JP2006026390A - Ring artifact removing method and x-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ring artifact removing method and an X-ray CT apparatus capable of easily and accurately removing ring artifact. <P>SOLUTION: Cartesian coordinates of each pixel of a reconstructive image on an X-ray CT apparatus are converted to polar coordinates where zero is set at isocenter (601), the each pixel is arranged in polar space based on its polar coordinates to generate an intermediate image (603), the intermediate image is processed for removing artifact parallel to θ axis (605), polar coordinates of each pixel of the processed intermediate image are converted to Cartesian coordinates (607), and the each pixel of polar coordinates of the processed intermediate image is arranged in Cartesian space based on its Cartesian coordinates to generate a revised image (609). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a ring artifact removal method and an X-ray CT (Computed Tomography) apparatus, and more particularly to a method for removing ring artifacts included in a reconstructed image of an X-ray CT apparatus, and ring artifact removal. The present invention relates to an X-ray CT apparatus.

  In the X-ray CT apparatus, ring artifacts may occur in the reconstructed image. The ring artifact is an annular artifact centered on an isocenter. This is due to a failure of a specific channel in the X-ray detector.

  In order to remove ring artifacts from the reconstructed image, the location and signal strength of the ring artifact in the reconstructed image are first determined, and then the pixel values of the corresponding part of the reconstructed image are corrected according to the signal strength of the artifact. .

Ring artifact location and signal strength detection are performed after the reconstructed image is moved from the orthogonal coordinate space to the polar coordinate space, and pixel value correction is performed after the artifact image is returned from the polar coordinate space to the orthogonal coordinate space. (For example, refer to Patent Document 1).
JP 2001-095793 A (page 3-4, FIG. 1)

  In the ring artifact removal method as described above, since the coordinate space for detecting artifacts and the coordinate space for correcting artifacts are separate, it is difficult to accurately remove artifacts due to errors caused by coordinate transformation.

  Accordingly, an object of the present invention is to realize a ring artifact removal method and an X-ray CT apparatus that can easily remove ring artifacts.

  (1) According to one aspect of the invention for solving the above-described problem, the orthogonal coordinates of each pixel of the reconstructed image of the X-ray CT apparatus are converted into coordinates of a polar coordinate system having an isocenter as an origin, and each of the pixels Is arranged in a polar coordinate space in which one of two coordinate axes orthogonal to each other based on the polar coordinates is the θ axis and the other is the r axis, and an intermediate image is created, and artifacts parallel to the θ axis are removed from the intermediate image For processing, converting polar coordinates of each pixel of the processed intermediate image into coordinates of an orthogonal coordinate system, and arranging each pixel of the processed intermediate image in an orthogonal coordinate space based on the orthogonal coordinates A ring artifact removal method characterized by creating a corrected image.

  (2) In another aspect of the invention for solving the above problems, an X-ray source, an X-ray detector arranged to face the X-ray source with a subject interposed therebetween, and the X-ray Image reconstruction means for reconstructing a tomographic image of a subject based on projection data of a plurality of views collected through a detector, and transforming orthogonal coordinates of each pixel of the reconstructed image into coordinates of a polar coordinate system with an isocenter as an origin. Polar coordinate conversion means; intermediate image creation means for creating an intermediate image by arranging each pixel in a polar coordinate space in which one of two coordinate axes orthogonal to each other based on the polar coordinates is the θ axis and the other is the r axis; Processing means for performing processing for removing artifacts parallel to the θ-axis for the intermediate image, orthogonal coordinate conversion means for converting polar coordinates of each pixel of the processed intermediate image into coordinates of an orthogonal coordinate system, and the processing Intermediate picture An X-ray CT apparatus comprising: corrected image generating means for generating a corrected image by arranging each pixel of an image in an orthogonal coordinate space based on the orthogonal coordinates.

  It is preferable to use a one-dimensional filter in the r-axis direction for the processing in terms of easily removing artifacts. The matrix size of the one-dimensional filter is preferably 1 × 7 from the viewpoint of appropriately removing artifacts.

