JP3204651B2 - Computer tomography equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a computer tomography apparatus for performing continuous scanning, and more particularly to a computer tomography apparatus having a simple configuration capable of reducing the amount of calculation from the collection of projection data to reconstructing a tomographic image and shortening the calculation time. The present invention relates to a computer tomography apparatus.

[0002]

2. Description of the Related Art A third method using a typical X-ray as radiation.
In a next generation X-ray computed tomography apparatus, an X-ray source and an arc-shaped detector are arranged so as to face each other with an imaging region where a subject is arranged.

In this X-ray computed tomography apparatus,
Rotating the X-ray source and the detector around a predetermined rotation center while maintaining the opposed arrangement relationship between the X-ray source and the detector, and exposing the X-ray from the X-ray source at predetermined rotation angles With this, projection data based on transmitted X-rays from each direction with respect to a certain slice of the subject in the imaging region is converted into a data collection unit D having a detector, an integrator, a multiplexer, an A / D converter, and the like.
Collected by AS (Data Acquisition System).
Then, this data is taken into an image reconstruction unit, and a calculation for image reconstruction is performed to obtain a tomographic image of a slice of the subject, and this image is displayed on a TV monitor or the like.

Recently, there has been a demand to view images in real time while performing continuous scanning. In order to meet such demands, various real-time image reconstruction systems have been developed. If an image can be displayed in real time during continuous scanning as described above, to what extent should the image be taken to rotate or to what time should the image be taken so that the subject is not exposed to X-rays more than necessary. There is a diagnostic merit that it can be determined while looking at the image.

[0005] As an example of a conventional real-time image reconstruction system, a plurality of reconstruction units are used to reconstruct an image continuously and in real time based on projection data acquired by continuous scanning, and to display the image. Some have. Here, time (angle) little by little
Each of the projection data with the deviation is taken into each reconstruction unit,
Image reconstruction is performed by these reconstruction units,
The obtained images are displayed continuously. However, this device requires a large number of reconfiguration units, and has disadvantages that the device becomes large and the cost is high.

As another conventional example, an apparatus using one reconstructing unit is described in Japanese Patent Publication No. 23136/1990. The outline of image creation by this device is shown in FIG.
This will be described with reference to FIG. First, as shown in FIG. 10A, during continuous scanning, an image A is created from 360 ° projection data collected for each Δα (view pitch). That is, as shown in FIG.
An image A is created using the projection data up to 0 ° -Δα, and the image A is displayed on a TV monitor.

Next, as shown in FIG. 10C, an image B from Δα to 360 ° is created. The image B is created by backprojecting (backprojecting) the image A at 0 ° with the projection data at 0 ° being negative and backprojecting the projection data at 360 °.

Next, as shown in FIG. 10D, the image C from 2Δα to 360 ° + Δα is similarly back-projected from the image B by subtracting the projection data at Δα from the image B, and the projection data at 360 ° + Δα. Is created by back-projecting.

[0009]

However, the above-mentioned second conventional apparatus has the following problems. Backprojection data for each pixel based on projection data at a certain rotation angle is expressed as follows.

BPi = ai · {li (xn + 1−xn) + xn} where BPi is back projection data, ai is a weighting coefficient, li is an interpolation coefficient, and xn + 1 and xn are projection data. is there.

For this reason, for example, when creating an image C,
In order to obtain backprojection data for each of the angles Δα and 360 ° + Δα, it is necessary to perform addition and subtraction processing twice and multiplication processing twice. Further, the angle Δα (for example, 0.36
When the projection data is collected for each angle (°), for example, there are 27 pieces of projection data in the angle range of 10 °. Even in the angle range of 10 ° alone, 108 addition and subtraction processes and 108 multiplication processes are performed.
It had to be done many times, and the amount of calculation was very large. Therefore, in order to display an image in real time, it was necessary to perform a large amount of calculations at high speed. However, this still has the disadvantage that the device becomes large and the cost increases.

