JP4007928B2 - X-ray CT system - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to a technique that can reduce exposure during imaging of a motion site such as a heart without lowering the diagnostic value of an X-ray CT apparatus.
[0002]
[Prior art]
The heart measurement of the subject of the conventional X-ray CT apparatus will be described from the viewpoints of (1) CT and cardiac imaging, (2) ECG synchronous reconstruction, (3) Prospective measurement, and (4) tube current control.
(1) CT and cardiac imaging: The mainstream X-ray CT system is a mechanical scanning system, and as a result of the increased scanner rotation speed, a scan time of about 0.5 seconds has been achieved. However, cardiac scans require a scan time of about 0.1 seconds, which is not sufficient. An electron beam scanning CT (EBCT) that exceeds the mechanical scanning type is a scanning time of 50 milliseconds at the shortest.
[0003]
(2) ECG synchronous reconstruction: With a multi-slice CT system that can measure multiple projection positions at the same time by preparing multiple detectors in the body axis direction, the table speed can be slowed down to make redundant measurements. Attempts have also been made to improve effective time resolution. This uses a reconstruction method called segment reconstruction, and is ideal by measuring the same cardiac time phase (for example, diastolic phase) at the same slice position multiple times (number of segments) in each detector row during a helical scan. The effective time resolution can be reduced to a fraction of the number of segments. In the case of 4-row multi-slice, the view range required for reconstruction (180 degrees + fan angle in the case of half scan) is divided into 4 segments, and table feed, scan time, etc. so that each segment can be measured in a different row Set the shooting conditions. In the case of segment reconstruction, the optimal scan time depends on the heart rate of the patient, but if the 0.6 second scan is used, an image with an effective time resolution of about 0.1 second, which is a quarter of the half scan, can be acquired. .
[0004]
Since the number of segments can be increased by increasing the number of columns, the resolution can be further increased. For example, with 8 rows, the number of segments can be doubled up to 1/8 of a half can. In order to realize this, it is necessary to send the 4-row system and the patient table at the same speed, and since the time resolution is emphasized, the throughput is not improved. As a typical example, the spiral pitch is about 1.
[0005]
(3) Prospective measurement: As a problem of the segment reconstruction mode, there is a problem of increased patient exposure due to redundant measurement. On the other hand, with the aim of reducing exposure, there is a method in which X-rays are exposed and imaged in synchronization with a specific cardiac phase of the electrocardiogram. This is an imaging method used in EBCT for the purpose of calculating the Coronary Calcium Score, and X-rays are applied only for the time phase such as end diastole. In the case of EBCT, for example, X-rays are exposed only for 0.1 seconds to obtain a half-scan image. In a general mechanical scanning method, in the case of 0.5 second / rotation, data of about 2/3 rounds necessary for image reconstruction is required, so the time resolution is 0.33 second. Since this method only captures specific time phases, it is possible to evaluate calcifications and coronary arteries, but it is not possible to evaluate cardiac function with multiple time phases such as heart wall movement. was there.
[0006]
(4) Tube current control: As a means to reduce exposure, taking into account that the transmission length (attenuation) when the subject is a uniform attenuation body varies depending on the angular position of the radiation source, and the tube according to the rotation angle during scanner rotation. There is a tube current control system that modulates the current. There are two types of transmission length, one using a scanogram taken in advance and the other that feeds back the output from the detector during measurement. In any case, since the tube current differs depending on each measurement view, for example, when an object on a cylinder is photographed, the noise amount of the measured projection data changes. Actually, the tube current is increased when the transmission length of the object is long, and is decreased when the transmission length is short, so that the detector output is always at a constant level so that the amount of noise in the projection data is uniform. To control.
[0007]
This method is most effective in reducing artifacts in areas where there is a large difference in attenuation in the vertical and horizontal directions (front and back, left and right of the patient), such as the shoulder and pelvis, and imaging from the lung field to the abdomen with a single spiral scan. This is effective in reducing total exposure in cases such as when performing exposure, but it did not consider exposure in specific organs such as the heart.
[0008]
Therefore, the conventional exposure reduction is, for example, as described in [Patent Document 1], a rotation mechanism that rotates and drives the X-ray tube in a state of facing the X-ray detector with the subject interposed therebetween, Multi-directional projection data required to reconstruct a single tomographic image by inputting projection data detected by an X-ray detector and an X-ray controller that supplies X-ray exposure power to the tube A reconstruction device that reconstructs tomograms in a shorter time than the time required to collect the images, a switch for selectively inputting ON and OFF, and a switch while rotating the X-ray tube by controlling the rotation mechanism By providing the X-ray control unit and the control scan control unit so that power is supplied from the X-ray control unit only when is in the ON state, the exposure dose can be reduced without affecting the diagnosis.
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-190547
[Problems to be solved by the invention]
However, in the above prior art, since the subject penetration length is not taken into account, if the subject penetration length is short, no consideration is given to reducing the tube current, which may increase unnecessary exposure. .
On the other hand, if the subject transmission length becomes longer, the magnitude of the tube current must be made appropriate. However, since the subject transmission length is not taken into account, there is a possibility that the image noise will increase excessively.
