Patent Publication Number: US-7583791-B2

Title: X-ray tube target assembly and method of manufacturing same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 11/161,778, filed on Aug. 16, 2005, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   This disclosure relates generally to an X-ray tube, and more particularly to an injection molded segmented target assembly for an X-ray tube that is designed for high-speed operation. 
   Modern medical imaging systems have increased in complexity and imaging capabilities. As computed tomography (CT) imaging systems increase gantry speed in order to image organs and other structures with increasing detail, X-ray tube requirements must increase as well. At these higher gantry speeds, parameters like peak power and anode target rotational speed must be optimized in order to meet the high demands of next generation X-ray tubes. An X-ray tube generally includes a cathode assembly and an anode assembly disposed within a vacuum vessel. The anode assembly includes an X-ray tube target assembly. The X-ray tube target assembly typically consists of a rectangular cross-section target disk that is machined at its periphery to include an angled surface creating an impact zone for an electron beam from the cathode assembly for X-ray generation. The target disk is commonly a rotating disk. With higher peak power requirements, higher rotational speeds and thermal loads on the target disk, the simple rectangular cross-section is no longer sufficient. The increased rotational speeds of the target disk may result in high stresses to the hub portion of the target disk that exceeds present design criteria. The hub portion is the center portion of the target disk that is coupled to a drive shaft. As target disk geometries become more complex, the typical manufacturing and machining processes, such as pressing, sintering and forging used today results in a highly inefficient process. The manufacturing and machining operations are more numerous and more complicated with the cost of parts increasing significantly. 
   The cathode assembly is positioned at some distance from the anode assembly creating a vacuum gap between the cathode assembly and the anode assembly, and a high voltage potential difference is maintained therebetween. The cathode assembly emits electrons in the form of an electron beam that are accelerated across the potential difference and impact the target disk at a focal spot of the impact zone at a high velocity. As the electrons impact the impact zone of the target disk, the kinetic energy of the electrons is converted to high-energy electromagnetic radiation, or X-rays. The X-rays are then transmitted through a window in the X-ray tube to an object such as the body of a patient and are intercepted by a detector that forms an image of the object&#39;s internal anatomy. 
   In any X-ray tube target assembly design it is likely that the target disk or portions thereof will suffer damage during prolonged usage. This is simply a result of the target disk being impacted by an electron beam to facilitate the generating of X-rays. When the wear or damage becomes too great, existing designs require complete replacement of the target disk. Disassembly and repair is not contemplated by existing designs and may be impractical based on design configurations and associated costs. Since such wear and damage may only occur on certain portions of the target disk, a design where only those portions of the target disk are replaced would be beneficial. In addition, where repair is still not cost effective, a design that allowed reuse of certain portions of the target disk would provide desirable cost benefits. 
   Therefore, it would be highly desirable to have an X-ray tube target assembly that allows for simplified replacement of worn or damaged portions of the target disk. It would also be highly beneficial to have an X-ray tube target assembly that was manufactured under new manufacturing processes and is capable of withstanding high peak power requirements, high rotational speeds and increased thermal requirements of modern anode assembly performance. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In an embodiment, an X-ray tube target assembly comprising an injection molded target disk. 
   In an embodiment, an X-ray tube target assembly comprising a substantially planar circular-shaped hub member with a central opening extending therethrough for attachment to a drive shaft; a substantially planar circular-shaped outer member removably attachable to an outer perimeter of the hub member; and a target track formed on an outer surface on one side of an outer periphery of the outer member. 
   In an embodiment, an X-ray tube target assembly comprising a substantially planar circular-shaped hub member with a central opening for attachment to a drive shaft; a plurality of substantially planar pie-shaped outer member segments removably attached together and removably attached to an outer perimeter of the hub member; and a target track formed on an outer surface on one side of an outer periphery of each of the plurality of outer member segments. 
   In an embodiment, an X-ray tube target assembly comprising an injection molded hub member with a central opening extending therethrough for attachment to a drive shaft; a plurality of injection molded outer member segments removably attachable together and removably attachable to an outer perimeter of the hub member; and a target track formed on an outer surface on one side of an outer periphery of the outer member. 
   In an embodiment, a method of constructing an X-ray tube target assembly comprising injection molding a substantially planar circular-shaped hub member with a central opening attachable to a drive shaft; injection molding a substantially planar circular-shaped outer member; removably attaching the injection molded outer member to the injection molded hub member; and forming a target track on an outer surface of one side of an outer periphery of the injection molded outer member. 
