Diagnostic ultrasound ( PDFDrive )

1524 PART V Pediatric Sonography
Germinal Matrix
he germinal matrix develops deep to the ependyma and consists
of loosely organized, proliferating cells that give rise to the neurons
and glia of the cerebral cortex and basal ganglia (Fig. 45.17). Its
vascular bed is the most richly perfused region of the developing
brain. Vessels in this region form an immature vascular rete of
ine capillaries, extremely thin-walled veins, and larger irregular
vessels. 42 he capillary network is best developed on the periphery
of the germinal matrix and becomes less well developed toward
the central glioblastic mass. Although the germinal matrix is
not visualized on sonography, it is important as the typical
anatomic site over the caudate nucleus where GMH occurs in
premature infants.
Early in gestation, the germinal matrix forms the entire wall
of the ventricular system. Ater the third month of gestation,
the germinal matrix regresses, irst around the third ventricle,
then around the temporal and occipital horns and trigone. By
24 weeks’ gestation, the germinal matrix persists only over the
head of the caudate nucleus and to a lesser extent over the body
of the caudate. By 32 weeks’ gestation, it is unusual to see GMH
because these cells migrate out to the cerebral cortex. his
regression continues until 40 weeks’ gestation, when the germinal
matrix ceases to exist as a discrete structure, and the immature
vascular rete has been remodeled to form adult vascular
patterns.
Calcar Avis
On posterior fontanelle views, a normal gyrus, the calcar avis,
frequently protrudes into the medial aspect of the lateral ventricle
at the junction of the trigone and occipital horn (see Fig. 45.16B).
Although this normal brain gyrus may mimic intraventricular
clot, slightly turning the transducer will show its continuity with
GM
FIG. 45.17 Germinal Matrix. Drawing shows germinal matrix (GM)
at 30 to 32 weeks’ gestation, with largest amount near the caudate
nucleus. (With permission from Rumack CM, Manco-Johnson ML.
Perinatal and infant brain imaging: role of ultrasound and computed
tomography. St Louis: Mosby; 1984. 7 )
the brain. It can be recognized because of a central echogenic
sulcus (calcarine issure), its continuity with the adjacent brain,
and normal vascularity on color Doppler ultrasound. 41
Cerebellar Vermis
Because the cerebellum develops late in gestation, a mistaken
fetal diagnosis of cerebellar vermian hypoplasia is occasionally
made. If axial scans are done below the normal level of the fourth
ventricle, the vallecula between the cerebellar hemispheres may
be mistaken for a Dandy-Walker variant. Pseudoabsence of the
inferior vermis 41 or pseudo–vermian hypoplasia can be appreciated
on axial views through the posterior fossa. here may appear to
be a small or absent cerebellar vermis and a wide communication
between the fourth ventricle and cisterna magna. Slightly more
superior axial views will show the normal cerebellar vermis. he
appearance of a missing or hypoplastic vermis can be evaluated
carefully by moving the transducer superiorly to depict the normal
vermis, and by obtaining sagittal midline views. he cerebellar
vallecula is a variably sized subarachnoid space below and not
continuous with the fourth ventricle. he foramen of Magendie
is thinner than the vermian clet in a Dandy-Walker variant 14
(see Fig. 45.9C).
Cisterna Magna
Cisterna magna septa are typically seen inferior and posterior
to the cerebellar vermis, usually straight and parallel (see Fig.
45.9). hese septa arise at the cerebellovermian angle and continue
to the occipital bone. Robinson and Goldstein 43 proposed that
these septa are a remnant of Blake pouch cysts and thus a marker
of normal cerebellar development. 44,45
CONGENITAL BRAIN
MALFORMATIONS
Congenital brain malformations are the most common anomalies
in humans. 42,46,47 Malformations can be classiied based on brain
development and the types of anomalies that result when development
is altered. Brain development can be divided into three
stages. 46 Cytogenesis involves the formation of cells from
molecules. Histogenesis is the formation of cells into tissues
and involves neuronal proliferation and diferentiation. Organogenesis
is the formation of tissues into organs.
Organogenesis can be subdivided into further stages 48
(Fig. 45.18). he irst stage, neural tube formation and closure,
occurs at 3 to 4 weeks’ gestation. he neural plate folds in on
itself, fusing dorsally and giving rise to the earliest recognizable
brain and spinal cord. In the next stage, segmentation and
diverticulation of the forebrain occur at 5 to 6 weeks. he single
central fetal ventricle separates into two lateral ventricles, and
the brain divides into two cerebral hemispheres. Anterior
diverticulation results in the formation of the olfactory bulbs
and optic vesicles and the induction of facial development. he
pituitary and pineal gland also develop by diverticulation from
the ventricle at this stage.
Neuronal proliferation and migration occur at 8 to 24 weeks’
gestation. Tremendous cellular proliferation is necessary to provide