JP4010802B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT (Computed Tomography) apparatus, and more particularly to an X-ray CT apparatus suitable for cardiac ECG gate imaging.
[0002]
[Prior art]
In general, when an image is taken using an X-ray CT apparatus due to the pulsation of a human heart, motion artifacts appear in a tomographic image, which is not preferable for diagnosis. In order not to generate motion artifacts, it can be solved by increasing the scanning speed.
[0003]
In an X-ray CT apparatus with a current scan speed of about 1 second, the X-ray exposure is performed intermittently based on the electrocardiographic information of the subject, so that projections at different projection angles with the same heartbeat time phase are performed. Data is measured for one scan, and the image is reconstructed using this data, which is generally called an ECG trigger, prospective, or the like. In addition, a method has been proposed in which projection data is obtained (taken) without synchronizing with the cardiac cycle, and after obtaining the projection data, an image is reconstructed by combining projection data having the same heartbeat time phase. This is generally called ECG gate photography or retrospective.
[0004]
Note that the cardiac imaging method described above is mainly a method in which imaging is performed with the table stopped (referred to as normal scanning).
[0005]
In general, in order to observe the state of the heart beat, the cardiac tomogram obtained as described above or a three-dimensional image obtained from a plurality of cardiac tomograms in the order of cardiac phases. Therefore, means for continuously displaying moving images of heart tomograms has been taken, and means for achieving synchronization with the heartbeat cycle by finely adjusting the scan time has been taken.
[0006]
[Problems to be solved by the invention]
However, since the cardiac imaging method described above is premised on imaging (normal scan) with the table stopped, in the case of spiral scanning in which the table is moved and imaged, there is discontinuity in the projection data. This will result in clinically unfavorable images.
[0007]
In addition, when obtaining a moving image of a cardiac tomography, as described above, means for finely adjusting the scan time to synchronize with the heartbeat cycle has been adopted. There is a limit to the range of scan time that can be adjusted, and in the above-mentioned conventional technique, since one heartbeat was divided and the images in each cardiac phase were arranged, it was possible to create a smooth movie could not.
[0008]
Accordingly, an object of the present invention is to provide an X-ray CT apparatus capable of obtaining a wide range of tomographic images with reduced motion artifacts even when using a helical scan.
[0009]
Furthermore, in the present invention, even when a deviation occurs between the heart rate and the set value of the scan time, it is possible to obtain a tomographic image of a heart with little motion artifact (motion artifact) due to the heartbeat, and Another object of the present invention is to provide an X-ray CT apparatus capable of obtaining a smoother heart tomographic moving image than in the prior art.
[0010]
[Means for Solving the Problems]
The present invention relates to an X-ray CT apparatus having an ECG gate function for a heart, a synchronization unit for obtaining a synchronization signal of a heartbeat period and a scan period from a heart rate of a subject and a scan speed of a CT scanner, By adjusting at least one of the head projection angle of the projection data, the number of projection data, and the data width based on the synchronization signal, the projection equal to an arbitrary heartbeat time phase and necessary for image reconstruction An X-ray CT apparatus comprising: a projection data calculation unit that forms projection data in an angle range is disclosed.
Further, according to the present invention, when the projection data calculation unit collects projection data having a heartbeat time phase from each scan cycle during the helical scan, the interpolation that compensates the discontinuous region in the bed direction caused by the heartbeat and the scan cycle with the opposing data is performed. There is disclosed an X-ray CT apparatus having means for calculating discontinuous projection data in the bed direction by this interpolation.
Further, according to the present invention, when the projection data calculation unit collects projection data having a matching heartbeat time phase from each scan cycle during the helical scan, the discontinuous region in the bed direction caused by the heartbeat and the scan cycle is detected as a nearby heartbeat time phase. Discloses an X-ray CT apparatus having interpolation means that compensates by interpolation using the same projection data, and that calculates discontinuous projection data in the bed direction by this correction.
[0011]
Furthermore, the present invention is characterized in that the projection data calculation unit further obtains and displays a cardiac tomographic movie by creating a cardiac tomographic image at an arbitrary slice position from the collected projection data having a matching heartbeat time phase. An X-ray CT apparatus is disclosed.
Furthermore, the present invention is characterized in that the projection data calculation unit further obtains and displays a cardiac tomographic video by creating a cardiac tomographic image of an arbitrary heartbeat time phase from the collected projection data with a matching heartbeat time phase. An X-ray CT apparatus is disclosed.
Further, according to the present invention, the projection data calculating unit further obtains a three-dimensional image of the heart by collecting a plurality of obtained cardiac tomographic images of any heartbeat time phase in the body axis direction for each heartbeat time phase. An X-ray CT apparatus is disclosed.
