Sodium MRI in Pediatric Brain Tumors

Direct sodium MRI (²³Na-MRI) derives its signal from spin-manipulation of the ²³Na nucleus itself and not the more conventional and familiar ¹H-MRI. Although present at much lower concentrations in the human body than the ¹H nuclei in the water molecule H₂O, advances in coil design and pulse sequence development have enabled the feasibility of human in vivo ²³Na-MRI. Additionally, ²³Na-MRI has the potential to offer nuanced physiologic insights not available to conventional MRI; this feature forms the basis of interest in its development and optimism for its novel clinical utility. ²³Na-MRI has the potential to be a useful noninvasive imaging technique to assess biochemical and physiologic cellular changes in tissues, eg, cell integrity and tissue viability. Pathologically, the concentration of total sodium is elevated in tumors relative to normal counterparts due to increased intracellular sodium and/or an increased proportion of extracellular space (reflecting changes in cell morphology and anomalies of homeostasis). Here we review the technological advancements with improved pulse sequences and reconstruction methods that counter the inherent challenges of measuring sodium concentrations in the pediatric brain (in particular, its short-tissue T2 value) and present detailed imaging approaches to quantifying sodium concentrations in the pediatric brain that can be assessed in various CNS pathologies, with the focus on pediatric brain tumors.