X-ray tubes for CT scanners

Abstract: A toroidal x-ray tube (I) is supported and selectively positioned by a CT gantry (II). The x-ray tube includes a toroidal housing (A) in which a rotor (30) 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 passes through a window (20) and strikes an annular ring of detectors (160). A grid bias control circuit (100) selectively applies a continuously adjustable bias to a grid (32) for regulating the electron current, and hence the intensity of the x-ray beam. A scintillating optical fibre (110) extends around the exterior of the window. The scintillation optical fibre includes fluorescent dopant (116) which converts a very small fraction of the x-rays into light which is transmitted along the fibre to an optoelectric transducer (118). The opto-electric transducer is connected with the grid bias control circuit. The opto-electrical transducer (118) can also be connected with an intensity compensator (162) for adjusting the signals from the detector ring before they are reconstructed (164) into an image representation.

Description: This invention relates to x-ray tubes. It finds particular application in conjunction with annular x-ray tubes for CT (computed tomography) 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 the x-ray tubes 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 would 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. In an x-ray tube with a fixed anode and a fixed cathode or with a rotating anode and fixed cathode, accurate control of the radiation dose delivered to the patient was relatively simple. Because the quality of the reconstructed image is dependent upon the number of x-ray photons that are captured by the radiation detector, accurate control of the number of x-ray photons generated is important for good CT images. In order to control the amount of radiation generated, the x-ray tube current is typically monitored and the cathode current is controlled accordingly. Typically, the cathode is fixed at a high negative voltage relative to the housing and the anode similarly fixed to a high positive voltage. Control of the tube current passing between the anode and the cathode (actually an electron flow cathode to anode) is typically done by controlling the temperature of a directly heated cathode filament. More specifically, the electron density from the filament surface is a function of the temperature, the applied anode/cathode voltage, the geometry of the cathode structure, and the distance from the cathode to the anode. The filament temperature is traditionally controlled by controlling a filament heating current. 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. One problem with rotating cathode x-ray tubes resides in the difficulty of monitoring the x-ray tube current. The tube current is generally more easily measured at the anode portion of the tube. Measuring the x-ray tube current with an end grounded anode is difficult. Measuring the x-ray tube filament current on the rotating cathode side within the vacuum is also difficult because the filament is rotating and there is not direct means of measurement. Another problem is controlling the cathode filament temperature accurately. The filament current is supplied to the rotating cathode by a transformer, capacitive coupling, or the like across a vacuum. Any wobble or variation in the gap between transformer or capacitive elements tends to vary the x-ray tube current. This causes difficulties in controlling the filament current in the rotating cathode assembly. The present invention contemplates a toroidal x-ray tube and CT scanner which provides for improved x-ray intensity measurement and/or control. EP-A-0564292 discloses a toroidal x-ray tube supported for rotation about a horizontal axis, translation along a vertical axis, and translation along a horizontal axis. The x-ray tube includes a toroidal housing, an annular anode, and a cathode which rotates a beam of electrons around the annular anode. A plurality of parallel connected voltage sources provide a sufficiently high bias voltage between the electron source and the anode that x-rays are generated. The x-ray beam passes through a compensator crystal, an annular window, a collimator, through a subject received in a central bore of the x-ray tube, and impacts an arc segment of radiation detectors. The x-ray detectors are stationarily mounted outside of the plane of the annular window, nutate into the plane of the windows opposite of the origin of the x-ray beam, rotate in part or rotate in full. Angular position monitors determine the angular position of the cathode assembly, hence the x-ray beam, and the angular position of the detectors in the rotating detector embodiment. GB-A-2034149 discloses, in an X-ray apparatus for a computed tomography scanner, a plurality of combined X-ray tube segments arranged