Further, the present invention provides the X-ray CT apparatus, wherein the projection data calculation unit further obtains a moving image of a three-dimensional image of the heart by displaying the obtained three-dimensional image in order of heartbeat time phases. Disclose.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a basic configuration of the present invention, and a patient displaying a tomographic image of a subject 12 and a projection data calculation unit 11 that collects projection data that matches a heartbeat time phase from each scanning cycle with an X-ray CT scanner 10. A monitor 13 is shown.
[0013]
FIG. 2 shows the waveform names of the electrocardiograms used for explanation in future examples. The ECG with the highest value is called the R wave, and the front and back are called the Q and S waves. Please refer to medical books for details.
[0014]
FIG. 3 illustrates a retrospective ECG gate imaging method. FIG. 3 shows the case where the bed does not move during scanning and the X-ray detector is in one row. As shown in FIG. 3, when the scan cycle is 1 second and the heart rate cycle (RR time) of the subject is 0.75 seconds, the scan time phase and the heart rate after 3 scans (after 4 heartbeats in terms of heartbeat) The phase will be the same. Since the heart repeats the heartbeat four times during the three-scan cycle, projection data having the same heartbeat time phase is present four times in the three scans. In the retrospective ECG gate, data having the same heartbeat time phase and different projection angles may be collected for one scan period. Since the projection data for one scan period is collected from the four heartbeat periods, the projection data collected per one heartbeat period can be collected by ½ redundancy in terms of the projection angle.
[0015]
That is, electrocardiographic waveforms R1 and R2 are generated in the first period, R3 is generated in the second period, and R4 and R5 are generated in the third period. The motion of the heart becomes nearly stationary when it expands, which is when the electrocardiographic waveforms R1-R5 occur. Starting from this point in time, projection data of ½π is created at projection angles, and these are connected in the order of projection angles, so that almost stationary projection data of one heart can be obtained. The projection data of a, b, c, and d in FIG. 3 indicate partial projection data when images are taken at a projection angle of 1 / 2π with R1, R2, R3, and R4 as starting points, respectively. When this is executed, projection data as shown in FIG. This data is collected from each heartbeat cycle by 1 / 2π after the R wave, and the projection angles are all different. That is, it is equivalent to scanning 360 degrees in a time for scanning a projection angle of 1 / 2π. As shown in FIG. 4B, the data a and b are collected from the first scan period, the data d is collected from the third scan period, and the data C is collected from the second scan period.
[0016]
That is, FIG. 4A shows a case where partial projection data a, b, c, and d in FIG. 3 are arranged in the order of projection angles 0 to 2π7 to create one projection data. Accordingly, the partial projection data a, d, c, b are arranged in the order of the electrocardiogram waveforms R1, R4, R3, R2, R5. FIG. 4B shows an example in which partial projection data a, b, c, and d are collected to form projection data of a single heart, and the first 1 / 4π with respect to projection angles 0 to 2π. That is, 0 to 1 / 2π is the partial projection data a, then 1 / 2π to π is the partial projection data d, and c and d are π to 3 / 2π and 3 / 2π to 2π, respectively. This is partial projection data that is located, and thus, projection data for one sheet is created.
[0017]
This retrospective ECG gate scans 360 degrees with a scan time of 1 / 2π projection angle (in the example of Fig. 3), so the theoretical scan time was taken at 0.25 seconds. Even in the current third generation CT of about 1 second per scan, the heart can be imaged at high speed. Also, faster scanning can be performed depending on the scan time and the heartbeat time.
[0018]
FIG. 5 shows a rough flow when performing cardiac imaging. First, in step 1, based on the scan time and the heart rate of the subject, the scan cycle required in step 3 is obtained. That is, in step 1, from the scan time of 1 second shown in FIG. 3 and the subject heart rate of 0.75 seconds, the number of scans in which the scan and the heart beat are synchronized as the required scan number = 3, and the projection data division number = 1/4 division [FIGS. 4A, 4B] is determined. In step 2, photographing is performed based on the number of scans obtained in step 1. In step 3, the projection data calculation unit 11 is based on the electrocardiogram information (R1 to R5 in FIG. 3) obtained from the subject 12, and the projection data (ad in FIG. 3) that matches the heartbeat time phase from various scan cycles. To collect. In step 4, an image is reconstructed by various processing devices based on the projection data obtained in step 3. This image is a tomographic image that makes the heart appear to stop.
[0019]
FIG. 6 shows a case where the letoscopic ECG gate scan of FIG. 3 is applied to a spiral scan in which the table moves during the scan. In FIG. 6, the detector will be described using a four-row detector. In FIG. 6, the scan cycle is 1 second and the heartbeat cycle is 0.75 seconds. The vertical axis in the figure indicates the bet direction, and the horizontal axis indicates the projection angle. The locus of the detector measures the center of each detector. As the helical scan pitch, a pitch 1 is used in which the table for one detector moves when one scan is completed. That is, the position of the one-row detector when the third scan ends is the same as the position of the four-row detector in the first scan (when the projection angle is zero). In FIG. 6, it is assumed that the bet is fixed and the X-ray source and the X-ray detector move (actually, the X-ray source and the X-ray detector do not move and the bet moves). The trajectory of the line detector can be expressed as a straight line waiting for inclination.
