Abstract:
A method and apparatus for a rotatable anode of an x-ray tube. The anode having an axis of rotation and includes a solid thin plate target having a substantially planar base surface extending from the axis of rotation to a periphery outlining the base surface, wherein the plate target includes target material for generating x-rays selected from a group of high-Z materials. The plate target has a thickness of about 1 mm or less. The method includes fabricating the thin plate target using silicon wafer processing technology using suitable materials for such technology in forming the plate target selected from the group of high-Z materials.

Description:
BACKGROUND OF INVENTION 
     The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical x-ray tubes are built with a rotating anode structure that is rotated by an induction motor comprising a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The x-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces x-rays upon impact with the anode target. The target typically comprises a disk made of a refractory metal such as tungsten, molybdenum or alloys thereof and the x-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. High speed rotating anodes can reach 9,000 to 11,000 RPM. 
     Only a small surface area of the target is bombarded with electrons. This small surface area is referred to as the focal spot, and forms a source of x-rays. Thermal management is critical in a successful target anode, since over 99 percent of the energy delivered to the target anode is dissipated as heat, while significantly less than 1 percent of the delivered energy is converted to x-rays. Given the relatively large amounts of energy which are typically conducted into the target anode, it is understandable that the target anode must be able to efficiently dissipate heat. The high levels of instantaneous power delivered to the target, combined with the small size of the focal spot, has led designers of x-ray tubes to cause the target anode to rotate, thereby distributing the thermal flux throughout a larger region of the target anode. There are various techniques for distributing thermal flux, for example, faster rotation speeds or greater target anode diameters, that allow for decreasing the thermal energy at any given location along the focal track. 
     However, there is a practical limitation regarding a maximum speed at which the target anode can be rotated, and in the size of practical target anode diameters. The materials of the target anode will eventually shatter at certain speeds and larger diameters. 
     Operating conditions for x-ray tubes have changed considerably in the last two decades. U.S. Pat. No. 4,119,261, issued Oct. 10, 1978, and U.S. Pat. No. 4,129,241, issued Dec. 12, 1978, were both devoted to joining rotating anodes made from molybdenum and molybdenum-tungsten alloys to stems made from columbium and its alloys. Continuing increases in applied energy during tube operation have led to a change in target composition to titanium zirconium molybdenum (TZM) TZM is a trademark of Metalwork Plansee or other molybdenum alloys, to increased target diameter and weight, as well as to the use of graphite as a heat sink in the back of the target. Future computerized tomography (CT) scanners will be capable of decreasing scan time from a one second rotation to a 0.5 second rotation or lower. However, such a decrease in scan time will quite possibly require a modification of the current CT anode design. The current CT anode design comprises two disks, one of a high heat storage material such as graphite, and the second of a molybdenum alloy such as TZM. These two concentric disks are bonded together by means of a brazing process. 
     A thin layer of refractory metal such as tungsten or tungsten alloy is deposited to form a focal track. Such a composite substrate structure may weigh in excess of 4 kg. 
     With faster scanner rotation rates, heavy targets will increase not only mechanical stress on the bearing materials but also a focal spot sag motion causing image artifacts. 
     Furthermore, there is a demonstrated need for multi-energy or multiple target material sources of x-radiation. In mammography, for example, the image contrast is enhanced by using Mo and Rh target tracks with two separate electron beam sources. However, using two tracks with two electron beam sources increases mechanical complexity of high voltage, high power x-ray tubes due to the size of the resulting target and the consequent design choices that must be made: the size and mass of the rotor, stator, and certain features of the vacuum enclosure which act as the support frame. In addition, there are certain limitations to this design, for example, only two materials may be employed and two electron beam sources may be required, as in mammography. The large mass anode assembly makes changing target materials unfeasible or inconsistent with present design goals. 
     Accordingly, it would be desirable over the state of the art to provide a target anode structure and material which is capable of high speeds of rotation, and which is less sensitive to thermal stresses. It would also be desirable to provide a new method of creating a layer of x-ray emissive material on a target anode substrate which would not be subject to delamination. It would be desirable then to replace the present CT target design with a lightweight design comparable in thermal performance, particularly suited for use in x-ray rotating anode assemblies. 
