Plaque region extracting method and apparatus therefor

According to one embodiment, a plaque region extracting apparatus includes a blood vessel wall data extracting unit, an intermediate image data generating unit, an enhancement processing unit, a plaque extracting unit. The blood vessel wall data extracting unit extracts first image data including a blood vessel wall from image data acquired by imaging a subject including blood vessels. The intermediate image data generating unit filters an intermediate region in the first image data to generate intermediate second image data. The enhancement processing unit processes the difference between the first image data and the second image data to generate third image data. The plaque extracting unit extracts a plaque in the blood vessel on the basis of the third image data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2011-066796, filed Mar. 24, 2011, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a plaque region extracting method and an apparatus therefor for automatically extracting, for example, a plaque region within a blood vessel wall in a CT image.

BACKGROUND

For example, in the diagnosis of coronary arteries, the possibility of cardiac disease associated with plaques in the coronary blood vessel is diagnosed. When broken, the plaques in the coronary blood vessel block or extremely narrow the blood vessel, and induce serious ischemic heart disease. Thus, the coronary arteries are diagnosed. In the coronary artery diagnosis, a narrowed part of the coronary arteries is specified or a plaque in the coronary arteries which has not yet caused the narrowing of a blood vessel is specified from the CT image acquired by X-ray CT scanning. In the coronary artery diagnosis, the plaque amount in a coronary blood vessel wall is measured from a CT image acquired by X-ray CT scanning, and the risk of a coronary disease is diagnosed by the measured plaque amount. Under the present situation, a doctor observes the CT image, and manually extracts plaques, and then diagnoses the risk of a coronary disease in accordance with the amount of extracted plaques. The plaques are extracted by a technique which cuts the CT image by a preset threshold.

DETAILED DESCRIPTION

In general, according to one embodiment, a plaque region extracting apparatus includes a blood vessel wall data extracting unit, an intermediate image data generating unit, an enhancement processing unit, and a plaque extracting unit. The blood vessel wall data extracting unit extracts first image data including a blood vessel wall from image data acquired by imaging a subject including blood vessels. The intermediate image data generating unit filters an intermediate region in the first image data to generate intermediate second image data. The enhancement processing unit processes the difference between the first image data and the second image data to generate third image data. The plaque extracting unit extracts a plaque in the blood vessel on the basis of the third image data.

One embodiment of a plaque region extracting apparatus will be hereinafter described with reference to the drawings.

FIG. 1is a block configuration diagram of a medical image diagnostic apparatus1to which the plaque region extracting apparatus is applied. The medical image diagnostic apparatus1comprises a medical image pickup device2, a medical image storage device3, and a medical image processing device4. The medical image pickup device2, the medical image storage device3, and the medical image processing device4are connected to one another, for example, via a local area network (LAN)5. The medical image pickup device2is, for example, an X-ray CT scanner (hereinafter referred to as an X-ray CT scanner2).

The X-ray CT scanner2applies X-rays to a subject, detects the X-rays which have passed through the subject, acquires, for example, three-dimensional (3D) volume data for a CT image of the inside (part of interest) of the subject, and displays the 3D volume data CT image, for example, on a display. Parts of the subject include, for example, the coronary arteries of the heart, the lungs, and the stomach.

The medical image storage device3comprises a database. The 3D volume data for CT images acquired by the X-ray CT scanner2, for example, the 3D volume data for CT images of the coronary arteries of the heart are saved in the medical image storage device3. The 3D volume data for the CT images saved in the medical image storage device3are sent to the medical image processing device4via the LAN5.

The medical image processing device4reads the 3D volume data for the CT images saved in the medical image storage device3, subjects the read 3D volume data to image processing, and automatically extracts a plaque region within a blood vessel wall in the CT image. The extracted image is used to diagnose the possibility of, for example, cardiac disease associated with the plaque in the coronary blood vessel.

FIG. 2shows a block configuration diagram of the medical image processing device4. The medical image processing device4comprises a control unit11such as a central processing unit (CPU), a memory12such as a read only memory (ROM) and a random access memory (RAM), a display unit13, an operation unit14, a storage unit15, and a communication unit16.

