Abstract:
An improved design for a solid state X-ray detector that decreases the amount of moisture diffusion that occurs through seals used to attach a cover to a glass panel substrate, thereby protecting the scintillator from moisture damage. In one embodiment, a second hermetic or semi-hermetic seal is introduced between the scintillator and the outside environment to increase the path moisture must travel to reach the scintillator. In another embodiment, a metal frame, preferably a Kovar® frame, is hermetically or semi-hermetically sealed to the cover and glass panel substrate, thereby decreasing the amount of moisture diffusion through the semi-hermetic seal of the prior art.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to a solid state X-ray detector and more specifically to an improved scintillator sealing for a solid state X-ray detector.  
           [0002]    The X-ray detectors have become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. One category of X-ray detectors uses scintillator materials to convert X-ray photons into visible-spectrum photons as part of the energy detection process. These scintillator materials are ionic salts such as CsI, which are hygroscopic. CsI is a crystalline material, with needle-shaped crystals. The crystals are oriented perpendicular to the plane of an adjacent glass substrate panel and act as short optical fibers to ensure that light photons originating in a crystal exit the crystal at its end and into an adjacent photodetector, rather than propagating within the CsI layer. The detector is sealed to prevent moisture from being absorbed into the scintillator. This moisture could adversely affect the crystal structure of the scintillator and degrade the image quality of the image detector. Additionally, the solid state electronics which convert the visible-spectrum photons to electrical signals in the image detector also should be protected from moisture to prevent their corrosion and consequent performance degradation.  
           [0003]    A true hermetic seal, allowing effectively zero diffusion of moisture, generally requires an inorganic material such as metal or glass to act as the barrier to moisture. Organic materials, such as epoxy adhesives and sealants, do not offer true hermecity, but rather offer a low diffusion rate of moisture, which is dependent upon their formation, the path length required for moisture to penetrate through diffusion, and the quality of their adhesion to the surfaces they are sealing. Epoxy sealants and adhesives are referred to as semi-hermetic seals.  
           [0004]    Current methods used to create a semi-hermetic seal use an epoxy sealant to attach a cover to the top layer of the image detector. The cover consists of a composite structural plate made of graphite fiber cloth in an epoxy matrix, with thin aluminum layers on one or both sides of the fiber cloth. The aluminum layers are positioned adjacent to the detector and provide a hermetic barrier over the detection surface area. This cover is bonded to the glass detector substrate with an epoxy seal, providing a semi-hermetic barrier at each edge of the cover. The X-ray image detector thus consists of a flat panel, with one face sealed by glass, one face sealed by aluminum, and the edges sealed by epoxy. Contained within the cover and detector layer are a scintillator and an Opticlad layer. The Opticlad layer consists of a plastic backing sheet with a layer of metal (typically silver or gold) and a layer of titanium oxide (TiO) and serves to reflect visible spectrum that would otherwise be wasted back to the diode layer of the detector where it is detected.  
           [0005]    As the thickness of the scintillator layer is increased, the area over which the epoxy provides a semi-hermetic layer increases in direct proportion. Since the epoxy seal is not truly hermetic, this increases the probability of penetration by sufficient moisture to damage the detector. Also, application of the epoxy sealant required for a thicker scintillator layer is time-consuming.  
           [0006]    It is therefore highly desirable to improve the method for sealing a scintillator for a solid state X-ray detector between the cover and substrate.  
         SUMMARY OF INVENTION  
         [0007]    The present invention proposes several different methods by which to improve the hermetic sealing of the scintillator for a solid state X-ray image detector.  
           [0008]    In one embodiment, a portion of the Opticlad layer that is free of its TiO coating is extended. The metal outer layer of this portion of the Opticlad layer is flexed towards and bonded to the glass substrate panel with an epoxy sealant, thereby creating a second semi-hermetic seal between the scintillator and outside moisture.  
           [0009]    In another embodiment, an insulating layer is deposited onto the panel in the area to be used for the seal. Over that, a layer of metal that can be reflowed during laser welding is deposited. The metal layer of the Opticlad layer is then laser welded to the metal layer on the top surface of the glass panel, thereby creating a second hermetic seal between the scintillator and outside moisture.  
           [0010]    In a third embodiment, a metal frame is fabricated and sealed to the inner aluminum face of the graphite cover. The metal frame, preferably a metal alloy such as Kovar®, has a length and width of the required seal, and of a rectangular section approximately equal to that of the scintillator. The metal frame replaces much of the volume of the epoxy seal, resulting in a smaller cross-sectional area of epoxy for moisture to diffuse through.  
