JPS6336775B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a so-called computer tomography system that collects X-ray projection data from multiple directions on the same tomographic plane of a subject and reconstructs the distribution of X-ray transmission and absorption coefficients of the tomographic plane based on the data. Law (Computerized
We are involved in computerized tomography devices (hereinafter abbreviated as "CT devices") using
In particular, the present invention relates to a so-called fourth generation CT device, that is, a rotating fan beam, fixed detector type CT device, which is equipped with a scattered ray removal collimator that removes scattered X-rays from a subject.

FIG. 1 shows the basic configuration of this type of CT apparatus. A large number of X-ray detectors 2 are fixedly arranged on the circumference around the subject 1, and an X-ray source 3
rotates on a circumference concentric with the detector 2 around the subject 1. The X-rays from the X-ray source 3 are so-called fan beams that have a fan shape that can cover the subject 1 . This X-ray can be pulsed or continuous
A line is used. In the case of continuous X-rays, the output circuit of detector 2 has a sampling function, and
The detector 2 corresponding to the rotational angular position of the radiation source 3 is collected as data.

A feature of this method is that the mutual geometrical relationship between the X-ray source 3 and the detector 2 changes over time.

On the other hand, Fig. 2 shows the so-called third generation CT equipment, namely rotating fan beam X-ray and rotating detector type.
The principle configuration of the CT device is shown. In this method,
The geometrical relationship between the X-ray source 3 and the detector 4 is fixed, and the X-ray fan beam and a number of detectors 4 arranged opposite to it rotate around the subject to detect the X-ray beam of the subject. Measure the linear absorption coefficient.

In the method of measuring the X-ray absorption coefficient of the subject using a thin linear X-ray beam called a pencil beam and a detector with a narrow aperture, as in the so-called first generation CT equipment, The geometry and dimensions of the ray beam and detector are designed so that only direct X-rays from the X-ray source enter the detector, and scattered X-rays scattered by the object do not enter the detector. has been done.

Furthermore, in the third generation CT apparatus shown in FIG. 2, the aperture of the detector 4 is wide because the X-ray beam is fan-shaped, and there is a risk that scattered X-rays may enter the detector 4. When the scattered X-rays enter the detector 4, the original target X-ray source 3 and its detector 4
Scattered X-rays from other parts give an error to the measurement of the amount of X-ray absorption on the straight line connecting the two points.

For this reason, in order to prevent X-rays from being scattered from other parts of this type of CT device, a collimator 5 made of a thin plate of heavy metal is generally provided in front of the detector 4, as shown in FIG. Scattered X-rays from a range other than the direct X-rays connected to the detector ₄ X ₂
is absorbed by this collimator 5 so that it does not enter the detector 4.

In contrast, the fourth generation shown in Figure 1 above
In a CT apparatus, the mutual relationship between the X-ray source 3 and the detector 2 changes over time. Therefore, the direction in which a particular detector 2 faces the X-ray source 3 also changes as the X-ray source 3 moves. Therefore, it is not possible to provide a collimator made of a heavy metal thin plate on the detector 2 side for removing scattered radiation similar to that shown in FIG. 3 above. In other words, since the position of the X-ray source 3 relative to the detector 2 moves,
This is because the direction of the collimator cannot be kept constant.

A collimator 6 is provided facing the X-ray source 3 and is arranged as appropriate on the condition that the plate-shaped X-ray shielding part (of the collimator 6) is parallel to the spreading plane of the X-ray beam. A configuration in which the X-ray source 3 and the X-ray source 3 are rotated simultaneously has been considered. However,
In this case, the shadow of the collimator 6 enters the detector 2 and is detected due to changes in the mutual relationship between the X-ray source 3, collimator 6 system, and detector 2 system (changes in the mutual angle between the X-ray source 3 and detector 2). Conventionally, this method has not been adopted because it has the disadvantage that the output of the second circuit changes.

Therefore, in the fourth generation CT device, detector 2
It was virtually impossible to install a collimator between the object and the subject to remove scattered radiation. Therefore, in order to reduce the influence of scattered radiation as much as possible, the tube voltage of the X-ray tube was changed from 120KV, which was used before the third generation.
Although efforts have been made to increase the voltage to 140KV, it is still affected by scattered radiation.

The present invention has been made based on the above circumstances, and is a fourth generation CT apparatus in which a collimator for removing scattered radiation that rotates in synchronization with the X-ray source is provided between the subject and the detector. The object of the present invention is to provide a CT apparatus that does not cause fluctuations in detector output due to shadows.

In other words, the present invention is characterized in that the angular interval of the plate-like X-ray shielding portions constituting the collimator is the same as the effective angle of the detector or an integer fraction thereof.
It is to do so.

An embodiment of the present invention will be described below with reference to the drawings.

