Influence of displayed image size on radiographic detection of approximal caries

To evaluate the validity of approximal caries detection on digital bitewing radiographs displayed at different image sizes on either a cathode ray tube (CRT) monitor or a thin film transistor (TFT) monitor.

Five observers assessed digital radiographs of a charge-coupled device (CCD)-based sensor system (Sidexis) of 160 unrestored premolars and molars for approximal caries using a six category caries rating scale. Images were displayed at ratios of 1:1, 1:2 and 1:7 on a CRT monitor (Nokia 446 XS) and a TFT display (Panasonic LC 50S). Histological assessments of serial sections were used as the validation standard. Diagnostic accuracy was expressed as area under the receiver operating characteristic (ROC) curve (AUC) and was calculated at two levels of caries penetration: presence of caries (I) and presence of a lesion in the dentine (II). The influence of the factors "monitor type", "image size" and "validation threshold" were analysed with repeated measures analysis of variance.

The ROC curve areas for approximal caries detection at both histological penetration levels were not influenced by the type of monitor display, whereas image size had a significant impact (P<0.01). AUCs for image size 1:7 (I, 0.62; II, 0.65) were smaller compared with ratios of 1:1 and 1:2 (P<0.01). No differences were observed between image size ratios 1:1 (I, 0.69; II, 0.74) and 1:2 (I, 0.68; II, 0.73).

In this study, the type of monitor did not influence approximal caries detection on digital radiographs. Image sizes with a display ratio of 1:1 and 1:2 resulted in better diagnostic validity than those with a ratio of 1:7.