An ionography imaging chamber for use in the cassette of an X-ray apparatus has a flat prismatic pressure vessel defining an interelectrode gap for compressed high Z gas and a dielectric charge-receiving sheet which is to be exposed to a pattern of X-rays. The pressure vessel fits into the compartment of a strongly deformation-resistant jacket having a centrally located sleeve consisting primarily of polyurethane foam, a thin tubular inner envelope which consists at least in part of convoluted fibrous material, and a thin tubular outer envelope which also consists at least in part of convoluted fibrous material. That envelope which takes up compressive stresses has a layer of carbon epoxy fibers. The other envelope takes up tensional stresses and contains Kevlar fibers. The pressure vessel has two flat tray-shaped main portions which consist of polyurethane foam. The main portions of the pressure vessel and/or the sleeve of the jacket may be strengthened by inserts consisting of metal or a synthetic plastic material with fibrous reinforcements embedded therein.