- Email: [email protected]
Gas scintillation glass GEM detector for high-resolution X-ray imaging and CT
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Gas scintillation glass GEM detector for high-resolution X-ray imaging and CT
MARK
⁎
T. Fujiwaraa, , Y. Mitsuyab, T. Fushiec, K. Muratad, A. Kawamurad, A. Koishikawad, H. Toyokawaa, H. Takahashie a Research Institute for Measurement and Analytical Instrumentation, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan b Nuclear Professional School, The University of Tokyo, Tokai, Naka, Ibaraki 319-1188, Japan c Radiment Lab. Inc., Setagaya, Tokyo 156-0044, Japan d XIT Co., Naruse, Machida, Tokyo 194-0045, Japan e Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8654, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Glass GEM MPGD X-ray detector CT Gaseous detector
A high-spatial-resolution X-ray-imaging gaseous detector has been developed with a single high-gas-gain glass gas electron multiplier (G-GEM), scintillation gas, and optical camera. High-resolution X-ray imaging of soft elements is performed with a spatial resolution of 281 µm rms and an effective area of 100×100 mm. In addition, high-resolution X-ray 3D computed tomography (CT) is successfully demonstrated with the gaseous detector. It shows high sensitivity to low-energy X-rays, which results in high-contrast radiographs of objects containing elements with low atomic numbers. In addition, the high yield of scintillation light enables fast X-ray imaging, which is an advantage for constructing CT images with low-energy X-rays.
1. Introduction Digital X-ray imaging techniques have developed rapidly and are applied in various detectors such as X-ray image intensifiers, X-ray charged-couple devices (CCDs), complementary metal–oxide–semiconductor (CMOS) sensors, semiconductor detectors, and flat-panel detectors [1–3]. These devices are capable of high spatial resolution, high detection efficiency, and good stability; however, they have limited active area and low sensitivity to low-energy X-rays. On the other hand, large active areas can be easily achieved by gaseous detectors [4]. In addition, recently developed micro-patterned gaseous detectors (MPGDs) enable amplifying small amounts of charge with high spatial resolution. Therefore, low-energy radiation such as βrays and low-energy X-rays can be easily detected with MPGDs. These have been demonstrated with charge-readout electronics [5,6]. However, to realize detectors with high spatial resolution and large sensitive area, huge numbers of readout circuits would be required and the complexity is an issue. Meanwhile, as opposed to the conventional method of reading charges from gaseous, radiography with an optical readout can be performed using scintillation gas [7–12]. With this method, large numbers of channels can be easily read out by an optical camera.
⁎
Radiography can be obtained either integrating the small amount of scintillation light for a long time with a low noise camera (such as a cooled CCD), or very high gas gain is required for a gaseous detector to achieve high enough photon yield. Hence, simultaneously satisfying the requirements of large sensitive area, high spatial resolution, and stable operation is required for gaseous detectors. To meet these requirements, we developed a new type of GEM using photo-etchable glass, which we refer to as the Glass GEM (GGEM) [13–15]. The most attractive parts of the G-GEM for X-ray imaging are high gain and low electrical resistance of the substrate. These two important characteristics of G-GEM are highly advantageous for stable operation in high-count-rate X-ray imaging. In addition, a photon yield is obtained using a single G-GEM and Ar/CF4 as scintillation gas, which is highly advantageous for coupling with optical cameras [16]. In the course of this study, we have developed a two-dimensional digital radiation imager with G-GEM, using Ar/CF4 as the scintillation gas, and an optical camera read-out. We have successfully addressed the traditional issues of gaseous-detector-based X-ray imagers; moreover, high-resolution X-ray imaging is demonstrated and 3D CT is performed.
Corresponding author. E-mail address: [email protected] (T. Fujiwara).
http://dx.doi.org/10.1016/j.nima.2017.01.013 Received 5 October 2016; Received in revised form 4 January 2017; Accepted 4 January 2017 Available online 15 January 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
Fig. 1. Schematic representation of the scintillating G-GEM with mirror and optical camera readout. Gas chamber Glass GEM
Table 1 Experimental condition and setups.
70 mm
Cathode Optical window
Sample object X-ray tube
D D
Gas multiplier
Mirror
X-ray
= 65 ~ 600 mm
Scintillation Gas Sensitive area Camera
530 ~ 700 mm
= 300 ~ 600 mm E = 2 mm Scintillation light Dark box
Lens Optical camera
Fig. 2. Schematic and detection principle of the G-GEM detector. Photoelectrons created by X-rays excitation are multiplied by the G-GEM in the chamber. Scintillation light emitted during the avalanche process is detected with a mirror-lens-optical camera configuration, and forms an X-ray transmission image.
