JP4421917B2 - Cone beam X-ray CT system - Google Patents

Description

  The present invention relates to a cone beam X-ray CT apparatus, and more particularly to a cone beam X-ray CT apparatus that can more easily determine the geometric parameters of an apparatus necessary for image reconstruction.

  The cone beam X-ray CT apparatus is an X-ray source that irradiates X-rays in a conical shape and a two-dimensional X-ray detector arranged opposite to the X-ray source (this is a combination of an X-ray image intensifier and a TV camera). The X-ray transmission image of the subject is rotated while rotating together on the same circular orbit. At this time, the subject is positioned on the rotation center axis in rotational imaging. A three-dimensional X-ray CT image is obtained by performing an image reconstruction calculation based on the two-dimensional X-ray transmission image acquired by the two-dimensional X-ray detector. As an image reconstruction calculation algorithm in a cone beam X-ray CT apparatus using such a two-dimensional X-ray detector, the Feldkamp method is known as a representative (for example, Non-Patent Document 1).

  Image reconstruction in the X-ray CT apparatus requires coordinates for image reconstruction. The coordinates are rotational trajectories obtained by virtually projecting a rotational trajectory plane (also called a midplane), which is a plane including a circular trajectory in which both the X-ray source and the two-dimensional X-ray detector rotate, onto the projection plane of the X-ray transmission image. The horizontal axis is given based on the surface projection, and the vertical axis is given based on the rotation center axis projection obtained by similarly projecting the rotation center axis. The correspondence between the rotation trajectory plane projection and the horizontal axis and the accuracy of the correspondence between the rotation center axis projection and the vertical axis greatly affect the accuracy of image reconstruction.

  Therefore, it is necessary to obtain the rotation orbit plane and the rotation center axis (these are also called geometric parameters in the X-ray CT apparatus necessary for image reconstruction) with high accuracy. In principle, the rotation orbit plane and the rotation center axis can be obtained from the design data of the cone beam X-ray CT apparatus. However, in practice, there are manufacturing errors, deformation of the members of the apparatus, and the like, so the actual rotation track surface and the rotation center axis appear differently from the design data for each apparatus. For this reason, it is necessary to obtain the actual rotation track surface and the rotation center axis by taking a test image for each apparatus.

  An example disclosed in Patent Document 1 is known as such a technique. In the technique of Patent Document 1, test photographing using a correction phantom is performed, and the rotation orbital plane projection and the rotation center axis projection are obtained from the image data of the obtained phantom. More specifically, it is as follows. The phantom is configured by arranging and supporting a plurality of spheres as a reference point for formation with a material having a large absorption with respect to X-rays on a support formed with a material with a relatively low absorption with respect to X-rays. In test radiography, this phantom is placed in the vicinity of the subject placement position so as to be substantially parallel to the rotation center axis, and in this state, the X-ray source and the two-dimensional X-ray detector are rotated in the same manner as in normal radiography. The X-ray transmission image is acquired at every fixed rotation angle. When the two-dimensional X-ray transmission images obtained by the test imaging are synthesized, each sphere in the phantom draws a specific locus on the synthesized image. The trajectory is an ellipse for a sphere away from the rotational trajectory plane. The short axis of the ellipse correlates with the distance of the sphere from the rotation orbital plane, and the short axis of the sphere on the rotation orbital plane is zero. In other words, the trajectory of a sphere on the rotational orbit surface is a straight line. For such an elliptical trajectory image of a sphere in a phantom, if a line segment connecting the centers of the trajectory ellipses of each sphere is obtained, it becomes a rotation center axis projection, and a linear trajectory whose minor axis is 0 from each elliptical trajectory image If an image is obtained, it becomes a rotating orbital plane projection.

  Another method for obtaining the rotation center axis is also known (for example, Patent Document 2). In a cone beam X-ray CT apparatus, it is necessary to correct image distortion caused by non-planarity of an X-ray incident surface in an X-ray image intensifier constituting a two-dimensional X-ray detector. For example, a technique disclosed in Patent Document 3 is known as an appropriate correction.

JP 9-173330 A JP 2000-201918 A JP-A-8-24248 Practical Cone-Beam Algorithm; L.A. Feldkamp, et al .; J. Optical Society of America, A / Vol. 1 (6), (1984), pp.612-619

  As described above, in the technique disclosed in Patent Document 1, a phantom formed by arranging a plurality of spheres in a straight line is used, and a geometric parameter is obtained based on an elliptical locus drawn by each sphere of the phantom. . This method is effective when, for example, 360-degree rotation imaging data can be acquired and the trajectory of the phantom sphere becomes a closed ellipse like a gantry rotating type or subject rotating type cone beam X-ray CT apparatus. . However, some cone beam X-ray CT apparatuses, such as a C-arm structure, cannot acquire 360-degree rotation imaging data, and the technique disclosed in Patent Document 1 is applied to such a cone beam X-ray CT apparatus. Can not do it.

