EP2637564A1

EP2637564A1 - Measurement arrangement for a computer tomograph

Info

Publication number: EP2637564A1
Application number: EP11772959.0A
Authority: EP
Inventors: Daniel Weiss
Original assignee: Carl Zeiss Industrielle Messtechnik GmbH
Current assignee: Carl Zeiss Industrielle Messtechnik GmbH
Priority date: 2010-11-10
Filing date: 2011-10-18
Publication date: 2013-09-18
Also published as: US20130230150A1; WO2012062543A1; DE102010050949A1

Abstract

The invention relates to a method for operating a measurement arrangement for a computer tomograph, which measurement arrangement has a radiation source (2) of invasive radiation and a flat image detector (3; 13) with scintillation layer (15) and a photocell array of photocells (4) for detection of radiation from the radiation source (2), wherein a calibration object (16) is arranged between the radiation source (2) and the flat image detector (3; 13), and at least one radiation image of the calibration object (16) is recorded with the flat image detector (3; 13), and, from known dimensions of the calibration object (16) and from the at least one radiation image, a distortion error, which occurred as a result of a distortion of the flat image detector (3; 13), is determined as a function of the location in the photocell array (device 6). In particular, a radiation image of an object to be measured, recorded with the flat image detector (3; 13), is corrected on the basis of the determined distortion error.

Description

Measuring arrangement for a computer tomograph

The invention relates to a method for operating a measuring arrangement for a computer tomograph, wherein the measuring arrangement comprises a radiation source of invasive radiation and a flat image detector. The invention also relates to such a measuring arrangement. The invention relates to the field of image formation,

in particular for the measurement or determination of coordinates of a measurement object, with a radiation source generating invasive radiation, i. Radiation (eg

X-radiation or particle radiation), which penetrates a measuring object, thereby weakened by absorption and / or scattering and impinges on an image forming device.

Furthermore, the invention relates to the field of computer tomographs, in particular those computer tomographs, in which a cone-shaped radiation beam propagates from the radiation source (for example a microfocus X-ray tube) through the measurement object onto the imaging device. In order to measure the measurement object, such radiation beams are transmitted from different directions one after the other through the measurement object, usually by rotating the measurement object about an axis of rotation. The computer tomograph calculates a three-dimensional image of the measurement object by means of so-called reconstruction (for example, filtered rear projection).

Previously, radiographic images, which are the result of irradiation of a measurement object with invasive radiation, were made visible with the aid of a so-called image intensifier. In this case, the radiation to be detected strikes a combination of a scintillator and a photocathode, which generates photoelectrons, which are imaged by means of an electric field on a screen or a field of photocells. By converting the incident radiation into visible light first and then into photoelectrons, a scattering occurs which worsens the sharpness and resolution of the images. Also a distortion, i. systematic deviation of the location in the generated image from the location that would have been reached by a straight path from the radiation source to the screen is observed. The severity of the distortion usually varies significantly with the location on the screen or photocell field, so that depending on the relative position of the measurement object to the detector array different dimensions of the measurement object in the

Radiographic image revealed. Therefore, it is common to calibrate the detector assembly and correct the distortion. In contrast, the invasive radiation according to the present invention meets after

Passing through the measurement object to a flat image detector. The flat image detector has, in particular in a known manner, a scintillation layer, which is preferably carried by a photocell array of photocells for detecting the radiation. At a minimum, the scintillation layer is immediately adjacent to the photocell array, which is also referred to as a flat image detector, and is preferably adjacent to the photocell array.

In contrast to the above image intensifiers, the path the light photons produced by the scintillator material (eg, cesium iodide) travel to the associated photocell is very small, and preferably extends exclusively through the material of the scintillator and the adjacent material the photocell. The

Scintillation layer in the flat image detector can, for. B. from a variety of

Scintillator bodies (eg, needle-shaped bodies). In this case, many scintillator bodies are arranged on each photocell whose diameter is in directions transverse to the direction of radiation in each case 5 to 10 μηι. In contrast, the edge length of the associated photocell and thus the pixel size of the generated image z. B. 100 to 150 μηι. Such flat image detectors are generally considered to be free from distortion errors.

However, it has been found by experiments which the inventor has evaluated that even in a measuring device with flat image detector of the type described above, a distortion occurs in which the location in the recorded transmission image of the expected in radiation propagation and radiation detection in the straight line location deviates, with the severity of the distortion of the place in the

recorded radiographic image depends. The severity of the distortion in the evaluated case is up to 20 μηι and varies over the surface of the image detector in a manner that lead to fluctuations of measured lengths in the measurement object, which may be up to 5 μηι, i. depending on the relative position of the DUT and the

Image detector can vary the measuring length by up to 5 μηι.

It is an object of the present invention to operate a measuring arrangement of the type mentioned in such a way that the most accurate measurement results, especially in the determination of coordinates of a measured object and dimensions of a Object to be achieved. It is a further object of the present invention to provide a corresponding measuring arrangement.

