WO2005000122A1

WO2005000122A1 - X-ray ct device

Info

Publication number: WO2005000122A1
Application number: PCT/JP2004/008900
Authority: WO
Inventors: Hiroto Kokubun; Osamu Miyazaki
Original assignee: Hitachi Medical Corporation
Priority date: 2003-06-25
Filing date: 2004-06-24
Publication date: 2005-01-06
Also published as: JP2005013378A

Abstract

An X-ray CT device which collects the electrocardiographic information of a subject obtained by an electrocardiogram concurrently with the detection of an X-ray dose, calculates the point in time of a specific pulse rate phase to be image processed based on an R wave position in the electrocardiographic information, and re-constructs an image using the detection data about an X-ray dose at that point in time to capture and display the tomographic image of the subject, wherein reliability indicating whether the point in time of the specific pulse rate phase gives a targeted phase or not is calculated by a reliability calculating means to be able to display it on a display unit along with a tomographic image.

Description

Specification

X-ray CT device

Technical field

The present invention relates to an X-ray CT apparatus suitable for imaging a target part that moves periodically in a predetermined phase.

Background art

[0002] An X-ray CT apparatus transmits an X-ray from an X-ray source toward a subject while rotating an X-ray source and an X-ray detector, which are arranged to face each other around the subject in a circumferential direction. An X-ray detector that emits radiation and has a plurality of detection elements arranged in an arc shape facing the X-ray source detects an attenuated X-ray dose that has passed through the subject, and an image based on the detected attenuation data. By performing a reconstruction operation, a tomographic image of a cross section substantially perpendicular to the body axis of the subject is obtained.

[0003] Patent Document 1: JP-A-2002-330961

[0004] According to the electrocardiographic synchronized radiography disclosed in Patent Document 1, such an X-ray CT apparatus is used, and a multi-slice X-ray detector is used as the detector, and the subject is rotated while being rotated. Spiral trajectory scanning is performed by sending the table on which the specimen is mounted in the direction of the rotation axis. At this time, data of X-ray transmission at a predetermined slice position (hereinafter referred to as measurement data) can be measured a plurality of times during one rotation scan. By selectively extracting the projection data necessary for reconstruction of the target tomographic image from the measurement data collected in this way and providing it for ECG-gated reconstruction, it is possible to reconstruct a tomographic image of the heart at a predetermined heartbeat phase.

[0005] However, in the above-described ECG-gated imaging and reconstruction, the following problem may occur. The problem was that the R wave of the electrocardiographic waveform mainly used as electrocardiographic information corresponding to the projection data could not be correctly detected. This problem will be described with reference to FIGS. Figure 8 shows an example of correctly creating a tomographic image at a heartbeat phase of 70% during a heartbeat cycle. Time is plotted on the horizontal axis, and ECG information and projection data collected during imaging are shown side by side. The projection data extraction range is determined based on the R-wave position, and the projection data necessary for image reconstruction is extracted. By performing an image reconstruction operation on the extracted projection data, a tomographic image at a heartbeat phase of 70% can be created. [0006] However, when noise is mixed in the electrocardiogram information, or when the R wave is weakened for some reason, the R wave may not be accurately specified. Figure 9 shows a comparison between the case where the ECG information is normal and the case where an abnormality has occurred. For example, if noise is mixed in the electrocardiogram information and becomes as shown in FIG. 9B, the noises 61 and 62 may be erroneously recognized as R waves. Further, when the signal strength of the R wave 53 is weak as shown in FIG. 9C, the signal may not be recognized as the R wave. In such a case, non-R-waves 61 and 62 may be detected as R-waves, or detection of R-waves 53 may be missed. In such a case, it becomes impossible to accurately extract projection data of the heartbeat phase, and as a result, it becomes impossible to obtain an appropriate tomographic image.

[0007] When such an inappropriate tomographic image is completed, the projection data used to create the tomographic image includes projection data corresponding to a phase different from the target phase. To know this, the operator himself looks at the tomographic image and judges the appearance of the image. Alternatively, the operator directly looks at the electrocardiographic information so as to avoid erroneous recognition of noise, a weak R wave, or the like when grasping the electrocardiographic information, and judges that the electrocardiographic information is noise and not an R wave. It takes time. However, such judgment requires visual observation by the operator, and is burdensome for the operator, and it is difficult to reduce oversight of noise and weak R waves.

