Abstract:
An x-ray tube for emitting x-rays which includes an anode assembly and a cathode assembly is disclosed herein. The x-ray tube includes a vacuum vessel, an anode assembly disposed in the vacuum vessel and including a target, a cathode assembly disposed in the vacuum vessel at a distance from the anode assembly, and a heat pipe is supported relative to the anode assembly. The cathode assembly is configured to emit electrons which hit the target of the anode assembly and produce x-rays. The heat pipe transfers thermal energy away from the target through the vacuum vessel. The heat pipe provides for greater thermal transfer down the bearing shaft of the anode assembly, thereby providing greater cooling of the anode assembly.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to imaging systems. More particularly, the present invention relates to the cooling of rotating anode x-ray tubes. 
     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 across 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. Ultimately, the back scattered electrons are absorbed by components within the vacuum vessel as heat energy. 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 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C. Additionally, all of the components of a conventional x-ray tube insert must be able to withstand the high temperature exhaust processing when the vacuum vessel is evacuated, at temperatures that may exceed very high temperatures for a relatively long duration. 
     To cool the x-ray tube insert, the thermal energy generated during tube operation must be radiated from the anode to the vacuum vessel and be ultimately removed by a cooling fluid circulating over the exterior of the x-ray tube insert vacuum vessel. 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. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bipolar configuration. 
     Additionally, this conventional approach becomes even more problematic when combined with new techniques in x-ray computed tomography, such as fast helical scanning, that require vastly more x-ray flux than previous techniques. Due to the inherent poor efficiency of x-ray production, the increased x-ray flux is purchased at the expense of greatly increased heat load that must be dissipated. As the power of x-ray tubes continues to increase, novel cooling techniques must be developed to remove heat from the rotating anode structures. 
     Rotating anode x-ray tubes are used in mammography, vascular, and computed tomography x-ray systems. Rotating anode x-ray tubes are also ultimately limited in performance by their heat dissipation rate. The bearing components of the rotating anode typically have a temperature limit which is significantly less than the operating temperature of the rotating anode target. Typically, the rotating anode target operates at temperatures over 1000° C. at the target ID. Consequently, the anode target must be thermally isolated from the bearing shaft by a long thermal barrier such that the temperature drop to the bearings closest to the heat source drops the temperature to below the bearing temperature design limit. 
     In a conventional rolling element x-ray tube bearing assembly, very little power is removed down the bearing shaft by design. If too much heat is allowed to go down the shaft, the temperature of the bearing races and solid lubricated ball bearings drastically increases and can exceed an acceptable limit. Such conditions lead to premature failure. Therefore, it is necessary to limit the maximum temperature in the bearings. Conversely, it is also desirable if more power could be transferred down the bearing shaft and out of the tube insert to aid in cooling the target. This would ultimately increase the power available from x-ray tube systems and, consequently, would provide greater subject (e.g., patient) throughput by the x-ray tube systems. 
     Another problem with conventional rotating anode x-ray tubes is that the internal diameter (ID) of the anode target can be extremely hot during operation, thereby reducing the strength of the anode material. This reduction in strength lowers the peak rotational operating speeds of the target. As a result, the peak power at which the x-ray tube can operate is reduced. The limit of anode rotational speed is caused by the peak temperatures under the electron beam. As the target spins faster, the local instantaneous heating under the electron beam is reduced. 
     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 increased performance by more effective heat dissipation. Even further, there is a need for an x-ray tube which operates with a cooler anode, providing the capability of faster anode rotation and greater x-ray tube power. 
     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 vacuum vessel, an anode assembly disposed in the vacuum vessel and including a target, a cathode assembly disposed in the vacuum vessel at a distance from the anode assembly, and a heat pipe supported relative to the anode assembly. The cathode assembly is configured to emit electrons which hit the target of the anode assembly and produce x-rays. The heat pipe transfers thermal energy away from the target to the exterior of the vacuum vessel. 
     Another embodiment of the invention relates to an x-ray tube for emitting x-rays having improved heat dissipation. The x-ray tube includes an electron source emitting electrons, an x-ray source providing x-rays from a bombardment of electrons from the electron source onto a target, and means for locally removing heat energy from the x-ray source. 
     Another embodiment of the invention relates to a method for dissipating heat from an anode including an electron target in an x-ray tube during operation of the x-ray tube. The method includes bombarding the electron target with electrons, the bombardment producing heat, and transferring heat away from the target with a heat pipe. 
