Patent Publication Number: US-9430855-B2

Title: X-ray CT apparatus and optimal cardiac phase determining method

Description:
TECHNICAL FIELD 
     The present invention relates to an X-ray CT apparatus and the like to obtain a diagnostic image by irradiating an object with X-rays and in particular, to an X-ray CT apparatus and the like to determine an optimal cardiac phase in image reconstruction in cardiac scanning. 
     BACKGROUND ART 
     In cardiac scanning of the X-ray CT apparatus, in order to obtain a diagnostic image, a cardiac phase in which the motion of the heart is the slowest needs to be timely imaged. In recent years, scanning speed has been improved due to an increase in the rotation speed of the X-ray CT apparatus, an increase in the number of detectors arranged in rows, and the like. However, since the cross-sectional position of the coronary artery of the heart moves about 5 mm to 10 mm in 0.1 second, it is difficult to obtain a clear image without an afterimage even in the current situation. Therefore, it is necessary to create a diagnostic image by collecting the data of the heart rhythm in all cardiac phases and determining an optimal cardiac phase with slowest rhythm by data analysis. 
     As a technique of determining such an optimal cardiac phase, PTL 1 discloses a method of calculating the variation in the entire heart or a specific location using the data of adjacent cardiac phases and determining a cardiac phase with least motion as an optimal cardiac phase. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP-A-2003-204961 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, it is known that the heart rhythm differs depending on a location (for example, the right coronary artery and the left coronary artery). For this reason, in the method disclosed in PTL 1, a large motion occurred locally even in a cardiac phase in which it is estimated that the entire heart moves the slowest. In addition, in some cases, there has been a case where the motion of the entire heart or the motion of a local region other than the region of interest is large in a cardiac phase in which it is estimated that the local region of interest moves the slowest. In addition, in the method disclosed in PTL 1, there has been a case where it is difficult to specify the optimal cardiac phase since the rhythm of each region of the heart being significantly different is not taken into consideration. In particular, it has been difficult to specify the optimal cardiac phase for the rhythm of the unhealthy heart. 
     The present invention has been made in view of the above-described problems, and it is an object of the present invention to provide an X-ray CT apparatus and the like that can specify an optimal cardiac phase in a wide variety of cases. 
     Solution to Problem 
     In order to achieve the above-described object, a first invention is an X-ray CT apparatus including an X-ray irradiation unit that irradiates X-rays from periphery of an object, an X-ray detection unit that detects information of X-rays transmitted through the object, an electrocardiographic information acquisition unit that acquires electrocardiographic information of the object, an image creation unit that creates an image of the object from the X-ray information and the electrocardiographic information, and a display unit that displays the image. The X-ray CT apparatus includes: a tomographic data creation unit that creates a plurality of pieces of tomographic data with different cardiac phases from the X-ray information and the electrocardiographic information; a region data generation unit that generates region data by extracting a predetermined region for each piece of the tomographic data; a variation distribution calculation unit that calculates a variation distribution within the tomographic data by calculating a variation between cardiac phases regarding the region data; and a degree-of-harmony calculation unit that calculates a degree of harmony, which is an index indicating whether or not rhythms at respective locations of a target portion of the object are in harmony with each other, on the basis of the variation distribution. 
     A second invention is an optimal cardiac phase determining method in an X-ray CT apparatus including an X-ray irradiation unit that irradiates X-rays from periphery of an object, an X-ray detection unit that detects information of X-rays transmitted through the object, an electrocardiographic information acquisition unit that acquires electrocardiographic information of the object, an image creation unit that creates an image of the object from the X-ray information and the electrocardiographic information, and a display unit that displays the image. The optimal cardiac phase determining method includes: creating a plurality of pieces of tomographic data with different cardiac phases from the X-ray information and the electrocardiographic information; generating region data by extracting a predetermined region for each piece of the tomographic data, calculating a variation distribution within the tomographic data by calculating a variation between cardiac phases regarding the region data; and calculating a degree of harmony, which is an index indicating whether or not rhythms at respective locations of a target portion of the object are in harmony with each other, on the basis of the variation distribution. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide an X-ray CT apparatus and the like capable of specifying the optimal cardiac phase in a wide variety of cases. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing the overall configuration of an X-ray CT apparatus. 
