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120
Chapter 3 Biometry of the Fetal Brain
Table 3–11. NOMOGRAM FOR EVALUATION OF THE INNER AND OUTER ORBITAL
DIAMETERS IN THE FETUS VERSUS BIPARIETAL DIAMETER (CONTINUED)
Biparietal
Diameter (cm)
4.0 1.2 3.0
4.2 1.2 3.1
4.3 1.2 3.2
4.4 1.3 3.2
4.5 1.3 3.3
4.6 1.3 3.4
4.7 1.3 3.4
4.8 1.4 3.5
4.9 1.4 3.6
5.0 1.4 3.6
5.1 1.4 3.7
5.2 1.4 3.8
5.3 1.5 3.8
5.4 1.5 3.9
5.5 1.5 4.0
5.6 1.5 4.0
5.7 1.5 4.1
5.8 1.6 4.1
5.9 1.6 4.2
6.0 1.6 4.3
6.1 1.6 4.3
6.2 1.6 4.4
6.3 1.7 4.4
6.4 1.7 4.5
6.5 1.7 4.5
6.6 1.7 4.6
6.7 1.7 4.6
6.8 1.7 4.7
6.9 1.7 4.7
7.0 1.8 4.8
Inner Orbital
Diameter (cm)
Outer Orbital
Diameter (cm)
(continued)
Chapter 3 Biometry of the Fetal Brain
Table 3–11. NOMOGRAM FOR EVALUATION OF THE INNER AND OUTER ORBITAL
DIAMETERS IN THE FETUS VERSUS BIPARIETAL DIAMETER (CONTINUED)
121
Biparietal
Diameter (cm)
7.1 1.8 4.8
7.3 1.8 4.9
7.4 1.8 5.0
7.5 1.8 5.0
7.6 1.8 5.1
7.7 1.8 5.1
7.8 1.8 5.2
7.9 1.9 5.2
8.0 1.9 5.3
8.2 1.9 5.4
8.3 1.9 5.4
8.4 1.9 5.4
8.5 1.9 5.5
8.6 1.9 5.5
8.8 1.9 5.6
8.9 1.9 5.6
9.0 1.9 5.7
9.1 1.9 5.7
9.2 1.9 5.8
9.3 1.9 5.8
9.4 1.9 5.8
9.6 1.9 5.9
9.7 1.9 5.9
From Mayden and colleagues, 1982,
Inner Orbital
Diameter (cm)
31
with permission.
Outer Orbital
Diameter (cm)
122
Chapter 3 Biometry of the Fetal Brain
Table 3–12. NOMOGRAM FOR EVALUATION OF THE INNER AND OUTER ORBITAL DIAMETERS
IN THE FETUS VERSUS GESTATIONAL AGE
Inner Orbital Diameter (mm) Outer Orbital Diameter (mm)
Gestational
Age (weeks)
13 4 7 10 12 16 20
14 5 8 11 14 18 22
15 5 8 11 17 21 25
16 6 9 12 19 23 27
17 71013212529
18 81114242731
19 81114263034
20 91215283236
21 10 13 16 30 34 38
22 10 13 16 32 36 40
23 11 14 17 33 37 41
24 12 14 17 35 39 43
25 12 15 18 37 41 45
26 13 16 19 39 43 47
27 13 16 19 40 44 48
28 14 17 20 42 46 50
29 14 17 20 43 47 51
30 15 18 21 45 49 52
31 15 18 21 46 50 54
32 16 19 22 47 51 55
33 17 20 23 48 52 56
34 17 20 23 49 53 57
35 18 21 24 50 54 58
From Trout and colleagues, 1994,
5th
Percentile
33
with permission.
50th
Percentile
95th
Percentile
5th
Percentile
50th
Percentile
95th
Percentile
33
authors
felt that some patients could have pathologic
intracranial conditions that might alter the BPD, making
this measurement less reliable ( Table 3–12 ).
Microphthalmos can be suspected when the orbital
diameter falls below the fifth percentile for age ( Table 3–13 ).
Because this is a statistical definition, careful examination of the intraorbital anatomy, as well as detection of
associated anomalies, is warranted. Conditions associated
with microphthalmos include chromosomal (trisomies
13 and 18 and trisomy 9 mosaic), environmental (fetal
toxoplasmosis, rubella, varicella, and alcohol syndrome
and maternal phenylketonuria), and multiple syndromes
(eg, frontonasal dysplasia, Fraser syndrome, Lenz syndrome, and Fanconi syndrome).
