Abstract:
A method for the rectification of an X-ray picture ( 2   a   , 2   b ) of an item of luggage ( 2 ) during luggage screening with the following steps: 
       recording the X-ray picture ( 2   a   , 2   b ) of the item of luggage ( 2 ) by means of a X-ray radioscopy device ( 11, 11   a );    geometric rescaling ( 7 ) of the X-ray picture ( 2   a   , 2   b ) whilst taking into consideration only the geometric data of the mapping geometry of the X-ray radioscopy device ( 11, 11   a ) and also the image data of the recording of the X-ray picture ( 2   a   , 2   b ) of the item of luggage ( 2 ), the beam path ( 10 ) being tracked for each detector element onto a respective preset plane;    transmission of the rescaled, de-skewed X-ray picture ( 8   a   , 8   b ) to a display or image-processing apparatus ( 9 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 U.S.C. §119 to and incorporated by reference German Application No. 10352411.8-52/DE filed on Nov. 10, 2003.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to a method for de-skewing an X-ray picture of an item of luggage during luggage screening.  
       BACKGROUND  
       [0003]     Currently it is possible to analyze items of luggage completely for explosives. To do this, apparatuses are used which often work in the form of a line scanner. The X-ray beam transilluminates the item of luggage to be examined in slices. The fluoroscopic radiation is recorded with a line detector. L-shaped detectors are often used in the process. Due to their geometry there is a strong distortion of the recorded item of luggage, in particular in the region of the right angle. Such a skewed image is displayed on a monitor and examined by the security staff. As a result of the distortion it can happen that the items contained in the item of luggage cannot be correctly assessed. This leads to a security risk as dangerous items may not be recognized under certain circumstances.  
       SUMMARY  
       [0004]     The object of the invention is therefore to provide a method, which makes possible a better recognizability of the items contained in an item of luggage.  
         [0005]     The object is achieved by a method with the features of claim  1 . In the method according to the invention, a clear rectification of the previously strongly skewed pictures of the item of luggage is achieved by means of geometric rescaling. For the geometric resealing according to the invention only the skewed picture of the item of luggage and the geometric data of the mapping geometry of the X-ray radioscopy device, by means of which the skewed picture was obtained, are required. The then de-skewed, i.e. geometrically rescaled, picture of the item of luggage is passed on to a display or image-processing apparatus so that the security staff can see an essentially rectified image of the items inside the item of luggage and thus can also classify these better.  
         [0006]     An advantageous development of the invention provides that a line detector, in particular an L-shaped detector, is used as X-ray radioscopy device. Such an L-shaped detector has a very simple geometry with the result that the rectification can be determined very easily.  
         [0007]     A further advantageous development of the invention provides that the X-ray picture is subjected to an optical calibration before geometric resealing. In particular the full dynamic range of the intensity values of the picture is used and a so-called histogram adaptation carried out.  
         [0008]     A further advantageous development of the invention provides that the beam path is tracked for each detector element onto a respective preset plane, this preferably being carried out into a single plane. As a result a uniform reference surface is defined on which the geometric resealing is based.  
         [0009]     A further advantageous development of the invention provides that the height of the item of luggage is determined, in particular by means of a light barrier. As a result it is possible that the optimum plane on which the geometric resealing is based can be established. In particular with tall items of luggage it is useful to have the preset plane on which the geometric resealing is based at half the height of the item of luggage parallel to the support surface.  
         [0010]     Further advantageous developments of the invention are the subject of the dependent claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Advantageous designs of the invention are explained in more detail using the drawings. There are shown in detail:  
         [0012]      FIG. 1   a - b  two schematically represented X-ray radioscopy devices with different geometries,  
         [0013]      FIG. 2   a - b  representations of a respective X-ray picture of two different items of luggage with the X-ray radioscopy device of  FIG. 1   a,    
         [0014]      FIG. 3 a  block diagram of a device on which the method according to the invention takes place, and  
         [0015]      FIG. 4   a - b  representations of the two items of luggage  FIG. 2   a - b  after carrying out the method according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Two known X-ray radioscopy devices  11 ,  11   a  are shown schematically in  FIGS. 1   a  and  1   b . Each of these has an X-ray source  1  from which a beam path  10  sweeps an item of luggage  2 . The transmitted X-radiation is recorded by a line detector  3 ,  3   a . The geometric design of the line detectors  3 ,  3   a  is known in different forms. Only a common L-shaped detector unit is discussed below. Such a detector is called L-shaped detector  3 .  
         [0017]      FIGS. 2   a  and  2   b  show an X-ray picture  2   a ,  2   b  of different items of luggage  2  respectively which were recorded in a X-ray radioscopy apparatus  11  with L-shaped detector  3 . A distortion along the horizontal axis is clearly visible here. The origin of this is in the angle resolution. A beam of rays with an angle width covers different-sized areas of the L-shaped detector  3 . In particular at the point at which the horizontal detector  30  passes into the vertical detector  31 , a strong distortion of the picture  2   a ,  2   b  of the item of luggage  2  is observed. This is the case in the left-hand region of the respective picture  2   a ,  2   b  in  FIGS. 2   a  and  2   b.    
         [0018]     In addition, objects in the item of luggage  2  which are different heights are mapped under certain circumstances onto the same detector element. This distortion greatly hinders the analysis of the picture  2   a ,  2   b  of the item of luggage  2 . The information density is not evenly distributed over the picture  2   a ,  2   b . In some regions, in particular in the strongly compressed regions in the left-hand section of  FIGS. 2   a ,  2   b , a lot of data is stored in a very small area. Added to this is the optical distortion which occurs for the reasons described above which considerably hinder an interpretation of such pictures  2   a ,  2   b . It can be emphasized that a comparison between X-ray pictures  2   a ,  2   b  of the same item of luggage  2  which were carried out by different line scanners is greatly impeded.  
         [0019]     To date, attempts have not been made to solve this problem by carrying out a rectification of the pictures  2   a ,  2   b  of the item of luggage  2 , but by means of dual-energy images. Such apparatuses serve to convey a suspicion, which is then examined by other analysis methods. To date another possibility was to change the position of the item of luggage  2  so that the strongly skewed region is moved into a less skewed region.  
         [0020]     The schematic course of a method according to the invention for the rectification of a X-ray picture  2   a ,  2   b  of an item of luggage  2  is shown in  FIG. 3 . The method according to the invention is based on the method described above for the skewed recording of an X-ray picture  2   a ,  2   b  of an item of luggage  2  within an X-ray radioscopy device  11  with an X-ray source  1  and an L-shaped detector  3 . The line-scan images obtained were regularly subjected to an optical calibration  4  or another known calibration or an image processing. With the dual-energy systems a colour coding takes place in the process, which corresponds to the order number of the object seen inside the item of luggage  2 . Suspicious regions were marked and the data of this preprocessing then transmitted to a display or image-processing apparatus  9 . For example an image distribution computer was used which sends the image data to one or various operators.  
         [0021]     The optical calibration  4  is still advantageously used. In addition however a transmission  5  of the geometric data of the mapping geometry of the X-ray radioscopy device  11  is carried out. In addition the determination  6  of the position of the projection plane is carried out by measuring the height of the item of luggage  2  and these data likewise transmitted. The height of the item of luggage  2  can preferably be measured using a light barrier. The skewed X-ray picture  2   b  of the item of luggage  2  obtained due to the optical calibration  4  is subjected to the geometric rescaling  7 . It is assumed that the X-ray picture  2   b  is the projection of an areal object. Whilst the actual intensity value of a detector element can be calculated from  
         I   =       ∫   0   1     ⁢       ⅆ   s     ⁢           ⁢     f   ⁡     (   s   )             ,       
 
