IMPACT OF LOSSY IMAGE COMPRESSION ON ACCURACY OF CARIES DETECTION IN DIGITAL IMAGES TAKEN WITH A STORAGE PHOSPHOR SYSTEM

Image compression may reduce storage needs whether in the lossless (re versible) or lossy (irreversible) form. The aims of the study were to evaluate (1) storage needs, (2) subjective image quality, and (3) accu racy of caries detection in digital radiographs compressed to various levels by a lossy compression method. The material consisted of 116 ex tracted human premolars and molars. The teeth were mounted three in a line and radiographed by the Digora system (Sorodex Medical Systems, H elsinki, Finland). The images were exported in tagged image file forma t and compressed with the Lempel-Ziv-Welch reversible and the joint Ph otographic Experts Group irreversible compression algorithm on four le vels. The total of 580 images were assessed by five observers on a 5-r ank confidence scale for caries diagnosis. The observers subjectively judged image quality on an 11-point rank scale. With the reversible co mpression, images could be compressed to less than 50% of the original storage needs whereas the four irreversible compression factors compr essed to 20%, 8%, 5%, and 3%, respectively. For occlusal surfaces, the re was no relationship between accuracy and image compression (p > 0.3 ); for approximal surfaces, when receiver operating characteristic cur ve areas were increasingly smaller and the compression rate was higher . The difference between the original and the most compressed images w as 14% (p = 0.1). The median quality score was above middle on the 11- point rank scale for all except the most compressed images (median sco re = 1). in conclusion, for caries diagnosis, compression rates of 1:1 2 can be justified before accuracy and image quality is significantly affected.