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10
m
Chapter 1 Prenatal Development of the Brain
A
Thalamus
B
C
Commissure
of fornix
Dentate
gyrus
D
Figure 1–11. The development of the corpus callosum ( shown in black )
as seen on the medial surface of the left cerebral hemisphere at postmenstrual weeks 14, 15, 19, and 30. The great increase in its length throughout
the fetal period is evident. (Modified from O’Rahilly and Müller, 2006,
with permission.)
Choroid fissure
Fornix
Corpus callosum
Anterior
commissure
Stria
terminalis
Septum
pellucidu
Column
of fornix
Indusium
griseum
considerably during the second trimester ( Figure 1–11C ),
and the underlying portion of the commissural plate
becomes thinned as the septum pellucidum.
By the middle of prenatal life, the rostrum, genu,
central part, and splenium can be distinguished clearly
( Figure 1–11D ). During the second and third trimesters,
the corpus callosum (except in agenesis) gradually forms
a solid covering over the roof of the third ventricle. The
corpus callosum continues to grow into the third decade
of life.
A narrow cavity appears within the septum pellucidum and is known as the cavum septi pellucidi. It has
been described by some researchers as arising as a pocket
that at first opens into the longitudinal fissure but later
becomes sealed by the rostrum of the corpus callosum.
According to others, however, the cavum is formed by
necrosis within the commissural mass and was never open
to the subarachnoid space. The cavum can be identified
during the second trimester, after which its posterior
portion becomes obliterated before birth, as occurs to its
anterior portion usually within a few months after birth. If
the posterior portion persists, it is frequently termed the
cavum Vergae (see Chapter 2 ).
The gyrus cinguli, or cingulate gyrus, is so named
because it forms a partial girdle around the corpus callosum and follows the callosal curve. It can be distinguished
on the medial surface of the cerebral hemisphere during
the second trimester. The hippocampal formation (dentate
gyrus, hippocampus, subiculum, and parahippocampal
gyrus) begins early as bilateral hippocampal thickenings
(5 weeks), and its characteristic S-shape becomes clear during the second trimester.
Agenesis of the corpus callosum is assumed to arise
already in the commissural plate or field ( Figure 1–4 ),
which is a thickening of the embryonic lamina terminalis
that appears very early (stage 12).
THE VENTRICULAR SYSTEM 20
The ventricles are a significant feature on ultrasonography,
and their development is described briefly here. The ventricular system, including the central canal of the medulla
and the spinal cord, is derived from the cavity of the neural
tube. During the embryonic period and the early portion of
the fetal period, the walls of the brain are very thin in most
regions; hence, the ventricular system is relatively very
large ( Figure 1–12A and B ). Later, as the walls increase in
thickness, the ventricles occupy relatively less of the volume of the brain ( Figure 1–12C and D ).
In the region of the rhombencephalon, the roof of
the brain is already noticeably thin at about 4 to 5 weeks
after fertilization, thereby indicating the fourth ventricle
( Figure 1–3C ), the floor of which has become rhomboid.
Extensions of the ventricle that are already detectable early
become the lateral recesses ( Figure 1–7C ) of the fourth
ventricle. The median aperture of the fourth ventricle
6
appears, at the earliest, at the end of the embryonic period
and is almost constant from early in the fetal period. The
lateral apertures appear later, probably during the second
trimester.
The cavity of the midbrain (the mesencephalic ventricle) remains relatively wide throughout the embryonic
period ( Figure 1–5 ), at the end of which its ends become
slightly constricted.
during the fetal period and would seem to justify the name
aqueduct at the end of the first trimester.
