WO2009036983A1 - Method for determining a corrective value of a position of the focal spot of an x-ray source in a measuring array, and measuring array for generating radiographs - Google Patents

Abstract

The invention relates to a method for correcting a position of the focal spot of an x-ray source in a measuring array (1) for generating radiographs of a test object (4), especially radiographs for generating a three-dimensional image of a test object (4) by means of back projection. Said measuring array (1) comprises the x-ray source, an object support (3) for accommodating the test object (4), and a detector (5). Said method encompasses the following steps: a reference image of a reference object (42) is generated in a known position of the focal spot, wherein the reference object (42) can be reproducibly placed in a reference position within a beam path of the measuring array (1); the reference object (42) is placed in a reproduced manner in the reference position within the beam path; a corrective image of the reference object (42) is recorded; geometrical parameters are determined in the reference image and the corrective image; and at least one corrective value for the position of the focal spot is derived from the determined parameters. Especially the distance of the focal spot is determined from the enlargement ratios of the reference object in the reference image and the corrective image. The invention further relates to a measuring array (1) and a measuring method.

Description

 Method for determining a correction value of a focal spot position of an X-ray source of a measuring arrangement and a measuring arrangement for generating radiographic images

The invention relates to a method for determining at least one correction value for compensating a focal position change of an X-ray source of a measuring arrangement for generating radiographic images of a measurement object, in particular radiographic images for generating a three-dimensional image of the measurement object by means of backprojection. The invention further relates to a measuring arrangement for generating radiographic images of a measurement object, in particular radiographic images for generating a three-dimensional image of the measurement object by means of backprojection, wherein the measurement arrangement is an x-ray source with a focal spot, on which an electromagnetic radiation used for imaging is generated, a slide for Recording the measurement object and a detector for detecting the transmission images comprises. In particular, the invention also relates to the field of investigation of industrially and / or artificially manufactured articles by means of electromagnetic invasive radiation.

The use of invasive radiation to inspect workpieces is known. In computed tomography (CT), for example, the workpiece is usually placed on a turntable and irradiated by rotation of the turntable in different representations from different directions of X-radiation. However, other geometries of the examination arrangement are possible and known. The attenuated by extinction in the material of the workpiece radiation is spatially and temporally resolved detected by a sensor device. In practice, for example, between 600 and 1200 projection images of the measurement object are recorded, wherein between each of the projections the rotational position is changed by a constant angle of rotation. By using one of several known methods of tomographic reconstruction, e.g. the filtered reconstruction, a three-dimensional (3D) image of the workpiece is calculated from this. The 3D image gives the local linear extinction coefficient for individual small volume areas (voxels). An example of a CT is described in DE 39 24 066 A1.

The 3D image can then be used eg for the qualitative or quantitative characterization of the measurement object. In industrial applications, for example, all dimensions of a part can be tested nondestructively, or qualitative tests, such as eg on blowholes. Components of a typical computer tomography system are in particular a microfocus X-ray tube and an area detector for X-radiation. In the X-ray tube, an X-ray source is realized with a very small diameter (typically 5 to 100 microns in diameter). The X-ray source generates polyenergetic X-rays in the energy range from about 10 to several 100 kilo-electron volts. The radiation penetrates the object, is thereby attenuated (by absorption, but also in another way, eg scattering) and generates an X-ray image of the object on the detector device. The detector device typically includes a scintillator that converts x-radiation into visible radiation, and a photodiode array extending over a surface for two-dimensional, spatially-resolved measurement of the visible radiation. Further components of such a CT system are adjusting units for accurately positioning and aligning the measuring object, the X-ray source and / or the detector. The adjustment units provide signals by which the relative position of source, object and detector to each other at any time with sufficient accuracy are known and / or can be determined to ensure accurate reconstruction.

The generation of the X-ray radiation in the X-ray tube takes place in that electrically charged particles are slowed down (negatively accelerated) on a target. In order to obtain a point source as possible radiation source, the electrically charged particles, usually electrons, focused on a so-called focal spot of the target. The electromagnetic X-radiation is generated in the target in the region of the focal spot.

One difficulty in operating microfocus X-ray tubes is keeping the individual physical parameters constant so that the focal spot position and size does not change. In particular, beam energy, focus voltages and target temperature are to be kept as constant as possible or so regulated that the position of the focal spot on the target and its size do not change. Despite great efforts and much progress in this area, it has not been possible to produce microfocus X-ray tubes in which no change in the focal spot occurs during operation. In particular, a change in the focal spot position leads to a change in the image quality with regard to scaling and / or edge sharpness and thus to inaccuracies of properties derived from the images, in particular dimensions, of the imaged object to be measured. In principle, a focal spot position can be determined by a complex calibration measurement in which a measurement object which is exactly known is measured. However, as the focal spot position may change over time, recalibrate periodically. It is unsatisfactory, however, that when measuring an object only after taking a plurality of individual images and the reconstruction calculation, it is shown in the reconstruction data and / or during the evaluation of the measurement object that the focal spot position has changed. This change is z. B. in a reconstruction image to recognize a deteriorated edge sharpness of the reconstruction image.

