Thanks Ed (and Jesper),
but your answer did not satisfy. If the mean <l>=0 that can only happen if all
eigenvalues are zero (unless you allow negative eigenvalues which is exactly
my point: Negative eigenvalues should not be permitted no matter what algorithm
you are going to use to calculate them - they have no physical meaning
in the diffusion tensor).
If all li=0 than also <l>=0 but now the ratio of numerator and denominator
in the equation of FA becomes ill-defined sqrt(0/0). Unless you have another
explanation. Jesper mentioned something about the "true FA" implicating
that FSL calculates FA-values by statistical means (which by the way is nowhere
explained or mentioned in the documents).
I simply find that FA can only approach sqrt(3/2) if one allows negative eigenvalues
for the calculation of FA and I would consider this an oversight or bug.
Does FSL use the LaPAck SVD or something different?
If this question was alraedy adressed in the blog - my appologies but I can't
Thanks for your patients,