Abstract:
A novel method of making a crystal block array (configured for coupling with photodetectors as part of an integrated detector module useful in advanced PET scanner systems) is disclosed herein. The novel method comprises a series of cutting, polishing, and assembling steps that utilize reflective sheet material. The crystal block arrays disclosed herein may be of various dimensions and geometries and are amenable to mass production.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 14/051,328 filed Oct. 10, 2013, which claims the benefit of priority to U.S. Provisional Application No. 61/712,181 filed on Oct. 10, 2012. These applications are incorporated herein by reference in their entireties for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to scintillation crystals and related crystal block assemblies used in Positron Emission Tomography (PET) scanners and, more particularly, to scintillation crystal block arrays (configured for coupling with photodetectors as part of an integrated detector module useful in advanced PET scanner systems), as well as to methods of making and using high performance scintillation crystal block arrays. 
       BACKGROUND 
       [0003]    In nuclear medicine, scintillation crystals have become important components of medical imaging devices. The performance of these medical imaging devices, including Positron Emission Tomography (PET) scanners, largely depends on the quality and uniformity of scintillation crystals and on related crystal block array assemblies. The cost of making such medical imaging devices is generally expensive. Thus, there is a need to reduce manufacturing costs by simplifying the procedures for making scintillation crystal block arrays. 
         [0004]    In a general sense, positron emission tomography is a medical imaging technique in which a patient ingests a radioactively tagged compound that mimics a naturally occurring compound. For reasons relating to the body&#39;s metabolism, the compound tends to accumulate in tumors. The radioactively tagged compounds tend to emit gamma rays. The gamma rays can be detected outside of the patient&#39;s body. In particular, when the scintillation crystals are struck by a gamma ray, they are likely to emit a photon (“scintillation”). The photon is in turn recognized by a photodetector, which generates an electronic signal. Various hardware and software components use the electronic signal to reconstruct the likely position (within a known tolerance) of the original gamma ray emission. 
         [0005]    In accordance with known methods of making a crystal block array, a scintillation crystal boule may be cut and polished to generate a plurality of individual scintillation crystal pixels that are then each surrounded by Teflon tape and grouped into a crystal block array. In accordance with other known methods, a reflective sheet material matrix may be defined, and individual scintillation crystal pixels can be disposed within slots defined by the sheet material matrix. However, these methods produce crystal block arrays that include gaps and inconsistently packed pixels. 
         [0006]    Better crystals and more uniform crystal block arrays provide better information about the gamma rays and thus provide a better image, and help lead to a better diagnosis, and potentially better medical treatment. Accordingly, and although some progress has made with respect to the development of crystal block arrays, there is still a need in the art for new crystal block arrays and related methods of manufacture to overcome the deficiencies and obstacles discussed above. 
       SUMMARY 
       [0007]    The present invention in an embodiment is directed to a novel method of making a crystal block array. The inventive method comprises at least the steps of: providing a selected crystal having a generally cylindrical form; cutting the selected crystal crosswise a plurality of times to yield a plurality of crystal pucks, with each crystal puck having a selected height; cutting at least one of the plurality of crystal pucks along a heightwise direction a plurality of times to yield a plurality of first crystal slabs; polishing the flat surfaces of at least two of the plurality of first crystal slabs to an optical finish; applying a first reflective sheet material on at least one face of the at least two of the plurality of first crystal slabs, thereby defining a first layered optical block assembly; cutting the first layered optical block assembly along a lengthwise direction a plurality of times to yield a plurality of second crystal slabs of sandwich construction; polishing the flat surfaces of at least two of the plurality of second crystal slabs of sandwich construction to an optical finish; and applying a second reflective sheet material on at least one face of the at least two of the plurality of second crystal slabs of sandwich construction, thereby defining a crystal block array. 
