Disclosed is a registration object for computed radiographic tomography. The object has a body portion defining a void for including at least part of a target object. The registration object comprises one or more relatively radiopaque or relatively radiolucent features. Said feature or features provide, in a suitable proportion of radiographic projections of the registration object with respect to angle about a predetermined axis, a pair of identifiable registration points which are spaced apart in a direction parallel to the predetermined axis by a first distance, and whose positions in that direction are each either a constant or a function only of the angle of the projection about the predetermined axis. Said feature or features define, in the suitable proportion of radiographic projections of the object with respect to angle about a predetermined axis, a pair of identifiable registration points which are spaced apart in a direction perpendicular to the predetermined axis by a second distance whose positions in that direction are each either a constant or a continuous function only of the angle of the projection about the predetermined axis. The suitable proportion can be substantially all or all radiographic projections. Also disclosed are a method, an apparatus, a storage medium and a signal for correcting computerized radiographic tomography data, optionally using such an object.