Abstract:
An X-ray collimator for collimating X-rays from an X-ray source that illuminate an array of columns and rows of X-ray detectors, the collimator having a first side that faces the X-ray source and a second side opposite the first side that faces the detector array, the collimator comprising: a plurality of strips formed from an X-ray absorbing material, wherein each strip is corrugated so that it has rectangular and/or square corrugations; and means for maintaining the plurality of strips one next to the other with the corrugations of one strip aligned with corrugations of an adjacent strip to form an array of rows and columns of square/and or rectangular wells corresponding to the X-ray detectors in the array.

Description:
FIELD OF THE INVENTION 
   The present invention relates to computerized tomography (CT) X-ray imaging and in particular to shielding for X-ray detectors in a CT imaging system that protects the detectors from scattered X-rays. 
   BACKGROUND OF THE INVENTION 
   A multislice CT scanner for imaging a region of interest (ROI) of a patient comprises an X-ray source that provides a cone-shaped X-ray beam radiated from a focal spot of the X-ray source and a detector array comprising rows and columns of X-ray detectors. The detector array is positioned facing the X-ray source and receives X-rays from the X-ray source that pass through the patient&#39;s body. The X-ray source and detector array are mounted on a rotor of a gantry and the patient is supported on an appropriate support couch. The couch is moveable axially, along an axis referred to as a “z-axis”, relative to the gantry and the rotor is rotatable to rotate the X-ray source about the z-axis to position the X-ray source at a plurality of different “cone beam view angles”. Generally the rows of detectors in the detector array are perpendicular to the z-axis and the columns are parallel to the z-axis. 
   To image the ROI, the couch is moved along the z-axis to translate the ROI through a field of view (FOV) of the scanner, which is located between the scanner&#39;s X-ray source and detector array. As the ROI moves through the FOV the X-ray source is rotated around the z-axis to illuminate the ROI with X-rays at a plurality of different view angles. At each view angle and different axial positions of the ROI, detectors in the array of detectors measure intensity of X-rays from the source that pass through the ROI. The intensity of X-rays measured by a given detector in the array of detectors is a function of an amount by which X-rays are attenuated by material in the slice along a path length from the X-ray source, through the ROI to the given detector. The measurement provides information on composition and density of tissue in the ROI along the path-length. 
   The attenuation measurements for the ROI provided by the detectors are processed using algorithms known in the art to provide a map of the absorption coefficient of material in the ROI as a function of position. The map is used to display and identify internal organs and features of the region. 
   Ideally, each detector in a CT scanner measures intensity of X-rays that reach the detector after passage along a substantially straight-line path from the X-ray source to the detector. Therefore, ideally, the detector measures intensity of only those X-rays that are neither absorbed by the material along the path from the X-ray source to the detector nor scattered by the material at angles that prevent the X-rays from being incident on the detector. However, X-rays that are scattered out of a path from the X-ray source to one X-ray detector in the detector array of the CT scanner may be scattered in directions towards other X-ray detectors in the scanner&#39; detector array. If these scattered X-rays are incident on the other X-ray detectors, they can generate error in measurements provided by the other detectors and degrade quality of an image provided by the CT scanner. 
   To reduce “scattering errors” in a CT scanner, X-ray detectors in the scanner&#39;s detector array are generally shielded from scattered X-rays by an anti-scattering (AS) collimator. The collimator generally comprises thin planar AS lamellae formed from a suitable X-ray absorbing material. In multi-slice scanners having a relatively small number of detector rows, the AS lamellae are generally located between columns of detectors but not between rows of detectors. The AS lamellae have their respective planes parallel to the z-axis and are oriented so that they intersect the focal spot of the X-ray source. In multislice scanners having a relatively large number of rows of detectors and a cone beam having a relatively large extent parallel to the z axis it has been found advantageous for an AS collimator to comprise AS lamellae between columns of detectors and also between rows of detectors. The AS lamellae between both the rows and columns have their respective planes oriented so that they intersect the focal spot of the X-ray source. An AS collimator for which the lamellae are located only between columns of detectors are referred to as a “1D” AS collimator. An AS collimator having lamellae between columns and rows of detectors are referred to as a “2D” AS collimator. A space in a 2D AS collimator through which X-rays may pass unhindered by the collimator&#39;s AS lamellae, which is surrounded on all sides by AS lamella and doesn&#39;t contain a smaller space surrounded on all sides by AS lamellae, is referred to as an “anti-scattering (AS) well”. 
