MRI · reads
Diffusion tensor imaging is an MRI method that measures how water diffuses in brain tissue. Diffusion-encoding magnetic field gradients make the scan sensitive to diffusion along at least six directions, and a tensor fitted in each voxel gives the overall diffusion rate and how strongly diffusion prefers one direction. This is an indirect proxy for tissue microstructure, not neural activity; white-matter anisotropy is attributed mainly to cell membranes, with contributions from myelin and axon packing. Tractography uses the data to estimate the paths of white-matter pathways.
Diffusion MRI is magnetic resonance imaging that measures the diffusion of water molecules in tissue. Diffusion-encoding gradients are applied during the scan. For diffusion tensor imaging (DTI) the standard setting is a diffusion weighting of b = 1000 s/mm² along at least six non-collinear directions, plus one minimally T2-weighted image at b = 0. The measured quantity is the water diffusion coefficient, in mm²/s. No concentration gradient is present, so this is self-diffusion. Fitting a tensor in each voxel gives an indirect measure of how fast and in which direction water diffuses. Mean diffusivity (MD), also called the apparent diffusion coefficient (ADC), is the overall diffusion rate. Fractional anisotropy (FA) is the directional preference of diffusion. Axial diffusivity is the rate along the main axis of diffusion, and radial diffusivity is the rate across it. In the white matter of the brain, diffusion anisotropy is caused mainly by cellular membranes, with some contribution from myelination and the packing of the axons. Tractography is a computational reconstruction method that uses the diffusion-weighted data to estimate the trajectories of white-matter pathways in vivo and to infer the structural connectome. Methods beyond DTI generally use a higher b-value than 1000 s/mm², or several b-values acquired as multiple shells.
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