A comparison of two registration techniques for digital subtraction radiography.

Abstract

The dental radiograph is a diagnostic tool that is used to view internal structures without any invasive procedures. A difference radiograph, i.e. one showing the changes that occurred over time, is the best tool for diagnosing slow or small changes. The difficulty in creating a meaningful difference radiograph, or its digital image, is that the imaging geometry of the image pair must be identical. This paper addresses the problem of reproducing imaging geometry. A new technique for registering digital images of radiographs based on the correspondence of 3D anatomical structures (such as root apices and the cemento-enamel junction) was compared with a registration method using occlusal stents. In a pilot study of 30 volunteers, difference images were created by three different registration techniques (no registration, mathematical methods and occlusal stents). The standard deviations of the difference images were used to compare the registration techniques. The results show that this new mathematical technique applied to digital images of radiographs can be used to establish correspondence between pairs of clinical images taken at different projection angles and to produce reconstructed images comparable with images taken with occlusal stents.

Assessment of apical root resorption using digital reconstruction.

“…This was rejected on ethical grounds. In a previous study, however, Dunn et al 9 made double exposures from volunteers. They showed that the mathematical technique as implemented in the Emago/Advanced software that we applied, can be used to establish correspondence between pairs of clinical images taken at dierent projection angles and to produce reconstructed images comparable with images taken with occlusal stents.…”

“…This procedure was repeated for each of the 50 radiographs. Emago used the reconstruction algorithm described by Dunn et al 9 to identify the co-ordinates of each pixel of the ®rst image in the second image. The second image was reconstructed and subtracted from the initial image using linear subtraction.…”

“…If (almost) no root and crown structures could now be discerned (Figures 1d and 2d) the reconstruction was considered successful. 9 If, on the other hand, root structures and/ or the crown could still be distinguished individually, the subtraction was considered to have failed. The evaluators were allowed one further attempt to get a better ®t between the original and the reconstructed images.…”

“…One of the consequences of orthodontic movement is that a mechanical device cannot be used to preserve the imaging geometry. Dunn et al 9 have shown that a mathematical technique can be applied to digital images of radiographs to establish correspondence between pairs of images taken at dierent projection angles. Orthodontic movement causes a change in angulation of one or more single teeth in relation to the radiograph.…”

Assessment of three methods of geometric image reconstruction for digital subtraction radiography

“…4 In the early 1980s, new software applications for DSR were introduced with reconstruction algorithms to derive a plain image with approximate geometric projection of a reference image obtained at a different geometric configuration. Geometric image reconstruction in DSR is based on the principle developed by Dunn et al, 5 who applied mathematical equations to find coordinates of corresponding points of a reference image in a secondary image. This results in a stable alignment between pairs of points and improves the performance of DSR.…”