- Email: [email protected]
A simple scanner for Compton tomography
Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
A simple scanner for Compton tomography Roberto Cesareoa,*, Cesare Cappio Borlinoa, Antonio Brunettia, Bruno Golosioa, Alfredo Castellanob a
Instituto di Matematica e Fisica, Universita" di Sassari, Via Vienna 2, 07100 Sassari, Italy b Dip. di Scienza dei Materiali, Universita" di Lecce, Lecce, Italy
Abstract A first generation CT-scanner was designed and constructed to carry out Compton images. This CT-scanner is composed of a 80 kV, 5 mA X-ray tube and a NaI(Tl) X-ray detector; the tube is strongly collimated, generating a X-ray beam of 2 mm diameter, whilst the detetor is not collimated to collect Compton photons from the whole irradiated cylinder. The performances of the equipment were tested contemporaneous transmission and Compton images. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Compton scattering; Compton tomography; Transmission tomography
1. Introduction A first generation ‘‘home made’’ transmission CT-scanner, using a collimated beam of bremsstrahlung X-rays, was implemented with a second X-ray detector located at 901 with respect to the incident beam, to collect Compton scattered Xrays and to carry out Compton images. Compton X-rays are approximately scattered in an isotrope manner by an irradiated object, and those emitted at 901 by the whole irradiated cylinder are collected by an uncollimated X-ray detector. By rotating and translating the object the same reconstruction procedures can be applied as those employed for transmission tomography, except for the auto attenuation effects in the object. Alternatively both source and detector can be strongly collimated, in order to delimit, with the intersection of collimated incident and secondary *Corresponding author. Fax: +39-079-229482. E-mail address: [email protected] (R. Cesareo).
Compton radiation, a small volume element (voxel). In this case each measurement directly gives the value of the scattering cross section of the voxel. The energy or energy interval of incident radiation should be selected for each element or material in such a manner that Compton scattering largely prevails over the other interaction modes, i.e. photoelectric effect and Rayleigh scattering. This ‘‘Compton region’’ for water, concrete, aluminium or iron is given approximately by energies larger than 40, 100, 120 and 200 keV, respectively. With these conditions both transmission and Compton tomographs are Compton tomographs.
2. Physical principles When a monoenergetic and well collimated X or g-ray beam of intensity N0 crosses a homogeneous
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 9 6 4 - 6
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
object of thickness x (cm), then part of the incident radiation, N; crosses the object and can be recorded by a collimated X-ray detector, while the remaining part, (N0 N) interacts with the object. The following well known equation is valid: N ¼ N0 emx
ð1Þ
where m indicates the ‘‘linear attenuation coefficient’’ (in cm1). When the object is not homogeneous, then the irradiated cylinder can be divided into many small cylinders of height Dx; where the linear attenuation coefficient can be assumed to be constant. Then Eq. (1) can be written as N ¼ N0 eðSi mi DxÞ :
ð2Þ
Eq. (2) is the basis for radiology and tomography. Tomographic images will give, therefore, a distribution of the linear attenuation coefficient. When bremsstrahlung radiation is employed, Eqs. (1) and (2) are still valid, when a proper mean energy value is used. As observed above, part of the incident radiation, ðN0 NÞ ¼ N0 ð1 emx Þ; interacts with the irradiated cylinder of the object and when the energy of the incident radiation is properly selected, then Compton scattering largely prevails over the other effects. That generally happens at sufficient high energies, for example above 40 keV for water, plastics and biological materials. Compton scattering occurs when an incident photon interacts with a free and at rest electron [1]. Then it loses part of its energy and is deflected from its original direction. Considering a monoenergetic beam of photons of energy E0 ; the Compton effect gives rise to an electron of energy Ee scattered at an angle f and a secondary photon of energy EC scattered through an angle W with respect to its original direction. The energy of the secondary photon is uniquely related to that of the primary one and the scattering angle by EC ¼ E0 =½1 þ að1 cos WÞ
ð3Þ
where a ¼ E0 ðkeVÞ=511: The energy shift is given by DE ¼ E0 EC ; and at 901, which corresponds to the geometrical arrangement of our measure-
189
ments: EC ¼ E0 =ð1 þ aÞ;
DE ¼ E0 ½a=ð1 þ aÞ:
ð30 Þ
The energy shift DE for E0 ¼ 40; 60, 80 and 100 keV is 2.9, 6.3, 10.9, 16.4 keV, respectively. The Compton ‘‘peak’’ is broader than a normal photoelectric peak, and its energy broadening is determined by several factors [2]: a. the range of accepted scattering angle, which gives rise to a ‘‘geometrical’’ broadening; b. the energy resolution of the X-ray detector; c. the movement of the orbital electrons, i.e. the momentum distribution of the electrons in the scatterer. This phenomenon is called ‘‘Compton profile’’, and gives the possibility to recognize the scattering material from its Compton profile, especially when the effects of points a and b are reduced [3]. The total Compton cross-section/atom is proportional to atomic number Z; and therefore the linear attenuation coefficient in the Compton region is proportional to the physical density r (g/cm3) (Fig. 1). A Compton tomography will give, therefore, a distribution of the physical density in the irradiated area.
