Patent Application
20090279668
The invention relates generally to x-ray tubes and, more particularly, to an x-ray tube constructed to address kV-dependent artifacts that result from primary beam interaction with an electron collector of the x-ray tube.
X-ray systems typically include an x-ray tube, a detector, and an assembly to support the x-ray tube and the detector. In some applications, the assembly is rotatable. In operation, an object is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object such that the radiation typically passes through the object to impinge 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 detector then emits data received, and the system 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 positioned in a medical imaging scanner and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes typically include an anode having a high density track material, such as tungsten, that generates x-rays when high energy electrons impinge thereon. The anode structure typically includes a target cap and a heat storage unit, such as graphite, attached thereto. X-ray tubes also include a cathode that has a filament to which a high voltage is applied to provide a focused electron beam. The focused electron beam comprises electrons that emit from the filament, which is typically constructed of tungsten, and are accelerated across an anode-to-cathode vacuum gap to produce x-rays upon impact with the track material. As the electrons impinge upon the track material and rapidly decelerate, a spectrum of x-rays is generated. X-rays generated within the anode emit therefrom and pass to the detector through, typically, a low density or low atomic number material such as beryllium, which is typically referred to as a “window.”
X-ray generation results in a large amount of heat being generated within the anode. Much of the energy is dissipated via conduction into the target, where it is stored in the heat storage unit and radiated to the surrounding walls from the heat storage unit. Coolant surrounding the walls transfers the heat out of the tube. However, much of the energy, including up to 40% or more, may be back-scattered from the anode to impinge upon other components within the x-ray tube. Much of this back-scatter energy is deposited in and around the window, which can overheat the window and the joints that attach the window to the x-ray tube.
An electron collector, or back-scatter electron reduction apparatus, which is typically fabricated of copper and has coolant circulated therethrough, is designed to be thermally coupled to the window and to have an aperture aligned with the window to allow passage of electrons therethrough. Accordingly, the coolant removes the heat load from the window and the surrounding region, thus maintaining the window and its attachment joints at low temperatures during operation of the x-ray tube.
However, the electron collector typically includes a substantial amount of mass and volume in order to both sink the heat and house the coolant lines therein. Thus, the walls of the aperture typically have a substantial depth, such as a few centimeters or more. And, because the x-rays emit from the focal spot in all directions, some of the x-rays impinge upon the walls of the aperture. The material of the electron collector is typically a polycrystalline material such as copper having, therefore, a large grain structure in a number of crystal orientations. Thus, interaction of the x-ray beam with the walls of the aperture can result in lattice diffraction (i.e., Bragg diffraction), and if the incident beam strikes a crystal at the Bragg angle relative to a diffracting plane, a portion of the incident beam will be redirected from its original vector. The Bragg diffraction condition for 1st order diffraction is given as L=2*d*sin(T), where L is the x-ray wavelength, d is the spacing between crystalline planes, and T is the diffraction angle. The diffracted beam will therefore result in an area of locally increased intensity that, when impacting on the detector, may give rise to an area of increased intensity, resulting in an image artifact.
A rotating anode x-ray tube generates a polychromatic spectrum of x-radiation. If the accelerating potential is below the K-edge of the anode track material, a Bremsstrahlung spectrum is generated. However, if the accelerating potential exceeds the K-edge for the track material, then characteristic radiation is also generated. The characteristic x-ray peaks increase dramatically in intensity relative to the Bremsstrahlung radiation as the tube accelerating potential is increased above the K-edge energy. In contrast, the intensity of the Bremsstrahlung increases gradually with increasing potential. Therefore, if x-rays of characteristic wavelength cause diffraction from the aperture, an image artifact can be generated that worsens as the accelerating potential increases above the K-edge energy, and any image artifact created cannot be easily calibrated out of the system due to the strong dependence on tube accelerating potential.
Therefore, it would be desirable to design a system and apparatus to reduce diffraction of x-rays within an electron collector of an x-ray tube without compromising the thermal performance of the electron collector.
The invention provides a method and apparatus for reducing kV dependent image artifacts in an x-ray tube.
According to one aspect of the invention, an x-ray tube includes a cathode positioned within a vacuum chamber and configured to emit electrons. The x-ray tube also includes an anode positioned within the vacuum chamber to receive electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons, a window positioned to pass the beam of x-rays therethrough, and an electron collector structure attached to the x-ray tube having an aperture formed therethrough to allow passage of x-rays therethrough. The aperture is shaped to prevent diffracted x-rays from combining with the beam of x-rays passing through the window.
