Hi Jeff
The numbers you are after are probably the ones contained in fdt_paths. The “waytotal” is just a normalisation factor and its absolute value mostly reflects your choices for different algorithmic options in probtrackx. Depending on how you run tractography this can be very similar across subjects for a given seed or quite different. It makes more sense to normalise the fdt_paths of every subject with the respective waytotal to get directly path probabilities (now normalised from 0 to 1).
Another way to compare connectivity for given tracts is to use classification targets. Have a look at the online documentation and proj_thresh. Seed_to_target results are more easily comparable across subjects rather than fdt_paths, because they have some anatomical priors on. These values can be further transformed to proportions (using proj_thresh) that further reduces undesired variability across subjects (e.g. due to noise, partial volume effects).
Cheers
Stam
On 11 Feb 2014, at 21:01, Jeff Waugh <[log in to unmask]> wrote:
> Hello experts,
> I am interested in assessing the absolute number of tracts generated
> from a given seed ROI, and comparing that number between my patient and
> control groups. Why? I want to know whether my tractography results
> represent an anatomically-isolated change in tract number, or merely
> the area of largest change in tract number. Probtrackx generates a file
> that might be used in this manner, the waytotal. From the FSL page for
> probtrackx: "waytotal - a text file containing a single number
> corresponding to the total number of generated tracts that have not
> been rejected by inclusion/exclusion mask criteria."
>
> Have any of you tried to use the waytotal in this manner?
>
> One interesting/confounding thing I've found so far: when I correct the
> waytotal number by the volume of the native-space ROI used to seed
> probtrackx, all subjects correct to almost exactly the same number (625
> +/- 0.1). This would lead me to believe that the total number of tracts
> generated by a voxel is always the same, regardless of subject
> characteristics. Is this right? I have done the same waytotal and ROI
> correction for over 40 regions and always come up with this same number,
> 625.
>
> Thank you for any insight you can provide.
>
> - Jeff
>
>
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