Myelin water imaging of in vivo and ex vivo human brains using multi-echo gradient echo at 3T and 7T

Purpose: To compare the myelin water fraction (MWF) measurements between 3 T and 7 T and between in vivo and ex vivo human brains, and to investigate the relationship between multi-echo gradient-echo (mGRE)-based 3D MWF and myelin content using histological staining, which has not been validated in the human brain. Methods: In this study, we performed 3D mGRE-based MWF measurements on five ex vivo human brain hemispheres and five healthy volunteers at 3 T and 7 T with 1mm isotropic resolution. The data were fitted with the T-2* based on a three compartment complex-valued model to estimate MWF. We obtained myelin basic protein (MBP) staining from two tissue blocks and co-registered the MWF map and histology image for voxel-wise correlation between the two. Results: The MWF values measured from 7 T were overall higher than 7 T, but data between the two field strength demonstrated high correlations both in vivo (r=0.88) and ex vivo (r=0.83) across 19 white matter regions. Moreover, the MWF measurements showed a good agreement between in vivo and ex vivo assessments at 3 T (r=0.61) and 7 T (r=0.54). Based on MBP staining, the MWF values exhibited strong positive correlations with myelin content on both 3 T (r=0.68 and r=0.78 for the two tissue blocks) and 7 T (r=0.64 and r=0.82 for the two tissue blocks). Conclusion: The findings demonstrated that the mGRE-based MWF mapping can be used to quantify myelin content in the human brain, despite the field-strength dependency of the measurements.