Patent Number: 
Section: description

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows an schematic top view of an X-ray inspection system 10. The system 10 includes an X-ray source 12 which produces a fan-shaped X-ray beam 16 having its center at the focal spot 14 of the source 12. An arc-shaped detector assembly 20 receives the X-ray radiation after it passes through a target 13. The X-ray source 12 may be any known X-ray source which is capable of producing X-rays having the energy level required for the particular application. The collimator assembly of the present invention is especially useful in high-energy applications, that is applications having an output of about 1 MeV or higher. One suitable X-ray source is a Linatron M6 linear accelerator of 6 MeV output, available from Varian Industrial Products, 3100 Hansen Way, Palo Alto, Calif., 84104 USA. The detector assembly 20 includes an X-ray detector 19, for example a linear array detector 19, and a collimator assembly 22. Referring to FIG. 2, the collimator assembly 22 generally comprises a carrier 26, an arcuate base 27 including a plurality of radio-opaque arcuate bar sections 28, and a plurality of radio-opaque collimator plates 30 arranged in a radial array. It is noted that, as used herein, the term xe2x80x9cradialxe2x80x9d means a direction parallel to a line extending from the focal spot 14 of the X-ray source 12. An example of one such line is line labeled R in FIG. 1. Also, as used herein, the term xe2x80x9ccircumferentialxe2x80x9d means a direction along the arc between first and second ends 15 and 17 of the detector assembly 20 (in other words, tangent to a line extending from the focal spot 14 of the X-ray source 12). One or more wires 70 may also be used to stabilize and align the collimator plates 30, as described below. The carrier 26 is an arc-shaped structure which provides a unified foundation for the collimator assembly 22. In the illustrated example the carrier 26 is constructed of steel plate, although other materials could be used. The carrier 26 has a generally planar top surface 32 which receives the bar sections 28 that constitute the arcuate base 27 and includes means for aligning the bar sections 28, such as dowel pins 34 which fit into holes in the carrier 26 and corresponding holes 33 in the bar sections 28. FIG. 3 shows a top view of an exemplary bar section 28. Each bar section 28 is a plate which is arcuate in plan view and comprises a radio-opaque material such as tungsten. In the illustrated example the bar section 28 is about 12 mm (0.47 in.) thick. The bar section 28 has an arcuate inner edge 36 and an arcuate outer edge 38. The distance between the inner edge 36 and the outer edge 38 (i.e. the depth) is selected to be sufficient to stop the beam 16 from passing through the bar section 28. This protects the active elements of the detector array 19, which are mounted behind the bar sections 28, from direct exposure to X-rays. The actual depth depends upon the output of the X-ray source 12 used in the particular application. In the illustrated example the curve of the inner edge 36 has a radius of about 235 cm (93 in.), while the curve of the outer edge 38 has a radius of about 244 cm (96 in.) A plurality of parallel slots 40 are formed in the inner edge 36, extending vertically between the top and bottom surfaces 37 and 39 of the bar section 28. The width of the slots 40 are approximately equal to the thickness of the collimator plates 30 (described below), while the lands 42 separating the slots 40 are of about the same width as the slots 42. In the illustrated example the slot and land width is about 0.5 mm (0.02 in.) A similar plurality of parallel slots 41 is formed in the outer edge 38. The slots in the inner and outer edges are positioned and spaced so that when the collimator plates 30 are mounted on the bar sections 28, each of the collimator plates 30 will be aligned along a radial line extending from the focal spot 14 of the X-ray source 12. Each of the bar sections 28 has first and second circumferential edges 44 and 46 which abut the adjoining bar sections on either side. The circumferential edges are disposed at an angle such that the joints between adjacent bar sections 28 are not parallel to a radial line extending from the focal spot 14 of the X-ray source 12. This prevents X-rays from having a straight line path of travel between the adjacent bar sections 28. Each of the bar sections 28 includes one or more holes 33 for receiving means for aligning the bar sections 28 during machining and during assembly to the carrier 26, such as dowel pins 34 (see FIG. 2). The bar section 28 located at each circumferential end of the collimator assembly 22 is configured as an end plate 29 (see FIG. 4). Each of the end plates 29 includes one edge 52 which is disposed at an angle so as to mate with the adjacent bar section 28, and a second edge 54 which is radially aligned with respect to the base 27. The end plates 29 are otherwise identical to the other bar sections 