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 http://www.ers.ibm.com _____________________________________________________________________________