Dear Susana and GATE users, Thank you very much for your solutions. I have tested Susana's ideas by following 5 quick simulations: 1. air + back-to-back(object+source): 3409/2/20/20  (true/scatter/random/delay) 2. vacuum + back-to-back: 3483/0/20/23 3. water + back-to-back: 2208/383/17/18 4. no object + back-to-back: 29472/0/11/16 5. no object + F-18: 5818/0/19/28 All simulations are applied on ECAT 962, the object (if have one) is same shape to the source (cylinder with 4 cm radius and 3.88 cm long). Activity is 0.01 mCi and scan time is 1 second. (see the attached .mac) Obviously, simulation 4 is definitely wrong!! By using the absolute sensitivity of this system (about 9 cps/kBq according to Daube-Witherspoon, J Nucl Med 2002, 43: 1398-1409 ), the recorded true counts should be around 3300. Does this mean that we cannot use this combination for simulations? Simulation 5 is also questionable. When there is no medium around the source, how can Gate simulate annihilations without electrons? And the total counts is also too high. Simulation 1 & 2 are reasonable from my point of view. Did I do something wrong, or miss something here ? Any comments are very welcome. Thank you very much. Sincerely, Hongwei <table width=100%> <tr valign=top> <td width=40%>"Susana Branco Silva" <susana.silva@fc.ul.pt> 04/25/2008 07:14 PM <td width=59%> <table width=100%> <tr valign=top> <td> <div align=right>To</div> <td>"'Hongwei Ye'" <HYe@tmriusa.com>, <gate-users@lists.healthgrid.org> <tr valign=top> <td> <div align=right>cc</div> <td> <tr valign=top> <td> <div align=right>Subject</div> <td>RE: [Gate-users] about attenuation and sensitivity in GATE</table> <table> <tr valign=top> <td> <td></table> </table> Hi Hongwei,   If you want to “turn off” the attenuation information you don’t need to define the phantom! You just need to define the source (shape; type of source; distribution of the emission; placement of the source…). And, if you want to “turn off” the physical effects (positron range, scattering and photon accolinearity) you may use the gamma/gamma emission source.   This choice (of these 2 turn off) allow to produce simulated data representing an “acquisition best case scenario”.     Susana Branco Silva   <img src=cid:_1_08D5940008D591AC00601B6B8625743A alt=untitled> Faculdade de Ciências da Universidade de Lisboa Campo Grande, 1749-016 Lisboa, Portugal Phone: (+351) 21 750 01 77 ext.: 20515 Fax:  (+351) 21 750 00 30 <a href=mailto:susana.silva@fc.ul.pt>email: susana.silva@fc.ul.pt</a> url: <a href=www.ibeb.fc.ul.pt>www.ibeb.fc.ul.pt</a>     From: gate-users-bounces@lists.healthgrid.org [mailto:gate-users-bounces@lists.healthgrid.org] On Behalf Of Hongwei Ye Sent: sábado, 26 de Abril de 2008 0:14 To: gate-users@lists.healthgrid.org Subject: [Gate-users] about attenuation and sensitivity in GATE   Dear All, I'm wondering if there is any way to turn off the attenuation in GATE (because I'd like to use a simulation without attenuation in reconstruction). What I did right now is to use "Air" to replace "Water" (or something else) for the object material. But I met a problem here: I have tested two simulations: same objects with same activity, same scan time, but simulation 1 is made by "Air" and simulation 2 is made by "Water". Guess what I got? Simulation 2 (13920 counts) had more coincidences than simulation 1 (8646 counts) even if simulation 2 was supposed to be attenuated more by water. (I attached .mac file here) Can anyone explain this phenomena or guide me how to turn off the attenuation ? Should I use "Vacuum" instead of "Air" and set up "back to back gamma source"? One more question is about the sensitivity: what is its range for ECAT 962 system? By roughly calculating, I got 20~50 cps/kBq depending on different objects. It seems to me those number are too high. Thanks a lot. Regards, Hongwei _____________________________________________________________________________ Scanned by IBM Email Security Management Services powered by MessageLabs. For more information please visit <a href=http://www.ers.ibm.com/>http://www.ers.ibm.com _____________________________________________________________________________</a> _____________________________________________________________________________ Scanned by IBM Email Security Management Services powered by MessageLabs. For more information please visit <a href=http://www.ers.ibm.com/>http://www.ers.ibm.com _____________________________________________________________________________</a> _____________________________________________________________________________ Scanned by IBM Email Security Management Services powered by MessageLabs. For more information please visit http://www.ers.ibm.com _____________________________________________________________________________