Patent Publication Number: US-2021193426-A1

Title: Aligned grain structure targets, systems, and methods of forming

Description:
X-ray tubes may include a target material that generates x-rays in response to incident electrons. The target material may be subjected to cyclical thermal stress during operation. The target material may crack and/or separate from a mounting surface within the x-ray tube due to the thermal stress, leading to failure of the x-ray tube. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a target having a grain structure according to some embodiments. 
         FIGS. 1B and 1C  are block diagrams of target having a grain structure different from that of  FIG. 1A . 
         FIG. 2  is a block diagram of an x-ray system including a target having a grain structure according to some embodiments. 
         FIG. 3  is a block diagram of an anode of the x-ray system of  FIG. 2  according to some embodiments. 
         FIGS. 4A-4C  are block diagrams illustrating orientations of a grain structure relative to the support structure of  FIG. 3  according to some embodiments. 
         FIG. 5  is an overhead view of an anode of an x-ray system according to some embodiments. 
         FIG. 6  is a block diagram of a rotating anode of an x-ray system according to some embodiments. 
         FIG. 7  is flowchart of a technique of forming an x-ray system according to some embodiments. 
         FIG. 8A-8C  are block diagrams illustrating the formation of a target for an x-ray system according to some embodiments. 
         FIG. 9  is a block diagram of a computerized tomography (CT) gantry according to some embodiments. 
         FIG. 10  is a block diagram of a 2D x-ray imaging system according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments relate to an aligned grain structure target, systems including such a target, and methods of forming the same. In some embodiments, a tungsten, tungsten-rhenium, or any other material suitable for generating x-rays may be used or a target for a stationary anode. Some of these materials may improve the strength of the target material especially under cyclical thermal stresses. However, cyclical thermal stresses may still cause the target to crack, delaminate, or otherwise fail. Embodiments described herein include a target material having a grain structure that may reduce the likelihood of delamination, cracking, or the like that may cause the system to fail. 
       FIG. 1A  is a block diagram of a target having a grain structure according to some embodiments. The target  100  may be formed of a variety of materials. For example, the target may include tungsten, rhenium, rhodium, palladium, combinations of alloys of such materials, or the like. The target  100  may have properties designed for generating X-rays from electron emissions and/or maintaining structural integrity due to high temperatures generated from heat from the electron bombardment. As will be described in further detail below, the target  100  may be more easily manufactured than other targets with different grain structures. 
     In some embodiments, the target  100  has a grain structure  102  that is elongated along axis D 1 . Here, the elongated grain structure  102  is illustrated with line showing a general direction of the major axis of the grains. Each grain of a target material may be oriented such that the major axis is aligned in a slightly different direction. However, a combination of the different directions results in a direction illustrated by the lines. 
       FIGS. 1B and 1C  are block diagrams of target having a grain structure different from that of  FIG. 1A . Referring to  FIG. 1B , the target  100 ′ has a target material having grains elongated along axis D 2 , perpendicular to axis D 1 . Such a target  100 ′ may be formed by rolling or forging a target material into a sheet. Although the orientation of the grain structure  102 ′ of the target  100 ′ may be similar the target  100  of  FIG. 1A , the grain structure  102 ′ is aligned along a different axis D 2 . Referring to  FIG. 1C , the target  100 ″ includes a grain structure  102 ″ where the grains are substantially equiaxed. Accordingly, a number of grains per unit area at a surface  103  of target  100  may be greater than that at a surface  103 ′ of target  100 ′ or a surface  103 ″ of target  100 ″. 
     As will be described in further detail below, in some embodiments, the target  100  may be formed by pressing, sintering, and forging the target material. However, in other embodiments, the target  100  may be formed using a different technique. The processing of the material forming the target  100  may result in the grain structure described herein. Pressing or hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting the material to the point of liquefaction, often used in powder metallurgy. Creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The combinations of pressing, sintering, and forging can also be used to remove impurities from the target material. 
       FIG. 2  is a block diagram of an x-ray system including a target having a grain structure according to some embodiments.  FIG. 3  is a block diagram of an anode of the x-ray system of  FIG. 2  according to some embodiments. Referring to  FIGS. 2 and 3 , the x-ray system  200  includes a cathode  201  and an anode  202 . The cathode  201  is configured to generate a particle beam  204 , such as an electron beam. The cathode  201  may include an emitter such a bulk emitter, planar emitter, a filament, or the like. The cathode  201  may include other components such as grids, focusing/steering components, or the like. 
