Detection of simulated periodontal bone gain by digital subtraction radiography with tuned-aperture computed tomography. The effect of angular disparity

Objectives: To investigate the influence of angular disparity on observer detection of simulated bone gain in digital subtraction radiography using tuned-aperture computed tomography (TACT*). Materials and methods: Simulated periodontal defects were created in interproximal and buccal or lingual (tooth-obscured) areas of the premolar and molar regions of a dry human skull. Radiographs were obtained before and after known weights of amorphous bone were added to the defects to simulate bone gain. The skull was positioned in a multidirectional tomographic unit to achieve reproducibility. A series of nine basis images were acquired with a CMOS intra-oral receptor and repeated using angular disparities of 10, 20 degrees, and 30 degrees. Stacks of TACT slices generated from the basis images were paired for image-registration, histogram-equalization and subtraction using TACT* Workbench. Eight calibrated observers randomly assessed the presence or absence of bone gain using a 5-point confidence scale. ROC curves were generated and A(t) values were analysed using ANOVA. Results: There were significant differences in the performance of the observers (P=0.034), defect location (P = 0.005), amount of bone gain (P<0.001), angular disparity (P = 0.003) and angular disparity x defect location interaction (P = 0.019). Mean A(t) values in detecting bone gain were 0.90, 0.85, 0.79 for angular disparities of 10<degrees>, 20 degrees, and 30 degrees respectively. Conclusions: Smaller angular disparity provided better detection of bone gain with TACT-subtraction using nine basis-projections. This effect of angular disparity was especially evident with tooth-obscured defects.