Mammography method and mammography X-ray tube

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: This invention relates to methods and apparatus for x-ray mammography diagnostics. 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 60 KV/cm to 120KV/cm 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 12.7 to 25.4mm. 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 symmetrical cups which are displaced and rotated toward one another. 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 12.7mm 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'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 7.5mm, the fields are 44 kV/cm to 52kV/cm. 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 foci typically could deliver only 25-30 mA and for a typical 0.3 mm foci could deliver only approximately 100 mA. Since the high voltage employed is 25 kV, the target anodes are not fully loaded. A rotating anode of 7 cm to 10 cm 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. EP-A-322 260 discloses a mammography X-ray tube with an anode and cathode in a vacuum envelope and filaments in a conventional arrangement for providing electron beams. The present invention provides an improved arrangement including a specialised cathode assembly, as set out in claim 1. Alternatively it provides a method of using such an arrangement, as set out in claim 10. FR-A-2411487 and EP-A-0440532 disclose a cathode structure incorporating at least one pair of slot structures including a three slot structure. Each pair of the slot structures can contain more than a pair of filaments but the largest slots of the structures do not intersect in a manner that the adjacent interior sidewalls of the largest slots are shorter than the outer sidewalls of the largest slots. Examples of the prior art and of the invention will now be described with reference to the accompanying drawings in which: 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 12.7 mm. 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. 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 7.5mm. 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 > 12.7mm, 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. All the slots are parallelepiped shaped with right rectangular cross sections. In the preferred embodiments of FIG. 3A, 3B and 3C, matching filament 32 and 34 are mounted in 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 essentially to 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' is superimposed in a mammography X-ray tube on the same focii as a larger filament 27'. 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' and 27' but provides the alternate selection capability of the large focii or small focii on the same spot in a mammography X-ray tube.

Claims: A mammography X-ray tube comprising: a vacuum envelope (1) containing a rotating anode (3), a cathode assembly (2), a high voltage circuit having two insulated terminals for connecting a high voltage near 27.5 kV ± 15% from an external voltage generator via one (7) of said terminals to said rotating anode relative to said cathode assembly and a plurality of filament current connector terminals for providing external filament current sources to said cathode assembly; said cathode assembly including two cathode cups connected to the other of said terminals, said cups each containing a pair of filaments (31 and 32, or 33 and 34) connected in parallel to respective filament current terminals (35 and 39, or 36 and 40) for simultaneous excitation of the filaments of a pair, said filaments being 7.5 mm or less displaced from said rotating anode, the electric field between said filaments and said rotating anode being, in operation, on the order of 48 kV/cm; and said cathode cups further including means for shaping said electric field between said filaments and said rotating anode so that electron beams produced by said pairs of filaments, in operation, are focused to be superposed on respective fixed rectangular regions in the space overlying said rotating anode, said means comprising a three slot structure (41-46) for each cup in which the largest slots (41, 44) of said three slot structures of the two cups intersect, such that said largest slot interior sidewall is shorter than the outer sidewall of said largest slot. A tube as claimed in claim 1 wherein said cathode assembly comprises a plurality of said cathode cups, each said cup containing at least one of said filaments, said cup being a triple slotted cup, each said slot being a groove having a right rectangular cross section; and wherein each said slot of each triple slotted cup is coaxial and symmetrical about an imaginary plane, which plane is parallel to the longer walls of said groove; said plurality of cathode cups being two axially displaced cups such that the larger grooves of each said cup intersect, and wherein each of said cups is rotated toward the other about the line of intersection. A tube as claimed in claim 2 wherein the angle in which the said cups are rotated is on the order of 20 to 25 degrees. A tube as claimed in claim 2 or claim 3 wherein each of said plurality of filament cups contains two thermal filaments. A tube as claimed in claim 4 where said two filaments are connected at one end to a common electrical terminal (37, 38) and wherein said two thermal filaments are of unequal electron beam producing capacity for the same excitation current. A tube as claimed in claim 5 wherein at least one thermal filament in each cup matches the electron beam producing capacity at the same exciting current as a filament in said other cup and wherein each said matching filament is electrically connected in parallel to be simultaneously excited. A tube as claimed in claim 6 wherein each filament in each cup has a matching capacity electron beam capacity filament in said intersecting cup and wherein each said matching capacity filament is connected in parallel to its said matching filament for simultaneous excitation therewith. A tube as claimed in any one of claims 1 to 7 wherein each said cup is configured to cause, in operation, the simultaneously produced electron beams to be superpositioned on the same rectangular region in space in the plane of the face of said rotating anode target. A tube as claimed in any one of claims 1 to 8 wherein said cups are formed in a solid member, each said slot being parallelepiped shaped with a right rectangular cross section, the length L of each said slot being greater than the depth or width of said slot cross section, each said three parallelepiped shaped slots of said cup being parallel, and each said slot being contiguous to one of the other of said three parallelepiped shaped slots, each of said slots of a said cup being aligned in respect to the other slots of said cup so that there is a plane which is parallel to the longest side of each said slot which is coplanar with and also passes through the centre of the cross section of each of said slots of a said cup; the outer slot (41, 44) of a said cup having the largest cross sectional area, the intermediate slot (42, 45) having an intermediate cross sectional area and the most interior slot (43, 46) having the smallest cross sectional area; and said displaced cups being aligned so that the longest dimensions of said slots are parallel, and said slots having the largest cross sectional area intersect. A method of using an X-ray tube as claimed in any one of the preceding claims comprising simultaneously exciting said plurality of filament cathodes (31 and 32, or 33 and 34) in a said cup to each produce a beam of electrons; shaping the electric fields in said shortened space between a rotating anode target and said filaments cathodes simultaneously to superpose each said produced electron beam onto the same region on said rotating anode target thereby increasing the intensity of X-rays produced; decreasing the exposure time of said patient such that the integral of X-ray intensity times the exposure time is equal to the standard dose. A method as claimed in claim 10 wherein said step of simultaneously exciting a plurality of filaments includes the ability to switch between a first plurality of excited filaments (31, 32) in one cup producing a large spot to a second plurality of excited filaments (33, 34) in the other cup producing a smaller spot, wherein the exposure time of the patient in said smaller spot mode is able to be reduced by a factor five to a time on the order of 1 second while providing the standard X-ray dose.

Inventors: KOLLER THOMAS J, TRESEDER ROBERT C, KOLLER, THOMAS J., TRESEDER, ROBERT C.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1993
Main classification: H01J35
IPC codes: H01J35