EP1784837A2

EP1784837A2 - Shield structure and focal spot control assembly for x-ray device

Info

Publication number: EP1784837A2
Application number: EP05794388A
Authority: EP
Inventors: Gregory C. Andrews; James R. Boye
Original assignee: Varian Medical Systems Technologies Inc
Current assignee: Varian Medical Systems Inc
Priority date: 2004-09-03
Filing date: 2005-09-02
Publication date: 2007-05-16
Also published as: EP1784837A4; WO2006029026A3; WO2006029026A2; JP5226312B2; JP2008512831A

Abstract

A shield structure (200) and focal spot control assembly (250) is provided for use in connection with an X-ray device (100) that includes an anode (114) and cathode (112)disposed in a vacuum enclosure (102) in a spaced apart arrangement so that a target surfac of the anode is positioned to receive electrons emitted by the anode.

Description

SHIELD STRUCTURE AND FOCAL SPOT CONTROL ASSEMBLY

FOR X-RAY DEVICE

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to x-ray systems and devices. More particularly, embodiments of the invention concern an x-ray device shield structure and focal spot control assembly that contributes to improved x-ray device performance, through enhanced heat management within the x-ray device, and by way of focal spot control. Related Technology

X-ray systems and devices are 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 are produced when electrons are produced and released, accelerated, and then stopped abruptly. A typical x-ray device includes an x-ray tube having a vacuum enclosure collectively defined by a cathode cylinder and an anode housing. An electron generator, such as a cathode, is disposed within the cathode cylinder and includes a filament that is connected to an electrical power source such that the supply of electrical power to the filament causes the filament to generate electrons by the process of thermionic emission. The anode is disposed in the anode housing in a spaced apart arrangement with respect to the cathode. The anode includes a target surface, sometimes referred to as a "target track" or "focal track," oriented to receive electrons emitted by the cathode. Typically, the target surface is composed of a material having a relatively high atomic number, such as tungsten, so that a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, namely, x-rays.

In operation, the electrons are rapidly accelerated from the cathode to the anode under the influence of a high electric potential between the cathode and the anode that is created in connection with a suitable voltage source. The accelerating electrons then strike the target surface at a high velocity. 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 the body of a patient. The x-rays that pass through the object can then 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 relatively large percentage of the electrons that strike target surface of the anode do not cause the generation of x-rays however and, instead, simply rebound from the target surface. Such electrons are sometimes referred to as "back-scatter" or "rebound" electrons. In some x-ray tubes, some of these rebounding electrons are blocked and collected by an electron collector that is positioned between the cathode and the anode so that rebounding electrons do not re-strike the target surface of the anode. In general, the electron collector thus prevents the rebounding electrons from re-impacting the target anode and producing "off-focus" x-rays, which can negatively affect the quality of the x-ray image.

Typically, such electron collectors define an aperture through which the emitted electrons pass from the cathode to the target surface of the anode. To this end, the aperture includes or defines an inlet positioned near the cathode, as well as an outlet positioned near the target surface of the anode. In at least one implementation, the aperture is configured so that the inlet has a diameter that is relatively larger than the diameter of the outlet.

While such electron collectors have proven useful in some applications, some problems nonetheless remain. For example, the geometry of some electron collectors is such that the electron collector experiences undesirable heat concentrations. Such heat concentrations can cause, among other things, thermal stress and strain that may ultimately contribute to structural failure of the collector. More particularly, non-uniform thermal expansion of structural elements, such as is produced by high temperature differentials, induces destructive mechanical stresses and strains that can ultimately cause a mechanical failure in the part.

Yet other concerns with some typical electron collectors relate to the heat flux distribution associated with the electron collector. In particular, the heat flux distribution within typical electron collectors is generally non-uniform. As a result, such electron collectors are prone to heat concentrations that impose harmful, and potentially destructive, thermally-induced stresses and strains on the electron collector, as well as on other components of the x-ray device. Further, such heat concentrations tend to dimmish the efficiency and effectiveness with which heat can be removed from typical electron collectors.

Additionally, x-ray devices that incorporate or include an electron collector typically lack devices or systems that are effective in guiding an electron beam through the electron collector and/or adjusting the position of the focal spot on the target track of the anode.

Consequently, the tomographic, and other, information that can be obtained in connection with such fixed focal spot type devices is somewhat limited. Moreover, the target track of the anode may experience premature wear and failure as a result of the continued presence of the focal spot at the same location on the target track.

Yet other problems with known electron collectors concern the relatively close proximity between the small diameter outlet of the aperture and the target surface of the anode. Particularly, this spatial relationship sometimes results in undesirable heat concentrations at the outlet. Such heat concentrations can cause, among other things, thermal stress and strain that may ultimately contribute to structural failure of the collector. More particularly, non-uniform thermal expansion of structural elements, such as is produced by high temperature differentials, induces destructive mechanical stresses and strains that can ultimately cause a mechanical failure in the part. Further, because the inlet of typical electron collectors is relatively larger than the inlet, backscattered electrons can readily rebound through the inlet and strike the cathode, thereby damaging the cathode and/or interfering with the electrons emitted from the cathode. Further concerns with some typical electron collectors relate to anomalous current flows, such as arcing, within the x-ray device. More particularly, outgassing of metal and glass x-ray device components is generally employed to remove gases adsorbed to the surfaces of those components. The removal of these gases enables a relatively higher vacuum to be achieved in the evacuated enclosure of the x-ray device. In general, outgassing involves heating the x-ray device components to a high temperature for a predetermined period of time. However, typical outgassing processes do not remove all of the adsorbed gas, and some gases, whether present on or under the surfaces of such components, often remain even after outgassing has been performed. As discussed below, these remaining gases, as well as gases that may be produced during normal x-ray device operations, tend to accentuate certain shortcomings associated with typical electron collectors.

