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Moxel: A molar tooth voxel model for dosimetric studies
Radiation Measurements 45 (2010) 234–236
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Short communication
Moxel: A molar tooth voxel model for dosimetric studies P. Ferrari a, *, G. Gualdrini a, P. Fattibene b, c, I. Veronese d, e a
ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, ION-IRP, Radiation Protection Institute, 16 Via dei Colli, 40136 Bologna, Italy `, Department of Technology and Health, Viale Regina Elena 299, I-00161 Roma, Italy Istituto Superiore di Sanita c `, Viale Regina Elena 299, I-00161 Roma, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Gruppo Collegato Sanita d ` degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, 20133 Milano, Italy Universita e INFN, Sezione di Milano, Via Celoria 16, 20133 Milano, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2009 Received in revised form 26 November 2009 Accepted 5 January 2010
Stylized numerical models of the tooth are usually employed in qualification procedure related to Electronic Paramagnetic Resonance in long-term accidental contamination dose reconstruction. In this work a voxel model was developed from the microCT image data set of a human non contaminated molar tooth. A stylized model, reproducing the characteristics of the voxel model, was also created in order to investigate the level of accuracy that can be obtained in this kind of study. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Voxel models EPR Retrospective dosimetry Internal contamination Tooth models for dosimetry
1. Introduction Electronic Paramagnetic Resonance (EPR) of tooth enamel is a technique of retrospective dosimetry of long-term protracted ionizing radiation exposures (Wieser et al., 2005; IAEA, 2002; Onori et al., 2000). The EPR measurement provides the cumulated absorbed dose in tooth enamel generated by the lifetime environmental, medical and occupational exposures, both internal and external, but it does not offer any possibility to distinguish among the different contributions. In case of internal contamination from beta-emitters, depending on the mineralization status of tooth at the time of radioactive intake, the radionuclides may be predominantly located in the tooth dentine, producing damage in dentine and enamel (Veronese et al., 2008). A possible solution to distinguish between the dose to enamel due to the internal contamination and external exposure is to complement EPR measurements with Monte Carlo simulations of the radiation transport within the tooth structure and a reliable numerical model representing the tooth is needed (Seltzer et al., 2001; Tolsykh et al., 2000). In the present work a human, non contaminated, molar was scanned with a microCT at ISS (Istituto Superiore di Sanita` – Italian National Institute of Health) laboratories. The CT data were used to
* Corresponding author. E-mail address: [email protected] (P. Ferrari). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.009
obtain a voxel model of the molar, segmenting dentine, enamel and the void regions (roots where cut before the CT exam). We translated the obtained model for the Monte Carlo code MCNPX (Pelowitz, 2005) simulating a homogeneous Strontium Yttrium in secular equilibrium (90Sr–Y) contamination in the dentine. The energy deposited in the enamel was the investigated quantity and the results, obtained in this complex geometry were compared with a typical stylized geometry. The stylized model was created approximating the molar structure with simple coaxial cylindrical surfaces. We carried out a parametric study of the energy deposition as a function of the stylized model enamel thickness to assess at what extent the voxel model could be proficiently approximated by such simplified scheme. The leading parameter to correlate the analytical models with the real tooth was the enamel volume that was maintained constant. This means that for increasing thicknesses in the axial direction the radius of the crown was reduced to guarantee a constant volume given by the union of a cylinder and a cylindrical ring around the tooth top structure. 2. Materials and methods 2.1. The molar voxel model: moxel In the present work we choose a human non contaminated first lower molar. The pulp was dried and the roots were cut away in order to adapt the tooth to the rotating support of the employed
P. Ferrari et al. / Radiation Measurements 45 (2010) 234–236
tomographer. We employed a microCT Skyscan 1072 (SkyScan, Kartuizersweg 3B, 2550 Kontich, Belgium) to scan the molar at 100 kVp and 98 mA. The image set is made of 67 slices, of 191 mm thickness, with a pixel of 20 mm. The data set was treated with ImageJ, a free open source imaging software (rsbweb.nih.gov/ij/) and with IDL, Interactive Data Language (www.ittvis.com), in order to obtain a matrix of 660 660 66 voxel. A segmentation procedure, with the application of the smoothing and thresholding operators, allowed distinguishing the void regions, including pulp, from the dentine and the enamel layers. The segmentation and the conversion of the numerical matrix into an MCNPX input were obtained through ad hoc C programs. In order to speed-up calculation we scaled the 1:4 the original matrix obtaining a reduced voxel model, called moxel (see in Fig. 1 some 3-D moxel images), made of 150 150 66 elements. In this model a 90Sr–Y contamination was created as homogeneous source in the dentine. The aim of the simulations was to tally the energy deposition in the enamel of the moxel model. 