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]
|