JPH06186341A - Positron computer tomography system - Google Patents

Abstract

PURPOSE:To provide a positron computer tomography system comprising small number of detectors in which effective visual view is widened while making uniform the sampling density. CONSTITUTION:In a positron computer tomography system comprising at least a pair of detectors 10a, 10b disposed oppositely with each detector comprising a plurality of gamma-ray detecting elements 11, and a stage 20 for placing a specimen in the center of visual view of the detector, a rotary drive motor for rotating one or both of the detector and the specimen stage relatively about the center of visual view is provided and the center line CL connecting the centers of the detectors disposed oppositely is spaced apart by a predetermined distance from the center of visual view.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positron computed tomography apparatus (hereinafter referred to as "positron CT apparatus").

[0002]

2. Description of the Related Art Positron CT systems are available for ¹¹ C, ¹³ N, ¹⁵
Positron-emitting nuclides such as O and ¹⁸ F are administered to a subject, and they are emitted in opposite directions as the positron disappears.
This is a CT apparatus that uses a pair of gamma rays, and detects the gamma ray pairs in the subject by a coincidence counting method with a gamma ray detecting element that is arranged facing each other.

A conventional general positron CT apparatus is shown in FIG.
As schematically shown in FIG. 1, a large number of gamma ray detecting elements 1 are arranged in a ring shape. These gamma ray detecting elements 1 are divided into several groups, and each group is treated as one detector 2. Then, simultaneous counting of gamma rays is performed between the specific pair of detectors 2 arranged facing each other.

However, in order to collect the data necessary for constructing a tomographic image, the detectors 2 do not necessarily have to be arranged around the subject 3 over the entire circumference, and the required minimum number of detectors 2 is required. There is enough. For example, as shown in FIG. 11, even in a positron CT device having a configuration in which only one pair of detectors 2 among detectors arranged in a ring are left, the detector 2 or the subject 3 can be rotated. It is possible to collect the necessary data.

[0005]

A conventional positron CT apparatus composed of a small number of detectors 2 as described above,
Although there is an unavoidable increase in measurement time, there is an advantage that the entire device can be simplified and cost can be reduced.

However, such a positron CT apparatus has a problem that the effective field of view is narrow because the detector 2 is arranged only in a part in the circumferential direction. Since this effective field of view is determined by the width of the detector 2, it is necessary to increase the number of detectors 2 in order to expand the field of view. However, merely increasing the number of detectors 2 will reduce advantages such as simplification of the device. become.

Further, since the center line CL connecting the respective centers of the detectors 2 passes on the center of the visual field (in this case, the center of rotation of the detectors 2) VC, the sampling density distribution becomes non-uniform. There are also problems. Before describing this problem in more detail, a sinogram, which is a diagram showing the sampling distribution state of the positron CT apparatus, will be described.

The data acquired by the positron CT apparatus is obtained by detecting a pair of gamma rays emitted in opposite directions due to pair annihilation of a positron and an electron by the coincidence counting method,
It is the number pair of the gamma ray detecting element 1. In addition, FIG.
As will be clearly shown in FIG. 3, the gamma ray source 4 exists on a straight line connecting the gamma ray detecting elements 1 ′ detecting the gamma ray pair 5, and this straight line is generally called a simultaneous measurement line.

The simultaneous measurement line can be replaced with projection data. The projection data is a group of simultaneous measurement lines grouped according to their directions. For example, in FIG. 12, when the data of only the lines in the vertical direction on the paper surface of the drawing are collected and the frequency distribution is obtained as a function of the distance T between the visual field center VC and the simultaneous measurement line, a graph (a) shown in FIG. 12 is obtained. Projection data can be obtained. In other words, this means that the radiation source 4 in the subject 3 is projected in the vertical direction. Similarly, what is projected in the horizontal direction is the graph (b) shown in FIG.

The sinogram is a representation of such projection data, in which the horizontal axis represents the distance T between the visual field center VC and the simultaneous measurement line, and the vertical axis represents the projection angle θ.

Here, a sinogram of a positron CT apparatus including a pair of detectors 2 as shown in FIG. 11 will be created.

