Physics Case of the Day History : Patient presented for a follow-up mammogram. The cranio-caudal view was suboptimal due to very low contrast and was repeated by the technologist. The repeat mammogram showed the same poor image quality as the original. After consulting with the radiologist, a change was made to the acquisition protocol used for the mediolateral view, producing a result that was adequate for diagnostic interpretation. Explain the physical basis for the poor quality of the cranio-caudal images (images 1 and 2). Authors: Eric L. Gingold, Barbara C. Cavanaugh Thomas Jefferson University Hospital, Philadelphia, PA 25 kV, 25 mAs, Mo/Mo a. Poor image postprocessing b. Incorrect window/level setting c. Beam hardening d. Poor scatter control e. General underexposure 25 kV, 25 mAs, Mo/Mo29 kV, 82 mAs, Rh/Rh

Diagnosis: e. General underexposure

Discussion: The craniocaudal images are examples of underexposure in digital mammography. These images were obtained using a manual exposure technique (25 kVp, 25 mAs, Mo/Mo), a technique that was selected and used for a specimen radiograph on this mammography unit immediately before this patient was imaged. The technologist failed to recognize that the mammography machine was still in the manual exposure control mode before the images were acquired. After the error was recognized, the mediolateral view was obtained using AOP (Automatic Optimization of Parameters) mode (29 kVp, 82 mAs, Rh/Rh). In this mode, the target/filter combination, kVp and mAs are all selected automatically after a “test” exposure using a short radiation pulse that precedes the primary exposure. The underlying cause of poor digital image quality can be difficult to determine, especially by those who received mammography training using screen-film technology. The inherently limited latitude of the screen-film image receptor system results in improper optical density when errors occur in radiographic technique selection. Digital radiography and mammography, on the other hand, have wide inherent latitude and employ computerized post-processing that compensates for exposure variation. This results in consistent image brightness regardless of the detector exposure, but variability in the level of quantum noise in the image. Underexposed images feature high levels of quantum noise (mottle), and dense anatomy can appear “washed-out” or with poor contrast due to a lack of adequate beam penetration.