Abstract:
Certain embodiments of the present invention include methods and systems for improved motion and angulation profiles in tomosynthesis. A method includes designating a target with a first and second dimension. An x-ray beam is projected onto at least a portion of the target. The x-ray beam has an origin with a position along the first dimension. The x-ray beam also has a beam axis, a projection area, and an angle ∅ representative of an angular distance between the beam axis and the at least a portion of the target. The method further includes varying the angle ∅ based at least in part on the position of the origin along the first dimension. The angle ∅ is varied to substantially maintain the projection area.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to tomosynthesis. More specifically, the invention relates to angulation and motion profiles in a tomosynthesis system.  
         [0002]     Tomography involves obtaining a two-dimensional image slice (or tomogram) from a three-dimensional volume. A variety of tomographic imaging techniques exist today, such as conventional linear tomography, computed axial tomography (CT), and positron emission tomography (PET).  
         [0003]     A relatively new and promising tomographic imaging technique is tomosynthesis. Tomosynthesis allows retrospective reconstruction of an arbitrary number of tomographic planes of anatomies from a set of projection images acquired over a variety of angles. Compared to conventional linear tomography, tomosynthesis provides premium image quality and enhanced depth information at a lower x-ray dose. Image quality and depth information is, of course, important when diagnosing patients. Additionally, tomosynthesis is relatively fast and cost-effective.  
         [0004]      FIG. 1  shows, generally, a tomosynthetic imaging system. In a tomosynthetic imaging system  10 , a target  12  may be stationary while an x-ray source  14  moves along at least a first dimension  16 . The first dimension  16  may be horizontal, vertical, or may be along any orientation useful for tomosynthetic imaging. In some configurations, the x-ray source  14  may move along two or more dimensions. For instance, the x-ray source  14  may move along an arc. As the x-ray source  14  moves, an x-ray beam  18  is projected towards the target  12  over a variety of angulations  20 . The x-ray beam  18  originates, or has its origin at or near the x-ray source  14 . The intersection of the x-ray beam  18  and a plane defined by the target 12 forms an x-ray projection area  28 . The target  12  may be an x-ray detector. If an x-ray detector is used, the x-ray detector may be digital, in that the detector may generate digital images. Digital x-ray detectors have advantages over film-based detectors, but digital x-ray detectors may be relatively expensive.  
         [0005]     Using the technique shown in  FIG. 1 , a series of x-ray projection images may be acquired over a variety of x-ray source angulations  20 . The series of x-ray projection images may subsequently be processed with image processing techniques to reconstruct planar images. The resulting reconstructed planar images provide a high degree of clarity and structure resolution. The clarity and resolution may be attributed, in part, to information contained in the series of x-ray projection images acquired over a variety of angulations.  
         [0006]     When performing x-ray imaging, it may be important to limit x-ray exposure. For instance, x-ray exposure on a person is presently regulated by the Food and Drug Administration as set forth in 21 C.F.R. 1020.30. It is therefore desirable to reduce excess x-rays in x-ray imaging systems.  
         [0007]     For example, excess x-rays in tomosynthetic systems may result if part of the projection area  28  lands outside of the target  12 . If the target is an x-ray detector, then x-rays that fall outside of the detector cannot be used for tomosynthetic imaging. Therefore, these x-rays may be considered unessential for tomosynthetic imaging.  
         [0008]     The size of the x-ray beam  18  may be adjusted with a collimator. Certain collimators may be adjusted by electro-mechanical systems, for example. Some of these collimators have two or more moveable blades that adjust the x-ray beam size. One type of collimator has four blades. The blades may be moved to adjust a size of the x-ray beam  18 . The adjusted x-ray beam cross-section may be a variety of rectangular shapes. Other shapes are possible as well. For instance, a four-blade collimator may form an x-ray beam cross-section into a polygon that has more than four sides, such as an octagon. The projection area  28  of an x-ray beam increases as the x-ray beam travels farther from an x-ray source. As a tomosynthesis system operates, a variety of projection area  28  sizes and shapes may be possible.  
         [0009]     Because the size and shape of the x-ray beam projection area  28  varies during the operation of a tomosynthetic imaging system, it may be preferable to adjust the size of the x-ray beam  18  so that the projection area  28  falls substantially within a perimeter of the target  12 . There may be at least two reasons for adjusting the x-ray beam in this manner. First, x-rays that do not fall on the target  12  may not be detected by the imaging system, and therefore may be unessential. Unessential x-rays increase the amount of x-ray dose received by an x-ray subject without increasing the performance of the system. Second, digital x-ray detectors may be relatively expensive. It may be preferable, therefore, to adjust the size of the x-ray beam  18  to efficiently use of the surface area of the target  12 .  
         [0010]     Thus, there is a need for a tomosynthetic imaging system that may adjust an x-ray beam according to movement of an x-ray source and angulations of an x-ray beam. Additionally, there is a need for a tomosynthetic imaging system that may adjust an x-ray beam such that the x-ray beam falls substantially within a target, such as a digital x-ray detector. Moreover, there is a need for a tomosynthetic imaging system that may adjust an x-ray beam to efficiently exploit a digital x-ray detector when used in combination with a tomosynthesis projection system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Certain embodiments of the present invention provide a system and method for motion and angulation profiles in tomosynthesis. In an embodiment, a method of tomosynthesis includes designating a target. The target has a first and second dimensions. An x-ray beam is projected onto at least a portion of the target. The x-ray beam has an origin, and the origin has a position along the first dimension. The x-ray beam also has a beam axis, a projection area, and an angle ∅ that is representative of an angular distance between the target and the x-ray beam. The angle ∅ is varied based at least in part on the position of the origin along the first dimension. The angle ∅ is varied so that the x-ray beam projection area is substantially maintained.  