As the cerebral hemispheres begin to develop, the
cav ities become the lateral ventricles ( Figure 1–3B ),
and the cavity of the telencephalon medium and the
diencephalon becomes delimited as the third ventricle
( Figure 1–3B and D ). The temporary cavities of the optic
7
It becomes gradually more tubular
Chapter 1 Prenatal Development of the Brain
B
11
A
I
30
C
T
P
100
Figure 1–12. (A ) Right lateral outlines of the brain postmenstrual weeks 9½, 12, and 13. (B) , (C ), and ( D ) Right lateral ventricle at postmenstrual weeks
10, 13, and 19. A comparison of views B , C , and D shows that the wall of the hemisphere ( shaded ) is becoming thicker; hence, the ventricle appears
relatively smaller as age advances. The small evagination from the anterior horn is the temporary “olfactory ventricle.” A, anterior horn; C, central part;
I, inferior horn; P, posterior horn; T, trigonum. ( A is based on Hochstetter,
C
I
A
100
50
25
D
A
P
150
17
B on O’Rahilly and Müller,
C
T
I
A
6
and C and D on Westergaard. 22 )
cups (the optic ventricles) and their stalks are originally
evaginations of the diencephalon. The openings between
the third and lateral ventricles become very gradually narrowed ( Figure 1–3B and D ) to form the interventricular
foramina ( Figure 1–5 ).
Growth of the corpus striatum during the last week of
the embryonic period further narrows the interventricular foramen ( Figure 1–9A ) and transforms the formerly
spherical lateral ventricle into a characteristic C-shaped
cavity ( Fig. 1–12B ), which extends from anterior (its
frontal horn) to inferior (its temporal horn).
6 , 21 , 22
The posterior (occipital) horn ( Figure 1–12C ) develops at about
12 postmenstrual weeks and is very obvious early in the
second trimester. The following subdivisions can now be
distinguished: the anterior or frontal horn, the central part
(which is not really a “body”), and the trigone (frequently
referred to as the atrium), from which proceed the posterior or occipital horn and the inferior or temporal horn
( Figure 1–12C and D ). (Strictly speaking, the collateral
trigone is the ventricular floor between the posterior and
inferior horns, but the term trigone is commonly used for
a portion of the cavity.)
Three different liquids are successively in contact
with the developing brain: the amniotic fluid until closure
of the neuropores, the ependymal fluid after closure, and
the cerebrospinal fluid after the formation of the choroid
plexuses.
The choroid plexuses, first that of the fourth ventricle
and then those of the lateral ventricles ( Figure 1–3D ),
develop between postmenstrual weeks 8 and 9. The plexuses are noticeably voluminous in the lateral ventricles
( Figure 1–7 ) during the embryonic period and on into
fetal life, as is readily seen on ultrasonography. The choroid plexus of the third ventricle develops early in the fetal
period, and the interthalamic adhesion (formerly termed
the massa intermedia ) may develop, in some instances,
before the middle of prenatal life.
Although indications of the subarachnoid space
appear much earlier, the space and most of the cisternae
are distinct at the end of the embryonic period, at which
time the cerebellomedullary cistern (the cisterna magna) is
also discernible.
23
MYELINIZATION
Reflexes can occur while nerve fibers are still unmyelinated, and embryonic and fetal movements during the
first trimester take place before the onset of myelinization. Myelinization in the CNS begins during the second
trimester, although the cerebral hemispheres contain little
myelin at birth. Myelin is deposited more rapidly during
the first two postnatal years, but the process continues into
adulthood.
Fibers associated with related functions tend to
become myelinated at the same time, and cortical association fibers are the last to be involved. It is believed
that the state of myelinization indicates the functional
maturity of the brain and is correlated with psychomotor
12
Figure 1–13. Left lateral views of the developing head, showing the orbitomeatal plane and examples of the many possible coronal planes. (A ) End of
the embryonic period (stage 23) showing the orbitomeatal plane as determined from graphic reconstructions by the authors. ( B) A fetus at postmenstrual
week 12 illustrating that transverse and “coronal” planes through the trunk differ considerably from horizontal and coronal planes through the head.
( C) Neonatal skull showing a coronal plane through the anterior fontanelle. (D ) Adult skull showing a coronal plane through the bregma.
Chapter 1 Prenatal Development of the Brain
C
AB
“Coronal”
Transverse
D
Coronal
Orbitomeatal plane
development. Magnetic resonance imaging is particularly
suitable for assessing the progress of myelinization.
