Patent Number: 
Section: description

Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system 10 is shown as including a gantry 12 representative of a xe2x80x9cthird generationxe2x80x9d 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 16xc3x9716 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.