[Gate-users] backprojection of coincidences

Peter Bloomfield peter.bloomfield at camhpet.ca
Thu Sep 13 17:49:12 CEST 2007

On Thursday 13 September 2007 11:08, Marlen Priegnitz wrote:
> Dear Gate-Users,
> I found some strange result for the backprojection of coincidences and
> cannot find any explanation for it. Maybe somebody can help me with this?
> This is what I did:
> I was simulating a PETscanner and saving the coincidences in an ASCII-File.
> My source is a voxelized cylinder (diameter 180 mm) of water with some
> background activity (0.01 Bq per voxel) and inside there are some
> spheres (water) with activity 0.04 Bq per voxel.
> I have chosen this low activity to get in principle no randoms.
> I'm simulating Positrons.
> When I do the backprojection of the registered coincidences, then I
> found, that approx. 25 % of the registered coincidences lie outside of
> the source. (Also if I set the background activity to zero and have only
> the hot spheres, I get this result)
> How can this be? I don't believe that this are randoms, because when I
> simulate a delayed time window, then I got nearly no coincidences.
> Can anybody help me with this?
> Did anybody observed something similar?
> I'm grateful for each hint.
> Thanks in advance.
> Marlen



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