JP2002122671A - Scintillation camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the adjusting manhour, to improve the precision, and to provide a high picture quality image by digitally removing effects of superimposition (pile-up) of signals without using a clipping circuit by a delay line and differentiating and integrating circuits. SOLUTION: An output signal of a photomultiplier 4 is converted into a digital signal by an A/D converting circuit 5 and cumulatively added by an integrating circuit 7. This scintillation camera is provided with a shifter, which makes a prescribed time delay by shifting the output signal of the A/D converting circuit to a rear step synchronously with a reference clock signal outputted by an oscillation circuit 13, and a digital clipping circuit 6, which comprises a divider reducing the signal time-delayed by the shifter at a prescribed rate and a subtracter subtracting the signal of the divider from the output signal of the A/D converter, between the A/D converter and the integrating circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scintillation camera, and more particularly to a scintillation camera which eliminates the influence of signal overlap (pile-up) to reduce adjustment man-hours, improve adjustment accuracy, and obtain high-quality images. It relates to a scintillation camera.

[0002]

2. Description of the Related Art A conventional scintillation camera is shown in FIG.
As shown in FIG. 1, a collimator 1 that passes only gamma rays that are incident on a detector surface in a direction perpendicular to the gamma rays emitted from a radioisotope in a subject, and the collimator 1
A scintillator 2 that emits light by gamma rays incident therethrough, a light guide 3 that transmits light of the scintillator 2, and the scintillator 2 through the light guide 3.
A photomultiplier tube (hereinafter abbreviated as “PMT”) 4 that is optically coupled to the PMT 4 and converts the light into an electric signal;
Conversion circuit 5 for converting an output signal from the A / D converter into a digital signal, integration circuits 7 for accumulatively adding the signals output from the A / D conversion circuit 5 to perform integration, and an output signal of each integration circuit 7 Calculation circuit 8 (CAL) for calculating the gamma ray incident position on the scintillator 2 according to the difference in the size of
And a memory circuit 9 (Memory) for accumulating and storing the position signals output from the position calculation circuit 8 and storing it as a scintigram (gamma ray image), and a display circuit for displaying the scintigram stored in the memory circuit 9 10 (CRT)
And an addition circuit 1 for adding the electric signals of all the PMTs 4 to control the A / D conversion circuit 5 and the integration circuit 7
1 (Summing), and a timing circuit 12 (Timing) for judging that a gamma ray has entered the scintillator 2 based on the output signal of the adding circuit 11 and controlling start / stop of the cumulative addition in the integrating circuit 7. Oscillation occurs in a sufficiently short time as compared with the integration time of the integration circuit 7, and the A / D conversion circuit 5,
An oscillation circuit 13 (Clock) for outputting a reference clock signal for obtaining synchronization of the integration circuit 7 and the timing circuit 12 is provided.

Here, the light emission of the scintillator 2 is shown in FIG.
As shown in (a), an exponential function ex
Take a waveform that attenuates with p (-t / τ). τ is an emission decay time constant, which is about 250 ns (nanosecond) in the case of a sodium iodide (Nal) scintillator used in a scintillation camera. The light emitted from the scintillator 2 is originally weak, and is further distributed via the light guide 3 to each of the PMTs 4 around the gamma ray incident position.
The waveform has fluctuations due to statistical noise as shown in FIG. The reason that the output signal of the A / D conversion circuit 5 is integrated by the integration circuit 7 in the configuration of the scintillation camera described above is that a signal from which fluctuation due to this statistical noise is removed (S / N ratio is increased) is input to the position calculation circuit 8. This is because the position of the gamma ray incident on the scintillator 2 is determined with high accuracy.

Normally, the integration time of the integration circuit 7 is 3 to 4 times the emission decay time constant τ, that is, 750 ns to
It takes 1 μs (microsecond).

The details of this integration operation will be described below.
The A / D conversion circuit 5 outputs the PM signal irrespective of the incidence of the gamma ray.
The output of T4 is continuously converted to a digital signal, and is output for each clock of the oscillation circuit 13. FIG. 5C shows the digital signal output of the A / D conversion circuit 5 in a time-series manner. Here, in order to make it easier to see, fluctuations due to statistical noise are displayed, and in the following figures, fluctuations due to statistical noise are also displayed. On the other hand, the timing circuit 12 adds the signals of all the PMTs 4 to each other.
From the gamma ray based on the signal from the
Then, a control signal for instructing integration for a certain period of time is output to the integration circuit 7 after counting by a built-in counter.
The integration circuit 7 accumulates and outputs the digital signal from the A / D conversion circuit 5 according to the control signal for each clock of the oscillation circuit 13 as shown in FIG. The position calculating circuit 8 obtains the gamma ray incident position on the scintillator 2 with high accuracy by using the last signal data that has been cumulatively added, that is, the integrated data.

