Dual filament x-ray tube used in production of fluoroscopic images

Abstract

Method and apparatus is disclosed for providing a beam of X-radiation having an intensity value which is rapidly switchable between nonzero levels. An X-ray tube having one or more electron-emitting thermionic filaments is provided with cathode bias control circuitry for limiting electron flow between the tube's cathode and anode structures. The bias control circuitry applies one of a plurality of predetermined values of negative potential to a cathode cup structure for allowing predetermined quanta of electrons emitted from an energized filament to reach the anode. One value of the negative potential bias is a relatively large value for cutting off all flow of electrons; a second or intermediate value of negative voltage bias allows a predetermined fraction of the total quanta of emitted electrons to flow; and a third potential value allows a greater number of electrons to flow. Because the bias potential can be rapidly switched to rapidly change the number of electrons striking the anode, the intensity value of the X-ray beam produced is rapidly switchable. The invention finds particularly advantageous application in fluoroscopic examination systems wherein the radiation level is rapidly switched between a relatively low television level and a relatively high spot camera or cine photographic level.