Patent Publication Number: US-11037751-B2

Title: X-ray tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of PCT Application No. PCT/JP2017/020312, filed May 31, 2017 and based upon and claiming the benefit of priority from Japanese Patent Application No. 2016-195589, filed Oct. 3, 2016, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an X-ray tube. 
     BACKGROUND 
     X-ray tubes are used for X-ray image diagnosis, non-destructive inspection, and the like. Examples of the X-ray tubes include a stationary anode type X-ray tube and a rotation anode type X-ray tube, and the X-ray tube is used according to its use. An X-ray tube comprises an anode target, a cathode, and a vacuum envelope. On the anode target, a focal spot on which an electron beam is made incident to emit X-rays is formed. 
     The cathode comprises a filament coil and an electron convergence cup. The filament coil can emit electrons. A high tube voltage in the range of several tens to one hundred and several tens of kilovolts (kV) is applied between the anode target and the cathode. For this reason, the electron convergence cup can play a role of an electron lens, i.e., can urge an electron beam traveling toward the anode target to be converged. The rotation anode X-ray tube is generally used for medical diagnosis. In general, the X-ray tube has two focal spots, i.e., a large focal spot having large dimensions and enabling a large current to be input, and a small focal spot having small dimensions, and a small input but a high resolution. One of X-ray tubes has three focal spots. Dimensions of each focal spot depend on each of the shape and positional relationship of the filament coil and the electron converging cup and are fixed in general. When a large focal spot or a small focal spot is used, imaging conditions are determined by determining a spatial resolution and an input current (influenced by contrast and noise) in accordance with application of diagnosis, and the large focal spot and the small focal spot are used separately. 
     With only two focal spots, however, imaging conditions are discontinuous and images necessary for x-ray image diagnosis can hardly be obtained in some cases. In particular, when imaging is performed continuously in an axial direction of a subject in a manner such as helical scanning in an X-ray CT device, continuity in image quality cannot be maintained and accurate image diagnosis cannot be performed due to variation in input at two focal spots that are discontinuous, in some cases. Thus, a method of varying dimensions of the focal spot by varying voltages of a plurality of electrodes in accordance with electric signals has been provided. 
     In such a focal spot dimension varying method, however, the control and the structure are complicated or complicated control to adjust a ratio between a tube current and dimensions of a focal spot is required. In addition, a tube current which can be input is limited due to the dimensions of the focal spot but, if the control of the current and that of the dimensions of the focal spot are exerted by different systems, there is a possibility that if those controls are in nonconformity to each other an overcurrent will be caused, resulting in breakage of an X-ray tube. 
     Furthermore, when the dimensions of the focal spot are varied, the focal spot can hardly be controlled to be set to desired dimensions. For example, variations in length and width of the focal spot are greatly different with respect to a variation in a bias voltage applied to an electron convergence cup. For this reason, the ratio of the dimensions of the focal spot and the amount of current can hardly be adjusted to appropriate values at the same time. Thus, a technique of preparing an electrode which controls the length of the focal spot and an electrode which controls the width of the focal spot and controlling the focal spot to have desired dimensions has been proposed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing an X-ray tube assembly according to one of embodiments. 
         FIG. 2  is an enlarged view showing a cathode of an example of the embodiment, including (a) a plan view, (b) a cross-sectional view, (c) another cross-sectional view, and (d) another cross-sectional view. 
         FIG. 3  is a schematic view showing the cathode and an anode target of the example as viewed in two directions perpendicular to a tube axis of the X-ray tube, and also showing a state in which an electron beam is emitted from a first filament coil toward the anode target in a case where a bias voltage to be applied to an electron convergence cup is set to 0V similarly to a filament voltage. 
         FIG. 4  is a schematic view showing the cathode and the anode target of the example as viewed in two directions perpendicular to the tube axis of the X-ray tube, and also showing a state in which an electron beam is emitted from a filament coil toward the anode target in a case where a bias voltage which is negative to the filament voltage is applied to an electron convergence cup. 
         FIG. 5  is a graph showing a variation in a tube current to a filament current supplied to the first filament coil in a case where the bias voltage of the example is set to 0V. 
         FIG. 6  is a graph showing a variation in a tube current to a filament current supplied to the first filament coil in a case where a negative bias voltage is applied to the electron convergence cup of the example. 
