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3D-CT imaging using characteristic X-rays and visible lights produced by ion micro-beam bombardment
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 726–729 www.elsevier.com/locate/nimb
3D-CT imaging using characteristic X-rays and visible lights produced by ion micro-beam bombardment K. Ishii *, S. Matsuyama, H. Yamazaki, Y. Watanabe, Y. Kawamura, T. Yamaguchi, G. Momose, Y. Kikuchi, A. Terakawa, W. Galster Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Aoba-Ku, Aramaki-Aza-Aoba 6-6, Sendai 980-8579, Japan Available online 5 June 2006
Abstract We improved the spatial resolution of a 3D-CT imaging system consisting of a micro-beam and an X-ray CCD camera of 1 mega pixels (Hamamatsu photonics C8800X), whose element size is 8 lm · 8 lm providing an image size of 8 mm · 8 mm. A small ant of 6 mm body length was placed in a small tube, rotated by a stepping motor, and a spatial resolution of 4 lm for X-ray micron-CT using characteristic Ti-K-X-rays (4.558 keV) produced by 3 MeV proton micro-beams was obtained. We applied the X-ray micron-CT to a small ant’s head and obtained the fine structures of the head’s interior. Because the CCD is sensitive to visible light, we also examined the capability of light micron-CT using visible red light from an Al2O3(Cr) ruby scintillator and applied the micron-CT to a small red tick. Though the red tick is highly transparent to Ti-K-X-rays, visible red light does not penetrate through the red tick. The most serious problem was dispersion of lights due to Thomson scattering resulting in obscure projection images. 2006 Elsevier B.V. All rights reserved. PACS: 87.59.e; 87.59.Fm; 41.50.+h Keywords: 3D-CT; PIXE; Monochromatic X-ray point source; X-ray CCD camera; Micron-CT; Ion micro-beam
1. Introduction When a metal target is bombarded with ion beams, characteristic X-rays of the metal element and continuous X-rays are produced. The intensity of characteristic X-rays is about three orders of magnitude larger than the one of continuous X-rays because nuclear bremsstrahlung cross sections are much smaller than inner shell ionization cross sections [1]. Therefore, the characteristic X-rays produced by ion beam bombardment can be used as quasi-monochromatic X-ray source. By using ion micro-beams, a point source of quasi-monochromatic X-rays is provided. Placing a small object close to the X-ray point source and detecting the X-rays passing through the object with a X-ray detector *
Corresponding author. Tel./fax: +81 22 795 7931. E-mail address: [email protected] (K. Ishii).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.127
array, then rotating the object, a three dimensional image reflecting the photo-ionization cross sections of elements in the object is obtained. Based on these thoughts, we have developed a 3D-CT imaging system consisting of a microbeam and an X-ray CCD camera [2], named X-ray micronCT. The spatial resolution of this micron-CT depends strongly on the geometrical relation between the positions of point X-ray source, sample and CCD camera. Here, we report an improvement of spatial resolution of micron-CT and an application to the 3D image of a small ant’s head. The X-ray CCD camera is sensitive not only to X-rays but also to visible light. Therefore, with luminiferous material (i.e. scintillator) as target, 3D imaging using light transmission data can be achieved. We name it Light micron-CT and report on the performance of Light micron-CT for 3D imaging a small red tick whose body is semi-transparent.
