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

Authors

  • Yi-Cheng Hsu,

    1. Department of Mathematics, National Taiwan University, Taipei, Taiwan
    2. Department of Biomedical Engineering and Computational Science, Aalto University School of Science, Espoo, Finland
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  • I-Liang Chern,

    1. Department of Mathematics, National Taiwan University, Taipei, Taiwan
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  • Wei Zhao,

    1. A. A. Martinos Center, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA
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  • Borjan Gagoski,

    1. Center for Fetal-Neonatal Neuroimaging and Developmental Science, Boston Children's Hospital, Boston, Massachusetts, USA
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  • Thomas Witzel,

    1. A. A. Martinos Center, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA
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  • Fa-Hsuan Lin

    Corresponding author
    1. Department of Biomedical Engineering and Computational Science, Aalto University School of Science, Espoo, Finland
    2. Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
    • Correspondence to: Fa-Hsuan Lin, Ph.D., Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan. E-mail: fhlin@ntu.edu.tw

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Abstract

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 B1+ remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B1+, 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 B1+ inhomogeneity in high-field MRI experiments. Magn Reson Med 71:1458–1469, 2014. © 2013 Wiley Periodicals, Inc.

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