JP2007000348A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the image quality of the peripheral part of a tomographic image of an X-ray CT apparatus with a two-dimensional X-ray area detector of the matrix structure such as a multi-row X-ray detector or a flat panel X-ray detector. <P>SOLUTION: The X-ray CT apparatus is for two-dimensional image reconstruction without using three-dimensional image reconstruction (also called as cone-beam reconstruction). In case of partly enlarging reconstruction without having the center of rotation of an X-ray data collection system as the center, the center of the image display is the center point of the reverse projection of reconstruction, the center point at which X-ray beams used in the image reconstruction are converged in the reverse projection process. Alternatively, the center of image reconstruction of the helical reconstruction is set not only at the center of the image reconstruction flat plane but also in the periphery of the tomographic image, so that the image quality in the periphery of the tomographic image can be improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and relates to improving the image quality of a tomographic image and improving the image quality of a partially enlarged tomographic image.

Conventionally, in an X-ray CT apparatus (for example, Patent Document 1) using a two-dimensional X-ray area detector having a matrix structure represented by a multi-row X-ray detector or a flat panel X-ray detector, two-dimensional image reconstruction is performed. When the center of the backprojection processing is one, as shown in FIG. 6A, there are areas with good image quality and areas with poor image quality. Note that the center of backprojection processing in two-dimensional image reconstruction is the projection data of each column of X-ray beams or each column in two-dimensional image reconstruction of a multi-row X-ray detector as shown in FIG. This is one point at which correct projection data is backprojected in the z direction when two-dimensional backprojection is performed on projection data obtained by weighted addition of projection data of X-ray beams. In other words, the correct projection data is back-projected so as to focus on the xy plane and the z-direction only at one central point in the back-projection processing.
In the center of the tomographic imaging field, which is the center of the back projection processing, and in the vicinity thereof, there were few artifacts, the slice thickness was thin, and the image quality was good. However, as the distance from the center of back projection processing goes to the periphery of the tomographic imaging field, as shown in FIG. 14, the projection data is back projected to each pixel in the tomographic image at a point away from the z direction, resulting in contradiction of back projection. However, there is a problem that the image quality is deteriorated, the artifact is increased, the slice thickness is increased, and the image quality is deteriorated.
JP 2003-159244 A

In an X-ray CT apparatus for two-dimensional image reconstruction that does not use three-dimensional image reconstruction (also called cone beam reconstruction), artifacts increase and slice thickness increases when going to the periphery of the tomogram. The reason why the image quality deteriorates is that it is away from the center of the back projection process of the two-dimensional image reconstruction. Further, in general, when performing two-dimensional image reconstruction in an X-ray CT apparatus of a multi-row X-ray detector, the center of back projection processing is arranged only at the center of the entire imaging region. Instead of setting the center of the entire field of view of the image reconstruction plane as the center of the backprojection process, the center of the backprojection process other than the center is arranged at the periphery of the entire field of view of the tomographic image. The image quality of the partial reconstruction area is improved, and the image quality of the peripheral part of the tomographic image can be improved.
In this way, in an X-ray CT apparatus using a multi-row X-ray detector, the uniformity of image quality can be controlled by placing the center position of back projection processing at an optimal position. Therefore, the back projection processing center can be increased as long as the performance of the image reconstruction processing device permits.
Also, in conventional scan (axial scan) or cine scan when the scanning gantry of an X-ray CT apparatus using a multi-row X-ray detector is tilted, back projection is performed so that the uniformity of image quality can be maintained in the z direction. The center of processing can also be arranged.
Further, in “partially enlarged image reconstruction” in which a part of the tomographic image is enlarged and image reconstruction is performed, if the center of the back projection process is located at the center of the entire imaging region, as shown in FIG. If the magnified field of view for image reconstruction is not in the center of the entire field of view but in the periphery, the tomographic image reconstructed from the partially magnified image will have image quality in the vicinity near the center of the entire field of view and the distant peripheral part. The non-uniformity is remarkable. However, by rearranging the center of the back projection process at or near the center of the partially enlarged imaging field of view, the center of the back projection process in which the X-ray beam is correctly at the center of the partially enlarged field of view as shown in FIG. The overall image quality of the partially enlarged field of view can be improved by being backprojected to pass through.
Further, in this case, by arranging the center of the back projection processing not only in the center of the partial magnified field of view but also in the peripheral portion, the X-ray beam is back so that it passes through the center of each back projection processing as shown in FIG. Since the image is reconstructed after the projection processing, the overall image quality can be further improved.
In addition, there may be a problem that the boundary line is seen and the image quality is discontinuous at the boundary between the plurality of partial reconstruction regions as shown in FIG. 11 existing around the back projection process. It is possible to continuously change the image quality by performing.
Further, the projection data of a plurality of X-ray beams acquired at different positions in the z direction by the X-ray data acquisition system is back-projected to one partial reconstruction area, thereby improving the S / N of image noise. You can also.
In addition, the projection data of the multi-row X-ray detector can be weighted and added in the column direction (z direction), or the z filter can be superimposed in the z direction to control the beam thickness in the z direction of the X-ray beam. Thus, the slice thickness in the z direction can also be controlled.
Accordingly, an object of the present invention is to provide uniform image quality of tomographic images in X-ray CT using a two-dimensional X-ray area detector having a matrix structure typified by a multi-row X-ray detector or a flat panel X-ray detector. To improve.
Further, in another object of the present invention, the uniformity of the image quality in the z-axis direction of an inclined tomogram obtained from an inclined gantry and a data acquisition system is improved.
Another object of the present invention is to improve the overall image quality of the partially enlarged image reconstructed tomographic image and to improve the uniformity of the image quality.
Further, in another object of the present invention, the continuity of the image quality of tomographic images in all fields of view is maintained.
In another object of the present invention, S / N improvement of image noise in a tomographic image of conventional scan (axial scan), cine scan, helical scan, or variable pitch helical scan that collects data at different positions in the z direction is performed. .
In another object of the present invention, the slice thickness in the z direction is also controlled.

The present invention can control the uniformity and improvement of tomographic image quality by controlling the position and number of backprojection processing centers.
In a first aspect, the present invention relates to a multi-row X-ray detector that detects X-rays relative to an X-ray generator while rotating around a center of rotation between them, X-ray data collection means for collecting X-ray projection data transmitted through the specimen, image reconstruction means for reconstructing an image of projection data collected from the X-ray data collection means by two-dimensional back projection, and image-reconstructed tomogram In an X-ray CT apparatus provided with an image display means for displaying an image, the image reconstruction means can correctly perform backprojection processing on pixels through which projection data has passed when two-dimensional backprojection of projection data is performed ( (Hereinafter referred to as the center pixel of the backprojection process) is divided into a plurality of partial reconstruction areas located in the vicinity of the center pixel of the plurality of backprojection processes. Exists and its partial reconstruction area To provide an X-ray CT apparatus for performing two-dimensional backprojection process.
In the X-ray CT apparatus according to the first aspect, if a plurality of back projection processing center pixels are arranged in the reconstruction area, the image quality can be improved in terms of artifacts and slice thicknesses in the vicinity of the back projection processing center pixels. good. For this reason, the more the plurality of central pixels of the backprojection processing are arranged in the reconstruction area, the better the image quality of the reconstruction area is in terms of artifacts and slice thicknesses. In addition, as shown in FIG. 6B, a pixel that is correctly backprojected to a pixel through which projection data has passed is selected and used for backprojection processing by selecting X-ray beam data passing through a predetermined pixel. This is the predetermined pixel. In addition, the partial reconstruction area located in the vicinity of a plurality of backprojection processing center pixels has an average distance between the center pixel of the backprojection processing and each pixel of the partial reconstruction area, and the center of the entire photographing field and the partial reconstruction area. If the distance is set to be shorter than the distance to each pixel on average, an artifact reduction effect can be obtained.

