Abstract:
An x-ray tube for emitting x-rays includes a housing, an anode assembly disposed in the housing and including a target surface, a cathode assembly mounted in the housing at a distance from the anode assembly, and a target body extending from the target surface of the anode assembly. The cathode assembly includes an electron emitter which emits electrons. The electrons hit the target surface of the anode assembly and produce x-rays. The target body has a cavity containing a working fluid and is configured to transfer thermal energy away from the target surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to imaging systems. More particularly, the present invention relates to an x-ray tube anode with enhanced thermal performance. 
     Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube. 
     Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. 
     The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies to accelerate the electrons. The hot cathode filament emits thermal electrons that are accelerated by the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. 
     In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports. 
     Since the production of x-rays in an x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2500° C., while the temperature in the other parts of the anode may range up to about 1800° C. Additionally, the components of the x-ray tube insert must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450° C. for a relatively long duration. 
     To cool the x-ray tube insert, the thermal energy generated during tube operation must be transferred from the anode through the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. 
     In conventional systems, extra performance from increased heat dissipation is achieved by increasing the diameter and mass of the target to increase the heat storage and radiating surface area of the target. Nevertheless, increasing the diameter and mass of the target is not easily done for the following reasons: (1) Increasing the diameter of the target is limited due to space constraints on the scanning system. Space constraints are particularly applicable to x-ray systems due to the desire to have good angulation capability. (2) Faster scanning on the CT gantry increases the mechanical loads on the entire x-ray tube. Hence, faster scanning tends to drive the mass of the target downward, which conflicts with the thermal performance of the x-ray tube. (3) Thickening the target will provide little benefit for high power scans since there is a finite amount of time required for the heat to conduct from the track of the target (i.e., the region where the electron beam hits the target) to other regions of the target. As such, the heat energy may not even reach the back of the target until the scan has ended. Therefore, adding extra mass to the back of the target will give little to no extra benefit with respect to thermal performance. 
     Thus, there is a need for an improved method of dissipating heat from the anode of the x-ray tube. Further, there is a need for an x-ray tube which provides greatly enhanced heat dissipation at the track and for the entire target, resulting in the capability to do longer and more powerful x-ray scans. Such an x-ray tube would beneficially operate with lower track temperature. Even further, there is a need for an x-ray tube which provides lower mass and smaller targets for a given power rating, enabling higher gantry speeds on CT systems or better angulation on x-ray systems. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to an x-ray tube for emitting x-rays which includes an anode assembly and a cathode assembly. The x-ray tube includes a casing, an anode assembly disposed in the casing and including a target surface, a cathode assembly mounted in the casing at a distance from the anode assembly, and a target body extending from the target surface of the anode assembly. The cathode assembly includes an electron emitter which emits electrons. The electrons hit the target surface or track of the anode assembly and produce x-rays. The target body includes a cavity containing a working fluid and is configured to transfer thermal energy away from the target surface or track. Advantageously, the x-ray tube may include a large surface target to radiate thermal energy to a cooled surface. 
     Another embodiment of the invention relates to an x-ray tube for emitting x-rays with increased performance by effective heat dissipation. The x-ray tube includes an electron source emitting electrons, which strike the track in order to produce x-rays, and means for transferring thermal energy away from the track. 
     Another embodiment of the invention relates to a method for dissipating heat from an anode bombarded with electrons in an x-ray tube during operation of the x-ray tube. The method includes rotating a target surface to distribute the heat from the impact of electrons on the target surface and transferring heat away from the target surface using a target body with a cavity configured to transfer thermal energy away from the target surface. 
     Another embodiment of the invention relates to a method of assembling an x-ray tube having an x-ray tube casing, an anode assembly, a cathode assembly, and a target body. The method includes locating an x-ray tube casing, orienting an anode assembly and cathode assembly within the casing, and fastening a target body to the anode assembly. The target body has a cavity containing a working fluid for transferring thermal energy away from the anode assembly. 
     Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals denote like elements, in which: 
     FIG. 1 is a perspective view of a housing having an x-ray tube in accordance with the present invention; 
     FIG. 2 is a sectional perspective view with the stator exploded to reveal a portion of an anode assembly of the x-ray tube of FIG. 1; 
     FIG. 3 is a cross sectional view of the anode assembly of the x-ray tube of FIG. 1; 
     FIG. 4 is a perspective view of an alternative embodiment of a target of the anode assembly of the x-ray tube of FIG. 1; 
     FIG. 5 is a partial view of a cross section of the target of the anode assembly of FIG. 4; 
     FIG. 6 is a partial view of a cross section of a third embodiment of the target of the anode assembly of FIG. 4; 
     FIG. 7 is a partial view of a cross section of a fourth embodiment of the target of the anode assembly of FIG. 4; and 
     FIG. 8 is an exploded view with partial cutout of the x-ray tube of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a housing unit  10  for an x-ray generating device or x-ray tube insert  12 . Housing unit  10  includes an anode end  14 , cathode end  16 , and a center section  18  positioned between anode end  14  and cathode end  16 . X-ray generating device  12  is enclosed in a fluid chamber  20  within a casing  22 . 
