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Hi Stam, Thanks for the answer :) Cheers, Marius På Tue, 27 Apr 2010 16:56:21 +0300, skrev Stamatios Sotiropoulos <[log in to unmask]>: > Hi Marius, > > FSL fits a linear model to the log of the signal to obtain the tensor. > It does not impose any positive definiteness constraints on the tensor. > Therefore, negative eigenvalues can be obtained in the presence of > noise, leading to a larger than 1 FA value. > > Cheers, > Stam > > > On 27 Apr 2010, at 14:15, Marius Widerøe wrote: > >> I have used DTIFIT to calculate FA-maps from diffusion data of a rat >> brain. >> On the border between the brain tissue and surrounding tissue (CSF, >> meninges etc) FA values higher than 1 is reported. How is this possible? >> Example of one voxel: Reported FA-value: 1.22429. Corresponding >> L1=0.00033, L2=0.000018 and L3=0.00029. When I calculate the FA myself >> in this voxel based on the reported L1,L2 and L3 I get 0.668767. >> I am aware that these voxels are probably noise and should be >> disregarded, but it worries me that FSL actually reports FA-values >> higher than 1. >> >> Can anyone explain what has happened here? >> -- ------------------------------------------------------------------ Marius Widerøe, MD, PhD-student Manager, FUGE MIC Trondheim Department of Circulation and Medical Imaging Norwegian University of Science and Technology MR-centre, Ragnhildsgt., 7006 Trondheim, Norway Phone: +47 73 86 76 80 Mobile: +47 40 23 19 23 e-mail: [log in to unmask]