JPH0528353B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field This invention is applicable to scintillation cameras, etc.
A scintillator and multiple photomultiplier tubes (hereinafter referred to as
This invention relates to a device for stabilizing the amplification degree of the PMT in a radiation position detector configured with the PMT.

Conventional technology Radiation position detectors such as scintillation cameras use multiple PMTs, and when the amplification degree of each PMT changes over time, various performances such as uniformity, spatial resolution, and energy resolution deteriorate. . Therefore, as a countermeasure to this problem, the following proposals have been made in the past.

The first proposal is to install a reference light source such as a light emitting diode that emits reference light in the light guide, use that light to generate an output from the PMT, and adjust the PMT based on this output. This stabilizes the amplification degree of the PMT by
9082).

In the second proposal, a radiation source is sequentially placed on the central axis of each PMT to obtain data for correcting amplification fluctuations (Japanese Patent Laid-Open No. 57-59184).

The third proposal is to find the change in amplification degree of each PMT from the energy signal of a scintillation camera (Japanese Patent Application Laid-Open No. 143981/1983).

The fourth proposal has already been filed by the present applicant (Japanese Patent Application No. 59-172590), in which a standard output is determined in advance for each PMT output, and when light emission occurs at a certain position, the position PMT closest to
The purpose is to compare the standard output that is supposed to be output with the actual output to obtain data regarding the amplification variation of the PMT.

Problems to be Solved by the Invention In the first conventional proposal, the reference light itself from a reference light source such as a light emitting diode is easily affected by temperature changes, etc., and there are also problems such as aging.

The second proposal has the disadvantage that data collection is complicated when performed manually, and a jig is required when performed automatically, and furthermore, data collection requires time.

The third proposal has the disadvantage that it is necessary to repeat data collection and correction, and convergence may not occur in some cases.

The fourth proposal is excellent as it eliminates the shortcomings of the first to third proposals, but there are still some points that need to be improved, such as a large amount of circuitry and problems with counting rate characteristics. Existing.

This invention solves these conventional problems and allows each light source to be used without using a reference light source such as a light emitting diode.
In addition to automatically correcting changes in PMT amplification over time to stabilize various characteristics, data collection regarding the amplification of each PMT is performed in parallel with a relatively small amount of circuitry, improving count rate characteristics and shortening the time. The purpose of the present invention is to provide a PMT amplification stabilization device that allows correction over time.

Means for Solving the Problems The PMT amplification stabilizing device according to the present invention has the following features:
In a radiation position detector such as a scintillation camera, an adding means for classifying a group of PMTs into a plurality of groups according to a rule that mutually adjacent PMTs belong to different groups, and adding signals related to PMT outputs for each group; means for selecting the largest sum signal from among the sum signals for each group; means for analyzing the wave height of the selected sum signal; means for determining the PMT closest to the light emitting point based on the position signal; means for collecting statistical data of the pulse height analysis of the signal on a PMT determined to be closest to the light emitting point; and means for collecting statistical data on the PMT determined to be closest to the light emitting point, and on the basis of the statistical data obtained for each PMT or the corresponding amplifier system; and means for controlling the degree of amplification.

Effect When scintillation light emission occurs near the central axis of a certain PMT, that light will be incident on that PMT and the surrounding PMTs, but the amount of incident light will depend on that PMT.
It is smaller for the largest neighboring PMT, and even smaller for surrounding PMTs that are further adjacent to this neighboring PMT. Therefore, we classify the PMT group into multiple groups according to the rule that PMTs that are adjacent to each other belong to different groups, obtain the summed signals for each group, and then calculate the largest summed signal among these groups. By selecting the sum signal of the group to which the PMT closest to the light emitting point belongs, we can obtain the sum signal of the group to which the PMT closest to the light emitting point belongs.
The added signal of the group to which a PMT belongs can be regarded as approximately equal to the output of only that PMT. Therefore, the maximum summed signal selected in this way can be treated as the output of the PMT closest to the light emitting point, so statistical data on output fluctuations for each PMT can be obtained from the summed signal of each group. . Processing such as time adjustment, integration, waveform shaping, sample/hold, etc. need only be performed on one system of the selected maximum summed signals. Each obtained as above
The amplification degree of the PMT can be stabilized by controlling the amplification degree of the PMT or the corresponding amplifier system based on statistical data of output fluctuations for the PMT.

