Device, system and method for controlling the position of an anti-scatter grid in an X-ray image acquisition system

Abstract

The present invention relates to a device for controlling the position of an anti-scatter grid in an X-ray image acquisition system, the device (10) comprising: a measurement unit (12); a control unit (14); and a shift unit (16); wherein the measurement unit (12) is configured to determine an X-ray beam focus position (37) of an X-ray radiation source of the X-ray image acquisition system relative to an X-ray detector of the X-ray image acquisition system; wherein the control unit (14) is configured to generate a shift signal based on a displacement (18) between the X-ray beam focus position (37) and a grid focus position (35) of the anti-scatter grid; and wherein, based on the shift signal, the shift unit (16) is configured to shift the anti-scatter grid of the X-ray image acquisition system in at least one direction to align the anti-scatter grid with the X-ray beam focal spot position (37), providing an improved anti-scatter grid for the X-ray acquisition system. The present invention provides for the use of an improved anti-scatter grid (26) in an X-ray acquisition system (20).

Description

Field of the Invention

[0001] The present invention relates to an apparatus, a system and a method for controlling the position of an anti-scatter grid in an X-ray image acquisition system. Background Art

[0002] To acquire an X-ray image, an X-ray radiation source emits X-ray radiation from a focal spot defined by the position where an electron beam strikes the anode of the X-ray radiation source. The X-ray radiation passes through an object and a grating and reaches an X-ray detector. On the traveling path, the X-ray radiation is scattered, which results in significant noise in the X-ray image. An anti-scatter grid can be used to reduce the scattering. The anti-scatter grid is a plate in front of the X-ray detector. The plate has lead or a similar highly absorbent material (such as tungsten strips), which is positioned such that only the X-ray radiation originating from the focal spot can pass through, and other (scattered) X-ray radiation will be absorbed.

[0003] In a computed tomography system, a two-dimensional anti-scatter grid with a height of several centimeters is used, which absorbs a significant amount of scattered radiation. However, in a C-arm system, since the C-arm X-ray system is not infinitely rigid, the height of the anti-scatter grid is only 2-3 mm. Depending on the orientation, speed and acceleration, the exact position of the focal spot relative to the X-ray detector may vary.

[0004] US 5469429 A relates to an apparatus for aligning a focal spot to a predetermined position in a computed tomography system. The apparatus detects the focal spot of the X-ray radiation on the anode of the X-ray tube. Then, the apparatus adjusts the position of the focal spot to a predetermined position by repositioning the anode or by changing the path of the electron beam. Summary of the Invention

[0005] There may be a need for an apparatus and a method that provide an improved use of an anti-scatter grid for an X-ray acquisition system.

[0006] The object of the present invention is solved by the subject matter of the independent claims; further embodiments are incorporated in the dependent claims. It should be noted that the aspects described below of the present invention also apply to a system, an X-ray acquisition system, a method, a computer program unit and a computer-readable medium.

[0007] According to one aspect of the present invention, there is provided an X-ray image acquisition system, which includes: an X-ray radiation source and an X-ray detector, which are connected to at least one C-arm as a support structure, and an anti-scatter grid, which is arranged between the object receiving space and the X-ray detector. The system further includes a device for controlling the position of the anti-scatter grid, and the device includes a measurement unit, a control unit, and a displacement unit. The measurement unit is configured to determine the X-ray beam focusing position of the X-ray radiation source relative to the X-ray detector. The control unit is configured to generate a displacement signal based on the displacement between the X-ray beam focusing position and the grid focusing position of the anti-scatter grid. The displacement unit is configured to displace the anti-scatter grid in at least one direction based on the displacement signal to align the anti-scatter grid with the X-ray beam focusing position.

