JPH0572554B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a scintillation camera that uses radiation to perform medical diagnosis of a subject. This invention relates to a scintillation camera that can automatically vary the integration time.

Conventional technology As shown in FIG. 7, a conventional scintillation camera includes a scintillator 1 and a photomultiplier tube 2.
, an X, Y conversion circuit 3, an integration circuit 4, a position calculation circuit 5, and a display device 6.
The operation is as follows: First, radiation such as gamma rays emitted from the subject 7 to which a radioactive isotope has been administered is
The light passes through the collimator 8 and enters the scintillator 1. Then, the scintillator 1 emits a flash of light in an amount proportional to the energy of the incident radiation, and the light is converted into an electrical signal by the photomultiplier tube 2 optically coupled to the scintillator 1. This electric signal is input to the X, Y conversion circuit 3 and converted into a matrix signal of coordinates in the X and Y directions, and the electric signals of each photomultiplier tube 2 are added. The matrix signal S ₁ output from the X, Y conversion circuit 3 is input to the integrating circuit 4 , and the addition signal S ₂ is input to the signal detection circuit 9 .
The addition signal S ₂ input to the signal detection circuit 9 is compared with an internal reference voltage, and only when it exceeds the reference voltage, the signal detection circuit 9 outputs the radiation incident signal S ₃ . This radiation incident signal S ₃
is input to the integral control circuit 10, and the integral control circuit 10 sends an integral signal _S4 for a certain period of time to the above-mentioned integral circuit 4. Upon input of the integral signal S ₄ , the integrating circuit 4 integrates the matrix signal S ₁ input from the X, Y conversion circuit 3 for the time period of the integral signal S ₄ . The output of the integration circuit 4 after this integration is the position calculation circuit 5
, and this position calculation circuit 5 calculates
For each direction, the light emitting point on the scintillator 1 is determined from the signal intensity of each row and each column, and its position is displayed on the display device 6. By repeating the above operations, a distribution image of the radioisotope administered to the subject 7 was displayed on the screen of the display device 6.

Here, the reason why the scintillation camera has the integrating circuit 4 as described above is because the flash of light emitted by the scintillator 1 includes statistical fluctuations due to changes in the energy intensity of the incident radiation, and this statistical fluctuation needs to be removed. It is. In other words, the matrix signal of the X, Y conversion circuit 3 containing this statistical fluctuation
The output waveform of _S1 becomes jagged as shown in FIG. 9a. When this is integrated by the integrating circuit 4 for a specific period of time, the influence of statistical fluctuations becomes smaller as shown in FIG. 9b. And the longer the time to perform the integration, the smaller the influence of statistical fluctuations will be.
The removal of this statistical variation will be explained below.

FIG. 8 is a circuit diagram showing details of the integration circuit 4, signal detection circuit 9, and integration control circuit 10 shown in FIG. 7. The addition signal _S2 output from the X, Y conversion circuit 3 is input to the signal detection circuit 9, and is converted into a bipolar signal by a differential integrator consisting of an operational amplifier 11, a resistor _R1 , and a capacitor _C1 , and this bipolar signal is sent to a comparator. 12 and is compared with a reference voltage E, and only when it exceeds the reference voltage E, the signal detection circuit 9 outputs a radiation incident signal _S3 . The integral control circuit 10 is composed of a monostable multivibrator 13, and receives the radiation incident signal _S3 from the signal detection circuit 9 and outputs an integral signal _S4 for a certain period of time. This integral signal S ₄
The time is determined by the time constant provided by the capacitor C ₂ and the resistor R ₂ of the monostable multivibrator 13. The integrating circuit 4 includes an operational amplifier 14,
Field effect transistor 15 and integrating capacitor
_C3 , inputs the matrix signal _S1 from the X, Y conversion circuit 3, and integrates the integral signal _S4 output from the integral control circuit 10 for a certain period of time. That is, while the integration signal _S4 is output, the field effect transistor 15 is turned off, charge is stored in the integration capacitor _C3 , and integration is performed, and when the integration signal _S4 stops, the field effect transistor 15 is turned off.
Turns ON and stops integration. The output waveforms of each part in such an operation are as shown in FIG. Here, if the time length of the integral signal _S4 shown in FIG. 10d is appropriate, the output waveform of the integrating circuit 4 will be a clean waveform with statistical fluctuations removed, as shown in FIG. 10f. .

