JPH07209426A - Scintillation camera - Google Patents

Abstract

PURPOSE:To correct the offset generated according to the incident quantity of gamma ray in a circuit for A/D converting the signal from the photomultiplier of a scintillation camera, and provide a stable image which is never changed by the quantity of the incident gamma ray. CONSTITUTION:An A/D converting circuit is formed of an A/D converter 14 for converting the signal outputted from a preamplifier 5 into a digital signal; a latch 16 for holding the lowest value of the output of the A/D converter 14; a comparator 15 for comparing the lowest value held by the latch 16 with the output value of the A/D converter 14 to renew the lowest value held by the latch 16; and a subtracter 17 for subtracting the lowest value from the output value of the A/D converter 14, so that the standard level of the input signal is regularly conformed to the lowest value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention administers a substance labeled with a radioisotope (hereinafter referred to as RI) to a subject, detects radiation such as gamma rays emitted from the RI, and detects the radiation inside the subject. The present invention relates to a scintillation camera that obtains the distribution of RI, and particularly to a plurality of photomultiplier tubes (hereinafter referred to as PM
Write T. The present invention relates to a scintillation camera capable of constantly correcting the reference level of a signal that changes according to the amount of incident signal in a circuit for A / D converting the signal output from (1).

[0002]

2. Description of the Related Art A conventional scintillation camera is shown in FIG.
It is configured as shown in. That is, a scintillator 2 that emits light when a gamma ray is incident, a collimator 1 that passes only a gamma ray that is incident on the lower surface of the scintillator 2 at a right angle among the gamma rays emitted from RI in a subject (not shown), A light guide 3 for transmitting an optical signal of the scintillator 2, and a plurality of PMTs 4 optically coupled to the scintillator 2 via the light guide 3 for converting the light into an electric signal and for amplifying and outputting the electric signal; This PMT4 is installed in the number of P
A plurality of preamplifiers 5 for amplifying the output signal of MT4,
Similarly, a plurality of A / D conversion circuits 40, which are installed by the number of PMTs 4 and convert the output signals of the respective PMTs 4 amplified by the preamplifier 5 into digital signals, and the magnitude of the output signals of the respective A / D conversion circuits 40. The scintillator 2 due to the difference in height
A position calculation circuit 7 for obtaining the position of the gamma ray incident on
The display device 8 displays bright spots according to the position signal of the gamma ray obtained by the position calculation circuit 7 and creates and displays a scintigram.

In the conventional scintillation camera having such a structure, in particular, the A / D conversion circuit 40 is shown in FIG.
As shown in, the capacitor 11 for AC coupling the signal from the preamplifier 5, the ideal diode 21 for reproducing the reference level of the AC coupled signal, and the signal suitable for A / D conversion by amplifying the signal. An amplifier 13 to be sized,
A / D converter 1 for A / D converting the signal from the amplifier 13
It is composed of 4.

Here, the capacitor 11 is used to separate the preamplifier 5 and the subsequent circuits in terms of direct current. The signal input from the preamplifier 5 is shown in FIG. If the amount of gamma rays incident on the scintillator 2 is small, even if it is a unipolar signal, there is no difference between the reference level of the signal and the ground level of the direct current, and the following circuits consider the reference level of the signal and the ground level to be the same. There is no problem in processing.

However, when the amount of gamma rays incident on the scintillator 2 increases and the count becomes high (see FIG. 6B),
Since AC coupling is performed by the capacitor 11, a difference occurs between the reference level of the signal and the ground level. For this reason, it becomes necessary to reproduce the reference level of the signal before A / D conversion by the A / D converter 14, but the ideal diode 21 performs this reference level reproduction. Ideal diode 21
Is an operational amplifier 23 and a diode 2 as shown in FIG.
The reference level of the signal does not always take a negative value.

As a result, even when the count is high, the reference level of the signal can be matched with the ground level and shaped into the signal of the ideal diode (see FIG. 6C), which is input to the A / D converter 14. Even when the signal reference level is normally counted,
The same can be maintained even at high counts.

[0007]

However, in such a conventional scintillation camera as described above, the ideal diode 21 has the following characteristics depending on the frequency characteristic of the operational amplifier 23 used, the offset, the forward voltage of the diode 22, and the like.
As shown in FIG. 6D, the reference level of the signal cannot be made equal to the ground level, and the reference level of the signal input to the A / D converter 14 varies depending on the amount of gamma rays incident on the scintillator 2. Was there.

