JP5389965B2 - Scattered ray correction method and X-ray CT apparatus - Google Patents

Description

  The present invention relates to a scattered radiation correction method and an X-ray CT apparatus for correcting scattered radiation of an X-ray beam (beam) irradiated on a subject.

  In recent years, with the widespread use of X-ray CT apparatuses, the image quality required for tomographic image information acquired by X-ray CT apparatuses is becoming increasingly high quality. Here, as a method for improving the image quality of the X-ray CT image, for example, correction of scattered X-rays generated inside the subject can be mentioned.

  According to this method, the scattered X-ray dose determined theoretically or experimentally is removed from the projection information of the subject. The projection information that is the basis of image reconstruction is made up of only the transmitted X-ray beam.

  Here, the scattered X-ray dose of the subject increases in proportion to the transmission distance of the X-ray beam that passes through the subject (hereinafter referred to as the projection length). Therefore, even when correcting the scattered X-rays, if this projection length is taken into consideration and the projection length is long, in other words, if the subject has a large thickness in the projection direction, the corrected scattered X-ray dose is also large. It will be a thing.

Japanese Patent Application Laid-Open No. 2005-09597, (first page, FIG. 1)

  However, according to the above background art, correction of scattered X-rays based on the projection length is not sufficient. That is, when a boundary where the X-ray absorption rate changes exists on the path through which the X-ray beam passes, a phenomenon different from the X-ray beam scattering that occurs in a uniform medium occurs, and the projection length is taken into consideration. It is not corrected only by itself.

  In particular, when a tomographic image is formed by image reconstruction, an artifact may be generated in an image near the boundary where the X-ray absorption rate changes. This artifact becomes a factor of deteriorating the quality of the tomographic image and is not preferable.

  From these facts, it is important how to realize the scattered radiation correction method and the X-ray CT apparatus when the boundary including the X-ray absorption rate of the subject is changed.

  The present invention has been made in order to solve the above-described problems caused by the background art, and provides a scattered radiation correction method and an X-ray CT apparatus in the case of including a boundary where the X-ray absorption rate of a subject changes. Objective.

  In order to solve the above-described problems and achieve the object, the scattered radiation correction method according to the first aspect of the invention is an X-ray projection information of a subject or a tomographic image generated by image reconstruction of the projection information. Using the information, the boundary position information including the boundary position including the boundary position inside the subject and the boundary position information including the boundary position information indicating the magnitude of the change is extracted. The projection information or the tomographic image information is corrected using the scattered X-ray correction amount in the projection information or the tomographic image information obtained based on the information included.

  Further, the scattering correction method according to the invention of the second aspect is the scattered radiation correction method according to the first aspect, wherein the amount of scattered X-ray correction is also the transmission length of X-rays passing through the subject. Based on.

  The scattering correction method according to the invention of the third aspect is the scattered radiation correction method according to the first or second aspect, wherein the correction is a scattered X-ray correction amount in the projection information or the tomographic image information. Is a correction performed by subtracting from the projection information or the tomographic image information.

  The scattering correction method according to the invention of a fourth aspect is the scattered radiation correction method according to any one of the first to third aspects, wherein the extraction is performed by using a projection value of the projection information as the projection. Differentiating in the direction corresponding to the channel direction and / or column direction of the X-ray detector for acquiring information, extracting the position where the magnitude of the differentiated differential value exceeds a threshold value as the boundary position information, the differential value Is extracted as the size information.

  The scattering correction method according to the invention of the fifth aspect is the scattered radiation correction method according to any one of the first to third aspects, wherein the extraction is a differential processing of the tomographic image of the tomographic image information. The differential image subjected to the differential processing is subjected to absolute value processing to create a boundary image, and the boundary information is extracted using the boundary image.

  The scattering correction method according to the invention of a sixth aspect is the scattered radiation correction method according to the fifth aspect, wherein the extraction calculates boundary projection information using the boundary image, and the boundary projection information A position where the boundary projection value is not zero is extracted as boundary position information, and the boundary projection value is extracted as the size information.

  Further, the scattering correction method according to the seventh aspect of the invention is the scattered radiation correction method according to any one of the first to sixth aspects, wherein the correction comprises a multi-order function of the magnitude information. A gain function is used.

  Further, the scatter correction method according to the invention of the eighth aspect is the scattered radiation correction method according to the seventh aspect, wherein the correction is a scattered X-ray dose at the boundary position in the projection information or the tomographic image information. Is multiplied by the function value of the gain function, and the multiplied scattered X-ray dose is subtracted from the projection information or the tomographic image information.

  An X-ray CT apparatus according to the ninth aspect of the invention is configured to relatively rotate an X-ray generator and an X-ray detector disposed so as to face the X-ray generator around the subject. In an X-ray CT apparatus having an X-ray data collection unit for collecting X-ray projection data and an image information generation unit for generating image information of a subject using the X-ray projection data, the image generation unit includes: Using the X-ray projection information of the specimen or tomographic image information generated by reconstructing the projection information, the boundary position information of the boundary position where the X-ray absorption rate including the boundary position inside the subject changes, and the The means for extracting boundary information including size information indicating the magnitude of change, and the scattered X-ray correction amount in the projection information or tomographic image information obtained based on the information including the boundary information, Projection information or said Characterized in that it comprises a correction means for correcting the layer image information.

