Abstract:
A slotted window collector assembly for an x-ray tube having an anode and a cathode includes a collector having an anode side and a cathode side, which opposes the anode side. The anode side and the cathode side define an internal bore there between for receiving x-rays from the anode. The slotted collector further includes a window side common to both the anode side and the cathode side and having a window coupled thereto, wherein a window aperture is defined extending from the window to the internal bore. Within the collector assembly, a slot is defined intersecting the window aperture and extending trans-axially thereto and beyond a set length. The slot also extends circumferentially with the window such that plastic strain on the window and heat transferred to the window are reduced.

Description:
BACKGROUND OF INVENTION  
       [0001]     The present invention relates generally to thermal energy management systems within electron beam generating devices and, more particularly, to an assembly for cooling and relieving stress from an x-ray tube window.  
         [0002]     There is a continuous effort to increase x-ray imaging system scanning capabilities. This is especially true in computed tomography (CT) imaging systems. Customers desire the ability to increase the peak power to reduce X-ray doses. The increase in peak power also allows physicians to get improved CT images of vascular applications with high speed CT imaging systems. Although the increase in imaging speed generates improved imaging capability, it causes new constraints and requirements for the functionality of the CT imaging systems.  
         [0003]     CT imaging systems include a gantry rotating at various speeds in order to generate a 360° image. The gantry includes an x-ray tube, which composes a large portion of the rotating gantry mass. The CT tube generates x-rays across a vacuum gap between a cathode and an anode. For generating the x-rays, a large voltage potential is applied across the vacuum gap allowing electrons to be emitted, in the form of an electron beam, from the cathode to the anode target. During the releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target, whereby they are abruptly slowed down to emit x-rays. The high voltage potential generates a large amount of heat within the x-ray tube, especially within the anode.  
         [0004]     Typically, a small portion of energy within the electron beam is converted into x-rays; the remaining electron beam energy is converted into thermal energy within the anode. The thermal energy radiates to other components within a vacuum vessel of the x-ray tube and is removed from the vacuum vessel via a cooling fluid circulating over an exterior surface of the vacuum vessel. Additionally, electrons within the electron beam are back-scattered from the anode and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result, the x-ray tube components are subject to high thermal stresses decreasing component life and reliability of the x-ray tube.  
         [0005]     The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bi-polar configuration.  
         [0006]     High temperatures at an interface between the vacuum vessel and a transmissive window in the casing cause the cooling fluid to boil, which may degrade the performance of the cooling fluid. Bubbles may form within the fluid and cause high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images.  
         [0007]     Prior art cooling methods have primarily relied on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. The coolant fluid is often a special fluid for use within the vacuum vessel, as opposed to the cooling fluid that circulates about the external surface of the vacuum vessel. Other methods have been proposed to electromagnetically deflect back-scattered electrons so that they do not impinge on the x-ray window.  
         [0008]     These approaches, however, are limited with regard to energy storage and dissipation. Due to inherent poor efficiency of x-ray generation and desire for increased x-ray flux, heat load is increased that must be dissipated. As power of x-ray tubes continues to increase, heat transfer rate to the coolant can exceed heat flux absorbing capabilities of the traditional cooling system design.  
         [0009]     A thermal energy storage device or electron collector, coupled to an x-ray window, has been used to collect back scattered electrons between the cathode and the anode. In using this device, the collector and window need to be properly cooled to prevent high temperature and thermal stresses, which can damage the window and joints between the window and collector.  
         [0010]     High temperature on the window and collector can induce boiling of coolant. Bubbles from boiling coolant obscure the window and thereby compromise image quality. Further boiling of the coolant results in chemical breakdown of the coolant and sludge formation on the window, which also results in poor image quality.  
         [0011]     Previously, a heat exchange chamber has been coupled to the electron collector, including a cooling channel, which allows coolant to flow in the channel across each of four walls of the electron collector. Although, the heat exchange chamber aids in cooling the electron collector, it is difficult to effectively manufacture due to its complexity and large number of seams, which each need to be properly sealed. Also, the heat exchange chamber has limited effectiveness in cooling of and preventing deposits from forming on the x-ray tube window. Also, portions of the window have been known to crack due to cyclic thermal loadings.  
