Hi, This effect is not something new. One or two years ago someone already found this and reported here. At that time I was also curious about this so I spent some time looking into it. After reading the source code, my conclusion is that this result is caused by the GATE code about how it processing the energy deposit from hits. When two gamma ray photons travel inside the material, GATE records all the energy deposit in an array. Then in the "adder" process, GATE will add up all the energy deposits from the same gamma photon and stored in one of the cells in the same array. During this step, the gamma photon with a higher total deposited energy will always stored in front of the other gamma photon (or after, I can remember clearly). So in the output file the photon1 and photon2 are just a label, it does not represents the original photon1 or photon2. So if you really want the spectrum of the coincidences, just add the photon1 and photon2 together. Yuxuan <table width=100%> <tr valign=top> <td> <td>Irène Buvat <buvat@imed.jussieu.fr> Sent by: gate-users-bounces@lists.healthgrid.org 08/26/2007 09:19 AM <td>                To:        gate-users@lists.healthgrid.org         cc:                 Subject:        [Gate-users] Fwd: FW: Difference in energy spectrum for first and second event in         cylindrical PET system</table> X-Ids: 165 X-Greylist: delayed 857 seconds by postgrey-1.21 at ns2919.ovh.net;   Sat, 25 Aug 2007 00:17:05 CEST X-IronPort-AV: E=Sophos;i="4.19,305,1183352400";  d="mac'?jpg'145?scan'145,208,217,145";a="310266" Subject: FW: Difference in energy spectrum for first and second event in        cylindrical PET system Date: Fri, 24 Aug 2007 16:54:28 -0500 Thread-Topic: Difference in energy spectrum for first and second event in    cylindrical PET system Thread-Index: Acfmlfu0s1jG3B4WQ1WCyGHMNxLkBQAAzp2w From: "Andrew Goertzen" <AGoertzen2@exchange.hsc.mb.ca> To: <gate-users-owner@lists.healthgrid.org> X-OriginalArrivalTime: 24 Aug 2007 21:54:28.0267 (UTC)        FILETIME=[57E9BFB0:01C7E699] Sender: mailman-bounces@lists.healthgrid.org X-Virus-Scanned: ClamAV 0.88.7/4054/Fri Aug 24 18:55:59 2007 on shiva.jussieu.fr X-Virus-Status: Clean X-Miltered: at shiva.jussieu.fr with ID 46CF5710.007 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)! The following message was rejected by the address 'gate-users@lists.healthgrid.org'.  Could you please post it on my behalf? Thanks, Andrew Hi All, I am simulating a cylindrical PET system in GATE 3.0.0 with the geometry of a microPET R4 and I am hoping someone can help explain the behaviour I am seeing.  My source is a 1.25 MBq line source at the centre of the FOV.  Data is written to an ASCII text file.  When I create an energy histogram from the coincidence data output file I notice that the events identified as 'single 1' in a coincidence event (i.e. columns 1-21 of a coincident event in ascii format as described in section 10.1 of the user manual) have a different energy spectrum than events identified as 'single 2'.  I have included an example of this in the attached jpg image.  I do not think it is time related because I get this behaviour with and without 1.5ns of time blurring.  Also, the timestamp for single 1 is not always less than for single 2.  I do not think it is energy related because the energy of single 1 is neither always greater than or always less than single 2. So I am hoping that someone can answer the following questions: How does GATE determine the order of singles that make up a coincident event? Has anyone else observed this sort of difference between the first and second single? I have included the .mac file I am using. Regards, Andrew Goertzen Department of Radiology University of Manitoba <<EnergyHistogram.jpg>> <<microPETR4.mac>> Content-Type: image/jpeg;       name="EnergyHistogram.jpg" Content-Description: EnergyHistogram.jpg Content-Disposition: attachment;      filename="EnergyHistogram.jpg" Content-Type: application/octet-stream;   name="microPETR4.mac" Content-Description: microPETR4.mac Content-Disposition: attachment;        filename="microPETR4.mac" This email and/or any documents in this transmission is intended for the addressee(s) only and may contain legally privileged or confidential information.  Any unauthorized use, disclosure, distribution, copying or dissemination is strictly prohibited.  If you receive this transmission in error, please notify the sender immediately and return the original. Ce courriel et tout document dans cette transmission est destiné à la personne ou aux personnes à qui il est adressé. Il peut contenir des informations privilégiées ou confidentielles. Toute utilisation, divulgation, distribution, copie, ou diffusion non autorisée est strictement défendue. Si vous n'êtes pas le destinataire de ce message, veuillez en informer l'expéditeur immédiatement et lui remettre l'original. <tt>-- </tt> Irene Buvat, PhD Equipe "Imagerie de la perfusion et des échanges moléculaires" U678 INSERM CHU Pitié Salpêtrière 91 Boulevard de l'Hôpital 75634 Paris Cedex 13 France Tel : 01 53 82 84 19 Fax : 01 53 82 84 48 email : buvat@imed.jussieu.fr http://www.guillemet.org/irene<tt>_______________________________________________ Gate-users mailing list Gate-users@lists.healthgrid.org http://lists.healthgrid.org/mailman/listinfo/gate-users </tt>