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Investigating white matter fibre density and morphology using fixel-based analysis

Raffelt, DA; Tournier, J-D; Smith, RE; Vaughan, DN; Jackson, G; Ridgway, GR; Connelly, A; (2017) Investigating white matter fibre density and morphology using fixel-based analysis. NeuroImage , 144 (Part A) pp. 58-73. 10.1016/j.neuroimage.2016.09.029. Green open access

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Abstract

Voxel-based analysis of diffusion MRI data is increasingly popular. However, most white matter voxels contain contributions from multiple fibre populations (often referred to as crossing fibres), and therefore voxel-averaged quantitative measures (e.g. fractional anisotropy) are not fibre-specific and have poor interpretability. Using higher-order diffusion models, parameters related to fibre density can be extracted for individual fibre populations within each voxel (‘fixels’), and recent advances in statistics enable the multi-subject analysis of such data. However, investigating within-voxel microscopic fibre density alone does not account for macroscopic differences in the white matter morphology (e.g. the calibre of a fibre bundle). In this work, we introduce a novel method to investigate the latter, which we call fixel-based morphometry (FBM). To obtain a more complete measure related to the total number of white matter axons, information from both within-voxel microscopic fibre density and macroscopic morphology must be combined. We therefore present the FBM method as an integral piece within a comprehensive fixel-based analysis framework to investigate measures of fibre density, fibre-bundle morphology (cross-section), and a combined measure of fibre density and cross-section. We performed simulations to demonstrate the proposed measures using various transformations of a numerical fibre bundle phantom. Finally, we provide an example of such an analysis by comparing a clinical patient group to a healthy control group, which demonstrates that all three measures provide distinct and complementary information. By capturing information from both sources, the combined fibre density and cross-section measure is likely to be more sensitive to certain pathologies and more directly interpretable.

Type: Article
Title: Investigating white matter fibre density and morphology using fixel-based analysis
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.neuroimage.2016.09.029
Publisher version: https://doi.org/10.1016/j.neuroimage.2016.09.029
Language: English
Additional information: Copyright © 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: Diffusion; MRI; Fixel; Fibre; Density; Cross-section
UCL classification: UCL
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences
URI: https://discovery-pp.ucl.ac.uk/id/eprint/1524973
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