Journal of Medical Imaging and Radiation Oncology 59 (2015) 586–589
MEDIC AL I MAG I N G —O R I G I N AL A RTICLE
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Computed tomography overexposure as a consequence of extended scan length Mohamed Khaldoun Badawy,1,2 Michael Galea,3 Kam Shan Mong1 and Paul U1 1 Department of Medical Physics, Austin Health, Heidelberg, Victoria, Australia 2 School of Medical Sciences, RMIT, Bundoora, Victoria, Australia 3 Department of Radiology, Austin Health, Heidelberg, Victoria, Australia
MK Badawy MAppSci; M Galea BAppSci; KS Mong MAppSci; P U MAppSci. Correspondence Mr Mohamed Khaldoun Badawy, Department of Medical Physics, Austin Health, 145 Studley Road, Heidelberg, Vic. 3084, Australia. Email: [email protected] Conflict of interest: No conflict of interest to declare. Submitted 3 November 2014; accepted 26 May 2015. doi:10.1111/1754-9485.12339
Abstract Introduction: This study aimed to raise awareness around the increased effective dose as scan length chosen is increased from standard protocol Methods: The Monte Carlo-based software CT-Expo (G. Stamm (Medizinische Hochschule Hannover, Hannover, Germany) and H.D. Nagel (SASCRAD, Buchholz, Germany)) was used to simulate the effective dose increase as the scanned region of the standard protocol increased. Results: The results of this study show that for scans with a high computed tomography dose index (CTDI)vol the patient could be exposed to an extra 1 mSv within 6 cm of overscan. Protocols that investigated large scan areas may not see a significant relative dose reduction because of the use of a lower CTDIvol; however, radiation exposure should be kept as low as reasonably achievable. Conclusion: There is significant dose optimisation potential when strictly adhering to appropriate scan lengths within each imaging protocol wherever possible. Key words: computed X-ray tomography; CT dose optimisation; CT dose reduction; radiation dose; radiography.
Introduction X-ray CT is a powerful tool used by physicians to assist in patient diagnosis. CT scanners make use of ionising radiation, in the form of X-rays, to generate an image of patient anatomy. In the period between July 2012 and June 2013, 2,540,546 CT scans were ordered in Australia according to Medicare records.1 The use of ionising radiation is not without its inherent risk. The dose measurement used to determine the radiation risk as a weighted sum of the tissues irradiated is known as the effective dose. The units of effective dose is Sievert (Sv). The study of low dose radiation risk is still unclear; however, it is accepted that the ionising radiation associated with diagnostic imaging may be carcinogenic.2,3 The diagnostic benefits of CT scans with appropriate clinical reasoning far outweigh the potential risks and this has seen an increased number of CT scans ordered yearly in Australia.4 As a result, a greater emphasis on dose optimisation needs to be made to ensure the patients undergoing diagnostic procedures are not subjected to unnecessary additional radiation dose. The risk of cancer formation5–7 requires methods to be in place to optimise the radiation dose associated with
586
medical imaging. Diagnostic quality images should be acquired while keeping the dose as low as possible. Many dose optimisation techniques, such as kV/mA modulation and improved noise reduction methods have been reviewed in the literature.8–10 Another simple method of dose optimisation is ensuring the correct scan length is chosen for the corresponding scan type, that is imaging only the anatomy that is required and nothing more. Initially, a scout image is obtained to ensure the patient is set up correctly for their scan. Adjustments to scan lengths based on anatomical landmarks and radiologist protocol should be made on the scout image before performing the CT scan on the patient. This study aimed to quantify the extra dose a patient will receive from a CT scan that utilises an incorrect scan length.
