JPH0532058B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention belongs to the technical field of X-ray tomography devices (hereinafter referred to as X-ray CT devices).

(Prior art and problems to be solved) Conventionally, when obtaining an X-ray tomographic image (hereinafter referred to as a tomographic image) of a desired slice plane of a subject, for example, a patient, an X-ray CT apparatus fixes the position of the patient. While rotating the X-ray tube around the patient in the vertical plane having the slice plane,
By irradiating the line, all projection data from all directions on the slice plane is collected, image reconstruction is performed based on this total projection data, and a tomographic image of the desired slice plane is displayed on the display device. was configured to display.

Then, when trying to obtain a plurality of tomographic images for a plurality of different slice planes of a patient using the X-ray CT device, the X-ray tube for the first slice plane is
After rotating 180° or 360° and collecting all projection data for the first slice plane, the X-ray tube is deactivated and the tube is positioned in a vertical plane with the second slice plane. To do this, one must spend time horizontally moving the patient, then rotating the x-ray tube and exposing the x-ray to the second slice plane.

Therefore, conventional X-ray CT devices have the problem that when obtaining multiple tomographic images for different slice planes, the patient is restrained for a long time, which reduces the operating efficiency of the X-ray CT device. . Furthermore, when obtaining multiple tomographic images for different slice planes of a patient injected with a contrast agent, conventional X-ray CT systems have two methods: when acquiring projection data for the first slice plane and when acquiring projection data for the last slice plane. There is also the problem that it is not possible to obtain multiple tomographic images under the same physiological state because the physiological state of the patient changes depending on when the excision data is collected.

The present invention has been made in view of the above circumstances, and aims to shorten the time required to transport the subject during data collection, reduce the time required to restrain the subject, and shorten the time required to collect multiple slice planes. The purpose is to provide an X-ray CT device that can perform

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention provides a device that rotates around a subject and has a predetermined thickness in a direction perpendicular to the rotation plane. X that passes through the specimen
an X-ray source that emits radiation; an X-ray detector that detects the X-rays that have passed through the subject; a subject moving means for moving the subject in the body axis direction during rotational movement of the radiation source; a data collecting means for collecting data obtained from the detector; and an image capturing means based on the data supplied from the data collecting means. and an image reconstruction means for reconstructing the image.

(Operation) It is possible to scan so as to move an amount equivalent to the thickness of one slice tomographic plane in a short time.

(Example) The present invention will be specifically explained below using Examples.

FIG. 1 is a system block diagram of an X-ray CT apparatus showing one embodiment of the present invention. 1 is a pedestal;
It is equipped with an insertion hole 6 into which the subject M placed on the bed top plate 2 is inserted, and the inserted subject M
An X-ray tube 3 and an X-ray detector 4 as an X-ray source are sandwiched between
are arranged opposite to each other. Here, the X-ray tube 3 is controlled to generate X-rays by a high-pressure generator 7, and the insertion hole 6 is controlled by an X-ray tube drive control device 5.
The X-ray detector 4 has a plurality of individual detectors arranged in an array along the circumferential surface of a diagonally arranged cylindrical holding member. The detector 4 is configured such that a part of the detector 4 always receives the X-rays transmitted through the subject from the X-ray tube 3. Further, the bed top 2 is configured to be able to continuously move along the body axis direction of the subject M by a bed drive control device 8 (sometimes referred to as a subject moving means). ). 9 is a data collection device that collects data obtained by the X-ray detector 4; 10 is a correction calculation device for converting the data in the data collection device into appropriate reconstruction data; and 11 is a correction calculation device. It is an image reconstruction device that reconstructs an image based on data sent from the arithmetic device 10, and 12 is an image reconstruction device 11.
This is a display device that performs display based on image data from. Reference numeral 13 denotes a system control device that controls each of the above-mentioned devices.

Next, the operation will be explained.

In the above apparatus, it is assumed that the X-ray tube 3 generates a fan beam-like X-ray (hereinafter also simply referred to as a fan beam), and the X-ray detector 4 detects this fan beam as a unit. Furthermore, this fan beam does not stop at 360° rotation, but is capable of continuous rotation of 10 rotations or infinite rotations in this embodiment. Such continuous rotation can be achieved using known slip rings or by employing electron beam scanning as disclosed in USP No. 4,158,142. While such a fan beam is continuously rotating and collecting data, the subject M is continuously moved by the bed drive control device 8. Assume that this amount of movement is, for example, an advance of Pmm per rotation of the fan beam. With this configuration, for example, it is equivalent to the fan beam rotating and translating in the body axis direction with respect to the stationary subject M, and the fan beam moves in a spiral around the subject M. Data will be collected (spiral scan).
That is, a spiral scan is performed by a combination of movements of the X-ray source and the subject. Data obtained in this manner can be defined as spiral data. Therefore, not only a CT device (4th generation CT device) that rotates only the X-ray tube 3 with the X-ray detector 4 fixed as in this embodiment, but also a CT device that rotates only the X-ray tube 3 with the X-ray detector 4 fixed, as well as Drive the detector in relative rotation
The aforementioned spiral data can also be obtained using a CT device (third generation CT device). If the positions of the helical scan X-ray source and fan beam are observed from the direction perpendicular to the body axis direction, a sinusoidal waveform XL with a period Pmm as shown in FIG. 2 will be drawn.