First, the invention is based on the finding that the observed

Although distortion is significantly smaller than the usual edge length of a pixel of the flat image detector (the edge length is typically 100 to 150 μηι, but also up to 400 μηι). However, it is still important to avoid errors in the sub-pixel area, especially in the cone-beam computed tomography. In particular, the various radiographic images taken for reconstruction as a whole contribute to the three-dimensional image computed by the computed tomography scanner. Within the individual radiographic images are z. B. information about the course of outer surfaces and material boundaries of the measurement object, which is in the preferred scope of the invention, a handcrafted or industrially manufactured article. Although a single point of the measurement object can not be located with sub-pixel resolution, this is e.g. by local interpolation or compensation calculation for structures of the measurement object possible. Therefore, a systematic error due to distortion already has an effect on any preprocessing of individual radiographic images (eg in order to determine structures by said interpolation or compensation calculation) and has an overall effect on the three-dimensional image which is calculated from the individual radiographic images by reconstruction , In particular, based on the three-dimensional image, the said interpolation or

Compensation calculation to be carried out.

The fact that the severity of the distortion is in the sub-pixel range is a possible reason why the distortion has not been detected, especially as CT scanners have other systematic errors, e.g. due to the effect of the so-called beam hardening, due to artefacts during the reconstruction (eg so-called Feldkamp artifacts) and

Misalignment of the axis of rotation about which the object to be measured is rotated

To obtain radiographic images at different transmission direction.

In fact, the inventor could not determine the effect of the distortion until all other possible causes had been excluded. The geometric distortion could only be analyzed by a plurality of three-dimensional

Total images are determined, the same measurement object in different

Depict relative positions to the detector. In particular, a measuring object with was exactly known dimensions required, for. B. a measurement object with a plurality of characteristic structures whose relative positions are known (in particular an arrangement with a plurality of spheres, the ball centers are located in known locations).

There are several possible reasons for sub-pixel distortion. One possible cause is a mechanical bending of the flat image detector. Also, the internal structure of the scintillator material could be responsible for the distortion. Furthermore, a stress caused by the scintillator structures could be the cause of the distortion. Also combinations of the mentioned possible

Causes are the reason for the distortion in question.

It is now proposed to determine the distortion as a function of the location in the photocell field of the flat image detector. For this purpose, a calibration object with known dimensions is used.

In particular, it is proposed: A method of operating a measuring device for a computed tomography device comprising a radiation source of invasive radiation and a scanned image and a photocell array of photocells for detecting radiation from the radiation source, wherein a calibration object between the radiation source and the Flat image detector is arranged and the flat image detector at least one transmission image of the calibration object is taken and wherein from known dimensions of the calibration and the at least one transmission image distortion error, which has arisen due to distortion of the flat image detector, as a function of the location in the photocell field is determined. Preferably, a transmission image is taken by the flat image detector of an object to be measured and under

Correction of the determined distortion error corrected.

The calibration object is preferably arranged directly adjacent to the flat image detector. In particular, the calibration object can be arranged such that structures of the calibration object extend along the flat image detector and in the at least one transmission image of the image generated by the flat image detector

Kalibrierobjekts appear as juxtaposed and / or partially overlapping pattern structure. In this case, the calibration object may preferably have an arrangement with a plurality of separate structures whose size and their position relative to each other are known. The size and relative position is used in determining the distortion error. The use of a plurality of structures which generate structured images distributed over the area of the photocell field makes it possible to determine the distortion at least at correspondingly many places in the photocell field. In addition, it can be assumed that the intensity of the distortion does not vary abruptly over the course of the surface of the photocell field, but varies constantly. Therefore, it is possible to determine the distortion not only at locations of the photocell array where structural images have been taken, but also at locations lying between the recorded structural images. For example, you can interpolate,

For example, by linear interpolation, or it can be a model function adjusted by compensation calculation. As a result, e.g. a

Two-dimensional map of the distortion, where each location on the map corresponds to a location in the photocell field and thus a location. Preferably, at least for each photocell of the field, and thus for each pixel of a transmission image, there exists a value of the distortion error. The distortion error is a two-dimensional vector and consists of an x and a y component. In particular, that can

Calibration object are arranged in different position and / or orientation relative to the flat image detector and in each case at least one transmission image recorded and evaluated. From the radiographs of

different relative positions and relative orientations can the

Distortion errors are detected with higher resolution. In particular, this gives a higher number of nodes for interpolation or

Compensation calculation that determines the distortion error for any location in the photocell field.

A calibration object with a plurality of structures is understood in particular to mean an arrangement of a plurality of objects whose relative position is fixed. Preferred is an arrangement having a plurality of objects which are spaced apart and which have a first attenuation coefficient. Of the

Attenuation coefficient is the material coefficient that describes the weakening of the invasive radiation passing through the material. The objects are, as mentioned, spaced from each other, wherein the gap may be wholly or partially filled by material having a different attenuation coefficient. For example, the first attenuation coefficient (the coefficient of the objects) can be very large, so that the absorbed by the objects passing radiation to a very high percentage and / or scattered in directions other than in the direction of the photocell field. For example, the objects are steel balls. By contrast, it is preferred that the material (eg, glass) in the interstices between the objects have a weakening coefficient that differs significantly from the weakening coefficient of the material of the objects. Glass-ceramic is well suited because its shape remains stable over a long time and with temperature changes. The result is a picture of the entire arrangement of objects, a projection image with alternating dark and light areas, the outer edges of the dark areas correspond to the outer contours of the objects with high attenuation coefficient.