  The X-ray CT apparatus according to the invention of the tenth aspect is the X-ray CT apparatus according to the ninth aspect, wherein the scattered X-ray correction amount is also determined by a transmission length of X-rays passing through the subject. Based on.

  An X-ray CT apparatus according to an eleventh aspect of the invention is the X-ray CT apparatus according to the ninth or tenth aspect, wherein the correction is a scattered X-ray correction amount in the projection information or the tomographic image information. Is a correction performed by subtracting from the projection information or the tomographic image information.

  An X-ray CT apparatus according to the invention of a twelfth aspect is the X-ray CT apparatus according to any one of the ninth to tenth aspects, wherein the means for extracting the boundary information is the extraction Differentiating the projection value of the projection information in a direction corresponding to the channel direction and / or the column direction of the X-ray detector for acquiring the projection information, and the position where the magnitude of the differentiated differential value exceeds a threshold value is the boundary position It includes a means for extracting as information.

  An X-ray CT apparatus according to a twelfth aspect of the present invention is the X-ray CT apparatus according to any one of the ninth to eleventh aspects, wherein the means for extracting the boundary information includes: It includes means for differentiating a tomographic image, performing absolute value processing on the differentiated differential image, creating a boundary image, and extracting the boundary information using the boundary image.

  An X-ray CT apparatus according to a thirteenth aspect of the invention is the X-ray CT apparatus according to any one of the ninth to eleventh aspects, wherein the means for extracting the boundary information includes: It includes means for differentiating a tomographic image, performing absolute value processing on the differentiated differential image, creating a boundary image, and extracting the boundary information using the boundary image.

  The X-ray CT apparatus according to the fourteenth aspect of the invention is the X-ray CT apparatus according to the thirteenth aspect, wherein the means for extracting the boundary information calculates boundary projection information using the boundary image. A position where the boundary projection value of the boundary projection information is not zero is extracted as boundary position information, and the boundary projection value is extracted as the size information.

  An X-ray CT apparatus according to the invention of a fifteenth aspect is the X-ray CT apparatus according to any one of the ninth to fourteenth aspects, wherein the correction is made of a multi-order function of the magnitude information. A gain function is used.

  The X-ray CT apparatus according to the sixteenth aspect of the invention is the X-ray CT apparatus according to the fifteenth aspect, wherein the correction is performed by the scattered X-ray dose at the boundary position in the projection information or the tomographic image information. Is multiplied by the function value of the gain function, and the multiplied scattered X-ray dose is subtracted from the projection information or the tomographic image information.

According to the present invention, when the projection information or tomographic image information is included in the boundary position where the X-ray absorption rate changes, the scattered X-ray correction is made accurate, and at the same time, the artifact of the tomographic image is reduced, A quality tomographic image can be acquired.

It is a block diagram which shows the whole structure of a X-ray CT apparatus. 3 is a functional block diagram illustrating a functional configuration of the data processing device according to the first embodiment. FIG. 3 is a flowchart illustrating a correction operation according to the first embodiment. It is a figure which shows the tomographic image of the subject, projection information, and boundary information. It is a figure which shows the scattered X-ray information of the subject 1, the corrected scattered X-ray information, and the corrected projection value. 6 is a functional block diagram illustrating a functional configuration of a data processing device according to a second embodiment. FIG. 10 is a flowchart illustrating a correction operation according to the second embodiment. It is a figure which shows the tomographic image of the subject 2, a boundary image, and a boundary projection value. It is a figure which shows the scattered X-ray information of the subject 2, the corrected scattered X-ray information, and the corrected projection value. FIG. 10 is a functional block diagram illustrating a functional configuration of a data processing device according to a third embodiment. It is a figure which shows the tomographic image of the subject 2, a scattered X-ray image, and a subtraction image.

The best mode for carrying out a scattered radiation correction method and an X-ray CT apparatus according to the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited thereby.
(Embodiment 1)
First, the overall configuration of the X-ray CT apparatus according to the first embodiment will be described. FIG. 1 shows a block diagram of an X-ray CT apparatus. As shown in FIG. 1, the apparatus includes a scanning gantry 10, an operation console 6, and an imaging table 4.

  The scanning gantry 10 has an X-ray tube 20. X-rays (not shown) radiated from the X-ray tube 20 which is an X-ray generator are shaped by a collimator 22 so as to be, for example, a cone-shaped X-ray beam (beam) spreading in a fan shape with a thickness. Then, the X-ray detector 24 disposed opposite to the X-ray tube 20 is irradiated.

  The X-ray detector 24 has a plurality of scintillators arranged in a matrix in the spreading direction of the fan beam X-rays. The X-ray detector 24 is a wide multi-channel detector in which a plurality of scintillators are arranged in a matrix.

  The X-ray detector 24 as a whole forms an X-ray incident surface curved in a concave shape. The X-ray detector 24 is a combination of, for example, a scintillator made of an inorganic crystal and a photodiode that is a photoelectric converter.