[0020]
6, when projection data having the same heartbeat time phase is collected as in FIG. 3, the hatched area in FIG. 6 corresponds.
[0021]
In the case of FIG. 6 as well, since the heartbeat cycle is performed four times in the three scan cycles, it is possible to obtain projection data for one scan if ½π projection data is collected. In the case of FIG. 6, since a four-row detector is used, the projection data to be collected waits for the width of the detector, and this is indicated by a hatched rectangle in the figure. In other words, if it is within a rectangle, projection data can be calculated at an arbitrary slice position by interpolation between projection data obtained by neighboring detectors. That is, the scan is 1 second per cycle, and the projection angle is 0 to 2π. This scan is performed for three cycles to obtain projection data a, b, c, d for each of the electrocardiographic waveforms R1 to R4.
[0022]
In FIG. 7, the rectangle is translated into the first scan so that the projection data obtained in each scan period of FIG. 6 can be drawn in one scan. That is, the projection data are arranged in the order of a, d, c, and b between the projection angles 0 to 2π. In FIG. 7, the projection data collected from each period is drawn in a staircase pattern because the collected data is shifted in the bet direction because of the spiral scan. For example, the projection data at the slice position SP shown in FIG. 7 can be calculated because it is included in the hatched rectangle at any projection angle. The interpolation method can be realized by using linear interpolation using a weight corresponding to a simple distance as shown in FIG.
[0023]
In FIG. 8, for the sake of explanation, the projection data of the portion π to 3 / 2π of FIG. 7 is extracted. In this case, projection data at a desired slice position is calculated from the 3-row and 4-row detectors. That is, FIG. 8A shows a slice position 83 calculated by a known general linear interpolation method using the detector 80 in the third column and the detector 81 in the fourth column. FIG. 8B shows an example in which the slice position 83 ′ is calculated. For example, 1.0 is set between the locus 80 ′ and 81 ′ of the detector, and the slice position 83 ′ and the locus 80 ′. For example, the slice position is calculated by setting the weight between the positions w = 0.6, for example, and the weight between the slice position 83 ′ and the trajectory 81 ′, for example, w = 0.4. The slice position 70 in FIG. 7 is calculated in this way.
[0024]
6 and 7 represent up to the third scan, but in FIG. 9, the projection data that can be collected in the case of further scanning is represented by translation in the first scan. If scanning is continued, projection data having the same heartbeat time phase and different projection angles can be calculated at various slice positions as shown in FIG. That is, the projection data a, b, c, and d are projection data corresponding to the first to third periods, respectively, and e, f, g, and h are projection data corresponding to the fourth to sixth periods. It is shown. In this case, since the slice positions 90 to 93 extend over any projection data, image data can be created at an arbitrary slice position.
[0025]
Next, a case where heartbeats are different will be described. When the scan cycle is 1 second, the heartbeat cycle is 0.8 seconds, and the spiral scan pitch is 1, the scan is synchronized with 4 cycles, and the heartbeat cycle is synchronized with 5 cycles. In this case, the collected projection data is 2 / 5π each. In the present invention, the width of projection data to be collected and the like is set by a projection data calculation unit.
[0026]
As described with reference to FIGS. 6, 7, and 8, the above example is illustrated in FIG. 10. FIG. 10 When calculating projection data at the slice position SP, a discontinuous area 100 of projection data in which the slice data is not included because the slice position is not included in the rectangle in the interval 2 / 5π to 4 / 5π. Has occurred. In this case, projection data cannot be calculated. In the example of FIG. 9, discontinuous regions are generated, but since the section is extremely narrow compared to the example of FIG. 10, projection data can be calculated at various slice positions. In FIG. 10, since the width of the discontinuous section is wide, slice positions that cannot be calculated greatly increase. FIG. 11 shows an extreme example of this, assuming a scan cycle of 1 second and a heartbeat cycle of 0.9 seconds. In the case of FIG. 11, the width of the projection data collected is 1 / 5π, and the required projection data is 9 cycles. Projection data for one scan can be collected gradually after 9 cycles, but in the case of a spiral scan during this 9 scan cycle, the bet moves steadily. The continuous area 110 becomes significantly larger. This makes it impossible to calculate projection data at a desired slice position.
[0027]
In the present invention, the above-described problems are solved as follows. FIG. 12A shows a retrospective ECG gate scan method in the case where the scan is repeated for 1 second and the cardiac cycle is repeated at 0.8 seconds. As is clear from FIG. 12A, for example, discontinuity of projection data occurs in the section of 4 / 5π to 6 / 5π (discontinuous region A) at the slice position SP1. Further, at the slice position SP2, a discontinuous region of projection data is generated in a section of 8 / 5π to 2π (discontinuous region B). If there is discontinuity in projection data, projection data at a desired slice position cannot be calculated. Therefore, if an image is obtained by performing reconstruction processing as it is, an artifact or a significantly deteriorated image is obtained.
