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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