Abstract:
The present embodiments relate to efficient electron beam steering within X-ray tubes, for example X-ray tubes used in CT imaging. In one embodiment, and X-ray tube with enhanced electron beam steering is provided. The X-ray tube includes an electron beam source, a target configured to generate X-rays when impacted by an electron beam from the electron beam source, and a steering magnet assembly having a plurality of ferrite cores and a plurality of litz wire coils wound on the ferrite cores.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to X-ray tubes and, in particular, to electron beam steering within an X-ray tube. 
         [0002]    In non-invasive imaging systems, X-ray tubes are used in fluoroscopy, projection X-ray, tomosynthesis, and computer tomography (CT) systems as a source of X-ray radiation. Typically, the X-ray tube includes a cathode and a target. A thermionic filament within the cathode emits a stream of electrons towards the target in response to heat resulting from an applied electrical current, with the electrons eventually impacting the target. A steering magnet assembly within the X-ray tube may control the size and location of the electron stream as it hits the target. Once the target is bombarded with the stream of electrons, it produces X-ray radiation. 
         [0003]    The X-ray radiation traverses a subject of interest, such as a human patient or baggage, and a portion of the radiation impacts a detector or photographic plate where the image data is collected. In a medical diagnostic context, tissues that differentially absorb or attenuate the flow of X-ray photons through the subject of interest produce contrast in a resulting image. In some X-ray systems, the photographic plate is then developed to produce an image which may be used by a radiologist or attending physician for diagnostic purposes. In other contexts, parts, baggage, parcels, and other subjects may be imaged to assess their contents and for other purposes. In digital X-ray systems, a digital detector produces signals representative of the received X-ray radiation that impacts discrete pixel regions of a detector surface. The signals may then be processed to generate an image that may be displayed for review. In CT systems, a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is displaced around a patient. 
         [0004]    One method of imaging in CT systems includes dual energy imaging. In a dual energy application, data is acquired from an object using two operating voltages of an X-ray source to obtain two sets of measured intensity data using different X-ray spectra, which are representative of the X-ray flux that impinges on a detector element during a given exposure time. Since projection data sets corresponding to two separate energy spectra must be acquired, the operating voltage of the X-ray tube is typically switched rapidly so that the same anatomy is sampled at both high and low x-ray energy to prevent image degradation due to object motion. 
         [0005]    For X-ray systems using the fast voltage switching methods as well as X-ray systems that have wobble capabilities, eddy currents may be induced into the beam pipe through which the electron beam passes, the core of the magnets used to steer the beam, and the windings of the steering magnet assembly. Such induction may slow response time for deflection of the electron stream, and thus may result in increased transition time and reduced exposure at a required power level. Accordingly, a need exists for improved response times within the steering magnet assembly. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one embodiment, an X-ray tube is provided. The X-ray tube includes an electron beam source, a target configured to generate X-rays when impacted by an electron beam from the electron beam source, and a steering magnet assembly disposed between the electron beam source and the target. The steering magnet assembly has a plurality of ferrite cores and a plurality of litz wire coils wound on the ferrite cores. 
         [0007]    In another embodiment, a method for making an X-ray tube is provided. The method includes forming a steering magnet assembly comprising four substantially identical ferrite cores including two cathode side cores and two target side cores. Additionally, a plurality of cathode side quadrupole coils comprising litz wire is wound on the cathode side cores and coupled in series. Also, a plurality of target side quadrupole coils comprising litz wire is wound on the target side cores and coupled in series. The steering magnet assembly is disposed between an electron beam source and a target. Additionally, the coils are coupled to power supplies configured to switch current in the coils at a frequency of at least 100 kHz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  is a perspective view of an X-ray tube, in accordance with an embodiment of the present invention; 
           [0010]      FIG. 2  is a cross-sectional side view of a portion of the X-ray tube depicted in  FIG. 1 ; 
           [0011]      FIG. 3  is a perspective view of a steering magnet sub-assembly; and 
           [0012]      FIG. 4  is schematic illustration of the position of the beam pipe, magnetic poles, and the electromagnet coils within an X-ray tube. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The present embodiments are directed towards a system and method for enhancing response time of a steering magnet assembly. For example, in embodiments of an X-ray tube wherein the steering magnet assembly controls steering and wobble of an electron stream through the use of electromagnets, eddy currents induced into the beam pipe, magnet core, and magnet windings may be reduced by selecting an appropriate core material, selecting an appropriate material for the electromagnet coil windings, and defining proper positioning of the magnet poles with respect to the electron beam pipe. The reduction in eddy currents may considerably reduce the response time of the steering magnet assembly. 
