JPH0436352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scintillation camera with a wide field of view.

An overview of a conventional scintillation camera will be explained based on FIG. First, γ emitted from a radioactive isotope (hereinafter referred to as RI) inside the subject 1
The collimator 2 limits the direction of the line to only a specific direction, and the line enters the scintillator 3. The scintillator 3 emits light upon incidence of γ-rays, and the light is transmitted to a photoelectric conversion element, generally a photomultiplier tube (hereinafter referred to as RMT).
) 4, but the output signal of each PMT 4 changes depending on the light emitting position and the position of the PMT 4. The output signal of each PMT 4 is further amplified by a preamplifier 5, and then input to a position calculation circuit 6 and a pulse height analyzer (hereinafter referred to as PHA) 7. The position calculation circuit 6 determines the light emitting position in the scintillator 3, that is, γ, based on the difference in the magnitude of the output signal of each PMT 4.
Find the incident position of the line, and PHA7 calculates all PMT4
The output signals of are added together, and based on that value, it is determined whether the incident γ-rays were emitted from the intended RI, thereby eliminating the generation of bright spots due to unnecessary γ-rays, which will be described later.
Then, the position calculation circuit 6 outputs the γ-ray incident position signals X, Y, and the PHA 7 outputs the unbundling signal UB.
are output, and a bright spot is generated at a position corresponding to the position where the gamma rays are incident on the cathode ray tube (hereinafter referred to as CRT) 8, and the integrated image of this bright spot is the RI inside the subject 1. This becomes a distribution image.

One of the methods of calculating the position with the conventional camera mentioned above is to A/D convert the output signal of each PMT 4 and calculate the position digitally, but this will be explained below with reference to Fig. 2. do. That is, the light emitted by the scintillator 3 is incident on the PMT 4, converted into an electrical signal, and amplified. The magnitude of the output signal of this PMT4 is the scintillator 3
This is related to the light emitting position in the center, and the output signal of PMT4 close to the light emitting position is large, the output signal of PMT4 far away is small, and when light emission occurs at the center of PMT4, the output signal of PMT4 becomes maximum. The output signals of each PMT 4 are digitally converted by an A/D conversion circuit 9, then added by an addition circuit 10, and inputted to a standardization circuit 11. The normalization circuit 11 receives the output signal of the adder circuit 10, that is, the total
Add signal 2 of PMT4 output signals to each PMT4.
The output signal of is divided and normalized. By comparing the output signals of the PMTs 4 on both sides of the PMT 4 that outputs the maximum signal, it is determined whether the standardized signal is a signal to the right or left of the center of the PMT 4 that outputs the maximum signal. The memory circuit 12 outputs a position signal corresponding to the magnitude of the output signal of each PMT 4 for which this determination has been made. In this case, the position signal corresponding to the magnitude of the output signal of each PMT 4 after the above determination is such that when light emission occurs at a certain position of the scintillator 3, the magnitude of the output signal of each PMT 4 becomes a certain specific magnitude. Taking advantage of this fact, the relationship can be obtained by measuring the relationship over the entire surface of the scintillator 3 in advance and then calculating it (conversion), and the memory circuit 12 stores the value of the reverse calculation (conversion). Each of the above obtained
By averaging the position signals according to the magnitude of the output signal of the PMT 4 in the position calculation circuit 13, the light emitting position of the scintillator 3, that is, the γ-ray incident position is determined, and is sent to the CRT 8 as the γ-ray incident position signals X, Y. It will be done. Furthermore, the output signal 2 of the adder circuit 10 is
It is also sent to the PHA 14, and the PHA 14 analyzes the wave height to determine whether or not the incident γ-ray was emitted from the target RI, and only when it is determined that the incident γ-ray was radiated from the target RI, an unblank signal is sent.
Outputs UB to eliminate bright spots on the CRT8 due to unnecessary gamma rays.

However, with such a scintillation camera, (1) When performing wave height analysis with PHA14,
is configured to allow only signals within a certain level range to pass through, and since the signal entering the PMT4 is reduced due to leakage in the vicinity and outside of the PMT array, the signal from such a position is
Cannot pass through PHA14.

(2) Therefore, it is impossible to create images around and outside the PMT array. To make this possible, smaller signals can also be
If the light is allowed to pass through the PHA 14, a bright spot due to γ rays other than the target γ rays will be generated on the CRT 8 at the center of the PMT array, resulting in a noisy image.

(3) In addition, the output signal of each PMT4 is normalized by the output signal 2 of the adder circuit 10, which is obtained by adding the signals obtained by A/D converting the output signal of the PMT4, but the output signal of each PMT4 is When normalizing the optical signal generated by light emission in the same way as for the central part of the PMT array, the following problem arose. In other words, in this case, when light is emitted in the vicinity of each PMT 4 on the periphery and outside, the relatively strong light in the surrounding area, especially towards the outside, is a PMT 4 because there is no PMT 4 there.
4, it is not detected (leakage occurs). For this reason,
The output signal Z of the adder circuit 10 becomes smaller, and the output signals of the peripheral and outer PMTs 4 are normalized to become larger by the amount of decrease due to the leakage. Therefore, there is a problem that the position signal read from the memory circuit 12 is shifted inward (toward the center) from the actual position.

