Analysis of CT Hounsfield units for fetal imaging: Basic data for creating dosimetry phantoms of pregnant women

Purpose: The computed tomography (CT) numbers of key organs and biological structures are important for phantom production. This study aimed to provide basic data on the CT numbers of actual organs, including the placenta, amniotic fluid, fetal bones, and fetal organs. Additionally, the abdominal circumference of pregnant women and skin surface-to-fetus distance were measured. Methods: Maternal and fetal anatomical data were obtained from the imaging data of 14 pregnant patients who had previously undergone abdominal and pelvic CT examinations. All anatomical measurements were performed on a workstation using the CT images of each patient. The imaging data of the 14 pregnant patients did not include repeated data from the same individuals. Results: For each organ, the average CT numbers did not significantly differ across tube voltage or gestational age groups. The average CT numbers of the fetal brain, lungs, liver, and caput femoris were 22.2, 28.9, 55.3, and 504.9 HU, respectively. The average values for the fetal depth and maternal abdominal circumference were 28.0 and 861.5 mm, respectively. Conclusions: The CT numbers of the placenta, amniotic fluid, and fetal organs are similar to those of adult soft tissues and should be represented by equivalent materials. However, the fetal lung phantom should not be made of the same material used for adult lung phantoms; instead, it should consist of materials simulating soft tissue. Because the position of the fetus varies among individuals, determining the measurement point inside the phantom requires a customizable design, particularly for pregnant women with minimal subcutaneous fat.