     Another embodiment of the invention relates to a method of assembling an x-ray tube having a vacuum vessel, an anode assembly, a cathode assembly, and a heat pipe. The method includes locating a vacuum vessel, orienting an anode assembly and a cathode assembly within the vacuum vessel, and fastening a heat pipe to 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 casing enclosing an x-ray tube insert in accordance with an exemplary embodiment of 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 insert 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 cross sectional view of a secondary embodiment of the anode assembly of the x-ray tube of FIG. 1; 
     FIG. 5 is a perspective view with partial cross section of a heat pipe included in the anode assembly of the x-ray tube of FIG. 1; and 
     FIG. 6 is an exploded view of the x-ray tube insert of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an x-ray tube assembly unit  10  for an x-ray generating device or x-ray tube insert  12 . X-ray tube assembly 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 tube insert  12  is enclosed in a fluid-filled chamber  20  within a casing  22 . 
     Fluid-filled chamber  20  generally is filled with a fluid  24 , such as, dielectric oil, which circulates throughout casing  22  to cool x-ray tube insert  12 . Fluid  24  within fluid-filled chamber  20  is cooled by a radiator  26  positioned to one side of center section  18 . Fluid  24  is moved throughout fluid-filled 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 it. 
     Electrical connections to x-ray tube insert  12  are provided through an anode receptacle  32  and a cathode receptacle  34 . X-rays are emitted from x-ray generating device  12  through a casing window  36  in casing  22  at one side of center section  18 . 
     As shown in FIG. 2, x-ray tube insert  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 thermal energy. The residual energy is absorbed by the components within x-ray tube insert  12  as heat. The x-rays are directed out through an x-ray transmissive window pane  48  and casing window  36 , which allows the x-rays to be directed 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 bearing support  62 , bearings  64 , corrugated bellows  66 , a plug  68 , and a heat pipe  70 . Target  60  is a metallic disk made of a refractory metal with graphite possibly brazed to it. Target  60  provides a surface against which electrons from cathode assembly  42  strike. In the exemplary embodiment, target  60  rotates by the rotation of a bearing shaft  72  coupled to target  60  by a connector  74 . The rotation of target  60  distributes the area on target  60  which is impacted by the electrons. 
     Bearing support  62  is a cylindrical shaft which provides support for target anode assembly  40 . Bearing balls  64  and bearing races  63  are located within bearing support  62  and provide for the rotational movement of target  60  by providing for rotational movement of bearing shaft  72 . Bearing balls  64  and bearing races  63  are made of metal and can become softened and even deformed by excessive heat. As such, distributing the heat away from bearing balls  64  and bearing races  63  is important to the proper rotational movement of target  60  and, hence, the proper operation of the x-ray generating device  12 . 
     Corrugated bellows  66  is a metal structure located at the opposite end of bearing support  62  from target  60 . Plug  68  is a structure made of a heat conducting material, such as, copper. Corrugated bellows  66  and plug  68  are designed to help dissipate heat away from target  60  and bearings  64 . Corrugated bellows  66  and plug  68  define a cavity which is filled with a heat conducting liquid, such as, gallium. Corrugated bellows  66  and plug  68  form a thermal bridge  76  between condenser end  82  of heat pipe  70  and cooling fluid  24  exterior to the vacuum vessel  44 . 
     Heat pipe  70  is an evacuated, sealed metal pipe partially filled with a working fluid. As shown in FIG. 5, the internal walls of heat pipe  70  contain a capillary wick structure  84  extending from an evaporator end  80  to a condenser end  82 . Capillary wick structure  84  allows heat pipe  70  to operate against gravity by transferring the liquid form of the working fluid to the opposite end of heat pipe  70  where it is vaporized by heat. In general, heat pipe  70  conducts heat away from a source of heat such as target  60 . 
     Heat pipes have found wide application in space-based applications, electronic cooling, and other high-heat-flux applications. For example, heat pipes can be found in satellites, laptop computers, and solar power generators. A wide variety of working fluids have been used with heat pipes, including, nitrogen, ammonia, alcohol, water, sodium, and lithium. Heat pipes have the ability to dissipate very high heat fluxes and heat loads through small cross sectional areas. Heat pipes have a very large effective thermal conductivity and can move a large amount of heat from source to sink. A typical heat pipe can have an effective thermal conductivity more than two orders of magnitude larger than a similar solid copper conductor. Advantageously, heat pipes are totally passive and are used to transfer heat from a heat source to a heat sink with minimal temperature gradients, or to isothermalized surfaces. 
