Apparatus and method for improved transient response in an electromagnetically controlled X-ray tube

An x-ray tube assembly includes a vacuum enclosure including a cathode portion, a target portion, and a throat portion. The throat portion includes a magnetic field section, upstream section, and downstream section. The magnetic field section has a first susceptibility to generate eddy currents in the presence of a magnetic field intensity. The upstream section is coupled to the cathode portion and the magnetic field section and has a second susceptibility to generate eddy currents in the presence of the magnetic field intensity. The downstream section is coupled to the magnetic field section and has a third susceptibility to generate eddy currents in the presence of the magnetic field intensity. The first susceptibility to generate eddy currents is less than the second and third susceptibilities to generate eddy currents. The assembly includes a target within the target portion, and a cathode within the cathode portion.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to an apparatus and method for improved transient response in an electromagnetically controlled x-ray tube.

X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.

X-ray tubes include a rotating target structure for the purpose of distributing the heat generated at a focal spot. The target is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating target assembly is driven by the stator.

One skilled in the art will recognize that the operation described herein need not be limited to a single X-ray tube configuration, but is applicable to any X-ray tube configuration. For instance, in one embodiment the target and frame of the X-ray tube may be held at ground potential and the cathode may be maintained at the desired potential difference, while in another embodiment the X-ray tube may operate in a bipolar arrangement having a negative voltage applied to a cathode and a positive voltage applied to an anode.

An x-ray tube cathode provides an electron beam that is accelerated using a high voltage applied across a cathode-to-target vacuum gap to produce x-rays upon impact with the target. The area where the electron beam impacts the target is often referred to as the focal spot. Typically, the cathode includes one or more cylindrical coil or flat filaments positioned within a cup for providing electron beams to create a high-power, large focal spot or a high-resolution, small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application. Typically, an electrically resistive emitter or filament is positioned within a cathode cup, and an electrical current is passed therethrough, thus causing the emitter to increase in temperature and emit electrons when in a vacuum.

The shape of the emitter or filament and the shape of the cathode cup that the filament is positioned within affects the focal spot. In order to achieve a desired focal spot shape, the cathode may be designed taking the shape of the filament and cathode cup into consideration. However, the shape of the filament is not typically optimized for image quality or for thermal focal spot loading. Conventional filaments are primarily shaped as coiled or helical tungsten wires for reasons of manufacturing and reliability. Alternative design options may include alternate design profiles, such as a coiled D-shaped filament. Therefore, the range of design options for forming the electron beam from the emitter may be limited by the filament shape, when considering electrically resistive materials as the emitter source.

Electron beam (e-beam) wobbling is often used to enhance image quality. Wobble may be achieved using electrostatic e-beam deflection or magnetic deflection (i.e., spatial modulation), which utilizes a rapidly changing magnetic field to control the e-beam. Likewise, a rapidly changing magnetic field may be used to rapidly change the focusing of the electron beam (i.e., change the cross-sectional size of the electron beam in width and length directions). Typically, a pair of quadrupole magnets are used to achieve electron beam focusing in both width and length directions. For certain scan modes, such as rapid kV modulation, or so-called dual-energy scanning, the ability to rapidly adjust the focusing magnetic field is advantageous to maintain the focal spot size constant between the kV levels. Such electromagnetic e-beam control may achieve a high image quality by ensuring that the electron beam moves from one position to the next or refocuses as quickly as possible while staying in the desired position or at the desired focus without straying. However, when current in the electromagnets is rapidly changed to generate the changing magnetic field, eddy currents are generated in the vacuum vessel wall that opposes the magnetic field penetration inside the x-ray tube. The eddy currents increase the rise time of the magnetic field inside the throat of the x-ray tube, which slows the deflection or refocusing time of the e-beam. Accordingly, it would be desirable to design an x-ray tube having a throat portion that minimizes eddy current losses to optimize the transient magnetic field developed at the electron beam.

The configuration of the x-ray tube throat is subject to a number of design constraints. During operation, the throat experiences significant heat fluxes in the x-ray tube environment due to backscattered electrons from the target, for example. Further, the throat should be easy to manufacture and easy to join with interface components while still being capable of maintaining a hermetic vacuum and withstanding atmospheric pressure.

