JP2009178493A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus for capturing a substance discriminating image with an excellent signal-to-noise ratio. <P>SOLUTION: In reconstructing a substance discriminating image from two kinds of images captured by the X-ray CT apparatus using two kinds of X-rays with different energies, a combination of the two kinds of pixel values crossing over the two kinds of images is found for each same pixel position (551), and pixels are classified based on the characteristic value of the combination of the pixels (552-554). According to the classified pixels, pixels in at least one of the two kinds of images are classified (555). The characteristic value is found as the ratio of the two kinds of pixel values, and the reference for the pixel classification is found from the histogram of the characteristic values. Alternatively, the reference for the pixel classification can be set on the two-dimensional space having two coordinate axes corresponding to the two kinds of pixel values. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and in particular, takes two types of images using two types of X-rays having different energies, and forms a substance separation image from these two types of images. The present invention relates to an X-ray CT apparatus to be reconfigured.

  When an X-ray CT apparatus captures a material separation image, that is, an MD (material decomposition) image, at least two types of X-rays having different energies are used. Two types of X-ray scans with different energies are also called dual energy scans.

Due to the energy-dependent characteristics of the X-ray absorption coefficient, X-ray projection data (data) obtained with low-energy X-rays or pixel values of tomographic images and X-ray projection data obtained with high-energy X-rays or pixel values of tomographic images Have different values, and the ratio of these data varies depending on the substance. Such characteristics of the X-ray absorption coefficient are used for taking a substance-sorted image (see, for example, Patent Document 1).
Japanese Patent Laying-Open No. 2004-65975 (paragraph numbers 0019-0023, FIGS. 3 and 4)

For example, when taking a substance-separated image of a contrast medium mainly composed of iodine (I) injected into a subject as a contrast medium, two types of X-rays with the tube voltage of the X-ray tube set to 80 kV and 140 kV, for example. The two images are respectively taken and the calcium (Ca) image included in both images is erased. Calcium elimination is performed by weighted subtraction of the 140 kV image from the 80 kV image. As the load (weight) w, a ratio of CT values of calcium at 80 kV and 140 kV (CT ₈₀ / CT ₁₄₀ ) is used. The subtraction result may be normalized by 1 / (w−1).

  The CT value ratio of calcium is given as 1.4 to 1.5, and the CT value ratio of iodine is given as 1.6 to 2.0. Since the CT value ratio of calcium (eg, 1.45) is closer to 1 than the CT value ratio of iodine (eg, 1.9), the normalized substance separation image is S / N (signal-to-noise ratio). Will be bad. Further, when image processing for making the iodine stand out white is performed, an area having a minus CT value becomes white and it becomes difficult to identify the iodine. The same problem occurs not only in iodine and calcium, but also in the taking of separate images of other substances.

  Therefore, an object of the present invention is to realize an X-ray CT apparatus that captures a substance-separated image with a good S / N.

  In a first aspect, the present invention as a means for solving the problem is that two types of images are captured using two types of X-rays having different energies, and a substance separation image is imaged from these two types of images. A reconfigurable X-ray CT apparatus, a unit for obtaining a combination of two types of pixel values spanning the two types of images for each same pixel position, and classifying characteristic values of the combination of the pixel values according to a predetermined criterion X-ray CT comprising: means for classifying pixels based on the characteristic value; and means for classifying pixels in at least one of the two types of images corresponding to the classification of the pixels Device.

According to a second aspect of the present invention, the characteristic value is a ratio of the two types of pixel values. The X-ray CT apparatus according to the first aspect is characterized in that: It is.
In a third aspect, the present invention as means for solving the problem is characterized in that the classification of the pixels is performed based on a criterion set on a histogram of characteristic values of the combination of the pixel values. The X-ray CT apparatus according to the first aspect or the second aspect.

  According to a fourth aspect of the present invention, as the means for solving the problem, the characteristic value is shown in a two-dimensional space having two coordinate axes corresponding to the two types of pixel values, and the classification of the pixels is The X-ray CT apparatus according to the first aspect, which is performed based on a reference indicated on the two-dimensional space.

  In a fifth aspect, the present invention as means for solving the problem is characterized in that the pixel separation is performed by at least one of color overlay, gradation provision, opacity provision, and clipping. The X-ray CT apparatus according to any one of the fourth aspect and the fourth aspect.

According to a sixth aspect of the present invention, as a means for solving the problem, the color overlay, gradation provision, opacity provision, and clipping can be switched to each other. X-ray CT apparatus.

