MBFEX tube

Abstract

A MBFEX tube (1) for an x-ray device comprises, in a vacuum tube (20), an anode (30) designed as a cooling finger and securely arranged in the vacuum tube, and a plurality of securely arranged cathodes (40, 41, 42), wherein the vacuum tube (20) comprises a plurality of cathode feed lines (50) and no more than two high-voltage bushings (51, 52), in a high-voltage bushing (52) a coolant pipe (31) is passed through by an internal coolant inner pipe (32), the coolant pipe (31) and the coolant inner pipe (32) are provided for cooling the anode (30) with a liquid coolant, the cathodes (40, 41, 42) are provided for field emission of electrons and are arranged on the anode (30) for generating x-ray sources (Q).

Claims

1. A multibeam field emission X-ray (MBFEX) tube for an x-ray device which comprises, in a vacuum tube, an anode designed as a cooling finger and securely arranged in the vacuum tube, and a plurality of securely arranged cathodes, wherein the vacuum tube comprises a plurality of cathode feed lines and no more than two high-voltage bushings, in a high-voltage bushing a coolant pipe is passed through by an internal coolant inner pipe, the coolant pipe and the coolant inner pipe are provided for cooling the anode with a liquid coolant, the cathodes are provided for field emission of electrons and are in each case oriented toward the anode for generating x-ray sources.

2. The MBFEX tube of claim 1, wherein the cathode feed lines and the high-voltage bushings are arranged in a row and lying opposite the anode on the vacuum tube.

3. The MBFEX tube of claim 2, wherein the x-ray sources are arranged in a row arrangement on the anode.

4. The MBFEX tube of claim 3, wherein the x-ray sources are each located on a surface section of the anode which is slanted with respect to the center axis of the anode.

5. The MBFEX tube of claim 4, wherein the slanted surface sections are formed by at least one of projections of the anode or ground sections in the anode.

6. The MBFEX tube of claim 5, wherein the slanted surface sections are coated.

7. The MBFEX tube of claim 1, wherein the cathodes comprise nanorods.

8. The MBFEX tube of claim 7, wherein the nanorods are designed as at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, single-walled hetero nitrogen carbon nanotubes, or multi-walled hetero nitrogen carbon nanotubes.

9. The MBFEX tube of claim 7, wherein at least some of the nanorods contain at least one of rare earth borides, metal oxides, metal sulfides, nitrides, carbides or silicon.

10. The MBFEX tube of claim 7, wherein the nanorods have a length of less than 20 μm and a diameter of less than 10 nm, and wherein a density with respect to the surface area of the cathodes is at least 10.sup.6 nanorods per cm.sup.2.

11. The MBFEX tube of claim 1, wherein focusing electrodes are arranged between at least one extraction grid, located above the cathodes, and the anode.

12. The MBFEX tube of claim 11, wherein the focusing electrodes are grounded separately from the extraction grid.

13. The MBFEX tube of claim 11, wherein at least one of the focusing electrodes and extraction grids are produced from steel.

14. The MBFEX tube of claim 11, wherein the extraction grid has a rectangular form with mutually parallel edge strips, which are connected to one another by grid strips to form a single piece, wherein, at the transitions between the grid strips and the edge strips, rounded transition regions are formed, with which the grid strips in each case has an elongate S form.

15. The MBFEX tube of claim 1, wherein the vacuum tube comprises different types of cathodes which differ by at least one parameter from a group of parameters, wherein the group of parameters comprises geometric parameters and material parameters.

16. The MBFEX tube of claim 1, wherein a layer designed for the emission of electrons having a thickness of less than 20 μm and an average roughness (Ra) of less than 2.5 μm is formed by at least one type of cathode.

17. The MBFEX tube of claim 1, wherein the plurality of cathodes is arranged on a flat support element.

18. The MBFEX tube of claim 17, wherein the flat support element comprises corundum.

19. The MBFEX tube of claim 17, wherein the flat support element comprises strip-shaped openings of a first type and strip-shaped openings of a second type, wherein a group of strip-shaped openings of the first type is arranged closer to a cathode than a group of strip-shaped openings of the second type, and wherein the strip-shaped openings of the first type are smaller than the strip-shaped openings of the second type.

