The prospect of single-photon counting (SPC) detectors for x-ray image acquisition has identified a number of potential benefits over the usual approach in which the detector signal is proportional to total energy deposited during an image-acquisition interval. While a generalized approach to describing transfer of signal and noise through medical imaging systems has been developed over the past several years, in which image-forming processes are represented in terms in cascades of amplified point processes, theis approach describes conventional detectors. We extend cascaded systems analysis (CSA) to the description of signal and noise in SPC x-ray detectors. Expressions describing transfer of the mean value and noise power spectrum (NPS) through a signal thresholding stage are derived and used to describe the effects of conversion to secondary quanta, collection of secondary quanta, additive noise, and thresholding on the zero-frequency DQE of a hypothetical flat-panel SPC detector. We found that when the mean number of secondary quanta collected is 2 to 3 times greater than the threshold value, the
zero-frequency DQE for an SPC detector was 5-10% greater than that of a conventionaldetector. We also found that when additive noise is not thresholded out, DQE(0) can be degraded 30-50 %. However, when thresholding techniques are implemented, in some situations this reduction can be largely recovered. It is concluded that in some situations SPC x-ray imaging has the potential to provide equal or better DQE values than conventional energy-integrating detectors.