Using Low-dose Fetal CT to Diagnose Skeletal Dysplasias

Skeletal dysplasias (SD) are a heterogeneous and complex group of conditions that affect bone and cartilage and result in abnormal shape, growth and integrity of the skeleton. Prenatal suggestion of a dysplasia typically occurs during the second trimester ultrasound, but the exact diagnosis is often difficult to confirm prenatally because of the rarity of each SD, the multitude of differential diagnoses encountered with similar boney abnormalities, the fact that many SDs have overlapping features and marked phenotypic variability, and the limited sensitivity of ultrasound.

Inspired by groups in Europe, our center began using low-dose fetal CT in July 2008 to assess severe prenatal osseous abnormalities in an effort to better evaluate these complex entities. We are one of very few centers in the United States to use this technique. In our experience, we’ve found that low-dose fetal CT depicts the fetal bones in exquisite detail and improves the final prenatal diagnosis markedly, which allows for more precise parental counseling and optimizes antepartum and intrapartum management and postnatal care.

Left: MIP rendering of a fetal CT demonstrating multiple segmentation anomalies of the thoracolumbar spine and a focal lumbosacral scoliosis. In addition, there are multiple rib anomalies including fusion and bony bridging. Collectively these findings represent spondylocostal dysostosis.

Right: 3-D volumetric rendering of a fetus demonstrating lack of ossification at the apex of the skull and an otherwise normal skeleton. These CT findings suggested a diagnosis of Adams Oliver syndrome, which was later confirmed by genetic markers.

How low-dose fetal CT is performed

Patients referred to us with a prenatal diagnosis of osseous abnormalities undergo a highly detailed ultrasound, which is interpreted by an experienced team of radiologists, maternal-fetal medicine specialists and geneticists familiar with the normal osseous structures for gestational age. If the exam demonstrates severe osseous abnormalities but the exact diagnosis is still in question, fetal CT is performed.

The mother is placed supine on the scanner table, the top and bottom of the uterus are sonographically marked with radiopaque markers, and a CT topogram of the uterus is obtained to confirm fetal position. Unenhanced CT of the maternal pelvis is then performed covering only the area between the markers using a 64-detector Siemens SOMATOM® Sensation CT scanner.

Three-dimensional volume-rendered images of the fetus are generated from the original 2-D data, allowing complete visualization of the fetal skeleton. Our team evaluates every study for bone mineralization, presence of fractures or bowing, relative length of long bones, and overall morphology of the bones. We take a team approach to prenatal diagnosis, with maternal-fetal medicine specialists, geneticists, radiologists and a host of surgical specialists weighing in on the prenatal and postnatal implications.

We recently published our experience with low-dose fetal CT for evaluation of these potential lethal or severe osseous abnormalities. The study found that five of 21 cases were interpreted correctly with CT and incorrectly with ultrasound (whereas the reverse did not occur); in 17 cases, CT revealed additional osseous findings not in the ultrasound report, which then allowed increased diagnosis confidence.

When performed for appropriate indications, with proper technical parameters and the knowledge that fetuses and children are far more sensitive to radiation than adults, the benefits of low-dose fetal CT may far exceed the relatively small individual risks. Knowing that fetuses are particularly sensitive to radiation, our imaging protocols conform to the “as low as reasonably achievable” (ALARA) principle, a concept endorsed by the Society for Pediatric Radiology. Our team is currently working to develop an ultra-low dose fetal CT protocol that further decreases the radiation dose we currently use in fetal CT.

Reference

Victoria T, Epelman M, Bebbington M, Johnson AM, Kramer S, Wilson RD, Jaramillo D. Low-dose fetal CT for evaluation of severe congenital skeletal anomalies: preliminary experience. Pediatr Radiol. 2012;42 Suppl 1:S142–149.