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Development of a panorama coded-aperture gamma camera for radiation detection
Radiation Measurements 77 (2015) 34e40
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Development of a panorama coded-aperture gamma camera for radiation detection Shifeng Sun a, b, c, Zhiming Zhang a, b, Lei Shuai a, b, Daowu Li a, b, Yingjie Wang a, b, Yantao Liu a, b, Xianchao Huang a, b, Haohui Tang a, b, Ting Li a, b, Pei Chai a, b, Xiaopan Jiang a, b, Bo Ma a, b, Meiling Zhu a, b, Xiaoming Wang a, b, Yiwen Zhang a, b, c, Wei Zhou a, b, c, Fanjian Zeng a, b, c, Jing Guo a, b, Liyang Sun a, b, c, Mingjie Yang a, b, c, Yubao Zhang a, b, Cunfeng Wei a, b, Chuangxin Ma a, b, Long Wei a, b, * a
Division of Nuclear Technology and Applications, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Beijing Engineering Research Center of Radiographic Techniques and Equipment, Beijing 100049, China c University of Chinese Academy of Sciences, Beijing 100049, China b
h i g h l i g h t s Development of a coded-aperture gamma camera. Panoramic gamma ray imaging in real time. Mitigate the partial encoding of coded aperture imaging (CAI). Experimental validation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 21 March 2015 Accepted 17 April 2015 Available online 18 April 2015
For radiation detection, the imaging system should have a large field of view (FOV) and high detection efficiency because it has to be used in a radiation environment where the quantity and direction of radioactive sources are unknown. A panorama coded-aperture gamma camera optimized for use in complex nuclear environment has been developed and evaluated with an angular resolution of 3.5 . Typical gamma cameras have the limited field of view ranging from 10 to 60 in both horizontal and vertical direction. The system presented in this paper extends the field of view to 360 in the horizontal direction and 60 in the vertical direction. The partial encoding of coded aperture imaging is mitigated by convolving the data of diverse adjacent modules with a partial transmission function. The experimental feasibility of measuring multiple sources in the 360 horizontal field of view was demonstrated with a panoramic image. The results showed that the system could clearly identify the direction of multiple radiation sources in an unknown extended radiation environment. The system can help to simplify the clean up and decommissioning of nuclear sites. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coded aperture Gamma-ray imaging Modified uniformly redundant arrays Radiation detection
1. Introduction The gamma camera, which was first developed in the 1950s by Hal Anger, has been applied in many fields, including nuclear medicine (Olcott et al., 2014), industrial surveys (Bake et al., 2013),
* Corresponding author. Division of Nuclear Technology and Applications, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. E-mail addresses: [email protected] (S. Sun), [email protected] (L. Wei). http://dx.doi.org/10.1016/j.radmeas.2015.04.014 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
and homeland security (Penny et al., 2011). Early gamma cameras obtained a distribution of gamma rays by using a pinhole collimator, which are inefficient due to the low open fractions (Guru et al., 1996). In nuclear medicine, a pinhole collimator is used because the detector is placed very close to the source; however, when searching for hot spots during radiation detection, which is characterized by far-field geometry, the pinhole gamma camera has limited use. When the detected counting rate is low or dominated by background from non-FOV sources, the pinhole system has very low sensitivity (Gal et al., 2006).
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Coded aperture imagers (Skinner, 1984) extend the pinhole camera concept by employing a mask with many pinholes distributed with a predefined pattern. The coded aperture has more transparent regions, which ensures that the detector can receive more radiation from the radiation sources. Thus, coded aperture imaging has higher performance in terms of detection efficiency and sensitivity (Gmar et al., 2004). Unlike random pinhole distribution, which has the limitation of numerical instability due to the existence of zeros in the inverse-mask Fourier-domain transfer function, uniformly redundant arrays (URAs) (Fenimore and Cannon, 1978) and modified URAs (MURAs) (Gottesman and Fenimore, 1989) are considered to have near-delta-function system point-spread functions. By placing the pinholes in prime number-based patterns determined by sampling theory to form URAs and MURAs, the quality of the reconstructed images are greatly improved. Anti-masks of MURAs can be obtained by 90 rotations around their central element. The mask and anti-mask subtraction of MURAs can significantly reduce the loss of contrast in the image, which is caused by background noise and inconsistent detection efficiency of the detector. This enhanced the ability of the imaging system to detect weak sources at large distances in a relatively short time. The typical coded-aperture gamma camera (Caroli et al., 1987) is composed of a position-sensitive detector (PSD) and an active mask. The length of the active mask is two times that of the detector. As shown in Fig. 1, the detector's field of view (FOV, the fully encoded region) is defined by the mask size and the distance between the mask and the PSD. The partially coded field of view (PCFV) is defined as comprising all the directions for which only a fraction of the detected gamma rays is coded by the mask. In some conditions of a nuclear survey, the gamma-ray sources may distribute radiation in wide ranges that exceed the FOV of a gamma camera. Thus, the radiation sources distributing in a PCFV are partially coded, and normal decoding will lead to an incorrect imaging result. For radiation detection to search for missing radioactive sources or hot spots, the quantity and direction of radioactive sources are always unknown and the gamma camera has to perform with blind imaging (Hu et al., 2014). One way to mitigate these problems is to expand the FOV of the system. Decreasing the maskePSD separation can expand the FOV, while it will require a corresponding decrease in the PSD pixel size to give a similar angular resolution; however, this does not eliminate the partial encoding. Some attempts have been made to exempt the system from the typical limitations of partial encoding. SORIS (Zelakiewicz et al., 2011) employed a nonplanar mask to obtain a view of about 180 , but this suffered from a relatively low angular resolution. Kaye et al. (2008) demonstrated near-4p gamma-ray imaging by using multiple three-dimensional (3D) position-sensitive pixilated detectors by combing both Compton and coded aperture imaging techniques; however, the detection
Fig. 1. Field of view of the gamma camera.