Methods Using the anatomical landmarks suggested by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) in the National Diagnostic Reference Levels survey user guide,11 a reference scan was acquired using the CTU-41 CT Torso Phantom (Kyoto Kagaku Co., Ltd., Kyoto, Japan) on a Toshiba Aquilion Prime CT scanner
© 2015 The Royal Australian and New Zealand College of Radiologists
CT overexposure due to incorrect scan length
Table 1. Scan protocols and anatomical landmarks investigated in this study Protocol Head Neck Chest Abdomen/pelvis Chest/abdomen/pelvis Lumbar spine
Scan location Between base of skull to vertex Between external auditory meatus to T2 Between lung apices to adrenal glands From above diaphragm to below symphysis pubis From above lung apices to below symphysis pubis Between T12 to S1
(Toshiba Co., Tokyo, Japan; SN: 1CA11Y2079). This was done in order to determine the CT scanner parameters used for an average patient in each of the tested protocols. The scan location for each protocol is listed in Table 1 and the scanned regions on the phantom are shown in Figure 1. The scan parameters from these images including kV, mA, rotation time, collimation width, table feed and slice thickness were recorded. The computed tomography dose index (CTDI) and dose length product (DLP) on the CT scanner were checked using the Nested Head/ Body CTDI Phantom Model L-007n (Ludlum Medical Physics, Sweetwater, TX, USA) and Piranha CT Dose Profiler (RTI, Mölndal, Sweden) and were found to be within 5% of the displayed value.
All scans, except the head scan, were acquired using spiral mode and dose modulation on the CT scanner. The head scan was performed using manual mA selection as the use of mA modulation on head scans often results in the scanner attempting to increase the mA past what is set manually without necessary improvement to image quality; therefore, this is an unjustified increase in patient dose. The SURE Exposure™12 settings utilised in this scanner have been altered from factory settings to further reduce dose while still providing image quality to radiologist satisfaction. CT-Expo V2.2 (G. Stamm (Medizinische Hochschule Hannover, Hannover, Germany) and H.D. Nagel (SASCRAD, Buchholz, Germany))13 is a widely used CT dose assessment software tool that utilises a mathematical phantom and Monte Carlo data to estimate organ absorbed dose and effective dose. Brix et al. have validated the accurateness of CT-Expo for dose calculation with thermoluminescent dosimeter measurements in an anthromorphic phantom study.14 Using this software tool, a relative measure of the effective dose was made for each of the scan protocols using the scan parameters that were recorded when the CT torso phantom was used. Additionally, the mA value displayed in the CT-Expo software was adjusted so that the calculated
Fig. 1. The anatomical regions scanned in each protocol.
© 2015 The Royal Australian and New Zealand College of Radiologists
587
MK Badawy et al.
Table 2. CTDIvol, DLP and effective dose for each standard scan protocol Scan protocol
Head Neck Chest Abdomen/pelvis Chest/abdomen/pelvis Lumbar spine
CTDIvol (mGy)
DLP (mGy.cm)
Effective dose (mSv)
36.2 5.4 5.1 8.5 9.0 14.3
416 130 174 426 653 319
0.8 1.3 2.8 6.9 10.2 6.4
CTDIvol, computed tomography dose index; DLP, dose length product.
CTDIvol as displayed in the software matched the CTDIvol displayed during the CT scan of the phantom. This adjustment was done as it is assumed that the displayed CTDIvol can be used as a relative indication of the X-ray output of the CT scanner. For most of the simulations, the scanner model ‘Toshiba Prime (BS medium)’, spiral mode and dose modulation were selected in the CT-Expo settings. For the neck scan, ‘Body mode for head/neck region’ was selected in addition to the settings mentioned earlier. For the head scan, the scanner model was changed to ‘Toshiba Prime (BS small)’. The excess dose because of overscan was simulated by varying the scan length on the reference image using CT-Expo. The scan length was changed by 0.5 cm inferiorly and 0.5 cm superiorly. This was repeated 10 times until a total overscan length of 10 cm was achieved. The effective dose calculation was based on the tissueweighting factors found in International Commission on Radiological Protection publication 103.15
Results
Impact of CT parameters on effective dose increase Protocols that employ a larger CTDIvol, particularly over a relatively short standard scan length, such as head and lumbar spine, will have a greater effective dose increase with each centimetre of overscan. The overexposure because of an extended scan length of 10 cm in the head and lumbar scan is an increase of 250% and 46%, respectively. This is due to the slow pitch and high tube current required to provide low levels of noise in order to distinguish the anatomy in these scans. Conversely, the protocols used in the neck, chest, abdomen and pelvis regions are obtained using a faster pitch and lower tube currents, for an average-sized patient. The relative increase in dose as the scan length is increased is not as significant as the head and lumbar region. For instance, in a chest, abdomen and pelvis scan, the overexposure with an extended scan length of 10 cm is only a 15%.
