JP2005103254A - Method for reconstituting image, method for evaluating projection data, and x-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reconstituting an image with excellent quality from projection data for the portion of partial scan even when no perfect facing data are available, and to provide an X-ray CT apparatus for reconstituting the image. <P>SOLUTION: In a view channel space where actually measured projection data for the portion of partial scan are collected, the projection data for the portion of full scan is generated by supplementing a data point A where the actually measured projection data does not exist with the actually measured projection data in a mirror point A'. When the image is reconstituted on the basis of the projection data for the portion of full scan, data interpolated from the actually measured projection data in data points B, C near the mirror point A' are utilized in the case no actually measured projection data are available in the mirror point A'. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image reconstruction method, a projection data (data) evaluation method, and an X-ray CT (Computed Tomography) apparatus, and more particularly, to a method and projection data for reconstructing an image based on partial scan projection data. The present invention relates to a method for evaluating the normality of X-ray, and an X-ray CT apparatus for performing such image reconstruction and projection data evaluation.

In the X-ray CT apparatus, the measured projection data is collected for the half scan (half scan) in the view-channel space, and the portion that is not sufficient for the full scan is the counter data included in the measured projection data. The image may be reconstructed by replenishing the image (see, for example, Patent Document 1).
JP-A-4-84940 (page 3-4, Fig. 2-3)

  Since the data points in the view channel space are discrete, there is no guarantee that there is always correct data in the measured data. In such a case, the best facing data among the actually measured data is used, but the quality of the reconstructed image is not good because it is not complete facing data.

  Accordingly, the object of the present invention is to realize a method for reconstructing a high-quality image from projection data for partial scan even when there is no complete counter data, and an X-ray CT apparatus capable of such image reconstruction. It is to be. Another object of the present invention is to realize a method for determining the normality of projection data in the view channel space and an X-ray CT apparatus for performing such determination.

  (1) One aspect of the invention for solving the above problem is that, in a view channel space where measured projection data for partial scan is collected, a data point for which measured projection data does not exist is at the mirror point. A method of forming projection data for a full scan by supplementing with actual projection data, and reconstructing an image based on the projection data for the full scan, in the vicinity when there is no actual projection data at the mirror point An image reconstruction method characterized by using data interpolated from actually measured projection data at a plurality of data points.

  (2) Another aspect of the invention for solving the above problems is a data collection means for collecting projection data for image reconstruction in a view channel space for an imaging target using X-rays, and a partial scan. In the view channel space in which the actual projection data for the minute is collected, the data points for which the actual projection data does not exist are supplemented with the actual projection data at the mirror point to form the projection data for the full scan, and the full scan An X-ray CT apparatus having a reconstructing means for reconstructing an image based on projection data for a minute, wherein the reconstructing means includes a plurality of data in the vicinity thereof when there is no measured projection data at the mirror point. An X-ray CT apparatus is characterized by using data interpolated from measured projection data at a point.

  In the invention according to each aspect described in (1) or (2), when there is no actual projection data at the mirror point, data interpolated from the actual projection data at a plurality of data points in the vicinity thereof is used. It can be supplemented with data, whereby a reconstructed image with good quality can be obtained. The partial scan is preferably a half scan from the viewpoint of obtaining necessary and sufficient projection data by a minimum scan.

  (3) Another aspect of the invention for solving the above-described problem is a method for evaluating the normality of projection data collected in a view channel space, which is a mirror point mutually in the view channel space. The projection data evaluation method is characterized in that a difference in projection data at a pair of data points having the relationship is obtained, and normality of the projection data is evaluated based on the difference.

  (4) According to another aspect of the invention for solving the above-described problem, there is provided data collection means for collecting projection data for image reconstruction in a view channel space for an imaging target using X-rays, and the projection An X-ray CT apparatus having reconstruction means for reconstructing an image based on data, and obtaining a difference in projection data between a pair of data points that are in a mirror point relationship with each other in the view channel space; An X-ray CT apparatus comprising an evaluation means for evaluating the normality of projection data based on the evaluation data.

  In the invention according to each aspect described in (3) or (4), a difference in projection data in a data point pair mutually having a mirror point relationship in the view channel space is obtained, and the normality of the projection data is determined based on the difference. evaluate. Since the projection data in the data point pair that are in the relationship of mirror points with each other becomes mutually opposing data and should originally have the same value, normality can be evaluated based on the difference between them.

  When there is no actual projection data at the mirror point, it is preferable to use data interpolated from the actual projection data at a plurality of data points in the vicinity of the mirror point in terms of enhancing the evaluation system.

  According to the present invention, it is possible to realize a method for reconstructing a high-quality image from projection data for partial scan even when there is no complete facing data, and an X-ray CT apparatus capable of such image reconstruction. Can do. In addition, it is possible to realize a method for determining the normality of projection data in the view channel space and an X-ray CT apparatus that performs such determination.

