Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1167
important issues for the examiner are familiarity with the strengths
and limitations of all imaging modalities, expertise in their use,
and collaboration with specialists in other disciplines. 15,18-20
DEVELOPMENTAL ANATOMY
Embryology
Knowledge of fetal gestational age is particularly important when
evaluating anatomy in early pregnancy. In this chapter we use
menstrual age and gestational age as typically used clinically
and with ultrasound studies to mean “age from last menses” or
“postmenstrual weeks.” For consistency will convert “fertilization
age” or “conception age” used in publications to menstrual age
by adding 2 weeks.
CNS development is controlled by numerous genes with
functions not only in the CNS but also in the rest of the body,
and the understanding of the molecular basis of development
and malformations is occurring at an explosive rate. 21 Morphologic
changes start at about the ith menstrual week when cells destined
to form the notochord iniltrate into the embryonic disc and
induce the overlying embryonic tissue to thicken, fold over, and
fuse as the neural tube. Zipperlike fusion starts in the midtrunk
of the embryo and then extends to the cranial and caudal ends
(Table 34.1). Final anterior closure, at the rostral neuropore,
occurs by about 5 1 2 menstrual weeks, and a few days later at
the caudal neuropore. By the sixth week, the cephalic end enlarges
and lexes to become the brain. 22,23 Transvaginal ultrasound and
in vitro MRI can demonstrate many of the stages. 8,9,24 By 12 to
15 menstrual weeks, almost all structures and processes are in
their inal form except for the corpus callosum, cerebellar vermis,
neuronal migration (from the periventricular germinal matrix),
development of the sulci and gyri, and myelination. hese latter
structures and processes develop from about 15 weeks onward.
he corpus callosum starts developing at about 12 weeks. Its
development induces the formation of the two septi pellucidi
and the intervening space, the cavum septi pellucidi (CSP) and
cavum Vergae (ater Andrea Verga in 1851). 8,25
TABLE 34.1 Differentiation of Brain
Regions From Primary Vesicles
Primary Vesicle
Secondary
Vesicle
Mature
Structure
Forebrain Telencephalon Cerebral
hemispheres
Basal ganglia
Olfactory system
Diencephalon
Thalamus
Hypothalamus
Midbrain Mesencephalon Midbrain
Hindbrain Metencephalon Pons
Myelencephalon
Cerebellum
Medulla
Modiied from Moore K. Essentials of human embryology. Toronto,
Ontario: BC Decker; 1988. 26
he dorsal aspect of the neural tube at the hindbrain (rhombencephalon)
is very thin and membranous (membranous area
or area membranosa). In the irst trimester, the enclosed central
neural canal, the rhombencephalic cavity, is very large and forms
a conspicuous cystic cavity that should not be mistaken for an
abnormality. 8 his space eventually becomes the fourth ventricle.
he membrane is divided into an anterior rostral area (anterior
membranous area or area membranosa rostralis) and posterior
caudal area (posterior membranous area or area membranosa
caudalis) by a fold, the plica choroidea, which will become the
choroid of the fourth ventricle. he cerebellum and vermis
develop as lateral and rostral proliferations into the anterior
membranous area. he posterior membranous area eventually
fenestrates to form the foramina of Magendie and Luschka. he
posterior membranous area can bulge into the cisterna magna
to a variable extent, forming the Blake pouch. With highresolution
equipment, the Blake pouch can be seen in most
fetuses, where it is oten mistaken for arachnoid strands. 27-29 he
cerebellum and vermis are essentially formed by 22 weeks. Care
must be taken before 22 weeks to avoid mistaking the incompletely
developed early vermis for vermian dysplasia or hypoplasia. 30
Cortex and neuron development starts at about 5 weeks and
is complete by 28 weeks. here are three complex overlapping
stages: proliferation, migration, and organization. Neurons
proliferate and develop from glial stem cells at the surface of the
ventricles and ganglionic eminence. Migration occurs via two
pathways, projectional and tangential. In projectional migration,
neurons follow a radially oriented glial scafold directly to the
cortical surface, with later-forming neurons passing through the
earlier layers to the pial surface. From 17 to 34 weeks, this
migration can be seen as bands or laminations that are visible
on MRI and ultrasound as the ventricular zone, intermediate
zone, subplate zone, and cortical plate. his regular pattern is
disturbed with migrational disorders, malformations, and infections
such as cytomegalovirus (CMV). 31-33 In tangential migration,
GABAergic cells from the ganglionic eminence take a tangential
route through the cortex and provide neurons with controlling
functions. he ganglionic eminence overlying the thalamus is
more evident on MRI than ultrasound. 34,35 he cortex undergoes
folding into gyri to accommodate the extra cells. Finally, neurons
in the cortex organize local connections and send axons remotely
as large tracts such as the corpus callosum to connect the
hemispheres. Development requires normally functioning genes
and is easily disrupted by intrinsic and extrinsic insults such as
fetal and maternal metabolic disorders, hypoxia, infections, and
teratogens. Note that with teratogens, the inal morphologic
appearance relects the gestational age and stage at which disruption
occurred, rather than the speciic agent. 21,36
Sonographic Anatomy
he early embryo is best examined transvaginally. he cephalic
end is identiiable by about 8 weeks. By 10 or 11 weeks, bones
of the vault show mineralization. At this age, the brain mantle
is very thin. he ventricles are large and illed with choroid,
which provides nourishment for the developing brain. 37 A large,
echo-free space behind the hindbrain represents the rhombencephalic
cavity, which decreases in size as the cerebellum forms