Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields

Yi Cheng Hsu, I. Liang Chern, Wei Zhao, Borjan Gagoski, Thomas Witzel, Fa Hsuan Lin*

*Corresponding author for this work

    Research output: Contribution to journalArticleScientificpeer-review

    6 Citations (Scopus)


    Purpose High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B1+, and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult. Method We propose a two-step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the B1+ map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this B 1+ remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B 1+, a simple pulse sequence design using time-varying SEMs can achieve a homogenous flip-angle distribution efficiently. Results We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B1+ inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple-pulse design, a similar average flip-angle distribution with less RF power, and/or a more homogeneous flip-angle distribution. Conclusion Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the B 1+ inhomogeneity in high-field MRI experiments.

    Original languageEnglish
    Pages (from-to)1458-1469
    Number of pages12
    JournalMagnetic Resonance in Medicine
    Issue number4
    Publication statusPublished - 2014
    MoE publication typeA1 Journal article-refereed


    • 7T
    • fast imaging
    • nonlinear gradient
    • RF inhomogeneity
    • SAR

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