In a second aspect, the present invention provides the X-ray CT apparatus according to the first aspect, wherein the multi-row X-ray detector is a two-dimensional X-ray area detector having a matrix structure represented by a flat panel X-ray detector. Or an X-ray CT apparatus that is a two-dimensional X-ray area detector configured by combining a plurality of the two-dimensional X-ray area detectors.
In the X-ray CT apparatus according to the second aspect, the central pixel of the back projection process is reconstructed in the tomographic image of each column in the two-dimensional X-ray area detector having a matrix structure represented by the flat panel X-ray detector. If a plurality of them are arranged, the image quality is good in terms of artifacts and slice thicknesses in the vicinity of the center pixel of the back projection process. For this reason, when the central pixels of the back projection process are evenly arranged in the reconstruction area, the image quality of the reconstruction area is further improved in terms of artifacts and slice thickness.

In a third aspect, the present invention provides the X-ray CT apparatus according to the first or second aspect, wherein the image reconstruction means uses projection data for 360 degrees or approximately 360 degrees for projection data to be used. An X-ray CT apparatus for performing a two-dimensional backprojection process using the same is provided.
In the X-ray CT apparatus according to the third aspect, it is possible to set the center pixel of the back projection process at an arbitrary position in the reconstruction area using normally used projection data of 360 degrees. For this reason, it is possible to make the image quality uniform and improved by the number and position of the center pixels of back projection.

In a fourth aspect, the present invention provides an X-ray CT apparatus according to the first or second aspect,
The image reconstruction means includes an X-ray CT apparatus that performs two-dimensional backprojection processing using projection data whose projection data is 180 degrees + detector fan angle, or approximately 180 degrees + detector fan angle. provide.
In the X-ray CT apparatus according to the fourth aspect, using projection data for 180 degrees + detector fan angle of half scan used when time resolution is required in CT imaging such as cardiac imaging, It is possible to set the center pixel of the back projection process at an arbitrary position in the reconstruction area. For this reason, it is possible to make the image quality uniform and improved by the number and position of the center pixels of back projection.

In a fifth aspect, the present invention provides the X-ray CT apparatus according to the first to fourth aspects, wherein at least one of the plurality of back projection center pixels of the tomographic image is the center of the imaging field. An X-ray CT apparatus is provided.
In the X-ray CT apparatus according to the fifth aspect, normally, when trying to set the center pixel of the back projection process that is the smallest in the imaging field, assuming that the circle with the radius r is in the imaging field, When trying to minimize the maximum value of the distance to all the pixels, it is most efficient to bring the center pixel of the back projection process to the center of the shooting field circle. In this case, the maximum value of the distance to all the pixels is r, and the image quality and its uniformity are the best. For this reason, when a plurality of center pixels for backprojection processing are arranged, it is efficient to bring one of them to the center of the field of view and the image quality and uniformity thereof can be improved.

In a sixth aspect, the present invention relates to the X-ray CT apparatus according to any one of the first to fourth aspects, wherein at least one of the plurality of back projection processing center pixels of the tomographic image is the rotation center of the data acquisition system. An X-ray CT apparatus is provided.
In the X-ray CT apparatus according to the sixth aspect, usually, the field of view of the X-ray CT apparatus extends in a circle determined by the X-ray focal point and the fan angle of the X-ray detector around the rotation center of the data acquisition system. . For this reason, when arranging a plurality of back projection processing center pixels, it is efficient to bring one of them to the rotation center of the data collection system, and the image quality and uniformity thereof can be improved.

In a seventh aspect, the present invention provides the X-ray CT apparatus according to the first to fourth aspects, wherein the image reconstruction means is obtained by a conventional scan (axial scan) using a multi-row X-ray detector. An X-ray CT apparatus is provided in which the tomograms of all the columns to be reconstructed are image reconstructed, and at least one of the plurality of backprojection center pixels of each tomogram is the center or almost the center of the imaging field.
In the X-ray CT apparatus according to the seventh aspect, usually, as shown in FIG. 15, when the rotation axis of the X-ray data acquisition system in the scanning gantry is perpendicular to the xy plane, a multi-row X-ray detector or a flat The center of rotation of the tomographic image of each column in a two-dimensional X-ray area detector having a matrix structure typified by a panel X-ray detector is the central pixel of back projection processing. However, when the scanning gantry is inclined, the rotation axis of the X-ray data collection system in the scanning gantry is not perpendicular to the xy plane. In this case, as shown in FIG. 16, in the conventional scan (axial scan) or cine scan, if the center of rotation is aligned with the center of the reconstruction area, the conventional scan (axial scan) is continuously performed at different positions in the z direction. Or, when a cine scan is performed, the continuity in the z direction of the tomographic image is lost as shown in FIG. For this reason, as shown in FIG. 18, if the continuity in the z direction of the tomographic image is not lost, the center of the imaging region is not the center of the rotation data collection system. Instead, the center of the imaging region is the intersection of the tomographic image and the z axis. For this reason, by aligning the center of the back projection process with the center of the imaging region, the continuity in the z direction of the tomographic image and the continuity of the image quality that the image quality is the best at the center of the back projection process can be obtained.

In an eighth aspect, the present invention provides the X-ray CT apparatus according to the first to fourth aspects, wherein the image reconstruction means is obtained by a conventional scan (axial scan) using a multi-row X-ray detector. X-ray CT that is an intersection of the z-axis passing through the rotation center and each tomographic image among at least one of the central pixels of back projection processing present in each tomographic image. Providing equipment.
In the X-ray CT apparatus according to the eighth aspect, as in the seventh aspect, in the conventional scan (axial scan) or the cine scan, as shown in FIG. Is not the center of the rotation axis, and the center of the imaging region is the intersection of the tomographic image of each row and the z axis passing through the rotation center. In this case, if the center of the back projection process is the center of the imaging region, the center of the back projection process is the intersection of the tomographic image of each column and the z axis passing through the rotation center.

In a ninth aspect, the present invention is the X-ray CT apparatus according to the first to fourth aspects, comprising a multi-row X-ray detector for detecting X-rays relative to the X-ray generator, between them. X-ray data collecting means for collecting X-ray projection data transmitted through the subject in between while rotating around the center of rotation, and projection data collected from the X-ray data collecting means by two-dimensional back projection In an X-ray CT apparatus provided with image reconstruction means for reconstructing an image and image display means for displaying an image reconstructed tomographic image, a two-dimensional inverse in the case of partially reconstructing projection data by the image reconstruction means In projection, an X-ray CT apparatus is provided in which the center pixel of back projection processing is located at a position shifted from the center of the entire imaging region toward the center of the reconstruction region of partial enlargement reconstruction.
In the X-ray CT apparatus according to the ninth aspect, when the center of the back projection process is the center of the entire imaging field in the two-dimensional backprojection that performs the partially enlarged image reconstruction, the center of the partially enlarged image reconstruction field of view is If it does not coincide with the center of the entire field of view, as shown in FIG. 7, the area close to the center of the entire field of view of the partially enlarged reconstruction field of view is good, and the area far from the center of the entire field of view is There was a problem in terms of poor image quality and uniformity of image quality. For this reason, by setting the center of the back projection process to the center of the partially enlarged reconstruction area, that is, by setting the center of the back projection process to a position that is not the center of the entire field of view, the image quality of the partially enlarged image reconstructed tomographic image Can be ensured.

In a tenth aspect, the present invention provides the X-ray CT apparatus according to the ninth aspect, wherein the central pixel of the backprojection processing is located at the center or substantially the center of the reconstruction area of the partial enlargement reconstruction. provide.
In the X-ray CT apparatus according to the tenth aspect, similarly to the ninth aspect, the image quality of the partially enlarged image reconstructed tomographic image is made uniform by setting the center of the back projection process as the center of the partially enlarged image reconstruction area. Sex can be secured.

In an eleventh aspect, the present invention provides the X-ray CT apparatus according to the ninth or tenth aspect, wherein a plurality of center pixels of the backprojection processing exist in a reconstruction area of partial enlargement reconstruction, and the partial enlargement reconstruction is performed. A plurality of reconstructed regions are divided into a plurality of divided reconstructed regions located in the vicinity of the central pixel of the backprojection process, and a two-dimensional backprojection process is performed for each divided reconstructed region X A line CT apparatus is provided.
In the X-ray CT apparatus according to the eleventh aspect, by arranging a plurality of back projection processing centers in the partially enlarged image reconstruction region, the average distance of each pixel in the imaging region to the back projection processing center is reduced. Since the deterioration of the image quality can be prevented, the uniformity of the image quality of the partially enlarged image reconstruction tomographic image can be secured.

