Abstract:
A dual filament x-ray tube assembly ( 16 ) includes an evacuated envelope ( 52 ) having an anode ( 54 ) disposed at a first end of the evacuated envelope ( 52 ) and a cathode assembly ( 62 ) disposed at a second end of the evacuated envelope ( 52 ). The cathode assembly includes a variable-length filament assembly ( 72, 74; 100 ) which emits electron beams for impingement on the anode ( 54 ) at focal spots having varying lengths. The cathode assembly ( 62 ) further includes a cathode cup ( 64, 66, 68; 110, 112 ) which is subdivided into a plurality of electrically insulated deflection electrodes ( 64, 66, 68; 110, 112 ). A filament select circuit ( 80 ) selectively and individually heats a portion of the variable-length filament assembly ( 72, 74 ). Electron beams emitted from the filament assembly ( 72, 74 ) are electrostatically focused and controlled by applying potentials to different ones of the deflection electrodes ( 64, 66, 68; 110, 112 ). The x-ray tube assembly ( 16 ) provides longer focal spots for thick-slice scanning applications and shorter focal spots for thin-slice scanning applications along with the benefit of electrostatic focusing and control.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with conventional x-ray diagnostic systems and other penetrating radiation systems for medical and non-medical examinations.  
           [0002]    Typically, a high power x-ray tube includes an evacuated envelope or housing which holds a cathode filament through which a heating or filament current is passed. A high potential, typically on the order of 100-200 kV, is applied between the cathode and an anode which is also located within the evacuated envelope. This potential causes a tube current or beam of electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area or focal spot of the anode with sufficient energy to generate x-rays.  
           [0003]    In order to increase the resolution of a CT scanner, it is desirable to modulate the position or size of the focal spot between two or more positions or sizes, creating two distinct point sources of radiation. Conventionally, two different methods have been employed to control the position and/or width of the focal spot. One method of focal spot control employs electrostatic grids or biasing electrodes referenced to a common leg of a single filament. The voltages on the two electrostatic grids are varied to change the location, as well as the width, of the electron beam impinging on the focal track of the anode. While the electrostatic method yields greater focal position control, it is limited to providing a focal spot of a single length.  
           [0004]    Another method of focal spot control employs a magnetic yoke in order to create a magnetic field that affects the path of the electron beam emitted from the cathode. While the magnetic yoke method employs two filaments, therefore providing two focal spot lengths and widths, it is disadvantageous for a number of reasons. The magnetic yoke tube requires two additional connections to be passed through the x-ray tube housing, making it incompatible with many CT systems. In addition, the magnetic fields employed to deflect and focus the electron beam cannot be moved in a square wave fashion between the two focal spot positions, creating a gap in the collected data.  
           [0005]    Therefore, a need exists for an x-ray tube assembly that provides multiple focal spot lengths and widths to create a system having a high modulation transfer function as well as a high x-ray flux in order to limit exposure times. The present invention contemplates a new and improved x-ray tube having an adjustable focal spot length and width, which overcomes the above-referenced problems and others.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with one aspect of the present invention, an x-ray tube assembly includes an evacuated envelope and an anode disposed at a first end of the evacuated envelope for rotation about an anode axis. A cathode assembly disposed at a second end of the evacuated envelope emits an electron beam which strikes the anode at a focal spot, having a focal spot length and a focal spot width. The cathode assembly includes a variable-length filament assembly which emits electron beams, which impinge on the anode at focal spots having variable lengths. A cathode cup defines a plurality of electrostatic deflection electrodes which are electrically insulated from each other. Further, potentials are individually and selectively applied to different ones of the electrostatic electrodes of the cathode cup for controlling the width and location of the focal spot on the anode.  
           [0007]    In accordance with another aspect of the present invention, an x-ray tube includes a cathode assembly having a long filament portion and a short filament portion and a common electrostatic deflection electrode disposed between the long and short filament portions. A first electrostatic deflection electrode is disposed adjacent the long filament portion opposite the common electrode and a second electrostatic deflection electrode is disposed adjacent the short filament portion opposite the common electrode. The x-ray tube further includes an anode and a vacuum enclosure which encloses the cathode assembly and the anode.  