         [0014]    The electromagnet steering techniques described herein may be utilized in an X-ray tube, such as X-ray tubes utilized in digital and photographic projection X-ray systems, fluoroscopy imaging systems, tomosynthesis imaging systems, CT imaging systems, and so on.  FIG. 1  illustrates such an X-ray tube  10  for obtaining X-rays useful for imaging systems designed to acquire X-ray data, to reconstruct an image based upon the data, and to process the image data for display and analysis. 
         [0015]    In the embodiment illustrated in  FIG. 1 , the X-ray tube  10  includes a cathode assembly. The cathode assembly  12  accelerates a stream of electrons through the X-ray tube  10 , including through the steering magnet assembly  14 , designed to control steering and size of the electron stream. The steering magnet assembly may include two sub-assemblies with multiple quadrupole and dipole magnets configured to provide steering and wobble capabilities for the stream of electrons within the X-ray tube  10 . As a result of a collision of the electrons with a target within the X-ray tube  10 , X-rays are produced. Focal X-ray radiation is emitted through the window  16 , where it may be useful in obtaining X-ray imaging data. 
         [0016]      FIG. 2  depicts a cross-sectional view of the X-ray tube embodiment of  FIG. 1 , in an effort to more clearly explain the current techniques. As previously discussed, cathode assembly  12  may accelerate an electron stream  18  through the X-ray tube  10 . The electron stream  18  may pass through a throat, or electron beam pipe,  20  of the steering magnet assembly  14 . As the electron stream  18  passes through the electron beam pipe  20 , the steering magnet assembly  14  may provide electromagnetic fields through electromagnets  22 , controlling the size and position of electron stream  18 . Thus, the steering magnet assembly  14  provides for steering of the electron stream as well as the ability to quickly change the position of the electron stream, for wobble. The electromagnets  22  may be encased in epoxy, to create a path around the electron beam pipe  20  of the steering magnet assembly  14  as well as provide structural integrity for the steering magnet assembly  14 . Next, the electron stream may pass through an electron collector  24  and collide with a target  26 . The collision of the electron stream  18  with the target may result in some electrons or secondary radiation bouncing back into the beam pipe. As illustrated, the electron collector  24  may be disposed in facing relation to the target  26 , allowing the electron collector  24  to capture and contain electrons and radiation that may be directed from the target  26  back into the electron collector  24 . Focal X-ray radiation is produced and emitted through the window  16 . Off-focal X-ray radiation  28  may be directed inwardly, back through the X-ray tube  10 , reaching the steering magnet assembly  14 . The off-focal X-ray radiation  28  may be attenuated by X-ray shielding materials. The steering magnet assembly  14  may obtain structural support by being in support base  30 , which extends to exterior walls  32 . The support base  30  may be designed to receive and couple magnetic sub-assemblies making up the steering magnet assembly  14 . 
         [0017]      FIG. 3  illustrates an embodiment of one magnet subassembly  36 , or one half of a full magnet assembly utilized in a steering magnet assembly  14 . The full magnet assembly may consist of two substantially identical magnet subassemblies  36 . The magnet subassembly  36  may include a frame  38 , capable of uniting the various elements of the magnet assembly  36 . The magnet subassembly  36  may include a plurality of cores (i.e., cathode side cores  40  and target side cores  42 ). The inventors have found that selecting an appropriate core material may have a considerable effect on steering response times within the steering magnet assembly  14 . For example, core materials with lower permeability and higher bulk resistivity may reduce eddy currents in the core material, thus decreasing response time. Examples of such core materials may include ferrites. More specifically, the use of soft ferrites such as nickel zinc (Ni—Zn) or (Mn—Zn) may be warranted. The cathode side cores  40  and the target side cores  42  may include radial extensions  44 , which may act as poles for the magnet subassembly  36 . 
