Tournier, Jacques-Donald; (2003) Diffusion tensor magnetic resonance imaging and fibre tractography in the brain. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Diffusion tensor magnetic resonance imaging allows detailed studies of the diffusion of water to be carried out non-invasively in-vivo. Brain white matter consists of coherently oriented fibre bundles, in which the diffusion coefficient is larger when measured along the direction of fibres rather than across them. Fibre-tracking techniques attempt to use this information to establish connectivity within the brain, by assuming that the direction of fastest diffusion corresponds to the orientation of the underlying fibre tract. Fibre-tracking requires the acquisition of a large number of images with the diffusion weighting applied in different directions. A number of technical difficulties had to be overcome before such data could be acquired on our Siemens Vision 1.5T system. In particular, eddy-current problems needed to be addressed, and all image reconstruction was implemented offline to allow a more rapid acquisition of the raw data. Fibre-tracking techniques are susceptible to noise, partial volume effects, diffusion anisotropy and other properties of the underlying structure. In this work, simulations were carried out to assess the reliability of a popular fibre-tracking algorithm when applied to artificial data sets designed to model in-vivo structures. Partial volume effects in particular were found to have a detrimental effect on the reliability of fibre tracking. To address the issues raised above, a novel fibre-tracking technique is proposed, based on the evolution of a front. It is capable of branching and produces estimates of the likelihood of the connections established. The algorithm is designed to be more robust to noise and to deal with the problem of crossing fibres more adequately. Fibre tracts reconstructed using this technique are presented and discussed. To illustrate a practical application of diffusion tensor MRI, the relationship between diffusion anisotropy and language deficits in patients with unilateral basal ganglia infarctions was investigated.

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