Understanding renal DTI at 3T: FA and MD changes with water loading

Introduction: Diffusion weighted MR (DWI) primarily measures restriction to the motion of water molecules in the voxel being interrogated, but can also be sensitized to active, incoherent flow processes. DWI has been explored in kidney imaging because both parenchymal structure and fluid flow in the kidney are important reflections of renal function and may change in disease states (1-3). Most of these studies have measured changes in the directionally averaged mean diffusivity (MD) / apparent diffusion coefficient (ADC) (1-4). However, renal medulla is composed of closely packed radially oriented tubules carrying directional flow of the gloemerular filtrate from the corticomedullary junction to the renal papilla. The directional sensitivity obtained with diffusion tensor imaging (DTI) may provide better insight in the structure and function of the renal tubules. Fractional anisotropy (FA) is measure of directionality of the diffusion and has been shown to be higher in medulla with respect to the cortex (5-7). It is unclear if medullary FA is predominantly a measure of tubular structural arrangement or if it is significantly influenced by the tubular flow; antiparallel flow patterns in the tubular loop of Henle could conceivably induce an anisotropic incoherent motion effect. In response to this question, our goal for this study was to investigate the effect of tubular flow on the renal FA measures in normal healthy volunteers by evaluating changes in response to water loading (which increases tubular flow). We would expect increase in FA with water-loading if tubular flow contributed to the renal anisotropy measurements.