Patent Publication Number: US-8126116-B2

Title: Anode plate for X-ray tube and method of manufacture

Description:
The present invention relates to x-ray tubes, and to anode plates employed in X-ray tubes and their corresponding method of manufacture. 
     An anode plate (typically in the form of a rotating disk) is implemented in an X-ray tube used in diagnostic medical equipment, such as computed tomography (CT) systems. Under normal operating conditions, the anode plate is subjected to large mechanical compression and tensile stresses resulting from the anode&#39;s high rotational speed, as well as extreme thermal loading resulting from heat generated from an incident electron beam impinging the anode&#39;s surface. These mechanical and thermal stresses degrade the anode surface, leading to, for example, cracking or warping of the anode plate over time. The usable lifetime of the anode, and accordingly, the X-ray tube, is reduced by these effects. 
       FIG. 1A  illustrates a top view of one conventional rotating anode plate  100  showing thermal gradient and tangential stress distribution. The outer diameter  110  represents the target area in which an electron beam strikes the anode plate  100 . About 99% of the kinetic energy of the incident electron bean is transferred into heat, forming a thermal gradient between the outer and inner diameters  110  and  130 . Due to the thermal expansion coefficient, mechanical compression stress in the tangential direction is generated at the outer diameter while tensile stress in the tangential direction is generated at the inner diameter. 
       FIGS. 1B and 1C  illustrate a conventional anode plate design in which radial slots  140  are used to reduce the aforementioned tensile and compression stresses. In particular, the radial slots  140  extend from the anode&#39;s outer edge toward the inner region  130 , the radial slots  140  having rounded slot ends  142  for further reducing mechanical stresses on the anode  100 .  FIG. 1B  further illustrates the tangential stress distribution across the anode plate during rotation and thermal loading. As can be seen therefrom, the radial slots  140  operate to reduce the stresses at the outer edge of the anode plate, but high compression stress is exhibited at the slot ends  142 .  FIG. 1C  illustrates the tangential stress distribution across the anode plate during anode rotation without thermal loading, which shows a high degree of tensile stress is imparted to the slot end  142 . 
     It may be desirable to provide an anode plate with reduced tensile and compression stresses, so as to extend the usable lifetime of the X-ray tube in which the anode plate is used. 
     This need may be met by an anode plate for an X-ray tube according to the independent claims. 
     In one embodiment of the invention, an anode plate for an X-ray tube is provided and includes slots disposed along the outer edge and extending toward the center region, each of the slots terminating in a slot end. The anode plate further includes slot termination material disposed around at least a portion of the periphery of one or more of the slot ends. The slot termination material is operable to reduce the tension stress or compression stress which may be developed at the slot end as a result of the rotation and/or heating of the anode as described above. 
     In another embodiment of the invention, a method for manufacturing an anode plate for an X-ray tube includes the operation forming the anode plate having an outer edge and a center region, the anode plate including a plurality of slots disposed along the outer edge and extending toward the center region, each of the plurality of slots including a slot end, the manufacturing method further includes depositing slot termination material around at least a portion of the periphery of one or more of the slot ends, the slot termination material operable to reduce the tension stress or compression stress at the slot end. 
     In a further embodiment of the invention, an X-ray tube is presented having a cathode operable to provide a stream of electrons for bombarding an anode, and an anode plate in accordance with the present invention. 
     It may be seen as a gist of an exemplary embodiment of the present invention that slot termination material is deposited at the slot ends to reduce the compression and tensile stress developed at the slot ends during operation, thus extending the usable lifetime of the anode plate, and accordingly, the X-ray tube in which it is employed. 
     The following describes exemplary features and refinements of the anode of an X-ray tube in accordance with the invention, although these features and refinements will apply to the manufacturing system as well. 
     In optional embodiments, the anode plate and the slot ends may be of a generally circular shape. Further exemplary, the slot termination material ( 230 ) is disposed around at least one-half of the periphery of the slot end, and further optionally around substantially the entire periphery of the slot end. As a further exemplary embodiment, the slot termination material may be formed within an inner ring of the anode plate, whereby the slot ends of one or more slots intersects the inner ring of slot termination material. Exemplary embodiments of the slot termination material may be selected from a group of ductile refractory metals consisting of Ti, V, Ta, Nb, Re and alloys thereof. Further optionally, the slot termination material may be formed from Ni-based super alloy, fiber reinforced materials or materials with high fracture toughness. 
     The following describes exemplary features and refinements of a method of manufacturing the X-ray tube anode in accordance with the invention, although these features and refinements may also apply to the aforementioned manufacturing method. 
     In one embodiment of the manufacturing method, the anode plate and the slot ends may be formed in a generally circular shape. Further exemplary, the slot termination material is optionally deposited around at least one-half of the periphery of one or more of the slot ends. In a further optional embodiment, slot termination material is deposited on the anode plate in the form of an inner ring, whereby the slot end of one or more of the slots intersect the inner ring of slot termination material. In another optional embodiment, a first hole is provided in the anode plate at a location in which a slot end is intended. Next, slot termination material is deposited within the first hole. Next, a second hole within the deposited slot termination material is provided, the second hole forming a slot end. Next, a slot is extended from the slot end to the outer edge of the anode plate. The slot termination material may be composed of ductile refractory metals consisting of Ti, V, Ta, Nb, Re and alloys thereof, or a Ni-based super alloy. 
     The operations of the foregoing methods may be realized by a computer program, i.e. by software, or by using one or more special electronic optimization circuits, i.e. in hardware, or in hybrid/firmware form, i.e. by software components and hardware components. The computer program may be implemented as computer readable instruction code in any suitable programming language, such as, for example, JAVA, C++, and may be stored on a computer-readable medium (removable disk, volatile or non-volatile memory, embedded memory/processor, etc.), the instruction code operable to program a computer of other such programmable device to carry out the intended functions. The computer program may be available from a network, such as the WorldWideWeb, from which it may be downloaded. 
     These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiment described hereinafter. 
    