     The anode  202  includes a support structure  206  and target  100  similar to the target  100  of  FIG. 1A . The support structure  206  may be formed of a variety of materials. For example, the support structure  206  may include copper, Glidcop, combinations of alloys of such materials, or the like. The support structure  206  may have properties designed for dissipating heat (a high thermal conductivity, cooling structures, or the like) generated by the target and/or maintaining structural integrity due to high temperatures generated from heat. In some embodiments, the support structure  206  may have a thermal conductivity greater than 100 or 200 watts per meter-Kelvin (W/(m·K)) at 20° Celsius (C). The target  100  is attached to a mounting surface  206   a  of the support structure  206 . A target material may have a different rate or coefficient of thermal expansion rate from a support structure material. Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature. An interface between the target  100  and the mounting surface  206   a  may be susceptible to delamination and/or cracking due to thermal cycling and the different coefficients of thermal expansion rate between the target material and the support structure. In some embodiments, the mounting surface  206   a  is angled relative to the particle beam  204 ; however, in other the mounting surface  206   a  may have a different orientation. In some embodiments, the anode  202  may be a stationary anode; however, as will be described in further detail below, the anode  202  may be a rotating anode. 
     The grain structure  102  has a particular orientation relative to the mounting surface  206   a . Axis D 1  is perpendicular to the mounting surface  206   a . Axis D 2  is parallel to the mounting surface  206   a . The grain structure  102  has a first dimension along the axis D 1  perpendicular to the mounting surface  206   a  that is longer than a longest dimension along any axis parallel to the mounting surface  206   a  such as axis D 2 . Here, axis D 2  is used as an example of an axis parallel to the mounting surface  206   a , however, those axes may include different axes, such as axis D 3  that extends out of the plane of the figure. In some embodiments, at least 80% or 95% to all of the target  100  has a grain structure  102  with a first dimension along the axis D 1  perpendicular to the mounting surface  206   a  that is longer than a longest dimension along any axis parallel to the mounting surface  206   a  such as axis D 2 . 
     A result of the grain orientation relative to the mounting surface  206   a  is that for a given grain size, a number of grains per unit area at the interface between the target  100  and the mounting surface  206   a  may be relatively increased. This increase in the number of grains per unit area may reduce a probability that the target  100  delaminates from the support structure  206 . A lower probability of delamination may lower a probability of cracking of the target  100  as the support structure  206  may be able to conduct heat from the target  100  more efficiently due to the maintained contact. 
       FIGS. 4A-4C  are block diagrams illustrating orientations of a grain structure relative to the support structure of  FIG. 3  according to some embodiments. Referring to  FIG. 4A , axes D 1  and D 2  are the same as those of  FIG. 3 . A single grain  102   a  is used as an example of the general orientation of the grain structure  102 . The grain  102   a  has a length  400  along axis D 1  and a length  402  along axis D 2 . The length  400  is greater than the length D 2 . 
     As the length  400  along axis D 1  may be greater than any length along an axis D 2  or another axis perpendicular to axis D 1 , i.e., parallel to the mounting surface  206   a , a number of grains per unit area at the interface between the target  100  and the mounting surface  206   a  may be larger in a plane perpendicular to axis D 1  than in a plane perpendicular to axis D 2  or other axis perpendicular to axis D 1 . In addition, as long as the length along axis D 1  is greater, then the grain structure  102  of the target  100  may be oriented relative to the mounting surface  206   a  in a manner to improve the number of grains contacting the mounting surface  206   a . In an example, the length  400  along axis D 1  may be twice, four times, or ten times as great than any length along an axis D 2  or another axis perpendicular to axis D 1 . In another example, the length  400  along axis D 1  may be twice, four times, or ten times as great than any length along an axis D 2  or another axis perpendicular to axis D 1  for at least 80% or 95% to all of the target  100 . In another example, the length  400  along axis D 1  may be twice, four times, or ten times as great than any length along an axis D 2  or another axis perpendicular to axis D 1  for at least 80% or 95% to all of the interface between the target  100  and the mounting surface  206   a.    
