Cathode for high emission x-ray tube

#The present invention relates generally to x-ray tubes and, more particularly, to a cathode configuration therefor.

#Presently available medical x-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an x-ray tube anode, or target, which is typically a planar metal or composite structure. The space between the cathode and anode is evacuated.

#A disadvantage of typical cathode designs is that the emitter, which typically comprises a helically coiled tungsten wire filament, tends to be rather large and electrons are emitted radially outward from all surfaces of the filament surface. The cup, therefore, must be designed to produce a very tailored electric potential distribution in the vacuum such that all electron trajectories are redirected from their initial divergent motion toward a very small focal spot on the anode surface. This is commonly done by configuring a uniformly biased cathode cup having a carefully machined profile in close proximity to the filament(s) for passively shaping the electric field leading to the focal spot. For design purposes it is usually sufficient to treat the coiled filament as a solid emitting cylinder, and to neglect detail at the level of individual turns of the coil. It is also usually sufficient to be concerned only with the focal spot width, rather than its complete two-dimensional shape because the focal spot length can be more-or-less independently set by emitter-cup changes which do not strongly alter the width. However, even with this design freedom, it is difficult in practice to design a cup which produces such tailored electric fields and leads to a small focal spot width. The present state-of-the-art is represented by filament coils of major diameter around 1 millimeter which can be focused onto a 0.1 millimeter-wide focal spot on the anode, i.e., a beam compression ratio of 10.

#FIG. 1 is a perspective view of a conventional x-ray tube cathode design;

#FIG. 2 is a cross sectional view of the x-ray tube of FIG. 1;

#FIG. 3 graphically illustrates a focal spot profile showing the spatial distribution of electron current at the anode surface of a conventional x-ray tube such as that illustrated in FIGS. 1 and 2;

#FIGS. 1 and 2 illustrate a conventional x-ray tube 10 including a cathode 12 having an emitter 14 and a cup 16. Cathode 12 is oriented to face an x-ray tube anode 18, or target, which is typically a planar metal or composite structure. For many applications wherein high x-ray flux is required, the anode itself is a disk which is rotated at a high speed (typically 1000 to 10,000 revolutions per minute) in order to keep the peak anode temperature in the focal spot to an acceptable value. The cathode assembly is typically held from 20 to 200 kilovolts negative with respect to the anode. The space, or air gap, between the cathode and anode is evacuated to improve the voltage standoff capability of the gap and reduce scattering by electronatom collisions. Emitter 14 is typically a helically coiled tungsten wire filament which is heated by passing an electric current of several amperes through the wire to a temperature sufficient for thermionic emission of electrons. Emitter 14 is set into cup 16. The potential difference between the cathode and anode accelerates the thermionically emitted electrons to the desired kinetic energy, and guides them to a suitable line focus on the anode, where x-rays are then generated by bremsstrahlung and other processes which are characteristic of the anode material. The shape of the cup is chosen so as to form the desired electron beam cross section as it impacts the anode, i.e., the focal spot size and shape. The electric potential in the vacuum may be altered further through the application of an electric potential, or bias, between the emitter and the cup. Practical cathode assemblies are designed to produce the best compromise between total emission current, focal spot line width, and other measures of performance.

#FIG. 3 graphically illustrates the double-peaked focal spot current distribution typical of conventional filament-cup designs such as that illustrated in FIG. 1. As explained hereinabove, this is the result of the highly nonlaminar nature of the electron beam created by such conventional filament-cup cathode designs which makes the formation of a gaussian focal spot current distribution quite difficult in practice.

#In accordance with exemplary embodiments of the present disclosure, an emitter-cup cathode configuration is provided which produces an approximately flat focal spot current distribution. FIG. 4 graphically illustrates such a desirable gaussian focal spot current distribution in a computer simulation using an exemplary embodiment of the present disclosure described below which would lead to a better modulation transfer function and hence the best image quality for x-ray imaging.