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Reduction of breast dose in abdominal CT examinations: Effectiveness of automatic exposure control system
Radiation Measurements 46 (2011) 2056e2059
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Reduction of breast dose in abdominal CT examinations: Effectiveness of automatic exposure control system Kosuke Matsubara a, *, Kichiro Koshida a, Kimiya Noto b, Tadanori Takata b a b
Department of Quantum Medical Technology, Faculty of Health Sciences, Kanazawa University, 5-11-80 Kodatsuno, Kanazawa, Ishikawa 920-0942, Japan Department of Radiological Technology, Kanazawa University Hospital, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8641, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2010 Received in revised form 5 July 2011 Accepted 7 July 2011
We evaluated the effectiveness of the automatic exposure control (AEC) system for reducing the radiation dose to the breasts in abdominal computed tomography (CT). Acquisitions from the upper abdomen of an anthropomorphic phantom were acquired using multiple parameter combinations after placing radiophotoluminescent glass dosimeters at the breasts and abdominal regions to measure the breasts and abdominal radiation dose for each parameter combination. In addition, the level of image noise was evaluated using the images obtained from the anthropomorphic phantom for each parameter combination. When identical beam width and pitch factor were selected, the breast dose tended to be low using z-axis modulation compared with fixed mA acquisition, and it also tended to be low using xyz-axis modulation compared with using z-axis modulation. The level of image noise in general was almost equal to the level that was preset when the AEC system was applied, but the level of image noise at the liver region under the right diaphragm tended to be low. Thus, the AEC system is effective for the reduction of the radiation dose to the breasts while maintaining appropriate image quality in abdominal CT examinations. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Computed tomography Automatic exposure control Radiophotoluminescent glass dosimeter Breast dose
1. Introduction Considering the dynamic study of the liver and pancreas, there have been reports about usability of three-phase (single-arterial phase), four-phase (double-arterial phase), and five-phase acquisitions (triple-arterial phase) (Laghi et al., 2003; McNulty et al., 2001; Murakami et al., 2001). However, the total radiation dose to the patient tends to be high when multiphase acquisition is performed in abdominal computed tomography (CT). In addition, overbeaming and over-ranging, which are characteristic of a multidetector CT, affect patient doses (Valentin, 2007b). Overbeaming occurs when the X-ray beam incident on a patient extends beyond the active detector area, and over-ranging occurs when there is an increase in dose-length product due to additional rotation required for the interpolation algorithm. In abdominal CT examinations, these issues cause increasing absorbed radiation doses for organs out of the acquisition range, such as the breasts. The breasts are one of the most radiation-sensitive organs in the human body. The tissue weighting factor has been recently revised from 0.05 to 0.12 (Valentin, 2007a). Therefore, radiation dose reduction for the breasts is important to avoid stochastic risk when
* Corresponding author. Tel.: þ81 76 265 2530; fax: þ81 76 234 4366. E-mail address: [email protected] (K. Matsubara). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.07.018
diagnostic X-ray examinations such as CT, mammography, and thoracic radiography are performed. Improved automatic exposure control (AEC) systems have been introduced in modern CT scanners to facilitate the optimization of image quality and radiation exposure, while eliminating the arbitrary selection of tube current by operators (Kalra et al., 2004; Matsubara et al., 2008). It is classified into angular (xy-axis), longitudinal (z-axis), and combined (xyz-axis) mA modulations. Previous studies have shown that the xyz-axis modulation is the most effective in reducing patient radiation dose and can reduce the absorbed radiation dose when the AEC system is applied (Matsubara et al., 2008). However, to our knowledge, evaluation of the radiation dose reduction in organs out of the acquisition range when the AEC system is applied has not been reported. This can be evaluated by measuring the radiation dose reduction for the breasts in abdominal CT examinations when the AEC system is applied. The aim of this study was to evaluate the effectiveness of the AEC system in reducing the radiation dose to the breasts in abdominal CT examinations. 2. Materials and methods The multidetector CT scanner used was LightSpeed VCT (GE Healthcare, Milwaukee, WI, USA). First, a pencil-shaped ionization
Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
2057
Fig. 1. Geometrical setup for evaluating the performance of the AEC system. (a) First, a frontal radiograph of the calibration phantom was obtained. (b) Then, the phantom was removed from the bed and a pencil-shaped ionization chamber was placed in the center of the CT gantry.