Impact of organs scanned on effective dose increase The effect of the overscan varies greatly with each region imaged. Regions that contain more radiosensitive organs, such as the lungs and colon, will exhibit a greater increase in effective dose. As the head scan is increased inferiorly, a greater dose contribution from the salivary glands and the thyroid results in an overall increase in effective dose. An overscan in the neck region results in a greater exposure of the lungs, oesophagus, thymus, eye lens and the brain. For a chest scan, the contribution to effective dose is increased as more of the liver and thyroid are imaged. Scatter dose
The results of this study show potential for dose optimisation from ensuring the correct scan length is chosen. Table 2 gives a summary of the effective dose, DLP and CTDIvol, for each standard protocol investigated in this study using a phantom to simulate the average patient size. The CTDIvol is constant for each scan as the only parameter that is changed is the scan length. Figure 2 shows the additional effective dose received as the scan length is increased.
Discussion Effective dose is only an estimate of the relative biological risk of each exposure, and these estimates can vary greatly between methods and phantoms used.16 However, the relative increase of dose in each region as the scan length is increased is indicative of the need for strict adherence to the landmarks of optimised protocols unless specified by the radiologist. This is to ensure the radiation exposure is kept to as low as reasonably achievable and any patient risk is minimised.
588
Fig. 2. The additional effective dose in mSv as the scan length increased. , head; , neck; , chest; , abdomen + pelvis; , chest + abdomen + pelvis; , lumbar spine.
© 2015 The Royal Australian and New Zealand College of Radiologists
CT overexposure due to incorrect scan length
to the stomach also becomes an issue as the scan length is increased inferiorly. The increase in effective dose around the pelvic region becomes significant as the scanned area approaches the reproductive organs. The amount of tissue present in the beam in this region also requires an increased dose output to produce consistent image quality.
Clinical limitations Certain protocols that use long scan lengths, such as a routine intravenous contrast enhanced scan of the chest, abdomen and pelvis, are performed in two separate acquisitions to ensure adequate contrast timing for the relevant body areas. Although, the relative increase in effective dose in this type of scan may not be great, there is some overlap between the two acquisitions. If the two scans are treated separately, attention to optimisation in scan length per acquisition will result in a lower patient effective dose. Furthermore, some scans will be acquired on suspended respiration, which can result in a change in position of internal bodily structures. This introduces a degree of difficulty when attempting to plan scan lengths to particular soft tissue organs. This can inevitably result in some unavoidable overscan for certain body areas.
Conclusion There are many factors to take into account when selecting the scan length of each patient study. The results mentioned earlier quantify an estimate of the dose increase when deviating from the standard protocol. The main emphasis of CT dose optimisation has been focused on new scanner technologies; however, the operator can still play a key role in reducing patient dose. Ensuring the correct scan length is specified by the radiologist and selected by the radiographer can potentially reduce the effective dose of each scan by up to 2 mSv. High-dose protocols, such as head and spine, can benefit significantly in dose reduction with more stringent adherence to the anatomical region to be imaged.
Acknowledgement We would like to acknowledge the medical physics staff at the Australian Radiation Protection and Safety Agency for providing the phantom that was used in this study.
