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National survey of doses from CT in the UK: 2003

¹ Radiation Protection Division (formerly NRPB), Health Protection Agency, Chilton, Didcot, Oxon OX11 0RQ, ² ImPACT, Department of Medical Physics, St George's Hospital, London SW17 0QT, ³ Medical Physics Department, Queen's Medical Centre, Nottingham NG7 2UH, UK

Correspondence: Dr Paul C Shrimpton, Radiation Protection Division, Health Protection Agency, Chilton, Didcot, Oxon OX11 0RQ, UK. E-mail: paul.shrimpton{at}hpa-rp.org.uk

	Abstract

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
A review of patient doses from CT examinations in the UK for 2003 has been conducted on the basis of data received from over a quarter of all UK scanners, of which 37% had multislice capability. Questionnaires were employed to collect scan details both for the standard protocols established at each scanner for 12 common types of CT examination on adults and children, and for samples of individual patients. This information was combined with published scanner-specific CT dose index (CTDI) coefficients to estimate values of the standard dose indices CTDI_w and CTDI_vol for each scan sequence. Knowledge of each scan length allowed assessment of the dose–length product (DLP) for each examination, from which effective doses were then estimated. When compared with a previous UK survey for 1991, wide variations were still apparent between CT centres in the doses for standard protocols. The mean UK doses for adult patients were in general lower by up to 50% than those for 1991, although doses were slightly higher for multislice (4+) (MSCT) relative to single slice (SSCT) scanners. Values of CTDI_vol for MSCT were broadly similar to European survey data for 2001. The third quartile values of these dose distributions have been used to derive UK national reference doses for examinations on adults (separately for SSCT and MSCT) and children as initial tools for promoting patient protection. The survey has established the PREDICT (Patient Radiation Exposure and Dose in CT) database as a sustainable national resource for monitoring dose trends in CT through the ongoing collation of further survey data.

	Introduction

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
The relatively high doses to patients from CT were first highlighted for the radiology community by the publication in 1991 of systematic estimates of organ and effective doses for some common CT procedures on adults that arose from a national survey conducted in the UK [1]. Data from this early study, which related to practice with single slice scanners operating largely in slice-by-slice, axial scanning mode, also provided the basis for reference doses subsequently published in 1999 as part of European guidelines on quality criteria for CT [2]. Continuing advances in CT technology, such as the development of fast, helical scanning using multidetector rows (MDCT), also known as multislice CT (MSCT), have promoted the increasing clinical application of CT and undoubted improvements in health care for populations [3–6]. In the UK, for example, CTs contribution to the collective effective dose from medical X-ray examinations has more than doubled over the last 10 years to about 47% [7]. A total of some 2.0 million CT examinations were reported for the National Health Service (NHS) in England for the year 2003–2004, representing about 9% of all X-ray examinations [8]. Similar growth in CT practice has been observed in many other countries [9].

Contemporary CT scanners facilitate the rapid imaging of large volumes of the patient and are therefore promoting the development of new and complex diagnostic and interventional procedures, with clear potential for further increases in the doses to individuals and populations [10–12]. At the same time, increasing attention is also being paid to the optimization of patient protection through improvements in CT technology and practice, with some possibilities for dose reduction [13–16]. In order to meet the challenge of such significant developments in CT practice, a new UK national survey has been carried out [17] by the Radiation Protection Division of the Health Protection Agency (formerly the National Radiological Protection Board), in collaboration with the CT Users Group (http://www.ctug.org.uk/) and the national CT evaluation centre ImPACT (Imaging Performance and Assessment of CT) (http://www.impactscan.org/) of the NHS Purchasing and Supplies Agency. This survey provides a snapshot of UK practice for 2003, with updated information relevant to helical and multislice CT on adults and children, and updated national reference doses for comparison with previous UK data [1] and the recently revised European quality criteria for MSCT [18]. The study has also established a sustainable database, known as PREDICT (Patient Radiation Dose and Exposure in CT), for the ongoing collation of further data from local surveys of CT. This will facilitate the analysis of trends and the periodic review of national reference doses for CT in a similar way to the national patient dose database already operating successfully for conventional X-ray examinations [19].