[0008] Therefore, the device adjusts the position of the anti-scatter grid to align the grid focusing position of the anti-scatter grid with the X-ray beam focusing position. The device first measures the focusing position of the X-ray radiation beam in the X-ray radiation source. In addition, the displacement between the measured X-ray beam focusing position and the grid focusing position of the anti-scatter grid of the X-ray image acquisition system is determined. Depending on the displacement, a displacement signal is generated, and the displacement signal includes information on how much the anti-scatter grid should be displaced to align the grid focusing position with the X-ray beam focusing position. Then, the displacement unit displaces the anti-scatter grid according to the information of the displacement signal. Therefore, the displacement between the X-ray radiation source and the anti-scatter grid that occurs, for example, due to the deformation of the structure of the X-ray image acquisition system can be compensated by moving the anti-scatter grid to a position where the X-ray radiation can pass through the anti-scatter grid. Since the misalignment between the X-ray beam focal spot position and the grid focal spot position is compensated, a thick anti-scatter grid, that is, an anti-scatter grid with a high grid ratio, can be used. The use of an anti-scatter grid with a high grid ratio results in an improvement in image quality, which is due to the reduction of scattering and the improvement of the contrast of the X-ray image.

[0009] In one example, the anti-scatter grid is arranged between the object receiving space and the X-ray detector, and the object receiving space is the space between the X-ray radiation source and the X-ray detector. The anti-scatter grid can be arranged, for example, in front of the X-ray detector. In another example, the anti-scatter grid can be arranged on the X-ray detector.

[0010] In one example, the displacement unit is connected to the anti-scatter grid of the X-ray image acquisition system.

[0011] According to an example, the control unit is configured to determine the displacement based on the analysis of the X-ray image acquired by the X-ray detector.

[0012] In one example, the analysis of the X-ray image includes contrast analysis.

[0013] In one example, the determination of the displacement is performed based on the position of the anti-scatter grid relative to the X-ray radiation source.

[0014] In one example, the control unit is configured to determine whether the X-ray image depicts a shadow of the anti-scatter grid, and the shadow indicates displacement.

[0015] According to an example, the shifting unit is configured to shift the anti-scatter grid in at least one direction in a plane arranged parallel to the X-ray impact surface of the X-ray detector.

[0016] In one example, the anti-scatter grid is a one-dimensional grid.

[0017] According to an example, the shifting unit is configured to shift the anti-scatter grid in two directions. In one example, the anti-scatter grid is a 2D grid. This further improves scatter removal.

[0018] According to an example, the shifting unit includes at least one control member configured to move the anti-scatter grid.

[0019] In one example, the control member is a motor.

[0020] According to an example, the measurement unit is configured to determine the X-ray beam focus position during acquisition of the X-ray image by the X-ray image acquisition system. Thus, during acquisition of the X-ray image, the alignment deviation between the X-ray beam focus position and the grid focus position can be compensated. Thus, repeated image acquisition at the same position due to low contrast is avoided.

[0021] In one example, the measurement unit is configured to determine the actual position during a calibration process before acquisition of the X-ray image by the image acquisition system.

[0022] According to an example, the X-ray radiation source and the X-ray detector are mounted to opposite parts of a C-arm as at least one support structure. And wherein, the support structure is configured to rotate two opposite attachment parts around the object receiving space.

[0023] In one example, the X-ray image acquisition is one of the following group: a two-dimensional image acquisition system, a three-dimensional image acquisition system or a mobile system.

[0024] According to an example, the control unit is configured to generate the shift signal based on the angular position, velocity and / or acceleration of the X-ray detector.

[0025] In one example, the anti-scatter grid has a grid ratio in the range of 8:1 to 16:1, preferably in the range of 10:1 to 16:1, and most preferably in the range of 12:1 to 16:1. The grid ratio is defined as the ratio of the grid height to the grid gap width.

[0026] In one example, the anti-scatter grid is a high-ratio grid, i.e., a "thick" anti-scatter grid.

[0027] Other examples and advantages of the X-ray image acquisition system can be derived from the above description. Therefore, refer to the above description.

[0028] According to the present invention, there is also provided a method for controlling the position of an anti-scatter grid in a C-arm X-ray image acquisition system, the method comprising the steps of: a) using a measurement unit to determine the X-ray beam focusing position of the X-ray radiation source of the X-ray image acquisition system relative to the X-ray detector of the X-ray image acquisition system; b) using a control unit to generate a shift signal based on the displacement between the X-ray beam focusing position and the grid focusing position; c) using the shift unit to shift the position of the anti-scatter grid in at least one direction based on the shift signal so that the anti-scatter grid is aligned with the X-ray beam focusing position in the X-ray radiation source.