References 1. Medicare. Medicare group reports – diagnostic imaging [report]. Australia: Australian Government, Department of Human Services; 2014. Updated 25/09/2014 [Cited 3 Oct 2014.] Available from URL: http://medicarestatistics.humanservices.gov.au/ statistics/do.jsp?_PROGRAM=%2Fstatistics%2Fmbs _group_standard_report&DRILL=on&GROUP=5&VAR =services&STAT=count&RPT_FMT=by+state&PTYPE =finyear&START_DT=201207&END_DT=201306
© 2015 The Royal Australian and New Zealand College of Radiologists
2. Upton AC. The state of the art in the 1990’s: NCRP Report No. 136 on the scientific bases for linearity in the dose-response relationship for ionizing radiation. Health Phys 2003; 85: 15–22. 3. Pierce DA, Preston DL. Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res 2000; 154: 178–86. 4. Valenti L, Miller G, Charles J. Use of diagnostic imaging in Australian general practice. Aust Fam Physician 2006; 35: 280–1. 5. Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 2001; 176: 289–96. 6. Smith-Bindman R, Lipson J, Marcus R et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med 2009; 169: 2078–86. 7. Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA 2007; 298: 317–23. 8. Kalra MK, Maher MM, Toth TL et al. Strategies for CT radiation dose optimization 1. Radiology 2004; 230: 619–28. 9. McCollough CH, Bruesewitz MR, Kofler JM Jr. CT dose reduction and dose management tools: overview of available options 1. Radiographics 2006; 26: 503–12. 10. Strauss KJ, Goske MJ, Kaste SC et al. Image gently: ten steps you can take to optimize image quality and lower CT dose for pediatric patients. AJR Am J Roentgenol 2010; 194: 868–73. 11. Australian Radiation Protection and Safety Agency. National Diagnostic Reference Level Survey User Manual. ARPANSA, 2011. 12. Angel E. Sure exposure: low dose diagnostic image quality. [Brochure] (Tustin, CA: Toshiba America Medical Systems). 2009. 13. Stamm G, Nagel H. CT-expo – a novel program for dose evaluation in CT. Rofo 2002; 174: 1570–6. 14. Brix G, Lechel U, Veit R et al. Assessment of a theoretical formalism for dose estimation in CT: an anthropomorphic phantom study. Eur Radiol 2004; 14: 1275–84. 15. International Commission on Radiological Protection. 2007 recommendations of the international commission on radiological protection. Ann ICRP 2007; 37: 64–5. Publication 103. 16. Christner JA, Kofler JM, McCollough CH. Estimating effective dose for CT using dose–length product compared with using organ doses: consequences of adopting international commission on radiological protection publication 103 or dual-energy scanning. AJR Am J Roentgenol 2010; 194: 881–9.
589
MEDIC AL I MAG I N G —O R I G I N AL A RTICLE
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Computed tomography overexposure as a consequence of extended scan length Mohamed Khaldoun Badawy,1,2 Michael Galea,3 Kam Shan Mong1 and Paul U1 1 Department of Medical Physics, Austin Health, Heidelberg, Victoria, Australia 2 School of Medical Sciences, RMIT, Bundoora, Victoria, Australia 3 Department of Radiology, Austin Health, Heidelberg, Victoria, Australia
MK Badawy MAppSci; M Galea BAppSci; KS Mong MAppSci; P U MAppSci. Correspondence Mr Mohamed Khaldoun Badawy, Department of Medical Physics, Austin Health, 145 Studley Road, Heidelberg, Vic. 3084, Australia. Email: [email protected] Conflict of interest: No conflict of interest to declare. Submitted 3 November 2014; accepted 26 May 2015. doi:10.1111/1754-9485.12339
Abstract Introduction: This study aimed to raise awareness around the increased effective dose as scan length chosen is increased from standard protocol Methods: The Monte Carlo-based software CT-Expo (G. Stamm (Medizinische Hochschule Hannover, Hannover, Germany) and H.D. Nagel (SASCRAD, Buchholz, Germany)) was used to simulate the effective dose increase as the scanned region of the standard protocol increased. Results: The results of this study show that for scans with a high computed tomography dose index (CTDI)vol the patient could be exposed to an extra 1 mSv within 6 cm of overscan. Protocols that investigated large scan areas may not see a significant relative dose reduction because of the use of a lower CTDIvol; however, radiation exposure should be kept as low as reasonably achievable. Conclusion: There is significant dose optimisation potential when strictly adhering to appropriate scan lengths within each imaging protocol wherever possible. Key words: computed X-ray tomography; CT dose optimisation; CT dose reduction; radiation dose; radiography.