Therefore, it would be desirable to design an apparatus and method for improving the transient response in an electromagnetically controlled x-ray tube that satisfies the above-described design constraints and overcomes the aforementioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, an x-ray tube assembly includes a vacuum enclosure that has a cathode portion, a target portion, and a throat portion. The throat portion includes a magnetic field section having an upstream end and a downstream end. The magnetic field section has a first susceptibility to generate eddy currents in the presence of a magnetic field intensity. The throat portion also has an upstream section having a first end and a second end. The first end of the throat portion is coupled to the cathode portion and the second end of the throat portion is coupled to the upstream end of the magnetic field section. The upstream section has a second susceptibility to generate eddy currents in the presence of the magnetic field intensity. The throat portion also has a downstream section that has a first end and a second end. The first end of the downstream section is coupled to the downstream end of the magnetic field section. The downstream section has a third susceptibility to generate eddy currents in the presence of the magnetic field intensity. The first susceptibility to generate eddy currents is less than the second and third susceptibilities to generate eddy currents. The x-ray tube assembly also includes a target positioned within the target portion of the vacuum enclosure, and a cathode positioned within the cathode portion of the vacuum enclosure, the cathode configured to emit a stream of electrons toward the target.

In accordance with another aspect of the invention, an x-ray tube assembly includes a housing having a vacuum formed therein. The housing includes a cathode portion, a target portion, and a throat portion. The throat portion includes a first section having a first wall thickness, a second section having a second wall thickness, and a first magnetic field section positioned between the first and second sections. The first magnetic field section has a third wall thickness that is thinner than the first and second wall thicknesses. The x-ray tube assembly also includes a target positioned in the target portion of the vacuum housing, and a cathode positioned in the cathode portion of the vacuum housing to direct a stream of electrons toward the target.

In accordance with another aspect of the invention, an imaging system includes a rotatable gantry having an opening therein for receiving an object to be scanned, a table positioned within the opening of the rotatable gantry and moveable through the opening, and an x-ray tube coupled to the rotatable gantry. The x-ray tube includes a vacuum chamber having a target portion housing a target, a cathode portion housing a cathode, and a throat portion. The throat portion has a first section having a first wall thickness, a second section having a second wall thickness, and a first magnetic field section coupled to the first and second sections. The first magnetic field section has a third wall thickness that is thinner than the first and second wall thicknesses. The imaging system also includes a first electron manipulation coil mounted on the x-ray tube and configured to generate a first magnetic field to manipulate a stream of electrons emitted from the cathode. The first electron manipulation coil is mounted on the x-ray tube and aligned with the first magnetic field section of the throat portion of the vacuum chamber such that a rise time of the first magnetic field is faster in the first magnetic field section than in the first and second sections.

DETAILED DESCRIPTION

The operating environment of embodiments of the invention is described with respect to a computed tomography (CT) system. It will be appreciated by those skilled in the art that embodiments of the invention are equally applicable for use with any multi-slice configuration. Moreover, embodiments of the invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that embodiments of the invention are equally applicable for the detection and conversion of other high frequency electromagnetic energy. Embodiments of the invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems, surgical C-arm systems, and other x-ray tomography systems as well as numerous other medical imaging systems implementing an x-ray tube, such as x-ray or mammography systems.

FIG. 1is a block diagram of an embodiment of an imaging system10designed both to acquire original image data and to process the image data for display and/or analysis in accordance with the present invention. It will be appreciated by those skilled in the art that the present invention is applicable to numerous medical imaging systems implementing an x-ray tube, such as x-ray or mammography systems. Other imaging systems such as computed tomography systems and digital radiography systems, which acquire image three dimensional data for a volume, also benefit from the present invention. The following discussion of x-ray system10is merely an example of one such implementation and is not intended to be limiting in terms of modality.

Referring toFIG. 1, a computed tomography (CT) imaging system10is shown as including a gantry12representative of a “third generation” CT scanner. Gantry12has an x-ray tube assembly or x-ray source assembly14that projects a cone beam of x-rays toward a detector assembly or collimator16on the opposite side of the gantry12. Referring now toFIG. 2, detector assembly16is formed by a plurality of detectors18and data acquisition systems (DAS)20. The plurality of detectors18sense the projected x-rays22that pass through a medical patient24, and DAS20converts the data to digital signals for subsequent processing. Each detector18produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient24. During a scan to acquire x-ray projection data, gantry12and the components mounted thereon rotate about a center of rotation26.