[0029] Other examples and advantages of the method can be derived from the above description. Therefore, refer to the above description.

[0030] According to the present invention, there is also provided a computer program unit for controlling the device according to the above description, the computer program unit being adapted to execute the method according to the above description when executed by a processing unit.

[0031] According to the present invention, there is also provided a computer-readable medium storing the program unit according to the above description.

[0032] With reference to the embodiments described below, these and other aspects of the present invention will become apparent and will be elucidated. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings:

[0034] Figure 1a Figs. a and b show schematic diagrams of an X-ray image acquisition system.

[0035] Figure 2a Figs. -c show schematic diagrams of the movement of the anti-scatter grid.

[0036] Figure 3A schematic diagram of a device for controlling the position of an anti-scatter grid in an X-ray image acquisition system is shown.

[0037] Figure 4 shows a schematic diagram of a two-dimensional anti-scatter grid.

[0038] Figure 5 A schematic flowchart of the method is shown. Detailed implementation

[0039] Figure 1a and 1b An X-ray image acquisition system 20 is shown, which includes: an X-ray radiation source 22, an X-ray detector 24, an anti-scatter grid 26, at least one support structure 32, and a device 10 for controlling the position of the anti-scatter grid in the X-ray image acquisition system. The device 10 includes a measurement unit 12, a control unit 14, and a shift unit 16.

[0040] In an embodiment of the present invention, the X-ray image acquisition system 20 may be a two-dimensional X-ray image acquisition system.

[0041] In another embodiment of the present invention, the X-ray image acquisition system 20 may be a three-dimensional X-ray image acquisition system.

[0042] In another embodiment of the present invention, the X-ray image acquisition system 20 may be a mobile X-ray image acquisition system.

[0043] The X-ray radiation source 22 emits X-ray radiation 30 from the X-ray beam focusing position 37. The X-ray radiation 30 travels through the object receiving space 36, in which, for example, a patient to be examined may be present, and then reaches the X-ray impact surface 43 of the X-ray detector 24. The anti-scatter grid 26 filters the scattered X-ray radiation, allowing only the X-ray radiation 30 emitted from the grid focusing position 35 of the anti-scatter grid 26 to pass through.

[0044] The anti-scatter grid 26 may include a grid ratio in the range of 8:1 to 16:1, preferably in the range of 10:1 to 16:1, and most preferably in the range of 12:1 to 16:1. The grid ratio is defined as the ratio of the grid height to the grid gap width. The anti-scatter grid 26 with a high grid ratio may be referred to as a thick anti-scatter grid. The thick anti-scatter grid will greatly reduce the scatter in the X-ray image, which will greatly improve the quality of the X-ray image due to the reduced signal-to-noise ratio.

[0045] In Figure 1aIn [the device], at least one support structure 32 is a C-arm 38. The C-arm includes two opposite parts 31, 33 which are arranged at opposite ends of the C-arm 38. The X-ray radiation source 22 is attached to the first opposite part 31. The X-ray detector 24 is attached to the second opposite part 33. At least one support structure 32 can cause the two opposite parts 31, 33, where the X-ray radiation source 22 and the X-ray detector 24 respectively, to rotate together around the object receiving space 36. The rotation can be performed around an axis 34 which provides a rotary bearing for the support structure 32.

[0046] In Figure 1b [the device], at least one support structure 32 includes two robotic arms 39, 41. The X-ray radiation source 22 is attached to the first robotic arm 39. The X-ray detector 24 is attached to the second robotic arm 41. The robotic arms 39, 41 can cause the X-ray radiation source 22 and the X-ray detector 24 to rotate around the object receiving space 36. In addition, the robotic arms 39, 41 are configured to arrange the X-ray radiation source 22 and the X-ray detector 24 on opposite sides of the object receiving space 36 during an image acquisition process. Thus, the X-ray radiation 30 emitted by the X-ray radiation source 22 can pass through the object receiving space 36 to reach the X-ray detector 24.

[0047] As long as the X-ray beam focal spot position 37 and the grid focusing position 35 match, the X-ray radiation 30 passing through the anti-scatter grid 26 provides a high-quality object image in the object receiving space 36. This is shown in Figure 2a [the figure].