The display unit13displays various images such as CT images. The operation unit14receives input operation from an operator. An image processing program and various kinds of data are stored in the storage unit15. The image processing program functions so that the control unit11subject the 3D volume data for a CT image acquired by the X-ray CT scanner2to image processing and the processed CT image is displayed on the display unit13. The communication unit16communicates with external devices. An image processing unit17processes various images of, for example, the 3D volume data for a CT image, and extracts a plaque region from the 3D volume data for the CT image. The control unit11, the memory12, the display unit13, the operation unit14, the storage unit15, the communication unit16, and the image processing unit17are electrically connected to one another via a bus line18.

In addition to the image processing program and the various kinds of data, a plaque region extracting program is stored in the storage unit15. The plaque region extracting program functions so that the control unit11extracts a plaque region from the 3D volume data for the CT image.

The plaque region extracting program is executed by the control unit11during the diagnosis of cardiac disease associated with plaques in the coronary blood vessel. The plaque region extracting program includes the following functions: a blood vessel wall data extracting function for extracting first image data for a blood vessel wall from the 3D volume data for the CT image acquired by imaging the subject including coronary blood vessels, an intermediate image data generating function for filtering an intermediate region in the first image data to generate intermediate second image data, an enhancement processing function for generating, from the difference between the first image data and the generated second image data, third image data in which image regions lower in pixel value than peripheral portions in the first image data are enhanced, and a plaque extracting function for finding a standard deviation from the pixel value of the first image data and extracting a plaque, for example, in the coronary arteries in accordance with levels of thresholds based on the standard deviation.

The standard deviation is a statistical analytic value that represents the variation of the pixel value.

The storage unit15is, for example, a magnetic disk device or a semiconductor disk device (flash memory).

In accordance with the image processing program and the various kinds of data stored in the storage unit15, the control unit11receives input operation from the operation unit14, controls, for example, the display unit13, the communication unit16, and the image processing unit17, and subjects the three-dimensional volume data for the CT image to image processing and displays the processed data on the display unit13.

The control unit11executes the plaque region extracting program stored in the storage unit15. Accordingly, the control unit11extracts first blood vessel wall image data from the 3D volume data for the CT image acquired by imaging the subject including coronary blood vessels in the image processing unit17. The control unit11filters an intermediate region in the first image data to generate intermediate second image data. From the difference between the first image data and the second image data, the control unit11generates third image data in which image regions lower in pixel value than peripheral portions in the first image data are enhanced. The control unit11finds a standard deviation from the pixel value of the first image data, and extracts a plaque, for example, in the coronary arteries in accordance with levels of thresholds based on the standard deviation.

For example, a startup program to be executed by the control unit11is stored in the memory12. The memory12also functions as a work area of the control unit11. The startup program is read and executed by the control unit11at the startup of the medical image processing device4.

The display unit13displays various images such as 2D images and 3D images in color. The display unit13is, for example, a liquid crystal display or a CRT display.

The operation unit14is operated by the operator for inputting. The operation unit14receives various input operations including, for example, the start of image display, the switch of images, and the change of settings. The operation unit14is an input device such as a mouse or a keyboard.

The communication unit16communicates with external devices via the LAN5, and is, for example, a LAN card or a modem. The external devices are, for example, the medical image pickup device2and the medical image storage device3.

The image processing unit17executes the plaque region extracting program stored in the storage unit15, and extracts, for example, a plaque in the coronary blood vessel from the 3D volume data for the CT image.

More specifically, the image processing unit17functions as a plaque extracting device, and reads the 3D volume data for the CT image saved in the medical image storage device3, for example, heart volume data. The image processing unit17uses the 3D volume data to cut out a region of interest, for example, a part of the coronary arteries from the read heart volume data. The image processing unit17extracts the center line of the coronary blood vessel from the cut coronary artery 3D volume data, and extracts first blood vessel wall image data from the coronary artery 3D volume data along the direction of the extracted center line. The image processing unit17filters an intermediate region in the extracted first image data to generate intermediate second image data. From the difference between the first image data and the second image data, the image processing unit17generates third image data in which image regions lower in pixel value than peripheral portions in the first image data are enhanced. The image processing unit17finds a standard deviation from the pixel value of the third image data, and extracts a plaque, for example, in the coronary arteries in accordance with levels of thresholds based on the standard deviation.