20. The MBFEX tube of claim 17, wherein the flat support element is part of a layered emitter arrangement, which moreover comprises a metal intermediate plate, a grid plate including an extraction grid, as well as an upper insulating layer.

21. The MBFEX tube of claim 20, wherein strip-shaped openings of the flat support element are at least partially aligned with openings in the metal intermediate plate.

22. The MBFEX tube of claim 1, wherein the anode is designed for two-way feeding and discharging of coolant, wherein, at the two ends of the anode, in each case a coolant feed line and an associated coolant discharge line are arranged.

23. The MBFEX tube of claim 1, wherein the anode at least partially encloses an examination region, wherein the x-ray sources also at least partially surround the examination region.

24. The MBFEX tube of claim 23, wherein the anode has an arcuate design.

25. The MBFEX tube of claim 1, wherein the anode is designed as a rotating anode.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

(1) Below, the proposed MBFEX tube is explained in further detail in reference to drawings in which different embodiments are summarized. In the drawings, in part in a roughly simplified representation:

(2) FIG. 1 shows a first embodiment example of a MBFEX tube 1 in a diagrammatic view onto an anode 30 formed as a circular arc.

(3) FIG. 2 shows the first embodiment example of a MBFEX tube 1 in a diagrammatic side view.

(4) FIG. 3 shows a second embodiment example of a MBFEX tube 1 with an anode 30 of straight linear design.

(5) FIG. 4 shows the second embodiment example of a MBFEX tube 1 with a sectional view of the anode 30.

(6) FIG. 5 shows a high-voltage bushing 52 of the MBFEX tube 1 according to FIG. 3.

(7) FIGS. 6, 7 show partial views of a grid device 43 of the MBFEX tube 1 of the first embodiment example of a computer tomograph.

(8) FIGS. 8, 9 show partial views of a grid device 43 of the MBFEX tube 1 of the second embodiment example of a computer tomograph.

(9) FIGS. 10, 11 show partial views of an alternative design of a grid device 43 of a MBFEX tube 1.

(10) FIG. 12 shows an emitter arrangement 33 of a MBFEX tube 1 in an exploded representation.

(11) FIG. 13 shows an upper insulating layer 48 of the emitter arrangement 44 according to FIG. 12.

(12) FIG. 14 shows a grid plate 47 of the emitter arrangement 44 according to FIG. 12,

(13) FIG. 15 shows an extraction grid electrode 71 of the grid plate 47 according to FIG. 14.

(14) FIG. 16 shows a metal intermediate plate 46 of the emitter arrangement 44 according to FIG. 12.

(15) FIGS. 17, 18 show the front side of a ceramic plate 45 of the emitter arrangement 44 according to FIG. 12.

(16) FIG. 19 shows the back side of the ceramic plate 45 of the emitter arrangement 44 according to FIG. 12.

(17) FIG. 20 shows a detail of the ceramic plate 45.

(18) FIG. 21 shows a detail of a MBFEX tube 1 with two different types of cathodes 41, 42.

(19) FIGS. 22, 23 show an example of an overall annular arrangement of multiple MBFEX tubes 1 in two different views.

(20) FIGS. 24, 25 show an example of an overall polygonal arrangement of multiple MBFEX tubes 1 in two views analogous to FIGS. 22 and 23,

(21) FIGS. 26, 27 show an anode 30 of a MBFEX tube 1, comprising multiple projections 33, each functioning as x-ray source,

(22) FIG. 28 shows, in a three-dimensional representation, the form of a cathode 40 of a MBFEX tube 1 as well as, for comparison, a conventional cathode form,

(23) FIG. 29 shows, in a diagram, current and voltage pulses during the operation of the MBFEX tube 1.

