Fetal MRI: thoracic, abdominal and pelvic pathology

Transcription

Obstetric
Revision article
Internacional colaboration
Fetal MRI: thoracic, abdominal and
pelvic pathology
Manuel Recio Rodríguez (1), Pilar Martínez Ten (2), Javier Pérez Pedregosa (2),
Carmina Bermejo López (3), Inés Tamarit Degenhardt (4), Ignacio Pastor Abascal (1).
Resumen
Abstract
Aunque la ecografía (US) es el método de elección en
la evaluación del feto, la resonancia magnética (RM) es
una técnica complementaria a la US en el diagnóstico
de las anomalías fetales. Entre las ventajas de la RM se
destacan un excelente contraste tisular, un campo de
visión grande y una relativa operador-independencia.
La mayoría de los trabajos previos de RM fetal han
estudiado el sistema nervioso central (SNC). Sin
embargo, la RM es útil en la evaluación de las anomalías torácicas y abdominales. En este artículo se muestran los diferentes aspectos por RM de las anomalías
fetales torácicas y abdominales y se discuten las indicaciones y ventajas de la RM fetal.
Palabras clave. Anomalías fetales. Ecografía fetal.
Estudios comparativos, resonancia magnética (RM).
RM embarazo. RM fetal.
Fetal MRI: thoracic, abdominal and pelvic pathology.
Ultrasonography (US) is the method of choice in fetal examination. However, magnetic resonance (MR) imaging is a
complementary technique that contributes to the accurate
diagnosis of fetal anomalies. The benefits of MR include
excellent tissue contrast, large field of view and relative operator independence. Most previous reports on fetal MR have
focused on central nervous system (CNS). However, MR is a
useful tool for the examination of fetal thoracic and abdominal anomalies. This article illustrates the different features of
fetal thoracic and abdominal anomalies on MR, and further
discusses the indications and benefits of fetal MR.
Keywords. Comparative studies. Fetal anomalies. Fetal
MR. Fetal US. Magnetic resonance (MR) imaging.
Pregnancy MR. Rapid imaging.
INTRODUCTION
IMAGING PROTOCOL
Ultrasound (US) is the routine screening test for
fetal anomalies but, even when performed by experienced personnel, it has technical limitations. Even if
most ultrasound examinations are diagnostic, such
limitations may require an alternative imaging
method in more complex cases to confirm or complete ultrasound findings, to guide management of pregnancy and to plan intrauterine interventions, delivery,
and postnatal care.
With the advent of ultrafast sequences in the
1990’s, magnetic resonance imaging (MRI) is becoming a non-invasive method complementary to ultrasound, useful in the detection of fetal anomalies,
which is helpful in formulating prognosis and perinatal management. However, most published research
has focused on brain pathology and only a few studies
report the use of fetal MRI in thoracic, gastrointestinal
or genitourinary abnormalities. This article reviews
fetal thoracic anomalies (with an especial emphasis on
congenital diaphragmatic hernia, where MRI plays
the most important role) and describes the main indications for MRI in abdominal pathology, as well as the
advantages and disadvantages of MRI as compared to
ultrasound.
Scans should be performed using high-field (1.5T)
resonators with multi-channel coils of great spatial
resolution. Three orthogonal planes with respect to
the mother are identified to obtain sagittal, coronal
and axial slices of the fetus, always taking as reference the last sequence used to plan the following
sequence because of fetal movements. The main
sequences used are (Fig. 1):
• T2-weighted images --such as Single Shot Fast Spin
Echo T2 (SSFSE T2)-- )-- and balanced Steady-State
Free Precession sequences—such as FIESTA
sequence, with high tissue contrast, showing hyperintense amniotic fluid. Both sequences are useful
to study the airways, lung, urinary tract and gastrointestinal tract—from the esophagus to the loops
of the proximal ileum. All these structures are hyperintense on T2-weighted images. The FIESTA
sequence is also useful in assessing vascular anatomy without intravenous contrast, showing hyperintense fetal vessels (vessels appear hypointense
on SSFSE T2- and T1-weighted images).
• T1-weighted images (3D gradient dual echo, 2D
FSPGR or 3D LAVA): provide less tissue contrast
than SS FSE T2 or FIESTA. They are useful in the
(1)
Hospital Universitario Quirón- Madrid, España.
