A generalized ratiometric chemical exchange saturation transfer (CEST) MRI approach for mapping renal pH using iopamidol

PURPOSE: To extend the pH detection range of iopamidol-based ratiometric chemical exchange saturation transfer (CEST) MRI at sub-high magnetic field and establish quantitative renal pH MRI. METHODS: Chemical exchange saturation transfer imaging was performed on iopamidol phantoms with pH of 5.5 to 8.0 and in vivo on rat kidneys (n = 5) during iopamidol administration at a 4.7 T. Iopamidol CEST effects were described using a multipool Lorentzian model. A generalized ratiometric analysis was conducted by ratioing resolved iopamidol CEST effects at 4.3 and 5.5 ppm obtained under 1.0 and 2.0 µT, respectively. The pH detection range was established for both the standard ratiometric analysis and the proposed resolved approach. Renal pH was mapped in vivo with regional pH assessed by one-way analysis of variance. RESULTS: s > 0.99). The proposed approach extends the in vitro pH detection range to 5.5 to 7.5 at 4.7 T. In vivo renal pH was measured to be 7.0 ± 0.1, 6.8 ± 0.1, and 6.5 ± 0.2 for cortex, medulla and calyx, respectively (P < 0.05). CONCLUSIONS: The proposed ratiometric approach extended the iopamidol pH detection range, enabling the renal pH mapping in vivo, which is promising for pH imaging studies at sub-high or low fields with potential clinical applicability. Magn Reson Med 79:1553-1558, 2018. © 2017 International Society for Magnetic Resonance in Medicine.