Fetal CT - General Information

Indications: 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

1. Exposure of the fetus to radiation
2. Limited soft tissue detail fetus


 


 

 

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. 

Regarding the radiation dose effect on the conceptus, there is a robust body of knowledge on the effects of radiation derived mostly from the sequelae of the atomic bombings at the end of World War II and from children whose mothers received medical irradiation (diagnostic or therapeutic) during pregnancy. The American College of Radiology, in its guidelines for imaging of the pregnant patient, summarizes the suspected in utero deterministic radiation effects of radiation at a dose of less than 50 mSv as none. It is postulated that the absolute risk of fetal effects, including fetal cancer induction, are small at a conceptus dose of 100 mSv and negligible at doses of less than 50 mSv. Assuming a normal 0.07% incidence of childhood cancer and a natural incidence of malformation risk on the order of 4% in the healthy population, the risk to a conceptus given a fetal dose of approximately 50 mSv results in a 95.999% probability of birth of a child with no malformation, or a 99.926% chance that the child will not have cancer, or a 95.928% chance that the child will have no malformation and no cancer. The American College of Radiology stipulates that a dose of 20 mGy represents an additional projected lifetime risk of approximately 40 additional cases of cancer or fewer per 5000 infants, or approximately 0.8%. Despite this, no radiation risk is acceptable for any fetus unless there are strong reasons to perform the diagnostic study.

At CHOP, when a mother is counseled before undergoing fetal CT, she is told that the aim of the study is to keep the radiation dose to the fetus at a minimum, usually approximately 5 mSv. To place this information in context, she is told that a dose of 5 mSv is the upper limit of what is acceptable for a woman in the medical profession during the course of the pregnancy and that  3.6 mSv is the radiation dose estimated to be background as measured in Colorado. 

When CT is performed for appropriate indications and with proper technical parameters and, most important, with the knowledge that fetuses and children are far more sensitive to radiation than adults are, the benefits of low-dose fetal CT may far exceed the relatively small, yet real, individual risk.

Recommended Readings

Use of low dose computed tomography with 3D reconstructions for the prenatal evaluation of suspected skeletal dysplasia.

Adler-Levy Y, Yagel S, Nadjari M, Bar-ziv Y, Simanovsky N, Hiller N.

Isr Med Assoc J. 2015 Jan;17(1):42-6.

 

Radiation dose reduction at MDCT with iterative reconstruction for prenatal diagnosis of skeletal dysplasia: preliminary study using normal fetal specimens.

Tani C, Funama Y, Fujioka C, Nakamura Y, Komoto D, Aoyama H, Awai K.

AJR Am J Roentgenol. 2014 Dec;203(6):1249-56. doi: 10.2214/AJR.13.11578.

 

Low-dose fetal CT in the prenatal evaluation of skeletal dysplasias and other severe skeletal abnormalities.

Victoria T, Epelman M, Coleman BG, Horii S, Oliver ER, Mahboubi S, Khalek N, Kasperski S, Edgar JC, Jaramillo D.

AJR Am J Roentgenol. 2013 May;200(5):989-1000. doi: 10.2214/AJR.12.9722.

 

Three-dimensional helical computed tomography in prenatal diagnosis of fetal skeletal dysplasia.

Macé G, Sonigo P, Cormier-Daire V, Aubry MC, Martinovic J, Elie C, Gonzales M, Carbonne B, Dumez Y, Le Merrer M, Brunelle F, Benachi A.

Ultrasound Obstet Gynecol. 2013 Aug;42(2):161-8. doi: 10.1002/uog.12298. Epub 2013 Jul 14.

 

Prenatal diagnosis of fetal skeletal dysplasia with 3D CT.

Miyazaki O, Nishimura G, Sago H, Horiuchi T, Hayashi S, Kosaki R.

Pediatr Radiol. 2012 Jul;42(7):842-52. doi: 10.1007/s00247-012-2381-7. Epub 2012 Apr 25.

 

Low-dose fetal CT for evaluation of severe congenital skeletal anomalies: preliminary experience.

Victoria T, Epelman M, Bebbington M, Johnson AM, Kramer S, Wilson RD, Jaramillo D.

Pediatr Radiol. 2012 Jan;42 Suppl 1:S142-9. doi: 10.1007/s00247-011-2175-3. Epub 2012 Mar 6.

 

Prenatal diagnosis of skeletal dysplasias: contribution of three-dimensional computed tomography.

Ulla M, Aiello H, Cobos MP, Orioli I, García-Mónaco R, Etchegaray A, Igarzábal ML, Otaño L.

Fetal Diagn Ther. 2011;29(3):238-47. doi: 10.1159/000322212. Epub 2011 Jan 4.

 

Fetal Imaging Index

Fetal CT Image

Protocols

Technique

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.

Image Interpretation

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. [1], 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” [2] 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.

 

References

  • 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.