X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2001-05-14
2004-02-17
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S119000, C378S133000, C378S125000, C378S127000, C378S130000
Reexamination Certificate
active
06693990
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to x-ray tubes that employ a target anode rotatably supported by a bearing assembly. More particularly, embodiments of the present invention relate to systems and structures concerned with improving the rate that heat is transferred away from the x-ray tube bearing assembly and thereby minimize destructive thermal conditions that occur during operation of the x-ray tube.
2. The Relevant Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such devices are commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various applications, the different x-ray devices share the same basic underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron source, or cathode, and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, x-ray medical diagnostic examination, material analyses, or other procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be generated. Some of this heat is often conducted to other areas of the x-ray tube and, as discussed further below, can result in thermal stresses that damage the tube.
In addition to the heat generated as a result of the primary electron stream, other sources of destructive heat are present within the operating x-ray tube. For example, a percentage of the electrons that strike the target surface of the anode do not generate x-rays, and instead simply rebound from the surface and then impact other surfaces and structures within the x-ray tube evacuated enclosure. These are often referred to as “secondary” electrons. These secondary electrons retain a large percentage of their kinetic energy after rebounding, and when they impact non-target surfaces, a significant amount of heat is generated that is conducted to various other elements, such as the bearing assembly, of the x-ray device. Thus, non-target structures, as well as the anode, are routinely exposed to extremely high operating temperatures.
The heat produced by secondary electrons combined with the high temperatures generated at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure and components. In fact, the resulting thermal stresses often shorten the operational life of the x-ray device, affect its efficiency and performance, and/or render it inoperable. These high temperatures can be especially problematic in rotating anode type x-ray tubes.
In a typical rotating anode type x-ray tube, the anode is mounted to a shaft of a bearing assembly confined within a bearing housing. Generally, the bearing assembly includes front and rear bearings having respective sets of balls confined within front and rear races disposed circumferentially with respect to the shaft. Because the balls are free to travel along the races, the shaft of the bearing assembly can freely rotate but is desirably constrained from any substantial axial movement. A stator serves to impart rotational movement to the shaft and the connected anode. As the anode rotates, each point on the focal track is rotated into and out of the path of the electron beam generated by the cathode. In this way, the electron beam is in contact with a focal spot on the focal track for only short periods of time, thereby allowing the remaining portion of the focal track to cool during the time that it takes such given portion to rotate back into the path of the electron beam.
The rotating anode x-ray tube of this sort is used in a variety of applications, some of which require that the anode be rotated at relatively high speeds so as to maintain an acceptable heat distribution along the focal track. For instance, x-ray tubes used in mammography equipment have typically been operated with anode rotation speeds around 3500 revolutions per minute (rpm). However, the demands of the industry have continued to change and high-speed machines for mammography and other applications are now being produced that operate at anode rotation speeds of around 10,000 rpm and higher. Moreover, the rotation must be exact; any wobble or non-uniform rotation of the anode greatly reduces the operating efficiency of the x-ray tube, or may render it imoperable. These high rotational speeds, coupled with the need for rotational precision, make the rotating anode structure—especially the bearing assembly—especially susceptible to the high operating temperatures.
For example, high operating temperatures can result in undesirable temperature differentials in the bearing assembly. Because the front bearing is located relatively closer to the anode than the rear bearing, the front bearing is exposed to relatively higher temperatures than is the rear bearing. Since the heat transmitted to the bearing assembly from the anode is not evenly distributed and dissipated, such an arrangement results in a temperature differential between the front and rear bearings. The relatively higher temperature experienced at the front bearing effectively reduces the maximum bulk operating temperature of the anode to a point somewhat lower than what the anode could be safely exposed if at least some of the heat experienced at the front bearing was more evenly distributed or otherwise dissipated. This effectively limits the operating power of the x-ray tube.
One solution to this problem is to use a relatively larger anode having a higher heat absorption capability. However, larger anodes are undesirable due to higher costs and because they are heavier and more difficult to balance and rotate at higher speeds.
In addition to acting as a limitation on the maximum operational temperature of the anode, the temperature differential between the front and rear bearings also has negative implications with respect to the operation of the bearings, and thus, the x-ray device as a whole. In particular, because thermal expansion is at least partially a function of temperature, the relatively greater temperature at the front bearing results in a relatively greater expansion of the front bearing, considered with respect to the expansion of the rear bearing. A thermal expansion differential between the front and rear bearings, can cause unbalanced, or otherwise improper, rotation and operation of the shaft which is supported by the bearings. Unbalanced shaft rotation, or similar defects, may cause, among other things, undesirable drifting or movement of the focal spot and degradation of resulting x-ray image quality.
Not only are temperature differentials in the bearings associated with various undesirable and destructive effects, but excessively high temperatures, in general, have a variety of undesirable consequences with respec
Church Craig E.
Varian Medical Systems Technologies, Inc.
Workman Nydegger
Yun Jurie
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