    
     
       An exemplary embodiment of the present invention will be described in the following, with reference to the following drawings. 
         FIGS. 1A-1C  illustrates top views of a conventional anode plate for an X-ray tube and corresponding showing thermal gradient and tangential stress distribution thereacross. 
         FIG. 2A  illustrates a first exemplary embodiment of an anode plate for an X-ray tube in accordance with the present invention. 
         FIG. 2B  illustrates a second exemplary embodiment of an anode plate for an X-ray tube in accordance with the present invention. 
         FIG. 3  illustrates an exemplary embodiment for manufacturing an anode plate for an X-ray tube in accordance with the invention. 
         FIGS. 4A-4B  illustrate exemplary processes by which an inner ring of slot termination material is formed on the anode plate for an X-ray tube in accordance with the invention. 
         FIG. 5  illustrate a computed tomography system having an X-ray tube employing an anode plate in accordance with the present invention. 
     
    
    
     For clarity, previously-identified features retain their reference numerals in subsequent drawings. 
       FIG. 2A  illustrates a first exemplary embodiment of an anode plate for an X-ray tube in accordance with the present invention. The anode plate  210  includes slots  220  disposed along the outer edge  210   a  and extending toward the center region  210   b , each of the slots  220  terminating in a slot end  222 . The anode plate  210  further includes slot termination material  230  disposed around at least a portion of the periphery of one or more of the slot ends  222 . The slot termination material  230  is operable to reduce the tension stress or compression stress which may be developed at the slot end  222  as a result of the rotation and/or heating of the anode as described above. 
     In a particular embodiment of the invention, the anode plate  210  is generally circular shape, although other shapes may be alternatively employed. Further exemplary, the slot ends  222  may be of a generally circular shape, although different geometry may be implemented as well in other embodiments under the invention. 
     The slot termination material  230  is disposed at least partially around the periphery of one or more of the slot ends  222 . In one embodiment, the slot termination material  230  extends at least half way around the periphery of one or more of the slot ends  222 , and in another embodiment, the slot termination material extends substantially around the entire slot end periphery, as shown in  FIG. 2A . The term “slot end periphery” refers to the periphery of the slot end  222  around which a portion of the anode plate is located, excluding the slot  220  itself. The anode plate  210  may be constructed from conventional materials such as Mo-alloys. The slot termination material  230  may be ductile refractory metals such as Ti, V, Ta, Nb, Re, or alloys thereof. Alternatively, Ni-based super alloy may be used for the slot termination material  230 . Further exemplary, materials which exhibit high ductility, high fracture toughness, and low Young&#39;s modulus or fiber reinforced materials may be employed as the slot termination material  230 . 
       FIG. 2B  illustrates a second exemplary embodiment of an anode plate for an X-ray tube in accordance with the present invention, with previously recited feature retaining their reference numerals. In this embodiment, the anode plate  210  includes an inner ring  250  of slot termination material  230 , whereby the slot end  222  of one or more of the slots intersects the inner ring  250  of slot termination material  230 . In the particular embodiment shown, the slot termination material  230  extends around the entire periphery of the slot end  222 . In alternative embodiments, the positioning and/or width of the inner ring  250  is such that less than the entire periphery of the slot end  222  is covered, for example, half of the periphery, one quarter of the periphery, or less. Exemplary slot and hole dimensions for a generally circular anode plate of radius R would be as follows: width of slot  220 : 0.001*R to 0.02*R; length of slot  220 : 0.2-0.8*R; radius of slot end  222 : less than 0.02*R; radius of slot termination material  230  disposed around at least a part of the slot end  222 : 0.005 to 0.2*R; width of the inner ring of slot termination material ( 250 , when employed) 0.005 to 0.2*R. 
       FIG. 3  illustrates an exemplary embodiment for manufacturing an anode plate for an X-ray tube in accordance with the invention. Initially at  312 , an anode plate  210  is formed having a plurality of slots ( 220 ) extending from an outer edge  210   a  of the anode plate toward a center region  210   b . In an exemplary embodiment, the anode plate is formed in a generally circular shape, although other shapes may be used in accordance with the present invention. Further exemplary, the slot ends  222  are formed in a generally circular shape, although other shapes may be used in accordance with the present invention. 
