British Journal of Radiology 74 (2001),259-261 © 2001 The British Institute of Radiology
Calibration frequency of dose–area product meters
M T Crawley, MSc, MIPEM1,
S Mutch, MSc, MIPEM2,
M Nyekiova, MSc, MIPEM2,
C Reddy, MSc2 and
H Weatherburn, PhD, MIPEM2
1 Radiology Department, Stoke Mandeville Hospital, Mandeville Road, Aylesbury, Buckinghamshire HP21 8AL
2 Medical Physics Department, Oxford Radcliffe Hospital, Old Road, Headington, Oxford, OX3 7LJ, UK
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Abstract
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Calibration of patient dose monitoring devices in diagnostic radiology has become increasingly important in the light of new legislation that requires monitoring of patient dose against local and national diagnostic reference levels. An investigation was conducted into the long-term stability of 41 dose–area product (DAP) meters over a period of approximately 5 years, to assess the suitability of an annual calibration regimen. For DAP meters fitted to overcouch X-ray tubes, 77% of calibrations were within 10%, whilst for undercouch tubes only 50% of calibrations were within 10%. These findings suggest that annual calibration may be too infrequent. Suitable calibration frequencies for different clinical workloads are discussed.
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Introduction
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Legislation requires that patient doses are monitored and checked against local and national diagnostic reference levels [1]. The purpose of such monitoring is to promote optimization of radiological procedures with respect to patient dose and image quality by detecting unplanned changes in dose levels or when local or national diagnostic reference levels are exceeded, and taking remedial action in such circumstances. Individual patient doses provide a poor indicator for comparison against reference levels; however, dose distributions acquired over time and involving many patients to average out effects due to variables such as patient size, examination complexity and operator technique provide a more suitable indicator. Dose–area product (DAP) meters are a common, readily available method of providing a patient dose index and acquiring such distributions, from which median and quartile values can be derived for comparison against equivalent local and national values.
The Departments of Medical Physics in Oxford and Aylesbury are responsible for the calibration of DAP meters in eight hospitals. The number of DAP meters has increased steadily from 18 in 1995 to 41 in 2000. 17 DAP meters have been fitted to equipment retrospectively, the remaining 26 being installed as an integral part of the radiological system. With the exception of three systems, all DAP systems in use are of the sametype, namely the PTW Diamentor DAP meter (PTW, Freiburg, Germany), and most of these DAP meters are harnessed to equipment from two major radiological equipment manufacturers. We have analysed the calibration records to investigate the stability of these 41 DAP meters over a 5-year period from 1995 to 2000, during which time 153 calibrations were carried out (Table 1
).
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Method
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On installation, all 41 DAP meters were adjusted as necessary to give a DAP reading within 10% of the product of true air kerma and field area, relative to the patient, across the clinical tube potential (kVp) range [2]. A DAP meter fitted to an overcouch X-ray tube, a mobile X-ray unit or a mobile C-arm fluoroscopic unit was calibrated by irradiating an independent ion chamber supported above the couch and measuring the air kerma and field size at that point. For an undercouch X-ray tube, these same measurements were made, again with the ion chamber above the couch, so that air kerma measurements included couch attenuation. Thus, the DAP was again calibrated relative to the patient and not to the couch. A 20–30% difference was observed between calibrations including and excluding the couch. Without exception, undercouch tube DAP calibrations required adjustment at acceptance.
Where a deviation in calibration of more than 10% was observed at subsequent annual inspection, a further adjustment was made to bring the calibration back to within 10%. All DAP calibrations were corrected for the calibration of the reference dosemeter used for cross-calibration. In all instances, the reference dosemeter was one of two Keithley 35050A dosemeters with 15 cm3 ion chamber (Keithley Instruments Inc., Cleveland, OH). These are tertiary standards with calibration renewed annually by an accredited laboratory.
The DAP meter calibration records from 1995–2000 were then analysed. DAP calibration at calibration points between 60–80 kVp was recorded as satisfactory if the deviation observed between indicated and measured values was within 10%, and unsatisfactory if it was greater than 10%. Calibrations that deviated by more than 15% were also recorded separately. For each calibration, a note was made of tube configuration (namely undercouch or overcouch, with mobiles being classed as overcouch). Average coefficients of variation on the calibration factors at 80 kVp were then determined for overcouch and undercouch configurations. A note was also made whether the DAP meter was integral to the radiological equipment or whether it had been fitted retrospectively. The data were then analysed to determine whether there was any difference in DAP stability between the two tube configurations, or between integral DAP meters and those fitted retrospectively.