[0006]

In such a conventional scintillation camera, when a plurality of gamma rays are incident on the scintillator 2 at short time intervals, the accuracy of position calculation is caused by signal overlap (hereinafter referred to as pile-up). Problems such as reduced image quality and the formation of false images occur.

The output of the A / D conversion circuit 5 in this case is shown in FIG.
FIG. 6A shows the output of the integration circuit 7 in FIG. FIG.
In (a), since the tail part of the previous signal remains in the base part of the second and subsequent signals, the time-series output of the integrating circuit 7 that integrates this is the second and subsequent signals in FIG. Signal is larger than that obtained by a single gamma ray indicated by a broken line.

When the position calculation circuit 8 calculates a position based on this signal, the accuracy of the calculated position is degraded and a pseudo image is generated. The pile-up phenomenon described above becomes more likely to occur when the number of gamma rays incident on the scintillator 2 increases (incidence count rate increases), and has been an obstacle to improving the measurement accuracy in the field of gamma ray measurement for a long time.

Therefore, in the field of gamma ray measurement, a clipping circuit (waveform shaping circuit) for shortening a pulse width by using a delay line (delay line) or a calculus circuit (CR) for removing this pile-up has conventionally been employed. I have. In the scintillation camera, if this clipping circuit is placed before the A / D conversion circuit 5, the output of the A / D conversion circuit 5 can be shortened as shown in FIG. .

However, a clipping circuit using a delay line (delay line) or a calculus circuit (CR) is considerably large in size, and a large number of PMs such as a scintillation camera are used.
It was difficult to apply to T4 due to the structure of the detector. In addition, a delay line (delay line) or a calculus circuit (C
The clipping circuit according to R) needs to individually adjust a time constant and a gain, and performing the adjustment for each PMT 4 has problems in terms of labor and adjustment work accuracy. For this reason, although the effect is recognized, it is difficult to apply the clipping circuit with the conventional scintillation camera.

Therefore, an object of the present invention is to provide a clipping circuit using a conventional delay line or a calculus circuit without using a conventional clipping circuit.
It is an object of the present invention to provide a scintillation camera capable of reducing the number of adjustment steps, improving adjustment accuracy, and obtaining a high-quality image by digitally eliminating the influence of signal overlap (pile-up).

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a scintillator which emits light by an incident gamma ray, a photomultiplier tube optically coupled to the scintillator and converting the light into an electric signal, and an output of the photomultiplier tube. An A / D conversion circuit for converting a signal into a digital signal, an integration circuit for cumulatively adding the output signals of the A / D conversion circuit, an addition circuit for adding the electric signals of all the photomultiplier tubes, and the addition circuit A timing circuit for controlling the start / stop of the cumulative addition of the integration circuit with the output signal of the integration circuit; an oscillation circuit for outputting a reference clock signal for synchronizing the timing circuit and the A / D conversion circuit with the integration circuit; A position calculating circuit for calculating an incident position of a gamma ray incident on the scintillator using a digital signal output from the integrating circuit; and a position signal output from the position calculating circuit. A scintillation camera having a memory circuit and a display circuit for displaying a scintigram stored in the memory circuit, wherein the A / D conversion circuit is connected between the A / D conversion circuit and the integration circuit. A shifter that shifts an output signal to a subsequent stage in synchronization with a reference clock signal output from the oscillation circuit to create a designated time delay, a divider that reduces a signal delayed by the shifter at a fixed rate, This is achieved by providing a digital clipping circuit including a subtractor for subtracting the signal of the divider from the output signal of the / D conversion circuit.

In the scintillation camera thus configured, the influence of the immediately preceding signal is removed from the continuously incident gamma ray signal by the subtractor of the digital clipping circuit, so that the fluctuation of the output signal of the integration circuit due to pile-up. Can be eliminated.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to FIGS. 1, 2 and 3. FIG. FIG. 1 is a circuit block diagram showing an embodiment of a scintillation camera according to the present invention. In FIG. 1, a collimator 1 that passes only gamma rays that are incident on a detector surface in a direction perpendicular to the gamma rays emitted from a radioisotope in a subject,
A scintillator 2 that emits light by gamma rays incident through the collimator 1, a PMT 4 that is optically coupled to the scintillator 2 and converts the light into an electric signal, and an A / D that converts an output signal of the PMT 4 into a digital signal. (Clip) a conversion circuit 5 and a digital clipping circuit 6 for shortening the output signal of the A / D conversion circuit 5 in time series by shifting, dividing and subtracting a fixed number of clock signals;
An integrating circuit 7 for cumulatively adding the output signals of the digital clipping circuit 6, an adding circuit 11 (Summing) for adding all the electric signals of the PMT 4, and a cumulative addition of the integrating circuit 7 using the output signals of the adding circuit 11; A timing circuit 12 (Timing) for controlling start / stop, an oscillation circuit 13 for outputting a reference clock signal for synchronizing the timing circuit 12, the A / D conversion circuit 5, the digital clipping circuit 6, and the integration circuit 7; (Clock), a position calculation circuit 8 (CAL) for calculating the incident position of the gamma ray incident on the scintillator 4 using the digital signal output from the integration circuit 7, and a position signal output from the position calculation circuit 8 It comprises a memory circuit 9 (Memory) for storing and a display circuit 10 (CRT) for displaying scintigrams stored in the memory circuit 9.

