Abstract The influence of detection threshold on the performance of ideal photon counting sensors is investigated in mammography for several X-ray tube anode/filter combinations and various breast thicknesses. Based on linear X-ray system theory, a method is proposed to take into consideration detector contrast modulation in the domain of X-ray energy as well as effects related to scattered radiation. Breast screening procedures are modelled as radiographic tasks in which low-contrast objects (tumours or microcalcifications) have to be detected in a noisy image background. For ideal photon counting detectors, results at zero spatial frequency show that higher detection thresholds must be implemented when imaging a thick compressed breast, compared to a thin compressed breast. In practice, the photon counting threshold also affects the image noise and the spatial resolution of hybrid pixel detectors, by tuning the amount of charge-shared events. To be generalized to more realistic imaging situations, this analysis must be performed over the full range of spatial frequencies and include additional detector noise parameters.