Radiation Protection Dosimetry (2014), Vol. 162, No. 3, pp. 316 – 321 Advance Access publication 19 November 2013
doi:10.1093/rpd/nct280
RADIATION DOSE ASSOCIATED WITH CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION: AN EXPERIMENTAL PHANTOM STUDY
*Corresponding author: [email protected] Received 23 July 2013; revised 26 September 2013; accepted 16 October 2013 A study on the radiation dose associated with cerebral CT angiography (CTA) and CT perfusion (CTP) was conducted on an anthropomorphic phantom with the aim of estimating the effective dose (E) and entrance skin dose (ESD) in the eyes and thyroid gland during different CTA and CTP protocols. The E was calculated to be 0.61 and 0.28 mSv in CTA with 100 and 80 kVp, respectively. In contrast, CTP resulted in an estimated E of 2.74 and 2.07 mSv corresponding to 40 and 30 s protocols, respectively. The eyes received a higher ESD than the thyroid gland in all of these protocols. The results of this study indicate that combining both CTA and CTP procedures are not recommended in the stroke evaluation due to high radiation dose. Application of modified techniques in CTA (80 kVp) and CTP (30 s) is highly recommended in clinical practice for further radiation dose reduction.
INTRODUCTION Acute stroke assessment using magnetic resonance (MR) imaging has been widely established due to its ability in evaluating cerebral perfusion. The mismatch concept of diffusion and perfusion studies in MR imaging to indicate penumbra zone as a significant indicator for stroke has been widely accepted(1). However, MR imaging technology has limited accessibility and infers several contraindications, such as patient agitation, haemodynamic instability(2), implanted cardiac pacemaker or any metallic fragments(3). The latest technological development of multi-slice CT scanner makes assessment of acute stroke possible due to rapid image acquisition, which may be more practical in many centres that deal with acute stroke using CT as a first-line emergency imaging modality. Apart from reduced imaging time, CT is better tolerated by many patients and all relevant diagnostic information can be provided with only one imaging sequence(4). Non-contrast CT of the brain is routinely performed to evaluate the cerebral circulation and surrounding tissues in addition to assessment of underlying disease such as intracranial hemorrhage. CT perfusion (CTP) and CT angiography (CTA) of the brain are commonly performed in the assessment of acute stroke(4 – 7). CTP procedure is used to measure cerebral blood flow, cerebral blood volume and mean transit time. CTP has been reported to correlate well with MR imaging and it provides additional information of pathophysiology compared with MR perfusion imaging(8). To assess
cerebral perfusion, the first pass of intravenously administered non-ionic contrast agent is monitored using several continuous images acquired usually between 30 and 50 s without changing the table position(9). On the other hand, CTA of the circle of Willis detects stenosis or occlusion of intracranial arteries(4). CTA provides useful information such as the site of vascular occlusion and delineates vascular anatomy and is considered very useful for guiding therapeutic approaches(2). A comprehensive CT imaging technique, which involves a combination of CTA and CTP, is of paramount importance in the evaluation of acute stroke(10 – 13). However, the combination of these two procedures also raises a major concern due to increased radiation exposure to the patient. Assessment of acute stroke with intracranial CTA includes anatomic regions ranging from the carotid bifurcation to the brain as well as the venous sinuses(6, 14), whereas CTP involves consecutive imaging of the same brain area repetitively, thus producing high radiation dose to the patient (5). Previous studies investigating the radiation dose in combining standard CT of the head, CTA and CTP with different scanning protocols suggested the necessity of dose reduction in comprehensive stroke CT imaging as these combined CT imaging protocols could result in effective doses up to 9.5 mSv(9, 15, 16). The use of lower tube voltage 100 or 80 kVp and a narrow scan range was recommended by these studies for dose reduction. However, the total scan time of 50– 90 s was reported in these studies, which consequently
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Nanyang Technological University on June 13, 2015
Akmal Sabarudin1,*, Mohd Zaki Yusof1, Mazlyfarina Mohamad1 and Zhonghua Sun2 1 Diagnostic Imaging and Radiotherapy Program, School of Diagnostic and Applied Health Sciences, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur 50300, Malaysia 2 Discipline of Medical Imaging, Department of Imaging and Applied Physics, Curtin University, GPO Box U1987 Perth, WA 6845, Australia
CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION
increased the radiation dose significantly. Thus, it is also important to consider reducing the scan time during the stroke CT imaging, which was not well addressed in the previous studies. Therefore, the purpose of this study was to determine the radiation dose resulting from different protocols of intracranial CTA and brain CTP in the acute stroke assessment based on standard clinical settings with a focus on the effective dose (E) and entrance skin dose (ESD) in the selected radiosensitive organs.