           [0008]    In accordance with another aspect of the present invention, an x-ray tube with an adjustable length and width focal spot includes an anode and a cathode assembly, which includes at least two filament segments and electrostatic deflection electrodes. A vacuum envelope surrounds the cathode assembly and the anode. Not more than four leads pass through the vacuum envelope to apply electrical power to the filament sections and bias potentials to the electrodes. A filament selection circuit is disposed inside the vacuum envelope in connection with the four leads passing through the vacuum envelope. The filament selection circuit is connected with the filament segments for applying electric current selectively through a long section of filament and a short section of filament in order to control focal spot length. Further, the filament selection circuit is connected with the electrodes in order to select focal spot width end position.  
           [0009]    In accordance with another aspect of the present invention, an x-ray tube assembly includes an evacuated envelope having an electron-emitting cathode assembly spaced apart from a rotating anode, where the cathode assembly includes at least a first filament and a second filament for emitting electrons in a beam which impinges on the anode at a focal spot having a variable length and a variable width. A cathode cup is sub-divided into at least three electrically insulated deflection electrodes. A filament select circuit is disposed adjacent the evacuated envelope. The filament select circuit includes means for selectively and individually electrically heating one of the first and second filaments and means for individually and selectively applying potentials to different ones of the electrostatic deflection electrodes in order to control a width and a location of a focal spot on the anode.  
           [0010]    In accordance with another aspect of the present invention, a computerized tomographic system includes a source of penetrating radiation for transmitting radiation through a subject disposed in a subject receiving aperture. The source includes at least two point sources of radiation, each providing beams of radiation having different focal lengths. Detector means are coupled to the source for detecting radiation emitted from the source after passage of the radiation through the subject. The source and detector means are mounted on a rotatable gantry. The system further includes means for processing the detected radiation into a tomographic image representation.  
           [0011]    In accordance with a more limited aspect of the present invention, the source of penetrating radiation includes an evacuated envelope and an anode disposed at a first end of the evacuated envelope. A cathode assembly is disposed at a second end of the evacuated and includes a cathode base portion and at least a first filament and a second filament, where the first filament is longer than the second filament. At least three deflection electrodes are attached to and electrically insulated from the cathode base portion. The source further includes means for individually and selectively applying potentials to different ones of the deflection electrodes.  
           [0012]    In accordance with another aspect of the present invention, an x-ray tube includes an evacuated envelope having a cathode spaced apart from an anode adapted to be maintained at a positive voltage relative to the cathode. The cathode includes a filament assembly for selectively emitting electrons in a beam which impinges on the anode at a focal spot having at least one of a long focal spot length and a short focal spot length and a variable focal spot width, and a cathode cup having a plurality of parts electrically insulated from each other. A method of operating the x-ray tube includes the steps of selectively heating a portion of the variable filament assembly to emit electrons in the beam having one of the short focal spot length and the long focal spot length. The method further includes individually and selectively applying potentials to different ones of the cathode cup parts for controlling the width and location of the focal spot on the anode.  
           [0013]    One advantage of the present invention resides in obtaining a higher x-ray flux without overheating the anode track.  
           [0014]    Another advantage of the present invention is that it produces x-ray radiation having multiple focal spot lengths.  
           [0015]    Another advantage of the present invention resides in the presence of multiple filaments without additional external connections between the x-ray tube and the CT system.  
           [0016]    Another advantage of the present invention resides in the combination of filament length selection and electrostatic focusing.  
           [0017]    Yet another advantage of the present invention resides in selective excitation of one of multiple filaments.  
           [0018]    Still another advantage of the present invention is that it modulates the focal spot between two or more positions providing greater sampling density.  