LIST OF SYMBOLS

(24) 1 MBFEX tube 6 Support Transformer 10 X-ray arrangement 20 Vacuum tube 21 X-ray window 30 Anode 31 Coolant discharge pipe 32 Coolant feed pipe 33 Projection 34 Surfaces 40 Cathode 41 Cathode 42 Cathode 43 Grid device 44 Emitter arrangement 45 Ceramic plate 46 Intermediate plate 47 Grid plate 48 Upper insulating layer 49 Opening 50 Cathode feed line 51 High-voltage bushing 52 High-voltage bushing 61 Rectangular opening 62 Strip-shaped opening 63 Connection strip 64 Opening 65 Opening 66 Conductor structures 71 Extraction grid 72 Focusing electrode 73 Extraction grid electrode 74 Extraction grid electrode 75 Focusing electrode 76 Focusing electrode 77 Grid strip 78 Edge strip 79 Transition region 80 Ceramic support 81 Metal layer AC Anode current E Electron beam e Main electron emission direction EC Emitter current GEV Grid emitter voltage Q X-ray source X X-ray beam x Main x-ray emission direction U Examination region

DETAILED DESCRIPTION

(25) All the embodiment examples of the proposed MBFEX tube 1 explained below are provided for a computer tomograph and comprise a vacuum tube 20 with an x-ray window 21. In the vacuum tube 20 of all the embodiment examples, an anode 30 designed as a cooling finger is securely arranged. The anode 30 contains tungsten.

(26) The first two embodiment examples of the proposed MBFEX tube comprise, in the vacuum tube 20, a plurality of cathodes 40 of a uniform type arranged in a row arrangement, and the embodiment example according to FIG. 21 comprises such cathodes 41, 42 of two different types, wherein the cathodes 40, 41, 42 are provided for field emission of electrons. The cathodes 40, 41, 42 are each oriented with respect to the main electron emission direction e of the electron beams E which can be generated toward the common anode 30 for generating x-ray sources Q The cathodes 40, 41, 42 are securely arranged in a row arrangement in such a manner that an arrangement of x-ray sources Q which is also in a row arrangement, can be generated on the anode 30. The cathodes 40, 41, 42 are provided for a sequential electric actuation. The x-ray beams X each have a main x-ray emission direction x.

(27) In all the embodiment examples, in each case a grid device 43 is oriented toward each x-ray source Q The grid devices 43 are securely arranged between the cathodes 40, 41, 42 and the anode 30 in the vacuum tube 20. Each grid device 43 comprises an extraction grid. The extraction grids are arranged with small spacing in front of the cathodes 40, 41, 42 and are provided for extraction of electrons in the form of an electron beam E from the cathodes 40, 41, 42. The extraction grids are not drawn in the FIGS. 1 to 4.

(28) The vacuum tube 20 of all the embodiment examples in turn comprises a plurality of cathode feed lines 50 and two high-voltage bushings 51, 52. The cathode feed lines 50 are provided as connections of the cathodes and of the grid devices 43 for an electric voltage of a few kV and are designed as wire feed lines. The high-voltage bushings 51, 52 are provided for the respective end-side connection of the anode to a high electric voltage of several 10 kV. Typically, the high voltage is in the range of 10 kV to 420 kV. Values in the upper range of this interval are selected, for example, for x-ray installations for examining large objects in the non-medical sector.

(29) In a high-voltage bushing 52, a coolant discharge pipe 31 is passed through by an internal coolant feed pipe 32. The coolant discharge pipe 31 and the coolant feed pipe 32 are provided for cooling the anode 30 with a liquid, electrically non-conductive coolant by means of a circulation device.

(30) In all the embodiment examples of the proposed MBFEX tube 1, by means of the cathodes 40, 41, 42, in cooperation with the anode 30, x-ray pulses of uniform or alternatingly varying energy can be generated. For example, in FIG. 29, the temporal course of the emitter current EC, of an anode current AC, and of the grid emitter voltage GEV is drawn. The diagram according to FIG. 29 shows actual measurement data. The high transmission degree of approximately 90%, which indicates the ratio of anode current AC to emitter current EC, should be emphasized. In the present case, the anode current AC determined from the measured voltage values is 52.2 mA, and the emitter current EC is 58.2 mA. This extremely favorable ratio between anode current AC and emitter current EC results essentially from the high quality of the emitter arrangement 44 of the x-ray tube 1 to be explained in further detail below.

