Patent Application
20090279669
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 vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube further includes a window positioned to pass the beam of x-rays therethrough, an electron collector structure having an aperture formed therein to allow passage of x-rays therethrough, and a layer attached to the electron collector structure and configured to at least partially absorb or reduce diffraction of x-rays that contact the layer.
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, positioning an anode within the vacuum chamber to receive electrons emitted from the cathode and generate a beam of x-rays, and positioning a window proximately to the anode to receive the beam emitted from the anode. The method further includes attaching a first structure to the x-ray tube having an aperture therein that is positioned to allow passage of the primary beam of x-rays to the window, and attaching a second structure to a wall of the aperture.
Yet another aspect of the invention 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 further includes an electron collector attached to the x-ray tube and having an opening therein, the opening positioned to permit the x-rays to pass therethrough, and a material positioned in the opening, the material configured to attenuate or directionally deflect the x-rays that impinge thereon.
According to a further aspect of the invention, an x-ray tube includes a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube further includes a window positioned to pass the beam of x-rays therethrough, and a structure having an aperture formed therein to allow passage of x-rays therethrough and configured to at least partially absorb or reduce diffraction of x-rays that contact the 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.
Referring now to
The thickness of the attenuating layer 106 typically ranges from 5-50 micrometers and is selected based on the angle of incidence of primary x-rays 14 and based on the characteristics of the attenuating material. Because the aperture 104 is typically located close to the track material 86, the angle of the incident beam relative to the wall 108 of the material 102 is typically 3-7°. As such, for an attenuating thickness, t, and an incident angle, A, the length of attenuating material through which x-rays 14 pass before reaching material 102 is given as t/sin(A). Thus, for 1st order diffraction, given as L=2*d*sin(T) as discussed above, a diffracted beam (after being diverted from its original vector by an angle 2T due to diffraction) exiting the base material 102 of the aperture 104 travels through the attenuating layer 106 an additional distance of t/sin(2T-A), further amplifying the attenuation effect of the attenuating layer 106.
As an example, for tungsten x-rays of Kα wavelength that impact the wall 108 of the aperture 104 coated with a 10 micron layer of gold and having a 3° incident beam angle, 97% attenuation of the beam intensity is realized (for the strongest diffraction condition of copper, along the (111) lattice. And, for a 5° incident beam angle, the attenuation is 99.9%.
Additionally, although
Therefore, according to one embodiment of the invention, an x-ray tube includes a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube further includes a window positioned to pass the beam of x-rays therethrough, an electron collector structure having an aperture formed therein to allow passage of x-rays therethrough, and a layer attached to the electron collector structure and configured to at least partially absorb or reduce diffraction of x-rays that contact the layer.
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, positioning an anode within the vacuum chamber to receive electrons emitted from the cathode and generate a beam of x-rays, and positioning a window proximately to the anode to receive the beam emitted from the anode. The method further includes attaching a first structure to the x-ray tube having an aperture therein that is positioned to allow passage of the primary beam of x-rays to the window, and attaching a second structure to a wall of the aperture.
Yet another embodiment of the invention 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 further includes an electron collector attached to the x-ray tube and having an opening therein, the opening positioned to permit the x-rays to pass therethrough, and a material positioned in the opening, the material configured to attenuate or directionally deflect the x-rays that impinge thereon.
According to a further embodiment of the invention, an x-ray tube includes a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube further includes a window positioned to pass the beam of x-rays therethrough, and a structure having an aperture formed therein to allow passage of x-rays therethrough and configured to at least partially absorb or reduce diffraction of x-rays that contact the 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.