Further, in this conventional example, since the second and subsequent tomographic images can be generated by back projection processing of one projection data of Δα, it is not necessary to store a large amount of projection data. In order to generate one tomographic image, it is necessary to store 360 ° projection data. Storing 360 ° of projection data requires a large-capacity storage medium, since there is a considerable amount of projection data alone, and also has the disadvantage of increasing the size of the apparatus and increasing the cost. .

Accordingly, an object of the present invention is to provide a simple and simple method for reconstructing an image with a small amount of calculation and for obtaining a tomographic image continuously and almost in real time from projection data obtained by continuous scanning. An object of the present invention is to provide a computed tomography apparatus having the above configuration.

[0014]

In order to achieve the above object, the present invention collects projection data on a subject while periodically changing a projection direction, and forms a partial image from projection data for a predetermined angle. In a computed tomography apparatus that obtains n partial images necessary for reconstructing a tomographic image, each time the projection direction changes by a predetermined angle, a partial image corresponding to the change is reconstructed. The image processing apparatus further comprises image generating means for sequentially generating a tomographic image composed of the latest n partial images including the minute partial images.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a computer tomography apparatus according to the present invention will be described below. FIG. 1 is a schematic configuration diagram illustrating a third-generation X-ray computed tomography apparatus, and FIG. 2 is a diagram illustrating projection data P acquired by continuous rotation of an X-ray source 1 and a detector 3.

In FIG. 1, an X-ray source 1 emits an X-ray beam FB having a predetermined angle β into an imaging region where a subject 2 exists. Plurality of detector elements 3-1 to 3-N
The detector 3 is disposed to face the X-ray source 1 with the subject 2 interposed therebetween, and detects X-rays transmitted through the subject 2. The driving device 10 continuously rotates the X-ray source 1 and the detector 3 around the subject 2 while maintaining the arrangement relationship between the X-ray source 1 and the detector 3 facing each other. Note that X
The source 1 rotates the view pitch Δα during continuous rotation (for example, 0.
X-rays every 36 °).

As shown in FIG. 2, the angle .theta.
The rotation angle of the beam FB, the X-ray source 1 and the detector 3
Increases by 360 ° for each rotation.

The DAS 4 comprises an integrator (not shown), a multiplexer 11 for taking in X-ray intensity data from each of the detector elements 3-1 to 3-N at high speed and serially,
An A / D converter 12 for converting the line intensity data into a digital signal; and a pitch Δα of the X-ray source 1 with respect to the subject 2.
Projection data P (m, ψ,
θ). Here, m is the number of rotations of the X-ray source 1, the rotation angle θ is the rotation angle (view) of the X-ray source 1, and the angle ψ is the X-ray source 1 and the rotation center O1 as shown in FIG.
And a straight line connecting the X-ray source 1 and each of the detector elements 3-k, from the angle −β / 2 to the angle + β.
/ 2 range. View pitch Δ as shown in FIG.
The projection data P (m, ψ, θ) is collected by the DAS 4 for each α.

The reconstructing unit 6 stores the projection data P from the DAS 4
(M, ψ, θ), and has a computer 6a for performing backprojection calculation processing for image reconstruction based on the projection data, and a memory 6b for storing the image. Constitute. FIG. 3 is a diagram for explaining the back projection calculation processing of the reconstruction unit 6.

First, as shown in FIG. 3A, the computer 6a in the reconstruction unit 6 backprojects the projection data P (m, ψ, θ) at an arbitrary rotation angle θ, Get BP. Then, the obtained back projection data BP
Is initially set to the backprojection data Q (the initial value of the backprojection data Q is set to zero).

Next, as shown in FIG. 3B, projection data P (m, ψ, θ) at a rotation angle obtained by adding an angle of Δα to an arbitrary rotation angle is back-projected, and the obtained inverse data is obtained. The projection data BP is added to the previously obtained back projection data Q,
New back projection data Q is obtained. The computer 6a repeatedly performs the above-described addition processing on the projection data P (m, ψ, θ) having the rotation angle θ of 0 ° to 360 °.