In a twelfth aspect, the present invention provides the X-ray CT apparatus according to the ninth to eleventh aspects, wherein the image reconstruction means uses projection data for 360 degrees or approximately 360 degrees as projection data to be used. An X-ray CT apparatus for performing a two-dimensional backprojection process using the same is provided.
In the X-ray CT apparatus according to the twelfth aspect, it is possible to set the center pixel of the back projection processing at an arbitrary position in the partially enlarged image reconstruction area using normally used 360-degree projection data. is there. For this reason, it is possible to make the image quality uniform and improved by the number and position of the center pixels of back projection.

In a thirteenth aspect, the present invention relates to the X-ray CT apparatus according to the ninth to eleventh aspects, wherein the image reconstruction means uses projection data of 180 degrees + detector fan angle or approximately 180 degrees + An X-ray CT apparatus that performs two-dimensional backprojection processing using projection data corresponding to a detector fan angle is provided.
In the X-ray CT apparatus according to the thirteenth aspect, by using projection data for 180 degrees + detector fan angle of half scan used when time resolution is required in CT imaging such as cardiac imaging, It is possible to set the center pixel of the back projection process at an arbitrary position in the partially enlarged image reconstruction area. For this reason, it is possible to make the image quality uniform and improved by the number and position of the center pixels of back projection.

In a fourteenth aspect, the present invention provides the X-ray CT apparatus according to the first to thirteenth aspects, wherein the image reconstruction means includes a plurality of image reconstruction areas that overlap with adjacent image reconstruction areas. Provides an X-ray CT apparatus for performing weighted addition.
In the X-ray CT apparatus according to the fourteenth aspect, when there are a plurality of back projection processing centers and partial image reconstruction areas existing in the vicinity thereof, an image is not displayed at the boundary line between the partial image reconstruction areas. It can be continuous. For this reason, as shown in FIG. 12, image reconstruction is performed by overlapping a plurality of partial image reconstruction areas existing in the entire imaging area in the vicinity of the boundary line between the adjacent partial image reconstruction areas. As shown in FIG. 12, the overlapped region can be successively transferred to adjacent reconstructed images by performing weighted addition.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In a fifteenth aspect, the present invention provides the X-ray CT apparatus according to the first to fourteenth aspects, wherein the X-ray data collection unit collects X-ray projection data by a conventional scan (axial scan) or a cine scan. An X-ray CT apparatus is provided. In conventional scan (axial scan) or cine scan, a multi-row X-ray detector that detects X-rays relative to the X-ray generator and a single rotation while rotating around the center of rotation between them. While collecting X-ray projection data for minutes or multiple rotations, the subject in between stays in a position in the body axis direction (hereinafter referred to as the z direction), and the subject is in a different z direction position. After the movement, the X-ray generator and the multi-row X-ray detector collect X-ray projection data for the next one rotation or a plurality of rotations.
In the X-ray CT apparatus according to the fifteenth aspect, in a conventional scan (axial scan) or a cine scan at a plurality of positions in the z direction, a plurality of partial image reconstruction regions and a central pixel for back projection processing are provided on a tomographic image of each column. By making it exist, the image quality in the entire photographing region can be made more uniform.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In a sixteenth aspect, the present invention relates to an X-ray CT apparatus according to any one of the first to fourteenth aspects, wherein the X-ray data collection means collects X-ray projection data by helical scanning or variable pitch helical scanning. Providing equipment. In the helical scan or the variable pitch helical scan, the multi-row X-ray detector that detects the X-rays relative to the X-ray generator is in between while rotating around the rotation center between them. X-ray projection data of the subject is collected while moving the subject in the z-axis direction.
In the X-ray CT apparatus according to the sixteenth aspect, while moving the subject in the z direction, X-ray data collection of the subject's helical scan or variable pitch helical scan is performed, and a tomographic image at each z-direction position is imaged. When reconstructing, the presence of a plurality of partial image reconstruction regions and the central pixel of the backprojection processing makes it possible to make the image quality in the entire photographing region more uniform.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In a seventeenth aspect, the present invention provides the X-ray CT apparatus according to the fifteenth or sixteenth aspect, wherein the X-ray data collection means includes the X-ray generator, the multi-row X-ray detector, and the subject. Are relatively moved in the body axis direction (hereinafter referred to as the z direction) of the subject, and X-ray projection data is collected at a plurality of positions in the z direction. An X-ray CT apparatus that performs image reconstruction by using projection data that passes through each other in the z-direction and has different positions in the z direction for back projection processing is provided.
In the X-ray CT apparatus according to the seventeenth aspect, a conventional scan (axial scan), a cine scan, a helical scan, or a variable pitch helical scan is different in the z direction passing through a partial image reconstruction area as shown in FIG. By performing image reconstruction using projection data of a plurality of X-ray beams acquired at positions, it is possible to make image quality uniform and at the same time use more X-ray beams for image reconstruction. / N can be improved.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In an eighteenth aspect, the present invention relates to the X-ray CT apparatus according to any of the fourteenth to seventeenth aspects, wherein the X-ray data collecting means inclines the scanning gantry from the xy plane perpendicular to the z direction to collect X-ray data. An X-ray CT apparatus for performing the above is provided.
In the X-ray CT apparatus according to the eighteenth aspect, even if the scanning gantry is tilted in the z direction and tilted from the xy plane, the effects of the first to seventeenth aspects can be made effective.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In a nineteenth aspect, the present invention provides the X-ray CT apparatus according to any one of the first to eighteenth aspects, wherein the image reconstructing means applies to X-ray projection data by an X-ray beam facing when performing back projection processing. Provided is an X-ray CT apparatus that performs back projection by applying a weighting coefficient depending on an angle formed between each tomographic image and an X-ray beam.
In the X-ray CT apparatus according to the nineteenth aspect, each tomographic image is obtained by applying a weighting coefficient depending on an angle formed by a plurality of X-ray beams passing through each partial image reconstruction area and a tomographic image plane (for example, an xy plane). Can improve the image quality.
Of course, the above is also effective in the case of partially enlarged image reconstruction.

In a twentieth aspect, the present invention provides the X-ray CT apparatus according to any one of the first to nineteenth aspects, wherein the image reconstruction means is an X-ray detector for one column in the z direction of a multi-row X-ray detector. When it is desired to reconstruct the z-direction thickness (hereinafter referred to as slice thickness) of the tomographic image corresponding to the channel width wider than the X-ray detector channel width, the projection data of each X-ray detector channel is converted to the z-direction. An X-ray CT apparatus is provided that reconstructs a tomographic image in which the slice thickness in the z direction is controlled by weighted addition or superimposing a z filter in the z direction.
In the X-ray CT apparatus according to the twentieth aspect, when it is desired to control the slice thickness of each tomographic image or the slice thickness of each partial image reconstruction region to a certain thickness, the projection data of each X-ray detector channel is converted to the z direction. By performing weighted addition or filtering in the z direction, the slice thickness of each tomographic image or each partial image reconstruction region can be controlled to make the image quality of the tomographic image uniform. Of course, the above is also effective in the case of partially enlarged image reconstruction.

According to the X-ray CT apparatus or the X-ray CT image reconstruction method of the present invention, the effect thereof is a two-dimensional X-ray area having a matrix structure represented by a multi-row X-ray detector or a flat panel X-ray detector. In the X-ray CT using the detector, the uniformity of the image quality of the tomographic image can be improved.
Further, as another effect of the present invention, it is possible to improve the uniformity of the image quality in the z-axis direction of the tilted tomogram obtained from the tilted gantry and the data collection system.
Further, as another effect of the present invention, it is possible to improve the overall image quality of the partially enlarged image reconstructed tomographic image and to improve the uniformity of the image quality.
As another effect of the present invention, it is possible to maintain the continuity of image quality of tomographic images in the entire field of view.
As another effect of the present invention, S / N improvement of image noise in a tomographic image of conventional scan (axial scan), cine scan, helical scan, or variable pitch helical scan that collects data at different positions in the z direction is performed. be able to.
As another effect of the present invention, the slice thickness in the z direction can also be controlled.

Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
In the present embodiment, the following six embodiments are shown.
Example 1: An example in which there are few artifacts, slice thickness is reduced, uniformity of image quality is improved, and overall image quality is improved because there are a plurality of back projection processing centers and partial reconstruction regions.
Example 2: In the case of a conventional scan (axial scan) or cine scan in which the scanning gantry is inclined, the center of back projection processing is not brought on the rotation center of the data acquisition system, but each tomogram and z-axis An example where the consistency of the image quality in the z direction can be improved by bringing it to the intersection.
Embodiment 3 In the case of partial enlarged image reconstruction, the back projection processing is not performed at the center of the entire imaging area, but is performed at the center of the display area of the partial enlarged image reconstruction, that is, the center of the imaging area. By placing the center of, an example where the overall image quality has few artifacts and the slice thickness can be reduced.
Example 4: When there are a plurality of back projection processing centers and partial image reconstruction regions, image reconstruction is performed by overlapping at the boundaries between adjacent partial image reconstruction regions, and two tomographic images are obtained in the overlapped region. An example in which images are subjected to weighted addition, and continuous tomographic images are successively transferred to realize continuous image quality on the xy plane of tomographic images.
Example 5: Back projection processing is performed using projection data of a plurality of X-ray beams acquired at different positions in the z direction passing through a certain partial image reconstruction area, and the angle between the tomographic image and the X-ray beam is set. An example in which back-projection processing with reduced artifacts is performed by applying dependent weights, and S / N of image noise can be improved and image quality can be made uniform by back-projecting more X-ray beams.
Example 6: In order to control the slice thickness of a tomographic image, a weighted addition in the column direction (z direction) of a multi-row X-ray detector or a two-dimensional X-ray area detector, or a z filter is superimposed in the z direction, An example in which a tomographic image in which the slice thickness in the z direction is controlled can be reconstructed by performing back projection processing of the X-ray beam onto a partial image reconstruction region after controlling the beam width of the image.

  FIG. 1 is a configuration block diagram of an X-ray CT apparatus according to an embodiment of the present invention. The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 collects X-ray detector data acquired by the input device 2 that receives input from the operator, the central processing device 3 that executes preprocessing, image reconstruction processing, postprocessing, and the like, and the scanning gantry 20. A data acquisition buffer 5, a monitor 6 for displaying a tomographic image reconstructed from projection data obtained by preprocessing X-ray detector data, and a storage device 7 for storing programs, data and X-ray CT images It has.

  The imaging table 10 includes a cradle 12 on which a subject is placed and put into and out of the cavity of the scanning gantry 20. The cradle 12 is moved up and down and linearly moved by the motor built in the imaging table 10.

  The scanning gantry 20 rotates around an X-ray tube 21, an X-ray controller 22, a collimator 23, a multi-row X-ray detector 24, a DAS (Data Acquisition System) 25, and the body axis of the subject. A rotating unit controller 26 for controlling the X-ray tube 21 and the like, and a control controller 29 for exchanging control signals and the like with the operation console 1 and the imaging table 10. Further, the scan gantry 20 can be tilted about ± 30 degrees forward and backward in the z direction by the tilt controller 27 (tilt controller).

FIG. 2 is an explanatory diagram of the geometric arrangement of the X-ray tube 21 and the multi-row X-ray detector 24.
The X-ray tube 21 and the multi-row X-ray detector 24 rotate around the rotation center IC. When the vertical direction is the y direction, the horizontal direction is the X direction, and the table traveling direction perpendicular thereto is the z direction, the rotation plane of the X-ray tube 21 and the multi-row X-ray detector 24 is the xy plane. The moving direction of the cradle 12 is the z direction.
The X-ray tube 21 generates an X-ray beam called a cone beam CB. A view angle of 0 degree is defined when the central axis direction of the cone beam CB is parallel to the y direction.
The multi-row X-ray detector 24 has, for example, 256 X-ray detector rows. Each X-ray detector array has, for example, 1024 channels of X-ray detector channels.
The projection data collected by the X-ray irradiation is A / D converted by the DAS 25 from the multi-row X-ray detector 24 and input to the data acquisition buffer 5 via the slip ring 30. Data input to the data collection buffer 5 is processed by the central processing unit 3 according to a program in the storage device 7, converted into a tomographic image, and displayed on the monitor 6.

FIG. 3 is a flowchart showing an outline of the operation of the X-ray CT apparatus 100 of the present invention.
In step S1, the helical scan operation or variable is performed while rotating the X-ray tube 21 and the multi-row X-ray detector 24 around the subject and moving the cradle 12 on the imaging table 10 linearly on the table. A pitch helical scan operation is performed, and the table linear movement position z (view) is added to the projection data DO (view, j, i) represented by the view angle view, the detector row number j, and the channel number i. Collect line detector data.
Alternatively, a conventional scan (axial scan) operation or a cine scan operation is performed while the cradle 12 on the imaging table 10 is fixed to obtain X-ray detector data.

  In step S2, pre-processing is performed on the projection data DO (z, view, j, i). As shown in FIG. 4, the preprocessing includes step S21 offset correction, step S22 logarithmic conversion, step S23 X-ray dose correction, and step S24 sensitivity correction.

  In step S3, beam hardening correction is performed on the preprocessed projection data DO (z, view, j, i). In the beam hardening correction step S3, the projection data subjected to the sensitivity correction step S24 in the preprocessing step S2 is D1 (view, j, i), and the data after the beam hardening correction step S3 is D11 (view, j, Assuming i), the beam hardening correction step S3 is expressed, for example, in a polynomial form as follows.

この時、検出器の各ｊ列ごとに独立したビームハードニング補正を行なえるため、撮影条件で各データ収集系の管電圧が異なっていれば、各列ごとの検出器の特性の違いを補正できる。

At this time, since independent beam hardening correction can be performed for each j column of the detector, if the tube voltage of each data acquisition system differs depending on the imaging conditions, the difference in the detector characteristics for each column is corrected. it can.

In step S4, a z-filter convolution process for applying a filter in the z direction (column direction) is performed on the projection data D11 (view, j, i) subjected to beam hardening correction.
In step S4, the projection of the multi-row X-ray detector D11 (ch, row) (ch = 1 to CH, row = 1 to ROW) subjected to beam hardening correction after preprocessing in each view angle and each data acquisition system. For example, a filter having a column filter size of 5 columns as shown below is applied to the data in the column direction.
(W1 (ch), w2 (ch), w3 (ch), w4 (ch), w5 (ch)),
However,

とする。
補正された検出器データＤ１２（ｃｈ，ｒｏｗ）は以下のようになる。

And
The corrected detector data D12 (ch, row) is as follows.