Experiments set-up This study was performed on an anthropomorphic whole-body phantom (Pixy, USA), which mimicked an adult with a height of 1.75 m and weight of 74 kg. Two intracranial CTA protocols with use of different tube voltages and two protocols of brain CTP with different scanning times were performed on a 64-slice CT scanner (Somatom Sensation, Siemens Medical Solutions, Germany). These protocols were selected for comparison of the radiation dose (E and ESD), consisting of 100 kVp (CTA protocol I) versus 80 kVp (CTA protocol II) and 40 s (CTP protocol I) versus 30 s (CTP protocol II) scanning time. The ESD was measured using thermoluminescence dosemeter (TLD) chips, while the E was determined by calculating the dose length product (DLP) with a
CTA and CTP protocols Unlike routine CTA, iodinated contrast medium was not used in this study due to the fact that the anthropomorphic phantom was unable to show contrast enhancement on the CT images. This also applies to brain CTP procedure without injection of contrast medium. Therefore, no image quality assessment was conducted in this study. Both CTA and CTP procedures were performed with a detector collimation of 64` 0.625 mm, z-flying focus techniques with 64` 0.6 mm slice acquisition, 330 ms rotation time and a pitch of 1.2. In the CTA procedure, two sets of tube voltage were used at 100 kVp (protocol I) and 80 kVp ( protocol II), while the tube voltage of 80 kVp was used in the CTP procedure. Anatomical-based tube current modulation was used with the tube current (500– 800 mA s) adjusted automatically according to the phantom’s size and weight in both procedures. In the CTP procedure, the scan was continuously performed at two different scanning time 40 s ( protocol I) and 30 s ( protocol II) based on the standard CTP
Figure 1. Images show the anthropomorphic phantom and TLDs were securely placed (marked with ‘X’) on the skin surface of the phantom at the targeted radiosensitive organs (eyes and thyroid gland).
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MATERIALS AND METHODS
conversion coefficient factor. The dose values from DLP measurements were recorded during the procedure and then compared between CTA and CTP. TLD chips were securely placed above the selected radiosensitive organs (eyes and thyroid) on the phantom skin surface during the scan (Figure 1).
A. SABARUDIN ET AL. Table 1. CTA and CTP of the brain protocols. Parameters
CTP
Protocol I
Protocol II
Protocol I
Protocol II
64` 0.625 3.0 1.20 0.33 100 300– 500 160 10.98 158.0 N/A N/A 48.0 Head first
64` 0.625 3.0 1.20 0.33 80 300– 500 160 10.98 158.0 N/A N/A 48.0 Head first
64` 0.625 9.6 N/A 0.33 80 300– 500 270 40.00 50.0 345 15 N/A Head first
64` 0.625 9.6 N/A 0.33 80 300– 500 270 30.00 50.0 345 15 N/A Head first
N/A, not available.
procedure at the institution. The protocols for both CTA and CTP are detailed in Table 1.
Radiation dose measurements Identification of the radiation dose at the highest point received by organs from the overall dose distribution was of paramount importance in radiation dose monitoring during intracranial CTA and brain CTP procedures. The radiation dose may include exposure from the primary beam and the secondary radiation beams, depending on the location of these organs. The ESD was measured and recorded in milligray (mGy) after undergoing a series of procedures including annealing, calibration, radiation dose exposure and read-out process. The annealing and dose read-out was performed with a Harshaw-5500 reader (Thermo Electron Corp., USA) while calibration was performed with a 64-slice CT scanner. TLD calibration was designed to create a graph pattern for radiation dose conversion from nanocoulombs to mGy by exposing it to a known dose, which was measured with a digital radiation survey meter (model 660) having an ion-chamber model 660-3 beam measurement probe and a readout/logic unit (model 451P). These TLDs were interpreted 24 h after the exposure. The TLD used in this study was micro-square shaped and sealed in numbered, with specifically designed plastic wrap. A total of 12 TLDs were securely taped on the phantom skin surface over the radiosensitive organs for each set of procedure, where they were placed on the bilateral sides of the thyroid and eyes for radiation dose measurements. Similar to the intracranial CTA procedure, 12 different TLDs were used and placed at the same locations during each brain CTP protocol
for dose comparison. The TLD readings of a specific organ were averaged to calculate the dose for that organ. The E was obtained by a straightforward calculation based on the following formula: E ¼ DLP DCC where E is the effective dose in mSv, DLP the dose length product in mGy.cm and dose conversion coefficient (DCC) factor in mSv mGy21.cm21. The DLP values were displayed on the CT console once the scan is completed. Therefore, the DLP values were recorded from CT scan procedures. The DCC factor was derived from the specific body region that was being scanned. The DCC for an adult head with a conversion factor of 2.2 mSv.mGy21.cm21 was used(17) for both CTA and CTP procedures in the present study.
