Introducing axonal myelination in connectomics: A preliminary analysis of g-ratio distribution in healthy subjects

Abstract

Please cite this article as: Mancini, M., Giulietti, G., Dowell, N., Spanò, B., Harrison, N., Bozzali, M., Cercignani, M., Introducing axonal myelination in connectomics: A preliminary analysis of g-ratio distribution in healthy subjects, NeuroImage (2017), doi: 10.1016/j.neuroimage.2017.09.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Introducing axonal myelination in connectomics: a preliminary analysis of g-ratio distribution in healthy subjects Matteo M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 AbstractMicrostructural imaging and connectomics are two research areas that hold great potential for investigating brain structure and function. Combining these two approaches can lead to a better and more complete characterization of the brain as a network. The aim of this work is characterizing the connectome from a novel perspective using the myelination measure given by the g-ratio. The g-ratio is the ratio of the inner to the outer diameters of a myelinated axon, whose aggregated value can now be estimated in vivo using MRI. In two different datasets of healthy subjects, we reconstructed the structural connectome and then used the g-ratio estimated from diffusion and magnetization transfer data to characterise the network structure. Significant characteristics of g-ratio weighted graphs emerged. First, the g-ratio distribution across the edges of the graph did not show the power-law distribution observed using the number of streamlines as a weight. Second, connections involving regions related to motor and sensory functions were the highest in myelin content. We also observed significant differences in terms of the hub structure and the rich-club organization suggesting that connections involving hub regions present higher myelination than peripheral connections.Taken together, these findings offer a characterization of g-ratio distribution across the connectome in healthy subjects and lay the foundations for further investigating plasticity and pathology using a similar approach.

The R1-weighted connectome: complementing brain networks with a myelin-sensitive measure

“…We also found that the hub regions, defined in the NOS-connectome, do not necessarily have a high R1-weighted average. Similar results have been previously reported in (Mancini et al, 2018) for a gratio-weighted connectome.…”

“…Several studies (Kamagata et al, 2019;Mancini et al, 2018;Martijn P van den Heuvel et al, 2010) used such myelin-sensitive MR measures in brain network models. A recent study (Mancini et al, 2018) used the g-ratio, which is defined as the ratio between the inner and outer axonal diameters, derived from MRI as a myelin weighting scheme for the connectome. Van den Heuvel and colleagues (Martijn P van den Heuvel et al, 2010) used magnetization transfer ratio (MTR) to assess network structure in schizophrenia.…”

Evolution of white matter tract microstructure across the life span

“…We estimated the normalization factor, α, by following the previous literature (Cercignani et al, 2016;Mancini et al, 2017;Mohammadi et al, 2015). We estimated the normalization factor, α, by following the previous literature (Cercignani et al, 2016;Mancini et al, 2017;Mohammadi et al, 2015).…”

“…where MVF is the myelin volume fraction, AVF is the axonal volume fraction, MT is the quantitative MT measurement, and ν iso /ν ic are, respectively, the ISO and ICVF estimates from the NODDI diffusion model. We estimated the normalization factor, α, by following the previous literature (Cercignani et al, 2016;Mancini et al, 2017;Mohammadi et al, 2015). A region of interest (ROI) was selected in the splenium of 11 subjects (mean age 26.5 ± 1.41 years) and a value of α = 0.23 chosen as the median normalization factor required to constrain the splenium g-ratio to a value of 0.7 across subjects, mirroring reported g-ratio values made in the splenium using electron microscopy (Graf von Keyserlingk & Schramm, 1984).…”

Constructing an Axonal‐Specific Myelin Developmental Graph and its Application to Childhood Absence Epilepsy

“…Combining the structural information from DTI with information on the myelin content from MWI might provide a more comprehensive description of the white matter organization in the brain, compared to only the DTI‐derived tract trajectories. Previously, it was already shown that myelin‐water maps possess distinct patterns along DTI‐derived tracts 18,19 . Ideally, it should be feasible to follow the myelin development of various white matter tracts over time, and differentiate tracts that reflect abnormalities during healthy development.…”