Abstract:
A method and apparatus are provided for adjusting a filament set height in a cathode of an x-ray tube. The method for adjusting a filament set height of a cathode comprises providing a cathode cup of an x-ray tube, the cathode cup comprising at least one bore extending therethrough; inserting a filament post through at least one bore such that the filament set height is below a desired filament set height; measuring an actual filament set height that results from the step of inserting; determining a filament set height adjustment distance in which the filament set height adjustment is generally equal to a difference between the actual filament set height and the desired filament set height; contacting an end of the filament lead with an adjustment tool; and moving the adjustment tool a distance substantially equal to the filament set height adjustment distance. Therefore, the filament is positioned at the predetermined filament set height.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to x-ray tubes used in medical imaging. In particular, the invention relates to a method and apparatus for adjusting a filament set height in a cathode cup assembly for an x-ray tube. 
     An x-ray source is often used in medical imaging systems, such as, but not limited to, computed tomography, fluoroscopy and mammography systems. The x-ray source typically includes an evacuated vessel, known as the frame, containing an anode and a cathode. X-rays are produced by applying a high voltage across the anode and cathode, and accelerating electrons from the cathode toward a focal track on the anode. 
     Known cathode assemblies for such x-ray sources typically include a cathode cup and a plurality of current carrying filaments. The filament leads extend through the cup via the filament feed-through assembly, which typically comprises an electrical insulator and a metallic sleeve used for securing the leads at the desired location. 
     At least one known filament geed-through assembly includes a tubular filament post, a substantially cylindrical insulator, and a sleeve. The filament post may be positioned within the sleeve. Precise positioning of the filaments with respect to the cathode cup is important because the positioning affects operational characteristics of the x-ray tube, such as the focal spot size and position and the emission current. Accordingly, it is desirable to properly position the filament leads and thus the filament within the cathode cup. 
     Conventional filament setting methods for adjusting the filament set height may use pliers and a hammer to pull, twist, or push the filament lead to its desired location. The extent of lead movement is difficult to control, and the desired filament alignment is attained by iteratively adjusting and measuring the filament position, until the filament set height is within a desired tolerance. This process is time-consuming and requires multiple steps to position the filament within the desired filament set height tolerance and may result in damage to the filament assembly. 
     Accordingly, a need exists to enhance the filament setting operations. In particular, a need exists for adjusting filament positions in cathodes and similar devices. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention provides a method for adjusting a filament set height in a cathode of an x-ray tube. The method for adjusting a filament set height in a cathode comprises providing a cathode cup of an x-ray tube, the cathode cup comprising at least two feed-throughs extending therethrough; inserting a filament lead through the at least two feedthroughs to a filament set height below a desired filament set height; measuring an actual filament set height that results from the step of inserting; comparing actual filament set height that results from the step of inserting to the desired filament set height; determining an adjustment to the filament set height in which the adjustment is generally equal to a difference between the actual filament set height and the desired filament set height; contacting an end of the filament post with an adjustment tool; and moving the adjustment tool a distance substantially equal to said adjustment filament set height distance. Therefore, the filament is positioned at the predetermined filament set height. 
     A further aspect of the invention provides a device for adjusting a filament set height of a filament in a cathode. The device comprises a fixture comprising a central bore having a first open end and a second open end; an axial calibrator received in the first open end of the fixture and disposed relative to the second open end; and a clamp adjacent the second open end for securing the fixture to a portion of the cathode such that a filament post extends through the second opened end and is capable of being moved along its axis. The calibrator is capable of being brought into contact with an end of the filament lead. 
