Patent Number: 060758406
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION The fragmentary perspective view of FIG. 1 illustrates a cellular air cross grid typical of the grids of the present invention with the partition walls defining the cells passing through the grid being shown with an exaggerated convergence angle between adjacent walls to emphasize the fact that the cells are focused, i.e. aligned with their central axes converging at a point corresponding to the focal source of the X-rays produced by an X-ray tube in a mammography X-ray unit for example. These air passages or cells are unobstructed by any transverse interleaved layers of plastic or metal, and no surface covering layers of protective sheet material are needed with the air cross grids of the present invention. As shown in FIGS. 1 and 2, these cross grids are formed of stacked layers of metallic foil sheet, preferably beryllium-copper alloy or brass. Aligned portions of each sheet are removed by chemical milling or photo-etching to create the open air cell portions of each sheet, separated only by the partition wall portions thereof formed in a cross grid pattern. These chemically milled metal layers are then bonded together in the stacked configurations illustrated in FIGS. 1 and 2 where the convergent focussing orientation of the radiation axes forming the center line of each air passage or cell are easily seen to be converging at a focal point above the cross grids shown in these figures. The general proportions of the chemically milled metallic sheets and the dimensions of the partition wall portions thereof and the air cell passageways formed therethrough are best seen in FIGS. 1, 2, 5 and 14, which are intended to illustrate the very large volume of air cells extending entirely through the cross grids of the present invention, as contrasted with the very thin partition walls between the air cells formed by the portion of these metal sheets remaining after the photo etching process is completed on each sheet. Thus in FIGS. 1, 2 and 5 it will be seen that the stack of eight sheets 21, 22, 23, 24, 26, 27, 28 and 29 are assembled and laminated together to define the large plurality of open air cell passages 31. These passages 31 all pass through the entire depth of the assembled laminated multi-layer grid assembly 32, and each air cell passage 31 may be considered to have a central axis 33 defining the direction it extends through the cross grid assembly 32. As indicated in FIGS. 2, 10 and 11, these axes 33 for adjacent cells 31 preferably converge at a focal point 52 corresponding to the source of the X-rays delivered through this grid 32. The partition walls defining and separating the adjacent air cells 31 are formed of individual partition segments 34 which are the only part of the metal foil sheet remaining after the chemical milling operation has removed the air cell portions thereof, aligned to create the through air cell passageways 31. These partition segments 34 are thus aligned with each other in successive sheets of the laminated assembly, as shown in FIGS. 1 and 2, to create partitions or sidewalls 36 shown at the right hand sides of FIGS. 1 and 2. The width of these partition segments 34 forming the sidewalls 36 measured in the plane of each metal sheet lamination is preferably between 0.0010 inches and 0.0020 inches, and in a preferred embodiment a partition width or wall thickness of 0.0012.+-.0.00015 inches. The resulting grid structure 32 is a sturdy and highly useful implement in the mammography field, and provides the desired absorption of scattered secondary radiation entering the air cell passages 31 at a considerable angle from the axes 33. As a result, when from 8 to 20 or more layers of the cross grid laminations 21, 22 etc. are bonded together in the laminated assembly 32, this cellular air grid is unusually well adapted to perform its scatter-reducing function while transmitting substantially all of the desired primary radiation without attenuation, significantly improving mammogram contrast and image quality at comparable radiation dosages. The beryllium-copper foil sheet material employed to form the laminations of 21, 22, 23 etc. is referably about 0.004 inches thick, and is preferably formed of a layer of beryllium-copper alloy known as Alloy No. 190, having a temper specified as "XHMS", sold by Brush Wellman of Fairfield, N.J. The air cell openings formed by photo-etching between the partition segments 34 in each lamination may be from 0.010 inches to 0.030 inches in internal width. Center to center distances between the center lines of the partition segments 34 forming the walls defining each air cell, have been selected as 0.017 inches in a preferred embodiment of the cellular air grids of the present invention illustrated in the figures, as shown in FIG. 5, where the thickness of the partition