Statistical analysis All data were entered into SPSS V17.0 (SPSS, version 17.0 for Windows, Chicago, IL, USA) for statistical analysis. Continuous variables were expressed as mean values+standard deviation. A p-value of ,0.05 was considered to indicate statistically significant differences. All of the doses from each protocol were presented in box plot. One-way ANOVA and repeated measures ANOVA were used in ESD analysis to determine the differences in mean ESD of radiosensitive organs (thyroid and eyes) during these two CT procedures. With regard to the effective dose, the ANOVA test was performed to determine whether there is any significant difference in the mean values between the protocols.
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Collimation (mm) Slice thickness (mm) Pitch Rotation time (s) Tube voltage (kV) Tube current (mA) Tube current.time (mA s) Scan time (s) Scan length (mm) Scan angle (8) Tilt (8) Table speed (mm/s) Orientation
CTA
CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION
RESULTS
Figure 3. Box plot shows the mean ESD of the thyroid and the eyes in CTP with different scanning time settings as protocol I (40 s) and protocol II (30 s). It also shows that the eyes received the highest dose distribution compared with the thyroid in both protocols. The boxes indicate the first to third quartiles; each midline indicates the median (second quartile) and the whiskers represent the maximum and minimum values of ESD.
did not differ significantly ( p¼0.83) when compared with that in the eyes.
Entrance skin dose The ESD measured at the eyes was 9.57+0.84 and 3.32+0.50 mGy corresponding to CTA protocol I (100 kVp) and protocol II (80 kVp), respectively, which is significantly higher than the values 1.08+0.20 and 0.39+0.09 mGy, which was measured in the thyroid in both protocols (100 versus 80 kVp) (Figure 2). However, the difference in ESD measurements in the thyroid between those two protocols was not statistically significant as tested with ANOVA ( p¼0.11). The radiation dose measured on each organ was performed bilaterally. However, the doses measured in each side were calculated to represent as an entire dose for that particular organ (eyes and thyroid). Similar to the findings observed in CTA protocols, ESDs measured in the eyes were significantly higher than that in the thyroid in both CTP protocols. The ESDs measured in the eyes for protocols I (40 s) and II (30 s) were 7.01+0.64 and 5.52+0.42 mGy respectively, while the ESDs values recorded for the thyroid in protocols I and II were 1.43+0.24 and 1.23+0.12 mGy, respectively (Figure 3). However, among both protocols, the ESD values in the thyroid
Effective dose estimation Statistically significant differences were found between the mean E values in all CTA and CTP protocols. The highest E was found in CTP protocol 1 with dose being 2.74+0.03 mSv, followed by CTP protocol 2 (2.07+0.02 mSv). The mean estimated E values were found significantly lower in CTA protocols with dose being 0.61+0.05 and 0.28+0.02 mSv, corresponding to the 100 and 80 kVp protocols. DISCUSSION This study highlights three important findings of the radiation dose associated with intracranial CTA and brain CTP procedures. First, the estimated effective dose in brain CTP was 3.5 –9 times higher than that calculated in the intracranial CTA. Secondly, although brain CTP shows significant higher radiation dose than CTA, the ESD measured at the eyes was the highest in CTA. Lastly, the effective dose of CTP and CTA can be further reduced with some
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Figure 2. Box plot shows the mean ESD of the thyroid and the eyes in CTA with different tube voltage settings as protocol I (100 kVp) and protocol II (80 kVp). It shows that the eyes received the highest dose distribution compared with the thyroid in both protocols. The boxes indicate the first to third quartiles; each midline indicates the median (second quartile) and the whiskers represent the maximum and minimum values of ESD.
A. SABARUDIN ET AL.
eyes received very high radiation dose from CTA procedure, it still remains ,3 mGy which does not exceed the dose limit for the eyes as set by the European Commission DIMOND III project (2003)(20). Although lower tube voltage was introduced in the CTA protocol in this study (80 kVp), it is assumed to provide good image quality for the vessel assessment. The aim of the CTA study in patients with acute stroke is to provide better visualisation of the cerebral blood vessels(14). Schueller-weidekamm et al.(21) reported that a low tube voltage CTA study provided similar diagnostic image quality to the higher tube voltage with administration of iodinated contrast medium. This is because the low-energy photon increases the attenuation index due to the contrast material introduced to the stroke patient. Thus, the use of lower tube voltage in CTA not only significantly reduces the radiation dose to the patient, but also sustains the quality of diagnostic value of contrastenhanced CT images(22). This study has its limitations. The study was based on an anthropomorphic phantom to simulate the radiation dose quantification on a standardised patient. No contrast medium was used in any of the angiography protocols. Therefore, the authors could not compare and assess image quality, since no contrast enhancement was present in the vascular system. Further studies are necessary to verify the accuracy of the results through administration of contrast medium with qualitative and quantitative assessment of diagnostic image quality. In conclusion, this study shows that CTA results in significantly lower radiation dose than CTP protocols, which is demonstrated by effective dose and ESD. Modification of imaging techniques in CTA (lower tube voltage) and shorter scanning time in CTP is recommended in daily clinical practice for further radiation dose reduction in stroke CT examinations.
