Abstract:
In an X-ray tube having an anode supported for rotation and an annular target track mounted upon the anode, a cathode spaced apart from the anode projects a beam of electrons onto the target track within a focal spot. The cathode is designed to normalize the impact temperature across the focal spot, as a function of length. In accordance therewith, the cathode comprises a filament and a cathode cup, wherein the filament is disposed to project the electron beam onto the target track to generate X-rays, when a high voltage potential difference is established between the filament and the anode. The filament and the cathode cup are respectively configured to selectively form the electron beam so that the beam provides an electron distribution within the focal point which maintains each point within the focal spot at substantially the same temperature.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein generally pertains to design of the focusing cathode or filament geometry of a rotating anode X-ray tube. More particularly, the invention pertains to a cathode design that normalizes impact temperature along the focal spot length. Even more particularly, the invention pertains to a design of the above type which effectively varies the width of the X-ray tube focal spot, as a function of position along the length thereof, to normalize impact temperature over the focal spot length. 
     In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum and across a very high voltage, on the order of 100 kilovolts, from a cathode to a focal spot position on an annular tungsten target track. X-rays are produced as electrons strike the focal spot on the target track, which is mounted on a disk-shaped anode rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, so that very little of the total power input, typically less than 1%, is converted to X-radiation. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, the effective management of heat is a major concern in X-ray tube design. 
     As used herein, the term “impact temperature” refers to the temperature of the target track within the focal spot, resulting from impacting electrons of the electron beam. In view of the thermal considerations referred to above, the impact temperature must not exceed the melting temperature of tungsten, at any point within the focal spot, to avoid damage to the target track. At present, this temperature constraint limits the maximum power that an X-ray tube can deliver in high current applications such as computed tomography (CT) and cine angiography. More particularly, if electric power applied to the cathode is increased, in order to increase electron emissions and thereby produce a higher output of X-rays, it may be necessary to enlarge the area of the focal spot. The increased number of impacting electrons are thereby spread over a greater area, to enhance cooling and maintain a specified level of loadability. (As used herein, loadability refers to the ability of the target track, within the focal spot, to tolerate a given quantity of heat.) 
     As is well known by those of skill in the art, while increasing focal spot size tends to improve loadability, such increase also reduces the quality of images produced by X-rays derived from the focal spot. Accordingly, the temperature limitation referred to above has, in the past, required a trade-off in the design of X-ray tubes. That is, increasing X-ray output could result in diminished image quality, and improvements in image quality could require reduced X-ray output. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus for normalizing the impact temperature across an X-ray tube focal spot, as a function of length. In accordance therewith, the invention is directed to apparatus for producing X-rays which comprises an anode supported for rotation within the tube, an annular target track mounted upon the anode for rotation therewith, and a cathode spaced apart from the anode. The cathode comprises a filament and a cathode cup, which cooperatively project a beam of electrons onto the target track, within the focal spot, to generate X-rays. The filament and cathode cup are respectively configured to selectively form the electron beam so that the beam provides an electron distribution within the focal point which maintains each point within the focal spot at substantially the same temperature. 
     In a preferred embodiment, the filament has an associated axis and the focal spot has length and width dimensions. The length dimension is measured between two focal spot end points along a direction which is parallel to the axis, and the width dimension is measured along a direction which is orthogonal to the filament axis and the length direction. The filament and cathode cup are respectively configured to form the beam so that the beam defines a focal spot having width dimensions at its end points which are substantially less than the focal spot width at a location midway between the two end points. Preferably also, the target track comprises tungsten, the anode comprises a rotatable disk formed of a refractory metal, and a potential difference on the order of 100 kilovolts is maintained between the cathode and the anode to produce X-rays. 
     In a useful embodiment, the cathode cup is provided with a planar surface having a channel formed therein, and the filament comprises a helical filament disposed for insertion into the channel, the helical filament having a central portion and opposing end portions. The filament is selectively curved, so that its end portions are recessed deeper into the channel than the central portion thereof, relative to the planar surface of the cup. 
     In another useful embodiment, the filament comprises a linear helical filament having a central portion and opposing end portions. The cathode cup is provided with a selectively curved surface having a channel formed therein, the filament being inserted into the channel so that the opposing end portions of the filament are recessed further into the channel than the central portion thereof, as a result of the curvature of the cup. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view with a section broken away, showing an X-ray tube which may employ an embodiment of the invention. 
     FIG. 2 is a perspective view showing prior art components which may be employed in the X-ray tube of FIG. 1 in further detail. 
     FIG. 3 is a graph showing the relationship between temperature and length of the focal spot depicted in FIG.  2 . 
     FIG. 4 shows an X-ray tube focal spot provided by an embodiment of the invention. 
     FIG. 5 is a graph which compares parameters associated with the focal spots of FIG.  2  and FIG. 4, respectively. 
     FIG. 6 is a perspective view showing a cathode constructed in accordance with an embodiment of the invention. 
