Abstract:
An apparatus for focusing and deflecting the electron beam of an x-ray device is disclosed herein. The apparatus includes a vacuum enclosure, and a cathode assembly disposed within the vacuum enclosure. The cathode assembly is adapted to transmit an electron beam comprising a plurality of electrons. The cathode assembly is generally maintained at a first voltage. The apparatus also includes an anode disposed within the vacuum enclosure. The anode is generally maintained at a second voltage. The apparatus also includes a member disposed within the vacuum enclosure between the cathode assembly and the anode. The member defines an aperture through which the electron beam is passed. The member is generally maintained at the second voltage. A corresponding method for focusing and deflecting the electron beam of an x-ray device is also disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates generally to a method and apparatus for focusing and deflecting the electron beam of an x-ray device. 
       BACKGROUND OF THE INVENTION 
       [0002]    X-ray tubes generally include a cathode assembly and an anode assembly disposed within a vacuum vessel. The anode assembly includes an anode having a target track or impact zone that is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. The cathode assembly emits electrons that form of an electron beam and that impact the target track of the anode assembly at high velocity. As the electrons impact the target track, the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays. The x-rays are then transmitted through an object such as the body of a patient and are intercepted by a detector that forms an image of the object&#39;s internal anatomy. 
         [0003]    In a conventional x-ray device, a voltage differential is maintained between the cathode assembly and the anode assembly in order to accelerate the electrons therebetween. This voltage differential generates an electric field having a strength defined as the voltage differential between the anode and the cathode divided by the distance between the anode and the cathode. While it may be beneficial in some applications to increase the distance between the anode and the cathode, it should be appreciated that doing so can diminish electric field strength. Diminishing the electric field strength can reduce the emission of electrons from the cathode assembly which may reduce the life of the filament. Diminishing the electric field strength can also produce a larger influence of space charge on electron beam size, referred to as “blooming”, and can thereby degrade x-ray image quality. 
         [0004]    The cathode assembly generally includes a pair of electrodes positioned on opposite sides of the electron beam. A bias voltage is independently applied to each of the electrodes to focus and/or deflect the electron beam. It is generally preferable to perform a desired command to either focus or move the electron beam with a minimal bias voltage at the electrodes. For example, minimizing bias voltage requirements at the electrodes reduces the risk of insulation breakdown in the x-ray tube to improve reliability; reduces insulation requirements to save cost; and reduces heat generation in bias voltage switching components which both improves reliability and saves cost otherwise required for cooling. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0006]    In an embodiment, an x-ray apparatus includes a vacuum enclosure, and a cathode assembly disposed within the vacuum enclosure. The cathode assembly is adapted to transmit an electron beam comprising a plurality of electrons. The cathode assembly is generally maintained at a first voltage. The x-ray apparatus also includes an anode disposed within the vacuum enclosure. The anode is generally maintained at a second voltage. The x-ray apparatus also includes a member disposed within the vacuum enclosure between the cathode assembly and the anode. The member defines an aperture through which the electron beam is passed. The member is generally maintained at the second voltage. 
         [0007]    In another embodiment, an x-ray apparatus includes a vacuum enclosure, and a cathode assembly disposed within the vacuum enclosure. The cathode assembly is adapted to transmit an electron beam comprising a plurality of electrons. The cathode assembly includes a first and second electrode configured to selectively focus and deflect the electron beam. The cathode assembly is generally maintained at a first voltage. The x-ray apparatus also includes an anode disposed within the vacuum enclosure. The anode is adapted to receive the electron beam from the cathode assembly. The anode is generally maintained at a second voltage. The x-ray apparatus also includes a member disposed within the vacuum enclosure between the cathode assembly and the anode. The member defines an aperture through which the electron beam is passed. The member is generally maintained at the second voltage. An electric field adapted to accelerate the electrons is generated substantially between the cathode assembly and the member, and a field free region through which the electrons drift is defined substantially between the member and the anode. 