Embodiment FIG. 1 shows an embodiment in which the present invention is applied to a scintillation camera. In FIG. 1, the scintillator 1, light guide 2, PMT 3, and preamplifier 4 are the same as in a normal scintillation camera, and the position calculation circuit 5 is also almost the same as in a normal scintillation camera, except for some parts. Performs position calculations, wave height analysis of energy signals, timing control, etc. That is, the γ-rays are made to enter the scintillator 1 from the surface side of the scintillator 1 via a collimator (not shown), and the scintillator 1 absorbs the incident γ-rays to generate scintillation light emission, and this light is transmitted to the scintillator 1. The light is guided through a light guide 2 to a large number of PMTs 3 arranged on the back side of the . The intensity of scintillation light is proportional to the energy of incident gamma rays.
In each of the PMTs 3, light is converted into electrons and then amplified to produce an output corresponding to the amount of incident light, and this output is converted into a voltage signal by the preamplifier 4. This signal is sent to the position calculation circuit 5, and the output of the PMT 3 is based on the principle that the output of the PMT 3 is larger as the closer to the light emitting point, the greater the amount of light. ) is required. Further, a signal Z representing energy is obtained by adding all the outputs of a large number of PMTs 3, and it is determined that this energy signal is within a desired energy range to obtain an unblank signal which is a timing signal.

A large number of PMT3 are classified into four groups, A, B, C, and D, as shown in Figure 2, for example, according to the rule that adjacent PMT3 belong to different groups, and for each of these groups, The outputs of the preamplifier 4 are added by an adder 6, and summed signals SUM A to SUM D for each group are obtained. These signals SUM A to SUM D are sent to the adder 7
are added to form a SUM Z signal, which is input to the position calculation circuit 5 and converted into an energy signal Z. Also, each of SUM A to SUM D is the maximum detection circuit 21.
to determine which signal is the largest,
The signal representing the largest group is applied to analog multiplexer 9. This analog multiplexer 9 has SUM A~
SUM D is input, and the maximum signal among them is selected by the signal from the maximum detection circuit 21. The output of this analog multiplexer 9
MPX OUT is input to the integration and delay circuit 22, and is subjected to processing such as integration (or waveform shaping, etc.), delay, sample and hold, etc. while being appropriately controlled by the timing signal etc. from the position calculation circuit 5. , and are sent to the pulse height analyzer 14, where the pulse heights are compared with the signals L1 to L3 from the level supply circuit 13. Therefore, using this pulse height analyzer 14, MPX
The OUT wave height is L from this level L1 to L2 range.
Signal U/L and timing signal +1 that indicate which range is in the range of 2 to L3
is output.

On the other hand, the analog position signals X and Y output from the position calculation circuit 5 are sample-hold and A/
It is sent to the D converter 11, where the digital position signal is
Converted to Dig.X, Dig・Y. These digital position signals Dig.X and Dig.Y are sent to the conversion memory 12. The conversion memory 12 is composed of a P-ROM, etc., and determines the position and number of the nearest PMT 3 when the scintillation light emitting point is within a predetermined tuning spot (set on the central axis of each PMT 3). do. In other words, signals representing the position and number of the nearest PMT 3 addressed by Dig.X and Dig.Y are read out.

The above level supply circuit 13 is connected to the conversion memory 12
The level signals L1 to L3 take different values depending on the position of the PMT3 (such as whether it is in the center or the periphery of the field of view). is being used.
That is, MPX at a certain tuning spot
If the distribution of the OUT signal wave height should be as shown in Figure 3 if the amplification degree of the PMT3 amplification system closest to the tuning spot is normal, then the levels L1 to L3 should be as shown in Figure 3. The settings are as shown in the figure, and the level L2 is set to correspond to the peak. Note that the level signals L1 to L3 may be modified and set according to the nuclide.

