Image Analysis

Renal tissue sodium content maps were calculated voxel-wise. First, a calibration curve was calculated as the least-squares linear regression fit of the mean <sup>23</sup> Na signal intensity in each standard solution to their known concentrations (40, 70, 140 mmol/L). The calibration curve was applied voxel-wise to calculate tissue sodium content (TSC, mmol/L) in the imaged kidney. *T*<sup>2</sup> relaxation time maps were calculated voxel-wise for each kidney using a nonlinear least-squares fit of the 1H signal intensity at each echo time, normalized by 1H signal in a standard water phantom, to a monoexponential decay function. Anatomical images were used to segment regions of interest (ROIs) manually in renal compartments consisting of the cortex, medulla, and papilla. Mean TSC (mmol/L) and *T*<sup>2</sup> relaxation time (ms) metrics were calculated in each ROI and preserved for statistical analyses.
