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150
Chapter 3 Biometry of the Fetal Brain
Table 3–36. RATIO OF THE OCCIPITAL (POSTERIOR) HORN HEIGHT TO THE CHOROID PLEXUS THICKNESS
Estimated Gestational
Age (weeks) 5th Percentile (mm) 50th Percentile (mm) 95th Percentile (mm)
14 0.02 0.85 1.68
15 0.04 0.86 1.69
16 0.05 0.88 1.71
17 0.07 0.90 1.72
18 0.84 0.91 1.74
19 0.10 0.93 1.75
20 0.12 0.94 1.77
21 0.13 0.96 1.79
22 0.15 0.97 1.80
23 0.16 0.99 1.82
24 0.18 1.00 1.83
25 0.19 1.02 1.85
26 0.21 1.04 1.87
27 0.23 1.05 1.88
28 0.24 1.07 1.90
29 0.26 1.09 1.91
30 0.27 1.10 1.93
31 0.29 1.12 1.94
32 0.31 1.13 1.96
33 0.32 1.15 1.98
34 0.24 1.16 1.99
35 0.35 1.18 2.00
36 0.37 1.20 2.02
37 0.38 1.21 2.04
38 0.40 1.23 2.06
39 0.42 1.24 2.07
40 0.43 1.26 2.09
From Monteagudo et al, 1993, 60 by permission.
Figure 3–20.
CSP
OW
Chapter 3 Biometry of the Fetal Brain
A
I
CP
P
T
AH
AH
I
BN/I
P
T
CP
A
151
How to Measure It ( Figure 3–20 )
These structures can be readily outlined in the standard horizontal BPD plane previously described.
hypoechoic thalamus (T) is measured at the anterior tip
of the ambient cistern. The basal nuclei (BN) and insula
(I) can be measured as a whole from the edge of the thalamus to the echo of the cistern of the lateral sulcus. The
opercular width (OW) of the temporal lobe corresponds
sonographically to the distance between the inner surface of the temporal bone and the cistern of the lateral
sulcus at the point of its invagination into the cerebral
51
cortex.
5 , 8
The
Comments
The cerebral hemispheres buckle as their growth increases,
creating the sulci and the gyri. The largest infolding
corresponds to the developing lateral sulcus, where the
caudal end of the cerebral hemispheres grows to form
the operculum of the temporal lobe. With the described
measurement method, the OW is overestimated in the
second trimester because the gap that exists between the
inner bone surface and the insular cortex is not taken into
consideration. On the other hand, in the third trimester
Table 3–37. MEASUREMENTS OF FETAL INTRACRANIAL STRUCTURES
this measurement is underestimated because the distance
between the cistern of the lateral sulcus and the calvarium
does not represent exactly the full width of the developing
temporal operculum. Nevertheless, by 35 postmenstrual
weeks, the OW has approximately doubled from the time
of its first appearance.
By 18 postmenstrual weeks, the basal nuclei and the
insular cortex can be identified. Siedler and Filly
51
51
proposed that with transabdominal sonography, the basal
nuclei appear as two zones of different echogenicity:
(1) an echogenic curvilinear strip marginating the thalamus
and posterolateral to the frontal horns that corresponds to
the caudate and lentiform nuclei; and (2) a sonolucent
band lateral to the previously described, corresponding to
the external capsule, claustrum, and extreme capsule. The
insular cortex is included when measuring this hypoechoic
band.
The thalami, in the BPD plane, appear sonographically as two hypoechoic oval structures separated in the
midline by the third ventricle. Their width increases
from 7 ± 0.8 mm at 15 to 20 postmenstrual weeks to
9 ± 0.7 mm at 31 to 35 postmenstrual weeks, undergoing
considerably less growth than the temporal operculum and
the basal nuclei-insula ( Table 3–37 ).
