AJR 2003; 180:433-437
© American Roentgen Ray Society
Prenatal Sonography and MR Imaging of Pulmonary Sequestration
Rajpal Dhingsa1,
Fergus V. Coakley1,
Craig T. Albanese2,
Roy A. Filly1 and
Ruth Goldstein1
1 Department of Radiology, University of California San Francisco, Box 0628,
M-372, 505 Parnassus Ave., San Francisco, CA 94143-0628.
2 Department of Surgery, Fetal Treatment Center, University of California San
Francisco, San Francisco, CA 94143-0628.
Received June 7, 2002;
accepted after revision July 24, 2002.
Address correspondence to F.V. Coakley.
A pulmonary sequestration is a developmental mass of nonfunctioning
bronchopulmonary tissue that is separate from the tracheobronchial tree and
receives arterial blood from the systemic circulation. Pulmonary
sequestrations account for as many as 23% of prenatally detected lesions in
the lung [1]. Sonography is the
primary modality for prenatal imaging, but the quality of sonographic findings
may be affected by poor acoustic contrast, a small field of view, beam
attenuation in the mother's adipose tissue, or the skillfulness of the
operator [2]. Because it is
sometimes difficult to distinguish a sequestration from other congenital
pulmonary abnormalities, MR imaging is increasingly used as a supplement to
obstetric sonography in prenatal imaging of fetuses with complex anomalies,
including thoracic lesions such as a sequestration
[3]. Surface coils and rapid
sequences produce high-resolution MR imaging with no significant fetal motion
artifacts. The aim of our pictorial essay is to review the prenatal imaging
findings of pulmonary sequestration, with particular emphasis on the
supplementary role of MR imaging.
Pathology and Embryology
Postnatally, a pulmonary sequestration is classified as extralobar (15-25%
of patients) or intralobar (75-85% patients), depending on whether the
sequestration is a separate pleural investment or is within the pleura of the
lung. Most, if not all, prenatally diagnosed sequestrations are extralobar.
Therefore, the imaging findings described in this review are essentially
confined to extralobar sequestration; we are unaware of published data
describing any distinctive prenatal imaging findings of intralobar
sequestration.
An extralobar sequestration is characterized pathologically by diffuse
dilatation of bronchioles, alveoli, and subpleural lymphatic vessels. Cystic
areas are occasionally present. The arterial supply is nearly always from the
descending aorta and likely represents persistence of primitive splanchnic
arteries that supply the early foregut. Venous drainage is typically through
the azygos system or the inferior vena cava, but in 25% of cases, the venous
drainage is through the pulmonary veins. Although the lesion is believed to
result from abnormal budding of the primitive foregut (resulting in an
"accessory lung"), the lesion typically has no visible
communication with the tracheobronchial tree. Sometimes a fibrous pedicle is
seen accompanying the feeding and draining vessels and may represent the
involuted foregut bud.
An intralobar sequestration is characterized pathologically by chronic
inflammation, fibrosis, and cystic changes. The developmental nature of an
intralobar sequestration is controversial. It is possible that, at least in
some fetuses, intralobar sequestration is acquired rather than congenital and
is due to bronchial obstruction that results in a distal infection and the
recruitment of a systemic arterial supply through pleural granulation tissue.
This hypothesis would explain the frequency of diagnosis of intralobar
sequestration in adult patients, the rarity of additional associated
congenital anomalies, the occasional communication with the bronchial tree,
and the characteristic pattern of venous drainage through the pulmonary
veins.
Prenatal Imaging
Sonography
More common in males (male-to-female ratio, 4:1), an extralobar
sequestration can be seen on fetal sonography as early as 16 weeks' gestation
and typically appears as a solid well-defined triangular echogenic mass
[4]
(Fig. 1). The extralobar
sequestration may be supradiaphragmatic (90%) or subdiaphragmatic (10%). Most
supradiaphragmatic sequestrations occur between the left lower lobe and
diaphragm; most subdiaphragmatic sequestrations are also left-sided. Cysts can
be seen, particularly in hybrid lesions that combine elements of sequestration
and congenital cystic adenomatoid malformation
(Fig. 2). Visualization of a
systemic feeding artery (Fig.
3) arising from the thoracic or abdominal aorta is a useful
finding that distinguishes a sequestration from other masses such as a
congenital cystic adenomatoid malformation or bronchial atresia. Color and
spectral Doppler sonography can be helpful in visualizing the feeding artery,
but visualization may still be difficult. In one study, a feeding systemic
artery was identified on Doppler sonography in only four of 10 cases of
pathologically proven sequestration
[5].
