JP3333469B2 - False image detection method and apparatus, and radiation tomography apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for detecting a false image and a radiation tomography apparatus, and more particularly to a method and apparatus for detecting a false image contained in a reconstructed image, and a false image contained in the reconstructed image. The present invention relates to a radiation tomography apparatus having a means for detecting an image.

[0002]

2. Description of the Related Art As an example of a radiation tomography apparatus, for example, an X-ray computed tomography (CT) is used.
y) There is a device. In an X-ray CT apparatus, X-rays are used as radiation. An X-ray tube is used for X-ray generation. An X-ray irradiator including an X-ray tube irradiates an X-ray beam having a width encompassing an imaging range and having a predetermined thickness in a direction perpendicular thereto. The thickness of the X-ray beam can be changed by adjusting the opening of the X-ray passing aperture (aperture) of the collimator, thereby adjusting the thickness of the slice for imaging. . X-rays are detected by a multi-channel X-ray detector in which a large number (for example, about 1000) of X-ray detection elements are arranged in an array in the direction of the width of the X-ray beam.

An X-ray irradiation / detection system is rotated (scanned) around an object to be photographed, and projection data of the object to be photographed by X-rays is projected in a plurality of directions around the object to be photographed.
a) is collected by a data collection device. A tomographic image is reconstructed by the data processing device based on the projection data of a plurality of views collected by the data collection device.

[0004] Since the measurement sensitivity of the projection data in the data collection apparatus differs for each channel, calibration is performed in advance to correct the sensitivity of each channel, and a false image due to uneven sensitivity is generated in the reconstructed image. I try not to. Calibration of the data acquisition device is performed even after the start of use of the X-ray CT device at the time of periodic inspection or the like, so as to cope with a temporal change in channel sensitivity.

[0005]

In the above, since calibration is not performed until the next periodic inspection, even if the channel sensitivity changes during that period, the use is continued as it is, and for example, a ring artifact (ri) is added to the reconstructed image.
NG artifacts and the like are included. In such a case, it is difficult to detect the false image while the image is relatively mild, so that there is a problem that the photographer may continue to use the camera without noticing it and continue shooting with poor quality.

The present invention has been made to solve the above problems, and has as its object to provide a method and apparatus for effectively detecting a false image, and a radiation tomography provided with such false image detecting means. That is, to realize a photographing device.

[0007]

Means for Solving the Problems (1) According to a first aspect of the invention for solving the above-mentioned problems, an average value of pixel values at pixel positions substantially equidistant from the center of a matrix of a reconstructed image is calculated. A false image detection method, wherein a false image is detected based on a ratio of a pixel value of a band-like region including the pixel position to a standard deviation.

(2) According to a second aspect of the invention for solving the above-mentioned problems, an average value of pixel values belonging to a first range having pixels substantially equidistant from the center of a matrix of a reconstructed image as an outer periphery is provided. And the average value of pixel values belonging to a second range larger than the first range with pixels substantially equidistant from the center of the matrix of the reconstructed image being determined as an outer periphery, and calculating the second value from the second range. The standard deviation of the pixel values belonging to the remaining range excluding the range of 1 is obtained, and the ratio of the difference between the average value of the first pixel value and the average value of the second pixel value and the standard deviation is obtained. A false image detection method characterized by detecting a false image based on the ratio.

(3) The invention according to a third aspect for solving the above-mentioned problem is characterized in that an average value calculating means for obtaining an average value of pixel values at pixel positions substantially equidistant from the center of a matrix of a reconstructed image; A standard deviation calculating unit that calculates a standard deviation of a pixel value of a band-like region including the pixel position, and a false image detecting unit that detects a false image based on a ratio between the average value and the standard deviation. This is a false image detection device that is a feature.

