JP2007333596A - X-ray ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT system unnecessary to move a center-of-rotation axis projecting position calibrating fixture by an operator even if the positioning of the calibrating fixture is bad and the X-ray projection data of the wire thereof is not obtained over 360° and capable of calibrating a center-of-rotation projecting position with respect to each axis without using a fixture, which is finished in the position of the wire especially with high precision, as the calibrating fixture. <P>SOLUTION: When the projection data of the wire Jw of the center-of-rotation axis projecting position calibrating fixture Jc obtained by rotating a turntable 3 is not obtained over 360°, the already obtained data is allowed to approximate by a torigonometric function and the amplitude center and phase thereof are calculated to approximately calculate the position of a center-of-rotation axis R while a stage 5 is moved on the basis of the calculation result to allow the wire Jw to approach the center-of-rotation axis R and the projection data of the wire Jw is employed by subsequently again rotating the turntable 3. By this constitution, the wire Jw can be set at a position where the projection data over 360° is automatically obtained without positioning the calibrating fixture by the operator. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an industrial X-ray CT apparatus for investigating the internal structure of various industrial parts and the like, the presence or absence of defects, etc., in a non-destructive manner.

  In an industrial X-ray CT apparatus, generally, as schematically shown in FIG. 6, an X-ray focal point and an X-ray detector of an X-ray generator are placed between an X-ray generator 61 and an X-ray detector 62. A structure in which a rotating table 63 rotating around a rotation center axis R (z-axis direction) orthogonal to the X-ray optical axis L (x-axis direction) connecting the centers of the rotating table 63 is used. In addition, a stage 64 is often provided on which a subject W is mounted and moved on a plane (xy plane) orthogonal to the rotation center axis R (see, for example, Patent Document 1).

  In this type of X-ray CT apparatus, the X-ray detector 62 captures the output of the X-ray detector 62 every time the subject W is irradiated with X-rays and is rotated by a minute angle around the rotation center axis R. A tomographic image along a plane orthogonal to the rotation center axis R of the subject W is reconstructed using the line projection data. For this reconstruction calculation, it is essential to obtain coordinate information of the projection position of the rotation center axis R on the X-ray detector 62.

  As a method for obtaining coordinates on the X-ray detector 62 of the projection position of the rotation center axis R, a method using a rotation center axis projection position calibration jig Jc as shown in a perspective view in FIG. ing. The rotation center axis projection position calibration jig Jc has a structure in which a wire Jw made of a material having a high X-ray absorption rate such as tungsten is arranged inside a support member Js made of a material that easily transmits X-rays such as acrylic. X-ray projection data of the wire Jw is collected while rotating while the rotation center axis projection position calibration jig Jc is placed on the stage 64 on the rotary table 63 (see, for example, Patent Document 2).

If the data collected in this way is represented by a graph in which the horizontal axis represents the element row arranged in the y-axis direction of the X-ray detector 62 and the vertical axis represents the rotation angle, the data is illustrated in FIG. A simple sinogram. From this sinogram, the coordinates of the projection position of the rotation center axis R on the element array of the X-ray detector 62 can be obtained. Such calibration may be performed for each CT imaging of the subject, or a calibration table or the like may be created and referred to when CT imaging of the subject.
JP 2005-351879 A JP 2004-184122 A

  By the way, in the calibration operation using the rotation center axis projection position calibration jig described above, if the amount of deviation from the rotation center of the calibration jig used increases, the image of the wire is rotated from the field of view of the X-ray detector. It deviates and the X-ray projection data of the wire for 360 ° cannot be obtained as illustrated in FIG. 8B, and the rotation center axis cannot be calibrated.

  Even when the enlargement factor is increased, the amount of wire displacement increases on the X-ray detector, so that even if the actual deviation amount of the wire with respect to the rotation center axis is small, depending on the enlargement factor, data for 360 ° is obtained. May not be obtained.

  In such a case, in the conventional X-ray CT apparatus, the calibration jig is moved on the horizontal plane (plane orthogonal to the rotation center axis) by the operator moving the stage, and the center of the calibration jig is rotated. After positioning so as to approach the central axis, it is necessary to rotate the rotary table again to obtain the projection data of the wire, which is troublesome.

