Radiation Protection Dosimetry Advance Access published February 23, 2014 Radiation Protection Dosimetry (2014), pp. 1–7
doi:10.1093/rpd/ncu022
DESIGN AND MANUFACTURING OF ANTHROPOMORPHIC THYROID-NECK PHANTOM FOR USE IN NUCLEAR MEDICINE CENTRES IN CHILE A. Hermosilla1, G. Dı´az London˜o1,*, M. Garcı´a1, F. Ruı´z2, P. Andrade2 and A. Pe´rez3 1 Departament of Physics, Universidad de La Frontera, Temuco, Chile 2 Departament of Mechanical Engineering, Universidad de La Frontera, Temuco, Chile 3 Nuclear Medicine Service, National Cancer Institute, Santiago, Chile
Received 3 December 2013; revised 27 January 2014; accepted 29 January 2014 Anthropomorphic phantoms are used in nuclear medicine for imaging quality control, calibration of gamma spectrometry system for the study of internal contamination with radionuclides and for internal dosimetric studies. These are constructed of materials that have radiation attenuation coefficients similar to those of the different organs and tissues of the human body. The material usually used for the manufacture of phantoms is polymethyl methacrylate. Other materials used for this purpose are polyethylene, polystyrene and epoxy resin. This project presents the design and manufacture of an anthropomorphic thyroid-neck phantom that includes the cervical spine, trachea and oesophagus, using a polyester resin (r 5 1.1 g cm23). Its linear and mass attenuation coefficients were experimentally determined and simulated by means of XCOM software, finding that this material reproduces the soft tissue ICRU-44 in a range of energies between 80 keV and 11 MeV, with less than a 5 % difference.
INTRODUCTION In the diagnostic and therapeutic procedures of nuclear medicine, a radiopharmaceutical is administered orally or intravenously to the patient, which implies a potential risk of external exposure and internal contamination for occupational staff, patients and the general public. Currently in Chile, there are 43 nuclear medicine centres, which mainly use 99m Tc and 131I as radionuclides in diagnosis or treatment. In most of them, the staff perform various tasks (avoidance, fractionation, labelling and injection), which increase the risk of incorporating 131I especially because this has more radiotoxicity. This needs the implementation of routine monitoring programmes for calculating internal doses in the workers(1, 2). In patient-specific cases, it is necessary to evaluate the risk of prescribed dose of therapeutic procedures. Therefore, it is imperative to establish a standard procedure for the estimation of internal doses in patients(3, 4). The evaluation of internal doses of radionuclide incorporation depends both on the total number of nuclear transformations of radionuclide in the source regions and on the energy absorbed per unit mass in the target tissue(5). The nuclear transformations of radionuclide are determined from activity in the source regions, which can be measured by quantitative imaging, direct measurement in the whole body or specific organs or by indirect measurements in biological samples, especially in urine(3). The energy absorbed per unit mass in the target tissue
can be calculated by Monte Carlo simulations(5, 6) or Kernel Point doses(6). In order to measure activity, calibration factors, which relate net counts in the source region to activity, must be established(3, 4). Anthropomorphic phantoms are used that are made of materials that reproduce the radiation attenuation coefficients that human tissue has and that replicate the size and shape of anatomical dimensions of the body and human organs(1 – 4, 7). The anthropomorphic phantoms can also be used for the evaluation and validation of properties of the gamma camera system as well as reconstruction and correction methods(3). Thyroid-neck phantoms commercially available or designed by other authors are commonly made of polymethyl methacrylate (PMMA)(8, 9). This project presents the design and manufacture of an anthropomorphic thyroid-neck phantom using a polyester resin (r ¼ 1.1 g cm23). As this material is not commonly used in the fabrication of phantoms, a study of the characteristics of the linear and mass attenuation coefficients of polyester resin was made necessary. These were compared with data of PMMA and the soft tissue ICRU-44 reported by Berger and Hubbell(10), showing that polyester resin is equivalent to human soft tissue. The phantom includes cervical spine, trachea and oesophagus. Its design was based on images of computed tomography and magnetic resonance as well as the dimensions proposed by Bouchet et al.(11) The phantom could be used in the calibration of gamma cameras or gamma spectrometry for in vivo measurements in order
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Northeastern University Libraries on April 6, 2014
*Corresponding author: [email protected]
A. HERMOSILLA ET AL.
to determine patient-specific therapeutic activity and the incorporation of the occupational worker. It could also be used for imaging quality control of the gamma spectrometry system used in nuclear medicine.
