Hi Amelia,
1. As mentioned in a recent post, the dispersion is equal to 1 minus the
average dot product of every sample orientation with the main dyad. As the
bedpostx effectively produces in each voxel a distribution for any modelled
fibre orientation, the dispersion simply characterizes how wide this
distribution is around the dyad, which represents the mean of the
distribution.
2. Have you been using different FSL versions for different subjects? The
dispersion file is only produced by relatively recent FSL versions.
3. I am not sure I understand the question here. After running probtrackx,
you should observe more coherent results when tracking through regions with
low dispersion. Going through regions with high dispersion will produce more
spatially distributed results and potentially smaller connection
probabilities.
Hope that helps,
Stam
----- Original Message -----
From: "Versace, Amelia" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, August 01, 2010 1:39 PM
Subject: [FSL] Dyads Dispersion
Dear FSL experts,
I ran probabilistic tractography on a quite large sample of HC and I
realized that in some of them the derived fdt_paths are much more
informative (more vs few fibers reconstructed) than in others. So, going
back to the bedpost folders, I realized that some of my subjects, but not
all of them, have an extra output, i.e.= dyads*_ dispersion. I searched the
FSL archive, but I found just one post about this. So, can anybody
1. provide a better description of or references to learn more about this
output?
2. suggest a possible explanation of why just some of my subjects have this
output (is this noise-related??) and confirm that the presence of dyads*_
dispersion can explain the difference seen in the estimated fdt_paths
3. suggest a way to implement the use of this output in the estimation of my
fdt_paths, I order to confidently include these "different" subjects in one
group
Thank you very much!!!
Kind Regards
Amelia
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