FIG. 4 shows a predetermined rotation angle (here, 10 degrees).
°) is a diagram for explaining a process of creating a partial image by adding back projection data obtained by back projection of projection data collected within a range. As shown in FIG.
Each projection data P (m, ψ, θ) within a predetermined rotation angle range
Are back-projected (BP in the figure), and the above-mentioned FIGS. 3 (a) and 3 (b)
By adding each of the backprojection data BP according to the above, partial images a1 to a37 can be obtained. The memory 6b stores the partial images a1 to a37 at the corresponding addresses. TV monitor 7 is 1
An image corresponding to 360 ° based on the partial image for each 0 ° is displayed in real time at a time corresponding to an angle range of 10 °.

Next, the operation of the embodiment configured as described above will be described. The periphery of the subject 2 is continuously scanned while the X-ray source 1 and the detector 3 are kept facing each other, and X-rays are emitted from the X-ray source 1 toward the subject 2 at every view pitch Δα. . The X-rays transmitted through the subject 2 with the continuous scan are detected by the detectors 3-1 to 3-N, and based on the X-ray intensity data, the multiplexer 11, the DAS 4 and the multiplexer 11,
The A / D converter 12 collects projection data P (m, ψ, θ) for each view pitch Δα. The projection data P (m, ψ, θ) for each pitch Δα of this view is
And the computer 6a performs the following arithmetic processing. Here, a description will be given of a calculation process in a case where an image is displayed on the TV monitor 7 at intervals corresponding to, for example, a rotation angle of 10 °.

First, according to the arithmetic processing shown in FIG. 3, the projection data P (m, ψ, 0) at 0 ° is back-projected to obtain back-projection data Q 0, and then the projection data P at Δα is obtained. (M, ψ, Δα) is back-projected and back-projected data BP1
Is obtained. Then, the back projection data BP1 is added to the back projection data Q0 to obtain back projection data Q1. Further, similarly, 2Δα projection data P (m, ψ, 2Δα)
Is back-projected to obtain back-projection data BP2, and back-projection data BP2 is added to back-projection data Q1 to obtain back-projection data Q2. Such addition processing is performed from 0 ° to 10 ° for each Δα to obtain the partial image a1. For example, if Δα is 0.36 °, the partial image a1 is expressed as follows.

A1 = ΣBPj Here, Σ is a sum operation where j is 1 to 27. This partial image a1 is stored in the memory 6b.

Similarly, each projection data within a predetermined rotation angle range from a rotation angle of 10 ° is back-projected according to the arithmetic processing shown in FIG. 3, and a partial image obtained by adding each back-projection data BP is obtained. a2 to a37 are stored in the memory 6b.

Next, the partial images a1 to a36 are read from the memory, and the computer 6a performs the following calculation to obtain the image A. Then, an image A corresponding to 360 ° is displayed on the TV monitor 7 at a time corresponding to the rotation angle of 360 °.

A = Σai where Σ is the summation operation for i = 1 to 36. Next, the image B is obtained by the following equation, and the rotation angle 3
An image B corresponding to 360 ° is displayed on the TV monitor 7 at a time corresponding to 70 °.

B = A-a1 + a37 By performing such arithmetic processing, 54 addition processings and 54 multiplication processings for forming the partial image a37, the partial image a1 is subtracted from the image A, and the image The image B can be obtained by performing only one addition / subtraction process of adding the partial image a37 to A. In the conventional apparatus, the backprojection data is subtracted for each Δα in order to create the image B. In the present embodiment, however, it is only necessary to add and subtract two partial images, and a tomographic image can be efficiently formed with a small amount of calculation. Can be reconfigured. Further, since the memory 6b only stores a partial image obtained by back-projecting the projection data, the memory 6b requires a smaller storage capacity than storing the projection data, and can be realized by a semiconductor memory. Can be performed at high speed.

In normal continuous rotation, one rotation per second is performed.
36 partial images can be obtained every second. Therefore, in the case of the present embodiment, since 36 partial images are displayed per second, the partial images are smoothly connected, and images can be displayed continuously in real time. In the embodiment, ai is created every 10 °. However, if one rotation takes 2 seconds, if ai is created every 5 °, the same real-time property can be secured.

