CT-Expositionspraxis in der Bundesrepublik Deutschland

In 1999, a nation-wide survey on CT practice in Germany was conducted in a collaborative study by the German Roentgen Society (DRG) and the Association of Manufacturers of Electromedical Equipment (ZVEI). The survey was based on questionnaires sent to all users of CT scanners in hospitals and private practices, asking for dose-relevant data on 14 standard CT examinations (scanner data, protocol settings, examination-related parameters, and frequencies of examination). With a return rate of approximately 50%, representative results were obtained for the entire spectrum of areas of application, groups of users, and scanner models. Dose values were assessed individually for each scanner from the protocol settings given in the returned questionnaires. As this study was the first large-scale survey performed since 1989, numerous trends resulting from changes in the design and the use of CT scanners, and their consequences on radiation exposure, could be deduced. The most important results of this study are: • Existing dose recommendations that were introduced in 1992 by the German Federal Chamber of Physicians in terms of the dose free-in-air on the axis of rotation (a maximum of 50 mGy) are often exceeded and are met on average only in examinations of the chest region and in CTA. This is mainly caused by the choice of an inadequate dose quantity for the recommendations which overly handicaps the users of certain scanner models. • The mean effective dose per examination amounts to approximately 8 mSv. • Effective doses are significantly higher than 10 years ago (NRPB survey in the UK) for most examinations, except for those of the chest and spine. This particularly holds true for general scans of the abdomen, where the effective dose has increased by a factor of three. • Increases in effective dose are primarily a consequence of improved tube loading capacity and shorter rotation times, which allow not only the use of higher dose settings per scan and the scanning of larger volumes but also new indications and multiphasic examinations. • A comparison with the results from a projection made by the German Federal Bureau on Radiation Protection in 1990 – 92, however, reveals a 30% reduction in effective dose per examination. This reduction is mostly due to the introduction of spiral CT which allows to perform scans with increased pitch settings. • Significantly increased dose values are regularly found for scanners that were installed in the period 1988 1992. These models are characterized by shorter geometries, reduced beam filtration and lack of spiral CT capability. • The frequency of annual CT examinations increased by 60% compared to 1990/92 and amounts now to 90 examinations per 1000 inhabitants per year. • Provided that the contributions from other modalities have not changed significantly since 1990/92, the contribution of CT to the collective effective dose has slightly increased from 35% to approximately 40%. Based on the results of this survey, a proposal is made how to set up reference dose values for standard CT examinations as required from the European directive issued in 1997 for the purposes of radiation protection of patients. A comparison with the proposal given in the European guidelines on quality criteria for CT reveals that these values are often not representative for the actual state of examination techniques and scanner technology. The proposal given in this report is specified in terms of weighted CTDI and dose-length product and includes pairs of dose values which indicate the range of good radiological practice.