     Referring to  FIG. 4B , another orientation of the grain  102   a  is illustrated as an example of the general orientation of the grain structure  102 . Here, the major axis of the grain structure  102  is substantially parallel with the axis D 1 . That is, the grain structure  102  may aligned to axis D 1 . The length along axis D 2  may be a minimum. Referring to  FIG. 4C , the orientation of the grain  102   a  may be similar to the orientation of  FIG. 4B  relative to axes D 1  and D 3 . Axis D 3  may be perpendicular to both axes D 1  and D 2 . The length  404 ′ along axis D 3  may also be a minimum. Accordingly, the grains of the grain structure are generally oriented to be elongated parallel to the axis D 1 . This orientation may maximize the grains per unit area at the interface between the target  100  and the mounting surface  206   a . For example, the grains per unit area at the interface may substantially the greatest, greater than the grains per unit area on another surface of the target  100 , and/or greater than the grains per unit area of any cross-section of the target. 
     Some applications of a target material in an x-ray system include a sheet material. The sheet material may be formed by pressing and sintering to form a blank. The blank may be rolled or forged into a sheet. As a result, the grain structure has a major axis that is generally in the plane of the sheet and aligned in the direction of the rolling used to form the sheet. As a result, when the sheet is used as a target, the grain structure may result in a long side of the grains contacting a support structure. A grain structure with the long side of the grains contacting a support structure will reduce the relative grains per unit area in contact with the support structure. This may increase the probability of the sheet material delaminating, which may lead to the failure of the x-ray system. Other techniques of forming a target include pressing and sintering to form a disc blank. The disc blank may be forged to a desired thickness. While the grain structure may be smaller and/or less elongated than when the blank is rolled into a sheet, the grain structure is expanded in the plane of the disk due to the forging, reducing the grains per unit area. In addition, a process of forming such a disc may be difficult to perform with an acceptable reliability and/or cost. 
     Using a target  100  as described herein results in a grain structure with a higher grain per unit area at a mounting interface between the target  100  and the mounting surface  206   a . As a result, the interface of the target to the mounting surface  206   a  may be more resistant to stress induced by thermal cycling, such as that from a cyclical and/or pulsed operation of an x-ray system  200 . As x-ray systems may be operated with thousands to millions of cycles over a lifetime, an improved resistance to thermal cycling may improve reliability of the overall system. 
     In some embodiments the orientation of the grain structure may be substantially the same throughout the target  100 . However, in other embodiments, the grain structure may be oriented as described above, only at the interface between the target  100  and the mounting surface  206   a . That is, the orientation of the grain structure may be different throughout the target  100  and/or may deviate from the orientation described above further from the mounting surface  206   a.    
       FIG. 5  is an overhead view of an anode of an x-ray system according to some embodiments. In some embodiments, the mounting surface  206   a  may be similar to the mounting surface  206   a  of the support structure  206  described above. The mounting surface  206   a  may have a circular cross-section. However, in other embodiments, the cross-section of the mounting surface  206   a  may have a different shape. The target  100   a  may include a disc. The disc may have a minor axis perpendicular to the mounting surface  206   a . That is, the disc  100   a  may have a relatively low aspect ratio where the diameter may be much larger than the thickness of the disc  100   a . While a disc has been used as an example of the shape of the target  100   a , in other embodiments, the target  100   a  may have a different shape. In addition, while the mounting surface  206   a  and the target  100   a  may have similar cross-sections, such as the illustrated circular cross-sections, the cross-sections of the mounting surface  206   a  and the target  100   a  may be different. 
       FIG. 6  is a block diagram of a rotating anode of an x-ray system according to some embodiments. In some embodiments, an x-ray system includes a rotating anode  600 . The rotating anode  600  includes a support structure  602  and a bearing assembly  610 . In some embodiments, the support structure  602  and the bearing assembly  610  are rotatably coupled by a hydrodynamic bearing  612 . In other embodiments, the support structure  602  and the bearing assembly  610  may be rotatably coupled in other ways such as through ball bearings. 
     A target  100   b  is attached to a mounting surface  606   a . The target  100   b  may include a grain structure aligned similar to the relationship between the grain structure of the target  100  and the mounting surface  206   a  described above. For example, the grain structure of the target  100   b  may be generally perpendicular to the mounting surface  606   a . This relationship may be maintained even though the mounting surface  606   a  is a curved annular shape. 
