T1-weighted divided by T2-weighted (T1w/T2w) myelin maps were initially developed for neuroanatomical analyses such as identifying cortical areas, but they are increasingly used in statistical comparisons across individuals and groups with other variables of interest. Existing T1w/T2w myelin maps contain radiofrequency transmit field (B1+) biases, which may be correlated with these variables of interest, leading to potentially spurious results. Here we propose two empirical methods for correcting these transmit field biases using either explicit measures of the transmit field or alternatively a ‘pseudo-transmit’ approach that is highly correlated with the transmit field at 3T. We find that the resulting corrected T1w/T2w myelin maps are both better neuroanatomical measures (e.g., for use in cross-species comparisons), and more appropriate for statistical comparisons of relative T1w/T2w differences across individuals and groups (e.g., sex, age, or body-mass-index) within a consistently acquired study at 3T. We recommend that investigators who use the T1w/T2w approach for mapping cortical myelin use these B1+ transmit field corrected myelin maps going forward.
Bibliographical noteFunding Information:
Supported in part by HCP-YA: Mapping the Human Connectome: Structure, Function, and Heritability U54MH091657, HCD: Mapping the Human Connectome During Typical Development U01MH109589 , HCA: Mapping the Human Connectome During Typical Aging U01AG052564 , Connectome Coordination Facility (CCF) I: R24MH108315, Connectome Coordination Facility II : R24MH122820 , the McDonnell Center for Systems Neuroscience at Washington University, NIH R01MH60974 (DCVE, MFG), NIH F30 MH097312 (MFG), NIH T32 EB021955, Brain/MINDS‐beyond from Japan Agency of Medical Research and Development (AMED) ( JP22dm0307004 , P22dm0307006 ) (TH), and the FreeSurfer Maintenance Grant ( R01EB023281 ) (DG).