Selected subjects will return for a third visit at CHUV (V3): they will start at home with an oral hydration protocol (load dose of 3 ml / kg at 8:00 am, followed by 1 ml / kg every hour between 9:00 am and 3:00 pm to avoid as much as possible changes in kidney perfusion). Patients will arrive at the Department of Nephrology and Hypertension (CHUV) at 11.00 am. In this occasion, two catheters will be placed into each patient's arm. Later, participants will be escorted to the Radiology Department to undergo four renal oxygenation imaging (between 1:00 pm and 2:00 pm) by the mean of BOLD-MRI technique. During this period, patients and control subjects will lie down and are not allowed to stand up. At T0, they will receive a bolus (0.75 ml / kg) of glucose 20%. Subsequently, four BOLD-MRI scans, together with blood tests, will be performed at T0, T1 (+10 min), T2 (+20 min), T3 (+30 min). Sodium intake will be measured by 24 hours urinary collection the day before V3 (sodium is known to influence R2*).