[0048] Figure 2b [The figure] shows a situation where the X-ray beam focusing position 37 deviates from the grid focusing position 35 by a displacement 18. Then, the anti-scatter grid 26 will filter some of the X-ray radiation 30 emitted from the X-ray beam focusing position 37, thus reducing the image quality on the X-ray detector 24. Such a deviation may be caused by deformation of at least one support structure 32 during image acquisition. The deformation can occur, for example, due to acceleration and / or gravity during rotation of at least one support structure 32 around the object receiving space 36.

[0049] To align the grid focusing position 35 of the anti-scatter grid 26 with the X-ray beam focusing position 37, according to Figure 2c [the figure], the moving unit 16 can move the anti-scatter grid 26 by a distance 19 such that the X-ray radiation 30 emitted from the X-ray beam focusing position 37 can pass through the anti-scatter grid 26. The movement of the anti-scatter grid 26 can be performed parallel to the X-ray impact surface 43.

[0050] Figure 3Device 10 is shown in more detail. The measuring unit 12 may include two elements that may be arranged at the X-ray radiation source 22 and the X-ray detector 24. The measuring unit 12 is configured to determine the X-ray beam focusing position 37 in the X-ray radiation source 22 relative to the X-ray detector 24. The X-ray beam focusing position 37 is determined by the position at which the electron beam 28 emitted from the cathode 21 arrives at the anode 23 in the X-ray radiation source 22.

[0051] The measuring unit 12 determines the position of the X-ray detector 24 and compares it with the position of the X-ray beam focusing position 37 to determine the displacement between the X-ray beam focusing position 37 and the X-ray detector 24. The measuring unit 12 may provide this determination during the image acquisition process.

[0052] In order to determine the displacement between the X-ray beam focusing position 37 and the X-ray detector 24, an initial alignment between the X-ray beam focusing position 37 in the X-ray radiation source 22 and the grid focusing position 35 of the anti-scatter grid 26 may be provided. The anti-scatter grid 26 may be arranged near or on the X-ray detector 24 respectively. The measuring unit 12 may determine the deviation between the initial spatial alignment of the X-ray radiation source 22 and the X-ray detector 24 during image acquisition (i.e., during the rotation of the X-ray radiation source 22 and the X-ray detector 24 around the object receiving space 36). Then, the measuring unit 12 may determine any displacement relative to the initial alignment during image acquisition.

[0053] Additionally, in an example, the displacement 18 between the X-ray beam focusing position 37 and the grid focusing position 35 may be determined during image acquisition by analyzing the acquired X-ray image of the X-ray detector 24. A reduction in the X-ray image quality will indicate a misalignment between the X-ray beam focusing position 37 and the grid focusing position 35. The reduction in quality may be due to a reduction in the contrast of the X-ray image and / or the presence of the shadow of the anti-scatter grid 26 in the X-ray image. This may improve the determination of the displacement between the X-ray beam focusing position 37 and the grid focusing position 35.

[0054] The control unit 14 analyzes the measured displacement 18 between the X-ray position 37 and the grid focusing position 35. Based on the displacement 18, the control unit 14 generates a shift signal. The shift signal includes information about the distance 19 by which the anti-scatter grid 26 must be shifted in order to align the grid focusing position 35 with the X-ray beam focusing position 37.

[0055] The control unit 14 may also base the shift signal on the angular position, velocity, and / or acceleration of the X-ray detector 24. The angular position, velocity, and acceleration may be acquired by sensors on at least one support structure 32.

[0056] The shift unit 16 can be attached to the anti-scatter grid 26. Additionally, the shift unit 16 can include at least one control member which can be a motor. The control member is configured to relocate the anti-scatter grid 26 in one dimension, i.e., in one direction.

[0057] The shift unit 16 receives a shift signal from the control unit 14 and shifts the anti-scatter grid 26 by a distance 19. This moves the grid focus position 35 towards the X-ray beam focus position 37. As a result, the X-ray beam focus position 37 is realigned with the grid focus position 35 of the anti-scatter grid 26.

[0058] Figure 4a and 4b Different types of anti-scatter grids 26 are shown. Figure 4a A one-dimensional anti-scatter grid 26 is shown. The one-dimensional anti-scatter grid 26 includes a single row of X-ray radiation transparent portions 25 separated by X-ray radiation absorbing portions 27. The X-ray radiation transparent portions 25 are arranged such that they are aligned with the grid focus position 35. The one-dimensional anti-scatter grid 26 can reduce scatter in one dimension.

