Hi there.
First things first: I've always hated probability calculations and set theory!
Say you had supperb quality DTI (which I don't) and wanted to track fibers
(generate a probabilistic streamline in fsl-speak) from the chiasma to the visual
cortex (vc). Easy enough: I'd set up the seed in the chiasma and a waypoint
mask in the vc, giving me "waytotals_all". Now I'd like to see those paths going
from the chiasma to the vc and! passing through the lateral geniculate nucleus
(lgn) (i.e. Meyer's loop; i.e. "waytotals_lgn"). If my reasoning is correct, the
proportion of samples connecting from the chiasma to the vc and passing
through the lgn in respect to all samples connecting the chiasma to the vc is
waytotals_lgn/waytotals_all.
Now the hard part: I'd like to find the proportion of samples that originate in
the chiasma "en route" to the vc, but that terminate in the lgn
(e.g. "waytotals_stop_in_lgn". In deterministic tractography thats a simple
(albeit theoretically unsound) excercise: look at the fibers running to your
waypoint (i.e. lgn) and visually inspect if they continue or not. But using a
termination mask in probrackx won't do the trick, because it forces termination
of the streamline independently of if it had continued or not. Now is there
some way of using e.g. unions/intersections of sets of streamlines that would
indirectly give you waytotals_stop_in_lgn? I recall that there was a somewhat
similar discussion on the list some time ago, I just can't find it.
Regards.
Stefan Kreisel
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