X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-09-07
2003-02-11
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S121000, C378S119000, C378S130000
Reexamination Certificate
active
06519318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cooling system that increases the rate of heat transfer from the x-ray tube to a cooling system medium, thereby significantly reducing heat-induced stress and strain in x-ray tube structures and extending the operating life of the device.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing.
While used in a number of different applications, the basic operation of x-ray devices is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. The typical basic x-ray tube has a cathode cylinder with an electron generator, or cathode, at one end. Electrical power applied to a filament portion of the cathode generates electrons by thermionic emission. A target anode is axially spaced apart from the cathode, and is oriented so as to receive electrons emitted by the cathode. Also present is a voltage source that is used to apply a high voltage potential between the cathode and the anode.
In operation, the high voltage potential is applied between the cathode and the anode, which causes the thermionically emitted electrons to accelerate away from the cathode and towards the anode in an electron stream The accelerating electrons then strike the target anode surface (or focal track) at a high velocity. The target surface on the anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thereby converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray device for penetration into an object, such as a patient's body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
A percentage of the electrons that strike the anode target surface do not generate x-rays, and instead simply rebound from the surface. These are often referred to as “back-scatter” electrons. In some x-ray tubes, some of these rebounding electrons—still traveling at relatively high velocities—are blocked and collected by a shield structure that is positioned between the cathode and the anode so the rebounding electrons do not re-strike the target surface of the anode. In this way, the rebounding electrons are prevented from reimpacting the target anode and producing “off-focus” x-rays, which can negatively affect the quality of the x-ray image. Some of the rebounding electrons may also impact the interior of the cathode cylinder.
While such a shield structure may prevent rebounding electrons from re-striking the anode target, its use can result in additional problems that can ultimately damage the x-ray tube device, and shorten its operational life. In particular, the high kinetic energy of the rebounding electrons is converted to thermal energy by the impact of those electrons on the shield structure or on the interior of the cathode cylinder. Due to the high level of kinetic energy of the electrons, the thermal energy produced by these impacts is significant and typically results in very high temperatures in the x-ray tube structures. These thigh temperatures, in combination with the high temperatures also being generated at the target anode, cause thermal stresses in the structures (including the cathode cylinder and the shield) and structure joints that can, especially over time, lead to various structural failures in the x-ray tube assembly. Moreover, because the rebounding electrons impact some portions of the cathode cylinder and shield structure with relatively greater frequency than other portions the heat produced by the rebounding electrons is not evenly distributed. Accordingly, the different heat regions are collectively characterized by varying rates of thermal expansion, resulting in mechanical stresses that can also damage the x-ray tube device, especially over numerous operating cycles.
For instance, mechanical stress and strain is induced when the cooler part of the structure resists the expansion of the hotter portion of the structure. The level of stress and strain is relatively insignificant at low temperature differentials. However, non-uniform expansion produced by high temperature differentials induces destructive mechanical stresses and strains that can ultimately cause a mechanical failure in the part. Moreover, these stresses are especially damaging to joints between attached components.
Because such high temperatures can cause destructive thermal stresses and strains in the shield structure, the cathode cylinder, and in other parts of the x-ray device, attempts have been made to minimize thermal stress and strain through the use of various types of cooling systems. However, previously available x-ray tube cooling systems have not been entirely satisfactory in providing effective and efficient cooling—especially in the regions of the shield structure and cathode cylinder.
In order to dissipate the high heat present, x-ray tubes have typically utilized some type of liquid cooling arrangement. In such systems, at least some of the external surfaces of the cathode cylinder are placed in direct contact with a circulating coolant, which facilitates a convective cooling process. Often however, this approach is not satisfactory for cooling an adjacent shield structure, which has a limited external surface area, and, because it is exposed to extremely high temperatures from rebounding electrons, is unable to efficiently transfer significant amounts of heat by convection to the coolant.
To address this problem, shield structures have been fashioned with internal cooling passages through which a coolant stream is circulated. Thus, the shield structure gives up heat primarily by convection to the coolant which flows through its interior. This approach has not been entirely satisfactory either. Due to the limited size of such cooling passages, only a limited amount of heat can be absorbed by the coolant, and consequently the shield structure may not be adequately cooled. Thus, x-ray devices of this sort may experience greater failure rates and shorter operating lives due to repeated exposure to higher temperatures and resultant stresses.
Also, in systems of this sort, the coolant must be capable of absorbing significant amounts of heat in order to preclude harmful thermal stresses and strain in the shield structure and cathode cylinder. However, with current designs, the circulated coolant eventually, and often prematurely, experiences thermal breakdown and is no longer able to effectively remove heat from the x-ray tube. Again, this translates into an x-ray device that is more subject to failure and that typically has an overall shorter operating life.
Currently available cooling system designs are lacking in another respect as well. As noted, heat produced within the x-ray tube is not evenly distributed. However, currently available cooling systems are not capable of removing heat from certain higher-temperature areas of the x-ray tube faster than cooler areas. Instead, the rate of heat transfer is fairly constant throughout the x-ray tube in existing systems. As such, those regions that are exposed to higher temperatures are not adequately cooled, and experience a greater failure rate.
There are additional problems in existing x-ray tube designs caused by excessive operating temperatures. In particular, the high operating temperatures are especially destructive to the connection points b
Hobden Pamela R.
Porta David P.
Varian Medical Systems Inc.
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