Download Evaluation of Diagnostic Reference Levels for CT scan in Isfahan

Global Journal of Medicine Researches and Studies, 1(4) 2014, Pages: 130-134
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Global Journal of Medicine Researches and Studies
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2345-6094
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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.
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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
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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. Centers have been studied in this research are equipped with newer scanners and have
experienced and trained technicians that cause diagnostic reference level in Isfahan to be less than other mentioned studies.
Acknowledgments
We would like to thank all those who helped us during this study especially all CT canters personnel.
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