Hi Saad,
>assuming the two events are independent
which they are certainly not, don't you think?
Anyway, how about normalizing that output (i.e. the sum of the two fdt_path minus their product) to a meaningful waytotal? Would you have to simply have to add the two waytotals for that?
Thanks, hope all is well!
Cheers-
Andreas
-----Ursprüngliche Nachricht-----
Von: FSL - FMRIB's Software Library im Auftrag von Saad Jbabdi
Gesendet: Mi 01.10.2008 10:55
An: [log in to unmask]
Betreff: Re: [FSL] probtrackx in MNI space
Hi -
It is important to keep in mind what is the probability that you want
to calculate, it will tell you whether you want to add, substract,
multiply or divide.
Probtrackx calculates the probability that the path of least hindrance
to diffusion from A passes through x: P(A->x). Using a waypoint B, the
values become now the probability that the path of least hindrance to
diffusion from A passes through x and B, i.e. p(A->x->B).
Now combining p(A->x->B) and p(A<-x<-B), what you want is the
probability that either the path from A to B or from B to A passes
through x, which means:
p( [A->x->B] or [A<-x<-B] ) = p( [A->x->B] ) + p( [A<-x<-B] ) - p( [A-
>x->B] and [A<-x<-B] )
Assuming the two events are independent (i.e. passing through x coming
from A to B, or from B to A), the last probability is the product of
both maps.
So in order to calculate the probability that you want, you would need
to add both maps, and substract their product.
Cheers,
Saad.
On 30 Sep 2008, at 18:25, Cherif Sahyoun wrote:
> Hi Markus,
>
> Matt and Saad will correct me if I'm wrong, but I'd say the way
> you're proposing will cause you to lose any meaningful relationship
> between the waytotal and your image. i.e. if you add, then
> essentially the statement that waytotal is the total number of
> streamlines that "made it" between your masks is preserved, but once
> you multiply your images, then it would seem like your waytotal is
> not as helpful...
> (i like the idea of multiplying to get rid of outliers though...
> i've been thresholding as per previous posts).
>
> Cherif.
> ------------------------------------------------------------------------------------------
> Cherif P. Sahyoun HST-
> MEMP
>
> Developmental Neuroimaging of Cognitive Functions
>
> C: 617 688 8048
> H: 617 424 6956
> [log in to unmask]
>
> "Live as if this were your last day. Learn as if you'll live forever"
> Ghandi
> -------------------------------------------------------------------------------------------
>
>
> On Tue, Sep 30, 2008 at 3:41 AM, Markus Gschwind <[log in to unmask]
> > wrote:
> Hi!
> I am following your discussion with a lot of interest!
> Here I have a question:
> Why wouldn't you MULTIPLY (instead of add) the paths AB and BA?
> I thought it is a more conservative measure of what is in common and
> trancking outliers will dorp out. (And take the mean of waytotals).
>
> Thanks,
> Markus
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