  In addition, the rotational orbital surface, which is one of the geometric parameters, may be deviated every time shooting is performed at a predetermined rotation angle. However, the method disclosed in Patent Document 1 takes this into consideration. Absent.

  The present invention has been made in the background as described above, and for a cone beam X-ray CT apparatus, even when rotational imaging data of 360 degrees cannot be acquired as in a C-type arm structure, for example, geometric parameters, It is an object to make it possible to easily obtain the rotation track surface, and to obtain a shift of the rotation track surface for each photographing at a predetermined rotation angle.

The present invention relates to the X-ray source and the two-dimensional X-ray while rotating the X-ray source for irradiating the X-ray and the two-dimensional X-ray detector arranged opposite to the X-ray source on the same circular orbit. While irradiating the subject disposed between the detectors with X-rays from the X-ray source, an X-ray transmission image of the subject is taken with a two-dimensional X-ray detector in units of a predetermined rotation angle in the rotation, and In a cone beam X-ray CT apparatus that performs image reconstruction calculation based on the X-ray transmission image obtained by this imaging to generate a three-dimensional X-ray CT image,
The rotation track surface calculation processing is performed by separating a rotation track surface calculation phantom configured by arranging metal balls in a straight line from the rotation center axis by a predetermined distance, and the linear array direction of the phantom is the rotation center axis. The X-ray projection image of the rotational trajectory plane calculation phantom in a state of being arranged in parallel with each other and rotating to collect a plurality of projection images, and superimposing the plurality of projection images obtained in the step, a composite projection image Generating a gap line relating to a trajectory that does not overlap each other in a trajectory drawn on the composite projection image on the composite projection image, and a gap line extracted in the step A cone beam X-ray CT apparatus including a step of obtaining coordinate values on a projected image of the rotating orbital surface based on That.

  Furthermore, the present invention is configured to perform a rotation track surface movement amount calculation process that is obtained by correlating the movement amount of the rotation track surface for each rotation angle unit in the photographing with the coordinate value obtained by the rotation track surface calculation process. The rotational trajectory plane movement amount calculation processing is performed in a state where a rotational trajectory plane movement amount calculation phantom composed of a single metal sphere is arranged at the rotation center that is the intersection of the rotational trajectory plane and the rotation center axis. An X-ray projection image of the rotation trajectory surface movement amount calculation phantom is taken for each rotation angle unit, and the rotation trajectory surface of the rotation trajectory surface is based on a projection area of the index body in the projection image obtained in the step. A cone beam X-ray CT apparatus is disclosed that includes a step of calculating a displacement.

  In the present invention, the rotational trajectory plane is obtained using gap lines relating to trajectories that do not overlap each other in the trajectory drawn by each index body in the composite projection image of the rotational trajectory plane calculation phantom. For this reason, using the same phantom, the trajectory of the reference point (corresponding to the index body in the present invention) in the phantom is an elliptical closed trajectory, and the total rotation angle is 360. Even if it is less than the degree, there is an advantage that the rotating track surface can be obtained with high accuracy, and an advantage that the process for calculating the rotating track surface can be simplified.

  In the present invention, since the amount of movement of the rotating track surface for each rotation angle unit in photographing can be obtained, the rotating track surface can be determined with higher accuracy.

  Hereinafter, modes for carrying out the present invention will be described. FIG. 1 shows a schematic configuration of a C-arm type cone beam X-ray CT apparatus according to an embodiment of the present invention. As shown in FIG. 1, the cone beam X-ray CT apparatus captures an X-ray transmission image of the subject 3 to obtain measurement data, a measurement unit control unit 100 that controls the measurement unit 50, and a measurement unit 50. The three-dimensional reconstruction means 200 for reconstructing a three-dimensional X-ray CT image based on the measurement data obtained in the above, and the geometric parameters such as a rotation center axis and a rotation orbit plane described later in the measurement unit 50 are obtained. The geometric parameter calculation means 300 is provided as a main element, and the three-dimensional X obtained by the image collecting means 110 and the three-dimensional reconstruction means 200 for collecting and recording the X-ray transmission images obtained by the measurement unit 50. Image display means 210 for displaying a line CT image is provided. Hereinafter, each of these elements will be described more specifically.

