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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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Fig. 5.16 Three-vessel view. Axial view superiorly shows three vessels: the pulmonary
artery (P) and its continuation via the ductus arteriosus, the aorta (Ao) at the level of the
aortic arch, and the superior vena cava (SVC). (P)Ao, proximal or ascending aorta; (D)Ao,
descending aorta; D, ductus arteriosus; LT, left; RT, right.
right atrium can also be seen on a longitudinal view through the right atrium.
This plane shows the inferior vena cava entering the right atrium and the tricuspid valve between the right atrium and ventricle. The four-chamber view shows
the atrial–ventricular connections.
Demonstration of the ventricular–arterial connections and great vessels requires
familiarity with normal anatomy. The pulmonary artery arises anteriorly, close to
the chest wall, and is directed straight back towards the spine while the aorta arises
centrally within the heart from the left ventricle and ascends to the right, posterior to the pulmonary artery (see Figs 5.15, 5.17). The right ventricular outflow
tract and branching of the pulmonary artery can be identified on an axial plane
just above the level for a four-chamber view with angling slightly toward the left
shoulder (see Fig. 5.15) while the left ventricular outflow tract can be visualized
by angling toward the right shoulder from the four-chamber view (see Fig. 5.14).
Normal fetal anatomy at 18–22 weeks
Fig. 5.17 Ductal arch. Midsagittal scan shows the ductal arch. Note the arch is relatively
flattened and has no systemic vessels arising from it. Also note that the ductus arteriosus
(DA) is a direct extension of the main pulmonary artery (MPA) and so receives most of the
blood pumped through the right ventricle (RV). RPA, right pulmonary artery; DA, ductus
arteriosus; Ao, ascending aorta in cross-section; Ao(D), descending aorta; PV, pulmonic valve.
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Fig. 5.18 Aortic arch. The aorta (Ao) arises from the left ventricle. It is rounder in shape than
the ductus arteriosus and also gives rise to the neck vessels (arrows). Ao(D), descending aorta.
Each of these views should also show the other great artery in cross-section. The
left ventricular outflow view should also show continuity of the aortic wall and
the ventricular septum.
The ductal arch view is obtained on longitudinal midline plane which shows
continuation of the ductus arteriosus from the pulmonary artery and its connec-
Ultrasound in obstetrics and gynaecology
tion with the aorta just below the aortic arch (see Fig. 5.17). The aortic arch can
be seen on transverse views above the heart as the aorta curves from right to left
(see Fig. 5.16). However, the entire aortic arch and origin of the neck vessels can
best be seen on an aortic arch view (see Fig. 5.18). This is an oblique longitudinal plane oriented from the right anterior chest to the left posterior chest. The
descending aorta can be seen on longitudinal views, as well as transverse views
where it is seen in cross-section just to the left and anterior to the spine.
In summary, a checklist of the great vessels should make note of the following.
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The aorta arises from the centre of the heart and ascends as the aortic arch
•
which can be confirmed by showing the origin of head and neck vessels.
The pulmonary artery arises from the right ventricle and gives rise to the
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pulmonary arteries and the ductus arteriosus.
The great arteries are similar in size but the pulmonary artery at the valve
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ring may be slightly bigger than the aorta.
The great arteries cross each other at their origin.
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The ventricular septum is continuous with the aortic wall.
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The recent introduction of 3D and 4D fetal echocardiography has opened new
possibilities of studying normal and abnormal fetal cardiac anatomy.
89
LUNGS AND THORAX
In addition to views of the heart, images directed to the heart will also demonstrate
surrounding structures, including the lungs, great vessels, bony thorax and extrathoracic structures. The lungs are observed as homogeneously echogenic structures surrounding the heart. They should be roughly equal in size. They should surround the
pulmonary arterial branches and pulmonary veins. Lung size has been evaluated by
various ratios, lung length, and more recently by volume using 3D ultrasound.
58–60
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The osseous components of the thorax are readily identified due to their inherent
subject contrast with the adjacent soft tissue structures. In this location the rib cage is
composed of cartilage, which is sonographically hypoechoic and allows good sound
transmission. The scapulae, clavicles, ribs and dorsal spine can all be easily identified
on directed scanning. Since shadowing caused by the overlying bones makes examination of the intrathoracic contents difficult, especially with advancing gestational
age, scan planes that avoid the bony thorax are employed when feasible.
