FETAL DOSE ESTIMATION IN ABDOMINAL AND PELVIC CT EXAMINATIONS

Abstract

Purpose: The purpose of this is study was to compare different methods for estimating the fetal dose, how much the different methods differ from each other and on which dose levels the fetal doses are estimated to. Theory: A radiological procedure should always be justified and in the case of a pregnant patient the justification should include both the mother and the fetus. The absorbed dose to the fetus should be considered in the planning procedure, which means that you need to be aware of the potential fetal dose in advance. There are some risks associated with ionizing radiation for the fetus during pregnancy. These risks can be related to which stage of the pregnancy the woman is in and to the absorbed dose of the fetus. The associated risks are the deterministic effects which have a 100-200 mGy threshold and increase with the dose, these effects include abnormalities and mental impairments, and the stochastic effects which is the increased risk of inducing cancer. The doses in radiology are under normal circumstances low, but during CT examinations there is a risk that the fetal dose increases either in a single examination or repeated examinations, and for those cases a more accurate fetal dose estimation is necessary. If the dose is high and there is a high uncertainty in the dose calculations, there is a risk that the dose might come near or exceed the threshold value. In those cases, it is important to know which dose estimation method to use. Method: To estimate the fetal dose, data from female pregnant patients CT examinations and phantom measurements were retrospectively used to estimate the fetal dose using different methods and programs. The patients were categorized into groups depending on the scan protocol and trimester, because the scan range and parameters are similar for each protocol. The different estimation methods and quantities that were used was CTDIvol, SSDE, CT-Expo, CODE and VirtualDoseCT. The methods differ in their calculations, some of them take consideration to the patient’s size and some of them the trimester or pregnancy week. The results from these methods and quantities were approximated to the fetal absorbed dose for comparison. The phantom measurement was made to compare the different dose estimation methods on the same “patient” during the same circumstances and to compare the results of fetal dose estimations from the patient measurements with a reference.