         [0018]    The cathode side core  40  and the target side core  42  may include several coils created by winding wire around portions of the cathode side core  40  and target side core  42 . By utilizing litz wire instead of solid conductors for the windings, inductance in the coils may be reduced, thus decreasing response time. As illustrated, the cathode side core may include litz wire coils formed along the radial extensions  44  of the cathode side core  40 . Litz wire is made in different sizes with varying numbers of conductors within the wire. In a preferred embodiment, the litz wire may be approximately an 18 gauge wire and may include at least 100 conductors. The target side core  42  may also include a plurality of coils (i.e., inner target side quadrupole coils  48 , outer target side quadrupole coils  50 , and additional target side coils  52 ). The inner target side quadrupole coils  48  may be formed on the radial extensions  44  of the target side core  42 . The outer target side quadrupole coils  50  may be formed over the inner target side quadrupole coils  48 . Additional target side coils  52  may be formed on spans of the target side core  42 . The dipole and quadrupole windings are formed on the same pole piece to make the assembly compact by utilizing the same poles for both focusing and deflection. 
         [0019]    As previously mentioned, the magnet sub-assembly  36  depicted in  FIG. 3  represents one half of the full magnet assembly. The other one half of the full magnet assembly may be substantially identical to magnet sub-assembly  36 . Thus, the full magnet assembly, in accordance with the embodiment of magnet subassembly  36  of  FIG. 3 , may include two cathode side cores  40 , two target side cores  42 , eight radial extensions  44  (four on the cathode side cores  40  and four on the target side cores  42 ), four cathode side quadrupole coils  46 , four inner target side quadrupole coils  48 , four outer target side quadrupole coils  50 , and two additional target side coils  52 . The coils may be coupled in series based upon their groupings. For example, the cathode side quadrupole coils may be coupled in series by connecting the first coil with the second, the second with the third, and the third with the fourth. This coupling is represented by the dashed line in  FIG. 3 . Additionally, the inner target side quadrupole coils  48  may be coupled in series, the outer target side quadrupole coils  50  may be coupled in series, and the additional target side coils  52  may be coupled in series. 
         [0020]    System control circuitry  54  may be coupled to a plurality of power supplies  56 . The plurality of power supplies  56  may be coupled to each set of coils coupled in series. For example, as depicted in the embodiment of  FIG. 3 , a first power supply  56  may be coupled to the cathode side quadrupole coils  46 , a second power supply  56  may be coupled to the inner target side quadrupole coils  48 , a third power supply  56  may be coupled to the additional target side coils  52 , and a forth power supply  56  may be coupled to the outer target side quadrupole coils  50 . The system control circuitry may control current switching in the coils. In some embodiments, the current switching will be at a frequency of at least 1 kHz. 
         [0021]    As previously mentioned, proper positioning of the electromagnet poles (i.e., radial extensions  44 ) with respect to the electron beam pipe may further reduce response time within the steering magnet assembly  14 . Enhanced magnetic field uniformity may be obtained by providing less spacing between the beam pipe diameter  58  and the cores (i.e., cathode side cores  40  and target side cores  42 ). Additionally field uniformity may be increased by extending the cores (i.e., cathode side cores  40  and target side cores  42 ) beyond the coils (i.e., cathode side quadrupole coils  46 , inner target side quadrupole coils  48 , outer target side quadrupole coils  50 , and additional target side coils  52 ).  FIG. 4  provides an illustration of positioning of the poles, coils and beam pipe, in accordance with an embodiment of the current techniques. 
         [0022]    Depicted are the two target side cores  42 , representing placement that would be achieved by coupling two magnet subassemblies  36 . The target side cores  42  include radial extensions  44 , acting as magnetic poles. As the distance  60  between the radial extensions  44  and the beam pipe diameter  58  decreases, the electromagnetic fields resulting from electrical current being supplied to the coils (i.e., outer target side quadrupole coils  50 ) may obtain increased coupling. While reducing distance  60  between the radial extensions  44  and the beam pipe diameter  58  creates increased coupling, it may not, in some embodiments, be feasible to obtain a distance of zero. Indeed, in some embodiments the cores (i.e., target side cores  42 ) may be encased in epoxy or other materials for structural support, cooling purposes, etc. In some embodiments, an example of a typical distance  60  between the radial extensions  44  and the beam pipe diameter may be less than 5 millimeters, leaving space around the pipe for oil/coolant circulation and for an epoxy encasment of the magnet assembly. 
         [0023]    In addition to minimizing the distance  60  between radial extensions  44  and the beam pipe diameter  58 , extending the distance  62  between the coils (i.e., the outer target side quadrupole coils  50 ) and the end of the radial extensions  42  may increase field uniformity, and thus increase effectiveness of the steering magnet assembly  14 . The radial extensions  42  protrude substantially inward to reduce distance  60  and the coils are formed either flush with the face of the radial extensions  42  or further backwards, away from the beam pipe diameter  58 , leaving distance  62 . 
         [0024]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.