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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5786_Библиотеки_им_академика_М_И_Перельмана.pdf
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Normal fetal anatomy at 18–22 weeks
Fig. 5.3 Axial sequence images from superior to inferior as seen with tomographic
ultrasound imaging (TUI). The images are set at standard 3 mm intervals in this case. Note
that the choroid (CH) fills the lateral cerebral ventricles and this is located at a plane superior
to the transthalamic view. TH, thalamus; C, cerebellar hemispheres; CM, cisterna magna.
prominent anechoic anterior components of the lateral ventricles. In contrast, the
surrounding cerebral cortex is hypoechoic with scarcely more echogenicity than
the CSF. After about 18 weeks, the posterior aspect of the lateral ventricles (atria
and occipital horns) is drawn laterally with development of the temporal horns.
Simultaneously, the ventricles and choroid appear less pronounced with growth
of the cerebral hemispheres.
Fig. 5.4 Transcerebellar view. Slightly oblique scan through the posterior fossa shows
cerebellar hemispheres (C) which show a normal biconvex shape, outlined by the cisterna
magna (CM). This single normal view excludes nearly all open spinal defects.
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The standard measurement of the cerebral ventricle is obtained in an axial
plane through the atrium. Most studies have found the mean size of the ventricular atrium to be in the range of 5.4–6.5 mm, with 10 mm considered to be
a cut-off for abnormal.
10,11
However, even lesser degrees of ventricular dilation
may prove to be significant during the second trimester.12 Although 10 mm is a
commonly accepted cut-off for normal size, others have found that ventricle size
over 8 mm is unusual before 25 weeks.13 Mild or borderline degrees of cerebral
ventricular dilation pose a difficult dilemma since most fetuses are perfectly normal but some have underlying abnormalities or experience adverse outcome.
Idiopathic lateral ventricular dilation is more common among male fetuses16 and
in our experience, this is more common in those who are large for gestational age.
Detection of borderline or mildly dilated cerebral ventricles is controversial.
Careful techniques should be used to evaluate the ventricles. Off-axis or angled
planes can overestimate the size of the lateral ventricles.17 Reverberation artifact
from bone normally obscures the proximal hemisphere. A technique employing
an oblique scan plane angled superiorly through the temporal bone affords markedly improved visualization of the proximal hemisphere.
Ultrasound in obstetrics and gynaecology
18,19
A common ‘normal variant’ on transventricular views is a choroid plexus cyst.
These are identified as often as 3–4% at 15–18 weeks20 and approximately 1%
at 18–22 weeks. Choroid plexus cysts have been the subject of much study and
21–24
debate.
There is now generalized consensus that they are of no direct consequence themselves. They appear to slightly increase the risk of fetal chromosome
abnormality, especially trisomy 18. However, as an isolated finding in a low-risk
patient, this risk is considered to be low and amniocentesis is not recommended.
14,15
25
84
Transcerebellar view
The transcerebellar or posterior fossa view is obtained by slightly angling the scan
plane down posteriorly from the axial image for BPD determination until the cerebellum and cisterna magna are delineated (see Fig. 5.4).
26,27,86
This view is important for identification of the Dandy–Walker malformation and cerebellar agenesis
and can provide diagnostic information regarding the presence or absence of the
Arnold–Chiari malformation, seen with nearly all cases of open spina bifida. This
scan plane is also useful for evaluating the nuchal soft tissue thickness.
The cerebellar hemispheres can easily be seen on each side of the echogenic
midline vermis, anterior to the cisterna magna. The cerebellum appears slightly
more echogenic than the cerebral hemispheres and is separated from supratentorial
structures by the tentorium. The cerebellar hemispheres are biconvex in shape.
The normal fourth ventricle can be occasionally visualized within the midbrain by high-resolution scans. It appears as a triangular-shaped small fluid space
between the cerebellar hemispheres.
The cisterna magna appears as a sonolucent space just posterior to the cerebral hemispheres and vermis near the base of the brain. The presence of a normal cisterna magna (CM) excludes nearly all open spinal defects. Standardized
measurements of the cisterna magna, with a normal range of roughly 3–10 cm,
are taken in the midline from the vermis to the occipital bone.28 Measured values
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at the lowest end of this range are expected only early in gestation, while conversely a cisterna magna of large dimension is generally seen in the third trimester.
