Flexible CW-fNIRS system based on Silicon Photomultipliers: In-Vivo characterization of sensorimotor response

We developed and tested in vivo a flexible multi-channel Continuous Wave (CW) functional Near Infrared Spectroscopy (fNIRS) system based on Silicon Photomultipliers (SiPMs). SiPMs are inexpensive, low voltage and resilient semiconductor light detectors with sensitivity analogous to PhotoMultiplierTubes (PMTs). In contrast with PMTs, SiPMs allow contact with the head avoiding optical fibers. The coupling of SiPMs and Light Emitting Diodes (LEDs) located on the scalp made the optical probe wearable and robust against motion artifacts. After characterization of SiPMs performances and system SNR, the apparatus was compared through an in vivo experiment to a commercial system based on Laser Diodes, PMTs and optical fibers. The optical probes were positioned over the sensorimotor cortex and the similarities between the estimated hemodynamic responses to the same contralateral motor task were evaluated. When compared to other state of the art wearable fNIRS systems that employ photodiodes detectors, the single photon sensitivity and significant dynamic range of SiPMs can exploit the long and variable inter-optode distances needed for estimation of brain hemodynamics through CW-fNIRS.