35
efficiency is very poor. The purpose of this paper is to represent a novel hexagonal coded-aperture gamma camera that is able to measure multiple sources horizontally with a 360 FOV in real time. Different from the existing gamma cameras, which always require multiple times measurements, the system often needs only one measurement to survey entire room. The imaging system is optimized for nuclear facilities security inspection, the clean up and decommissioning of nuclear sites. These nuclear industrial applications require the imaging system should meet some criteria. These criteria should include: energy range 50 keV-1.5 MeV; energy resolution (<15% at 662 keV); anger resolution (<4 ); activity of radioisotopes range 10 mCi-10 Ci; distance between the system and radioisotopes 1e50 m at least; 360 field of view in the horizontal direction; compact, easy to use; short image acquisition times which implies maximized sensitivity (Durrant et al., 1999). To meet the detailed criteria above, those design parameters of the detectors, coded aperture, shield and etc were studied, characterized, and optimized accordingly. 2. Design and methods The panorama coded-aperture gamma camera is show in Fig. 2 with its photograph and the schematic diagram of the detector system. The overall dimensions of the gamma camera are about 42 42 80 cm3. The detector system is designed based on the typical coded-aperture gamma camera. Six pixilated detector modules are placed in a hexagonal shape with a 10 mm thick copper shield. Every detector module is separated from its adjacent modules by a 12 mm-thick rectangular lead for radiation shielding. Three masks and three anti-masks are placed in front of the six detector modules at intervals, and the corresponding aperture of each detector module is the anti-mask of the adjacent detector modules. These masks form a mask layer, which is mechanical isolated from the detector modules. The mask layer can rotate 60 clockwise and then turn back, which makes mask and anti-mask acquisition and subtraction possible. One of the benefits of this structure is that only one set of rotating mechanisms can achieve mask and anti-mask switching of all six detector modules. The selection of MURA rank depends on the required mask performance characteristics, which will determine angular resolution of the system. With consideration of the detector design parameters, rank 19 MURA was selected as the system mask. The mosaicked array of rank 19 MURA is shown in Fig. 3. The pixel pitch of the mask is 2.5 mm. The mask's dimensions are 92.5 92.5 6 mm3, and the primary component material is tungstenecopper alloy. Fig. 4 shows the detector module, analog and digital electronics of the panorama coded-aperture gamma camera. Each one of the detectors consists of a 19 19 pixilated CsI(Tl) scintillation crystal array and a position-sensitive photomultiplier tube (PSPMT). The crystal arrays have a thickness of 8.0 mm and an area of 50.0 mm 50.0 mm and are optically coupled to a Hamamatsu H8500 PSPMT. The 64 anode outputs of the H8500 are read into a discretized version of a single-wire, position-sensitive proportional counter readout (DPC) circuit (Siegel et al., 1996), and after amplification and shaping, the four signals are fed into the data acquisition system. The digital electronics complete 4 6 channel parallel sampling with a sampling frequency of 40 MHz and a sampling precision of 12 bits. After sampling, the digital pulses are differentially input into a field-programmable gate array (FPGA) in the form of serial signals. The FPGA controls the integration and digitization and then passes the data to the computer through a 1000-M base-T interface. The imaging system has a small vision-blind region due to the
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S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Fig. 2. Panorama coded-aperture camera. (a) Photograph and (b) schematic diagram of the detector system.
Fig. 3. The coded aperture pattern of MURA type. The aperture is a mosaic of a 19 19 MURA.
small gap between the detector modules. As radiation detection is characterized by far-field geometry, the small vision-blind region is ignored here. As shown in Fig. 5, the placement of the system is drawn at a certain scale and the different parts of the system are defined. The radiation sources in the FOV of one detector are very likely to be in the PCFV of the side (left or right) detector. This will
contribute some counts to the sideward detector and reduce the contrast of the image. The wrapped shielding of the detector and the thick lead shield between different detectors attenuate this effect. In addition, by dividing the horizontal field of view into different regions and processing the data of adjacent detector modules, the quality of the reconstructed image is improved. The usual reconstruction technique of coded aperture imaging is to convolve the recorded data with a decoding array G (Caroli et al., 1987). A method has been developed for the panorama gamma camera to mitigate the partial encoding without modifying the reconstruction algorithms. In order to eliminate partially coded data, we expand the definition of the source and observation spaces. A partial transmission function is defined to connect sources in PCFV and detectors on observation spaces. Each element of the function gives the probability to observe a particle emitted from a particular source pixel in a particular detector pixel. For instance, FOV of detector module 1 is divided into suspect regions and a confidence region. The 1#FOV is 60 and the confidence region is 30 . Suspect region 1 and 2 are divided into two equal parts, part A and B separately. Source spaces of detector module 1 are expanded to include 1#FOV, suspect region 1 part A and suspect region 2 part B. An initial estimate image can be obtained by the recorded data of all six detectors with decoding array G. Then, the projection counts of the sources in suspect region 1 part A and suspect region 2 part B are estimated by convolving the initial image with the partial transmission function. The filtered counts can be expressed
Fig. 4. Components of the panorama coded-aperture gamma camera. (a) Detector module with DPC circuit and connector; (b) analogous circuits and on-board FPGA, DAQ.
