JPS6239904B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides a coincidence circuit for a positron CT device,
In particular, the positron makes it possible to detect coincidental coincidence events without increasing the dead time of the circuit.
Concerning coincidence counting circuit of CT device. The coincidence events obtained by the coincidence circuit of a positron CT device include not only true coincidence events but also coincidental coincidence events. This random coincidence event does not have positional information of the positron extinction, and therefore contributes to the deterioration of image quality. However, a chance coincidence event that occurs between opposing detectors can be quantitatively determined by delaying one of the input signals to the coincidence circuit and detecting a coincidence event with this delayed signal. You can know. Therefore, by subtracting the random coincidence events obtained by the above method from the coincidence events obtained by the coincidence circuit, only true coincidence events can be easily extracted. Here, the former coincidence event will be referred to as a total coincidence event. FIG. 1 is a diagram showing an example of a circuit in which a function for detecting the above-mentioned coincidental coincidence event is added to the conventional coincidence circuit. In the figure, detectors 1 and 11 are detectors provided facing each other, delay element 2 is a delay element that delays the output of one detector 1, and AND gates 3 and 30 are coincidence counters, respectively. It is a gate that detects an event. Output 4 of AND gate 3 is the total coincidence event detection signal, AND gate 3
Output 5 of 0 becomes a coincidental coincidence event detection signal. On the other hand, in a positron CT device in which multiple detectors are arranged in a ring shape, in order to simplify the circuit configuration, the multiple detectors are divided into several groups and coincidence events are detected between the groups. method is adopted. That is, a plurality of detectors arranged in a ring shape are divided into several groups, and each group is associated with several other groups. Coincidence counting is performed between this group and other groups that have a corresponding relationship. FIG. 2 is a convenient explanatory diagram thereof. The detectors arranged in a ring shape are divided into nine groups a to i. In the figure, 6 is a detector ring, 7 is a detector group, and each detector group has a corresponding relationship with four opposing groups. 8 is a coincidence pair obtained between detector groups. In the figure, group a(7) is associated with four opposing groups, groups d, e, f, and g. Similarly, groups other than group a have corresponding relationships with the four opposing groups. Now, group a will be called group A, and the four groups d, e, f, and g will be called group B. Groups having other correspondence relationships are also called A group and B group. Therefore, all of the detector groups a to i can become the A group and the B group. FIG. 3 shows a conventional example in which it is possible to detect an accidental coincidence event in a coincidence circuit having detectors arranged in a ring shape. This FIG. 3 shows an example of a circuit in relation to a detector group a and d, e, f, and g facing the group a. In this circuit, an OR gate 9 is provided which takes in the outputs of opposing groups d, e, f, and g.
The output of is used as one-way input of AND gates 3 and 30. Although not shown in FIG. 3, when a grouped configuration is adopted, the output of the OR gate 9 is input to the OR gate 9 in parallel, so the output of the OR gate 9 is input to the OR gate 9. Which detector is used) cannot be determined from the output of the OR gate 9 alone. As this determination means, an encoder is provided, and means is provided for inputting a detector output to the encoder and causing the encoder to generate a code corresponding to the input detector number. The detector number is a so-called detector address. This detector address and the outputs 4 and 5 at that time are associated and taken into a processing device, where appropriate processing is performed. FIG. 4 shows some examples of output pulse trains 10, 10A of detector groups a and g of FIG. 3. Table 1 shows the relationship between the groups and output numbers at that time.

