Hi Irina
> Dear experts,
> I have a couple of questions regarding the results of probabilistic tractography:
> 1. Too low intencity values in the fdt_paths.
> After normalization of the paths (I divided the fdt_paths by (#voxels in the seed-mask * #permutations per voxel) and multiplied by 100 to get the resulting paths in %) I get the intencity values between 0.02 and 2% (when the paths are clearly seen). This is far too low, considering that the seed is practically within the corpus callosum. What are the usual values for the transcallosal paths?
Those numbers don’t surprise me given that streamlines can travel in 3D (and therefore, e.g. under isotropic conditions, probabilities will vanish with 1/r^2). If you normalise by waytotal the numbers will be higher because you will be excluding sample streamlines that do not contribute to the fdt_paths distribution (having been rejected by the inclusion/exclusion criteria)
> 2. FA vs. waytotal values between a seed and a target.
> I compared the waytotal values (to waypoint mask, normalized) between the 2 groups (15 subjects in each). I found that in the group of patients both the #paths between a seed and a target (normalized waytotal values) and the FA values within the same tract in the skeleton were significantly lower, which was expected. But for another tract I've got significantly lower FA values in patients and higher #paths between a seed and a target (and also higher intencity values in the resulting path). Is this possible at all and how to explain this paradox?
It’s not necessarily a paradox. The fdt_paths values depend on the uncertainty in fibre orientations, which relate to but are not the same as FA. For example, FA can drop considerably in regions of crossing fibres, but uncertainty in fitting the crossing orientation can still be low if you have sufficient sensitivity to crossings.
Cheers,
Saad
> Thank you very much.
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