Occasionally a measurement exceeds these standards, which is usually still normal if the vermis is well seen and intact and the transverse cerebellar diameter is
normal for gestational age. An inappropriate scan plane can also lead to an abnormally large measurement of the CM or even the appearance of a Dandy–Walker
variant.29 Caution should be exercised in diagnosis of incomplete closure of the
vermis in the early second trimester;30 however, the normal vermis should be
closed by 18 weeks.
In most fetuses, 1–3 linear echoes can be seen traversing the CM posteriorly from
the cerebellum. Although originally mistaken for the straight sinus, these lines are
now attributed to ‘subarachnoid septa’31 or ‘dural folds’.32 Occasionally they may
appear ‘cyst-like’ and simulate a posterior fossa cyst.
FACE AND NECK
Examination of the face is not included in basic examination guidelines; however,
it is generally easy to perform and may provide important information. Parents
also desire and easily recognize views of the face. Views of the face are particularly important when other anomalies are suspected. For all these reasons, we
believe views of the face should be included in any fetal survey at 18–22 weeks.
Imaging of the fetal face can be accomplished in coronal, sagittal and axial planes
as well as three-dimensional (3D) multiplanar ultrasound (Figs 5.5–5.8). Each scan
plane has advantages and disadvantages, and a combination of scan planes is often
desired, when positioning is favourable, to adequately delineate all anatomy. A midline sagittal scan, or profile view, is one of the most recognizable images of any fetus
(see Fig. 5.5). It is useful for evaluation of size and position of the mandible for
exclusion of micrognathia, the nose and nasal bridge, and the tongue.
Coronal imaging of the soft tissues of the nose and upper lip can be easily
shown at 18–22 weeks (see Fig. 5.6). This view is most useful for exclusion of
cleft lip, with or without cleft palate. Although mild forms of cleft lip/cleft palate
can be missed earlier, the majority of clefts should be visualized by 18–22 weeks.
Normal fetal anatomy at 18–22 weeks
Fig. 5.5 Midline sagittal view of the face shows normal structures including a normal nasal
bone (NB).
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Fig. 5.6 Coronal view through the superficial soft tissues shows a normal upper lip (L) and
nose (N).
On the other hand, cleft palate without associated cleft lip (aetiologically distinct
from cleft lip with or without cleft palate) remains undetected throughout preg-
Ultrasound in obstetrics and gynaecology
nancy, with rare exceptions. Although the soft tissue view is most useful, coronal
views more posteriorly within the face may show the oral cavity and deep nasal
structures. This can be useful for showing the involvement of cleft palate when
cleft lip is suspected.
Transverse or axial views are also useful when cleft lip/palate is suspected. The
upper lip and anterior maxilla can be imaged simultaneously in this plane, and
the integrity of the alveolar ridge can be demonstrated.33 The oral cavity may also
be nicely imaged in the axial plane. When the upper lip is evaluated on coronal
images, the nose also comes into plane. The nose should be normal in size and
shape; two ala should be demonstrated. Some familial and racial differences are
probably evident in the appearance of the nose.
The orbits are best imaged in the axial or coronal plane (see Fig. 5.7).
Measurement of the outer orbital diameter (OOD), which correlates with gestational age,34 allows for detection of hypo/hypertelorism. Intraorbital anatomy
that can be identified includes the globe, lens and hyaloid artery.
The fetal ears are not frequently targeted for imaging, but when necessary
can be easily identified as complex soft tissue protrusions external to the skull.35
86
Fig. 5.7 Axial image through the orbits shows normal measurements of the orbital diameter
(OD), binocular diameter (BOD), and interocular diameter (IOD).
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Fig. 5.8 Face, 3D multiplanar ultrasound. A variety of normal structures on a single image,
including normal mouth, jaw, lips, orbits and ear.
Because of their complex shape, they are best imaged using 3D multiplanar ultrasound (see Fig. 5.8). Normal ear size has been documented in an effort to detect
small ears as a sign of fetal chromosome abnormality.36 Fetal hair may be seen
during the third trimester37 but is not seen at this time.
Evaluation of the soft tissue of the back of the neck (nuchal fold or nuchal
thickness) has proven to be one of the best sonographic ‘markers’ for trisomy 21
during the second trimester.
38-40
Although detection of sonographic markers
is generally performed earlier, with nuchal translucency assessed as early as
10–14 weeks, nuchal thickness can still be evaluated before 20 weeks, and probably
as late as 22 weeks. It should be included on any routine fetal anatomical survey
during the second trimester. Because nuchal thickness increases with gestational
age, absolute cut-offs will be useful throughout the pregnancy.
