Diagnostic ultrasound ( PDFDrive )

1168 PART IV Obstetric and Fetal Sonography
*
C
C
A
B
FIG. 34.1 Early Normal Fetal Head Images Obtained With Transvaginal Probe. (A) At 8 menstrual weeks the head is clearly differentiated
from the trunk and limb buds. The intracranial cystic structure is the fetal rhombencephalic cavity (arrow), a normal space that eventually becomes
the fourth ventricle. (B) Scan at 12 1 2 weeks. Note that the cerebral cortex is very thin (at tip of arrow). The choroid plexuses (C) are very large
and ill the ventricles (*) from side to side. Ossiication is already visible in the skull bones.
and is destined to become the fourth ventricle. In the irst trimester
the normal rhombencephalic cavity appears especially large and
prominent and should not be mistaken for an abnormality 8,38
(Fig. 34.1A).
Ater 13 or 14 weeks, most cerebral structures can be identiied
ultrasonographically (see Fig. 34.1B). In the second trimester,
three standard transaxial planes or views (thalamic, ventricular,
and cerebellar) can lead to the detection of more than 95% of
sonographically detectable cerebral anomalies. 2,3,5,39 hese three
views (thalamic view, ventricular view, and cerebellar view) form
the starting point for routine scanning. If abnormality is suspected,
then additional views including transvaginal scan and the midline
view to assess the corpus callosum and vermis are useful to
clarify problems 5,20,40 (Fig. 34.2, Video 34.1, Video 34.2, and Video
34.3).
Cranial Structures to Note at Routine
Anatomic Scan
Measurement of biparietal diameter and head
circumference
Head shape
Bone density
Falx and interhemispheric issure
Ventricle size and appearance
Cavum of the septi pellucidi (CSP)
Thalamus
Cerebellum and vermis
Cisterna magna
Nuchal fold
(Median view for corpus callosum and vermis—not part of
routine scanning, but increasingly being done)
he thalamic view displays the thalamus, third ventricle,
fornices, basal ganglia, insula, and ambient cistern and is used
to measure the biparietal diameter (BPD), occipitofrontal diameter
(OFD), and head circumference (HC) (see Fig. 34.2A). he smooth
straight interhemispheric issure is visible on this and the ventricular
view. If the issure appears irregular, then abnormalities
of the corpus callosum and other cerebral structures should be
suspected. 41,42 he ventricular view is slightly higher than the
thalamic view and shows the bodies and, more important, the
atria of the lateral ventricles as well as the interhemispheric
issure (see Fig. 34.2B).
Atrial width (occipital horn width) is the most useful and
accepted measurement of ventricular size. 3,5,6 he atrium of the
lateral ventricles is the site of conluence of the bodies, occipital
horns, and temporal horns. Fortuitously, with abnormalities this
is the part of the ventricle that undergoes the earliest and most
marked enlargement. Ventricular measurement is a required
element of routine scans and is easily performed during routine
obstetric scanning. Usually only the distal ventricle is measured
because the closer ventricle is obscured by artifact; it is assumed
that the ventricles are symmetrical, but mild asymmetry is
common (Fig. 34.3). Measurements should be performed in a
very standardized manner on true axial views. Calipers should
be placed touching the inner walls of the ventricles opposite the
deepest part of the parieto-occipital issure; a normal value is
taken as below 10 mm. 39 Ventricular abnormality, especially
enlargement, is a key inding in the initial detection of CNS
abnormalities and is seen in about 88% of fetuses with cerebral
abnormalities. 39,43
he cerebellar view is obtained by rotating the transducer
into a suboccipitobregmatic plane centered on the thalamus to
show the cerebellar hemispheres. his view shows the cerebellum,