JP2006130060A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a determination method for a geometrical parameter for reconstructing a clear three-dimensional X-ray CT image from rotation imaging data by a two-dimensional X-ray detector, especially rotation imaging data with a C-shaped arm. <P>SOLUTION: This X-ray CT apparatus is provided with an X-ray source 11, the two-dimensional X-ray detector 12 outputting X-ray image data, a rotating means for rotating movement, and an image processor performing image reconstruction by computing based on the X-ray image data. The image processor is provided with a rotation axis projection position computing means computing a relative displacement between the position of a reference rotation axis and the position of the rotation axis showing the actual rotating movement based on characteristic quantity of the reference rotation axis and the projection image of a phantom positioned near the rotating trajectory plane of the two-dimensional X-ray detector 12, and a back projection means correcting the X-ray image data based on the relative displacement and reconstructing the image. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an X-ray CT apparatus, and in particular, the present invention relates to a technique effective for reconstructing a three-dimensional X-ray CT image from rotational imaging data obtained by a two-dimensional X-ray detector.

  A conventional cone beam X-ray CT apparatus includes an X-ray source and a two-dimensional X-ray detector disposed opposite to the X-ray source (including a combination of an X-ray image intensifier and a television camera). Are rotated on a circular orbital plane having the same rotation center, and an X-ray fluoroscopic image of the subject located on the rotation central axis is taken with a two-dimensional X-ray detector, and further, based on the X-ray fluoroscopic image. A three-dimensional X-ray CT image is obtained by performing an image reconstruction operation.

  As an image reconstruction calculation algorithm of a cone beam X-ray CT apparatus using such a two-dimensional X-ray detector, the Feldkamp method described in Non-Patent Document 1 is representative.

  In addition, in the cone beam X-ray CT apparatus, in order to obtain a three-dimensional X-ray CT image free from artifacts due to mechanical manufacturing errors, a shift from the design position of the rotation center axis and the trajectory of the focus of the X-ray source are included. There are cases where a rotational orbital surface, which is a plane, is obtained as a parameter (hereinafter referred to as a geometric parameter) and is corrected while being corrected in an image reconstruction calculation. There are conventional techniques described in Patent Documents 1 and 2 as methods for obtaining geometric parameters, and Patent Document 3 as a method for accurately obtaining the rotation center axis of a gantry type cone beam X-ray CT apparatus.

  Patent Document 1 discloses a method of photographing a small metal sphere having a large X-ray absorption coefficient and performing geometric correction of the rotation center using a deviation from the sinogram of the projected image.

  In Patent Document 2, a phantom in which a plurality of small metal spheres having a large X-ray absorption coefficient are placed on a rod-like support is arranged in parallel to the rotation center axis, and rotated 360 degrees, and the metal spheres are two-dimensional X-rays. Disclosed is a method for obtaining geometric parameters such as a rotation orbit plane, a rotation center axis, and a detector mounting correction angle using an elliptical orbit drawn on a transmission image.

In Patent Document 3, a wire line having a large X-ray absorption coefficient is photographed, and the geometric parameters and rotation center axis projection used in the reconstruction operation are evaluated while evaluating the sharpness of the three-dimensional X-ray CT image by the evaluation function. A method for obtaining with high accuracy is disclosed.
"Practical Cone-Beam Algorithm; LAFeldkamp, et al .; J. Optical Society of America, A / Vol. 1 (6), (1984), pp.612-619" Japanese Patent Laid-Open No. 7-16220 Japanese Patent Laid-Open No. 9-173330 JP 2000-201918 A

  As a result of studying the method for determining the imaging system geometric parameter according to the conventional technique, the present inventor has found the following problems.

  The geometric parameter correction method described in Patent Document 1 is a geometric parameter adjustment method using interpolation calculation on a projected image. Therefore, there is a problem that the accuracy required for CT reconstruction may not be obtained.

  The geometric parameter correction method described in Patent Document 2 is also a geometric parameter adjustment method using an interpolation operation on a projected image, and thus has a problem that the accuracy necessary for CT reconstruction cannot be obtained.

  In addition, the rotation center axis calculation method described in Patent Document 3 is a method of obtaining geometric parameters while evaluating the sharpness with an evaluation function including the artifacts in the reconstructed image of the wire cross section. The cone beam X-ray CT apparatus having a high roundness of the rotation trajectory can calculate the rotation center axis with high accuracy. However, in the case of the rotation trajectory with deflection and shake like the C-type arm, the correct geometry There is a problem that parameters cannot be obtained.

  The present invention has been made in view of such circumstances. From the rotational imaging data by the two-dimensional X-ray detector, in particular, from the rotational imaging data of 180 ° + detector opening angle by the C-type arm, a clear three-dimensional X-ray is obtained. It is an object of the present invention to provide a geometric parameter determination method for reconstructing a line CT image.

  In order to achieve the object, an X-ray CT apparatus according to the present invention includes an X-ray source that irradiates a subject with X-rays, and the X-ray that is disposed to face the X-ray source and transmits the subject. A two-dimensional X-ray detector that detects a line and outputs X-ray image data of the subject, and the X-ray source and the two-dimensional X-ray detector are rotated about the same rotation center axis. An X-ray CT apparatus comprising: a rotation unit; and an image processing apparatus including a back projection unit that performs an image reconstruction operation based on the X-ray image data. The X-ray source and the two-dimensional X-ray detector Is designed to rotate about a reference rotation center axis, and the image processing apparatus calculates a relative displacement amount between a position of the reference rotation center axis and a position of the rotation center axis indicating an actual rotation movement. In the vicinity of the reference rotation center axis and the rotation orbit surface of the two-dimensional X-ray detector A rotation center axis projection position calculation means for calculating based on the feature amount of the projected image of the placed phantom, and the back projection means corrects the X-ray image data based on the relative displacement amount and re-images the image. Constitute.

