Ring tube X-ray source

Abstract: A toroidal x-ray tube housing (A) has an evacuated interior. An annular anode (B) is connected with the housing closely adjacent the window such that a cooling fluid passage (12) is defined in intimate thermal communication with the anode. A cathode assembly (32) is mounted within the evacuated housing or an annular ring (30) that rotates an electron beam (22) around the large diameter annular anode. The annular ring is magnetically levitated (40) and rotated by a motor (50) . A collimator (62) and filter (64) are rotated with the cathode assembly closely adjacent an electron emitter or cathode cup (32) such that the generated x-rays are collimated and filtered within the x-ray tube. Preferably, a plurality of cathode cups (120) are provided, whose operation is selected by a series of magnetically controlled switches (76) . The cathode cup is insulated (106) from the annular ring and isolated by a transformer (104, 112) from the filament current control switches. The cathode assembly (C) can include a multiplicity of stationarily mounted electron cups which are selectively actuated to rotate the electrode beam by a switch. An electron beam scan control may bias the potential applied to grids to scan the electron beam generated by electron emitter over a commensurate arc length of the anode with the arc length of the emitter. Multiple anode surfaces as well as multiple cathode cups may be provided.

Description: The present invention relates to x-ray tubes. Typically, a patient is positioned in a prone 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 rate of speed. For faster scans, the x-ray tube is rotated more quickly. However, rotating the x-ray more quickly decreases the net radiation per image. As CT scanners have become quicker, larger x-ray tubes which generate more radiation per unit time have been required, which, 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 stronger 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. This requires more detectors for the same resolution and provides a longer path length between the x-ray tube and the detectors. The longer path length can cause more radiation divergence and other degradation of the image data. 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. However, unless the tubes rotate only limited data is generated and only limited image resolution is achieved. If the x-ray tubes rotate, similar mechanical problems are encountered trying to move all the tubes quickly. 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 in the well of the tube. 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°. Another problem is that the very large evacuated space required for containing the scanning electron beam is difficult to maintain in an evacuated state. Troublesome and complex vacuum pumping systems are required. Another problem is that no provision can be made for off-focus radiation. Another problem resides in its large physical size. Messrs. Mayden, Shepp, and Cho in "A New Design For High-Speed Computerized Tomography", IEEE Transactions on Nuclear Science, Vol. NS-26, No. 2, April 1979, proposed reducing the size of the conical or bell-shaped tube discussed above by rotating the cathode around the large diameter anode ring. However, their design had several engineering deficiencies and was never commercially produced. EP-A-456114 discloses an x-ray tube for a CT apparatus which comprises a ring-shaped vacuum tube containing a fixed cathode having a thermion emitting surface, a ring-shaped fixed anode, and a ring-shaped rotatable cathode interposed between the fixed cathode and fixed anode. The rotatable cathode defines a thermion receiving surface opposed to the thermion emitting surface, and a thermion emitting portion opposed to the fixed anode. Thermions are emitted from the thermion emitting portion toward the fixed anode while the rotatable cathode is suspended to a non-contact state and rotated at high speed. With the thermions being accelerated and colliding on the fixed anode, an x-ray is generated toward the centre of the vacuum tube. The x-ray generating position moves at high speed along a circumferential surface of the fixed anode with rotation of the rotatable cathode. EP-A-377534 discloses an x-ray tube including a vacuum containment vessel; an anode disposed within the vacuum containment vessel and which is stationary relative thereto, a cathode disposed within the vacuum containment vessel in operative relationship with the anode means for rotating the vacuum containment vessel and the anode together relative to a fixed reference and relative to the cathode such that the cathode is stationary relative to the fixed reference. According to the invention there is provided an x-ray tube comprising: a generally toroidal housing having an evacuated interior and a central bore; an annular anode surface mounted in the toroidal housing interior, the anode surface being in thermal communication with a cooling fluid passage such that cooling fluid can be circulated contiguous to the anode surface for removing heat; a cathode assembly disposed within the toroidal housing and including a plurality of electron emitting means supported by an annular ring rotatably disposed within the housing, each said electron emitting means being capable of forming an electron beam that strikes the anode surface; a coupling means