Radiation detector

Abstract

To provide a radiation detector capable of improving time resolution for detecting the interaction time of radiation in a scintillator.SOLUTION: A radiation detector 10 comprises a scintillator 11, a photodetector 12, and a processing section 13. The processing section 13 includes a signal waveform acquisition section 14, a deconvolution computation section 15, and an interaction time detection section 16. The signal waveform acquisition section 14 acquires a waveform f(t) of an electric signal output from the photodetector 12. The deconvolution computation section 15 performs deconvolution computation on the electric signal waveform f(t) acquired by the signal waveform acquisition section 14 with an impulse response function i(t) of the photodetector 12. The interaction time detection section 16 detects, as the interaction time, the time when the waveform obtained by the computation of the deconvolution computation section 15 reaches a threshold.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a radiation detector. [Background technology]

[0002] Radiation detectors that detect high-energy radiation (e.g., gamma rays, X-rays, alpha rays, etc.) can be used in a variety of devices that detect radiation, as well as radiation tomography devices such as Positron Emission Tomography (PET) devices and Single Photon Emission Computed Tomography (SPECT) devices.

[0003] A radiation detector used in such an apparatus includes a scintillator that generates scintillation light by interacting with radiation, a photodetector that receives the scintillation light and outputs an electrical signal, and a processing unit that detects the time when the radiation interacts with the scintillator based on the electrical signal. The processing unit detects the time when the value of the electrical signal output from the photodetector reaches a predetermined threshold as the time when the radiation interacts with the scintillator. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] MVNemallapudi et al. “Single photon time resolution of state of the art SiPMs,” JINST 11 P10016 (2016) [Non-Patent Document 2] S. Gundacker et al. “Experimental time resolution limits of modern SiPMs andTOF-PET detectors exploring different scintillators and Cherenkov emission,” Phys.Med. Biol., 65 (2020) 025001 Summary of the Invention [Problem to be solved by the invention]

[0005] In radiation detectors, there is a demand for improved time resolution in detecting the interaction time of radiation in the scintillator. However, since the interaction time is detected in the manner described above, there is a limit to how much the time resolution can be improved.

[0006] The present invention has been made to solve the above problems, and has an object to provide a radiation detector that can improve the time resolution of detecting the interaction time of radiation in a scintillator. [Means for solving the problem]

[0007] A first aspect of the radiation detector of the present invention includes (1) a scintillator that generates light by interacting with radiation, (2) a photodetector that receives light generated by the scintillator and outputs an electrical signal, and (3) a processing unit that detects the time when radiation interacts with the scintillator based on the waveform of the electrical signal. The processing unit includes (a) a signal waveform acquisition unit that acquires the waveform of the electrical signal output from the photodetector, (b) a deconvolution calculation unit that deconvolutes the impulse response function of the photodetector with the waveform of the electrical signal, and (c) an interaction time detection unit that detects the time when the waveform obtained by calculation by the deconvolution calculation unit reaches a threshold as the time when radiation interacts with the scintillator.

[0008] In a second embodiment of the radiation detector of the present invention, in addition to the first embodiment, the photodetector is a silicon photomultiplier.

[0009] In a third aspect of the radiation detector of the present invention, in addition to the first or second aspect, the scintillator is made of BGO.

[0010] In a fourth embodiment of the radiation detector of the present invention, in addition to the first or second embodiment, the scintillator is made of PbWO4.