The image processing unit17generates curved planer reconstruction (CPR) images of sections from the cut coronary artery 3D volume data. The CPR images are, for example, slice images of image data for an A-A section, a B-B section, and a C section of a coronary blood vessel100, for example, as shown inFIG. 3. The A-A section is a section along the direction of a center line (blood flow direction) E of the blood vessel100. The B-B section is a section which extends along the direction of the center line E of the blood vessel100and which is perpendicular to the A-A section. The C section is a section perpendicular to the direction of the center line E of the blood vessel100.

FIG. 4Ashows a schematic diagram of sectional image data AD for the A-A section.FIG. 4Bshows a schematic diagram of sectional image data BD for the B-B section.FIG. 4Cshows a schematic diagram of sectional image data CD for the C section.FIG. 5shows an example of how to display, on the display unit13, the sectional image data AD for the A-A section, the sectional image data BD for the B-B section, and the sectional image data CD for the C section.FIG. 3toFIG. 5shows a plaque P and a calcified part K in the blood vessel100. J indicates a lumen, and L indicates a wall.

FIG. 6shows a specific functional block diagram in the image processing unit17. The image processing unit17comprises a blood vessel wall data extracting unit17-1, an intermediate image data generating unit17-2, an enhancement processing unit17-3, and a plaque extracting unit17-4.

The blood vessel wall data extracting unit17-1cuts out, for example, a portion of the coronary arteries from CT image heart volume data D shown inFIG. 5in the form of 3D volume data, and extracts two-dimensional (2D) image data (first image data) W for the coronary blood vessel wall from the 3D volume data.

FIG. 7shows an example of how the blood vessel wall image data W is displayed. ADW indicates blood vessel wall image data in the sectional image data AD for the A-A section shown inFIG. 5. BDW indicates blood vessel wall image data in the sectional image data BD for the B-B section shown inFIG. 5. CDW indicates blood vessel wall image data in the sectional image data BD for the C section shown inFIG. 5.

The intermediate image data generating unit17-2filters the intermediate region in the image data W for the coronary blood vessel wall, for example, in the blood vessel wall image data ADW corresponding to the A-A section, the blood vessel wall image data BDW corresponding to the B-B section, and the blood vessel wall image data CDW corresponding to the C section that are shown inFIG. 7, thereby generating intermediate image data (second image data) M, for example, shown inFIG. 8.

The intermediate region filtering includes the filtering of the intermediate region for all the regions of CT values of the blood vessel wall image data ADW, BDW, and CDW along the center line of the blood vessel100in the blood vessel wall image data ADW, BDW, and CDW. For example, a median filter is used for the filtering of the intermediate region.

InFIG. 8, ADM indicates intermediate image data in which the blood vessel wall image data ADW shown inFIG. 7is filtered. BDM indicates intermediate image data in which the blood vessel wall image data BOW shown inFIG. 7is filtered. CDM indicates intermediate image data in which the blood vessel wall image data CDW shown inFIG. 7is filtered.

The enhancement processing unit17-3generates differential image data (third image data) S in which image regions lower in pixel value than the peripheral portions are enhanced in accordance with the difference between the blood vessel wall image data ADW, BDW, and CDW shown inFIG. 7and the intermediate image data ADM, BDM, and CDM shown inFIG. 8.

More specifically, the enhancement processing unit17-3finds a difference of CT values between the blood vessel wall image data ADW, BDW, and CDW shown inFIG. 7and the intermediate image data ADM, BDM, and CDM shown inFIG. 8. From the found difference, the enhancement processing unit17-3generates the differential image data S shown inFIG. 9in which the image regions lower in CT value than the peripheral portions are enhanced. From the generated differential image data S, the enhancement processing unit17-3generates a differential value distribution (histogram) H shown inFIG. 10that indicates the number of pixels versus the difference of CT values.

In the differential image data S shown inFIG. 9, ADS indicates differential image data between the blood vessel wall image data ADW shown inFIG. 7and the intermediate image data ADM shown inFIG. 8. BDS indicates differential image data between the blood vessel wall image data BDW shown inFIG. 7and the intermediate image data BDM shown inFIG. 8. CDS indicates differential image data between the blood vessel wall image data CDW shown inFIG. 7and the intermediate image data CDM shown inFIG. 8.

