Computed Tomography (CT) was introduced into clinical practice in 1972 and revolutionised x-ray imaging by providing high quality images which reproduced transverse cross sections of the body. The technique offered in particular improved low contrast resolution for better visualization of soft tissue, but with relatively high absorbed radiation dose. The initial potential of the imaging modality has been realised by rapid technological developments, resulting in a continuing expansion of CT practice. As a result, the numbers of examinations are increasing to the extent that CT has made a substantial impact on not only patient care but also patient and population exposure from medical x-rays. Today it accounts for up to 40% of the resultant collective dose from diagnostic radiology in some countries of the European Union (EU) (1,2). Special measures are consequently required to ensure optimisation of performance in CT, and of patient protection.

In comparison with conventional radiology, the relative complexity, range and flexibility of scanner settings in CT may adversely affect the levels of image quality and patient dose achieved in practice. There is, therefore, a need to establish quality criteria for CT which will provide the required clinical information in its optimal form, with minimum dose to the patient.

The quality criteria concept, as developed for conventional x-ray examinations of adult and paediatric patients by the European Commission's (EC) research actions, has proved to be an effective method for optimising the use of ionising radiation in medical imaging procedures. The purpose of quality criteria for CT was therefore also to provide an operational framework for radiation protection initiatives for this modality, in which technical parameters required for image quality are considered in relation to patient dose.

CT continues to evolve and the research base for guidance is limited. The study group on "Development of Quality Criteria for CT" has drawn extensively on the results of the projects carried out in the EC's Research Action on Optimisation of Radiation Protection of the Patient. It has also gained inspiration from the guidelines of the German Federal Chamber of Physicians on Quality Assurance in Computed Tomography (3). The primary working document of April 1997 has been commented on by external experts from countries in Europe and was presented at the EC workshop on reference dose and quality in medical imaging, October 1997, Luxembourg (4). A revised document dated May 1998 was posted on the Internet (http://www.drs.dk/CT/document/) and advertised to all national delegates of the European Association of Radiology (EAR) and the European Federation of Organisations for Medical Physics (EFOMP), in addition to the European National Boards of Health and the associations of radiographers. A notification was also given to these bodies of a Workshop on Quality Criteria for Computed Tomography that was held in Aarhus, Denmark, 13-14th November 1998. The document was open for discussion at the workshop and consequently revised to the present final guidelines. Furthermore, the study group performed a pilot study in 1997-1998 to test the image quality criteria, with simultaneous registration of the radiation dose, for five types of examination: 1) face and sinuses, 2) vertebral trauma, 3) HRCT of the lung, 4) liver and spleen and 5) osseous pelvis (5). The results have been taken into account in the final guidelines, including the specification of diagnostic reference dose values.

These guidelines on Quality Criteria for Computed Tomography provide guidance on the definition and introduction of quality criteria for diagnostic images and equipment performance, as well as for dose to the patient. The report contains four chapters.

The first chapter presents general principles associated with good imaging technique and lists the Quality Criteria for six groups of CT examination: cranium, face and neck, spine, chest, abdomen and pelvis, and bones and joints. Each group of examinations is subdivided into the most common examinations of specific organs or parts of the body. The chapter defines Diagnostic Requirements by specifying anatomical image criteria; indicates Criteria for the Radiation Dose to the Patient; and gives Examples of Good Imaging Technique by which the Diagnostic Requirements and Dose Criteria can be achieved.

The second chapter summarises available research results as well as the ongoing experiments which have supported the establishment of the Quality Criteria listed in Chapter 1, and suggests directions for future research.

The third chapter outlines a procedure for implementing and auditing the Quality Criteria and a model for image quality assessment.

The fourth chapter contains a glossary of terms used in the guidelines.

This initiative in CT will continue within the framework of forthcoming research programmes and is reflected by the Council Directive on health protection of individuals against dangers of ionising radiation in relation to medical exposure (6). For techniques such as CT the new Directive requires the establishment of quality assurance measures which include criteria that can be employed and checked in a comparable way so that the radiation dose to the patient can be linked to the required image quality and to the performance of the chosen technique.

Emerging techniques such as multislice CT and fluoro-CT have not been specifically addressed. With the continuing evolution of CT technology there will be a need for regular updating of the guidelines.

It is the hope of the European Commission's services that the elaboration of the Quality Criteria for CT will stimulate the professionals concerned to look for improvements in the criteria in such a way that day-to-day practice achieves optimal diagnostic information and fulfils at the same time the requirements for optimization of radiation protection in the 1997 Council Directive.