 f(s) representing the absorption properties of the transilluminated medium along the beam, the assumption is made here that the intensity value can be calculated from  
       I   =       ∫   0   1     ⁢       ⅆ   s     ⁢           ⁢     δ   ⁡     (     s   -     s   0       )       ⁢         f   ′     ⁡     (   s   )       .             
 
         [0022]     The point s 0  lies on a particular plane. This plane can optionally be modified. For each detector element the beam path  10  is tracked onto the s 0  plane. The chosen s 0  plane is at half the height of the item of luggage  2  which has been previously established for example by means of a light barrier. It goes without saying that every other s 0  plane can also be used. The advantage of using the s 0  plane at half the height of the item of luggage  2  is that the maximum distortion is lowest as the distance of each object within the item of luggage  2  to the s 0  plane is at most half the height of the item of luggage.  
         [0023]     The rectified X-ray picture  8   b  established by the geometric resealing  7  is passed to and displayed on the display or image-processing apparatus  9 . It is thus possible for the security staff to obtain a predominantly rectified X-ray picture  8   b  of the item of luggage  2  and to be able to better recognize the objects contained therein.  
         [0024]     The rectified X-ray pictures  8   a ,  8   b  of the two items of luggage  2   a ,  2   b  shown in  FIGS. 2   a  and  2   b  are shown in  FIGS. 4   a  and  4   b . The rectified X-ray pictures  8   a ,  8   b  look clearer and the objects contained in the item of luggage  2  can be more easily recognized. The problem of distortion depending on the distance of an object inside the item of luggage  2   a ,  2   b  from the s 0  plane can be observed particularly well in  FIG. 4   b . The projection plane here was not set at half the height of the item of luggage  2 , but at the height of the conveyor belt. Therefore objects, which are located near the conveyor belt are only slightly skewed. Objects, which are higher up, in particular near the top of the item of luggage  2 , are more strongly skewed. However this can be corrected by changing the projection plane.  
         [0025]     The regions in the left-hand halves of the images, which are strongly compressed in  FIGS. 2   a  and  2   b  are greatly de-skewed in  FIGS. 4   a  and  4   b . As a result the analysis of objects, which are located in these regions is much simplified.  
         [0026]     The method according to the invention can also be carried out in real time because of its low computational outlay. In fact, only the image data and the detector geometries are required. Thus every existing line-scan system can be equipped with such a method according to the invention without difficulty.  
       LIST OF REFERENCE NUMBERS  
       [0000]    
       
           1  X-ray source  
           2  Item of luggage  
           2   a ,  2   b  X-ray picture of the item of luggage  
           3 ,  3   a  Line detector  
           4  Optical calibration  
           5  Transmission of the geometric data of the mapping geometry  
           6  Determination of the position of the projection plane  
           7  Geometric rescaling  
           8   a ,  8   b  De-skewed X-ray picture of the item of luggage  
           9  Display or image-processing apparatus  
           10  Beam path  
           11 ,  11   a  X-ray radioscopy device  
           30  Horizontal detector  
           31  Vertical detector