- Email: [email protected]
Thermoluminescence of zirconium oxide nanostructured to mammography X-ray beams
Applied Radiation and Isotopes 70 (2012) 1400–1402
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence of zirconium oxide nanostructured to mammography X-ray beams L.L. Palacios a, T. Rivera a,n, J. Roman a, J. Azorı´n b, E. Gaona c a
´n en Ciencia Aplicada y Tecnologı´a Avanzada-Legaria, IPN. Av. Legaria 694, 11500 Me´xico D.F., Mexico Centro de Investigacio ´noma Metropolitana-Iztapalapa. Av. San Rafael Atlixco 187, 09340 Me ´xico D.F., Mexico Universidad Auto c ´noma Metropolitana-Xochimilco. Calz. Del Hueso 1100, 04960 Me ´xico D.F., Mexico Universidad Auto b
a r t i c l e i n f o
a b s t r a c t
Available online 24 February 2012
In the present work thermoluminescent (TL) response of zirconium oxide (ZrO2) nanostructured induced by mammography X-ray radiation was investigated. Measurements were made of the response per unit air kerma of ZrO2 with mammography equipment parameters (semiautomatic exposure control, 24 kVp and 108 mAs). The calibration curves were obtained by simultaneously irradiating ZrO2 samples and ion chamber. Samples of ZrO2 showed a linear response as a function of entrance skin air kerma. The observed results in TL properties suggest that ZrO2 nanostructured could be considered as an effective material for X-ray beams dosimetry if appropriate calibration procedures are performed. & 2012 Elsevier Ltd. All rights reserved.
Keywords: X-rays Mammography Thermoluminescent dosimetry X-ray low energy
1. Introduction The ionizing radiation sources in medicine has been increasing worldwide ever since the discovery of X-rays and radioactivity. Wide varieties of imaging and treatment procedures relying on ionizing radiation have been developed and the trend will continue in order to meet the involving needs of diagnostic and therapeutic practices (Azorı´n and Rivera, 2002; Rivera, 2011). At present, mammography is considered to be the most effective method for early detection of small malignant lesions in the female breast (Palacios et al., 2008; Palacios, 2008). Mammary cancer detection by means diagnostic radiology at an early stage improves the probability of survival considerably (Yaffe, 2000). However, the use of ionizing radiation implies the risk of induction of breast cancer. In case of digital detectors (Gaona et al., 2005; Kimme et al., 1990), the images are digital and can either be printed or viewed on a monitor. The examinations by mammography devices are performed by compressing the breast between a plastic plate and the bucky that contains the radiographic film. Breast dose determination requires entrance surface air kerma (ESAK) measurements and the use of conversion factors (ACR, 1981; Byng et al., 1998; ICRP, 1987; Dance et al., 2000; Dance, 1990). The entrance surface dose (ESD) is the absorbed dose in air, including the contribution from backscatter, measured at a point on the entrance surface of a specific object in this case the breast of patient (ACR, 1994). A simple direct method of determining
n
Corresponding author. Tel.: þ525553956738. E-mail address: [email protected] (T. Rivera).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.02.005
ESD is the use of thermoluminescent detectors (TLDs) positioned on top of a compressed breast or a breast phantom. Thermoluminescent dosimetry (TLD) system presents some advantages over other systems, i.e. high sensitivity, small size, linear response for a wide dose interval, low cost and easy handle. Amongst the most frequently utilized thermoluminescent materials is the lithium fluoride doped with magnesium and titanium (LiF: Mg, Ti) which exhibits some important characteristics such as the effective atomic number (Zeff ¼8.2) close to that of human tissue. The aim of the present work was to investigate thermoluminescent characteristics of ZrO2 nanostructured irradiated with mammography X-ray beams and the implications in the entrance skin dose estimation in patients submitted to mammography procedures.