[0008] Therefore, an object of the present invention is to provide an X-ray CT apparatus capable of displaying the reliability of a tomographic image at a target heartbeat phase. Disclosure of the invention

[0009] In order to achieve the above object, according to the present invention, an X-ray source that emits X-rays and is arranged to face each other with a subject interposed therebetween, and attenuated through the subject. An X-ray detector for detecting an X-ray dose, means for sequentially rotating and moving the irradiation and detection directions of the X-ray source and the X-ray detector substantially on the circumference of the subject, and the inside of the subject. A periodic motion measuring means for measuring a periodic motion of a part which performs a periodic motion, and a means for storing the measurement time of the periodic motion, the detection time of the X-ray dose, and the detection data of the X-ray dose in association with each other. Means for specifying a time point of a first specific phase of the periodic motion based on a time point of the periodic motion measurement of the subject obtained by the periodic motion measuring means; and Means for setting a time point of the second specific phase based on the time point; Means for performing image reconstruction using the detection data of the X-ray dose in the X-ray CT apparatus and obtaining a tomographic image of the subject along the rotationally moving surface.

Means for calculating a reliability related to the time point of the second specific phase used to obtain the tomographic image, and display means for displaying the reliability together with the tomographic image. X-ray CT apparatus is provided. Brief Description of Drawings

FIG. 1 is a block diagram showing an overall configuration of an X-ray CT apparatus according to the present invention.

FIG. 2 is a flowchart showing a procedure from completion of measurement to display of an image.

FIG. 3 is a diagram showing a dropout in an R-wave position.

FIG. 4 is an example of a list of each tomographic image displayed on the display device.

FIG. 5 is an example of a tomographic image display screen displayed on the display device.

FIG. 6 is a flowchart showing a procedure for increasing the reliability of a tomographic image while re-specifying an R-wave position specifying parameter.

FIG. 7 is a diagram showing re-specification of an R-wave position.

FIG. 8 is a view showing a method of extracting projection data using electrocardiographic information.

FIG. 9 is a diagram showing an abnormal example of electrocardiographic information.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the X-ray CT apparatus according to the present invention will be described with reference to the accompanying drawings.

Example 1

FIG. 1A is a block diagram showing the overall configuration of an X-ray CT apparatus according to one embodiment of the present invention. The X-ray CT device creates a scanner gantry unit 2 that irradiates and detects X-rays, and creates projection data to be used for reconstruction calculation from the measurement data detected by the scanner gantry unit 2 and converts the projection data into a CT image signal. An image processing device 7 for processing and a display device 5 for outputting a CT image are provided. The scanner gantry unit 2 includes a rotating disk 8 driven by a rotation driving device 10 controlled by a measurement control device 11, an X-ray tube 1 mounted on the rotating disk 8, and an X-ray tube 1 attached to the rotating disk 8. Collimator to control the direction and size of X-ray flux 9 And an X-ray detector 4 mounted on a rotating disk 8. The X-ray intensity generated from the X-ray tube 1 is controlled by a measurement control device 11, which is controlled by a computer 12 having an input device 13. On the other hand, the image processing device 7 is connected to the electrocardiograph 6 for acquiring an electrocardiographic waveform of the subject, and further connected to a storage medium 14 for storing the processing results of the image processing device 7.

With reference to FIG. 1B, details of the image processing device 7 of FIG. 1A will be described. The image processing device 7 is configured to have various functions. The R-wave position specifying means 7a, which specifies the R-wave position (unit: time) using the R-wave position specifying parameter, from the electrocardiographic information collected at the same time as the imaging, is obtained by the R-wave position specifying means 7a Projection data extraction means 7b for extracting projection data necessary for image reconstruction at a desired heartbeat phase and slice position based on the R-wave position obtained, and image reconstruction from the projection data extracted by the projection data extraction means 7b. Image reconstruction means 7c for creating a tomographic image according to the configuration, reliability calculating means 7d for calculating the reliability of the tomographic image created by the tomographic image creating means 7c, a plurality of tomographic images and names of the tomographic images Alternatively, a reliability group selection means 7e for selecting a combination of a number and a reliability into a plurality of groups using the reliability as a reference value, and a parameter for updating the R-wave position identification parameter for better R-wave position identification. Update means 7f etc. Te, Ru.