  The measurement unit 50 includes a bed 4 on which the subject 3 is placed, an X-ray source 1 that irradiates the subject 3 placed on the bed 4 with X-rays, and a two-dimensional X image that is captured by X-rays from the X-ray source 1. The line detector 5, the C-type arm 10 for mechanically connecting the X-ray source 1 and the two-dimensional X-ray detector 5, the C-type arm holding body 11 for holding the C-type arm 10, and the C-type arm holding body 11 for the ceiling And a ceiling rail 13 that supports the ceiling support 12 so as to be movable in the front-rear and left-right two-dimensional directions in the state shown in FIG. The X-ray source 1 includes an X-ray tube 1t that generates X-rays and a collimator 1c that controls the irradiation direction of the X-rays from the X-ray tube 1t in a conical shape, and the two-dimensional X-ray detector 5 includes An X-ray image intensifier 5i that converts an X-ray transmission image into a visible light image and a TV camera 5c that captures a visible light image by the X-ray image intensifier 5i.

  The X-ray source 1 and the two-dimensional X-ray detector 5 in the measuring unit 50 are both rotated on the same circular orbit by the rotation of the C-arm 10 when the subject 3 is imaged. For this rotational movement, there are geometric parameters used in image reconstruction. That is, a rotation orbit plane (midplane) 7 which is a plane including a circular orbit in rotation of the X-ray source 1 and the two-dimensional X-ray detector 5 by rotation of the C-type arm 10, and a rotation center axis 8 in rotation of the C-type arm 10. , And the rotation center 9 which is the intersection of the rotation center axis 8 and the rotation track surface 7. As described above, these are determined in principle from the design data of the measuring unit 50, but in reality, there are manufacturing errors of the measuring unit 50, deformation of members, etc. Each part 50 appears differently from the design data. For this reason, it is necessary to obtain the rotation track surface 7 and the rotation center axis 8 in the actual measurement unit 50 by taking a test image for each apparatus.

  The measuring unit control means 100 includes an imaging system rotation control means 101 that controls the rotation of the C-arm 10 that rotates on the rotation center axis 8 (this rotation is called propeller rotation), and the ceiling support 12 on the ceiling rail 13. An imaging system position control means 102 that controls the position of the C-arm 10 in the direction of the body axis of the subject 3 in a two-dimensional manner, and an X that controls ON / OFF of the tube current flowing through the X-ray tube 1t. X-ray transmission images are taken by the irradiation control means 103, the object position control means 104 for controlling the position of the subject 3 with respect to the rotation center axis 8 by controlling the position of the bed 4, and the two-dimensional X-ray detector 5. The imaging system control means 105 to control is provided as a main element.

  The three-dimensional reconstruction unit 200 performs an image reconstruction calculation based on the X-ray transmission images captured by the measurement unit 50 and collected and recorded by the image collection unit 110 under the control of the measurement unit control unit 100. A dimensional X-ray CT image is obtained. For this purpose, the three-dimensional 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 pre-processing unit 201 is a unit that converts the X-ray transmission image collected and recorded by the image collection unit 110 into a distribution image of the X-ray absorption coefficient. A well-known method can be used for the processing in the preprocessing unit 201. In the present embodiment, first, an X-ray transmission image of air is taken in a state where the subject 3 and the bed 4 are not placed in the imaging field of view, and natural logarithmic conversion calculation is performed on each pixel data in the X-ray transmission image. Then, an X-ray transmission image of the subject 3 placed on the bed 4 is taken, and a natural logarithmic conversion operation is performed on each pixel data in the X-ray transmission image. And the X-ray absorption coefficient distribution image of the subject 3 and the bed 4 is obtained by taking the difference between the two images subjected to the natural logarithmic transformation calculation.

  When the X-ray image intensifier 5i converts the X-ray transmission image into a visible light image, the image distortion correction unit 202 has a non-planar X-ray incident surface (detection surface) of the X-ray image intensifier 5i. This is a means for correcting distortion caused in the image. For the processing in the image distortion correction unit 202, for example, the technique described in Patent Document 3 can be used. The specific contents will be described later.

  The filtering means 203 is a well-known means for performing filtering processing in X-ray CT image reconstruction.

  The backprojection means 204 is a means for performing a backprojection operation on the image data after filtering processing using the Feldkamp method of Non-Patent Document 1 and generates a three-dimensional X-ray CT image. The generated three-dimensional X-ray CT image is displayed on the image display unit 210.

