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

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Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields. / Hsu, Yi Cheng; Chern, I. Liang; Zhao, Wei; Gagoski, Borjan; Witzel, Thomas; Lin, Fa Hsuan.

In: Magnetic Resonance in Medicine, Vol. 71, No. 4, 2014, p. 1458-1469.

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Hsu, Yi Cheng ; Chern, I. Liang ; Zhao, Wei ; Gagoski, Borjan ; Witzel, Thomas ; Lin, Fa Hsuan. / Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields. In: Magnetic Resonance in Medicine. 2014 ; Vol. 71, No. 4. pp. 1458-1469.

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@article{f181712fed754dc09a667476e3294f44,
title = "Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields",
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 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.",
keywords = "7T, fast imaging, nonlinear gradient, RF inhomogeneity, SAR",
author = "Hsu, {Yi Cheng} and Chern, {I. Liang} and Wei Zhao and Borjan Gagoski and Thomas Witzel and Lin, {Fa Hsuan}",
year = "2014",
doi = "10.1002/mrm.24801",
language = "English",
volume = "71",
pages = "1458--1469",
journal = "Magnetic Resonance in Medicine",
issn = "0740-3194",
publisher = "Wiley",
number = "4",

}

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TY - JOUR

T1 - Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields

AU - Hsu, Yi Cheng

AU - Chern, I. Liang

AU - Zhao, Wei

AU - Gagoski, Borjan

AU - Witzel, Thomas

AU - Lin, Fa Hsuan

PY - 2014

Y1 - 2014

N2 - 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.

AB - 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.

KW - 7T

KW - fast imaging

KW - nonlinear gradient

KW - RF inhomogeneity

KW - SAR

UR - http://www.scopus.com/inward/record.url?scp=84896547186&partnerID=8YFLogxK

U2 - 10.1002/mrm.24801

DO - 10.1002/mrm.24801

M3 - Article

VL - 71

SP - 1458

EP - 1469

JO - Magnetic Resonance in Medicine

JF - Magnetic Resonance in Medicine

SN - 0740-3194

IS - 4

ER -

ID: 9028859