|Year : 2020 | Volume
| Issue : 2 | Page : 76-81
Estimating reference dose measurements during common computed tomographic procedures
Hussain M Almohiy1, E Elshiekh2, Mohammed S Alqahani1, Khalid I Hussein3, Madshush M Alshahrani4, Mohammed Saad1
1 Department of Radiological Sciences, College of Applied Medical Sciences, King Khalid University, Abha, Saudi Arabia
2 Department of Radiological Sciences, College of Applied Medical Sciences, King Khalid University, Abha, Saudi Arabia; Radiation Safety Institute, Sudan Atomic Energy Commission, Khartoum 1111, Sudan
3 Department of Radiological Sciences, College of Applied Medical Sciences, King Khalid University, Abha, Saudi Arabia; Department of Medical Physics and Instrumentation, National Cancer Institute, University of Gezira, Wad Medani 20, Sudan
4 Department of Radiology, KMGH, Khamis Mushayt, Saudi Arabia
|Date of Submission||04-Sep-2020|
|Date of Decision||14-Sep-2020|
|Date of Acceptance||30-Sep-2020|
|Date of Web Publication||25-Feb-2021|
Dr. E Elshiekh
Department of Radiological Sciences, College of Applied Medical Sciences, King Khalid University, Abha, Saudi Arabia
Source of Support: None, Conflict of Interest: None
Background: Medical X-rays are the largest man-made source of public exposure to ionizing radiation. In CT examination, the probability of radiation-induced cancer is more than other x-ray examinations. Objective: The aim of this research is to estimate the reference dose values for some common procedures (head and abdomen) and compare the findings with those of a previous research. Methods: Dose measurements were taken from the scanner readings, and using the periphery of the PMMA phantom, the body phantom and head were found to be about 32 cm and 16 cm in diameter, respectively. The volume computed tomography (CT) dose index (CTDIvol) carefully chosen in the small phantom was used as a benchmark for a CT head, and the measure in large phantom was utilized as a benchmark for body CT. The results of the average estimated CTDIvol for the head-and-abdomen scans in the emergency department of a large hospital and large diagnostic clinic scanner were compared with international reference dose levels. Results: In this study, the average values of CTDIvol were 53.02 mGy and 16.95 mGy for the head and abdomen, respectively. The dose for the head phantom, 53.02 mGy, was perceived to be inferior in differentiation with international diagnostic reference level (DRL) doses. The estimated dose for the abdomen scan was elevated in comparison with 2004 European DRLs by 13%, but it was lower than the DRL for ACR by 32%. Conclusion: The results presented here will assist by collectively providing a fixed reference point for determining reference doses for CT examinations.
Keywords: Computed tomography, radiation safety, radiology, volume computed tomography dose index
|How to cite this article:|
Almohiy HM, Elshiekh E, Alqahani MS, Hussein KI, Alshahrani MM, Saad M. Estimating reference dose measurements during common computed tomographic procedures. King Khalid Univ J Health Sci 2020;5:76-81
|How to cite this URL:|
Almohiy HM, Elshiekh E, Alqahani MS, Hussein KI, Alshahrani MM, Saad M. Estimating reference dose measurements during common computed tomographic procedures. King Khalid Univ J Health Sci [serial online] 2020 [cited 2021 Jun 12];5:76-81. Available from: https://www.kkujhs.org/text.asp?2020/5/2/76/309615
| Introduction|| |
One of the most routinely employed diagnostic imaging methods is computed tomography (CT), and its use is continuously increasing., For example, up to 6.5 crore CT examinations are carried out in the USA per year, constituting over 50% of the total allocated radiation doses. The International Atomic Energy Agency estimated that in around 25% of all radiological investigations, CT procedures are used, contributing to about 60% to 70% of the total medical imaging examination doses.
The quantity of ionizing radiation in CT procedures is measured utilizing the CT dose index (CTDI). Most CTDIs are nowadays assessed with a 100-mm pencil-patterned ionization chamber (i.e., CTDI100) that merges radiation intensity outlines in the orchestration perpendicular with the scanning plane.
