Standard sonographic evaluation of the fetal brain includes three transverse (axial) views. The transthalamic view is used to measure the BPD and HC and includes the midline falx, cavum septum pellucidum (CSP), and thalami (see Fig. 10-IA). The CSP is the space between the two laminae that separate the frontal horns of the lateral ventricles. Inability to visualize a normal CSP may indicate a midline brain abnormality such as agenesis of the corpus callosum, lobar holoprosencephaly, or septo-optic dysplasia (de Morsier syndrome). The transventricular view includes the lateral ventricles, which contain the echogenic choroid plexus (Fig. 10-6). The ventricles are measured at their atrium, which is the confluence of the temporal and occipital horns. The transcerebellar view is obtained by angling the transducer back through the posterior fossa (Fig. 10-7). In this view, the cerebellum and cisterna magna are measured, and between 15 and about 20 weeks, the nuchal skinfold thickness may also be measured. From 15 until 22 weeks' gestation, the cerebellar diameter in millimeters is roughly equivalent to the gestational age in weeks (Goldstein, 1987). The cisterna magna normally measures between 2 and 10 mm. Efacement of the cisterna magna is present in the Chiari II maormation, discussed later (p. 193).
Imaging of the spine includes evaluation of the cervical, thoracic, lumbar, and sacral regions (Fig. 10-8). Representative spinal images for record keeping are often obtained in the sagittal or coronal plane, but real-time imaging of each spinal segment in the transverse plane is more sensitive for anomaly detection. Transverse images demonstrate three ossification centers. The anterior ossiication center is the vertebral body, and the posterior paired ossification centers represent the
FIGURE 10-8 Normal fetal spine. In this sagittal image of a 21-week fetus, the cervical (, thoracic (, lumbar (, and sacral spine (5) are depicted. Arrows denote the parallel rows of paired posterior ossification centers-representing the junction of vertebralolamina and pedicles.
FIGURE 10-9 Anencephaly/acrania. A. Acrania. This ll-week fetus has absence of the cranium, with protrusion of a disorganized mass of brain tissue that resembles a "shower cap" (arrows) and a characteristic triangular facial appearance. B. Anencephaly. This sagittal image shows the absence of forebrain and cranium above the skull base and orbit. The long white arrow points to the fetal orbit, and the short white arrow indicates the nose.
junction of vertebral laminae and pedicles. Ossiication of the spine proceeds in a cranial-caudal fashion, such that ossification of the upper sacrum (51-52) is not generally visible sonographically before 16 weeks' gestation, and ossification of the entire sacrum may not be visible until 21 weeks (De Biasio, 2003). Thus, detection of some spinal abnormalities can be challenging in the early second trimester.
If a brain or spinal abnormality is identified, targeted sonography is indicated. he International Society of Ultrasound in Obstetrics and Gynecology (2007) has published guidelines for a "fetal neurosonogram." Fetal MR imaging may also be helpful (p. 217).