- Email: [email protected]
CCD system
Nuclear Instruments and Methods in Physics Research B 142 (1998) 425±431
Development and characterization of a neutron tomography system based on image intensi®er/CCD system Amar Sinha a
a,*
, A.M. Shaikh b, A. Shyam
a
High Pressure Physics Division, Bhabha Atomic Research Centre Trombay, Purnima Laboratory, Mumbai 400 085, India b Solid State Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400 085, India Received 19 January 1998; received in revised form 20 March 1998
Abstract A neutron tomography system has been developed using a low cost electronic imaging system which uses commercially available Image intensi®er, CCD, Frame grabber. This setup has been used for a series of experiments on neutron tomography at the 400 kW swimming pool type reactor APSARA which has a neutron ¯ux of 106 n/cm2 /s at L/D 90. The main feature of this tomography system is integrated computer controlled imaging and stepper motor control system with online data acquisition, analysis and display of images for each projection angle. The density pro®le of the projection images and ray sum calculation can be carried out in oine or online manner at any slice and a series of reconstructed image for any slice can be obtained. An algorithm for reconstruction using Convolution Back Projection (CBP) technique has been developed and implemented on a Pentium system. The imaging system is modular in design. The reconstructed image has shown good quality and holes of less than 0.6 mm ®lled with wax can be revealed in a 60 mm aluminum matrix or 1 mm holes in 40 mm solid brass matrix have been resolved. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Neutron tomography; CCD; Image intensi®er; CBP method
1. Introduction Computed tomography is used to provide a reconstructed image of 2D slice or 3D volume of an object. The image is a map of the linear attenuation coecient averaged over a pixel or voxel size used in reconstruction. Though radiography is being routinely used for non-destructive testing,
* Corresponding author. Fax: +91 22 5560 750; e-mail: [email protected].
computed tomography is a recent addition to NDE technique. It overcomes many of the limitations of radiography and makes possible visualization of physical structures in their actual relative spatial positions and orientations. We have developed a low cost neutron tomography system based on Image intensi®er/CCD technique. The system is fully automated with integrated computer controlled stepper motor control for sample manipulation and image acquisition system. The main advantage of this technique compared to conventional translate±rotate tomography
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 6 3 - 8
426
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
is that in the CCD method the only operation required is rotation of the sample and from each projection data set, not only 2D slice can be constructed but 3D information corresponding to different slices can also be obtained. This results in considerable saving of data acquisition time. However, use of multidetector system has certain limitations like the problem of cross-talk due to scattering of radiation from one channel to another and special techniques are required to correct for such cross-talk eect. The low cost electronic imaging system uses an intensi®er, CCD camera, frame grabber-cum-processor, PC/AT and video monitor and is completely modular in design which enables one to use the same setup with change of scintillator for gamma/ X-ray imaging. We have chosen a frame grabber with special input circuitry having software programmable video gain, oset and reference which allows us to zoom on any particular gray range. This overcomes some of the limitations of 8 bit digitizer by eectively focussing on available gray range. A series of software have been developed which can analyze these images such as density pro®le, projection integrals for input to reconstruction
program. We present in this paper description of the basic hardware and software used, results of characterization of the system and some sample images. 2. Methods and equipment The main tomography system has been assembled by us using an opto-electronic two-dimensional position sensitive detector which consists of a second generation high gain hybrid image intensi®er tube, a lens coupled CCD, frame grabbercum-processor having 8 bit digitization and 16 bit processor circuitry with provision of oset, reference and programmable gain. This provision of programmable gain oers the possibility to zoom on any desired range of gray scale and thus eectively add extra bits to the digitization of video signal. The electronic imaging system is described in Fig. 1. The neutron beam passing through the sample are absorbed in a scintillator screen (NE426). Photons generated by the scintillating screen are re¯ected by 90° and focussed onto the input ®bre optic face of an image intensi®er tube. The output image is focussed onto a CCD camera using a
Fig. 1. Schematic diagram of setup used at APSARA reactor for neutron tomography work.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
F1.4 lens. The video output is digitized using a frame grabber card and processed using a onboard processor. The CCD camera used has 756 (H) ´ 581 (V) pixel array and image intensi®er used has 30 lp/mm resolution with gain of 105 Cd/m2 /lx. An image acquisition software with online thresholding, contrast stretching, integration, averaging, etc., operation has been developed. The sample to be imaged is placed on a rotating platform which is rotated using a stepper motor. The stepper motor is controlled using a PC based stepper motor controller card. The image acquisition and processing and sample rotation control program has been interfaced within a single control program so that after each image acquisition operation, the sample can be rotated by the desired angle for the next image acquisition. This way the entire data acquisition for tomography has been automated.
