Abstract:
A portable radiation system is described that employs transmissive x-ray imaging for the inspection of an object positioned between a source and a detector. In one embodiment, the source is kept stationary and the detector is rotated about an axis that passes through the center of source to scan the object. This rotational scan eliminates the degradation in scanned images that would have otherwise resulted due to shaking of the detector if the source and detector were both moved over a rough terrain. Multiview scans are obtained by moving source and detector to additional locations around the object. In another embodiment, multiview scans are obtained by changing the angular position of the detector. Multview scans are then additionally combined to generate 3D information of the object.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A portable high fidelity scanner especially suited for inspection of suspicious packages is described. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    U.S. Pat. No. 8,734,013 B2 to Singh describes a small mobile x-ray scanning system for inspection of packages left in buildings or outdoors. The system described has an x-ray generator mounted on a mobile platform and a detector suspended at a distance from the source such that the package to be inspected is situated in between the source and detector during the scanning operation. One problem encountered with such systems is that as the mobile platform drives over an uneven surface, the scanned image gets distorted due to shaking of the detector. Since the detector arm is pivoted on the mobile platform and extends outwards approximately three feet, even very small vibrations of the platform get amplified at the detector end. This situation is similar to fixing a long pole at one end while the other end is free floating. Now if small vibrational displacement is given at the fixed end of the pole, the longer the pole, greater is the displacement at the free end of the pole. Likewise, in the system described by Singh, even tiny vibrations of the platform result in substantial vibrations of the detector that lead to blurring and loss of resolution of the scanned image. Therefore, a method is desired where the loss of resolution due to vibrations can be eliminated or reduced. 
         [0003]    Suspicious package scanning requires that the package not be moved as it might contain a motion triggered bomb. It is also desired to estimate the size of threat and the location with 3D coordinates inside the package so that counter threat measures could be taken. However, the current methods employed to scan left behind packages produce 2D images. It is therefore desired to have a method that is suitable for 3D imaging of leave behind packages. 
         [0004]    The objects of this invention are therefore to overcome some of the above problems and are listed next. 
       OBJECTS AND ADVANTAGES OF THE INVENTION 
       [0005]    It is, accordingly, an object of the invention to develop a portable inspection system suitable for inspection of leave behind packages and capable of producing high fidelity images. 
         [0006]    Another object of the present invention is to generate 3D images without moving the package that has been left on the floor of a building or outdoors. 
         [0007]    There are several embodiments and advantages that will become apparent in the description that follows. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with one embodiment, a portable scanner is presented that is especially suited for x-ray inspection of packages. An x-ray source and a detector are used to implement a transmissive x-ray imaging of an object or a package. The source is mounted on a small mobile platform close to the ground level. A vertical member rising above the source supports a linear detector array assembly at a predetermined distance from the source. This detector assembly rotates about a vertical axis that is coincident with the vertical member supporting it. To implement the scan, the mobile platform is moved to a location such that the package or object to be inspected gets positioned in between the source and detector. The mobile platform is then kept stationary and the detector arm is rotated over an arc to scan the object. 
         [0009]    To further inspect the object from another angle or view, the mobile platform is moved a small predetermined distance and another angular or rotational scan of the detector arm implemented to generate a scanned image. Moving the mobile platform repeatedly along a straight path or to different locations around the object, multiple views of the object from different angles are generated. These multiple angle views are then combined in a computer using the methods of tomosynthesis or tomography to generate 3D information of the object. 
         [0010]    In another embodiment, the detector assembly is held fixed at a predetermined angle while the mobile platform moves so that the object to be inspected passes in between the source and the detector. This generates a scan with a view of the object looking along the angle of incidence at which the radiation beam from the source to the detector intercepts the object. Next, the detector arm is rotated by a predetermined angle, this changes the angle of incidence at which the radiation beam intercepts the object. The angle of the detector arm is then kept fixed while the mobile platform is moved again to implement a second scan of the object from a different angle. By repeatedly rotating the detector assembly by predetermined angles, and then moving the mobile platform to scan the object, multiple angle views of the object are generated which are then combined in a computer using tomosynthesis or tomographic methods to generate 3D information of the object. 