     Another aspect of the invention sets forth an adjustment apparatus for adjusting an x-ray tube filament. The adjustment apparatus comprises a fixture, a central bore having a first open end and a second open end; a micrometer received in the first open end of the fixture and disposed relative to the second open end; and a clamp adjacent the second open end for securing the fixture to a portion of the cathode such that a filament lead of the filament extends through the second opened end. The micrometer is capable of being brought into contact with an end of the filament post and adjusting the position of the filament post so as to affect the filament set height. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a filament insulator and cathode cup; 
     FIG. 2 is a side elevation partial cross-section illustration of the filament insulator and cathode cup of FIG. 1; 
     FIG. 3 is an illustration of a device for filament set height adjustment in a cathode cup assembly, as embodied by the invention; 
     FIG. 4 is an illustration of the device of FIG. 3 used in connection with the filament insulator and cathode cup of FIG. 2; 
     FIGS. 5-7 are sequential illustrations of a method for filament set height adjustment of a cathode cup assembly, as embodied by the invention; 
     FIG. 8 is an illustration of a fixture used in FIGS. 3-7; 
     FIG. 9 is an illustration of another fixture, as embodied by the invention; and 
     FIG. 10 is an illustration of another filament set height adjustment tool, as embodied by the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a plan illustration of a known cathode cup assembly  50  to which the present invention may be readily adapted for use. The cathode cup assembly  50  comprises a cathode cup  52 , four filament insulators  54 A,  54 B,  54 C, and  54 D and two filament assemblies  56 A,  56 B (illustrated in phantom). The filament assemblies  56 A,  56 B each include a filament (not illustrated) and posts  58 A,  58 B,  58 C, and  58 D that extend from respective ends of the filaments. The filament insulators  54 A,  54 B,  54 C, and  54 D each comprise a respective insulating member  60 A,  60 B,  60 C, and  60 D, a respective post sleeve  62 A,  62 B,  62 C, and  62 D, and a respective flange  64 A,  64 B,  64 C, and  64 D. The insulating members  60 A,  60 B,  60 C, and  60 D each can comprise a bore therein (not illustrated in FIG.  1 ), and filament lead tubes  62 A,  62 B,  62 C, and  62 D that are inserted within the respective bores. The filament lead tubes  62 A,  62 B,  62 C, and  62 D can be brazed to respective insulating members  60 A,  60 B,  60 C, and  60 D. NEED? 
     Each flange  64 A,  64 B,  64 C, and  64 D comprises an aperture (not illustrated) sized so that the respective insulating members  60 A,  60 B,  60 C, and  60 D can extend therethrough. The flanges  64 A,  64 B,  64 C, and  64 D can be brazed to respective insulating members  60 A,  60 B,  60 C, and  60 D so that each flange portion  66 A,  66 B,  66 C, and  66 D extends radially outwardly from respective insulating members  60 A,  60 B,  60 C, and  60 D. The flanges  64 A,  64 B,  64 C, and  64 D can be welded, for example spot welded, to cathode cup  52  at weld  68  to secure cathode insulator  54 A,  54 B,  54 C, and  54 D to cathode cup  52 . Prior to welding, however, the flanges  64 A,  64 B,  64 C, and  64 D should be trimmed so that the flanges  64 A,  64 B,  64 C, and  64 D do not overlap. If such flanges  64 A,  64 B,  64 C, and  64 D were to overlap, then it would be very difficult to securely weld each flange  64 A,  64 B,  64 C, and  64 D to the cathode cup  52 . The following description may refer to the welding as spot welding, however, this description is merely exemplary and is not intended to limit the invention in any manner. 
     Filament  56 A can be inserted in cathode cup  52 , so that the filament rests within a filament receiving portion  70  of the cathode cup  52 . The filament post  58 A can extend through a filament feedthrough assembly comprising an insulating member  74 A, and the filament feedthrough sleeve  62 A. The filament post  58 A extends from the filament feedthrough sleeve  62 A. The distance from a specified cathode cup surface to the emitting portion of the filament is also referred to herein as a “filament set height”. 