walls 34 is shown to be 0.0012 inches, falling within the range of 0.0010 inches to 0.0020 inches in useful embodiments of the device. With these dimensions as illustrated in FIG. 5, the very large volume of the overall assembly 32 occupied by air cell passages 31 is evident, as compared to the very small volume of the overall assembly 32 occupied by the partition walls 36. For these dimensions, the partition walls occupy only about 14% of the total surface area of this air cross grid 32, leaving 86% of the total cross-sectional area available to receive and transmit the desired information-bearing primary radiation directly to the imaging unit, the X-ray film or digital imaging device. A cross grid 32 having these dimensions actually achieves transmission of about 82% of the desired primary radiation, while substantially blocking scattered secondary radiation. This effectiveness is considerably greater than has ever been achieved by any other form of commercially available fixed or moving grid. While a total number of eight laminations are illustrated in the figures, as many as 18 or 20 or more laminations may be employed if desired. To simplify the fabrication process, the minimum number of laminations required to achieve the desired scatter reduction is customarily employed. The bonding of the sheet metal foil layers 21, 22, 23 etc. together to form the laminated assembly 32 may be achieved by brazing, soldering, diffusion bonding or adhesive bonding techniques. It has been found that adhesive bonding with 3M adhesive coatings produced by Minnesota Mining and Manufacturing Company ("3M") have produced highly effective air cross grid assemblies 32, using the techniques hereinafter described for the fabrication assembly methods. These methods apply a thin coating of epoxy adhesive 37 to the surfaces of the partition segments 34 and particularly to those portions of the surface planes of the metal foil sheets which are juxtaposed in abutting contact as the chemically milled layers are laminated together. These production methods also eliminate almost all excess adhesive which might otherwise project into air passages 31, and might reduce the size of these air cell passages. An alternate fabrication method employs a thin plated coating of lead, lead-tin or other metal on the partition segments, which are stacked in accurate registration, clamped together under pressure, and heated to melt and fuse the plated coatings, bonding all of the stacked layers to form an integral laminated air grid. In the plan view of FIG. 5 showing an enlarged portion of the air cross grid 32 of the present invention, the air cell passages 31 and the partition walls 36 defining them are clearly illustrated. In the lower portion of FIG. 5 is a circle 38 identifying a corner junction where perpendicular partition walls 36 join, and the photoresist pattern forming such junctions is illustrated in enlarged detail in FIG. 6. In this figure, a possible problem sometimes encountered with the chemical milling or photo etching process is also illustrated by a dashed line 39 showing a corner fillet which may be formed at the junction 40 where the partition walls 36 join. Such a fillet 39 is undesirable because it reduces the overall area of the air grid cell 31 available for passing primary radiation, and it increases the cross sectional area of the junction regions, which are carefully dimensioned for optimum elimination of image shadows by grid movement during each X-ray exposure. In order to reduce the likelihood that such fillet 39 will appear, the mask or masking tool employed in the chemical milling or photo-etching process is preferably provided with square cornered diagonal projections 41, reducing the effective cross-sectional area of the junction regions to the desired value. Diagonal projections 41 protrude into the junction area 40 from each of the four air cell passage regions 31 surrounding junction 40, all as illustrated in FIG. 7. Projection 41 has the effect of ensuring that any "filleting" left behind by the chemical milling process will be concentrated at the inside corners of the resulting diagonal groove, leaving substantially the entire square area of each air cell passage region 31 open directly to its corner at each junction 40, and reducing the junction area to the desired value. FIGS. 8 and 9 show the general appearance of the cellular air cross grids of the present invention with the outer edges mounted in protective frames 42. In FIG. 8 the pattern of partitions 34 and cells 31 is aligned with the rectangular edges of the frame 42. Also shown in FIG. 8 is a point F midway along the lower longer edge of the cross grid at the frame, close to the patient's chest wall for mammography, and this is the point designed to be positioned directly under the focal source point 52 of the X-rays delivered through the