  The geometric parameter calculation unit 300 is a unit for obtaining a geometric parameter of the measurement unit 50 that is necessary when the three-dimensional reconstruction unit 200 performs image reconstruction. The geometric parameters handled by the geometric parameter calculation means 300 include the image distortion caused by the rotation orbit plane 7 and the rotation center axis 8 and the X-ray image intensifier 5i in FIG. For this purpose, the geometric parameter calculation unit 300 includes an image distortion correction table generation unit 320, an image distortion correction table storage unit 330, a rotation track surface calculation unit 350, a rotation track surface movement amount calculation unit 370, and a rotation center axis calculation unit 390. I have. Hereinafter, each of these elements in the geometric parameter calculation means 300 will be specifically described.

  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 generation of the image distortion correction table is described in detail in Patent Document 3 above. The outline will be described below. In generating the image distortion correction table, a hole chart 20 as shown in FIG. 2 is used. The hole chart 20 is formed by drilling a large number of small holes 20h in a lattice arrangement on a plate made of a material that absorbs a large amount of X-rays. In order to generate the image distortion correction table, first, the hole chart 20 is fixed to the front surface of the X-ray image intensifier 5i, and the X-ray projection image of the hole chart 20 is photographed as a hole chart distortion projection image. Next, the pre-processing unit 201 performs processing on the hall chart distortion projection image obtained by this photographing, and then detects the projection position of the hole 20h on the projection surface of the projection image. Then, a calculation for converting the detected projection position of the hole 20h so that the hole chart 20 is projected onto the projection plane without distortion so as to be located at the virtual projection position of the hole 20h is performed for each hole 20h. A 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 in a magnetic disk or the like, and the image distortion correction unit 202 stores the image distortion correction table in the X-ray transmission. It has a function to read out when correcting image distortion.

  The rotating track surface calculating unit 350 is a unit for obtaining the rotating track surface 7 of the measuring unit 50. Here, as described above, the rotational trajectory plane 7 can be defined as a plane including a circular trajectory in the rotation of the X-ray source 1 and the two-dimensional X-ray detector 5, but can also be defined as follows. it can. Pay attention to the point (x, y, z) on the subject. Since the distance between the X-ray source and the point (x, y, z) changes due to the rotation, the projection point on the detection surface (projection surface) of the two-dimensional X-ray detector can be seen from the perspective principle. Generally, it moves up and down. However, in the plane that includes the focal point of the X-ray source and is perpendicular to the rotation center axis, even if the distance between the X-ray source and the point (x, y, z) changes due to rotation, the detection surface of the two-dimensional X-ray detector There is no vertical movement of the projection point. The plane that includes the focus of the X-ray source and is perpendicular to the rotation center axis is the rotation orbit plane, and thus the rotation orbit plane is defined as the plane that includes the focus of the X-ray source and is perpendicular to the rotation center axis. You can also.

  In order to obtain the rotation track surface 7, a metal ball array phantom 30 having a shape in which metal balls are connected as shown in FIG. 3 is used as a rotation track surface calculation phantom. The metal sphere array phantom 30 includes a plurality of metal spheres 30b, which are index bodies made of a material having a large absorption with respect to X-rays, specifically, a metal material such as tungsten, platinum, or iron-nickel-chromium alloy. A material that absorbs relatively little with respect to the wire, specifically a synthetic resin material such as acrylic resin or vinyl chloride resin, is linearly arranged and supported on a support 30s formed in a rod shape, a linear shape, or a chain shape. Composed.

  In order to obtain the rotation orbit plane 7 using such a metal sphere array phantom 30, first, an X-ray projection image of the metal sphere array phantom 30 arranged in a predetermined positional relationship with respect to the rotation center axis 8 is rotated and photographed. . The rotational imaging is performed for each predetermined rotation angle unit (rotation angle pitch) while rotating the X-ray source 1 and the two-dimensional X-ray detector 5 to the full rotation angle of the measurement unit 50. The positional relationship of the metal sphere array phantom 30 with respect to the rotation center axis 8 is parallel to the rotation center axis 8 and appropriate for the rotation center axis 8 so that each metal sphere 30b can draw a locus as described later. It is a position separated by a small interval S.

