Dear FSL-community,

I recognize that the histogram output from calculated DTI-FA data with FSL

always produces a value range that exceeds 1.0 (typical max values for FA

are on the order of 1.2).

According to the calculation for FA (Conturo MRM1916)

FA=sqrt(3/2)*sqrt((l1-<l>)^2+(l2-<l>)^2+(l3-<l>)^2)/(sqrt(l1^2+l2^2+l3^2))

with: <l>=(l1+l2+l3)/3

this seems to be impossible no matter what actual numerical value the eigenvalues

li have, unless one allows negative eigenvalues.

Since negative li don't have physical meaning they should be omitted and constraint

to : {li}>=0 !

Has somebody an explanation that causes this strange "normalization"

behaviour for FA in FSL?

Thanks Burkhard.