If such a poor or degraded resolution is detected, the prior art recalibrates the focal spot position. The numerous recorded X-ray images, which led to the poorly resolved reconstruction image, can not be used as a rule. Thus, it is desirable to be able to detect a shift of the focal spot in a simple manner and to make a correction of the focal spot position or to take into account in an evaluation with only small changes.

The object of the present invention is therefore to provide a method for determining at least one correction value for compensation of a focal position change of an X-ray source and a measuring arrangement, with which an improved correction with respect to a focal position change is possible.

The object is achieved by a method with the features of claim 1 and a measuring arrangement with the features of claim 8 according to the invention. Advantageous embodiments of the invention will become apparent from the dependent claims.

For this purpose, a method is proposed for determining at least one correction value for compensating a focal position change of an X-ray source, in particular a microfocus X-ray source, a measuring arrangement for generating radiographic images of a DUT, in particular radiographic images for generating a three-dimensional image of the DUT by means of backprojection, wherein the measuring arrangement X-ray source, and comprising a detector, the method comprising the steps of: generating a reference image of a reference object, which can be arranged reproducibly in a reference position in a beam path of the measuring arrangement, at a known focal spot position; reproducing the reference object in the reference position in the beam path; Taking a correction image of the reference object; Determining geometric parameters in the reference image and in the correction image and deriving at least one correction value for the focal spot position based on the determined parameters. The reference position is understood to mean both a reference position and a reference orientation of the reference object in space. This means that the reference object, when it is arranged reproducibly in the reference position, is moved in a fixed orientation, a reference orientation, into a reference position. Only if the reference object has an excellent geometry, it may be possible that the reference object can be arranged in several orientations in the reference position and is in the reference position. On the basis of the determined geometric parameters of the reference object in the reference image and in the correction image, changes can be detected. From these changes can be deduced a change in the focal spot position. At least as long as the change in the focal spot position is small, it is usually possible to derive the change in the focal spot position from the changes in the geometric parameters and to determine a correction value that can be taken into account when evaluating the recorded fluoroscopy images of a test object To compensate for change in the focal spot position, ie to correct the determined from a calibration focal spot position. A focal spot position is known if, for example, it has been determined in immediate temporal relation by means of a calibration of the measuring arrangement. In a measuring arrangement according to the invention for generating radiographic images of a measurement object, in particular through images for generating a three-dimensional image of the measurement object by means of backprojection, which comprises an X-ray source with a focal spot on which an electromagnetic radiation used for imaging is generated and a detector for detecting the radiographic images In addition, a reference object image for reproducibly recording a reference object in a reference position in the beam path and an evaluation unit for determining geometric parameters in a reference image representing an image of the reference object in the reference position at a known focal spot position, and a correction image representing an image of the reference object in FIG of the reference representation at a focal spot position to be corrected, and for deriving at least one correction value for the focal spot position. The method and the measuring arrangement offer the advantage that they use a simple measurement of a reference object in the beam path in order to determine a change of the focal spot position. Furthermore, it is possible to use the current focal spot position by means of at least derive or determine a correction value from a time previously exactly determined focal spot position. Thus, it can be determined from a simple measurement whether a focal spot position change has occurred to the previously known focal spot position and, if so, how large is the focal spot position change. This reference measurement can preferably be recorded before and / or after detecting the transmission images associated with a 3D image of a measurement object. In particular, taking a reference image prior to preparing the fluoroscopic images for a CT-3D image ensures that after a focal position change has occurred, a large number of fluoroscopic images are not taken, which are later unusable or must be reworked consuming. If only a small change of the focal spot position is determined on the basis of the reference image, which enables a reasonable correction of the focal spot position based on the at least one correction value, then this at least one correction value can be used to evaluate the subsequently acquired fluoroscopic images in an evaluation

Correct focal spot position change. Furthermore, at least one correction value can be used in order to position the focal spot on the primary side by means of centering coils back to the reference position.

While in the prior art it is often assumed that removal of the focal spot from the detector is constant or at least not required to be corrected, it has been found that for high-resolution measurements, for example, the dimensions of measurement objects based on simple radiographic images or CT 3D images are determined, a precise knowledge of the removal of the focal spot position of the detector is necessary. The at least one correction value therefore includes an indication of the focal spot position with respect to a distance of the focal spot from the detector, for example a deviation indication. On the basis of the determined geometric parameters, a magnification ratio of the reference object in the reference image and correction image is determined and derived from the magnification ratio of the indication of the focal spot position with respect to a distance of the focal spot of the detector. The distance of the focal spot position from the detector is considered to be a distance of the focal spot position to the detector measured along a direction perpendicular to a detection plane of the detector representing an imaging plane of the acquired transmission images. Associating this direction with a z-direction of a Cartesian coordinate system, so includes in this embodiment of the at least one correction value is a correction for the z-coordinate of the focal spot position.