(31) The first embodiment example of the proposed MBFEX tube 1 is explained in further detail below in reference to FIG. 1 and FIG. 2. In the first embodiment example, the anode 30 is designed as a circular arc.

(32) FIG. 1 shows a diagrammatic view onto the anode 30, wherein the vacuum tube 20, the grid devices 43 and the high-voltage bushings 51, 52 cannot be seen. FIG. 1 is not true to scale. The anode 30, the cathodes 40 and the grid devices 43 are arranged within the vacuum tube 20. Here, the cathodes 40 are on a support 6 made of metallized ceramic. The anode 30 is fastened independently of the cathodes 40 in the vacuum tube 20. The x-ray sources Q are arranged so that the x-ray beams X generated are oriented in their respective main x-ray emission directions x toward an examination region U.

(33) The examination region U is provided for positioning an examination object, in particular a patient.

(34) FIG. 2 shows the proposed MBFEX tube 1 in its first embodiment example in a side view in cross section. In FIG. 2, the coolant feed pipe 32, the cathode feed lines 50 and the high-voltage bushings 51, 52 cannot be seen. The cathodes 40 comprise, on their surface, multi-walled carbon nanotubes in a perpendicular preferential direction. “Perpendicular” in this context is understood to mean an orientation directed toward the anode 30.

(35) The second embodiment example of the proposed MBFEX tube 1 is explained in further detail below in reference to FIG. 3 and FIG. 4. The second embodiment example differs from the first embodiment example only in that the anode 30 is of linear design.

(36) FIG. 3 shows a partial section view onto the MBFEX tube 1 of the second embodiment example. In FIG. 3, the coolant feed pipe 32, the cathodes 40 and the grid devices 43 cannot be seen. As in the first embodiment example of the MBFEX tube 1, the cathode feed lines 50 and the high-voltage bushings 51, 52 are arranged in a row and lie opposite the anode 30 on the vacuum tube 20.

(37) FIG. 4 shows the proposed MBFEX tube 1 in its second embodiment example with a sectional view of the anode 30. In FIG. 3, the cathodes 40 and the grid devices 43 can also not be seen. Individual features of the high-voltage bushing 52 are apparent from FIG. 5.

(38) A grid device 43 present in all the embodiment examples, which is represented in detail in different variants in FIGS. 5 to 11, is oriented toward the anode 6, that is to say arranged between the cathodes 40, 41, 42 and the anode 6 in the vacuum tube 20. The grid device 43 comprises by definition at least one extraction grid electrode 71, 73, 74 and at least one form of focusing electrodes 72, 75, 76.

(39) The extraction grid electrodes 71, 73, 74 are securely arranged directly above the cathodes 40, 41, 42 and are provided for field extraction of electrons from the cathodes 40, 41, 42. The focusing electrodes 72, 75, 76 are also securely arranged above each extraction grid electrode 71, 73, 74, face the anode 6 and are provided for the focusing the extracted electrons as an electron beam E onto the respective x-ray source Q to be generated. The extraction grid electrodes 71, 73, 74 are grounded independently of focusing electrodes 72, 75, 76. The focusing electrodes 72, 75, 76 can be operated as passive or active focusing electrodes.

(40) In the first embodiment example, the grid device 43 comprises an extraction grid electrode 71 common to all the cathodes 40, wherein an individual focusing electrode 72 is associated separately with each individual cathode 40. In the second embodiment example, the grid device 43 comprises an extraction grid electrode 73 of a first form, which is common to the cathodes 41 of the first type, and an extraction grid electrode 74 of a second form, which is common to the cathodes 42 of the second type, wherein in each case an individual focusing electrode 75 of a first form is separately associated with each individual cathode 41 of the first type, and in each case an individual focusing electrode 76 of a second form is associated with each individual cathode 42 of the second type. The extraction grid electrodes 71, 73, 74 and the focusing electrodes 72, 75, 76 are not drawn in FIGS. 1 to 4.