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
2. Micron-CT system using ion micro-beams The micron-CT consists of an accelerator, a micro-beam system, and a target chamber system with an X-ray CCD camera, whose details are shown in Ref. [2]. The microbeam system was newly developed for this purpose [3,4]. The typical performance of our micro-beam system is a beam spot size of 1 lm in diameter with a 40 pA beam of 3 MeV protons [4]. The focal field of depth is about 100 lm for a micro-beam spot of 1 lm. The principle of micron-CT is shown in Fig. 1, where Xray point source and CCD are fixed and the object is rotated. In this system, 2D transmission data of X-rays are obtained by the X-ray cone beam and complete data for 2D image reconstruction are obtained only in the plane where the object’s axis of rotation is vertical and the X-ray point source is contained. Therefore, a usual image reconstruction method (Filtered Back Projection Method (FBP)) is applicable to this vertical plane. By using this vertical plane image and an iteration method, we can reconstruct images in other planes. We adopted the Simple gradient algorithm [5], which belongs to the group of modified Maximum Likelihood-Expectation Maximization methods [6]. A target producing X-rays is set at 30 with respect to the horizontal axis and the bombarding micro-beam produces a X-ray cone beam. The point X-ray source is positioned 0.4 mm above the bottom of the CCD. The axis of rotation is supported by two bearings and is rotated with an accuracy of ±0.7 degrees by a stepping motor. For the micron-CT, we adopted a CCD of Hamamatsu photonics Co. Ltd. (C8800X) with a pixel size of
727
8 lm · 8 lm and a number of pixels 1000 · 1000. The data transfer mode employed was frame transfer (30 frames/ sec). The CCD starts exposure by an outside trigger and it stops when beam currents have been accumulated up to a constant value; readout of data is finished while the target is rotated. A 100 lm Mylar foil was placed in front of the CCD thus preventing scattered protons from entering. We measured the detection efficiency of the CCD for Sn-L-X-rays (3.44 keV), Ti-K-X-rays (4.558 keV) and CuK-X-rays (8.048 keV) produced by 3 MeV proton bombardment, which were 0.33, 0.15 and 0.05, respectively. Considering attenuation in the 100 lm Mylar foil, we adopted Ti as the best X-ray source target element. 3. Spatial resolution of micron-CT The spatial resolution dD of the micron-CT is given by [2], dD ¼
LS D; Ld
ð1Þ
where Ld and LS are shown in Fig. 1 and D is the pixel size of the CCD. In our case, Ld = 8.8 mm and D = 8 lm. In accordance with the above equation, the spatial resolution of the micron-CT can be improved by shortening LS. We examined the spatial resolution as a function of LS using a Tungsten wire of 20 lm in diameter. The sampling number with respect to projection angle was 250 and the beam spot size was 1.5 lm in diameter. The spatial resolutions of the micron-CT are shown in Fig. 2 as a function of LS. It is apparent that the spatial resolution is improved by decreasing LS as shown in Fig. 2. The experimental resolutions are worse than those calculated by Eq. (1). The difference is mainly due to uncertainties in the accuracy of positioning and rotating the sample axis.
14
Exp.
Spatial resolution [µm]
12 10 8 6 Cal.
4 2 0
Fig. 1. Principle of micron-CT using PIXE with micro-beams.
0
1000
2000
3000 Ls [µm]
4000
5000
6000
Fig. 2. Spatial resolution of micron-CT as a function of the distance between the center of rotation of the sample stage and the X-ray point source.
728
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
4. Application of X-ray micron-CT to the head of a small ant
5. Application of Light micron-CT to a small red tick
The head of a small ant was placed in a polyimide tube whose outside diameter was 1000 lm and its wall thickness 25 lm (see Fig. 3(a)). LS was set at 2.0 mm, the beam spot size was 1.5 lm in diameter and beam current was 300 pA. The measurement time was 90 min and 3D image reconstruction for 256 · 256 · 182 voxels was performed with unit pixels of 5 · 5 · 5 lm3. Fig. 3(b) shows the cross sectional image of the ant’s head obtained by the FBP method. This image just corresponds to the cut plane along by a solid line in Fig. 3(a). It is apparent that the image is well reconstructed and details of the head’s interior are seen. Fig. 4 shows the 3D images obtained by iterations. The left side shows a stereoscopic image and the right side shows horizontal cross sectional views.
In general, blue visible light is strongly absorbed by materials, whereas red light penetrates deeper. For this reason, we tested Al2O3(Cr) ruby scintillators as a point light source for Light micron-CT, as the An Al2O3(Cr) ruby scintillator emits red light. Because this scintillator is an insulator, it is charged up by ion beam irradiation. To avoid beam charge up, the surface of the scintillator is coated with a thin gold layer. A small red tick (0.5 mm) was used as test sample (see Fig. 5(a)) and the intensity of red light was monitored with a Si-PIN photodiode detector at each projection angle. The projection images of the red tick by X-rays and light are shown in Fig. 5(b) and (c), respectively. A clear image was obtained by Ti X-rays. In the case of red light imaging,
Fig. 3. Picture of small ant’s head (a) and cross sectional view of ant’s head obtained by the FBP method (b).
Fig. 4. Stereoscopic image and horizontal cross sectional views of small ant’s head.
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
729
Fig. 5. Photograph of small red tick (a), projection images of small red tick by X-ray (b) and lights (c). (For interpretation of colour in this figure, the reader is referred to the web version of this article.)
