Patent Publication Number: US-6993110-B2

Title: Collimator for imaging systems and methods for making same

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods for making a collimator used in an imaging system, and to the collimator made from these methods. 
     In at least one known CT 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 of the object. 
     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 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 backprojection 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. 
     Detector elements are configured to perform optimally when impinged by x-rays travelling a straight path from the x-ray source to the detector elements. Particularly, detector elements typically include scintillation crystals which generate light events when impinged by an x-ray beam. These light events are output from each detector element and directed to photoelectrically responsive materials in order to produce an electrical signal representative of the attenuated beam radiation received at the detector element. Typically, the light events are output to photomultipliers or photodiodes which produce individual analog outputs. Detector elements thus output a strong signal in response to impact by a straight path x-ray beam. 
     X-rays often scatter when passing through the object being imaged. Particularly, the object often causes some, but not all, x-rays to deviate from the straight path between the x-ray source and the detector. Therefore, detector elements are often impinged by x-ray beams at varying angles. System performance is degraded when detector elements are impinged by these scattered x-rays. When a detector element is subjected to multiple x-rays at varying angles, the scintillation crystal generates multiple light events. The light events corresponding to the scattered x-rays generate noise in the scintillation crystal output, and thus cause artifacts in the resulting image of the object. 
     To reduce the effects of scattered x-rays, scatter collimators are often disposed between the object of interest and the detector array. Such collimators are constructed of x-ray absorbent material and positioned so that scattered x-rays are substantially absorbed before impinging upon the detector array. For one known collimator, it is important that the scatter collimator be properly aligned with both the x-ray source and the detector elements so that only x-rays travelling on a substantially straight path impinge on the detector elements. 
     Known collimators are complicated and cumbersome to construct. In addition, it is difficult to satisfactorily align known collimators with the x-ray source and the detector elements to both absorb scattered x-rays and shield sensitive portions of the detector elements. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, a method for fabricating a collimator is provided. The method includes mixing an x-ray absorbent material with at least one of a temporary binder and a temporary gel, and extruding the mixed x-ray absorbent material through a die to form a unitary collimator structure that is at least one of substantially honeycomb in shape and substantially rectangular in shape. 
     In another embodiment, a collimator for an imaging system is provided. The collimator includes an extruded x-ray absorbent material, and is unitary and at least one of substantially honeycomb in shape and substantially rectangular in shape. 
     In a further embodiment, a computed tomographic (CT) imaging is provided. The CT system includes a detector array, at least one radiation source, and a collimator including an extruded x-ray absorbent material wherein the collimator is unitary and at least one of substantially honeycomb in shape and substantially rectangular in shape. 
    
    
     
       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 side view of a detector array including a plurality of scintillator elements and a collimator positioned proximate to the scintillator elements. 
         FIG. 4  is a perspective view of the collimator and scintillator elements shown in  FIG. 3 . 
         FIG. 5  is a perspective view of another embodiment of the collimator shown in  FIG. 3 . 
         FIG. 6  is a flow chart illustrating an exemplary method for fabricating a collimator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 from a plurality of detector elements  20  including a plurality of scintillators (not shown), 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. Each detector element  20  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 . 
     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 . 
     In one embodiment, computer  36  includes a device  50 , for example, a floppy disk drive or CD-ROM drive, for reading instructions and/or data from a computer-readable medium  52 , such as a floppy disk or CD-ROM. In another embodiment, computer  36  executes instructions stored in firmware (not shown). Computer  36  is programmed to perform functions described herein, accordingly, as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits. 
       FIG. 3  is a side view of detector array  18  including a plurality of scintillator elements  58 , and a collimator  60  positioned proximate to scintillator elements  58  such that x-rays  16  pass through object  22 , and collimator  60  before impinging on detector array  18 .  FIG. 4  is a perspective view of an exemplary collimator  60  positioned proximate scintillator elements  58 .  FIG. 5  is a perspective view of another exemplary collimator shown in  FIG. 3 . In the exemplary embodiment, collimator  60  includes a length  62  and a width  64  that defines a collimator area, and a thickness  68 . Collimator  60  also includes a plurality of sidewalls  70  surrounding a plurality of openings  72 . In one embodiment, sidewalls  70  are formed such that openings  72  define a substantially honeycomb shape (i.e. a plurality of interconnected hexagons). In another embodiment, sidewalls  70  are formed such that openings  72  form a shape that is substantially rectangular in shape. Each scintillator element  58  includes a length  82  and a width  84  that defines a scintillator area, and a thickness  88 . In the exemplary embodiment, the scintillator area is substantially greater than a single collimator opening  72  such that a random distribution of collimator openings  72  are positioned proximate scintillator elements  58  and such that at least two partial openings  72  are positioned proximate a single scintillator element  58 . For example, a single scintillator element  58  may have a single opening  72  and several partial openings or a single scintillator element  58  may have a plurality of openings  72  and a plurality of partial openings  72  proximate each scintillator element  58 . 
       FIG. 6  is a method  90  for fabricating collimator  60 . Method  90  includes mixing  92  an x-ray absorbent material with at least one of a temporary binder or a temporary gel. In one embodiment, the x-ray absorbent material includes a metallic material having a high atomic weight (Z number). In one embodiment, the metallic material has a Z number greater than seventy-two such as, but not limited to, tungsten, tantalum, and lead. 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, hydroxyethyl-cellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. The collimator is partially solidified or dried to make a flexible “cake”. Method  90  also includes extruding  94  the mixed x-ray absorbent material through a die to form a unitary collimator structure that is substantially honeycomb in shape or substantially rectangular in shape, and sintering  96  the unitary structure at an appropriate temperature to form a substantially solid honeycomb structure. The substantially solid honeycomb structure is then sliced to a length dependent upon the embodiment to form collimator  60 . 
     In use, collimator  60  is positioned proximate a scintillator x-ray incidence side  100  (shown in  FIG. 4 ) such that x-rays  16  pass through object  22  and collimator  60  before impinging on scintillator elements  58 . Scattered x-rays passing through object  22  are substantially reduced using collimator  60  prior to being sensed by detector  18 . In the exemplary embodiment, collimator  60  is mechanically attached to detector  18  using at least one mechanical fastener (not shown). 
     In the exemplary embodiment, collimator  60  is positioned to form a series of high aspect ratio channels proximate each scintillator element  58  to facilitate attenuating scattered x-rays. Additionally, the accuracy of openings  72  can be reduced since a specific collimator opening  72  is not aligned with a specific scintillator element  58 , but rather openings  72  form a random distribution of collimating openings proximate the scintillator array. Collimator  60  also facilitates reducing an aspect ratio since collimation is accomplished in two directions. 
     The above-described collimator provides for alignment with both the focal spot and the detector elements without the collimator being precisely aligned between the detector and the radiation source. Also, the collimator is not complex, and is more simple to fabricate than some known collimators. In addition, the scatter collimator sufficiently shields the detector elements from undesirable scattered x-rays and other radiation, thereby facilitating a reduction in x-rays, that are not travelling on a substantially straight path, from impinging on the detector elements and thus reducing artifacts in the resulting image of the object. Accordingly, the herein described collimator is believed to provide improved system performance as compared to known collimators. 
     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.