ABDOMEN
The basic ultrasound examination of the fetal abdomen and pelvis includes identification of the stomach, kidneys, bladder, umbilical cord insertion and adjacent
anterior abdominal wall, and measurement of an abdominal circumference for
dating/growth. Other organs that can easily be documented, and in some cases
measured, include the liver,61 spleen,62 adrenals,63 large and small bowel and gallbladder,64 as well as major vascular structures. Both axial (Fig. 5.19) and longitu-
dinal (Fig. 5.20) views are useful for evaluating the abdomen.
The abdominal circumference (AC) measurement, while useful as an adjunctive parameter for fetal dating, finds its greatest value in the evaluation of fetal
growth in the latter part of pregnancy. Fetal growth disturbances are generally
detected by a change in the size of the fetal liver. This is reflected sonographically on the AC measurement, obtained on an axial image through the fetal liver
where the midline umbilical vein joins the portal venous system. Only a short
portion of the umbilical vein deep within the liver should be imaged, since visualization of the vein more anteriorly is only possible with oblique scans as it
passes inferiorly towards the umbilicus (see Fig. 5.19).
The stomach is a variably sized fluid-filled structure in the left upper quadrant.
For confirmation of normal solitus, the stomach should be confirmed to be on the
same side as the apex of the heart. Situs inversus seen prenatally usually reflects
one of the cardiosplenic syndromes (asplenia or polysplenia). Absence of a visible
Normal fetal anatomy at 18–22 weeks
Fig. 5.19 Abdomen, axial view, at the level where the abdominal circumference
measurement is obtained. St, stomach; L, liver; UV, umbilical vein; IVC, inferior vena cava;
Ao, aorta; Sp, spine.
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Ultrasound in obstetrics and gynaecology
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Fig. 5.20 Longitudinal view in the right abdomen shows the normal liver, diaphragm and
more echogenic lung.
stomach after 14 weeks is unusual. This may indicate underlying fetal abnormality or transient decrease in fetal swallowing. In this situation, a short-interval
follow-up scan is indicated for confirmation.
65,66
A more common presentation is
a small or prominent stomach. However, it is often difficult to know what is too
small or too prominent. To help in this situation, Pekindil et al67 proposed a ratio
of stomach circumference to abdominal circumference expressed as a percent
(SC/AC ratio). They found this was normally distributed from 15 to 39 weeks at
a mean of 20.4% and ranged between 14.8% and 27.03% throughout pregnancy.
Although the fetal stomach is a dynamically changing organ, the SC/AC ratio can
be considered as a potentially useful parameter in assessing fetal stomach size.
The fetal duodenum can be identified but dilated duodenum suggests underlying
obstruction68 (double bubble sign).
The gallbladder is often seen in the right abdomen near the inferior edge of
the liver (Fig. 5.21). Care should be taken not to mistake the gallbladder for the
umbilical vein. While both structures extend to the region of the porta hepatis,
the umbilical vein is of uniform calibre, midline in position, and courses inferiorly
to penetrate the abdominal wall; the gallbladder is clearly not in the midline, usually is somewhat teardrop shaped, and does not penetrate the abdominal wall.
Persistent intrahepatic right umbilical vein is a relatively common normal variant in which the right umbilical vein persists rather than the left.69 Blazer et al69
observed this finding in 69 of 30,240 consecutive pregnancies at 14–26 weeks.
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Fig. 5.21 Oblique scan shows a normal gallbladder in the right abdomen. This can be easily
confused with the umbilical vein, which is more central in location. St, stomach.
Normal fetal anatomy at 18–22 weeks
In this situation, the persistent right umbilical vein enters the right lobe of the
liver, lateral to the gallbladder rather than medial to it. In the absence of other
abnormalities, this variation is associated with a favourable outcome.
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The liver occupies the majority of the upper abdomen, with a prominent left
lobe extending well into the left upper quadrant in the fetus. The smaller spleen
is identified as a solid organ posterior to the stomach. Much of the remaining
abdominal cavity is filled with bowel. Early in the second trimester bowel appears
as an area of midlevel to increased echogenicity filling the abdomen from the liver
to the bladder. The large bowel progressively enlarges with meconium throughout
pregnancy, measuring 3–5 mm at 20 weeks.
