Abstract:
An electron beam tomography (EBT) scanning system comprising an electron source generating an electron beam, a target ring that receives the electron beam and emits an x-ray fan beam upon impingement of the electron beam on the target ring, a pair of detector arrays arranged opposite the target ring, and a collimator arranged concentrically between the target ring and the pair of detector arrays. The collimator has interior and exterior walls concentrically arranged with one another and surrounding a patient examination area. The interior and exterior walls have a first set of apertures aligned to collimate the x-ray fan beam into a first collimated beam having a first width and a second collimated beam having a second width. Each collimated beam may form a single or double tomographic slice. The collimated beams are detected by the pair of detector arrays.

Description:
BACKGROUND OF INVENTION 
     Certain embodiments of the present invention generally relate to an electron beam tomography system, and more particularly to a collimator for a dual-slice electron beam tomography system. 
     Computerized tomography (CT) systems produce planar images along imaginary cuts, or slices, through a patient. CT systems typically include an x-ray source, which revolves about an imaginary axis through a subject. After passing through the subject, the x-rays impinge on an opposing array of detectors. 
     Typical CT patient scans are executed in either an axial mode or in a helical mode. In axial mode, the table that supports the patient stops, the scan is executed, and then the table moves to a new location. In helical mode, the patient table continuously moves throughout the course of the scan. Single slice scanners (scanners having one detector array) are common, and dual slice CT systems (systems having two detector arrays) are known. 
     Some CT scanners include a scanning electron beam x-ray source, such that an electron beam is magnetically deflected so as to rotate in a generally arcuate path, and in doing so, impinges upon an arc-shaped target. As the electron beam impinges on the target, a source of x-rays is generated. As the electron beam moves, so does the source of x-rays. The x-rays encounter a collimator which passes a portion and blocks a portion of the x-rays. The x-rays are shaped into a fan beam by the collimator and then intercepted by a ring-shaped detector array on an opposite side of the patient. U.S. Pat. No. 4,352,021 (“the &#39;021 patent”), issued Sep. 28, 1982, discloses such an electron beam scanner. However, in order to collimate the x-ray fan beam at different widths, multiple collimators having different sized apertures were typically needed, thereby increasing the cost of the system. 
     U.S. Pat. No. 5,442,673, issued Aug. 15, 1995 (“the &#39;673 patent”), discloses an x-ray collimator for use within an electron beam computed tomography (EBT) scanner, in which a rotating electron beam is directed to impinge upon a ring shaped target. The &#39;673 patent discloses variable tomographic slice width for a single slice EBT system. Single slice EBT systems, however, take longer to scan a given number of slices than corresponding dual slice systems. 
     Thus, a need exists for a more efficient method and apparatus for achieving EBT scanning. 
     SUMMARY OF INVENTION 
     Embodiments of the present invention provide an electron beam tomography (EBT) scanning system comprising an electron source, a target ring, first and second detector arrays, and a collimator. The electron source generates an electron beam, which is received by the target ring. The target ring emits an x-ray fan beam upon impingement of the electron beam on the target ring. The first and second detector arrays are arranged opposite the target ring and detect the x-ray fan beam. The collimator is arranged concentrically between the target ring and the first and second detector arrays. The collimator has interior and exterior walls concentrically arranged with one another and surrounding a patient examination area. The interior and exterior walls have first and second sets of apertures. The first set of apertures are aligned to collimate the x-ray fan beam into a first collimated beam having a first width. The first collimated beam may be detected by first and second detector arrays when the collimator is in a first position. The first collimated beam may be detected by one of the first and second detector arrays when the collimator and detector are moved to a second position. The second set of apertures are aligned to collimate the x-ray fan beam into a second collimated beam having a second width. The second collimated beam may be detected by the first and second detector arrays when the collimator is moved to a third position. The collimator is moved between the first, second, and third positions with respect to the target ring to define the first and second collimated beams having the first and second widths, respectively. 
