On the transmit field inhomogeneity correction of relaxation-compensated amide and NOE CEST effects at 7 T.

High field MRI is beneficial for chemical exchange saturation transfer (CEST) in terms of high SNR, CNR, and chemical shift dispersion. These advantages may, however, be counter-balanced by the increased transmit field inhomogeneity normally associated with high field MRI. The relatively high sensitivity of the CEST contrast to B-1 inhomogeneity necessitates the development of correction methods, which is essential for the clinical translation of CEST. In this work, two B-1 correction algorithms for the most studied CEST effects, amide-CEST and nuclear Overhauser enhancement (NOE), were analyzed. Both methods rely on fitting the multi-pool Bloch-McConnell equations to the densely sampled CEST spectra. In the first method, the correction is achieved by using a linear B-1 correction of the calculated amide and NOE CEST effects. The second method uses the Bloch-McConnell fit parameters and the desired B-1 amplitude to recalculate the CEST spectra, followed by the calculation of B-1-corrected amide and NOE CEST effects. Both algorithms were systematically studied in Bloch-McConnell equations and in human data, and compared with the earlier proposed ideal interpolation-based B-1 correction method. In the low B-1 regime of 0.15-0.50 T (average power), a simple linear model was sufficient to mitigate B-1 inhomogeneity effects on a par with the interpolation B-1 correction, as demonstrated by a reduced correlation of the CEST contrast with B-1 in both the simulations and the experiments.