Application of draft European Commission reference levels to a regional CTdose survey

A survey of CT doses in Northern Ireland in the period between October 1995 and March 1997 was carried out. The survey included all but one of the 10 scanners in use at the time, and, additionally, two others that were replac ement machines. The method used was to study standard protocols and calcula te doses to the NRPB mathematical phantom, so that a direct comparison coul d be made with other surveys carried out in a similar fashion elsewhere. Th e survey addressed the patient radiation dose but not image quality or clin ical outcomes. It is estimated that in Northern Ireland the contribution to collective dose to the population from CT is about 40% of that from all me dical X-rays. The proposed European Commission reference quantities, weight ed CT dose index and dose-length product were computed and their potential use evaluated. A full study of mean values of effective dose per examinatio n revealed the average dose per examination was not significantly different from that found in the 1989 UK survey, although several procedures gave ri se to doses that were high enough to be investigated with a view to justifi cation or reduction. One of the scanners was found to give consistently hig h doses. It is likely that a revision of the mAs values used on this scanne r will produce a significant reduction in patient doses without compromisin g image quality. When compared with the draft EC reference levels, fewer pr ocedures were found to have excessively high dose values. The proposed EC r eference levels would therefore be useful for continual monitoring of CT do se status, but do not appear to provide as comprehensive an assessment of p atient exposure as that given by consideration of effective doses.