Variation of patient dose in head CT

CT dose varies with both equipment related and operator dependent factors. Thermoluminescence dosimetry (TLD) was employed in two phantoms to investig ate the variation in absorbed dose for head CT scans, using a cylindrical h ead CT dose phantom. Dose profiles were plotted and the computed tomography dose index (CTDI) calculated for a single 10 mm thick slice on 14 CT scann ers. An anthropomorphic head phantom was also scanned from the base-of-skul l to the vertex using 10/10 mm slices. The absorbed dose measured at the ce ntre of the scan series is reported (D-mid) The mean CTDIW for the 14 scann ers was 60.0 mGy, while the mean D-mid was 45.8 mGy. D-mid better represent s the absorbed dose in human tissues. The CTDIW and D-mid normalized to mAs varied by up to a factor of 2.2 for the different scanners. Equipment rela ted factors contribute to such variations. However, variations due to opera tor dependent factors such as the choice of exposure factors, scanning prot ocol and positioning technique must also be considered. When such factors a re taken into account the absorbed dose received by the patient can vary co nsiderably, by its much as 16.2 for lens dose. Increased awareness of the f actors influencing CT dose and the standardization of scanning protocols is recommended.