         FIG. 2  is a diagram showing components of the X-ray CT apparatus. 
         FIG. 3  is a flow chart showing an optimal cardiac phase determining process in a first embodiment. 
         FIG. 4  is a diagram showing an example of electrocardiographic waveform data. 
         FIG. 5  is a flow chart showing a process of extracting a region of interest. 
         FIG. 6  is a flow chart showing a variation calculation process. 
         FIG. 7  is a flow chart showing a process of calculating the degree of harmony. 
         FIG. 8  is a schematic diagram of a graph of the variation and the degree of harmony. 
         FIG. 9  is a schematic diagram of a screen that displays an optimal cardiac phase image. 
         FIG. 10  is a flow chart showing a process of calculating the degree of harmony in a second embodiment. 
         FIG. 11  is a flow chart showing a process of calculating the degree of harmony in a third embodiment. 
         FIG. 12  is a schematic diagram of a screen for displaying the association between blocks. 
         FIG. 13  is a schematic diagram of a screen for instructing the integration or separation of blocks by change in the threshold value. 
         FIG. 14  is a schematic diagram of a screen for instructing the change in the display range by change in the upper and lower limits of the threshold value. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     First, referring to  FIGS. 1 and 2 , the configuration of an X-ray CT apparatus according to all embodiments will be described. 
     As shown in  FIG. 1 , the X-ray CT apparatus is configured to include an scanning unit  1 , an operating unit  2 , and an electrocardiograph  3 , and the like. The scanning unit  1  includes a gantry  100  having a scanner body thereinside and a bed  101 . The operating unit  2  operates and controls the scanning unit  1 . In addition, the operating unit  2  performs input of the scanning conditions, image processing, and the like. The electrocardiograph  3  acquires the electrocardiographic waveform of an object  4 . 
     As shown in  FIG. 2 , the gantry  100  is configured to include an X-ray generator  102  that irradiates X-rays from the periphery of the object  4 , a collimator device  104  that narrows the range of the X-ray flux generated from the X-ray generator  102 , an X-ray detector  103  that detects X-rays transmitted through the object  4 , a high voltage generator  105  that applies a high voltage to the X-ray generator  102 , a data acquisition system  106  that collects data detected by the X-ray detector  103 , a driving device  107  that rotates a scanner around the object  4 , and the like. In addition, the data acquisition system  106  also collects electrocardiographic information from the electrocardiograph  3  simultaneously. 
     The operating unit  2  is configured to include an input/output device  201 , a calculation device  202 , a central control device  200 , and the like. 
     The input/output device  201  is configured to include a display device  211  that displays data such as an image, an input device  212  used when an operator inputs scanning conditions and the like, a storage device  213  that stores data required for scanning, such as a program and a device parameter, and the like. 
     The calculation device  202  is configured to include an image reconstruction device  221  that creates a reconstructed image of the object  4  on the basis of the data obtained from the scanning unit  1 , an image processing device  222  that performs analysis of image data, and the like. 
     The central control device  200  controls each device of the scanning unit  1  and the operating unit  2  according to an operation instruction from the operator. 
     As scanning using an X-ray CT apparatus, there are rotation scanning in which scanning is performed while the X-ray generator  102  and the data acquisition system  106  are rotating within the gantry  100  and stationary scanning in which scanning is performed in a state where the X-ray generator  102  and the data acquisition system  106  stand still within the gantry  100 . Tomographic scanning for obtaining a tomographic image of the object  4  is based on the rotation scanning. In addition, scanogram imaging for determining the scanning position of the tomographic scanning is based on the stationary scanning. In addition, the scanning trajectory of the tomographic scanning may be any of a circular trajectory, a spiral trajectory, and a combination of a circular trajectory and a spiral trajectory, and is not particularly limited. 