     The collimator also includes a detector-only region, a source-only region and a source/detector overlap region. The detector-only region has a first set of post-patient x-ray apertures for shielding the detector from scattered x-rays when the beam has a first width; and a second set of post-patient x-ray apertures for shielding the detector from scattered x-rays when the beam has a second width. The source/detector overlap region has a first set of pre-patient x-ray apertures for collimating the x-ray fan beam into the first collimated beam at the first width and a first set of post-patient x-ray apertures for shielding the detector from scattered x-rays. Additionally, the source/detector overlap region has a second set of pre-patient x-ray apertures for collimating the x-ray fan beam into the second collimated beam at the second width and a second set of post-patient x-ray apertures for shielding the detector from scattered x-rays. The source-only region has a first set of pre-patient x-ray apertures for collimating the x-ray fan beam into the first collimated beam at the first width; and a second set of pre-patient x-ray apertures for collimating the x-ray fan beam into the second collimated beam at the second width. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a transverse cross-sectional view of an electron beam tomography (“EBT”) system, formed in accordance with an embodiment of the present invention. 
     FIG. 2 is an axial cross-sectional view of an electron beam tomography (“EBT”) system, formed in accordance with an embodiment of the present invention. 
     FIG. 3 is a cross-sectional view in a plane that contains axis line X in FIG. 1 of the collimator in the source-only region, formed in accordance with an embodiment of the present invention. 
     FIG. 4 is a cross-sectional view in a plane that contains axis line X in FIG. 1 of the collimator in the detector-only region, formed in accordance with an embodiment of the present invention. 
     FIG. 5 is a cross-sectional view in a plane that contains axis line X in FIG. 1 of the collimator in the detector/source overlap region, formed in accordance with an embodiment of the present invention. 
     FIG. 6 is a cross-sectional view in a plane that contains axis line X in FIG. 1 of both sides of the collimator in the overlap region, formed in accordance with an embodiment of the present invention. 
     FIG. 7 is a cross-sectional view in a plane that contains axis line X in FIG. 1 of both sides of the collimator in the overlap region, formed in accordance with an embodiment of the present invention. 
    
    
     The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, certain embodiments. It should be understood, however, that the present invention is not limited to the arrangements and instrumentalities shown in the attached drawings. 
     DETAILED DESCRIPTION 
     FIG. 1 is a transverse cross-sectional view of an electron beam tomography (“EBT”) system  10 , formed in accordance with an embodiment of the present invention. The EBT system  10  includes an electron beam scan tube  12  having a cylindrical portion  14  and a semi-circular conical portion  17 ; and x-ray detector arrays  20  and  21 . The scan tube  12  develops and projects an electron beam  26  towards a semi-circular ring-shaped target (“target ring”)  16 . The target ring  16  generates x-rays at portions thereof where the electron beam  26  impinges. The x-rays, after being collimated and subsequently passed through the patient  18  lying along a patient axis denoted by line X, are intercepted and detected by at least one of the detector arrays  20  and/or  21 . A data output of the detector array  20  and/or  21  is processed by a computer system (not shown) to form diagnostic images and other information of interest to a physician and the patient. 
     Scan tube  12  includes a vacuum envelope  22 , which houses an electron gun  24  in the cylindrical portion  14 . The electron gun  24  projects the axial electron beam  26  through the semi-circular conical portion  17 . Focus coils  28  magnetically focus the electron beam  26  to a spot, which impinges on the target ring  16 . Bending coils  30  provide a magnetic field to bend the electron beam  26  so that it is directed through the semi-circular conical portion  17  toward the target ring  16 . 
     The bending coils  30  not only deflect the electron beam  26 , but also rapidly and repeatedly sweep the electron beam  26  arcuately along the target ring  16  so as to create a source of x-rays that rotates substantially within a single plane. A collimator assembly  36  (shown in FIGS. 3-7) is disposed in the beam path between the target ring  16  and the detector arrays  20  and  21  so as to block the unwanted x-rays emitted by the target ring  16  and to define an x-ray beam projected as a one to ten millimeter thick planar fan beam. A sector of the x-ray fan beam is detected by a portion of the x-ray detector array  20  and/or  21 , which provide measured values to the computer in response thereto. 
