Abstract:
An x-ray tube assembly ( 16 ) includes a housing ( 40 ) and an insert frame ( 52 ) supported within the housing ( 40 ), such that the insert frame ( 52 ) defines a substantially evacuated envelope in which cathode ( 56 ) and anode ( 54 ) assemblies operate to produce x-rays. An x-ray transmissive window assembly ( 70 ) extends between and in a fluid-tight relationship with the housing ( 40 ) and the insert frame ( 52 ). The window assembly ( 70 ) includes an insert window ( 72 ) brazed to the insert frame ( 52 ), a top plate ( 76 ), which is connected to and substantially surrounded by a flange ( 78 ), where the flange ( 78 ) is fastened to the x-ray tube housing ( 40 ). An annular side plate ( 86 ) is connected to a fluid-tight relationship with both the insert frame/window interface ( 74 ) and the flange ( 78 ). The window assembly ( 70 ) cools the window through enhanced heat transfer while preventing housing coolant from contacting the insert window ( 72 ), thereby eliminating coolant carbonization on the window ( 72 ) and enhancing x-ray beam quality.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the x-ray tube art. It finds particular application in conjunction with metal insert frame x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with conventional x-ray diagnostic systems and other penetrating radiation systems for medical and non-medical examinations. 
     Typically, a high power x-ray tube includes an evacuated envelope made of metal or glass, which holds a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e., thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and anode, which is also located within the evacuated envelope. This potential causes the electrons to flow from the cathode to the anode through the evacuated region within the interior of the evacuated envelope. A cathode focusing cup, which houses the cathode filament, focuses the electrons onto a focal spot on the anode. The electron beam impinges the anode with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through an x-ray transmissive window of the envelope to a beam limiting device or collimator, which is attached to an x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination, thereby allowing images of the patient or subject to be reconstructed. 
     During the production of x-rays, many electrons from the electron beam striking the anode are reflected from the anode and strike other regions of the x-ray tube. The reflected electrons are often referred to as secondary electrons, and the act of such reflected electrons striking other regions of the x-ray tube is often referred to as secondary electron bombardment. Accordingly, the temperature of the x-ray transmissive window or insert window rises rapidly once the anode power is applied. The rise in window temperature is caused by both the thermal radiation from the anode inside the insert frame and the secondary electron bombardment. Excessive window temperatures may destroy the window braze joints due to thermal stress caused by expansion differences between the window and the insert frame at operating temperatures. 
     Due to its excellent ability to withstand high voltage, oil is the preferred cooling fluid for an x-ray tube. The oil is circulated between the housing and the x-ray tube passing directly across the window of the x-ray tube. However, for a metal frame x-ray tube, the insert window receives extensive heat flux and oil cooling may not be sufficient. As a result, oil may boil locally at the insert window, depositing a layer of carbon on the window surface. Carbonization of the x-ray window significantly reduces window cooling and also deteriorates x-ray image quality due to x-ray absorption in the carbon layer. 
     One prior method for protecting the window from overheating includes a window heat shield, which shields a junction between the window and a metal envelope from secondary electron bombardment. Unfortunately, the heat shield method requires a heat shield material having properties of both high thermal conductivity and excellent x-ray transparency. Materials with these properties may be costly and not easily obtainable. Another prior device for protecting the x-ray window from overheating includes the use of a refrigeration cooled window joint. This method requires that a refrigeration system be attached to the x-ray tube. While this solution may serve to cool the window braze, it may be ineffective for solving the problem of oil carbonization at the center of the window. Another prior method for protecting the x-ray window from overheating includes the use of an electrode window, which is intended to deflect the back-scattered electrons, i.e., the secondary electron bombardment, from the window, therefore reducing window heat flux. However, an effective design of such a window is still unavailable. 
     The present invention contemplates a new and improved x-ray tube assembly having a liquid-free x-ray insert window, which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an x-ray tube assembly includes an x-ray tube housing, a cathode assembly, and a rotating anode assembly. An insert frame, which is supported within the x-ray tube housing, defines a substantially evacuated envelope in which the cathode and anode assemblies operate to produce x-rays. A dielectric liquid coolant flows between the x-ray tube housing and the insert frame. An x-ray transmissive window assembly extends between and in a fluid-tight relationship with the x-ray tube housing and the insert frame. 
     In accordance with a more limited aspect of the present invention, the x-ray transmissive window assembly includes an x-ray transmissive insert window, which is hermetically connected to the insert frame. An x-ray transmissive top plate is connected to and substantially surrounded by a flange, which is fastened to the x-ray tube housing. An annular side plate has a first end hermetically connected to at least one of the insert frame and insert window, and a second end which is connected to a bottom surface of the flange. 
     In accordance with a more limited aspect of the present invention, the annular side plate includes an inner surface and an extended outer surface having a plurality of fins in contact with the dielectric liquid coolant. 