   Unpublished PCT application PCT/IL02/00729 filed Sep. 4, 2002, the disclosure of which is incorporated herein by reference describes different configurations of 2D AS collimators useable for multislice CT scanners. 
   U.S. Pat. No. 6,363,136 describes 2D AS collimators for a CT scanner formed from a plurality of shielding elements each comprising a base plate and a plurality of lamellae mounted perpendicular to the base plate. A plurality of the shielding elements are abutted one to the other so that the lamellae of one plate butt against the base plate of a next shielding element. The butted base plates are held in position by support plates having grooves that receive the base plates. 
   U.S. Pat. No. 4,054,800 entitled “Methods of Collimator Fabrication” shows various methods of producing “a collimator for radiation receiving and imaging devices”. All the methods employ tenon and mortise like joints to couple components of the collimator.  FIG. 6  in the patent shows foils of radiation absorbing material corrugated with slightly trapezoidal like corrugations to facilitate tenon and mortise joining of the corrugated foils one to the other. The joined strips are held together by compression in a suitable frame. 
   U.S. Pat. No. 3,943,366 entitled “Collimator for Ray Diagnosing Device” describes forming a honeycomb structure having hexagonal AS wells, very much like a bee honeycomb. The structure is formed from strips corrugated with corrugations having trapezoidal edge-on profiles so that the edge-on profile of each corrugation appears as three sides of a hexagon. The strips are glued together to form the honeycomb collimator. 
   U.S. Pat. No. 4,450,706 entitled “Method and Apparatus For Forming Collimator Strips” describes and shows a method of forming strips suitable for fabricating a honeycomb collimator similar to that described in U.S. Pat. No. 3,943,366. The method comprises pressing a strip of suitable deformable radiation absorbing material between matching male and female dies. 
   SUMMARY OF THE INVENTION 
   An aspect of some embodiments of the present invention relates to providing a 2D AS X-ray collimator and method of making the same for a CT scanner in which lamellae in the collimator form rectangular AS wells. 
   In accordance with an embodiment of the invention a strip of suitable heavy metal such as Tungsten (W) or Molybdenum (Mo) is precision “corrugated” to generate a strip of the metal, which when seen edge on might for example exhibit a shape of a train of rectangular and/or square pulses. That is, the “edge-on” profiles of the corrugations are rectangular or square. A plurality of the corrugated strips are aligned so that the protruding, “convex”, sides of the corrugations of one strip are opposite the depression, “concave”, sides of corrugations in an adjacent strip and bonded together to form a honeycomb structure of rectangular and/or square AS wells. 
   In an embodiment of the invention corners of the corrugations are chamfered to facilitate mortise joining the corrugations and providing lands for a bonding agent used to bond the strips together. 
   In an embodiment of the invention, each strip is aligned with and bonded to a flat strip of an X-ray absorbing metal so that the flat strip covers the concave sides of the corrugations on one side of the corrugated strip. The bonded corrugated and flat strips form an “intermediate unit” that is planar on one side and corrugated on the other. The units are aligned with the planar side one unit facing the corrugated side on a next unit and bonded together to form the 2D AS collimator. 
   There is therefore provided in accordance with an embodiment of the present invention, An X-ray collimator for collimating X-rays from an X-ray source that illuminate an array of columns and rows of X-ray detectors, the collimator having a first side that faces the X-ray source and a second side opposite the first side that faces the detector array, the collimator comprising: a plurality of strips formed from an X-ray absorbing material, wherein each strip is corrugated so that it has rectangular and/or square corrugations; and means for maintaining the plurality of strips one next to the other with the corrugations of one strip aligned with corrugations of an adjacent strip to form an array of rows and columns of square/and or rectangular wells corresponding to the X-ray detectors in the array. 