(4) According to a fourth aspect of the invention for solving the above-mentioned problem, an average value of pixel values belonging to a first range having pixels substantially equidistant from the center of a matrix of a reconstructed image as an outer periphery. A first average value calculating means for calculating the average value of pixel values belonging to a second range larger than the first range and having pixels substantially equidistant from the center of the matrix of the reconstructed image as an outer periphery. 2, an average value calculating means, a standard deviation calculating means for calculating a standard deviation of pixel values belonging to the remaining range excluding the first range from the second range, and an average value of the first pixel values. A false image, comprising: a ratio calculating unit that calculates a ratio between the difference between the average value of the second pixel values and the standard deviation; and a false image detecting unit that detects a false image based on the ratio. It is a detection device.

(5) According to a fifth aspect of the invention for solving the above-mentioned problems, a data collection means for collecting projection data of a plurality of views of an imaging target by radiation, and reconstructing a tomographic image based on the projection data Radiation tomography apparatus having image reconstruction means to perform,
Mean value calculation means for calculating an average value of pixel values at pixel positions substantially equidistant from the matrix center of the tomographic image, and standard deviation calculation means for calculating a standard deviation of pixel values of a band-like region including the pixel position And a false image detecting means for detecting a false image based on a ratio between the average value and the standard deviation.

(6) The invention according to a sixth aspect that solves the above-mentioned problems is a data collection unit that collects projection data of a plurality of views of an imaging target by radiation, and reconstructs a tomographic image based on the projection data. Radiation tomography apparatus having image reconstruction means to perform,
First average value calculation means for calculating an average value of pixel values belonging to a first range having pixels substantially equidistant from the center of the tomographic image as an outer periphery; Second average value calculating means for calculating an average value of pixel values belonging to a second range larger than the first range with pixels at the same distance as an outer periphery, and excluding the first range from the second range. A standard deviation calculating means for calculating a standard deviation of pixel values belonging to the remaining range; and a ratio calculating for calculating a ratio between a difference between an average value of the first pixel value and an average value of the second pixel value and the standard deviation. And a false image detecting means for detecting a false image based on the ratio.

(7) The invention according to a seventh aspect for solving the above-mentioned problem comprises a calibration means for calibrating the data collection means based on a false image detection signal of the false image detection means. (5) or (6).

(8) The invention according to an eighth aspect for solving the above-mentioned problems is characterized in that a tomographic image of a phantom is used as the tomographic image. A radiation tomography apparatus according to item 1.

(9) The invention according to a ninth aspect for solving the above-mentioned problems is characterized in that X-rays are used as the radiation, as described in any one of (5) to (8). It is a radiation tomography apparatus.

(Operation) In the present invention, the ratio between the average value of the pixel values at pixel positions substantially equidistant from the center of the matrix of the reconstructed image and the standard deviation of the pixel values of the band-like region including this pixel position is calculated. A false image is detected based on this. Further, a false image is detected based on the ratio of the difference between the average pixel value between the center of the matrix of the reconstructed image and the periphery thereof and the pixel value standard deviation of the peripheral region. In addition, the data collection device is calibrated with the detection of the false image.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiment. FIG. 1 shows a block diagram of the radiation tomography apparatus. This device is an example of an embodiment of the present invention. The configuration of the present apparatus shows an example of an embodiment relating to the apparatus of the present invention.

As shown in FIG. 1, this apparatus comprises a scanning gantry 2 and a photographing table 4.
And an operation console (console) 6. The scanning gantry 2 is an example of an embodiment of a data collection unit according to the present invention. The scanning gantry 2 has an X-ray tube 20 as a radiation source. The X-rays (not shown) emitted from the X-ray tube 20 are, for example, fan-shaped X-ray beams, that is, fan beams by the collimator 22.
m) and irradiates the detector array 24. X-rays are an example of a radiation embodiment in the present invention. The detector array 24 has a plurality of X-ray detection elements arranged in an array in the direction of the width of the fan-shaped X-ray beam. The configuration of the detector array 24 will be described later.

The X-ray tube 20, collimator 22 and detector array 24 constitute an X-ray irradiation / detection device. The X-ray irradiation / detection device will be described later. A data collection unit 26 is connected to the detector array 24.
The data collection unit 26 collects detection data of the individual X-ray detection elements of the detector array 24.

