JPH0359488A - Single photon emission tomographic device - Google Patents

Abstract

PURPOSE:To make new correction through data processing even if the position relation between a ceiling plate and a center of rotation changes by reading the position relation between the ceiling plate and the center of rotation out and feeding it back to absorption correction data, and making the correction. CONSTITUTION:A plane type gamma-ray detector 2 detects gamma rays emitted by a body 3 to be inspected while rotating around the ceiling plate 40 which can move right and left, and up and down and mounts the body 3 to be inspected. Then absorption values of the ceiling plate 40 at respective rotational positions are stored as correction data in an absorption correction table circuit 18. A stand and head circuit 9 detects the height (h) of the ceiling plate 40 and the deviation 1 between the center of the ceiling plate 40 and the center of rotation and if there is variation from those at a starting reference position, a CPU 12 calculates their shift quantities and corrects the table in the circuit 18. When a gamma-ray incidence position signal outputted by a position calculating circuit 6 is the signal of a position to be corrected by a ceiling absorption correcting circuit 14, a correction value is added to the signal and the resulting signal is stored on a memory 7 according to the corrected circuit 18.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an emission CT apparatus, and particularly to an emission CT apparatus that corrects the influence of a top plate.

[Conventional technology]

A block diagram of a conventional emission CT apparatus is shown in FIG.

The CT main body is mounted on the detector stand 1. Detector 2. bed 3
．． In addition to the rail 5, the bed 3 further has a top plate 40, and the subject 3 is mounted on the top plate 40.

The detector 2 is a planar radiation detector, and is attached to a detector stand 1, and the stand 1 extends around 360 degrees around the subject 3.
' When rotated, it is integrated into 3600 objects 3
The gamma rays emitted from the entire circumference of the subject 3 are detected. This rotation of the detector stand 1 is performed by a rotation mechanism.

The processing section includes a position calculation circuit 6. Memory 7, tomographic image reconstruction circuit 8. Stand and bed control circuit 9° display control circuit 1
0. CRTII, CPU12. It consists of 13 discs.

Now, the rotation angle of the detector stand 1 is read by the CPU 12 through the control circuit 9. Further, the detector stand 1 can be moved horizontally on the rail 5, and the bed top plate 40 can also be moved up and down, and these movement positions are read by the CPU 12.

When the gamma rays emitted from R1 in the subject 3 are incident on the detector 2, the signal is sent to the position calculation circuit 6, which calculates which position on the detector 2 (which position on the plane) the gamma rays have entered. ,
+1 is added to the address in memory 7 corresponding to that position. By repeating this addition every time γ-rays are incident, the memory 7
At the top, an R1 distribution image within the subject 3 is obtained.

After storing R1 distribution images from a plurality of directions in the memory 7 while rotating the detector 2 around the subject 3 at a certain angle, the tomographic image reconstruction circuit 8 reconstructs the tomographic image. The tomographic image is displayed in C, RTll and used for diagnosis.

Note that the tomographic image reconstruction circuit 8 may be high-speed dedicated hardware, or may be realized by software. disc 1
3 has the role of auxiliary memory for storing data and programs.

The problem with this conventional example is that when the detector 2 rotates below the top plate 40, the γ rays emitted from the body are absorbed by the top plate 40, and the detector 2 is able to accurately emit γ rays. This is the point where the line cannot be detected.

As a countermeasure to this problem, it is possible to obtain correction data and use this to correct the value detected by the detector under the large root. Specifically, as shown in FIG. 6, a Flemish source phantom (made of acrylic or the like) 15 having a uniform radiation source 15A inside is installed instead of the subject, and this phantom 15 is connected to a top plate 40. The top plate 40 is sandwiched to face the detector 2, and the area around the top plate 40 is rotated in this facing state. An absorption distribution image of the top plate is obtained for each direction of rotation. Furthermore, detector 2
There is a collimator at the input section of the detector, which allows only gamma rays incident at right angles to the detection surface to be taken in.