     Fluid chamber  20  generally is filled with a fluid  24 , such as, dielectric oil, which circulates throughout housing unit  10  to cool x-ray generating device  12 . Fluid  24  within fluid chamber  20  is cooled by a radiator  26  positioned to one side of center section  18 . Fluid  24  moves throughout fluid chamber  20  and radiator  26  by a pump  31 . Preferably, a pair of fans  28  and  30  are coupled to radiator  26  for providing cooling air flow over radiator  26  as hot fluid flows through radiator  26 . 
     Electrical connections to x-ray generating device  12  are provided through an anode receptacle  32  and a cathode receptacle  34 . X-rays emitted from x-ray generating device  12  pass through an x-ray transmissive window  36  in casing  22  at one side of center section  18 . 
     As shown in FIG. 2, x-ray generating device  12  includes a target anode assembly  40  and a cathode assembly  42  disposed in a vacuum within a vessel  44 . A stator  46  is positioned over vessel  44  adjacent to target anode assembly  40 . Upon the energization of the electrical circuit connecting target anode assembly  40  and cathode assembly  42 , which produces a potential difference of, e.g., 60 kV to 140 kV, electrons are directed from cathode assembly  42  to target anode assembly  40 . The electrons strike target anode assembly  40  and produce high frequency electromagnetic waves, or x-rays, and residual energy. The residual energy is absorbed by the components within x-ray generating device  12  as heat. The x-rays are directed out of casing  22  through window  36 , which helps direct the x-rays toward the object being imaged (e.g., the patient). 
     FIG. 3 illustrates a cross sectional view of target anode assembly  40 . Target anode assembly  40  includes a target  60 , a frame  62 , fins  86 , and a cooling chamber  68 . Target  60  is a metallic disk made of a dense material. Target  60  is preferably made of tungsten or a tungsten alloy. Target  60  includes a track  66  which provides a surface against which electrons from cathode assembly  42  strike. Frame  62  is a rigid casing which envelopes target  60 . 
     In the production of x-rays by x-ray generating device  12 , an electron beam is accelerated into track  66  of x-ray tube target  60 . The electron bombardment deposits large amounts of heat into the track  66  region of target  60 . The heat then is spread in the outer rim of target  60 . When the heat makes contact with vapor chamber  64 , the heat causes working fluid  87  in vapor chamber  64  to evaporate into vapors  88 . Evaporator region  80  is a location with relatively higher vapor pressure, causing vapors  88  to move to condenser regions  82 , where pressure is relatively lower. 
     Target  60  and fins  86  define a vapor chamber  64 . Vapor chamber  64  is an annular hollow cavity within target anode assembly  40  containing a working fluid  87 . Working fluid  87  of vapor chamber  64  is preferably sodium or lithium. Alternatively, working fluid  87  is potassium, water or other fluids. The main structure of vapor chamber  64  is made with a material compatible with working fluid  87 . For example, if sodium or lithium is used as working fluid  87 ; tungsten, molybdenum or one of their alloys is preferably used for the vapor chamber walls. 
     Vapor chamber  64  transfers heat by vaporizing working fluid  87  at an evaporator region  80  near track  66 , and liquefying the vaporized fluid at condenser regions  82  further from track  66 . The walls of vapor chamber  64  taper from evaporator region  80  to condenser regions  82 . As target  60  spins, the tapered structure helps to centrifuge working fluid  87  back to evaporator region  80  (i.e., the area near track  66 ). In addition, the inner surface of vapor chamber  64  may include a wick structure enhancing the surface area of vapor chamber  64  and, consequently, improving the ability to evaporate and condense working fluid  87 . The large volume of vapor chamber  64  gives relatively little resistance to the flow of the vapor. Hence, vapor chamber  64  has a relatively uniform pressure and the evaporation and condensation will take place at nearly the same temperature. Thus, the entire vapor chamber is essentially isothermal. 
