A method and apparatus for monitoring a kidney that consists of monitoring the corticomedullary sodium concentration gradient in a kidney by an imaging technique selected from the group consisting of MRI, optical imaging, computed tomography (CT), ultrasound or positron emission tomography (PET), to obtain dynamic images; processing the obtained images to quantitatively determine, pixel by pixel of the images, the concentration of sodium along the corticomedullary axis of the kidney; and mapping the sodium distribution at high resolution to indicate the sodium concentration gradient of the corticomedullary axis of the kidney. Preferably the monitoring is carried out using .sup.23Na MRI. Maps of the sodium distribution are displayed.