       FIG. 7  is flowchart of a technique of forming an x-ray system according to some embodiments.  FIGS. 8A-8C  are block diagrams illustrating the formation of a target for an x-ray system according to some embodiments. The structures of  FIGS. 8A-8C  will be used as an example; however, in other embodiments, the operations may result in different structures. 
     Referring to  FIGS. 7 and 8A , in  700  a blank  800  of a target material is formed. For example, a powder material such as tungsten, rhenium, rhodium, palladium, combinations of alloys of such materials, or the like may be pressed into the blank  800 . The blank  800  may be sintered. In some embodiments, the material may be formed into a blank  800  in the shape of a rod. Regardless of the shape of the blank  800 , the grains  802  of the blank may have lengths that are substantially the same along any axis. This shape of the grains  802  is represented by the circular shapes. The grain of a material can also be referred to as a crystallite, which is a small or microscopic crystal structure which can form during the cooling of many materials. The initial orientation of crystallites is typically random with no preferred direction, but can be directed through growth and processing conditions. The areas where crystallites meet are known as grain boundaries. The powder material used in the blank can include grains or crystallites of the material. 
     Referring to  FIGS. 7 and 8B , in  702 , the blank  800  is processed to extend a dimension of a grain structure  802  of the target material along an axis  804  of the blank. The elongated grain structure  802 ′ is represented by lines to illustrate the elongation along the axis  804 . 
     The elongation may be performed in a variety of ways. For example, the blank  800  may be forged, rolled, drawn, pulled, extruded, compressed, or the like to extend the length along axis  804 . As described above, the blank  800  may have a rod shape. The resulting processed blank  800 ′ may still have a general shape of an elongated rod, wire, or the like. 
     In some embodiments, a dimension  808  of the processed blank  800 ′ may be at or near a final dimension of the target  100 . For example, a diameter of a rod may be substantially the same as the diameter of the disc  100   a  described above. The processing in  702  may be performed until a diameter of the rod is less than or equal to a corresponding dimension of the mounting surface. 
     In  704 , a portion  810  of the processed blank  800 ′ may be separated. For example, the portion  810  may be separated by cutting, machining, or the like. The separation operation may be performed in a plane  814  that results in the grain structure as described above. For example, the plane  814  may be substantially perpendicular to the elongation of the grain structure  802 ′. That is, the portion  812  of the processed blank  800 ′ may be separated such that the grain structure  802 ′ of the portion  812  has a first dimension along an axis perpendicular to the mounting surface that is greater than a dimension along an axis parallel to the mounting surface similar to the target  100  described above. In some embodiments, the resulting portion  812  may be a slice of a pressed sintered forged rod. 
     Further processing, such as forging, may be performed on the portion  812 . For example, the portion  812  may be forged to achieve a desired thickness. In other embodiments, the portion  812  may have a thickness  810  that is at or near a final thickness of the target  100 . 
     In some embodiments, the portion of the processed blank that is separated may have a thickness less than about 0.125 inches (in.) or 3.18 millimeters (mm). In other embodiments, the thickness may be less than about 0.050 in or 1.27 mm. In other embodiments, the thickness may be less than about 0.016 in or 0.41 mm. When the thickness of the portion is as thin as those described above, the interface may be more susceptible to delamination due to thermal cycling. The susceptibility may increase with decreasing thickness with the thicknesses of less than about 0.050 in. and less than about 0.016 in. being more susceptible. 
     In  706 , the portion  812  is mounted on a mounting surface of a support structure. For example, the portion  812  may be mounted on an anode. The portion  812  may be mounted in a variety of ways, such as by back casting, brazing, welding (e.g., e-beam welding), or the like. The resulting structure may be similar to that of  FIGS. 2, 3, 5, 6 , or the like. 
       FIG. 9  is a block diagram of a computerized tomography (CT) gantry according to some embodiments. In some embodiments, the CT gantry includes an x-ray source  902 , a cooling system  904 , a control system  906 , a motor drive  908 , a detector  910 , an AC/DC converter  912 , a high voltage source  914 , and a grid voltage source  916 . The x-ray source  902  may include an x-ray tube including a target  100  or the like as described above. Although particular components have been used as examples of components that may be mounted on a CT gantry, in other embodiments, the other components may be different. Although a CT gantry is used as an example of a system that includes an x-ray tube including a target  100  or the like as described above, an x-ray tube including a target  100  or the like as described above in may be used in other types of systems. 