   Optionally, the corrugations are aligned so that convex side of corrugations on one strip are aligned opposite the concave sides of corrugations of an adjacent strip. Optionally, corners of corrugations in a given strip butt up against corners of corrugations in the adjacent strip. Additionally or alternatively, the corners of corrugations are chamfered. Additionally or alternatively, the means for maintaining the plurality of strips aligned comprises a bonding agent that bonds corners of corrugations that butt up against each other together. 
   In some embodiments of the present invention, the convex sides of corrugations of one strip are aligned opposite and contiguous with the convex sides of corrugations of an adjacent strip. Optionally, the means for maintaining the plurality of strips aligned comprises a bonding agent that bonds contiguous regions of corrugations together. 
   In some embodiments of the invention, the means for maintaining the plurality of strips together comprises a frame having two parallel sides that face each other and are formed with mirror image slots that receive ends of the corrugated strips. 
   In some embodiments of the present invention, the X-ray collimator comprises a flat strip formed from an X-ray absorbing material sandwiched between every two corrugated strips. Optionally, the means for maintaining the plurality of strips aligned comprises a bonding agent that bonds each flat strip to the corrugated strips between which it is sandwiched. Additionally or alternatively, the means for maintaining the plurality of strips together comprises a frame having two parallel sides that face each other and are formed with mirror image slots that receive ends of the corrugated and flat strip. 
   In some embodiments of the present invention, the X-ray collimator comprises two flat strips aligned and parallel to the other flat strips each of which is contiguous to a different one of an outermost corrugated strip in the collimator. Optionally, the outermost flat strips protrude beyond the corrugated strips on the second side of the collimator. 
   In some embodiments of the present invention, the flat strips protrude beyond the corrugated strips on the first side of the collimator. 
   In some embodiments of the present invention, each corrugation comprises three planar lamellae having four edges and wherein lines coincident with the edges of the lamellae intersect substantially at a same point on a first side of the collimator. Optionally, the intersection point substantially coincides with a focal spot of the X-ray source. 
   In some embodiments of the present invention, the X-ray detector array is comprised in a CT scanner 

   
     BRIEF DESCRIPTION OF FIGURES 
     Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto, which are listed following this paragraph. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
       FIG. 1  schematically shows a CT scanner having a 2D AS collimator; 
       FIG. 2  schematically shows a detector module, which is a component of an X-ray detector array, comprising a plurality of X-ray detectors and its associated 2D AS collimator; 
       FIGS. 3A-3E  schematically illustrate production of a 2D AS collimator in accordance with an embodiment of the present invention; 
       FIG. 4  schematically shows another configuration of a 2D AS collimator, in accordance with embodiments of the present invention; 
       FIGS. 5A and 5B  schematically show another configuration of a 2D AS collimator, in accordance with embodiments of the present invention; 
       FIGS. 6A and 6B  schematically show an exploded and assembled view of another AS collimator, in accordance with an embodiment of the present invention; and 
       FIG. 7  schematically shows another AS collimator in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  schematically shows a multislice CT scanner  20  comprising an X-ray source  22  controllable to provide an X-ray cone beam  24 , schematically indicated by dashed lines  26 , and an array  28  of X-ray detectors  30 . Cone beam  24  emanates from a focal spot  32  of X-ray source  22 . X-ray source  22  and detector array  28  are mounted to a rotor  40 , which in turn is rotatably mounted to a stator  42  so that the rotor can be rotated about an axis  44  conveniently labeled as the z-axis of a coordinate system  45 . Stator  42  and rotor  40  are components of a gantry  46  of CT scanner  20 . Only those features and components of CT scanner  20  germane to the present discussion are shown in  FIG. 1 . 
   Array  28  has rows  50  and columns  52  of X-ray detectors  30 . The number of rows  50  and columns  52  of detectors  30  and the relative size of X-ray detectors  30  shown in detector array  28  is arbitrary and chosen for convenience and clarity of presentation. Detectors  30  in array  28  are shielded by a 2D AS collimator  60  which is shown partially cut away to show the detectors. AS collimator  60  comprises “row” lamellae  62  that are located between rows  50  of detectors  30  and “column” lamellae  64  that are located between columns  52  of detectors. 