[0028]
As shown in FIG. 12A, 4 / 5π to 6 / 5π and 8 / 5π to 2π are in a 180-degree opposed relationship. The facing relationship is an X-ray path that passes through the same region, and the X-ray incidents are shifted from each other by 180 degrees. That is, the 180-degree facing data is theoretically exactly the same in the scan where the bet is stationary.
[0029]
In the facing relationship in FIG. 12A, since the spiral scan is used, even in the facing relationship, the bet directions are different, and thus they are not the same. However, it is a very effective correction method to eliminate the discontinuous area by replacing the discontinuous area with projection data facing 180 degrees. This is because retrospective ECG gate imaging is based on the premise that projection data having the same heartbeat time phase is collected in order to suppress heart motion artifacts as much as possible. For this reason, when replacing some projection data with the discontinuous area A shown in FIG. 12A, projection data having a 180-degree opposed positional relationship and the same heartbeat time phase must be used. Since the square hatched areas shown in FIG. 12 (a) are all data having the same heartbeat time phase, if projection data having an opposing relationship is used from these, the 180 ° opposing positional relationship and the heartbeat Projection data having the same time phase is used.
[0030]
FIG. 12B shows an example in which projection data having the above-mentioned facing relationship and the same heartbeat time phase is embedded in the discontinuous region. In FIG. 12B, the discontinuous region has disappeared due to the embedding process, and projection data can be calculated at the slice position SP1. Conversely, in the discontinuous region B, the section of 8 / 5π to 2π is discontinuous, but this is also solved by embedding data of 4 / 5π to 6 / 5π that are in a 180-degree facing relationship, and the slice position SP2 Projection data can be calculated.
[0031]
Although the above uses projection data having an opposing relationship, the projection data can be calculated even if the projection data of the discontinuous region at the slice position SP1 is calculated from the rectangular hatched regions C and D by linear interpolation. Although the reliability is lower than that of the method using the 180-degree opposed data, data having the same heartbeat time phase is used, which can be said to be suitable for cardiac imaging.
[0032]
In the above description, a method using data having the same heartbeat time phase with 180 degrees opposite relationship and a method using linear interpolation or the like using adjacent projection data are employed. Since the same data is used, the obtained cardiac tomogram can obtain an image with few heart motion artifacts.
[0033]
Next, an X-ray CT apparatus having a cardiac ECG gate function according to another embodiment of the present invention, which is obtained from the above-described continuous region projection data obtained by the present invention, that is, from each scan period. An X-ray CT apparatus that can obtain a smoother heart tomographic movie by collecting projection data that matches heartbeat time phases and obtaining a heart tomogram will be described.
[0034]
First, as shown in FIG. 13, this apparatus uses a scanner gantry unit 202 that performs X-ray irradiation and detection, and projection data formation that creates projection data from measurement data detected by the scanner gantry unit 202. A device 207, an image processing device 208 that processes the created projection data into a CT image signal, and a display device 205 that outputs a CT image are provided.
[0035]
That is, the scanner gantry unit 202 includes a rotating disk 209, an X-ray tube 201 mounted on the rotating disk 209, a collimator 210 attached to the X-ray tube 201 for controlling the direction of the X-ray bundle, The X-ray detector 204 mounted on the rotating disk 209 is also provided. The X-ray detector 204 is a multi-slice detector that can simultaneously acquire projection data at a plurality of positions by arranging a plurality of detection elements in the body axis direction of the subject. The rotating disk 209 is rotated by the rotation driving device 11, and the rotation driving device 211 is controlled by the measurement control device 212.
[0036]
The X-ray intensity generated from the X-ray tube 201 is controlled by the measurement control device 212, and this measurement control device 212 is operated and controlled by the computer 213. Further, the projection data forming apparatus 207 is connected to an electrocardiograph 206 in order to acquire a patient's electrocardiographic waveform.
[0037]
In the X-ray CT apparatus according to the present invention, the schematic configuration of which has been described above, X-rays are irradiated from the X-ray tube 201 with the patient lying on the patient table. The X-ray is obtained by the collimator 210 and is detected by the X-ray detector 204. At this time, while rotating the rotating disk 209 around the patient, the X-ray irradiation direction is changed. X-rays are detected using the X-ray detector 204 (while scanning). The detected measurement data is transferred to the projection data forming device 207, and here, the patient's electrocardiogram information measured by the electrocardiograph 206 (see FIG. 2 and the description related thereto) and the measurement control device 212. From the obtained imaging conditions, projection data with less motion artifact is formed by the method described in the above embodiment. Further, the projection data obtained in this way is further reconstructed into a CT image by the image processing device 208 and output as a three-dimensional moving image on the display device 205.
