JPH0546513B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field This invention relates to a radiation positioning device such as a scintillation camera, which is composed of a scintillator and a plurality of photomultiplier tubes (abbreviated as PMT in this specification). This invention relates to a device for stabilizing the output of PMT in a detector.

(b) Conventional technology In scintillation cameras, the output of the PMT is kept constant by installing a light emitting diode in the light guide that emits reference light and changing the high voltage applied to the PMT based on the PMT output caused by that light. It is proposed to do so. but,
In this case, there are disadvantages in that the output of the reference light from the light emitting diode itself is susceptible to changes in temperature, etc., and maintenance is extremely complicated when the light emitting diode or the like deteriorates.

(c) Purpose This invention eliminates the problem of fluctuations in the reference itself without using a light generator that emits reference light, and the PMT is always maintained regardless of temperature changes or secular changes.
The purpose of the present invention is to provide a PMT output stabilization device for a radiation position detector that can keep the output constant.

(D) Configuration According to the present invention, there is a scintillator, a plurality of PMTs arranged on the back surface of the scintillator to which scintillation light from the scintillator is guided, and a position of a light emitting point in the scintillator based on the outputs of the plurality of PMTs. In the radiation position detector, the radiation position detector has a position calculation circuit that integrates the output of each PMT, a circuit that selects the integral output of the PMT closest to the light emitting point based on the output of the position calculation circuit, and the position calculation circuit. a reference output generation circuit that generates a reference output that the PMT output should take when scintillation light is generated at the light emitting position by the output of the above; a comparison circuit that compares the integrated output with the reference output; and a comparison result. A PMT output stabilizing device is configured by having a control circuit that controls a high voltage applied to the PMT based on the above.

(E) Embodiment FIG. 1 shows an embodiment in which the present invention is applied to a scintillation camera. In this figure, γ-rays enter from the front side of scintillator 1 via a collimator (not shown), and when γ-rays enter scintillator 1, scintillation occurs, and the light is arrayed on the back side of scintillator 1. Guided by many PMT2. The scintillation light is proportional to the energy of the incident gamma rays.
In each of the PMTs 2, light is converted into electrons and then amplified, an output corresponding to the amount of incident light is generated, and this output is converted into a voltage signal by the preamplifier 3.
This signal is sent to the position calculation circuit 4, and based on the principle that the output of the PMT2 is larger as it is closer to the light emitting point, a position signal Xa in the X direction and a position signal Ya in the Y direction are obtained. By adding all the outputs, the energy signal Za is obtained,
Further, by determining that this energy signal falls within a desired energy range, an unblank signal, which is a timing signal, is obtained. The above configuration is the same as that of a normal scintillation camera.

According to this invention, each output of the preamplifier 3 is sent to an integrating circuit (pulse shaper) 6, integrated, and held until an unblank signal is generated. This integrated output is selected by multiplexer 7, and the address of this multiplexer 7 is given by address generation circuit 15. That is, after the analog position signals Xa, Ya and energy signal Za outputted from the position calculation circuit 4 are converted into digital position signals X, Y and digital energy signal Z via the A/D converter 5, the address generation circuit 15 address generation circuit 15.
Then, from these position signals X and Y, find the one closest to the light emission point.
Generates the address of PMT2 and multiplexer 7
give to As a result, as shown in Figure 2, the light emitting point (X, Y) is located at one of the many circular PMTs 2.
When entering the hexagonal region 2na circumscribing the PMT 2n, the multiplexer 7 selects the integral output of the PMT 2n. The integrated output of one PMT 2 selected in this way is sent to the comparator 9.

On the other hand, a reference voltage is sent to the comparator 9 from the D/A converter 8, and this reference voltage is compared with the above-mentioned integrated output. This reference voltage is created as follows. The address generation circuit 15 is closest to the light emitting point based on the position signals X and Y.
Find the distance r from the center of PMT2 to the light emitting point (X, Y), and convert the data corresponding to this distance r to D/A.
The D/A converter 8 generates a reference voltage that must be applied to the converter 8 and generated from the PMT 2 when light emission occurs at a position at a radius r from the center of the PMT 2. That is, as shown by the solid line in FIG. 4, the average value of the reference voltage at each radius r for each PMT 2 is measured in advance, and the reference voltage is calculated from the D/A converter 8 based on the solid line in FIG. 4 from the distance r. The address generation circuit 15 calculates such data. Alternatively, you can actually measure the reference voltage for the entire field of view of the scintillation camera (that is, for all possible positions The signal may also be supplied from the generation circuit 15 to the D/A converter 8.

