Patent Number: 
Section: description

X-ray imaging uses the fact that x-rays xe2x80x9cRxe2x80x9d are extremely penetrating but are absorbed by the material xe2x80x9cBxe2x80x9d (such as a patient""s body) through which they pass. An x-ray image is the two-dimensional map of the x-ray absorption of the material xe2x80x9cBxe2x80x9d lying between an x-ray source located at a focal point xe2x80x9cFPxe2x80x9d and an X-ray detector located at a detector plane xe2x80x9cDPxe2x80x9d. FIG. 1 shows a typical medical x-ray imaging situation. The quality of the image depends on the fact that a significant fraction of the x-rays R are absorbed rather than scattered. Referring to FIG. 2, Ray R is emitted from the source located at the focal point FP and detected at point P by the X-ray detector located at the detector plane DP. Ray R1 scatters and is also detected at the point P. Ray R2 is totally absorbed and, therefore, not detected. In the making of an image, occurrences such as these happen many millions of times. The fact that R1 scattered and was detected at P causes density along the ray R1 to be appropriately assigned to the point P1. However, the point P receives radiation from the ray R1 and, therefore, the density along the ray R is measured to be lower than it actually is. Since scattering occurs in all directions, there is very little spatial information contained in the scattered radiation. The scattered radiation tends to blur the image and lower the measured absorption of localized regions of high absorption. This problem can be ameliorated by placing a grid 10 of plates 11, 12 in front of the X-ray detector DP which prevents the scattered radiation from reaching the detector, as shown in FIG. 3. The grid 10, which is also shown in FIGS. 4 and 5, is formed of a radiopaque material, such as lead, tungsten, copper or nickel. Each of these plates 11, 12 should be positioned so that the focal spot FP lies in the plane of the plate 11, 12. As illustrated in FIG. 3, it is clear that scattered radiation emanating from outside region (a) will not be detected; a fraction of the radiation emanating from the two regions labeled (b) will be detected; and all the radiation emanating from (c) will be detected. Furthermore, it is clear that this grid 10 will remove some of the unscattered radiation because the plates 11, 12 have a finite thickness xe2x80x9ctxe2x80x9d. For a one-dimensional grid, the geometric efficiency of the grid 10 is (pxe2x88x92t)/p where xe2x80x9cpxe2x80x9d is the period of the grid. For a two-dimensional grid, the geometric efficiency of the grid is ((pxe2x88x92t)/p)2. It is also clear that the effectiveness of the grid 10 in removing scattered radiation increases as the ratio h/p increases, where xe2x80x9chxe2x80x9d is the height of the grid 10 in the direction of the x-ray beam. Exemplary embodiments of the present invention provide techniques for making the focused anti-scatter grid 10 of FIGS. 3 through 5 efficiently and with high precision in those attributes which are important. The resulting grid structure is a sturdy and highly useful implement in the X-ray patient diagnostic imaging field, and provides the desired absorption of scattered secondary radiation. In addition, techniques conducted in accordance with the present invention can go to smaller characteristic dimensions, are relatively easier, less time-consuming and less expensive than existing techniques for making focused anti-scatter grids. One exemplary embodiment of a method (the exemplary embodiment of the method is illustrated as a flow chart labeled as reference numeral xe2x80x9c20xe2x80x9d in FIG. 6) according to the present invention for manufacturing the anti-scatter grid 10 having a desired height h (with reference to FIG. 3) and includes positioning a bottom surface of a mask of dielectric material, with a depth at least equal to the desired height of the anti-scatter grid, on a sheet of metal, as shown at 22 of FIG. 6. First and second series of intrinsically focused slots are then cut through a top surface of the mask to the sheet of metal, as shown at 24 of FIG. 6, and the sheet of metal is plated at the bottom of each of the slots of the mask with a radiopaque material to form partition walls of the anti-scatter grid, as shown at 26 of FIG. 6. Plating the radiopaque material into the slots of the mask is continued, as shown at 28 of FIG. 6, until the desired height h of the anti-scatter grid 10 (with reference to FIG. 3) is achieved. FIG. 7 a schematic illustration showing an exemplary embodiment of a method of plating the anti-scatter grid 10 of FIGS. 3 through 5 using the mask in accordance with the method of FIG. 6. The metal plate 1, on which the dielectric plating mask 2 is bonded, is