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20
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
ab c d e
10.1
7/2
Figure 2–5. In vivo 3D ultrasound reconstructions of embryos and early fetuses. (From Blaas HG, 1998,
mentally re-create the necessary planes and sections. Thus,
knowledge of neuroanatomy remains an important issue.
Another technique that can be used to better describe
the anatomy of the fetal brain is color Doppler flow studies. If color flow studies are used, a textbook on neuroanatomy, particularly on the arterial and venous network
of the fetal brain, should initially be studied. Color and
power Doppler flow studies may become important when
anatomical structures such as space-occupying lesions,
degenerative changes, or hemorrhages of the brain are
examined. Another area still under investigation is the
physiology of the blood supply to the brain. This is covered in Chapter 15 .
17 21.5 29 38
8/2
8/6
9/6 10/4
scanned, such a change in the fetal presenting part most of
the time can be brought about without effort or can occur
spontaneously.
Transvaginal 2D and 3D neuroscanning can be used
as early as 10 to 14 postmenstrual weeks. Its technique
is relatively simple.
pared in the customary fashion, covering it with a clean
condom or one of the digits of a surgical rubber glove
after contact gel has been applied to the tip of the probe
and, finally, applying some lubricating (K-Y) gel onto the
covered tip, making it ready for vaginal insertion. Due
to increasing reports of latex allergies in the population,
special prelubricated polyvinyl vaginal probe covers are
21 , 24 , 31 , 32
available. The patient should be in the lithotomy posi-
SCANNING TECHNIQUE
Because TAS is easy to perform and is used routinely
by those who engage in ultrasonography of the fetus, it
seems redundant to describe its technique. Transvaginal
two-dimensional (2D) and 3D scanning of the fetal brain,
however, requires more experience and is skill dependent;
therefore, we describe it here in detail.
Obviously, due to its small size, the embryonic and
early fetal CNS requires the use of a high-resolution transvaginal US probe.
trimester are described later.
Scanning the fetal brain in the second and third
trimesters via the vaginal route requires the same safety
guidelines used for the customary speculum and palpatory
examinations during pregnancy. If these can be performed
safely in the second trimester, there is no contraindication
to insertion of the vaginal probe for scanning the fetal
brain. Of course, as stated before, the fetus must be in the
cephalic presentation. At times, when it is of the utmost
importance to obtain accurate and more detailed information regarding a disease state, or if an additional sagittal
view would significantly contribute to the diagnostic process, it may be important to consider the external cephalic
version of the fetus. When a second-trimester fetus is
22 – 30
The CNS structures seen in the first
tion, preferably lying on a gynecologic examination table.
Constantly following the image created by the advancing
probe on the monitor, the first structure to be viewed (and
evaluated as well as measured) is the cervix. The tip of the
probe is typically placed on top of the anterior cervical lip.
If the patient’s bladder is full and displaces the fetal head
upward, the patient should be asked to void before the
examination proceeds.
To obtain a clear image of the fetal brain, it may be
necessary to maneuver the probe and/or the fetal head
into the most convenient position. This will be achieved
if the axis of the probe and the median plane of the
fetal brain (ie, the falx cerebri) are in line ( Figure 2–3 ).
Usually, the operator must use both hands to point the
probe to the anterior fontanelle or along the metopic
and/or sagittal sutures and hold the fetal head in the
desired position. Unless skilled help is available to freeze
the image and trigger the recording device, a foot pedal
is necessary to perform all of the necessary tasks at the
same time. An active fetus may be extremely hard to
stabilize by the abdominally placed second hand of the
operator. To wait for the fetus to enter a quiet sleep state
(state IF) may be a time-consuming option. However,
the scanning of an almost motionless fetus in deep sleep
does have its dividends.
40
30
20
10
0
mm
weeks
21
with permission.)
The transvaginal probe is pre-
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
21
Crystal
2.3 mm
1.5 cm
5 mm
A. Sagittal B. Axial
Figure 2–6. The effect of slice thickness, or the third dimension of ultrasound imaging, is demonstrated. (A) Due to the slice thickness, even at the focal
point, a scan in the median plane may include the entire width of the embryonic head. (B), (C) In axial or coronal planes, two or three sections may be
possible. Starting with the fetal period, an increasing number of sections become feasible.
