In vivo measurement of the myelin g-ratio with histological validation

PURPOSE: The myelin g-ratio, defined as the ratio between the inner and the outer diameter of the myelin sheath, is a fundamental property of white matter that can be computed from a simple formula (Fig. 1) relating the myelin volume fraction (MVF) to the fiber volume fraction (FVF) or the axon volume fraction (AVF). Recent studies have suggested that the sexual dimorphism in white matter development is due to a higher g-ratio (relatively thinner myelin) in adolescent boys. Additionally, in vivo imaging of the myelin thickness in multiple sclerosis could provide a real-time tool for tracking myelination in lesions, facilitating the development and evaluation of new therapeutic agents that promote remyelination. In this abstract, a unique combination of magnetization transfer, diffusion imaging and histology is presented, providing a novel method for validating the in vivo measurements of the myelin g-ratio.