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.