Patent Application
20140105365
Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an apparatus for forming an expansion joint and a method of constructing same.
Computed tomography (CT) X-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support 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 converts the received radiation to electrical signals and 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.
A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of cooling the target. Components of the x-ray tube are placed in a ultra-high vacuum which is maintained by a frame that is typically made of metal or glass.
The x-ray tube also includes a rotating subsystem that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an 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 subsystem assembly is driven by the stator. Typically, the target is supported by a bearing assembly in a cantilever type arrangement. The bearing assembly is comprised of a front inner/outer bearing race and a rear inner/outer bearing race, ball bearings, and a shaft extends therefrom to support the target. The bearing assembly is axially anchored on one end such that, in a typical design the shaft supporting the target is able to expand and contract freely during operation and as a result of the extreme temperatures experienced during operation.
In recent years, it has been desired within the CT industry to increase gantry speeds to 0.4 seconds gantry rotation and faster. As the industry drives to faster gantry speeds, the mechanical loading on x-ray tubes has increased as well. Generally the mechanical loading on an x-ray tube increases as the square of the gantry rotational speed, thus increased gantry speeds have lead to enormous g-loading on the x-ray tube and particularly on the target. Accordingly, the mechanical loading on the support bearing assembly of the target has increased dramatically as well.
As such and in order to accommodate the increased gantry speeds, in some known designs the target is supported by a single shaft, but a flange is incorporated that enables the target to be positioned between the front and rear races of the bearing assembly (sometimes referred to as a reentrant design). This positions the target proximate to both the front and rear races, and in some known designs the target is positioned such the center of gravity of the rotating subsystem is centered between the front and rear races, which enables equal load sharing between the front and rear races. In other known designs a spiral groove bearing (SGB) may be incorporated, in lieu of ball bearing-based bearing assemblies, that provides a much broader distribution of stress over a low vapor fluid (liquid metal fluid) that is positioned between inner and outer components that rotate with respect to one another under a relatively small gaps, approximately 15 microns in one known embodiment. One known fluid in a SGB is gallium.
However, it has been desired in recent years to increase gantry speeds yet more, to 0.25 gantry speeds and faster. As such, known bearing designs may fail either catastrophically or through a shortened life due to wear in these increased g-load conditions. Increased gantry speeds can also cause relatively large mechanical deflections of the target support structure (shaft, bearing & target) that can cause focal spot motion or other sources of image quality problems. Thus, in order to enable operation in 0.25 seconds gantry speed and faster, recent x-ray tube designs have included a shaft that is supported on both axial sides of the target. That is, the rotatable shaft to which the target and rotor are attached may include a bearing stationary support (ball-bearing or SGB, as examples) that is hard connected to a plate or other support structure of the x-ray tube. In other words, in order to accommodate the dramatically increased loads for gantry speeds of 0.25 seconds or greater, it is desirable to support the target with supports that are positioned on both sides of the target, providing a ‘straddle’ support that significantly reduces the concentrated load and deflection on the bearing and removes the cantilever affect of a cantilever-mounted target.
However, in order to do so (that is, to provide the second support) the second support is typically hard mounted to the frame of the x-ray tube. As such, the support mechanically constrains the shaft axially on its second end as well, precluding the shaft from being able to freely expand and contract during operation and during other heating and cooling events.
Typically the components of the x-ray tube are made of different materials for different reasons. For instance, the shaft itself is often made of molybdenum (because of its ability to sustain high temperatures during operation), while the support plate and frame to which the shaft is attached is typically made of a far less expensive material such as stainless steel. Because of the mismatched coefficients of thermal expansion (CTE) and weldability, as examples, kovar is typically included as an interim material between the shaft and the support plate. The frame itself, attached to the support plate and used to enclose the target, rotor, and other components, may be made of 304L, for example. As such, for a variety of reasons that include but are not limited to material cost, processing and machining expense, performance (i.e., high temperature operation), and weldability, a variety of materials is typically used to form the shaft, plate, frame, and other components that support and enclose the target. Because each material has its own axial length, CTE, overall operating temperature and because the shaft is hard mounted at both ends, differential thermal growth can induce high stresses at interfaces (in welds and brazes) and component parts for the variety of thermal conditions experienced.
