Patent Application
20080310593
The subject matter described herein generally relates to a radiation generator and more particularly to a method of manufacturing and using an X ray tube assembly in a radiation generator.
Various types of radiation generators have been developed so as to generate electromagnetic radiation. The electromagnetic radiation thus generated can be utilized for various purposes including medical imaging. One such example of a radiation generator is an X ray generator. A typical X ray generator generally comprises an X ray tube assembly for generating electromagnetic radiation (For example, X rays) and a power supply circuit configured to energize the X ray tube assembly in a conventional manner so as to emit X rays through a port and toward a target. The X ray tube assembly generally comprises an evacuated envelope, an anode disposed at a first end of the evacuated envelope and a cathode assembly disposed at a second end of the evacuated envelope. The cathode assembly is configured for emitting an electron beam, which strikes the anode at a focal spot to generate X rays.
The position of the focal spot can be dynamically controlled through electrostatic or electromagnetic means. When using an electrostatic deflection means, it may be desired to have multiple electrically isolated deflection electrodes within close proximity to each other. This allows a wide range of focal spot sizes in length and width, as well as many deflection options. Conventional methods of constructing deflection electrodes typically use metal-ceramic-metal sandwich design. One limitation associated with the conventional methods is difficulties arising due to metal-ceramic brazing, alignment issues, surface contamination, and issues with high voltage standoff.
Hence, there exists a need to provide a system and method for effective control of the focal spot in a radiation generator.
The above-mentioned drawbacks and limitations described above are addressed by the present invention.
In one embodiment, a method of manufacturing an X ray tube assembly is provided. The method comprises steps of providing an evacuated envelope, disposing an anode at a first end of the evacuated envelope, disposing a cathode assembly at a second end of the evacuated envelope. The method of disposing the cathode assembly comprises providing a cathode filament configured for emitting an electron beam to impinge on the anode at a focal spot to generate X rays, providing a cathode cup comprising at least two portions, a first portion comprising an electrically conductive material and a second portion comprising an electrically insulating material, drilling at least one slot in the cathode cup and subjecting the cathode cup to a process of electrical discharge machining to form a plurality of electrically isolated deflection electrodes.
In another embodiment, an X ray tube assembly is provided. The X ray tube assembly comprises an evacuated envelope, an anode disposed at a first end of the evacuated envelope and a cathode assembly disposed at a second end of the evacuated envelope. The cathode assembly comprises a cathode filament and a cathode cup defining a plurality of electrically isolated deflection electrodes. Further, the cathode cup comprises at least two portions. A first portion comprises an electrically conductive material and a second portion comprises an electrically insulating material.
In yet another embodiment, a method of operating an X ray tube assembly is provided. The method comprises steps of selectively heating at least a portion of a cathode filament to emit an electron beam, maintaining a voltage potential between an anode and a cathode assembly to cause the electron beam to strike the anode at a focal spot to generate X rays and applying voltage potential individually and selectively to a plurality of electrically isolated deflection electrodes in a cathode cup for controlling the width and location of the focal spot on the anode. Further, the cathode cup comprises at least two portions, a first portion comprising an electrically conductive material and a second portion comprising an electrically insulating material.
In yet another embodiment, a cathode cup for a radiation generator is provided. The cathode cup defining a plurality of electrically isolated deflection electrodes comprises at least two portions, a first portion comprising an electrically conductive material and a second portion comprising an electrically insulating material.
Systems and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and with reference to the detailed description that follows.
In the following detailed description, reference is made to the accompanying drawings that form a part thereof, 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.
An imaging apparatus such as a computed tomography system and an X ray imaging device, configured for imaging objects comprises a radiation generator, a radiation detector and a data acquisition system. The radiation generator generates electromagnetic radiation for projection towards the object to be scanned. The electromagnetic radiation includes X rays, gamma rays and other HF electromagnetic energy. The X rays incident on the object being scanned are attenuated by the object. The radiation detector comprises multiple detector elements for converting the attenuated X rays into electrical signals. The electrical signals so formed are named as projection data. The data acquisition system (DAS) samples the projection data from the detector elements and converts the projection data into digital signals for computer processing.
The cathode assembly 125 is located opposite the anode 120 in general alignment along a longitudinal axis of the X ray tube assembly 105. The cathode assembly 125 includes an electron-emitting cathode filament 130 that is capable in a conventional manner of emitting electron beams. The electron beams emitted by the cathode filament 130 are incident on a focal spot on the surface of an anode target 140 in which they generate X rays that can emanate from the X ray tube assembly 105 via a window 145.
The power supply circuit 200 comprises two principal power sections; a kV drive circuit 210 and a mA drive circuit 215. The kV drive circuit 210 provides power to the high voltage source 205 to enable the high voltage source 205 to develop the high voltage potentials necessary to generate X rays. The mA drive circuit 215 provides power to the cathode filament 130 for heating the cathode filament 130 so as to emit electrons. The mA drive circuit 215 allows control of the number of electrons boiled off by the cathode filament 130, and thus provides control of the current flow in the X ray tube assembly 105. The power supply circuit 200 also houses a plurality of low voltage power supplies, which are used to furnish biasing voltages to an internal circuitry within the power supply circuit 200.