     FIG. 7 is a sectional view showing a portion of FIG. 6 in greater detail. 
     FIGS. 8,  9  and  10  are sectional views taken along lines  8 — 8 ,  9 — 9 , and  10 — 10 , respectively, of FIG.  7 . 
     FIG. 11 is a perspective view showing a cathode constructed in accordance with a second embodiment of the invention. 
     FIG. 12 is a sectional view showing the embodiment of FIG. 11 in greater detail. 
     FIGS. 13,  14  and  15  are sectional views taken along lines  13 — 13 ,  14 — 14  and  15 — 15 , respectively, of FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown an X-ray tube  10 . In accordance with conventional practice, tube  10  generally includes a metal housing  12  which supports other X-ray tube components including a cathode  14 , and also provides a protective vacuum enclosure therefor. Cathode  14  directs a high energy stream of electrons  16  onto a target track  18  of an anode  20 , which consists of a refractory metal disk, or alternatively a graphite disk, and is continually rotated by means of a conventional mounting and drive mechanism  22 . Target track  18  has an annular or ring-shaped configuration and typically comprises a tungsten based alloy integrally bonded to the anode disk  20 , or may comprise tungsten-rhenium if anode  20  is formed of graphite. As anode  20  rotates, the stream of electrons from cathode  14  impinges upon a continually changing portion of target track  18  to generate X-rays, at a focal spot position. A beam of X-rays  24  generated thereby is projected from the anode focal spot through an X-ray transmissive window provided in the side of housing  12 . 
     In order to produce X-rays as described above, there must be a potential difference on the order of 100 kilovolts between cathode  14  and anode  20 , to accelerate the electrons in the space therebetween. In a common arrangement, this is achieved by coupling the anode to a ground connection (not shown), and applying power at the required 100 kilovolt range to cathode  14  through an electric cable  26  and a cathode coupling  28 . 
     Referring to FIG. 2, there is shown a cathode  14  constructed in accordance with the prior art. Such cathode has a cup provided with a surface  32 , having a slot or channel  34  formed therein. An elongated helical filament  36 , extending along an axis A, is placed within channel  34  and emits the electrons of a beam  16  when electric power is applied to the cathode as described above. The electrons are accelerated through the vacuum space between cathode  14  and anode  20 , by the 100 kilovolt potential difference, and impact the target track  18  of the anode within a focal spot  30 . While the tungsten target track is continually rotating, the boundaries of the focal spot are fixed, and are defined by the electron beam. 
     Referring further to FIG. 2, there is shown focal spot  30  having a length dimension l measured along a direction which is parallel to filament axis A, and a width dimension w which is measured along a direction orthogonal to the direction of the length measurement. The length l of the focal spot is determined by and is substantially equal to the length of filament  36 . The width of the focal spot, at a particular position along the length thereof, is determined by the extent to which electrons emitted by a corresponding portion of filament  36  spread outward therefrom, between the filament and the target track. FIG. 2 shows that electron distribution is greater along the sides  30   a  and  30   b  of focal spot  30  than along the center thereof. 
     Referring to FIG. 3, there is shown a graph depicting variation of focal spot impact temperature T imp  over the length of the focal spot  30 . More particularly, FIG. 3 shows that the temperature of the focal spot is substantially lower at its ends than in its center. As illustrated by filament  36  placed above the graph of FIG. 3, the temperature of the coils along the central portion of the filament is greater than the temperature at the two ends thereof. Electron emission increases rapidly with temperature. Moreover, locations along the central portion of focal spot  30  receive electron emissions  38  from many more filament coils than locations proximate to the ends of the focal spot. Accordingly, electron density is greater along the corresponding central portion of focal spot  30  than at the ends thereof. This is also illustrated in FIG. 2, wherein end regions  30   c  and  30   d  are shown to have lower electron densities than other regions of focal spot  30 . The lower electrons densities in turn result in lower impact temperatures at the ends of the focal spot. 
     In accordance with the invention, it has been recognized that the condition depicted in FIG. 3 enables certain adaptations of the focal spot, which can provide significant benefits in X-ray tube operation. More specifically, it has been recognized that the cathode may be designed to provide a focal spot having the configuration of focal spot  40 , shown in FIG. 4, rather than the configuration of conventional focal spot  30 . Focal spot  40  is widest at its central region  40   a , and tapers therefrom to its two ends  40   b  and  40   c , which are both of substantially less width than the central region  40   a . The overall area of focal spot  40  is significantly reduced, by reducing the width of focal spot  40  at its two ends, whereby electron density is more uniformly distributed over the width of the focal spot, and the image quality of X-rays produced from the focal spot  40  is substantially improved. Reducing the area of focal spot  40  proximate to its end regions  40   b  and  40   c  will, of course, raise the temperature of such regions, since there will be less area for distributing the heat of electrons impacting thereupon. This is acceptable, however, since the focal spot temperature at its end regions was lower to begin with, as described above in connection with FIG.  3 . It is only necessary to ensure that the end region temperatures do not exceed a maximum allowable temperature for the tungsten target track. Generally, it would be desirable to vary the width of the focal spot along its length to normalize impact temperature with respect to length, i.e., to have substantially the same temperature at each point along the focal spot length. 
