- Email: [email protected]
Development of a chest phantom for testing in Computed Tomography scans
Radiation Physics and Chemistry 140 (2017) 275–277
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of a chest phantom for testing in Computed Tomography scans W.N. Aburjailea, A.P. Mouraoa,b, a b
MARK
⁎
Department of Nuclear Engineering, UFMG, Belo Horizonte, Brazil Biomedical Engineering Center, CEFET-MG, Belo Horizonte, Brazil
A R T I C L E I N F O
A BS T RAC T
Keywords: Chest phantom Computed Tomography Dosimetry
To optimize the process of Computed Tomography (CT) scans, it is important to know the dose distribution in order to vary the acquisition parameters. The aim of this study is to evaluate the variation of CT Dose Index (CTDI) in chest scans, by using two chest phantoms. In this study it was used a cylindrical chest phantom made in polymethylmethacrylate (PMMA) and a second phantom was developed in PMMA with the same volume and an oblong shape, based on the dimensions of an adult human chest. The two phantoms have five openings, one central and four peripheral out of phase 90°, which allow placing a pencil ionizing chamber in five positions. In a CT scanner GE, Discovery model with 64 channels, the central slice of the two phantoms were irradiated successively to obtain air kerma in PMMA measurements using the pencil chamber. From the measurements obtained in these five positions in the phantoms, it was possible to obtain the volumetric CTDI for both chest phantoms. By comparison of the results, it was demonstrated that the oblong phantom, with a similar human chest shape, have received higher doses, mainly in the anterior, posterior and central region. The volumetric CTDI indicates a value 5.1% higher for the oblong phantom.
1. Introduction The technology of Computed Tomography (CT) scanners has had a great development since the early 1970s. This evolution that allows data acquisition in helical scans with multidetector arches (MDCT) has revolutionized the role of images in medical diagnosis. State of the art of CT equipment allows providing diagnostics through images with few artifacts in full body scans realized in less than 5 s. These scans provide in vivo images that have information similar to those that could be obtained by visual inspection of an anatomical dissection. The CT scans may be performed quickly with a minimum of patient preparation in virtually all types of clinical situations. There are reduced restrictions imposed by an implantable medical device or medical monitoring equipment. Finally CT is well accepted by patients and doctors because the images acquisitions require minimal patient compliance, it has a small discomfort and are usually very accurate. These conditions promote every year a significant increase in the number of CT examinations (Thakur et al., 2013). Standards for dosimetry in radiology were launched during the Malaga Conference in 2001, aimed at radiation protection of patients undergoing diagnostic tests or therapy, including radiotherapy, diagnostic radiology and nuclear medicine. The use of ionizing radiation in
⁎
medicine has grown due to the benefits associated with it and its technological development, such as the application of new radiopharmaceuticals, digital radio-graphic images and the new generations of CT scanners (IAEA, 2007). Currently, CT scanners used in diagnostic radiology departments allow axial, helical and helical multislice scans. The variety of manufacturers, different types of X-ray tubes, the use of different electrical current values, tube rotation times, etc., It follows that each diagnostic radiology service, regardless of the used CT scanner, adopt its own scan protocols for routine CT examinations (Dance et al., 2014; Mourao, 2015). Many countries have introduced in their legislation the obligation to report the doses received by patients undergoing radiodiagnostic exams. Brazilian law provides reference levels for diagnosis (DRL) only in terms of multislice average dose (MSAD) of 25 mGy to the abdomen and chest, considering a standard adult patient (Brazil, 1998). As part of an optimization program, DRLs should be used for the quality control of CT scans, to review and adjust procedures and techniques when doses exceed the limits specified in Brazil (1998). DRL values were adopted from international recommendations of the IAEA (1996), and they may not represent the actual conditions of the tests in Brazil. In Minas Gerais state, CT quality control tests have been
Corresponding author. E-mail address: [email protected] (A.P. Mourao).
http://dx.doi.org/10.1016/j.radphyschem.2017.01.031 Received 21 June 2016; Received in revised form 20 January 2017; Accepted 26 January 2017 Available online 28 January 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Radiation Physics and Chemistry 140 (2017) 275–277
W.N. Aburjaile, A.P. Mourao
Fig. 1. Images of PMMA chest phantoms, cylindrical (a) and oblong (b), placed in the CT scanner.
