X-ray tube assemblies

Abstract: A toroidal x-ray tube assembly (I) includes a toroidal housing (A) in which a rotor (20) is rotatably mounted. One or more cathodes (C) are mounted on the rotor for generating an electron beam which strikes an anode (B) to generate a beam of x-rays which pass through a patient aperture (62) to strike a detector (60). The assembly includes precollimators (70, 74) having slots (72, 76) for passage of the x-ray subsequent to generation thereof and prior to being collimated by a ring collimator (90). The ring collimator, which collimates the x-rays into a fan shaped beam, includes a first ring (92) and a second ring (94) which are concentric but of different diameters. The distance between the first and second rings (92, 94) is adjustable to adjust the slice thickness of the final image and cause the beam to cut-off when the smaller ring is received within the larger.

Description: This invention relates to x-ray tube assemblies. It finds particular application in conjunction with annular x-ray tubes for computed tomography (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with x-ray tube assemblies for other applications. Typically, a patient is positioned in a supine position on a horizontal couch through a central bore of a CT scanner. An x-ray tube is mounted on a rotatable gantry portion and rotated around the patient at a high speed. For faster scans, the x-ray tube is rotated more rapidly. However, rotating the x-ray tube more rapidly decreases the net radiation per image. As CT scanners have become faster, larger x-ray tubes have been developed which generate more radiation per unit time to maintain the desired radiation dose at higher speeds. Larger tubes, of course, cause high inertial forces. High performance x-ray tubes for CT scanners and the like commonly include a stationary cathode and a rotating anode disk, both enclosed within an evacuated housing. As more intense x-ray beams are generated, there is more heating of the anode disk. In order to provide sufficient time for the anode disk to cool by radiating heat through the vacuum to surrounding fluids, x-ray tubes with progressively larger anode disks have been built. The larger anode disk requires a larger x-ray tube which does not readily fit in the small confined space of an existing CT scanner gantry. Particularly in a fourth generation scanner, incorporating a larger x-ray tube and heavier duty support structure requires moving the radiation detectors to a larger diameter. A longer radiation path between the x-ray tube and the detectors would require that the detectors be physically larger to subtend the required solid angle. Larger detectors would be more expensive. Not only is a larger x-ray tube required, larger heat exchange structures are required to remove the larger amount of heat which is generated. Rather than rotating a single x-ray tube around the subject, others have proposed using a switchable array of x-ray tubes, e.g. five or six x-ray tubes in a ring around the subject. See, for example, U.S. Patent No. 4,274,005 to Yamamura. However, unless the tubes rotate, only limited data is generated and only limited image resolution is achieved. If multiple x-ray tubes are rotated, similar mechanical problems are encountered trying to move all the tubes quickly and remove all of the heat. Still others have proposed constructing an essentially bell-shaped, evacuated x-ray tube envelope with a mouth that is sufficiently large that the patient can be received a limited distance in the well of the tube. See, for example, U.S. Patent No. 4,122,346 issued October 24, 1978 to Enge or U.S. Patent No. 4,135,095 issued January 16, 1979 to Watanabe. An x-ray beam source is disposed at the apex of the bell to generate an electron beam which impinges on an anode ring at the mouth to the bell. Electronics are provided for scanning the x-ray beam around the evacuated bell-shaped envelope. One problem with this design is that it is only capable of scanning about 270°. Still others have proposed open bore x-ray tubes. See, for example, U.S. Patent No. 5,125,012 issued June 23, 1992 to Schittenhelm and U.S. Patent No. 5,179,583 issued January 12, 1993 to Oikawa. These large diameter tubes are constructed analogous to conventional x-ray tubes with a glass housing and a sealed vacuum chamber. Such tubes are expensive to fabricate and are expensive to repair or rebuild in case of tube failure. EP-A-0 676 911 published after the priority date of the present application discloses a ring anode disposed in a toroidal housing. An annular rotor is rotatably received in the toroidal housing. At least one cathode is mounted on the rotor for generating an electron beam which strikes the anode target. The rotor and the cathode are rotated such that the electron beam is rotated around the ring anode. X-rays are emitted from the ring anode when struck by energetic electrons from a selected cathode on the rotor. The more precisely the x-rays are collimated into a fan or other preselected shaped beam, the sharper and more artifact free the resultant CT images are. One consideration is the amount of off-focal radiation produced during generation of the x-ray beam. Off-focal radiation is produced primarily due to energetic backscattered electrons whose energy is comparable to x-rays in locations off of the focal spot. The backscattered electrons tend to cause x-rays to be generated from broad areas of the x-ray anode and any surrounding material that may be at a positive potential relative to the cathode. Off-focal radiation has a negative effect on the image quality of x-ray images, particularly the reconstructed CT images. The off-focal radiation is a broad source