  The X-ray CT apparatus according to the third aspect or the fourth aspect, wherein the present invention as means for solving the problem, in the seventh aspect, is that the classification of the pixels can be corrected. It is.

  According to an eighth aspect of the present invention, as a means for solving the problem, in the eighth aspect, the correction of the classification is performed by determining whether the classification of the characteristic values is appropriate according to a shape feature amount of the group of classified pixels in an image space. The X-ray CT apparatus according to the seventh aspect, which is performed by making a determination and correcting based on the determination result.

  According to a ninth aspect of the present invention, as a means for solving the problem, in the ninth aspect, the correction is performed by adjusting a standard used for the classification. Is an X-ray CT apparatus.

  In a tenth aspect of the present invention as a means for solving the problem, in the tenth aspect, the image is a tomographic image or a 3D image in which tomographic images are arranged in the z direction. The X-ray CT apparatus according to any one of the above aspects.

  According to an eleventh aspect of the present invention as a means for solving the problem, the shape feature amount includes the number of segments (number of region numbers), sphericity, volume, surface area, average / maximum / minimum pixel value, average / The eighth aspect is characterized in that it is at least one of maximum / minimum standard deviation values, three-dimensional Ferret diameter, circumscribed cuboid volume fraction, Euler number, primary / secondary moment, and approximate ellipsoidal diameter. X-ray CT apparatus.

According to the present invention, an X-ray CT apparatus that captures two types of images using two types of X-rays having different energies and reconstructs a substance-separated image from these two types of images, Means for obtaining a combination of two types of pixel values across the image for each identical pixel position, and means for classifying the pixels based on the characteristic values by classifying the characteristic values of the pixel value combinations according to a predetermined criterion And means for classifying pixels in at least one of the two types of images corresponding to the classification of the pixels, so that an X-ray CT apparatus for imaging a substance-separated image with a good S / N can be realized. .

  The best mode for carrying out the invention will be described below with reference to the drawings. Note that the present invention is not limited to the best mode for carrying out the invention. FIG. 1 shows a schematic configuration of an X-ray CT apparatus. This apparatus is an example of the best mode for carrying out the invention. An example of the best mode for carrying out the invention related to the X-ray CT apparatus is shown by the configuration of the apparatus.

  The apparatus has a gantry 100, a table 200, and an operator console 300. The gantry 100 scans the subject 10 carried by the table 200 with the X-ray irradiation / detection device 110, collects projection data of a plurality of views, and inputs it to the operator console 300. A contrast agent is injected into the subject 10 in advance. As the contrast agent, a contrast agent mainly composed of iodine is used.

  The operator console 300 performs image reconstruction based on the projection data input from the gantry 100, creates a substance classification image from the reconstructed image, and displays the substance classification image on a display 302. Image reconstruction and material classification image creation are performed by a dedicated computer in the operator 300. Hereinafter, the material separation image is referred to as an MD image. In addition, shooting of an MD image is also referred to as MD shooting.

  The operator console 300 also controls the operation of the gantry 100 and the table 200. Control is performed by a dedicated computer in the operator 300. Under the control of the operator console 300, the gantry 100 scans under a predetermined scanning condition, and the table 200 positions the subject 10 so that a predetermined part is scanned. Positioning is performed by adjusting the height of the top plate 202 and the horizontal movement distance of the cradle 204 on the top plate by a built-in position adjustment mechanism.

  An axial scan can be performed by scanning the subject with the cradle 204 stopped. A cine scan can be performed by continuously performing an axial scan over a predetermined time.

  A helical scan can be performed by continuously performing a plurality of scans while continuously moving the cradle 204. During the helical scan, the cradle 204 is continuously moved reciprocally, and the x-ray data is collected together with the coordinate position information of the cradle in the z direction to perform image reconstruction processing. shuttle scan).

  The height adjustment of the top plate 202 is performed by swinging the support column 206 around the attachment portion to the base 208. The top plate 202 is displaced in the vertical direction and the horizontal direction by the swing of the column 206. The cradle 204 moves in the horizontal direction on the top plate 202 to cancel the horizontal displacement of the top plate 202. Depending on the scan conditions, the scan is performed with the gantry 100 tilted. The gantry 100 is tilted by a built-in tilt mechanism.

