Chronic Kidney Disease (CKD) is characterized by progressive renal fibrosis that leads to end-stage renal failure and the need for dialysis or kidney transplantation¹. There is a compelling need for the development of biomarkers to monitor CKD progression and help guide the evaluation of experimental treatment strategies. We hypothesize that characteristics of water diffusion and flow will serve as a biomarker for renal fibrotic burden. The objective of this study is to develop and evaluate the utility of magnetic resonance imaging based diffusion tensor² and non-parametric q-space imaging techniques³ as biomarkers of renal fibrosis in an animal model of CKD. Preparation of the animal CKD model: An ischemia/reperfusion model was used to create hypoxia induced renal fibrosis in Wistar rats (N=4). In each animal the renal artery in one kidney was clamped for 50 minutes to induce ischemia and hypoxia (IHK), followed by restoration of blood flow. The contralateral kidney served as a control reference (CON). MRI imaging: MRI was performed 2 days following surgery. During each imaging session, rats were sedated and placed in a head-first prone position. All animal handling followed institutional Animal Care and Use Committee (IACUC) guidelines. The MRI diffusion pulse sequence was a single-shot spin-echo echo-planar imaging (SS-SE-EPI) sequence with multiple diffusion-weighting b-values (i.e. 3 shells with b-values of 150, 300 and 450 s/mm²) and multiple diffusion-weighting directions at each shell (i.e., 10, 19 and 30, respectively). Diffusion directions in each shell and in the projected sphere with all directions (i.e., total 59) were optimized for uniform diffusion sampling in the spherical space⁴. The repetition time (TR) is 2200 ms and echo time (TE) is 73.6 ms. A total of four signal averages was performed. The imaging parameters were field-of-view (FOV) = 128 x 64 mm, matrix size = 128 x 64, isotropic voxel size of 1 mm³, and 20 oblique coronal slices. Image Processing: DTI derived parameters including axial diffusivity (AD), radial diffusivity (RD), mean diffusivity (MD), and fractional anisotropy (FA) were computed⁵. A non-parametric q-space approach was used to compute the probability density function (PDF), a marker of very slow water diffusion (P₀)³. ROIs: In the b0 image, anatomically defined layers of kidney are clearly identified. Two distinct ROIs were placed on medulla and cortex (Figure 1). Statistics: Student’s paired two tailed t-test were performed on individual ROI’s and multiple comparison Bonferroni corrections were implemented, two-tail p value < 0.01 was considered significant.All DTI indices (AD, RD, MD, and FA) showed statistically significant reductions in the medulla of the IHK kidneys (Figure 2A and B). The tissue restriction index, P₀, increased in the medulla of the IHK kidneys (p < 0.01). No significant changes in DTI parameters or tissue restriction index were observed in the renal cortex of the IHK kidneys (Figures 2C and D).The observed reduction in mean diffusivity measured with DTI and increased P₀ (population of very slowly diffusing water molecules) in renal medulla are consistent with the formation of fibrotic regions within the tissue. It is thought that DTI axial diffusivity is an indicator of intra-tubular flow in the renal medulla^6-9 and radial diffusivity is an indicator of water reabsorption rate⁶. The changes in AD and RD observed in this study suggest that intra-tubular flow and water reabsorption rate decreased in the IHK kidney. The dramatic decrease in FA suggests the impact of intra-tubular flow is much higher than the impact of water reabsorption rate in our renal hypoxia induced fibrosis model. In addition, the medulla appear more sensitive ischemia induced hypoxia than renal cortices.