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
37
without masks placed in front of the detectors. Fig. 6 illustrates the flood images of the six detectors. As the 64 anode signals of PSPMT are routed to the on-board DPC circuit that generates four anger signal outputs, the Anger logic (Siegel et al., 1996) was used to calculate the position of the incident gamma rays. All 361 crystals in each detector module can be identified, and the peak-to-valley ratio of the flood images is about 4:1. A lookup table was made to identify the pixels and determine the positions of the incident gamma rays. The majority of detector pixels give an energy resolution between 10% and 15% at 662 keV. After energy calibration, the detectors' energy resolution was measured at about 15% for 662 keV. The inconsistent detection efficiency of crystal pixels among detector modules was normalized. 3.2. Imaging performance test
Fig. 5. Definition of different regions of the system.
as a subtraction of the recorded data and the estimated projection counts. Finally, the reconstructed image is decoded by convolving the filtered counts with decoding array G. 3. Measurement set-up and performance test 3.1. Detector performance test To evaluate the performance of the detectors, flood images and energy resolution measurements of the detectors were taken. The raw images were created using a distantly placed cs-137 source
In order to evaluate the system response to a single radiation source, one 25 cm long 2 mCi Ge-68 rod source with a 2 mm lead shielding shell was placed in the FOV of one detector with a distance of about 4 m from the center line of the imaging system. The detectors are named in clockwise order from 1 to 6, and the radiation source was placed in the FOV of detector 4. Before the sources were placed, the background was measured for 600 s; then the system acquired data for 600 s without the rotation of the mask layer. The experiment was performed with an energy window of 200 keVe1000 keV, and results are listed in Table 1. As Table 1 indicates, the radiation source was placed in the FOV of detector 4. Detectors 3 and 5, which are on the left and right side of the detector 4, respectively, also have a relative high count because the radiation source was not in their FOV but in their PCFV. Detectors 1, 2, and 6 are not as affected by the radiation source. To evaluate the imaging performance of the system, three 25 cm long Ge-68 rod sources with a 15.5 mm lead shielding shell were placed in three separate directions of the system with distances of about 3 m, 3 m and 3.5 m from the center line of the imaging system. The activities of the sources are 1.5, 2.5 and 2 mCi. As shown in Fig. 7, the radiation sources were placed in the FOV of
Fig. 6. Two-dimensional flood images measured of the system's six detectors. The resolution of the flood images are 512 512 pixels.
38
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Table 1 System response to a single radiation source. Detector number
Counts (N)
Background (Nb)
Net counts (N0)
Net counting rate (n/s)
1 2 3 4 5 6
12,660 11,772 31,474 73,670 28,387 12,920
12,033 11,580 13,261 12,128 11,102 11,829
627 192 18,213 61,542 17,28 1091
1.04 0.32 30.36 102.57 28.81 1.82
performed twice with an energy window of 200 keVe1000 keV. Counts are listed in Table 2. It is difficult to infer from Table 2 where the radiation sources are as there is no clear count rate distribution. The lower counting rates compared to the previous experiment is because the radiation sources are in a thick lead shielding shell.
4. Results
Fig. 7. Experimental arrangement of system response to multiple radiation sources.
detectors 1, 3, and 5. Before the sources were placed, the background was measured for 600 s. The system first acquired data for 300 s as the mask mode, after which the mask layer rotated 60 clockwise to the anti-mask mode and acquired data for 300 s. After the completion of data acquisition, the mask layer rotated 60 anticlockwise back to the mask mode. The experiment was
The first test of the system response to a single radiation source verified that the detectors are affected by the source in the PCFV. This causes a loss of contrast in the reconstructed images. Fig. 8 shows the reconstructed gamma image of one Ge-68 source. As the source is relative strong, the mask and anti-mask subtraction of MURAs was not used to improve the quality of the reconstructed images. From this figure, we can identify the precise location of the radiation source. The loss of contrast in the image was mainly caused by inconsistent detection efficiency of the detectors. In the multiple radiation sources imaging experiment, the thick lead shielding shells were used to reduce the degree of activeness of the radiation source. This also simulated the search of unknown strong shield radioactive sources, which may occur in a real sense. Whether to use the mask and anti-mask subtraction of MURAs or not depends on the complexity of the radiation environment. The mask and anti-mask subtraction of MURAs was used here. The image of multiple radiation sources was reconstructed, and displayed in two ways to make the results more intuitive. Fig. 9 shows the 3D gamma image of three Ge-68 sources, which was plotted by superposing the reconstructed image with a unit sphere.
Table 2 System response to multiple radiation sources. Detector number
Counts first time
Counts second time
Background
Net counting rate (n/s)
1 2 3 4 5 6
20,588 16,451 35,276 23,254 29,124 16,008
21,045 17,106 34,004 23,308 29,065 16,227
11,818 11,766 13,354 12,284 11,253 11,612
15.00 8.35 35.48 18.32 29.74 7.51
Fig. 8. Reconstructed gamma image of one Ge-68 source.