  Also, the rotation center axis projection position calibration jig is basically arranged so that the wire is positioned as close as possible to the rotation center axis when placed at a specified position on the stage. There is also a problem that it is necessary to finish the position of the wire with respect to the reference surface with high accuracy.

  The present invention has been made in view of such a situation, and even if the positioning of the rotation center axis projection position calibration jig is not good and the X-ray projection data of the wire is not obtained over 360 °, Thus, the operator does not need to move the calibration jig, and the calibration jig can be used without using a wire with a particularly high precision with respect to the reference surface such as the outer peripheral surface. Another object of the present invention is to provide an X-ray CT apparatus capable of calibrating the rotation center axis projection position.

  In order to solve the above-described problems, an X-ray CT apparatus according to the present invention includes an X-ray focal point of an X-ray generator and an X-ray detector between the X-ray generator and the X-ray detector arranged to face each other. A rotating table that rotates around a rotation center axis that is orthogonal to the X-ray optical axis that connects the center of the sensitive surface is provided, and a subject is mounted on the rotation table so that it is at least orthogonal to the rotation center axis. The X-ray projection data acquired at every minute rotation angle by rotating the rotary table while irradiating X-rays with a subject mounted on the stage is used for the rotation axis. In an X-ray CT apparatus provided with reconstruction calculation means for constructing a tomographic image of a subject along an orthogonal plane, a rotation center axis projection position calibration jig comprising a wire on the stage is used as the rotation center. Along the axis By rotating the rotary table 360 ° by mounting the rotary table, a rotation center axis on the X-ray detector is obtained from a sinogram based on X-ray projection data of the wire obtained from the output of the X-ray detector. And a rotation center axis projection position calculating means for calculating the projection coordinates of the wire, and in the calibration operation, a part of the X-ray projection data for 360 ° of the wire was not obtained from the output of the X-ray detector. In this case, the X-ray projection data of the wire obtained from the output of the X-ray detector is approximated by a trigonometric function to calculate the amplitude center and phase, and the calculation result is used to approximate the position of the rotation center axis. Approximate calculation means for calculating, and control means for finely moving the stage on the plane so that the position of the wire coincides with the position of the approximate rotation center axis. Accordingly characterized (claim 1).

  Here, in the present invention, with the rotation center axis projection position calibration jig mounted on the stage, the rotary table is rotated 360 ° to acquire X-ray projection data, and the X-ray projection data of the wire is acquired. If the approximate calculation means and the stage control means are automatically operated, the rotation table is rotated again by 360 degrees, the X-ray projection data is taken in, and the rotation center axis projection is performed. A configuration (Claim 2) including control means for operating the position calculation means can be employed.

  According to the present invention, the rotation center axis projection position calibration jig is rotated 360 ° because the distance between the rotation center axis projection position calibration jig wire arranged on the stage and the rotation center axis is relatively large. If the X-ray projection data of the wire collected in this way is not aligned for 360 °, the obtained projection data is approximated by a trigonometric function, and the approximate position of the rotation center axis is obtained from the amplitude and phase of the trigonometric function, The problem is solved by automatically moving the stage to that position.

  That is, the sinogram obtained by rotating the rotation center axis projection position calibration jig by projecting the wire onto the X-ray detector is different from the trigonometric function, but both are relatively similar and calibrate the sinogram. Even if the data for 360 ° is not available, it can be approximated by a trigonometric function. Using the approximate amplitude and phase of the trigonometric function, SID (the distance between the focal point of the X-ray generator and the sensitive surface of the X-ray detector) and SOD (also the focal point of the X-ray generator and the subject) are used. From the distance), the position of the rotation center axis can be estimated approximately. The estimation result of the rotation center axis by the approximate calculation using the trigonometric function does not accurately represent the rotation center axis, but almost correctly represents the rotation center axis.

  Therefore, in the present invention, when the X-ray projection data of the wire is collected by rotating the rotation center axis projection position calibration jig on the stage and rotating 360 °, the projection data of the wire is not aligned for 360 °. In addition, the obtained projection data is approximated by a trigonometric function, the position of the rotation center axis is approximately estimated from the amplitude and phase, SID and SOD, and the wire position matches the estimated position. Move the stage automatically. Thereby, a wire will be in the state very close to the rotation center axis. In that state, by rotating the rotary table 360 ° and collecting the X-ray projection data of the wire again, a sinogram based on the projection data of the wire without 360 ° is obtained, and by a known calculation using the sinogram The projection position coordinates of the rotation center axis can be accurately obtained.