         FIG. 7  is a view illustrating an orbit of an electron beam emitted from the first filament coil toward the anode target to form a first focal spot and an orbit of an electron beam emitted from a second filament coil toward the anode target to form a second focal spot, in the example, and also illustrating a state in which the first focal spot and the second focal spot overlap. However, emitting electrons from the first filament coil and the second filament coil at the same time is not intended. 
         FIG. 8  is a cross-sectional view showing a first modified example of the cathode of the X-ray tube assembly according to the example. 
         FIG. 9  is a cross-sectional view showing a second modified example of the cathode of the X-ray tube assembly according to the example. 
         FIG. 10  is a cross-sectional view showing a third modified example of the cathode of the X-ray tube assembly according to the example. 
         FIG. 11  is an enlarged view showing a cathode of a comparative example, including (a) a plan view, (b) a cross-sectional view, (c) another cross-sectional view, and (d) another cross-sectional view. 
         FIG. 12  is a schematic view showing the cathode and an anode target of the comparative example as viewed in two directions perpendicular to a tube axis of the X-ray tube, and also showing a state in which an electron beam is emitted from a first filament coil toward the anode target in a case where a bias voltage to be applied to an electron convergence cup is set to 0V, i.e., the electron convergence cup and the filament are set at the same electric potential. 
         FIG. 13  is a schematic view showing the cathode and the anode target of the comparative example as viewed in two directions perpendicular to the tube axis of the X-ray tube, and also showing a state in which an electron beam is emitted from a filament coil toward the anode target in a case where a bias voltage which is negative to the filament voltage is applied to an electron convergence cup. 
         FIG. 14  is a graph showing a variation in a tube current to a filament current supplied to the first filament coil in a case where the bias voltage of the comparative example is set to 0V. 
         FIG. 15  is a graph showing a variation in a tube current to a filament current supplied to the first filament coil in a case where a negative bias voltage is applied to the electron convergence cup of the comparative example. 
         FIG. 16  is a view illustrating an orbit of an electron beam emitted from the first filament coil toward the anode target to form a first focal spot and an orbit of an electron beam emitted from a second filament coil toward the anode target to form a second focal spot, in the comparative example, and also illustrating a state in which the first focal spot and the second focal spot overlap. However, emitting electrons from the first filament coil and the second filament coil at the same time is not intended. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided an X-ray tube comprising: an anode target emitting X-rays by electrons made incident thereon; a cathode comprising a first filament emitting electrons and a converging electrode urging the electrons emitted from the first filament to be converged; and a vacuum envelope accommodating the anode target and the cathode. The converging electrode comprises a flat front surface which is the closest to the anode target, a flat first surface located on a side opposite to the anode target with respect to the front surface, a first trench portion opening to the first surface, accommodating the first filament, and having a first length direction along a longer axis of the first filament, and a pair of first protruding portions formed to protrude from the first surface toward the front surface side and sandwiching the first trench portion in the first length direction, and an upper surface of each of the first protruding portions on a side opposed to the anode target is formed of a plurality of flat inclined surfaces inclined in directions different from one another. 
     One of embodiments will be described hereinafter with reference to the accompanying drawings. The disclosure is merely an example, and proper changes in keeping with the spirit of the invention, which are easily conceivable by a person of ordinary skill in the art, come within the scope of the invention as a matter of course. In addition, in some cases, in order to make the description clearer, the widths, thicknesses, shapes, and the like, of the respective parts are illustrated schematically in the drawings, rather than as an accurate representation of what is implemented. However, such schematic illustration is merely exemplary, and in no way restricts the interpretation of the invention. In addition, in the specification and drawings, the same elements as those described in connection with preceding drawings are denoted by like reference numbers, and detailed description thereof is omitted unless necessary. 
       FIG. 1  is a cross-sectional view showing an X-ray tube assembly according to one of embodiments. In the embodiment, the X-ray tube assembly is a rotation anode X-ray tube assembly. 
     As shown in  FIG. 1 , the X-ray tube assembly comprises a rotation anode X-ray tube  1 , a stator coil  2  serving as a coil which generates a magnetic field, a housing  3  which accommodates the X-ray tube and the stator coil, an insulating oil  4  serving as a coolant filled in the housing, and a controller  5 . 
