Background The countercurrent arrangement of capillary blood flow in the medulla of mammalian kidneys generates a gradient of oxygen tension between the renal cortex and the papillary tip that results in a state of relative hypoxia within the renal medulla. Exploration of the pathophysiological implications of medullary hypoxia has been hampered by the absence of a noninvasive technique to estimate intrarenal oxygenation in different zones of the kidney. In the present study, we demonstrate the feasibility of such a method on the basis of blood oxygenation level–dependent (BOLD) MRI, which allows sequential measurements in humans in response to a variety of physiological/pharmacological stimuli in health and disease.

Methods and Results BOLD MRI measurements were obtained in healthy young human subjects (n=7), and the effects of three different pharmacological/physiological maneuvers that induce diuresis were studied. Spin-spin relaxation rate, R₂*, was measured, which is directly related to the amount of deoxyhemoglobin in blood and in turn to tissue Po₂. Furosemide but not acetazolamide (n=6 each) increased medullary oxygenation (ΔR₂*=7.62 Hz; P<.01), consistent with the separate sites of action of these diuretics in the nephron and with previous direct measurements of their effects in anesthetized rats with oxygen microelectrodes. A new finding is that water diuresis improves medullary oxygenation (ΔR₂*=6.43 Hz; P<.01) in young human subjects (n=5).

Conclusions BOLD MRI can be used to monitor changes in intrarenal oxygenation in humans in a noninvasive fashion.