28. An exemplary collimator plate 30 is illustrated in FIG. 5. The collimator plate 30 has spaced-apart inner and outer edges 56 and 58 and spaced-apart upper and lower edges 60 and 62. A first alignment tab 64 extends downward from the corner formed by the inner edge 56 and the lower edge 62. A second alignment tab 66 extends downward from the corner formed by the outer edge 58 and the lower edge 62. A plurality of notches 68 are formed in the upper edge 60 for receiving wires 70 (described below). The notches 68 are shown with exaggerated dimensions in FIG. 5 for clarity. In the illustrated embodiment, the collimator plate 30 has a length L of about 76 mm (3 in.), a height H of about 12 mm (0.47 in.), and a thickness of about 0.5 mm (0.02 in.). These dimensions are related to the dimensions of the particular detector array 19 used and the power of the X-ray source 12, and may be varied to suit a particular application. The wires 70 (short sections of which are shown in FIG. 2) serve to stabilize and align the upper edges 60 of the collimator plates 30. Each of the wires 70 extends continuously from one circumferential end 15 of the detector assembly 20 to the other circumferential end 17. The wires 70 span the spaces between the collimator plates 30 and are received in the corresponding notches 68 of each adjacent collimator plate 30. The wires 70 are secured to the collimator plates 30, for example with an adhesive, and therefore prevent relative movement of the collimator plates 30. In the illustrated embodiment, the wires 70 are made of tungsten. The wires 70 are of a rectangular cross-section to increase the surface area available for the adhesive, with dimensions of about 0.27 mm (0.011 in.) by about 0.43 mm (0.017 in.) FIG. 6 shows a perspective view of an exemplary alignment fixture 72 used to assemble the collimator assembly 22. The view is oriented from below looking upward at the underside of the alignment fixture 72. In the exemplary embodiment illustrated, the alignment fixture 72 is made from three main parts: a body 74, a first end cap 76, and a second end cap 78, each of which is machined from stainless steel. Other materials which are stable and machinable may be used. Also, the components of the alignment fixture 72 could be arranged differently, or the alignment fixture could be a one piece integral structure. The body 74 is a generally planar and includes inner and outer edges 80 and 82, a top surface 84 (see FIG. 7), and a bottom surface 86. A plurality of ribs 88 are formed in the bottom surface 86. The ribs 88 are disposed in three rows 90, 92, and 94. The spaces between the ribs 88 have a width approximately equal to the thickness of the collimator plates 30. The spaces have a slight taper in the vertical direction to ease installation of the collimator plates 30. The ribs 88 are disposed in a radial array, that is, each of the ribs 88 is aligned along a line extending from the focal spot 14 of the X-ray source 12. Accordingly, the ribs 88 are not parallel to each other. On the contrary, they diverge from the inner edge 80 to the outer edge 82 so as to match the intended positioning of the collimator plates 30. The body 74 also includes slots 96 formed through its thickness to allow access to the collimator assembly 22 and the wires 70 during the assembly process so that adhesive can be applied to the needed areas. The first end cap 76 has a horizontal portion 98 and a vertical portion 100. The two portions define a generally L-shaped cross section. A slot 102 is formed in the first end cap 76 to allow access to the collimator assembly 22 during the assembly process. The horizontal portion 98 of the first end cap has a bottom surface 97 which protrudes below the bottom surface 86 of the body 74. The lower part of the vertical portion 100 includes a radially facing internal surface 104. A pair of pads 108 are formed on opposite ends of the internal surface 104. The pads 108 contact the outer edges 38 of the bar sections 28 during assembly. Also, a locating rib 110, used to position the alignment fixture 72 in the circumferential direction during the assembly process by engaging slots 41 in the outer edge 38 of the bar sections 28, is formed in the center of the internal surface 104. The horizontal portion 98 of the first end cap 76 is attached to the outer edge 82 of the body 74, for example with cap screws 112 and dowel pins 114 (see FIG. 7). A second end cap is generally in the shape of a rectangular bar. The second end cap 78 is attached to the inner edge 80 of the body 74, for example with cap screws 118 and dowel pins 120. The second end cap 78 has a bottom surface 116 which protrudes below the bottom surface 86 of the body 74. This bottom surface 116 works in conjunction with the bottom surface 97 of the first end cap 76 to properly position the alignment fixture 72 in the vertical direction with respect to the arcuate base 27, as explained more fully below. The assembly process of the collimator assembly 22 is now explained in