of radiation that tends to blur in CT images and to cause more pronounced artifacts in the region of the interface of high and low contrast objects. A fixed collimator attached to the rotating frame within the vacuum enclosure is one method that can create the required fan beam of x-rays. This approach is most effective for fixed single or multiple slice applications in which each cathode assembly has an individual collimator. However, it is difficult and inconvenient to adjust collimators within a high vacuum and mechanical adjustment mechanisms increase the risk of vacuum contamination. EP-A-0 564 292 discloses the prior art portion of claim 1. It is an object of the present invention to provide a toroidal x-ray tube assembly including a collimator arrangement which alleviates the above referenced difficulties. According to the invention there is provided an x-ray tube assembly including a generally toroidal housing that defines an evacuated, annular interior, an annular anode mounted in the toroidal housing evacuated interior, a rotating frame rotatably mounted within the toroidal housing evacuated interior, at least one electron source mounted to the rotating frame for forming an electron beam that strikes the anode along an annular path as the frame rotates to generate x-rays, a ring collimator supported by the toroidal housing for collimating the x-rays into a beam, the collimator including a first ring and a second ring between which the x-ray beam is defined, the first and second rings having a common central axis, at least one of the first and second rings being adjustably mounted such that a distance between the first and second rings is adjustable, and one ring having a smaller diameter than the other ring such that the one ring is telescopically receivable into the other ring to provide blocking of the x-ray beam in an overlapping position of the rings. The invention also provides a method of controlling an x-ray beam emitted by an annular x-ray tube assembly in a toroidal housing in which the emitted x-ray beam passes between a pair of collimator rings having a common central axis, crosses the central axis, and strikes a ring of radiation detectors, the first and second rings having different diameters such that one ring is telescopically receivable in the other ring, and the method including the steps of: shifting the rings into an overlapping position to prevent x-rays from passing therebetween; selectively positioning a subject within the toroidal housing generally along said central axis; moving the rings apart such that the rings define an x-ray beam of a preselected width; conducting an x-ray diagnostic examination with the x-ray beam; and after conducting the x-ray diagnostic examination, moving the first and second rings to the overlapping position to prevent x-rays from striking the subject. One x-ray tube assembly in accordance with the invention and its method of use in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a perspective view of a CT scanner system incorporating the assembly; Figure 2 is a cross-sectional diagrammatic view of the assembly; Figure 3 is an enlarged diagrammatic view of an anode of the assembly; Figure 4 is an enlarged diagrammatic view of a ring tube collimator of the assembly; Figure 5 is a partial cross sectional view of the ring tube collimator in an open state; and Figure 6 is a partial cross sectional view of the collimator in a closed state. Referring to FIGURE 1, the CT scanner includes a toroidal ring x-ray tube assembly 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 FIGURE 2, the ring tube assembly 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 (Figure 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. Alternatively, 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 FIGURE 3, along with FIGURE 2, an off-focal radiation shield, or precollimator, 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 precollimator, shields, or precollimators, 70 are used. In the precollimator 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 precollimator 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 precollimator shield 70 is held at a negative potential such that it repels backscattered electrons. This prevents back scattered electrons from interacting with the precollimator 70 to produce x-rays. Further, however, it is recognized that the proximity of the precollimator 70 to the anode is limited due to their potential difference. A precollimator 74 which is constructed of a high z radiation blocking material is directly mounted to the anode B. The precollimator 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 precollimator 74 extending beyond the slot 76. The precollimator 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 precollimator 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 precollimator 70. The rear anode shield 80 is directly mounted on the anode B in a known manner. The high z material used in the precollimator 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 precollimators 70 and 74 and rear anode shield 80 are used. For example, in one embodiment, only the precollimator 70 is provided. In another embodiment, only precollimator 74 is utilized. In still yet other embodiments, the rear anode shield 80 is selectively used with the aforenoted, and other, configurations of the precollimators. Referring now to FIGURE 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 FIGURE 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 (1 inch = 2.54 cm) accuracy. Referring to FIGURES 5 and 6, in accordance with the invention the 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 FIGURES 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.