In particular, it was noted earlier herein that typical electron collectors are arranged with the large diameter portion of the aperture located near the cathode. Thus, a relatively large portion of the electron collection surface, where adsorbed gases are commonly present, is located in relatively close proximity to the cathode, where the electrical field strength is at or near a maximum. When an exposure commences, adsorbed gases tend to desorb, or ionize, as a result of the heat generated. The presence of the ionized gas in the strong electrical field near the cathode results in gas arcing and/or other anomalous current flow in the high field region. Among other things, such current effects compromise the performance and service life of the x-ray device and can damage or destroy the components of the x-ray device.

BRIEF SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION In general, embodiments of the invention are concerned with a shield structure and focal spot control assembly having a shield structure configured to contribute to the attenuation of heat concentrations in x-ray devices. The shield structure and focal spot control assembly can include an electron steering mechanism, such as a magnetic device configured and arranged to guide an electron beam through the shield structure and, further, to enable control of the location of the electron beam focal spot on a target track of the anode.

In a first embodiment of the invention, a shield structure and focal spot assembly is provided that includes a shield structure configured to be interposed between a cathode and anode, such as a rotating anode. In an illustrated embodiment, the environment is in the form of an anode-grounded x-ray device. The shield structure defines a chamber through which the electrons are passed from the cathode to the target surface of the anode, and the shield structure further defines an inlet passageway and an outlet passageway in communication with the chamber. In an example configuration, the inlet and outlet passageways each have a maximum diameter that is less than a maximum diameter of the chamber. In an illustrated embodiment, the shield structure and focal spot control assembly includes a magnetic device, such as a magnetic coil, that is situated so that a field can be generated to influence the travel path of electrons emitted by the cathode of the x-ray device.

The shield structure of the shield structure and focal spot assembly may take various other forms as well, each of which can be employed in the aforementioned x-ray device. For example, the shield structure can include extended surfaces attached to a body of the shield structure.

In another embodiment, the shield structure defines an electron collection surface that at least partially defines an aperture through which electrons can be passed from a cathode to a target surface of an anode. The collection surface can be oriented towards the target surface of the anode. Also, the aperture includes an inlet and an outlet configured such that the inlet has a relatively smaller area than the outlet.

Exemplary embodiments of the invention thus facilitate, among other things, attenuation of heat concentrations in the shield structure, and effective and reliable control of the focal spot location on the target track of the anode. These and other, aspects of embodiments of the present invention will become more fully apparent from the following description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other aspects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1 is top view illustrating aspects of an exemplary shield structure and focal spot control assembly as employed in connection with an x-ray device;

Figure 2 is a perspective view illustrating aspects of an exemplary implementation of a shield structure that includes a plurality of extended surfaces; Figure 3 is a section view of the shield structure illustrated in Figure 2;

Figure 4 is a partial section view illustrating aspects of an alternative implementation of a shield structure and focal spot control assembly as employed in an x-ray device;

Figure 5 is a perspective view illustrating aspects of an alternative implementation of a shield structure that includes a plurality of extended surfaces; Figure 6 is a section view of the shield structure illustrated in Figure 5;

Figure 7 is a section view illustrating aspects of an alternative implementation of a shield structure and focal spot control assembly as employed in connection with an x-ray device;

Figure 8 is a section view illustrating further aspects of the alternative shield structure and focal spot control assembly disclosed in Figure 8; and

Figure 9 is a perspective view of the alternative shield structure implementation disclosed in Figures 7 and 8.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

OF THE INVENTION Reference will now be made to the drawings to describe various aspects of exemplary embodiments of the invention. It is to be understood that the drawings are diagrammatic and schematic representations of such exemplary embodiments, and are not limiting of the present invention, nor are they necessarily drawn to scale. In general, embodiments of the invention are concerned with a shield structure and focal spot control assembly having a shield structure configured to contribute to the attenuation of heat concentrations in x-ray devices, such as anode-grounded x-ray tubes for example. As discussed in further detail below, it is desirable in some applications and operating environments to be able to achieve a relatively even heat flux distribution over the interior surface of the shield structure chamber. Among other things, a relatively even heat flux distribution contributes to a relative improvement in heat transfer associated with the electron collector, since heat concentrations are attenuated or eliminated.

Exemplary implementations of the shield structure and focal spot control assembly may also include a steering mechanism, such as a magnetic device, configured and arranged to guide an electron beam through the shield structure and, further, to enable control of the location of the electron beam focal spot on a target track of the anode. Among other things, the ability to control, and adjust, the location of the focal spot enables generation of tomographic information beyond that which can be readily obtained with known x-ray devices configured for fixed focal spot operations. This additional tomographic information enables the user of the x-ray device to obtain improved radiological information that can then be employed in performing various analyses and evaluations.

I. Aspects of an Exemplary Operating Environment for the Shield Structure and Focal Spot Control Assembly Directing particular attention now to Figure 1, details are provided concerning various aspects of an x-ray device, denoted generally at 100, wherein exemplary embodiments of a shield structure and focal spot control assembly 150 may be employed. The illustrated implementation of the shield structure and focal spot control assembly 150 includes a shield structure 200 and magnetic device 250, both of which are discussed in further detail below. In at least some implementations, the x-ray device 100 takes the form of an anode-grounded x-ray device where the anode is held at ground potential and the cathode has a potential of - 140KV, for example. Of course, embodiments of the invention may be employed in connection with anode-grounded devices of other potentials as well and, further, may be employed in other than anode-grounded x-ray devices. Accordingly, the scope of the invention should not be construed to be limited to any particular type(s) of x-ray device.

