Properties and performance of digital X-ray detectors for medical imaging can be studied by Monte Carlo simulations. Most simulations of such detectors simplify the setup by only taking the conversion layer into account neglecting everything behind. For hybrid detectors with Si as the conversion layer, such as the Medipix2 chip less photons are absorbed at higher photon energies in the conversion layer and thus may reach the detector ASIC including its bump bonds. For photon energies above the K-edges of the backscatter materials, fluorescence may occur. The fluorescence photons can have relatively long ranges and thus have a great impact on the MTF of the detector decreasing its spatial resolution. They also add noise to the detector decreasing the overall signal-difference-to-noise-ratio (SDNR). In our study we simulated the line spread functions (LSF) for photon-counting pixel detectors by Monte Carlo simulations, implementing the detectors in detail. We used the program ROSI (ROentgen SImulation) which is based on the well-established EGS4 algorithm. The appropriate MTFs were calculated by FFT. We show that internal backscattering, especially from Sn bump bonds, contributes to the so-called low-frequency drop of the MTF. For a 300 μm Si absorber on the Medipix2 chip, backscattering contributes up to 10% to the detected signal. This strongly decreases contrast by adding additional noise. Therefore, we also investigated the amount of noise added by internal backscattering.