The counter memory 15 is composed of a RAM, etc., and the contents of the memory addressed by the number of PMT3 outputted from the conversion memory 12 and each U/L signal outputted from the pulse height analyzer 14 are transmitted to the pulse height analyzer 15. Each time a +1 signal from 14 is input, +1 is added. In addition, the information determined by the conversion memory 12
In order to prevent the counter memory 15 from counting when the number of PMT3 and the group that produces the maximum addition signal detected by the maximum detection circuit 21 are inconsistent, the signal indicating the number of PMT3 and the group of the maximum addition signal are indicated. A coincidence detection circuit 23 is provided to detect coincidence with the signal, and thereby the counter memory 1
5 counting operation is controlled. PMT
If the amplification degree of the amplification system 3 deviates from the normal value, the spectrum in Figure 3 will shift left and right, so L
From the difference or ratio between the count value in the range 1 to L2 and the count value in the range L2 to L3, the change in amplification degree can be known.

Digital data related to the high voltage to be applied to each PMT 3 is recorded in advance in the register 17, and this digital data is converted into an analog signal by the D/A converter 18 and then input to the high voltage control circuit 19, and is then applied to each PMT 3. The supply voltage is controlled. The contents of this register 17 are modified based on the data collected in the counter memory 15 by a data processing circuit 16 constituted by, for example, a microcomputer.

To actually correct the amplification degree of PMT3, for example, a flat source is used to uniformly irradiate the entire field of view with gamma rays. Then, in the count memory 15, counting is performed for each PMT 3. When this count reaches a predetermined value or when a certain period of time has elapsed, γ-ray irradiation is terminated.
In this way, the change in the amplification degree in the amplification system for each PMT 3 can be statistically measured from the count value obtained by the counter memory 15, and the data processing circuit 16
The contents of the register 17 can be modified so that the amplification degree of the PMT 3 becomes the optimum amplification degree.

Here, MPX OUT is the sum signal of each group, but it can be regarded as the output of PMT3 closest to the light emitting point. This is because in the optical system of scintillator 1, light guide 2, and PMT 3 in a normal scintillation camera, one
If light emission occurs on the central axis of PMT3, that PMT
3 and surrounding PMT 3, each of these PMTs
Output is generated from PMT 3, but if the output of PMT 3 in the center is 100%, the output of adjacent PMT 3 is 18 to 20.
%, and furthermore, the output of PMT3 adjacent to this adjacent PMT3 is about 3 to 3.5%, and the output of PMT3 adjacent to this adjacent PMT3 is about 3 to 3.5%.
PMT3s are grouped according to the rule that they belong to different groups, and the sum signal of that group is obtained. Therefore, the output of the nearest PMT3 makes a large contribution to the summation signal of this group, and Other PMT3 belonging to
This is because the contribution rate of the output is small. Furthermore, by comparing the magnitudes of the summed signals for each group, the summed signal of the group containing the PMT3 closest to the light emitting point is selected.
For each group, it is sufficient to simply provide a delay circuit 8 for timing adjustment with the maximum detection circuit 21, as well as time adjustment and integration (or waveform shaping, etc.) until the output is generated from the conversion memory 12, sample and hold, etc. Since only one integration and delay circuit 22 is required to perform the processing, the count rate characteristics can be improved, consideration for the count rate characteristics can be reduced, and the circuit configuration can be simplified.

Although one embodiment has been described above, it goes without saying that various changes can be made without departing from the spirit of the invention.

For example, first of all, the PMT 3 may be grouped in combinations other than the four groups A to D in FIG. 2.

Second, instead of controlling the amplification degree of the PMT 3 itself by adjusting the high voltage of the PMT 3, the amplification degree of an amplifier system such as the preamplifier 4 may be controlled.

Third, the conversion memory 1
2 not only detects the PMT3 closest to the light emitting point, but also adjusts the level signals L1 to L3 (or amplifies the MPX OUT signal) depending on the distance from the central axis of the PMT3 to the light emitting point.
Functions may be added.