  Further, an X-ray CT apparatus according to the present invention includes an X-ray source that irradiates a subject with X-rays, and an X-ray source that is disposed to face the X-ray source and transmits the subject to detect the X-ray. A two-dimensional X-ray detector for outputting X-ray image data of a subject; a rotating means for rotating the X-ray source and the two-dimensional X-ray detector around the same rotation center axis; An X-ray CT apparatus comprising back projection means for performing image reconstruction calculation based on line image data, wherein the X-ray source and the two-dimensional X-ray detector have a reference rotation center axis The two-dimensional X-ray detector is designed to be mounted at a predetermined reference angle with respect to the reference rotation center axis, and the image processing apparatus The actual mounting angle of the two-dimensional X-ray detector with respect to the The correction angle necessary for correcting the angle difference from the angle is based on the feature amount of the projected image of the phantom located near the reference rotation center axis and at a predetermined distance from the rotation orbit plane of the two-dimensional X-ray detector. And a detector mounting correction angle calculation means for calculating the X-ray image data based on the correction angle to reconstruct the image.

  In addition, an X-ray CT apparatus according to the present invention includes an X-ray source that irradiates a subject with X-rays, and an X-ray source that is disposed so as to face the X-ray source and that has transmitted through the subject. A two-dimensional X-ray detector that outputs X-ray image data of a subject; a rotating means for rotating the X-ray source and the two-dimensional X-ray detector around the same rotation center axis; An X-ray CT apparatus including a back projection means for performing image reconstruction calculation based on line image data, wherein the X-ray source and the two-dimensional X-ray detector have a reference rotation center axis And the two-dimensional X-ray detector is designed to be mounted at a predetermined reference angle with respect to the reference rotation center axis, and the image processing apparatus includes the reference rotation center axis. And a position of the two-dimensional X-ray detector located in the vicinity of the rotational orbital surface. Based on the feature amount of the projected image of Tom, the feature amount of the projected image of the phantom located in the vicinity of the reference rotation center axis and at a predetermined distance from the rotation orbit plane of the two-dimensional X-ray detector, and the predetermined distance The relative displacement amount between the position of the reference rotation center axis and the position of the rotation center axis indicating the actual rotational movement, the actual mounting angle of the two-dimensional X-ray detector with respect to the reference rotation center axis, and the reference angle And a geometric parameter calculation means for calculating a correction angle required to correct the angle difference between the first and second projections, and the back projection means corrects the X-ray image data based on the relative displacement amount and the correction angle. To reconstruct the image.

  The X-ray CT apparatus according to the present invention further includes a bed on which the subject is placed, and the phantom is a circle arranged along the body axis direction of the subject placed on the bed. It is configured as a columnar or hollow pipe phantom.

  According to the present invention, it is possible to reconstruct a clear three-dimensional X-ray CT image from rotational imaging data by a two-dimensional X-ray detector, in particular, rotational imaging data of 180 ° + detector opening angle by a C-arm. A method for determining geometric parameters can be provided.

  Hereinafter, preferred embodiments of an X-ray CT apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus to which the present invention is applied. FIG. 2 is a conceptual diagram showing a hall chart 18 used for generating an image distortion correction table. FIG. 3 is a view for explaining a detector mounting angle generated when the Hall chart 18 is mounted on the X-ray incident surface of the two-dimensional X-ray detector.

  The cone beam X-ray CT apparatus 1 in FIG. 1 irradiates a subject 40 with X-rays, captures an X-ray transmission image of the subject 40, and obtains X-ray image data. And a control calculation unit 20 that controls each of the above components and reconstructs a three-dimensional X-ray CT image of the subject 40 based on X-ray image data.

(Shooting unit 10)
The imaging unit 10 is installed at a position facing the bed 17 on which the subject 40 is placed, the X-ray source 11 that irradiates the subject 40 placed on the bed 17 with X-rays, and the X-ray source 11. A two-dimensional X-ray detector 12 that outputs X-ray image data by detecting transmitted X-rays and a C-arm 13 that mechanically connects the X-ray source 11 and the two-dimensional X-ray detector 12 are provided. . In addition, the photographing unit 10 includes a C-type arm holding body 14 for holding the C-type arm 13, a ceiling support 15 for attaching the C-type arm holding body 14 to the ceiling, and the ceiling support 15 in the state of FIG. And a ceiling rail 16 that is movably supported in the two-dimensional direction.

  The X-ray source 11 includes an X-ray tube 11t that generates X-rays, and a collimator 11c that controls the direction of X-ray irradiation from the X-ray tube 11t in a cone or quadrangular pyramid shape.

  The two-dimensional X-ray detector 12 includes an X-ray image intensifier 12i that converts an X-ray transmission image into a visible light image, and a television camera 12c that captures a visible light image by the X-ray image intensifier 12i. In the present embodiment, the two-dimensional X-ray detector 12 includes an X-ray image intensifier 12i and a television camera 12c, but the two-dimensional X-ray detector 12 may be a flat panel detector (FPD). Good. Further, the shape may be any shape such as a circle and a rectangle. The two-dimensional X-ray detector 12 is installed such that its detector element array is parallel (0 °) or 90 ° to the rotation center axis 31. For example, when a rectangular flat panel detector (FPD) is used as the two-dimensional X-ray detector 12, if the long side is installed at an angle of 90 ° with the central axis of rotation, a cross-sectional image of a large field of view such as the chest and abdomen can be obtained. It is suitable for photographing, and it is useful for photographing the head and neck, limbs, etc. if the long side is installed at an angle parallel to the rotation center axis (0 °). The two-dimensional X-ray detector 12 may be configured such that its detector element array can be rotated manually or electrically by a predetermined reference angle with respect to the rotation center axis 31.