となる。なお、チャネルの最大値はＣＨ，列の最大値はＲＯＷとすると、

It becomes. If the maximum channel value is CH and the maximum column value is ROW,

とする。
また、列方向フィルタ係数を各チャネルごとに変化させると画像再構成中心からの距離に応じてスライス厚を制御できる。一般的に断層像では画像再構成中心に比べ周辺部の方がスライス厚が厚くなるので、列方向フィルタ係数を中心部と周辺部で変化させて、列方向フィルタ係数を中心部チャネル近辺では列方向フィルタ係数の幅を広く変化させると、周辺部チャネル近辺では列方向フィルタ係数の幅をせまく変化させると、スライス厚は周辺部でも画像再構成中心部でも一様に近くすることもできる。
このように、多列Ｘ線検出器２４の中心部チャネルと周辺部チャネルの列方向フィルタ係数を制御してやることにより、スライス厚も中心部と周辺部で制御できる。列方向フィルタでスライス厚を弱干厚くすると、アーチファクト、ノイズともに大幅に改善される。これによりアーチファクト改善具合、ノイズ改善具合も制御できる。つまり、画像再構成された断層像つまり、ｘｙ平面内の画質が制御できる。また、その他の実施例として列方向（ｚ方向）フィルタ係数を逆重畳（デコンボリューション）フィルタにすることにより、薄いスライス厚の断層像を実現することもできる。

And
Further, when the column direction filter coefficient is changed for each channel, the slice thickness can be controlled in accordance with the distance from the image reconstruction center. In general, in the tomogram, the slice thickness is thicker in the peripheral part than in the image reconstruction center, so the column direction filter coefficient is changed between the central part and the peripheral part, and the column direction filter coefficient is changed in the vicinity of the central channel. If the width of the directional filter coefficient is changed widely, the slice thickness can be made close to uniform in both the peripheral part and the image reconstruction center part by changing the width of the column direction filter coefficient in the vicinity of the peripheral part channel.
In this way, by controlling the column direction filter coefficients of the central channel and the peripheral channel of the multi-row X-ray detector 24, the slice thickness can also be controlled in the central portion and the peripheral portion. When the slice thickness is slightly reduced with the row direction filter, both artifacts and noise are greatly improved. Thereby, artifact improvement and noise improvement can also be controlled. That is, the image reconstructed tomographic image, that is, the image quality in the xy plane can be controlled. As another embodiment, a thin slice thickness tomogram can be realized by using a deconvolution filter with the column direction (z direction) filter coefficients.

  In step S5, reconstruction function superimposition processing is performed. That is, the Fourier transform is performed, the reconstruction function is multiplied, and the inverse Fourier transform is performed. In reconstruction function convolution processing step S5, assuming that the data after the z filter convolution processing is D12, the data after the reconstruction function convolution processing is D13, and the reconstruction function to be superimposed is Kernel (j), the reconstruction function convolution processing is performed. It is expressed as follows.

つまり、再構成関数kｅｒｎｅｌ（ｊ）は検出器の各ｊ列ごとに独立した再構成関数重畳処理を行なえるため、各列ごとのノイズ特性、分解能特性の違いを補正できる。
ステップＳ６では、再構成関数重畳処理した投影データＤ１３(ｖｉｅｗ,ｊ,ｉ)に対して、２次元逆投影処理を行い、逆投影データＤ３(ｘ，ｙ)を求める。部分画像再構成領域が複数ある場合は、その各々の部分画像再構成領域にある逆投影処理の中心画素を中心にして、逆投影を各々の部分画像再構成領域ごとに２次元逆投影処理を行う。この逆投影処理については後述する。
ステップＳ７では、逆投影データＤ３(ｘ，ｙ，ｚ)に対して画像フィルタ重畳、ＣＴ値変換などの後処理を行い、断層像Ｄ３１（ｘ，ｙ）を得る。
後処理の画像フィルタ重畳処理では、２次元逆投影後のデータをＤ３１（ｘ，ｙ，ｚ）とし、画像フィルタ重畳後のデータをＤ３２（ｘ，ｙ，ｚ）、画像フィルタをＦｉｌｔｅｒ（ｚ）とすると、

That is, since the reconstruction function kernel (j) can perform the reconstruction function convolution process independently for each j column of the detector, the difference in noise characteristics and resolution characteristics for each column can be corrected.
In step S6, two-dimensional backprojection processing is performed on the projection data D13 (view, j, i) subjected to reconstruction function superposition processing to obtain backprojection data D3 (x, y). When there are a plurality of partial image reconstruction regions, back projection is performed for each partial image reconstruction region by performing back projection around the center pixel of the back projection processing in each partial image reconstruction region. Do. This back projection process will be described later.
In step S7, post-processing such as image filter superimposition and CT value conversion is performed on the backprojection data D3 (x, y, z) to obtain a tomographic image D31 (x, y).
In post-processing image filter superimposition processing, the data after two-dimensional backprojection is D31 (x, y, z), the data after image filter convolution is D32 (x, y, z), and the image filter is Filter (z). Then,

ステップＳ８では、画像再構成された断層像はモニタ６に表示される。

In step S8, the reconstructed tomographic image is displayed on the monitor 6.

  In the two-dimensional backprojection process in step S6, first, a field-of-view field (FOV: Field Of View) is divided into a plurality of partial image reconstruction areas as shown in FIG. The divided partial image reconstruction area is divided so that one back projection processing center exists.

The explanatory diagram of the flow of the two-dimensional backprojection process in FIG. 5 shows the flow of the two-dimensional backprojection process when there is no overlapping region.
In step S61, i = 1 is performed.
In step S62, processing for extracting the backprojection processing center in the i-th partial image reconstruction area is performed.

  In step S63, the data of the column of the multi-row X-ray detector 24 or the two-dimensional X-ray area detector 24 corresponding to the center of the backprojection process is obtained from 360-degree projection data or 180-degree + detector fan angle projection data. Extract. If there is no corresponding detector row, two neighboring detector rows are extracted, and for example, a process of obtaining the weighted addition of the projection data of the two detector rows according to the distance ratio is performed. An X-ray beam passing through the center of the back projection process that selects the projection data of the column corresponding to the X-ray beam that passes through the center of each back projection process of each partial image reconstruction region, or projection data of the column that corresponds to the X-ray beam. If not, the two X-ray beams closest to the center of the back projection process or the projection data in the column corresponding thereto are selected, and the weighted addition in the z direction is performed and the X-ray beam passing through the center of the back projection process or the equivalent is selected. Create projection data for columns.

In step S64, a process of two-dimensional backprojecting the projection data extracted in step S62 to the i-th partial image reconstruction is performed. From the selected projection data or a plurality of selected projection data, the projection data weighted and added in the z direction is two-dimensionally backprojected onto the partial image reconstruction area.
In step S65, it is determined whether i = N. However, N is the number of partial image reconstruction areas. If YES, the process ends via step S67. If NO, i = i + 1 is set in step S66, the number of the partial image reconstruction area is updated, the process returns to step S62, and the process of continuing the back projection process is performed again. In the first embodiment, steps S67 and S68 are omitted.

  The two-dimensional backprojection process in step S6 described above is a backprojection process performed in a partial image reconstruction area near the center of one backprojection process. That is, after performing the processing from step S1 to step S5, step S6 is repeated by the number of partial image reconstruction regions. After the back projection process in step S6 is repeated for the entire partial image reconstruction area, steps S7 and S8 are performed to reconstruct a tomographic image. In the vicinity of the center of the back projection process, the image quality is improved because there are few artifacts and the slice thickness is thin. If the centers of backprojection processing are uniformly scattered in the entire field of view, the overall image quality becomes more uniform and the image quality is improved.

  In this case, the larger the distance to the center of the back projection process and the farthest point in the divided partial image reconstruction area to which the center of the back projection process belongs, or each of the divided partial image reconstruction areas The greater the average distance to the pixel, the greater the contradiction in the z direction of image reconstruction due to the effect of cone angle due to the tilt of the x-ray beam in the z direction, the greater the artifact and the thicker the slice thickness. In other words, the pixel at the center of the backprojection process is “the projection data of the X-ray beam that correctly passes through the pixel is selected when backprojected and backprojected”, but is away from the center point of the backprojection process. Then, “projection data of X-ray beam passing through the pixel correctly” is not selected, and projection data of the X-ray beam shifted in the z direction is selected and back-projected. An error due to a deviation in the z direction due to the cone angle in the z direction causes an artifact. FIG. 14 shows the deviation in the z direction. For this reason, if the image reconstruction area is finely divided and the “distance between the image reconstruction center of the back projection process and the farthest point in the divided image reconstruction area” can be reduced, the artifact of the tomographic image can be reduced.

  In the above case, various methods may be used for dividing the image reconstruction area. FIG. 13 shows an example of a method for dividing the image reconstruction area. In FIG. 13, the imaging visual field region is set to include the entire tomographic image.