         [0008]    These and other aspects of the present invention will become more evident upon reference to the following detailed description and accompanying drawings. It is to be understood, however, that various changes, alterations, and substitutions may be made to the specific embodiments disclosed herein without departing from their essential spirit and scope. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The drawings are intended to be illustrative and symbolic representations of certain exemplary embodiments of the present invention (namely, the manufacture of an exemplary 4×4 pixelated crystal block array useful for operations within a PET scanner) and as such are not necessarily drawn to scale. In addition, the relative dimensions and distances depicted in the drawings are exemplary and may be varied in numerous ways. Finally, like reference numerals have been used to designate like features throughout the views of the drawings. 
           [0010]      FIG. 1A  is a perspective view of a selected crystal boule. 
           [0011]      FIG. 1B  is a perspective exploded view of the crystal boule shown in  FIG. 1A , but wherein the crystal boule has had its top and bottom portions cut-off and removed. 
           [0012]      FIG. 1C  is a perspective view of one of the crystal pucks shown in  FIG. 1B , but wherein the crystal puck has been cut lengthwise a plurality of times to yield a plurality of crystal slabs in accordance with an embodiment of the present invention. 
           [0013]      FIG. 2  is a top plan view of the sliced crystal puck shown in  FIG. 1C . 
           [0014]      FIG. 3  is a top plan view of the sliced crystal puck shown in  FIGS. 1C and 2 , but wherein the two outermost crescent-shaped slabs have been removed and a plurality of first wires (spacers) have been placed between the slabs and at their outer edges in accordance with an embodiment of the present invention. 
           [0015]      FIG. 4  is a top plan view of the sliced and spaced apart slabs shown in  FIG. 3 , but wherein a bonding agent has been placed in between each of the slabs, and between first and second glass end plates positioned adjacent to the outermost slabs to form a “sandwich” block in accordance with an embodiment of the present invention. 
           [0016]      FIG. 5  is a top plan view of the bonded together slabs and glass end plates shown in  FIG. 4 , but wherein UV light is being applied to facilitate curing of the bonding agent in accordance with an embodiment of the present invention. 
           [0017]      FIG. 6  is a top plan view of the bonded together slabs and glass end plates shown in  FIG. 5 , but after exposure to UV light and wherein the slabs have been cut lengthwise a plurality of times and in a transverse direction to the first plurality of lengthwise cuts to yield a plurality of crystal pixels (second slabs of sandwich construction) and a pair of outer pixel end pieces in accordance with an embodiment of the present invention. 
           [0018]      FIG. 7  is a top plan view of the plurality of crystal pixels (second slabs of sandwich construction) shown in  FIG. 6 , but wherein a plurality of second wires (spacers) have been placed between the second segmented slabs of sandwich construction and at their outer edges in accordance with an embodiment of the present invention. 
           [0019]      FIG. 8  is a top plan view of the unfinished crystal block array of  FIG. 7 , but wherein opposing third and fourth protective glass plates have been added and further bonding agent has been added. 
           [0020]      FIG. 9  is a top plan view of a crystal block array in accordance with an embodiment of the present invention. 
           [0021]      FIG. 10  is a perspective view of the crystal block array shown in  FIG. 9 . 
           [0022]      FIG. 11  is a top plan view of the sliced crystal puck shown in  FIGS. 1C and 2 , but wherein the two outermost crescent-shaped slabs have been removed. 
           [0023]      FIG. 12  is a top plan view of reflective sheet material stacked between crystal slabs of  FIG. 11 . 
           [0024]      FIG. 13  is a top plan view of the stack of  FIG. 12  that has been cut lengthwise a plurality of times and in a transverse direction to the first plurality of lengthwise cuts to yield a plurality of crystal pixels (second slabs of sandwich construction). 
           [0025]      FIG. 14  is a top plan view of the second slabs of sandwich construction of  FIG. 13  with crescent-shaped slabs removed. 
           [0026]      FIG. 15  is a top plan view of a crystal block array in accordance with an embodiment of the present invention where reflective sheet material is stacked between the second slabs of sandwich construction of  FIG. 14  to define the crystal block array. 
