Patent Publication Number: US-2015063531-A1

Title: Collimator-detector structure for a ct imaging system

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to collimators for use in diagnostic imaging and, more particularly, to a collimator construction and arrangement that provides increased alignment tolerance between the collimator and a scintillator array and reduces spectral and thermal non-linearity issues related to the interaction of the collimator and the scintillator array. 
     Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image. 
     Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator, currently made of highly absorbing material such as tungsten or lead, for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction. 
     As stated above, typical x-ray detectors include a collimator for collimating x-ray beams such that collection of scattered x-rays is minimized. As such, the collimators operate to attenuate off-angle scattered x-rays from being detected by a scintillator cell. Reducing this scattering reduces noise in the signal and improves the final reconstructed image. Therefore, it is necessary that the scintillator array and the collimator—which is formed of plates extending along one or two dimensions above the scintillator array—is uniformly aligned at scintillator cell boundaries defined by a cast reflector material and have a plate in every channel between each scintillator cell. That is, currently exact mechanical alignment is required between the collimator plates and the cast reflector channels in the array of scintillators. 
     Known manufacturing processes attempt this exact alignment by constructing a continuous collimator that is sized to dimensionally match the width and length of the entire detector array. That is, the collimator plates are arranged or arrayed in a continuous consistent pattern or pitch that spans the entire detector length and is placed and attached to the detector rail structure. As such, individual scintillator arrays or packs must then be exactly aligned to the continuous collimator to ensure that all scintillator cells and collimator cells are aligned exactly. This process requires tight tolerancing and requires great operator skill and patience to assemble. 
     A known CT detector  1  fabricated according to known manufacturing processes is shown in  FIG. 1 . The CT detector  1  includes a series of tungsten collimator plates  2  configured and positioned to collimate x-rays projected toward scintillator cells  3  of a scintillator array  4 . As shown, each of the collimator plates  2  is generally aligned with a reflector channel  5  disposed between adjacent scintillator cells  3  that prevents light from being emitted between adjacent scintillators—with an air gap  6  being present between the collimator plates  2  and the scintillator cells  3  due to the manufacturing process whereupon the collimator plates  2  are formed as a single collimator assembly that accepts and aligns an array of scintillators  4 . The scintillator array  4  is coupled to a photodiode array  7  that detects light emissions from the scintillator array and transmits corresponding electrical signals to a data acquisition system for signal processing. 
     As shown in  FIG. 1 , the collimator plates  2  are generally constructed such that they are wider than a width of the reflector channels  5 . This increased thickness of the collimator plates  2  relative to the reflector channels  5  further aggravates the already tight tolerancing required between the collimator plates  2  and the scintillator cells  3 , as any misalignment of the collimator plates  2  with the scintillator cells  3  will result in the collimator plates  2  covering a substantial portion of scintillator cells  3 , such that performance of the detector can be compromised. In addition, the increased thickness of the collimator plates  2  and the alignment thereof with the reflector channels  5  increases spectral non-linearity due to the interaction of the plates  2  and the scintillator cell edges (which occurs especially during calibration) and increases thermal non-linearity due to the interaction of plate shadows with scintillator cell edges (and the thermal expansion caused thereby). 
     Therefore, it would be desirable to design a detector assembly and method of manufacturing thereof that provides for easy alignment between the scintillator array and collimator assembly having relaxed alignment tolerances therebetween, that effectively reduces spectral and thermal non-linearities, and that reduces manufacturing and testing costs for CT detectors. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with one aspect of the invention, a detector assembly for a CT imaging system includes a scintillator array comprising a plurality of scintillator cells, and configured to detect high frequency electromagnetic energy attenuated through an object, the scintillator array including a reflective material positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The detector assembly also includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels. The reflector channels in the scintillator array are formed to have a first thickness and the collimator plates of the collimator are formed to have a second thickness that is equal to or less than the first thickness. 