  As shown in FIG. 2, the table 200 may be of a type in which the top plate 202 moves up and down vertically with respect to the base 208. The top plate 202 is moved up and down by a built-in lifting mechanism. In this table 200, the horizontal movement of the top plate 202 accompanying the raising and lowering does not occur.

  FIG. 3 schematically shows the configuration of the X-ray irradiation / detection device 110. The X-ray irradiation / detection device 110 detects an X-ray 134 emitted from the focal point 132 of the X-ray tube 130 with an X-ray detector 150.

  The X-ray 134 is shaped by a collimator mechanism (not shown) and becomes a cone beam X-ray. The X-ray detector 150 has an X-ray detector light receiving surface 152 that expands two-dimensionally corresponding to the spread of X-rays. The X-ray detector light receiving surface 152 is curved so as to constitute a part of a cylinder. The central axis of the cylinder passes through the focal point 132.

  The X-ray irradiation / detection device 110 rotates around a central axis passing through an imaging center, that is, an isocenter O. The central axis is parallel to the central axis of the partial cylinder formed by the X-ray detector 150.

  The direction of the center axis of rotation is the z direction, the direction connecting the isocenter O and the focal point 132 is the y direction, and the direction perpendicular to the z direction and the y direction is the x direction. These x, y, and z axes are three axes in a rotating coordinate system with the z axis as the central axis.

  FIG. 4 schematically shows a plan view of the X-ray detector light receiving surface 152 of the X-ray detector 150. The X-ray detector light receiving surface 152 has detector elements 154 arranged two-dimensionally in the x and z directions. That is, the detector elements 154 are two-dimensionally arranged on the X-ray detector light receiving surface 152.

  Each detector element 154 constitutes a detection channel of the X-ray detector 150. Thereby, the X-ray detector 150 becomes a multi-channel X-ray detector. The detector element 154 is composed of, for example, a combination of a scintillator and a photodiode.

  FIG. 5 shows a flow diagram of the operation of this apparatus during MD shooting. MD shooting is performed under the control of the operator console 300. As shown in FIG. 5, the scan position is set in step 501. The scan position is set through the operator console 300 by the operator. Thereby, for example, the scan position for the abdomen is set. The scan position is not limited to the abdomen, but can be set for a desired part such as the head or abdomen.

  In step 502, a scan protocol is set. The scan protocol is set through the operator console 300 by the operator. As a result, the required imaging conditions such as the tube voltage and tube current of the X-ray tube, the scan speed and duration, the image reconstruction conditions, the MD image creation conditions, etc. are set.

  As the tube voltage, for example, two kinds of voltages of 80 kV and 140 kV are set. As a result, two types of X-ray energies for dual energy scanning are set.

  The two types of tube voltages are not limited to 80 kV and 140 kV, and may be a combination of appropriate voltages. Hereinafter, an example of 80 kV and 140 kV will be described, but the same applies to other combinations.

  In step 503, a dual energy scan is performed. The dual energy scan is performed, for example, by switching the tube voltage between 80 kV and 140 kV alternately. The tube voltage is switched every view, every several views, or every scan.

  One scan is performed by a full scan or a half scan. The full scan is performed by rotating the X-ray irradiation / detection device 110 by 360 degrees. The half scan is performed by rotating the X-ray irradiation / detection device 110 by 180 + γ degrees. Note that γ is an opening angle of the X-ray beam 134 in the xy plane. Further, 360 degrees or 180 degrees may be divided by an integer value to form a segment scan unit.

  The dual energy scan may be performed by simultaneous X-ray irradiation with two X-ray irradiation / detection devices 110 provided, with one system having a tube voltage of 80 kV and the other system having a tube voltage of 140 kV. In that case, the X-ray irradiation direction is varied by, for example, 90 degrees between the two systems.

  By dual energy scanning, two types of data corresponding to two types of X-rays having different energies are obtained. One of the two types of data is data acquired by X-rays having a maximum energy of 80 keV, and the other is data acquired by X-rays having a maximum energy of 140 keV.

  In step 504, image reconstruction is performed. Image reconstruction is performed by the operator console 300 based on one of the two types of data and the other. As a result, two types of images are obtained. Both types of images are obtained as tomographic images or 3D (3-dimensional) images.

  Of the two types of images, the pixel value in one image is a CT value under an X-ray with a maximum energy of 80 keV, and the pixel value in the other image is a CT value under an X-ray with a maximum energy of 140 keV. Value.

