Detective quantum efficiency provides a useful measure of the imaging efficiency of imaging systems. Methods for measuring the exposure and the spatial-frequency dependence of the contrast transfer function, the noise power spectrum, and the detective quantum efficiency are developed for x-ray imaging systems. These are applied to a high-resolution screen-film combination exposed to a 30-kV-peak x-ray spectrum. The major component sources of screen-film noise in this system are identified and quantified. These are interpreted in terms of a simple model to predict the screen-film noise power spectrum and detective quantum efficiency. Reasonable agreement is found between model predictions and experimental measurements.