  When the metal sphere array phantom 30 is photographed under the above conditions, a certain number of X-ray projection images are obtained, and these are superimposed to create a composite projection image. FIG. 4 shows an example of a composite projection image when the C-arm type cone-beam X-ray CT apparatus of the present embodiment has a total rotation angle of 200 degrees, but the image is taken up to 360 degrees. In the projected image of FIG. 4, the horizontal axis 31 is u and the vertical axis 32 is v. As seen in FIG. 4, each metal sphere 30 b draws a trajectory 33 in the projected image. As will be described later, the rotating track surface 7 moves at every photographing angle. For this reason, the trajectory 33 is not actually a clean line segment as shown in the figure, but is depicted ideally here.

  The trajectory 33 becomes an ellipse or a part of an ellipse based on the same principle as the vertical movement of the projection point described with respect to the definition of the rotational trajectory plane 7 described above, and the short axis length thereof is in the positional relationship with the rotational trajectory plane 7 of the metal ball 30b. Correlates and becomes 0 on the rotating track surface 7. In other words, the trajectory 33 is a straight line for the metal sphere 30b on the rotating raceway surface 7. Therefore, when the distance between the metal balls 30b in the metal ball array phantom 30 is appropriately set, the trajectory 33 can be overlapped with each other except in the vicinity of the rotating track surface 7. That is, a gap line (separation line) 34 is generated in the vicinity of the rotation track surface 7 so that the trajectories 33 are separated from each other by the gap lines 34, and the trajectories 33 can overlap each other in other portions. In the present invention, this is used to determine the rotating track surface 7. Specifically, the gap lines 34 and 34 are detected from the projection image as in the example of FIG. 4, and the rotational track surface 7 is obtained as an intermediate line between the gap lines 34 and 34. In order to enable such a method of the present invention, it is necessary to appropriately set the interval between the metal balls 30b as described above. The appropriate interval can be defined as an interval in which about several gap lines 34 are generated in the vicinity of the rotating track surface 7 and the tracks 33 overlap each other in other portions. In the case of the specification of the cone beam X-ray CT apparatus of the form, for example, about 4 mm is an appropriate interval.

  When such a method in the present invention is compared with the method in Patent Document 1, the following can be said. In the method of Patent Document 1, a phantom similar to the metal sphere array phantom 30 in the present invention is used to obtain the rotation trajectory plane, but the locus of the reference point (corresponding to the metal sphere 30b in the present embodiment) in the phantom is By utilizing the fact that the locus becomes an elliptical closed locus, the short axis length is calculated for each elliptic locus, and the elliptic locus where it becomes 0 is obtained as the rotation locus surface. For this reason, the locus of the reference point needs to be a closed ellipse, and when the total rotation angle is less than 360 degrees as in the C-arm type cone beam X-ray CT apparatus of this embodiment, It cannot be obtained with high accuracy. In addition, the calculation process for obtaining the rotating track surface is complicated. On the other hand, in the method according to the present invention, since the gap line is used, even when the total rotation angle is less than 360 degrees, the rotational track surface can be obtained with high accuracy, and the calculation process is patented. This makes it easier than the method of Document 1.

  The rotational trajectory plane obtained as described above is obtained through the synthesis of a plurality of X-ray projection images obtained by repeated imaging for each predetermined rotation angle pitch, and is the average rotational trajectory plane for each imaging, Actually, the rotational orbital surface in each photographing with different rotation angle states slightly moves (displaces) with respect to the average rotational orbital surface. For this reason, in order to obtain the rotation track surface with higher accuracy, it is necessary to measure the movement of the rotation track surface for each rotation angle in the rotation imaging and reflect it. The rotational track surface movement amount calculation means 370 performs processing necessary for this.

  In the calculation of the movement amount of the rotation track surface by the rotation track surface movement amount calculation means 370, a rotation track surface movement amount calculation phantom is used. The rotational track surface movement amount calculation phantom is composed of one metal sphere (index body) similar to the metal sphere 30 b in the metal sphere array phantom 30. In order to obtain the amount of movement of the rotating track surface using such a single metal sphere phantom with one metal sphere, a metal sphere that is a phantom is placed at the rotation center 9 in FIG. The reason why the metal sphere is arranged at the rotation center 9 is to prevent the distance between the X-ray tube 1t and the metal sphere from changing depending on the imaging angle direction.

  FIG. 5 shows a conceptual diagram about the calculation of the amount of movement of the rotating track surface. The projected image 41 is a case where there is no movement with respect to the average rotational trajectory plane obtained by the rotational trajectory plane calculating means 350, and the projected image 40 of the metal sphere is on the average rotational trajectory plane. On the other hand, the projection image 42 is a case where movement occurs with respect to the average rotation orbit plane, and the projection image 43 of the metal sphere moves relative to the average rotation orbit plane by a movement amount mv. By such a comparison, the movement amount mv can be calculated for each projection. In each projection image, there may be a movement in the projection position of the projection image of the metal sphere also in the lateral direction u. This is because the arrangement position of the metal sphere is shifted from the actual rotation center 9. Therefore, there is no need to consider it because it is not related to the movement of the rotating raceway surface.

