JPH0264496A - Triple coincidence counting circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a triple coincidence counting circuit suitable for a positron ECT device.

[Conventional technology]

The positron ECT device uses a positron-emitting nuclide R1.
By administering a radioisotope (radioactive isotope) to a subject, detecting the radiation emitted outside the subject, and processing the data with a computer, a concentration distribution image (tomographic image) of R1 in a certain cross section is created. ). With positron-emitting nuclides, when the positron disappears, it emits two gamma rays in a 180° direction.
It is necessary to arrange a large number of radiation detectors in a ring shape at 0° and detect that these two γ-rays are incident on the two detectors at the same time. By the way, two gamma rays from one annihilation positron and one unrelated gamma ray may coincidentally be incident on a large number of radiation detectors arranged in a ring shape. Conventionally, in the case of such triple coincidences, they are rejected as multiple events and are deliberately not counted (counted down).

[Problem to be solved by the invention]

However, since the triple coincidence also includes the position information of the annihilation positron, discarding it as a matter of law would result in a disadvantage in terms of the S/N ratio. In particular, when the counting rate is high, the number of accidental coincidences increases and the probability of triple coincidence also increases, leading to more missed counts, lowering sensitivity, and lowering quantitative performance (
linearity) may deteriorate. However, if all these triple coincidences are counted, accidental triple coincidences will also be included, which will actually lower the S/N ratio. SUMMARY OF THE INVENTION An object of the present invention is to provide a triple coincidence counting circuit that can count l·ribricoincidences and subtract accidental triple coincidences therefrom to obtain a count of true one-edge pull coincidences.

[Means to solve the problem]

In order to achieve the above object, the triple coincidence counting circuit according to the present invention includes a first coincidence detection circuit that receives three signals and generates a detection output when the three signals occur within a certain time interval. , 3 after delaying two of the above three signals with different delay times.
A second simultaneous detection circuit that generates a detection output when two signals are inputted and occur within a certain time interval, and the difference between the detection outputs of the first and second simultaneous detection circuits is counted. A counting circuit is provided.

[For production]