   Various other features, aspects, and advantages will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cut away perspective view of an exemplary embodiment of an X-ray tube assembly; 
       FIG. 2  is a partial cross-sectional view of an exemplary embodiment of an X-ray tube anode assembly; 
       FIG. 3  is a top plan view of a portion of an exemplary embodiment of an X-ray tube target assembly; 
       FIG. 4  is flow diagram of an exemplary embodiment of a method of constructing an X-ray tube target assembly; and 
       FIG. 5  is flow diagram of an exemplary embodiment of a method of constructing an X-ray tube target assembly. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings,  FIG. 1  illustrates an exemplary embodiment of an X-ray tube assembly  10 . The X-ray tube assembly  10  includes a tube casing  12  enclosing a cathode assembly  14  and an anode assembly  18 . The tube casing  12  provides a vacuum housing for the anode assembly  18  and the cathode assembly  14 . The anode assembly  18  includes a drive assembly  20  rotating a drive shaft  22  that rotates an X-ray tube target assembly  24 . The X-ray tube target assembly  24  includes a target disk  28  that is attached to the drive shaft  22  and driven by the drive assembly  20  to rotate the target disk  28  at high speeds. In an exemplary embodiment, the X-ray tube target assembly  24  may include a plurality of injection molded parts that are removably attached together. 
   The target disk  28  includes a hub member  26  attached to the drive shaft  22 , an outer member  32  attached to the hub member  26 , and a target track  30  formed on an outer surface  34  on one side  36  of the outer periphery  38  of the outer member  32 . The target track  30  is designed for receiving a bombardment of electrons from an electron beam generated by the cathode assembly  14  for the generation of X-rays. 
   In an exemplary embodiment, the outer member  32  may be removably attached to the hub member  26  such that if the target track  30  experiences undesirable levels of wear or damage, the outer member  32  may be replaced while the hub member  26  remains. In an exemplary embodiment, the outer member  32  may comprise a plurality of outer member segments that are removably attached together and attached to the hub member  26 , as will be further discussed with reference to  FIGS. 3 and 4  below. In addition to replacement, the present design allows to cost savings through reuse of non-damaged portions of the X-ray tube target assembly  24  in new assemblies. The material used to manufacture the target disk  28  is very expensive. This design allows the reuse of portions of the target disk  28  to provide beneficial cost savings. In addition, the hub member  26  may be optimized to withstand the stresses transmitted to it by the drive assembly  20 , while the outer member  32  may be optimized to withstand the thermal energy associated with electron bombardment. 
   The cathode assembly  14  is positioned at some distance from the anode assembly  18  in the vacuum housing, creating a vacuum gap therebetween, and having a high voltage potential difference maintained therebetween. The cathode assembly  14  generates and emits electrons in the form of an electron beam from a cathode  16 . The electrons in the electron beam are accelerated from the cathode  16  across the high voltage potential difference towards the target disk  28  of the anode assembly  18  to impact the target track  30  at a focal spot at high velocity. As the electrons impact the target track  30  of the target disk  28 , the kinetic energy of the electrons is converted to high-energy electromagnetic radiation or X-rays. The X-rays are then transmitted through a window (not shown) in the X-ray tube casing  12  to an object such as the body of a patient and are intercepted by a detector that forms an image of the object&#39;s internal anatomy. The impact of electrons on the target track  30  generates considerable heat and considerable wear on the target disk  28 . 
     FIG. 2  illustrates an exemplary embodiment of an X-ray tube anode assembly  48 . The anode assembly  48  includes a drive assembly  50  rotating a drive shaft  52  that rotates an X-ray tube target assembly  54 . The X-ray tube target assembly  54  includes a target disk  58  that is attached to the drive shaft  52  and driven by the drive assembly  50  to rotate the target disk  58  at high speeds. 
   In an exemplary embodiment, the target disk  58  may be an integral injection molded part that includes curved outer surfaces with an angled outer surface  64  on one side  66  of an outer portion  68  creating an area for a target track  60  to be applied or formed thereon. The target track  60  formed on the outer surface  64  on one side  66  of the outer portion  68  is designed for receiving a bombardment of electrons from an electron beam generated by a cathode assembly for the generation of X-rays. The target disk  58  is capable of withstanding high rotational speeds and increased thermal requirements of modern X-ray tube anode assembly performance. 
