by Teresa Victoria MD, Children’s Hospital of Philadelphia
Congenital skeletal abnormalities are a group of rare abnormalities of the fetus that affect bone growth and development. The prenatal diagnosis of these entities is challenging because of the relative rarity of each skeletal dysplasia, the multitude of differential diagnoses encountered when the bony abnormalities are identified, lack of precise molecular diagnosis and the fact that many of these disorders have overlapping features and marked phenotypic variability. In addition, our main imaging modality in the obstetric world, ultrasound, only has 40-60% sensitivity in the diagnosis of such malformations, and prenatal MR has not been show to shed significant light in the evaluation of these abnormalities.
Prenatal low-dose CT is an imaging modality that emerged to better evaluate these entities. This imaging study is ONLY done during the second and third trimester of gestation, in the fetus with severe skeletal abnormalities, when the diagnosis is still in question after performing an ultrasound. It is also done at low dose such that the fetal skeleton is well seen, but the fetal body is not.
The main risk to this examination is the radiation dose. We aim to keep it as low as reasonably achievable. Our mean radiation dose is <5 mSv. To put things in context, the American College of Radiologists describes the suspected in-utero deterministic radiation dose of <50 mSv as negligible. Note that this study is only done in a highly selective group of fetuses with severe skeletal abnormalities, as discussed above.
Once the patient is on the CT table, the top and bottom of the uterus are sonographically marked with radiopaque markers. The topogram, which only includes the maternal abdomen between the markers, confirms fetal position. Our protocol for this unenhanced CT is: 80-100 kVp, modulated mAs, pitch 1.2 mm, slice thickness 1.5 mm on a 1.2 mm detector, rotation time 0.5 sec. Images are then evaluated in a 3D console, where the maternal abdomen can be selectively excluded and the fetal skeleton can be reconstructed in 3D for complete evaluation.
Although this is a radiology exam, we usually do our image interpretation in conjunction with the obstetricians, geneticists, and when appropriate, orthopedic colleagues, and then reach a team-approach diagnosis.
Evaluation of bone mineralization requires a learning curve. We found the fetal atlas of Schumacher et al. , which shows postmorten radiographs of normal fetuses up to a gestational age of 23 weeks, valuable. The reconstructed fetal skeleton is then evaluated as expected for these group of diseases, including description of shape of skull, ribs, vertebral bodies, and pelvis, and full evaluation for the presence of fractures, bone bowing and segmentation anomalies. Referral to the always helpful Taybi and Lachmans’s “Radiology of Syndromes, Metabolic Disorders and Skeletal Dysplasias”  is strongly encouraged.
In summary, low-dose fetal CT affords exquisite detail of the fetal bones, allowing improved prenatal diagnosis, parental counseling and predelivery planning in a selective group of fetuses.
- Schumacher R, Spranger JW, Seaver LH. Fetal radiology: a diagnostic atlas. Berlin; New York: Springer, 2004: viii, 194 p.
- Lachman RS, Taybi H. Taybi and Lachman's radiology of syndromes, metabolic disorders, and skeletal dysplasias, 5th ed. Philadelphia: Mosby Elsevier, 2007: xxiii, 1365 p.