X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-09-14
2002-08-20
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S121000, C378S119000, C378S140000, C378S130000
Reexamination Certificate
active
06438207
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 having the capability to control the position, size and shape of focal spots on an anode target.
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 structure and operation of x-ray devices is similar. X-rays, or x-radiation, are produced when electrons are produced, accelerated to a high speed, and then stopped abruptly. Typically, this entire process takes place within a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode assembly, which is the source of electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode. Usually, the cathode assembly is composed of a metallic cathode head having a cathode cup. Disposed within the cathode cup is a filament that, when heated via an electrical current, emits electrons.
The three x-ray tube elements are usually positioned within an evacuated glass tube and connected within an electrical circuit. The electrical circuit is connected so that the voltage (generation element can apply a very high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode and the cathode. This high voltage differential causes the electrons that are emitted from the cathode filament to accelerate at a very high velocity towards an x-ray “target” positioned on the anode in the form of a thin stream, or beam. The x-ray target has a target surface (referred to as the focal track) that is comprised of a refractory metal. When the electrons strike the target surface, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target surface, and are then collimated through a window formed in the x-ray device for penetration into an object, such as an area of 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.
The area upon which the electron beam is concentrated when it strikes the anode target surface, or focal track, is referred to as the “focal spot.” In most x-ray applications, it is important that the local spot have a specific size and/or shape so as to result in the generation of an x-ray signal that provides an acceptable image quality. This “focusing” of the electron beam is provided primarily at the cathode, which constrains the emitted electron cloud and accelerated electron stream in a manner so as to result in a focal spot having a specific size and shape.
In addition to the need for a focused electron beam, in some applications—such as diagnostic radiology for example—there is a need to generate two or more different x-ray beams having different energy characteristics, and/or two or more x-ray beams that have different angles of incidence upon the area being analyzed, such as the patient's body. In general, this can be achieved by providing two or more separate focal spots on the focal track. Each focal spot (i.e., point of impact of electrons) will thus generate a separate and distinct x-ray signal, and each signal can thus have a desired characteristic (e.g., energy characteristic, angle of incidence, etc.).
In general, providing an x-ray tube that is capable of generating multiple focal spots of specific size and shape has proven difficult. One approach is to utilize an x-ray tube having multiple cathode head structures. With this approach, a separate cathode with its own cathode cup, heated filament and electrical circuit, is provided. Each cathode is then physically oriented with respect to the anode target surface in a manner so as be capable of generating a separate focal spot. While this approach does result in the generation of multiple x-ray signals, it is not entirely satisfactory for several reasons. It requires additional structural components within the x-ray tube, which increases manufacturing cost and complexity, and increases the likelihood of component failure. Moreover, the number of focal spots that can be produced is limited by the number of cathode structures provided, thereby limiting the number and types of x-ray signals that can be produced.
Another approach for producing multiple x-ray signals is to provide some facility for redirecting or displacing the point of impact of the electron beam (i.e., the focal spot) to different positions on the focal track. These approaches typically utilize a voltage potential to deflect the electron beam after it has been emitted from the cathode filament. However, x-ray tubes using these approaches have not been entirely satisfactory either. For instance, in designs of this sort a deflection mechanism, such as multiple deflection plates, is usually disposed external to the cathode. In operation, a voltage potential is applied to the deflection plates, which creates a deflection region between the cathode and the anode target. Typically, one plate is placed at a much higher negative voltage with respect to the other deflection plate. This voltage bias acts to deflect and alter the direction of the accelerating electron beam, and thus causes it to impinge on a different focal spot location on the anode target surface.
The use of these deflection plates cause several problems that can negatively affect the quality of the resulting x-ray signal. First, in some designs the deflection plates are positioned external to the focusing structure of the cathode cup. Thus, the electron beam has already been formed and focused, and is accelerating towards the anode before it reaches the deflection region. At this point, the electrons are already traveling at a high rate of speed and have therefore achieved an appreciable amount of energy. As such, deflection of the electron beam to alter its direction requires that a high voltage potential be applied to the deflection plates. However, higher voltage can result in arcing between the deflection mechanism and the anode structure, which can render the tube inoperable. To alleviate this problem, the anode must be physically spaced farther from the cathode structure. However, moving the target farther from the anode results in lower x-ray emission, thereby decreasing the quality of the x-ray image. This is not acceptable in many applications. Designs utilizing external deflection plates must thus limit the amount of voltage potential used to steer the electron beam (to maintain the stability of the tube and avoid electrical arcing). This limits the degree to which the electron beam can be deflected. Alternatively, such designs must increase the distance between deflection plates and the anode, which decreases the x-ray emission quality due to the resulting increase in distance between the anode and the cathode.
Another problem with the use of such external deflection plates is that the physical position of the plates relative to one another and relative to the cathode cup and filament, can greatly affect the ability to precisely steer the electron beam. However, each of the plates is typically supported by a separate support structure. Thus, mechanical precision is difficult to achieve, and can result in an expensive and time consuming, manufacturing and assembly process. Moreover, repeated use of the x-ray tube—especially in the extreme thermal and
Chidester Charles Lynn
Heber Mark Alan
Hobden Pamela R.
Porta David P.
Varian Medical Systems Inc.
Workman & Nydegger & Seeley
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