   In an exemplary embodiment, the target disk  58  includes a hub portion  70  with a cross-sectional width  72  to reduced stresses due to loading transferred from the drive shaft  52 , an inner portion  78  with a cross-sectional width  74  that is smaller that the cross-sectional width  72  of the hub portion  70 , and an outer portion  68  with a cross-sectional width  76  that is larger that the cross-sectional width  74  of the inner portion  78 . The inner portion  78  cross-sectional width  74  is smaller than the outer portion  68  cross-sectional width  78  to prevent thermal transfer from the target track  60  to the hub portion  70 . 
     FIG. 3  is a top plan view of a portion of an exemplary embodiment of an X-ray tube target assembly  80 . The X-ray tube target assembly  80  includes a target disk  82 . The target disk  82  includes an injection molded hub member  84  and an injection molded outer member  86 . The injection molded outer member  86  is horizontally in-line with and extends radially outwardly from an outer perimeter  112  of the injection molded hub member  84 . The injection molded outer member  86  includes a target track  88  formed on an outer surface  90  on one side  92  of an outer periphery  94  of the outer member  86  for receiving a bombardment of electrons from an electron beam generated by a cathode assembly for the generation of X-rays. In an exemplary embodiment, the target track  88  may be a toroidal-shaped target track. While it is contemplated that the outer member  86  may be formed as a single integral injection molded part, the advantages of simplified manufacturing, assembly, and repair are further increased if the outer member  86  is comprised of a plurality of injection molded outer member segments  96  that are removably attached together and removably attached to the injection molded hub member  84 . 
   In an exemplary embodiment, the injection molded hub member  84  may be a substantially planar circular-shaped hub member with a central opening extending therethrough for attachment to a drive shaft. In an exemplary embodiment, the injection molded outer member  86  may be a substantially planar circular-shaped outer member removably attachable to an outer perimeter of the hub member  84 . 
   In an exemplary embodiment, the outer member  86  may comprise a plurality of injection molded pie-shaped outer member segments  96  that are removably attached together and removably attached to an outer perimeter  112  of the injection molded hub member  84 . If the target track  88  experiences undesirable levels of wear or damage, the outer member segments  96  may be replaced while the hub member  84  remains. This allows for simplified replacement of worn or damaged portions of the target disk  82 . In an exemplary embodiment, the X-ray tube target assembly  80  comprises a target disk  82  including an injection molded hub member  84  and a plurality of injection molded outer member segments  96  removably attached together and removably attached to the injection molded hub member  84 .  FIG. 3  illustrates an outer member segment  96  being separated from the hub member  84 . 
   It is contemplated that the target disk  82  may be formed in a variety of configurations such that the plurality of outer member segments  96  are removably attached together and removably attached to the hub member  84 . 
   In an exemplary embodiment, each outer member segment  96  may include at least one side tab  98  formed on and extending from a first radial side  102  of the outer member segment  96 , and at least one corresponding side slot  100  formed within a second radial side  104  of the outer member segment  96  for accepting at least one mating side tab  98 . The first radial side  102  being opposite the second radial side  104 . In an exemplary embodiment, each outer member segment  96  may also include at least one end tab  106  extending axially outwardly from an inner perimeter  108  of each outer member segment  96 , and a plurality of corresponding end slots  110 , one for each end tab  106 , formed within an outer perimeter  112  of the hub member  84  for accepting at mating end tab  106 . As illustrated in  FIG. 3 , each side tab  98  of an outer member segment  96  fits within and engages a side slot  100  of a neighboring outer member segment  96 , and each end tab  106  of an outer member segment  96  fits within and engages an end slot  110  of the hub member  84  to secure the outer member segments  96  together and secure the plurality of outer member segments  96  to the hub member  84  to form a solid target disk  82 . 
   Other structures and methods of removably interlocking the plurality of outer member segments  96  to the hub member  84  are contemplated within the present disclosure. 
     FIG. 4  is flow diagram of an exemplary embodiment of a method of constructing an X-ray tube target assembly. The X-ray tube target assembly includes a target disk. In order to achieve higher power levels, a geometrically optimized and faster spinning target disk may be required. This disclosure alleviates the problems described above by using a more cost effective and manufacturable method for making a higher power, faster spinning target disk. 
