Abstract:
A CT scanner comprises an x-ray window mounted on an x-ray tube, a cooling fluid circulation line, and a cooling fluid return line. A cold-plate is operatively mounted on the x-ray tube around the x-ray window. The cold plate includes an elongated shell and corrugated fins for rapidly removing heat from the x-ray window. The circulation line is in fluid communication with an inlet of the cold-plate, a cooling fluid reservoir defined between the x-ray tube and a surrounding housing, and a heat exchanger. The return line is in fluid communication with an outlet of the cold-plate, the cooling fluid reservoir, and the heat exchanger. A pump circulates the cooling fluid through the heat exchanger, the suction and return lines, the cold-plate, and the x-ray tube housing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the radiographic arts. It finds particular application in conjunction with x-ray tubes for computerized tomographic (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention may also be amenable to x-ray tubes for other applications. 
     CT scanners have commonly included a floor-mounted frame assembly which remains stationary during a scan. An x-ray tube is mounted to a rotatable frame assembly which rotates around a patient receiving examination region during the scan. Radiation from the x-ray tube traverses the patient receiving region and impinges upon an array of radiation detectors. Using the position of the x-ray tube during each sampling, a tomographic image of one or more slices through the patient is reconstructed. 
     The x-ray tube assembly typically comprises a lead lined housing containing a vacuum envelope or x-ray insert which holds a rotating anode and a stationary cathode. Cooling oil is flowed between the x-ray insert and the housing. In large, high performance x-ray tubes, the x-ray insert may be a metal shell or frame with a beryllium window mounted or brazed thereon for allowing the transmission of x-rays from the x-ray insert. Likewise, the housing defines an x-ray output window that is in alignment with the beryllium window of the x-ray insert such that x-rays may pass directly through the beryllium window and the x-ray output window. 
     During x-ray generation, electrons are emitted from a heated filament in the cathode and accelerated to a focal spot area on the anode. Upon striking the anode, some portion of the electrons, or secondary electrons, are bounced to the surrounding frame and converted into heat. The beryllium window receives the highest intensity of the secondary electron heating because the window is closer to the focal spot on the anode. The heat is undesirable and is commonly termed waste heat. One of the persistent problems in CT scanners and other radiographic apparatus is dissipating the waste heat created while generating x-rays. 
     In order to remove the waste heat, a cooling oil is often circulated through the housing and around the x-ray insert forming a cooling jacket around the x-ray insert. For example, oil may be drawn through an output aperture located at one end of the housing, circulated through a radiator or heat exchanger and returned to an inlet aperture in the opposite end of the housing. The returned cooled fluid flows axially through the housing toward the outlet aperture, absorbing heat from the x-ray insert. 
     Removing waste heat in this manner is not always completely effective. More specifically, waste heat removal by merely forcing coolant to flow between the x-ray insert and the housing is particularly ineffective around the x-ray output window. The beryllium window and its environs, being the recipient of the secondary electrons and heat from the closely adjacent focal spot, is preferentially heated. Further, the beryllium window protrudes out from the frame and generally disrupts the flow of coolant around the window preventing optimal cooling. Additionally, the configuration of the x-ray output window on the housing disrupts coolant flow and, by its proximity to the beryllium window, limits the amount of coolant capable of passing over the beryllium window. 
     When the window is not sufficiently cooled, the heat can damage the braze joint between the beryllium window and the x-ray insert causing the x-ray tube to fail. Further, the coolant adjacent to the beryllium window may boil and leave a carbon residue on the beryllium window. Such a coating is undesirable as it may degrade the quality of the x-ray image. 
     The present invention provides a new and improved cooling system and technique for overcoming the above-reference drawbacks and others. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the use of a cold-plate on the x-ray window of an x-ray insert to provide for the removal of undesirable waste heat from an x-ray tube. 
