Abstract:
An x-ray source with an x-ray source target are provided. The x-ray source includes an electron source. The x-ray source also includes an x-ray transmission window. The x-ray source also includes an x-ray source target located between the electron source and the window, wherein the target is arranged to receive electrons from the electron source to generate x-rays in the x-ray source target, and a rotational mechanism adapted to rotate the x-ray source target. A method of producing x-rays and an x-ray target are also provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention is related generally to an x-ray source, an x-ray source target, and a method of operating the same.  
           [0002]    CT (computed tomography) scanning typically uses X-rays to gain two-dimensional (2D) or three-dimensional (3D) information on a scanned object. The X-rays are generated when an electron beam hits a target with a high atomic number, i.e., a target including a high density material. These electrons are typically produced by a hot filament and they are accelerated to the target by a large potential, typically 80 to 120 kV for CT scanning. When the electrons strike the target they interact with the target atoms and generate the x-rays needed for a CT scan.  
           [0003]    CT scanning allows a physician to obtain a 2D or planar cross sectional image of a patient. CT scanning can thus reveal anatomical detail for diagnostic purposes. Many such 2D images can be added together to generate a volume in helical or step-and-shoot modes. However, tradeoffs between axial coverage (i.e., the coverage of the patient along the axis of the CT system in a single rotation) and image quality (spatial resolution and noise) limit this coverage cone beam artifacts to about 80 mm because of cone beam artifacts. To provide coverage larger than this with good image quality, x-ray sources with multiple focal spots (i.e., the x-ray source target is impinged by electron beams in multiple spots) must be used.  
           [0004]    U.S. Pat. No. 6,125,167 to Picker discloses a multiple spot target design. Picker discloses a conventional reflection x-ray design, wherein the x-rays are reflected from the x-ray generating material, using multiple discs. A multiple spot target design is also disclosed in U.S. Pat. No. 6,118,853 to Hansen et al. The target in this design is stationary and the incident electron beam angle is roughly 90 degrees.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with one aspect of the present invention, there is provided an x-ray source. The x-ray source comprises an electron source; an x-ray transmission window; an x-ray source target located between the electron source and the window, wherein the target is arranged to receive electrons from the electron source to generate x-rays in the x-ray source target; and a rotational mechanism adapted to rotate the x-ray source target.  
           [0006]    In accordance with another aspect of the present invention, there is provided a method of producing x-rays. The method comprises rotating an x-ray source target; directing electrons from an electron source to the x-ray source target to generate x-rays in the x-ray source target while the x-ray source target is rotating; and transmitting the x-rays through the x-ray source target through an x-ray window.  
           [0007]    In accordance with another aspect of the present invention, there is provided an x-ray source target comprising a high density material for generating x-rays; and a support structure supporting the high density material, wherein the support structure is generally shaped as a hollow cylinder with a central axis and has a plurality of notches extending generally radially to the central axis. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is side cross sectional view of an x-ray source according to an exemplary embodiment of the invention.  
         [0009]    [0009]FIG. 2 is an enlarged view of a portion of the x-ray source of FIG. 1.  
         [0010]    [0010]FIG. 3 is a side view of a notch in an x-ray source target according to an embodiment of the invention.  
         [0011]    [0011]FIG. 4 is a side view of a notch in an x-ray source target according to another embodiment of the invention.  
         [0012]    [0012]FIG. 5 is a front view of the x-ray source target and plate of the source of the embodiment of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    Reference will now be made in detail to presently preferred embodiments of the present invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0014]    The present inventors have realized that prior art multiple spot x-ray target designs may be limited in output of x-rays if not designed appropriately. When electrons from an electron beam hit a target and are deflected, over 99% of the electron&#39;s energy is dissipated as heat. Thus, the challenge is to design an x-ray target and source such that the source produces sufficient x-rays while not overheating the target surface.  
         [0015]    The present inventors have realized that a solution to overheating of the target for a multiple spot target design, and/or maintaining good x-ray parameters, can be accomplished through any one or more of the following three different avenues: (i) developing a source wherein multiple x-ray generating locations can be turned on simultaneously, (ii) continually rotating the target so that new, cooler material is continually being introduced into the electron beam(s), and (iii) angling the surface of the target with respect to the electron beam(s) so that it has a long thermal length yet retaining a small x-ray focal spot dimension.  
         [0016]    [0016]FIG. 1 illustrates a side cross-sectional view of an x-ray source  10  according to one preferred embodiment of the invention. The x-ray source  10  includes a grounded anode frame  12  which encloses a cathode assembly  14 . The cathode assembly  14  comprises an electron source  16  which includes a number of individual electron sources  16   a,    16   b,    16   c,    16   d,    16   e,    16   f,    16   g,    16   h,    16   i,    16   j.  The number of individual electron sources is shown as numbering ten for ease of illustration. The number of individual electron sources of the electron source  16  may of course be more or less than ten.  