  A data collection unit 26 is connected to the X-ray detector 24. The data collection unit 26 collects detection information of individual scintillators of the X-ray detector 24. X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. Note that the connection relationship between the X-ray tube 20 and the X-ray controller 28 and the connection relationship between the collimator 22 and the collimator controller 30 are not shown. The collimator 22 is controlled by a collimator controller 30.

  The above-described components from the X-ray tube 20 to the collimator controller 30 are mounted on the rotating unit 34 of the scanning gantry 10. Here, the subject or phantom is placed on the imaging table 4 in the bore 29 located at the center of the rotating unit 34. The rotator 34 rotates while being controlled by the rotation controller 36, bombards X-rays from the X-ray tube 20, and transmits X-rays of the subject and the phantom in the X-ray detector 24 according to the rotation angle. view) is detected as projection information for each number (hereinafter represented by j) and each channel number (hereinafter represented by i) of the X-ray detector array in the rotation direction. The connection relationship between the rotating unit 34 and the rotation controller 36 is not shown.

  The operation console 6 includes 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 scanning gantry 10 is connected to the control interface 62. The data processing device 60 controls the scanning gantry 10 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 10 are controlled through a control interface 62. The individual connections between these units and the control interface 62 are not shown.

  In addition, a data collection buffer 64 is connected to the data processing device 60. The data collection buffer 64 is connected to the data collection unit 26 of the scanning gantry 10. Data collected by the data collection unit 26 is input to the data processing device 60 through the data collection buffer 64.

  The data processing device 60 performs image reconstruction using a transmission X-ray signal collected through the data collection buffer 64, that is, projection information. A storage device 66 is connected to the data processing device 60. The storage device 66 stores projection information collected in the data collection buffer 64, reconstructed tomographic image information, a program (program) for realizing the functions of the present device, and the like.

  Further, a display device 68 and an operation device 70 are connected to the data processing device 60, respectively. The display device 68 displays the tomographic image information and other information output from the data processing device 60. The operation device 70 is operated by an operator and inputs various instructions and information to the data processing device 60. An operator operates the apparatus interactively using the display device 68 and the operation device 70. The scanning gantry 10, the imaging table 4, and the operation console 6 acquire a tomographic image by imaging a subject or a phantom.

  FIG. 2 is a functional block diagram showing a functional configuration of the data processing device 60. The data processing device 60 includes preprocessing means 71, image reconstruction means 72, postprocessing means 73, boundary information extraction means 74, correction means 75, and scattered X-ray correction means 76. The data processing device 60 is an example of the image generation device of the present invention.

  The preprocessing unit 71 performs correction such as offset correction and X-ray detector sensitivity correction on the projection information input from the data collection buffer 64.

  The image reconstruction unit 72 performs image reconstruction using the preprocessed projection information P (i, j), and generates tomographic image information. In this image reconstruction, when the projection information is acquired using an axial scan or a helical scan, an FBR (Filtered Back Projection) method or the like is used, and projection information having a thickness is also obtained. A three-dimensional image reconstruction method can be used by treating it as volume data.

  The post-processing means 73 performs post-processing such as CT value conversion on the tomographic image information. The post-processed tomographic image information is transmitted to the display device 68.

The boundary information extraction unit 74 uses the pre-processed projection information P (i, j), the boundary position information of the boundary position where the X-ray absorption rate is discontinuously different, and the magnitude of the change in the X-ray absorption rate at the boundary position. Boundary information consisting of size information indicating the height is obtained. Here, the boundary information extraction means 74 has a differential operation means for differentiating projection information in a direction corresponding to the channel direction of the X-ray detector (hereinafter simply referred to as channel direction), and the differential value exceeds a threshold value. In this case, the boundary position is determined. Here, if the projection value at the position of the view number j and the channel number i is P (i, j), the magnitude of the differential value D (i, j) is
| D (i, j) | = | [P (i + Δi, j) −P (i, j)] / Δi |
And the threshold is th,
│D (i, j) │> th
The position of the channel number becomes the boundary position.

  The scattered X-ray correction means 76 calculates the scattered X-ray dose S (i, j) for each view number and channel number included in the projection information. This scattered X-ray dose S (i, j) is calculated based on scattered X-ray information acquired by using, for example, a phantom, and further, taking into account the projection length in the subject through which X-rays pass, It is considered high (see, for example, Patent Document 1).

The correcting means 75 uses the boundary information acquired by the boundary information extracting means 74 to correct the scattered X-ray dose calculated by the scattered X-ray correcting means 76 at the boundary position where the X-ray absorption rate changes. . Here, the correcting means 75 has a gain function G (i, j) for multiplying the scattered X-ray dose and correcting the scattered X-ray dose. Here, when the gain function is G (i, j), for example, f is a multi-order function,
G (i, j) = 1 + f [| D (i, j) |]
It is represented by Then, the scattered X-ray dose S (i, j) for each channel number is multiplied by the gain function G (i, j) to obtain a scattered X-ray dose including correction at the boundary position. This gain function adjusts the magnitude of change and the amount of correction for it. This multi-order function is experimentally determined so that the correction amount is optimized.

Further, the correction means 75 subtracts the corrected scattered X-ray dose from the projection information to correct the scattered X-rays. That is, when the corrected projection value is CP (i, j),
CP (i, j) = P (i, j) -S (i, j) * G (i, j)
It is said. Then, the corrected projection information CP (i, j) is output to the image reconstruction unit 72 and is reconstructed.

