Simultaneous mapping of water shift and B₁ (WASABI)-Application to field-Inhomogeneity correction of CEST MRI data

Together with the development of MRI contrasts that are inherently small in their magnitude, increased magnetic field accuracy is also required. Hence, mapping of the static magnetic field (B₀ ) and the excitation field (B₁ ) is not only important to feedback shim algorithms, but also for postprocess contrast-correction procedures.

A novel field-inhomogeneity mapping method is presented that allows simultaneous mapping of the water shift and B₁ (WASABI) using an off-resonant rectangular preparation pulse. The induced Rabi oscillations lead to a sinc-like spectrum in the frequency-offset dimension and allow for determination of B₀ by its symmetry axis and of B₁ by its oscillation frequency.

Stability of the WASABI method with regard to the influences of T₁ , T₂ , magnetization transfer, and repetition time was investigated and its convergence interval was verified. B₀ and B₁ maps obtained simultaneously by means of WASABI in the human brain at 3 T and 7 T can compete well with maps obtained by standard methods. Finally, the method was applied successfully for B₀ and B₁ correction of chemical exchange saturation transfer MRI (CEST-MRI) data of the human brain.

The proposed WASABI method yields a novel simultaneous B₀ and B₁ mapping within 1 min that is robust and easy to implement. Magn Reson Med 77:571-580, 2017. © 2016 International Society for Magnetic Resonance in Medicine.