Most CTDI measurements consist of two phantom dimensions known as the head (diameter = 16 cm) and body (diameter = 32 cm). Phantom quantifications are traditionally acquired at the center and on the periphery of a specific phantom, which helps to ensure that the consequences of beam-shaping filters are appropriately considered. The quantified CTDI (CTDIw) is calculated by using Equation (1) and described as a third of the central CTDI and 67% of the peripheral CTDI.
Most of the CT examinations are executed in helical scanning, distinguished by the CT pitch, which is acquired by dividing the interspace traveled by the patient table per X-ray tube rotation by the nominal X-ray beam width taken at the CT isocenter. The volume CTDI (CTDIVol) is explained as CTDIw divided by CT pitch and can be evaluated by utilizing Equation (2), yielding an estimate of the mean phantom dose for a total helical scan. Understanding these parameters is crucial to becoming thoroughly acquainted with CT terminology, including tube voltage, tube current, pitch, and extra factors because they influence patient dose and image quality. CTDIvol is recognized internationally and is conceivably the most significant tool of the radiation output of the CT scanner, and its unit is milligray. CTDIvol is a quantity estimated and utilized exclusively by CT. It is estimated utilizing a pencil type ionization chamber, which is positioned at the center and at periphery of the PMMA phantom.
Tube current–time product (milliampere-seconds [mAs]) is the multiplication of the CT scanner exposure time per rotation (in seconds) and the current of the X-ray tube (in mAs); the outcome of enhancing tube current or tube current–time product is a consistent increment in ionizing radiation dose. Tube voltage peak (kVp) is the X-ray tube potential designating the pinnacle energy of the X-ray photons (in kilovolts) in a range of X-ray energies.
The objective of this research was to estimate the reference dose measurements for common CT procedures (namely, the head and abdomen) and to compare the results with those of other similar studies. Furthermore, the accuracy of the estimates of the reference dose measurements was evaluated in this study, and the results were compared with the doses displayed in the console CT scanner.
| Materials and Methods|| |
The ethical community of scientific research waived this study (ECM#2019-42-HA-06-B-001). This study was performed on two CT systems, manufactured by the GE Healthcare company, Beijing, China; both CT scanners provided 16 slices per gantry rotation (i.e., GE Bright Speed 16 CT scanners). One of these systems was used in the emergency department of large hospital in Asir region, Kingdom of Saudi Arabia, averaging more than 150 examinations per week. The other system was used in a large diagnostic clinic at King Khalid University, Abha city, Kingdom of Saudi Arabia, averaging more than 60 examinations per week.
CT of the head (diameter of 16 cm) and body (diameter of 32 cm) with drillings of Polymethylmetacrylate (PMMA) dosimetry phantoms was utilized in the research to represent the head portion and body portion of an adult (model number 76-419-4150). In addition, a calibrated CT sensor (Unfors XI CT with S/N: 205469) manufactured by the Unfors RaySafe AB company, Billdal, Sweden with a 10-cm pencil ion chamber linked to the electrometer (Unfors RaySafe AB company, Billdal, Sweden) was used in taking dose measurements.
The evaluation for the CT radiation doses was accomplished by calculating CTDI, which may be regarded as the absorbed dose extending toward the CT scanner's longitudinal axis (z-axis), calculated in the course of a single rotation of the X-ray source.
Measurements of the doses were generated at the peripheral area and at the center of the PMMA phantom, as shown in [Figure 1], and these points were then added together utilizing an average weighted CTDI (CTDIw). This process resulted in a single conjecture of the dose of radiation specified for the phantom. The respective measurements of the body and head of the phantom were 32 cm and 16 cm in diameter, respectively. The measurement of the small phantom through CTDIvol is utilized as a benchmark for the CT head, and the measurement of the large phantom is a guide for the CT of the body for many manufacturers of the scanner. The CTDIvol quantified in the large phantom is also utilized as a guide for performing adult CT procedures including pelvis, abdomen, and chest. The value of CTDIvol is stated in mGy units.
|Figure 1: The positions of computed tomography during the dose measurements|
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The head phantom was the first arrangement to be installed on the CT couch and centered at the scanner's isocenter along the phantom's long axis, aligned with the scanner's z-axis. The single and scout views of the phantom image, which equaled 1 mm, were obtained for reasons of alignment.
