Abstract:
An improved x-ray tube is disclosed that eliminates the use of a fluid-filled outer housing for cooling tube components. The present x-ray tube includes a cathode housing in which is disposed a cathode assembly having a filament for producing electrons, and an anode housing in which is disposed a rotary anode for receiving electrons produced by the filament. An adapter plate is sized to hermetically receive therein the cathode assembly and the anode housing, thereby forming a unitary vacuum enclosure and tube housing. The improved x-ray tube is cooled by a combination of air and fluid cooling to reduce the buildup of damaging heat within the vacuum enclosure. The features of the present invention are preferably directed to an anode grounded-type x-ray tube, though other x-ray tube types may benefit therefrom.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention generally relates to x-ray tube devices. More specifically, the present invention relates to an x-ray tube wherein the need for a fluid-filled outer housing is eliminated. 
     2. The Relevant Technology 
     X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis. 
     Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. The entire outer housing is typically covered with a shielding layer composed of lead for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium, such as a dielectric oil, is typically disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. The oil may be cooled by circulating it to an external heat exchanger via a pump and fluid conduits disposed in the outer housing. 
     In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. In anode grounded x-ray tubes, a high negative electric potential is placed on the cathode while the anode is electrically grounded. This causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon approaching and striking the target surface, many of the electrons convert their kinetic energy and either emit, or cause the target surface material to emit, electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The x-rays are then collimated so that they exit the x-ray device through windows disposed in the vacuum enclosure and outer housing, and enter the x-ray subject, such as a medical patient. 
     A recurrent problem encountered with the operation of x-ray tubes deals with the removal of heat therefrom. In general, only a small percentage of the electrons that impact the anode target surface during x-ray production do, in fact, produce x-rays. The majority are instead absorbed into the anode target surface and surrounding areas, thereby creating large quantities of heat. This heat must be continuously and reliably removed from the anode and surrounding areas in order to prevent damage to critical tube components. To the extent that the heat is efficiently removed, less thermal and mechanical stress is imposed upon the x-ray tube, and its operation and performance will be enhanced. If the heat is allowed to buildup to detrimental levels, however, it can damage the anode and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube. 
     Many approaches have been implemented to help alleviate the problems created by heating within the x-ray tube. For instance, in many x-ray tubes the anode, which typically comprises a substrate and a target surface disposed thereon, is formed in the shape of a disk. The rotary anode (also referred to as the rotary target or the anode disk) is then mounted on a supporting shaft and rotor assembly that can then be rotated by some type of motor, such as a stator. During operation of the x-ray tube, the rotary anode is rotated at high speeds, which causes successive portions of the target surface to continuously rotate into and out of the path of the electron beam produced by the cathode filament. In this way, the electron beam is in contact with any given point on the target surface for only short periods of time. This allows the remaining portion of the surface to cool during the time that it takes to rotate back into the path of the electron beam, thereby spreading the heat absorbed by the anode. 
     While the rotating nature of the anode reduces the amount of heat present at the target surface, a large amount of heat is still absorbed by the anode substrate, the rotor assembly, the cathode, and other components within the vacuum enclosure. This heat must be continuously and reliably removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube&#39;s efficiency and overall service life. 
     One approach has been to place the vacuum enclosure within an outer housing, as mentioned above. This outer housing must serve several functions. First, it must act as a radiation shield to prevent radiation leakage resulting from the production of x-rays within the vacuum enclosure. To do so, the can must include a radiation shield, which must be constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which surrounds and envelops the vacuum enclosure, and which may be continuously circulated by a pump about the outer surface thereof As heat is emitted from the x-ray tube components (anode, support shaft, etc.), it is radiated to the outer surface of the vacuum enclosure, and then at least partially absorbed by the dielectric oil. The heated oil is then passed to some form of heat exchange device, such as a radiative surface, and then cooled. The oil is then recirculated by the pump back through the outer housing and the process repeated. 
     While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquid coolants/dielectrics that surround and envelop the vacuum enclosure can be problematic in several respects. For example, use of large amounts of cooling fluid adds complexity to the construction and operation of the x-ray generating device. Use of fluid to envelop the vacuum enclosure requires that there be an outer housing as outlined above to retain the fluid. This outer housing must be constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing to retain the cooling fluid. This increases the cost and manufacturing complexity of the device. Also, the outer housing requires a large amount of physical space, resulting in the need for a larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner vacuum enclosure, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates. 