First, for the sake of simplicity, each detector 2 is shown in FIG.
5 gamma ray detecting elements 1a to 1e, 1f
˜1j. The simultaneous measurement lines 6 created by the gamma ray detecting element 1a are 1a-1f, 1a-1g, and 1a.
-1h, 1a-1i, and 1a-1j. Looking at the lines 1a-1f, θ is 0 ° (the vertical direction is 0 °), and T is −2 pitch when the interval between adjacent gamma ray detecting elements is 1 pitch. Also, line 1a-
For 1 g, θ is the angle increased by one unit in the plus direction, and T is about −1.5 pitch. That is, when the mating element shifts one by one to the right, θ increases by one unit in the plus direction and T also increases by −0.5 pitch. When this is expressed on a sinogram, it is expressed by five points that descend to the right as shown in FIG.

Next, looking at the gamma ray detecting element 1b, θ of the lines 1b-1f is one unit in the minus direction,
T becomes -1.5 pitch, and thereafter the same behavior as above is exhibited. When this is expressed on a sinogram, five points in the lower right direction are plotted above the above-mentioned five points. Similarly, if you plot all the simultaneous measurement lines,
Finally, a rhombic pattern as shown in FIG.

When the detector 2 is rotated, the field center VC and the center of rotation are the same, so the value of T on each coincidence counting line does not change, and only θ changes by the rotation angle.
Therefore, each sampling point is detected by the detector 2 on the sinogram.
With the rotation of No. 1, the movement moves upward or downward in FIG.

Looking at the sampling points of a certain θ (data of a certain projection direction, that is, one row on the sinogram), the interval between the sampling points at the time of rest is the pitch of the gamma ray detecting element 1 and each θ value. The sampling points are shifted by half pitch for each (each row). Therefore, when the detector 2 is rotated, the sampling points of each θ value fill the adjacent empty sampling points, and sampling is performed at intervals of half the pitch of the gamma ray detecting element 1.

FIG. 15 shows the sampling density distribution in the T direction of the finally obtained sampling distribution. From this figure, the sampling density distribution is T = 0 (center of visual field).
It can be seen that the distribution becomes triangular with vertices as the apex, the sampling density at the center of the visual field is maximum, and the sampling density decreases toward the periphery of the visual field. As described above, the positron CT apparatus including a pair of detectors according to the conventional method has a non-uniform sampling density distribution, and the S / N ratio of an image is not so good.

The present invention has been made in view of such circumstances, and an object thereof is to provide a positron CT apparatus composed of a small number of detectors, which has a wide effective field of view and a uniform sampling density. To provide.

[0018]

In order to achieve the above-mentioned object, the present invention provides at least one pair of detectors which are arranged to face each other and each of which is provided with a plurality of gamma ray detecting elements arranged in parallel. In a positron computed tomography apparatus having a subject setting stage for arranging the body in the center of the field of view of the detector, the detector and the stage for setting the subject are relative to each other with the center of the field of view as the center of rotation. It is characterized in that it is provided with a rotation driving means for rotating the center of the detector and the center line connecting the centers of the detectors arranged facing each other and the center of the visual field are separated by a constant distance.

When a plurality of pairs of detectors arranged facing each other are provided, the distance between the center line and the center of the visual field between the detectors of each pair is made different.

Further, in order to adjust the distance between the center line between the detectors and the center of the field of view, detector position changing means for changing the position of the detector is provided, or the position of the stage for setting the subject is changed. It is preferable to provide a stage position changing means for controlling the stage position.

[0021]

In the configuration of the present invention as described above, since the center line between the detectors of each pair is displaced from the center of the field of view, as shown in FIG. It will be shifted to the left or right.
Therefore, the sampling density distribution obtained when the detector or the stage for setting the subject is rotated is in a state of being widened in the T direction, the effective field of view is expanded, and the sampling density is made uniform.

When a plurality of pairs of detectors arranged to face each other are provided, the distance between the center line of each pair of detectors and the center of the field of view is made different to further expand the effective field of view. Also, it is possible to make the sampling density uniform. In addition, when the difference in the distance is set finely,
The sampling density is increased and the resolution of the device is improved.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described in detail below with reference to the drawings. In the following description, the same or corresponding parts will be designated by the same reference numerals.