When X-rays are emitted from the X-ray tube 1 with the subject lying on the subject table 3 as shown in FIG. 1 (a), the X-rays are set by the collimator 9. The irradiated X-ray irradiation field is detected by the X-ray detector 4. At this time, by rotating the rotating disk 8 around the subject, X-rays are measured using the X-ray detector 4 while changing the direction in which the subject is irradiated with X-rays. The measurement data thus detected is once transferred to the storage medium 14 and stored. At the same time, electrocardiographic information of the subject is also measured by the electrocardiograph 6, and is once transferred to the storage medium 14 and stored.

[0015] Next, after measuring, collecting, and storing X-rays and electrocardiographic information at the same time, how to perform image reconstruction and the like, and calculate the reliability of a tomographic image obtained by image reconstruction. The procedure for displaying the data on the display device 5 will be described with reference to the flowchart of FIG.

[0016] In step 1, the electrocardiographic information obtained simultaneously with the X-ray measurement is read into the image processing device 7. In step 2, the input device 13 inputs, for example, an initial value of a threshold value of the wave height as the R wave position specifying parameter.

In step 3, the R-wave position specifying means 7a specifies the position (time) of the R-wave of the electrocardiographic information using the R-wave position specifying parameter input in step 2.

In step 4, parameters required for image creation are input from the input device 13. Here, it is assumed that the heartbeat phase is, for example, 70% of the R-wave interval, and the slice position is, for example, the center of the heart.

[0020] In step 5, the projection data forming means 7b uses the position of the R wave specified in step 3 and the parameters necessary for image re-creation input in step 4 to obtain an image from the X-ray measurement data. The projection data necessary for image reconstruction of the target heartbeat phase and slice position is formed.

In step 6, the image reconstruction means 7c performs an image reconstruction operation at a desired heartbeat phase and a desired slice position based on the projection data formed in step 5. Further, the tomographic image obtained as a result is stored in the storage medium 14. At the same time, the image name of each tomographic image is also stored in the storage medium 14.

In step 7, the reliability calculating means 7d calculates the reliability of the tomographic image obtained in step 6. The reliability is determined, for example, by calculating the difference between the average heart cycle Re of the subject measured before imaging and the R-wave cycle Ra at the time of measurement of each tomographic image (the ratio is different). Here, the R-wave period Ra is the R-wave interval substantially at the center on the time axis of the data for reconstruction. For example, as shown in Fig. 3, if the R-wave period Ra at the time of measurement of each tomographic image is significantly different from the average heartbeat period Re of the subject, it is not possible to identify a certain R-wave. If the force that has been applied, or the noise is not actually an R-wave, the noise is erroneously determined to be an R-wave, a tomographic image of a desired heartbeat phase is obtained, and the possibility is high. The reliability is low. Specifically, the reliability is calculated using a calculation formula such as Equation (1) so that the higher the reliability, the higher the numerical value.

Reliability = (Re / Ra) x 100 where Ra> Re (1)

Using such a calculation formula, the value is 100 when the reliability is the highest (when Ra and Re are equal).

As the difference between Ra and Re increases and the reliability decreases, the number gradually decreases from 100. The calculated reliability is stored in the storage medium 14 together with the corresponding tomographic image. [0024] In step 8, the reliability group selection means 7e classifies the tomographic images based on the reliability obtained in step 7. For example, A rank, B rank, and C rank are ranked in descending order of reliability. Which rank is assigned to which range of reliability may be arbitrarily determined.