3. Neutron source The experiments on neutron tomography were performed using the neutron radiography facility at 400 kW swimming pool type reactor APSARA at Bhabha Atomic Research Centre, Trombay, Mumbai. The available thermal ¯ux in this radiography facility is 1 ´ 106 n/cm2 /s at L/D ratio of 90 [1]. The beam size is 15 ´ 15 cm2 .
427
5. Data correction procedure From the acquired images projection data sets were made. The raw data from CCD camera contains various two-dimensional distortions which mainly consist of shading of the optical system and dark current of the electronic system. For this purpose dark current image without neutron beam and bright image with full neutron beam but without sample were obtained and the shading corrections were done after subtracting the dark current image. These images were then preprocessed to calculate ray-sum at each pixel. These preprocessed ray-sum data were subjected to the reconstruction algorithm. 6. Result and discussion Several sets of samples were fabricated to assess the quality of tomography system. Fig. 2 is the schematic diagram of sample 1 which contains a set of 19 rods (9 of SS and 10 of brass) each of 3 mm diameter embedded in a 60 mm diameter aluminum matrix. On one side of this sample one of the brass rod is broken. The tomography reconstruction has been done at both the ends of this sample. Fig. 3 shows tomography reconstruction of this sample at the top end where all the rods are intact. Fig. 4 shows a thresholded image of
4. Reconstruction algorithm and data collection methodology Convolution Back Projection (CBP) [2±5] technique with Shepp Logan Filter [6] was used for the reconstruction of images. In most of the cases 101 projection images were used for reconstruction. The pixel size was 0.25 mm/pixel. Data acquisition time required for each projection image was typically 10 s. After obtaining images for each angle, these images were preprocessed for ray-sum calculation at various slices. The resulting raysum data was used in a reconstruction program based on CBP method for obtaining CT scan at various slices.
Fig. 2. Schematic diagram of a sample # 1 containing 19 rods (10 of brass and 9 of SS) embedded in a matrix of aluminum rod dia 3 mm; aluminum matrix dia 60 mm.
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A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Fig. 3. Neutron CT image of sample # 1 (top side).
Fig. 4. Thresholded CT image of sample # 1 to highlight the dierent attenuation coecient of SS and brass.
the same CT scan to highlight the dierence between SS and brass rod. Fig. 5 shows CT reconstruction at the other end of sample where one of the rods is broken and seen in the ®gure as missing. Fig. 6 shows the schematic diagram of another aluminum sample of 60 mm diameter but with holes of various sizes (0.3±5 mm) ®lled with wax, SS rods and indium wire. This sample contained wax ®lled holes of diameter 0.6 mm and holes containing indium wire of 300 lm. Fig. 7 shows CT
Fig. 5. Neutron CT image of sample # 1 (bottom side) (one rod is broken on bottom side).
