20 Chapter 1 Diffusion Tensor Imaging To investigate the microstructure of the white matter of the brain, we will use Diffusion Tensor Imaging (DTI). DTI is a MRI technique that uses movement of water molecules in the brain to produce neural tract images. By evaluating and quantifying diffusion restriction and limited movement of water in white matter of the brain the pattern of neural networks in the brain could be revealed (Figure 7). In this way, the microarchitecture of the white matter tracts in the brain can be investigated. Figure 7. Tractography of bundles of axons in the brain. The main direction of diffusion is encoded in the colour of tracks with blue representing tracts coursing up and down, green representing tracts coursing from front-to-back and left representing tracts coursing left to right. To understand the technique behind DTI, we have to zoom into diffusion. In general, diffusion movement in all directions (for example in a glass of pure water) is called isotropy. Diffusion in tissues varies with direction, which is called anisotropy. In white matter of the brain, the diffusion anisotropy is primarily caused by cellular membranes, with some contribution frommyelination and the packing of axons. Anisotropic diffusion can indicate the underlying tissue orientation (Figure 8).
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