Rotation of gantry12and the operation of x-ray source assembly14are governed by a control mechanism28of CT system10. Control mechanism28includes an x-ray controller30that provides power and timing signals to an x-ray source assembly14and a gantry motor controller32that controls the rotational speed and position of gantry12. An image reconstructor34receives sampled and digitized x-ray data from DAS20and performs high speed reconstruction. The reconstructed image is applied as an input to a computer36which stores the image in a mass storage device38. Computer36also has software stored thereon corresponding to electron beam positioning and magnetic field control, as described in detail below.

Computer36also receives commands and scanning parameters from an operator via console40that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display42allows the operator to observe the reconstructed image and other data from computer36. The operator supplied commands and parameters are used by computer36to provide control signals and information to DAS20, x-ray controller30and gantry motor controller32. In addition, computer36operates a table motor controller44which controls a motorized table46to position patient24and gantry12. Particularly, table46moves patient24through a gantry opening48ofFIG. 1in whole or in part.

FIG. 3illustrates a cross-sectional view of x-ray tube assembly14according to an embodiment of the invention. X-ray tube assembly14includes an x-ray tube50that includes a vacuum chamber or enclosure52having a cathode assembly54positioned in a cathode portion56thereof. A rotating target58is positioned in a target portion60of vacuum enclosure or housing52. Cathode assembly54includes a number of separate elements, including a cathode cup (not shown) that supports a filament62and serves as an electrostatic lens that focuses a beam of electrons64emitted from heated filament62toward a surface66of target58. A stream of x-rays68is emitted from surface66of target58and is directed through a window70of vacuum enclosure52. A number of electrons72are backscattered from target58and impact and heat an inner surface74of vacuum enclosure52. A coolant is circulated along an outer surface76of vacuum enclosure52, as illustrated by arrows78,80to mitigate heat generated in vacuum enclosure52by backscattered electrons72.

A magnetic assembly82is mounted in x-ray tube assembly14at a location near the path of electron beam64within a throat portion84of vacuum enclosure52, which is downstream from cathode portion56and upstream from target portion60. Magnetic assembly82includes a first coil assembly86. According to one embodiment, coil86is wound as a quadrupole and/or dipole magnetic assembly and is positioned over and around throat portion84of vacuum chamber52such that a magnetic field generated by coil86acts on electron beam64, causing electron beam64to deflect and move along either the x- and/or y-directions. The direction of movement of electron beam64is determined by the direction of current flow though coil86, which is controlled via a control circuit92coupled to coil86. According to another embodiment, coil86is configured to control a focal spot size or geometry. Optionally, a second coil assembly94(shown in phantom) may also be included in magnetic assembly82, as shown inFIG. 3. Coil assemblies86,94may have dipole and/or quadrupole configurations, according to various embodiments and based on a desired electron beam control.

Embodiments of the invention set forth herein reduce the generation of eddy currents within the section of the x-ray tube throat84that is aligned with coil assemblies86,94, which allows the desired magnetic field to develop more rapidly. Eddy currents are developed in throat section84whenever the magnetic field is changing in magnitude, spatially or temporally. Eddy currents are not present when the magnetic field is unchanging. Consequently, the embodiments set forth herein are directed toward reducing the eddy current generation that would take place in a baseline metal throat section that is of a uniform cross-sectional thickness and volume, while simultaneously maintaining desired design specifications of throat section84. Such design specifications may be, for example, that throat section84is hermetic, structurally robust to resist atmospheric pressure and other applied forces, thermally robust to heating primarily due to backscattered electrons, electrically conducting on an inside surface to provide a conduction path for collected charge, and joinable to cathode section56and target section60of vacuum enclosure52.