Introduction X-ray CT is a powerful tool used by physicians to assist in patient diagnosis. CT scanners make use of ionising radiation, in the form of X-rays, to generate an image of patient anatomy. In the period between July 2012 and June 2013, 2,540,546 CT scans were ordered in Australia according to Medicare records.1 The use of ionising radiation is not without its inherent risk. The dose measurement used to determine the radiation risk as a weighted sum of the tissues irradiated is known as the effective dose. The units of effective dose is Sievert (Sv). The study of low dose radiation risk is still unclear; however, it is accepted that the ionising radiation associated with diagnostic imaging may be carcinogenic.2,3 The diagnostic benefits of CT scans with appropriate clinical reasoning far outweigh the potential risks and this has seen an increased number of CT scans ordered yearly in Australia.4 As a result, a greater emphasis on dose optimisation needs to be made to ensure the patients undergoing diagnostic procedures are not subjected to unnecessary additional radiation dose. The risk of cancer formation5–7 requires methods to be in place to optimise the radiation dose associated with
586
medical imaging. Diagnostic quality images should be acquired while keeping the dose as low as possible. Many dose optimisation techniques, such as kV/mA modulation and improved noise reduction methods have been reviewed in the literature.8–10 Another simple method of dose optimisation is ensuring the correct scan length is chosen for the corresponding scan type, that is imaging only the anatomy that is required and nothing more. Initially, a scout image is obtained to ensure the patient is set up correctly for their scan. Adjustments to scan lengths based on anatomical landmarks and radiologist protocol should be made on the scout image before performing the CT scan on the patient. This study aimed to quantify the extra dose a patient will receive from a CT scan that utilises an incorrect scan length.
Methods Using the anatomical landmarks suggested by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) in the National Diagnostic Reference Levels survey user guide,11 a reference scan was acquired using the CTU-41 CT Torso Phantom (Kyoto Kagaku Co., Ltd., Kyoto, Japan) on a Toshiba Aquilion Prime CT scanner
© 2015 The Royal Australian and New Zealand College of Radiologists
CT overexposure due to incorrect scan length
Table 1. Scan protocols and anatomical landmarks investigated in this study Protocol Head Neck Chest Abdomen/pelvis Chest/abdomen/pelvis Lumbar spine
Scan location Between base of skull to vertex Between external auditory meatus to T2 Between lung apices to adrenal glands From above diaphragm to below symphysis pubis From above lung apices to below symphysis pubis Between T12 to S1
(Toshiba Co., Tokyo, Japan; SN: 1CA11Y2079). This was done in order to determine the CT scanner parameters used for an average patient in each of the tested protocols. The scan location for each protocol is listed in Table 1 and the scanned regions on the phantom are shown in Figure 1. The scan parameters from these images including kV, mA, rotation time, collimation width, table feed and slice thickness were recorded. The computed tomography dose index (CTDI) and dose length product (DLP) on the CT scanner were checked using the Nested Head/ Body CTDI Phantom Model L-007n (Ludlum Medical Physics, Sweetwater, TX, USA) and Piranha CT Dose Profiler (RTI, Mölndal, Sweden) and were found to be within 5% of the displayed value.
All scans, except the head scan, were acquired using spiral mode and dose modulation on the CT scanner. The head scan was performed using manual mA selection as the use of mA modulation on head scans often results in the scanner attempting to increase the mA past what is set manually without necessary improvement to image quality; therefore, this is an unjustified increase in patient dose. The SURE Exposure™12 settings utilised in this scanner have been altered from factory settings to further reduce dose while still providing image quality to radiologist satisfaction. CT-Expo V2.2 (G. Stamm (Medizinische Hochschule Hannover, Hannover, Germany) and H.D. Nagel (SASCRAD, Buchholz, Germany))13 is a widely used CT dose assessment software tool that utilises a mathematical phantom and Monte Carlo data to estimate organ absorbed dose and effective dose. Brix et al. have validated the accurateness of CT-Expo for dose calculation with thermoluminescent dosimeter measurements in an anthromorphic phantom study.14 Using this software tool, a relative measure of the effective dose was made for each of the scan protocols using the scan parameters that were recorded when the CT torso phantom was used. Additionally, the mA value displayed in the CT-Expo software was adjusted so that the calculated
Fig. 1. The anatomical regions scanned in each protocol.
