JP3617424B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus, and in particular, an X-ray CT apparatus for performing a leveling operation of an X-ray tube while changing the thickness of a slice or not irradiating an X-ray to a detector. About the collimator.
[0002]
[Prior art]
As shown in FIG. 4, the X-ray CT apparatus is configured so that the X-ray tube 17 and the X-ray detector 16 rotate from the X-ray tube 17 around the body axis of the subject 14 while rotating in the circumferential direction. X-rays are focused and radiated by a collimator 12 into a fan-shaped X-ray beam 13, and the X-ray dose transmitted through the subject 14 is detected by a plurality of detectors 16 arranged in a circle opposite to the X-rays. An image is formed by processing.
As shown in FIG. 5, the collimator 12 includes a fixed slit 8 that determines the maximum slice width, and two blades 2 a that are provided above the fixed slit 8 and can change the slice width in the body axis direction small. 3a.
A collimator 15 that shields scattered X-rays incident obliquely on each detector element is provided on the front surface of the detector 16, and X-rays transmitted through the subject 14 enter the detector 16.
The detector 16 has an X-ray tube 17 centered on an X-ray detection element composed of a scintillator element that converts X-rays into light and a photodiode that detects light converted by the scintillator element and outputs it as an electrical signal. About 500 to 1000 channels are arranged in an arc shape.
Even if the X-ray tube 17 has a cathode-anode tube axis installed in the body axis direction of the subject 14 and includes the collimator 12 immediately before the X-ray tube 17, the radiation X-ray intensity distribution at that time is The subject 14 is not uniform in the body axis direction but has a certain distribution.
The X-ray intensity distribution has a steep rise on the cathode side of the X-ray tube 17, and the anode side has a smooth slope. As described above, the X-ray intensity distribution changes in the body axis direction, that is, in the opening direction of the fixed slit 8 and the blades 2a and 3a constituting the collimator 12, and when the X-ray emission is continued, the X-ray tube 17 The temperature rises, the anode and the like extend forward, and the X-ray tube focus moves in the tube axis direction (body axis direction). Therefore, the X-ray dose passing through the fixed slit 8 and the blades 2a and 3a changes. That is, the X-ray dose entering the detector 16 changes even when the X-ray passes through the same subject 14 at the same position of the opening of the fixed slit 8 or the blades 2a, 3a immediately before the X-ray tube 17.
If the input data has fluctuations or deviations even at 0.1%, artifacts are generated in the reconstructed image, and the detection system is required to have high accuracy and stability. Since the accuracy and stability of the detection system are limited in design and manufacture, a process for correcting it is necessary. Many detectors 16 require high geometric accuracy and it is almost impossible to make the sensitivity characteristics uniform across all channels. Therefore, calibration for scanning the reference phantom and updating various correction amounts for the detection system is performed to maintain the image quality.
In the X-ray CT apparatus, it is necessary to obtain correction data before imaging the subject 14, and at this time, the X-ray tube 17 is sufficiently warm in order to reduce the influence of the focal point movement of the X-ray tube 17. Generally, it is acquired in a state, that is, in a state where the focal point has moved sufficiently. For this reason, it is necessary to continue X-ray irradiation continuously for a certain period of time.
At this time, if the detector 16 is continuously irradiated with X-rays while the blades 2a and 3a, which are components of the front collimator 12, are opened, a fluorescent element is mounted as a photoelectric conversion element in each channel of the detector 16. In particular, so-called radiation degradation occurs in the fluorescent element, in which the light emission output gradually decreases due to the irradiated X-rays. In order to prevent this radiation deterioration, the blades 2a and 3a constituting the collimator 12 are generally closed and continuous X-ray irradiation is performed.
[0003]
[Problems to be solved by the invention]
The conventional X-ray CT apparatus is configured as described above. However, when X-rays are continuously emitted for the purpose of warming the X-ray tube 17, the blades 2a and 3a are placed on the fixed slit 8 as shown in FIG. The opening was closed by rotating the shaft 9 and moving the blades 2 and 3 uniformly from both sides. However, even if the two blades 2a and 3a are brought into contact with each other as much as possible, it is difficult to completely eliminate the gap, and the X-rays cannot be cut completely. Moreover, if it contacts too much, blade 2a, 3a may be damaged, or a drive mechanism may be damaged.
Further, as shown in FIG. 7A, the step blade 2b is attached to the blade 2, the step blade 3b is attached to the blade 3, and both the step blades 2b and 3b are rotated on the fixed slit 8 by rotating the shaft 9. As shown in (b), the opening is also closed so as to engage with each other. However, if the two step blades 2b and 3b are stepped, the blades will be displaced up and down, and a slice thickness that is symmetric with respect to the center cannot be provided.
In addition, as shown in FIG. 8, a mechanism having a shutter 10 is separately provided, and the opening is closed by taking it in and out. However, if a separate shutter mechanism is provided, a new motor is required, and there is a problem that the control and mechanism are complicated.
