  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 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.
