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> <td>"Su, Yi" <suy@mir.wustl.edu> Sent by: gate-users-bounces@lists.healthgrid.org 01/04/2008 01:31 PM <td>                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   <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.<tt>_______________________________________________ Gate-users mailing list Gate-users@lists.healthgrid.org http://lists.healthgrid.org/mailman/listinfo/gate-users </tt>