Hi Kenji,
See below:
On 15 May 2009, at 09:00, Kenji wrote:
> Dear FSL user.
>
> I have a question about "anistropy to constrain tracking".
>
> Use anisotropy to constrain tracking:
> Use this option if you want the fractional anisotropic volumes to
> influence the
> tractography.
> The tracts stop if the anisotropy is lower than a random variable
> between 0 (low anisotropy) and 1 (high anisotropy).
>
> Why is the threshold decided by not a fixed value but random numbers?
> What is the advantage?
There is no "threshold", in the common sense. The method used here is
formally equivalent to integrating over all thresholds between 0 and
1, using rejection sampling on the following distribution:
probability(stop tract) = int_0^1( prob( f < t | Y) dt
where prob( f < t | Y ) is equal to the area under the curve of the
posterior on f between 0 and t.
You can have a feel about how this works in the following cases:
when p(f|Y) = delta(0), the integral is 0, as prob(f<t|Y) is always 0.
so the tract would never stop.
when p(f|Y) = delta(1), the integral is 1, as prob(f<t|Y) is always 1.
so the tract would always stop.
when p(f|Y) is flat, the integral is 1/2, as prob(f<t|Y) = t. so there
is a 50/50 percent chance to stop.
Cheers,
Saad.
> I would like to reconstruction white matter pathways, but,
> if the value of random number is large, the tracking will stop in
> spite of areas
> of high anisotropy (white matter).
> Or if the value of random number is very small, the tracking will
> not stop in
> spite of areas low anisotropy (gray matter).
>
>
> Thanks
> Kenji.
>
Saad Jbabdi
Oxford University FMRIB Centre
JR Hospital, Headington, OX3 9DU, UK
+44 (0) 1865 222545 (fax 717)
www.fmrib.ox.ac.uk/~saad
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