chamber type 77336 of 4.73 cm3 volume and 15 cm sensitive length(PTW-Freiburg, Freiburg, Germany) which was connected with Ramtec 1000plus dosimeter (Toyo Medic, Tokyo, Japan) was placed in the center of the CT gantry. Then, the electric current was measured when z- and xyz-axis modulations and fixed mA acquisition were performed after obtaining a frontal radiograph of the calibration phantom (Catphan 600 and CTP579, Phantom Laboratory, Salem NY, USA) which consisted of cylindrical and elliptic cylindrical regions (Figs. 1 and 2). For the timeescale analysis, the electric current was measured using a software (Toyo Medic) that was installed in a laptop that was connected to the dosimeter. The acquisition parameters were: tube voltage, 120 kVp; tube current, 10e700 mA (noise index of 8.0); tube rotation time, 1.0 s; longitudinal beam width (BW), 20 mm; and pitch factor (PF), 0.531 with helical acquisition type. Second, acquisitions from the upper abdomen of an anthropomorphic phantom (RAN-110; Phantom Laboratory) were acquired under the following parameters: tube voltage, 120 kVp; tube current, 10e700 mA (noise index of 8.0); gantry rotation time, 0.5 s; slice thickness, 2.5 mm; and 12 patterns of parameter combinations that changed the longitudinal BW (20 mm or 40 mm), PF (0.516, 0.531, or 1.375), and modulation type (z- or xyz-axis modulations or fixed mA acquisition) after placing six radiophotoluminescent glass dosimeters (RPLDs, GD-302M; Chiyoda Technol, Tokyo, Japan) at the breasts and two RPLDs at the abdominal region, were used to measure the breasts and abdominal radiation dose for each parameter combination (Fig. 3). Each acquisition was performed four times to reduce random error. After each acquisition, the absorbed doses for the breasts and abdomen were calculated by
Fig. 2. The calibration phantom consisted of cylindrical and elliptic cylindrical regions. The phantom was made from solid cast material. The cylindrical region was 200 mm in diameter and 140 mm in total length, and the elliptic cylindrical region had a shortaxis diameter of 250 mm, long-axis diameter of 350 mm, and length of 50 mm. The acquisition range was set from the cylindrical region to the elliptic cylindrical region.
multiplying the mean dose values that were calibrated in terms of air kerma for each region by the mass energy coefficient ratio of the breasts or soft tissue to air. RPLDs were annealed before each use. After each exposure, they were heated to 70 C for 30 min and read after 24 h using the FGD1000 reader (Chiyoda Technol) according to the manufacturer’s recommended protocol. Air kerma calibration for RPLD was performed in free air against a Ramtec 1500B dosimeter (Toyo Medic) using an ion chamber with 3 cm3 volume (DC300; Toyo Medic) attached to a 120 kVp diagnostic X-ray beam that had an effective energy adjusted to approximately 50 keV using aluminum plates of 6.0 mm total thickness. The ionization chamber and RPLDs were placed side-by-side at the same distance from the X-ray tube in an irradiated field. The ionization dosimeter had been calibrated at a laboratory in the Japan Quality Assurance Organization. In addition, the level of image noise was evaluated using the images of the anthropomorphic phantom for each combination by measuring standard deviations of CT values within 10 regions of interest (ROIs) at the liver region. The size of each ROI was 112 mm2. Each measurement was performed four times to reduce random error.
Fig. 3. An anthropomorphic phantom of the upper abdomen and the acquisition range.
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Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
Fig. 6. Comparison of the abdominal dose between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
Fig. 4. Comparison of the timeescale analysis between z- and xyz-axis modulations and fixed mA acquisition. The quantity of electric charge every 0.1 s was measured and expressed in terms of electric current.
