Hi
Just to add a clarification to (2):
- deterministic doesn't necessarily mean fitting a single fibre. you
can use multiple fibres in deterministic tracking
- having too few gradient directions increases the uncertainty on the
fibre orientations, so probabilistic will look *less* like
deterministic, there will be more uncertainty on streamline trajectories
(but it is true bedpost will tend to fit a single fibre).
Saad
> Hi Stefano,
>
>
>
> 1) Yes that seems fine.
>
> 2) Yes, you can run bedpostx -n 1. Also, if you don't have enough
> fiber orientations (e.g. 12 only) bedpostx starts to look like
> deterministic
> tractography because it cannot fit more than one fiber. I did some
> unpublished comparisons of this nature. ~30 directions is where you need
> to
> be at a minimum (but more is obviously better).
>
> 3) Yes, I looked at it a little bit in my arcuate paper (it is a
> supplementary figure). Also I think there is some in Heidi and Tim's
> diffusion book from Andreas Bartsch. Since you have so many questions
> about
> diffusion stuff, I highly recommend owning a copy. :-)
>
>
>
> Peace,
>
>
> Matt.
>
>
>
> _____
>
> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
> Of Marenco, Stefano (NIH/NIMH) [E]
> Sent: Friday, December 16, 2011 10:58 AM
> To: [log in to unmask]
> Subject: [FSL] 3 questions about tractography
>
>
>
> Hello, we are interested in comparing results from deterministic and
> probabilistic tractography. In order to do this, we need to do the fiber
> tracking in dti space, but we would like to maintain compatibility with
> results in MNI space which are more efficient. We have a single spherical
> seed ROI in MNI space. We thought that we could transform the centroid of
> the sphere to diffusion space, then construct a sphere of the same
> dimension
> in each subject around it and the result should be almost identical to the
> MNI tractography result, at least with probtrackx. Is this a correct
> assumption?
>
> The second question I have in this vein is this. Deterministic
> tractography
> differs in at least two respects from probtrackx results: 1) the
> probabilistic nature of the tractography and 2) the fact that probtrackx
> recognizes a secondary fiber. Is it possible to alter probtrackx to
> recognize only the principal fiber?
>
> Are you aware of any systematic work that has compared deterministic and
> probabilistic tractography in clinical populations?
>
>
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