A comparison of static and dynamic B-0 mapping methods for correction of CEST MRI in the presence of temporal B-0 field variations

PurposeTo assess the performance, in the presence of scanner instabilities, of three dynamic correction methods which integrate B-0 mapping into the chemical exchange saturation transfer (CEST) measurement and three established static B-0-correction approaches. MethodsA homogeneous phantom and five healthy volunteers were scanned with a CEST sequence at 7 T. The in vivo measurements were performed twice: first with unaltered system frequency and again applying frequency shifts during the CEST acquisition. In all cases, retrospective voxel-wise B-0-correction was performed using one intrinsic and two extrinsic [prescans with dual-echo gradient-echo and water saturation shift referencing (WASSR)] static approaches. These were compared with two intrinsic [using phase data directly generated by single-echo or double-echo GRE (gradient-echo) CEST readout (CEST-GRE-2TE)] and one extrinsic [phase from interleaved dual-echo EPI (echo planar imaging) navigator (NAV-EPI-2TE)] dynamic B-0-correction approaches [allowing correction of each Z-spectral point before magnetization transfer ratio asymmetry (MTRasym) analysis]. ResultsAll three dynamic methods successfully mapped the induced drift. The intrinsic approaches were affected by the CEST labeling near water ( < |0.3| ppm). The MTRasym contrast was distorted by the frequency drift in the brain by up to 0.21%/Hz when static B-0-corrections were applied, whereas the dynamic B-0 corrections reduced this to <0.01%/Hz without the need of external scans. The CEST-GRE-2TE and NAV-EPI-2TE resulted in highly consistent MTRasym values with/without drift for all subjects. ConclusionReliable correction of scanner instabilities is essential to establish clinical CEST MRI. The three dynamic approaches presented improved the B-0-correction performance significantly in the presence of frequency drift compared to established static methods. Among them, the self-corrected CEST-GRE-2TE was the most accurate and straightforward to implement.