     The filament post  58 A can then be connected to the filament feedthrough sleeve  62 A. A similar process is carried out for the remaining filament posts  58 B,  58 C, and  58 D. It should be appreciated that the aforementioned cathode cup arrangement is merely exemplary of the invention, and may be readily adapted to numerous other types and styles of cathode cup arrangements. 
     filaments need to be adjusted to prescribed set height tolerances, for example, but not limited to, about 20 microns. Previously, filament set heights were adjusted using pliers to pull the filament posts to decrease the set height or using a hammer to push the post through the filament feedthrough sleeve in order to increase the set height. Therefore, the set height adjustment distance was an estimation and several iterations were generally required before a desired filament set height was achieved. 
     An apparatus for filament set height adjustment, as embodied by the invention, is illustrated in FIG.  3 . The apparatus (referred to hereinafter as a “filament adjustment tool”)  100  comprises a calibrator, such as micrometer  102 . The following description will refer to the calibrator as a micrometer; however, this is merely exemplary and is not intended to limit the invention. The digital micrometer illustrated in FIG. 3, which satisfactorily performs the process, as embodied by the invention, is manufactured by Mitutoyo Corporation of Kawasaki, Japan Model No. 350-714-30. 
     The micrometer  102  includes a measuring range of up to about 25 mm and can read distances to a precision of approximately 1 micron. The micrometer  102  comprises an axial pushrod  106 , which is moveable, for example, by rotation of a spindle  108 . An adjustment fixture  112  is secured to a housing  110  of the micrometer  102 . This fixture  112  is illustrated in FIG.  8 . The adjustment fixture  112  is secured relative to the housing  110  by way of a set screw  114  received in a bore  115  of the adjustment fixture  112 . The adjustment fixture  112  includes a bore  113  extending through a central region of the adjustment fixture  112 , with the bore  113  comprising a first section  116  having a first diameter  117 . The first section  116  may accommodate the housing  110  of the micrometer  102 ; a second section  118  having a second diameter  119 , which is less than the first diameter  117 , for accommodating axial movement of the rod  106 ; and a third section  120  having a third diameter  121  for accommodating a filament lead tube of a cathode cup. The third diameter  121  is less than that of the second diameter  119 . A set screw  122  received in bore  123  for fixedly securing the adjustment fixture  112  relative to the filament lead tube of the cathode cup, extends into the third section  120 . 
     View  124  is provided along the length of the adjustment fixture  112 . The adjustment fixture  112  may take on a number of possible configurations. For example, the adjustment fixture  112  may have an axial slot to facilitate the viewing of the contact point between the pushrod  106  and the filament post. The pushrod  106  may have rounded tip or a spherical insert  126  in order to reduce friction between the pushrod  106  and filament post. The adjustment fixture  112  may be made of any suitable rigid material, such as, but not limited to, steel or aluminum. 
     The adjustment fixture  112  (FIG. 4) can readily receive the filament feedthrough sleeve  62 A of the cathode cup  52  through a central bore  113 . The adjustment fixture  112  can be secured to the filament lead tube  62 A using a set screw  122  or any equivalent securing device. The filament post  58 A is inserted through the filament feedthrough sleeve  62 A to an initial location, such that the filament set height is below the desired value. The filament feedthrough sleeve  62 A and filament post  58 A can then be lightly crimped to temporarily hold the filament post  58 A in place with respect to the cup  52 . Because the filament post  58 A is located such that it extends a greater distance through the filament feedthrough sleeve  62 A than needed for the desired filament set height, the filament post  58 A needs to be pushed along its axis to achieve the desired filament set height. 
     Once each of the filament leads is inserted into its respective filament feedthrough sleeve and secured using a light crimp, the filament set height is measured. For example, the cup  52  is placed under a microscope, which comprises a digital height read-out. The difference between the actual set height and the desired set height is determined for each filament lead. The cup  52  may be removed from the microscope and the adjustment tool  100  can then be clamped to a respective filament lead tube, as discussed herein with respect to FIG.  4  and as illustrated in FIG.  5 . Each of the filament leads can then be individually adjusted to obtain the desired set height. 
