Patent Publication Number: US-8542801-B2

Title: X-ray tube with secondary discharge attenuation

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to X-ray tubes and, in particular, to attenuation features for secondary discharges of X-rays within an X-ray tube. 
     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. Once the target is bombarded with the stream of electrons, it produces focal and off-focal X-ray radiation. 
     The focal X-ray radiation traverses a subject of interest, such as a human patient, and a portion of the radiation impacts a detector or photographic plate where the image data is collected. Generally, 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 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. 
     Despite the electron stream colliding with the target in the proper location, some X-rays do not exit through the window, but instead are projected back through the X-ray tube, and may result in secondary radiation. This off-focal X-ray radiation generated in the X-ray tube must be contained within the unit so that the X-rays do not exit to the environment. Traditionally, X-ray attenuation has been provided through the use of lead linings placed along the outer periphery of the tube unit. Environmental awareness and regulation has made these techniques less desirable. Furthermore, full enclosure shielding can be bulky, requiring a large amount of shielding material. Accordingly, a need exists from improved off-focal X-ray shielding in X-ray tubes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, an X-ray tube is provided. The X-ray tube includes a cathode configured to output an electron beam and a target configured to receive the electron beam and to generate X-rays. Additionally, the X-ray tube includes a magnetic focal spot control unit disposed between the cathode and the target. The magnetic focal spot control unit may generate electromagnetic fields to affect the electron beam. The magnetic focal spot control unit includes at least one electromagnet encased in a resin loaded with an X-ray attenuating material. 
     In another embodiment, an electromagnet for an X-ray tube is provided. The electromagnet includes an electromagnet assembly for a magnetic focal spot control unit designed to be disposed between a cathode and a target of an X-ray tube. The electromagnet assembly may generate electromagnetic fields to affect the electron beam. Additionally, the electromagnet is encased in a resin loaded with an X-ray attenuating material. 
     In a further embodiment, a method of forming an electromagnet is provided. The method generally includes doping a resin with an X-ray attenuating material, winding a coil around a magnet core, and encasing the magnet core and the coil in the loaded resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a perspective view of an X-ray tube, in accordance with present embodiments of the invention; 
         FIG. 2  is a cross-sectional side view of a portion of the X-ray tube depicted in  FIG. 1 ; 
         FIG. 3  is a perspective view of a part of the magnet assembly, depicting various features of the electromagnet. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present approach is directed towards a system and method for attenuating off-focal X-rays produced in an X-ray tube. For example, in embodiments of an X-ray tube wherein a magnetic focal spot control unit is present, attenuation materials surrounding the electromagnets within the magnetic focal spot control unit may provide the attenuation desired to contain the off focal, or secondary, X-rays. 
     The secondary discharge attenuation techniques discussed herein may be utilized in an X-ray tube, such as X-ray tubes utilized in projection X-ray imaging systems, fluoroscopy 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. 
     In the embodiment illustrated in  FIG. 1 , the X-ray tube  10  includes a cathode assembly  12 . The cathode assembly  12  accelerates a stream of electrons through the X-ray tube  10 , including through the magnetic focal spot control unit  14 , designed to control steering and size of the electron stream. The magnetic focal spot control unit may comprise two subassemblies with multiple quadrupole and dipole magnets configured to provide steering and wobble abilities for the stream of electrons. As a result of a collision of the electrons 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. The electron stream collision within the X-ray tube  10  may also result in off-focal X-ray radiation occurring within the X-ray tube. In an effort to decrease exposure to unnecessary radiation by the X-ray system operator and to decrease interference with the X-ray imaging system using the focal X-ray radiation, the off-focal X-ray radiation must be contained within the X-ray tube  10 . 
