We compared the radiation dose values of 10 CT protocols of two 16-detector CT scanners in our department. CT scanner A (Somatom Sensation 16 CT Scanner, Siemens) was installed in our department in 2008 while CT scanner B (Optima CT540, GE Healthcare) was installed in 2017. Radiation dose values were compared in terms of DLP (mGy cm) values. A comparison of radiation dose values of these types of equipments was accomplished using a radiation dose monitorization system (GE DoseWatch).
We also recorded CT acquisition parameters used in 10 CT protocols in order to depict the effect of CT acquisition parameters in radiation dose change between CT scanners.
CT protocols used in radiation dose comparison and radiation dose values of most frequent protocols are shown in Table 1. CT acquisition parameters of CT protocols are summarized in Table 2.
According to comparison results, DLP values in 8 of 10 CT protocols performed in CT B (newer) were lower than CT A (older). Dose reduction rates were in the ranges of 2.98% - 39.33%. Dose reduction was highest in head CT (DLP difference 386 mGycm, 39.33%) and lowest in chest + abdomen CT (DLP difference: 20.68 mGycm, 2.98%).
Dose differences between the CT equipments were interrogated in terms of CT acquisition parameters. Comparison of head CT protocols yielded that tube voltage values (kV) of CT scanners were same (120 kV) while head CT protocols were obtained with higher collimated slice thickness (CSL) and lower tube rotation time interval (TI) values in CT B than CT A. Dose reduction in head CT performed in CT B was attributed to an increase in CSL and a decrease in TI values.
There was no significant difference between dose values of paranasal sinus CT protocols in CT A and CT B. Although tube voltage value in CT B (100 Kv) was lower than CT A (120 Kv) CSL was lower and TI value was higher. Low tube voltage causes a decrease in radiation dose however decreased CSL and increased TI values balanced the effect of tube voltage and resulted in an unchanged radiation dose level in this protocol.
Neck CT protocol presented with a lower dose level in CT B with a 7.56% decrease rate. Decreased radiation dose in neck CT was attributed to lower TI value in CT B.
Chest CT dose was higher in CT B although a tube voltage levels were the same and higher CSL was used in CT B. Lower radiation dose in CT A was caused by lower TI value in this protocol.
Low radiation dose values in Chest + Abdomen CT and Abdomen CT protocols obtained in CT B were attributed to increased CSL values in CT B.
Lower DLP values of multiphasic urinary CT, urinary stone protocol and multiphasic liver and pancreas protocols in CT scanner B were attributed to higher collimation thickness and lower tube rotation time values.
Overall dose reduction in CT scanner B compared to CT scanner A was also attributed to the usage of newer iterative reconstruction techniques in CT scanner B and optimized detector configuration which enables lowering tube current leading to dose reduction.
Table 1: The name and number of CT protocols performed in CT scanner A and B with radiation dose values and radiation dose differences.
CT Protocols
|
CT A
Number
|
CT A
DLP
|
CT B
Number
|
CT B
DLP
|
Radiation Dose Change
|
Difference
%
|
Head
|
48
|
981,62
|
473
|
595,48
|
-386,14
|
39,33
|
Paranasal Sinus
|
264
|
154,72
|
26
|
155,46
|
+ 0,74
|
0,47
|
Neck
|
233
|
264,73
|
399
|
284,75
|
-20,02
|
7,56
|
Chest
|
1802
|
235
|
408
|
289
|
+ 54
|
22
|
Chest + Abdomen
|
517
|
693,35
|
1578
|
672,67
|
-20,68
|
2,98
|
Abdomen+Pelvis
|
750
|
466,79
|
1406
|
429,14
|
-37,65
|
8,06
|
Multiphasic (Urinary protocol)
|
90
|
987
|
124
|
722,08
|
-264,2
|
26,76
|
Urinary stone protocol
|
281
|
344,15
|
246
|
277,06
|
-67,09
|
19,49
|
Multiphasic abdomen (Liver)
|
98
|
988,67
|
131
|
751
|
-237,67
|
24,03
|
Multiphasic abdomen (Pancreas)
|
91
|
723
|
73
|
681
|
-42
|
5,80
|
DLP: Dose Length Product (mGy-cm)
Table 2: CT acquisition parameters used in CT scanner A and CT scanner B for 10 CT protocols.
CT Protocols
|
kV
CT A CT B
|
CSL (mm)
CT A CT B
|
TI (s)
CT A CT B
|
Head
|
120
|
120
|
0.8
|
1.25
|
0.8
|
0.6
|
Paranasal Sinus
|
120
|
100
|
0.8
|
0.625
|
0.5
|
0.7
|
Neck
|
120
|
120
|
1.5
|
1.25
|
0.7
|
0.6
|
Chest
|
120
|
120
|
0.8
|
1.25
|
0.5
|
0.6
|
Chest + Abdomen
|
120
|
120
|
0.8
|
1.25
|
0.5
|
0.6
|
Abdomen+Pelvis
|
120
|
120
|
1.25
|
1.5
|
0.5
|
0.5
|
Multiphasic Urinary CT
|
120
|
120
|
1.25
|
1.5
|
0.6
|
0.6
|
Urinary stone protocol
|
120
|
120
|
1.25
|
1.5
|
0.6
|
0.5
|
Multiphasic abdomen (Liver)
|
120
|
120
|
1.25
|
1.5
|
0.8
|
0.5
|
Multiphasic abdomen (Pancreas)
|
120
|
120
|
1.5
|
1.5
|
0.8
|
0.5
|
kV: kilovoltage (tube potential)
CSL: Collimated slice thickness
TI: Tube rotation time interval