Abstract:
A method for making a detector array comprising a photosensor and scintillators, including placing a thermoplastically encased reflective film between scintillators of a scintillator array and optically coupling the scintillators with the photosensor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation In Part of U.S. patent application Ser. No. 09/751,872 filed Dec. 29, 2000 now abandoned. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to a computer tomograph (CT) imaging system and more particularly to a CT detector module and reflector useful therewith and to methods for preparing and using the detector module and reflector. 
   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. 
   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. 
   At least one known detector in CT imaging systems comprises a plurality of detector modules, each having a scintillator array optically coupled to a semiconductor photodiode array that detects light output by the scintillator array. The scintillator array in at least one known CT detector is produced with individual diced scintillator elements with the gap between scintillators elements filled with a reflector material applied through a casting operation. This cast reflector is made of a two part epoxy and a chrome pigment used in one known typical CT semiconductor photodiode detector array. Such cast reflectors are sometimes damaged by X-ray exposure which causes color center formation, reduces reflectivity and causes lower light output from each photodiode detector cell. Cast reflectors are protected by tungsten wires and plates in known CT systems and have a certain thickness to sufficiently reduce cross talk. Cast reflectors are typically produced in a casting process whereby the scintillator array and epoxy is cast in molds. 
   Accordingly it would be desirable to provide an improved reflector which is comparatively less susceptible to radiation damage, offers enhanced resistance to color center formation and thereby provides improved light collection efficiency along with potentially lower cross talk. 
   BRIEF SUMMARY OF THE INVENTION 
   There is therefore provided in one embodiment of this invention, a computed tomograph (CT) imaging system having a rotating gantry, a radiation source, a detector array on the rotating gantry and configured to detect radiation from the radiation source and the detector array. The detector array includes a photosensor array and an array of scintillators optically coupled to the photosensor array and a thermoplastic encased reflective film between and in front of the scintillators of the scintillator array. The reflector is thin and is less susceptible to x-ray damage. 
   These and other embodiments of the invention provide various combinations of additional advantages, including lower manufacturing cost due to use of a lamination reflector fabrication process, and lower cross talk. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial view of a CT imaging system. 
       FIG. 2  is a block schematic diagram of the system illustrated in FIG.  1 . 
       FIG. 3  is a perspective view of a CT system detector array. 
       FIG. 4  is a perspective view of a detector module shown in FIG.  3 . 
       FIG. 5  presents an overview of several embodiments of a process for preparing an improved reflector and further illustrates placement of a reflector within a scintillator array. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , a computed tomography (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 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. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . Detector array  18  may be fabricated in a single slice or multi-slice configuration. In a multi-slice configuration, detector array  18  has a plurality of rows of detector elements  20 , only one of which is shown in FIG.  2 . 
   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 . 
   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 . 
   As shown in  FIGS. 3 and 4 , detector array  18  includes a plurality of detector modules  50 , each module comprising an array of detector elements  20 . Each detector module  50  includes a high-density photosensor array  52  and a multidimensional scintillator array  54  positioned above and adjacent to photosensor array  52 . Particularly, scintillator array  54  includes a plurality of scintillators  56 , while photosensor array  52  includes photodiodes  58 , a switch apparatus  60 , and a decoder  62 . A material such as a titanium dioxide-filled epoxy fills the small spaces in front of and between scintillator elements. Photodiodes  58  are individual photodiodes. In another embodiment, photodiodes  58  are a multidimensional diode array. In either embodiment, photodiodes  58  are deposited or formed on a substrate. Scintillator array  54 , as known in the art, is positioned over or adjacent photodiodes  58 . Photodiodes  58  are optically coupled to scintillator array  54  and have electrical output lines for transmitting signals representative of the light output by scintillator array  54 . Each photodiode  58  produces a separate low level analog output signal that is a measurement of beam attenuation for a specific scintillator of scintillator array  54 . Photodiode output lines (not shown in  FIG. 3  or  4 ) may, for example, be physically located on one side of module  20  or on a plurality of sides of module  20 . In the embodiment illustrated in  FIG. 4 , photodiode outputs are located at opposing sides of the photodiode array. 