Advantageously, in the case of reproducible positioning in the reference position, the reference object is arranged adjacent to a boundary surface, preferably an exit surface or an exit window, of the x-ray source. Adjacent means here that the reference object as close as possible to a boundary surface, which comprises the outlet opening of the X-ray radiation from the X-ray source is arranged. An outlet opening in this case is an opening through which X-ray radiation can penetrate. Since the charged particles which generate the electromagnetic radiation on the target in the focal spot can propagate undisturbed only in a vacuum, the interior of the x-ray tube is enclosed by a vacuum-compatible vessel relative to the rest of the measuring arrangement. A boundary surface, from which the X-ray radiation used for imaging the measurement object emerges, is hereby regarded as the exit aperture. This means that the reference object is arranged as close as possible to the focal spot in the beam path. This offers the advantage that a change of the focal spot position results in a large change of the imaged reference object and thus of the determined geometrical parameters between the reference image and the correction image. A preferred measuring arrangement is thus preferably designed such that the reference object receptacle is mechanically connected to the X-ray source, preferably an exit aperture.

A reference object receptacle, in a preferred embodiment of the invention, has three pairs of cylindrical surfaces respectively aligned parallel to a respective central axis, the central axes intersecting at a point. Cylindrical surfaces are parallel to an axis when the cylinder axes are parallel to the central axis. The intersection of the central axes is chosen so that it lies on a main beam axis, ie on a central beam of a cone-shaped emanating from the focal point of electromagnetic radiation, which is perpendicular to the desired detection surface. This means that a straight line that passes through a desired focal spot position and the intersection of the central axes, perpendicular and usually centered on the detector. A reference object receptacle configured in this way ensures that a reference object, which comprises three spherical sector surfaces, which are designed for engagement with the cylindrical surfaces for centering the reference object in a centered manner, enables a simple, optimally reproducible arrangement of the reference object in the reference position. Preferably, therefore, the reference object in the reproducible placing in the reference position by means of a Three-point bearing against the X-ray source, preferably pressed against the exit aperture of the X-ray source. This means that the reference object receptacle is preferably incorporated in the boundary surface (which is generally an exit surface of the x-ray radiation and preferably includes an exit window) of the x-ray source or is at least mechanically connected thereto. A pressing against the boundary surface is therefore also present when the reference object holder is mechanically connected to the boundary surface of the X-ray source and the reference object is pressed against the reference object holder.

An arrangement of the reference object in the reference position and a removal from the beam path preferably takes place by means of a movable unit. This means that in a preferred embodiment of the measuring arrangement, the reference object is arranged on a movable unit with which the reference object can be moved either into the reference position or removed from the beam path.

In one embodiment, it is provided that the reference object is moved with a positioning system for the measurement object in the reference position. Preferably, in such a case, the movable unit is a positioning system for the measurement object. In such an embodiment, a slide on which measurement objects for computed tomography are arranged can be coupled to the reference object so that, when the slide is removed from the beam path, the reference object is moved to the reference position.

The reference object is preferably designed such that it comprises a geometric figure in a plane perpendicular to the main radiation direction when it is arranged in the reference position. This means that the geometric figure is formed in a plane which is parallel to a desired detection area.

This makes it easy to determine a change in the position of the geometric figure and / or an altered magnification of the geometric figure between the reference image and the correction image. From this, a focal spot position change in a plane parallel to the detection plane or correspondingly perpendicular to the detection plane can be determined accordingly.

To determine the change in the distance of the focal spot position from the detector, the magnification of the geometric figure is evaluated. In order to determine the magnification correctly, it may be advantageous to evaluate a distortion. Includes that For example, a reference object such as a circular geometric figure may result in distortion to an elliptical image in the reference image. If this distortion is taken into account, a diameter of the image of the geometric figure and, therefrom, an enlargement of the image of the geometric figure can be determined correctly.

In a preferred embodiment, the geometric parameters thus include an extent, in particular one or more diameters and / or a circumference, one or more excellent positions, in particular one or more geometric center position (s) of one or more figures of the reference object, and / or a distortion of an object representation. These parameters are particularly easy to determine based on a plurality of pixels. If a plurality of pixels are used to determine the parameters, an error associated with the determination is reduced.

In order to be able to record a plurality of pixels in a calculation of a parameter, it is advantageous if the reference object has one or more figures in a plane parallel to the image plane, i. perpendicular to a main beam axis or having a high symmetry. A particularly high symmetry has, for example, a circle. A particularly preferred embodiment of the invention therefore provides that the reference object comprises a conical bore whose cone angle is greater than an aperture angle of the radiation of the X-ray source. The reference object is arranged in the beam path when arranged in the reference position so that the conical bore widens in the propagation direction of the X-radiation. The reference object is preferably to be selected from a material as high as possible X-ray attenuation. It follows that a sharp circular edge of the borehole, which faces the X-ray source, is imaged on the detector. This means that the conical bore in the reference object is preferably to be designed so that the smaller circular opening has a diameter which is smaller than a diameter of the radiation cone at the location at which the smaller circular opening of the conical bore of the reference object in the reference position located. Furthermore, the size of the conical bore is preferably to be designed such that it preferably leads to a complete imaging on the detector when the reference position is selected.

