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50
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
C
1
23
5
4
Figure 2–38. (continued) (C) Serial “coronal” brain sections at 18 postmenstrual weeks. (1) Frontal–1 section through the white matter. (2) Frontal–2
section through the anterior horns (AH). (3) Midcoronal–2 section through the choroid plexus (CP) and the interventricular foramina (two small
arrows), and the thalamus (T). (4) Midcoronal–3 section through the choroid plexus (C) and the thalamus (T). (5) Occipital–1 section through the
posterior (occipital) horn (OH). The arrows indicate the tentorium. C, Cerebellum; f, falx. (Modified from Timor-Tritsch, Monteagudo, 1991,
permission.)
24
with
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
51
AH
T
CSP
A
T
D
B
T
E
C
FM FM
OH
C
T
OH
C
AH
E
A
D
B
C
Figure 2–39. Serial transvaginal coronal sections at 25 postmenstrual weeks: (A) Frontal–1, (B) Frontal–2, (C) Midcoronal–2, (D) Midcoronal–3, and
(E) Occipital–1. The longitudinal fissure is indicated by small arrows. CSP, cavum septi pellucidi; AH, anterior horn; T, thalamus; FM, interventricular
foramina; OH, posterior horn; C, cerebellum. The long arrow in C indicates the choroid plexus within the third ventricle between the thalami; the
arrowheads point to the subarachnoid space.
52
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
SS
S
A
S
OH
T
A
Figure 2–40. At 18 postmenstrual weeks, this brain was scanned in (A) the left Oblique–1 and (B) Occipital–1 planes. The conventional measurements
that can be taken of the lateral ventricle and the posterior horn are shown. These measurements are within the normal range. T, Thalamus; CP, choroid
plexus; OH, posterior (occipital) horn; SAS, subarachnoid space; F, falx; SS, sagittal sinus. The small arrows indicate the tentorium.
CP
OH
B
SAS
F
SAS
SS
The lateral ventricles are situated in parallel fashion
within both cerebral hemispheres. They have three horns
(anterior, posterior, and inferior), a body, and a triangular
atrium. Even though this is the correct nomenclature of the
three horns of the lateral ventricles, on some images the
old nomenclature (frontal, occipital, and temporal horns)
may still appear. The lateral ventricles are the most obvious
when ultrasonography of the fetal brain is undertaken.
The different parts of the lateral ventricles undergo
extensive change in their shape and size. The lateral
ventricles are at first relatively very large ( Figure 1–7
in Chapter 1 ) and gradually become more slender during the fetal period. The posterior horn is the last to
appear ( Figure 1–7 in Chapter 1 ) and is the most variable. Examples of casts by Day
44
of fetal lateral ventricles
are shown in Figure 2–50 . They are from fetuses at 12,
18, and 32 weeks, respectively. It seems that they match,
in general, the sonographic evaluation of the lateral
ventricles performed with high-frequency transvaginal
transducers. The conclusions of Day’s study were (1) the
posterior horn develops late in relation to the anterior
and inferior horns, (2) the lateral ventricles become
progressively more slender in proportion, and (3) the
difference in size between homologous ventricles is not
as great in the fetus as in the adult, especially in the
posterior horn. As the largest of all ventricles, they were
readily seen by the relatively low-frequency transabdominal probes. The diagnosis of ventriculomegaly and hydrocephaly was established by measuring the size of the body
of the lateral ventricle on the axial transabdominal picture.
The term lateral ventricle–hemisphere width ratio was
coined to refer to objective measurement of ventricular
size. The change in this ratio throughout normal gestation was followed up and reported.
45 – 53
By looking at the
published graphs, it is obvious that the relative size of the
lateral ventricular width decreases rapidly from ∼70% at 18
postmenstrual weeks to 30% at around 28 weeks and stays
constant at this level thereafter.
On an axial (horizontal) plane, the normal lateral
ventricle should not measure >9 mm. The correct measurements should be taken at a place that contains the
choroid plexus.
