Abstract:
A method for determining geometric imaging properties of a flat panel detector in an x-ray inspection system includes the steps arranging a calibration phantom between an x-ray source and the flat panel detector, wherein the calibration phantom comprises at least one discrete geometric object; recording at least one x-ray image of the calibration phantom with the flat panel detector, wherein at least one discrete geometric shape is generated in the x-ray image by imaging the at least one discrete geometric object of the calibration phantom; and determining a location-dependent distortion error of the flat panel detector from the at least one x-ray image on the basis of at least one characteristic of the at least one discrete geometric shape. All characteristics of the at least one discrete geometric shape used for determining the location-dependent distortion error are independent of the dimensions of the calibration phantom.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    The present invention relates to a method for determining geometric imaging properties of a flat panel detector in an x-ray inspection system for non-destructive material testing, including the steps arranging a calibration phantom between an x-ray source and the flat panel detector, the calibration phantom comprising at least one discrete geometric object; recording at least one x-ray image of the calibration phantom with the flat panel detector, with at least one discrete geometric shape being recorded by imaging the at least one discrete geometric object of the calibration phantom; and determining a location-dependent distortion error of the flat panel detector from the at least one x-ray image on the basis of at least one characteristic of the at least one discrete geometric shape. An embodiment of the present invention further relates to a correspondingly adapted x-ray inspection system and a corresponding calibration phantom. 
         [0002]    The testing accuracy of an x-ray inspection system for non-destructive material testing with a flat panel detector inter alia depends on the geometric model of the detector taken as the basis in reconstruction and evaluation matching the dimensions of the real detector as exactly as possible. Contrary to former assumptions it has turned out that flat panel detectors within the bounds of the desired high testing accuracies are not flat but show a bend or curvature of the detector surface. It is also possible that for example the pixel size of flat panel detectors is not constant but is a function of the location, i.e. depends on the line number and column number. Distortion errors are thus caused, i.e. the coordinate of the real image of a characteristic, for example of one point, compared to the ideal image is dislocated due to the curvature or the nonconstant pixel size. 
         [0003]    For the correction of distortion errors of a flat panel detector, document WO 2012 062543 A2 proposes a method for operating a measurement arrangement for a computer tomograph, with a calibration phantom being arranged between the radiation source and the flat panel detector, and at least one x-ray image of the calibration phantom being recorded with the flat panel detector, and a distortion error of the flat panel detector being determined from known dimensions of the calibration phantom and from the at least one x-ray image as a function of the location. The calibration phantom comprises a plurality of separate structures, for example spheres, the dimensions of which, i.e. size and distances, must be known exactly. This usually requires a high-precision measurement of the calibration phantom, for example by use of a coordinate measuring instrument, thus involving a lot of time and high costs. 
       SUMMARY OF THE INVENTION 
       [0004]    Embodiments of the present invention include a x-ray inspection method, an x-ray inspection system and a calibration phantom, with simple means allowing an exact determination of geometric imaging properties of the flat panel detector, to enable a correction of a corresponding distortion error, and thus, to improve the measuring precision of the x-ray inspection system. 
         [0005]    An embodiment of the present invention achieves this goal through the features of the independent claims. In an embodiment of the present it is not necessary to have dimensions of the calibration phantom for determining the location-dependent distortion error. In fact, characteristics of the at least one discrete geometric shape, which are independent of the dimensions of the calibration phantom, are sufficient for this purpose, without an adverse effect on the resulting calibration accuracy. Compared to conventional methods, an embodiment of the present invention provides an advantage of dispensing with a previously time-consuming, high-precision measurement of the calibration phantom, for example by use of a coordinate measuring instrument. 
         [0006]    In an embodiment of the present invention, various characteristics of the at least one discrete geometric shape are suited for the determination of the location-dependent distortion error. In the following, different characteristics are explained in more detail. 
         [0007]    In an embodiment, a plurality of discrete geometric shapes is recorded at different positions by imaging the at least one discrete geometric object of the calibration phantom. This can be done in particular by use of a calibration phantom comprising a plurality of discrete geometric objects which can be imaged simultaneously by the x-ray device. Alternatively, for example it is also possible that the calibration phantom comprises just one discrete geometric object which is imaged by the x-ray device at different positions, one by one, of the detector surface. 
