British Journal of Radiology (2004) 77, 931-933
© 2004 British Institute of Radiology
doi: 10.1259/bjr/62185486
Exposure variations under error conditions in automatic exposure controlled film–screen projection radiography
C Walsh, BA, MSc
A Larkin, BSc
S Dennan, BRad, MSc
and
G O'Reilly, MSc, PhD
St. James's Hospital, Dublin 8, Ireland
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Abstract
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Improper automatic exposure control (AEC) termination may result in high overexposures on some radiographic systems. Under AEC, X-ray factors are adjusted automatically to compensate for differences in patient thickness and density. In radiography, AEC is implemented using ionization chambers placed in the film bucky. In this study we deliberately chose incorrect set-up conditions and assessed the response of the AEC system. Two types of incorrect set-up were studied: (1) incorrect selection of bucky radiation detector and (2) simulated misalignment between the X-ray field and light field. The systems tested varied in age from 1 year to in excess of 10 years. In the first test, overexposures of 90 mGy were recorded. Two systems did not meet EC guidelines for improper AEC termination. The second test, misalignment of the X-ray field, was observed to affect the exposure delivered by approximately ±22%. The maximum dose increase observed, with a chest phantom in the beam, was 165 µGy. Misalignments also resulted in reduced exposures, which may impact on image quality.
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Introduction
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The purpose of this study was to assess the impact of set-up errors on automatic exposure control (AEC) controlled exposures in plain film radiographic X-ray systems. AEC is a radiographic density control device that terminates the exposure when a predetermined amount of radiation is detected [1]. The AEC loop automatically controls the output of the high voltage generator and is used to regulate film density and hence image quality during radiographic imaging. While AEC is an efficient method of obtaining homogeneous image quality, it may result in increased dose under certain circumstances. The variation and complexity of AEC facilities, even between systems supplied by the same manufacturer, can give rise to incorrect operation [2]. Increased X-ray exposures due to technical faults in AEC systems have been documented by Eder [3].
Our objective was to assess the effect on exposures when the AEC set-up is incorrect. Two types of incorrect set-up were studied: (1) incorrect selection of bucky and (2) minor misalignment between X-ray field and film bucky, simulating X-ray field and light field misalignment. In the systems studied the operator aligns the tube and bucky detectors manually. Selection of the bucky detector to be monitored is also made manually.
As the first test may prevent normal termination of AEC controlled exposures, we assessed whether European Guidelines for dose termination in the event of improper AEC operation were met [4]. These guidelines state that the maximal focal spot charge should be less than 600 mAs, and the exposure time for a single exposure should be limited to a maximum of 6 s [4].
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Materials and methods
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It was verified that all systems were within tolerance as recommended in the guidelines of the Institute of Physics and Engineering in Medicine (IPEM) and National Council on Radiation Protection (NCRP) [5–7], before the tests were performed.
Incorrect bucky selection
The X-ray tube was positioned over the table, but the radiation detectors in the chest bucky were selected. The X-ray tube was warmed up before testing commenced. Dose measurements, indicating radiation exposure at a point, were taken at 100 cm focus to chamber distance (FCD), with a calibrated Radcal Radiation output meter (Model number 9010; Radcal, Monrovia, CA) using the 6 cm3 chamber. The meter was within calibration. Times and mAs supplier values displayed by the control panel were also noted. Dose–area product (DAP) meter readings were taken if available. 60 kVp was selected, and considerable time was allowed between successive exposures as a precaution against tube failure. The effect of varying tube potential was also investigated. Four general radiographic systems were tested. Details of the systems are shown in Table 1
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X-ray field misalignments
The second part of the experiment measured the effect on exposure when the correct bucky was selected, but the X-ray field was incorrectly aligned. In order to simulate X-ray to light field misalignment, we deliberately offset the light field to film alignment by 1 cm and 2 cm. Otherwise, the set-up was based on local protocols for chest film, which are consistent with European guidelines [8]. Offsets were in the horizontal (i.e. left and right) and vertical (i.e. up and down) directions from the centre point. Measurements were taken in both the lateral and anterior–posterior (AP) positions. For the lateral set up the centre radiation detector was selected. For the AP set-up both the left and right radiation detectors were selected. The set-up was examined with both homogeneous material (polymethylmethacrylate, PMMA) in the beam (15 cm water equivalent material in three blocks with dimensions of 34.5 cm x 34.5 cm x 5 cm) and inhomogeneous material in the beam (chest phantom, approximate dimensions 46 cm x 32 cm x 16 cm). The chest phantom gives an indication of performance when varying anatomical features are in the beam. The water phantom removes this variable and measures performance simply as a result of misalignment over bucky ionization chambers. The experiment was performed on three machines (Super 50 CP, Super 80 CP and Polydoros).