     In condenser regions  82  of target  60 , vapors  88  of working fluid  87  condense because the walls are slightly cooler. During the condensation process, heat is given up to the walls of vapor chamber  64  and the heat is subsequently radiated to the walls of frame  62 . The condensation process results in a relatively lower vapor pressure in condenser regions  82 . Due to the pressure gradient in vapor chamber  64 , the evaporated fluid (i.e., the vapors  88 ) will flow to the condenser regions  82  of fins  86 . 
     Hence, heat is effectively transferred from the track region of target  60  to the slightly cooler condenser regions  82 . The condensed fluid is then transferred back to evaporator region  80 , closer to track  66 . The transfer of fluid back to evaporator region  80  is aided by the spinning of target  60  during operation. 
     The fact that evaporation and condensation occur at approximately the same temperature effectively makes vapor chamber  64  isothermal. As such, the process utilized by vapor chamber  64  can quickly transfer the heat from the heated region of target  60  to condenser regions  82  with minimal thermal gradients in the walls of vapor chamber  64 . This results in lower track  66  temperatures because the thermal storage of the rest of the anode is more effectively used. 
     Fins  86  provide a material surrounding condenser regions  82  to aid in the condensation of vapors  88 . Fins  86  can be lengthened as necessary to develop the desired heat dissipation capability. If necessary, mass can be added to target  60  as necessary to aid in extremely high power transient x-ray exposures which exceed the average power rating of vapor chamber  64 . The extra mass will temporarily store the heat energy for later dissipation. 
     The heat radiated by the outer surface of the vapor chamber walls is collected by frame  62 . Frame  62  includes walls which closely conform to vapor chamber  64  at condenser regions  82 . The vacuum side of both target  60  and frame  62  (i.e., the side opposite vapor chamber  64 ) can be modified to enhance the thermal emissivity of the surfaces. Frame  62  is cooled by either a water based, oil based or special thermal fluid liquid in cooling chamber  68 . To enhance the heat transfer capability at the frame/coolant interface, extended surfaces are alternatively built in the structure to enhance mixing of the coolant and to increase the surface area used in the convection process. Coolant is forced through the walls of frame  62 , passing through a coolant inlet  70  and exiting through a coolant outlet  72 . 
     Advantageously, vapor chamber  64  provides greatly enhanced heat dissipation at target  60 , resulting in the capability to do longer and more powerful x-ray scans. Further, vapor chamber  64  provides lower target temperatures. Even further, vapor chamber  64  provides lower mass and smaller targets for a given power rating, enabling higher gantry speeds on CT systems or better angulation on x-ray systems. 
     Referring now to the alternative embodiment shown in FIG. 4, target  60  now includes an extension  63  extending from track  66  in parallel to the rotational axis of target  60 . Extension  63  provides an increased mass of material aiding in the storage of heat from track  66 . Further, this concept may be easier to fabricate. In FIG. 5, a vapor chamber  165  is included as an integral part of extension  63 , and is partially filled with a working fluid. Vapor chamber  165  operates to transfer thermal energy away from track  66  in much the same manner as vapor chamber  64  (i.e., by evaporating the working fluid at evaporator region  80  and condensing the resulting vapors at condenser region  82 ). 
     Vapor chamber  165  can be integrated into extension  63  by different methods. In an exemplary method, vapor chamber  165  is placed within a groove in extension  63 . The groove is created by an electro-discharge machine (EDM). Such a method minimizes the number of brazes required in fabrication. In an alternative method, a series of individual heat pipes  165  are machined into extension  63 . Heat pipe  165  is created within extension  63  by drilling or EDM&#39;ing axially holes which accept heat pipe fluid. Such an alternative method aids in the fabrication process. 
     Referring now to FIGS. 6 and 7, alternative embodiments are shown wherein heat pipes  164  are brazed into extension  63 . Extension  63  is preferably graphite and provides greater thermal storage for a given mass compared to tungsten and TZM. In the embodiment shown in FIG. 7, one long coiled heat pipe  164  is provided. In the embodiment shown in FIG. 8, multiple heat pipes  164  are provided. A person of skill in the art would understand that a variety of such heat pipe configurations are possible. 
     FIG. 8 illustrates a portion  11  of unassembled x-ray tube insert  12 . Portion  11  includes target anode assembly  40 , cathode assembly  42 , vacuum vessel  44 , and stator  46 . The assembly of x-ray tube insert  12  includes locating vacuum vessel  44 , orienting target anode assembly  40  and cathode assembly  42  within vacuum vessel  44 , and fastening a target body  61  to anode assembly  40 . 
     While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Other embodiments may include vapor chambers or heat pipes in different sizes, numbers, and locations. The invention is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations that nevertheless fall within the scope and spirit of the appended claims.