From the above, conventionally, the field of view of a scintillation camera is located at the outermost periphery of the PMT array.
It is restricted to the inside of the line connecting the centers of PMT4, and in order to widen the field of view, increase the number of PMT4 etc.
The scintillator 3 also had to be made larger accordingly, resulting in a disadvantage of being larger.

The present invention was made to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a scintillation camera that can widen the field of view without increasing its size.

An embodiment of the present invention will be described below with reference to FIG. FIG. 3 is a block diagram showing an embodiment of the scintillation camera according to the present invention. In FIG. 3, the same reference numerals as in FIG. 2 indicate the same or corresponding parts. Further, 15 is a comparator, and the standardization circuit 11
Of the output signals of PMT4 standardized by PMT
The output signals of the PMT4 located at the outermost circumference of the array and the PMT4 located one row inside are compared, and the light emission of the scintillator 3 is compared to the PMT4 located at the outermost circumference of the PMT array.
It is determined whether the event occurred inside or outside the intermediate position between PMT4 and PMT4 located one row inside. If it is determined that the occurrence occurred outside, the changeover switch 16
is switched to the dotted line side, and the output signal 2 of the adder circuit 10 is input to the ratio calculation circuit 17.

The ratio calculation circuit 17 compares the magnitude of the target gamma ray energy with the output signal 2 of the addition circuit 10,
Calculate the ratio of the output signals of all PMTs 4 to what percentage of the gamma ray energy when it is assumed that the target gamma ray is incident. Then, if the result (ratio) is a certain reference value, here 1, or more, the signal "L" is input to one input terminal of the AND circuit, which will be described later, to 1.
If it is less than that, a signal "H" is applied to each of the above input terminals. Further, the above calculation result (ratio) is given to a multiplication circuit, which will be described later. 18 is the multiplication circuit, which calculates the ratio calculated by the ratio calculation circuit 17 and the normalized PMT4.
This is to eliminate the problem mentioned in (3) above (correct positional deviation). Note that the reference value 1 or more specifically means the incidence of noise light (scattered radiation) from the outside. Also, less than 1 means that the leakage occurs.

Reference numeral 19 denotes an image mask, which is located further outside a preset fixed position outside of PMT 4 at the outermost periphery of the PMT array (limit position of the field of view expanded by the present invention (set as appropriate); hereinafter referred to as the expanded field of view limit position). This position prevents an image from being created on the CRT8. Therefore, the image mask 19 outputs a signal "H" when the γ-ray incident position signals X and Y from the position calculation circuit 13 represent a position inside the expanded field of view limit position, and a signal "L" when they represent a position outside the expanded field of view limit position. ” are described below.
Apply to the other input terminal of the AND circuit.

20 is the AND circuit, and the signals from the ratio calculation circuit 17 and the image mask 19 are both “H”.
Only when the ratio is less than 1 and is inside the expanded field of view limit position, the signal “H” is applied.
That is, the unblank signal UB is output to the CRT 8, and at this time, a bright spot is generated on the CRT 8 at a position corresponding to the γ-ray incident position signals X and Y given from the position calculation circuit 13.

Note that setting the enlarged field of view limit position using the image mask 19 eliminates the occurrence of bright spots outside the effective setting range for positional deviation correction by the comparison calculation circuit 17 and the multiplication circuit 18, and also eliminates the occurrence of unnecessary spots on the outer circumference side. This is to eliminate the occurrence (operation) of bright spots.

Next, the operation of the scintillation camera of the present invention described above will be explained. That is, scintillator 3
The emitted light enters the PMT 4, where it is converted into an electrical signal and amplified. After the output signal of each PMT4 is digitally converted by the A/D converter 9,
The signals are input to an adder circuit 10 and a normalization circuit 11, respectively. The adder circuit 10 adds the output signals of all the PMTs 4, and the standardization circuit 11 standardizes the output signal of each PMT 4 with the sum signal of the output signals of all the PMTs 4. Among the standardized PMT4 output signals,
PMT4 at the outermost periphery of the PMT array and PMT inside it
4 are compared by a comparator 15, and it is determined whether the light emission of the scintillator 3 occurs inside or outside of the intermediate position between the outermost PMT 4 of the PMT array and the inner PMT 4.

Now, if it is determined that it occurred outside,
The changeover switch 16 is switched to the dotted line side (left side in the figure), and the output signal of the adder circuit 10 is switched to the ratio calculation circuit 1.
7, and the ratio calculation described above is performed. As a result of the calculation, if the ratio is 1 or more, it is assumed that γ rays other than the target γ rays have been incident, and a signal "L" is output to the AND circuit 20, but if the ratio is less than 1, the signal "L" is output to the AND circuit 20. Outputs a signal “H” to

At this time, the multiplication circuit 18 receives the calculation result (ratio) of the ratio calculation circuit 17 and outputs the standardized PMT.
Multiply the output signal of 4 to correct the positional deviation mentioned above due to normalization (particularly the positional deviation when the emission of scintillator 3 occurs outside the intermediate position between the outermost PMT4 of the PMT array and the inner PMT4). and provides it to the memory circuit 12.

