X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-01-19
2001-09-18
Kim, Robert H. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S138000
Reexamination Certificate
active
06292538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray tube of the type having a vacuum housing in which a cathode and an anode are arranged, wherein the cathode emits electrons forming a beam and the electrons are accelerated by an electrical field, so as to strike the anode, at a focus (focal spot) and having a magnet system for deflecting and focusing the electron beam formed by several coil elements with current flowing therein, so as to produce a flying focus on the anode from which X-rays emanate.
2. Description of the Prior Art
In computed tomography with a single-line detector, the electron beam of the X-ray tube can be deflected in the &phgr;-direction, i.e. in the direction of the perimeter of the outer annular margin of the anode (known as a flying focus), for improving the resolution. This is achieved by a magnet system whose dipole field deflects the beam at high speed. Varying focal spot positions are thus obtained, and the number of projections thus can be increased.
For future application with a multi-line detector and/or surface detector, it would be desirable to simultaneously displace the focal spot in the z direction in order to achieve an enhancement of the resolution in spiral scans.
To achieve these different flying focusings, magnet systems that are spatially separated in the z-direction, which corresponds to the direction parallel to the longitudinal axis of the x-ray tube, can be used, but this increases the length of the tube appreciably, which in turn causes problems in the focusing. Therefore, such a design is extremely undesirable in computed tomography.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an X-ray tube of the type described above wherein a flying focus deflection of the electron beam is possible in both the &phgr; and z directions, with a compact structure and with simple control.
This object is inventively achieved in an X-ray tube of the type described above wherein the magnet system has a carrier constructed as a yoke, preferably an iron yoke, and which has four pole projections that are arranged at an angular offset to one another, preferably an offset of 90°, two of which are arranged opposite one another, preferably in one plane. The coils of two coil pairs, for instance z coils and &phgr; coils, (of a cylindrical (r, &phgr;, z) coordinate system) which are preferably offset by 90° to one another, are arranged on the carrier, with the coils of a pair being arranged opposite one another between different pole projections. The coils of each pair are energized by a common current, preferably a high-frequency alternating current that deflects the electron beam in a pulsing manner, so that the electron beam can be deflected by one pair of coils in a first direction, for instance the z-direction, that is different from a second direction, for instance the &phgr;-direction, in which the electron beam can be deflected by the other pair of coils.
The inventive construction in which the coils of the pairs, for instance z coils and &phgr;-coils, are arranged on the carrier, which is preferably constructed as a closed yoke, results in a rather compact realization of the magnet system for the flying focus deflection of the electron beam.
In X-ray tubes having a centrally arranged cathode in which the electron beam must be deflected from the center axis of the tube onto the radially outer margin or edge region of the anode, as is the case in basically all rotary-piston tubes, r-coils that are connected to a separate current supply should be provided in addition to the aforementioned z-coils. These r-coils are supplied with a constant current in order to deflect the beam onto the desired path of the edge region of the anode. The z-coils and r-coils do not differ in their basic construction apart from the fact that they must be laid out for alternating current in some cases and for direct current in others. The deflection of the electron beam they respectively cause is in the same direction. For the r-coils this is primarily a radially fixed deflection of the electron beam onto the outer edge of the anode; for the z-coils the deflection modifies the focal spot in the z-direction, which can be functionally realized only by pulsing the beam in the re-direction. Due to the beveled edge region of the anode, this radial displacement simultaneously produces an axial change of location of the focal spot in the z-direction.
Specifically in rotary-piston tubes, due to the deflection of the electron beam by the r-coils onto the edge region of the anode, an other possibility is needed for shaping the focal spot, for which purpose a quadrupolar system is particularly suitable. In an embodiment of the invention, the magnet system can be as described in German OS 196 31 899. This quadrupolar magnet system utilizes the same carrier as is set forth in the present invention for realizing a flying focus, this being constructed as an iron yoke, whereby the four q-coils (quadrupole coils) for generating the quadrupole field are preferably arranged on the pole projections.
In this embodiment it is possible to arrange only four coils on the pole projections, which are selectively driven as q-coils, z-coils and r-coils by simultaneously using three different current supply systems which respectively supply the corresponding deflection currents to the drive coils. Driving the same coils with different currents, that is, in part with direct currents of high amplitude and in part with high-frequency alternating currents of low amplitude, entails a considerable wiring outlay, however, so that nevertheless it is simpler and less expensive to provide separate coil systems for each magnetic field, which are individually driven completely independently from each other.
REFERENCES:
patent: 4748650 (1988-05-01), Ammann
patent: 5528658 (1996-06-01), Hell
patent: 5751784 (1998-05-01), Enck
patent: 5812632 (1998-09-01), Schardt et al.
patent: 5822395 (1998-10-01), Schardt et al.
patent: 5883936 (1999-03-01), Hell et al.
patent: 6091799 (2000-07-01), Schmidt
patent: 6181771 (2000-01-01), Hell et al.
Hell Erich
Mattern Detlef
Schardt Peter
Kiknadze Irakli
Kim Robert H.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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