Abstract:
An x-ray tube for emitting x-rays which includes an anode and a cathode is disclosed herein. The x-ray tube includes a housing, an anode disposed in the housing and including a target, a cathode disposed in the housing at a distance from the anode, and a heat pipe thermally coupled to the cathode and extending away from the electron emitter. The cathode includes an electron emitter which is configured to emit electrons which hit the target of the anode and produce x-rays. The heat pipe provides transfer of thermal energy away from the electron emitter and into a heat sink.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to imaging systems. More particularly, the present invention relates to x-ray tube cathodes 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 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. 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 a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. To cool the x-ray tube, 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 x-ray system or 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. The performance of the cooling fluid may be degraded, however, by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the vacuum vessel and/or the transmissive window. The boiling fluid may produce bubbles within the fluid that may allow high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images. Thus, the current method of relying on the cooling fluid to transfer heat out of the x-ray tube may not be sufficient. 
     As X-ray tubes continue to grow in heat storage capability, the duration of an X-ray scan increases and the cooling time between scans decreases. The longer scans and shorter cool times require that the filaments in the cathode be held at high temperatures for a greater percentage of time. As a result, the cup that holds the filaments experiences higher temperatures than that of prior x-ray tubes. 
     In current high performance CT tubes, it has been observed that these higher temperatures can result in braze failures and distortions in the cathode arm. This results in image quality degradation. A conventional approach to the problem is to make a more conductive thermal path from the cathode cup to the cooler oil that lies in the X-ray tube. However, adding greater thermal conduction typically results in higher mass in the cathode support structure, while only marginally improving thermal performance. The higher mass often results in cathode vibration problems which compromise the x-ray tube&#39;s image quality. 
     Thus, there is a need for an apparatus which significantly increases the heat flow away from the cathode cup, resulting in cooler cathode assembly temperatures. Further, there is a need for a cathode design with greater ability to produce long duration scans without sacrificing image quality or long term reliability of the X-ray tube due to joint failure or mechanical component distortions. Even further, there is a need for a cathode design which greatly increases the heat flow from the cathode cup without producing a lower natural frequency in the cathode design due to added mass, resulting in good image quality while still giving good thermal performance of the cathode assembly. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to an x-ray tube for emitting x-rays which includes an anode and a cathode. The x-ray tube includes a housing, an anode disposed in the housing and including a target, a cathode disposed in the housing at a distance from the anode, and a heat pipe thermally coupled to the cathode and extending away from the electron emitter. The cathode includes an electron emitter which is configured to emit electrons which hit the target of the anode and produce x-rays. The heat pipe provides transfer of thermal energy away from the electron emitter. 
     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, an x-ray source, and heat pipe means for selectively directing heat energy away from the electron source. The x-ray source provides x-rays from a bombardment of electrons from the electron source. 
     Another embodiment of the invention relates to a method for dissipating heat from a cathode in an x-ray tube during operation of the x-ray tube. The method includes providing electrons using an electron emitter in the cathode and transferring heat away from the electron emitter with at least one heat pipe. The electrons produce x-rays and heat upon impact with a target. 
    
    
     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 a cathode assembly of the x-ray tube of FIG. 1; 
     FIG. 3 is a cross sectional view of the cathode assembly of the x-ray tube of FIG. 1; 
     FIG. 4 is a cross sectional view of the cathode assembly of a second embodiment of the x-ray tube of FIG. 1; 
     FIG. 5 is a perspective with partial cross-section of a heat pipe included in the cathode assembly of the x-ray tube of FIG. 1; and 
     FIG. 6 is a perspective view with partial cross-section of a second heat pipe included in the cathode assembly 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  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  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 it. 
     Electrical connections to x-ray generating device  12  are provided through an anode receptacle  32  and a cathode receptacle  34 . X-rays emit from x-ray generating device  12  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 through an x-ray transmissive window pane  48  and window  36 , which help direct the x-rays toward the object being imaged (e.g., the patient). In one embodiment, target anode assembly  40  includes a rotating target which distributes the area impacted by the electrons from the cathode assembly  42 . 
