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60
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
PCA TC
V
CSP
CC
29
The
3V
4V
CM
(CSF): the choroid plexuses. These structures are found
in all lateral ventricles.
The choroid plexuses consist of villi in large numbers,
the outer (ventricular) surface of which is covered by a
single-layered, modified cuboidal epithelium (ependyma).
The inner layer is a stromal core derived from the pia. The
capillaries found in each of these villi are the major source
of CSF production.
Embryologically, the choroid plexuses develop from
the anteriorly located ventricles, more precisely, from
their medial and upper wall. The stroma and the covering
pia are derived from the mesenchyme. Even though the
choroid plexuses are present from 6 to 7 postmenstrual
weeks on, it takes several weeks until it becomes large
and echogenic enough to be detected by a high-frequency
transvaginal probe. At 8 to 8½ postmenstrual weeks, it is
small in size and already significantly echogenic ( Figure
2–18 ). Starting from the ninth postmenstrual week, it is
consistently seen on the two sides of the falx within the
lateral ventricles. It can be said that it is the most striking
sonographic intracranial structure well beyond the end
of the first trimester. At 9 to 11 postmenstrual weeks,
AH
TH
CP
OH
IH
Figure 2–48. The targeted or more detailed fetal neurosonogram contains other planes and sections; the sagittal planes are displayed in this figure.
The brain specimen supports the ultrasound image.
scanning sound waves if an extremely echogenic structure, namely, the choroid plexus, did not highlight them.
These connections were reportedly detected as early as
8½ postmenstrual weeks by the Trondheim group.
interventricular foramina (Monro) are clearly visible
from 14 to 16 postmenstrual weeks on and mark the most
typical coronal (midcoronal–2) section of the fetal brain
( Figures 2–38C and 2–39C ).
The cerebral aqueduct (Sylvius), the connection
between the third and fourth ventricles, was not reported
to be detected by ultrasonography of the normal fetal
brain. This may soon change using 3D imaging when a
special plane to detect and study this extremely thin structure can be created by the multiplanar capability of this
technique.
Using a posterior (occipital) axial or median approach,
the median aperture (Magendie) can be seen ( Figure 2–44 ).
This aperture is wide open before 16 postmenstrual weeks
and is more easily detected before 20 postmenstrual weeks
than after. This aperture becomes wide open in cases of
abnormal dilations of the cerebellomedullary cistern (the
cisterna magna) and the fourth ventricle (see Chapter 4 ).
the choroid plexuses fill both lateral ventricles ( Figures
Choroid Plexuses
An integral part of the ventricular system is a complex
tissue dedicated to the production of cerebrospinal fluid
2–21 , and 2–23 ). As pregnancy progresses, their relative
size compared with the lateral ventricular size decreases.
They “move” posteriorly and assume their permanent
anatomical site within the atrium, “hugging” the thalami
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Anterior horns
Interventricular
foramen
(Monro)
61
Body of lateral ventricle
Atrium
Posterior horns
Third ventricle
A
Inferior horns
Lateral aperture
(Luschka)
Central canal
Cerebral aqueduct
(Sylvius)
Fourth ventricle
Median aperture
(Magendie)
B
Figure 2–49.
the brain, seen from the left side.
The ventricular system of
Figure 2–50.
C
(A) 12, (B) 18, and (C) 32 postmenstrual weeks. (From Day, 1959,
permission.)
The development of the human fetal lateral ventricles at
44
with
62
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
AB
Figure 2–51. ( A, B) Two axial sections of the brain at 26 postmenstrual weeks, showing the obscured near field due to ineffective transabdominal
imaging of the hemisphere close to the transducer (open arrows).
Cisterna
Magna
4th Ventricle
28 Week fetus
Anterior
Horn
Stored
Image #8
A
Third
Ventricle
Thalamus
Cerebellum
Stored
Image #4
B
Figure 2–52.
fourth and third ventricles at 28 postmenstrual weeks. A and B axial sections tilted slightly backward.
