Abstract: In the illustrated embodiments, a pair of primary X-ray diaphragm plates have cooperating front edges each with angled sections forming a generally V-shaped edge configuration and providing an included aperture angle facing the symmetry axis, of from 50.degree. to 150.degree., the angled sections being connected by a definitely rounded central section for conforming to rounded boundary contours of different organs such as the heart. The diaphragm plates are advantageously oppositely inclined and crossed so as to be capable of assuming a symmetrical overlapping relation in defining small diaphragm apertures. Further the diaphragm plate front edges may exhibit progressively reduced absorption values to provide a penumbra effect about the freely transmitted cone of rays particularly so as to facilitate examination of marginal zones of an organ which is surrounded by regions of greatly reduced absorption value.