Hi Chen Ju, Now I am confused, I think you are write but there are still missing informations ... I don't really know what thinking about it now ... Hope an expert will clarify that too. <div> 2007/12/13, Chen, Yu <<a href="mailto:Yu.Chen@umassmed.edu">Yu.Chen@umassmed.edu</a>>:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div> Hi, Simon, In the following message, you mentioned: >So if your hits have scattered many >times in your crystal, I think the position, energy and time stamp mentioned >are for the last one. And so for your singles, there is a sort of average >applied by these modules that change the position, energy and time stamp. Are you indicating that only last scattered photon infomation be recorded and initial incident photon information just simply lost?</div></blockquote><div>Here I want to say that the Singles informations (energy, time stamp, and position) are for the last hit where occurred a photoelect effect in the crystal (moreover the digitizer modules : adder, readout and blurrings, are applied on its too). For scatter effect, there is no energy deposit, the energy is just transferred to an other particle. But all informations about that are recorded in the hits collection (where no digitizer is applied). But I am not sure of anything now ... </div> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div> If this had been the case for GATE, I think it would be problematic since one single crystal detector could not be able to distinguish incident and scattered photons deposited in it. My understanding for GATE is the following as sent two days ago:  >From GATE manual, what I can learn is that Singles is from Pulse, and the latter is given by centroid of hits (weighted by energy). >What I am not clear is if a Pulse is only related to individual detector. i.e., Can a photon generate and be recorded several Pulses >in different detectors (crystals) if scatter (and/or penetration) occurs? >1)If yes, then it is possible to record the first pulse information besides that single (I assume it is centroid of pulses) information. >2)If no, (that is, one photon, one pulse, one single) we can try to record the first hit information (global X,Y,Z and crystal ID) into >corresponding Single though they are not experiment-detectable. >I prefer solution 2), which can give us precise information on how big the scatter and penetration effects are.  And I tend to believe that above 2) is the case for current GATE: i.e., even an incident photon occurs many scatterings in different crystals the final output is still one single that takes into account all hits from initial and scattered photons. Can any expert clarify this? Thanks, 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</div></blockquote><div> </div> </div> Thank you for the discussion, Simon