Furthermore, instead of the conical bore, the reference object can be applied as a quasi 2D structure as a circular surface on a suitable support. The fat the layer and the choice of the material of the absorbent structure depend on the chosen magnification and the energy spectrum of the X-ray source. Such structures can be produced by various lithography processes with high accuracy.

The arrangement of the reference object in the reference position and a choice of the one or more figures of the reference object is preferably such that the highest possible utilization of the detection surface is achieved in imaging the one or more figures of the reference object. As a result, a plurality of pixels is available to determine the geometric parameters.

The arrangement of the reference object in the reference position or the execution of the conical bore in the reference object take place so that the cone axis of the bore is aligned parallel to a central imaging axis when the reference object is moved to the reproducible reference position. Preferably, the tapered bore is centered with respect to the spherical sector surfaces provided for centering the reference object in the reference object receptacle. This ensures that the circular bore is centered in the reference object shot.

It is desirable for a figure of the reference object to be centered as possible about a major axis of the central imaging axis of the x-ray radiation used for fluoroscopy, i. an axis of symmetry of an X-ray cone, is aligned. Since the focal spot position on the target is subject to small variations, it is advantageous if the reference object shot can be centered with respect to the known focal spot position, which was determined for example by means of a complex calibration. Therefore, in a preferred embodiment it is provided that the reference object receptacle is movably arranged with respect to the target on which the focal spot is produced, in order to align the reference object receptacle with respect to the known focal spot position, preferably centered. If the reference object and the reference position are known precisely, the reference image can be generated on the basis of a calculation and does not necessarily have to be detected as a fluoroscopy image by means of the detector. As a result, a reference measurement can be saved.

In a preferred embodiment it is provided that the method as a correction method is part of a measuring method in which the measuring arrangement is calibrated and the known focal spot position is determined for which the reference image is generated and correction images at temporal intervals, in particular before and / or after a measurement of an object, wherein the object is not in the beam path when recording one of the correction images and images taken during the measurement of the object are automatically corrected on the basis of the at least one correction value. Here, a plurality of transmission images, which are combined to form a CT-3D image, can be recorded between two executions of the correction process.

The invention can be used to verify a quality of centering and focusing. Furthermore, a correction of the scaling can be carried out in an evaluation software for determining features of the measurement object. In addition, a focal spot shape can be determined for adjustment and for control purposes.

The invention will be explained in more detail with reference to a drawing. Hereby show:

1 shows a greatly simplified schematic representation of a measuring arrangement for generating radiographic images of a measurement object;

Fig. 2 is a schematic representation of a beam path for imaging a

Measurement object;

Fig. 3 is a schematic plan view of an exit region of a

X-ray source with a reference object recording and a reference object removed from the beam path;

4 shows a schematic plan view of the exit region of the X-ray source according to FIG. 3, in which the reference object is moved into the reference position;

Fig. 5 is a detail of a sectional view taken along a line A-A through the

Reference object, the reference receptacle and a boundary wall of the X-ray source of Fig. 4;

6 shows a schematic sectional view of an exit region of the X-ray source with a reference object in the reference position; and

7 is a schematic flow diagram of a measurement and correction method. In Fig. 1, a measuring arrangement 1 for recording radiographic images is shown schematically. The relative size ratios of the displayed objects do not correspond to realistic size ratios. Rather, the individual elements are shown only to illustrate their principal function. In particular, the elements of an X-ray tube 2 are shown greatly enlarged. The measuring arrangement 1 comprises the X-ray tube 2, which is preferably designed as a microfocus X-ray tube, a slide 3, on which a measuring object 4 is arranged, a detector 5 and an evaluation unit 6 preferably designed as a computer. These individual components described so far are basic to a person skilled in the art are known and therefore not described here in detail, unless it is necessary for the invention. The X-ray source 2 comprises a particle-generating unit 7, which is preferably designed as a hairpin cathode, although a filament is shown here in the drawing. An acceleration diaphragm 8 arranged in front of the particle-generating unit 7 and having an electric potential difference with respect to the particle-generating unit 7 accelerates the charged particles emerging from the particle-generating unit 7, which is preferably a hairpin cathode, in particular thermally emitted electrons. A particle beam 9 produced in this way is accelerated onto a target 11 by means of a focusing unit 10. The focusing unit 10 may comprise different focusing elements, for example coils, baffles, etc. At a point of impact of the focused particle beam 9 on the target 11, a focal spot is formed at which X-radiation is generated by braking the charged particles, ie a negative acceleration.

A housing 13 of the X-ray tube 2 comprises an exit window 14, which is transparent to X-radiation. This can be made for example of beryllium, aluminum or diamond. The remainder of the housing 13 is designed to absorb x-ray radiation. A detector wall 5 facing housing wall 15 of the X-ray tube 2 thus acts together with the exit window 14 as an exit aperture. It will be apparent to one skilled in the art that in a particular embodiment of a microfocus X-ray tube, apertures may be provided within the X-ray tube that affect propagation of X-radiation within the X-ray tube.

