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CT--an increasingly important slice of the medical exposure of patients.

It is now 21 years since the revolutionary imaging technique of computed tomography (CT) was introduced into clinical practice. In coming of age, CT has found increasing use in the diagnosis and assessment of cancer and other pathological conditions and has undoubtedly led to significant advances in patient care. As a measure of this success, perhaps, CT now represents a major source of exposure from diagnostic X-rays for the UK population. This importance was firmly established by a national survey relating to the 200 scanners in clinical use in 1989 (Shrimpton, 1992). Information on CT practice was provided by questionnaires completed by over 80% of National Health Service scanners (Shrimpton et al, 1991).