Therefore, the magnitude of the signal input to the position calculation circuit 7 changes depending on the amount of incident gamma rays, which makes it difficult to obtain a stable image.

It is an object of the present invention to provide a scintillation camera capable of obtaining a stable image which does not change depending on the amount of incident gamma rays.

[0010]

The above object is to provide a scintillator which emits light by incident radiation and a PMT which is optically coupled to the scintillator and converts the light into an electric signal.
A preamplifier that amplifies the signal output from the PMT, an A / D conversion circuit that converts the signal output from the preamplifier into a digital signal, and a signal output from the A / D conversion circuit to the scintillator. In a scintillation camera including a position calculation circuit for calculating the position of incident radiation and a display device for displaying the input position of the radiation calculated by the position calculation circuit, a signal output from the preamplifier is converted into a digital signal. A / D
A converter, a latch for holding the minimum value of the output of this A / D converter, the minimum value held by this latch, and the above A / D
A comparator for comparing the output value of the converter and updating the minimum value held in the latch, and a subtracter for subtracting the minimum value from the output value of the A / D converter are provided. This is achieved by configuring a D conversion circuit.

[0011]

In the scintillation camera configured as above, the reference level of the input signal is held by the latch, and the signal amount of the reference level is subtracted by the subtractor, so that the amount (quantity) of the incident gamma ray is increased. Keep the signal magnitude constant regardless of. As a result, a stable image that does not change depending on the amount of incident gamma rays can be obtained.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a scintillation camera according to the present invention. In FIG. 1, 1 is a collimator 1, 2 is a scintillator, 3 is a light guide,
4 is PMT, 5 is preamplifier, 6 is A / D conversion circuit, 7
Is a position calculation circuit, and 8 is a display device.

In this case, the collimator 1 allows only the gamma rays which are incident on the lower surface of the scintillator 2 at right angles among the gamma rays emitted from the RI in the subject (not shown) to pass through, and the scintillator 2 is placed on the upper surface thereof. Is provided.

This scintillator 2 comprises the above collimator 1
It emits light by gamma rays incident through it and is formed in a circular or rectangular flat plate shape. A light guide 3 is provided on the back surface of the scintillator 2. The light guide 3 transmits the light emitted from the scintillator 2 to the PMT 4, and is formed in a circular or rectangular flat plate shape like the scintillator 2.

On the back surface of the light guide 3, a plurality of PMTs 4 are arranged over the entire surface. This P
The MT 4 is connected to the scintillator 2 via the light guide 3.
Is optically coupled to the scintillator 2, detects the light emitted from the scintillator 2, converts it into an electrical signal proportional to the intensity of the light, and amplifies and outputs the electrical signal.

The signal output from the PMT 4 is input to the preamplifier 5. This preamplifier 5 is the above PMT
The output signal from 4 is amplified. The signal output from the preamplifier 5 is input to the A / D conversion circuit 6. This A
The / D conversion circuit 6 converts the signal from the preamplifier 5 into a digital signal and outputs it.

The signal output from the A / D conversion circuit 6 is input to the position calculation circuit 7. The position calculation circuit 7 determines the light emitting point on the scintillator 2 from the difference in the magnitudes of the signals from the A / D conversion circuits 6 and determines the position of the gamma ray incident on the scintillator 2.

The signal output from the position calculation circuit 7 is input to the display device 8. The display device 8 displays the incident position of the gamma ray obtained by the position calculation circuit 7 and creates a scintigram.

FIG. 2 shows a specific example of the A / D conversion circuit 6 which is a main part of the present invention. In FIG. 2, the signal output from the preamplifier 5 passes through a capacitor 11 that performs AC coupling, and is input to an amplifier 13 that amplifies the signal and has a size suitable for A / D conversion. The signal amplified by the amplifier 13 is
The A / D converter 14 produces a digital signal.

The output signal of the A / D converter 14 is the lowest value, that is, the latch 16 which holds the reference level of the signal.
And a comparator 15 that always compares the output of the A / D converter 14 with the reference level held by the latch 16, and a subtracter 17 that subtracts the lowest value from the output of the A / D converter 14. Enter each.

The comparator 15 constantly compares the output of the A / D converter 14 with the output of the latch 16, and the A / D converter 14
When the new minimum value is output, the holding signal is output to the latch 16 to update the minimum value.