[0004]
The present invention has been made in view of such circumstances, and prevents radiation of the detector more than necessary when the X-ray is irradiated toward the detector to warm the X-ray tube. Therefore, an object of the present invention is to provide an X-ray CT apparatus capable of reliably closing an opening with a simple mechanism.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the X-ray CT apparatus of the present invention rotates between two movable blades provided on the top of a plate having a fixed slit for providing a maximum slice thickness, and the two blades. The intervening part is formed with a part having a plurality of thicknesses in the rotating direction, and the rotating surface is moved to such an extent that the two blades are moved so that one of the blades covers the fixed slit. A cam having a portion formed at a position eccentric from the center, and a motor for rotating the cam. The motor is appropriately rotated, and a plurality of thickness portions of the cam are interposed between the two blades. Thus, the slit width of the two blades can be made variable, and a portion formed at an eccentric position in the cam is interposed in the two blades, so that the two blades are inserted. And it has a collimator which is adapted to move eccentrically slit to produce the over de outside the fixed slit.
[0006]
The X-ray CT apparatus of the present invention is configured as described above, and the two blades are shifted from the slice position above the fixed slit, and the gap between the two blades is moved eccentrically outside the fixed slit. This constitutes the shutter mechanism. Therefore, since the gap created by the two blades is outside the fixed slit, the X-rays can be completely cut. And it is not necessary to add a step to the two blades. Further, there is no need to provide a separate shutter mechanism, and the opening can be reliably and inexpensively closed with a simple mechanism. Accordingly, even if X-rays are continuously emitted to warm the X-ray tube, the shutter mechanism can be operated during that time to prevent radiation deterioration of the detector.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of the collimator of the X-ray CT apparatus of the present invention will be described with reference to FIGS. FIG. 1 is a plan view of a collimator of an X-ray CT apparatus of the present invention, FIG. 2 is a view showing a cross section in the body axis direction, and (a) is a blade 2a, 3a provided on an upper part of a fixed slit 8. (B) shows the state at the time of 5 mm slice. The collimator of this X-ray CT apparatus is provided with a fixed slit 8 that provides the maximum slice thickness, a shaft 9 that is provided above the fixed slit 8, and is rotated by a motor 5 via a belt and a pulley 4. The fixed and circumferential thickness corresponds to each predetermined slice thickness, and when the X-ray is shielded, the slit created by the two blades 2a and 3a is moved out of the fixed slit 8 while being eccentric. A cam 6, a cam follower 7 that moves back and forth along the thickness of the cam 6 while pressing the circumferential portion of the cam 6 from both sides, and the cam follower 7 is attached and pulled together by a spring 11 and sliced on the shaft 9. The blades 2 and 3 each have two X-ray shielding blades 2a and 3a that move at an interval of.
The difference between the collimator of the present X-ray CT apparatus and the conventional collimator is that two movable blades 2a and 3a provided on the upper part of a plate having a fixed slit 8 for providing a maximum slice thickness have a predetermined slit width. The width can be set, and the slits created by the two blades 2a, 3a are eccentrically moved out of the fixed slit 8, and a mechanism for shielding X-rays is provided.
The collimator of this X-ray CT apparatus is located just before the X-ray tube 17 shown in FIG. 4 as in the prior art, and moves from both sides in the body axis direction to shield X-rays and scatter in that direction. It has a function of removing lines and determines the slice thickness.
A fixed slit 8 that determines the maximum value of the slice width is provided, and a blade 2a and a blade 3a are provided as movable slits above the fixed slit 8. The slice width is smaller than the width of the fixed slit 8.
[0008]
The blades 2 and 3 have blades 2 a and 3 a made of X-ray shielding plates such as lead attached to the surfaces of the blades 2 and 3 that are close to each other above the fixed slit 8, and are attached to both sides of the blades 2 and 3. It moves on the two shafts 9 provided. The blade 2 and the blade 3 are attracted to each other by the spring 11, and the distance and position thereof are determined by the thickness of the cam 6 sandwiched by the cam followers 7 attached to the both blades 2 and 3 and the front and rear positions.
FIG. 3A shows a development view in the thickness direction when the cam 6 is viewed from the A direction, FIG. 3B shows a left side view, and FIG. 3C shows a right side view. As shown in (a), the position in the thickness direction of the periphery of the cam 6 is eccentric from the right at the Shutter position, and as shown in the side view of (b), the original position is rotated counterclockwise from that position. Returning to the center position, the thickness is equivalent to 2 mm slice, 5 mm slice, 10 mm slice, 7 mm slice, 3 mm slice, and 1 mm slice. The cam 6 has a hole through which the shaft 9 is passed in the center, and is fixed to the shaft 9 with a screw.
The cam follower 7 is attached to the blade 2 and the blade 3, and is brought into contact with both side surfaces of the cam 6 by springs 11 mounted on the blade 2 and the blade 3. 2 and blade 3 are moved.