In a concrete embodiment of such an arrangement, the objects may be balls, which are preferably distributed according to a regular grid in rows and columns, each with equal distances between the balls. In this case, preferably both the row direction and the column direction extend parallel to the surface of the photocell field. The projection images of spheres generated by a cone-like diverging radiation beam are generally ellipses and only in special cases circles which in turn are a special case of ellipses. From the outer contours of the corresponding structural images (i.e., ellipses), the center of the ellipse can be easily determined. The term "ellipse" is therefore understood in this description not the outline, but the area with elliptical outline.

On the other hand, balls make it possible, in a simple way, e.g. through a

Coordinate measuring device tactile, i. touching the surface, touches to determine the relative positions of the balls in the ball assembly.

In particular, the positions of the ball centers of the arrangement can thus be determined in a coordinate system which is assigned to the arrangement. These

Determination succeeds with an accuracy of less than a micrometer. In addition, balls with very little roundness error can be made, i. the spherical surface is at a constant distance from the surface with high accuracy

Sphere. Therefore, after correction due to the fact that the projected sphere center does not coincide with the center of the ellipse, a comparison can be made which gives the distortion error with high precision. Preferably, not only for the separate measurement of such an arrangement of calibration balls, but also for any other calibration objects

Coordinate measuring device used, which contacts the surface of objects of the arrangement in order to determine therefrom the required information about the shape, relative position and arrangement of the objects. However, there are other methods of

Obtaining the coordinates of the calibration possible. For example, can also be one

Coordinate measuring device can be used with an optically scanning sensor.

Instead of spheres, another type of object can be assembled into a calibration arrangement. It is also possible to assemble various objects into a calibration arrangement. Besides spheres, e.g. Cylinder, hollow cylinder, circular discs and / or checkered structures suitable. In the checkerboard-like structures, areas of high and low alternate according to the arrangement of white and black fields in the chessboard

Attenuation coefficients. In the radiation image of the chessboard, the locations where four fields meet can be determined robustly and with subpixel accuracy. In particular, in various calibration arrangements, the number, the material and / or the size of the objects used can also be varied, as well as their relative arrangement and possibly relative orientation.

In particular, the objects of the calibration arrangement need not, as is the case with spheres, have a spatial extension in the direction of propagation direction of the invasive radiation which is of the same order of magnitude as that transverse to

Radiation propagation direction extending width and height of the objects. Rather, a suitable calibration arrangement may be an array having a plurality of flat objects. For example, For example, such calibration objects can consist of a material layer which is arranged on the surface of a carrier material. The material of the material layer and the material of the carrier have different

Attenuation coefficients for invasive radiation. In particular, it is the

Material layer structured such that the individual objects of the arrangement arise. A suitable carrier for such a structured material layer is

in particular a plate-shaped carrier. Such a plate-shaped carrier having a planar surface on which the structured material layer is applied can be arranged very close to the flat-panel image detector. Such an arrangement in which the patterned material layer is disposed on the side of the disc-shaped carrier facing the flat-panel image detector is preferable. For example, while the structured Material layer contact the surface of the flat image detector or - if a touch could damage the structured material layer - is preferably provided a small distance of at least a hundredth of a millimeter and at most one millimeter between the structured material layer and the surface of the flat image detector.

A structured material layer is preferably produced as a calibration arrangement on a plate-shaped carrier similar to manufacturing methods for producing structured semiconductor components for microelectronic components by carrying out at least one lithographic process. Using at least one mask corresponding at least to each of a part of the outer contours of an object, and using radiation representing the shape of the respective mask on the surface of the support material, the structures of the structured material layer are defined. A suitable material for the structured material layer is e.g. Chromium, which can be applied to a plate-shaped support made of glass or glass ceramic. For example, Suitable structures are crosses from the material. For each cross can then be analogous to the determination of the projected

Ball center at the o.g. spherical calibration object the center of the cross or the center of the projection of the cross can be determined. All other steps described in this description with regard to an arrangement of balls for determining the distortion error can also be carried out in a manner analogous to a calibration object having a plurality of crosses.

A preferred embodiment for a cross as a calibration object and for a calibration arrangement with a plurality of crosses, which is arranged adjacent to a flat image detector, will be described in the description of the figures.

In particular, the calibration object or the calibration arrangement on a

Moving means may be mounted, which is arranged and configured such that the moving means can move the calibration object relative to the flat image detector. In this way, the calibration object can be brought into different relative positions to the flat image detector.

The scope of the invention optionally includes the correction of the by

Distortion caused error in at least one transmission image of a measured object. By performing the correction, therefore, the distortion is corrected. When correcting a transmission image, that of one to be measured

Specifically, the entire transmission image can be corrected according to the distortion error, z. B. every pixel of the

Radiograph.

