Purpose: In CT exams of pregnant women, fetus (or embryo) may be encompassed by, partly inside, or completely outside scan range. This study is to compare two fetal dose evaluation methods (Monte Carlo simulation, z-axis dose profile based direct calculation) for abdominopelvic, abdominal, and chest exams.
Methods: This work considered three whole-body models of a pregnant patient (3, 6, 9 gestational months; 62, 67, 73 kg) undergoing chest, abdominal, and abdominopelvic exams on Siemens Somatom Definition CT with fixed mA, 120 kV, 19.2-mm beam width, helical pitch at 1, body bowtie filter, and CTDIvol of 15.6 mGy. Maternal water equivalent diameter (WED) at the fetus centroid was 30.7, 35.3, and 36.3 cm, respectively. A previously validated Monte Carlo simulation program (Geant4) was used to evaluate fetal dose, where helical scan was simulated with 360 projections and 360 patient table translations per gantry rotation, each translation by pitch × (collimator width)/360. In parallel, another previously reported method was used to evaluate z-axis dose profile of a water phantom, with inputs of scan range, CTDIvol and WED. Fetal average dose was calculated over the fetal volume in the cranial-caudal direction, and was compared with that of simulation.
Results: Fetal dose from Geant4 simulation was 18.7, 18.3 and 17.7 mGy (3, 6, 9 months) in abdominopelvic exam; 0.69, 1.88 and 3.2 mGy (3, 6, 9 months) in abdominal exam; 0.10, 0.24 and 0.38 mGy (3, 6, 9 months) in chest exam. The results of profile-based evaluation were consistent with those of simulation, where their difference in magnitude was (8.8±5.9)% (maximum, 17.1%) for three exams.
Conclusion: The dose profile based fetal dose calculation is easier to implement than Monte Carlo simulation and is suited for body CT of pregnant women at all gestational ages.