JPS5832749A - Ct apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a computed tomography apparatus (hereinafter referred to as C
The present invention relates to a CT key device (abbreviated as T device), and particularly to a CT key device whose purpose is to obtain a tomographic image of only a portion of a subject.

In recent years, in order to take a detailed image of a part of the subject, for example, the inside of the marrow, a CT key device with a so-called target scan function that obtains an image of only that part (hereinafter referred to as target) has been developed, hereinafter referred to as Targetera 1-CT device. ) is used. This CT key device is used to reduce radiation exposure of the subject and to increase the spatial resolution of the target area.

Plan views of a conventional CT apparatus are shown in FIGS. 1 and 2. Both use a so-called 6th generation OCT device in which the X-ray tube and detector face each other and rotate at the same time.
There is a T-key device.

In the target CT apparatus shown in FIG. 1, two X-rays are generated from an X-ray tube 1 and are made into a fan beam shape by a collimator 6 and transmitted through a wedge 4. As shown in the figure, the wedge 4 is made so that the wall thickness is thinner at the center and thicker at both peripheral areas.
, 116-1[! 6) i is smaller than the X-rays (beams 11a to 116) fjt that pass through the center. Therefore, the target 3 of the object 2 has X-rays (beams 11a to 116) that pass through the center of the wedge 4. 11b), a sufficient amount of X-rays are irradiated, and the portions of the subject 2 other than the target 3 are exposed to the X-rays transmitted through the periphery of the wedge 4.
Since the rays (beams 10e to 11i, 116 to 10b) are irradiated, the amount of X-rays is small and the dose to which the subject is exposed is reduced. As described above, the X-rays that have completely passed through the object 2 are sent to the detector 5.
is detected and used to reconstruct the CT image.

The CT device shown in Fig. 1 obtains data from X-rays that have passed through the entire subject, so reconstruction of the CT image can be performed in almost the same way as with a normal OCT device, but the spatial resolution of the three parts of the target is difficult to raise. In other words, in order to increase the spatial resolution, it is necessary to increase the number of detection elements in the detector 5 and increase the number of detection channels, but increasing the number of detection channels increases the electronic circuit of the detector and makes the device expensive. As the amount of data increases, image reconstruction becomes complicated. In order to improve these defects, as shown in Figure 1, increase the number of detection elements in the part corresponding to target 6 to make the detection channel denser, and reduce the number of detection elements in parts other than target 6 to make the detection channel coarser. There is a method of using a detector 5 made of It is possible to increase the spatial resolution by increasing the distance of 5 and increasing the magnification of the target part B, but in this case, the overall length of the detector becomes longer and the equipment becomes shorter. At the same time as it became larger, there were also structural limitations of the detector itself, and there were limits to the improvement of spatial resolution.

The Targera CT apparatus shown in FIG. 2 overcomes the above-mentioned drawbacks of the apparatus shown in FIG. That is,
In the target CT device shown in Fig. 2, X-rays are irradiated only to the target target L-3 of the object, so the detector 5 only needs to be located in the part corresponding to the target B, so the number of detection elements is increased to detect Even if the channels are densely arranged, the number of channels is the same as that of a normal CT device. The length of the device 5 is usually comparable to the length of the OCT device. In this way, the spatial resolution can be increased with this device. However, in general, in order to reconstruct both CT and CT images, data of X-rays that pass through the entire area of the subject is required.
CT equipment that can only obtain # data
There is a drawback that both images cannot be reconstructed, and in order to reconstruct, data that is not detected in data other than the target (hereinafter referred to as missing data) must be estimated by some method.

The present invention was made in order to eliminate the drawbacks of the conventional CT apparatus.It is possible to reconstruct both CT and CT images using a detector with a small number of detection elements, and to obtain high spatial resolution. The purpose of the company is to provide CT girders (Tagera).

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 3 is a plan view of an embodiment of the invention. In the figure,
1 is an X-ray tube 6 that generates X-rays, and the X-rays generated from the X-ray tube 1 are directed toward the rotation center 7 of the X-ray tube 1 by a collimator B installed in front of the X-ray tube 1. It is shaped like an asymmetrical fan beam.

4 is a wedge that transmits X-rays while attenuating them;
The collimator 6 has a left-right asymmetrical shape as shown in the figure.
The fan-beam-shaped X-rays transmitted through O are the central and right-hand portions of the wedge 4, and the central portion is thin and the right-hand portion is thick. The X-rays that have passed through the thin central part of the wedge 4 are irradiated onto the target section 6 of the subject 2. Fan-shaped X-rays that have passed through the target 3 (
As shown in the figure, the viewing angles of the beams 11α to 11b) are −90 degrees and +90 degrees in the left and right directions as a straight line connecting the X-ray tube 1 and the center of rotation 7. 5 is the X that passes through the target 3 of the subject 2 and the right fi11 part of the subject 2.
(This is a detector that detects the beams 11α to 11b, 1oα to 11th), and the detection element of the detector 5 is the target 3t.
- The area corresponding to the transmitted X-rays (beams 11α to 11b) is large in number and the detection channels are dense), and the X-rays transmitted to the right side of the subject 2 (beams 10α to 118)
The number of parts corresponding to 1 is small, and the detection channel is rough.