correct position of the phantoms and to demarcate the central slice position. The central slice irradiation was performed using the cylindrical and oblong phantoms in axial mode. The parameters of the protocol used in this irradiation are shown in Table 1. An UNFORS dosimetry set with a pencil ionizing chamber model 10×5-3CT, n. 8202041-B Xi CT, and an electrometer Ray Safe n. 8201023-C Xi has been used, for the air kerma in PMMA measurement in the central (CPMMA,100,c) and in the periferical (CPMMA,100,p) regions. The calibrated pencil ionizing chamber was placed in the central and peripheral openings of each phantom. When the pencil chamber has been placed in one opening for acquiring data, the others have been filled by rods of PMMA. The central slice was irradiated successively in order to obtain air kerma values (mGy). Seven readings were taken per measurement point (IAEA, 2007). The metrological reliability of the ionizing chamber was demonstrated through reproducibility test and by calibrating it in a reference radiation for CT (RQT9) that were reproduced in Calibration Laboratory of the CDTN/CNEN.
mandatory since July 2009, but a better understanding of the methodology to perform such tests is still needed (Mourão et al., 2014). 2. Materials and methods In this work, experimental measurements were performed on two chest phantoms, both made in PMMA. The cylindrical phantom is the standard used to obtain the Computed Tomography Dose Index (CTDI) to the body. A second object, in oblong shape, was developed based on the size of the chest, in the axillary region, of an adult human body. The oblong chest phantom has the same flat surface area and the same length of 15 cm, and therefore, the same volume of the cylindrical phantom. Fig. 1 shows an image of the two phantoms placed in isocenter of the gantry CT scanner. The two chest phantoms have five openings with a diameter of 12.67 mm for the placement of the pencil chamber, one central and four peripheral lagged of 90o. The center of the peripheral openings is positioned 10 mm from the phantom edge. In analogy to the display of an analog clock, peripheral openings were named 3, 6, 9 and 12, according to position within the gantry during the scanning of the central slice. The first phantom has a cylindrical shape with 160 mm in diameter and 150 mm in length and the second is an oblong phantom with dimensions of 43 cm in width, 22 cm in height and 15 cm in length. The cut area of the oblong phantom is defined by two semi ellipses generated from an ellipse of 30 cm by 22 cm and a rectangle of 22 cm by 13 cm. Fig. 2 shows a sectional image of the central slice of the two chest phantoms. The experiment was conducted in a GE scanner, Discovery model of 64 channels. The phantoms were positioned using optical alignment aids, in the isocenter of the CT scanner. A scout was done to check the
3. Results Table 2 shows the CPMMA,100 values obtained for each one of the phantoms in each of the five observed positions, and the respective standard deviations for each measurement. The minimum recorded value was 5.34 mGy and the maximum was 14.35 mGy. Fig. 3 presents a graphic representation of the CPMMA,100 values in each region of the phantoms. This illustration allows comparing the dose variation inside the phantoms and between them. All the measurements obtained were converted to CT dose index (CTDI100) using a ratio of 1.042 for the attenuation coefficients PMMA/ air. The routine chest scan protocol has a pitch of 0.984, so the
Fig. 2. Axial cutting images of the phantom central slice with positions measurements in cylindrical (a) and oblong (b) phantoms.
276
Radiation Physics and Chemistry 140 (2017) 275–277
W.N. Aburjaile, A.P. Mourao
the central region are smaller than the periphery CPMMA,100 values for the two phantoms, ranging from 46.33% to 58.76% smaller, for the cylindrical phantom, and 37.19–60.81% smaller for the oblong chest phantom. Increasing the CTDI100 has occurred in the short axis of the oblong phantom, region 6–12. On the other hand, there was a reduction in the lateral extremities, 3 and 9 points, of 22.48% and 15.97%. In both irradiation conditions, the CTDI100 recorded in position 6 is smaller than that recorded in position 12, and this is due to the X-ray beam filtration by the table. The oblong phantom, being a object which is more similar to chest surface showed that the dose distribution in the chest occurs differently from that obtained with the cylindrical object and the CTDIvol is greater than what can be found in a circular region as the abdomen, for example. When considering the CTDIvol, the oblong object presented an index 5.12% greater than the cylindrical object. This happened because of the increased dose in the positions 6, 12 and Central. These points were higher in 19.50%, 24.59% and 22.29%, respectively. This means that the CTDIvol estimated by CT systems in chest scans for adult patient should be corrected by a factor of at least 1.05. The experiment showed that in addition to an average dose increased, according the CTDIvol, mediastinum region receives much higher doses than those provided when the cylindrical phantom is used. In mediastinum is the heart, the thoracic spine and sternum bone. In future work, the results will be obtained in other CT scanners will help to understand better the changes in the dose distribution with the shape of the cut area in CT scans.