  Hereinafter, the former is referred to as a low energy image, and the latter is referred to as a high energy image. Both images are also referred to as original images. The pixel value of the low energy image is referred to as a low energy pixel value, and the pixel value of the high energy image is referred to as a high energy pixel value.

  In step 505, an MD image is created. The MD image creation is performed by the operator console 300 using the low energy image and the high energy image. As will be described later, the MD image is an image obtained by separating body substances such as iodine, calcium, soft tissue, and fat. The MD image is also reconstructed as a tomographic image or a 3D image in which tomographic images are arranged in the z direction. Details of the MD image reconstruction will be described later.

  In step 506, the MD image is displayed. As a result, the MD image is displayed on the display 302, and the distribution in the body of iodine, calcium, soft tissue, fat and the like can be visually recognized.

  The processing after the image reconstruction in step 504 may be performed by taking scan data from the X-ray CT apparatus by a work station or the like. In the following description, an example using the X-ray CT apparatus will be described.

  FIG. 6 shows a flowchart of MD image reconstruction. This flowchart shows the operation of the operator console 300 in step 505 of FIG. 5 in an exploded manner into sub steps.

  As shown in FIG. 6, in sub-step 551, a combination of a low energy pixel value and a high energy pixel value is obtained. The combination of pixel values is obtained for each identical pixel position across the low energy image and the high energy image.

As a result, for all the pixels of the low energy image and the high energy image, a combination of pixel values of pixels having the same x and y coordinate position is obtained. Hereinafter, a combination of a low energy pixel value and a high energy pixel value is simply referred to as a combination of pixel values. A combination of pixel values is represented by (CT ₈₀ , CT ₁₄₀ ).

The operator console 300 for obtaining a combination of pixel values (CT ₈₀ , CT ₁₄₀ ) is an example of means for obtaining a combination of two types of pixel values spanning two types of images for each identical pixel position in the present invention.

In sub-step 552, the pixels are classified. The pixel classification is performed based on the characteristic value of the combination of pixel values (CT ₈₀ , CT ₁₄₀ ). The operator console 300 that classifies pixels is an example of a unit that classifies pixels based on the characteristic value of the combination of pixel values in the present invention.

As the characteristic value of the combination of pixel values (CT ₈₀ , CT ₁₄₀ ), for example, a ratio between a low energy pixel value and a high energy pixel value is used. Hereinafter, the ratio between the low energy pixel value and the high energy pixel value is simply referred to as a pixel value ratio. The pixel value ratio is represented by (CT ₈₀ / CT ₁₄₀ ).

The pixel value ratio (CT ₈₀ / CT ₁₄₀ ) is usually such that iodine is generally in the range of 2.00-1.55, calcium is generally in the range of 1.55-1.35, and soft tissue is generally in the range. The value is in the range of 1.35 to 0.95, and the fat is generally in the range of 0.95 to 0.80.

The classification of the pixels is performed using such characteristics of the pixel value ratio (CT ₈₀ / CT ₁₄₀ ). That is, a pixel having a pixel value ratio (CT ₈₀ / CT ₁₄₀ ) in the range of 2.00 to 1.55 is classified as iodine, and a pixel in the range of 1.55 to 1.35 is classified as calcium. Pixels in the range of .35-0.95 are classified as soft tissue, and pixels in the range of 0.95-0.80 are classified as fat.

When a histogram of the pixel value ratio (CT ₈₀ / CT ₁₄₀ ) is obtained over all pixels of the original image, for example, a histogram as shown in FIG. 7 is obtained. As shown in FIG. 7, the histogram has a plurality of peaks. The plurality of peaks correspond to a plurality of substances in the subject, and are, for example, iodine, calcium, soft tissue, and fat, respectively.

  On the horizontal axis, the iodine peak is in the ab range, the calcium peak is in the bc range, the soft tissue peak is in the cd range, and the fat peak is in the de range. Is in range. The values of a, b, c, d, and e are approximately 2.00, 1.55, 1.35, 0.95, and 0.80, respectively. Fluctuates due to machine differences.

  For this reason, the values of a, b, c, d, and e obtained from the histogram of the original image are values corresponding to the actual effective mass number of the substance. Therefore, if these values a, b, c, d, and e are used as pixel classification criteria, pixel classification that matches the actual situation can be performed.

Instead of the histogram, a two-dimensional distribution of pixel value combinations (CT ₈₀ , CT ₁₄₀ ) may be obtained. A two-dimensional distribution of pixel value combinations (CT ₈₀ , CT ₁₄₀ ) is obtained in a two-dimensional space having two coordinate axes corresponding to a low energy pixel value and a high energy pixel value.