[Table] In the figure, output 10 shows the output of detector a before passing through delay element 2, and output 10A shows the output after passing through delay element 2. In the figure, the case is when a and g occur simultaneously, and in this case, an output indicating simultaneous occurrence is output from output 4. Cases and cases are
This is a case where the a and g outputs happen to be at the same time,
Output 5 of AND gate 30 provides an indication to that effect. In such a circuit configuration, the case in which output 4 occurs is when all simultaneous occurrences occur, and output 5 means an output when coincidental occurrences occur. In addition, a and g are A group,
The reason why the B group is taken alternately is that
This means that both a and g can be in group A or group B. Therefore, since there are always two cases in which groups a and g become group A and group B, a coincidental coincidence event will be detected in both cases. As a result, when subtracting a chance coincidence event from all coincidence events, twice the normal number is subtracted. Correcting for coincidental coincidence events therefore requires the above-mentioned modifications. Furthermore, it is clear that due to the detection of this extra chance coincidence event, the dead time increases, resulting in a reduction in the high count rate characteristics of the circuit. Furthermore, the amount of hardware is also enormous. SUMMARY OF THE INVENTION An object of the present invention is to provide a coincidence counting circuit for a positron CT apparatus that reduces the number of omissions during high counting. The gist of the present invention is that when one or more groups facing one group are selected between opposing detectors or detector groups (hereinafter simply referred to as groups), a group belonging to the latter is placed in an opposing group. The purpose of this method is to remove duplicates from the coincidence pairs without selecting the former group, thereby constructing a coincidence circuit. below,
The present invention will now be described in detail. Figure 5 I to I are groups a to i shown in Figure 6.
It shows all the combinations among all the groups up to. The reason why coincidental coincidence events are detected twice with the circuit configuration shown in FIG. 3 is that the same event is detected twice. for example,
It becomes like a → d and d → a. Therefore, it is sufficient to remove duplicates from among the above-mentioned coincidence pairs and determine opposing groups so that only the remaining ones are detected by the actual circuit. Figure 5 I
In Figure 2, the pairs that are unnecessary in order to eliminate duplicates from among the above-mentioned coincidence pairs are shown by broken lines. That is,
Conventionally, the B group includes, for example, d, e, f, g.
, we chose two groups, d and e, and f
It is only necessary to select such that and g are in group A for a. FIG. 6 is a diagram showing this situation, and the direction of the arrow means A group→B group. An embodiment of the present invention made from this viewpoint is shown in FIG. This embodiment shows a detection circuit configuration between group a and groups d and e. It is assumed that groups d and e are selected as groups opposing group a. However, since it is necessary to take the four groups d, e, f, and g as opposing groups, for f and g, f
A circuit in which g is selected as the detector 1, and a and i are selected as the detector 11A, and a circuit in which g is selected as the detector 1 and a and b are selected as the detector 11A are provided, and the coincidence pairs a to f, a are provided.
~g will be detected. Now, according to the circuit configuration shown in Fig. 7, the output of detector 1 (group a) is
This becomes an input to an AND gate 3 and an input to a delay element 2, and the output of the detector 1 delayed by the delay element 2 becomes an input to an AND gate 30. On the other hand, the output of the detector 11A (groups d and e) passes through the OR gate 9A and becomes the input to the AND gates 3 and 30.
As a result, a total coincidence output 4 of a and d or e is obtained from the AND gate 3, and a coincidental coincidence output 5 as a result of being delayed by the delay element 2 is obtained from the AND gate 5. It turns out. According to this embodiment, without redundantly detecting coincidence pairs between groups, as a result, especially when detecting a coincidental coincidence event, the dead time of the circuit due to the above-mentioned overlap is removed, We were able to improve the high count rate characteristics. Furthermore, since the number of opposing detector groups is halved (when the number of opposing groups is an even number), the counting rate within the opposing detector group is reduced, further improving the high counting rate characteristic. Further, regarding the duplication of all coincidence outputs (output 4), the duplication can be removed, and the circuit configuration as a whole can be simplified. Although this embodiment shows an example of application between opposing groups when the detector groups are grouped, the present invention can also be applied to cases where the detector groups are not grouped.

[Brief explanation of the drawing]

1 and 3 are diagrams of the conventional example, FIGS. 2 and 4 are explanatory diagrams thereof, FIGS. This is an example diagram. 1, 11A...Detector, 2...Delay element, 3,
30...AND gate, 9A...OR gate.

Claims (1)

[Claims] 1. A plurality of positron CTs arranged in a ring shape
A positron CT device that captures and processes the output signal of a detector corresponding to a combination of detectors formed by each detector of a detector and one of a plurality of detectors facing the detector. In the coincidence circuit, a combination of detectors AB formed by one detector A of each of the detectors arranged in a ring shape and detector B which is one of the plurality of detectors facing the detector , and the detector combination BA formed by the detector B and the detector A, which is one of the plurality of detectors facing the detector, a detection corresponding to one of the detector combinations. A coincidence counting circuit for a positron CT device, which is characterized by capturing and processing the output signal of the device. 2. The coincidence circuit for a positron CT apparatus according to claim 1, wherein each of the detectors A and B is constituted by a detector group consisting of a plurality of detectors.