     In light of the above discussion, therefore, a need exists for an x-ray tube that eliminates the problems associated with fluid-filled outer housings. Further, a need exists to provide an x-ray tube whereby sufficient cooling of the vacuum enclosure is efficiently attained, thereby improving the performance and longevity of the x-ray tube. Moreover, an x-ray tube having a simple construction and flexible design would be an advancement in the art. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the needs outlined above, an improved x-ray tube is provided wherein the housing thereof comprises a unitary vacuum enclosure in which is disposed the cathode, anode, and associated components. The heat created by components of the present x-ray tube is cooperatively dissipated by way of limited fluid and air cooling systems. In this way, problems associated with an outer housing and a cooling fluid disposed therein are avoided. 
     Generally, the present x-ray tube comprises an adapter plate to which is connected a cathode assembly and an anode housing. These three components are hermetically attached such that they form the unitary vacuum enclosure of the tube. The vacuum enclosure is designed in such a way as to not require a cooling fluid to envelop it; rather a directed fluid cooling system is combined with an air cooling system to efficiently cool the x-ray tube components during operation. 
     In one presently preferred embodiment, the present x-ray tube comprises an adapter plate composed of stainless steel. A first hole is defined on the adapter plate to receive a portion of a cathode assembly, which comprises a filament and a cathode shield. A cathode housing is preferably disposed about the cathode assembly. The cathode assembly and adapter plate are sealably attached to one another, with the cathode assembly being disposed on a first side of the adapter plate. On a second side of the adapter plate, an anode housing, in which is disposed an anode assembly, is sealably attached. The anode assembly is rotatably supported within the anode housing via a support shaft, which is in turn received by and fixedly attached to a second hole defined in the adapter plate. 
     A circular cavity is preferably defined on the second side of the adapter plate. The inner surface of the cavity defines a volume that is sized to receive therein a portion of the target surface of the rotary anode. A window is disposed in a hole defined on the edge of the adapter plate such that x-rays produced on the target surface of the rotary anode are emitted through the window. 
     Specific portions of the x-ray tube of the present invention are cooled by a fluid, such as water, thereby dissipating heat from those areas of the tube where heat buildup is most likely to occur. Unlike the cooling that occurs with fluid-filled outer housings, however, the fluid cooling system of the present invention utilizes fluid passageways defined in portions of the cathode and the anode housing to direct the cooling fluid. During tube operation, heat produced in the anode and cathode portions of the present x-ray tube is conducted to the regions immediately surrounding the fluid passageways. The heat is then absorbed by the circulating cooling fluid and removed from the tube by a pump. The heated fluid then enters a cooling unit, such as a heat exchanger or radiator, where the fluid is cooled and conditioned before being re-circulated into the fluid passageways within the x-ray tube. 
     In addition to fluid cooling, the present x-ray tube also utilizes air cooling to remove heat from tube components such as the stator. In a preferred embodiment, a fan shroud is disposed about the stator, which in turn is disposed about and affixed to a portion of the anode housing. The shroud has defined in its outer surface air inlet holes and air outlet holes. The holes provide a supply and escape for air that is circulated past the stator by way of a fan disposed near the bottom of the fan shroud. Heat that is produced by the stator during tube operation is transmitted to the air circulating past it, which air then exits the shroud via the outlet holes. 
     Portions of the anode housing, adapter plate, and cathode housing are preferably shielded to prevent the escape of x-rays from within the vacuum enclosure. Such shielding may be provided by a layer of lead or other suitable material disposed on the exterior of the vacuum enclosure. 
     Alternative embodiments include modifications to the adapter plate such that the window is disposed in the anode housing instead of the plate, and disposing an extension ring between the anode housing and adapter plate in order to more easily join the two components. 
     The improved x-ray tube of the present invention is simpler and smaller than previous tubes having a fluid-filled outer housing. The simplicity of the present tube reduces manufacturing costs, while its smaller size provides additional options with respect to the physical placement of the tube in a particular application, or possible enhancements thereto (such as the placement therein of a larger diameter anode) that were impossible before because of space restrictions within the tube caused by the outer housing and the cooling fluid disposed therein. Further, the present x-ray tube is lighter than the prior design, which improves ease of handling and operating. Though features of the present invention are preferably directed to x-ray tubes having electrically grounded anodes, other x-ray tube configurations may also benefit where a fluid-filled outer housing is not desired. 
