1. A phantom for a volumetric computed tomography system, said phantom comprising: an essentially transparent or partially transparent supporting structure; and a plurality of discrete, essentially opaque markers spaced apart from one another on or in said supporting structure and configured so as not to overlap when scanned over a range of view angles by a volumetric computed tomography system, and said markers are configured on said supporting structure so as to permit separate identification of each said marker in a collection of projection images.