70,71
Normal small bowel is less distinct
since it is smaller, circuitous in course, and changes with peristalsis. Small bowel
segments can be transiently identified with small quantities of fluid, particularly
with higher-resolution scanners. Echogenic bowel may be seen as a normal variant.72 However, moderate to markedly echogenic bowel has been associated with
adverse outcome including chromosome abnormality, in utero infection, growth
retardation and fetal demise.
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ANTERIOR ABDOMINAL WALL
The site of the umbilical cord insertion into the abdominal wall must be evaluated to confirm a normal-sized cord penetrating into the abdomen. The adjacent
abdominal wall must also be examined to confirm its integrity. The musculature
of the fetal abdominal wall appears hypoechoic, and may be confused with fetal
ascites.74 Knowledge of the hypoechoic nature of fetal musculature and close attention to anatomical detail should easily differentiate the normal from abnormal.
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Confirmation of a normal three-vessel cord may be made by direct imaging of the
cord to delineate the two smaller umbilical arteries and the larger umbilical vein.
Alternatively, the paired umbilical arteries can be imaged within the fetus, extending along the anterior abdominal wall from the umbilicus to a position lateral to
the fetal bladder (Fig. 5.22). This can easily be confirmed with colour flow imag-
ing (Fig. 5.23), a valuable technique early in gestation when direct visualization of
the arteries within the cord is suboptimal. The umbilical cord insertion site should
be visualized on all routine fetal surveys. Detection of a normal cord insertion
excludes the vast majority of anterior abdominal wall defects.
URINARY TRACT
By 18–22 weeks, the kidneys can be clearly seen as oval masses lateral to the psoas
muscles and inferior to the adrenal glands (Figs 5.24, 5.25). Before this time, the
kidneys may be difficult to identify with certainty so that guidelines refer to scanning through the kidney regions. Use of colour flow Doppler can be helpful for
confirming the presence of two kidneys when they are difficult to visualize on
Ultrasound in obstetrics and gynaecology
standard grey-scale imaging (Fig. 5.26).
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Fig. 5.22 Cord insertion site. Axial view shows the umbilical cord inserting into the
umbilicus. The paired umbilical arteries are seen on this view; the umbilical vein deviates
from the arteries immediately on entering the abdomen and courses cephalad to the liver.
Because the cord insertion is inferior on the abdominal wall, the urinary bladder (B) can
often be seen on this view.
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Normal fetal anatomy at 18–22 weeks
Fig. 5.23 Urinary bladder. A slightly inferior plane with colour flow Doppler shows the
paired umbilical arteries coursing around the urinary bladder. LUA, left umbilical artery;
RUA, right umbilical artery.
The kidneys grow throughout gestation and standard measurements for renal
circumference, volume, thickness, width and length have been reported as a function of menstrual age.75 The ratio of kidney circumference to abdominal circumference remains constant at 0.27 to 0.30 throughout pregnancy.76 In general, the
normal kidney length spans approximately 4–5 vertebral bodies.
Fig. 5.24 Kidneys. Axial view with the spine anterior in position shows the normal
paraspinal kidneys (K), outlined by arrows. A tiny amount of fluid is seen within the central
renal pelvis of each kidney.
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Ultrasound in obstetrics and gynaecology
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Fig. 5.25 Kidneys, coronal view. Coronal view of the abdomen shows both kidneys (K).
Again note the small amount of fluid within the central renal pelvis of each kidney.
High-resolution scans can identify normal renal architecture. The medullae are
arranged in anterior and posterior rows around the pelvic sinus. The medullae appear
hypoechoic, probably because the tubules are thin-walled and fluid-filled, compared
to the more peripheral renal cortex. Recognition of this normal renal architecture is
important in distinguishing normal kidneys from those with cystic dysplasia.
Fig. 5.26 Normal renal arteries. Colour Doppler, using the power mode, with the fetus in
coronal plane shows both renal arteries arising from the aorta. This can be helpful when the
kidneys are difficult to visualize on standard grey-scale imaging.