In this embodiment, the right side of the wedge 4 is thicker and the radiation dose of the subject 2 is reduced because it attenuates more xi, and the detector 5 has a coarser detection channel on the right side. As a result, the number of channels in the entire detector is reduced. The reason why the detection channel of the detector 5 can be configured in this manner is that, in accordance with the principle of re-44M reconstruction of the 01 image, there is little shadow change of data in the non-target portion compared to the reconstruction of the target portion.

By the way, with the configuration of this embodiment, there is no loss of transmitted X-ray data regarding the target part, but half of the data in the non-target part (data of the part corresponding to beams 116 to 10b) is missing data. There is. However, when considering all the X-ray data emitted from each rotational imaging position in a series of rotational imaging, it can be seen that there is transmitted X-ray data corresponding to the missing portion. This will be explained using FIG. 4.

In Fig. 4, when the X-ray tube 1 is at point A, which is 8 degrees away from its initial position, the angle at which the left side of the object is transmitted beyond the target (-ψ is radiated by the ship, and the collimator 6
Data on the imaginary trajectory a of the beam blocked by the beam b, and the beam b emitted at an angle (+9 degrees) that passes through the right side of the subject when it is at point B, rotated by π12ψ degrees from point A. The data is the same as that of Data from beam a is missing data, but data from beam b can be detected. Therefore, the data from the missing beam α can be obtained from the data from the beam only. Expressing this mathematically, let φ(θ, ψ) be the transmission data of a beam with a viewing angle of ψ degrees from the X-ray tube at a rotation angle of 0 degrees from the initial position. φ(θ, -ψ) = φ(0+π+2ψ , ψ) ・・・・・・
...It can be expressed as (1). Assuming that the angle at which the outermost part of the subject target is viewed is 90 degrees (see Figures 6 and 4) -
Data φ(θ,
-·ψ) is missing data, this data can be supplemented (interpolated) with φ(01π+2ψ, ψ) using the m(]) formula.

To explain this relationship using the rhinobram in Figure 5,
The hatched area on the left side of the figure is missing data, and the data φG in this area can be created by interpolating the detected data φb and φC in the shaded area on the right side.

There are various methods for creating missing data by beam interpolation, but for the sake of simplicity, we will explain the primary interpolation using two points. In equation (1), if θ and ψ are continuous, there is always an opposing beam. However, in general, θ and ψ take discrete values in a CT rack. Considering the missing data in the sinobram of FIG. 5, the discrete values of ψ become symmetrical about the center of rotation. Therefore, ψ has opposing discrete values. Now, considering the case of finding φα,
As shown in Fig. 6, the opposing beams φα' of φα are φb and φ
It exists between 0 and 0. The discrete value of θ is every 0.6° (36
600 at 0°), it can be defined as P −1−1=(π+2ψ)10.6. Here, P is an integer value and t is a decimal number or less. φα is calculated for one fire interval using the following equation.

φα(θ, −ψ)=φ(θ10.6 X P #ψ)X
(1-/-) 10φ (θ+〇, 6 X (P-1-1)
, ψ)x7 If θ+06×P≧660°, an angle of θ10.6XP−660° is used.

The interpolation method is not limited to the above method, and four or more points may be used. The interpolation is not limited to between one hiboke and may be interpolated several times.

In this way, if radiation data at all rotational positions of the X-ray tube are obtained, missing data can be interpolated with data at other rotational positions to obtain a complete data set, so reconstruction of the 01 image can be performed using conventional OCT. It can be done in the same way as the device.

As explained above, in the CT rack according to the present invention, the spread of the X@ beam and the arrangement of the X-ray detectors are configured asymmetrically with respect to the X-ray beam passing through the center of rotation of the subject, so By using a detector with a small number of channels, it is possible to obtain a low-cost CT apparatus that has high spatial resolution, is capable of reconstructing both CT and CT images, and is inexpensive.

[Brief explanation of the drawing]

Figures 1 and 2 are plan views of the conventional Targetera CT device;
FIG. 6 is a plan view of an embodiment of the present invention, and FIGS. 4, 5, and 6 are explanatory diagrams of interpolation of missing data. 1... X-ray tube, 2... Subject, 6... Target, 4... Wedge, 5... Detector,
6...Collimator, 7...Rotation center. 159.1. °′

Claims (1)

[Claims] In an eT device in which an X-ray tube and an X-ray detector both rotate around the circumference of an object, and an image of a part of the object is reconstructed using data from emitted X-rays at each rotational position, the X-ray tube The shape of the spread of emitted X-rays and the arrangement of the XW detectors are both asymmetrical with respect to the X-ray beam transmitted through the center of rotation of the A CT key device characterized in that X-rays are always irradiated, and the remaining part of the subject is irradiated at some rotational positions so that only 7'C X-rays are irradiated.