A method for correction of radiographic errors in serial three-dimensional cephalometry.

OBJECTIVES: To improve the accuracy of location of anatomical structures in serial three-dimensional (3D) cephalometric radiography. METHODS: A new method was developed to correct for geometrical errors in the calculation of the 3D coordinates of a point viewed on any two of three frontal, lateral and axial cephalometric radiographs. A computer-based method was used to reduce measurement errors. The methods were tested on a phantom containing metallic markers and on a dried skull. RESULTS: The mean corrected geometric error was 0.43 mm (SD 0.25 mm) compared with a maximum of 8 mm (SD 0.2 mm) when calculated directly from the radiographs. The mean computer-based measurement error was 0.46 mm (SD 0.34 mm) compared with 1.38 mm (SD 0.74 mm) when made directly from the radiographs. CONCLUSIONS: The new method for the correction of radiographic errors in 3D serial cephalometric radiography appears to be sufficiently accurate to justify clinical evaluation.

[Radiographic images: appearance and reality ( a method of measurement of the 3D coordinates of anatomic points)]. / Cliches RX: apparence et realite (methode de mesure des coordonees 3D de points anatomiques).

The catch of stereotypes X-ray introduces a conical projection phenomenon which introduce errors from 1 to 5 millimetres. To obtain the real position of anatomical point with precision, we show an original method of co-ordinate 3D calculation which takes account of system radiographic dimensions. A point of space is located compared to the frame of reference of center O (medium of the segment materialized by olives). The frontal stereotype is selected like image of reference after having placed, a 4 mms metal ball on midface of patient. The side stereotype is obtained while making turn the patient of 90 degrees. The axial stereotype is obtained by rotation of the head, around horizontal olives axis. We suppose that the Source-Stereotype unit turns around the patient's head (considered fixed). The protocol comprises a stage of catch of stereotypes X-ray, a stage of digitalization of the sights and initialization of the system and a stage of measurement itself. We describe the setting in equation of real co-ordinate point determination. We apply to the apparent coordinate, the correction formulas we obtain the real co-ordinates of the point.

[The effect of conical radiographic projection: a study in 3D]. / Effet de la projection radiographique cônique: etude en 3D.

Since 1925, the work of BROADBENT and the invention of the cephalostat (in 5), radiographic cephalometry has enabled us to have a detailed knowledge of craniofacial structures, and to better understand the form of the skull. At the same time it enabled us to understand the maxillo-facial growth and dental maturation. However, they have given rise to many controversies because of magnifying power and distortions. The question posed by magnification is still debated, even if some authors have attempted to solve the problems brought forward by distortion effect which seem to neglect radiographic conical projection. This study aims to appreciate not only the magnifying and its distortion consequences but equally the effect of conical X ray. The authors make use of mathematical equations and attempt to bring forward a correction based on 3D projection.

[The pterygoclival trihedron: its 3D reconstruction, stability and possible variations during growth]. / Le trièdre ptérygo-clivien: reconstruction en 3D, stabilité ou variations possibles au cours de la croissance.

The growth's study, from skull's radiographics, needs the use of superposition's structures. Three X-ray radiographics pictures from the front, from profile and from under are retalling by a soft. This orthogonalization's step has been realized for every child at two different ages. The reconstruction of the "clivus" straight and "pterygoïde" straights from three views gives the "pterygo-clivus triedre" which stability is studied for time.

[A method of orthogonalization of X-ray negatives by image processing permitting a 3D reconstruction of the elements of the head]. / Méthode d'orthogonalisation de clichés radio X par traitement d'images autorisant la reconstruction 3D d'éléments de la tête.

In order to know better the unrolling of the growth, we dispode several radiographics elements which is sometimes difficult to connect with themselves. Using the computer, may give us the solution to this problem. We describe here a soft which authorizes the 3D reconstruction dating from X-ray radiographic pictures. During step called orthogonalization, the three pictures from the front, from profile and from under, are retalling by the soft. When it's realized, any visible point from two views will automatically be visible in the third view.