  Next, a specific operation of correction of the data processing device 60 will be described with reference to FIG. FIG. 3 is a flowchart showing the correction operation. First, the operator places the subject 1 in the bore 29 and acquires projection information of the subject 1 (step S301). FIG. 4A is a diagram schematically showing an imaging cross section of the subject 1. In the example shown in FIG. 4A, the subject 1 includes a generally elliptical low X-ray absorption rate portion 2 and a circular high X-ray absorption rate portion 3 located near the center of the elliptical shape. It has a cross section. FIG. 4B is a diagram showing projection information obtained by projecting the cross section of the subject 1 shown in FIG. The projection information has a projection image of the low X-ray absorption rate portion 2 that is generally semicircular in the channel direction, and a small semicircular projection including the high X-ray absorption rate portion 3 near the center position of the semicircle. The image is superimposed. Note that such projection information is acquired for each view number (j) corresponding to the rotation angle of the rotation unit 34 over 360 degrees around the subject 1.

  After that, the boundary information extraction unit 74 differentiates the projection information and extracts the boundary position as the boundary information and the magnitude of the change in the X-ray absorption rate (step S302). FIG. 4C is an example in which a differential operation is performed on the projection information shown in FIG. The boundary position where the projection value of the projection information changes sharply has a large value.

  Thereafter, the correcting unit 75 corrects the scattered X-ray dose S (i, j) for each view number and channel number calculated by the scattered X-ray correcting unit 76 (step S303). At this time, the correction means 75 uses a gain function to adjust the differential value, that is, the amount of correction multiplied by the magnitude of the change in the X-ray absorption rate and the scattered X-ray dose.

  FIG. 5A shows the projection direction of the view number j similar to FIG. 4B calculated by the scattered X-ray correction means 76 when the subject 1 has a cross section as shown in FIG. It is a figure which shows the scattered X dose S (i, j) in. The scattered X-ray dose has a large value at a position near the center where the high X-ray absorptivity portion 3 exists and the projection length is long, and gradually becomes small at a peripheral position where the projection length is short. FIG. 5B shows the scattered X-ray dose using the magnitude | D (i, j) | of the differential value at the channel direction position shown in FIG. 4C, that is, the magnitude of the change in the X-ray absorption rate. This is an example in which S (i, j) is corrected.

  Thereafter, the correcting unit 75 performs scatter correction on the projection information using the corrected scattered X-ray dose (step S304). In this scatter correction, the scattered X-ray dose is subtracted from the projection value P (i, j) of the projection information. FIG. 5C is an example in which the projection information of FIG. 4B is scatter corrected using the corrected scattered X-ray dose S (i, j) × G (i, j) shown in FIG. is there. A diagram indicated by a solid line in FIG. 5C is the projection information CP (i, j) subjected to the scattering correction. In addition, the figure shown with the broken line in FIG.5 (C) was shown in figure for the comparison with the projection information P (i, j) same as FIG.4 (B).

  Thereafter, the image reconstruction unit 72 performs image reconstruction using the corrected projection information (step S305), and the reconstructed tomographic image information is displayed on the display device 68 (step S306).

As described above, in the first embodiment, from the projection information P (i, j) of the subject 1, boundary information D (i, j, which includes the boundary position where the X-ray absorption rate changes and the magnitude of the change. j) is extracted, and using this boundary information, the scattered X-ray dose S (i, j) is multiplied by the amount G (i, j) corresponding to the magnitude of the change at the boundary position, and the X-ray at the boundary position is obtained. Since correction is performed, artifacts generated in the tomographic image can be reduced.
(Embodiment 2)
In the first embodiment, the boundary information in which the X-ray absorption rate changes is extracted by differentiating the projection information in the channel direction. However, in the present embodiment, the direction orthogonal to the rotation direction of the X-ray detector. An example in which boundary information in the column direction is extracted by differentiating projection information in a direction corresponding to the column direction of the X-ray detector (hereinafter simply referred to as the column direction) that is the width of the X-ray detector will be described. In the present embodiment, the boundary information extracting unit 74 and the correcting unit 75 will be mainly described.

In the present embodiment, the boundary information extracting means 74 has a differential operation means for differentiating the projection information in the column direction, and determines that this is the boundary position when this differential value exceeds a threshold value. Here, if the projection value at the position of view number j and column number k is P (i, k, j), the magnitude of the differential value Dk (i, k, j) is
| Dk (i, k, j) | = | [P (i, k + Δk, j) −P (i, k, j)] / Δk |
And the threshold is th_k,
| Dk (i, k, j) |> th_k
The position of the column number that becomes is the boundary position.

  The scattered X-ray correction means 76 calculates the scattered X-ray dose S (i, k, j) for each view number, channel number, and column number included in the projection information. This scattered X-ray dose S (i, k, j) is calculated based on scattered X-ray information acquired using, for example, a phantom, for example, in the subject through which X-rays pass, as in the first embodiment. Considering the projected length and projected area, the accuracy is raised.

