Reference doses in computed tomography

Computed tomography (CT) is firmly established as a major source of population exposure from diagnostic X ray examinations. National surveys have highlighted significant variations in dose between individual scanners and potential scope for improvement in practice. Reference doses are already recognised as a practical way of promoting optimisation of patient protection and accordingly this concept has been elaborated for CT by the European Commission in Working Document EUR 16262 (1997) on Quality Criteria for CT. Two reference dose quantities have been defined on the basis of measurements of the computed tomography dose index (CTDI) made with a pencil ionisation chamber in standard head or body CT dosimetry phantoms: weighted CTDI for a single slice and, with due account of the thickness and number of slices in a complete examination, dose-length product. Pending trials of the quality criteria, some initial reference doses have been proposed using survey data from the UK for 1989.