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X-ray imaging using a single plastic scintillating fiber
Nuclear Instruments and Methods in Physics Research B 225 (2004) 617–622 www.elsevier.com/locate/nimb
X-ray imaging using a single plastic scintillating fiber Mohammad M. Nasseri a
a,b,*
, Zejie Yin a, Xiaoyi Wu a, Da-Ming Zhu
a,c
Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China b Atomic Energy Organization of Iran, P.O. Box 11365-3486, Tehran, Iran c Department of Physics, University of Missouri – Kansas City, Kansas City, MO 64110, USA Received 17 November 2003; received in revised form 15 May 2004
Abstract In this work, we demonstrated that the data acquisition for either computerized tomography (CT) or digital radiography (DR) could be achieved by using a single plastic scintillation fiber coupled with a photomultiplier. The method is simple and particularly useful for imaging small objects. We describe the experimental set-up and procedure used in obtaining images. We show that the results obtained by using this method are impressive and the method can be implemented in many laboratories to demonstrate the concept of radiation imaging. 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Mc Keywords: Scintillating fiber; Imaging; Radiography; Tomography
1. Introduction The term ‘‘Digital X-ray Imaging’’ refers to a variety of technologies that electronically capture X-ray images and one of those technologies is CT scan. Computerized tomography (CT) has been widely used in the fields of medicine and many other technological industries since earlier 1970s of last century [1], and has been advanced further in the past two decades, along with the development of computer and digital imaging techniques. The technique relies on a combination of radiation and detection, mathematics, and computer technolo*
Corresponding author. Tel.: +86-551-3665426; fax: +86551-3601164. E-mail address: [email protected] (M.M. Nasseri).
gies. Among these, the radiation and detection is relatively involved and could be expensive. Usually, the radiation source can be an X-ray generator or a c-ray source such as Cs-137, Ir-192. Radiation detection part consists of an array of detectors that can produce secondary particles like electrons or photons. Using a proportional readout part; these secondary particles can be measured [2]. Each detector is accompanied by a series of electronics components, which could be costly. To reduce the cost, CT systems with single pixel detector were developed and commercialized [3]. A commonly used single pixel detector CT system uses cadmium zinc telluride (CZT) detector with a collimator of a few hundred micrometer diameter. The system can achieve a resolution as good as that achieved by array detectors.
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.05.032
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M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
In this study, we explored a new method that employ a single plastic scintillation fiber (PSF) coupled with a photomultiplier (PMT) as a radiation detector to obtain radiographic or tomographic images. Scintillation fibers have been widely used in detecting high energy charge particles and radiations [4]. The PSF is an organic scintillator and it is composed of a scintillation core, generally made of polystyrene doped with fluorescent materials. When X-ray, c-ray or ionizing radiation, in general, impinge on the fiber core, the energy lost by incident particles excites mainly molecules of the solvent or doped material [5]. The relaxation is followed by an isotropic light emission. The fundamental mechanisms of the excitation and scintillation properties of an organic scintillators is, roughly speaking, the same as that in the inorganic scintillators, except in the organic case the excitations occur between bound states of molecules which typically give rise to an emission spectrum in the ultra-violet range. When the organic material is doped with a low concentration of wavelength shifter (WLS), the emission spectrum would be shifted to longer wavelength around visible range and the absorption is greatly reduced. The main advantages of PSF are their fast response (typical the decay time is 2–3 ns), flexibility, and electromagnetic immunity [6]. The core of a PSF is surrounded with a cladding layer with a lower refraction index. A small fraction of emitted light generated by incoming ionized radiation would be trapped and travels along the fiber, identical to the case of optical fibers used in telecommunications. It is thus possible to detect the light from one end of PSF using PMT while using the other, the open end of the PSF as a single pixel in performing X-ray or c-ray imaging. This idea has been explored in building PSF arrays as imaging elements in applications such as positron emission tomography (PET) and radiographic imaging [7,8]. For steady radiations, instead of using PSF arrays, the idea can also be implemented by either scan a single PFS across an object or moving the object across the open end of PSF steps. The main advantage of this approach over the PSF array is that cross talk (emission from one fiber enter the other) between neighbor-
ing fibers can be eliminated and, thus, the resolution may be increased.