         [0011]    There are several embodiments, objects and advantages to this invention that will be apparent to one skilled in the art. The accompanying figures and description herein should be considered illustrative only and not limiting or restricting the scope of invention, the scope being indicated by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows one embodiment. 
           [0013]      FIGS. 2A to 2D  demonstrate the scanning operation in accordance with embodiment shown in  FIG. 1 .  FIG. 2A  shows the top view with the detector arm at the center position.  FIG. 2B  shows the rotation of the detector arm to implement the scan.  FIG. 2C  shows the fan beam shaped geometry traced by the detector arm.  FIG. 2D  shows the radiation paths within the fan beam shaped scan geometry. 
           [0014]      FIGS. 3A to 3D  demonstrate multiview scanning using the embodiment of  FIG. 1 . With reference to the central position of the detector arm,  FIG. 3A  shows object to one side of detector arm,  FIG. 3B  shows the object in the center and  FIG. 3C  shows object on the other side of the detector arm.  FIG. 3D  shows the superimposition of the scan geometries for the scans for  FIGS. 3A to 3C . 
           [0015]      FIGS. 4A to 4C  demonstrate multiview scanning using another embodiment. With reference to the central position of the detector arm,  FIG. 4A  shows scans taken with detector arm positioned at a fixed angle to one side of the object,  FIG. 4B  shows scan taken with detector arm fixed at zero angle, and  FIG. 4C  shows scan taken with detector arm positioned at a fixed angle to the other side of the object. 
           [0016]      FIGS. 5A to 5C  show the respective ray paths or ray trajectories for scans of  FIGS. 4A, 4B and 4C . 
           [0017]      FIG. 5D  shows the superimposition of ray paths of  FIGS. 5A, 5B and 5C  for the reconstruction of point A using the method of back projection. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    In describing the first embodiment and its alternatives, specific terminology will be used for the sake of clarity. However, the invention is not limited to the specific terms so used, and it should be understood that each specific term includes all its technical equivalents which operate in a similar manner to accomplish similar purpose. 
         [0019]    A simplified assembly of one or the first embodiment is shown in  FIG. 1  and its top view is shown in  FIG. 2A . A mobile platform  14  mounted on wheels  20  is shown. Mounted on the mobile platform  14  is an x-ray generator  30  with a source or focal point at  31  and emitting a radiation beam  32 . A “L” shaped detector or detectors or detector assembly  1211  comprising of a horizontal detector arm  12  and vertical detector arm  11  is used to detect the radiation beam  32 . A means to support detector  1211  at a predetermined distance from source  31  allows an object or a package  10  to be interposed between source  31  and detector  1211 . Further, the means to support detector  1211  at a predetermined distance includes a vertical member  13  situated vertically above the source  31 . A rotational means is used to swivel or rotate the detector assembly of  1211  around a vertical axis  22  or about the vertical member  13  as indicated by a circular arrow  23 . This rotation provides a means to change the angle of incidence at which the radiation beam  32  intercepts the object  10 . 
         [0020]    It should be noted that since the detector arm  11  is connected to one end of detector arm  12  as shown in  FIG. 1 , rotating the detector arm  12  is the same as rotating the detectors or detector assembly  1211 . Further, since the top view of  FIG. 1  would show only the detector arm  12  and not the vertical detector arm  11 ,  FIGS. 2-4  show only the detector arm  12 . Therefore, whenever a reference is made to the angle or position or rotation of the detector arm  12 , it should be understood that the reference also applies to the entire assembly of detectors  1211 . 
         [0021]    To scan a package or object  10 , the mobile platform  14  is moved to a location that is next to or in the vicinity of the object  10  as shown in  FIG. 1  and  FIG. 2A . The mobile platform  14  is then kept stationary and the detector  12  or equivalently detectors  1211  are rotated as indicated by circular arrow  23 . The distal end of detector  12 , and hence the vertical detector arm  11  trace an arc as indicated by a circular arrow  25  shown in  FIG. 2B . The half angle of rotation is shown in  FIG. 2B  as α, and the dotted line  24  shows the central position of the detector  12  which serves as a reference for the measure of angle α. At the start of the scan, the detector  12  is shown on one side of the object  10 , and at the end of scan it has moved or rotated to a position indicated by  12   b  on the other side of object  10 . By keeping the mobile platform stationary, there are no motion induced vibrations of the detectors  1211 , this results in a high fidelity or high resolution image which is free from blurring which might have resulted otherwise due to motion over uneven terrain. 