Because of the very high processing and operating temperatures in x-ray tubes, x-ray tube components such as the target and its supporting shaft are made with refractory metals such as Molybdenum. Molybdenum is characterized by a low coefficient of thermal expansion (CTE) compared to ferrous metals. The supporting shaft is itself supported and enclosed by the vacuum frame and a support plate, which are generally made from an austenitic stainless steel (304), which has a CTE that is approximately three times that of Molybdenum or alloys thereof. Thus, although the target, the supporting shaft, and the vacuum frame and the support plate may not be made of these specific materials, they are nevertheless typically made of materials in which a large CTE difference occurs at interfaces. The differences of material CTEs and the overall length of the relatively large parts can cause large differential thermal growth between the shaft and its linked components. When combined also with a typically relatively high component stiffness for load capability and deflection control, high internal stresses can be induced at the component interfaces that may include weld and braze joints. The weld or braze joints therefore can present modes of failure that may include a vacuum leak at the joint or a mechanical joint failure that can even lead to a catastrophic tube failure.
As such, one known method of reducing stresses in the components and interfaces is to selectively design the components such that the changes in lengths, that result from temperature changes, balance one another (zero differential thermal growth). That is, based on a thermal model, temperature distributions of the component parts may be predicted and then materials and component related geometric length can be selected such that they balance the changes in lengths that can occur as a result of the predicted temperature distributions. For instance, during operation the center shaft made of Molybdenum, although having a lower expansion coefficient than the 304L frame material, the center shaft may nevertheless expand more than the frame because of the much higher temperature at which it the center shaft operates. Thus, in this example, in order to counteract the effect, a material having a higher CTE than 304 L can be included in a portion of the frame (reentrant rotor) such that the parts expand the same amount when the component parts reach their steady state operating temperature. Also nickel based alloys such as Ni42 with lower CTE than SS304L or a hybrid frame assembly made of ceramic, kovar, or nickel base alloys could be used in the frame construction to reduce the overall component thermal growth.
However, although component parts can be designed that minimize the stresses that result at temperature, not all thermal conditions are the same for the x-ray tube. For instance, x-ray tubes operate at a wide range of steady state or average powers, thus one set of assumed steady state thermal conditions may not suffice to minimize stress in the components when a different steady state occurs. One day may see a lot of high power imaging with a heavy patient load, while on other days only low power scans may be conducted. Further and regardless, while heating and cooling, the components experience transient thermal responses (temperature distributions) that can cause stresses to occur, due to differential dynamic expansion during the transients, that can cause stresses to occur even if the stresses are reduced to near zero when they do reach steady state.
In addition, aside from the extreme temperatures experienced during typical x-ray tube operation, during manufacture the x-ray tube may go through significant temperature excursions during processing such as bakeout and seasoning. As one example, during bakeout the entire x-ray tube (frame, support plate, shaft, etc . . . ) is brought to a high temperature (approximately over 400° C.). Typically, the x-ray tube is baked in an oven in order to bring all component parts up to sufficient temperature so as to clean all the exposed surfaces and provide longterm high voltage stability. During bakeout the frame in particular experiences a much higher temperature excursion that typically occurs during normal operation in an x-ray tube. As such, even if component parts are designed in order to survive various steady state and transient conditions, bakeout and other processing steps can cause worse differential thermal growth than those under tube operating conditions.
Thus, when both ends of the stationary shaft of the rotating subsystem are hard mounted to the frame, enormous stresses can result at the component interfaces and at the component itself as the overall system heats due to processing or operating thermal condition from room temperature. The stresses can be reduced to an extent by designing components appropriately such that interfaces and component stresses are within design limits for a given set of thermal conditions. However, an x-ray tube can see a wide variety of steady state and transient conditions, as well as different operating conditions. As such, not all possible sets of thermal conditions can be designed for, and component stresses can occur that can lead to fatigue cycling and/or catastrophic component failure.
