[Insight-users] problems with eigenvalues and fractional anisotropy

[Garin, Alba]

Dear all,
I'm developing a tractography application based on ITK-VTK. Our immediate aim is to create a visualization of an RGB map of the water diffusion of the brain.
ITK is used in order to read the metaimage file, get the image and the gradients, and, as result, we get an image of tensors, with DiffusionTensor3D as pixel type. Eigenvalues, eigenvectors and FA are computed for each voxel using the ComputeEigenAnalysis() and GetFractionalAnisotropy() methods of DiffusionTensor3D class. In some cases, we realized that the FA was greater than 1.0, as some voxels contained negative eigenvalues, although they had a very low order of magnitude (around 1e-4).
Is there any way to correct a tensor, ensuring that all its eigenvalues are positive, before calling GetFractionalAnisotropy ? Or should I just avoid computing the FA on a voxel with a negative eigenvalue?
Thanks in advance,

Alba Garin Muga

Computer Science PhD Student

Applied Mechanics Department

Paseo Manuel Lardizabal, 15
20018 Donostia-San Sebastian. Spain
Tel.  +34 943 212 800  (Ext. 2113)
Fax: +34 943 213 076
Web: www.ceit.es<http://www.ceit.es/>


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