         [0022]    The rotation of detector  12  about vertical member  13  as seen from the top view of  FIG. 2B , traverses a fan shaped or a pie shaped or sector shaped geometry  250   b  shown in  FIG. 2C  with the center of curvature or apex at  13   b.  This center of curvature  13   b  is coincident with the center of rotation of detector  12  about the vertical member  13  as apparent by looking at  FIG. 2B . It is not necessary but to simplify the explanation, the center of curvature  13   b  is vertically above and collinear with the x-ray source  31  shown in  FIG. 1 . The radiation paths or ray paths from the x-ray source  31  to the detectors  1211  as seen from the top view of  FIG. 2B , appear as radial lines  251   b  shown in  FIG. 2D . As apparent to a person skilled in the art,  FIG. 2D  represents a fan beam shaped projection of the object  10  if a radiation source were placed at or below  13   b.  With reference to  FIG. 2D , it is seen that the radiation paths or rays  251   b  strike the object  10  at different incidence angles. Therefore the rotational means to rotate the detector  1211  about  13  is effectively a means to change the angle of incidence at which the radiation path from source  31  to detector  1211  intercepts object  10 . 
         [0023]    It is well known to a person skilled in the art that a computing means is used to analyze the data collected or received or detected by detectors, hence no further details of computing means are being provided. 
         [0024]    In order to further inspect the object  10  from different angles, multiview scanning is implemented as shown in  FIGS. 3A to 3C . The first scan or view is obtained by positioning the mobile platform  14  such that the object  10  is to one side of the detector  12  as shown in  FIG. 3A . In a manner similar to that explained with reference to  FIG. 2B , the detector arm  12  is rotated about the vertical member  13  as indicated by an arc  25   a  in  FIG. 3A . 
         [0025]    To obtain a second view of object, the mobile platform is next moved in the direction of arrow  21  in  FIG. 3A  so that object is now positioned in the central position of the detector  12  as shown in  FIG. 3B . In a manner as explained with reference to  FIG. 2B , the detector arm  12  is now rotated about the vertical member  13  as indicated by an arc  25   b  in  FIG. 3B . 
         [0026]    To obtain a third view of object, the mobile platform is next moved in the direction of arrow  21  in  FIG. 3B  so that object is now positioned on the other side of the detector  12  as shown in  FIG. 3C . In a manner as explained with reference to  FIG. 2B , the detector arm  12  is now rotated about the vertical member  13  as indicated by an arc  25   c  in  FIG. 3C . 
         [0027]    The three scans obtained above can be analyzed to further examine the object  10 .  FIG. 3D  shows one method employed to determine the 3D coordinates and the density reconstruction by the method of back projection of point A within the object  10 . As explained with reference to FIG.  2 C, the fan shaped scan or beam geometries for the scans of  FIGS. 3A, 3B and 3C  are shown in  FIG. 3D  as  250   a,    250   b  and  250   c  respectively and their center of curvatures are shown as  13   a,    13   b  and  13   c  respectively. In  FIG. 3D , the distance D 1  between  13   a  and  13   b  is the distance the mobile platform  14  is moved from its scan position of  FIG. 3A  to the scan position of  FIG. 3B . Likewise, the distance D 2  between  13   b  and  13   c  is the distance the platform  14  is moved from the scan position of  FIG. 3B  to the scan position of  FIG. 3C . Arrows  251   a,    251   b  and  251   c  are coincident at point A within the object  10  and are the respective radiation or ray paths for the scans of  FIGS. 3A, 3B and 3C . As is well known to the person skilled in the art, the intersection of just any two of the rays  251   a,    251   b  or  251   c  will suffice to determine the 3D coordinates of point A. Further, as is well known to a person skilled in the art, the density of point A can be approximated by just summing up the gray scale values of the scanned images corresponding to rays  251   a,    251   b  and  251   c.  Repeating the above process for all points A within the object  10  thus reconstructs the 3D density of object  10 . 
         [0028]    In the above illustration, only three views or three scans were used to reconstruct point A within the object. However, only two or several more views can be used by repeatedly moving the platform  14  either in a straight line or to different locations around the object  10 . 