     As noted above, the present embodiments are directed towards attenuation of the off-focal X-ray radiation produced within the X-ray tube  10 . In accordance with the embodiments disclosed herein, attenuation may be performed by placing attenuation materials within the magnetic focal spot control unit  14 .  FIG. 2  depicts a cross-sectional view of the X-ray tube embodiment of  FIG. 1  to more clearly explain the current techniques. As previously discussed, cathode assembly  12  may accelerate an electron stream  18  through a common bore in the X-ray tube  10 . The electron stream  18  may pass through a throat  20  of the magnetic focal spot control unit  14 . As the electron stream  18  passes through the throat  20 , the magnetic focal spot control unit  14  may provide electromagnetic fields through electromagnets  22 , controlling the size and position of electron stream  18 . Thus, the magnetic focal spot control unit  14  provides for steering of the electron stream as well as the ability to quickly change the position of the electron stream, such as for wobble. The electromagnets  22  may include a resin encasement, which creates a path around the throat  20  of the magnetic focal spot control unit  14  as well as provides mechanical integrity to the magnet assembly. Additionally, as will be described in more detail below, the resin may be configured to provide X-ray attenuation characteristics within the X-ray tube  10 . 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 electrons bouncing back into the X-ray tube. 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 that bounce from the target  26  back into the electron collector  24 . Additionally, the collision may produce resultant X-ray radiation. Focal X-ray radiation is produced and emitted through the window  16 . Off-focal X-ray radiation  28  may be directed inward, back through the X-ray tube  10 , reaching the magnetic focal spot control unit  14 . As will be discussed in more detail below, the electromagnets  22  within the magnetic focal spot control unit  14  may be configured to attenuate the off-focal X-ray radiation, so that the X-ray radiation does not pass through a support base  30 , and more specifically through external surfaces  32  of the support base  30 . 
     In some embodiments, the electromagnets  22  within the magnetic focal spot control unit  14  may be formed into a magnet assembly.  FIG. 3  illustrates a partial cross-sectional view of one embodiment of a magnet assembly  36 , which may be incorporated into the magnetic focal spot control unit  14 .  FIG. 3  depicts one half of an electromagnet  22 . In some embodiments, the magnet assembly  36  may include a pair of substantially identical electromagnets  22 . The magnet assembly  36  may include a frame  38 , capable of uniting the various elements of the magnet assembly  36 . As is commonly found in electromagnets, the magnet assembly  36  may contain magnet cores  40 . The magnetic cores  40  may be contained within the magnet assembly  36  by resting on nests  42 . Windings  44  may surround the magnetic cores  40  in various locations of the core. As electrical current flows through the windings  44 , the cores  40  become magnetic and an electromagnetic field is formed. 
     As previously discussed, the electromagnets within the electromagnetic focal spot control unit  14  may attenuate the off-focal X-ray radiation  28 . Providing attenuation within the magnetic focal spot control unit  14  may provide more efficient X-ray shielding than shielding external to the X-ray tube, by attenuating the off-focal X-rays at a sight of greater flux. The attenuation features of the electromagnets  22  and ultimately the electromagnet assembly  36  may be achieved by providing a resin encasement for the electromagnets  22 , where the resin  46  is loaded with X-ray attenuating materials. The X-ray attenuating materials incorporated into the resin  46  may consist of high-density, non-magnetic materials that have a low magnetic permeability. Additionally, it may be desirable that the attenuating materials have little to no electrical conductance, as conductive materials may affect the electromagnetic field generated by the electromagnets  22 . For example, tungsten, while high density and capable of X-ray attenuation, is also conductive and thus may interfere with the electromagnetic field produced by the electromagnets  22 . Examples of a few suitable attenuating materials may include bismuth oxide, lead oxide, or barium sulfate. The ratio of resin  46  to attenuating materials may affect the attenuation characteristics of the electromagnets  22 . Increasing the percent by volume of attenuating materials may increase the attenuation capabilities of the resin. Furthermore, the percent by volume of attenuating materials may be controlled based upon the desired thickness of the resin  46  loaded with the attenuating material or based upon the amount of attenuation desired by the encased electromagnets  22 . For example, in one embodiment, the resin may have a thickness of 9 mm. At a 9 mm thickness level, to obtain full attenuation, it may beneficial for the resin  46  to contain at least approximately 50% bismuth oxide by volume. The percent by volume may be reduced if full attenuation is not required. For example, if full attenuation is not necessary, the amount of bismuth oxide may be reduced to approximately 40% by volume, providing approximately 99% attenuation. 
     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.