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

PurposeTogether 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 (B-0) and the excitation field (B-1) is not only important to feedback shim algorithms, but also for postprocess contrast-correction procedures. MethodsA novel field-inhomogeneity mapping method is presented that allows simultaneous mapping of the water shift and B-1 (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 B-0 by its symmetry axis and of B-1 by its oscillation frequency. ResultsStability of the WASABI method with regard to the influences of T-1, T-2, magnetization transfer, and repetition time was investigated and its convergence interval was verified. B-0 and B-1 maps obtained simultaneously by means of WASABI in the human brain at 3T and 7T can compete well with maps obtained by standard methods. Finally, the method was applied successfully for B-0 and B-1 correction of chemical exchange saturation transfer MRI (CEST-MRI) data of the human brain. ConclusionThe proposed WASABI method yields a novel simultaneous B-0 and B-1 mapping within 1min that is robust and easy to implement. Magn Reson Med 77:571-580, 2017. (c) 2016 International Society for Magnetic Resonance in Medicine