It is useful to utilise a computed radiography system with a combination of storage phosphor plates and therapy cassettes for portal images. However, the therapy cassette has some disadvantages. The objective of the present study was to determine whether the disadvantages of a diagnostic cassette could be overcome to obtain clinically useful portal images. A storage phosphor plate has 0.3 mm of lead foil on the distal surface to reduce backscattered radiation. When the plate is reversed, the lead foil acts as a front intensifying screen, similar to the metal plate of a therapy cassette. We reversed a combination of the diagnostic cassette and storage phosphor plate and obtained portal images of the contrast-detail phantom and the Rando phantom (The Phantom Laboratory, Salem, NY) using three types of cassettes: a reversed diagnostic cassette, a therapy cassette and a diagnostic cassette. The image quality was subjectively evaluated. The Friedman and Wilcoxon tests were applied for statistical analyses. Evaluation of images for the contrast-detail phantom revealed that the reversed diagnostic cassette was inferior to the therapy cassette and superior to the diagnostic cassette, and that the differences were significant (p<0.017). There was no significant difference between the reversed diagnostic cassette and therapy cassette (p>0.05) for the Rando phantom. Portal images obtained using the reversed diagnostic cassette were of sufficiently high quality for clinical use as determined by Rando phantom studies.