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
39
Fig. 9. Three-dimensional display of a gamma image of three Ge-68 sources. The figure was plotted by superposing the reconstructed image with a unit sphere.
The origin represents the center point of the imaging system. The pseudo color of every point on the spherical surface represents the relative radiation intensity value. Fig. 10 shows the polar intensity distribution of the g-ray. The polar diagram was plotted by adding the row values of the three identified hot spots. In addition to the imaging tests, we conducted the angular resolution tests using a 30 mCi Cs-137 point source. Given the dimensions of mask/detector elements, the angular resolution of the system in theory is 3.1. The average test angular resolution is 3.5 ; it's close to the theoretical value. Table 3 lists the test angular
resolution of all six detector modules. 5. Discussion and conclusions A panorama coded-aperture gamma camera, which has an FOV of 360 in the horizontal direction and 60 in the vertical direction, was developed and evaluated. The panorama gamma image was obtained by combining the recorded data of the hexagonally placed detector arrays. The FOV of coded-aperture gamma camera was expanded to 360 , and the partial encoding effect of CAI was attenuated. Compared to the standard coded-aperture gamma camera, the system has higher detection efficiency and is applicable to more complex radiation environment. The MURAs has mask and anti-mask subtraction, which can significantly reduce the loss of contrast in the image. That's a pivotal reason why it is ubiquitously used. However, to a rank N MURA, only the prime number N meets N ¼ 4m 1, can the antimasks of MURAs be obtained by 90 rotations around their central element. The unique rotating mechanisms structure of the system can achieve mask and anti-mask switching of all six detector modules without the condition above. All MURAs or even random arrays can be employed as the mask of the imaging system while maintaining mask and anti-mask subtraction. Meanwhile, the detector's pixel number can be selected more flexible. The experimental feasibility of measuring multiple sources in a 360 FOV was demonstrated with the panoramic image. The results showed that the panorama coded-aperture gamma camera could
Table 3 Angular resolution of the panorama coded-aperture gamma camera at 662 keV.
Fig. 10. Polar intensity distribution of g-rays, with the polar axis length indicates the relative radiation intensity value and polar axis angle shows the direction of radiation sources in the horizontal direction.
Detector number
Position resolution
Angular resolution
1 2 3 4 5 6
17.8 18.8 18.3 18.3 18.8 17.8
3.4 3.6 3.5 3.5 3.6 3.4
cm cm cm cm cm cm
at at at at at at
3 3 3 3 3 3
m m m m m m
40
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
clearly show the direction of multiple radiation sources in an extended radiation environment with only one measurement. The results can be further improved by applying the maximum likelihood expectation maximization (MLEM) method, which is iterative procedure to find the maximum probability estimates of parameters in statistical model (Dempster et al., 1977). While, for MLEM to perform well, a better forward model must be devised. In future work we will superimpose the gamma and CCD images and test the system in a more complex radiation environment. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2011YQ120096) and the National Natural Science Foundation of China (Grant no. 11205170, 11175200). References Bake, C.H., An, S.J., Kim, H., Kwak, S.W., Chung, Y.H., 2013. Development of a pinhole gamma camera for environment monitoring. Radiat. Meas. 59, 114e118. Caroli, E., Stephen, J.B., Cocco, G.D., Natalucci, L., Spizzichino, A., 1987. Coded aperture imaging in x- and gamma-ray astronomy. Space Sci. Rev. 45, 349e403. Dempster, A.P., Laird, N.M., Rubin, D.B., 1977. Maximum likelihood from incomplete data via the EM algorithm. J. R. Stat. Soc. Ser. B 39 (1), 1e38. Durrant, P.T., Dallimore, M., Jupp, I.D., Ramsden, D., 1999. The application of pinhole and coded aperture imaging in the nuclear environment. Nucl. Instrum. Methods Phys. Res. A 422, 667e671. Fenimore, E.E., Cannon, T.M., 1978. Coded aperture imaging with uniformly