           [0019]    Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0021]    [0021]FIG. 1 is a diagrammatic illustration of a prior art computerized tomographic (CT) diagnostic system employing the x-ray tube assembly in accordance with the present invention;  
         [0022]    [0022]FIG. 2 is a diagrammatic illustration of a preferred embodiment of the x-ray tube assembly in accordance with the present invention;  
         [0023]    FIGS.  3 A- 3 E are diagrammatic illustrations of preferred embodiments of the cathode assembly in accordance with the present invention;  
         [0024]    [0024]FIG. 4 is a diagrammatic illustration of a filament select circuit in accordance with the present invention;  
         [0025]    [0025]FIGS. 5A and 5B are diagrammatic illustrations of electrical switching by the filament select circuit in accordance with the present invention;  
         [0026]    [0026]FIG. 6 is an alternate embodiment of the cathode assembly in accordance with the present invention; and  
         [0027]    [0027]FIG. 7 is an alternate embodiment of the cathode assembly in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    With reference to FIG. 1, a computerized tomographic (CT) scanner  10  radiographically examines and generates diagnostic images of a subject disposed on a patient support  12 . More specifically, a volume of interest of the subject on the support  12  is moved into an examination region  14 . An x-ray tube assembly  16  mounted on a rotating gantry projects one or more beams of radiation through the examination region  14 . A collimator  18  collimates the beams of radiation in one dimension. In third generation scanners, a two-dimensional x-ray detector  20  is disposed on the rotating gantry across the examination region  14  from the x-ray tube. In fourth generation scanners, a ring or array of two-dimensional detectors  22  is mounted on the stationary gantry around the rotating gantry.  
         [0029]    Each of the two-dimensional x-ray detectors  20 ,  22  includes a two-dimensional array of photodetectors connected or preferably integrated into an integrated circuit. The detectors generate electrical signals indicative of the intensity of the received radiation which is indicative of the integrated x-ray absorption along the corresponding ray between the x-ray rube and the scintillation crystal segment.  
         [0030]    The electrical signals, along with information on the angular position of the rotating gantry, are digitized by analog-to-digital converters. The digital diagnostic data is communicated to a data memory  30 . The data from the data memory  30  is reconstructed by a reconstruction processor  32 . Various known reconstruction techniques are contemplated including spiral and multi-slice scanning techniques, convolution and back projection techniques, cone beam reconstruction techniques, and the like. The volumetric image representation generated by the reconstruction processor  32  is stored in a volumetric image memory  34 . A video processor  36  withdraws selective portions of the image memory to create slice images, projection images, surface renderings, and the like and reformats them for display on a monitor  38 , such as a video or LCD monitor.  
         [0031]    With reference to FIG. 2 and continuing reference to FIG. 1, the x-ray tube assembly  16  includes an anode  50  and a cathode assembly  62 , which are located at opposite ends of an evacuated envelope  52 . The evacuated envelope  52  is evacuated such that an electron beam passes from the cathode assembly  62  to a focal spot on an annular, circumferential face  54  of the anode  50 . The anode  50  includes a rotor  56 , which is driven by a rotational driver  58 , for rotation about an anode axis  60 . Preferably, the evacuated envelope  52  is disposed in a dielectric medium  70 , such as an oil-based dielectric fluid, which is circulated to a cooling means.  
         [0032]    The cathode assembly  62  is located on the other end of the evacuated envelope  52 . In one embodiment, the cathode assembly  62  includes a cathode cup, which is subdivided into three voltage biasing or deflection electrodes  64 ,  66 ,  68 . In one embodiment, the two side deflection electrodes  64 ,  68  and one center deflection electrode  66  are electrically insulated from each other, as shown in FIG. 2. In an alternate embodiment, shown in FIG. 3A, the two side deflection electrodes  64 ,  68  are electrically connected to one another and to a common voltage source through electrical lead  69 . As is described more fully below, the deflection electrodes  64 ,  66 ,  68  are selectively powered, through a filament select circuit  80 , by a pair of deflection electrode power supplies  82 ,  84  and a filament power supply  86 , all of which are switchably connected to a high voltage supply  90 .  
         [0033]    With reference to FIGS. 3B and 3C and continuing reference to FIG. 2, the cathode assembly  62  includes a variable-length filament assembly. The variable-length filament assembly emits electron beams which impinge on the anode  50  at focal spots of varying lengths and widths. In one embodiment, shown in FIG. 3B, the variable-length filament assembly includes two filaments  72 ,  74  of different lengths, each producing focal spots of different lengths. Each filament  72 ,  74  of the filament assembly is electrically insulated from the deflection electrodes  64 ,  66 ,  68 . As is described more fully below, the filaments  72 ,  74  are selectively excited based on the desired imaging application. Although thin wire filaments are illustrated, it is to be appreciated that the filaments can also be thin metallic layers deposited on an insulating substrate.  