  The rotation center axis calculation means 390 obtains the rotation center axis 8 in FIG. As the method in the rotation center axis calculation means 390, the method described in Patent Document 2 is preferably used. The outline is as follows. A wire phantom formed by stretching a single metal wire having a large X-ray absorption coefficient on a rod-like support is placed in parallel to the rotation center axis 8 to perform rotational imaging, and the sharpness of the metal wire in the reconstructed image is determined. By setting the evaluation function as the evaluation function and optimizing the evaluation function, the projection coordinate value on the projection plane of the rotation center axis 8 is obtained.

  Here, a specification example of the C-arm type cone beam X-ray CT apparatus in the present embodiment is as follows. The distance between the X-ray tube 1t and the rotation center axis 8 is 800 mm, the distance between the rotation center axis 8 and the X-ray incident surface of the X-ray image intensifier 5i is 400 mm, the detector surface size is 400 mm, and the image size is 1024 ×. 1024 (number of scan lines) and detector pitch is 0.4 mm. The rotation angle of the C-arm 10 is 200 degrees at the maximum. That is, the state of FIG. 1 is 0 degree with respect to the subject 3, the rotation start position is at −100 degrees with respect to the subject 3, and the rotation end position is +100 degrees with respect to the subject 3. Has been. Further, the rotational speed of the C-arm 10 is 40 degrees per second, and therefore the time for one scan is 5 seconds.

  Hereinafter, an outline of operations in imaging by the C-arm type cone beam X-ray CT apparatus of the present embodiment will be described. When shooting is started, first, the shooting system rotation control means 101 starts to rotate the C-arm 10. After the rotation acceleration period, the X-ray irradiation control unit 103 generates X-rays in the X-ray tube 1t, and the imaging system control unit 105 starts capturing a visible light image with the television camera 5c. The X-ray irradiated from the X-ray tube 1t passes through the subject 3 and then enters the X-ray image intensifier 5i. The X-ray transmission image is converted into a visible light image by the X-ray image intensifier 5i and is captured by the television camera 5c. The television camera 5c converts the X-ray transmission image into a video signal, and the video signal undergoes A / D conversion and is then recorded as digital image data in the image collecting unit 110. The standard scanning mode of the television camera 5c is 30 frames per second and 1024 scanning lines. The rotation angle pitch of the C-arm 10 is 1.33 degrees, and 150 X-ray transmission images are acquired in 5 seconds. When the rotation imaging at 200 degrees is completed, the X-ray irradiation control means 103 ends the X-ray generation of the X-ray tube 5t, and the imaging system rotation control means 101 stops the rotation of the C-arm 10.

  The three-dimensional reconstruction unit 200 reads the digital image data from the image collection unit 110 in parallel with the above-described imaging or after the imaging is completed, and performs a three-dimensional reconstruction of the subject 3 by a three-dimensional reconstruction process described later. X-ray CT image reconstruction calculation is performed. The three-dimensional X-ray CT image obtained in this way is displayed on the image display means 210. Note that the X-ray transmission image recorded in the image collecting unit 110 can be directly displayed on the image display unit 210.

  Next, the contents of processing in the geometric parameter calculation means 300, which is a main characteristic item in the present invention, will be described more specifically. FIG. 6 shows a flow of geometric parameter calculation processing in the geometric parameter calculation means 300. First, the Hall chart 20 is fixed to the front surface of the X-ray image intensifier 5i to perform rotational imaging, and the image collection means 110 collects a Hall chart distortion projection image at each imaging angle (step S310). In the case of this embodiment, the number of hall chart distortion projection images to be collected is 150 as described above.

  Next, based on the 150 hall chart distortion projection images obtained in step S310, the image distortion correction table is calculated by the image distortion correction table generating means 320 (step S320), and the image distortion correction table storage means 330 is calculated. (Step S330). The image distortion correction table stored in the image distortion correction table storage unit 330 is used for correcting image distortion of the X-ray transmission image by the image distortion correction unit 202.