     The X-ray tube  1  comprises a cathode (cathode electron gun)  10 , a sliding bearing unit  20 , an anode target  60 , and a vacuum envelope  70 . 
     The sliding bearing unit  20  comprises a rotor  30 , a fixed shaft  40  serving as a fixed body, and a metallic lubricant (not shown) serving as a lubricant, and uses sliding bearings. 
     The rotor  30  is formed in a cylindrical shape, and one end portion of the rotor  30  is closed. The rotor  30  extends along the axis of rotation, which is a central axis of a rotating operation of the rotor. In the embodiment, the axis of rotation is the same as a tube axis a 1  of the X-ray tube  1 , and will be hereinafter explained as the tube axis a 1 . The rotor  30  can be rotated about the tube axis a 1 . The rotor  30  includes a connection portion  31  located at this end portion. The rotor  30  is formed of a material such as iron (Fe) or molybdenum (Mo). 
     The fixed shaft  40  is formed in a cylindrical shape having dimensions smaller than those of the rotor  30 . The fixed shaft  40  is provided coaxially with the rotor  30  and extends along the tube axis a 1 . The fixed shaft  40  is fitted in an inner part of the rotor  30 . The fixed shaft  40  is formed of a material such as Fe or Mo. One end portion of the fixed shaft  40  is exposed to the outside of the rotor  30 . The fixed shaft  40  supports the rotor  30  in such a manner as to permit the rotor  30  to be rotated. 
     The metallic lubricant is filled in a gap between the rotor  30  and the fixed shaft  40 . 
     The anode target  60  is located opposite to the other end portion of the fixed shaft  40  in a direction along the tube axis a 1 . The anode target  60  includes an anode body  61  and a target layer  62  provided on part of an outer surface of the anode body. 
     The anode body  61  is fixed to the rotor  30  by means of the connection portion  31 . The anode body  61  has a disk-like shape and is formed of a material such as Mo. The anode body  61  is rotatable around the tube axis a 1 . The target layer  62  is formed in an annular shape. The target layer  62  includes a target surface  62 S which is opposed to and spaced apart from the cathode  10  in the direction along the tube axis a 1 . In the anode target  60 , an electron beam is made incident on the target surface  62 S, and a focal spot is thereby formed on the target surface  62 S and X-rays are emitted from the focal spot. 
     The anode target  60  is electrically connected to a terminal  91  via the rotor  30 , the fixed shaft  40 , and the like. 
     As shown in  FIG. 1  and  FIG. 2 , the cathode  10  includes one or more filament coils and an electron convergence cup  15  serving as a converging electrode. In the embodiment, the cathode  10  includes a first filament coil  11  serving as a first filament, and a second filament coil  12  serving as a second filament. The first filament coil  11  and the second filament coil  12  are formed of a material containing tungsten as its main component. The first filament coil  11  and the second filament coil  12  are formed to extend linearly. The first filament coil  11 , the second filament coil  12 , and the electron convergence cup  15  are electrically connected to terminals  81 ,  82 ,  83 ,  84 , and  85 . 
     The electron convergence cup  15  includes one or more trench portions in which the filament coil or coils (electron emission sources) are set. In this embodiment, the electron convergence cup  15  includes a first trench portion  16  in which the first filament coil  11  is set, and a second trench portion  17  in which the second filament coil  12  is set. The first filament coil  11  is accommodated in the first trench portion  16  and is located to be spaced apart from inner surfaces (side and bottom surfaces) of the first trench portion  16 . The first filament coil  11  is supplied with a current (filament current). The first filament coil  11  thereby emits electrons (thermoelectrons). The second filament coil  12  is accommodated in the second trench portion  17  and is located to be spaced apart from inner surfaces (side and bottom surfaces) of the second trench portion  17 . The second filament coil  12  is supplied with a current (filament current). The second filament coil  12  thereby emits electrons (thermoelectrons). 
     The first trench portion  16  and the second trench portion  17  are inclined such that the electrons emitted from the first filament coil  11  and the electrons emitted from the second filament coil  12  collide at substantially the same position on the target surface  62 S. In addition, each of the first trench portion  16  and the second trench portion  17  includes a lower trench and an upper trench which is located on the target surface  62 S side from the lower trench and has dimensions larger than those of the lower trench. 
