Patent Publication Number: US-2022236426-A1

Title: Hybrid lased/air coupled pet block detector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to United States Provisional Application No. 62/903,257, filed on Sep. 20, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a method for fabricating a detector array for use in imaging applications such as X-ray imaging, fluoroscopy, positron emission tomography (PET), single photon emission computed tomography (SPECT), computed tomography (CT), gamma camera and digital mammography systems. 
     BACKGROUND 
     Imaging is widely used in many applications, both medical and non-medical. In the field of imaging, it is well known that imaging devices incorporate a plurality of scintillator arrays for detecting radioactivity from various sources. When constructing scintillator arrays composed of discrete scintillator elements, often the scintillator elements are packed together with a reflective medium interposed between the individual elements creating photon boundaries. Conventionally, the reflective medium serves to direct the scintillation light along the scintillator element into a light guide to accurately determine the location at which the radiation impinges upon the detector elements. The reflective medium further serves to increase the light collection efficiency from each scintillator element as well as to minimize the cross-talk, or light transfer (transmission of light), from one scintillator element to an adjacent element. The reflective media include reflective powders, films, paints, and adhesives doped with reflective powders, or a combination of materials. Reflective paints and powders contain one or more pigments such as MgO, BaSO 4 , and TiO 2 . Regardless of the approach, the conventional method of fabricating radiation detector arrays is a time- and labor-intensive process, with product uniformity dependent upon the skill level of the workforce. With the current market trend of higher spatially-resolute systems containing an order of magnitude more number of pixels than current designs, these process effects are even more pronounced. 
     Detector arrays are commonly integrated with photomultiplier tubes (PMTs) or solid-state photodetectors such as silicon photomultipliers (SiPMs), avalanche photodiodes (APDs), PIN diodes, and charge-coupled devices (CCDs). The incident high-energy photons absorbed by the scintillating material are converted to lower energy scintillation photons, which may be guided to the photodetectors via one or more of the following: the scintillator itself, a light guide, and other established means of light distribution. 
     In the arrangement wherein a light guide and/or other established means is used, commonly the light guide is formed by creating slits of various depths in a suitable substrate. Once packed with a reflective media, the light guide becomes an effective method to channel light and to enhance the position information of the scintillator. In the arrangement wherein paint or reflective tape is used, the paint or reflective tape is applied directly to the scintillators, achieving similar results. The height and placement of the applied reflective material varies according to design. 
     Conventionally, scintillator arrays have been formed from polished or unpolished crystals that are either: hand-wrapped in reflective PTFE tape and bundled together; glued together using a white pigment such as BaSO 4  or TiO 2  mixed with an epoxy or RTV; or glued to a glass light guide with defined spacing and afterwards filled with reflective material as discussed above. 
     Another approach utilizes individual reflectors bonded to the sides of certain scintillator elements with the aid of a bonding agent. An array is formed by arranging the individual elements spatially such that the impingement of the high-energy photon is decoded accurately. 
     SUMMARY 
     Provided herein is a method for fabrication of an optically-segmented detector array, such as a scintillator array. The method comprises: preparing a plurality of slabs of an optical medium of an imaging device; forming a plurality of optical boundaries within at least one of the slabs of optical medium, wherein the plurality of optical boundaries defining a 1×N array of non-contiguous, independent light-redirecting regions within the at least one slab; arranging the plurality of slabs into a stack with a reflective layer defined between each adjacent slab; and affixing the positions of the plurality of slabs with respect to each other. 
     Provided is a detector array comprising: a plurality of slabs of an optical medium arranged in a stack, wherein at least one of the slabs comprising: a 1×N array of non-contiguous, independent light-redirecting regions defined by N−1 optical boundaries, wherein each optical boundary is formed by an array of non-contiguous, independent micro-voids; a reflective layer defined between two adjacent slabs of the detector array, the reflective layer functioning to reflect light from the non-contiguous, independent light-redirecting regions, such that no other reflective material is present between adjacent slabs; and a mechanism for maintaining a relative position of each of the slabs of the array with respect to each other. 
