Hi John - This is a probabilistic tractography method, so it draws samples from the probability distribution of the particular tract, and then adds up these sample streamlines to estimate the probability distribution. The number that you see is the number of sample streamlines. It's a parameter that can be set in the configuration file (otherwise it'll use the default number), and it'll be the same for all subjects.
TRACULA is not an exploratory tractography method where you specify a seed and find how many of the streamlines originating in the seed end up in the target. It's a global tractography method, where all samples drawn go through both ROIs (both "seed" and "target" are treated in exactly the same way and it doesn't matter which is which).
Best, a.y On Tue, 6 Oct 2015, John Anderson wrote:
Hi Anastasia, How can I calculate the tract density from the output of tracula. For example in the final stats file for the cortico spinal tract I can see the number of sample paths in the WM tract. Is this number refer to the tract density ( number of DTI fibers comprising the tract - in other words the number of distinct tracts that pass through the manually constructed sets of ROIs) ? Bests, John Anderson Senior Research Associate Psychological and Brain Sciences Dept. Dartmouth College, 419 Moore Hall, Hinman Box 6207, Hanover, NH 03755 Phone: +1 (603) 646-9834 Fax: +1 (603) 646-1419
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