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Results
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Incorrect bucky selection
All systems allowed exposures with an incorrect bucky selected and both film buckys readied (i.e. film bucky returned to position). Table 2 summarizes doses obtained when the incorrect bucky was selected. It is noted whether systems comply with the European Commission Radiation Protection Report no. 91 [4]. The exposures recorded in Table 2 resulted from a setting of 60 kVp. The actual exposure obtained when the incorrect radiation detectors are selected will depend on system design and the tube potential selected. For example a measurement on one system (the Super 80 CP) at 109 kV produced factors of 850 mAs, 2.56 s and an exposure of 89.2 mGy. Dose/mAs values for this system, when not using AEC, were 0.034 mGy mAs–1 at 60 kVp, and 0.11 mGy mAs–1 at 109 kVp, giving predicted exposure values (for a 850 mAs exposure) of 28.9 mGy and 93.5 mGy, respectively. However, under AEC, with incorrect bucky selected, and at lower tube potential settings, the system increases tube potential during the exposure in an attempt to obtain signal. Thus, it is only at high kVp settings that dose/mAs data reliably predicts exposure. From our data, maximum predicted exposure on this system, based on 125 kVp and incorrect detector selection, is 120 mGy.
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Table 2. Exposure values at 60 kVp, with automatic exposure control enabled, but when incorrect bucky (radiation detectors) selected
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X-ray field misalignments
X-ray field offsets on the water phantom caused exposure variations of –6% to +16% from mean dose at correct alignment, with the maximum increase of 34 µGy for a 2 cm offset. Exposure variations on the chest phantom ranged from –22% to +22% from mean dose at correct alignment. The greatest variations were recorded in the lateral set-up, and the maximum dose increase was 165 µGy.
The maximum exposure increase from mean dose at correct alignment for a 1 cm misalignment on either the water phantom or the chest phantom was less than 10%.
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Discussion
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Incorrect bucky selection
Radiographic systems are operated by highly trained, skilled radiographers, which significantly reduces the possibility of set-up error. This study investigates the effect upon radiation patient dose of a single step set-up error or a machine failure.
Incorrect selection of bucky bypasses the dose termination mechanism, and thus has the potential to deliver considerable dose. Two of the systems tested did not meet European Dose Guidelines for incorrect termination of AEC [4]. The third system, the Shimadzu HD 150 (Duisburg, Germany), terminated exposure at 577 mAs. The Siemens Polydoros (Munich, Germany) terminated exposure after 96 ms thus providing a much safer response than the three other systems.
Draft UK Guidelines for equipment used in connection with medical exposure [9] note that if failure of a single component can give rise to an unintended exposure to the patient, the employer may need to have additional controls in place. The set-up error described here is the result of a single step error; although the bucky must be readied, this is a standard condition as the bucky is returned to position after a film is removed, so that the only requirement for exposure error is simply pressing an incorrect button on the control panel. Thus for systems of this type, additional controls when using AEC might be considered desirable or even necessary.