grid, as will be explained more fully with regard to FIGS. 10 and 11. For general radiography, the focal point F is normally placed in the center of the grid panel, and not near one edge. The air cell passages 31 close to this point F will have their cell axes 33 substantially parallel to the perpendicular axis of the cell directly at point F. As the observer examines cells further away from point F, the cell axes 33 of such cells will be seen to be progressively more slanting, to converge at the same focal point 52 directly above point F when the cross grid is in use. This successively greater converging slant of the axes 33 is illustrated in FIG. 2, somewhat exaggerated for clarity. When the cross grids of the present invention are used as moving or Bucky grids, provided with translation movement during the X-ray exposure, the effect is to avoid any shadow image of the partition walls 36 which might be created if the cross grid remained in a stationary position during the exposure. FIG. 9 shows a diagonal orientation of the partition walls 36 and the air cell passages 31 when the assembly 32 is mounted in a frame 42 for use as a moving or Bucky grid. By examining FIG. 5 with reference to FIG. 8 and FIG. 9, it will be evident that movement of the cross grid in the normal lateral direction, parallel to the long edge of frame 42 in which the focal point F is positioned--the normal translation movement direction for a moving or Bucky grid--would avoid shadow images of the partitions 36 shown as vertical partitions in FIG. 5, but would result in the horizontal partitions 36 of FIG. 5 moving edgewise, maintaining their same position and leaving a shadow on the X-ray image. Any such shadow is undesired and the purpose of using a moving or Bucky grid is to avoid any such shadows of lamellae or partitions in the grid which are required to absorb scattered radiation. Such shadow images would degrade the resulting X-ray image and might conceal calcifications or extremely small dots having important significance in mammography. Radiologists specializing in mammography generally recommend using Bucky grids to avoid any such shadow images, in order to produce the most detailed images possible. To avoid the edgewise translation movement which might leave such an image of a partition wall, the diagonal arrangement of the partition walls 36 and air cell passages 31 illustrated in FIG. 9 is preferred. This orientation produces the diagonal movement indicated in FIG. 5 from the full line position of the partitions 36 to the dotted line positions of partitions 36, in the direction of the arrows 43 and 44. Arrow 43 at the left hand side of FIG. 5 shows the direction of movement of a particular junction 40 to its new position 40A during the course of the translation movement. Arrow 44 shows the translation of a central axis 33 of one of the air cell passages 31 to its new position 33A following translation movement. These relative positions illustrated in FIG. 5 are examples of instantaneous intermediate positions in which the cross grid structure will be found at various times during its continuous diagonal translation movement during the entire X-ray exposure period. This is desired to avoid even the shadow image which might be created if the partition walls came to a stop at any time during the exposure either to reverse direction or to continue after an increment of movement. Continuous movement during exposure thus produces the most effective elimination of all such shadow images from the partition walls of a moving grid. The direction of movement for such a moving or Bucky grid as that shown in FIG. 9 is indicated in the schematic diagrams of FIGS. 10 and 11. FIG. 10 is a side view of a mammography X-ray procedure showing the patient's breast 46 held stable in the mammography device between an underlying breast tray 47 and an overlying clamping plate 48 gently moved into position to deter any unnecessary movement during the exposure and to minimize the thickness of breast tissue for best X-ray imaging. The imaging unit beneath the breast tray 47 includes the moving Bucky grid 50 of the kind illustrated in FIG. 9 with diagonal-orientation of the cross grid pattern, overlying a film cassette 49 incorporating the X-ray film 51 encased inside cassette 49. Point F is indicated at the left-hand edge of grid 50 close to the patient and directly beneath the focal source point 52 which is shown with a source to image distance or SID represented as a dash line extending from X-ray source 52 through the patient's breast 46 and through the Bucky grid 50 at point F into the film cassette 49 to expose the film 51. The SID dash line is perpendicular to the Bucky grid 50 and to the film 51 as indicated by the diagram of FIG. 10. The direction of movement of Bucky grid 50 is indicated by an X shown midway across the shorter dimension of