Patent Publication Number: US-7715529-B1

Title: X-ray tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Patent Application No. 10-2008-0130333, filed with the Korean Intellectual Property Office on Dec. 19, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
   BACKGROUND 
   1. Technical Field 
   The present invention relates to an X-ray tube. 
   2. Description of the Related Art 
   An X-ray tube is based on the principle of generating X-rays using a cathode made from a filament and an anode made from a metallic material. When a high voltage is applied between the cathode and the anode, thermal electrons generated in the cathode are made to collide with the anode to generate X-rays. 
   The inside of the X-ray tube may be kept in a vacuum state, in order to avoid reductions in kinetic energy and deflections, which may otherwise occur as electrons collide with air molecules while traveling towards the target. The target can be made of a thin layer of metal, the thickness of which can be determined in consideration of the penetration depth of the electrons and heat-absorbing capacity. 
   The X-ray tube can be divided into a fixed type and a rotating type, according to the operation of the anode. A rotating X-ray tube can be substantially the same as the fixed X-ray tube, except that the anode may rotate to better disperse the heat generated in the target. 
   A conventional X-ray tube, such as that illustrated in  FIG. 1 , may experience an anode heel effect, in which the intensity of the X-rays is higher in the direction of the cathode from the midpoint, so that the effective focal spot size is larger, while the intensity of the X-rays is lower in the direction of the anode, so that the effective focal spot size is smaller. 
   Thus, in practice, a technician may move towards the parts closer to the cathode when acquiring an image for a thick portion and move towards the parts closer to the anode when acquiring an image for a thin portion, when operating an X-ray machine. This uneven distribution of X-ray intensity is caused by the inclination of the anode. 
   SUMMARY 
   An aspect of the invention aims to provide an X-ray tube, in which the unevenness in X-ray intensity is improved. 
   Another aspect of the invention provides an X-ray tube that includes a cathode, an anode, and a guide. The cathode can be configured to emit electrons. The anode can have a surface arranged parallel to an emission direction of the electrons and can be configured to collide with the electrons to emit X-rays. The guide can be positioned between the cathode and the anode, to modify the direction in which the electrons travel such that the electrons collide with the surface of the anode. 
   The guide can include a magnet, and the anode can include many targets having different materials. Here, the targets may be aligned in a row along the direction in which the electrons are emitted. 
   The X-ray tube can further include a filter, which may be arranged in a path of the X-rays, to filter bremsstrahlung. The cathode can be made to include carbon nanotubes. 
   Additional aspects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the anode heel effect in an X-ray tube according to the related art. 
       FIG. 2  illustrates the structure of an X-ray tube according to an embodiment of the invention. 
       FIG. 3  is a plan view illustrating the target array in  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to particular modes of practice, and it is to be appreciated that all changes, equivalents, and substitutes that do not depart from the spirit and technical scope of the present invention are encompassed in the present invention. 
   The X-ray tube according to certain embodiments of the invention will be described below in more detail with reference to the accompanying drawings. Those components that are the same or are in correspondence are rendered the same reference numeral regardless of the figure number, and redundant explanations are omitted. 
     FIG. 2  is a drawing illustrating the structure of an X-ray tube according to an embodiment of the invention, and  FIG. 3  is a plan view illustrating the target array in  FIG. 2 . In  FIG. 2  and  FIG. 3 , there are illustrated a cathode  10 , an electron emitter  12 , an anode  20 , a target array  22 , a guide  30 , and a filter  40 . 
   As in the example shown in  FIG. 2 , an X-ray tube according to an embodiment of the invention can be composed mainly of a cathode  10 , an anode  20 , a guide  30 , and a filter  40 . 
   The cathode  10  can be arranged inside a vacuum housing (not shown) to generate electrons. The cathode  10  can include an electron emitter  12 , which emits electrons, and a focusing apparatus (not shown), which converges the electrons generated in the electron emitter  12  to move in a particular direction. 
   An example of an electron emitter  12  is a filament, which may be a coil made from a material such as tungsten. When an electric current is supplied to the filament, the filament may be heated, and the heated filament may emit electrons in every direction. Thus, a focusing apparatus (not shown) can be used, which converges the electrons to a particular direction, so that the electrons may be transferred precisely to the anode  20 . 
   Besides using the filament, the electron emitter  12  may also be implemented using carbon nanotubes. The use of carbon nanotubes makes it possible to obtain electron emission at normal temperature, greatly improving the expected life span of the radiation source. An electron emitter  12  using carbon nanotubes can also provide a very high efficiency in emitting electrons, so that X-rays may be generated with higher intensity and higher efficiency, and can be fabricated in compact sizes, so that the value of the final product may be increased. 
   The anode  20  can collide with the electrons emitted from the cathode  10  to emit X-rays  24  and  24 ′. For this, the anode  20  can include a target  22  made from a metallic material. Here, the target  22  can be arranged parallel to the general direction in which the electrons are emitted from the cathode  10 . In other words, the initial emission direction of the electrons can be parallel to the surface of the target  22 , as in the example shown in  FIG. 2 . It should be noted that the term “parallel” is not limited to an exact, mathematical meaning of the word, but is used to convey a meaning of a general parallel that allows for mechanical and design tolerances, etc. 
   A guide  30  can be positioned between the cathode  10  and the anode  20  and can control the path  14  of the electrons. A magnet having an N-pole and an S-pole, such as an electromagnet, etc., can be utilized as the guide  30 . By arranging the guide  30 , which uses an electromagnet, for example, at the front of the cathode  10 , the magnitude, direction, etc., of the magnetic field around the cathode  10  can be modified, whereby the path  14  of the electrons emitted from the cathode  10  may also be modified. 
   In this embodiment, the target  22  can be arranged parallel to the emission direction of the electrons emitted from the cathode  10 , and the guide  30  can refract the path  14  of the electrons towards the target  22 , so that the electrons may collide with the target  22 . 
   By modifying the magnitude of the magnetic field around the cathode  10 , the degree to which the path  14  of the electrons is refracted can be modified, and hence the position on the target where the electrons collide can also be modified. 
   Taking advantage of this fact, the target  22  in this embodiment can be formed as a target array  22 , which is made from a multiple number of targets  22   a ,  22   b ,  22   c ,  22   d , and  22   e  that are made from different materials. According to this embodiment, a variety of characteristic X-rays and bremsstrahlung can be obtained from a single X-ray tube by using several targets having different materials, instead of using one target made of a single material. 
   With the several targets  22   a ,  22   b ,  22   c ,  22   d , and  22   e  aligned in a row along the emission direction of the electrons, the target to which the electrons collide can be determined by changing the magnitude of the magnetic field around the cathode  10 . For example, the electrons can be made to collide with the surface of the target  22   a  farthest from the cathode  10  by lowering the magnitude of the magnetic field. Conversely, the electrons can be made to collide with the target  22   e  closest to the cathode  10  by increasing the magnitude of the magnetic field.  FIG. 3  illustrates an example of a target  22  that is composed of a row of targets  22   a ,  22   b ,  22   c ,  22   d , and  22   e  having different materials, and the table below presents the atomic numbers and K-alpha energies of a few typical target materials. 
   
