Venous contribution to sodium MRI in the human brain.

Purpose: Sodium MRI shows great promise as a marker for cerebral metabolic dysfunction in stroke, brain tumor, and neurodegenerative pathologies. However, cerebral blood vessels, whose volume and function are perturbed in these pathologies, have elevated sodium concentrations relative to surrounding tissue. This study aims to assess whether this fluid compartment could bias measurements of tissue sodium using MRI. Methods: Density-weighted and B-1 corrected sodium MRI of the brain was acquired in 9 healthy participants at 4.7T. Veins were identified using co-registered H-1 T-2*-weighted images and venous partial volume estimates were calculated by down-sampling the finer spatial resolution venous maps from the T-2*-weighted images to the coarser spatial resolution of the sodium data. Linear regressions of venous partial volume estimates and sodium signal were performed for regions of interest including just gray matter, just white matter, and all brain tissue. Results: Linear regression demonstrated a significant venous sodium contribution above the underlying tissue signal. The apparent venous sodium concentrations derived from regression were 65.8 +/- 4.5 mM (all brain tissue), 71.0 +/- 7.4 mM (gray matter), and 55.0 +/- 4.7 mM (white matter). Conclusion: Although the partial vein linear regression did not yield the expected sodium concentration in blood (similar to 87 mM), likely the result of point spread function smearing, this regression highlights that blood compartments may bias brain tissue sodium signals across neurological conditions where blood volumes may differ.