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1 Medical Physics Directorate and 2 Imaging Directorate, University Hospital of North Staffordshire NHS Trust, Princes Road, Hartshill, Stoke on Trent ST4 7LN, UK
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Abstract |
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Introduction |
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Current regulations [2, 3] also require that representative patient doses are assessed at appropriate intervals, and these should be compared with local diagnostic reference levels (DRLs). These local DRLs are to be set by the employer with due regard to national DRLs where available [4]. Where local DRLs are exceeded, reviews must be conducted and corrective action taken where appropriate. In addition, the European guidelines [5] draw attention to the fact that reference levels do not signify optimum levels of performance and that, with due caution as to the loss of clinical information, reduction of doses below the reference level should always be pursued in line with the ALARP principle.
In order to facilitate measurement and optimization of patient dose, the National Radiological Protection Board (NRPB) introduced the national protocol for patient dose measurements in diagnostic radiology in 1992 [6] and recently published a national review of patient doses recorded under this protocol [7]. The national protocol specifies two dose assessment quantities for comparison with national DRLs, namely entrance surface dose (ESD) and dose–area product (DAP). DAP meters are traditionally used for fluoroscopic and more complex examinations, whereas ESD is used for simple plain radiography examinations. The national protocol recommends that ESD is directly measured on a sample of patients using thermoluminescent dosemeters (TLDs), although free-in-air measurements of a tube's radiation output together with the calculation of ESD using standard factors can be employed in appropriate circumstances [6]. There has been some use of calculated ESD in patient dose assessment programs [8, 9], although the TLD measurement technique has been more widely employed.
When the national protocol was introduced, the computational method was not encouraged [6], partly due to difficulties in assessing the exposure factors where an automatic exposure control device is employed. However, since it is now a statutory requirement to have written exposure protocols, users of X-ray equipment must provide representative standard exposure factors for manually set exposures. In addition, a written record should be kept of any exposure values programmed via the console into the X-ray generator in case of equipment failure [10]. By implication, average tube current–exposure time product (mAs) values would also ideally be available for exposures normally undertaken with automatic exposure control. Some of the limitations of the computational method may therefore have been overcome.
The computational method has three potential advantages. First, it requires minimum additional work by the radiology department since exposure charts should already be in existence for every X-ray unit. Second, the computational method allows dose studies to be extended to a much larger number of examinations than would be cost effective with TLDs. Third, it allows assessment of low dose examinations, which may deliver doses below the practical sensitivity of TLDs and some DAP meters.
The work described here evaluates the use of computational estimates of ESD from a large number of X-ray examinations in a programme of dose optimization across several NHS Hospital Trusts.
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Method |
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As a minimum, for calculation of ESD, the required data included tube identification details and values for focus-to-film distance (FFD), tube potential and mAs for each type of examination. Where the procedure would normally be carried out under automatic exposure control, departments were asked to return typical average manual mAs settings for that particular X-ray tube.
Microsoft Excel and Access were used to store and manipulate the data, calculate doses and automatically produce reports for individual feedback to each participating hospital. Each Trust received a report containing results for their own X-ray units compared with anonymous data from the other Trusts.
Dose calculation
The ESD at a given tube potential was estimated from the measured radiation output of the tube, the relevant exposure factors and a mathematical model of the human body used to estimate radiation backscatter [11, 12]. 
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Experimental verification
In order to demonstrate that the computational technique was in agreement with practical measurements, comparisons were made of the computed ESD with doses measured using a lithium fluoride TLD chip attached to the entrance surface of the patient and placed in the centre of the X-ray field. TLD measurements, together with factors used in each case, were collected for the posteroanterior (PA) chest examination and for the anteroposterior (AP) abdomen examination for a limited sample of six X-ray units per examination at the time of the initial survey.
In the case of the PA chest examination, where doses were lower than the recording threshold of the TLD detector, a single TLD was exposed 10 times yielding an average dose for 10 patients. The average weight of the patients was within 5 kg of the 70 kg standard man. The computed dose was evaluated from the mean values of each exposure factor used for the examinations.
For the AP abdomen examination, the mean exposure factors and TLD reading for each X-ray tube were taken from a minimum of four patients. All individual patient weights were between 50 kg and 90 kg, and the average weight of patients for each room was within 5 kg of the 70 kg standard.
In addition to comparisons with TLD measurements and in order to assess whether the standard exposure factors matched those used in clinical practice, the ESD calculated from the factors listed on the exposure chart was compared with the ESD computed from the actual factors used during the TLD measurements for chest and abdomen examinations.
Dose optimization strategy
In order to demonstrate dose optimization, an initial study whose results were reported to the Trusts in August 2001 and a review study (using the same techniques) reported in June 2003 were conducted. In the interim period, Trusts were encouraged to review and optimize their exposure factors. In the initial study, exposure charts were received for 74 X-ray tubes out of 89 requested, giving details of more than 1500 individual examination settings. In the review study, exposure charts were received for 68 X-ray tubes out of 92 requested, giving details of almost 2000 examination settings.
The examinations were separated into 47 categories (Table 1) each involving a single projection. ESD was calculated as described above and suggested investigation levels (SILs) were set for each category. This was carried out for each of the two studies. SILs were set at twice the mean ESD for each examination unless the SIL exceeded the national DRL for that projection, in which case it was set at the value of the national DRL.
