JPH0141951B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a position calculation circuit for a scintillation camera.

NaI (Tl) used in scintillation cameras
The scintillator fluorescence decay time is approximately 250 ns. This fluorescence is converted into an electric current by a photomultiplier tube, and is output as a current pulse as schematically shown in FIG. 1a every time a gamma ray is detected. The position information of the γ-ray corresponds to the total amount of charge of this current pulse, and for convenience of position calculation, a process called waveform shaping is performed to express the amount of charge of this current pulse as the height of the voltage pulse. . Conventionally, the waveform shaping that has been generally performed is to first pass the first waveform through an integration circuit (for example, integration using C and R) with a time constant (for example, 10 μs) that is sufficiently larger than the width of the current pulse.
Create a voltage signal as shown in Figure b. The height of the voltage signal at this time is proportional to the total charge amount of the current pulse. Next, it is passed through a differentiating circuit to shorten the waveform.
The resulting signal becomes a voltage pulse with a shortened time width as shown in FIG. 1c. In this case, the differential is
It has a width of 1 to several μs by using a C and R differentiating circuit (for example, time constant 1 μs) or waveform shortening by a delay line.

The pulses thus obtained are input to a position calculation circuit that calculates the incident position of the γ-rays. The width of this pulse almost proportionally affects the time resolution of the position calculation circuit (the minimum time in which two successively detected gamma rays can be processed normally).

In the conventional waveform shaping described above, if the pulse width is too shortened with the aim of increasing the number of gamma rays counted per unit time (hereinafter referred to as the counting rate), the energy resolution of gamma rays decreases and This is known to be a factor that reduces the spatial resolution of images.

In the field of radiation measurement, not just scintillation cameras, the demand for high energy resolution and precision has led to various developments in circuit technology.
One of them is to create a narrow pulse signal in the amplification stage using a differentiator circuit using a delay line, so that high counting rates can be used without sacrificing energy resolution, and then A gate integrator that integrates only the signal by opening the gate has already been proposed. (N.
Karlovac and TVBlalock, IEEE Trans.
Nuclear Science, Vol.NS−22,
(February 1975, 452-456) The method of shortening the pulse width by minimizing the energy resolution described above improves the time resolution and can greatly improve the count rate characteristics (for example, by a factor of 3 or more), but it requires a complicated device. However, there are reliability and cost disadvantages due to this complexity. This difficulty has a particularly large effect on devices having a multi-channel circuit system of the same type of circuits, such as the embodiments described below.

The objects of the present invention are to improve the above-mentioned drawbacks of the conventional devices and provide: (1) a scintillation camera that has high time resolution and can handle high counting rates without reducing energy resolution;

(2) A scintillation camera with a simplified waveform shaping method for pulse shortening and superior cost and reliability.

Our goal is to provide the following.

In explaining the present invention below, points of interest obtained from the characteristics of conventional devices and their implementation will be explained first.

FIG. 2 is a block diagram of pulse signal processing in a conventional radiation measuring device whose count rate characteristics have been improved by the method described above. That is, a radiation detector (not shown) (for example, a sodium iodide scintillator and a scintillator that retains the luminescence characteristics of the scintillator)
photomultiplier tube and preamplifier that converts it into an electrical signal)
The pulse signal from the input terminal 1 is input to the input terminal 1, and one system has a differentiating circuit 2 that shortens the pulse signal by differentiating it with a delay line or CR, a delay circuit 3 that adjusts the time with other systems, and a pulse signal. a gated integration circuit 4 that integrates for a gate time to be described later; a sampler 5 that extracts the integration result in a short time width;
Another system provided to control the output terminal 6 and the entire system is a discriminator 7 that detects the arrival of a pulse signal at the input terminal 1 and that the pulse height exceeds a required certain level. It is comprised of a control circuit 8 which uses the output of the discriminator 7 as the origin of control time to determine the start and end of integration of the gated integration circuit 4 and the sampling time and sampling time width of the sampler 5. Further, waveforms and timing corresponding to the block diagram of FIG. 2 are shown in FIG. 1S is a pulse signal arriving at input terminal 1. 2S is an output waveform of the differentiating circuit 2, which is a differentiated waveform using a delay circuit with a differentiating time set to Tc. S3 is the output waveform of the delay circuit 3, which has a delay time corresponding to the output waveform of the discriminator 7. 4S is the output waveform of the gated integration circuit 4, and the integration start and end times are determined by a signal from the control circuit 8. 5S is sampler 5
Output waveform 4 of gated integrator circuit 5
S is sampled with the sampling start time delayed by Ts. 7S is the output waveform of the discriminator 7, and the falling side of the waveform is used as the origin of time. This output waveform determines the integration start time of the gated integration circuit 5. This is one output waveform of the 8Sa control circuit 8, which determines the integration start and end times of the gated integration circuit 5. Further, the falling side of 8Sa is used as a reset signal indicating the end of integration.
8Sb is the other output waveform of the control circuit 8, which determines the sampling start and end times of the sampler 5.
The former becomes Ts. Considering the above relationship between Ts and Tc, it is effective to sample at the point where the 4S integral waveform is saturated, so it is usually better that Ts≧Tc. However, when Tc and Ts are made small, the energy resolution of the sampling waveform deteriorates. Tc
Figure 4 was obtained by examining the relationship between Ts and energy resolution using Ts as a parameter. . K _R on the vertical axis is a value corresponding to energy resolution, and an increase in K _R corresponds to a decrease in energy resolution. When looking at Tc = 0.1, 0.2, and 0.4μs in Figure 4, the energy conversion ability decreases as the sampling start time Ts decreases, but Ts
It is shown that the decrease in energy resolution is the smallest in the range of the solid line that satisfies the condition of =Tc. Furthermore, when trying to suppress the decrease in energy resolution to 10% or less (corresponding to K _R ≦1.1), Ts cannot be made extremely small, and in the case of Fig. 4, Ts≧
It becomes 0.42μs. As described above, the lower limit value of Ts needs to be determined in relation to the allowable value of energy resolution.