[0009]
Next, the operation of the collimator of this X-ray CT apparatus will be described. First, when the slice thickness is set to, for example, a 5 mm thick slice on the control console, the motor 5 rotates according to the set value, and the driving rotational force is transmitted to the two pulleys 4 via the belt. Is transmitted to the shaft 9 attached to the motor. The motor 5 stops when it rotates a predetermined number of rotations. At that time, the cam 6 fixed to the shaft 9 rotates to a predetermined rotation angle, and the cam follower 7 comes into contact with a position where the thickness of the cam 6 shown in FIG. 3 is 5 mm slice. Since the cam follower 7 is attached to the blade 2 and the blade 3, and the blade 2 and the blade 3 are pulled by the spring 11, the distance between the blade 2 a attached to the blade 2 and the blade 3 a attached to the blade 3 is It becomes 5 mm determined by the thickness of the cam 6. Then, as shown in FIG. 2B, the opening is located at the position coincident with the center of the fixed slit 8.
Next, when closing the collimator, when Shutter is selected on the control console, the motor 5 rotates according to the command, and the driving rotational force is transmitted to the two pulleys 4 via the belts. Transmitted to the shaft 9. The motor 5 stops when it rotates a predetermined number of rotations. At that time, the cam 6 fixed to the shaft 9 rotates to a predetermined rotation angle, and the cam follower 7 comes into contact with the position of the shutter of the cam 6 shown in FIG. The position of the Shutter is formed at a position where the rotation surface of the cam 6 is eccentric from the center to the right, and the blade 2 and the blade 3 move to a position shifted by the eccentricity. Therefore, as shown in FIG. 2 (a), the center of the fixed slit 8 and the center of the blades 2 and 3 are shifted from each other, and the blade 2a and the blade 3a are closed. Can be shielded.
[0010]
Thus, when X-rays are irradiated toward the detector 16 for the purpose of warming the X-ray tube 17, the positions of the blades 2 a and 3 a of the collimator of the present X-ray CT are eccentric from the center of the fixed slit 8. To close the X-rays. Further, when taking a tomographic image of the subject 14, a predetermined slice width can be set and the blades 2 a and 3 a can be opened at the same position as the center of the fixed slit 8 to obtain normal data.
With the above-described structure, in order to warm the X-ray tube 17, X-rays entering the detector are surely blocked during the period even if X-rays continue to be emitted, thereby preventing radiation deterioration of the detector 16. be able to.
[0011]
【The invention's effect】
The X-ray CT apparatus according to the present invention is configured as described above, and two X-ray shielding blades are attracted by a spring and provided on the upper portion of the fixed slit so as to correspond to a predetermined slice thickness. When the disc rotates, its thickness and front / rear position change to change the spacing and position of the X-ray shielding blade. When shielding the X-ray, the gap between the two blades is moved out of the fixed slit. Close it. Therefore, X-rays can be completely cut. With such a mechanism, it is not necessary to break the blade or to apply a step process, and it is not necessary to provide a separate shutter mechanism. Therefore, the opening can be closed reliably and inexpensively with a simple mechanism, and even if X-rays are continuously emitted for the purpose of warming the X-ray tube, the shutter mechanism is operated during that time to prevent radiation deterioration of the detector. be able to.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of a collimator of an X-ray CT apparatus according to the present invention.
FIG. 2 is a view showing a cross section of a fixed slit and a blade of a collimator of the X-ray CT apparatus of the present invention.
FIG. 3 is a diagram showing the thickness of a cam that determines the blade width of a collimator of the X-ray CT apparatus of the present invention.
FIG. 4 is a diagram showing a structure of the X-ray CT apparatus viewed from the body axis direction.
FIG. 5 is a plan view showing a relationship among a detector, a fixed slit, and a blade of the X-ray CT apparatus.
FIG. 6 is a cross-sectional view showing a state in which a blade of a conventional X-ray CT apparatus is closed.
FIG. 7 shows a cross-sectional view of a conventional X-ray CT apparatus in a closed state using a step blade.
FIG. 8 is a cross-sectional view showing a state where a conventional X-ray CT apparatus is shielded using a shutter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Case 2, 2a ... Blade 2b ... Step blade 3, 3a ... Blade 3b ... Step blade 4 ... Pulley 5 ... Motor 6 ... Cam 7 ... Cam follower 8 ... Fixed slit 9 ... Shaft 10 ... Shutter 11 ... Spring 12 ... Collimator 13 ... X-ray beam 14 ... Subject 15 ... Collimator 16 ... Detector 17 ... X-ray tube

Claims (1)

An X-ray tube and an X-ray detector provided opposite to the X-ray tube are rotated around the subject to collect X-ray transmission data of the subject and reconstruct an X-ray image. In CT equipment,
Two movable blades on top of a plate with fixed slits that provide maximum slice thickness;
The two blades are rotatably interposed between the two blades, and the interposed portions are formed with portions having a plurality of thicknesses in the rotation direction, and the two blades cover the fixed slit. A cam having a portion formed at a position eccentric from the rotation surface to such an extent that the
A motor for rotating the cam,
By appropriately rotating the motor and interposing a plurality of thickness portions of the cam on the two blades, the slit width of the two blades can be made variable and formed at an eccentric position on the cam. An X-ray CT apparatus comprising a collimator in which a slit created by the two blades is eccentrically moved out of the fixed slit by interposing a portion between the two blades.

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