Definition of the Region of Interest for CBCT in Orthodontics

Objective: Cone-Beam Computed Tomography (CBCT) might improve orthodontic diagnosis and treatment planning. The aim of this cephalometric study was to define the minimum size of both a spherical and a cylindrical description of the region of interest enclosing and showing all anatomical structures needed for proper orthodontic diagnosis and orthognatic surgery planning. Methods: 95 lateral and anterior-posterior cephalographs of children (age 5-14) and adults (>15) were selected. Using a special cephalometric software we analyzed the cephalographs and determined the three dimensional Cartesian coordinates of key structures for cephalometric diagnosis such as Sella, Nasion, Menton, Tip of the Nose, Gonion angle, Basion and the Condyles. We computed the minimal cylindrical and spherical volumes enclosing all these points by optimizing a scalar-valued mathematical function with standard optimization software. In detail, we used the fminsearch algorithm of MATLAB which is based on the Nelder-Mead simplex method. Results: For children our results suggest a three dimensional imaging region of interest of 14,39 centimetres in diameter for spherical volumes and a field of view of 11,12 centimetres in height and 13,56 centimetres in diameter for cylindrical volumes, considering the upper border of the 95% interval of confidence. In adults the region of interest is described by the following values: 15,20 centimetres in diameter seem to be necessary for spherical volumes while 11,34 centimetres of height and 14,16 centimetres in diameter should be chosen for cylindrical volumes. The mean spherical volume for children/adults was 1,41 (SD 0,22)/ 1,79 (SD 0,30) litres, the minimum cylindrical volume 1,36 (SD 0,20)/ 1,70 (SD 0,26) litres. Conclusion: A CBCT system suitable for orthodontic use should provide a larger FOV than required in most other dental specialties. Cylindrical volumes including all needed anatomical structures proved to be smaller than spherical ones which might result in lower radiation dose for the patient.