Projected Digital Radiologic Images for Teaching: Balance of Image Quality with Data Size Constraints 1 Dawid Schellingerhout, MBChB, Felix S. Chew, MD, MEd, Mark E. Mullins, MD, PhD, R. Gilberto Gonzalez, MD, PhD Rationale and Objectives. The authors performed this study to determine, in the context of a teaching presentation with slides, the information content needed in a digitized radiologic image for it to be equivalent in quality to an analog image. Materials and Methods. Eleven sets of radiologic images were obtained from a teaching file collection and digitized. The images were scanned at high resolution and saved as image files. The information content of each image was then halved repeatedly with repeat sampling of the image. This procedure was repeated nine times to yield a total of 10 images with an information content that ranged from very low (32 kB) to high (12 MB). Each of these image files was made into a 35-mm slide by using a digital slide maker. The original radiographs were subsequently made into slides by using conven- tional photographic methods. Care was taken to make the images identical in every respect except digitization and infor- mation content. The slides were shown to radiologists, who filled out questionnaires to rate image quality. Results. Digitized slides that contain an average of 416 kB of data (the equivalent of a 750  570-pixel matrix with 8-bit gray levels) were equivalent to analog images for 90% of viewers. At 830 kB (1,060  800-pixel matrix with 8-bit gray levels), 100% of viewers found no difference between digitized and analog images. Conclusion. Digital images become indistinguishable from analog images if a sufficient amount of information is retained in the digital image. There is a defined relationship between the perception of quality and the information content of a digital image. Key Words. Education; images, processing; images, quality. © AUR, 2002 Education in diagnostic radiology is heavily dependent on images. The pairs of carousel slide trays filled with 35-mm slides have been, for many years, standard accou- trement for the radiology educator. As more presentations and teaching files change to a digital format, radiologists are faced with the dilemma of maximizing image resolu- tion and quality while minimizing storage space. To our knowledge, the optimum digital image quality, as defined by information content (and correlation of image file size), has not yet been determined. The advantages of dealing with images of optimized size, compared with excessively large images, are many and include (a) time savings when digitizing images, (b) time savings when digitally manipulating images with image processing software, (c) time and resource savings when digitally displaying images by using presentation software, (d) time and resource savings when printing images or distributing images by using e-mail (many servers have maximum size limits for e-mail attach- ments), (e) space savings in terms of hard drive or ar- chive space, and (f) cost and performance advantages in optimizing the specifications of digital projection equip- ment to meet the needs of radiology departments. Simi- larly, images with too little data will be of poor quality and may not be adequate for the purposes of teaching. Acad Radiol 2002; 9:157–162 1 From the Department of Radiology, Massachusetts General Hospital and Harvard Medical School, 32 Fruit St, Boston, MA 02112 (D.S., M.E.M., R.G.G.), and the Department of Radiology, Wake Forest University School of Medicine, Winston-Salem, NC (F.S.C.). Received July 31, 2001; revision requested August 28; revision received September 6; accepted September 17. Address correspondence to D.S. © AUR, 2002 157