The one-dimensional filter has a pixel value of a target pixel as Image (i, j), a sum of pixel values of two pixels at both ends in a 1 × 7 matrix, a Souter, and a sum of pixel values of five central pixels as a Scenter, When the two threshold values are Edge_mask and Value_limit, and the correction value is Ring = │ (Souter + Scenter) / 7-Image (i, j) │
When Value_limit <│2.5 × Souter-Scenter│ <Edge_mask
If (Souter + Scenter) / 7-Image (i, j) <0,
Image (i, j) = Image (i, j) −Ring
If (Souter + Scenter) / 7-Image (i, j)> 0,
Image (i, j) = Image (i, j) + Ring
This is preferable in terms of appropriately removing artifacts.

  In the inventions according to the above aspects, the orthogonal coordinates of each pixel of the reconstructed image of the X-ray CT apparatus are converted into the coordinates of a polar coordinate system with the isocenter as the origin, and each coordinate axis is orthogonal to each other based on the polar coordinates. Are arranged in a polar coordinate space in which one of them is the θ axis and the other is the r axis, an intermediate image is created, and the intermediate image is processed to remove artifacts parallel to the θ axis. Convert pixel polar coordinates to Cartesian coordinate system coordinates and place each pixel in the processed intermediate image in Cartesian coordinate space based on the Cartesian coordinates to create a modified image, thus eliminating ring artifacts precisely It is possible to realize a ring artifact removal method and an X-ray CT apparatus that are easy to implement.

  The best mode for carrying out the invention will be described below with reference to the drawings. Note that the present invention is not limited to the best mode for carrying out the invention. FIG. 1 shows a block diagram of an X-ray CT apparatus. This apparatus is an example of the best mode for carrying out the present invention. An example of the best mode for carrying out the present invention relating to an X-ray CT apparatus is shown by the configuration of the apparatus. The operation of this apparatus shows an example of the best mode for carrying out the present invention relating to the ring artifact removal method.

  As shown in FIG. 1, the apparatus includes a scanning gantry 2, an imaging table 4, and an operation console 6. The scanning gantry 2 has an X-ray tube 20. The X-ray tube 20 is an example of an X-ray source in the present invention.

  X-rays (not shown) emitted from the X-ray tube 20 are shaped (collimation) by a collimator 22 so as to be a cone-shaped X-ray beam, that is, a cone beam X-ray, The X-ray detector 24 is irradiated.

  The X-ray detector 24 has a plurality of detection elements arranged in an array according to the spread of the X-ray beam. The X-ray detector 24 is an example of the X-ray detector in the present invention. The configuration of the X-ray detector 24 will be described later.

  In the space between the X-ray tube 20 and the X-ray detector 24, an imaging target is mounted on the imaging table 4 and carried in. The X-ray tube 20, the collimator 22, and the X-ray detector 24 constitute an X-ray irradiation / detection device. The X-ray irradiation / detection apparatus will be described later.

  A data collection unit 26 is connected to the X-ray detector 24. The data collection unit 26 collects detection signals of individual detection elements of the X-ray detector 24 as digital data. The detection signal of the detection element is a signal representing the projection of the target by X-rays. Hereinafter, this is also referred to as projection data or simply data.

  X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. The connection relationship between the X-ray tube 20 and the X-ray controller 28 is not shown. The collimator 22 is controlled by a collimator controller 30. The connection relationship between the collimator 22 and the collimator controller 30 is not shown.

  The components from the X-ray tube 20 to the collimator controller 30 described above are mounted on the rotating unit 34 of the scanning gantry 2. The rotation of the rotating unit 34 is controlled by the rotation controller 36. The connection relationship between the rotating unit 34 and the rotation controller 36 is not shown.

  The operation console 6 has a data processing device 60. The data processing device 60 is configured by, for example, a computer. A control interface (interface) 62 is connected to the data processing device 60. The control gantry 2 and the imaging table 4 are connected to the control interface 62. The data processing device 60 controls the scanning gantry 2 and the imaging table 4 through the control interface 62.

