General
- DTI is a more sophisticated version of DWI that also measures the directionality of proton movement within each voxel in terms of fractional anisotropy (FA)
- DTI is based on DWI
- DWI: MRI imaging measuring random Brownian movement of water molecules in a voxel of tissue
- In DWI, three gradient-directions are applied to estimate average diffusivity
- Sequence of choice spin-echo EPI
- DTI scans derive directional information from data using 3D or multidimensional vector algorithms based on six or more gradient directions
- Diffusion anisotropy measures can be computed, such as fractional anisotrophy (FA)
- FA images are colour-coded maps that indicate the orientation of the direction of diffusion
- Red: right to left
- Green: antero-posterior
- Blue: cranio-caudal
- Three types of molecular motion
- Isotropic diffusion (molecular motion is random)
- Restricted diffusion (random motion is constrained)
- Bulk flow-anisotropic diffusion
Anisotropic diffusion
- In white matter, diffusion follows the pathway of least resistance along white-matter fibres
- Cell membranes (not myelin) are predominantly responsible for the anisotropy of molecular diffusion observed in white matter
- DTI measures anisotropic (directionally dependant) diffusion along white matter tracts
Diffusion MR Tractography
- A 3D reconstruction technique to improve depiction of data from DWI
- Aim is to clarify orientational architecture of tissues and show neural tracts
- Fibres are shown by following the direction of maximal diffusion voxel by voxel across the brain
- Different computer tracking algorithms are used to map connectivity of brain
Neurosurgical applications
- Neuro-oncology- assess white matter tracts (including cortico-spinal tract and optic radiation) pre-operatively
- Deviation, infiltration, destruction by tumours
- Pre-operative planning epilepsy surgery
- Traumatic brain injury
- (Normal brain development; neurodegenerative conditions)
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