The latch 16 holds the lowest value, that is, the reference level of the signal while updating it, and outputs the lowest value signal to the comparator 15 and the subtractor 17. The subtractor 17 subtracts the lowest value, which is the reference level of the signal held by the latch 16, from the output of the A / D converter 14, and always outputs only a value equal to or higher than the reference level of the signal to the position calculation circuit Output to.

As a result, the minimum value is always held in the latch 16, and the incident gamma ray signal can be reproduced regardless of the amount of gamma ray incident on the scintillator 2.

In the above arrangement, when the incidence of gamma rays decreases, it becomes impossible to follow up even if the minimum value becomes large. Therefore, the latch 16 portion may be configured by a counter instead of the latch 16, and the minimum value may be gradually increased by updating the data one by one at regular time intervals.

According to this, even if the amount of gamma rays incident on the scintillator 2 is reduced and the reference level of the signal approaches the ground level, the minimum value can be increased, so that the reference level is always set to the above. It can be held by a counter. Of course, when the comparator 15 determines a value smaller than the output value of the counter, the counter is caused to hold the minimum value.

Further, the A / D conversion circuit 6 may be constructed as shown in FIG. In FIG. 3, the adder 12 is provided in front of the amplifier 13, and the output signal of the D / A converter 18 and the signal from the preamplifier 5 are added to perform A / D conversion.

The CPU 19 reads the output signal from the latch 16 and sets a value for the D / A converter 18 at which the output signal of the latch 16 becomes small. That is, the CPU 19
Reads the output of latch 16 and when the value is 0, D /
The input of the A converter 18 is reduced by one. Latch 16
When the output becomes large, the input of the D / A converter 18 is increased one by one.

By repeating this, the reference level of the signal of the output of the D / A converter 18 becomes a value close to the lower limit of the conversion range of the A / D converter 14, and therefore the conversion of the A / D converter 14 is performed. The range can be effectively used, the number of effective bits of the A / D converter 14 can be reduced, and an inexpensive element can be used. Of course, it is also possible to change the A / D converter 14 to a unipolar type. In addition,
In this example as well, the latch 16 may be composed of a counter as in the above example.

[0029]

As described above, according to the present invention, even if the reference level of the input signal fluctuates depending on the amount of incident gamma rays, it is corrected, and the reference level of the signal and the ground level differ from each other as in the conventional case. It is possible to obtain a stable image that does not change depending on the amount of incident gamma rays.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a scintillation camera according to the present invention.

FIG. 2 is a block diagram showing a specific example of an A / D conversion circuit that is a main part of the present invention.

FIG. 3 is a block diagram showing another configuration example of the same A / D conversion circuit.

FIG. 4 is a block diagram showing a conventional scintillation camera.

FIG. 5 is a block diagram showing a configuration of an A / D conversion circuit of a conventional scintillation camera.

FIG. 6 is a waveform diagram for explaining an input signal of the A / D conversion circuit.

FIG. 7 is a diagram illustrating the configuration of the ideal diode in FIG.

[Explanation of symbols]

 1 Collimator 2 Scintillator 3 Light guide 4 Photomultiplier tube (PMT) 5 Preamplifier 6 A / D converter circuit 7 Position calculation circuit 8 Display device 11 Capacitor 12 Adder 13 Amplifier 14 A / D converter 15 Comparator 16 Latch 17 Pull Calculator 18 D / A converter 19 CPU 21 Ideal diode 22 Diode 23 Operational amplifier 40 A / D conversion circuit

Claims (1)

[Claims] 1. A scintillator that emits light by incident radiation, a photomultiplier tube that is optically coupled to the scintillator and converts the light into an electrical signal, and a preamplifier that amplifies the signal output from the photomultiplier tube. And an A / D that converts the signal output from this preamplifier into a digital signal
A conversion circuit, a position calculation circuit for calculating the position of the radiation incident on the scintillator by the signal output from the A / D conversion circuit, and a display device for displaying the input position of the radiation calculated by the position calculation circuit. In the scintillation camera, the A / D conversion circuit converts the signal output from the preamplifier into a digital signal.
/ D converter, a latch for holding the minimum value of the output of this A / D converter, the minimum value held in this latch and the above A
A comparator for comparing the output value of the A / D converter and updating the minimum value held in the latch; and a subtractor for subtracting the minimum value from the output value of the A / D converter. The scintillation camera is characterized in that the reference level of the input signal always matches the above-mentioned minimum value.