Next, a method of reconstructing an image from the data obtained as described above will be explained. First, generally speaking, there is a method in which the entire scan range is divided into small elements and reconstructed at once, for example, data obtained in one rotation of the X-ray tube from slice point S ₁ to S ₂ in Figure 2 is used. In this case, there is a known method (USP No. 4149247) that performs image reconstruction for each plane by approximating that the fan beam position is fixed at the center of the positions X ₁ and X ₂ in the X direction in Fig. 2. Conceivable. In addition, the data obtained from the two rotations from point S ₁ to S ₃ above are bundled (overlaid) using the following formula (1).
Slice position if it is data for one rotation
It can also be reconstructed as scan data represented by _X2 .

P=(θ, φ)P ₁₂ (θ, φ)+P ₂₃ (θ, φ)/2...
(1) Here, θ is the X-ray tube 3 and
It is the rotation angle of the linear fan beam FB and takes a value from 0 to 360°. P ₁₂ (θ _, φ) is the projection data obtained while the relative position of the X-ray tube 3 with respect to the subject M changes from S ₁ to S ₂ in FIG. Line tube relative position is from S ₂
This is projection data obtained during the period leading up to _S3 .

The above method can be used up to the point where an image for one slice, which is three or more rotations, is obtained.

Furthermore, the same effect as in the above case can be obtained by independently reconstructing the projection data obtained in each rotation and averaging the obtained plural images.

As mentioned above, creating a single image by bundling data for a plurality of consecutive rotations is useful in reducing artifacts. That is, generally X-rays
In a CT apparatus, the thickness of the X-ray fan beam when viewed from the side is approximately proportional to the thickness of the X-ray tube since it is not parallel X-rays. When examining a subject with an X-ray beam in this way, if the subject has large changes in the slice thickness direction, the projection data will contain slightly contradictory parts at each angle θ, which is often referred to as the clipping effect. This will result in artifacts. A similar phenomenon occurs in the case of the present invention, which will be explained with reference to FIG. In Figure 4, A is X
This is an intensity profile in the slice thickness direction of the X-ray fan beam obtained when the ray tube is at the S ₁ position (ie, X=X ₁ ) in FIG. 2. The slice thickness at this time is tmm. As the x-ray tube rotates,
The slice plane moves in the direction of the subject's body axis, and for example, at θ = 180°, the X-ray tube position remains at the initial position.
It is not located at X ₁ , but is located at a position that is advanced by P/2 from there (X = X ₁ + P/2). Here, if P=t, the intensity profile and position of the X-ray fan beam in the slice thickness direction at θ=180° will be as shown in FIG. 4B. Here, A and B
In the waveform, the hatched portions mean that different subjects are being measured. Image reconstruction calculations are performed on the premise that all projection data are the results of measuring the exact same subject, so the hatched portions in the A and B waveforms may cause some distortion to the image. Seem. This is true not only for the relationship between θ=0° and 180°, but also for the entire range of θ.
Particularly in the data collection method of this embodiment, phenomena similar to the clipping effect described above are likely to occur.

The present invention solves these problems using the following principle. For example, when you want to obtain an effective slice width of tmm, the subject M is sent at a rate of t/2mm per rotation of the X-ray fan beam, and the X-ray fan beam FB is focused to t/2mm using a collimator, etc. That's what I do. As a result, the fifth
The intensity profile and location as shown is obtained. In the same figure, A and B are the intensity profile and position in the slice thickness direction of the X-ray fan beam obtained when the X-ray tube relative positions are X = X ₁ and X = X ₂ , respectively, and C and D are similarly X. =X ₁ +X ₂ /2=X ₁ +t/4, X=X ₂ +X ₃ /2=X ₁ +3/4t. As a result, by bundling the projection data as shown in equation (1) above, a profile and position as shown in FIG. 6 can be obtained. That is, waveforms E and F are obtained in the slice thickness direction of projection data obtained at θ=0° and 180°, respectively. The hatched portion is relatively small compared to the case of FIG. 4 described above. In other words, image distortion is reduced.

Furthermore, when performing the spiral skiving as described above, there are the following problems. If we start collecting projection data at θ=0° and complete collecting one image's worth of projection data at a position θmax almost close to θ=360°, P(0, φ) and P
(θmax, φ), the scan plane to be measured is shifted, so the content of the data will be quite different. It is well known that when there is a discontinuous difference between adjacent data, artifacts are more likely to occur than when there is a continuous shift. In order to solve such problems, the present invention includes a correction calculation device 10 that performs the following processing. The principle of this correction calculation device is that one tomographic plane (slice plane)
One part obtained at the beginning or one part obtained at the end of the data used for image reconstruction is obtained at the same rotation angle in the data of one tomographic plane obtained before or after that. This is corrected using the data obtained.