round the central axis of the apparatus on which a subject to be examined is disposed. Each segment includes at least one electron beam generating means, a strip-like anode target extending along a circular arc round the central axis, and an envelope containing these components. A control grid, arranged between the electron source and the target anode, selectively controls the area of the anode which is to be bombarded at any instant. Along the surfaces of the X-ray tube segments facing the central axis lie a large number of X-ray detector elements to detect doses of fan-shaped X-ray beams emitted from the target and transmitted through the to-be-examined subject. The electron beam generating means successively produce electron beams, and fan-shaped X-ray beams are applied one after another to the subject from around the same. According to the present invention there is provided an x-ray tube comprising: a generally toroidal housing having an evacuated interior and an annular x-ray transmissive window; an annular anode mounted in the interior of the toroidal housing, the anode being in thermal communication with a circulated cooling fluid passage such that the cooling fluid removes heat from the anode; a rotor rotatably disposed within the interior of the toroidal housing; at least one cathode assembly mounted to the rotor for rotation therewith, the cathode assembly including a means for emitting electrons to form an electron beam that strikes the anode to generate an x-ray beam; and means for rotating the annular rotor such that the electron beam is rotated around the anode; characterised by a radiation intensity monitoring means mounted adjacent the window such that the same radiation which exits the window also passes through the radiation intensity monitoring means, the monitoring means including a scintillating optical fibre which itself both converts a small amount of incident x-rays to optical light and transmits the light therealong. The invention also provides CT apparatus incorporating x-ray tubes in accordance with the invention. One advantage of the present invention is that it monitors actual x-ray intensity with a very fast response time without introducing extraneous noise into the x-ray receiving subsystem or degrading resultant CT images. Another advantage of the present invention is that it yields improved images by providing for real time adjustments to the image reconstruction algorithm to compensate for any x-ray intensity fluctuations. Another advantage of the present invention is that the x-ray tube current is accurately controlled. Another advantage of the present invention is that it enables the tube current to be controlled relatively independently of the filament current. One CT scanner in accordance with the invention incorporating an x-ray tube 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 the CT scanner; Figure 2 is a cross-sectional view of a toroidal, rotating cathode x-ray tube of the scanner of Figure 1; Figure 3 is a circuit diagram of a circuit for adjusting and controlling the potential bias of a cathode of the tube of Figure 2; and Figure 4 is a view of a radiation intensity monitoring means of the scanner. Referring to Figure 1, the CT scanner includes a toroidal ring x-ray tube I which is mounted to a gantry or mounting assembly II and an electronics section III. The electronic section III includes a high voltage power supply 80 for the x-ray tube I and further receives data from the gantry and reconstructs the received data into an electronic image representation. With reference to FIGURE 2, the ring tube I includes a toroidal housing A which defines a large, generally donut-shaped interior volume. An anode B is mounted within the toroidal housing interior volume and extends circumferentially therearound. A cathode 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 ring having a tungsten face 10 upon which the electron beam impinges. The anode assembly defines an annular anode adjacent 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 window means 20 is mounted in the housing A in radial alignment with the tungsten face 10 of the target anode. The window is positioned such that x-rays generated by interaction of the electron beam and the tungsten target anode 10 are directed transverse to a central axis 22 of a bore 24 of the toroidal tube. An annular ring or rotor 30 extends around an interior of the toroidal housing A . The rotor supports a corresponding plurality of cathode assemblies C . Each of the cathode assemblies includes a cathode cup 32 which contains a filament or other electron source 34 and a cathode control circuit 36 . The filament 34 and the anode 10 are maintained at a high relative voltage relative to each other, e.g. 150 kV. The housing A and the rotor 30 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 30 