[0011]
An object of the present invention is to provide an X-ray CT apparatus capable of performing a functional diagnosis and evaluating a function of a moving part by one imaging without increasing X-ray exposure to a subject as much as possible. is there.
[0012]
[Means for Solving the Problems]
The purpose is to irradiate the subject with X-rays from an X-ray source from multiple directions to obtain projection data, reconstruct the tomographic image data of the subject from the projection data, and reconstruct the tomographic image. In the X-ray CT apparatus for displaying data, a means for setting a target time phase of the periodic motion when imaging an imaging range involving the periodic motion of the subject, and the set target time phase X The X-ray source using a modulation control pattern in which the line intensity is relatively large as a motion cycle dependent control pattern and a transmission thickness dependent control pattern that keeps the detector output level constant depending on the transmission thickness of the subject. And the segment data in the set target time phase are obtained from the projection data photographed by the X-rays irradiated from the controlled X-ray source, and the obtained segment data is used. Said covered It is accomplished by comprising a means for reconstructing a tomographic image of the imaging range of the body.
The reconstruction means further includes a filter means for reducing a noise level difference between the segment data or between the tomographic images.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the X-ray CT apparatus of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing an example of the configuration of the X-ray CT apparatus of the present invention, FIG. 2 is a diagram for explaining an example of a tube current control pattern adopted in the present invention, and FIG. 3 is a flowchart for explaining a mode for inserting a filter process It is.
[0014]
As shown in FIG. 1, the X-ray CT apparatus is equipped with a host computer 107 that controls the entire system, an X-ray generation system that includes an X-ray tube 101, and a detector system that includes a detector 102 (rotational) scanning mechanism 103, a patient table 104 for conveyance at the time of patient positioning and spiral scanning, an image processing device 106 for performing various image processing, an external storage device 110, a display device 109, and an input device 108 for inputting operator instruction information Become. In addition, electrocardiographic waveform information can be input from an external electrocardiograph 111 to this X-ray CT apparatus.
[0015]
An X-ray control device 101C is mounted on the rotating disk of the scanner 100 to control the X-ray intensity. Prior to the start of shooting, shooting preparations (setting of shooting conditions, reconstruction conditions, etc.) of each device are made. The rotary scanning mechanism 103 rotates the rotary disk of the scanner 100, and the scanning control device 103C notifies the host computer 107 of the preparation completion information when the rotation speed reaches a desired rotational speed. In the case of the helical scan, control is performed in advance so that the patient table 104 is moved to a position that takes into account the acceleration time and the X-ray exposure start position is at a steady speed. When X-rays are emitted and imaging is started, the X-rays with the intensity instructed by the host computer 107 (or the tube current control pattern registered in advance) are emitted toward the opposing detector 102 To do. The detector 102 detects X-rays that have passed through the subject (not shown), converts them into electrical signals, and then obtains projection data as digital data in the measurement circuit 105. The projection data is subjected to image processing such as preprocessing, filter processing, and back projection processing by the image processing device 106 to reconstruct a tomographic image. The reconstructed image is displayed on the display device 109 and provided to the observer as a diagnostic image.
[0016]
Next, the flow of cardiac imaging will be described.
This X-ray CT apparatus has two control modes: a heartbeat-dependent control mode and a coexistence control mode of a heartbeat-dependent control mode and a subject equivalent thickness-dependent control mode. Prior to cardiac imaging, the operator selects one of the aforementioned control modes, and inputs the selected control mode, the average heartbeat of the subject (patient), the target time phase, and the maximum / minimum tube current. The host computer 107 calculates the tube current control cycle from the input average heart rate, creates a tube current control pattern 1 that fluctuates between the maximum and minimum tube currents, and transfers the control pattern to the X-ray controller. . In the heartbeat-dependent control mode, imaging is started using the tube current control pattern 1. In the heart rate + subject equivalent thickness dependent control mode, for example, as shown in JP-A-2002-263097, a control pattern 2 at a rotation angle θ and a body axis direction position z is created from a scanogram. A modulated control pattern 3 is used.
[0017]
FIG. 2 shows (a) ECG waveform data and (b) the tube current control pattern of the present invention. FIG. 2C shows a control pattern for comparison when the minimum tube voltage is ½ in the subject transmission length dependent method. However, here, in order to simplify the explanation, it is assumed that there is no change in the body axis direction. Further, the pattern is controlled in a sine wave shape as shown in the figure, but the time width of the target time phase may be relatively large, and it is desirable that the pattern take into account fluctuations in the heartbeat.
[0018]
When the tube current is controlled by the heartbeat cycle-dependent method of the present invention, the control pattern becomes a broken line in FIG. Here, the minimum tube voltage was set to 1/2 as in the conventional control. In this case, the target time phase is an example in which the time phase 1 is set, and imaging is performed in which the maximum tube current is applied in accordance with the time phase 1 to be viewed with the highest image quality in the ECG waveform. Since the reconstruction unit reconstructs using only segment data in the target time phase, it can be seen that a good image with the least noise can be acquired. Considering the case of obtaining an image of another time phase, for example, in time phase 2 shown in the figure, only data with a tube current close to the minimum value is used, so that an image with much noise is obtained. A typical method for determining the target time phase is, for example, a time phase of 60 to 70% (so-called diastole) is set as the target time phase. Diastolic images are used to calculate the calcification index or to evaluate coronary stenosis, and other time phases are used to see the motion of the heart wall. Although the motion of the heart wall is observed with, for example, a moving image, since noise is generally not conspicuous compared with a still image, sufficient observation can be performed. Further, even when analyzing using a still image, a higher resolution is not required compared to evaluation of blood vessels. Therefore, the exposure can be minimized and an examination that can be performed by the X-ray CT apparatus can be performed by one imaging.
