title: Including diffusion time dependence in the extra-axonal space improves in vivo estimates of axonal diameter and density in human white matter.
abstract: Axonal density and diameter are two fundamental properties of brain white matter. Recently, advanced diffusion MRI techniques have made these two parameters accessible in vivo. However, the techniques available to estimate such parameters are still under development. For example, current methods to map axonal diameters capture relative trends over different structures, but consistently over-estimate absolute diameters. Axonal density estimates are more accessible experimentally, but different modeling approaches exist and the impact of the experimental parameters has not been thoroughly quantified, potentially leading to incompatibility of results obtained in different studies using different techniques. Here, we characterise the impact of diffusion time on axonal density and diameter estimates using Monte Carlo simulations and STEAM diffusion MRI at 7 T on 9 healthy volunteers. We show that axonal density and diameter estimates strongly depend on diffusion time, with diameters almost invariably overestimated and density both over and underestimated for some commonly used models. Crucially, we also demonstrate that these biases are reduced when the model accounts for diffusion time dependency in the extra-axonal space. For axonal density estimates, both upward and downward bias in different situations are removed by modeling extra-axonal time-dependence, showing increased accuracy in these estimates. For axonal diameter estimates, we report increased accuracy in ground truth simulations and axonal diameter estimates decreased away from high values given by earlier models and towards known values in the human corpus callosum when modeling extra-axonal time-dependence. Axonal diameter feasibility under both advanced and clinical settings is discussed in the light of the proposed advances.
authors.affiliation: CUBRIC, School of Psychology, Cardiff University, Cardiff CF10 3AT, UK; Faculty of Psychology & Neuroscience, Maastricht University, Maastricht, The Netherlands. Electronic address: desantiss@cardiff.ac.uk.
authors.forename: Silvia
authors.initials: S
authors.lastname: De Santis
authors.affiliation: CUBRIC, School of Psychology, Cardiff University, Cardiff CF10 3AT, UK; Neuroscience & Mental Health Research Institute, Cardiff University, CF10 3AT, UK.
authors.forename: Derek K
authors.initials: DK
authors.lastname: Jones
authors.affiliation: Faculty of Psychology & Neuroscience, Maastricht University, Maastricht, The Netherlands.
authors.forename: Alard
authors.initials: A
authors.lastname: Roebroeck
mesh_terms: D001369:Axons / Q000648:ultrastructure*; D001921:Brain / Q000648:ultrastructure*; D001931:Brain Mapping / Q000379:methods*; D003198:Computer Simulation; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008962:Models, Theoretical; D009010:Monte Carlo Method; D066127:White Matter / Q000648:ultrastructure*
keywords: Axonal density; Axonal diameters; CHARMED; Diffusion time; STEAM diffusion MRI; White matter microstructure
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