       FIG. 10  is a block diagram of a 2D x-ray imaging system according to some embodiments. The imaging system  1000  includes an x-ray source  1002  and a detector  1010 . The x-ray source  1002  may include an x-ray tube including a target  100  or the like as described above. The x-ray source  1002  is disposed relative to the detector  1010  such that x-rays  1020  may be generated to pass through a specimen  1022  and detected by the detector  1010 . 
     Some embodiments include an x-ray system  200 , comprising: a support structure  106 ,  206 ,  606  including a mounting surface  106   a ,  206   a ,  606   a ; a target  100 ,  100   a ,  100   b  attached to the support structure  106 ,  206 ,  606  on the mounting surface  106   a ,  206   a ,  606   a ; wherein the target  100 ,  100   a ,  100   b  has a grain structure  102 ,  802 ′ having a first dimension along an axis perpendicular to the mounting surface  106   a ,  206   a ,  606   a  is longer than a longest dimension along any axis parallel to the mounting surface  106   a ,  206   a ,  606   a . As described above with respect to  FIG. 4A , the major axis of a grain  102   a  may be rotationally offset relative to the axis D 1  perpendicular to the mounting surface  106   a ,  206   a ,  606   a . The rotational offset may be less than 45 degrees as a result of the dimension along the axis D 1  being longer than the dimension along another, perpendicular axis such as axes D 2  and D 3 . Thus, in some embodiments, the major axis of the grains may not be substantially parallel to the axis D 1 . 
     In some embodiments, a major axis of the grain structure  102 ,  802 ′ is substantially parallel with the axis perpendicular to the mounting surface  106   a ,  206   a ,  606   a.    
     In some embodiments, the target  100 ,  100   a ,  100   b  is a disc having a minor axis perpendicular to the mounting surface  106   a ,  206   a ,  606   a.    
     In some embodiments, the target  100 ,  100   a ,  100   b  comprises a pressed sintered material. 
     In some embodiments, the target  100 ,  100   a ,  100   b  comprises at least one of tungsten, rhenium, rhodium, and palladium or an alloy of at least two of tungsten, rhenium, rhodium, and palladium. 
     In some embodiments, the target  100 ,  100   a ,  100   b  comprises a slice of a pressed sintered forged rod. 
     In some embodiments, a thickness of the target  100 ,  100   a ,  100   b  is less than about 0.050 inches. 
     In some embodiments, a location where the grain structure  102 ,  802 ′ has the first dimension along the axis perpendicular to the mounting surface  106   a ,  206   a ,  606   a  that is longer than the longest dimension along any axis parallel to the mounting surface  106   a ,  206   a ,  606   a  is at an interface between the target  100 ,  100   a ,  100   b  and the mounting surface  106   a ,  206   a ,  606   a.    
     In some embodiments, the x-ray system  200  further comprises a cathode; and an anode  202 ,  600 ; wherein the support structure  106 ,  206 ,  606  is part of the anode  202 ,  600 . 
     In some embodiments, the anode  202  is a stationary anode  202 . 
     In some embodiments, the anode  202  is a rotating anode  600 . 
     In some embodiments, a surface of the target  100 ,  100   a ,  100   b  contacting the mounting surface  106   a ,  206   a ,  606   a  comprises a greatest number of grains per unit area of surfaces of the target  100 ,  100   a ,  100   b.    
     Some embodiments include an x-ray system  200  formed by a process comprising: forming a blank  800  of a target  100 ,  100   a ,  100   b  material; processing the blank  800  to extend a dimension of a grain structure  102 ,  802 ′ of the target  100 ,  100   a ,  100   b  material along an axis of the blank; separating a portion  810  of the processed blank  800 ′; and mounting the portion on a mounting surface  106   a ,  206   a ,  606   a  of a support structure  106 ,  206 ,  606  of an anode  202 ; wherein the portion  812  of the processed blank  800 ′ is separated such that the grain structure  102 ,  802 ′ of the portion  812  has a first dimension along an axis perpendicular to the mounting surface  106   a ,  206   a ,  606   a  that is greater than a dimension along an axis parallel to the mounting surface  106   a ,  206   a ,  606   a.    