MATERIALS AND METHODS The linear attenuation coefficients of polyester resin
Comparison between mass attenuation coefficients Two comparisons were carried out in order to verify whether the polyester resin could be used as material in the fabrication of a phantom. In the first, relative percentage differences between mass attenuation coefficients of polyester resin and PMMA were calculated in the energy range of 10 keV to 20 MeV. In the second, the relative percentage differences of mass attenuation coefficients between polyester resin and soft tissue ICRU-44 were determined as well as between PMMA and soft tissue ICRU-44 for the energies of the common radionuclides used in nuclear medicine centres, given that the phantom will be used mainly in nuclear medicine applications. The mass attenuation coefficients were calculated with the XCOM software. Thyroid-neck phantom design The anthropomorphic thyroid-neck phantom was designed using the CATIA V5R16 program. For the
Figure 1. Experimental set: (a) linear attenuation coefficient per line of molybdenum Ka ¼ 17.48 keV of the X-ray equipment; (b) linear attenuation coefficient for gamma energies of 99mTc, 22Na, 137Cs, 54Mn and 60Co.
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The linear attenuation coefficients of polyester resin were measured for energies between 17 keV and 1.33 MeV from exponential attenuation law, and polyester resin samples were used, set with various thicknesses. The measurements were performed with an Si detector for X ray and NaI(Tl) detector of 1.500` 1.500 for gamma spectrometry. The experimental mounting used to gauge the linear attenuation coefficient per line of molybdenum Ka ¼ 17.48 keV of the X-ray equipment is shown in Figure 1a and that for the gamma energies of 99mTc, 22Na, 137Cs, 54Mn and 60 Co is shown in Figure 1b. In addition, the linear attenuation coefficients of polyester resin were determined from the XCOM software(10), and the relative percentage differences between the theoretical and measured values were calculated. The composition of the polyester resin is based on a homogeneous mixture of orthophthalic acid and ethylene glycol dissolved in styrene at 40 %. To solidify, the resin is added a cobalt octoate at 1 % as accelerator substance and methyl ethyl ketone
peroxide as a catalyst substance for the mixture polymerisation. The simulation carried out in the XCOM program has not considered the concentration of cobalt octoate at 1 %, because its concentration can vary from one preparation to another; for this case, it represents ,0.01 % of the total mixture, which does not modify significantly the linear attenuation coefficient values.
DESIGN OF AN ANTHROPOMORPHIC THYROID-NECK PHANTOM
x2 þ y2 6:002 ; 4:00 z 4:00
ð1Þ
Cervical spine: x2 þ y2 1:352 ; 4:00 z 4:00
equation: x 2 y 2:152 þ 1:00; 4:00 z 4:00 0:50 0:15 ð4Þ The thyroid lobes are represented by two ellipsoids that are joined by two concentric cylinders that represent the thyroid isthmus. The thyroid lobes are determined by the following equation: x + 1:50 2 y 2:67 2 z 1:00 2 þ þ 1:00 1:00 1:00 2:50 ð5Þ The isthmus is made up of two concentric cylinders, cut by two horizontal planes, and the thyroid lobes. Their equations are as follows: x + 1:50 2 y 2:67 2 z 1:00 2 þ þ 1:00 1:00 1:00 2:50 ð6Þ
ð2Þ
The trachea is defined as a circular cylinder cut by a vertical plane and two horizontal planes as follows: x2 þ ðy 3:35Þ2 0:652 ð3Þ 4:00 z 4:00; y 2:95 The oesophagus is an elliptical cylinder cut by two horizontal planes calculated with the following
2
ð7Þ
2
ð8Þ
x2 þ ðy 3:35Þ 1:652 ; 4:00 z 2:00 x2 þ ðy 3:35Þ 1:152 ; 4:00 z 2:00
The thyroid is within a separate piece that is inserted at the neck; this piece is delimited by the following equations: x2 þ y2 6:002 ; y 1:36 ð9Þ x 2:99; x 2:99 x2 þ ðy 3:35Þ2 0:902 ; y 1:13 ð10Þ x 0:90; x 0:90 Figure 3 presents some 3D images of the neck phantom design that includes the thyroid gland, cervical spine, trachea and oesophagus. Subsequently, the thyroid design is imported to Dimension uPrint Plus 3D printer where the silicone rubber models are made, the polyester resin in liquid state is poured into moulds and five hours after, this reaches its solid state. RESULTS AND DISCUSSION Linear attenuation coefficients of polyester resin
Figure 2. Axial section of the neck anatomy at the level of the C6 vertebral body where the location of the thyroid gland is visible. Th, thyroid gland; Tr, trachea; E, oesophagus; VB, vertebral body(12).