  The invention according to claim 2 automatically performs the above-described series of operations, that is, when the X-ray projection data of the wire is first collected by rotating the rotary table, and the data is not aligned for 360 °. Approximate the obtained projection data with trigonometric functions, approximate the position of the rotation center axis, move the stage based on the calculation result, then rotate the rotation table again to reconstruct the X-ray projection data of the wire The operation to obtain the rotation center axis from the acquired sinogram is automatically executed, so that the operator first mounts the rotation center axis projection position calibration jig and rotates the turntable. Even if the wire position is shifted, the projection position coordinates of the rotation center axis are automatically obtained.

  According to the present invention, in the calibration operation of the rotation center axis using the rotation center axis projection position calibration jig provided with the wire, the projection data of the wire onto the X-ray detector obtained by rotating the rotary table is 360. Even if they are not aligned, the apparatus automatically moves the stage so that the wire approaches the center of the rotation axis, so that the positioning operation by the operator is not required as in the prior art, and the labor can be saved.

  At the same time, it is not necessary to finish the position of the wire with respect to a reference surface such as the outer peripheral surface of the rotation center axis projection position calibration jig with high accuracy as in the prior art, and the cost can be reduced.

  According to the invention of claim 2, when the projection data for 360 ° is not obtained when the rotation center axis projection position calibration jig is mounted on the rotary table and the projection data is first collected. Approximate calculation of the rotation center axis using the obtained projection data, stage positioning operation based on the calculation result, and a series of operations to rotate the rotary table again after the positioning and collect projection data Therefore, the operator only needs to first mount the rotation center axis projection position calibration jig on the stage and give a command without determining whether or not the projection data of the wire is aligned by 360 °. Thus, the rotation center axis projection position coordinates are obtained, and the calibration operation of the rotation center axis projection position can be greatly simplified.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of an embodiment of the present invention, and is a diagram illustrating a schematic diagram showing a mechanical configuration and a block diagram showing a system configuration.

  The X-ray generator 1 generates cone beam-shaped X-rays centering on the X-ray focal point 1a in the horizontal direction (x-axis direction). A two-dimensional X-ray detector 2 is disposed opposite to the X-ray generator 1, and a turntable 3 is provided therebetween.

  The rotary table 3 rotates around a rotation center axis R along a vertical direction (z-axis direction) perpendicular to the X-ray optical axis L connecting the X-ray focal point 1a and the center of the sensitive surface of the X-ray detector 2. Given. The rotary table 3 can be moved in the x-axis direction and the z-axis direction by the rotary table moving mechanism 4. A stage 5 that moves on a plane (xy plane) perpendicular to the rotation center axis R is placed on the turntable 3. The rotary table 3 is rotationally driven by a drive control signal from the rotary table controller 11, and the rotary table moving mechanism 4 is driven and controlled by a signal supplied from the rotary table moving mechanism controller 12. The stage 5 is driven and controlled by a signal supplied from the stage controller 13.

  The rotary table controller 11, the rotary table moving mechanism controller 12, and the stage controller 13 described above are placed under the control of the control unit 20. An operation unit 21 including a joystick, a mouse, a keyboard, and the like is connected to the control unit 20, and the operator operates the operation unit 21 to rotate or move the rotary table 3 and further move the stage 5. It can be moved at will. In the rotation center jig projection position calibration routine described later, the stage 5 automatically moves to a required position in response to a command from the control unit 20.

  Now, the output of the X-ray detector 2 is captured by the image data capturing circuit 14 and stored in the storage unit 15. During normal CT imaging, the output of the X-ray detector 2 is stored in the storage unit 15 each time the rotary table 3 is rotated by a minute angle while the subject W is mounted on the stage 5 and irradiated with X-rays. Then, the stored data is reconstructed by the reconstruction calculation unit 16, thereby constructing a tomographic image of the subject W along a plane orthogonal to the rotation center axis R and displaying it on the display 17. As described above, this reconstruction calculation requires the projection coordinates of the rotation center axis R onto the sensitive surface of the X-ray detector 2, and the rotation center axis projection position calibration is performed before the reconstruction calculation. Action is required.