Moreover, while exemplary embodiments of the shield structure and focal spot control assembly 150 are well-suited for use in connection with rotating anode type x-ray devices, the scope of the invention is not so limited. Rather, embodiments of the shield structure and focal spot control assembly 150 may be employed in any application where the functionality disclosed herein would prove useful.

The illustrated implementation of the x-ray device 100 includes a vacuum enclosure 102 cooperatively defined, at least in part, by a cathode can 104 and an anode housing 106. A window 108, substantially composed of beryllium or other suitable material, in the vacuum enclosure 102 allows generated x-rays to pass out of the x-ray device 100.

An adapter 110 is also provided that is configured to mate with the open end of the cathode can 104. In the illustrated implementation, the adapter 110 defines a socket HOA configured to receive a portion of the cathode can 104. The adapter 110 and cathode can 104 may be joined together by any suitable process including, but not limited to, brazing, butt welding, or socket welding. As indicated in Figure 1, the socket HOA in this exemplary embodiment has a diameter relatively larger than the diameter of the necked portion 11OB of the adapter 110. Further details concerning the diameter of the necked portion HOB of the adapter 110 as such diameter relates to the shield structure 200 are provided below. Within continuing reference to Figure 1, a cathode 112 is provided that is disposed within the cathode can 104. The cathode 112 includes a filament (not shown) configured for connection to an electrical power source (not shown) such that when power from the electrical power source is supplied to the filament, electrons are emitted from the filament by thermionic emission. The cathode 112, as well as the anode (discussed below), is also configured for connection with a high voltage source.

The x-ray device 100 further includes a rotating type anode 114 that includes a substrate 114A upon which is disposed the target surface 114B, exemplarily composed of tungsten or other suitable material(s). The anode 114 is rotatably supported by a bearing assembly 116, and a stator 118 is provided that, when energized, causes the anode 114 to rotate at high speed. In the exemplary illustrated arrangement, only the anode 114 and bearing assembly 116 are disposed in the anode housing 106, while the stator 118 is positioned outside the anode housing 106.

With continuing attention to Figure 1, the shield structure 200 is interposed between the cathode 112 and the anode 114. In the exemplary illustrated implementation, the shield structure 200 cooperates with the cathode can 104 and the anode housing 106 to define the vacuum enclosure 102. In at least some implementations, the shield structure 200 is substantially circular, but may be implemented in other shapes as well such as a square, rectangle, or oval for example. In general, the shield structure 200 is configured to pass electrons emitted by the cathode 112 to the target surface 114B of the anode 114. At least some implementations of the shield structure 200 define, or otherwise incorporate or include, one or more fluid passageways through which coolant is passed so as to remove heat from the shield structure 200. In particular, exemplary implementations of the shield structure 200 additionally, or alternatively, include various structural elements, such as extended surfaces 204, configured and arranged to cooperate with other structures such as, but not limited to, the housing 202, adapter 110, anode housing 106 and/or other structures, to define one or more fluid passageways 206 through which a coolant is circulated. Finally, and as suggested above, an external cooling system 120 is provided that is in fluid communication with a coolant reservoir 122 containing coolant wherein at least a portion of the vacuum enclosure 102 is immersed. The external cooling system 120 is also configured and arranged for fluid communication with the shield structure 200, as discussed in further detail elsewhere herein. II. Aspects of Exemplary Implementations of the Shield Structure and Focal Spot Control Assembly

Directing attention now to Figures 2 and 3, and with continuing attention to Figure 1, further details are provided concerning an exemplary implementation of a shield structure, denoted generally at 300 in Figures 2 and 3. Exemplary embodiments of the shield structure 300 are substantially composed of copper or a copper alloy. Any other suitable material(s) may likewise be employed however. Moreover, the shield structure 300 is, in some exemplary implementations, integral with the cathode can 104, adapter 110 or the anode housing 106. Accordingly, the scope of the invention should not be construed to be limited to any particular implementation of the shield structure 300. Embodiments of the shield structure may be manufactured in a variety of different ways. For example, some implementations of the shield structure are formed by casting. Yet other implementations of the shield structure are produced with a milling machine, such as a 4 axis milling machine for example.

The shield structure 300 includes a body 302 that defines a chamber 304 having an interior surface 305. The chamber 304 generally is configured to allow the electron stream to pass from the cathode 112 to the target surface 114B of the anode 114 (see Figure 1). The chamber 304 communicates with an inlet passageway 304A and an outlet passageway 304B, also defined by the body 302. Adjacent the inlet passageway 304A, a socket 304C is defined that is configured to receive a portion of the adapter 110. In other implementations, no socket 304C is required.

In the illustrated implementation of the shield structure 300, the chamber 304, inlet passageway 304A, outlet passageway 304B and socket 304C each have a substantially circular cross-sectional shape, although alternative geometries may be employed. For example, in some implementations, one or more of the chamber 304, inlet passageway 304 A, outlet passageway 304B and socket 304C have a non-circular geometry, such as an oval shape. Further, while the illustrated embodiment indicates an arrangement where the chamber 304, inlet passageway 304A, outlet passageway 304B and socket 304C are each substantially coaxial with each other, the scope of the invention is not so limited. Rather, one or more of the chamber 304, inlet passageway 304 A, outlet passageway 304B and socket 304C may be arranged to be non-coaxial relative to the other(s).