     A relatively positive voltage is applied to the anode target  60  from the terminal  91  via the fixed shaft  40 , the rotor  30 , and the like. A relatively negative voltage is applied to the first filament coil  11 , the second filament coil  12 , and the electron convergence cup  15  from the terminals  81  to  83 . In the embodiment, the X-ray tube  1  is an anode grounding type 
     X-ray tube, the anode target  60  is set to a ground potential, and a negative high voltage is applied to the cathode  10 . 
     Unlike the embodiment, however, the X-ray tube  1  may be a neutral grounding type X-ray tube or a cathode grounding type X-ray tube. In the neutral grounding type X-ray tube, a positively high voltage is applied to the anode target  60  and a negatively high voltage is applied to the cathode  10 . In the cathode grounding type X-ray tube, a positively high voltage is applied to the anode target  60 , and the cathode  10  is set to a ground potential. 
     Since an X-ray tube voltage (hereinafter referred to as a tube voltage) is applied between the anode target  60  and the cathode  10 , the electrons emitted from the first filament coil  11  are accelerated and made incident on the target surface  62 S as an electron beam. Similarly, electrons emitted from the second filament coil  12  are accelerated and made incident on the target surface  62 S as an electron beam. The electron convergence cup  15  urges the electron beam traveling from the first filament coil  11  toward the anode target  60  through an opening  16   a  of the first trench portion  16  to be converged on one hand, and urges the electron beam traveling from the second filament coil  12  toward the anode target  60  through an opening  17   a  of the second trench portion  17  to be converged on the other hand. 
     As shown in  FIG. 1 , the vacuum envelope  70  is formed in a cylindrical shape. The vacuum envelope  70  is formed of a combination of insulating materials such as glass and ceramics, metals and the like. In the vacuum envelope  70 , a diameter of a portion opposed to the anode target  60  is larger than a diameter of a portion opposed to the rotor  30 . The vacuum envelope  70  includes an opening  71 . The opening  71  is tightly attached to one end of the fixed shaft  40  in order to maintain the sealed state of the vacuum envelope  70 . The vacuum envelope  70  fixes the fixed shaft  40 . In the vacuum envelope  70 , the cathode  10  is mounted on an inner wall. The vacuum envelope  70  is sealed, and accommodates the cathode  10 , the sliding bearing unit  20 , the anode target  60 , and the like. The inside of the vacuum envelope  70  is maintained in a vacuum state. The stator coil  2  is provided to be opposed to the side surface of the rotor  30  and to surround the outside of the vacuum envelope  70 . The stator coil  2  has an annular shape. The stator coil  2  is electrically connected to the terminals  92  and  93  and is driven via the terminals  92  and  93 . 
     The housing  3  includes an X-ray transmission window  3   a  which transmits X-rays, at a position close to the target layer  62  opposed to the cathode  10 . The housing  3  accommodates the X-ray tube  1  and the stator coil  2 , and is filled with the insulating oil  4 . 
     The control unit  5  is electrically connected to the cathode  10  via the terminals  81 ,  82 ,  83 ,  84 , and  85 . The control unit  5  can drive and control the first filament coil  11 , the second filament coil  12 , and the electron convergence cup  15 . The control unit  5  selectively drives the first filament coil  11  and the second filament coil  12 . 
     Next, an operation of the X-ray tube assembly for emitting X-rays will be described. As shown in  FIG. 1 , when the X-ray tube assembly is in operation, first, the stator coil  2  is driven via the terminals  92  and  93  to generate a magnetic field. That is, the stator coil  2  produces a rotating torque to be applied to the rotor  30 . For this reason, the rotor rotates and the anode target  60  also rotates. 
     Next, the control unit  5  supplies a current to drive the first filament coil  11  or the second filament coil  12  via the terminals  81  to  85 . Then, a negative high voltage (common voltage) is applied to the first filament coil  11  (second filament coil  12 ) and the electron convergence cup  15 . The negative high voltage is, for example, in a range of negative tens of kilovolts (kV) to approximately −150 kV. A current is further supplied to the first filament coil  11  (second filament coil  12 ). A bias voltage (superimposed voltage based on the filament voltage) in a range of −5kV to 0V is applied to the electron convergence cup  15 . The anode target  60  is grounded via the terminal  91 . 