54
One of the problems of ventricular measurements by
TAS is the lack of standardization. “Obviously normal”
and “clearly abnormal” lateral ventricles do not seem to
require measurements. However, borderline cases would
probably benefit from a quantitative determination of
size. Continuous follow-up of a case with suspected ventriculomegaly would also require the values to be put on
a conventional graph. There is, however, another pitfall,
namely, that different authors measure distances from and
to different echogenic “lines” within the head.
The last and probably most important drawback of
conventional transabdominal imaging of the fetal brain
is the problem of ineffective imaging of the hemisphere
close to the transducer ( Figure 2–51 ). This incomplete
picture is the reason for a large number of referrals to
imaging centers.
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
CC
T
T
AC
SAS
OH
AH
53
CSP
B
A
Figure 2–41. At 32 postmenstrual weeks: (A) Frontal Oblique–1, (B) Midcoronal–1, (C) Midcoronal–2, and (D) Occipital Oblique–1 sections are
shown. Note that the longitudinal fissure (arrowheads) in C displays the branching of the cingulate gyrus (two arrows). In D, the tentorium is highlighted
with small double arrows. SAS, subarachnoid space containing the superior sagittal sinus, CSP, cavum septi pellucidi; AH, anterior horn; T, Thalamus;
CC, corpus callosum; OH, posterior (occipital) horn.
D
D
C
B
54
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Table 2–3. SUMMARY OF THE BRAIN STRUCTURES IMAGED ON EACH OF THE
CONSECUTIVE MEDIAN, OBLIQUE–1, AND OBLIQUE–2 SECTIONS
MEDIAN OBLIQUE-1 OBLIQUE-2
Corpus callosum
Cavum septi pellucidi
Caudate nucleus
Thalamus
Tela choroidea
Te c tu m
Corpora quadrigemina
Ver mi s
4th ventricle
Cisterna magna
From Timor-Tritsch, et al, 1996,41 with permission.
Lateral ventricle
Anterior horn
Posterior horn
Atrium
Choroid plexus
Thalami
If more sophisticated and better US machines are
used (eg, compound scanning transducers) and operated
by knowledgeable examiners, the transabdominal images
have the capability to produce pictures of the fetal brain
with a high degree of resolution ( Figure 2–52 ).
Hertzberg and colleagues
55
questioned the validity
of these echogenic “lines” mentioned above, postulating
that they do not correspond to the walls of the lateral ventricles. In a more recent article, the same author suggests
that for a correct measurement of the lateral ventricle on
an axial view, the examiner should make “a direct attempt
to find the medial wall of the ventricle.”
Cardoza and coworkers
48
tried to measure selectively
56
the width of the lateral ventricular atrium according
to increasing fetal age. These measurements remained
relatively constant throughout gestation ( Table 2–4 ), at
a value of 7.6 ± 0.6 mm. This group suggested that atrial
diameters >10 mm (>4 standard deviations) should raise
suspicion of ventriculomegaly. Other graphs, tables, and
nonograms to measure distances from the lateral and
Insula
Parietal operculum
Temporal operculum
Lateral sulcus
medial walls of the lateral ventricles are now available
57 – 60
(see also Chapter 5 ). All of these, however, still use
the axial views of the head obtained by TAS. Indeed,
newer equipment has helped in identifying the abovementioned components of the lateral ventricles to serve
as reproducible landmarks for the measurements. Reece
and Goldstein
62
tried to standardize the axial planes
obtained by TAS by introducing three successive scanning planes (levels I, II, and III) at the intersection of
different intracranial brain structures. Unfortunately (as
in the case of all transabdominal scanning approaches),
abdominal thickness of the patient, mounting bone thickness, and low transducer frequencies will almost always
yield less resolution, hence, a relatively poor fetal neuroscan as opposed to TVS of the brain. However, once the
technique of transvaginal neurosonography is observed
and mastered, there is no doubt that it will be increasingly used until it almost entirely replaces the transabdominal route, provided that the fetus is in the vertex
presentation.
20 , 23,24,31,32
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
55
1
2
1
2
CC
CSP
CN
T
TC
ah
CP
oh
Figure 2–42.
left Oblique–1 sections at 18 post-
th
menstrual weeks. CC, Corpus callosum; CSP, cavum septi pellucidi;
CN, caudate nucleus; T, thalamus; ah,
anterior horn; CP, choroid plexus; oh,
posterior (occipital) horn; lh, lateral
horn. (Modified from Timor-Tritsch
and Monteagudo, 1991,
mission.)