In step 9, a list of each tomographic image is displayed on the display device 5 as a list. Figure 4 shows an example. FIG. 4 shows, from left to right, image number No, image name name, shooting position, heartbeat phase, reliability, reliability group, group icon, and the like. Then, the tomographic image numbers are shown in order from the top. Note that in FIG. 4, the group icons and the darkness of the black in each column are displayed as ancillary information so that the operator can easily understand the reliability grouping. Instead of the density of the ink, the color or the density may be applied. In addition, a force at which the most calculation execution button 75 is arranged at the lower right, which will be described in step 11.

In step 10, the operator can select a desired tomographic image with a cursor and click on the series of tomographic images of FIG. 4 displayed as a list in step 9, thereby displaying the tomographic image on the display device 5. it can. The above series of operations is performed by the computer 12.

FIG. 5 shows No. 2 of FIG. 4 as an example. In Fig. 5, it is attached to the X-ray CT image 71 displayed in the center, the reliability is placed at the position 72, the reliability group is placed at the position 73, the group icon is placed at the position 74, and recalculation is performed if necessary. The execute button 75 can also be displayed.

In this embodiment, when the reliability of the tomographic image calculated in step 7 of the flow in FIG. 2 is low, the R-wave position identification parameter is updated by the parameter updating means 7f, thereby increasing the reliability. Work to obtain a high-quality image. Therefore, the flowchart of FIG. 6 is executed.

Hereinafter, the procedure will be described with reference to FIG. In step 11, when the reliability of the tomographic image at a certain slice position calculated in step 7 is low, for example, when the reliability of the image in FIG. 4 is low, the corresponding image is selected with the cursor on the list in FIG. In this state, select the recalculation execution button 75. Then, recalculation of the selected image with low reliability is started.

In step 12, the operator inputs recalculation allowable parameters. The recalculation allowable parameter gives a calculation index when recalculating the image selected in step 11. Here, how high the reliability should be to judge that the R wave is correctly detected Let P be the target value of reliability and N be the maximum number of calculation operations as the index to determine. In the following calculation steps, when the reliability reaches the target value P and the number of calculation operations reaches the maximum value N, the calculation is stopped. These parameters can be optimized empirically with repeated use experience.

In step 13, the R-wave position is re-specified by changing the R-wave position specifying parameter. An example of the calculation at the time of re-specifying the R wave will be described with reference to FIGS. FIG. 3 shows an example in which the R-wave position specifying parameter is the peak value of the R-wave, and if the peak value exceeds an arbitrary threshold Th, the R-wave is determined to be an R-wave. Here, the average cardiac cycle of the subject measured before imaging is Re, and the cardiac cycle at the time of image measurement is Ra, and the difference between the ratio of the two and 1 is also calculated. In this case, the threshold value Th of the R wave height can be updated using the following equation (2).

Th = Th + B * (1— Ra / Re) (2)

n + l n

[0032] Equation 2 gives a new peak height threshold (Th) by subtracting the ratio of Ra and Re from 1 with respect to the previous peak height threshold Th _n and further multiplying B by a weighting factor. When you calculate! /

n + l

The According to Equation 2, since the cardiac cycle is longer than the average value when collecting the measurement data, when it is considered that the R wave may have been overlooked, the pulse height threshold Th is lowered, and when the measurement data is collected, the cardiac cycle is increased by the average value. Since it is shorter, when it is considered that noise was picked up by mistake, an operation such as raising the peak height threshold Th can be performed.

As another example of the R-wave re-identification, the R-wave identification is repeatedly performed using a value obtained by sequentially reducing the re-specification threshold to an initial value, for example, 10%.ぅ Adopt a method.

In step 14, a change in the pulse height threshold Th performed in step 13 determines whether the specific position of the R wave has changed, for example, the force when the previous time was performed in step 3, for example. If yes, go to step 15 with Yes; if no, go back to step 13 with No.

In step 15, the same steps as the creation of the projection data in step 5, the image reconstruction of the tomographic image in step 6, and the calculation of the reliability in step 7 are executed again.

In step 16, it is determined whether or not the obtained image satisfies the allowable condition obtained in step 12. Specifically, if the reliability has reached the target value P, or if the number of calculation operations has exceeded the maximum value N, proceed to step 17 with Yes. Return to step 13 with No. When returning to step 13, the wave height threshold Th is raised and lowered again to try to change the specific position of the R wave.