     Since the tube voltage is applied between the cathode  10  and the anode target  60 , the electrons emitted from the filament coil are converged and accelerated and collide with the target layer  62 . That is, an X-ray tube current (hereinafter referred to as a tube current) flows from the cathode  10  to a focal spot on the target surface  62 S. 
     The electron beam is made incident on the target layer  62 , which thereby emits X-rays, and the X-rays emitted from the focal spot are emitted to the outside of the housing  3  through the X-ray transmission window  3   a . The electron beam is made incident on the focal spot, and the focal spot thereby has a length corresponding to the longer axis of the filament coil and a width corresponding to the shorter axis of the filament coil. X-ray imaging can be thereby performed. 
     Next, the structure and operation of the X-ray tube assembly according to the example of the embodiment and the structure and operation of an X-ray tube assembly of a comparative example will be described. The X-ray tube assemblies of the example and comparative example are manufactured similarly except for the electron convergence cup  15 . 
     EXAMPLE 
     As shown in  FIG. 1  and  FIG. 2 , the electron convergence cup  15  comprises a front surface Sf, a first surface S 1 , the first trench portion  16 , a pair of first protrusions P 1 , a second surface S 2 , and the second trench portion  17 . In  FIG. 2( a ) , hatch lines are drawn on the first protrusions P 1 . The front surface Sf is a flat surface and is the surface closest to the anode target  60  of the electron convergence cup  15 . Each of the first surface S 1  and the second surface S 2  is a flat surface located on the side opposite to the anode target  60  with respect to the front surface Sf. 
     The first trench portion  16  opens to the first surface S 1  and accommodates the first filament coil  11 . The first trench portion  16  has a first length direction dL 1  along a longer axis of the first filament coil  11 , a first depth direction dD 1 , and a first width direction dW 1  perpendicular to the first depth direction dD 1  and the first length direction dL 1 . 
     The second trench portion  17  opens to the second surface S 2  and accommodates the second filament coil  12 . The second trench portion  17  has a second length direction dL 2  along a longer axis of the second filament coil  12 , a second depth direction dD 2 , and a second width direction dW 2  perpendicular to the second depth direction dD 2  and the second length direction dL 2 . 
     A pair of first protruding portions P 1  is formed to protrude from the first surface S 1  toward the front surface Sf side and are provided to sandwich the first trench portion  16  in the first length direction dL 1 . The first protruding portions P 1  do not protrude toward the anode target  60  side beyond the front surface Sf. The pair of first protruding portions P 1  has first side surfaces Ss 1  opposed to each other in the first length direction dL 1 . In the example, the first side surfaces Ss 1  are surfaces which are flat and parallel to a first virtual plane defined by the first depth direction dD 1  and the first width direction dW 1 . The first side surfaces Ss 1  of the respective pair of first protruding portions P 1  have the same dimensions. However, the configuration of the pair of first protruding portions P 1  is not limited to the example but can be variously modified. For example, the first side surfaces Ss 1  may not be surfaces parallel to the first virtual plane or may not be flat surfaces. 
     Each of the first protruding portions P 1  has an upper surface SU 1  on the side opposed to the anode target  60 . The upper surface SU 1  is formed of a plurality of flat inclined surfaces that are inclined in directions different from one another. In the example, the upper surface SU 1  is formed of two flat inclined surfaces. 
     Comparative Example 
     As shown in  FIG. 1  and  FIG. 11 , the X-ray tube assembly of a comparative example is different from the X-ray tube assembly of the above example with respect to the configuration of the electron convergence cup  15 . More specifically, the electron convergence cup  15  of the comparative example is different from the above example with respective of features that the electron convergence cup is formed without a pair of protruding portions P 1 , the first trench portion  16  (upper trench of the first trench portion  16 ) is formed to be deep, and the first trench portion  16  and the first filament coil  11  are formed to be short in the first length direction dL 1 . 
     The present inventors conducted simulation of emitting the X-rays by using the X-ray tube assembly according to the example and simulation of emitting the X-rays by using the X-ray tube assembly according to the comparative example. At this time, the simulations were conducted by adjusting the bias voltage applied to the electron convergence cup  15 . A focal spot formed on the target surface  62 S was a single focal spot. The simulations were conducted under the same conditions. 
