Patent Publication Number: US-7903787-B2

Title: Air-cooled ferrofluid seal in an x-ray tube and method of fabricating same

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to x-ray tubes and, more particularly, to an air-cooled ferrofluid seal in an x-ray tube and a method of assembling same. 
     X-ray systems typically include an x-ray tube, a detector, and a bearing 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 then emits 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 a computed tomography (CT) package scanner. 
     X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. In addition, the rotor, as well, is placed in the vacuum region of the x-ray tube. Wear of the silver and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation. Placement of the bearing assembly in the vacuum region prevents lubricating with wet bearing lubricants, such as grease or oil, and performing maintenance on the bearing assembly to replace the solid lubricant. 
     In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode run speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions. Placing the bearing assembly and rotor outside the vacuum region of the x-ray tube by use of a hermetic rotating seal such as a ferrofluid seal allows the use of wet lubricants, such as grease or oil, to lubricate the bearing assembly. 
     A ferrofluid seal typically includes a series of annular regions between a rotating component and a non-rotating component. The annular regions are occupied by a ferrofluid that is typically a hydrocarbon-based or fluorocarbon-based oil with a suspension of magnetic particles therein. The particles are coated with a stabilizing agent, or surfactant, which prevents agglomeration of the particles and allows the particles to remain in suspension in the matrix fluid. When in the presence of a magnetic field, the ferrofluid is polarized and is caused to form a seal between each of the annular regions. The seal on each annular region, or stage, can separately withstand pressure of typically 1-3 psi and, when each stage is placed in series, the overall assembly can withstand pressure varying from atmospheric pressure on one side to high vacuum on the other side. 
     The ferrofluid seal allows rotation of a shaft therein designed to deliver mechanical power from the motor to the anode. As such, the motor rotor may be placed outside the vacuum region to enable a conventional grease-lubricated or oil-lubricated bearing assembly to be placed on the same side of the seal as the rotor to support the target. Furthermore, such bearings may be larger than those typically used on the vacuum side. 
     During operation, liquid coolant passing through the shaft may serve as coolant for the conventional bearings or for cooling the ferrofluid seal to operate in its designed range. The target, too, may be cooled via the liquid coolant in the shaft. However, although liquid cooling provides benefits due to enhancement in energy diffusion to and within the working fluid, such a solution typically includes a rotating liquid seal between a rotating shaft and a stationary supply line. Because of the high G loads during operation and the high speed rotation of the shaft, such seals introduce a reliability risk to the system and may lead to a leak of liquid, causing damage to the x-ray tube or the equipment in which it is installed. Further, rotating liquid seals add cost and complexity, to not only the x-ray tube, but also to the equipment needed to supply the liquid coolant. 
     Therefore, it would be desirable to design an x-ray tube having a ferrofluid assembly therein that is cooled without the need for a rotating liquid seal. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention provides an apparatus for improving an x-ray tube with a ferrofluid seal that overcomes the aforementioned drawbacks. 
     According to one aspect of the invention, an x-ray tube includes a rotatable shaft having a first end and a second end, a target coupled to the first end of the rotatable shaft, the target positioned to generate x-rays toward a subject upon impingement of electrons thereon, and an impeller coupled to the second end of the rotatable shaft and positioned to blow a gas into an inlet of an aperture passing into the rotatable shaft. 
     In accordance with another aspect of the invention, a method of fabricating an x-ray tube includes attaching a target to a first end of a rotatable shaft, forming a passageway in the rotatable shaft, the passageway configured to pass a fluid therein, and coupling a bladed wheel to the passageway at a second end of the rotatable shaft, the bladed wheel configured to pressurize a gas at an inlet of the passageway. 
     Yet another aspect of the invention includes an imaging system includes a detector and an x-ray tube. The x-ray tube includes a rotatable shaft having a first end and a second end, and having a cooling passage therein, and an anode attached to the rotatable shaft at the first end and configured to emit x-rays toward the detector. The imaging system includes a pressurizing device configured to force a gas into an inlet of the cooling passage. 
     Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate preferred embodiments presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a block diagram of an imaging system that can benefit from incorporation of an embodiment of the invention. 