     
       
         
             
             
             
             
             
           
             
                 
                 
             
             
                 
                 
                 
               Atomic 
               K X-ray 
             
             
                 
                 
               Chemical 
               Number 
               Energy 
             
             
                 
               Element 
               Symbol 
               (Z) 
               (KeV) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               Tungsten 
               W 
               74 
               69 
             
             
                 
               Lead 
               Pb 
               82 
               75 
             
             
                 
               Molybdenum 
               Mo 
               42 
               20 
             
             
                 
               Iodine 
               I 
               53 
               28 
             
             
                 
               Rhodium 
               Rh 
               45 
               23 
             
             
                 
               Silver 
               Ag 
               47 
               22 
             
             
                 
               Copper 
               Cu 
               29 
               8 
             
             
                 
               Tantalum 
               Ta 
               73 
               57 
             
             
                 
               Rhenium 
               Re 
               75 
               61 
             
             
                 
               Osmium 
               Os 
               76 
               63 
             
             
                 
               Iridium 
               Ir 
               77 
               64 
             
             
                 
               Platinum 
               Pt 
               78 
               66 
             
             
                 
               Gold 
               Au 
               79 
               68 
             
             
                 
               Uranium 
               U 
               92 
               98 
             
             
                 
                 
             
          
         
       
     
   
   A target base  26  can be coupled to the target  22 , where a material having a high thermal conductivity and a low atomic number (Z&lt;10) may be selected for the target base  26  in consideration of its heat-releasing effect. 
   After positioning the components such that the emission direction of the electrons and the surface of the target  22  are parallel, as described above, forming the incident direction of the electrons closer to the direction normal to the target can improve the anode heel effect, as illustrated in  FIG. 1 , that is caused by the inclination of the anode&#39;s surface. In other words, 1) the increasing of the effective focal spot size in the direction of the cathode can be reduced, 2) the changes in the effective focal spot size due to the inclination of the anode surface can be reduced, and 3) the uneven distribution of X-ray intensity, where the intensity increases towards the cathode and decreases towards the anode, can be reduced. 
   A filter  40  for filtering bremsstrahlung can be positioned in a path of the X-rays  24  and  24 ′, which are generated at the anode  20  when the electrons collide with the target  22 . In general, the radiation intensity that contributes to obtaining an X-ray image includes less than 20% from characteristic X-rays and more than 80% from bremsstrahlung. By using the filter  40  to filter the bremsstrahlung, monochromatic X-rays, which only use characteristic X-rays, can be implemented. This results in an image that has higher sharpness and higher contrast. 
   As described above, an X-ray tube according to an embodiment of the invention can be used to improve the uneven distribution of X-ray intensity, as well as to provide X-rays having different energy properties. Furthermore, the X-ray tube can be used to implement monochromatic X-rays consisting only of characteristic X-rays, to provide an image that has higher sharpness and higher contrast. 
   While the spirit of the invention has been described in detail with reference to particular embodiments, the embodiments are for illustrative purposes only and do not limit the invention. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the invention. 
   Many embodiments other than those set forth above can be found in the appended claims.