The irradiation of X-rays from the X-ray tube 20 is controlled by an X-ray controller (controller) 28. The illustration of the connection relationship between the X-ray tube 20 and the X-ray controller 28 is omitted. The collimator 22
It is controlled by the collimator controller 30. The illustration of the connection relationship between the collimator 22 and the collimator controller 30 is omitted.

The components from the X-ray tube 20 to the collimator controller 30 are mounted on the rotating unit 32 of the scanning gantry 2. The rotation of the rotation unit 32 is controlled by a rotation controller 34. The illustration of the connection relationship between the rotation unit 32 and the rotation controller 34 is omitted.

The photographing table 4 is adapted to carry a photographing object (not shown) into and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the imaging target and the X-ray irradiation space will be described later.

The operation console 6 has a central processing unit 60. The central processing unit 60 is, for example, a computer (c)
and the like. The central processing unit 60 is an example of an embodiment of the false image detection device of the present invention. The configuration of the present false image detecting apparatus shows an example of an embodiment relating to the apparatus of the present invention. The operation of the false image detection apparatus shows an example of an embodiment of the method of the present invention.

The central processing unit 60 includes, for example, an average value calculating means, a first average value calculating means, a second average value calculating means, a standard deviation calculating means, and a ratio calculating means which are realized by a computer program or the like. , A false image detecting unit, an image reconstructing unit, and a calibration unit.

The average value calculating means is an example of an embodiment of the average value calculating means in the present invention. The first average value calculating means is an example of an embodiment of the first average value calculating means in the present invention. The second average value calculating means is an example of an embodiment of the second average value calculating means in the present invention.

The standard deviation calculating means is an example of an embodiment of the standard deviation calculating means in the present invention. The ratio calculating means is an example of an embodiment of the ratio calculating means in the present invention. The false image detecting means is an example of an embodiment of the false image detecting means in the present invention. The image reconstructing means is an example of an embodiment of the image reconstructing means in the present invention. The calibration means is an example of an embodiment of the calibration means in the present invention.

A control interface 62 is connected to the central processing unit 60. The scanning gantry 2 and the imaging table 4 are connected to the control interface 62. The central processing unit 60 controls the scanning gantry 2 and the imaging table 4 through the control interface 62.

The data collection unit 26 in the scanning gantry 2, X
The line controller 28, collimator controller 30 and rotation controller 34 are controlled through a control interface 62. It should be noted 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 central processing unit 60. Data collection buffer 6
4 is connected to the data collection unit 26 of the scanning gantry 2. The data collected by the data collection unit 26 is input to the data collection buffer 64. Data collection buffer 6
4 temporarily stores the input data.

The central processing unit 60 performs image reconstruction based on the projection data of a plurality of views collected through the data collection buffer 64. Image reconstruction includes, for example, filtered back projection (filt).
For example, an erased back projection method is used. The central processing unit 60 also has a storage device 66.
Is connected. The storage device 66 stores various data, reconstructed images, programs, and the like.

The central processing unit 60 also includes a display device 6
8 and the operating device 70 are connected to each other. The display device 68 displays a reconstructed image output from the central processing unit 60 and other information. The operation device 70 is operated by an operator and transmits various instructions and information to the central processing unit 60.
To enter.

FIG. 2 shows a schematic configuration of the detector array 24. The detector array 24 includes a large number of X-ray detection elements 24.
This is a multi-channel X-ray detector in which (i) is arranged. The large number of X-ray detection elements 24 (i)
An X-ray incident surface curved in a cylindrical concave shape is formed. i is a channel number, for example, i = 1 to 1000.

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

FIG. 3 shows the mutual relationship between the X-ray tube 20, the collimator 22, and the detector array 24 in the X-ray irradiation / detection device. 3A is a diagram illustrating a state viewed from the front, and FIG. 3B is a diagram illustrating a state viewed 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 40 by a collimator 22 and irradiated to a detector array 24.

FIG. 3A shows a fan-shaped X-ray beam 40.
, Ie, the width of the X-ray beam 40. The width direction of the X-ray beam 40 matches the arrangement direction of the channels in the detector array 24. (B) shows the thickness of the X-ray beam 40.

