Hi,
That will depend on the details of your acquisition, and the details of
how Philips writes out the diffusion vectors into its .PAR file. e.g.,
did you acquire the dMRI obliquely in each subject, relative to that
subject's own anatomical landmarks, or were the dMRI gradients always
applied relative to the gradient axes? And in what "space" does Philips
write out the vectors -- in the frame of the imaging axes or relative to
the gradient axes.
The specification of gradient directions is a bit of a wild-west across
different platforms, so you'll either need the assistance of your local
Philips engineer, or hope that a Philip's expert chimes in here.
At a minimum, if the directions are the same for all your subjects, that
means that the effect of the imaging gradients on the effective bval/bvec
is not being accounted for (unless you somehow acquired all the data with
the FOV in the exact same position relative to the gradient axes for all
subjects).
cheers,
-MH
--
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO 63110 Email: [log in to unmask]
On 5/15/14 2:18 AM, "FT" <[log in to unmask]> wrote:
>Dear FSL users,
>
>I'd like to have your opinion about a problem with my DTI data. For a set
>of subjects the information about the bvecs has been lost. What I'm left
>with is a .PAR file with a field "diffusion" which contains three vectors
>(one for each of my 64+1 volumes), but they are equal among all subjects.
>So my question is: is this information correct to be used in the --bvecs
>field, when calling the dtifit command? Or is it a bad approximation of
>the gradients' orientation?
>
>Thank you.
>
>Best,
>
>FT
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