Fig. 1. Correlation between tabulated values of linear attenuation coefficient m (in cm1) and physical density r (g/cm3) for various materials: from left to right: 1: H2O; 2: C2H4 (polyethylene); 3: C8H8 (polystirene); 4: C6H11NO (nylon); 5: C5H8O2 (lucite); 6: C43H38O7 (bakelite); 7: C2F4 (teflon); 8: C; 9: Al; 10: Li; 11: Ca; 12: Be; 13: B, and 14: Si. For hydrogen m=r=1, for low-Z non-hydrogenated substances (or with low H content) m=rE0:5; for high-Z materials m=rE0:38:
190
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
3. Experimental set-up The experimental set-up for contemporaneous transmission and Compton tomography is composed of the following components: a. an X-ray tube with a W-anode working at 80 kV, 5 mA maximum voltage and current. The output of the tube is collimated with a brass cylinder 50 mm long and with an internal hole of 2 mm; b. a translation–rotation table for the movement of the object; c. an NaI(Tl) X-ray detector for transmission measurements; this detector is strongly collimated with a 2 mm diameter internal hole collimator, and is coupled to a pulse amplifier; d. an NaI(Tl) X-ray detector of 2.5 cm diameter and 5 mm thickness for Compton tomography; this detector is simply shielded and collimated, to collect photons from the whole irradiated volume; e. electronics for the NaI(Tl)-detector, i.e. a pulse amplifier and a single channel analyser to select a proper energy interval; f. alternatively to point c, a CdZnTe detector with 9 mm2 area and 2 mm thickness ; this detector is strongly collimated with a 3 mm
long, 2 mm internal diameter brass collimator, to delimit a small volume voxel; g. a software for image reconstruction and auto attenuation corrections, in the hypothesis that the matrix of the material is known. Data of transmission tomography can be also used to this purpose. Fig. 2 shows the experimental set-up.
4. Results The performances of the described equipment were tested for contemporaneous transmission and Compton tomographic images. Fig. 3 shows a comparison of transmission and Compton images for various test objects: a hollow Lucite cylinder, the same cylinder with a graphite cylinder at the interior, and a walnut. Auto attenuation corrections were carried out on the walnut. Fig. 4 shows a cork cylinder with three small bores, one void, and the others filled with a 0.8 and 0.05 mm diameter Fe and Ag wire, respectively. From Figs. 3 and 4, it may be deduced that Compton tomography provides good results when low atomic number materials are involved. Transmission images are generally of higher quality than the Compton ones. This may depend on various factors: a. due to the statistics of the processes; in fact, the number of transmitted photons is approximately 2 orders of magnitude larger than the number of Compton scattered photons; b. due to multiple Compton scattering; c. due to approximate auto attenuation corrections; d. due to the contribution of additional photons in the detector, due to background, Rayleigh photons, photoelectric photons, etc.
Fig. 2. Experimental set-up for contemporaneous transmission and Compton tomography. Two possible solutions for Compton tomography are shown. 1: ‘‘Line’’ Compton tomography, where Compton scattered photons from the whole irradiated volume are collected and 2: ‘‘Single voxel’’ tomography, where Compton scattered photons from a single voxel are collected.
Factors a, c and d can be easily, at least partially, eliminated. Multiple Compton scattering can be limited by improving the energy of the incident radiation, by putting a proper collimation system in front of the detector, or by selecting only the high-energy tail of the spectrum.
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
191
Fig. 4. Compton tomography of a cork cylinder of 2.5 cm diameter with four bores: void, filled with a 0.8, 0.05 and 0.15 mm diameter Fe, Ag and Cu wires, respectively. A noncollimated NaI(Tl) X-ray detector was employed.
