Patent Publication Number: US-2005117704-A1

Title: X-ray tube system and apparatus with conductive proximity between cathode and electromagnetic shield

Description:
BACKGROUND OF INVENTION  
      The present invention relates generally to the high-voltage stability of computed tomography x-ray sources. More particularly, the present invention relates to the minimization of electrostatic field line bending within the triple point areas of an x-ray tube.  
      High-voltage stability of high power and high-voltage computed tomography (CT) x-ray sources, such as an x-ray tube, is essential to constructing, seasoning, testing, and placing of the x-ray sources in service. During manufacturing of an x-ray tube, the x-ray tube is assembled and tested. Following the manufacturing of the x-ray tube, the x-ray tube is further tested and calibrated during system assembly. Many of the test protocols and calibration procedures are more aggressive than the typical or anticipated protocols and procedures in actual endpoint customer use. A desire to withstand the rigorous protocols and procedures in addition to a desire for the quick and efficient execution thereof, results in a need for a highly robust x-ray source that satisfies rigorous high-voltage x-ray tube design requirements.  
      In single-ended or monopolar high-voltage x-ray tubes x-rays are generated by accelerating an electron beam across a vacuum gap between a cathode and a rotating anode. The cathode and the anode reside within a vacuum vessel, which is sometimes referred to as an insert or frame. High voltage is supplied to the cathode via a high voltage cable through a single high voltage insulator. In the case of anode-grounded x-ray tubes, the high voltage insulator can be at a negative potential with respect to the potential of a ground reference.  
      The high-voltage insulator isolates and separates the cathode from the walls of the insert, which are often approximately at the ground potential. In so doing, the insulator provides a vacuum seal between the cathode and the walls. The high-voltage cable penetrates the insert or vacuum vessel, via conductor pins, to provide high-voltage to the cathode. The high-voltage cable is coupled to the insert by a connector having a Faraday cage. The Faraday cage is typically in the form of a cylinder that encompasses and prevents high-voltage stress on and breakdown of the conductor pins, which provide conduction between the high-voltage cable and the cathode.  
      There are generally two main design features that aid in the high-voltage stability of the insert. The two main features are the design of a vacuum side and of an atmospheric-side of the high-voltage insulator. Vacuum-tight sealing techniques are used on the vacuum side of the insulator to prevent atmospheric gas leakage into the x-ray tube. The atmospheric-side includes the use of the connector having the Faraday cage. Since the connector is typically at ground potential, the Faraday cage is used to isolate and separate the conductor pins and the connector.  
      The insulator designs are hybrid in nature. The insulator provides high-voltage potential isolation and separation through use of air gaps and insulating material. The insulator also provides mechanical strength to maintain certain physical distances to sub-millimeter tolerances over a wide range of temperatures. The insulator provides a solid surface for the establishment of electrostatic potential, across which arcing can occur. The arc path may, for example, exist between a pair of high-voltage terminals, such as between the cathode and the insert walls.  
      The areas within the vacuum vessel along which the conductors and the insulator are adjacent to or are in contact with each other are referred to collectively as “triple point areas”. High electric field stress is experienced both externally from and internally to the insulator near the cathode and conductors in the triple point areas.  
      The high electric field stress in the triple point areas can produce punctures in the insulator and electron emission through field emission effects and other hybrid microscopic mechanisms. Once the charges from the electron emission are separated from a solid surface, such as the cathode, and reside within the vacuum or the insulator they can accelerate under the effects of the electric fields and cascade to initiate arcs. The arcs can occur along the above stated paths. The arcing can damage, breakdown, and cause cracking of the insulator. Breakdown of the insulator can eventually cause air leaks and render the x-ray tube inoperable. The arcing can also result in atmosphere side flashovers, which can cause damage to other x-ray system componentry.  
      Thus, there exists a need for an improved x-ray tube design that minimizes high electric field stresses experienced within the triple point areas, while maintaining and satisfying present voltage potential differences and electric field performance standards and tolerances of an x-ray tube.  
     SUMMARY OF INVENTION  
      The present invention provides an imaging tube that includes a vacuum vessel and an atmospheric-side supply line assembly. The vacuum vessel has an internal vacuum. The supply line assembly has an electromagnetic shield. An insulator separates the internal vacuum from an external atmosphere. A cathode post resides within the vacuum vessel. The cathode post is in conductive proximity with the electromagnetic shield and prevents bending of electrostatic field lines within the imaging tube.  