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Results |
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for the PA chest and Figure 1b for the AP abdomen. Each pair of bars represents the results for a single piece of X-ray equipment. The national protocol for patient dose measurement states that the uncertainty in TLD measurement should be within ±25% and this is indicated by the error bars in Figure 1. Errors in the calculated doses included possible drift in the radiation output of the X-ray tube over time, uncertainty in the mean thickness of the body part being imaged and uncertainty in the level of backscatter. For each of the X-ray tubes, radiation output is measured annually and remedial action recommended whenever the output drifts from baseline values by more than ±20%. It is unlikely that typical variations in output from year to year are more than half this value. Errors in the representative patient thickness and in assigned BSFq may be of a similar order and therefore a nominal ±25 % error (taken to match that assigned to the TLD results) is also shown in Figure 1.
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The results of the comparison of ESD computed from the factors given in the exposure charts and the ESD computed from actual exposure factors used are given in Figure 2a and 2b. Each pair of bars represents a particular X-ray unit. Errors in assigned BSFq and representative patient thickness were systematic rather than random for a given examination leaving only the estimated ±10% random error in tube output, which is shown in Figure 2. There was broad agreement between the two sets of results.
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as a typical example, and the shaded bars represent equipment held by one Trust. The results showed good compliance with national DRLs.
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for all 47 examination categories used in the study. Including data only for Trusts participating in both studies for a given examination, the computed ESD in the review study was less than the corresponding value in the initial survey for 37 of the 47 examinations (Figure 4). The SIL values were based on all data and were presented to only one decimal place and with these limitations, the SIL in the review study was less than the corresponding value in the initial study for 22 of the 47 examinations. On average examinations showed around a 10% reduction in ESD between the studies. Examinations showing a large percentage change typically comprised data from a small number of units in both studies. For instance, the C spine peg examination showed a 64% increase in dose between the studies, but there were only six examination settings in the initial study and four examination settings in the review study, and only two X-ray units were represented in both studies. When the doses for the C spine peg examination were reviewed for just the two units represented in both studies, a dose reduction of 12% was observed.
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). For AP pelvic doses across all Trusts, the standard deviation as a percentage of the mean reduced from ±47% in the initial study to ±41% in the review study. Two exposures in the first study and one exposure in the second exceeded the SIL. In both studies, all AP pelvic exposures were within the national DRL.
The AP pelvic examination (Figure 3) can be taken as a typical example to demonstrate the reasons for the decrease in calculated doses which were as follows:
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Discussion |
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) with evidence to suggest that this resulted not only through equipment replacement, but also through optimization of exposure factors. In some cases, although the calculated ESD changed between the studies, this did not reflect a real change in the mean patient dose, but was due to improvements in the accuracy of the data contained in the exposure charts. Nevertheless, even improvements in chart accuracy could lead to some actual dose reduction by limiting any errors made in the factors selected.
The computational technique employed here showed good agreement with the TLD measurement technique (Figure 1) and may produce smaller errors compared with TLDs for comparative assessments. The accuracy of the computational technique is particularly important where it is used to estimate individual patient doses. The relative importance of accurate agreement with TLD measurements is less where, as in this study, the technique is used to monitor trends, to provide broad comparison with national DRLs, and to promote optimization of patient dose.
There was broad agreement between the exposure settings given on the charts and those used in practice although in a few cases agreement was poor (Figure 2). These few results suggested a need for improvements to the accuracy of data contained in some exposure charts. This may have been particularly true for exposures where automatic exposure control was normally used. As attention is drawn by studies such as this to the accuracy of data contained in exposure charts, the correlation between the factors given on charts and those used in practice should improve.
Setting local DRLs using data from individual Trusts has been discouraged in the past due to lack of statistical significance of the data [4]. The NRPB has set national reference doses for those examinations where data was available from more than 20 rooms [7]. In the review study conducted here, 28 of the 47 examinations were conducted in and comprised data from more than 20 rooms. Because of the good agreement between the computational technique and the TLD measurements, it is suggested that the results for these 28 examinations could be used to assist in setting local DRL values.
This method of dose assessment has enabled a greater range of examinations to be studied than would have been possible using a TLD method, due to time constraints and the costs involved. Although DAP meters are quicker to use than TLDs, many of the general X-ray units participating in the survey, particularly older radiographic or mobile equipment, did not have DAP meters fitted. In addition the sensitivity of some DAP meters may not have been adequate for comparative purposes when used for the lowest dose, small field size examinations (such as some extremity examinations) included in the two studies.
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Conclusions |
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Acknowledgments |
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Received for publication December 18, 2003. Revision received April 5, 2004. Accepted for publication May 11, 2004.
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This article has been cited by other articles:
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  H. Liu, W. Zhuo, B. Chen, Y. Yi, and D. Li Patient doses in different projections of conventional diagnostic X-ray examinations Radiat Prot Dosimetry, December 1, 2008; 132(3): 334 - 338. [Abstract] [Full Text] [PDF]
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 V Tsapaki, I A Tsalafoutas, I Chinofoti, A Karageorgi, E Carinou, V Kamenopoulou, E N Yakoumakis, and E D Koulentianos Radiation doses to patients undergoing standard radiographic examinations: a comparison between two methods Br. J. Radiol., February 1, 2007; 80(950): 107 - 112. [Abstract] [Full Text] [PDF]
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B M Moores Radiation dose measurement and optimization Br. J. Radiol., September 1, 2005; 78(933): 866 - 868. [Full Text] [PDF]
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