  The data acquisition unit 26, the X-ray controller 28, the collimator controller 30 and the rotation controller 36 in the scanning gantry 2 are controlled through the control interface 62. Note that illustration of individual connections between these units and the control interface 62 is omitted.

  A data collection buffer 64 is connected to the data processing device 60. A data collection unit 26 of the scanning gantry 2 is connected to the data collection buffer 64. Data collected by the data collection unit 26 is input to the data processing device 60 through the data collection buffer 64.

  A storage device 66 is connected to the data processing device 60. The storage device 66 stores projection data input to the data processing device 60 through the data collection buffer 64 and the control interface 62, respectively. The storage device 66 also stores a program for the data processing device 60. When the data processing device 60 executes the program, the operation of this device is performed.

  The data processing device 60 performs image reconstruction using the projection data collected in the storage device 66 through the data collection buffer 64. For the image reconstruction, for example, a filtered back projection method or the like is used.

  A display device 68 and an operation device 70 are connected to the data processing device 60. The display device 68 is configured by a graphic display or the like. The operation device 70 includes a keyboard having a pointing device.

  The display device 68 displays the reconstructed image and other information output from the data processing device 60. The operation device 70 is operated by a user and inputs various instructions and information to the data processing device 60. The user operates the present apparatus interactively using the display device 68 and the operation device 70.

  FIG. 2 shows a schematic configuration of the X-ray detector 24. As shown in the figure, the X-ray detector 24 is a multi-channel X-ray detector in which a plurality of X-ray detection elements 24 (ik) are arranged in a two-dimensional array. The plurality of X-ray detection elements 24 (ik) as a whole form an X-ray light receiving surface curved in a cylindrical concave shape.

  i is a channel number, for example, i = 1, 2,. k is a column number, for example, k = 1, 2,. The X-ray detection elements 24 (ik) each have the same column number k and constitute a detection element array. Note that the number of detection element rows of the X-ray detector 24 is not limited to 32 rows, and may be an appropriate plural number or a single number.

  The X-ray detection element 24 (ik) is configured by a combination of, for example, a scintillator and a photodiode. However, the present invention is not limited to this. For example, a semiconductor X-ray detection element using cadmium tellurium (CdTe) or the like, or an ionization chamber type X-ray detection element using xenon (Xe) gas may be used.

  FIG. 3 shows the interrelationship among the X-ray tube 20, the collimator 22, and the X-ray detector 24 in the X-ray irradiation / detection apparatus. 3A is a diagram showing a state seen from the front of the scanning gantry 2, and FIG. 3B is a diagram showing a state seen from the side. As shown in the figure, the X-rays emitted from the X-ray tube 20 are shaped by the collimator 22 into a cone-shaped X-ray beam 400 and irradiated to the X-ray detector 24.

  FIG. 3A shows the spread of the cone-shaped X-ray beam 400 in one direction. Hereinafter, this direction is also referred to as a width direction. The width direction of the X-ray beam 400 coincides with the channel arrangement direction in the X-ray detector 24. (B) shows the expansion of the X-ray beam 400 in the other direction. Hereinafter, this direction is also referred to as a thickness direction of the X-ray beam 400. The thickness direction of the X-ray beam 400 coincides with the direction in which a plurality of detection element rows are arranged in the X-ray detector 24. The two spreading directions of the X-ray beam 400 are perpendicular to each other.

  As shown in FIG. 4, for example, the object 8 placed on the imaging table 4 is carried into the X-ray irradiation space by intersecting the body axis with such an X-ray beam 400. The scanning gantry 2 has a cylindrical structure including an X-ray irradiation / detection device inside.

  The X-ray irradiation space is formed in the inner space of the cylindrical structure of the scanning gantry 2. An image of the object 8 sliced by the X-ray beam 400 is projected onto the X-ray detector 24. The X-ray detector 24 detects X-rays transmitted through the object 8 for each detector row. The thickness th of the X-ray beam 400 irradiated to the object 8 is adjusted by the opening degree of the aperture of the collimator 22.