  The C-arm 13 rotates around the rotation center axis 31 at every predetermined projection angle when the subject 40 is imaged. Thereby, the X-ray source 11 and the two-dimensional X-ray detector 12 perform X-ray imaging while rotating and moving on substantially the same circular orbit. For this rotational movement, there are geometric parameters used for the image reconstruction operation. That is, as the C-arm 13 rotates, a rotation orbit plane (midplane) 30 that includes a circular orbit drawn by the X-ray source 11 and the two-dimensional X-ray detector 12, a rotation center axis 31, And the deviation of the detector mounting angle from the reference angle (0 ° or 90 °).

  These geometric parameters are determined in principle from the design data of the imaging unit 10, but in reality, due to manufacturing errors of the imaging unit 10 and deformation of members, individual cone beam X-rays are used. This value is unique to the CT apparatus 1.

(Control operation unit 20)
The control calculation unit 20 is based on the imaging unit control unit 100 that controls the imaging unit 10, the image collection unit 110 that collects and stores the X-ray image data output by the imaging unit 10, and the collected X-ray image data. The reconstructing means 200 for reconstructing a three-dimensional X-ray CT image and an error in mechanical production of the imaging unit 10 are numerically expressed and used as correction data when the reconstructing means 200 performs three-dimensional reconstruction. Geometric parameter calculation means 300 for obtaining geometric parameters. Furthermore, an image display unit 210 that displays a three-dimensional X-ray CT image generated by the reconstruction unit 200 is provided.

  The imaging unit control unit 100 includes an imaging system rotation control unit 101 that controls the rotational movement of the C-arm 13 around the rotation center axis 31 (hereinafter referred to as “propeller rotation”), and the ceiling support 15. An imaging system position control means 102 is provided for controlling the position of the C-arm 13 relative to the subject 40 in a two-dimensional manner by controlling the position on the ceiling rail 16. Further, the imaging unit control means 100 adjusts the position of the subject 40 by controlling the position of the bed 17 and the X-ray irradiation control means 103 that controls ON / OFF of the tube current flowing through the X-ray tube 11t. A couch control unit 104 and a detection system control unit 105 that controls imaging of an X-ray transmission image by the two-dimensional X-ray detector 12 are provided.

(Reconstruction means 200)
The reconstruction unit 200 includes a preprocessing unit 201, an image distortion correction unit 202, a filtering unit 203, and a back projection unit 204.

  The preprocessing means 201 is means for converting the X-ray image data collected by the image collection means 110 into an X-ray absorption coefficient distribution image. In the present embodiment, first, a natural logarithmic transformation calculation is performed on each pixel data of an X-ray transmission image of air that has been captured in advance without the subject 40 and the bed 17 being placed in the field of view. Next, a natural logarithmic conversion operation is performed on each pixel data of an X-ray transmission image taken with the subject 40 placed on the bed 17. By taking the difference between the two X-ray transmission images, an X-ray absorption coefficient distribution image of the subject 40 and the bed 17 is obtained.

  The image distortion correction unit 202 corrects the image distortion of the X-ray absorption coefficient distribution image generated by the preprocessing unit 201. This image distortion is an image distortion generated when an X-ray transmission image is converted into a visible light image by the X-ray image intensifier 12i, and is stored in an image distortion correction table storage unit 330 described later. Is used to correct the image distortion of the X-ray absorption coefficient distribution image obtained by the pre-processing means 201.

  The filtering unit 203 performs a filtering process in X-ray CT image reconstruction.

  The back projection unit 204 is a unit that performs a back projection operation based on the filtered X-ray image data, and generates a three-dimensional X-ray CT image.

(Geometric parameter calculation means 300)
The geometric parameter calculation unit 300 is a unit that calculates a geometric parameter required when the reconstruction unit 200 performs image reconstruction. The geometric parameters handled by the geometric parameter calculation means 300 are the rotation trajectory plane 30, the rotation center axis 31, the image distortion caused by the X-ray image intensifier 12i, and the mounting angle of the two-dimensional X-ray detector 12. These are the rotation trajectory 30 and the rotation center axis 31, the image distortion caused by the X-ray image intensifier 12i in FIG. 1, and the detector mounting correction angle described later. In order to calculate these geometric parameters, the geometric parameter calculation means 300 includes an image distortion correction table generation means 320, an image distortion correction table storage means 330, a rotation trajectory plane calculation means 350, a rotation center axis projection position. Calculation means 370 and detector mounting correction angle calculation means 380 are provided. Each component of the geometric parameter calculation means 300 described above will be described later.

  The image distortion correction table generation unit 320 generates an image distortion correction table used by the image distortion correction unit 202 described above. The image distortion correction table generating means 320 generates an image distortion correction table using the hall chart 18 shown in FIG. The hole chart 18 in FIG. 2 is formed by drilling a large number of small holes 18h in a lattice arrangement on a plate material made of a material that absorbs a large amount of X-rays. In order to generate the image distortion correction table, first, the Hall chart 18 is fixed to the front surface of the X-ray image intensifier 12i, and the X-ray projection image of the Hall chart 18 (hereinafter referred to as “Hall chart distortion projection image”). Shoot. Next, preprocessing means 201 preprocesses the whole chart distortion projection image. Then, the projection position of each hole 18h in the hall chart distortion projection image is detected. Next, the virtual projection position of each hole 18h is calculated assuming that the hole chart 18 is projected onto the X-ray incident surface without distortion. Then, a calculation for converting the detected projection position of each hole 18h so as to coincide with the virtual projection position is performed for each hole 18h, and an image distortion correction table is generated.

  The image distortion correction table storage unit 330 stores the image distortion correction table generated by the image distortion correction table generation unit 320 on a magnetic disk or the like. When the image distortion correction unit 202 corrects the distortion of the X-ray transmission image, the stored image distortion correction table is read out.

