A registration object 10 for computed radiographic tomography 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 which provide, in substantially all or all radiographic projections with respect to angle about a predetermined axis Z, a pair of identifiable registration points 15a, 15b 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. The features also define a pair of identifiable registration points 15c, 15d which are spaced apart a second distance. Also computerised radiographic tomography data can be corrected using such a registration object. The registration object may be a cylindrical sleeve having a void 16 and two annular grooves 13 and 14 formed in a circumferential direction around the cylindrical surface 12.
