Patent Publication Number: US-2011064202-A1

Title: Method and system for generating an x-ray beam

Description:
FIELD OF THE INVENTION 
     The present invention relates to computed tomography methods and apparatus, in particular, to methods and apparatus for performing computed tomography (CT) scans using one X-ray Source. 
     BACKGROUND OF THE INVENTION 
     Conventional multislice X-ray computed tomography (CT) employs an X-ray source to produce a collimated cone-shaped beam directed along the transverse plane through a patient and to be received by a detector array. The X-ray source and the detector array are mounted on a gantry to be rotated about a patient to obtain “projections” measuring X-ray attenuation at the different gantry angles along various X-ray paths through the patient. Projections obtained over a gantry rotation range of at least 180 degrees plus the fan angle subtended by the beam, are used to reconstruct a tomography image. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved X-ray generating apparatus and corresponding methods for scanning. 
     The basic idea of the present invention is providing a moveable X-ray tube to perform scans having different anode angles. When the moveable X-ray tube is moved to a different position, a scan is performed at a different anode angle. The word “moveable” not only means that the X-ray tube comprises a rotatable anode disk, but also that the X-ray tube can be moved among a plurality of predefined positions, especially along the axial and radial directions of the CT gantry. 
     By improving the scan geometry, particularly the trajectory of the corresponding focal spots, the so-called cone beam artifacts, are suppressed in the images. By using the apparatus and methods proposed in the present invention, the trajectory of focal spots can be improved by performing different axial scans where the focal spot is located at different Z-positions. Moreover, the advantages of adapting the tilt of the X-ray cone beam to scan the object from different angles can be achieved by utilizing the proposed apparatus having variable anode angles. 
     In a first aspect, according to an embodiment of the present invention, there is provided a device comprising an anode disk, wherein the anode disk comprises a plurality of focal tracks being cone-shaped with different anode angles. 
     It is advantageous to generate X-ray beams having different anode angles when different focal tracks are successively bombarded by electron beams. 
     Optionally, according to an embodiment of the present invention, the device further comprises an X-ray tube, wherein the X-ray tube comprises the anode disk and a first cathode, wherein the first cathode is configured to generate electron beams targeting at least one of the plurality of focal tracks. The device can be a scanner. 
     Optionally, in another embodiment, the device further comprises a movement controller, which is configured to move the X-ray tube among a plurality of predefined positions. 
     It is advantageous to adjust the X-ray tube&#39;s position, and bombard corresponding focal tracks, based on the position of the X-ray tube, to generate X-ray beams with a corresponding anode angle so as to scan the object from different angles. Another advantage resides in that more projections are generated when the X-ray tube scans the object from different positions. Generating more projections is helpful for mitigating the generation of artifacts in the subsequent image process. Moreover, by arranging the locations of the plurality of predefined positions, the field of view of the scanner, i.e., the Z-range of a scanner, can be extended. It is noted that when the X-ray is moved, the anode disk is also moved. So, the meaning of “move the X-ray tube among a plurality of predefined positions” also covers the situation that the anode disk is moved among a plurality of corresponding positions, while the shell of the X-ray tube does not move. 
     Optionally, in an embodiment, the movement controller is further configured to move the X-ray tube along its axial and radial directions and to ensure (?) that the movement of corresponding focal spots is parallel to the axis of rotation of the gantry, e.g. the gantry of the scanner. A focal spot is formed when the anode disk is rotating and a corresponding focal track is bombarded by an electron beam. In this embodiment, a plurality of focal spots, formed when the X-ray tube is successively at the plurality of predefined positions, are in a line parallel to the axis of rotation of the gantry. 
     Optionally, in an embodiment, the device further comprises a focal track selector, which is configured to direct the electron beam to bombard different focal tracks of the plurality of focal tracks, based on different positions of the X-ray tube. For example, the focal track selector, e.g., electron lenses, generates different electronic fields and/or magnetic fields on the electron beam and directs it to bombard different focal tracks. By using the focal track selector, it is advantageous to use only one cathode to generate electron beams to bombard different focal tracks, based on the X-ray tube&#39;s different positions. Alternatively, the scanner further comprises a second cathode or more cathodes, wherein different cathodes are each configured to generate electron beams targeted at a different focal track of the plurality of focal tracks. 
     Optionally, according to an embodiment, the device further comprises a plurality of collimators, wherein each collimator&#39;s position corresponds to one corresponding position of the plurality of predefined positions of the X-ray tube. When the X-ray tube is at one of the predefined positions, the corresponding collimator forms the X-ray generated by the X-ray tube. Optionally, at least one collimator has a size different from that of other collimators, which provides the advantage that a different size and/or shape of cone beam can be obtained. 