‘‘Compton profile’’ to identify materials, and to carry out Compton tomographs. The bremsstrahlung X-ray beam was monochromatized by using an external target and the quasi monoenergetic fluorescence X-rays emitted by the target [4]. Scattering peaks of the quasi monoenergetic X-rays emitted by a Sn secondary target is shown in Fig. 5. Unfortunately the photon output was too low for Compton tomography, and additional measurements are needed, by using a more powerful X-ray tube. 5. Conclusions
Fig. 3. Comparison between transmission (left) and Compton CT-images for various test objects. From the top: a hollow lucite cylinder; the same cylinder with a graphite cylinder at the interior; a lucite box with nine holes; a walnut. In the last case autoattenuation corrections for Compton scattering and background subtraction were introduced, that improve the quality of the image. NaI(Tl) X-ray detectors were employed, both for transmission and for Compton tomography.
Further, the Compton scattering apparatus was tested with monoenergetic or quasi monoenergetic radiation, to verify the possibility to use the
A simple apparatus for Compton tomography has been constructed, and some potentialities of the technique have been explored, for low atomic number materials. The advantages of Compton tomography are the following: *
*
It can be employed even when the object is not accessible from all sides, i.e. when transmission tomography cannot be carried out; It should give, at least theoretically, a better contrast between air (voids and fractures) and material.
The disadvantages of Compton tomography are the following:
192
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
Fig. 5. Scatter spectrum of tin-secondary targets X-rays (25.1 and 28.8 keV) by graphite and zinc. Elastic (Rayleigh) and 901 Compton scattered peaks (at 24.3 and 27.3 keV) are evident. Compton peaks are clearly larger than elastic scattered peaks, partially due to the Compton profile. Identification of materials from their Compton profile is therefore possible. A CdZnTe detector was employed. *
*
Less photon counts in the detector, due to the object–detector solid angle. More powerful Xray tubes are, therefore, required; Multiple scattering in the object, making worse the image.
Acknowledgements This work was partially financed by INFN, Istituto Nazionale di Fisica Nucleare.
References [1] R. Cesareo, in: X-ray Physics, La Rivista del Nuovo Cimento, Ed. Compositori, Bologna, 2000, pp. 1–231. [2] R. Cesareo, A.L. Hanson, G.E. Gigante, L.J. Pedraza, S.Q.G. Mahtaboally, Phys. Rep. 213 (3) (1992) 118. [3] M.J. Cooper, Rad. Phys. Chem. 50 (1997) 63. " A. [4] R. Cesareo, A. De Almeida, C. Costanza, E. Tarrago, Brunetti, The use of elemental filters for the monochromatization of bremsstrahlung radiation, Internal Report, CISB 02.94, Rome, 1994.
A simple scanner for Compton tomography Roberto Cesareoa,*, Cesare Cappio Borlinoa, Antonio Brunettia, Bruno Golosioa, Alfredo Castellanob a
Instituto di Matematica e Fisica, Universita" di Sassari, Via Vienna 2, 07100 Sassari, Italy b Dip. di Scienza dei Materiali, Universita" di Lecce, Lecce, Italy
Abstract A first generation CT-scanner was designed and constructed to carry out Compton images. This CT-scanner is composed of a 80 kV, 5 mA X-ray tube and a NaI(Tl) X-ray detector; the tube is strongly collimated, generating a X-ray beam of 2 mm diameter, whilst the detetor is not collimated to collect Compton photons from the whole irradiated cylinder. The performances of the equipment were tested contemporaneous transmission and Compton images. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Compton scattering; Compton tomography; Transmission tomography
1. Introduction A first generation ‘‘home made’’ transmission CT-scanner, using a collimated beam of bremsstrahlung X-rays, was implemented with a second X-ray detector located at 901 with respect to the incident beam, to collect Compton scattered Xrays and to carry out Compton images. Compton X-rays are approximately scattered in an isotrope manner by an irradiated object, and those emitted at 901 by the whole irradiated cylinder are collected by an uncollimated X-ray detector. By rotating and translating the object the same reconstruction procedures can be applied as those employed for transmission tomography, except for the auto attenuation effects in the object. Alternatively both source and detector can be strongly collimated, in order to delimit, with the intersection of collimated incident and secondary *Corresponding author. Fax: +39-079-229482. E-mail address: [email protected] (R. Cesareo).
Compton radiation, a small volume element (voxel). In this case each measurement directly gives the value of the scattering cross section of the voxel. The energy or energy interval of incident radiation should be selected for each element or material in such a manner that Compton scattering largely prevails over the other interaction modes, i.e. photoelectric effect and Rayleigh scattering. This ‘‘Compton region’’ for water, concrete, aluminium or iron is given approximately by energies larger than 40, 100, 120 and 200 keV, respectively. With these conditions both transmission and Compton tomographs are Compton tomographs.