Now, depending on the position in FIG. 6, in the detection area not affected by the top plate 40, the γ rays from the uniform source are detected almost as they are, resulting in characteristic A (count value). - On the other hand, when the light passes through the top plate 40, the characteristic C is obtained instead of the characteristic B, and the count value becomes a smaller value than the characteristic A.

The difference between this characteristic B and characteristic C is the amount absorbed by the top plate 40, and becomes an error.

The shape of characteristic C depends on the rotation angle. Therefore, using the rotation angle as a parameter, the sixth
The characteristic C is measured by the method shown in the figure, and correction data based on the characteristic C is obtained using the detection channel (coordinates) and rotation angle as parameters.

This correction data is stored in a table in memory. Corresponding correction data is read from the memory using the rotation angle and detection channel as parameters for the measurement value from the actual object, and this correction data is added to the measurement value that has an adverse effect. Become. This will be explained in detail below.

The correction data table is shown in FIG. This correction data table 18 contains the original rotation reference angle of the opposing relationship between the phantom and the detector (for example, θ=O).

6', 12", etc.).

An example of a divided table is shown in FIG. (A) is the rotation angle θ-0°, (O) is the rotation angle θ=θ. This is an example of °. The rotation pitch is arbitrary, and for example, the basic rotation unit is 6 degrees. In the example of 6″, if the rotation range is 1806, then
There are 30 correction data tables.

In FIG. 8, tables 100 and 101 are shaded portion 100a.

101a is the area affected by the top plate, non-shaded area 10
0b and 101b indicate areas not affected by the top plate.

When θ=O°, the lateral width of the shaded portion matches the lateral width l of the top plate, and the lateral center position W C2 of the shaded portion coincides with the lateral center position C1 of the detector.

However, as the rotation progresses (counterclockwise) and approaches θ-90', the width W of the shaded area decreases to W,<W, and becomes zero at θ=90', and the center of the shaded area is θ° −
Move to the right as you approach 90°, 0”-10°
Move to the left as you approach.

This change in width is due to the transmission distance of the top plate varying depending on the rotation angle, and the movement of the center position is due to the fact that the width of the top plate substantially changes depending on the rotation angle (
(changes when viewed from the detector).

If the top plate is made of a uniform material and has a rectangular shape with uniform length and width, the correction data in the shaded area will be the same value determined for each rotation angle. However, in general, the top plate is not necessarily made of a uniform material, and its shape is often not perfectly rectangular (for example, the edges are rounded). Therefore, even if the rotation angle is the same, the correction data and date will be different.

FIG. 9 shows an example of a correction data table for an example of 16 4×4 detection elements (channels) at an arbitrary angle. however,
The actual number is 64 x 64, 128 x 128, etc., which is much larger.The correction table 102 sets a flag IF indicating that no correction is necessary in the area that is not affected by the top plate, and removes the influence of the top plate. Correction data is stored in the affected area. In the figure, (1, 1), (2, 1).

(3,1), (4,1), (1,4), (
An example is shown in which IF is set up at the positions 2, 4), (3, 4), and (4, 4), and correction data is put in the other parts. The reason why the value of the correction data differs for each coordinate is because the above-mentioned example of the top plate non-uniformity is assumed. still,
Coordinates correspond to detection channels.

The correction data may be the amount of gamma rays absorbed by the top plate, but in FIG. 9, the absorption rate is shown numerically. For example, the correction data for the detection element coordinates (1, 3) is rlOJ.
3) shows that the absorption rate on the top plate is 10% (1/10). 6.7% (1/15) at coordinates (4,2)
It was assumed that

Correction using such correction data involves adding +1 to the γ-ray count value at coordinates (1, 3) every time 10 γ-rays are input. At coordinates (4, 2), a correction is performed by adding +1 to the count value at coordinates (4, 2) every time 15 gamma rays are input.

On the other hand, at coordinates (1, 2), the correction data is IF and no correction is necessary.

In this way, it is determined whether or not correction is necessary for each rotational position using the rotation angle and the detection channel (coordinates) as parameters, and if correction is necessary, correction is performed according to the correction data.

