X-ray or gamma ray systems or devices – Source support – Shielding
Reexamination Certificate
2000-01-26
2003-09-16
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source support
Shielding
C378S140000
Reexamination Certificate
active
06619842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to x-ray generating devices and their method of manufacture. More particularly, the present invention relates to an x-ray tube having an evacuated housing assembly that provides enhanced thermal stability and improved x-ray shielding characteristics. The invention also relates to methods of manufacturing the improved housing assembly.
2. The Prior State of the Art
X-ray generating devices are extremely valuable tools for use in a variety of medical and industrial applications. For example, such equipment is commonly used in areas such as medical diagnostic and therapeutic radiology.
Regardless of the particular application involved, the basic operation of x-ray devices is similar. In general, an x-ray generating device is formed with a vacuum housing that encloses an anode assembly and a cathode assembly. The cathode assembly includes an electron emitting filament that is capable of emitting electrons. The anode assembly provides an anode target that is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode. In operation, electrons emitted by the cathode filament are accelerated towards a focal spot on the anode target by placing a high voltage potential between the cathode and the anode target. These accelerating electrons impinge on the focal spot area of the anode target. The anode target is constructed of a high refractory metal so that when the electrons strike, at least a portion of the resultant kinetic energy generates x-radiation, or x-rays. The x-rays then pass through a window that is formed within a wall of the vacuum enclosure, and are collimated towards a target area, such as a patient. As is well known, the x-rays that pass through the target area can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, only a very small portion—approximately one percent in some cases—of an x-ray tube's input energy results in the production of x-rays. In fact, the majority of the input energy resulting from the high speed electron collisions at the target surface is converted into heat of extremely high temperatures. In addition, a percentage of the electrons that strike the anode will rebound from the target surface and strike other areas within the x-ray tube assembly. The collisions of these secondary electrons (sometimes referred to as “back scattered electrons) also create heat and/or result in the production of errant x-rays. This excess heat is absorbed by the anode assembly and is conducted to other portions of the anode assembly, and to the other components that are disposed within the vacuum housing. Over time, this heat can damage the anode, the anode assembly, and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of the high operating temperatures in the x-ray tube. For example, in some x-ray devices the x-ray target, or focal track, is positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is then mounted on a supporting shaft and rotor assembly, that can then be rotated by some type of motor. During operation of the x-ray tube, the anode disk is rotated at high speeds, which causes the focal track to continuously rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with any given point along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still present within the anode, the anode drive assembly, and other components within the evacuated housing. This heat must be continuously removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube's efficiency and overall service life.
One approach has been to place the housing that forms the evacuated envelope within a second outer metal housing, which is sometimes referred to as a “can.” This outer housing must serve several functions. First, it must act as a radiation shield to prevent radiation leakage, such as that which results from back-scattered electrons previously discussed. To do so, the can must include a radiation shield, which must be constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which can be continuously circulated by a pump over the outer surface of the inner evacuated housing. As heat is emitted from the x-ray tube components (anode, anode drive assembly, etc.), it is radiated to the outer surface of the evacuated housing, and then at least partially absorbed by the coolant fluid. The heated fluid is then passed to some form of heat exchange device, such as a radiative surface, and then cooled. The fluid is then re-circulated by the pump back through the outer housing and the process repeated.
The dielectric oil (or similar fluid) may also provide additional functions. For example, the oil serves as an electrical insulator between the high voltage potential that exists at the anode and cathode assemblies and the inner evacuated housing, and the outer housing, which is typically comprised of a conductive metal material that is at a different potential, typically ground.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquid coolants/dielectrics can be problematic in several respects. For example, use of a fluid adds complexity to the construction and operation of the x-ray generating device. Use of fluid requires that there be a second outer housing or can structure to retain the fluid. This outer housing must be constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing to retain the coolant fluid. This increases the cost and manufacturing complexity of the overall device. Also, the outer housing requires a large amount of physical space, resulting in the need for an overall larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner evacuated housing, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates.
Moreover, construction of the outer housing adds expense and manufacturing complexity to the overall device in other respects. If the liquid is used as a coolant, the device may also be equipped with a pump and a radiator or the like, that in turn must be interconnected within a closed circulation system via a system of tubes and fluid conduits. Also, since the fluid expands when it is heated, the closed system must provide a facility to expand, such as a diaphragm or similar structure. Again, these additional components add complexity and expense to the x-ray device's construction. Moreover, the tube is more subject to fluid leakage and related catastrophic failures attributable to the fluid system.
The presence of a liquid coolant/dielectric is also detrimental because it does not function as an efficient noise insulator. In fact, the presence of a liquid may tend to increase the mechanical vibration and resultant noise that is emitted by the operating x-ray tube. This noise can be distressing to the patient and/or the operator. The presence of liquid also lim
Artig Christopher F.
Salmon Deborah L.
Dunn Drew A.
Varian Medical Systems Inc.
Workman, Nydegger & Seelay
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