Dear Gate User, I have some questions about the acolinearity of the two 511 keV gamma photons in Gate simulation. According to GATE manual and the output of root file, if a positron source was used, the acolinearity is taken into account in the calculation. Then my questions are:  1. If a backtoback gamma source is used, is the acolinearity be simulated?  2. If a isotope source is used, (F18, O15, etc), is the acolinearity be simulated? As for the second question, in the 3.2 part of the paper "GATE:a simulation toolkit for PET and SPECT" (Phys. Med. Biol. 49, 2004), it is said "The second modules deals with the acollinearity of the two annihilation photons, which is not accounted for in Geant4...", but in the Physics Reference Manual of Geant4, the acolinearity should be calculated in Geant4. Anyone can give some explainations about that? Thanks. Yuxuan ZHANG