(1 mm) in detail. The X-ray micron-CT will be tested in in vivo experiments and may be applicable to a wide range of research in biology and medicine. The possibility of using less destructive light micron-CT with red light produced by an Al2O3(Cr) scintillator was explored and applied to a red tick. The projection data faded compared to X-rays and the tick’s size was expanded by Thomson scattering. This effect strongly degrades the capability of light micron-CT using a CCD camera. Fig. 6. Horizontal cross sectional views of small red tick with X-ray micron-CT (a) and light micron-CT (b). (For interpretation of colour in this figure, the reader is referred to the web version of this article.)
the tick’s shape fades and expands, and internal details are not observed. This is the consequence of scattering of light by the object and it causes a serious problem to the image reconstruction. Fig. 6 shows the CT images obtained by the FBP method. The spherical structure in Fig. 6(b) is due to noises of the CCD. The interior of the red tick is not observed in the case of Light micron-CT because light does not penetrate the tick’s body. Though an image of the cross sectional view was obtained, because the red tick is transparent to Ti K-X-rays, the structure of the interior is not visible in contrast to the small ant’s head. 6. Conclusion The spatial resolution of micron-CT was improved by shortening the distance between the X-ray point source and the object, and allowed to observe the small ant’s head
Acknowledgement This study was supported by a Grant-in-Aid for Scientific Research (S) No. 13852017 (K. Ishii) of the Ministry of Education, Culture, Science, Sports and Technology. References [1] K. Ishii, S. Morita, Int. J. PIXE 1 (1992) 1. [2] K. Ishii, S. Matsuyama, H. Yamazaki, Y. Watanabe, T. Yamaguchi, G. Momose, T. Amartaivan, A. Suzuki, Y. Kikuchi, W. Galster, Int. J. PIXE 15 (2005) 111. [3] S. Matsuyama, K. Ishii, H. Yamazaki, R. Sakamoto, M. Fujisawa, Ts. Amartaivan, Y. Oishi, M. Rodriguez, A. Suzuki, T. Kamiya, M. Oikawa, K. Arakawa, N. Matsumoto, Nucl. Instr. and Meth. In Phy. Res. B 210 (2003) 117. [4] S. Matsuyama, K. Ishii, H. Yamazaki, Y. Barbotteau, Ts. Amartaibvan, D. Izukawa, K. Hotta, K. Mizuma, S. Abe, Y. Ohishi, M. Rodrigeez, A. Suzuki, R. Sakamoto, M. Fujisawa, T. Kamiya, M. Oikawa, Int. J. PIXE 14 (2004) 1. [5] K. Lange, J.A. Fessler, IEEE Trans. Image Process. 14 (1995) 1430. [6] L.A. Shepp, Y. Vardi, IEEE Trans. Med. Imag. MI-1 (1982) 113.
Nuclear Instruments and Methods in Physics Research B 249 (2006) 726–729 www.elsevier.com/locate/nimb
3D-CT imaging using characteristic X-rays and visible lights produced by ion micro-beam bombardment K. Ishii *, S. Matsuyama, H. Yamazaki, Y. Watanabe, Y. Kawamura, T. Yamaguchi, G. Momose, Y. Kikuchi, A. Terakawa, W. Galster Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Aoba-Ku, Aramaki-Aza-Aoba 6-6, Sendai 980-8579, Japan Available online 5 June 2006
Abstract We improved the spatial resolution of a 3D-CT imaging system consisting of a micro-beam and an X-ray CCD camera of 1 mega pixels (Hamamatsu photonics C8800X), whose element size is 8 lm · 8 lm providing an image size of 8 mm · 8 mm. A small ant of 6 mm body length was placed in a small tube, rotated by a stepping motor, and a spatial resolution of 4 lm for X-ray micron-CT using characteristic Ti-K-X-rays (4.558 keV) produced by 3 MeV proton micro-beams was obtained. We applied the X-ray micron-CT to a small ant’s head and obtained the fine structures of the head’s interior. Because the CCD is sensitive to visible light, we also examined the capability of light micron-CT using visible red light from an Al2O3(Cr) ruby scintillator and applied the micron-CT to a small red tick. Though the red tick is highly transparent to Ti-K-X-rays, visible red light does not penetrate through the red tick. The most serious problem was dispersion of lights due to Thomson scattering resulting in obscure projection images. 2006 Elsevier B.V. All rights reserved. PACS: 87.59.e; 87.59.Fm; 41.50.+h Keywords: 3D-CT; PIXE; Monochromatic X-ray point source; X-ray CCD camera; Micron-CT; Ion micro-beam
1. Introduction When a metal target is bombarded with ion beams, characteristic X-rays of the metal element and continuous X-rays are produced. The intensity of characteristic X-rays is about three orders of magnitude larger than the one of continuous X-rays because nuclear bremsstrahlung cross sections are much smaller than inner shell ionization cross sections [1]. Therefore, the characteristic X-rays produced by ion beam bombardment can be used as quasi-monochromatic X-ray source. By using ion micro-beams, a point source of quasi-monochromatic X-rays is provided. Placing a small object close to the X-ray point source and detecting the X-rays passing through the object with a X-ray detector *
Corresponding author. Tel./fax: +81 22 795 7931. E-mail address: [email protected] (K. Ishii).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.127
array, then rotating the object, a three dimensional image reflecting the photo-ionization cross sections of elements in the object is obtained. Based on these thoughts, we have developed a 3D-CT imaging system consisting of a microbeam and an X-ray CCD camera [2], named X-ray micronCT. The spatial resolution of this micron-CT depends strongly on the geometrical relation between the positions of point X-ray source, sample and CCD camera. Here, we report an improvement of spatial resolution of micron-CT and an application to the 3D image of a small ant’s head. The X-ray CCD camera is sensitive not only to X-rays but also to visible light. Therefore, with luminiferous material (i.e. scintillator) as target, 3D imaging using light transmission data can be achieved. We name it Light micron-CT and report on the performance of Light micron-CT for 3D imaging a small red tick whose body is semi-transparent.