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Fig. 1. —Axial sonogram of chest obtained at 20 weeks' gestation in
fetus with extralobar sequestration. Sequestration is visualized as large
solid echogenic mass in left (LT) side of fetal chest, with cardiomediastinal
shift (arrow) to right (RT). Aorta (Ao) and spleen (Sp) are also
visible. This is typical sonographic appearance of extralobar
sequestration.
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Fig. 2. —Axial sonogram of chest obtained at 22 weeks' gestation in
fetus with extralobar sequestration (asterisk). Cysts
(arrows) are visible. Findings of cysts are not unusual, particularly
in hybrid lesions that combine elements of sequestration and congenital cystic
adenomatoid malformation.
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Fig. 3. —Sagittal sonogram of chest obtained at 32 weeks' gestation in
fetus with extralobar sequestration (mass). Systemic feeding artery (vessel)
is seen supplying sequestration and arising from descending thoracic aorta
(Ao). Visualization of systemic feeding artery is helpful in accurately
distinguishing sequestration from other congenital thoracic lesions.
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An ipsilateral pleural effusion is seen in 6-10% of fetuses with extralobar
sequestration and may be related to the common pathologic finding of dilated
subpleural lymphatics or to torsion around the connecting vasculature and
fibrous pedicle. The finding of a unilateral pleural effusion in association
with a prenatal thoracic mass is suggestive of an extralobar sequestration
(Fig. 4).
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Fig. 4. —Axial sonogram of chest obtained at 30 weeks' gestation in
fetus with extralobar sequestration (mass). Sequestration is in left (LT) side
of fetal chest, with cardiomediastinal shift to right (RT). Large effusion
(asterisk) is present. Finding of unilateral pleural effusion in
association with prenatal thoracic mass is suggestive of extralobar
sequestration.
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Extralobar sequestration can occupy between one and two thirds of the
hemithorax and may cause mediastinal shift and even fetal hydrops. Detailed
sonographic evaluation of the entire fetus is important when an extralobar
sequestration is suspected because associated anomalies have been reported to
occur in 11-65% of fetuses (although the higher number may represent an
overestimation due to selection bias)
[6,
7]. Associated anomalies that
have been described include congenital diaphragmatic hernias
(Fig. 5), congenital cystic
adenomatoid malformations, diaphragmatic eventrations or paralysis,
bronchogenic cysts, pericardial defects, foregut duplications or diverticula,
ectopic pancreas, vertebral anomalies, and pectus excavatum
[4,
5,
7].
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Fig. 5. —Axial sonogram of chest obtained at 26 weeks' gestation in
fetus with left-sided extralobar sequestration (arrow). Heart (hrt)
is displaced to right. Fetal stomach (st) is seen in left side of fetal thorax
because of coexisting left-sided congenital diaphragmatic hernia. Associated
anomalies, including congenital diaphragmatic hernia, are common in fetuses
with sequestration.
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MR Imaging
On MR imaging, normal fetal lungs are homogenous and have a relatively high
T2 signal intensity because they are filled with amniotic fluid
[8]. A sequestration typically
appears as a well-defined mass in the chest that has a T2 signal intensity
that is higher than that of the normal lung
[3] but lower than that of the
free amniotic fluid (Fig.
6A,6B).
The frequency with which MR imaging reveals feeding vessels has not been
systematically established. Hubbard et al.
[3] did not visualize a feeding
artery in either of their patients, but at our institution, we have seen such
vessels on MR imaging in fetuses with a sequestration
(Fig. 7).
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Fig. 6A. —Prenatal MR imaging appearances of typical extralobar
sequestration in fetus at 22 weeks' gestation. Axial T2-weighted single-shot
rapid acquisition with relaxation enhancement (RARE) MR image (TR/effective
TE, infinite/100) of fetal chest. Sequestration (black asterisk) is
visible as large left-sided triangular mass of increased signal intensity,
relative to displaced and compressed normal lungs (arrows). Lungs and
heart (white asterisk) are displaced to right.
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Fig. 6B. —Prenatal MR imaging appearances of typical extralobar
sequestration in fetus at 22 weeks' gestation. Coronal T2-weighted single-shot
RARE MR image (infinite/100) of fetal chest. Sequestration (asterisk)
is again seen as large left-sided triangular mass of increased signal
intensity, with some extension across midline into right hemithorax.
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Fig. 7. —Coronal T2-weighted single-shot rapid acquisition with
relaxation enhancement MR image (TR/effective TE, infinite/100) of fetus
obtained at 23 weeks' gestation with left-sided extralobar sequestration.
