Towards in vivo g-ratio mapping using MRI: Unifying myelin and diffusion imaging

TY - JOUR

T1 - Towards in vivo g-ratio mapping using MRI: Unifying myelin and diffusion imaging

AU - Mohammadi, Siawoosh

AU - Callaghan, Martina F

N1 - Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.

PY - 2021/1/15

Y1 - 2021/1/15

N2 - BACKGROUND: The g-ratio, quantifying the comparative thickness of the myelin sheath encasing an axon, is a geometrical invariant that has high functional relevance because of its importance in determining neuronal conduction velocity. Advances in MRI data acquisition and signal modelling have put in vivo mapping of the g-ratio, across the entire white matter, within our reach. This capacity would greatly increase our knowledge of the nervous system: how it functions, and how it is impacted by disease.NEW METHOD: This is the second review on the topic of g-ratio mapping using MRI.RESULTS: This review summarizes the most recent developments in the field, while also providing methodological background pertinent to aggregate g-ratio weighted mapping, and discussing pitfalls associated with these approaches.COMPARISON WITH EXISTING METHODS: Using simulations based on recently published data, this review reveals caveats to the state-of-the-art calibration methods that have been used for in vivo g-ratio mapping. It highlights the need to estimate both the slope and offset of the relationship between these MRI-based markers and the true myelin volume fraction if we are really to achieve the goal of precise, high sensitivity g-ratio mapping in vivo. Other challenges discussed in this review further evidence the need for gold standard measurements of human brain tissue from ex vivo histology.CONCLUSIONS: We conclude that the quest to find the most appropriate MRI biomarkers to enable in vivo g-ratio mapping is ongoing, with the full potential of many novel techniques yet to be investigated.

AB - BACKGROUND: The g-ratio, quantifying the comparative thickness of the myelin sheath encasing an axon, is a geometrical invariant that has high functional relevance because of its importance in determining neuronal conduction velocity. Advances in MRI data acquisition and signal modelling have put in vivo mapping of the g-ratio, across the entire white matter, within our reach. This capacity would greatly increase our knowledge of the nervous system: how it functions, and how it is impacted by disease.NEW METHOD: This is the second review on the topic of g-ratio mapping using MRI.RESULTS: This review summarizes the most recent developments in the field, while also providing methodological background pertinent to aggregate g-ratio weighted mapping, and discussing pitfalls associated with these approaches.COMPARISON WITH EXISTING METHODS: Using simulations based on recently published data, this review reveals caveats to the state-of-the-art calibration methods that have been used for in vivo g-ratio mapping. It highlights the need to estimate both the slope and offset of the relationship between these MRI-based markers and the true myelin volume fraction if we are really to achieve the goal of precise, high sensitivity g-ratio mapping in vivo. Other challenges discussed in this review further evidence the need for gold standard measurements of human brain tissue from ex vivo histology.CONCLUSIONS: We conclude that the quest to find the most appropriate MRI biomarkers to enable in vivo g-ratio mapping is ongoing, with the full potential of many novel techniques yet to be investigated.

KW - Axons

KW - Brain/diagnostic imaging

KW - Diffusion Magnetic Resonance Imaging

KW - Humans

KW - Magnetic Resonance Imaging

KW - Myelin Sheath

KW - White Matter

U2 - 10.1016/j.jneumeth.2020.108990

DO - 10.1016/j.jneumeth.2020.108990

M3 - SCORING: Review article

C2 - 33129894

VL - 348

SP - 108990

JO - J NEUROSCI METH

JF - J NEUROSCI METH

SN - 0165-0270

ER -