Abstract:
One embodiment of the present invention is a method for constructing a post-patient collimator for a computed tomographic (CT) imaging system, the method including steps of: edge welding collimator plates to a top rail using at least one directed energy beam welder; and edge welding the collimator plates to a bottom rail, using the at least one directed energy beam welder. 
     The above described embodiment provides an efficient and less expensive method for manufacturing a post-patient collimator for a CT imaging system than embodiments requiring use of precision combs for accurately positioning the plates.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to computed tomography imaging systems, and more particularly to post-patient collimators used in such systems and methods for making such collimators. 
     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. 
     In a multislice imaging system, the detector comprises a plurality of parallel detector rows, wherein each row comprises a plurality of individual detector elements. A multislice detector is capable of providing a plurality of images representative of a volume of an object. Each image of the plurality of images corresponds to a separate “slice” of the volume. The thickness or aperture of the slice is dependent upon the thickness of the detector rows. It is also known to selectively combine data from a plurality of adjacent detector rows (i.e., a “macro row”) to obtain images representative of slices of different selected thicknesses. 
     It is known to provide multislice CT detectors with a post-patient collimator. These collimators include many precisely aligned plates and wires to collimate x-rays impinging on and to attenuate x-rays impinging between individual scintillating detector elements. In one known system, alignment of the collimator plates and attachment of the wires is accomplished with slots and notches in various components for alignment, and adhesives for bonding. The manufacturing steps presently required for precision alignment of the collimator plates and wires add considerably to manufacturing costs. For example, to manufacture one known collimator, upper and lower combs with precision slots, slot spacings, and slot alignments are required for insertion of collimator plates. Welding has not been practical in known post-patient collimators because of induced distortions in collimator plates resulting from the welding process itself. 
     It would therefore be desirable to provide precision-aligned post-patient collimators for CT imaging systems and methods for manufacturing them that are more efficient and less expensive than those that require precision combs. 
     BRIEF SUMMARY OF THE INVENTION 
     There is thus provided, in one embodiment of the present invention, a method for constructing a post-patient collimator for a computed tomographic (CT) imaging system, the method including steps of: edge welding collimator plates to a top rail using at least one directed energy beam welder; and edge welding the collimator plates to a bottom rail, using the at least one directed energy beam welder. 
     The above described embodiment provides an efficient and less expensive method for manufacturing a post-patient collimator for a CT imaging system than embodiments requiring use of precision combs for accurately positioning the plates. 
    
    
     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 drawing of a multislice detector array of the system illustrated in FIG.  1 . 
     FIG. 4 is a drawing of a detector module of the detector array illustrated in FIG.  3 . 
     FIG. 5 is a schematic cross-sectional view of the welding of a collimator plate to rails of a collimator in one embodiment of the present invention. 
     FIG. 6 is a schematic cross-sectional view of a post-patient collimator embodiment of the present invention that is constructed in sections. 
     FIG. 7 is an illustration of the radial arrangement of the sections of a post-patient collimator embodiment of the present invention. 
     FIG. 8 is an enlargement of a region of FIG. 5, showing how steel wire is used in one embodiment to take up spacing tolerance in a z-direction. 
     FIG. 9 is a top view of the collimator and welder configuration shown in FIG.  5 . 
     FIG. 10 is an illustration of laser welding of a collimator in one embodiment in conjunction with a comb and optional molybdenum spacers. 
    
    
     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 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 . 
     In one embodiment, and referring to FIGS. 3 and 4, detector array  18  comprises a plurality of modules  50 . Each module  50  includes a scintillator array  52  and a photodiode array  54 . Detector elements  20  include one photodiode of photodiode array  54 , and a corresponding scintillator of scintillator array. Each module  50  of detector array  18  comprises a 16×16 array of detector elements  20 , and detector array  18  comprises fifty-seven such modules  50 . Dectector array  18  is thus capable of acquiring projection data for up to 16 image slices simultaneously. 
     In one embodiment and referring to FIG. 5, to collimate x-rays  16  after they have passed through an object or patient  22 , a post-patient collimator  56  is disposed over detector array  18 . Post-patient collimator  56  comprises a top rail  58  and a bottom rail  60  spaced from and parallel to top rail  58 . A plurality of collimator plates  62  (e.g., tungsten plates) are arranged radially between each rail  58 ,  60 . (FIG. 5 is a cross-sectional view of post-patient collimator  56  through one collimator plate  62 .) To attach collimator plates to rails  58  and  60 , collimator plates  62  are each edge-welded at opposite ends to rails  58  and  60  using at least one directed energy beam welder  64 . The use of edge welding prevents warping of collimator plates out of the plane of FIG.  5 . Distortion inherent in other welding methods, including laser welding not specifically directed at edges of collimator plates  62 , is avoided. Suitable types of directed energy beam welders  64  include those utilizing directed energy beams  65  comprising photons (e.g., laser beam welders) and those utilizing particles (e.g., electron beam welders). Directed energy beams  65  are thin beams of energy that concentrate their energy at a single point. (FIG. 5 is intended to show narrow beams  65  directed at different locations, i.e.,  66 ,  68 ,  70 , and  72  rather than two fan beams of energy.) 