C/Diego de Velázquez 1, Pozuelo de Alarcón (28223)- Madrid, España.
(2)
Delta Ecografía. Centro de Diagnóstico por la Imagen en Obstetricia
y Ginecología, Madrid.
(3)
Gabinete Médico Velázquez. Madrid.
(4)
Servicio de Ginecología y Obstetricia. Hospital La Moraleja. Madrid.
Correspondencia: Dr. Recio Rodríguez- machamama@telefonica.net
Recibido: agosto 2011; aceptado: diciembre 2011
Received: august 2011; accepted: december 2011
©SAR
RAR - Volumen 76 - Número 1 - 2012
Página 1
assessment of the liver, the loops of the distal
ileum and colon, which are visualized as hyperintense structures. 3D sequences offer the possibility
of volume reconstructions of the entire colon. As
in brain pathology, these sequences are useful to
determine the presence of subacute bleeding, calcifications or lipomas (1).
• Diffusion-weighted imaging: its applications are
evolving and it is currently being used in the
assessment of lung parenchyma maturation (2) and
in the study of renal pathology, such as renal
venous thrombosis or the Twin-Twin Transfusion
Syndrome (TTTS) (3).
THORACIC PATHOLOGY
Fetal lungs are filled with fluid, and are therefore
hyperintense on T2-weighted imaging, easily differentiated from other organs. MRI will not replace US
as a first-line screening technique, but is useful in
cases of oligohydramnios, maternal obesity, and unfavorable fetal position. Levine et al. (4) examined by US
74 fetuses diagnosed with thoracic abnormalities. MRI
yielded additional information in 38% of cases and
affected care in 8%.
Congenital diaphragmatic hernia
Congenital diaphragmatic hernia (CDH) is the
main indication for fetal MRI in thoracic pathology.
The incidence of CDH is 1 in 2,500 to1 in 5,000 live
births (5), with 85% occurring on the left side
(Bochdalek hernia), 15% on the right and 2% are bilateral (6). Approximately 40% of patients with CDH
have other congenital malformations (mainly cardiac
and of the central nervous system –CNS-), chromosomal anomalies (trisomy 21, trisomy 18 and trisomy 13)
and genetic syndromes (Fryns syndrome, Lange
syndrome or Marfan syndrome) (7). Associated anomalies are considered an independent survival factor
(survival is less than 15%) (8).
The degree of pulmonary hypoplasia and liver
herniation are the main prognostic factors. To calculate lung volume on ultrasound, the lung-to-head ratio
(LHR) (contralateral lung area / head circumference)
is used, and it is measured in the second trimester. If
LHR is >1.6, survival is ≥83%; if LHR is ≥ 1 and < 1.6,
survival is 66%; and with values ≥0.8 and <1, survival
is 16% (9). Volumetric evaluation of the lung using 3D
ultrasound for calculation of lung volume is not better
than LHR (10).
With MRI, both the ipsilateral and contralateral
lung volumes can be measured, thus obtaining the
observed total fetal lung volume (TFLV).
Rypens et al (11) conducted a multicenter (seven
hospitals in France and Belgium) prospective study
with MRI in 336 fetuses suspected of having CNS
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disorders, to establish the correlation between total
lung volume and gestational age. Predicted or expected lung volume (ELV) is calculated by the equation:
ELV = 0.0033g2.86 (where g is gestational age in
weeks).
In their retrospective study of 46 fetuses from
American women, Coakley et al (12) found a strong
correlation between total lung volume measured at
MRI and liver volume measured at MRI, fetal weight
estimated at US, head circumference measured at US
and gestational age. The ELV is calculated by the
equation: ELV = (0.47 ≥ liver volume in milliliters) +
(0.76 ≥ biparietal diameter in millimeters) ≥ (0.39 ?
femur length in millimeters) ≥ 18.9.
In addition, Cannie et al. (13) conducted a study in
200 fetuses without abnormalities at University
Hospital Gasthuisberg (Belgium). Total lung volume
correlated best with fetal body volume (FBV) than
with all other biometric variables. The ELV is calculated by the equation: ELV = (2.0 x 10-9 x FBV3) – (1.19
x 10 -5 x FBV2) + (0.0508 x FBV) – 1.79.