2. Experimental set-up The experimental set-up is illustrated in Fig. 1(a). A dental X-ray tube with continuous tungsten anode was used as a simple X-ray source. The peak voltage and current of the tube respectively are 65 kVp and 1.5 mA. The X-ray beam was focused to an area of dimension of 0.3 mm · 0.3 mm. The X-ray beam was pre-collimated through a hole of 2 mm in diameter. An aluminum plate with 1.5 mm in thickness was used as a filter to the X-ray. The filter eliminates those photons that have energy less than 20 keV almost entirely. So, because of the filtering, the width of the spectrum becomes narrow (20–65 keV). The beam-hardening artifact due to polychromatic X-ray beam is almost negligible, because of the narrowed spectrum in X-ray and because the relatively small thickness of the objects being imaged in this study. A BCF-10 type scintillation fiber with a diameter of 1 and 20 cm in length, purchased from Bicron Corporation [9], was used in the experiment. The core material of the fiber is polystyrene [1% butyl–phenyl–byphenylyloxadiazole (PBD) doped] and refractive index of 1.60 and density of 1.05 g/cm3 . The cladding material is polymethylmethacrylate (PMMA) with refractive index of 1.49. The thickness of the cladding layer is about 3% of core diameter. The numerical aperture of the fiber is 0.58, so the trapping efficiency is 3.44%. The number of emission light per MeV is about 8000 photon. The light emission spectrum of the PSF is peaked at 432 nm. The decay time is 2.7 ns. The fiber was connected to a photomultiplier in one end; the other end was placed through a small hole in a Pb, X-ray shielding block, as shown in Fig. 1(b). A portion (10 cm) of the fiber near the open end was aligned with the X-ray beam to assure the maximum detection efficiency. The other end was polished and was optically coupled directly at the entrance of the PMT. Great care was taken to maximize light-collection efficiency. The whole fiber was covered by a black masking tape to protect the system from outside light. Also, the
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
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Fig. 1. (a) Photograph of the experimental set-up. (b) A schematic of the experimental set-up.
open end of the fiber was fixed slightly within the Pb block (post-collimator) to reduce the influence of scattered radiation. The photomultiplier tube (PMT) used is a R1166 from HAMAMATSU, Inc. It has a cathode sensitivity of 105 lA/lm for radiation with wavelength in the range between 300 and 650 nm. The peak wavelength of the spectral response curve of the PMT is centered at 420 nm. At the emission peak wavelength of PSF, the quantum efficiency of the PMT is 25%, which is also the maximum efficiency of the PMT. A manipulator which can move an object freely in both the vertical and horizontal direction is placed in between the X-ray generator and the open end of PSF. A Tektronix oscilloscope (TDS 3032) was used for recording signal from the PMT measure signal amplitude with high precision. We have imaged four simple objects to demonstrate the effectiveness of this method. The four objects imaged are Sample A – a thick wall aluminum cylinder with the outer diameter being 14 mm and inner diameter being 6 mm and has
11 mm in length. The cylinder is surrounded by a 1 mm thick plastic host. Sample B – a ceramic cylinder that has outer and inner diameters of 9 and 5.5 mm, respectively. Sample C – a single pin electronic connector made by Al–Cu alloy, the outer and inner diameters of the outside cylinder are 5.5 and 4.5 mm, respectively. The pin in the center has a diameter of 1 mm. Sample D – an electronic connector made of an Al cylinder with the outer and inner diameters being 11.5 and 9 mm, respectively. The four pins are made of Cu and have a diameter of 1.5 mm. The basic principle of this experiment is as follows: X-rays coming out from the X-ray generator passing through the object for imaging enter the PSF. The X-ray would then produce electromagnetic wave with wavelength in the visible range in the PSF. As described above, a portion of the produced light would propagate along the fiber and produced a signal at PMT. The amount of the photons that reach to PMT depends on X-ray energy, radiation intensity and optical properties
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M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