ACKNOWLEDGEMENTS The author would like to thank Mohamad Norman Mohd Nordin from UKM Medical Centre, Khadijah Mohamad Nassir, Fakhrur Razi Kuffian, Norjana Abd Rahman and Hafizah Abd Rashid for their assistance in CT data collection. REFERENCES
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1. Neumann-Haefelin, T. et al. Diffusion- and perfusionweighted MRI. The DWI/PWI mismatch region in acute stroke. Stroke 30, 1591–1597 (1999). 2. Mnyusiwalla, A., Aviv, R. I. and Symons, S. P. Radiation dose from multidetector row CT imaging for acute stroke. Neuroradiology 51, 635– 640 (2009). 3. Singer, O. C., Sitzer, M., du Mesnil de Rochemont, R. and Neumann-Haefelin, T. Practical limitations of acute
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adjustments on the technical parameters such as lowering the tube voltage in CTA and decreasing the acquisition time in CTP. According to these findings, the thyroid revealed with lower ESD compared with that in the eyes for overall protocol due to the organ location and its distance to the scanning area(18, 19). Since the organ is situated away from the primary scanning area, the radiation absorbed by both thyroid and eyes were likely to be the secondary radiation, which was generated by the interaction of the primary radiation with matter. Although CTP protocols in the present study were performed with adequate scanning time in a range of 30–40 s, which was shorter than that reported in the previous literature (50–90 s)(9, 15, 16), the tube current used in this study was much higher (270 mA s) than that used in the Yamauchi-Kawara et al. study(16) (100 mA s). Consequently, the effective dose calculated in the present study was doubled (2.07–2.74 mSv) the effective dose determined in their study (1.2–1.4 mSv). Moreover, the CTA procedure revealed that the effective dose was slightly lower in this study compared with that found in the Yamauchi-Kawara et al. study with 0.3–0.6 versus 0.8–1.3 mSv. This may be due to the use of different multi-slice CT scanners (4-, 16- and 64-slice CT) in their study, while the present study focused only on the 64-slice CT. In CTP, the scanning area is at the level of basal ganglia in the brain where it contains territories supplied by the anterior, middle and the posterior cerebral arteries, thus offering the opportunity to interrogate each of the major vascular regions. Furthermore, the axial scan is performed in situ where it is commonly carried out for neuroimaging using dynamic sequential scanning of a pre-selected region of the brain during the injection of a bolus of iodinated contrast media as its travels through the vasculature. The slices are produced by repeatedly scanning the same region at the same table position(2). Although both eyes and thyroid are excluded from the scanning area, they are still at risk of receiving high radiation dose. In fact, the eyes are closer to the scanning region than thyroid, which explained the finding of having higher ESD measurement throughout the procedure. In CTA, the scanning area ranges from the second cervical vertebra to the skull vertex(6). Therefore, eyes are anatomically located in the scanning field, while the thyroid remains excluded from the region of interest. This explains the discrepancies in the radiation dose measurements in both eyes and thyroid. In fact, the CTA protocol of 100 kVp is recorded with the largest difference in the radiation dose between both eyes and thyroid. The use of DLP in this study as a radiation measurement method is certainly practical because the scan length was calculated to determine the total radiation dose received by each patient based on a particular CT procedure. Although the
CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION
4. 5. 6. 7. 8.
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14. Leclerc, X. et al. Internal carotid arterial stenosis: CT angiography with volume rendering. Radiology 210, 673–682 (1999). 15. Hirata, M. et al. Measurement of radiation dose in cerebral CT perfusion study. Radiat. Med. 23, 97– 103 (2005). 16. Yamauchi-Kawara, C., Fujii, K., Aoyama, T., Yamauchi, M. and Koyama, S. Radiation dose evaluation in multidetector-row CT imaging for acute stroke with an anthropomorphic phantom. Br. J. Radiol. 83, 1029–1041 (2010). 17. Huda, W., Ogden, K. M. and Khorasani, M. R. Converting dose-length product to effective dose at CT. Radiology 248, 995– 1003 (2008). 18. Brady, Z., Cain, T. and Johnston, P. Differences in using the international commission on radiological protection’s publications 60 and 103 for determining effective dose in paediatric CT examinations. Radiat. Meas. 46, 2031–2034 (2011). 19. Suzuki, S. et al. Evaluation of skin exposure during cerebral CT perfusion studies on a phantom. Eur. J. Radiol. 80, 851–855 (2011). 20. Neofotistou, V. et al. Preliminary reference levels in interventional cardiology. Eur. Radiol. 13, 2259–2263 (2003). 21. Schueller-Weidekamm, C., Schaefer-Prokop, C. M., Weber, M., Herold, C. J. and Prokop, M. CT angiography of pulmonary arteries to detect pulmonary embolism: improvement of vascular enhancement with low kilovoltage settings. Radiology 241, 899– 907 (2006). 22. Viteri-Ramı´rez, G. et al. Low radiation and low-contrast dose pulmonary CT angiography: Comparison of 80 kVp/60 ml and 100 kVp/80 ml protocols. Clin. Radiol. 67, 833–839 (2012).