     These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a cross-sectional side view of the x-ray tube as provided in one presently preferred embodiment of the present invention; 
     FIG. 2A is perspective view of the adapter plate included in the presently preferred embodiment of the present invention; 
     FIG. 2B is a cross-sectional side view of the adapter plate of FIG. 2A, taken along the line B—B; 
     FIG. 3A is a top view of a cathode sleeve as disclosed in accordance with a presently preferred embodiment of the present invention; 
     FIG. 3B is a cross-sectional side view of the cathode sleeve of FIG. 3A, taken along the line B—B; 
     FIG. 4 is a cross-sectional side view of another embodiment of the x-ray tube of the present invention; 
     FIG. 5A is a perspective view of the adapter plate included in another embodiment of the present invention; 
     FIG. 5B is a cross-sectional side view of the adapter plate of FIG. 5A, taken along the line B—B; and 
     FIG. 6 is a cross-sectional side view of yet another embodiment of the x-ray tube of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of presently preferred embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale. FIGS. 1-6 depict several embodiments of the present invention, which is directed to an improved x-ray tube having a unitary vacuum enclosure and housing. An x-ray tube constructed in this manner is smaller, lighter, and less problematic than prior tube designs utilizing a fluid-filled outer housing. 
     Reference is first made to FIG. 1, wherein is depicted one preferred embodiment of the present x-ray tube, designated generally at  10 . The present x-ray tube  10  generally comprises a cathode portion  50 , an anode portion  100 , an adapter plate  150 , a fan portion  200 , and a fluid circulation system  250 . A part of the cathode portion  50 , together with the anode portion  100  and the adapter plate  150  hermetically attach together to form a unitary vacuum enclosure  300 . The unitary enclosure  300 , in conjunction with the complementary components to be described further below, provides the necessary envelope for housing the critical components of the tube  10  while providing the shielding and cooling necessary for proper x-ray tube operation. 
     Continuing reference is made to FIG. 1, wherein details of various portions of the improved x-ray tube  10  are discussed. The cathode portion  50 , generally comprising a cathode assembly  52  and a cathode housing  54 , is responsible for supplying an stream of electrons for producing x-rays as previously described. The cathode housing  54  preferably comprises a hollow cylindrical tube having an open end  54 A and a closed end  54 B, which includes an opening to accommodate a high voltage connector assembly  55 . The housing  54  preferably comprises stainless steel or similar material. A shielding layer  54 C of lead or the like is preferably disposed on the inner surface of the housing  54 . 
     The interior volume of the cathode housing has disposed within it the cathode assembly  52 . The cathode assembly  52  includes a filament  56  supported by a filament support structure  56 A. The filament  56  is appropriately connected to an electrical power source (not shown) to enable the production by the filament of high-energy electrons. A cathode aperture shield  58  defines an aperture  60  that is positioned between the filament  56  and the anode target surface  112  to allow electrons  62  to pass. The aperture shield  58  is affixed to the support structure  56 A such that a hermetic seal is formed between the aperture shield  58  and the support structure. The aperture shield  58  is preferably cooled via a fluid cooling system  250 , described further below. Such cooling is preferable in order to remove heat that is created in the aperture shield  58  as a result of errant electrons impacting the aperture shield surface. 
     Continuing reference is made to FIG. 1, wherein details of the anode portion  100  are disclosed. The anode portion  100 , generally comprising an anode assembly  102  and an anode housing  104 , is responsible for receiving the electrons  62  produced by the filament  56  of the cathode assembly  52  and converting them into x-rays  160  to be emitted by the improved x-ray tube  10 . The anode housing  104  comprises a hollow cylindrical body, preferably composed of stainless steel or similar material, having an open end  104 A and a closed end  104 B. The central portion of the closed end  104 B further defines a hollow cylindrical extension  104 C that extends below the plane in which the rest of the closed end  104 B is disposed. It is noted here that words such as above, below, upper, and lower, are merely descriptive terms used to enable a sufficient description of the present invention to be made. Indeed, it is appreciated that the present improved x-ray tube  10  could be disposed in a variety of spatial orientations without affecting the functionality thereof. Accordingly, such words are not meant to limit the scope of the present invention in any way. 