The differential value distribution H shown inFIG. 10indicates the number of pixels versus the difference of CT values between the blood vessel wall image data ADW, BOW, and CDW shown inFIG. 7and the intermediate image data ADM, BDM, and CDM shown inFIG. 8. The difference of CT values is obtained, for example, by finding, over the entire image, the difference of CT values for the same coordinates of the blood vessel wall image data ADW and the intermediate image data ADM. The difference of CT values may otherwise be obtained from the number of pixels versus the difference of CT values between the blood vessel wall image data BDW and the intermediate image data BDM, or from the number of pixels versus the difference of CT values between the blood vessel wall image data CDW and the intermediate image data CDM.

The plaque extracting unit17-4finds an image noise standard deviation σ from CT values in a required part of the blood vessel100, for example, the beginning of the aorta. The plaque extracting unit17-4calculates thresholds −σ, −2σ, and −3σ by integrally multiplying levels of thresholds based on the found standard deviation σ, for example, the standard deviation σ by different numerical values. The plaque extracting unit17-4separates the difference of CT values in accordance with regions that can be sorted by the thresholds −σ, −2σ, and −3σ, thereby determining the certainty factor of the plaque region. The standard deviation σ of the image noise is found from the beginning of the aorta because the beginning of the aorta has a large volume and objects equal in CT value gather therein.

More specifically, the plaque extracting unit17-4sets the thresholds −σ, −2σ, and −3σ, and unconditionally determines, as the plaque region, a difference of CT values equal to or more than the threshold −3σ, that is, a difference of CT values included in a judgment range Q1of the CT values equal to or less than the threshold −3σ shown inFIG. 10.

The plaque extracting unit17-4determines, as the plaque region with a first condition, the difference of CT values included in a judgment range Q2between the threshold −3σ and the threshold −2σ. The first condition is to be adjacent to the plaque region which is unconditionally determined from the difference of CT values between the threshold −3σ and the threshold −2σ. Alternatively, the first condition is that an image portion in the coronary artery 3D volume data shown inFIG. 5corresponding to the difference of CT values included in the judgment range Q2has a given volume, for example, of 0.12 mm3or more.

In order to make a judgment with the first condition, the plaque extracting unit17-4labels the image portion corresponding to the difference of CT values included in the judgment range Q2, for example, the image portion in the three-dimensional volume data for the coronary arteries shown inFIG. 5.

The plaque extracting unit17-4determines, as the plaque region with a second condition, the difference of CT values included in the judgment range Q3between the threshold −2σ and the threshold −σ. The second condition is to be adjacent to the plaque region which is determined as described above.

In order to make a judgment with the second condition, the plaque extracting unit17-4labels the image portion corresponding to the difference of CT values included in the judgment range Q3, for example, the image portion in the three-dimensional volume data for the coronary arteries shown inFIG. 5.

When determining the certainty factor of the plaque region on the basis of the difference of CT values of the judgment ranges Q1, Q2, and Q3, the plaque extracting unit17-4excludes plaques having a wall thickness of a given value, for example, less than 1 mm, and also excludes outermost pixels in the wall. In other words, when determining the certainty factor of the plaque region on the basis of the difference of CT values of the judgment ranges Q1, Q2, and Q3, the plaque extracting unit17-4extracts plaques having a wall thickness of a given value, for example, 1 mm or more, and does not extract plaques in the outermost pixels in the wall.

FIG. 11shows plaque region candidates P and image noises N extracted by the threshold −σ from the differential image data S shown inFIG. 9.

FIG. 12shows results obtained by setting the thresholds −σ, −2σ, and −3σ in the plaque extracting unit17-4, separating the difference of CT values in accordance with the judgment ranges Q1, Q2, and Q3that can be sorted by the thresholds −σ, −2σ, and −3σ, and thereby determining plaques Ps from the plaque region candidates P.

Now, the plaque extracting operation by the apparatus having such a configuration is described.

The X-ray CT scanner2applies X-rays to a subject such as a human body to which a contrast media is injected, detects the X-rays which have passed through the subject, acquires, for example, 3D volume data for a CT image of a part of interest of the subject such as the coronary arteries of the heart, and displays the 3D volume data CT image, for example, on the display.

The medical image storage device3stores, in the database, the 3D volume data for CT images acquired by the X-ray CT scanner2, for example, the 3D volume data for CT images of the coronary arteries of the heart. The 3D volume data for the CT images saved in the medical image storage device3is sent to the medical image processing device4via the LAN5.

