Patent Number: 054935997
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, a CT scanner includes a toroidal ring x-ray tube I which is mounted to a gantry or mechanical mounting assembly II, and an electronics section III. The electronic section provides operating power and control signals to the gantry, receives data from the gantry, reconstructs the received data into an electronic image representation, and converts the electronic representation to human readable form. With reference to FIG. 2, the ring tube I includes a toroidal housing A which defines a large, generally donut-shaped interior volume. A ring anode B is mounted within the toroidal housing interior volume and extends circumferentially therearound. A cathode assembly C is disposed within the toroidal housing interior space for generating at least one beam 10 of electrons. A motor D selectively rotates the cathode or otherwise rotates the electron beam around the anode B. More specifically, the anode B is a tungsten ring having a tungsten face 12 upon which the electron beam 10 impinges. The interaction of the electron beam 10 and the anode face 12 generates a hemisphere x-ray flux 14 (FIG. 3) for collimation into a usable x-ray beam 16, as described below. The anode assembly defines an annular anode adjacent cooling fluid path or channel 18 in intimate thermal communication with the anode face, specifically along an opposite surface of the anode. Optionally, the anode can have internal passages, fins, and the like to promote thermal communication with the cooling fluid. A rotor, or rotating frame, 20, such as a rotating annular ring or frame is mounted for rotation around an interior of the toroidal housing A. The rotating frame 20 supports a corresponding plurality of cathode assemblies C. Each of the cathode assemblies includes a cathode cup 22 which contains a filament 24 or other electron source and a cathode control circuit 26. The filament 24 and the anode face 12 are maintained at a high relative voltage relative to each other, e.g. 150 kV. The housing A and the rotating frame 20 are maintained at a common potential, preferably ground potential. In the preferred embodiment, the anode is also maintained at ground potential and the cathode cup is insulated from the rotor 20 and maintained at about -150 kV. Alternately, the anode may be maintained at approximately +75 kV and the cathode at about -75 kV relative to ground. The rotating frame 20 is rotatably supported within the housing A for rotation about a central axis 28 on a bearing 30, a magnetic levitation bearing in the preferred embodiment. The magnetic levitation bearing 30 includes rings of silicon steel 32, which are stable within the vacuum, mounted along an inner radius of the rotating frame 20. Passive and active elements including permanent magnets 34 and electromagnets 36 are disposed closely adjacent the rings 32 of silicon steel. The housing A includes a magnetic window 38 which separates the vacuum region from the electromagnets 36. The magnetic window, such as an aluminum film, permits magnetic flux to pass but prevents epoxy or other polymers commonly used in coils from outgassing into the vacuum region. To maintain the alignment of the rotating frame 20, a pair of oppositely disposed magnetic levitation bearings 40 are mounted on opposite sides of the rotor. Each has rings of silicon steel 42 and permanent magnets 44 to provide opposing forces on the rotor. The magnetic levitation bearing on one side also has electromagnetic coils 46 to adjust the relative opposing forces. The electromagnet is again shielded from contaminating the vacuum by an aluminum film or other magnetic window. Position sensors, not shown but conventional in the art, are provided for controlling the electromagnetic coils to maintain the position of the rotor 20 precisely during rotation. The motor D is preferably a large diameter brushless ring motor 50. The motor includes rotor 52, preferably of permanent magnets, mounted to the rotating frame 20 within the vacuum region. A stator 54, including electromagnetic windings, is positioned directly opposite the rotor 52 but across the magnetic window 38 outside of the vacuum region. Mechanical roller bearings 56, normally out of contact with the rotor, are provided to support the rotor 20 in the event the magnetic levitation system should fail. The mechanical roller bearings prevent the rotor 20 from interacting directly with the stationary housing A and other associated structures. An angular position monitor 58 monitors the angular position of the rotation of the rotating frame 20, hence the angular position of the cathode assemblies and the apices of the x-ray beams precisely. A detector ring 60 is disposed around a patient aperture 62 that is surrounded by the housing A to detect x-rays that have exited the housing through an x-ray transmissive window 64. The detector ring 60 includes a ring of x-ray detectors 66, such as optically coupled scintillation crystals and photodiodes. A detector electronics section 68 includes preamplifiers, filters, analog-to-digital converters, and the like. Referring now more particularly to FIG. 3, along with FIG. 2, an off-focal radiation shield, or pre-collimator, 70 is mounted to the rotating frame 20 in alignment with each cathode C disposed thereon. In the preferred embodiment, eight cathodes and off-focal radiation, or pre-collimator, shields, or pre-collimators, 70 are used. In the pre-collimator shield 70, an aperture or slot 72 is cut slightly wider than the maximum selectable thickness of the x-ray beam 16. The length of the slot defines the width or arc of the x-ray beam 16. The pre-collimator shield 70 is preferably made of a high z material, such as tantalum or tungsten, and is placed as near to the focal spot as possible to shield the environment from x-rays. Preferably, the pre-collimator shield 70 is held at a negative potential such that it repells backscattered electrons. This prevents back scattered electrons from interacting with the pre-collimator 70 to produce x-rays. Further, however, it is recognized that the proximity of the pre-collimator 70 to the anode is limited due to their potential difference. A pre-collimator 74 which is constructed of a high z radiation blocking material is directly mounted to the anode B. The pre-collimator (74) has a window, such as an annular slot 76 that sets a maximum limit on the thickness of the x-ray beam 16. A support structure 78 such as a ring of low z material, supports the portion of the pre-collimator 74 extending beyond the slot 76. The pre-collimator 74, which is at ground potential and bonded to the anode, limits out-of-plane off-focal radiation seen by the x-ray