     The X-ray CT apparatus creates and displays an image of the object  4  on the basis of the X-ray information of the object  4  detected by the scanning unit  1  and the electrocardiographic information of the object  4  acquired by the electrocardiograph  3 . In particular, the X-ray CT apparatus creates and displays an image of the object  4  by collecting the data of all cardiac phases and determining the optimal cardiac phase with the slowest heart rhythm by data analysis. 
     [First embodiment] 
     Hereinafter, a first embodiment will be described with reference to  FIGS. 3 to 9 . In particular, the flow of the optimal cardiac phase determining process in the first embodiment will be described with reference to  FIG. 3 , and details of each process will be described while appropriately referring to other diagrams. Hereinafter, a case where an image is used as tomographic data will be described. In addition, it is possible to achieve the object of the present invention by the same processing even if a sinogram is used as tomographic data. The sinogram is obtained by arranging projection data in order for each projection angle, and the horizontal axis indicates the position of the X-ray detector  103  and the vertical axis indicates the projection angle. 
     Hereinafter, it is assumed that “images” include not only an image, which is displayed on the display device  211  to be visualized, but also an image, which is stored in an internal memory of the calculation device  202  as a group of pixel values. 
     &lt;Step S 11 : Cardiac Scanning&gt; 
     First, a waveform collection portion of the electrocardiograph  3  is attached to the object  4 , so that electrocardiographic information can be collected. Then, a contrast agent is administered to perform cardiac scanning using the scanning unit  1 , thereby collecting transmitted X-ray data (X-ray information) and electrocardiographic waveform data  5  (electrocardiographic information: refer to  FIG. 4 ). In addition, a contrast agent may not be administered depending on the scanning purposes. 
       FIG. 4  shows the typical electrocardiographic waveform data  5  obtained by the electrocardiograph  3 . The electrocardiographic waveform is a periodic waveform according to the heart rhythm. When creating images of a plurality of cardiac phases, a relative method may be used or an absolute method may be used as a method of expressing the time phase of the heart (cardiac phase). As the relative method, there is a method of expressing the relative position of the time phase as a percentage with an R-R interval as a reference. 
     As the absolute method, there is a method of expressing the time phase before and after an R wave as a time with the position of the R wave as a reference. 
     &lt;Step S 12 : Creation of Tomographic Images of a Plurality of Cardiac Phases&gt; 
     The image reconstruction device  221  reconstructs an image in each cardiac phase using transmitted X-ray data, thereby creating reconstructed images of a plurality of cardiac phases. Here, a plurality of cardiac phases are assumed to be included in the single R-R interval. 
     &lt;Step S 13 : Extraction of a Region of Interest&gt; 
     The image processing device  222  extracts a region of interest (contrast region or the like) on the basis of image information for each reconstructed image of the plurality of cardiac phases.  FIG. 5  shows the details of a process of extracting the region of interest. 
     As shown in  FIG. 5 , first, the image processing device  222  extracts a candidate region of interest by replacing the pixel value of a pixel, which has a CT value equal to or less than a threshold value, with the threshold value by threshold value processing (step S 21 ). 
     Then, the image processing device  222  extracts a region of interest as a region-of-interest image by removing a noise region, such as an isolated pixel (pixel whose surrounding pixels do not have pixel values) or a small region (small region compared with the area of the region of interest), from the candidate region of interest by noise removal processing (step S 22 ). 
     &lt;Step S 14 : Calculation of a Variation&gt; 
     The image processing device  222  calculates a variation for each region-of-interest image of the plurality of cardiac phases.  FIG. 6  shows the details of a process of calculating the variation. 
     As shown in  FIG. 6 , first, the image processing device  222  generates a difference image between cardiac phases by taking the amount of difference of each pixel for region extraction images of adjacent cardiac phases (step S 31 ). The amount of difference may be a difference value including a negative value (a previous cardiac phase and a later cardiac phase are distinguished), or may be a positive value, such as an absolute value or a square value of the difference value. For example, assuming that the pixel value of the previous cardiac phase is p i  and the pixel value of the later cardiac phase is p j , the amount of difference may be any of (p j −p i ), |p j −p i |, and (p j −p i ) 2 . In the following explanation, the amount of difference is assumed to be a positive value in order to make the explanation easily understood. 