Accordingly, it would be advantageous to have an x-ray tube having a robust design with joints between components that can maintain an ultra-high vacuum under a wide range of thermal conditions during operation and processing and overcome the aforementioned drawbacks.
Embodiments of the invention provide an apparatus and method of constructing an apparatus that overcomes the aforementioned drawbacks and maintains an ultra-high vacuum required by the x-ray tube to operate, with low mechanical stresses at the component interfaces.
According to one aspect of the invention, an x-ray tube includes a frame forming a first portion of a vacuum enclosure, a rotating subsystem shaft positioned within the vacuum enclosure and having a first end and a second end, wherein the first end of the rotating subsystem shaft is attached to a first portion of the frame, a target positioned within the vacuum enclosure and attached to the rotating subsystem shaft between the first end and the second end, the target positioned to receive electrons from an electron source positioned within the vacuum enclosure, and a thermal compensator mechanically coupled to the second end of the rotating subsystem shaft and to a second portion of the frame, the thermal compensator forming a second portion of the vacuum enclosure.
In accordance with another aspect of the invention, a method of manufacturing an x-ray tube includes forming a first portion of a vacuum enclosure with a frame, attaching a first end of a rotating subsystem shaft to the frame, coupling a second end of a thermal compensator to the frame, wherein the thermal compensator forms a second portion of the vacuum enclosure, and mechanically coupling a first end of the thermal compensator to a second end of the target support shaft by the rotor can or other component attachment.
Yet another aspect of the invention includes an imaging system that includes a support structure, a detector attached to the support structure, and an x-ray tube attached to the support structure. The x-ray tube includes a vessel forming a portion of a vacuum enclosure, a rotating subsystem shaft positioned within the vacuum enclosure and having a first end and a second end, wherein the first end of the shaft is attached to a portion of the vessel, a target in the vacuum enclosure that is attached to the rotating subsystem shaft between the first end and second ends, the target positioned to receive electrons from a cathode positioned within the vacuum enclosure, and a thermal compensator mechanically coupled to the second end of the shaft and to another portion of the vessel, the compensator forming another portion of the vacuum enclosure.
Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
As shown in
A processor 12 receives the signals from the detector 10 and generates an image corresponding to the object 8 being scanned. A computer 14 communicates with processor 12 to enable an operator, using operator console 16, to control the scanning parameters and to view the generated image. That is, operator console 16 includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the imaging system 2 and view the reconstructed image or other data from computer 14 on a display unit 18. Additionally, operator console 16 allows an operator to store the generated image in a storage device 20 which may include hard drives, flash memory, compact discs, etc. The operator may also use operator console 16 to provide commands and instructions to computer 14 for controlling a source controller 22 that provides power and timing signals to x-ray source 4.
X-rays 6 are produced when high-speed electrons from a primary electron beam are suddenly decelerated when directed from the cathode 60 to the target 56 via a potential difference therebetween. In high voltage CT applications, the potential difference between the cathode 60 and target 56 may be, for example, 60 thousand volts (keV) and up to 140 keV or more. In other applications, the potential difference may be lower. The electrons impact a material layer or target focal track 86 at a focal spot or point and x-rays 6 emit therefrom. The point of impact at focal point 61 is typically referred to in the industry as the focal spot. The x-rays 6 emit through the radiation emission passage 52 toward a detector array, such as detector 10 of
Bearing assembly 58 includes stationary inner shaft 59 and rotatable outer shaft 61 and, in the illustrated embodiment includes a gap 63 therebetween. Gap 63 is filled with a liquid metal such as gallium, and the gallium is maintained in gap 63, as known in the art, using spiral grooves (not shown) on inner and outer surfaces of respective outer shaft 61 and inner shaft 59. Outer shaft 61 includes an axial limiter or thrust bearing 65 that limits or prevents axial motion of outer shaft 61 and therefore of target 56. Inner shaft 59 is supported on a first end 67 by a supporting plate which, as stated, is stationary with respect to target 56. Inner shaft 59 is also supported on a second end 68, therefore the rotating subsystem target 56 is supported at both front and rear ends, causing solid support or ‘straddle’ to form the mechanical support of the rotating subsystem target 56 during operation. The straddle support provides smaller mechanical system deflection in contrast to conventional x-ray tube design in which the rotating subsystem target 56 is supported only on one axial end of outer shaft 61.