When correcting but not necessarily the entire radiographic image

corrected according to the distortion error. For example, there is

Applications in which only individual points can be identified in the radiograph of a measurement object. In this case, it is preferable to correct only the individual points corresponding to the distortion error. For example, a measurement object having an arrangement of a plurality of balls can be used not only as a calibration object for determining the distortion error, but also for adjusting the entire measurement arrangement.

In particular, (as described in DE 10 2005 033 187 A1) during the calibration of a measuring arrangement which generates images of test objects by means of invasive radiation, for. B. a computed tomography (CT) -Messanordnung, at least one transmission image of a calibration object are recorded. In this measuring arrangement, it may be z. B. to act the same measurement arrangement and optionally also the same calibration object, with the / the distortion error of the flat image detector is determined. The

Calibration object has known dimensions, wherein by evaluating the at least one radiographic image geometry parameters of a geometric model that describes a geometry of the measuring arrangement are determined. In a preferred embodiment of the method, the calibration object has at least one calibrating element with a spherical surface (spherical surface) and / or a calibrating element whose surface forms at least part of a spherical surface. The

Calibration element is imaged as an ellipse in the radiographic image and the

The center of the spherical projection is determined by determining the ellipse center point from the radiographic image and optionally also correcting a deviation from the position of the projected spherical center due to the projection (eg as described below in the description of the figures). Geometric information about the center in the radiograph, so z. As the position, the relative position and / or the distance are then used in determining the geometry parameters of the model.

According to the invention, the center point is a point passing through

Correction of distortion error was obtained. To determine the Geometry parameters therefore only single point, here midpoints, must be determined and corrected, not the entire radiographic image.

Generally speaking, at least one transmission image of the calibration object or another measurement object can be recorded by the flat image detector, individual points each corresponding to a structure of the calibration object or measurement object can be determined in the transmission image, and the distortion error can only be corrected with respect to these individual points so that an array of corrected individual points is formed.

The flat image detector, the calibration object or measurement object and a radiation source of the invasive radiation are part of the measuring arrangement. In particular, the arrangement of corrected individual points can then be used to determine geometry parameters of a geometric model that describes a geometry of the measurement arrangement.

Preferably, during calibration, as already described, a map of

Distortion error determined. This also means that for each location of the photocell field the information about the distortion error and / or the

Correspondingly to be executed correction of the distortion error is present. The

Information about the distortion error may, for example, be in the form of a vector, i. a length and a direction. In this case, the vector from the respective location in the radiographic image points to the location which the pixel would have assumed without the effect of the distortion. However, for performing the correction, the inverse distortion error vector is also advantageous, i. a vector of equal length and opposite orientation, which points from the location of a pixel of an image to be corrected to that location in the transmission image whose associated image value is to be included in the corrected image. Here, the inverse

Distortion error vector usually point to a point in the coordinate system of the recorded radiographic image, which lies between known pixels (each pixel corresponds to a pixel, for example). The image value (eg gray value), which is adopted in the corrected image, is then preferably determined by interpolation from the neighboring pixels in the transmission image. In the case of a bilinear interpolation, in the two coordinate directions of the coordinate system of the radiographic image, in each case the next adjacent pixels are used for the interpolation. In the case of a bicubic interpolation are also not immediate neighboring pixels are included in the interpolation, ie an environment with 4 x 4 pixels. In any case, an image value weighted according to the distance of the location from the neighboring pixels is determined by the interpolation. The method of interpolation takes into account the fact that the distortion error lies in the sub-pixel range.

More generally, to correct the distortion error for a pixel of the corrected image, a location corresponding to the distortion error is determined in a captured radiographic image, and the associated image value (e.g., gray scale) of the location is determined by interpolating the image values from adjacent landmarks in the radiographic image. The interpolation points are, in particular, the locations of the pixels of the radiographic image (for example the center of the respective pixel) assumed to be punctiform.

The determination of the distortion, in particular the determination of the above-mentioned distortion map, and / or the correction of the distortion can in particular be carried out by a computer which executes a corresponding computer program. The scope of the invention therefore includes a computer program comprising computer-executable steps for determining distortion errors from image data corresponding to at least one transmission image and information about structures of a calibration object imaged in the at least one transmission image will be described in this description.

Furthermore, the scope of the invention includes a computer program that of a

Computer executable steps with which the image data of at least one transmission image are corrected with respect to the distortion error in a manner described in this specification.

In addition, the scope of the invention includes a measuring arrangement for a

Computed tomography apparatus comprising a radiation source of invasive radiation and a flat image detector with a scintillation layer and a photocell array of photocells for detecting radiation from the radiation source. The measuring arrangement further comprises an error detection device connected to the flat image detector and configured from at least one transmission image of the calibration object between the radiation source and the flat image detector is arranged, and from known dimensions of the calibration object a

Detecting distortion errors due to distortion of the flat image detector, the distortion error being determined as a function of the location in the photocell field.