         [0012]     In another embodiment, the method of tomosynthesis further includes varying angle ∅ based at least in part on a position of the target along the first dimension, a distance along a third dimension between the target and the origin, and a size of the target along a first dimension.  
         [0013]     In another embodiment, the target of the method includes a digital x-ray detector. In yet another embodiment, the projection area remains substantially within the target. In yet another embodiment, the projection area substantially conforms to a size of the detector along the first dimension.  
         [0014]     In an embodiment, a method of tomosynthesis includes identifying a target. The target has a first and second dimensions. An x-ray beam is projected onto at least a portion of the target. The x-ray beam has an source, and the source has a position along the first dimension. The x-ray beam also has a beam axis, a projection area, and an angle ∅ that is representative of an angular distance between the target and the x-ray beam. The x-ray beam also has an angle γ representative of the x-ray beam width along the first dimension. The x-ray source position is varied along the first dimension. The angle ∅ is varied based at least in part on the position of the source along the first dimension. The angle ∅ is varied so that the x-ray beam projection area is substantially maintained. The angle γ is varied based at least in part on the position of the source along the first dimension. The angle γ is varied so that the x-ray beam projection area is substantially maintained.  
         [0015]     In another embodiment, the method of tomosynthesis further includes varying angle ∅ based at least in part on a position of the target along the first dimension, a distance along a third dimension between the target and the origin, and a size of the target along a first dimension.  
         [0016]     In another embodiment, the method of tomosynthesis further includes varying angle γ based at least in part on a position of the target along the first dimension, a distance along a third dimension between the target and the origin, and a size of the target along a first dimension.  
         [0017]     In another embodiment, the method may further include an x-ray beam with an angle α that represents the x-ray beam width along a second dimension. In yet another embodiment, the method may include the step of varying α based at least in part on angles ∅ and γ, so that the x-ray beam projection area is substantially maintained. In yet another embodiment the angle α is varied based at least in part on a size of the target along the second dimension, and a distance along a third dimension between the source and the target.  
         [0018]     In another embodiment, the method may further include adjusting sizes l and w of the projection area which represent sizes along the first and second dimensions respectively. The size l may be adjusted with respect to angle γ and a source-to-image distance SID representing a distance between the source and the target. The size w may be adjusted with respect to angles ∅ and γ and size l. In yet another embodiment, the target includes a digital x-ray detector and the projection area remains substantially within the target.  
         [0019]     In an embodiment, system for performing tomosynthesis is provided including an x-ray source capable of emitting an x-ray beam. The x-ray beam has a beam axis, a beam width, a projection area, and an angle γ representing the beam width along a first dimension. The system also has a target with a perimeter. Included is a first motion subsystem for moving the x-ray source along at least a portion of the first dimension to a first dimension position. Also, a second motion subsystem is provided for adjusting an angle ∅ representing an angular distance between the beam axis and the target. The system further includes at least one collimator capable of altering the angle γ based at least in part on the first dimension position of the x-ray source so that the projection area remains substantially within the perimeter of the target.  
         [0020]     In another embodiment, the method of tomosynthesis further includes varying angle ∅ based at least in part on a position of the target along the first dimension, a distance along a third dimension between the target and the origin, and a size of the target along a first dimension.  
         [0021]     In another embodiment, the method of tomosynthesis further includes varying angle γ based at least in part on a position of the target along the first dimension, a distance along a third dimension between the target and the origin, and a size of the target along a first dimension.  
         [0022]     In another embodiment, the method may further include an x-ray beam with an angle α that represents the x-ray beam width along a second dimension. In yet another embodiment, the method may include the step of varying α based at least in part on angles ∅ and γ, so that the x-ray beam projection area is substantially maintained. In yet another embodiment the angle α is varied based at least in part on a size of the target along the second dimension, and a distance along a third dimension between the source and the target. In yet another embodiment the target includes a digital x-ray detector.  
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0023]      FIG. 1  shows a tomosynthetic imaging system.  
         [0024]      FIG. 2  shows x-ray beam geometry.  
         [0025]      FIG. 3  shows x-ray beam geometry representative of a tomosynthetic imaging system used in accordance with an embodiment of the present invention.  
         [0026]      FIG. 4  shows x-ray beam projection geometry representative of a tomosynthetic imaging system used in accordance with an embodiment of the present invention.  
         [0027]      FIG. 5  shows a flow diagram for a method of tomosynthesis used in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 2  shows a geometry representative of an x-ray beam  18 . An x-ray beam  18  emanates from an origin  80 . The x-ray beam  18  that has a projection area  28 .  FIG. 2  shows a rectangular projection area  28 , but the x-ray beam projection area  28  may be in the shape of other geometric shapes, such as an octagon, a trapezoid, or a circle. The x-ray beam  18  also has a beam axis  34 , and two beam width angles—α and γ. The projection area  28  has a width w and a length l. The beam width angles may be adjustable by a collimator, which is not shown in  FIG. 2 . As the x-ray beam  18  travels a distance Source-to-Image-Distance (SID) along a dimension d, the x-ray beam projection area  28  expands.  
         [0029]     Equations (1) and (2) describe certain geometric relationships between α, γ, w, l, and SID:  
               tan   ⁢           ⁢     (   γ   )       =     l     2   ·   SID               (   1   )                 tan   ⁢           ⁢     (   α   )       =       w   l     ⁢   sin   ⁢           ⁢   γ             (   2   )             
 