     In accordance with another aspect of the invention, a CT imaging system includes a rotatable gantry having an opening to receive an object to be scanned, a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam toward the object, and a detector assembly positioned on the gantry opposite the high frequency electromagnetic energy projection source. The detector assembly further includes a scintillator array comprising a plurality of scintillator cells, and configured to detect high frequency electromagnetic energy attenuated through an object, wherein a reflective material is positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The detector assembly still further includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels. The reflector channels in the scintillator array are formed to have a first thickness and the collimator plates of the collimator are formed to have a second thickness that is equal to or less than the first thickness. 
     In accordance with yet another aspect of the invention, a detector assembly for a CT imaging system includes a scintillator array comprising a plurality of scintillator cells and configured to detect high frequency electromagnetic energy attenuated through an object, the scintillator array including a reflective material positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The detector assembly also includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels, and wherein each of the plurality of collimator plates is aligned with a centerline of a respective scintillator cell. 
     Various other features and advantages will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate preferred embodiments presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a cross-sectional view of a prior art CT detector having a collimator aligned with a scintillator array. 
         FIG. 2  is a pictorial view of a CT imaging system. 
         FIG. 3  is a block schematic diagram of the system illustrated in  FIG. 2 . 
         FIG. 4  is a perspective view of one embodiment of a CT system detector array. 
         FIG. 5  is perspective view of a collimator according to an embodiment of the invention. 
         FIG. 6  is a perspective view of one embodiment of a detector. 
         FIG. 7  is a cross-sectional view of a CT detector having a collimator aligned with a scintillator array according to an embodiment of the invention. 
         FIG. 8  is a cross-sectional view of a CT detector having a collimator aligned with a scintillator array according to an embodiment of the invention. 
         FIG. 9  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
     
    
    
     DETAILED DESCRIPTION 
     The operating environment of the invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the invention is equally applicable for use with other multi-slice configurations. Moreover, the invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems. 
     Referring to  FIG. 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 toward a detector assembly  18  on the opposite side of the gantry  12 . Referring now to  FIG. 3 , detector assembly  18  is formed in part by a plurality of detectors  20  and data acquisition systems (DAS)  32 . The plurality of detectors  20  sense the projected x-rays  16  that pass through a medical patient  22 , and DAS  32  converts the data to digital signals for subsequent processing. Each detector  20  produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the 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 . 
     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 an x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed 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 some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated 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  and gantry  12 . Particularly, table  46  moves patients  22  through a gantry opening  48  of  FIG. 2  in whole or in part. 
     As shown in  FIG. 4 , detector assembly  18  further includes rails  17  having collimating blades or plates  19  placed therebetween that collectively form a collimator  21 , with the collimator plates  19  being generally made of tungsten, molybdenum, or lead. Collimator plates  19  are positioned to collimate x-rays  16  before such beams impinge upon, for instance, detector  20  of  FIG. 6  positioned on detector assembly  18 . In one embodiment, detector assembly  18  includes  57  detectors  20 , each detector  20  having an array size of 64×16 of scintillator cells  50  (i.e., pixel elements). As a result, detector assembly  18  has  64  rows and  912  columns (16×57 detectors) which allows 64 simultaneous slices of data to be collected with each rotation of gantry  12 . 
     While collimator  21  is shown in  FIG. 4  as including collimator plates  19  being formed as linear blades extending in a single direction/dimension, it is recognized that the collimator could instead be constructed as a “two dimensional collimator”  49  such as shown in  FIG. 5 , with a blade/wall structure that forms a honeycomb structure defining a two dimensional array of channels that collimate x-rays attenuated by the subject  22 , for example, prior to the x-rays impinging upon detector  20  ( FIG. 6 ). 