The results obtained from the experimental determination of the linear attenuation coefficients, mM, of the polyester resin were compared with the respective values, mT, calculated by simulation with the XCOM program for the energy range of 17 keV to 1.33 MeV. Their values and the relative percentage differences,
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design, the geometric dimensions proposed by Bouchet et al. (11) were considered, who defined the neck as a cylinder with a diameter of 12 cm and the volume of the thyroid as being 20 cm3. The morphology of the thyroid, cervical spine, trachea and oesophagus were taken from the information reported in the Atlas of Human Anatomy(12), which was related with the images obtained by computed tomography and nuclear magnetic resonance imaging as shown in Figure 2. Subsequently, the geometric shape of each organ was defined from surface quadrics because the CATIA V5R16 program works with this type of mathematical equations. In this mathematical model, the neck, cervical spine, trachea and oesophagus are located between the same two horizontal planes. The neck and cervical spine are defined as circular cylinders cut by two planes as follows: Neck:
A. HERMOSILLA ET AL.
Table 1. Linear attenuation coefficients of polyester resin for specific energies between 17 keV and 1.33 MeV. Eg (keV) 17.48 140 364 662 835 1.173 1.275 1.333
mT (cm21)
mM (cm21)
Dl ðEg Þa (%)
8.06` 1021 1.62` 1021 1.17` 1021 9.09` 1022 8.17` 1022 6.92` 1022 6.63` 1022 6.48` 1022
7.84 + 0.30` 1021 1.56 + 0.05` 1021 1.19 + 0.05` 1021 8.88 + 0.02` 1022 7.92 + 0.06` 1022 6.77 + 0.05` 1022 6.66 + 0.05` 1022 6.55 + 0.05` 1022
2.73 3.70 1.71 2.31 3.06 2.17 0.45 1.08
a
Dl ðEg Þ : absolute percentage relative difference between simulation and experimental linear attenuation coefficients of polyester resin.
Dl ðEg Þ; between these values are presented in Table 1, which shows that differences were ,4 %. Comparison between mass attenuation coefficients As mentioned in Materials and methods, two comparisons between mass attenuation coefficients were
performed in this study. In the first, the relative percentage difference between the mass attenuation coefficients of polyester resin and PMMA was calculated. These results are presented in Figure 4, showing that the relative percentage differences are ,1 % for energies between 50 keV and 20 MeV and reach nearly 5 % for energies of ,50 keV. Therefore, the use of polyester resin may be considered as a substitute material for PMMA in the manufacturing of phantoms for applications in radiation protection, imaging quality control and clinical dosimetry in nuclear medicine. In the second comparison, the relative percentage difference, Dm ðEg Þ; of mass attenuation coefficient between polyester resin and soft tissue ICRU-44 was determined, as well as between PMMA and soft tissue ICRU-44, for the gamma energies of common radionuclides used in nuclear medicine. Differences of ,5 % are observed in Table 2, showing that both materials have a good relationship in regard to soft tissue ICRU-44, except in the gamma energy of 153 Sm; for this energy, the differences up to 13.31 % were obtained for both materials. In the particular case of 131I, there are differences of 2.75 and 1.83 %
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Figure 3. The 3D design of anthropomorphic thyroid-neck phantom in CATIA V5R16 program: (a) anthropomorphic phantom; (b) thyroid gland.
DESIGN OF AN ANTHROPOMORPHIC THYROID-NECK PHANTOM
Table 2. Mass attenuation coefficients of soft tissue ICRU-44 (st), polyester resin (pr) and PMMA obtained by simulation with the XCOM program for the energies of the common radionuclides used in Chilean nuclear medicine centres. Isotope 153
Sm Tl 67 Ga 99m Tc 131 I 18 F 201
a
Eg (keV)
ðm/rÞst (cm2 g21)
ðm/rÞpr (cm2 g21)
ðm/rÞPMMA (cm2 g21)
Dm ðEg Þa (%)
Dm ðEg Þb (%)
41 80 93 140 364 511
2.63` 1021 1.82` 1021 1.73` 1021 1.52` 1021 1.09` 1021 9.51` 1022
2.28` 1021 1.73` 1021 1.66` 1021 1.47` 1021 1.06` 1021 9.24` 1022
2.31` 1021 1.75` 1021 1.68` 1021 1.49` 1021 1.07` 1021 9.32` 1022
13.31 4.95 4.05 3.29 2.75 2.84
12.17 3.85 2.89 1.97 1.83 2.00
Dm ðEg Þ : absolute percentage relative difference between mass attenuation coefficients of polyester resin and soft tissue. Dm ðEg Þ : absolute percentage relative difference between mass attenuation coefficients of PMMA and soft tissue.