The projection data obtained from θ=0 to θX is data that has been subjected to calculation processing as shown in the following equation (2).
P'(θ, φ) is substituted (θX is arbitrarily set depending on the width of the required image reconstruction area and the degree of reduction of artifacts).

P'(θ, φ)=W(θ)・P ₁₂ (θ, φ) +(1−W(θ))・P ₂₃ (360°+θ, φ) ……(2) Here, W(θ ) is a function that is 0 at θ=0° and 1 at θ=θX, as shown in FIG. 7, and is connected, for example, by a straight line, without any steep changes.

Instead of such correction, θ=θY to θmax
The data obtained by the previous rotation is corrected by the data P′ (θ, φ), which is processed by the following equation (3) (θY is the same as θX). have meaning).

P'(θ, φ) = W(θ)・P ₁₂ (θ, φ) + (1−W(θ))・P ₂₃ (θ−360°, φ) ……(3) Here, W( θ) is a function that takes 1 when θ=θY and 0 when θmax, and connects them with, for example, a straight line without any steep changes.

By providing such a correction calculation device 10, adjacent data can be evaluated as continuous deviations, so that the occurrence of artifacts can be reduced. Note that the above correction was for creating an image with one rotation from S ₁ to S ₂ and a slight extension from that, but it is possible to create one image by making two or three rotations and then a slight extension. Needless to say, it is possible to do so.

In the above embodiment, the fan beam 1
If the relationship between the movement amount P of the bed per rotation and the slice width t is selected such that P<t, the deviation of the scan plane per one spiral rotation is small, and therefore the occurrence of artifacts is reduced.

The present invention is not limited to the embodiments described above, and various modifications are possible. For example, in the above embodiment, 0 to
1 from projection data obtained over 360°
Although we have described X-ray CT that generates images, it can also be applied to X-ray CT equipment such as the one shown in FIG. 8 that reconstructs images from scan data of less than 360 degrees. That is, X
The radiation source moves reciprocatingly or one-way at high speed on the orbit XL, and the detector group 4' is arranged along about 2/3 of the circumference, and continuously scans the object M during repeated scanning. You can send it to In this case as well, it is possible to obtain projection data for one image from point a to point b of the X-ray source 3'. According to such a device, a trajectory like a series of U-shaped scans can be obtained as shown in FIG. The data obtained thereby can be defined as deformed spiral data. In this case, in Figure 8,
The X-ray source 3' must be moved at such a high speed that the speed of movement from b to a (during return) can be ignored compared to the speed of movement from position a to b (during data collection). , this is fully possible by employing a known electron beam scan. According to the apparatus of this embodiment, since the moving time of the X-ray source 3' can be shortened, the slice interval Pmm can also be minimized, and therefore, by expanding the above formula (1), it is possible to save projection data for a large number of times. By superimposing the images to create one slice image, it becomes easy to reduce artifacts.

[Effects of the Invention] According to the present invention described in detail above, it is possible to shorten the time required to transport the specimen during data collection, reduce the time to restrain the specimen, and reduce the time required to collect multiple slice planes. It is possible to provide an X-ray CT apparatus that can be shortened and can reduce artifacts due to clipping effects.

[Brief explanation of the drawing]

FIG. 1 is a system block diagram showing one embodiment of the present invention, FIG. 2 is a schematic explanatory diagram showing the relative trajectory of the X-ray source according to the embodiment, and FIG. 3 shows the state of the fan beam according to the embodiment. A schematic explanatory diagram, Fig. 4 is an explanatory diagram of the reason why distortion occurs in an image, and Figs. 5 and 6 respectively show the reason why distortion occurring in an image can be reduced by adopting the embodiment device of the present invention. , FIG. 7 is an explanatory diagram of functions used in correction calculations, FIG. 8 is a schematic explanatory diagram showing another embodiment of the present invention, and FIG. 9 is an explanatory diagram of X according to the other embodiment.
It is an explanatory diagram of the relative trajectory of a radiation source. 1... Frame, 2... Bed top, 3,3'...X
Radiation source, 4...X-ray detector, 5...X-ray drive control device, 6...detector drive device, 7...high pressure generator, 8...bed drive control device, 9...data acquisition device 10... Correction calculation device, 11... Image reconstruction device, 12... Display device, 13... System control device.

Claims (1)

[Scope of Claims] 1. An X-ray source that rotates around the subject and emits X-rays that pass through the subject with a predetermined thickness in a direction perpendicular to the plane of rotation; an X-ray detector that detects X-rays transmitted through a specimen; and an X-ray detector that detects X-rays transmitted through a specimen; It has a subject moving means for moving in the axial direction, a data collecting means for collecting data obtained from the detector, and an image reconstructing means for reconstructing an image based on the data supplied from the data collecting means. An X-ray tomography device characterized by: