Abstract
A novel magnetic resonance imaging (MRI) acquisition and reconstruction method for obtaining a series of dynamic sodium 23Na-MRI acquisitions was designed to non-invasively assess the signal variations of brain sodium during a hand motor task in 14 healthy human volunteers on an ultra high field (7T) MR scanner. Regions undergoing activation and deactivation were identified with reference to conventional task-related BOLD functional MRI (fMRI). Activation observed in the left central regions, the supplementary motor areas and the left cerebellum induced an increase in the sodium signal observed at ultra short echo time and a decrease in the 23Na signal observed at long echo time. Based on a simple model of two distinct sodium pools (namely, restricted and mobile sodium), the ultra short echo time measures the totality of sodium whereas the long echo time is mainly sensitive to mobile sodium. This activation pattern is consistent with previously described processes related to an influx of Na+ into the intracellular compartments and a moderate increase in the cerebral blood volume (CBV). In contrast, deactivation observed in the right central regions ipsilateral to the movement, the precuneus and the left cerebellum induced a slight decrease in sodium signal at ultra short echo time and an increase of sodium signal at longer echo times. This inhibitory pattern is compatible with a slight decrease in CBV and an efflux of intracellular Na+ to the extracellular compartments that may reflect neural dendritic spine and astrocytic shrinkage, and an increase of sodium in the extracellular fraction. In conclusion, cerebral dynamic 23Na MRI experiments can provide access to the ionic transients following a functional task occurring within the neuro-glial-vascular ensemble. This has the potential to open up a novel non-invasive window on the mechanisms underlying brain function.
Highlights
Since its introduction in 1990 (Ogawa et al., 1990), functional magnetic resonance imaging has become an indispensable tool in neuroscience, providing insight into the localisation and organization of human brain function and task-related reorganization, and the intrinsic factors underling its integration in task and rest states, and their evolution over time (Shine et al, 2016)
Time-resolved 23Na magnetic resonance imaging (MRI) volumes obtained were very close in quality to the full sampled dataset but with an acquisition window corresponding to only one interleave (Figures 1d,e,f)
23Na signal variations in the BOLD activated regions As the primary goal was to characterize sodium variations in regions with significant BOLD variations, we studied 23Na MRI signal variations in the motor areas with significant clusters observed on BOLD functional MRI (fMRI), namely the bilateral central areas and the bilateral cerebellum
Summary
Since its introduction in 1990 (Ogawa et al., 1990), functional magnetic resonance imaging (fMRI) has become an indispensable tool in neuroscience, providing insight into the localisation and organization of human brain function and task-related reorganization, and the intrinsic factors underling its integration in task and rest states, and their evolution over time (Shine et al, 2016). We propose new methodology that enables the dynamic exploration of variations in sodium signals during brain activation/inhibition This approach consists of the combination of i) efficient acquisition schemes (radial density adapted sequences) (Nagel et al, 2009) and ultra high field MRI (7 Tesla) to counter the poor sensitivity inherent to 23Na , ii) a multi-TE approach to modulate the contributions of 23Na signals from various brain environments (i.e. restricted and mobile pools) (Rooney and Springer, 1991) (Ridley et al, 2018) and iii) a weighted least squares sliding window reconstruction method to extract sub-30 second temporal information from an extended total acquisition time (TA=20min). This innovation was applied to characterize variation in sodium signals following excitation/inhibition processes induced by a conventional right-hand motor task in human volunteers, and in particular the consequences on 23Na MRI signal of the variations in the proportions of sodium belonging to different environments (i.e. free isotropic environment as a hydrated ion or anisotropic and macromolecular environment as 'trapped' ion) within the neuro-gliavascular ensemble (Jolivet et al, 2015)
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