Abstract: A target is provided for calibrating and determining the spatial resolution, SNR and/or CNR associated with an X-ray computed tomography system having a rotation axis about which a beam of X-rays is rotated relative to the target to produce signals that are processable to generate an image on an imaging plane perpendicular to the rotation axis. The target comprises a radiopaque body and is configured to be locatable in the system with a centre of the body on the rotation axis. A thickness direction of the body parallel to the rotation axis such that the body extends radially from its centre in the imaging plane. The body contains a plurality of non-radiopaque columns extending longitudinally in the thickness direction of the body. The columns are arranged in first sub-groups of identically-shaped columns with the columns of each first sub-group sharing a respective predetermined transverse diameter.