Other aspects of the geometry of the exemplary shield structure 300 vary as well. For example, in the implementation illustrated in Figures 2 and 3, the shield structure 300 is configured to interface with an adapter 110 having an inside diameter "a." Further, the shield structure 300 defines or embodies various parameters, including at least three characteristic diameters whose values may be adjusted to suit the requirements of a particular application.

In particular, the shield structure 300 defines an inlet passageway diameter "b," an outlet passageway diameter "c," and a maximum chamber diameter "d." In at least some implementations, the respective values of the aforementioned diameters, as well as the ratio of one or more diameters relative to another, are selected so as to facilitate achievement of a desired effect, such as a relatively uniform heat flux distribution over the interior surface 305 of the chamber 304. Such diameters, and/or other aspects of the shield structure, may be selected and implemented to enable achievement of other thermal effects as well. For example, adjustment of the outlet passageway diameter enables control of the number of rebound electrons that will enter the chamber. Similarly, adjustment of the inlet passageway diameter enables control of the number of rebound electrons that will exit the chamber near the cathode. As another example, changes to the geometry and/or size of the interior surface of the chamber, either alone or in combination with changes to one or both of the passageway diameters, can be used to adjust the heat flux distribution within the chamber.

Thus, the particular values selected for design parameters such as the c/d ratio of the shield structure 300 for example, and the "a" and "b" dimensions, may depend upon a host of factors which include, but are not limited to, the operating temperature of the x-ray device, the amount of time taken to run up to operating temperature, the number of exposures made with a particular x-ray device over a predefined period of time, the intensity of the exposures made with the x-ray device, the operating time of the x-ray device, the age of the x-ray device, the material of the shield structure, the vacuum within the evacuated enclosure, and the rate at which heat can he transferred from the shield structure. As the foregoing suggests, the designer has considerable latitude as to the values selected for the various parameters of the shield structure. Accordingly, the scope of the invention should not be construed to be limited to any particular implementation of the shield structure, nor to any particular design parameter value or group of values.

In the illustrated implementation, for example, the inlet passageway diameter "b" is selected to be smaller than the adapter inside diameter "a." Additionally, the outlet passageway diameter "c" is selected to be greater than both the inlet passageway diameter "b" and the adapter diameter "a." Finally, the maximum chamber diameter "d" is greater than the adapter inside diameter "a," the inlet passageway diameter "b," and the outlet passageway diameter "c." The specific ratio of any given diameter to one or more other diameters may be selected as desired.

For example, the ratio of c/d may be adjusted as desired to facilitate achievement of a desired heat flux distribution within the chamber 304. As another example, Figures 5 and 6, discussed below, illustrate aspects of a shield structure implementation where the inlet passageway diameter "b" and outlet passageway diameter "c" are substantially equal, but are less than the maximum chamber diameter "d."

It should be noted that in the more general case, where one or more of the chamber 304, inlet passageway 304A, outlet passageway 304B and socket 304C has other than a substantially circular cross-sectional shape, the relationships between the adapter, inlet passageway, outlet passageway, and chamber can be expressed in terms of respective cross- sectional areas, rather than in terms of respective diameters.

With continuing reference now to Figures 2 and 3, the exemplary shield structure 300 further includes one or more extended surfaces 306 attached to the body 302. In the illustrated implementation, a plurality of extended surfaces 306 are provided that are substantially circular and are arranged annularly about the body 302. In the illustrated embodiment, each of the extended surfaces 306 defines a substantially rectangular cross- section, but the scope of the invention is not so limited. Rather, aspects such as, but not limited to, the size, shape, spacing, arrangement and orientation of the extended surface(s) 306 may be varied as necessary to suit the requirements of a particular application. As indicated in Figure 4, for example, the extended surfaces 306 cooperate with each other to at least partially define one or more fluid passageways 308. In at least some of such implementations, the fluid passageways 308 are cooperatively defined by the extended surfaces 306 of the shield structure 300 and the anode housing 106. In yet other implementations, a housing 310 is provided that cooperates with the extended surfaces 306 to at least partially define the fluid passageway(s) 308. The housing 310 comprises a discrete component in some implementations, but is integral with the anode housing 106 in other implementations .

In any case, the fluid passageways 308 are configured and arranged to allow a flow of coolant, generated and provided by a suitable cooling system (Figure 1) to be directed into contact with portions of the shield structure 300 so as to effect cooling, such as by convection and/or conduction for example, of the shield structure 300. To this end, exemplary implementations of the shield structure 300 further define, or otherwise include, at least one coolant inlet port and at least one coolant outlet port (not shown), both of which are in fluid communication with the fluid passageway(s) 308. As noted elsewhere herein, the shield structure 300 is connected with an external cooling system in some implementations.

Finally, the shield structure 300 may be constructed in a variety of different ways. In the exemplary implementation illustrated in Figure 3 (see Figure 6 also), the body 302 includes three discrete portions 302A, 302B and 302C which are formed, such as by machining and/or other suitable processes. After the three portions 302A, 302B and 302C have been constructed, they are stacked as shown, aligned, and then attached to each other by brazing, welding or any other suitable process.

With continuing attention to Figure 4, the illustrated implementation of the shield structure and focal spot control assembly 200 includes in addition to the shield structure 300, an electron steering component. In the illustrated embodiment, this is provided with a magnetic device 250, such as a B-field generator. As discussed in further detail below, the magnetic device 250 generally enables control and adjustment of the location of the focal spot on the target surface 114B of the anode 114.