2.2. Optimization of a stylized analytical tooth model Many of the works concerning EPR with teeth use stylized models, made of cylinders, allowing parametric analysis by changing layers thicknesses and radii. In order to study the level of accuracy required and the approximations induced in this case, an analytical numerical model (called atmo, analytical tooth model) was created reproducing the dimensions of the employed molar and its volumes, previously calculated using the voxel model. A critical parameter to be optimized was the thickness of the enamel crown which is not uniform but strongly varying depending on the considered position. Therefore a parametric analysis was performed: as for the voxel model, a 90Sr–Y contamination was simulated as homogeneous source in the dentine. A set of simulations was done increasing the crown enamel thickness, at the same time guaranteeing the constancy of the enamel volume as already explained. The energy deposition in dentine and in enamel was calculated. In Fig. 1 the stylized model is shown compared with the voxel real model. 3. Results The composition and densities of enamel and thickness were taken from a previous study (Wieser et al., 2001). In Table 1 the
235
Table 1 Absorbed doses in dentine and enamel for the two models in pGy per source particle (the precision is 6% for the moxel estimates and 0.2% for the atmo ones). For the atmo model doses are reported for the different enamel thicknesses (the mass of the dentine is always the same). Moxel Atmo Doses at different enamel thickness Dentine (pGy) 46.8 45.5 Enamel (pGy) 6.04 5.64 6.22 6.19 5.80 5.21 4.47 3.53 Atmo enamel thickness (cm) 0.047 0.097 0.147 0.197 0.247 0.297 0.347
results of the simulations are shown. Considering atmo and moxel, the absorbed doses calculated in dentine and enamel are about the same. The differences are of the order of 0.3% and 2% for dentine and enamel respectively, values inside the uncertainties of the evaluation, for the model with 0.147 cm enamel thickness. In Fig. 2 the results are presented as a function of the enamel thickness in the analytical model in the axial direction, with a condition of constancy of the enamel volume that was equal to that taken from the voxel model. The figure shows that there is a domain between 0.07 and 0.17 cm enamel thickness, in which the analytical model is in good agreement with the voxel model. This effect does not occur if the condition of constancy of the enamel volume is not fulfilled in the analytical approximation of the model. We evaluated also the effect of the presence of the pulp using the analytical model. No difference between the results was shown, as far as tallying is concerned. 4. Discussion In the present work a voxel model representing a molar tooth was created employing the data of a microCT of a human molar tooth. These preliminary studies were mainly addressed at investigating the possible usage of detailed voxel models of the tooth and assessing at what extent a simplified cylindrical model could provide similar results. The outcome of this study is intrinsically dependent on the fact that the simplified model was designed on the basis of the physical characteristics of the complex molar. Therefore the used simplified model is representative strictly only of the studied molar tooth. Of course, accepting the unavoidable uncertainties, it could be possible to try to introduce some analytical receipts to find a simplified model representing a family of real teeth (e.g. molars). Being the extrapolated range of the 90Y
Fig. 1. Images of (A) moxel and (B) atmo models, obtained with the code Moritz (K. van Riper, Moritz User Guide, 2006, White Rock Science Los Alamos, NM 87544, USA) (moxel image resolution is scaled to 1:3) and (C) a comparison between their coronal sections (with moxel on the left side).
236
P. Ferrari et al. / Radiation Measurements 45 (2010) 234–236
Acknowledgments The authors wish to thank Dr. Rossella Bedini and Dr. Raffaella Pecci (Department of Technology and Health, ISS) for providing the microCT scan of the molar. References
Fig. 2. Absorbed dose per unit starting particle in the enamel: comparison between a parametric study of the thickness dependency of the quantity for the analytical model (solid line) and the result of the voxel fixed model (dotted line). Note the possible range of different enamel thicknesses that guarantee a good agreement, at the condition that the enamel volume is maintained constant (equal to the real situation).
electrons of the order of nearly 7 mm, the energy deposition is in large part dominated by the primary electrons and only in a minor part by the bremsstrahlung photons put in motion by them, therefore the structure is quite important for the transport mechanism. The voxel model of the molar could be a good reference model for further studies for a more accurate development of simplified models that are necessary to carry out a comprehensive analysis, as it can be considered unfeasible to produce microCT images of every tooth to be analyzed.