         [0030]     Equations (1) and (2) may be solved for l and w respectively to arrive at equations (3) and (4):  
             l   =     2   ×   SID   ×   tan   ⁢           ⁢   γ             (   3   )               w   =       l   ×   tan   ⁢           ⁢     (   α   )         sin   ⁢           ⁢   γ               (   4   )             
 
         [0031]      FIG. 3  shows a geometry representing tomosynthetic imaging systems. Referring for a moment to  FIG. 1 , the tomosynthetic imaging system  10  features an x-ray beam  18  that moves over a range of angulations  20  with reference to a stationary target  12 . Turning back to  FIG. 3 , the angulation  20  may be represented by an angle ∅. Angle ∅ represents an angular distance between an x-ray beam axis  34  and a dimension  42  perpendicular to the x-ray detector, which may also be described as a third dimension.  
         [0032]     When angle ∅ is not 0°, the x-ray beam  18  forms a projection area  28  that is trapezoidal. Moreover, as angle ∅ varies, and as the position of the x-ray source  14  varies along a first dimension  16 , the projection area  28  will vary in size and shape.  
         [0033]     Beam width angle γ represents an angular distance of the x-ray beam  18  along a first dimension. Beam width angle α represents an angular distance of the x-ray beam  18  along a second dimension. Distance d represents a distance between an x-ray source  14  and a position of the target  12  along a third dimension  42 . A source-to-image distance SID represents a distance between the x-ray source and the target  12 . The SID may be represented by the following geometric relationship:  
             SID   =     d     cos   ⁢           ⁢   ϕ               (   5   )             
 
