Why did I obtain this (see attachment) strange (many black voxels) DTI map?
I noticed that the black voxels correspond to voxels in which
FA is about zero.
The AFNI commands that I used were:

- 3dDWItoDT -prefix eig -automask -eigs Grad_directions DIFF+orig
- 3dcalc -prefix DTI -a 'eig+orig.[9..11]' -c 'eig+orig.[18]' -expr 'c*255*ABS(a)'
- 3dThreetoRGB -prefix DTI_RGB -anat 'DTI+orig.[0]' 'DTI+orig.[1]' 'DTI+orig.[2]'

(DIFF=DWI brik file).

The same analysis computed with FSL returned good main diffusion eigenvectors map.
So I thought that data are good.
In FSL I must specify the b-value, but in AFNI no.
Is there a way to specify b-value in AFNI DTI analysis?

Thanks for any suggestion.
Ciao
Giovanni