2. Materials and methods 2.1. Thermoluminescent material For this investigation ZrO2 chips were used. ZrO2 powder was synthesized from propoxide of zirconia and propanol which is described in previous works (Rivera et al., 2005; Rivera et al., 2006a, 2006b). The amorphous powder is then submitted to different thermal treatments in an oxidying atmosphere in order to stabilize the trap structure. After this process, the powder is mixed with polytetrafluorethylene (PTFE) having a ZrO2 þPTFE chips of 5 mm in diameter and 0.8 mm in thickness.
L.L. Palacios et al. / Applied Radiation and Isotopes 70 (2012) 1400–1402
Table 1 Exposure parameters. Experiment [num]
Breast thickness [cm]
Voltage [kV]
Current [mAs]
Exposure time [s]
1 2 3 4
4.2 4.2 4.2 4.2
24 24 24 24
108 108 108 145
1.20 1.20 1.22 1.63
2.2. Phantom studies The phantom used in the present investigation was American College of Radiology (ACR) accreditation phantom (ACR, 1981). This phantom provides measures of image quality and is also used for dose measurements during physics testing. The use of the ACR accreditation phantom for dosimetry measurements gives certainly for both QC and measurements dose. This phantom is generally considered to be representative of an average compressed breast of 4.2 cm thickness and composition represented by 50% glandular tissue and 50% fat. Aluminum oxide specks within the phantom simulate microcalcifications. Six different sizes nylon fibrous simulate fibrous structures and five different size lens shaped masses simulate tumors. In Table 1 parameters of exposure are described.
1401
using a heating rate of 10 1C/s. All TL measurements were made in a nitrogen atmosphere in order to reduce the thermal noise resulting from the heating planchet of the TL reader. All measurements were carried out 24 h after irradiations.
3. Results and discussion Fig. 1 shows a typical TL glow curve of the ZrO2 exposed to X-rays mammography beam. TL glow curve of ZrO2 þPTFE exhibited two peaks which can be seen at around 160 and 260 1C respectively. TL intensity of the 260 1C peak is higher than the intensity of the 160 1C peak. This spectrum was obtained from the output of the mammography unit at 24 kVp. TL response of ZrO2 as a function of entrance air kerma is showed in Fig. 2. As it can be seen in Fig. 2, TL response of ZrO2 exhibits a linear response in the range studied. Experimental values of the entrance skin dose were obtained irradiating simultaneously ZrO2 phosphors and ionization chamber mammography T43014 Gammex. All irradiations were carried out at the phantom thickness of 4.2 cm; the entrance skin doses obtained by ionization chamber were used to plot calibration values. From the experimental results a straight line was adjusted with an equation as follows: TLIntensity ¼ 97:863AK þ 109:8
Measurements were performed with a molybdenum/molybdenum anode and filter material combination in a mammography unit model Glory Elscintec. The irradiations with diagnostic radiology clinical beams were using an automatic exposure control (AEC) with the average exposure time of 1.5 s. The breast anthropomorphic phantom with a thickness of 4.2 mm and Mo/Mo filter/ anode material combination was used. Images obtained by this technique were evaluated according the European Community quality criteria. The high voltage, anode/filter material, projection, compressed breast thickness, exposure time and tube current product were reported for each exposure. Kerma in air and absorbed dose for each exposition, exposure parameters were collected for all mammographic services.
Table 2 shows the entrance skin air kerma obtained by irradiating to the phantoms and calculated from before equation. All values are higher than recommended by the American College
600 500 TL Intensity [a.u.]