   In one embodiment, as shown in  FIG. 3 , detector array  18  includes fifty-seven detector modules  50 . Each detector module  50  includes a photosensor array  52  and scintillator array  54 , each having a detector element  20  array size of 16×16. As a result, array  18  is segmented into 16 rows and 912 columns (16×57 modules) allowing up to N=16 simultaneous slices of data to be collected along a z-axis with each rotation of gantry  12 , where the z-axis is an axis of rotation of the gantry. 
   Switch apparatus  60  is a multidimensional semiconductor switch array. Switch apparatus  60  is coupled between photosensor array  52  and DAS  32 . Switch apparatus  60 , in one embodiment, includes two semiconductor switch arrays  64  and  66 . Switch arrays  64  and  66  each include a plurality of field effect transistors (FETS) (not shown) arranged as a multidimensional array. Each FET includes an input electrically connected to one of the respective photodiode output lines, an output, and a control (not shown) arranged as a multidimensional array. 
   Each FET includes an input electrically connected to one of the respective photodiode output lines, an output, and a control (not shown). FET outputs and controls are connected to lines that are electrically connected to DAS  32  via a flexible electrical cable  68 . 
   Particularly about one-half of the photodiode output lines are electrically connected to each FET input line of switch  64  with the other one-half of photodiode output lines electrically connected to FET input lines of switch  66 . Flexible electrical cable  68  is thus electrically coupled to photosensor array  52  and is attached, for example, by wire bonding. 
   Decoder  62  controls the operation of switch apparatus  60  to enable, disable, or combine photodiode  58  outputs depending upon a desired number of slices and slice resolution for each slide. Decoder  62  in one embodiment, is a FET controller as known in the art. Decoder  62  includes a plurality of output and control lines coupled to switch apparatus  60  and DAS  32 . Particularly, the decoder outputs are electrically coupled to the switch apparatus control lines to enable switch apparatus  60  to transmit the proper data from the switch apparatus inputs to the switch apparatus outputs. 
   Utilizing decoder  62 , specific FET&#39;s within switch apparatus  60  are selectively enabled, disabled, or combined so that specific photodiode  58  outputs are electrically connected to CT system DAS  32 . Decoder  62  enables switch apparatus  60  so that a selected number of rows of photosensor array  52  are connected to DAS  32 , resulting in a selected number of slices of data being electrically connected to DAS  32  for processing. 
   As shown in  FIG. 3 , detector modules  50  are filled in a detector array  18  and secured in place by rails  70  and  72 .  FIG. 3  shows rail  72  secured in place, while rail  70  is positioned to be secured over electrical cable  68 , over module substrate  74 , flexible cable  68 , and mounting bracket  76 . Screws (not shown in  FIG. 3  or  4 ) are then threaded through holes  78  and  80  and into threaded holes  82  of rail  70  to secure modules  50  in place. Flanges  84  of mounting brackets  76  are held in place by compression against rails  70  and  72  (or by bonding, in one embodiment) and prevent detector modules  50  from “rocking”. Mounting brackets  76  also clamp flexible cable  68  against substrate  74 . In one embodiment, flexible cable  68  is also adhesively bonded to substrate  74 . If desired, photosensor array can be adhesively bonded to the substrate. Flexible cable  68  is also electrically and mechanically bonded to photosensor array  52 , for example, by wire bonding. 
   With reference to  FIG. 5 , to improve reflectivity and light collection efficiency of scintillator/diode detector elements (not shown) in detector array (not shown), a new composite thermoplastic film reflector is used between elements (not shown), the new composite thermoplastic film reflector including at least one thermoplastic film (not shown). Suitable encasement material for the reflective film used herein includes but is not limited to a thermoplastic. As used herein, the term “thermoplastic” includes any material which has in whole or part, a linear macromolecular structure that will repeatedly soften when heated and harden when cooled. In one embodiment, a thermoplastic is used that is readily malleable and flexible at room temperature for fabrication purposes, offers higher resistance to color center formation and provides resistance to X-ray radiation damage. Also in one embodiment, the thermoplastic either has an adhesive surface or is capable of adhering to a surface upon which an adhesive has been applied. 