X-ray tube 4 includes a support plate 69, a frame 71, and a rotor can 73, in part forming vacuum 54 in which the target 56, outer shaft 61, and rotor 62 of the rotating subsystem are positioned. Because inner shaft 59, support plate 69, frame 71, and rotor can 73 are hard-connected (i.e., physically hard-attached to one another by weld, braze or by a combination of both), it can be understood by one skilled in the art that, if rotor can 73 were also hard-connected to inner shaft 59, then temperature changes due to operation and/or processing of x-ray tube 4 can build enormous stresses between components and component interfaces. Such stresses can lead to component and component interfaces distortion and failure, as stated above.
As such, according to the invention, a thermal compensator assembly 75 is included in which a compensator 77 is used to allow for axial expansion and contraction of components of x-ray tube 4. Thermal compensator 77 is coupled to the frame by direct attachment in one embodiment, and formed as a frame component in another embodiment, as examples. According to one embodiment, thermal compensator 77 is coupled to a target support shaft by a rotor can or other component attachment. Thermal compensator 77 in this embodiment and subsequent embodiments has low mechanical stiffness and allows component thermal induced strains or displacement without high internal and interface component stresses, with a main structural support thru the casing structure in order to improve X ray tube reliability and performance. Thus, the main mechanical load path of the rotating subsystem is thru the casing support structure by a coupling component or shaft adapter and not thru the other tube components or thermal compensator to improve component reliability and tube performance. As such, mechanical stresses therein are significantly reduced as a result of the thermal compensator 77.
The thermal compensator 77 can be manufactured by forming a convolution into a thin wall component (or tube) or by welding individual convolutions together forming a welded assembly. Material selection depends upon mechanical (stiffness and allowable stress and temperature) and weldability or brazebility requirements but must be ultra high vacuum compatible such stainless steels for high voltage applications.
The thermal compensator 75 may be formed or manufactured (assembled) in a number of fashions, according to the invention. According to one embodiment, illustrated in
In other words, in the embodiment illustrated in
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According to an embodiment of the invention, an x-ray tube includes a frame forming a first portion of a vacuum enclosure, a rotating subsystem shaft positioned within the vacuum enclosure and having a first end and a second end, wherein the first end of the rotating subsystem shaft is attached to a first portion of the frame, a target positioned within the vacuum enclosure and attached to the rotating subsystem shaft between the first end and the second end, the target positioned to receive electrons from an electron source positioned within the vacuum enclosure, and a thermal compensator mechanically coupled to the second end of the rotating subsystem shaft and to a second portion of the frame, the thermal compensator forming a second portion of the vacuum enclosure.
According to another embodiment of the invention, a method of manufacturing an x-ray tube includes forming a first portion of a vacuum enclosure with a frame, attaching a first end of a rotating subsystem shaft to the frame, coupling a second end of a thermal compensator to the frame, wherein the thermal compensator forms a second portion of the vacuum enclosure, and mechanically coupling a first end of the thermal compensator to a second end of the target support shaft by the rotor can or other component attachment.
Yet another embodiment of the invention includes an imaging system that includes a support structure, a detector attached to the support structure, and an x-ray tube attached to the support structure. The x-ray tube includes a vessel forming a portion of a vacuum enclosure, a rotating subsystem shaft positioned within the vacuum enclosure and having a first end and a second end, wherein the first end of the shaft is attached to a portion of the vessel, a target in the vacuum enclosure that is attached to the rotating subsystem shaft between the first end and second ends, the target positioned to receive electrons from a cathode positioned within the vacuum enclosure, and a thermal compensator mechanically coupled to the second end of the shaft and to another portion of the vessel, the compensator forming another portion of the vacuum enclosure.
The invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.