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
2. Micron-CT system using ion micro-beams The micron-CT consists of an accelerator, a micro-beam system, and a target chamber system with an X-ray CCD camera, whose details are shown in Ref. [2]. The microbeam system was newly developed for this purpose [3,4]. The typical performance of our micro-beam system is a beam spot size of 1 lm in diameter with a 40 pA beam of 3 MeV protons [4]. The focal field of depth is about 100 lm for a micro-beam spot of 1 lm. The principle of micron-CT is shown in Fig. 1, where Xray point source and CCD are fixed and the object is rotated. In this system, 2D transmission data of X-rays are obtained by the X-ray cone beam and complete data for 2D image reconstruction are obtained only in the plane where the object’s axis of rotation is vertical and the X-ray point source is contained. Therefore, a usual image reconstruction method (Filtered Back Projection Method (FBP)) is applicable to this vertical plane. By using this vertical plane image and an iteration method, we can reconstruct images in other planes. We adopted the Simple gradient algorithm [5], which belongs to the group of modified Maximum Likelihood-Expectation Maximization methods [6]. A target producing X-rays is set at 30 with respect to the horizontal axis and the bombarding micro-beam produces a X-ray cone beam. The point X-ray source is positioned 0.4 mm above the bottom of the CCD. The axis of rotation is supported by two bearings and is rotated with an accuracy of ±0.7 degrees by a stepping motor. For the micron-CT, we adopted a CCD of Hamamatsu photonics Co. Ltd. (C8800X) with a pixel size of
727
8 lm · 8 lm and a number of pixels 1000 · 1000. The data transfer mode employed was frame transfer (30 frames/ sec). The CCD starts exposure by an outside trigger and it stops when beam currents have been accumulated up to a constant value; readout of data is finished while the target is rotated. A 100 lm Mylar foil was placed in front of the CCD thus preventing scattered protons from entering. We measured the detection efficiency of the CCD for Sn-L-X-rays (3.44 keV), Ti-K-X-rays (4.558 keV) and CuK-X-rays (8.048 keV) produced by 3 MeV proton bombardment, which were 0.33, 0.15 and 0.05, respectively. Considering attenuation in the 100 lm Mylar foil, we adopted Ti as the best X-ray source target element. 3. Spatial resolution of micron-CT The spatial resolution dD of the micron-CT is given by [2], dD ¼
LS D; Ld
ð1Þ
where Ld and LS are shown in Fig. 1 and D is the pixel size of the CCD. In our case, Ld = 8.8 mm and D = 8 lm. In accordance with the above equation, the spatial resolution of the micron-CT can be improved by shortening LS. We examined the spatial resolution as a function of LS using a Tungsten wire of 20 lm in diameter. The sampling number with respect to projection angle was 250 and the beam spot size was 1.5 lm in diameter. The spatial resolutions of the micron-CT are shown in Fig. 2 as a function of LS. It is apparent that the spatial resolution is improved by decreasing LS as shown in Fig. 2. The experimental resolutions are worse than those calculated by Eq. (1). The difference is mainly due to uncertainties in the accuracy of positioning and rotating the sample axis.
14
Exp.
Spatial resolution [µm]
12 10 8 6 Cal.
4 2 0
Fig. 1. Principle of micron-CT using PIXE with micro-beams.
0
1000
2000
3000 Ls [µm]
4000
5000
6000
Fig. 2. Spatial resolution of micron-CT as a function of the distance between the center of rotation of the sample stage and the X-ray point source.