X-ray source
2. Detector design Image enlarging
The design of the scintillation-gas-filled G-GEM detector used in this study is shown in Fig. 1. The high-gain G-GEM is mounted in a chamber filled with scintillating gas. The X-ray entrance was fabricated from Al-coated 25 μm-thick Kapton foil and performs as the cathode. X-rays interact at the 2-mm-thick drift gap. The gas filled chamber is attached to a dark box containing a mirror and optical camera. X-rays enter from the entrance window of the chamber, and ionize the gas. The electrons created in the drift gap move toward the G-GEM holes in which electron avalanches then occur. During the avalanche process, scintillation light is emitted by the de-excitation of electron-excited Ar/ CF4 molecules. This high-yield secondary-gas scintillation light can be easily detected using a mirror–lens–optical–camera system looking from the bottom side of the chamber through a transparent window [17,18]. The mirror also performs the role of preventing the camera from irradiation by X-rays.
1. X-ray Imaging
B. X-ray imaging (enlarging)
C. 3D CT (medicine tablet)
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm Cooled CCD Camera BITRAN BU−52LN 16 bit 4M pixels Nikon 50 mm F1.4
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm CMOS Camera
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm CMOS Camera
BITRAN CS−61 M 12 bit 3M pixels Nikon 50 mm F2.8
Hamamatsu ORCA-Flash 4.0 16 bit 4M pixels Nikon 85 mm F1.4
Micro-focus Xray tube Hamamatsu L9631 20 kV 450 µA ×1
Micro-focus Xray tube Hamamatsu L9631 30 kV 450 µA ×3
Micro-focus Xray tube Hamamatsu L10101 40 kV 200 µA ×4.6c
3.1. X-ray imaging The X-ray imaging capability of the gas scintillation G-GEM detector was investigated using X-ray beams from the micro-focus Xray tube. Fig. 3(b) shows an X-ray transmission image of a flower (Fig. 3(a)) obtained by the detector. The figure is formed by calibrating X-ray transmission image with an image calculation method as follows. The original X-ray image (with sample) is divided by gain correction Xray image (without the sample). This provides a high quality X-ray transmission image with an effective area of 100×100 mm. Overlapping petals can be clearly seen. Another X-ray absorption radiography image was obtained using an X-ray image-enlarging technique with micro-focus equipment. Fig. 4(a) shows the bee used as the sample and the magnified X-ray image of the bee is shown in Fig. 4(b). The radiograph of the bee shows excellent contrast of the soft tissue elements, such that even the thin wings can be observed. These X-ray images demonstrated the excellent imaging capability of this technique. In addition, the high photon yield of the detector means that X-ray images can be obtained very quickly. For example, an X-ray movie is demonstrated at 10 fps with G-GEM detector (Movie 1).
3. Detector performance Fig. 2 shows the X-ray-imaging experimental setup. A micro-focus X-ray tube is used as the X-ray source. The detector is operated in a gas flow mode at a pressure of 1 bar. The sample is placed between the Xray tube and the detector. A digital X-ray radiograph of a sample was simply obtained by taking a photograph with a digital optical camera connected to a computer. Each of the distances of the source-sampledetector-camera were adjusted for respective experiments and those experimental conditions are shown in Table 1.
3.2. Spatial resolution and energy spectrum In order to quantitatively evaluate the spatial resolution of the detector, the edge profiles (arrays of pixel brightness) were extracted 8
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
Fig. 3. (a). Optical image of the flower used as a sample for the X-ray imaging demonstration. Fig. 3(b). X-ray transmission image of the flower taken using the gas scintillation G-GEM
Fig. 4. (a). The bee used as a sample for X-ray magnified imaging technique. (b). X-ray transmission image of a bee.
3.3. 3D CT (medicine tablet) Movie 1. An X-ray movie (10 fps) of a moving plastic toy is demonstrated with G-GEM detector and relatively low energy X-ray sourcem (tube energy: 20 kV, current: 100 μΑ).Supplementary material related to this article can be found online at: http:// dx.doi.org/10.1016/j.nima.2017.01.013.