ORIENTATION AND SCANNING PLANES
In addition to the previously mentioned advantages and
disadvantages of TAS and TVS of the fetal CNS, another
caveat should be introduced. At times, it is hard or too
time-consuming to obtain perfect planes and classical sections of the brain using TVS. This is, of course, a result of
the somewhat limited mobility of the vaginal probe and/or
the almost constantly moving fetus. Sometimes the fetal
head position is such that it prevents the imaging of clear
planes. Complementing the scan with an attempt at TAS,
rescheduling the scan, or simply allowing the patient to
walk for some time may improve the results.
Figure 2–1B depicts the three well-known classical
planes of the body, in terms of its planes and sections.
These are described below. Additional terms used in the
orientation process are rostral or anterior (toward the face);
occipital , posterior , or nuchal (toward the back); left lat-
eral or right temporal (toward the respective ear); caudal,
basal , or inferior (toward the base of the skull); and medial
(toward the middle). It should be noted that the sagittal plane traversing the middle of the body is termed the
median (do not use “midsagittal”) plane (see Chapter 1 ).
Crystal
may include the entire width of the head due to the relatively thick imaging slice ( Figure 2–6A ). This is true even
for a high-frequency transvaginal transducer; in the axial
or coronal planes, it may be possible to obtain several
sections due to the somewhat larger size of the growing
embryo ( Figure 2–6B and C ).
Controlling the thickness of the slice becomes important in postprocessing of 3D US pictures derived from
the volume scan. Using the software of several 3D US
machines, it is possible to select the desired slice thickness,
thus enhancing the displayed image clarity.
Around 9 postmenstrual weeks the size of the embryonic head becomes large enough to yield several sections
in each of the three cardinal planes. Because of the small
head size, the sections can be obtained in almost a perfect
parallel fashion. This becomes increasingly difficult later in
pregnancy, as discussed in subsequent sections.
It should also be clear that the terms axial ( horizontal )
and coronal ( frontal ) planes refer to the trunk and become
somewhat unclear if applied to the anteflexed embryonic
head. Therefore, a coronal plane of the trunk would be
axial of the flexed head. It is our understanding, therefore,
that the term axial in an embryo at 8 to 10 postmenstrual
Crystal
C. Coronal
weeks is in a plane parallel with the base of the skull or the
Scanning before 12 Postmenstrual Weeks
Ultrasonographic images of the embryonic (up to 9 postmenstrual weeks) or fetal (at and after 9 postmenstrual
weeks) period is heavily dependent on several factors.
The first factor is the resolution, which, in turn, is a
function of the frequency at which the crystals operate.
The frequency and, in some ways, the diameter of the
piezoelectric crystal determine the slice thickness. The
orbitomeatal plane (see Chapter 1 ). A coronal plane of the
same fetus is one at 90° to the axial plane ( Figure 2–7 ).
If for any reason (eg, research or clinical observation)
it is necessary to image the brain in the first trimester, the
observer should place the region of interest rigorously into
the exact focal point of the transducer. Thus, it is essential to be knowledgeable about the respective transducer
specifications in order to generate high-quality images.
thicker the slice that is insonated and imaged, the more
information is “collapsed” into the 2D picture seen on
the screen. This slice thickness may, in many cases, be
several millimeters. In the case of an embryonic brain at
7 or 8 postmenstrual weeks, the median imaging plane
Scanning after 12 Postmenstrual Weeks
With its increasing size, the developing fetal brain becomes
progressively available for high-frequency 5 to 12 MHz
22
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
For the head
Axial
(horizontal)
D. Rosenzweig
Axial
(horizontal)
Coronal
For the body
Figure 2–7. Due to the flexed posture of the embryonic or early fetal
head, the plane that is coronal as far as the trunk is concerned becomes
an axial plane when applied to the brain. In the same way, if an axial or
transverse section of the trunk is applied to the head, it generates a coronal section of the brain.
Coronal
TVS. The pictures obtained in the different scanning
planes are thus of diagnostic quality. In other words, major
diseases of the CNS can be diagnosed, starting at 9 to 11
postmenstrual weeks.