	Survey methods

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
Data collection
Key data on local CT practice were collected for individual CT scanners by means of a questionnaire that was voluntarily completed following active promotion of the survey. The questionnaire was published in October 2002 on the CT Users website (http://www.ctug.org.uk/) for downloading, manual completion and postal submission. It was based on a form (Geleijns, 2002; Personal Communication) originally developed for a 2001 European Survey on CT [18].

The questionnaire sought two principal sets of scanning data. First, information was collected in relation to standard protocols established for some common CT examinations and standard (average-sized) patients, as listed in Table 1. Specific clinical indications were included for each type of CT examination since these could influence the scanning technique used. This approach was intended to make the data from different CT centres more comparable. These particular examinations were selected as representing the bulk of core practice for adult patients, whilst broadly characterizing the range in paediatric technique. Such standard protocols should form the basis for typical practice and its variants at each CT centre. Protocols consist of a number of separate scan sequences, each representing a single helical exposure or a series of similar axial exposures using identical scan conditions. Full details were requested of the scan settings applied for each sequence, including an indication of whether the sequence was performed routinely for every patient or only as required [17].

Data from each questionnaire were entered manually into an Excel spreadsheet (Microsoft Corporation, USA) for subsequent import into a dBASE V relational database (dataBased Intelligence Inc., NY). Manipulation of these raw data required the use of 10 supplementary data files. Structured quality assurance procedures were implemented at all stages to help ensure the accuracy of the calculations and analyses [17].

Dosimetry
The framework for CT dosimetry is already well established [2, 13, 20, 21]. Monitoring of performance in CT as part of routine quality assurance is based on the practical dose quantities: weighted CT dose index (CTDI_w), volume weighted CT dose index (CTDI_vol) [22] and dose–length product (DLP). These form the basis for reference doses (and diagnostic reference levels (DRLs)) set for the purposes of promoting optimization of patient protection [23, 24]. In addition, values of effective dose (E) [25] for complete CT examinations are also useful for comparison with other types of radiological procedure.

The strategy for the present survey ideally involved the calculation of values of CTDI_w, CTDI_vol and DLP for each sequence. This was accomplished on the basis of the scan settings provided in the questionnaires and the representative scanner-specific CTDI_w coefficients (mGy mA^–1 s^–1) published by ImPACT [26] as part of its CT patient dosimetry calculator. For those 15% of sequences in the survey performed with automatic tube current modulation [27–29], doses were calculated using reported values, where available, of (average) tube current or current–time product (mAs) that included the effects of modulation. DLPs were calculated for each sequence using either the scan length as directly reported or a standard scan length assumed on the basis of the recorded anatomical landmarks denoting the limits of the scan. These standard lengths were derived using a scheme developed for the survey that characterized the typical distances between common anatomical landmarks for four standard patient ages [17]. Table 2 summarizes the perpendicular distances between planes through landmarks in the head, relative to a plane through base of skull and superior orbital margin, and in the trunk, relative to a transverse plane through the lung bases. The typical data shown for the adult were based on measurements performed on scan projection radiographs for a series of average-sized patients. The paediatric data were derived from adult values using scaling factors based on anthropometric data published for the head and torso [30, 31]. Data were also derived (and used when appropriate) relevant to other angulations of the CT gantry for two further scan baselines: true transverse; and a plane through the base of skull and inner canthus.

	Results

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
Scanner sample
During the 6-month period to March 2003, 153 questionnaires were received as separate submissions of data from 126 scanners located at 118 hospitals spread widely around the UK. The mean value of 1.2 questionnaires per scanner reflects survey instructions not to delay submission of data for standard protocols pending the collation of any data for individual patients. Table 4 presents detailed analyses for the regional distributions of CT scanners both in the survey sample and the UK as a whole. Overall, the sample included over a quarter of the estimated total of 471 CT scanners in clinical service in the UK during 2003. Sampling rates for England (with about 70% of all UK scanners) and Wales (about 4% of the UK total) were broadly appropriate, although Scotland and Northern Ireland were somewhat over-represented in the sample, whereas scanners included in the "Other" category were under-represented.

View larger version (22K): [in this window] [in a new window]   Figure 1. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of CTDIw (±2 SE) for adult patients from UK 2003 review and comparison with survey data from Europe published in 1999 (EU99) [2].

View larger version (21K): [in this window] [in a new window]   Figure 2. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of CTDIvol (±2 SE) for adult patients from UK 2003 review and comparison with survey data from Europe published in 2004 (EU04) [18].

View larger version (26K): [in this window] [in a new window]   Figure 3. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of DLP (±2 SE) for adult patients from UK 2003 review and comparison with survey data from Europe published in 1999 (EU99) [2] and 2004 (EU04) [18].

View larger version (25K): [in this window] [in a new window]   Figure 4. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of CTDIw (±2 SE) for paediatric patients from UK 2003 review and comparison with survey data from Europe published in 2000 (EU00) [32].

View larger version (25K): [in this window] [in a new window]   Figure 5. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of CTDIvol (±2 SE) for paediatric patients from UK 2003 review and comparison with survey data from Europe published in 2004 (EU04) [18].

View larger version (25K): [in this window] [in a new window]   Figure 6. Analysis by examination type and scanner technology(single (SSCT) or multislice (MSCT)) of mean values of dose–length product (DLP) (±2 SE) for paediatric patients from UK 2003 review and comparison with survey data from Europe published in 2000 (EU00) [32] and 2004 (EU04) [18].

National reference doses recommended on the basis of rounded third quartiles for the dose distributions observed in 2003 are presented in relation to standard examination protocols for adults in Table 10 (separately for SSCT and MSCT scanners) and for children in Table 11.

	Discussion

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
The present survey provides a substantial snapshot of national practice at a time of significant evolution in the technology and clinical application of CT. Analyses of the data collected have helped demonstrate the robustness of the methods employed for dose assessment. Comparisons of calculated values of CTDI and DLP with those often reported in the questionnaires showed reasonable agreement overall [17]. This suggests that dose displays on the scanner console are probably sufficiently accurate for direct use in dose audit, including future national surveys, provided some initial checks are carried out locally to validate the readings. Reasonable agreement was also found between the standard scan lengths assumed for a given anatomical region (Table 2) and corresponding specific data reported for sequences when the latter were available, generally in the case of survey data for individual patients. The mean ratio of recorded length to standard length was 1.10 over all sequences [17].

Standard examination protocols provide the basic framework for typical practice, although every CT procedure should, of course, be tailored to meet the needs of the individual patient. Doses to individual patients were on average similar to those for the corresponding standard protocols established for each scanner, although significant variations were apparent for some patients. In particular, mean DLPs to patient groups from examinations of the abdomen for liver metastases were typically higher than those expected for standard protocols (mean ratio of 1.48 over all scanners in the survey), owing to the use in practice of additional sequences for further phases of contrast [17]. Optimization studies in CT should therefore include audit of doses to groups of patients as an important complement to the review of standard examination protocols and the assessment of typical practice.

Results from the 2003 review confirm that there are still wide variations in technique and dose between CT centres for similar examinations, although the mean doses for standard examination protocols for adult patients are in general lower by up to 50% than previous UK data for 1991 [2] when including all scanners together. As an apparent trend, however, doses appear lowest for the dual scanners and highest for the quad scanners (Tables 6 and 7) and this observation is similar to those reported for surveys of CT in East Anglia [35] and Germany [36]. The increased doses observed for quad scanners are consistent with X-ray beam penumbral effects being most pronounced for such scanners and this leads to reduced z-axis geometric efficiency [29, 37].

Differences in the doses from SSCT and MSCT are more marked for examinations on adults (Figures 1–3) than for examinations on children (Figures 4–6). Accordingly, national reference doses are presented separately for SSCT and MSCT for examinations on adults (Table 10), as representing an equitable approach in view of the observed differences in dose between these technologies. In contrast, single (general) values have been derived for examinations on children (Table 11) as the best practical option on the basis of the limited amount of survey data presently available for paediatric CT.

The relative increase in reference dose values for adults from MSCT compared with SSCT (Table 10) is largest for scans of the head and the chest (high-resolution). These examinations both involve axial scanning with narrow beam collimations, where beam penumbral effects and differences in z-axis geometrical efficiency between single and multislice scanners are most pronounced. Although dual slice scanners were excluded from this particular analysis, the national reference doses shown for SSCT on adults can in practice also be applied to these scanners. The values of CTDI_w and CTDI_vol broadly reflect pitches of about 1 for axial scan sequences of the head, about 10 for axial scan sequences of the chest (high-resolution), and about 1.4 for helical scan sequences on the trunk. Levels of CTDI are higher for specific sequences through the posterior fossa compared with the cerebrum and through the abdomen or pelvis relative to the lung.