The magnetic device 250 may be implemented in a variety of ways. For example, the magnetic device 250 is a permanent magnet in some implementations. Alternatively, the magnetic device 250 may be implemented as an electromagnet in other implementations. Further, the magnetic device 250 can be implemented as a single magnet, or multiple magnets. Additionally, aspects such as, but not limited to, the size, number, configuration, type and strength of magnetic device(s) 250 may be varied as necessary to suit the requirements of a particular application.

In the case where the magnetic device is implemented as a magnetic coil, for example, rapid energizing and de-energizing of the coil causes the position of the focal spot to change. Alternatively, the same result can be achieved by rapidly reversing the polarity of the voltage applied to the magnetic coil.

In connection with the foregoing, it should be noted that electromagnets, permanent magnets, magnetic coils and, more generally, the magnetic device, comprise exemplary structural implementations of a means for generating a magnetic field. Accordingly, any other structure(s) capable of implementing comparable functionality may likewise be employed.

As indicated in Figure 4, the magnetic device 250 is exemplarily disposed about the necked portion HOB of the adapter 110, proximate the inlet passageway 304A of the shield structure 300. Thus arranged, the magnetic device 250 is able to influence the travel path of electrons emitted by the cathode 112, and thereby facilitate control of the position of the focal spot. It should be noted that the arrangement in Figure 4 is exemplary only however. More generally, the magnetic device(s) 250 may be located and oriented in any other way that would be conducive to implementation of focal spot control.

Directing attention now to Figures 5 and 6, details are provided concerning various aspects of an alternative implementation of a shield structure, denoted generally at 500. As the shield structure 500 is similar in many regards to the shield structure 300 illustrated in

Figures 2 and 3, the discussion of Figures 5 and 6 will focus primarily on certain differences between the two embodiments.

Similar to the shield structure 300, the shield structure 500 includes a body 502 that defines a chamber 504 having an interior surface 505. Generally, the chamber 504 is configured to allow the electron stream to pass from the cathode 112 to the target surface

114B of the anode 114 (see Figures 1 and 4). The chamber 504 communicates with an inlet passageway 504A and an outlet passageway 504B, also defined by the body 502. Adjacent the inlet passageway 504A, a socket 504C is defined that is configured to receive a portion of the adapter 110 having an inside diameter "a."

As in the case of the shield structure 300, the shield structure 500 defines an inlet passageway diameter "b," an outlet passageway diameter "c," and a maximum chamber diameter "d." In at least some implementations, the respective values of the aforementioned diameters, as well as the ratio of one or more diameters relative to another, are selected so as to facilitate achievement of a relatively uniform heat flux distribution over the interior surface 505 of the chamber 504. Such diameters, and/or other aspects of the shield structure, may be selected and implemented to enable achievement of other thermal effects as well.

In the illustrated implementation, the inlet passageway diameter "b" is selected to be smaller than the adapter inside diameter "a." In contrast with the shield structure 300 however, the outlet passageway diameter "c" of the shield structure 500 is selected to be substantially the same size as the inlet passageway diameter "b," while both the outlet passageway diameter "c" and inlet passageway diameter "b" are smaller than the maximum chamber diameter "d." Of course, the specific ratio of any given diameter to one or more other diameters may be selected as desired. By way of example, the ratio of c/d may be adjusted as desired to facilitate achievement of a desired heat flux distribution within the chamber 504.

It should be noted that in the more general case, where one or more of the chamber 504, inlet passageway 504A, outlet passageway 504B and socket 504C has other than a substantially circular cross-sectional shape, the relationships between the adapter, inlet passageway, outlet passageway, and chamber can be expressed in terms of respective cross- sectional areas, rather than in terms of respective diameters.

III. Operational Aspects of the Exemplary Shield Structure and Focal Spot Control Assembly of Figures 1-6 With continuing reference to Figures 1-6, details are provided concerning various operational aspects of an exemplary implementation of a shield structure and focal spot control assembly, such as the shield structure and focal spot control assembly 200 (see Figures 1 and 4), as employed in an x-ray device operating environment.

In operation, power is applied to the cathode 112, and a high electric potential established between the cathode 112 and the anode 114. The power applied to the cathode 112 causes the thermionic emission of electrons from the cathode filament and the high voltage causes the electrons to accelerate rapidly toward the target surface 114B of the anode 114. As the electrons strike the target surface 114B, x-rays are produced that pass through the window 108. At least some of the electrons that strike the target surface 114B rebound from the target surface 114B toward the cathode 112 and/or other structures and elements of the x-ray device 100. As noted earlier, such rebound electrons still possess significant kinetic energy that is transformed to heat when the rebound electrons strikes a portion of the x-ray device 100. However, the geometry of the shield structure 300 (Figure 4) is such that selection of c/d ratio, in light of the applicable operating environment conditions and operational requirements, enables achievement of a substantially uniform heat flux distribution over a substantial portion of the interior surface of the chamber 304. For example, in some implementations, a c/d ratio of less than about 1.0 facilitates achievement of a substantially uniform heat flux distribution on the interior surface 305 of the chamber 304. Among other things, this substantially uniform heat flux attenuates undesirable heat concentrations within the shield structure 300 and also contributes to a relative improvement in the effectiveness and efficiency with which heat can be removed from the shield structure 300 by, for example, the external cooling system 120.