     Then, the image processing device  222  performs weighting processing on the amount of difference (step S 32 ). In the weighting processing on the amount of difference, weighting is performed separately for a case where the target pixel itself moves in a different image between cardiac phases and a case where a peripheral pixel of the target pixel moves. 
     For example, it is assumed that the pixel position of a target pixel is (x, y) and the pixel position of a peripheral pixel of the target pixel is (x′, y′) (where x′≠x and y′≠y). 
     The case where the target pixel itself moves is a case where (x, y) in the region-of-interest image of the previous cardiac phase moves to (x′, y′) in the region-of-interest image of the later cardiac phase. 
     In addition, the case where the peripheral pixel moves is a case where (x′, y′) in the region-of-interest image of the previous cardiac phase moves to (x, y) in the region-of-interest image of the later cardiac phase. 
     Hereinafter, variation calculation processing according to variation factors in step S 32  will be described in detail. 
     First, the image processing device  222  labels a pixel, which has an amount of difference equal to or greater than the threshold value, in a difference image between cardiac phases. Here, the threshold value may be a value (static value) set in advance in consideration of the extraction accuracy of the region of interest or an organ and tissue to which focus is given, or may be a value (dynamic value) based on the distribution of the amount of difference in a difference image between cardiac phases generated in step S 31 . 
     Then, the image processing device  222  determines the labeled pixel as a target pixel one by one, and performs iteration processing to be described later. In addition, the following processing may be performed not only using a single pixel as a target pixel but also using a plurality of pixels collectively as a target region. 
     In the iteration processing, the image processing device  222  gives a weighting to the difference between the target pixel and the labeled peripheral pixel (for example, a pixel closest to the target pixel) or the inter-pixel distance difference between the target pixel and the peripheral pixel, and calculates the result as a variation. In addition, the following processing may be performed not only using a single pixel as a peripheral pixel but also using a plurality of pixels collectively as a peripheral region. 
     Specifically, the image processing device  222  increases a weighting (for example, makes a weighting larger than the reference value) as a difference between the amount of difference of the target pixel and the amount of difference of the peripheral pixel decreases and the inter-pixel distance between the target pixel and the peripheral pixel increases. This case is equivalent to the “case where the target pixel itself moves” described above. 
     On the other hand, the image processing device  222  decreases a weighting (for example, makes a weighting smaller than the reference value) as a difference between the amount of difference of the target pixel and the amount of difference of the peripheral pixel increases and the inter-pixel distance between the target pixel and the peripheral pixel decreases. This case is equivalent to the “case where the peripheral pixel moves” described above. 
     Here, although it is necessary to determine the reference value of the weighting, the reference value of the weighting may be a static value or may be a dynamic value similarly to the threshold value of the labeling described above. 
     In addition, for a pixel that is not labeled, the image processing device  222  may calculate a lower variation than for a labeled pixel without considering the amount of difference of the peripheral pixel or the inter-pixel distance between the target pixel and the peripheral pixel, or may not calculate a variation assuming that there has been no variation. 
     In addition, in the variation calculation processing according to variation factors, it is not necessary to accurately determine whether the target pixel has moved or the peripheral pixel has moved. First of all, it is not possible to accurately determine in which direction and by which amount each cell of the object  4  has moved. In addition, in view of the purpose of the present invention, there is no problem even if the accuracy of the variation calculation processing according to variation factors is slightly low. 
     Then, the image processing device  222  generates a variation distribution image indicating the distribution of the variation calculated in S 32  (step S 33 ).  FIG. 9  shows an example of a variation distribution image  11 . Details of  FIG. 9  will be described later. 
     Then, the image processing device  222  calculates the sum of the variation of the entire region of interest as a region-of-interest variation on the basis of the variation distribution image generated in S 33  (step S 34 ). 