The more advanced technology of the transabdominal transducer allows visualization of very fine detail of the cerebellum as well as the
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Table 2–4. DIAMETER OF THE NORMAL LATERAL VENTRICULAR ATRIUM
AS A FUNCTION OF POSTMENSTRUAL AGE
Postmenstrual Weeks Mean ± SD (mm) Range (mm)
14–20 7.6 ± 0.7 6.0–9.0
21–25 7.7 ± 0.5 7.0–9.0
26–30 7.5 ± 0.7 6.5–9.0
31–38 7.6 ± 0.5 7.0–8.5
Modified from Cardoza and colleagues, 1988,51 with permission.
63
from posterior and above ( Figure 2–35 ). Their posterior
contour is smooth. If they lose their regular contour, one
should suspect an adjacent intraventricular hemorrhage.
An obviously thin or dangling choroid plexus should
arouse suspicion of ventriculomegaly or hydrocephaly.
The tela choroidea is the thin choroid plexus layer
covering the thalami, extending into the third ventricle
( Figures 2–39C and 2–43 ). The choroid plexus of the
fourth ventricle is extremely difficult to image. Therefore,
to our knowledge, it is not mentioned in the pertinent
literature.
It should be emphasized that the choroid plexus may
give rise to tumors, such as choroid plexus papilloma and
carcinoma.
THE CAVA
The cavum septi pellucidi and its posterior part, the
cavum Vergae, are not strictly part of the ventricular
system; if found at autopsies of infants and adults, they
are generally considered insignificant. The cavum septi
pellucidi and the cavum Vergae are, in fact, the same
structure, the former located anteriorly and the latter
positioned posteriorly to a vertical plane formed by the
columns of the fornix. The two usually communicate
with each other ( Figure 2–53A and B ). The presence or
absence of the cavum septi pellucidi as a function of age
was studied by Shaw and Alvord.
of 374 normal subjects, they found that 100% of the premature infants had a cavum septi pellucidi wider than 1
mm. The cavum Vergae closes first, well before term, and
the cavum septi pellucidi starts to close just before term.
Table 2–5 also shows that at 6 months of age only 12% of
the brains had a recognizable cavum. This group did not
report on the age of the premature infants.
Jou et al
68
measured the width of the cavum septi pellucidi from 19 to 42 weeks in 608 fetuses. They found that
this measurement increased gradually to 27 weeks when it
plateaued until term.
Larroche and Bandey
69
is typically absent at birth. This was observed following
pneumoencephalographic studies in neonates.
The cava forms simultaneously with the corpus callosum. The appearance of the cava on ultrasonographic
images is also limited to the first detection of the corpus
67
Dissecting the brains
noted that the cavum Vergae
callosum, that is, about 17 to 18 postmenstrual weeks. At
term these spaces become almost obliterated.
Even though it will be discussed in detail later in this
chapter when we describe the arteries of the brain, we have
to mention here the pericallosal artery. This artery closely
follows the corpus callosum. As a matter of fact, three
structures and their development are closely interrelated:
the corpus callosum; above it, the pericallosal artery; and
below it, the cavum septi pellucidi. This parallel development becomes important when deviant development of
the corpus callosum is discussed.
The cavum septi pellucidi is best imaged on the
median ( Figures 2–42 , 2–43 , 2–44 , 2–45 ) and the midcoronal–1 and –2 ( Figures 2–39 and 2–41B ) sections.
Two lateral walls separate this structure from the anterior horns of the lateral ventricles ( Figure 2–41B ). Failing
to detect these lateral walls should arouse suspicion
of disease, such as agenesis of the corpus callosum,
septo-optic dysplasia, schizencephaly, hydrocephaly, and
porencephaly.
70
Corpus Callosum
The corpus callosum is one of the important structures
of the brain, connecting the right and left hemispheres.
In fact, it is the largest interconnecting structure and
consists of myelin-coated nerve fibers that cross the
median plane. It forms the roof of the cavum septi pellucidi and the cavum Vergae. The development of these
structures (ie, the corpus callosum and the cava) is
closely linked. Because the roof of the cavum septi pellucidi is the corpus callosum itself, it is clear that if there
is no roof, there is no cavum.