As disclosed elsewhere herein, modifications to the heat flux distribution, and/or implementation of other desired thermal effects can be readily achieved with appropriate modifications to one or more of the parameters of the shield structure. For example, the shield structure 500 is constructed with a passageway outlet 504B having a relatively smaller diameter than the passageway outlet 304B of the shield structure 300. Thus, the shield structure 500 is configured to admit relatively fewer rebound electrons to the chamber 504, with an attendant decrease in heat flux through the interior surface 505.

With continuing reference to exemplary operational aspects of the shield structure and focal spot control assembly, the magnetic device generates a magnetic field of desired strength and orientation so that a substantial portion of the emitted electrons follow a prescribed path to the target surface of the anode. Because aspects such as the strength and orientation of the magnetic field exerted by the magnetic device can be adjusted, changes to the position of the focal spot can be readily implemented. Among other things, the ability to move the focal spot in this way enables the operator to gather relatively more tomographic information than would otherwise be possible. This additional information, in turn, contributes to a relative improvement in the evaluations and analyses that can be performed with the x-ray device. IV. Aspects of an Alternative Implementation of the Shield Structure and Focal Spot

Control Assembly With attention now to Figures 7 through 9, details are provided concerning an alternative implementation of a shield structure and focal spot control assembly, denoted generally at 600 in Figure 7. The shield structure and focal spot control assembly 600 differs somewhat from some other implementations disclosed herein in that the shield structure 602 does not include a chamber but, rather, has an interior surface that defines an electron collection surface defining an aperture 602A through which electrons pass from the cathode to the anode. In the illustrated embodiment, the surface is configured with a substantially concave shape, and is oriented towards the anode.

Directing particular attention first to Figure 7, the shield structure and focal spot control assembly 600 further includes one or more electron steering devices, such as magnetic device(s) 604 (implemented, for example, as a B-field generator), configured and arranged to implement focal spot control functionality as disclosed herein. As in the case of the other magnetic devices disclosed herein, the magnetic device 604 is implemented, for example, as an electromagnet, magnetic coil, or as a permanent magnet. Further, the magnetic device 604 is implemented as a single magnet in some cases, or as multiple magnets. Additionally, aspects such as, but not limited to, the size, number, configuration, type and strength of magnetic device(s) 604 may be varied as necessary to suit the requirements of a particular application. The magnetic device(s) 604 may be located and oriented in any way that would be conducive to implementation of focal spot control. Further details concerning the structure and operation of a similarly configured shield structure are provided below in connection with the discussion of Figures 8 and 9.

Directing attention now to Figures 8 and 9, further details are provided concerning an exemplary implementation of a shield structure, denoted generally at 700. Exemplary embodiments of the shield structure 700 (see Figures 8 and 9) are substantially composed of copper or a copper alloy. Any other suitable material(s) may likewise be employed however. Moreover, the shield structure 700 is, in some exemplary implementations, integral with the cathode can 104, adapter 110 or the anode housing 106. Accordingly, the scope of the invention should not be construed to be limited to any particular implementation of the shield structure 700. As best illustrated in Figures 8 and 9, the shield structure 700 includes a body 701 having an interior surface 702 that defines an aperture 704 that allows the electron stream to pass from the cathode 112 to the target surface 114B of the anode 114. The aperture 704 includes an inlet 704A and an outlet 704B. In this exemplary implementation, the aperture 704, inlet 704A and outlet 704B are all substantially circular in shape and, further, the inlet 704 A and outlet 704B are substantially coaxial with each other. However, the scope of the invention is not so limited. For example, in some implementations, one or more of the aperture 704, inlet 704A and outlet 704B have a non-circular geometry, such as an oval shape. Moreover, the inlet 704A and outlet 704B need not be coaxial with each other. In the illustrated implementation, the aperture 704 defined by the interior surface 702 includes a substantially tubular section configured and arranged to be attached to the adapter

108. This implementation is exemplary only and should not be construed to limit the scope of the invention in any way. For example, some implementations of the shield structure 700 do not include such a tubular section.

With continuing reference to Figures 8 and 9, the inlet 704A has an area that is less than the area of the outlet 704B. In the exemplary implementation of the shield structure 700 where the inlet 704A and outlet 704B are substantially circular in shape, this means that the diameter of the inlet 704 A is relatively smaller than the diameter of the outlet 704B. Additionally, in the illustrated embodiment the interior surface 702 is generally concave in form. In one exemplary implementation, best illustrated in the cross-section view of Figure 8, the interior surface 702 curves between the inlet 704A and the outlet 704B. In other exemplary implementations however, the interior surface 702 is implemented in another type of concave form. In particular, the interior surface 702 is also implemented in a substantially frustoconical cross-sectional shape such that the interior surface 702 describes a substantially straight line between the inlet 704A and the outlet 704B.

In either case however, the interior surface 702 of the shield structure is configured so as to be oriented toward the target surface 114B of the anode 114 and away from the cathode

112, as indicated in Figure 8 for example, when the shield structure 700 is positioned between the cathode 112 and the anode 114. Consequently, the inlet 704 A is located proximate the cathode 112, while the outlet 704B is located proximate the anode 114, as indicated in Figure 8. As discussed in further detail elsewhere herein, such arrangements have various useful implications. As best illustrated in Figure 8, the shield structure 700 also defines a socket 705 located near the inlet 704A. The socket 705 is generally configured and arranged to mate with a portion of the adapter 110, as shown.