         [0034]    In an alternate embodiment, shown in FIG. 3C, the variable-length filament assembly includes a single tapped filament  100  that is electrically insulated from two deflection electrodes  110 ,  112 . The tapped filament  100  includes three filament leads, a first filament lead  102 , a second or common filament lead  104 , and a third filament lead  106 . The first filament lead  102  is in electrical communication with opposite ends of the tapped filament  100 . The second or common filament lead  104  is in electrical communication with the center of the tapped filament  100 . When current flows through electrodes  102 ,  104 , the entire length of the filament is heated to emit electrons. As shown in FIG. 3C, the third filament lead  106  is in electrical communication with the tapped filament  100  at points between the first filament leads and symmetric about the common lead. In one embodiment, the filament leads  102 ,  104 ,  106  are electrically connected to the tapped filament  100  via solder joints or welds. However, it is to be appreciated that the filament leads may be electrically connected to the tapped filament in a variety of conventional manners.  
         [0035]    In the embodiment of FIG. 3C, either the entire filament length  100 , lying between filament leads  102 , or a portion of the filament length, lying between leads  106 , may be excited depending on the particular diagnostic application. With age, the filament resistance increases. Positioning the filament portion that is common to both the long and short modes in the center assures that if its resistance increases, the corresponding higher electron generation will be symmetric in the center of the beam.  
         [0036]    In an alternate embodiment, shown in FIG. 3D, the tapped filament  100  includes three filament leads, a first filament lead  122 , a second filament lead  126 , and a common filament lead  124 . The first filament lead  122  is in electrical communication with a first end of the tapped filament  100 . The common filament lead  124  is in electrical communication with the other end of the tapped filament  100 . As shown in FIG. 3D, the second filament lead  126  is in electrical communication with the tapped filament at a point between the first and second filament leads. When current flows through leads  122 ,  124 , the entire length of the filament is heated to emit electrons. When current flows through leads  126 ,  124 , only a portion of the filament is heated to emit electrons.  
         [0037]    In the alternate embodiment illustrated in FIG. 3E, the tapped filament  100  includes four filament leads  132 ,  134 ,  136 ,  138  in electrical communication therewith. When current flows through leads  132 ,  134 , the entire length of the filament is heated to emit electrons, resulting in x-rays having a longer focal length. Conversely, when current flows through leads  136 ,  138 , the center portion of the filament is heated to emit electrons, resulting in x-rays having a shorter focal length.  
         [0038]    Voltages are applied to the two deflection electrodes  110 ,  112  and varied in the form of a square wave having a 180° phase shift between the two electrodes. It is to be appreciated that the electrode voltages may be varied according to other waveforms as well. The oscillating voltages on the deflection electrodes cause the emitted electron beam to oscillate between two impingement positions on the rotating anode, hence the origin of the x-ray beam to shift between two origins.  
         [0039]    With reference to FIG. 4 and continuing reference to FIG. 2, the cathode assembly  62  is controlled by a filament select circuit  80 , which is located within the x-ray tube housing  76 . In one embodiment, the filament select circuit  80  includes four inputs  402 ,  406 ,  410 ,  414  and six outputs  420 ,  424 ,  428 ,  432 ,  436 ,  440  to the cathode assembly (not shown). It is to be appreciated that having four inputs to the x-ray tube assembly facilitates compatibility with a variety of conventional x-ray and CT systems. In other words, no external connections between the x-ray tube assembly and the x-ray system need to be changed or added.  
         [0040]    The filament select circuit  80  provides selective and individual heating of one of the two filaments  72 ,  74  depending upon the desired focal spot length necessary for a particular application. The desired filament is selected by the order in which the end deflection electrodes  64 ,  68  are turned on or powered. More particularly, powering the large deflection electrode  68  first (via input  414 ) enables the large filament  74 , while turning on the small deflection electrode  64  first (via input  402 ) enables the small filament  72 . In addition, the order in which the side deflection electrodes  64 ,  68  are powered determines to which side deflection electrode the center deflection electrode  66  is shorted.  