In accordance with another aspect of the invention, a method of manufacturing an x-ray tube includes the steps of positioning a cathode in a vacuum chamber and positioning an anode within the vacuum chamber to receive electrons emitted from the cathode and generate a beam of x-rays. The method further includes positioning a window proximate to the anode to receive the beam of x-rays emitted from the anode, and attaching an electron collector structure to the x-ray tube, the electron collector having an aperture therein that is positioned to allow passage of the beam of x-rays through the window.
Yet another aspect of the invention includes an x-ray system that includes an x-ray tube positioned to emit the x-rays toward an object. The x-ray tube includes an anode positioned to generate the x-rays from electrons that impinge thereon, and a window material positioned to receive the x-rays. The x-ray tube also includes an electron collector structure attached to the x-ray tube and having an opening therein, the opening positioned to allow passage of the x-rays therethrough, and an attenuating material attached to the electron collector structure.
Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
As shown in
A processor 20 receives the analog electrical 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, floppy discs, 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.
The bearing cartridge 58 includes a front bearing assembly 63 and a rear bearing assembly 65. The bearing cartridge 58 further includes a center shaft 66 attached to the rotor 62 at a first end 68 of center shaft 66, and a bearing hub 77 attached at a second end 70 of center shaft 66. The front bearing assembly 63 includes a front inner race 72, a front outer race 80, and a plurality of front balls 76 that rollingly engage the front races 72, 80. The rear bearing assembly 65 includes a rear inner race 74, a rear outer race 82, and a plurality of rear balls 78 that rollingly engage the rear races 74, 82. Bearing cartridge 58 includes a stem 83 which is supported by a back plate 75. A stator (not shown) is positioned radially external to rotor 62, which rotationally drives anode 56. The target shaft 59 is attached to the bearing hub 77 at joint 79. One skilled in the art will recognize that target shaft 59 may be attached to the bearing hub 77 with other attachment means, such as a bolted joint, a braze joint, a weld joint, and the like. In one embodiment a receptor 73 is positioned to surround the stem 83 and is attached to the x-ray tube 12 at the back plate 75. The receptor 73 extents into a gap formed between the target shaft 59 and the bearing hub 77.
Referring still to
The anode 56 has a re-entrant target design that serves to position the mass or center-of-gravity 67 of target 57 at a position between the front bearing assembly 63 and the rear bearing assembly 65 and substantially along centerline 64, about which center shaft 66 rotates. Additionally, both target shaft 59 and bearing hub 77 serve to increase a conduction path length between target 57 and bearing cartridge 58 such that a reduction in the peak operating temperature of front inner race 72, front balls 76, and front outer race 80 may be realized as compared to a direct connection of target 57 to second end 70 of center shaft 66. In one embodiment, as illustrated in phantom in
In operation, as electrons impact focal point 61 and produce x-rays 14, heat generated therein causes the target 57 to increase in temperature, thus causing the heat to transfer via radiation heat transfer to surrounding components such as, and primarily, casing 50. Heat generated in target 57 also transfers conductively through target shaft 59 and bearing hub 77 to bearing cartridge 58 as well, leading to an increase in temperature of bearing cartridge 58. The heat generated includes radiant thermal energy from the anode 56 and kinetic energy of back-scattered electrons that deflect off of the anode 56. The back-scattered electrons typically impinge upon an electron collector 95 positioned on and typically attached to the radiation emission passage 52. As such, back-scattered electrons that would otherwise impinge on the radiation emission passage 52, are intercepted by the electron collector 95. The electron collector 95 may include coolant lines 97 which carry coolant therethrough and reduce the operating temperature of the electron collector 95.
For Bragg diffraction, as is known in the art, the deviation of x-rays from an incident beam is 2× the Bragg angle (θ). In other words, incoming x-rays at the Bragg angle are diffracted from the lattice at the Bragg angle, hence the x-rays are re-directed by 2× the Bragg angle. Bragg diffraction is dependent on both 1) the material on which the diffraction occurs (i.e. its lattice structure), and 2) the type of radiation generated at the anode. As such, the configuration of aperture 104 may be selected based on at least the electron collector material 102 (i.e. copper) and the target track material 86 of target 57. Table 1 below summarizes results for Bragg diffraction in copper, where the most intense reflection is the (111) reflection, and for characteristic radiation of W, Mo, and Rh. Table 1 includes 2× the Bragg angle for a copper collector with respect to x-rays of the primary beam of x-rays.
As such, and referring to Table 1, because the track material 86 may include, as examples, W, Mo, and Rh, various types of characteristic radiation may be generated therein that, therefore, have differing Bragg angles against a copper collector. Furthermore, the primary beam of electrons, having a high energy, may penetrate below the surface of the collector and generate Bragg diffraction therein, which, if not attenuated in the collector, may emerge from the collector after being reflected by 2× the Bragg angle and cause image artifacts.