  The rotational trajectory plane calculating means 350 is a means for calculating coordinates on the two-dimensional X-ray transmission image of the rotational trajectory plane 30 which is a plane including the focal trajectory of the X-ray source. For example, a phantom in which a plurality of small metal spheres having a large X-ray absorption coefficient are placed on a rod-like support is arranged in parallel to the rotation center axis, and rotation imaging is performed. It can be obtained using the elliptical orbit drawn above.

  As shown in FIGS. 4 and 5, the cone beam X-ray CT apparatus 1 takes an X-ray transmission image (hereinafter referred to as “phantom projection image”) of a cylindrical or hollow pipe-shaped phantom 50, and phantom X-rays. Output image data.

  The rotation center axis projection position calculation means 370 calculates the projection coordinates on the two-dimensional X-ray transmission image of the rotation center axis (hereinafter referred to as “rotation center axis projection position”) from the reconstruction calculation of the phantom projection image. It is.

  The detector attachment correction angle calculation means 380 is a means for calculating a difference (detector attachment correction angle) between the coordinate axis on the image of the two-dimensional X-ray detector 12 and the actual rotation center axis.

  The detector mounting angle will be described with reference to FIG. 32 is a projection of the rotation center axis 31, and 30 is a rotation orbit plane. Reference numeral 22 denotes a vertical axis fixed to the Hall chart 18, and 21 denotes a horizontal axis of the Hall chart 18, which are attached to the projection 32 of the rotation center axis 31 and the rotation orbit plane 30 by an angle b (24). 24 is called a detector mounting angle, and if the angle b (24) is not zero, the image after image distortion correction by the image distortion correction means 202 is rotated by an angle b. The rotation angle b is obtained by the detector mounting correction angle calculation means 380, and the X-ray image is rotated in the direction opposite to the detector mounting angle b to correct it. However, in this embodiment, the X-ray image intensifier 12i and the television camera 12c are used as the X-ray imaging system. However, in the case of replacing with a two-dimensional X-ray detector using a TFT element or the like, an X-ray detector is used. The deviation of the mounting angle is angle b.

  A specification example of the C-arm type cone beam X-ray CT apparatus 1 is as follows. The distance between the X-ray tube 11t and the rotation center axis 31 is 800 mm, and the distance between the rotation center axis 31 and the X-ray input surface of the two-dimensional X-ray detector 12, that is, the X-ray incident surface of the X-ray image intensifier 12i is The size of the X-ray incident surface of the X-ray image intensifier 12i is 400 mm and the image size is 1024 × 1024 (the number of scanning lines). The pitch of the two-dimensional X-ray detector 12 is 0.4 mm.

  The imaging system rotation control means 101 moves the two-dimensional X-ray detector 12 in the direction of the left hand of the subject 40 (with the head of the subject 40 facing the C-arm 13 side on the bed 17 ( −100 °) passes the ceiling direction (0 °) and moves to the right hand direction (+ 100 °) of the subject 40 (at this time, the X-ray source 11 is under the bed from the right hand direction (+ 100 °) of the subject 40). By passing through the direction and moving to the left hand direction (−100 °) of the subject 40, a two-dimensional X-ray transmission image of the subject 40 is captured over a projection angle of 200 degrees. A typical example of the rotational speed of the C-arm 13 is 40 degrees per second, and the scan time is 5 seconds. The two-dimensional X-ray detector 12 is placed on the ceiling from the direction of the right hand (−100 °) of the subject 40 with the feet of the subject 40 placed on the bed 17 with the C-arm 13 side facing. There is also a case where the subject 40 is moved to the left hand direction (+ 100 °) through the direction (0 °).

  Next, an outline of operations in imaging by the C-arm type cone beam X-ray CT apparatus 1 of the present embodiment will be described.

  First, the imaging system rotation control means 101 starts the propeller rotation of the C-arm 13. After the rotation acceleration period, the X-ray irradiation control means 103 irradiates the X-ray tube 11t with X-rays. The detection system control means 105 starts capturing a visible light image by the television camera 12c. The X-ray irradiated from the X-ray tube 11t passes through the subject 40 and then enters the X-ray image intensifier 12i. The X-ray transmission image is converted into a visible light image by the X-ray image intensifier 12i and captured by the television camera 12c. The television camera 12c converts a visible light image into a video signal. The video signal undergoes A / D conversion, and is then recorded in the image collecting unit 110 as two-dimensional X-ray image data including a digital signal. The standard scanning mode for photographing by the television camera 12c is 30 frames per second and 1024 scanning lines. The rotation angle pitch is 1.33 degrees, and 150 X-ray transmission images are acquired in 5 seconds. When the 200 degree rotation imaging is completed, the X-ray irradiation control means 103 ends the X-ray irradiation of the X-ray tube 11t, and the imaging system rotation control means 101 stops the rotation.

  The reconstruction unit 200 reads the two-dimensional X-ray image data from the image collection unit 110 in parallel with the above-described imaging or after the imaging is completed, performs an image reconstruction calculation based on the X-ray image data, Reconstruction calculation of a three-dimensional X-ray CT image of the specimen 40 is performed. The image display unit 210 displays a three-dimensional X-ray CT image on a display device 80 including a CRT device or a liquid crystal display device. Note that the image display unit 210 may display a two-dimensional image based on the X-ray image data recorded in the image collection unit 110.

  Next, the contents of the processing in the geometric parameter calculation means 300 will be described based on FIGS. FIG. 4 is a conceptual diagram for explaining a state in which the cylindrical phantom 50 is rotated and photographed. FIG. 5 is a conceptual diagram showing a state of FIG. 4 as viewed from the bed 17 toward the C-arm 13. FIG. 6 is a flowchart showing a procedure in which the geometric parameter calculation means 300 performs a geometric parameter calculation process. Hereinafter, description will be made in the order of steps in FIG.

(Step S300)
The geometric parameter calculation process is started (S300).