         [0041]    For example, to selectively excite the large filament  74  (at output  424 ), the large deflection electrode  68  is powered up first (at input  414 ). This action controls a relay coil  450  opening contact  452  within the filament select circuit  80  to disable the small filament selection circuit. In addition, the common deflection electrode  66  (at output  436 ) is shorted to the small deflection electrode  64  (at output  420 ), as shown in FIG. 5A. It is to be appreciated that this allows for finer control of the electron beam position and width as it strikes the rotating anode. Preferably, the voltages on the now “two deflection electrodes,” the large deflection electrode  68  and the combination deflection electrode  64 ,  66 , are varied in the form of a square wave having a 180° phase shift between the two electrodes. It is to be appreciated that the electrode voltages may be varied according to other waveforms as well. Oscillating the voltages on the deflection electrodes causes the electron beam to oscillate between two impingement positions.  
         [0042]    To selectively excite the small filament  72  (at output  428 ), the small deflection electrode  64  is powered. This action powers the relay coil  460  opening normally closed contacts  462 ,  464  and  466  and closing normally open contacts  468  and  470  within the filament select circuit  80 . This routes the hot lead of the filament power supply (at input  406 ) to the small filament  72  (at output  428 ) and blocks the large filament  72  from receiving any current. In addition, contacts  470  short the common deflection electrode  66  (at output  436 ) to the large deflection electrode  68  (at output  440 ), as shown in FIG. 5B, allowing for finer control of the electron beam position and width. Preferably, the voltages on the now “two deflection electrodes,” the small deflection electrode  64  and the combination deflection electrode  66 ,  68 , are varied in the form of a square wave having a 180° phase shift between the two electrodes. It is to be appreciated that the electrode voltages may be varied according to other waveforms as well.  
         [0043]    [0043]FIG. 6 illustrates an alternative embodiment of the cathode assembly. More particularly, FIG. 6 provides a stair-stepped cathode base portion  500  housing two filaments  510 ,  514 , which are insulated from the base portion  500 . The side and center deflection electrodes  520 ,  524 ,  528  are electrically insulated from the base portion  500  by a plurality of insulating layers  530 ,  534 ,  538 . Alternatively, the last two steps of the base portion are suppressed and completely replaced by the electrically insulated side and center deflection electrodes.  
         [0044]    [0044]FIG. 7 illustrates an alternative embodiment of the cathode assembly which includes a metallic base portion  600  pierced with at least two bore  604 ,  608  and at least one additional bore (not shown) through which leads  610 ,  612  for supplying current to at least two filaments  614 ,  616  are passed. The leads are insulated from the metallic base portion by insulator sleeves  620 ,  626 . The metallic base portion  600  is shaped near the filaments so as to form stair-steps  630 ,  632 ,  634 ,  636 , which place the edges of the base portion at a distance from the filaments  614 ,  616 .  
         [0045]    Insulating elements  640 ,  642  are fixed on the external lateral faces  660 ,  662  of the metallic base portion. The insulating elements  640 ,  642  provide support for the side deflection electrodes  650 ,  652 . The insulating elements  640 ,  642  are shaped to have on the sides nearest the filaments two opposite faces  641 ,  643 , which are parallel to the steps  632 ,  636  of the base portion  600 . The side deflection electrodes  650 ,  652  are deposited on the opposite faces as well as on the top surfaces and bottom surfaces of the insulating elements  640 ,  642 . The side deflection electrodes are connected to voltages supplies (not shown) by means of conductors  670 ,  672 , which pass through the insulating elements  640 ,  642 .  
         [0046]    A central deflection electrode  656  is located between the two filaments  614 ,  616 . The central deflection electrode  656  is insulated from the base portion  600  by an insulating element  646 . The central electrode is connected to a voltage supply by means of a conductor  676  which passes through and is insulated from the base portion  600  and the insulating sleeve  646 .  
         [0047]    It is to be appreciated that all of the aforementioned embodiments may be constructed in a variety of ways without departing from the scope of the present invention. In one embodiment, the deflection electrodes and cathode base portion are formed through metal deposition on a ceramic substrate. Alternatively, the cathode assembly consists of machined metal, insulator spacers, and hermetically sealed feed-throughs which house the filament and electrode leads.  
         [0048]    The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of 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.