[0011] A radiation tomography apparatus according to the present invention includes the above-described radiation detector according to the present invention. Effect of the Invention

[0012] According to the present invention, it is possible to improve the time resolution of detecting the interaction time of radiation in a scintillator. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the configuration of a radiation detector 10. As shown in FIG. [Diagram 2] FIG. 2 is a diagram showing a measurement system for acquiring the impulse response function i(t) of a photodetector. [Diagram 3] FIG. 3 is a diagram showing an example of a waveform f(t) of an electrical signal output from a photodetector. [Figure 4] FIG. 4 is a diagram showing an example of the time response function s(t) of the scintillator. [Diagram 5] FIG. 5 is a diagram showing an example of a function p(t) representing the single-photon time resolution of a photodetector. [Figure 6] FIG. 6 is a diagram showing an example of the impulse response function i(t) of the photodetector. [Figure 7] FIG. 7 is a diagram showing an example of the result of the deconvolution operation of i(t) with respect to f(t). [Figure 8] FIG. 8 is a diagram showing a measurement system for acquiring the waveform f(t) of the electrical signal output from the photodetector in the embodiment. [Figure 9] FIG. 9 is a diagram showing the results obtained by evaluating the time response of the BGO in the example. [Figure 10] FIG. 10 is an enlarged view of a portion of FIG. [Figure 11] FIG. 11 is a diagram showing the results of evaluating the time response of the BGO in the embodiment in comparison with the results of evaluation by the TCSPC method. [Figure 12] FIG. 12 is a diagram showing the results of evaluating the time response of the BGO in the example, in comparison with the evaluation results described in Non-Patent Document 2. [Figure 13] FIG. 13 is a diagram showing the results obtained by evaluating the coincidence time resolution in the example in comparison with the evaluation results in the comparative example. [Figure 14] FIG. 14 is a flowchart illustrating the operation of the radiation detector 10. [Figure 15] FIG. 15 is a diagram showing a schematic configuration of a radiation tomography system 1 including a radiation tomography apparatus 2 and an image processing device 3. As shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions are omitted. The present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0015] 1 is a diagram showing the configuration of a radiation detector 10. The radiation detector 10 includes a scintillator 11, a photodetector 12, and a processing unit 13.

[0016] The scintillator 11 generates light (scintillation light and Cherenkov light) by interacting with radiation. The scintillator 11 may be made of any material, but is preferably made of a material that generates a large proportion of Cherenkov light, such as BGO (Bi4Ge3O 12 ) or PbWO4.

[0017] The photodetector 12 receives the light generated by the scintillator 11 and outputs an electrical signal. The photodetector 12 is preferably one with small fluctuations in impulse response, and is preferably, for example, a silicon photomultiplier (SiPM).

[0018] Processing unit 13 receives the electrical signal output from photodetector 12, performs required processing based on the received electrical signal, and detects the time when radiation interacts with scintillator 11 (interaction time). Processing unit 13 can be configured with an electronic circuit. Processing unit 13 includes a signal waveform acquisition unit 14, a deconvolution calculation unit 15, and an interaction time detection unit 16.

[0019] The signal waveform acquisition unit 14 acquires the waveform f(t) of the electrical signal output from the photodetector 12. t is a variable indicating time. The deconvolution calculation unit 15 performs deconvolution calculation of the impulse response function i(t) of the photodetector 12 on the electrical signal waveform f(t) acquired by the signal waveform acquisition unit 14. The interaction time detection unit 16 detects the time when the waveform acquired by calculation by the deconvolution calculation unit 15 reaches a threshold as the interaction time.

[0020] As described above, radiation detector 10 detects the interaction time by processing unit 13 based on the waveform f(t) of the electrical signal output from photodetector 12 that receives light generated by scintillator 11, and the impulse response function i(t) of photodetector 12. Next, an example of a measurement system for acquiring i(t) will be described.

[0021] FIG. 2 is a diagram showing a measurement system for acquiring the impulse response function i(t) of the photodetector. The photodetector 12 outputs an electric signal in response to the incidence of a photon ν. The electric signal output from the photodetector 12 is input to an oscilloscope 40, and the waveform of the electric signal is acquired. In the measurement system shown in this figure, the following is done, for example, to acquire the impulse response function i(t) of the photodetector 12. The applied voltage of the photodetector 12 is set as high as possible (for example, the overvoltage from the breakdown voltage to +8.0 V) so that a single photon can be visually recognized by the oscilloscope 40 without an amplifier. The range is set so that all of the waveforms of the single photon are within the voltage range of the oscilloscope 40 (for example, 1 mV / div). The trigger threshold of the oscilloscope 40 is set to 0.5 photons. Then, the waveform acquisition is repeated until statistical noise does not become a problem, and the waveform is averaged to acquire the impulse response function i(t) of the photodetector 12.