           [0027]      FIG. 16  is a perspective view of the crystal block array shown in  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Referring now to the drawings where like numerals have been used to designate like features throughout the views, and more specifically to  FIGS. 1A through 10 , the present invention in an embodiment is directed to a novel method of making a scintillation crystal block array  10  (as shown in Figures and  9  and  10 ) adapted for coupling with a planar photodetector array (not shown) as part of an integrated detector module useful in an advanced PET scanner system (not shown). In the inventive method and as an initial step, a crystal boule  7  ( FIG. 1A ) of an appropriately grown and sized scintillation crystal (such as, for example, a cerium-activated lutetium-based oxyorthosilicate scintillation crystal boule (Ce:LSO and/or Ce:LYSO) grown by the Czochralski method) is first selectively sliced (cut) into a plurality of “pucks” ( FIG. 1B ). In other words, a selected scintillation crystal boule  7  of a generally cylindrical form is cut a plurality of times along its width (perpendicular to its longitudinal axis) to yield a plurality of pucks  12 . The plurality of pucks  12  that are cut (sliced) from the selected cylindrical crystal boule  7  may or may not be of the same height. The cutting or slicing of the crystal boule  7  may be carried out with the aid of an appropriate cutting tool such as, for example, a diamond hardened saw. In addition, the height of each puck  12  is typically selected to be equal to the length of the individual scintillation crystal pixels  11  (utilized in a particular PET scanner) plus a minor allowance for grinding/polishing. Scintillation crystal pixels utilized in advanced PET scanners generally have dimensions of about 4×4×20 millimeters (mm) or about 4×4×22 millimeters (mm) or about 4.7×6.3×30 millimeters (mm), but are not limited to those exemplary dimensions. Accordingly, the present invention is not limited to any particular crystal or pixel dimension or range of dimensions and is applicable to the manufacture of scintillation crystal block arrays of various sizes and dimensions. For example, the inventive method is useful for making all types arrays having any number of rows and columns (2×2, 3×3, 4×4, 4×6, et cetera). 
         [0029]    As best shown in  FIGS. 1C and 2 , a puck  12  having a selected height (h) is further sliced (cut) along its height (parallel to its longitudinal axis) to yield a plurality of inner “slabs”  14  positioned between a pair of opposing end slab pieces  16 . The pair of opposing end slab pieces  16  are removed and set aside for other applications. The remaining inner slabs  14  are each further lapped and polished on all of their flat surfaces (preferably to at least an optical finish of it/8 or better) and readied for further assembly. 
         [0030]    As best shown in  FIG. 3 , the inner slabs  14  are reassembled back into the shape of the puck  12  (less the removed pair of opposing end slab pieces  16 ), but spaced apart from each other by means of first height-wise wires  18  selectively positioned between the slabs  14  and along their outer edges as shown. The first wires  18  are of a selected diameter that preferably ranges from about 0.1 mm to 1.0 mm, or preferably from about 0.5 mm to 0.6 mm. The diameter of the wires  18  corresponds to the distance between adjacent pixels  11  of the final crystal block array  10 . The wires  18  may be metallic or polymeric, and in some embodiments are composed of nylon. 
         [0031]    Next and as shown in  FIG. 4 , a curable liquid or semi-liquid bonding agent  19  (such as, for example, an optical cement made of a barium sulphate composition) is applied between each of the spaced apart slabs  14  and between first and second glass end plates  20 ,  22  that have been positioned adjacent to the two outermost slabs  14 , all of which together form a “sandwich” block. As shown in  FIG. 5 , the bonding agent (cement)  19  positioned within the sandwich block is subsequently cured (hardened) by exposure to UV light for a selected period of time. In this regard, curing times are generally a function of at least the type of bonding agent used, its applied thickness, and on the intensity of the light source. Accordingly, curing times may be as short as five minutes or as long as several hours. 