     First, the procedure and results of the simulation of emitting the X-rays by using the X-ray tube assembly according to the example will be described. 
     As shown in  FIG. 1 ,  FIG. 2 , and  FIG. 3 , first, the X-ray tube assembly of the above example was used to commonly apply a negative high voltage to the first filament coil  11  and the electron convergence cup  15 , to set the bias voltage applied to the electron convergence cup  15  to 0V, and to form the focal spot (large focal spot) F 1  on the target surface  62 S. Electrons were emitted from the whole region of the first filament coil  11  toward the target surface  62 S. The electron beam was converged by the effect of the electric field formed by the first trench portion  16  of the electron convergence cup  15  and the first protrusion P 1 . In the formed focal spot (effective focal spot) F 1 , the length is referred to as L 1  and the width is referred to as W 2 . 
     As shown in  FIG. 1 ,  FIG. 2 , and  FIG. 4 , next, the X-ray tube assembly of the above example was used to commonly apply a negative high voltage to the first filament coil  11  and the electron convergence cup  15 , to further apply a bias voltage which is negative to the filament voltage to the electron convergence cup  15 , and to form the focal spot (small focal spot) F 1  on the target surface  62 S. Electrons were emitted from a central portion of the first filament coil  11  toward the target surface  62 S. Due to the effect of the first protruding portion P 1 , the end portion of the first filament coil  11  was larger than the central portion of the first filament coil  11  with respect to the effect of the electric field. The quantity of the electrons from the end portion of the first filament coil  11  was not decreased. The electron beam was converged by the effect of the electric field formed by the first trench portion  16  of the electron convergence cup  15  and the first protrusion P 1 . 
     The width of the formed focal spot (effective focal spot) F 1  was W 2 . In addition, since the quantity of the electrons emitted from the end portion of the first filament coil  11  was not decreased as described above, the length of the focal spot (effective focal spot) F 1  was L 2 , i.e., slightly smaller than the above length L 1 . W 2  was smaller than W 1 , and L 2  was smaller than L 1 . It can be understood from comparison between the focal spot (large focal spot) F 1  in  FIG. 3  and the focal spot (small focal spot) F 1  in  FIG. 4  that the variation in length is smaller than the variation in width. 
     In addition, as shown in  FIG. 5  and  FIG. 6 , the filament current to be supplied to the first filament coil  11  was varied continuously and the tube current was measured. It can be understood that in the example, even though the bias voltage is 0V or −5 kV the tube current can be made larger as the filament current becomes larger. 
     Then, it can be understood that as shown in  FIG. 7 , the focal point F 1  formed by the electrons emitted from the first filament coil  11  and the focal point F 2  formed by the electrons emitted from the second filament coil  12  can be formed at substantially the same positions. For this reason, it can be understood that the first protruding portion P 1  does not affect superposition of the focal point F 1  and the focal point F 2 . 
     Next, the procedure and results of the simulation of emitting the X-rays by using the X-ray tube assembly according to the comparative example will be described. 
     As shown in  FIG. 1 ,  FIG. 11 , and  FIG. 12 , first, the X-ray tube assembly of the above comparative example was used to commonly apply a negative high voltage to the first filament coil  11  and the electron convergence cup  15 , to set the bias voltage applied to the electron convergence cup  15  to 0V, and to form the focal spot (large focal spot) F 1  on the target surface  62 S. The focal spot (effective focal spot) F 1  of the comparative example was formed in the same dimensions as the example, and its length is referred to as L 1  and its width is referred to as W 1 . However, since the first filament coil  11  in the comparative example is shorter than that in the example, the electron density of the focal point F 1  becomes lower and the tube current becomes smaller. 
     As shown in  FIG. 1 ,  FIG. 11 , and  FIG. 13 , next, the X-ray tube assembly of the above comparative example was used to commonly apply a negative high voltage to the first filament coil  11  and the electron convergence cup  15 , to further apply a negative bias voltage to the electron convergence cup  15 , and to form the focal spot (small focal spot) F 1  on the target surface  62 S. Electrons were emitted from a central portion of the first filament coil  11  toward the target surface  62 S. The quantity of the electrons from the end portion of the first filament coil  11  was not decreased. 