Bucky grid 50 from the patient to the outer free edge of the Bucky grid, thus indicating that the grid moves in the direction parallel to the patient's sternum, from right to left for example. The front elevation diagram of FIG. 11 further indicates the normal positioning of the air cross grid 50 of the present invention, when used as moving or Bucky grid, moving from the patient's right to the patient's left in the direction shown by the arrow 44 during the X-ray exposure. Here it is indicated that the point F is positioned under the middle of the breast at the midpoint of the translation movement of the Bucky grid during the exposure period. The SID dash line extending from X-ray source 52 directly perpendicular to film cassette 49 containing film 51 identifies the radiation travelling directly along this perpendicular radiation axis as it passes through Bucky grid 50 at point F and travels toward the film 51 in cassette 49. The actual dimensions of a preferred version of the movable Bucky grid 50 are 18 centimeters in width and 26 centimeters in length for an 18 cm.times.24 cm cassette, or 24 cm.times.32 cm for a 24 cm.times.30 cm cassette, and the SID distance from the X-ray film image to the X-ray source is 63 centimeters. The grids of this invention are preferably moved at a precise angle and distance during the X-ray exposure so that an integral number of grid lines in each of the two perpendicular partition directions will pass each point of the image. In addition, the ratio of the two directions is preferably set so that there is no repeating trace pattern of the intersections of grid partitions until at least 9 grid lines have passed a specific image point. Cross-Grids Cross grid partitions absorb scattered X-rays slanting in many directions. To eliminate imaging grid lines, the X-ray intensity variations produced by moving the grid should be reduced below about 1%. The usual technique with fine linear (non-cross) grids is to have the grid move with uniform velocity during the exposure. If the motion is fast enough so that about 100 lines cross each spot, the absorption difference due to the start and end positions during the exposure will be less than one line crossing or 1%. With cross grids the patterns are coarser, and the grid must be angled so that the motion translates both of the sets of perpendicular lines. Because the grid intersections represent a single line width, these areas will tend to be overexposed as compared to the in-between or open areas which are traversed by the two sets of cross lines. By choosing the grid translation angle so that as many intersection traces as possible are adjacent rather than coincident, the intersection traces can cover the entire space and become invisible. Thus there are two components to be optimized in the motion for the cross grid, first a linear transverse motion such that the grid lines uniformly cover the space, and second an angle, or diagonal motion such that the image-traces of all intersections of the cross lines cover as much area as possible, without trace overlaps. A suitable linear transverse motion would move the grid lines exactly one open space distance with uniform velocity during the exposure. Then each portion of the image will be obscured for exactly the same interval, and there will be no line image. For the cross lines to behave similarly the grid must be angled such that an integral number of lines will be moved in both sets. Thus angles giving line motion ratios of 1:1, 1:2, 5:4 etc. would be suitable. The more lines that are moved, the more tolerance allowed in the position, since any fixed error in the mechanism will be a smaller fraction of the total motion. The maximum travel is limited by both the focus shift and the constraints of the Bucky stage. A practical limit for travel is about 2.5 cm. With a 1 mm grid spacing and a 45 degree angle, the motion can cover about 18 lines. The angle of the grid determines the repeat pattern of the image-traces of intersections and to cover the space as uniformly as possible the number of lines to a repeat should be large. Ratios such as 9:1, 9:2:--9:8 appear to give adequate coverage. About 16:11 (34.6.degree.) is a suitable angle (FIG. 9). Two patterns then are 16 lines by 11 lines. Fabrication Methods The air cross grid 30 and the moving Bucky grid 50 shown in FIGS. 8 and 9, both incorporating the cross grid structure illustrated in FIGS. 1, 2 and 5, may be fabricated using the choice of bonding techniques previously mentioned for joining the stacked layers of chemically milled beryllium copper foil sheet in the desired registration, with the openings therein forming the air cell passages 31 illustrated in the figures. These techniques include brazing, soldering, diffusion bonding or adhesive bonding. Adhesive Bonding It has been found that an adhesive dip bath coupled with air