         [0032]    Next and as shown in  FIG. 6 , the cured sandwich block  24  is selectively sliced (cut) a plurality of times perpendicular to the first and second protective glass end plates  20 ,  22  (as well as to the inner bonded together first slabs  14 ) to yield a plurality of second slabs of sandwich construction  24 , and a pair of opposing end second slab pieces  26 . The pair of opposing end second slab pieces  26  are removed and set aside for other applications. The remaining inner second slabs of sandwich construction  24  are each further lapped and polished on all of their flat surfaces (preferably to at least an optical finish of A/8 or better) and readied for further assembly. 
         [0033]    As shown in  FIG. 7 , the inner second slabs of sandwich construction  24  are reassembled back into the shape of an unfinished crystal block array (less the removed pair of opposing end second slab pieces  26 ), but spaced apart from each other by means of second height-wise wires  28  selectively positioned between the second slabs of sandwich construction  24  and along their outer edges (and adjacent to the first and second glass end plates  20 ,  22 , which have now been cut as described above) as shown. The plurality of second wires  28  may or may not be the same diameter as the plurality of first wires  18 . 
         [0034]    Next and as shown in  FIG. 8 , a pair of opposing third and fourth protective glass end plates  30 ,  32  are similarly positioned adjacent to and spaced apart from the unfinished crystal block array, and thereafter additional bonding agent  19  is poured into the space therebetween and subsequently cured as before. The first, second, third, and fourth protective glass end plates  20 ,  22 ,  30 ,  32  are all removed by use a diamond plate tool, for example, and all of the surfaces are cleaned to thereby yield (as shown in  FIGS. 9 and 10 ) a crystal block array  10  having highly uniform and controlled spacing between individual pixels  11 . 
         [0035]    In some embodiments, alternative materials may be used to surround individual pixels  11  instead of bonding agent  19 . For example, pixels  11  may be surrounded by a reflective sheet material such as a film or tape. In one preferred embodiment, 3M Radiant Mirror Film VM 2000 (3M Company, Minnesota) is used. Manufacturing steps including the use of spacers or wires  18  may be absent in various embodiments where a reflective sheet material is used. 
         [0036]    Accordingly, referring to  FIG. 11  crescent-shaped slabs  16  can be removed from the sliced crystal puck  12  shown in  FIGS. 1C and 2 . As shown in  FIG. 12 , the remaining slabs  14  may be stacked with first reflective sheet material  42  between each of the slabs  14 . and with first reflective sheet material  40  at the ends of the stack. In various embodiments, an adhesive may be applied between respective faces of reflective sheet material  40 ,  42 , and respective faces of the slabs  14  so that the slabs  14  and reflective sheet material  40 ,  42  are rigidly coupled. Such a stack may be cut into second slabs of sandwich construction  44 ,  46  as depicted in  FIG. 13 . Crescent-shaped second slabs of sandwich construction  46  may be removed as shown in  FIG. 14  and cut faces of the second slabs of sandwich construction  44  may be polished to an optical finish. 
         [0037]    The second slabs of sandwich construction  44  may be further stacked with a second reflective sheet material  46  between each of the second slabs of sandwich construction  44  and with second reflective sheet material  48  at the ends of the stack to yield a crystal block array  10  as depicted in  FIGS. 15 and 16 . The first and second reflective sheet materials may be the same or different in some embodiments. 
         [0038]    In various embodiments, an adhesive may be applied between respective faces of reflective sheet material  46 ,  48 , and respective faces of the second slabs of sandwich construction  44  so that the second slabs of sandwich construction  44  and reflective sheet material  46 ,  48  are rigidly coupled. An optical adhesive may be selected having desirable qualities. In some embodiments, an adhesive may require curing or other treatment, which may include heating, drying, or exposure to light, including ultraviolet light. Accordingly, in some embodiments, a production method may include one or more step of curing or treating an applied adhesive. 
         [0039]    While the present invention has been described in the context of the embodiments illustrated and described herein, the invention may be embodied in other specific ways or in other specific forms without departing from its spirit or essential characteristics. Therefore, the described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.