Thereby, for example, a distribution as shown in FIG. 8 is obtained. As shown in FIG. 8, in the two-dimensional space, the low energy pixel value CT ₈₀ is the vertical axis, the high energy pixel value CT ₁₄₀ is the horizontal axis, and the combination of pixel values (CT ₈₀ , CT ₁₄₀ ) is (CT ₈₀ , CT ₁₄₀ ) is arranged in a two-dimensional space as a two-dimensional coordinate. Hereinafter, the distribution of pixel value combinations (CT ₈₀ , CT ₁₄₀ ) is simply referred to as pixel distribution.

  The iodine pixel distribution forms group I, the calcium pixel distribution forms group C, the soft tissue pixel distribution forms group S, and the fat pixel distribution forms group F. Pixel classification can be performed based on such a group. That is, the pixels belonging to the group I are classified as iodine, the pixels belonging to the group C are classified as calcium, the pixels belonging to the group S are classified as soft tissue, and the pixels belonging to the group F are classified as fat.

  The group I exists between the straight lines a and b, the group C exists between the straight lines b and c, the group S exists between the straight lines c and d, and the group F exists between the straight lines d and e. To do. The straight lines a, b, c, d, and e form a group boundary.

The inclinations of the boundary lines a, b, c, d, and e are, for example, 2.00, 1.55, 1.35, 0.95, and 0.80, respectively. Therefore, pixel classification can be performed using the boundary of the two-dimensional distribution of the combination of pixel values (CT ₈₀ , CT ₁₄₀ ).

The inclinations of the straight lines a, b, c, d, and e vary depending on individual differences among subjects, machine differences among X-ray CT apparatuses, and the like. For this reason, the values of a, b, c, d, and e obtained from the two-dimensional distribution of the pixel value combination (CT ₈₀ , CT ₁₄₀ ) of the original image are values in accordance with the actual effective mass number of the substance. . Accordingly, by using the values of a, b, c, d, and e as the pixel classification reference, pixel classification in accordance with the actual situation can be performed. The classification result is the same as when the histogram is used.

  The classified pixels are given a label corresponding to the classification for each pixel position. In each label, iodine is, for example, Id, calcium is, for example, Ca, soft tissue is, for example, So, and fat is, for example, Fa.

  A collection of pixel locations with the same label forms a block in the image space. The shape of the block is, for example, a circle, an ellipse, or an indeterminate surface in a two-dimensional image space, and is, for example, a sphere, an ellipsoid, or an indefinite solid in a three-dimensional image space. Hereinafter, a collection of pixel positions having the same label in the image space is simply referred to as a collection of pixel positions. It is also possible to extract a continuous area that is spatially continuous for each level, and use this continuous area as a collection of pixel positions.

  The shape of the collection of pixel positions can be specified based on the shape feature amount. In the two-dimensional image space, the shape feature amount is, for example, the number of segments (number of region numbers), circularity, area, average / maximum / minimum pixel value, average / maximum / minimum standard deviation value, circumscribed rectangle Volume ratio, first and second moment, approximate elliptical diameter, etc. In 3D image space, number of segments, sphericity, volume, surface area, average / maximum / minimum pixel value, average / maximum / Minimum standard deviation value, three-dimensional Ferret diameter, circumscribed cuboid volume fraction, Euler number, primary / secondary moment, approximate ellipsoidal diameter, etc.

  It is possible to specify the shape of a collection of pixel positions based on such shape feature amounts and determine whether or not pixel classification is appropriate according to the complexity of the shape. That is, the simpler the shape, the more likely the classification is appropriate, and the more complicated the shape, the more likely the classification is inappropriate.

  In sub-step 553, it is determined whether the classification is appropriate. A shape feature amount is used for determining the suitability of classification. If it is determined that the classification is not appropriate, the reference value for classification is corrected in sub-step 554.

The reference value for classification is the characteristic value of the combination of pixel values (CT ₈₀ , CT ₁₄₀ ), that is, the above-mentioned a, b, c, d, e. These characteristic values are corrected to new values. The correction of the characteristic value is performed manually or automatically. The processing of sub-steps 553 and 554 is repeated until an appropriate classification is obtained.

  If the classification is appropriate, in step 555 the pixels are separated. Pixel separation is performed on the original image. As the original image, an average value of either or both of the low energy image and the high energy image is employed.