With the body axis intersecting the fan surface of the X-ray beam 40, for example, as shown in FIG.
Is loaded into the X-ray irradiation space.
The scanning gantry 2 has a cylindrical structure including an X-ray irradiation / detection device inside.

The X-ray irradiation space is formed inside the cylindrical structure of the scanning gantry 2. An image of the imaging target 8 sliced by the X-ray beam 40 is projected on the detector array 24. The detector array 24 provides a measurement of the projection of the object 8 to be imaged. The thickness t of the X-ray beam 40 irradiating the imaging target 8 is set by adjusting the aperture of the aperture of the collimator 22.

The operation of the present apparatus will be described. FIG. 5 shows a flow chart of the operation of the present apparatus. As shown in the figure, at step 602, the operator inputs a scan plan through the operation device 70. The scan plan includes X-ray irradiation conditions, slice thickness, slice position, and the like. Hereinafter, the present apparatus operates under the operation of the operator and the control by the central processing unit 60 in accordance with the input scan plan.

Next, in step 604, scan positioning is performed. That is, the operator presses a table feed switch (not shown) of the operation device 70 to move the imaging table 4 so that the center of the imaging region of the imaging target 8 coincides with the rotation center (isocenter) of the X-ray irradiation / detection device. .

After performing such scan positioning, scanning is performed in step 606. That is, the X-ray irradiating / detecting device is rotated around the imaging target 8 while irradiating X-rays, and, for example, 1024 views of projection data are collected and transferred to the data collection buffer 64.

Next, in step 608, an image is reconstructed using the projection data by, for example, a filtered back projection method. The reconstructed tomographic image is displayed on the display device 68 in step 610.

Next, at step 612, a false image check (c
Heck) is determined. Whether a false image check is required
It is specified in advance by the operator. If the operator has not designated the false image check, the operation of the present apparatus ends.

When the false image check is designated, in steps 614 and 616, the ring artifact detection and the center artifact (center a) are performed, respectively.
(rtifact) detection. The ring artifact detection and the center artifact detection will be described later.

If no artifact is detected in steps 614 and 616, the operation of the apparatus ends. If any of the artifacts is detected, calibration of the data acquisition unit (DAS) 26 is performed in step 618. The calibration is performed according to, for example, the calibration performed at the time of periodic inspection or the like.
As a result, a change in sensitivity of each channel of the data collection unit 26 due to a change in sensitivity of the detector array 24 with time or the like is corrected. After the calibration, return to step 602,
Retake the picture if necessary. Since the data collection unit 26 has been calibrated, high-quality shooting without a false image can be performed subsequently.

FIG. 6 shows a flowchart of ring artifact detection. As shown in the figure, in step 642, a matrix of a reconstructed image is partitioned. The division of the matrix is performed, for example, by equally dividing the matrix with a plurality of radiations passing through the center O of the matrix as shown in FIG. The matrix size is X pixels (pixels) in the x direction and Y pixels in the y direction. Although FIG. 7 shows an example in which the sections A to D are divided into four equal parts, the sections are not limited to four equal parts, and may be divided into arbitrary n equal parts. It is preferable to increase the value of n because it is easy to detect a partial ring artifact or a band artifact having a width. Hereinafter, a band artifact is also represented by a ring artifact. The value of n is 1
That is, it is a matter of course that the whole may be defined as one section.

Next, in step 644, an average pixel value is calculated for one section (eg, section A) of the reconstructed image. The calculation of the average value will be described with reference to FIG. As shown in the figure, an average value of pixel values is calculated for a pixel group located at a position where the distance ir from the center O of the matrix (hereinafter referred to as the center O) is equal. Note that the distance ir is an integer in units of pixels. Since the distance ir is a discrete value, pixels having substantially the same distance are selected. Such an average value is sequentially obtained up to the farthest distance in the section while sequentially changing the distance ir, and the pixel average value P for each distance ir is obtained.
(Ir) is obtained.

