Patent Publication Number: US-7720200-B2

Title: Apparatus for x-ray generation and method of making same

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to x-ray tubes and, more particularly, to an apparatus for x-ray generation and a method of fabrication. 
   X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner. 
   X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. 
   Newer generation x-ray tubes have increasing demands for providing higher peak power. Higher peak power, though, results in higher peak temperatures occurring in the target assembly, particularly at the target “track,” or the point of impact on the target. Thus, for increased peak power applied, there are life and reliability issues with respect to the target. Such effects may be countered to an extent by, for instance, spinning the target faster. However, doing so has implications to reliability and performance of other components within the x-ray tube. As a result there is greater emphasis in finding material and fabrication solutions for improved performance and higher reliability of target structures within an x-ray tube. Furthermore, there is greater emphasis on repair and reuse of x-ray tube targets and other x-ray tube components. Thus there is a need to salvage what might otherwise be unrecoverable x-ray tube targets. 
   Therefore, it would be desirable to have a method and apparatus to improve target track fabrication and repair of an x-ray tube target. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides a method and apparatus that overcome the aforementioned drawbacks. The x-ray target track is fabricated or repaired using a laser beam to heat the substrate of the target while applying a material to the substrate in order to fuse the materials together. The process may be performed multiple times to form layered or graded structures or interfaces, and it may be performed to fabricate complex geometries of track material on the surface of the target. 
   According to one aspect of the present invention, a composite target for generating x-rays includes a target substrate and at least one material applied to the target substrate with a laser beam. 
   In accordance with another aspect of the invention, a method of fabricating an x-ray target assembly includes forming an x-ray target substrate, directing at least one laser beam toward a surface of the x-ray target substrate to create a first heated area, and applying a first layer of at least one powder to the first heated area. 
   Yet another aspect of the present invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray emission source includes an anode having a target base material and a track comprising at least one layer of a track material applied to the target base material using a laser process. 
   Still another aspect of the present invention includes a method of repairing a target for an x-ray tube. The method includes applying at least one laser beam to a surface of the x-ray tube target to create a heated area and applying a powder material to the heated area. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a block diagram of an imaging system that can benefit from incorporation of an embodiment of the present invention. 
       FIG. 2  is a cross-sectional view of an x-ray tube useable with the system illustrated in  FIG. 1  according to an embodiment of the present invention. 
       FIG. 3  is a flowchart of a target fabrication or repair process according to an embodiment of the present invention. 
       FIG. 4  is a cross-sectional view of an x-ray tube target according to an embodiment of the present invention. 
       FIG. 5  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a block diagram of an embodiment of an imaging system  10  designed both to acquire original image data and to process the image data for display and/or analysis in accordance with the present invention. It will be appreciated by those skilled in the art that the present invention is applicable to numerous medical imaging systems implementing an x-ray tube, such as a CT system, an x-ray system, a vascular system, and a mammography system. Other imaging systems such as computed tomography systems and digital radiography systems, which acquire image three dimensional data for a volume, also benefit from the present invention. The following discussion of x-ray system  10  is merely an example of one such implementation and is not intended to be limiting in terms of modality. 
   As shown in  FIG. 1 , x-ray system  10  includes an x-ray source  12  configured to project a beam of x-rays  14  through an object  16 . Object  16  may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source  12  may be a conventional x-ray tube producing x-rays having a spectrum of energies that range, typically, from 30 keV to 200 keV. The x-rays  14  pass through object  16  and, after being attenuated by the object, impinge upon a detector  18 . Each detector in detector  18  produces an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  16 . In one embodiment, detector  18  is a scintillation based detector, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, etc.) may also be implemented. 
   A processor  20  receives the signals from the detector  18  and generates an image corresponding to the object  16  being scanned. A computer  22  communicates with processor  20  to enable an operator, using operator console  24 , to control the scanning parameters and to view the generated image. That is, operator console  24  includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system  10  and view the reconstructed image or other data from computer  22  on a display unit  26 . Additionally, console  24  allows an operator to store the generated image in a storage device  28  which may include hard drives, floppy discs, compact discs, etc. The operator may also use console  24  to provide commands and instructions to computer  22  for controlling a source controller  30  that provides power and timing signals to x-ray source  12 . 
   Moreover, the present invention will be described with respect to use in an x-ray tube. However, one skilled in the art will further appreciate that the present invention is equally applicable for other systems that require operation of a target used for the production of x-rays wherein high peak temperatures are driven by peak power requirements. 