     FIG. 3 illustrates a cross sectional view of cathode assembly  42 . Cathode assembly  42  includes a cathode cup  50 , an arm  52 , a post  54 , a cathode insulator  56 , electrical connectors  58 , and a heat pipe  70 . Cathode cup  50  is made of a high temperature metal and contains filaments which heat up and provide electrons. The temperatures involved in the heating of the filaments are approximately 2600° C. 
     Arm  52  extends between cathode cup  50  and post  54 . Post  54  extends between the end of arm  52  distal to cathode cup  50  and cathode insulator  56 . Cathode insulator  56  is designed in a shape to provide electrical insulation of the high electrical potential cathode parts. Electrical connectors  58  electrically couple filaments in cathode cup  50  with x-ray generating device  12 . 
     Heat pipe  70  is preferably 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  channels or selectively directs heat away from a source of heat such as cathode cup  50 . 
     Heat pipes (as shown in FIGS. 5 &amp; 6) 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 generators. A wide variety of working fluids have been used with heat pipes, including, nitrogen, ammonia, alcohol, water, sodium, lithium, and other suitable fluids. 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 or some other material. Heat pipes can be manufactured using a wide range of materials and working fluids spanning the temperature range from cryogenic to molten lithium. Heat pipes suitable for this application are commercially available. 
     In operation, heat from cathode cup  50  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. 
     Heat pipe  70  greatly increases the heat flow from the source of the heat in the filaments back to the cooler oil that is in x-ray tube casing  22 . Referring now to FIG. 3, heat pipe  70  is coupled to post  54  at one end. The other end of heat pipe  70  is brazed to a braze plate at ceramic insulator  56 . The heat is then transferred from the top of post  54  to ceramic insulator  56  and ultimately is dissipated into the oil contained in vessel  44  and surrounding cathode assembly  42  by convection. 
     FIG. 4 illustrates a cross sectional view of a second embodiment of cathode assembly  42 , including a second heat pipe  72  brazed in arm  52 . Heat pipe  72  increases the transfer of heat away from cathode cup  50  toward the top of post  54 . In this embodiment, heat pipe  70  passes through cathode insulator  56  and is welded to a weld prep on cathode insulator  56  to make a vacuum seal. As such, heat pipe  70  is in direct contact with the cooling oil contained within vessel  44 . Advantageously, heat pipe  70  can also serve simultaneously as one of the electrical paths for the cathode (not shown), in which case heat pipe  70  would take the place of one of the electrical connectors  58 . In the embodiment of cathode assembly  42  shown in FIG. 4, heat pipe  70  can include fin structures  88  at condenser end  82  (FIG.  6 ). Fin structures  88  enhance convective heat transfer to the oil in order to assist in further cooling condenser end  82 . 
     The benefits of cathode assembly  42  with heat pipe  70  (and possibly heat pipe  72 ) include that cathode cup  50  runs significantly cooler. Cooler temperatures permit higher performance of the x-ray tube  12  without causing braze joint failures and cathode bolted joint failures. Cathode assembly  42  includes a greater ability to produce long duration scans and greater patient throughput, without sacrificing image quality or long term reliability of the x-ray tube due to joint failure or mechanical component distortions. In addition, thermal and plastic deformations of arm  52  are eliminated. Further, by removing the joint failures and component distortions, the image quality of the x-ray tube will not be compromised due to thermal issues with the cathode. The light weight of heat pipe  72  will also make it possible to obtain the greater heat transfer from the cathode cup without decreasing the natural frequency of the cathode assembly. Low natural frequencies of the cathode assembly are known to cause image quality problems due the wobbling of the focal spot in the x-ray tube. 
     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 cathode assembly  42 . 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.