detail with reference to FIG. 7. First, the bar sections 28 are placed on the carrier 26. The bar sections 28 are located in the proper position by means such as dowel pins 34 (see FIG. 2) which pass through holes in the bar sections 28 and the carrier 26. If desired, the bar sections 28 could also be attached to the carrier 26 by known means such as fasteners or adhesives (not shown). After the bar sections 28 are placed on the carrier 26, their top surfaces 37 are ground flat, using a known process, to provide a continuous, planar, arcuate surface 32. The collimator plates 30 are then placed in a radial array on top of the bar sections 28. The first and second alignment tabs 64 and 66 of the collimator plates 30 are received into the slots 40 and 41, in the inner and outer edges 36 and 38 respectively, of the bar sections 28. This ensures that the collimator plates 30 have the proper radial alignment and have the correct plate-to-plate spacing. The alignment fixture 72 described above is used to square and align the collimator plates 30, one section at a time. Beginning at the center of the collimator assembly 22, after the collimator plates 30 are placed on the surface 32, the wires 70 are laid over the notches 68 in the upper edges 60 of the collimator plates 30. The alignment fixture 72 is then placed on top of the collimator plates 30. The ribs 88 on the bottom surface of the alignment fixture 72 engage the upper edges 60 of the collimator plates 30. This ensures that the collimator plates 30 are in the proper radial alignment and that the individual plates are not xe2x80x9crackedxe2x80x9d with respect to each other, that is, each of the collimator plates 30 is perpendicular to the surface 32. The bottom surface 97 of the first end cap 76 and the bottom surface 116 of the second end cap 78 both rest on the upper edges 60 of the collimator plates 30. The dimensions of the alignment fixture 72, specifically the distances between the bottom surfaces 97 and 116 of the end caps and the bottom surface 86 of the body 74, are selected to position the alignment fixture 72 in a vertical direction with respect to the arcuate base 27 such that the collimator plates 30 will not fully engage or xe2x80x9cbottom outxe2x80x9d in the spaces between the ribs 88, in order to prevent binding and distortion of the collimator plates 30. The alignment fixture 72 is pushed in the radially inward direction, causing the locating rib 110 to engage one of the slots 41 in the outer edge 38 of one of the bar sections 28, and thus position the alignment fixture 72 in the circumferential direction with respect to the arcuate base 27. The pads 108 bear against the outer edges 38 of the bar sections 28 to prevent rocking of the alignment fixture 72. After the alignment fixture 72 is installed, the wires 70 are pushed down into the notches 68 in the upper edges 60 of the collimator plates 30. With the collimator plates 30 and the wires 70 are disposed in the proper position, the collimator plates 30 are secured to the bar sections 28, and the wires 70 are secured to the collimator plates 30, for example using a known industrial adhesive. One example of a usable adhesive is Loctite 499 thermal cycling adhesive gel, available from Loctite Corporation, 1001 Troutbrook Crossing, Rocky Hill, Conn. 06067. Other methods could also be used to secure the collimator plates 30 and the wires 70, for example, brazing or tack welding. The wires 70 are generally continuous for the entire length of the collimator assembly 22 and are therefore secured to the collimator plates 30 one section at a time, with the excess wire length hanging free, to be secured to a subsequent section of collimator plates 30. After the initial section of collimator plates 30 are secured to the base 27, the alignment fixture 72 is removed and the process described above is repeated using additional groups of collimator plates 30, working from the center of the assembly outward, until the entire collimator assembly 22 is complete. This system of modular assembly allows the construction of collimators of arbitrarily large sizes while maintaining precision and with reasonable assembly costs. This system also reduces the material costs of the collimator assembly 22 itself, because the use of the reusable precision alignment fixture 72 minimizes the amount of precision machining required in the components of the collimator assembly 22. The foregoing has described a collimator comprising a carrier having a planar top surface; an arcuate base disposed on the carrier, comprising at least one arcuate bar section made from a radio-opaque material; and A plurality of radio-opaque collimator plates disposed on the arcuate base in a radial array with a bottom edge of each collimator plate in contact with the top surface of the arcuate base. The foregoing has furthermore described a method for assembling such a collimator, as well as an alignment fixture useful for practicing the described method. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.