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My thesis advisor and I are wondering if it is even valid to compare mean FA, MD, and Volume when using: 2 roi's, an exclusion mask, and probability tracking. If one subject has a many fibers connecting the two roi's while another subject has many that do not connect and only a few that do, then the probabilities will be weighted very differently. Are these probabilities subjective and not comparable. Does this make comparisons between groups when using 2 roi's not valid? How does an exclusion mask effect probability when a tract hits the exclusion is it removed from the probability calculation? I am still wondering about the questions below(from july 7th) as well. Perhaps, my question should be... what is the best way to evaluate group differences in volume, FA (TBSS I'm guessing), and MD on a specific tract? What is probabilistic tractography best used for? Can I use it for group comparisons when trying to connect 2 regions of interest? Any enlightenment on this would be awesome. Thanks, _J Sheehan On Mon, 7 Jul 2008 03:39:45 +0100, James Sheehan <[log in to unmask]> wrote: >Hey all, > >just a few quick questions. > >1. After running a few probtrackX with 2 roi masks on different subjects. I >am wondering what the intensities mean. I have a few tracts which are 1000+ >max and some which are as low as 1 max. With 2 masks are these numbers more >subjective? What concerns should I have about the tracts with max >intensities of 1? > >2. Is the Volume calculation dependent on the probability of voxel to the tract? > >3. How is probability calculated with two masks, is it dependent on the roi >and seed points, or on the number of total tracts found to the number that >reach the 2nd roi, or something completely different? > >Thanks so much for any light that you can shed!! > >-J >========================================================================