     In addition, the region-of-interest variation may be a weighted average value in the region of interest or the like. 
     &lt;Step S 15 : Calculation of the Degree of Harmony&gt; 
     The image processing device  222  calculates the degree of harmony of each cardiac phase using a variation distribution image.  FIG. 7  shows the details of a process of calculating the degree of harmony. 
     Here, the degree of harmony is an index indicating whether or not the rhythms at respective locations of a target portion (for example, the heart) of the object  4  are in harmony with each other. In order to conform to this meaning, the image processing device  222  calculates a high value of the degree of harmony relevant to the cardiac phase when individual pixels move similarly, and calculates a low value of the degree of harmony relevant to the cardiac phase when individual pixels move apart. 
     The present invention determines an optimal cardiac phase in consideration of not only the entire motion but also the local motion on the basis of the findings that (1) there is a place of large motion locally even in a cardiac phase estimated that the entire heart moves most slowly, (2) motion of the entire heart or motion of a local region other than a region of interest may be large in a cardiac phase estimated that a local region of interest moves most slowly, and (3) influence of these phenomena on diagnosis cannot be neglected especially for the rhythm of the unhealthy heart. That is, in the present invention, the optimal cardiac phase is determined using at least the degree of harmony described above. 
     As shown in  FIG. 7 , as first division processing, the image processing device  222  divides a region of interest into blocks according to the magnitude of variation in the variation distribution image (step S 41 ). Here, the image processing device  222  divides a region of interest into several blocks on the basis of the threshold value of the static or dynamic value. 
     As a simplest example, the image processing device  222  divides a region of interest into two blocks of a region with a large variation and a region with a small variation. 
     Then, the image processing device  222  calculates a block variation of each block (step S 42 ). The block variation may be any of the sum of variations of all pixels in a block, the weighted average value, and the maximum value. 
     Then, the image processing device  222  calculates the degree of harmony for each cardiac phase (step S 43 ). 
     For example, the image processing device  222  calculates the degree of harmony H of a region of interest C using the following Expression. 
     
       
         
           
             
               
                 
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     Here, a i  is an amount relevant to the area of each block, and u i  is an amount relevant to the variation of each block. It is preferable that a i  be proportional to the area of a block, and it is preferable that u i  be inversely proportional to the variation of a block. As a simplest example, H=S×t is satisfied assuming that S is the area of a region of interest and t is an inverse of a region-of-interest variation. 
     In addition, when calculating the degree of harmony, the image processing device  222  may set the reference area and the reference block variation to normalize or score a and u in advance. In addition, when calculating the degree of harmony, the image processing device  222  may perform weighting in consideration of the block shape for the area. 
     In addition, the image processing device  222  may calculate the degree of harmony H using Expressions (2) to (5) shown below as specific examples of Expression (1). 
     
       
         
           
             
               
                 
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     In Expressions (2) to (5), S i  is the area of a block, and t i  is a block variation. 
     Expression (2) is the sum of (area of a block/block variation). 
     Expression (3) is a maximum value of the area of a block/(sum of the areas of blocks). 
     Expression (4) is the area of a block having a minimum block variation/(sum of the areas of blocks). 
     Expression (5) is a maximum value of (area of a block/block variation). 
     In addition, in processing to be described later, the image processing device  222  may use the inverse of the degree of harmony or a difference of the degree of harmony from a predetermined value as a degree of in harmony. The following explanation will be given on the assumption that the image processing device  222  uses the degree of harmony. 
       FIG. 8  shows a region-of-interest variation graph  6  in a solid line and a degree-of-harmony graph  7  in a dashed line.  FIG. 8  is a graph in which the cardiac phase is on the horizontal axis and the variation or the degree of harmony is on the vertical axis. 
     As can be seen from  FIG. 8 , the region-of-interest variation graph  6  has valleys (minimum values) in two places of cardiac phases  8  and  9 . Since there is almost no difference between the region-of-interest variations of the cardiac phases  8  and  9 , it is difficult to determine which of the cardiac phases  8  and  9  is to be selected as an optimal cardiac phase. In particular, in the case of a person having a heart disease (for example, a person with fast cardiac motion), a clear valley of the region-of-interest variation is not present, or the valley of the region-of-interest variation is present at a location different from a healthy person. 