[0022] Let s(t) be the time response function of the scintillator 11, p(t) be the function representing the single-photon time resolution of the photodetector 12, and i(t) be the impulse response function of the photodetector 12. The waveform f(t) of the electrical signal output from the photodetector 12 is expressed by the convolution of s(t), p(t), and i(t), as shown in the following equation (1).

[0023]

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[0024] Fig. 3 shows an example of the waveform f(t) of an electrical signal output from a photodetector. Since f(t) is expressed as a convolution of s(t), p(t), and i(t), it is difficult to obtain information on s(t), p(t), and i(t) from f(t) alone.

[0025] FIG. 4 is a diagram showing an example of the time response function s(t) of a scintillator. s(t) includes information on both the rise r and fall d of the scintillation light generated by the interaction with radiation. The rise r and fall d differ depending on the material of the scintillator. The rise r is often 1 ns or less. The fall d is 1 ns or less for BGO (Bi4Ge3O 12 ) is about 300 ns, and LYSO(Lu 2-x Y x In the case of SiO5, it is about 30 to 40 ns. s(t) can generally be expressed as a weighted sum of multiple basic equations (equation (2) below). t0 is the time when radiation interacts with the scintillator.

[0026]

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[0027] FIG. 5 shows an example of a function p(t) that represents the single-photon time resolution of a photodetector. p(t) can be approximately represented by a Gaussian function. The single-photon time resolution (SPTR) is given by the full width at half maximum of this function. The SPTR is about 100 to 300 ps, ​​which is short compared to the response of s(t).

[0028] Figure 6 shows an example of the impulse response function i(t) of a photodetector. i(t) can be expressed as the sum of two types of exponential functions. i(t) is a function specific to the photodetector, does not change for each interaction event of radiation in the scintillator, and can be easily measured. In SiPMs, the fluctuation of i(t) is small.

[0029] The deconvolution calculation unit 15 performs a deconvolution calculation of the impulse response function i(t) of the photodetector 12 with respect to the electrical signal waveform f(t) acquired by the signal waveform acquisition unit 14. Through this calculation, the deconvolution calculation unit 15 can obtain the convolution of s(t) and p(t), as shown in the following equation (3). The convolution of s(t) and p(t) is expressed by the model function shown in the following equation (4). This model function indicates the probability density distribution of the scintillation light component generated in the scintillator and detected by the photodetector. In this equation, σ corresponds to the SPTR of the photodetector. ρ i are the weighting coefficients when s(t) is expressed as a weighted sum of multiple basic equations (above equation (2)). A is a coefficient.

[0030]

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[0031]

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[0032] In addition, when the light generated by the scintillator contains a Cherenkov light component in addition to the scintillation light component, the result obtained by deconvoluting i(t) with f(t) can be expressed by the model function F(t) shown in the following equation (5). This model function F(t) is the probability density distribution F of the scintillation light component generated by the scintillator. s (t) and the probability density distribution F of the Cherenkov light component generated by the scintillator. c (t) and the probability density distribution F s (t) is expressed by the following equation (6). The probability density distribution F of the Cherenkov light component c(t) is expressed by the following equation (7) (see Non-Patent Document 1). In equation (7), erfc is a complementary error function expressed by the following equation (8). ρ3 is a coefficient indicating the proportion of the Cherenkov light component. λ is a parameter representing the interaction depth fluctuation of the photodetector. μ is a parameter representing the expected value of the emission time of Cherenkov light. Δ is a parameter representing the time fluctuation of the photodetector.