immersed in an electrolyte 3 containing ions of the desired radiopaque material. An anode 4 of the same radiopaque material is placed in the electrolyte. The anode is connected to the positive terminal of a power supply 5 and the metal plate 1 (cathode), with the plating mask, is connected to the negative terminal. Positive ions are driven to the negatively charged cathode. By this technique, radiopaque material will build up in the slots resulting in a grid 10 of radiopaque material being formed. FIG. 8 is a flow chart illustrating an exemplary embodiment of a method 30 of cutting the mask 2 of FIG. 7 in accordance with the present invention. As the flow chart illustrates, the mask is cut by attaching the top surface of the mask to a steel xe2x80x9ccombxe2x80x9d having teeth forming a plurality of parallel slots, as shown at 32, mounting a conductor, such as a stranded copper wire, at the focal spot of the grid, as shown at 34, positioning the bottom surface of the mask on the detector plane, as shown at 36, connecting a high-resistance wire to the conductor and insulating the wire from the comb, as shown at 38, and pulling the high-resistance wire taunt, applying a charge through the high-resistance wire, and cutting the first series of intrinsically focused slots in the mask by passing the taunt, charged high-resistance wire along each tooth of the comb, as shown at 40. FIG. 8A illustrates an exemplary embodiment of an apparatus 100 that can be utilized in performing the electric machining of the method 30 of FIG. 8. The apparatus 100 includes an electrically insulated xe2x80x9ccombxe2x80x9d 102 having teeth 104 forming a plurality of parallel slots, which can attached to a top surface of the mask 2. The mask 2 is positioned on an imaginary detector plane DP and the apparatus 100 includes a first electrical connector 110 positioned at an imaginary focal spot FP, with reference to the imaginary detector plane DP. The first electrical connector 110 is fixed in position. The apparatus 100 also includes a second electrical connector 120, which is movable with respect to the first electrical connector 110, and an elongated high-resistance electrical conductor 130, such as a stranded copper wire, connected and pulled taunt between the first and the second electrical connectors 110, 120. As described previously, during a procedure wherein intrinsically focused slots are cut in the mask 2 with the apparatus 100, a charge is applied through the high-resistance wire 130 so that the wire is heated. Then the second electrical connector 120 is moved so that the taunt, electrified, high-resistance wire 130 is passed along each tooth 104 of the comb 102. The method 30 further includes attaching the metal sheet to the bottom surface of the mask, as shown at 42, detaching the comb from the top surface of the mask, as shown at 44, rotating the comb 90xc2x0 from its original orientation on the mask, as shown at 46, and reattaching the comb to the top surface of the mask, as shown at 48. Then the metal sheet is removed from the bottom surface of the mask, as shown at 50, and the second series of intrinsically focused slots is cut in the mask by passing the high-resistance wire along each tooth of the comb, as shown at 52. The metal sheet is then reattached to the bottom surface of the mask, as shown at 54, and the comb is detached from the top surface of the mask, as shown at 56. FIG. 9 is a flow chart illustrating another exemplary embodiment of a method 60 of cutting the mask 2 of FIG. 7 in accordance with the present invention. The mask comprises dielectric material which can be cut with a laser and dissolved with a solvent, as shown at 62, and is attached to the metal sheet, as shown at 64. The mask is cut by positioning the bottom surface of the mask on a xe2x80x9cdetectorxe2x80x9d plane, as shown at 62, positioning a mirror mounted on a two-axis gimbals at a xe2x80x9cfocalxe2x80x9d spot, as shown at 68, directing a laser beam off the mirror and onto the top surface of the mask, as shown at 70, and operating the mirror so that the first and the second series of focused slots are cut by the laser beam in the mask, as shown at 72. This laser should have enough power to cut through the mask. The laser and optics should be suitable to cut slots which are 100 microns or smaller. It may be useful to use a beam which is wide in the direction of the cut and very narrow perpendicular to the cut. This would allow much greater power to be applied to the mask; however, it would add complexity to the optics. The computer-controlled gimbals can be moved using standard motion control techniques either with servomotors, stepper motors, or other techniques such as piezoelectric actuators. Both coordinates