           [0011]    In a fourth embodiment, which also utilizes a metal frame, a metal such as nickel or gold that can be easily welded is deposited on the aluminum of the graphite composite cover. The metal frame is then welded or soldered directly to the deposited metal layer to create a cover layer with the metal frame attached, as compared to epoxy seal as in the third embodiment described above. This eliminates approximately one-half of the epoxy as used in the third embodiment, thus again reducing the exposed cross-sectional area of epoxy for moisture to diffuse through.  
           [0012]    The fifth embodiment builds upon the principles of the third and fourth embodiments, and adds an insulating layer and metal layer that can be welded or soldered between the metal frame and glass substrate panel as well. In this method, the epoxy seal is completely eliminated, and thus the problem of moisture diffusion is also eliminated.  
           [0013]    Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    [0014]FIG. 1 is a perspective view of a imaging system according to one preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a close-up view of a portion of FIG. 1;  
         [0016]    [0016]FIG. 3 is a close-up view of the sealing mechanism of the X-ray detector according to the prior art;  
         [0017]    [0017]FIG. 4 is a close-up view of the sealing mechanism of the X-ray detector according to one preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is a close-up view of the sealing mechanism of the X-ray detector according to another preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 6 is a close-up view of the sealing mechanism of the X-ray detector according to another preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 7 is a close-up view of the sealing mechanism of the X-ray detector according to another preferred embodiment of the present invention; and  
         [0021]    [0021]FIG. 8 is a close-up view of the sealing mechanism of the X-ray detector according to another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Referring now to FIG. 1, an imaging system  10 , for example, an X-ray imaging system, is shown including a photodetector array  12  and an X-ray source  14  collimated to provide an area X-ray beam  16  passing through an area  18  of a patient  20 . Beam  16  is attenuated by an internal structure (not shown) of patient  20  to be received by detector array  12  which extends generally over an area in a plane perpendicular to the axis of the X-ray beam  16 .  
         [0023]    The detector array  12  is preferably fabricated in a solid-state panel configuration having a plurality of detector elements, or pixels (not shown in FIG. 1) arranged in columns or rows. As will be understood by those of ordinary skill in the art, the orientation of the columns and rows is arbitrary; however, for clarity of description, it will be assumed that the rows extend horizontally and the columns extend vertically. Each pixel includes a photosensor, such as a photodiode, that is coupled via a switching transistor (field effect transistor, or FET) to two separate address lines, a scan line and a data line (not shown in FIG. 1). The radiation incident on a scintillator material (shown as  54  in FIGS.  4 - 8 ) and the pixel photosensors measure, by way of change in the charge across the photodiode, the amount of light generated by X-ray interaction with the scintillator. As a result, each pixel produces an electrical signal that represents the intensity, after attenuation of patient  20 , of an impinging X-ray beam  16 .  
         [0024]    System  10  also includes an acquisition control and image-processing circuit  30  that is electrically connected to X-ray source  14  and detector array  12 . More specifically, circuit  30  controls X-ray source  14 , turning it on and off and controlling the tube current and thus the fluence of X-rays in beam  16  and/or the tube voltage and thereby altering the energy of the X-rays in beam  16 . In one embodiment, acquisitioning control and image processing circuit  30  includes a data acquisition system (DAS)  32  having at least one DAS module, or circuit (not shown in FIG. 1), which samples data from detector array  12  and transmits the data signals for subsequent processing. Each DAS module can include a plurality of driver channels or a plurality of readout channels. Acquisition control and image processing circuit  30  receives sampled X-ray data from DAS and generates image and displays the image on a monitor, or cathode x-ray tube display  36  based on the data in each pixel.  
         [0025]    As shown in FIG. 2, the photodetector array  12  consists of an amorphous silicon array  50  coupled to a glass substrate panel  52 . The amorphous silicon array  50  is comprised of a series of pixels, or detector elements, containing a photosensor and a switching transistor. The pixels produce an electrical signal that represents the intensity, after attenuation, of an impinging X-ray.  
         [0026]    A scintillator  54  and Opticlad layer (shown as  56  in FIG. 3) are stacked on top of the amorphous silicon array  50  and are contained within a cover (shown as  58  in FIG. 3). The scintillator  54  materials are ionic salts such as cesium iodide (CsI), which are hygroscopic. CsI is a crystalline material, with needle-shaped crystals. The crystals are oriented perpendicular to the plane of the glass substrate panel  52  and they act as short optical fibers to ensure that light photons originating in the crystals exit at its ends of the crystals and into its amorphous silicon array  50 , rather than propagating within the CsI layer. Absorption of moisture into the scintillator  54  will spoil the crystal structure of the CsI and degrade the image quality of the image detector.  