redundant arrays. Appl. Opt. 17 (3), 337e347. Gal, O., Gmar, M., Ivanov, O.P., Laine, F., Lamadie, F., Goaller, C.L., Mahe, C., Manach, E., Stepanov, V.E., 2006. Development of a portable gamma camera with coded aperture. Nucl. Instrum. Methods Phys. Res. A 563, 233e237. Gmar, M., Gal, O., Goaller, C. Le, Ivanov, O.P., Potapov, V.N., Stepanov, V.E., Laine, F., Lamadie, F., 2004. Development of coded-aperture imaging with a compact gamma camera. IEEE Trans. Nucl. Sci. 51 (4), 1682e1687. Gottesman, S.R., Fenimore, E.E., 1989. New family of binary arrays for coded aperture imaging. Appl. Opt. 28 (20), 4344e4352. Guru, S.V., He, Z., Wehe, D.K., Knoll, G.F., Redus, R.H., Squillante, M.R., 1996. Portable high energy gamma ray imagers. Nucl. Instrum. Methods Phys. Res. A 378, 612e619. Hu, T.T., Shuai, L., Wang, P.L., Feng, B.T., Zhang, Y.W., Sun, Y.H., Li, X.H., Wei, S.J., Shan, B.C., Wei, L., 2014. Design of data acquisition system and algorithm research for omnidirectional gamma-ray positioning equipment. Chin. Phys. C 38 (1), 018202. Kaye, S.J., Kaye, W.R., He, Z., 2008. 4p Coded Aperture Imaging Using 3D Positionsensitive CdZnTe Detectors. NSS '08. IEEE 711e713. Olcott, P., Pratx, G., Johnson, D., Mittra, E., Niederkohr, R., Levin, C.S., 2014. Clinical evaluation of novel intraoperative handheld gamma camera for sentinel lymph node biopsy. Phys. Med. 30, 340e345. Penny, R.D., Hood, W.E., Polichar, R.M., Cardone, F.H., Chavez, L.G., Grubbs, S.G., Huntley, B.P., Kuharski, R.A., Shyffer, R.T., Fabris, L., Ziock, K.P., Labov, S.E., Nelson, K., 2011. A dual-sided coded-aperture radiation detection system. Nucl. Instrum. Meth. Phys. Res. A 652, 578e581. Siegel, S., Silverman, R.W., Shao, Y.P., Cherry, S.R., 1996. Simple charge division readouts for imaging scintillator arrays using a multi-channel PMT. IEEE Trans. Nucl. Sci. 43 (3), 1634e1641. Skinner, G.K., 1984. Imaging with coded-aperture masks. Nucl. Instrum. Meth. Phys. Res. A 221, 33e40. Zelakiewicz, S., Hoctor, R., Ivan, A., Ross, W., Nieters, E., Smith, W., McDevitt, D., Wittbrodt, M., Milbrath, B., 2011. SORIS dA standoff radiation imaging system. Nucl. Instrum. Meth. Phys. Res. A 652, 5e9.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Development of a panorama coded-aperture gamma camera for radiation detection Shifeng Sun a, b, c, Zhiming Zhang a, b, Lei Shuai a, b, Daowu Li a, b, Yingjie Wang a, b, Yantao Liu a, b, Xianchao Huang a, b, Haohui Tang a, b, Ting Li a, b, Pei Chai a, b, Xiaopan Jiang a, b, Bo Ma a, b, Meiling Zhu a, b, Xiaoming Wang a, b, Yiwen Zhang a, b, c, Wei Zhou a, b, c, Fanjian Zeng a, b, c, Jing Guo a, b, Liyang Sun a, b, c, Mingjie Yang a, b, c, Yubao Zhang a, b, Cunfeng Wei a, b, Chuangxin Ma a, b, Long Wei a, b, * a
Division of Nuclear Technology and Applications, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Beijing Engineering Research Center of Radiographic Techniques and Equipment, Beijing 100049, China c University of Chinese Academy of Sciences, Beijing 100049, China b
h i g h l i g h t s Development of a coded-aperture gamma camera. Panoramic gamma ray imaging in real time. Mitigate the partial encoding of coded aperture imaging (CAI). Experimental validation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2015 Received in revised form 21 March 2015 Accepted 17 April 2015 Available online 18 April 2015
For radiation detection, the imaging system should have a large field of view (FOV) and high detection efficiency because it has to be used in a radiation environment where the quantity and direction of radioactive sources are unknown. A panorama coded-aperture gamma camera optimized for use in complex nuclear environment has been developed and evaluated with an angular resolution of 3.5 . Typical gamma cameras have the limited field of view ranging from 10 to 60 in both horizontal and vertical direction. The system presented in this paper extends the field of view to 360 in the horizontal direction and 60 in the vertical direction. The partial encoding of coded aperture imaging is mitigated by convolving the data of diverse adjacent modules with a partial transmission function. The experimental feasibility of measuring multiple sources in the 360 horizontal field of view was demonstrated with a panoramic image. The results showed that the system could clearly identify the direction of multiple radiation sources in an unknown extended radiation environment. The system can help to simplify the clean up and decommissioning of nuclear sites. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coded aperture Gamma-ray imaging Modified uniformly redundant arrays Radiation detection
1. Introduction The gamma camera, which was first developed in the 1950s by Hal Anger, has been applied in many fields, including nuclear medicine (Olcott et al., 2014), industrial surveys (Bake et al., 2013),
* Corresponding author. Division of Nuclear Technology and Applications, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. E-mail addresses: [email protected] (S. Sun), [email protected] (L. Wei). http://dx.doi.org/10.1016/j.radmeas.2015.04.014 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
and homeland security (Penny et al., 2011). Early gamma cameras obtained a distribution of gamma rays by using a pinhole collimator, which are inefficient due to the low open fractions (Guru et al., 1996). In nuclear medicine, a pinhole collimator is used because the detector is placed very close to the source; however, when searching for hot spots during radiation detection, which is characterized by far-field geometry, the pinhole gamma camera has limited use. When the detected counting rate is low or dominated by background from non-FOV sources, the pinhole system has very low sensitivity (Gal et al., 2006).
