JPH0528355B2 - - 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 method stabilizes the degree of amplification of the PMT (Japanese Patent Publication No. 55-19511, Japanese Unexamined Patent Publication No. 58-9082).

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 by the PMT 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.

Although the fourth proposal is excellent because it eliminates the drawbacks of the first to third proposals, there are still some points to be improved, such as the circuit being complicated and the amount of circuitry being large.

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, the circuit for collecting data regarding the amplification of each PMT is based on the conventional energy signal (or similar signal) circuit. PMT amplifier stabilization, which can be realized by reusing and partially modifying or adding, and also makes data collection easier and reduces data collection time compared to sequentially setting and selecting PMTs. The purpose is to provide a device for converting

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, the above PMT group is classified into multiple groups based on the rule that adjacent PMTs belong to different groups, and the PMT is divided into each group.
a first addition means for adding signals related to the output;
a switch means for selecting one or all of the addition signals for each group; a second addition means to which the output of the switch means is input; In the normal measurement mode, all the summed signals for each group mentioned above are transferred to the second adding means, and in the correction mode, the summed signals for each group are sequentially selected one by one. a mode setting means for controlling the switching means to transfer the signal to the second addition means; and an imperfection obtained from the selected one group of addition signals output from the second addition means in the correction mode. means for analyzing the wave height of the energy signal; and a means for analyzing the wave height of the energy signal, and in the correction mode, the PMT determined to be closest to the light emitting point for each event of radiation incidence belongs to the group of summation signals selected by the above-mentioned switch means. Under the conditions, the above wave height analysis results are
It has a data storage means for storing data regarding the PMT, and a means for controlling the amplification degree of the PMT or the corresponding amplifier system based on the statistical data of the pulse height analysis obtained for each PMT by the data storage means. configured.

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, if the PMT group is classified into multiple groups according to the rule that PMTs that are adjacent to each other belong to different groups, and the sum signal for each group is obtained, it is possible to The added signal can be considered approximately equal to the output of only that PMT. Therefore, each group was designated in turn, and the incomplete energy signal obtained from the summed signals of the designated groups was analyzed for wave height. If a PMT belongs to the above specified group, the wave height analysis result obtained from the added signal of the specified group can be treated as data regarding that PMT. Therefore, if the amplification degree of the PMT or the corresponding amplifier system is controlled based on the statistical data of the pulse height analysis regarding each PMT obtained in this way,
This makes it possible to stabilize the amplification degree of 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, energy signal wave height analysis, 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 PMT 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 a 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 closer to the light emitting point, the greater the amount of light, so the output is larger. ) 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 via the analog switch 8 and added to form the SUM Z signal, which is input to the position calculation circuit 5 and converted into the energy signal Z. This analog switch 8 is controlled by a mode setting device 9, and in the normal measurement mode (diagnosis mode), all switches are turned on and the sum signal SUM A of each group is turned on.
All of ~SUM D is transferred to the adder 7, and in the correction mode, the switches are turned on one by one at regular intervals, and each group's sum signal SUM A~
One of the SUM D is sequentially selected and transferred to the adder 7. Therefore, in the normal measurement mode, the energy signal Z obtained from the position calculation circuit 5 is a complete signal obtained from the summation of the output signals of all PMT3s, but in the correction mode, the energy signal Z obtained from the summation of the output signals of all PMT3s is a complete signal. It is an incomplete signal obtained from the summed signal of . In addition, in correction mode, SUM Z
Since the signal itself is incomplete, it is necessary to take precautions such as lowering or widening the window range of the pulse height analyzer in the position calculation circuit 5.

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. That is, the signal addressed by Dig.X and Dig.Y and representing the position and number of the nearest PMT 3 is read out.

The incomplete energy signal Z outputted from the position calculation circuit 5 is sent to the pulse height analyzer 14, and is compared in pulse height with the signals L1 to L3 from the level supply circuit 13. This pulse height analyzer 14 detects a signal U/L and a timing signal +1 indicating which range the wave height of the incomplete energy signal Z is in the range of levels L1 to L2 or the range of L2 to L3. is output. The level supply circuit 13 is controlled by a signal indicating the position of the nearest PMT 3 from the conversion memory 12, and the level supply circuit 13 is controlled by a signal indicating the position of the nearest PMT 3, and a level signal is output depending on the position of the PMT 3 (whether it is in the center or the periphery of the visual field, etc.). L1 to L3 are arranged to take different values. In other words, the signal wave height distribution of the incomplete energy signal Z for a certain tuning spot is
If the amplification degree of the PMT3 amplification system is normal, it should be as shown in Figure 3. Levels L1 to L3 are set as shown in this figure, and level L2 is set to correspond to the peak. be done. Note that the level signals L1 to L3 may be modified and set according to the nuclide. The timing control circuit 10 outputs a timing signal for controlling the sample/hold and A/D converter 11, the conversion memory 12, the pulse height analyzer 14, etc. using the unblank signal as a reference.

The counter memory 15 is composed of a RAM, etc., and when the PMT3 determined to be closest to the light emitting point for each incident of radiation belongs to the group specified by the mode setting device 9, the number of the PMT3 and The contents of the memory addressed by each signal U/L are incremented by +1 each time a +1 signal is input. If the amplification degree of the PMT3 amplification system deviates from the normal one, the spectrum in Figure 3 will shift to the left and right, so the count value in the range of L1 to L2 and L2 to L
The amplification change can be known from the difference or ratio with the count value in the range of 3.