FIG. 4is an enlarged view of a subportion96ofFIG. 3that includes coil assembly86(FIG. 3) and a throat wall98that is a portion of throat84of vacuum enclosure52(FIG. 3), according to one embodiment of the invention. Vacuum wall98includes a magnetic field section100, which has an upstream end102and a downstream end104. Magnetic field section100is defined as an area of throat portion84between coil assembly86and beam of electrons64that experiences the primary magnetic field generated by coil assembly86. In other words, magnetic field section100experiences the maximum magnetic flux density generated in throat portion84by coil assembly86. As shown inFIG. 4, magnetic field portion100has a wall thickness106that is smaller than a wall thickness108of an upstream section110of wall98, which is upstream of coil assembly86. A first end112of upstream section110is coupled to upstream end102of magnetic field section100, and a second end114of upstream section110is coupled to cathode portion56(FIG. 3). Likewise, wall thickness106of magnetic field section100is smaller than a wall thickness116of a downstream section118of wall98. Downstream section118includes a first end120and a second end122. As shown inFIG. 4, first end120is coupled to downstream end104of magnetic field section100.

The eddy current magnitude developed in throat section84is proportional to the amount or thickness of the throat. Therefore, a thinner throat section where the magnetic flux density is highest will generate less eddy currents and therefore the magnetic field rise rate will be faster. Accordingly, because wall thickness106is less than thicknesses108, a magnetic field generated by coil assembly86has a faster rise time in magnetic field section100than in upstream section110. Likewise, because wall thickness106is less than thickness116, the magnetic field generated by coil assembly86has a faster rise time in magnetic field section100than in downstream section118. According to one embodiment, the decreased thickness of section100may result in a 50% improvement in the magnetic field rise time in magnetic field section100as compared to a metallic throat wall through having a uniform thickness. The larger thickness116of sections110and118allow for a more thermal and structurally sound vacuum throat.

Further, the thicker wall thickness108of non-magnetic field sections110,118provides structural integrity to throat84and provides a larger mass of metal to absorb the heat from backscattered electrons124. According to one embodiment, magnetic field section100has a wall thickness106of approximately 0.5 mm and a wall length126of approximately 1 cm. An outer diameter128of wall98is the same throughout magnetic field section100and upstream and downstream sections110,118. The thinned window section106is shown formed by material removed from the vacuum-side111of throat84. This aids the throat cooling flow on the exterior of the vacuum throat by leaving a smooth outer surface113. In alternative embodiment, the thinned section may be formed in the opposite manner, that is, with a smooth inner surface115and material removed from the outer surface113. Wall98is a non-ferromagnetic material having a high electrical resistivity to minimize eddy current development, such as, for example, molybdenum alloys), stainless steel, or a titanium alloys, according to various embodiments. One skilled in the art will recognize that other materials of low electrical conductivity, high thermal conductivity and structural soundness may also be used.

Referring now toFIG. 5, an enlarged view of subportion96ofFIG. 3is shown according to an embodiment wherein magnetic assembly82(FIG. 3) includes two coil assemblies86,94. Wall130of throat portion84is configured in a similar manner as wall98ofFIG. 4such that a first magnetic field section132, corresponding to coil assembly86has a wall thickness134that is less than a wall thickness136of a first section138and less than a wall thickness140of a second section142, which are adjacent to first magnetic field section132. Likewise, second magnetic field section144has a wall thickness146that is less than wall thickness140of second section142and less than a wall thickness148of a third section150, which are adjacent to second magnetic field section144, as shown inFIG. 5.

FIG. 6illustrates an enlarged view of subportion96ofFIG. 3according to another embodiment of the invention. Subportion96includes a throat wall152that has a magnetic field section154aligned with coil assembly86. Unlike wall98(FIG. 4), wall152ofFIG. 6is constructed in two parts: a metal part156and a non-metal part158. Metal part156includes a metal magnetic field section160and first and second sections162,164, which are adjacent to and upstream and downstream of metal magnetic field section160, respectively, similar to wall98ofFIG. 4. Metal part156has a substantially uniform inner diameter166. An outer diameter168in first and second sections162,164is larger than an outer diameter170of throat wall152in metal magnetic field section160. Thus, wall152is thinner in magnetic field section160than in first and second sections162,164. In one embodiment, metal part156is a non-ferromagnetic material having a high electrical resistivity similar to prior described embodiments.

Non-metal part158of wall152comprises an insulator or electrically non-conducting material that is brazed or otherwise intimately joined onto an outside surface172of thinned areas of metal magnetic field section160. According to various embodiments, non-metal part158may be graphite, alumina, aluminum nitride, or silicon nitride, as examples. Because non-metal part158provides structural support and additional thermal storage capacity for the thinned metal magnetic field section174of wall152, metal magnetic field section174may be designed to be thinner than magnetic field portion100ofFIG. 4. For example, according to one embodiment metal magnetic field section160has a wall thickness174of approximately 0.1-0.2 mm. By thinning throat wall152in metal magnetic field section160, eddy current generation is minimized inside throat portion84. Further, the thinned wall of metal magnetic field section160minimizes the ramp rate of the magnetic field inside throat84, thereby improving deflection and/or focusing time of the electron beam.

