hi, I am recently trying to simulate a PET camera system, however, I got problem on defining the system. Two difficulties are: 1. My camera is not axial symmetric, thus I can not use "cylindricalPET" system, 2. the detector sectors consist of two type of blocks, the pixel size of these two    blocks are different. When I tried to define the geometry using "scanner" system,    I had to define two modules in the same level. But after that I cannot attach    them to the same level in "scanner" system. It seems that GATE only allow one    module to be attached to one level. Is there anyone can give me some advice in this situation (non-axial-symmetric and different block sizes)? It's enough for me even if only the singles output can be achieved, because I can do the coincidence sorting by myself. Thanks, Yuxuan