The correction unit 75 uses the boundary information acquired by the boundary information extraction unit 74 to correct the scattered X-ray dose calculated by the scattered X-ray correction unit 76 at the boundary position where the X-ray absorption rate changes. Here, the correcting means 75 has a gain function G2 (i, k, j) for multiplying the scattered X-ray dose and correcting the scattered X-ray dose. Here, if the gain function is G2 (i, k, j), for example, f2 is a multi-order function,
G2 (i, k, j) = 1 + f2 [| Dk (i, k, j) |]
It is represented by Then, the scattered X-ray dose S (i, k, j) is multiplied by the gain function G2 (i, k, j) to obtain a scattered X-ray dose including X-ray correction at the boundary position. This gain function adjusts the magnitude of change and the amount of correction for it. This multi-order function is experimentally determined so that the correction amount is optimized.

Further, the correcting means 75 subtracts the corrected scattered X-ray dose from the projection information, and corrects the scattered X-rays. That is, assuming that the corrected projection value is CP (i, k, j),
CP (i, k, j) = P (i, k, j) −S (i, k, j) * G2 (i, k, j)
It is said. Then, the corrected projection information CP (i, k, j) is output to the image reconstruction unit 72 to be reconstructed.

Thus, in the second embodiment, from the projection information P (i, k, j) of the subject 1, the boundary information D (i, k, j) is extracted, and the boundary information is used to multiply the scattered X-ray dose S (i, k, j) by the amount G2 (i, k, j) corresponding to the magnitude of the change at the boundary position, Since X-rays at the boundary position are corrected, artifacts generated in the tomographic image can be reduced.
(Embodiment 3)
In the first embodiment, the boundary direction information is extracted by differentiating the projection information in the channel direction and in the second embodiment by differentiating the projection information in the column direction. Describes an example of extracting boundary information in the channel direction and the column direction by differentiating both the channel direction and the column direction. In the present embodiment, the boundary information extracting means 74 and the correcting means 75 will be mainly described, and the description of the configuration is omitted since it is the same as that of the first embodiment.

In the present embodiment, the boundary information extracting means 74 has a differential operation means for differentiating the projection information in the channel direction and the column direction, and determines that this is the boundary position when this differential value exceeds a threshold value. Here, assuming that the projection value at the position of view number j and column number k is P (i, k, j), differential value Di (i, k, j) in the channel direction and differential value Dk (i, k) in the column direction. , J) is
| Di (i, k, j) | = | [P (i + Δi, k, j) −P (i, k, j)] / Δi |
| Dk (i, k, j) | = | [P (i, k + Δk, j) −P (i, k, j)] / Δk |
It is obtained by. Assuming that the threshold in the channel direction is th_i and the threshold in the column direction is th_k,
| Di (i, k, j) |> th_i
| Dk (i, k, j) |> th_k
The position of the column number that becomes is the boundary position.

  The scattered X-ray correction means 76 calculates the scattered X-ray dose S (i, k, j) for each view number, channel number, and column number included in the projection information. This scattered X-ray dose S (i, k, j) is calculated based on scattered X-ray information acquired using, for example, a phantom or the like, as in the first embodiment, and further in the subject through which X-rays pass. Considering the projected length and projected area, the accuracy is raised.

The correction unit 75 uses the boundary information acquired by the boundary information extraction unit 74 to correct the scattered X-ray dose calculated by the scattered X-ray correction unit 76 at the boundary position where the X-ray absorption rate changes. Here, the correction means 75 has a gain function G3 (i, k, j) that is multiplied by the scattered X-ray dose and corrects the scattered X-ray dose. Here, when the gain function is G3 (i, k, j), for example, f3 is a multi-order function,
G3 (i, k, j) = 1 + f3 [| Di (i, k, j) |, | Dk (i, k, j) |]
It is represented by Then, the scattered X-ray dose S (i, k, j) is multiplied by the gain function G3 (i, k, j) to obtain a scattered X-ray dose including correction at the boundary position. This gain function adjusts the magnitude of change and the amount of correction for it. This multi-order function is experimentally determined so that the correction amount is optimized.

Further, the correction means 75 subtracts the corrected scattered X-ray dose from the projection information to correct the scattered X-rays. That is, if the projection value after correction is CP (i, k, j),
CP (i, k, j) = P (i, k, j) −S (i, k, j) * G3 (i, k, j)
It is said. Then, the corrected projection information CP (i, k, j) is output to the image reconstruction unit 72 and is reconstructed.

Thus, in the second embodiment, from the projection information P (i, k, j) of the subject 1, the boundary information D (i, k, j) is extracted, and the boundary information is used to multiply the scattered X-ray dose S (i, k, j) by the amount G2 (i, k, j) corresponding to the magnitude of the change at the boundary position, Since X-rays at the boundary position are corrected, it is possible to correct the scattered X-ray correction of the projection information in the channel direction and the column direction, thereby reducing artifacts generated in the tomographic image.
(Embodiment 4)
In the first to third embodiments, the projection information P (i, j) is differentiated and the boundary information D (i, j) whose X-ray absorption rate changes is extracted. It is also possible to extract boundary information from the tomographic image information and correct X-rays at the boundary position. Therefore, the fourth embodiment shows a case where boundary information is extracted from tomographic image information reconstructed and X-ray correction is performed at the boundary position where the X-ray absorption rate changes. FIG. 6 is a functional block diagram of a functional configuration of the data processing device 61 according to the second embodiment. Here, the data processing device 61 corresponds to the data processing device 60 shown in the overall configuration of FIG. 1, and the other configuration is exactly the same as that shown in FIG.

