JPH0698883A - X-ray ct apparatus - Google Patents

Abstract

PURPOSE:To enable the lowering of scattered X rays to be generated from a specimen by arranging a soft ray filter in a structure of having a sheet with an even thickness stuck on a curved surface of a compensating body. CONSTITUTION:A compensating body 2 is inserted to make flat at the center and in a peripheral part an intensity distribution of X rays incident into a detector 6 after transmitted through a specimen 5 and is made thinner to be less in reduction at the center thereof with respect to X rays and thicker to be greater in reduction in the perimeter thereof with respect to the X rays so that a difference in the reduction of the X rays between the center and the perimeter becomes equal to the reduction in the specimen. A soft ray filter 3 using a plate with a uniform thickness is stuck on a curved surface of the compensating body 2. The material of the soft rays filter 3 requires a larger reduction with respect to X rays of lower energy and employs copper, a copper allow or the like. Thus, the X rays of lower energy is removed more toward the perimeter to allow the lowering of the scattering of energy of the X rays in the perimeter thereby enabling the measurement of the intensity of transmission X rays at higher accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus, and more particularly to an improvement of an X-ray compensator capable of reducing X-rays scattered by a patient and performing more accurate observation.

[0002]

2. Description of the Related Art FIG. 6 shows an outline of a measuring portion of an X-ray CT apparatus. The X-ray CT apparatus includes an X-ray tube 11 and an X-ray on a rotating plate 10.
The line detectors 16 are arranged so as to face each other, and measurement is performed while rotating the transmitted X-ray intensity of the subject 15 placed in the center. The X-ray beam radiated from the X-ray tube 11 is made of aluminum or Teflon, which is provided so that the X-ray intensity distribution at the soft ray removal 13 filter placed at the X-ray tube emission port and the detector entrance may be flat. After passing through the compensator 12 made of, for example, a collimator 14 for adjusting the X-ray beam width forms a fan beam. The fan beam passes through the subject and then enters the X-ray detector 16. Soft line removal filter 1
Reference numeral 3 is a plate made of a material such as copper or copper alloy and having a thickness of about 0.1 to 0.3 mm. The X-ray emitted from the X-ray tube does not pass through the object and is absorbed inside. It removes low soft rays. Since the soft ray removing filter 13 normally uses a flat plate as it is, the energy distribution of the X-rays after passing through the soft ray removing filter 13 is substantially constant regardless of the place. The X-ray beam emitted from the X-ray tube 11 is the subject 15 placed in the center at the center.
However, the intensity of the light does not decrease in the peripheral portion that does not pass through the subject 15. Therefore, if the intensity distribution of the X-rays incident on the subject 15 is uniform, the intensity of the X-rays incident on the detector 16 when the subject 15 is imaged is small in the central part and large in the peripheral part. I will end up. In the abdomen where the attenuation is large, the intensity ratio becomes several thousand.

The output current from the detector is input to the detection circuit, where it is subjected to current-voltage conversion and signal strength amplification, and then subjected to analog-digital conversion (A / D conversion). The digital numerical data is sent to the image processing circuit to reconstruct the tomographic image. Normally, the current-voltage conversion circuit and the amplification circuit are prepared for the channels of the detector, but the number of A / D conversion circuits is several and it is configured to handle a plurality of channels. Therefore, the A / D conversion circuit is also required to operate at high speed. When the intensity of the input signal greatly differs depending on the channel, the A / D conversion circuit is required to have a wide dynamic range in order to ensure measurement accuracy. Alternatively, it is necessary to make the amplification factor of the amplifier in the preceding stage of the A / D conversion circuit variable so that the intensity of the signal input to the A / D conversion circuit becomes appropriate. However, in this case, since the value of the adjustment amount itself is also related to the signal strength, it is necessary to correct the data by multiplying the numerical data after A / D conversion by the coefficient corresponding to the adjustment amount. Since an A / D conversion circuit which has a wide dynamic range and operates at high speed is expensive, in many cases, a method of varying the amplification factor of the amplification circuit in the preceding stage of the A / D conversion circuit is used. Even in this case, if the signal intensities of the respective channels are largely different, it is necessary to change the amplification factor frequently, and high-speed operation must be performed.