IAEA, 2002. Use of Electron Paramagnetic Resonance Dosimetry with Tooth Enamel for Retrospective Dose Assessment. IAEA-TECDOC-1331. IAEA, Vienna. Onori, S., Aragno, D., Fattibene, P., Petetti, E., Pressello, M.C., 2000. ISS protocol for EPR tooth dosimetry. Radiation Measurements 32, 787–792. Pelowitz, D.B., 2005. MCNPX User’s Manual. LA-CP-05-0369. Los Alamos Laboratory, USA. Seltzer, S.M., Romanyukha, A.A., Nagy, V., 2001. Monte Carlo calculations of the dose distribution in teeth due to internal exposure from 90Sr: application to EPR tooth dosimetry. Radiation Protection Dosimetry 93, 245–260. Tolsykh, E.I., Degteva, M.O., Kozheurov, V.P., Shishkina, E.A., Romanyukha, A.A., Wieser, A., Jacob, P., 2000. Strontium metabolism in teeth and enamel dose assessment: analysis of the Techa river data. Radiation Environment Biophysics 39, 161–171. Veronese, I., Fattibene, P., Cantone, M.C., De Coste, V., Giussani, A., Onori, S., Shishkina, E.A., 2008. EPR and TL-based beta dosimetry measurements in various tooth components contaminated by 90Sr. Radiation Measurements 43, 813–818. Wieser, A., Aragno, D., El-Faramawy, N., Fattibene, P., Meckbach, R., Onori, S., Pressello, M.C., Pugliani, L., Ulanovsky, A., Zankl, M., 2001. Monte Carlo calculation and experimental verification of the photon energy response of tooth enamel in head-sized plexiglas phantom. Radiation Protection Dosimetry 101, 549–552. Wieser, A., Debuyst, R., Fattibene, P., Meghzifene, A., Onori, S., Bayankin, S.N., Blackwell, B., Brik, A., Bugay, A., Chumak, V., Ciesielski, B., Hoshi, M., Imata, H., Ivannikov, A., Ivanov, D., Junczewska, M., Miyazawa, C., Pass, B., Penkowski, M., Pivovarov, S., Romanyukha, A., Romanyukha, L., Schauer, D., Scherbina, O., Schultka, K., Shames, A., Sholom, S., Skinner, A., Skvortsov, V., Stepanenko, V., Tielewuhan, E., Toyoda, S., Trompier, F., 2005. The 3rd international intercomparison on EPR tooth dosimetry: part 1, general analysis. Applied Radiation Isotopes 62 (2), 163–171.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Short communication
Moxel: A molar tooth voxel model for dosimetric studies P. Ferrari a, *, G. Gualdrini a, P. Fattibene b, c, I. Veronese d, e a
ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, ION-IRP, Radiation Protection Institute, 16 Via dei Colli, 40136 Bologna, Italy `, Department of Technology and Health, Viale Regina Elena 299, I-00161 Roma, Italy Istituto Superiore di Sanita c `, Viale Regina Elena 299, I-00161 Roma, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Gruppo Collegato Sanita d ` degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, 20133 Milano, Italy Universita e INFN, Sezione di Milano, Via Celoria 16, 20133 Milano, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2009 Received in revised form 26 November 2009 Accepted 5 January 2010
Stylized numerical models of the tooth are usually employed in qualification procedure related to Electronic Paramagnetic Resonance in long-term accidental contamination dose reconstruction. In this work a voxel model was developed from the microCT image data set of a human non contaminated molar tooth. A stylized model, reproducing the characteristics of the voxel model, was also created in order to investigate the level of accuracy that can be obtained in this kind of study. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Voxel models EPR Retrospective dosimetry Internal contamination Tooth models for dosimetry
1. Introduction Electronic Paramagnetic Resonance (EPR) of tooth enamel is a technique of retrospective dosimetry of long-term protracted ionizing radiation exposures (Wieser et al., 2005; IAEA, 2002; Onori et al., 2000). The EPR measurement provides the cumulated absorbed dose in tooth enamel generated by the lifetime environmental, medical and occupational exposures, both internal and external, but it does not offer any possibility to distinguish among the different contributions. In case of internal contamination from beta-emitters, depending on the mineralization status of tooth at the time of radioactive intake, the radionuclides may be predominantly located in the tooth dentine, producing damage in dentine and enamel (Veronese et al., 2008). A possible solution to distinguish between the dose to enamel due to the internal contamination and external exposure is to complement EPR measurements with Monte Carlo simulations of the radiation transport within the tooth structure and a reliable numerical model representing the tooth is needed (Seltzer et al., 2001; Tolsykh et al., 2000). In the present work a human, non contaminated, molar was scanned with a microCT at ISS (Istituto Superiore di Sanita` – Italian National Institute of Health) laboratories. The CT data were used to
* Corresponding author. E-mail address: [email protected] (P. Ferrari). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.009
obtain a voxel model of the molar, segmenting dentine, enamel and the void regions (roots where cut before the CT exam). We translated the obtained model for the Monte Carlo code MCNPX (Pelowitz, 2005) simulating a homogeneous Strontium Yttrium in secular equilibrium (90Sr–Y) contamination in the dentine. The energy deposited in the enamel was the investigated quantity and the results, obtained in this complex geometry were compared with a typical stylized geometry. The stylized model was created approximating the molar structure with simple coaxial cylindrical surfaces. We carried out a parametric study of the energy deposition as a function of the stylized model enamel thickness to assess at what extent the voxel model could be proficiently approximated by such simplified scheme. The leading parameter to correlate the analytical models with the real tooth was the enamel volume that was maintained constant. This means that for increasing thicknesses in the axial direction the radius of the crown was reduced to guarantee a constant volume given by the union of a cylinder and a cylindrical ring around the tooth top structure. 2. Materials and methods 2.1. The molar voxel model: moxel In the present work we choose a human non contaminated first lower molar. The pulp was dried and the roots were cut away in order to adapt the tooth to the rotating support of the employed