Embodiments of the measuring arrangement will become apparent from the description of the method and its embodiments. In particular, the measuring arrangement can also have an error correction device, which is connected to the error determination device and is designed, one of an object to be measured

to correct the received transmission image taking into account the distortion error determined by the error detection device.

Furthermore, the measuring arrangement can also be the calibration object and / or a tactile

Coordinate measuring device, which is configured to key the calibration object and to determine the size and location of structures of the calibration object and provide for the determination of the distortion error.

The error-determining device can also be configured from structural images, which have emerged from the separate structures in the at least one transmission image, at least in each case (in particular for each structure) a location in the

To determine radiographic image, so that a first arrangement of the determined locations is obtained, and to compare the first arrangement with the calibration object corresponding second arrangement of locations that correspond to the determined locations, and from deviations of the two arrangements the distortion error as a function of the location in the photocell field and / or in the radiographic image to determine. The function may be, in particular, the above-mentioned distortion map, e.g. A map of the above-mentioned distortion vectors or the above-mentioned inverse distortion vectors.

The second arrangement, which corresponds to the calibration object, can be obtained in particular by a mathematical calculation. In this case, the calculation corresponds in particular to a possible projection of the calibration object into the image plane of the radiographic image. In this embodiment, therefore, the first arrangement is obtained inter alia by a projection by means of invasive radiation and the second arrangement by a calculation of a projection. Information about the geometry of the calibration object, for example information about the relative position, is included in the calculation of ball centers, which were obtained in particular by means of the tactile coordinate measuring machine.

Furthermore, for the comparison of the two arrangements, the error determination device can be designed to adapt the position and orientation of the locations in the second arrangement (in particular as best as possible, for example by an optimization algorithm) to the position and orientation of the locations in the first arrangement. This serves to take into account different possible relative positions and orientations of the calibration object and the flat image detector. The locations in the second array may be determined by determining the best possible second arrangement, e.g. B. by varying the calculated projection corresponding to different relative positions and - orientations of the calibration object and the detector can be obtained. After the adjustment, remaining deviations of the locations in the first array from corresponding locations in the second array are identified as distortion errors, since the remaining deviation due to the adjustment is no longer false

assumed relative positions or orientations of the calibration object and the detector can be returned.

Embodiments of the invention will now be described with reference to the accompanying drawings. The individual figures of the drawing show:

Fig. 1 shows schematically the geometry of a measuring arrangement with a

X-ray source, a measurement object and a two-dimensionally spatially resolving detector device,

Fig. 2 shows schematically a flat-panel image detector with a scintillation layer and a

Photocell field behind the scintillation layer, wherein in the radiation direction in front of the scintillation layer, a calibration arrangement is arranged, with a plurality of regularly arranged in rows and columns spheres (for example steel balls), which are held for example by a glass matrix,

Fig. 3 shows schematically the image of a ball on an elliptical structure image of

Ball caused by a conical radiation beam,

FIG. 4 shows the representation of a noisy outline obtained from a transmission image section and of the compensating ellipse, with representation of the middle support, FIG.

Fig. 5 is a highly simplified example of an arrangement of a

Radiation image determined corrected ellipse centers, where in the Representation also shown a best possible adapted to the arrangement second arrangement of ball centers is shown,

6 is an illustration for explaining that the projected sphere center does not coincide with the center of the ellipse obtained by the projection;

Fig. 7 is a cross which is formed by a thin layer of material that can be applied to a plate-shaped support with a flat surface and

Fig. 8 shows an arrangement with a plate-shaped carrier, on the back of a

Plural of crosses is applied, and with an adjacent flat image detector.

1 has a measuring object 1 which is arranged in the rectilinear beam path between a radiation source 2, in particular an X-ray radiation source (for example a microfocus X-ray tube), and a detection device 3. A beam is denoted by S, a midpoint beam by MS. Of the

Midpoint beam impinges on the detection device 3 at point Z. The

Detection device 3 has a multiplicity of detection elements 4 (eg photocells with scintillation material lying in front of it in the radiation direction), so that a spatially resolved detection of radiation is possible. The detection signals of

Detection elements 4 are fed to a device 6 which determines a transmission image of the measurement object 1 in each case in a given rotational position of the measurement object 1. The measuring object 1 is combined with a rotating device 7, for example a turntable. The axis of rotation of the rotating device 7 is designated T. In addition, a positioning device 5 with holding jaws 8, 9 is optionally provided, which makes it possible to position the measuring object 1 relative to the rotating device. The device 6 is z. As a computer, preferably also digital image data at least one

Radiographic image of the device 3 evaluates and / or processed by the

Determine and / or correct distortion errors.

Preferably, the positioning device 5 is designed such that it separately enables the positioning of the measuring object 1 in the direction of three coordinate axes x, y, z of a Cartesian coordinate system. Alternatively or additionally, the

Positioning device 5 enable further positioning movements, e.g.