b
between the mass attenuation coefficients of polyester resin and PMMA with soft tissue ICRU-44, respectively. Due to polyester resin having attenuation properties equivalent to that of PMMA and soft tissue ICRU-44 for a wide range of energies, it is possible to consider their use in the manufacture of phantoms for application in radiodiagnosis and radiotherapy studies for energies from 80 keV to 11 MeV. One advantage of polyester resin is its low cost compared with PMMA, which allows for cost reduction in production. Design of anthropomorphic thyroid-neck phantom Figure 5 shows a photograph of the anthropomorphic thyroid-neck phantom. In its structure is included an aluminium cylinder of 2.7 cm diameter, which is equivalent material to the cervical spine and two air
chambers. The first corresponds to the trachea, and its dimensions are 1.0 cm in the anterior-posterior axis and 1.3 cm in the transverse axis. The second represents the oesophagus, which is defined by an ellipsoid of major and minor axes equal to 1.0 and 0.3 cm, respectively. As can be seen, the thyroid section was designed as an independent piece, removable from the rest of the structure, for better handling. Also, this design allows for the incorporation of different morphologies of the thyroid in order to evaluate several pathological thyroid conditions such as hypo and hyperthyroidism, in which both the size and the volume of the gland are altered. Finally, Figure 6 shows a thyroid scintigraphy of the anthropomorphic thyroid-neck phantom performed with a gamma camera SMV model DSM, in order to evaluate the morphology of the phantom. The image shows its resemblance of the anatomy of a human thyroid.
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Figure 4. Absolute percentage difference between mass attenuation coefficients of polyester resin in relation to PMMA, simulated using XCOM program, from 10 keV to 20 MeV.
A. HERMOSILLA ET AL.
ACKNOWLEDGEMENTS The development of this study was made possible thanks to: DI13-0035 Project, the Composite Materials Laboratory of Mechanical Engineering Department and the Masters of Medical Physics, all belonging to Universidad de La Frontera.
FUNDING Figure 5. Anthropomorphic thyroid-neck phantom manufactured in the Composite Materials Laboratory, Department of Mechanical Engineering, Universidad de La Frontera.
This work was carried out with financial support from Research Direction of Universidad de La Frontera (Grant Number DI13-0035). REFERENCES
Figure 6. Scintigraphy of anthropomorphic thyroid-neck phantom performed with gamma camera SMV model DSM using a high-energy collimator.
CONCLUSIONS Polyester resin can be used as tissue-equivalent in the manufacturing of different phantoms for an energy
1. Internacional Atomic Energy Agency. Direct methods for measuring radionuclides in the human body. Safety Series No. 114, IAEA, (1996). 2. Dantas, B. et al. A protocol for the calibration of gamma cameras to estimate internal contamination in emergency situations. Radiat. Prot. Dosim. 127(1–4), 253– 257 (2007). 3. Siegel, J. A. et al. Techniques for quantitative radiopharmaceutical biodistribution data acquisition and analysis for use in human radiation dose estimates. MIRD Pamphlet No. 5. J. NucI. Med. 40, 37S – 61S (1999). 4. Ha¨nscheid, H., Canzi, C., Eschner, W., Flux, G., Luster, M., Strigari, L. and Lassmann, M. EANM dosimetry committee series on standard operational procedures for pre-therapeutic dosimetry II. Dosimetry prior to radioiodine therapy of benign thyroid diseases. Eur. J. Nucl. Med. Mol. Imag. (2013). doi: 10.1007/s00259–013– 2387-x. 5. International Commission on Radiological Protection. Human alimentary tract model for radiological protection. ICRP Publication 100. Pergamon Press (2006). 6. Snyder, W. S., Ford, M. and Warner, G. Estimates of specific absorbed fractions for photon sources uniformly distributed in various organs of a heterogeneous phantom. MIRD Pamphlet No. 5 (revised). Oak Ridge National Laboratory (1978).
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range between 80 keV and 11 MeV, due to the fact that the relative percentage difference between the mass attenuation coefficients of this material and soft tissue ICRU-44 is ,5 %. Other advantages of polyester resin are its low cost and easy handling compared with PMMA. With the development and manufacturing of the anthropomorphic thyroid-neck phantom, its application in the calibration of gamma cameras or gamma spectrometry for in vivo measurements may be considered in order to determine patient-specific therapeutic activity and the incorporation of the occupational worker. Also, it could be used for imaging quality control of the gamma spectrometry system used in nuclear medicine in Chile.
DESIGN OF AN ANTHROPOMORPHIC THYROID-NECK PHANTOM 7. Dı´az, G., Puerta, A. and Morales, J. Simulador de tiroides de adulto. Revista Colombiana de Fı´sica. 38(2), 938 –941 (2006). 8. Cerqueira, R. A. D. and Maia, A. F. Development of thyroid anthropomorphic phantoms for use in nuclear medicine. Radiat. Phys. Chem. (2013) http://dx.doi.org/ 10.1016/j.radphyschem.201212.038i. 9. International Commission on Radiation Units and Measurements ICRU. Phantoms and computational models in therapy, diagnosis and protection. ICRU Report 48, 96– 101 (1992).
