Just a quick note -
I've been asked to point out that, although we would advise high angular
resolution diffusion data for probabilistic tractography, it is not
necessarily a requirement for voxel based Diffusion Tensor Imaging
studies.
You _can_ run dtifit on your 6 direction DTI data
Cheers
T
On Wed, 8 Dec 2004, Tim Behrens wrote:
> Russ - It is an absolute statement if you really only have 6 data points.
>
> Bedpost is estimating 6 parameters (including a variance) - imagine trying
> to estimate distributions on your parameter estimates in FMRI if you have
> 6 datapoints and 5 regressors !!
>
> If you have many averages, you can merge them to give Bedpost a little
> more freedom to estimate the variances, but the uncertainty in orientation
> will then be very dependent on the orientation of the fibre bundles with
> respect to the orientation of your bvectors. Derek Jones did a study on
> this at ISMRM in Toronto I believe. I think it may now be published in
> MRM. The uncertainty stops being dependent on orientation wrt bvectors
> when you get to about 25-30 isotropically oriented bvectors- hence we
> recommend this number as the minimum for probabilistic tractography.
>
>
> Hope this helps
>
> T
>
>
>
> -------------------------------------------------------------------------------
> Tim Behrens
> Centre for Functional MRI of the Brain
> The John Radcliffe Hospital
> Headley Way Oxford OX3 9DU
> Oxford University
> Work 01865 222782
> Mobile 07980 884537
> -------------------------------------------------------------------------------
>
> On Tue, 7 Dec 2004, Russ Poldrack wrote:
>
> > Thanks Andreas. Regarding the using a 6-direction data with bedpost,
> > is this an absolute statement (i.e, "One will never be able to use
> > 6-direction data to do probabilistic tracking") or a relative statement
> > ("One needs to be careful with the output of bedpost using 6-direction
> > data")? If the latter, can you suggest some metrics for robustness?
> >
> > cheers
> > russ
> >
> > On Dec 7, 2004, at 4:00 PM, Automatic digest processor wrote:
> >
> > >
> > > hi russ-
> > >
> > > 6 directions is generally too few for a robust probabilistic solution.
> > > w/r to 1, i would tend to average the coregistered /
> > > eddy-current-corrected runs and perform further computations on the
> > > average. (simply because it would be less time consuming.)
> > > w/r to 2, yes - flirt can register to the b0-image (corrratio puts up
> > > well with the liquor being bright on b0 but dark on the other images),
> > > i.e you can simply put it first to use it as the default reference
> > > volume.
> > >
> > > best regards-
> > > andreas
> > >
> > ---
> > Russell A. Poldrack, Ph.D.
> > Assistant Professor of Psychology, UCLA
> > Franz Hall, Box 951563
> > Los Angeles, CA 90095-1563
> > email: [log in to unmask]
> > phone: 310.794.1224
> > fax: 310.206.5895
> > web: http://www.poldracklab.org/
> >
>
--
-------------------------------------------------------------------------------
Tim Behrens
Centre for Functional MRI of the Brain
The John Radcliffe Hospital
Headley Way Oxford OX3 9DU
Oxford University
Work 01865 222782
Mobile 07980 884537
-------------------------------------------------------------------------------
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