Hi there.
First things first: I've always hated probability calculations and set theory!
Say you had superb quality DTI (which I don't) and wanted to track fibers
(generate probabilistic streamlines 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 running from the chiasma to the vc and passing through
the lgn is waytotals_lgn/waytotals_all.
Now the hard part: I like to find the proportion of samples running from the
chiasma “en route” to the vc, but terminating in the lgn
(e.g. “waytotals_stop_in_lgn”). In deterministic tractrography that’s a simple
(though theoretically not quite sound) exercise: Visually inspect (or count)
those paths reaching your waypoint (i.e. lgn), but not leaving it, in respect to
those continuing to the vc. In probtrackx you obviously can’t use a lgn
termination mask as that would just give you those samples going from the
chiasma to the lgn irrespective of if or if not they would have continued. Is
there some way of e.g. using unions/intersections of sets of samples that
would give you waytotals_stop_in_lgn? I recall that there was something of a
similar discussion on the list some time ago (I can’t find it…).
Regards.
Stefan Kreisel
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