Myelin water fraction mapping from multiple echo spin echoes and an independent B1+ map

Purpose Myelin water fraction (MWF) is often obtained from a multiple echo spin echo (MESE) sequence using multi‐component T2 fitting with non‐negative least squares. This process fits many unknowns including B1+ to produce a T2 spectrum for each voxel. Presented is an alternative using a rapid B1+...

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Bibliographic Details
Published in:Magnetic resonance in medicine Vol. 88; no. 3; pp. 1380 - 1390
Main Authors: Mehdizadeh, Nima, Wilman, Alan H.
Format: Journal Article
Language:English
Published: Hoboken Wiley Subscription Services, Inc 01.09.2022
John Wiley and Sons Inc
Subjects:
Brain
Coefficient of variation
Corpus callosum
Echoes
flip angle estimation
Mapping
Mathematical analysis
multi‐component T2 fitting
Myelin
myelin water fraction
stimulated echoes
Technical Note
Technical Notes–Computer Processing and Modeling
Thalamus
ISSN:0740-3194, 1522-2594, 1522-2594
Online Access:Get full text
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Summary:Purpose Myelin water fraction (MWF) is often obtained from a multiple echo spin echo (MESE) sequence using multi‐component T2 fitting with non‐negative least squares. This process fits many unknowns including B1+ to produce a T2 spectrum for each voxel. Presented is an alternative using a rapid B1+ mapping sequence to supply B1+ for the MWF fitting procedure. Methods Effects of B1+ errors on MWF calculations were modeled for 2D and 3D MESE using Bloch and extended phase graph simulations, respectively. Variations in SNR and relative refocusing widths were tested. Human brain experiments at 3 T used 2D MESE and an independent B1+ map. MWF maps were produced with the standard approach and with the use of the independent B1+ map. Differences in B1+ and mean MWF in specific brain regions were compared. Results For 2D MESE, MWF with the standard method was strongly affected by B1+ misestimations arising from limited SNR and response asymmetry around 180°, which decreased with increasing relative refocusing width. Using an independent B1+ map increased mean MWF and decreased coefficient of variation. Notable differences in vivo in 2D MESE were in areas of high B1+ such as thalamus and splenium where mean MWF increased by 88% and 31%, respectively (P < 0.001). Simulations also demonstrated the advantages of this approach for 3D MESE when SNR is <500. Conclusion For 2D MESE, because of increased complexity of decay curves and limited SNR, supplying B1+ improves MWF results in peripheral and central brain regions where flip angles differ substantially from 180°.
Bibliography:Funding information
Canadian Institutes of Health Research, Grant/Award Number: PS 180473; Natural Sciences and Engineering Research Council of Canada, Grant/Award Number: RGPIN‐2017‐04006
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ISSN:0740-3194
1522-2594
1522-2594
DOI:10.1002/mrm.29286