2.3. X-ray unit
400 300 200 100
2.4. TLD calibration To X-ray dosimeter application materials were firstly calibrated. Thermoluminescent dosimeters (TLDs) and ion chamber model T43014 Gammex were used to measure the output of mammography unit. The irradiations with diagnostic clinical beams were made using the accredited phantom (ACR). The TLD were put on top of the phantom and carefully pressed with the paddle. In all irradiations with clinical beams, the TLDs were positioned in the center of the radiation field, on the phantom, with two pairs of each thermoluminescent material (two TLD-100 and two ZrO2) and ion chamber being simultaneously irradiated. TLD-100 was used during the same exposition just as reference material. The irradiations parameters were made based on clinical practice: 24 kVp and 108 mAs (based on the most common mAs parameter). The dose quantity obtained from this measurement is entrance skin air kerma (ESAK). Patient dose calculation was made using TLD measurements (expressed in terms of ESAK) using backscatter correction factor provided by IAEA TRS457.
0 100
150
200 250 Temperature [°C]
300
350
Fig. 1. Thermoluminescent glow curve of ZrO2 X-ray exposed.
2.5. TL readings After the desired exposure, TL glow curves were recorded using a Harshaw TLD reader (Model 3500) connected to a PC in order to record and process the data to use in the future analysis. TL signal was integrated from room temperature up to 350 1C
Fig. 2. Thermoluminescent response as a function of entrance skin dose.
1402
L.L. Palacios et al. / Applied Radiation and Isotopes 70 (2012) 1400–1402
Table 2 ESAK determined by irradiating simultaneously ZrO2 and ionization chamber. Procedure Breast thickness [cm]
Voltage [kV]
Current [mAs]
Mammography exposure time [s]
ESAK [mGy]
1 2 3 4
24 24 24 24
108 108 108 145
1.20 1.20 1.22 1.63
9.12 9.19 9.42 13.17
4.2 4.2 4.2 4.2
of Radiology. The difference in the results of four procedures for the same thickness of the breast is possible due to the large number of variables that interact at the dose estimating time.
4. Conclusions For breast densities of 50% adipose and 50% glandular tissues, the incident air-kerma was measured and the absorbed dose was calculated considering the X-ray output during the diagnostic procedure. Entrance skin dose on the phantom was higher than the American College Recommendation. The variation could be attributed to the parameters exposure procedure, i.e. time and the applied current. To optimize entrance skin dose could be by means of the combination of mammography anode/filter and potential or current application. Taking into account the above experimental results, we conclude thermoluminescent dosimeters based on ZrO2 are very promising for use as a TL dosimeter for mammography X-ray beams due its very high sensitivity.
Acknowledgment This work has been supported under grant of the CGPI-IPN Project no. 20120758.
References ACR 1981. American College of Radiology, 1981. Mammography Accreditation Program. Testing Instruction. Preston White Drive, Reston, VA 20191-4397. ACR 1994. American College of Radiology, 1994. Mammography Quality Control Manual. American College of Radiology, Reston, VA. Azorı´n, N.J., Rivera, M.T., 2002. Dosimetrı´a Termoluminiscente Aplicada a la Medicina. Byng, J.W., Mainprize, Yaffe M.J., 1998. X-ray characterization of breast phantom materials. Phys. Med. Biol. 43, 1367–1377. Dance, D.R., Skinner, C.L., Young, K.C., Beckett, J.R., Kotre, C.J., 2000. Additional factors for the estimation of mean glandular