     SUMMARY OF INVENTION 
     The above discussed and other drawbacks and deficiencies are overcome or alleviated by a rotatable anode for x-ray tube comprising: a solid thin plate target selected from a group of high-Z materials selectively deposited onto a substrate material including silicon, silicon carbide, aluminum nitride, gallium arsinide, glass or other commercially available thin disk substrate material. The substrate material includes single crystal, polycrystalline and amorphous forms. The plate target includes a substantially planar base surface extending from the axis of rotation to a periphery outlining the base surface, wherein the plate target includes target material for generating x-rays. The plate target has a thickness of about 1 mm or less. 
     In an alternative embodiment, a method for manufacturing a rotatable anode for an x-ray tube is disclosed. The method comprising: fabricating a thin plate target with silicon wafer processing technology using suitable materials for such technology in forming the plate target selected from a group of high-Z materials. The plate target includes an axis of rotation and a thickness of about 1 mm or less. 
     The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 illustrates a high level diagram of an x-ray imaging system; 
     FIG. 2 is a profile cross sectional view of a state of the art target anode which includes a substrate, where the substrate is typically composed of a carbon material (e.g. graphite); 
     FIG. 3 is a perspective view of an exemplary embodiment of a target anode having two different target materials interleaved therein in an ABABAB pattern; 
     FIG. 4 is a schematic view of an x-ray tube illustrating a partial view of the target anode of FIG. 3; and 
     FIG. 5 is schematic view of the target anode of FIG. 3 illustrating two electromagnetic beam incident angles and an axis relative to rotation and translation of the target anode. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIG. 1, that figure illustrates an x-ray imaging system  100 . The imaging system  100  includes an x-ray source  102  and a collimator  104 , which subject structure under examination  106  to x-ray photons. As examples, the x-ray source  102  may be an x-ray tube, and the structure under examination  106  may be a human patient, test phantom or other inanimate object under test. 
     The x-ray imaging system  100  also includes an image sensor  108  coupled to a processing circuit  110 . The processing circuit  110  (e.g., a microcontroller, microprocessor, custom ASIC, or the like) couples to a memory  112  and a display  114 . 
     The memory  112  (e.g., including one or more of a hard disk, floppy disk, CDROM, EPROM, and the like) stores a high energy level image  116  (e.g., an image read out from the image sensor  108  after 110-140 kvp 5 mAs exposure) and a low energy level image  118  (e.g., an image read out after 70 kVp 25 mAs exposure). The memory  112  also stores instructions for execution by the processing circuit  110 , to cancel certain types of structure in the images  116 - 118  (e.g., bone or tissue structure). A structure cancelled image  120  is thereby produced for display. 
     Referring now to FIG. 2, a typical prior art CT anode target  122  suitable for use in x-ray tube  102  is illustrated. The current CT anode  122  design comprises two disks  124  and  126 . One disk  126  is of a high head storage material such as graphite, and the second disk  124  is of a molybdenum alloy such as titanium zirconium molybdenum (TZM) TZM is a trademark of Metalwork Plansee. These two concentric disks are bonded together by means of a brazing process. A thin layer of refractory metal such as tungsten or tungsten alloy is deposited to form a focal track  127 . Such a composite substrate structure may weigh in excess of 4 kg. With faster scanner rotation rates, heavy targets will increase not only mechanical stress on the bearing materials but also a focal spot sag motion causing image artifacts. 
     The present disclosure proposes tailored silicon wafer processing material structures to replace the graphite material in existing CT scanner systems. The present disclosure proposes the use of existing silicon wafer processes and technologies, well known in the art, applied to a rotable target, to achieve thin lightweight anode structures. 
