Correction of Radiographic Errors in Several Three Dimensional Cephalometry

In cephalometric radiography a reduced dimensions radiograph system introduces a parallax effect. With the aid of a five-sided plexiglass phantom, we show that when the distance between the X-ray sources is lower than 4 metres from film and subject, the effect of parallax produces coordinates errors of a point in the space greater than the coordinate errors due to the capture on the radiographs. The objective is to improve the accuracy of the calculation of 3D coordinates of dentomaxillofacial points when the distance between patient and X-ray source is less than 6 metres. Using the stenope model of radiographic system, we describe a new method and we develop formulas, which permits the calculation of 3D real coordinates of a point viewed on any two out of the 3 radiographs, frontal, lateral and axial, from only one X-ray source. We test this method in comparing the known real coordinates of metallic markers fixed on the phantom to the computed coordinates given by this method. We test also this method by using a dried skull, we compare the computed coordinate errors to those due to capture on the radiograph by the operator and compare the dispersion of computed coordinate errors to errors due re-positioning the object on the radiographic system