We asked a number of readers to evaluate 28 images of the ACR phantom acquired under a broad range of conditions (varying kVp, mAs, grid, no-grid, scatter materials). The phantom contains three types of structures: fibrils, microcalcification groups, and masses. The evaluation was performed according to the standard ACR criteria (i.e. counting the number of visible structures). The resulting scores were averaged across readers to obtain the average number of fibers, masses, and microcalcification groups seen for each image. The images were digitized and analzyed to obtain values for the noise level, background pixel value, and the contrast of image structures. We then found the linear combination of the image measurements which best predicted the reader scores. The variablity of the reader scores and variablity of the computer measures were also analyzed. We found that the computer measures of image contrast provide a good prediction of observer scores, have much less variablity than the observer scores, are straightforward to obtain, and are reproducible.