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stroke MRI due to patient-related problems. Neurology 62, 1848– 1849 (2004). Romans, L. E. Computed Tomography for Technologists: Exam Review. Lippincott Williams and Wilkins (2011). Hoeffner, E. G. et al. Cerebral perfusion CT: technique and clinical applications. Radiology 231, 632–644 (2004). Shrier, D. A. et al. CT angiography in the evaluation of acute stroke. Am. J. Neuroradiol. 18, 1011– 1020 (1997). Hopyan, J. et al. Certainty of stroke diagnosis: incremental benefit with CT perfusion over noncontrast CT and CT angiography. Radiology 255, 142–153 (2010). Wintermark, M. et al. Comparison of CT perfusion and angiography and MRI in selecting stroke patients for acute treatment. Neurology 68, 694– 697 (2007). Wintermark, M. et al. Dynamic perfusion CT: optimizing the temporal resolution and contrast volume for calculation of perfusion CT parameters in stroke patients. Am. J. Neuroradiol. 25, 720– 729 (2004). Scharf, J. et al. Improvement of sensitivity and interrater reliability to detect acute stroke by dynamic perfusion computed tomography and computed tomography angiography. J. Comput. Assist. Tomogr. 30, 105–110 (2006). Lev, M. H. et al. CT angiography in the rapid triage of patients with hyperacute stroke to intraarterial thrombolysis: accuracy in the detection of large vessel thrombus. J. Comput. Assist. Tomogr. 25, 520– 528 (2001). Aviv, R. I. et al. Early stroke detection and extent: impact of experience and the role of computed tomography angiography source images. Clin. Radiol. 62, 447– 452 (2007). Cohnen, M. et al. Radiation exposure of patients in comprehensive computed tomography of the head in acute stroke. Am. J. Neuroradiol. 27, 1741– 1745 (2006).
doi:10.1093/rpd/nct280
RADIATION DOSE ASSOCIATED WITH CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION: AN EXPERIMENTAL PHANTOM STUDY
*Corresponding author: [email protected] Received 23 July 2013; revised 26 September 2013; accepted 16 October 2013 A study on the radiation dose associated with cerebral CT angiography (CTA) and CT perfusion (CTP) was conducted on an anthropomorphic phantom with the aim of estimating the effective dose (E) and entrance skin dose (ESD) in the eyes and thyroid gland during different CTA and CTP protocols. The E was calculated to be 0.61 and 0.28 mSv in CTA with 100 and 80 kVp, respectively. In contrast, CTP resulted in an estimated E of 2.74 and 2.07 mSv corresponding to 40 and 30 s protocols, respectively. The eyes received a higher ESD than the thyroid gland in all of these protocols. The results of this study indicate that combining both CTA and CTP procedures are not recommended in the stroke evaluation due to high radiation dose. Application of modified techniques in CTA (80 kVp) and CTP (30 s) is highly recommended in clinical practice for further radiation dose reduction.
INTRODUCTION Acute stroke assessment using magnetic resonance (MR) imaging has been widely established due to its ability in evaluating cerebral perfusion. The mismatch concept of diffusion and perfusion studies in MR imaging to indicate penumbra zone as a significant indicator for stroke has been widely accepted(1). However, MR imaging technology has limited accessibility and infers several contraindications, such as patient agitation, haemodynamic instability(2), implanted cardiac pacemaker or any metallic fragments(3). The latest technological development of multi-slice CT scanner makes assessment of acute stroke possible due to rapid image acquisition, which may be more practical in many centres that deal with acute stroke using CT as a first-line emergency imaging modality. Apart from reduced imaging time, CT is better tolerated by many patients and all relevant diagnostic information can be provided with only one imaging sequence(4). Non-contrast CT of the brain is routinely performed to evaluate the cerebral circulation and surrounding tissues in addition to assessment of underlying disease such as intracranial hemorrhage. CT perfusion (CTP) and CT angiography (CTA) of the brain are commonly performed in the assessment of acute stroke(4 – 7). CTP procedure is used to measure cerebral blood flow, cerebral blood volume and mean transit time. CTP has been reported to correlate well with MR imaging and it provides additional information of pathophysiology compared with MR perfusion imaging(8). To assess
