Priorities in optimisation of medical X-ray imaging--a contribution to the debate

A simplistic approach to optimising medical imaging is to use the lowest effective dose to the patient that does not jeopardise a correct diagnosis. With limited resources and over 1000 different types of X-ray examinations, it is not always easy to set the right priorities and to decide how to perform the optimisation. Recent research shows that the 'Rose model' for the detection of specific structures does not hold for realistic backgrounds. A reasonable conclusion regarding methods for optimisation is therefore not to use contrast-detail phantoms. Phantoms producing clinically realistic background images or real clinical images-modified with respect to quantum noise levels-are preferred. The images should be evaluated using visual grading or receiver operating characteristic methods. The quality of many common X-ray investigations, performed with projection techniques, is not limited by quantum noise. For these, the radiation dose to the patient can be lowered without seriously affecting the outcome of the detection task. For computed tomography (CT) investigations, the obscuring effect of anatomical structures and anatomical noise is less pronounced than in projection techniques. For CT, true optimisation in terms of a trade-off between radiation dose and image quality is therefore more likely to be effective. Both the number of CT examinations performed per year and the effective dose per examination are increasing owing to the technical advances in CT--jointly leading to a steady increase in the collective dose from CT examinations. Moreover, the smaller influence of the anatomical background in CT gives a high correlation between detection tasks and radiation dose. Thus, a reasonable view to take on which examinations to optimise is to give priority to CT examinations. The recommended distribution of a full working week for optimisation, based on the relative lifetime risk of lethal cancer from diagnostic X rays and the total collective dose from CT, is to use three out of five days to optimise CT examinations, of which one day should be devoted to paediatric CT.