  In parallel with the rotation of the X-ray irradiation / detection device, the X-ray irradiation / detection device is moved relative to the target 8 by continuously moving the imaging table 4 in the body axis direction of the target 8 as indicated by an arrow 42. Thus, the object 8 turns along a spiral trajectory surrounding the object 8. As a result, a so-called helical scan is performed. If the X-ray irradiation / detection device is rotated while the imaging table 4 is stopped, an axial scan is performed.

  Projection data of a plurality of views (for example, about 1000) per scan rotation is collected. Projection data is collected by a series of X-ray detector 24 -data collection unit 26 -data collection buffer 64.

  The operation of this apparatus will be described. FIG. 5 shows a flow chart of the operation of this apparatus. As shown in the figure, shooting conditions are set on a stage 501. The photographing condition is set by the user through the operation device 70. As a result, various scan parameters such as an imaging region, tube voltage, tube current, and slice thickness are set.

  Next, scanning is performed at the stage 503. Scanning is performed by the scanning gantry 2 and the imaging table 4 according to the imaging conditions set in the stage 501. Thereby, projection data of the object 8 is collected.

  Next, image reconstruction is performed at stage 507. Image reconstruction is performed by the data processing device 60. The data processing device 60 is an example of an image reconstruction unit in the present invention. The data processing device 60 reconstructs an image based on, for example, a filtered back projection method based on projection data collected by scanning.

  FIG. 6 shows a schematic diagram of a reconstructed image. As shown in the figure, the reconstructed image is represented by a distribution of pixel values I (x, y) on the xy plane. The xy plane is a two-dimensional space of an orthogonal coordinate system, and each pixel has an orthogonal coordinate.

  As a reconstructed image, an example including a ring artifact inside the body surface contour is shown. The ring artifact is an annular artifact centered on isocenter 0. The isocenter 0 is the center of scanning.

  Ring artifact removal is performed on the reconstructed image at stage 509. The ring artifact removal is performed by the data processing device 60. The ring artifact removal will be described later.

  The reconstructed image after removing the ring artifact is displayed and stored in the stage 511. The display of the image is performed by the display device 68. The storage of the image is performed by the storage device 66.

  The ring artifact removal will be described. FIG. 7 shows a flow diagram of ring artifact removal. This flowchart is a flowchart of the operation of the data processing device 60. As shown in the figure, polar coordinate conversion is performed in the stage 601. Polar coordinate transformation is performed for each pixel of the reconstructed image. The data processing device that performs polar coordinate conversion is an example of polar coordinate conversion means in the present invention.

  By the polar coordinate conversion, as shown in FIG. 8, the orthogonal coordinates (x, y) of the pixel I (x, y) are converted into polar coordinates (r, θ). The origin of the polar coordinate system is isocenter 0. Conversion from rectangular coordinates to polar coordinates is performed by the following equation.

  At stage 603, an intermediate image is created. The data processing apparatus that performs intermediate image generation is an example of the intermediate image generation means in the present invention. The intermediate image creation is performed by rearranging each pixel in the polar coordinate space based on the polar coordinates. In the polar coordinate space, one of two coordinate axes orthogonal to each other is the r-axis, and the other is the θ-axis.

  As a result, an intermediate image as shown in FIG. 9 is obtained. As shown in the figure, in the intermediate image, the ring artifact is a line artifact parallel to the θ axis. This is because the radius r of the ring artifact is constant.

  At stage 605, line artifacts are removed. A data processing apparatus that performs line artifact removal is an example of processing means in the present invention. For line artifact removal, a one-dimensional filter in the r-axis direction as shown in FIG. 10 is used. The matrix size of the one-dimensional filter is, for example, 1 × 7. However, the present invention is not limited to this, and the matrix size may be appropriate.

  By using a one-dimensional filter in the r-axis direction, line artifacts can be easily removed as described below. Further, the artifact can be appropriately removed by setting the matrix size of the one-dimensional filter to 1 × 7.

