Evaluation of the reproducibility of intrarenal R2* and DeltaR2* measurements following administration of furosemide and during waterload.

It is now well recognized that the renal medulla operates at a low oxygenation level (renal medullary hypoxia) (1), which makes it highly susceptible to even mild reductions in blood flow. Renal ischemia accounts for almost 50% of the observed cases of acute renal failure (2). Over the years, in several in vitro and in vivo animal studies, it has been shown that this type of acute renal failure usually involves hypoxic injury to the renal medullary tubules (3). Renal dysfunction may also play a role in the development of all forms of hypertension in humans and laboratory animals (4). Many experiments have shown that medullary blood flow (and presumably medullary oxygenation status) is reduced in hypertension, and, more importantly, that reduced medullary blood flow is sufficient to produce hypertension (4,5). All of these studies were performed with the use of invasive microelectrodes or Doppler flow probes in rat kidneys. The availability of a noninvasive technique to monitor renal medullary blood flow in humans under normal conditions and during physiological and pharmacological stresses may allow for an extension of these observations to humans. Blood oxygenation level-dependent (BOLD) MRI has been used extensively in various organs, such as the brain (6,7). The BOLD MRI technique exploits the fact that the magnetic properties of hemoglobin vary depending on whether it is in the oxygenated or deoxygenated form. This affects the T2∗ relaxation time of the neighboring water molecules, and in turn influences the MRI signal on T2∗- weighted images. The rate of spin dephasing R2∗(=1/T2∗) is closely related to the tissue content of deoxyhemoglobin. Since the oxygen tension (pO2) of capillary blood is thought to be in equilibrium with the surrounding tissue, changes estimated by BOLD MRI can be interpreted as changes in tissue pO2 (8). A strong correspondence has been demonstrated between BOLD MRI measurements in humans and earlier animal data obtained with the use of invasive microelectrodes (9). In previous studies, we used BOLD MRI to measure the response of the renal medulla to furosemide in healthy volunteers (8,10). Furosemide is one of the so-called “loop” diuretics. These diuretics have been reported to improve medullary oxygenation, presumably because they selectively decrease oxygen use by blocking active transport in the thick ascending limbs (1,9). One of the main findings in previous studies was that the increased average level of medullary oxygenation in young healthy subjects following pharmacological maneuvers can be monitored noninvasively with BOLD MRI (8,11). Water diuresis is the result of reduced osmotic reabsorption of water, which leads to increased solute- free water clearance and hypoosmotic urine. Water diuresis can potentially “washes out” obstructing cellular debris and casts, and thus may protect against acute renal failure (12). Water diuresis can be induced by drinking large amounts of hypotonic fluid. During water diuresis, the amount of water reabsorbed in the proximal tubules is normal, and the maximal urine flow that can be produced is about 16 ml/min. Our previous studies showed that water diuresis decreased intrarenal R2∗ in young healthy human subjects evaluated by BOLD MRI (10,13,14). Since BOLD MRI is primarily used to assess changes following a physiological or pharmacological maneuver, it is essential that we understand the level of reproducibility of baseline R2∗ measurements (i.e., how close are the R2∗ measurements obtained in the same subject under similar conditions on two different days or times?). This is usually estimated by means of a test-retest analysis. Another significant measurement is the response to a maneuver ( ΔR2∗(preR2∗−postR2∗)). In this case, it is important to know how the magnitude of response compares to variations in baseline R2∗, and how much this magnitude varies from day to day. The reproducibility of baseline R2∗ and ΔR2∗ would depend not only on the variability associated with the measurement system, but also on the physiological status of the subject at these two different time points (especially in a water diuresis study). To minimize the effects of ingested substances on the BOLD test, an overnight fast is required to ensure that the volunteer is in the basal state. However, the pre- fast ingestion of a salty, sweet, or fatty food, or alcohol or coffee may still induce some physiological variations. Exercise and emotional or physical stress may also contribute to physiological variations. In this study, we investigated the degree of reproducibility of BOLD measurements in the renal medulla and cortex in the same group of healthy individuals before and after administration of furosemide and induction of water diuresis on different days.