(Step S310)
The Hall chart 18 is fixed to the X-ray incident surface of the X-ray image intensifier 12i to perform rotational imaging, and the image collection means 110 collects a Hall chart distortion projection image at each projection angle. In the case of the present embodiment, the number of hall chart distortion projection images to be collected is 150 as described above.

(Step S320)
The image distortion correction table generation unit 320 generates an image distortion correction table based on the 150 hole chart distortion projection images obtained in step S310.

(Step S330)
The image distortion correction table storage unit 330 stores the image distortion correction table generated and calculated in step S320.

(Step S340)
The rod-shaped phantom containing a plurality of small metal spheres is rotated and an X-ray transmission image of the metal sphere phantom is collected by the image collecting means 110.

(Step S350)
From the X-ray transmission image of the metal sphere phantom collected in step S340, an elliptical orbit drawn by the small metal sphere in the X-ray transmission image is obtained, and the coordinates on the two-dimensional X-ray transmission image of the rotating orbit plane are determined.

(Step S360)
The cylindrical phantom 50 is rotated and photographed. As shown in FIG. 4, the phantom holder 50 a is placed on the bed 17. Then, the phantom holder 50a places the cylindrical phantom 50 so as to protrude from the bed 17 at a position as close as possible to the rotation center axis 31, as shown in FIGS. Then, the X-ray source 11 and the two-dimensional X-ray detector 12 perform rotational imaging of the cylindrical phantom 50, take an X-ray transmission image of the cylindrical phantom 50, and output phantom X-ray image data. The X-ray transmission image of the cylindrical phantom 50 includes a phantom projection image that is a transmission image of the cylindrical phantom 50. The image collecting unit 110 collects phantom X-ray image data. When the photographing of the cylindrical phantom 50 is completed, the cylindrical phantom 50 and the cylindrical phantom holder 50a are removed from the bed 17.

(Step S370)
The rotation center axis projection position calculation means 370 uses the phantom projection image included in the phantom X-ray image data collected in step S360 and uses the rotation center as a reference for forming a three-dimensional X-ray CT image. Determine axial projection parameters. Details of specific processing of the calculation will be described later with reference to FIGS. In step S370, it is sufficient to perform processing using the phantom X-ray image data limited to the rotation orbit plane (midplane) 30. By doing so, the amount of calculation can be reduced.

(Step S380)
A detector mounting correction angle calculation unit 380 calculates a detector mounting correction angle from the X-ray transmission image of the cylindrical phantom 50 collected in step S360. Note that step S380 differs from step S370 in that a reconstructed image of the cylindrical phantom 50 is generated on a surface away from the rotational orbital surface.

  The geometric parameters calculated in steps S350, S370, and S380 are used to form a three-dimensional X-ray CT image in the back projection unit 204.

  Next, details of the rotation center axis projection position calculation means 370 and the detector mounting correction angle calculation means 380 will be described with reference to FIGS. FIG. 7 is a diagram showing a reconstructed cross section of a cylindrical phantom reconstructed from projection data in the first half of rotation when the projection parameter of the rotation center axis has an error in the + u direction. FIG. 8 is a diagram showing a reconstructed section of a cylindrical phantom reconstructed from projection data in the latter half of the rotation when the projection parameter of the rotation center axis has an error in the + u direction. FIG. 9 is a diagram showing a reconstructed cross section of a cylindrical phantom reconstructed from projection data of full rotation shooting when the projection parameter of the rotation center axis has an error in the + u direction. FIG. 10 is a diagram showing a reconstructed cross section of a cylindrical phantom reconstructed from projection data of full rotation imaging when the projection parameter of the rotation center axis has an error in the −u direction. FIG. 11 is a flowchart illustrating a procedure in which the rotation center axis projection position calculation unit 370 performs the rotation center axis projection position calculation process according to the embodiment of the present invention. FIG. 12 is a flowchart showing a procedure in which the detector attachment correction angle calculation means 380 performs the detector attachment correction angle calculation processing in the embodiment of the present invention.

  First, a phantom reconstruction image 53 obtained when the initial position of the rotation center axis 31 is different will be described with reference to FIGS.

  For easy understanding, FIG. 7 shows a state in which a reconstructed image is generated from the projection data in the first half of the rotation, and FIG. 8 shows a state in which a reconstructed image is generated from the projection data in the second half of the rotation. 7 and 8 show a case where the projection position of the rotation center axis 51 is reconstructed with a value larger in the u-axis direction than the geometrically correct value 52. FIG. 7 and 8 are viewed from the foot direction of the subject 40 (that is, from the bed 17 toward the C-arm 13), and only the X-ray image intensifier 12i and the television camera 12c are shown for simplicity. Others were omitted. The X-ray image intensifier 12 i and the television camera 12 c move from the direction of the left hand of the subject 40 to the direction of the right hand of the subject 40 through the direction of the ceiling.

  As shown in FIG. 7, the reconstructed cross-sectional image 54 from the projection data in the first half of the rotation does not have the correct size, and the phantom radius indicated by the reconstructed cross-sectional image 54 becomes small. The reduced radius of the reconstructed phantom is obtained by calculating the phantom end projection position 51 measured with the geometrically correct rotation center axis and the phantom end projection position 52 measured with the currently used rotation center axis. Equal to the difference.

  On the other hand, the phantom radius of the reconstructed sectional image 55 from the projection data in the latter half of the rotation shown in FIG. In this case, the radius of the increased reconstruction phantom is such that the projected position 51 of the phantom end measured with the geometrically correct rotation center axis and the projected position 52 of the phantom end measured with the currently used rotation center axis. Is equal to the difference.

  FIG. 9 shows a reconstructed image 56 that is actually obtained when the projection position of the rotation center axis is reconstructed with a value larger than the geometrically correct value in the u-axis direction. A reconstruction image 57 obtained when reconstruction is performed with the axial projection position smaller than the geometrically correct value in the u-axis direction is shown. The phantom reconstructed cross section is not circular but unbalanced from side to side. Note that FIGS. 9 and 10 are schematic for easy understanding, and the boundary between the left and right data does not actually protrude, and a smoothly connected reconstruction image is obtained.