If the corpus callosum is scanned through the anterior fontanelle, the sound waves meet this structure at
almost a right angle. The sonographic picture in the
median plane of the developed corpus callosum is that of
two parallel echogenic lines with a sonolucent strip of ∼3
to 5 mm between them. The upper line is generated by
the deepest points of the longitudinal fissure. The lower
echogenic line is the roof of the cavum septi pellucidi
and the cavum Vergae. The pericallosal artery closely
follows the upper boundary of the corpus callosum. The
midcoronal–1 and –2 sections depict this commissural
structure as semilunar.
64
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
A
F-1 MC-1 MC-2 MC-3 O-1
B
Figure 2–53.
images were generated along the white lines seen on the median plane. The anterior section “cuts” through the cavum septi pellucidi (CP). The posterior
coronal section highlights the cavum Vergae (CV). (B) The systematic scanning using the transfontanelle approach reveals the normal cavum septi pellucidi on the MC–2 plane and the still open cavum Vergae on the MC–3 plane. This study uses the sequential “coronal” planes.
The cavum Vergae in the brain of this normal 28-week old fetus is the posterior part of the cavum septi pellucidi. (A) The two lower
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Table 2–5. INCIDENCE OF OPEN CAVUM SEPTI
PELLUCIDI AS A FUNCTION OF AGE
Age Percentage Present
Premature 100
Full term to 7 days 97
8 days to 1 month 85
6 month to 16 years 12
Modified from Shaw and Alvord, 1969,67 with permission.
65
After understanding the anatomy and the sonographic presentation of the corpus callosum in the various
scanning planes, its development should be discussed.
The corpus callosum is a relatively late- developing
structure reported to develop between the 12th and
18th postmenstrual weeks.
develops in an anterior-to-posterior direction,
development can be followed sonographically.
71 , 72
It is also known that it
69
but this
20,24,72–74
(See Chapter 1 .)
In Figure 2–35 , which represent coronal and sagittal
sections of fetuses at 14 postmenstrual weeks, it is not yet
possible to discern the corpus callosum. At 16 postmenstrual weeks, the first hint of its sonographic appearance is
present ( Figure 2–38 ). At 18 postmenstrual weeks ( Figures
2–38 and 2–42 ), the anterior portion of the genu and
almost all of the central part are seen. Figure 2–44 depicts
images of the corpus callosum from 18 to 28 postmenstrual weeks.
At 22 to 23 postmenstrual weeks, the median section
shows the totally formed corpus callosum ( Figure 2–45 ).
This structure has four parts, from anterior to posterior:
the rostrum, the genu (knee), the trunk, and the splenium
(see Figure 2–45C ).
The importance of knowing this pattern is because,
at times and for various reasons, the development of the
corpus callosum is incomplete.
72 , 73 , 75
Such a partial agenesis
of the corpus callosum appears on the sonographic image
as if it had been “caught” at an early stage of development.
The lack of development of this structure is discussed in
Chapter 5 .
Normative measurements of the length as well as the
thickness of the corpus callosum were published.
76
We have to mention here that on some of axial sections there is a structure that can be mistaken for the
cavum septi pellucidi. These structures are the columns
of the fornix ( Figure 2–54 ). These are seen as parallel
echogenic lines that appear if the BPD plane is slightly
shifted (in a parallel fashion) toward the base of the skull.
In a retrospective study of 20 consecutive sonograms
on pregnant patients between 18 and 24 postmenstrual
weeks, the columns of the fornix could be identified in all
fetuses. To mistake the columns of the fornix in a case of
agenesis of the corpus callosum may lead to misdiagnosis
of this entity.
77
Figure 2–54. The columns of the fornix (arrow) appear as parallel echo-
genic lines in the “BPD plane” but slightly inferior.
Subarachnoid Spaces and Cisterns
In scanning the normal fetal brain, spaces of different
shapes and sizes can be seen between the cortex and the
bony skull or between structures of the brain itself. Fine
cords of the arachnoid, delicate blood vessels, and probably arachnoid granulations produce a variety of echoes
within this space ( Figures 2–39B – E , 2–55 , and 2–56 ).
