Abstract:
To provide simpler, more efficient methods for making scintillator arrays, one embodiment of the present invention is a method for making a scintillator array. The method includes extruding a mixture of a scintillator powder and a binder into rods; laminating the extruded rods with a sinterable reflector material; and sintering the laminated rods and reflector material into a scintillator block. Scintillator array embodiments of the present invention are useful in many types of pixelated radiation detectors, such as those used in computed tomography systems.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates generally to methods for making scintillator arrays used in radiation detectors, and to the scintillator arrays made from these methods.  
           [0002]    In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.  
           [0003]    In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.  
           [0004]    Detectors of CT and other types of x-ray imaging systems utilize scintillation detectors having pixelated scintillator arrays. It would therefore be desirable to provide simplified, inexpensive methods for making such arrays, and to provide inexpensive, pixelated scintillator arrays for CT and other imaging applications.  
         SUMMARY OF INVENTION  
         [0005]    To provide simpler, more efficient methods for making scintillator arrays, one embodiment of the present invention is a method for making a scintillator array. The method includes extruding a mixture of a scintillator powder and a binder into rods; laminating the extruded rods with a sinterable reflector material; and sintering the laminated rods and reflector material into a scintillator block. Scintillator array embodiments of the present invention are useful in many types of pixelated radiation detectors, such as those used in computed tomography systems. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    [0006]FIG. 1 is a pictorial view of a CT imaging system.  
         [0007]    [0007]FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.  
         [0008]    [0008]FIG. 3 is a flow chart illustrating two different method embodiments of the present inventive method for making a scintillator array.  
         [0009]    [0009]FIG. 4 is a flow chart illustrating another embodiment of the present invention for making a scintillator array.  
         [0010]    [0010]FIG. 5 is a simplified perspective representation of a scintillator block of the present invention during one stage of its fabrication.  
     
    
     DETAILED DESCRIPTION  
       [0011]    Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a radiation detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through an object  22 , for example a medical patient. Detector array  18  may be fabricated in a single slice or multi-slice configuration. In one embodiment of the present invention, and as described below, detector elements  20  comprise sintered scintillator elements. Each scintillator element produces light in response to x-ray radiation, which is converted to an electrical signal by a sensing region of a semiconductor array optically coupled thereto. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam on that detector element and hence the attenuation of the beam as it passes through patient  22  at a corresponding angle. During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 .  
         [0012]    Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
         [0013]    Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 .  
         [0014]    In one embodiment of the present invention and referring to FIG. 3, a scintillator precursor is prepared by mixing  10  a temporary organic binder or gel with a scintillator powder. Suitable organic binders include organic binders or gels used in ceramic molding, such as polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. Suitable scintillator powders include gadolinium oxysulfide (GdOS), Lumex ((YGd) 2 O 3 ), cadmium tungstate (CdWO 4 ), GGG (Gd 3 Ga 5 O 12  garnet), bismuth germanate (BGO, Bi 4  Ge 3  O 12 ) and mixtures thereof.  
         [0015]    The scintillator precursor is partially solidified or dried to make a flexible “cake,” that is extruded  12  through a die or multiple dies to make a square or round rod. The rods are then cut to a length dependent upon the embodiment. The cut parts are then assembled  14  into an array with sheets, laminates or layers that comprise a low melting point or easily dissolvable sacrificial material. Suitable materials for such sheets, laminates or layers include any low melting point polymer sheet coated with adhesive, such as polyester films, MYLAR® and polycarbonate films. The sheet, laminate, or layer forms a separator between individual scintillator elements or pixels. In one embodiment, the entire separator is made of a sacrificial material.  
         [0016]    In one embodiment, the scintillator structure is also supported  16 , for example, by bonding conforming plates at cut ends or faces of the scintillators to hold extrusions in place during subsequent operations. After building the structure, the sacrificial layer is removed  18 , for example, by heating the structure to a temperature at which the sacrificial layer material melts away. In one embodiment, the sacrificial layer is entirely burned out. In another embodiment, the sacrificial layer is removed by a solvent. The precursor material that remains is sintered  20  at an appropriate sintering temperature to form a scintillator array.  
         [0017]    After sintering and removal of the sacrificial layer, the gap left by the sacrifical layer is filled  22  with a reflective material which separates each scintillator array into individual channels or pixels. Examples of suitable reflective materials include titanium dioxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), and barium sulfate (BaSO 4 ) powders, and mixtures thereof. The conforming plates are then removed  24  and the long scintillator array is diced or cut  26  into thinner arrays suitable for the desired application.  