Table 1 Parameters of chest protocol scan. Voltage (kV)
Charge (mAs)
Current (mA)
Thickness (mm)
Time (s)
120
100
200
10
0,5
Table 2 CPMMA,100 values in mGy. Position
Phantom Cylindrical
C 3 6 9 12
Oblong
CPMMA,100
SD
CPMMA,100
SD
5.34 11.32 9.08 10.71 11.52
0.03 0.03 0.09 0.06 0.07
6.53 8.77 10.85 9.00 14.35
0.03 0.03 0.04 0.35 0.03
Acknowledgements This work was supported by the Fundação de Amparo à Pesquisa de Minas Gerais (FAPEMIG) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). References Brazil, 1998. Ministério da Saúde. Diretrizes de proteção radiológica em radiodiagnóstico médico e odontológico. Portaria no. 453. Brasília: Diário oficial da União. Dance, D.R., Christofides, S., Maidment, A.D., McLean, I.D., Ng, K.H., 2014. Diagnostic radiology physics: a handbook for teachers and students. 682 IAEA - International Atomic Energy Agency, 2007. Dosimetry in diagnostic radiology: an international code of practiceTRS Serie 457, (Vienna). IAEA - International Atomic Energy Agency, 1996. International basic safety standards for protection against ionizing radiation and for safety of radiation sources. (Vienna) Saf. Ser. 115, (Vienna). Mourão, A.P., 2015. Tomografia Computadorizada: tecnologias e aplicações. São Caetano do Sul: Difus.. Mourão, A.P., Gonçalves, R.G., Jr., Alonso, T.C., 2014. Dose profile variation with pitch in head CT scans using gafchromic films. Recent Adv. Biomed. Chem. Eng. Mater. Sci. 1, 51–54. NIST-National Institute of Standards and Technology, 2016. At 〈http://www.nist.gov/ pml/data/xraycoef/index.cfm〉. (Access 15.10.16). Thakur, Y., McLaughlin, P.D., Mayo, Jr., 2013. Strategies for Radiation Dose Optimization. Curr. Radio. Rep. 1, 1–10.
Fig. 3. Air kerma in PMMA (CPMMA,100) recorded in the cylindrical and oblong phantoms.
calculation of the Computed Tomography Dose Index (CTDIvol) has the value of 9.41 mGy for the cylindrical phantom and 9.89 mGy for the oblong phantom (NIST-National Institute of Standards and Technology, 2016). 4. Discussions and conclusions The CPMMA,100 values obtained allow to observe that the values in
277
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of a chest phantom for testing in Computed Tomography scans W.N. Aburjailea, A.P. Mouraoa,b, a b
MARK
⁎
Department of Nuclear Engineering, UFMG, Belo Horizonte, Brazil Biomedical Engineering Center, CEFET-MG, Belo Horizonte, Brazil
A R T I C L E I N F O
A BS T RAC T
Keywords: Chest phantom Computed Tomography Dosimetry
To optimize the process of Computed Tomography (CT) scans, it is important to know the dose distribution in order to vary the acquisition parameters. The aim of this study is to evaluate the variation of CT Dose Index (CTDI) in chest scans, by using two chest phantoms. In this study it was used a cylindrical chest phantom made in polymethylmethacrylate (PMMA) and a second phantom was developed in PMMA with the same volume and an oblong shape, based on the dimensions of an adult human chest. The two phantoms have five openings, one central and four peripheral out of phase 90°, which allow placing a pencil ionizing chamber in five positions. In a CT scanner GE, Discovery model with 64 channels, the central slice of the two phantoms were irradiated successively to obtain air kerma in PMMA measurements using the pencil chamber. From the measurements obtained in these five positions in the phantoms, it was possible to obtain the volumetric CTDI for both chest phantoms. By comparison of the results, it was demonstrated that the oblong phantom, with a similar human chest shape, have received higher doses, mainly in the anterior, posterior and central region. The volumetric CTDI indicates a value 5.1% higher for the oblong phantom.