reconstruction of this image. All the SS rods, wax ®lled holes and indium wire are clearly visible. Fig. 8 shows CT image of a Stainless steel gear of 37 mm outer diameter and 15 mm inner diameter with a notch of 1 mm. Fig. 9 shows the schematic diagram of a 40 mm diameter brass cylinder with holes ranging from 5 to 1 mm some of them ®lled with wax and air. Fig. 10 shows CT scan of this sample. All the holes either wax or air ®lled are visible in the image. Fig. 11 shows the schematic diagram of an ocentered cone made of SS embedded inside an aluminum matrix. At the base of the cone there is a hole of diameter 3 mm and length 10 mm. The diameter of cone is 34 mm and length 40 mm, embedded inside an aluminum matrix of diameter 50 mm and length 35 mm. Figs. 12±14 show tomography reconstruction at three slices all obtained from the same set of projection data. The ®rst slice taken near the base of the cone clearly shows presence of hole in that slice whereas the second and third slice taken beyond the hole do not show any such hole. This sample has been chosen for demonstrating the capability of the present technique for 3D tomography. A technique based on volume rendering [7] is being implemented to stack these slices in a three-dimensional matrix for volume visualization of this sample. The result of this visualization will be reported in a separate paper.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
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Fig. 6. Schematic diagram of sample # 2 made of aluminum having rods of dierent types and sizes embedded inside (dia of aluminum matrix 60 mm).
Fig. 7. Neutron CT image of sample # 2.
The tomography system can accept samples of sizes upto 150 mm without need for any translation provided the sample is not too neutron absorbing. In order to do quantitative evaluation of the data, scattering correction need to be accounted. In the present series of experiments eorts have been made to minimize scattering eect which leads to cross-talk by increasing detector to sample spacing. A technique [4] to account for this eect using a grid of cadmium in the form of a narrow strip of cadmium with a series of 1 mm holes interposed between sample and the beam is being implemented.
Fig. 8. Neutron CT image of a gear made of SS (outer dia 37 mm; inner dia 15 mm, notch size 1 mm).
Fig. 9. Schematic diagram of sample # 3 made of solid brass dia 40 mm having holes of 1 to 5 mm wax and air ®lled.
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A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Fig. 10. Neutron CT scan of sample # 3.
Fig. 13. Neutron CT image of sample # 4 away from hole.
Fig. 11. Schematic diagram of sample # 4; An SS cone inside a 50 mm dia aluminum matrix placed in ocentered manner. The cone has 3 mm dia and 10 mm length central hole at the base.
Fig. 14. Neutron CT image of sample # 4 at lower portion of cone.
7. Conclusion
Fig. 12. Neutron CT image of sample # 4 at the base of cone.
A low cost Neutron tomography system has been developed based on CCD/Image Intensi®er. This detector system has been used for obtaining neutron CT scan using APSARA reactor. The data acquisition system consisting of image acquisition and stepper motor control for sample manipulation is integrated resulting in a fully automated data acquisition setup. Experiments have been conducted to characterize the system and holes of 1 mm in 40 mm brass matrix have been resolved.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Wax ®lled hole of 0.6 mm and indium wire of about 0.3 mm have been identi®ed in a 60 mm aluminum matrix. The results obtained show that it may be possible to obtain good quality results even with the use of relatively low cost components.
References [1] Y.D. Dande, R.S. Uyavar, J.K. Ghosh, K.N. Chandrasekharan, J.P. Pannakal, Indian J. Pure Appl. Phys. 29 (1991) 721.
431
[2] G.T. Herman, Image Reconstruction from Projections: The Fundamentals of Computerised Tomography, Academic press, New York, 1980. [3] G.N. Ramchandran, A.V. Laxminarayanan, Proc. Natl. Acad. Sci. U.S.A. 68 (1971) 2236. [4] Proceedings of the 2nd, 3rd and 4th World Conference on Neutron Radiography, held respectively in Paris, 1986, Osaka, 1989 and San Francisco, 1992, Reidel, Dordrecht. [5] A. Macovski, Proc. IEEE 71 (3) (1983) 373. [6] L.A. Shepp, B.E. Logan, IEEE Trans. Nucl. Sci. NS 21 (1974) 21. [7] R.A. Derbin, L. Carpenter, P. Hanrahan, Comput. Graphics 22 (4) (1988) 65.
