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I think I'm starting to get the idea! You all are SO helpful! -Dianne On Wed, Oct 7, 2009 at 11:53 AM, Matt Glasser <[log in to unmask]> wrote: > There are a couple of things that cause these asymmetries. 1) As Scott > says, > one encounters a different pattern of local diffusion going in one > direction > than one does going in the other. 2) This is more complex because newer > versions of FDT model 2 or more fibers. Thus, tracts can become > non-symmetrical by choosing one fiber when tracking in one direction > through > a voxel, but the other when tracking the other direction. > > Two mask symmetric actually does an A to B and adds it to a B to A, so one > can use that or do them separately. I usually do them separately because > two mask symmetric didn't used to output a waytotal file (I don't know if > it > still doesn't). > > Peace, > > Matt. > > -----Original Message----- > From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf > Of Scott Kolbe > Sent: Tuesday, October 06, 2009 11:33 PM > To: [log in to unmask] > Subject: Re: [FSL] probtrackx conceptualization problem > > Dear Dianne > > I think about it in terms of /serial /sampling of the principal > direction of diffusion (PDD). when you use probtrack with seed/waypoint > your start sampling from all seed voxels and keep only the samples which > pass through the waypoint. when you use two mask symmetric you seed from > both regions and keep the samples which pass through the other (please > check this as i can't seem to access the FSL website at the mom to check > it myself). when you sample the PDD serially, errors will build up as a > function of distance from the seed. so when you seed from both "ends" of > a hypothesised tract you improve the SNR of your tract estimate. > > In the case of cortico-cortico connections, I think it's best to track > twice: first using a seed in region A and a waypoint in region B, then > from B to A. The intersection of the two results is a good estimate of > the tract location, and SNR should be reasonably good along the whole > length of the pathway. > > anyone please feel free to disagree. > > cheers > Scott > > > Dianne Patterson wrote: > > Dear Group, > > > > I'm trying to wrap my head around a problem...and it just isn't > > working. Perhaps someone can help reduce my confusion. > > > > If I am tracking between 2 masks, it seems to matter which is the seed > > and which is the target. > > In fact, having a seed mask and a waypoint mask yields different > > results than having 2 seed masks. > > > > There is one tract that I can get if I have the seed in the occipital > > lobe and a waypoint in the anterior temporal lobe, > > but if I use 2 seed rois, instead of a seed and waypoint, I don't > > generate a tract. > > > > How can I make sense of the apparent assymetrical character of these > > trackings? > > What does it mean for the underlying tracts? > > > > Thankyou for your time, > > > > Dianne > > > > > > -- > > Dianne Patterson, Ph.D. > > [log in to unmask] <mailto:[log in to unmask]> > > University of Arizona > > SLHS 328 > > 621-5105 > > -- > ======================== > Scott Kolbe > Neuroimaging Group > Florey Neuroscience Institutes and > Centre for Neuroscience > University of Melbourne > VIC, Australia, 3010. > > ph: +61 3 8344 1929 > email: [log in to unmask] > website: www.neuroimaging.org.au/index.php?id=383 > > -- Dianne Patterson, Ph.D. [log in to unmask] University of Arizona SLHS 328 621-5105