Abdominal CT during pregnancy: a phantom study on the effect of patient centring on conceptus radiation dose and image quality

To investigate the effect of patient centring on conceptus radiation dose and image quality in abdominal CT during pregnancy. Three anthropomorphic phantoms that represent a pregnant woman at the three trimesters of gestation were subjected to a routine abdominal CT. Examinations were performed with fixed mAs (mAs_f) and with the automatic exposure control system (AEC) activated. The percent reduction between mAs_f and modulated mAs (mAs_mod) was calculated. Conceptus dose (D_c) was measured using thermoluminescent dosimeters. To study the effect of misplacement of pregnant women on D_c, each phantom was positioned at various locations relative to gantry isocentre. Image quality was evaluated on the basis of image noise, signal-to-noise ratio, and contrast-to-noise ratio. The maximum reduction between mAs_f and mAs_mod was 59.8 %, while the corresponding D_C reduction was 59.3 %. D_C was found to decrease by up to 25 % and 7.9 % for phantom locations below and above the isocentre, respectively. Image quality deteriorated when AEC was activated, and it was progressively improved from lower to higher than the isocentre locations. Centring errors do not result in an increase in D_c. To maintain image quality, accurate centring is required. (orig.)