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Editors Selection IGR 11-4
Progression: New MRI technique and VF deterioration
Comment by Neeru Gupta on:
24689 Optic nerve and optic radiation neurodegeneration in patients with glaucoma: in vivo analysis with 3-T diffusion-tensor MR imaging, Garaci FG; Bolacchi F; Cerulli A et al., Radiology, 2009; 252: 496-501
Garaci et al. (1760) use diffusion tensor imaging (DTI), a relatively new MRI modality, to study optic nerve and optic radiation pathology in glaucoma patients. DTI provides unique clues into the architecture of white matter, and is used to map the orientation in space of the nerve fibers tracts in the brain.1 Tissues are scanned in at least 6 different orientations to measure water diffusion represented as a diffusion tensor coefficient. DTI parameters include mean diffusivity (MD) which looks at average molecular motion independent of tissue directionality, and is affected by cellular size and integrity.2,3 Fractional anisotropy (FA)4 is used to reflect the degree of alignment of cellular structures within fiber tracts, as well as their structural integrity. Both MD and FA are used as potential biomarkers in multiple sclerosis.5
Diffusion tensor imaging is a promising tool to evaluate neural degeneration in glaucoma
The authors performed good quality DTI measurements in spite of the CSF signal and orbital fat surrounding the optic nerve. Motion artifacts were reduced by limiting scan time to approximately 15 minutes. In glaucoma, DTI parameters were altered at various stages compared to controls, and FA in the optic nerve of glaucoma patients increased with visual field loss. It would have been interesting to have information regarding the optic nerve head or nerve fibre layer in these patients. Both eyes were used in the majority of patients, and were analyzed as independent data. Applying correction for multiple comparisons may have altered the statistical significance of some of the results. For example, using Bonferroni correction, multiplying the P values by six for the six comparisons for MD, makes the difference between stages 3 and 4 non-significant. For FA, the difference between stages 4 and 5 also loses significance. However, two other comparisons with P values of 0.002 for FA remain significant. DTI of optic radiations were performed in 12 patients with bilateral glaucoma and also showed changes. Optic radiations can be difficult to distinguish from surrounding white matter tracts, and show large interindividual variation in position and space.6 A detailed description of optic radiation measurements would have been helpful. In this interesting study, DTI shows promise as a tool to evaluate neural degeneration in glaucoma. Carefully designed, larger studies are needed to validate parameters of DTI, a potential biomarker for disease progression in glaucoma.
References
- Le Bihan D. Looking into the functional architecture of the brain with diffusion MRI. Nature Reviews Neuroscience 2003; 4: 469-480.
- Basser PJ, Mattiello J, Le Bihan D. Estimation of the effective self-diffusion tensor from the NMR spin-echo. J Magn Reson B 1994; 103: 247-254.
- Pierpaoli C, Jezzard P, Basser PJ, Blarnett A, Di Chiro G. Diffusion tensor MR imaging of the human brain. Radiology 1996; 201: 637-648.
- Basser PJ, Pierpaoli C. Microstructural features measured using diffusion tensor imaging. J Magn Reson B 1996; 111: 209-219.
- Hickman SJ, Wheeler-Kingshott CA, Jones SJ, Miszkiel KA, Barker GJ, Plant GT, Miller DH. Optic nerve diffusion measurement from diffusion-weighted imaging in optic neuritis. Am J Neuroradiol 2005; 26: 951-956.
- Burgel U, Schormann T, Schleicher A, Zilles K. Mapping of histologically identified long fiber tracts in human cerebral hemispheres to the MRI volume of a reference brain: Position and spatial variability of the optic radiation. NeuroImage 1999; 10: 489-499.
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