Referring now to
One skilled in the art will recognize that x-rays passing through the corner 112 may not be collected by electron collector 100 and may diffract at the Bragg angle within the collector material 102 to pass into the aperture 104, though the emission passage 52, and impinge on a detector such as, for instance, the detector 18 of
According to embodiments of the invention, the attenuating material 114 may include silver, gold, platinum, tungsten, and the like (and their alloys). Other materials that may be used for the attenuating material 114 may include, for example, hafnium, iridium, molybdenum, niobium, osmium, palladium, rhenium, rhodium, tantalum, etc. (and their alloys). The attenuating material 114 may be applied by plating and other deposition processes known within the art. Alternatively, one skilled in the art will recognize that the attenuating material 114 may be brazed, soldered, welded, or mechanically fastened to the aperture according to methods known within the art.
For the attenuating material thicknesses of Table 2 that are less than, for instance, 0.100 mm, one skilled in the art will recognize that the attenuating material 114 may be applied using a variety of deposition processes such as plasma vapor deposition (PVD) and chemical vapor deposition (CVD). Likewise, for attenuating material thicknesses that are greater than 0.100 mm, the attenuating material 114 may be an insert or attached piece that may be joined by brazing, soldering, welding, or mechanically fastening, as examples.
Instead of precluding x-rays from impinging upon the wall 108, as described with respect to the embodiment illustrated in
Referring now to
As described above, wall angle 133 is determined such that x-rays 14 that impinge the wall 134 at the Bragg angle (θ) are deflected into the aperture material 102. The deflected x-rays are, accordingly, absorbed or attenuated in the aperture material 102 after deflecting therefrom at the Bragg angle (θ). One skilled in the art will recognize that the wall angle 133 may be selected based on at least the characteristic radiation, the collector material, and the geometric relation of the target 57 with respect to the collector 102 such that substantially all x-rays diffracted in the electron collector 102 are diffracted into the collector 102, as illustrated in
Referring again to
Furthermore, one skilled in the art will recognize that the embodiments described herein are applicable to a wide range of design conditions related to an x-ray tube and its operation. As stated above, wall angle 133 is determined to be greater than the Bragg angle such that any x-rays 14 emitting from target 57 and impinging on the wall 134 at the Bragg angle are diffracted into the collector material 102. One skilled in the art will recognize that the x-rays 14 that impinge upon the wall 134 may have a widely varying and complex range of angles, and such angles are affected by a number of geometric and operating parameters of the x-ray source 12. Such parameters include, but are not limited to, the radial length of the target track material 86, the radial position of the target track material 86 with respect to the position of the electron collector 100, the characteristic radiation generated at the target track material 86, and the lattice orientation of the electron collector material 102 with respect to the central axis 135 of the aperture 104.
Furthermore, one skilled in the art will recognize that the position of the target 57 with respect to the electron collector 100 may change due to thermal growth of components within the x-ray tube 12 during operation, or due to physical deformation of the x-ray tube as it ages. For instance, one skilled in the art will recognize that the bearing cartridge 58 of
Therefore, according to one embodiment of the invention, an x-ray tube includes a cathode positioned within a vacuum chamber and configured to emit electrons. The x-ray tube also includes an anode positioned within the vacuum chamber to receive electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons, a window positioned to pass the beam of x-rays therethrough, and an electron collector structure attached to the x-ray tube having an aperture formed therethrough to allow passage of x-rays therethrough. The aperture is shaped to prevent diffracted x-rays from combining with the beam of x-rays passing through the window.
In accordance with another embodiment of the invention, a method of manufacturing an x-ray tube includes the steps of positioning a cathode in a vacuum chamber and positioning an anode within the vacuum chamber to receive electrons emitted from the cathode and generate a beam of x-rays. The method further includes positioning a window proximate to the anode to receive the beam of x-rays emitted from the anode, and attaching an electron collector structure to the x-ray tube, the electron collector having an aperture therein that is positioned to allow passage of the beam of x-rays through the window.
Yet another embodiment of the invention includes an x-ray system that includes an x-ray tube positioned to emit the x-rays toward an object. The x-ray tube includes an anode positioned to generate the x-rays from electrons that impinge thereon, and a window material positioned to receive the x-rays. The x-ray tube also includes an electron collector structure attached to the x-ray tube and having an opening therein, the opening positioned to allow passage of the x-rays therethrough, and an attenuating material attached to the electron collector structure.
The invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.
