B₁inhomogeneity corrected CEST MRI based on direct saturation removed omega plot model at 5T

Purpose: CEST can image macromolecules/compounds via detecting chemical exchange between labile protons and bulk water. B-1 field inhomogeneity impairs CEST quantification. Conventional B-1 inhomogeneity correction methods depend on interpolation algorithms, B-1 choices, acquisition number or calibration curves, making reliable correction challenging. This study proposed a novel B1 inhomogeneity correction method based on a direct saturation (DS) removed omega plot model. Methods: Four healthy volunteers underwent B1 field mapping and CEST imaging under four nominal B1 levels of 0.75, 1.0, 1.5, and 2.0 mu T at 5T. DS was resolved using a multi-pool Lorentzian model and removed from respective Z spectrum. Residual spectral signals were used to construct the omega plot as a linear function of 1/B-2(1) , from which corrected signals at nominal B-1 levels were calculated. Routine asymmetry analysis was conducted to quantify amide proton transfer (APT) effect. Its distribution across white matter was compared before and after B-1 inhomogeneity correction and also with the conventional interpolation approach. Results: B-1 inhomogeneity yielded conspicuous artifact on APT images. Such artifact was mitigated by the proposed method. Homogeneous APT maps were shown with SD consistently smaller than that before B-1 inhomogeneity correction and the interpolation method. Moreover, B-1 inhomogeneity correction from two and four CEST acquisitions yielded similar results, superior over the interpolation method that derived inconsistent APT contrasts among different B-1 choices. Conclusion: The proposed method enables reliable B-1 inhomogeneity correction from at least two CEST acquisitions, providing an effective way to improve quantitative CEST MRI.