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The central renal pelvis commonly contains small amounts of fluid (urine).
Obvious dilation of the renal pelvis may reflect an underlying obstructive process. Therefore, it is important to attempt to distinguish normal physiological
amounts of fluid from a true abnormality. An objective method of assessment is
measurements of the renal pelvis, best obtained in the anterior–posterior plane
with the fetus either spine anterior or posterior relative to the transducer. ‘Cutoffs’ for normal vary with gestational age and will also vary between centres. By
18–22 weeks, we use a cut-off of 5 mm or more for suggesting a possible renal
77
abnormality.
As a normal variant, mild degrees of renal pyelectasis occur more
commonly among fetuses that are large for gestational age and males are affected
more often than females. Kent et al78 found that 13 of 37 (35%) fetuses with renal
dilation of 4–8 mm at 16–21 weeks went on to require medical or surgical intervention for significant urinary tract anomalies. These anomalies included pelviureteric
junction obstruction, dysplastic kidney, vesicoureteric reflux and posterior urethral
valves. Follow-up evaluation is suggested at 28 weeks when renal pelvic dilation is
suggested.
Although not part of the genitourinary tract, the adrenal glands are usually
assessed at the same time as the kidneys due to their proximity. The adrenal glands
characteristically appear as triangular hypoechoic shadows which outline the
upper poles of the kidneys. The right gland is positioned immediately posterior
to the inferior vena cava, while the left lies lateral to the aorta. Sonographically
the adrenals are hypoechoic peripherally, with a central echogenic layer.
The urinary bladder is a fluid-filled structure located low within the pelvis,
in the midline. Changes in bladder volume over time are obvious and help differentiate the urinary bladder from other cystic pelvic structures. If in doubt, the
umbilical arteries course along the lateral walls of the bladder and confirm it as
the urinary bladder. Therefore, this view can confirm the presence of both the
urinary bladder and both umbilical arteries.
Normal fetal anatomy at 18–22 weeks
GENITALIA
Evaluation of the genitalia is often desired by the prospective parents in order to
determine gender, and is also sometimes medically indicated. Certainly fetal genitalia are well visualized by 18–22 weeks (Fig. 5.27), and fetal gender can probably be
accurately determined by the late first trimester.
The first finding with a male fetus is delineation of the penis, a solid structure
in contrast to the fluid-filled umbilical cord which may lie between the thighs.
The scrotum is a bulbous soft tissue structure increasingly apparent at the base
of the penis as gestation progresses. Although the scrotum is visualized, testicular
descent is not seen before 26 weeks. Longitudinal scans of the scrotum and penis
may produce a ‘turtle’ appearance.
Female genitalia are confirmed early in gestation via identification of several
parallel linear echos representing the margins of the labia. In the third trimester
the prominent soft tissues of the labia majora border the linear echoes of the labia
minora.
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Fig. 5.27 Normal genitalia. Axial images of the perineum show normal female (left) and
male (right) external genitalia.
SKELETON AND EXTREMITIES
Ultrasound in obstetrics and gynaecology
The bones of the extremities are readily identifiable due to their inherent subject
contrast with the surrounding soft tissues, from the late first trimester to term.
The femur is the only long bone which is routinely measured, however, being a
primary parameter for fetal dating as well as a screen for the skeletal dysplasias.
Humerus length is also commonly measured during the second trimester, especially as a potential marker for fetal Down syndrome (Fig. 5.28).
Mild contour variation in the normal femoral shaft is often apparent, with
a straighter appearance on the lateral aspect and a mild ‘bowed’ appearance
medially.80 Only the ossified portions of the bone are measured, excluding the
hypoechoic cartilaginous epiphyses of the femoral head and condyles distally.
It is recommended that a survey of all extremities be performed to confirm a
grossly normal appearance of the bones and soft tissues to the level of the feet
and hands (Figs 5.29, 5.30). Use of 3D multiplanar ultrasound can also help to
confirm normal extremities, including the hands and feet (Fig. 5.31). Imaging of
specific bones is accomplished by careful progression from one known structure
81
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Fig. 5.28 Longitudinal views of the femur (F, left) and humerus (H, right). These are similar
in size during the second trimester.