Claims: An x-ray tube assembly including a generally toroidal housing (A) that defines an evacuated, annular interior, an annular anode (B) mounted in the toroidal housing (A) evacuated interior, a rotating frame (20) rotatably mounted within the toroidal housing (A) evacuated interior, at least one electron source (C) mounted to the rotating frame for forming an electron beam (10) that strikes the anode (B) along an annular path as the frame (20) rotates to generate x-rays, a ring collimator (90) supported by the toroidal housing for collimating the x-rays into a beam, characterised in that the collimator includes a first ring (92) and a second ring (94) between which the x-ray beam is defined, the first and second rings (92, 94) having a common central axis (28), at least one of the first and second rings (92, 94) being adjustably mounted such that a distance between the first and second rings (92, 94) is adjustable, and one ring (94) having a smaller diameter than the other ring (92) such that the one ring (94) is telescopically receivable into the other ring (92) to provide blocking of the x-ray beam in an overlapping position of the rings. An x-ray tube assembly as set forth in Claim 1 including a precollimator (70 or 74) disposed within the toroidal housing (A) evacuated interior for performing a preliminary collimation of the x-rays before the x-rays reach the ring collimator (90). An x-ray tube assembly as set forth in Claim 2 wherein the precollimator (70) is supported on the rotatable frame (20) for rotation therewith. An x-ray tube assembly as set forth in Claim 2 wherein the precollimator (74) includes an annular ring (78) supported adjacent an inner circumference of the anode (B). An x-ray tube assembly as set forth in Claim 2, 3 or 4 wherein the annular path along the anode (B) travelled by the x-ray beam (10) defines a first plane orthogonal to the central axis (28) of the ring collimator (90) and further including a detector ring (60) mounted adjacent an inner diameter of the toroidal housing (A), the detector ring (60) having a ring of radiation detectors (66) that define a second plane orthogonal to the collimator ring central axis (28), the first and second planes being parallel to and offset from each other, and the precollimator (70 or 74) and ring (90) collimator being disposed relative to the anode (B) and the detector ring (66) such that a central plane of the x-ray beam extends from said annular path along the anode (B) at an angle to the first plane, across the central axis (28), and intersects the detector ring (66). An x-ray tube assembly as set forth in Claim 5 including means for adjustably positioning the first and second rings (92, 94) of the ring collimator (90) for selectively adjusting the width of the collimated x-ray beam by a selected amount and concurrently shifting the central plane of the x-ray beam by one half of said amount, such that the x-ray beam remains focussed centrally on the radiation detector ring (66). An x-ray tube assembly as set forth in any one of Claims 2 to 6 wherein the precollimator (74) includes a ring of radiation blocking material having an annular radiation passing ring (76) therein and a radiation passing material (78) disposed across the radiation passing ring (76). An x-ray tube assembly as set forth in any preceding claim including a shield ring (80) attached around an outer circumference of the anode (B) for blocking x-rays from passing radially outward from the anode (B). A method of controlling an x-ray beam emitted by an annular x-ray tube assembly (I) in a toroidal housing (A) in which the emitted x-ray beam passes between a pair of collimator rings (92, 94) having a common central axis (28), crosses the central axis (28) and strikes a ring of radiation detectors (66) the first and second rings (92, 94) having different diameters such that one ring (94) is telescopically receivable in the other ring (92), and the method including the steps of: shifting the rings (92, 94) into an overlapping position to prevent x-rays from passing therebetween; selectively positioning a subject within the toroidal housing (A) generally along said central axis (28); moving the rings (92, 94) apart such that the rings (92, 94) define an x-ray beam of a preselected width; conducting an x-ray diagnostic examination with the x-ray beam; and after conducting the x-ray diagnostic examination, moving the first and second rings (92, 94) to the overlapping position to prevent x-rays from striking the subject. A method as set forth in Claim 9 wherein, after moving the rings (92, 94) apart to a preselected width, the rings (92, 94) are shifted to change the width of the collimated beam and concurrently to shift the central plane of the collimated x-ray beam by one half the change in width such that the x-ray beam remains focussed centrally on the detector ring (66).

Inventors: MATTSON RODNEY A, MATTSON, RODNEY A.
Applicants: PICKER INT INC, PICKER INTERNATIONAL, INC.
Application year: 1995
Main classification: H05G1
IPC codes: A61B6, H05G1, H01J35, G21K1