Abstract:
A mammography X-ray tube providing increased X-ray intensity for shortening patient exposure times to eliminate motion artifacts. The cathode design permits superpositioning of electron beam from multiple filaments.

Description:
FIELD OF THE INVENTION 
     This invention relates to methods and apparatus for x-ray mammography diagnostics. 
     BACKGROUND OF THE INVENTION 
     Diagnostic X-ray equipment is well known for so called non-invasive examination. Equipment is available for industrial as well as medical applications. A most important element of such equipment is the generator of X-rays which is most typically a high vacuum tube with the capability of generating an electron beam and accelerating the beam toward a high speed rotating target where the impact produces X-rays which pass out of the vacuum envelope and are collimated and directed toward the patent or sample being studied. For standard X-ray diagnostic tubes, electric fields of 150 KV/inch to 300 KV/inch are employed which are produced in conjunction with DC voltages of 75 to 150 KV. Typically the distance between the cathode and the rotating target is on the order of 0.5 to 1 inch. It is known in such standard purpose X-ray tubes to superimpose electron beams produced from more than one filament onto the same focal spot on the anode target. In such standard purpose X-ray tubes this focussing is accomplished using a pair of cathode cups employing two and three slot designs. Typically, the slots have been machined grooves which form two cups which are symmetrically displaced about an axis. The cathode filaments are normally mounted adjacent the intersection of the smallest and next smallest slot. When the filament is mounted inside of the smallest slot, its emission is reduced because of space charge effects. The dimensions of the slots and the distance between the center of the slots to enable focusing of the beams from adjacent cups to a single spot has heretofore required at least 0.5 inch of anode to cathode spacing. 
     Mammography X-ray diagnostics is a special application for which a specific mammography X-ray tube has become standard. Specifically, the mammography tube is very much shorter in overall length than the standard X-ray tubes. The mammography tube is particularly designed to be able to have its X-ray exit port very close to the patient&#39;s breast to obtain the highest resolution and contrast picture possible. 
     Superimposition of electron beams from adjoining cathode cups has not been heretofore achieved in mammography X-ray tubes because the slot dimensions necessary in standard two slot cathode cup configurations required the center of the slots to be too far apart to allow the electron beams to become superimposed over the shorter anode to cathode distance employed in mammography tubes. For mammography tubes, the DC voltage employed is only 25 to 30 thousand volts. Because the shorter anode to cathode distances employed in these tubes, i.e. less than 0.3 inches, the fields are 110 KV/inch to 130 KV/inch. 
     In view of the above problems, currently designed mammography X-ray tubes are not capable of providing high intensity electron beams and are generally considered cathode emission limited. This requires the typical mammograph examination for large spot applications to take 1-2 seconds and for small spot, high resolution examinations to take approximately 5 seconds. The high resolution, 5 second examination time, introduces significant opportunity for picture blurring due to patient movement or other mechanical and environmental vibrations. Specifically, cathode filaments in mammography tubes with 0.1 mm focii typically could deliver only 25-30 ma and for a typical 0.3 mm focii could deliver only approximately 100 ma. Since the high voltage employed is 25 KV, the target anodes are not fully loaded. A three to four inch rotating anode can handle these power levels at 3000 RPM. Since the mammography X-ray tubes are capable of rotating their target anodes at speeds up to 9000 RPM, and the power handling capacity at this higher speed is 70% greater than at 3000 RPM, a technique to provide greater electron beam intensity can be accommodated by the existing mammography X-ray tube design by increasing the anode speed. 
     SUMMARY OF THE INVENTION 
     It is the object of this invention to enable shorter mammography patient exposure time and to avoid movement blurring effects. 
     It is a further object of this invention to provide a method and apparatus for increasing electron beam intensity in a mammography X-ray tube. 
     It is a feature of this invention to simultaneously excite a plurality of cathode filaments and to superimpose electron beams so formed on the same region of said X-ray tube rotating target anode. 
     It is a further feature of this invention that the cathode cups are formed in triple slot configurations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of a standard prior art mammography X-ray tube. 
     FIG. 2 is a schematic of electron optics for superimposing small filament and large filament beams for a standard diagnostic X-ray tube having anode to cathode distances greater than 0.5 inch. 
     FIG. 3A is the front view of a preferred cathode assembly of our invention. 
     FIG. 3B is a side view of Section A--A of FIG. 3A. 
     FIG. 3C is a schematic of filament connections of the cathode assembly of FIG. 3A. 
     FIG. 4 is the preferred cathode assembly of FIG. 3A showing its detailed dimensions. 
