In vivo pH was calculated in a similar manner to the in vitro pH calculation. The calibration equation used was pH = p1 × (ST ratio)3 + p2 × (ST ratio)2 + p3 × (ST ratio)1 + p4 with p1 = − 0.0317, p2 = 0.4463, p3 = − 2.5029, and p4 = 20.5146. The root mean square error of the fit is 0.0714. From the images corresponding to the two offsets, 4.2 and 5.5 ppm over time, ~16 images were selected, with the initial image occurring 40 to 50 min after injection for both Mut+/− and Mut−/− mice for time averaging. The pixel-by-pixel ST ratio was calculated for these images, and using the in vitro calibration equation, the pixel-by-pixel pH was calculated. This was masked using the >20% contrast criteria found for the FF pixels. Mean pH maps were generated, and the mean pH range observed was calculated by averaging the pH of the images selected. Histogram plots of pH for both RD and HP diet mice were calculated for the pH images by keeping the bin size of 200 between the pH values of 4 and 10. After producing the histograms, these data were fit to a normal distribution (Gaussian function was used) to obtain the mean or the most probable pH and range in pH for each mouse. The full width at half maximum of this Gaussian function was used to quantify the ΔpH values across the kidneys.

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