  The data processing device 61 includes pre-processing means 71, image reconstruction means 72, post-processing means 73, boundary information extraction means 84, correction means 85, and scattered X-ray correction means 76. Here, the preprocessing means 71, the image reconstruction means 72, the post-processing means 73, and the scattered X-ray correction means 76 are functionally the same as those shown in FIG. However, in the first embodiment, the preprocessing means 71 outputs the preprocessed projection information only to the boundary information extraction means 74, but in the second embodiment, the image reconstruction means 72 and the correction means 85. Output to. Then, the image reconstruction unit 72 performs image reconstruction using the projection information from the preprocessing unit 71 to form tomographic image information.

  The boundary information extraction unit 84 uses the tomographic image information IMG (x, y) acquired from the image reconstruction unit 72 (x and y indicate the position coordinates in the horizontal and vertical directions of the image) and absorbs X-rays. Boundary information consisting of boundary position information of boundary positions with different rates discontinuously and size information indicating the magnitude of the change in the X-ray absorption rate at the boundary position is obtained. Here, the boundary information extraction means 84 is a differential operation means for differentiating the tomographic image included in the tomographic image information, an absolute value means for taking the absolute value of the differentiated image and generating a boundary image, and each of the boundary images using this boundary image. Reprojecting means for obtaining a plurality of projection images from positions corresponding to the view angle is provided.

  Here, when generating a boundary image BIMG (x, y) from the tomographic image IMG (x, y),

Equation (1) in the relationship is used. Note that * represents a convolution operation, and HF (x, y) is a high-pass filter function. The numerator of this formula is the above-described differential calculation means, and the denominator is a normalization factor for not depending on the CT value.

  Then, the boundary information extracting unit 84 obtains the projection image BD (i, j) of the boundary image BIMG (x, y) for each view number and channel number. Then, when the projection image BD (i, j) has a non-zero projection value, the boundary information extraction unit 84 sets the position in the channel direction where the projection value exists as the boundary position, and sets the magnitude of the projection value as an X-ray. The magnitude of the change in absorption rate.

The correction unit 85 uses the boundary information acquired by the boundary information extraction unit 84 to correct X-rays at the boundary position where the X-ray absorption rate changes to the scattered X-ray dose calculated by the scattered X-ray correction unit 76. Add Here, the correction unit 85 has a gain function for multiplying the scattered X-ray dose similar to the correction unit 75 and correcting the scattered X-ray dose. This gain function is, for example, where f is a multi-order function,
G (i, j) = 1 + f [| BD (i, j) |]
It is represented by When the scattered X-ray dose is S (i, j), the scattered X-ray dose is corrected by S (i, j) × G (i, j) from the definition.

Further, the correcting means 85 subtracts the corrected scattered X-ray dose CP (i, j) from the projection information P (i, j) from the preprocessing means 71 to correct the scattered X-rays.
CP (i, j) = P (i, j) −S (i, j) × G (i, j)
Next, a specific operation of correction of the data processing device 61 will be described with reference to FIG. FIG. 7 is a flowchart showing the correction operation. First, an operator places the subject 2 in the bore 29 and acquires projection information of the subject 2 (step S701). This projection information is transmitted to the data processing device 61, pre-processed by the pre-processing unit 71, and then output to the image reconstruction unit 72. Then, the image reconstruction means 72 reconstructs the projection information P (i, j) (step S702) and acquires tomographic image information IMG (x, y). FIG. 8A is an example of a tomographic image of the subject 2. This tomographic image is composed of a low X-ray absorptivity part 8 occupying most of the subject 2 and two circular high X-ray absorptivity parts 9 in the low X-ray absorptivity part 8. A portion indicated by a broken line in this tomographic image schematically shows an artifact 7 at a boundary position where the X-ray absorption rate changes.

  Returning to FIG. 7, the data processing device 61 extracts boundary information by the boundary information extracting means 84 (step S703). In this boundary information extraction, a calculation using the equation (1) is performed on the tomographic image IMG (x, y) to obtain the boundary image BIMG (x, y). FIG. 8B is a diagram showing a boundary image obtained from the tomographic image shown in FIG. In this figure, only the boundary region between the low X-ray absorption rate portion 8 and the high X-ray absorption rate portion 9 of the subject 2 is extracted. Then, the boundary information extraction unit 84 calculates a boundary projection value BD (i, j), which is boundary information for each view number and channel number, from the boundary image BIMG (x, y). FIG. 8C shows an example of the boundary projection value projected in the vertical direction within the plane of the boundary image shown in FIG. In FIG. 8C, the boundary projection values are large at the edge of the low X-ray absorptivity part 8 and the edge of the high X-ray absorptivity part 9, and the high X-ray absorptivity part. The boundary projection value including 9 indicates a value larger than the boundary projection value including only the low X-ray absorption rate unit 8.

  Returning to FIG. 7, the data processing device 61 then corrects the scattered X-ray dose by the correcting means 85 (step S704). In this correction, a gain function G (i, j) using the boundary projection value BD (i, j) is obtained, and the scattered X-ray dose S (i, j) is multiplied by this gain function G (i, j).

