JPS6052787A - Scintillation camera - Google Patents

Abstract

PURPOSE:To correct always a spatial distortion and an ununiformity due to this distortion independently of energy by changing the nonlinear characteristic of an amplifying system of a PMT (photomultiplier) nonlinearly in accordance with an energy window level or an energy signal. CONSTITUTION:Weighting addition different in weight by the position of each PMT 3 and normalization due to division by an energy signal Z are performed in a position operating circuit 7 to obtain position signals X and Y. Respective outputs of waveform shaping circuits 5 are inputted to an adding circuit 8 also to obtain the energy signal Z, and this energy signal Z is sent to a pulse height analyzer 9, and it is discriminated by this pulse height analyzer 9 whether the energy signal Z is within a certain energy window or not, and an unblank signal is generated from the pulse height analyzer 9 if the energy signal is within the energy window. The energy signal Z is transformed by an energy signal nonlinear circuit 10 and is inputted to a nonlinear point control voltage supply circuit 11, and a nonlinear point control voltage is so generated that nonlinear points of nonlinear circuits 6 are changed in accordance with the energy for every event of radiation incidence, and this voltage is supplied to individual nonlinear circuits 6.

Description

[Detailed description of the invention] (b) Industrial application fields This invention relates to improvements in scintillation cameras.

(b) Conventional scintillation cameras, as is well known,
Radiation is incident on a scintillator to generate scintillation light, and this light is transferred to a large number of photoelectric converters (usually photomultipliers; PM) arranged on the back side of the scintillator.
(hereinafter referred to as PMT),
The output of each PMT is passed through an amplification system and inputted to a position calculating circuit consisting of a weighting and adding circuit, thereby calculating the position where scintillation light is generated and obtaining an image.

Conventionally, in order to improve spatial resolution or improve spatial distortion in such scintillation cameras, it has been possible to provide nonlinear characteristics to the amplification system connected between each PMT and the position calculation circuit. It has also been proposed to change the nonlinear characteristics in proportion to the energy wind level of the pulse height analyzer or the energy signal for each event (Japanese Patent Publication No. 51-29839,
(See Japanese Patent Publication No. 56-51312, Japanese Patent Application Publication No. 53-84378, Japanese Patent Application Publication No. 55-44998, etc.).

However, in the configurations of these conventional proposals, the nonlinear characteristics are changed linearly and proportionally to the energy wind level or energy signal. If the parameters of the nonlinear characteristics are selected to be optimal, there is a problem in that non-uniformity such as a hot spot directly above the PMT occurs during high-energy measurement.

That is, in the above proposal, the input/output characteristics of the above amplification system are
For example, as shown in FIG. 1A, the so-called suppressed nonlinear characteristic has a small amplification factor for a high input signal, and has two bending points (the lower one is LNL and the higher one is HNL). These bending points are changed linearly in proportion to the energy signal Z, as shown in FIG. Therefore, for example, low energy Zl, medium energy Z2. The characteristics for high energy Z3 are the 1st Ug
! The results are as shown by the dotted line, solid line, and dashed-dotted line of JA, respectively.

However, when measured under certain conditions, if the spatial distortion and resulting inhomogeneity are adjusted to be best improved for medium energy Z2, a cold spot appears directly above the PMT for low energy Zl. For the high energy Z3, the spatial distortion that causes a hot spot directly above the PMT tends to change somewhat. This is thought to be due to the fact that the position where radiation is absorbed within the scintillator and scintillation light is generated becomes deeper on average as the energy increases.

(C) Objective The object of the present invention is to provide a scintillation camera that always improves spatial distortion and non-uniformity caused by it, regardless of energy.

(d) Configuration According to this invention, radiation is absorbed within the scintillator.
Focusing on the fact that the position where scintillation light is generated becomes deeper on average as the energy increases, we calculated the nonlinear characteristics in the amplification system of each PMT into the energy wind level of the pulse height analyzer or the energy signal for each event of radiation incidence. It is made to change non-linearly.