With regard to scanning the fetal brain after 12 weeks,
especially after weeks 14 to 15, it is at this point that the
thickening skull bones attenuate the high-frequency sound
waves of the vaginal probe to a very low level, making
imaging from any randomly selected direction increasingly impossible. This is the time at which the window
to the fetal brain (ie, the fontanelle) becomes important.
This relatively narrow gateway for the sound waves forces
certain restrictions on the sonologist or sonographer. First,
the tip of the transducer must be kept over the fontanelles
or the sagittal (or metopic) suture, to achieve the full scanning potential of the sound output. Second, the different
scanning sections in the sagittal, coronal, and oblique
planes should be generated by tilting the transducer back
and forth ( Figure 2–8 ). Because of the reasons mentioned,
these successive sections are not parallel to each other
in any given plane, as, for example, with the shifting but
always parallel planes of computed tomography (CT) and
magnetic resonance imaging (MRI).
This transfontanelle scanning by TVS, common to
prenatal and neonatal brain scanning, is really performed
in a “radial” fashion. Some of the sagittal and coronal sections are therefore oblique-sagittal and oblique-coronal
sections. In the sagittal plane, then, left and right oblique
sections are generated; in the coronal plane, occipital and
frontal oblique sections are generated ( Figure 2–8 ).
However, as we will see later, if a 3D brain volume
is acquired, regardless of the transducer or the scanning
route used, the final display will use perfectly parallel
planes and sections, as in MRI and CT.
It is important to understand that in order to scan
the fetus presenting with the vertex, the free motion of
the vaginal probe is restricted by the anatomical limits
of the vagina. Only a few “classical” or “pure” planes can
therefore be achieved: the median and one coronal plane.
In the sagittal planes (to use the best approximation),
because of the fan-shaped, radial scanning sections,
several left and right oblique sections can be generated
on each side of the median plane ( Figure 2–8 ). This is
partially overcome by using 3D US. In front of or behind
the coronal plane (once again, to use the best approximation), several frontal and occipital oblique sections can
be generated ( Figure 2–8 ).
If several sections are created, the 3D anatomy should
be re-created mentally by the observer. This “mental processing” will be less and less necessary by the introduction
of 3D multiplanar imaging. Three-dimensional imaging of
the fetal brain will also eliminate the “radiating” planes, as
all planes will be parallel with each other.
If the second- or third-trimester fetus assumes an
asynclitic head position, it is sometimes possible to obtain
axial planes. The same is true if the head assumes a flexed
occipitoanterior position. A detailed study of the posterior
fossa should be performed as soon as this presentation is
detected. After some time, due to fetal movement, this
presenting anatomy may shift away, not to return for the
duration of the examination.
As the examination proceeds, we have observed certain conventions in orientation. This serves to introduce a
systematic way of displaying the images.
In the sagittal planes the fetal face or the frontal direction should be almost uniformly oriented to the left of the
picture. This will orient the posterior fossa and the occipital structures toward the right side of the screen. Likewise,
if the lateral ventricles are imaged using these planes, the
anterior horn should point toward the left and the posterior horn toward the right side of the monitor.
In the coronal planes the pictures should be taken in
such a manner that the fetus should face, or look right at,
the examiner. This means that by convention—as is customary in imaging laboratories—the right side of the brain
will appear on the left side of the screen, whereas the left
side of the prenatal brain will point toward the right of the
picture. If no annotation is to be seen on a hard copy, it is
assumed that the above-described orientations of the prenatal brain sections in the coronal planes were observed. It
seems best, however, to indicate the orientation of the left
or right side on at least one of the images. This annotation
of the left or right side becomes crucial if an asymmetrical
or unilateral lesion is detected.
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
23
A
Sagittal
Axial
horizontal
D. Rosenzweig
C
Figure 2–8. Scan through the anterior fontanelle. (A) The limited motion of the probe within the vagina allows the median plane to be visualized,
whereas on each side of it, the planes are oblique and not parallel, and therefore cannot be regarded as strictly sagittal. (B) Similarly, a coronal plane
can be obtained, whereas those in front of and behind it are oblique planes, not subsequent coronal planes. (C) The horizontal planes can be achieved
using the transabdominal approach.