[0033]

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[0034]

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[0035]

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[0036]

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[0037] 7 is a diagram showing an example of the result of the deconvolution of i(t) with respect to f(t). The result of the deconvolution based on f(t) and i(t) obtained by measurement is expressed by the model function of the above formula (4) or (5). Therefore, the value of each parameter in these formulas can be obtained by fitting the result of this deconvolution with the model function.

[0038] Next, an example will be described in which an experiment was conducted to confirm the validity of the model function expressed by the above formula (5).

[0039] 8 is a diagram showing a measurement system for acquiring the waveform f(t) of the electrical signal output from the photodetector in the embodiment. The scintillator 11 and the photodetector 12 constitute the radiation detector 10. The scintillator 21 and the photodetector 22 constitute the radiation detector 20. The photodetector 22 is, for example, a PMT or a SiPM.

[0040] The radiation detector 10 and the radiation detector 20 are disposed opposite to each other. An arbitrary RI radiation source (for example 22 A pair of gamma ray photons γ with energy of 511 KeV generated by annihilation of an electron and a positron in the RI source travel in opposite directions and are counted by the radiation detectors 10 and 20.

[0041] When one type of gamma ray photon γ is incident on the scintillator 11 of the radiation detector 10, scintillation light is generated in the scintillator 11. The scintillation light is received by the photodetector 12, and an electrical signal is output from the photodetector 12. The electrical signal output from the photodetector 12 is input to the oscilloscope 40 as a signal whose waveform f(t) is to be acquired.

[0042] When the other gamma ray photon γ is incident on the scintillator 21 of the radiation detector 20, scintillation light is generated in the scintillator 21. The scintillation light is received by the photodetector 22, and an electrical signal is output from the photodetector 22. The electrical signal output from the photodetector 22 is input to the oscilloscope 40 as a signal indicating the timing of coincidence counting of the photon pair.

[0043] In addition, the electrical signal output from the photodetector 22 is also input to a CFD (Constant Fraction Discriminator) circuit 30. An energy discrimination signal indicating whether or not the photon incident on the scintillator 21 of the radiation detector 20 is a gamma ray photon with energy of 511 KeV is output from the CFD circuit 30, and this energy discrimination signal is also input to the oscilloscope 40.

[0044] In the oscilloscope 40, when the energy discrimination signal indicates that the photon incident on the scintillator 21 of the radiation detector 20 is a gamma ray photon with energy of 511 KeV, the waveform f(t) of the electrical signal output from the photodetector 12 is acquired using the time indicated by the timing signal output from the photodetector 22 as the reference time.

[0045] In the measurement system shown in FIG. 8, the waveform f(t) of the electrical signal output from the photodetector 12 is obtained, for example, as follows: The amount of light is reduced to an extent that the saturation effect of the photodetector 12 is not visible, or the detection efficiency is reduced by reducing the applied voltage to the photodetector 12. i(t) is scaled according to the applied voltage. The applied voltage is reduced to an extent that the crosstalk probability of the photodetector 12 can be sufficiently ignored. When the oscilloscope 40 is operated in coincidence counting mode, the timing signal output from the photodetector 22 is delayed from the signal from the photodetector 12 so that the trigger time of the photodetector 12 does not depend on the rise time. The voltage range of the oscilloscope 40 is increased so that the entire waveform of the electrical signal output from the photodetector 12 can be seen.

[0046] In the embodiment, in the measurement system shown in FIG. 8, the scintillator 11 has a size of 3×3×15 mm 3 The BGO was used, and a SiPM (S13360-3050 manufactured by Hamamatsu Photonics K.K.) was used as the photodetector 12. The scintillator 21 was a 3×3×10 mm 3 LYSO was used, and a SiPM (S13360-3075 manufactured by Hamamatsu Photonics K.K.) was used as the photodetector 22. The electrical signal waveform f(t) was acquired 1000 times, and the average of these was calculated.

