The objectives of the guidelines are to achieve:

• adequate image quality, comparable throughout Europe
• reasonably low radiation dose per examination

The guidelines also provide a basis for accurate radiological interpretation of the image.

The guidelines are directed primarily at clinical and technical staff who perform CT and report on it. They will also be of interest to those responsible for the design of CT equipment and for the maintenance of its function. They will be helpful to those who have responsibility for equipment specification and purchase.

The guidelines represent an achievable standard of good practice which may be used as a basis for further development by the radiological community.

In support of these objectives, the guidelines provide structured advice on the following key areas:

The diagnostic requirements are presented as image criteria, which in CT are basically of two different types: anatomical and physical image criteria. The anatomical image criteria include requirements which must be fulfilled when specific clinical questions are posed. These criteria may be defined in terms of visualization or critical reproduction of anatomical features (see Description of Terms, p.12). Evaluation of image quality based on anatomical criteria takes into account both the anatomy of the area under examination and the contrast between different tissues which is essential for the detection of pathological changes.

The physical image criteria are measurable by objective means. They include noise; low contrast resolution; spatial resolution; linearity; uniformity and stability of the CT numbers; slice thickness and dose. It is mandatory for departments carrying out CT to employ a suitable quality assurance programme to maintain imaging performance at optimal levels. Routine tests have to specify physical image criteria.

Consideration of dose constraint has particular importance in CT, since this is recognised as a relatively high dose modality. ICRP (1) has recommended the dose constraint concept for medical exposure, that is translated to diagnostic reference levels for diagnostic radiography (3). The application of this concept is in line with the reference dose values for a standard sized patient indicated in the previous European Guidelines (5, 6). In the present guidelines tentative reference dose values for CT have been established for selected examinations in order to facilitate comparison of examination protocols used in different departments and with different types of equipment. The reference dose values are based on dose descriptors defined in Appendix 1. More detailed discussion of dosimetry is given in Chapter 2.

Diagnostic reference dose values provide quantitative guidance to help identify relatively poor or inadequate use of the technique rather than an indication of satisfactory performance.

Further dose reduction below reference values may be achievable without compromising the diagnostic value of an individual examination, and this should always be pursued.

Image quality in CT depends primarily on two types of scan parameter: dose-related parameters and those which are related to processing and viewing of the image. Both are hardware related. Dose-related parameters are the slice thickness, inter-slice distance, pitch factor, volume of investigation, exposure factors and gantry tilt. Processing parameters are field of view, number of measurements, reconstruction matrix size, reconstruction algorithm and window settings for viewing the image. Impact of these parameters on image quality and patient dose can be assessed quantitatively by measurement with test phantoms, which provide information essential to the definition of quality criteria related to the clinical objective.