Mapping regional brain total sodium concentration - using anatomically- guided reconstruction of dual echo Sodium-23 MRI: moving toward improved accuracy and precision.

Background And Purpose: Sodium (Na) MRI provides unique information about ionic homeostasis in the brain. However, in vivo quantification of regional brain sodium is highly challenging due to low SNR and limited spatial resolution. Here, we employ our novel anatomically guided reconstruction (AGR) method to overcome these challenges and enable precise quantification of regional brain total sodium concentration (TSC).

Materials And Methods: Thirty-four healthy subjects were studied using a 3T clinical MRI scanner with a dual-tuned (H-Na) birdcage coil. Sodium images were acquired using a TPI sequence (TR=100ms, TE/TE=0.5/5ms), while proton (H) images were obtained with a standard T1w MPRAGE sequence. AGR was performed with regularization parameters βr =0.67, 2.0, and 6.0. As a baseline comparison, standard reconstruction (SR) was also performed using a re-gridding algorithm with compensation for nonuniform sampling. To assess partial volume effects (PVE) on the reconstruction methods, an erosion experiment was conducted. Internal linear calibration using noise-only background and vitreous humor regions was applied to calculate TSC in ROIs including lobar cortical gray matter (GM), subcortical (including hippocampus, caudate, pallidum, putamen and thalamus), callosal, and whole brain white matter (WM). Bonferroni-corrected pairwise comparison was performed using MANOVA at a significance level < 0.05.

Results: The WM erosion experiments confirmed that TSCAGR was stabilized beyond one voxel erosion in the WM, but TSCSR was decreasing with erosion increasing, showing a reduced PVE in the AGR images. AGR also shows greater separation in TSC between GM and WM compared with SR (GM TSC=49.2±4.6mMol/L, WM TSC=38.1±3.0mMol/L; GM TSC= 48.6±4.9mMol/L, WM TSC=30.5±2.8mMol/L). We also found smaller variance of TSC in WM and GM compared with TSC.

Conclusions: The AGR helps sodium quantification in healthy human brains by reducing the partial volume effect and variance of TSC in non-cortical brain regions. Our normative values of TSC in the brain regions set the stage to better understand derangements of sodium metabolism and homeostasis in neurological disease.

Abbreviations: AGR＝ anatomically-guided reconstruction; SR= Standard reconstruction; Na= sodium-23; / - = sodium-potassium pump; PVC= partial volume correction; PVE= partial volume effect; TSC= total sodium concentration; VH= vitreous humor.