In step 17, the reliability is grouped in step 8, the tomographic image list is displayed in step 9, and the tomographic image is displayed in step 10 again.

[0038] It should be noted that the number of images for which recalculation is specified in step 11 may not be one. For example, it is also possible to collectively recalculate C groups with low reliability.

[0039] In addition, in the execution of the iterative calculation in steps 11 to 17, the reliability may not be improved so easily and may be executed indefinitely. For example, this may be the case where the cardiac cycle fluctuates due to the subject's arrhythmia, the case where the R-wave signal is completely missing, or the case where noise very similar to the R-wave signal is mixed. In this embodiment, in order to prevent such infinite execution, the maximum value N of the number of calculation operations can be set in step 12. In other words, it is possible to stop the calculation when the number of times that the reliability does not improve even if it is calculated further. Steps 11 to 17 are also performed by the computer 12.

[0040] According to the present embodiment, the R-wave position of the electrocardiographic information force read at the same time as the X-ray measurement is specified, and the heartbeat phase is determined based on the specified R-wave position. A plurality of images are obtained, and the R-wave position is repeatedly specified while changing the R-wave position specifying parameter used for the R-wave specifying process, for example, the threshold value Th of the R-wave height. Then, the image is obtained repeatedly, so that the target value with reliability can be achieved. As a result, it is possible to automatically determine the optimal R-wave position identification parameter at each measurement point without having to visually judge whether or not the appropriate reconstruction has been performed. Accuracy increases, and reliability of each tomographic image also increases.

[0041] In particular, when performing ECG-gated reconstruction of a person whose pulse is originally weak or a person whose pulse is disturbed due to tension, it is possible to prevent the dropping of R waves and collect appropriate projection data to reconstruct an image. This makes it possible to provide the configuration, and the reliability of using the reconstructed image for diagnosis can be improved as much as possible.

Example 2

FIG. 3 in Example 1 above shows that the R-wave 53 with the subject could not be identified as in FIG. 9 (c). In addition, this example shows how to deal with the case where the corresponding projection data is missed.In this embodiment, as shown in FIG. This relates to the case where an R wave is erroneously detected. The second embodiment uses the same apparatus configuration and the same processing as in the first embodiment unless otherwise specified. Therefore, the same components as those in the first embodiment are denoted by the same reference numerals.

A specific difference between the second embodiment and the first embodiment is that step 2 in FIG. 2 and equation 2 are used instead of equation 1 in the reliability calculation means 7d in FIG. 1B. As described above, in the case of Fig. 9 (b), the noise 61 is mistaken for the R wave, and the interval between the R wave 51 and the noise 61 is recognized as the R wave period Ra at the time of measuring each tomographic image. There is a risk that it will. In this case, the R-wave period Ra at the time of measurement is smaller than the average heartbeat period Re of the subject measured before the imaging. In this case, use Equation 3 below.

Confidence = (Ra / Re) X 100 where Re> Ra (3)

As described above, according to the present embodiment, particularly, an electrocardiograph or the like picks up electromagnetic waves or the like from peripheral devices. V. When noise is generated. Even when there is noise due to the arrhythmia of the subject, Proper projection data can be collected without dropping the R-wave and used for image reconstruction, and the reliability of using the reconstructed image for diagnosis can be further improved.

Example 3

The third embodiment uses the same apparatus configuration and the same processing as in the first embodiment unless otherwise specified. Therefore, the same components as those in the first embodiment are denoted by the same reference numerals.

In Embodiments 1 and 2 described above, in Step 11, when the reliability of the tomographic image at a certain slice position calculated in Step 7 is low, for example, when the reliability of the image in FIG. 5 is low, FIG. When the image shown was selected with the cursor on the list in FIG. 4, the recalculation execution button 75 was selected to recalculate the image.