Thalamus (mm) Basal Nuclel–Insula (mm) Temporal Operculum (mm)
Gestational
Age (weeks)
Mean Range SD Mean Range SD Mean Range SD
15–20 7 6–9 0.8 6 5–7 0.7 6 5–7 0.9
21–25 8 6–9 0.7 7 6–11 1.2 9 7–11 1.0
26–30 8 8–9 0.4 9 8–12 1.2 11 10–13 0.6
31–35 9 8–10 0.7 11 9–14 1.1 13 11–15 0.7
SD, standard deviation.
From Siedler and Filly, 1987,
51
with permission.
152
Chapter 3 Biometry of the Fetal Brain
POSTERIOR FOSSA: CEREBELLUM AND
CEREBELLOMEDULLARY CISTERN
(CISTERNA MAGNA)
Definitions
The cerebellum is a suprasegmental portion of the brain
located within the cranial posterior fossa that receives
input from virtually the entire nervous system, playing a
key role in movement coordination.
The cerebellomedullary cistern (CMC) corresponds
to a portion of the subarachnoid space that bathes the cranial posterior fossa in CSF. It arcs around the cerebellum
posteriorly, invaginating in the midline between the cerebellar hemispheres.
70
How to Measure It ( Figure 3–21 )
To evaluate the posterior fossa, a horizontal plane of
the fetal head equal to that used for determination of
the BPD must be obtained. Once the landmarks of the
thalami (T) and the cavum septum pellucidum (CSP)
are identified, a slight caudal rotation of the transducer
will bring the characteristic butterfly-like appearance
of the cerebellum into view. The transverse cerebellar
diameter (TCD) can then be measured as the widest
diameter across both hemispheres in an outer-to-outer
fashion.
71
The CMC depth is evaluated in the same plane and
measured in the median plane from the posterior aspect of
the cerebellum to the inner table of the occiput.
Comments
The fetal cerebellum can be visualized sonographically as
early as 10 to 11 postmenstrual weeks. It grows rapidly in
the second trimester following a linear relationship with
gestational age, so that during this period, measurements
in millimeters equal approximately the gestational age in
69
weeks. However, as pregnancy advances, the growth curve
of the cerebellum tends to flatten, showing a slower rate of
evolution ( Tables 3–38 and 3–39 ).
71
Because the cerebellum is located inside the posterior fossa and is surrounded by the dense petrous
ridges and the occipital bone, it should be able to
withstand deformation by extrinsic pressure better
than the parietal bone. Keeping this concept in mind,
several authors
71 – 73
proposed that the TCD, as opposed
to the BPD, can better predict gestational age in cases
in which variations of the fetal head shape, such as
dolichocephaly and brachycephaly, have been described
(eg, breech presentation, oligohydramnios, twins, and
uterine anomalies).
Intrauterine growth retardation remains a major cause
of perinatal morbidity and mortality, affecting 4% to 8% of
all deliveries in so-called developed countries.
74
In order to
better evaluate fetal biometry, when intrauterine growth
retardation is suspected, the TCD should also be used.
Cabbad and associates
75
found that 22 out of 23 asymmetrically growth-impaired fetuses had a TCD lower than
expected but within the normal range, suggesting that this
measurement could be used to help estimate gestational
age in these cases. On the other hand, Hill and colleagues
76
found in a group of 116 diabetic and nondiabetic singleton
gestations with an estimated fetal weight at or above the
90th percentile that the TCD did not overestimate gestational age in the nondiabetic group and overestimated
age in the diabetic group by only 0.5 postmenstrual weeks,
rendering it a useful tool for predicting age in this population. The TCD has also been used to evaluate fetal growth
in twin gestations.
77
Dilmen and colleagues
78
used the
TCD/AC ratio obtained by transabdominal sonography
to evaluate fetal growth. Ten of 11 fetuses with TCD/AC
ratios exceeding 2 SD (0.1648) were found to have asymmetrical intrauterine growth retardation upon neonatal
examination.
