A method of investigating a specimen using a tomographic imaging apparatus using a stage for producing relative motion of a source with respect to a specimen , so as to allow the source and a detector to image the specimen along a series of different viewing axes and a processing apparatus for assembling a tomographic image of at least part of the specimen . The investigation is carried out by considering a virtual reference surface that surrounds the specimen and is substantially centered thereon , considering an incoming point of intersection of each of said viewing axes with this reference surface , thereby generating a set of such intersec tion points corresponding to the series of viewing axes , choosing discrete viewing axes in the series so as to cause the set to comprise a two - dimensional lattice of points located areally on the reference surface in a substantially uniform distribution.