     In some embodiments, forming the blank  800  of the target  100 ,  100   a ,  100   b  material comprises forming a rod; and processing the blank  800  comprises extending a length of the rod. 
     In some embodiments, forming the rod comprises: pressing the target  100 ,  100   a ,  100   b  material into the blank  800 ; and sintering the blank  800 . 
     In some embodiments, extending the length of the rod comprises extending the length of the rod until a diameter of the rod is less than or equal to a corresponding dimension of the mounting surface  106   a ,  206   a ,  606   a.    
     In some embodiments, separating the portion  812  of the processed blank  800 ′ comprises cutting the portion  812  from the processed blank  800 ′ along a plane perpendicular to the extended dimension of the grain structure  102 ,  802 ′ of the target  100 ,  100   a ,  100   b  material. 
     In some embodiments, mounting the portion  812  on the mounting surface  106   a ,  206   a ,  606   a  of the support structure  106 ,  206 ,  606  of the anode  202  comprises one of: back casting the support structure  106 ,  206 ,  606  to the portion  812 ; brazing the portion  812  to the support structure  106 ,  206 ,  606 ; and welding the portion  812  to the support structure  106 ,  206 ,  606 . 
     Some embodiments include an x-ray system, comprising: means for generating a particle beam; means for supporting; and means for converting at least part of the particle beam including means for attaching the means for converting the at least part of the particle beam to the means for supporting with a number of grains per unit area greater than a number of grains per unit area in a plane perpendicular to the means for supporting. 
     Examples of the means for generating the particle beam include the cathode  201  or the like. Examples of the means for supporting include the support structures  206 ,  606 , or the like. Examples of the means for converting at least part of the particle beam include the target  100 ,  600 , or the like. Examples of the means for attaching the means for converting the at least part of the particle beam to the means for supporting with a number of grains per unit area greater than a number of grains per unit area in a plane perpendicular to the means for supporting include the portions of the targets  100 ,  600 , or the like having the grain structure described above. 
     In some embodiments, the means for attaching the means for converting the at least part of the particle beam to the means for supporting comprises means for attaching the means for converting the at least part of the particle beam to the means for supporting with a substantially greatest number of grains per unit area, where substantially greatest number of grains per unit area is within 5% of a possible greatest number of grains per unit area. Examples of the means for attaching the means for converting the at least part of the particle beam to the means for supporting with a greatest number of grains per unit area include a target  100 ,  600 , or the like having a grain structure as described with respect to  FIGS. 4B and 4C . 
     Although the structures, devices, methods, and systems have been described in accordance with particular embodiments, one of ordinary skill in the art will readily recognize that many variations to the particular embodiments are possible, and any variations should therefore be considered to be within the spirit and scope disclosed herein. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. 
     The claims following this written disclosure are hereby expressly incorporated into the present written disclosure, with each claim standing on its own as a separate embodiment. This disclosure includes all permutations of the independent claims with their dependent claims. Moreover, additional embodiments capable of derivation from the independent and dependent claims that follow are also expressly incorporated into the present written description. These additional embodiments are determined by replacing the dependency of a given dependent claim with the phrase “any of the claims beginning with claim [x] and ending with the claim that immediately precedes this one,” where the bracketed term “[x]” is replaced with the number of the most recently recited independent claim. For example, for the first claim set that begins with independent claim  1 , claim  3  can depend from either of claims  1  and  2 , with these separate dependencies yielding two distinct embodiments; claim  4  can depend from any one of claim  1 ,  2 , or  3 , with these separate dependencies yielding three distinct embodiments; claim  5  can depend from any one of claim  1 ,  2 ,  3 , or  4 , with these separate dependencies yielding four distinct embodiments; and so on. 
     Recitation in the claims of the term “first” with respect to a feature or element does not necessarily imply the existence of a second or additional such feature or element. Elements specifically recited in means-plus-function format, if any, are intended to be construed to cover the corresponding structure, material, or acts described herein and equivalents thereof in accordance with 35 U.S.C. § 112 ¶6. Embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.