The 600 mAs limit in the European Commission Radiation Protection Report no. 91 [4] is considerably higher than the factors set for many examinations. For example chest X-rays are often performed at 10 mAs or less. High mAs values are only required for a limited number of examinations and most exposures are based on values of <100 mAs. Arguably a limit of 600 mAs is not justified, and system designs should be reviewed to ensure termination at a much lower value in the event of a problem. This could easily be achieved by monitoring the dose at the radiation detectors and terminating if no signal is observed in a short period – a mechanism of this type is employed in the Siemens system (the only modern system tested), which terminated within 100 ms.
We argue that, given the additional exposure incurred as a result of a simple one-step-error on some X-ray designs, it is important that radiographic staff are aware of the potential for overexposure on each of the X-ray systems in their hospital. Exposure in the event of incorrect AEC termination should be identified for each X-ray system, as designs vary. Our results show that not all systems conform to the European guidelines [4]. Furthermore we argue that the limits in Report no. 91 [4] are high, and that operation within its limits should not automatically be taken as sufficient guarantee against overexposure.
Misalignment of X-ray field
X-ray field misalignment did not produce the dramatic overexposures observed in the case of incorrect bucky selection. The maximum error occurred with a 2 cm misalignment. Maximum dose increase was observed with the chest phantom in the beam, where anatomical variations compound the misalignment problem. The water phantom measurements give an indication of relative performance of the ionization chambers in the bucky, and the effect of misaligning the field over the chambers. Field sizes appropriate to chest X-ray examinations were employed in this study. Significant exposure variation is more likely for small fields. However, small fields are normally used for extremities, and AEC is not selected.
1 cm errors are more likely as this is the tolerance for X-ray field to light field alignment [5–7]. For all 1 cm misalignments dose variations were less than 10%.
Radiographic systems generally operate in high patient throughput areas. In our example patient throughput figures for the whole radiographic area per year are approximately 90 000. Thus any systematic increase in dose may be considered significant in the context of an increase in population dose. Reductions in exposures were also noted, which may have an impact on image quality.
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Conclusions
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The main overexposure hazard observed was due to incorrect bucky selection. Systems vary in their response to incorrect AEC termination and users should be aware of the exposure hazard for each system. This is important even if systems meet the European guidelines [1], as we argue that the thresholds in these guidelines are high.
Minor misalignment of the X-ray field did not have a large impact on exposure, with 1 cm misalignments producing exposure variations of <10%.
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Acknowledgments
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The authors wish to acknowledge the advice and assistance of the radiographers in the Diagnostic Imaging Department of St. James's Hospital, particularly Ms B Moran, Ms F Cashen and Ms L Kenny.
Received for publication November 6, 2003.
Revision received April 26, 2004.
Accepted for publication June 16, 2004.
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References
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- Sterling S. Automatic exposure control: a primer. Radiological Technology 1998;59:421–7.
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- Eder H. Erhohte Strahlenexposition von Patienten Infolge Geratebedingter Storfalle. Bayerisches Landesamt fur Arbeitsicherheit, Arbeitsmedizin und Sicherheitstechnik. www.lfas.bayern.de/publ/roentg/sb_roentgen.htm, 2001.
- European Commission. Criteria for acceptability of radiological (including radiotherapy) and nuclear installations. Radiation Protection Report No.91, 1997.
- Institute of Physics and Engineering in Medicine (IPEM). Measurement of the performance characteristics of diagnostic X-ray systems used in medicine, part 1, X-ray tubes and generators, Report no. 32, (2nd edn). York, UK: IPEM, 1995.
- IPEM. Recommended standards for the routine performance testing of diagnostic X-ray imaging systems, Report no. 77. York, UK: IPEM, 1997.
- National Council on Radiation Protection (NCRP). Quality assurance for diagnostic imaging, Report no. 99. Bethesda, MD: NCRP, 1997.
- European Guidelines on Quality Criteria for Diagnostic Radiographic Images, EUR 16260, CEC publication, 1996.
- Equipment used in connection with Medical Exposure, 2003, http://www.hse.gov.uk/hthdir/noframes/draft226.pdf
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Abstract
Introduction
