A theoretical study on optical quantum gain of granular scintillators used in detectors of medical imaging systems is presented. The fraction of optical gain due to K-characteristic radiation was also determined. The study was applied on two widely known scintillating materials, Gd2O2S:Tb and ZnSCdS:Ag, for various incident photon energies and scintillator coating thickness. Optical quantum gain due to K-characteristic photons varies with incident photon energy and scintillator coating thickness and may amount to up to 20% or 29% for Gd2O2S:Tb and ZnSCdS:Ag respectively