     FIG. 2  illustrates a cross-sectional view of an x-ray tube  12  that can benefit from incorporation of an embodiment of the present invention. The x-ray tube  12  includes a casing  50  having a radiation emission passage  52  formed therein. The casing  50  encloses a vacuum  54  and houses an anode  56 , a bearing assembly  58 , a cathode  60 , and a rotor  62 . X-rays  14  are produced when high-speed electrons are suddenly decelerated when directed from the cathode  60  to the anode  56  via a potential difference therebetween of, for example, 60 thousand volts or more in the case of CT applications. The electrons impact a material layer or target track  86  at focal point  61  and x-rays  14  emit therefrom. The point of impact is typically referred to in the industry as the focal spot, which forms a circular region or track on the surface of the target track  86 , and is visually evident on the target surface after operation of the x-ray tube  12 . According to an embodiment of the present invention, target track  86  may include a plurality of layers  92 ,  93 ,  94  applied according to the disclosed process. The x-rays  15  emit through the radiation emission passage  52  toward a detector array, such as detector  18  of  FIG. 1 . To avoid overheating the anode  56  from the electrons, the anode  56  is rotated at a high rate of speed about a centerline  64  at, for example, 90-250 Hz. 
   The bearing assembly  58  includes a center shaft  66  attached to the rotor  62  at first end  68  and attached to the anode  56  at second end  70 . A front inner race  72  and a rear inner race  74  rollingly engage a plurality of front balls  76  and a plurality of rear balls  78 , respectively. Bearing assembly  58  also includes a front outer race  80  and a rear outer race  82  configured to rollingly engage and position, respectively, the plurality of front balls  76  and the plurality of rear balls  78 . Bearing assembly  58  includes a stem  83  which is supported by the x-ray tube  12 . A stator (not shown) is positioned radially external to and drives the rotor  62 , which rotationally drives anode  56 . As shown in  FIG. 2 , a heat storage medium  90 , such as graphite, may be used to sink and/or dissipate heat built-up near the target track  63 . 
   Referring still to  FIG. 2 , the anode  56  includes a target substrate  84 , having target track  86  attached thereto according to an embodiment of the present invention. The target track  86  typically includes tungsten or an alloy of tungsten such as tungsten with rhenium ranging from 3-10%. The target substrate  84  typically includes molybdenum or an alloy of molybdenum such as TZM (Titanium, Zirconium, and Molybdenum). 
   According to embodiments of the present invention, the target track  86  may be applied to a base substrate such as target substrate  84  by a laser consolidation process  96  as illustrated in  FIG. 3 . In process  96 , the target substrate  84  is prepared at step  97 , which may include, but is not limited to: 1) heat treatment such as may be required for densification, stress relief, and the like; 2) surface preparation which may include cleaning, fusing, roughening, and the like; and 3) cleaning and mounting of the target substrate  84  in a fixture. At step  98 , one or more beams of laser energy are arranged to impinge an area of the target substrate  84 , thus heating a region of the target substrate  84 . In one embodiment of the present invention, the heating of the target substrate  84  is adequate to melt a region of the target substrate  84 . At step  99 , powdered material is typically simultaneously supplied through a feeder to the heated region of the target substrate  84  at a rate that is controlled so that the added material melts and bonds with the underlying material of the target substrate  84 . At step  100 , after application of a layer, it is determined whether another layer is desired and, if so, the process at  101  repeats steps  97 ,  98  and  99 , which may include changing the material of the powder to be applied as described above. If no further layers are desired, then at step  102 , the process calls for moving to a post-processing step at  103 , during which the target may be removed, cleaned, and otherwise prepared for further assembly with anode  56 . The target track  86  typically may range from thicknesses ranging from tens of microns in thickness to hundreds of microns in thickness. 
   Referring now to  FIG. 4 , a multi-layer target track  86  may be applied to the target substrate  84  according to an embodiment of the present invention employing process  96  as described in  FIG. 3 . A first layer  92  is applied to the target substrate  84  as described above. Then, each succeeding layer  93 ,  94  is applied on preceding layers  92 ,  93 , respectively, one at a time as described above such that layers  92 ,  93  serve as base substrates for layers  93 ,  94 , respectively. In one embodiment, layer  92  is tungsten, layer  93  is rhenium, and layer  94  is an alloy of tungsten and rhenium. It is recognized that target track  86  may include more or less than three layers, or that the layers  92 - 94  may include combinations and alloys thereof. It is further recognized that the layers  92 - 94  may be applied with powder that contains a mix of alloying components. As an example, layer  92 , for instance, may be applied using a powder having 5% rhenium and in a mixture. As such, layer  92  may be applied as an alloy that will form upon impingement with the heated region on the target substrate  98 . 
