*4.2. Magnetic Resonance Imaging Acquisition*

Imaging experiments were performed in the Vanderbilt University Institute of Imaging Science (VUIIS) Center for Small Animal Imaging. Live animals were anesthetized and respiration and temperature were continuously monitored during imaging. Sodium MRI was acquired with custom-built, single-tuned sodium (23Na) surface coil (approximately 2 cm in diameter) placed over the kidney and the animal positioned prone in a 63 mm quadrature proton (1H) volume coil in a 9.4T scanner (Agilent Technologies, Santa Clara, CA, USA). Sodium standards (NaCl in milli-q water with concentrations 40, 70, 140 mmol/L) were incorporated in the image field-of-view (FOV) to calibrate 23Na signal intensity to standard sodium concentrations. Sodium MRI was acquired using a gradient echo multi-slice sequence with repetition time (TR) = 150 ms, echo time (TE) = 1.45 ms, FOV = 80 × 80 mm2, matrix = 32 × 32 interpolated to 128 × 128, slice thickness = 20 mm, and the number of experiments (NEX) = 100. Anatomical T1-weighted images were acquired in an identical FOV as sodium MRI with a fast spin-echo sequence (TR/TE = 2000/20 ms, matrix = 128 × 128, number of slices = 10, slice thickness = 2 mm, and NEX = 4 respiratory-triggered gated acquisitions), sufficient to locate renal compartments.

Quantitative *T*<sup>2</sup> mapping, which measures the transverse relaxation rate of water protons in tissue, is a commonly used technique both clinically and pre-clinically for identifying and evaluating edema [34,35]. Proton MRI for *T*<sup>2</sup> relaxation time quantification was acquired at 7T (Bruker Avance III) in live animals with the kidneys centered in a 72 mm quadrature proton (1H) volume coil (Bruker Biospin). A water standard (3 mm NMR tube filled with 5 mM copper sulfate (CuSO4) in distilled water) was placed next to the left kidney. High spatial-resolution *T*2-weighted anatomical imaging was performed using a RARE (Rapid Acquisition with Relaxation Enhancement) sequence with TR/TE = 2000/45 ms, FOV = 70 × 70 mm2, slice thickness = 1 mm, matrix = 128 × 128, with 28–36 slices covering the kidneys bilaterally, and NEX = 8. In one axial slice through the center of each kidney, multi-spin-echo imaging was performed for *T*2-relaxation time mapping (TR = 2000 ms, echo spacing = 7 ms, 16 echoes, FOV = 70 × 70 mm2, matrix = 64 × 64, NEX = 4).