     In contrast, when the degree-of-harmony graph  7  is viewed, the degree-of-harmony graph  7  has mountains (maximum values) in two places. Since a difference between the degree of harmony of the cardiac phase  8  and the degree of harmony of the cardiac phase  9  is clear, it can be determined that it is preferable to select the cardiac phase  9  as an optimal cardiac phase. 
     &lt;Step S 16 : Optimal Cardiac Phase Determining Process&gt; 
     The image processing device  222  determines an optimal cardiac phase on the basis of the variation or the degree of harmony. As a simplest example, the image processing device  222  determines a cardiac phase, in which the region-of-interest variation is small and the degree of harmony is large, as an optimal cardiac phase. 
     In addition, the processing of step S 16  is not essential. For example, the image processing device  222  may display the graph shown in  FIG. 8  on the display device  211 , and the user may determine the optimal cardiac phase while referring to the graph displayed on the display device  211 . 
     &lt;Step S 17 : Display of an Optimal Cardiac Phase Image&gt; 
     The image reconstruction device  221  generates an optimal cardiac phase image, which is a reconstructed image in the optimal cardiac phase, and displays it on the display device  211 .  FIG. 9  shows a display example of the optimal cardiac phase and the like. 
     In  FIG. 9 , an optimal cardiac phase image  10 , a variation distribution image  11 , a region-of-interest variation and degree-of-harmony graph  12 , and an optimal cardiac phase  13  are schematically shown. In addition, the variation distribution image  11  shows places where the variations are different due to differences in the pattern or differences in shades of gray scale. 
     The optimal cardiac phase image  10  is generated in step S 17 . The variation distribution image is generated in step S 33 . The region-of-interest variation and degree-of-harmony graph  12  is the same as in  FIG. 8 . The optimal cardiac phase  13  is determined in step S 16 . 
     As described above, the optimal cardiac phase may be automatically determined by the image reconstruction device  221 , or may be determined through the input device  212  by the user. 
     When the user determines the optimal cardiac phase, the optimal cardiac phase is determined by moving a linear slide  14 , which is displayed on the region-of-interest variation and degree-of-harmony graph  12 , to the position of the desired cardiac phase. In this case, the image reconstruction device  221  may follow the movement of the linear slide  14  to update the display of the optimal cardiac phase image  10 , the variation distribution image  11 , and the optimal cardiac phase  13 . 
     As described above, the X-ray CT apparatus according to the first embodiment calculates the degree of harmony (index indicating whether or not the rhythms at respective locations of the heart are in harmony with each other) for each cardiac phase, and determines the optimal cardiac phase using the degree of harmony or displays the degree of harmony so that the user determines the optimal cardiac phase. 
     Thus, the X-ray CT apparatus according to the first embodiment can specify the cardiac phase, which is slow and in harmony, in consideration of the uniformity of the rhythm at each location of the heart. As a result, it is possible to provide an optimal diagnostic image. This will be described more specifically. In the related art, an image in which the total variation is minimal is selected as the optimal cardiac phase image (=diagnostic image). However, there has been a case where a diagnostic portion has an image quality, which is not suitable for diagnosis, since the uniformity of the rhythm at each location of the heart is not taken into consideration. On the other hand, since the X-ray CT apparatus according to the first embodiment determines the optimal cardiac phase on the basis of the degree of harmony, the uniformity of the rhythm at each location of the heart is taken into consideration. Therefore, in the first embodiment, the variation of the entire image may not be minimal, but there is no case where a diagnostic portion has an image quality unsuitable for diagnosis. 
     In addition, even if there is a plurality of time phases that has the same degree of variation, it is possible to determine the optimal cardiac phase. In addition, since a variation distribution image is displayed, a user can set reconstruction conditions (for example, weighting in a viewing direction) suitable for the variation. 