[0019]
Focusing on the time t1 in FIG. 2 (b), it can be seen that the tube current is high in the front and back (AP) direction where the transmission length is short and the tube current is desired to be lowered from the viewpoint of the subject transmission length. It can also be seen that at time t2, the tube current decreases in the left-right (LAT) direction where the tube current is to be increased. At t1, unnecessary exposure increases, and at t2, the tube current is lowered more than necessary, and image noise may increase excessively.
[0020]
The heartbeat + subject equivalent thickness-dependent control mode is a method in which a heartbeat cycle-dependent control pattern is further modulated with a subject transmission length-dependent pattern. Assuming that each is a broken line in FIG. 2 (b) and a solid line in FIG. 2 (c), the modulated pattern is a solid line in FIG. 2 (b). As a result, the best image quality in the target time phase and the image noise increase in other time phases, but noise fluctuations in the rotation angle direction depending on the subject transmission length are also suppressed, and the image quality is maintained within an allowable range. Image.
[0021]
Further, in this embodiment, since the noise level changes for each phase, as shown in FIG. 3, the standard deviation value of the projection data is calculated, the noise amount of each segment data is calculated, and the projection is performed according to the noise amount. Steps 304 and 305 for adjusting the filter applied to the data are provided. This filtering process is performed by the image processing apparatus. The filtering process may change the reconstruction filter process for correcting the blur of backprojection, but a known method such as a weighted average filter may be used separately in the channel direction, and is limited as long as the frequency characteristics can be adjusted. Is not to be done.
[0022]
According to the above embodiment, by setting a target time phase such as diastole and increasing the tube current in the target time phase, a good image quality of the target time phase can be obtained, and the coronary artery can be evaluated. In addition, since the tube current is low in the systole but the heart wall boundary can be traced sufficiently, the cardiac function can be evaluated from the volume ratio of the diastole and the systole. Therefore, the projection data can be reconstructed in one tomogram. All evaluations are possible.
[0023]
In addition, since the means for making the noise level of the projection data substantially constant is provided, the image quality is stabilized, and the exposure can be further reduced when further modulation is performed with the subject transmission length dependent pattern.
[0024]
Further, when the exposure is performed only in the diastole, the data may be insufficient due to arrhythmia or the like, but the imaging method according to the present invention can compensate for the data although the noise increases.
[0025]
Therefore, it is obvious that the same expectation can be expected regardless of the generation of the X-ray CT apparatus and the imaging mode, and the same exposure reduction effect can be obtained by single slice CT, cone beam CT, spiral scan, and dynamic scan.
[0026]
In addition, patient exposure when performing cardiac imaging can be reduced, and only an image with a specific cardiac phase can be obtained like an imaging method that exposes X-rays in synchronization with a specific cardiac phase of the electrocardiogram. In addition, it is possible to acquire images of all cardiac phases by measuring projection data that can reconstruct one tomogram.
Although the description has focused on the heart, it is apparent that the present invention can be applied to other periodic motion sites and imaging during intentional periodic motion.
[0027]
【The invention's effect】
The present invention has an effect of providing an X-ray CT apparatus capable of evaluating cardiac function without increasing X-ray exposure to a subject as much as possible.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration example of an X-ray CT apparatus according to the present invention.
FIG. 2 is a diagram for explaining an example of a tube current control pattern employed in the present invention.
FIG. 3 is a flowchart for explaining an aspect of inserting filter processing.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... X-ray tube, 101C ... X-ray control apparatus, 105 ... Measurement circuit, 106 ... Image processing apparatus, 108 ... Input device

Claims (2)

Projection data is obtained by irradiating a subject with X-rays from an X-ray source from multiple directions, tomographic image data of the subject is reconstructed from the projection data, and the reconstructed tomographic image data is displayed. In the X-ray CT apparatus, a means for setting a target time phase of the periodic motion in imaging the imaging range accompanied by the periodic motion of the subject, and an X-ray intensity relative to the set target time phase. The X-ray source is modulated and controlled using a motion cycle dependent control pattern and a transmission thickness dependent control pattern that keeps the detector output level depending on the transmission thickness of the subject constant. Gets means, the segment data in the set target time phase than the projection data captured by the X-rays emitted from the X-ray source that is the control, the object with the segment data the acquired X-ray CT apparatus characterized by comprising a means for reconstructing a tomographic image of the imaging range of the body. The X-ray CT apparatus according to claim 1, wherein the reconstruction unit further includes a filter unit that reduces a noise level difference between the segment data or between the tomograms.

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