Abstract
.With the aim of transitioning functional near-infrared spectroscopy (fNIRS) technology from the laboratory environment to everyday applications, the field has seen a recent push toward the development of wearable/miniaturized, multiwavelength, multidistance, and modular instruments. However, it is challenging to unite all these requirements in a precision instrument with low noise, low drift, and fast sampling characteristics. We present the concept and development of a wearable fNIRS instrument that combines all these key features with the goal of reliably and accurately capturing brain hemodynamics. The proposed instrument consists of a modular network of miniaturized optode modules that include a four-wavelength light source and a highly sensitive silicon photomultiplier detector. Simultaneous measurements with short-separation (7.5 mm; containing predominantly extracerebral signals) and long-separation (20 mm or more; containing both extracerebral and cerebral information) channels are used with short-channel regression filtering methods to increase robustness of fNIRS measurements. Performance of the instrument was characterized with phantom measurements and further validated in human in vivo measurements, demonstrating the good raw signal quality (signal-to-noise ratio of 64 dB for short channels; robust measurements up to 50 mm; dynamic optical range larger than 160 dB), the valid estimation of concentration changes (oxy- and deoxyhemoglobin, and cytochrome-c-oxidase) in muscle and brain, and the detection of task-evoked brain activity. The results of our preliminary tests suggest that the presented fNIRS instrument outperforms existing instruments in many aspects and bears high potential for real-time single-trial fNIRS applications as required for wearable brain–computer interfaces.
Highlights
Functional near-infrared spectroscopy is an optical technique for the noninvasive measurement of human brain activity.[1,2] Near-infrared (NIR) light, emitted from light sources placed on the scalp, travels through the tissues and is measured as diffusely reflected light by detectors
In our previous work,[24] we developed a first prototype of an silicon photomultipliers (SiPMs)-based functional near-infrared spectroscopy (fNIRS) instrument demonstrating a high signal-to-noise ratio (SNR) of more than 70 dB for source–detector separation (SDS) below 30 mm, as well as a high photosensitivity for small light intensities at larger SDS
The modules were manufactured from rigid-flex printed circuit board (PCB), which allowed the miniaturization of the modules to a size of 20.5 × 18 × 8 mm[3] (PCB only without mechanical casing) or 25 × 22 × 10 mm[3], and a corresponding weight of 3 and 5 g, respectively
Summary
Functional near-infrared spectroscopy (fNIRS) is an optical technique for the noninvasive measurement of human brain activity.[1,2] Near-infrared (NIR) light, emitted from light sources placed on the scalp, travels through the tissues (i.e., scalp, skull, and brain) and is measured as diffusely reflected light by detectors. When a person is mentally active (e.g., during the execution of a motor task), the concentrations of O2Hb (1⁄2O2Hb) and HHb ([HHb]) in the brain vary (functional hyperemia due to neurovascular coupling), thereby providing an estimate for local brain activity detectable by fNIRS. Through the development of fiberless and portable instruments, fNIRS opens new avenues for wearable brain imaging applications.[3,4,5,6,7,8,9,10] In particular, wearable fNIRS devices may find application in brain–computer interfaces (BCI) for “out of the lab” applications, e.g., to trigger robotic devices for assistance or rehabilitation of neurological patients in the home environment,[11,12] for the communication of locked-in patients,[13,14] or for neuroergonomic investigations
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