JPH04110792A - Scintillation camera - Google Patents

Abstract

PURPOSE:To improve counting rate characteristics by generating a timing signal in response to incidence of radiation having energy close to a specified energy- window, thereby reducing useless processing time. CONSTITUTION:Respective outputs of plural number of photoelectric transducers connected to a scintillator 22 are all added, and their addition signal is discriminated from a noise level by a discriminator 42. The addition signal is further sent to a discriminator 43, and it is discriminated that the signal exceeds a detection level close to a specified energy-window. Besides, when outputs are generated together from the discriminators 42, 43, a timing signal is generated. Then, an energy discriminating and position calculating circuit 31 begins to operate in response to the timing signal to count unblank signals for each position signal. Thus, the rate of processing events on which it becomes clear that they do not enter the energy-window, is reduced, and thereby useless processing time, unnecessary dead time are reduced to improve counting-rate characteristics.

Description

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

The present invention relates to a scintillation camera used in nuclear medicine, and particularly to improvements in its signal processing system.

[Conventional technology]

In a scintillation camera, scintillation light emission caused by gamma rays incident on the scintillator is converted into an electrical signal (pulse signal) using a photomultiplier, which is a photoelectric converter.
Convert to . Since a photomultiplier near a light emission position produces a large output, a plurality of photomultipliers are connected to a scintillator, and the light emission position, that is, the gamma ray incident position is calculated from the magnitude of each output signal. On the other hand, the subject is usually a human body, and RI is applied to the human body in advance.
When a drug containing (a radioactive isotope) is administered and the RI accumulates in a specific organ, gamma rays generated from this RI are made to enter a scintillator. Therefore, it is necessary to detect only radiation of a specific energy emitted from the RI. Therefore, the scintillation camera uses the fact that the peak value of the summed signal of all the outputs of many photomultipliers corresponds to the incident energy, and uses the peak value of the energy signal as shown in Figure 2. It is determined whether or not the energy is within a specific energy window W determined by the RI, and only when it is, the number of incident gamma rays is counted for each incident position, thereby creating an image of the incident gamma rays. In this scintillation camera, the output of the discriminator is used as a timing signal for starting signal processing such as position calculation and energy discrimination. That is, the sum signal of all photomultipliers obtained for energy discrimination is used as an energy signal, and this energy signal is input to the discriminator. The discriminator consists of a comparator that compares this energy signal with a predetermined low detection level for discriminating it from the noise level, and a timing signal is generated from the comparator output. Since this timing signal is used to determine the basic operation timing of the main circuits, it must be generated as early as possible in the radiation pulse signal generated from each photomultiplier.
As shown in FIG. 2, the detection level L mentioned above is set to a level lowered to the limit where the energy signal can be distinguished from the noise level.

[Problem to be solved by the invention]

However, if the detection level of the discriminator is lowered to the limit as in the past to accelerate the generation of the timing signal, even if an energy signal that does not fall within the specific energy window W is generated, position calculation or energy discrimination etc. This causes the problem that the counting rate characteristics deteriorate. In other words, if an operation such as position calculation or energy discrimination is started when an energy signal that does not fit into a specific energy window W is generated, it will not fit into the energy window W after all. , and the signal is not counted. The time required for processing such as position calculation and energy discrimination is 1μ.
It takes more than a second, and during this time, useless signal processing is performed, resulting in a dead time for necessary signal processing, resulting in poor counting rate characteristics. In view of the above, an object of the present invention is to provide an improved scintillation camera with improved count rate characteristics.

[Means to solve the problem]

In order to achieve the above object, the scintillation camera according to the present invention includes a scintillator, a plurality of photoelectric converters coupled to the scintillator, and energy discrimination and position calculation to which each output of the plurality of photoelectric converters is input. a counting circuit that performs counting for each position signal from the energy discrimination and position calculation circuit according to the unblank signal obtained by the energy discrimination and position calculation circuit; a first discriminator for discriminating the summed signal from a noise level; a second discriminator for discriminating that the summed signal exceeds a detection level close to a predetermined energy window;
and a circuit for generating a timing signal for starting the operation of the energy discrimination and position calculation circuit when both the discriminator output and the second discriminator output are generated.

[For production]

When the energy discrimination and position calculation circuit is started to operate in response to the timing signal and a position signal and an unblank signal are generated, the unblank signal is counted for each position signal. That is, each time radiation is incident on the scintillator and its energy is determined to be within a predetermined energy window, a count is made for each incident position, thereby forming a radiation image. This timing signal is generated when the outputs of the first and second discriminators are both generated. That is, the outputs of the plurality of photoelectric converters coupled to the scintillator are all summed, and the summed signal is discriminated from the noise level by the first discriminator. This results in
No matter what energy of radiation is incident, the incident radiation will be detected. The summed signal is also sent to a second discriminator to determine if it exceeds a detection level close to a predetermined energy window. Then, when outputs are generated from both the first and second discriminators, a timing signal is generated to start the operation of the energy discrimination and position calculation circuit. Therefore, a timing signal is generated not simply in response to incident radiation of any energy, but rather in response to incident radiation with energy close to a predetermined energy window. This timing signal causes the energy discrimination and position calculation circuit to start operating, which results in a reduction in the rate at which processing is performed for events that are determined not to fall within the energy window, reducing waste. It is possible to reduce unnecessary processing time, that is, unnecessary dead time, and improve count rate characteristics.