The x-ray radiation generated at the focal spot 12 ideally emanates from the exit window 14 of the x-ray tube 2 in a cone-shaped beam 16. An axis of symmetry of the idealized radiation beam 16 is referred to below as the main propagation direction 17. The X-rays penetrate this Measuring object 4 and generates an image on the detector 5. The individual pixels in this case represent an extinction of the X-ray radiation through the measurement object. In order to produce a 3D image of the measurement object, a plurality of fluoroscopic images of the object 4 are recorded successively in succession, wherein the measurement object 4 is rotated by the slide 3 about an axis of rotation 18 lying in the plane of the drawing by a predetermined angle increment. The main propagation direction here preferably crosses the axis of rotation 18 at a 90 ° angle. The evaluation unit 6 generates a CT-3D image from a multiplicity of these fluoroscopic images of the measurement object 4 recorded at different angles by means of a back projection. The skilled person is known for this purpose suitable algorithms.

FIG. 2 schematically shows a beam path for an image of a measurement object for generating a transmission image. The same technical features are identified in all figures of the description with the same reference numerals. X-rays are propagated from a focal spot assumed to be 12 to a detector 5. For the sake of clarity, only a central ray 21 penetrating the measurement object 4 along the main propagation direction 17 and edge rays 22 are plotted, which map the boundary points 23 of the measurement object 4 on the detector 5 as boundary pixels 23 " A distance of the image delimiting points 23 'correspondingly indicates an image object extent, for example an image object height, which are correspondingly drawn with double arrows 24 and 25. Furthermore, a distance SD 26 from the focal spot 12 to the detector 5 in the beam path , a distance SO 27 from the focal spot 12 to the object 4 and a distance OD 28 from the measuring object 4 to the detector 5. A ratio of the image object height 25 to the object height 24 is referred to as magnification V. This is from the distance SD 26 of the focal spot 12 from the detector 5 and the distance SO of the focal spot 12 from the measuring object 4 dependent. The following relationship applies:

V = SD. SO

It can easily be seen that a change in the focal spot position causes both a change in the distance SD 26 and the distance SO 27. For example, if the focal spot position is removed by a distance D from the detector, the new magnification V is given by:

_V , _ SD + D SO + D ^'

It turns out that the magnification V becomes smaller. On the other hand, if the focal spot position drifts towards the detector 5, the enlargement of the image becomes larger. If the focal spot position travels in a direction parallel to the detection surface of the detector 5, for example by a distance H along an arrow 30, an image 4 'of the measurement object 4 shifts on the detector 5 in the opposite direction, as indicated by the arrow 31. A change in the enlargement ratio does not occur.

In order to generate high-resolution CT-3D images as possible or to be able to derive exact dimensions of objects from simple radiographic images with high precision, in particular in addition to a change of the focal spot position in a plane parallel to the detection or imaging plane, based on an occurred and determined change of the enlargement ratio, a change of the focal spot position perpendicular to the detection and imaging plane and to use as a correction value in an image evaluation (compensation of the focal spot position change).

If a focal position change is not detected and compensated, the resolution and accuracy of a 3D computed tomography image are adversely affected. Measurements examined on the measurement object are incorrectly determined in simple radiographic images as well as in 3D images.

In order to avoid a complex calibration for determining the exact focal spot position, a reference object receptacle 41 is arranged on the housing wall 15 adjacent to the exit window 14 in the measuring arrangement according to FIG. This is designed so that it can record a reference object 42 reproducibly in a reference object position. Preferably, the reference object holder 41 and the reference object 42 are formed such that a self-centering three-point bearing results in the reference position. The reference object 42 is preferably connected to a slide 43 with which the reference object 42 can be removed from the beam path or moved into the reference position. In Fig. 1, the reference object 42 is shown in dashed lines in the reference position. A schematically illustrated spring 44 is shown to indicate that the reference object is pressed in the reference position against the reference object holder 41, so that it reliably remains in the reference position until it is removed by the slider 43 again from the beam path. FIG. 3 schematically shows a plan view of the housing wall 15 and a reference object receptacle 41 arranged thereon. The reference object receptacle has a recess 45 in order to allow the X-ray radiation emerging from the exit window 14 to pass unhindered. A center 46 of the exit window 14 lies on a central beam of X-radiation when the focal spot position coincides with a desired focal spot position. It can be seen that a center 47 of the recess 45 is offset from the center 46 of the exit window 14. The center 47 of the recess 45 of the reference object receptacle 41 is preferably centered so as to be in the main radiation direction of X-radiation emitted from the focal spot position determined at the most recent calibration. In order to move the reference object receptacle 41 relative to the housing wall 15 and the exit window 14, 41 actuators 48 are arranged on the reference object recording, which each support a movable member 49 against a projection 50 of the housing wall 15.