     Also provided is a detector assembly that includes the detector array of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the embodiments described herein will be more fully disclosed in the following detailed description, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts. All drawing figures are schematic and they are not intended to represent actual dimensions of the structures or relative ratios of their dimensions. 
         FIG. 1  is an illustration of a detector array according to an embodiment of the present disclosure. 
         FIG. 2  is an illustration showing a partial exploded view of the detector array of  FIG. 1 . 
         FIG. 3  is an illustration showing an array of non-contiguous, independent micro-voids that form the optical boundary between scintillator elements in each of the slabs that form the detector array of the present disclosure. 
         FIGS. 4A-4B  are illustrations of examples of detector assembly according to some embodiments. 
         FIG. 5  is a flowchart illustrating a method for fabricating a detector array according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. The drawing figures are not necessarily to scale and certain features may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. 
     Referring to  FIGS. 1 and 2 , a detector array  100  according to an embodiment is disclosed. The detector array  100  comprises a plurality of slabs  110  of optical medium arranged in a stack. At least one of the slabs  110  comprises a 1×N array of non-contiguous, independent light-redirecting regions  115  (i.e. detector elements) within the at least one slab  110 . Each of the non-contiguous, independent light-redirecting regions  115  is defined by N−1 number of optical boundaries  120 . 
     In some embodiments of the detector array  100 , each of the plurality of slabs  110  comprises a 1×N array of non-contiguous, independent light-redirecting regions  115  within each of the slab  110 . The stack of the slabs  110  would then form an M×N array of detector elements  115 , where M represents the number of the slabs  110  of optical medium. 
     The slabs  110  are assembled into the stack so that there is reflective layer  130  defined between two adjacent slabs  110  of the array. The reflective layer  130  functions as a reflector and reflects light from within a detector element  115  to travel within the detector element  115 . 
     In some embodiments, the reflective layer  130  can be an air-filled gap between the two adjacent slabs  110  and the detector array  100  has no other reflective material present between adjacent slabs  110 . The width of the air-filled gap  130  depends on the surface roughness of the slabs  110 . In preferred embodiments, the slabs  110  are configured to have surface roughness so that when two slabs  110  are brought together, the width of the air-filled gap  130  is greater than 1× to 2× of the wavelength of the light generated by the optical medium of the detector element  115  so that evanescent waves (frustrated total internal reflection) are suppressed. 
     In some embodiments the reflective layer  130  comprises at least one of the following reflective materials: reflective powder, reflective film, reflective paint, and an adhesive doped with reflective powder, or a combination of the reflective materials 
     The stacked slabs  110  are maintained in their relative positions in the detector array  100  with respect to each other by a mechanism such as a bonding agent or a retainer  150 . 
     In some embodiments, the optical medium is scintillator crystal and the scintillator slabs  110  are stacked to form the detector array  100 , which is a scintillator array. The air-filled gaps  130 , in conjunction with the surface finish of the scintillator slab  110 , define the light collection efficiency of the scintillator elements  115  as well as the amount of light sharing that occurs between the elements  115 . The significant change in the index of refraction (IOF) from a detector element  115  and air increases the angle of total refraction. Based on the ratio IOF(scintillator)/IOF(air) and the surface finish of the scintillator slabs  110  the amount of scintillation light photons is tuned such that a controlled amount of photons are collimated down through a scintillator element  115  and a controlled amount are transmitted to neighboring scintillator elements  115 . The optimal ratio is customized for each scintillator element  115  within the detector array  100  such that each element  115  in the detector array  100  is clearly identified. The ratio may be spatially variant. 
