Whole-Brain In-vivo Measurements of the Axonal G-Ratio in a Group of 37 Healthy Volunteers

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.

Analysis of White Matter Damage in Patients with Multiple Sclerosis via a Novel In Vivo MR Method for Measuring Myelin, Axons, and G-Ratio

“…Larger axons (eg, those in corticospinal tracts) are known to have greater g-ratios than smaller axons. 18,33 There are various approaches for estimating MVF in the brain. Magnetization transfer imaging is one of these, and quantitative magnetization transfer imaging has shown a strong correlation with quantitative histology.…”

“…G-ratio has a limited dynamic range in healthy WM 13 and depends on age [15][16][17] and region. 15,18 Stikov et al 12 developed a model that estimates "aggregate" g-ratio in a voxel via MR imaging. This model assumes that the g-ratios of all axons in a voxel are the same.…”

Synthetic quantitative MRI through relaxometry modelling

“…15, 38 Limitations in the validity of this assumption may be a source of bias. Models such as the one presented here are also a first step towards in vivo histology,2 in which quantitative maps are combined using biophysical models in order to extract descriptors of the underlying tissue such as myelin and iron levels3 or the degree of myelination of fibres 48. Any biases present in the parameter maps derived from this model would propagate through to these biological descriptors.…”

Computationale Neuroanatomie und Mikrostrukturbildgebung mit der Magnetresonanztomographie

“…Es wurde gezeigt, dass es mit der mikroskopischen G-Ratio aus der Ex-vivo-Histologie korreliert [58]. Erste in vivo G-Ratio-Messungen auf der Populationsebene deuten darauf hin, dass die G-Ratio signifikant unterschiedlich ist für verschiedene Faserbündel ( [59]; . Abb.…”

“…Abb. 5a zeigt, dass die hMRT-G-Ratio in einer Populationsstudie von 37 gesunden Probanden (Mohammadi et al [59] Mikrostrukturänderungen gemessen mittels qMRT oder hMRT sind insbesondere für klinische Studien interessant [66].…”