         [0012]     It would therefore be desirable to provide an apparatus and method of cooling an x-ray tube or x-ray tube window, that allows for increased scanning speed and power, that is relatively easy to manufacture, and that minimizes blurring and artifacts in a reconstructed image.  
       SUMMARY OF INVENTION  
       [0013]     A slotted collector for an imaging system having a cathode and an anode includes an anode side, a cathode side opposing the anode side, and a common window side. Defined in the anode side is a cooling slot, and further defined is an internal bore through which electrons from the anode pass. The anode side also defines an anode receiving area such that a typical rotating anode may be positioned adjacent the collector such that maximum flux is directed through the internal bore and toward a cathode.  
         [0014]     The cathode side further defines the internal bore such that the cathode is positioned near or inside of the internal bore for receiving electrons.  
         [0015]     The window side is common to both the opposing cathode and anode sides. The window side includes a window and defines a fin pack area for receiving a fin pack. A window aperture extends between the window and the internal bore.  
         [0016]     The slot defined within the anode side is further defined within the collector circumferentially along the window to reduce the thermal gradient across the window and reduce plastic strain in the window braze region. The slot is embodied as extending trans-axial to, intersecting the window aperture and extending beyond the aperture for a set length. Positioning of the slot is such that the heat from the back-scattered electrons flows through the area near the back of the fin-pack for dissipation. The slot also causes thermal isolation to reduce the temperature of the window aperture.  
         [0017]     The present invention has several advantages over existing x-ray tube cooling systems. One of several advantages of the present invention is that it provides a path to force the heat flex to flow more closely to the liquid cooled finback area to increase cooling efficiency. Another advantage of the present invention is that it relieves plastic strain on the window attachment.  
         [0018]     The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]     For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying figures and described below by way of examples of the invention wherein:  
         [0020]      FIG. 1  is a block diagrammatic view of a multi-slice CT imaging system utilizing a slotted window collector assembly in accordance with an embodiment of the present invention;  
         [0021]      FIG. 2  is a perspective view of an x-ray tube assembly incorporating the slotted window collector assembly in accordance with an embodiment of the present invention;  
         [0022]      FIG. 3  is a sectional perspective view of an x-ray tube incorporating the slotted window collector assembly in accordance with an embodiment of the present invention;  
         [0023]      FIG. 4  is a close-up sectional perspective view of the slotted window collector assembly in accordance with an embodiment of the present invention;  
         [0024]      FIG. 5  is a view of the slotted window collector assembly looking in the direction of line  5 - 5  of  FIG. 4 ;  
         [0025]      FIG. 6  is a top perspective view of half of the slotted window collector assembly of  FIG. 4 ; and  
         [0026]      FIG. 7  is a side perspective view of half of the slotted window collector assembly of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0027]     While the present invention is described with respect to an assembly for cooling an x-ray tube window within a computed tomography (CT) imaging system, the following apparatus and method is capable of being adapted for various purposes and is not limited to the following applications: MRI systems, CT systems, radiotherapy systems, flouroscopy systems, x-ray imaging systems, ultrasound systems, vascular imaging systems, nuclear imaging systems, magnetic resonance spectroscopy systems, and other applications known in the art.  
         [0028]     In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.  
         [0029]     Also, in the following description the term “impinge” refers to an object colliding directly with another object. For example, as known in the art, an electron beam impinges upon a target of an anode within an x-ray tube. The electron beam is directed at the target and electrons within the beam collide with the target.  
         [0030]     Referring now to  FIG. 1 , a block diagrammatic view of a multi-slice CT imaging system  10  utilizing a slotted collector assembly  11  in accordance with one embodiment of the present invention is illustrated. The slotted collector assembly  11  will be further discussed regarding  FIGS. 2-7 .  
