Abstract
Chemical exchange saturation transfer (CEST) provides a novel contrast based on the chemical environment of protons. It has been shown its ability to detect the pH change in MRI. Here we try to reproduce the pH-sensitive contrast using our MR scanning system. Our study involves three parts: parameters optimization, in vitro and in vivo experiments of imaging the pH change. To perform CEST imaging, concentration phantom experiments on a MRI scanner were first conducted to optimize the scanning parameters for CEST contrast. The pH phantom experiments were to verify the CEST effect with the optimal parameters. Then, in order to in vivo detect the pH changes, the brains of rats were scanned to monitor the biochemical variation under hypercapnia and cardiac arrested. However, the CEST effect is very sensitive to the magnetic field inhomogeneity. Concerning this interference, we developed the field map (FM) containing the local off-resonance frequency information in pixels to correct the measured CEST effect. The induced CEST effect was quantified by the asymmetry ratio of z-spectra (MTRasym) or proton transfer ratio (PTR) in this study. The results showed the CEST effect decreased upon a lower pH level. Finally, the quantifications of FM efficiency were also included. A technique of imaging pH by endogenous contrast would allow an immediate, noninvasive clinical use of physiological indicator as part of a multi-modality MRI exam.