of the PSF used. The amount of photons that can be detected by PMT depends on the X-ray energy and intensity, trapping and transmitting efficiency of the fiber and the quantum efficiency of the PMT. In our case, the number of observed photon by PMT is only about 0.6 per an X-ray quanta. Because of this small value, care must be taken to cover the side walls of the fiber to prevent any environmental light entering the fiber. Small amount of this undesirable light would not only create noise seriously but also it may cause damage to PMT. Images were taken by scanning the object in steps both in horizontal and vertical directions. The received data were then analyzed using a home developed computerized tomography software to generate images [10]. The image reconstruction algorithm used is based on filtered back projection method and Fourier Slice Theorem [1]. This algorithm has been widely used in tomography imaging [11]. The algorithm we used does not have the capacity of compensating for artifacts. However, as mentioned above, the artifacts involved are negligible because of the size of the samples and X-ray energy spectrum used in this study. In our study, instead of moving X-ray source and the detector in scanning an object, the sample was placed on the manipulator that can move both in vertical and in horizontal directions. For tomography imaging, the sample was scanned horizontally with step size of 1 mm. When this horizontal line scan was completed, the sample was rotated by a small angle about the vertical axis. The angle rotated was typically 2 or less. Then the line scan was repeated. For radiography imaging, when a line scan was completed, the sample was moved vertically by a small step, and the horizontal line scan was repeatedly, until the entire sample was scanned. The measuring time for each step is about 2 s. Then it took about the 10 s for displacement the sample accurately, so total time for both measuring and displacement of the object is 12 s/step. Depends on the size of the sample and resolution required, it takes different amount of time to complete an image. If a fast stepping motor is employed, such a time can be reduced to 3 s/step.
3. Results and discussion For sample A – an aluminum cylinder host by a plastic host, the tomography image was taken on a plane parallel to the cylindrical axis. For the rest of the three objects, the images were taken on planes perpendicular to the axis of the cylinders. The tomographic imaging planes are indicated in Fig. 2(a). Fig. 2(b) shows both photographic images and computerized tomographic images taken on the planes described above for the four objects. The photographic and computerized tomographic images were not shown on a same scale. As can be seen, all the details of the objects are reflected by the computerized tomographic images. For object A, if the contrast is increased, the plastic host and a small plastic tape used for fixing the object on the manipulator can be seen. The image for object B is relatively featureless, reflecting the uniform of the ceramic cylinder. For object D, the gap on the ring and pins are all clearly shown in the CT images. The quality of an imaging system can be characterized by its modulation transfer function (MTF). We have calculated the MTF of this imaging system by using an edge spread function (ESF) method, which is the result obtained by the imaging system in imaging an edge [12]. The MTF curve of the system is shown in Fig. 3. According to the curve, the MTF of 20% is achievable for objects with spatial frequency of 0.05 lp/mm. The resolution of our imaging system is about 10 times poorer than that described in [12] while the size of the fiber we used is 20 times larger than that used in the same study. This enhancement in resolution is due to the lack of cross-talk when a single fiber is used. In general, the smaller the scan step size and the diameter of PSF used, the better a resolution one can achieve. Noise presents another limitation on the ability to visualize both large and small features. We characterized the signal-to-noise ratio (SNR) of our system by measuring the average voltage signals for aluminum samples with different thickness. The results shown in Fig. 4 indicate that the SNR of the system is 5 when the aluminum thickness is 3 cm. To increase SNR, one needs to utilize X-ray sources which can provide higher intensity. The
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
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Fig. 2. (a) An illustration of the tomography imaging plane of the four objects imaged. (b) The top ones are photography images of the four objects imaged. The bottom ones are the tomography images taken on the imaging planes indicated in Fig. 2(a).
Fig. 3. MTF curve of the system.
Fig. 4. SNR versus aluminum thickness.
figure also shows that a better SNR has been achieved when imaging relatively thin objects.