The reference object receptacle 41 comprises three pairs of parallel aligned cylindrical surfaces 51. The cylindrical surfaces 51 are each aligned parallel to a central axis 52. The central axes 52 intersect at the center of the recess 45 of the reference object receptacle 41 and in each case preferably enclose an angle of 120 ° in pairs.

At the reference object holder 41, a guide 53 is provided, in which a slider 43 is guided. The slider 43 is fixed to a reference object 42 via a spring (not shown) that supports the reference object 42 against the reference object receptacle 41, i. towards the drawing plane, presses. The reference object is removed from the beam path in FIG. 3.

The reference object 42 is disc-shaped parallel to the plane of the drawing. It comprises a conical bore 54, wherein a cone opening is larger than an opening angle of an X-ray beam (compare 16 in Fig. 1), which emerges from the X-ray source. A borehole edge 55 with a smaller diameter than a larger borehole edge 56 of the conical bore 54 in this case faces the X-ray source. The circular borehole edge 55 represents a highly symmetrical figure which, in a reference position of the reference object 42 shown in FIG. 4, is aligned parallel to the detection plane and perpendicular to the main propagation direction which emerges perpendicularly from the plane of the drawing. Three ball sector surfaces attached to an underside of the reference object 42 are each on a pair of parallel to each other aligned cylindrical surfaces. This ensures that the reference object centered with respect to the center 47 of the recess 45 of the reference object holder 41 is centered. For this purpose, the spherical sector surfaces 57 are arranged on a circle concentric with the circular borehole edge 55, each at an angle of 120 °.

If a fluoroscopic image is taken of the reference object in the reference position, a reference image is obtained, provided that the focal spot position is known during the recording, for example due to a calibration performed shortly before. Taking at later times again a fluoroscopic image of the reference object in the reference position, we obtain a correction image. In the reference image and in the correction image, in each case identical geometric parameters, for example a diameter of the imaged circular drilling edge 55 'and its center, are determined. From a change in diameter, drift of the focal spot position in the direction of the main radiation direction, i. to the detector to or away from the detector. A shift of the center, however, indicates a change in the focal spot position parallel to the detection plane. Distortion of the image of the circular drilling edge 55 'indicates a tilt of the detector. The correction values determined in this case can be used in an evaluation of fluoroscopic images of a measurement object in order to correct the focal spot position. The determination of the geometric parameters and deriving of the correction values is generally carried out in the evaluation unit 6, in which the fluoroscopy images are also evaluated. In some embodiments, a separate evaluation unit may be provided. Furthermore, in an embodiment of the evaluation unit in software, an evaluation module for determining the correction values may be provided in an integrated evaluation software for an evaluation of the fluoroscopy images, including a correction with respect to a change in the focal spot position.

If deviations above tolerance limits are detected, it is advisable to carry out a renewed calibration of the measuring arrangement. Decisive here is that the reference object holder 41 with respect to the housing wall 15 of the X-ray tube between the creation of the reference image and the recording of correction images must not be changed. Only immediately after calibration of the focal spot position is the reference object shot centered with respect to the known focal spot position prior to generating the reference image. In Fig. 5 is a detail of a sectional view along the section line AA of Fig. 4 is shown schematically. It is easy to see how the ball sector surface 57 rests on the cylindrical surfaces 51 for forming the three-point bearing.

FIG. 6 shows a schematic sectional view of an exit region of a further embodiment of an x-ray tube 2 with a reference object 42 located in the reference position. Technically identical or similar features are provided with identical reference numerals as in the other figures. The reference object 42 abuts against a housing wall 15. Here, a three-point storage is realized. Two out of three ball sector surfaces 57 of the three-point bearing are recognizable. Good to see is also the conical bore 54, which widens in a main propagation direction 17 of a focal spot 12 outgoing beam 16. The beam 16 is bounded by the focal spot 12 facing the borehole edge 55. This is thus imaged on the detector (not shown). The dimensions of the conical bore 54 are to be selected such that the image of the borehole edge 55 fills as far as possible the entire detector surface.

FIG. 7 schematically shows a flow diagram of a method for determining fluoroscopic images, in particular CT-3D images, which comprises method steps of a correction method for compensating a focal position change. First, in a method step 101, a calibration of the measuring arrangement is carried out in order to determine a focal spot position exactly. Hereafter the focal spot position is known. In a preferred embodiment, in a method step 102, the reference object holder is centered with respect to the known focal spot position, so that a reference object arranged reproducibly in the reference position, which comprises a figure in a plane parallel to the detection plane and perpendicular to the main radiation axis, is preferably centered to the main radiation axis. This method step 102 may be omitted in other embodiments. For centering the reference object recording or hereafter, the reference object is moved to the reference position, as indicated in method step 103.

As indicated in a method step 104, a reference image is generated, which represents an image of the reference object, preferably a highly symmetrical figure in a plane parallel to the detection plane, with known focal spot position. From the reference image, in particular from a figure of the figure of the reference object In a plane perpendicular to the main propagation direction of the X-radiation, geometric parameters called reference parameters are determined. If the focal spot position is known and if the reference position and the reference object are also known exactly, then the reference image and / or the reference parameters can also be determined by calculation or with a simulation. In this case, the method step "moving the reference object to the reference position" 103 and the subsequent step 105 "removing the reference object from the reference position", which otherwise follows now, can be omitted. It should be noted that a recording of the reference image (or even later of a correction image) takes place in such a way that there is no further measurement object in the beam path.