PLANES 24
The three main sets of planes used in anatomy are the
horizontal and the two vertical series: coronal and sagittal. One of the sagittal planes is median. There is no limit
to the number of sagittal and coronal planes, so that sections are in a, not the, sagittal or coronal plane. Although
frontal is frequently used as a synonym for coronal, in strict
usage the term frontal should be reserved for the antonym
of occipital. A particularly important plane in the head
is the orbitomeatal ( Figure 1–13D ), because it is used to
ensure that the head is in the standard position. For this
purpose, the plane is kept horizontal. In other words, the
orbitomeatal and the numerous possible horizontal planes
in the adult are all parallel with each other. Strictly coronal
planes can be defined as those vertical planes that are at
a right angle to the orbitomeatal plane and also at a right
angle to the median plane. Strictly coronal planes are necessarily parallel with one another.
It needs to be emphasized that, in anatomy and
embryology in general, the unofficial word midsagittal
should not be used for median . Coronal and sagittal refer
to planes parallel to, but not necessarily through, the coronal and sagittal sutures, respectively. All planes parallel to
the median plane are sagittal, so that the unofficial term
parasagittal is redundant and should be eliminated. If
necessary, a plane particularly close to the median could
justifiably be termed paramedian.
A scheme has been prepared for the embryonic
( Figure 1–13A ), fetal ( Figure 1–13B ), and neonatal
( Figure 1–13C ) heads, positioned in relation to the orbitomeatal plane. The situation is complicated prenatally,
however, by the curvature of the body and particularly by
the flexion of the head. As a result, a transverse section
of the trunk ( Figure 1–13B ), which would correspond to
a horizontal section in the adult, is no longer parallel to
the orbitomeatal plane. Similarly, “coronal” planes (of the
adult type) in the prenatal trunk differ considerably from
those in the head, that is, from vertical planes at a right
angle to the orbitomeatal ( Figure 1–13B ).
The above-mentioned differences are important in
discussions of the imaging of the prenatal brain. A further
complication arises because many of the planes used in
prenatal ultrasonography are oblique, as will be explained
in Chapter 2 .
REFERENCES
1. O’Rahilly R, Müller F. Developmental Stages in Human Embryos,
Including a Revision of Streeter’s “Horizons” and a Survey of the
Carnegie Collection . Publication 637. Washington, DC: Carnegie
Institution of Washington, 1987.
2. O’Rahilly R, Müller F. Embryonic length and cerebral landmarks in
staged human embryos. Anat Rec. 1984;209:265–271.
3. Böhmer S, Bruhns T, Degenhardt F, et al. Vergleich von vagino- und
abdominosonographischen Messergebnissen mit embryologischen
Wachstumskurven der Frühschwangerschaft. Geburtsh Frauenheilk.
1993;53:792–799.
4. Wisser J, Dirschedl P, Krone S. Estimation of gestational age
by transvaginal sonographic measurement of greatest embryonic
length in dated human embryos. Ultrasound Obstet Gynecol.
1994;4:457–462.
5. O’Rahilly R, Müller F. Prenatal ages and stages: Measures and errors.
Teratology . 2000;61:382–384.
6. O’Rahilly R, Müller F. The Embryonic Human Brain: An Atlas of
Developmental Stages . 3rd ed. New York: Wiley-Liss; 2006.
7. O’Rahilly R, Müller F. Significant features in the early prenatal devel-
opment of the human brain. Ann Anat. 2008;105–118.
8. O’Rahilly R, Müller F. The two sites of fusion of the neural folds
and the two neuropores in the human embryo. Teratology . 2002;
162–170.
9. Müller F, O’Rahilly R. Cerebral dysraphia (future anencephaly) in a
human twin embryo at stage 13. Teratology . 1984;30:167–177.
10. Müller F, O’Rahilly R. The development of anencephaly and its vari-
ants. Am J Anat. 1991;190:193–218.
