6-Bit and 8-bit digital radiography for detecting simulated periodontal lesions

The purpose of this study was to compare the diagnostic performance of a digital radiography system that uses 6-and 8-bit displays with conventional D-speed film for the detection of simulated periodontal bone lesions. Eleven human hemimandibles were used as specimens. Simulated lesions were created at the buccal cortical plate in the marginal bone area with the use of a round bur 1.4 mm in diameter. Lesions were created in a defined sequence to preclude visual cues as to the depth of the lesions. Lesion size progressed in 0.5 mm increments. At each stage the mandibles were imaged with a Sens-A-Ray system (REGAM Medical Systems AB, Sundsvall, Sweden) and D-speed film. Exposure parameters for each specimen/receptor combination were standardized by either the mean optical density or mean gray value at the approximal crestal bone area. Film images and digital images displayed with 64 and 256 gray levels were presented to six observers for evaluation. Observers were ask to rate their confidence as to the presence or absence of a lesion using a 5-point confidence scale. A total of 96 lesion sites and 96 control sites were presented to the observers. Receiver operating characteristic curves were generated for each system. The area under the curve was used as the index of diagnostic accuracy. The mean receiver operating characteristic areas for 6-bit and 8-bit displays and D-speed film were 0.746 ± 0.043, 0.717 ± 0.056 and 0.742 ± 0.059, respectively. Analysis of variance was used to compare the means. No statistical difference was found between any of the three image displays ( p > 0.05). These findings are consistent with an analytic analysis of the viewing conditions and indicate that 6-bit (64 gray levels) are sufficient for the diagnosis of these simulated periodontal lesions.