  Next, a procedure in which the rotation center axis projection position calculation unit 370 determines the rotation center axis projection position will be described in detail with reference to FIG. As described above, in the following processing, it is sufficient for the rotation center axis projection position calculation means 370 to perform calculation processing using projection data limited to the rotation orbit plane (midplane) 30.

(Step S370)
The rotation center axis projection position calculation process is started (S370).

(Step S371)
A rotation center axis projection position (hereinafter referred to as center) is set as an initial position (design position).

(Step S372)
The preprocessing unit 201 performs preprocessing of the phantom X-ray image data collected in step S360.

(Step S373)
The image distortion correction unit 202 performs image distortion correction processing on the phantom X-ray image data that has been pre-processed in step S372.

(Step S374)
The filtering unit 203 performs a filtering process on the phantom X-ray image data that has been subjected to the image distortion correction process in step S373.

(Step S375)
The back projection unit 204 performs a back projection operation on the rotation orbital plane 30 with the value of the rotation center axis projection position (center) for the phantom X-ray image data subjected to the filtering process in step S374, and the phantom reconstruction image 53 is obtained. Is generated.

(Step S376)
The left half phantom reconstruction radius (rA) and the right half phantom reconstruction radius (rB) of the phantom reconstruction image 53 generated in step S375 are calculated. Here, for example, a method of parameter fitting to a circular curve equation can be used to calculate the reconstruction radius.

(Step S377)
It is determined whether the left and right phantom reconstruction radii (rA, rB) calculated in step S376 can be considered equal. If the difference between the reconstruction radius Ra and the reconstruction radius rB can be regarded as zero, the current center value is output and the process ends (S379). If it cannot be regarded as zero, the process proceeds to step S378.

(Step S378)
If the radius of the arc cannot be regarded as zero in step S377, it is determined whether it is the case of FIG. 9 or FIG. 10, and correction is performed by adding / subtracting so that the center value becomes correct accordingly. Then, the processing from step S375 is performed again.

  Next, with reference to FIG. 12, a procedure in which the detector mounting correction angle calculation means 380 determines the detector mounting correction angle will be described in detail.

(Step S380)
The detector mounting correction angle calculation process is started (S380).

(Step S381)
A detector mounting correction angle (hereinafter referred to as twist) is set to an initial position (design position, for example, 0 °).

(Step S382)
The preprocessing unit 201 performs preprocessing of the phantom X-ray image data collected in step S360, as in step S372.

(Step S383)
Similar to step S373, the image distortion correction unit 202 performs image distortion correction processing on the phantom X-ray image data that has been pre-processed in step S382.

(Step S384)
The filtering unit 203 performs the filtering process on the phantom X-ray image data that has been subjected to the image distortion correction process in step S383, as in step S374.

(Step S385)
The back projection unit 204 rotates the trajectory of the phantom X-ray image data subjected to the filtering process in step S384 with the detector mounting correction angle (twist) and the rotation center axis projection position (center) determined in step S380. A back projection operation is performed on one or more surfaces away from the surface 30 to generate a phantom reconstructed image 53.

(Step S386)
The left half phantom reconstruction radius (r _A ) and the right half phantom reconstruction radius (r _B ) of the phantom reconstruction image 53 generated in step S385 are calculated.

(Step S387)
It is determined whether the left and right phantom reconstruction radii (rA, rB) calculated in step S387 can be regarded as equal. If the difference between the phantom reconstruction radius rA and the phantom reconstruction radius rB can be regarded as zero, the current twist value is output and the process ends (S389). If it cannot be regarded as zero, the process proceeds to step S388.

(Step S388)
If the radius of the arc cannot be regarded as zero in step S387, it is determined whether it is the case of FIG. 9 or FIG. 10, and correction is performed by adding / subtracting so that the center value becomes correct accordingly. Then, the processing from step S385 is performed again.

  Next, a second embodiment of the present invention will be described with reference to FIGS. In the first embodiment, after the rotation center axis projection position calculation unit 370 calculates the projection position of the rotation center axis, the correction angle calculation by the detector mounting correction angle calculation unit 380 is performed. In the embodiment, processing equivalent to that of the rotation center axis projection position calculation unit 370 and the detector attachment correction angle calculation unit 380 is performed at a time. FIG. 13 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus 1 according to the second embodiment. FIG. 14 is a flowchart for explaining the second embodiment, and FIG. 15 is a diagram showing how the rotation center axis projection position and the detector mounting correction angle are obtained from the processing of FIG. The ellipse on the left side of FIG. 15 shows the rotation center axis projection position and the detector mounting angle, the reconstructed cross-sectional image obtained when the cylindrical phantom 50 is reconstructed without correcting both correctly, and the detector mounting angle b. It is a perspective view which shows typically a mode that a reconstruction cross section changes to a slice direction because is not zero. This figure shows an oblique view of the reconstructed cross-sectional image of 3 slices, but if both the rotation center axis projection position and detector mounting angle are not corrected correctly, the left and right imbalance changes continuously in the slice direction. It shows how it is reconstructed as a cylindrical body. As a special case of FIG. 15, when only correction of the detector mounting angle is correctly performed, a reconstructed cross-sectional image in which the left-right imbalance is constant regardless of the slice position is obtained.

  As shown in FIG. 13, the geometric parameter calculation means 300 of the X-ray CT apparatus 1 according to the second embodiment includes a rotation center axis projection position calculation means 370 in the geometric parameter 300 of the first embodiment, Further, instead of the detector mounting correction angle calculation means 380, a rotation center axis projection position and detector mounting correction angle calculation means 470 are provided. Other configurations are the same as those of the X-ray CT apparatus according to the first embodiment.