1. Introduction The technology of Computed Tomography (CT) scanners has had a great development since the early 1970s. This evolution that allows data acquisition in helical scans with multidetector arches (MDCT) has revolutionized the role of images in medical diagnosis. State of the art of CT equipment allows providing diagnostics through images with few artifacts in full body scans realized in less than 5 s. These scans provide in vivo images that have information similar to those that could be obtained by visual inspection of an anatomical dissection. The CT scans may be performed quickly with a minimum of patient preparation in virtually all types of clinical situations. There are reduced restrictions imposed by an implantable medical device or medical monitoring equipment. Finally CT is well accepted by patients and doctors because the images acquisitions require minimal patient compliance, it has a small discomfort and are usually very accurate. These conditions promote every year a significant increase in the number of CT examinations (Thakur et al., 2013). Standards for dosimetry in radiology were launched during the Malaga Conference in 2001, aimed at radiation protection of patients undergoing diagnostic tests or therapy, including radiotherapy, diagnostic radiology and nuclear medicine. The use of ionizing radiation in
⁎
medicine has grown due to the benefits associated with it and its technological development, such as the application of new radiopharmaceuticals, digital radio-graphic images and the new generations of CT scanners (IAEA, 2007). Currently, CT scanners used in diagnostic radiology departments allow axial, helical and helical multislice scans. The variety of manufacturers, different types of X-ray tubes, the use of different electrical current values, tube rotation times, etc., It follows that each diagnostic radiology service, regardless of the used CT scanner, adopt its own scan protocols for routine CT examinations (Dance et al., 2014; Mourao, 2015). Many countries have introduced in their legislation the obligation to report the doses received by patients undergoing radiodiagnostic exams. Brazilian law provides reference levels for diagnosis (DRL) only in terms of multislice average dose (MSAD) of 25 mGy to the abdomen and chest, considering a standard adult patient (Brazil, 1998). As part of an optimization program, DRLs should be used for the quality control of CT scans, to review and adjust procedures and techniques when doses exceed the limits specified in Brazil (1998). DRL values were adopted from international recommendations of the IAEA (1996), and they may not represent the actual conditions of the tests in Brazil. In Minas Gerais state, CT quality control tests have been
Corresponding author. E-mail address: [email protected] (A.P. Mourao).
http://dx.doi.org/10.1016/j.radphyschem.2017.01.031 Received 21 June 2016; Received in revised form 20 January 2017; Accepted 26 January 2017 Available online 28 January 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Radiation Physics and Chemistry 140 (2017) 275–277
W.N. Aburjaile, A.P. Mourao
Fig. 1. Images of PMMA chest phantoms, cylindrical (a) and oblong (b), placed in the CT scanner.
correct position of the phantoms and to demarcate the central slice position. The central slice irradiation was performed using the cylindrical and oblong phantoms in axial mode. The parameters of the protocol used in this irradiation are shown in Table 1. An UNFORS dosimetry set with a pencil ionizing chamber model 10×5-3CT, n. 8202041-B Xi CT, and an electrometer Ray Safe n. 8201023-C Xi has been used, for the air kerma in PMMA measurement in the central (CPMMA,100,c) and in the periferical (CPMMA,100,p) regions. The calibrated pencil ionizing chamber was placed in the central and peripheral openings of each phantom. When the pencil chamber has been placed in one opening for acquiring data, the others have been filled by rods of PMMA. The central slice was irradiated successively in order to obtain air kerma values (mGy). Seven readings were taken per measurement point (IAEA, 2007). The metrological reliability of the ionizing chamber was demonstrated through reproducibility test and by calibrating it in a reference radiation for CT (RQT9) that were reproduced in Calibration Laboratory of the CDTN/CNEN.
mandatory since July 2009, but a better understanding of the methodology to perform such tests is still needed (Mourão et al., 2014). 2. Materials and methods In this work, experimental measurements were performed on two chest phantoms, both made in PMMA. The cylindrical phantom is the standard used to obtain the Computed Tomography Dose Index (CTDI) to the body. A second object, in oblong shape, was developed based on the size of the chest, in the axillary region, of an adult human body. The oblong chest phantom has the same flat surface area and the same length of 15 cm, and therefore, the same volume of the cylindrical phantom. Fig. 1 shows an image of the two phantoms placed in isocenter of the gantry CT scanner. The two chest phantoms have five openings with a diameter of 12.67 mm for the placement of the pencil chamber, one central and four peripheral lagged of 90o. The center of the peripheral openings is positioned 10 mm from the phantom edge. In analogy to the display of an analog clock, peripheral openings were named 3, 6, 9 and 12, according to position within the gantry during the scanning of the central slice. The first phantom has a cylindrical shape with 160 mm in diameter and 150 mm in length and the second is an oblong phantom with dimensions of 43 cm in width, 22 cm in height and 15 cm in length. The cut area of the oblong phantom is defined by two semi ellipses generated from an ellipse of 30 cm by 22 cm and a rectangle of 22 cm by 13 cm. Fig. 2 shows a sectional image of the central slice of the two chest phantoms. The experiment was conducted in a GE scanner, Discovery model of 64 channels. The phantoms were positioned using optical alignment aids, in the isocenter of the CT scanner. A scout was done to check the
3. Results Table 2 shows the CPMMA,100 values obtained for each one of the phantoms in each of the five observed positions, and the respective standard deviations for each measurement. The minimum recorded value was 5.34 mGy and the maximum was 14.35 mGy. Fig. 3 presents a graphic representation of the CPMMA,100 values in each region of the phantoms. This illustration allows comparing the dose variation inside the phantoms and between them. All the measurements obtained were converted to CT dose index (CTDI100) using a ratio of 1.042 for the attenuation coefficients PMMA/ air. The routine chest scan protocol has a pitch of 0.984, so the
Fig. 2. Axial cutting images of the phantom central slice with positions measurements in cylindrical (a) and oblong (b) phantoms.
