Thank you to everyone that has responded!
This has been a big help.
Sandra
Sandra Woodman
Mood and Motor Control Laboratory
Athinoula Martinos Center for Biomedical Imaging
Massachusetts General Hospital
120 6th St.
Charlestown, MA 02129
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-----Original Message-----
From: FSL - FMRIB's Software Library on behalf of Stamatios Sotiropoulos
Sent: Tue 2/1/2011 1:07 AM
To: [log in to unmask]
Subject: Re: [FSL] bvals/bvecs
Hi Sandra,
On 31 Jan 2011, at 15:42, Woodman, Sandra wrote:
> Hello all,
>
> We have a question regarding DTI data we aquired a few years ago (2005-2006).
>
> For those of you who use DTI sequences,
>
> 1.) Do you use normalized (alter) bvals/bvecs files or do you use the files
that
> come direct from the scanner for each individual subject?
>
You can normalize the bvecs entries, but Fdt tools will normalize them anyway.
> 2.) When scanning on different machines, are the same bval/bvecs files
> used/can they be used? ie is there a difference between magnets when using the
> same
> sequence?
>
If you want to keep the same sequence, i.e. b value and gradient directions, you
can do that across different scanners. The way the b value is implemented though
might change (i.e. duration vs amplitude of the gradient) depending on the
gradient system available on the scanner.
> 3.) If the sequence is the same, can the same bval/bvec files be used for all
> individual subjects in a study?
>
Not in general, I would extract a different bvals/bvecs file for each subject
scanned, even if the sequence is exactly the same. The reason is that a) The
bvecs entries might be rotated to reflect slice angulation, which depends on the
specific scanning session and b) some scanners include in their b value
calculation the imaging gradients.
> Thanks for your help!
>
> Sandra
>
>
> Sandra Woodman
> Mood and Motor Control Laboratory
> Athinoula Martinos Center for Biomedical Imaging
> Massachusetts General Hospital
>
>
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Cheers,
Stam
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