The position calculation circuit 13 averages the position signals from the memory circuit 12 to determine the light emitting position of the scintillator 3, that is, the γ-ray incident position, and sends γ-ray incident position signals X and Y to the CRT 8 and the image mask 19.

The image mask 19 is configured such that the position signals X and Y from the position calculation circuit 13 are applied to the outermost PMT of the PMT array.
If the signal is for a position further outside the fixed position (expanded field of view limit position) outside 4, the signal "L" is output, and the ratio calculation circuit 17 at that time
Regardless of the signal state to the AND circuit 20, the output of the unblank signal UB (signal "H") from the AND circuit 20 is suppressed to prevent unnecessary bright spots from occurring. When the position signals X and Y from the position calculation circuit 13 represent a position inside the expanded field of view limited position, the image mask 19 outputs a signal “H”, and the signal “H” from the ratio calculation circuit 17 and Together, the AND circuit 20 outputs an unblank signal UB (signal "H"), and the CRT 8 responds to the position signals X and Y.
Generates a bright spot at the top position.

This makes it possible to widen the field of view without causing positional deviations in position signals due to light emission around and outside the PMT array, and without increasing the size of the device. Creation can be prevented.

Next, if the comparator 15 determines that the light emission of the scintillator 3 has occurred inside the intermediate position between the outermost PMT 4 of the PMT array and the inner PMT 4, the changeover switch 16 is switched to the right side as shown in the figure. , the output signal of the adder circuit 10 is PHA1
4, the wave height is analyzed, and it is determined whether the incident gamma rays are the intended gamma rays. If the judgment is negative, PHA14 will cause unnecessary gamma rays to damage CRT8.
Unblank signal to avoid bright spots on the
Does not output UB.

Note that when the changeover switch 16 is switched to the right side, the signal (ratio) from the ratio calculation circuit 17 input to the multiplication circuit 18 is fixed at 1.
That is, it is the same as when there is no multiplier circuit 18, and a position signal corresponding to the magnitude of the output signal of each PMT 4 standardized by the standardization circuit 11 is output from the memory circuit 12, and this is averaged by the position calculation circuit 13. are used as gamma ray incident position signals X, Y, and the CRT
8 and the image is created. At this time, the image mask 19 is not activated.

In the above embodiment, the scintillator 3 emits light from the outermost PMT4 of the PMT array and the innermost PMT4.
Although we have explained the case where the ratio is calculated when the occurrence occurs outside the middle position with PMT 4, it is not limited to this. Good too.

In addition, after calculating the ratio and calculating the light emission position, the loss of γ-ray energy at the calculated light-emission position (the amount of γ-ray energy that does not enter the PMT4) is determined from a predetermined table, and the ratio is calculated again. The light emitting position may also be corrected by performing positional calculations, and in this way the positional resolution around and outside the PMT array can be improved.

As described above, the present invention includes a circuit that determines whether light emission from a scintillator occurs inside or outside a predetermined position around a photoelectric conversion element located in a photoelectric conversion element array, When it is determined that light emission has occurred outside the predetermined position, it is assumed that the target γ-ray is incident, and the sum signal of the total output signal of the photoelectric conversion element at that time and the target γ-ray are incident. and a circuit that compares the sum signal of all the output signals of the photoelectric conversion element to be obtained and corrects each output signal of the photoelectric conversion element by the ratio of both, and Since the incident position of the γ-ray is calculated from the output signal, there is an effect that the field of view can be expanded without causing the problems of the conventional camera mentioned above and without increasing the size.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an overview of a conventional scintillation camera, and Figure 2 shows a conventional scintillation camera in which a method of digitally calculating the position by A/D converting the output signal of each PMT is applied. Block Diagram FIG. 3 is a block diagram showing one embodiment of a scintillation camera according to the present invention. 3...Scintillator, 4...PMT, 8...CRT, 9
...A/D conversion circuit, 10... Addition circuit, 11... Standardization circuit, 12... Memory circuit, 13... Position calculation circuit, 14... PHA, 15... Comparator, 16... Changeover switch, 17... Ratio calculation circuit, 18 ...Multiplication circuit,
19...Image mask, 20...AND circuit.

Claims (1)

[Claims] 1 In a scintillation camera that A/D converts the signal from the photoelectric conversion element and calculates the incident position of γ rays based on the signal, the scintillator's light emission is transmitted to the photoelectric conversion element located at the outermost periphery of the photoelectric conversion element array. A circuit that determines whether light emission from the scintillator occurs inside or outside a predetermined position around the conversion element; The sum signal of all the output signals of the photoelectric conversion element at that time is compared with the sum signal of all the output signals of the photoelectric conversion element that should be obtained when the target γ ray is incident, and both are calculated. and a circuit for correcting each output signal of the photoelectric conversion element at a ratio of 1 to 2, and the incident position of the γ-ray is calculated based on each corrected output signal of the photoelectric conversion element. camera.