Frontal
coronal
B
Anterior
fontanelle
If 3D US is used, one should be acquainted with
the initial display of the orthogonal planes inherent to
the different manufacturers’ systems. These orthogonal
planes are the customary initial planes appearing on the
screen after the acquisition of the volume is completed.
Determining the correct side is essential when unilateral
brain pathology is encountered.
BRAIN DEVELOPMENT AND SONOANATOMY
FROM 6 POSTMENSTRUAL WEEKS TO TERM
This section deals with the sonoembryology and sonoanatomy of the prenatal brain as it appears and as a continuous function of structural development. It should be clear
that parts of the prenatal brain exist and are in different
developmental stages even before they can be imaged by
ultrasonography. This is particularly true for brain structures revealed by a scan performed during the first 12 to
14 postmenstrual weeks of pregnancy. The structure may
well be in place, but due to its size and location, the angle
of insonation may not be obvious to the observer. Other
structures develop later as a normal process, and because
of this they are not seen on early scans.
We present here the normal embryonic and fetal
neurosonology in two sections. The first subsection deals
with the period up to 9 postmenstrual weeks. The second
section comprises the weeks following the ninth postmenstrual week. This is an entirely arbitrary partition. Starting
from the 13th postmenstrual week, some of the structures
imaged by high-frequency transvaginal transducers are of
clinically usable quality. This is also the time after which
multiple and clearly discernible brain sections can be
obtained using the standard planes.
23
Scanning the Embryonic Central
Nervous System (6 to 9 Postmenstrual Weeks)
To better understand the development of the embryonic
brain, the reader is referred to Chapter 1 . After reading
that chapter, a clearer perception of the sonographically
visualized structures will be possible.
As concerns the age of the embryos and fetuses,
the arguments and debates are well known and well
24
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Table 2–1. CONVERSION TABLE FOR EMBRYONIC AND FETAL AGE AND SIZE
Postmenstrual
Week
7th
8th
9th
10th
11th
12th
13th
14th
LMP, Last menstrual period; EES, early embryonic size.
a
After Robinson, 1973.
b
EES measurements of embryonic length are used up to 25 mm or 68 ± 3 base from LMP.
documented. Chapter 1 provides the tools to understand
the different views. Table 2–1 contains the embryonic
and fetal age conversions, which will allow quick reference for those who would like to rely on the date of the
last menstrual period (LMP) to calculate age, as well
as for those who are used to expressing age in terms
of weeks or days from fertilization (if such a date is
known).
33
O’Rahilly and Müller
C and R of the crown-rump length (CRL, Figure 2–9 )
are imprecise and frequently are difficult to determine.
They emphasized that the best single measurement is the
greatest length (GL), exclusive of the lower limbs, and
its determination is practicable in both the embryonic
and fetal periods. Subsequently, Goldstein
concern about measuring the longest diameter of the
developing embryo sonographically, calling it the CRL.
His measurement, according to the study, does not follow the curvature of the curled-up embryonic body.
Therefore, it actually measures the longest diameter
(ie, the GL of O’Rahilly and Müller
embryo. Goldstein
size (EES) for the sonographic measurement. Table 2–1
also contains the EES measurements and their conversions to embryonic age.
Returning to the work of O’Rahilly and Müller,
clear that the C point (the crown), which, in stages 13 to
20 (6w0d-9w1d from the LMP), is at the point where an
imaginary line drawn along the mesencephalic flexure
would touch the surface of the embryo, just above the
middle of the midbrain (see Figure 2–9 ), is hard to find
sonographically. Even if it can be determined, the CRL
would yield a smaller length than the GL or the EES of
Goldstein.
35
Figure 2–9 depicts how the C point changes
location throughout development from stage 13 to stage
20. Only at stage 23 (10w1d postmenstrual weeks) do the
Time From LMP
(Weeks and Days)
6 + 0 to 6 + 64 + 0 to 4 + 6 ∼28–34
7 + 0 to 7 + 65 + 0 to 5 + 6 ∼35–41
8 + 0 to 8 + 66 + 0 to 6 + 6 ∼42–48
9 + 0 to 9 + 67 + 0 to 7 + 6 ∼49–55
10 + 0 to 10 + 68 + 0 to 8 + 6 ∼56–62
11 + 0 to 11 + 69 + 0 to 9 + 6 ∼63–69
12 + 0 to 12 + 6 10 + 0 to 10 + 6 ∼70–76
13 + 0 to 13 + 6 11 + 0 to 11 + 6 ∼77–83
33
Time From Fertilization
(Weeks and Days)
35
CRL and GL (or EES) become identical as clinically used
measurements.
