EP2637564A1 - Dispositif de mesure pour un tomographe assisté par ordinateur - Google Patents

Dispositif de mesure pour un tomographe assisté par ordinateur

Info

Publication number
EP2637564A1
EP2637564A1 EP11772959.0A EP11772959A EP2637564A1 EP 2637564 A1 EP2637564 A1 EP 2637564A1 EP 11772959 A EP11772959 A EP 11772959A EP 2637564 A1 EP2637564 A1 EP 2637564A1
Authority
EP
European Patent Office
Prior art keywords
arrangement
calibration object
image detector
distortion
locations
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP11772959.0A
Other languages
German (de)
English (en)
Inventor
Daniel Weiss
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Carl Zeiss Industrielle Messtechnik GmbH
Original Assignee
Carl Zeiss Industrielle Messtechnik GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Carl Zeiss Industrielle Messtechnik GmbH filed Critical Carl Zeiss Industrielle Messtechnik GmbH
Publication of EP2637564A1 publication Critical patent/EP2637564A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T7/00—Details of radiation-measuring instruments
    • G01T7/005—Details of radiation-measuring instruments calibration techniques
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
    • A61B6/4208—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector
    • A61B6/4233—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector using matrix detectors
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/58—Testing, adjusting or calibrating thereof
    • A61B6/582—Calibration
    • A61B6/583—Calibration using calibration phantoms
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]

Definitions

  • 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,
  • a radiation source generating invasive radiation, i. Radiation (eg.
  • 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.
  • the radiation source for example a microfocus X-ray tube
  • the computer tomograph calculates a three-dimensional image of the measurement object by means of so-called reconstruction (for example, filtered rear projection).
  • 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.
  • 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.
  • a scattering occurs which worsens the sharpness and resolution of the images.
  • 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
  • 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.
  • 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.
  • Scintillation layer in the flat image detector can, for. B. from a variety of
  • Scintillator bodies eg, needle-shaped bodies.
  • 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 ⁇ .
  • Such flat image detectors are generally considered to be free from distortion errors.
  • the severity of the distortion in the evaluated case is up to 20 ⁇ 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 ⁇ , i. depending on the relative position of the DUT and the
  • Image detector can vary the measuring length by up to 5 ⁇ .
  • the invention is based on the finding that the observed
  • the edge length is typically 100 to 150 ⁇ , but also up to 400 ⁇ .
  • the various radiographic images taken for reconstruction as a whole contribute to the three-dimensional image computed by the computed tomography scanner.
  • 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.
  • 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
  • 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
  • a measuring object with was exactly known dimensions required, for.
  • 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).
  • sub-pixel distortion There are several possible reasons for sub-pixel distortion.
  • One possible cause is a mechanical bending of the flat image detector.
  • the internal structure of the scintillator material could be responsible for the distortion.
  • a stress caused by the scintillator structures could be the cause of the distortion. Also combinations of the mentioned possible
  • 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.
  • a transmission image is taken by the flat image detector of an object to be measured and under
  • the calibration object is preferably arranged directly adjacent to the flat image detector.
  • 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
  • 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.
  • 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,
  • linear interpolation or it can be a model function adjusted by compensation calculation.
  • a model function adjusted by compensation calculation.
  • the distortion error is a two-dimensional vector and consists of an x and a y component.
  • 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
  • Distortion errors are detected with higher resolution. In particular, this gives a higher number of nodes for interpolation or
  • 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.
  • 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.
  • 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.
  • the objects are steel balls.
  • 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.
  • 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.
  • 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.
  • 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.
  • the positions of the ball centers of the arrangement can thus be determined in a coordinate system which is assigned to the arrangement.
  • Determination succeeds with an accuracy of less than a micrometer.
  • balls with very little roundness error can be made, i. the spherical surface is at a constant distance from the surface with high accuracy
  • 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.
  • Obtaining the coordinates of the calibration possible can also be one
  • Coordinate measuring device can be used with an optically scanning sensor.
  • spheres 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.
  • 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
  • a suitable calibration arrangement may be an array having a plurality of flat objects.
  • 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
  • a plate-shaped carrier in particular a plate-shaped carrier.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the distortion is corrected.
  • the entire transmission image can be corrected according to the distortion error, z. B. every pixel of the
  • 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.
  • 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.
  • 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.
  • 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.
  • the center point is a point passing through
  • 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.
  • 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.
  • Distortion error determined. This also means that for each location of the photocell field the information about the distortion error and / or the
  • Information about the distortion error may, for example, be in the form of a vector, i. a length and a direction.
  • 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.
  • 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.
  • 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.
  • 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.
  • interpolation 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.
  • a bicubic interpolation are also not immediate neighboring pixels are included in the interpolation, ie an environment with 4 x 4 pixels.
  • 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.
  • 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.
  • the scope of the invention includes a computer program that of a
  • 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
  • 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
  • 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
  • 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.
  • the calculation corresponds in particular to a possible projection of the calibration object into the image plane of the radiographic image.
  • 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 is included in the calculation of ball centers, which were obtained in particular by means of the tactile coordinate measuring machine.
  • 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
  • Fig. 1 shows schematically the geometry of a measuring arrangement with a
  • 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
  • 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. 5 is a highly simplified example of an arrangement of a
  • 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
  • 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.
  • 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.
  • detection elements 4 eg photocells with scintillation material lying in front of it in the radiation direction
  • 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.
  • 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.
  • a computer preferably also digital image data at least one
  • Radiographic image of the device 3 evaluates and / or processed by the
  • 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.
  • 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.
  • 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.
  • 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.
  • Positioning device 5 enable further positioning movements, e.g.
  • Rotary device 7 coincides.
  • 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.
  • a homogeneous layer may be used, e.g. B. with
  • Scintillation material such as gadolinium oxide
  • a Scintillation material such as gadolinium oxide
  • 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.
  • a beam of invasive radiation is indicated by an arrow
  • 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
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the distance of the ellipse center ME from the projected sphere center MK P can be calculated in particular as follows:
  • d denotes the distance of the radiation source from the projected sphere center point MK P (ie the length of the center point beam)
  • ⁇ denotes the angle between the distance line AL and the center point beam
  • ⁇ denotes the angle between the tangential ray 67 and the center point beam
  • cot is the cotangent and sin the sinus.
  • 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
  • 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.
  • the position and orientation of the locations in the second arrangement are (in particular best possible) related to the location and orientation of the locations in the first
  • 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 ,
  • FIG. 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.
  • 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.
  • 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.
  • 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.
  • 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
  • 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.
  • the invention makes it possible to measure dimensions such as lengths, widths,
  • 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.
  • 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.
  • 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.
  • 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.