As shown in Fig. 5, in the fourth generation CT scanner, a heavy metal thin plate, for example, rotates in synchronization with the A collimator 7 consisting of a plurality of plate-shaped X-ray shield parts made of In FIG. 5, consider the case where the X-ray detectors 8 are arranged at equal intervals. It is assumed that the angular interval between two adjacent detectors when viewed from the rotation center 0 is A degree, and the effective angular range of each detector 8 (as viewed from the rotation center) is B degrees.
That is, (A-B) degrees is the angle seen from the rotation center 0 of the insensitive portion between two adjacent detectors 8.
Then, the two adjacent plate shapes X of the collimator 7
The angle of the line edge portion viewed from the rotation center 0 is uniform and is defined as C degrees. Here collimator 7
The plate-like X-ray beam angle pitch C of
be equal to the effective angular range B, or an integer fraction thereof. That is, since A>B, A>B
C and B=nC (however, n=1.2.3...). In this case, since it is BC, one or an integral number of the plate-like X-ray sensitive portions of the collimator 8 will always exist within the effective angle range of the detector 8. In order to make this situation easier to understand, the arc-shaped portion is expanded into a straight line and shown in FIG. Therefore, if the width of the shadow due to the thickness of the plate-shaped shield part of the collimator 7 is D (constant), then the detector 8
A shadow corresponding to D or nD is always created in the effective angle range of . This collimator 7
Although the direct X-rays from the X-ray source 3 are suppressed by the proportion of the shadow due to the thickness of the plate-like X-ray shielding part,
This shrinkage ratio is the thickness D or
Since it is always constant according to nD, the sensitivity of the detector 8 does not change due to this.

If the angular interval C of the shielding part of the collimator 7 is set to be the same as the effective angle B of the detector 8 or an integer fraction thereof, the area covered by the collimator to the incident surface of the detector 8 will always be constant. Prove using a diagram.

Figure 7 shows the length D (however, D
<C) X-ray shields are arranged in a row. At this time, let E=C−D.

Next, extend an arbitrary length B (where B
≧C) When placing a detector, the distances from both ends of the detector to the left end of the shielding object to the left are T1 and T2, respectively.
shall be.

Further, it is assumed that B1=B−T2 (1).

Now, considering when B=C, C=T1+B1
Therefore, C=T1+B-T2 holds from B1=B-T2. Substituting B=C into this results in T1=T2. This is not limited to the position of B when B=C.
This means that the distances D and E are always the same. Next, when B=nc (n is a natural number), nc=T1+B-T2 holds from nc=T1+B1 and equation (1).

If we substitute B=nc into this, then T1=T
It becomes 2.

Therefore, it can be seen that B=nc is sufficient.

For convenience of explanation, the above proof can be applied to the case where the X-ray source 3 or the rotation center 0 is at an infinite distance, but
Even when the detector 8 and collimator 7 are arranged in an arc shape, the above equation holds true by changing the vertical lines in FIG. 7 to radial shapes.

By providing such a collimator 7,
It is now possible to remove scattered radiation even in so-called 4th generation CT equipment, and there is no influence of scattered radiation.
It becomes possible to obtain CT images. This is the fourth
In the configuration shown in the figure, the interval between the narrow parts of the collimator 6 and the detector cell width are unrelated, whereas in the present invention, the angular interval between the narrow parts of the collimator 6 is determined by the effective angle of the detector 8. By making it the same as or an integer fraction thereof, the area where the collimator shields the incident surface of the detector 8 can always be kept constant, which has the effect of preventing fluctuations in the detector output. It is.

As described above, the present invention provides a CT apparatus in which a collimator that rotates in synchronization with an X-ray source is provided between a subject and a detector, and the detector output does not fluctuate due to shadows. be able to.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle configuration of a so-called 4th generation CT device, Fig. 2 is a diagram showing the principle configuration of a so-called 3rd generation CT device, and Fig. 3 is a diagram showing the principle configuration of a so-called 3rd generation CT device.
Figure 4 is a diagram showing an example of the configuration of a collimator used in a CT device.
A diagram for explaining problems when a collimator is provided in the device, FIG. 5 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 6 is a diagram for explaining the principle configuration of the embodiment in more detail. 7 are diagrams showing the relationship between a collimator and a detector for explaining an embodiment of the present invention. 3...X-ray source, 7...Collimator, 8...X-ray detector, 0...Rotation center.

Claims (1)

[Claims] 1 In a computerized tomography system in which a large number of X-ray detectors are arranged at equal intervals on the same circumference, an X-ray source is rotated on a concentric axis, and the subject is projected and scanned with a fan-shaped X-ray beam. A collimator is provided between the subject and the detector, the collimator being composed of a plurality of plate-shaped X-ray shielding parts arranged at equal intervals so as to point toward the X-ray source, and rotating together with the X-ray source. A computer tomography apparatus characterized in that the angular interval of the plate-shaped X-ray shielding parts of the collimator is approximately the same as the effective angle of the detector or approximately an integer fraction thereof.