FIG. 1 shows a positron C to which the present invention is applied.
It is a top view which showed the structure of T apparatus roughly. This Figure 1
In the figure, reference numerals 10a and 10b denote detectors each having a plurality (five in the illustrated embodiment) of gamma ray detecting elements 11 arranged in parallel. Each detector 10a, 10b
The gamma ray detecting element 11 of is fixed on the detecting element supporting plate 12, and the detecting element supporting plate 12 is
It is fixed to one surface (upper surface) of the annular rotary plate 14 rotatably installed above. In addition, the detector 10
The a and 10b are arranged to face each other with the central opening of the rotary plate 14 interposed therebetween. More specifically, each gamma ray detecting element 11 in one detector 10a is
Corresponding gamma ray detecting element 1 in the other detector 10b
It is arranged so as to be coaxial with 1. Furthermore, the detector 1
A center line CL connecting the centers of 0a and 10b is located at a constant distance from the center of rotation of the rotary plate 14. In this structure, the center of rotation of the rotating plate 14 is the visual field center VC.

In the illustrated embodiment, each detector 10a, 10b
The detection element support plate 12 is fixed to the rotary plate 14 by the fixing bolts 15. However, by appropriately selecting from the plurality of screw holes 16 formed in the rotary plate 14, the detector The fixed positions of 10a and 10b can be changed. The screw holes 16 on each side are formed at regular intervals along the normal direction of the rotary plate 14. Even when the fixed positions of the detectors 10a and 10b are changed, it is necessary to maintain the facing relationship between the detectors 10a and 10b.

The rotating plate 14 is a transmission mechanism 1 such as a belt.
It is adapted to be rotationally driven by a rotational drive motor 18 via 7. Further, a stage 20 for placing a subject is arranged in the central opening of the rotating plate 14 so that the subject 19 is placed at the center of rotation of the rotating plate 14, that is, the center VC of the visual field.

Next, the operation of the positron CT apparatus according to the present invention having the above structure will be described.

As shown in FIG. 2, which is a schematic diagram of the positron CT apparatus of the present invention, the center line C between the detectors 10a and 10b is shown.
L is separated by a fixed distance from the center of rotation of the rotating plate 14, that is, the center VC of the visual field. The rhombus pattern on the sinogram showing the sampling distribution at the position (angle 0 °) shown in FIG. 2A is shifted from the center to one side in the T direction as shown in FIG. 3A. Then, when the rotary plate 14 is rotated from 0 to 180 degrees, the detectors 10a and 10b also rotate, and the diamond pattern of FIG. 3A moves downward, resulting in the sampling density in the T direction. The distribution is as shown in FIG.

Further, the detector 10a after being rotated by 180 degrees,
As shown in FIG. 2B, the state of 10b is an arrangement shifted to the opposite when the angle is 0 degree. Further, the rhombic pattern on the sinogram at that time is also shifted to the side opposite to the pattern of FIG. 3A (see FIG. 3B). Then, rotate the rotary plate 14 from 180 degrees to 360 degrees.
When it is rotated up to 0 degree, the rhombus pattern similarly moves downward, and the obtained sampling density distribution is as shown in FIG. 4B, which is shifted in the opposite direction to the distribution from 0 degree to 180 degrees. Distribution. Therefore, the detector 10
T when a and 10b are rotated from 0 degree to 360 degrees
The sampling density distribution in the direction is shown in (a) and (b) of FIG.
4C, that is, the trapezoidal shape shown in FIG. Comparing this figure with FIG. 15 showing the sampling density distribution obtained when the center line CL between the detectors coincides with the visual field center VC, the effective visual field is expanded and
It can be seen that the part where the sampling density is uniform is large.

As described above, by shifting the center line CL between the detectors 10a and 10b from the visual field center VC at a constant distance, it is possible to expand the effective visual field and make the sampling density uniform.

Further, in this embodiment, the detectors 10a, 1
Since the fixed position of 0b can be changed to adjust the distance, the width of the effective visual field can be freely changed, and sampling can be performed at a desired density.

In the above embodiment, the detector has only one pair, but two detectors may be provided, as schematically shown in FIG. In this case, one pair of detectors 10a, 1
Distance t between the center line CL ₁ between 0b and the visual field center VC
₁ is the center line CL between the other pair of detectors 10c and 10d
_The size is different from the distance t ₂ between ₂ and the visual field center VC. Although not shown in FIG. 5, the detectors 10a to 10a ...
10d can be rotated by a rotary plate similar to that shown in FIG.

