British Journal of Radiology (2003) 76, 541-545
© 2003 British Institute of Radiology
doi: 10.1259/bjr/60897046
A contribution to the establishment of diagnostic reference levels in CT
K Hatziioannou, MSc, PhD1,
E Papanastassiou, MSc2,
M Delichas, MSc, PhD2 and
P Bousbouras, MSc3
1 Medical Physics Department, Papageorgiou Hospital, 56403 Thessaloniki, 2 Medical Physics Laboratory, Medical School, Aristotle University of Thessaloniki, 54006 Thessaloniki and 3 Social Security Institution, 19 Aristotelous st., 54624 Thessaloniki, Greece
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Abstract
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CT has become the major source of population exposure to diagnostic X-rays. CT dose index (CTDI) and dose–length product (DLP) have been proposed as the appropriate dose quantities for the establishment of diagnostic reference levels for optimizing patient exposure. Dose measurements on 27 CT scanners in Northern Greece involving six routine CT examinations have been performed in order to compare their performance with the currently proposed European reference dose values and to produce a preliminary set of data for the establishment of local diagnostic reference levels. All measurements were performed using a pencil shaped ionization chamber introduced into polymethyl methacrylate cylindrical head and body phantoms. The results revealed significant discrepancies in dose values among the CT scanners, which can be mainly attributed to variations in the examination protocols and the different kinds of scanners. Significant overdosing compared with the European reference levels has not been observed, with the exception of the routine head examination, where 47% of the scanners exceeded the corresponding CTDIw value. CT scans in the trunk region result in the higher effective doses, which can reach estimated maximal values of the order of 15 mSv.
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Introduction
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CT was introduced in the early 1970s and quickly became a very important tool in medical diagnosis. Technological developments which have improved the speed and the quality of the resulting images have also encouraged the growth of CT practice worldwide. The expanding use of CT has resulted in this modality becoming the major source of population exposure to diagnostic X-rays, contributing 40% of the resulting collective dose in the UK in 1997 [1]. Data from 1991 to 1996 show that, globally, CT was responsible for 34% of the annual collective dose from medical exposures. Considering countries belonging in healthcare level I, this percentage rose to 41% [2]. Following new biological information on radiation harm and modified risk estimates, the basic recommendations of the International Commission on Radiological Protection (ICRP) were revised in Publication 60 [3] and new quantities and concepts were introduced. In optimization there was the important new concept of constraint with the main implication in diagnostic radiology being that of dose constraints for the patient. The concept of diagnostic reference levels for patients was first introduced in ICRP Publication 73 [4] and it was adopted by the European Commission [5].
The complex conditions of irradiations in CT, involving highly collimated X-ray beams and sometimes also beam-shaping filters, necessitate the use of specially defined dose descriptors, such as CT dose index (CTDI). This is defined as the integral along a line parallel to the axis of rotation (z) of the dose profile (D(z)) for a single slice, divided by the nominal slice thickness T: 
In practice, a convenient assessment of CTDI can be made using a pencil ionization chamber with an active length of 100 mm so as to provide a measurement of CTDI100 expressed in terms of absorbed dose to air (mGyair) [5]. Such measurements may be carried out free-in-air on or parallel with the axis of rotation of the scanner (CTDI100,air), or at the centre (CTDI100,c) and 10 mm below the surface (CTDI100,p) of standard CT dosimetry phantoms. The subscript "n" (nCTDI) is used to denote when these measurements have been normalized to unit radiographic exposure (mAs).
The European Commission (EC) have suggested the use of a normalized dose index, nCTDIw, expressed as absorbed dose to air, which takes account of non-uniformities of CTDI values measured at the centre or the periphery of these phantoms:
where C is the radiographic exposure (mAs) and CTDI100,p represents an average of measurements at four different locations around the periphery of the phantom. The weighted CT dose index, CTDIw, which is the first of the two reference dose quantities proposed by the EC, for a single slice in serial scanning or per rotation in helical scanning is then simply:
The second reference quantity is the dose–length product (DLP), which includes the patient, or the phantom volume irradiated during a complete examination:
where i represents each serial scan sequence forming part of an examination and N is the number of slices, each of thickness T (cm).
In the case of helical (spiral) scanning:
where, for each of i helical sequences forming part of an examination, T is the nominal irradiated slice thickness (cm), A is the tube current (mA) and t is the total acquisition time (s) for the sequence. Table 1 presents a summary of the proposed reference levels. In order to estimate the radiation risk associated with CT examinations, a broad estimation of effective dose (E) may be derived from values of DLP using appropriately normalized coefficients:
where EDLP is the region specific normalized effective dose (mSv mGy-1 cm-1). General values of EDLP appropriate to different anatomical regions of the patient (head, neck, chest, abdomen or pelvis) are given in Table 1.
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Table 1. Proposed European Commission reference levels and region specific normalized effective doses for some routine CT examinations
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There are approximately 190 CT scanners operating in Greece. To our knowledge, there are no data currently available concerning nationwide patient doses for CT examinations. The aim of the present work is to contribute to the establishment of diagnostic reference levels for various CT examinations in Greece.
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Materials and methods
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This survey was performed on 27 CT scanners in 25 public and private clinics in Northern Greece. 10 scanners were helical. All measurements were performed during 2000 to 2001. Table 2 summarizes the scanners according to their manufacturer. Six typical CT examinations, namely routine head, cervical spine, chest, abdomen, lumbar spine and pelvis, were selected for the study. For each examination, data concerning examination parameters, such as kVp, mAs, number of slices, slice thickness and couch increment, for five consecutive average sized patients were recorded in every CT clinic that performed the corresponding examination. It was found that for each examination, each clinic used standard exposure technique factors (kVp, mAs and slice thickness remained constant) whereas only the number of slices varied slightly from patient to patient. The actual number of slices selected for the study was the average derived from these five patients.
CTDI100,air measurements were made free-in-air using a 10 cm pencil shaped ionization chamber (model 20 x 5 10.3 CT; Radcal Corporation, Monrovia, CA) connected to a Radcal (model 9015; Radcal Corporation, Monrovia, CA) dosimeter. The chamber was aligned parallel to the scanner axis of rotation.
Two polymethyl methacrylate (PMMA) cylindrical phantoms according to FDA specifications [6] were used for the measurements of CTDI100 in order to calculate nCTDIw. The head phantom was 16 cm in diameter and 16 cm long, with nine 13 mm diameter holes drilled parallel to its long axis, one at the centre and eight around the periphery, at 45° apart and 1 cm from the edge. All holes could be filled with retractable PMMA inserts. The body phantom was 32 cm in diameter and 16 cm long, and perforated similarly to the head phantom. CTDI100 measurements were performed with the ionization chamber positioned at the central and four peripheral holes after removing the corresponding insert.
nCTDIw and CTDIw were calculated using Equations (2)
and (3). When different slice widths were involved in an examination protocol, the mAs setting usually remained unchanged at each slice width. For those few cases involving several slice widths and different mAs settings at each slice width, a "partial" CTDIw was initially calculated at each slice width. The "overall" CTDIw was then calculated as a weighted average of the "partial" CTDIws with respect to the corresponding length scanned at each slice width.
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Results
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Table 3 presents the different examination protocols used among the 26 CT scanners for the routine head examination, just as an indication of the discrepancy of the examination parameters applied amongst the scanners. Similar discrepancies were observed for the other five examinations as well.
Table 4 summarizes the dosimetric quantity CTDIw calculated for each examination, as well as the 3rd quartile values deduced from the corresponding distributions. DLP and effective dose per examination, calculated using the appropriate conversion coefficients, are presented in Table 5.
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Table 4. Mean, range and 3rd quartile values for weighted CT dose index (CTDIw) for six typical CT examinations (values in parentheses denote the corresponding number of CT scanners studied)
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Table 5. Mean, range and 3rd quartile values for dose–length product (DLP) and effective dose for six typical CT examinations (values in parentheses denote the corresponding number of CT scanners studied)
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In Figure 1, all CTDIw values calculated for each examination are shown in ascending order. The dotted horizontal line shows the corresponding 3rd quartile value and a continuous horizontal line represents the corresponding proposed EC reference level. Figure 2 presents all the DLP values with the corresponding 3rd quartile value and the EC reference level.
