Dynamics of human cerebral fluids measured by near-infrared spectroscopy

Abstract

Obtaining parameters that characterize cerebral fluid interactions in the human brain is of high interest particularly as regards studies of the glymphatic system and in relation to neurodegeneration diseases. Near-infrared spectroscopy (NIRS) based techniques commonly measure cerebral hemodynamics using a combination of wavelengths approximately between 650 nm and 950 nm, where light is to a lesser amount attenuated by water, enabling light to reach the brain. By adding a wavelength that is dominantly absorbed by water, while still penetrating below skull, we may have a possibility to measure also cortical water concentration changes, particularly dynamics of the cerebrospinal fluid (CSF) volume, which have been connected to brain's waste removal system. In this study, we show based on in vivo human experiments that small dynamical variations in the CSF layer, between the human skull and brain cortex determined by MRI, correlate with nearinfrared (NIR) light intensity changes particularly above 960 nm when measured at long (> 3 cm) source-detector distance. In addition, based on the previously reported anti-correlation between total haemoglobin (HbT) and water signal fluctuations measured with NIRS, we further investigated the differences in the anti-correlations when using short (< 2 cm) and long source-detector distances. In general, at a short source-detector distance the NIRS measurement volume does not reach a depth below human skull. In consequence, our results from 12 healthy subjects show greater anti-correlation between HbT and water when using a long source-detector distance, supporting the idea that NIRS can be used to monitor also human cortical water fluctuations non-invasively.

Validation of Monro-Kellie doctrine during blood pressure lowering in mice using fNIRS

“…We validated the Monro-Kellie doctrine phenomenon in mice during BP lowering procedures using fNIRS. In our previous study on human cerebral fluid concentration using fNIRS 12 , we also found the same phenomenon about Monro-Kellie doctrine. Thus, fNIRS has potential as a bed-side device to monitor Monro-Kellie doctrine phenomenon for patients.…”

“…Previously, we measured human cerebral fluid concentrations using fNIRS and the results support the Monro-Kellie doctrine 12,13 . Then, we were interested in studying this doctrine further in mice during blood pressure lowering treatment because the physiological response due to cerebral autoregulation provides a good scenario.…”