Comparing to currently available commercial computed tomography systems, the method we
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M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
have developed here is very simple and requires only a few necessary instruments. If the system is stable and there is no environmental drift, the system we describe here should achieve to more acceptable results and also should find more application in radiation imaging fields. The disadvantage of the method is that it is relatively time consuming. Using a fast stepping motor might compensate this disadvantage.
References [1] A.C. Kak, M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, New York, 1999. [2] G.F. Knoll, Radiation Detection and Measurement, Wiley, New York, 1989. [3] M.J. Paulus, H. Sari-Sarraf, S.S. Gleason, M. Bobrek, J.S. Hicks, D.K. Johnson, J.K. Behel, L.H. Thompson, W.C. Allen, IEEE Trans. Nucl. Sci. 46 (3) (1999) 558.
[4] N.S.P. King et al., An 800-MeV proton radiography facility for dynamic experiments, Nucl. Instr. and Meth. A 424 (1999) 84. [5] T.O. White, Scintillating fiber, Nucl. Instr. and Meth. A 273 (1988) 820. [6] V. Billini et al., Experimental study of high-energy resolution lead/scintillating fiber calorimetry in the 600–1200 MeV energy region, Nucl. Instr. and Meth. A 386 (1997) 254. [7] A. Del Guerra et al., Latest achievements in PET techniques, Nucl. Instr. and Meth. A 513 (2003) 13. [8] A. Ikhlef, M. Skowronek, A.S. Beddar, X-ray imaging and detection using plastic scintillating fibers, Nucl. Instr. and Meth. A 442 (2000) 428. [9] Bicron Corporation web site (http://www.bicron.com), 2003. [10] Ma Kui, M.S. Thesis, University of Science and Technology of China (unpublished). [11] G. Michael, X-ray computed tomography, Phys. Educ. 36 (6) (2001) 442. [12] A. Ikhlef, M. Skowronek, Application of a plastic scintillating fiber array for low-energy X-ray imaging, Appl. Opt. 37 (34) (1998).
X-ray imaging using a single plastic scintillating fiber Mohammad M. Nasseri a
a,b,*
, Zejie Yin a, Xiaoyi Wu a, Da-Ming Zhu
a,c
Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China b Atomic Energy Organization of Iran, P.O. Box 11365-3486, Tehran, Iran c Department of Physics, University of Missouri – Kansas City, Kansas City, MO 64110, USA Received 17 November 2003; received in revised form 15 May 2004
Abstract In this work, we demonstrated that the data acquisition for either computerized tomography (CT) or digital radiography (DR) could be achieved by using a single plastic scintillation fiber coupled with a photomultiplier. The method is simple and particularly useful for imaging small objects. We describe the experimental set-up and procedure used in obtaining images. We show that the results obtained by using this method are impressive and the method can be implemented in many laboratories to demonstrate the concept of radiation imaging. 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Mc Keywords: Scintillating fiber; Imaging; Radiography; Tomography
1. Introduction The term ‘‘Digital X-ray Imaging’’ refers to a variety of technologies that electronically capture X-ray images and one of those technologies is CT scan. Computerized tomography (CT) has been widely used in the fields of medicine and many other technological industries since earlier 1970s of last century [1], and has been advanced further in the past two decades, along with the development of computer and digital imaging techniques. The technique relies on a combination of radiation and detection, mathematics, and computer technolo*
Corresponding author. Tel.: +86-551-3665426; fax: +86551-3601164. E-mail address: [email protected] (M.M. Nasseri).
gies. Among these, the radiation and detection is relatively involved and could be expensive. Usually, the radiation source can be an X-ray generator or a c-ray source such as Cs-137, Ir-192. Radiation detection part consists of an array of detectors that can produce secondary particles like electrons or photons. Using a proportional readout part; these secondary particles can be measured [2]. Each detector is accompanied by a series of electronics components, which could be costly. To reduce the cost, CT systems with single pixel detector were developed and commercialized [3]. A commonly used single pixel detector CT system uses cadmium zinc telluride (CZT) detector with a collimator of a few hundred micrometer diameter. The system can achieve a resolution as good as that achieved by array detectors.