Systemic feeding artery (black arrow) is seen supplying sequestration
(asterisk) and arising from descending thoracic aorta (white
arrow). Relative accuracy of MR imaging and sonography for revealing
feeding vessels in sequestration is unknown.
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The incremental benefit of MR imaging over sonography remains under
investigation. In our experience, MR imaging is helpful in complex cases in
which an associated anomaly, such as congenital diaphragmatic hernia, is
present (Fig.
8A,8B).
MR imaging is also valuable in distinguishing subdiaphragmatic sequestration
from a neuroblastoma or an adrenal hemorrhage; a sequestration is
characterized by an extremely high and uniform T2 signal intensity (Fig.
9A,9B,9C).
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Fig. 8A. —Prenatal MR imaging appearances of extralobar sequestration
associated with congenital diaphragmatic hernia in fetus at 27 weeks'
gestation. Axial T2-weighted single-shot rapid acquisition with relaxation
enhancement (RARE) MR image (TR/effective TE, infinite/100) of fetal chest.
Sequestration (black asterisk) is visualized as large left-sided
triangular mass of increased signal intensity. Lungs and heart (white
asterisk) are displaced to right. In addition, stomach (arrow)
and left hepatic lobe (L) are visible in anterior part of left hemithorax
because of displacement by associated left-sided congenital diaphragmatic
hernia.
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Fig. 8B. —Prenatal MR imaging appearances of extralobar sequestration
associated with congenital diaphragmatic hernia in fetus at 27 weeks'
gestation. Sagittal T2-weighted single-shot RARE MR image (infinite/100) of
fetal chest. Sequestration (white asterisk) is seen as mass of
increased signal intensity. Herniated stomach (black asterisk) and
left hepatic lobe (arrow) are visible above expected location of
diaphragm.
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Fig. 9A. —Role of MR imaging in prenatal distinction of
subdiaphragmatic sequestration from neuroblastoma in fetus at 28 weeks'
gestation with left-sided suprarenal mass detected on prenatal sonography.
Axial sonogram of upper abdomen shows mildly echogenic mass (arrows)
lying between aorta (Ao) posteriorly and stomach (St) anteriorly. Sonographic
differential diagnosis included both neuroblastoma and subdiaphragmatic
sequestration.
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Fig. 9B. —Role of MR imaging in prenatal distinction of
subdiaphragmatic sequestration from neuroblastoma in fetus at 28 weeks'
gestation with left-sided suprarenal mass detected on prenatal sonography.
Sagittal T2-weighted single-shot rapid acquisition with relaxation enhancement
(RARE) MR image (TR/effective TE, infinite/100) shows mass (asterisk)
lying superior relative to kidney and posterior relative to stomach
(arrow).
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Fig. 9C. —Role of MR imaging in prenatal distinction of
subdiaphragmatic sequestration from neuroblastoma in fetus at 28 weeks'
gestation with left-sided suprarenal mass detected on prenatal sonography.
Coronal T2-weighted single-shot RARE MR image (infinite/100) shows mass
(asterisk) to be of uniformly high signal intensity. Signal intensity
characteristics were considered to indicate diagnosis of subdiaphragmatic
sequestration rather than neuroblastoma or adrenal hemorrhage. Parents were
counseled accordingly, and elective postnatal resection of mass confirmed
diagnosis of subdiaphragmatic sequestration.
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Prognosis
Extralobar sequestration has an excellent prognosis and frequently can be
seen to regress spontaneously on serial prenatal sonograms. In rare cases, the
volume of fluid secreted by the mass may cause a tension hydrothorax, which
may cause hydrops as a result of vena caval obstruction and cardiac
compression. The detection of hydrops in a fetus with a sequestration may be
an indication of in utero drainage by thoracocentesis or thoracoamniotic
shunting.
In one series, substantial or complete regression of sequestrations was
observed on serial prenatal sonograms in 28 of 41 fetuses with the condition
[1], but the sequestrations in
all 28 infants were visible on postnatal CT. However, all these infants were
asymptomatic after birth, and none required resection. No regression was seen
in seven fetuses, and all seven infants required postnatal resection for
respiratory symptoms. Tension hydrothorax with secondary hydrops was seen in
four fetuses. Three of the fetuses with hydrops were successfully treated
prenatally by thoracoamniotic shunt placement (n = 2) or serial
thoracocenteses (n = 1). One fetus with hydrops was not treated
prenatally and died despite postnatal resection and ventilatory support. The
two remaining pregnancies were electively terminated.
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