(E) Embodiment As shown in FIG. 2, a large number of PMTs 3 are arranged on the back surface of a scintillator 1 via a light guide 2. Thickness of this light guide 2 and thickness of scintillator l)
In exchange for obtaining high resolution by thinning the PMT 3, the configuration is such that the spatial distortion that becomes a hot spot directly above each PMT 3 and the resulting non-uniformity become significant. Spatial distortion and non-uniformity caused by this are improved, and as a result, both spatial resolution and spatial distortion (and non-uniformity caused by this) can be improved.

When radiation is incident on the scintillator 1, scintillation light is generated within the scintillator 1, and this light is guided to each of the PMTs 3 via the light guide 2. In this way, an output is generated from each PMT 3, each output is linearly integrated by the corresponding preamplifier 4, shaped and amplified by each waveform shaping circuit 5, and then input to a nonlinear circuit 6 where it is nonlinearly amplified. (The nonlinear circuit 6 may be configured with an attenuator to nonlinearly attenuate it.) The output of each nonlinear circuit 6 is input to a position calculation circuit 7, and the position calculation circuit 7 calculates each PMT3
The weighting, which has different weights depending on the position, is added and normalized by dividing by the energy signal Z to obtain the position signals X and Y. The output of each of the waveform shaping circuits 5 is also input to an adding circuit 8 to obtain an energy signal Z. This energy signal Z is sent to a pulse height analyzer 9, which converts the energy It is determined whether the signal Z falls within a certain energy window, and if so, an unblank signal is generated from this pulse height analyzer 9. The energy signal Z is transformed by the energy signal non-linear circuit 10 and then inputted to the non-linear point control voltage supply circuit 11. A point control voltage is generated and supplied to each of the nonlinear circuits 6. The input/output characteristics of the nonlinear circuit 6 are so-called suppressed nonlinear characteristics with a small amplification factor for high input signals, as shown in FIG. ), and each of these bending points is controlled by a nonlinear point control voltage from a nonlinear point control voltage supply circuit 11, respectively.

Here, the bending point is changed linearly as before (
As shown in FIG. 1B), spatial distortion occurs due to the fact that the radiation absorption position within the scintillator l becomes deeper on average as the energy increases. so that the bending point changes non-linearly with respect to the energy signal Z. As a result, as shown in FIG. 3A, the input/output characteristics of the nonlinear circuit 6 become somewhat shallower for low energy Zl and somewhat deeper for high energy Z3 (compared to FIG. 1A). , it becomes possible to make adjustments so that spatial distortion and non-uniformity caused by it are most improved for any energy. Note that even if only one of the two bending points is changed nonlinearly and the other one linearly, an effect may be obtained.

Also, for example, the nonlinear circuit 6 may have a 1
If similar characteristic curves are not necessarily satisfied (this is an unavoidable problem when using a diode clamp to obtain nonlinear characteristics, since the curve at the bend is due to the threshold characteristics of the diode) ) or when a mask is placed inside the light guide 2, spatial distortion with the same tendency as above does not occur (when a mask is placed inside the light guide 2, Due to the energy dependence of the radiation absorption depth, there is a spatial distortion with a tendency opposite to the above, that is, a hot spot appears on PMT3 at low energy Z1, and a cold spot appears on PMT3 at high energy Z3. ) However, even in such a case, by doing as described above, spatial distortion can always be improved satisfactorily within a certain energy range regardless of the energy.

In the above embodiment, the input/output characteristic of the nonlinear circuit 6 was explained as a suppressed nonlinear characteristic having two bending points as shown in FIG. 3A.
It is also effective to apply it to various similar cases such as cases other than the location, cases where bending is smooth, cases with continuous nonlinear characteristics, etc.