breast dose using the UK mammography dosimetry protocol. Phys. Med. Biol. 45, 3225–3240. Dance, D.R., 1990. Monte Carlo calculation of conversion factors for the estimation of mean glandular breast dose. Phys. Med. Biol. 35, 1211–1219. ˜ eda-Perdigo´n, G.M., Casian-Castellanos, G.A., Azorı´n-Nieto, J., Gaona, E., Castan Ira´n-Dı´az Go´ngora, J.A., Arreola, M., 2005. Utilidad clı´nica de los programas de control de calidad en mamografı´a. Anales de Radiologı´a Me´xico 2, 133–140. ICRP 52, 1987. International Commission on Radiological Protection, ICRP. ICRP Publication 52. Ann. ICRP 17 (4) (Oxford: Pergamon Press). Kimme, C.S., Bassett, L.W., Gold, R., Zhentlin, J., Gornbein, J.A., 1990. New mammography screen/film combinations: imaging characteristics and radia¨ tion dose. Am. J. Roentgenol. 154 (1990), 713–719. Palacios, P.L., Rivera, M.T., Ortiz, H., Guzma´n, J., Azorı´n, N.J., Garcı´a, H.M., 2008. Dosimetrı´a de Rayos X de Baja Energı´a Usando ZrO2 en Mastografı´a. XIX Congreso Anual de la SNM, Me´rida, Me´xico. 6–9 de Julio. Palacios, L., 2008. Tesis de Maestrı´a. Determinacio´n de Dosis Glandular Media en Mama Utilizando Dosı´metros Termoluminiscentes de ZrO2. CICATA-LEGARIA, IPN. Rivera, T., Olvera, L., Azorin, J., Soto, A.M., Barrera, M., Furetta, C., 2005. Thermoluminescence (TL) characteristics of hydrogenated amorphous zirconia. J. Radiat. Eff. Defects Solids 160 (5), 181–186. Rivera, T., Olvera, L., Azorin, J., Sosa, R., Soto, A.M., Barrera, M., Furetta, C., 2006a. Preparation of luminescent nanocrystals started from amorphous zirconia prepared by sol–gel technique. J. Radiat. Eff. Defects Solids 161 (2), 91–100. Rivera, T., Olvera, L., Azorı´n, J., Sosa, R., Barrera, M., Soto, A.M., Furetta, C., 2006b. Preparation of luminiscent nanocrystals started from amorphous zirconia prepared by sol–gel. Radiat. Eff. Defects Solids 161 (2), 91–100. Rivera, T., 2011. Chapter 6. Advances in Ceramics—Synthesis and Characterization, Processing and Specific Applications. Costas Sikalidis, (Ed.), InTech Publication. Yaffe, M.J., 2000. Digital Mammography, Chapter 5. Handbook of Medical Imaging, Physics and Psychophisics vol. 1, In: J. Beutel, H.L. Kundel, R.L. Van Meter, (Eds.), SPIE. Bellingham, WA.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence of zirconium oxide nanostructured to mammography X-ray beams L.L. Palacios a, T. Rivera a,n, J. Roman a, J. Azorı´n b, E. Gaona c a
´n en Ciencia Aplicada y Tecnologı´a Avanzada-Legaria, IPN. Av. Legaria 694, 11500 Me´xico D.F., Mexico Centro de Investigacio ´noma Metropolitana-Iztapalapa. Av. San Rafael Atlixco 187, 09340 Me ´xico D.F., Mexico Universidad Auto c ´noma Metropolitana-Xochimilco. Calz. Del Hueso 1100, 04960 Me ´xico D.F., Mexico Universidad Auto b
a r t i c l e i n f o
a b s t r a c t
Available online 24 February 2012
In the present work thermoluminescent (TL) response of zirconium oxide (ZrO2) nanostructured induced by mammography X-ray radiation was investigated. Measurements were made of the response per unit air kerma of ZrO2 with mammography equipment parameters (semiautomatic exposure control, 24 kVp and 108 mAs). The calibration curves were obtained by simultaneously irradiating ZrO2 samples and ion chamber. Samples of ZrO2 showed a linear response as a function of entrance skin air kerma. The observed results in TL properties suggest that ZrO2 nanostructured could be considered as an effective material for X-ray beams dosimetry if appropriate calibration procedures are performed. & 2012 Elsevier Ltd. All rights reserved.