X-ray 3D CT of a medicine tablet was performed using the scintillating G-GEM detector. The tablet was placed on a rotating stage and 720 X-ray transmission images were taken at a scanning pitch of 0.5° to cover the entire 360°. Each image was recorded with an exposure time of 5 s, with approximately 60 min required to obtain a complete set of images. The 3D image was reconstructed from those images by a filtered back-projection algorithm. The reconstructed 3D images and xyz slices are shown in Fig. 7(b). During this experiment, the gas gain of the G-GEM detector was 2000 and the count rate at the detection point was evaluated as 16,000 cps/mm2. Through the entire measurement, the detector was stable; discharges, electrical charging and gain fluctuations were not observed in our detector. The detector demonstrated sufficient stability to take 720 images, and high resolution and high contrast X-ray 3D CT images of objects containing lowatomic-number elements were successfully reconstructed.
from the edge of the Al plate. The edge profiles were then fitted with the error function shown below:
⎛ x − μ⎞ f (x ) = a∙erf ⎜ ⎟ + c, ⎝ 2σ ⎠
(1)
where x is the position, μ is the expectation of the edge position, σ is the standard deviation, a is the scaling factor, and c is the offset. The edge profiles and fitted curves are shown in Fig. 5. The spatial resolution was σ=281 µm rms. Fig. 6 shows the energy spectra obtained with our GGEM detector of various energies of X-ray tube. The energy was calibrated with a 55Fe source (5.9 keV). Most of the detected X-rays were lower than 5 keV. 9
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
4. Summary A high-resolution X-ray imager was developed with scintillating gas and G-GEM and optical readout. A large effective area detector with 100×100 mm size was successfully demonstrated with which we obtained high-resolution X-ray transmission images and 3D CT images. Owing to the high gas gain, the detector shows high sensitivity to low-energy X-rays, which results in high-contrast radiographs for samples containing elements with low atomic numbers. In addition, high scintillation light yield enables fast X-ray imaging which is advantageous for CT imaging with low-energy X-rays. The high photon emission yield from the Ar/CF4 gas mixture means that the scintillation light from the G-GEM can be easily detected with a commercially available optical camera coupled to a conventional lens system. Although the data acquisition rate and dynamic range is restricted to the specification of the optical camera, the optical readout design is very simple and can image a large area with a very large number of readout channels. In addition, we have made a significant progress in improving the imaging performance of gaseous detectors: high resolution imaging, large effective area and contrast resolution in soft x-rays. Moreover, owing to the high charge multiplication ability, this gaseous detector is effective to detect low-energy deposit radiation and is useful for obtaining high contrast images of low-atomic-number materials using low-energy X-rays. We conclude that the gaseous detector based imaging/CT system is attractive for visualizing low-atomic-number objects such as medicine tablets. An imager with a size above 200×200 mm is required for practical application including CT. To satisfy this requirement, the development of a large size scintillation gaseous detector based on G-GEM fabrication technology is under way.
Fig. 5. Fitted result of an edge in the x-direction using the error function method.
5.9 keV peak
1.0 0.9 0.8 0.7 0.6 0.5
Ar escape peak 0.4 0.3 0.2 0.1
Acknowledgments 0.0
0
5,000
10,000
15,000
20,000
25,000
30,000
This work was supported by (i) JSPS KAKENHI Grant Number 15K18316 and (ii) the Adaptable and Seamless Technology Transfer Program through Target-driven R & D (A-STEP) Japan Science and Technology Agency (JST).
Fig. 6. Spectra of X-ray tubes with various energies detected using Ar/CF4 G-GEM.
Fig. 7. a Photograph of micro-sphere medicine tablet. b Reconstructed 3D CT image and each XYZ slices of medicine tablet taken with Glass GEM and gas scintillation. CT is reconstructed with standard FBP algorithm. Micro-spheres in the tab can be well seen.
