X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
2001-10-05
2004-06-15
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Target
C378S121000, C378S122000, C378S125000
Reexamination Certificate
active
06751293
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention generally relates to support systems for use in conjunction with components designed for rotary motion. More particularly, the present invention relates to a rotary component support system for a rotating anode of an x-ray tube.
2. Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are accelerated to high speeds and then impinged upon a material of a particular composition. This process typically takes place within an evacuated housing of an x-ray tube located in the x-ray generating device. The x-ray tube includes an electron source, or cathode, and an anode oriented to receive electrons emitted by the cathode. The anode, which typically comprises a graphite substrate and a heavy metallic target surface, can be stationary within the tube, or can be in the form of a rotating disk supported by a bearing assembly and a support shaft.
In operation, an electric current is supplied to a filament portion of the cathode, causing it to emit a stream of electrons by thermionic emission. A high electric potential placed between the cathode and anode causes the electron stream to accelerate toward a target surface located on the anode. Upon striking the target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten, are typically employed. The x-rays ultimately exit the x-ray device through a window formed in the x-ray tube housing so as to interact in or on material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic procedures.
In general, only a small portion of the kinetic energy contained in the electron stream is converted into x-rays. A majority of the energy is dissipated as heat in the anode target region and the rest of the anode. This heat can reach extremely high temperatures that can damage the anode structure over time, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube. To help alleviate this problem, the x-ray target, or focal track, is typically positioned on an annular portion of a rotatable anode disk. Typically, the anode disk (also referred to as the rotary target or the rotary anode) is mounted on a supporting shaft which, in turn, is supported by bearings contained a bearing housing. The shaft and disk are then appropriately connected to and rotated by a motor.
When the anode is rotated, the focal track is rotated into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam.
While the basic operational principles of x-ray devices have remained substantially unchanged, new uses and applications for x-rays have increased the performance demands placed on x-ray tubes. One response to such demands has resulted in the development of anodes that have increasingly larger sizes and/or that are suited for relatively higher operational speeds. While such anodes generally facilitate a desirable increase in the overall performance of the x-ray tube, the increased size and/or speed of those anodes often can cause other undesirable problems with respect to other portions of the x-ray tube. An area of particular concern in this regard is the mounting system used to rotatably support the anode.
In general, the quality of the images produced by a particular device is at least partially a function of the stability of the anode. In particular, image quality can depend on the changes in the relative position of the focal spot, or the point at which the electrons strike the target. Any migration of the focal spot due to vibration or other instabilities can reduce the image quality of the x-ray tube. Thus, the anode mounting system must be constructed and implemented in such a way that balanced mounting of the anode can be achieved and maintained, and undesirable movement of the anode during x-ray tube operation thereby minimized or prevented. As suggested above however, such results are not achieved in all cases due to relative increases in the size and weights of anodes now in use, the operational speeds at which such anodes are employed, and/or the extreme thermal stresses imposed on the anode and its mounting system.
In particular, the weights and operational speeds of many anodes can cause various components of the anode mounting system to loosen over time. This effect may be further exacerbated by thermal effects resulting from the high temperature operating environment typical of x-ray devices. Further, anodes typically accelerate at a high rate of revolutions per minute to achieve an operational speed. Such high rates of acceleration introduce various mechanical stresses and strains that often compromise the integrity of the anode mounting system components, and may cause such components to loosen over a period of time.
A related concern with anode mounting systems deals with the extent to which they can be disassembled. In particular, it is often desirable to remove an anode from an x-ray tube after a given amount of operating time, and then recondition and re-install the anode. Removal may also be needed to access other x-ray tube components. However, removal of the anode from the x-ray tube, has often proven difficult. These difficulties are often associated with the configuration and layout of the anode mounting system.
For example, some x-ray tubes employ an anode mounting system that includes a bearing assembly and stationary shaft configured so that so that the anode, rotatably supported by the bearing assembly, rotates about the stationary shaft. In such arrangements, the stationary support shaft is typically mounted to the vacuum enclosure within an aperture or sleeve. In such a configuration, a braze joint usually serves to secure the stationary shaft to the vacuum enclosure and also to seal the vacuum enclosure. This configuration can be especially difficult to disassemble. This is due in large part to the manner by which the stationary shaft is mounted to the vacuum enclosure. To maintain rigid support for the anode, the fit between the sleeve and the support shaft must be extremely tight. Typically, this is achieved with an interference fit, where the sleeve, for example, is heated while the shaft is cooled. Due to the expansion of the sleeve and the contraction of the shaft, the shaft can be inserted into the sleeve. As the two components reach thermal equilibrium, the gap between the shaft and sleeve is eliminated, resulting in a tight fit between the two.
While this is effective in tightly fitting the shaft and sleeve together, it also creates a bond between the sleeve and the support shaft that is very difficult to break. Further, when the bond is broken, damage may occur to the shaft, sleeve, anode, and/or vacuum enclosure. Consequently, disassembly can result in added expense when repairing or replacing tube components and may even render the entire x-ray tube inoperable. While it may be possible in some instances to minimize the damage that may occur during separation of the shaft and sleeve, the processes necessary to achieve such results are typically expensive
Church Craig E.
Varian Medical Systems Inc.
Workman Nydegger
Yun Jurie
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