[0038]
Next, an example in which the scan cycle and the cardiac cycle are completely synchronized will be described with reference to FIG. FIG. 14A shows the timing of collecting divided projection data in the projection data forming apparatus 207 of the X-ray CT apparatus. FIG. 14B shows an enlarged view of the divided projection data collection timing in the period of “180 degrees + fan angle≈240 degrees” shown in FIG.
[0039]
The horizontal axis of this FIG. 14 (A) is time (t), and the vertical axis | shaft is the position of the linear movement direction (Z-axis) of a patient table (refer the code | symbol 3 in FIG. 13). Further, below the horizontal axis, an ECG signal from the electrocardiograph 206 is shown, and the position of the heartbeat in the time direction (t) is shown. Note that the imaging conditions at this time were assumed to be detectors with a spiral pitch of 1 and 4 rows, and the ratio of the scan cycle to the cardiac cycle was 6: 7. Here, the helical pitch is defined as a ratio to the detection element arrangement pitch in the Z-axis direction.
[0040]
The vertical axis of FIG. 14A shows the trajectories of the four detection elements at the center of rotation when the four diagonal lines perform a helical scan. Further, the thick line on the center locus of the detection element indicates the divided projection data having the same cardiac phase as described in the above embodiment. Here, the divided projection data are represented by al, a2, a3, a4. It was. Further, here, in order to make it easy to understand the method of collecting the divided projection data, the projection data after collection is shown in the first scan, and the rectangle divided into the lower four parts in FIG. FIG. 4 is an enlarged view of the projection data, and each divided part shows the collected divided projection data, and in the figure, the detector data of each divided projection data, the number of scans from the start of scanning And the projection angle range is specified.
[0041]
Thus, when the ratio of the scan cycle to the cardiac cycle is 6: 7, the divided projection data a1 of the first scan projection angle of 0 to 60 degrees of the first row detector, and the second scan projection angle 60 of the second row detector. -120 degree divided projection data a2, third row detector third scan projection angle 120-180 degree divided projection data a3, and fourth row detector fourth scan projection angle 180-240 degree divided projection The data a4 is divided projection data within the same range in the ECG signal, and the fan angle of the X-ray source is added to 180 degrees necessary for image reconstruction from four divided projection data having the same data width. It will be apparent that the projection data for the projection angle (about 240 degrees) has been successfully collected.
[0042]
By reconstructing the projection data collected using the above algorithm, it is possible to obtain a reconstructed image with a time resolution of one-sixth of the scan time and few motion artifacts. Note that, since the projection data of FIG. 14 can originally form projection data with few motion artifacts, this imaging condition is an ideal condition here.
[0043]
However, the conditions under which the projection data collection method described with reference to FIG. 14 can be applied are limited to the case where the ratio of the scan cycle to the cardiac cycle is 6: 7. When the data collection method is applied, it is impossible to collect divided projection data having the same cardiac phase, and it is impossible to obtain a reconstructed image with less motion artifact.
[0044]
As a countermeasure against the above problems, for example, it is possible to change the scan time mechanically to approach the ideal condition, however, there is a limit to the range of scan time that can be finely adjusted by mechanical change. Therefore, it is impossible to collect divided projection data having the same cardiac phase, and it is impossible to obtain a reconstructed image with less motion artifacts.
[0045]
Therefore, the present invention proposes a projection data collection method for obtaining a reconstructed image with few motion artifacts by adjusting the number of divisions of divided projection data and the division projection data width according to the cardiac cycle of the patient. Will be described below with reference to FIG.
[0046]
The contents shown in FIG. 15 are the same as those in FIG. However, it is assumed that the imaging condition is a spiral pitch 1, 4-row detector and the ratio of the scan cycle to the cardiac cycle is 36:43. This ratio is a ratio when the cardiac cycle is longer than the scan time by the time required for the X-ray source to move 10 degrees compared to the ideal condition described above. Further, the divided projection data to be collected are b1, b2, b3, and b4, respectively.
[0047]
When the ratio of the scan cycle to the cardiac cycle is 36:43, the divided projection data b1 with the first scan projection angle of 0 to 70 degrees of the first row detector, and the second scan projection angle of 70 to 140 degrees with the second row detector. Divided projection data b2 of the third row detector, divided projection data b3 of the third scan projection angle 140 to 210 degrees of the third row detector, and divided projection data b4 of the fourth scan projection angle 210 to 240 degrees of the fourth row detector Are combined to collect projection data for a projection angle (about 240 degrees) obtained by adding the fan angle of the X-ray source to 180 degrees necessary for image reconstruction (see the enlarged view of FIG. 15B). .
[0048]
The divided projection data collection method of FIG. 15 increases the data widths of the divided projection data b1, b2, and b3 by 10 degrees, respectively, while the data width of b4 is b1, b2, The b3 data width is decreased by 30 degrees, which is the total increase angle of the data width, and is 30 degrees. In addition, the divided projection data start angles of b2, b3, and b4 are increased by 10 degrees, 20 degrees, and 30 degrees, respectively, so that b1, b2, b3, and b4 are divided projection data having the same cardiac phase. (See the ECG signal (heart rate signal) at the bottom of FIG. 15A).