  For classification of pixels, a classification of pixel positions, that is, a label assigned to each pixel position is used. The operator console 300 that performs pixel classification is an example of a unit that classifies pixels in at least one of two types of images in accordance with pixel classification in the present invention.

  The pixel separation in the original image is performed according to the label of the pixel position. That is, the pixel with the label Id at the pixel position is sorted into iodine, the pixel with the label Ca is sorted into calcium, the pixel with the label So is sorted into soft tissue, and the label Fa is attached. Pixels are classified into fat.

  For the separated substances, the color overlay, the gradation, the opacity, etc. are made different for each substance. Or, cut out each substance. Alternatively, they may be combined and arbitrarily selectable.

  An MD image is completed by such classification. The MD image is an image in which aspects such as color overlay, gradation, opacity, etc. are made different for each substance on the original image, or an original image is cut out for each substance. Therefore, the S / N of the original image is directly the S / N of the MD image.

  The MD image is displayed on the display 302. FIG. 9 schematically shows an example of MD image display. As shown in FIG. 9, in the original tomographic image, the iodine image ID is indicated by a color overlay, for example. A plurality of other material images can also be indicated by different color overlays. Not only the color overlay but also individual display by clipping is possible.

  When the original image is a 3D image, the material image can be displayed in a discriminable state due to differences in aspects such as gradation and opacity in addition to color overlay and clipping.

It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing invention. It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing invention. It is a figure which shows the structure of a X-ray irradiation / detection apparatus. It is a figure which shows the structure of the X-ray detector light-receiving surface of an X-ray detector. It is a flowchart which shows operation | movement of the X-ray CT apparatus of an example of the best form for implementing invention. It is a flowchart which shows MD image reconstruction. It is a figure which shows notionally the histogram of pixel value ratio. It is a figure which shows notionally the two-dimensional distribution of the combination of a pixel value. It is a figure which shows an example of MD image display typically.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100: Gantry 110: Detection apparatus 130: X-ray tube 132: Focus 134: X-ray beam 150: X-ray detector 152: X-ray detector light-receiving surface 154: Detector element 200: Table 202: Top plate 204: Cradle 206 : Post 208: Base 300: Operator console 302: Display

Claims (11)

An X-ray CT apparatus that takes two types of images using two types of X-rays having different energies, and reconstructs a material separation image from these two types of images,
Means for obtaining, for each identical pixel position, a combination of two types of pixel values spanning the two types of images;
Means for classifying the pixels based on the characteristic values by classifying the characteristic values of the combination of the pixel values according to a predetermined criterion;
An X-ray CT apparatus comprising means for separating pixels in at least one of the two types of images corresponding to the classification of the pixels. The X-ray CT apparatus according to claim 1, wherein the characteristic value is a ratio of the two types of pixel values. The X-ray CT apparatus according to claim 1, wherein the classification of the pixels is performed based on a reference set on a histogram of characteristic values of the combination of the pixel values. The characteristic value is indicated in a two-dimensional space having two coordinate axes corresponding to the two types of pixel values,
The X-ray CT apparatus according to claim 1, wherein the classification of the pixels is performed based on a criterion indicated on the two-dimensional space. The X-ray CT apparatus according to claim 1, wherein the pixel classification is performed by at least one of color overlay, gradation provision, opacity provision, and clipping. . The X-ray CT apparatus according to claim 5, wherein the color overlay, gradation provision, opacity provision, and clipping are switchable with each other. 5. The X-ray CT apparatus according to claim 3, wherein the classification of the pixels can be corrected. The classification correction is performed by determining suitability of the classification of the characteristic value according to a shape feature amount of the classified pixel group in the image space, and correcting based on the determination result. The X-ray CT apparatus according to claim 7. The X-ray CT apparatus according to claim 7, wherein the correction is performed by adjusting a reference used for the classification. The X-ray CT apparatus according to claim 1, wherein the image is a tomographic image or a 3D image in which tomographic images are arranged in the z direction. The shape feature amount is the number of segments (number of area numbers), sphericity, volume, surface area, average / maximum / minimum pixel value, average / maximum / minimum standard deviation value, three-dimensional ferret diameter, circumscribed cuboid volume fraction, Euler number The X-ray CT apparatus according to claim 8, wherein the X-ray CT apparatus is at least one of a primary / secondary moment and an approximate ellipsoidal diameter.

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