In such a configuration, the detector 10
The sampling density distribution by a and 10b is shown in FIG.
6B, and the sampling density distribution by the detectors 10c and 10d is as shown in FIG. 6B. Finally, the sampling density distribution of the entire apparatus is the sum of the sampling density distributions of the two pairs of detectors 10a to 10d, as shown in FIG. As can be seen from FIG. 6, by increasing the number of detectors to two pairs, the effective field of view can be expanded and the area of uniform sampling density can be widened as compared with the case where there is only one pair of detectors. From the above,
It will be easily understood that the number of detector pairs can be further increased to further expand the effective field of view and further homogenize the sampling density.

Here, the difference between the distances t ₁ and t ₂ between the center lines CL ₁ and CL ₂ and the visual field center VC is set to 1/4 of the pitch between the adjacent gamma ray detecting elements 11. In such a case, the sampling distributions obtained by the detectors 10a and 10b and the detectors 10c and 10d are represented by a sinogram,
White circles and black circles shown in FIG. 7, respectively, are obtained, which means that the distribution of sampling points is doubled. Also, FIG.
Shows the sampling density distribution in the T direction obtained when these detectors 10a to 10d are rotated.
As is apparent from FIGS. 7 and 8, the detectors 10a-10a
By adjusting the position of d, it becomes possible to collect sampling points in detail, and a fine image can be obtained.
As described above, by appropriately changing the difference between the distances t ₁ and t ₂ , the degree of freedom of the sampling density distribution is increased.

The pair of detectors 10a, 10 shown in FIG.
Even if the positron CT device is composed of only b, the detectors 10a and 10b are rotated every time the rotating plate 14 is rotated once.
By shifting, sampling points can be collected in detail.

In both of the above two embodiments, the detector 10
It is configured to rotate a to 10d by the rotating plate 14. However, since the detectors 10a to 10d only need to be rotated relative to the visual field center VC, the detector 1
The same effect can be obtained by fixing 0a to 10d and rotating the subject setting stage 20. Although various structures are conceivable for rotating the stage 20 for installing the subject, the structure shown in FIG. 9 is preferable.

As shown in FIG. 9, the subject-installed stage 20 is rotatably supported by a stage support plate 21, and a rotation drive motor 22 also fixed to the stage support plate 21 transmits a belt or the like. Mechanism 23
It is rotationally driven via. The stage support plate 21 is mounted on the base 24 with bolts 25.
In addition, a pair of detectors 10a, 10a including a plurality of gamma ray detecting elements 11 and a detecting element supporting plate 12 are provided on the base 24 so as to sandwich the subject setting stage 20 therebetween.
b is arranged facing and fixed. Detectors 10a, 10
The center line CL between b is separated from the visual field center VC that coincides with the rotation center of the subject setting stage 20 with a certain distance.

In such a structure, when the stage 20 for placing a subject is rotated, when viewed from above the stage 20, the detectors 10a and 10b are attached to the stage 20.
Will rotate. Therefore, the detectors 10a and 10b
Thus, the same measurement data as in the above case in which is rotated is obtained.

In this embodiment, the holes 26 of the stage support plate 21 through which the bolts 25 are inserted are the detectors 10a, 1
It is an elongated hole extending in a direction perpendicular to the center line CL between 0b, and the stage support plate 21 can be moved along the elongated hole 26 to change the distance between the center line CL and the visual field center VC. It is like this. By adjusting this distance, it is possible to change the size of the effective field of view and the uniformity of the sampling density.
This is similar to the case of the configuration of FIG. 1 in which b is rotated.

The position of the stage support plate 21 may be manually adjusted, but in this embodiment,
The stage support plate 21 is moved by a support plate driving motor 27 on the base 24 via a pinion rack transmission mechanism 28.

The positron CT apparatus of the above-mentioned embodiment is
In either case, the rotating plate 14 or the stage 20 for installing the subject is a vertical type in which the rotating axis is oriented in the vertical direction, but the present invention is applicable even if it is a horizontal type in which the rotating axis is oriented in the horizontal direction. is there.

Also, the positron C according to all the above embodiments
The T apparatus has a configuration in which a tomographic image of the subject 19 can be obtained in only one layer, but the detectors 10 to 10 located on the same plane.
If a plurality of d is laminated along the rotation axis of the rotating plate 14 or the stage 20 for placing a subject, a tomographic image of a plurality of layers can be obtained. Further, even with the configuration of the above-described embodiment, it is possible to obtain a plurality of layers of tomographic images by moving the rotating plate 14 or the subject installation stage 20 along the rotation axis.

[0043]

As described above, according to the present invention, the effective field of view can be expanded even with a positron CT apparatus having a small number of detectors.