10
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
References [1] K.G. Vosburgh, R.K. Swank, J.M. Houston, X-ray Image Intensifiers, Elsevier, 1977, pp. 205–244. http://dx.doi.org/10.1016/S0065-2539(08)60757-X. [2] J. Bian, J.H. Siewerdsen, X. Han, E.Y. Sidky, J.L. Prince, C.A. Pelizzari, et al., Evaluation of sparse-view reconstruction from flat-panel-detector cone-beam CT, Phys. Med. Biol. 55 (2010) 6575–6599. http://dx.doi.org/10.1088/0031-9155/55/ 22/001. [3] P.R. Granfors, R. Aufrichtig, Performance of a 41×41 cm amorphous silicon flat panel X-ray detector for radiographic imaging applications, Med. Phys. 27 (2000) 1324. http://dx.doi.org/10.1118/1.599010. [4] L. Coelho, J. Lopes, D.S. Covita, Xenon GPSC high-pressure operation with largearea avalanche photodiode readout, Nucl. Instr. Meth. Phys. Res. A (2007). http:// dx.doi.org/10.1016/j.nima.2007.02.111. [5] F. Sauli, The gas electron multiplier (GEM): operating principles and applications, Nucl. Instr. Meth. Phys. Res. A 805 (2016) 2–24. http://dx.doi.org/10.1016/ j.nima.2015.07.060. [6] F. Sauli, Gas detectors: achievements and trends, Nucl. Instr. Meth. Phys. Res. A 461 (2001) 47–54. http://dx.doi.org/10.1016/S0168-9002(00)01165-7. [7] F. Fraga, L. Maragato, S. Fetal, M. Fraga, R. Marques, A. Policarpo, Luminescence and imaging with gas electron multipliers, Nucl. Instr. Meth. Phys. Res. A 513 (2003) 379–387. http://dx.doi.org/10.1016/j.nima.2003.08.066. [8] F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, M.M.F.R. Fraga, R. Ferreira Marques, A.J.P.L. Policarpo, Optical readout of GEMs, Nucl. Instr. Meth. Phys. Res. A 471 (2001) 125–130. http://dx.doi.org/10.1016/S0168-9002(01)00972-X. [9] F.A.F. Fraga, S.T.G. Fetal, L.M.S. Margato, I. Ivaniouchenkov, R.F. Marques, A.J.P.L. Policarpo, Imaging detectors based on the gas electron multiplier scintillation light, in: Nssmic-99, IEEE (1999) 829–833. http://dx.doi.org/ 10.1109/NSSMIC.1999.845794. [10] S. Fetal, C.W.E. van Eijk, F. Fraga, J. de Haas, R. Kreuger, T.L. van Vuure, et al.,
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
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Dose imaging in radiotherapy with an Ar–CF4 filled scintillating GEM, Nucl. Instr. Meth. Phys. Res. A 513 (2003) 42–46. http://dx.doi.org/10.1016/S0168-9002(03) 02133-8. F. Tokanai, T. Atsumi, T. Endo, Y. Fujita, Y. Ohishi, T. Okada, et al., Developments of optical imaging capillary plate gas detector, Nucl. Instr. Meth. Phys. Res. A 567 (2006) 376–380. http://dx.doi.org/10.1016/j.nima.2006.05.148. E. Seravalli, J. Hendrikse, J. Huizenga, R. Kreuger, J.M. Schippers, A. Simon, et al., First results of a scintillating GEM detector for 2-D dosimetry in an alpha beam, IEEE Trans. Nucl. Sci. 54 (2007) 1271–1278. http://dx.doi.org/10.1109/ TNS.2007.903179. H. Takahashi, Y. Mitsuya, T. Fujiwara, T. Fushie, Development of a glass GEM, Nucl. Instr. Meth. Phys. Res. A 724 (2013) 1–4. http://dx.doi.org/10.1016/ j.nima.2013.04.089. T. Fujiwara, Y. Mitsuya, H. Takahashi, T. Fushie, S. Kishimito, B. Guerard, et al., The performance of glass GEM, J. Inst. 9 (2014). http://dx.doi.org/10.1088/17480221/9/11/P11007. Y. Mitsuya, T. Fujiwara, T. Fushie, T. Maekawa, Development of large-area glass GEM, Nucl. Instr. Meth. Phys. Res. A 795 (2015) 156–169. http://dx.doi.org/ 10.1016/j.nima.2015.05.039. T. Fujiwara, Y. Mitsuya, T. Yanagida, T. Saito, H. Toyokawa, H. Takahashi, Highphoton-yield scintillation detector with Ar/CF4 and glass gas electron multiplier, Jpn. J. Appl. Phys. 55 (2016) 106401. http://dx.doi.org/10.7567/JJAP.55.106401. T. Fujiwara, S. Tanaka, Y. Mitsuya, H. Takahashi, K. Tagi, J. Kusano, et al., Development of a scintillating G-GEM detector for a 6-MeV X-band linac for medical applications, J. Inst. 8 (2013). http://dx.doi.org/10.1088/1748-0221/8/ 12/C12020. T. Fujiwara, U. Bautista, Y. Mitsuya, H. Takahashi, N.L. Yamada, Y. Otake, et al., Microstructured boron foil scintillating G-GEM detector for neutron imaging, Nucl. Instr. Meth. Phys. Res. A (2016). http://dx.doi.org/10.1016/j.nima.2016.09.005.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Gas scintillation glass GEM detector for high-resolution X-ray imaging and CT
MARK
⁎