35
Chapter 3 Biometry of the Fetal Brain
Table 3–13. NOMOGRAM FOR EVALUATION OF THE OCULAR
DIAMETER VERSUS GESTATIONAL AGE
Ocular Diameter (mm)
123
Gestational
Age (weeks)
5th
Percentile
11 ———
12 136
13 247
14 358
15 469
16 579
17 5 8 10
18 6 9 11
19 7 9 12
20 8 10 13
21 8 11 13
22 9 12 14
23 10 12 15
24 10 13 15
25 11 13 16
26 12 14 16
27 12 14 17
28 13 15 17
29 13 15 18
30 14 16 18
31 14 16 19
32 14 17 19
33 15 17 19
34 15 17 20
35 15 18 20
36 16 18 20
37 16 18 21
38 16 18 21
39 16 19 21
40 16 19 21
Adapted from Romero and colleagues, 1988,
34
with permission.
50th
Percentile
95th
Percentile
124
Chapter 3 Biometry of the Fetal Brain
THE VENTRICULAR SYSTEM
For practical purposes, we have divided the analysis of the
ventricular system depending on the sonographic modality
used, transabdominal or transvaginal.
TRANSABDOMINAL SONOGRAPHY
Lateral Ventricular Width–Hemispheric
Width Ratio
Definition
The lateral ventricular width–hemispheric width (LVW/
HW) ratio represents the percentage of the whole cerebral
hemisphere that corresponds to the lateral ventricle, measured in an axial plane.
How to Measure It ( Figure 3–4 )
The widths of the lateral ventricles and the hemispheres
are measured in a plane parallel to the BPD, but slightly
closer to the top of the head. In this section, the lateral
ventricles (LVs) appear as two linear echoes, roughly
parallel to the midline. The LVW is then measured as
the distance from the midline to the first echoes of the
LVs. This measurement is thus slightly greater than that
of the actual LV, because it does not use its medial wall
as the internal landmark. The HW is the largest distance
between the midline and the inner edge of the skull,
measured perpendicular to the midline. To avoid tilting
errors, one should be able to recognize both LVs as being
equal in size.
Comments
The normal LVW/HW ratio at 15 weeks can be as high as
71%, with a mean of 56% and a range of 40% to 71%. By 37
postmenstrual weeks, the mean is 29%, with a range of 24%
to 34% ( Table 3–14 ). These data reflect the rapid growth of
the cerebral hemispheres as pregnancy progresses, making
the LVW/HW decrease with advancing age.
The LVW/HW ratio was developed to monitor ventricular growth in an attempt to provide an early diagnosis
of hydrocephaly at a time when the classical diagnosis
36
37
of intrauterine hydrocephaly relied on the finding of a
BPD > 11 cm or a head-to-abdomen ratio > 2.
this ratio provided a way to diagnose hydrocephaly up to
2 months before the BPD was pathologically enlarged,
other parameters, such as the width of the lateral ventricular atrium were able to do that too.
37
Transvaginal neuro-
38
Although
36
sonography can evaluate the fetal ventricular system in a
much more accurate way and somewhat earlier.
One of the major drawbacks of the LVW/HW ratio
is that echogenic “lines” previously used to delineate the
ventricular walls are instead reflections from small venous
structures deep in the fetal white matter.
SD of this ratio renders it ineffective for detecting early
ventricular dilation.
40
39
Also, the wide
ANTERIOR (FRONTAL) HORN OF
THE LATERAL VENTRICLES AND
CAVUM SEPTI PELLUCIDI
Definition
The anterior horn corresponds to the portion of the lateral
ventricles anterior to the interventricular foramen.
The cavum septi pellucidi (CSP) is a closed cavity in
the brain, located on the midline of the transverse plane
between the two leaves of septum pellucidum, which separate the lateral ventricles.