[0049]
As shown in FIG. 15, according to the image reconstruction of the projection data collected using the divided projection data collection method for adjusting the data width of the divided projection data, the time resolution is 7/36 of the scan time, In addition, it is possible to obtain a reconstructed image with less motion artifact.
[0050]
Thus, in FIG. 15 above, when the ratio of the scan time to the cardiac cycle is 36:43 (compared to the ideal condition, the cardiac cycle is longer than the scan time by the time required for the X-ray source to move 10 degrees. Even when the difference between the scan time and the cardiac cycle is larger, similarly, the cardiac phase is equal by changing the number of divided projection data to be collected. It is possible to collect projection data.
[0051]
FIG. 16 shows the case where the ratio of the scan time to the cardiac cycle is 18:23 (compared to the case where the cardiac cycle is longer than the scan time by the time required for the X-ray source to move 20 degrees compared to the ideal condition). ) Shows the method of collecting divided projection data. Here, in order to avoid duplication of explanation, only enlarged views are illustrated, and the divided projection data are c1, c2, and c3, respectively. Then, by increasing the data width of each of the divided projection data c1, c2, and c3 by 20 degrees and reducing the collected divided projection data to three, the projection data with few motion artifacts is obtained as described above. It becomes possible.
[0052]
By reconstructing the image of the projection data collected using the divided projection data collection method for adjusting the data width of the divided projection data in FIG. 16, the time resolution is two-ninths of the scan time and the motion artifact is obtained. It is possible to obtain a reconstructed image with less.
[0053]
Further, even in the case of the ratio between the scan cycle and the cardiac cycle other than those described in FIGS. 14 to 16 described above, the width of the divided projection data and the number of the divided projection data are set in accordance with the cardiac cycle of the patient as described above. By adjusting and collecting divided projection data having the same cardiac phase, it is possible to form projection data with less motion artifacts.
[0054]
However, when the present invention is used in a condition that deviates from an ideal condition (that is, the ratio of the scan cycle to the cardiac cycle is 6: 7), the number of divided projection data obtained is reduced and the divided projection data width is obtained. And the resulting reconstructed image has more motion artifacts. Therefore, in order to obtain a reconstructed image with fewer motion artifacts, it is desirable to use the present invention by setting the scan time at the time of measurement to be close to an ideal condition in advance.
[0055]
Further, when the divided projection data is collected, the cardiac time phase changes abruptly at the boundary portion of the divided projection data, so that a motion artifact occurs when a cardiac tomogram is created. In order to solve this phenomenon, the data width of each divided projection data is set wider, weighting processing is performed on the data near the boundary of each divided projection data, and the boundary portions of adjacent divided projection data are added together. Means such as overlapping can be considered.
[0056]
Further, even when the heartbeat of the patient becomes irregular, such as an arrhythmia, the data width of the divided projection data and the data collected by using the heartbeat information obtained from the electrocardiograph (that is, the above ECG signal) The present invention can be used by adjusting the number.
[0057]
Next, an image reconstruction method after creating projection data obtained as described above will be described with reference to FIGS. That is, in order to obtain a cardiac tomogram at an arbitrary slice position, it is necessary to perform some processing on the created projection data. First, FIG. 17 shows the flow of processing until a cardiac tomography is obtained. As described above, the division number of divided projection data and the data width are obtained from the patient's heart rate and imaging conditions (step 11), and the projection data is created by collecting the divided projection data (step 12). In FIGS. 14, 15, and 16 described above, an example of a method for collecting one projection data is shown. However, actually, a plurality of projection data having different slice positions can be collected. Therefore, the creation of the plurality of projection data will be described below with reference to FIG.
[0058]
First, FIG. 18 shows an example of a method for creating projection data under the ideal conditions shown in FIG. Three projection data with different slice ranges are generated from the data of the detectors in four rows. The obtained projection data are R1, R2, and R3, and for each of the collected divided projection data, the detector data used, the number of scans from the start of scanning, and the projection angle range are shown. Further, with the first scan as a reference, the previous scan is indicated by minus. Two methods for obtaining a cardiac tomogram at a certain slice position from these three projection data will be described.
[0059]
The first method will be described with reference to FIG. The vertical axis in FIG. 19 indicates the slice direction position, and a tomographic image of the heart at the slice position indicated by the dotted line is obtained. Therefore, first, weighting processing in the Z-axis direction is performed on each of the three projection data R1, R2, and R3 (step 13 in FIG. 17), thereby generating projection data at a certain slice position. Next, image reconstruction is performed on each of the processed projection data R1 ′, R2 ′, and R3 ′ (step 14), and three cardiac tomographic images having different slice positions are obtained. Then, from the finally obtained three cardiac tomograms img1, img2, img3, interpolation processing (Z-axis direction interpolation processing) of interpolation or extrapolation is used (step 15), and the heart at an arbitrary slice position A tomographic image img can be obtained.