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.05.032
618
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
In this study, we explored a new method that employ a single plastic scintillation fiber (PSF) coupled with a photomultiplier (PMT) as a radiation detector to obtain radiographic or tomographic images. Scintillation fibers have been widely used in detecting high energy charge particles and radiations [4]. The PSF is an organic scintillator and it is composed of a scintillation core, generally made of polystyrene doped with fluorescent materials. When X-ray, c-ray or ionizing radiation, in general, impinge on the fiber core, the energy lost by incident particles excites mainly molecules of the solvent or doped material [5]. The relaxation is followed by an isotropic light emission. The fundamental mechanisms of the excitation and scintillation properties of an organic scintillators is, roughly speaking, the same as that in the inorganic scintillators, except in the organic case the excitations occur between bound states of molecules which typically give rise to an emission spectrum in the ultra-violet range. When the organic material is doped with a low concentration of wavelength shifter (WLS), the emission spectrum would be shifted to longer wavelength around visible range and the absorption is greatly reduced. The main advantages of PSF are their fast response (typical the decay time is 2–3 ns), flexibility, and electromagnetic immunity [6]. The core of a PSF is surrounded with a cladding layer with a lower refraction index. A small fraction of emitted light generated by incoming ionized radiation would be trapped and travels along the fiber, identical to the case of optical fibers used in telecommunications. It is thus possible to detect the light from one end of PSF using PMT while using the other, the open end of the PSF as a single pixel in performing X-ray or c-ray imaging. This idea has been explored in building PSF arrays as imaging elements in applications such as positron emission tomography (PET) and radiographic imaging [7,8]. For steady radiations, instead of using PSF arrays, the idea can also be implemented by either scan a single PFS across an object or moving the object across the open end of PSF steps. The main advantage of this approach over the PSF array is that cross talk (emission from one fiber enter the other) between neighbor-
ing fibers can be eliminated and, thus, the resolution may be increased.
2. Experimental set-up The experimental set-up is illustrated in Fig. 1(a). A dental X-ray tube with continuous tungsten anode was used as a simple X-ray source. The peak voltage and current of the tube respectively are 65 kVp and 1.5 mA. The X-ray beam was focused to an area of dimension of 0.3 mm · 0.3 mm. The X-ray beam was pre-collimated through a hole of 2 mm in diameter. An aluminum plate with 1.5 mm in thickness was used as a filter to the X-ray. The filter eliminates those photons that have energy less than 20 keV almost entirely. So, because of the filtering, the width of the spectrum becomes narrow (20–65 keV). The beam-hardening artifact due to polychromatic X-ray beam is almost negligible, because of the narrowed spectrum in X-ray and because the relatively small thickness of the objects being imaged in this study. A BCF-10 type scintillation fiber with a diameter of 1 and 20 cm in length, purchased from Bicron Corporation [9], was used in the experiment. The core material of the fiber is polystyrene [1% butyl–phenyl–byphenylyloxadiazole (PBD) doped] and refractive index of 1.60 and density of 1.05 g/cm3 . The cladding material is polymethylmethacrylate (PMMA) with refractive index of 1.49. The thickness of the cladding layer is about 3% of core diameter. The numerical aperture of the fiber is 0.58, so the trapping efficiency is 3.44%. The number of emission light per MeV is about 8000 photon. The light emission spectrum of the PSF is peaked at 432 nm. The decay time is 2.7 ns. The fiber was connected to a photomultiplier in one end; the other end was placed through a small hole in a Pb, X-ray shielding block, as shown in Fig. 1(b). A portion (10 cm) of the fiber near the open end was aligned with the X-ray beam to assure the maximum detection efficiency. The other end was polished and was optically coupled directly at the entrance of the PMT. Great care was taken to maximize light-collection efficiency. The whole fiber was covered by a black masking tape to protect the system from outside light. Also, the
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
619
Fig. 1. (a) Photograph of the experimental set-up. (b) A schematic of the experimental set-up.
