Here, you can download the source code to read the coincidences from a ROOT file and generate the corresponding michelogram and projection files that can be used as an input for STIR. Courtesy of Ross Schmidtlein and Sadek Nehmeh from the Memorial Sloan-Kettering Cancer Center, New York.
PET data generated using GATE in the ECAT7 or LMF format can be reconstructed using STIR. STIR can be downloaded for free. The current version is maintained by Hammersmith Imanet: Hammersmith Imanet Ltd