Ray-summation (raysum) images reconstructed from computed tomography (CT) volume data resemble digital radiography (DR) images. Therefore, they have a potential to be used instead of DR images.The aim of this study was to compare the physical image quality evaluated by signal-difference-to-noise ratio (SDNR) and clinical usefulness between raysum and DR images. We employed an oval water phantom simulating adult abdomen for image quality measurement. Raysum images were reconstructed from CT volume data using an assumed x-ray quality of 70 keV. DR images were obtained using an indirect-type flat panel detector system. The normalized noise-power spectrum (NNPS) for various same dose indices (DR: entrance surface dose, CT: CT dose index volume) were measured from raysum and DR images. SDNRs were calculated from the results of NNPSs, modulation transfer function (MTF), and cartilage material contrast. Five experienced observers visually compared each pair of a clinical raysum image and a DR image for nine clinical cases (head, finger, pelvis, and foot). MTF of raysum was significantly lower than that of DR. SDNRs of DR were superior to those of raysum for each dose index, by an average factor of 1.24. For head and pelvis images, raysum images were comparable or a little superior compared with the DR images, because the radiation doses of raysum was much higher than those of DR. For finger and foot cases, the raysum images were inferior to DR images due to its lower resolution. Our results indicated a limited clinical usefulness of raysum compared with DR.