【Example】

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. In Figure 1, the human body 1 that is the subject
A drug containing RI (radioactive isotope) is administered to 1 in advance, and the RI accumulates in a specific organ 12, and this R
Assume that gamma rays are emitted from 1. This gamma ray passes through the collimator 21 and enters the scintillator 22, causing scintillation light emission. This light is guided by a light guide 23 to each of a number of photomultipliers 24. Each photomultiplier 24 generates an electrical signal according to the light incident thereon. This electrical signal becomes pulsed in response to scintillation light emission due to the incidence of gamma rays, and the magnitude of the signal increases as it approaches the light emission position. The output of each photomultiplier 24 is sent to an energy discrimination and position calculation circuit 31. This energy discrimination and position calculation circuit 31 starts operating when a timing signal is given, adds all the outputs to create an energy signal, and determines whether the wave height of this energy signal is within a predetermined energy window. perform discrimination,
Generates an unblank signal when the Further, since the magnitude of each output of the photomultiplier 24 depends on the gamma ray incident position as described above, the incident position is calculated from the magnitude of each output, and position signals X and Y are obtained. These position signals X and Y are sent to the memory 32 and used as address signals. unblank signal and position signal
When Y occurs simultaneously, this unblank signal is added to the adder 33 with the value read from that address in the memory 32.
, and stored at that address again. In this way, the number of incident gamma rays is counted for each incident position at each address in the memory 32. When counting for a certain period of time is completed, a gamma ray image is reproduced in the memory 32 and taken into the CPU 34. A timing signal for starting the operation of one of the energy discrimination and position calculation circuits 31 is generated as follows. First, each output of a large number of photomultipliers 24 is sent to an adder 41 and added. This addition output is an energy signal, which is the discriminator 4
Sent to 2. The discriminator 42 is constituted by a comparator, and its detection level L is set close to the limit so that the energy signal can be discriminated from the noise level as shown in FIG. That is, since the peak value of the added signal corresponds to the energy of the incident radiation, a similar addition signal is separately generated in the energy discrimination and position calculation circuit 31, and the peak value of this signal is determined by a predetermined energy window. In addition to this, the adder 41 and the discriminator 42 are used to detect whether the radiation is in the pr range or not. Further, the output signal of the adder 41 is sent to another discriminator 43 which is also configured by a comparator,
It is determined whether the added signal (energy signal) exceeds a predetermined detection level L' as shown in FIG. This detection level L. is smaller than the energy window W and is set to correspond to W, which is slightly below the lower limit of W. Therefore, the discriminator 43 can roughly check the possibility that the energy signal wave height will fall within the predetermined energy window W. Then, the output of the discriminator 42 through the delay circuit 44 and the output of the discriminator 43 are combined into an AND circuit 4.
5, and when both occur, a timing signal is generated from this AND circuit 45 and sent to the energy discrimination and position calculation circuit 31. Here, considering that the delay circuit 44 generates a signal later than the discriminator 43 or the discriminator 42, the output of the discriminator 42 is generated while the output of the discriminator 43 is generated. This is to ensure that. Since the output of the discriminator 42, which serves as a timing signal, is delayed through the delay circuit 44, the amount of delay of the input signal in the energy discrimination and position calculation circuit 31 is adjusted accordingly. In this way, the discriminator 42 obtains an accurate timing signal at an early stage of the radiation incident event, and the discriminator 43 receives the timing signal from the AND circuit 45 for energy discrimination when there is no possibility of entering the energy window W. By preventing the signals from being sent to the energy discrimination and position calculation circuit 31, unnecessary signal processing in the energy discrimination and position calculation circuit 31 can be prevented. Note that in this embodiment, the discriminators 42, 43
The energy discrimination and position calculation circuit 31 is equipped with a similar adder for obtaining an energy signal, so it can be used to obtain an energy signal from the adder 41. The discriminator 42, 43 may be configured to provide the obtained energy signal to the discriminator 42, 43. Further, the detection level L° of the discriminator 43 may be configured to change in proportion to the energy of the radiation to be measured. Furthermore, although the discriminators 42 and 43 have a two-stage configuration in the above example, they can also have a three-stage configuration or more.

【Effect of the invention】

According to the scintillation camera of the present invention, the rate of signal processing for radiation incident events that are determined not to fall within the energy window is reduced,
Reduce wasted processing time, reduce unnecessary dead time,
Count rate characteristics can be improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a signal waveform diagram. DESCRIPTION OF SYMBOLS 11... Human body, 12... Organ, 21... Collimator, 22... Scintillator, 23... Light guide, 24... Photo multiplier, 31... Energy discrimination and position calculation circuit, 32...Memory, 33.
...adder, 34...cpu, 41...adder, 4
2.43...Discriminator, 44...Delay circuit, 45...AND circuit.

Claims (1)

[Claims] (1) A scintillator, a plurality of photoelectric converters coupled to the scintillator, an energy discrimination and position calculation circuit to which each output of the plurality of photoelectric converters is input, and a signal obtained by the energy discrimination and position calculation circuit. A counting circuit that counts each position signal from the energy discrimination and position calculation circuit according to the unblank signal, and a first counting circuit that discriminates the sum signal of all outputs of the plurality of photoelectric converters from the noise level.
a discriminator, a second discriminator for discriminating that the summed signal exceeds a detection level close to a predetermined energy window, an output of the first discriminator, and an output of the second discriminator. and a circuit that generates a timing signal for starting the operation of the energy discrimination and position calculation circuit when both occur.