Purpose or Case Report: Fetal CT has almost the same utility as a postnatal skeletal survey. Despite this benefit, the associated radiation exposure is disadvantageous and radiation dose reduction is mandatory. It is however impossible to measure the actual radiation dose to the fetus directly. Several previous reports have described the CT dose index (CTDI) volume and dose length product (DLP) as representing an imagined fetal dose. The actual fetal radiation dose needs to be confirmed using a phantom that practically corresponds to a pregnant woman.
Methods & Materials: We created a custom-made phantom that corresponds to a pregnant woman, using acrylic resin (Polymethyl methacrylate: PMMA; 340 (Width) x 260 (Height) x 300 (Length)). It could contained three artificial fetal skeletons with different densities of calcium fluoride (300 HU: 1.90 mol/L, 500 HU: 3.63 mol/L, 700 HU: 5.37 mol/L). Dose measurements were obtained using four CT scanners (GE, TOSHIBA, SIEMENS, PHILIPS) at three different institutions with the same scan parameters (CTDIvol: 3.0 mGy; tube voltage: 80, 100, 120 kV) at four measurement points (distance from skin surface: 4 cm, 7 cm, 10 cm, and center). We compared the radiation dose for each tube voltage, at the different measurement points. The differences in dose between CTDIvol and measured data were assessed.
Results: The mean measured dose was 2.3 mGy at the center, and 3.73 mGy, 4 cm from the skin surface, across all vendors and tube voltages, corresponding to doses between 76% and 124% of the displayed CTDIvol (3 mGy). The maximum deviation was 1.58 fold (at 120 kV, distance from skin surface: 4 cm). Doses in the peripheral part of the pelvis showed 1.5–1.8 fold greater exposure in comparison with the central position. Doses increased in proportion to tube voltage settings and mean doses at 120 kV were 1.03–1.18 fold greater than at 80 kV. There were discrepancies in the measured dose among four CT scanners that ranged from 61 to 120% (around the mean of 120 kV).
Conclusions: Fetuses undergoing CT for suspected skeletal dysplasia may be exposed to approximately 0.8–1.2 times the displayed CTDIvol on the CT console. We suggest that the CTDIvol roughly represents fetal dose. There is however heterogeneity and the wide range of fetal doses depends on the position of the fetus, the selection of tube voltage, and CT units. As pediatric radiologists, we should be aware of these characteristics so that we can prevent excess radiation.