     Referring to  FIG. 3 , each optical boundary  120  in the scintillator slab  110  is formed by an array of non-contiguous, independent micro-voids  12  formed in the optical medium. The plurality of micro-voids  12  is defined to collectively function to channel scintillation light through optically-segmented portions  115  of the scintillator. The micro-voids  12  are positioned in a spatial plane  14  in the optical medium to define the optical boundaries  120  of the optically-segmented portions  115  of the detector array  100 . The micro-voids  12  can be disposed in varying sizes in a specific uniform pattern, or randomly placed. The micro-voids may be disposed in single or multiple layers, or may be randomly scattered within a given volume. The micro-voids can be disposed in planar, curvilinear, or other geometrically-arranged configurations on the spatial plane  14 . To this extent, the optically-segmented portions of the scintillator can define various cross-sectional configurations other than square. For example, the optically-segmented portions can define triangular, trapezoidal, or hexagonal geometries. Alternatively, the optically-segmented portions can define a combination of configurations, such as octagons and squares. 
     The micro-voids are formed using a laser source  30 . The laser source  30  is used to generate and focus a beam of light  32  into an optical medium at each selected location (x,y,z) in sequence. The laser source  30  yields a laser beam  32  of sufficient power to ablate the targeted optical medium at the focal point, resulting in a damage to the crystalline structure of the optical medium at that location which will be referred to herein as a micro-void  12 . The micro-void causes photon that encounters it to scatter. The micro-voids are sometime referred to as micro-cracks in the industry. 
     As mentioned herein, the optical medium used for the detector array  100  can be a scintillator or a light-transmitting block, or light guide. The optical media is fabricated from a material that does not absorb in the wavelength of the laser. The intense energy collecting at the focal point of the laser beam creates a micro-void within the targeted optical media that extends outward in all directions from the point of origin. More details of the laser process for forming the micro-void can be found in U.S. Pat. No. 8,470,214, the contents of which are incorporated by reference herein. 
     Illustrated in  FIG. 4A  is an example of a detector assembly  100 A according to some embodiments. The detector assembly  100 A comprises the detector array  100  and at least one photodetector  18 . The detector array  100  is coupled to the at least one photodetector  18  and the array of scintillator elements  115  are optically coupled to the at least one photodetector  18 . The at least one photodetector  18  can be selected from, but not limited to, PMTs, position sensitive PMTs, SiPMs, APDs, PIN diodes, CCDs, and other solid state detectors. 
     In this arrangement, the scintillator elements  115  disposed within the detector array  100  serve to detect an incident gamma ray and thereafter produce a light signal corresponding to the amount of energy deposited from the initial interaction between the gamma ray and the scintillator element  115 . The detector array  100  serves to reflect and channel the light down the scintillator element  115  to the photodetector  18 . The signal generated by the photodetector  18  is then post-processed and utilized in accordance with the purpose of the imaging device. 
     In some embodiments, a light guide  20  can be selectively placed between the detector array  100  and the receiving photodetectors  18  if necessary. The light guide  20  defines a selected configuration, such as being segmented or continuous. The light guide  20 , when employed, is optimized depending on the choice of scintillator elements  115  and photodetectors  18 . 
     In embodiments utilizing the light guide  20 , the detector array  100  serves to reflect and channel the light generated within a scintillator element down the scintillator element  115  to the coupled light guide  20  and to the photodetector  18 . The signal generated by the photodetector  18  is then post-processed and utilized in accordance with the purpose of the imaging device. 
     In the detector array  100  of the present disclosure, the relative positions of the individual detector elements  115  are maintained by maintaining the relative positions of the scintillator slabs  110  within the detector array  100  by the use of a retaining mechanism  16 . In some embodiments, the retaining mechanism  16  can be a retainer that wraps around the outer perimeter of the detector array  100 . The detector assembly  100 A in  FIG. 4A  is illustrated with such retainer  16 . Such retainer  16  can be fabricated from conventional materials such as shrink wrap, rubberized bands, tape or a combination of like materials may be used to enclose or hold the slabs  115  together in a tight, uniform fashion. Although illustrated in  FIG. 4A  as spanning the entire height of the detector array  100 , the retainer  16  can in some applications include one or more retainers which span only a portion of the height of the detector array  100 . 