10. Berger, M. J. and Hubbell, J. H. XCOM: photon cross section on a personal computer. Report NBSIR 87– 3597. National Bureau of Standards (1987). http:// physics.nist.gov/PhysRefData/XrayMassCoef/tab2.html 11. Bouchet, L., Bolch, W., Weber, D., Atkins, H. and Poston, J. MIRD pamphlet no. 15: radionuclide S values in a revised dosimetric model of the adult head and brain. J. NucI. Med. 40(3), 62– 101 (1999). 12. Moeller, T. and Reif, E. Pocket Atlas of Sectional Anatomy: Computed Tomography and Magnetic Resonance Imaging. 3rd Edition, Vol. I, Thieme Medical, pp. 184–197 (2007).
Downloaded from http://rpd.oxfordjournals.org/ at Northeastern University Libraries on April 6, 2014
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doi:10.1093/rpd/ncu022
DESIGN AND MANUFACTURING OF ANTHROPOMORPHIC THYROID-NECK PHANTOM FOR USE IN NUCLEAR MEDICINE CENTRES IN CHILE A. Hermosilla1, G. Dı´az London˜o1,*, M. Garcı´a1, F. Ruı´z2, P. Andrade2 and A. Pe´rez3 1 Departament of Physics, Universidad de La Frontera, Temuco, Chile 2 Departament of Mechanical Engineering, Universidad de La Frontera, Temuco, Chile 3 Nuclear Medicine Service, National Cancer Institute, Santiago, Chile
Received 3 December 2013; revised 27 January 2014; accepted 29 January 2014 Anthropomorphic phantoms are used in nuclear medicine for imaging quality control, calibration of gamma spectrometry system for the study of internal contamination with radionuclides and for internal dosimetric studies. These are constructed of materials that have radiation attenuation coefficients similar to those of the different organs and tissues of the human body. The material usually used for the manufacture of phantoms is polymethyl methacrylate. Other materials used for this purpose are polyethylene, polystyrene and epoxy resin. This project presents the design and manufacture of an anthropomorphic thyroid-neck phantom that includes the cervical spine, trachea and oesophagus, using a polyester resin (r 5 1.1 g cm23). Its linear and mass attenuation coefficients were experimentally determined and simulated by means of XCOM software, finding that this material reproduces the soft tissue ICRU-44 in a range of energies between 80 keV and 11 MeV, with less than a 5 % difference.
INTRODUCTION In the diagnostic and therapeutic procedures of nuclear medicine, a radiopharmaceutical is administered orally or intravenously to the patient, which implies a potential risk of external exposure and internal contamination for occupational staff, patients and the general public. Currently in Chile, there are 43 nuclear medicine centres, which mainly use 99m Tc and 131I as radionuclides in diagnosis or treatment. In most of them, the staff perform various tasks (avoidance, fractionation, labelling and injection), which increase the risk of incorporating 131I especially because this has more radiotoxicity. This needs the implementation of routine monitoring programmes for calculating internal doses in the workers(1, 2). In patient-specific cases, it is necessary to evaluate the risk of prescribed dose of therapeutic procedures. Therefore, it is imperative to establish a standard procedure for the estimation of internal doses in patients(3, 4). The evaluation of internal doses of radionuclide incorporation depends both on the total number of nuclear transformations of radionuclide in the source regions and on the energy absorbed per unit mass in the target tissue(5). The nuclear transformations of radionuclide are determined from activity in the source regions, which can be measured by quantitative imaging, direct measurement in the whole body or specific organs or by indirect measurements in biological samples, especially in urine(3). The energy absorbed per unit mass in the target tissue
can be calculated by Monte Carlo simulations(5, 6) or Kernel Point doses(6). In order to measure activity, calibration factors, which relate net counts in the source region to activity, must be established(3, 4). Anthropomorphic phantoms are used that are made of materials that reproduce the radiation attenuation coefficients that human tissue has and that replicate the size and shape of anatomical dimensions of the body and human organs(1 – 4, 7). The anthropomorphic phantoms can also be used for the evaluation and validation of properties of the gamma camera system as well as reconstruction and correction methods(3). Thyroid-neck phantoms commercially available or designed by other authors are commonly made of polymethyl methacrylate (PMMA)(8, 9). This project presents the design and manufacture of an anthropomorphic thyroid-neck phantom using a polyester resin (r ¼ 1.1 g cm23). As this material is not commonly used in the fabrication of phantoms, a study of the characteristics of the linear and mass attenuation coefficients of polyester resin was made necessary. These were compared with data of PMMA and the soft tissue ICRU-44 reported by Berger and Hubbell(10), showing that polyester resin is equivalent to human soft tissue. The phantom includes cervical spine, trachea and oesophagus. Its design was based on images of computed tomography and magnetic resonance as well as the dimensions proposed by Bouchet et al.(11) The phantom could be used in the calibration of gamma cameras or gamma spectrometry for in vivo measurements in order
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Northeastern University Libraries on April 6, 2014
*Corresponding author: [email protected]
