Abstract
Silicon photomultipliers are novel solid state photodetectors that recently became commercially available. The goal of this paper was to investigate their suitability for low light level detection in miniaturized functional near-infrared spectroscopy instruments. Two measurement modules with a footprint of 26×26 mm2 were built, and the signal-to-noise ratio was assessed for variable source-detector separations between 25 and 65 mm on phantoms with similar optical properties to those of a human head. These measurements revealed that the signal-to-noise ratio of the raw signal was superior to an empirically derived design requirement for source-detector separations up to 50 mm. An arterial arm occlusion was also performed on one of the authors in vivo, to induce reproducible hemodynamic changes which confirmed the validity of the measured signals. The proposed use of silicon photomultipliers in functional near-infrared spectroscopy bears large potential for future development of precise, yet compact and modular instruments, and affords improvements of the source-detector separation by 67% compared to the commonly used 30 mm.
Highlights
Near-infrared spectroscopy (NIRS) is an optical and non-invasive method that can be used to determine the concentration of oxy- and deoxyhemoglobin (O2Hb and HHb, respectively) in human tissue in vivo [1, 2]
In continuous-wave functional NIRS (fNIRS), steady-state NIR light of at least two discrete wavelengths is guided to the scalp, and its diffuse reflectance is measured at some centimeters from the source site [1,2,3]
For phantom A, the critical source-detector separation (SDS), up to which the design criteria are fulfilled for both wavelengths, was roughly 50 mm, while for phantom B, it was 55 mm
Summary
Near-infrared spectroscopy (NIRS) is an optical and non-invasive method that can be used to determine the concentration of oxy- and deoxyhemoglobin (O2Hb and HHb, respectively) in human tissue in vivo [1, 2]. In functional NIRS (fNIRS), this principle is used to assess the cortical activation of brain tissue [1, 3]: increased cortical activity alters the oxygenation state of brain tissue through neurovascular coupling [3,4,5], which causes a focal increase (decrease) of O2Hb (HHb) [1, 3, 5]. A fraction of the detected photons follows a path that passes through cortical tissue, thereby carrying information about the brain. This fraction depends on the source-detector separation (SDS): the larger the SDS, the deeper the detected photons have penetrated the tissue and the larger the probed cortical volume, resulting in improved measurement signals [1,2,3,7]. Increasing the SDS dramatically increases the light attenuation (approx. 20 dB per cm SDS [8])
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