     Referring to FIG. 5, there are shown curves  42  and  44  depicting X-ray density over width, for X-rays produced by the electrons impacting focal spots  30  and  40 , respectively. Each curve was generated by integrating the X-ray density of its corresponding focal spot over length, at each point along the focal width. Curve  42  indicates that X-rays are concentrated along the sides of focal spot  30 . In contrast, curve  44  indicates that X-rays produced in connection with focal spot  40  are much more uniformly distributed across the width thereof. This enhances the quality of images acquired by the X-rays of focal spot  40 , as stated above. 
     Referring to FIG. 6, there is shown cathode  14  adapted to generate a focal spot having the configuration of focal spot  40 . Cathode  14  is provided with a cup or cup-shaped structure  46  at its forward end having a planar surface  48  and a slot or channel  52  formed therein. A helical filament  50  is inserted into channel  52 , to project an electron  54  onto target track  18  when energized by a high voltage electric current. The electrons impact track  18  within boundaries defining focal spot  40 , as shown in FIG.  4 . 
     As best shown by FIG. 7, filament  50  is selectively curved, so that the end portions or segments  50   a  thereof are set more deeply into channel  52  than other portions thereof. Intermediate portion  50   b  of filament  50  is proximate to planar surface  48  of cup  46 , and central filament portion  50   c  is positioned above surface  48 . Thus, the set heights of respective portions or segments of filament  50  gradually increase from the ends thereof to the center thereof, wherein set height indicates the level of a filament portion with respect to the cup surface  48 . 
     Referring to FIG. 8, it is seen that because end portions  50   a  of filament  50  are recessed deeply into channel  52 , the width of the beam portions  54   a  produced thereby are narrowed by the walls of channel  52 . Thus, the beam portions  54   a  provide focal spot  40  with reduced width at its end regions  40   b  and  40   c . Referring to FIGS. 9 and 10, it is seen that as set height of the filament portions is increased, the widths of the beam portions respectively produced thereby are likewise increased. Accordingly, the width of beam portion  54   b , produced by intermediate filament portion  50   b , is greater than the width of beam portion  54   a , and beam portion  54   c , produced by central filament portion  50   c  has the greatest width. Thus, curved filament  50  and cup  46  cooperate to generate focal spot  40 , which is widest at its center and tapers to its ends, as described above. It is to be noted that as the filament set height in the cup is reduced, the emission will decrease. However, around the typical operating point of cathodes with helical filaments the loss of emission is less than the reduction in width, so that there is an overall gain. It is to be noted further that since set height of a filament portion determines the width and electron density of a corresponding position or region of the focal spot, the set height also determines focal spot impact temperature at such region. Referring to FIG. 11, there is shown an alternative embodiment of the invention, wherein cathode  14  is provided with a cup  56  having a curved surface  58  and a channel  62  formed therein. A linear helical filament  60 , having its coils oriented along an axis A 1 , is inserted into channel  62  of cup  58  to project an electron beam  64  onto target track  18 , within a focal spot  40 . As best shown by FIG. 12, the curved surface  58  of the cup results in the set heights of filament  60 , with respect to channel  62 , progressively increasing from the filament end portions  60   a  to the central filament portion  60   c . In like manner with filament  50  described above, and as shown by FIGS. 13-15, as the set height of respective filament portions is increased, the widths of the beam portions respectively produced thereby are likewise increased. Accordingly, the width of beam portion  64   b , produced by intermediate filament portion  60   b , is greater than the widths of beam portions  64   a , produced by end filament portions  60   a , and beam portion  64   c , produced by central filament portion  60   c , has the greatest width. 
     In order to provide cathode and filament geometries for the embodiments disclosed above, to generate focal spot  40 , it is to be understood that electron emission and impact temperature can be normalized along the anode-cathode axis by altering focal spot width as a function of position along the length of the focal spot. Thermal analysis indicates that impact temperature is proportional to the inverse of the square root of focal spot width when peak current density is constant. Accordingly, the cathode and filament should be designed to provide a focal spot  40  having a width that decreases along the length of the focal spot, from the center to the ends thereof, wherein width at a given point along the length is proportional to the square root of the current density at the given point. 
     While the above embodiments have been directed to cathodes with helical filaments, other embodiments of the invention may employ other types of filaments, including flat filaments and circular filaments. In yet other embodiments of the invention, instead of decreasing focal spot size to improve image quality, electron emission may be increased to increase X-ray output. It is anticipated that an embodiment of the invention can increase X-ray output on the order of 11%, for a focal spot size of 1.0 millimeters, while maintaining constant resolution and maximum impact temperature, which is normalized along the focal spot length. It will be understood that X-ray output is a function of focal spot size, and increases with focal spot size. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.