knife removal of excess liquid adhesive provides a highly workable fabrication technique in the production of these air cross grids. This production step is illustrated in FIGS. 13 and 14. The overall method employed to produce the adhesive-bonded air cross grids of this invention are summarized in the flow diagram of FIG. 12. Each of the multiple layers of metallic foil sheet material is first chemically milled to remove the air cell passage portions 31, leaving only the partition segments 34 in place. For this purpose, a mask tool is employed and for each metal foil layer a pair of mask tools or web negatives are applied to the opposite faces of the foil sheet material, which has been coated by a photoresist on each of its surfaces. The photoresist application step is identified as step 53 in FIG. 12 and the application of the aligned mask tools or web negatives positioned in registration on opposite surfaces of the foil sheet is identified as step 54. Identical mask tools applied to both surfaces of the metal foil will produce partition segments 34 photo-etched in planes perpendicular to the foil surface. A mask tool sized for the slightly wider "focused" spacing of the next adjacent foil sheet will produce converging "focused" partition segments 35 (FIGS. 4 and 4A). The sandwich of two web negative mask tools aligned in registration flanking both faces of the sheet of the foil sheet blank is next exposed to ultraviolet light projected on both surfaces through the web negatives to expose the photoresist coatings to ultra-violet radiation in step 56. The web negatives are then removed and a developer solution is applied in step 57. The foil sheet blanks are next etched in step 58 to perform the "chemical milling" removal of the air space sections 31 forming the air cell passages, leaving only the cross grid partition segments 34 as illustrated in FIG. 14. This cross grid structure formed only of partition segments 34 is referred to as a fine web structure, and after all remaining photoresist is removed, this web is next dipped in liquid adhesive in the step 59. The liquid adhesive bath 61 is illustrated in FIGS. 13 and 14, where the web is being removed edgewise in an upward movement illustrated by the arrow 62 in FIG. 13. The upward movement of the web identified as metal sheet 21 in FIGS. 13 and 14 passes it upward between a pair of air knives 63, delivering thin wide fast moving curtains 64 of compressed air or nitrogen through an elongated narrow slot 66 formed in each of the air knives 63. These wide flat streams of compressed gas are directed diagonally at the two faces of the web structure of sheet 21, as indicated in FIG. 13, removing any bubble or film 67 of liquid adhesive which might be carried upward on the ascending web, spanning some of its passages 31 as shown in FIG. 13, for example. All such bubbles and films of adhesive are blasted from the surfaces and from spaces 31 in the ascending web structure by the action of the compressed gas stream 64 from air knives 63 and the displaced liquid adhesive thereby removed from the web structure drains back into the liquid adhesive bath 61 below. This leaves a sufficient thin coating of liquid adhesive on each of the parallel faces of the web structure sheet 21, these being the surfaces ready for abutting engagement and aligned registration with the next sheet of metal foil being fabricated and dipped in liquid adhesive to form the succeeding sheet 22. A small quantity of liquid adhesive may adhere to the internal wall surfaces of partition segments 34. Such a small amount of liquid adhesive cures during the assembly process and does not intrude significantly into the spaces 31 of the assembled cross grid 32. This air knife adhesive removal step is identified by the number 68 in FIG. 12 and the assembly of the successive layers 21, 22, 23 etc. aligned by registration indicia on the edge of each layer, to form the laminated assembled air grid 32 illustrated in FIGS. 1 and 2, is identified as step 69 in FIG. 12. The preferred bonding adhesive used in these fabrication methods is 3M SCOTCH-GRIP.RTM. plastic adhesive Model 1099L, diluted with acetone in accordance with the manufacturer's recommendations to decrease its viscosity. This adhesive is air curable, requiring no heat to be supplied during curing. Another suitable adhesive is Beacon Chemical Company's MAGNA-TAC.RTM. Model E645 two part epoxy adhesive, diluted with a 50--50 mixture of acetone and isopropyl alcohol for reduced viscosity. The curing of the adhesive between the layers identified as step 71 in FIG. 12 is performed while these layers are maintained in precise registration in the alignment illustrated in FIGS. 1, 2 and 3 to create the desired convergence of the axes 33 and provide a focused cross grid with all of these axes 33 converging at the X-ray source 52 illustrated in FIGS. 