Radiation dose in computed tomography of the pelvis: comparison of helical and axial scanning

An anthropomorphic Rando phantom was used to compare radiation doses sustained during helical and conventional axial CT of the pelvis. The values obtained with the Rando phantom were validated against cadaveric phantoms, and show good agreement. For the authors' particular CT unit, helical scanning was found to deliver a lower radiation dose than conventional axial scanning. This was most prominent at 1.0-s tube rotation times (average dose ratio 1.24). For realistic scanning parameters and exposure factors, the ratio of radiation dose to pelvic organs can be expected to lie in the range of 40-100 mGy. The whole-body effective dose (ED) depends on selection of scanning parameters and patients anatomy. In a favourable case scenario, the ED for CT scanning of the pelvis in a male can be expected to be between 10 and 20 mSv if the scrotum is not included in the radiation field, while the ED in a female will be approximately 20 mSv. An examination of scatter radiation fall-off curves from a single slice shows that the spread of scatter radiation is only marginally affected by slice thickness. A total of 10-12 cm of human soft tissue acts as a good barrier against internal scattered radiation. The use of such scatter fall-off curves, together with manufacturers' dosimetry specifications, allows a fast estimate of absorbed dose.