The medical image processing device4reads the 3D volume data for the CT images saved in the medical image storage device3, subjects the 3D volume data to image processing, and automatically extracts a plaque region within a blood vessel wall in the CT image. The image of the extracted plaque region is used to diagnose the possibility of, for example, cardiac disease associated with the plaque in the coronary blood vessel.

As basic processing, the image processing unit17subjects the 3D volume data for the heart to image cutting processing, calcification extracting processing, lumen (contrast media) extracting processing, external wall extracting processing, and plaque extracting processing.

More specifically, the image processing unit17reads the 3D volume data for the CT image saved in the medical image storage device3, for example, heart volume data. The image processing unit17cuts out 3D volume data for a region of interest, for example, a part of the coronary arteries from the heart volume data.

The image processing unit17acquires, for example, a CT value of the beginning of the aorta from the volume data for the coronary arteries, and extracts, as calcification (calcified plaque), a region having a CT value which is more than 1.2 times the CT value of the beginning of the aorta. In this case, the image processing unit17uses the fact that the CT value of the calcification is higher than the CT value of the contrast media, in particular, uses the CT value of the beginning of the aorta having a low partial volume effect.

From the 3D volume data for the cut part of the coronary arteries, the image processing unit17generates CPR images of sections, for example, the sectional image data AD for the A-A section, the sectional image data BD for the B-B section, and the sectional image data CD for the C section shown inFIG. 5. The image processing unit17sets a tube ROI (three-dimensional region of interest) around the center line from the sectional image data AD, BD, and CD, and extracts and removes calcification in this tube ROI.

The image processing unit17digitizes, for example, the sectional image data CD shown inFIG. 5, and scans in a direction toward the internal wall of the coronary blood vessel. The image processing unit17finds the center of gravity within the coronary blood vessel from the scanning results, and finds the center point of the coronary blood vessel based on the center of gravity.

The image processing unit17scans in the extending directions of the coronary blood vessels to generate sectional image data CD, and finds the center points of the coronary blood vessel from the sectional image data CD.

The image processing unit17finds a center line E of the coronary blood vessel by connecting the center points.

The plaque region extraction operation is described below with reference to a plaque region extracting flowchart shown inFIG. 13.FIG. 14shows the flow of image data processing in the plaque region extraction operation.

In step #1, the blood vessel wall data extracting unit17-1cuts out, for example, a portion of the coronary arteries from the CT image heart volume data D shown inFIG. 5in the form of 3D volume data. From the cut 3D volume data, the blood vessel wall data extracting unit17-1extracts 2D image data W for the coronary blood vessel wall shown inFIG. 7, that is, the blood vessel wall image data ADW, BDW, and CDW.

In step #2, the intermediate image data generating unit17-2filters the intermediate region in the image data W for the coronary blood vessel wall, for example, in the blood vessel wall image data ADW, BDW, and CDW shown inFIG. 7along the direction of the center line E of the blood vessel100, thereby generating the intermediate image data M, for example, shown inFIG. 8. For example, the median filter is used for the filtering of the intermediate region.

In step #3, the enhancement processing unit17-3finds a difference of CT values between the blood vessel wall image data ADW, BDW, and CDW shown inFIG. 7and the intermediate image data ADM, BDM, and CDM shown inFIG. 8. From the found difference, the enhancement processing unit17-3generates the differential image data S shown inFIG. 9in which the image regions lower in pixel value than the peripheral portions are enhanced. The differential image data S is generated by subtracting the intermediate image data ADM, BDM, and CDM shown inFIG. 8from the blood vessel wall image data ADW, BDW, and CDW shown inFIG. 7, respectively.

From the differential image data S, the enhancement processing unit17-3generates the differential value distribution H shown inFIG. 10that indicates the number of pixels versus the difference of CT values.

In step #4, the plaque extracting unit17-4finds the image noise standard deviation σ from CT values in a required part of the blood vessel100, for example, the beginning of the aorta. The plaque extracting unit17-4calculates the thresholds −σ, −2σ, and −3σ by integrally multiplying levels of thresholds based on the standard deviation σ, for example, the standard deviation σ by different numerical values, respectively. The plaque extracting unit17-4separates the difference of CT values in accordance with regions that can be sorted by the thresholds −σ, −2σ, and −3σ, thereby determining the certainty factor of the plaque region.