Chapter 1 Prenatal Development of the Brain
13
11. Müller F, O’Rahilly R. Mediobasal prosencephalic defects, including
holoprosencephaly and cyclopia, in relation to the development of
the human forebrain. Am J Anat. 1989;185:391–414.
12. O’Rahilly R, Müller F. Interpretation of some median anomalies as
illustrated by cyclopia and symmelia. Teratology . 1989;40:409–421.
13. Hoving E W. Frontoethmoidal Encephaloceles . Groningen: Rijksuni-
versiteit; 1993.
14. Padget DH. The development of the cranial arteries in the human
embryo. Contr Embryol Carnegie Instn . 1948;32:205–261.
15. O’Rahilly R, Müller F. Human Embryology and Teratology . 3rd ed.
New York: Wiley-Liss; 2001.
16. Müller F, O’Rahilly R. The timing and sequence of appearance of
neuromeres and their derivatives in staged human embryos. Acta
Anat . 1997;158:83–99.
17. O’Rahilly R, Müller F. Somites, spinal ganglia, and centra. Cells
Tissues Organs. 2003;173:75–92.
18. Müller F, O’Rahilly R. The human chondrocranium at the end of the
embryonic period proper, with particular reference to the nervous
system. Am J Anat. 1980;159:33–58.
19. Müller F, O’Rahilly R. Segmentation in staged human embryos: The
occipitocervical region revisited. J Anat. 2003;203:297–315.
20. O’Rahilly R, Müller F. Ventricular system and choroid plexuses of
the human brain during the embryonic period proper. Am J Anat.
1990;189:285–302.
21. Hochstetter F. Beiträge zur Entwicklungsgeschichte des menschlichen
Gehirns. I. Teil. Vienna: Deuticke; 1919.
22. Westergaard E. The lateral cerebral ventricles of human fetuses with
a crown rump length of 26–178 mm. Acta Anat . 1971;79:409–421.
23. O’Rahilly R, Müller F. The meninges in human development.
J Neuropathol Exp Neurol . 1986;45:588–608.
24. O’Rahilly R. Making planes plain. Clin Anat . 1996;10:128–129.
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Chapter 2
NORMAL TWO- AND THREEDIMENSIONAL NEUROSONOGRAPHY
OF THE PRENATAL BRAIN
Ilan E. Timor-Tritsch ● Ana Monteagudo ● Maria Del Rio
KEY POINTS
1. Neurosonography of the fetus and the neonate is an
informative and noninvasive as well as inexpensive
modality that is of great benefit in clinical diagnosis.
2. As technology improves and our understanding of
sonoanatomy of the fetal and neonatal brain widens,
the distinction between normal and abnormal, as
far as anatomy is concerned, becomes increasingly
possible.
3. Anomalies and disease of the developing brain area
are common. In order to diagnose a deviation from
what is normal, it is important to understand and
recognize the developmental milestones of the
normal brain how it grows, and matures, reaching its
almost final anatomy at birth.
4. This chapter presents the sonographic landmarks
of developing embryonic and fetal CNS and also
suggested new ways to study it systematically.
5. The introduction of TVS in fetal neurosonography
adds an additional powerful tool to the diagnostic
algorithm, increasing diagnostic precision in patients
suspected of an anatomically abnormal fetal CNS.
6. In the future, 2D as well as 3D fetal neurosonography
will be used routinely in all pregnant patients to
examine the developing brain, the same way we now
examine other fetal organs and organ systems.
Problems of the central nervous system (CNS) can range
from very simple, that is, merely a variance of the normal,
to the most devastating diseases incompatible with life. It
is important to recognize these anomalies as the fetus is
scanned throughout gestation, starting very early in the
first trimester to late in the third.
The prerequisite for differentiating normal from
abnormal structures is a thorough knowledge of the CNS
anatomy. It is beyond the scope of this and the following
chapters to teach the reader advanced neuroanatomy;
however, special emphasis is placed on describing basic
but sufficiently detailed sonographic neuroanatomy to
recognize structures seen by transabdominal (TAS) or
transvaginal sonography (TVS). Those interested in scanning the fetal brain to detect deviations from the norm
should first refresh their knowledge of neuroanatomy.
It is not sufficient to know only the anatomy of the
full-term fetal or neonatal brain. Dealing with the sonographic anatomy and pathology of the fetal brain requires
an additional dimension that is of the utmost importance
for correctly evaluating the CNS at various prenatal ages of
the fetus, that is, knowledge of the evolution of structures
from about 6 to 7 postmenstrual weeks to the time of birth.
Almost all organs and organ systems are in place by the end
of the embryonic period. However, only at approximately
14 to 16 postmenstrual weeks would some of them—the
heart and the kidneys, for example—perform almost at the
level of perfection in the final month.
All that most organs do during the fetal period is
increase in size. The brain, on the other hand, undergoes major developmental changes almost until the last
several postmenstrual weeks of intrauterine life. Good
examples of this are the changes in the size of the ventricles; the appearance and completion of the development
of the corpus callosum; and the deepening, branching,
multiplying, and growth of the sulci and the gyri. It is
therefore essential to understand the development of
various parts of the prenatal brain in order to evaluate it
and differentiate abnormal from normal development.
We deal with the prenatal brain in this book. This
chapter covers normal anatomy as viewed by sonography,
emphasizing the development of the fetal brain from 6 to
7 postmenstrual weeks to term. Of course, the CNS starts
to develop at much earlier prenatal ages. However, at these
very early prenatal ages, probably up to 7 postmenstrual
weeks, the tiny structures still cannot be recognized
by presently used ultrasonographic technologies. These
methods were dealt with in Chapter 1 , on the embryology and development of the early prenatal brain. This
chapter therefore will begin with the description of the
sonographic appearance of the CNS starting at 6 to 7 postmenstrual weeks.
The reader’s expectation for this chapter should be
limited to enhancing his or her understanding of the normal prenatal brain anatomy before engaging in the diagnostic process and describing its pathology.
16
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
ULTRASOUND EQUIPMENT
The customary ultrasound (US) equipment is used in
imaging the prenatal brain. The two types of US probes
employed are the transabdominal and transvaginal US
transducer probes. Imaging the fetal brain depends
on penetration of the sound waves as well as acoustic
impedance of the tissues along the sound path. The
acoustic impedances of most biologic tissues are similar.
Therefore, only a small fraction of the sound is bounced
back at each of these interfaces. The sound is thus successfully transmitted to deeper tissues, which can then
be imaged. The soft tissue–bone interface is an exception to this rule. Bone has much higher impedance; thus,
at the interface, due to the reflected sound, a very strong
echo is produced. The sound energy reflected back is
significant, and only a fraction of the attenuated sound
waves are transmitted to deeper structures. To illustrate
the magnitude of this effect, the acoustic impedance of
bone is about 7 times that of water or soft tissues in the
fetal body. Imaging of structures behind bone therefore
becomes problematic because an acoustic shadow is created. As the fetal skull bones thicken and calcify during
the course of gestation, fewer and fewer sound waves
penetrate to enable imaging of the brain.
Imaging is also frequency dependent. The higher
the frequency is, the better the resolution of the picture.
However, the price we pay for increased picture resolution is reduced penetration, or the reduced half-intensity
depth. The latter describes the ability of sound to penetrate
and is expressed as the thickness of the tissues at which
the sound intensity is reduced by half. This half-intensity
depth decreases with increased frequency, correlates well
with the attainable imaging depth, and is greatly dependent on the medium in which it travels. Excellent through
transmission of sound in fluids such as blood or even
in body tissue results from their weak sound-absorbing
properties. However, bone and air strongly attenuate the
intensity of sound. It is clear that the high acoustic impedance of the skull bone, as well as its property to attenuate
the intensity of sound, greatly influences the way the brain
can be imaged inside its bony case.
It was necessary to find imaginative ways to scan the
fetal as well as the neonatal brain. By nature, transabdominal probes use lower frequencies to obtain deeper
penetration and greater half-intensity depth. This is one
way to penetrate beyond the bony skull. Another way is
to scan through “windows” of the skull. These windows
are, of course, the fontanelles. The younger the fetus
is, the larger the fontanelles and sutures. Because these
fontanelles measure ∼1 to 2 cm in width, it is natural
that sector or curvilinear scanners having a small footprint yield a better picture of the fetal and neonatal
brain. These special transducers yield images that are of
diagnostic value, even if they are acquired via the transabdominal route using the relatively open sutures, and
fontanelles ( Figure 2–1A ).
Another inventive way to scan the fetal brain is to use
extremely high-frequency probes, such as 6.5 to 7.5 or
even 9 MHz. Such probes are used in transvaginal gynecologic scanning. If the fetus is in the vertex presentation, it
is relatively easy to maneuver the fetal head and the vaginal transducer into a position from which the device can
“see” through a fontanelle. If this is successfully achieved,
extremely clear pictures of the fetal brain from 13 postmenstrual weeks to term can be obtained.
Since we became aware of the possibility of obtaining
limited access to the brain using the transvaginal probe,
we have been using it whenever possible (ie, when the
fetus is in the vertex presentation). The rather narrow
but still open “window” or space between the two parietal
bones; namely, the sagittal suture enables by placing the
footprint of the vaginal probe over the sagittal suture,
a satisfactory picture of the mid-brain structures in the
median plane can be obtained. Because of the narrow
nature of this space, it is almost impossible to obtain an
image of lateral structures.
Transabdominal scanning of the fetal brain using the
anterolateral fontanelle or the squamosal suture can, in
certain cases, yield clear and clinically diagnostic images.
However, the younger the fetus is, or the smaller the structure in question is that must be scrutinized, the more the
transabdominal probes will be at handicap as opposed to
the higher-resolution transvaginal probes. Under matched
conditions the transvaginal probes, which operate at
higher frequency, produce better and clinically more useful pictures.
image quality obtained by the abdominal route. This
modality employs compound scanning techniques.
has become more readily available; therefore, a short
explanation of these probes and their use is in place.
Both the transabdominal and the transvaginal 3D US
probes (at this time) are mechanical transducers. They
are controlled by a timing module that determines the
number of continuously advancing US pictures acquired
and saved into a volume. The acquisition speed ultimately
determines the picture resolution and quality. For moving structures (eg, a moving fetal part), high acquisition
speed is required; for stationary organs (eg, in gynecology), lower acquisition speed is adequate, which enables
obtaining many more sections, hence a higher-resolution
end product. In view of the fact that the fetal brain is
a stationary organ (except when the fetal head moves),
when scanning the fetal brain, one can select a lower
acquisition speed. The technique of 3D volume acquisition is relatively simple, and given that an increasing
number of new US machines offer 3D technology, it is
essential that those readers interested in enhancing their
fetal neurosonography skills get acquainted with this
powerful scanning technique.
1
Lately, electronically steered transducers can improve
Over the last several years, three-dimensional (3D)
SCANNING CONCEPT
Fetal brain scanning has emerged from the vast experience gained with neurosonographic imaging of the
neonate. The fetal as well as the neonatal head is
scanned using the three main body coordinates: the
sagittal, coronal (frontal), and horizontal axial planes
( Figure 2–1B ). Initially, scanning of the neonatal brain
was done through the temporal region, obtaining axial
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
B
17
Antero-
lateral
fontanelle
Sagittal
suture
Metopic
suture
Anterior
fontanelle
A
Parietal
bone
Occipital
bone
Parietal
bone
Frontal
bone
Anterior fontanelle
Squamosal suture
Parietal
bone
Temporal
bone
Parietal
bone
Parietal
bone
Posterior fontanelle
Metopic suture
Suture
Frontal
bone
C
Parietal
bone
Temporal
bone
Occipital
bone
Figure 2–1A. Schematic and 3D ultrasound illustration of the fetal skull showing its bones, sutures, and fontanelles. The spaces between the bones
can be used as access to the brain. (A) Lateral, posterior and superior views (www.uprightdoctor.files.worldpress.com), (B) and (C) are 3D maximum
mode displays of cranial bones and sutures.
sections.
2 – 9
To achieve this imaging, lower-frequency
transducers were used. Two factors contributed significantly to the improved resolution of one neonatal scan:
the higher-frequency US transducers (mainly the sector
and the small-footprint curvilinear probes) and use of
the anterior fontanelle as an acoustic window. The pictures obtained are in the median, paramedian, and different coronal sections, as well as oblique, sections.
10 – 19
should be stated at the outset that TAS of the fetal brain
usually provides axial and coronal sections. However,
it is extremely difficult or almost impossible to obtain
sagittal sections ( Figure 2–2 ). At times, though, sagittal
sections are needed for imaging of different pathognomonic features and diseases of the brain. Transvaginal
scanning through the anterior fontanelle provides us
with such median, paramedian, coronal, and oblique
sections, much like neonatal scanning ( Figure 2–3 ).
Another advantage of scanning the fetal brain using TVS
is that the scanning planes obtained are identical and
therefore comparable to those performed in the neonate. Continuity of follow-up and comparison between
fetal and neonatal scans are then possible by pediatric
neurologists and neurosurgeons. The input of these
consultants is therefore relevant even in the prenatal
period.
An issue of some importance is that fetal neuroimaging requires the use of an end-firing, symmetrical, in-axis
(or in-line) vaginal probe. Using an end-firing, off-axis
vaginal probe makes symmetrical imaging of the brain
and maneuvering of the probe extremely cumbersome
It
( Figure 2–4 ). The orientation process is also affected,
with regard to its speed, simplicity, and teaching. Most
off-axis probes require constant use of the left-right
orientation key on the control panel to correctly display
orientation on the picture.
Blaas et al
21
described the use of the 3D US probe.
The development of the three different structures in the
brain was defined using this technique. The information
20
obtained by this “spatial scan” was stored in the computer
and enabled the user to obtain different pictures, creating
not only coronal, sagittal, and axial planes but also selected
planes to highlight the spatial embryonic brain anatomy
( Figure 2–5 ) and pathology. Until the 3D technology
becomes universally available and widely used, we must
18
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Figure 2–1B. The “classical” planes
used in the imaging of the fetal brain.
The vertical planes are either coronal or
sagittal. One of the latter planes is the
median plane. There are infinite numbers of coronal and sagittal (paramedian)
planes. As far as fetal neurosonography
is concerned, the coronal and sagittal
planes are reached through the anterior
fontanelle in a fetus presenting with the
vertex. Several sections through each
plane can be generated. The horizontal
(axial) planes are achieved classically by
a transabdominal 3D scanning.
Horizontal
(Axial)
Median
Sagittal
(Paramedian)
Coronal
(Frontal)
Transabdominal
Transvaginal
Axial
SagittalCoronal (?)
Coronal
90°
Vaginal transducer
Figure 2–2. Schematic illustration of fetal neuroscanning routes. The transabdominal and the 3D routes classically provide axial or coronal views, but
rarely sagittal views. The transvaginal approach rarely yields axial views. However, sagittal and coronal planes typically are obtained.
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Sagittal
19
Coronal
Pressure
Coronal
Pressure
Figure 2–3. Schematic drawing depicting the technique of transvaginal sonography during the second and third trimesters. Inset: The relationship of
the anterior fontanelle to the transvaginal transducer is demonstrated. (From Monteagudo and colleagues, 1991,
20
with permission.)
AB
Figure 2–4. The different scanning planes of the transvaginal probes most often used. (A) Transvaginal fetal neuroscans are imaged best with an end-
firing, symmetrical, in-axis probe. (B) The tilted, off-axis scanning plane generated by a curvilinear transvaginal probe. In order to scan hemispheres at
the same time, the shaft of the probe must be moved from side to side. This may be uncomfortable for the patient, or it may simply be impossible. At
times, the probe must be rotated 180° to direct the scanning plane to the other hemisphere.