© 2015 The Royal Australian and New Zealand College of Radiologists
587
MK Badawy et al.
Table 2. CTDIvol, DLP and effective dose for each standard scan protocol Scan protocol
Head Neck Chest Abdomen/pelvis Chest/abdomen/pelvis Lumbar spine
CTDIvol (mGy)
DLP (mGy.cm)
Effective dose (mSv)
36.2 5.4 5.1 8.5 9.0 14.3
416 130 174 426 653 319
0.8 1.3 2.8 6.9 10.2 6.4
CTDIvol, computed tomography dose index; DLP, dose length product.
CTDIvol as displayed in the software matched the CTDIvol displayed during the CT scan of the phantom. This adjustment was done as it is assumed that the displayed CTDIvol can be used as a relative indication of the X-ray output of the CT scanner. For most of the simulations, the scanner model ‘Toshiba Prime (BS medium)’, spiral mode and dose modulation were selected in the CT-Expo settings. For the neck scan, ‘Body mode for head/neck region’ was selected in addition to the settings mentioned earlier. For the head scan, the scanner model was changed to ‘Toshiba Prime (BS small)’. The excess dose because of overscan was simulated by varying the scan length on the reference image using CT-Expo. The scan length was changed by 0.5 cm inferiorly and 0.5 cm superiorly. This was repeated 10 times until a total overscan length of 10 cm was achieved. The effective dose calculation was based on the tissueweighting factors found in International Commission on Radiological Protection publication 103.15
Results
Impact of CT parameters on effective dose increase Protocols that employ a larger CTDIvol, particularly over a relatively short standard scan length, such as head and lumbar spine, will have a greater effective dose increase with each centimetre of overscan. The overexposure because of an extended scan length of 10 cm in the head and lumbar scan is an increase of 250% and 46%, respectively. This is due to the slow pitch and high tube current required to provide low levels of noise in order to distinguish the anatomy in these scans. Conversely, the protocols used in the neck, chest, abdomen and pelvis regions are obtained using a faster pitch and lower tube currents, for an average-sized patient. The relative increase in dose as the scan length is increased is not as significant as the head and lumbar region. For instance, in a chest, abdomen and pelvis scan, the overexposure with an extended scan length of 10 cm is only a 15%.
Impact of organs scanned on effective dose increase The effect of the overscan varies greatly with each region imaged. Regions that contain more radiosensitive organs, such as the lungs and colon, will exhibit a greater increase in effective dose. As the head scan is increased inferiorly, a greater dose contribution from the salivary glands and the thyroid results in an overall increase in effective dose. An overscan in the neck region results in a greater exposure of the lungs, oesophagus, thymus, eye lens and the brain. For a chest scan, the contribution to effective dose is increased as more of the liver and thyroid are imaged. Scatter dose
The results of this study show potential for dose optimisation from ensuring the correct scan length is chosen. Table 2 gives a summary of the effective dose, DLP and CTDIvol, for each standard protocol investigated in this study using a phantom to simulate the average patient size. The CTDIvol is constant for each scan as the only parameter that is changed is the scan length. Figure 2 shows the additional effective dose received as the scan length is increased.
Discussion Effective dose is only an estimate of the relative biological risk of each exposure, and these estimates can vary greatly between methods and phantoms used.16 However, the relative increase of dose in each region as the scan length is increased is indicative of the need for strict adherence to the landmarks of optimised protocols unless specified by the radiologist. This is to ensure the radiation exposure is kept to as low as reasonably achievable and any patient risk is minimised.
588
Fig. 2. The additional effective dose in mSv as the scan length increased. , head; , neck; , chest; , abdomen + pelvis; , chest + abdomen + pelvis; , lumbar spine.
© 2015 The Royal Australian and New Zealand College of Radiologists
CT overexposure due to incorrect scan length
to the stomach also becomes an issue as the scan length is increased inferiorly. The increase in effective dose around the pelvic region becomes significant as the scanned area approaches the reproductive organs. The amount of tissue present in the beam in this region also requires an increased dose output to produce consistent image quality.