Keywords: X-rays Mammography Thermoluminescent dosimetry X-ray low energy
1. Introduction The ionizing radiation sources in medicine has been increasing worldwide ever since the discovery of X-rays and radioactivity. Wide varieties of imaging and treatment procedures relying on ionizing radiation have been developed and the trend will continue in order to meet the involving needs of diagnostic and therapeutic practices (Azorı´n and Rivera, 2002; Rivera, 2011). At present, mammography is considered to be the most effective method for early detection of small malignant lesions in the female breast (Palacios et al., 2008; Palacios, 2008). Mammary cancer detection by means diagnostic radiology at an early stage improves the probability of survival considerably (Yaffe, 2000). However, the use of ionizing radiation implies the risk of induction of breast cancer. In case of digital detectors (Gaona et al., 2005; Kimme et al., 1990), the images are digital and can either be printed or viewed on a monitor. The examinations by mammography devices are performed by compressing the breast between a plastic plate and the bucky that contains the radiographic film. Breast dose determination requires entrance surface air kerma (ESAK) measurements and the use of conversion factors (ACR, 1981; Byng et al., 1998; ICRP, 1987; Dance et al., 2000; Dance, 1990). The entrance surface dose (ESD) is the absorbed dose in air, including the contribution from backscatter, measured at a point on the entrance surface of a specific object in this case the breast of patient (ACR, 1994). A simple direct method of determining
n
Corresponding author. Tel.: þ525553956738. E-mail address: [email protected] (T. Rivera).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.02.005
ESD is the use of thermoluminescent detectors (TLDs) positioned on top of a compressed breast or a breast phantom. Thermoluminescent dosimetry (TLD) system presents some advantages over other systems, i.e. high sensitivity, small size, linear response for a wide dose interval, low cost and easy handle. Amongst the most frequently utilized thermoluminescent materials is the lithium fluoride doped with magnesium and titanium (LiF: Mg, Ti) which exhibits some important characteristics such as the effective atomic number (Zeff ¼8.2) close to that of human tissue. The aim of the present work was to investigate thermoluminescent characteristics of ZrO2 nanostructured irradiated with mammography X-ray beams and the implications in the entrance skin dose estimation in patients submitted to mammography procedures.
2. Materials and methods 2.1. Thermoluminescent material For this investigation ZrO2 chips were used. ZrO2 powder was synthesized from propoxide of zirconia and propanol which is described in previous works (Rivera et al., 2005; Rivera et al., 2006a, 2006b). The amorphous powder is then submitted to different thermal treatments in an oxidying atmosphere in order to stabilize the trap structure. After this process, the powder is mixed with polytetrafluorethylene (PTFE) having a ZrO2 þPTFE chips of 5 mm in diameter and 0.8 mm in thickness.
L.L. Palacios et al. / Applied Radiation and Isotopes 70 (2012) 1400–1402
Table 1 Exposure parameters. Experiment [num]
Breast thickness [cm]
Voltage [kV]
Current [mAs]
Exposure time [s]
1 2 3 4
4.2 4.2 4.2 4.2
24 24 24 24
108 108 108 145
1.20 1.20 1.22 1.63
2.2. Phantom studies The phantom used in the present investigation was American College of Radiology (ACR) accreditation phantom (ACR, 1981). This phantom provides measures of image quality and is also used for dose measurements during physics testing. The use of the ACR accreditation phantom for dosimetry measurements gives certainly for both QC and measurements dose. This phantom is generally considered to be representative of an average compressed breast of 4.2 cm thickness and composition represented by 50% glandular tissue and 50% fat. Aluminum oxide specks within the phantom simulate microcalcifications. Six different sizes nylon fibrous simulate fibrous structures and five different size lens shaped masses simulate tumors. In Table 1 parameters of exposure are described.
1401
using a heating rate of 10 1C/s. All TL measurements were made in a nitrogen atmosphere in order to reduce the thermal noise resulting from the heating planchet of the TL reader. All measurements were carried out 24 h after irradiations.
3. Results and discussion Fig. 1 shows a typical TL glow curve of the ZrO2 exposed to X-rays mammography beam. TL glow curve of ZrO2 þPTFE exhibited two peaks which can be seen at around 160 and 260 1C respectively. TL intensity of the 260 1C peak is higher than the intensity of the 160 1C peak. This spectrum was obtained from the output of the mammography unit at 24 kVp. TL response of ZrO2 as a function of entrance air kerma is showed in Fig. 2. As it can be seen in Fig. 2, TL response of ZrO2 exhibits a linear response in the range studied. Experimental values of the entrance skin dose were obtained irradiating simultaneously ZrO2 phosphors and ionization chamber mammography T43014 Gammex. All irradiations were carried out at the phantom thickness of 4.2 cm; the entrance skin doses obtained by ionization chamber were used to plot calibration values. From the experimental results a straight line was adjusted with an equation as follows: TLIntensity ¼ 97:863AK þ 109:8
Measurements were performed with a molybdenum/molybdenum anode and filter material combination in a mammography unit model Glory Elscintec. The irradiations with diagnostic radiology clinical beams were using an automatic exposure control (AEC) with the average exposure time of 1.5 s. The breast anthropomorphic phantom with a thickness of 4.2 mm and Mo/Mo filter/ anode material combination was used. Images obtained by this technique were evaluated according the European Community quality criteria. The high voltage, anode/filter material, projection, compressed breast thickness, exposure time and tube current product were reported for each exposure. Kerma in air and absorbed dose for each exposition, exposure parameters were collected for all mammographic services.
Table 2 shows the entrance skin air kerma obtained by irradiating to the phantoms and calculated from before equation. All values are higher than recommended by the American College
600 500 TL Intensity [a.u.]
2.3. X-ray unit
400 300 200 100
2.4. TLD calibration To X-ray dosimeter application materials were firstly calibrated. Thermoluminescent dosimeters (TLDs) and ion chamber model T43014 Gammex were used to measure the output of mammography unit. The irradiations with diagnostic clinical beams were made using the accredited phantom (ACR). The TLD were put on top of the phantom and carefully pressed with the paddle. In all irradiations with clinical beams, the TLDs were positioned in the center of the radiation field, on the phantom, with two pairs of each thermoluminescent material (two TLD-100 and two ZrO2) and ion chamber being simultaneously irradiated. TLD-100 was used during the same exposition just as reference material. The irradiations parameters were made based on clinical practice: 24 kVp and 108 mAs (based on the most common mAs parameter). The dose quantity obtained from this measurement is entrance skin air kerma (ESAK). Patient dose calculation was made using TLD measurements (expressed in terms of ESAK) using backscatter correction factor provided by IAEA TRS457.
0 100
150
200 250 Temperature [°C]
300
350
Fig. 1. Thermoluminescent glow curve of ZrO2 X-ray exposed.
2.5. TL readings After the desired exposure, TL glow curves were recorded using a Harshaw TLD reader (Model 3500) connected to a PC in order to record and process the data to use in the future analysis. TL signal was integrated from room temperature up to 350 1C
Fig. 2. Thermoluminescent response as a function of entrance skin dose.
1402
L.L. Palacios et al. / Applied Radiation and Isotopes 70 (2012) 1400–1402
Table 2 ESAK determined by irradiating simultaneously ZrO2 and ionization chamber. Procedure Breast thickness [cm]
Voltage [kV]
Current [mAs]
Mammography exposure time [s]
ESAK [mGy]
1 2 3 4
24 24 24 24
108 108 108 145
1.20 1.20 1.22 1.63
9.12 9.19 9.42 13.17
4.2 4.2 4.2 4.2
of Radiology. The difference in the results of four procedures for the same thickness of the breast is possible due to the large number of variables that interact at the dose estimating time.
4. Conclusions For breast densities of 50% adipose and 50% glandular tissues, the incident air-kerma was measured and the absorbed dose was calculated considering the X-ray output during the diagnostic procedure. Entrance skin dose on the phantom was higher than the American College Recommendation. The variation could be attributed to the parameters exposure procedure, i.e. time and the applied current. To optimize entrance skin dose could be by means of the combination of mammography anode/filter and potential or current application. Taking into account the above experimental results, we conclude thermoluminescent dosimeters based on ZrO2 are very promising for use as a TL dosimeter for mammography X-ray beams due its very high sensitivity.