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 rotor 30 is rotatably supported within the housing A on a bearing means 40 , a magnetic levitation bearing in the embodiment of FIGURE 2. The magnetic levitation bearing includes rings of silicon steel 42 , which are stable within the vacuum, mounted along an inner radius of the rotor 30 . Passive and active elements including permanent magnets 44 and electromagnets 46 are disposed closely adjacent the rings 42 of silicon steel. The housing A includes a magnetic window 48 which separates the vacuum region from the electromagnets 46 . The magnetic window 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 rotor ring 30 , a pair of oppositely disposed magnetic levitation bearings 50 are mounted on opposite sides of the rotor. Each has rings of silicon steel 52 and permanent magnets 54 to provide opposing forces on the rotor. The magnetic levitation bearing on one side also has electromagnetic coils 56 to adjust the relative opposing forces. Position sensors, not shown but conventional in the art, are provided for controlling the electromagnetic coils to maintain the position of the rotor 30 precisely. The rotating means D includes a large diameter brushless motor 60. A rotor 62 , preferably of permanent magnets, is mounted to the rotor 30 within the vacuum region. A stator 64 including electromagnetic windings, is positioned directly opposite the rotor 62 but across the magnetic window 48 outside of the vacuum region. Mechanical roller bearings 66 , normally out of contact with the rotor, are provided to support the rotor 30 in the event the magnetic levitation system should fail. The mechanical roller bearings prevent the rotor 30 from interacting directly with the stationary housing A and other associated structures. An angular position monitor 68 monitors the angular position of the rotation of the rotor 30 , hence the angular position of the cathode assemblies and the apices of the x-ray beams precisely. Each of the cathode assemblies C include insulation for insulating the cathode assembly from the rotor 30 . An electrical conductor 72 extends from one end of the filament 34 to a first electrical power transfer means 70 for transferring the cathode biasing potential from the stationary housing A to the cathode assemblies. The first power transfer means includes a toroidal ring 74 which is supported by but insulated from the rotor 30 by a series of mounting brackets 76 . A hot cathode filament 78 is connected with a high voltage supply system 80 . The hot cathode filament 78 is preferably of a lower work function type. The toroidal channel 74 which partially surrounds the hot cathode filament 78 is maintained near the potential of the hot filament by the transfer of electrons therebetween. A second power transfer means 82 transfers a filament current to the cathode assemblies. The second power transfer means includes a filament control circuit 84 , a horse-shoe shaped primary coil 86 , and an annular secondary coil 88 . A third power transfer means 90 transfers a bias potential for adjustably controlling a cathode bias. The third power transfer means includes a first bias control circuit 92 off the rotor for designating a selected bias. A horse-shoe shaped primary 94 transfers the bias potential to an annular secondary 96 . With reference to FIGURE 3 and continuing reference to FIGURE 2, the cathode control circuit 36 connects the filament current from the secondary coil 88 of the second or filament power transfer means 82 to the filament 34 . Optionally, the cathode control circuit may include a rectifier circuit for rectifying the filament current. The cathode control circuit 36 also connects the bias potential from the hot filament 78 of the first or cathode electrical power transfer means 70 to the cathode filament 34 . This causes the filament to be at about 150 kV negative, for example, relative to the anode. The cathode control circuit 36 further includes a second bias control circuit 100 for biasing the cathode cup 32 relative to the filament 34 by the bias potential indicated by the first bias control circuit 92 . Biasing the cup over a continuum of potentials relative to the filament controls the flow of electrons from the filament to the anode analogous to biasing potential on a grid. The bias on the potential cup relative to the filament is sometimes called a "grid bias". For controlling which of the cathode assemblies are generating x-rays, a bandpass circuit 102 is connected between the secondary coil winding 88 and the filament. The bandpass frequency of the band pass circuit associated with each of the cathode assemblies is distinctly different. In this manner, by adjusting the frequency of the signal applied to the primary winding 86 , the operator can select which bandpass circuit will pass the signal, hence which cathode filament will be heated. With