         [0031]     The imaging system  10  includes a gantry  12  that has an x-ray tube assembly  14  and a detector array  16 . The x-ray tube assembly  14  has an x-ray generating device or x-ray tube  18 . The tube  18  projects a beam of x-rays  20  towards the detector array  16 . The tube  18  and the detector array  16  rotate about an operably translatable table  22 . The table  22  is translated along a z-axis between the assembly  14  and the detector array  16  to perform a helical scan. The beam  20  after passing through a medical patient  24 , within a patient bore  26 , is detected at the detector array  16  to generate projection data that is used to generate a CT image.  
         [0032]     The tube  18  and the detector array  16  rotate about a center axis  28 . The beam  20  is received by multiple detector elements  30 . Each detector element  30  generates an electrical signal corresponding to intensity of an impinging x-ray beam. As the beam  20  passes through the patient  24  the beam  20  is attenuated. Rotation of gantry  12  and the operation of tube  18  are governed by a control mechanism  32 . Control mechanism  32  includes an x-ray controller  34  that provides power and timing signals to the tube  18  and a gantry motor controller  36  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  38  samples analog data from the detector elements  30  and converts the analog data to digital signals for subsequent processing. An image reconstructor  40  receives sampled and digitized x-ray data from the DAS  38  and performs high-speed image reconstruction. A main controller or computer  42  stores the CT image in a mass storage device  44 .  
         [0033]     The computer  42  may also receive commands and scanning parameters from an operator via an operator console  46 . A consol  48  allows the operator to observe the reconstructed image and other data from the computer  42 . The operator supplied commands and parameters are used by the computer  42  in operation of the DAS  38 , the x-ray controller  34 , and the gantry motor controller  36 . In addition, the computer  42  operates a table motor controller  50 , which translates the table  22  to position patient  24  in gantry  12 .  
         [0034]     The x-ray controller  34 , the gantry motor controller  36 , the image reconstructor  40 , the computer  42 , and the table motor controller  50  are preferably microprocessor-based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The x-ray controller  34 , the gantry motor controller  36 , the image reconstructor  40 , the computer  42 , and the table motor controller  50  may be a portion of a central control unit or may each be stand-alone components as illustrated.  
         [0035]     Referring now to  FIG. 2 , a perspective view of the x-ray tube assembly  14  incorporating the slotted window collector assembly  11 , in accordance with one embodiment of the present invention, is illustrated. The tube assembly  14  includes a housing unit  52  and may include a coolant pump  54 , an anode end  56 , a cathode end  58 , and a center section  60  positioned between the anode end  56  and cathode end  58 , which contains the x-ray tube  18 . The x-ray tube  18  is enclosed in a fluid chamber  62  within lead-lined casing  64 . The chamber  62  is typically filled with fluid, such as dielectric oil, but other fluids including water or air may be utilized. The fluid circulates through housing  52  to cool the x-ray tube  18  and may insulate casing  64  from high electrical charges within the x-ray tube  18 .  
         [0036]     Referring now to  FIGS. 2 and 3 , sectional perspective views of the x-ray tube  18  incorporating the slotted window collector assembly  11  in accordance with an embodiment of the present invention is illustrated. The x-ray tube  18  includes a rotating anode  80 , having a target  82 , and a cathode assembly  84  disposed in a vacuum within vessel  86 . The slotted window collector assembly  11  is interposed between the anode  80  and the cathode  84 .  
         [0037]     Referring now to  FIGS. 4-7 , the slotted window collector assembly  11  is illustrated.  FIG. 4  is a close-up sectional perspective view of the slotted window collector assembly;  FIG. 5  is a view of the slotted window collector assembly looking in the direction of line  5 - 5  of  FIG. 4 ;  FIG. 6  is a top perspective view of half of the slotted window collector assembly of  FIG. 4 ; and  FIG. 7  is a side perspective view of half of the slotted window collector assembly of  FIG. 4 .  
         [0038]     As is illustrated, the collector  11  includes an anode side  100 , a cathode side  150  opposing said anode side  100 , an internal bore  140  defined between said anode side  100  and said cathode side  150 , and a window side  200  common to both said anode side  100  and said cathode side  150 . The present embodiment of the collector  11  is cubical in shape and also includes three other sides  250 ,  300 ,  350  common to both the anode side  100  and the cathode side  150 .  