     The anode assembly  102  is disposed within the volume created by the anode housing  104 . The anode assembly  102  comprises an anode  106 , and an anode support assembly  108 . The anode  106  comprises a substrate  110  preferably composed of graphite, and a target surface  112  disposed thereon. The target surface  112  preferably comprises TZM or a similar alloy. The graphite substrate  110  has defined on its lower surface a plurality of concentric annular grooves  114 . The grooves  114  cooperatively interleave with concentric annular extended surfaces  116  defined on the closed end  104 B of the anode housing  104 . The grooves  114  and the extended surfaces  116  are designed to continuously remove heat from the anode  106  during tube operation. As mentioned above, large quantities of heat accumulate in the anode  106  during x-ray production. Much of that heat is conducted to the graphite substrate  110  surrounding the grooves  114 . The spacing between the grooves  114  and the extended surfaces  116  is minimal, thereby allowing the heat created in the anode  106  during tube operation to be radiated from the grooves to the extended surfaces despite the rotation of the anode. The heat is then removed from the extended surface  116  via the fluid cooling system  250  as discussed further below. Stainless steel is a preferred material from which to form the extended surfaces  116  because it possesses a low coefficient of thermal expansion, that is, it tends not to greatly expand when heated. This is a critical quality for the extended surfaces to possess, given the minimal clearance that exists between the extended surface  116  and the grooves  114 . Advantageously, graphite, in which the grooves  114  are defined, also possesses a low thermal expansion coefficient. 
     The anode  106  is supported by the anode support assembly  108 , which generally comprises a bearing assembly  118 , a support shaft  120 , and a rotor sleeve  122 . The support shaft  120  is fixedly attached to a portion of the adapter plate  150 , as described further below. The anode  106  is rotatably disposed about the stationary support shaft  120  via the bearing assembly  118 , thereby enabling the anode to rotate with respect to the support shaft  120 . The bearing assembly  118  and the rotor sleeve  122  substantially occupy the volume created by the cylindrical extension  104 C of the anode housing  104 . A stator  124  is circumferentially disposed about the anode housing extension  104 C and the rotor sleeve  122  disposed therein. As is well known, the stator utilizes rotational electromagnetic fields to cause the rotor sleeve  122  to rotate. The rotor sleeve  122  is fixedly attached to the anode  106 , thereby providing the needed rotation of the anode during tube operation. 
     Attention is now directed to FIGS. 1,  2 A, and  2 B, wherein details concerning the adapter plate  150  are disclosed. Preferably, the adapter plate  150  comprises a circular stainless steel slab having a first side  150 A, a second side  150 B, and a circumferential edge  150 C. Alternatively, however, the plate, instead of having a circular shape, may comprise one of a variety of shapes and dimensions, such as a square or oval, as may be appreciated by one of skill in the art. In place of stainless steel, similar materials, such as tungsten alloys, could also be utilized to form the adapter plate  150 . The adapter plate  150  may be formed using known techniques, such as casting, machining, or milling. 
     Preferably, an inner cavity  152  is defined by an inner surface  153  that inwardly extends from the second side  150 B of the plate  150 . The cavity  152  is sized to receive therein a portion of the anode  106 , as explained below. Two holes  154  and  156  are defined on the first side  150 A of the plate  150  and extend to the cavity  152 . The first hole  154  is defined off-center on the plate  150  and is sized to fixedly receive therein a portion of the cathode assembly  52 . Specifically, a hermetic seal is formed between the cathode aperture shield  58  and the adapter plate  150  by known methods, such as brazing or welding. About the first hole  154  may be defined a circular depression  154 A for receiving therein the open end  54 A of the cathode housing  54 . The housing is also preferably brazed or welded to the adapter plate  150 . 
     The second hole  156  is defined by the adapter plate  150  and enables the anode assembly  102  to be rotatably supported by the plate. Preferably, the hole  156  is sized to receive therein, and have brazed thereto, a hollow, cylindrical mounting sleeve  158 . The sleeve  158 , in turn, fixedly receives one end of the support shaft  120 . The attachment of the support shaft  120  to the sleeve  158  may be accomplished by brazing, welding, interference fitting, or by other known methods. Additionally, it is appreciated that there exist other ways by which the anode assembly  102  may be supported within the unitary vacuum enclosure  300 . Such other ways are accordingly contemplated as residing within the claims of the present invention. 