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Results
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Over the 5-year interval, 31% of the 153 calibrations analysed were unsatisfactory, i.e. the calibration showed a deviation of greater than 10%. There was no difference in consistency of calibration between integral DAP meters and those fitted retrospectively (Table 1
). However, a notable difference in frequency of miscalibration was observed between overcouch and undercouch tube configurations (Table 2), the frequency of miscalibration being greater for undercouch X-ray tube configurations (50%) than for overcouch configurations (23%). Moreover, undercouch DAP calibrations were observed to deviate by more than 15% in 40% of cases. Mean coefficients of variation on the calibration factors at 80 kVp were 7% (overcouch) and 16% (undercouch).
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Discussion
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Our investigation of the apparent increased instability of undercouch DAP meters highlighted an important problem, namely the adjustment of DAP meters by radiological equipment manufacturers' service engineers during routine maintenance. The PTW DAP meter has a test function in which a calibrated signal is used to produce a test reading. The sensitivity of the DAP meter can be adjusted by altering the reading given for this test signal. This has the advantage that radiography staff can record the meter reading directly, thereby simplifying subsequent analyses of DAP data and facilitating direct comparison with diagnostic reference levels without first having to make a correction for calibration. For DAP meters that required recalibration on installation, the value of the test reading was altered by medical physics staff when this adjustment was made. However, a crucial disadvantage of this calibration tactic has been demonstrated in that service engineers subsequently checking the unit during routine service visits may re-set the test reading to the original value, thereby re-setting the calibration to the installation value. If the original installation test value was based on a factory calibration carried out without a couch in the X-ray beam, then a deviation of about 30% from the desired calibration might be expected. This provides the most likely explanation for the increased frequency of miscalibration observed with undercouch tube configurations.
Frequency of instrument calibration depends on legal requirements, the stability of the instrument and the importance of the calibration to the work being undertaken. Institute of Physics and Engineering in Medicine Report 77 [3], which represents the Health and Safety Executive's inspection standard for testing radiological equipment, advocates calibration at least annually. Other guidance also suggests calibration at annual intervals [4]. Our experience suggests that using our current calibration procedures, an annual calibration regimen may not be frequent enough, as about one-third of our DAP meters were out of calibration at any given time. This could be a consequence of our chosen method of calibration, in which adjustments are made to DAP meter sensitivity by medical physics staff, as this could lead to problems due to service engineers re-setting the calibration.
The overall uncertainty in the medical physics calibration is ±7% at the 95% confidence level. Fluctuations in temperature and pressure could typically account for an additional 1–2%, with a maximum deviation (between a high pressure hot day and a low pressure cold day) of ±5%, although most of the X-ray rooms in question would not have been subject to such extremes. However, the additional 1–2% correction, if made, is unlikely to alter our findings significantly. A similar observation can be made regarding intrinsic uncertainty in the calibration chain when two different dosemeters are used to calibrate the DAP meter setting.
In the light of these findings, it may be prudent to undertake DAP meter calibration at intervals of no more than 6 months, or indeed at intervals determined by service frequency. For X-ray rooms with a high screening workload, involving larger patient doses, calibration checks may be warranted at more frequent intervals, perhaps quarterly. Similarly, if radiation dose is a key element of a clinical trial, DAP calibration becomes critically important and the frequency of calibration checks should be increased. One way of achieving a greater level of consistency may be to incorporate a monthly or even weekly check in the radiographer-based quality control programme, using the DAP system test function, although this test facility is not always readily accessible on DAP meters integral to the installation.
Legislation imposes a requirement that a medical physics expert is involved as appropriate for consultation on optimization, including patient dosimetry and quality assurance [1]. Adjustments to DAP meters should only be made by a person who is, or who is acting under the direction of, such an expert. In NHS Trusts, this role often falls to the Radiation Protection Adviser or a member of their staff. It is recommended that managers of radiological equipment agree with the relevant manufacturers that DAP meters are not adjusted by service personnel unless specifically requested and authorized by the appropriate staff. Alternatively, DAP calibration procedures based on the calibration factors set by service personnel at installation could be adopted. This could remove the problem of unauthorized adjustment defeating the medical physics calibration. However, there would be accompanying consequences for radiology department procedures for monitoring DAP data against relevant diagnostic reference levels, as radiography staff would require the calibration factors pertinent to different items of equipment to make the necessary comparisons.
Received for publication July 26, 2000.
Revision received November 17, 2000.
Accepted for publication December 21, 2000.
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References
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Statutory Instrument 2000, No. 1059. The Ionising Radiation (Medical Exposure) Regulations 1999. London: HMSO.
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Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National protocol for patient dose measurements in diagnostic radiology. Chilton: NRPB, 1992.
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Institute of Physics and Engineering in Medicine. Recommended standards for routine testing of diagnostic X-ray imaging systems, Report 77. York, UK: Institute of Physics and Engineering in Medicine, 1997.
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Wall BF. Quality control of dose–area product meters. In: Moores BM, Stieve FE, Eriskat H, Schibilla H, editors. Technical and physical parameters for quality assurance in medical diagnostic radiology, BIR Peport 18. London: British Institute of Radiology, 1989:140–2.
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Abstract
Introduction