The present invention utilizes a conventional method in which a narrow signal is created from a pulse signal using a differentiating circuit, and then only the pulse signal to be measured is integrated by opening the gate, and then sampled after the integration is completed.
The purpose of the present invention is to provide a radiation measuring device that has a high counting rate and excellent time resolution without reducing the energy resolution by focusing on the fact that the decrease in energy resolution is the smallest when Ts and the differential time Tc are equal. . Embodiments of the present invention are shown in FIG. 5 showing the basic configuration, FIG. 6 showing a time chart, FIG. 7 showing a specific circuit example, and a block diagram in which the invention is applied to a scintillation camera. This will be explained with reference to FIG.

The basic structure and operation of an embodiment of the present invention will be explained using FIG. 5 and FIG. 6. FIG. 5 is a block diagram of a pulse shortening means showing an embodiment of the present invention. A pulse signal from a radiation detector is input to an input terminal 21, and a delay circuit 22 adjusts the time between one system and another system, an integration circuit 23 with a gate that integrates the pulse signal by a gate time to be described later, and an integration result. a sampler 24 that takes out the data in a short time span;
Another system provided to control the output end 27 and the entire system includes a discriminator 25 that detects the arrival of a pulse signal at the input end 21 and that the pulse height exceeds a required certain level. The gated integration circuit 23 uses the rising side of the output waveform of the discriminator 25 as the origin of the control time.
The control circuit 26 determines the integration start and end times of the sampler 24, the sampling start time of the sampler 24, and the sampling time width.

FIG. 6 is a time chart of output waveforms of each part in FIG. 5. 21S is a pulse signal waveform input to the input terminal 21. 22S is a signal pulse delayed by the delay circuit 22 in order to adjust the timing of control by the control circuit 26. 25S is the output waveform of the discriminator 25, and the origin of the time on the rising side of this signal, that is, the pulse signal 22S
Let the integration start time of . 26Sa is the control circuit 26
This is one output waveform of the gated integration circuit 24, which determines the integration start and end times of the gated integration circuit 24. 26Sb
is the other output waveform of the control circuit 26 and determines the sampling start time Ts and the sampling end time of the sampler 24. Further, the falling side of 26Sa is used as a signal for resetting the integral value up to that point after the completion of sampling. 23S is the output waveform of the gated integration circuit 23, and the integration time is controlled by the control signal 26Sa of the control circuit 26.
24S is the output waveform of the sampler 24, and the sampling time is controlled by the control signal 26Sb of the control circuit 26. From the results shown in FIG. 4, the upper limit of the sampling start time Ts is preferably 0.8 μs or less, and preferably 1 μs or less. Even if it is made longer than this, no improvement in energy resolution can be expected and it will be a waste of time. Further, as for the lower limit of Ts, the shorter the better, but since this causes a decrease in energy resolution, it is selected in relation to the allowable value of energy resolution as described above.

The present invention is characterized in that the function of shortening the pulse width by the differentiating circuit provided before or after the delay circuit in the conventional configuration is not added. Another feature is that by setting the sampling time Ts to a time corresponding to the conventional differential time Tc, it is possible to
In addition to integrating by effectively using information up to
By continuing integration during sampling, the effective integration time is extended, which increases the utilization rate of the information contained in the input pulse signal, contributing to improved energy resolution.

Next, an embodiment based on this operating principle will be explained using a specific circuit shown in FIG.