When there is a ring artifact, that is, an annular stripe pattern, all pixel values at the same distance as the radius have substantially the same value far from other parts. For this reason, the pixel average value of the portion having the ring artifact has a significant difference from the pixel value average value of the other portions. Therefore, it is possible to use the absolute value of the pixel average value P (ir) for determining the presence or absence of a ring artifact.

Next, at step 646, the standard deviation of the pixel value is determined. That is, as shown in FIG. 8, the standard deviation of the pixel values is determined for the pixels in the band region having the width w including the pixel group for which the average value has been determined as described above. Such a standard deviation is sequentially obtained up to the one corresponding to the longest distance in the section while sequentially changing the distance ir to obtain a pixel value standard deviation S (ir) for each distance ir.

The pixel value standard deviation S (ir) is determined by the band region w (i
r) represents the degree of variation of the pixel value, and the magnitude of the pixel value standard deviation S (ir) corresponds to the magnitude of the variation of the pixel value. The band region w (ir) is, as it were, a background (back) against ring artifacts occurring therein.
ground), the pixel value standard deviation S (i
The fact that r) is small indicates that the uniformity of the density of the background image is good. On the other hand, a large pixel value standard deviation S (ir) indicates that the uniformity of the density of the background image is poor.

Next, at step 648, the pixel average value P
(Ir) and the pixel value standard deviation S (ir), the distance ir
Are determined and the ratio P (ir) / S (ir) is calculated for
This is defined as Q (ir). Since the ratio Q (ir) is a value obtained by dividing the pixel average value P (ir) by the pixel value standard deviation S (ir), the ratio Q (i) for the same pixel average value P (ir)
Becomes larger as the pixel value standard deviation S (ir) becomes smaller, and becomes smaller as the pixel value standard deviation S (ir) becomes larger.

In other words, even if the pixel average value P (ir) is the same, Q (ir) increases as the uniformity of the background image density increases, and Q (ir) increases as the uniformity of the background image density decreases. Become smaller. This is consistent with the tendency that ring artifacts are more noticeable as the background image density is more uniform, and that ring artifacts are less noticeable as the background image density is less uniform. It is very convenient as a numerical value to represent.

Next, at step 650, threshold processing is performed on Q (ir). The threshold processing will be described with reference to FIG. The figure shows the distribution of Q (ir) along the distance ir, that is, the profile (pro) of Q (ir) in the direction of the distance ir.
file). Since the pixel value is a CT number and may be a negative value, the profile of Q (ir) may have a negative portion. Therefore, a threshold value ± Qt is set,
Based on this, threshold processing is performed.

The threshold processing is performed according to the flowchart shown in FIG. As shown in the figure, in step 702, the count value ir is set to 0. ir is used to read data Q (ir) on the profile as described later. Next, in step 704, the count value k is set to 0. As described later, when Q (ir) exceeding the threshold value is continuous, the Q (ir) is integrated, and the number of integrations at that time is k.

Next, at step 706, 1 is added to ir,
In step 708, it is determined whether or not ir exceeds irm-1. irm is a distance to the outermost edge of the image of the section A, and when irm-1 is exceeded, the threshold processing is terminated. The value of irm is known in advance.

Now, since ir is 1, it is No, and it is determined in step 710 whether or not the absolute value of Q (ir) at ir = 1 exceeds the threshold value Qt. As shown in FIG. 9, when ir = 1, the absolute value of Q (ir) does not exceed the threshold value Qt, and thus the result is No, and the process returns to step 706 to add 1 to ir. Is determined, and if No, it is determined in step 710 whether or not the absolute value of Q (ir) at ir = 2 exceeds the threshold value Qt. Here, No is returned to step 706. Similarly, i
The value of Q (ir) is sequentially determined in the r direction based on the threshold value Qt.