A. HERMOSILLA ET AL.
to determine patient-specific therapeutic activity and the incorporation of the occupational worker. It could also be used for imaging quality control of the gamma spectrometry system used in nuclear medicine.
MATERIALS AND METHODS The linear attenuation coefficients of polyester resin
Comparison between mass attenuation coefficients Two comparisons were carried out in order to verify whether the polyester resin could be used as material in the fabrication of a phantom. In the first, relative percentage differences between mass attenuation coefficients of polyester resin and PMMA were calculated in the energy range of 10 keV to 20 MeV. In the second, the relative percentage differences of mass attenuation coefficients between polyester resin and soft tissue ICRU-44 were determined as well as between PMMA and soft tissue ICRU-44 for the energies of the common radionuclides used in nuclear medicine centres, given that the phantom will be used mainly in nuclear medicine applications. The mass attenuation coefficients were calculated with the XCOM software. Thyroid-neck phantom design The anthropomorphic thyroid-neck phantom was designed using the CATIA V5R16 program. For the
Figure 1. Experimental set: (a) linear attenuation coefficient per line of molybdenum Ka ¼ 17.48 keV of the X-ray equipment; (b) linear attenuation coefficient for gamma energies of 99mTc, 22Na, 137Cs, 54Mn and 60Co.
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The linear attenuation coefficients of polyester resin were measured for energies between 17 keV and 1.33 MeV from exponential attenuation law, and polyester resin samples were used, set with various thicknesses. The measurements were performed with an Si detector for X ray and NaI(Tl) detector of 1.500` 1.500 for gamma spectrometry. The experimental mounting used to gauge the linear attenuation coefficient per line of molybdenum Ka ¼ 17.48 keV of the X-ray equipment is shown in Figure 1a and that for the gamma energies of 99mTc, 22Na, 137Cs, 54Mn and 60 Co is shown in Figure 1b. In addition, the linear attenuation coefficients of polyester resin were determined from the XCOM software(10), and the relative percentage differences between the theoretical and measured values were calculated. The composition of the polyester resin is based on a homogeneous mixture of orthophthalic acid and ethylene glycol dissolved in styrene at 40 %. To solidify, the resin is added a cobalt octoate at 1 % as accelerator substance and methyl ethyl ketone
peroxide as a catalyst substance for the mixture polymerisation. The simulation carried out in the XCOM program has not considered the concentration of cobalt octoate at 1 %, because its concentration can vary from one preparation to another; for this case, it represents ,0.01 % of the total mixture, which does not modify significantly the linear attenuation coefficient values.
DESIGN OF AN ANTHROPOMORPHIC THYROID-NECK PHANTOM
x2 þ y2 6:002 ; 4:00 z 4:00
ð1Þ
Cervical spine: x2 þ y2 1:352 ; 4:00 z 4:00
equation: x 2 y 2:152 þ 1:00; 4:00 z 4:00 0:50 0:15 ð4Þ The thyroid lobes are represented by two ellipsoids that are joined by two concentric cylinders that represent the thyroid isthmus. The thyroid lobes are determined by the following equation: x + 1:50 2 y 2:67 2 z 1:00 2 þ þ 1:00 1:00 1:00 2:50 ð5Þ The isthmus is made up of two concentric cylinders, cut by two horizontal planes, and the thyroid lobes. Their equations are as follows: x + 1:50 2 y 2:67 2 z 1:00 2 þ þ 1:00 1:00 1:00 2:50 ð6Þ
ð2Þ
The trachea is defined as a circular cylinder cut by a vertical plane and two horizontal planes as follows: x2 þ ðy 3:35Þ2 0:652 ð3Þ 4:00 z 4:00; y 2:95 The oesophagus is an elliptical cylinder cut by two horizontal planes calculated with the following
2
ð7Þ
2
ð8Þ
x2 þ ðy 3:35Þ 1:652 ; 4:00 z 2:00 x2 þ ðy 3:35Þ 1:152 ; 4:00 z 2:00
The thyroid is within a separate piece that is inserted at the neck; this piece is delimited by the following equations: x2 þ y2 6:002 ; y 1:36 ð9Þ x 2:99; x 2:99 x2 þ ðy 3:35Þ2 0:902 ; y 1:13 ð10Þ x 0:90; x 0:90 Figure 3 presents some 3D images of the neck phantom design that includes the thyroid gland, cervical spine, trachea and oesophagus. Subsequently, the thyroid design is imported to Dimension uPrint Plus 3D printer where the silicone rubber models are made, the polyester resin in liquid state is poured into moulds and five hours after, this reaches its solid state. RESULTS AND DISCUSSION Linear attenuation coefficients of polyester resin
Figure 2. Axial section of the neck anatomy at the level of the C6 vertebral body where the location of the thyroid gland is visible. Th, thyroid gland; Tr, trachea; E, oesophagus; VB, vertebral body(12).