for coupling the electron emitting means to a current supply exterior to said toroidal housing; a switching means for selectively switching one or more of said plurality of electron emitting means to said current supply; motor means for rotating said annular ring and hence said plurality of electron emitting means so that the electron beam formed by the or each selected electron emitting means travels around said annular anode surface to produce x-rays; and a window defined in said toroidal housing and positioned such that the x-rays produced are directed into said central bore transverse to a central axis of the bore. One advantage of the embodiments of the present invention is to increase the power over conventionally available 125 mm and 175 mm anode x-ray tubes. Another advantage of the embodiments of the present invention is to provide for efficient cooling of the anode. Another advantage of the embodiments of the present invention is to facilitate higher speed scans. Another advantage of the embodiments of the present invention resides in low bearing wear and long tube life. Another advantage of the embodiments of the present invention is that the tubes are field repairable. The invention will now be further described by way of example with reference to the accompanying drawings in which: FIGURE 1 is a cross-sectional view of a toroidal, rotating cathode x-ray tube in accordance with the present invention; FIGURE 2 is a front view of the x-ray tube of FIGURE 1; FIGURE 3 is a detailed view of an embodiment in which the cathode is isolated from the rotating structure; FIGURE 4 is a sectional view of the anode/cathode cup portion of a multiple anode tube; and FIGURE 5 is a sectional view of the anode/cathode cup portion of a movable anode tube. With reference to FIGURES 1 and 2, a toroidal housing A defines a large, generally donut-shaped interior volume. An anode B is mounted within the toroidal housing interior volume and extends circumferentially therearound. A rotor means C is disposed within the toroidal housing interior space for generating at least one beam of electrons. A means D selectively rotates the electron beam around the anode B. More specifically, the anode B is a tungsten disk having a tungsten face 10 upon which the electron beam impinges. The housing and the anode define an annular cooling fluid path or channel 12 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 fluid circulating means 14 circulates the fluid through the stationary anode and housing to a heat exchanger 16 to keep the target anode cool. A window 20 is defined in the housing closely adjacent to the target anode B. The window is positioned such that x-rays 22 generated by interaction of the electron beam and the tungsten target anode are directed transverse to a central axis 24 of a central bore 26 of the toroidal tube. A vacuum means, preferably one or more ion pumps 28, is interconnected with the housing to maintain the vacuum within the housing. In the embodiment of FIGURES 1 and 2, the cathode assembly includes an annular ring 30 which extends around the interior of the toroidal housing. A plurality of cathode cups including cups 32a and 32b are mounted on the cathode ring. The cathode cups 32 each include a cathode filament 34 and a grid assembly 36 . Preferably, the grid assembly includes a grid for gating the electron beam on and off, a grid assembly for focusing the width of the electron beam in the radial direction, and a grid assembly for focusing the dimension of the electron beam in the circumferential direction. In the preferred embodiment, each of the cathode cups 32 has a grid assembly with one of a variety of preselected focus characteristics. In this manner, different dimensions of the x-ray beam focal spot are chosen by selecting among the cathode cups. Optionally, there are multiple cathode cups focused with the most commonly used dimensions to provide a back-up cathode cup in the event the first cathode cup should burn out. The cathode ring 30 is rotatably supported within the housing by a bearing means 40 . In the preferred embodiment, the bearing means is a magnetic levitation bearing. Thin rings 42 of silicone iron or other material, suitably prepared to be insulating in vacuum, are longitudinally stacked to form cylinders for the radial portion of the bearing. Thin hoops of silicon iron or other material, also suitably prepared for use in vacuum, are assembled to form tightly nested cylinders for the axial portion of the bearing. Passive and active elements, i.e. permanent magnets 44 and electromagnets 46 , are controlled by proximity sensors and suitable feedback circuits to balance attractive forces and suspend the cathode ring accurately in the center of the toroidal vacuum space and to center the cathode ring axially. Ceramic insulation 48 isolates the iron rings 42 from the cathode and any portions of the annular ring 30 that may be at the potential of the cathode. The isolation permits the iron rings to be held at the potential of the housing to prevent arcing between the rings 42 and the magnets 44 , 46 and the housing. A brushless, large diameter induction motor 50 includes a stator 52 stationarily mounted to the housing and a rotor 54 connected with the cathode ring. The motor causes the cathode assembly C to rotate at a selected speed through the toroidal vacuum of the housing. Mechanical roller