Fast correction of B 0 field inhomogeneity for pH‐specific magnetization transfer and relaxation normalized amide proton transfer imaging of acute ischemic stroke without Z‐spectrum

Purpose The magnetization transfer and relaxation normalized amide proton transfer (MRAPT) analysis is promising to provide a highly pH‐specific mapping of tissue acidosis, complementing commonly used CEST asymmetry analysis. We aimed to develop a fast B 0 inhomogeneity correction algorithm for acute stroke magnetization transfer and relaxation normalized amide proton transfer imaging without Z‐spectral interpolation. Methods The proposed fast field inhomogeneity correction describes B 0 artifacts with linear regression. We compared the new algorithm with the routine interpolation correction approach in CEST imaging of a dual‐pH phantom. The fast B 0 correction was further evaluated in amide proton transfer imaging of normal and acute stroke rats. Results Our phantom data showed that the proposed fast B 0 inhomogeneity correction significantly improved pH MRI contrast, recovering over 80% of the pH MRI contrast‐to‐noise‐ratio difference between the raw magnetization transfer ratio asymmetry and that using the routine interpolation‐based B 0 correction approach. In normal rat brains, the proposed fast B 0 correction improved pH‐specific MRI uniformity across the intact tissue, with the ratio of magnetization transfer and relaxation normalized amide proton transfer ratio being 10% of that without B 0 inhomogeneity correction. In acute stroke rats, fast B 0 inhomogeneity–corrected pH MRI reveals substantially improved pH lesion conspicuity, particularly in regions with nonnegligible B 0 inhomogeneity. The pH MRI contrast‐to‐noise ratio between the ipsilateral diffusion lesion and contralateral normal tissue improved significantly with fast B 0 correction (from 1.88 ± 0.48 to 2.20 ± 0.44, P < .01). Conclusions Our study established an expedient B 0 inhomogeneity correction algorithm for fast pH imaging of acute ischemia.