Patient doses from CT in New Zealand and a simple method for estimating effective dose

A 1992 survey of doses to patients from CT scanning in New Zealand found that CT contributes 81 microSv per capita per annum, representing about 17% of the collective dose from medical irradiation. Because of the significance of CT, estimates of patient dose are now included in the National Radiation Laboratory (NRL) routine surveys of CT scanners. For some models of CT scanner, normalized organ dose data from Monte Carlo studies are available, enabling the calculation of the effective dose. For scanners for which there are no normalized organ dose data, it is shown that the computed tomography dose index (CTDI), determined at 1 cm depth in Perspex phantoms and normalized for mAs, may be used to estimate the effective dose. Estimates of effective dose derived from the CTDI may then be used to formulate reference dose levels, against which individual scanner doses may be compared.