Diffusion tensor imaging and fiber tractography

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

Fiber tractography (FT) is a 3D reconstruction technique to assess neural tracts using data collected by diffusion 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 fibers is projected in the final image.

For many, diffusion tensor imaging is synonymous with MR imaging of the CNS. However, its use for the assessment of highly-organized body systems outside the CNS, where anisotropy can facilitate early detection of pathology, has been gaining favor in recent years. Applications include:

  • assessment of the deformation of white matter by tumors - deviation, infiltration, destruction of white matter
  • delineation of the anatomy of immature brains
  • pre-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
  • ROI (region of interest) based
  • voxel-based
  • histogram analysis
  • tractography/fiber tracking 

Some terms 

  • Diffusion tensor imaging  provides a quantitative analysis of the magnitude and directionality of water molecules
  • the word tensor indicates the use of a 3x3 matrix with eigenvalues and eigenvectors as its constituents
  • the two main parameters derived from DTI data are mean diffusivity (MD), also referred to as apparent diffusion coefficient (ADC), and fractional anisotropy (FA)
  • FA reflects the directionality of molecular displacement by diffusion and varies 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 higher the MD value, the more isotropic the medium
  • AD - axial diffusivity represents the longest eigenvector 
  • RD - radial diffusivity represents the average of two shorter eigenvectors
  • 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)

In fiber tractography imaging, the convention for color coding is a follows:

  • red: transverse fibers
  • green: anteroposterior fibers
  • blue: craniocaudal fibers

 

Physics and Imaging Technology: MRI
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Article information

rID: 22702
Section: Physics
Tag: spine
Synonyms or Alternate Spellings:
  • DTI
  • Diffusion tensor imaging (DTI)
  • Diffusion tensor imaging
  • Tractography
  • Diffusion-tensor imaging
  • Fiber tractography
  • MRI tractography

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Cases and figures

  • Case 1: thalamic radiations
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  • Case 2: trigeminal nerve
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  • Case 3: superior cerebellar peduncle decussation
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  • Case 4: multiple sclerosis
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  • Case 5: glioblastoma NOS
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  • Case 6: pontine cavernoma
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