A new electron source has radiological significance if it increases the current density available at the x-ray tube anode. The field emission electron source provides an increase of approximately one million in electron current density as compared with the thermal emitter (heated metal) used in conventional tubes. The field emitter has recently been applied in several new x-ray tubes which offer advantages, particularly with respect to high dose rates, high information rates, and miniaturization of the equipment. X-rays are generated when high-energy electrons strike a metal target. The dose rate is proportional to the electron current; the radiographic resolution depends on the cross-sectional area of the electron beam (x-ray source size). Hence the electron current density (ratio of current to area) is an important radiographic parameter. In fact, it can be shown that the maximum information per unit time that can be obtained radiographically, with an x-ray tube, is dependent on the current density in the electron beam. In turn, the electron beam is dependent on the current density of its source. The very large current density of field emitters thus has direct radiological significance. The field emitter is a sharp metallic needle, often of tungsten. From a tip of micron diameter it is possible to draw currents of the order of one ampere (108 amp.∕cm.2). Larger currents have been drawn from several needles in parallel. It is not necessary to heat the needle; electrons are drawn from its cold tip in the presence of a high electric field. The field emission mechanism is an old one; it was discovered by R. W. Wood (1) in 1897. Early attempts were made by Lillienfeld (2) and others to apply it to x-ray tubes. Recent success depends on the later demonstration of the aforementioned high current densities (3) and the development of technics for stability and longevity of field emission electron sources (4, 5). Two surveys of the state of the field emission art have been published recently (6, 7). When the field emitter is heated, one observes a further increase in current density which depends both on the temperature T and electric field F; hence this mechanism has been called T-F emission (8). In this case, current densities of the order of 1,000 amp.∕cm.2 are emitted from more gross metallic structures of simple configuration, such as tungsten wires. Currents up to 2,000 amp. are obtained with T-F emitters in the Fexitron x-ray tubes to be described. It should be noted that in both field and T-F emission the metallic cathode is conserved; emitted electron current and applied voltage are in phase and their wave shapes are reproducible on a pulse-to-pulse basis. Corresponding advantages are the reliability of the x-ray yield and potentially long tube life.