         [0017]    The electron source  16  directs electrons to an x-ray source target  20 . The x-ray source  10  includes a motor assembly  24  that acts to rotate the x-ray source target  20 . The motor assembly  24  includes a motor  26  that drives and rotates a drive shaft  28 . The drive shaft  28  in turn is attached to, and drives, a plate  30 . The x-ray source target  20  is coupled to plate  30  such that when the motor is driven, the x-ray source target  20  can be rotated about the cathode assembly  14 .  
         [0018]    The x-ray source  10  also includes an x-ray transmission window  34 . The x-ray transmission window may comprise any x-ray transmissive material, such as, for example, beryllium or aluminum.  
         [0019]    The x-ray source target  20  includes a plurality of notches  36 . The target  20  is positioned such that the individual electron sources of the electron source  16  each provide an individual electron beam that is directed into a respective one of the notches  36 . X-rays are generated in the x-ray source target  20  and these x-rays are transmitted through the region of the target  20  near where the electrons impinge and then onto and out of the x-ray window  34 . The target  20  is thus arranged as a target with the electron source  16  on one side of the region of the target  20  where the electrons impinge, and the x-ray window  34  arranged on the other side.  
         [0020]    The x-ray source  10  also includes an insulator  40  that surrounds and supports the cathode assembly  14  and insulates the cathode assembly  14  from the grounded anode frame  12 . The insulator  40  in turn is supported by the grounded anode frame  12 .  
         [0021]    The cathode assembly  14  includes a number of control connections  42  that provide control for respective of the individual electron sources  16   a,    16   b,    16   c,    16   d ,  16   e,    16   f,    16   g,    16   h,    16   i,    16   j  (see FIG. 2) through electronics (not shown). The individual electron sources  16   a,    16   b,    16   c,    16   d,    16   e,    16   f,    16   g,    16   h,    16   i,    16   j  may be electron emitters, such as for example, thermionic heated tungsten filaments or field emission sources.  
         [0022]    [0022]FIG. 2 is an enlarged view of a portion of the x-ray source showing the cathode assembly  14 , x-ray source target  20  and plate  30 . The x-ray source target  20  preferably comprises a support structure  50  and a high density material film  52 . The support structure  50  or a tungsten film acts to support the high density material film  52 , such as a tungsten film, but need not be of a high density material. It is preferable that the support structure  50  comprise a material that is not a high density material, such as graphite for example, so that x-rays are generated substantially only in the high density material film  52 . The x-rays generated in the high density material  52  may pass through the support structure  50  and onto the x-ray window  34  (shown in FIG. 1). Preferably films  52  are located only in notches  36 . Alternatively, the support structure  50  may be made of a high density material and high density material films may be eliminated. The high density material  52  may be, for example, tungsten or a tungsten alloy, molybdenum, tantalum or rhenium.  
         [0023]    The length of the electron source  16 , and also the length of the region of the target  20  containing the notches  36 , will depend upon the particular application. A longer length will provide an x-ray source that provides x-rays over a greater axial length without cone beam CT artifacts, and thus a greater axial length of an object may imaged using this extended x-ray source. The length of object which can be imaged without significant cone beam CT artifacts from a single-spot x-ray source in the axial scanning mode is limited to about 40 mm.  
         [0024]    [0024]FIGS. 1 and 2 are side cross sectional views of the x-ray source  10  and a portion of the source  10 , respectively. Thus, the x-ray source target  20  is also shown in side cross sectional view. The x-ray source target  20  is preferably arranged to rotate such that the electrons from the electron source  16  continually impinge in the notches  26 . The target is preferably shaped as a hollow cylinder which rotates about its rotational axis. The rotational axis is substantially the same as the central axis  100  of the cylinder. The notches  36  may extend generally radially to this central axis  100 , on the interior surface of cylinder  20 . The cathode assembly  14  including the electron source  16  is positioned inside the cylinder. Other configurations can be used if desired. For example, target  20  may comprise a flat rotating disk located above the window  34  with a line of electron beams impinging on its top surface.  
         [0025]    [0025]FIG. 5 is a front view of a portion of the source  10  of FIG. 1 illustrating the x-ray source target  20  and plate  30 . The central axis  100  of the x-ray source target  20  points out of the page in FIG. 5.  