particular reference to FIGURE 4 and continuing reference to FIGURE 2, a stationary radiation detector 110 is mounted across the window 20 . More specifically to the preferred embodiment, the stationary radiation detector includes a band or stripe of scintillating optical fibers. In the embodiment of FIGURE 4, three optical fibers with circular cross sections are shown for illustration purposes only. The preferred embodiment is composed of a ribbon of scintillating fibers with a total thickness of 1 mm or less. The individual fibers could be composed of round or square cross sections. Each of the fibers has an inner core 112 which is optically transmissive and an outer cladding 114 . The relative indices of refraction of the inner core 112 and the outer sheathing 114 are such that light within the inner core that strikes the outer sheathing is reflected back into the inner core as in conventional optic fiber or light pipe. The inner core 112 includes fluorescent dopants 116 which convert x-rays into optical light of a characteristic wavelength. In this manner, when x-rays come through the window 20 , the x-rays pass through the scintillation fibers causing the inner core to transform a small fraction of the x-rays into the light which is conveyed along the fibers to an opto-electrical transducer 118 . Preferably, an optical sheathing 120 coats the fibers such that ambient light cannot enter the optical fibers creating erroneous readings or noise. In the illustrated embodiment, the scintillation fibers are folded back on themselves out of the window 20 and parallel to the central axis 24 . In the preferred embodiment in which an annular radiation opaque shutter 122 is mounted slidably within the bore for selectively closing the window 20 , the scintillating fibers pass between the window and the shutter. With reference to FIGURES 1 and 3, the output of the opto-electrical transducer 118 is connected with a comparing means 124 which compares the amount of detected light, hence the intensity of the radiation, with a preselected radiation intensity from a selected radiation intensity memory 126 . Based on the comparison, the comparing means 124 causes the first or grid bias control circuit 92 to increase or decrease the bias, hence the x-ray tube current, hence the x-ray intensity. It is to be appreciated that the selected intensity may be cathode angular position dependent as determined by the angular position of the patient. That is, the radiation intensity can be increased through the thicker dimension of the subject and decreased through the thinner dimensions. The selected intensity memory 126 may be in the form of a look-up table which is indexed or addressed by the rotation monitor 68 to retrieve the selected intensity corresponding to the current angular position of the apex of the x-ray beam. The rotor also carries a collimator means, preferably a box collimator 130 . Opposite side walls of the collimator box 130 in a direction parallel to the central axis control the width or thickness of the x-ray beam. Oppositely disposed walls of the collimator box 130 in the transverse direction control the fan angle of the x-ray beam. The collimator assembly also blocks any scattered x-rays from merging with the x-ray beam. To maintain the vacuum within the vacuum region of the housing A , an active vacuum pumping means 140 is provided, e.g, an ion pump. An annular ring of radiation detectors 160 is disposed around the interior of the bore 24 . The detectors are disposed closely adjacent but slightly offset from the window 20 . The detectors 160 produce signals which are indicative of radiation intensity received. The signals from the radiation detectors 160 and from the reference detector 110 mounted on the patient side of window 20 are connected to an intensity compensation circuit 162 which adjusts the detector signals in accordance with fluctuations in radiation intensity indicated by the output of the opto-electric transducer 118 . The radiation intensity compensation function may also be performed by software in the reconstruction module. The intensity data is digitized and reconstructed by a volume image reconstruction means 164 into an image representation. The volume image representation is stored by volume image memory 166 from which selected information is withdrawn and converted into appropriate video format for display on a monitor 168 . Alternately, a plurality of discrete radiation detectors is disposed in a substantially uniform manner near the periphery of the exit beam. These discrete detectors are placed between the x-ray source and the annular slide collimator and are totally immersed in the incident x-ray beam and do not protrude into the defined slice. Since the thickness of the swath of x-rays incident on the collimator exceeds the width of the collimator, this condition is ensured.