     FIG. 2 is an axial cross-sectional view of an electron beam tomography (“EBT”) system  10 , formed in accordance with an embodiment of the present invention. By way of example, the collimator assembly  36  may be donut or circular shaped to surround the scan field  39 . The collimator assembly  36  collimates x-rays projecting from the target ring  16  and projecting onto the detector arrays  20  and  21 . As shown in FIG. 2, only detector array  21  is visible, as detector array  20  is positioned behind and adjacent to detector array  21 . The x-ray fan beam  38  is shown emanating from beam spot  40 . That is, the electron beam  26  impinges on the target ring  16  at the beam spot  40 , which in turn generates the x-ray fan beam  38 . The target ring  16  and the detector arrays  20  and  21  overlap at an overlap region A. The source-only region of the EBT system  10  is denoted by the source-only region B; while the detector-only region is denoted by the detector-only region C. 
     FIG. 3 is a cross-sectional view in a plane that contains axis line X of the collimator assembly  36  in the source-only region B, formed in accordance with an embodiment of the present invention. FIG. 4 is a cross-sectional view in a plane that contains axis line X of the collimator assembly  36  in the detector-only region C, formed in accordance with an embodiment of the present invention. FIG. 5 is a cross-sectional view in a plane that contains axis line X of the collimator assembly  36  in the overlap region A, formed in accordance with an embodiment of the present invention. 
     Turning now to FIG. 3, the portion of the collimator assembly  36  in the source-only region B includes first and second rings  41  and  43 , a cover  42  over the first ring  41 , a first pre-patient x-ray surface  44 , a second pre-patient x-ray surface  46  and an inner cavity  48  therebetween. The first pre-patient x-ray surface  44  includes a first x-ray inlet aperture  50  and a second x-ray inlet aperture  52 . The second pre-patient x-ray surface  46  includes a first x-ray outlet aperture  51  and a second x-ray outlet aperture  53 . The first and second pre-patient x-ray surfaces  44  and  46  may be covered by Lexan®, or any other material that allows x-rays to pass through, while at the same time, maintaining the structural integrity of the collimator assembly  36 . The x-ray fan beam  38  is generated from the target ring (not shown in FIG. 3) toward the patient axis X, as shown by the arrows of beams E and F. The x-ray fan beam is collimated through the apertures formed between the blocking portions  54 . The blocking portions  54  may be formed of steel, lead, brass, or other materials that impede the progress of x-rays. The x-ray fan beam  38  may pass through the apertures  50 - 53 , but is blocked by the blocking portions  54 . Also, the x-ray fan beam  38  is collimated through the source-only region B before the x-ray fan beam  38  passes through the patient. That is, as discussed below, the x-ray fan beam  38  passes through the collimator assembly  36  in the source-only region B before the x-ray fan beam  38  encounters the patient along the axis denoted by line X and the portion of the collimator assembly  36  in the detector-only region C. 
     The collimator assembly  36  may be positioned such that the following tomographic slices (“slices”) may be used to image a patient: (1.) one intermediate slice on one detector array  21  (“one intermediate slice”); (2.) one thin slice on detector array  20  and one thin slice on detector array  21  (“two thin slices”); (3.) one thick slice on both detector arrays  20  and  21  (“one thick slice”); or (4.) one intermediate slice on detector array  20  and one intermediate slice on detector array  21  (“two intermediate slices”). The “one thick slice” is obtained by using the same collimator position as the “two intermediate slices,” but the outputs of the two detectors  20  and  21  are added either electrically or digitally. By way of example only, the thick, intermediate and thin slices may range in width from 10 mm to 1 mm. The slice widths depend on the widths of the apertures  51  and  53 . The apertures  50 - 53  may be sized differently to accommodate different sized slices. 
