Diffusion tensor imaging

Diffusion tensor imaging (DTI) is an MRI technique that measures the diffusion of water molecules through brain tissue. Because water diffuses more freely along white matter fibres than across them, DTI gives an indirect picture of the brain's wiring at a microscopic scale.

It is the standard method for studying white matter integrity in vivo.

What it measures

DTI produces several derived metrics per voxel:

• Fractional anisotropy (FA) — how directional the diffusion is. High FA means water flows mostly in one direction (typical of intact white matter); low FA means diffusion is more random (typical of grey matter, CSF, or damaged white matter).
• Mean diffusivity (MD) — overall diffusion magnitude. Goes up when tissue is damaged or when CSF leaks into a region.
• Axial and radial diffusivity — separate parallel and perpendicular components, sensitive to axon damage versus myelin damage.

What it's used for

• Tractography — reconstructing the major white matter tracts (corpus callosum, corticospinal tract, arcuate fasciculus) by following directional diffusion patterns
• Ageing and disease — FA declines with age and drops sharply in conditions affecting white matter (multiple sclerosis, traumatic brain injury, leukoaraiosis)
• Pre-surgical planning — mapping tracts near a tumour or epileptic focus

Strengths and limits

• Strength — the only non-invasive way to image white matter microstructure in living humans
• Limit — DTI's tensor model breaks down in regions where fibres cross. Newer techniques (HARDI, NODDI) handle this better but are less standard.