More specifically, the plaque extracting unit17-4unconditionally determines, as the plaque region, the difference of CT values equal to or more than the threshold −3σ in the differential value distribution H shown inFIG. 10, that is, the difference of CT values included in the judgment range Q1of the CT values equal to or less than the threshold −3σ shown inFIG. 10.

The plaque extracting unit17-4determines, as the plaque region with a first condition, the difference of CT values included in the judgment range Q2between the threshold −3σ and the threshold −2σ. The first condition is to be adjacent to the plaque region which is unconditionally determined from the difference of CT values between the threshold −3σ and the threshold −2σ. Alternatively, the first condition is that an image portion in the coronary artery 3D volume data shown inFIG. 5corresponding to the difference of CT values included in the judgment range Q2has a given volume, for example, of 0.12 mm3or more.

The plaque extracting unit17-4determines, as the plaque region with a second condition, the difference of CT values included in the judgment range Q3between the threshold −2σ and the threshold −σ. The second condition is to be adjacent to the already determined plaque region.

When determining the certainty factor of the plaque region on the basis of the difference of CT values of the judgment ranges Q1, Q2, and Q3, the plaque extracting unit17-4excludes plaques having a wall thickness of a given value, for example, less than 1 mm, and also excludes outermost pixels in the wall. In other words, when determining the certainty factor of the plaque region on the basis of the difference of CT values of the judgment ranges Q1, Q2, and Q3, the plaque extracting unit17-4extracts plaques having a wall thickness of a given value, for example, 1 mm or more, and does not extract plaques in the outermost pixels in the wall.

As a result of the plaque extraction, the plaque Ps is determined from the plaque region candidates P, as shown inFIG. 12. It is apparent from the contrast with the result of extraction using the threshold −σ shown inFIG. 11that the determined plaque Ps are free of the image noises N, and the plaques Ps having high accuracy are only extracted.

Simulation results of the plaque extraction operation by the present device is described.

A numerical phantom shown inFIG. 15is used in the plaque extraction simulation. There are numerical phantoms without image noises and with image noises. The threshold is, for example, 60 HU. The plaque P is at, for example, 30 HU. The calcified part K is at 400 HU. The lumen J is at 350 HU. The wall L is at 130 HU.

A simulator reconstructs an image from the numerical phantom to generate raw data for the coronary blood vessel, and uses this raw data to run the plaque extraction simulation. As a result of the simulation, the plaque Ps can be extracted, for example, as shown inFIG. 16AandFIG. 16B.FIG. 16Aincludes no image noises.FIG. 16Bincludes image noises.

Thus, according to the embodiment described above, the blood vessel wall image data W are extracted, for example, from the coronary artery 3D volume data cut out from the CT image heart volume data D. The intermediate regions in the blood vessel wall image data W are filtered to generate the intermediate image data M. The differential image data S in which the image regions lower in CT value than the peripheral portions are enhanced is generated from the difference of CT values between the blood vessel wall image data W and the intermediate image data M. The differential value distribution H that indicates the number of pixels versus the difference of CT values is generated from the differential image data S, and the thresholds −σ, −2σ, and −3σ are calculated from the image noise standard deviation σ found from CT values in, for example, the beginning of the aorta. The difference of CT values is separated in accordance with regions that can be sorted by the thresholds −σ, −2σ, and −3σ, and the certainty factor of the plaque region is thereby determined. This ensures that the plaque Ps can be extracted, and regions other than the plaque Ps are not extracted as plaques. Thus, the extracted plaques can be matched to the plaques in the coronary blood vessel wall extracted by doctor's interpretation.

In the plaque extraction, the difference of CT values included in the judgment range Q1of the CT value difference equal to or less than the threshold −3σ in the differential value distribution H shown inFIG. 10is unconditionally determined as the plaque region. The difference of CT values included in the judgment range Q2between the threshold −3σ and the threshold −2σ is determined as the plaque region with the first condition. The difference of CT values included in the judgment range Q3between the threshold −2σ and the threshold −σ is determined as the plaque region with the second condition. This allows higher accuracy of the extraction of the plaque Ps.

The accuracy of the extraction of the plaque Ps can also be increased by excluding plaques having a wall thickness of a given value, for example, less than 1 mm and also excluding the outermost pixels in the wall in the difference of CT values of the judgment ranges Q1, Q2, and Q3.