According to one embodiment, non-metal part158is a continuous ring or donut of material surrounding non-magnetic field portion154of metal part156. Alternatively, as shown inFIG. 7, non-metal part158may be a number of individual sections of non-metallic material inserted at locations on throat wall152proximate to individual poles176of a coil assembly, such as, for example coil assembly86.

While embodiments of subportion96ofFIG. 3are described inFIGS. 6 and 7as including one coil assembly, one skilled in the art will recognize that such embodiments may be modified for an x-ray tube assembly having a pair of, or more, coil assemblies in a similar manner as described with respect toFIGS. 4 and 5for focusing the electron beam in length and width directions and deflecting the electron beam along two axes.

Referring now toFIG. 8, a package/baggage inspection system242includes a rotatable gantry244having an opening246therein through which packages or pieces of baggage may pass. The rotatable gantry244houses a high frequency electromagnetic energy source248as well as a detector assembly250having detectors similar to those shown inFIG. 2. A conveyor system252is also provided and includes a conveyor belt254supported by structure256to automatically and continuously pass packages or baggage pieces258through opening246to be scanned. Objects258are fed through opening246by conveyor belt254, imaging data is then acquired, and the conveyor belt254removes the packages258from opening246in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages258for explosives, knives, guns, contraband, etc.

Therefore, in accordance with one embodiment, an x-ray tube assembly includes a vacuum enclosure that has a cathode portion, a target portion, and a throat portion. The throat portion includes a magnetic field section having an upstream end and a downstream end. The magnetic field section has a first susceptibility to generate eddy currents in the presence of a magnetic field intensity. The throat portion also has an upstream section having a first end and a second end. The first end of the throat portion is coupled to the cathode portion and the second end of the throat portion is coupled to the upstream end of the magnetic field section. The upstream section has a second susceptibility to generate eddy currents in the presence of the magnetic field intensity. The throat portion also has a downstream section that has a first end and a second end. The first end of the downstream section is coupled to the downstream end of the magnetic field section. The downstream section has a third susceptibility to generate eddy currents in the presence of the magnetic field intensity. The first susceptibility to generate eddy currents is less than the second and third susceptibilities to generate eddy currents. The x-ray tube assembly also includes a target positioned within the target portion of the vacuum enclosure, and a cathode positioned within the cathode portion of the vacuum enclosure. The cathode is configured to emit a stream of electrons toward the target.

In accordance with another embodiment, an x-ray tube assembly includes a housing having a vacuum formed therein. The housing includes a cathode portion, a target portion, and a throat portion. The throat portion includes a first section having a first wall thickness, a second section having a second wall thickness, and a first magnetic field section positioned between the first and second sections. The first magnetic field section has a third wall thickness that is thinner than the first and second wall thicknesses. The x-ray tube assembly also includes a target positioned in the target portion of the vacuum housing, and a cathode positioned in the cathode portion of the vacuum housing to direct a stream of electrons toward the target.

In accordance with yet another embodiment, an imaging system includes a rotatable gantry having an opening therein for receiving an object to be scanned, a table positioned within the opening of the rotatable gantry and moveable through the opening, and an x-ray tube coupled to the rotatable gantry. The x-ray tube includes a vacuum chamber having a target portion housing a target, a cathode portion housing a cathode, and a throat portion. The throat portion has a first section having a first wall thickness, a second section having a second wall thickness, and a first magnetic field section coupled to the first and second sections. The first magnetic field section has a third wall thickness that is thinner than the first and second wall thicknesses. The imaging system also includes a first electron manipulation coil mounted on the x-ray tube and configured to generate a first magnetic field to manipulate a stream of electrons emitted from the cathode. The first electron manipulation coil is mounted on the x-ray tube and aligned with the first magnetic field section of the throat portion of the vacuum chamber such that a rise time of the first magnetic field is faster in the first magnetic field section than in the first and second sections.