276
Radiation Physics and Chemistry 140 (2017) 275–277
W.N. Aburjaile, A.P. Mourao
the central region are smaller than the periphery CPMMA,100 values for the two phantoms, ranging from 46.33% to 58.76% smaller, for the cylindrical phantom, and 37.19–60.81% smaller for the oblong chest phantom. Increasing the CTDI100 has occurred in the short axis of the oblong phantom, region 6–12. On the other hand, there was a reduction in the lateral extremities, 3 and 9 points, of 22.48% and 15.97%. In both irradiation conditions, the CTDI100 recorded in position 6 is smaller than that recorded in position 12, and this is due to the X-ray beam filtration by the table. The oblong phantom, being a object which is more similar to chest surface showed that the dose distribution in the chest occurs differently from that obtained with the cylindrical object and the CTDIvol is greater than what can be found in a circular region as the abdomen, for example. When considering the CTDIvol, the oblong object presented an index 5.12% greater than the cylindrical object. This happened because of the increased dose in the positions 6, 12 and Central. These points were higher in 19.50%, 24.59% and 22.29%, respectively. This means that the CTDIvol estimated by CT systems in chest scans for adult patient should be corrected by a factor of at least 1.05. The experiment showed that in addition to an average dose increased, according the CTDIvol, mediastinum region receives much higher doses than those provided when the cylindrical phantom is used. In mediastinum is the heart, the thoracic spine and sternum bone. In future work, the results will be obtained in other CT scanners will help to understand better the changes in the dose distribution with the shape of the cut area in CT scans.
Table 1 Parameters of chest protocol scan. Voltage (kV)
Charge (mAs)
Current (mA)
Thickness (mm)
Time (s)
120
100
200
10
0,5
Table 2 CPMMA,100 values in mGy. Position
Phantom Cylindrical
C 3 6 9 12
Oblong
CPMMA,100
SD
CPMMA,100
SD
5.34 11.32 9.08 10.71 11.52
0.03 0.03 0.09 0.06 0.07
6.53 8.77 10.85 9.00 14.35
0.03 0.03 0.04 0.35 0.03
Acknowledgements This work was supported by the Fundação de Amparo à Pesquisa de Minas Gerais (FAPEMIG) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). References Brazil, 1998. Ministério da Saúde. Diretrizes de proteção radiológica em radiodiagnóstico médico e odontológico. Portaria no. 453. Brasília: Diário oficial da União. Dance, D.R., Christofides, S., Maidment, A.D., McLean, I.D., Ng, K.H., 2014. Diagnostic radiology physics: a handbook for teachers and students. 682 IAEA - International Atomic Energy Agency, 2007. Dosimetry in diagnostic radiology: an international code of practiceTRS Serie 457, (Vienna). IAEA - International Atomic Energy Agency, 1996. International basic safety standards for protection against ionizing radiation and for safety of radiation sources. (Vienna) Saf. Ser. 115, (Vienna). Mourão, A.P., 2015. Tomografia Computadorizada: tecnologias e aplicações. São Caetano do Sul: Difus.. Mourão, A.P., Gonçalves, R.G., Jr., Alonso, T.C., 2014. Dose profile variation with pitch in head CT scans using gafchromic films. Recent Adv. Biomed. Chem. Eng. Mater. Sci. 1, 51–54. NIST-National Institute of Standards and Technology, 2016. At 〈http://www.nist.gov/ pml/data/xraycoef/index.cfm〉. (Access 15.10.16). Thakur, Y., McLaughlin, P.D., Mayo, Jr., 2013. Strategies for Radiation Dose Optimization. Curr. Radio. Rep. 1, 1–10.
Fig. 3. Air kerma in PMMA (CPMMA,100) recorded in the cylindrical and oblong phantoms.
calculation of the Computed Tomography Dose Index (CTDIvol) has the value of 9.41 mGy for the cylindrical phantom and 9.89 mGy for the oblong phantom (NIST-National Institute of Standards and Technology, 2016). 4. Discussions and conclusions The CPMMA,100 values obtained allow to observe that the values in
277