     In other embodiments, the retaining mechanism  16  can be a bonding agent that is applied between the detector array  100  and the continuous light guide  20 . 
     Illustrated in  FIG. 4B  is another example of a detector assembly  100 B disposed above a continuous light guide  20 . In this example, the relative positions of the individual scintillator elements  115  are maintained by a mechanism  16  that is a bonding agent applied between the detector array  100  and the continuous light guide  20 . The continuous light guide  20  is disposed above an array of photodetectors  18 , such as in a panel detector. 
     In some embodiments of the detector assembly  100 A,  100 B, the at least one photodetector  18  can be a PMT, a position sensitive PMT, an SiPM, an APD, a PIN diode, a CCD, or other type of solid state detector. 
     In the embodiments of the detector assembly  100 A,  100 B comprising a light guide  20  disposed between the detector array  100  and the at least one photodetector  18 , the scintillator elements  115  are optically coupled to the at least one photodetector  18  via the light guide  20 . 
     In some embodiments of the detector assembly  100 A,  100 B, the light guide  20  is configured to be continuous over a plurality of the array of scintillator elements  115  and a plurality of the at least one photodetector  18 . 
     Referring to the flowchart  200  in  FIG. 5 , disclosed herein is a method for fabricating the detector array  100  of the present disclosure. The method comprises preparing a plurality of slabs  110  of an optical medium of an imaging device (step  210 ); forming a plurality of optical boundaries  120  within at least one of the slabs  110  of optical medium, wherein the plurality of optical boundaries  120  defining a 1×N array of non-contiguous, independent light-redirecting regions (i.e. detector elements) within the at least one of the slabs, where N represents the number of individual detector elements  115  (step  220 ); arranging the plurality of slabs into a stack with a reflective layer  130  defined between two adjacent slabs  110  (step  230 ); and affixing the positions of the plurality of slabs with respect to each other (step  240 ). 
     In some embodiments of the method, the reflective layer  130  can be an air-filled gap between each adjacent slab  110 . In some embodiments, the reflective layer comprises at least one of the following reflective materials: reflective powder, reflective film, reflective paint, and an adhesive doped with reflective powder, or a combination of the reflective materials. 
     In some embodiments of the method, a plurality of optical boundaries  120  are formed within each of the slabs  110  of optical medium, whereby the stack of the plurality of slabs  110  form a M×N array of non-contiguous, independent light-redirecting regions in the detector array, wherein M represents the number of slabs  110 . 
     In some embodiments of the method, affixing the positions of the plurality of slabs  110  comprise using a bonding agent  16  to attach each of the slabs  110  to at least one photodetector  18 . In some embodiments, the detector assembly  100 A,  100 B comprises a light guide  20  and the method comprises affixing the positions of the plurality of slabs  110  comprises using a bonding agent  16  to attach each of the slabs  110  to the light guide  20 , then attaching the light guide  20  to at least one photodetector  18 . 
     In some embodiments of the method, forming the plurality of optical boundaries  120  comprise forming a plurality of non-contiguous, independent light-redirecting regions  12  in a spatial surface  14  within each of the slabs  110  by focusing a laser beam  32  at a selected wavelength at a focal point at a different selected location on the spatial surface  14 , thereby changing the optical properties of the optical medium at the focal point. 
     In some embodiments of the detector array  100 , the optical medium is a scintillator and wherein the optical boundary  120  defines a boundary between separate optically-segmented resolution elements  115  of the scintillator. 
     In some embodiments of the detector array  100 , the optical medium is a light-transmitting object and wherein the optical boundary  120  defines a portion of a light guide within the light-transmitting object. 
     In some embodiments of the detector array  100 , the plurality of optically-segmented resolution elements are rectilinear in shape. 
     In some embodiments of the detector array  100 , the spatial surface  14  is curvilinear. 
     It will be understood that the foregoing description is of exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown. Modifications may be made in the design and arrangement of the elements without departing from the scope of the invention.