open end of the fiber was fixed slightly within the Pb block (post-collimator) to reduce the influence of scattered radiation. The photomultiplier tube (PMT) used is a R1166 from HAMAMATSU, Inc. It has a cathode sensitivity of 105 lA/lm for radiation with wavelength in the range between 300 and 650 nm. The peak wavelength of the spectral response curve of the PMT is centered at 420 nm. At the emission peak wavelength of PSF, the quantum efficiency of the PMT is 25%, which is also the maximum efficiency of the PMT. A manipulator which can move an object freely in both the vertical and horizontal direction is placed in between the X-ray generator and the open end of PSF. A Tektronix oscilloscope (TDS 3032) was used for recording signal from the PMT measure signal amplitude with high precision. We have imaged four simple objects to demonstrate the effectiveness of this method. The four objects imaged are Sample A – a thick wall aluminum cylinder with the outer diameter being 14 mm and inner diameter being 6 mm and has
11 mm in length. The cylinder is surrounded by a 1 mm thick plastic host. Sample B – a ceramic cylinder that has outer and inner diameters of 9 and 5.5 mm, respectively. Sample C – a single pin electronic connector made by Al–Cu alloy, the outer and inner diameters of the outside cylinder are 5.5 and 4.5 mm, respectively. The pin in the center has a diameter of 1 mm. Sample D – an electronic connector made of an Al cylinder with the outer and inner diameters being 11.5 and 9 mm, respectively. The four pins are made of Cu and have a diameter of 1.5 mm. The basic principle of this experiment is as follows: X-rays coming out from the X-ray generator passing through the object for imaging enter the PSF. The X-ray would then produce electromagnetic wave with wavelength in the visible range in the PSF. As described above, a portion of the produced light would propagate along the fiber and produced a signal at PMT. The amount of the photons that reach to PMT depends on X-ray energy, radiation intensity and optical properties
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M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
of the PSF used. The amount of photons that can be detected by PMT depends on the X-ray energy and intensity, trapping and transmitting efficiency of the fiber and the quantum efficiency of the PMT. In our case, the number of observed photon by PMT is only about 0.6 per an X-ray quanta. Because of this small value, care must be taken to cover the side walls of the fiber to prevent any environmental light entering the fiber. Small amount of this undesirable light would not only create noise seriously but also it may cause damage to PMT. Images were taken by scanning the object in steps both in horizontal and vertical directions. The received data were then analyzed using a home developed computerized tomography software to generate images [10]. The image reconstruction algorithm used is based on filtered back projection method and Fourier Slice Theorem [1]. This algorithm has been widely used in tomography imaging [11]. The algorithm we used does not have the capacity of compensating for artifacts. However, as mentioned above, the artifacts involved are negligible because of the size of the samples and X-ray energy spectrum used in this study. In our study, instead of moving X-ray source and the detector in scanning an object, the sample was placed on the manipulator that can move both in vertical and in horizontal directions. For tomography imaging, the sample was scanned horizontally with step size of 1 mm. When this horizontal line scan was completed, the sample was rotated by a small angle about the vertical axis. The angle rotated was typically 2 or less. Then the line scan was repeated. For radiography imaging, when a line scan was completed, the sample was moved vertically by a small step, and the horizontal line scan was repeatedly, until the entire sample was scanned. The measuring time for each step is about 2 s. Then it took about the 10 s for displacement the sample accurately, so total time for both measuring and displacement of the object is 12 s/step. Depends on the size of the sample and resolution required, it takes different amount of time to complete an image. If a fast stepping motor is employed, such a time can be reduced to 3 s/step.
