Assessment and correction of macroscopic field variations in 2D spoiled gradient‐echo sequences

Purpose To model and correct the dephasing effects in the gradient‐echo signal for arbitrary RF excitation pulses with large flip angles in the presence of macroscopic field variations. Methods The dephasing of the spoiled 2D gradient‐echo signal was modeled using a numerical solution of the Bloch e...

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Bibliographic Details
Published in:Magnetic resonance in medicine 2020-08, Vol.84 (2), p.620-633
Main Authors: Soellradl, Martin, Lesch, Andreas, Strasser, Johannes, Pirpamer, Lukas, Stollberger, Rudolf, Ropele, Stefan, Langkammer, Christian
Format: Article
Language:English
Subjects:
Algorithms
Computer simulation
field inhomogeneities
Full Papers—Imaging Methodology
Magnetic Resonance Imaging
Mapping
Myelin
Myelin Sheath
myelin water fraction
Phantoms, Imaging
Phase variations
Polarity
R 2
relaxometry
slice profile
T 2
Two dimensional models
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Summary:Purpose To model and correct the dephasing effects in the gradient‐echo signal for arbitrary RF excitation pulses with large flip angles in the presence of macroscopic field variations. Methods The dephasing of the spoiled 2D gradient‐echo signal was modeled using a numerical solution of the Bloch equations to calculate the magnitude and phase of the transverse magnetization across the slice profile. Additionally, regional variations of the transmit RF field and slice profile scaling due to macroscopic field gradients were included. Simulations, phantom, and in vivo measurements at 3 T were conducted for R2∗ and myelin water fraction (MWF) mapping. Results The influence of macroscopic field gradients on R2∗ and myelin water fraction estimation can be substantially reduced by applying the proposed model. Moreover, it was shown that the dephasing over time for flip angles of 60° or greater also depends on the polarity of the slice‐selection gradient because of phase variation along the slice profile. Conclusion Substantial improvements in R2∗ accuracy and myelin water fraction mapping coverage can be achieved using the proposed model if higher flip angles are required. In this context, we demonstrated that the phase along the slice profile and the polarity of the slice‐selection gradient are essential for proper modeling of the gradient‐echo signal in the presence of macroscopic field variations.
ISSN:0740-3194
1522-2594
DOI:10.1002/mrm.28139