10 and 11. If identical web negative mask tools are employed on both faces of the foil sheet material being treated by the fabrication method described with reference to FIG. 12, as previously noted partition segments result which are perpendicular to the parallel faces of each of the foil sheets as illustrated in FIGS. 1 and 2, and the slight convergence of their axes 33 produces a slight stepwise misalignment between the successive partition segments 34 in each of the layers as illustrated in FIG. 3, where it is shown that adhesive 37 is effective only in the overlapping juxtaposed end portions of such stepwise aligned partition segments 34. If a slightly different web negative mask tool such as the one for the underlying foil layer is employed on the under surface of the overlying foil layer, the resulting slanted partition segments 35 may be produced by the etching operation, with a slight converging slant parallel to the desired cell axes 33, as indicated in FIG. 4. This convergent slant partition segment etching operation thus minimizes the stepwise alignment of the successive partition segments making up each partition wall 36, avoiding any risk of total misalignment at the grid corners most remote from focal point F, and thus allows the adhesive 37 to be bonded to all or a larger portion of the facing abutting surfaces of the partition segments 34, all as shown in FIG. 4. When all of the layers to be assembled or laminated in the cross grid assembly 32 have been dipped in the adhesive bath and withdrawn through the air knife clearing step 68, these web structure sheets are all aligned in careful registration to produce the desired convergent axes 33 for all of the air cell passages 31, utilizing optical or mechanical registration indicia or both impressed on all of the stacked foil webs, and the entire assembly is then secured between clamping surfaces under the clamping pressure desired, 100 pounds, for example, avoiding any unwanted separation of the layers. This allows the assembly in its clamped position to be held at room temperature or to be exposed to curing heat if desired, for the time required to cure the liquid adhesive between the layers. This bonds all of the partition segments in each partition wall to form a solid integral structure, having much greater strength than prior art grids employing sheets of glass as the partition material. When heated clamping plates are employed, they are formed of material, such as selected aluminum alloys, having a coefficient of thermal expansion matching that of the beryllium-copper foil sheets. The grids of this invention also avoid the disadvantage of interleaved "transparent" material designed to transmit X-rays through the cell passages. Most such materials are incapable of transmitting X-rays without absorbing some portion of the radiation. In this way, the radiation dose to which the patient is exposed can be held to the absolute minimum while achieving high quality mammograms with good detail, excellent contrast and no undesired shadow images of crossed partitions. If desired, outer dustcover sheets or films of polycarbonate may be applied to protect the exposed surfaces of these air grids. Diffusion Bonding of Metal Plated Partition Segments A plated coating of lead or a lead-tin mixture on each photo-etched foil sheet web can be employed, if desired, to enhance the absorption of secondary scattered radiation, and aligned plated partition segments 35 are shown in FIGS. 4 and 4A, and fabricated using the method shown in the flow sheet diagram of FIG. 15. These plated partition segments 35 are formed of beryllium-copper foil, chemically machined by photo-etching as described above in the converging, slanted segment orientation. After all photoresist is removed in step 70, a coating 72 of intermediate metal, such as copper, is applied by plating to the web's surface, in step 73 (FIG. 15). An outer bonding metal coating 74 of lead or lead-tin mixture is then plated over coating 72 in step 76. The plated web structures, after being dipped in a liquid flux, are precisely aligned in step 77 using mechanical or optical registration indicia or both marked on each foil sheet, and then clamped in an assembled stack and heated between clamping plates in step 78, in a hydrogen atmosphere in a hydrogen pusher furnace, to a temperature just sufficient to melt and fuse the bonding metal plating layers 74 at the bond zones 79 where the slanted "convergent" partition segments 35 of successive adjacent foil sheets are engaged in coincident alignment (FIGS. 4 and 4A). After final cooling in step 81, and removal of remaining flux in a vapor de-greasing operation, these plated air cross grids are sturdy, rigid panels, well adapted for use in absorbing secondary scattered radiation in mammography procedures. 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.