         [0008]    The geometric objects forming the basis of the geometric shapes can be uniform, with a characteristic taken as the basis for the determination of the location-dependent distortion error being the deviation of the geometric shapes from uniformity. For example, the geometric objects forming the basis of the geometric shapes can be of equal size, with a characteristic taken as the basis for the determination of the location-dependent distortion error being a deviating size among the geometric shapes. In this embodiment, it is only required that the geometric objects of the sample forming the basis of the geometric shapes are of equal size with high precision, however it is not necessary to have knowledge of the size itself. The geometric objects forming the basis of the geometric shapes, for example can also have an equal shape, with a characteristic taken as the basis for the determination of the location-dependent distortion error being the deviating shape among the geometric shapes. In an embodiment, the geometric objects forming the basis of the geometric shapes can be arranged at regular intervals and/or periodically, with a characteristic taken as the basis for the determination of the location-dependent distortion error being the arrangement of the geometric shapes deviating from regularity and/or periodicity. 
         [0009]    It is not compulsory that a plurality of discrete geometric shapes is recorded at different positions of the detector surface. Embodiments in which just one discrete geometric shape covering a significant area of the detector surface is recorded are also possible. 
         [0010]    In an embodiment, the geometric object forming the basis of the geometric shape can be at least one straight line or can be arranged in at least one straight line, with a characteristic taken as the basis for the determination of the location-dependent distortion error being the deviation of the geometric shape from straightness. In an embodiment of the present invention, the geometric object forming the basis of the geometric shape can be at least one cylindrical object having a constant diameter, with a characteristic taken as the basis for the determination of the location-dependent distortion error being a deviation of the geometric shape from the constant diameter. 
         [0011]    The location-dependent distortion error is not determined from one or more two-dimensional radiographic images but from a three-dimensional x-ray image in particular reconstructed using computer tomography. In particular, in a computer tomography system, this provides the advantage that the present reconstruction algorithms can also be used for the calibration. Furthermore, a three-dimensional x-ray image in particular reconstructed using computer tomography allows a more exact determination of the location-dependent distortion error. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein; 
           [0013]      FIG. 1  a schematic illustration of a computer tomography system; 
           [0014]      FIG. 2  a schematic longitudinal cut through a calibration phantom in an embodiment; 
           [0015]      FIG. 3  a schematic reproduction of an x-ray image of a calibration phantom according to  FIG. 2 ; 
           [0016]      FIG. 4  an illustration of the diameter variation of the spheres of the calibration phantom over the detector extension; 
           [0017]      FIG. 5  a two-dimensional illustration of the diameter variation of the spheres of the calibration phantom over both detector extensions; 
           [0018]      FIG. 6  a schematic longitudinal cut through a calibration phantom in a further embodiment; 
           [0019]      FIG. 7  a schematic cross-sectional cut through the calibration phantom from  FIG. 6 ; 
           [0020]      FIG. 8  a schematic reproduction of an x-ray image of a calibration phantom according to  FIG. 6 ; 
           [0021]      FIG. 9  a schematic illustration of a calibration phantom in an alternative embodiment; and 
           [0022]      FIG. 10   a  schematic reproduction of an x-ray image of a calibration phantom according to  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The computer tomography (“CT”) system shown in  FIG. 1  includes an x-ray device  10  for recording x-ray projections of a sample  13 . For this purpose, the x-ray device  10  comprises an x-ray source  11 , in particular an x-ray tube emitting an x-radiation cone  14 , and an imaging x-ray detector  12 . Furthermore, an only schematically sketched sample manipulator  20 , which can be adapted to rotate the sample  13  around a vertical axis, is provided. In an embodiment, the x-ray device  10  can be rotated around the fixed sample  13 . The sample  13  can be displaced by the sample manipulator  20  linearly in x-, y- and/or z-direction. In an embodiment, the x-ray device  10  and the sample  13  can appropriately be adjusted relatively to each other, in each case including rotation and/or translation around one or more axes. 
