Hi Marc,
If all your bvecs have the same norm "n", then the estimate of the ADC
will be ADC/n^2.
This means that if the norm of the bvecs is higher than 1, you will
underestimate ADC (and MD, L1,2,3).
It shouldn't affect FA though..
if the bvecs have different norms, then it is not easy to predict what
will happen.
Note that from FSL3.3 onwards, both dtifit and bedpostx automatically
normalise the bvecs.
Cheers,
Saad.
On 11 May 2009, at 16:46, Marc Dubin wrote:
> Dear FSL Users,
>
> Does anyone know the consequences of not normalising all directions
> in the bvecs file?
> Mine all have magnitude 2 and I was wondering if this might have
> somehow inverted the
> magnitude of fractional anisotropy in the dtifit output.
>
> Best,
> Marc
>
> --
> Marc Dubin, MD PhD
> Clinical Research Fellow
> Brain Imaging Laboratory
> NYSPI & Columbia University
>
> Office: 212-543-6702
> Mobile: 646-831-8886
Saad Jbabdi
Oxford University FMRIB Centre
JR Hospital, Headington, OX3 9DU, UK
+44 (0) 1865 222545 (fax 717)
www.fmrib.ox.ac.uk/~saad
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