Rotational movements about an axis of rotation, not with the axis of rotation T of

Rotary device 7 coincides. In this way, the positioning device 5 can also be used to position a calibration object (not shown in FIG. 1) immediately in front of the detection device 3. Fig. 2 shows a flat image detector 13 having a scintillation layer 15 and a layer 14 arranged behind it, which is formed by a field of photocells. The layer 14 preferably carries the layer 15 which is not a homogeneous layer but consists for example of a plurality of needle-shaped scintillating bodies.

Instead, however, a homogeneous layer may be used, e.g. B. with

Scintillation material (such as gadolinium oxide), for example, in a

Embedded polymer matrix.

FIG. 2 also shows in the direction of radiation (a beam of invasive radiation is indicated by an arrow) in front of the scintillation layer 15 a calibration object 16 which has a regular grid with balls arranged in rows and columns, some of which are designated by reference numeral 17. As the beam of invasive radiation indicates, one of the spheres 17 is struck by the beam, the intensity of the beam is weakened and, as a result, an ellipsoidal image 18 is created in the photocell array 14 behind the scintillation layer 15. The flat image detector 13 shown in FIG. 2 is, for example, the detector 3 shown in the arrangement of FIG. 1.

Fig. 3 shows a radiation source 2 for invasive radiation (for example the

1), one of the spheres 17 from the arrangement according to FIG. 2, and the elliptical radiation image 18 which arises in an image 19 which is generated by the photocell field 14. Since the midpoint beam, which of the

Radiation source 2 radiates from the spherical center MK, not perpendicular to the detection surface of the photocell field, the Figure 18 is elliptical. The spherical center MK projected by the center beam in the elliptical image 18 is designated by the reference character MK _P.

Fig. 4 shows a cutout area 20 of a radiographic image, for. 2 with a plurality of calibration balls, the region 20 containing a noisy outline 21 of one of the balls. As can be seen, the contour line 21 shows a serrated pattern with a plurality of outwardly directed tips 22 and with a plurality of inwardly directed tips 23, on each of which a support point is located. Further interpolation points lie between the tips 22, 23. Each interpolation point corresponds to a point determined from the radiographic image on the edge of the image of the sphere. ME denotes the ellipse center of z. B. by Equalization calculation fitted ellipse line 28. The storage of the support points of the ellipse line 28 is shown greatly enlarged in the figure.

In the same way, the centers of the ellipses of the other spherical images of the calibration arrangement shown in FIG. 2 can be determined. Furthermore, in a further step, the overall arrangement of the ellipse center points determined and corrected from the radiographic image is compared with the known arrangement of the ball centers of the calibration object.

Generally speaking, the calibration object has a plurality of separate structures whose size and location relative to each other are known, and the size and relative location of which is used in determining the distortion error. Out

Structural images, which originated from the separate structures in the radiograph, at least one location in the radiograph is determined. The arrangement of the determined locations is then compared with an arrangement corresponding to the calibration object.

In the specific example described here (calibration arrangement with a large number of balls), the arrangement of the ball centers (in particular by a calculated projection of the calibration arrangement) into the coordinate system of the determined ellipse centers (which form the first arrangement) is introduced for the comparison and by positional compensation calculation and alignment adjusted.

However, before carrying out the actual comparison of the in the

Radiation image specific location with the appropriate location of the calibration optionally optionally carried out a correction that takes into account geometric effects of the projection. This optional correction is performed before the compensation calculation is executed. Such a compensation calculation can not only at a

Calibration arrangement can be performed with a plurality of balls, but also in other calibration objects that have multiple shape features or are composed of several individual objects. In the case of a sphere, the concept of correction due to geometric effects of the projection will be explained below.

FIG. 6 shows in a two-dimensional representation the projection of a sphere 17 with a center MK onto a screen or a detector field 13, eg the detector 3 from FIG. 1 or the detector 13 from FIG. 2. The projection corresponds to the case of FIG cone-shaped radiation beam KS, which emanates from a point-shaped radiation source 2, for example, the source 2 of Fig. 1.

The radiation beam KS impinges on the screen in the area between the vertically spaced apart points 66 and 68. The points 66 and 68 correspond to beams 65 and 67 of the radiation beam KS, which run tangentially to the ball 17. Since the illustration in FIG. 6 is selected such that the distance line AL of the radiation source 2 from the screen 13 extends in the horizontal direction and is thus perpendicular to the vertical line of the profile of the screen 13 shown in FIG. 6, the distance is projected points 66, 68 also equal the length of the major axis of the projected ellipse. Therefore, in FIG. 6, the center ME of the ellipse, which has equal distances to the points 66, 68, is also marked. Also shown in Fig. 6, the position of the projected center point MK _{P of} the ball center point MK. It can be seen that the center of the ellipse ME is spaced from this projected center of the sphere MK _P by a length δ. This positional deviation is corrected before the comparison of the arrangement of the projected sphere centers with the arrangement of the determined ellipse centers ME is performed. In particular, using information about the geometry of the measuring arrangement (ie in particular the position of the radiation source assumed as a point relative to the photocell field), the respective ellipse center ME determined from the transmission image can first be corrected such that it coincides with the projected sphere center with this correction, the effect of the distortion is still disregarded. For this correction, it is therefore assumed that the situation corresponds exactly to the situation illustrated in FIG.

