www.fmrib.ox.ac.uk/~behrens/old_fdt_lectures.tar.gz
T
On 19 Sep 2007, at 22:25, Marenco, Stefano (NIH/NIMH) [E] wrote:
> Sorry for the last couple emails that were not intended for the list.
>
> Tim, I was looking for your excellent animated tutorial on the physics
> of diffusion imaging, which I was using for teaching purposes b/c
> it is
> very clear and instructive, even for the not-inclined. You had shown
> this material at the FSL course in Boston, but it appears to have
> disappeared from the website? Could you point me to it, please?
>
> Stefano Marenco, MD
> GCAP, Clinical Brain Disorders Branch,
> NIMH
> 10 Center Drive, room 4S235
> Bethesda, MD 20892
> tel. (301) 435-8964
> fax. (301) 480-7795
> email: [log in to unmask]
>
>
> -----Original Message-----
> From: Tim Behrens [mailto:[log in to unmask]]
> Sent: Wednesday, September 19, 2007 4:17 PM
> To: [log in to unmask]
> Subject: Re: [FSL] Bedpostx Question
>
> Yes - absolutely as expected. There are 2 reasons -
>
> 1. some voxels have 2 fibres, but neighbours have 1, so the % of the
> 1st population will be different between them.
> 2. Even in the voxels with 2 voxels, in some voxels the each chain of
> samples always stays with one population (e.g. f1 is always the same
> population), but in other voxels f1 jumps between the two
> populations, so "mean_f1" is a mixture of "f1 and f2". This obviously
> has no effect on tractography as each sample is treated
> independently, but it does make each image look a bit speckly. One
> day we will get round to writing a program that unwraps the samples
> so that f1 is always the same fibre population in each of the
> samples, but as I say, it will make no difference to porbtrackx.
>
> You can check all this by looking at the sum of mean_f1 and mean_f2.
> This should be nice and smooth,
>
> Cheers
>
> T
>
>
>
>
> On 19 Sep 2007, at 20:55, Timothy Laumann wrote:
>
>> Dear FSL experts,
>>
>> We have been comparing the results of the new bedpostx to the old
>> bedpost
>> and have a question which is illustrated in the example jpeg I have
>> attached (I hope it is large enough to see). Both sets of images
>> are from
>> the same subject with the same preprocessing. The left side
>> represents the
>> dyadic_vectors modulated by mean_fsamples from the original
>> bedpost, the
>> right side represents dyads1 modulated by mean_f1sample from
>> bedpostx.
>>
>> We noticed that the bedpostx results have considerably more
>> speckling in
>> the image, i.e. scattered voxels which appear to represent a primary
>> direction significantly different from the predominant direction
>> which
>> surrounds them. In this regard, the original bedpost image is much
>> smoother. Is this speckling to be expected with the crossing fiber
>> calculations or is this a result of the insufficient resolving
>> power of
>> our data acquisition scheme?
>>
>> Thanks so much for your help!
>>
>> Timothy Laumann
>>
>>
>>
>> <Example.jpg>
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