(1) Median and (2)
24
with per-
CP
cc
CN
AH
T
OH
T
I
C
A
B
C
C
A
Figure 2–43.
and extremely lateral section through the still-gaping lateral sulcus, showing the insula. CC, corpus callosum; CN, caudate nucleus; TC, tela choroidea;
T, thalamus; C, vermis of the cerebellum; C, choroid plexus; AH, anterior horn; OH, posterior horn; l, insula.
B
Serial transvaginal “sagittal” sections at 25 postmenstrual weeks. (A) Median section. (B) Left Oblique–1 section. (C) Left Oblique –2
56
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
1
2
2
4
1
T
C
CM
B
2
3
4
1
T
A
3
C
2
T
4
1
4
T
C
C
2
4
1
3
T
C
1
D
E
F
Figure 2–44. Transvaginal median images depicting the development of the corpus callosum at (A) 18, (B, C) 22, (D) 23, and (E, F) 28 postmenstrual
weeks. C, cerebellum; 1, genu of the corpus callosum; 2, central part (trunk) of the corpus callosum; 3, splenium of the corpus callosum; 4, cavum septi
pellucidi; 5, cavum Vergae; T, thalamus; CM, cisterna magna. The white arrows in C and F indicate the fourth ventricle. (Modified from Timor-Tritsch
and Monteagudo, 1991,
24
with permission.)
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
CSP
57
T
CC
A
Septal nuclei
Diagonal band of Broca
Corpus callosum (genu)
Anterior commissure
Mammillothalamic tract
Preoptic region
Hypothalamic nuclei
Optic chiasm
Mammillary nucleus
Habenulointerpeduncular tract
Ventral tegmental area
Fascicles of oculomotor nerve (III)
Superior cerebellar peduncle decussation
C
C
CM
4V
Corpus callosum
(body)
Septum pellucidum
Fornix
Red nucleus
Oculomotor
nucleus (III)
Interpeduncular nucleus
Medial longitudinal fasciculus
Dorsal medial nucleus
Pontine nuclei
B
Periventricular gray matter
and midline thalamic nuclei
Corpus callosum
(splenium)
Stria medularis
Pulvinar
Posterior commissure
Superior colliculus
Inferior colliculus
Periaqueductal gray matter
Trochlear nucleus (IV)
Decussation of
trochlear nerve (IV)
Periventricular
gray matter
IV ventricle
Nodulus
Prepositus nucleus
Hypoglossal nucleus (XII)
Dorsal longitudinal fasciculus
Gracile nucleus
Gracile fascicle
Central gray matter
Pyramid and decussation
Medial
lemniscus
Figure 2–45. Sonographically identifiable central structures of the fetal brain at 25 and 28 postmenstrual weeks. (A) This transvaginal median section
shows the fully developed corpus callosum (CC), the cavum septi pellucidi (CSP), the thalamus (T), the tela choroidea of the third ventricle (small arrow),
the vermis of the cerebellum (c), the fourth ventricle (4V), and the cisterna magna (CM) at 28 postmenstrual weeks’ gestation. (B) Focused median
section of the fetal brain at 25 postmenstrual weeks. No annotations were made to identify structures. The image is presented for comparison with the
drawing and properly annotated picture in C. (C) The anatomic structures of the midbrain. (From Martin, 2003, 61 with permission.)
58
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
Transventricular plane
Transthalamic plane
Transcerebellar plane
CSP
TH
AH
PH
CP
HG
C
CM
Figure 2–46. The basic brain scan is performed by transabdominal sonography and its objective is to evaluate the fetal brain only in the general axial
(horizontal) planes. It includes three planes: transventricular, transthalamic, and trancerebellar.
At times, it may become important to perform a
cephalic version of a fetus presenting with breech presentation for more accurate studies.
Measurements of the anterior horn–hemispheric
width ratio were reported by Campbell in 1979.