[0060]
Next, the second method will be described with reference to FIG. In this method, interpolation in the slice direction using interpolation or extrapolation is performed from the three created projection data R1, R2, and R3 to obtain projection data R ′ at an arbitrary slice position (Z-axis direction interpolation processing). (Step 16 in FIG. 17 above). Next, by performing image reconstruction on the projection data R ′ (step 17), a cardiac tomogram img at an arbitrary slice position can be obtained.
[0061]
Furthermore, a method for forming projection data in a projection angle range necessary for image reconstruction in an arbitrary cardiac time phase designated by the operator will be described with reference to FIG. FIG. 21 shows a method of collecting divided projection data for each cardiac phase under the same conditions as in FIG. 14. That is, the imaging condition is a spiral pitch 1, 4-row detector, and the scan cycle and cardiac cycle. The ratio is assumed to be 6: 7. As described above, under this ideal condition, it is possible to obtain a reconstructed image with few motion artifacts using four divided projection data having the same cardiac phase and the same data width.
[0062]
In the figure, an electrocardiogram waveform (ECG signal) is shown in the uppermost part, and below that, each cardiac time phase when one heartbeat is divided into seven cardiac time phases is represented by alphabetic characters from A to G. did. Below that, the projection data of each of the four detectors is shown as a band-shaped square. One scan is divided by the projection angle width of the divided projection data, and each divided projection data is divided into 1 in ascending order of the projection angle. It is represented by the number 6. Also, in order to make the divided projection data collection method easy to understand, the projection data of the second row detector is from the second scan, the projection data of the third row detector is from the third scan, and the projection data of the fourth row detection loss is A state of being collected from the fourth scan is illustrated. Further, from this figure, since the ratio of the scan cycle to the cardiac cycle is 6: 7, the cardiac time phase of the divided projection data 1 to 6 of each detector is 1 heart as the scan proceeds. You will see that they are out of phase. In the lowermost part of the figure, the projection data after collecting the divided projection data is shown. For example, when forming the projection data of the cardiac phase of A, four divided projection data (portions surrounded by double lines) arranged in series vertically are collected to obtain projection data necessary for image reconstruction. It is possible. Similarly, an example of projection data formation is shown for B and G cardiac time phases.
[0063]
Next, as an example of forming projection data of an arbitrary cardiac phase, let us consider forming projection data having a cardiac phase just between cardiac phase A and cardiac phase B. In this case, first, the head projection angle of the divided projection data in the projection data of each detector is moved in the advancing direction of the X-ray source by half the angle (30 degrees) of the projection angle range of the divided projection data. The projection data after the change of the leading projection angle in each detector is shown below the projection data string before the change, and each divided projection data is 1.5, 2.5,... 6.5.
[0064]
Also in this case, similarly to the case where the projection data of the cardiac phase A is formed, by collecting the divided projection data after moving the head projection angle of the divided projection data, the cardiac phase A and the cardiac phase are collected. Projection data in the middle cardiac phase of B can be formed. The projection data after collection is shown as projection data of cardiac phases A to B below the FIG. In addition, it is possible to collect projection data of a cardiac phase that is intermediate between the cardiac phases, such as projection data of a cardiac phase that is intermediate between the cardiac phases B and C, by a similar collection method.
[0065]
As described above, the method for collecting the projection data of the cardiac phase intermediate between the cardiac phases by moving the leading projection angle of the divided projection data has been described. It can be seen that projection data of an arbitrary cardiac time phase can be formed by changing the amount of movement of the angle.
[0066]
In addition, by creating multiple projection data in which the amount of movement of the leading projection angle of the divided projection data differs by an arbitrary projection angle, and reconstructing each image, multiple cardiac tomographic images in the cardiac time phase at any time interval It can also be seen that it can be created.
[0067]
In addition, a reconstructed image in the cardiac phase of an arbitrary time interval is created at multiple positions in the body axis direction (slice direction) by the same method, and a reconstructed image having the same cardiac phase is created. It will also be understood that by gathering in the axial direction, it is possible to create a tomographic image of the entire heart, ie, a three-dimensional image, in the cardiac phase at any time interval.
[0068]
Further, by displaying the three-dimensional image obtained as described above on the display device (see reference numeral 206 in FIG. 13) in the order of cardiac phases, the three-dimensional heart that beats smoothly without interruption. It can be seen that a moving image, that is, a four-dimensional image can be obtained.
[0069]
【The invention's effect】
As described above in detail, according to the X-ray CT apparatus of the present invention, the discontinuity of projection data is eliminated particularly during a helical scan, and in particular, the tomographic image obtained thereby is a motion of the heart. A good tomographic image with few artifacts can be obtained.