  FIG. 9A is a diagram showing the scattered X-ray dose S (i, j) in the vertical direction of the subject 2 obtained by the scattered X-ray correction means 76. FIG. 9B shows the scattered X-ray dose corrected by multiplying the scattered X-ray dose S (i, j) by the gain function G (i, j) of the boundary projection value BD (i, j). is there.

  Returning to FIG. 7, the data processing device 61 performs scattering correction by the correcting means 85 (step S705). In this scatter correction, the correction means 85 corrects the scattered X-ray dose S (i, j) × G (i, j) from the projection information P (i, j) of the subject 2 acquired from the preprocessing means 71. Is subtracted. FIG. 9C shows the projection information P (i, j) of the subject 2 with a broken line in the drawing, and the projection information CP (i, j) of the subject 2 subjected to scattering correction with the solid line in the drawing. Is shown.

  Returning to FIG. 7, the data processing device 61 uses the image reconstruction unit 72 to reconstruct the projection information CP (i, j) subjected to the scattering correction (step S706), and displays the reconstructed image on the display device 68. It is displayed (step S707). In this reconstructed image, the artifact 7 shown in the tomographic image of FIG. 8A can be reduced.

As described above, in the second embodiment, the boundary image BIMG (x, y) consisting only of the boundary position where the X-ray absorption rate changes is extracted from the tomographic image IMG (x, y) of the subject 2. The boundary projection value BD (i, j) of this boundary image is calculated for each view number and channel number, the scattered X-ray dose S (i, j) is corrected using this boundary projection value, and this scattered X-ray dose is calculated. Since the scatter correction of the projection information P (i, j) of the subject 2 is used, the X-ray scatter correction at the boundary position where the X-ray absorption rate is different is corrected, and the artifact 7 generated in the tomographic image is eventually corrected. Can be reduced.
(Embodiment 5)
In the fourth embodiment, the boundary image BIMG (x, y) is extracted from the tomographic image, and the projection information of the subject 2 is corrected using the boundary projection value BD (i, j) of the boundary image. Then, the scattered X-ray dose corrected based on the boundary projection value is reconstructed to generate a scattered X-ray image, and the scattered X-ray image is subtracted from the tomographic image to reduce scattered X-rays and artifacts. You can also. Therefore, the fifth embodiment shows a case where X-rays at the boundary position where the X-ray absorption rate of the tomographic image changes are corrected using the scattered X-ray image corrected with the boundary projection value. FIG. 10 is a functional block diagram of a functional configuration of the data processing device 63 according to the fifth embodiment. Here, the data processing device 63 corresponds to the data processing device 60 shown in the overall configuration of FIG. 1, and the other configuration is exactly the same as that shown in FIG.

  The data processing device 63 includes preprocessing means 71, image reconstruction means 72, postprocessing means 73, boundary information extraction means 84, correction means 95, and scattered X-ray correction means 76. Here, the pre-processing means 71, the image reconstruction means 72, the post-processing means 73, the boundary information extraction means 84, and the scattered X-ray correction means 76 are functionally completely different from those of the second embodiment shown in FIG. Since it is the same, detailed description is abbreviate | omitted. However, in the third embodiment, the preprocessing unit 71 does not output the preprocessed projection information to the correction unit 95, and the correction unit 95 uses only the reconstructed tomographic image and scattered X-ray image. To correct the X-rays at the boundary position. Further, the image reconstruction unit 72 does not directly output the reconstructed image to the post-processing unit 73 but outputs it via the correction unit 95.

  FIG. 11A is an example of a tomographic image IMG (x, y) reconstructed from the projection information of the subject 2. The subject 2 has a low X-ray absorptivity part 8 and two circular high X-ray absorptivity parts 9 therein. The artifacts 7 are indicated by broken lines at the edge portions of the low X-ray absorption rate portion 8 and the high X-ray absorption rate portion 9.

  The correction means 95 is calculated by the scattered X-ray correction means 76 using the boundary projection value BD (i, j) acquired from the boundary image BIMG (xy) by the boundary information extraction means 84 as in the correction means 85. The scattered X-ray dose S (i, j) to be corrected is corrected. When generating a boundary image BIMG (x, y) from the tomographic image IMG (x, y),

Equation (2) in the relationship is used. Note that * represents a convolution operation, and HF (x, y) is a high-pass filter function. Similarly to the correction unit 85, the boundary projection value BD (i, j) is converted using the gain function G (i, j).

  Thereafter, the correction unit 95 outputs the corrected scattered X-ray dose S (i, j) × G (i, j) to the image reconstruction unit 72. The image reconstruction unit 72 reconstructs the corrected scattered X-ray dose to generate a scattered X-ray image SIMG (x, y). FIG. 11B is a diagram schematically showing a scattered X-ray image reconstructed. In general, the artifact 7 shown in FIG. 11A is imaged.

Thereafter, the correction unit 95 inputs the scattered X-ray image SIMG (x, y) from the image reconstruction unit 72 and performs subtraction with the tomographic image IMG (x, y) to obtain the subtraction image DIMG (x, y). Generate. That is,
DIMG (x, y) = IMG (x, y) −SIMG (x, y)
Is calculated. FIG. 11C shows a subtracted image obtained by subtraction. The subtracted image is generally obtained by removing the artifact 7 portion from the tomographic image.