Acknowledgment This work has been supported under grant of the CGPI-IPN Project no. 20120758.
References ACR 1981. American College of Radiology, 1981. Mammography Accreditation Program. Testing Instruction. Preston White Drive, Reston, VA 20191-4397. ACR 1994. American College of Radiology, 1994. Mammography Quality Control Manual. American College of Radiology, Reston, VA. Azorı´n, N.J., Rivera, M.T., 2002. Dosimetrı´a Termoluminiscente Aplicada a la Medicina. Byng, J.W., Mainprize, Yaffe M.J., 1998. X-ray characterization of breast phantom materials. Phys. Med. Biol. 43, 1367–1377. Dance, D.R., Skinner, C.L., Young, K.C., Beckett, J.R., Kotre, C.J., 2000. Additional factors for the estimation of mean glandular breast dose using the UK mammography dosimetry protocol. Phys. Med. Biol. 45, 3225–3240. Dance, D.R., 1990. Monte Carlo calculation of conversion factors for the estimation of mean glandular breast dose. Phys. Med. Biol. 35, 1211–1219. ˜ eda-Perdigo´n, G.M., Casian-Castellanos, G.A., Azorı´n-Nieto, J., Gaona, E., Castan Ira´n-Dı´az Go´ngora, J.A., Arreola, M., 2005. Utilidad clı´nica de los programas de control de calidad en mamografı´a. Anales de Radiologı´a Me´xico 2, 133–140. ICRP 52, 1987. International Commission on Radiological Protection, ICRP. ICRP Publication 52. Ann. ICRP 17 (4) (Oxford: Pergamon Press). Kimme, C.S., Bassett, L.W., Gold, R., Zhentlin, J., Gornbein, J.A., 1990. New mammography screen/film combinations: imaging characteristics and radia¨ tion dose. Am. J. Roentgenol. 154 (1990), 713–719. Palacios, P.L., Rivera, M.T., Ortiz, H., Guzma´n, J., Azorı´n, N.J., Garcı´a, H.M., 2008. Dosimetrı´a de Rayos X de Baja Energı´a Usando ZrO2 en Mastografı´a. XIX Congreso Anual de la SNM, Me´rida, Me´xico. 6–9 de Julio. Palacios, L., 2008. Tesis de Maestrı´a. Determinacio´n de Dosis Glandular Media en Mama Utilizando Dosı´metros Termoluminiscentes de ZrO2. CICATA-LEGARIA, IPN. Rivera, T., Olvera, L., Azorin, J., Soto, A.M., Barrera, M., Furetta, C., 2005. Thermoluminescence (TL) characteristics of hydrogenated amorphous zirconia. J. Radiat. Eff. Defects Solids 160 (5), 181–186. Rivera, T., Olvera, L., Azorin, J., Sosa, R., Soto, A.M., Barrera, M., Furetta, C., 2006a. Preparation of luminescent nanocrystals started from amorphous zirconia prepared by sol–gel technique. J. Radiat. Eff. Defects Solids 161 (2), 91–100. Rivera, T., Olvera, L., Azorı´n, J., Sosa, R., Barrera, M., Soto, A.M., Furetta, C., 2006b. Preparation of luminiscent nanocrystals started from amorphous zirconia prepared by sol–gel. Radiat. Eff. Defects Solids 161 (2), 91–100. Rivera, T., 2011. Chapter 6. Advances in Ceramics—Synthesis and Characterization, Processing and Specific Applications. Costas Sikalidis, (Ed.), InTech Publication. Yaffe, M.J., 2000. Digital Mammography, Chapter 5. Handbook of Medical Imaging, Physics and Psychophisics vol. 1, In: J. Beutel, H.L. Kundel, R.L. Van Meter, (Eds.), SPIE. Bellingham, WA.