When ir = ir1, the absolute value of Q (ir) exceeds the threshold Qt, as shown in FIG. Therefore, the process branches from step 710 to step 712. Then, in step 712, the value of R (ir) is set to Q (ir). Next, in step 714, 1 is added to ir, and in step 716, it is determined whether or not ir exceeds irm-1. If ir exceeds irm-1, the threshold processing ends, but since it is No, i
The absolute value of Q (ir) at r = ir1 + 1 is the threshold Qt
Is determined. As shown in FIG.
From r1 to ir2, since the absolute value of Q (ir) exceeds the threshold value Qt, it is now Yes and step 720
Branch to

In step 720, R (ir-1-k)
Is added to Q (ir), and is added to a new R (ir-1−
k). The meaning of this processing is as follows. Now i
Since r = ir1 + 1 and k = 0, ir-1-k = i
r1. Therefore, by the processing in step 720, Q (ir1 + 1) is added to R (ir1), and this becomes a new value of R (ir1). Since the original value of R (ir1) was Q (ir1), the value of R (ir1) is eventually Q (ir1) + Q (ir1 + 1). That is, integration of Q (ir) exceeding the threshold is performed.

After this integration, 1 is added to k in step 722. This results in k = 1 and Q (i
r) is counted once. Then, the process returns to step 714. In step 714, 1 is added to ir.
Thus, ir becomes ir1 + 2.

After checking the value of ir in step 714, in step 718, the Q in ir = ir1 + 2
It is determined whether or not the absolute value of (ir) exceeds the threshold value Qt. Since the absolute value of Q (ir) exceeds the threshold value Qt from ir1 to ir2, the answer is Yes, and step 72
At 0, Q (ir) is added to R (ir-1-k) to make it new R (ir-1-k).

Since ir = ir1 + 2 and k = 1, ir-1-k = ir1. Therefore, by the processing in step 720, R (ir1) is added to Q (ir1 +
2) is added, and this becomes a new value of R (ir1). The value before addition of R (ir1) is Q (ir1) + Q (i
r1 + 1), the value of R (ir1) is Q (ir
1) + Q (ir1 + 1) + Q (ir1 + 2). That is, integration of Q (ir) exceeding a threshold value that is continuous in the ir direction is performed. Then, at step 722, 1 is added to k. As a result, k = 2, and counting that Q (ir) has been integrated twice is counted. Then, the process returns to step 714.

Thereafter, the above processing is repeated until ir exceeds ir2. As a result, an integrated value of Q (ir) from ir1 to ir2, that is, a value corresponding to the area of the portion where the oblique line is lowered is obtained as the value of R (ir1). It is preferable to integrate Q (ir) exceeding a threshold value that is continuous in the ir direction in order to increase the detection sensitivity of the ring artifact. When it is not necessary to increase the detection sensitivity of the ring artifact, Q (ir) exceeding the threshold may be individually set to the value of R (ir) without performing the above-described integration.

When ir exceeds ir2, the process branches from step 718 to step 724 because the absolute value of Q (ir) becomes equal to or smaller than the threshold value. In step 724, k = 0
And This resets the integration count (rese
t). Thereafter, returning to step 706, the same operation as described above is repeated.

By such threshold processing, R (ir
As the value of 3), the integrated value of Q (ir) from ir3 to ir4 is obtained, and as the value of R (ir5), ir5 to ir
An integrated value of Q (ir) up to 6 is obtained.

When ir> irm-1, the above threshold processing ends, and the flow branches from step 708 or 716 to step 652 in the flowchart shown in FIG. In step 652, the ring value R is calculated. As the ring value R, R (ir) obtained by the threshold processing in step 650 is used,
It is calculated by the following equation.

[0065]

(Equation 1)

That is, the mean square of R (ir) is set as the ring value R. In addition, instead of the mean square, R (ir) is 2
The sum of the powers may be used as the ring value R. By the processing up to this point, the ring value R for the section A is obtained.

Next, at step 654, it is determined whether the calculation of the ring value R has been completed for all the partitions. Since it is the stage at which the section A has been completed, the answer is No.
4, the same processing is performed for the next section B, and the ring value R of the section B is obtained. Similarly, ring values R are obtained for sections C and D, respectively. By determining the ring value for each section in this way, in addition to complete ring artifacts, incomplete ring or band-like partial artifacts can be effectively detected.