3. Results In the timeescale analysis, it was confirmed that the AEC system could reduce the radiation dose, and it modulated mA well according to the shape of the phantom (Fig. 4). At the circular region of the phantom, the electric current decreased considerably when the AEC system was applied. At the elliptic circular region of the phantom, the electric current was nearly equal to the fixed mA acquisition when z-axis modulation was applied, and it modulated according to the transverse shape of the phantom when xyz-axis modulation was applied. In the anthropomorphic phantom study, the radiation dose to the breasts tended to be quite high if a combination of high PF (1.375) and narrow BW (20 mm) was selected and the AEC system was not applied, and it tended to be relatively high if a combination of high PF and wide BW (40 mm) was selected and the AEC system was either applied or not applied. When an identical BW and nearly identical PF were selected, the radiation dose to the breasts tended to be low using z-axis modulation compared with fixed mA acquisition except for the combination of high PF and wide BW, and it also tended to be low using xyz-axis modulation compared with using z-axis modulation in all parameter combinations (Fig. 5). The abdominal dose tended to be high if wide BW was selected and the AEC system was not applied. However, the radiation dose
Fig. 5. Comparison of the radiation dose to the breasts between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
could be considerably reduced by applying the AEC system. Even the breast dose tended to be low using xyz-axis modulation compared with using z-axis modulation in all parameter combinations (Fig. 6). The level of image noise in general was quite low when a narrow BW was selected and the AEC system was applied. However, it was almost equal to the level that we preset (noise index of 8.0) when the AEC system was applied (Fig. 7), but the level of image noise at the liver region under the right diaphragm tended to be low even if the AEC system was applied (Fig. 8). 4. Discussion We confirmed that the AEC system could modulate mA appropriately according to the shape of the object. In addition, we confirmed that z-axis modulation was effective for radiation dose reduction when elliptic objects were acquired. Tube current was chosen by the AEC system according to a localizer radiograph in this CT system. In abdominal CT examinations, the AEC system was effective for the reduction of the absorbed radiation dose not only for the abdomen but also for the breasts. When the AEC system was not applied, the radiation dose to the breasts tended to be high if the combination of high PF and narrow BW was selected. We believe this is because long acquisition time and over-ranging are unavoidable in the mentioned parameter combination. Here, overranging increases as PF or BW increases (Schilham et al., 2010). On the other hand, when the AEC system was applied, it tended to be high if the combination of high PF and wide BW was selected. We believe this is because overbeaming and over-ranging were
Fig. 7. Comparison of the level of image noise at the liver region between z- and xyzaxis modulations and fixed mA acquisition for each parameter combination.
Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
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words, the optimization is insufficient at the liver region under the right diaphragm even if the AEC system is applied. It is more effective for the optimization of image quality as well as for the reduction of the radiation dose to the breasts to improve the accuracy of the AEC system. 5. Conclusion
Fig. 8. Comparison of the level of image noise at the liver region under the right diaphragm between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
unavoidable in the mentioned parameter combination. In other words, over-ranging increases as PF or detector collimation increases and overbeaming depends on detector collimation and focal spot size (Perisinakis et al., 2009). When the AEC system is applied, the effect of long acquisition time for breast dose is small because tube current is modulated appropriately for long or short acquisition time. In addition, when identical BW and nearly identical PF were selected, the radiation dose to the breasts could be reduced using z-axis or xyz-axis modulation compared with fixed mA acquisition. Therefore, xyz-axis modulation should be applied when abdominal CT examination is performed to reduce radiation dose to the breasts, which is radiation-sensitive and located outside the acquisition range. On the other hand, the abdominal radiation dose could be reduced using z-axis or xyz-axis modulation compared with fixed mA modulation. This result indicates that it is more effective when xyz-axis modulation is applied in abdominal CT for reducing the radiation dose not only for the abdomen but also for the breasts. When the AEC system was applied, it was also effective in optimizing the image quality. The level of image noise was quite low when narrow BW was selected and the AEC system was not applied. This indicates that there is a possibility that the optimization of tube current is difficult without applying the AEC system. On the other hand, when the AEC system was applied, it was almost equal to the level that we preset (noise index of 8.0). However, the level of image noise at the liver region under the right diaphragm tended to be low even if the AEC system was applied. In other