Development and characterization of a neutron tomography system based on image intensi®er/CCD system Amar Sinha a
a,*
, A.M. Shaikh b, A. Shyam
a
High Pressure Physics Division, Bhabha Atomic Research Centre Trombay, Purnima Laboratory, Mumbai 400 085, India b Solid State Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400 085, India Received 19 January 1998; received in revised form 20 March 1998
Abstract A neutron tomography system has been developed using a low cost electronic imaging system which uses commercially available Image intensi®er, CCD, Frame grabber. This setup has been used for a series of experiments on neutron tomography at the 400 kW swimming pool type reactor APSARA which has a neutron ¯ux of 106 n/cm2 /s at L/D 90. The main feature of this tomography system is integrated computer controlled imaging and stepper motor control system with online data acquisition, analysis and display of images for each projection angle. The density pro®le of the projection images and ray sum calculation can be carried out in oine or online manner at any slice and a series of reconstructed image for any slice can be obtained. An algorithm for reconstruction using Convolution Back Projection (CBP) technique has been developed and implemented on a Pentium system. The imaging system is modular in design. The reconstructed image has shown good quality and holes of less than 0.6 mm ®lled with wax can be revealed in a 60 mm aluminum matrix or 1 mm holes in 40 mm solid brass matrix have been resolved. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Neutron tomography; CCD; Image intensi®er; CBP method
1. Introduction Computed tomography is used to provide a reconstructed image of 2D slice or 3D volume of an object. The image is a map of the linear attenuation coecient averaged over a pixel or voxel size used in reconstruction. Though radiography is being routinely used for non-destructive testing,
* Corresponding author. Fax: +91 22 5560 750; e-mail: [email protected].
computed tomography is a recent addition to NDE technique. It overcomes many of the limitations of radiography and makes possible visualization of physical structures in their actual relative spatial positions and orientations. We have developed a low cost neutron tomography system based on Image intensi®er/CCD technique. The system is fully automated with integrated computer controlled stepper motor control for sample manipulation and image acquisition system. The main advantage of this technique compared to conventional translate±rotate tomography
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 6 3 - 8
426
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
is that in the CCD method the only operation required is rotation of the sample and from each projection data set, not only 2D slice can be constructed but 3D information corresponding to different slices can also be obtained. This results in considerable saving of data acquisition time. However, use of multidetector system has certain limitations like the problem of cross-talk due to scattering of radiation from one channel to another and special techniques are required to correct for such cross-talk eect. The low cost electronic imaging system uses an intensi®er, CCD camera, frame grabber-cum-processor, PC/AT and video monitor and is completely modular in design which enables one to use the same setup with change of scintillator for gamma/ X-ray imaging. We have chosen a frame grabber with special input circuitry having software programmable video gain, oset and reference which allows us to zoom on any particular gray range. This overcomes some of the limitations of 8 bit digitizer by eectively focussing on available gray range. A series of software have been developed which can analyze these images such as density pro®le, projection integrals for input to reconstruction
program. We present in this paper description of the basic hardware and software used, results of characterization of the system and some sample images. 2. Methods and equipment The main tomography system has been assembled by us using an opto-electronic two-dimensional position sensitive detector which consists of a second generation high gain hybrid image intensi®er tube, a lens coupled CCD, frame grabbercum-processor having 8 bit digitization and 16 bit processor circuitry with provision of oset, reference and programmable gain. This provision of programmable gain oers the possibility to zoom on any desired range of gray scale and thus eectively add extra bits to the digitization of video signal. The electronic imaging system is described in Fig. 1. The neutron beam passing through the sample are absorbed in a scintillator screen (NE426). Photons generated by the scintillating screen are re¯ected by 90° and focussed onto the input ®bre optic face of an image intensi®er tube. The output image is focussed onto a CCD camera using a
Fig. 1. Schematic diagram of setup used at APSARA reactor for neutron tomography work.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
F1.4 lens. The video output is digitized using a frame grabber card and processed using a onboard processor. The CCD camera used has 756 (H) ´ 581 (V) pixel array and image intensi®er used has 30 lp/mm resolution with gain of 105 Cd/m2 /lx. An image acquisition software with online thresholding, contrast stretching, integration, averaging, etc., operation has been developed. The sample to be imaged is placed on a rotating platform which is rotated using a stepper motor. The stepper motor is controlled using a PC based stepper motor controller card. The image acquisition and processing and sample rotation control program has been interfaced within a single control program so that after each image acquisition operation, the sample can be rotated by the desired angle for the next image acquisition. This way the entire data acquisition for tomography has been automated.
