Abstract
Diffusion-weighted imaging has pushed the boundaries of neuroscience by allowing us to examine the white matter microstructure of the living human brain. By doing so, it has provided answers to fundamental neuroscientific questions, launching a new field of research that had been largely inaccessible. We briefly summarize key questions that have historically been raised in neuroscience concerning the brain's white matter. We then expand on the benefits of diffusion-weighted imaging and its contribution to the fields of brain anatomy, functional models and plasticity. In doing so, this review highlights the invaluable contribution of diffusion-weighted imaging in neuroscience, presents its limitations and proposes new challenges for future generations who may wish to exploit this powerful technology to gain novel insights.
Highlights
Local measures of white matter microstructureVoxel-wise measures of diffusion properties, such as fractional anisotropy or mean diffusivity, are modulated by local tissue microstructure 30
As noted by Steno, post mortem dissections of the brain reveal an astonishing level of complexity, within the white matter fibre pathways, composed of trillions of axons
Anatomical exploration of the white matter connections of the human brain becomes a new challenge, mixing medical knowledge with advanced practical skills of dissection and illustration (Figure 1). These anatomical descriptions were a stepping-stone for the elaboration of new theories of brain function. This can be seen for example toward the end of the XIXth century, when Meynert extended Newton’s vision even further, by introducing the possibility of reasoning occurring through the association of specialised areas of the brain via their white matter connections
Summary
Voxel-wise measures of diffusion properties, such as fractional anisotropy or mean diffusivity, are modulated by local tissue microstructure 30. Diffusion parameters provide useful noninvasive measures of local tissue microstructure, with sensitivity to features including membrane integrity, myelin thickness, axon diameter and packing density These physical characteristics of the white matter fibre bundle will have consequences for the physiological functioning of that bundle, affecting properties such as conduction time, refractory time, probability of transmission or even synchronisation of signals across a distributed corticocortical network. The ActiveAx acquisition and analysis pipeline, not measuring the entire axonal distribution probability function, provides an approach to estimate the mean axon diameter for the entire brain Another limitation to measuring axonal diameter properties with diffusion imaging is the need to have high diffusion gradient amplitude in order to have comparable accuracy over a wide spread of axon diameters. These two problems hamper progress in the depiction of the projection and commissural pathways 114116
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