The input to the power supply circuit 200 is generally a direct current (DC) voltage. However, when the input voltage is an AC voltage, the AC voltage is rectified and then applied to the power supply circuit 200. Accordingly, the radiation generator may further comprise a line rectifier (not shown) configured to provide DC voltage to the power supply circuit 200. An input line power is supplied to the line rectifier (not shown), which converts AC voltage to an unregulated DC voltage. The unregulated DC voltage from the line rectifier (not shown) is applied to the kV drive circuit 210 and the mA drive circuit 215.
The high voltage source 205 is designed to receive an AC waveform from the kV drive circuit 210 and condition the AC waveform to provide a high voltage DC potential to the double-pole supply of the X ray tube assembly 105, where the anode 120 and the cathode assembly 125 carry equal voltages of different polarity. The high voltage source 205 comprises a voltage multiplier assembly 220 and a transformer assembly 225 coupled to the voltage multiplier assembly 220. The voltage multiplier assembly 220 configured to provide the high voltage DC power supply comprises a cathode multiplier and an anode multiplier. The transformer assembly 225 coupled to the voltage multiplier assembly 220 comprises a high voltage transformer 230 and a filament transformer 235 (shown at
The AC voltage from the kV drive circuit 210 is applied to the primary winding of the high voltage transformer 230 within the high voltage source 205. The high voltage transformer 230 increases the amplitude of the AC square wave voltage at the secondary winding. The high voltage AC signal is applied in turn to the voltage multiplier assembly 220. The voltage multiplier assembly 220 comprises a plurality of serially connected voltage multiplying-rectifying stages having a low voltage potential end and a high voltage potential end. The low voltage potential end is connected to the secondary winding of the high voltage transformer 230 and the high voltage potential end is connected to the tube electrodes 120 and 125 of the X ray tube assembly 105. The voltage multiplier assembly 220 converts the AC signal to two equal DC voltages of different polarities and increases the voltage level. The DC voltage is then applied to the tube electrodes 120 and 125 of the X ray tube assembly 105.
In parallel with the multiple-stage voltage multiplier assembly 220 is the filament transformer 235 producing AC filament heating output currents for the cathode filament 130. The AC voltage generated by the mA drive circuit 215 is applied to the input of the filament transformer 235. The filament transformer 235 provides voltages appropriate for driving the cathode filament 130 in the X ray tube assembly 105.
In one embodiment, the cathode assembly 125 may comprise one or more cathode filaments. Generally, the cathode filaments are used individually to provide a choice of multiple operating focal spots. Referring now to
Prior to being focused on the focal spot, the electrons beams emitted by the cathode filaments 302 and 304 are formed into a narrow, uniform stream by a cathode cup 135 (shown at
The electrically insulating material comprises an electrically insulating ceramic selected from a group consisting of aluminum oxide, aluminum nitride, silicon nitride zirconium oxide, mullite, and magnesium oxide.
Referring back to
The filament select circuit provides selective and individual heating of one of the two filaments 302 and 304, depending upon the desired focal spot length for a particular application. The desired filament 302 or 304 is selected by the order in which the deflection electrodes 306, 308 and 310 are turned on or powered. More particularly, powering the deflection electrode 306 enables the filament 302, while turning on the deflection electrode 310 enables the filament 304.
The voltages are applied to the two deflection electrodes 306 and 310, and varied in the form of a square wave having a 180.degree phase shift between the two deflection electrodes 306 and 310. It is to be appreciated that the electrode voltages may be varied according to other waveforms as well. The oscillating voltages on the deflection electrodes 306 and 310 cause the emitted electron beam to oscillate between two impingement positions on the anode 120, hence the origin of the X ray beam shifts between two focal spots. Thus selective application of the electrical potential at each deflection electrode 306 and 310 can alter the focal point of the X ray beam generated therefrom.
As described above, the second portion 410 of the cathode cup 135 may comprise a plurality of insulating layers. The insulating layers are used to insulate the deflection electrodes 306, 308 and 310 from an electrical potential developed on the second portion 410 of the cathode cup 135. As the deflection electrodes 306, 308 and 310 are electrically isolated from the second portion 410, as well as from each other, each electron beam being emitted from the cathode cup 135 can be steered individually, according to an electrical field generated when the electrical potential is applied to the deflection electrodes 306, 308 and 310.
In one embodiment a shown in
Further, a flow diagram of the method of providing the cathode cup 135 (step 525) is shown at
Referring to
The method of manufacturing the X ray tube assembly, as described in various embodiments, comprises a method of making a plurality of electrically isolated deflection electrodes in a limited space for the electrostatic control of the focal spot.
The process of making electrically isolated deflection electrodes, as described in various embodiments, does not include brazing, thereby avoiding braze overflow, voids and ceramic cracking, etc. Thus, the process may allow for the use of materials, such as various types of ceramics, that exhibit inability to withstand stresses incurred during brazing.
The method makes use of material property gradients built into a single cathode cup in order to obtain desired properties. This allows the electro statically deflecting cathodes to be electrically conductive and insulating when desired.
In various embodiments of the invention, a cathode assembly for a radiation generator and a radiation generator using a cathode assembly are described. However, the embodiments are not limited and may be implemented in connection with different applications. The application of the invention can be extended to other areas, for example medical imaging systems, industrial inspection systems, security scanners, particle accelerators, etc. The invention provides a broad concept of designing a cathode assembly, which can be adapted in similar radiation generators. The design can be carried further and implemented in various forms and specifications.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. 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 languages of the claims.
