You may also want to have a look at Jones et al, MRM 2004
It is looking at the stability in the estimates of FA etc. for
different gradient orientations settings.
Not exactly what you want, but it shows that 20 is a good number where
things start stabilising. (although the paper recommends 30)
Saad.
On 10 Oct 2008, at 15:51, Steve Smith wrote:
> Hi,
>
> Hard to quantify to be sure on this - but I would have thought that
> a well-spaced set of 21 directions should not make too much
> difference if the exact locations of the bvecs are changed a little
> between different acquisitions - Tim or Saad can comment further if
> they disagree.
>
> Cheers.
>
>
>
> On 9 Oct 2008, at 15:28, Jenifer Juranek wrote:
>
>> Thanks, Steve.
>> Could you help me understand "reasonably large"?
>> Data has 21 directions (and a single b0).
>> Reasonable to pool data across bvec variants?
>> Interested in both quantitative metrics (FA, MD) and probabilistic
>> tractography (in case definition of reasonably large varies by
>> objective).
>>
>> Jenifer
>>
>
>
> ---------------------------------------------------------------------------
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> ---------------------------------------------------------------------------
>
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Saad Jbabdi,
Postdoctoral Research Assistant,
Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222545 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~saad
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