The observed-expected (O/E) ratio for TFLV x 100
establishes the relative lung volume (RLV) and values
below 80% are considered as hypoplasia (14). All fetuses with values < 14.3% have a 100% mortality rate,
those with values >32.8 have a 100% survival rate and
those with values >44% do not need extracorporeal
membrane oxygenation (ECMO) (15) (Figs. 2, 3 and 4).
MRI is also useful in the assessment of lung maturation by lung signal intensity: lung maturation has
been correlated with a high signal intensity of the
lung on T2-weighted images, calculating signal intensity ratios of lung/spinal fluid (16), lung/gastric fluid
or lung/liver (17). Signal intensity increases with gestational age (18).
Moore et al (19) have studied fetal lungs using diffusion-weighted imaging and have shown that the ADC
increases with gestational age (as from week 18), probably reflecting the increase in alveolar fluid secretion
and pulmonary vascularization (20,21).
Spectroscopy has theoretical potential for evaluation of lung maturation. Surfactant is composed of
90% phospholipids (basically 70% phosphatidylcholine—lecithin) and 10% protein. The amount of choline,
the ratio of choline/creatine, lecithin and lactate
might be correlated with the amount of pulmonary
surfactant (22,23). Unfortunately, this technique has several limitations (especially fetal motion artifacts).
Fetuses with liver herniation have 50% survival.
The supradiaphragmatic liver position is difficult to
visualize on US, while MRI allows identification of
diaphragmatic defect (defect in the low-signal-intensity band on T2-weighted sequences), especially in
sagittal and coronal planes, and the abnormal position
of the liver (with hyperintense signal on T1-weighted
images and hypointense signal on T2-weightes images). The most commonly herniated structures in leftsided diaphragmatic hernias include omental fat, the
small bowel, the left hepatic lobe and the stomach.
Manuel Recio Rodríguez et al.
Fig. 1: (a) Sagittal SSFSE T2. (b) Coronal FIESTA. (c) SS FSE T2 (URO-MRI). (d) diffusion-weighted imaging. (e) Sagittal 3D T1-weighted gradient echo. (f) Volume reconstruction (VR) of the colon with 3D LAVA.
The kidney and pancreas are rarely herniated. In
right-sided diaphragmatic hernias, the most commonly herniated organ is the liver with herniation of
the right hepatic lobe.
Fetal endoluminal tracheal occlusion (FETO) is
indicated for liver herniation (LHR <1, at week 26-29
and in the absence of chromosomal anomalies or associated abnormalities). FETO is a minimally invasive
procedure in which an endotracheal balloon is inflated for 3 or 6 weeks. Retention of fluid within the lung
accelerates lung maturation and reduces the risk of
pulmonary hypertension. FETO should be performed
prior to 29 weeks of gestation, and the balloon is then
removed via fetoscopy --after being punctured at
week 34-- either by ex-utero intrapartum treatment
(EXIT) or post-natal (either by tracheoscopy or percutaneous puncture). When FETO is performed before
29 weeks of gestation, a significant lung expansion
occurs as a result of a marked production of fluids.
When the balloon is removed, the RLV decreases by
nearly 50%, but it is 40% higher than the RLV prior to
balloon insertion. When FETO is performed after 29
weeks of gestation, the RLV does not increase after
balloon removal (24).
In the study conducted by Jani et al the main complications were: prelabor rupture of membrane (47%),
chorioamnionitis and premature delivery. FETO increased survival in left-sided CDH from 24.1% to 49.1%
and in right-sided CDH from 0% to 35.3%. Ninetyseven per cent of fetuses undergoing FETO were live
born and 50% of those babies resisted surgery and
were discharged from the hospital alive.