The distance of the ellipse center ME from the projected sphere center MK _P can be calculated in particular as follows:

5 = d / [sin a (cot ² a cot ² β - 1)]

Where d denotes the distance of the radiation source from the projected sphere center point MK _P (ie the length of the center point beam), α denotes the angle between the distance line AL and the center point beam, β denotes the angle between the tangential ray 67 and the center point beam, "cot" is the cotangent and sin the sinus. If the correction is carried out for all ellipse centers, a corrected first arrangement is produced. In the following, "first arrangement" refers in particular to the corrected first arrangement With regard to the calibration object 16 from FIG. 2, the compensation calculation can now be carried out, for example by the sum of the amounts of the deviations or the sum of the squared deviations between each corrected one In a further step, the deviation remaining after this minimization (also referred to as storage) is between each of the corrected determined ellipse centers and the associated ellipse center

Ball center determined as a result of distortion. This deviation, which can also be referred to as a distortion error, is then present, for example, in relation to the coordinate system of the transmission image.

In general terms, for the comparison of the first and second arrangements, the position and orientation of the locations in the second arrangement (corresponding to the calibration object) are (in particular best possible) related to the location and orientation of the locations in the first

Places adapted (where the actual arrangement of the structures of the calibration object and resulting possible illustrations in the

Transmission image) and remaining deviations of the locations of the second array from corresponding locations in the first array are identified as distortion errors.

There are six degrees of freedom of pose (position and orientation)

Kalibrierobjektes relative to the image detector and are therefore adapted by the adjustment and can be determined in particular by compensation calculation. Therefore, it is preferred that the calibration object has at least four and preferably at least 10 separate structures, for which reason at least four or 10 points each having two coordinates (ie eight or 20 values for the determination of the pose) in the transmission image can be determined ,

5 shows, for a greatly simplified example with four corrected ellipse centers E1, E2, E3, E4 and four sphere centers K1, K2, K3, K4, the state after execution of the compensation calculation. In Fig. 5, the outer edge of the image is designated by reference numeral 59. It can be seen that in each case between the pairs of associated corrected ellipse centers and ball centers E1, K1; E2, K2; E3, K3; E4, K4 a distance which is the filing or distortion error. It can be seen that the tray can be different in size and can act in different directions.

In yet another optional step, a distortion map may now be made by interpolating the distortion errors at at least three locations of corrected ellipse centers for each other in the radiographic image other than locations of a corrected ellipse center

Distortion error is determined for the other location. In particular, the three locations correspond to the vertices of a triangle and therefore within the triangle can interpolate an interpolated value (and in particular vector) of each location

Distortion error can be assigned. In particular, a partial area or the entire radiographic image can have a plurality of such triangles which completely cover the partial area or the radiographic image, so that

Interpolation a full-scale distortion map of the subarea or the

Radiation image is obtained. Directory errors at locations outside the area covered by the triangles can be determined by extrapolation.

The measuring arrangements with attached computer tomographs can be, for example, the X-ray computer tomographs METROTOM 800 or METROTOM 1500, which are offered and distributed by Carl Zeiss Industrial Metrology GmbH, Germany.

In particular, the invention makes it possible to measure dimensions such as lengths, widths,

Diameter, radii of curvature of handmade or industrially manufactured

Detect objects accurately and independently of the relative position and orientation of the DUT and the detector.

Fig. 7 shows a cross 73 in plan view. The thickness of the cross 73 (measured in the direction perpendicular to the plane of the figure) is, for example, only a few hundred micrometers when the cross is made of chromium or lead and is arranged on a plate-shaped glass-ceramic carrier.

The four arms 72a, 72b, 72c, 72d of the cross 73 have no constant width when viewed from the center point MC to the free ends of the arms 72, but the width of the arms 72 gradually decreases toward the free end. In the illustrated embodiment, the thickness is three stages, ie there are three Sections in each arm 72, which have a different width. The center of the projection image of such a cross can with high precision from a

Radiation image can be determined.

Fig. 8 shows schematically an arrangement of a plate-shaped carrier 71, e.g. of glass ceramic, on whose backside a plurality of crosses 73 is applied, which are formed by a structured material layer. The crosses 73 in Fig. 8 may be e.g. each act around a cross, as shown in Fig. 7. Preferably, the crosses 73 are arranged so that their centers are arranged in rows next to each other and in columns one above the other.

The backside of the carrier 71 abuts directly on the surface of a flat-panel image detector 13, e.g. how the flat image detector 13 shown in Fig. 2 is constructed. When generating a radiographic image of the arrangement of the crosses 73, the invasive radiation first penetrates the side of the carrier 71, on which there are no crosses 73, penetrates the material of the carrier 71, is additionally weakened at the points where the crosses are located and enters the adjacent surface of the flat image detector 13 therein.

