Brainmap: In vivo magnetic resonance imaging of the myelin g-ratio
The myelin g-ratio, defined as the ratio between the inner and the outer diameter of the myelin sheath, has been proposed as an important biomarker for studying normal brain development, as well as brain pathology. Recent studies have suggested that the sexual dimorphism in white matter development is due to a higher g-ratio (thinner myelin) in adolescent boys. Variations in the g-ratio have also been detected in the frontal white matter of patients with schizophrenia, and have been associated with differences in brain connectivity in autism. Finally, in vivo imaging of the myelin thickness in multiple sclerosis can provide a real-time tool for tracking myelination in lesions, facilitating the development and evaluation of new therapeutic agents that promote remyelination. In this lecture, a unique combination of quantitative magnetic resonance imaging, histology, and numerical simulations will be presented, resulting in a novel method for in vivo measurement of the myelin g-ratio. The ramifications of this method will be discussed in the context of applications and validation in animal models.