A detector 31 is composed of a photomultiplier tube PMT and a preamplifier _A1 , which are the circuit parts of the detector, an input buffer _B1 that receives the output signal of the detector 31, and a delay line D amplifier _A2 that creates a delay time. A delay circuit consisting of a delay circuit and a baseline stabilizer to stabilize the baseline necessary to make it practical at high counting rates.
Gate G ₁ that controls BLR and integration time,
A pulse shortening circuit 32 includes a gated integrator consisting of a gate G ₂ , an integrating capacitor C, and an integrating amplifier A ₃ , and a sampler consisting of a gate G ₃ for sampling and a gate amplifier A ₄ , and the output signal of the detector output 31. A discriminator that creates the time origin is composed of an input buffer _B2 , an amplifier _A5 , and a comparator CO, and a monostable multivibrator that creates gate time signals for the gates _G1 and _G2 .
It consists of a timing control circuit 33 composed of MM ₁ and a monostable multivibrator MM ₂ that creates the gate time of the gate G ₃ . Furthermore, output waveforms 41S to 46 of main points 41 to 46
S is shown in FIG. In addition, the circuit of this example has the following:
A compensation circuit is added to eliminate switching noise generated in gates G ₁ , G ₂ , and G ₃ .
For G ₁ , the differential input characteristics of switch G ₁ ′ and its additional resistance and amplifier A ₃ are determined, and for gate G ₂ , the differential input characteristics of amplifier A ₃ are determined by switch G ₂ ′ and its additional resistance and capacitance, and gate G ₃ As for its additional capacity C
The switching noise is compensated for by the differential input characteristics of the resistor and amplifier _A4 . This compensation circuit is essential for preventing artificial artifacts caused by so-called switching noise when the waveform shaping circuit of the present invention is applied to radiation imaging.

Furthermore, as an example of a further expanded application of the previous embodiments, there is a scintillation camera that calculates the incident position of radiation and displays a radiation image.

Next, an embodiment in which the present invention is applied to a scintillation camera will be described with reference to the block diagram of FIG. In FIG. 9, scintillation due to γ rays incident on a planar sodium iodide crystal scintillator 51 is detected by 61 photomultiplier tubes 52 (more than one is possible in principle) densely arranged on the scintillator. It is converted into a current pulse and further amplified by a preamplifier 53. In this case, the circuit of the photomultiplier tube 52 and preamplifier 53 can be used as is for the detector 31 shown in FIG. 7 of the aforementioned embodiment. Furthermore, the outputs of these preamplifiers 53 are input to a row/column matrix 54 for position calculation. The output of this matrix 54 is input to a pulse shortening circuit 55, which is the same as the pulse shortening circuit 32 of FIG. 7 shown in the previous embodiment, and is subjected to waveform shaping processing mainly to compress pulses. On the other hand, another output Z signal of the row/column matrix 54, which provides information on all the scintillations for one gamma ray event, is the seventh signal shown in the above embodiment.
The signal is input to a timing control circuit 57 that is the same as the timing control circuit 33 shown in the figure, and its output is input to a pulse shortening circuit 55 as a control signal for the pulse shortening circuit. Furthermore, another system of the Z signal is input to a wave height splitter 58, and the position is calculated using this output and the output signal of the above-mentioned pulse shortening circuit 55 in a position calculation circuit 56 of a conventional general scintillation camera. After converting it into a two-dimensional position signal, it is input to the CRT 59 to reproduce and display the incident position of the γ-ray on the scintillator 51.

As described above, compared to conventional devices, the device of the present invention can (1) provide a radiation measuring device that can handle high count rates with even higher time resolution without reducing energy resolution;

(2) By eliminating the need for differentiating pulse signals, the waveform shaping method for pulse shortening is simplified, and a radiation measurement device that is superior in terms of cost and reliability can be provided.

It has the following advantages.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the waveform of a conventional gamma ray detection signal, Fig. 2 is a block diagram showing an example of a pulse shortening method for a conventional scintillation camera, and Fig. 3 is a time chart of the waveforms of various parts in Fig. 2. 4 is a characteristic diagram of pulse shortening according to the conventional method shown in FIG. 2, FIG. 5 is a block diagram of a pulse shortening method showing an embodiment of the present invention, and FIG.
The figure is a time chart of the output waveforms of each part in Figure 5, Figure 7 is a specific circuit diagram showing an embodiment of the present invention, Figure 8 is an output waveform diagram of the main points of Figure 7, and Figure 9 is the main point of the main points in Figure 7. FIG. 2 is a block diagram in which the invention is applied to a scintillation camera. 21...Input terminal, 22...Delay circuit, 23...
...Integrator circuit with gate, 24...Sampler, 25
... Discriminator, 26 ... Timing control circuit.

Claims (1)

[Scope of Claims] 1. A scintillation camera consisting of a plurality of photomultiplier tubes optically coupled to a plate-shaped scintillator and a position calculation circuit for determining the light emitting position of the scintillator, which maintains the emission characteristics of the scintillator caused by radiation and generates electricity. Pulse current conversion means consisting of a photomultiplier tube and a current amplification circuit for converting into a signal, detection means for detecting the current pulse outputted by the pulse current conversion means, and integration means for integrating the output signal of the pulse current conversion means. a sampling means for sampling the integral output of the integrating means in a preset time width after the elapse of a preset time after the start of the integration; and a reset means for terminating the integration operation by the integrating means in synchronization with the end of the sampling. By using the output signal of the detection means as the time origin and controlling the time of the integration means, the sampling means, and the reset means, the time width of the signal waveform that is the basic input of the position calculation circuit can be determined. A scintillation camera characterized by having a shortening means. 2. The scintillation camera according to claim 1, wherein the preset time is 1 μs or less.