     The open end  104 A of the anode housing  104  is affixed to the second side  150 B of the adapter plate  150  in such a way as to form a hermetic seal therebetween. Such a seal may be formed by brazing or welding the two components together. In this way, the unitary vacuum enclosure  300  is formed by the hermetic junction of the cathode assembly  52 , the adapter plate  150 , and the anode housing  104 , as shown in FIG.  1 . The filament  56  and the target surface  112 , both being disposed within the vacuum enclosure  300  of the anode  106 , are aligned such that the electrons  62  that are produced and emitted by the filament  56  during tube operation are able to pass through the cathode aperture shield  58 , the cathode assembly mounting hole  154 , and into the inner plate cavity  152 , where they strike the rotating target surface  112 , thereby producing x-rays  160 , as previously outlined. 
     Preferably, the anode  106  is disposed within the space created by the interior volume of the anode housing and the inner plate cavity  152  such that the target surface  112  of the anode  106  resides within the inner plate cavity. A tunnel  162  is defined in the circumferential edge  150 C of the adapter plate  150  that extends to the inner cavity surface  153 . A window  164 , preferably comprising beryllium, is disposed at the outer end of the tunnel  162  such that x-rays produced by the impingement of electrons on the target surface  112  during tube operation pass through the tunnel and exit the window. Alternatively, the window may be disposed in other locations in the improved x-ray tube  10 , as discussed further below. 
     In the presently preferred embodiment, portions of the improved x-ray tube  10  are air cooled by the fan portion  200  to remove heat from the tube. Specifically, the stator  124  is air-cooled by continuously circulating air thereabout. To accomplish this, the fan portion  200  preferably comprises a fan shroud  202 , preferably composed of stainless steel and disposed both about the stator  124  and about the extension  104 C of the anode housing  104 . The shroud  202  preferably has a hollow cylindrical shape and is fixedly attached to the closed end  104 B of the anode housing  104 via welding, brazing, or the like. A fan  204  is disposed near the bottom of the fan shroud  202 , as depicted in FIG.  1 . The fan  204  is powered by a motor (not shown) and is oriented to direct a continuous stream of air over the outer surface of the stator  124 . One or more air inlet ports  206  are disposed near the bottom of the fan shroud, as are one or more air outlet ports  208  disposed near the top thereof. During tube operation, air is drawn in through the air inlet ports  206  by the circulation of the fan  204 , which air is then forced over the stator  124 . The heat created by the operation of the stator  124  is radiated to the passing air before the air exits the air outlet ports  208 . In this 
     In order to prevent the emission of x-rays from the unitary vacuum enclosure  300  during tube operation, the presently preferred embodiment of the improved x-ray tube  10  has disposed about portions of the tube a layer of shielding  210  capable of absorbing an x-ray  160  that may escape. Preferably composed of lead, the shielding  210  may comprise two parts. A first shielding portion  210 A comprises a hollow cylindrical tube circumferentially disposed about the anode housing  104  between the open end  104 A and the closed end  104 B. The shielding portion  210 A has one end thereof fixedly attached to the top end of the fan shroud  202 . A brazement or weld may be utilized to attach the shielding portion  210 A to the fan shroud  202 . The shielding portion  210 A preferably possesses an inner diameter that is larger than the outer diameter of the anode housing  104  such that a small spacing exists therebetween. Alternatively, the shielding portion  210 A may disposed about the anode housing by other methods, including affixing it to the outer surface of the anode housing  104 , or chemical deposition to the anode housing. No shielding is required on the closed end  104 B or the extension  104 C of the anode housing  104  because the anode substrate  110  sufficiently absorbs any x-rays  160  that are emitted in that direction. 
     A second shielding portion  210 B is disposed about the outer surface of the adapter plate  150 . As can be seen in FIG. 1, the shielding portion  210 B preferably extends as a covering around the interface of the adapter plate  150  with the end  54 A of the cathode housing  54 , around the window  164 , over the mounting sleeve  158  and the end of the support shaft  120 , and to the edge of the shielding portion  210 A. Like the shielding portion  210 A, the shielding portion  210 B preferably comprises lead. The portion  210 B is preferably affixed to the adapter plate  150  by brazing, welding, or the like. Alternatively, the portion  210 B may be deposited on the outer surface of the adapter plate  150  by chemical deposition or similar processes. Moreover, shielding portions  210 A and  210 B might be integrally formed such that they comprise a unitary structure. Or, several shielding portions could comprise the shielding  210 . Such and other modifications to the shielding  210  that may be appreciated by one who is skilled in the art are accordingly appreciated as residing within the claims of the present invention. 
