Hi, Angela, I spent some time read the source code of GATE recently and found out why the spectrum looks different. The GATE program first generates two gamma photons, let's say S0 and S1. Then GATE will traces down the two photons respectively, and generates a Hits list. After that, the program will generates Adder_list and Readout_list one after another before generates the Single_list. If you check the ASCII output files, you will find that in the Singles_Adder file, for each photon, if a Compton backscatter event happened, then the event of energy deposit after backscatter will be listed in front of all other events in the Adder_list. So for each photon-pair with one  Compton backscatter event, we should have two possible event lists as: Case 1: (1). S0 backscatter in detector B (or A), but deposits energy (0.17MeV) in detector A (or B) (2). S0 deposits remained energy (0.34MeV) in detector B (or A) (3). S1_photoelectric_absorption (0.511MeV) in detector A (or B) or Case 2: (1). S0_photoelectric_absorption (0.511MeV) in detector A (or B) (2). S1 backscatter in detector B (or A), but deposits energy (0.17MeV) in detector A (or B) (3). S1 deposits remained energy (0.34MeV) in detector B (or A) when GATE process the Adder_lsit, in Case1, it will add the energy from enevt3 to enevt1 and update the other information of event1 with event3, and keep the event2, then generats the Readout_list with two events: (1). S1 (energy 0.17+0.511MeV) in detector A (or B) (2). S0_remained_energy (0.34MeV) in detector B (or A) for Case2, GATE will add the energy from enevt2 to enevt1 and keep the original information of event1, then generats the Readout list as: (1). S0 (energy 0.17+0.511MeV) in detector A (or B) (2). S1_remained_energy (0.34MeV) in detector B (or A) Therefore, no matter the Compton backscatter event was happened with photon S0 or S1, or in detector A or detector B, in the Singles_Readout List, the high energy Single always comes first. Then later, this Single will appears in the first column in the coincidence list. Which means that the first and the second photons in the coincidence pairs do not always correspond with the original two gamma photons S0 and S1. Best regards, Yuxuan Zhang Dr. Yuxuan ZHANG Dept. Experimental Diagnostic Imaging Univ. Texas, MD Anderson Cancer Center 1515 Holcombe Blvd, Unit 217 Houston, TX 77030-4095 Tel: +1-713-745-1671 Fax: +1-713-745-1672 <table width=100%> <tr valign=top> <td> <td>Angela M K Foudray <afoudray@stanford.edu> Sent by: gate-users-bounces@lphe1pet1.epfl.ch 09/24/2004 04:37 PM Please respond to GATE feedback and helpline for Users <td>                To:        GATE feedback and helpline for Users <gate-users@lphe1pet1.epfl.ch>         cc:                 Subject:        Re: [gate-users]  Energy Spectrum Woes</table> <tt>A few people have written back to the list (and to me) about the energy spectrums from photon one and photon two that are generated from the Coincidence file in a sphericalPET system model and their ideas for why these spectra are different.  I feel my question comes down to the specifics of how the two photons are modelled, i.e.: - How are energies assigned for each event? - How is time assigned for each event? Specifically, to use some nomenclature already used in this list, let's call photon one S0 and photon two S1.  I am assuming the following: 1 - Since we are dealing with a simulation that presumably models the two annihilation photons independently, the first photon should have a random chance of being initially directed anywhere in the 4pi solid angle.   2 - We arbitrarily chose this first photon to model first - I assume we can't and shouldn't automatically label this photon S0, we assign it some time stamp which we have "jittered" based on a gaussian distribution whose standard deviation is the time resolution. 3 - Every event depositing energy in the detectors is written in the singles list.  A time window and energy window, specified by the user, is used to comb the singles data, looking for pairs of events fitting this criteria.  S0 is assigned to the event with the earlier time of the pair (not necessarily the first photon modelled in a event pair of trues), and S1 assigned to the one with the later time stamp. If 1, 2 and 3 are true, (particularly the time jittering on the time stamp), S0 and S1 have an equally likely chance of being the first photon modelled.  And since we generally have scattering media in the system, they obviously need not be even of the same annihilation event (they could both be the first photon modelled or both be the second).   Therefore, S0 and S1 cannot have different energy spectra (qualitatively with low statistics, or "at all" with enough statistics).  A) S0's event location is randomly distributed about the detection system, and B) it can be either the first or second photon modelled (this shouldn't make a difference anyway, right?).  S0 and S1 for all intents and purposes, if 1, 2 and 3 are correct, are the "same" photon for true events, or have statistically averaged in differences for scatter and random events. Even with backscatter, this should add just as often to our slightly earlier time jittered event or the later (and really this shouldn't happen much at all, right, because this requires that the photon traverses on average half of the phantom without other-than-forward scatter, backscatters at pretty much 180 degrees, traverses the entirety of the phantom without other-than-forward scatter as well as the second photon traversing half the phantom without other-than-forward scatter - seems like we shouldn't notice this effect much). So, how could these photons have different physics (i.e. different energy spectra)?   It can't even be that the first photon modelled is automatically labelled S0 - how do we get randoms then?  And how would the first photon have difference physics than the second anyway?! (on average) Ange _______________________________________________ gate-users mailing list gate-users@lphe1pet1.epfl.ch http://lphe1pet1.epfl.ch/mailman/listinfo/gate-users </tt>