Hi Fred - As with any probabilistic method, "volume" here is a measure of the spread of the probability distribution of the pathway (i.e. of path.pd.nii.gz), so it can be interpreted as the amount of uncertainty in the data. A more spread out distribution means more uncertainty, a tighter distribution means less uncertainty.

Please note that TRACULA performs tractography in each subject's native DWI space, so volume/length are not calculated in a common/normalized space for all subjects. So beware when comparing volume/length across subjects.

If you use the weighted FA/MD/etc measures, these are weighted by the value of the probability distribution at every voxel, so they are the mean values in a probabilistic sense.

Hope this helps, a.y

On Wed, 19 Jun 2013, Fred Sanders wrote:

Dear DTI experts,

I am trying to understand how to interpret tract length & volume measurements that TRACULA provides. For example, let’s say a clinical population differs from a control group in mean FA of a particular tract, and tract length & volume are comparable across the 2 groups. I think this would support the notion that the tract’s diffusion properties are indeed different across the two groups and that the FA finding are unlikely to be an artifact of probabilistic tractography. Is this a correct interpretation? Conversely, if group differences in tract volumes were detected, would you recommend comparing mean FA across the two group while covarying for tract volume?

Thanks!

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