     In addition, although the case where the degree of harmony in tomographic data (two-dimensional region) is calculated has been described above, the application range of the present invention is not limited to the two-dimensional region, and can be extended to a three-dimensional region. In the case of a three-dimensional region, the X-ray CT apparatus calculates the variation and the degree of harmony in consideration of a variation in the axial direction of a plurality of cross-sections. 
     [Second embodiment] 
     The first embodiment is suitable for a case where the contrast ratio of a contrast region and a non-contrast region is high as in contrast scanning since the correlation of the variation in the entire region of interest is taken into consideration. In a second embodiment, a preferred form in cardiac scanning in which no contrast medium is administered will be described. 
     Processing of an X-ray CT apparatus according to the second embodiment is the same as that in the first embodiment except for the details (refer to  FIG. 10 ) of processing of step S 15 , and repeated explanation thereof will be omitted. In the second embodiment, however, a region of interest is extracted on the basis of a CT value corresponding to a real portion instead of the CT value corresponding to the contrast region in step S 13 , and is set as a region-of-interest image. 
     Details of processing of step S 15  in the second embodiment will be described with reference to  FIG. 10 . 
     As second division processing, the image processing device  222  divides a region-of-interest image mechanically (step S 51 ). Here, the second division processing is division processing in which the magnitude of variation is not taken into consideration. In the second division processing, an overlapping region is allowed, and the region-of-interest image is divided into a plurality of blocks of three or more blocks. The shape of a block may be a circle or may be a rectangle. As a simplest example, the image processing device  222  divides a region-of-interest image into grid-like blocks. 
     In addition, the image processing device  222  calculates a provisional block variation for each block by the same processing as in the first embodiment. 
     Then, the image processing device  222  calculates a correlation coefficient of variation between blocks using the provisional block variation (step S 52 ). In addition, any value may be used as long as it indicates the degree of association between blocks on the basis of the variation between blocks, without being limited to the correlation coefficient. 
     Then, as section arrangement processing, the image processing device  222  re-classifies blocks, which have a correlation coefficient equal to or greater than the threshold value, into the same block (step S 53 ). The image processing device  222  continues the processing of re-classification, for example, until the number of blocks becomes 2 or less. 
     Then, the image processing device  222  calculates a block variation of each of arranged blocks by the same processing as in the first embodiment (step S 54 ). 
     Then, the image processing device  222  calculates the degree of harmony in consideration of the block variation and the block area of each of the arranged blocks by the same processing as in the first embodiment (step S 55 ). 
     Also in the second embodiment, the same effects as in the first embodiment are obtained. In addition, in the second embodiment, the region-of-interest image is mechanically divided into blocks. Accordingly, even if the contrast of a region of interest and a peripheral region is low as in cardiac scanning in which no contrast medium is used, it is possible to specify the slow and harmonious cardiac phase in consideration of the correlation of the motion within a cross-section of the heart. 
     [Third embodiment] 
     The first and second embodiments are suitable for a case where the scanning result is good. In a third embodiment, a case where contrast scanning is not good and contrast spots and the like cannot be neglected will be described. In addition, the third embodiment can also be applied to a case where the extraction of a contrast region is not good. 
     Processing of an X-ray CT apparatus according to the third embodiment is the same as that in the first embodiment except for the details (refer to  FIG. 11 ) of processing of step S 15 , and repeated explanation thereof will be omitted. In the third embodiment, however, in step S 13 , a good contrast region and a poor contrast region including a region of contrast spots or a region of poor extraction are extracted, and a region-of-interest image in which the good contrast region and the poor contrast region are distinguished is generated. 
     Details of processing of step S 15  in the third embodiment will be described with reference to  FIG. 11 . 
     The image processing device  222  performs first division processing on the good contrast region as in step S 41  in the first embodiment (step S 61 ). 
     Then, the image processing device  222  performs second division processing on the poor contrast region as in step S 51  in the second embodiment (step S 62 ). 
     Then, the image processing device  222  calculates a correlation coefficient of variation between a block of the good contrast region and a block of the poor contrast region (step S 63 ). 