     FIG. 3 illustrates an exemplary embodiment of a thin plate target anode  122  in a perspective view. The target anode  122  is comprised of a substrate  130 . An x-ray emissive target material  128  is deposited on a substantially planar base surface  132  of substrate  130 . Base surface  132  is preferably configured with micro-channels  134  to provide cooling when plate target  122  rotates. Cooling micro-channels  134  are capable of handling about 10 to about 100 kW and can be machined into substrate structures by etching or photoresist, for example, a silicon substrate  130  that acts as the target material support. This cooling technique makes it possible to dissipate large thermal fluxes away from target anode  122 . The x-ray emissive target material  128  in this type of target anode is deposited using a technique such as chemical vapor deposition (CVD) or physical vapor deposition (PVD); both are well known techniques in silicon wafer processing. Two different x-ray emissive materials, A and B, are preferably deposited in an alternating manner with respect to each other forming alternating materials in one focal track. In this manner, when anode  122  rotates about an axis of rotation  136 , an electron beam (not shown) focused on base surface  132  will strike either emissive material A or B providing differing spectral content of x-ray generation from a respective focal track. In alternative embodiments, emissive material A and B may be disposed concentrically with respect to each other and more than one electron beam may be used, where each beam is focused to strike one of the two emissive materials A or B. Preferably, however, one electron beam is used and target anode  122  is translatable in a direction  138  perpendicular to axis  136  for focusing a beam on a number of different focal tracks concentrically disposed on base surface  132  as anode  122  translates in direction  138 . In addition, when emissive material A and B is interleaved as illustrated in FIG.  3  and rotatable target anode  122  is translatable in direction  138 , a focused electron beam can be directed on substantially all of the target material  128  disposed on base surface  132 . It will be appreciated that more than two emissive materials may be used as the target material  128  as well. Likewise, it will also be recognized that substrate  130  and emissive target material  128  may be one and the same providing a unitary substrate thin plate target anode  122  made from a high-Z material. Substrate  130  may be composed of one of the following, including combinations of at least one of the following materials: silicon, silicon carbide, aluminum nitride, carbon, and Gas. 
     Referring now to FIG. 4, the plate target  122 , with multiple materials A and B deposited on the surface  132  shown in FIG. 3, is illustrated in cooperation with a generic arrangement of a cathode  140 , and a surrounding frame surface  142  of an x-ray tube insert  146  as the x-ray source  102 . Cathode  140  generates an electron beam  148  that is incident upon the base surface  132  of thin rotating plate target  122 . As shown in FIG. 3, the target has two focal tracks (i.e., A and B) that are separated on the target surface  132  in radius from the center of rotation  136 . In a preferred embodiment, the different target materials A and B are interleaved, A, B, A, B as shown in FIG.  3 . In this embodiment, the electron beam  148  is gated by means of gridding or pulsing the high voltage, as is the case in present x-ray tube designs, to match the arrival of the track portions that are exposed to the focal spot of the electron beam  148 . This arrangement of the two materials A and B allows for the advance of the target rotation axis to permit the use of the entire thin target disk by a means for translation of the disk  122  in a direction substantially perpendicular to the axis of rotation  136 . 
     FIG. 4 illustrates back-scattering x-ray generation, e.g. electrons are incident upon the target material (i.e., A and B) and the x-rays  152  escape from the material&#39;s top layer base surface  132  to exit the insert  146  by means of a beryllium window  154  disposed in frame  142 . The thin rotating target  122  can be used for generating x-radiation in transmission mode as well. Instead of massive layers of target material  128 , it is also possible to deposit thin layers of high-Z material. The incident electrons impinge upon the material  128 , generate the x-rays  152  by bremsstrahlung process, and the x-rays  152  emerge from the back side of the thin layer of material  128 . It will be appreciated that there is an associated filtration due to the thickness of the substrate  130 , density, atomic number and energy. For example, thin layers of target material  128  can be directly deposited onto a substrate  130  like silicon. Silicon has a Z=14 and as such submits the x-rays  152  to much less filtration than typically tolerated total filtration for an x-ray insert, of the order of 0.15 mm of Cu (for CT tubes). It will be recognized that silicon is commonly used as a semiconductor substrate material. As such, well-known techniques of etching, photoresist, and architecture of microscopic structures are easily employed to the deposition of any desired configuration of target zones. 
