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Hi Saeideh There isn't a deterministic tractography option in probtrackx. So you cannot calculate the number of streamlines between two regions. Note that this measure, while it may or may not be correlated with something of interest like connection strength, is not easy to interpret. We prefer to calculate the spatial probability of a streamline. Remember that in tractography, we are calculating the route of least hindrance to diffusion. The question of "how many such routes there are" is a very strange one. Imagine that you have a situation where all your axons are pointing in the same direction, how many routes of least hindrance to diffusion there are? The answer is of course one, but you will still get as many streamlines as your tractography algorithm allows you to trace! In contrast, probabilistic algorithms will tell you that you have 100% confidence on this route. Saad. On 14 Jul 2011, at 19:01, Saeideh Bakhshi wrote: > Dear experts, > > I have 18 ROIs and I performed probtrackx to find the probability of connection from each seed to all targets. > Now I am interested to know the actual number of fibers between each ROIs. If I understand correctly, probtrackx does not give you any information about the count of fibers. > > Should I use deterministic tractography to obtain this information? If so which tool and tractography algorithm do you suggest? > > Appreciate your help. > > Saeideh Bakhshi, > http://www.cc.gatech.edu/~sbakhshi/ <http://www.cc.gatech.edu/%7Esbakhshi/> > PhD Student, > Georgia Institute of Technology > College of Computing > 266 Ferst Drive, Suite 3337 > Atlanta, Georgia 30332 > -- Saad Jbabdi University of Oxford, FMRIB Centre JR Hospital, Headington, OX3 9DU, UK (+44)1865-222466 (fax 717) www.fmrib.ox.ac.uk/~saad