3. Neutron source The experiments on neutron tomography were performed using the neutron radiography facility at 400 kW swimming pool type reactor APSARA at Bhabha Atomic Research Centre, Trombay, Mumbai. The available thermal ¯ux in this radiography facility is 1 ´ 106 n/cm2 /s at L/D ratio of 90 [1]. The beam size is 15 ´ 15 cm2 .
427
5. Data correction procedure From the acquired images projection data sets were made. The raw data from CCD camera contains various two-dimensional distortions which mainly consist of shading of the optical system and dark current of the electronic system. For this purpose dark current image without neutron beam and bright image with full neutron beam but without sample were obtained and the shading corrections were done after subtracting the dark current image. These images were then preprocessed to calculate ray-sum at each pixel. These preprocessed ray-sum data were subjected to the reconstruction algorithm. 6. Result and discussion Several sets of samples were fabricated to assess the quality of tomography system. Fig. 2 is the schematic diagram of sample 1 which contains a set of 19 rods (9 of SS and 10 of brass) each of 3 mm diameter embedded in a 60 mm diameter aluminum matrix. On one side of this sample one of the brass rod is broken. The tomography reconstruction has been done at both the ends of this sample. Fig. 3 shows tomography reconstruction of this sample at the top end where all the rods are intact. Fig. 4 shows a thresholded image of
4. Reconstruction algorithm and data collection methodology Convolution Back Projection (CBP) [2±5] technique with Shepp Logan Filter [6] was used for the reconstruction of images. In most of the cases 101 projection images were used for reconstruction. The pixel size was 0.25 mm/pixel. Data acquisition time required for each projection image was typically 10 s. After obtaining images for each angle, these images were preprocessed for ray-sum calculation at various slices. The resulting raysum data was used in a reconstruction program based on CBP method for obtaining CT scan at various slices.
Fig. 2. Schematic diagram of a sample # 1 containing 19 rods (10 of brass and 9 of SS) embedded in a matrix of aluminum rod dia 3 mm; aluminum matrix dia 60 mm.
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A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Fig. 3. Neutron CT image of sample # 1 (top side).
Fig. 4. Thresholded CT image of sample # 1 to highlight the dierent attenuation coecient of SS and brass.
the same CT scan to highlight the dierence between SS and brass rod. Fig. 5 shows CT reconstruction at the other end of sample where one of the rods is broken and seen in the ®gure as missing. Fig. 6 shows the schematic diagram of another aluminum sample of 60 mm diameter but with holes of various sizes (0.3±5 mm) ®lled with wax, SS rods and indium wire. This sample contained wax ®lled holes of diameter 0.6 mm and holes containing indium wire of 300 lm. Fig. 7 shows CT
Fig. 5. Neutron CT image of sample # 1 (bottom side) (one rod is broken on bottom side).