         [0018]    In one embodiment, after preparing  10  the precursor material, extruding  12  the cake, the rods are laminated  14  with a material that is not removed (or not completely removed) when the rods are sintered  28 . Thus, it is not necessary to use conforming plates to hold the laminated structure together during sintering. After sintering  28 , conforming plates are then applied  30  to the ends of the rods and the laminate material is then removed  32 . Removal  32  of the laminate forming the sacrificial layers is accomplished, for example, by use of heat or a solvent. The gaps left by removal of the sacrificial laminate material are then filled  22 , the conforming plates are removed  24 , and the resulting structure cut or sliced into sections  26 , as in an embodiment described above[0009] In embodiments in which the separator layer serves as a permanent part of the array, there are no gaps to fill with reflector, so the extruded and sintered array is simply diced into a thickness appropriate for the desired application. More particularly, and referring to FIG. 4, powder and binder are mixed  10  and extruded  12  into rods, as in the embodiments of FIG. 3. The rods are then laminated (i.e., coated and joined)  34  with a sinterable reflector material. Suitable sinterable reflector materials include, for example, titanium dioxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), and barium sulfate (BaSO 4 ) powders, and other high temperature inorganic reflectors capable of surviving the scintillator sintering temperature, as well as mixtures of sinterable reflector materials. The rods and reflector material are then heated and sintered  36  in one operation, leaving a sintillator block that can simply be sliced  38  into sections of desired dimensions.  
         [0019]    [0019]FIG. 5 is a simplified view of one embodiment of a “laminated” scintillator block  40  comprising a plurality of rods  42  of extruded scintillator material, a sheet of sacrificial laminate material  44 , and additional sacrificial material  46 . Although FIG. 5 shows only four rods  42 , it is illustrative of embodiments having a larger number of rods  42 .  
         [0020]    Extruded rods  42  comprising a scintillator powder and an organic binder are assembled with elongate axes parallel to one another. In FIG. 5, rods  42  have a square cross-section transverse to their elongate (extruded) dimension, having been extruded through a square die. However, other embodiments utilize round rods or rods having other geometrical shapes. Laminate  40  is assembled using sacrificial materials  44  and  46 . For example, rods  42  are assembled parallel to one another, in layers  48  parallel to one another, using a sheet  44  of laminate material between layers  48 . In one embodiment, sheet  44  has adhesive properties (e.g., it is coated with an adhesive) so that rods  42  adhere to sheet  44 . In another embodiment, rods  42  are dipped in a sacrificial adhesive (not shown) to adhere rods  42  to sheet  44 . A liquid or solid (e.g., powdered) sacrificial material  46  is applied between rods  42  in each layer. Although not shown in FIG. 5, the embodiment described herein is scalable, so that laminated block  40  embodiments of the present invention can comprise any number of layers  48 , and a layer  48  can comprise any number of blocks  42 . Opposite faces  50 ,  52  of rods  42  are then joined or bonded to conforming plates  54  (only one of which is shown in FIG. 5). In one embodiment, opposite faces  50  and  52  of rods  48  are flat and parallel to one another, so conforming plates  54  are also flat and parallel to one another.  
         [0021]    Sacrificial laminate sheets  44  and additional sacrificial material  46  are removed by heating or by dissolution in a solvent. However, because rods  42  are bonded to conforming plates  54  at faces  50  and  52 , rods  42  maintain their separation from one another, and gaps remain where sacrificial laminate sheets  44  and additional sacrificial material  46  is removed. The luminescent powder comprising rods  42  is then sintered in the rods by further heating. Gaps between rods  42  are then filled with a reflector material. Conforming plates  54  are removed, and the resulting scintillator block  40  is sliced in a direction perpendicular to the length of rods  42  and parallel to faces  50  and  52 . Each slice is useful as a scintillator assembly for a detector array.  
         [0022]    In one embodiment, a sacrificial material  46  is applied by dipping each rod  42  into a sacrificial material  46 . In this embodiment, no sacrificial laminate material  44  is required. Instead, sacrificial material  46  separates rods  42  both within layers  48  and between layers  48 .  
         [0023]    In one embodiment, rods  42  are sintered prior to removal of sacrificial material  46 , or  44  and  46 . After sintering, sacrificial material  46 , or  44  and  46  is removed and the resulting gaps filled with a reflector material (not shown in FIG. 5). Conforming plates  54  are removed and the resulting scintillator assembly  40  is diced into sections as above.  
         [0024]    In yet another embodiment, after extrusion of rods  42 , rods  42  are assembled into an array  40  using a sinterable reflector material (not shown in FIG. 5) instead of sacrificial laminate material  44  and additional laminate material  46 . For example, the sinterable reflector material is provided in the form of a sheet, a coating (e.g., a liquid), or a powder. Rods  42  of sintillator powder mixture and the reflector material are then sintered together, so that there is no gap filling required, and thus, no conforming plates  54  are required. The resulting sintered assembly  40  is simply sliced into sections using cuts perpendicular to the direction of the rods.  
         [0025]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.