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Coded aperture imagers (Skinner, 1984) extend the pinhole camera concept by employing a mask with many pinholes distributed with a predefined pattern. The coded aperture has more transparent regions, which ensures that the detector can receive more radiation from the radiation sources. Thus, coded aperture imaging has higher performance in terms of detection efficiency and sensitivity (Gmar et al., 2004). Unlike random pinhole distribution, which has the limitation of numerical instability due to the existence of zeros in the inverse-mask Fourier-domain transfer function, uniformly redundant arrays (URAs) (Fenimore and Cannon, 1978) and modified URAs (MURAs) (Gottesman and Fenimore, 1989) are considered to have near-delta-function system point-spread functions. By placing the pinholes in prime number-based patterns determined by sampling theory to form URAs and MURAs, the quality of the reconstructed images are greatly improved. Anti-masks of MURAs can be obtained by 90 rotations around their central element. The mask and anti-mask subtraction of MURAs can significantly reduce the loss of contrast in the image, which is caused by background noise and inconsistent detection efficiency of the detector. This enhanced the ability of the imaging system to detect weak sources at large distances in a relatively short time. The typical coded-aperture gamma camera (Caroli et al., 1987) is composed of a position-sensitive detector (PSD) and an active mask. The length of the active mask is two times that of the detector. As shown in Fig. 1, the detector's field of view (FOV, the fully encoded region) is defined by the mask size and the distance between the mask and the PSD. The partially coded field of view (PCFV) is defined as comprising all the directions for which only a fraction of the detected gamma rays is coded by the mask. In some conditions of a nuclear survey, the gamma-ray sources may distribute radiation in wide ranges that exceed the FOV of a gamma camera. Thus, the radiation sources distributing in a PCFV are partially coded, and normal decoding will lead to an incorrect imaging result. For radiation detection to search for missing radioactive sources or hot spots, the quantity and direction of radioactive sources are always unknown and the gamma camera has to perform with blind imaging (Hu et al., 2014). One way to mitigate these problems is to expand the FOV of the system. Decreasing the maskePSD separation can expand the FOV, while it will require a corresponding decrease in the PSD pixel size to give a similar angular resolution; however, this does not eliminate the partial encoding. Some attempts have been made to exempt the system from the typical limitations of partial encoding. SORIS (Zelakiewicz et al., 2011) employed a nonplanar mask to obtain a view of about 180 , but this suffered from a relatively low angular resolution. Kaye et al. (2008) demonstrated near-4p gamma-ray imaging by using multiple three-dimensional (3D) position-sensitive pixilated detectors by combing both Compton and coded aperture imaging techniques; however, the detection
Fig. 1. Field of view of the gamma camera.
35
efficiency is very poor. The purpose of this paper is to represent a novel hexagonal coded-aperture gamma camera that is able to measure multiple sources horizontally with a 360 FOV in real time. Different from the existing gamma cameras, which always require multiple times measurements, the system often needs only one measurement to survey entire room. The imaging system is optimized for nuclear facilities security inspection, the clean up and decommissioning of nuclear sites. These nuclear industrial applications require the imaging system should meet some criteria. These criteria should include: energy range 50 keV-1.5 MeV; energy resolution (<15% at 662 keV); anger resolution (<4 ); activity of radioisotopes range 10 mCi-10 Ci; distance between the system and radioisotopes 1e50 m at least; 360 field of view in the horizontal direction; compact, easy to use; short image acquisition times which implies maximized sensitivity (Durrant et al., 1999). To meet the detailed criteria above, those design parameters of the detectors, coded aperture, shield and etc were studied, characterized, and optimized accordingly. 2. Design and methods The panorama coded-aperture gamma camera is show in Fig. 2 with its photograph and the schematic diagram of the detector system. The overall dimensions of the gamma camera are about 42 42 80 cm3. The detector system is designed based on the typical coded-aperture gamma camera. Six pixilated detector modules are placed in a hexagonal shape with a 10 mm thick copper shield. Every detector module is separated from its adjacent modules by a 12 mm-thick rectangular lead for radiation shielding. Three masks and three anti-masks are placed in front of the six detector modules at intervals, and the corresponding aperture of each detector module is the anti-mask of the adjacent detector modules. These masks form a mask layer, which is mechanical isolated from the detector modules. The mask layer can rotate 60 clockwise and then turn back, which makes mask and anti-mask acquisition and subtraction possible. One of the benefits of this structure is that only one set of rotating mechanisms can achieve mask and anti-mask switching of all six detector modules. The selection of MURA rank depends on the required mask performance characteristics, which will determine angular resolution of the system. With consideration of the detector design parameters, rank 19 MURA was selected as the system mask. The mosaicked array of rank 19 MURA is shown in Fig. 3. The pixel pitch of the mask is 2.5 mm. The mask's dimensions are 92.5 92.5 6 mm3, and the primary component material is tungstenecopper alloy. Fig. 4 shows the detector module, analog and digital electronics of the panorama coded-aperture gamma camera. Each one of the detectors consists of a 19 19 pixilated CsI(Tl) scintillation crystal array and a position-sensitive photomultiplier tube (PSPMT). The crystal arrays have a thickness of 8.0 mm and an area of 50.0 mm 50.0 mm and are optically coupled to a Hamamatsu H8500 PSPMT. The 64 anode outputs of the H8500 are read into a discretized version of a single-wire, position-sensitive proportional counter readout (DPC) circuit (Siegel et al., 1996), and after amplification and shaping, the four signals are fed into the data acquisition system. The digital electronics complete 4 6 channel parallel sampling with a sampling frequency of 40 MHz and a sampling precision of 12 bits. After sampling, the digital pulses are differentially input into a field-programmable gate array (FPGA) in the form of serial signals. The FPGA controls the integration and digitization and then passes the data to the computer through a 1000-M base-T interface. The imaging system has a small vision-blind region due to the
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S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Fig. 2. Panorama coded-aperture camera. (a) Photograph and (b) schematic diagram of the detector system.
