Patent Application
20090080615
The present invention relates generally to x-ray tubes and, more particularly, to a high emissive coating on a target shaft of an x-ray tube.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support 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 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 in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed.
Newer generation x-ray tubes have increasing demands for providing higher peak power, thus higher average power as well. Higher peak power, though, results in higher peak temperatures occurring in the target assembly, particularly at the target “track,” or the point of impact on the target. Thus, for increased peak power applied, there are life and reliability issues with respect to the target. Such effects may be countered to an extent by, for instance, spinning the target faster. However, doing so has implications to reliability and performance of other components within the x-ray tube such as the target and the bearing assembly. And, although spinning the target faster may reduce the peak focal track temperature, there is limited enhancement or improvement in the overall average power capability of the x-ray tube. As such, spinning faster has minimal impact on decreasing the operating temperature of the bearing assembly for a given average power.
Therefore, it would be desirable to have a method and apparatus to improve thermal performance and reliability of an x-ray tube target and bearing.
The present invention provides a high emissive coating on a target shaft of an x-ray tube anode.
According to one aspect of the present invention, a target for generating x-rays includes a target substrate, a target shaft attached to the target substrate, and a radiation emissive coating applied to at least one of the target substrate and the target shaft, wherein a center-of-gravity of the target is positioned between a front bearing assembly and a rear bearing assembly of an x-ray tube.
In accordance with another aspect of the invention, a method of fabricating an x-ray target assembly includes forming a substrate, and attaching a target shaft to the substrate. The method further includes forming a radiation emissive coating on at least one of the substrate and the target shaft and positioning a center-of-gravity of the substrate between a front bearing assembly and a rear bearing assembly of an x-ray tube.
Yet another aspect of the present invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray emission source includes a cathode, a front bearing assembly, a rear bearing assembly, and an anode having a center-of-gravity between the front bearing assembly and the rear bearing assembly. The anode includes a target base material, a shaft attached to the target base material, and a radiation emissive coating attached to at least one of the target base material and the shaft.
Various other features and advantages of the present 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.
Moreover, the present invention will be described with respect to use in an x-ray tube. However, one skilled in the art will further appreciate that the present invention is equally applicable for other systems that require operation of a target used for the production of x-rays wherein high peak temperatures are driven by peak power requirements.
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 the x-ray tube 12. A stator (not shown) is positioned radially external to and drives the 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 would 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, 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. To reduce conductive heat transfer into bearing cartridge 58 and increase the amount of radiation heat transfer to the surrounding components, an emissive coating 92 may be applied to an outer surface 93 of target shaft 59. Without an emissive coating, target shaft 59 may have an emissivity of 0.18. One skilled in the art would recognize that the emissive coating 92 may be applied by, for instance, plasma spray, chemical vapor deposition, electro-plating or physical vapor deposition, and the like, according to an embodiment of the present invention. In one embodiment the emissivity may be increased to, for instance, greater than 0.18. In one embodiment a coating having an emissivity of 0.75 is applied. An emissive coating 97, furthermore, may be applied to a substrate, or surface 99 of the target 57. One skilled in the art will recognize that an emissive coating may be applied to an outer circumference of the target 57, or to a surface parallel to and opposite that of surface 99. With the increase in radiative emissivity on outer surface 93 or surface 99, an increase in heat transferred out of target shaft 59 via radiation will thus reduce heat transferred out of target shaft 59 via conduction, and, as a consequence, the operating temperature of the target shaft 59, the bearing hub 77, and the bearing cartridge 58 may be reduced.
In like fashion an emissive coating 94 may likewise be applied to an inner surface 95 of target shaft 59. Accordingly, the operating temperature of the target shaft 59 may likewise be reduced in temperature as compared to a surface 95 without an emissive coating. And, although the inner surface 95 of the target shaft 59 views, in part, the bearing hub 77, the target shaft 59 also views stem 83. Thus, an increase in the emissivity of surface 95 will likewise increase the amount of radiation heat transfer to the stem 83, thus avoiding some of the heat that would otherwise conduct down the bearing hub 77 to the front bearing assembly 63. One skilled in the art will recognize that as for a longer target shaft 59 length, more of the surface 95 would likewise be exposed to the outer surface of stem 83, thus increasing the rate of radiation heat transfer to the stem 83, thereby avoiding some heat transfer via conduction directly into the front bearing assembly 63. Furthermore, one skilled in the art would recognize that, in an embodiment that includes a receptor 73, then heat transfer from the target shaft 59 to the receptor 73 will likewise be increased for increased emissivity of target shaft 59 at surface 95.
According to one embodiment of the present invention, a target for generating x-rays includes a target substrate, a target shaft attached to the target substrate, and a radiation emissive coating applied to at least one of the target substrate and the target shaft, wherein a center-of-gravity of the target is positioned between a front bearing assembly and a rear bearing assembly of an x-ray tube.
In accordance with another embodiment of the invention, a method of fabricating an x-ray target assembly includes forming a substrate, and attaching a target shaft to the substrate. The method further includes forming a radiation emissive coating on at least one of the substrate and the target shaft and positioning a center-of-gravity of the substrate between a front bearing assembly and a rear bearing assembly of an x-ray tube.
Yet another embodiment of the present invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray emission source includes a cathode, a front bearing assembly, a rear bearing assembly, and an anode having a center-of-gravity between the front bearing assembly and the rear bearing assembly. The anode includes a target base material, a shaft attached to the target base material, and a radiation emissive coating attached to at least one of the target base material and the shaft.
The present 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.