cerebral perfusion, the first pass of intravenously administered non-ionic contrast agent is monitored using several continuous images acquired usually between 30 and 50 s without changing the table position(9). On the other hand, CTA of the circle of Willis detects stenosis or occlusion of intracranial arteries(4). CTA provides useful information such as the site of vascular occlusion and delineates vascular anatomy and is considered very useful for guiding therapeutic approaches(2). A comprehensive CT imaging technique, which involves a combination of CTA and CTP, is of paramount importance in the evaluation of acute stroke(10 – 13). However, the combination of these two procedures also raises a major concern due to increased radiation exposure to the patient. Assessment of acute stroke with intracranial CTA includes anatomic regions ranging from the carotid bifurcation to the brain as well as the venous sinuses(6, 14), whereas CTP involves consecutive imaging of the same brain area repetitively, thus producing high radiation dose to the patient (5). Previous studies investigating the radiation dose in combining standard CT of the head, CTA and CTP with different scanning protocols suggested the necessity of dose reduction in comprehensive stroke CT imaging as these combined CT imaging protocols could result in effective doses up to 9.5 mSv(9, 15, 16). The use of lower tube voltage 100 or 80 kVp and a narrow scan range was recommended by these studies for dose reduction. However, the total scan time of 50– 90 s was reported in these studies, which consequently
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Nanyang Technological University on June 13, 2015
Akmal Sabarudin1,*, Mohd Zaki Yusof1, Mazlyfarina Mohamad1 and Zhonghua Sun2 1 Diagnostic Imaging and Radiotherapy Program, School of Diagnostic and Applied Health Sciences, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur 50300, Malaysia 2 Discipline of Medical Imaging, Department of Imaging and Applied Physics, Curtin University, GPO Box U1987 Perth, WA 6845, Australia
CEREBRAL CT ANGIOGRAPHY AND CT PERFUSION
increased the radiation dose significantly. Thus, it is also important to consider reducing the scan time during the stroke CT imaging, which was not well addressed in the previous studies. Therefore, the purpose of this study was to determine the radiation dose resulting from different protocols of intracranial CTA and brain CTP in the acute stroke assessment based on standard clinical settings with a focus on the effective dose (E) and entrance skin dose (ESD) in the selected radiosensitive organs.
Experiments set-up This study was performed on an anthropomorphic whole-body phantom (Pixy, USA), which mimicked an adult with a height of 1.75 m and weight of 74 kg. Two intracranial CTA protocols with use of different tube voltages and two protocols of brain CTP with different scanning times were performed on a 64-slice CT scanner (Somatom Sensation, Siemens Medical Solutions, Germany). These protocols were selected for comparison of the radiation dose (E and ESD), consisting of 100 kVp (CTA protocol I) versus 80 kVp (CTA protocol II) and 40 s (CTP protocol I) versus 30 s (CTP protocol II) scanning time. The ESD was measured using thermoluminescence dosemeter (TLD) chips, while the E was determined by calculating the dose length product (DLP) with a
CTA and CTP protocols Unlike routine CTA, iodinated contrast medium was not used in this study due to the fact that the anthropomorphic phantom was unable to show contrast enhancement on the CT images. This also applies to brain CTP procedure without injection of contrast medium. Therefore, no image quality assessment was conducted in this study. Both CTA and CTP procedures were performed with a detector collimation of 64` 0.625 mm, z-flying focus techniques with 64` 0.6 mm slice acquisition, 330 ms rotation time and a pitch of 1.2. In the CTA procedure, two sets of tube voltage were used at 100 kVp (protocol I) and 80 kVp ( protocol II), while the tube voltage of 80 kVp was used in the CTP procedure. Anatomical-based tube current modulation was used with the tube current (500– 800 mA s) adjusted automatically according to the phantom’s size and weight in both procedures. In the CTP procedure, the scan was continuously performed at two different scanning time 40 s ( protocol I) and 30 s ( protocol II) based on the standard CTP
Figure 1. Images show the anthropomorphic phantom and TLDs were securely placed (marked with ‘X’) on the skin surface of the phantom at the targeted radiosensitive organs (eyes and thyroid gland).
317
Downloaded from http://rpd.oxfordjournals.org/ at Nanyang Technological University on June 13, 2015
MATERIALS AND METHODS
conversion coefficient factor. The dose values from DLP measurements were recorded during the procedure and then compared between CTA and CTP. TLD chips were securely placed above the selected radiosensitive organs (eyes and thyroid) on the phantom skin surface during the scan (Figure 1).
A. SABARUDIN ET AL. Table 1. CTA and CTP of the brain protocols. Parameters
CTP
Protocol I
Protocol II
Protocol I
Protocol II
64` 0.625 3.0 1.20 0.33 100 300– 500 160 10.98 158.0 N/A N/A 48.0 Head first
64` 0.625 3.0 1.20 0.33 80 300– 500 160 10.98 158.0 N/A N/A 48.0 Head first
64` 0.625 9.6 N/A 0.33 80 300– 500 270 40.00 50.0 345 15 N/A Head first
64` 0.625 9.6 N/A 0.33 80 300– 500 270 30.00 50.0 345 15 N/A Head first
N/A, not available.
procedure at the institution. The protocols for both CTA and CTP are detailed in Table 1.