[0047] Figures 9 and 10 are diagrams showing the results obtained by evaluating the time response of the BGO in the embodiment. Figure 10 shows an enlarged view of the area enclosed by the dashed rectangle in Figure 9. Figures 9 and 10 show the results of the deconvolution of i(t) with f(t) and the fitting results using the model function (Equation (5) above). Figure 10 also shows the probability density distributions of the Cherenkov light component and the scintillation light component in the fitting results. The probability density distributions of the Cherenkov light component and the scintillation light component were obtained by fitting the results of the deconvolution of i(t) with f(t) using the model function F(t) in Equation (5) above.

[0048] The results obtained in this example are as follows. The time response of the BGO was d1 = 339ns, d2 = 24.7ns, and the rise time was r1 = r2 = 0.050ns. The SPTR of the SiPM was 0.498ns. In addition, the ratio of Cherenkov light components to scintillation light components of the light incident on the SiPM (ρ3 / (ρ1 + ρ2)) was 0.53%.

[0049] Fig. 11 shows the results of evaluating the time response of the BGO in the embodiment in comparison with the results of evaluation by the TCSPC method. This figure shows the fitting results in the embodiment and the distribution of the time difference between the signal pulse and the synchronization pulse obtained for each of a large number of photon pairs in the Time-Correlated Single Photon Counting (TCSPC) method. As shown in this figure, the probability density distribution of the scintillation light components obtained by this embodiment can accurately reproduce the evaluation results by the TCSPC method.

[0050] In the TCSPC method, when one of a pair of gamma-ray photons generated by an RI source traveling in opposite directions is incident on a scintillator to be evaluated, the light generated by the scintillator is detected by a photodetector and a signal pulse is output from the photodetector, and the other photon is detected by a reference radiation detector and a synchronization pulse is output from the radiation detector. The time difference between the signal pulse and the synchronization pulse is determined for each of the many photon pairs generated by the RI source, and the time response of the scintillator to be evaluated is evaluated based on a histogram of these time differences.

[0051] FIG. 12 is a diagram showing the results obtained by evaluating the time response of BGO in the embodiment in comparison with the evaluation results described in Non-Patent Document 2. In this figure, the evaluation results described in Non-Patent Document 2 are shown in a histogram, and the fitting results in the embodiment are shown as a solid line superimposed thereon. Non-Patent Document 2 describes that when radiation interacts with a scintillator, the photon detected by the photodetector first has the highest probability of being Cherenkov light. As shown in this figure, the results of this embodiment are in good agreement with the results described in Non-Patent Document 2, and the peak surrounded by the dashed ellipse in the figure is considered to indicate Cherenkov light detection.

[0052] FIG. 13 shows the results of evaluating the coincidence time resolution in the embodiment in comparison with the results of evaluation in the comparative example. (a) shows a histogram of the measurement time difference between the signal from the photodetector 22 and the signal from the photodetector 12 in an experiment equivalent to that in FIG. 8. In this case, the detection time of the photodetector 12 is defined as the time when a certain threshold is exceeded for the waveform after deconvolution as shown in FIG. 7. The certain threshold is, for example, 0.1 photons / 50 ps. (b) shows a histogram in the case where the detection time of the photodetector 12 is determined as the time when a conventional voltage threshold is exceeded. The voltage threshold is, for example, 0.5 mV. The Cherenkov light component buried in the electrical noise is emphasized by the deconvolution, so the coincidence time resolution is 461 ps, which is improved from 660 ps of the conventional method.

[0053] As shown by the results of this embodiment, the waveform obtained by deconvoluting the impulse response function i(t) of the photodetector 12 with the electrical signal waveform f(t) acquired by the signal waveform acquisition unit 14 reflects with high accuracy not only the probability density distribution of the scintillation light components generated by the interaction of radiation in the scintillator 11, but also the probability density distribution of the Cherenkov light components that have a high probability of being generated first as a result of that interaction.