     For example, if the collimator assembly  36  is set in a first position, the x-ray fan beam (the center of which is represented by reference line E) may pass through the first x-ray inlet aperture  50 , through the inner cavity  48 , and then through the first x-ray outlet aperture  51 . Then, the x-ray fan beam  38  passes through the patient  18  lying along the axis denoted by line X, then through the collimator assembly  36  at the detector-only region A, until it impinges on one or both of the detector arrays  20  or  21 . Because FIG. 3 only shows the collimator assembly  36  in the source only region B, the x-ray fan beam  38  that passes through the collimator assembly  36  in the source-only region B has yet to pass through the patient  18 . If the collimator assembly  36  is positioned to obtain one intermediate slice the collimated x-ray fan beam  38  impinges on detector array  21  when the cone angle of the x-ray fan is minimized. If, however, the collimator assembly  36  is positioned to obtain two equal width slices, such as two thin slices, one half of the collimated x-ray fan beam  38  impinges on the first detector array  20 , while the second half of the collimated x-ray fan beam  38  impinges on the second detector array  21 . 
     The collimator assembly  36  may also be mechanically shifted, either through an actuator, an operator, or the like, to a second position such that the x-ray fan beam  38  may pass through the second x-ray inlet aperture  52  to the second x-ray oulet aperture  53  (with the center of the x-ray fan beam  38  being denoted by reference line F). The second x-ray inlet aperture  52  may be a different size than the first x-ray inlet aperture  50  and the second x-ray outlet aperture  53  may be a different size than the first x-ray outlet aperture  51 . Thus, different size slices may be obtained depending upon whether the first or second x-ray inlet and outlet apertures  50  and  51  or  52  and  53  are used, which is determined by the position of the collimator assembly  36 . That is, the collimator assembly  36  may be in a first position to obtain a first single slice (such as the single intermediate slice if the x-ray fan beam  38  passes through the first x-ray inlet aperture  50  and first x-ray outlet aperture  51 ), a second position to obtain a first double slice (such as two thin slices if the x-ray fan beam  38  passes through the first x-ray inlet and outlet apertures  50  and  51 , respectively); a third position to obtain a second single slice (such as the single thick slice if the x-ray fan beam  38  passes through the second x-ray inlet aperture  52  and the second x-ray outlet aperture  53 ); and the third position to obtain a second double slice (such as two intermediate slices if the x-ray fan beam  38  passes through the second x-ray inlet and outlet apertures  52  and  53 , respectively). Thus, the collimator assembly  36  may be moved, actuated, or otherwise shifted through multiple positions to obtain different slice thicknesses and combinations. The detector assembly (including detector arrays  20  and  21 ) is also positioned differently for each collimator assembly  36  position. 
     The collimator assembly  36  may be shifted through three different positions, while the detector arrays  20  and  21  are shifted through two different positions to provide four different slice configurations. That is, the collimator assembly  36  may be in a first position, while the detector arrays are in a first position to provide a first slice configuration. The collimator assembly  36  may be in a second position, while the detector arrays  20  and  21  are in a second position to provide a second slice configuration. Further, the collimator assembly  36  may be in a third position, while the detector arrays  20  and  21  are in the second position to provide a third and fourth slice configurations. 
     With respect to FIG. 4, the portion of the collimator assembly  36  in the detector-only region C includes the first ring  41 , the cover  42  and second ring  43 . Additionally, the collimator assembly  36  in the detector-only region C includes a first post-patient x-ray surface  58 , a second post-patient x-ray surface  60  and an inner cavity  62 . The first post-patient x-ray surface  58  includes a first x-ray inlet aperture  64  and a second x-ray inlet aperture  66 . The second post-patient x-ray surface  60  includes a first x-ray outlet aperture  65  and a second x-ray outlet aperture  67 . The first and second post-patient x-ray surfaces  58  and  60  may be covered with Lexan®, or any other material that allows x-rays to pass through, while at the same time, maintaining the structural integrity of the collimator assembly  36 . The x-ray fan beam  38  passes through the collimator assembly  36  in the detector-only region C after the x-ray fan beam  38  passes through the source-only region B and the patient lying along the axis X. It is noted that E′ and F′ represent that the x-ray fan beam  38  has passed through the patient lying along the axis X; whereas E and F, as shown in FIG. 3, represent that the x-ray fan beam  38  has not yet passed through the patient. Additionally, once the x-ray fan beam  38  has encountered the collimator assembly  36  in the source-only region B, the x-ray fan beam  38  is a collimated beam. That is, the collimator assembly  36  collimates the x-ray fan beam  38  into a collimated beam. 