     A detector  20  is shown in  FIG. 6  for use with embodiments of the invention. Each detector  20  includes a number of detector elements  50  (i.e., scintillator cells or pixels) forming an array or pack  51  (i.e., scintillator array). Scintillator array  51  is optically coupled to a photodiode array  53  having a plurality of diodes  59 , with backlit diode array  53  in turn being positioned on, and electrically coupled to, multi-layer substrate  54 . As further shown in  FIG. 6 , detectors  20  also include pins  52  positioned relative to scintillator array  51  and spacers  55  positioned on multi-layer substrate  54 . Flex circuits  56  are attached to face  57  of multi-layer substrate  54  and to DAS  32 . Detectors  20  are positioned within detector assembly  18  by use of pins  52 . In the operation of one embodiment, x-rays impinging within detector elements  50  generate photons which traverse scintillator array  51 , thereby generating an analog signal which is detected on a diode within backlit diode array  53 . The analog signal generated is carried through multi-layer substrate  54 , through flex circuits  56 , to DAS  32  wherein the analog signal is converted to a digital signal. 
     Referring now to  FIGS. 7 and 8 , a cross-sectional view of a portion of detector assembly  18  are shown according to embodiments of the invention. As shown in  FIGS. 7 and 8 , detector assembly  18  includes a collimator  60  having a plurality of collimator plates  62  positioned proximate detector  20  that define collimator channels  64 . While collimator  60  is shown as having a plurality of plates  62  defining only three channels  64 , it is noted that  FIG. 7  is for illustrative purposes only and that collimator  60  of detector assembly  18  would be formed to include a plurality of plates  62  that defines a greater number of channels  64  arranged in one dimension or in a two-dimensional array. As shown in  FIGS. 7 and 8 , the plurality of plates  62  of collimator  60  are positioned proximate to and stacked in a vertical arrangement with a scintillator array  66  of the detector  20 , with the scintillator array  66  in turn being coupled to a photodiode array  68 . The scintillator array  66  includes a plurality of scintillator cells  70  that are separated from one another by a reflective material that is cast around each of the scintillator cells to form reflector channels  72 . The reflector channels  72  separate individual scintillator cells  70  from each other to prevent cross-talk therebetween. That is, as collimated x-rays pass through channels  64  created by collimator plates  62  of collimator  60  and impinge on the scintillator material of scintillator cells  70  housed in scintillator array  66  of detector  20 , photons are generated. The reflector channels  72  formed around scintillator cells  68  act to reflect these photons, such that they are trapped within a particular scintillator cell  70 , allowing for readout thereof by photodiode array  68  without cross-talk interference from adjacent scintillator cells. 
     According to embodiments of the invention, and as shown in  FIGS. 7 and 8 , the plates  62  of collimator  60  are formed so as to have a thickness  74  that is reduced as compared to conventional collimator plates. Specifically, collimator plates  62  are formed so as to have a thickness  74  that is equal to or less than a thickness  76  of the reflector channels  72 . As one example, the collimator plates  62  may have a thickness of 100 μm, so as to have a thickness  74  that is less than the thickness  76  of reflector channel  72 . By controlling a thickness of the collimator plates  62  to be equal to/less than that of the reflector channels  72 , the alignment tolerance of the collimator  60  to the scintillator array  66  can be relaxed, as can the alignment tolerances between adjacent scintillator arrays  66  (i.e., the pack-to-pack spacing) in the detector array  18 . The controlling of the thickness of the collimator plates  62  also improves detector performance, as it serves to minimize the impact of any misalignment of the collimator plates  62  with the scintillator cells  70  by reducing the area of the scintillator cells  70  that might be covered by the plates  62 . 
     Referring now to  FIG. 7 , in one embodiment, the plurality of collimator plates  62  are aligned with (or approximately with) centerlines  78  of scintillator cells  70  along at least one dimension. Alignment of the collimator plates  62  with the centerlines  78  of scintillator cells  70  in this manner allows for a relaxation of the alignment tolerance between the collimator  60  and the scintillator array  66  to a range of one half a pitch (indicated as  80  in  FIG. 7 ) of the scintillator cells  70  without affecting performance of the detector  20 . The alignment of the collimator plates  62  with the centerlines  78  of scintillator cells  70  also reduces the spectral non-linearity in the scintillator cells  70 , as the interaction of the plates  62  and the edges of scintillator cells  70  (which occurs especially during calibration) is minimized. Still further, the alignment of the collimator plates  62  with the centerlines  78  of scintillator cells  70  reduces the thermal non-linearity in the scintillator cells  70 , as the interaction of plate shadows with the edges of scintillator cells  70  (and the thermal expansion caused thereby) is also minimized. 