bearings 56 are provided for supporting the cathode ring in the event the magnetic levitation system should fail. The mechanical roller bearings prevent the cathode ring from interacting with stationary housing and other structures. An angular position monitor 58 monitors the angular position of the cathode assembly, hence the angular location of an apex of the x-ray beam. The ceramic insulation 48 also isolates the rotor 54 and the angular position monitor from the potential of the cathode. Adjacent each cathode cup assembly 32 , there is a support 60 which rotates with the cathode cup. The support 60 carries an off-focal radiation limiting means or collimator 62 , e.g. pairs of lead plates which limit length and width of the x-ray beam. Alternately, the off-focal radiation limiting means may include one or more apertured lead or tungsten-tantalum plates. A filter or compensator 64 is mounted to the support in or adjacent to the window for filtering the generated x-ray beams to provide beam hardness correction or the like. A preferred compensator material is beryllium oxide. A current source 70 provides an AC current for actuating the selected cathode cup. The AC current is passed to a stationary, annular capacitor plate or inductive coil 72 mounted inside the housing. A matching, rotating capacitor plate or inductive coil 74 supported by the cathode ring is mounted closely adjacent to the stationary cathode plate. The rotating cathode plate or inductive coil is electrically connected with a series of magnetically controlled switches 76 . Each of the switches 76 is connected with one of the cathode cups. A plurality of annular electromagnets 78 are stationarily mounted along the housing. An electrical control means 80 selectively actuates one or more of the electromagnets for selectively opening and closing the magnetically controlled switches to select among the cathode cups. Alternately, external switches provide power to one of a plurality of stationary capacitor ring. Each of a matching plurality of rotating rings is connected with a different cathode cup. As yet another alternative, the capacitive coupling may be replaced by an inductive coupling, such as a stationary annular primary winding which is mounted closely adjacent and across an air gap from the rotating annular secondary winding. The anode and the cathode are maintained at a high relative voltage differential, typically on the order of 100 kV. In the FIGURE 1 embodiment, the stationary housing and the anode are held at ground, for user safety. The rotating cathodes are biased on the order of -100 to -200 kV relative to the housing. To this end, a high voltage section 90 generates a relatively high voltage which is applied to a hot cathode 92 of a vacuum diode assembly. Preferably, the high voltage supply is of a compact, high frequency type that is directly attached to the hot cathode to avoid the problems of high voltage cables and terminations. The hot cathode filament 92 is preferably of a low work function type. A circular channel of a toroidal or donut-shaped plate 94 partially surrounds the hot cathode filament 92 . The toroidal plate is mounted to the cathode assembly for rotation therewith. Preferably, a ceramic or other thermally isolating plate or means 96 isolates the toroidal plate 94 from the rotating cathode. The current is conducted by a thin wire or metal film 98 from the toroidal plate to the remainder of the rotating cathode assembly to limit heat transfer. One or more grids 99 surround the hot filament 92 for grid control, mA regulation, and active filtering. In the embodiment of FIGURE 3, the cathode cups 32 , which are held at a -100 to -200 kV relative to the housing A , is completely isolated from the remainder of the rotating annular ring 30 which is held at the same potential as the housing, preferably ground. More specifically, the toroidal ring 94 is connected by a metal strap 100 with a bayonet or other quick connector 102 . The cathode assembly has a mating connector which is received into the connector 102 . In this manner, the cathode cup is held at the same potential as the toroidal ring 94 . The filament 34 has one end connected with the cathode cup and the other end connected with the windings of a secondary coil 104 . The secondary coil is wrapped around a tubular portion of a ceramic insulator 106 which insulates the conductive strap 100 , the cathode cup, and the toroidal ring 94 from the remainder of the annular ring 30 . The ceramic tube 106 in the voltage isolation transformer is preferably a ferrite material, due to its good magnetic flux transfer properties and electrical insulation properties. A tubular insulating member 110 surrounds the secondary winding 104 to support a primary winding 112 . In this manner, a voltage isolation transformer is constructed which isolates the voltage of the filament from the filament current control. One end of the primary winding is connected with a toroidal conductive portion 114 of the rotor C and the other end is connected with one of the reed switches 76 . By selectively opening and closing the reed switch 76 , power from the inductive or capacitive power transfer means 72 , 74 is selectively conveyed to the primary. Preferably, the primary and secondary have different turns ratios such that the current flow is boosted by the isolation transformer. The isolation transformer