Non-invasive measurement of in vivo total sodium concentration (TSC) has been possible due to advances in sodium MRI at clinical field strengths. Changes in white matter (WM) and grey matter (GM) concentrations have been reported in a number of different diseases like, Multiple Sclerosis (MS). However, the presence of regional differences in normal healthy brain TSC has not been yet investigated. Here we use Geodesic Information Flow¹ (GIF) technique for computing per subject brain parcellations to allow differentiation of these areas and subsequently characterization of regional TSC in healthy controls.

Sodium Protocol: 10 healthy controls were scanned on a 3T Philips Achieva and underwent two protocols in the same session using: a 32-channel head coil and a fixed tuned sodium volume coil (Rapid Biomedical). The first protocol was composed of PD/T2 (1x1x3 mm³) and 3DT1-weighted (1x1x1 mm³) MRI. The second 23Na-MRI protocol was a 3D-Cones UTE sequence² (3x3x3 mm³) (see Figure 1) and a 1H PD/T2w (1x1x3 mm³) sequence. Two agar phantoms with 40 and 80 mM NaCl were placed either side of the head for quantification.

Processing: Probabilistic brain tissue segmentation was performed on the 3DT1 images using GIF (see Figure 2). The segmentation masks were registered to the TSC maps. To preserve mask integrity, masks were resampled to sodium space using a point-spread function³ with a linear interpolation. The resample step was performed using the concatenation of three affine transformations from the registrations of: 3DT1 to the pseudo-T1 images (PDw-T2w images⁴), the T2w to the 1H PD/T2w images, and finally 1H PD-T2w image to the TSC map.

Partial volume correction: With the aim to remove the CSF sodium contribution in the brain tissues, we applied a voxel-by-voxel modified partition-based subject-specific correction⁵. The average TSC for the voxels with 100% probability of being CSF were computed. Then the final tissue sodium concentration was expressed as $$$C_{tissue}=\frac{(C_{voxel}-C_{CSF}*VF_{CSF})}{VF_{tissue}}$$$ where C represents TSC and VF is the volume fraction provided by GIF. As in Paling et al., only voxels with at least 20% tissue VF were included to produce a mask threshold to 0.2 for each area. In order to reject unrealistically high TSC values, a threshold was set to 2 standard deviations above the mean of the TSC in voxels with at least 95% of tissue probability (TSC_{mean_95%}). This was used as an additional threshold so that only voxels within the 0.2 threshold mask, which also had TSC values within two standard deviations of the TSC_{mean_95%}, were included.

Selected brain areas: We selected and compared the following brain areas from both the left and right hemispheres: Frontal Lobe, Occiptal Lobe, Sensorimotor Cortex, Hippocampus, Limbic System, Thalamus, Cerebral White Matter, Brainstem, Cerebellum White Matter, Cerebellum Grey Matter and Cerebellar Vermis. Following this the Sensorimotor Cortex area was split into two groups depending on whether the subjects were left or right handed.

Table 1 shows the sodium concentration per brain tissue type, and Table 2 per brain area. Most of the brain regions have similar concentrations in both hemispheres and differences are not significant. Only the Occipital Lobe (visual areas) has significant differences between hemispheres. The Sensorimotor Cortex for left handed subjects has significant differences from the TSC of right-handed subjects, but this could be due to the lower sample size. All left handed subjects have less sodium concentration in the right hemisphere and vice versa with right handed subjects.We have presented the first regional sodium concentration study in human healthy brain. Future work will analyse a larger cohort and will explore the correlation with clinical scales for different diseases.