     In particular, a top rear corner  66 , a top front corner  68  a bottom rear corner  70 , and a bottom front corner  72  of collimator plates  62  are edge welded by directed energy beam welding in the plane of FIG.  5 . Top rear corner  66  and bottom rear corner  70  are edge welded towards a rear  74  of top rail  58  and towards a rear  76  of bottom rail  60 , respectively. Top front corner  68  and bottom front corner  72  are edge welded towards a front  78  of top rail  58  and towards a front  80  of bottom rail  60 , respectively. 
     In one embodiment and referring to FIG. 6, a collimator is prepared by assembling a plurality of sections. For each collimator section, a plurality of collimator plates  62  are edge welded, using at least one directed energy beam welder, to curved metal (e.g., steel) top and bottom segments  82  and  84 , respectively. Each segment  82  and  84  has a cross sectional area and length smaller than that of rails  58 ,  60  to form sections  86  of a collimator. Sections  86  are then radially arrayed between and fastened to top and bottom rails  58  and  60 . (The radial arrangement of sections  86  is illustrated in FIG. 7, which shows collimator plates  62  that are not actually visible in a top view as hidden lines.) Top segments  82  are affixed to top or upper rail  58  and bottom segments  84  are affixed to bottom or lower rail  60 . Wires  92  (such as tungsten wires) are also affixed to collimator plates  62  in a direction transverse to rear edges  88  of the collimator plates  62 . 
     A fixture (not shown) is used to hold collimator plates  62  and rails  58 ,  60  (or segments  82 ,  84 ) in position relative to one another. This fixture serves essentially the same purpose as a comb in a conventional post-patient collimator. However, unlike a comb, a fixture is needed only during welding of post-patient collimator  56 . The fixture is not, and does not become a part of collimator  56 , and can be re-used as needed. It is not necessary to use spacers, such as the molybdenum spacers used in at least one known post-patient collimator. 
     In one embodiment, two directed energy beam welders  64 ,  90  are used to weld collimator plates  62  to rails  58  and  60 . In another embodiment, two welders  64 ,  90  are used to weld collimator plates  62  to segments  82  and  84 . One of the welders produces the rear welds, while the other produces the front welds. 
     For a multislice detector array  18 , attenuating wires  92  (e.g., tungsten wires) are strung across collimator  56  in spaced notches  94  on rear edges  88  of collimator plates  62 . Wires  92  provide x-ray attenuation between detector rows. In one embodiment of the present invention, a directed energy beam welder  64  is used to weld wires  92  onto collimator plates  62 . In another embodiment, the precision of directed energy beam welders allows the use of collimator plates  62  without notches  94 . Wires  92  are strung across collimator plates  62  transverse to rear edges  88  and are accurately positioned against the collimator plates, for example, by using a fixture. Wires  94  are then welded to collimator plates  62  using a directed energy beam welder  64 . 
     In one embodiment, laser welders are used as welders  64  and  90  and their welds are accurately aimed and operated by computers (not shown) under program control. 
     FIG. 8 is an enlargement of region  96  of FIG. 5, showing how a wire  98  (for example, steel wire) is used in one embodiment to take up collimator plate  62  height and/or rail  58 ,  60  spacing tolerance in a z-direction. Wire  98  is inserted in chamfered gaps  100  between at least one of top rail  58  or bottom rail  60  and collimator plates  62 . (The selection of which one or both of rails  58  and  60  is a design choice.) Wire  98  is welded on one side to the selected rail  58  (or  60 ) and on the other side to collimator plate  62 . The welds of wire  98  to the selected rail  58  (or  60 ) are at least in chamfered gaps  100 . In one embodiment using welded wire  98 , a weld at  68  is omitted. Also in a segmented embodiment of the present invention, chamfered gaps  100  are provided between at least one segment  82  or  84  and collimator plates  62  rather than between rail  58  or  60  and plate  62 . Chamfers forming chamfered gap  100  can be in either plate  62  or the opposing segment or rail, or both. 
     FIG. 9 is a top view in an x-y plane of the collimator and laser welder configuration shown in FIG. 5 (or FIG. 6) showing a phantom outline of a segment  82  (if used) and the location of one collimator plate  62  welded to rail  58  (or segment  82 ). (Neither segment  82 , if used, nor collimator plate  62  would actually be visible from the top of collimator  56 .) FIG. 9 illustrates the curvature of collimator  56 , which corresponds to that of detector array  18 . The arrangement of collimator plates  62  in collimator  56  is such as to provide collimation between detector elements  20  that are adjacent one another in the same row or slice of detector array  18 . 
     In another embodiment and as shown in FIG. 10, laser welding is used in conjunction with a comb  102  affixed to at least one of rail  58  or  60  and optional spacers  104 ,  106 ,  108 , for example, molybdenum spacers. In the embodiment illustrated in FIG. 10, collimator plates  62  are positioned in slots of combs  102 ,  110  and directed energy beam welders  64 ,  90  weld areas  112 ,  114  and  116 . In one embodiment, welder  64  is also used to weld wires  92  into wire notches  94 . 
     It is clear that the various embodiments of the invention provide more efficient and less expensive manufacturing methods for producing post-patient collimators. The welded collimators themselves are less expensive and potentially more durable than collimators having adhesive bonds, whether or not a comb is part of the collimator. 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.