FIG. 2 is a circuit block diagram showing the internal configuration of the digital clipping circuit 6 according to the present invention. In FIG. 2, a shifter 21 (Shift) shifts an output signal of the A / D conversion circuit 5 to a subsequent stage in synchronization with a reference clock signal output from the oscillation circuit 11 to create a specified time delay.

The time delay is usually set to be equal to or slightly shorter than the integration time of the integration circuit 7. This is to reduce the output signal from the A / D conversion circuit 5 to a necessary minimum and keep the S / N ratio of the output of the integration circuit 7 at the subsequent stage large.
Next, the signal delayed by the shifter 21 is reduced at a fixed rate by the divider 22 (Divide). FIG. 3A shows the output of the A / D conversion circuit 5, and FIG. 3B shows the output of the divider 22. The output of the A / D conversion circuit 5 is an exponential function exp
(−t / τ), so that the divider 22
Is divided by the time lag time. Subtractor 23
(Sub) subtracts the output of the divider 22 from the output of the A / D conversion circuit 5. The result is shown in FIG. The output of FIG. 3C is shortened in time series,
The output of the integration circuit 7 is such that the effect of pile-up has been removed as shown in FIG.

[0017]

As described above, according to the present invention, the influence of the previous signal is removed from the continuously incident gamma ray signal by the subtracter 23 of the digital clipping circuit 6, and the output of the integration circuit 7 by pile-up is obtained. Since the signal fluctuation can be eliminated, even if the number of gamma rays incident on the scintillator 2 increases (incident count rate increases), a scintillation camera that does not cause a decrease in position calculation accuracy due to pile-up or a pseudo image is generated. Can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of a scintillation camera according to the present invention.

FIG. 2 is a block diagram of a digital clipping circuit of the scintillation camera according to the present invention.

FIG. 3 shows A of the present invention using a digital clipping circuit.
FIG. 4 is an output waveform diagram of a / D conversion circuit and an integration circuit.

FIG. 4 is a circuit block diagram of a conventional scintillation camera.

FIG. 5 is an output waveform diagram of a photomultiplier tube, an A / D conversion circuit, and an integration circuit in a conventional scintillation camera.

FIG. 6 is an output waveform diagram of an A / D conversion circuit and an integration circuit when a pile-up occurs in a conventional scintillation camera.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 collimator 2 scintillator 3 light guide 4 photomultiplier tube (PMT) 5 A / D conversion circuit 6 digital clipping circuit (Clip) 7 integration circuit 8 position calculation circuit (CAL) 9 memory circuit (Memory) 10 display circuit (CRT) Reference Signs List 11 Addition circuit (Summing) 12 Timing circuit (Timing) 13 Oscillation circuit (Clock) 21 Shifter 22 Divider (Divide) 23 Subtractor (Sub)

Claims (1)

[Claims] 1. A scintillator which emits light by an incident gamma ray, a photomultiplier tube which is optically coupled to the scintillator and converts the light into an electric signal, and an A which converts an output signal of the photomultiplier tube into a digital signal. / D conversion circuit;
An integration circuit for cumulatively adding the output signals of the D conversion circuit, an addition circuit for adding the electric signals of all the photomultiplier tubes, and a start of the cumulative addition of the integration circuit with the output signal of the addition circuit;
A timing circuit for controlling stop, an oscillation circuit for outputting a reference clock signal for synchronizing the timing circuit and the A / D conversion circuit with the integration circuit, and a digital signal output from the integration circuit. A position calculating circuit for calculating an incident position of a gamma ray incident on the scintillator, a memory circuit for storing a position signal output from the position calculating circuit, and a display circuit for displaying a scintigram stored in the memory circuit. In the scintillation camera comprising: the A / D conversion circuit and the integration circuit;
A shifter that shifts an output signal of the A / D conversion circuit to a subsequent stage in synchronization with a reference clock signal output by the oscillation circuit to create a designated time delay, and reduces a signal delayed by the shifter at a fixed rate. And a digital clipping circuit comprising a subtractor for subtracting the signal of the divider from the output signal of the A / D conversion circuit.