T. Fujiwaraa, , Y. Mitsuyab, T. Fushiec, K. Muratad, A. Kawamurad, A. Koishikawad, H. Toyokawaa, H. Takahashie a Research Institute for Measurement and Analytical Instrumentation, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan b Nuclear Professional School, The University of Tokyo, Tokai, Naka, Ibaraki 319-1188, Japan c Radiment Lab. Inc., Setagaya, Tokyo 156-0044, Japan d XIT Co., Naruse, Machida, Tokyo 194-0045, Japan e Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8654, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Glass GEM MPGD X-ray detector CT Gaseous detector
A high-spatial-resolution X-ray-imaging gaseous detector has been developed with a single high-gas-gain glass gas electron multiplier (G-GEM), scintillation gas, and optical camera. High-resolution X-ray imaging of soft elements is performed with a spatial resolution of 281 µm rms and an effective area of 100×100 mm. In addition, high-resolution X-ray 3D computed tomography (CT) is successfully demonstrated with the gaseous detector. It shows high sensitivity to low-energy X-rays, which results in high-contrast radiographs of objects containing elements with low atomic numbers. In addition, the high yield of scintillation light enables fast X-ray imaging, which is an advantage for constructing CT images with low-energy X-rays.
1. Introduction Digital X-ray imaging techniques have developed rapidly and are applied in various detectors such as X-ray image intensifiers, X-ray charged-couple devices (CCDs), complementary metal–oxide–semiconductor (CMOS) sensors, semiconductor detectors, and flat-panel detectors [1–3]. These devices are capable of high spatial resolution, high detection efficiency, and good stability; however, they have limited active area and low sensitivity to low-energy X-rays. On the other hand, large active areas can be easily achieved by gaseous detectors [4]. In addition, recently developed micro-patterned gaseous detectors (MPGDs) enable amplifying small amounts of charge with high spatial resolution. Therefore, low-energy radiation such as βrays and low-energy X-rays can be easily detected with MPGDs. These have been demonstrated with charge-readout electronics [5,6]. However, to realize detectors with high spatial resolution and large sensitive area, huge numbers of readout circuits would be required and the complexity is an issue. Meanwhile, as opposed to the conventional method of reading charges from gaseous, radiography with an optical readout can be performed using scintillation gas [7–12]. With this method, large numbers of channels can be easily read out by an optical camera.
⁎
Radiography can be obtained either integrating the small amount of scintillation light for a long time with a low noise camera (such as a cooled CCD), or very high gas gain is required for a gaseous detector to achieve high enough photon yield. Hence, simultaneously satisfying the requirements of large sensitive area, high spatial resolution, and stable operation is required for gaseous detectors. To meet these requirements, we developed a new type of GEM using photo-etchable glass, which we refer to as the Glass GEM (GGEM) [13–15]. The most attractive parts of the G-GEM for X-ray imaging are high gain and low electrical resistance of the substrate. These two important characteristics of G-GEM are highly advantageous for stable operation in high-count-rate X-ray imaging. In addition, a photon yield is obtained using a single G-GEM and Ar/CF4 as scintillation gas, which is highly advantageous for coupling with optical cameras [16]. In the course of this study, we have developed a two-dimensional digital radiation imager with G-GEM, using Ar/CF4 as the scintillation gas, and an optical camera read-out. We have successfully addressed the traditional issues of gaseous-detector-based X-ray imagers; moreover, high-resolution X-ray imaging is demonstrated and 3D CT is performed.
Corresponding author. E-mail address: [email protected] (T. Fujiwara).
http://dx.doi.org/10.1016/j.nima.2017.01.013 Received 5 October 2016; Received in revised form 4 January 2017; Accepted 4 January 2017 Available online 15 January 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
Fig. 1. Schematic representation of the scintillating G-GEM with mirror and optical camera readout. Gas chamber Glass GEM
Table 1 Experimental condition and setups.