     Then, the image processing device  222  performs block arrangement by integrating a poor contrast block, which has a correlation coefficient equal to or greater than a threshold value, with a good contrast block (step S 64 ). 
     Then, the image processing device  222  calculates the block area and the block variation after block arrangement (step S 65 ). 
     Then, the image processing device  222  calculates the degree of harmony in consideration of the block area and the block variation after block arrangement by the same processing as in the first embodiment (step S 66 ). 
     Also in the third embodiment, the same effects as in the first embodiment are obtained. In addition, in the third embodiment, processing suitable for each of the good contrast region and the poor contrast region is performed. Accordingly, even if a poor contrast region is included, the influence of motion depending on the position of the heart can be taken into consideration. As a result, it is possible to determine the cardiac phase that is slow and in harmony. 
     [Fourth embodiment] 
     In a fourth embodiment, details of the interface of an input and output screen will be described with reference to  FIGS. 12 to 14 . In the fourth embodiment, the threshold value of the division processing in step S 41  of the first embodiment is changed to correct the division processing. 
       FIG. 12  schematic shows a screen for displaying the association between blocks. In  FIG. 12 , ellipses having symbols, such as “A 1 ” and “A 2 ”, thereinside show regions of respective blocks. In addition, a straight line (connecting line) connecting the ellipses is given to blocks that are integrated when the threshold value is increased by one step. In addition, the elliptical insides of the blocks that are integrated when the threshold value is increased by one step are shown by the same pattern. 
     The image processing device  222  displays the screen shown in  FIG. 12  on the display device  211 . The user determines whether or not to change the threshold value while referring to the screen shown in  FIG. 12 . When an instruction to change the threshold value is given, the image processing device  222  performs division processing on the basis of the changed threshold value. 
       FIG. 13  schematically shows a screen for instructing the integration or separation of blocks by the change in the threshold value. In  FIG. 13 , the association shown in  FIG. 12  is expressed in a tree form. Rectangles having symbols, such as “A 1 ” and “A 2 ”, thereinside show regions of respective blocks. 
     The user can fold up (not display) or expand (display) the tree below the target block by specifying the black circle through the input device  212 . 
     In addition, the user can fold up (not display) or expand (display) the tree of lower order for each hierarchy by specifying the white circle through the input device  212 . 
       FIG. 14  schematically shows a screen for instructing the change the display range by changing the upper and lower limits of the threshold value. In  FIG. 14 , ellipses having numeric values, such as “13%” and “10%”, thereinside show regions of respective blocks. The numeric value inside the ellipse indicates an optimal cardiac phase for each block calculated from only the variation. In addition, the elliptical insides of blocks that are highly associated with each other are shown by the same pattern. 
     The user can change the width of the cardiac phase to be displayed by moving the upper limit or the lower limit of a slide bar on the right side. 
     For example, when the lower limit is set to 30%, ellipses to which “10%”, “13%”, and “15%” are given are no longer displayed. In addition, for example, when the upper limit is set to 79%, an ellipse to which “80%” is given is no longer displayed. 
     According to the fourth embodiment, user operability is improved. As a result, it is possible to quickly specify the optimal cardiac phase. 
     While the preferred embodiments of the X-ray CT apparatus according to the present invention have been described with reference to the accompanying drawings, the present invention is not limited to such examples. It is apparent to those skilled in the art that various changes and modifications can be made within the range of the technical idea disclosed in this specification, and it should undoubtedly be understood that they also belong to the technical range of the present invention. 
     REFERENCE SIGNS LIST 
       1 : scanning unit 
       2 : operating unit 
       3 : electrocardiograph 
       4 : object 
       5 : electrocardiographic waveform data 
       6 : region-of-interest variation graph 
       7 : degree-of-harmony graph 
       8 ,  9 : cardiac phase 
       10 : optimal cardiac phase image 
       11 : variation distribution image 
       12 : region-of-interest variation and degree-of-harmony graph 
       13 : optimal cardiac phase 
       14 : linear slide