         [0026]    The rotation of the x-ray source target  20  prevents the region of the target  20  which is receiving the electrons from overheating, because the region of the target  20  receiving the electrons is continually changing. The rotational speed of the x-ray source target  20  will depend upon the particular application. In applications where the rate of electrons impinging upon the target  20  is lower, the rotational speed of the target  20  may also be lowered without risk of overheating the target  20 . An exemplary speed range is 3,000 to 10,000 rpm.  
         [0027]    [0027]FIG. 3 is a side view of a notch  36  of the plurality of notches  36  according to an embodiment of the invention. In this embodiment the notch  36  includes a side surface  60 . The high density material film  52  is preferably located on the side surface  52  but not the bottom  63  of notch  36 . However, film  52  may cover every surface of notch  36 . The individual electron beam  62  from one of the individual electron sources (see FIGS.  1  or  2 ), impinges upon the side surface  60 . Preferably the electron beam  62  impinges only upon the side surface  60 , and not substantially upon a bottom  63  of the notch. Preferably the electron beam  62  is directed at an angle θ with respect to a normal  64  (the normal  64  is a line that is perpendicular to the side surface  60 ) in a range of between 80 and 90 degrees. A radial line from the side surface  60  to the central axis  100  (See FIG. 1) makes an angle θ 2  with respect to the normal  64  which is the same as the angle θ.  
         [0028]    Because the angle θ is relatively large, i.e. somewhere near 90°, the electron beam  62  impinges over a substantial portion of the side surface  60 , and the electron beam focal spot size, i.e., the area of the side surface  60  upon which the electron beam is impinged, is relatively large. This increase in the electron beam focal spot size reduces the temperature locally at the side surface  60  because the electrons scattered by the high density material film  52  will tend to be absorbed over a wider spread out area by the support  50 . Thus, the heat will also be spread out over a larger volume of the target  20 .  
         [0029]    [0029]FIG. 3 also illustrates the size of the x-ray beam  70  emerging from the support  50 . While the electron beam focal spot size is increased by increasing the angle between the direction of the electron beam  62  and the normal  64 , the x-ray beam  70  spot size, i.e., the cross-sectional area of the x-ray beam, is not substantially increased. This embodiment provides good heat spreading properties, thus beneficially lowering temperature of the region of the high density material upon which the electron beam is impinging, while at the same time the spot size of the x-ray beam is not substantially increased.  
         [0030]    FIGS.  1 - 3  illustrate an x-ray source according to a transmission design, where the x-rays produced in the high density material film are substantially transmitted through the high density material  52  to the x-ray transmission window. In this case the thickness of the high density material  52  may be less than about 20 μm, and a radial line from the side surface  60  to the central axis  100  (See FIG. 1) makes an angle θ 2  with respect to the normal  64  which is less than 90° . The high density material  52  in this embodiment should be thin enough not to substantially absorb the x-rays generated so that they may be transmitted therethrough.  
         [0031]    [0031]FIG. 4 illustrates another embodiment where the x-rays produced in the high density material film are x-rays are substantially reflected from the high density material, and not substantially transmitted through the high density material to the x-ray transmission window. In this embodiment the notch has a side surface  80 . The high density material film  52  is preferably located on the side surface  80  but not the bottom  83  of notch  36 . The individual electron beam  82  from an individual electron sources, impinges upon the side surface  80 . In this embodiment the electron beam  82  from the individual electron source is oriented at a non-normal angle to the x-ray transmission window. Preferably the electron beam  82  impinges only upon the side surface  80 , and not substantially upon a bottom  83  of the notch. Preferably the electron beam  62  is directed at an angle θ with respect to a normal  84  in a range of between 80 and 90 degrees. A radial line from the side surface  60  to the central axis  100  (See FIG. 1) makes an angle θ 2  with respect to the normal  64  which is greater than the angle θ, and is greater than 90°.  
         [0032]    In the embodiment of FIG. 4, the x-ray source  10  shown in FIG. 1 is implemented with the individual electron sources are oriented so that they impinge at the angle shown in FIG. 4.  
         [0033]    In the embodiment of FIG. 4, the thickness of the high density material  52  may be greater than about 30 μm, and a radial line from the side surface  80  to the central axis  100  (See FIG. 1) makes an angle θ 2  with respect to the normal  84  which is greater than 90°. The high density material  82  in this embodiment should be thick enough to substantially absorb the x-rays generated so that are not substantially transmitted therethrough.  
         [0034]    The x-ray source and target described above provides a number of advantages when implemented in a CT scanner system. This target allows the CT scanner to provide the quantity of x-rays needed to generate good CT images without melting the target. It also allows for many focal spots to be stacked in a line over a large axial range. This increased axial range allows whole body organs to be scanned for perfusion studies and volumetric CT imaging. However, the x-ray source  10  may be used in suitable applications other than a CT scanner system.  
         [0035]    While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.