     More than two materials can be deposited for a wider choice of procedures and protocols and energy-dependent digital image subtraction methods, such as used currently in angiography. Many different materials can be deposited onto the surface or into wells or depressions designed for the materials and the particular deposition techniques, preferably including but not limited to, W, Mo, Rh, U, Pb. In other exemplar embodiments the list of other suitable materials include metals such as, Ta, Hf, Pt, Au, Ti, Zr, Nb, Ag, V, Co, Cu, in descending order of Z, atomic number. In other target technology applications, high performance ceramics are optionally used. Whether one, two or more materials are used in the target, the electron beam voltage and current can be varied to produce the optimal contrast-to-dose and spectral content depending upon the desired image, modality, physiology and associated pathology 
     In an exemplary embodiment referring again to FIG. 3, the target anode is composed of 1 mm thick 160 silicon having a diameter of about 300 mm for mechanical stability necessary to survive fabrication, loading and mechanical stresses associated with acceleration/deceleration and thermal loads. Current automated semiconductor fabrication techniques can be applied to mass-produce such targets. The mass of such an exemplary silicon target  122  is about 0.14 kg, which is approximately 40 times less than currently known high-power CT x-ray tube targets. The light weight of the target disks  122  permit using high speed spindle technology routinely used in rotating mechanisms for semiconductor manufacturing. These spindle mechanisms involve conventional (hybrid) bearing technology through a ferrofluidic feedthrough, or (in vacuum) bearings with low-vapor pressure vacuum grease. The light weight of the targets also permits throw-away or single-procedure or protocol use for a target. For example, carousels loaded with several targets can optionally be used in an x-ray tube insert  146 . Alternatively, a load-lock arrangement can be used to shuttle targets into and out of the x-ray tube. 
     Referring to FIG. 5, electron beam  148  is incident upon the target material  128  at an angle relative to base surface  132  ranging from about 20 degrees to about 90 degrees (i.e., normal incidence). It has been found through experimentation that optimization of the x-ray output per unit heat deposited in the target occurs at about 20 degrees. 
     In an alternate embodiment, laser ablation plasma x-ray generation is optionally used with the thin rotating target  122 . This use of the thin rotating disk target  122  with a mechanical axis advance mechanism as a means for translation of anode  122  in a direction depicted with arrows  166  is particularly well suited for the ablation techniques of x-ray production. The ablation method is destructive and management of pressure excursions and target ejecta is a concern. Sufficient pumping (whether by active means or by means of bulk or surface getter technology) will alleviate the problems with pressure. Baffles are typically employed to limit the straight-line paths that target molecules follow which can result in fouling of x-ray transparent windows  154 . Once the target has been used, it can be swapped out either by the load-lock method or by the carousel advance method discussed above. 
     While it is understood that there is a certain amount of mechanical rigidity demanded by the aiming system for the electron beam or laser beam, the light weight anode and target presents a number of significant advantages. Lower mass targets imply lower mass motor elements to drive target rotation. Thus, the rotor and stator need not be as large as in traditional 4 to 6 kg target assemblies. This lowers total material costs as well as costs related to manufacture and processing. Semiconductor manufacturing technology can be leveraged to accomplish this particular technical task. The power supply that is required in order to rotate the target is smaller and less power is required at the x-ray tube insert  146 . Smaller power supplies cost less to begin with and occupy less space in high voltage generators. Furthermore, the wires, connectors, and associated hardware costs are lower. The bearing will be lighter in weight, have reduced wear, and be much quieter. Smaller bearings cost less to produce in terms of materials, and cost less to process. High-speed rotation is implied by the target weight reduction. This means lower peak focal spot temperatures as analyzed by traditional track temperature calculation algorithms. While the distribution of track/target material is different compared to a traditional thick target, any significant reduction in temperature while maintaining x-radiation output is an important gain. The bearing can be of the sealed bearing type. Since the bearing itself is not exposed to the chamber where relatively low pressure is necessary, a variety of lubricants and noise-abatement strategies can be adopted for optimized bearing performance. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.