JPH0426072B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to improvements in scintillation cameras.

(b) Prior art As is well known, scintillation cameras generate scintillation light by injecting radiation into a scintillator, and convert this light into a large number of photoelectric converters (usually photoconverters) arranged on the back of the scintillator. The scintillation light generation position is calculated by inputting the output of each PMT to a position calculation circuit consisting of a weighted addition circuit through an amplification system. to obtain an image.

Conventionally, in order to improve spatial resolution or improve spatial distortion in such scintillation cameras, it has been necessary to provide nonlinear characteristics to the amplification system connected between each PMT and the position calculation circuit. has been proposed, and it has also been proposed to change its nonlinear characteristics in proportion to the energy window level of the pulse height analyzer or the energy signal for each event (Tokuko Sho 51-29839,
56-51312, JP-A-53-84378, JP-A-55-44998
(Refer to each of the publications, etc.) However, in the configurations according to these conventional proposals, there was a problem in that specific spatial distortion and non-uniformity remained in the peripheral part of the visual field.

In other words, in one of the above proposals, the amplification factors of each PMT and the amplification system connected to them are all equal, and in this case, there is essentially no PMT outside the outermost PMT. Due to the local asymmetry of the field of view, there is a drawback that the spatial distortion and the positional dependence of the energy signal wave height are significant in the periphery of the field of view.

Another proposal uses a configuration in which the weight in the adder circuit is increased only for the outermost PMT, making it different from the weights of other PMTs, but this configuration certainly increases the energy signal wave height. Although the positional dependence is improved, it still has the drawback that spatial distortion is significant in the periphery.

Yet another proposal adopts a configuration in which the amplification factor of the PMT or amplification system is made different (for example, increased) only for the outermost PMT, and by this configuration, the position dependence of the energy signal wave height is Although the peripheral effect of spatial distortion (for example, distortion toward the center in the periphery) is improved to some extent, the PMT in the periphery
A disadvantage arises in that spatial distortion appears, such that a hot spot or a cold spot appears directly above.

(c) Purpose The purpose of the present invention is to improve spatial distortion and non-uniformity in the periphery of the field of view in a scintillation camera in which the amplification factors of each PMT and each corresponding amplification system are nonlinear.

(D) Configuration In the scintillation camera of the present invention, only the PMTs in the periphery have different outputs from each amplification system for the same input compared to the PMTs in the center.
The amplification factor of the PMT or amplification system is adjusted, and the nonlinear points (bending points and/or threshold levels) in each amplification system are also adjusted for each amplification system output, with respect to the peripheral PMT and with respect to the center PMT. They are configured differently depending on their size.

(e) Example In this example, 61 PMTs 3 are arranged in a hexagonal dense arrangement as shown in Fig. 1, and in this way, as shown in Fig. installed. 61 pieces
1 to 61 as shown in Figure 1 for each of PMT3.
number, classify them into three types, and classify them into 3 types.
No. 4, No. 6, No. 11, No. 12, No. 18, No. 19
No. 26, No. 36, No. 43, No. 44, No. 50,
The 51st, 56th, and 58th to 60th PMT3 are called the outermost PMT302, and the 1st, 5th, and
PMT3 No. 27, No. 35, No. 57, and No. 61 are called PMT303 at the top of the outermost circumference, and other PMTs
3 will be referred to as the central PMT 301. 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 of PMT3,
These outputs are input to the position calculation circuit 7 via the corresponding preamplifier 4, waveform shaping circuit 5, and nonlinear amplifier 6, where they are weighted and added, and division (normalization) with respect to energy is performed. Position signals X and Y are obtained.
These preamplifiers 4, waveform shaping circuits 5, and nonlinear amplifiers 6 are also classified into three types according to the PMTs 301, 302, and 303, and each has a corresponding number 4.
01-403, 501-503, 601-603
is attached. The output of each of the waveform shaping circuits 5 is also input to an adder circuit 8 to obtain an energy signal Z. This energy signal Z is sent to a pulse height analyzer 9, and the energy signal Z is It is determined whether or not the energy falls within a certain energy window, and if so, an unblank signal is generated from the pulse height analyzer 9. The energy signal Z is also input to the nonlinear point control voltage generation circuit 10, which generates a nonlinear point control voltage so that the nonlinear point of the nonlinear amplifier 6 changes according to the energy for each event of radiation incidence, and this is applied to the nonlinear point control voltage generation circuit 10. 6
send to each of them. The input/output characteristics of the nonlinear amplifier 601 corresponding to the central PMT 301 are, for example, so-called suppressed nonlinear characteristics with a small amplification factor for high input signals, as shown by the solid line in FIG. The higher one is LNL1, the higher one is HNL
1), which are controlled by nonlinear point control voltages L1 and H1, respectively. The same goes for the nonlinear amplifiers 602 and 603 corresponding to the PMTs 302 and 303 at the outermost edge and apex, as shown by the dashed line in FIG. 3 for the former and the dotted line for the latter, respectively. 2 bending points LNL2, HNL2 and LNL3, HNL3
These bending points are controlled by nonlinear control voltages L2, H2 and L3, H3, respectively. By providing the input/output characteristics with suppressed nonlinear characteristics in this way, it is possible to improve the spatial distortion (and the non-uniformity caused by it) that causes hot spots directly above each of the PMTs 3.
As a result, the light guide 2 and the scintillator 1 can be made thinner, which indirectly improves the spatial resolution.