  In step S340, rotational imaging using the metal sphere array phantom 30 is performed. As shown in FIG. 3, this rotational shooting is performed by arranging the metal sphere array phantom 30 in a position parallel to the rotation center axis 8 and at an appropriate distance S from the rotation center axis 8. The projection image of the metal sphere array phantom 30 obtained in the above is collected by the image collecting means 110. The appropriate distance S depends on the geometric system (the distance between the X-ray tube 5t and the rotation center axis 8, the distance between the rotation center axis 8 and the two-dimensional X-ray detector 5, and the size of the detector surface). In the case of this embodiment, it is appropriate that the rotation center axis 8 is 70 to 120 mm.

  Next, based on the projected image of the metal sphere array phantom obtained in step S340, the coordinate value on the projected image of the rotating track surface 7 is determined by the rotating track surface calculating means 350 (step S350). The projection coordinate value of the rotation orbit plane 7 obtained here is used when the back projection means 204 reconstructs a three-dimensional X-ray CT image from the X-ray transmission image.

  In step S360, rotational imaging using the single metal sphere phantom described above is performed. This rotational imaging is performed by placing a single metal sphere phantom at the rotation center 9 in FIG. 1, and a projection image of the single metal sphere phantom obtained by this is collected by the image collecting means 110. Then, based on the projection image of the single metal sphere phantom obtained in step S360, the moving amount of the rotating track surface 7 is calculated for each projection by the rotating track surface moving amount calculating means 370 (step S370). The amount of movement of the rotational orbital surface 7 obtained here is combined with the average value on the projected image of the rotational orbital surface 7 obtained in step S350, and the three-dimensional X-ray image is obtained from the X-ray transmission image by the back projection means 204. Used when reconstructing a line CT image.

  In step S380, the above-described wire phantom is rotated in parallel with the rotation center axis 8, and the projected image is collected by the image collecting means 110. Then, based on the projection image of the wire phantom obtained in step S380, the coordinate value on the projection image of the rotation center axis 8 is determined by the rotation center axis calculation means 390. The projection coordinate value of the rotation center axis 8 obtained here is used when the back projection means 204 reconstructs a three-dimensional X-ray CT image from the X-ray transmission image.

  In the following, the contents of the treatment in each of the rotating track surface calculating unit 350 and the rotating track surface moving amount calculating unit 370 will be described more specifically. FIG. 7 shows a flow of details of the processing performed by the rotary orbital plane calculating means 350 in step S350 in FIG. First, in step S351, the projection image of the metal sphere array phantom obtained in step S340 in FIG. 6 is preprocessed by the preprocessing unit 201 to generate an X-ray absorption coefficient distribution image. Then, the image distortion correction unit 202 performs image distortion correction on the projected image of the metal sphere array phantom (step S352).

  In step S353, the 150 projection images collected for the metal sphere array phantom are superimposed. Here, as a method of superimposing the projected images, there is a method of taking an average of 150 projected images, and 150 projected images for each corresponding pixel in order to make the pixels darkened by the metal sphere stand out. There is a method of putting the minimum value of.

  In step S354, a threshold value process is applied to the projected superimposed image of the metal sphere array phantom obtained in step S353, and a portion where the trajectories of the metal sphere do not overlap, that is, the gap line 34 in FIG. 4 is extracted. In this extraction, the number of gap lines 34 corresponding to the arrangement interval of the metal balls 30b in the metal ball arrangement phantom 30 (which is usually two or more) is extracted. Then, the centroid value of the v coordinate in the plurality of extracted gap lines 34 is obtained, and this is output as the coordinate value on the projected image of the rotating orbital plane 7 (step S355).

  FIG. 8 shows a flow of details of the processing performed by the rotational track surface movement amount calculation means 380 in step S370 in FIG. First, in step S371, an X-ray absorption coefficient distribution image is generated by performing preprocessing by the preprocessing unit 201 on the projection image of the single metal sphere phantom obtained in step S360 in FIG. Then, the image distortion correction unit 202 performs image distortion correction on the projected image of the single metal sphere phantom (step S372).

  In step S373, threshold processing is performed on the projection image of the single metal sphere phantom obtained in step S372, and the projection area of the metal sphere is extracted for each projection. Next, a centroid value of the v coordinate is obtained for each of the extracted projection areas of the metal sphere (step S374). Finally, the v-coordinate centroid value of the metal sphere projection area for each projection obtained in step S374 is the coordinate value on the projection image of the rotation orbit plane 7 for each projection number or on the projection image of the rotation orbit plane 7. Saved in the table as the amount of movement from the coordinate average value at.

