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Jones, D.K.: Studying connections in the living human brain with diffusion MRI. Cortex 44(8), 936-952

Cardiff University Brain Research Imaging Centre, School of Psychology, Cardiff University, Cardiff, Wales, UK.
Cortex (Impact Factor: 6.04). 09/2008; 44(8):936-52. DOI: 10.1016/j.cortex.2008.05.002
Source: PubMed

ABSTRACT The purpose of this article is to explain how the random walks of water molecules undergoing diffusion in living tissue may be exploited to garner information on the white matter of the human brain and its connections. We discuss the concepts underlying diffusion-weighted (DW) imaging, and diffusion tensor imaging before exploring fibre tracking, or tractography, which aims to reconstruct the three-dimensional trajectories of white matter fibres non-invasively. The two main classes of algorithm - deterministic and probabilistic tracking - are compared and example results are presented. We then discuss methods to resolve the 'crossing fibre' issue which presents a problem when using the tensor model to characterize diffusion behaviour in complex tissue. Finally, we detail some of the issues that remain to be resolved before we can reliably characterize connections of the living human brain in vivo.

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    • "Anatomical connectivity between brain regions can be measured (or rather approximated) using diffusion magnetic resonance imaging. It delineates the likelihood of white-matter fiber bundles traced to link brain regions (Johansen-Berg and Rushworth, 2009; Jones, 2008). The number of samples reaching any voxel/vertex in the gray matter or, more frequently, the likelihood of passing through brain white matter then provides the connectivity profile of a particular voxel/vertex or node in the ROI. "
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    • "In myelinated axons, which make up white matter tracts, the direction of diffusion is restricted due to the presence of myelin sheaths, neurofilaments, microtubules, and cell membranes (Beaulieu, 2002). DW-MRI captures the degree of restriction, called anisotropy, and provides measures of the microstructural properties of white matter, such as the orientation and magnitude of diffusion within each voxel of the brain (Alexander et al., 2007, 2011; Jones, 2008; Tournier et al., 2011). Tractography allows for the visualization of white matter tracts and can be combined with DTI to calculate microstructural indices specific to particular white matter tracts. "
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    • "Whilst voxel-based global search techniques have their advantages, they do not allow a search to be constrained to a specific anatomical pathway of interest and effectively assume that the properties of neighbouring voxels are independent, even though this will not be the case if they are contained within the same white matter pathway. Diffusion MRI-based tractography (summarised in Jones, 2008)allowsthree- dimensional reconstruction of WM fibre bundles from diffusion MRI data, permitting integration of diffusion properties along the entire length of specific, anatomically-defined WM pathways. Tractography therefore potentially gives greater power to detect an effect which might be indiscernible using voxelwise approaches. "
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