Patent Grant
10614989
The invention relates generally to emitters for x-ray imaging systems and more particularly to improvements to the structures of emitters of this type.
Presently available medical x-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an x-ray tube anode, or target, which is typically a annular metal or composite structure. The space between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth. In thermionic cathode systems, a filament is included that may be induced to release electrons through the thermionic effect, i.e. in response to being heated. However, the distance between the cathode and the anode must be kept short so as to allow for proper electron bombardment. Further, thermionic X-ray cathodes typically emit electrons throughout the entirety of the surface of the emitter. Accordingly, it is very difficult to focus all electrons into a small focal spot.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode.
Typically, the cathode includes one or more cylindrically wound filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
In these prior art x-ray tubes, the wire(s) forming the filaments are formed of drawn wire formed into coiled shape to function as the emitter. The formation of the wire in a suitable drawing process provides sufficient deformation processing to the material in order to result in a creep resistance imparted through subsequent annealing of the material. This processing, in addition to other manners of strengthening the emitter material, such as carbide-, oxide-, and/or void-strengthening the emitter material, allows the wire to have significant resistance to creep as a result of the high operating temperatures for the emitter.
Conventional cylindrically wound filaments, however, emit electrons in a complex pattern that is highly dependent on the circumferential position from which they emit toward the anode. Due to the complex electron emission pattern from a cylindrical filament or wire, focal spots resulting therefrom can have non-uniform profiles that are highly sensitive to the placement of the filament within the cup. As such, cylindrically wound filament-based cathodes are required to be manufactured having their filament positioned with very tight tolerances in order to meet the exacting focal spot requirements in an x-ray tube.
In order to generate a more uniform profile of electrons toward the anode to obtain a more uniform focal spot, cathodes having an approximately flat emitter surface have been developed, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament.
In addition, recent developments in diagnostic x-ray tubes made it desirable to provide high emission at reduced tube voltages. For example in vascular x-ray tubes it is desirable to reduce tube voltages to 60 kV from the typical lower limit of 80 kV while ideally maintaining the power delivered to the target. For large focal spots, emission currents between 1000 mA and 1500 mA at 60 kV are desirable. For small focal spots, especially in fluoroscopic mode, emission currents up to 400 mA are desirable.
These current emitters are formed from rolled sheets of the emissive or emitter material. These sheets are formed from the same metals and/or materials utilized for the wound emitters, but are rolled into flat sheets instead of being drawn or worked into wires. These flat sheets are then cut into emitters having the desired shapes and configurations for use in x-ray tubes for more precise direction of the electrons from the emitters onto the anode/target for x-ray generation.
In rolling the material into the sheets, the amount of deformation created in the sheets is less than that created in the formation of the wires. As a result, the sheets formed of the emitter material do not have the same high temperature property benefit as found in the wound emitters. As such, under the high operating temperatures for the flat emitters, these emitters become subject to creep at lower accumulated operational times, thereby decreasing the life span of the flat emitter.
One prior art attempt to overcome this issue with flat emitters is disclosed in Falce et al. U.S. Pat. No. 7,545,089 entitled Sintered Wire Cathode, the entirety of which is expressly incorporated herein by reference for all purposes. In this reference, wires formed an emitter metal, i.e., tungsten, are wound about a bobbin and sintered in order to form the wires into a porous cathode structure including a number of desired uniform pores formed within the cathode structure as a result of the wire diameter and sintering parameters utilized.
However, the presence of the voids in the resulting porous cathode structure significantly limits the effectiveness of the cathode as a thermionic electron emitter including voids or pores in the emitter structure is detrimental to the desired emission of electrons from the emitter/cathode as well as the detrimental structural integrity impacts.
Accordingly, it is desirable to provide an emitter for an x-ray tube cathode having a flat, non-porous structure that includes yield strength, tensile strength and creep-resistance properties similar to that of wire formed or wound emitters.
There is a need or desire for a flat emitter that has increased creep-resistance properties in order to improve the useful life of the flat emitter from that of flat, rolled emitters. The above-mentioned drawbacks and needs are addressed by the embodiments described herein in the following description.
According to one exemplary aspect of the invention, a flat emitter is formed by the formation of emitter material preforms formed of the emitter material, such as wires or any other the preforms that possess sufficient work history and performance attributes to achieve the creep resistance, e.g., doping and/or ion implantation into foils, into a unitary non-porous flat emitter structure. The preforms, e.g., wires, are formed with increased thermomechanical deformation properties, including but not limited to creep resistance, yield and tensile strength as a result of the manner of the formation of the emitter material or metal into the preform(s) that is transferred to the flat emitter formed from the preform material. To form the flat emitter, the preform(s) are encapsulated and subjected to sufficient a process that applies sufficient temperatures and pressure to increase the density of the preform into a solid component, rod, sheet plate, etc. without the presence of voids or pores in the resulting component. In forming the emitter in this manner, pressures applied in conjunction with the high temperatures, the beneficial creep resistance and other high temperature microstructure/morphology and thermomechanical deformation properties from the preform(s) are retained within the resulting component to provide the emitter with increased creep resistance and a consequently longer useful life in the x-ray tube.
According to another exemplary embodiment of the invention, an emitter with enhanced creep-resistant properties for an x-ray tube includes an assembly of wires having a defined creep resistance, each wire including at least one component formed of an electron emitter material, wherein the emitter does not include a work function lowering material.
According to another aspect of the invention, a method for forming an emitter with enhanced creep-resistant properties for an x-ray tube includes the steps of: providing a preform having a defined creep resistance, the preform including at least one component formed of an electron emitter material and subjecting the assembly of wires to a consolidation process to form an emitter,
According to a further aspect of the invention, a method for forming an emitter for an x-ray tube having enhanced creep-resistant properties includes the steps of providing a preform having a desired creep resistance, the preform including at least one component formed of an electron emitter material, subjecting the preform to a consolidation process to form a rod, slicing the rod to form a number of sheets; and cutting each of the number of sheets to form the emitter.
