Publication date: Feb 07, 2020
Na provides the second strongest MR-observable signal in biological tissue and exhibits bi-exponential T relaxation in micro-environments such as the brain. There is significant interest in developing Na biomarkers for neurological diseases that are associated with sodium channel dysfunction such as multiple sclerosis and epilepsy. We have previously reported methods for acquisition of multi-echo sodium MRI and continuous distribution modelling of sodium relaxation properties as surrogate markers of brain microstructure. This study aimed to compare Na T relaxation times to more established measures of tissue microstructure derived from advanced diffusion MRI at 7?T. Six healthy volunteers were scanned using a 3D multi-echo radial ultra-short TE sequence using a dual-tuned H/Na birdcage coil, and a high-resolution multi-shell, high angular resolution diffusion imaging sequence using a 32-channel H receive coil. Na T relaxation parameters [mean T (T) and fast relaxation fraction (T)] were calculated from a voxel-wise continuous gamma distribution signal model. White matter (restricted anisotropic diffusion) and grey matter (restricted isotropic diffusion) density were calculated from multi-shell multi-tissue constrained spherical deconvolution. Sodium parameters were compared with white and grey matter diffusion properties. Sodium T and T showed little variation across a range of white matter axonal fibre and grey matter densities. We conclude that sodium T relaxation parameters are not greatly influenced by relative differences in intra- and extracellular partial volumes. We suggest that care be taken when interpreting sodium relaxation changes in terms of tissue microstructure in healthy tissue.
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Kolbe, S.C., Syeda, W., Blunck, Y., Glarin, R., Law, M., Johnston, L.A., and Cleary, J.O. Microstructural correlates of Na relaxation in human brain at 7 Tesla. 20189. 2020 Neuroimage.