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Multiple interleaved mode saturation (MIMOSA) for B 1 + inhomogeneity mitigation in chemical exchange saturation transfer
Author(s) -
Liebert Andrzej,
Zaiss Moritz,
Gumbrecht Rene,
Tkotz Katharina,
Linz Peter,
Schmitt Benjamin,
Laun Frederik B.,
Doerfler Arnd,
Uder Michael,
Nagel Armin M.
Publication year - 2019
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.27762
Subject(s) - nuclear magnetic resonance , imaging phantom , magnetization transfer , saturation (graph theory) , flip angle , amplitude , physics , electromagnetic coil , materials science , magnetic field , chemistry , computational physics , magnetic resonance imaging , optics , mathematics , medicine , combinatorics , quantum mechanics , radiology
Purpose To mitigate B 1 + inhomogeneity in quantitative CEST MRI at ultra‐high magnetic field strengths (B 0 ≥ 7 Tesla) using a parallel transmit system. Methods Multiple interleaved mode saturation employs interleaving of 2 complementary phase sets during the saturation pulse train. Phase differences of 45° (first mode) and 90° (second mode) between 2 adjacent transmitter coil channels are used. The influence of the new saturation scheme on the CEST contrast was analyzed using Bloch‐McConnell simulations. The presented method was verified in phantom and in vivo measurements of the healthy human brain. The relayed nuclear Overhauser effect was evaluated, and the inverse magnetic transfer ratio metric was calculated. Results were compared to a published B 1 + correction method. All measurements were conducted on a whole‐body 7 Tesla MRI system using an 8 transmitter and 32 receiver channel head coil. Results Simulations showed that the inverse magnetic transfer ratio metric contrast of relayed nuclear Overhauser effect shows a smaller dependency on the relative amplitudes of the 2 different modes than the contrasts of Cr and amide proton transfer. Measurements of an egg white phantom showed markedly improved homogeneity compared to the uncorrected inverse magnetic transfer ratio metric (relayed nuclear Overhauser effect) images and slightly improved results compared to B 1 + corrected images. In vivo multiple interleaved mode saturation images showed similar contrast compared to B 1 + corrected images. Conclusion Multiple interleaved mode saturation can be used as a simple method to mitigate B 1 + inhomogeneity effects in CEST MRI at ultra‐high magnetic field strengths. Compared to previous B 1 + correction methods, acquisition time can be reduced because an additional scan, usually required for B 1 + correction, can be omitted.