(41) For a computer-assisted x-ray imaging by tomosynthesis, a temporally constant potential of typically 40 kV is applied to anode 6, wherein between the anode 6 and the respective switched cathode 40, 41, a uniform pulsed direct electric current of 30 mA flows. For computer-assisted x-ray imaging by HPEC tomosynthesis, on the other hand, on the anode in question, a temporally constant potential of typically 120 kV is applied, wherein, between the anode 6 and the respective switched cathode 40, 42, a common pulsed direct electric current of the order of magnitude of 0.5 mA flows.

(42) In all the embodiment examples, the proposed computer tomograph comprises a current controller, a device control, an electronic control system (ECS=Electric Control System), a cathode high-voltage source (CPS=Cathode Power Supply), an anode high-voltage source (APS=Anode Power Supply), and a device control. The current controller, the device control, the electronic control system, the cathode high-voltage source, the anode high-voltage source and the device control are part of an electronic closed-loop control device. The current controller, the device control and the electronic control system represent an electronic control system.

(43) The electronic closed-loop control device comprises an electric main circuit and a control loop, wherein the main circuit and the control loop are integrated in a direct-current circuit. In the main circuit, the anode high-voltage source is electrically connected to the anode 6 and the current controller, the current controller is electrically connected to the device control, the device control is electrically connected to the electronic control system, the electronic control system is electrically connected to the cathode high-voltage source, and the cathode high-voltage source is connected in parallel connection to the cathodes 40, 41, 42 and also to the respective grid device 43. In the control loop, the anode high-voltage source is electrically linked by feedback to the control system. Here, the control system can be provided both for the sequential switching of the cathodes 40, 41, 42, for the closed-loop control of the extraction grid electrodes 71, 73, 74, and of the focusing electrodes 72, 76, 56 of the respective grid device 43, and also for the closed-loop control of the main circuit current, wherein the electric voltage of the cathode high-voltage source can be adapted to the main circuit current predetermined by the control system.

(44) In FIG. 21, as examples, eight cathodes 41, 42 of the MBFEX tube 1 are outlined. Both the cathodes 41 of the first type and also the cathodes 42 of the second type comprise carbon nanotubes that differ however by their geometry. The cathodes 41, 42 are arranged in the vacuum tube 20 in a row arrangement alternatingly offset, wherein the number of the cathodes 41 of the first type is equal to the number of the cathodes 42 of the second type. In each case, a cathode 41 of the first form and in each case a cathode 42 of the second form can be associated with a grid device 43 and thus with an x-ray source Q In the MBFEX tube 1 according to FIG. 21, the cathodes 41 of the first type or the cathodes 42 of the second type can be actuated sequentially as desired. In this manner, the dual dose x-ray image acquisitions with the MBFEX tube 1 can be implemented.

(45) As is apparent from FIGS. 22 to 25, multiple MBFEX tubes 1 can be combined to form a rigid annular or polygonal arrangement which in a computer tomograph replaces a rotating arrangement. This applies to any design of MBFEX tubes 1 already described or to be explained below.

(46) A layered structure of an emitter arrangement 44 of a MBFEX tube 1 is illustrated in FIGS. 12 to 20. The emitter arrangement 44 comprises, as lowermost layer, a ceramic plate 45 made of corundum. The cathodes 40 are located on a conductive coating of the ceramic plate 45 and are produced in the silk screen printing method with high geometric precision. On the back side of the ceramic plate 45, conductor structures 66 can be seen.

(47) On the ceramic plate 45, a metal intermediate plate 46 is positioned. This metal intermediate plate 46 comprises rectangular openings 61 for the cathodes 40. In addition, in the metal intermediate plate 46, strip-shaped openings 62 which are smaller and longer in comparison to the openings 61 are located on the long sides of the openings 61. The strip-shaped openings 62 have a function of degassing the vacuum tube 20. This applies both to the preparation for the operation and also for the running operation of the x-ray tube 1, in each case in cooperation with the ceramic plate 45.

