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 con ventional 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 bon e area with the use of a round bur 1.4 mm in diameter. Lesions were cr eated 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 sta ge the mandibles were imaged with a Sens-A-Ray system (REGAM Medical S ystems AB, Sundsvall, Sweden) and D-speed film. Exposure parameters fo r each specimen/receptor combination were standardized by either the m ean optical density or mean gray value at the approximal crestal bone area. Film images and digital images displayed with 64 and 256 gray le vels were presented to six observers for evaluation. Observers were as k to rate their confidence as to the presence or absence of a lesion u sing a 5-point confidence scale. A total of % lesion sites and 96 cont rol sites were presented to the observers. Receiver operating characte ristic curves were generated for each system. The area under the curve was used as the index of diagnostic accuracy. The mean receiver opera ting characteristic areas for 6-bit and 8-bit displays and D-speed fil m were 0.746 +/- 0.043, 0.717 +/- 0.056 and 0.742 +/- 0.059, respectiv ely. Analysis of variance was used to compare the means. No statistica l difference was found between any of the three image displays (p > 0. 05), These findings are consistent with an analytic analysis of the vi ewing conditions and indicate that 6-bit (64 gray levels) are sufficie nt for the diagnosis of these simulated periodontal lesions.