     In addition to the air cooling of the present x-ray tube  10  as described above, the improved x-ray tube is also preferably fluid cooled in selected areas where specific heat removal is desired. Specifically, the cathode aperture shield  58  and the anode  106  are cooled via the fluid circulation system  250 . The fluid circulation system  250  is used to continuously circulate a cooling fluid, such as water, through areas of the x-ray tube during operation thereof, in order to cool those areas. As contemplated herein, “fluid” is understood to comprise liquids and gases, or a mixture thereof. 
     As seen in FIG. 1, the fluid circulation system  250  first comprises fluid passageways defined in the cathode aperture shield  58  and the anode housing  104 . The aperture shield  58  preferably includes on its exterior surface a plurality of extended surfaces, or cooling surfaces  64 . The outer ends of the cooling surfaces  64  may abut against a cathode sleeve  66  that is circumferentially disposed about the aperture shield  58  such that multiple fluid passageways  68  are created between adjacent cooling surfaces. The fluid passageways  68  are in fluid communication with fluid inlet and outlet ports  70 A and  70 B defined in the cathode sleeve  66  (see FIGS.  3 A and  3 B). Further details concerning this portion of the fluid circulation system  250  are disclosed in an application for United States patent filed on Jul. 12, 1999, Ser. No. 09/351,579, which is hereby incorporated by reference in its entirety. The fluid inlet port  70 A is in fluid communication with a fluid input conduit  252 A, which connects to a fluid pump  254  and a cooling unit  256 , as described more fully below. 
     The closed end  104 B of the anode housing  104  also preferably defines a plurality of interconnected annular fluid passageways  258 . The fluid passageways  258  are preferably in fluid communication with the fluid passageways  68  of the cathode aperture shield  58  such that cooling fluid may circulate in series first through the passageways  68  of the cathode aperture shield, then through the passageways  258  of the anode housing  104  during the fluid cooling/circulation process described below. 
     The aforementioned fluid circulation system  250  is utilized to provide critical tube cooling during the production of x-rays. One area where cooling is needed is the cathode aperture shield  58 . During tube operation, a significant portion of electrons produced by the filament  56  impacts the inner portion of the cathode aperture shield  58 , thereby creating heat. This heat is conducted through the aperture shield  58  to the cooling surfaces  64 . Cooling fluid is pumped through the inlet port  70 A of the cathode sleeve  66  and into the fluid passageways  68  of the aperture shield  58  via the fluid pump  254 . As the cooling fluid circulates past the cooling surfaces  64 , heat is transferred from the surfaces to the fluid, thereby removing the heat from the cathode aperture shield  58 . The cooling fluid is still capable of accepting more heat, however, and thus it is pumped through the outlet port  70 B toward the anode housing  104 . 
     As discussed above, heat created in the anode  106  during tube operation is conducted to the area of the anode substrate  110  immediately surrounding the annular grooves  114 . This heat is continuously radiated from the substrate  110  to the annular extended surfaces  116  defined on the anode housing closed end  104 B that interleave with the grooves  114 . The heat is then conducted to the area immediately surrounding the fluid passageways  258 . Cooling fluid received from the outlet port  70 B of the cathode sleeve  66  is introduced through an inlet port (not shown) in the anode housing  104 . The cooling fluid circulating through the fluid passageways  258  remove the heat that has been conducted to the area of the anode housing closed end  104 B immediately surrounding the fluid passageways  258 . The cooling fluid, after receiving the heat, then exits the anode housing  104  through a fluid outlet port (not shown), where it is transported via the outlet conduit  252 B to the pump  254  and cooling unit  256 , where the fluid is cooled and conditioned. After conditioning, the cooling fluid is recirculated by the pump  254  to the interior of the x-ray ray tube  10  via the input conduit  252 A to again cool the cathode aperture shield  58  and the anode assembly  102 . In this way, excessive heat is continuously and reliably removed from the x-ray tube  10  by a cooling fluid, thereby protecting the tube components from excessive heat damage. 