     In the second aspect, according to an embodiment of the present invention, there is provided a method of scanning an object, the method comprises the steps of: rotating an anode disk, wherein the anode disk comprises a plurality of focal tracks being cone-shaped with different anode angles; bombarding one of the plurality of focal tracks for generating an X-ray beam, wherein the anode angle of the generated X-ray beam is determined by the anode angle of the bombarded focal track. 
     Optionally, according to an embodiment, the method further comprises the steps of: moving the anode disk to a first predefined position; performing a first scan with a first focal track of the plurality of focal tracks being bombarded by a first electron beam; moving the anode disk to a second predefined position; and performing a second scan with a second focal track of the plurality of focal tracks being bombarded by a second electron beam. 
     By using the provided method, it is advantageous to perform two or more scans having different z-positions of the x-ray source, which further provides advantages for suppressing artifacts in the subsequent image processing. Optionally, an X-ray tube comprising the anode disk can be moved as a whole. In such an embodiment, a cathode comprised in the X-ray tube is also moved in the same way. 
     These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular aspects of the invention will now be explained with reference to the embodiments described hereinafter and considered in connection with the accompanying drawings, in which identical parts or sub-steps are designated in the same manner: 
         FIG. 1   a  depicts a front view of an anode disk comprising a plurality of focal tracks, according to an embodiment of the present invention; 
         FIG. 1   b  depicts a side view of the anode disk of  FIG. 1   a;    
         FIG. 1   c  depicts a side view of an anode disk, according to an embodiment of the present invention; 
         FIG. 2   a  depicts an X-ray tube comprising one cathode, according to an embodiment of the present invention; 
         FIG. 2   b  depicts an X-ray tube comprising two cathodes, according to an embodiment of the present invention; 
         FIG. 3  depicts the block diagram of a scanner according to an embodiment of the present invention; 
         FIG. 4  depicts the working principle of a scanner, according to an embodiment of the present invention; 
         FIG. 5  depicts the movement of X-ray tube, according to an embodiment of the present invention; 
         FIG. 6  depicts the generation of different cone beams having different cone angles, according to an embodiment of the present invention; 
         FIG. 7  illustrates an embodiment to realize different Z-ranges; 
         FIG. 8  depicts a working flowchart of a method used in a scanner according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1   a  illustrates an anode disk according to an embodiment of the present invention. The anode disk  100  comprises a plurality of focal tracks, e.g., a first focal track  110  and a second focal track  120 . Each focal track is cone-shaped, the axis of the cones corresponding to the center of the 3 circular lines, and different focal tracks have different anode angles. The anode angle of a focal track affects the X-ray beam, which is generated when the focal track  110  or  120  is bombarded by an electron beam. In other words, when different focal tracks are bombarded by electron beams, the anode disk  100  generates X-ray beams having different cone angle ranges. The anode disk can be an element of an X-ray scanner, for example a CT scanner. 
       FIG. 1   b  illustrates a side view of the anode disk, in particular the different anode angles of different focal tracks. The anode angle is the angle between a vertical line in  FIG. 1   b  and the anode surface in the region of the focal track, for example, the focal track  110  has the anode angle  112 , and the focal track  120  has the anode angle  122 . The anode disk  100  may comprise more than two focal tracks for generating X-ray beams having different anode angles, each focal track having an anode angle different from the anode angle of other focal tracks. Optionally, as shown in  FIG. 1   c , the plurality of focal tracks can form a convex area, which allows a continuously selectable anode angle. In  FIGS. 1   a - 1   c , the arrows  130  and  140  respectively represent the axial direction and the radial direction of the anode disk when the anode disk is moved from a first position to a second position. 
       FIG. 2   a  illustrates an X-ray tube  200  comprising an anode disk  100  (in a first position on the left side, in a second position on the right side) and a first cathode  210 . The first cathode  210  is configured to generate an electron beam targeting at least one focal track of the anode disk  100 . When the anode disk is rotating and one focal track is bombarded by the electron beam generated from the first cathode  210 , an X-ray beam is generated with a corresponding anode angle. 
     For example, when the first focal track  110  having anode angle  114  is bombarded, X-ray beam  112  is generated; and when the second focal track  120  with anode angle  124  is bombarded, X-ray beam  122  is generated. 