Radiation dose measurements Identification of the radiation dose at the highest point received by organs from the overall dose distribution was of paramount importance in radiation dose monitoring during intracranial CTA and brain CTP procedures. The radiation dose may include exposure from the primary beam and the secondary radiation beams, depending on the location of these organs. The ESD was measured and recorded in milligray (mGy) after undergoing a series of procedures including annealing, calibration, radiation dose exposure and read-out process. The annealing and dose read-out was performed with a Harshaw-5500 reader (Thermo Electron Corp., USA) while calibration was performed with a 64-slice CT scanner. TLD calibration was designed to create a graph pattern for radiation dose conversion from nanocoulombs to mGy by exposing it to a known dose, which was measured with a digital radiation survey meter (model 660) having an ion-chamber model 660-3 beam measurement probe and a readout/logic unit (model 451P). These TLDs were interpreted 24 h after the exposure. The TLD used in this study was micro-square shaped and sealed in numbered, with specifically designed plastic wrap. A total of 12 TLDs were securely taped on the phantom skin surface over the radiosensitive organs for each set of procedure, where they were placed on the bilateral sides of the thyroid and eyes for radiation dose measurements. Similar to the intracranial CTA procedure, 12 different TLDs were used and placed at the same locations during each brain CTP protocol
for dose comparison. The TLD readings of a specific organ were averaged to calculate the dose for that organ. The E was obtained by a straightforward calculation based on the following formula: E ¼ DLP DCC where E is the effective dose in mSv, DLP the dose length product in mGy.cm and dose conversion coefficient (DCC) factor in mSv mGy21.cm21. The DLP values were displayed on the CT console once the scan is completed. Therefore, the DLP values were recorded from CT scan procedures. The DCC factor was derived from the specific body region that was being scanned. The DCC for an adult head with a conversion factor of 2.2 mSv.mGy21.cm21 was used(17) for both CTA and CTP procedures in the present study.
Statistical analysis All data were entered into SPSS V17.0 (SPSS, version 17.0 for Windows, Chicago, IL, USA) for statistical analysis. Continuous variables were expressed as mean values+standard deviation. A p-value of ,0.05 was considered to indicate statistically significant differences. All of the doses from each protocol were presented in box plot. One-way ANOVA and repeated measures ANOVA were used in ESD analysis to determine the differences in mean ESD of radiosensitive organs (thyroid and eyes) during these two CT procedures. With regard to the effective dose, the ANOVA test was performed to determine whether there is any significant difference in the mean values between the protocols.
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Collimation (mm) Slice thickness (mm) Pitch Rotation time (s) Tube voltage (kV) Tube current (mA) Tube current.time (mA s) Scan time (s) Scan length (mm) Scan angle (8) Tilt (8) Table speed (mm/s) Orientation
CTA
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RESULTS
Figure 3. Box plot shows the mean ESD of the thyroid and the eyes in CTP with different scanning time settings as protocol I (40 s) and protocol II (30 s). It also shows that the eyes received the highest dose distribution compared with the thyroid in both protocols. The boxes indicate the first to third quartiles; each midline indicates the median (second quartile) and the whiskers represent the maximum and minimum values of ESD.
did not differ significantly ( p¼0.83) when compared with that in the eyes.
Entrance skin dose The ESD measured at the eyes was 9.57+0.84 and 3.32+0.50 mGy corresponding to CTA protocol I (100 kVp) and protocol II (80 kVp), respectively, which is significantly higher than the values 1.08+0.20 and 0.39+0.09 mGy, which was measured in the thyroid in both protocols (100 versus 80 kVp) (Figure 2). However, the difference in ESD measurements in the thyroid between those two protocols was not statistically significant as tested with ANOVA ( p¼0.11). The radiation dose measured on each organ was performed bilaterally. However, the doses measured in each side were calculated to represent as an entire dose for that particular organ (eyes and thyroid). Similar to the findings observed in CTA protocols, ESDs measured in the eyes were significantly higher than that in the thyroid in both CTP protocols. The ESDs measured in the eyes for protocols I (40 s) and II (30 s) were 7.01+0.64 and 5.52+0.42 mGy respectively, while the ESDs values recorded for the thyroid in protocols I and II were 1.43+0.24 and 1.23+0.12 mGy, respectively (Figure 3). However, among both protocols, the ESD values in the thyroid
Effective dose estimation Statistically significant differences were found between the mean E values in all CTA and CTP protocols. The highest E was found in CTP protocol 1 with dose being 2.74+0.03 mSv, followed by CTP protocol 2 (2.07+0.02 mSv). The mean estimated E values were found significantly lower in CTA protocols with dose being 0.61+0.05 and 0.28+0.02 mSv, corresponding to the 100 and 80 kVp protocols. DISCUSSION This study highlights three important findings of the radiation dose associated with intracranial CTA and brain CTP procedures. First, the estimated effective dose in brain CTP was 3.5 –9 times higher than that calculated in the intracranial CTA. Secondly, although brain CTP shows significant higher radiation dose than CTA, the ESD measured at the eyes was the highest in CTA. Lastly, the effective dose of CTP and CTA can be further reduced with some
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Figure 2. Box plot shows the mean ESD of the thyroid and the eyes in CTA with different tube voltage settings as protocol I (100 kVp) and protocol II (80 kVp). It shows that the eyes received the highest dose distribution compared with the thyroid in both protocols. The boxes indicate the first to third quartiles; each midline indicates the median (second quartile) and the whiskers represent the maximum and minimum values of ESD.