Here, according to the present invention, PMT3 in the peripheral area
Regarding 02 and 303, the amplification factors of the PMTs 302 and 303 or the preamplifiers 402 and 403 (or the amplification system including the waveform shaping circuits 502 and 503, etc.) are different from the amplification factor of the PMT 301 in the center, and the same radiation each source
Even when the nonlinear amplifiers 6 are placed directly above the PMT 3 and the same radiation input is applied, the outputs input to each of the nonlinear amplifiers 6 are different. In addition, in accordance with the difference in the inputs of the nonlinear amplifier 6, the nonlinear amplifiers 602 and 603 corresponding to the peripheral part are made to have different input/output characteristics from the nonlinear amplifier 601 corresponding to the central part. There is.

Specifically, for example, the PMT at the outermost edge
The PMT 3 or preamplifier 4 (or waveform shaping circuit 5), and the nonlinear amplifiers 601, 602, 6
The input/output characteristics of each of 03 are shown by the solid line in FIG. 3 for the center part, that is, the nonlinear amplifier 601, and by the dashed line in the figure for the outermost side part, that is, the nonlinear amplifier 602, and for the outermost vertex part, that is, the nonlinear amplifier. 603 is adjusted as shown by the dotted line. Of course, the above value is scintillator 1
It should be determined depending on the thickness of the light guide 2, the diameter and spacing of the PMT 3, etc., and depending on these factors, the above-mentioned values of 1.5 times and 0.7 to 0.8 times are not optimal.

Since the amplification factor and nonlinear characteristics are changed between the periphery and the center in this way, the positional dependence and spatial distortion of the energy signal wave height in the periphery are improved, and as a result, the effective field of view is relatively Can be expanded.

In the above embodiment, the input/output characteristics of the nonlinear amplifier 6 are suppressed nonlinear characteristics having two bending points as shown in FIG. In the case of smooth bending, in the case of continuous nonlinear characteristics, for example, in the case of having a threshold characteristic in addition to the suppression type characteristic as shown in FIG. 4, in the case of only a threshold characteristic as shown in FIG. The same applies to other nonlinear characteristics.

Furthermore, in the above embodiment, the input/output characteristics of the nonlinear amplifier 6 were configured to change according to the energy signal Z for each event of radiation incidence, but instead, the input/output characteristics of the nonlinear amplifier 6 are changed depending on the energy window level of the pulse height analyzer 9. It can also be configured.

Furthermore, in the above example, PMT3, which is arranged in large numbers,
were classified into three types according to their position, but it is also possible to classify them into two or four or more types.
Further, the present invention is similarly applicable to configurations other than the configuration in which 61 PMTs 3 are arranged in a hexagonal dense arrangement shown in FIG.

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 5, and performs peak detection, or performs integration during position calculation) may be used. The present invention is applicable even if this is adopted.

In addition, the weight of the position calculation circuit 7 is not necessarily proportional to the position of each PMT 3, and the weight of the signal related to the energy on the denominator side of the division circuit for standardization regarding energy is not necessarily proportional to the position of each PMT 3.
It is also applicable to unequal configurations with respect to PMT3.

(F) Effect According to the present invention, spatial distortion inherent in the peripheral part of the visual field and non-uniformity caused by it are improved, and position dependence of the energy signal wave height specific to the peripheral part and non-uniformity caused by it are improved. sex is improved. Furthermore, the effective field of view of the scintillation camera is relatively expanded.

[Brief explanation of drawings]

FIGS. 1 and 2 relate to an embodiment of the present invention; FIG. 1 is a plan view showing the arrangement of PMTs;
FIG. 2 is a block diagram, FIG. 3 is a graph showing the input/output characteristics of this embodiment, and FIGS. 4 and 5 are graphs showing the input/output characteristics of modified examples. 1...Scintillator, 2...Light guide, 3...
PMT, 4...Preamplifier, 5...Waveform shaping circuit, 6
...Nonlinear amplifier, 7. Position calculation circuit, 8. Addition circuit, 9. Wave height analyzer, 10. Nonlinear point control voltage generation circuit.

Claims (1)

[Claims] 1. A scintillator, a large number of photoelectric converters arranged on the back side of this scintillator, an amplification system with nonlinear characteristics that amplifies the output of each photoelectric converter, and an output of each photoelectric converter after passing through each of these amplification systems. In a scintillation camera having a position arithmetic circuit to which a
The amplification factor of the photoelectric converter or the amplification system is adjusted, and the nonlinear points in each of the amplification systems are also determined depending on the magnitude of the output of each amplification system, depending on whether the output is related to the photoelectric converter in the periphery or the photoelectric converter in the center. A scintillation camera characterized by being configured differently depending on the situation.