     FIG. 5 is a schematic of the electron optics of an embodiment of our invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, the mammography X-ray tube has a vacuum envelope 1 containing a rotating anode 3, a motor rotor coil 4 for providing high speed drive power for said anode in conjunction with stator coils 5 of said motor. Cathode assembly 2 is offset from the axis 10 for providing a beam of electrons 8 which are accelerated to impact the sloped surface of the target anode in a fixed rectangle line in space which provides an output rectangular X-ray beam 11. The high voltage standoff 7 connects high voltage to the anode, i.e., 25 to 30 kv, through a bearing (not shown) between the rotor support 12 and the rotor 4 for coupling the high voltage to said rotating target 3 to create an accelerating field between the anode and cathode. Because the X-ray tube for mammography applications employs a lower energy X-ray, the accelerating voltage is considerably lower than in standard X-ray. The distance between the cathode assembly and the target in such mammography tubes is less than 0.3 inches. The cathode assembly 2 filament current is supplied to the cathode assembly from connector 14 via conductors 13. One side of each filament is normally grounded to the housing. Space 15 on the inside of the housing which is not within the vacuum envelope is filled with a dielectric oil. The elastomeric cup 16 is able to deform to accommodate temperature induced changes in the oil and to maintain oil pressure. 
     In the prior art standard X-ray tube, the distance between the cathode assembly 2 and the target is long enough, as shown in FIG. 2, i.e. D&gt;0.5 inch, in cooperation with the higher electric field gradient and the double slot and triple slot cathode cups to superimpose the beams from the small filament 26 and the large filament 27 to a single region 29 on the target anode. In the prior art standard X-ray tube, the two filaments are not excited simultaneously but rather they provide the ability to select a high or a low resolution focused X-ray beam which will exist the X-ray tube on exactly the same center line. As indicated in FIG. 2, a symmetrical triple slot 21, 22 and 23 filament cup configuration for the smaller diameter filament is coupled together with a symmetrical double slotted 24 and 25 filament cup configuration for the larger diameter helix filament 27. Note that the prior art cups are each completely symmetrical and separated somewhat, 12 at their closest contact. 
     In contrast, the Mammography X-ray tubes have not been able to superimpose both the large and small filaments using the double and triple slot design because the distance D is smaller and the field gradient is lower. Electron optics computer modeling is not successful to provide adequate calculations to solve this problem in the X-ray tube because the helical cathode filaments do not emit electrons either uniformly in energy or direction. Accordingly, we have empirically discovered a technique that makes it possible to focus different size beams as well as equal size beams to superimpose beams on the same region of the anode of a mammography X-ray tube. 
     With reference to FIG. 3A and FIG. 3B is disclosed a novel cathode assembly for use with a mammography X-ray tube which enables superposition of a plurality of electron beams on a common anode region. The novel cathode assembly, with reference to FIG. 3B, comprises a first triple slot 44, 45 and 46 filament cup which intersects a second triple slot 41, 42 and 43 filament cup. Neither cup is symmetrical since the intersection of the two cups along line 56 interrupts the slots 44 and 41. Slots 45 and 46 are parallelepiped shaped with rectangular cross sections, and slots 41 and 44 are prismoids with trapezoid cross section. 
     In the preferred embodiments of FIG. 3A, 3B and 3C, matching filament 32 and 34 are mounted is slots 46 and 43 respectively and are matching in diameter and all other characteristics. As shown, in FIG. 3C, there are two filaments in each slot. In slot 43, filaments 34 is the large diameter filament and filament 33 is a small diameter filament. In slot 46, as stated, filament 32 is a large diameter filament matching filament 34 and filament 31 is a smaller diameter filament matching the smaller diameter filament 33. 
     Filaments 34 and 32 are connected electrically in parallel by connecting terminals 40 and 39 together. Terminals 37 and 38 are common and are also connected together. Filaments 31 and 33 are also connected electrically in parallel by connecting terminals 36 and 35 together. 
     External controls connected via connector 7 enables the selection of the pair of larger diameter filaments or the pair of small diameter filaments to be simultaneously excited to create electron beams which are superimposed. 
     The two larger diameter beams will superimpose at a first focal rectangle and the two smaller diameter beams will superimpose at a second displaced focal rectangle. 
     By combining via superposition the electron beams from two filaments simultaneously, we are able to essentially double the beam current and substantially increase the X-ray intensity in both the small spot 0.1 mm focii and in the larger 0.3 mm focii mode. This substantially reduces the amount of exposure time required for a picture which greatly enhances the ability to avoid motion artifacts. 
     FIG. 4 gives the exact dimensions of the preferred cathode cup configuration for use with the Varian mammography X-ray tube Model M143-SP according to this invention. 
     With reference to FIG. 5, an alternate embodiment is illustrated in which a small diameter filament 26&#39; is superimposed in a mammography X-ray tube on the same focii as a larger filament 27&#39;. In FIG. 5, both filament cups are triple slotted configuration. However, the cup slot dimensions in FIG. 5 are not identical as is the configuration of FIG. 3B. Also, the two cups are not equally displaced from the center line. In FIG. 3B, both cups are tipped 25° inward which will not be the case for FIG. 5. The FIG. 5 embodiment is not intended to simultaneously excite the two filaments 26&#39; and 27&#39; but provides the alternate selection capability of the large focii or small focii on the same spot in a mammography X-ray tube. 
     The invention herein has been described in conjunction with the specific embodiments of the drawings. It is not our intention to limit our invention to any specific embodiment, and the scope of our invention should be determined by our claims.