     The x-ray fan beam  38  passes through the patient lying along the axis X. After passing through the patient, the x-ray fan beam  38  passes through the first post-patient x-ray surface  58 , through the inner cavity  62 , and then through the second post-patient x-ray surface  60 . As mentioned above, the blocking portions  54  may be formed of steel, lead, brass, or other materials that impede the progress of x-rays. The collimated x-ray fan beam  38  may pass through the apertures  64 - 67 . The blocking portions  54  prevent scattered x-rays from reaching the detector arrays  20  and  21 . 
     As discussed above, the collimator assembly  36  may be positioned such that the following tomographic slices may be used to image a patient: (1.) one intermediate slice; (2.) two thin slices; (3.) one thick slice; or (4.) two intermediate slices. The slice thickness depends on the width of the apertures  51  and  53 . The apertures  64 - 67  may be sized differently to accommodate different size slices. For example, if the collimator is set in a first position, the x-ray fan beam (the center of which is represented by reference line E′) may pass through the first x-ray inlet aperture  64 , then through the inner cavity  62 , and through the x-ray outlet aperture  65 . Then, the x-ray fan beam  38  impinges on one or both of the detector arrays  20  or  21 . Because FIG. 4 only shows the collimator assembly  36  in the detector-only region C, the x-ray fan beam  38  that passes through the collimator assembly  36  in the detector-only region C has already passed through the patient  18 . If the collimator assembly  36  is positioned to obtain one slice, such as a 3 mm slice, the collimated x-ray fan beam  38  impinges on one detector array  21 . If, however, the collimator assembly  36  is positioned to obtain two equal width slices, such as two thin slices, one half of the collimated x-ray fan beam  38  impinges on the first detector array  20 , while the second half of the collimated x-ray fan beam  38  impinges on the second detector array  21 . 
     The collimator assembly  36  may also be shifted to a second position such that the x-ray fan beam  38  may pass from the second x-ray inlet aperture  66  to the second x-ray outlet aperture  67  (with the center of the x-ray fan beam  38  being denoted by reference line F′). The second x-ray inlet aperture  66  may be a different size than the first x-ray inlet aperture  64 ; while the second x-ray outlet aperture  67  may be a different size than the first x-ray outlet aperture  65 . Thus, different sized slices may be accommodated depending on the position of the collimator assembly  36 . That is, the collimator assembly  36  may be in a first position to obtain one intermediate slice (when the x-ray fan beam  38  passes through the first x-ray inlet aperture  64  and the first x-ray outlet aperture  65 ), a second position to obtain two thin slices (when the x-ray fan beam  38  passes through the first x-ray inlet and outlet apertures  64  and  65 , respectively); a third position to obtain one thick slice (when the x-ray fan beam  38  passes through the second x-ray inlet aperture  66  and second x-ray outlet aperture  67 ); and the same third position to obtain two intermediate slices (when the x-ray fan beam  38  passes through the second x-ray inlet and outlet apertures  66  and  67 , respectively). 
     Additionally, the apertures  64 - 67  are wider than the apertures  50 - 53  to accommodate the width of the x-ray fan beam  38 . That is, the collimated x-ray fan beam  38  is wider near the detector arrays  20  and  21  than by the target ring  16 , which is the x-ray source. 
     With respect to FIG. 5, the portion of the collimator assembly  36  in the overlap region A includes the first ring  41 , the second ring  43 , and the cover  42  over the first ring  41 . Additionally, the collimator assembly  36  in the overlap region A includes a first x-ray surface  70 , a second x-ray surface  72  and an inner cavity  73 . The x-ray surface  44 ,  60  and  70  are physically the same cylindrical surface; and x-ray surface  46 ,  58  and  72  are physically a second cylindrical surface. Cavities  48 ,  62  and  73  are the same donut shaped cavity. Each aperture pair  50 ,  74 ;  51 ,  75 ;  52 ,  76 ;  53 ,  77 ;  64 ,  78 ;  65 ,  79 ;  66 ,  80  and  67 ,  81  is physically a single continuous aperture. The first x-ray surface  70  includes a first pre-patient x-ray inlet aperture  74 , a second pre-patient x-ray inlet aperture  76 , a first post-patient x-ray outlet aperture  79 , and a second post-patient x-ray outlet aperture  81 . The second x-ray surface  72  includes a first pre-patient x-ray outlet aperture  75 , second pre-patient x-ray outlet aperture  77 , a first post-patient x-ray inlet aperture  78  and a second post-patient x-ray inlet aperture  80 . Because the collimator assembly  36  is positioned within the overlap region A, the portion of the collimator within the overlap region A includes x-ray inlet and outlet apertures on both x-ray surfaces  70  and  72  to accommodate the sweeping of the x-ray fan beam  38 . That is, at a first radial angle, the x-ray fan beam  38  passes through the collimator assembly  36  in the overlap region A before it passes through the patient  18  lying along the axis X. However, when the x-ray fan beam  38  is radially rotated toward the other side of the collimator assembly  36 , the same portion of the collimator assembly  36  in the overlap region receives the x-ray fan beam  38  after the x-ray fan beam  38  passes through the patient  18  lying along the axis X. For example, as shown in FIG. 2, the beam spot (i.e., the point from which the x-ray fan beam  38  emanates) may be at the position marked by reference numeral  84 . The beam spot may then be swept to a position denoted by reference numeral  86 . Thus, the collimator assembly  36  includes corresponding apertures to accommodate both pre and post patient x-ray fan beams. 
     Referring again to FIG. 5, the first and second x-ray surfaces  70  and  72  may be covered with Lexan®, or any other material that allows x-rays to pass through, while at the same time, maintaining the structural integrity of the collimator assembly  36 . The x-ray fan beam  38  passes between the apertures as described above with respect to FIG. 3 and 4. For example, for one intermediate slice or two thin slices, the x-ray fan beam  38  (the center of which is denoted by line E, for a pre-patient x-ray fan beam  38 ) passes from the first pre-patient x-ray inlet aperture  74  to the first pre-patient x-ray outlet aperture  75 . The x-ray fan beam  38  then passes through the patient and encounters the other side of the collimator assembly  36  in the overlap region, such that the x-ray fan beam passes between an analogous first post-patient x-ray inlet aperture  78 ′ to an analogous first post-patient x-ray outlet aperture  79 ′ (the center of the post-patient beam is denoted by reference line E′). 
     For one thick slice or two intermediate slices, the collimator assembly  36  is shifted such that the x-ray fan beam  38  passes from the second pre-patient x-ray inlet aperture  76  to the second pre-patient x-ray outlet aperture  77 . The x-ray fan beam  38  then passes through the patient lying along the axis X. After passing through the patient, the x-ray fan beam  38  encounters the corresponding other side of the collimator assembly  36  in the overlap region A such that the post patient x-ray fan beam  38  passes from an analogous second post-patient x-ray inlet aperture  80 ′ to an analogous second post-patient x-ray outlet aperture  81 ′. In general, if a pre-patient x-ray fan beam  38  impinges on the collimator assembly  36  in the overlap region A, the resulting post-patient x-ray fan beam  38  impinges on the other side of the collimator assembly  36  in the overlap region A. 
     FIG. 6 is a cross-sectional view in a plane that contains axis line X of FIG. 1 of both sides of the collimator assembly  36  in the overlap region A, formed in accordance with an embodiment of the present invention. As noted in FIG. 6, the horizontal scale is approximately 4:1, while the vertical scale is approximately 1:10. FIG. 6 represents the one intermediate slice and two thin slice portion of the EBT system  10 . The narrower the slice width, the better the axial resolution (narrower slice widths also result in reduced dosage to the patient). FIG. 6 illustrates the EBT system  10  on the left side of reference line G, shown in FIG.  5 . The patient  18  lies along the axis X. The x-ray fan beam  38  emanates from the beam spot  40 . The x-ray fan beam  38  has sides  138  and  238 . The x-ray fan beam  38  passes from the first pre-patient x-ray inlet aperture  74  through the first pre-patient x-ray outlet aperture  75 . The x-ray fan beam  38  then passes through the patient lying along the axis X. The x-ray fan beam  38  then passes through the first post-patient x-ray inlet aperture  78 ′ on through the first post-patient x-ray outlet aperture  79 ′. 