         [0008]    In yet another embodiment, a method for focusing and deflecting an electron beam of an x-ray device includes providing a vacuum enclosure, and applying a first voltage potential to a cathode assembly disposed within the vacuum enclosure. The cathode assembly is adapted to transmit an electron beam comprising a plurality of electrons. The method also includes applying a second voltage potential to an anode disposed within the vacuum enclosure. The anode is spaced apart from the cathode assembly by an amount selected to allow more efficient focusing and deflection of the electron beam. The method also includes applying the second voltage potential to a member disposed within the vacuum enclosure between the cathode assembly and the anode. The member defines an aperture through which the electron beam passes. The method also includes applying a bias voltage to a first electrode and a second electrode in order to selectively focus and/or deflect the electron beam. 
         [0009]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective sectional diagram of an x-ray tube in accordance with an embodiment; 
           [0011]      FIG. 2  is schematic sectional diagram illustrating the transmission of an electron beam from a cathode assembly to an anode; 
           [0012]      FIG. 3  is a schematic sectional diagram illustrating the deflection of the electron beam of  FIG. 2 ; and 
           [0013]      FIG. 4  is schematic sectional diagram illustrating an x-ray tube in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
         [0015]    Referring to  FIG. 1 , a perspective sectional view of an x-ray tube  10  in accordance with an embodiment is shown. The x-ray tube  10  includes an anode  12  and a cathode assembly  14  which are at least partially disposed in a vacuum  16  within a vacuum enclosure or vessel  18 . A member  20  defining an aperture  22  is interposed between the anode  12  and the cathode assembly  14 . It should be appreciated that the x-ray tube  10  is shown for exemplary purposes, and that the member  20  may be implemented with other x-ray tube configurations. 
         [0016]    The cathode assembly  14  generates and emits an electron beam  24  comprising a stream of electrons  26  that are accelerated toward the anode  12 . The electrons  26  pass through the aperture  22  of the member  20  and strike a focal spot  28  on the anode  12  such that high frequency electromagnetic waves, or x-rays  30 , are produced. A portion of the emitted x-rays  30  are directed out of a window  32  for penetration into an object such as the body of a patient (not shown). The window  32  is hermetically sealed to the vessel  18  in order to maintain the vacuum  16 . The window  32  is transmissive to x-rays  30 , and preferably only allows the transmission of x-rays having a useful diagnostic amount of energy. 
         [0017]    The anode  12  is generally disc-shaped and includes a target track or impact zone  34  that is generally fabricated from a refractory metal with a high atomic number such as tungsten or tungsten alloy. Heat is generated in the anode  12  as the electrons  26  from the cathode assembly  14  impact the target track  34 . For example, the temperature of the anode at the focal spot  28  can run as high as about 2,700 degrees C. The anode  12  is preferably rotated so that the electron beam  24  from the cathode assembly  14  does not focus on the same portion of the target track  34  and thereby cause the accumulation of heat in a localized area. 
         [0018]    In a conventional x-ray tube, a voltage differential is maintained between the cathode and the anode. In an exemplary conventional monopolar x-ray tube design, the cathode may be held at −200 kilovolts (kV) and the anode is grounded. In an exemplary conventional bipolar x-ray tube design, the cathode may be held at −100 kV and the anode may be held at +100 kV. The voltage differential between the cathode and the anode generates an electric field having a field strength defined as ΔV ca /L ca , where the term ΔV ca  is the voltage differential between the cathode and the anode, and the term L ca  is the distance between the cathode and the anode. The electric field in a conventional x-ray tube accelerates the electrons from the cathode toward the anode at a rate proportional to the electric field strength. As will be described in detail hereinafter it may be beneficial to increase the distance between the cathode and the anode (L ca ), however, it should be appreciated that increasing this distance can also diminish electric field strength. 