Using the AEC system in abdominal CT examination is effective for reducing the radiation dose not only for the abdomen but also for the breasts while maintaining appropriate image quality (except for the liver region under the right diaphragm). An improvement in the AEC system is required to achieve further optimization of image quality and to achieve further reduction in radiation dose to the breasts. Acknowledgments We thank Yoshinari Sakamoto of Toyo Medic Co., Ltd for providing the expansion software of Ramtec 1000plus dosimeter. References Kalra, M.K., Maher, M.M., Toth, T.L., Schmidt, B., Westerman, B.L., Morgan, H.T., Saini, S., 2004. Techniques and applications of automatic tube current modulation for CT. Radiology 233, 649e657. Laghi, A., Iannaccone, R., Rossi, P., Carbone, I., Ferrari, R., Mangiapane, F., Nofroni, I., Passariello, R., 2003. Hepatocellular carcinoma: detection with triple-phase multi-detector row helical CT in patients with chronic hepatitis. Radiology 226, 543e549. Matsubara, K., Koshida, K., Suzuki, M., Tsujii, H., Yamamoto, T., Matsui, O., 2008. Comparison between 3-D and z-axis automatic tube current modulation technique in multidetector-row CT. Radiat. Prot. Dosim. 128, 106e111. McNulty, N.J., Francis, I.R., Platt, J.F., Cohan, R.H., Korobkin, M., Gebremariam, A., 2001. Multiedetector row helical CT of the pancreas: effect of contrastenhanced multiphasic imaging on enhancement of the pancreas, peripancreatic vasculature, and pancreatic adenocarcinoma. Radiology 220, 97e102. Murakami, T., Kim, T., Takamura, M., Hori, M., Takahashi, S., Federle, M.P., Tsuda, K., Osuga, K., Kawata, S., Nakamura, H., Kudo, M., 2001. Hypervascular hepatocellular carcinoma: detection with double arterial phase multi-detector row helical CT. Radiology 218, 763e767. Perisinakis, K., Papadakis, A.E., Damilakis, J., 2009. The effect of X-ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging. Med. Phys. 36, 1258e1266. Schilham, A., van der Molen, A.J., Prokop, M., de Jong, H.W., 2010. Overranging at multisection CT: an underestimated source of excess radiation exposure. Radiographics 30, 1057e1067. Valentin, J., 2007a. The 2007 recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann. ICRP 37, 1e332. Valentin, J., 2007b. Managing patient dose in multi-detector computed tomography(MDCT). ICRP Publication 102. Ann. ICRP 37, 1e79. iii.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Reduction of breast dose in abdominal CT examinations: Effectiveness of automatic exposure control system Kosuke Matsubara a, *, Kichiro Koshida a, Kimiya Noto b, Tadanori Takata b a b
Department of Quantum Medical Technology, Faculty of Health Sciences, Kanazawa University, 5-11-80 Kodatsuno, Kanazawa, Ishikawa 920-0942, Japan Department of Radiological Technology, Kanazawa University Hospital, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8641, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2010 Received in revised form 5 July 2011 Accepted 7 July 2011
We evaluated the effectiveness of the automatic exposure control (AEC) system for reducing the radiation dose to the breasts in abdominal computed tomography (CT). Acquisitions from the upper abdomen of an anthropomorphic phantom were acquired using multiple parameter combinations after placing radiophotoluminescent glass dosimeters at the breasts and abdominal regions to measure the breasts and abdominal radiation dose for each parameter combination. In addition, the level of image noise was evaluated using the images obtained from the anthropomorphic phantom for each parameter combination. When identical beam width and pitch factor were selected, the breast dose tended to be low using z-axis modulation compared with fixed mA acquisition, and it also tended to be low using xyz-axis modulation compared with using z-axis modulation. The level of image noise in general was almost equal to the level that was preset when the AEC system was applied, but the level of image noise at the liver region under the right diaphragm tended to be low. Thus, the AEC system is effective for the reduction of the radiation dose to the breasts while maintaining appropriate image quality in abdominal CT examinations. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Computed tomography Automatic exposure control Radiophotoluminescent glass dosimeter Breast dose
