Hiya Stefano
On 27 Mar 2007, at 16:06, Marenco, Stefano (NIH/NIMH) [E] wrote:
> This is an interesting question to me, as we just finished processing
> data with multiple shells (with less than 60 directions at the highest
> b-value) through the probabilistic fiber tracking method. The
> results we
> get for thalamic segmentation look very similar to those published in
> your Nature Neuroscience paper,
good-oh...
> and appear to be fairly reproducible, at
> least for frontal and temporal connections to the thalamus, however
> I am
> not sure if the estimates are more noisy than otherwise. We do not
> have
> a comparison with one b-shell for the same subjects. Tim, would you
> like
> to look at a sample of these data?
I'm not sure what I would be looking for unless you had an equivalent
dataset with the same number of images from a single b-shell in
between the two. It would certainly be hard to do anything quantitative.
> Would it be possible to compute a
> bi-exponential fit in bedpost?
>
Yes it would be, but you'd need to do a bit of coding :)
T
> 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: Tuesday, March 27, 2007 9:07 AM
> To: [log in to unmask]
> Subject: Re: [FSL] intermediate b-value: how useful?
>
> Hi - I guess this is not an easy question - here is my take.
>
> If you are fitting a single diffusion tensor, the diffusivity (and
> FA) values that you measure will depend on b-value, as apparent
> diffusion is not mono-exponential in the brain, as the diffusion
> tensor model assumes.
>
> Therefore, if you collect at more than a single b-value, your model
> fit to a diffusion tensor model will be worse. You will measure
> something between what you would have measured with each one of the b-
> values. My guess is that the error in FA/MD will be higher than for a
> single b-value shell acquisition, but I have not spent a long time
> thinking about it. However, remember that for single shell
> experiments, your measurements are necessarily conditioned on you
> choice of b-value.
>
> If all you are interested in is diffusion orientation (for
> tractography), I again think you are better off spreading your
> orientations equally on a single shell. The model in bedpost also
> assumes mono-exponential decay of each compartment, so I expect that
> the multi b-value experiment will look like more noise to the
> bayesian estimation (as the model fit will be worse) and therefore
> will inflate uncertainty.
>
> Having said this, we do not have practical experience.
>
> T
>
>
> On 26 Mar 2007, at 20:43, Stephen Towler wrote:
>
>> For TBSS and probtrack analysis, what benefit (if any) does one
>> derive from acquiring three b-values (b0, bmax/2, bmax) instead of
>> two b-values (b0 & bmax)? When collecting 2-3 mm isovoxels at 32
>> or sixty directions, omitting the intermediate b-value creates
>> substantial time savings which is a huge help with compromised
>> subjects.
>>
>> Thanks!
>> Stephen
>>
>>
>> --
>> (please note email change from mbi.ufl.edu to ufl.edu)
>>
>> Stephen Towler
>> [log in to unmask]
>> 352-294-0048 office
>> 352-258-6409 mobile
>> 352-392-8347 fax
>>
>> Leonard Lab
>> Department of Neuroscience
>> PO Box 100244
>> University of Florida HSC
>> Gainesville, FL 32610
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