title: 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.
authors.affiliation: Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf Hamburg, Germany ; Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, UK.
authors.forename: Siawoosh
authors.initials: S
authors.lastname: Mohammadi
authors.affiliation: Birkbeck/UCL Centre for NeuroImaging, Birkbeck College London, UK.
authors.forename: Daniel
authors.initials: D
authors.lastname: Carey
authors.affiliation: Birkbeck/UCL Centre for NeuroImaging, Birkbeck College London, UK.
authors.forename: Fred
authors.initials: F
authors.lastname: Dick
authors.affiliation: UCL Institute of Cognitive Neurology, University College London London, UK.
authors.forename: Joern
authors.initials: J
authors.lastname: Diedrichsen
authors.affiliation: Birkbeck/UCL Centre for NeuroImaging, Birkbeck College London, UK.
authors.forename: Martin I
authors.initials: MI
authors.lastname: Sereno
authors.affiliation: Medical Physics, Department of Radiology, University Medical Center Freiburg Freiburg, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Reisert
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, UK.
authors.forename: Martina F
authors.initials: MF
authors.lastname: Callaghan
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, UK ; Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences Leipzig, Germany.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
keywords: DTI; diffusion MRI; fiber volume fraction; g-ratio; in-vivo histology; magnetization transfer imaging; multi-parameter mapping; myelin volume fraction