Fig. 3. The coded aperture pattern of MURA type. The aperture is a mosaic of a 19 19 MURA.
small gap between the detector modules. As radiation detection is characterized by far-field geometry, the small vision-blind region is ignored here. As shown in Fig. 5, the placement of the system is drawn at a certain scale and the different parts of the system are defined. The radiation sources in the FOV of one detector are very likely to be in the PCFV of the side (left or right) detector. This will
contribute some counts to the sideward detector and reduce the contrast of the image. The wrapped shielding of the detector and the thick lead shield between different detectors attenuate this effect. In addition, by dividing the horizontal field of view into different regions and processing the data of adjacent detector modules, the quality of the reconstructed image is improved. The usual reconstruction technique of coded aperture imaging is to convolve the recorded data with a decoding array G (Caroli et al., 1987). A method has been developed for the panorama gamma camera to mitigate the partial encoding without modifying the reconstruction algorithms. In order to eliminate partially coded data, we expand the definition of the source and observation spaces. A partial transmission function is defined to connect sources in PCFV and detectors on observation spaces. Each element of the function gives the probability to observe a particle emitted from a particular source pixel in a particular detector pixel. For instance, FOV of detector module 1 is divided into suspect regions and a confidence region. The 1#FOV is 60 and the confidence region is 30 . Suspect region 1 and 2 are divided into two equal parts, part A and B separately. Source spaces of detector module 1 are expanded to include 1#FOV, suspect region 1 part A and suspect region 2 part B. An initial estimate image can be obtained by the recorded data of all six detectors with decoding array G. Then, the projection counts of the sources in suspect region 1 part A and suspect region 2 part B are estimated by convolving the initial image with the partial transmission function. The filtered counts can be expressed
Fig. 4. Components of the panorama coded-aperture gamma camera. (a) Detector module with DPC circuit and connector; (b) analogous circuits and on-board FPGA, DAQ.
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
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without masks placed in front of the detectors. Fig. 6 illustrates the flood images of the six detectors. As the 64 anode signals of PSPMT are routed to the on-board DPC circuit that generates four anger signal outputs, the Anger logic (Siegel et al., 1996) was used to calculate the position of the incident gamma rays. All 361 crystals in each detector module can be identified, and the peak-to-valley ratio of the flood images is about 4:1. A lookup table was made to identify the pixels and determine the positions of the incident gamma rays. The majority of detector pixels give an energy resolution between 10% and 15% at 662 keV. After energy calibration, the detectors' energy resolution was measured at about 15% for 662 keV. The inconsistent detection efficiency of crystal pixels among detector modules was normalized. 3.2. Imaging performance test
Fig. 5. Definition of different regions of the system.
as a subtraction of the recorded data and the estimated projection counts. Finally, the reconstructed image is decoded by convolving the filtered counts with decoding array G. 3. Measurement set-up and performance test 3.1. Detector performance test To evaluate the performance of the detectors, flood images and energy resolution measurements of the detectors were taken. The raw images were created using a distantly placed cs-137 source
In order to evaluate the system response to a single radiation source, one 25 cm long 2 mCi Ge-68 rod source with a 2 mm lead shielding shell was placed in the FOV of one detector with a distance of about 4 m from the center line of the imaging system. The detectors are named in clockwise order from 1 to 6, and the radiation source was placed in the FOV of detector 4. Before the sources were placed, the background was measured for 600 s; then the system acquired data for 600 s without the rotation of the mask layer. The experiment was performed with an energy window of 200 keVe1000 keV, and results are listed in Table 1. As Table 1 indicates, the radiation source was placed in the FOV of detector 4. Detectors 3 and 5, which are on the left and right side of the detector 4, respectively, also have a relative high count because the radiation source was not in their FOV but in their PCFV. Detectors 1, 2, and 6 are not as affected by the radiation source. To evaluate the imaging performance of the system, three 25 cm long Ge-68 rod sources with a 15.5 mm lead shielding shell were placed in three separate directions of the system with distances of about 3 m, 3 m and 3.5 m from the center line of the imaging system. The activities of the sources are 1.5, 2.5 and 2 mCi. As shown in Fig. 7, the radiation sources were placed in the FOV of
Fig. 6. Two-dimensional flood images measured of the system's six detectors. The resolution of the flood images are 512 512 pixels.
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S. Sun et al. / Radiation Measurements 77 (2015) 34e40
Table 1 System response to a single radiation source. Detector number
Counts (N)
Background (Nb)
Net counts (N0)
Net counting rate (n/s)
1 2 3 4 5 6
12,660 11,772 31,474 73,670 28,387 12,920
12,033 11,580 13,261 12,128 11,102 11,829
627 192 18,213 61,542 17,28 1091
1.04 0.32 30.36 102.57 28.81 1.82
performed twice with an energy window of 200 keVe1000 keV. Counts are listed in Table 2. It is difficult to infer from Table 2 where the radiation sources are as there is no clear count rate distribution. The lower counting rates compared to the previous experiment is because the radiation sources are in a thick lead shielding shell.