     In accordance with another aspect of the present invention, an x-ray tube assembly includes a housing, a cathode assembly, and an anode assembly. A metal insert frame, which is disposed within the housing, defines an evacuated envelope in which the cathode and anode assemblies operate to produce x-rays. A cooling system circulates a dielectric liquid coolant between the housing and the metal insert frame. An x-ray transmissive window, through which x-rays produced by the cathode and anode assemblies pass, is brazed to the insert frame at a braze joint in a vacuum-tight manner. A cooling assembly, which is in thermal contact with the x-ray transmissive window at the braze joint, removes heat from the window without liquid coolant passing over the window. 
     In accordance with another aspect of the present invention, an x-ray tube assembly includes an outer housing, a metal insert frame supported within the outer housing, which defines an evacuated envelope in which cathode an anode assemblies operate to produce x-rays, and an x-ray transmissive insert window brazed to the insert frame at a braze joint. A method of cooling the x-ray transmissive insert window, in which no liquid coolant contacts the x-ray transmissive window, includes forming a fluid-free cooling chamber around and above the x-ray transmissive window and circulating a cooling fluid around the fluid-free cooling chamber. 
     One advantage of the present invention is that it relieves overheating of the window joint. 
     Another advantage of the present invention resides in increased image intensity. 
     Another advantage of the present invention resides in the elimination of oil carbonization on the x-ray window. 
     Yet another advantage of the present invention resides in reduced input power to achieve a selected x-ray output. 
     Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     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 preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a prior art computerized tomographic (CT) diagnostic system employing the x-ray tube assembly in accordance with the present invention; 
     FIG. 2 is a diagrammatic illustration of a preferred embodiment of the x-ray tube assembly in accordance with the present invention; 
     FIG. 3 is a diagrammatic illustration of a preferred embodiment of the liquid-free x-ray window in accordance with the present invention; 
     FIG. 4 is a side perspective view of the liquid-free x-ray window in accordance with the present invention; 
     FIG. 5 is a diagrammatic illustration of another preferred embodiment of the liquid-free x-ray window in accordance with the present invention; 
     FIG. 6 is a diagrammatic illustration of another preferred embodiment of the liquid-free x-ray window in accordance with the present invention; and 
     FIG. 7 is a diagrammatic illustration of another preferred embodiment of the liquid-free x-ray window in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a computerized tomographic (CT) scanner  10  radiographically examines and generates diagnostic images of a subject disposed on a patient support  12 . More specifically, a volume of interest of the subject on the patient support  12  is moved into an examination region  14 . An x-ray tube assembly  16  mounted on a rotating gantry projects one or more beams of radiation through the examination region  14 . A collimator  18  collimates the beams of radiation in one dimension. In third generation scanners, an x-ray detector  20  is disposed on the rotating gantry across the examination region  14  from the x-ray tube. In fourth generation scanners, a ring or array of detectors  22  is mounted on the stationary gantry around the rotating gantry. 
     Each of the x-ray detectors  20 ,  22  preferably includes a two-dimensional array of photodetectors connected to or preferably integrated into an integrated circuit. The detectors generate electrical signals indicative of the intensity of the received radiation, which is indicative of the integrated x-ray absorption along the corresponding ray between the x-ray tube and the scintillation crystal segment. 
     The electrical signals, along with information on the angular position of the rotating gantry, are digitized by analog-to-digital converters. The digital diagnostic data is communicated to a data memory  30 . The data from the data memory  30  is reconstructed by a reconstruction processor  32 . Various known reconstruction techniques are contemplated including spiral and multi-slice scanning techniques, convolution and back projection techniques, cone beam reconstruction techniques, and the like. The volumetric image representations generated by the reconstruction processor are stored in a volumetric image memory  34 . A video processor  36  withdraws selective portions of the image memory to create slice images, projection images, surface renderings, and the like, and reformats them for display on a monitor  38  such as a video or LCD monitor. 
     With reference to FIG.  2  and continuing reference to FIG. 1, the x-ray tube assembly  16  includes a housing  40  filled with a heat transfer and electrically insulating cooling fluid, such as oil. More particularly, the cooling fluid (represented by the arrows) is circulated within the housing  40  through a heat exchanger  42  and through circulation and return lines  46 ,  48  by a pump  44 . An insert frame or envelope  52 , preferably comprised of metal, within which an evacuated chamber or vacuum  53  is defined, is supported within the housing  40 . An anode assembly  54  and a cathode assembly  56  are disposed at opposite ends of the evacuated envelope  52 . An electron beam passes from the cathode assembly  56  to a focal spot on an annular, circumferential race  58  of the anode  54 . The anode assembly is mounted to a rotor assembly  60 , which is driven by a rotational drive  62 , for rotation about an anode axis. The anode assembly includes a target area along a peripheral edge of the anode assembly, which is comprised of a tungsten composite or other suitable material capable of producing x-rays. 
     The cathode assembly  56  is stationary and includes a cathode focusing cup  64  positioned in a spaced relationship with respect to the target area of the anode assembly for focusing electrons to a focal spot on the target area. A cathode filament  66  mounted to the cathode focusing cup  64  is energized to emit electrons  67 , which are accelerated to the target area of the anode assembly in order to produce x-rays. The electrons from the cathode filament  66  are accelerated toward the anode assembly  54  by a large DC electrical potential difference between the cathode and the anode assembly. In one embodiment, the cathode is at an electrical potential of −75,000 volts with respect to ground, and the anode assembly is at an electrical potential of +75,000 volts with respect to ground, thereby providing a bipolar configuration having a total electrical potential difference of 150,000 volts. Impact of the accelerated electrons from the cathode filament  66  onto the focal spot of the anode assembly  54  causes the anode assembly  54  to be heated to a range of between 1100°-1400° C. 