Cystic adenomatoid malformation (CAM) or
congenital pulmonary airway malformation
CAM is the most commonly diagnosed fetal lung
mass of the newborn period. It consists of lung hamartoma with proliferation of terminal bronchioles and
lack of normal alveoli. They are solid or cystic masses,
vascularized by the pulmonary artery and with drainage via the pulmonary veins. Most CAMs are unilateral and affect one entire lung lobe, although there
are hybrid lesions associated with pulmonary sequestration with systemic vascularization (26,27). Stocker et al
(28)
classified them as: type I (one or more cysts > 2 cm),
type II (multiple cysts 2-0.5 cm) and type III (large
microcystic lesion <0.5 cm). CAMs are usually detected by ultrasound at 20 weeks of gestation (29) and they
appear as cystic or solid echogenic lung masses. They
may be occasionally misdiagnosed on ultrasound as
CDH or vice versa (30).
Isolated CAMs are associated with a good prognosis, with 97% survival (independent of histological
type), excluding CAMs with associated abnormalities,
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Fig. 2: (a), (b) and (c) Sagittal SS FSE T2. (d) Axial SS FSE. (e) VR of the right lung (5.311 cm3) and (f) VR of the left lung (1.425 cm3). Leftsided diaphragmatic hernia with herniated bowel loops (white arrow), stomach (white arrowhead) and left hepatic lobe (curved arrow). Right lung
(broken arrow). Heart (black arrowhead). Hypoplasia of the left lung (black arrow) and contralateral cardiomediastinal shift. Expected total fetal
lung volume: 18.2 cc; observed total fetal lung volume: 6.73 cc; relative lung volume: 36.9%. Gestational Age (GA): 20 weeks.
Fig. 3: (a) Coronal FIESTA and (b) Coronal T1weighted dual echo gradient. (c) and (d) Sagittal
SS FSE T2. Right-sided diaphragmatic hernia
with herniated bowel loops (black arrow) and
right hepatic lobe (white arrow). Hypoplasia of
right lung (arrowhead). Contralateral cardiomediastinal shift. GA: 22.5 weeks. Right lung
volume: 0.884 cc; left lung volume: 5.905 cc;
observed total lung volume: 6.79 cc; expected
total lung volume: 20.32 cc; relative lung volume: 33%.
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Fig.4: (a) Sagittal SS FSE T2, (b) Sagittal T1-weighted dual echo gradient and (c) Coronal SSFSE T2. Left-sided diaphragmatic hernia with partially herniated stomach (black arrow), bowel loops (broken arrow), colon’s splenic flexure (curved arrow) and spleen (white arrowhead). Left lung
(white arrow). Contralateral cardiomediastinal shift. GA: 21 weeks. Right lung volume: 6.077 cc; left lung volume: 3.063 cc; observed total lung
volume: 9.14 cc; expected total lung volume: 19.2 cc; relative lung volume: 48.05%.
which account for 3-12% (most of them corresponding
to Stocker type II lesions). In over 50%, there is spontaneous resolution during pregnancy (29). Typically,
CAMs with less than 57% of total lung volume regress
completely, while those with a volume greater than
84% show partial regression.
The most important prognostic factor is the presence or absence of hydrops (31). CAM may cause compression of the heart and the inferior vena cava and, secondarily, impair cardiac contractility and venous return
(32)
. If hydrops is left untreated, over 90% of fetuses will
die before being born (33). To estimate the risk of developing hydrops, the CAM volume / head circumference ratio (CVR) is calculated at ultrasound. CAM volume is calculated based on ultrasound measurements
obtained in three dimensions of the mass. The CAM
volume is divided by the head circumference, which is
assessed by ultrasound (occipital frontal diameter +
biparietal diameter) x 1.57. If the CVR is > 1.6, the fetus
will develop hydrops in 75% of cases; if the CVR is
<1.6, hydrops will occur in less than 3% of cases (35).
MRI findings vary according to the type of CAM:
type I lesions manifest as hyperintense unilocular
lesions on T2-weighted images; type II lesions appear
as hyperintense multilocular lesions on T2-weighted
images; type III lesions manifest as hyperintense solid
masses with normal adjacent lung, which is more
hypointense, on T2-weighted images. In addition, the
volume of lesions and lung parenchyma can be calculated, thus allowing quantification and monitoring of
the lesions growth or resolution (31) (Figs. 5 and 6).
CAMs require postnatal surgery, after one month
of life, because of the risk of infection and the low risk
of malignant transformation --annual rate of 3%-- into
pleuropulmonary blastoma, rhabdomyosarcoma or
myxosarcoma in infants and into bronchoalveolar carcinoma in children and adults (36).