Next, a second embodiment of the present invention will be described. In the first embodiment described above, the X-ray source 1 is rotated at a rotation angle of 0 °.
In the second embodiment, as shown in FIG. 5, the X-ray source 1 is set to a position at a rotation angle of 25 °, and the rotation angle is set to 25 °. Rotation angle 2
It is designed to rotate continuously from 5 °. FIG. 6 is a view for explaining a process of creating a partial image by adding back projection data obtained by back projection of projection data collected within a predetermined rotation angle range in the continuous rotation shown in FIG. is there. The memory 6b stores the partial images a3 to a39 at the time of continuous rotation from a rotation angle of 25 °.

For example, when continuous scanning is performed from a rotation angle of 25 °, each partial image is created by the computer 6a as in the first embodiment. However, here, the partial image a3 'is created by adding backprojection data obtained by backprojecting the projection data from the rotation angle of 25 ° to 30 °. Next, from a rotation angle of 380 ° to 385
A partial image a39 'is created by adding backprojection data obtained by backprojecting the projection data up to °. Then, from the rotation angle of 30 ° to the rotation angle of 380 °, the partial image a is obtained by adding back projection data obtained by back projection of each projection data collected at 10 ° within a predetermined rotation angle range.
4 to a38 are read from the memory 4b. By adding each partial image created in this way using the following equation, 1
An image C is created.

C = a 3 '+ Σai + a 39' where Σ is a summation operation for i = 4 to 38. As described above, also in the second embodiment, an image corresponding to 360 ° can be obtained with a small amount of calculation.

Next, a third embodiment of the present invention will be described.
In this embodiment, artifacts due to body motion are reduced by applying a weight function to projection data at the time of back projection. In the first and second embodiments, the rotation angle is 0 ° to 10 °.
Is back-projected to obtain the partial image a1, but in the third embodiment, the projection data P (m,
{, θ) is a partial image obtained by performing a convolution operation on a weighting function ω1 (θ) as shown in FIG.
Let it be 1. The weighting function ω1 (θ) is ω1 (θ) + ω1 (θ + 10 °) = when the rotation angle θ is 0 ° to 10 °.
1 is satisfied.

Similarly, weighting function ω1 is added to projection data P (m, ψ, θ) for rotation angle θ of 10 ° to 30 °.
(Θ−10 °) is subjected to a convolution operation, and the operation result is back-projected to create a partial image b2.

Generally, when the rotation angle θ is from n × 10 ° to (n
+2) × 10 ° projection data P (m, ψ, θ)
Convolution operation of the weighting function ω1 (θ−n × 10 °) and backprojecting the operation result to obtain the partial image bn
Create +1. Here, n is an integer.

Using the partial images b 1, b 2,..., B n +1 created by performing such arithmetic processing, the following arithmetic processing can be performed to obtain one complete image. A '= Σbi where Σ is the summation operation of i = 1 to 36.

The image A 'has a rotation angle θ of 0 ° to 370 °.
Is equivalent to a backprojected image obtained by performing a convolution operation on the projection data P (m, ψ, θ) with respect to the following weight function ω (θ).

Ω (θ) = Σω (θ−n × 10 °) Here, Σ represents a range of n from 0 to 35 and a range of θ from 0 ° to 3
This is a summation operation for 70 °.

The weight function ω (θ) is a function as shown in FIG. Using such a weighting function ω (θ), the projection data P (m,
It is known that an image (equivalent to A ′) obtained by (ψ, θ) convolution and back projection has few artifacts due to body motion. Therefore, it is understood that the tomographic image A ′ of the present embodiment obtained by Δbi is not affected by the artifact.

In the third embodiment, an image B ′ at a rotation angle of 370 ° similar to the image B created in the first embodiment described above is created by the following equation. B '= A'-b1 + b37 The image B' created by the above equation also convolves the weighting function ω1 (θ-10 °) with the projection data P (m, ψ, θ) for the rotation angle from 10 ° to 380 °. It is equivalent to an image created by calculating and backprojecting the result of the calculation.

