16-slice CT: achievable effective doses of common protocols in comparison with recent CT dose surveys

The aim of the study was to investigate achievable dose levels in 16-slice CT by evaluating CT dose indices (CTDI) and effective doses of dose-optimized protocols compared with 4-slice dose surveys. Normalized CTDI free in air and in 16 cm and 32 cm diameter phantoms were measured on four different 16-slice CT scanners in the Netherlands. All collimation and tube potential settings were analysed. Volume CTDI was calculated for adult protocols for brain, chest, pulmonary angiography (CTPA), abdomen and biphasic liver CT. Effective doses were calculated first using volume CTDI with conversion factors and second from CTDI air values using the ImPACT dose calculator. Average results of the 16-slice scanners were correlated to results of dose surveys with predominantly 4-slice scanners. Statistical analysis was done with Student t-tests with a Bonferroni correction; therefore p < 0.017 was significant. The results of CTDlair and weighted CTDI were documented for all scanners. Effective doses averaged over four scanners for brain, chest, CTPA, abdomen and biphasic liver protocols were 1.9 +/- 0.4, 3.8 +/- 0.4, 3.0 +/- 0.2, 7.2 +/- 0.9 and 10.2 +/- 1.3 mSv, respectively. Compared with dose surveys achievable effective doses were equal (p=0.069) to significantly lower (p < 0.017) for chest and abdomen protocols. For 16-slice spiral brain CT there was a trend of equal doses compared with sequential brain CT in the dose surveys. Thus, with dose-optimized protocols 16-slice CT can achieve equal to lower effective doses in examinations of the chest and abdomen compared with 4-slice CT, while doses can remain stable in the brain.