     Referring now to  FIG. 8 , in another embodiment, the plurality of collimator plates  62  are aligned with centerlines  82  of the reflector channels  72  in scintillator array  66 . As shown in  FIG. 8 , the reflector channels  72  in scintillator array  66  are significantly thicker than collimator plates  62  (e.g., 2× to 3× thicker), and thus small misalignments of the collimator plates  62  with the reflector channels  72  has a minor affect on performance of the detector  20 . That is, the alignment tolerance between the collimator  60  and the scintillator array  66  can be relaxed, as small misalignments of the collimator plates  62  with the reflector channels  72  will still result in the collimator plates  62  being positioned/aligned with the reflector channels  72 —such that the plates  62  do not cover a portion of a scintillator cell  70 , thereby reducing the spectral and thermal non-linearity in the scintillator cells  70  in the same manner described above. 
     Referring now to  FIG. 9 , a package/baggage inspection system  100  is shown that can incorporate a detector assembly having a collimator construction and collimator-scintillator alignment as shown and described in  FIGS. 7 and 8 . The system  100  includes a rotatable gantry  102  having an opening  104  therein through which packages or pieces of baggage may pass. The rotatable gantry  102  houses a high frequency electromagnetic energy source  106  as well as a detector assembly  108  having a collimator construction and collimator-scintillator array arrangement similar to that shown in  FIGS. 7 and 8 . A conveyor system  110  is also provided and includes a conveyor belt  112  supported by structure  114  to automatically and continuously pass packages or baggage pieces  116  through opening  104  to be scanned. Objects  116  are fed through opening  104  by conveyor belt  112 , imaging data is then acquired, and the conveyor belt  112  removes the packages  116  from opening  104  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  116  for explosives, knives, guns, contraband, etc. 
     Beneficially, embodiments of the invention thus provide a detector assembly having a collimator construction that provides for an alignment between the collimator and a scintillator array having relaxed alignment tolerances. The collimator construction/alignment therefore reduces manufacturing and testing/calibration costs for CT detectors. The collimator—and the alignment thereof with the scintillator array—also serves to effectively reduce spectral and thermal non-linearities in the scintillator array. 
     Therefore, according to one embodiment of the invention, a detector assembly for a CT imaging system includes a scintillator array comprising a plurality of scintillator cells, and configured to detect high frequency electromagnetic energy attenuated through an object, the scintillator array including a reflective material positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The CT imaging system also includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels. The reflector channels in the scintillator array are formed to have a first thickness and the collimator plates of the collimator are formed to have a second thickness that is equal to or less than the first thickness. 
     According to another embodiment of the invention, a CT imaging system includes a rotatable gantry having an opening to receive an object to be scanned, a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam toward the object, and a detector assembly positioned on the gantry opposite the high frequency electromagnetic energy projection source. The detector assembly further includes a scintillator array comprising a plurality of scintillator cells, and configured to detect high frequency electromagnetic energy attenuated through an object, wherein a reflective material is positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The detector assembly still further includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels. The reflector channels in the scintillator array are formed to have a first thickness and the collimator plates of the collimator are formed to have a second thickness that is equal to or less than the first thickness. 
     According to yet another embodiment of the invention, a detector assembly for a CT imaging system includes a scintillator array comprising a plurality of scintillator cells and configured to detect high frequency electromagnetic energy attenuated through an object, the scintillator array including a reflective material positioned around each of the plurality of scintillator cells to form reflector channels between each of the plurality of scintillator cells. The detector assembly also includes a collimator positioned proximate the scintillator array and configured to filter the high frequency electromagnetic energy attenuated through the object prior to impinging on the scintillator array, the collimator comprising a plurality of collimator plates arranged to form a plurality of channels, and wherein each of the plurality of collimator plates is aligned with a centerline of a respective scintillator cell. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.