         [0019]    Referring to  FIG. 2 , a schematic sectional diagram illustrating the transfer of electrons  26  from the cathode assembly  14  to the anode  12  is shown. The electron beam  24  from the cathode assembly  14  passes through the aperture  22  of the member  20  and hits the focal spot  28  on the anode  12 . Advantageously, the member  20  can act as a “false anode” for purposes of calculating electric field strength. By interposing the member  20  between the cathode assembly  14  and the anode  12 , and by maintaining predetermined voltage potentials at the cathode assembly  14 ; the anode  12 ; and the member  20 , an electric field  36  is generated between the cathode assembly  14  and the member  20  and a field free region  38  is generated between the member  20  and the anode  12 . More precisely, to generate the electric field  36  and the field free region  38 , the cathode assembly  14  is held at a first voltage potential V 1 , the member  20  is held at a second voltage potential V 2  which may be zero or ground for monopolar tubes, and the anode  12  is also held at the second voltage potential V 2 . The electrons  26  are accelerated by the electric field  36  from the cathode assembly  14  to the member  20 , and thereafter the electrons  26  drift through the field free region  38  from the member  20  to the anode  12 . 
         [0020]    The strength of the electric field  36  is a function of the distance between the cathode assembly  14  and the member  20 , and is independent of anode  12  location. This distance is labeled in  FIG. 2  as L cfa  which stands for the distance between the cathode assembly  14  and the false anode or member  20 . It should therefore be appreciated that, by incorporating the member  20  configured to act as a false anode, the anode  12  can be moved farther away from the cathode assembly  14  without diminishing the electric field strength. 
         [0021]    The cathode assembly  14  preferably includes an emitter  40  positioned between a pair of electrodes  42 ,  44 . The emitter  40  is the portion of the cathode assembly  14  that emits the electrons  26  which form the electron beam  24 . A bias voltage is independently applied to the electrodes  42 ,  44  in order to focus and deflect the electron beam  24 . By increasing the magnitude of a common bias voltage applied to both electrodes  42 ,  44 , the electron beam  24  can be made to either converge or diverge more rapidly. More precisely, by increasing the magnitude of a negative bias voltage applied equally to each electrode  42 ,  44 , the electron beam  24  converges with an increasing convergence angle α and, by increasing the magnitude of a positive bias voltage applied to each electrode  42 ,  44 , the electron beam  24  diverges with an increasing divergence angle (not shown). Application of an asymmetrical bias voltage to the two electrodes  42 ,  44  deflects the electron beam  24 , and the amount of angular deflection θ (shown in  FIG. 3 ) is directly proportional to the magnitude of the voltage differential between the two electrodes  42 ,  44 . It is generally preferable to perform a desired command to either focus or move the electron beam  24  with a minimal bias voltage at the electrodes  42 ,  44 . While the present invention has been described as including one pair of electrodes  42 ,  44  adapted to focus and/or deflect the electron beam  24  along a single axis, it should be appreciated that alternate embodiments may implement additional electrode pairs (not shown) in order to focus and/or deflect an electron beam in other axial directions. 
         [0022]    As previously indicated, it can be beneficial to move the anode  12  farther away from the cathode assembly  14 . One such benefit relates to a reduction in the electrode  42 ,  44  bias voltage required to focus and/or deflect the electron beam  24 . It can be seen with respect to  FIG. 2  that by increasing the distance (L ca ) between the cathode assembly  14  and the anode  12 , the convergence angle θ required to produce a focal spot of a given size L fs , decreases. Decreasing the convergence angle α correspondingly reduces the requisite amount of bias voltage at the electrodes  42 ,  44 . Similarly, it can be seen with respect to  FIG. 3  that by increasing the distance (L ca ) between the cathode assembly  14  and the anode  12 , the deflection angle θ required to produce a given amount of focal spot movement ΔX fs  decreases. Decreasing the deflection angle θ correspondingly reduces the requisite bias voltage differential between the electrodes  42 ,  44 . 