must be controlled at the same time. Alternatively, the laser can remain fixed and the mask can be moved relative to the laser beam. A second option is to place a photomask in the laser beam, which will cast a shadow on the mask. This shadow is precisely in the form of the desired final plating mask. Using this technique, a much larger laser beam can be used which will cut many slots simultaneously. This beam will also be scanned from a single spot so that the slots, which are cut in the mask, converge on that spot. FIG. 9A illustrates an exemplary embodiment of an apparatus 200 that can be utilized in performing the laser cutting of the method 60 of FIG. 9. The apparatus 200 includes a laser source 202 and a mirror 210 mounted on a two-axis gimbal positioner 220. A two-axis gimbal positioner 220 provides titling movement with respect to two perpendicular axes, such as the x and y axes, as shown (two-axis gimbal positioners with motorized actuators are available, for example, from Microwave Instrumentation Technologies, LLC of Duluth Ga., http://www.mi-technologies.com, and Newport Corporation of Irvine Calif., http://www.newport.com). The mask 2 is cut by positioning the bottom surface of the mask on a detector plane DP, positioning the mirror 210 at the focal spot FP, directing a laser beam 204 from the laser source 202 off the mirror 210 and onto the top surface of the mask 2, and operating the two-axis gimbal positioner 220 so that first and second series of slots are cut in the mask 2 by the laser beam 204. A stainless steel (or other suitable metal) frame is attached to the aluminum sheet to provide a mounting means for the anti-scatter grid. The frame is connected electrically to the aluminum sheet so that during plating the anti-scatter grid is attached to the frame. The surface of the frame, which should not be plated, must be masked with a thick coat of wax. According to one exemplary embodiment, the metal sheet comprises aluminum and the mask comprises a fine grain styrene foam. The mask is secured to the metal sheet using hot wax, and the wax is scraped from the metal sheet at the bottom of each slot of the mask prior to plating. A lower surface of the metal sheet is coated with wax prior to plating. The mask is secured to the comb using hot wax, and the comb is heated to remove the comb from the top surface of the mask The surface of the aluminum sheet and the frame should be clean and free of contaminants so that a good bond can be achieved between the plated structure and the frame. If the surface of the metal to be plated is not perfectly clean, it may be necessary to etch it or clean it chemically or electrochemically in some way. When the aluminum plate with the plating mask and the frame are completed, they are placed in a plating bath. At this point, a radiopaque material is plated through the slots in the plating mask on to the aluminum of the backing plate and the stainless steel (or other suitable metal) of the frame. The plating continues until the grid is thick enough. At this point, the radiopaque material of the grid may be smooth and uniform, in which case the aluminum backing electrode may be dissolved in sodium hydroxide, or other agent for dissolving the metal sheet without dissolving the grid, the plating mask dissolved in an organic solvent, and the grid carefully cleaned. Alternatively, the metal sheet, can be provided as a very thin layer secured onto a thicker layer of radiolucent material, such as carbon fiber. In this manner, the combination of the thin layer of the metal sheet and the thicker layer of the radioluscent material can remain attached to the grid without substantially interfering the passage of x-rays through the grid. The metal sheet can also be provided as a very thin layer of a metal grid secured to a thicker support layer of radiolucent material. If the radiopaque material of the grid is uneven, the grid should be machined in some fashion to make it uniform. This is probably best done while the plating mask is still supporting the grid. After this, the aluminum electrode and plating mask are removed as explained above. When the grid is completely clean, a very thin layer of carbon fiber laminate or other suitable material may be glued to each face of the grid and the frame to protect and stabilize the grid. Alternately, the surface of the radiopaque material may be left rough so long as it is entirely within the slots of the plating mask. Furthermore, under some circumstances, the plating mask may be left in place since its absorption of x-rays is very small compared to that of the radiopaque material. It will thus be seen that the objects set forth above, and those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.