Other neck structures are not routinely sought during the anatomical survey.
Normal thyroid size has been documented since 20 weeks.41 However, cystic
hygromata or other neck masses can be detected.
Normal fetal anatomy at 18–22 weeks
SPINE
The fetal spine is well developed at 18–22 weeks. Each vertebral segment is composed of three ossification centres which appear echogenic sonographically.42 The
anterior centre is the developing vertebral body while the posterior centres are
formed at the junction of the lamina and the pedicle on each side.43 The ossification
centres lie in a symmetrical triangular configuration, with the posterior centres
oriented towards the midline.
Imaging of the fetal spine can be accomplished in three planes: parasagittal, coronal and transverse. While the parasagittal and coronal images give the
best overall views of the spine (Fig. 5.9), transverse images permit simultaneous evaluation of both the ossification centres and the overlying soft tissues (Fig.
5.10). Although transverse images look only at a single level, the entire spine can
be quickly evaluated with transverse images with real-time ultrasound. Threedimensional multiplanar ultrasound can also be used to evaluate the spine and
other bony structures (Fig. 5.11).
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Fig. 5.9 Longitudinal view shows normal spine (Sp) extending to the sacral spine.
Parasagittal imaging of the entire spine demonstrates two rows of roughly parallel ossification centers, one being the vertebral bodies and the other one a row
of posterior centres. While this plane of section nicely demonstrates the overall
appearance of the spine and delineates the posterior soft tissues adjacent to the
midline, it theoretically might be less sensitive for subtle widening between the
Ultrasound in obstetrics and gynaecology
paired posterior ossification centres. Coronal images may be oriented to show
both posterior ossification centres. However, this plane does not show visualization of the posterior soft tissues.
HEART
The 18–22-week scan is an ideal time to evaluate the fetal heart.87 Indeed, the
heart can usually be evaluated in great detail at this time. In additional to the
requisite four-chamber view, most centres have now adopted the policy of additional views of the outflow tracts and great vessels.
using specific views will result in optimal detection of cardiac defects. However,
specific views are not a substitute for understanding normal anatomical relationships. Also, it should be stressed that static images are not sufficient for evaluating complex anatomical structures. Nowhere is this more true than when
evaluating the dynamic fetal heart.
44,87,88
A systematic approach
88
Fig. 5.10 Spine. Transverse view shows normal anterior (Ant) and posterior (Post)
ossification centres and overlying skin posteriorly.
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Normal fetal anatomy at 18–22 weeks
Fig. 5.11 3D multiplanar view shows normal spine (Sp), ribs (R) and scapula (Sc).
Obtaining a good four-chamber view is usually not difficult at 18–22 weeks
although it requires good ultrasound technique (Fig. 5.12).
45-47
This view is best
obtained from an anterior or left lateral approach to avoid the spine and ribs.
The scan plane is transverse through the lower thorax; however, the transducer
is actually tilted slightly cephalad toward the spine to include the atria which
are located posterior and superior to the ventricles. This can be accomplished by
beginning the plane inferior to the heart and angling up slightly to the thorax.
Using the four-chamber view, evaluation of the heart begins by overall assess-
ment of the position, axis and size of the heart.
48,49
The heart lies anteriorly
within the thorax, slightly to the left of midline. The cardiac apex, as determined by a line through the ventricular septum, is directed to the left side of the
hemithorax at about a 45 ° angle from the midline or roughly equidistant from
the anterior and lateral aspects of the chest.50 The heart should occupy about
one-third of the thorax; an abnormal heart to thoracic ratio can help identify a
variety of cardiac defects.51 Cardiac rate and rhythm should be noted, with a normal range of 120–160 beats per minute from the second trimester to term.
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Fig. 5.12 Normal four-chamber axial view of the heart. LV, left ventricle; RV, right ventricle;
LA, left atrium; RA, right atrium; PV, pulmonary veins; Ao, aorta.
Ultrasound in obstetrics and gynaecology
A checklist of normal anatomical relationships can be confirmed on the four-
chamber view, including the following.
There are two atria of approximately equal size. The left atrium is the most
•
posterior chamber.
The two atrioventricular valves (mitral and tricuspid) show normal
•
mobility.
There are two ventricles of approximately equal size and thickness. Both
•
show normal contractility. The right ventricle is anterior in the midline, just
behind the sternum, and the left ventricle is positioned left and posterior
to the right ventricle.
