This project was aimed at investigating the lesion detectability of an optically-coupled CCD x-ray imaging system. The `lesion-matched filter' signal-to-noise ratio (SNR) was calculated for various clinical applications. This analysis differs from that of previous investigations in that we have analyzed jointly the effect of quantum noise and additive noise; we have also analyzed the combined effect of the contrast transfer property and the resolving power of the system. Numerical calculations were performed to determine lesion detectability and contrast-detail curves under mammographic and other clinical conditions. A standard mammographic phantom and a Leeds phantom were imaged to validate the calculation. The results demonstrate that the lesion detectability of a well designed optically-coupled CCD can be comparable to that of the state-of-the-art screen-film system under the same conditions.