  Here, just obtaining the v-coordinate value for each projection in the rotation trajectory plane movement amount calculation processing in step S370 does not mean that the rotation trajectory plane calculation processing in step S350 is unnecessary. In other words, what is calculated in the rotation track surface movement amount calculation process is the movement amount from the average value of the rotation track surface 7 calculated in the rotation track surface calculation process. A rotating track surface calculation process for accurately determining the average value of the track surface 7 is required.

  In the present invention, as described above, the geometric parameter calculation means 300 can determine the geometric parameters of the apparatus more easily and accurately, and by using this, a sharper three-dimensional image can be obtained. An X-ray CT image can be generated. Further, by mounting the geometric parameter calculation means 300 in this embodiment or a means equivalent thereto, 360-degree rotational imaging cannot be performed unlike the C-arm type cone beam X-ray CT apparatus in this embodiment. The apparatus can reconstruct a clear three-dimensional X-ray CT image. Thereby, for example, contrast imaging of the head, abdomen, etc., or the diagnostic performance in the shaping field such as the tooth jaw, the lumbar vertebra, and the 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 two-dimensional X-ray detector using a TFT element is used. You may make it comprise. The present invention can also be applied to cone beam X-ray CT apparatuses other than the C-arm system, and can also be applied to cases where the entire angle of rotational imaging is other than 200 degrees.

  The cone beam X-ray CT apparatus of the present invention can easily determine the rotational orbital plane with high accuracy even when 360-degree rotation imaging cannot be performed, and therefore, a clearer three-dimensional X-ray CT image can be obtained. Can be reconfigured. For this reason, it can contribute to improving diagnostic performance in the field of orthopedic imaging such as the jaws, lumbar vertebrae, and extremities, for example, imaging of the head and abdomen.

1 is a diagram showing a schematic configuration of a C-arm cone-beam X-ray CT apparatus according to an embodiment. FIG. It is a figure which shows the example of a whole chart. It is a figure which shows the example of a metal ball arrangement | sequence phantom with the arrangement | positioning relationship with respect to a rotation center axis | shaft. It is a figure which shows the example of the composite projection image of a metal ball arrangement | sequence phantom. It is a conceptual diagram about calculation of a rotation track surface movement amount. It is a figure which shows the flow of a geometric parameter calculation process. It is a figure which shows the flow of a rotation track surface calculation process. It is a figure which shows the flow of a rotation track surface movement amount calculation process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 X-ray source 3 Subject 5 Two-dimensional X-ray detector 7 Rotating track surface 8 Rotation center axis 30 Rotating track surface calculation phantom 30b Metal sphere (index body)
30s Support 33 Trajectory 34 Gap line

Claims (2)

An X-ray source for irradiating X-rays, a two-dimensional X-ray detector arranged opposite to the subject with the X-ray source interposed therebetween, and means for rotating both of them around the rotation center axis on the same circular orbit , And irradiating the subject disposed between the X-ray source and the two-dimensional X-ray detector with X-rays from the X-ray source, while the X of the subject is in units of a predetermined rotation angle in the rotation. Cone-beam X-ray CT that captures a line-transmission image with a two-dimensional X-ray detector and generates a three-dimensional X-ray CT image by performing an image reconstruction operation based on the X-ray transmission image obtained by the imaging In the device
A rotating phantom surface calculation phantom configured by linearly arranging metal spheres, the phantom spaced apart from the rotation center axis by a predetermined distance, and the linear arrangement direction of the phantom arranged parallel to the rotation center axis In this state, the X-ray projection image of the rotational trajectory plane calculation phantom is rotated to collect a plurality of projection images, and a plurality of projection images obtained by the acquisition unit are superimposed to generate a composite projection image. means for, extracting means for extracting a gap line is a portion not I overlap if one another in a path which the are the respective metal balls drawn for the synthesis projected image on the composite projection image, extracted by the extraction means A cone beam X-ray CT apparatus comprising: means for obtaining a coordinate value on a projected image of the rotating orbit plane based on the gap line. Rotating orbital plane movement amount calculation processing for obtaining a rotational orbital plane movement amount calculation phantom composed of a single metal sphere and correlating the moving amount of the rotating orbital plane for each rotation angle unit in the photographing with the coordinate value. A rotating phantom surface movement amount calculation phantom disposed in a rotation center that is an intersection of the rotation track surface and the rotation center axis, and the X-ray projection image of the rotation track surface movement amount calculation phantom is rotated. 2. The cone according to claim 1, further comprising means for photographing for each angle unit, and means for calculating a movement amount of the rotating orbit plane based on a projection area of the metal sphere in the projection image obtained by the means. Beam X-ray CT system.

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