      The embodiments of the present invention provide several advantages. One such advantage provided by multiple embodiments of the present invention is the provision of configuring an x-ray tube such that a cathode post is in conductive proximity with an electromagnetic shield of a high-voltage supply line assembly. In so doing, the stated embodiments prevent bending of electrostatic field lines within the x-ray tube. Prevention of the electrostatic field lines prevents arcing and breakdown of a high-voltage x-ray tube insulator, thus increasing life of the x-ray tube.  
      Furthermore, the present invention increases high-voltage stability of an x-ray tube, which in turn minimizes the manufacturing time of the x-ray tube. A decrease in the manufacturing time results in a reduction in x-ray tube cost and cycle time. The present invention increases ease in discriminating between a high-voltage stable tube and an unstable tube, such as a tube with contamination, insufficient exhaust or seasoning, loose foreign material, or a tube having surface contaminating films; all of which can compromise the high-voltage stability or performance of an x-ray tube.  
      Moreover, the present invention provides multiple techniques, which may be applied in multiple applications, for the configuration of a cathode post in conductive proximity with an electromagnetic shield.  
      The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying figures and described below by way of examples of the invention wherein:  
       FIG. 1  is a close-up cross-sectional view of a high-voltage insulator portion of a traditional x-ray tube.  
       FIG. 2  is a quarter close-up cross-sectional electrostatic field line representation view of the high-voltage insulator portion of  FIG. 1 .  
       FIG. 3  is a schematic block diagrammatic view of a multi-slice CT imaging system utilizing an imaging tube in accordance with an embodiment of the present invention.  
       FIG. 4  is a block diagrammatic view of the multi-slice CT imaging system of  FIG. 1  in accordance with an embodiment of the present invention.  
       FIG. 5  is a close-up cross-sectional view of a high-voltage insulator portion of an x-ray tube having a cathode tube in conductive proximity with an atmospheric-side electromagnetic shield and in accordance with an embodiment of the present invention.  
       FIG. 6  is a quarter close-up cross-sectional electrostatic field line representation of the high-voltage insulator portion of  FIG. 5  in accordance with an embodiment of the present invention and  
       FIG. 7  is a close-up cross-sectional view of a high-voltage insulator portion of an x-ray tube with a cathode cup in conductive contact with an atmospheric-side electromagnetic shield and in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Referring now to  FIG. 1 , a close-up cross-sectional view of a high-voltage insulator portion  10  of a traditional x-ray tube  12  is shown. The x-ray tube  12  has a vacuum vessel  14  with an internal vacuum  16 . A cathode post  18  resides within the vacuum  16  and receives power from a high-voltage cable  20  via a high-voltage connector assembly  22 . The connector assembly  22  includes a main connector  24  that is coupled to the vacuum vessel  14  and a Faraday cage  26 . The Faraday cage  26  provides an electromagnetic shield around and prevents breakdown of connector connections  30 .  
      A high-voltage insulator  32  is coupled between the cathode post  18  and walls  34  of the vacuum vessel  14 , and along side the connector assembly  22 . Notice that the cathode post  18  and the Faraday cage  26  are separated by the insulator  32  and the connector  24 . A triple point area exists at a connection  44  between the cathode post  18  and the insulator  32  near the vacuum  16 . A high field stress area exists in a region between the cage  26  and the insulator  32 . The triple point area is designated by a dashed circle  38  and the high field stress area is designated by a dashed circle  40 , is a region of high electric field non-uniformity. Areas  38  and  40  are areas of the triple point area  38  and the high field stress area  40  are shown in  FIG. 2 .  
      Referring now to  FIG. 2 , a quarter close-up cross-sectional electrostatic field line representation view of the insulator portion  10  is shown. Electrostatic field lines  42  are shown as equipotential lines that generally extend along the cathode post  18  and the Faraday cage  26  and through the insulator  32 . Notice that the field lines  42  bend within the insulator  32  around the end  44  of the cathode post  18  and the end  46  of the Faraday cage  26 . This bending of the field lines  42  causes high electric field stress within the triple point area  38  and the high field stress area  40 . The tighter the curvatures of the field lines  42  the higher the electric field stress. In general, tight bends of electric field lines exist at sharp corners and discontinuities in metallic shapes. Electrons are released from the solid into the vacuum from the end  44  and across the surface of the insulator  32 , as represented by arrows  48 . This is referred to as field effect emission. Over time, the field effect emission across the insulator  32  causes cracking in the insulator  32  and eventually causes the x-ray tube  12  to become inoperable. The multiple embodiments of the present invention prevent the bending of the electrostatic field lines within an x-ray tube, such as around a cathode post and a Faraday cage. The stated embodiments are described in detail below.  