1. Introduction Considering the dynamic study of the liver and pancreas, there have been reports about usability of three-phase (single-arterial phase), four-phase (double-arterial phase), and five-phase acquisitions (triple-arterial phase) (Laghi et al., 2003; McNulty et al., 2001; Murakami et al., 2001). However, the total radiation dose to the patient tends to be high when multiphase acquisition is performed in abdominal computed tomography (CT). In addition, overbeaming and over-ranging, which are characteristic of a multidetector CT, affect patient doses (Valentin, 2007b). Overbeaming occurs when the X-ray beam incident on a patient extends beyond the active detector area, and over-ranging occurs when there is an increase in dose-length product due to additional rotation required for the interpolation algorithm. In abdominal CT examinations, these issues cause increasing absorbed radiation doses for organs out of the acquisition range, such as the breasts. The breasts are one of the most radiation-sensitive organs in the human body. The tissue weighting factor has been recently revised from 0.05 to 0.12 (Valentin, 2007a). Therefore, radiation dose reduction for the breasts is important to avoid stochastic risk when
* Corresponding author. Tel.: þ81 76 265 2530; fax: þ81 76 234 4366. E-mail address: [email protected] (K. Matsubara). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.07.018
diagnostic X-ray examinations such as CT, mammography, and thoracic radiography are performed. Improved automatic exposure control (AEC) systems have been introduced in modern CT scanners to facilitate the optimization of image quality and radiation exposure, while eliminating the arbitrary selection of tube current by operators (Kalra et al., 2004; Matsubara et al., 2008). It is classified into angular (xy-axis), longitudinal (z-axis), and combined (xyz-axis) mA modulations. Previous studies have shown that the xyz-axis modulation is the most effective in reducing patient radiation dose and can reduce the absorbed radiation dose when the AEC system is applied (Matsubara et al., 2008). However, to our knowledge, evaluation of the radiation dose reduction in organs out of the acquisition range when the AEC system is applied has not been reported. This can be evaluated by measuring the radiation dose reduction for the breasts in abdominal CT examinations when the AEC system is applied. The aim of this study was to evaluate the effectiveness of the AEC system in reducing the radiation dose to the breasts in abdominal CT examinations. 2. Materials and methods The multidetector CT scanner used was LightSpeed VCT (GE Healthcare, Milwaukee, WI, USA). First, a pencil-shaped ionization
Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
2057
Fig. 1. Geometrical setup for evaluating the performance of the AEC system. (a) First, a frontal radiograph of the calibration phantom was obtained. (b) Then, the phantom was removed from the bed and a pencil-shaped ionization chamber was placed in the center of the CT gantry.
chamber type 77336 of 4.73 cm3 volume and 15 cm sensitive length(PTW-Freiburg, Freiburg, Germany) which was connected with Ramtec 1000plus dosimeter (Toyo Medic, Tokyo, Japan) was placed in the center of the CT gantry. Then, the electric current was measured when z- and xyz-axis modulations and fixed mA acquisition were performed after obtaining a frontal radiograph of the calibration phantom (Catphan 600 and CTP579, Phantom Laboratory, Salem NY, USA) which consisted of cylindrical and elliptic cylindrical regions (Figs. 1 and 2). For the timeescale analysis, the electric current was measured using a software (Toyo Medic) that was installed in a laptop that was connected to the dosimeter. The acquisition parameters were: tube voltage, 120 kVp; tube current, 10e700 mA (noise index of 8.0); tube rotation time, 1.0 s; longitudinal beam width (BW), 20 mm; and pitch factor (PF), 0.531 with helical acquisition type. Second, acquisitions from the upper abdomen of an anthropomorphic phantom (RAN-110; Phantom Laboratory) were acquired under the following parameters: tube voltage, 120 kVp; tube current, 10e700 mA (noise index of 8.0); gantry rotation time, 0.5 s; slice thickness, 2.5 mm; and 12 patterns of parameter combinations that changed the longitudinal BW (20 mm or 40 mm), PF (0.516, 0.531, or 1.375), and modulation type (z- or xyz-axis modulations or fixed mA acquisition) after placing six radiophotoluminescent glass dosimeters (RPLDs, GD-302M; Chiyoda Technol, Tokyo, Japan) at the breasts and two RPLDs at the abdominal region, were used to measure the breasts and abdominal radiation dose for each parameter combination (Fig. 3). Each acquisition was performed four times to reduce random error. After each acquisition, the absorbed doses for the breasts and abdomen were calculated by
Fig. 2. The calibration phantom consisted of cylindrical and elliptic cylindrical regions. The phantom was made from solid cast material. The cylindrical region was 200 mm in diameter and 140 mm in total length, and the elliptic cylindrical region had a shortaxis diameter of 250 mm, long-axis diameter of 350 mm, and length of 50 mm. The acquisition range was set from the cylindrical region to the elliptic cylindrical region.
