Prenatal Imaging

Erin Simon Schwartz and David M. Mirsky

Vast improvements have been made in the diagnosis and therapy of fetal anomalies as a result of advances in imaging and surgical technology. Nowhere is this more applicable than in the fetal spine. Ultrasonography is the primary imaging modality for fetal evaluation and helps distinguish normal spine development from abnormal spine development while providing valuable information about spinal anomalies. Magnetic resonance imaging (MRI) is complementary to ultrasonography for assessment of spinal malformations, as it has the advantage of better detection and depiction of associated central nervous system (CNS) and non-CNS anomalies that may have a significant impact on postnatal neurologic function and quality of life.

The major lesion of the CNS currently amenable to fetal diagnosis and intervention is open spinal dysraphism, commonly referred to as myelomeningocele (MMC); this lesion is a significant driving force behind the use of highly detailed prenatal imaging. The osseous detail and real-time evaluation of lower extremity configuration, position, and motion detectable with ultrasonography, combined with soft tissue and parenchymal assessments with the use of MRI, provide the maternal-fetal medicine specialist and pediatric neurosurgeon with highly detailed anatomic information. Because the surgery is not without risk to the mother and the fetus, obtaining extremely accurate information is essential for determining surgical appropriateness and presurgical planning. When significant congenital anomalies are present outside of the CNS, information about associated spinal abnormalities is valuable for counseling and parental decision making regarding the management of the pregnancy, labor, and delivery or interruption of the pregnancy.

Imaging Techniques

The accuracy of identifying spinal anomalies during nontargeted screening ultrasonography varies, depending on the skill and experience of the operator. The accuracy of a referral center performing detailed targeting studies for a suspected neural tube defect (elevated maternal serum α-fetoprotein) is close to 100%. A detailed protocol should be performed. Because fetuses with open neural tube defects typically have Chiari II malformations, the fetal brain should be scanned initially. A small cisterna magna with rounded small cerebellum is termed the “banana sign” and is 99% sensitive in the diagnosis of a Chiari II malformation (Fig. 42-1). The frontal bones may be concave, and this is termed the “lemon sign.” This sign is less specific and may be present in 1% to 2% of normal fetuses and may resolve by the third trimester. Ventriculomegaly may be present. Axial and longitudinal views of the spine should be obtained. Spine ossification progresses from 10 to 22 weeks’ gestation. By 16 weeks’ gestation, neural arch ossification is complete to L5. By 19 weeks, S1 is completely ossified, and by 22 weeks, S2 is ossified. Splayed pedicles are best visualized in the transverse plane. An overlying sac may be imaged in transverse and longitudinal planes, with higher-frequency transducers showing cord tethering and placode contents (Fig. 42-2).

Figure 42-1 Banana sign.
An axial ultrasonographic image through the posterior fossa in a 21-week fetus with a Chiari II malformation shows the crowding of the cerebellum around the brainstem that has been termed the “banana sign” (demarcated by arrows).

Figure 42-2 Myelomeningocele.
Longitudinal ultrasonography through the lumbosacral spine in a 20-week fetus shows the defect in the posterior elements, with the abnormally low spinal cord extending to the level of the defect (arrows).

Fetal MRI is typically performed on 1.5 Tesla (T) magnets, although some centers are now performing fetal MRI at 3.0 T. A surface coil (torso, cardiac, or body phased-array) is used to maximize image quality. A standard localizer sequence generally facilitates quick identification of fetal position. This is used to guide the initial imaging plane, which should be appropriately aligned with the fetal anatomy in question. Subsequent imaging sequences are prescribed in orthogonal planes with respect to the fetal spine, each adjusted from the preceding image set, to account for changes in fetal position.

Ultrafast sequences are used to minimize image degradation by maternal and fetal movements. T2-weighted images provide most of the diagnostic information for the examination: single-shot fast spin echo (SSFSE) or half-Fourier acquisition single-shot turbo spin echo (HASTE) sequences at minimal slice thickness (2 to 4 mm). Fast T1-weighted gradient echo imaging is frequently attempted but seems to show satisfactory contrast resolution and signal-to-noise ratio only after approximately 26 weeks’ gestation. Extremely rapid gradient echo, echo planar imaging, with its high sensitivity to paramagnetic susceptibility may be used to identify osseous and vascular structures and hemorrhage. Diffusion-weighted imaging is increasingly being used to assess for acute ischemia.

Abnormalities in neurulation may result from defects in disjunction, the process by which the neural tube separates from the overlying ectoderm. A large site of complete failure of disjunction may result in MMC. Prenatally, the findings of MMC are clearly detectable with ultrasonography and MRI. In the presence of elevation of the neural placode, secondary to expansion of the subarachnoid space, the lesion is referred to as MMC (Fig. 42-3). This is distinguished from myeloschisis, also known as myelocele, where an open neural tube defect exists, but the subarachnoid space is not expanded, and the neural placode remains within the confines of the dysraphic spinal canal (Fig. 42-4). MMC occurs during the formation of the primitive neural tube (neurulation) in the third week of gestation, when a localized failure of neural tube closure occurs. This failure may occur anywhere along the length of the spinal cord, but it is most common in the lumbar region. The resulting lesion is an open spinal canal with a flat neural placode instead of a cylindrical spinal cord. Imaging reveals the failure of neurulation in the MMC as a posterior osseocutaneous defect. When present, the expansion of the subarachnoid space is readily apparent (see Fig. 42-3). The neurologic deficits sustained by the fetus are postulated to occur in stages—a “two-hit” hypothesis. The first “hit” is the original defect in neurulation that creates the dysraphism and any associated myelodysplasia. The second “hit” is the secondary chemical or physical trauma (or both) to the neural tissue as a result of its exposure to the intrauterine environment.