Subsequently, measured objects can be measured. For this purpose, the measurement object is first of all positioned on the measurement object carrier 106. Subsequently, a fluoroscopic image is taken 107. A query 108 queries whether the measurement object has been completely measured. If this is not the case, then, according to a method step 109, the measurement object is rotated by a predefined angle increment in order to enable a back projection of a plurality of fluoroscopy images for creating a 3D image. Subsequently, the method is continued with method step 107, taking a fluoroscopic image.

If the query 108 indicates that the measurement object has been measured, then the measurement object is removed from the beam path 110 and an evaluation of the fluoroscopy images is carried out, optionally taking into account correction values for the focal spot position. For example, a back projection is performed to generate a CT-3D image of the measurement object. From this, for example, dimensions of the measurement object can be derived. Likewise, qualitative features of the measurement object can be analyzed.

In a query block 112, it is determined whether a measurement of another measurement object should take place. If this is not the case, the method is finished 113. If, however, another measurement object is to be measured, the reference object is moved to the reference position in a method step 114.

Subsequently, a fluoroscopic image is taken 115. The recorded fluoroscopic image is referred to as a correction image. On the basis of the image of the reference object in the correction image, geometric parameters which are referred to as correction parameters are determined 116. In a method step 117 the geometric parameters, ie the reference parameters and correction parameters, are compared with one another and from this at least one correction value for the focal spot position is derived.

The reference object is removed again from the beam path 118.

A query 119 determines whether one of the determined correction values exceeds a corresponding tolerance value. If this is the case, the user of the measuring system is proposed a new calibration 120. If the user follows this proposal, the method is continued with the method step 101, the calibration for determining the focal spot position. In some embodiments, it may be provided that the calibration is performed automatically.

If query 119 has shown that none of the correction values exceeds a tolerance value, then the method is continued with method step 106 "Positioning of the new measurement object on the measurement object carrier".

It will be clear to the person skilled in the art that the described measuring method and the correction method contained therein can be modified. For example, it may be provided to carry out a determination of a correction image even after a measurement of a measurement object has been completed in order to check whether the focal spot position has changed during the measurement of the measurement object. If such a change is detected, which is, for example, above further tolerance limits, an evaluation of the fluoroscopy images can be omitted. In another embodiment, it may be provided that a correction image is not taken after each object to be measured. Many other embodiments of the measuring method are possible. The advantage of the correction method described here is that the focal spot position can be reliably controlled with respect to changes with simple, quickly executed measurements, and additional correction values can be determined in order to compensate for a focal spot position change. In this case, it is of decisive advantage if the reference object is arranged as close as possible to the focal spot position. LIST OF REFERENCE NUMBERS

1 measuring arrangement

2 x-ray tube

3 slides

4 measuring object

5 detector

6 evaluation unit

7 particle generation unit

8 accelerator

9 particle beam

10 focusing unit

11 target

12 focal spot

13 Housing of the X-ray tube

14 exit window

15 housing wall

16 beams

17 main propagation direction

18 axis of rotation

21 central beam

22 edge beam 3 boundary point 3 ¹ boundary pixel 4 object height 5 image object height 6 distance focal spot (source) - detector 7 distance focal spot (source) - object 8 distance object detector 0 arrow 1 further arrow 1 reference object holder 2 reference object 3 slider 4 spring 5 recess 6 center of exit window Center of the recess Actuator Moving element Projection Cylinder surface Center axis Guide Conical hole Drill hole edge (smaller diameter) Drill hole edge (smaller diameter) Drill hole edge (larger diameter) Spherical sector surface Calibrate - Determine focal spot position Center the reference object pickup Move the reference object to the reference position Generate a reference image and determine of geometric reference parameters Removal of the reference object from the beam path Positioning a measurement object on the slide Recording a fluoroscopy image Query: Measure the measurement object completely? Rotate the measurement object by a predetermined angle increment Remove the measurement object from the beam path Evaluate the fluoroscopy image (s), taking account of correction values for the focal spot position if necessary Query: Measuring another measurement object? End Moving the reference object to the reference position Recording a correction image Determining geometric correction parameters Comparing reference and correction parameters and deriving at least one correction value for the focal spot position Removing the reference object from the beam path Query: Is a correction value greater than a corresponding tolerance value? Suggestion: new calibration

