An electronic portal imaging device (EPID) for use in radiotherapy with high energy photons has been under development since 1985 and has been in clinical use since 1988. The x-ray detector consists of a metal plate/fluorescent screen combination, which is monitored by a charge-coupled device (CDD)-camera. This paper discusses the physical quantities governing image quality. A model which describes the signal and noise propagation through the detector is presented. The predicted contrasts and signal-to-noise ratios are found to be in agreement with measurements based on the EPID images. Based on this agreement the visibility of low contrast structures in clinical images has been calculated with the model. Sufficient visibility of relevant structures (4-10 mm water-equivalent thickness) has been obtained down to a delivered dose of 4 cGy at dose maximum. It is found that the described system is not limited by quantum noise but by camera read-out noise. In addition we predict that with a new type of CCD sensor the signal-to-noise ratio can be increased by a factor of 5 at small doses, enabling high quality imaging, for most relevant clinical situations, with a patient dose smaller than 4 cGy. The latter system would be quantum noise limited.