The results obtained from the experimental determination of the linear attenuation coefficients, mM, of the polyester resin were compared with the respective values, mT, calculated by simulation with the XCOM program for the energy range of 17 keV to 1.33 MeV. Their values and the relative percentage differences,
Page 3 of 7
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design, the geometric dimensions proposed by Bouchet et al. (11) were considered, who defined the neck as a cylinder with a diameter of 12 cm and the volume of the thyroid as being 20 cm3. The morphology of the thyroid, cervical spine, trachea and oesophagus were taken from the information reported in the Atlas of Human Anatomy(12), which was related with the images obtained by computed tomography and nuclear magnetic resonance imaging as shown in Figure 2. Subsequently, the geometric shape of each organ was defined from surface quadrics because the CATIA V5R16 program works with this type of mathematical equations. In this mathematical model, the neck, cervical spine, trachea and oesophagus are located between the same two horizontal planes. The neck and cervical spine are defined as circular cylinders cut by two planes as follows: Neck:
A. HERMOSILLA ET AL.
Table 1. Linear attenuation coefficients of polyester resin for specific energies between 17 keV and 1.33 MeV. Eg (keV) 17.48 140 364 662 835 1.173 1.275 1.333
mT (cm21)
mM (cm21)
Dl ðEg Þa (%)
8.06` 1021 1.62` 1021 1.17` 1021 9.09` 1022 8.17` 1022 6.92` 1022 6.63` 1022 6.48` 1022
7.84 + 0.30` 1021 1.56 + 0.05` 1021 1.19 + 0.05` 1021 8.88 + 0.02` 1022 7.92 + 0.06` 1022 6.77 + 0.05` 1022 6.66 + 0.05` 1022 6.55 + 0.05` 1022
2.73 3.70 1.71 2.31 3.06 2.17 0.45 1.08
a
Dl ðEg Þ : absolute percentage relative difference between simulation and experimental linear attenuation coefficients of polyester resin.
Dl ðEg Þ; between these values are presented in Table 1, which shows that differences were ,4 %. Comparison between mass attenuation coefficients As mentioned in Materials and methods, two comparisons between mass attenuation coefficients were
performed in this study. In the first, the relative percentage difference between the mass attenuation coefficients of polyester resin and PMMA was calculated. These results are presented in Figure 4, showing that the relative percentage differences are ,1 % for energies between 50 keV and 20 MeV and reach nearly 5 % for energies of ,50 keV. Therefore, the use of polyester resin may be considered as a substitute material for PMMA in the manufacturing of phantoms for applications in radiation protection, imaging quality control and clinical dosimetry in nuclear medicine. In the second comparison, the relative percentage difference, Dm ðEg Þ; of mass attenuation coefficient between polyester resin and soft tissue ICRU-44 was determined, as well as between PMMA and soft tissue ICRU-44, for the gamma energies of common radionuclides used in nuclear medicine. Differences of ,5 % are observed in Table 2, showing that both materials have a good relationship in regard to soft tissue ICRU-44, except in the gamma energy of 153 Sm; for this energy, the differences up to 13.31 % were obtained for both materials. In the particular case of 131I, there are differences of 2.75 and 1.83 %
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Figure 3. The 3D design of anthropomorphic thyroid-neck phantom in CATIA V5R16 program: (a) anthropomorphic phantom; (b) thyroid gland.