     As shown in FIG. 6, the apertures  74 ,  75 ,  78 ,  79  (and  74 ′,  75 ′,  78 ′ and  79 ′) are formed such that the x-ray fan beam  38  impinges on the detector array(s) at a first intermediate width. When the collimator assembly  36  is in a first position, as shown in FIG. 6, one half of the x-ray fan beam  38  impinges on the first detector  20 , while the other half of the x-ray fan beam  38  impinges on the second detector  21  (thus, two thin slices are detected). However, the collimator assembly  36  and detector arrays  20  and  21  may be shifted to a second position such that x-ray fan beam  38  impinges only on the second detector  21  (resulting in one intermediate slice). For example, reference slot  222  shows where the first pre-patient x-ray outlet  75  shifts (thus, the other apertures would also shift), for the x-ray fan beam defined by sides  338  and  438  to impinge solely on the second detector  21 . Additionally, the x-ray fan beam  38  may be swept through a 180 degree radial arc such that the x-ray fan beam  38 ′ emanates from beam spot  40 ′. In this case, the electron fan beam  38 ′ is defined by sides  138 ′ and  238 ′. 
     FIG. 7 is a cross-sectional view of both sides of the collimator assembly  36  in the overlap region A, formed in accordance with an embodiment of the present invention. As noted in FIG. 6, the horizontal scale is approximately 4:1, while the vertical scale is approximately 1:10. FIG. 7 represents the one thick slice and two intermediate slice portions of the EBT system  10 . That is, FIG. 7 illustrates the EBT system  10  on the right side of reference line G, shown in FIG.  5 . The patient  18  lies along the axis X. The x-ray fan beam  38  emanates from the beam spot  40 . The x-ray fan beam  38  has sides  138  and  238 . The x-ray fan beam  38  passes from the second pre-patient x-ray inlet aperture  76  through the first pre-patient x-ray outlet aperture  77 . The x-ray fan beam  38  then passes through the patient lying along the axis X. The x-ray fan beam  38  then passes through the second post-patient x-ray inlet aperture  80 ′ on through the second post-patient x-ray outlet aperture  81 ′. As shown in FIG. 7, the apertures  76 ,  77 ,  80  and  81 ; (and  76 ′,  77 ′,  80 ′ and  81 ′) are formed such that the x-ray fan beam  38  impinges on the detector array(s) at a thick width. When the collimator assembly  36  is in the position, as shown in FIG. 7, one half of the x-ray fan beam  38  impinges on the first detector  20 , while the other half of the x-ray fan beam  38  impinges on the second detector  21  (resulting in two intermediate slices). However, data from the two detector arrays  20  and  21  may be added resulting in one thick slice. Additionally, the x-ray fan beam  38  may be swept through a 180 degree arc such that the x-ray fan beam  38 ′ emanates from beam spot  40 ′. In this case, the electron fan beam  38 ′ is defined by sides  138 ′ and  238 ′. 
     The collimator assembly  36  of the EBT system  10  is not limited to the slice configurations described above. A wide variety of slice configurations, in addition to the one thick, two intermediate, one intermediate, and two thin slice configurations, may be implemented within the system. Additionally, the collimator may include more apertures that may allow for an increased number of slice configurations. For example, the collimator may include more than two sets of x-ray inlet and outlet apertures. That is, the collimator may include a set of apertures for one thick/two intermediate slices, another set of apertures for one intermediate/two thin slices, another set of apertures for one thin slice/two very thin slices, etc. Also, additionally, more than two detector arrays may be used. For example, instead of having two detector arrays aligned with, and adjacent one another, three or more detector arrays may be utilized. Also, multiple target rings may be used within the EBT system. 
     Thus, embodiments of the present invention provide a more efficient (and less expensive) dual-slice EBT scanner, because a single collimator may be used to collimate an x-ray fan beam at a plurality of slice widths. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.