enables the reed switch 76 to operate at less than an amp, much lower than the 4-5 amps and possibly as high as 10 amps that are induced in the secondary 104 and cathode filament 34 . Further, the isolation transformer allows the switches 76 to operate at only a few hundred volts AC, much lower than the -100 to -200 kV of the secondary 104 . It is to be appreciated, that even with the ceramic insulation tubes 106 and 110 , the conductive portion 114 of the rotor will tend to become charged, eventually reaching the potential of the cathode. This is due in part to the finite resistance of the ceramic insulators. To create a potential equilibrium between the housing A and the conductive rotor portion 114 , a filament 116 is connected between the power transfer means 72 , 74 and the conductive portion 114 , i.e. ground. This causes a current flow through the filament 116 , causing electrons to be boiled off carrying any excess charge on the annular ring 30 to the housing. In this manner, the potential of the rotating portion is held at ground. Flux shields 118 , preferably a ferrite material, surround the cathode assembly 32 and the toroidal ring 94 to provide magnetic flux isolation. Alternately, the flux shields 118 may be constructed of a metallic, conductive material. With reference to FIGURE 4, multiple anodes 10, 10' , and 10" are mounted in stair/step fashion, each adjacent a corresponding window 20, 20' , and 20" . A cathode cup 32 , 32 ', and 32 " are mounted to the annular ring 30. Preferably, the annular ring 30 is rotatably mounted on magnetic bearings as described above. Each cathode cup is controlled by the magnetic switch control 80 such that the operator can select among a plurality of modes of operation. For example, all three cathode cups can be operated simultaneously for multi-slice imaging. As another alternative, collimators 62, 62' and 62" can be associated with each of the anode/cathode cup combinations. Each collimator can have a different aperture size to produce a different size or shape x-ray beam. As another alternative, each anode/cathode cup combination can have a different filter or compensator 64', 64", associated with it. With reference to FIGURE 5, the anode assembly has a face 10 which is movable relative to the electron source 32. In the embodiment illustrated in FIGURE 5, the anode surface 10 along with the surrounding structure that defines the cooling fluid channel 12 is selectably rotatable or tippable as illustrated, to an exaggerated degree, in phantom. Instead of rotating, the surface may be flexed. Also, the anode surface may be other than a single plane such that shifting its position alters the characteristics of the anode surface which receives the electron beam.

Claims: An x-ray tube comprising: a generally toroidal housing (A) having an evacuated interior and a central bore (26); an annular anode surface (10) mounted in the toroidal housing interior, the anode surface (10) being in thermal communication with a cooling fluid passage (12) such that cooling fluid can be circulated contiguous to the anode surface (10) for removing heat; a cathode assembly (C) disposed within the toroidal housing (A) and including a plurality of electron emitting means (32a, 32b) supported by an annular ring (30) rotatably disposed within the housing (A), each said electron emitting means (32a, 32b) being capable of forming an electron beam that strikes the anode surface (10); a coupling means (72, 74) for coupling the electron emitting means (32a, 32b) to a current supply (70) exterior to said toroidal housing (A); a switching means (76, 78) for selectively switching one or more of said plurality of electron emitting means (32a, 32b) to said current supply (70); motor means (50) for rotating said annular ring (30) and hence said plurality of electron emitting means (32a, 32b) so that the electron beam formed by the or each selected electron emitting means (32a, 32b) travels around said annular anode surface (10) to produce x-rays (22); and a window (20) defined in said toroidal housing (A) and positioned such that the x-rays (22) produced are directed into said central bore (26) transverse to a central axis (24) of the bore (26). An x-ray tube according to Claim 1 wherein the annular ring (30) is mounted on a bearing (40) and wherein the motor means (50) includes an annular stator (52) mounted stationarily to the housing (A) and a rotor (54) mounted to the annular ring (30). An x-ray tube according to Claim 1 further including a magnetic levitation bearing means (40) for rotatably supporting the annular ring (30) in the housing (A). An x-ray tube according to Claim 3 further including a mechanical bearing means (56) for supporting the annular ring (30) in the event of a failure of the magnetic levitation bearing means (40). An x-ray tube according to Claim 1 further including an annular rotating capacitor plate (74) mounted to the annular ring (30) in a capacitively coupled relationship to a stationary capacitor plate (72) mounted to the housing (A), the rotating capacitor plate (74) being connected with the electron emitting means (32a, 32b) for controlling electrical power thereto and the stationary capacitor plate (72) being connected with an AC power source (70). An x-ray tube according to Claim 1 further including an annular rotating inductor (74) mounted to the annular ring (30) in an