Promise and pitfalls of g-ratio estimation with MRI

Neuroimage. 2018 Nov 15:182:80-96. doi: 10.1016/j.neuroimage.2017.08.038. Epub 2017 Aug 16.


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.

Keywords: Diffusion MRI; MRI; Microstructure; Myelin imaging; White matter; g-ratio.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Diffusion Magnetic Resonance Imaging / methods*
  • Diffusion Magnetic Resonance Imaging / standards
  • Humans
  • Nerve Fibers, Myelinated* / ultrastructure
  • Neuroimaging / methods*
  • Neuroimaging / standards
  • White Matter / anatomy & histology*
  • White Matter / diagnostic imaging*