However, as the integration time in the integrating circuit 4 becomes longer, as _shown in FIG _. ′ was input, and this signal S ₁ ′ was also integrated. Then,
As shown in FIG. 11b, the output of the integration circuit 4 is such that the integration result I ₁ of the signal S ₁ ' is superimposed on the integration result I ₁ of the signal S ₁ . When such a phenomenon occurs, the position calculation circuit 5 cannot calculate the correct position of the light emitting point of the incident radiation.
If a different position was specified, an artifact would occur. Also, the superposition of the integration results shown in Figure 11b occurs stochastically, and when the counting rate per unit time of incident radiation is low, superposition does not occur much because the average time interval between the incidences of radiation is long. . However, when the counting rate per unit time of the incident radiation is high, the average time interval between the incidences of the radiation becomes short, and when it becomes less than the integration time of the integrating circuit 4, superimposition is likely to occur. Therefore,
Suspicious images were also likely to occur.

Problems to be Solved by the Invention However, in the conventional scintillation camera, the integration time for the matrix signal _S1 in the integration circuit 4 is fixed to a constant time by the CR time constant of the monostable multivibrator 13 of the integration control circuit 10. is,
Furthermore, since the integration time was set to be a little long in order to increase the accuracy of position calculation, false images were likely to occur especially when the count rate of incident radiation was high. To deal with this, if the operator judges that the counting rate of incident radiation is high, the mode is switched so that the integration time of the integrating circuit 4 is shortened, and if the counting rate is judged to be low, the integration time is changed to a longer one. Some models are designed to switch modes, but in this case, due to operator error, the integration time may be too short, resulting in a decrease in position calculation accuracy, or the integration time may be too long, resulting in an increase in false images. Something happened. In addition, since the counting rate had to be judged and operated each time, it was troublesome to use. Therefore, an object of the present invention is to solve such problems.

Means for Solving the Problems The means of the present invention for solving the above problems consists of a scintillator that emits a flash of light due to incident radiation, and a photomultiplier that is optically coupled to the scintillator and converts the light into an electrical signal. , an integrating circuit that integrates the electric signal from the photomultiplier tube, a position calculation circuit that calculates the incident position of radiation based on the output from this integrating circuit, and a display device that displays the incident position of the incident radiation. In the scintillation camera, the number of displayed radiation per unit time is counted by the signal from the position calculation circuit, or the number of incident radiation per unit time is counted by the electric signal from the photomultiplier tube. A counting circuit is provided, and an integral time control circuit is provided which inputs a counting rate signal from the counting circuit and outputs an integral signal having a time inversely proportional to the counting rate signal, and the integral time of the integrating circuit is varied by this integral signal. This is accomplished using a scintillation camera that is characterized by the following.

Function: A scintillation camera configured in this manner counts the number of displayed radiations per unit time or counts the number of incident radiations per unit time, and generates an integral signal having an integration time inversely proportional to the counting rate. The integration time of the integration circuit that integrates the electrical signal from the photomultiplier tube can be automatically varied.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a scintillation camera according to the present invention. This scintillation camera includes a scintillator 21 and a photomultiplier tube 22.
, an integration circuit 23 , a position calculation circuit 24 , a display device 25 , a counting circuit 26 , and an integration time control circuit 27 .

The scintillator 21 emits a flash of light at the point of incidence when radiation such as gamma rays emitted from the subject 7 to which a radioactive isotope has been administered is incident. Only radiation emitted perpendicularly to the bottom surface of the tube is allowed to enter. A plurality of photomultiplier tubes 22 are arranged on the back surface of the scintillator 21 over the entire surface thereof. This photomultiplier tube 22 detects the flash of light from the scintillator 21, converts it into an electric signal of a magnitude proportional to the amount of incident light, amplifies it, and outputs it.
It is provided optically coupled to the scintillator 21.