Further, the non-uniformity of the sampling density distribution is improved, and the S / N ratio of the image is improved.

Furthermore, by using two or more pairs of detectors,
Finer sampling is possible and the resolution of the image is improved.

When the position of the detector can be changed, there is an effect that a desired effective field of view and sampling density distribution can be freely selected.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of a positron computed tomography apparatus according to the present invention.

2A and 2B are schematic explanatory views showing the positional relationship between the detector and the center of the visual field in the positron computed tomography apparatus of FIG. 1, where FIG. 2A is an initial position and FIG.

3 (a) and (b) are respectively FIG. 2 (a).
3 is a sinogram showing patterns of sampling points obtained at positions (b) and (b).

4A is a graph showing a sampling density distribution obtained when the detector is rotated from 0 to 180 degrees, and FIG. 4B is a graph obtained when the detector is rotated from 180 to 360 degrees. FIG. 3C is a graph showing a sampling density distribution obtained, and FIG. 7C is a graph showing a sampling density distribution obtained when the detector is rotated from 0 degree to 360 degrees.

5 is a schematic explanatory view similar to FIG. 2, showing another embodiment of the positron computed tomography apparatus according to the present invention.

6A is a graph showing a sampling density distribution obtained by one pair of detectors in FIG. 5, and FIG. 6B is a graph showing a sampling density distribution obtained by the other pair of detectors in FIG. (C) is a graph showing a sampling density distribution obtained by the entire apparatus of FIG.

FIG. 7 shows the difference between the distance from the center line between the detectors of one pair to the center of the visual field and the distance from the center line between the detectors of the other pair to the center of the visual field in the positron CT apparatus shown in FIG. When the pitch between the gamma ray detecting elements is set to 1/4, the distribution of sampling points obtained by each pair of detectors is a sinogram that is divided into white circles and black circles.

FIG. 8 is a graph showing a sampling density distribution in the T direction obtained under the same conditions as in FIG.

FIG. 9 is a plan view showing still another embodiment of the positron computed tomography apparatus according to the present invention.

FIG. 10 is a schematic explanatory view showing a conventional general positron computed tomography apparatus.

FIG. 11 is a schematic explanatory view showing another example of a conventional positron computed tomography apparatus.

12 is a schematic explanatory view showing the positron computed tomography apparatus of FIG. 10 together with projection data obtained by the apparatus.

FIG. 13 is a schematic explanatory view showing a simultaneous measurement line in a conventional positron computed tomography apparatus including a pair of detectors.

14 (a) is a sinogram showing sampling points obtained by one gamma ray detecting element in FIG.
13B is a sinogram showing sampling points obtained by all the gamma ray detecting elements in FIG.

15 is a graph showing a sampling density distribution obtained by the positron computed tomography apparatus shown in FIG.

[Explanation of symbols]

10a, 10b, 10c, 10d ... Detector, 11 ... Gamma ray detecting element, 12 ... Detection element supporting plate, 14 ... Rotating plate, 15 ... Fixing bolt, 16 ... Screw hole, 18
... rotational drive motor, 19 ... subject, 20 ... subject installation stage, 21 ... stage support plate, 22 ... rotational drive motor, 27 ... support plate drive motor.

Claims (4)

[Claims] 1. At least one pair of detectors, which are arranged to face each other and each of which has a plurality of gamma ray detecting elements arranged side by side, and a subject for placing the subject in the center of the visual field of the detector. In a positron computed tomography apparatus having a stage for rotation, a rotation drive means for relatively rotating both or either of the detector and the stage for setting a subject with the center of the field of view as a rotation center is provided, and they are arranged facing each other. A positron computed tomography apparatus, wherein a center line connecting the respective centers of the detected detectors and a center of the visual field are separated by a constant distance. 2. When a plurality of pairs of the detectors arranged to face each other are provided, the distance between the center line and the center of the visual field between the detectors of each pair is different. The positron computed tomography apparatus according to claim 1. 3. A detector position changing means for changing the position of the detector in order to adjust the distance between the center line and the center of the visual field between the detectors arranged opposite to each other. The positron computed tomography apparatus according to claim 1 or 2. 4. A stage position changing means for changing the position of the stage for placing the subject to adjust the distance between the center line and the center of the visual field between the detectors arranged opposite to each other. The positron computed tomography apparatus according to claim 1 or 2.