         [0034]      FIG. 4  shows a geometry with respect to a first dimension and a second dimension used in accordance with an embodiment of the present invention. X-ray source position (x 2 ,y s )  58  is shown. The x-ray source position  58  also has a location along a third dimension  42  which is shown in  FIG. 3 , but not in  FIG. 4 . An x-ray beam projection area  28  is formed between the intersection of an x-ray beam  18  (shown in  FIG. 3 ) and a plane defined by a target  12 . The target  12  may be a digital x-ray detector, for example. When the x-ray beam  18  is rectangular, a trapezoidal projection area  28  is formed.  
         [0035]     The projection area  28  has a first side  60  and a second side  62 . As depicted in  FIG. 3 , the “fatter” side of the trapezoidal projection area  28  is on the second side  62 . As angle ∅ varies from positive to negative (or vice versa), the trapezoidal projection area  28  changes shape as depicted in  FIG. 3 . A position x 1  represents a distance along a first dimension between the x-ray source position  58  and the first side  60  of the projection area  28 . A position x 2  represents a distance along the first dimension between the x-ray source position 58 and the second side  62  of the projection area  28 . A position X d    64  represents a position along the first dimension of a center of the target  12 . A size D l  represents a size of the target  12  along a first dimension, which may also be described as a length dimension. A size D w  represents a size of the target  12  along a second dimension, which may also be described as a width dimension. Looking at both  FIGS. 3 and 4 , the positions x 1  and X 2  may be represented by equation (6):  
             {             x   1     =     d   ⁢           ⁢   tan   ⁢           ⁢     (     ϕ   -   γ     )                     x   2     =     d   ⁢           ⁢   tan   ⁢           ⁢     (     ϕ   +   γ     )                       (   6   )             
 
         [0036]     A position y 1  represents a distance along the second dimension between the x-ray source position  58  and a corner of the first side  60  of the projection area  28 . A position Y 2  represents a distance along the second dimension between the x-ray source position  58  and a corner of the second side  62  of the projection area  28 . The positions y 1  and Y 2  may be described by equation (7):  
             {             y   1     =       d     cos   ⁢           ⁢     (     ϕ   -   γ     )         ⁢   tan   ⁢           ⁢   α                   y   2     =       d     cos   ⁢           ⁢     (     ϕ   +   γ     )         ⁢   tan   ⁢           ⁢   α                     (   7   )             
 
         [0037]     In order for the x-ray beam projection area  28  to fall within the target  12  along a first dimension, the assumption represented by equation (8) may be used.  
             {               x   s     -     x   d       =       x   1     +       D   l     2                       x   s     -     x   d       =       x   2     -       D   l     2                       (   8   )             
 
         [0038]     If the target  12  is, for instance, a digital x-ray detector, then the assumption represented by equation (8) has an additional advantage of efficiently utilizing the digital x-ray detector along a first dimension. In other words, the assumption in equation (8) represents an x-ray beam projection along a first dimension that substantially covers the digital x-ray detector along the first dimension.  
         [0039]     Equation (9) may be obtained by solving equations (6) and (8) for angle ∅.  
             ϕ   =       1   2     [       arctan   ⁢       (       x   s     -     x   d     +       D   l     2       )     d       +     arctan   ⁢       (       x   s     -     x   d     -       D   l     2       )     d         ]             (   9   )             
 
         [0040]     Equation (10) may be obtained by solving equations (6) and (8) for angle γ.  
             γ   =       1   2     [       arctan   ⁢       (       x   s     -     x   d     +       D   l     2       )     d       -     arctan   ⁢       (       x   s     -     x   d     -       D   l     2       )     d         ]             (   10   )             
 
         [0041]     Equation (11) represents an angular velocity profile for angle ∅, and may be obtained by taking a derivative an angular position of angle ∅ as represented by equation (9).  
                 ⅆ   ϕ       ⅆ   t       =       1     2   ⁢   d       ⁢     {             1     1   +       (         x   s     -     x   d     +       D   l     2       d     )     2         +               1     1   +       (         x   s     -     x   d     -       D   l     2       d     )     2               }     ⁢       ⅆ     x   s         ⅆ   s                 (   11   )             
 