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 A/D 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.

In order to actually correct the amplification degree of the PMT 3, the mode setter 9 is set to correction mode, and the entire field of view is uniformly irradiated with γ-rays using, for example, a flat source. Then, the mode setter 9 selects groups A to D.
are selected sequentially. First, if group A is selected, the counter memory 15 counts each of the PMTs 3 belonging to this group A. When this count reaches a predetermined value or when a certain period of time has elapsed, the mode setter 9 switches from group A to group B in response to a signal from the counter memory 15 or data processing circuit 16. Then, data collection for group A is completed, and data collection for the next group B is performed in the same manner. In this way, the change in the amplification degree in the amplification system for each PMT3 can be statistically measured from the count value obtained by the counter memory 15, and the data processing circuit 16 registers the amplification degree so that the amplification degree of each PMT3 becomes the optimum amplification degree. 17 can be modified.

Here, the incomplete energy signal Z is obtained from the summed signals of each group, but it can be regarded as the output of the PMT 3 closest to the light emitting point belonging to that group. This is because in the optical system of scintillator 1, light guide 2, and PMT3 in a normal scintillation camera, one PMT3
When light emission occurs on the central axis of , light enters that PMT 3 and surrounding PMT 3 and output is generated from each PMT 3, but the output of the central PMT 3 is 100
%, the output of the adjacent PMT 3 is 18 to 20%, and the output of the PMT 3 adjacent to this adjacent PMT 3 is about 3 to 3.5%, and the output of the adjacent PMT 3 is about 3 to 3.5%.
The configuration is such that the groups are divided according to the rule that they belong to different groups, and the sum signal of that group is obtained, so the output of the nearest PMT3 contributes greatly to the sum signal of this group, and This is because the contribution rate of the output of other PMTs 3 to which it belongs is small. Therefore, the wave height analysis result of this incomplete energy signal Z can be stored in the counter memory 15 as data regarding the amplifier of the PMT 3 closest to the light emitting point belonging to that group.

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, rather than adjusting the high voltage of PMT3 to control the amplification degree of PMT3 itself, the preamplifier 4
The amplification degree of the amplifier system may be controlled.

Third, it is possible to separate the complete energy signal system in the normal measurement mode from the incomplete energy signal system in the correction mode, and to send the output of the preamplifier 4 to the adder 6 via the threshold circuit. The addition signals SUM A to SUM D may also be created. When a threshold circuit is used, the outputs of the PMTs 3 other than the output of the nearest PMT 3 can be removed from each of the summed signals SUM A to SUM D of the group, so that a more precise change in the amplification degree of the PMT 3 is required.

Fourth, there is a scintillation camera system in which spatial nonlinearity correction and energy nonuniformity correction are further performed on the outputs X, Y, and Z of the position calculation circuit 5; When applying this invention, the sample hold and A/D converter 11, and the pulse height analyzer 14 are
It is possible to use the sample hold, A/D converter, and pulse height analyzer used in that system.

Fifth, 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 incomplete energy signal) depending on the distance from the central axis of the PMT3 to the light emitting point. Functions may be added.

Sixth, 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.

Seventhly, the present invention applies not only to scintillation cameras, but also to other radiation position detectors with 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.

In addition, data regarding the amplification degree of each PMT is collected in parallel within the group to which that PMT belongs (although each group must be set and selected in sequence), so data can be collected in a short time. , it is possible to shorten the time for correction.

Furthermore, a circuit for obtaining data for PMT amplification correction can be configured by only partially modifying a conventional position calculation circuit (particularly the energy signal system) or adding a simple circuit.

[Brief explanation of the drawing]

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 incomplete energy signal wave height. 1...Scintillator, 2...Light guide, 3
...PMT, 4 ... Preamplifier, 5 ... Position calculation circuit, 6, 7 ... Adder, 8 ... Analog switch, 9 ... Mode setter, 10 ... Timing control circuit, 11 ... Sample hold and A/
D converter, 12... Conversion memory, 13... Level supply circuit, 14... Wave height analyzer, 15... Counter memory, 16... Data processing circuit, 17... Register, 18... D/A converter , 19...High voltage control 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 on 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 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 groups. a first addition means for adding a signal related to the photomultiplier tube output; and any one of the above addition signals for each group.
a switch means for selecting one or transferring all; a second addition means to which the output of the switch means is input; and means for determining the photomultiplier tube closest to the light emitting point based on the position signal; mode setting means for controlling the switch means to sequentially select the addition signals of each group one by one and transfer them to the second addition means in the correction mode;
means for analyzing the wave height of the incomplete energy signal obtained from the selected one group of summed signals output from the second adding means in the correction mode; and means for analyzing each event of radiation incidence in the correction mode. On the condition that the photomultiplier tube determined to be closest to the light emitting point in each case belongs to the group of addition signals selected by the above switch means, the above pulse height analysis result is applied to the photomultiplier tube closest to the light emitting point. A data accumulation means for accumulating data regarding the multiplier, and controlling the amplification degree of the photomultiplier or the corresponding amplifier system based on statistical data of pulse height analysis obtained for each photomultiplier by the data accumulation means. 1. An amplification degree stabilizing device for a photomultiplier tube, comprising means for stabilizing the amplification degree of a photomultiplier tube.