4. Results
Fig. 7. Experimental arrangement of system response to multiple radiation sources.
detectors 1, 3, and 5. Before the sources were placed, the background was measured for 600 s. The system first acquired data for 300 s as the mask mode, after which the mask layer rotated 60 clockwise to the anti-mask mode and acquired data for 300 s. After the completion of data acquisition, the mask layer rotated 60 anticlockwise back to the mask mode. The experiment was
The first test of the system response to a single radiation source verified that the detectors are affected by the source in the PCFV. This causes a loss of contrast in the reconstructed images. Fig. 8 shows the reconstructed gamma image of one Ge-68 source. As the source is relative strong, the mask and anti-mask subtraction of MURAs was not used to improve the quality of the reconstructed images. From this figure, we can identify the precise location of the radiation source. The loss of contrast in the image was mainly caused by inconsistent detection efficiency of the detectors. In the multiple radiation sources imaging experiment, the thick lead shielding shells were used to reduce the degree of activeness of the radiation source. This also simulated the search of unknown strong shield radioactive sources, which may occur in a real sense. Whether to use the mask and anti-mask subtraction of MURAs or not depends on the complexity of the radiation environment. The mask and anti-mask subtraction of MURAs was used here. The image of multiple radiation sources was reconstructed, and displayed in two ways to make the results more intuitive. Fig. 9 shows the 3D gamma image of three Ge-68 sources, which was plotted by superposing the reconstructed image with a unit sphere.
Table 2 System response to multiple radiation sources. Detector number
Counts first time
Counts second time
Background
Net counting rate (n/s)
1 2 3 4 5 6
20,588 16,451 35,276 23,254 29,124 16,008
21,045 17,106 34,004 23,308 29,065 16,227
11,818 11,766 13,354 12,284 11,253 11,612
15.00 8.35 35.48 18.32 29.74 7.51
Fig. 8. Reconstructed gamma image of one Ge-68 source.
S. Sun et al. / Radiation Measurements 77 (2015) 34e40
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Fig. 9. Three-dimensional display of a gamma image of three Ge-68 sources. The figure was plotted by superposing the reconstructed image with a unit sphere.
The origin represents the center point of the imaging system. The pseudo color of every point on the spherical surface represents the relative radiation intensity value. Fig. 10 shows the polar intensity distribution of the g-ray. The polar diagram was plotted by adding the row values of the three identified hot spots. In addition to the imaging tests, we conducted the angular resolution tests using a 30 mCi Cs-137 point source. Given the dimensions of mask/detector elements, the angular resolution of the system in theory is 3.1. The average test angular resolution is 3.5 ; it's close to the theoretical value. Table 3 lists the test angular
resolution of all six detector modules. 5. Discussion and conclusions A panorama coded-aperture gamma camera, which has an FOV of 360 in the horizontal direction and 60 in the vertical direction, was developed and evaluated. The panorama gamma image was obtained by combining the recorded data of the hexagonally placed detector arrays. The FOV of coded-aperture gamma camera was expanded to 360 , and the partial encoding effect of CAI was attenuated. Compared to the standard coded-aperture gamma camera, the system has higher detection efficiency and is applicable to more complex radiation environment. The MURAs has mask and anti-mask subtraction, which can significantly reduce the loss of contrast in the image. That's a pivotal reason why it is ubiquitously used. However, to a rank N MURA, only the prime number N meets N ¼ 4m 1, can the antimasks of MURAs be obtained by 90 rotations around their central element. The unique rotating mechanisms structure of the system can achieve mask and anti-mask switching of all six detector modules without the condition above. All MURAs or even random arrays can be employed as the mask of the imaging system while maintaining mask and anti-mask subtraction. Meanwhile, the detector's pixel number can be selected more flexible. The experimental feasibility of measuring multiple sources in a 360 FOV was demonstrated with the panoramic image. The results showed that the panorama coded-aperture gamma camera could
Table 3 Angular resolution of the panorama coded-aperture gamma camera at 662 keV.
Fig. 10. Polar intensity distribution of g-rays, with the polar axis length indicates the relative radiation intensity value and polar axis angle shows the direction of radiation sources in the horizontal direction.
Detector number
Position resolution
Angular resolution
1 2 3 4 5 6
17.8 18.8 18.3 18.3 18.8 17.8
3.4 3.6 3.5 3.5 3.6 3.4
cm cm cm cm cm cm
at at at at at at
3 3 3 3 3 3
m m m m m m
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clearly show the direction of multiple radiation sources in an extended radiation environment with only one measurement. The results can be further improved by applying the maximum likelihood expectation maximization (MLEM) method, which is iterative procedure to find the maximum probability estimates of parameters in statistical model (Dempster et al., 1977). While, for MLEM to perform well, a better forward model must be devised. In future work we will superimpose the gamma and CCD images and test the system in a more complex radiation environment. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2011YQ120096) and the National Natural Science Foundation of China (Grant no. 11205170, 11175200). References Bake, C.H., An, S.J., Kim, H., Kwak, S.W., Chung, Y.H., 2013. Development of a pinhole gamma camera for environment monitoring. Radiat. Meas. 59, 114e118. Caroli, E., Stephen, J.B., Cocco, G.D., Natalucci, L., Spizzichino, A., 1987. Coded aperture imaging in x- and gamma-ray astronomy. Space Sci. Rev. 45, 349e403. Dempster, A.P., Laird, N.M., Rubin, D.B., 1977. Maximum likelihood from incomplete data via the EM algorithm. J. R. Stat. Soc. Ser. B 39 (1), 1e38. Durrant, P.T., Dallimore, M., Jupp, I.D., Ramsden, D., 1999. The application of pinhole and coded aperture imaging in the nuclear environment. Nucl. Instrum. Methods Phys. Res. A 422, 667e671. Fenimore, E.E., Cannon, T.M., 1978. Coded aperture imaging with uniformly
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