[0059] The shift unit 16 attached to the anti-scatter grid 26 can relocate the anti-scatter grid 26 in the direction indicated by the arrow.

[0060] Figure 4b A two-dimensional anti-scatter grid 26 is shown. The two-dimensional anti-scatter grid 26 has several rows of X-ray radiation transparent portions 25 separated by X-ray radiation absorbing portions 27. Similarly, the rows of X-ray radiation transmitting portions 25 are separated by X-ray radiation absorbing portions 27. The two-dimensional anti-scatter grid 26 can reduce scatter in two dimensions.

[0061] The shift unit 16 includes a first shift member 162 and a second shift member 164. The first and second shift members 162, 164 can be control members.

[0062] The first shift member 162 can shift the anti-scatter grid 26 in a first dimension, wherein the second shift member 164 can shift the anti-scatter grid 26 in a second dimension. The first dimension and the second dimension can be orthogonal to each other as indicated by the arrow. However, the first dimension and the second dimension can also be non-orthogonal but not parallel.

[0063] Figure 5 A flowchart of a method 100 for controlling the position of an anti-scatter grid in an X-ray image acquisition system is shown.

[0064] In step a), a measuring unit is used to determine an X-ray beam focal position of an X-ray radiation source of the X-ray image acquisition system relative to an X-ray detector of the X-ray image acquisition system. The position of the X-ray detector and the X-ray radiation source can be determined in order to determine a change in the alignment of an anti-scatter grid arranged close to or on the X-ray detector, respectively, with the position of the X-ray beam focal spot. This means that in step a), a displacement between the grid focal position and the position of the X-ray beam focal spot can be determined.

[0065] In one example, the acquired X-ray images may also be analyzed 102 to determine a reduction in image quality, wherein the reduction may be due to a reduction in contrast in the X-ray images due to a change in alignment and / or shadowing of the anti-scatter grid. This may improve the determination of the displacement between the grid focus position and the X-ray beam focal spot position.

[0066] In step b), a shift signal is generated 103 according to the displacement between the X-ray beam focus position and the grid focus position. The generation 102 may be performed by a control unit. The shift signal comprises information how much the anti-scatter grid must be repositioned in order to align the grid focus position of the anti-scatter grid to the X-ray beam focus position.

[0067] In step c), the position of the anti-scatter grid may be shifted 104 in at least one direction based on the shift signal. The shift may be performed by a shift unit. This will align the grid focus position of the anti-scatter grid with the X-ray beam focal spot position in the X-ray radiation source.

[0068] In a further exemplary embodiment of the present invention, a computer program or a computer program element 40 as shown in Fig. 1 is provided, characterized in that it is adapted to perform the method steps of the method according to one of the preceding embodiments on a suitable system.

[0069] The computer program unit 40 can therefore be stored on a computing unit, which can also be part of an embodiment of the present invention. The computing unit can be suitable for executing the steps of the above method or causing the execution of the steps of the above method. In addition, it can be suitable for operating the components of the above device. The computing unit can be suitable for automatically operating and / or executing the user's command. The computer program can be loaded into the working memory of a data processor. The data processor can therefore be equipped to implement the method of the present invention.

[0070] This exemplary embodiment of the invention covers both a computer program that right from the beginning uses the invention and a computer program that by means of an update turns an existing program into a program that uses the invention.

[0071] Furthermore, the computer program element may be able to provide all necessary steps to implement the procedures of an exemplary embodiment of the method as described above.

[0072] According to another exemplary embodiment of the present invention, there is provided a computer-readable medium 1 according to FIG. 1, such as a CD-ROM, wherein the computer-readable medium has a computer program unit stored thereon, and the computer program unit is as described in the previous part. The computer program can be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium provided with or as part of other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0073] However, the computer program can also be provided via a network such as the World Wide Web and can be downloaded from such a network into the working memory of a data processor. According to a further exemplary embodiment of the present invention, there is provided a medium for making a computer program unit available for download, the computer program unit being arranged to execute one of the previously described embodiments of the present invention.