DESIGN OF AN ANTHROPOMORPHIC THYROID-NECK PHANTOM
Table 2. Mass attenuation coefficients of soft tissue ICRU-44 (st), polyester resin (pr) and PMMA obtained by simulation with the XCOM program for the energies of the common radionuclides used in Chilean nuclear medicine centres. Isotope 153
Sm Tl 67 Ga 99m Tc 131 I 18 F 201
a
Eg (keV)
ðm/rÞst (cm2 g21)
ðm/rÞpr (cm2 g21)
ðm/rÞPMMA (cm2 g21)
Dm ðEg Þa (%)
Dm ðEg Þb (%)
41 80 93 140 364 511
2.63` 1021 1.82` 1021 1.73` 1021 1.52` 1021 1.09` 1021 9.51` 1022
2.28` 1021 1.73` 1021 1.66` 1021 1.47` 1021 1.06` 1021 9.24` 1022
2.31` 1021 1.75` 1021 1.68` 1021 1.49` 1021 1.07` 1021 9.32` 1022
13.31 4.95 4.05 3.29 2.75 2.84
12.17 3.85 2.89 1.97 1.83 2.00
Dm ðEg Þ : absolute percentage relative difference between mass attenuation coefficients of polyester resin and soft tissue. Dm ðEg Þ : absolute percentage relative difference between mass attenuation coefficients of PMMA and soft tissue.
b
between the mass attenuation coefficients of polyester resin and PMMA with soft tissue ICRU-44, respectively. Due to polyester resin having attenuation properties equivalent to that of PMMA and soft tissue ICRU-44 for a wide range of energies, it is possible to consider their use in the manufacture of phantoms for application in radiodiagnosis and radiotherapy studies for energies from 80 keV to 11 MeV. One advantage of polyester resin is its low cost compared with PMMA, which allows for cost reduction in production. Design of anthropomorphic thyroid-neck phantom Figure 5 shows a photograph of the anthropomorphic thyroid-neck phantom. In its structure is included an aluminium cylinder of 2.7 cm diameter, which is equivalent material to the cervical spine and two air
chambers. The first corresponds to the trachea, and its dimensions are 1.0 cm in the anterior-posterior axis and 1.3 cm in the transverse axis. The second represents the oesophagus, which is defined by an ellipsoid of major and minor axes equal to 1.0 and 0.3 cm, respectively. As can be seen, the thyroid section was designed as an independent piece, removable from the rest of the structure, for better handling. Also, this design allows for the incorporation of different morphologies of the thyroid in order to evaluate several pathological thyroid conditions such as hypo and hyperthyroidism, in which both the size and the volume of the gland are altered. Finally, Figure 6 shows a thyroid scintigraphy of the anthropomorphic thyroid-neck phantom performed with a gamma camera SMV model DSM, in order to evaluate the morphology of the phantom. The image shows its resemblance of the anatomy of a human thyroid.
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Figure 4. Absolute percentage difference between mass attenuation coefficients of polyester resin in relation to PMMA, simulated using XCOM program, from 10 keV to 20 MeV.
A. HERMOSILLA ET AL.
ACKNOWLEDGEMENTS The development of this study was made possible thanks to: DI13-0035 Project, the Composite Materials Laboratory of Mechanical Engineering Department and the Masters of Medical Physics, all belonging to Universidad de La Frontera.
FUNDING Figure 5. Anthropomorphic thyroid-neck phantom manufactured in the Composite Materials Laboratory, Department of Mechanical Engineering, Universidad de La Frontera.
This work was carried out with financial support from Research Direction of Universidad de La Frontera (Grant Number DI13-0035). REFERENCES
Figure 6. Scintigraphy of anthropomorphic thyroid-neck phantom performed with gamma camera SMV model DSM using a high-energy collimator.
CONCLUSIONS Polyester resin can be used as tissue-equivalent in the manufacturing of different phantoms for an energy
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range between 80 keV and 11 MeV, due to the fact that the relative percentage difference between the mass attenuation coefficients of this material and soft tissue ICRU-44 is ,5 %. Other advantages of polyester resin are its low cost and easy handling compared with PMMA. With the development and manufacturing of the anthropomorphic thyroid-neck phantom, its application in the calibration of gamma cameras or gamma spectrometry for in vivo measurements may be considered in order to determine patient-specific therapeutic activity and the incorporation of the occupational worker. Also, it could be used for imaging quality control of the gamma spectrometry system used in nuclear medicine in Chile.
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10. Berger, M. J. and Hubbell, J. H. XCOM: photon cross section on a personal computer. Report NBSIR 87– 3597. National Bureau of Standards (1987). http:// physics.nist.gov/PhysRefData/XrayMassCoef/tab2.html 11. Bouchet, L., Bolch, W., Weber, D., Atkins, H. and Poston, J. MIRD pamphlet no. 15: radionuclide S values in a revised dosimetric model of the adult head and brain. J. NucI. Med. 40(3), 62– 101 (1999). 12. Moeller, T. and Reif, E. Pocket Atlas of Sectional Anatomy: Computed Tomography and Magnetic Resonance Imaging. 3rd Edition, Vol. I, Thieme Medical, pp. 184–197 (2007).
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