The electric signal from the photomultiplier tube 22 is
Input to Y conversion circuit 29. This X, Y conversion circuit 29 inputs the electric signal and converts it into a matrix signal S ₁ of coordinates in the X and Y directions, and also adds the electric signals of each photomultiplier tube 22 to generate a sum signal S ₂ . Output. The addition signal S ₂ output from the X, Y conversion circuit 29 is input to the signal detection circuit 30 . This signal detection circuit 30 has a circuit configuration similar to that of the conventional signal detection circuit ₉ shown in FIG. Only when this is exceeded, the signal detection circuit 30 outputs the radiation incident signal _S3 .

Radiation incident signal S ₃ from the signal detection circuit 30
is input to the integral time control circuit 27. This integral time control circuit 27 inputs a count rate signal S5 from a counting circuit 26, which will be described later, and outputs an integral signal _S4 having a time inversely proportional to _the count rate signal S5.As shown in FIG. , an operational amplifier 32, an integrating capacitor C ₄ and a field effect transistor 3
an integrator 34 consisting of 3, a comparator 35,
It consists of an RS flip-flop 36.
The amplifier 31 is used to appropriately amplify the count rate signal _S5 inputted from the counting circuit 26. Integrator 3
4, when the RS flip-flop 36 is activated or deactivated, its field effect transistor 33 is activated or deactivated.
It is turned OFF or ON to start or stop integrating the output signal from the amplifier 31. Comparator 3
5 inputs the output from the integrator 34 and compares it with the reference voltage E', and inverts the output only when the output of the integrator 34 exceeds the reference voltage E'. The RS flip-flop 36 is operated by inputting the radiation incident signal _S3 from the signal detection circuit 30 to its set terminal S, and is stopped by inputting the inverted output from the comparator 35 to its reset terminal R. It is. The output signal from this RS flip-flop 36 becomes the integral signal _S4 .

Integral signal S ₄ from the integration time control circuit 27
is input to the integrating circuit 23. This integrating circuit 23
inputs the matrix signal _S1 outputted from the X, Y conversion circuit 29 and integrates it for the time of the integral signal _S4 . The output of the integrating circuit 23 after this integration is:
Input to position calculation circuit 24. This position calculation circuit 24 performs the following in each of the X direction and the Y direction.
From the signal strength of each row and each column, the scintillator 21
The upper light emitting point is determined and its position is displayed on the display device 25.

A counting circuit 26 is connected to the position calculation circuit 24. This counting circuit 26 counts the number of displayed radiations per unit time based on the signal _S6 from the position calculation circuit 24. The number of radiations displayed on the display device 25 per second is output to the rate meter 37, and a counting rate signal _S5 having the same value as the number of radiations displayed is sent to the integration time control circuit 27. It is now being sent out.