inductively coupled relationship to a stationary inductor (72) mounted to the housing (A), the rotating inductor (74) being connected with the electron emitting means (32a, 32b) for controlling electrical current flow therethrough. An x-ray tube according to Claim 1 further including a supporting means (60) mounted to the annular ring (30) adjacent the electron emitting means (32a, 32b), the supporting means (60) supporting at least one of an off-focal radiation collimator means (62) and a filter means (64) for filtering the x-ray beam (22), the supporting means (60) supporting the collimator means (62) and the filter means (64) closely adjacent the anode surface (10) such that the filter means (64) and the collimating means (62) rotate with the electron beam (22). An x-ray tube according to Claim 1 wherein the annular ring (30) includes an electrically conductive portion (114) and a means (106, 110, 116) for holding the electrically conductive portion (114) at substantially the same potential as the housing (A). An x-ray tube according to Claim 8 wherein the means (106, 110, 116) for holding the conductive annular ring portion (114) at the same potential as the housing (A) includes a filament (116) which is heated to boil off electrons which are conducted to the housing (A). An x-ray tube according to Claim 8 further including an isolation transformer (104, 106, 110, 112) for isolating the cathode assembly (C) from circuitry (76) for controlling a current flow therethrough. An x-ray tube according to Claim 1 wherein the switching means (76, 78) includes a plurality of magnetically controlled switches (76) which are mounted for rotation with the annular ring (30) and a plurality of annular electromagnets (78) mounted to the housing (A), each annular electromagnet (78) being disposed closely adjacent to a path of rotation of one of the magnetically controlled switches (76) for selectively supplying a controlling magnetic field thereto. An x-ray tube according to Claim 1 further including a high voltage power supply means (90, 92, 94) for biasing the cathode assembly (C) to a high negative voltage relative to the housing (A). An x-ray tube according to Claim 12 wherein the high voltage power supply means (90, 92, 94) includes at least one hot cathode (92) supported by the housing (A) and a partially toroidal electron receiving plate (94) at least partially encompassing the hot cathode (92) and supported by the annular ring (30) such that the toroidal plate (94) remains closely adjacent to the hot cathode (92) as the annular ring (30) rotates. An x-ray tube according to Claim 13 further including a grid (99) between the hot cathode (92) and the receiving plate (94). An x-ray tube according to Claim 12 wherein the high voltage power supply means (90, 92, 94) includes a means (94) which is biased to the high voltage, the high voltage biased means (94) being electrically connected with the cathode assembly (C, 32); and further including an electrical insulation means (106) for insulating the high voltage biased means (94), the cathode assembly (C, 32), and an electrical connection (100) therebetween from other portions (114) of the annular ring (30). An x-ray tube according to Claim 15, wherein the cathode assembly (C, 32) includes a cathode cup (32) and further including a quick connect coupling (102) for electrically and mechanically connecting the cathode cup (32) and the electrical connection (100). An x-ray tube according to Claim 15 further including: a secondary winding (104) extending around at least a portion of the insulation means (106), the secondary winding (104) being connected at one end with the electrical connection (100), and at its other end with the cathode assembly (C, 34); a second electrical insulation means (110) surrounding the secondary winding (104); a primary winding (112) surrounding the second insulation means (110) which surrounds the secondary winding (104), whereby an electrical isolation transformer (104, 106, 110, 112) is defined. An x-ray tube according to Claim 17 wherein the primary winding (112) is connected with a means (76) for controlling current flow through the cathode assembly (C). An x-ray tube according to Claim 1 further including a position encoder (58) for providing an encoded signal indicative of an angular position of the annular ring (30) relative to the housing (A). An x-ray tube according to Claim 1 further including: a second anode surface (10') mounted in the toroidal housing interior in thermal communication with a second cooling fluid passage (12'); a second means (32') for emitting electrons mounted on the cathode assembly (C, 30) for selectively forming a second electron beam which strikes the second anode surface (10'). An x-ray tube according to Claim 20 wherein the first and second anode surfaces (10, 10') are concentric circular annuli of different radius. An x-ray tube according to Claim 20 further including: a first filter (64) and collimator (62) assembly mounted to the cathode assembly (C) and disposed adjacent the first anode surface (10); a second filter (64') and collimator (62') assembly mounted to the cathode assembly (C) adjacent the second anode surface (10').

Inventors: BURKE JAMES E, MILLER LESTER, BURKE, JAMES E., MILLER, LESTER
Applicants: PICKER INT INC, PICKER INTERNATIONAL, INC.
Application year: 1993
Main classification: H01J35
IPC codes: H01J35