Patent Number: 060552964
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a schematic arrangement of a mammography apparatus 10. An x-ray source 12 emits a cone-shaped x-ray beam 14 towards the mammography apparatus 10. An upper compression plate 16 and a lower compression plate 18 compress a woman's breast 20 (shown in hatching). In this position, the breast 20 is exposed to the incident x-ray beam 14. The x-ray beam 14 is shaped by an operator (not shown) as required to fully illuminate the breast 20, but ideally does not extend beyond an outer diameter of the breast 20. Resulting scattered x-rays from the breast 20 are indicated by arrows 22. The upper compression plate 16 and the lower compression plate 18 are formed from polyester sheets having a thickness of 0.1778 mm. The upper compression plate 16 and the lower compression plate 18 generate little secondary radiation and exhibit negligible scattered radiation. A reciprocating radiographic grid 24 is disposed between the lower compression plate 18 and a film/screen cassette 26 for preventing transmission of scattered x-ray radiation to the film/screen cassette 26. The radiographic grid 24 and the film/screen cassette 26 are positioned closely to the lower compression plate 18 to minimize magnification effects. The radiographic grid 24 has a reciprocating travel indicated by double headed arrow "A" and as fully described in U.S. Pat. No. 4,901,335. Radiographic grids are taught more fully by U.S. Pat. No. 4,901,335, which is incorporated herein by reference. Generally speaking, and as shown in FIG. 2, the radiographic grid 24 includes a grid housing 28, a plurality of x-ray radiation absorbing lamellae 30 disposed in the grid housing 28, a top polymeric sheet 32 sealing an upper side of the grid housing 28 and a bottom polymeric sheet 34 sealing a lower side of the grid housing 28. The grid housing 28 includes a first side wall 28A, a second side wall 28B, a front wall 28C and a back wall 28D. The side walls 28A and 28B each include a plurality of longitudinal slots 36 therein, facing an interior of the grid housing 28 and corresponding to the number of the plurality of lamellae 30. The side walls 28A and 28B are preferably arc-shaped or bent along a circumference of a desired cylindrical section for the radiographic grid 24. The longitudinal slots 36 are positioned on the side walls 28A and 28B such that the plurality of lamellae 30, when inserted therebetween, are focused to a convergent line at the x-ray radiation source (12 in FIG. 1) spaced above the radiographic grid 24. Each of the plurality of lamellae 30 are typically lead strips having a thickness between 0.075 mm and 0.25 mm. However, other metals can be used. As described in greater detail below, each of the plurality of lamellae 30 has a thin foil strip (not shown) applied to its outer walls as shown in (FIG. 3) or applied to its lower end portions (as shown in FIGS. 7 and 8). The plurality of lamellae 30 are placed in the longitudinal slots 36 along the length of the side walls 28A and 28B. Between each of the plurality of lamellae 30 is an air gap or slot 38. The ratio of the height of each of the slots 38 (i.e. the height of each of the plurality of lamellae 30) to its width (i.e. the distance between each of the plurality of lamellae 30) is preferable a minimum of 5:1 and is potentially as large as 30:1. Each of the plurality of lamellae 30 have a preferred height of 3 to 20 mm. The top polymeric sheet 32 and the bottom polymeric sheet 34 have a thickness preferably between 0.0225 and 0.127 mm. The polymeric sheets 32 and 34 are preferably made of a mylar material. However, any other type of flexible, dimensionally stable plastic is equally acceptable. Finally, both of the top polymeric sheet 32 and the bottom polymeric sheet 34 have an adhesive along a peripheral border thereof for application of the polymeric sheet 32 or 34 to the grid housing 28. As an aide for alignment, and as shown in FIG. 2, the plurality of lamellae 30 preferably include top tabs 50 and bottom tabs 52. The top polymeric sheet 32 includes slits 54 which correspond to the top tabs 50. Similarly, the bottom polymeric sheet 34 includes slits 56 which correspond to the bottom tabs 52. During assembly, once the plurality of lamellae 30 have been positioned within the longitudinal slots 36 of the grid housing 28, the top polymeric sheet 32 and the bottom polymeric sheet 34 are adhered to the grid housing 28. More particularly, the top polymeric sheet 32 is placed on to the grid housing 28 such that the upper tabs 50 of one of the plurality of lamellae 30 pass through one of the slits 54 in the top polymeric sheet 32. Likewise, the