  FIG. 13A illustrates a case where a rectangular photographing field area is divided by rectangular partial image reconstruction areas having the same size. In this case, since the field of view and the partial image reconstruction area are divided by a simple shape and the area becomes smaller, the average distance from the center of the back projection processing of each divided area to each pixel is shortened, and the artifact is Small and thin slice thickness, improving image quality.

  FIG. 13B illustrates a case where a circular photographing field area is divided by rectangular partial image reconstruction areas having the same size. In this case, by dividing the image by the partial image reconstruction area having a simple shape, the artifact is reduced, the slice thickness is reduced, and the image quality is improved. By omitting the back projection processing of the corners of the rectangle in the circular outer portion where image reconstruction is not performed, the amount of calculation can be reduced.

  FIG. 13C illustrates a case where a partial image reconstruction area is set at the center of the photographing visual field area and a plurality of partial image reconstruction areas are set around the partial image reconstruction area. Specifically, a circular partial image reconstruction area is set at the center, and the periphery thereof is divided by concentric circles and is also divided by radial straight lines. The pixel center of the back projection process of the central partial image reconstruction area is set to, for example, the center of the photographing visual field area (for example, the center of gravity of the photographing visual field area) or the rotation center IC (see FIG. 2). The surrounding partial image reconstruction area is set so that the reconstruction area on the center side is larger in the radial direction and the circumferential direction than the reconstruction area on the outer circumference side, that is, the area is larger. In this case, an effect of reducing artifacts can be obtained by distributing the pixel centers of the back projection process in the peripheral area while maintaining the conventional accuracy with respect to the center of the imaging field area with small artifacts. Furthermore, the distance from the pixel center of the back projection process of each partial image reconstruction area to the peripheral edge of each partial image reconstruction area becomes shorter as the outer peripheral side where artifacts tend to be larger, and the image quality is improved and uniformized. .

  FIG. 13D illustrates an example in which a partial image reconstruction area is set at the center of the photographing visual field area and a plurality of partial image reconstruction areas are set around the same as in FIG. 13C. Yes. In FIG. 13D, the central partial image reconstruction area is rectangular, the surrounding partial image reconstruction area is polygonal, or the peripheral partial image reconstruction area is the outer peripheral one. This indicates that the shape and area of the central partial image reconstruction area and the surrounding partial image reconstruction area may be set as appropriate, for example, by reducing the area.

The number, position, shape, and the like of the pixel center and the partial image reconstruction area in the back projection process may be stored in advance in the storage device 7 or may be appropriately set by the user via the input device 2. The central processing unit 3 may set according to a predetermined algorithm.
The same effect can be obtained even if the projection data used is projection data of 360 degrees or half-scan 180 degrees + detection fan angle projection data.
A similar effect can be obtained with a two-dimensional X-ray area detector typified by a flat panel X-ray detector.

  In the first embodiment, the entire image reconstruction area is divided into a plurality of partial image reconstruction areas, and the center point (focal point) of image reconstruction of back projection processing is provided in each area. One pixel of the image may be used as the center of the back projection process as an image reconstruction area.

Next, the case where the scanning gantry 20 is tilted in conventional scanning (axial scanning) or cine scanning will be described. FIG. 15 shows the positional relationship between a normal X-ray data acquisition system and a tomogram. With respect to this state, as shown in FIG. 16, when the rotation center of the data acquisition system is used as the center of back projection processing of the tomographic images in each column, and all the tomographic images are tilted together with the scanning gantry 20, the tomographic images are in the z direction. The coordinate position in the y-axis direction is different between the larger coordinate and the smaller coordinate, and a tomogram in the z direction as shown in FIG. 17 between conventional scans (axial scans) or cine scans at a plurality of z-direction positions. The continuity of the position cannot be obtained.
Therefore, as shown in FIG. 18, if the position of the center of the tomographic image is set as the intersection with the z axis, the continuity of the position of the tomographic image in the z direction can be obtained. At this time, if the center of the back projection process is also set at the intersection of the tomographic image and the z axis, the uniformity of image quality in the z direction can be obtained.

The projection data used in this case can be the same effect whether it is projection data for 360 degrees or half-scan 180 degrees + detector fan angle projection data.
A similar effect can be obtained with a two-dimensional X-ray area detector represented by a flat panel X-ray detector.
Also in the second embodiment, as in the first embodiment, the entire image reconstruction area is divided into a plurality of partial image reconstruction areas for the tomographic image of the conventional scan (axial scan) of each column, and each partial image reconstruction area is divided. You may provide the pixel center of a back projection process. In this case, if the pixel center of back projection processing of one image reconstruction area is the intersection of an axis parallel to the z axis passing through the rotation center and the tomographic image of each column, the pixels of the entire image reconstruction area described above Similar to the center, the continuity of the partial image reconstruction area in the z direction is obtained. At this time, similarly to the first embodiment, the smaller the distance between the central pixel of the back projection process of the partial image reconstruction area and each pixel of the partial image reconstruction area, the fewer the tomographic image artifacts. The slice thickness is also reduced and the image quality is improved.

  Next, in the third embodiment, an example of efficient use of the center of back projection processing in the case of partial enlarged image reconstruction will be described. In the following, partial enlarged image reconstruction refers to image reconstruction in which a tomographic image is traced back to projection data using a part of the entire imaging field area (all imaging area) as an imaging field area. The partial enlarged image reconstruction area is an area in which a tomographic image is reconstructed by partial enlarged image reconstruction. The entire imaging visual field region (total imaging region) is a range including the entire tomographic imaging visual field of the subject.

  For example, as shown in FIG. 7, when the partially enlarged image reconstruction area is not in the central portion of the entire imaging visual field area (for example, the center of gravity of the entire imaging visual field area), for example, the entire imaging visual field image captures both lung fields. If the field of view of the partially magnified image reconstruction area is a single lung field, even if the center of the back projection process is at the center of the entire field of view field of view, the partially magnified image reconstruction area The center of the back projection process is located at the periphery of the constituent area, and this does not contribute to improving the image quality of the tomographic image reconstructed from the partially enlarged image. Rather, the center of the back projection process is centered on the center and center of the partially enlarged image reconstruction area (for example, the center of gravity or the outer or inner center of the partially enlarged image reconstruction area). There is no need. In this case, assuming that the partial enlarged image reconstruction area has a radius r, the distance between the peripheral part of the partial enlarged image reconstruction area and the center of the back projection process is at most r, and even in the peripheral part of the partial enlarged image reconstruction area. Artifacts are minimized, slice thickness can be minimized, and image quality can be optimized.

Of course, in this case, not only the center of the back projection process is placed at the center of the partially enlarged image reconstruction area, but also the center of a plurality of back projection processes is arranged at the periphery of the partially enlarged image reconstruction area as shown in FIG. And the partial image reconstruction area in the vicinity of the center of the backprojection processing are evenly disposed as shown in FIG. 11, the image quality of the tomographic image further reconstructed by the partially enlarged image is further improved.
The projection data used in this case is the same for 360-degree projection data or for half-scan 180-degree + projection fan angle projection data.
The same effect can be obtained with a two-dimensional X-ray area detector represented by a flat panel X-ray detector.

Next, in a fourth embodiment, an example of how to obtain continuity of image quality in a tomographic image that is an xy plane when a plurality of partial image reconstruction regions exist will be described.
The explanatory diagram of the flow of the two-dimensional backprojection process in FIG. 5 shows the flow of the two-dimensional backprojection process when there are overlapping regions.
Steps S61 to S66 are as described in the first embodiment.

In step S67, it is determined whether or not there is an overlapping area at the boundary between each of the N partial image reconstruction areas. If YES, the process proceeds to step S68, and if NO, the process is terminated.
In step S68, in the overlap region, two overlapping tomographic images are subjected to weighted addition with a linear weighting coefficient corresponding to the distance.

  Similar to the first embodiment, after performing the preprocessing from step S1 to step S5, the two-dimensional backprojection processing in step S6 is repeated by the number of partial image reconstruction regions. Thereafter, post-processing in step S7 and image display in step S8 are performed. Usually, when partial tomographic images of a plurality of partial image reconstruction regions are directly joined to a single tomographic image as they are, boundary lines of the partial image reconstruction regions often appear. Therefore, as shown in FIG. 12, the two-dimensional backprojection processing is performed as a large partial image reconstruction region that overlaps the backprojection processing of each adjacent partial image reconstruction region beyond the boundary. In the overlapped region, as shown in FIG. 12, for example, the weighting coefficient is gradually changed in the linearly overlapped region and weighted addition is performed.

