Glioma grade and the extent of local infiltration are important factors for guiding management. Imaging tumor heterogeneity may also improve diagnosis and therapy planning. 23Na-MRI has been used here to demonstrate a gradient in sodium concentration across gliomas: necrosis greater than viable tissue greater than edema. This gradient was evident in all 17 tumors analyzed and is consistent with the expected underlying histopathology; concentration is increasing throughout the evolution from edema, dominated by the extracellular compartment, to the necrotic core, dominated by dead cells and broken sodium pumps. 23Na-MRI may therefore represent an imaging biomarker of tumor heterogeneity in glioma.