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Assessment and correction of macroscopic field variations in 2D spoiled gradient‐echo sequences
Author(s) -
Soellradl Martin,
Lesch Andreas,
Strasser Johannes,
Pirpamer Lukas,
Stollberger Rudolf,
Ropele Stefan,
Langkammer Christian
Publication year - 2020
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.28139
Subject(s) - dephasing , flip angle , context (archaeology) , physics , signal (programming language) , nuclear magnetic resonance , field (mathematics) , imaging phantom , phase (matter) , scaling , computational physics , optics , condensed matter physics , magnetic resonance imaging , mathematics , geometry , computer science , pure mathematics , medicine , paleontology , quantum mechanics , radiology , programming language , biology
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 R 2 ∗ and myelin water fraction (MWF) mapping. Results The influence of macroscopic field gradients on R 2 ∗ 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 R 2 ∗ 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.