   The method comprises injection molding a hub member at step  122  and injection molding an outer member at step  124 . Injection molding is used in a variety of industries as a cost effective method of producing parts having complex geometries. In an exemplary embodiment, the injection molding process may include a metal injection molding process, which combines the versatility of plastic injection molding with the strength and integrity of machined, pressed or otherwise manufactured small complex metal parts. 
   In a metal injection molding process, metal powders are blended and mixed with polymer binders and additives, which allow the metal to be injected into a mold. The polymer serves as a binder that allows the metal powders to be injection molded. This blend is then processed on a conventional injection molding machine to form molded parts. The binder is removed from the molded parts in a continuous process under a highly defined and controlled temperature and time profile. During this debinding process, the polymer binder breaks down and dissipates while the metal particles retain all of their molded features. The resultant metal part is then sintered. During sintering, the metal particles fuse together to form a solid metal part. The advantage of metal injection molded parts is such that the complexity and small size of a part or perhaps the difficulty of fabrication through other means may make it cost inefficient or even impossible to manufacture small complex parts using other methods. 
   The target disk is generally fabricated from a refractory metal with a high atomic number such as molybdenum, tungsten or a tungsten alloy. In an exemplary embodiment, spherical molybdenum powders with an average size of 1-20 μm may be injection molded into complex geometries and sintered in a hydrogen atmosphere at times and temperatures within the range of conventional sintering furnaces. Metal injection molding has been shown to reach a physical and economical limitation as part size increases. Since current X-ray technology calls for target disks in excess of 200 mm in diameter, the capability to injection mold a target disk in a single molding cycle does not conventionally exist. In order to facilitate the injection molding of such a large component, a segmented target disk design is proposed. In an exemplary embodiment, the outer member may include a plurality of outer member segments. In this case, each of the plurality of outer member segments are made from an injection molding process. With this combination of techniques, complex geometries can be produced in a cost effective manner, with a minimum of final machining. The plurality of outer member segments are removably attached together at their sides and removably attached to the hub member at an inner perimeter to form an interlocking target disk. 
   At step  126 , the injection molded outer member is removably attached to the injection molded hub member. In an exemplary embodiment, the outer member may include a plurality of outer member segments. In this case, each of the plurality of outer member segments are removably attached together and removably attached to the hub member. 
   At step  128 , a target track of target material is applied or formed on an outer surface of one side of an outer periphery of the outer member. In an exemplary embodiment, the outer member may include a plurality of outer member segments. In this case, a target track of target material is applied or formed on an outer surface of one side of an outer periphery of each of the plurality of outer member segments. Therefore, the injection molded outer member includes a target track formed on an outer surface on one side of an outer periphery of the outer member for receiving a bombardment of electrons from an electron beam generated by a cathode assembly for the generation of X-rays. 
   The method described above allows more complex target disk geometries to be easily manufactured at lower machining and scrap cost. These more complex target disk geometries enable higher peak power, faster gantry speeds, and higher target disk spin speeds through geometric optimization. Segmented target disks also have the effect of eliminating hoop stresses. 
     FIG. 5  is flow diagram of an exemplary embodiment of a method of constructing an X-ray tube target assembly  130 . The X-ray tube target assembly includes a target disk. The method comprises injection molding a hub member at step  132  and injection molding a plurality of outer member segments at step  134 . Each of the plurality of outer member segments is made from an injection molding process. At step  136 , a target track of target material is applied or formed on an outer surface of one side of an outer periphery of each of the plurality of outer member segments. Therefore, each of the plurality of injection molded outer member segments includes a target track formed on an outer surface on one side of an outer periphery thereof for receiving a bombardment of electrons from an electron beam generated by a cathode assembly for the generation of X-rays. At step  138 , the plurality of injection molded outer member segments are removably attached together and removably attached to the injection molded hub member. The plurality of outer member segments are removably attached together at their sides and removably attached to the hub member at an inner perimeter to form an interlocking target disk. 
   Several embodiments are described above with reference to drawings. These drawings illustrate certain details of exemplary embodiments that implement the systems and methods of this disclosure. However, the drawings should not be construed as imposing any limitations associated with features shown in the drawings. 
   The foregoing description of exemplary embodiments is presented for purposes of illustration and explanation of the disclosure. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principals of the disclosure and its practical application to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. 
   While this disclosure has been described with reference to various exemplary embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the disclosure. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the following claims.