         FIG. 2  illustrates a cross-sectional view of an x-ray tube according to an embodiment of the invention. 
         FIG. 3  illustrates a cross-sectional view of a ferrofluid seal assembly according to the invention. 
         FIG. 4  illustrates a cross-sectional view of an x-ray tube according to an embodiment of the invention. 
         FIG. 5  illustrates a cross-sectional view of an x-ray tube according to an embodiment of the invention. 
         FIG. 6  illustrates an assembly procedure according to an embodiment of the invention. 
         FIG. 7  is a pictorial view of an x-ray system for use with a non-invasive package inspection system incorporating embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an embodiment of an x-ray imaging system  2  designed both to acquire original image data and to process the image data for display and/or analysis in accordance with the invention. It will be appreciated by those skilled in the art that the 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 (CT) systems and digital radiography (RAD) systems, which acquire image three dimensional data for a volume, also benefit from the invention. The following discussion of imaging system  2  is merely an example of one such implementation and is not intended to be limiting in terms of modality. 
     As shown in  FIG. 1 , imaging system  2  includes an x-ray tube or source  4  configured to project a beam of x-rays  6  through an object  8 . Object  8  may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source  4  may be a conventional x-ray tube producing x-rays having a spectrum of energies that range, typically, from 30 keV to 200 keV. The x-rays  6  pass through object  8  and, after being attenuated by the object, impinge upon a detector  10 . Each detector in detector  10  produces an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  8 . In one embodiment, detector  10  is a scintillation based detector, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, etc.) may also be implemented. 
     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 . In one embodiment, imaging system  2  includes a pressurizing device  24  (shown in phantom) that is external to x-ray source  4  and configured to pressurize a coolant and feed it to x-ray source  4 , as will be described. 
       FIG. 2  illustrates a cross-sectional view of x-ray source  4  incorporating embodiments of the invention. The x-ray source  4  includes a frame  26 , a mount structure  28 , and an anode backplate  30 . Mount structure  28  is configured to attach x-ray source  4  to an imaging system, such as imaging system  2  of  FIG. 1 . A radiation emission passage  32  allows x-rays  6  to pass therethrough. Frame  26  and anode backplate  30  enclose an x-ray tube vacuum volume  34 , which houses a target, or anode  36 , a bearing assembly  38 , and a cathode  40 . A center shaft  42  includes a flange  44  attached to anode  36  via welding, brazing, a bolted joint, and the like. 
     X-rays  6  are produced when high-speed electrons are suddenly decelerated when directed from the cathode  40  to the anode  36  via a potential difference therebetween of, for example, 60 thousand volts or more in the case of CT applications. The x-rays  6  are emitted through radiation emission passage  32  toward a detector array, such as detector  10  of  FIG. 1 . To avoid overheating the anode  36  from the electrons, a rotor  46  and center shaft  42  rotate the anode  36  at a high rate of speed about a centerline  48  at, for example, 90-250 Hz. Anode  36  is attached to center shaft  42  at a first end  50 , and the rotor  46  is attached to center shaft  42  at a second end  52 . 
     The bearing assembly  38  includes a front bearing  54  and a rear bearing  56 , which support center shaft  42  to which anode  36  is attached. In a preferred embodiment, front and rear bearings  54 ,  56  are lubricated using grease or oil. Front and rear bearings  54 ,  56  are attached to center shaft  42  and are mounted in a stem or bearing housing  58 , which is supported by anode backplate  30 . A stator  60  rotationally drives rotor  46  attached to center shaft  42 , which rotationally drives anode  36 . 
     A mounting plate  62 , a stator housing  64 , a stator mount structure  66 , stem  58 , and a ferrofluid seal assembly  68  surround an antechamber  70  into which bearing assembly  38  and rotor  46  are positioned and into which the second end  52  of center shaft  42  extends. Center shaft  42  extends from antechamber  70 , through ferrofluid seal assembly  68 , and into x-ray tube vacuum volume  34  and may include a coolant line or passageway therein (not shown in  FIG. 2 ), and center shaft  42  may include an impeller attached thereto, as will be discussed below. The ferrofluid seal assembly  68  hermetically seals x-ray tube vacuum volume  34  from antechamber  70 . A cooling passage  72  carries coolant  74  through anode backplate  30  and into stem  58  to cool ferrofluid seal assembly  68  thermally connected to stem  58 . 
     In addition to the rotation of the anode  36  within x-ray source  4 , in a CT application, the x-ray source  4  as a whole is caused to rotate about an object at rates of, typically, 1 Hz or faster. The rotational effects of both cause the anode  36  weight to be compounded significantly, hence leading to large operating contact stresses in the bearings  54 ,  56 . 
       FIG. 3  illustrates a cross-sectional view of the ferrofluid seal assembly  68  of  FIG. 2 . A pair of annular pole pieces  76 ,  78  abut an interior surface  80  of stem  58  and encircle center shaft  42 . An annular permanent magnet  82  is positioned to include a magnet or pole spacer  83  between annular pole piece  76  and annular pole piece  78 . In embodiments of the invention, pole pieces  76 ,  78  and magnet spacer  83  are brazed, welded, or machined as a single piece, forming a hermetic assembly. In a preferred embodiment, center shaft  42  includes annular rings  84  extending therefrom toward annular pole pieces  76 ,  78 . Alternatively, however, annular pole pieces  76 ,  78  may include annular rings extending toward center shaft  42  instead of, or in addition to, annular rings  84  of center shaft  42 . A ferrofluid  86  is positioned between each annular ring  84  and corresponding annular pole pieces  76 ,  78 , thereby forming cavities  88 . Magnetization from annular permanent magnet  82  retains the ferrofluid  86  positioned between each annular ring  84  and corresponding annular pole pieces  76 ,  78  in place. In this manner, multiple stages of ferrofluid  86  are formed that hermetically seal the pressure of gas in the antechamber  70  of  FIG. 2  from a high vacuum formed in x-ray tube vacuum volume  34 . As shown,  FIG. 3  illustrates 8 stages of ferrofluid  86 . Each stage of ferrofluid  86  withstands 1-3 psi of gas pressure. Accordingly, one skilled in the art will recognize that the number of stages of ferrofluid  86  may be increased or decreased, depending on the difference in pressure between the antechamber  70  and the x-ray tube vacuum volume  34 . 
       FIG. 4  illustrates an x-ray tube according to an embodiment of the invention. X-ray tube  90  includes a vacuum enclosure or frame  92  that contains a vacuum  94  and encloses an anode or target  96  and a cathode  98 . Target  96  is coupled to and supported by a shaft  100  at a first end  102  thereof, and in embodiments of the invention, the coupling is via a bolted joint, a welded joint, a braze joint, and the like. Shaft  100  is coupled to target  96  via a rim or flange  104 . In one embodiment, flange  104  and shaft  100  are fabricated from a single material, and in another embodiment, flange  104  is attached to shaft  100  via a braze joint, a weld joint, and the like. 
     Shaft  100  is supported by bearings  106  that are housed in a stem  108 . A single-stage or multi-stage ferrofluid seal assembly  110  includes an aperture  112  therein, the aperture having a diameter  114 . Ferrofluid seal assembly  110  is positioned between target  96  and bearings  106  and is configured to fluidically separate vacuum  94  from an environment  116 . Thus, ferrofluid seal assembly  110  includes a vacuum end  118  and an atmospheric pressure or pressurized end  120 , the pressure end  120  in fluidic contact with environment  116 . Environment  116  contains bearings  106  and a rotor  122 , and rotor  122  is attached to shaft  100  at a second end  124 . A stator  126  is positioned proximately to rotor  122 . In one embodiment, shaft  100  includes an opening, passageway or aperture  128 , and a diffuser or tube wall  130  that is stationary with respect to frame  92  of x-ray tube  90  or rotating having a shaft internally supported by annular supports  131  that form partial axial passages and which allow cooling fluid to pass therethrough. Wall  130  is positioned to separate flow such that an inlet is formed inside wall  130  and an outlet is formed outside wall  130 . An impeller  132  is attached to rotor  122  via an impeller mounting structure  134 , and a region  136  proximate impeller  132  is fed by a coolant or gas (such as air or an inert gas such as nitrogen, argon, and the like) via a coolant supply line  138 . In an embodiment of the invention, impeller  132  causes coolant to be pressurized and to flow into aperture  128  as will be discussed below. While impeller  132  is illustrated as being attached to rotor  122  via mounting structure  134 , impeller  132  may be attached to any of the rotating components therein, thus being caused to rotate and pressurize the coolant. 
     Thus, in operation, as anode  96  is caused to rotate via rotor  122 , impeller  132  rotates therewith, causing the coolant to pressurize and pass into aperture  128  at an inlet  140  and to flow along shaft  100  and along an inner diameter  142  of stationary or rotatable wall  130  to first end  102 . The coolant then passes along an outer diameter  144  of stationary or rotatable wall  130  and out to environment  116  and therebeyond. In one embodiment, impeller  132  is foregone, and an impeller external to x-ray tube  90  (such as pressurizing device  24  of  FIG. 1 ) is used as the motive mechanical power behind the coolant, causing it to flow therein. As such, coolant passing therein causes ferrofluid seal assembly  110  and bearings  106  to decrease in temperature, while drawing heat from anode  96  via flange  104 . In one embodiment, stationary or rotatable wall  130  includes jets or apertures  146  therein that are positioned to impinge coolant and enhance turbulence in preferred locations of shaft  100 , such as in the region of the ferrofluid seal assembly  110  or in the region of the bearings  106 . Thus, as coolant passes through aperture  128  of shaft  100 , convective heat transfer occurs which increases rates of heat transfer above that of typical conduction in metal. The convection may be increased by increasing the heat transfer coefficients therein by providing jets or apertures  146 . In another embodiment, gas is pressurized prior to entering coolant supply line  138  via a pressurizing device  24  that is external to x-ray source  4  and may be part of imaging system  2 . 
       FIG. 5  illustrates x-ray tube  90  according to another embodiment of the invention. As with  FIG. 4 , x-ray tube  90  includes ferrofluid seal assembly  110  having shaft  100  passing therethrough, shaft  100  having flange  104  at first end  102  and rotor  122  at second end  124 . Shaft  100  includes bearings  106  that are housed in stem  108 . Impeller  132  is attached to shaft  100  via impeller mounting structure  134 , and target  96  is attached to flange  104 . However, in this embodiment, shaft  100  includes a tapered aperture  148 , which increases in diameter in a direction from the first end  102  to the second end  124 . Tapered aperture  148  is configured to ease flow of a coolant to pass therethrough due to coolant buoyancy, and shaft  100  includes stationary or rotatable wall  130  passing therein. 
     Thus, in operation, anode  96  is caused to rotate via rotor  122  and impeller  132  rotates therewith, causing coolant to pressurize and pass into tapered aperture  148 . The coolant passes along shaft  100  and along inner diameter  142  of stationary wall  130  to first end  102 , then passes along outer diameter  144  of stationary wall  130  and out to environment  116  and therebeyond. However, in this embodiment, because of the taper of tapered aperture  148 , coolant passes therethrough having a reduced pressure drop when compared to, for instance, coolant passing through aperture  128  of  FIG. 4  and takes advantage of coolant buoyancy, as understood by those skilled in the art. In addition, because of the tapered nature of tapered aperture  148  and the resulting variable thickness of shaft  100  along its length, one skilled in the art will recognize that favorable rotordynamic behavior may result, as well, such that a natural frequency of shaft  100  may be different from a runspeed of shaft  100 . 
     Referring back to  FIG. 4 , x-ray tube  90  is configured to be assembled by inserting second end  124  of shaft  100  through ferrofluid seal assembly  110  in a direction  150 , wherein shaft  100  first passes through ferrofluid seal assembly  110  and then through stem  108 . As such, a maximum diameter  152  of shaft  100  is selected such that shaft  100  is insertable through aperture  128  of ferrofluid seal assembly  110  without interference. 
       FIG. 6  illustrates an assembly procedure  154  for anode  36  of x-ray tube  90  according to an embodiment of the invention. According to this embodiment, shaft  100  is fabricated having flange  104  attached thereto at step  156 . According to one embodiment of the invention, shaft  100  is first fabricated having flange  104  attached thereto via a weld joint, a braze joint, and the like. According to another embodiment of the invention, the shaft/flange combination  100 / 104  is fabricated from a single piece of material, such as a stainless steel. The target  96  may be attached to flange  104  at  158 . However, it is contemplated that target  96  may be instead be attached to flange  104  after any of steps  160 - 166  in process  154 . At step  160 , stem  108  is provided having ferrofluid seal assembly  110  attached thereto. Ferrofluid is applied to the shaft  100  at step  162 , and the shaft  100  is inserted through the ferrofluid seal assembly  110  from the vacuum end  118  toward the pressure end  120  at step  164 . After the shaft is inserted at step  164 , bearings  106  and rotor  122  are attached to shaft  100  at step  166 . Thus, because shaft  100  is inserted from the vacuum end  118  toward the pressure end  120 , target  96  may be attached to flange  104  prior to or after inserting shaft  100  through ferrofluid seal assembly  110  at step  164 . 
       FIG. 7  is a pictorial view of an x-ray system  500  for use with a non-invasive package inspection system. The x-ray system  500  includes a gantry  502  having an opening  504  therein through which packages or pieces of baggage may pass. The gantry  502  houses a high frequency electromagnetic energy source, such as an x-ray tube  506 , and a detector assembly  508 . A conveyor system  510  is also provided and includes a conveyor belt  512  supported by structure  514  to automatically and continuously pass packages or baggage pieces  516  through opening  504  to be scanned. Objects  516  are fed through opening  504  by conveyor belt  512 , imaging data is then acquired, and the conveyor belt  512  removes the packages  516  from opening  504  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  516  for explosives, knives, guns, contraband, etc. One skilled in the art will recognize that gantry  502  may be stationary or rotatable. In the case of a rotatable gantry  502 , system  500  may be configured to operate as a CT system for baggage scanning or other industrial or medical applications. 
     Thus, because of the improved assembly procedure, x-ray tube  90  includes a flange  104  that is larger than the aperture  112  that passes through ferrofluid seal assembly  110 . Flange  104  may include a diameter having an increased amount of surface contact area with target  96  as compared with prior art devices and may also accommodate a bolted joint, as an example. Such an increase in surface contact area improves conduction heat transfer through the joint, allowing an increased amount of heat to conduct to shaft  100 . Thus, coolant passing through shaft  100  may not only serve to cool the ferrofluid seal assembly  110  and the bearings  106 , but also to extract additional heat from the target  96 . 
     In addition, because the target  96  may be attached to flange  104  prior to assembly of the shaft  100  into aperture  112 , target  96  may be attached to flange  104  via high temperature processes such as brazing and welding, as examples, to minimize negative effects to the ferrofluid of ferrofluid seal assembly  110 . 
     Further, because of the impeller  132  mounted at second end  124  of shaft  100 , air or other coolant may be forced or pressurized into a cavity or aperture  128  during operation of x-ray tube  90  and rotation of target  96 , thus further enhancing the cooling of target  96  and heat transfer along shaft  100 . 
     Therefore, according to one embodiment of the invention, an x-ray tube includes a rotatable shaft having a first end and a second end, a target coupled to the first end of the rotatable shaft, the target positioned to generate x-rays toward a subject upon impingement of electrons thereon, and an impeller coupled to the second end of the rotatable shaft and positioned to blow a gas into an inlet of an aperture passing into the rotatable shaft. 
     In accordance with another embodiment of the invention, a method of fabricating an x-ray tube includes attaching a target to a first end of a rotatable shaft, forming a passageway in the rotatable shaft, the passageway configured to pass a fluid therein, and coupling a bladed wheel to the passageway at a second end of the rotatable shaft, the bladed wheel configured to pressurize a gas at an inlet of the passageway. 
     Yet another embodiment of the invention includes an imaging system includes a detector and an x-ray tube. The x-ray tube includes a rotatable shaft having a first end and a second end, and having a cooling passage therein, and an anode attached to the rotatable shaft at the first end and configured to emit x-rays toward the detector. The imaging system includes a pressurizing device configured to force a gas into an inlet of the cooling passage. 
     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.