The 2003 reference values for CTDI_w are 15–60% lower compared with previous European recommendations from 1999 [2], particularly so in relation to SSCT, although the MSCT reference values for scans through the posterior fossa and chest (high-resolution) are larger than previously. The values of CTDI_vol are broadly consistent with updated European recommendations for 2004 [18]. DLP values are lower than the previous 1999 European values by 30–70% for SSCT and 10–50% for MSCT. Unfortunately, reference values for DLP have not been included in the updated 2004 European Quality Criteria [18]. The 2003 UK national reference doses are also lower than similar data for Germany [38].

With regard to the general national reference doses for examinations on children (Table 11), values of CTDI_w and CTDI_vol broadly increase as might be expected with increasing patient size (standard age) and doses are again higher for scans of the posterior fossa relative to the cerebrum. There are also no reference dose values for paediatric CT in the 2004 European Quality Criteria [18], but in comparison with previous recommendations from a European survey published in 2000 [32], the UK 2003 values for CTDI_w are in general 10–30% lower and the values of DLP up to 40% lower.

The 2003 UK national reference dose values for CTDI_vol and DLP are summarized in Tables 12 and 13, respectively. These data are intended for general application in patient protection over the next few years during which CT practice will no doubt continue to evolve and SSCT scanners will decline in number. As an initial step in the process of optimization, local levels of dose determined as a routine part of clinical audit should normally be below relevant national reference doses. However, since these values are clearly not optimum doses, further dose reductions should always be pursued where clinically compatible. These 2003 national reference doses are likely to underpin any national DRLs subsequently set by the Department of Health [23, 39].

Typical (mean) effective doses for adult patients (Table 7) are less than those previously assumed for the UK in the 1990s (i.e. 2 mSv, 8 mSv and 10 mSv for examinations of the "head", "chest" and "abdomen", respectively [40, 41]), although there are still wide variations in practice. For examinations on children, typical values of the dose descriptors CTDI_w, CTDI_vol and DLP decrease with decreasing age (and size), whereas the corresponding effective dose increases. Indeed, effective doses to children aged 0–1 years from examinations of the "head" and the "chest" were typically higher than those for adults. Since children are potentially more susceptible to radiation effects, special efforts should be made in clinical practice to reduce their doses by the use of size-specific scan protocols for optimized CT imaging [34, 42–45].

	Conclusions

Top Abstract Introduction Survey methods Results Discussion Conclusions References

 
A new and substantial national CT dose survey for 2003 has established a robust methodology for the wide-scale assessment of dose and provided a wealth of updated dose data in relation to 12 common types of CT examination on adults and children in the UK [17]. Doses to adults are in general lower by up to 50% than previous UK data from 1991, but with an apparent trend for slightly increased doses from multislice (4+) compared with single slice scanners. Third quartile values from the survey have been used to set national reference doses for CTDI and DLP, with separate values for single and multislice CT on adults, and general values for paediatric CT. The single slice values are in general lower than previous European recommendations [2, 32], whereas the multislice values are more consistent with European survey data and recommendations for 2004 [18]. Effective doses to very young patients (aged 0–1 year) were typically higher than corresponding values for adults.

The survey provides essential data on dose and technique to facilitate further initiatives in the optimization of patient protection in CT. In particular, the study has established the PREDICT database for the ongoing collation of local survey data as the basis for further periodic reviews of practice and timely revisions of UK national reference doses. These doses will help inform the setting of national diagnostic reference levels for CT by the Department of Health in accordance with IRMER 2000 [39].

M Dunn is a Past Chair of CT Users Group.

	Acknowledgments

 
We are pleased to acknowledge the generous support and co-operation of numerous colleagues that has ensured the success of this large-scale survey. In particular, we are most grateful to Nick Keat and Sue Edyvean (ImPACT), and Barry Wall (HPA-RPD) for their ongoing wise counsel, detailed comments and helpful suggestions; Koos Geleijns and the European CT study group for kindly sharing their survey questionnaire; and, most importantly, everyone who kindly submitted invaluable data on local CT practice.

Received for publication January 3, 2006. Revision received April 3, 2006. Accepted for publication April 12, 2006.

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Top Abstract Introduction Survey methods Results Discussion Conclusions References

 

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