Abstract: It is described a method for determining values of geometry parameters of a cone beam computer tomography apparatus, the method comprising: (a) obtaining x-ray projection data captured by the detector from at least three calibration objects arranged at mutually different positions, the x-ray projection data comprising for each calibration object plural projections at different rotation angles; (b) determining for each calibration object a respective ellipse representation from the respective plural projections; (c) performing a random search across candidate values of the geometry parameters for determining the values of the geometry parameters, wherein a cost function depending on the ellipse representations and the geometry parameters is optimized.