The first two question, Nicolas already has a clear answer and explanation for that. From my experience, few warning messages per second is not a big deal as long as the simulation goes on without crash. Usually if  the source is partially not surrounded by any phantom, the simulation will crash in a short while. I also have the same problem with voxelized phantom/source such as the MOBY phantom. I am thinking maybe we can suggest the developer of MOBY phantom to put a "skin" outside the MOBY, so we can set the activity in the "skin" as 0. Anyway even with current version of MOBY phantom, the escaped positrons are very small portion of the total events and should not have any significant effect on your result. yuxuan <table width=100%> <tr valign=top> <td> <td>"Su, Yi" <suy@mir.wustl.edu> 01/04/2008 03:59 PM <td>                To:        "'yuxuan.zhang@di.mdacc.tmc.edu'" <yuxuan.zhang@di.mdacc.tmc.edu>         cc:        "'gate-users@lists.healthgrid.org'" <gate-users@lists.healthgrid.org>         Subject:        RE: [Gate-users] GATE running problem--WARNING : positron found and photonID vector size 1</table> Thanks a lot for the information. So exactly what triggers the warning message, when the positron escapes the 'world' or the phantom volume? Would these positrons interact with the detector and messing up the simulation? Right now I am seeing a couple of such warning messages per second. Another question is if we are simulating a mouse PET scan using voxelized phantom/source, I assume the same volume is phantom and source at the same time, would this cause more such warning messages?   Thanks again.   Yi   From: yuxuan.zhang@di.mdacc.tmc.edu [mailto:yuxuan.zhang@di.mdacc.tmc.edu] Sent: Friday, January 04, 2008 3:44 PM To: Su, Yi Cc: 'gate-users@lists.healthgrid.org'; gate-users-bounces@lists.healthgrid.org Subject: Re: [Gate-users] GATE running problem--WARNING : positron found and photonID vector size 1   Usually it is because that the size of your phantom is not big enough therefore some positron can escape from the phantom without annihilation. If there are not a lot of warning messages, (less than a few per second) then that's fine. Otherwise you need to check your phantom and source definition. The common error is that the part of the source is outside the phantom. yuxuan <table width=100%> <tr valign=top> <td width=1%> <td width=30%>"Su, Yi" <suy@mir.wustl.edu> Sent by: gate-users-bounces@lists.healthgrid.org 01/04/2008 01:31 PM <td width=68%>               To:        "'gate-users@lists.healthgrid.org'" <gate-users@lists.healthgrid.org>        cc:                Subject:        [Gate-users] GATE running problem--WARNING : positron found and photonID vector size 1</table> Hi there,   I am running a GATE simulation and the following warning message keeps popping up:   [GateTrajectoryNavigator::FindAnnihilationGammasTrackID]  WARNING : positron found and photonID vector size: 1   Does anyone know what is going on, and what might be the problem? I am using Gate 3.1.2 and I am simulating an ecat system with a simple cylinder phantom and source.   Thanks a lot,   Yi __________________________________________ Yi Su, PhD Division of Radiological Sciences Mallinckrodt Institute of Radiology Washington University School of Medicine Campus Box 8225 510 S. Kingshighway Blvd St. Louis MO 63110 Phone: 314-362-8363 Fax:       314-362-9940 Email: suy@mir.wustl.edu     <div align=center> <hr></div> The materials in this message are private and may contain Protected Healthcare Information. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail._______________________________________________ Gate-users mailing list Gate-users@lists.healthgrid.org http://lists.healthgrid.org/mailman/listinfo/gate-users <hr>The materials in this message are private and may contain Protected Healthcare Information. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail.