Abstract:
In a preferred embodiment, an X-ray producing device, including: an X-ray tube; a cathode disposed in the X-ray tube to produce a stream of electrons; and a spherical anode disposed in the X-ray tube to have impinged on a point thereon the stream of electrons and to produce thereby a stream of X-rays.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to X-ray tubes generally and, more particularly, but not by way of limitation, to a novel X-ray tube having a spherical anode. 
     2. Background Art 
     Originally, an X-ray tube had a fixed tungsten target, or anode, that produced X-rays at an angle out of the tube the X-rays being were emitted when electrons impinged on the tungsten anode. This arrangement created a problem with heating of the fixed tungsten target. The life of an X-ray tube is generally proportional to the available surface area of the target, with small target areas resulting in lower life and concomitant higher overcall costs for an X-ray installation. 
     In a further development, a rotating disk is used to produce a greater target area and to reduce problems with heating and heat dissipation. In yet a further development, X-ray tubes with rotating cone-shaped targets were developed. Such X-ray tubes greatly increase surface area, but still suffer from problems with limited surface area. 
     The increased surface area does allow the heat and energy load to be distributed over a larger surface area (focal track), thus increasing tube capacity. Additionally, the larger mass serves as a better heat sink compared with a stationary anode. Rotor speeds are usually in the range of from about 3000 to about 10,000 rpm. 
     The rotating shaft of such X-ray tubes is bearing mounted and usually constructed of molybdenum, a poor conductor of heat. Nevertheless, the heat from the anode is transferred to the rotor bearings and they are often the cause of X-ray tube failure. 
     Another limitation with conventional X-ray tubes is that there is a pronounced “heel effect” that reduces the intensity of the X-ray beam on the anode side of the tube. Such effect causes the X-rays produced in the area of the target to be attenuated, or partially absorbed, by the target itself before being able to be used for diagnostic purposes. This makes for a variable image, darker on the cathode side of the film. 
     The relatively small anode surface areas of conventional X-ray tubes limits the degree of heat removal therefrom and concomitantly limits the rate of use of such X-ray tubes, in order to prevent damage to the X-ray tubes due to overheating. Radiologists and radiologic technicians must go to some effort to make sure that an X-ray tube is not damaged by overheating. For example, radiologists and radiologic technicians have to analyze X-ray tube heating and cooling charts to determine X-ray tube output limits and cooling times before it is safe to use a tube again and must adhere to the results of the analysis to avoid overheating of the X-ray tube. 
     In summary, some of the limitations of conventional X-ray tubes are: 
     Heat loading limits tube output. 
     Heat build-up requires cooling time before next use. 
     Heat loading damages targets and shortens tube life. 
     Heat loading is transferred to bearings, shortening tube life. 
     Molybdenum stem (in rotating disk tubes) may warp, causing wobbling of the anode. 
     There is a pronounced heel effect. 
     Some devices that address increasing target area and the cleaning thereof are disclosed in the following patents: 
     U.S. Pat. No. 3,334,228, issued Aug. 1, 1967, to Mattson, and titled X-RAY SPECTROMETER HAVING AN X-RAY SOURCE WITH A CONTINUOUSLY CLEANED X-RAY TARGET, describes an X-ray source having a rotating cylindrical target. Cleaning of the target is effected by sputtering off contaminants with ionized gas. 
     U.S. Pat. No. 4,107,563, issued Aug. 15, 1978, to Oddell, and titled X-RAY GENERATING TUBES, describes in one embodiment a rotating cone-shaped target that is reciprocated along its axis of rotation to vary the path of the X-rays. In another embodiment, the surface of the target is shaped so that the path varies as the target is rotated without having to reciprocate the target. 
     U.S. Pat. No. 4,399,551, issued Aug. 16, 1983, to Grady, and titled X-RAY TUBE HAVING ROTATABLE TRANSVERSELY OSCILLATORY ANODE, describes a rotating disk-shaped target that is movable transversely with respect to its axis of rotation to provide a spirally arranged focal track on the surface of the target to facilitate heat dissipation. 
     U.S. Pat. No. 4,413,356, issued Nov. 1, 1983, to Hartl, and titled FLAT ROTARY-ANODE X-RAY TUBE, describes a rotating disk that has a cone-shaped target surface on the outer periphery thereof. The patent is directed to the bearing arrangement for the disk. 
     U.S. Pat. No. 4,799,249, issued Jan. 17, 1989, to Paulikas, and titled SELF-CLEANING ROTATING ANODE X-RAY SOURCE, describes a rotating cylindrical target including a self-cleaning feature similar to the 1228 patent above. 
     None of the foregoing patents discloses an X-ray tube having a target with a greatly increased available target area and reduced heel effect. 
     Accordingly, it is a principal object of the present invention to provide an X-ray tube having a target with a greatly increased area. 
     It is a further object of the invention to provide such an X-ray tube that facilitates cooling of the target. 
     It is another object of the invention to provide such an X-ray tube that reduces the heel effect. 
     It is an additional object of the invention to provide such an X-ray tube that is economical. 
     Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures. 
     SUMMARY OF THE INVENTION 
     The present invention achieves the above objects, among others, by providing, in a preferred embodiment, an X-ray producing device, comprising: an X-ray tube; a cathode disposed in said X-ray tube to produce a stream of electrons; and a spherical anode disposed in said X-ray tube to have impinged on a point thereon said stream of electrons and to produce thereby a stream of X-rays. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Understanding of the present invention and the various aspects thereof will be facilitated by reference to the accompanying drawing figures, submitted for purposes of illustration only and not intended to define the scope of the invention, on which: 
     FIG. 1 is a side elevational view, in cross-section, of one type of conventional X-ray tube. 
     FIG. 2 is side elevational view, in cross-section, of another type of conventional X-ray tube. 
     FIG. 3 is a schematic, side elevational view, in cross-section, of an X-ray tube constructed according to the present invention, employing a spherical tungsten anode. 
     FIG. 4 is a schematic isometric view of the spherical tungsten anode of the present invention, showing a means of providing motion to the spherical anode. 
     FIGS.  5 (A),  5 (B), and  5 (C) are schematic top plan, side elevational, and front elevational views, respectively, of a mounting arrangement for the spherical tungsten anode of the present invention. 
     FIG. 6 is a schematic isometric view of an arrangement whereby the spherical tungsten anode of the present invention can be rotated and turned by a single motor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference should now be made to the drawing figures, on which similar or identical elements are given consistent identifying numerals throughout the various figures thereof, and on which parenthetical references to figure numbers direct the reader to the view(s) on which the element(s) being described is (are) best seen, although the element(s) may be seen also on other views. 
     The X-ray tube was invented in 1895 by a German physicst named Wilhelm Conrad Roentgen whose invention won him the Nobel Prize for Physics in 1901. 
     An early X-ray tube is illustrated on FIG. 1 where the X-ray tube is generally indicated by the reference numeral  20 . Tube  20  includes an evacuated glass housing  30  in which is disposed a cathode  32  and a tungsten target  34  set in a copper anode  36 . Suitable connections  40  and  42  are provided at the ends of the tube for connection to a source of electrical power (not shown). 
     In use, a voltage imposed across tube  20  causes cathode  32  to emit electrons which accelerate across the tube towards and striking tungsten target  34  on anode  36 . This produces X-rays that are then directed through a window area  50  in tube  20  and toward a patient (not shown) to produce a diagnostic image on a suitable medium (not shown). The general path of the electrons and the X-rays is indicated generally by the broken line arrow on FIG.  1 . 
     In time, more efficient generators were produced, providing large amounts of power and making the X-ray tube itself the limiting factor in diagnostic X-ray production. About five percent of the energy transferred to the anode goes into the production of X-rays, leaving 95% of the transferred energy to produce heat. This limited the rate of X-ray production, so as not to damage the tungsten anode through cracking or pitting. In stationary tubes, the small target area limits heat dissipation and, thus, X-ray output. 
     As noted above, a second type of X-ray tube has an anode that is rotating. Such an X-ray tube assembly is shown on FIG. 2, generally indicated by the reference numeral  60 . X-ray tube assembly  60  has an outer, transformer-oil-filled, housing  70  in which is disposed an evacuated X-ray tube  72 . Disposed within X-ray tube  72  is a cathode  80  and a rotating tungsten disk anode, or target,  82 . Tungsten disk anode is operatively connected to, and rotated by, a rotor  90  that is in electromagnetically cooperating relationship with a stator  92 . As was the case with X-ray tube  20  (FIG.  1 ), a stream of electrons generated by cathode  80  strikes tungsten disk anode  82  which emits X-rays, the X-rays exiting X-ray tube assembly  60  through a window area 100. Ports  110  and  112  for electrical connections (not shown) are provided at the upper end of X-ray tube assembly  60 . An expansion bellows  120  is provided at one end of oil-filled housing  70  to accommodate expansion and contraction of transformer oil  122 . 
     In the improvement of X-ray assembly  60  (FIG. 2) over X-ray tube  20  (FIG.  1 ), a focal spot became a focal track. Assuming a five centimeter average focal track radius, rotating anode  82  provides an annular area over 300 times that of a stationary anode with an equivalent target area. However, even that increase has the disadvantages noted above. 
     FIG. 3 illustrates an X-ray tube assembly, constructed according to the present invention, and generally indicated by the reference numeral  150 . Most of the components of X-ray tube assembly  150  are the same as similar components of X-ray tube assembly  60  (FIG. 2) and include a housing  160  filled with transformer oil  162 , an evacuated X-ray tube  164 , an expansion diaphragm  166 , electrical connection ports  168  and  170 , and a stator  172 . Disposed within X-ray tube  164  are a cathode  180 , a rotor  182 , and, according to the present invention, a spherical tungsten anode, or target,  184 . Means of support of spherical tungsten anode  184  are not indicated on FIG.  3 . 
     As was the case with X-ray assembly  60  (FIG.  2 ), a stream of electrons emitted by cathode  180  strikes anode  184  and exits a stream of X-rays from assembly  150  through a window area  190 . Also shown on FIG. 3, for completeness, is a collimator assembly  200 . Collimator assembly  200  is conventional and includes therein a collimator light  210 , a mirror  212  and collimator blades  214 . Collimator assembly  200  is provided so that an operator of X-ray assembly  150  will have a visible light field  220  coextensive with the field of the stream of X-rays so that the non-visible stream of X-rays can be located on a patient or other target. 
     It will be understood that spherical tungsten anode  184  is rotated and turned such that substantially the entire surface of the anode is used. This provides a much larger surface area over the arrangements of conventional X-ray tubes. For example, if the radii of a rotating disk and a sphere are both 50 mm, then replacing the disk with the sphere will increase the surface area by a factor of about 100. In actuality, the increase will be something greater, since the sphere is using its outer surface as a target area, while the disk is using a smaller inner circumference. 
     This increased surface area will greatly improve tube life, as well as tube output. If tube life is often limited by target pitting and cracking from prolonged bombardment with electrons, more target area will spread out the damage and, therefore, lengthen tube life. Tube output, which is often a limiting factor in diagnostic imaging today, also will be improved with the increased surface area. Increase current capabilities will allow shorter exposure times, thereby decreasing artifact from patient motion. Additionally, with the electron beam exposure spread out over a larger area, tube cooling time will be shorter, as each spot on the sphere&#39;s surface is “hit” less often with electrons, therefore not accumulating as much heat. Also, the mass of the sphere is larger than that of a disk and, therefore, provides a larger heat sink to assist in cooling. 
     Tube life will be extended, due to reduced bearing failure. In rotating anode tubes, heat is transferred to the bearings via the molybdenum shaft, causing bearing and, therefore tube failure. Additionally, without the molybdenum shaft, there will be no warping thereof which causes “wobble”. 
     The heel effect, as noted above, is caused by the anode itself absorbing, or attenuating, a portion of the X-ray beam. With a spherical instead of a disk target, there should be a smaller percentage of the diagnostic beam that is attenuated. This is attributable to the different shapes of the anode-spherical vs. squared off. 
     In summary, the advantages of the present invention include: 
     Greater heat loadability, resulting in: 
     increased tube output, and 
     improved anode cooling. 
     Larger heat sink. 
     Much larger surface area. 
     Reduced bearing failure. 
     No shaft warpage due to heating. 
     Less heal effect. 
     FIG. 4 illustrates a method of rotating and turning spherical tungsten anode  184 . Here a spindle  300  contacting spherical tungsten anode  184  has its ends  302  and  304  rotatingly journaled, respectively, in resilient mountings  520  and  522  attached to the ends of a U-shaped yoke  306 . As shown on FIG. 4, rotation of spindle  300  will cause spherical tungsten anode  184  to rotate about axis “A—A”. Rotation of spindle  300  is indicated by arrow “B” on FIG. 4 However, rotation of shaft  310  to which yoke  306  is fixedly attached will cause spherical tungsten anode  184  to rotate about other axes. Rotation of shaft  310  is indicated by arrow “C” on FIG.  4 . It will be understood, therefore, that proper selection of the rates of the rotations of spindle  300  and yoke  306  will cause the spot (not shown) at which an electron beam (not shown) is focused on spherical tungsten anode  184  to change such that substantially the entire surface area of the spherical tungsten anode can be a target. 
     FIGS.  5 (A),  5 (B), and  5 (C) illustrate spherical tungsten anode  184  supportingly mounted in a cylindrical support block  400 . Eight casters  410 ,  412 ,  414 ,  416 ,  418 ,  420 , and  422  (only seven shown on the present figures) support spherical tungsten anode  184  within support block  400 , the casters being spring loaded and having the ability to swivel in any direction required by the spherical tungsten anode. The casters&#39; positions should correlate to the eight corners of an equivalent cube and the angles thereof will be perpendicular to the tangent of the sphere at any point. This should allow good support while X-ray tube  182  (FIG. 3) is in any position or orientation. 
     Spindle  300  and yoke  306  extend into support block  400  through an opening  440  (FIGS.  5 (A) and  5 (B)) defined in the rear thereof. A window opening  450  defined in the front of support block  400  permits the entrance of an electron beam and the exit of an X-ray beam to and from the support block without any attenuation thereof. The casters may be able to relay heat to support block  400 , making the support block something of a heat sink and the casters may also be employed to give spherical tungsten anode its charge. 
     FIG. 6 illustrates a method by which a single motor may be employed to rotate spindle  300  and yoke  306 . Here, a motor  480 , which may be assumed to be a schematical representation of stator  172  and rotor  182  (FIG. 3) is connected to rotate shaft  310 . End  302  of spindle  300  has fixedly attached thereto a gear  500  that engages a complementarily shaped ring gear disposed on the inner periphery of opening  440 . Thus, rotation of shaft  310  causes gear  500  to rotate spindle  300 . Proper selection of gears  500  and  502  permits the entire surface of spherical tungsten anode  184  to serve as a target. 
     Also shown on FIG. 6 are resilient mountings  520  and  522  for ends  304  and  302 , respectively, of spindle  300  and control circuitry  530  operatively connected to motor  480  to control the speed thereof. 
     The point on which the electron bean hits the target determines the anode angle. In conventional X-ray tubes, the cathode includes two filaments disposed in a focusing cup, a small filament to produce a small focal spot and a large filament to produce a large focal spot. Only one filament is used at a time and the electron beams produced by the filaments strike the tungsten target at the same point. This can easily be done, as well, in the X-ray tube assembly of the present invention. Additionally, the use of a spherical target permits the angle of the X-ray beam to be changed. This can be accomplished by providing two focusing cups, each with one or more filaments, directed at different points on the sphere, thus selectively producing one of two different angles of X-rays. Alternatively, a single focusing cup could be provided and the spherical target could be moved to change the anode angle. 
     The non-conventional elements of the present invention may be economically formed of suitable materials by conventional manufacturing means. 
     In the embodiments of the present invention described above, it will be recognized that individual elements and/or features thereof are not necessarily limited to a particular embodiment but, where applicable, are interchangeable and can be used in any selected embodiment even though such may not be specifically shown. 
     Terms such as “upper”, “lower”, “inner”, “outer”, “inwardly”, “outwardly”, and the like, when used herein, refer to the positions of the respective elements shown on the accompanying drawing figures and the present invention is not necessarily limited to such positions. 
     It will thus be seen that the objects set forth above, among those elucidated in, or made apparent from, the preceding description, are efficiently attained and, since certain changes may be made in the above construction and/or method without departing from the scope of the invention, it is intended that all matter contained in the above description or shown on the accompanying drawing figures shall be interpreted as illustrative only and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.