<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <META NAME="Generator" CONTENT="MS Exchange Server version 6.5.7651.59"> <TITLE>RE: [Gate-users] backprojection of coincidences</TITLE> </HEAD> <BODY>  What about positron range? Which positron have you simulated? Can positron range and scatter account for 25%? You should be able to look at list file to figure out different kinds of events in your recorded coincidences. Yu Chen, Ph.D. University of Massachusetts Medical School Division of Nuclear Medicine 55 Lake Avenue North Worcester, MA 01655-0243 Phone:  (508) 856-6123 Fax:    (508) 856-4572 -----Original Message----- From: gate-users-bounces@lists.healthgrid.org on behalf of Angela Foudray Sent: Thu 9/13/2007 12:36 PM To: Peter Bloomfield Cc: gate-users@lists.healthgrid.org Subject: Re: [Gate-users] backprojection of coincidences Yep, sounds like scatter to me.  Some fraction of the photons are scattering in your water medium, some of those at small enough angles that they would still be included in your energy window, but large enough to move the line or response away from its original line. Ange On Thu, 2007-09-13 at 11:49 -0400, Peter Bloomfield wrote: > 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 > > Scatter? > > Peter > _______________________________________________ > Gate-users mailing list > Gate-users@lists.healthgrid.org > <A HREF="http://lists.healthgrid.org/mailman/listinfo/gate-users">http://lists.healthgrid.org/mailman/listinfo/gate-users</A> _______________________________________________ Gate-users mailing list Gate-users@lists.healthgrid.org <A HREF="http://lists.healthgrid.org/mailman/listinfo/gate-users">http://lists.healthgrid.org/mailman/listinfo/gate-users</A> </BODY> </HTML>