Using currently available US machines, the earliest
scan to depict any brain structure can be performed at 7
postmenstrual weeks (5 weeks, or 35 days, from fertilization). At this age one or several sonolucent areas can be
34
indicated that points
detected in the rostral and cephalic pole of the embryo.
The detail of the image depends on the resolution of the
equipment used.
If attempts are made to image details of the CNS
before this embryonic age, a clear fetal pole, at times
adjacent to the yolk sac, can be seen ( Figures 2–10 , 2–11
35
expressed
and 2–12A and B ).
sections do not yield distinct structures. At times, a tiny
sonolucency is seen in the rostral part of the embryo. The
nature of this structure is unknown. The parallel lines of
the somites (from which the vertebrae are derived) are
discernible at or slightly before 7 postmenstrual weeks
34
35
proposed the term early embryonic
) or the size of the
( Figures 2–12C ).
Blaas and colleagues
starting with an embryo with a CRL of 12 mm (7 weeks,
3 days from the LMP). They described the possibility of
imaging the cerebral hemispheres (telencephalic vesicles),
34
it is
rhombencephalon, and diencephalons ( Figure 2–13 ). In
embryos with a CRL of 16 mm (8 weeks from the LMP),
these structures become better defined. Thus, the interventricular foramina (Monro) can be seen ( Figure 2–14 ).
Our group has scanned well-dated embryos from 7
to 10 weeks from the LMP and evaluated the structures
imaged. A 6.12 MHz mechanical 3D transvaginal transducer was used. At 8 weeks, 1 day (from the LMP) in
the sagittal and axial/coronal planes, we could see all the
primitive ventricular suctum ( Figure 2–15A , B , and C ).
Figure 1–4 in Chapter 1 represents a sagittal section of the
embryonic brain at a comparable age.
Days From
Fertilization
Crown–Rump
Length (cm)
0.42–0.81 1–6
0.89–1.38 7–13
1.47–2.08 14–20
2.19–2.92
3.05–3.89
4.04–5.00
5.17–6.25
6.43–7.63
36
However, sagittal as well as coronal
29 , 30
studied the brain structures,
a
EES (mm)
b
b
b
b
b
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
25
16
13
1mm
G.L. C
R
20
C
17 18
15 17
C
2023
19
C
R
18
17
16
15
14
13
R
Figure 2–9. The outlines of four embryos (right lateral views) with the brain superimposed. The embryos are shown at stages 13, 15, 17, and 20, respec-
tively, and are at 6w1d, 6w5d, 7w5d, 9w1d postmenstrual weeks. In the first three examples, the greatest length (GL) is larger than the crown-rump (C–R)
length, whereas the two measurements coincide in the fourth. The last drawing (stages 13 to 23) is a scheme to summarize the “ascent” of point C until
the C–R length comes to equal the GL. (From O’Rahilly and Müller, 1984,
At 8 weeks, 1 day and 8 weeks, 3 days from the LMP
( Figure 2–15 ), the structures appear clearer, and flexures of
the brain, such as the mesencephalic and pontine flexures,
were located. The coronal sections revealed the rhombencephalon with regard to the major divisions. Figure 1–4
in Chapter 1 depicts a median section of the brain at
8 weeks, 3 days after the LMP. The subdivisions of the
brain ( Figure 2–16 ) could not yet be sufficiently discerned
by TVS.
embryo starts to “unfold.”
37
At 8 weeks, 5 days from the LMP ( Figure 2–17 ), the
23
The mesencephalic flexure is
almost in line with the longitudinal body axis. The cavities of the ventricular system are clearly identifiable.
Subdivisions such as the telencephalon, diencephalons,
34
with permission.)
metencephalon, and myelencephalon are seen sonographically not only on sagittal but also on coronal sections.
Observations by Blaas and associates
29 , 30
are consistent
with those described above.