Because many congenital alterations of the cranial
posterior fossa can modify the normal size of the CMC,
Figure 3–21.
CSP
AH
AH
T
P
TCP
P
T
CMC
Chapter 3 Biometry of the Fetal Brain
Table 3–38. PREDICTED GESTATIONAL AGES FOR TRANSVERSE CEREBELLAR DIAMETERS OF 14 TO 56 MM
153
Cerebellar
Diameter (mm)
14 15.2 35 29.4
15 15.8 36 30.0
16 16.5 37 30.6
17 17.2 38 31.2
18 17.9 39 31.8
19 18.6 40 32.3
20 19.3 41 32.8
21 20.0 42 33.4
22 20.7 43 33.9
23 21.4 44 34.4
24 22.1 45 34.8
25 22.8 46 35.3
26 23.5 47 35.7
27 24.2 48 36.1
28 24.9 49 36.5
29 25.5 50 36.8
30 26.2 51 37.2
31 26.9 52 37.5
32 27.5 54 38.0
33 28.1 55 38.3
34 28.8 56 38.5
From Hill and colleagues, 1990,
76
with permission.
Gestational
Age (weeks)
Cerebellar
Diameter (mm)
Gestational
Age (weeks)
its evaluation deserves special consideration when searching for infratentorial anomalies. The mean normal CMC
depth has been reported to be 5 mm (range 1 to 10 mm),
with an SD of ±3 mm. The CMC can be enlarged in DandyWalker malformation, as well as in posterior fossa arachnoid cysts. Joubert syndrome also should be considered
in the differential diagnosis of an enlarged CMC. In both
conditions, the cerebellar hemispheres and fourth ventricles can be of normal size, the inferior and posterior
vermian dysplasia is common to both disorders, and, in
both, the CMC communicates with the fourth ventricle.
Joubert syndrome, however, is associated with bilaterally
enlarged echogenic kidneys, agenesis of the corpus callo-
sum, occipital encephalocele, facial anomalies, and poly-
79
dactyly.
However, it is important to keep in mind that
in the absence of other findings (eg, hydrocephaly, shift
of the midline, or dysgenesis of the cerebellar vermis), a
prominent CMC is unlikely to be of clinical significance.
On the contrary, in Arnold-Chiari malformation, the
CMC is diminished in size, typically measuring ≤2 mm.
70
In this case, the cerebellum can have a flattened, wedged
appearance, giving the impression of the so-called banana
80
sign.
The cerebellar vermis should also be adequately
imaged when evaluating the sonographic appearance of
the cerebellum. Vermian agenesis is commonly found
154
Chapter 3 Biometry of the Fetal Brain
Table 3–39. NOMOGRAM OF THE TRANSVERSE CEREBELLAR DIAMETER ACCORDING
TO PERCENTILE DISTRIBUTION
Cerebellar Diameter (mm)
Gestational
Age (weeks)
15 10 12 14 15 16
16 14 16 16 16 17
17 16 17 17 18 18
18 17 18 18 19 19
19 18 18 19 19 22
20 18 19 20 20 22
21 19 20 22 23 24
22 21 23 23 24 24
23 22 23 24 25 26
24 22 24 25 27 28
25 23 21.5 28 28 29
26 25 28 29 30 32
27 26 28.5 30 31 32
28 27 30 31 32 34
29 29 32 34 36 38
30 31 32 35 37 40
31 32 35 38 39 43
32 33 36 38 40 42
33 32 36 40 43 44
34 33 38 40 41 44
35 31 37 40.5 43 47
36 36 29 43 52 55
37 37 37 45 52 55
38 40 40 48.5 52 55
39 52 52 52 55 55
Modified from Goldstein and colleagues, 1987,
10th
Percentile
25th
Percentile
71
with permission.
50th
Percentile
75th
Percentile
90th
Percentile
with Dandy-Walker malformation and variants, as well
as in association with other malformations and syndromes. Although uncommon, agenesis if the cerebellar
vermis can be part of the Dandy-Walker malformation
or an isolated finding. It may also present as complete or
partial agenesis.