Claims

claims 1. A method for correcting a focal spot position of an X-ray source of a measuring arrangement (1) for generating radiographic images of a measurement object (4), in particular radiographic images for generating a three-dimensional image of the measurement object (4) by means of backprojection, wherein the measurement arrangement (1) the X-ray source and a detector (5) comprising the steps of: Generating a reference image of a reference object (42), which in a Beam path of the measuring arrangement (1) can be arranged reproducibly in a reference position, at a known focal spot position, reproducing the reference object (42) in the reference position in the beam path, Taking a correction image of the reference object (42), Determine geometric parameters in the reference image and in the Correction picture and Deriving at least one correction value for the focal spot position based on the determined parameters, wherein the at least one correction value comprises an indication of the focal spot position with respect to a distance of the focal spot from the detector (5), wherein based on the geometric parameters Magnification ratio of the reference object (42) in the reference image and Correction image is determined and derived from the magnification ratio of the indication of the focal spot position with respect to a distance of the focal spot of the detector. 2. The method according to claim 1, characterized in that the reference object (42) in the reproducing arrangement in the reference position adjacent to a boundary surface, in particular an exit aperture or an exit window (14), the X-ray source is arranged. 3. The method according to claim 1 or 2, characterized in that the reference object (42) is pressed in the reproducing arrangement in the reference position by means of a three-point bearing against the X-ray source, preferably the boundary surface of the X-ray source. 4. The method according to any one of the preceding claims, characterized in that the reference object is moved with a positioning system for the measurement object (4) in the reference position. 5. The method according to any one of the preceding claims, characterized in that the geometric parameters include an expansion, in particular a diameter, a circumference, an excellent position, in particular a center position of a geometric center, and / or a distortion of the object representation. 6. The method according to any one of the preceding claims, characterized in that the reference image is calculated based on a knowledge of the reference object (42) and the reference position for the known focal spot position. 7. The method according to any one of the preceding claims, characterized in that it is part of a measuring method as a correction method in which the measuring arrangement (1) is calibrated and the known focal spot position is determined for the reference image is generated and correction images at intervals, in particular before and / or after a measurement of a measurement object (4) are recorded, wherein the object is not in the beam path when recording one of the correction images and images taken during measurement of the measurement object (4) are automatically corrected on the basis of the at least one correction value. 8. Measuring arrangement (1) for generating radiographic images of a measurement object (4), in particular radiographic images for generating a three-dimensional image of the measurement object (4) by means of backprojection, comprising an X-ray source with a focal spot (12) on which an electromagnetic used for imaging Radiation is generated, and a detector (5) for detecting the radiographic images, characterized in that in addition a reference object recording (41) for reproducibly recording a reference object (42) in a reference position in the beam path and an evaluation unit for determining geometric parameters in a reference image, representing an image of the reference object (42) in the reference position at a known focal spot position, and a Correction image, which is an image of the reference object (42) in the reference position at a focal spot position to be corrected, and for deriving at least one correction value for the focal spot position, wherein the evaluation unit is designed as at least one correction value an indication of the focal spot position with respect to a distance of the focal spot from the detector (5), wherein based on the determined geometric parameters, a magnification ratio of the reference object (42) in the reference image and correction image determined and derived from the magnification ratio, the indication of the focal spot position with respect to a distance of the focal spot of the detector. 9. Measuring arrangement (1) according to claim 8, characterized in that the reference object receptacle (41) is mechanically connected to the X-ray source, preferably a boundary surface, in particular an exit aperture or an exit window (14). 10. measuring arrangement (1) according to claim 8 or 9, characterized in that the reference object receptacle (41) comprises three pairs of each to a corresponding central axis (52) parallel aligned cylindrical surfaces (51), wherein the central axes (52) are in one point to cut. 11. Measuring arrangement (1) according to one of claims 8 to 10, characterized in that the reference object (42) comprises three ball sector surfaces (57) adapted for engagement with the cylindrical surfaces (51) for centered reproducible positioning of the reference object (42) are. 12. Measuring arrangement (1) according to one of claims 8 to 11, characterized in that the reference object (42) is arranged on a movable unit with which the reference object (42) can be arranged either in the reference position or removed from the beam path. 13. Measuring arrangement (1) according to one of claims 8 to 12, characterized in that the movable unit is a positioning system for the measuring object (4). 14. Measuring arrangement (1) according to one of claims 8 to 13, characterized in that the geometric parameters include an expansion, in particular a diameter, a circumference, an excellent position, in particular a center position of a geometric center, and / or a distortion of the object representation , 15. Measuring arrangement (1) according to one of claims 8 to 14, characterized in that the reference object (42) comprises a conical bore (54) whose cone angle is greater than an opening angle of the radiation of the X-ray source. 16. Measuring arrangement (1) according to claim 15, characterized in that the conical bore (54) in the reference object (42) is arranged so that its cone axis is aligned parallel to a main propagation direction (17) of the electromagnetic radiation when the reference object ( 42) is moved to the reproducible reference position. 17. Measuring arrangement (1) according to claim 15 or 16, characterized in that the conical bore (54) centered with respect to the ball sector surfaces (57). 18. Measuring arrangement (1) according to one of claims 8 to 17, characterized in that the reference object receptacle (41) with respect to a target on which the focal spot (12) is generated, is movably arranged to the reference object recording (41) with respect to the known focal spot position , preferably centered, to align.