The Children’s Hospital of Philadelphia is expanding fetal imaging abilities beyond ultrasound, MRI and echocardiography to also include fetal computed tomography (CT). The low-dose CT program was initiated by CHOP pediatric radiology in order to evaluate skeletal dysplasia.
Traditionally, ultrasound (US) is used to image these rare abnormalities, with sensitivity in the range of 40 percent to 60 percent. At present, the protocol for fetal CT imaging is limited to second and third trimester gestations with severe bony abnormalities, without a clear diagnosis by US.
The CT scan captures only the uterus, previously mapped by US. The acquired axial CT images undergo post-imaging computer aided renderings to create various two- and three-dimensional depictions of the fetal skeletal system in exquisite detail. The radiation dose to the fetus is below 5 mSv, well below the radiation dose thought to cause fetal anomalies (100 mSv), and below the radiation dose permitted in patients working in a radiation-exposed environment.
Full body fetal CT showing shortened long bones and lack of ossification of fetal spine
Interpretation of the fetal CT is performed by pediatric radiologists with expertise in prenatal diagnosis. The differential diagnosis is reached through a multidisciplinary team including pediatric radiologists, perinatologists, geneticists and pediatric orthopaedic surgeons.
Role of fetal CT in diagnosing skeletal dysplasias
Fetal CT has significantly helped to refine our diagnostic abilities with skeletal dysplasias and bony abnormalities. The images to the right are of a 21-week fetus found by ultrasound to have a skeletal dysplasia consistent with a lethal form. Parental counseling using only ultrasound was limited due to the lengthy differential diagnosis.
Fetal chest noting absence of vertebral bodies
The availability of fetal CT refined the findings to include what is seen in the images — lack of ossification of the fetal spine, marked shortening of the long bones, bowed femora, shortened ribs and a very small chest. These findings were most consistent with achondrogenesis. This diagnosis allowed improved counseling for the parents regarding prognosis, and recurrence risk, and also efficiently guided the approach to molecular diagnosis, which confirmed the CT diagnosis.
Not every fetus with a skeletal dysplasia is a candidate for fetal CT studies. This is determined at the time of evaluation, on a case-by-case basis, dependent on ultrasound results. In certain instances, low-dose fetal CT has been extremely useful to assist in prenatal diagnosis of skeletal dysplasias and refine counseling and subsequent evaluation.