2. Physical principles When a monoenergetic and well collimated X or g-ray beam of intensity N0 crosses a homogeneous
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 9 6 4 - 6
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
object of thickness x (cm), then part of the incident radiation, N; crosses the object and can be recorded by a collimated X-ray detector, while the remaining part, (N0 N) interacts with the object. The following well known equation is valid: N ¼ N0 emx
ð1Þ
where m indicates the ‘‘linear attenuation coefficient’’ (in cm1). When the object is not homogeneous, then the irradiated cylinder can be divided into many small cylinders of height Dx; where the linear attenuation coefficient can be assumed to be constant. Then Eq. (1) can be written as N ¼ N0 eðSi mi DxÞ :
ð2Þ
Eq. (2) is the basis for radiology and tomography. Tomographic images will give, therefore, a distribution of the linear attenuation coefficient. When bremsstrahlung radiation is employed, Eqs. (1) and (2) are still valid, when a proper mean energy value is used. As observed above, part of the incident radiation, ðN0 NÞ ¼ N0 ð1 emx Þ; interacts with the irradiated cylinder of the object and when the energy of the incident radiation is properly selected, then Compton scattering largely prevails over the other effects. That generally happens at sufficient high energies, for example above 40 keV for water, plastics and biological materials. Compton scattering occurs when an incident photon interacts with a free and at rest electron [1]. Then it loses part of its energy and is deflected from its original direction. Considering a monoenergetic beam of photons of energy E0 ; the Compton effect gives rise to an electron of energy Ee scattered at an angle f and a secondary photon of energy EC scattered through an angle W with respect to its original direction. The energy of the secondary photon is uniquely related to that of the primary one and the scattering angle by EC ¼ E0 =½1 þ að1 cos WÞ
ð3Þ
where a ¼ E0 ðkeVÞ=511: The energy shift is given by DE ¼ E0 EC ; and at 901, which corresponds to the geometrical arrangement of our measure-
189
ments: EC ¼ E0 =ð1 þ aÞ;
DE ¼ E0 ½a=ð1 þ aÞ:
ð30 Þ
The energy shift DE for E0 ¼ 40; 60, 80 and 100 keV is 2.9, 6.3, 10.9, 16.4 keV, respectively. The Compton ‘‘peak’’ is broader than a normal photoelectric peak, and its energy broadening is determined by several factors [2]: a. the range of accepted scattering angle, which gives rise to a ‘‘geometrical’’ broadening; b. the energy resolution of the X-ray detector; c. the movement of the orbital electrons, i.e. the momentum distribution of the electrons in the scatterer. This phenomenon is called ‘‘Compton profile’’, and gives the possibility to recognize the scattering material from its Compton profile, especially when the effects of points a and b are reduced [3]. The total Compton cross-section/atom is proportional to atomic number Z; and therefore the linear attenuation coefficient in the Compton region is proportional to the physical density r (g/cm3) (Fig. 1). A Compton tomography will give, therefore, a distribution of the physical density in the irradiated area.
Fig. 1. Correlation between tabulated values of linear attenuation coefficient m (in cm1) and physical density r (g/cm3) for various materials: from left to right: 1: H2O; 2: C2H4 (polyethylene); 3: C8H8 (polystirene); 4: C6H11NO (nylon); 5: C5H8O2 (lucite); 6: C43H38O7 (bakelite); 7: C2F4 (teflon); 8: C; 9: Al; 10: Li; 11: Ca; 12: Be; 13: B, and 14: Si. For hydrogen m=r=1, for low-Z non-hydrogenated substances (or with low H content) m=rE0:5; for high-Z materials m=rE0:38:
190
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
3. Experimental set-up The experimental set-up for contemporaneous transmission and Compton tomography is composed of the following components: a. an X-ray tube with a W-anode working at 80 kV, 5 mA maximum voltage and current. The output of the tube is collimated with a brass cylinder 50 mm long and with an internal hole of 2 mm; b. a translation–rotation table for the movement of the object; c. an NaI(Tl) X-ray detector for transmission measurements; this detector is strongly collimated with a 2 mm diameter internal hole collimator, and is coupled to a pulse amplifier; d. an NaI(Tl) X-ray detector of 2.5 cm diameter and 5 mm thickness for Compton tomography; this detector is simply shielded and collimated, to collect photons from the whole irradiated volume; e. electronics for the NaI(Tl)-detector, i.e. a pulse amplifier and a single channel analyser to select a proper energy interval; f. alternatively to point c, a CdZnTe detector with 9 mm2 area and 2 mm thickness ; this detector is strongly collimated with a 3 mm
long, 2 mm internal diameter brass collimator, to delimit a small volume voxel; g. a software for image reconstruction and auto attenuation corrections, in the hypothesis that the matrix of the material is known. Data of transmission tomography can be also used to this purpose. Fig. 2 shows the experimental set-up.