Fourth, in the above, the output of each PMT3 is
The sum signal of the group to which PMT3 belongs is used, which means that the sum signal of that group is the sum signal of the group to which PMT3 belongs.
This is because the output of 3 can be approximated, but its PMT
It cannot be denied that in addition to the output of PMT3 in the same group, the output of other PMT3 in the same group makes a small contribution. Therefore, the inverse matrix of the matrix representing the contribution effect may be convolved with the data obtained by the counter memory 15 to more precisely determine the amplification change of each PMT 3.

Fifth, instead of adding the outputs of the preamplifier 4 as they are in the adder 6, they may be passed through a threshold circuit and added to obtain SUM A to SUM D. In this way, the sum signal of the group
From each of SUM A to SUM D, the nearest PMT3
Since the outputs of the PMT3 other than the output of the PMT3 can be removed, a more precise change in the amplification degree of the PMT3 is required.

Furthermore, sixthly, the present invention applies not only to scintillation cameras but also to other radiation position detectors involving one or more scintillators and an array of multiple PMTs, such as multi-slice ECT devices (emission-type computer It can also be applied to tomography devices).

Effects of the Invention According to the PMT amplification stabilizing device of the present invention,
Since no reference light source is used, the amplification degree of each PMT can be kept constant regardless of temperature changes or aging. Therefore, it is possible to prevent deterioration of various characteristics of the radiation position detector, such as uniformity, spatial resolution, and energy resolution.

Furthermore, since data regarding the amplification degree of each PMT is collected in parallel, correction can be made in a short time.

Furthermore, the PMT group is classified into multiple groups according to the rule that PMTs that are adjacent to each other belong to different groups, the summed signals for each group are obtained, and the largest summed signal of these groups is calculated. By selecting, the sum signal of the group to which the PMT closest to the light emitting point belongs is obtained, and the sum signal of the group to which the PMT closest to the light emitting point belongs is approximately equal to the output of only that PMT. By utilizing the fact that it can be considered as Since the integration and delay circuit only needs to be provided for one system of the maximum summation signal selected as described above, the count rate characteristics can be improved, and there is no need to consider the count rate characteristics. can be done easily.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a plan view showing the arrangement and grouping of PMTs 3, and FIG. 3 is a graph showing the spectrum of the analog multiplexer output signal wave height. 1...Scintillator, 2...Light guide, 3
...PMT, 4 ... Preamplifier, 5 ... Position calculation circuit, 6, 7 ... Adder, 8 ... Delay circuit, 9 ...
... Analog multiplexer, 10 ... Timing control circuit, 11 ... Sample/hold and A/D converter, 12 ... Conversion memory, 13 ... Level supply circuit, 14 ... Pulse height analyzer, 15 ... Counter memory, 16...data processing circuit, 17...
... Register, 18 ... D/A converter, 19 ... High voltage control circuit, 21 ... Maximum detection circuit, 22 ... Integration and delay circuit, 23 ... Coincidence detection circuit.

Claims (1)

[Claims] 1 A scintillator, a plurality of photomultiplier tubes arranged on the back side of the scintillator to which scintillation light from the scintillator is guided, and the position of the light emitting point in the scintillator calculated from the outputs of these plurality of photomultiplier tubes. In a radiation position detector having a position calculation circuit that generates a position signal, the photomultiplier tubes are classified into a plurality of groups according to the rule that adjacent photomultiplier tubes belong to different groups, and each group is divided into two or more groups. an adding means for adding a signal related to the photomultiplier tube output to the
means for selecting the largest sum signal from among the sum signals for each group; means for analyzing the wave height of the selected sum signal; and means for determining the photomultiplier tube closest to the light emitting point based on the position signal. And the statistical data of the wave height analysis of the above addition signal,
means for collecting statistical data on the photomultiplier tubes determined to be closest to the light emitting point, and on the basis of the statistical data obtained for each photomultiplier tube or the corresponding amplifier system; 1. An amplification degree stabilizing device for a photomultiplier tube, comprising means for controlling the amplification degree.