reconstruction of this image. All the SS rods, wax ®lled holes and indium wire are clearly visible. Fig. 8 shows CT image of a Stainless steel gear of 37 mm outer diameter and 15 mm inner diameter with a notch of 1 mm. Fig. 9 shows the schematic diagram of a 40 mm diameter brass cylinder with holes ranging from 5 to 1 mm some of them ®lled with wax and air. Fig. 10 shows CT scan of this sample. All the holes either wax or air ®lled are visible in the image. Fig. 11 shows the schematic diagram of an ocentered cone made of SS embedded inside an aluminum matrix. At the base of the cone there is a hole of diameter 3 mm and length 10 mm. The diameter of cone is 34 mm and length 40 mm, embedded inside an aluminum matrix of diameter 50 mm and length 35 mm. Figs. 12±14 show tomography reconstruction at three slices all obtained from the same set of projection data. The ®rst slice taken near the base of the cone clearly shows presence of hole in that slice whereas the second and third slice taken beyond the hole do not show any such hole. This sample has been chosen for demonstrating the capability of the present technique for 3D tomography. A technique based on volume rendering [7] is being implemented to stack these slices in a three-dimensional matrix for volume visualization of this sample. The result of this visualization will be reported in a separate paper.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
429
Fig. 6. Schematic diagram of sample # 2 made of aluminum having rods of dierent types and sizes embedded inside (dia of aluminum matrix 60 mm).
Fig. 7. Neutron CT image of sample # 2.
The tomography system can accept samples of sizes upto 150 mm without need for any translation provided the sample is not too neutron absorbing. In order to do quantitative evaluation of the data, scattering correction need to be accounted. In the present series of experiments eorts have been made to minimize scattering eect which leads to cross-talk by increasing detector to sample spacing. A technique [4] to account for this eect using a grid of cadmium in the form of a narrow strip of cadmium with a series of 1 mm holes interposed between sample and the beam is being implemented.
Fig. 8. Neutron CT image of a gear made of SS (outer dia 37 mm; inner dia 15 mm, notch size 1 mm).
Fig. 9. Schematic diagram of sample # 3 made of solid brass dia 40 mm having holes of 1 to 5 mm wax and air ®lled.
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A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Fig. 10. Neutron CT scan of sample # 3.
Fig. 13. Neutron CT image of sample # 4 away from hole.
Fig. 11. Schematic diagram of sample # 4; An SS cone inside a 50 mm dia aluminum matrix placed in ocentered manner. The cone has 3 mm dia and 10 mm length central hole at the base.
Fig. 14. Neutron CT image of sample # 4 at lower portion of cone.
7. Conclusion
Fig. 12. Neutron CT image of sample # 4 at the base of cone.
A low cost Neutron tomography system has been developed based on CCD/Image Intensi®er. This detector system has been used for obtaining neutron CT scan using APSARA reactor. The data acquisition system consisting of image acquisition and stepper motor control for sample manipulation is integrated resulting in a fully automated data acquisition setup. Experiments have been conducted to characterize the system and holes of 1 mm in 40 mm brass matrix have been resolved.
A. Sinha et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 425±431
Wax ®lled hole of 0.6 mm and indium wire of about 0.3 mm have been identi®ed in a 60 mm aluminum matrix. The results obtained show that it may be possible to obtain good quality results even with the use of relatively low cost components.
References [1] Y.D. Dande, R.S. Uyavar, J.K. Ghosh, K.N. Chandrasekharan, J.P. Pannakal, Indian J. Pure Appl. Phys. 29 (1991) 721.
431
[2] G.T. Herman, Image Reconstruction from Projections: The Fundamentals of Computerised Tomography, Academic press, New York, 1980. [3] G.N. Ramchandran, A.V. Laxminarayanan, Proc. Natl. Acad. Sci. U.S.A. 68 (1971) 2236. [4] Proceedings of the 2nd, 3rd and 4th World Conference on Neutron Radiography, held respectively in Paris, 1986, Osaka, 1989 and San Francisco, 1992, Reidel, Dordrecht. [5] A. Macovski, Proc. IEEE 71 (3) (1983) 373. [6] L.A. Shepp, B.E. Logan, IEEE Trans. Nucl. Sci. NS 21 (1974) 21. [7] R.A. Derbin, L. Carpenter, P. Hanrahan, Comput. Graphics 22 (4) (1988) 65.