     In the exemplary embodiment, heat pipe  70  is made of copper and includes water as a working fluid. Alternatively, heat pipe  70  is made of monel, tungsten, stainless steel or some other high temperature material. Heat pipes can be manufactured using a wide range of materials and working fluids spanning the temperature range from cryogenic to molten lithium. High temperature heat pipes, such as, tungsten tube with lithium as the working fluid can be coupled directly to the ID of the anode to transfer heat from the anode. Heat pipes suitable for this application are commercially available. 
     In operation and as illustrated in either FIG. 3 or  4 , heat from target  60  enters evaporator end  80  of heat pipe  70  where the working fluid is evaporated, creating a pressure gradient in the pipe. The pressure gradient forces the resulting vapor through the hollow core of heat pipe  70  to the cooler condenser end  82  where the vapor condenses and releases its latent heat of vaporization to the heat sink. The liquid is then wicked back by capillary forces through capillary wick structure  84  to evaporator end  80  in a continuous cycle. For a well designed heat pipe, effective thermal conductivities can range from 10 to 10,000 times the effective thermal conductivity of copper depending on the length of the heat pipe. Due to the cooling effect of the target heat pipe, the bore temperature is reduced. As a result, the yield stress in the material of target  60  is increased. As a result, greater hoop stresses caused by rotating target  60  can be accommodated. 
     In the exemplary embodiment, evaporator end  80  is attached to the target bore internal diameter at connector  74  (FIG.  4 ). Heat pipe  70  is thermally isolated from bearing balls  64  and bearing races  63  such that heat conducted through heat pipe  70  does not effect the bearings. Condenser end  82  is located on the opposite side of the bearing support  62 . In one embodiment, a thermal bridge is made between the rotating heat pipe and the stationary frame via a liquid metal, such as, gallium. The thermal bridge allows for conductive and convective cooling of condenser end  82 . One example of such a thermal bridge is corrugated bellows  66  (FIG.  4 ). 
     With heat pipe  70  located at the internal diameter of target  60 , the bore of target  60  runs cooler. As such, target anode assembly  40  is capable of faster rotation, providing greater power. Higher scanning power enables faster scans or thinner slices on a CT scanner. This design also allows for more scanning in a given period of time. For vascular x-ray tubes, the cooling provided by heat pipe  70  allows higher power and longer fluoroscopy and cine operation. In the embodiment illustrated in FIG. 3, heat pipe  70  is located within the ID of bearing shaft  72 . Such a location for heat pipe  70  is particularly advantageous for reducing bearing temperatures. 
     X-ray generating device  12  has the benefits of heat pipe  70  integrated with the bearing shaft of a rotating anode x-ray tube. Heat pipe  70  provides greater heat transfer from the anode target, improving the thermal performance of the x-ray tube. Further, heat pipe  70  provides thermal isolation of the bearing balls  64  and bearing races  63  because the center section of heat pipe  70  is adiabatic through the heat pipe wall and isothermal along its length. Heat pipe  70  also provides improved life of the bearing assembly due to lower operating temperatures. Heat pipe  70  provides direct cooling of the joint between the anode and bearing shaft assembly, preventing it from overheating. Additionally, heat pipe  70  provides for greater rotational speeds of the anode, resulting in higher peak power capability of the x-ray tube. Even further, heat pipe  70  provides less focal spot motion due less thermal growth of the bearing shaft assembly. 
     FIG. 6 illustrates a portion  11  of unassembled x-ray tube assembly unit  10 . Portion  11  includes target anode assembly  40 , cathode assembly  42 , vacuum vessel  44 , and stator  46 . The assembly of x-ray tube assembly unit  10  includes locating vacuum vessel  44 , orienting anode assembly  40  and cathode assembly  42  within vacuum vessel  44 , and fastening heat pipe  70  to anode assembly  40 . X-ray tube assembly unit  10  can be repaired or reconstructed by the assembling of portion  11 . 
     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 heat pipes in other locations of the anode assembly. Although not preferred, heat pipe  70  may alternatively be made at least partially of a solid thermally conductive material, such as, copper. 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.