[0074] It must be noted that the embodiments of the present invention are described with reference to different subjects. In particular, some embodiments are described with reference to method-type claims, while other embodiments are described with reference to device-type claims. However, as can be derived by those skilled in the art from the above and the following descriptions, unless otherwise indicated, any combination between features related to different subjects is also considered to be disclosed by this application, in addition to any combination of specific features belonging to the same type of subject. However, all features can be combined to provide a synergistic effect that is more than the simple sum of the said features.

[0075] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary and not restrictive. The present invention is not limited to the disclosed embodiments. Those skilled in the art can, by studying the drawings, the disclosure and the dependent claims, understand and realize other variations of the disclosed embodiments when practicing the claimed invention.

[0076] In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality. A single processor or other unit can implement the functions of several items recited in the claims. Although specific measures are recited in mutually different dependent claims, this does not indicate that the combination of these measures cannot be used advantageously. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An X-ray image acquisition system (20), comprising: an X-ray radiation source (22) and an X-ray detector (24), the X-ray radiation source and the X-ray detector being connected to at least one C-arm (38) serving as a support structure (32); an anti-scatter grid (26), which is arranged between the object receiving space (36) and the X-ray detector (24), and a device (10) for controlling the position of the anti-scatter grid, comprising: a measurement unit (12); a control unit (14); and a shifting unit (16); wherein the measurement unit (12) is configured to determine the X-ray beam focusing position (37) of the X-ray radiation source relative to the X-ray detector; wherein the control unit (14) is configured to generate a shift signal based on the displacement (18) between the X-ray beam focusing position (37) and the grid focusing position (35) of the anti-scatter grid, and wherein, based on the shift signal, the shifting unit (16) is configured to shift the anti-scatter grid in at least one direction to align the anti-scatter grid with the X-ray beam focusing position (37).

2. The system according to claim 1, wherein the anti-scatter grid has a grid ratio in the range of 8:1 to 16:

1.

3. The system according to claim 1 or 2, wherein the control unit (14) is configured to determine the displacement based on an analysis of the X-ray image acquired by the X-ray detector.

4. The system according to claim 1 or 2, wherein the shifting unit (16) is configured to shift the anti-scatter grid in at least one direction in a plane arranged parallel to the X-ray impact surface of the X-ray detector.

5. The system according to claim 1 or 2, wherein the shifting unit (16) is configured to shift the anti-scatter grid in two directions.

6. The system according to claim 1 or 2, wherein the shifting unit (16) comprises at least one control member configured to move the anti-scatter grid.

7. The system according to claim 1 or 2, wherein the measurement unit (12) is configured to determine the X-ray beam focusing position (37) during acquisition of an X-ray image using the X-ray image acquisition system.

8. The system according to claim 1 or 2, wherein the X-ray radiation source (22) and the X-ray detector (24) are mounted on opposite parts (31, 33) of the at least one C-arm (38); and wherein the C-arm (38) is configured to rotate two opposite attachment parts (31, 33) around the object receiving space (36).

9. The system according to claim 1 or 2, wherein the control unit (14) is configured to generate the shift signal based on the angular position, speed, and / or acceleration of the X-ray detector (24).

10. A method for controlling the position of an anti-scatter grid in a C-arm X-ray image acquisition system, the method (100) comprises the following steps: a) using a measurement unit to determine (101) the X-ray beam focusing position of the X-ray radiation source of the C-arm X-ray image acquisition system relative to the X-ray detector of the C-arm X-ray image acquisition system; b) using a control unit to generate (103) a shift signal based on the displacement between the X-ray beam focusing position and the grid focusing position; c) using a shift unit to shift (104) the position of the anti-scatter grid in at least one direction based on the shift signal, so as to align the anti-scatter grid with the X-ray beam focusing position in the X-ray radiation source.

11. A computer program product comprising a computer program unit (40) for controlling the system according to one of claims 1 to 9, the computer program unit being adapted to execute the steps of the method according to claim 10 when run by a processing unit.

12. A computer-readable medium (50) storing a computer program unit (40) for controlling the system according to one of claims 1 to 9, the computer program unit being adapted to execute the steps of the method according to claim 10 when run by a processing unit.

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