Figures 2–55 and 2–56 show an ultrasonographic image
and a drawing of the subarachnoid space.
Early in pregnancy, it is rare to detect subarachnoid spaces. However, around 14 to 16 postmenstrual
weeks, they can be revealed using high-frequency TVS
( Figures 2–40 , 2–55 , and 2–57 ). The relative sizes of these
spaces decrease as pregnancy advances. In a study by Laing
and colleagues,
weeks were scanned transabdominally. A subdural space
was sought on a transaxial scan. Eighty-three percent of
the fetuses with this subdural space were less than 30 postmenstrual weeks, whereas 77% lacking this sign were older
than 30 weeks. All of the fetuses were normal at birth.
These investigators concluded that if a wide subarachnoid
space is seen in late pregnancy, it should be investigated
following birth, as a prominent space may predispose a
neonate to subdural hematoma.
It is possible that Laing’s group
immediately adjacent to the lateral sulcus (of Sylvius).
This fluid-filled space was also studied by Jeanty and collaborators.
structure (ie, the cortex) below the internal table of the
bone overlapping this area in which the pulsations of
the middle cerebral artery is seen should be termed the
insula and not the sylvian fissure . Such an area is shown
in Figure 2–58 .
The subarachnoid space overlying the insula is also
shown in the subsection dealing with the sulci, fissures,
and gyri in this chapter.
It is worth discussing the echoes appearing within the
subarachnoid space seen in Figure 2–55 . These may be
78
122 fetuses from 21 to 40 postmenstrual
79
78
studied the space
80
The latter came to the conclusion that the
66
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Figure 2–55. The subarachnoid space is shown on this Frontal–1 section. The small arrows indicate the hyperechoic structures thought to repre-
sent cross-sections of blood vessels or arachnoid granulations in the subarachnoid space. The open arrow indicates the falx; the arrowhead marks
the sagittal sinus.
Arachnoid granulations Sagittal sinus
Figure 2–56.
into the areas supplied by blood and which drain the cerebrospinal fluid.
Schematic drawing of a similar section depicted in Figure 2–55, showing the subarachnoid space and the arachnoid granulations bulging
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
21
CM
C
V
C
4V
Figure 2–57. Axial section of the posterior fossa in a normal
fetus at 21 postmenstrual weeks. CM, cisterna magna; C, cerebellum; V, vermis; 4V, fourth ventricle.
67
cross sections of vessels; however, on color Doppler and
power Doppler scans, these do not demonstrate flow. An
additional hypothesis is that they may represent cross sections of arachnoid granulations ( Figure 2–56 ).
Several cisterns in the subarachnoid space can be
seen by neurosonography of the fetal brain. The cisterns
are scattered around as well as within the folds of the
brain. We could not detect cisterns such as that of the
lamina terminalis or the chiasmatic, interpeduncular,
and pontine cisterns (cisterna magna); at times, the
superior (interpeduncular) cistern below the cerebellum
and the quadrigeminal cistern and the bilateral cisterna
ambiens above the cerebellum have been imaged by
ultrasonography.
81
The cisterna magna has been seen as early as 12 to
14 postmenstrual weeks ( Figures 2–36 , and 2–57 ). From
Insula
Figure 2–58. Left Oblique-2 section through the insula. Pulsations of the middle cerebral artery may be detected scanning through this view.
68
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
AB
E
Figure 2–59. Structural evaluation of the brain at 15 postmenstrual weeks. The posterior fossa and the cerebellum are studied on these serial sections.