         [0029]    Another embodiment of the invention is illustrated in  FIGS. 4A to 4C . Multiview scans are obtained for three different angular positions of the detector  12 . The angular position of the detector  12  is defined by the angle of the detector with reference to the central position  24  shown in  FIG. 2B . In the first scan, the detector arm  12  is rotated about the vertical member  13  so as to position it to one side of object  10  as shown in  FIG. 4A . This position of the detector  12  is similar to that shown in  FIG. 2B  where the angle of the detector  12  to the central position  24  is denoted as α. The detector arm  12  is then kept fixed at this angle and the mobile platform  14  is moved in the direction of arrow  21  to scan the object  10 , the corresponding ray paths  251   a  are shown in  FIG. 5A . For the second view, the detector arm is rotated back to its central position as shown in  FIG. 4B  and now the angle α, which was explained with reference to  FIG. 2B , is zero. The detector arm  12  is then kept fixed at this angle and the mobile platform  14  is moved in the direction of arrow  21  to scan the object  10 , the corresponding ray paths  251   b  are shown in  FIG. 5B . For the third view, the detector arm  12  is rotated about the vertical member  13  so as to position it to the other side of object  10  as shown in  FIG. 4C  and now the angle α, which was explained with reference to  FIG. 2B , is negative. The angle of the detector arm  12  is then kept fixed while the platform  14  is moved in the direction of arrow  21  to scan the object  10 , the corresponding ray paths  251   c  are shown in  FIG. 5C . 
         [0030]    It should be noted that the angle α for the three scans for  FIGS. 4A-4C  are predetermined before the start of the scans and held constant during the scans. It should be further noted that the angles at which the ray paths  251   a  in  FIG. 5A, 251   b  in  FIG. 5B, and 251   c  in  FIG. 5C  intercept the object  10  are also the angles of incidence at which the radiation beam  32  from source  31  to detector  1211  intercepts object  10 . 
         [0031]    As is well known to a person skilled in the art of laminography and tomographic image reconstructions, the density of a point A within object  10  can be approximated by the summation of the rays  251   a,    251   b  and  251   c  shown in  FIG. 5D . Again, as is well understood by the person skilled in art, by adjusting the delays between scans of  FIGS. 5A, 5B and 5C  and summing them, a 3D reconstruction of the entire volume of the object  10  can be realized. 
         [0032]    In the above illustration, only three views were used, however, just two or several more views can be used by repeatedly scanning the object  10  with different angles of the detector arm  12  with reference to its central position, and by orienting the platform  14  at different angles to the object  10  for example by positioning the platform  14  at different locations around the object  10 . 
         [0033]    There are several embodiments possible as would be apparent to a person skilled in the art. For example, only the vertical detector arm  11  is used and the arm  12  would then be just a mechanical member without any detectors used only to support detector  11 . In such a situation, the vertical member  13  and the horizontal arm  12  are then just a means to support the detector  11  at a predetermined distance from the source  31 . 
         [0034]    In another embodiment, the axis of rotation  22  for the detectors  1211  need not be directly above the focus  31  of the x-ray tube. 
         [0035]    In another embodiment, the assembly of radiation source  30 , vertical member  13 , and detectors  12  and  11  need not be mounted on mobile platform  14 , but just manually or by a robot placed on the ground next to the object  10  to be scanned. In another variation of this embodiment, there may not be any member  13  connecting the detector assembly  1211  to the x-ray generator  30 . In such an embodiment, the detectors  11  and  12  could be translated linearly on the other side of object  10  by a suitable translational means to implement the scan of object  10 . 
         [0036]    In yet another embodiment, the radiation source  30  can be placed on the ground or floor under a desk for example with the radiation beam  32  pointing up. This configuration would be used to image a bag left on the desk. In such an embodiment, one detector arm  11  would be swept by a suitable means above the bag to be inspected. 
         [0037]    In yet other alternative embodiments, the x-ray source  30  may be replaced by a radioactive source, or an electromagnetic source. 
         [0038]    The foregoing description of the invention and its embodiments should be considered as illustrative only of the concept and principles of the invention. The invention may be configured in a variety of ways, shapes and sizes and is not limited to the description above. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is desired that the scope of the present invention not be limited by the description above but by the claims presented herein.