     Upon striking the target area, a portion of the electrons reflect from the target area and scatter within the evacuated chamber of the envelope. The electrons which are absorbed, as opposed to reflected, by the anode assembly  54  serve to produce x-rays  68 . A portion of the x-rays pass through an x-ray transmissive window assembly  70 , which is coupled to the envelope  52  towards a patient or subject under examination. 
     With reference to FIGS. 3 and 4 and continuing reference to FIG. 2, the x-ray window assembly  70  includes an x-ray transmissive insert window  72 , which is hermetically connected to the insert frame  52 . The insert window  72  is made of beryllium, titanium, or another suitable x-ray transmissive material. Preferably, the x-ray transmissive insert window  72  is brazed to the insert frame  52  at a braze joint  74 , providing an excellent thermal connection between the insert window  72  and the insert frame. The x-ray. window assembly  70  further includes an x-ray transmissive top plate  76  which is connected to and substantially surrounded by a flange  78 . The flange  78  is fastened to the x-ray tube housing  40  by a plurality of fasteners  80 . The fasteners  80  may include bolts, rivets, screws as well as any other suitable fastener. A sealing member  84 , such as an O-ring, is positioned between the x-ray tube housing  52  and the flange  78  in order to prevent the housing from leaking housing cooling fluid. 
     An annular side plate  86 , preferably made of copper or any other highly thermally conducted material, is connected between the flange  78  and the insert frame/insert window  72  at the braze joint  74 . The side plate  86  includes a smooth inner surface  90 , which is coated with a thermally emissive coating, and an extended outer surface  94 , which includes a plurality of cooling fins in thermal contact with the dielectric liquid coolant. As shown in FIG. 3, the insert window  72 , top plate  76 , the flange  78 , and the inner surface  90  of the annular side plate form a window chamber  100 . The window chamber  100  is defined by a window chamber surface, which is coated with a thermally emissive coating. In one embodiment, the window chamber  100  contains a non-oxygen gas having a high thermal conductivity. It is to be appreciated that the non-oxygen gas within the window chamber both prevents the window from oxidizing and aids in carrying heat from the window to the side plates. 
     During operation of the x-ray tube assembly  16 , the cooling system  42 ,  44  circulates the dielectric liquid coolant between the housing  40  and the metal insert frame  52 . The x-ray window assembly serves to remove heat from the insert window  72  in such a way that no dielectric liquid coolant comes into direct contact with the insert window  72 . It is to be appreciated that preventing liquid coolant from contacting the insert window serves to avoid oil carbonization on the window surface, which provides enhanced x-ray output. More particularly, heat from the insert window  72  is dispersed through thermal conduction along the window  72  to the insert frame  52  and radiation to the side plates  86  and the top plate  76 . The heat transferred to the side plates  86  is dissipated through the extended outer surfaces  94  or cooling fins, which are in contact with the dielectric liquid coolant. In one embodiment, an air flow means  104  introduces cold air (shown by the arrow) toward a top surface of the top plate  76  in order to provide additional heat dissipation. 
     With reference to FIGS. 5 and 6 and continuing reference to FIG. 3, in alternate embodiments of the present invention, auxiliary cooling means are placed in thermal contact with the outer extended surface of the annular side plate. In one embodiment, illustrated in FIG. 5, the auxiliary cooling means includes a liquid coil  112 , which is embedded within the side plate. In this embodiment, water, oil, or another highly thermal conductive fluid is used as the liquid within the liquid coil. More particularly, a heat exchanger and pump means are placed in fluid communication with the cooling fluid circulation coil. The cooling fluid circulation coil or liquid coil aids in reducing the temperature of the side plate, thus increasing the heat conduction away from the insert window  72 . 
     In an alternate embodiment illustrated in FIG. 6, the auxiliary cooling means takes the form of a cooling jacket  114  which surrounds the extended outer surface  94  of the side plate  86 . Again, oil, water or another highly thermal conductive fluid is introduced to the cooling jacket through a heat exchanger and pump means in fluid communication with the cooling jacket. By helping to lower the temperature of the side plate, the cooling jacket  114  increases heat conduction away from the insert window. 
     With reference to FIG.  7  and continuing reference to FIG. 3, in an embodiment in which oxidation on the insert window  72  does not effect the quality of the x-ray image, the top plate is eliminated. Rather than filling the window chamber with non-oxygen gas, a cold air jet is introduced to the top surface of the insert window  72  through a pair of thin channels  120 , which are built adjacent the side plate. Cold air is forced against the top surface of the insert window  72  through the openings at the end of the thin channels  120 . In this embodiment, heat transfer away from the insert window  72  is accomplished by both the cold air injection and the thermal radiation/heat conduction achieved via the side plate  86 . 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the detailed description. Is it intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalence thereof.