     The width of the formed focal spot (effective focal spot) F 1  was W 2 . In addition, since the quantity of the electrons emitted from the end portion of the first filament coil  11  was not decreased as described above, the focal spot (effective focal spot) F 1  of the comparative example was formed in the same dimensions as the example, and its length was L 2  and its width was W 2 . As described above, however, since the first filament coil  11  in the comparative example is shorter than that in the example, the electron density of the focal spot F 1  becomes lower and the tube current becomes smaller. 
     In addition, as shown in  FIG. 14  and  FIG. 15 , the filament current to be supplied to the first filament coil  11  was varied continuously and the tube current was measured. It can be understood that in the modified example, if the bias voltage is 0V the tube current can be made larger as the filament current becomes larger. It can be understood that when the bias voltage is set to several hundreds of V, for example, −1kv the tube current can hardly be made larger even if the filament current is made larger as shown in  FIG. 15 . 
     Then, it can be understood that in the comparative example, as shown in  FIG. 16 , the focal spot F 1  formed by the electrons emitted from the first filament coil  11  and the focal spot F 2  formed by the electrons emitted from the second filament coil  12  can be formed at substantially the same positions similarly to the example ( FIG. 7 ). 
     According to the X-ray tube assembly of the example of the embodiment having the above-described structure, the X-ray tube  1  comprises the anode target  60 , the cathode  10  including the first filament coil  11  and the electron convergence cup  15 , and the vacuum envelope  70 . The electron convergence cup  15  comprises a front surface Sf, the first surface S 1 , the first trench portion  16 , and a pair of first protruding portions P 1 . A pair of first protruding portions P 1  is formed to protrude from the first surface S 1  toward the front surface Sf side and sandwich the first trench portion  16  in the first length direction dL 1 . 
     For this reason, it can be understood that even if the long first filament coil  11  is used the tube current can be made larger while setting the length of the focal point F 1  to a desired value. Alternatively, even if the first trench portion  16  is made shallow the tube current can be made larger while setting the dimensions of the focal point F 1  to desired values. 
     Based on the above, the X-ray tube  1  capable of performing the focal spot dimension variation control and the tube current control easily and stably, and suppressing enlargement of the electron convergence cup  15 , and the X-ray tube assembly comprising the X-ray tube  1  can be obtained. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 
     For example, the upper surface SU 1  of the first protruding portion P 1  can be formed of three or more flat inclined surfaces. 
     As shown in  FIG. 8 , the upper surface SU 1  of the first protruding portion P 1  is formed of three flat inclined surfaces. 
     As shown in  FIG. 9 , the upper surface SU 1  of the first protruding portion P 1  may be formed of an arcuately curved surface. 
     As shown in  FIG. 10 , the electron convergence cup  15  may further comprise a pair of second protruding portions P 2 . A pair of second protruding portions P 2  is formed to protrude from the second surface S 2  toward the front surface Sf side and sandwich the second trench portion  17  in the second length direction dL 2 . The second protruding portion P 2  comprises an upper surface SU 2  on the side opposed to the anode target  60 , and is formed similarly to the first protruding portion P 1 . 
     In the above embodiment, the first filament coil  11  is shorter than the second filament coil  12 . However, when the cathode  10  comprises a plurality of filament coils, the plurality of filament coils may be of the same type or different types. A plurality of focal spots of different dimensions can be selected by making the types different. When the filament coils are of the same type, the lifetimes of the filaments can be extended by alternately using the filaments. 
     If the electron convergence cup  15  comprises a plurality of trench portions, at least one trench portion may be formed in a set with the protruding portion similarly to the trench portion of the above example, and the other trench portions may be formed without using the protruding portions. 
     The filaments serving as electron emission sources are not limited to the filament coils but various types of filaments can be used. For example, the cathode  10  may comprise flat filaments instead of the filament coils. In this case, too, the same advantages as those of the above embodiment can be obtained. The flat filament is a flat plate-like filament having a flat upper surface of filament (electron emission surface) as a plane and a back surface. 
     The X-ray tube and the X-ray tube assembly of the present invention are not limited to the above X-ray tube and X-ray tube assembly but can be variously modified, and can be applied to various types of X-ray tubes and X-ray tube assemblies. For example, the X-ray tube of the present invention can also be applied to a stationary anode type X-ray tube assembly.