Additionally, at least some implementations of the shield structure 700 include one or more extended surfaces 706 that are substantially annular. In the illustrated embodiment, each of the extended surfaces 706 defines a substantially rectangular cross-section, but the scope of the invention is not so limited. Rather, aspects such as, but not limited to, the size, shape, spacing, arrangement and orientation of the extended surface(s) 706 may be varied as necessary to suit the requirements of a particular application.

As the Figures indicate, the extended surfaces 706 cooperate with each other to at least partially define one or more fluid passageways 708. In some implementations, a housing 710 (see Figure 8) is also provided within which the shield structure 700 is received. In at least some of such implementations, the fluid passageways 708 are cooperatively defined by the extended surfaces 706 of the shield structure 700 and the housing 710. In any case, the fluid passageways 708 are configured and arranged to allow a flow of coolant, generated and provided by a suitable cooling system (not shown) to be directed into contact with portions of the shield structure 700 so as to effect convective and conductive cooling of the shield structure 700. To this end, exemplary implementations of the shield structure further define, or otherwise include, at least one coolant inlet port and at least one coolant outlet port, both of which are in fluid communication with the fluid passageway(s) 708.

With more particular attention now to the housing 710, some exemplary embodiments of the shield structure 700 are configured so that the shield structure 700 is integral with the housing 710 which, in turn, is configured to be attached to one, or both, of the cathode can 104 and the anode housing 106, such as by welding or brazing. In yet other implementations however, the shield structure 700 and housing 710 are discrete structures. The housing 710, similar to the shield structure 700 is exemplarily composed of copper or a copper alloy, but other suitable materials may be employed as well in the construction of the housing 710.

As noted earlier, some implementations of the housing 710 cooperate with the shield structure 700 to define one or more fluid passageways that facilitate cooling of the shield structure. In order to further facilitate such cooling, some exemplary implementations of the housing 710 additionally include a plurality of extended surfaces (not shown) on the exterior portion of the housing 710 so that in implementations where the x-ray device 100 is immersed in a coolant reservoir (see Figure 1), the extended surfaces of the housing 710 contact the coolant and transfer heat from the shield structure 700 to the coolant in the coolant reservoir.

In yet other implementations, the housing 710 is constructed so that the shield structure 700 is only partially received in the housing 710. In some of these implementations, the portion of the shield structure 700 that remains outside the housing 710 includes a plurality of extended surfaces configured and arranged for substantial contact with coolant contained in a coolant reservoir. V. Operational Aspects of the Alternative Shield Structure and Focal Spot Control Assembly of Figures 7-9

With continuing reference to Figures 7-9, and with attention to Figure 1 also, details are provided concerning various operational aspects of an exemplary implementation of a shield structure 700 as employed in an x-ray device 100 operating environment. In operation, power is applied to the cathode 112, and a high potential established between the cathode 112 and the anode 114. The power applied to the cathode 112 causes the thermionic emission of electrons from the cathode filament and the high voltage causes the electrons to accelerate rapidly toward the target surface 114B of the anode 114. As the electrons strike the target surface 114B, x-rays are produced that pass through the window 108.

At least some of the electrons that strike the target surface 114B rebound from the target surface 114B toward the cathode 112 and/or other structures and elements of the x-ray device 100. As noted earlier, such rebound electrons still possess significant kinetic energy that is transformed to heat when the rebound electrons strikes a portion of the x-ray device 100. However, because the inlet 704A of the shield structure 700 is relatively small, as compared with the outlet 704B, many of the rebound electrons harmlessly strike the interior surface 702 instead of the cathode 112. Thus, the configuration and positioning of the shield structure 700 reduces the number of rebound or backscatter electrons that are able to strike sensitive elements of the x-ray device 100, such as the cathode 112, thereby reducing the heat load on the cathode 112 and, accordingly, the likelihood that the cathode 112 will be damaged as a result of excessive heat.

Additionally, the positioning of the interior surface 702 of the shield structure 700 toward the anode 114 and away from the cathode 112 attenuates heat concentrations that occur at the inlet of some typical shield structures. More particularly, the rebound electrons tend to strike the interior surface 702 at various locations, so that the heat load produced by the impacts of such rebound electrons is distributed relatively evenly over the interior surface 702.

In contrast, typical shield structures are disposed in the opposite orientation with respect to the cathode 112 and the anode 114 so that, in such arrangements, a large portion of the rebound electrons strike the structure immediately adjacent to the relatively small aperture outlet, thereby concentrating the heat load near this aperture outlet. As noted elsewhere herein, this problem is aggravated by the fact that the small size of the aperture outlet permits only a relatively limited number of rebound electrons to pass through the aperture. A related aspect of the configuration and arrangement of the shield structure 700 indicated in Figures 8 and 9 is that because the inlet 704A is located relatively further from the target surface 114B of the anode 114, as compared with the configuration and arrangement of typical shield structures, the heat load imposed on the inlet 704A by x-ray generation at the target surface 114B is reduced. Among other things, the distribution and/or reduction of heat loads that is effectuated by embodiments of the shield structure 700 contributes to a relative reduction in destructive thermal stresses and strain, and attendant effects, in the x-ray device 100 and associated structures.

Yet other aspects of the configuration and arrangement of the shield structure 700 concern anomalous current effects, such as arcing, that sometimes occur in x-ray devices. It was noted earlier herein that outgassing of adsorbed gases often occurs in x-ray devices and, further, that such outgassing often contributes to gas arcing when ionized gas is present in the high strength area of the electrical field between the cathode and the anode. More particularly, outgassing commonly occurs during exposure operations performed by the x-ray device. Between exposures, the gas collects on x-ray component surfaces, such as the interior surface of the shield structure. When an exposure is initiated, the gas is ionized and, as a result of the arrangement of typical shield structures, the ionized gas tends to be concentrated in the high strength area of the electrical field, namely, near the cathode. The presence of the ionized gas, in combination with the strong electrical field causes arcing and/or other undesirable anomalous current effects.