bottom polymeric sheet 34 is placed on the grid housing 28 such that the bottom tabs 52 of one of the plurality of lamellae 30 pass through one of the slits 56 in the bottom polymeric sheet 34. It should be emphasized that the tabs 50, 52 and the slits 54, 56 are utilized only in the preferred embodiment to assist in assembly and alignment of the radiographic grid 24. They are not required elements. In other words, the radiographic grid 24 will function without the tabs 50, 52 or the slits 54, 56. The radiographic grid 24 shown in FIG. 2 is generally known in the prior art. While the radiographic grid 24 is quite functional, it still results in the undesirable lamellae line artifact previously described. The present invention overcomes this problem by providing an improved lamella 58 shown in FIG. 3. The lamella 58 includes a first side wall 60 and a second side wall 62. Additionally, the lamella 58 has a thin foil strip 64a applied to the first side wall 60 and a thin foil strip 64b applied to the second side wall 62. The foil strips 64a and 64b can be made from a variety of elements, and are preferably tin. However, copper, lead, or any other metal or combination of metals which can be manufactured as a foil are equally acceptable substitutes. Whatever the composition of the foil strip 64a and 64b, it must be able to "block" the shadow density effect of the lamella 58. The thickness of the foil strip 64a, 64b will vary depending upon the type of material used. So long as the metal used is manufactured to industry standards as a "foil", the resulting thickness will be acceptable. Therefore, for example, where tin is used for the foil strip 64a and 64b, a thickness of 0.003 mm produced highly successful results. The foil strip 64a or 64b can be pre-cut to a shape conforming to the lamella 58 and then attached to the appropriate side wall 60, 62 with an adhesive 65. In the preferred embodiment, the adhesive 65 is an acrylic based, pressure sensitive adhesive. However, other adhesives or forms of attaching the foil strips 64a or 64b to the lamella 58 are acceptable. For example, the foil strip 64a, 64b can be electrochemically coated on the first side wall 60 and the second side wall 62. Finally, a single piece of foil can be wrapped around the lamella 58. FIGS. 4A, 4B, 5A, 5B and 6 represent various tests and results of the foil strips 64a and 64b placed on the plurality of lamellae 30. FIG. 4A represents a first test performed with uncoated lamellae 30. In particular, a radiographic grid, including the plurality of lamellae 30 which were not coated with the foil strip (64a and 64b in FIG. 3), was placed on a film 70. Notably, the outer walls (28A-28D in FIG. 2) of the radiographic grid have been omitted from FIG. 4a to better show the test. A lead strip 72 was placed on top of the plurality of lamellae 30. A 4 cm piece of plastic 74, representing a human breast, was placed between an x-ray source (12 in FIG. 1 for example) and the radiographic grid. The x-ray source (12 in FIG. 1) was run at an energy radiation of 28 keV. Notably, mammographies are normally run at an energy radiation level in the range of 24-28 keV. During the test, the lead strip 72 blocked primary radiation from reaching the film 70 so as to better demonstrate the effects of the lamellae 30. FIG. 4B is a representation of an x-ray image 80 formed with the test described with reference to FIG. 4A. The image 80 depicts the strip of lead (72 in FIG. 4A) as an area of different density 82. Each of the plurality of lamellae (30 in FIG. 4A) also produced a definable image 84. Finally, each of the plurality of lamellae (30 in FIG. 4A) emitted line artifacts 86. These artifacts 86 appeared as shadows on the edges of the lamellae images 84. Between each lamella image 84, there is one artifact 86. FIG. 5A represents a second test performed with uncoated lamellae 30. Once again, a radiographic grid, including the plurality of lamellae 30 which were not coated with the foil strips (64a and 64b in FIG. 3), was placed on a film 90. The outer walls (28A-28D of FIG. 2) of the radiographic grid have been omitted to better show the test. A lead sheet 92 was placed on top of the plurality of lamellae 30. The lead sheet 92 included a rectangular opening 94. A piece of plastic 96, representing a human breast, was placed between an x-ray source (12 in FIG. 1 for example) and the radiographic grid. The x-ray source was run at an energy radiation of 28 keV. The rectangular opening 94 in the lead sheet 92 allowed primary radiation to pass through to the film 90. FIG. 5B is a representation of an x-ray image 100 formed with the test described with reference to FIG. 5A. The rectangular