Abstract:
Methods and apparatus for generating x-ray beams are described. In one embodiment, the method includes operating a cathode to generate an electron beam, directing the electron beam from the cathode through an aperture in an accelerating electrode, and impinging the electron beam on an anode surface to form a focal spot on the anode surface.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates generally to x-ray generating equipment, and more particularly to methods and apparatus for maintaining an electron beam incident angle and focus on an x-ray target anode.  
           [0002]    In medical x-ray imaging, an x-ray tube is utilized for generating x-ray beams that pass through an object being imaged. More specifically, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The xbeam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.  
           [0003]    In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator.  
           [0004]    Known x-ray tubes include a cathode aligned with a rotating target anode. An electron beam generated at a cathode emitter is directed towards the anode and forms a focal spot on an anode surface. As a result, x-ray beams are emitted from the anode.  
           [0005]    The shape and focus of the electron beam emitted from the cathode emitter are defined by the cathode. In spite of the shaping and focusing within the cathode, as the beam travels to the anode, electric fields within the x-ray tube can accelerate the electrons and possibly even deflect and defocus the beam. If the electron beam does not have the desired shape and focus, the resulting x-ray beam also will lack such characteristics. As a result, the image quality of an image generated based on projection data collected utilizing such x-ray beams may not be as high as desired.  
         SUMMARY OF INVENTION  
         [0006]    In one aspect, a method for generating an x-ray beam is provided. In an exemplary embodiment, the method includes the steps of operating a cathode to generate an electron beam, directing the electron beam from the cathode through an aperture in an accelerating electrode, and impinging the electron beam on an anode surface to form a focal spot on the anode surface. The accelerating electrode facilitates shaping and focusing the electron beam.  
           [0007]    In another aspect, an x-ray source for generating an x-ray beam is provided. In an exemplary embodiment, the x-ray source includes a cathode for generating an electron beam, an accelerating electrode having an aperture through which the electron beam from the cathode passes, and an anode positioned so that the electron beam impinges thereon. Again, the accelerating electrode facilitates shaping and focusing the electron beam.  
           [0008]    In yet another aspect, an imaging system is provided. The imaging system includes a gantry, and a detector and an x-ray source are coupled to the gantry. The x-ray source is configured for radiating an x-ray beam along an imaging plane toward the detector. The x-ray source includes a cathode for generating an electron beam, an accelerating electrode having an aperture through which the electron beam from the cathode passes, and an anode positioned so that the electron beam impinges thereon. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 is a pictorial view of a CT imaging system;  
         [0010]    [0010]FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1;  
         [0011]    [0011]FIG. 3 is a schematic illustration of an x-ray tube;  
         [0012]    [0012]FIG. 4 is a schematic illustration of an x-ray source assembly including an accelerating electrode; and  
         [0013]    [0013]FIG. 5 is a schematic illustration of another embodiment of an x-ray source assembly including an accelerating electrode. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Various embodiments of anode and cathode assemblies are described herein. Although such assemblies are sometimes described in the context of a computed tomography (CT) machine, and more specifically, a third generation CT machine, such assemblies are not limited to practice in such CT machines and can be utilized in other applications as well. Therefore, the description of such assemblies in the context of CT machines is exemplary only.  
         [0015]    Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through an object, such as a medical patient  22 . Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through object or patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . In one embodiment, and as shown in FIG. 2, detector elements  20  are arranged in one row so that projection data corresponding to a single image slice is acquired during a scan. In another embodiment, detector elements  20  are arranged in a plurality of parallel rows, so that projection data corresponding to a plurality of parallel slices can be acquired simultaneously during a scan.  
         [0016]    Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
         [0017]    Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 .  
         [0018]    [0018]FIG. 3 is a schematic illustration of an x-ray tube  50 . Tube  50  includes a glass or metal envelope  52  which at one end has a cathode support  54  sealed into it. The electron emissive filament of a cathode  56  is mounted on insulators located in a focusing cup  58  which focuses an electron beam  60  against a beveled annular focal track area  62  of a rotating x-ray target  64 . Target  64  is supported on a rotor shaft  66  that extends from a rotor assembly  68 .  
         [0019]    During operation, a rotating magnetic field is induced in the rotor of assembly  68  to cause rotor shaft  66  to rotate. In addition, electron beam  60  is emitted from cathode cup  58  and is focused on beveled annular focal track area or surface  62  of x-ray target  64 . The electrons of beam  60  collide with anode  64  and as a result, x-ray beams are generated. A focal spot is formed on anode surface  62  by electron beam  60 , and the x-ray beams emanate from the focal spot. The x-ray beams are through a window in envelope  52  and pass through an object being imaged, such as a patient.  