How to Measure It ( Figure 3–5 )
To allow proper visualization of the anterior horn with
transabdominal sonography, a horizontal (axial) scanning
plane parallel and slightly anterior to that for the BPD
should be used. In this section one should be able to recognize, in an anterior-to-posterior fashion, the anterior
horns (AHs), the CSP, and the atria of the lateral ventricles (A). The cerebrofrontal horn distance (CFHD) can
then be measured from the midline echo to the lateral
wall of the anterior horn distal to the transducer. It should
be kept in mind that from about 30 postmenstrual weeks
on, the lumen of the anterior horns is very hard to visualize, and only its lateral aspect is defined. The frontal HW
is measured from the leading edge of the midline echo
Figure 3–4.
LW
LVW
LW
HW
Chapter 3 Biometry of the Fetal Brain
Table 3–14. NOMOGRAM FOR EVALUATION OF THE LATERAL VENTRICULAR WIDTH–HEMISPHERIC WIDTH RATIO
125
Menstrual Age
(weeks)
15 0.75 1.4 56 (40–71)
16 0.86 1.5 57 (45–69)
17 0.85 1.5 52 (42–62)
18 0.83 1.8 46 (40–52)
19 — — —
20 0.82 1.9 43 (29–57)
21 0.76 2.2 35 (27–43)
22 0.82 2.6 32 (26–38)
23 0.83 2.5 33 (24–42)
24 0.83 2.7 31 (23–39)
25 1.1 3.0 34 (26–42)
26 0.9 3.0 30 (24–36)
27 0.9 3.0 28 (23–34)
28 1.1 3.3 31 (18–45)
29 1.0 3.4 29 (22–37)
30 1.0 3.4 30 (26–34)
31 1.0 3.4 29 (23–36)
32 1.1 3.6 31 (26–36)
33 1.1 3.4 31 (25–37)
34 1.1 3.8 28 (23–33)
35 1.1 3.8 29 (26–31)
36 1.1 3.9 28 (23–34)
37 1.2 4.1 29 (24–34)
Term 1.2 4.3 28 (22–33)
From Johnson and colleagues, 1980,
Lateral Ventricular
Width (LVW) (cm)
37
with permission.
Hemispheric Width
(HW) (cm)
Ratio LVW/HW
(% ± 2 SD)
CFHD
CSP
A
CP
AH
AH
HW
CP
A
Figure 3–5.
126
Chapter 3 Biometry of the Fetal Brain
Table 3–15. NOMOGRAM OF THE CEREBROFRONTAL HORN DISTANCE THROUGHOUT
PREGNANCY (IN CENTIMETERS)
Percentile
Gestational Age (weeks) Mean ± 2 SD 10th 50th 90th
15 0.7 ± 0.10 0.6 0.7 0.9
16 0.8 ± 0.12 0.5 0.8 1.0
17 0.9 ± 0.16 0.7 0.9 1.2
18 0.8 ± 0.05 0.8 0.9 0.9
19 0.8 ± 0.10 0.7 0.8 0.9
20 0.8 ± 0.10 0.7 0.8 1.0
21 0.8 ± 0.08 0.7 0.8 0.9
22 0.8 ± 0.07 0.7 0.8 0.9
23 0.9 ± 0.10 0.7 0.8 1.0
24 0.8 ± 0.07 0.7 0.9 0.9
25 0.8 ± 0.05 0.7 0.8 0.8
26 0.9 ± 0.11 0.8 1.0 1.1
27 1.0 ± 0.17 0.8 1.0 1.2
28 0.9 ± 0.13 0.8 0.9 1.1
29 1.0 ± 0.09 0.9 1.0 1.1
30 1.0 ± 0.12 0.8 1.0 1.2
31 1.0 ± 0.16 0.8 1.0 1.3
32 1.1 ± 0.08 1.0 1.1 1.2
33 1.1 ± 0.12 0.9 1.1 1.2
34 1.1 ± 0.13 0.9 1.0 1.2
35 1.2 ± 0.15 1.0 1.1 1.4
36 1.2 ± 0.10 1.1 1.2 1.3
37 1.1 ± 0.00 1.1 1.1 1.1
38 1.2 ± 0.06 1.2 1.2 1.3
39 1.3 ± 0.19 1.0 1.3 1.4
40 1.2 ± 0.00 1.2 1.2 1.2
From Goldstein and colleagues, 1988,
41
with permission.
to the inner aspect of the fetal calvarium at the point of
maximal HW.