In step 656, the sum of the ring values R of each section is determined. Then, at step 658, it is determined whether or not the sum of the ring values R exceeds the threshold value R0. Threshold value R0
Is preset as an allowable limit value of the ring artifact. If the determination result is Yes, the flow branches to step 618 in the flowchart shown in FIG. 5 to perform the calibration. If the determination result is No, the flow branches to step 616 to detect the center artifact.

FIG. 11 is a flowchart showing the detection of the center artifact. As shown in the figure, in step 662, the average value Ma of the pixel values of ROIa is calculated. ROIa
Is an area having a radius ira including the center O of the matrix as shown in FIG. 12, for example, and an average value of pixel values belonging to this area is obtained. ROIa is an example of the first embodiment of the present invention.

Next, at step 664, the average value Mb of the pixel values of ROIb is calculated. The ROIb has a radius irb (> i) including the center O of the matrix as shown in FIG.
The average value of the pixel values belonging to this area is obtained. ROIb is an example of a second embodiment of the present invention.

Next, at step 666, ROIb
The standard deviation S of the pixel value of the difference area excluding OIa is calculated. Next, at step 668, the center value C is calculated.
The center value C is calculated by the following equation.

[0072]

(Equation 2)

As shown in the equation (2), the center value C is 2
It is the ratio between the absolute value of the difference between the average pixel values of two regions and the pixel value standard deviation of the difference region. The absolute value of the difference between the average pixel values represents the density difference between the pixel value of the pixel near the matrix center O and the pixel values around the pixel value. By dividing this by the standard deviation of the surrounding pixel values, it is possible to obtain a numerical value representing the conspicuousness of the false image, as in the case of the ring artifact described above.

Next, at step 670, it is determined whether or not the center value C has been obtained for all of a plurality of predetermined ROIs. ROIa and ROIb are radiuses ira and ir
Since a plurality of values in which b is sequentially increased are set in advance, the flow returns to step 662, and the same calculation as above is performed for the next combination of the radii ira and irb. This is repeated until the center values C of all ROIs are obtained.

After obtaining the center values C of all the ROIs, the maximum value Cmax of the center values C is extracted in step 672. Then, in step 674, Cmax is equal to the threshold C
It is determined whether or not it exceeds 0. The threshold value C0 is set in advance as an allowable limit value of the center artifact. If the determination result is Yes, step 6 shown in FIG.
Calibration is performed by branching to 18, and in the case of No, the operation is terminated.

As described above, by analyzing the reconstructed image, a false image included in the reconstructed image can be effectively detected. By using a tomographic image obtained by actually photographing the imaging target 8 as a reconstructed image used for analysis, it is possible to detect a false image that is realistic. Also, a uniform phantom as a reconstructed image used for analysis
By using the tomographic image obtained by capturing the image, accurate false image detection can be performed.

The above description has been made of the example in which X-rays are used as radiation. However, the radiation is not limited to X-rays, but may be other types of radiation such as γ-rays. However,
At present, X-rays are preferred because practical means for generating, detecting, controlling, and the like are the most substantial.

[0078]

As described above in detail, according to the present invention, a method and an apparatus for effectively detecting a false image and a radiation tomography apparatus having such a false image detecting means are realized. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to an example of an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a detector array in the device shown in FIG.

FIG. 3 is a schematic configuration diagram of an X-ray irradiation / detection device in the device shown in FIG.

FIG. 4 is a schematic configuration diagram of an X-ray irradiation / detection device in the device shown in FIG.

FIG. 5 is a flowchart of the operation of the apparatus shown in FIG. 1;

FIG. 6 is a flowchart showing details of a part of the flowchart shown in FIG. 5;

FIG. 7 is a conceptual diagram of a matrix of a reconstructed image.

FIG. 8 is a conceptual diagram of a part of the matrix shown in FIG.