[Problem to be solved by the invention]

In the conventional example described above, the positional relationship between the center of rotation and the top plate must always be the same when collecting correction data and when collecting data of the subject. For example, if the height of the top plate (the distance between the center of rotation O and the top plate) changes, the position of the shadow of the top plate relative to the detector changes, so the correction data cannot be accurate correction data for that position.

An example of a change in the height of the top plate is when it is desired to match the center of rotation with the center of the subject depending on the thickness of the subject. That is, if the thickness of the subject is large, the top plate is lowered, and if the thickness of the subject is small, the top plate is raised to align the center of rotation with the center of the subject.

If the height of the top plate changes, each time the height changes,
The supplementary data table can be recreated by positioning the flood source phantom and measuring the absorption value. However, it is inefficient to have to perform position setting/measurement each time.

An object of the present invention is to provide an emission CT apparatus that enables new corrections in terms of data processing even if the positional relationship between the top plate and the center of rotation changes.

[Means to solve the problem]

The present invention reads the positional relationship between the top plate and the center of rotation, feeds it back to the absorption correction data to correct it, and uses the correction data after this feedback as the true correction data.

[Effect]

In the present invention, the positional relationship between the top plate and the center of rotation is read, and absorption correction data is corrected by feedbanking based on this positional relationship. This corrected correction data reflects the position of the top plate, and is used as true correction data for the top plate absorption correction.

〔Example〕

FIG. 1 shows an embodiment of the present invention. The features of this embodiment are as follows:
Top plate absorption correction circuit 14. The point is that an absorption correction table circuit 18 is added.

According to this additional configuration, the T-line input position signal output from the position calculation circuit 6 enters the top plate absorption correction circuit 14, and the position to be corrected is determined based on the correction data read from the absorption correction table circuit i8. If it is a signal, the correction amount is added and stored in the memory 7.

During this addition, the correction data is corrected. The variation in the height of the top plate and other positional relationships during this correction will be explained.

The height h of the top plate changes depending on the thickness of the subject. This is to align the center of the subject and the center of rotation.

Furthermore, measurements are taken by rotating the detector in an elliptical shape along the subject's body part. In order to rotate this detector in an elliptical shape, it is necessary to move the detector stand horizontally and move the top plate vertically. Therefore, the height h of the top plate and the difference fl between the center of the top plate and the center of rotation change depending on the rotation angle θ of the detector.

The principle of correction will be explained with reference to FIG.

When the detector 2 is at the position indicated by the two-dot chain line perpendicular to the top plate 40, θ-0, and there is no influence of absorption by the top plate 40. Therefore, there is no need for correction. Although the method of determining θ is different from that in FIG. 8, formally it is simply a method of determining the coordinate system, and which coordinate system is determined is arbitrary.

When the detector 2 goes under the top plate and θ≠O, correction is required, and the amount of correction changes with θ.

Furthermore, as is clear from the figure, the height h of the top plate, the rotation center O, and the center of the top plate 0. The amount of correction also changes depending on the deviation l.

The range and size to be corrected are shown below.

First, the range that requires correction, that is, the area of the effective field of view that is in the shadow of the top plate, is the range from l) 1 to P2 when considered on the center line in the thickness direction of the top plate, to make the explanation easier to understand. , the width of the top plate is W (mm), and the rotation angle of the detector is θ, then Wsin θ is obtained. This is the absolute value of the range affected by absorption, and the actual range can be determined by finding the positions of P and P2. Therefore, 9. The height of the top plate h (mu),
If the deviation between the top plate center O0 and the rotation center O is J (mm+), P, , P2 can be expressed by the following equation.

pl -h'-ff'→W' = hcos θ-#sin θ+ sin
θ-(1)Pz =h'-e'-w'
-hcos θ-fsin θ--5inθ ・
f2) Here, the coordinate axis (PI pz direction) is centered on C0, the P side is t, and the p2 side is -. C+ and Cz are the projection positions of the deviation e.

(1), the conclusion based on the f21 formula is as follows.

(a) (W/2) sin θ is a value that is not affected by h and l, and is a value that depends only on the rotation angle θ. W sin θ is the absolute value of the range that requires correction.