41
Comments
The relative size of the anterior horns decreases with
advancing gestational age. This was demonstrated by
Goldstein and coworkers,
41
who found that, in spite of
an increasing CFHD throughout pregnancy, the CFHD/
HW ratio decreases throughout gestation, from 50% at
15 menstrual weeks to 28% at term ( Tables 3–15 and 3–16 ).
These findings correlate with previous reports
42
and are
related to the fact that the internal lumina of the ventricles
are progressively reduced and molded by the growth of
Chapter 3 Biometry of the Fetal Brain
Table 3–16. NOMOGRAM OF THE FRONTAL HEMISPHERIC WIDTH THROUGHOUT PREGNANCY
Percentile
127
Gestational Age (weeks) Mean ± 2 SD (cm)
15 1.5 ± 0.11 1.3 1.5 1.6
16 1.6 ± 0.12 1.5 1.7 1.8
17 1.7 ± 0.10 1.6 1.8 1.9
18 2.0 ± 0.07 1.9 2.0 2.1
19 2.1 ± 0.09 2.0 2.1 2.2
20 2.2 ± 0.10 2.1 2.3 2.3
21 2.3 ± 0.09 2.2 2.3 2.5
22 2.5 ± 0.07 2.4 2.5 2.6
23 2.7 ± 0.13 2.5 2.7 2.9
24 2.8 ± 0.10 2.6 2.9 2.9
25 3.0 ± 0.04 3.0 3.0 3.1
26 3.1 ± 0.11 3.0 3.3 3.4
27 3.3 ± 0.12 3.1 3.3 3.4
28 3.4 ± 0.09 3.3 3.5 3.5
29 3.6 ± 0.15 3.4 3.6 3.8
30 3.7 ± 0.20 3.4 3.8 3.9
31 3.9 ± 0.19 3.6 3.9 4.2
32 3.9 ± 0.17 3.5 3.9 4.1
33 4.0 ± 0.10 3.8 4.0 4.1
34 4.1 ± 0.18 3.8 4.2 4.2
35 4.4 ± 0.34 3.9 4.4 4.8
36 4.4 ± 0.13 4.2 4.4 4.5
37 4.4 ± 0.00 4.4 4.4 4.4
38 4.3 ± 0.22 4.1 4.3 4.6
39 4.6 ± 0.14 4.5 4.5 4.8
40 4.4 ± 0.00 4.4 4.4 4.4
From Goldstein and colleagues, 1988,
41
with permission.
10th 50th 90th
the basal nuclei, the corpus striatum, and the knee of the
corpus callosum.
The septi pellucidi are two thin, translucent leaves that
extend from the anterior part of the body, the genu, and
the rostrum of the corpus callosum to the superior surface
of the fornix. They begin to develop at 10 to 12 weeks of
gestation and reach an adult form by the 17th week of
gestation ( Table 3–17 ).
43
They are also part of the limbic
system and are important relay stations, which are linked
with the main hippocampus and hypothalamus. For a
more extensive review of the sonographic diagnosis of CSP
abnormalities, see Chapter 5 .
128
Chapter 3 Biometry of the Fetal Brain
Table 3–17. MEAN WIDTH AND STANDARD DEVIATION OF THE CAVUM SEPTI PELLUCIDI AT VARIOUS
GESTATIONAL AGES IN 608 FETUSES
Gestational Age (weeks) −2 SD −1 SD Mean Width (mm) +1 SD +2 SD n
19–20 2.08 2.74 3.40 4.06 4.7 43
21–22 2.60 3.33 4.06 4.81 5.52 104
23–24 3.02 3.88 4.74 5.60 6.46 92
25–26 3.96 4.76 5.56 6.36 7.16 36
27–28 4.12 5.27 6.42 7.57 8.72 18
29–30 4.37 5.29 6.11 7.13 8.05 24
31–32 4.43 5.47 6.51 7.55 8.59 77
33–34 4.04 5.26 6.48 7.70 8.92 116
35–36 4.37 5.41 6.45 7.49 8.53 55
37–38 3.81 5.09 6.37 7.65 8.93 27
39–40 4.64 5.47 6.30 7.13 7.96 10
41–42 3.62 4.55 5.48 6.41 7.34 6
CSP, cavum septi pellucidi; SD, standard deviation.
From Jou and colleagues, 1998,43 with permission.