728
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
4. Application of X-ray micron-CT to the head of a small ant
5. Application of Light micron-CT to a small red tick
The head of a small ant was placed in a polyimide tube whose outside diameter was 1000 lm and its wall thickness 25 lm (see Fig. 3(a)). LS was set at 2.0 mm, the beam spot size was 1.5 lm in diameter and beam current was 300 pA. The measurement time was 90 min and 3D image reconstruction for 256 · 256 · 182 voxels was performed with unit pixels of 5 · 5 · 5 lm3. Fig. 3(b) shows the cross sectional image of the ant’s head obtained by the FBP method. This image just corresponds to the cut plane along by a solid line in Fig. 3(a). It is apparent that the image is well reconstructed and details of the head’s interior are seen. Fig. 4 shows the 3D images obtained by iterations. The left side shows a stereoscopic image and the right side shows horizontal cross sectional views.
In general, blue visible light is strongly absorbed by materials, whereas red light penetrates deeper. For this reason, we tested Al2O3(Cr) ruby scintillators as a point light source for Light micron-CT, as the An Al2O3(Cr) ruby scintillator emits red light. Because this scintillator is an insulator, it is charged up by ion beam irradiation. To avoid beam charge up, the surface of the scintillator is coated with a thin gold layer. A small red tick (0.5 mm) was used as test sample (see Fig. 5(a)) and the intensity of red light was monitored with a Si-PIN photodiode detector at each projection angle. The projection images of the red tick by X-rays and light are shown in Fig. 5(b) and (c), respectively. A clear image was obtained by Ti X-rays. In the case of red light imaging,
Fig. 3. Picture of small ant’s head (a) and cross sectional view of ant’s head obtained by the FBP method (b).
Fig. 4. Stereoscopic image and horizontal cross sectional views of small ant’s head.
K. Ishii et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 726–729
729
Fig. 5. Photograph of small red tick (a), projection images of small red tick by X-ray (b) and lights (c). (For interpretation of colour in this figure, the reader is referred to the web version of this article.)
(1 mm) in detail. The X-ray micron-CT will be tested in in vivo experiments and may be applicable to a wide range of research in biology and medicine. The possibility of using less destructive light micron-CT with red light produced by an Al2O3(Cr) scintillator was explored and applied to a red tick. The projection data faded compared to X-rays and the tick’s size was expanded by Thomson scattering. This effect strongly degrades the capability of light micron-CT using a CCD camera. Fig. 6. Horizontal cross sectional views of small red tick with X-ray micron-CT (a) and light micron-CT (b). (For interpretation of colour in this figure, the reader is referred to the web version of this article.)
the tick’s shape fades and expands, and internal details are not observed. This is the consequence of scattering of light by the object and it causes a serious problem to the image reconstruction. Fig. 6 shows the CT images obtained by the FBP method. The spherical structure in Fig. 6(b) is due to noises of the CCD. The interior of the red tick is not observed in the case of Light micron-CT because light does not penetrate the tick’s body. Though an image of the cross sectional view was obtained, because the red tick is transparent to Ti K-X-rays, the structure of the interior is not visible in contrast to the small ant’s head. 6. Conclusion The spatial resolution of micron-CT was improved by shortening the distance between the X-ray point source and the object, and allowed to observe the small ant’s head
Acknowledgement This study was supported by a Grant-in-Aid for Scientific Research (S) No. 13852017 (K. Ishii) of the Ministry of Education, Culture, Science, Sports and Technology. References [1] K. Ishii, S. Morita, Int. J. PIXE 1 (1992) 1. [2] K. Ishii, S. Matsuyama, H. Yamazaki, Y. Watanabe, T. Yamaguchi, G. Momose, T. Amartaivan, A. Suzuki, Y. Kikuchi, W. Galster, Int. J. PIXE 15 (2005) 111. [3] S. Matsuyama, K. Ishii, H. Yamazaki, R. Sakamoto, M. Fujisawa, Ts. Amartaivan, Y. Oishi, M. Rodriguez, A. Suzuki, T. Kamiya, M. Oikawa, K. Arakawa, N. Matsumoto, Nucl. Instr. and Meth. In Phy. Res. B 210 (2003) 117. [4] S. Matsuyama, K. Ishii, H. Yamazaki, Y. Barbotteau, Ts. Amartaibvan, D. Izukawa, K. Hotta, K. Mizuma, S. Abe, Y. Ohishi, M. Rodrigeez, A. Suzuki, R. Sakamoto, M. Fujisawa, T. Kamiya, M. Oikawa, Int. J. PIXE 14 (2004) 1. [5] K. Lange, J.A. Fessler, IEEE Trans. Image Process. 14 (1995) 1430. [6] L.A. Shepp, Y. Vardi, IEEE Trans. Med. Imag. MI-1 (1982) 113.