(b) hcos θ−/sin θ is influenced by the rotation angle θ and h and 2. Therefore, as the address shift amount for the correction data table, h cos θ−1s
:n θ, it is possible to obtain correction data that is not affected by h and x. This is the basic idea of this embodiment. However, although this shift amount is a function of h, l, and θ, θ is fixed at the time of the shift, so it is only necessary to determine the shift amount using l as a parameter. The shift may be either the address itself or the data itself.

FIG. 3 shows an example of this. Set the table as shown in the left figure of (A) at the angle θ, and now shift to the left 11s = hc
If os θ-j2sin θ is calculated, the address is shifted by the shift amount s as shown on the right side of (a).

Figure 3 ([1) is an example of a shift for the table in Figure 9. The left side is an example of no shift when h = 0.1 = 0, and the right side is an example of a shift to the left by one column when S = 1. Give an example. According to this example, one column of data is shifted to the left, and, for example, the coordinates (1, 1) become data "7" at the pre-shift coordinates (1, 2) instead of the original data IF. Also, the coordinates (
1, 3) is not the original data "IO" but the data IF at the coordinates (1, 4) before shifting.

Next, a method of creating a correction data table (the table shown in FIG. 7) for the circuit 18 will be described. First, when setting theoretically, the correction data C is as follows, assuming that the thickness of the top plate is t (m) and the γ-ray absorption coefficient μ (m*-') depends on the material of the top plate.

C=1/exp (it t /sinθ')
-(3) Here, μ is the energy of γ-ray (K
eV).

Incidentally, if the top plate is made of a plurality of materials or if μ is unknown, the table may be created by actually measuring using a flat source table as described above.

A detailed embodiment of the present invention for correction using correction data is shown in FIG. The top plate absorption correction circuit 14 includes a comparison circuit 14A, a subtraction circuit 14B, and correction memories 14C and 14D.
, the correction control circuit 14E warns. The memory 7 consists of a memory circuit 7A and an adder circuit 7B.

Stand and head! 11111 circuit 9 to CPtJ1
2 receives the rotation angle θ and reads a correction data table corresponding to the rotation angle θ from the absorption correction table 18.

This correction data table corresponding to the rotation angle θ is stored simultaneously in the correction memories 14C and 14D. This memory 14C
, 14D and the memory circuit 7A have the same capacity, (X,
Y) and are given the same address.

On the other hand, the stand and head control circuit 9 detects 1, and the CPU 12 reads this.

If h and z do not change compared to the original reference position, the correction data table is not corrected. If there is a change in h, 1, the CPU 12 changes the shift amount (h cos θ-1 sin
θ) and modify the correction data table accordingly.

Describe specific operations.

(a) Regarding addition processing into the memory circuit 7A.

When γ-rays occur at arbitrary coordinates (X, Y), the coordinates (X, Y) are input as a position signal and stored in the memory circuit 7A.
accesses the address (X, Y) of and reads the stored value. This stored value is the added value at the address (X, Y) up to that point.

Assuming that the stored value is M, this stored value M is sent to the adder circuit 7B and added by +1, resulting in (M+1).

This (M+1) is again returned to (XY) of the memory circuit 7A. Thereafter, similarly, each time a position signal of (X, Y) is input, the stored value of the (X, Y) coordinate is incremented by +1. Thus, the number of occurrences of gamma rays for each coordinate is stored in the memory circuit 7A as a count value.

This operation is the same as before.

(0) Regarding correction processing.

Having read the rotation angle θ, the CPU 12 reads a correction data table corresponding to the rotation angle θ from the absorption correction table circuit 18. The CPU 12 sends this correction data table to the correction memories 14C and 14D.

The correction memory 14C is used directly for correction, and the correction memory 140 is used for setting the initial value of this correction memory 14c.

