Imaging pH phantoms using magnetization transfer technology at 1.5 Tesla

Magnetization transfer (MT) is an important source of contrast in magnetic resonance imaging (MRI), and is the basis of chemical exchange saturation transfer (CEST) imaging, which does not exist in our clinical MR scanner at 1.5 Tesla (T). Amide proton transfer (APT) imaging, a variant of CEST imaging, has been shown capable of detecting tissue acidosis during stroke at above 3.0 T. According to the theory of CEST and APT imaging developed by MT, our study is to make full use of MT technology in the clinic by modifying MT parameters to acquire clinical pH-weighted MT imaging. We modified the source codes of the MT sequence to enable the frequency offset to reach 121 Hz and 224 Hz for saturating the amine protons of creatine. The results showed that MT imaging at a frequency offset of 121 Hz could not be acquired clearly for free water saturation, and MT imaging at 224 Hz could reflect slightly different pH values. We analyzed MT imaging of the same and different pH phantoms, and gave reasonable explanation for the results. We conclude that MT imaging at clinical 1.5 T could differentiate pH phantoms by choosing appropriate parameters.