ATRIUM OF THE LATERAL VENTRICLES
Definition
The atrium, or the trigone, is the triangular portion of the
lateral ventricle that is connected anteriorly to the body,
posteriorly to the posterior horn, and inferiorly to the inferior horn. The lateral ventricular atrium width (LVAW) is
the widest dimension of the atrium of the lateral ventricles
that can be measured in an axial plane.
HW
Figure 3–6.
How to Measure It ( Figure 3–6 )
The atrium (A) of the lateral ventricles can be measured
near the axial plane previously described for the BPD.
It can be easily recognized by the presence of the highly
echogenic choroid plexus (CP) within it, marking the lateral wall of the ventricle farther from the transducer. The
electronic calipers are then placed using an outer-toinner (leading edge-to-leading edge) technique. Other
CAD
LVAW
39
Chapter 3 Biometry of the Fetal Brain
Table 3–18. DIAMETER OF THE LATERAL VENTRICULAR ATRIUM ACCORDING
TO GESTATIONAL AGE
129
Gestational
Age (weeks)
14–20 7.6 ± 0.7 6.0–9.0
21–25 7.7 ± 0.5 7.0–9.0
26–30 7.5 ± 0.7 6.5–9.0
31–38 7.6 ± 0.5 7.0–8.5
All 7.6 ± 0.6 6.0–9.0
SD, standard deviation.
Modified from Cardoza and colleagues, 1988,
Lateral Ventricular Atrium Diameter (mm)
Mean ± SD Range
39
with permission.
parameters that can be used to evaluate the ventricular
atrium are the cerebroatrial distance (CAD), atrial width/
HW ratio, atrial width/CAD ratio, and CAD/HW ratio.
The CAD is measured in the same plane as the distance
between the midline and the outer border of the atrial
lumen. The HW corresponds to the distance between
the midline and the inner border of the calvarium.
Comments
It is important to place the calipers across the ventricle
that contains the choroid plexus just across the indentation of the cortex marking the parietooccipital fissure. This
anatomic landmark becomes increasingly evidence and
marked as the gestational age increases.
The ventricular atrium is a structure easily recognized
due to the presence within it of the choroid plexus, which
is among the most noticeable intracranial landmarks.
During the second and third trimesters, the choroid plexus
normally fills the atrium of the lateral ventricles, touching the lateral ventricular walls. Ventricular enlargement
can thus be suspected when a separation ≥ 3 mm can be
measured between the ventricular surface of the choroid
plexus and the adjacent ventricular wall.
The LVAW is an excellent measurement for verifying the state of the ventricular system. It is age independent, with a mean of 7.6 mm, which remains stable
with little change (SD 0.6 mm) throughout gestation,
certainly a very useful feature for any parameter used to
evaluate a normal fetal structure ( Table 3–18 ). For prac-
tical purposes, a normal upper limit of 10 mm for atrial
width has been established.
measurement is its low reported intra- and interobserver
variability.
39
Other authors
45 , 46
Another advantage of this
47
prefer morphological criteria,
such as the shrunken appearance of the choroid plexus
in hydrocephaly, rather than absolute measurements for
estimation of the cerebral ventricles.
44
Typically, a reverberation artifact from the fetal
calvarium obscures the ventricle closest to the transducer, but variations in the angle of the beam and maternal position can make visualization of this ventricle
possible.
mon but reported
44
Both atria should be visualized if the uncom-
48
possibility of unilateral ventriculomegaly is to be ruled out in the atria closest to the
transducer.
The CAD increases as pregnancy progresses, reflecting the steady growth of the cerebral hemispheres
( Table 3–19 ). The decreasing atrial width/HW, atrial
width/CAD, and CAD/HW ratios ( Tables 3–20 , 3–21 ,
Table 3–19. NOMOGRAM OF THE CEREBROATRIAL
DISTANCE THROUGHOUT PREGNANCY
Gestational Age (weeks) Mean ± SD (cm)
15–17 1.16 ± 0.08
18–20 1.35 ± 0.09
21–23 1.45 ± 0.10
24–26 1.59 ± 0.13
27–29 1.81 ± 0.14
30–32 1.96 ± 0.17
33–35 2.04 ± 0.15
36–38 2.15 ± 0.21
39–40 2.36 ± 0.15
From Pilu and colleagues, 1989,
49
with permission.