The first coincidence detection circuit inputs the three signals as they are, so it detects true triple coincidence (simultaneous incidence of positron annihilation gamma rays) and cases where three gamma rays are incident completely coincidentally. be done. On the other hand, the second simultaneous detection circuit inputs three signals after two of the three signals described above are delayed by different delay times, so if three gamma rays are accidentally incident, A detection output is generated only in the case of true triple coincidence, and no detection output is generated in the case of true triple coincidence. Therefore, by counting the difference between the detection outputs of the first and second simultaneous detection circuits, it is possible to exclude accidental triple coincidences and count only true triple coincidences. Since true triple coincidences can be counted in this way, the S/N ratio can be increased especially at high counting rates. That is, the count rate characteristics of the positron ECT device are as shown in FIG. Here, the curve T is related to true coincidence (that is, the coincidence of positive 1-ron annihilation gamma rays), the curve R is coincidental coincidence, and the curve TC is related to true triple coincidence (the shadow of true coincidence). are shown respectively. As shown in FIG. 5, when the counting rate becomes high, accidental coincidences become dominant and triple coincidences also increase. Since this triple coincidence includes true coincidence, the S/N ratio can be increased by using this information. Especially at high counting rates, it is possible to prevent a decrease in sensitivity and improve quantitative performance.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, three signals A, B, and D are input as they are to AND gate 1, and when the pulse width of these three signals A, B, and D is τ, these signals are input within a time interval of 3τ. When the signals are input simultaneously, a simultaneous detection signal is obtained from the AND gate 1. On the other hand, the AND gate 2 receives the undelayed signal A, the signal B passed through the delay circuit 3, and the signal R passed through the delay circuit 4.5. The delay time of delay circuit 3.4.5 is set to 6τ, for example. As a result, the signal B is input to the AND gate 2 after being delayed by a time of 6τ and the signal B by twice that time. The output of AND gate 1.2 is memory 6
．． 7 and the contents of memory 6.7 are incremented by +1 when they occur. The contents of this memory 6.7, that is, the count value of the output of the AND gate 1.2, are sent to the arithmetic circuit 8, and the count value of the memory 7 is subtracted from the count value of the memory 6. The above signals A, B, and D are group A in FIG. Sent from B and D. That is, a large number of radiation detectors 9
are arranged in a ring shape, which are divided into six groups A~
When a gamma ray is incident on any one of the detectors 9 belonging to the F groups and an output is generated, it is sent as a signal for that group. As shown in FIG. 2, the positron disappears at point P, and two gamma rays 21 and 22 are simultaneously emitted in a 180° direction, one detector 9 belonging to group A and one detector belonging to group D. Suppose that they are incident on 9 at the same time. At this time, if by chance a completely unrelated gamma ray 23 also simultaneously enters one detector 9 belonging to group B, then AN
An output is generated from the D gate 1, and the contents of the memory 6 are incremented by +1. In other words, although omitted in the figure,
The memory 6 has an address designated for each combination of radiation detectors 9, and the above-mentioned counting is performed at that address. In this way, as shown in FIG. 4, the signals are added at addresses corresponding to straight lines 41, 42, and 43 connecting the three detectors 9 that are simultaneously incident. On the other hand, if three gamma rays 31, 32, and 33, which are completely unrelated to each other, coincidentally enter one detector 9 belonging to groups A, B, and D at the same time as shown in FIG. An output is generated from gate 1, and the contents of memory 7 are incremented by +1 in the same way as memory 6 described above. The case in Figure 2 includes simultaneous incidence of positron annihilation gamma rays, so this is called a true triple coincidence, and the case in Figure 3 does not include simultaneous incidence of positron annihilation gamma rays. Therefore, when they are called accidental triple coincidences, the memory 6 counts both these true triple coincidences and accidental triple coincidences. On the other hand, AND game 1-2 produces an output only in response to random triple coincidences. In other words, in the case of true triple coincidence as shown in Figure 2, two of the three signals are delayed by different delay times, so the simultaneity of the three input signals to the AND gate 2 is Without being satisfied, no output is produced from AND gate 2. The output from the AND gate 2 is generated when three gamma rays are incident completely unrelatedly as shown in Fig. 3, and by delaying two of the signals B and D by different times, the three signals are This is when the simultaneity of is satisfied. Then, the frequency when simultaneity is satisfied by chance by delaying in this way is three γ
This is considered to be the same frequency as when lines coincidentally satisfy simultaneity. Therefore, the counted value of memory 7 is the counted value of the accidental triple coincidence, and by subtracting the counted value of memory 7 from the counted value of memory 6 in the arithmetic circuit 8, the true triple coincidence excluding the accidental triple coincidence is calculated. It is possible to obtain the count value of . In an actual positron ECT device, there are six groups A to F.
The circuit of FIG. 1 is provided for every three combinations of
A true triple coincidence count is performed for the two combinations. Therefore, the data obtained is a collection of position information represented by three straight lines as shown in FIG. 4, two of which do not represent the position of the positron. Therefore, image reconstruction from this collected data requires the use of a different image reconstruction algorithm than usual. For example, the following can be considered (of course, it is not limited to this). First, we collect triple coincidence data using a point source and simply back-project it. Then, the image becomes spatially expanded and blurred compared to the original image of a point source, so by comparing this with a known image of a point source, a spread inverse function is used to return the image to the original image. (two-dimensional filter) is determined in advance. Then, the triple coincidence data collected for the actual subject is simply back-projected, and the spread inverse function described above is applied.6 A normal Posi-L-ron ECT device is configured to obtain coincidence data between two groups. Since the image is reconstructed from that data,
If an image based on triple coincidence data as described above is added to this image, it is possible to improve the sensitivity of the image at a high counting rate, improve the S/N ratio, and improve quantitative performance (linearity). In the above embodiment, the output of the AND gate 1 is counted in the memory 6, the output of the AND gate 2 is counted in the memory 7, and later the arithmetic circuit 8 converts the former counted value to the latter counted value. However, one memory is used, and when there is an output from AND gate 1, +1 is added to this memory, and when there is an output from AND gate 2, it is subtracted by 1. You can also do that. Also, as a simultaneous detection circuit, a circuit using a flip-flop other than AND gate 1.2 or F? , a circuit using OM, etc. can be considered. [Effect of the invention] According to the triple coincidence counting circuit of this invention,
Since it is possible to count only true triple coincidences, excluding accidental triple coincidences, it is possible to improve the sensitivity of images at high counting rates, improve the S/N ratio, and improve quantitative performance.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a schematic diagram showing true triple coincidence, Fig. 3 is a schematic diagram showing accidental triple coincidence, and Fig. 4 is a schematic diagram showing 1 to ripple coincidence detection. A schematic diagram showing the obtained data, and FIG. 5 is a graph showing counting rate characteristics. 1.2...AND gate, 3.4.5...Delay circuit, 6.7...Memory, 8...Arithmetic circuit, 9...Radiation detector, 21-23.31-33 ...γ rays.

Claims (1)

[Claims] (1) A first simultaneous detection circuit that generates a detection output when three signals are input and occur within a certain time interval, and two of the three signals mentioned above are delayed at different times. A second simultaneous detection circuit that receives three signals after being delayed by time and generates a detection output when they occur within a certain time interval, and a first and second simultaneous detection circuit. A triple coincidence counting circuit consisting of a counting circuit that counts the difference between the detection outputs of .