  Based on FIG. 14, the rotation center axis projection position and detector mounting correction angle calculation processing means 470 in the second embodiment will be described.

(Step S470)
The rotation center axis projection position and detector mounting correction angle calculation processing is started (S470).

(Step S471)
As in step S371, the rotation center axis projection position CM on the rotation orbit plane is set to the initial position (design position).

(Step S472)
The preprocessing unit 201 performs preprocessing of the phantom X-ray image data collected in step S360, as in step S372.

(Step S473)
Similar to step S373, the image distortion correction unit 202 performs image distortion correction processing on the phantom X-ray image data that has been pre-processed in step S472.

(Step S474)
The filtering unit 203 performs the filtering process on the phantom X-ray image data that has been subjected to the image distortion correction process in step S473, as in step S374.

(Step S475)
The back projection unit 204 performs back projection operation on a plurality of cross sections (distance from the rotation orbit plane is D) parallel to the rotation orbit plane with respect to the phantom X-ray image data subjected to the filtering process in step S474, A phantom reconstruction image 53 is generated.

(Step S476)
The left half phantom reconstruction radius (r _A ) and the right half phantom reconstruction radius (r _B ) of the plurality of phantom reconstruction images 53 generated in step S475 are calculated. For the calculation of the reconstruction radius, a method of parameter fitting to a circular curve equation or the like can be used as in the first embodiment.

(Step S477)
From the left and right phantom reconstruction radii (r _A , r _B ) calculated in step S476, the correction amount C (D) of the rotation center axis projection position is calculated for each reconstruction section.

(Step S478)
Using the correction amount C (D) of the rotation center projection position calculated in step S477, as shown in FIG. 15, the rotation center axis projection position CM and the detector mounting angle b are expressed by a relational expression: C (D) = CM + D · tanb Determine by fitting to. The determined rotation center axis projection position CM and detector mounting angle b are output and the process ends (S479).

  In the cone beam X-ray CT apparatus 1 for generating and displaying a three-dimensional X-ray CT image by the geometric parameter processing calculation means 300 described above, a geometric parameter for generating a clear three-dimensional X-ray CT image The projection position of the rotation center axis 31 and the detector mounting angle 24 can be obtained.

  As described above, a cone that generates a three-dimensional X-ray CT image from a two-dimensional X-ray image collected from rotational imaging of a C-type arm or the like by mounting the geometric parameter calculation means of the present embodiment. In the beam X-ray CT apparatus, a clear three-dimensional X-ray CT image can be generated and displayed even when the rotation center axis and detector mounting angle are displaced from correct design values. As a result, contrast imaging of the head, abdomen, and the like, and diagnostic performance in the field of shaping of the teeth, jaws, lumbar vertebrae, and limbs can be improved.

  In addition, this invention is not limited to the said embodiment, It can deform | transform variously in the range which does not deviate from the summary of this invention. For example, in the above embodiment, the two-dimensional X-ray detector is formed by a combination of an X-ray image intensifier and a TV camera. Instead, for example, a flat panel detector (FPD) is used. A two-dimensional X-ray detector may be configured. The present invention can also be applied to a cone beam X-ray CT apparatus other than the C-arm system, and of course, can also be applied to cases where the entire angle of rotational imaging is other than 200 degrees.

1 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus 1 to which the present invention is applied. FIG. It is a conceptual diagram which shows the hole chart 18 used for the production | generation of an image distortion correction table. It is a figure for demonstrating the detector attachment angle which arises when attaching the Hall chart 18 to the X-ray entrance plane of a two-dimensional X-ray detector. It is a conceptual diagram which shows the state by which the cylindrical phantom 50 is mounted in the bed 17 using the phantom holding body 50a, and is rotationally imaged. It is a conceptual diagram which shows the state by which the phantom of FIG. 4 is rotationally imaged. It is a flowchart which shows the procedure in which the geometric parameter calculation means 300 in embodiment of this invention performs a geometric parameter calculation process. It is the figure which showed about the reconstruction cross section of the cylindrical phantom reconstructed from the projection data of the first half of rotation when the projection parameter of the rotation center axis has an error in the + u direction. It is the figure which showed about the reconstruction cross section of the cylindrical phantom reconstructed from the projection data of the latter half of rotation when the projection parameter of the rotation center axis has an error in the + u direction. It is the figure which showed about the reconstruction cross section of the cylindrical phantom reconfigure | reconstructed from the projection data of all rotation imaging | photography when the projection parameter of a rotation center axis | shaft has an error in + u direction. It is the figure which showed about the reconstruction cross section of the cylindrical phantom reconfigure | reconstructed from the projection data of all the rotation imaging | photography when the projection parameter of a rotation center axis | shaft has an error in -u direction. It is a flowchart which shows the procedure in which the rotation center axis | shaft projection position calculation means 370 in embodiment of this invention performs a rotation center axis | shaft projection position calculation process. It is a flowchart which shows the procedure in which the detector attachment correction angle calculation means 380 in embodiment of this invention performs a detector attachment correction angle calculation process. It is a block diagram which shows schematic structure of the C-arm type cone beam X-ray CT apparatus 1 in 2nd embodiment of this invention. It is a flowchart which shows the procedure in which rotation center axis projection position and detector attachment correction angle calculation means 470 in the second embodiment of the present invention performs rotation center axis projection position and detector attachment correction angle calculation processing. FIG. 10 is a diagram showing how the rotation center axis projection position CM and the detector attachment angle b are obtained by fitting to the relational expression: C (D) = CM + D · tanb in the rotation center axis projection position and detector attachment correction angle calculation processing. is there.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Cone beam X-ray CT apparatus, 10 ... Imaging | photography part, 11 ... X-ray source, 11t ... X-ray tube, 11c ... Collimator, 12 ... Two-dimensional X-ray detector, 12i ... X-ray image intensifier, 12c ... TV camera, 13 ... C-type arm, 14 ... C-type arm holder, 15 ... Ceiling support, 16 ... Ceiling rail, 17 ... Sleeper, 18 ... Hall chart, 18h ... Hall, 20 ... Control operation unit, 21 ... Hall A horizontal axis fixed to the chart 18, 22... A vertical axis fixed to the Hall chart 18, 24... Detector mounting angle, 30... Rotating track surface (midplane), 31. Projection, 40 ... subject, 50 ... cylindrical phantom, 50a ... phantom holder, 51 ... projection position of phantom end measured with geometrically correct center axis, 52 ... rotation currently used Projection position of the phantom end measured by the central axis, 53... Phantom reconstruction image, 54... Cylindrical phantom reconstructed from projection data of the first half of rotation when the projection parameter of the rotation center axis has an error in the + u direction Reconstructed section of 55: When the projection parameter of the rotation center axis has an error in the + u direction, the reconstruction section of the cylindrical phantom reconstructed from the projection data in the latter half of the rotation, 56: The projection parameter of the rotation center axis is When there is an error in the + u direction, the reconstructed section of the cylindrical phantom reconstructed from the projection data of the full rotation shooting, 57... When the projection parameter of the rotation center axis has an error in the −u direction Reconstructed cross section of cylindrical phantom reconstructed from projection data, 80 ... display device, 100 ... imaging unit control means, 101 ... imaging system rotation control means, 102 ... imaging system position 103, X-ray irradiation control means, 104 ... Bed control means, 105 ... Detection system control means, 110 ... Image collection means, 200 ... Reconstruction means, 201 ... Preprocessing means, 202 ... Image distortion correction means, 203 DESCRIPTION OF SYMBOLS ... Filtering means, 204 ... Back projection means, 210 ... Image display means, 300 ... Geometric parameter calculation means, 320 ... Image distortion correction table generation means, 330 ... Image distortion correction table storage means, 350 ... Rotary orbital plane calculation means, 370 ... Rotation center axis projection position calculation means, 380 ... Detector mounting correction angle calculation means

