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Editors Selection IGR 8-4

Basic Research: DTI - a new MRI modality

Comment by Yeni Yucel on:

22891 Assessing optic nerve pathology with diffusion MRI: From mouse to human, Xu J; Sun S-W; Naismith RT et al., NMR in Biomedicine, 2008; 21: 928-940

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Xu et al. (206) discuss diffusion tensor imaging (DTI), a relatively new MRI modality, as applied to optic nerve pathology in mouse and human. Structural MRI can detect optic nerve damage or optic nerve atrophy, however, it does not accurately assess the underlying pathology preceding the irreversible structural damage. DTI provides unique clues to the fine architecture of neural tissues and to the changes associated with various physiological and pathological states. DTI can be used to map the orientation in space of the white matter tracts in the brain. Tissues are scanned in at least six different orientations to measure water diffusion represented as a diffusion tensor coefficient. Axial and radial diffusivities provide information regarding pathology along axons and in the surrounding tissue components, respectively. In experimental models of optic nerve injury, axial and radial directional diffusivities have been shown to be specific to axonal and myelin damage, respectively. In addition, progressive Wallerian degeneration along anterior visual pathways has been detected using directional diffusivities in a model of retinal injury.
Challenges in applying this technique to humans include a large size of the human optic nerve, an increasing CSF signal surrounding the optic nerve as it atrophies, in addition to orbital fat and motion artifacts due to increased total scan time compared to structural MRI. Clinical studies with DTI will need to address the increased need for total scan time compared to structural MRI methods, in addition to the lack of consensus on the optimal scan sequence or protocol for optic nerve DTI, or a normative database.
In spite of these challenges, directional diffusivities by DTI show great potential to be specific biomarkers for axonal and myelin injury. In glaucoma, pathology at the level of optic nerve is mainly axonal, however in early phase of pathological process, damage to non-neuronal cell types such as astrocytes and oligodendrocytes may contribute more complex structural changes. Therefore, detection of changes in both axial and radial directional diffusivities would be important.² DTI can be also used to detect changes in optic radiations.³ DTI imaging with correlative neuropathology in experimental glaucoma models will help to determine biomarkers to assess the evolution of glaucomatous optic nerve pathology. Among the potential biomarkers of glaucoma, neuroimaging parameters appear promising.

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