70 mm
Cathode Optical window
Sample object X-ray tube
D D
Gas multiplier
Mirror
X-ray
= 65 ~ 600 mm
Scintillation Gas Sensitive area Camera
530 ~ 700 mm
= 300 ~ 600 mm E = 2 mm Scintillation light Dark box
Lens Optical camera
Fig. 2. Schematic and detection principle of the G-GEM detector. Photoelectrons created by X-rays excitation are multiplied by the G-GEM in the chamber. Scintillation light emitted during the avalanche process is detected with a mirror-lens-optical camera configuration, and forms an X-ray transmission image.
X-ray source
2. Detector design Image enlarging
The design of the scintillation-gas-filled G-GEM detector used in this study is shown in Fig. 1. The high-gain G-GEM is mounted in a chamber filled with scintillating gas. The X-ray entrance was fabricated from Al-coated 25 μm-thick Kapton foil and performs as the cathode. X-rays interact at the 2-mm-thick drift gap. The gas filled chamber is attached to a dark box containing a mirror and optical camera. X-rays enter from the entrance window of the chamber, and ionize the gas. The electrons created in the drift gap move toward the G-GEM holes in which electron avalanches then occur. During the avalanche process, scintillation light is emitted by the de-excitation of electron-excited Ar/ CF4 molecules. This high-yield secondary-gas scintillation light can be easily detected using a mirror–lens–optical–camera system looking from the bottom side of the chamber through a transparent window [17,18]. The mirror also performs the role of preventing the camera from irradiation by X-rays.
1. X-ray Imaging
B. X-ray imaging (enlarging)
C. 3D CT (medicine tablet)
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm Cooled CCD Camera BITRAN BU−52LN 16 bit 4M pixels Nikon 50 mm F1.4
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm CMOS Camera
Glass GEM (ϕ=180 µm hole, 280 µm pitch, 680 µm thick) Ar/CF4 (90:10) 100×100 mm CMOS Camera
BITRAN CS−61 M 12 bit 3M pixels Nikon 50 mm F2.8
Hamamatsu ORCA-Flash 4.0 16 bit 4M pixels Nikon 85 mm F1.4
Micro-focus Xray tube Hamamatsu L9631 20 kV 450 µA ×1
Micro-focus Xray tube Hamamatsu L9631 30 kV 450 µA ×3
Micro-focus Xray tube Hamamatsu L10101 40 kV 200 µA ×4.6c
3.1. X-ray imaging The X-ray imaging capability of the gas scintillation G-GEM detector was investigated using X-ray beams from the micro-focus Xray tube. Fig. 3(b) shows an X-ray transmission image of a flower (Fig. 3(a)) obtained by the detector. The figure is formed by calibrating X-ray transmission image with an image calculation method as follows. The original X-ray image (with sample) is divided by gain correction Xray image (without the sample). This provides a high quality X-ray transmission image with an effective area of 100×100 mm. Overlapping petals can be clearly seen. Another X-ray absorption radiography image was obtained using an X-ray image-enlarging technique with micro-focus equipment. Fig. 4(a) shows the bee used as the sample and the magnified X-ray image of the bee is shown in Fig. 4(b). The radiograph of the bee shows excellent contrast of the soft tissue elements, such that even the thin wings can be observed. These X-ray images demonstrated the excellent imaging capability of this technique. In addition, the high photon yield of the detector means that X-ray images can be obtained very quickly. For example, an X-ray movie is demonstrated at 10 fps with G-GEM detector (Movie 1).
3. Detector performance Fig. 2 shows the X-ray-imaging experimental setup. A micro-focus X-ray tube is used as the X-ray source. The detector is operated in a gas flow mode at a pressure of 1 bar. The sample is placed between the Xray tube and the detector. A digital X-ray radiograph of a sample was simply obtained by taking a photograph with a digital optical camera connected to a computer. Each of the distances of the source-sampledetector-camera were adjusted for respective experiments and those experimental conditions are shown in Table 1.
3.2. Spatial resolution and energy spectrum In order to quantitatively evaluate the spatial resolution of the detector, the edge profiles (arrays of pixel brightness) were extracted 8
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
Fig. 3. (a). Optical image of the flower used as a sample for the X-ray imaging demonstration. Fig. 3(b). X-ray transmission image of the flower taken using the gas scintillation G-GEM
Fig. 4. (a). The bee used as a sample for X-ray magnified imaging technique. (b). X-ray transmission image of a bee.