In this embodiment, in step 11, a group having low reliability and a group having low reliability instead of an image is collectively selected. For example, the system is configured so that those in the C group with low reliability are collectively selected and recalculated. When selecting a group, for example, if the part of the reliability C is selected from the reliability groups in FIG. 4, the group with C is selected all at once. Then, by pressing the recalculation execution button, the one with reliability C is continuously recalculated. [0048] Images having the same reliability group often have similar R-wave determination values shown in Fig. 7. For this reason, for the images belonging to the same group, the number of repetitions of the loop for re-specifying the R wave position between steps 13 and 14 can be almost the same in many cases.

In this embodiment, since the images in the group are collectively recalculated with substantially the same number of re-specific loop iterations, the reliability can be quickly reviewed.

[0050] As described above, the present invention is not limited to the above-described embodiment, and can be variously modified and implemented without departing from the gist of the present invention.

[0051] For example, the X-ray CT apparatus is not limited to a method in which an X-ray source and an X-ray detector are mechanically rotated to face each other. You can switch to X-ray irradiation from a point and use the X-ray method!

[0052] The X-ray detector is not limited to one having a single detector row in the body axis direction of the subject, but also includes one having a plurality of detector rows.

[0053] The reliability is a numerical value obtained by determining how much the average heart cycle of the subject matches the R-wave cycle at the time of tomographic image measurement. And Equation 3, the force using Equation 2 as an update equation for the R-wave position identification parameter. The present invention is not limited to these.

The present invention is considered to be suitable for an ECG gate or a retrospective which is reconstructed after photographing as disclosed in JP-A-2002-330961. Embodiment 13 is a method in which an ECG gate or a retroactive image to be reconstructed after imaging is considered. However, Example 13 is not limited to this. The present invention is also useful in an ECG trigger or a prospective case, for example, when extending the time between X-ray irradiation and measurement.

[0055] Hereinafter, the ECG gate or retroactive and the ECG trigger or prospective will be described. The feature of the ECG trigger or prospective is that only the data necessary for image reconstruction with less exposure is collected. is there. For example, in the electrocardiographic waveform information before the start of the scan, 70% of the position when the interval between two adjacent R waves (hereinafter referred to as the heartbeat cycle) is 100%, that is, for example, in the 1-second scan, the position from the R wave to 0. The time of 7 seconds (middle diastole) is determined as the scan start timing, and the gantry is rotated in synchronization with this R-wave, and X-rays are emitted at the view angle required for image reconstruction to project projection data. Get and move the couch A spiral scan is performed. ECG gates or retrospectives do not specifically consider short-term exposures. Thus, it is intended to prevent a loss of projection data and to reconstruct a tomographic image of a heart at an arbitrary slice position later. Therefore, a spiral trajectory scan is performed, and projection data is continuously collected instead of at a predetermined timing. At the same time, electrocardiographic information such as an electrocardiograph is recorded by matching the time axis with the projection data acquisition. After that, the data related to the same heartbeat time phase is taken out from the finally obtained projection data and used for image reconstruction. In this method, since extra projection data is acquired for reconstruction, even if the data cannot be acquired well or the cardiac phase cannot be read, some other projection data other than that part can be used. A target tomographic image can be created. As a result, the occurrence of a situation in which the shooting is redone is reduced. As described above, the present invention is particularly suitable for a case where projection data is acquired in combination with electrocardiographic information, such as an ECG gate or a retrospective. Furthermore, it is suitable for long-time imaging even in the case of ECG trigger or prospective, or when the projection data can be used in combination with ECG information.

Claims

The scope of the claims

[1] an X-ray source that emits X-rays and is opposed to each other with the subject interposed therebetween, an X-ray detector that detects the attenuated X-ray dose that has passed through the subject,

Means for sequentially rotating and moving the irradiation and detection directions of the X-ray source and the X-ray detector on a substantially circumference around the subject;

A periodic motion measuring means for measuring the periodic motion of a part of the subject that performs a periodic motion,

Means for storing the measurement time of the periodic motion, the detection time of the X-ray dose, and the detection data of the X-ray dose in association with each other;

Means for specifying a time point of a first specific phase of the periodic motion based on a time point of the periodic motion measurement of the subject obtained by the periodic motion measuring means;

Means for setting a time point of a second specific phase based on the time point of the first specific phase, and performing image reconstruction using the detection data of the X-ray dose at the time point of the second specific phase. Means for obtaining a tomographic image of the subject along the rotational movement plane, the X-ray CT apparatus comprising:

Means for calculating a reliability associated with the time point of the second specific phase used to obtain the tomographic image;

Display means for displaying the reliability together with the tomographic image, and an X-ray CT apparatus.