      In the following figures the same reference numerals will be used to refer to the same components. While the present invention is described with respect to an apparatus for minimizing the bending of electrostatic field lines within triple point areas of an x-ray tube, the following apparatus is capable of being adapted for various purposes and is not limited to the following applications: computed tomography (CT) systems, radiotherapy systems, x-ray imaging systems, and other applications known in the art. The present invention may be applied to x-ray tubes, CT tubes, and other imaging tubes known in the art. The present invention may be applied in monopolar and bipolar imaging tubes.  
      In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.  
      Also, the term “triple point area” refers to areas within a vacuum vessel, of an imaging tube, along which high-voltage connections and a high-voltage insulator are adjacent to, proximate to, or are in contact with each other. The triple point areas may include areas that are external or internal to the insulator. Example triple point areas are shown in  FIGS. 1, 2 ,  5 , and  6 .  
      Referring now to  FIGS. 3 and 4 , perspective and block diagrammatic views of a multi-slice CT imaging system  50  utilizing an imaging tube  52  in accordance with an embodiment of the present invention is shown. The imaging system  50  includes a gantry  54  that has an x-ray tube assembly  56  and a detector array  58 . The assembly  56  has an x-ray generating device, such as the imaging tube  52 . The tube  52  projects a beam  60  of x-rays towards the detector array  58 . The tube  52  and the detector array  58  rotate about an operably translatable table  62 . The table  62  is translated along a z-axis between the assembly  56  and the detector array  58  to perform a helical scan. The beam  60  after passing through a medical patient  64 , within a patient bore  66 , is detected at the detector array  58 . The detector array  58  upon receiving the beam  60  generates projection data that is used to create a CT image.  
      The tube  52  and the detector array  58  rotate about a center axis  68 . The beam  60  is received by multiple detector elements  70 . Each detector element  70  generates an electrical signal corresponding to the intensity of the impinging x-ray beam  60 . As the beam  60  passes through the patient  64  the beam  60  is attenuated. Rotation of the gantry  54  and the operation of tube  52  are governed by a control mechanism  71 . The control mechanism  71  includes an x-ray controller  72  that provides power and timing signals to the tube  52  and a gantry motor controller  74  that controls the rotational speed and position of the gantry  54 . A data acquisition system (DAS)  76  samples the analog data, generated from the detector elements  70 , and converts the analog data into digital signals for the subsequent processing thereof. An image re-constructor  78  receives the sampled and digitized x-ray data from the DAS  76  and performs high-speed image reconstruction to generate the CT image. A main controller or computer  80  stores the CT image in a mass storage device  82 .  
      The computer  80  also receives commands and scanning parameters from an operator via an operator console  84 . A display  86  allows the operator to observe the reconstructed image and other data from the computer. The operator supplied commands and parameters are used by the computer  80  in operation of the control mechanism  71 . In addition, the computer  80  operates a table motor controller  88 , which translates the table  62  to position patient  64  in the gantry  54 .  
      Referring now to  FIG. 5 , a close-up cross-sectional view of a high-voltage insulator portion  90  of the x-ray tube  52  having a cathode post  92  in conductive proximity with an atmospheric-side electromagnetic shield  94  and in accordance with an embodiment of the present invention is shown. The x-ray tube  52  has a vacuum vessel  96  with an internal vacuum  98  and a center axis  100 . A cathode assembly  102  resides within the vacuum  98  and receives power from a high-voltage atmospheric-side supply line assembly  104 . A high-voltage insulator  106  is coupled between the cathode assembly  104 , walls  108  of the vacuum vessel  96 , and the supply line assembly  104 . The cathode post  92  extends through the insulator  106  such that it is in contact with the supply line assembly  104 . The extension of the cathode post  92  minimizes the separation distance between the cathode post  92  and the shield  94 . The minimal separation distance between the cathode post  92  and the shield  94  allows for electrical conductance therebetween.  
      The cathode assembly  102  includes the cathode post  92  that has an outer housing  110 . Multiple cathode connections  112  reside within the outer housing  110  and are coupled to the supply line assembly  104 .  
      The supply line assembly  104  includes a main connector  114  that is coupled to the vacuum vessel  96 . The main connector  114  includes the shield  94  that may be in the form of a Faraday cage. The shield  94  encompasses and prevents breakdown of connector connections  116 , within the connector  114 , and at the interface between the insulator  106  and the connector  114  at the point of connection. The connector connections  116  receive power from a high-voltage cable  118  and supply power to the cathode connections  112 . The main connector  114  and the shield  94  may be in various forms, shapes, and sizes.  