  Assuming that the tomographic images G1 (x, y) and G2 (x, y) reconstructed from the partial images, the weighting coefficients w1 and w2, and the tomographic images G (x, y) joined together are as follows.

これにより、連続的に隣り合う断層像に乗り移る際に、ｘｙ平面内で充分に連続的な画質を実現できる。この場合、断層像の各画素のＣＴ値が不連続にならないように、加重係数ｗ１（ｘ，ｙ），ｗ２（ｘ，ｙ）の和は常に１にしておく必要がある。

As a result, it is possible to realize sufficiently continuous image quality in the xy plane when changing to adjacent tomographic images. In this case, the sum of the weighting coefficients w1 (x, y) and w2 (x, y) must always be 1 so that the CT value of each pixel of the tomographic image does not become discontinuous.

  Note that the position and size of the overlapped region may be set as appropriate. Further, the position and size of the overlapped area may be stored in the storage device 7 in advance, or may be appropriately set by the user via the input device 2, or the central processing unit 3 may have a predetermined value. You may set according to an algorithm.

  Next, in Example 5, back projection processing is performed using projection data of a plurality of X-ray beams acquired at different positions in the z direction passing through a certain partial image reconstruction region, thereby improving S / N of image noise. An example of reducing artifacts and making the image quality uniform will be described.

  When performing a scan over a wide range in the z direction, one or a plurality of partial image reconstruction areas are obtained by image reconstruction of tomographic images in each column in a conventional scan (axial scan) or a cine scan at a plurality of positions in the z direction. , Back projection is performed on the center of the back projection processing of each partial image reconstruction area to perform two-dimensional image reconstruction.

  Further, at this time, as shown in FIG. 14, in the X-ray detector rows at both ends in the z direction, a tomographic image relating to a missing cone in which X-ray beams are reduced is reconstructed. In the tomographic image of the missing cone, artifacts are large and image noise tends to be large. As a countermeasure against this, as shown in FIG. 20, when performing back projection processing, not only projection data at one z-direction position is used, but projection data at a plurality of z-direction positions at other z-direction positions is used. If there is an X-ray beam that passes through the partial image reconstruction area that has undergone backprojection, backprojection can be performed effectively.

  In the case of helical scan or variable pitch helical scan, back projection is also performed in the helical scan or variable pitch helical scan at the start of acceleration or deceleration as shown in FIG. 21 where the helical pitch is 1 or less. When processing, only the projection data of one z-direction position is not used, but the partial image reconstruction area subjected to back projection processing among the projection data of a plurality of z-direction positions of other z-direction positions is used. If there is an X-ray beam that passes, back projection can be performed effectively.

  Next, in Example 6, as shown in FIG. 22, weighted addition is performed in the column direction (z direction) of the multi-row X-ray detector 24 or the two-dimensional X-ray area detector 24 in order to control the slice thickness of the tomographic image. Alternatively, a z-filter is superimposed in the z direction to control the X-ray beam width for two-dimensional backprojection of the partial image reconstruction area, and then two-dimensional backprojection processing is performed to determine the slice thickness and image reconstruction of the partial image reconstruction area. It is also possible to control the slice thickness of the tomographic image.

For example, the projection data D11 (ch, j) is filtered in the column direction with, for example, the following column direction filter size of 5 columns.
(W1 (ch), w2 (ch), w3 (ch), w4 (ch), w5 (ch)),
However,

とする。
補正された検出器データＤ１２（ｃｈ，ｒｏｗ）は以下のようになる。

And
The corrected detector data D12 (ch, row) is as follows.

となる。なお、チャネルの最大値はＣＨ， 列の最大値はＲＯＷとすると、

It becomes. If the maximum channel value is CH and the maximum column value is ROW,

とする。

And

  Further, when the column direction filter coefficient is changed for each channel, the slice thickness can be controlled in accordance with the distance from the image reconstruction center. In addition, the slice thickness can be freely controlled by changing the weighting coefficient of the z filter for each of the plurality of partial image reconstruction regions. In general, in the tomogram, the slice thickness is thicker in the peripheral part than in the image reconstruction center, so the column direction filter coefficient is changed between the central part and the peripheral part, and the column direction filter coefficient is changed in the vicinity of the central channel. If the width of the directional filter coefficient is changed widely, the slice thickness can be made close to uniform in both the peripheral part and the image reconstruction center part by changing the width of the column direction filter coefficient in the vicinity of the peripheral part channel.

  In this way, by controlling the column direction filter coefficients of the central channel and the peripheral channel of the multi-row X-ray detector 24, the slice thickness can also be controlled in the central portion and the peripheral portion. When the slice thickness is slightly reduced with the row direction filter, both artifacts and noise are greatly improved. Thereby, artifact improvement and noise improvement can also be controlled. That is, the image reconstructed tomographic image, that is, the image quality in the xy plane can be controlled. As another embodiment, a thin slice thickness tomogram can be realized by using a deconvolution filter with the column direction (z direction) filter coefficients.

In the X-ray CT apparatus 100 described above, according to the X-ray CT apparatus or the X-ray CT imaging method of the present invention, it is conventionally possible to reduce deterioration in image quality at the periphery of the image reconstruction area far from the image reconstruction center. effective. Also, the image quality can be further improved by increasing the number of image reconstruction centers.

The image reconstruction method may be another two-dimensional image reconstruction method as long as it is a conventionally known two-dimensional image reconstruction method.
In this embodiment, column direction (z-direction) filters having different coefficients are superimposed on each column to adjust image quality variation, thereby realizing uniform slice thickness, artifact, and noise image quality. However, various filter coefficients are conceivable for this. However, in either case, the same effect can be obtained.
In the present embodiment, the medical X-ray CT apparatus is written based on the X-ray CT-PET apparatus, the X-ray CT-SPECT apparatus, etc. combined with the industrial X-ray CT apparatus or other apparatuses. The effect can be produced similarly.
In this embodiment, several image reconstruction divided region dividing methods have been described. However, the same effect can be obtained with various image dividing methods. Further, the same effect can be obtained by using one pixel of each tomographic image as one image reconstruction divided region as the center of back projection processing.
In the present embodiment, the format of the projection data was not described in detail, but the projection data for 360 degrees, the projection data for 180 degrees + detector fan angle for half scan, and the projection data exceeding 360 degrees. But you can achieve the same effect.
In this embodiment, the multi-row X-ray detector is used as the X-ray detector. However, the same applies to an X-ray II (X-ray image intensifier), a flat panel two-dimensional X-ray area detector, and the like. Can produce an effect.

