The Resistive Plate Chambers (RPC) are low-cost charged-particle detectors with good timing resolution and potentially good spatial resolution. Using RPC as gamma detector provides an opportunity for application in positron emission tomography (PET). In this work, we use the GEANT4 simulation package to study various methods improving the detection efficiency of a realistic RPC-based PET model for 511 keV photons, by adding more detection units, changing the thickness of each layer, choosing different converters and using the multi-gaps RPC (MRPC) technique. The balance among these factor is discussed. It's found that although RPC with materials of high atomic number can reach a higher efficiency, they may contribute to a poor spatial resolution and higher background level.