4. Results The performances of the described equipment were tested for contemporaneous transmission and Compton tomographic images. Fig. 3 shows a comparison of transmission and Compton images for various test objects: a hollow Lucite cylinder, the same cylinder with a graphite cylinder at the interior, and a walnut. Auto attenuation corrections were carried out on the walnut. Fig. 4 shows a cork cylinder with three small bores, one void, and the others filled with a 0.8 and 0.05 mm diameter Fe and Ag wire, respectively. From Figs. 3 and 4, it may be deduced that Compton tomography provides good results when low atomic number materials are involved. Transmission images are generally of higher quality than the Compton ones. This may depend on various factors: a. due to the statistics of the processes; in fact, the number of transmitted photons is approximately 2 orders of magnitude larger than the number of Compton scattered photons; b. due to multiple Compton scattering; c. due to approximate auto attenuation corrections; d. due to the contribution of additional photons in the detector, due to background, Rayleigh photons, photoelectric photons, etc.
Fig. 2. Experimental set-up for contemporaneous transmission and Compton tomography. Two possible solutions for Compton tomography are shown. 1: ‘‘Line’’ Compton tomography, where Compton scattered photons from the whole irradiated volume are collected and 2: ‘‘Single voxel’’ tomography, where Compton scattered photons from a single voxel are collected.
Factors a, c and d can be easily, at least partially, eliminated. Multiple Compton scattering can be limited by improving the energy of the incident radiation, by putting a proper collimation system in front of the detector, or by selecting only the high-energy tail of the spectrum.
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
191
Fig. 4. Compton tomography of a cork cylinder of 2.5 cm diameter with four bores: void, filled with a 0.8, 0.05 and 0.15 mm diameter Fe, Ag and Cu wires, respectively. A noncollimated NaI(Tl) X-ray detector was employed.
‘‘Compton profile’’ to identify materials, and to carry out Compton tomographs. The bremsstrahlung X-ray beam was monochromatized by using an external target and the quasi monoenergetic fluorescence X-rays emitted by the target [4]. Scattering peaks of the quasi monoenergetic X-rays emitted by a Sn secondary target is shown in Fig. 5. Unfortunately the photon output was too low for Compton tomography, and additional measurements are needed, by using a more powerful X-ray tube. 5. Conclusions
Fig. 3. Comparison between transmission (left) and Compton CT-images for various test objects. From the top: a hollow lucite cylinder; the same cylinder with a graphite cylinder at the interior; a lucite box with nine holes; a walnut. In the last case autoattenuation corrections for Compton scattering and background subtraction were introduced, that improve the quality of the image. NaI(Tl) X-ray detectors were employed, both for transmission and for Compton tomography.
Further, the Compton scattering apparatus was tested with monoenergetic or quasi monoenergetic radiation, to verify the possibility to use the
A simple apparatus for Compton tomography has been constructed, and some potentialities of the technique have been explored, for low atomic number materials. The advantages of Compton tomography are the following: *
*
It can be employed even when the object is not accessible from all sides, i.e. when transmission tomography cannot be carried out; It should give, at least theoretically, a better contrast between air (voids and fractures) and material.
The disadvantages of Compton tomography are the following:
192
R. Cesareo et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 188–192
Fig. 5. Scatter spectrum of tin-secondary targets X-rays (25.1 and 28.8 keV) by graphite and zinc. Elastic (Rayleigh) and 901 Compton scattered peaks (at 24.3 and 27.3 keV) are evident. Compton peaks are clearly larger than elastic scattered peaks, partially due to the Compton profile. Identification of materials from their Compton profile is therefore possible. A CdZnTe detector was employed. *
*
Less photon counts in the detector, due to the object–detector solid angle. More powerful Xray tubes are, therefore, required; Multiple scattering in the object, making worse the image.
Acknowledgements This work was partially financed by INFN, Istituto Nazionale di Fisica Nucleare.
References [1] R. Cesareo, in: X-ray Physics, La Rivista del Nuovo Cimento, Ed. Compositori, Bologna, 2000, pp. 1–231. [2] R. Cesareo, A.L. Hanson, G.E. Gigante, L.J. Pedraza, S.Q.G. Mahtaboally, Phys. Rep. 213 (3) (1992) 118. [3] M.J. Cooper, Rad. Phys. Chem. 50 (1997) 63. " A. [4] R. Cesareo, A. De Almeida, C. Costanza, E. Tarrago, Brunetti, The use of elemental filters for the monochromatization of bremsstrahlung radiation, Internal Report, CISB 02.94, Rome, 1994.