         [0027]    The Opticlad layer  56  consists of a plastic backing sheet with a layer of metal (typically silver or gold) and a layer of titanium oxide (TiO) and serves to reflect visible spectrum that would otherwise be wasted back to the diode layer of the amorphous silicon array  50  where it is detected.  
         [0028]    The cover  58  consists of a composite structural plate made of graphite fiber cloth in an epoxy matrix, with thin aluminum layers on one (inner layer shown as  65  in FIG. 6) or both sides of the graphite fiber cloth.  
         [0029]    [0029]FIG. 3 illustrates the sealing mechanism for coupling the cover  58  to the glass substrate panel  52  according to the prior art. The cover  58  is sealed to the glass substrate  52  using a polymer sealant, preferably an epoxy sealant  59 . Together, the aluminum layers of the cover  58  and the epoxy sealant  59  provide a moisture barrier to protect the scintillator  54  material contained within the cover  58  and glass substrate  52 . However, because the epoxy sealant  59  is semi-hermetic, it is possible for a certain amount of moisture to diffuse through the epoxy sealant over time to damage the crystalline structure of the scintillator  54  material. The amount of diffusion of moisture through the sealant is dependent upon numerous factors, including but not limited to the type of polymer material used in the sealant as well as the cross-sectional area of the sealant. Epoxy sealants  59  are preferred for their low diffusion rate.  
         [0030]    FIGS.  4 - 8  illustrate five preferred embodiments of the present invention, in which the hermetic sealing between the cover  58  and glass substrate panel  52  is improved, thereby minimizing or preventing the diffusion of moisture within the cover  58  to damage the scintillator  54 .  
         [0031]    In the preferred embodiment as shown in FIG. 4, a portion  70  of the Opticlad layer  56  that is free of a TiO coating is flexed and bonded to the surface of the glass substrate panel  52  using an epoxy sealant  74 , thereby creating a second semi-hermetic seal between the scintillator  54  and outside moisture.  
         [0032]    In another embodiment, as shown in FIG. 5, an insulating layer  76  is deposited onto the glass substrate panel  52  in the area to be used for the seal. Over that, a layer of metal  78  that can be reflowed during laser welding is then deposited. The portion  70  of the Opticlad layer  56  is then laser welded to the metal layer  78  on the top surface of the glass panel  52 , thereby creating a second hermetic seal between the scintillator  54  and outside moisture.  
         [0033]    In a third embodiment, as shown in FIG. 6, a metal frame  90  of length and width of the required seal, and of a rectangular section approximately equal to that of the scintillator  54 , is fabricated and sealed to the inner aluminum face  65  of the cover  58  and to the glass substrate using an epoxy sealant  92 . The metal frame  90  thus replaces much of the volume of the epoxy seal, resulting in a smaller cross-sectional area of epoxy sealant  92  for moisture to diffuse through.  
         [0034]    In a fourth embodiment, as shown in FIG. 7, which also utilizes the metal frame  90 , a metal  93  such as nickel or gold that can be welded or soldered is deposited on the inner aluminum face  65  of the cover  58 . The metal frame  90  is then welded or soldered directly to that deposited metal layer  93  to create a cover layer with the metal frame  90  attached, as compared to epoxy seal as in FIG. 6 described above. This eliminates approximately one-half of the epoxy as used in the third embodiment, thus reducing the exposed cross-sectional area of epoxy for moisture to diffuse through.  
         [0035]    The fifth embodiment, as shown in FIG. 8, builds upon the principles of the third and fourth embodiment, and adds an insulating layer  94  and metal layer  96  that can be welded or soldered to the glass substrate panel  52  as well. In this method, the epoxy seal is completely eliminated, and thus the problem of moisture diffusion through the epoxy seal is also eliminated.  
         [0036]    The metal used in the metal frames  90  of FIGS.  6 - 8  should have a similar coefficient of thermal expansion to glass (3.85 ppm/C) to reduce thermal-induced stresses when attached to glass. The metal should also be weldable and solderable. Metal alloys are preferred for this type of application. One preferred metal alloy is Kovar® (5.86 ppm/C), manufactured by Carpenter Technology Corporation. Kovar® is a vacuum formed, iron-nickel-cobalt, low expansion metal alloy material. Of course, other metal alloys having similar physical and thermal attributes may be used in place of the Kovar® in the metal frame  90  as is contemplated in the art.  
         [0037]    The hermecity of the sealing within the detector array can be greatly improved by utilizing one of the design techniques disclosed in FIGS.  4 - 8 . These designs minimize or eliminate moisture diffusion through the sealing mechanisms, thereby protecting the scintillator  54  from moisture damage. The methods proposed in FIGS.  4 - 8  offer simple, inexpensive solutions that can be readily incorporated into known detector designs.  
         [0038]    While one particular embodiment of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.