The two slanted arrows on both sides of the cerebellum throughout the six images point to the two cerebral hemispheres. (A) Low axial section depicting the cisterna magna (small arrow). (B) The higher axial plane reveals the widening cisterna magna (small arrow) and the lower pole of the fourth
ventricle (arrowhead). (C) A somewhat “higher” axial section shows the hemispheres and the fourth ventricle (arrowhead). (D) This is the highest of axial
sections depicting the cerebellum at the level where the bicerebellar diameter measurement is usually taken. Note the hyperechoic cortex surrounded
by sonolucent cerebrospinal fluid and the low echogenicity of the medulla. (E) Coronal section showing the cisterna magna (small white arrow in the
median plane), the inverted-funnel-shaped tentorium (two black arrows), and the choroid plexus (cp). (F) A combined oblique axial-coronal section
showing the bicerebellar diameter, which measures 1.4 cm. The small arrow in the midline indicates the cisterna magna. (From Timor-Tritsch and colleagues, 1995,
42
with permission.)
14 postmenstrual weeks on, it is easy to visualize this
relatively large sonolucent structure in the posterior fossa
( 2–45 , 2–51 , and 2–57 ).
As mentioned before, using a posterior (occipital)
approach and a transvaginal probe, some of the fine details
of the posterior fossa and the cisterna magna, as well as the
quadrigeminal cistern, can be revealed ( Figure 2–59 ). The
connection between the fourth ventricle and the cisterna
magna through the median aperture can also be seen if the
correct section and plane are applied ( Figures 2–60 , 2–61 ,
2–62 , 2–63 , and 2–64 ).
The exact anatomy of the posterior fossa, but more
so the connections between the fourth ventricle and the
cisterns around the cerebellum (eg, the cisterna magna),
should be well known to those engaging in neurosonography of the fetal posterior fossa. Variations in the size
of these CSF-filled spaces are not uncommon. The size
of the cisterna magna in normal neonates was measured
and ranges between 3 and 8 mm, with a mean value of
CD
+
+
F
4.5 mm.
78
As opposed to an increase in the size of this
space due to disease, there is also restriction of this cistern
due to pathology (Arnold-Chiari type II malformation). It
is therefore of the utmost importance that this space be
examined in the context of the entire CNS.
The use of high-frequency (possibly through the
transvaginal route) transducers is probably warranted,
along with sound knowledge of the anatomy and neuropathology of this region.
At times, only TAS of the ventricular system is feasible
(eg, with breech or transverse positions of the fetus). In
such cases one should be familiar with nomograms developed for atrial measurements. One of these nomograms
was suggested by Pilu and coworkers.
59
After conducting a
prospective study of 171 normal pregnancies from 15 postmenstrual weeks to term, the following were found: (1) the
atrial width remained fairly constant during pregnancy
(0.69 ± 0.13 cm = 2 standard deviations); and (2) significant relationships were found between the cerebroatrial
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
C
CM
A
69
C
V
CM
B
Figure 2–60. The cisterns around the cerebellum in a normal fetus at 17 postmenstrual weeks. (A) Median section. The two arrows indicate the
quadrigeminal cisterns. C, cerebellum; CM, cisterna magna. (B) Coronal view obtained along the white line in A. The two arrows point to the cisterna
ambiens above the cerebellar hemispheres. Note the generous amount of cerebrospinal fluid around the cerebellar hemispheres (C). A small segment of
the cisterna magna (CM) is also shown below the vermis (V).
distance ( Figure 2–65 ) and age ( R
2
= 0.936; P = 0.0001)
and between the cerebroatrial distance and the biparietal
diameter, as well as between the atrial width–cerebroatrial
distance ratio and age.
It is hard to compare ventricular measurements taken
on axial sections using TAS with those obtained on paramedian sagittal sections using TVS. In spite of this, it
seems that the general trend, for example, the very slow
increase of fetal atrial size seen using the two methods, is
comparable.
52
C
Posterior Fossa and Upper Spinal Cord
Transvaginal sonographic examination of the posterior
fossa is feasible using a variety of approaches and scanning
planes. All of them are useful and informative.
After 10 to 12 postmenstrual weeks, several larger
structures of this anatomical area, such as the cerebellar hemispheres and the cisterna magna, are discernible
( Figures 2–35A, 1B, 2A, 2B ). At 16 to 18 postmenstrual
weeks, the posterior fossa lends itself to excellent imaging.