In addition to the already described structures, the
choroids fold and slightly later, at 8 weeks, 5 days from
LMP, the first sighting of the choroid plexus was reported.
Only days later, at 9 weeks, 3 days from the LMP (with a
CRL of 25 mm), a better image of the choroid plexus and
the cerebellum was seen ( Figure 2–18 ).
From about 9 weeks from the LMP, there is a significant change in the TVS evaluation of the fetus as a
whole, particularly of the CNS. Because of their gradual
development and increase in size, more structures are
26
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Figure 2–10. Pregnancy of 6 weeks 2 days (postmen-
strual). The chorionic sac contains the yolk sac and the
embryo measuring 3.7 mm. Inlay: 3D rendering of the
yolk sac/embryo complex.
ys
Figure 2–11. An embryo at 7 weeks 0 days (postmenstrual). The
CR is 10 mm. The amnion (arrows) starts to be seen “separately”
the first time by TVS (ys = yolk sac).
CRL:10 mm = 7 weeks 0 days
ys
ys
ACB
Figure 2–12. An embryo at 7 weeks and 2 days (postmenopausal). The CRL is 12 to 13 mm. By the Carnegie classification, this corresponds to a stage
16 embryo at 37
ical flow. (The patient was scheduled for a termination of the pregnancy.) (C) The parallel lines of the spinal structures are evident (ys = yolk sac).
th
day postfertilization. The arrows point to the rhombencephalon. (A) Sagittal image. (B) Sagittal image highlighting the heart and umbil-
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
H
3
4
Figure 2–13. Transverse/oblique section through the rhombencepha-
lon (4), diencephalon (3), and cerebral hemispheres (H) of an embryo
(crown-rump length of 12 mm at 7 postmenstrual weeks 3 days). The
bilateral evaginations of the hemispheres are clearly seen. There is still a
wide opening to the third ventricle. (From Blaas and colleagues, 1994,
with permission.)
H
better seen. The development of the tortuous, fluid-filled
ventricular system is readily imaged in the median plane
( Figure 2–19 ). The subdivisions, as well as the serially
connected cavities, appear on both the sagittal and coronal planes. Suddenly, it seems that many more sections
can be generated in almost each of the three cardinal
planes ( Figure 2–19A ).
23
The different parts of the ventricular system are visible
at this time. A study of the median section of the embryo at
9 weeks, 5 days from the LMP is depicted in Figure 2–19B .
The sonolucent chain of the ventricular system winds
itself around the two most prominent solid structures: the
echogenic pontine flexure and the mesencephalic flexure.
This age marks the stage (Carnegie stage 23) at which the
latter points at the “crown” of the head and the true CRL
can be measured reliably.
34 , 35
The first rostral sonolucent
structure on this image is the diencephalon. Following
the diencephalon, in the caudal direction, are the mesencephalon, the metencephalon, the myelencephalon, and
the part of the medulla that contains the central canal. On
a paramedian section, the relatively large choroid plexus
in the lateral ventricle is depicted. At this age the choroid
plexus fills the entire lateral (telencephalic) ventricle. On
coronal and axial sections, the falx is seen as an echogenic
structure. If the choroid plexus and the falx are imaged by
TVS, the age of the embryo must be at least 9 postmenstrual weeks. A reasonably high-quality, high-frequency
vaginal probe is necessary to detect the falx, the different
ventricles, and the choroid plexuses at or around 9 to 9½
29
postmenstrual weeks.
27
H
Figure 2–14.
lon (4), diencephalon (3), and cerebral hemispheres (H) of an embryo
(crown-rump length of 16 mm at 8 postmenstrual weeks 0 days). The borders between the cerebral hemisphere and the third ventricle (3) remain
relatively small and start to develop into the interventricular foramina.
(From Blaas and colleagues, 1994,
Transverse/oblique section through the rhombencepha-
H
3
4
29
with permission.)
The Developing and Maturing Fetal Brain
The development of the embryonic brain was previously
described. As detailed in Chapter 1 , the embryonic period
lasts up to 8 weeks from fertilization, or roughly 10 postmenstrual weeks. Following this, the fetal period starts;
this lasts until delivery takes place. From a sonographic
standpoint, there is definitely no visible quantitative
reason for this qualitative change. An increasing number
of detectable structures are seen from 8 postmenstrual
weeks on, without any significant increase at or around
the cutoff age at which the designation changes from
embryo to fetus.
