Diffusion tensor imaging and fiber tractography

Diffusion-tensor tensor imaging (DTI) is an MRI technique that uses anisotropic diffusion to estimate the axonal (white matter) organisation of the brain.

FiberFibre tractography (FT) is a 3D reconstruction technique to accessassess neural tracts using data collected by DTIdiffusion tensor imaging.

Diffusion-weighted imaging (DWI) is based on the measurement of thermal Brownian motion of water molecules. Within cerebral white matter, water molecules tend to diffuse more freely along the direction of axonal fascicles rather than across them. Such directional dependence of diffusivity is termed anisotropy. This direction of maximum diffusivity along the white-matter fibres is projected in the final image.

Colour coding Clinical applications

• red for fibres crossing from left to rightassessment of the deformation of white matter by tumours - deviation, infiltration, destruction of white matter
• green for fibres traversing in anteroposterior directiondelineation of the anatomy of immature brains
• blue for fibres going from superior to inferiorpre-surgical planning
• Alzheimer disease - detection of early disease
• schizophrenia
• focal cortical dysplasia
• multiple sclerosis - plaque assessment
• early identification of musculoskeletal and peripheral nerve pathology ^7-10

Quantitative analysis methods

• ROI (region of interest) based
• voxel-based
• histogram analysis
• tractography/fibre tracking 

Physics of DTI

• DTI providesDiffusion tensor imaging  provides a quantitative analysis of the magnitude and directionality of water molecules
• the word tensor indicates the use of a 3 x 33x3 matrix used here with eigen valueseigenvalues and eigen vectorseigenvectors as its constituents
• the two main parameters derived from DTI data are mean diffusivity (MD) or in another term, also referred to as apparent diffusion coefficient (ADC), and fractional anisotropy (FA)
• FA reflects the directionality of molecular displacement by diffusion and varyvaries between 0 (isotropic diffusion) and 1 (infinite anisotropic diffusion). FA value of CSF is 0
• MD reflects the average magnitude of molecular displacement by diffusion. The morehigher the MD value, the more the isotropic is the medium
• AD - axial diffusivity represents the longest eigen vectoreigenvector 
• RD - radial diffusivity represents the average of two shorter eigen vectorseigenvectors
• DTI data acquisition is done by SE-EPI with the application of diffusion gradients in multiple directions (SE-EPI = spin-echo echo-planar imaging, using pairs of 90-180 degree pulses, giving T2-weighting)