[0070]
Moreover, it is possible to form projection data of an arbitrary slice position and cardiac phase using the continuous division projection data obtained by the X-ray CT apparatus according to the present invention, and appropriately combine them. Or, by gathering, a tomographic image of the entire heart in a cardiac time phase at an arbitrary time interval, its three-dimensional image, and further, a three-dimensional moving image of the heart, etc. can be created smoothly and seamlessly with reduced motion artifacts. It becomes possible.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a basic configuration of an embodiment of the present invention.
FIG. 2 is an explanatory diagram of an electrocardiogram waveform.
FIG. 3 is an explanatory diagram of a retrospective ECG gate.
FIG. 4 is an explanatory diagram of a retrospective ECG gate.
FIG. 5 is a diagram showing a flow of the present invention.
FIG. 6 is an explanatory diagram of a retrospective ECG gate in a multi-row detector.
FIG. 7 is an explanatory diagram of a retrospective ECG gate in a multi-row detector.
FIG. 8 is an explanatory diagram of an interpolation method.
FIG. 9 is an explanatory diagram of a retrospective ECG gate in a multi-row detector.
FIG. 10 is an explanatory diagram of discontinuous projection data.
FIG. 11 is an explanatory diagram of discontinuous projection data.
FIG. 12 is an explanatory diagram of interpolation of discontinuous regions.
FIG. 13 is a diagram showing a schematic configuration of a multi-slice X-ray CT apparatus according to an embodiment of the present invention.
FIG. 14 is a diagram showing an example of divided projection data collection of the projection data forming apparatus in the X-ray CT apparatus of the present invention.
FIG. 15 is a diagram showing another example of the divided projection data collection of the projection data forming apparatus in the X-ray CT apparatus of the present invention.
FIG. 16 is a view showing still another example of the divided projection data collection of the projection data forming apparatus in the X-ray CT apparatus of the present invention.
FIG. 17 is a diagram showing a flow of processing performed by the projection data forming apparatus and the image processing apparatus in the X-ray CT apparatus of the present invention.
FIG. 18 is a diagram for explaining the details of the processing shown in FIG. 17;
FIG. 19 is also a diagram for explaining the details of the processing shown in FIG.
20 is also a diagram for explaining the details of the processing shown in FIG.
FIG. 21 is a diagram showing an example of divided projection data collection having an arbitrary cardiac time phase in the projection data forming apparatus in the X-ray CT apparatus of the present invention.
[Explanation of symbols]
10 X-ray CT system
11 Calculation unit
12 Subject
13 Processing equipment
80, 81 Detector trajectory
202 Scanner gantry
206 ECG
207 Projection data forming apparatus

Claims (7)

In an X-ray CT apparatus having an ECG gate function for the heart, a synchronization unit for obtaining a synchronization signal of the heartbeat period and the scan period from the heart rate of the subject and the scan speed of the CT scanner, and a synchronization signal from the synchronization unit By adjusting at least one of the head projection angle of projection data, the number of data of the projection data, and the data width, projection of a projection angle range equal to an arbitrary heartbeat time phase and necessary for image reconstruction is performed. An X-ray CT apparatus comprising: a projection data calculation unit that forms data.   The projection data calculation unit has interpolation means for compensating for discontinuous regions in the bed direction caused by the heartbeat and the scan cycle with the opposing data when collecting projection data in which the heartbeat time phase is matched from each scan cycle during the spiral scan, The X-ray CT apparatus according to claim 1, wherein projection data discontinuous in the bed direction is calculated by the interpolation.   When the projection data calculation unit collects projection data that matches the heartbeat time phase from each scan cycle during the helical scan, the projection data that has the same heartbeat time phase in the discontinuous region in the bed direction caused by the heartbeat and the scan cycle. The X-ray CT apparatus according to claim 1, further comprising: an interpolation unit that compensates by interpolation using a signal, and calculating discontinuous projection data in the bed direction by the correction.   2. The projection data calculating unit further obtains and displays a cardiac tomographic moving image by creating a cardiac tomographic image at an arbitrary slice position from the collected projection data with matching heartbeat time phases. The X-ray CT apparatus described.   2. The projection data calculation unit further obtains and displays a cardiac tomographic moving image by creating a cardiac tomographic image of an arbitrary heartbeat time phase from the collected projection data with a matching heartbeat time phase. X-ray CT apparatus described in 1.   The projection data calculation unit further obtains and displays a three-dimensional image of the heart by collecting a plurality of obtained cardiac tomographic images of any heartbeat time phase in the body axis direction for each heartbeat time phase. The X-ray CT apparatus according to claim 4, wherein the apparatus is an X-ray CT apparatus.   7. The X-ray according to claim 5, wherein the projection data calculation unit further obtains a moving image of a three-dimensional image of the heart by displaying the obtained three-dimensional image in order of heartbeat time phases. CT device.

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