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

  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. X-rays (not shown) emitted from the X-ray tube 20 are formed into a fan-shaped X-ray beam, that is, a fan beam by a collimator 22 and irradiated to the X-ray detector 24. The

  The X-ray detector 24 has a plurality of detection elements arranged in an array in the direction of expansion of the fan-shaped X-ray beam. The configuration of the X-ray detector 24 will be described later. 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.

  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 imaging table 4 carries an imaging target (not shown) in and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the object and the X-ray irradiation space will be described later.

  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 also 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 data collected by the scanning gantry 2. The portion comprising the scanning gantry 2, the data collection buffer 64, the data processing device 60, and the storage device 66 is an example of an embodiment of the data collection means in the present invention.

  The storage device 66 also stores a program for the data processing device 60. When the data processing device 60 executes the program stored in the storage device 66, various data processing related to photographing is performed.

  The data processing device 60 performs image reconstruction using projection data of a plurality of views 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. The data processing device 60 is an example of an embodiment of the reconstruction means in the present invention.

  A display device 68 and an operation device 70 are also connected to the data processing device 60. 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 large number of X-ray detection elements 24 (i) are arranged in a one-dimensional array. i is a channel number, for example, i = 1 to 1000. The X-ray detection element 24 (i) as a whole forms an X-ray incident surface curved in a cylindrical concave shape.

  As shown in FIG. 3, the X-ray detector 24 may include a plurality of X-ray detection elements 24 (ik) arranged in a two-dimensional array. The plurality of X-ray detection elements 24 (ik) as a whole form an X-ray incident surface curved in a cylindrical concave shape. k is a column number, for example, k = 1, 2, 3, 4. 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 four rows, and may be more or less than that.

  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. 4 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. 4A is a diagram showing a state seen from the front of the scanning gantry 2, and FIG. 4B is a diagram showing a state seen from the side. As shown in the figure, the X-rays radiated from the X-ray tube 20 are shaped into a fan-shaped X-ray beam 400 by the collimator 22 and irradiated to the X-ray detector 24. Hereinafter, the fan-shaped X-ray beam is also referred to as a fan beam X-ray.

  In FIG. 4A, the spread of the fan beam X-ray 400 is shown. The spreading direction of the fan beam X-ray 400 matches the channel arrangement direction in the X-ray detector 24. The collimator 22 has a pair of collimator blades 222 and 224 facing each other with a space therebetween. A space between the collimator blades 222 and 224 becomes an opening (aperture) in a direction in which the fan beam X-ray 400 spreads. This aperture determines the opening angle of the fan beam X-ray 400. The opening angle is also called a fan angle.

  FIG. 4B shows the thickness of the fan beam X-ray 400. The thickness direction of the fan beam X-ray 400 matches the direction in which a plurality of detection element rows in the X-ray detector 24 are arranged. The collimator 22 has a pair of collimator blades 226 and 228 that face each other with a space therebetween. A space between the collimator blades 226 and 228 becomes an aperture in the direction of the thickness of the fan beam X-ray 400. This aperture determines the thickness of the fan beam X-ray 400.

  As shown in FIG. 5, for example, the object 8 placed on the imaging table 4 is carried into the X-ray irradiation space with the body axis intersecting the fan surface of the fan beam X-ray 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 fan beam X-ray 400 is projected onto the X-ray detector 24. X-rays transmitted through the object 8 are detected by the X-ray detector 24. The thickness th of the fan beam X-ray 400 irradiated to the object 8 is determined by the aperture of the collimator 22.

  The X-ray irradiation / detection device including the X-ray tube 20, the collimator 22, and the X-ray detector 24 rotates (scans) around the body axis of the object 8 while maintaining their mutual relationship. As a result, an axial scan is performed. Multiple views of projection data are collected by axial scanning. A memory space in which projection data is stored is called a view channel space.

  FIG. 6 shows a conceptual diagram of the view channel space. In the figure, the vertical axis is a view and the horizontal axis is a channel. The view is represented by the rotation angle β of the center beam of the fan beam X-ray 400. The central beam is a beam that passes through the center of rotation. The channel is represented by an angle γ of the X-ray beam with respect to the center beam. Such a view channel space is formed for each detection element array.

When full scan is performed, projection data in the range of β from 0 to 2π can be obtained by actual measurement, but when half scan is performed, only projection data in the range of β from 0 to π + 2γ _max is actually measured. There is no data in the range from π + 2γ _max to 2π. 2γ _max is the fan angle of the fan beam X-ray 400.

When forming full-scan projection data from half-scan projection data, the half-scan projection data is supplemented in a range from π + 2γ _max to 2π. The replenishment of data is performed using the relationship of mirror points.

For data point A in the range from π + 2γ _max to 2π, there is a mirror point A ′. The view channel (β _A , γ _A ) of the data point A and the view channel (β _{A ′} , γ _{A ′} ) of the mirror point A ′ have the following relationship.

  Data at the data points having such a relationship become so-called opposite data and have the same value. Therefore, by copying the data at the data point A ′ to the data point A, data equivalent to that actually scanned is obtained. be able to.