Bronchopulmonary sequestration
Bronchopulmonary sequestration is the second
most common lesion in prenatal diagnosis.
Bronchopulmonary tissue is disconnected from the
bronchial tree and pulmonary arteries and receives
arterial blood from the systemic circulation, via the
thoracic or abdominal aorta. The most common location is the left lower lobe (>2/3) (37), with 90% being
supradiaphragmatic (Fig. 7) and less than 10% infradiaphragmatic (38) (Fig. 8).
Extralobar sequestration occurs in the fetus, has its
own pleural covering and venous drainage is through
the azygos system or the inferior vena cava (in 25% of
cases through the pulmonary veins) (39). Intralobar
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Fig. 5 (a) Coronal SS FSE T2. (b) Sagittal SS FSE T2. (c) Axial SS FSE T2, (d) VR reconstruction of CAM (e) VR reconstruction of the right lung, (f) VR
reconstruction of the left lung. CAM type II in lower left lobe (curved arrow), upper left lobe (arrowhead) and right lung (white arrow). GA: 22.5 weeks.
Right lung volume: 9.307 cc; left lung volume: 4.244 cc; total lung volume: 13.55 cc. CAM volume: 13.942 cc. CVR index: 0.75 (<1.6 good prognosis). As
from 32 weeks of gestation, the heart occupied its place on the left side of the chest and follow-up ultrasound confirmed a reduction in the pulmonary area
affected by CAM. Normal Full-term delivery, with newborn weighing 3200 g, not requiring respiratory assistance. Normal post-natal chest x-ray.
Fig. 6: (a) Sagittal SS FSE T2, (b) Coronal SS
FSE T2, (c) Axial SS FSE T2 and (d) Coronal
T1-weighted dual echo gradient. CAM type II
in posterior segment of lower right lobe (white
arrow). GA: 28 weeks. Normal liver (arrowhead) and colon's hepatic flexure (broken arrow).
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Fig. 7: (a) Coronal SS FSE T2. (b) Axial SSFSE T2. (c) Coronal FIESTA. (d) and (e) Ultrasounds. (1) Pulmonary sequestration presenting with
a hyperintense solid component on T2, similar to type III-CAM. (2) Anomalous systemic vessel of the distal thoracic aorta. (3) Distal thoracic
aorta. (4) Collapsed left lung parenchyma. (5) Right lung. GA: 28 weeks. Evolution: cesarean section at 38 weeks; Apgar scorer 8/10; 3100 g.
Postnatal confirmation.
sequestration does not have its own pleura and drainage is through pulmonary veins (40). They are rarely
diagnosed prenatally and are usually associated with
type II-CAM (hybrid lesions) (41). They have an excellent prognosis when isolated and over 50% resolve in
utero (29). Large lesions may compress the esophagus
and thoracic veins, and subsequently cause hydrops
(an indication for fetal intervention and early delivery)
(42)
. Prenatal treatment with thoracoamniotic shunting
is performed in cases of tension hydrothorax.
On MRI, pulmonary sequestration appears as a
solid, hyperintense lesion on T2-weighted images,
very similar to type III CAM. Identification of systemic blood supply helps in the diagnosis, mainly with
balanced sequences. MRI can detect small lesions and
associated anomalies.
Postnatally, most sequestrations are asymptomatic
and may present with respiratory distress or cyanosis.
Postnatal treatment consists of embolization of systemic blood supply or surgical resection, given the risk
of infection, hemorrhage and questionable malignancy (mainly in hybrid lesions) (42).
Bronchogenic cyst
Bronchogenic cyst is the solitary cystic lesion most
commonly found in the fetal chest. Most bronchogenic cysts appear as single lesions typically located in
the mediastinum, in the carinal region, and less frequently, in the hilar region (43). On MRI, it appears as a
well-defined cyst, hyperintense on T2-weighted images when compared to the surrounding lung tissue
(Fig. 9). In addition, the MRI can detect bronchial obstruction due to mass effect of some cysts with hyperintensity on T2-weighted images of the obstructed
lobe (44). The role of MRI is limited to the cases of bronchial obstruction, which will benefit from the EXITECMO procedure with resection of the obstructive
lesion, followed by reconstruction of the airway.
