A linear model‐based, B 1 inhomogeneity–corrected, glutamate‐weighted chemical exchange saturation transfer MRI at 5 T

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Abstract Purpose Glutamate‐weighted chemical exchange saturation transfer (gluCEST) MRI shows promise in the diagnosis and evaluation of neurological disorders. GluCEST must be conducted at high magnetic field strengths, in which pronounced B 1 inhomogeneity compromises gluCEST measurements. Conventional B 1 inhomogeneity correction methods rely on interpolation algorithms, B 1 choices, acquisition numbers, or calibration curves, making correction unrobust. This study proposes a linear model‐based method for B 1 inhomogeneity correction in gluCEST MRI. Methods Four healthy volunteers and 4 brain‐tumor patients underwent B 1 field mapping and chemical exchange saturation transfer (CEST) imaging at three nominal B 1 levels of 3.5, 4.0, and 4.5 μT at 5 T. The linear relationship between 1/Z(±3 ppm) and B 1 was investigated using numerical simulations and human‐brain data. The 1/Z(±3 ppm) values at nominal B 1 were calculated based on the linear function constructed from either two or three CEST acquisitions. GluCEST signals, quantified with 1‐Z(+3 ppm)/Z(−3 ppm), were compared before and after B 1 inhomogeneity correction, including those corrected with the conventional interpolation method. Results A linear dependency of 1/Z(±3 ppm) on B 1 within a B 1 range of 2–5 μT was identified. Evident B 1 inhomogeneity artifacts were observed in the uncorrected gluCEST images. GluCEST signals corrected using the conventional interpolation method varied with CEST acquisition numbers, particularly in regions with severe B 1 inhomogeneity. In contrast, the linear model yielded consistent gluCEST contrasts with less dependence on CEST acquisition numbers, superior to the interpolation method. Conclusion The proposed linear model allows robust B 1 inhomogeneity correction from at least two CEST acquisitions, providing an effective way for improved gluCEST MRI.

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