   Lamellae  62  and  64  are optionally oriented so that their respective planes substantially intersect focal spot  32 . Typically, lamellae  62  and  64  have thickness in a range from about 20 microns to about 100 microns and extend from array  28  towards focal spot  32  to a height in a range from about 20 millimeters to about 40 millimeters. Row and column lamellae  62  and  64  form AS wells  66 . Optionally, a single X-ray detector lies at the “bottom” of a well  66 . In some configurations, a plurality of X-ray detectors  30  lie at the bottom of a well  66 . Row and column lamellae  62  and  64  and wells  66  of a portion of collimator  60  are shown enlarged for convenience of viewing in inset  67 . 
   In practice, an X-ray detector array in a typical multi-slice CT scanner may comprise many thousands of small X-ray detectors  30  having areas typically equal to about a square millimeter configured in an array  28  comprising tens of detector rows  50  and many hundreds of detector columns  52 . The X-ray detector array is generally formed from detector-modules each comprising a relatively small number of X-ray detectors  30  configured in a usually rectangular “mini-array” of rows  50  and columns  52  of detectors. Typically, the mini-array is a few centimeters long and a few centimeters wide and is mounted with to its own AS collimator. The modules are positioned one adjacent to and contiguous with the other to form the X-ray detector array. 
     FIG. 2  schematically shows a detector module  80  and its associated 2D AS collimator  82 , which is partially cut away to show detectors  30 . AS collimator  82  comprises row lamellae  83  between rows  50  of detectors  30  and column lamellae  84  between columns  52  of the detectors. AS collimator  82  also comprises “outside” row lamella  86  that protrude on either side of detector module  80  below row and column lamellae  83  and  84 . End lamellae  86  bracket detector module  80  between them and aid in aligning and mounting AS collimator  82  to the detector module. 
     FIGS. 3A-3E  schematically show steps in a method of forming a 2D AS collimator for a detector module similar to detector module  80  shown in  FIG. 2 , in accordance with an embodiment of the present invention. 
   In a first step, each of a plurality of substantially rectangular strips of a material having a relatively high X-ray absorption coefficient, such as for example W or Mo, is precision “corrugated” so that it exhibits a plurality of corrugations. Each corrugation comprises three adjacent lamellae and viewed “edge on” appears as a square or rectangular upright or inverted “U”. Each inverted U corrugation is adjacent an upright U corrugation and share a lamella. After corrugation, a cross-section through the corrugated strip parallel to its length resembles a train of rectangular or square pluses. 
     FIG. 3A  schematically shows a strip  100  after it is corrugated so that it exhibits corrugations  102 , in accordance with an embodiment of the invention. By way of example, in  FIG. 3A  each corrugation  102 , seen edge-on, appears as a rectangular inverted or upright U, has three lamellae  104 , edges  106  and has a length L and height H. Strip  100  has a width “W”. Optionally, all corrugations  102  have a same length L. 
   Inset  108  schematically shows two adjacent corrugations  102  comprised in corrugated strip  100 . A left most corrugation  102  in inset  108  seen edge on resembles an inverted U and a right most corrugation  102  seen edge on resembles an upright U. For convenience of presentation and to reduce clutter, only some lamellae  104  and edges  106  are labeled with their identifying numerals. Optionally, strip  100  is corrugated so that straight lines coincident with edges  106  of any corrugation  102  meet substantially at a same intersection point. The intersection point is located at a distance from a corrugation  102  that is substantially equal to a distance from an X-ray detector to a focal spot of a CT scanner in which an AS collimator formed from strip  100  is used. In  FIG. 3A  dashed lines  110  coincident with edges  106  of two corrugations  102  are drawn to indicate orientation of the edges and that they meet at a common intersection point  112 . As a result of the orientation of edges  106  of a corrugation  102 , the planes of lamella  104  of the corrugation pass substantially through the focal spot of the CT scanner in which the AS collimator formed from strip  100  is used. A side of a corrugated strip  100  facing intersection point  112  is referred to as an “X-ray source side” of the strip. A side of a corrugated strip facing away from intersection point  112  is referred to as a detector side of the strip. 