     For selecting the focal track which is to be bombarded, in an embodiment of the present invention, a focal track selector  220  is provided. The focal track selector  220  is configured to direct the electron beam generated by the first cathode  210  so as to bombard one focal track, e.g. the first focal track  110  or the second focal track  120 . The focal track selector  220  generates an electronic field and/or magnetic field on the electron beam to direct it to the desired focal track. 
     In the exemplary embodiment of  FIG. 2   a , the focal track selector  220  is a pair of electron lenses. The skilled person should understand that other means capable of directing an electron beam to a selected focal track are also applicable for this invention.
     By using the focal track selector  220 , it is advantageous to use only one cathode to bombard a succession of different focal tracks.   

     Alternatively, in the exemplary embodiment shown in  FIG. 2   b , the X-ray tube  200 ′ further comprises a second cathode  230  to generate an electron beam for bombarding one of the plurality of focal tracks  110 - 120 . The first cathode  210  and the second cathode  230  are arranged to bombard different focal tracks. In this embodiment, there is no need for a focal track selector. It is feasible for the X-ray tube  200  to comprise more than two cathodes. 
       FIG. 3  illustrates a block diagram of a scanner  300 , according to an embodiment of the present invention. 
     The scanner  300  comprises an X-ray tube  200  and a movement controller  310 . The movement controller  310  is configured to move the X-ray tube  200  among a plurality of predefined positions. For example, the plurality of predefined positions can be arranged along the Z-axis of the CT scanner. Once moved to a predefined position, the X-ray tube  200  performs a scan, in which a corresponding focal track is selected and bombarded by an electron beam, for generating a corresponding X-ray beam with a corresponding anode angle. When the X-ray tube is placed at different positions, different focal tracks are selected and different X-ray beams having different anode angles are generated. 
     Optionally, the scanner  300  further comprises a plurality of collimators  320 . Each collimator is located with respect to a predefined position of the X-ray tube  200 , and is configured to collimate a corresponding X-ray beam when the X-ray tube  200  is placed at the predefined position. The cone angle of an X-ray cone beam traversing a scanned object is determined by the anode angle of the bombarded focal track and the position and/or size of the collimator. 
     Optionally, to generate different X-ray cone beams having different cone angles, in an embodiment of the present invention, at least one of the collimator sizes is different from that of other collimators. Furthermore, in another embodiment, the collimators&#39; positions can be arranged such that some collimators can shape the X-ray beams so as to be symmetric cone beams, while other collimators shape the X-ray beams so as to be asymmetric cone beams. 
       FIG. 4  illustrates the working principle of a scanner according to an embodiment of the present invention. 
     Reference sign  430  represents the axis direction of the gantry of the scanner, and reference sign  440  represents the axis of rotation of the gantry of the scanner, also referred to as z-axis. 
     The provided scanner  300  first places the X-ray tube  200  at the first predefined position  410 , and performs a first scan. A first focal spot  416  is formed when one focal track of the anode disk  100 , for example, the first focal track  110 , is bombarded. In the first scan, the X-ray tube  200  generates a cone beam  412  with a cone angle  414  to scan an object, which is not shown in this Figure. 
     After the first scan is performed, the scanner  300  moves the X-ray tube  200  to the second predefined position  420 , and performs a second scan. In the second scan, the second focal track  120  is bombarded, and a second focal spot  426  is formed. Cone beam  422  having a cone angle  424  is generated. 
     In this embodiment, a scan is performed when the X-ray tube  200  is at one of the plurality of predefined positions. 
       FIG. 5  illustrates an exemplary embodiment of the movement of the X-ray tube. 
     First, the X-ray tube  200  is placed at a first position  510 , and the first cathode  210  generates an electron beam. The focal track selector  220  exerts a force on, and directs, the electron beam to bombard the first focal track  110 . When the first focal track  110  having the anode angle  516  is bombarded while the anode disk  100  is rotating, a first focal spot  512  is formed, and a first X-ray beam  514  is formed. The generated X-ray beam  514  is collimated by a corresponding first collimator  520  to a first cone angle  518 , and targets the scanned object. 
     After a first scan is performed with the first X-ray beam  514 , the X-ray tube  200  is moved to a second position  530 . At the second position  530 , the focal track selector  220  exerts a different force on the electron beam generated by the first cathode  210 , and directs the electron beam to bombard the second focal track  120  having the anode angle  536 . While the anode disk  100  is rotating, a second focal spot  532  is formed and a second X-ray beam  534  is formed. The second X-ray beam  534  is collimated by a corresponding second collimator  540  to a second cone angle  538 . A second scan is thus performed by using the second X-ray beam. 