81
It is generally accepted that vermian
development should be completed by 18 weeks of gestation; however, other authors
82
have suggested that the
diagnosis of the different types of vermian hypoplasia
should not be performed before 24 weeks of gestation.85
At this point, the vermis should have completed its
development to allow accurate measurements. Vermian
25 weeks
Chapter 3 Biometry of the Fetal Brain
155
Frontal Lobe
Definition
The frontal lobe corresponds to the portion of the cerebral
hemisphere anterior to the central sulcus and an imaginary
line drawn at the level of the lateral sulcus up to the
circular sulcus of the insula.
How to Measure It ( Figure 3–23 )
Frontal lobe measurements are accomplished in the plane
in which the BPD is measured (eg, the horizontal plane).
The frontal lobe distance (FLD) is measured between the
anterior margin of the medial wall of the frontal horn
of the lateral ventricles and the middle hyperechogenic
frontal bone. The thalamic frontal lobe distance (TFD) is
measured from the most posterior landmark of the thalami
to the middle hyperechogenic frontal bone.
68
Figure 3–22.
of the thalamus, showing the width of the cerebellar vermis at 25 week
of gestation. (Reproduced, with permission, from Ultrasound Obstet
Gynecol. 2002;19:136–139. Blackwell Science Ltd. The development of
the fetal vermis: an in-utero sonographic evaluation.
Transverse (axial) sonogram of the fetal head at the level
84
)
width and height are commonly evaluated in the transverse (axial) plane for width and sagittal plane for height
( Figure 3–22 ).
More recently,
88, 92
vermian development and anatomy have been described using multiplanar 3D ultrasound. With this technique, the characteristic features
of the cystic malformations of the posterior fossa (ie,
upward displacement of the tentorium, counterclockwise
rotation, and hypoplasia of the cerebellar vermis) can be
more easily evaluated (Tables 3–40 through 3–49).
A
I
TFD
AH
CSP
FLD
AH
x
I
x
CP
P
T
P
T
CP
A
Comments
Frontal lobe measurements (ie, FLD and TFD) ( Tables
3–50 and 3–51 ) can be used as an adjunct for the diag-
nosis of microcephaly, as several investigators agree that
this entity is associated with a decreased size of the frontal
fossa and flattening of the frontal bone, with other lobes
of the brain remaining unchanged.
coworkers
29
reported on three cases of postnatally confirmed microcephaly in which the FLD and the TFD were
below the 10th percentile. Although this is a small series,
measuring the frontal lobe seems to be a logical suggestion in cases where microcephaly is suspected, as it adds
only a few seconds to the scanning session. Frontal lobe
measurements (especially the TFD) have also been used
to aid in the antenatal midtrimester diagnosis of Down
syndrome. Bahado-Singh and collaborators
among 19 fetuses with Down syndrome, 10 (52%) had
a TFD below the 10th percentile for gestational age. It
81 – 83
Goldstein and
83
found that
Figure 3–23.
156
Chapter 3 Biometry of the Fetal Brain
Table 3–40. VERMIS SIZE (WIDTH AND HEIGHT) ACCORDING TO GESTATIONAL AGE (MEAN ± SD)
Gestational
Week
Number of
Patients
Vermis Width
(mean, mm, (SD))
Vermis Height
(mean, mm, (SD))
18–20 8 5 (0.76) 5.88 (0.85)
21 17 5.76 (0.83) 6.47 (0.94)
22 17 6.24 (0.66) 6.88 (0.60)
23 31 6.90 (0.54) 7.71 (0.90)
24 25 8.12 (0.67) 8.44 (0.71)
25 26 8.58 (0.81) 8.62 (0.75)
26 18 9.11 (0.96) 9.17 (0.71)
27 18 9.78 (0.81) 10.00 (0.91)
28–29 17 10.4 (1.17) 10.5 (0.87)
30–31 12 11.3 (1.22) 11.6 (0.79)
32 14 12.3 (1.54) 12.1 (1.27)
33 16 11.8 (1.29) 12.2 (1.11)
34 10 13.0 (1.05) 13.0 (0.94)
35–36 18 14.2 (1.25) 14.2 (1.20)
37–38 9 15.4 (1.01) 15.3 (0.87)
Total 256
SD, standard deviation.