Bronchial obstruction by mucous plug
Obstruction by a mucous plug of a main or lobar
bronchus produces an image similar to that of type-III
CAM or a sequestration with intrathoracic mass, hyperintense on T2-weighted imaging, and which may
cause mediastinal shift (Fig. 10). It may resolve spontaneously in utero or by postnatal bronchoaspiration (45).
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Fig. 8: (a) and (b) Sagittal SS FSE T2. (c)
Coronal SS FSE T2. (d) Axial SS FSE T2.
Infradiaphragmatic sequestration (arrow) and
systemic artery of abdominal artery (arrowhead). GA: 31.5 weeks.
Fig. 9: (a) Sagittal FSE T2. (b) Sagittal FIESTA. (c) Axial SS FSE T2. (d) Coronal FIESTA. (e) Axial ultrasound and (f) sagittal ultrasound.
Bronchogenic cyst in left lung (white arrow). GA: 22 weeks. Bichorial biamniotic twins.
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Fig. 10: (a) and (b) Coronal SS FSE T2. (c) and
(d) Sagittal SS FSE T2. Affected LLL (white
arrow), healthy LUL (arrowhead) and healthy
right lung (broken arrow) with normal x-ray
at birth, corresponding to transient atelectasis
due to mucous plug. GA: 20.5 weeks.
Fig. 11: (a) and (b) Sagittal SS FSE T2. (c) Coronal SS FSE T2. (d) oblique slice SS FSE T2. (e) Axial SS FSE T2 (f) T1 gradient echo. Omphalocele:
herniated liver (white arrow), stomach (curved arrow), bowel loops without meconium (broken arrow) and bowel loops with meconium (arrowhead). GA: 20 weeks.
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Fig. 12: (a) 3D Ultrasound. (b) 2D Ultrasound
(c) Sagittal SS FSE T2. (d) sagittal gradientecho T1-weighted. Omphalocele (white arrow).
Bowel loops with meconium (white arrowhead).
GA: 28 weeks. Evolution: normal delivery and
subsequent surgical correction.
Other less common indications
Omphalocele
Thoracic abnormalities less commonly assessed by
MRI include: congenital airway obstruction (by extrinsic factors or tracheal and bronchial stenotic or atresic
malformations), congenital lobar emphysema, pleural
effusion, pericardial effusion, pericardial tumors (teratoma), mediastinal tumors (teratoma, lymphangioma)
and cardiac tumors (rhabdomyoma).
An omphalocele is a midline supraumbilical abdominal wall defect with herniation of abdominal content, covered by the peritoneum and amnion, with
umbilical cord insertion in the apex of the omphalocele. The most commonly herniated organs are the liver,
stomach and loops of the small bowel, which are easily
identified on MRI. Omphalocele is associated with
other malformations in 40 to 70% of cases and the incidence of chromosomal anomalies is 10 to 40%, including trisomy 13, 14, 15, 18 and 21 (47).
The presence of liver in herniated organs has been
considered as the element that differentiates between
large and small omphalocele. Large omphaloceles
have a high mortality rate, as several surgical procedures are required to obtain closure of the defect , with
their main limitation being a small thoracic cavity,
associated pulmonary hypoplasia and the need for
prolonged mechanical ventilation (Fig. 11). On MRI,
the presence of the liver in herniated organs is easily
identified and the degree of pulmonary hypoplasia
can be established. Small omphaloceles (Fig. 12) show
higher association with chromosomal abnormalities (48).
ABDOMINAL PATHOLOGY
Published studies on the usefulness of MRI in
abdominal pathology are much less than those on thoracic MRI. Even if most ultrasounds are diagnostic,
they have limitations that create a need for an alternative imaging method.
Anterior abdominal wall defects
The incidence of anterior abdominal wall defects is
1 in 2000 live births (4). The commonest abdominal wall
defects are gastroschisis and omphalocele and the less
common are the pentalogy of Cantrell (midline abdominal wall defect, anterior diaphragmatic hernia,
diaphragmatic pericardial defect, lower sternal defect
and cardiac abnormalities) and the cloacal and bladder
exstrophy.