   Optionally, each corrugated strip  100  is aligned with and bonded to a flat strip, hereinafter referred to as a “closure strip”, formed from a material having a relatively high X-ray absorption coefficient, which is optionally the same material from which the corrugated strip is formed.  FIG. 3B  schematically shows a corrugated strip  100  and a flat closure strip  114  to which it is to be bonded.  FIG. 3C  schematically shows strip  100  bonded to closure strip  114  to form an “intermediate assembly”  116 . A side of an intermediate assembly  116  on which a closure strip  114  is located is referred to as “closed side” of the assembly. A side opposite the closed side, which is not bonded to a closure strip  114 , is referred to as an “open side” of the assembly. An appropriate boding agent, such as an epoxy that is not readily degraded by exposure to X-rays, is used to bond corrugated strip  100  to closure strip  114 . Optionally, closure strip  114  and corrugated strip  100  have substantially a same width W. Optionally, closure strip  114  and corrugated strip  100  have a thickness in a range from about 20 microns to about 80 microns. Optionally closure strip  114  and corrugated strip  100  have a same thickness. 
   A plurality of intermediate assemblies  116  are aligned and bonded together with the closed side of one intermediate assembly facing the open side of an adjacent intermediate assembly and their respective X-ray source sides facing a same direction to form an AS collimator  120  shown in  FIG. 3D . By way of example, intermediate assemblies  116  comprised in AS collimator  120  form rectangular wells  66 . An additional closure strip  122  is bonded to the open side of a first intermediate assembly  124  in array  120  to “close off” open corrugations  102  in the intermediate assembly. Additional closure strip  122  and closure strip  114  ( FIG. 3D ) comprised in a last intermediate assembly  126  function as end row lamellae similar to end row lamellae  86  shown in  FIG. 2 . Closure strip  114  comprised in intermediate assembly  126  and additional closure strip  122  are optionally wider than the respective corrugated strips  100  to which they are bonded ( FIG. 3C ) and protrude slightly on the detector side of the corrugated strips. 
   After trimming of protruding lamellae  128 , collimator  120 , as schematically shown in  FIG. 3E , is suitable for mounting as a 2D AS collimator to an X-ray detector module, similar to module  80  shown in  FIG. 2 . Each rectangular well  66  in collimator  120  is suitable, for example, for providing X-ray collimation for a rectangular shaped X-ray detector or two square X-ray detectors having dimensions such that they fit at the bottom of the well. 
   It is noted that closure strips  114  and  122  used in producing AS collimator  120  provide ample lands for a bonding agent used to bond intermediate assemblies  116  together. In addition, depending upon an orientation of AS collimator  120  relative to rows and columns of detectors  30  in a detector module mounted with the AS collimator, closure strips  114  provide added shielding material between rows  50  or columns  52  of the X-ray detectors. In general, X-ray detectors in a detector array are exposed to a larger number of scattered X-rays from directions substantially transverse to the columns of the detector array than from directions substantially transverse to rows of the array. As a result it is usually advantageous to provide relatively more shielding material between columns of detectors than between rows of detectors. 
   In the above exemplary embodiment, closure strips  114  and  122  do not protrude beyond corrugated strips  100  on the X-ray source side of the collimator. However, in some collimators, as described in PCT Application PCT/IL02/00729 cited above, it can be advantageous to have lamellae between columns of detectors that extend on the X-ray source side of the collimator further than lamellae between rows of detectors. 
     FIG. 4  schematically shows a collimator  190  in which flat closure strips  114  extend on the X-ray source side of the collimator further than corrugated plates  100  in the collimator. As a result, collimator  190  has lamellae between columns of detectors in a corresponding detector array (not shown) that extend on the X-ray source side of the collimator further than lamellae between rows of detectors in the corresponding array. In the above description, AS collimator  120  is produced by first forming intermediate units  116  and then bonding the intermediate units together. In some embodiments of the present invention, corrugated strips  100 , closure strips  114  and  122  are mounted to an appropriate jig and bonded together simultaneously to form AS collimator  120 . 
