Empirical Transmit Field Bias Correction of T1w/T2w Myelin Maps

Abstract

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 residual radiofrequency transmit field (B1+) biases, which may be correlated with these variables of interest, leading to potentially spurious results. Here we propose multiple 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. 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 across individuals and groups (e.g., sex, age, or body-mass-index). We recommend that investigators who use the T1w/T2w approach for mapping cortical myelin use these B1+ transmit field corrected myelin maps going forward.

Graded Variation In Cortical T1w/T2w Myelination During Adolescence

“…We analyzed high-resolution cortical T1w/T2w myelin maps from 628 participants (ages 8-21 years old; Figure 1A and Figure 1B) to characterize the development of cortical T1w/T2w myelin content during youth after correcting for transmit field biases (Glasser et al, 2021). The posterior smooth function of T1w/T2w myelin on age and the posterior derivative of the age function were estimated by fitting Bayesian thin-plate spline models across 180 cortical areas in each hemisphere, thereby obtaining measures of the overall effect size, rate, and nonlinearity of age-related change in T1w/T2w myelin across the neocortical sheet.…”

“…Further, a lifespan study of 8-85 year olds characterized an early age of peak T1w/T2w myelin growth in sensorimotor areas, and a later age of peak myelination in association areas (Grydeland et al, 2019). Despite their strengths, these studies did not control for B1+ transmit field bias, which impacts T1w/T2w myelin estimates in a manner that is systematically related to age, sex, and body size (Glasser et al, 2021;MacLennan et al, 2022). Our study extends prior work by rigorously characterizing the pace and timing of cortical myelin development in youth, using sub-millimeter resolution, B1+ bias-corrected T1w/T2w myelin mapping, and mapping data to a multimodal cortical parcellation in a large, demographically diverse sample.…”

“…For further neuroimaging protocol description, see Harms et al, 2018. Additionally 2mm isotropic gradient echo and spin echo images were acquired and used for computing the pseudo-transmit field described below (Glasser et al, 2021).…”

“…Computing a measure of cortical myelin from the ratio of a T1-weighted to a T2-weighted MRI acquisition (T1w/T2w) has emerged as a highly accessible non-invasive measure that correlates with histological measures of cortical myelin content (Glasser et al, 2014;Glasser and Van Essen, 2011). However, prior studies evaluating age-related differences in T1w/T2w myelin content (Grydeland et al, 2019;Norbom et al, 2020) have not accounted for residual radiofrequency transmit field (B1+) biases, which impact T1w/T2w estimates in an age-dependent manner (Glasser et al, 2021;MacLennan et al, 2022) through their relationship to body size. Because studies that neglected B1+ bias field effects may be prone to spurious results, it is critical to characterize age-related differences in T1w/T2w myelin after mitigating these confounds.…”

“…We test whether cortical areas exhibit graded differences in the rate of T1w/T2w myelination according to their position along the S-A axis. We use a novel approach for mitigating B1+ confound effects in T1w/T2w myelin mapping that enables us to detect unbiased age-related differences in T1w/T2w myelin (Glasser et al, 2021). We fit Bayesian spline models across 180 cortical areas in each hemisphere (Glasser et al, 2016a) to characterize different properties of age-related changes in T1w/T2w myelin across the cerebral cortex, including estimates of the variance in T1w/T2w myelin explained by age, rate of change, age of peak growth, and degree of nonlinearity in T1w/T2w myelin development from 8 to 21 years old.…”