Clinical limitations Certain protocols that use long scan lengths, such as a routine intravenous contrast enhanced scan of the chest, abdomen and pelvis, are performed in two separate acquisitions to ensure adequate contrast timing for the relevant body areas. Although, the relative increase in effective dose in this type of scan may not be great, there is some overlap between the two acquisitions. If the two scans are treated separately, attention to optimisation in scan length per acquisition will result in a lower patient effective dose. Furthermore, some scans will be acquired on suspended respiration, which can result in a change in position of internal bodily structures. This introduces a degree of difficulty when attempting to plan scan lengths to particular soft tissue organs. This can inevitably result in some unavoidable overscan for certain body areas.
Conclusion There are many factors to take into account when selecting the scan length of each patient study. The results mentioned earlier quantify an estimate of the dose increase when deviating from the standard protocol. The main emphasis of CT dose optimisation has been focused on new scanner technologies; however, the operator can still play a key role in reducing patient dose. Ensuring the correct scan length is specified by the radiologist and selected by the radiographer can potentially reduce the effective dose of each scan by up to 2 mSv. High-dose protocols, such as head and spine, can benefit significantly in dose reduction with more stringent adherence to the anatomical region to be imaged.
Acknowledgement We would like to acknowledge the medical physics staff at the Australian Radiation Protection and Safety Agency for providing the phantom that was used in this study.
References 1. Medicare. Medicare group reports – diagnostic imaging [report]. Australia: Australian Government, Department of Human Services; 2014. Updated 25/09/2014 [Cited 3 Oct 2014.] Available from URL: http://medicarestatistics.humanservices.gov.au/ statistics/do.jsp?_PROGRAM=%2Fstatistics%2Fmbs _group_standard_report&DRILL=on&GROUP=5&VAR =services&STAT=count&RPT_FMT=by+state&PTYPE =finyear&START_DT=201207&END_DT=201306
© 2015 The Royal Australian and New Zealand College of Radiologists
2. Upton AC. The state of the art in the 1990’s: NCRP Report No. 136 on the scientific bases for linearity in the dose-response relationship for ionizing radiation. Health Phys 2003; 85: 15–22. 3. Pierce DA, Preston DL. Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res 2000; 154: 178–86. 4. Valenti L, Miller G, Charles J. Use of diagnostic imaging in Australian general practice. Aust Fam Physician 2006; 35: 280–1. 5. Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 2001; 176: 289–96. 6. Smith-Bindman R, Lipson J, Marcus R et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med 2009; 169: 2078–86. 7. Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA 2007; 298: 317–23. 8. Kalra MK, Maher MM, Toth TL et al. Strategies for CT radiation dose optimization 1. Radiology 2004; 230: 619–28. 9. McCollough CH, Bruesewitz MR, Kofler JM Jr. CT dose reduction and dose management tools: overview of available options 1. Radiographics 2006; 26: 503–12. 10. Strauss KJ, Goske MJ, Kaste SC et al. Image gently: ten steps you can take to optimize image quality and lower CT dose for pediatric patients. AJR Am J Roentgenol 2010; 194: 868–73. 11. Australian Radiation Protection and Safety Agency. National Diagnostic Reference Level Survey User Manual. ARPANSA, 2011. 12. Angel E. Sure exposure: low dose diagnostic image quality. [Brochure] (Tustin, CA: Toshiba America Medical Systems). 2009. 13. Stamm G, Nagel H. CT-expo – a novel program for dose evaluation in CT. Rofo 2002; 174: 1570–6. 14. Brix G, Lechel U, Veit R et al. Assessment of a theoretical formalism for dose estimation in CT: an anthropomorphic phantom study. Eur Radiol 2004; 14: 1275–84. 15. International Commission on Radiological Protection. 2007 recommendations of the international commission on radiological protection. Ann ICRP 2007; 37: 64–5. Publication 103. 16. Christner JA, Kofler JM, McCollough CH. Estimating effective dose for CT using dose–length product compared with using organ doses: consequences of adopting international commission on radiological protection publication 103 or dual-energy scanning. AJR Am J Roentgenol 2010; 194: 881–9.
589
