17.2 Fetal Anatomic Survey: Head, CNS, Face & Spine

The head and spine survey is the highest-stakes portion of the second-trimester anatomy scan: open neural tube defects (NTDs) are among the most common and consequential fetal anomalies, and most are detectable through a handful of reproducible cranial signs. The ARRT exam tests the exact measurement thresholds and the named signs.

Standard Intracranial Anatomy

Three axial planes organize the intracranial survey:

1. Transventricular plane — the highest cut, showing the lateral ventricles. Each ventricle is largely filled by the echogenic choroid plexus. The ventricle is measured at the atrium (the junction of the body, temporal, and occipital horns), placing calipers inner-edge to inner-edge at the level of the glomus of the choroid plexus, perpendicular to the ventricular axis. The atrial diameter should be ≤10 mm and remains stable from about 15 weeks to term.
2. Transthalamic (BPD) plane — slightly lower, showing the cavum septi pellucidi (CSP), thalami, and third ventricle. The CSP is a key normal landmark visible from about 18 to 37 weeks; its absence raises concern for agenesis of the corpus callosum, septo-optic dysplasia, or holoprosencephaly.
3. Transcerebellar plane — angled slightly downward to show the cerebellum (a dumbbell shape with two echogenic hemispheres), the cisterna magna behind it, and the nuchal skin fold.

Key measurements on these planes:

Ventriculomegaly

Ventriculomegaly is defined by an atrial diameter >10 mm and is graded:

• Mild: 10–12 mm
• Moderate: 13–15 mm
• Severe: >15 mm (often termed hydrocephalus when progressive)

A classic real-time clue is the dangling choroid sign: the choroid plexus, normally filling the ventricle, hangs dependently within the enlarged fluid space. Mild ventriculomegaly may be isolated but warrants a full survey, fetal karyotype discussion, and follow-up imaging.

Neural Tube Defects and Cranial Signs

The lemon sign and banana sign are the exam-favorite indirect markers of an open spina bifida with Chiari II malformation:

• Lemon sign — bilateral scalloping/flattening of the frontal bones gives the calvarium a lemon shape on the transthalamic view. It is most reliable in the second trimester and resolves by the third.
• Banana sign — on the transcerebellar view, the cerebellum is pulled down through the foramen magnum, wraps around the brainstem like a banana, and the cisterna magna is effaced (obliterated).

Specific NTDs:

• Anencephaly — absence of the cranial vault (calvarium) and cerebral hemispheres above the orbits, with exposed disorganized tissue (area cerebrovasculosa) and prominent "frog-eye" orbits; frequently accompanied by polyhydramnios. It is lethal and is reliably diagnosed after 12 weeks.
• Cephalocele (encephalocele) — a midline calvarial defect, most often occipital, herniating a sac that may contain brain (encephalocele) or only meninges (cranial meningocele). Prognosis depends on sac contents.
• Spina bifida — failure of posterior neural arch closure. On transverse views the normal closed triangle of ossification centers becomes a splayed, U- or V-shaped posterior complex; on sagittal views there is loss of the normal parallel lines with a bulging sac or skin defect. The level of the defect (sacral vs lumbar vs thoracic) predicts lower-limb function and ambulation, and maternal serum alpha-fetoprotein (MSAFP) is typically elevated in open defects.

Face

The facial survey includes a coronal view of the nose and lips — the essential plane for detecting a cleft lip, which appears as a defect extending from the lip toward the nostril. Cleft lip may be unilateral or bilateral and is commonly associated with cleft palate, which is harder to image and is suggested by loss of the intact alveolar ridge. A mid-sagittal profile assesses the forehead, nasal bone, and chin: micrognathia (a small, recessed chin) is associated with trisomy 18 and skeletal syndromes, and frontal bossing or a flattened profile may accompany skeletal dysplasias. Orbits and lenses are documented on an axial orbital view; hypotelorism suggests holoprosencephaly, and hypertelorism may accompany midline facial defects.

Spine

The spine is examined in three orthogonal planes:

• Sagittal — two parallel echogenic lines (the vertebral bodies anteriorly and the posterior arches behind them) that converge normally toward the sacrum; the overlying skin line must be intact
• Transverse — each vertebra shows a closed triangle of three ossification centers pointing posteriorly
• Coronal — the parallel rows of posterior ossification centers widen progressively from the cervical spine to the lumbar spine and converge in the sacrum

Any splaying of the posterior elements, interruption of the skin line, kyphosis, scoliosis, or a cystic/sac-like mass over the spine indicates an open defect and prompts a careful cranial survey for the lemon and banana signs, plus documentation of the uppermost level of involvement. A sacral defect carries a far better prognosis for ambulation than a thoracic defect, so the level should always be stated in the report.