Comparison of multi echo T relaxation and steady state approaches for myelin imaging in the central nervous system.

The traditional approach for measuring myelin-associated water with quantitative magnetic resonance imaging (MRI) uses multi-echo T relaxation data to calculate the myelin water fraction (MWF). A fundamentally different approach, abbreviated "mcDESPOT", uses a more efficient steady-state acquisition to generate an equivalent metric (f). Although previous studies have demonstrated inherent instability and bias in the complex mcDESPOT analysis procedure, f has often been used as a surrogate for MWF. We produced and compared multivariate atlases of MWF and f in healthy human brain and cervical spinal cord (available online) and compared their ability to detect multiple sclerosis pathology. A significant bias was found in all regions (p < 10), albeit reversed for spinal cord (f-MWF =  - 3.4%) compared to brain (+ 6.2%). MWF and f followed an approximately linear relationship for regions with MWF <  ~ 10%. For MWF >  ~ 10%, the relationship broke down and f no longer increased in tandem with MWF. For multiple sclerosis patients, MWF and f Z score maps showed overlapping areas of low Z score and similar trends between patients and brain regions, although those of f generally had greater spatial extent and magnitude of severity. These results will guide future choice of myelin-sensitive quantitative MRI and improve interpretation of studies using either myelin imaging approach.