title: Characterizing axonal myelination within the healthy population: a tract-by-tract mapping of effects of age and gender on the fiber g-ratio.
abstract: The g-ratio, equal to the ratio of the inner-to-outer diameter of a myelinated axon, is associated with the speed of conduction, and thus reflects axonal function and integrity. It is now possible to estimate an "aggregate" g-ratio in vivo using MRI. The aim of this study was to assess the variation of the MRI-derived fiber g-ratio in the brain of healthy individuals, and to characterize its variation across the lifespan. Thirty-eight healthy participants, aged between 20 and 76, were recruited. Whole-brain g-ratio maps were computed and analyzed voxel-wise. Median tract g-ratio values were also extracted. No significant effect of gender was found, whereas age was found to be significantly associated with the g-ratio within the white matter. The tract-specific analysis showed this relationship to follow a nearly-linear increase, although the slope appears to slow down slightly after the 6th decade of life. The most likely interpretation is a subtle but consistent reduction in myelin throughout adulthood, with the density of axons beginning to decrease between the 4th and 5th decade.
authors: Department of Neuroscience, Clinical Imaging Sciences Centre, Brighton and Sussex Medical School, University of Sussex, Brighton, East Sussex, UK; Neuroimaging Laboratory, Santa Lucia Foundation, Rome, Italy. Electronic address: m.cercignani@bsms.ac.uk.
authors: Mara
authors: M
authors: Cercignani
authors: Neuroimaging Laboratory, Santa Lucia Foundation, Rome, Italy.
authors: Giovanni
authors: G
authors: Giulietti
authors: Department of Neuroscience, Clinical Imaging Sciences Centre, Brighton and Sussex Medical School, University of Sussex, Brighton, East Sussex, UK.
authors: Nick G
authors: NG
authors: Dowell
authors: Department of Neuroscience, Clinical Imaging Sciences Centre, Brighton and Sussex Medical School, University of Sussex, Brighton, East Sussex, UK.
authors: Matt
authors: M
authors: Gabel
authors: Neuroimaging Laboratory, Santa Lucia Foundation, Rome, Italy.
authors: Rebecca
authors: R
authors: Broad
authors: Neuroimaging Laboratory, Santa Lucia Foundation, Rome, Italy.
authors: P Nigel
authors: PN
authors: Leigh
authors: Department of Neuroscience, Clinical Imaging Sciences Centre, Brighton and Sussex Medical School, University of Sussex, Brighton, East Sussex, UK.
authors: Neil A
authors: NA
authors: Harrison
authors: Neuroimaging Laboratory, Santa Lucia Foundation, Rome, Italy.
authors: Marco
authors: M
authors: Bozzali
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000378:metabolism / Q000473:pathology*; D001369:Axons / Q000378:metabolism / Q000473:pathology* / Q000502:physiology; D001921:Brain / Q000000981:diagnostic imaging / Q000473:pathology*; D001931:Brain Mapping / Q000379:methods*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009186:Myelin Sheath / Q000378:metabolism; D009413:Nerve Fibers, Myelinated / Q000378:metabolism / Q000473:pathology* / Q000502:physiology; D009431:Neural Conduction; D059906:Neuroimaging; D066127:White Matter / Q000000981:diagnostic imaging / Q000473:pathology; D055815:Young Adult
keywords: Axonal conduction; Lateralization; Myelination; White matter