Abstract
We report the development of a compact probe for time-domain (TD) functional near-infrared spectroscopy (fNIRS) based on a fast silicon photomultiplier (SiPM) that can be put directly in contact with the sample without the need of optical fibers for light collection. We directly integrated an avalanche signal amplification stage close to the SiPM, thus reducing the size of the detection channel and optimizing the signal immunity to electromagnetic interferences. The whole detection electronics was placed in a plastic screw holder compatible with the electroencephalography standard cap for measurement on brain or with custom probe holders. The SiPM is inserted into a transparent and insulating resin to avoid the direct contact of the scalp with the 100-V bias voltage. The probe was integrated in an instrument for TD fNIRS spectroscopy. The system was characterized on tissue phantoms in terms of temporal resolution, responsivity, linearity, and capability to detect deep absorption changes. Preliminary in vivo tests on adult volunteers were performed to monitor hemodynamic changes in the arm during a cuff occlusion and in the brain cortex during a motor task.
Highlights
Functional near-infrared spectroscopy is used for research and for preliminary clinical tests for the noninvasive assessment of tissue hemodynamic parameters such as oxy-hemoglobin concentration (O2Hb) and deoxy-hemoglobin concentration (HHb).[1,2] Because of its relatively low cost and compactness with respect to conventional neuroimaging methods such as functional magnetic resonance imaging and the possibility to be implemented in portable and wearable devices, functional near-infrared spectroscopy (fNIRS) has the potential to be used in both normal life conditions and clinical settings
In 2015, we demonstrated the possibility of employing silicon photomultiplier (SiPM) for TD diffuse optics,[8,11,12] obtaining timing resolutions down to 57 ps full-width at half maximum (FWHM)
We have reported, for the first time, the possibility of employing fiber-free SiPM detectors for in vivo TD fNIRS acquisitions
Summary
Functional near-infrared spectroscopy (fNIRS) is used for research and for preliminary clinical tests for the noninvasive assessment of tissue (e.g., muscle and brain cortex) hemodynamic parameters such as oxy-hemoglobin concentration (O2Hb) and deoxy-hemoglobin concentration (HHb).[1,2] Because of its relatively low cost and compactness with respect to conventional neuroimaging methods such as functional magnetic resonance imaging (fMRI) and the possibility to be implemented in portable and wearable devices, fNIRS has the potential to be used in both normal life conditions and clinical settings. In 2015, we demonstrated the possibility of employing SiPMs for TD diffuse optics,[8,11,12] obtaining timing resolutions down to 57 ps full-width at half maximum (FWHM) Since it was used in contact with the sample, we obtain high diffused photon harvesting efficiency thanks principally to the possibility of avoiding cumbersome fibers and lenses, exploiting the whole device NA (close to 1). SiPMs are inexpensive and require very compact and cheap front-end electronics, allowing the possibility of directly integrating the detector together with the electronics on the probe In this way, the detection chain of the instrument can be very compact, rugged, easy to use, and low cost.
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