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Abstract

L'invention concerne un procédé pour le fonctionnement d'un dispositif de mesure pour un tomographe assisté par ordinateur, qui présente une source de rayonnement (2) d'un rayonnement invasif et un détecteur d'images à plat (3 ; 13) muni d'une couche de scintillation (15) et d'un champ de cellules photo (4) pour la détection du rayonnement provenant de la source de rayonnement (2). Un objet de calibrage (16) est disposé entre la source de rayonnement (2) et le détecteur d'images à plat (3 ; 13). Au moins une radiographie de l'objet de calibrage (16) est enregistrée par le détecteur d'images à plat (3; 13) et une aberration, formée en raison d'une distorsion du détecteur d'images à plat (3), est déterminée en fonction de l'endroit dans le champ de photocellules à partir des dimensions connues de l'objet de calibrage (16) et à partir de ladite au moins une radiographie (Dispositif 6). En particulier, une radiographie prise d'un objet à mesurer par le détecteur d'images à plat (3; 13) est corrigée en tenant compte de l'aberration déterminée.
EP11772959.0A 2010-11-10 2011-10-18 Dispositif de mesure pour un tomographe assisté par ordinateur Withdrawn EP2637564A1 (fr)

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DE102010050949A DE102010050949A1 (de) 2010-11-10 2010-11-10 Messanordnung für einen Computertomographen
PCT/EP2011/068201 WO2012062543A1 (fr) 2010-11-10 2011-10-18 Dispositif de mesure pour un tomographe assisté par ordinateur

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