This is performed for all data points in the range from π + 2γ _max to 2π to obtain data for the full scan. However, in the actual scan, the value of view β is discrete, so the data point Depending on A, there is no data at the position of the mirror point A ′, and there are often only data points B and C in the vicinity thereof.

  Therefore, in such a case, the data at the data point A ′ is obtained by interpolation from the data at the data points B and C, and this is used as the data at the data point A. That is,

here,
β _B : view of data point B β _C : view of data point C The channels of data points B and C are the same.

  By doing so, correct data for a full scan can be obtained. Note that the interpolation is not limited to the primary interpolation as described above, and appropriate higher-order interpolation such as quadratic or cubic may be performed.

  The data processing device 60 performs image reconstruction after performing full scan data formation as described above. With the above processing, correct data for the full scan can be obtained from the data for the half scan, so that the reconstructed image has a good quality. In addition, since the data used for image reconstruction is originally data for half scan, it is possible to obtain an image with better time resolution than when full scan is performed. The scan is not limited to a half scan, and may be an appropriate partial scan that is greater than or equal to a half scan and less than a full scan.

  When it is not particularly necessary to increase the time resolution, a full scan may be performed. When a full scan is performed, projection data in the range of β from 0 to 2π is obtained, and the data at data point A is also an actual measurement value.

  In such data for a full scan, the data at the data point A should ideally be the same value as the data at the mirror point A ′, but it differs depending on noise, body movement of the object 8, etc. May occur. When this difference is large, the quality of the reconstructed image is deteriorated due to the contradiction between the opposing data.

  Therefore, the data processing device 60 obtains a difference between the data at the data point A and the data A ′ at the mirror point, and detects the presence / absence of an abnormality based on the difference. Also in this case, when there is no accurate mirror point A 'corresponding to the data point A, data interpolated from data of neighboring data points B and C is used. As a result, the evaluation system can be enhanced.

  When a data abnormality is detected by such determination, the fact is displayed and the image reconstruction is stopped, or the abnormality notification is given and the reconstructed image is displayed. As a result, the user can take appropriate measures such as re-scanning.

It is a block diagram of an X-ray CT apparatus. It is a figure which shows the typical structure of an X-ray detector. It is a figure which shows the typical structure of an X-ray detector. It is a figure which shows the typical structure of a X-ray irradiation / detection apparatus. It is a figure which shows the typical structure of a X-ray irradiation / detection apparatus. It is a figure which shows view channel space.

Explanation of symbols

2 Scanning gantry 20 X-ray tube 22 Collimator 24 X-ray detector 26 Data acquisition unit 28 X-ray controller 30 Collimator controller 32 Rotation unit 36 Rotation controller 4 Imaging table 6 Operation console 60 Data processing device 62 Control interface 64 Data acquisition buffer 66 Storage Device 68 Display device 70 Operating device

Claims (8)

In the view channel space where the actual projection data for the partial scan is collected, the data points for which the actual projection data does not exist are supplemented with the actual projection data at the mirror point to form the projection data for the full scan. A method for reconstructing an image based on scan projection data,
When there is no actual projection data at the mirror point, use data interpolated from the actual projection data at a plurality of data points in the vicinity thereof,
An image reconstruction method characterized by the above. The partial scan is a half scan.
The image reconstruction method according to claim 1. A method for evaluating the normality of projection data collected in a view channel space, comprising:
Determining a difference in projection data in a data point pair having a mirror point relationship with each other in the view channel space, and evaluating the normality of the projection data based on the difference.
A projection data evaluation method characterized by that. When there is no actual projection data at the mirror point, use data interpolated from the actual projection data at a plurality of data points in the vicinity thereof,
The projection data evaluation method according to claim 3, wherein: Data collection means for collecting projection data for image reconstruction in a view channel space for an imaging object using X-rays;
In the view channel space where the actual projection data for the partial scan is collected, the data points for which the actual projection data does not exist are supplemented with the actual projection data at the mirror point to form the projection data for the full scan, Reconstruction means for reconstructing an image based on projection data for a full scan;
An X-ray CT apparatus having
The reconstruction means includes
When there is no actual projection data at the mirror point, use data interpolated from the actual projection data at a plurality of data points in the vicinity thereof,
An X-ray CT apparatus characterized by that. The partial scan is a half scan.
The X-ray CT apparatus according to claim 5. Data collection means for collecting projection data for image reconstruction in a view channel space for an imaging object using X-rays;
Reconstructing means for reconstructing an image based on the projection data;
An X-ray CT apparatus having
An evaluation means for determining a difference of projection data in a data point pair having a mirror point relationship with each other in the view channel space, and evaluating the normality of the projection data based on the difference
An X-ray CT apparatus comprising: The evaluation means includes
When there is no actual projection data at the mirror point, use data interpolated from the actual projection data at a plurality of data points in the vicinity thereof,
The X-ray CT apparatus according to claim 7.

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