In order to eliminate such inconvenience, a compensator is provided between the X-ray tube and the subject. The compensator is made of a material such as aluminum or Teflon that appropriately attenuates X-rays, and has a shape such that the attenuation in the central portion is small and the attenuation in the peripheral portion is large with respect to the transmitted X-rays.
In other words, the X-ray passing path length is short at the center and becomes longer toward the periphery. The intensity of the X-ray beam after passing through the subject is small at the center of the subject where the X-ray passing path length is long, and becomes X as it goes to the periphery.
The strength of the line is increased. When the compensator as described above is inserted in the passage of the X-ray beam, the X-ray intensity distribution due to the compensator and the intensity distribution due to the subject work in the direction of canceling each other, and the X-rays incident on the detector are The intensity becomes flat in the channel direction of the detection element, and the detection circuit can be efficiently operated.

Further, the X-ray used in the CT apparatus is a monochromatic X-ray.
It is a continuous spectrum X-ray rather than a line. Therefore, when passing through an object, low energy X-rays (soft rays) are absorbed more and the energy distribution shifts to the higher side. This is the beam hardening effect. When there is a compensator, this beam hardening effect works in a direction that makes it uniform in the channel direction, and the energy distribution of the X-ray beam incident on the detector is similar to that when there is no compensator. As described above, the compensator has a function of flattening not only the intensity of the transmitted X-ray but also the energy distribution.

[0006]

When X-rays penetrate an object, they are not only attenuated and absorbed by the substance, but also
Part of it becomes scattered X-rays. The detection element measures the attenuation of X-rays of a portion of the subject on a line (measurement path) connecting the focus of the X-ray tube and the center of the detection element, but if there is scattered X-rays from other portions of the subject, An error will occur in this measurement. When scattered X-rays are incident, the output of the detection element is increased and the attenuation of the object on the measurement path is measured to be apparently small. When such an error increases, the resolution of the CT image reconstructed using these data will decrease. In particular, deterioration of low contrast resolution called density resolution becomes a problem.

Such scattering of the X-ray beam occurs in the entire portion of the subject irradiated with the X-rays, but the proportion of scattered X-rays increases toward the periphery of the subject. This is because scattered X-rays generated inside the object must pass through the inside of the object before reaching the detection element, but the path length inside the object is long at the center of the object, and as it goes to the periphery, This is because the passing path length becomes short. This is because the longer the pass length inside the subject, the greater the attenuation with respect to scattered X-rays. After coming out of the subject, there is no great difference in the attenuation due to the path length for passing through the air with little attenuation.

The phenomenon of scattered X-rays has the property that the lower the energy of X-rays, the greater the proportion of scattering in the lateral and backward directions. On the contrary, when X-rays with high energy are used, the ratio of scattered X-rays contained in transmitted X-rays can be reduced.

As described above, the X used in the CT apparatus
The ray is not a monochromatic X-ray but a continuous spectrum X-ray, and when passing through an object, the low-energy portion is more absorbed, and the energy distribution after passing through the object has been positively utilized. Is a soft line removal filter. Generally, the greater the atomic number of a substance and the higher the density, the greater the attenuation of the substance with respect to X-rays. Also, the rate of attenuation on the low energy side is high. Therefore, the soft ray removal filter using a material having a large atomic number and high density has a higher effect of removing the soft rays, and the energy distribution of the X-rays after passing through the filter shifts to the higher side. Further, when the same material is used, the longer the X-ray passing path length is, the higher the effect of removing the soft rays becomes, and the higher the energy shifts. Although the above-mentioned compensator also has an effect of removing soft rays, it cannot obtain sufficient attenuation for low energy X-rays because it is made of aluminum or the like. For this reason, copper or copper alloy is mainly used as the material of the soft wire removal filter.

By using such a soft ray removing filter using copper or a copper alloy, low energy X-rays emitted from the X-ray tube are removed to eliminate ineffective exposure to the object and scattered X-rays. Has been reduced. However, in the conventional soft ray removal filter, the soft rays are removed only at the same rate in the central portion and the peripheral portion of the subject, and scattered X-rays reaching the detector after passing through the subject are generated in the peripheral portion of the subject. It will be big. In order to reduce the scattered X-rays generated in the peripheral portion of the subject, it is necessary to use a soft ray removal filter that attenuates low energy X-rays in the peripheral portion of the subject. To realize a soft ray elimination filter that attenuates low-energy X-rays from the center toward the periphery, use a material with a higher atomic number and a higher density toward the periphery, or if the same material is used, the closer it is to the periphery. The shape must be such that the X-ray passing path becomes long.