multiplying the mean dose values that were calibrated in terms of air kerma for each region by the mass energy coefficient ratio of the breasts or soft tissue to air. RPLDs were annealed before each use. After each exposure, they were heated to 70 C for 30 min and read after 24 h using the FGD1000 reader (Chiyoda Technol) according to the manufacturer’s recommended protocol. Air kerma calibration for RPLD was performed in free air against a Ramtec 1500B dosimeter (Toyo Medic) using an ion chamber with 3 cm3 volume (DC300; Toyo Medic) attached to a 120 kVp diagnostic X-ray beam that had an effective energy adjusted to approximately 50 keV using aluminum plates of 6.0 mm total thickness. The ionization chamber and RPLDs were placed side-by-side at the same distance from the X-ray tube in an irradiated field. The ionization dosimeter had been calibrated at a laboratory in the Japan Quality Assurance Organization. In addition, the level of image noise was evaluated using the images of the anthropomorphic phantom for each combination by measuring standard deviations of CT values within 10 regions of interest (ROIs) at the liver region. The size of each ROI was 112 mm2. Each measurement was performed four times to reduce random error.
Fig. 3. An anthropomorphic phantom of the upper abdomen and the acquisition range.
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Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
Fig. 6. Comparison of the abdominal dose between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
Fig. 4. Comparison of the timeescale analysis between z- and xyz-axis modulations and fixed mA acquisition. The quantity of electric charge every 0.1 s was measured and expressed in terms of electric current.
3. Results In the timeescale analysis, it was confirmed that the AEC system could reduce the radiation dose, and it modulated mA well according to the shape of the phantom (Fig. 4). At the circular region of the phantom, the electric current decreased considerably when the AEC system was applied. At the elliptic circular region of the phantom, the electric current was nearly equal to the fixed mA acquisition when z-axis modulation was applied, and it modulated according to the transverse shape of the phantom when xyz-axis modulation was applied. In the anthropomorphic phantom study, the radiation dose to the breasts tended to be quite high if a combination of high PF (1.375) and narrow BW (20 mm) was selected and the AEC system was not applied, and it tended to be relatively high if a combination of high PF and wide BW (40 mm) was selected and the AEC system was either applied or not applied. When an identical BW and nearly identical PF were selected, the radiation dose to the breasts tended to be low using z-axis modulation compared with fixed mA acquisition except for the combination of high PF and wide BW, and it also tended to be low using xyz-axis modulation compared with using z-axis modulation in all parameter combinations (Fig. 5). The abdominal dose tended to be high if wide BW was selected and the AEC system was not applied. However, the radiation dose
Fig. 5. Comparison of the radiation dose to the breasts between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
could be considerably reduced by applying the AEC system. Even the breast dose tended to be low using xyz-axis modulation compared with using z-axis modulation in all parameter combinations (Fig. 6). The level of image noise in general was quite low when a narrow BW was selected and the AEC system was applied. However, it was almost equal to the level that we preset (noise index of 8.0) when the AEC system was applied (Fig. 7), but the level of image noise at the liver region under the right diaphragm tended to be low even if the AEC system was applied (Fig. 8). 4. Discussion We confirmed that the AEC system could modulate mA appropriately according to the shape of the object. In addition, we confirmed that z-axis modulation was effective for radiation dose reduction when elliptic objects were acquired. Tube current was chosen by the AEC system according to a localizer radiograph in this CT system. In abdominal CT examinations, the AEC system was effective for the reduction of the absorbed radiation dose not only for the abdomen but also for the breasts. When the AEC system was not applied, the radiation dose to the breasts tended to be high if the combination of high PF and narrow BW was selected. We believe this is because long acquisition time and over-ranging are unavoidable in the mentioned parameter combination. Here, overranging increases as PF or BW increases (Schilham et al., 2010). On the other hand, when the AEC system was applied, it tended to be high if the combination of high PF and wide BW was selected. We believe this is because overbeaming and over-ranging were
Fig. 7. Comparison of the level of image noise at the liver region between z- and xyzaxis modulations and fixed mA acquisition for each parameter combination.
