1. A method for monitoring a kidney comprising the steps of: a. 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; b. 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 c. mapping the sodium distribution at high resolution to indicate the sodium concentration gradient of the corticomedullary axis of the kidney.