P. Ferrari et al. / Radiation Measurements 45 (2010) 234–236
tomographer. We employed a microCT Skyscan 1072 (SkyScan, Kartuizersweg 3B, 2550 Kontich, Belgium) to scan the molar at 100 kVp and 98 mA. The image set is made of 67 slices, of 191 mm thickness, with a pixel of 20 mm. The data set was treated with ImageJ, a free open source imaging software (rsbweb.nih.gov/ij/) and with IDL, Interactive Data Language (www.ittvis.com), in order to obtain a matrix of 660 660 66 voxel. A segmentation procedure, with the application of the smoothing and thresholding operators, allowed distinguishing the void regions, including pulp, from the dentine and the enamel layers. The segmentation and the conversion of the numerical matrix into an MCNPX input were obtained through ad hoc C programs. In order to speed-up calculation we scaled the 1:4 the original matrix obtaining a reduced voxel model, called moxel (see in Fig. 1 some 3-D moxel images), made of 150 150 66 elements. In this model a 90Sr–Y contamination was created as homogeneous source in the dentine. The aim of the simulations was to tally the energy deposition in the enamel of the moxel model. 2.2. Optimization of a stylized analytical tooth model Many of the works concerning EPR with teeth use stylized models, made of cylinders, allowing parametric analysis by changing layers thicknesses and radii. In order to study the level of accuracy required and the approximations induced in this case, an analytical numerical model (called atmo, analytical tooth model) was created reproducing the dimensions of the employed molar and its volumes, previously calculated using the voxel model. A critical parameter to be optimized was the thickness of the enamel crown which is not uniform but strongly varying depending on the considered position. Therefore a parametric analysis was performed: as for the voxel model, a 90Sr–Y contamination was simulated as homogeneous source in the dentine. A set of simulations was done increasing the crown enamel thickness, at the same time guaranteeing the constancy of the enamel volume as already explained. The energy deposition in dentine and in enamel was calculated. In Fig. 1 the stylized model is shown compared with the voxel real model. 3. Results The composition and densities of enamel and thickness were taken from a previous study (Wieser et al., 2001). In Table 1 the
235
Table 1 Absorbed doses in dentine and enamel for the two models in pGy per source particle (the precision is 6% for the moxel estimates and 0.2% for the atmo ones). For the atmo model doses are reported for the different enamel thicknesses (the mass of the dentine is always the same). Moxel Atmo Doses at different enamel thickness Dentine (pGy) 46.8 45.5 Enamel (pGy) 6.04 5.64 6.22 6.19 5.80 5.21 4.47 3.53 Atmo enamel thickness (cm) 0.047 0.097 0.147 0.197 0.247 0.297 0.347
results of the simulations are shown. Considering atmo and moxel, the absorbed doses calculated in dentine and enamel are about the same. The differences are of the order of 0.3% and 2% for dentine and enamel respectively, values inside the uncertainties of the evaluation, for the model with 0.147 cm enamel thickness. In Fig. 2 the results are presented as a function of the enamel thickness in the analytical model in the axial direction, with a condition of constancy of the enamel volume that was equal to that taken from the voxel model. The figure shows that there is a domain between 0.07 and 0.17 cm enamel thickness, in which the analytical model is in good agreement with the voxel model. This effect does not occur if the condition of constancy of the enamel volume is not fulfilled in the analytical approximation of the model. We evaluated also the effect of the presence of the pulp using the analytical model. No difference between the results was shown, as far as tallying is concerned. 4. Discussion In the present work a voxel model representing a molar tooth was created employing the data of a microCT of a human molar tooth. These preliminary studies were mainly addressed at investigating the possible usage of detailed voxel models of the tooth and assessing at what extent a simplified cylindrical model could provide similar results. The outcome of this study is intrinsically dependent on the fact that the simplified model was designed on the basis of the physical characteristics of the complex molar. Therefore the used simplified model is representative strictly only of the studied molar tooth. Of course, accepting the unavoidable uncertainties, it could be possible to try to introduce some analytical receipts to find a simplified model representing a family of real teeth (e.g. molars). Being the extrapolated range of the 90Y
Fig. 1. Images of (A) moxel and (B) atmo models, obtained with the code Moritz (K. van Riper, Moritz User Guide, 2006, White Rock Science Los Alamos, NM 87544, USA) (moxel image resolution is scaled to 1:3) and (C) a comparison between their coronal sections (with moxel on the left side).