63
This
ratio decreases from 60% at week 14 to 40% at 21 postmenstrual weeks. Another ratio—that of the frontal horn to the
hemispheric width—was measured by Goldstein and collaborators.
58
This ratio diminishes from 50 to 28% from 15
postmenstrual weeks to term. In addition, the size of the
frontal lobe can be measured on the TAS picture. Because
this measurement correlates with fetal size, it was used to
detect microcephaly.
64
The atrium of the lateral ventricles has also been the
subject of numerous studies and serial measurements by
various authors. Sonographically, it is easy to recognize the
atrium because it contains a large part of the choroid plexus
present in the lateral ventricular system. The distance,
measured typically on an axial plane, is that from the falx
to the lateral wall of the atrium.
distance and the hemispheric width was proposed as a sensitive indicator of abnormality. This ratio decreases from
60 to 30% from 15 postmenstrual weeks to 24 weeks.
From 27 postmenstrual weeks to term, the same ratio
remains fairly constant, at values of 0.56 to 0.51.
associates
59
suggested that the size of the atrium remains
relatively constant across gestation, at about 7 ± 1.3 mm,
due to the thickening of the parenchyma. According to this
group, this increase in the brain mass is expressed by the
slow but constantly increasing distance between the falx
and the lateral atrial wall.
The posterior horn is an extremely important structure. It is considered to be the most sensitive indicator
of incipient ventriculomegaly. This horn of the lateral
ventricle is somewhat neglected in the literature. The
reason may be that it is hard to obtain a consistently
good-quality image of the posterior horn for purposes of
measurement. It is interesting that in a study concentrating
59
The ratio between this
48
Pilu and
65
Chapter 2 Normal Two- and Three-Dimensional Neurosonography of the Prenatal Brain
59
IHF
T
CM
IHF
CC
CSP
INS
Figure 2–47. The targeted or more detailed fetal neurosonogram contains other planes and sections; the coronal planes are displayed in this figure.
AH
CN
TH
OH
C
on several measurements of the lateral cerebral ventricles
to detect impending poor fetal outcome, the most significant increase in size was that of the posterior horn.
However, this was not given great importance.
66
Chapter
3 discusses the importance of measuring the size of the
posterior horn as well as two ratios in which the size of
the posterior horn is compared with the thickness of the
choroid plexus within the atria.
The inferior horn extends from the atrium into the
temporal lobe. After emerging from the atrium, the horn
turns slightly toward the inferior and lateral direction, ending in the center of the temporal lobe ( Figure 2–49 ). The
lateral position of this horn is less obvious before 14 to 16
postmenstrual weeks, when the oblique–1 section may
include all three horns (ie, anterior, posterior, and inferior).
After 16 postmenstrual weeks, the oblique-1 section “cuts”
through the anterior and occipital horns but definitely
does not include the inferior horn, which is slightly lateral
to this plane. Based on our experience, if after 16 postmenstrual weeks all three horns are clearly imaged on the
paramedian sagittal section, ventriculomegaly should be
seriously considered.
The third ventricle is well imaged in the first and
early second trimesters ( Figures 2–21 , 2–22 , and 2–23 ).
However, as gestation progresses, it becomes filled with
the choroid plexus (tela choroidea) of the third ventricle
and is considered a virtual space. The two contralateral
thalami touch each other at the point of the interthalamic adhesion (massa intermedia). Denkhaus and
Winsberg
45
claimed to be able to measure the width of
the third ventricle on axial TAS images. They suggested
a table that lists the width of this ventricle at 2.5 mm as
a biparietal diameter of 2.3 cm, increasing to 8.2 mm at
term. It is unclear from this report whether or not they
saw the choroid plexus within the third ventricle. They
also attributed no importance to the clinical value of a
change in the size of this ventricle with respect to the
diagnosis of antenatal hydrocephaly. Our observation is
to the contrary; this is touched on in Chapter 4 .
The fourth ventricle can easily be seen on a median
plane or an axial section using the occipital approach
( Figures 2–44 and 2–45 ).
Foramina and the Aqueduct
A pair of narrow interventricular foramina connects
the body of the lateral ventricles with the third ventricle. These tiny connections would certainly elude the