A method for operating an inspection system is disclosed in which an item under inspection may be moved between radiation sources and detectors illuminated by the sources. The radiation sources may be positioned such that radiation from at least some of the sources impinge on the radiation detectors, forming acute angles with respect to a plane having a normal direction coinciding with the first direction that are substantially in excess of three degrees. Data accumulated by the radiation detectors may be processed to form a three-dimensional tomographic data image of at least a portion of the item under inspection. The processing may be performed using an algebraic reconstruction technique using an inverse system matrix. The inverse matrix can be derived without first computing a transpose of the system matrix.