  In the 1 × 7 matrix, as shown in FIG. 11, the sum of the pixel values of the two pixels at both ends of the matrix is set as Souter, and the sum of the pixel values of the five central pixels is set as Scenter. These Souter and Scenter are used to detect and remove line artifacts.

  A threshold Value_limit is provided for determining line artifacts. Further, a threshold Edge_mask is provided for determining an edge of the image. The values of the threshold Value_limit and the Edge_mask are system constants. These are preferably provided corresponding to a plurality of filter functions used for image reconstruction, from the viewpoint of appropriately performing artifact and edge determination according to the reconstructed image.

  FIG. 12 shows main patterns (patterns) of the pixel value distribution in the 1 × 7 matrix. In the figure, the shade of the pixel represents the magnitude of the pixel value. (A) and (b) of the figure show line artifact patterns. The difference between them is the difference between bright line artifacts and dark line artifacts. (C) and (d) both show edge patterns, of which (c) is the case where the edge is a body surface contour, and (d) is the case of a tissue contour inside the body.

Apply 1x7 matrix to each pixel of the intermediate image one by one,
Value_limit <│2.5 × Souter-Scenter│ <Edge_mask
Whether or not there is an artifact in the central pixel of the matrix is determined depending on whether or not is established. When the above condition is satisfied, it is determined that there is an artifact, and the pixel value of the center pixel is corrected.

The value used for correction is
Ring = │ (Souter + Scenter) / 7-Image (i, j) │
Given in
If (Souter + Scenter) / 7-Image (i, j) <0,
The pixel value of the center pixel is Image (i, j) = Image (i, j) −Ring,
If (Souter + Scenter) / 7-Image (i, j)> 0,
The pixel value of the center pixel is set to Image (i, j) = Image (i, j) + Ring.

By such correction, the pixel value of the center pixel is changed to (Souter + Scenter) / 7, that is, the average value of the pixels of the 1 × 7 matrix.
In addition,
Value_limit> │2.5 × Souter-Scenter│
In this case, it is determined that there is no artifact. Therefore, the above correction is not performed.

Also,
│2.5 × Souter-Scenter│> Edge_mask
In this case, it is determined that it is not an artifact but an edge. At this time, no correction is performed. As a result, line artifacts are removed from the intermediate image as shown in FIG.

  In stage 607, orthogonal coordinate transformation is performed. The data processing apparatus that performs orthogonal coordinate transformation is an example of the orthogonal coordinate transformation means in the present invention. The orthogonal coordinate transformation is performed for each pixel of the intermediate image. Conversion from polar coordinates to Cartesian coordinates is performed by the following equation.

  In stage 609, a corrected image is created. The data processing apparatus that performs the correction image generation is an example of the correction image generation means in the present invention. The corrected image is created by arranging the pixel value of the intermediate image converted into the orthogonal coordinates in the space of the orthogonal coordinate system.

  As a result, an image on the xy plane is obtained as shown in FIG. This image corresponds to the reconstructed image shown in FIG. 6, but does not include ring artifacts due to the removal of line artifacts in the polar coordinate space.

  In this way, detection and removal of ring artifacts are completed in the polar coordinate space, so that ring artifacts can be accurately removed without being affected by errors during coordinate conversion as in the prior art.

1 is a block diagram of an X-ray CT apparatus as an example of the best mode for carrying out the present invention. It is a figure which shows the structure of a X-ray detector. It is a figure which shows the structure of a X-ray irradiation / detection apparatus. It is a figure which shows the relationship between a X-ray irradiation / detection apparatus and object. It is a flowchart which shows operation | movement of a X-ray CT apparatus. It is a figure which shows the reconstruction image containing a ring artifact. It is a flowchart of ring artifact removal. It is a figure which shows the conversion from a rectangular coordinate to a polar coordinate. It is a figure which shows the intermediate image containing a line artifact. It is a figure which shows a one-dimensional matrix. It is a figure which shows a one-dimensional matrix. It is a figure which shows the pattern of the pixel value distribution in a one-dimensional matrix. It is a figure which shows the intermediate image from which the line artifact was removed. It is a figure which shows the reconstruction image from which the ring artifact was removed.