     FIG. 5A  schematically shows another AS collimator  180  in accordance with an embodiment of the present invention. Collimator  180  is formed by aligning strips  100  one next to the other so that lamellae  104  ( FIG. 3A ) of protruding, convex, sides of corrugations  102  of one strip are opposite and contiguous with protruding lamellae  104  of convex sides of corrugations  102  in an adjacent strip. Contiguous lamellae  104  of adjacent strips  100  are bonded together using a suitable bonding agent and excess lamellae trimmed to form collimator  180  having wells  182 . 
   Along a first outer corrugated strip  183  and a last outer corrugated strip  184  of collimator  180 , corrugations  102  that are concave are open and do not form complete wells. In some embodiments of the invention the open corrugations are not used to form wells and collimator  180  is used to collimate X-rays in an array having detectors that correspond to wells  182 . 
   In some embodiments of the invention flat closure strips  114  are bonded to outer corrugated strips  183  and  184 , as shown in  FIG. 5B  to close open concave corrugations  102  in the outer corrugated strips. When closed by closure strips  114  bonded to outer corrugated strips  183  and  184 , the concave corrugations in the outer strips form “complete” wells  186  having cross sectional areas that are half that of wells  182 . 
   In some embodiments of the invention, a plurality of collimators  180  are positioned side by side and contiguous with the other so that open concave corrugations  102  of adjacent collimators are directly opposite each other. Opposite open corrugations “close” each other and form complete wells that are substantially identical to wells  182 . A plurality of collimators  180  placed side by side may thus be used to form a relatively long homogeneous collimator having substantially identical wells along its length. 
     FIGS. 6A-6B  schematically show an exploded view and an assembled view of another 2D AS collimator  150 , in accordance with an embodiment of the present invention. 
   Collimator  150  is produced from a plurality of corrugated strips  152  having corrugations  154  comprising lamellae  156 . Corrugated strips  152  are similar to corrugated strips  100  except that lamellae  156  have chamfered corners  158 . In addition, corrugated strips  152  are not bonded to closure strips to provide lands for bonding the corrugated strips together and which remain between the corrugated strips in a collimator matrix. Instead, corrugated strips  152  are aligned so that the protruding, convex, sides of the corrugations of one strip are opposite the depressed, concave, sides of corrugations in an adjacent strip and chamfered corners  158  of the convex corrugations butt up against chamfered corners of the concave corrugations. The aligned corrugated strips  152  are bonded together using a suitable bonding agent applied to the chamfered corners, which provide lands for the bonding agent. Closure strips  160  and  162  are, optionally, bonded to a first and last corrugated strip  164  and  166  respectively in AS collimator  150  to “close” corrugations  154  in the strips. 
   After trimming excess material from matrix  150  the matrix, as shown in  FIG. 6B  is suitable for mounting to a matching X-ray detector module. 
   In some embodiments of the present invention corrugated strips, such as strips  100  ( FIG. 3A ) that are not formed with chamfered edges are aligned and bonded together similarly to the manner in which corrugated strips  152  are bonded together to form a collimator. When properly aligned with convex sides of the corrugations  102  of one strip  100  opposite concave sides of corrugations  102  in an adjacent strip  100  un-chamfered corners of the convex corrugations butt up against un-chamfered corners of the concave corrugations. While the un-chamfered corners provide smaller lands for a bonding agent than chamfered corners  158  of corrugated strips  152 , the un-chamfered corners provide sufficient lands for a bonding agent used to bond the strips together. 
   In some embodiments of the present invention, corrugated strips, such as strips  100  shown in  FIG. 3A  or strips  152  shown in  FIG. 6A  are held in position by a suitable frame to from a collimator.  FIG. 7  schematically shows, by way of example. a collimator  200  comprising strips  152  held together in a frame  202 . Frame  202  comprises two registration bars  204  and optionally end lamellae  206 , which registration bars and end lamellae are shown shaded in the figure. Registration bars  204  are formed with mirror image slots  208  that receive ends  210  of strips  152  and secure the strips properly positioned relative to each other. 
   A collimator in accordance with an embodiment of the invention comprising corrugated strips and flat strip between pairs of adjacent corrugated strips may also comprise a frame, which holds flat strips as well as corrugated strips in position. Slots in registration bars receive ends of both the corrugated and the flat strips. 
   In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
   The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.