         [0023]    The reduction in the electrode  42 ,  44  bias voltage required to deflect the electron beam  24  is particularly advantageous for applications that implement “double sampling”. “Double sampling” is a technique used in computed tomography (CT) systems to prevent aliasing effects in image reconstruction and thereby improve image quality. Double sampling can be achieved by numerically evaluating two separate images. The two images are generally obtained by moving the focal spot  28  between two different positions on the target track  34  of the anode  12 . The process of rapidly moving the focal spot  28  back and forth to obtain two images may be referred to as “wobbling”. Wobbling is produced by rapidly changing the bias voltage applied to each of the electrodes  42 ,  44  in order to deflect the electron beam  24  by a predetermined amount in a manner similar to that described hereinabove. The process of rapidly changing the bias voltage generates heat in the electronic bias voltage switching components by an amount proportional to the magnitude of bias voltage change. Therefore, by minimizing the requisite bias voltage differential for a given amount of electron beam deflection, less heat is generated during wobbling which improves durability of the bias voltage power supplies and minimizes the expense associated with cooling the power supplies. 
         [0024]    Advantageously, the incorporation of the member  20  has the effect of relocating the focal spot  28  from a position within the electric field  36  to a position within the field free region  38 . As will be appreciated by those skilled in the art, high voltage instability is often precipitated by localized outgassing of the anode  12  due to focal spot  28  overheating. By moving the focal spot  28  into the field free region  38 , a high voltage breakdown event can no longer originate at the focal spot  28  thus enabling more stable tube operation. Improved high voltage stability enables better image quality. 
         [0025]    Referring again to  FIG. 1 , the member  20  is shown in accordance with a preferred embodiment as being generally disc shaped with a rectangular aperture  22 . The aperture  22  is preferably conformal meaning that it conforms to the size and shape of the electron beam  24  which is also preferably rectangular. According to an embodiment of the invention, the size of the aperture  22  is just large enough to accommodate the electron beam  24  when the beam  24  is largest and most deflected. By minimizing the size of the aperture  22  in the manner described, the member  20  is better adapted to maintain separation between the electric field  36  (shown in  FIG. 2 ) and the field free region  38  (shown in  FIG. 2 ). While the member  20  and aperture  22  have been shown and described in accordance with a preferred embodiment, it should be appreciated that alternate member and/or aperture configurations may be also envisioned. 
         [0026]    Referring to  FIG. 4 , a schematic sectional diagram illustrates a member  50  in accordance with an embodiment. Like reference numbers are used to describe like components from the embodiment of  FIG. 2 . The electron beam  24  from the cathode assembly  14  passes through the aperture  52  of the member  50  and hits the focal spot  28  on the anode  12 . Advantageously, the member  50  can act as a “false anode” for purposes of calculating electric field strength. By interposing the member  50  between the cathode assembly  14  and the anode  12 , and by maintaining predetermined voltage potentials at the cathode assembly  14 ; the anode  12 ; and the member  50 , an electric field  56  is generated between the cathode assembly  14  and the member  20  and a field free region  38  is generated between the member  20  and the anode  12 . More precisely, to generate the electric field  56  and the field free region  38 , the cathode assembly  14  is held at a first voltage potential V 1  , the member  50  is held at a second voltage potential V 2  which may be zero or ground for monopolar tubes, and the anode  12  is also held at the second voltage potential V 2 . The electrons  26  are accelerated by the electric field  56  from the cathode assembly  14  to the member  20 , and thereafter the electrons  26  drift through the field free region  38  from the member  20  to the anode  12 . 
         [0027]    The member  50  includes a first surface  58  generally facing the cathode  14 , and a second surface  60  generally facing the anode  12 . The first surface  58  includes a radially inner end  62  and a radially outer end  64 . It has been observed that altering the orientation of the first surface  58  relative to the cathode  14  can make the electron beam  24  either converge or diverge more rapidly. More precisely, by configuring the member  50  as shown in  FIG. 4  such that the radially inner end  62  of the first surface  58  is closer to the cathode  14  than radially outer end  64  of the first surface  58 , the electric field  56  is distorted in a manner tending to make the electron beam  24  converge more rapidly. Similarly, although not shown in the figures, the electron beam  24  can be made to diverge more rapidly by configuring the member  50  so that the radially outer end  64  of the first surface  58  is closer to the cathode  14  than radially inner end  62  of the first surface  58 . Therefore, the first surface  58  of the member  50  may be shaped or oriented in a predetermined manner in order to control the focus of the electron beam  24 . 
         [0028]    While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the invention as set forth in the following claims.