It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
The drawings illustrate the best mode presently contemplated of carrying out the disclosure. In the drawings
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments, which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken in a limiting sense.
As shown in
A processor 20 receives the signals from the detector 18 and generates an image corresponding to the object 16 being scanned. A computer 22 communicates with processor 20 to enable an operator, using operator console 24, to control the scanning parameters and to view the generated image. That is, operator console 24 includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system 10 and view the reconstructed image or other data from computer 22 on a display unit 26. Additionally, console 24 allows an operator to store the generated image in a storage device 28 which may include hard drives, flash memory, compact discs, etc. The operator may also use console 24 to provide commands and instructions to computer 22 for controlling a source controller 30 that provides power and timing signals to x-ray source 12.
Feedthrus 77 pass through an insulator 79 and are electrically connected to electrical leads 71 and 75. X-ray tube 12 includes a window 58 typically made of a low atomic number metal, such as beryllium, to allow passage of x-rays therethrough with minimum attenuation. Cathode assembly 60 includes a support arm 81 that supports cathode cup 73, flat emitter 55, as well as other components thereof. Support arm 81 also provides a passage for leads 71 and 75.
In operation, target 56 is spun via a stator (not shown) external to rotor 62. An electric current is applied to flat emitter 55 via feedthrus 77 to heat emitter 55 and emit electrons 67 therefrom. A high-voltage electric potential is applied between anode 56 and cathode 60, and the difference therebetween accelerates the emitted electrons 67 from cathode 60 to anode 56. Electrons 67 impinge target 57 at target track 86 and x-rays 69 emit therefrom at a focal spot 89 and pass through window 58.
To form the emitter 55, looking at
The assembly 102 is the positioned within a suitable containment vessel (not shown) and subjected to selected temperatures and pressures in order to form a component, such as a rod 108 (
In one exemplary embodiment of the invention using the hot isostatic pressing treatment or process, within the containment vessel the assembly 102 is subjected to temperatures between 600° C.-3000° C., and in other embodiments between 1000° C.-2500° C., and pressures sufficient to achieve consolidation of the wires 100 in the assembly 102, such as greater than approx. 5 ksi for HIP or flow stress above 50 MPa, while simultaneously having the pressure maintained in isostatic manner within the containment vessel against the entire exterior surface of the assembly 102. The pressures exerted against the assembly 102 can be generated by introducing a gas, such as an inert gas, into the containment chamber until the desired pressure within the chamber is reached. In this manner the desired pressure is exerted on all surfaces of the assembly 102 equally to achieve the desired effect in conjunction with the application of the desired temperature to the assembly 102. After completion of the process, the encapsulating material 106, which forms a skin around the assembly 102 and the resulting rod 108, can be removed for further processing of the rod 108.
In this manner, the hot isostatic pressing process alters the wires 100 within the assembly 102 by increasing the density of the rod 108 formed from the wires 100, thereby compressing the wires 100 into a solid component, e.g., the rod 108, while additionally eliminating the voids 110 (
After formation of the rod 108 in the hot isostatic pressing treatment or other suitable process, the rod 108 can be sliced into sheets 112 that are ultimately utilized to form the emitters 55. As shown in
In alternative embodiments of the processes used to form the sheets 112 from the preform A, such as the hot isostatic pressing process, eliminating gas between the elements of the preform A, e.g., the wires 100 in the assembly 102, enables consolidation and elimination of voids between the elements/wires 100. Further, the materials forming the wires 100 can be strengthened during their initial formation in order to enable the enhancements to the strength and/or thermomechanical properties of the wires 100 to be carried through to the sheet 112 formed from the wires 100 in the formation process. In some exemplary embodiments, these enhancements include, but are not limited to, oxide doping such as potassium-doped, alkali-doped, or dispersion of the refractory metal(s) forming the wires 100, such as lanthanum oxide dispersion, and/or carbide doping or dispersion of the refractory metal(s) forming the wires 100, such as hafnium carbide or zirconium carbide dispersion. In any embodiment of the assembly 102, the individual composite microstructure of the wires 100 forming the assembly 102 is retained within as the microstructure for the sheet 112 formed from the assembly 102. This provides significant benefits to emitters 55 that are formed from the sheet 112, as the tensile strength and creep resistance of potassium doped tungsten wires is much higher at elevated temperatures than that of a flat sheet of potassium doped tungsten at the same temperatures. In one exemplary embodiment of the sheet 112, as doped tungsten wire is known to have dramatically better creep properties than doped tungsten sheet, due to better distribution and reduced size of the potassium bubbles, a wire 100 formed of that material can be drawn down to very small sizes, giving an even better distribution and size reduction of bubbles within the wire 100. This bubble distribution would be retained in the rod 108 and/or sheet 112 formed of the wires 100 in the formation process, such that a sheet 112 formed from the wires 100 in the process of the invention would have similar density to and better creep properties than a prior art rolled sheet.
Further, in another exemplary embodiment, after the formation of the rod 108 and/or the sheet 112 from the wires 100 in the selected process, the rod 108 and/or the sheet 112 can be subjected to additional mechanical working, such as extrusion, rolling and/or swaging, among other suitable processes. This added work to the rod 108 and/or the sheet 112 further increases the density of the sheet 112, and can further enhance the deformation of the microstructure of the material forming the rod 108 and/or the sheet 112, thereby further increasing the creep resistance of the material forming the rod 108 and/or the sheet 112.
The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
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| Number | Date | Country | |
|---|---|---|---|
| 20190066963 A1 | Feb 2019 | US |
