X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-09-08
2002-08-20
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S161000
Reexamination Certificate
active
06438208
ABSTRACT:
BACKGROUND 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 so as to significantly reduce heat-induced stress and strain in the x-ray tube structures and thereby extend the operating life of the device
THE PRIOR STATE OF THE ART
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 tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within an evacuated enclosure, or “can.” Disposed within the can is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then placed between the anode and the cathode, which causes the emitted electrons accelerate towards a target surface positioned on the anode. Typically, the electrons are “focused” into a primary electron beam towards a desired “focal spot” located at the target surface. In addition, some x-ray tubes employ a deflector device to control the direction of the primary electron beam. For example, a deflector device can be a magnetic coil disposed around an aperture that is disposed between the cathode and the target anode. The magnetic coil is used to produce a magnetic field that alters the direction of the primary electron beam. The magnetic force can thus be used to manipulate the direction of the beam, and thereby adjust the position of the focal spot on the anode target surface. A deflection device can be used to control the size and/or shape of the focal spot.
During operation of an x-ray tube, the electrons in the primary electron beam strike the target anode surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thus 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 tube for penetration into an object, such as a patient's body. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
A percentage of the electrons that strike the target anode target surface rebound from the surface and then either impact at other random areas on the target surface, or at other “non-target” surfaces within the x-ray tube can. The electrons within this secondary electron beam are often referred to as “secondary” electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated. In fact, as many as half the electrons generated by the cathode, representing as much as one third of the total energy of the electron beam, rebound from the target as secondary electrons. As discussed in further detail below, the heat thus generated can ultimately damage the x-ray tube, and shorten its operational life.
In particular, the temperatures generated by secondary electrons, in conjunction with the high temperatures generated by the primary electrons at the focal spot of the target surface, often reach levels high enough to damage portions of the x-ray tube structure. The window of the x-ray tube, and the joints and connection points between x-ray tube structures, are examples of areas where the x-ray tube can be weakened when repeatedly subjected to such thermal stresses. In some instances, the resulting temperatures can even melt portions of the x-ray tube, such as lead shielding disposed on the can. Such conditions can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
Further, because the trajectories of secondary electrons cause them to impact some interior surface locations with relatively greater frequency than other areas, the resulting heat distribution can be uneven. The varying rates of thermal expansion cause mechanical stresses and strains when the cooler part of the structure resists the expansion of the hotter portion of the structure. Ultimately, this can cause a mechanical failure in the part, especially over numerous operating cycles.
While the aforementioned problems are cause for concern in all x-ray tubes, these problems become particularly acute in the new generation of high-power x-ray tubes (generally, those x-ray tubes with operating powers exceeding 20 kilowatts (kw)) which have relatively higher operating temperatures than the typical devices.
Note that the problems herein described are also cause for concern where long exposures, or exposure chains, are being performed, regardless of the power of the x-ray tube performing the exposures. Some examples of these types of exposures include helical computed tomography scanning, and angiography.
Attempts have been made to reduce temperatures in such areas of high heat concentration, and 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, and have been especially ineffectual in those particular regions of the tube that are subjected to high temperatures, such as from rebounding electrons. Moreover, the inadequacies of known x-ray tube cooling systems are further exacerbated by the increased heat levels that are characteristic in high-powered x-ray tubes.
For example, conventional x-ray tube systems often utilize some type of liquid cooling arrangement. In such systems, at least some of the external surfaces of the vacuum enclosure are placed in contact with a circulating coolant, which facilitates a convective cooling process. While these types of processes are adequate to cool some portions of the x-ray tube, they may not adequately cool areas of localized heat—such as those that are particularly susceptible to heating from secondary electrons, including the window area of the tube, the window itself, and portions of the can structure that are proximate to the window area. The joint where the x-ray tube window is attached to the can is also particularly vulnerable to thermally induced damage, due largely to the relatively close proximity of this joint to the cathode and anode, and may not be adequately cooled by conventional cooling systems and processes.
Not only does its close proximity to the cathode and anode render the window especially susceptible to thermally induced damage, but certain characteristics of the window itself also make the window vulnerable to such damage. For example, because the window is relatively thin and is typically constructed of a material having a low atomic number, such as beryllium, it is relatively more susceptible to heat damage.
As suggested above, the window area of the x-ray tube, and the window itself, are particularly susceptible to heat induced structural damage, due at least in part to their proximity to the target anode, and the cathode. The damage caused by high temperatures is not limited solely to destructive structural effects however. For example, even in relatively low-powered x-ray tubes, the window area can become sufficiently hot to boil coolant that is adjacent to the window. Heat levels such as this can induce potentially dest
Church Craig E.
Varian Medical Systems Inc.
Workman & Nydegger & Seeley
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