Matsubara et al. / Radiation Measurements 46 (2011) 2056e2059
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words, the optimization is insufficient at the liver region under the right diaphragm even if the AEC system is applied. It is more effective for the optimization of image quality as well as for the reduction of the radiation dose to the breasts to improve the accuracy of the AEC system. 5. Conclusion
Fig. 8. Comparison of the level of image noise at the liver region under the right diaphragm between z- and xyz-axis modulations and fixed mA acquisition for each parameter combination.
unavoidable in the mentioned parameter combination. In other words, over-ranging increases as PF or detector collimation increases and overbeaming depends on detector collimation and focal spot size (Perisinakis et al., 2009). When the AEC system is applied, the effect of long acquisition time for breast dose is small because tube current is modulated appropriately for long or short acquisition time. In addition, when identical BW and nearly identical PF were selected, the radiation dose to the breasts could be reduced using z-axis or xyz-axis modulation compared with fixed mA acquisition. Therefore, xyz-axis modulation should be applied when abdominal CT examination is performed to reduce radiation dose to the breasts, which is radiation-sensitive and located outside the acquisition range. On the other hand, the abdominal radiation dose could be reduced using z-axis or xyz-axis modulation compared with fixed mA modulation. This result indicates that it is more effective when xyz-axis modulation is applied in abdominal CT for reducing the radiation dose not only for the abdomen but also for the breasts. When the AEC system was applied, it was also effective in optimizing the image quality. The level of image noise was quite low when narrow BW was selected and the AEC system was not applied. This indicates that there is a possibility that the optimization of tube current is difficult without applying the AEC system. On the other hand, when the AEC system was applied, it was almost equal to the level that we preset (noise index of 8.0). However, the level of image noise at the liver region under the right diaphragm tended to be low even if the AEC system was applied. In other
Using the AEC system in abdominal CT examination is effective for reducing the radiation dose not only for the abdomen but also for the breasts while maintaining appropriate image quality (except for the liver region under the right diaphragm). An improvement in the AEC system is required to achieve further optimization of image quality and to achieve further reduction in radiation dose to the breasts. Acknowledgments We thank Yoshinari Sakamoto of Toyo Medic Co., Ltd for providing the expansion software of Ramtec 1000plus dosimeter. References Kalra, M.K., Maher, M.M., Toth, T.L., Schmidt, B., Westerman, B.L., Morgan, H.T., Saini, S., 2004. Techniques and applications of automatic tube current modulation for CT. Radiology 233, 649e657. Laghi, A., Iannaccone, R., Rossi, P., Carbone, I., Ferrari, R., Mangiapane, F., Nofroni, I., Passariello, R., 2003. Hepatocellular carcinoma: detection with triple-phase multi-detector row helical CT in patients with chronic hepatitis. Radiology 226, 543e549. Matsubara, K., Koshida, K., Suzuki, M., Tsujii, H., Yamamoto, T., Matsui, O., 2008. Comparison between 3-D and z-axis automatic tube current modulation technique in multidetector-row CT. Radiat. Prot. Dosim. 128, 106e111. McNulty, N.J., Francis, I.R., Platt, J.F., Cohan, R.H., Korobkin, M., Gebremariam, A., 2001. Multiedetector row helical CT of the pancreas: effect of contrastenhanced multiphasic imaging on enhancement of the pancreas, peripancreatic vasculature, and pancreatic adenocarcinoma. Radiology 220, 97e102. Murakami, T., Kim, T., Takamura, M., Hori, M., Takahashi, S., Federle, M.P., Tsuda, K., Osuga, K., Kawata, S., Nakamura, H., Kudo, M., 2001. Hypervascular hepatocellular carcinoma: detection with double arterial phase multi-detector row helical CT. Radiology 218, 763e767. Perisinakis, K., Papadakis, A.E., Damilakis, J., 2009. The effect of X-ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging. Med. Phys. 36, 1258e1266. Schilham, A., van der Molen, A.J., Prokop, M., de Jong, H.W., 2010. Overranging at multisection CT: an underestimated source of excess radiation exposure. Radiographics 30, 1057e1067. Valentin, J., 2007a. The 2007 recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann. ICRP 37, 1e332. Valentin, J., 2007b. Managing patient dose in multi-detector computed tomography(MDCT). ICRP Publication 102. Ann. ICRP 37, 1e79. iii.