[Mristudio-users] second and tertiary eigenvalue equal to 0

[Yi Jiang]

Dear All,

 

I computed diffusion tensors and FA, va0 (primary eigenvalue), va1 (secondary eigenvalue), and va2 (tertiary eigenvalue) by linear fitting in DTIStudio. Then I save them into raw matrix and read them in matlab.

 

I found the number of non-zero (the background is all zero) voxels for va1 and va2 is less than that of va0 or FA. Such as:

 

number of non-zeros voxels for va0 =     6311454
number of non-zeros voxels for va1 =     6310767
number of non-zeros voxels for va2 =     6248757
number of non-zeros voxels for va0 =     6311454

 

This happens in all my datasets, consistently.

 

The voxels where va0~=0 & va1==0 are not a lot (a few hundred), and they mostly are at the edge of the brain, not inside white mattter.

 

But the voxels where va0~=0 & va2==0 are quite a lot (tens of thousand), and they resides mostly inside white matter, such as corpus callosum.

 

I would like to know why this happens. 



Moreover, when I try to, for example, calculate the mean va2 within corpus callosum, how can I deal with those voxels where va2==0 ? I think to treat them as equal to 0 sounds not correct physiologically. But because there are a lot, I want to count those points in, especially for small white matter structures (sometimes those va2==0 voxels can take up to 10% of a white matter structure). 



Also, when I calculate radial diffusivity (va1+va2)/2, the va2==0 points generate incorrect radial diffusivity.

 

Thank you very much!

 

Best,

Yi

 
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