of fiberless technology. To optimize the signal-to-noise ratio and avoid fNIRS-EEG interference, a digital lock-in was implemented for fNIRS signal acquisition. A benchtop characterization of the fNIRS component showed its high performances with a noise equivalent power below 1 pW. Moreover, the fNIRS-EEG device was tested in vivo during tasks stimulating visual, motor and pre-frontal cortices. Finally, the capabilities to perform ecological recordings were assessed in clinical settings on one Alzheimer’s Disease patient during long-lasting cognitive tests. The system can pave the way to portable technologies for accurate evaluation of multimodal brain activity, allowing their extensive employment in ecological environments and clinical practice.
Highlights
Investigation of brain function is becoming increasingly important in studying neurophysiological and neuropathological status
Single subject stimulation was modelled through a boxcar function that was convolved with a canonical hemodynamic response function to provide an ideal hemodynamic response to the task
The results presented here showed the capabilities of silicon photomultipliers (SiPMs) for fiberless functional near-infrared spectroscopy (fNIRS) imaging and their possibility to represent state-of-the-art detectors for wearable optical neuroimaging
Summary
Investigation of brain function is becoming increasingly important in studying neurophysiological and neuropathological status. FNIRS measures are sensitive to optical phenomena occurring within small volumes (of lateral dimension analogous to the source-detector distance) that have the shape of curved spindles (“bananas”); by changing the distance between the source and detector, different depth sensitivities can be obtained These characteristics, potentially providing a better spatial and depth resolution than EEG, require many overlapping channels with high optode density to obtain a large field of view and spatially resolved brain monitoring, making standard sparse fNIRS systems not appropriate. Considering injectable light source power, average light attenuation within the head, and SiPMs detection area, this sensitivity allows to perform fNIRS measurements at 70–80 mm of interoptode distance These distances can enable the investigation of brain regions up to a depth of ~30 mm from the scalp and to utilize SiPMs in multidistance high density optical arrays for tomographic approaches (i.e., Diffuse Optical Tomography, DOT) using fiberless wearable fNIRS technology [34,35], and with possible integration with EEG. Disease (AD) patient during long-lasting (~1 h) cognitive test routinely performed for AD diagnosis
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