   In one embodiment, the thermoplastic material is capable of being bonded to by being laminated with or stuck with an adhesive to the reflective film. 
   In one embodiment, reflective films useful herein include 3M visible mirror film, Hitachi, and NKK TiO2 doped plastic films. The thickness of the “encased” reflector is such that an outside dimension is accommodated by an interior fit or placement within the scintillator array. Upon insertion of the reflector of this invention into a gap of the wafer stack, the stack is thereafter heated, pressure is applied if necessary to effectively bonded the wafer stack together sufficiently adherent to accommodate a subsequent slicing. The other dimensions of the encased reflective film will be in general accordance with the dimensions of the wafers with which the film is to be utilized. 
   With reference to  FIG. 5 , in one embodiment, a scintillator wafer is stacked vertically upon one another of several others of similar kind and similar size, creating a 0.002 inch vertical gap  106  between adjacent stacked wafers  108 . Gaps  106  are then filled with a reflector of this invention which in turn comprises a thermoplastic film (not shown), a reflector film (not shown) and another thermoplastic film (not shown) with reflector film (not shown) sandwiched therebetween by laminating reflector film (not shown) between thermoplastic films (not shown). 
   With further reference to  FIG. 5 , in this embodiment, in more detail, a first thermoplastic film is inserted in gap  106 , a reflective film is then inserted between the inserted thermoplastic film and an adjacent wafer face in gap  106  remaining open, and thereafter a second thermoplastic film is inserted in gap  106  remaining open, between the inserted reflective film face and an adjacent wafer face thus closing gap  106 . Suitable heat and pressure are applied to the wafer stack to carry out effective lamination of the thermoplastic and reflective film and achieving adherence to the wafers in stack  108 . Lamination of a thermoplastic to a reflective film in embodiments of the present invention are readily carried out according to lamination processes known in the art. 
   A series of parallel vertical cuts  110  is made on stack  108  using a suitable cutting or dicing process creating a section  112 . In one embodiment, cuts are made to provide a stack  112  having a thickness of about 1.0 mm. A series of parallel horizontal cuts  110  are then made on stack  108  using a suitable cutting or dicing process creating sections  114 . These sections  114  can then be cast together into a final scintillator array or pack. 
   In an embodiment and with continued reference to  FIG. 5 , the thermoplastic material can be any suitable thermoplastic adhesive material. In this embodiment additional heat and pressure are not necessary to carry out effective bonding between the reflective film and thermoplastic films creating an encased reflective film. The additional heat and pressure are not necessary because the thermoplastic and reflective film are held together by an adhesive which is applied to either the thermoplastic or reflective film. 
   In an embodiment, a reflective film is deposited directly onto a thermoplastic sheet and is then laminated with another layer of thermoplastic material. The laminated reflective material is then inserted into the gap between the wafers. Additionally, in an exemplary embodiment, the reflective film is a low x-ray damageable material. As used herein a low x-ray damageable material refers to a material in which each cell suffers no more than a ten percent loss of output due to discoloration after sustaining a cumulative dose of 1 megarad (MR). 
   In an embodiment, a bar  114  is created by a second cut in a stack of wafers. Bars  114  are assembled into a scintillator array. The gaps between bars are then filled with the reflector material. 
   Embodiments of the present invention use thin reflective film in one or two dimensions of the scintillator array, that are less susceptible to radiation damage and more resistant to color center formation than known cast reflectors, and more resistant to cross talk between scintillator elements. Use of improved reflector embodiments of the present invention in one or more dimensions makes it possible to avoid the use of collimator wires, thereby providing a beneficial cost savings. The elimination of collimator wires and use of potentially thinner reflectors allows use of detectors to acquire data representation of thinner slices in the z-direction. Laminates can be thinner than current cast materials and advantageously provide for more exposed scintillator thereby improving the detector quantum detection efficiency. 
   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.