      The insulator  106  has a cathode post internal section  120 , a cathode post channel  122 , and an external section  124 . The internal section  120  may reside entirely within the cathode post  92 . The cathode post  92  resides within the channel  122 . The insulator  106  isolates and separates the vacuum  98  from an atmosphere  126 , which is external to the vacuum vessel  96 . The insulator  106  also isolates and separates voltage potential between the cathode post  92 , the supply line assembly  104 , and the walls  108 . The insulator  106  may be in the form of dielectric insulation, such as a thick ceramic insulator having high dielectric strength or may be in some other form known in the art. The insulator  106  may also be in various forms, shapes, and sizes.  
      A triple point area and a high field stress area, within the x-ray tube  52 , are designated by dashed circles  130  and  131 , respectively. Electrostatic field bending, within the triple point area  130 , and in the high electric field stress area  131 , is minimized due to the conductive proximity of the cathode post  92  with the shield  94 . This can be seen in further detail in  FIG. 6 .  
      Referring now to  FIG. 6 , a quarter close-up cross-sectional electrostatic field line representation of the insulator portion  90  of  FIG. 5  in accordance with an embodiment of the present invention is shown. Notice that there is minimal bending of the electrostatic field lines  132  along the cathode post  92  and within the insulator  106 . A minimal amount of bending exists between and around the end  134 , of the cathode post, and the end  136 , of the shield  94 . The electromagnetic field stress within the x-ray tube  52 , in the triple point area  130  and the high electric field stress area  131 , is substantially smaller than the electromagnetic field stress within the x-ray tubes of prior art, such as that shown in  FIG. 1 . The field lines  132  more closely follow a true coaxial arrangement such that the field lines  132  are approximately parallel relative to the center axis  100  and terminate perpendicular to any solid metallic surfaces contained within the vessel  96 , such as the cathode post  92  and the shield  94 . The minimal amount of bending remaining is further eliminated by the embodiment of  FIG. 7 .  
      Referring now to  FIG. 7 , a close-up cross-sectional view of a high-voltage insulator portion  90 ′ of an x-ray tube  52 ′, with a cathode post  92 ′ in conductive contact with an atmospheric-side electromagnetic shield  94 ′, is shown in accordance with an embodiment of the present invention.  FIG. 7  illustrates an alternative embodiment of the present invention. The x-ray tube  52 ′ includes a cathode assembly  102 ′, an insulator  106 ′, and a supply line assembly  104 ′. The insulator  106 ′ has a conducting element  140  that resides in a center portion  142  of the insulator  106 ′. The conductive element  140  is in conductive contact with the cathode post  92 ′ and the supply line assembly  104 ′. Also, the shield  94 ′ is extended along the center axis  100 , further than that of the shield  94 , such that it is in contact with the conductive element  140 .  
      The conducting element  140  resides and conducts current between the cathode post  92 ′ and the shield  94 ′. Although, the conducting element  140  is shown in the form of a conductive ring, the conducting element  140  may be in various forms, shapes, and sizes. The conducting element  140  may be formed of a metallic material or other conductive material known in the art.  
      The embodiment of  FIG. 7  provides a continuous conductive connection between the cathode post  92 ′ and the shield  94 ′. The continuous conductive connection eliminates the bending of electrostatic field lines along the cathode post  92 ′ and the shield  94 ′ within and external to the insulator  106 ′. The continuous conductive connection even minimizes the small amount of bending  150 , shown in  FIG. 6 , between the cathode post  92  and the shield  94 , by elimination of a gap  152  therebetween.  
      The present invention provides an x-ray tube with a minimal gap between a cathode post and an electromagnetic shield of a high-voltage supply line assembly. The reduction in the gap therebetween reduces the electric field stress in triple point areas and high electric field stress areas of the x-ray tube. The reduction in the electric field stress minimizes spit activity and increases high-voltage stability of the x-ray tube. The present invention minimizes the charge mobility due to the electric field acceleration along insulator surfaces and cascade-enhanced discharge initiation. The present invention also increases dielectric field strength of a high-voltage insulator of the x-ray tube.  
      The above-described apparatus and method, to one skilled in the art, is capable of being adapted for various applications and systems known in the art. The above-described invention can also be varied without deviating from the true scope of the invention.