Abstract
The g-ratio, quantifying the ratio between the inner and outer diameters of a fiber, is an important microstructural characteristic of fiber pathways and is functionally related to conduction velocity. We introduce a novel method for estimating the MR g-ratio non-invasively across the whole brain using high-fidelity magnetization transfer (MT) imaging and single-shell diffusion MRI. These methods enabled us to map the MR g-ratio in vivo across the brain's prominent fiber pathways in a group of 37 healthy volunteers and to estimate the inter-subject variability. Effective correction of susceptibility-related distortion artifacts was essential before combining the MT and diffusion data, in order to reduce partial volume and edge artifacts. The MR g-ratio is in good qualitative agreement with histological findings despite the different resolution and spatial coverage of MRI and histology. The MR g-ratio holds promise as an important non-invasive biomarker due to its microstructural and functional relevance in neurodegeneration.
Highlights
Understanding the normal and diseased human brain crucially depends on reliable knowledge of its anatomical microstructure
The susceptibility-related distortions in the original diffusion MRI (dMRI) data led to a spatial mismatch with respect to the magnetization transfer (MT) data
The largest inter-individual variation as measured by the CoV of the MR g-ratio occurred toward the edges of white-matter tracts (Figure 3B)
Summary
Understanding the normal and diseased human brain crucially depends on reliable knowledge of its anatomical microstructure. An important microstructural property is the g-ratio of fibers, which is defined as the ratio of the axonal diameter to the outer fiber diameter including the myelin sheath (Rushton, 1951; Hodgkin, 1964; Stikov et al, 2011). The g-ratio is related to the conduction velocity of nerve fibers (e.g., Rushton, 1951; Johansen-Berg and Behrens, 2009) and of significant functional relevance. G-ratio maps could be used in conjunction with structural connectivity maps (Knösche and Tittgemeyer, 2011) to assess the importance of connective pathways. The g-ratio can change due to functional stimulation (Gibson et al, 2014) and might be an important indicator of structural plasticity (Zatorre et al, 2012). The cortical g-ratio in multiple sclerosis patients is higher as compared to cortical g-ratio in controls, probably because of de- and re-myelination processes (Albert et al, 2007)
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