A linear model-based, B₁ inhomogeneity-corrected, glutamate-weighted chemical exchange saturation transfer MRI at 5 T

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 B₁ inhomogeneity compromises gluCEST measurements. Conventional B₁ inhomogeneity correction methods rely on interpolation algorithms, B₁ choices, acquisition numbers, or calibration curves, making correction unrobust. This study proposes a linear model-based method for B₁ inhomogeneity correction in gluCEST MRI.

Four healthy volunteers and 4 brain-tumor patients underwent B₁ field mapping and chemical exchange saturation transfer (CEST) imaging at three nominal B₁ levels of 3.5, 4.0, and 4.5 μT at 5 T. The linear relationship between 1/Z(±3 ppm) and B₁ was investigated using numerical simulations and human-brain data. The 1/Z(±3 ppm) values at nominal B₁ 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 B₁ inhomogeneity correction, including those corrected with the conventional interpolation method.

A linear dependency of 1/Z(±3 ppm) on B₁ within a B₁ range of 2-5 μT was identified. Evident B₁ 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 B₁ inhomogeneity. In contrast, the linear model yielded consistent gluCEST contrasts with less dependence on CEST acquisition numbers, superior to the interpolation method.

The proposed linear model allows robust B₁ inhomogeneity correction from at least two CEST acquisitions, providing an effective way for improved gluCEST MRI.