     Next at  314 , slot termination material  230  is deposited around at least a portion of the periphery of one of one or more of the slot ends  220 , the slot termination material  230  operable to reduce the tension stress or compression stress at the one or more slot ends  222 . In a particular embodiment of this process, slot termination material is deposited around the periphery of each of the slot ends  222 , although in other embodiments, one or more slot ends may exclude the slot termination material. Further exemplary, the slot termination material  230  may be deposited around at least one half of the periphery of one or more of the slot ends  222 , e.g., extending around substantially the entire periphery of the slot ends  222 , as illustrated in  FIGS. 2A and 2B . Of course, other embodiments are also possible, for example, the slot termination material may extend around less than half of the periphery of the slot ends  222 , e.g., one quarter of the slot end periphery. 
     In a first specific process of  314 , an inner ring of slot termination material  250  is formed on the anode plate, whereby one or more slot ends  222  intersects the inner ring  250 . The inner ring of slot termination material  250  may be deposited using, e.g. power metallurgy, plasma spraying, or such similar techniques known in the art. 
       FIGS. 4A-4B  illustrate a second specific process of  314  in which slot termination material  230  is formed around at least a portion of the periphery of a slot end  222 . Initially, a first hole  410  is provided (e.g., drilled, etched, machined, or the like) in the anode plate  210  at a location in which the slot end is intended. Next, the first hole  410  is filled with the slot termination material  230 . Further exemplary a bolt made from slot termination material  230  is put into hole  410  and connected to plate  210  by e.g. brazing.  FIG. 4A  illustrates the resulting structure. 
     Next, a second hole  420  is provided within the slot termination material  230 , the second hole  420  forming a slot end  222 . Subsequently, a slot  220  is extended (e.g., by drilling, etching, machining, or the like.) from the slot end  222 / 420  to the outer edge  210   a  of the anode plate.  FIG. 4B  illustrates the resulting structure. 
       FIG. 5  illustrate a computed tomography (CT) system (cone beam) having an X-ray tube  530  employing an anode plate in accordance with the present invention. The CT system  500  includes a gantry  501 , within which a X-ray tube  530  and an opposing detector  515  rotate to provide x-ray images of a patient  510  or object positioned therebetween. Within the X-ray tube  530 , a cathode is operable to generate a steam of electrons for bombarding an anode plate, the anode plate in response emitting X-rays through an X-ray transparent material/window for illuminating the patient  510  or object. Motor control units  520  and  525  control movement of the X-ray tube  530  and the patient platform  512 . As noted above, the anode&#39;s high rotational speed and surface heat produces significant compression and tension stresses on the anode. The present invention provides an anode plate having decreased compression and tension stresses, thus extending the usable lifetime of the X-ray tube, and in turn enabling less maintenance, and greater reliability of the CT system. 
     In summary, it may be seen as one aspect of the present invention that a slotted anode plate for an X-ray tube is presented which is operable with decreased compression and tension stress forces on the slot ends. The anode includes a plurality of slots extending from the plate&#39;s outer edge toward the center region, each of the slots including a slot end. Slot termination material is disposed on the slot ends, the slot termination material operable to reduce the tension stress or compression stress at the slot end. 
     As readily appreciated by those skilled in the art, the described processes may be implemented in hardware, software, firmware or a combination of these implementations as appropriate. In addition, some or all of the described processes may be implemented as computer readable instruction code resident on a computer readable medium (removable disk, volatile or non-volatile memory, embedded processors, etc.), the instruction code operable to program a computer of other such programmable device to carry out the intended functions. 
     It should be noted that the term “comprising” does not exclude other features, and the definite article “a” or “an” does not exclude a plurality, except when indicated. It is to be further noted that elements described in association with different embodiments may be combined. It is also noted that reference signs in the claims shall not be construed as limiting the scope of the claims. 
     The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the disclosed teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined solely by the claims appended hereto.