         [0024]    The imaging x-ray detector  12  is a flat panel detector, in an embodiment a solid-state detector or semiconductor detector, which in an embodiment comprises a scintillation layer for transforming the incident x-radiation into light and a light-sensitive layer in particular formed by photo cells or photo diodes for transforming the light into an electric signal. In an embodiment, an x-radiation-sensitive photo conductor, for example on the basis of Selenium, is provided instead of a scintillation layer and a light-sensitive layer. 
         [0025]    A set of x-ray projections of the sample  13  is recorded by the manipulator  20  being rotated by one small angular step at a time and one x-ray projection being recorded at each angular position. An x-ray projection  18 , as for example shown in  FIG. 1 , is a two-dimensional image, with the detected density value of a pixel  17 , typically a grey tone, indicating the attenuation of the corresponding x-ray beam  15  from the focal spot  16  of the x-ray source  11  through the sample  13 , by which an attenuated x-ray beam  19  results in the corresponding pixel  17 . Due to a curvature of the sensitive surface of the detector  12  it is possible that an object point is not imaged on the ideal pixel  17  but on another position or another pixel. 
         [0026]    The recorded x-ray projections are read out from the x-ray detector  12  and are transmitted to a computer device  40 , where they are stored in a storage device  44  for further evaluation and processing. The computer device  40  includes a programmable computer  41 , in particular with a micro processor or a micro controller, and an operation terminal  42  with a display  43 . The computer  41  includes a software for performing an appropriate ct reconstruction algorithm to determine a three-dimensional reconstructed image (volume image) of the sample  13  from the recorded x-ray projections. In an embodiment, a separate computer can be provided for carrying out the reconstruction. In an embodiment according to  FIG. 1 , the computer  41  is adapted to control the x-ray device  10 , in particular the x-ray source  11 , the x-ray detector  12  and the sample manipulator  20 . In an embodiment, a separate control device can be provided for controlling the x-ray device  10 . 
         [0027]    For the calibration of the flat panel detector  12  a calibration phantom  13  is placed into the beam path  14  of the x-ray device  10 , then x-ray images or x-ray projections of the calibration phantom  13  are recorded and the volume density of the calibration phantom  13  is reconstructed. 
         [0028]    An embodiment of a calibration phantom  13  is shown in  FIG. 2 . In a tube  29  made of a suitable, particularly radiation-transparent material, in an embodiment a plastic material or aluminum, a single row of spheres  30  made of a suitable radiation-absorbing material, in an embodiment steel or ceramic, particularly is arranged in such a way that the spheres contact each other. The spheres  30  form a plurality of separate calibration objects not connected with each other. The inner diameter of the tube  29  is larger than the diameter of one sphere  30 , however it is smaller than the double diameter of one sphere  30 . The tube  29  can be closed at both ends via elastic closing objects  31 , for example made of a foam material clamping and thus fixing the spheres  30  in the tube  29  to prevent the spheres  30  from shaking With a desired high precision, for example in the range of ±1 μm, the spheres  30  have equal dimensions, i.e. an equal diameter, which does not have to be known with the same precision for carrying out the calibration process, and usually is just known with a significantly higher tolerance. For example, the diameter of the spheres  30  can be in a manufacturing tolerance range of ±100 μm (or more) about a nominal value, provided that the diameter variation among the spheres  30  is small (for example in the range of ±1 μm). For example, it is possible to use comparatively cost-effective ball bearing spheres as calibration objects  30 . The diameter of ball bearing spheres indeed can vary significantly from one lot to the other, however ball bearing spheres within one lot usually with a very high precision have an equal diameter which however is not known exactly. For this reason, for example ball bearing spheres of the same lot can simply be used as calibration objects  30  for the present calibration process; a high-precision measurement of the diameter of the spheres  30  is not necessary. 