The chamber of the CT was set in the middle of the phantom hole 1, and other unused holes (2, 3, 4, and 5) were plugged with acyclic rods, and a scout view image was utilized to select the slice or volume to be imaged. Axial mode exposure was conducted through the CT brain scan technique (120 kV, 300 mAs, slice thickness = 5 mm), the reference dose measurements as shown in [Table 1] were used with a tube voltage range of (80–140 kVp), and different products of the time–tube current (mAs) were made ready for the head of the phantom study. The CTDI was calculated at the central Hole 1 and peripheral holes 2, 3, 4, and 5 by changing the position of the ion chamber from one hole to the other, as depicted in [Figure 1].
|Table 1: Setup for reference dose measurements for some common procedures (pencil computed tomography chamber in peripheral and at the center of phantom)|
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Following the head phantom measurements, the process was replicated multiple times in the body phantom utilizing pelvis CT scanning techniques (120 kV, 100 mAs, and slice thickness = 5 mm).
The CTDIvol was estimated from CTDIw, according to Equations (1) and (2). The approximate values were then compared with the head-and-body console-displayed doses (CTDIvol).
The reference dose measurements of the CTDIvol, for the common procedure of head and abdomen, were calculated according to the setup presented in Table 1. Results from the dose measurement were compared with the CTDIvol displayed in the CT scanner console. Different setting of tube voltage (kVp), and tube current time products (mAs) were used to investigate dose measurements for a wide range of exposure.
The weighted CTDI (CTDIw) was calculated as follows:
where the second term (with brackets) is the average CTDI calculated from the average, measured CTDI in phantom peripheral – holes 2, 3, 4, and 5.
The volume CTDIvol was calculated as follows:
| Results|| |
In [Table 2],[Table 3],[Table 4],[Table 5], the displayed CTDIvol, estimated CTDIvol, percentage differences for CTDIvol, exposure factor kVp, and mAs are presented from the axial and helical scans, respectively, for both CT scanners in the EDH and LDC units. The findings described in [Table 2],[Table 3],[Table 4],[Table 5] indicate that the approximate CTDIvol differed by the tube current factor of 2 in the axial and helical tests. The results shown in [Table 2] were obtained when using 120 kVp, and mAs varied from 50 to 100 mAs by a factor of two. In addition, the results of the estimated CTDIvol varied by the same factor, as did the given values (8.87 and 17.86 mGy), with a deviation equal to 5%. The results obtained from the EDH scanner are shown in [Table 2] and [Table 4], given the values of estimated CTDIvol of 6.52, 11.66, 17.88, and 24.49 mGy and 6.33, 12.51, 17.86, and 24.15 mGy for the helical and axial scans, respectively, with varied tube voltages of 80–100, 80–120, and 80–140 kVp, when other exposure factors were constant.
|Table 2: The data collected during the axial scan to measure computed tomography dose indexvol from the emergency department of a large hospital scanner|
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|Table 3: The data collected during the axial scan to measure computed tomography dose indexvol from the large diagnostic clinic scanner|
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|Table 4: The data collected during the helical scan to measure the computed tomography dose indexvol from the emergency department of a large hospital scanner|
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|Table 5: The data collected during the helical scan to measure computed tomography dose indexvol from the large diagnostic clinic scanner|
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The same results were achieved using the LDC scanner, given the values of estimated CTDIvol of 6.87, 12.08, 16.96, and 23.52 mGy and 6.54, 11.90, 18, 28, and 25.4 mGy for the helical and axial scans, respectively. When reducing the tube voltage in the EDH scanner from 140 to 120 kVp by a factor of 14%, the estimated CTDIvol was also reduced by a factor of 37% to 35% for the helical and axial scans, respectively.
[Table 6] presents the result of estimated reference dose measurements of the CTDIvol for routine head-and-abdomen sequences for both CT scanners in the EDH and LDC units. The exposure factors of 120 kVp and 300 mAs and 120 kVp and 100 mAs for the head and abdomen, respectively, were selected, and these values were set to the head-and-body phantoms to mimic the clinical conditions of the adult patients.