     In the embodiment of  FIG. 5 , a plurality of collimators are shown, each collimator corresponding to a predefined position of the X-ray tube. The function of each collimator is to collimate the X-ray beam when the X-ray tube is at the corresponding position. 
     As the first focal spot  512  and the second focal spot  532  are formed on different focal tracks, the distance between the first focal spot  512  and the axis of rotation  560  of the gantry of the scanner may be different from the distance between the second focal spot  532  and the axis  560 . The difference is compensated by moving the X-ray tube in a radial direction between both positions. The X-ray tube is moved such that the distance between the first focal spot  512  and axis  560  is the same as the distance between the second focal spot  532  and the axis  560 . In other words, the dotted line  570  defined by the two focal spots  512 / 532  remains parallel to the axis of rotation  560 . In this embodiment, the movement of the X-ray tube  580  does not stay parallel to axis  560 , which means the X-ray tube is moved, by the movement controller, not only along its axial direction, but also along its radial direction. 
       FIG. 6  depicts the generation of different cone beams having different cone angles, according to an embodiment of the present invention. 
     When the X-ray tube is placed at position  610 , a first focal track  612  is bombarded and a first focal spot  614  is formed. After being collimated by a corresponding collimator  616 , a cone beam  618  having a cone angle  619  is formed. 
     When the X-ray tube is placed at position  620 , a second focal track  622  is bombarded and a second focal spot  624  is formed. A symmetric cone beam  628  having a cone angle  629  is formed, due to the position and size of the second collimator  626 . 
     When the X-ray tube is at position  630 , a third focal track  632  is bombarded and a third focal spot  634  is formed. 
     It is noted that the three focal spots  614 ,  624  and  634  are along a dotted line parallel to the axis of rotation of the gantry of the scanner, which means that the distance between each focal spot and the axis of rotation of the gantry is the same. The X-ray tube itself is moved along its axial direction and radial direction. It is noted that the size of collimators could be different. At least one collimator may have a different size compared to the size of other collimators. 
       FIG. 7  illustrates an embodiment for realizing different Z-ranges. Scanning with the X-ray tube in positions  710  and  720  yields a Z-range larger than scanning with the X-ray tube in positions  730  and  740 . By having different distances among the plurality of predefined positions, different Z-ranges can be realized. 
       FIG. 8  depicts a working flowchart of a method used in a scanner according to an embodiment of the present invention. 
     The method  800  first comprises a step S 810  to move the X-ray tube to a first predefined position. 
     Then the method further comprises a step S 820  of performing a first scan of a given object of interest. During the first scan, a first focal track of the X-ray tube is bombarded, and a first X-ray beam having a first anode angle is formed. 
     The method further comprises a step S 830  to move the X-ray tube to a second predefined position. The X-ray tube is moved along its axial and radial directions, to make sure that a first focal spot formed in the first scan and a second focal spot formed in the second scan are in a line parallel to the axis of rotation of the gantry of the scanner. In another word, the X-ray tube is moved in such a way as to keep the distance between the first focal spot and the axis of rotation of the gantry the same as the distance between the second focal spot and the axis of rotation of the gantry. 
     In step S 840 , a second scan is performed, and a second focal track is bombarded. During the second scan, a second X-ray beam is formed with a second anode angle. Generally, the first anode angle and the second anode angle are different. 
     Optionally, in one embodiment, the step S 820  further comprises a step S 822  of generating a first electron beam by a cathode, and a step S 824  of directing the first electron beam to bombard the first focal track by exerting a first force on the first electron beam. 
     The step S 840  further comprises a step S 842  of generating the second electron beam by the cathode, and a step S 844  of directing the second electron beam to bombard the second focal track by exerting a second force on the second electron beam. 
     Alternatively, in another embodiment, the step S 820  further comprises a step of generating the first electron beam by a first cathode, and the step S 840  further comprises a step of generating the second electron beam by a second cathode. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention and that those skilled in the art will be able to design many alternative embodiments without departing from the spirit or scope of the invention. Therefore, the scope of the invention shall be limited only by the appended claims. 
     The remarks made hereinbefore demonstrate that the detailed description with reference to the drawings, illustrates rather than limits the invention. There are numerous alternatives, which fall within the scope of the appended claims. Any reference sign in a claim should not be construed as limiting the claim. The word “comprising” does not exclude the presence of other elements or steps than those listed in a claim. The word “a” or “an” preceding an element or step does not exclude the presence of a plurality of such elements or steps.