         [0029]    The calibration phantom  13  is arranged along or parallel to the rotational axis (vertical axis or y-axis in  FIG. 1 ), so that the spheres  30  are distributed over the extension (here the height) of the detector  12 , and then x-ray projections are recorded. An x-ray image or an x-ray projection of the calibration phantom  13  from  FIG. 2  is shown for example in  FIG. 3 . The row of spheres  30  of the calibration phantom  13  results in a corresponding row of sphere shapes  32  in the x-ray image. In the computer  41 , a volume image of the calibration phantom  13  with three-dimensional sphere shapes  32  is reconstructed from all projections. The diameter of the three-dimensional sphere shapes  32  is determined from the reconstructed volume image in the computer  41  via evaluation or image processing. The relative diameter variation  33  of the three-dimensional sphere shapes  32  for example is plotted in  FIG. 4  over the sphere row (here ten measurements corresponding to ten spheres  30 ). An embodiment illustrated by  FIG. 4  shows the relative diameter deviation  33 , for example in mm over the detector extension (here height) for example in pixel. As the diameter deviation or the diameter variation is not constant, conclusions with respect to the detector curvature can be drawn after an allocation to the corresponding detector positions. 
         [0030]    Comparable measurements are carried out over the whole or a large part of the sensitive surface of the detector  12  by the calibration phantom  13  successively being displaced perpendicular to its longitudinal extension and corresponding x-ray images being recorded. If the calibration phantom  13  for example is arranged parallel to the rotational axis, the displacement is expediently carried out perpendicular to the rotational axis. The resulting two-dimensional diameter deviation  34  for example in mm over the detector height and detector width, each for example in pixel, said embodiment is an embodiment illustrated in  FIG. 5 . The two-dimensional curvature of the detector  12  can be determined accordingly. A location-dependent distortion error, i.e. a pixel-precise distortion error depending on the x- and y-coordinate of each pixel, can be determined from the curvature of the detector  12 , and in particular can be stored in the computer  41 . Each subsequently measured x-ray projection can then be corrected to the determined distortion error, by which the precision of the measured projections and thus also of the reconstructed data can be improved significantly. In an embodiment, the distortion error can be taken into account during the reconstruction, without the need to correct the x-ray projections themselves. In addition or as an alternative to the curvature of the sensitive detector surface, the pixel size or the local pixel size deviation can be determined in a pixel-precise manner. 
         [0031]    An embodiment of the calibration phantom  13  is shown in  FIGS. 6 and 7 . The inner diameter of the tube  29  is larger than the double, particularly larger than 2.1547 times the diameter of one sphere  30 . In this way, a plurality of spheres  30 , particularly at least three spheres  30 , can be arranged in a plane perpendicular to the rotational axis. In the next sphere plane, the three-sphere group is turned by 60°, as is apparent from the cross-sectional cut according to  FIG. 7 . In this embodiment is that for each axial position several measured values are available allowing an averaging and thus an improvement in the measuring precision. Embodiments with two spheres or more than three spheres per plane perpendicular to the rotational axis are also possible. 
         [0032]    An embodiment of the calibration phantom  13  is shown in  FIGS. 9 and 10 . The rod-shaped calibration phantom  13  includes a radiation-transparent rod  35 , for example made of CFRP, to each end of which one sphere-shaped radiation-absorbing calibration object  30  made of a suitable material, for example ruby, is fixed. The length of the rod does not have to be known and for example is in the range between 2 mm and 200 mm. The rod-shaped calibration phantom  13  can be recorded at different positions of the detector  12  and a curvature of the detector  12  can be derived from the relative deviations of the lengths with respect to each other. In a corresponding x-ray image shown in  FIG. 10  a holder  36  not yet shown in  FIG. 9  can be seen. 
         [0033]    The application is not only applicable to computer tomography systems but also to non-CT x-ray inspection systems based on transmission. The application is particularly applicable to inspection systems for the non-destructive inspection of non-biological test objects. 
         [0034]    The calibration phantom  13  is not limited to the shown embodiments. The calibration phantom, for example can be a plate-shaped calibration phantom  13  in the form of a mask with calibration objects, for example in the form of parallel or grid-like arranged lines, deviations of the lines from the straightness being determined in the x-ray image. The calibration objects, for example can also be equally spaced circles, crosses or the like, where, however, the exact distance does not have to be known. An embodiment is a cylinder, for example made of steel, having a high diameter constancy along the axis, deviations of the diameter along the cylinder axis being determined in the x-ray image. Various alternative embodiments of the calibration phantom  13  are possible. 
         [0035]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural element with insubstantial differences from the literal languages of the claims.