LAUSR

Abstract The fiber g‐ratio is the ratio of the inner to the outer diameter of the myelin sheath of a myelinated axon. It has a limited dynamic range in healthy white matter, as it is optimized for speed of signal conduction, cellular energetics, and spatial constraints. In vivo imaging of the g‐ratio in health and disease would greatly increase our knowledge of the nervous system and our ability to diagnose, monitor, and treat disease. MRI based g‐ratio imaging was first conceived in 2011, and expanded to be feasible in full brain white matter with preliminary results in 2013. This manuscript reviews the growing g‐ratio imaging literature and speculates on future applications. It details the methodology for imaging the g‐ratio with MRI, and describes the known pitfalls and challenges in doing so. HighlightsThis article reviews the recently developed g‐ratio imaging framework.Confounds in the methodology are detailed.Recent progress and applications are reviewed.