I'm not seeing an attached image, but I can guess you mean either one of two things:
A) an FA map colorized by local direction of first eigenvector (blue, superior-inferior; etc.); or
B) little, tiny overlay lines per voxel showing local orientation of the first eigenvector.
For case (A), see for example using 3dThreetoRGB:
[
afni.nimh.nih.gov]
For case (B), this is really a larger AFNI/SUMA-viewing question, to which I don't know the answer, and hopefully someone more authoritative on such things can supply information.
--pt
