1. An apparatus for reconstructing a 3D image of at least a first region of an object from truncated cone beam projection data acquired by means of an x-ray device having a source-detector assembly moved along an acquisition trajectory such that a cone beam between the source and the detector encompasses said first region, the apparatus comprising: extrapolation means for processing the truncated projection data to derive pseudo-projection data associated with projection directions outside the detector; first 3D reconstruction means for reconstructing an intermediate CT image covering at least the first region of the object based on the truncated projection data completed with the pseudo-projection data; registration means for computing registration data representing a spatial transformation selected to register a prior CT image, representing a second region of the object including the first region and larger than the first region, with the intermediate CT image; forward projection means for processing the truncated projection data and the registered prior CT image to derive forward projection data associated with projection directions outside the detector; and second 3D reconstruction means for synthesizing a reconstructed CT image covering at least the first region of the object based on the truncated projection data completed with the forward projection data.