opening (94 in FIG. 5A) produced a definable image 102. Similarly, the plurality of lamellae (30 in FIG. 5A) produced definable images 104. Finally, several of the plurality of lamellae (30 in FIG. 5A) emitted line artifacts 106. As expected, no line artifacts were produced by the plurality of lamellae (30 in FIG. 5A) not aligned with the rectangular opening (94 in FIG. 5A). Notably, the line artifacts 106 extended far beyond the rectangular opening image 102. Thus, the line artifacts 106 appear to be carefully transmitted to extend beyond an expected angle of acceptance. In other words, as x-rays pass through the piece of plastic (96 in FIG. 5A), scattering takes place. The x-ray source (12 in FIG. 1) produces x-rays which pass into the piece of plastic (96 in FIG. 5A). The scatter resulting from the primary rays striking the plastic at an angle leaves the piece of plastic (96 in FIG. 5A) at a resulting angle of acceptance. Some of these scattered x-rays pass through the rectangular opening (94 in FIG. 5A) and then contact the plurality of lamellae (30 in FIG. 5a) aligned with the rectangular opening (94 in FIG. 5A) at an angle. Thus, the resulting lamellae line artifacts 106 do not terminate at the angle of acceptance of the rectangular opening (96 in FIG. 5A), but instead extend "beyond" the image 102. FIG. 6 is a representation of an x-ray image 110 formed with a radiographic grid having the plurality of lamellae (30 in FIG. 2) lined with a tin foil (shown in FIG. 3 as 64a, 64b). Similar to the test shown in FIG. 4A, a strip of lead (72 in FIG. 4A) was placed across the radiographic grid prior to activating the x-ray source. The strip of lead (72 in FIG. 4A) produced a definable image 112. Similarly, the plurality of lamellae (30 in FIG. 2) produced a definable image 114. However, as is shown in FIG. 6, the lamellae line artifacts are no longer present. Thus, the foil (64a, 64b in FIG. 3) eliminated the unilateral, well-defined density emanating from the plurality of lamellae. Numerous tests have produced consistent results. For example, foil comprised of tin, copper or lead all eliminated the lamellae line artifacts from the x-ray image. Further tests, similar to those described with respect to FIG. 6, were performed with a foil coating on only one of the lamella. This approach did not eliminate the line density artifact. Thus, the complete elimination of the lamellae line artifact appears to depend upon coating both adjacent lamellae with foil strips. However, coating only a single lamella with foil strips will still reduce the line density artifact. The radiographic grid of the present invention provides a significant improvement over past grids. By applying a foil coating to the side walls of the lamellae, the lamella line artifacts are eliminated. As a result, a more accurate x-ray image is produced. FIG. 7 and 8 show another embodiment of the present invention, which is based upon the surprising discovery that a foil (or coating) on lower end portions of each lamella is effective in eliminating the lamellae line artifacts from the x-ray image. As shown in FIG. 7, lamella 58' has foil 64' covering its lower or bottom end portion. Foil 64' covers a small portion of each side of lamella 58', as well as the bottom edges of lamella 58'. Tabs 52' are also covered by the foil. It has been found that using foil only on the bottom portions of each lamella (as opposed to covering both sides entirely) achieves the same elimination of line density artifacts. The embodiment shown in FIGS. 7 and 8 offers the further advantage of using far less foil. As in the previous embodiment shown in FIG. 3, foil 64' can be attached by adhesive or can be formed by coating processes, such as electrochemical coating. When the distance between the lamellae is decreased to less than 2 mm the linear artifact becomes a solid density in the interspaces. This density may or may not be present in the particular interspace. There is no pattern; it may alternate. The energy that produces this effect appears to be equilibrated and related to the variance in the interspace. By analogy, immersing a varied interspace grid partway into a fluid with standing wave vibrations would induce a wave in some but not all interspaces depending on the spacing and frequency of the wave. The application of the foil as described above will eliminate this density. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, the radiographic grid has been described as including lamellae with tabs. However, these tabs are not required. Further, the use of foil coated lamellae has other applications with radiographic grids. For example, the grid can be used with digital radiography.