The use of the effective dose equivalent,HE, as a risk parameter in computed tomography

Conventional approaches to computed tomography (CT) dosimetry include the specification of the absorbed dose at given locations in the radiation field (Shope et al, 1982), the use of dose descriptors such as the Computed Tomography Dose Index (CTDI) (Shope et al, 1981), estimates of the total energy imparted in the CT examination (Huda, 1984) and an explicit evaluation of the somatic and genetic stochastic radiation risks associated with a given CT examination (Huda & Sandison, 1985). A problem with all these approaches is the difficulty of comparing the radiation risks associated with the CT procedure with the comparable risks associated with other diagnostic procedures that use ionising radiation. It is also difficult to compare the non-uniform radiation doses encountered in CT examinations with the whole-body dose-equivalent* limits imposed for non-medical exposures for members of the public (5 mSv/year) and for radiation workers (50 mSv/year), or with the risks from more familiar everyday activities such as smoking cigarettes and driving automobiles. This note employs the concept of the effective dose equivalent, HE, to overcome these problems. This parameter was introduced by the International Commission on Radiological Protection (ICRP, 1977a) to deal with partial-body radiation exposure of radiation workers. Simple numerical radiation risk estimates from CT examinations performed on an EMI 5005 CT scanner are used to generate corresponding HE values and this parameter is then adopted as the means for intercomparing total risk.