As described above, in the third embodiment, the partial images b1, b2,... Are created using the weighting function, and these partial images are added to create one image. This has the same effect as the first embodiment. Further, the third embodiment has an advantage that artifacts on an image caused by a patient's body movement can be reduced.

The weight function may be, for example, a weight function ω 2 (θ) as shown in FIG. This weight function ω2 (θ) is expressed as ω when the rotation angle θ is from 0 ° to β.
2 (θ) = {1−cos (θπ / β)} / 2, when the rotation angle θ is from β to Δα, ω2 (θ) = 1, and when the rotation angle θ is from Δα to Δα + β, , Ω2
(Θ) = 1−ω (θ−Δα), and when the rotation angle θ is a value other than the above, ω2 (θ) = 0.

This ω2 (θ) is ω2 (θ) + ω2 (θ
-Δα) = 1. In this case, the partial image b1 is the projection data P for the rotation angle θ of 0 ° to Δα + β.
It is created by performing a convolution operation on the weight function ω2 (θ) for (m, ψ, θ) and backprojecting the operation result. Generally, the partial image bk has a rotation angle θ of (k-1)
The projection data P (m, ψ,
θ) is created by performing a convolution operation on the weight function ω2 {θ- (k-1) Δα} and backprojecting the operation result.

A tomographic image obtained by adding partial images created using such a weighting function ω 2 (θ) also has less artifact due to body motion. The present invention is not limited to the embodiments described above, and can be implemented with various modifications. For example, although X-rays are used as radiation, γ-rays may be used. Although the reconstruction has been described as the full reconstruction using the projection data of 360 °, the half reconstruction may be performed based on the projection data of 180 ° + fan angle. The so-called third-generation computed tomography apparatus has been described.
It is also applicable to the generation and fifth generation devices. Further, not limited to pulsed X-rays, continuous X-rays may be used.

[0046]

According to the present invention, there is provided a computer tomography apparatus having a simple configuration capable of reconstructing a tomographic image continuously and almost in real time from data obtained by continuous scanning.

[Brief description of the drawings]

FIG. 1 shows a first example of a computed tomography apparatus according to the present invention.
FIG. 1 is a schematic configuration diagram illustrating an embodiment.

FIG. 2 is a diagram showing projection data P acquired by continuous rotation of an X-ray source and a detector.

FIG. 3 is a diagram for explaining an addition process of backprojection data obtained by backprojecting projection data.

FIG. 4 is a diagram for explaining a process of creating a partial image by adding backprojection data obtained by backprojecting projection data collected within a predetermined rotation angle range.

FIG. 5 is a diagram illustrating a rotation angle when a continuous scan is performed from an arbitrary angle.

FIG. 6 is a view for explaining a process of creating a partial image by adding backprojection data obtained by backprojecting projection data collected within a predetermined rotation angle range in the continuous rotation shown in FIG. 5;

FIG. 7 is a diagram illustrating an example of a weight function ω1 (θ) used when back-projecting projection data.

FIG. 8 is a diagram showing a weight function ω0 (θ).

FIG. 9 is a diagram showing a weighting function ω2 (θ).

FIG. 10 is a view for explaining image reconstruction processing by a conventional computed tomography apparatus.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 X-ray source, 2 subject, 3 detector, 4 DAS, 6
... reconstruction unit, 7 ... TV monitor, 11 ... multiplexer,
12: A / D, FB: X-ray beam, PD: projection data,
BP: Back projection data.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) A61B 6/00-6/14

Claims (2)

(57) [Claims] 1. An imaging device comprising: a projection data acquisition unit that collects projection data on a subject while periodically changing a projection direction, and reconstructs a partial image from projection data for a predetermined angle range to form a tomographic image; In the computed tomography apparatus for obtaining a partial image of the above, each time the projection direction changes by a predetermined angle, a partial image of the change is reconstructed, and the partial image is composed of the latest n partial images including the partial image of the change A computer tomography apparatus, comprising: an image generating unit that sequentially generates tomographic images. 2. The computer tomography apparatus according to claim 1, further comprising display means for sequentially updating and displaying the tomographic image generated by said image generating means each time said predetermined angle changes.