     In accordance with one aspect of the present invention, a CT scanner comprises an x-ray tube assembly mounted on a rotating frame portion. The x-ray tube assembly includes a housing, an x-ray tube operatively mounted within the housing, and a cooling fluid reservoir defined between the x-ray tube and the housing. The cooling fluid reservoir includes an inlet aperture through the housing and an outlet aperture through the housing. The CT scanner also comprises an x-ray window mounted on the x-ray tube, a cooling fluid circulation line, and a cooling fluid return line. The circulation line is in fluid communication with the inlet aperture of the cooling fluid reservoir and with a heat exchanger. The return line is in fluid communication with the heat exchanger and the outlet aperture of the cooling fluid reservoir. The CT scanner additionally comprises a pump and a cold-plate mounted on the x-ray tube around the x-ray window. The pump circulates the cooling fluid through the heat exchanger, the suction and return lines, and the x-ray tube housing assembly. 
     In accordance with another aspect of the present invention, an x-ray tube assembly comprises a housing, an x-ray tube, and a cold-plate. The housing has an x-ray window and defines a housing cavity therein. The x-ray tube includes a vacuum envelope which holds an anode and a cathode. The vacuum envelope has an x-ray translucent window adjacent the anode. The x-ray tube is mounted in the housing cavity spaced from the housing to define a cooling fluid reservoir therebetween and the x-ray translucent window is aligned with the x-ray window. The cold-plate is operatively mounted on the x-ray translucent window. 
     In accordance with another aspect of the present invention, an x-ray tube comprises a cold plate and a vacuum envelope having an anode and a cathode with an x-ray window mounted thereon adjacent the anode. The cold plate includes an elongated shell and a plurality of heat transfer elements positioned therein. The shell is operatively mounted around the x-ray window and circumferentially oriented relative to the vacuum envelope. The shell includes an inlet defined in a first end, an outlet defined in an opposite end, and an expansion section disposed therebetween. 
     In accordance with another aspect of the present invention, a method of cooling an x-ray tube is provided. A first portion of a cooling fluid is circulated over an x-ray tube to remove heat. A second portion of the cooling fluid forced around an x-ray translucent window disposed on the x-ray tube removes heat from the window. The cooling fluid is cooled and recirculated around the window and over the x-ray tube. 
     The advantages of the present invention include the ability to prevent or reduce the risk of thermal damage to the joint between the beryllium window and the x-ray insert. 
     Another advantage resides in reducing or preventing failure of the x-ray tube due to overheating. 
     Another advantage of the present invention resides in reducing or preventing carbon build-up on the beryllium window due to overheating of the cooling fluid. 
     Another advantage of the present invention resides in maintaining the dielectric characteristics of the cooling fluid to decrease the possibilities of high-voltage instabilities. 
     Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a CT scanner in accordance with the present invention; 
     FIG. 2 is a diagrammatic cross-sectional illustration of the x-ray tube assembly of FIG. 1; 
     FIG. 3 is a perspective view of the cold-plate with a portion of a shell removed to show corrugated fins; and 
     FIG. 4 is a diagrammatic illustration of a cooling system according to an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a CT scanner includes a floor mounted or stationary frame portion A whose position remains fixed during data collection. An x-ray tube B is mounted on a rotating frame C rotatably mounted within the stationary frame portion A. Heat generated by the x-ray tube B is transferred to a heat exchanger D by a cooling fluid, such as oil, water, refrigerant gas, other fluids and combinations thereof. 
     The stationary frame portion A includes a bore  10  that defines a patient receiving examination region  12 . An array of radiation detectors  14  are disposed concentrically around the patient receiving region  12 . The stationary frame A with the rotating frame C can be canted or tipped to scan slices at selectable angles. A control console  16  contains an image reconstructing processor  18  for reconstructing an image representation of output signals from the detector array  14 , performing image enhancements, and the like. A video monitor  20  converts the reconstructed image representation into a human readable display. The console  16  also includes appropriate digital recording memory media for archiving the image representations. Various control functions, such as initiating a scan, selecting among different types of scans, calibrating the system, and the like are also performed at the control console  16 . 
