EVALUATION OF A DENTAL SUBTRACTION RADIOGRAPHY SYSTEM

The aim of this in vitro study was to evaluate the ability of a dental subtraction radiography system to quantitatively detect differences i n density between radiographic image pairs. Four periapical radiograph s were taken of the upper first permanent molars on 5 human skulls usi ng the Digora radiographic imaging system. The 4 images were a ''basel ine'' image and 3 containing test objects consisting of either 0.5, 1 or 2 mm thick aluminium cylinders, 2.5 mm in diameter. Semi-automated image processing software was used to ''warp'' the 3 images with test objects into the same geometric/density registration as the correspond ing baseline image using a process called patch minimization. ''Differ ence'' images were then produced and their contrast stretched. For reg ions of interest, with and without test objects present, the differenc e in density between the baseline and ''test object'' images was calcu lated using a reference aluminium step wedge. The test objects were cl early visible in all the ''difference'' images. The mean difference be tween the actual and estimated volume of the test object was 0.31 (95% CI [-0.55, 1.17])mm(3) Al. There was a strong association (r = 0.83) between the actual and estimated aluminium volumes. It is concluded th at this system provides adequate precision for clinical evaluation.