The atrial and ventricular septa meet the two atrioventricular valves
•
(mitral and tricuspid) to form the crux of the heart. This crux has a
slightly offset cross appearance since the septal leaflet of the tricuspid
valve inserts slightly lower in the ventricular septum than the mitral valve.
The left atrium is the most posterior chamber and is similar in size to the
right atrium. The atrial septum is often difficult to image, but appears as a
continuous thin structure with the exception of a physiological opening,
the foramen ovale, through which blood flows in the fetus from right to
left atrium.
The interventricular septum should appear intact.
•
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The ventricular septum appears as a continuous thick muscular structure separating the ventricles, except for its short, thinner membranous portion near the
atrioventricular (AV) valves. In some scan planes the ventricular walls or interventricular septum may have a component which is quite hypoechoic relative
to the remaining muscle. This is a normal variant secondary to the complex
orientation of the cardiac muscle fibres interacting variably as the scan plane
changes.52 Fetal rotation or probe position changes may also alter the appearance
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and identification of some intracardiac structures, such as the ventricular septum
which can vary from appearing relatively thin to rather thick when imaging in
orthogonal planes. Colour flow Doppler can help confirm an intact ventricular
septum (Fig. 5.13).
The moderator band may be seen in the right ventricle. A commonly seen
normal variant, usually within the left ventricle, is an echogenic focus caused by
a specular reflection from the papillary muscles and chordae tendinae.
53–55
This
has been referred to as an echogenic intracardiac focus or echogenic chorda tendinae. It is observed in approximately 3–4% of the normal population before
20 weeks, and appears to be even more common among Asian populations. It
typically resolves later in gestation and has no direct functional significance.
However, an echogenic intracardiac focus does appear to increase the risk of
fetal chromosome abnormality, especially trisomy 21, at least in non-Asian
populations.
In addition to the four-chamber view, other views of the heart are required to
show normal relationships (Figs 5.14–5.18).
42,44,56,57
However, it is important to
understand the normal anatomy and circulatory pattern while obtaining these
views. This sequential segmental approach is most useful in the evaluation of the
heart and especially in the diagnosis of congenital heart disease. In brief, blood
enters the right atrium through the superior and inferior vena cavae. Oxygenated
blood from the inferior vena cava is preferentially directed across the foramen
ovale to the left atrium while deoxygenated blood from the superior vena cava is
directed to the right ventricle through the tricuspid valve. Deoxygenated blood is
pumped through the pulmonary artery and much of this continues through the
ductus arteriosus to the aorta, returning to the placenta and lower fetus. A portion of pulmonary arterial flow goes to the lungs, returning via the pulmonary
veins. This blood enters the left atrium where it mixes with oxygenated blood
crossing the foramen ovale and then enters the left ventricle through the mitral
valve. It is then pumped through the left ventricle into the aorta.
Normal fetal anatomy at 18–22 weeks
Fig. 5.13 Four-chamber view without (left) and with (right) colour flow. Colour flow helps
to confirm an intact ventricular septum.
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Fig. 5.14 Left ventricular outflow tract. Plane angled toward the right shoulder from
the standard four-chamber view shows normal ascending aorta (Ao). Note the wall of
the ascending aorta is continuous with the ventricular septum. RV, right ventricle; LV, left
ventricle; LA, left atrium.
Other views help show these normal relationships including the venous–atrial,
atrial–ventricular and ventricular–arterial junctions of both the left and right side
Ultrasound in obstetrics and gynaecology
of the heart. In addition to the four-chamber view described above, these include
a view of the upper abdomen to show normal solitus, and views of the great vessels. Left and and right ventricular outflow views (see Figs 5.14, 5.15) should be
obtained, if possible. A ‘three-vessel’ view (see Fig. 5.16) should also be attempted
by continuing to angle the transducer superiorly from the four-chamber view.
A more complete study would include the venous–atrial connections and longitudinal views of the ductal arch (see Fig. 5.17) and aortic arch (see Fig. 5.18).
The venous–atrial connections of both the left and right heart can be seen on
transverse views sweeping from the abdomen to the four-chamber view. As an
optional view, the relationship of the inferior and superior vena cavae with the
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Fig. 5.15 Right ventricular outflow tract. Plane angled toward the left shoulder from the
standard four-chamber view shows the main pulmonary artery (MPA), and its continuation
by way of the ductus arteriosus (DA) which joins the aorta. RPA, right pulmonary artery; RV,
right ventricle; Ao, ascending aorta seen in cross-section.