[2] The X-ray CT apparatus according to claim 1, wherein the reliability is displayed on the display device as a numerical value, a symbol, or a combination thereof.

[3] The reliability calculating means is configured to calculate the reliability based on a difference between an adjacent first specific phase interval at the time of measuring the corresponding tomographic image and an average period of a portion of the subject that periodically moves.

The X-ray according to any one of claims 1 and 2, wherein the reliability is calculated.

CT device.

[4] The apparatus further comprises storage means for acquiring and storing an average period of the part of the subject which periodically moves before the tomographic image measurement, and storing the average period, wherein the reliability calculation means is stored in the storage means. The X according to claim 3, wherein a heart cycle is used as the average cycle. Ray CT device.

[5] If the first specific phase interval is smaller than the average period, the reliability calculation means sets the reliability as a ratio of the average period to the first specific phase interval, 5. The method according to claim 3, wherein when the first specific phase interval is larger than the average period, the reliability is a ratio of the first specific phase interval to the average period. 6. The X-ray CT apparatus described in 1.

[6] For the tomographic image with low reliability, the time point of the first specific phase is re-detected, and the second specific phase is again detected based on the re-detected time point of the first specific phase. Means for obtaining the tomographic image by performing image reconstruction again by using the detected X-ray dose data at the time point of the reset second specific phase. The X-ray CT apparatus according to any one of claims 1 to 5.

[7] The display means displays a name of the tomographic image, the reliability corresponding to the tomographic image, and a switch for specifying a time point of a first specific phase of the periodic motion,

The reliability calculating means, when the name of the tomographic image and the switch on the display means are selected, re-specifies the time of the first specific phase with respect to the tomographic image, and then the second The X-ray CT apparatus according to any one of claims 1 to 6, wherein the reliability at the time of the specific phase is calculated again.

[8] The display means displays the name of the tomographic image, the corresponding reliability, a result of grouping the reliability in a predetermined range, and a time point of a first specific phase of the periodic motion. And a switch to be specified.

When the reliability group and the switch are selected on the display means, the reliability calculation means determines the time of the first specific phase for all images belonging to the group. The X-ray CT apparatus according to any one of claims 1 to 6, wherein the identification is performed again, and then the reliability regarding the time point of the second specific phase is calculated again.

[9] The reliability calculation means calculates a peak value of the periodic motion measurement data in the first specific phase for each calculation within a range of a target value of the predetermined reliability and the number of recalculations. While shifting the threshold for detection, the time point of the first specific phase is specified again. 9. The X-ray CT apparatus according to claim 7, wherein the X-ray CT apparatus is used.

[10] The X-ray detector according to any one of claims 1 to 9, wherein the X-ray detector is a multi-row detector configured by arranging a plurality of detectors in a body axis direction of a subject. The X-ray CT apparatus described in 1.

[11] An X-ray source that emits X-rays and is opposed to each other with the subject interposed therebetween, an X-ray detector that detects the attenuated X-ray dose transmitted through the subject,

Means for sequentially rotating and moving the X-ray source and the X-ray irradiation and detection directions of the X-ray detector on a substantially circumference around the subject;

An electrocardiograph attached to the subject to obtain electrocardiographic information from the subject, and an electrocardiographic information collected at the same time as the detection of the X-ray dose. R wave position specifying means for specifying a wave position;

Projection data for extracting the X-ray dose required for image reconstruction at a desired heartbeat phase and a desired slice position of the subject as projection data based on the R-wave position obtained by the R-wave position specifying means. Extraction means;

Image reconstruction means for creating a tomographic image by image reconstruction from the projection data extracted by the projection data extraction means,

A reliability calculating means for calculating the reliability of the created tomographic image,

An X-ray CT apparatus, comprising: means for displaying the reliability calculated by the reliability calculation means.