Diffusion Tensor Imaging (DTI) - Fiber Tracking

Diffusion Tensor Imaging (DTI) is a MRI-based neuroimaging technique which makes it possible to visualize the location, orientation, and anisotropy of the brain's white matter tracts.

DTI Technique

The architecture of the axons in parallel bundles and their myelin shield facilitate the diffusion of the water molecules along their main direction. If we apply diffusion gradients in at least 6 non-collinear directions, it is possible to calculate, for each pixel, a tensor (i.e. a 3*3 matrix) that describes this diffusion anisotropy. The fiber's direction is indicated by the tensor’s main eigenvector. This vector can be color-coded, yielding a cartography of the tracts’ position, direction (red for right-left, blue for foot-head, green for anterior-posterior), and anisotropy (as indicated by the tract's brightness). In addition, the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) can be quantified.

Fiber Tracking - Tractography

Fiber tracking uses the diffusion tensor to track fibers along their whole length. Starting from a seed ROI, generally defined manually, the fiber tracking algorithm looks for adjacent voxels whose main diffusion direction is in the continuity of the previous one. The most tracked fiber bundle is the cortico-spinal (and associated) tract, but fiber tracking can identify most of the brain's white matter tracts.

Limitations of DTI-based Tractrography

The diffusion tensor imaging model assumes that, in each voxel, there is a unique orientation of the fibers, the direction of which is represented by the tensor’s main eigenvector (Mori and Tournier, 2013). This assumption is not valid in case of crossing fibers. The term “crossing fibers” generally refers to regions in which the fibers’ orientation is not unique, i.e. when the fibers are interdigitating, brushing past each other, curving, bending or diverging (Mori and Tournier, 2013). Higher order models, such as constrained spherical deconvolution, have been developed in order to address the issue of crossing fibers.

Clinical Applications of Diffusion Tensor Imaging

  • Tract-specific localization of white matter lesions
  • Localization of tumors in relation to the white matter tracts (infiltration, deflection, oedema, destruction)
  • Localization of the main white matter tracts for neurosurgical planning
  • Assessment of the white matter maturation

DTI Papers and Website by Laurent Hermoye

  • Hermoye, Mori et al. Pediatric diffusion tensor imaging: normal database and observation of the white matter maturation in early childhood. Neuroimage 2006, 29:493-504.
  • Lemaire, Hermoye et al. White matter anatomy of the human deep brain revisited with high resolution DTI fibre tracking. Neurochirurgie 2011, 57:52-67.
  • White Matter Atlas

Other References about Diffusion Tensor Imaging

  • Le Bihan. Looking into the functional architecture of the brain with diffusion MRI. Nat Rev Neurosci. 2003, 4:469-80.
  • Mori and Tournier Introduction to Diffusion Tensor Imaging: And Higher Order Models. Academic Press 2013.
  • Mori and van Zijl. Fiber tracking: principles and strategies - a technical review. NMR Biomed. 2002,15:468-80.
  • Wakana et al. Fiber tract-based atlas of human white matter anatomy. Radiology 2004, 230:77-87.
  • Tournier et al. Diffusion tensor imaging and beyond. Magn Reson Med. 2011, 65:1532-56.

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