X-ray tube having improved focal spot control

Abstract

An x-ray tube having a cathode and an anode disposed within an evacuated housing is disclosed. The cathode is spaced apart from a target surface formed on the anode and the anode is placed at a positive voltage relative to the cathode so that electrons emitted from the cathode accelerate towards and strike the target surface at a focal spot. The resulting collision produces x-rays. The cathode assembly includes a cathode support base, upon which is mounted a filament for emitting electrons. A first focusing mechanism focuses the emitted electrons into an electron beam. In illustrated embodiments, a pair of deflector plates are also supported upon the cathode support base. Voltage potentials are applied to the deflector plates so as to create a deflection region which alters the trajectory of the electron beam and thereby reposition the focal spot on the anode target. The cathode assembly also includes as secondary focusing mechanism, formed as a focusing aperture, that further focuses the electron beam before it exits the cathode assembly. The focusing aperture has a size and a shape that provides controls the size and the shape of the focal spot. In preferred embodiments, the aperture is formed within a housing that substantially encloses the filament, the deflector plates and the cathode cup. Preferably the housing is at the same electrical potential as the cathode cup. The housing electrically and physically isolates the cathode assembly from the anode, thereby reducing arcing between the two and reducing the amount of heat that is radiated to the cathode.