Claims (4)

An X-ray source for irradiating the subject with X-rays;
A two-dimensional X-ray detector that is disposed opposite to the X-ray source, detects the X-ray transmitted through the subject, and outputs X-ray image data of the subject;
Rotating means for rotating the X-ray source and the two-dimensional X-ray detector around the same rotation center axis;
An image processing apparatus comprising back projection means for performing image reconstruction calculation based on the X-ray image data;
In an X-ray CT apparatus equipped with
The X-ray source and the two-dimensional X-ray detector are designed to rotate around a reference rotation center axis;
The image processing apparatus includes:
The relative displacement amount between the position of the reference rotation center axis and the position of the rotation center axis indicating the actual rotational movement is determined by the phantom positioned in the vicinity of the reference rotation center axis and the rotation track surface of the two-dimensional X-ray detector. A rotation center axis projection position calculation means for calculating based on the feature amount of the projection image;
The back projection means corrects the X-ray image data based on the relative displacement amount, and reconstructs an image;
An X-ray CT apparatus characterized by that. An X-ray source for irradiating the subject with X-rays;
A two-dimensional X-ray detector that is disposed opposite to the X-ray source, detects the X-ray transmitted through the subject, and outputs X-ray image data of the subject;
Rotating means for rotating the X-ray source and the two-dimensional X-ray detector around the same rotation center axis;
An image processing apparatus comprising back projection means for performing image reconstruction calculation based on the X-ray image data;
In an X-ray CT apparatus equipped with
The X-ray source and the two-dimensional X-ray detector rotate around a reference rotation center axis, and the two-dimensional X-ray detector forms a predetermined reference angle with respect to the reference rotation center axis. Designed to be installed
The image processing apparatus includes:
A correction angle necessary for correcting an angle difference between an actual mounting angle of the two-dimensional X-ray detector with respect to the reference rotation center axis and the reference angle is determined in the vicinity of the reference rotation center axis and the two-dimensional X-ray detection. A detector mounting correction angle calculating means for calculating based on the feature amount of the projected image of the phantom located at a predetermined distance from the rotating orbital surface of the detector,
The back projection means corrects the X-ray image data based on the correction angle, and reconstructs an image;
An X-ray CT apparatus characterized by that. An X-ray source for irradiating the subject with X-rays;
A two-dimensional X-ray detector that is disposed opposite to the X-ray source, detects the X-ray transmitted through the subject, and outputs X-ray image data of the subject;
Rotating means for rotating the X-ray source and the two-dimensional X-ray detector around the same rotation center axis;
An image processing apparatus comprising back projection means for performing image reconstruction calculation based on the X-ray image data;
In an X-ray CT apparatus equipped with
The X-ray source and the two-dimensional X-ray detector rotate around a reference rotation center axis, and the two-dimensional X-ray detector forms a predetermined reference angle with respect to the reference rotation center axis. Designed to be installed
The image processing apparatus includes:
From the feature amount of the projected image of the phantom located in the vicinity of the reference rotation center axis and the rotation orbit plane of the two-dimensional X-ray detector, and from the rotation orbit plane of the two-dimensional X-ray detector in the vicinity of the reference rotation center axis Based on the feature amount of the projected image of the phantom located at a predetermined distance and the predetermined distance, the relative displacement amount between the position of the reference rotation center axis and the position of the rotation center axis indicating the actual rotation movement, Geometric parameter calculation means for calculating a correction angle required to correct an angle difference between an actual mounting angle of the two-dimensional X-ray detector with respect to the reference rotation center axis and the reference angle;
The back projection means corrects the X-ray image data based on the relative displacement amount and the correction angle, and reconstructs an image;
An X-ray CT apparatus characterized by that. Further comprising a bed on which the subject is placed;
The phantom is configured as a cylindrical or hollow pipe phantom arranged along the body axis direction of the subject placed on the bed,
The X-ray CT apparatus according to claim 1, wherein:

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