Dear All, I'm kind of stuck with "back-to-back" simulation since I met a crazy problem about sensitivity. I'm using ECAT 962 system which comes with radius=41.2cm, axial FOV=15.5cm, 32 rings. If I used solid angle to estimate the sensitivity under perfect conditions, I get the maximum sensitivity: sensitivity = solid angle/(4*PI) = area/(4*PI*r*r)= 2PI*41.2*15.5/(4*PI*41.2*41.2)= 18.8% (of course, you can more accurate formula to calculate solid angle. But I think it is already good enough) I simulated a sphere with radius 1mm at center of FOV, back-to-back gammas at 511 keV with activity 1000 Bq and scan time 100 seconds. I got a crazy result: true coincidences is 48903! It means the sensitivity is over 48.9% !! I don't understand why this number is larger than the maximum sensitivity. I also attached .mac file here for your checking. Could any experts point out what is wrong with my simulation (I may just make some stupid mistakes since I'm new with GATE)? Or I misused "back-to-back"? I use it for attenuation-free sinograms since "back-to-back" doesn't need any media to make annihilations happen. Thank you very very much. Hongwei _____________________________________________________________________________ Scanned by IBM Email Security Management Services powered by MessageLabs. For more information please visit http://www.ers.ibm.com _____________________________________________________________________________