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Global Journal of Medicine Researches and Studies, 1(4) 2014, Pages: 130-134 Academic Journals Global Journal of Medicine Researches and Studies ISSN 2345-6094 www.academicjournalscenter.org Evaluation of Diagnostic Reference Levels for CT scan in Isfahan Mohamad Bagher Tavakoli *1, Kianoosh Heydari 2 , Salman Jafari 3 1. Professor of Medical Physics, Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran 2. MSc Student in Medical Physics, Department of Medical Physics , School of Medicine,Isfahan University of Medical Science,Isfahan, Iran 3. PhD Student in Medical Physics ,Department of Medical Physics , School of Medicine, Isfahan University of Medical Science Isfahan, Iran ARTICLE IN FO ABSTRACT Keywords: Background: Computed Tomography (CT) Scanning as a sectional medical imaging modality plays an important role in the diagnosis of various diseases. Spatial and temporal resolution of new CT Scanners, as a result of improvements in CT technology, has ameliorated and ability to achieve images with more diagnostic details has increased. Survey of the patients absorbed dose and reducing it is important when using this imaging modality. Determination of diagnostic reference levels (DRLs) for CT examinations is a dose reduction strategy. The aim of this study was to investigate CT diagnostic reference levels in Isfahan city. Methods and materials: Based on statistical, between sixteen CT scan centers in Isfahan DRLs were investigated for seven centers using two standard phantoms of head and body with, respectively, diameter of 16 and 32 cm and length of 15 cm made of Perspex. Head, sinus, lung and pelvic-abdominal scans were selected to be investigated. For each exam in all under study centers Weighted Computed Tomography Dose Index (CTDIw) was calculated and third quartile of this quantity was determined as DRL. Results: DRLs for head, sinus, lung and pelvic-abdominal CT scans in Isfahan city were found to be 28/76, 26/86, 12/9, 12/58 mGy respectively. Conclusion: Results showed that DRL for head scans is higher than other scans; it is less for new spiral and multislice scanners than conventional scanners. DRLs in Isfahan are less compared with Iran’s national reference dose. By application of the DRLs resulted from this study, it is expected to obtain acceptable diagnostic images while minimizing patients absorbed dose. CT scan Diagnostic Reference Level (DRL) Weighted Computed Tomography Dose Index (CTDIw) Dose Length Product (DLP) © 2014 Global Journal of Medicine Researches and Studies. All rights reserved for Academic Journals Center. Introduction Since introduction in 1972 until now, there have been many improvements in computed tomography(CT) scanning caused clinical use of this medical imaging modality to increase (1-3).Nowadays CT scan plays an important role to the diagnosis of the diseases. Sectional images of the body are obtained based on the measurement of the attenuation coefficient of the passing radiation beam in different angles around the patient. X-ray beam passes through patient at different angles and is detected in opposite side by detector, then sectional images are reconstructed by using of the various algorithms (4-6). Better spatial and contrast resolution of the CT images were enabled by advances in CT scanners manufacture technology, so that dynamic imaging of the moving tissues such as heart is possible by newer CT scanners such as 64 rows detector CT scan (7-12). The proportion of CT scan to the patient’s diagnostic absorbed dose has increased due to increasing use of the new CT examinations such as coronary CT angiography, extremities angiography and four dimensional CT(13). Despite advances in the manufacture technology of the new CT scanners used for the mentioned above new examinations, absorbed dose to the patients has increased, for example absorbed dose from coronary CT angiography is higher than conventional angiography (14). Evaluation of the absorbed does in CT examinations and using dose reduction strategies are important from radiation protection aspect. Survey and determination of the diagnostic reference level is one way to investigate the absorbed dose to the patients (15-19). Diagnostic reference level determines the