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T2 – weighted MRI images


Fractional anisotropy is a method that is used to emphasize and evaluate white matter fiber tracts. This method has been applied to MRI scans in diffusion weighted images. If full tensor data acquisition is applied, the fractional anisotropy (FA) is calculated from the eigenvalues λ1, λ2, and λ3 of the diffusion tensor:

$${\rm FA = }{{\sqrt {3} } \over {\sqrt {2} }}{{\sqrt {{\rm (\lambda }_{1} {\rm - \lambda )}^{2} {\rm + (\lambda}_{2} {\rm - \lambda )}^{2} {\rm + (\lambda }_{3} {\rm - \lambda )}^{2} } } \over {\sqrt {{\rm \lambda }_{1} ^{2} {\rm + \lambda }_{2}^{2} {\rm + \lambda }_{3} ^{2} } }},$$

where λ is the mean diffusivity (trace/3). FA takes on values between 0 (perfectly isotropic diffusion) and 1 (the hypothetical case of an infinite cylinder) and is thus directly comparable between subjects. Here FA is multiplied by 1,000. A fractional process is a procedure that separates mixtures into their components. For example, the total...

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References and Readings

  • O’Donnell, L. J., Kubicki, M., Shenton, M. E., Dreusicke, M. H., Grimson, W. E., & Westin, C. F. (2006). A method for clustering white matter fiber tracts. American Journal of Neuroradiology, 27, 1032–1036.

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Kaplan, P.E. (2011). Fractional Anisotropy. In: Kreutzer, J.S., DeLuca, J., Caplan, B. (eds) Encyclopedia of Clinical Neuropsychology. Springer, New York, NY.

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