A comparison of exogenous and endogenous CEST MRI methods for evaluating in vivo pH

Purpose: Extracellular pH (pHe) is an important biomarker for cancer cell metabolism. Acido-chemical exchange saturation transfer (CEST) MRI uses the contrast agent iopamidol to create spatial maps of pHe. Measurements of amide proton transfer exchange rates (k_ex ) from endogenous CEST MRI were compared to pHe measurements by exogenous acido-CEST MRI to determine whether endogenous k_ex could be used as a proxy for pHe measurements.

Methods: Spatial maps of pHe and k_ex were obtained using exogenous acidoCEST MRI and an endogenous CEST MRI analyzed with the omega plot method, respectively, to evaluate mouse kidney, a flank tumor model, and a spontaneous lung tumor model. The pHe and k_ex results were evaluated using pixelwise comparisons.

Results: The k_ex values obtained from endogenous CEST measurements did not correlate with the pHe results from exogenous CEST measurements. The k_ex measurements were limited to fewer pixels and had a limited dynamic range relative to pHe measurements.

Conclusion: Measurements of k_ex with endogenous CEST MRI cannot substitute for pHe measurements with acidoCEST MRI. Whereas endogenous CEST MRI may still have good utility for evaluating some specific pathologies, exogenous acido-CEST MRI is more appropriate when evaluating pathologies based on pHe values. Magn Reson Med 79:2766-2772, 2018. © 2017 International Society for Magnetic Resonance in Medicine.