   Process  96  may be altered from that described above, according to embodiments of the present invention, to use other materials such as rhodium and its alloys, alloys of tungsten, alloys of molybdenum, alloys of tantalum, alloys of rhenium, and other refractory and non-refractory metals. For instance, one skilled in the art will recognize that specific properties of the target track  86  may be affected according to the thicknesses of individual layers  92 - 94  applied to the substrate  84 , how many layers  92 - 94  are applied overall, and the selection of powders and their mixtures during process  96  at step  99 . Material properties that may be affected by appropriate selection of process  96  parameters include but are not limited to surface emissivity, coefficient of thermal expansion (CTE), thermal conductivity, fatigue strength and crack resistance, and elastic modulus. For instance, one skilled in the art will recognize that tantalum, having a relatively high CTE and a relatively low elastic modulus as compared generally to other metals, may be applied as one or more layers to affect the overall CTE and elastic modulus of target track  86 . Furthermore, such materials may not be limited to use as x-ray emission materials, but may also be applied according to an embodiment of this invention as braze materials including, but not limited to, zirconium, titanium, vanadium, and platinum. Such materials may also be used for surface emissivity enhancement. Additionally, one skilled in the art would recognize that layers of materials  92 ,  94 ,  96  may be applied to the target substrate  84  to protrude or extend from a surface of the substrate  84 . 
   One skilled in the art will further recognize that many combinations of materials may be applied in powder form at step  99  of process  96 . For instance, a gradient of materials may be applied to fabricate target track  86  by applying, for instance, first layer  92  having 75% tungsten and 25% rhenium, and second layer  93  having 90% tungsten and 10% rhenium. As such, target track  86  may be formed having a gradient, or varying concentration of elements, therein, by appropriately selecting and varying the alloying elements from one layer to the next. 
   Materials applied using the process described herein need not be limited to those described above. One skilled in the art will recognize that, in addition to metals, oxides including oxides of lanthanum, yttrium, aluminum, and zirconium may be applied according to embodiments of the present invention. Furthermore, carbides, such as carbides of titanium, hafnium, and boron may be applied as well. 
   The process  96  disclosed herein can likewise be performed on pre-formed target cap materials. Accordingly, the materials deposited thereon may include wrought materials as well. Additionally, the process described herein allows the deposition of graded structures of track material, as well as complex geometries. 
   The process described herein need not be limited to new x-ray target fabrication, but may be applicable to repair and reuse of targets as well. Accordingly, targets may be salvageable by disassembling them from the x-ray tube and reprocessing them by using the method described herein. Targets having track material  86  damaged after use may be recovered by having the target track  86  replaced or repaired. Additionally, new targets fabricated with defects that may include but are not limited to pits, cracks, and voids may be recoverable via this method as well. As such, target preparation step  97  of process  96  may include but is not limited to target disassembly from an anode  56 , and machining or grinding of the target track  86  to expose the substrate  84  prior to applying a first layer  92 . 
   High-density coatings may be fabricated with this method as well. Density problems inherent in, for instance, a plasma-spray process may be mitigated by use of this process to apply high-density coatings to increase mechanical properties such as spallation and fatigue resistance. For some materials and material combinations, post-processing including but not limited to hot isostatic pressing (HIP) processing may be required. 
   Referring now to  FIG. 5 , package/baggage inspection system  510  includes a rotatable gantry  512  having an opening  514  therein through which packages or pieces of baggage may pass. The rotatable gantry  512  houses an x-ray energy source  516  as well as a detector assembly  518  having scintillator arrays comprised of scintillator cells similar to that shown in  FIG. 4  or  5 . A conveyor system  520  is also provided and includes a conveyor belt  522  supported by structure  524  to automatically and continuously pass packages or baggage pieces  526  through opening  514  to be scanned. Objects  526  are fed through opening  514  by conveyor belt  522 , imaging data is then acquired, and the conveyor belt  522  removes the packages  526  from opening  514  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  526  for explosives, knives, guns, contraband, etc. 
   According to one embodiment of the present invention, a composite target for generating x-rays includes a target for generating x-rays and includes a target substrate and at least one material applied to the target substrate with a laser beam. 
   In accordance with another embodiment of the invention, a method of fabricating an x-ray target assembly includes forming an x-ray target substrate, directing at least one laser beam toward a surface of the x-ray target substrate to create a first heated area, and applying a first layer of at least one powder to the first heated area. 
   Yet another embodiment of the present invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray emission source includes an anode having a target base material and a track comprising at least one layer of a track material applied to the target base material using a laser process. 
   Still another embodiment of the present invention includes a method of repairing a target for an x-ray tube. The method includes applying at least one laser beam to a surface of the x-ray tube target to create a heated area and applying a powder material to the heated area. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.