         [0020]    As explained above, the shape and focus of the electron beam emitted from the cathode emitter are defined by the cathode, e.g., by the cathode filament. As the beam travels to the anode, however, electric fields within the x-ray tube can accelerate the electrons and possibly even deflect and defocus the beam. Such deflection and defocusing of the electron beam adversely impacts generation of a desired x-ray beam.  
         [0021]    [0021]FIG. 4 is a schematic illustration of an exemplary x-ray source assembly  100  including an accelerating electrode  102 . More specifically, an electron gun  104  including a cathode cup  106  is positioned to emit an electron beam  108  that impinges on a beveled surface  110  of an anode  112 . Cathode cup  106 , in the exemplary embodiment, contains numerous filaments selectable to provide different focal spot sizes and/or shapes. In an exemplary embodiment, cathode cup  106  and/or the filaments have a concave shape to facilitate focusing of the resulting electron beam on anode  112  as well as to reduce sensitivity of gun  104  to motion.  
         [0022]    Anode  112 , or target, is disk shaped and includes beveled target surface  110  at its outer periphery. Anode  112  also includes a cut-out center portion  114  which facilitates locating accelerating electrode  102  near the focal spot of electron beam  108 . Anode  112  can have many different shapes and is not limited to the exemplary shape illustrated in FIG. 4.  
         [0023]    Accelerating electrode  102  is positioned to reduce the electric fields that might otherwise be present between accelerating electrode  102  and target  112 , i.e., a space where the electrons of electron beam  108  from gun  104  experience very little or no forces that can perturb their motion. Generally, accelerating electrode  102  provides that the region or area between accelerating electrode  102  and target  112  has a low electric field so that the effects on the transiting electron beam are not of significance. More specifically, accelerating electrode  102  is maintained at a positive potential with respect to the cathode of gun  104  thus imparting acceleration to electrons of electron beam  108  in the direction away from the cathode.  
         [0024]    Accelerating electrode  102  includes an opening or aperture  116 , and electron beam  108  from gun  104  passes through opening  116  and impinges on anode  112 . The shape of aperture  116  at input  118 , output  120 , or both, can be selected to provide focusing and control of an incident angle, i.e., the angle at which beam  108  impinges on anode  112 . In addition, removable inserts can be located in aperture  116  to provide for an easy change in focusing/incident angle, replacement, and/or reconditioning.  
         [0025]    Accelerating electrode  102  can be cooled by convection cooling. Specifically, cooling fluid can be supplied to electrode  102  for maintaining a temperature of electrode  102  with a pre-set range. To facilitate cooling, electrode  102  can include fins or have a geometric shape which facilitates cooling. Electrode  102  also can be coupled to the x-ray source frame and cooled by cooling fluid that circulates in the frame casing.  
         [0026]    Accelerating electrode  102  can also function as an electron collector. Specifically, accelerating electrode  102  can have a geometric shape to facilitate capturing back scattered electrons. The actual shape selected depends on the trajectories of the back scattered electrons. Surfaces which collect the majority of the back scattered electrons can be coated with a low atomic number material  122  such as carbon (e.g., graphite) to limit spurious radiation influences, as shown in FIG. 4.  
         [0027]    Accelerating electrode  102  also can be configured to intercept only a low fraction of the electron back scattered flux and/or thermal radiation flux. As a result, accelerating electron heating is not as great as when accelerating electrode  102  is specifically configured to capture back scattered electrons. Again, the specific geometric shape depends on the trajectories of the back scattered electrons.  
         [0028]    In addition, accelerating electrode  102  can be operated at ground potential or raised to a negative or positive potential. The specific circuit arrangement for providing the desired potential depends, of course, on the x-ray tube arrangement. Controlling the potential of accelerating electrode  102  facilitates focusing electron beam  108  from gun  104 .  
         [0029]    In a bi-polar configuration, accelerating electrode can be located close to target anode, i.e., accelerating electrode and anode are separated only by a distance required to maintain mechanical clearance between the rotating anode and the stationary accelerating electrode. The anode and electrode can be located closely together in such a configuration because both the anode and the electrode are at a same voltage and require no dielectric standoff. To lower localized accelerating electrode hot spots, the accelerating electrode surfaces facing the focal spot on the target anode can be located at a greater distance than required for mechanical and dielectric clearance in order to avoid concentration of electron back scatter and/or thermal radiation flux.  
         [0030]    [0030]FIG. 5 is a schematic illustration of another embodiment of an x-ray source assembly  150  including accelerating electrode  102 . As shown in FIG. 5, assembly  150  includes electron gun  104  and a target anode  152 . Target anode  152  is disk shaped and includes a beveled target surface  154  at its outer periphery. Anode  152  also includes a cut-out center portion  156 . By selecting dimensions A and B of anode  152 , a shorter or longer electron beam path from electron gun  104  to the focal spot on anode  152  is provided. Anode  152  can have many different shapes and is not limited to the exemplary shape illustrated in FIG. 4.  
         [0031]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.