After Zalel et al, 2002,
84
with permission.
Table 3–41. CORRELATION OF VERMIS HEIGHT
AND GESTATIONAL AGE ( r = 0.937 )
5
4
3
2
Square root of vermis height
1
15 20 25
Gestational age (weeks)
Reproduced from Zalel et al, 2002,
84
with permission.
r = 0.937
30
35
40
Table 3–42. CORRELATION OF VERMIS WIDTH
AND GESTATIONAL AGE ( r = 0.934 )
5
4
3
2
Square root of vermis width
1
15 20 25
Gestational age (weeks)
Reproduced from Zalel et al, 2002,
84
with permission.
r = 0.934
30
35
40
Chapter 3 Biometry of the Fetal Brain
157
Table 3–43. CORRELATION OF VERMIS HEIGHT AND
BIPARIETAL DIAMETER ( r = 0.937 )
5
4
3
2
Square root of vermis height
1
30 40 50
Biparietal diameter (mm)
Reproduced from Zalel et al, 2002,
60
84
with permission.
r = 0.937
80 9070
100
Table 3–44. CORRELATION OF VERMIS WIDTH
AND BIPARIETAL DIAMETER ( r = 0.936 )
5
4
3
2
Square root of vermis width
1
30 40 50
Reproduced from Zalel et al, 2002,
60
Biparietal diameter (mm)
84
with permission.
Table 3–45. MEAN ± SD MEASUREMENTS OBTAINED BY VOLUME CONTRAST IMAGING IN THE CORONAL
PLANE OF THE CEREBELLAR VERMIS IN 203 NORMAL FETUSES
80 9070
r = 0.936
100
Gestational
Age (weeks) Patients ( n )
18–19 10 10.5 ± 1.3 8.3 ± 0.8 0.6 ± 0.05
20–21 19 12.7 ± 1.4 9.1 ± 1.6 0.7 ± 0.3
22–23 46 14.2 ± 1.6 10.5 ± 1.7 1.2 ± 0.2
24–25 45 15.8 ± 1.6 12 ± 1.4 1.5 ± 0.3
26–27 28 17.6 ± 1.7 13.5 ± 1.8 1.7 ± 0.3
28–29 19 19.6 ± 1.7 13.9 ± 1.1 2.1 ± 0.2
30–31 16 20.9 ± 1.5 15.5 ± 1.6 2.4 ± 0.06
32–33 20 22.8 ± 1.6 18.2 ± 1.7 3.4 ± 0.2
From Vinals et al, 2005,
94
with permission.
Carniocaudal
Diameter (mm)
Anteroposterior
Diameter (mm)
Surface
Area (cm
2
)
158
Chapter 3 Biometry of the Fetal Brain
Table 3–46. CORRELATION BETWEEN VERMIS
SUPEROINFERIOR DIAMETER AND
GESTATIONAL AGE
25
20
15
10
Vermis diameter (mm)
5
0
192021
From Achiron et al, 2004,
23
24
22
25 262728
Gestational age (weeks)
93
with permission.
29 30
31 32
33 34
Table 3–47. CORRELATION OF CEREBELLAR VERMIS
CRANIOCAUDAL (CC) DIAMETER WITH
GESTATIONAL AGE (GA)
26
24
22
20
18
16
14
CC diameter (mm)
12
10
8
16
From Vinals et al, 2005),
18
20
94
with permission.