expected radiation dose for the different CT examination while considering image optimization principle. Based on the diagnostic reference levels, extra dose in one examination must be evaluated to prevent patient from receiving unnecessary radiation dose (20). The purpose of this study was to determine the diagnostic reference levels for CT examination to make supervision on the patient absorbed dose. Methods and materials Weighted computed tomography dose index (CTDIW) is usually used to determine the diagnostic reference level for CT examinations. Third quartile of this quantity is considered as the diagnostic reference level (20). CTDIW is calculated from Evaluation of Diagnostic Reference Levels for CT scan … Global Journal of Medicine Re searches and Studie s, 1(4) 2014 computed tomography dose index (CTDI). CTDI is defined as the proportion of the integral dose along the Z axis of the patient to beam collimation. It is measured for only one gantry rotation or one axial scan (21).The amount of CTDI equals to the following equation: CTDI 1 NT D( z )dz (1) Where D(z) is the radiation dose profile along the z-axis, N is the number of tomographic sections imaged in a single axial scan and T is the width of the tomographic section along the z-axis imaged by one data channel (21). CTDI is measured by using of two cylindrical phantoms of head and body with 15 cm in length and 16 and 32 cm in diameter, respectively, and an ionization chamber. Head and body phantoms are made of Perspex and there are five one centimeter radius holes, one in center and four on the peripheral area, along with longitudinal axis of each of them. Measured CTDI on the central and peripheral axis are called CTDIcand CTDIp respectively. CTDIw is calculated using CTDIc and CTDIp by the following equation: 1 2 CTDIw CTDIC CTDI p (2) 3 3 When CTDIw is calculated, third quartile of this quantity is considered as diagnostic reference level. To determine DRLs, first active CT centers in Isfahan city were identified. Among 16 CT centers in Isfahan, seven centers were selected. Most current CT examinations performed in those CT centers were chosen to evaluate diagnostic reference level for them. These examinations included head, sinus, lung and pelvic-abdominal CT scans. For CTDI measurement a pencil ionization chamber type RTI CT Chamber DCT 10RS and an electrometer type RTI Piranha 654 made in Sweden as well as two head and body phantoms made of Perspex were used. For head and sinus examinations head phantom and for lung and pelvic-abdominal body phantom was used. To measure CTDIc and CTDIp the pencil ionization chamber placed in the central and peripheral hole of phantoms respectively. For each exam the common parameters of the same exam in the clinical conditions were used. Then CTDIw were calculated based on the equation 2. A quantity called dose length product (DLP) is usually used to describe total energy released in the scan volume. The amount of this quantity is obtained according to the following: DLP CTDIv L (3) Where CTDIv is the volume CTDI and represents the average absorbed radiation dose over the x and y directions at the center of the scan from a series of axial scans where the scatter tails are negligible beyond the 100-mm integration limit, CTDIv represents the average absorbed radiation dose over the x, y, and z directions. L is the length of the scanned volume(21). This quantity can be used to evaluate the effective dose that reflects the risk of a non-uniform exposure in terms of an equivalent whole-body exposure (21). For each examination under study third quartile of the CTDIw was determined as diagnostic reference level. Results CTDIw(mGy) CTDIw for CT centers under study are shown in the diagram 1. For private reasons it is avoided to bring the actual name of the centers and they are shown here by A, B, C, D, E, F and G. As it is seen, center D and C has the maximum and minimum values of CTDIw respectively. 40 35 30 25 20 15 10 5 0 head sinus lung abdominal-pelvic A B C D E F G Centers Diagram 1: CTDIw for CT centers under study As shown in Table 1, Diagnostic reference levels for head, sinus, lung and pelvic-abdominal examinations were found to be 28.76, 26.86, 12.9 and 12.85 cGy respectively. 