First, when the position (X, Y) is input, the correction memory 14C
Access (X, Y) of Correction memory 14C (
X, Y) contains correction data, which is read out. This correction data is correction data at the position (X, Y), and if it is TF, no correction is required; if it is not IF, +1 is added to the white value stored in the memory circuit 7A for each count. . The subtraction circuit 14B calculates the number of times (X, Y) is manually applied, and for each (X, Y) 1 personal effort, (
The contents of the memory 14G at the position (X, Y) are read out, subtracted by -1, and the result of the subtraction is sent to the comparison circuit 14A and returned to the (X, Y) position of the memory 14C.

The comparator circuit 14A determines whether the subtraction result is rOJ or not, and if it is not "0", it does not output an output, and if it is "0", it outputs a signal 14a indicating that.

Upon receiving this signal 14a, the correction control circuit 14E issues a +1 addition command 14e for correction, and adds +1 to the value stored in the memory circuit 7 at the (X, Y) position.

This +1 addition is a correction process.

Note that at this correction position (X, Y), since the original +1 addition is performed as described in (A), an addition of "+2" is performed in conjunction with the correction process (+1 addition).

On the other hand, if the correction data is TF, no subtraction is performed by the subtraction circuit 14B, and the comparison result by the comparison circuit 14A also indicates that correction is not necessary.

The subtraction result of the subtraction circuit 14B is returned to the (XY) position in the memory 14C, which allows the number of occurrences of the T line at the (X, Y) position up to that point to be determined and the T line at the next (X, Y) position to be determined. Used as a subtraction target for line occurrences. However, the subtraction result is rO
If it is J, the correction process will be performed at that point, so the initial value will need to be set again from the next time onwards. For this purpose, a correction memory 14D is provided, and the subtraction result is rOJ
If so, the correction control circuit 14E detects this and stores the correction data of (X, Y) in the correction memory 14D in the (X, Y) position of the correction memory 14c. Then, it is ready for the next correction process.

(c) Regarding correction data correction processing.

The CPU 12 reads θ, h, and l, and finds that h≠0゜l≠O
The shift amount 5=h cos θ−1 sin θ is calculated under the condition. Therefore, software is applied to the correction data table for the angle θ using the shift amount S. The shift result is as explained in FIG.

The CPU 12 stores this shift result in the correction memories 14C and 1.
Write to 4D. The subsequent processing follows the correction processing described in (+]).

Although the processes (A), (II), and (A) have been described separately above, this is for convenience; in this embodiment, in which correction is performed simultaneously while collecting the R1 distribution, the addition process and the correction process are
The correction process is performed simultaneously and in parallel for two (X, Y) position inputs, and the correction process is performed at θ.

Do this every time h and l change. It is also possible to perform the correction process (0) all at once by software after data collection.

〔Effect of the invention〕

According to the present invention, the amount of shift of the correction data table can be determined based on the height of the top plate and the amount of deviation l, making it possible to perform correct correction.

[Brief explanation of drawings]

FIG. 1 is an embodiment of the T-ray tomography apparatus of the present invention, FIG. 2 is a diagram explaining the principle of correction data correction processing of the present invention, FIG. 3 is a diagram showing an example of shift in this embodiment, and FIG. 5 is a diagram showing a conventional T-ray tomography apparatus, FIG. 6 is an explanatory diagram of conventional correction, and FIG.
The figure shows a conventional correction data table, Figures 8 (A) and (0) show the relationship between conventional correction data and rotational position, and Figure 9 is a specific example of a correction data table. . 14... Top plate absorption correction circuit, 18... Absorption correction table circuit, 40... Top plate.

Claims (1)

[Scope of Claims] 1. A top plate on which a subject is mounted and is movable vertically and horizontally, and a planar γ that detects γ-rays emitted from the subject while rotating around the subject and the top plate. a line detector; a data table for storing the absorption value by the top plate for each rotational position as correction data; and means for detecting the height of the top plate and the deviation l of the center of rotation, and calculating a corrected shift amount. a means for shifting the data or address of the correction data table according to the correction shift amount; and a means for correcting the measured value obtained by detection by the gamma ray detector for each position according to the shifted correction data table. A single photon emission tomography apparatus comprising: means for reconstructing an image using the data of the correction results;