     Once the filament set height adjustment tool  100  has been secured to the filament lead tube  62 A, the spindle  108  of the micrometer  102  is rotated until the tip  126  of the pushrod  106  contacts the filament lead  58 A, as illustrated in FIG.  6 . Initial contact of the rod  106  with the filament lead  58 A and movement of the filament lead  58 A after contact can be inspected and visualized through the view  124  in the adjustment fixture  112 . Once the pushrod  106  contacts the filament lead  58 A, the read-out  104  of the micrometer  102  is noted. The spindle  108  of the micrometer  102  can then be rotated to push the filament lead  58 A toward the cathode cup  52 . This pushing increases the filament set height until the desired distance is reached to provide a desired filament set height, as illustrated in FIG. 7, as is readily determined by the read-out  104  of the micrometer  102 . 
     The distance by which the pushrod  106  is moved after contact with the filament lead  58 A is essentially equal to the respective set height difference previously determined. Once the desired filament set height is achieved, the filament set height adjustment tool  100  is removed from the filament lead tube  62 A, for example by loosening of the set screw  122 . The above process may be carried out for each of the filament leads in a cathode cup. Further, the process may be carried out on any cathode cup having a filament post Additionally, the adjustment fixture  112  may be configured so a plurality of filament leads may be adjusted simultaneously, to further reduce time associated with such an adjustment process. 
     FIG. 8 illustrates the adjustment fixture  112 , which is utilized to fix the relative position of the micrometer  102  with respect to the filament post of the cathode cup. The adjustment fixture  112  may be made of any suitable rigid material, including but not limited to steel, aluminum, or machinable ceramic. The adjustment fixture  112  includes the central bore  113  extending therethrough and comprises the first section  116 , second section  118 , and third section  120 . The first section  116  can receive the micrometer  102  and the third section  120  can receive the at least one of filament feedthrough sleeve and post of the cathode cup. The adjustment fixture  112  also includes the bore  115  for receiving the set screw  114  for securing the adjustment fixture  112  to the micrometer, and the bore  123  for receiving set screw  122  for securing the adjustment assembly relative to a filament feedthrough sleeve. An optional view port  124  is included in a sidewall of the second section  118  for allowing the operator to assess contact between the pushrod  106  of the micrometer  102  and the filament post. 
     With reference to FIGS. 9 and 10, an alternative configuration of a filament adjustment tool, as embodied by the invention, is illustrated. In FIG. 9, a cylindrical fixture  210  includes a central bore  212 , with the central bore  212  comprising a threaded interior wall for receiving an adjustment bolt  214 , as illustrated in FIG.  10 . The central bore  212  comprises a first section  216  having a first diameter and a second section  218  having a second diameter, which is generally less than that of the first diameter. A bore  220  intersects the second section  218  in which the bore  220  can receive a set screw  222  (similar to the set screw  122 ) for securing the fixture  210  with respect to a filament feedthrough sleeve. A view port  224  is provided to allow the operator to verify the contact between the bolt  214  and a filament post extending from the filament feedthrough sleeve. 
     As illustrated in FIG. 10, the adjustment bolt  214  comprises a threaded pushrod  226 , which engages the central bore  212  of the fixture and is movable therein in an axial direction in response to rotation of a head  228  of the bolt  214 . The fixture  210  is selectively secured to, for example, but not limited to, the filament feedthrough sleeve of a cathode cup by way of a set screw received in the bore  220 , with a portion of the filament feedthrough sleeve and the filament post of the cathode cup extending into the first section  216 . Once secured thereto, rotation of the bolt  214  causes the end  230  of the rod  226  to contact the filament lead. Continued rotation of the bolt  214  causes the post to be moved relative to the filament feedthrough sleeve, thus allowing the filament set height to be adjusted within the cathode cup. This process is generally similar to the above-described embodiment, and reference is made thereto for a description of the process. 
     In the description, the terms are provided with their normal meaning to a person of ordinary skill in the art, unless otherwise specified. For example, the terms “substantially” and “generally” are relative terms with variances as understood in the art. 
     While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention.