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30
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Cerebral
Divisions Subdivisions Cavities
hemisphere
Telencephalon
Lateral
Prosencephalon
Diencephalon
Mesencephalon
Metencephalon
3rd
Aqueduct
4th
Ep.
Orbitomeatal
plane
Hypophysis
Rhombencephalon
Myelencephalon
4th
Brain
stem
Spinal cord
Central
canal
Figure 2–16. The divisions, subdivisions, and brain cavities seen in an infant’s brain. The metencephalon consists of the cerebellum and the pons. The
myelencephalon is the medulla oblongata. The brain stem comprises the midbrain, pons, and medulla. The third ventricle is mainly diencephalic, but it
is partly telencephalic. The central canal is mainly in the spinal cord, but it is partly in the medulla. All of these parts are present by 5 postfertilizational
(7 postmenstrual) weeks. The subdivisions and the cavities can be detected by high-frequency transvaginal sonography, but only from about 8 to 8½
postmenstrual weeks. (From O’Rahilly and Müller, 2001,
37
with permission.)
b
Di
Mes
Mes
Met(R)
Te
PF
Met
My(R)
CF
My
BA
Figure 2–17. Median and coronal (posterior oblique) sections of an embryo at 8 postmenstrual weeks and 5 days (crown-rump length, 20 mm). (A)
Median section showing the following structures: telencephalon (Te), diencephalon (Di), and mesencephalon (Mes). The metencephalon (Met) and
myelencephalon (My) are parts of the rhombencephalon (R). The two main flexures, pontine (PF) and mesencephalic (CF), are also seen. (B) Coronal
oblique section through line b. (Modified from Timor-Tritsch and colleagues, 1991,
23
with permission.)
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
waves that, until this age, have created images of the fetal
CNS in a rather unrestricted fashion. The solution to this
problem is to use the anterior fontanelle ( Figure 2–37 )
as an acoustic window to continue scanning of the fetal
brain. As mentioned previously, scans obtained via the
MESENCEPHALON
CEREBELLUM
CHOROID
PLEXUS
anterior fontanelle do not provide parallel sections in the
coronal or sagittal plane. These sections are generated
radiating in an oblique fashion with their apex at the fontanelle, as shown in Figure 2–8 .
Coronal Planes
Using the anterior fontanelle as a sonic window, a few perfect midcoronal and a series of oblique coronal sections,
from anterior to posterior, can be obtained.
For easier understanding, but even more so for clearer
clinical use, three main groups of sections through the
above-mentioned planes are described. The three main
groups are the frontal, midcoronal, and occipital sections
( Table 2–2 ).
sible subsections. Their importance is discussed later.
41 , 42
Each of these may have two or three pos-
31
Figure 2–18. Coronal section at 9 postmenstrual weeks and 3 days,
showing the rhombencephalon (crown-rump length, 25 mm). The mesencephalon, cerebellum, and choroid plexus are marked. (Courtesy of
Blaas, Department of Obstetrics and Gynecology, National Center for
Fetal Medicine, Trondheim University Hospital, Trondheim, Norway.)
In general terms it is clear that at very early ages
(7 to 8 postmenstrual weeks) the rhombencephalon is
the dominant structure. Toward the 12th postmenstrual
week, the rhombencephalon appears to be small in
comparison to the telencephalon and the hemispheres
containing the choroid plexus, which at 7 to 8 weeks are
relatively small.
The composite picture at the end of the series
( Figure 2–29 ) allows viewers to clearly follow the changes
in size of each ventricle (and thus the surrounding brain
tissue) and their relationships. It is also evident in the
composite that the choroid plexuses of the lateral ventricles become increasingly large, occupying a larger
proportion of the lateral ventricles.
A significant change first noted at the 11th to 12th
postmenstrual week is the relative increase of the anterior
tip of the anterior horns of the lateral ventricles. Note
the anterior sonolucent area in front of the choroid plexuses ( Figure 2–23 ) in a fetus at 12 postmenstrual weeks.
This is the anterior horn of the lateral ventricle, and it
is difficult to overlook. For the next 2 to 3 weeks, it will
remain relatively large ( Figures 2–34, 2–35 , and 2–36 ).
Two simultaneous processes occur from 11 to 16 to 18
postmenstrual week: the choroid plexuses of the lateral
ventricles “move back” on top of the thalamus into its
final place—the body and the atrium of the lateral ventricle—and progressive growth of the cerebral cortex slowly
decreases the size of the horn. Indeed, toward term the
anterior horns become slitlike.
At around 14 to 16 postmenstrual weeks, the slowly
but constantly growing and thickening skull bones present an increasing obstacle for the high-frequency sound
Sonographic Anatomical Landmarks
Because of extensive changes in the size and location of
several anatomical landmarks that are well seen sonographically, it is practical to clearly separate two age
groups. One is the age group from 12 to 18 postmenstrual weeks; the second is above at least 18 but clearly
above 20 weeks. The groups require partitioning because
before 18 postmenstrual weeks, the frontal horn extends
frontally and is evident on any of the anterior (frontal)
coronal sections.
The sonographically identifiable anatomical landmarks in the coronal sections group are listed in Table 2–2 .
These landmarks are listed as subdivisions of the following
structures: skull, brain, ventricles, cavities, choroid plexus,
midbrain, cerebellum, and meninges. Their presence on all
of these sections is indicated.
41
There are several cardinal landmarks, that, if present
on a specific plane, are clear markers of the plane in question. Such anatomical markers are the orbits, the passage
of the choroid plexus into the third ventricle through
the interventricular foramina (Monro), and the posterior
horns. The presence of these structures indicates unequivocally that the section was taken at the frontal–2,
midcoronal–2, and occipital–1 sections, respectively.
These cardinal landmarks are highlighted in Table 2–2 for
easier understanding.
There is also a typical and unique clustering of several structures throughout each of the sections. Such
a clustering indicates that the section was obtained at
a specific and well-defined section. For example, if on a
coronal section the orbits and the anterior horns are seen
(without the choroid plexus), this can exclusively apply to
the Frontal–2 section. On the other hand, if one seeks out
the midcoronal–2 section, one would search for an image
containing the cross section of the corpus callosum, the
choroid plexus containing lateral ventricles, the cavum
septi pellucidi, the thalami, and the falx, all seen at the
same time. Only this one specific section is consistent
32
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
A
S
My
PF
Met
Mes
CF
Di
B
Figure 2–19. ( A) At 9 postmenstrual weeks and 1 day, it is possible to acquire multiple sections in the clanical planes. This picture illustrates five
coronal sections obtained using a high frequency transvaginal probe at this age. (B) Median section at 9 postmenstrual weeks and 5 days (crown-rump
length, 28 mm). The sonolucent and progressively tortuous ventricular system is imaged. Di, diencephalon; Mes, mesencephalon; Met, metencephalon;
My, myelencephalon; S, the entrance to the central canal; CF, mesencephalic flexure; PF, pontine flexure. (Modified from Timor-Tritsch and colleagues,
1991, 23 with permission.)
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
33
M
D
H
Umbilical cord
A
H
Heart
R
D
H
1 mm 1 mm
B
M
R
M
R
R
H
M
I R
R
C
Figure 2–20. Three-dimensional representation of the embryonic and fetal ventricular systems. (A) Lateral view of the brain cavities in an embryo of
13-mm crown-rump length (CRL). The outline shows the embryonic shape, with head and the umbilical cord. (B) Lateral view of the brain cavities in an
embryo of 24-mm CRL. The outline shows the embryonic head and eye. (C) Lateral view of the brain cavities in a fetus of 40-mm CRL. The outline shows
the fetal head and eye. (D) Oblique view of the brain cavities in a fetus of 40-mm CRL. H, cerebral hemisphere; D, diencephalon; M, mesencephalon;
R, rhombencephalon; IR, isthmus rhombencephali. (From Blaas and colleagues, 1995,
with the midcoronal–2 section. This concept holds true
for all the specific and unique sections obtained in the
two general planes.
38
Based on these easily recognized sonographic landmarks and the typical picture generated, we refer to
the frontal–2 section as the “steer’s head” configuration
( Figures 2–38A-O1 , 2–38C and 2–39E , and 2–39A ). The
occipital–1 section resembles the “owl’s eye” configuration
( Figures 2–38A , 2–39E , 2–40B , and 2–41D ).
1 mm
D
38
with permission.)
Frontal–2 is the typical steer’s head configuration
and should show only continuous white matter with
the anterior horns. The longitudinal fissure is seen
( Figures 2–39A–E and 2–41A–D ).
2. MIDCORONAL SECTIONS: Three distinct midcoronal sections can be separated. The common
denominator of all three is that the anterior horn,
with or without the choroid plexus, and the corpus
callosum are seen on all three. These are almost true
coronal sections because they are close to each other
Structures Seen on Coronal Sections
1. FRONTAL SECTIONS: At 12 to 18 postmenstrual
weeks, both frontal–1 and –2 images reveal the
widely open anterior horns of the lateral ventricles.
The frontal–2 section contains the orbits (steer’s head
configuration). Later during gestation, the frontal–1
(more anterior frontal) section “cuts” through the
white matter only. The longitudinal fissure and the
subarachnoid space containing the superior sagittal
sinus are present ( Figure 2–38A – C ).
and are generated by a very slight tilt of the probe
from the classical coronal plane.
Structures seen in the midcoronal–1 plane are
the laterally and upward slanted frontal horn of
the lateral ventricle, the corpus callosum, the
interventricular septum, and the cavum septi pellucidi between the heads of the caudate nuclei
( Figures 2–39B–C , 2–41B , 2–47 and 2–53A–B ).
Structures seen in the midcoronal–2 plane are
the lateral ventricles (body) containing the echogenic choroid plexuses, their extension through the
34
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Fx
Fx
CP
CP
Fx
3V
A
BC
FM
D
Figure 2–21. Serial horizontal (axial) sections at 11 postmenstrual weeks 3 days, from the top (far left) to the base of the skull (far right). The
third ventricle (3V) and the foramen magnum (FM) are seen. CP, choroid plexus; fx, falx. (Modified from Timor-Tritsch and colleagues, 1991,
permission.)
36
with
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
35
M
3
Figure 2–22. Horizontal (axial) section through the head of a fetus at 11 postmenstrual weeks and 6 days (crown-rump length, 49 mm). The view is
of the diencephalon (bold arrows) and its narrow third ventricle (3), and through the mesencephalon (large open arrows; M), with its wie cavity (small
open arrows). AH, anterior horns. (From Blaas and colleagues, 1984,
29
with permission.)
T
cpcp
f
T
cp
cp
f
3V
f
SS
1
12
AB C
4 cm
Figure 2–23. At 12 postmenstrual weeks 1 day, a frontal oblique section (A) shows the falx and the choroid plexus. Axial sections (B, C) reveal the
hyperechoic choroid plexus (cp) on top of the thalamus (T) in the lateral ventricles. Only a thin cerebral cortex mantle is still seen between the small
arrows. The third ventricle (3v), the falx (f), and the frontal part of the sagittal sinus (SS) are also seen. (Modified from Timor-Tritsch and colleagues,
23
with permission.)
1991,
36
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
AH
IH
CM
IH
1a
5b
+
+
CM
1b
1
3
2
5
4
5a
CP
T
T
234
Figure 2–24. Serial brain sections at 14 postmenstrual weeks. 1A, 1B. Frontal oblique sections, showing the longitudinal fissure (lH) and the anterior
horns (AH). Somewhat more posteriorly (1B), the choroid plexus appears (C). In sections 2 through 4 (midcoronal sections) the changing relationship of
the choroid plexus (C) to the thalamus (T) can be seen. 5A and 5B are more posterior occipital oblique sections showing the cisterna magna (CM) and
a bicerebellar diameter measurement of the cerebellum. (From Timor-Tritsch and Monteagudo 1991,
24
with permission.)
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
16w
CP
AH
37
CP
A
B
C
D
CP
C
C
t
t
T
C
CP
F
O
F
E
F
G
H
F
A
B
C
D
E
G H
Figure 2–25. Serial scans of the fetal brain at 16 postmenstrual weeks. (A) An extreme anterior frontal–1 section (see Table 2–2 ) almost tangential
to the fetal skull, showing the wide opening of the anterior fontanelle) (open arrow). (B) Frontal–2 section. The cortex and the white matter around the
anterior horns are becoming progressively thicker. The sonolucency of the anterior horn (AH) is outstanding. The subarachnoid space is marked by the
white arrow. (C) Midcoronal section. The choroid plexus (CP) and its connection through the interventricular foramina (small white arrows) are shown.
The midline arrow indicates the falx. (D) Midcoronal–3 section through the antrum of the lateral ventricles, which are filled entirely by the choroid
plexus (CP). The arrow indicates the falx. (E) Occipital–1 section. The posterior horns are marked by the small white arrows. The tentorium (t) is also
indicated. (F) Horizontal section. The sonolucent cisterna magna is highlighted by the large white arrow. The cerebellum (C) and the fourth ventricle
(small white arrow) are shown. The anterior horns (F) and the inferior horns (T) are visible using this section. (G) An oblique section highlighting the
choroid plexus (C) and one of the cerebellar hemispheres (C) (H) Right oblique–1 section with the anterior horn (F), posterior horn (O), and choroid
plexus (C) situated above the thalamus (T). The thick mass of the brain tissue is indicated by the two small arrows.
38
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
C
B
A
F
D&E
CM
C
V
C
P
P
C
CP
C
A
B
C
C
CM
MO
CMB
MO
D
E
F
Figure 2–26. These series of horizontal and sagittal sections are part of the workup of the posterior fossa in a normal fetus at 16 postmenstrual weeks.
(A) “Low” horizontal section. The cerebellum (C) measures 1.78 cm, and the cisterna magna measures 0.66 cm. Note the inferior pedunculi (P), the
hyperechoic lower portion of the vermis (V ), and the fourth ventricle (small arrow). (B) The somewhat higher horizontal section of the posterior fossa,
highlighting the small, threadlike structures of the arachnoid membrane (small arrows). (C) This is a higher, composite horizontal/coronal section that
shows the tentorium (small arrows). The cerebellum (C) and the choroid plexus (C), and the choroid plexus (C) with a tiny choroid plexus cyst (small
white arrow). (D) Almost median section showing the midbrain (MB), medulla oblongata (MO), hyperechoic vermis (small single arrow), and cerebellum (C). The tiny arrow points toward the medulla. This section clearly shows the upper portion of the medulla (multiple small arrows). (E) Paramedian
section. The structures are similar to those indicated in D. (F) Median section of the suboccipital region, highlighting the cerebellum (C), cisterna magna
(CM), medulla oblongata (MO), and spinal cord (small arrows).
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
39
A H
A
AH
B
AH
AH
OH
CP
CP
CP
C C
FM
T
C
T
D
Figure 2–27. Additional views of the same fetus imaged in Figure 2–25. In addition, all of these images highlight the subarachnoid space. (A) Frontal–2.
(B) Midcoronal–1 section. (C) Midcoronal–2 section. (D) Left oblique–1 section. The subarachnoid space is highlighted by small white arrows in all of
these sections. AH, Anterior horn; F, falx; CC, corpus callosum; FM, interventricular foramina; CP, Choroid plexus; T, thalamus; OH, posterior horn.
The two measurements in C are the distance from the midline to the tip of the anterior horn (1) and the depth of the anterior horn (2). These are normal
for this age.