Sodium MRI: a new frontier in imaging in nephrology

Purpose of review This review focuses on the recent technological advances in quantitative sodium (23Na) MRI to provide a noninvasive measure of tissue viability for use in clinical studies of patients with kidney disease. 23Na MRI is the only noninvasive imaging technique that allows for the absolute spatial quantification of tissue sodium concentration (TSC), providing assessment of the corticomedullary sodium gradient (CMSG) in the kidney, and allowing measures of TSC in the skin and muscle. Recent findings 23Na MRI of the kidney has demonstrated the sensitivity to measure the CMSG, providing the normal range in healthy individuals and demonstrating a reduction in CMSG in kidney disease and transplanted kidneys. Studies using 23Na and 1H MRI have shown that in humans, skeletal muscle and skin can store sodium without water retention, and that sodium concentrations in muscle and skin increase with advancing age. Recent studies have shown that TSC can be mobilised during haemodialysis, and that skin sodium content links closely to left ventricular mass in patients with chronic kidney disease. Summary 23Na MRI is currently a research technique, but with future advances, 23Na MRI has potential to become a noninvasive renal biomarker and a measure of tissue sodium storage for clinical studies.