detection system 60. In the preferred embodiment, the annular sections that define the slot 76 are bonded with a beryllium sheet 78. The gradient of the electric field generated near the anode focal track allows backscattered electrons to be attracted to the stationary pre-collimator 74. The backscattered electrons that return to the anode B is accordingly reduced. The number of backscattered electrons that strike the beryllium surface produce only 5.5% of bremsstrahlung x-rays usually produced by a tungsten surface. Beryllium of the thickness required is highly transmissive to x-rays of diagnostic energies and does not adversely affect the x-ray spectrum. The heat produced from the bombardment of backscattered electrons is conducted directly to the anode and is removed from the x-ray tube by the anode cooling water. A rear or outer peripheral anode shield 80 blocks portions of the x-ray flux 14 from escaping radially outward. Rear anode shield 80 is preferably formed of a high z material, i.e., a high atomic number material, and is disposed on an opposite side of the anode B relative to the pre-collimator 70. The rear anode shield 80 is directly mounted on the anode B in a known manner. The high z material used in the pre-collimator 74 and shield 80 near the annular anode B is an effective x-ray shield. This strategically located x-ray shield reduces the amount of shielding material needed at more distant locations. The overall weight of the x-ray tube is thus reduced. In alternative embodiments, different combinations of the pre-collimators 70 and 74 and rear anode shield 80 are used. For example, in one embodiment, only the pre-collimator 70 is provided. In another embodiment, only pre-collimator 74 is utilized. In still yet other embodiments, the rear anode shield is selectively used with the aforenoted, and other, configurations of the pre-collimators. Referring now to FIG. 4, a ring collimator 90 includes a fixed ring 92 and a movable ring 94. The movable ring 94 moves toward and away from the fixed ring 92 to adjust a distance therebetween. The distance between the fixed ring 92 and the movable ring 94 determines the x-ray beam thickness hence the slice thickness. The slice thickness is adjusted by translating the movable ring 94 toward the fixed ring 92 for thinner slices and by translating the movable ring 94 away from the fixed ring 92 for thicker slices. Advantageously, the central position of the x-ray beam is changed one-half the distance of the total adjustment of the movable ring 94. Such repositioning is accomplished using mechanical, electrostatic, and/or electromagnetic adjustment mechanisms and structures. The fixed ring 92 is positioned on mounting pads 96 with its center coincident with the central axis 28. The geometric plane of the fixed ring is adjusted to be parallel with the geometric plane of tungsten ring anode surface 12, both of which are perpendicular to the central axis. A plane alignment for any one of the cathodes C is typically satisfactory for all the cathodes. Variations in the alignment of the cathodes C on the rotating frame 20 will affect the beam landing track on the ring anode B. The x-ray plane and the fixed ring plane of fixed ring 92 has a constant spacing to preserve the angle of incidence of x-rays with the fixed ring 92. The angle is preferably constant for any cathode C that is selected. One preferred manner of suitably adjusting the movable ring 94 is to utilize a fine pitch screw thread drive. As shown in FIG. 4, a fine pitch screw thread 98 is machined in the outer periphery of the movable ring 94. A fine pitched screw thread, not shown, is also machined on a mating assembly on a mounting pad 100 attached to the scanner. The linear motion for slice thickness selection is preferably accomplished by rotating the movable ring 94 relative to the mounting pad 100 with a rotary motor drive mechanism 102. Conventional worm or helical gear configurations are preferred for this operation. Alternatively, an annular ring constructed of low density and low atomic materials can be used to join the threaded fixed ring 92 and movable ring 94. It is recognized that thread configurations on the fixed ring 92 and movable ring 94 can be reversed such that the forward ring is movable and the rearward ring is not movable. As yet another option, both rings can move in coordination with each other. In a further alternative, outer screw threads can be machined directly into the ring tube housing. In an alternative embodiment, the adjustment means is a linear motion drive mechanism. The linear motion drive mechanism adjusts the gap between the two rings by controlling one or more identical mechanisms that translate the movable ring to suitable positions without the use of threads or the rings. Generally, three or more linear drive mechanisms effectively drive the adjustable ring 94. It is preferred that the linear drive mechanism be capable of positioning the ring to 0.0005 of an inch accuracy. In the alternate embodiment of FIGS. 5 and 6, movable ring 94 has a smaller diameter than the fixed ring 92. The two rings, 92 and 94 overlap to function as a shutter to absorb all x-radiation. The movable ring 94 is translated to a position beyond a center line 104, as shown. The shutter function is primarily useful for service applications in which the x-ray tube is energized but radiation in the examination room is undesirable. The shutter effect described in connection with FIGS. 5 and 6 provides a significant advantage over known systems in which a separate shutter and collimator are used. The instant development reduces the necessity of incorporating two separate mechanisms into the x-ray thus increasing efficiency and reducing the number of mechanisms needed. The present development has been described placing particular emphasis on an x-ray tube using a stationary ring anode and a rotating cathode. However, as shown in FIG. 7, features of the present application are also applicable to an x-ray tube utilizing a rotating anode 110 having an anode face 112 and a stationary cathode 114. An election beam is directed by cathode 114 to the anode face 112 to generate x-rays which are transmitted through a pre-collimator 116. The pre-collimator 116, which is formed of a material having a high atomic number, is disposed on support structure, or ring, 118 so that a slot 120 can be formed in the pre-collimator 116 without the need for additional structure to support portions of the pre-collimator extending beyond the slot. The support structure 118 is preferably made of a material having a low atomic number. The invention has been described with reference to preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.