  Thereafter, the correction means 95 outputs the subtraction image to the post-processing means 73, and the post-processing means 73 outputs the subtraction image to the display device 68 after the post-processing.

  As described above, in the third embodiment, the boundary projection value BP (i, j) is viewed from the boundary image BIMG (x, y) including only the boundary of the tomographic image IMG (x, y) of the subject 2. The scattered X-ray dose S (i, j) is corrected using the boundary projection value, and the scattered X-ray dose is reconstructed to obtain a scattered X-ray image SIMG (x, y). Since it is generated and subtracted from the tomographic image, the artifact 7 generated in the tomographic image can be reduced.

1, 2 Subject 2, 8 Low X-ray absorption rate unit 3, 9 High X-ray absorption rate unit 7 Artifact 4 Imaging table 6 Operation console 10 Scanning gantry 20 X-ray tube 22 Collimator 24 X-ray detector 26 Data acquisition unit 28 X-ray controller 29 Bore 30 Collimator controller 34 Rotating unit 36 Rotation controller 60, 61, 63 Data processing device 62 Control interface 64 Data collection buffer 66 Storage device 68 Display device 70 Operation device 71 Preprocessing means 72 Image reconstruction means 73 Post processing Means 74 Boundary information extraction means 75 Correction means 76 Scattered X-ray correction means 84 Boundary information extraction means 85, 95 Correction means

Claims (12)

Using the X-ray projection information of the subject or tomographic image information generated by image reconstruction of the projection information, boundary position information of the boundary position where the X-ray absorption rate including the boundary position inside the subject changes, and The projection information or the tomographic image information obtained based on the information including the boundary information including the size information indicating the magnitude of the change and the transmission length of the X-ray passing through the subject and the information including the boundary information. The scattered X-ray correction amount is subtracted from the projection information or the tomographic image information to correct the scattered X-ray correction amount. In the extraction, the projection value of the projection information is differentiated in a direction corresponding to the channel direction and / or the column direction of the X-ray detector that acquires the projection information, and the magnitude of the differentiated differential value sets a threshold value. 2. The scattered radiation correction method according to claim 1, wherein a position exceeding the position is extracted as the boundary position information, and a size of the differential value is extracted as the size information. In the extraction, the tomographic image of the tomographic image information is subjected to differential processing, the differential image subjected to the differential processing is subjected to absolute value processing to create a boundary image, and the boundary information is extracted using the boundary image. The scattered radiation correction method according to claim 1 or 2. In the extraction, boundary projection information is calculated using the boundary image, a position where the boundary projection value of the boundary projection information is not zero is extracted as boundary position information, and the boundary projection value is extracted as the size information. The scattered radiation correction method according to claim 3. The scattered radiation correction method according to claim 1, wherein the correction uses a gain function including a multi-order function of the magnitude information. The correction is performed by multiplying the scattered X-ray dose at the boundary position in the projection information or the tomographic image information by a function value of the gain function, and subtracting the multiplied scattered X-ray dose from the projection information or the tomographic image information. The scattered radiation correction method according to claim 5, wherein: An X-ray data collection means for collecting X-ray projection data by relatively rotating an X-ray generator and an X-ray detector disposed so as to face the X-ray generator around the subject;
In an X-ray CT apparatus having image information generating means for generating image information of a subject using the X-ray projection data,
The image generating means includes
Using the X-ray projection information of the subject or tomographic image information generated by image reconstruction of the projection information, boundary position information of the boundary position where the X-ray absorption rate including the boundary position inside the subject changes, and Means for extracting boundary information including size information indicating the magnitude of the change;
The scattered X-ray correction amount in the projection information or the tomographic image information obtained based on the information including the transmission length of the X-ray passing through the subject and the boundary information is subtracted from the projection information or the tomographic image information. And an X-ray CT apparatus including correction means for correcting the X-ray CT. The means for extracting the boundary information is obtained by differentiating the projection value of the projection information in a direction corresponding to a channel direction and / or a column direction of an X-ray detector that acquires the projection information. 8. The X-ray CT apparatus according to claim 7, further comprising means for extracting, as the boundary position information, a position where the magnitude of the differential value exceeds a threshold value. Means for extracting the boundary information, the tomographic image before Symbol tomographic image information by differentiating processing, to create a boundary image of the differential processed differential image by absolute value processing, the boundary information using said boundary image The X-ray CT apparatus according to claim 7, further comprising: means for extracting The means for extracting boundary information calculates boundary projection information using the boundary image, extracts a position where the boundary projection value of the boundary projection information is not zero, and extracts the boundary projection value as the size. The X-ray CT apparatus according to claim 9, wherein the X-ray CT apparatus is extracted as information. The X-ray CT apparatus according to claim 7, wherein the correction uses a gain function including a multi-order function of the magnitude information. The correction is performed by multiplying the scattered X-ray dose at the boundary position in the projection information or the tomographic image information by a function value of the gain function, and subtracting the multiplied scattered X-ray dose from the projection information or the tomographic image information. The X-ray CT apparatus according to claim 11.

Images