Meetings » 5th CTUG Meeting » Findings from a CT dose survey

Findings from a CT dose survey

Sarah Morris and Lesley Leavesley

Southampton General Hospital


Aim: To determine and analyse doses delivered during frequent scans performed on six CT scanners within the catchment area served by Southampton Radiological Physics Group. Comparisons were to be made between the centres in the study; with European reference levels and with previous results obtained in 2001.

Method: Exposure factors were determined for frequent examinations, where 'frequent' has been defined as more than 10 per month. To obtain the information needed radiographers were interviewed and images were analysed on remote workstations. Measurements of dose were made for each of the scanners in either a head or body sized Perspex phantom, using a 10cc, 10cm long ionisation chamber calibrated in terms of absorbed dose in air. Calculations of CTDIw were made for a combination of exposure factors suited to the individual centres. In each case kilovoltage, mAs and slice thickness were varied. Effective dose and dose length product were calculated using the ImPACT CT dosimetry calculator.

Results: The scanners in the survey varied in age and design. For example a multislice scanner was included as well as a single slice scanner employing gas filled detectors. Despite the poorer efficiency of gas filled detectors the doses delivered were never greater then 50% of the European reference levels (ERL's) and were comparable to doses delivered in the other centres in the study. There was a vast range in the percentage of ERL's delivered, from 4% to 82%. 4% was attributable to the three-year-old multislice scanner in the study whilst 82% was attributable to a 10-year-old scanner in continuous use taking emergency cases.

Conclusions: Reference levels are a good way of ensuring doses do not creep up over time however the results of this study suggest that the European reference levels are too high to be useful. Therefore setting local reference levels that take into consideration local factors such as the age of the scanner; whether it is used for routine cases or emergencies; whether protocols are rigidly followed by the consultants or not, could prove to be a more helpful.

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