Explanation of symbols

2 Scanning gantry 4 Imaging table 6 Operation console 20 X-ray tube 24 X-ray detector 60 Data processing device

Claims (8)

Converting the orthogonal coordinates of each pixel of the reconstructed image of the X-ray CT apparatus into coordinates in a polar coordinate system with the isocenter as the origin,
An intermediate image is created by arranging each pixel in a polar coordinate space in which one of two coordinate axes orthogonal to each other based on its polar coordinates is the θ axis and the other is the r axis,
Perform processing for removing artifacts parallel to the θ-axis for the intermediate image,
Converting polar coordinates of each pixel of the processed intermediate image into coordinates of an orthogonal coordinate system;
Arranging each pixel of the processed intermediate image in an orthogonal coordinate space based on the orthogonal coordinates to create a modified image;
The ring artifact removal method characterized by the above-mentioned. Using a one-dimensional filter in the r-axis direction for the processing,
The ring artifact removal method according to claim 1, wherein: The matrix size of the one-dimensional filter is 1 × 7.
The ring artifact removing method according to claim 2, wherein: The one-dimensional filter is
The pixel value of the target pixel is Image (i, j), the sum of the pixel values of the two pixels at both ends in the 1 × 7 matrix is Souter, the sum of the pixel values of the five central pixels is Scenter, and the two threshold values are Edge_mask. And Value_limit, when the correction value is Ring = │ (Souter + Scenter) / 7-Image (i, j) │
When Value_limit <│2.5 × Souter-Scenter│ <Edge_mask
If (Souter + Scenter) / 7-Image (i, j) <0,
Image (i, j) = Image (i, j) −Ring
If (Souter + Scenter) / 7-Image (i, j)> 0,
Image (i, j) = Image (i, j) + Ring
The ring artifact removal method according to claim 3, wherein: An X-ray source;
An X-ray detector disposed so as to face the X-ray source across the subject;
Image reconstruction means for reconstructing a tomographic image of a subject based on projection data of a plurality of views collected through the X-ray detector;
Polar coordinate conversion means for converting the orthogonal coordinates of each pixel of the reconstructed image into coordinates of a polar coordinate system with the isocenter as the origin;
Intermediate image creation means for creating an intermediate image by arranging each pixel in a polar coordinate space in which one of two coordinate axes orthogonal to each other based on its polar coordinates is the θ axis and the other is the r axis;
Processing means for performing processing for removing artifacts parallel to the θ-axis for the intermediate image;
Orthogonal coordinate conversion means for converting polar coordinates of each pixel of the processed intermediate image into coordinates of an orthogonal coordinate system;
Modified image creation means for creating a modified image by arranging each pixel of the processed intermediate image in an orthogonal coordinate space based on the orthogonal coordinates;
An X-ray CT apparatus comprising: Using a one-dimensional filter in the r-axis direction for the processing,
The X-ray CT apparatus according to claim 5. The matrix size of the one-dimensional filter is 1 × 7.
The X-ray CT apparatus according to claim 6. The one-dimensional filter is
The pixel value of the target pixel is Image (i, j), the sum of the pixel values of the two pixels at both ends in the 1 × 7 matrix is Souter, the sum of the pixel values of the five central pixels is Scenter, and the two threshold values are Edge_mask. And Value_limit, when the correction value is Ring = │ (Souter + Scenter) / 7-Image (i, j) │
When Value_limit <│2.5 × Souter-Scenter│ <Edge_mask
If (Souter + Scenter) / 7-Image (i, j) <0,
Image (i, j) = Image (i, j) −Ring
If (Souter + Scenter) / 7-Image (i, j)> 0,
Image (i, j) = Image (i, j) + Ring
The X-ray CT apparatus according to claim 7.