Although it is theoretically possible to use a material whose atomic number and density are changed from the central portion to the periphery, it is actually possible to stably supply an industrial product with uniform characteristics at a low cost. With technical difficulties. In addition, in order to increase the X-ray passing path length toward the periphery,
It is possible to use a material having a thicker plate at the periphery, but there are some problems in stably supplying a product having uniform characteristics at a low cost. Uniform material
These problems can be solved if the X-ray passing path length can be increased toward the periphery by using a thick plate. Even if a plate having a uniform thickness is used, the effective X-ray path length differs if the X-ray passing path and the angle formed by the plate differ. By utilizing this fact, the X-ray passing path and the plate are orthogonal to each other at the center, and if the angle formed by the X-ray passing path and the plate decreases toward the periphery, the effective X-ray passing path becomes The longer it gets around the long, the longer it gets.

An object of the present invention is to provide an X-ray CT apparatus which removes soft rays with a simple structure from such a viewpoint.

[0013]

According to the present invention, an X-ray tube and a multi-element detector are arranged so as to face each other around a subject, and the intensity of X-rays incident on the detector in the channel direction of the detector. With a compensator placed between the X-ray tube and the subject so that the distribution is flat, X-ray transmission data from each direction of the subject is measured while rotating, and the X-ray of the cross section of the subject is measured from the data. In a CT device for reconstructing a distribution image of linear transmittance, a thin plate for removing soft rays having a uniform thickness is stuck to the concave portion of the surface of the compensating object or the hollow portion of the inner surface of the compensating object (claim 1). .

[0014]

According to the present invention, by using a plate having a uniform material and a thickness as described above, and by using a soft ray removing filter that becomes longer as it gets closer to the effective X-ray passing path length, Lower energy X-rays can be removed as the temperature increases. It is possible to reduce energy scattering of surrounding X-rays.

[0015]

EXAMPLE An example of the present invention is shown in FIGS. 1 (a) and 1 (b). FIG. 1B shows a view of FIG. 1A viewed from the side. The X-ray radiated from the X-ray tube 1 passes through the compensator 2 and the soft ray removal filter 3, is made into a fan beam by the slice collimator 4 for adjusting the X-ray beam width, and then passes through the subject 5 and the detector 6 Is incident on. Compensation 2
Is an object that is inserted so that the intensity distribution of the X-rays that enter the detector 6 after passing through the subject 5 is made flat in the central portion and the peripheral portion. ,
At the periphery, the thickness is thick and the attenuation with respect to X-rays is large, and the difference in the amount of X-ray attenuation between the center and the periphery is equal to the attenuation of the subject 5. FIG. 2 is a diagram showing how the compensator 2 and the subject 5 are attenuated by X-rays. FIG. 2A shows the subject 5
2B shows the positional relationship between the X-ray tube 1, the compensator 2, and the subject 5 when only the subject 5 is placed without the compensator 2. . 2 (c),
2D shows the transmission X in the case of FIGS. 2A and 2B, respectively.
It is a figure showing the intensity distribution of a line, and the horizontal axis corresponds to the position of incidence on the detector 6, and the vertical axis corresponds to the X-ray intensity. Compensation 2
When only the object is placed, the intensity distribution is such that the center is strong and becomes weaker toward the periphery, and when only the object 5 is placed, the intensity distribution is such that the center is weaker and the periphery is stronger. The intensity distribution of the transmitted X-rays when the compensator 2 and the subject 5 are placed at the same time is a combination of the respective intensity distributions, and is as shown in FIG. The compensator 2 is for adjusting the amount of attenuation of the detector 6 in the channel direction. The object can be achieved by using any material, but in reality, it is placed in a limited space. Considering that it is required to be stable without being deteriorated even when exposed to X-rays for a long period of time, it can be said that aluminum and Teflon are suitable materials. On the curved surface of the compensator 2, a soft ray removing filter 3 using a plate having a uniform thickness is attached. As the material of the soft wire removal filter 3,
Materials such as copper and copper alloys are used because they are required to be greatly attenuated by low energy X-rays. When copper is used, its thickness must be at least 0.1 mm. FIG. 3 is a diagram showing the path length of X-rays passing through the soft ray removal filter 3. The path length of the X-ray passing through the soft ray removing filter 3 attached to the curved surface is the plate thickness itself used in the central portion, but the X-ray obliquely passes through the plate material toward the periphery. When the path length is p, p is expressed by the following equation.