     With further reference to FIG. 2, the x-ray tube B includes a housing  22  having an x-ray permeable window  24  directed toward the patient receiving region  12  and an x-ray insert  26  mounted in the housing  22 . The x-ray insert  26  can be made of glass, ceramic or metal. A rotary anode  28  is rotatably mounted in the x-ray insert  26  by bearings and a cathode  30  is mounted adjacent the rotary anode  28 . Electrons from the cathode  30  are propelled by high voltage against the rotating anode  28  causing the emission of x-rays and a large amount of heat. The x-ray insert  26  includes a beryllium or other low Z metal window  32  mounted adjacent the cathode  30  and the x-ray permeable window  24  of the housing  22 . The beryllium window  32  passes x-rays generated by the cathode  30  and the anode  28  out of the x-ray insert  26  through the x-ray permeable window  24  and into the patient receiving area  12 . The beryllium window  32  is attached to the x-ray insert  26  by bending, brazing, or by any other suitable manner. Electrical leads for supplying current to the cathode  30  and leads for biasing the cathode  30  to a large, negative potential difference relative to the anode  28  pass through the envelope in a cathode well  34 . 
     Once the x-rays pass through the x-ray permeable window  24  and across the patient receiving region  12 , appropriate x-ray collimators focus the radiation into one or more planar beams which span the examination region  12  in a fan or cone pattern, as is conventional in the art. Other equipment associated with the x-ray tube B, such as a high voltage power supply  36  and a pump  38 , are also mounted on the rotating frame C. 
     During operation of the x-ray tube B, the temperature of the beryllium window  32  tends to rise quickly. The rapid increase of the window temperature is caused not only by thermal radiation from the hot anode  28  inside the x-ray insert  26 , but also by the kinetic energy from the secondary electrons impinging on the beryllium window  32  and its neighboring x-ray insert area. The dissimilar coefficients of thermal expansion of the beryllium window  32 , the insert  26 , and the bonding materials used to mount the window  32  to the x-ray insert  26  tend to create mechanical stresses that escalate as the temperature increases. Excessive window temperature is potentially dangerous for cracking the window joint, which can destroy the vacuum within the insert and cause failure of the x-ray tube B. A high window temperature can also overheat the cooling fluid near an outer surface of the window  32 , deteriorating the dielectric characteristics of the cooling fluid, and increasing possibilities of high-voltage instabilities. Overheated cooling fluid near the window  32  is also detrimental because it could be carbonized and form particles. Electrically conductive carbon particles floating inside the x-ray tube B can deteriorate the stability of the fluid and cause arcing. This effect may decrease the quality of the x-ray image produced by the CT scanner. 
     With additional reference to FIG. 3, a cold-plate  40  is integrated onto the beryllium  32  window for removing excess heat. The cold-plate  40  comprises a plurality of corrugated fins  42 , a shell  44 , an inlet  46 , and an outlet  48 . The corrugated fins  42  of thermally conductive material, such as beryllium or aluminum, are built on the rim area of the beryllium window  32  and its neighboring x-ray insert area. The shell  44  encloses the fins  42  and defines a fluid channel in a circumferential direction around the x-ray insert. The inlet  46 , and the outlet  48  are oriented to direct flow along the longitudinal direction of the window  32 . To reduce the pressure drop across the cold-plate  40 , the inlet  46  contains a smooth expansion section  50  and the outlet  48  is wide open. When cooling fluid is provided to the cold-plate  40 , the cooling fluid discharges through the outlet  48  and mixes with cooling fluid inside the x-ray tube housing  22 . The shell  44  can be made of aluminum. Therefore, this aluminum shell can be also used as an x-ray filtration plate by setting its thickness as the required filtration thickness. 
     Alternatively, the cold-plate cover  40  can be made of titanium instead of aluminum. The advantage of using titanium is that this alloy has excellent x-ray transparent features. Further, the cover  40  can also be made of thermally conductive and x-ray transmissible plastics. 
     To cool the x-ray tube B, heated cooling fluid is circulated from the x-ray tube housing  22  through a first cooling fluid duct to the heat exchanger D on the rotatable frame C. Circulation of the cooling fluid is effected by the fluid pump  36 . Cooled cooling oil exiting from the heat exchanger D is returned to the housing  22  via a second cooling fluid duct. The cooling fluid enters the housing  22  through an inlet aperture  52 . The cooling fluid flows through the x-ray tube B absorbing heat created during x-ray generation. The fluid exits the housing  22  through an outlet aperture  54  into the first cooling fluid duct and recirculates back to the heat exchanger D. 