From 10 postmenstrual weeks on, three axial, three
sagittal, and three or four coronal sections in each of the
cardinal planes of the fetal head can be obtained.
A 3D model created by computer and based on serial
2D slice scans of fetuses at 8 to 10 postmenstrual weeks
is depicted in Figure 2–20 . The Norwegian group from
Trondheim produced such computer-generated “casts” of
the well-delineated ventricular system, which enabled a
close study of this system as it changed shape between 8
and 12 postmenstrual weeks.
38
Until a US system becomes
widely available to re-create the different brain structures
in a 3D fashion, we will have to rely on our own mental
re-creation of fetal structures in general, and the brain in
particular. To perform such a complex task in our own
brain, serial sections of the structure in question must be
generated and examined during every scanning session.
A similar series of horizontal (axial) slices is depicted in
31
28
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Rhombencephalon
Aqueduct
Ant
Diencephalon
A
Myelencephalic vesicle
Cephalic flexure
Umbilical cord
Amnion
Median
(sagittal
Coronal
Lower limb buds
B
Figure 2–15. An 8 weeks and 1 day (postmenopausal) embryo with the crown-rump length of 16.9 mm. By the Carnegie classification, this corresponds
to a stage 18 embryo at 43 days postfertilization. (A) Median picture showing parts of the anechoic ventricular system. (B) The orthogonal planes; the
median axial and coronal planes. The lower limb buds and the telencephalic vesicles are highlighted by the arrows.
Telencephalic vesicles
Axial
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
29
C
Figure 2–15. (continued) (C) Tomographic, consecutive sagittal sections along the lines shown in the upper left image square. The picture with the
white frame is the median plane of the embryo.
Figure 2–21 . From these images it is obvious that the choroid plexus of the fetus at 11 to 11½ postmenstrual weeks
almost fills the available space within the lateral ventricle.
If the gain control is increased, the high brightness of the
choroid enables the detection of cysts as small as 2 to
3 mm, should they exist. The falx is sufficiently echoreflective and therefore easily seen. In addition, the third ventricle, the tentorium, the posterior fossa, and the cerebellum
and its peduncles leading to the midbrain can be imaged
( Figures 2–21 and 2–22 ). On a very low axial section the
foramen magnum can also be seen ( Figure 2–21C ).
It seems that the presence of the third ventricle
can clearly be seen on axial scans at 11, 12, and even 13
postmenstrual weeks ( Figures 2–21 , 2–22 , and 2–23 ).
However, on serial scans at 14 and 16 postmenstrual weeks
( Figure 2–30 ), the space is progressively taken up by the
expanding thalamus.
Trondheim group.
developments, new tools are available to study the early
fetal brain. One of these methods is the 3D inversion rendering of fluid-filled spaces utilized by our group.
The steps of 3D inversion rendering are the following. First, the volume is obtained, in this case using
an ultra-high-frequency transvaginal probe, and stored.
Then an inversion algorithm (in our case, the GE-Kretz
(GE Healthcare, Milwaukee, WI) 4D view laptop-based
software is applied, which inverts the anechoic spaces
(fluid) into white echoes. At the conclusion of this process,
the fluid-filled ventricular system appears as a cast.
This technique allows the primitive, developing
embryonic/fetal ventricular system to be displayed. By
performing this display sequence, applying it to the brain
of fetuses with increasing gestational ages, we can follow
the development of the ventricular system and the brain
38
Since then, thanks to 3D software
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containing it.
Three-Dimensional Inversion Rendering
of the Developing Ventricular System
in the First Trimester
We saw previously the 3D rendering of some brain
structures obtained by segmentation techniques by the
Figures 2–28 , 2–30 , 2–31 , 2–32 , 2–33, 2–34 and 2–38
contain the orthogonal display and/or the tomographic
(sequential) images of entry of fetuses of increasing gestational ages. The anatomical structures pertaining to each
gestational age are indicated with arrows. Similar 3D inversion rendering techniques were also used by Kim et al.
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