Abstract
We present a multi-channel, multi-distance broadband near-infrared spectroscopy (NIRS) system with the capability of measuring changes in haemoglobin concentrations (Δ[HbO2], Δ[HHb]), oxidation state of cytochrome-c-oxidase (Δ[oxCCO]) and tissue oxygen saturation (TOI) in the adult human brain. The main components of the instrument are two customized spectrographs and two light sources. Each spectrograph is lens-based to improve light throughput, has a grating enhanced to optimise reflection in the near-infrared (NIR) spectral region and uses a front illuminated cooled CCD camera (−70° C) with a square chip dimension of 12.3 x 12.3 mm (512 x 512 pixels). Each light source uses a 50W halogen bulb with a gold plated mirror to increase the intensity of the NIR light. Each light source was connected to a custom-built bifurcated fibre bundle to create two source fibre bundles (3.2 mm diameter each). Each spectrograph received light input from another custom-built fibre bundle comprised of six individual bundles (one with 0.6 mm diameter and the other five with 1.5 mm diameter). All fibre bundles were fixed on a 3D printed optode holder (two light sources x two fibre bundles each = four probes; and two spectrographs x six fibre bundles each = 12 probes) that allowed 24 multi-distance channels across the forehead (six channels at 20 mm, three channels at 30 mm and 15 channels at 35 mm) and six TOI measurements. We demonstrated the use of the system in a cohort of nine healthy adult volunteers during prefrontal cortex functional activation using the Stroop task. We have observed functional responses identified as significant increase in Δ[HbO2], decrease in Δ[HHb] and increase in Δ[oxCCO] in five channels (out of 12), that overlay the left and right dorsolateral prefrontal cortices. There was no observable TOI functional response and we have shown small variations in TOI across different sites within the same subject and within the same site across subjects.
Highlights
Near-infrared spectroscopy (NIRS) has for many years been used to investigate changes in haemodynamics and metabolism of various types of biological tissue, including the adult human brain [1,2,3]
To demonstrate the capability of the system, we used it to monitor the prefrontal cortex during functional activation induced by the Stroop task; and we investigate the functional response of the oxCCO signal by identifying channels with significant change during the functional task
The distribution has a flat shape with no prominent features and low intensity value as expected when there is no light input into the spectrograph
Summary
Near-infrared spectroscopy (NIRS) has for many years been used to investigate changes in haemodynamics and metabolism of various types of biological tissue, including the adult human brain [1,2,3]. Most commercial continuous wave (CW) functional NIRS (fNIRS) devices rely on dual wavelength approaches, limiting their capabilities to the measurements of concentration changes of oxygenated (Δ[HbO2]) and deoxygenated haemoglobins (Δ[HHb]) [4]. These measurements are not directly related to changes in metabolic activity during neuronal activation. A recent review by Bale and colleagues provides details on the spectral features of CCO, the methodology, analysis techniques as well as a summary of human brain studies regarding the NIRS measurement of cerebral oxCCO in both the healthy and injured brain [11]
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