Effect of manual compared with reference point superimposition on image quality in digital subtraction radiography.

The aim of this study was to compare a new subtraction program based on positioning of reference points (RP) in the two images with the classic manual (M) superimposition of the images during recording. The experiments were performed on 22 dry mandibles from domestic pigs. A bone chip was prepared from the cortex with seven edges: 0.26, 0.30, 0.36, 0.42, 0.49, 0.55 and 0.72 mm thick respectively. Each mandible was radiographed at 0 degrees vertical angulation without the bone chip. The bone chip was then fixed to the lingual plate for a series of radiographs during which the vertical angulation was varied from 0 degrees to 10 degrees at 1 degree intervals. After completion of this series of exposures, amalgam fillings were placed in the two molars on the right side and a second series obtained. The radiographs were digitized (512 x 512 x 8 bit resolution), stored in a personal computer and then subtracted in a random order by the two methods. The RP method was found to be superior to the M method for all the angulations evaluated; as the images were more homogeneous as shown by the smaller SD in the grey-scale histogram. The best results were obtained on images with amalgam-filled teeth. Significantly more bone edges were seen with the RP method, especially when amalgam fillings were present. The RP method may therefore be superior to the M in clinical trials using subtraction radiography.