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Figure 1. The weighted CT dose index (CTDIw) values calculated for each examination. The dotted horizontal line shows the corresponding 3rd quartile value and the continuous horizontal line represents the corresponding proposed European Commission reference level.
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Figure 2. Dose–length product (DLP) values calculated for each examination. The dotted horizontal line shows the corresponding 3rd quartile value and the continuous horizontal line represents the corresponding proposed European Commission reference level.
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Discussion
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The CT scanners examined cover a wide range of manufacturers and models. It should be noted that, in the present study, no quality control procedures concerning radiation beam and geometrical parameters were performed prior to dose measurements. Furthermore, no assessment of the resulting image quality in each scanner was included.
A significant discrepancy was observed in some of the examination parameters among the radiology departments. For example, in the routine head examination, most departments use similar high voltages (120–130 kVp) and slice thicknesses (10 mm and 5 mm) whereas the scanned volume length and the mAs values seem to differ by factors ranging from 1.5 to 3.0. Although mAs variation can be attributed to intrinsic differences in scanner design (long vs short geometry), the significant variation observed in scanned length is less straightforward and could be due to respondents not providing data for exactly the same examination, meaning that, for example, "routine head" could have been interpreted differently among the CT clinics. The fact that the majority of the clinics that participated in the study had not yet incorporated the EC guidelines on quality criteria for CT in their routine practice reinforces the validity of this remark. Significant discrepancies have also been observed in the CTDIw and DLP values calculated for the six different examinations. In some cases the differences between the highest and the lowest value reach one order of magnitude and they can be attributed to the large discrepancy in the examination protocols and the different kinds of CT scanner. No significant differences in doses due to helical scanning were observed. The 3rd quartile values for CTDIw and DLP which result from the corresponding distributions among the 27 scanners (Figures 1 and 2) are below the proposed EC reference levels, with the exception of the routine head examination, where 47% of the scanners (12 out of 26) exceeded the CTDIw reference level. This shows that the higher CTDIw values can be probably attributed to scanner type and protocol parameters selected. The percentage of scanners that exceeded the EC reference values for CTDIw and DLP for all the other examinations did not exceed 19% (4 out of 21).
Given that the local quartile values derived from this survey, although generally below the EC reference values, reflect the non-compliance of some clinics with the EC guidelines, it is suggested that the European values should be used to test the dosimetric performance of individual scanners. The local quartile values should be used in order to test future reductions in CT doses after full incorporation of EC recommendations by the majority of CT clinics. Full compliance with the EC guidelines and the development of national examination protocols based on these guidelines will also help to reduce the observed discrepancies in radiation doses.
CT scans in the trunk region result in the higher effective doses, which can reach estimated maximal values of the order of 15 mSv.
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Acknowledgments
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The authors wish to thank the medical physicists Mrs E Psarouli, Mr G Komisopoulos and Mrs H Kodona for their valuable assistance in data collection.
Received for publication August 16, 2002.
Revision received April 11, 2003.
Accepted for publication May 19, 2003.
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References
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- Shrimpton PC, Edyvean S. CT scanner dosimetry. Br J Radiol 1998;71:1–3.[Medline]
- United Nations. Sources and Effects of Ionizing Radiation, Volume I: Sources. United Nations Scientific Committee on the Effects of Atomic Radiation. 2000 Report to the General Assembly, with scientific annexes. United Nation Sales Publication, Sales No E.00.IX.3, New York, 2000.
- International Commission on Radiological Protection. 1990 recommendations of the International Commission on Radiological Protection. Publication 60. Annals of the ICRP 21 (1–3), Pergamon Press, Oxford, 1991.
- International Commission on Radiological Protection. Radiological Protection and Safety in Medicine. Publication 73. Annals of the ICRP 26 (2), Pergamon Press, Oxford, 1996.
- European Commission. European guidelines on quality criteria for computed tomography. Report EUR 16262. Luxembourg, 1999.
- Department of Health and Human Services, Food and Drug Administration, 21 CFR Part 1020.33 b (6).
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Abstract
Introduction