         [0039]     As was mentioned, the collector  11  includes an anode side  100  having an anode receiving area  102 . The anode side  100  defines the anode receiving area such that a typical rotating anode may be positioned adjacent to the collector  11  such that maximum flux reflected from the anode  82  is directed through the internal bore  140  and toward the cathode  84 . One embodiment of the present invention includes the slot  104  extending through the anode side  100 . Also defined in the anode side  100  is a bore  106  through which electrons from the anode  82  pass, as will be understood by one skilled in the art.  
         [0040]     The cathode side  150  further defines the internal bore  140  such that the cathode  84  is positioned near or inside of the internal bore  140  to receive electrons.  
         [0041]     The window side  200  is common to both the opposing cathode and anode sides  100 ,  150 . The window side  200  includes a window  202  and defines a fin pack area  204  for receiving a fin pack and cooling the anode  84 . One skilled in the art will realize that a window aperture  106  extends between the window  202  and the internal bore  140 .  
         [0042]     The slot  104  is defined within the collector  11  circumferentially along the window  202  to reduce the thermal gradient across the window  202  and reduce plastic strain in the window braze region. The slot  104  is embodied as extending trans-axial to, intersecting and extending beyond the window aperture  106 . Positioning of the slot  104  is such that the heat from the anode  82  flows through the internal bore  140  and then along the slot  104  prior to flowing to the window  202  (i.e. the slot  104  generates thermal isolation). The window temperatures are decreased significantly as a result of the slotted design.  
         [0043]     The slot  104  adds flexibility to the collector system  10  and mechanically isolates the window braze joint  204  from the non symmetric heat load and associated thermal growth of the collector system, thereby reducing the plastic strain in the window attachment joint.  
         [0044]     The slot  104  further has allowed a wider internal bore  140 , reducing the material required in the collector and simplifying the manufacture of the assembly, due to the decrease in temperature of the window.  
         [0045]     In operation, an electron beam  90  is directed through central cavity  92  and accelerated toward the anode  80 . The electron beam  90  impinges upon a focal spot  94  on the target  82  and generates high frequency electromagnetic waves or x-rays and residual energy. The residual energy is absorbed by components within the x-ray tube  18 . x-rays are directed through the vacuum toward the window aperture  106  in slotted window collector assembly  11 .  
         [0046]     The residual energy includes radiant thermal energy from the anode  80  and kinetic energy from back scattered electrons that deflect off the anode  80 . The kinetic energy is converted into thermal energy upon impact with components in the vessel  86 . A portion of the kinetic energy is reduced by the slotted window collector assembly  11  because of the increased surface area over which the kinetic energy flows.  
         [0047]     Disposed at the exterior of the aperture  106  is the x-ray tube window  202 , formed of a material that efficiently allows passage of x-rays. The window  202  is hermetically sealed to slotted window collector assembly  11  at seal  204 , such as by vacuum brazing or welding. Seal  204  serves to maintain the vacuum within vessel  86 . Thus, x-ray tube  18  generates residual energy and x-rays that are directed out of the x-ray tube  18  through the window  202 .  
         [0048]     In operation, a method of operating the x-ray tube  18  in accordance with an embodiment of the present invention is illustrated. The electron beam is generated as stated above and is directed to impinge upon the target anode  82  as to generate the x-rays.  
         [0049]     The x-rays are directed through the window  202 , which increases temperature of the window  202 . Back-scattered electrons, from the electron beam, are also impinging upon the window  202  further increasing temperature of the window  202 . Heat from the anode  82  on the window  202  is reduced however because the heat must now flow around the increased surface area of said slot  104  and is directed toward the cooling fins.  
         [0050]     The above-described steps are meant to be an illustrative example; the steps may be performed synchronously or in a different order depending upon the application.  
         [0051]     The present invention provides an x-ray generating device window cooling system that provides improved cooling and is relatively simple to manufacture. The window is efficiently cooled, thus minimizing blurring and artifacts in a reconstructed image.  
         [0052]     The above-described apparatus and method, to one skilled in the art, is capable of being adapted for various applications and systems known in the art. The above-described invention can also be varied without deviating from the true scope of the invention.