Next, the operation of the scintillation camera configured as described above will be explained. First, similar to a conventional scintillation camera, radiation emitted from the subject 7 to which a radioactive isotope has been administered enters the scintillator 21, and the flash of light emitted at the incident site is detected by the photomultiplier tube 22 and electrically generated. converted into a signal. This electric signal is transmitted to the X, Y conversion circuit 29
The signal is input into the integrator circuit 23 and converted into a matrix signal S ₁ of coordinates in the X and Y directions. This integrating circuit 2
3 is controlled by the integral signal S ₄ output from the integral time control circuit 27. Here, the operations of the integration time control circuit 27 and the integration circuit 23 will be explained with reference to FIG. 2. In addition, in FIG. 2, details of the signal detection circuit 30 and the integration circuit 23 are shown in the eighth section.
Conventional technology detection circuit 9 and integration circuit 4 shown in the figure
It is exactly the same. First, the signal detection circuit 30
The addition signal S ₂ from the X, Y conversion circuit 29 is input and compared with the internal reference voltage, and a radiation incident signal S ₃ is generated.
Output. This radiation incident signal S ₃ is transmitted to the gate 3
8 to the set terminal S of the RS flip-flop 36 of the integral time control circuit 27 to operate the RS flip-flop 36. On the other hand, the count rate signal S ₅ output from the counting circuit 26 is input to the amplifier 31 of the integration time control circuit 27 and amplified. Due to the operation of the RS flip-flop 36,
Field effect transistor 33 of integrator 34 is OFF
Then, the integration of the output signal from the amplifier 31 is started. The output of this integrator 34 is input to a comparator 35 and compared with a reference voltage E', and when it exceeds the reference voltage E', the output of the comparator 35 is inverted. This inverted output is input to the reset terminal R of the RS flip-flop 36 and resets the RS flip-flop 36.
6 is stopped, thereby turning on the field effect transistor 33 of the integrator 34, and the operation of the integrator 3 is turned on.
4 stops the integration. That is, the RS flip-flop 36 operates by inputting the radiation incident signal _S3 from the signal detection circuit 30 to its set terminal S, and then inputs the inverted output from the comparator 35 to its reset terminal R to operate. Until it stops, an output signal is sent out, and this output signal is input to the integration circuit 23 as an integration signal _S4 to control the integration time. In this case, the count rate signal _S5 input to the amplifier 31 changes according to the count rate of the number of radiations displayed on the display device 25, so the output of the amplifier 31 increases as the count rate increases, and The lower the rate, the smaller it will be. According to the magnitude of the output of the amplifier 31, the output of the integrator 34 also increases or decreases, and the time at which the output of the integrator 34 exceeds the reference voltage E' changes. In other words, when the output of the integrator 34 is large, it quickly exceeds the reference voltage E' and the comparator 35
When the output of the integrator 34 is small, it is slow to exceed the reference voltage E', and it takes time until the output of the comparator 35 is inverted. As a result, the operating time of the RS flip-flop 36 becomes shorter as the counting rate of the number of displayed radiations is higher, and becomes longer as the counting rate is lower. Since the operating time of the RS flip-flop 36 is the transmission time of the integral signal _S4 , the integral time control circuit 27 automatically creates an integral signal _S4 having an integral time inversely proportional to the counting rate of the number of displayed radiations. I will do it.

Timing diagrams for the above operations are shown in FIGS. 3 and 4. Here, FIG. 3 shows a case where the radiation counting rate is low, and FIG. 4 shows a case where the radiation counting rate is high. When the counting rate of radiation is high, as shown in FIG. 4b, the integrator 34
The slope of the output becomes steep, and it exceeds the reference voltage E' in a shorter time than in the case of FIG. 3b.
Then, the output of the comparator 35 shown in FIG. 2 is inverted, the RS flip-flop 36 stops operating, and the sending time of the integral signal _S4 becomes shorter as shown in FIG. Becomes shorter. Therefore, as shown in Figure 4d, when integrating the matrix signal S1 due to a certain incident radiation, the matrix signal _S1 due to the next incident radiation is integrated.
Even if S ₁ ' is input, the integral signal S ₄ at that time is the fourth
Since it is stopped as shown in FIG. 4c, the output of the integrating circuit 23 is the integration result _I1 of only the signal _S1 , as shown in FIG. 4e, and no superimposition occurs. Incidentally, I ₁ ' shown by a broken line in FIG. 4e indicates the integration result that would be superimposed if the integration time were constant as in the conventional case (see the broken line in FIG. 4c).

FIG. 5 is a block diagram showing a second embodiment. In this embodiment, a counting circuit 26' for sending a counting rate signal S ₅ to the integral time control circuit 27 is provided between the signal detection circuit 30 and the integral time control circuit 27. In this case, by counting the radiation incident signal _S3 output from the signal detection circuit 30, the number of incident radiations per unit time to the photomultiplier tube 22 is counted to create a count rate signal _S5 , It can be sent to the integral time control circuit 27. Note that at this time, the other counting circuit 26 connected to the position calculation circuit 24 only outputs the number of displayed radiations to the rate meter 37 and does not send a signal to the integration time control circuit 27.