3. Results and discussion For sample A – an aluminum cylinder host by a plastic host, the tomography image was taken on a plane parallel to the cylindrical axis. For the rest of the three objects, the images were taken on planes perpendicular to the axis of the cylinders. The tomographic imaging planes are indicated in Fig. 2(a). Fig. 2(b) shows both photographic images and computerized tomographic images taken on the planes described above for the four objects. The photographic and computerized tomographic images were not shown on a same scale. As can be seen, all the details of the objects are reflected by the computerized tomographic images. For object A, if the contrast is increased, the plastic host and a small plastic tape used for fixing the object on the manipulator can be seen. The image for object B is relatively featureless, reflecting the uniform of the ceramic cylinder. For object D, the gap on the ring and pins are all clearly shown in the CT images. The quality of an imaging system can be characterized by its modulation transfer function (MTF). We have calculated the MTF of this imaging system by using an edge spread function (ESF) method, which is the result obtained by the imaging system in imaging an edge [12]. The MTF curve of the system is shown in Fig. 3. According to the curve, the MTF of 20% is achievable for objects with spatial frequency of 0.05 lp/mm. The resolution of our imaging system is about 10 times poorer than that described in [12] while the size of the fiber we used is 20 times larger than that used in the same study. This enhancement in resolution is due to the lack of cross-talk when a single fiber is used. In general, the smaller the scan step size and the diameter of PSF used, the better a resolution one can achieve. Noise presents another limitation on the ability to visualize both large and small features. We characterized the signal-to-noise ratio (SNR) of our system by measuring the average voltage signals for aluminum samples with different thickness. The results shown in Fig. 4 indicate that the SNR of the system is 5 when the aluminum thickness is 3 cm. To increase SNR, one needs to utilize X-ray sources which can provide higher intensity. The
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
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Fig. 2. (a) An illustration of the tomography imaging plane of the four objects imaged. (b) The top ones are photography images of the four objects imaged. The bottom ones are the tomography images taken on the imaging planes indicated in Fig. 2(a).
Fig. 3. MTF curve of the system.
Fig. 4. SNR versus aluminum thickness.
figure also shows that a better SNR has been achieved when imaging relatively thin objects.
Comparing to currently available commercial computed tomography systems, the method we
622
M.M. Nasseri et al. / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 617–622
have developed here is very simple and requires only a few necessary instruments. If the system is stable and there is no environmental drift, the system we describe here should achieve to more acceptable results and also should find more application in radiation imaging fields. The disadvantage of the method is that it is relatively time consuming. Using a fast stepping motor might compensate this disadvantage.
References [1] A.C. Kak, M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, New York, 1999. [2] G.F. Knoll, Radiation Detection and Measurement, Wiley, New York, 1989. [3] M.J. Paulus, H. Sari-Sarraf, S.S. Gleason, M. Bobrek, J.S. Hicks, D.K. Johnson, J.K. Behel, L.H. Thompson, W.C. Allen, IEEE Trans. Nucl. Sci. 46 (3) (1999) 558.
[4] N.S.P. King et al., An 800-MeV proton radiography facility for dynamic experiments, Nucl. Instr. and Meth. A 424 (1999) 84. [5] T.O. White, Scintillating fiber, Nucl. Instr. and Meth. A 273 (1988) 820. [6] V. Billini et al., Experimental study of high-energy resolution lead/scintillating fiber calorimetry in the 600–1200 MeV energy region, Nucl. Instr. and Meth. A 386 (1997) 254. [7] A. Del Guerra et al., Latest achievements in PET techniques, Nucl. Instr. and Meth. A 513 (2003) 13. [8] A. Ikhlef, M. Skowronek, A.S. Beddar, X-ray imaging and detection using plastic scintillating fibers, Nucl. Instr. and Meth. A 442 (2000) 428. [9] Bicron Corporation web site (http://www.bicron.com), 2003. [10] Ma Kui, M.S. Thesis, University of Science and Technology of China (unpublished). [11] G. Michael, X-ray computed tomography, Phys. Educ. 36 (6) (2001) 442. [12] A. Ikhlef, M. Skowronek, Application of a plastic scintillating fiber array for low-energy X-ray imaging, Appl. Opt. 37 (34) (1998).