B1 inhomogeneity corrected CEST MRI based on direct saturation removed omega plot model at 5T

Abstract 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 B 1 inhomogeneity correction method based on a direct saturation (DS) removed omega plot model. Methods Four healthy volunteers underwent B 1 field mapping and CEST imaging under four nominal B 1 levels of 0.75, 1.0, 1.5, and 2.0 μT 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/, 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.