1. A method for operating a positron emission tomography (PET) scanner, the method comprising: acquiring, at a plurality of detector blocks of the PET scanner, emission data of gamma photons of a first energy level originating from annihilation events associated with radioactivity of a phantom in a field of view of the PET scanner; based on the emission data, generating an emission block-pair scattering model; acquiring counts of gamma photons of a second energy level originating from intrinsic background radiation of scintillator crystals of the detector blocks, without any phantom in the field of view, to provide blank scan data for the second energy level; generating a sinogram based on the blank scan data for the second energy level; and adding the emission block-pair scattering model to a scaled version of the sinogram, to yield a composite model.