  In the rotation center axis projection position calibration operation, a rotation center axis projection position calibration jig Jc equivalent to that illustrated in FIG. 7 is mounted on the stage 5, the rotation table 3 is rotated, and the projection of the wire Jw is performed. Data is captured into the storage unit 15 via the image data capturing circuit 14. The data is sent to the rotation center axis projection coordinate calculation unit 18 to be used for rotation center axis projection coordinate calculation using a sinogram. The feature of this embodiment is that, when the projection data of the wire is not aligned for 360 °, the data is sent to the rotation center axis approximation calculation unit 19 and used for the approximation calculation of the rotation center axis R. Based on the approximate calculation result, the stage 5 is automatically moved by a signal from the control unit 20 so that the position of the wire Jw on the xy plane approaches the rotation center axis R. In FIG. 1, the control unit 20 and various arithmetic units are shown in block diagrams for each function, but these are actually configured by a computer and its peripheral devices and realized by executing an installed program. Functions are indicated by blocks.

  FIG. 2 is a flowchart showing the contents of the rotation center axis projection position calibration operation according to the embodiment of the present invention. The details of the operation will be described below with reference to this figure.

  When obtaining the projection coordinates of the rotation center axis, first, the rotation center axis projection position calibration jig Jc shown in FIG. 7 is mounted on the stage 4 and a calibration operation start command is given. Thereby, the rotary table 3 is rotated while irradiating X-rays, and X-ray projection data of the rotation center axis projection position calibration jig Jc, specifically, projection data of the wire Jw is captured. It is determined whether or not the projection data is aligned for 360 °.

  When the projection data of the wire Jw is equal to 360 °, the rotation center axis projection coordinate calculation unit 17 converts the projection data from the sinogram of the projection data onto the X-ray detector 2 on the rotation center axis R as in the conventional case. Calculate projected coordinates.

  When the projection data of the wire Jw is not aligned for 360 °, approximation is performed using a trigonometric function using the obtained projection data. That is, when the projection data of the wire Jw partially deviates from the field of view of the X-ray detector 2 as illustrated in FIG. 3A, the projection data of the wire Jw is obtained as indicated by the broken line in FIG. Approximate projection data with trigonometric functions. Then, the amplitude and phase of the trigonometric function are obtained, and the distance SID formed between the X-ray focal point 1a and the X-ray detector 2 at that time, and the distance SOD formed between the X-ray focal point 1a and the rotary table 3 (rotation center). As shown in a schematic plan view in FIG. 4, the position of the rotation center axis R of the turntable 3 on the xy plane is obtained. In FIG. 4, θ is the rotation angle of the turntable 3, and r is the amount of deflection with respect to the rotation center axis R of the wire Jw calculated from the amplitude of the approximate trigonometric function, SID and SOD.

  While the rotation angle of the turntable 3 is θ and the rotation of the turntable 3 is stopped, the stage 5 is moved by rsinθ in the x direction and rcosθ in the y direction. That is, it is estimated that the position of the rotation center axis R is shifted by rsin θ in the x direction and rcos θ in the y direction with respect to the position of the wire Jw at that time, and the rotation center axis projection position toward the estimated position The calibration jig Jc is moved. Although the position of the rotation center axis R estimated by approximating the trajectory from the projection data of a part of the wire Jw with a trigonometric function includes an error in the strict sense, it is used for calibration of the rotation center axis projection position toward the estimated position. By moving the jig Jc, the distance between the wire Jw and the rotation center axis R becomes closer than before the movement. Accordingly, by rotating the rotary table 3 again while irradiating X-rays in this state and collecting the projection data of the wire Jw, the projection data for 360 ° of the wire Jw is obtained as shown in FIG. can get. From the sinogram based on the projection data for 360 °, the projection coordinates of the rotation center axis R can be calculated by a method equivalent to the conventional method.