[0054] Therefore, in this embodiment, the deconvolution calculation unit 15 performs deconvolution calculation of the impulse response function i(t) of the photodetector 12 with respect to the electrical signal waveform f(t) acquired by the signal waveform acquisition unit 14. The interaction time detection unit 16 detects, as the interaction time, the time at which the waveform acquired by calculation by the deconvolution calculation unit 15 reaches a threshold value. In this manner, it is possible to improve the time resolution of detection of the interaction time of the radiation in the scintillator.

[0055] 14 is a flowchart illustrating the operation of the radiation detector 10. The operation of the radiation detector 10 includes an impulse response function acquisition step S1, a signal waveform acquisition step S2, a deconvolution calculation step S3, and an interaction time detection step S4.

[0056] In the impulse response function acquisition step S1, the impulse response function i(t) of the photodetector 12 is acquired. If the impulse response function i(t) of the photodetector 12 is known, the impulse response function acquisition step S1 does not need to be performed. When the photodetector 12 receives the light generated by the scintillator 11, in the signal waveform acquisition step S2, the signal waveform acquisition unit 14 acquires the waveform f(t) of the electrical signal output from the photodetector 12.

[0057] In the deconvolution calculation step S3, the deconvolution calculation unit 15 performs deconvolution calculation of the impulse response function i(t) of the photodetector 12 with respect to the electrical signal waveform f(t). In the interaction time detection step S4, the interaction time detection unit 16 detects the interaction time based on the calculation result in the deconvolution calculation step S3. Specifically, the time when the value of the waveform of the calculation result in the deconvolution calculation step S3 reaches a predetermined threshold value (e.g., 0.1 photons / 50 ps) is detected as the interaction time.

[0058] The radiation detector 10 is preferably used in radiation tomography devices such as PET devices and SPECT devices, and can also be preferably used in various devices that detect radiation. FIG. 15 is a diagram showing a schematic configuration of a radiation tomography system 1 including a radiation tomography device 2 and an image processing device 3. The radiation tomography device 2 may be, for example, a PET device in which a plurality of radiation detectors 10 are arranged in a ring shape around a measurement space in which a subject 4 is placed, or a direct positron emission imaging device in which the radiation detectors 10 are arranged opposite each other across a measurement space. The image processing device 3 performs required image processing based on an event in which the radiation detector 10 detects radiation in the radiation tomography device 2, and reconstructs a tomographic image of the subject 4. The processing unit 13 of the radiation detector 10 may be included in the image processing device 3. The radiation tomography system 1 can reconstruct a high-precision tomographic image by using the radiation detector 10 that can improve the time resolution of detection of the interaction time of radiation in the scintillator. [Explanation of symbols]

[0059] Reference Signs List 1...Radiation tomography system, 2...Radiation tomography device, 3...Image processing device, 10...Radiation detector, 11...Scintillator, 12...Photodetector, 13...Processing unit, 14...Signal waveform acquisition unit, 15...Deconvolution calculation unit, 16...Interaction time detection unit, 20...Radiation detector, 21...Scintillator, 22...Photodetector, 30...CFD circuit, 40...Oscilloscope.

Claims

1. a scintillator that generates light by interacting with radiation; a photodetector that receives the light generated by the scintillator and outputs an electrical signal; a processing unit that detects a time when radiation interacts with the scintillator based on a waveform of the electrical signal; Equipped with The processing unit includes: a signal waveform acquiring unit for acquiring a waveform of an electrical signal output from the photodetector; a deconvolution calculation unit that performs a deconvolution calculation of an impulse response function of the photodetector with respect to a waveform of the electrical signal; an interaction time detection unit that detects a time when the waveform obtained by the calculation by the deconvolution calculation unit reaches a threshold as a time when radiation interacts with the scintillator; Including, Radiation detector.

2. the photodetector is a silicon photomultiplier; The radiation detector according to claim 1 .

3. The scintillator is made of BGO. The radiation detector according to claim 1 .

4. The scintillator is PbWO 4 Consists of: The radiation detector according to claim 1 .

5. A radiation tomography apparatus comprising the radiation detector according to any one of claims 1 to 4.

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