         [0042]     Looking at  FIGS. 3 and 4 , notice that when angle ∅ is positive, y 1  is greater than Y2, and when when angle ∅ is negative, y 1  is less than Y 2.  In other words, the “fatter” side of the trapezoid changes sides as angle ∅ goes from positive to negative, or from negative to positive. In order for the x-ray beam projection area  28  to fall within the target  12  along the second dimension, an assumption may be made: a maximum value of y 1  or Y 2  should not exceed half the size of the detector—or D w /2. If the target  12  is, for instance, a digital x-ray detector, then it may be preferable to assume a maximum value of y 1  or y 2  equal to D w /2. If a maximum value of y 1  or y 2  is substantially equal to D w /2, a digital x-ray detector target may be efficiently utilized. Applying the above discussed assumptions to equation (7), the following equations (12), (13), and (14) may be obtained.  
                 D   w     /   2     =       d     cos   ⁢           ⁢     (          ϕ        -   γ     )         ⁢   tan   ⁢           ⁢     (   α   )               (   12   )                 tan   ⁢           ⁢     (   α   )       =         D   w     ·   cos     ⁢           ⁢       (          ϕ        -   γ     )     /     (     2   ⁢   d     )                 (   13   )               α   =     arctan   ⁢           ⁢     (         D   w     ·   cos     ⁢           ⁢       (          ϕ        -   γ     )     /     (     2   ⁢   d     )         )               (   14   )             
 
         [0043]     The set of equations (9), (10), and (14) represent a set system angulations for ∅, α, and γ during tomosynthesis. By constraining a tomosynthesis system behavior to the assumptions represented by equations (8) and (12), the system angulations represented by equations (9), (10), and (14) may be derived.  
         [0044]     A tomosynthesis system that adjusts system angulations for ∅, α, and γ based at least in part on equations (9), (10), and (14) may effectively project an x-ray beam projection area  28  on a target  12 . Additionally, equations (9), (10), and (14) may assist a tomosynthesis system to project an x-ray beam projection area  28  such that the projection area  28  falls substantially within a target  12 . If the target is, for instance, a digital x-ray detector, then equations (9), (10), and (14) may assist a tomosynthesis system to efficiently use the surface area of a digital x-ray detector.  
         [0045]      FIG. 5  shows a flow diagram for a method of tomosynthesis used in accordance with an embodiment of the present invention. At step  110 , a target is identified. At step  120 , an x-ray beam is projected from a source towards at least part of the target. The x-ray beam has an angle ∅ representing an angular distance between the x-ray beam axis and the target. The x-ray beam also has an angle γ representing a beam width angle along a first dimension. The x-ray beam may also have additional angles. At step  130 , the x-ray source position is varied along the first dimension. At step  140 , angle ∅ is varied so that the x-ray beam projection area is substantially maintained. At step  150 , angle γ is varied so that the x-ray beam projection area is substantially maintained. Step  150  is optional.  
         [0046]     The embodiments disclosed herein may also be useful to improve techniques for image pasting and auto-positioning. Image pasting is a technique for imaging an area larger than a detector size. In image pasting, a detector is movable. Different images are generated at various detector locations. The images are then pasted together. For example, using image pasting, it is possible to generate a single image of the whole spinal column using only a 41 cm square detector. As one with ordinary skill in the art would appreciate, the techniques disclosed herein may be useful for image pasting.  
         [0047]     Auto-positioning is a technique that assists operators of x-ray imaging equipment to accurately position a tube source. Instead of manually positioning a tube source, auto-positioning may use motors or other automated motion subsystems to position the tube source in an appropriate location for imaging. As one with ordinary skill in the art would appreciate, the techniques disclosed herein may be useful for auto-positioning.  
         [0048]     Thus certain embodiments provide a system and method for adjusting an x-ray beam according to movement of an x-ray source. Certain embodiments provide a system and method for adjusting an x-ray beam according to movement of an x-ray source and system angulations. Certain embodiments provide a system and method to adjust an x-ray beam to fall substantially within a target, such as a digital x-ray detector. Certain embodiments provide a system and method to adjust an x-ray beam to efficiently exploit the usable surface area of a target such as a digital x-ray detector. Certain embodiments may extend to other functions, such as image pasting, auto tracking, auto positioning, as well as field of view centering.  
         [0049]     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.