  Here, when the projection data of the wire Jw is approximated by a trigonometric function, it is conceivable that two or more trigonometric functions having high correlation exist because the obtained data is not a visit function. In such a case, as shown in FIG. 5, the average c of the two upper trigonometric functions a and b having high correlation can be employed.

  In addition, when the projection data of the wire Jw cannot obtain data of 360 ° only in part in the z-axis direction, the calibration can be performed by moving the rotary table 3 in the z-axis direction and adopting the above method. It becomes possible.

In the configuration diagram of the embodiment of the present invention, a schematic diagram showing a mechanical configuration and a block diagram showing a system configuration are shown together. It is a flowchart showing the content of the rotation center axis | shaft projection position calibration operation | movement in embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining the operation of an embodiment of the present invention, in which (A) is a sinogram showing an example of projection data obtained by rotating a rotation center axis projection position calibration jig, and (B) is a trigonometric function. A graph showing an example of approximation, (C) is a sinogram showing an example of wire projection data obtained after moving the stage toward the estimated position of the rotation center axis R obtained by approximation calculation. FIG. 6 is a schematic plan view for explaining the moving amount and direction of the stage from the result of approximating the wire projection data with a trigonometric function in the embodiment of the present invention. It is explanatory drawing of the example of the method of a process when two trigonometric functions with high correlation are obtained in approximating the projection data of the wire in embodiment of this invention by a trigonometric function. It is explanatory drawing of the example of arrangement | positioning of each member of an industrial X-ray CT apparatus. It is a perspective view which shows the example of the jig | tool for rotation center axis | shaft projection position calibration used for an industrial X-ray CT apparatus. FIG. 8 is an example of a sinogram based on the projection data of a wire obtained using the jig of FIG. 7, (A) is an example in the case where data for 360 ° is obtained, and (B) is an example in which a part thereof is missing. It is explanatory drawing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 X-ray generator 1a X-ray focus 2 X-ray detector 3 Rotating table 4 Rotating table moving mechanism 5 Stage 11 Rotating table controller 12 Rotating table moving mechanism controller 13 Stage controller 14 Image data capturing circuit 15 Storage unit 16 Reconfiguration calculating unit 17 Display 18 New axis projection coordinate calculation unit during rotation 19 New axis approximation calculation unit during rotation 20 Control unit 21 Operation unit Jc Jig for rotation center axis projection position calibration Jw Wire Js Support member

Claims (2)

A rotation center orthogonal to the X-ray optical axis that connects the X-ray focal point of the X-ray generator and the center of the sensitive surface of the X-ray detector between the X-ray generator and the X-ray detector arranged opposite to each other. A rotating table that rotates around an axis, a stage on which the subject is mounted and movable on at least a plane orthogonal to the rotation center axis, and the subject is mounted on the stage. Reconstruction calculation means for constructing a tomographic image of a subject along a plane orthogonal to the rotation axis using X-ray projection data obtained by rotating the rotary table and taking in every minute rotation angle while irradiating X-rays In an X-ray CT apparatus equipped with
By mounting a rotation center axis projection position calibration jig comprising a wire on the stage so that the wire is in a posture along the rotation center axis and rotating the rotary table by 360 °, the X-ray Rotation center axis projection position calculating means for calculating projection coordinates of the rotation center axis on the X-ray detector from a sinogram based on X-ray projection data of the wire obtained from the output of the detector, and in the calibration operation, When a part of the X-ray projection data for 360 ° of the wire is not obtained from the output of the X-ray detector, the X-ray projection data of the wire obtained by the output of the X-ray detector is Approximation with a trigonometric function to calculate the amplitude center and phase, and using the calculation result, approximate calculation means for calculating the approximate position of the rotation center axis, and the position of the approximate rotation center axis An X-ray CT apparatus comprising stage control means for finely moving the stage on the plane so that the position of the wire coincides with the position.   When the rotation center axis projection position calibration jig is mounted on the stage, the rotary table is rotated 360 ° to acquire X-ray projection data, and the X-ray projection data of the wire is not aligned for 360 °. In addition, after the approximate calculation means and the stage control means are automatically operated, the rotation table is rotated again by 360 °, the X-ray projection data is taken in, and the rotation center axis projection position calculation means is operated. The X-ray CT apparatus according to claim 1, comprising:

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