236
P. Ferrari et al. / Radiation Measurements 45 (2010) 234–236
Acknowledgments The authors wish to thank Dr. Rossella Bedini and Dr. Raffaella Pecci (Department of Technology and Health, ISS) for providing the microCT scan of the molar. References
Fig. 2. Absorbed dose per unit starting particle in the enamel: comparison between a parametric study of the thickness dependency of the quantity for the analytical model (solid line) and the result of the voxel fixed model (dotted line). Note the possible range of different enamel thicknesses that guarantee a good agreement, at the condition that the enamel volume is maintained constant (equal to the real situation).
electrons of the order of nearly 7 mm, the energy deposition is in large part dominated by the primary electrons and only in a minor part by the bremsstrahlung photons put in motion by them, therefore the structure is quite important for the transport mechanism. The voxel model of the molar could be a good reference model for further studies for a more accurate development of simplified models that are necessary to carry out a comprehensive analysis, as it can be considered unfeasible to produce microCT images of every tooth to be analyzed.
IAEA, 2002. Use of Electron Paramagnetic Resonance Dosimetry with Tooth Enamel for Retrospective Dose Assessment. IAEA-TECDOC-1331. IAEA, Vienna. Onori, S., Aragno, D., Fattibene, P., Petetti, E., Pressello, M.C., 2000. ISS protocol for EPR tooth dosimetry. Radiation Measurements 32, 787–792. Pelowitz, D.B., 2005. MCNPX User’s Manual. LA-CP-05-0369. Los Alamos Laboratory, USA. Seltzer, S.M., Romanyukha, A.A., Nagy, V., 2001. Monte Carlo calculations of the dose distribution in teeth due to internal exposure from 90Sr: application to EPR tooth dosimetry. Radiation Protection Dosimetry 93, 245–260. Tolsykh, E.I., Degteva, M.O., Kozheurov, V.P., Shishkina, E.A., Romanyukha, A.A., Wieser, A., Jacob, P., 2000. Strontium metabolism in teeth and enamel dose assessment: analysis of the Techa river data. Radiation Environment Biophysics 39, 161–171. Veronese, I., Fattibene, P., Cantone, M.C., De Coste, V., Giussani, A., Onori, S., Shishkina, E.A., 2008. EPR and TL-based beta dosimetry measurements in various tooth components contaminated by 90Sr. Radiation Measurements 43, 813–818. Wieser, A., Aragno, D., El-Faramawy, N., Fattibene, P., Meckbach, R., Onori, S., Pressello, M.C., Pugliani, L., Ulanovsky, A., Zankl, M., 2001. Monte Carlo calculation and experimental verification of the photon energy response of tooth enamel in head-sized plexiglas phantom. Radiation Protection Dosimetry 101, 549–552. Wieser, A., Debuyst, R., Fattibene, P., Meghzifene, A., Onori, S., Bayankin, S.N., Blackwell, B., Brik, A., Bugay, A., Chumak, V., Ciesielski, B., Hoshi, M., Imata, H., Ivannikov, A., Ivanov, D., Junczewska, M., Miyazawa, C., Pass, B., Penkowski, M., Pivovarov, S., Romanyukha, A., Romanyukha, L., Schauer, D., Scherbina, O., Schultka, K., Shames, A., Sholom, S., Skinner, A., Skvortsov, V., Stepanenko, V., Tielewuhan, E., Toyoda, S., Trompier, F., 2005. The 3rd international intercomparison on EPR tooth dosimetry: part 1, general analysis. Applied Radiation Isotopes 62 (2), 163–171.