3.3. 3D CT (medicine tablet) Movie 1. An X-ray movie (10 fps) of a moving plastic toy is demonstrated with G-GEM detector and relatively low energy X-ray sourcem (tube energy: 20 kV, current: 100 μΑ).Supplementary material related to this article can be found online at: http:// dx.doi.org/10.1016/j.nima.2017.01.013.
X-ray 3D CT of a medicine tablet was performed using the scintillating G-GEM detector. The tablet was placed on a rotating stage and 720 X-ray transmission images were taken at a scanning pitch of 0.5° to cover the entire 360°. Each image was recorded with an exposure time of 5 s, with approximately 60 min required to obtain a complete set of images. The 3D image was reconstructed from those images by a filtered back-projection algorithm. The reconstructed 3D images and xyz slices are shown in Fig. 7(b). During this experiment, the gas gain of the G-GEM detector was 2000 and the count rate at the detection point was evaluated as 16,000 cps/mm2. Through the entire measurement, the detector was stable; discharges, electrical charging and gain fluctuations were not observed in our detector. The detector demonstrated sufficient stability to take 720 images, and high resolution and high contrast X-ray 3D CT images of objects containing lowatomic-number elements were successfully reconstructed.
from the edge of the Al plate. The edge profiles were then fitted with the error function shown below:
⎛ x − μ⎞ f (x ) = a∙erf ⎜ ⎟ + c, ⎝ 2σ ⎠
(1)
where x is the position, μ is the expectation of the edge position, σ is the standard deviation, a is the scaling factor, and c is the offset. The edge profiles and fitted curves are shown in Fig. 5. The spatial resolution was σ=281 µm rms. Fig. 6 shows the energy spectra obtained with our GGEM detector of various energies of X-ray tube. The energy was calibrated with a 55Fe source (5.9 keV). Most of the detected X-rays were lower than 5 keV. 9
Nuclear Instruments and Methods in Physics Research A 850 (2017) 7–11
T. Fujiwara et al.
4. Summary A high-resolution X-ray imager was developed with scintillating gas and G-GEM and optical readout. A large effective area detector with 100×100 mm size was successfully demonstrated with which we obtained high-resolution X-ray transmission images and 3D CT images. Owing to the high gas gain, the detector shows high sensitivity to low-energy X-rays, which results in high-contrast radiographs for samples containing elements with low atomic numbers. In addition, high scintillation light yield enables fast X-ray imaging which is advantageous for CT imaging with low-energy X-rays. The high photon emission yield from the Ar/CF4 gas mixture means that the scintillation light from the G-GEM can be easily detected with a commercially available optical camera coupled to a conventional lens system. Although the data acquisition rate and dynamic range is restricted to the specification of the optical camera, the optical readout design is very simple and can image a large area with a very large number of readout channels. In addition, we have made a significant progress in improving the imaging performance of gaseous detectors: high resolution imaging, large effective area and contrast resolution in soft x-rays. Moreover, owing to the high charge multiplication ability, this gaseous detector is effective to detect low-energy deposit radiation and is useful for obtaining high contrast images of low-atomic-number materials using low-energy X-rays. We conclude that the gaseous detector based imaging/CT system is attractive for visualizing low-atomic-number objects such as medicine tablets. An imager with a size above 200×200 mm is required for practical application including CT. To satisfy this requirement, the development of a large size scintillation gaseous detector based on G-GEM fabrication technology is under way.
Fig. 5. Fitted result of an edge in the x-direction using the error function method.
5.9 keV peak
1.0 0.9 0.8 0.7 0.6 0.5
Ar escape peak 0.4 0.3 0.2 0.1
Acknowledgments 0.0
0
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This work was supported by (i) JSPS KAKENHI Grant Number 15K18316 and (ii) the Adaptable and Seamless Technology Transfer Program through Target-driven R & D (A-STEP) Japan Science and Technology Agency (JST).
Fig. 6. Spectra of X-ray tubes with various energies detected using Ar/CF4 G-GEM.
Fig. 7. a Photograph of micro-sphere medicine tablet. b Reconstructed 3D CT image and each XYZ slices of medicine tablet taken with Glass GEM and gas scintillation. CT is reconstructed with standard FBP algorithm. Micro-spheres in the tab can be well seen.
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