1. A method for reconstructing a CT image of a subject from data acquired from the subject, comprising the steps of: operating a C-arm x-ray apparatus, having a rotatable C-arm on which an x-ray source and a radiation detector are mounted, said x-ray source having a focus from which x-ray emanate in a cone beam, by rotating said focus around a subject through an actual focus trajectory consisting of an actual incomplete circle and an actual straight-line segment attached at an end of said actual incomplete circle, and detecting radiation emanating from said focus and attenuated by said subject, for each focus position in said actual focus trajectory, on said radiation detector; for each position of said focus in said actual focus trajectory, electronically calculating a projection matrix that, for that focus position, describes a perspective cone beam projection of the subject on the radiation detector; and reconstructing an image of the subject using a reconstruction algorithm based on an ideal trajectory consisting of an ideal incomplete circle and an ideal straight-line segment attached at an end of said ideal incomplete circle and adapting said ideal trajectory in said reconstruction algorithm to said actual trajectory of said C-arm apparatus using said projection matrices.