FIG. 9 is a graph showing a profile of data obtained by the apparatus shown in FIG. 1;

FIG. 10 is a flowchart showing details of a part of the flowchart shown in FIG. 6;

FIG. 11 is a flowchart showing details of a part of the flowchart shown in FIG. 5;

FIG. 12 is a conceptual diagram of a part of the matrix shown in FIG. 7;

[Explanation of symbols]

 2 scanning gantry 4 imaging table 6 operation console 8 imaging target 20 X-ray tube 22 collimator 24 detector array 26 data collection unit 28 X-ray controller 30 collimator controller 32 rotation unit 34 rotation controller 40 X-ray beam 60 central processing unit 62 control interface 64 data collection buffer 66 storage device 68 display device 70 operating device

Continuation of the front page (56) References JP-A-11-128218 (JP, A) JP-A-61-231476 (JP, A) JP-A-8-266530 (JP, A) (58) Fields studied (Int .Cl. ⁷ , DB name) A61B 6/00-6/14 G06T 1/00

Claims (9)

(57) [Claims] 1. A false image is formed based on a ratio between an average value of pixel values at pixel positions substantially equidistant from a matrix center of a reconstructed image and a standard deviation of pixel values of a band-like region including the pixel position. Detecting a false image. 2. An average value of pixel values belonging to a first range having pixels substantially equidistant from the center of the matrix of the reconstructed image as an outer periphery, and substantially equal distance from the center of the matrix of the reconstructed image. The average value of the pixel values belonging to a second range larger than the first range with the pixel as the outer periphery is obtained, and the standard deviation of the pixel values belonging to the remaining range excluding the first range from the second range Calculating a ratio of a difference between the average value of the first pixel value and the average value of the second pixel value and the standard deviation, and detecting a false image based on the ratio. False image detection method. 3. An average value calculating means for calculating an average value of pixel values at pixel positions substantially equidistant from a center of a matrix of a reconstructed image; and a standard deviation of pixel values of a band-like region including the pixel position. A fake image detection device comprising: a standard deviation calculation means; and a fake image detection means for detecting a fake image based on a ratio between the average value and the standard deviation. 4. A first average value calculating means for calculating an average value of pixel values belonging to a first range having pixels substantially equidistant from a center of a matrix of the reconstructed image as an outer periphery; A second average value calculating means for calculating an average value of pixel values belonging to a second range larger than the first range with pixels substantially equidistant from the center of the matrix as an outer periphery; A standard deviation calculating means for calculating a standard deviation of pixel values belonging to a range excluding the range of 1; a difference between an average value of the first pixel values and an average value of the second pixel values; And a false image detecting means for detecting a false image based on the ratio. 5. A radiation tomography apparatus comprising: a data collection unit that collects projection data of a plurality of views of a target to be imaged by radiation; and an image reconstruction unit that reconstructs a tomographic image based on the projection data. Average value calculating means for calculating an average value of pixel values at pixel positions substantially equidistant from the center of the matrix of the tomographic image; and standard deviation calculating means for calculating a standard deviation of pixel values of a band-like area including the pixel position. A radiation tomography apparatus, comprising: a false image detecting unit configured to detect a false image based on a ratio between the average value and the standard deviation. 6. A radiation tomography apparatus comprising: a data collection unit that collects projection data of a plurality of views of an imaging target by radiation; and an image reconstruction unit that reconstructs a tomographic image based on the projection data. First average value calculating means for obtaining an average value of pixel values belonging to a first range having pixels substantially equidistant from the center of the matrix of the tomographic image, and substantially from the center of the matrix of the tomographic image A second average value calculating means for calculating an average value of pixel values belonging to a second range larger than the first range with pixels at the same distance as an outer periphery, and excluding the first range from the second range. A standard deviation calculating means for calculating a standard deviation of pixel values belonging to the remaining range; a difference between an average of the first pixel values and an average of the second pixel values; Radiation tomography apparatus characterized by comprising a ratio calculating means for calculating the ratio, and artifact detection means for detecting an artifact based on the ratio, a. 7. The radiation according to claim 5, further comprising: a calibration unit configured to calibrate the data collection unit based on a false image detection signal of the false image detection unit. Tomography equipment. 8. The radiation tomography apparatus according to claim 5, wherein a tomographic image of a phantom is used as the tomographic image. 9. The method according to claim 5, wherein X-rays are used as the radiation.
A radiation tomography apparatus according to any one of the above.