(48) In the ceramic plate 45, in addition to the cathodes 40, different strip-shaped openings 64, 65 can be seen. Here, in each case, three short small openings 64 lie directly adjacent to the long sides of each cathode 40. In addition, the cathodes 40 are flanked by somewhat farther lying openings 65 which are also strip-shaped. Here, in each case, two strip-shaped openings 65 are arranged in a line one after the other. Two pairs of such lines of strip-shaped openings 65, together with the arrangement lying in between consisting of cathode 40 and a total of six smaller strip-shaped openings 64, overall describe an H-shape. This applies to all the cathodes 40 on the ceramic plate 45 with the exception of the two outermost cathodes 40 which are flanked only on one side by strip-shaped openings 65 of the longer type.

(49) In particular the internal openings 64 which lie very close to the cathodes 40 here contribute to the fact that, during the emission of electrons, gas at an extremely low concentration of only a few particles can also be discharged toward the back side of the emitter arrangement 44. Thus, an essential contribution is made for preventing arcing within the vacuum tube 20. For removing gas by suctioning during the production of the x-ray tube 1, in particular during heating, the relatively large strip-shaped openings 65 are needed to a greater extent.

(50) The metal intermediate plate 46 comprises as an integral part a connection strip 63 as an electric connection leading outward from the emitter arrangement 44. On the metal intermediate plate 46, a grid plate 47 is located, which encloses the extraction grid electrodes 71 which are each put in front of a cathode at an exactly defined spacing of 0.224 mm (in the example according to FIG. 12).

(51) Details of the extraction grid electrode 71 are apparent from FIG. 15. Overall, the extraction grid electrode 71 has a rectangular form, the long sides of which are formed by completely straight edge strips 78. The two edge strips are connected to one another by a plurality of grid strips 77, resulting overall in the grid structure. However, in contrast to the edge strips 78, the grid strips 77 are not completely straight. Instead, at the two ends of each grid strip 77, that is to say at the transition to the edge strip 78, a rounded transition region 79 is formed. The rounded transition regions 79 essentially ensure that thermally caused deformations do not lead to a change in the spacing between the cathode 40 and the extraction grid 71, but instead are absorbed within the extraction grid 71 lying in a plane, without effects on the emission properties of the emitter arrangement 44.

(52) The grid plate 47 is covered by an upper insulating layer 48 in the form of a plate made of a ceramic material, whereby the emitter arrangement 44 is completed. The upper insulating layer 48 comprises, as is apparent from FIG. 12, openings 49 which are adapted to the shape of the cathodes 40 in order to enable the passage of electrons.

(53) Geometric features of the cathode 40, as are repeatedly contained in the emitter arrangement 44, are represented in FIG. 28. With good approximation, the cathode 40 has a cuboid structure. Over the entire electron-emitting surface of the cathode 40, there are thus hardly any changes in the spacing between the cathode 40 and the extraction grid electrode 71 which is not drawn in FIG. 28. For comparison, FIG. 28 shows, drawn with a dashed line, the surface structure of a conventional cathode produced by the method of electrophoretic deposition (EPD). In this comparison example, one can no longer speak of a smooth surface. Instead, particularly at the edges of the cathode produced by the EPD method, there are pronounced points within the surface of the emission cathode. The electrons are emitted mainly at these points. This limits, on the one hand, the life span, and on the other hand, the transmission rate of electrons. In contrast, the cathode 40, as used in the x-ray tube 1 according to the invention, emits electrons in each surface section of its surface at a nearly constant release rate.

(54) An embodiment example of an anode 30 cooperating with the emitter arrangement 44 is illustrated in FIGS. 26 and 27. On the cylindrical base body of the anode 30, multiple projecting pieces 33 are located, which are also referred to as anode projections or in brief as projections. Each of these projections 33 has a surface 34 which is slanted with respect to the base body and coated with tungsten or another material suitable for x-ray sources. The slants of the different surfaces 34 differ from one another in such a manner that—as indicated in FIG. 27—the emitted x-ray radiation X is focused in the direction of the isocenter of the x-ray arrangement 10 lying in the examination region U.

(55) 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.

(56) 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 6 can depend from any one of claim 1, 2, 3, or 4, with these separate dependencies yielding four distinct embodiments; and so on.

(57) 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(f). Embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.