For example, as shown in FIG. 4A, when there is a threshold characteristic in addition to the suppression type characteristic, when there is a tendency to generate a hot spot on the PMT3 at high energy, the fourth
As shown by the solid line in FIG. B, the same improvement as described above can be achieved by making the threshold point somewhat shallower for the high energy Z3 (compared to the linear case shown by the dotted line). At this time, both the bending point and the threshold point are
It may be changed non-linearly as shown in FIG. B and FIG. 4B, or only one may be changed non-linearly and the other linearly.

Further, in the above embodiment, the input/output characteristics of the nonlinear circuit 6 are configured to change according to the energy signal Z for each event of radiation incidence, but instead of this, a configuration is adopted in which the input/output characteristics of the nonlinear circuit 6 are changed depending on the energy window level of the pulse height analyzer 9. You can also.

Alternatively, a configuration in which the position calculation method is slightly different from that in FIG. 2 (for example, a configuration in which the preamplifier 4 is a non-integrating type, does not have the waveform shaping circuit 15, and performs peak detection, or performs integration during position calculation) may be used. The same applies even if it is adopted.

In addition, a mask is placed in the light guide 2, so that the input input characteristic of the nonlinear circuit 6 has only a so-called threshold effect with a small amplification factor for low input signals, as shown in FIG. 5A, and is similar to the above embodiment. Even in the case of a configuration that does not have a suppressing effect, this characteristic is shown by the solid line in FIG. 5B,
A similar effect can be obtained by nonlinearly changing the energy signal for each event or the energy window level of the pulse height analyzer 9. In this case, the nonlinear circuit 6 may be incorporated into the preamplifier 4.

Furthermore, the input/output characteristics of the nonlinear circuit 6 are fixed, and each PMT
3 or the amplification factor of the amplifier (or attenuator) inserted between each PMT 3 and each nonlinear circuit 6 is changed nonlinearly with respect to the energy signal for each event or the energy wind level of the pulse height analyzer 9. The same is true if you let it.

(Effect) According to the present invention, focusing on the fact that the position where radiation is absorbed in the scintillator and scintillation light is generated becomes deeper on average as the energy increases, PM is used to compensate for this.
Since the nonlinear characteristics of the T amplification system are changed nonlinearly according to the energy wind level or the energy signal, spatial distortion and nonuniformity caused by it can always be improved regardless of the energy, and The above improvements can be achieved while always maintaining high resolution regardless of energy.

[Brief explanation of drawings]

Figures 1A and B are graphs showing the characteristics of the conventional example;
The figure is a block diagram showing one embodiment of the present invention, FIG. 3A,
B is a graph showing the characteristics in this example, FIG. 4A,
FIGS. 5A and 5B are graphs showing characteristics of modified examples, respectively. l...Scintillator 2...Light guide 3...
PMT 4... Preamplifier 5... Waveform shaping circuit 6... Nonlinear circuit 7... Position calculation circuit 8... Addition circuit 9... Wave height analyzer 10... Energy signal nonlinear circuit 11...・Nonlinear point control voltage supply circuit Answer 1 Curtain 2 Eye Clear Answer 3 Vertex A Mathematical beads: tl:12 Z31S

Claims (1)

[Claims] (1) A scintillator, a large number of photoelectric converters arranged on the back side of the scintillator, an amplification system having nonlinear characteristics that amplifies the output of each of the photoelectric converters, and all of the outputs of the above-mentioned large number of photoelectric converters. An adder circuit that obtains an energy signal by adding , and a wave height that determines whether the wave height of the energy signal obtained from this adder circuit is within a preset energy window and generates an unranked signal when it is within a preset energy window. In a scintillation camera having an analyzer and a position calculation circuit into which the output of each photoelectric converter that has passed through each of the amplification systems is input, the energy wind level of the wave height analyzer or the energy signal for each event of radiation incidence is input. By applying a control signal to the amplification system, the nonlinear characteristics of the amplification system can be controlled by
A scintillation camera characterized by comprising a nonlinear characteristic control circuit that nonlinearly changes the energy wind level of the wave height analyzer or the energy signal for each event of radiation incidence.