Claims

claims

A method for operating a measuring device for a computed tomography, comprising a radiation source (2) of invasive radiation and a flat-image detector (3; 13) with scintillation layer (15) and a photocell array of photocells (4) for detecting radiation from the Radiation source (2), wherein

a calibration object (16) is arranged between the radiation source (2) and the flat image detector (3; 13) and at least one transmission image of the calibration object (16) is recorded with the flat image detector (3;

- From known dimensions of the calibration object (16) and from the

at least one transmission image, a distortion error, which has arisen due to a distortion of the flat image detector (3; 13), is determined as a function of the location in the photocell field.

2. The method according to claim 1, wherein a transmission image is taken by the flat image detector (3; 13) from an object to be measured and corrected taking into account the determined distortion error.

3. Method according to the preceding claim, wherein

At least one transmission image of the calibration object (16) or another measurement object (1) is recorded by the flat image detector (3; 13),

In the radiographic image, individual points are determined, each corresponding to a structure of the calibration object (16) or measuring object (1), and the distortion error is corrected only with respect to these individual points, so that an arrangement of corrected individual points is formed.

4. Method according to the preceding claim, wherein the flat image detector (3), the calibration object (16) or measuring object (1) and a radiation source (2) of the invasive radiation are part of the measuring arrangement and wherein the arrangement of corrected individual points is used for this purpose To determine geometric parameters of a geometric model describing a geometry of the measuring arrangement.

5. The method of claim 1, wherein the calibration object is arranged directly adjacent to the flat image detector such that structures of the calibration object (16) extend along the flat image detector (13) and in which at least one transmission image of the calibration object (16) produced by the flat image detector (13) appears as juxtaposed and / or partially overlapping structural images.

6. The method according to any one of the preceding claims, wherein the calibration object (16) has a plurality of separate structures (17) whose size and their position relative to each other is known, and wherein the size and relative position is used in the determination of the distortion error.

7. Method according to the preceding claim, wherein at least one location in the radiographic image is determined from structural images which have arisen from the separate structures (17) in the at least one radiographic image, so that a first arrangement of the determined locations is obtained, and the first arrangement is compared with a second arrangement of locations corresponding to the calibrated object (16) corresponding to the determined locations and determined from deviations of the two arrangements of the distortion errors as a function of the location in the photocell field and / or in the transmission image.

8. The method according to the preceding claim, wherein for the comparison of

Arrangements the position and orientation of the locations in the second arrangement is adapted to the location and orientation of the locations in the first array to different possible relative positions and orientations of

Calibration object (16) and the flat image detector (13) to be considered, and remaining deviations of the locations in the first array of corresponding locations in the second array are identified as distortion errors.

A measuring device for a computer tomograph comprising a radiation source (2) of invasive radiation and a flat image detector (3; 13) with scintillation layer (15) and a photocell array of photocells (4) for detecting radiation from the radiation source (2) said measuring arrangement further comprising an error detecting means (6) connected to the flat image detector (3; 13) and configured from at least one transmission image of the calibration object (16) located between the radiation source (2) and the flat image detector (3; 13) is arranged, and known dimensions of the

Kalibrierobjekts (16) to determine a distortion error due to a Distortion of the flat image detector (3; 13) has arisen, wherein the

Distortion error is determined as a function of the location in the photocell field.

10. Measuring arrangement according to claim 9, wherein the measuring arrangement also has a

Error correcting means (6) connected to the error detecting means (6) and configured to correct a transmission image taken by an object to be measured taking into account the distortion error detected by the error detecting means.

1 1. Measuring arrangement according to one of the preceding claims, wherein the

Measuring arrangement also the calibration object (16) and the calibration object (16) directly adjacent to the flat image detector (3; 13) is arranged so that structures (17) of the calibration object (16) on the flat image detector (3; along and in which at least one transmission image of the calibration object (16) generated by the flat image detector (3; 13) appears as juxtaposed and / or partially overlapping structure images.

12. Measuring arrangement according to one of the preceding claims, wherein the

Measuring arrangement also has the calibration object (16) and wherein the

Calibration object (16) has a plurality of separate structures (17) whose size and their position relative to each other is known, and wherein the size and relative position is used in the determination of the distortion error.

13. Measuring arrangement according to the preceding claim, wherein the error-determining device (6) is configured to determine at least one location in the radiographic image from structural images which have emerged from the separate structures (17) in the at least one radiographic image, so that a first arrangement of the determined locations is obtained comparing the first arrangement with a second arrangement of locations corresponding to the calibration object (16) corresponding to the determined locations, and from deviations of the two arrangements the distortion error as a function of the location in the photocell field to investigate.

14. Measuring arrangement according to the preceding claim, wherein the error detection device (6) is designed to adapt the position and orientation of the locations in the second arrangement to the position and orientation of the locations in the first arrangement to compare different arrangements possible relative positions and orientations of the calibration object (16) and the flat image detector (13) to take into account, and remaining deviations of the locations in the first array of corresponding locations in the second

To identify the arrangement as a distortion error.

15. Measuring arrangement according to one of the preceding claims, wherein the

Measuring arrangement also has a tactile coordinate measuring machine, which is configured to key the calibration object (16) and to determine the size and location of structures of the calibration object (16) and to provide for the determination of the distortion error.