Glioma grade and the extent of local infiltration are important for guiding management. Imaging tumor heterogeneity may improve diagnosis and therapy planning. 23Na-MRI has been used here to demonstrate a gradient in sodium concentration across gliomas: necrosis > viable tissue > edema. This gradient was evident in all the tumors analyzed and is consistent with the expected underlying cellular microstructure where the sodium concentration is dominated by the extracellular fluid in edema and by an absence of cells in the necrotic core. The study provides evidence that 23Na-MRI represents an imaging biomarker of tumor heterogeneity and tissue microstructure in glioma.