131 Evaluation of Diagnostic Reference Levels for CT scan … Global Journal of Medicine Re searches and Studie s, 1(4) 2014 Table 1: Diagnostic reference levels for head, sinus, lung and pelvic-abdominal examinations Examination Head Sinus Lung Pelvic-abdominal 28.76 Diagnostic reference level (mGy) 26.86 12.9 12.85 Values of average, maximum, minimum and third quartile of the CTDIw in Isfahan CT centers are shown in Table 2. Table 2: Values of average, maximum, minimum and third quartile of the CTDIw in Isfahan CT centers CTDIw (mGy) CT Protocol Average Max Min 3D Quqrtile Head 19.54 34.15 4.86 28.76 Sinus 18.83 34.31 4.21 26.86 Lung 9.60 17.38 2.80 12.90 Pelvicabdominal 10.29 18.31 3.10 12.85 Diagram 2 shows the values of diagnostic reference levels from this study compared to that for Iran’s national and European diagnostic reference levels (22, 23). As shown diagnostic reference levels in Isfahan is less than two others. Scan parameters influence on diagnostic reference levels including kVp ,mAs , time and number of sections for each scan are shown in Table 2. DRL (mGy) 80 60 40 This study 20 European Iran 0 Head Sinus Lung Abdominal-Pelvic Scan Diagram 2: Isfahan CT diagnostic reference levels compared to Iran’s national and European diagnostic reference levels. Center A B C D Table 2: Scan parameters influence on diagnostic reference levels CT scan type and CT KVp mA scanning mode examination Head 110 130 Simens-Spiral General Electric (GE)-Spiral General Electric (GE)-Multislice 64 rows detector General Electric (GE)Conventional s Slies 1 10 Sinus 110 120 1 10 Lung Abdominalpelvic Head 110 70 1 10 110 100 1 10 120 60 1 10 Sinus 120 50 1 10 Lung Abdominalpelvic Head 120 50 1 10 120 50 1 10 120 30 1 10 Sinus 120 30 1 10 Lung Abdominalpelvic Head 120 25 1 10 120 30 1 10 120 60 3 10 Sinus 120 60 3 10 Lung Abdominalpelvic 120 50 2 10 120 50 2 10 132 Evaluation of Diagnostic Reference Levels for CT scan … Global Journal of Medicine Re searches and Studie s, 1(4) 2014 E F G Philips- Neusoft c3000 Shimadzu-Spiral Shimadzu-Spiral Head 120 130 1 10 Sinus 120 130 1 10 Lung Abdominalpelvic Head 120 130 1 10 120 130 1 10 120 100 1 10 Sinus 120 100 1 10 Lung Abdominalpelvic Head 120 100 1 10 120 110 1 10 120 110 1 10 Sinus 120 100 1 10 Lung Abdominalpelvic 120 70 1 10 120 100 1 10 Discussion Scan parameters such as kVp, mA, scan time, pitch factor, sections thickness influence on the absorbed dose. It increases with Increasing kVp , mA and scan time (24). Only different parameter for all examinations for specific centers is mA and other parameters are the same. For center C the scan time of the head and sinus is different from the lung and pelvic-abdominal scans. Results show that in the all centers CTDIw for the head phantom is higher than the body phantom. This is because of the smaller diameter of the head phantom rather than body phantom as well as m As used for head and sinus scans is higher than that used for lung and abdominal-pelvic. Smaller diameter of the head causes the dose to distribute more uniform in a smaller volume. As shown in the result for all examinations CTDIw for center D that uses a conventional scanner is higher than spiral and multislice scanners. In these newer scanners table is moved during scan process. Proportion of the table translation per one gantry rotation to the slice thickness is called pitch factor. It effects on the absorbed dose. If pitch factor is less than one overlap there is between two adjacent slices and the absorbed dose decreases (24). In the conventional scanner table is fix during scan and then the pitch factor equals to one. Due to pitch factor usually used in the spiral scanners is more than one, absorbed dose is less than conventional scanners. Among all the scanner studied multislice scanner of General Electric (GE) belongs to center C has the least CTDIw for all scans except for abdominal-pelvic scans. In this scanner used m As is less than other scanners that causes CTDIw to be less. Diagnostic reference level determined in this study is less than national diagnostic reference level of Iran and Europe. In general, it can be concluded that various parameters such as exposure parameters, CT technician experience, scanner type influence on the absorbed dose. 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