CT Users Group

P C Shrimpton

Medical Dosimetry Group, National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK

Abstract

The concept of reference doses is recognised as a useful and practical way of promoting optimisation of patient protection for diagnostic medical exposures. A robust methodology has been developed for establishing reference doses for the peculiar conditions of exposure in CT. This initiative represents an essential component of the European Guidelines on Quality Criteria for Computed Tomography published by the European Commission (EUR 16262, 1999) and is available on the Internet (http://www.drs.dk/guidelines/ct/quality).

The reference dosimetry is based on measurements in standard head and body CT dosimetry phantoms of the computed tomography dose index made with a pencil ionisation chamber (CTDI₁₀₀) and expressed in terms of absorbed dose to air. Appropriate combination of measurements made centrally and peripherally yields the weighted CTDI (CTDI_w, mGy). This characterises the conditions of exposure for a single slice in serial scanning or per rotation in helical scanning, while the dose-length product (DLP, mGy.cm) takes due account of the scope of serial or helical scanning in a complete examination.

Reference dose values for these two quantities are intended to represent thresholds to trigger investigations by radiology departments where typical practice, as indicated by mean values of the dose descriptors observed for representative groups of patients, is likely to be well away from the optimum. Initial reference dose values for some common procedures on adult patients have been set as the third quartiles of survey data from the UK and from a European pilot study of the quality criteria.

This practical system of dosimetry is also being further developed to allow comparison of dose during critical review of CT practice involving paediatric patients.

In order to provide support to the wide-scale implementation of reference dosimetry for CT, a database is being established of standard dosimetric data for a wide range of scanners. This will include normalised dose data (mGy.(mAs)^-1) from measurements of CTDI in air and in the standard dosimetry phantoms for different models of CT scanner. The database will be accessible via the Internet at : http://www.efomp.org.

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