Microstructural correlates of ²³Na relaxation in human brain at 7 Tesla

²³Na provides the second strongest MR-observable signal in biological tissue and exhibits bi-exponential T₂^∗ relaxation in micro-environments such as the brain. There is significant interest in developing ²³Na 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 ²³Na T₂^∗ 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 ¹H/²³Na birdcage coil, and a high-resolution multi-shell, high angular resolution diffusion imaging sequence using a 32-channel ¹H receive coil. ²³Na T₂^∗ relaxation parameters [mean T₂^∗ (T₂^∗_mean) and fast relaxation fraction (T₂^∗_ff)] 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 T₂^∗_mean and T₂^∗_ff showed little variation across a range of white matter axonal fibre and grey matter densities. We conclude that sodium T₂^∗ 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.