FIG. 6 is a block diagram showing a third embodiment. This embodiment is based on the integration circuit 2 of the embodiment shown in FIG.
After analog-to-digital conversion of the output of 3, a radiation distribution image is created digitally, and on the output side of the integrating circuit 23, an A/D converter 38, a normalization circuit 39, and a digital position calculation circuit 40 are installed. and is provided. Here, the output of the integration circuit 23 is input to the A/D converter 38 and converted into a digital signal, but the A/D converter 38 takes in the integration signal S4 from the integration time control circuit 27, and receives the integration signal _S4 from the integration time control circuit 27. The integration circuit 23 performs digital conversion at the same time as the integration ends. The digital signal from this A/D converter 38 is input to the standardization circuit 39 and is standardized by the signal from the all-photomultiplier tube 22, and the position calculation accuracy varies depending on the magnitude of each digital signal. can be suppressed. The signal output from this standardization circuit 39 is input to a digital position calculation circuit 40,
The light emitting point on the scintillator 21 is calculated and its position is digitally displayed on the display device 25. In the case of this technique, even when direct analog-to-digital conversion is performed after integration in the integrating circuit 23, the integration time can be changed in inverse proportion to the radiation counting rate, as in FIG. 1.

Effects of the Invention As described above, the present invention includes a counting circuit 26 that counts the number of displayed radiations per unit time or counts input radiations per unit time.
26' and the counting circuits 26, 2
Since an integral time control circuit 27 is provided which outputs an integral signal _S4 having a time inversely proportional to the count rate signal _S5 from 6', the integral signal _S4 controls the integral circuit 2.
The integration time in step 3 can be automatically varied depending on the level of the radiation counting rate. Therefore, even when the counting rate of radiation is particularly high, the integration time is shortened, so that the integrated outputs do not overlap, and it is possible to prevent the generation of false images and obtain a good distribution image. Furthermore, since the integration time is lengthened when the counting rate of radiation is low, the accuracy of position calculation does not decrease. Furthermore, since the time required for integration by the integrating circuit 23 as a whole is shorter than before, more signals can be processed, and the maximum number of radiations that can be used to calculate the position in a given time can be increased, thereby improving the counting rate. I can do it. Furthermore, since the above-described integration time is automatically performed within the variable integration device, there is no deterioration in the image quality of the distribution image due to operator error.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a scintillation camera according to the present invention, FIG. 2 is a circuit diagram showing details of its integration time control circuit, and FIG. 3 is a timing diagram showing the operation when the counting rate of radiation is low. Fig. 4 is a timing diagram showing the operation when the counting rate of radiation is high, Fig. 5 is a block diagram showing the second embodiment, Fig. 6 is a block diagram showing the third embodiment, and Fig. 7. 8 is a block diagram showing a conventional scintillation camera, FIG. 8 is a circuit diagram showing details of its integration circuit, signal detection circuit, and integration control circuit, and FIG. 9 is a diagram showing a matrix signal containing statistical fluctuations and the integration circuit. FIG. 10 is a graph showing the output waveform. FIG. 10 is a timing diagram showing the operation of a conventional scintillation camera. FIG. 11 is a graph showing the superposition of output signals when the integration time of the integrating circuit is long. 21...Scintillator, 22...Photomultiplier tube, 2
3... Integration circuit, 24... Position calculation circuit, 25... Display device, 26, 26'... Counting circuit, 27... Integration time control circuit, 29... X, Y conversion circuit, 30... Signal detection circuit, S ₁ ... Matrix signal , S ₄ ... integral signal, S ₅ ... counting rate signal, S ₆ ... signal from the position calculation circuit.

Claims (1)

[Claims] 1. A scintillator that emits a flash of light due to incident radiation, a photomultiplier tube that is optically coupled to the scintillator and converts light into an electrical signal, an integrating circuit that integrates the electrical signal from the photomultiplier tube, and an integrating circuit that integrates the electrical signal from the photomultiplier tube. In a scintillation camera that has a position calculation circuit that calculates the incident position of radiation based on the output from the circuit and a display device that displays the incident position of the incident radiation, A counting circuit is provided to count the number of displayed radiation per unit time or to count the number of incident radiation per unit time by an electric signal from a photomultiplier tube, and a counting rate signal from this counting circuit is inputted and is inversely proportional to it. 1. A scintillation camera, comprising: an integral time control circuit that outputs an integral signal having time; and the integral time of the integral circuit is varied by the integral signal.