However, embodiments of the shield structure 700 are configured and arranged so that a significant portion of the interior surface 702, where gas is likely to be present, is relatively closer to the anode 114 than to the cathode 112. In connection with the foregoing, it was noted earlier that the strength of the electrical field generally diminishes as the distance from the cathode 114 increases. Among other things then, implementations of the shield structure 700 contrast with typical shield structures in that the configuration and arrangement of exemplary embodiments of the shield structure 700 are such that the concentration of ionized gas generated as a result of offgassing tends to be relatively higher in the low strength area of the electrical field. Consequently, implementations of the shield structure 700 contribute to a relative reduction in gas arcing in the x-ray device 100.

Embodiments of the shield structure 700 contribute to improvements in the performance of the x-ray device 100 in other ways as well. In particular, during the operation of the x-ray device 100, circulation of coolant through the fluid passageways defined in connection with exemplary embodiments of the shield structure 700 removes heat from the x- ray device 100, thereby reducing the likelihood of thermally induced damage to the x-ray device 100 and its components. The presence of extended surfaces or similar structures in some embodiments of the shield structure 700 further enhances and contributes to such heat removal. The described embodiments are to be considered in all respects only as exemplary and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

CLAIMSWhat is claimed is:

1. A shield structure suitable for use in connection with an x-ray device having an evacuated enclosure within which are disposed a cathode and anode, the cathode and anode being arranged so that a target surface of the anode is positioned to receive electrons emitted by the cathode, the shield structure being configured to be interposed between the cathode and the target surface of the anode, and the shield structure comprising: a body defining a chamber through which the electrons are passed from the cathode to the target surface of the anode, and the body further defining an inlet passageway and an outlet passageway in communication with the chamber, both the inlet passageway and the outlet passageway having a cross-sectional area less than a maximum cross-sectional area of the chamber; and at least one extended surface attached to the body.

2. The shield structure as recited in claim 1, wherein the cross-sectional area of the inlet passageway is substantially the same as the cross-sectional area of the outlet passageway.

3. The shield structure as recited in claim 1, wherein the cross-sectional area of the inlet passageway is less than the cross-sectional area of the outlet passageway.

4. The shield structure as recited in claim 1, wherein the shield structure substantially comprises one of: copper; and, copper alloy.

5. The shield structure as recited in claim 1, wherein the shield structure at least partially defines a fluid passageway.

6. The shield structure as recited in claim 1, wherein the body and the at least one extended surface cooperate with each other to at least partially define a fluid passageway.

7. The shield structure as recited in claim 1, wherein the at least one extended surface comprises a plurality of substantially annular extended surfaces.

8. An x-ray device, comprising: a vacuum enclosure; an anode and cathode substantially disposed in the vacuum enclosure in a spaced apart arrangement so that a target surface of the anode is positioned to receive electrons emitted by the cathode; a shield structure interposed between the anode and the cathode, the shield structure defining a chamber through which the electrons are passed from the cathode to the target surface of the anode, and the shield structure further defining an inlet passageway and an outlet passageway in communication with the chamber, both the inlet passageway and the outlet passageway having a cross-sectional area less than a maximum cross-sectional area of the chamber; and a means for generating a magnetic field, the means operating to permit control and adjustment of the location of a focal spot on the target surface of the anode.

9. The x-ray device as recited in claim 8, wherein the anode is at about ground potential during operation of the x-ray device.

10. The x-ray device as recited in claim 8, wherein the cross-sectional area of the inlet passageway is substantially the same as the cross-sectional area of the outlet passageway.

11. The x-ray device as recited in claim 8, wherein the cross-sectional area of the inlet passageway is less than the cross-sectional area of the outlet passageway.

12. The x-ray device as recited in claim 8, wherein the shield structure substantially comprises one of: copper; and, copper alloy.

13. The x-ray device as recited in claim 8, wherein the shield structure at least partially defines a fluid passageway.

14. The x-ray device as recited in claim 8, wherein the shield structure includes at least one extended surface.

15. The x-ray device as recited in claim 8, wherein the means for generating a magnetic field comprises at least one magnetic device disposed proximate the inlet passageway of the shield structure.

16. An x-ray device, comprising: a vacuum enclosure; an anode and cathode substantially disposed in the vacuum enclosure in a spaced apart arrangement so that a target surface of the anode is positioned to receive electrons emitted by the cathode; a shield structure interposed between the anode and the cathode, the shield structure including an interior electron collection surface that at least partially defines an aperture through which the electrons are passed from the cathode to the target surface of the anode, the aperture having an inlet and an outlet, and the shield structure being situated such that the interior electron collection surface is oriented towards the target surface of the anode; and a means for generating a magnetic field, the means operating to permit control and adjustment of the location of a focal spot on the target surface of the anode.

17. The x-ray device as recited in claim 16, wherein the shield structure substantially comprises one of: copper; and, copper alloy.

18. The x-ray device as recited in claim 16, wherein the shield structure at least partially defines a fluid passageway.

19. The x-ray device as recited in claim 16, further comprising a housing within which at least a portion of the shield structure is received.

20. The x-ray device as recited in claim 16, wherein the shield structure includes at least one extended surface.

21. The x-ray device as recited in claim 16, wherein the electron collection surface is formed with a substantially concave shape.