Claims: An x-ray tube comprising: a generally toroidal housing (A) having an evacuated interior and an annular x-ray transmissive window (20); an annular anode (B) mounted in the interior of the toroidal housing, the anode (B) being in thermal communication with a circulated cooling fluid passage (12) such that the cooling fluid removes heat from the anode (B); a rotor (30) rotatably disposed within the interior of the toroidal housing (A); at least one cathode assembly (C) mounted to the rotor (30)for rotation therewith, the cathode assembly (C) including a means (34) for emitting electrons to form an electron beam that strikes the anode (B) to generate an x-ray beam; and means (60) for rotating the annular rotor (30) such that the electron beam is rotated around the anode (B); characterised by a radiation intensity monitoring means (110) mounted adjacent the window (20) such that the same radiation which exits the window also passes through the radiation intensity monitoring means (110), the monitoring means (110) including a scintillating optical fibre which itself both converts a small amount of incident x-rays to optical light and transmits the light therealong. An x-ray tube according to Claim 1 further including an opto-electrical transducer for converting the light conveyed through the scintillating optical fibre into an electrical radiation intensity signal. An x-ray tube according to Claim 1 or Claim 2 further including an electron beam adjusting means (92, 100) for controlling the electron beam to adjust the intensity of the generated x-ray beam. An x-ray tube according to Claim 3, wherein the electron emitting means comprises a filament (34) mounted in a supporting cup (32) and the electron beam adjusting means (92, 100) includes: a biasing means (92) disposed to provide an electric field between the filament (34) and the supporting cup (32); and, a means (100) for adjusting the biasing means. An x-ray tube according to Claim 4, wherein the bias adjusting means (100) is mounted to the rotor (30) for rotation therewith. A CT scanner comprising: a patient support for supporting a selected portion of a patient in an examination region (24); an x-ray tube acccording to any one of Claims 1 to 5: a gantry (II) which supports and selectively positions the x-ray tube; a ring of radiation detectors (160) for receiving x-rays from the x-ray tube window (20) which have traversed the examination region (24); and an image reconstruction means (164) for reconstructing signals from the ring of radiation detectors (160) into image representations. A CT scanner according to Claim 6, wherein the scintillating optical fibre extends annularly around an exterior of the x-ray transmissive window (20). A CT scanner according to Claim 7 when dependent on Claim 2 wherein the opto-electrical transducer (118) is connected with the image reconstruction means (164) for adjusting image reconstruction for monitored variations in radiation intensity. An x-ray tube according to claim 1, further comprising a voltage biasing means (32) disposed adjacent the cathode for establishing an electric field adjacent the electron emitting means (34) through which the electron beam passes to strike the anode (B); and a bias adjusting means (100) for continuously adjusting the electric field for adjustably controlling the electron beam for adjustably controlling an intensity of the generated x-ray beam. An x-ray tube according to Claim 9, wherein the radiation intensity monitoring means (110) is connected with the bias adjusting means (100) for providing an indication of actual x-ray beam intensity thereto. An x-ray tube according to Claim 9 or Claim 10 further including: a first power transfer means (70) for transferring a bias potential from the toroidal housing (A) to the rotor (30) to provide a potential to the cathode assembly (C) relative to the anode (B); a second electrical power transfer means (82) for transferring a current from the housing (A) to the rotor (30) for providing an electrical current for heating the electron emitting means (34) of the cathode assembly (C); and a third electrical power transfer means (90) for transferring power from the toroidal housing (A) to the voltage biasing means (32). An x-ray tube according to Claim 11 including a plurality of cathode assemblies (C) mounted to the rotor (30) and a respective bandpass circuit (102) connected between each cathode assembly (C) and the second power transfer means (82), each of the bandpass circuits (102) having a distinctly different bandpass frequency such that the frequency of the power transferred by the second power transfer means (82) determines which cathode assembly (C) receives an electron emitting means heating current. An x-ray tube according to any one of Claims 9 to 12 wherein the bias adjusting means (100) includes a biasing circuit which is sealed in a vacuum tight container and mounted to the rotor (30) for rotation therewith. A CT scanner comprising: a patient support for supporting a selected portion of a patient in an examination region (24); an x-ray tube according to any one of Claims 9 to 13; a gantry (II) which supports and selectively positions the x-ray tube around the examination region (24); a ring of radiation detectors (160) for receiving radiation from the x-ray tube which has traversed the examination region (24); and an image reconstruction means (164) for reconstructing signals from the radiation detector ring (160) into image representations. An x-ray tube according to claim 1 further comprising: a plurality of said cathode assemblies (C) mounted to the rotor for rotation therewith, each cathode assembly including said means (34) for emitting electrons to form an electron beam that strikes the anode (B) to generate an x-ray beam; cathode assembly electrical power transfer means (82) for transferring cathode assembly electrical power of a selectable frequency from the housing (A) to the rotor (30); and a plurality of bandpass circuits (102) each having a different bandpass frequency, each of the circuits (102) being connected between the cathode assembly electrical power transfer means (82) and one of the cathode assemblies (C) such that one of the cathode assemblies (C) is selected to receive electrical power by the frequency of the transferred electrical power.

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