24
22
Gestational age (weeks)
26 28
30
32
34
Table 3–48. CORRELATION OF CEREBELLAR VERMIS ANTEROPOSTERIOR (AP) DIAMETER
WITH GESTATIONAL AGE (GA)
22
20
18
16
14
12
10
AP diameter (mm)
8
6
4
From Vinals et al, 2005,
94
with permission.
16
18
20
24
22
Gestational age (weeks)
26 28
30
32
34
Chapter 3 Biometry of the Fetal Brain
Table 3–49. PERCENTILE MEASUREMENTS CEREBELLAR VERMIS (SUPEROINFERIOR DIAMETER)
ACCORDING TO GESTATIONAL AGE
Superoinferior Diameter (mm)
159
GA (weeks)
19–20 18 6.5 9.1 9.5 10 11.1
21–22 114 10.1 10.9 11.7 12.1 13.2
23-24 82 11.4 12.3 13 13.6 15.2
25–26 20 13.1 14.3 14.8 15.4 16.4
27–28 15 15.3 16 16.8 17.9 18.6
29–30 11 15.6 17.5 18.5 20.3 20.9
31–32 13 17.2 19.6 20.1 20.7 21
33–34 14 18.3 20.6 21.5 22.8 24
GA, gestational age.
From Achiron et al, 2004,
is possible that the combination of these measurements
with other reported signs (eg, enlarged nuchal fold,
cardiovascular anomalies, hyperechogenic bowel, and
hydronephrosis) could further enhance the ability of
ultrasonography to diagnose this condition in utero.
n
84
with permission.
5 th 25 th 50 th 75 th 95 th
of the interhemispheric communication in the brain is
conducted across the corpus callosum. The fetal corpus callosum can be used as a marker for normal brain
development and maturation. During the prenatal period,
changes in the length of the corpus callosum could be
used as an indicator of abnormal development
( Tables 3–52 , 3–53 , 3–54 , and 3–55 ; Figures 3–24 , 3–25 ,
CORPUS CALLOSUM
Definition
The corpus callosum is a brain commissure composed of
fibers that connect the cerebral hemispheres with each
87 – 92
other.
How to Measure It
The corpus callosum can be evaluated in both the midsagittal and coronal planes, where it can be found between
the cingulate gyrus above and the cavum septi pellucidi
below. The length of the corpus callosum is measured
from the most anterior aspect of the genu to the most posterior aspect of the splenium along a straight rostrocaudal
line.
fetal corpus callosum by gestational age
Comments
The corpus callosum is a telencephalic structure that
connects the left and right cerebral hemispheres. It is the
largest white matter structure in the brain, consisting
of 200 million to 250 million axonal projections. Much
87 , 95
Tables 3–52 to 3–53 Measurements of the normal
3–26 , 3–27 , 3–28 , and 3–29 ).
The corpus callosum is composed of four parts
(from front to back): rostrum, genu, body, and splenium
( Figure 3–24 ). The formation of the corpus callosum
starts with the development of the genu; the body
and splenium develop at a later stage. If the normal
developmental process is disturbed, the corpus callosum
may be completely or partially absent.
posterior fashion of its development, it is usually the
posterior body and splenium that get affected. Prenatal
evaluation of the corpus callosum can be accomplished
via the transvaginal-transfontanellar approach (in the
fetus in the vertex presentation) or transabdominally
when the fetal presentation is breech.
evaluation of the corpus callosum requires its visualization both in the median and coronal planes, which are
easily obtainable using the transvaginal-transfontanellar
.88
More recently, the use of the 3D multiplanar
route
technique has been proposed
92
for the evaluation of the
fetal brain. The corpus callosum can be fully evaluated in
the fetus from the 18th gestational week onward, reaching its final adultlike configuration closer to the 28th
week of gestation. A full discussion of corpus callosum
abnormalities is beyond the scope of this chapter and
will be discussed further in this book.
86 , 88 – 91
89
Given the antero-
90 , 91
Adequate