     Cooling fluid flowing to the inlet  52  of the x-ray tube B is distributed into two streams. A first stream of the fluid goes generally into the housing  22 , whereas a second stream flows through a tube  56  to the cold-plate  40 . The tube  56  fluidly connects to the inlet  46  of the cold-plate  40  and can be made of plastic or any other non-metallic material. Thus, the tube  56  provides cooling fluid directly to the beryllium window  32  via the cold-plate  40 . The fluid exiting the tube  56  into the cold-plate  40  flows perpendicularly relative to the general flow of cooling fluid through the housing  22  around the cold-plate  40 . The inlet  52  and outlet  54  of the x-ray tube housing  22  are at a first end of the housing  22 , and separated by a first flow divider  54 . 
     A second flow divider  58  is installed in the middle section of the housing  22  along an axial plane of the x-ray insert  26  and perpendicular to the direction of the inlet  52  of the housing  22 . The second flow divider  58  is used for forcing the fluid to flow through the housing  22  in two passes. More specifically, the second flow divider  58  divides the housing  22  into a beryllium window cavity and an opposing cavity. The cavities are fluidly connected at the cathode side of the housing  22 . The upper half of the x-ray insert  26 , the upper half of the housing  22 , and the second flow divider  58  generally define the beryllium window cavity. The lower half of the x-ray insert  26 , the lower half of the housing  22 , and the second flow divider  58  generally define the opposing cavity. 
     In operation, cooling fluid supplied from the heat exchanger D enters the inlet  52  of the x-ray tube housing  22 . The cooling fluid is divided into first and second streams. The first stream enters generally into the x-ray housing  22  into the beryllium window cavity to cool the top half of the x-ray insert  26 . The second stream flows to the cold-plate  36  through a tube  56  fluidly connecting the flow inlet  52  of the housing  22  and the inlet  46  of the cold-plate  40 . The cooling fluid directed into the cold-plate  40  engages in vigorous heat transfer inside the cold-plate  40  while washing through the cold-plate  40 . The cooling fluid exits the cold-plate  40  and mixes with the fluid flowing in the beryllium window cavity. The joined cooling fluid flows continuously towards the cathode end of the housing  22  before making a one-hundred-eighty degree turn over the second flow divider  58 . The cooling fluid then flows into the opposing cavity and back to the outlet  54  of the housing  22  while cooling the bottom half of the x-ray insert  26 . Cooling fluid exits the outlet  54  of the housing  22  and goes to the heat exchanger D to release the heat that it has absorbed from inside the x-ray tube housing  22 . 
     To integrate the cold-plate  40  to the beryllium window  32 , corrugated fins  42  are built around the rim area of the window  32  and the x-ray insert area neighboring the window  32 . A shell  44  is brazed on the x-ray insert  26  thereby covering the window  32  and the fins  42  to form the cold-plate  40 . A high volume of cooling fluid is driven into the cold-plate  40  to enhance the heat transfer from the fins  42  and the window  32 . The cooling fluid to the cold-plate can be regulated and supplied through a flow director that may be placed at the inlet  52  to the x-ray housing  22 . 
     With reference to FIG. 4, in an alternate embodiment of the present invention, a second and independent cooling loop is used to provide cooling fluid to the cold-plate  40 . Cool cooling fluid is provided from a second heat exchanger E to the cold-plate  40  through a conduit  60 . While flowing through the cold-plate  40 , the cooling fluid removes heat from the beryllium window  32  and the area on the x-ray insert  26  surrounding the beryllium window  32 . The heated cooling fluid discharges from the cold-plate  40  into a return conduit  62  and is circulated back to the heat exchanger E by a second pump  64 . The first heat exchanger D continues to cool heated cooling fluid exiting the housing  22  of the x-ray tube B and provide cooled cooling fluid for circulation through the housing  22  by the pump  38 . 
     The cooling fluid exiting the cold-plate  40  no longer merges with the cooling fluid flowing through the housing  22 . Further, the cooling fluid flowing through the cold-plate  40  is not in fluid communication with the cooling fluid flowing through the x-ray housing. As a result, it is possible to introduce a non-dielectric and water-based fluid to cool the cold-plate  40 . Use of such a cooling fluid will enhance the heat transfer of the cold-plate  40  while keeping the beryllium window  32  clean. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they are within the scope of the appended claims or the equivalents thereof.