Dear Stam,
Sorry again I am not clear about one aspect If i choose single fiber setting as you suggested (setting the number of fibres to 1) even for that is the 12 directional data sufficient my question is whether the probabilistic method in itself is sensitive to number of directions and SNR.
Thanks very much
Venkat
-----Original Message-----
From: FSL - FMRIB's Software Library on behalf of Stamatios Sotiropoulos
Sent: Fri 10/15/2010 11:28 AM
To: [log in to unmask]
Subject: Re: [FSL] probabilistic tractography -general question
Dear Venkat,
the bedpostx model has 8 parameters for resolving two crossing populations. Using only 12 data points to estimate those will lead to estimates with high uncertainty, which will be reflected to the tractography results (i.e. very distributed connectivity maps). Furthermore, 12 measurements are not enough to reliably resolve crossings in the presence of noise (see Fig. 8 of the following paper http://www.ncbi.nlm.nih.gov/pubmed/17070705 to get an idea of what you can resolve, given the b value, SNR and the number of directions).
If you would like to use the data, I would recommend setting the number of fibres to 1 (both in bedpostx and in probtrackx), but be cautious when interpreting the results.
Cheers,
Stam
On 15 Oct 2010, at 15:58, Rajagopalan, Venkateswaran wrote:
>
> Dear All,
>
> We have DTI data set acquired in control and ALS subjects along 12 direction using SS-EPI sequence using 1.5 T(with no field map corrections done and signal to noise ratio is not good in some of the cases since these data sets are from our routine clinical scans) so the images are greatly distorted in some cases. We were studying changes in DTI metrics along corticospinal tract (CST) reconstructed using deterministic tractography approach and we saw CST truncation in some of our ALS patients above the level of top lateral ventricle. We were thinking of using probabilistic approach to reconstruct CST since it has the ability to pierce through the lesion (which we think may be stopping the tracts while using deterministic approach), I read somewhere that probabilistic tractography in FSL requires more than 25 directions will this be a constraint in our case. Futhermore, When i tried to do dti-fit in our DTI data using FSL since it requires brain extraction step and brain extraction fails in our subjects in areas where there is distortion caused by susceptibility artifacts. So I am wondering is it correct to do probabilistic tractography with this data set?
>
>
> Thanks
>
> Venkat
>
>
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Visit us online at http://www.clevelandclinic.org for
a complete listing of our services, staff and
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Confidentiality Note: This message is intended for use
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law. If the reader of this message is not the intended
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