It is a block diagram which shows the X-ray CT apparatus concerning one Embodiment of this invention. It is explanatory drawing which shows rotation of a X-ray generator (X-ray tube) and a multi-row X-ray detector. It is a flowchart which shows schematic operation | movement of the X-ray CT apparatus which concerns on one Embodiment of this invention. It is a flowchart which shows the detail of pre-processing. It is a flowchart which shows the flow of a two-dimensional back projection process. It is a figure which shows the relationship between the image reconstruction plane in a two-dimensional image reconstruction, and an image quality, and the figure which shows the relationship between the center of the back projection process in a two-dimensional image reconstruction, and an X-ray beam. It is a figure which shows a partial expansion image reconstruction area | region and all the imaging | photography areas. It is a figure which shows the X-ray beam which passes through the center pixel of the back projection process which shifted | deviated from the tomographic image center, and the center pixel of the back projection process. It is a figure which shows the X-ray beam which passes through the center pixel of several back projection processes, and the center pixel of the back projection process. It is a figure which shows the center point of the back projection process which exists in a whole imaging | photography area plane other than a center. It is a figure which shows the division | segmentation of a partial reconstruction area | region. It is a figure which shows the synthesis | combination of the image reconfigure | reconstructed by the weighting coefficient of the dividing line vicinity. It is a figure which shows the example of a division | segmentation of the partial image reconstruction area | region of an imaging | photography visual field area | region. It is a figure which shows the shift | offset | difference of the z direction of the pixel back-projected with an X-ray beam. It is a figure which shows the positional relationship of a normal X-ray data acquisition system and a tomogram. It is a figure which shows the positional relationship of an X-ray data acquisition system and a tomogram when a scanning gantry inclines. It is a figure which shows the positional relationship of an X-ray data acquisition system and a tomogram when the scanning gantry inclines in the several z direction position. It is a figure which shows the positional relationship which aligned the center of the X-ray data collection system when a scanning gantry inclines, and the back projection process of a tomogram in the z direction. It is a figure which shows the positional relationship which aligned the center of the back projection process of the X-ray data acquisition system and tomographic image in case the scanning gantry inclines in the several z direction position. It is a figure which shows the several X-ray beam which passes a partial image reconstruction area | region in the conventional scan (axial scan) or cine scan adjacent to az direction. It is a figure which shows the several X-ray beam which passes a partial image reconstruction area | region in the helical scan whose helical pitch is pitch 1 or less in a helical scan. It is a figure which shows control of the slice thickness of the tomogram which superimposes the weighting coefficient or z filter of a column direction on projection data, and is image-reconstructed.

Explanation of symbols

1 Operation Console 2 Input Device 3 Central Processing Unit 5 Data Collection Buffer 6 Monitor 7 Storage Device 10 Imaging Table 12 Cradle 15 Rotating Unit 20 Scanning Gantry 21 X-ray Tube 22 X-ray Controller 23 Collimator 24 Multi-row X-ray Detector 25 DAS ( Data collection device)
26 Rotating part controller 27 Tilt controller (tilt controller)
29 Controller 30 Slip ring dP Detector surface P Reconstruction area PP Projection surface
IC center of rotation (ISO)

Claims (20)

Collecting X-ray projection data transmitted through the subject while rotating the multi-row X-ray detector that detects X-rays relative to the X-ray generator around the center of rotation. X-ray data collection means
Image reconstruction means for reconstructing the projection data collected from the X-ray data collection means by two-dimensional back projection;
In an X-ray CT apparatus provided with image display means for displaying a tomographic image reconstructed,
When the image reconstruction unit performs two-dimensional backprojection of projection data, a pixel that is correctly backprojected to a pixel through which the projection data has passed (hereinafter referred to as a central pixel of backprojection processing) is a reconstruction region. A plurality of imaging field areas are divided into a plurality of partial reconstruction areas located near the central pixel of the plurality of back projection processes, and two-dimensional back projection processing is performed for each partial reconstruction area. Perform X-ray CT system. The X-ray CT apparatus according to claim 1,
The multi-row X-ray detector is a two-dimensional X-ray area detector having a matrix structure typified by a flat panel X-ray detector, or a combination of two or more two-dimensional X-ray area detectors. X-ray CT system that is a dimensional X-ray area detector. In the X-ray CT apparatus according to claim 1 or 2,
The image reconstruction unit performs two-dimensional backprojection processing using projection data for 360 degrees or about 360 degrees for projection data to be used. X-ray CT apparatus. In the X-ray CT apparatus according to claim 1 or 2,
The image reconstruction means performs two-dimensional backprojection processing using projection data whose projection data is 180 degrees + detector fan angle, or approximately 180 degrees + detector fan angle. X-ray CT apparatus. In the X-ray CT apparatus according to claims 1 to 4,
An X-ray CT apparatus in which at least one of the plurality of tomographic back-projection center pixels is the center of the field of view. In the X-ray CT apparatus according to claims 1 to 4,
An X-ray CT apparatus, wherein at least one of the plurality of back-projection center pixels of the tomographic image is a rotation center of a data acquisition system. In the X-ray CT apparatus according to claims 1 to 4,
The image reconstruction means reconstructs tomographic images of all columns obtained by a conventional scan (axial scan) using a multi-row X-ray detector,
An X-ray CT apparatus in which at least one of the plurality of back-projection center pixels of each tomographic image is the center or almost the center of the field of view. In the X-ray CT apparatus according to claims 1 to 4,
The image reconstruction means reconstructs tomographic images of all columns obtained by a conventional scan (axial scan) using a multi-row X-ray detector,
Among the plurality of back projection processing center pixels in each tomographic image, at least one is an intersection of the z axis passing through the rotation center and each tomographic image. X-ray CT apparatus. Collecting X-ray projection data transmitted through the subject while rotating the multi-row X-ray detector that detects X-rays relative to the X-ray generator around the center of rotation. X-ray data collection means
Image reconstruction means for reconstructing the projection data collected from the X-ray data collection means by two-dimensional back projection;
In an X-ray CT apparatus provided with image display means for displaying a tomographic image reconstructed,
In the two-dimensional backprojection when the projection data is partially enlarged and reconstructed by the image reconstruction means, the position where the center pixel of the backprojection process is shifted from the center of the entire photographing region to the center of the reconstruction region of the partially enlarged reconstruction X-ray CT system located in The X-ray CT apparatus according to claim 9.
An X-ray CT apparatus in which a central pixel of the backprojection processing is positioned at or substantially at the center of a reconstruction area for partial enlargement reconstruction. The X-ray CT apparatus according to claim 9 or 10,
Divided reconstruction in which a plurality of center pixels for the backprojection processing exist in a reconstruction area for partial enlargement reconstruction, and a plurality of reconstruction areas for the partial enlargement reconstruction are located in the vicinity of the plurality of center pixels for backprojection processing An X-ray CT apparatus in which a region is divided into a plurality of regions, and two-dimensional backprojection processing is performed for each divided reconstruction region. In the X-ray CT apparatus according to claims 9 to 11,
The image reconstruction means performs two-dimensional backprojection processing using projection data whose projection data is for 360 degrees or about 360 degrees. X-ray CT apparatus. In the X-ray CT apparatus according to claims 9 to 11,
The image reconstruction means performs a two-dimensional backprojection process using projection data whose projection data is 180 degrees + detector fan angle or approximately 180 degrees + detector fan angle. X-ray CT apparatus. In the X-ray CT apparatus according to claims 1 to 13,
The image reconstruction means performs a weighted addition on a portion overlapping with an adjacent image reconstruction region in a plurality of image reconstruction regions. The X-ray CT apparatus according to claim 1 to 14,
The X-ray data collection means collects X-ray projection data by a conventional scan (axial scan) or a cine scan. The X-ray CT apparatus according to claim 1 to 14,
X-ray data collection means collects X-ray projection data by helical scanning or variable pitch helical scanning. The X-ray CT apparatus according to claim 15 or 16,
The X-ray data collection means moves the X-ray generator, the multi-row X-ray detector, and the subject relative to each other in the body axis direction (hereinafter referred to as the z direction) of the subject, thereby moving in the z direction. X-ray projection data is collected at a plurality of positions,
The X-ray CT apparatus, wherein the image reconstruction means performs projection reconstruction using projection data passing through the central pixel of the backprojection processing and having different z-direction positions for backprojection processing. The X-ray CT apparatus according to claim 14 to 17,
The X-ray data collection means collects X-ray data by tilting a scanning gantry from an xy plane perpendicular to the z direction. The X-ray CT apparatus according to claim 1 to claim 18,
The image reconstruction means performs back projection on the X-ray projection data of the opposite X-ray beam during back projection processing by applying a weighting factor depending on the angle formed between each tomographic image and the X-ray beam. CT device. The X-ray CT apparatus according to any one of claims 1 to 19,
The image reconstruction means uses the z-direction thickness (hereinafter referred to as slice thickness) of the tomographic image corresponding to the X-ray detector channel width for one column in the z direction of the multi-row X-ray detector. If you want to reconstruct an image that is wider than the channel width, add projection data of each X-ray detector channel in the z direction or add a z filter in the z direction, and create a tomographic image in which the slice thickness in the z direction is controlled. X-ray CT system for image reconstruction.

Images