A. SABARUDIN ET AL.
eyes received very high radiation dose from CTA procedure, it still remains ,3 mGy which does not exceed the dose limit for the eyes as set by the European Commission DIMOND III project (2003)(20). Although lower tube voltage was introduced in the CTA protocol in this study (80 kVp), it is assumed to provide good image quality for the vessel assessment. The aim of the CTA study in patients with acute stroke is to provide better visualisation of the cerebral blood vessels(14). Schueller-weidekamm et al.(21) reported that a low tube voltage CTA study provided similar diagnostic image quality to the higher tube voltage with administration of iodinated contrast medium. This is because the low-energy photon increases the attenuation index due to the contrast material introduced to the stroke patient. Thus, the use of lower tube voltage in CTA not only significantly reduces the radiation dose to the patient, but also sustains the quality of diagnostic value of contrastenhanced CT images(22). This study has its limitations. The study was based on an anthropomorphic phantom to simulate the radiation dose quantification on a standardised patient. No contrast medium was used in any of the angiography protocols. Therefore, the authors could not compare and assess image quality, since no contrast enhancement was present in the vascular system. Further studies are necessary to verify the accuracy of the results through administration of contrast medium with qualitative and quantitative assessment of diagnostic image quality. In conclusion, this study shows that CTA results in significantly lower radiation dose than CTP protocols, which is demonstrated by effective dose and ESD. Modification of imaging techniques in CTA (lower tube voltage) and shorter scanning time in CTP is recommended in daily clinical practice for further radiation dose reduction in stroke CT examinations.
ACKNOWLEDGEMENTS The author would like to thank Mohamad Norman Mohd Nordin from UKM Medical Centre, Khadijah Mohamad Nassir, Fakhrur Razi Kuffian, Norjana Abd Rahman and Hafizah Abd Rashid for their assistance in CT data collection. REFERENCES
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adjustments on the technical parameters such as lowering the tube voltage in CTA and decreasing the acquisition time in CTP. According to these findings, the thyroid revealed with lower ESD compared with that in the eyes for overall protocol due to the organ location and its distance to the scanning area(18, 19). Since the organ is situated away from the primary scanning area, the radiation absorbed by both thyroid and eyes were likely to be the secondary radiation, which was generated by the interaction of the primary radiation with matter. Although CTP protocols in the present study were performed with adequate scanning time in a range of 30–40 s, which was shorter than that reported in the previous literature (50–90 s)(9, 15, 16), the tube current used in this study was much higher (270 mA s) than that used in the Yamauchi-Kawara et al. study(16) (100 mA s). Consequently, the effective dose calculated in the present study was doubled (2.07–2.74 mSv) the effective dose determined in their study (1.2–1.4 mSv). Moreover, the CTA procedure revealed that the effective dose was slightly lower in this study compared with that found in the Yamauchi-Kawara et al. study with 0.3–0.6 versus 0.8–1.3 mSv. This may be due to the use of different multi-slice CT scanners (4-, 16- and 64-slice CT) in their study, while the present study focused only on the 64-slice CT. In CTP, the scanning area is at the level of basal ganglia in the brain where it contains territories supplied by the anterior, middle and the posterior cerebral arteries, thus offering the opportunity to interrogate each of the major vascular regions. Furthermore, the axial scan is performed in situ where it is commonly carried out for neuroimaging using dynamic sequential scanning of a pre-selected region of the brain during the injection of a bolus of iodinated contrast media as its travels through the vasculature. The slices are produced by repeatedly scanning the same region at the same table position(2). Although both eyes and thyroid are excluded from the scanning area, they are still at risk of receiving high radiation dose. In fact, the eyes are closer to the scanning region than thyroid, which explained the finding of having higher ESD measurement throughout the procedure. In CTA, the scanning area ranges from the second cervical vertebra to the skull vertex(6). Therefore, eyes are anatomically located in the scanning field, while the thyroid remains excluded from the region of interest. This explains the discrepancies in the radiation dose measurements in both eyes and thyroid. In fact, the CTA protocol of 100 kVp is recorded with the largest difference in the radiation dose between both eyes and thyroid. The use of DLP in this study as a radiation measurement method is certainly practical because the scan length was calculated to determine the total radiation dose received by each patient based on a particular CT procedure. Although the
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