Validation of pH measurements. (A) The pH measured with CEST of Yb–DO3A–oAA at 38.3°C had outstanding agreement with the pH measured with MR spectroscopy of IEPA (slope = 1.00, R2 = 0.99, standard deviation = 0.09 pH units) at the same temperature. (B) The pH measured with MR spectroscopy of IEPA had excellent agreement with the pH measured with an electrode (slope =0.92, R2 = 0.996, standard deviation = 0.079 pH units), indicating that the presence of Yb–DO3A–oAA did not affect the chemical shift of IEPA.