In this way, each time a gamma ray is incident, we actually obtain
A comparison is made between the PMT output and a reference voltage, and as a result, a memory (RAM) 1
The data stored in 1 is increased or decreased. That is, data that determines the high voltage of each PMT 2 is stored in the memory 11 in advance, and this data is determined according to the output of the comparator 9 at one of the γ-ray incident events.
can be increased or decreased individually. By the way, energy discrimination is performed in the position calculation circuit 4, and the third
This operation is performed only when the energy falls within the range indicated by the dotted line in the energy spectrum shown in the figure. Therefore, if the above operation is performed for a certain period of time, a large number of γ rays will be incident, and the frequency distribution of the PMT output actually obtained for each position X and Y will be as shown in Figure 3. , the memory 11 according to the area difference represented by the diagonal lines on both sides of the reference value (represented by the solid line in FIG. 3).
The data stored in the data will be corrected. The solid line and dotted line in Figure 4 are plots of the energy represented by the solid line and dotted line in Figure 3 in the radial direction, so each line
Regarding PMT2, data is corrected according to the area difference between the solid line and each dotted line in FIG. 4.

The corrected data in the memory 11 is sequentially read out and sent to the register 12, and further converted into an analog quantity via the D/A converter 13 and sent to the high voltage control circuit 14. The respective timings in this operation are as shown in FIG.
Reading of data from the memory 11, calculation (increase/decrease of data) in the control circuit 10 based on the output of the comparator 9, and writing of the calculation result to the memory 11 are performed. High voltage control circuit 14
is for controlling the high voltage to be applied to each of the PMTs 2 according to the input voltage, and is comprised of, for example, a chopper type high voltage generator.

The above-mentioned correction operation of the high voltage applied to each PMT2 is preferably performed for a certain period of time by the operator only when γ-ray irradiation is performed using a ⁹⁹ mTc flat source, for example.
Since the correction operation is performed statistically based on the energy spectrum actually obtained at each position, it is also possible to perform the correction operation with the collimator attached during the actual examination of a subject.

Note that although the energy spectrum is obtained for all positions in the above example, it is also possible to increase the accuracy by limiting the corresponding area of each PMT 2 shown in FIG. 2 to a narrow range. Furthermore, narrowing the energy discrimination range only during the correction operation also improves accuracy. Furthermore, in the above example, the D/A converter 8
The data to be given to the D/A converter 8 is generated from the X and Y signals by the address generation circuit 15, but also takes into account not only the X and Y signals but also the energy signal Z.
If the reference voltage is generated by generating data to be applied to the scintillation position, it is effective because if the energy signal Z fluctuates depending on the scintillation position, it will not be affected by the fluctuation.

Furthermore, the present invention can be applied not only to scintillation cameras but also to other radiation position detectors configured with a scintillator and a plurality of PMTs.

(f) Effects According to this invention, the PMT output can always be kept constant regardless of temperature changes or secular changes.
In the radiation position detector, it is possible to always maintain position detection accuracy above a certain level. In particular, with a scintillation camera, the quality of the images obtained can be kept constant. In addition, with scintillation cameras that have energy correction and linearity correction, if the PMT output changes, these corrections may not function properly and artifacts such as moiré may occur. It is effective when applied to a digital camera. moreover,
Because it uses a statistical method, there is no need to use a special reference radiation source, and the same radioisotope as used in clinical settings can be used, simplifying the equipment and making maintenance easier. Furthermore, even if the amount of light emitted by the scintillator fluctuates due to temperature changes, the PMT output can be kept constant.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a diagram showing the PMT arrangement in the field of view, Fig. 3 is a graph showing the energy spectrum of incident radiation at a certain position, and Fig. 4 is a graph showing the energy spectrum of the incident radiation from the center of the PMT. A graph showing the relationship between PMT output and distance (radius), and FIG. 5 is a time chart showing operation timing. 1...Scintillator, 2...PMT, 3...Preamplifier, 4...Position calculation circuit, 5...A/D converter, 6...
Integration circuit, 7...Multiplexer, 8, 13...D/
A converter, 9... comparator, 10... control circuit,
11...Memory, 12...Register, 14...High voltage control circuit.

Claims (1)

[Claims] 1. A scintillator, a plurality of PMTs arranged on the back side of this scintillator to which the scintillation light of the scintillator is guided, and these plural PMTs.
A radiation position detector that has a position calculation circuit that calculates the position of the light emitting point in the scintillator from the output of the PMT, a circuit that integrates the output of each PMT,
closest to the light emitting point according to the output of the position calculation circuit above.
a circuit that selects the integral output of the PMT; a reference output generation circuit that generates a reference output that the PMT output should take when scintillation light is generated at the light emitting position by the output of the position calculation circuit; A PMT output stabilizing device that includes a comparison circuit that compares the output with a reference output, and a control circuit that controls a high voltage applied to the PMT based on the comparison result.