Comparative performance of linear and logarithmic subtraction for caries diagnosis

Objective: to compare the diagnostic performance of linear and logarithmically contrast enhanced subtraction images, acquired with digital and digitized radiographs, for the detection of approximal enamel caries lesions. Methods: in this investigation, 50 caries-free human third molars were immersed in a demineralizing solution in order to induce artificial carious lesions. Standardized radiographs of the teeth were taken prior to and after the demineralization period with three digital systems, CygnusRay MPS®, DenOptix® and DIGORA®, as well as InSight® film. Conventional radiographs were then digitized. Linear and logarithmically contrast enhanced subtraction images were acquired and, subsequently, examined by an experienced radiologist. For the validation of the radiographic diagnosis, the enamel test areas were submitted to cross-sectional Knoop microhardness profiling. Radiographic interpretation data was evaluated using ROC analysis. Results: the areas under the ROC curves ranged from 0.9474 to 0.9890, and were compared by the chi-square test. The level of significance was set at P=0.05. No significant differences between the subtraction images, acquired with the four studied modalities, were found. Accuracy values ranged from 92.9 to 98%. Conclusions: linear and logarithmically contrast enhanced subtraction images, acquired with digital and digitized radiographs, were diagnostically comparable; digital subtraction is a useful image processing tool and may potentially improve radiographic diagnosis of approximal enamel caries lesions.