     Though the above fluid cooling process circulates the cooling fluid first to the fluid passageways defined in the cathode aperture shield, then to the fluid passageways defined in the anode housing, the reverse process could also be utilized. Also, other or additional areas of the x-ray tube could have defined therein fluid passageways through which the cooling fluid could flow to remove heat therefrom. Moreover, additional components could comprise the fluid circulation system  250 , as may be appreciated by one of skill in the art. Accordingly, the fluid circulation system  250  disclosed herein is exemplary of a preferred embodiment thereof, and should not be considered as limiting the scope of the present invention in any way. 
     Reference is now made to FIGS. 4,  5 A, and  5 B, wherein details of another embodiment of the present invention are disclosed. As depicted in the figures, the improved x-ray  10  includes an adapter plate  350  having a first side  350 A, a second side  350 B, and a circumferential edge  350 C. The adapter plate is hermetically attached to both the cathode assembly  52  and the anode housing  104 , as before. Also as in the preferred embodiment, two holes  154  and  156  are defined on the first side  350 A of the plate  350  for receiving a portion of the cathode assembly  52  and a portion of the anode support assembly  108 . In contrast to the previous preferred embodiment, however, the second side  350 B of the adapter plate  350  is planar and does not include a cavity  152  defined therein for receiving any portion of the target surface  112  of the anode  104 . Instead, the anode  104  is suspended via the anode support assembly  108  such that the target surface  112  is disposed a small distance below the second side of the adapter plate  350 . In this embodiment, then, the two holes  154  and  156  extend to the second side  350 B of the adapter plate  350 , and the window  164  is disposed in the anode housing  104  below the adapter plate  350 . 
     Reference is now made to FIG. 6, wherein is depicted yet another embodiment of the present x-ray tube  10 . As in the previous embodiment, the x-ray tube  10  includes an adapter plate  350  having hermetically attached thereto the cathode assembly  52 . In this embodiment, an extension ring  400  is also hermetically attached to the adapter plate  350  via brazing, welding, or the like. The extension ring  400  comprises an annular band preferably composed of stainless steel, and has disposed within it the window  164  through which the x-rays  160  may pass. The anode housing  104  is hermetically attached (by brazing, welding, or the like) to the opposite end of the band to which the adapter plate  350  is attached. Thus the unitary vacuum enclosure  300  in this embodiment comprises the cathode assembly  52 , the adapter plate  350 , the extension ring  400 , and the anode housing  104 . The extension ring  400  of the present embodiment may be utilized to more easily accommodate the mating of the anode housing  104  to the adapter plate  350 . To this end, the extension ring  400  may comprise a variety of shapes and configurations in order to provide a joining surface for both the anode housing  104  and the adapter plate  350  that facilitates brazing or welding. The extension ring  400  may also be utilized to facilitate the joining of an existing anode housing to an adapter plate, thereby making possible the retrofitting of existing anode housings to adapter plates in order to produce the present improved x-ray tube  10 . 
     In summary, the present invention features a means for joining a cathode assembly to an anode housing to form the unitary vacuum enclosure and housing of the present improved x-ray tube. One such means is provided in FIG. 1, wherein an adapter plate hermetically receives the cathode assembly and the anode housing. This arrangement makes possible the elimination of a fluid-filled outer housing typically used to contain and cool the vacuum enclosure. Limited fluid and air cooling of the vacuum enclosure is achieved through a fan assembly disposed near the tube stator, and fluid passageways defined in the cathode aperture shield and the anode housing. Other means for joining the cathode assembly to the anode housing are provided in FIGS. 3 and 6, wherein the adapter plate comprises different physical dimensions and features, and wherein the plate may also include an extension ring for facilitating the joining of the anode housing thereto. Indeed, the present invention contemplates that the size, shape and physical features of the adapter plate and connected components of the present invention could be modified as appreciated by one of skill in the art, while still preserving the functionality disclosed herein. Embodiments of the present x-ray tube in addition to those discussed herein, then, are accordingly contemplated as residing within the present invention. 
     The improved x-ray tube of the present invention presents a simpler and cheaper design than known tubes. Because of the elimination of the fluid-filled outer housing normally disposed about known x-ray tubes, the present x-ray tube may be manufactured to be smaller and lighter than prior devices. As a result, the present x-ray tube may be manufactured at a lower cost than previously possible. Further, the space saved by the improved design enables the utilization of that space by other tube components, thereby further enhancing the performance of the present x-ray tube. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.