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A model‐based framework for correcting B 1 + inhomogeneity effects in magnetization transfer saturation and inhomogeneous magnetization transfer saturation maps
Author(s) -
Rowley Christopher D.,
Campbell Jennifer S. W.,
Wu Zhe,
Leppert Ilana R.,
Rudko David A.,
Pike Gilbert Bruce,
Tardif Christine L.
Publication year - 2021
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.28831
Subject(s) - magnetization transfer , saturation (graph theory) , physics , magnetization , range (aeronautics) , nuclear magnetic resonance , computational physics , mathematics , statistical physics , magnetic resonance imaging , materials science , combinatorics , magnetic field , quantum mechanics , medicine , composite material , radiology
Purpose In this work, we propose that Δ B 1 + ‐induced errors in magnetization transfer (MT) saturation (MT sat ) maps can be corrected with use of an R 1 and B 1 + map and through numerical simulations of the sequence. Theory and Methods One healthy subject was scanned at 3.0T using a partial quantitative MT protocol to estimate the relationship between observed R 1 (R 1,obs ) and apparent bound pool size ( M 0 , a p p B ) in the brain. MT sat values were simulated for a range of B 1 + , R 1,obs , and M 0 , a p p B . An equation was fit to the simulated MT sat , then a linear relationship between R 1,obs and M 0 , a p p B was generated. These results were used to generate correction factor maps for the MT sat acquired from single‐point data. The proposed correction was compared to an empirical correction factor with different MT‐preparation schemes. ResultsM 0 , a p p B was highly correlated with R 1,obs (r > 0.96), permitting the use of R 1,obs to estimate M 0 , a p p B for B 1 + correction. All B 1 + corrected MT sat maps displayed a decreased correlation with B 1 + compared to uncorrected MT sat and MT sat corrected with an empirical factor in the corpus callosum. There was good agreement between the proposed approach and the empirical correction with radiofrequency saturation at 2 kHz, with larger deviations seen when using saturation pulses further off‐resonance and in inhomogeneous (ih) MT sat maps. Conclusion The proposed correction decreases the dependence of MT sat on B 1 + inhomogeneities. Furthermore, this flexible framework permits the use of different saturation protocols, making it useful for correcting B 1 + inhomogeneities in ihMT.