◎

## EQU00001 ## p = t / sin .theta.t: thickness of the plate material used for the soft ray elimination filter 3 .theta .: pass of the passing X-rays and the soft ray elimination filter 3 in that portion
The angle θ formed by and is 90 ° at the center, and becomes smaller toward the periphery. For this reason, the path length of the soft ray elimination filter 3 through which the X-rays pass becomes longer as it goes closer to the periphery. As the pass length of the soft ray removal filter 3 becomes longer, the passing X-rays shift to the higher energy side, and the energy distribution of the X-rays incident on the subject 4 is higher in the peripheral portion than in the central portion. Therefore, the scattering of the subject 4 is reduced.

FIG. 4 shows another embodiment 2. The arrangement of the X-ray tube 1, the compensator 2 ′, the soft ray removing filter 3 ′, the slice collimator 4, the subject 5, and the detector 6 is basically the same as in the first embodiment.
(Fig. 1). The shape of the compensator 2'is different. The compensator 2'is made of the same material as that of the compensator 2 of Example 1, and has a structure in which a circular or elliptical hole is formed in the central portion. A soft wire removing filter 3'is formed by attaching a plate having a uniform thickness to the inner surface of the hole and using the same material as that of the soft wire removing filter 3 of the first embodiment. Also in this embodiment, it is understood that the attenuation of the transmitted X-rays by the compensator 2'is small in the central part and becomes large as it goes to the peripheral part. Also, regarding the X-ray passing path length of the soft ray removing filter 3 ', it is understood that the central portion is shortest and becomes longer toward the periphery, and the same effect as that described in the first embodiment can be obtained. The soft line removal filter 3'attached to the inner surface of the compensator 2'can also achieve the object by installing only the half circumference of the upper half or the lower half. When the soft wire removing filter 3'is provided all around the inner surface, the plate thickness may be half that in the first embodiment.

FIG. 5 shows another embodiment. The arrangement of the X-ray tube 1, the compensator 2 ″, the soft ray removing filter 3 ″, the slice collimator 4, the subject 5, and the detector 6 is basically the same as in the first embodiment.
(Fig. 1). In the first embodiment, the cutout portion of the compensator 2 is on the subject 5 side, but in this embodiment, the cutout portion is on the X-ray tube 1 side. Also in this embodiment, the pass path length of the X-ray compensator 2 ″ becomes longer toward the periphery as compared with the central portion, and the same effect can be obtained.

[0019]

As described above, by carrying out the present invention, when the object is imaged by the X-ray CT apparatus, the scattered X-rays generated from the object are reduced, so that the transmission X-ray can be accurately transmitted. Since the intensity of the line can be measured, it is possible to obtain a CT image with high diagnostic ability without degrading the resolution of the image.

[Brief description of drawings]

1A and 1B are views showing an embodiment of the present invention, in which FIG. 1A is a front view and FIG. 1B is a side view.

FIG. 2 is a diagram illustrating a function of a compensator.

FIG. 3 is a diagram illustrating that the path length of an X-ray passing through a soft ray removal filter differs depending on the location.

FIG. 4 is a diagram showing a second embodiment.

FIG. 5 shows a diagram of a third embodiment.

FIG. 6 is a diagram showing a configuration of a transmitted X-ray intensity measurement portion of the X-ray CT apparatus.

[Explanation of symbols]

 1 X-ray tube 2, 2 ', 2 "Compensator 3, 3', 3" Soft ray removal filter 4 Slice collimator 5 Subject 6 Detector 11 X-ray tube 12 Compensator 13 Soft ray removal filter 14 Slice collimator 15 Subject 16 detection

Claims (1)

[Claims] 1. An X-ray tube and a multi-element detector are arranged so as to face each other around a subject, and X-rays are incident on the detector so that the intensity distribution in the channel direction of the detector becomes flat. With the compensator placed between the X-ray tube and the subject, measure the X-ray transmission data from each direction of the subject while rotating and reconstruct the X-ray transmittance distribution image of the subject's cross section from the data. In the CT apparatus to be constructed, an X-ray CT apparatus characterized in that a thin plate for removing soft rays having a uniform thickness is attached to a concave portion of the surface of the compensating object or a hollow portion of the inner surface of the compensating object.