In the EDH scanner, the estimated CTDIvol values with the percentage difference displayed in the console CT scanner for the head and abdomen were 53.57 ± 9.13 and 16.60 ± 7.7, respectively. While in the LDC, we found the values of 52.46 ± 8.58 and 17.30 ± 8.6, respectively, for the head and pelvis.
The results of the average estimated CTDIvol for the head-and-abdomen scans, shown in [Table 7], from the EDH and LDC scanners, were compared with international reference dose levels. In this study, the average values of CTDIvol were found to be 53.02 mGy and 16.95 mGy for the head and abdomen, respectively. The dose in the phantom head of 53.02 mGy was seen to be inferior when compared with the international diagnostic reference level (DRL) amounts by 29.30% and 11.63% for American College of Radiology (ACR) (2008) and Europe (2004), respectively. The approximated dose for the abdomen scan in this study was higher than those of European DRLs (2004) by 13% but lower than the DRL for ACR by 32%.
|Table 7: Comparison of the estimated computed tomography dose indexvol (mGy) with international reference dose levels|
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The relations between the helical and axial scan data from the EDH scanner and the LDC scanner are presented in [Figure 2] and [Figure 3], respectively. Moreover, the relations between the estimated CTDIvol and mAs from the EDH and LDC scanners are presented in [Figure 4] and [Figure 5], respectively. They showed very strong linear correlations, with R2 = 0.992 and 0.999 for the axial scans from LDC and EDH, respectively.
|Figure 2: The relation between the helical and axial scans, data from the emergency department of a large hospital scanner|
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|Figure 3: The relation between the helical and axial scans, data from the large diagnostic clinic scanner|
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|Figure 4: The relation between the estimated volume computed tomography dose index and milliampere-seconds from the emergency department of a large hospital scanner|
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|Figure 5: The relation between the estimated volume computed tomography dose index and milliampere-seconds from the large diagnostic clinic scanner|
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| Discussion|| |
In this study, the average values of CTDIvol found were 53.02 mGy for the head and 16.95 mGy for the abdomen. Kalra et al. stated that the tube current is proportional to the amount of X-ray photons emitted and the delivered dose. If tube current decreased by a half, the dose will be lowered by around 50%. Therefore, the output CT dose in terms of CTDIvol could be estimated by utilizing tube current–time product.
The dose in the head of the phantom, 53.02 mGy, was seen to be inferior when compared with the international DRL doses. In the current study, the calculated results of the estimated CTDIvol were acceptable for the head and abdomen in both scanners because the percentage differences from the displayed CTDIvol were more than 10%. For some consoles, displayed doses of certain CT systems are theoretically calculated on approximations from international software packages, which are accepted throughout the world, whereas doses of other computer consoles of CT systems are approximated by measuring actual phantoms. Research has depicted that for systems bearing theoretically approximated console doses, the correctness of dose measurement may go beyond ±10%.
The estimated dose for the abdomen scan in this study was elevated higher than European DRLs (2004) and the UK DRLs (2019) by 13% but lower than the DRL for ACR by 32%., Furthermore, the reported results were the result of this study of the relation between the tube voltage and estimated CTDIvol, which was agreed with another study reported by Huda and Mettler which found the difference in patients' radiation exposure (output from the tube) to be nonlinear. In addition, a decline of 14% in tube voltage from 140 to 120 kV will decrease patient risk, and the radiation dose will decrease, preferably by 30%–35%. Nevertheless, the quality of the produced images could be affected by lowering the tube voltage.
Finally, scan length has an important influence on patient dose and must be limited to areas that help with the diagnosis process. Other dose reduction approaches include decreasing kVp, increasing pitch factor, and using automatic exposure control. Staff awareness and their training in CT examination technical parameters also have important effects on the doses given to patients.
| Conclusion|| |
The results in this study were found to be comparable with those of other studies. The results of average estimated CTDIvol for the head scan were found to be inferior to international DRL doses by 29.30% and 11.63% for ACR (2008) and European countries (2004), respectively. The predicted dose for the abdomen scan in this study was higher than the DRLs for European countries (2004) by 13% but lower than the DRL for ACR by 32%. The results presented will serve as guidelines for obtaining reference doses for CT examination.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]