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Gastroschisis
Gastroschisis is a typically right-sided paraumbilical anterior abdominal wall defect (49). The loops of the
small bowel are most frequently prolapsed and float in
Fig. 13: (a) and (b) Coronal SS FSE T2. (c) and
(d) Sagittal SS FSE T2. Single umbilical
artery (white arrow). Esophageal atresia with
small-sized stomach (arrowhead) and small
esophageal pouch (broken arrow). Trachea
(curved arrow). GA: 31.5 weeks.
Fig. 14: (a) and (b) Sagittal FIESTA. (c) Coronal FIESTA. (d) and (e) SS FSE T2-weighted urography images. (f) Coronal T1-weighted dual echo
gradient. Left renal agenesis. Hypertrophic right kidney (white arrow). Bladder (white arrowhead). Loops of small bowel (broken arrow) and colon’s splenic flexure (curved arrow) in the left renal fossa. GA: 32 weeks.
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Fig. 15: (a) Coronal SS FSE T2. (b) Axial SS FSE T2. (c) Sagittal SS FSE T2. Multicystic right kidney (arrow). Normal left kidney (arrowhead).
GA: 20 weeks.
Fig. 16: Bichorial biamniotic twin pregnancy. Cloacal malformation. (a) Ultrasound at 15 weeks of gestation: rectal-sigmoid dilation (white arrow)
and fecal impaction (white arrowhead) in one twin, without ascites. (b) Ultrasound at 20 weeks of gestation: cystic structure with incomplete septum (broken arrow), anterior to rectosigmoid and posterior to the bladder (black arrow), with ambiguous genitalia. MRI was performed at 26.5
weeks of gestation. (c) Sagittal SS FSE T2 and (d) Sagittal SS FSE T2 (URO-MRI). Ascites (black arrowhead) of the twin in longitudinal position and left breech presentation. Cystic structure (broken arrow)anterior to the rectum-sigmoid. (e) and (f) Coronal FIESTA. Ascites (black arrowhead) and cystic structure with incomplete septum (broken arrow) with content inside (curved arrow), suggesting detritus from fistula of the rectosigmoid (white arrow). Bladder cannot be identified. G SS FSE T2 (URO-RM). Grade III/IV hydronephrosis on the right side (1) grade II/IV
hydronephrosis on the left side (2), suggesting urinary ascites. Postnatal follow-up by x-ray: retrograde filling of contrast material through single
perineal opening (3). Filling of bladder (4), vagina and partial septate uterus (5) containing hydrometrocolpos and gas because of rectosigmoid fistula. The rectosigmoid is not filled with contrast (6).
the amniotic fluid without covering membrane.
Extracorporeal colon may be present in gastroschisis,
but not sigmoid or rectum. Prolapse of the liver less
frequently occurs. A higher incidence is found in
mothers using vasoactive substance (cocaine, nicotine,
decongestants or aspirins) and in mothers younger
than 25 years old. These defects are not generally associated with other malformations, although they may
affect the gastrointestinal tract by 10-15% (atresia, stenosis, short bowel syndrome) probably due to ische-
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mic damage by loop obstruction (50). They are associated with oligohydramnios and intrauterine growth
retardation (IUGR). When associated with polyhydramnios, bowel atresia or obstruction should be ruled
out. MRI allows visualization in cases of oligohydramnios or maternal obesity, or associated bowel complications.
In 4-5% of cases, gastroschisis is associated with
cardiac malformations (51). The incidence of chromosomal anomalies in gastroschisis is below 3% (52).
Fig. 17: Cystic lymphangioma (a) and (b) Axial SS FSE T2. (c) Sagittal SS FSE T2. (d) Coronal FIESTA. Multiseptated cystic lymphangioma of
the mesenterium, greater omentum and ileocecal region, hyperintense on T2-weighted images (white arrow). The lesion is independent of the right
kidney (arrowhead), displaces the small bowel loops to the left side of the abdomen (curved arrow) indenting the bladder dome (broken arrow). (3)
3D LAVA. The lymphangioma appears hypointense on T1-weighted images (white arrow) with no evidence of bleeding and indenting the bladder
dome (broken arrow). (f) 3D LAVA. Coronal MIP reconstruction. Cranial displacement of the transverse colon (black arrow). GA: 33.3 weeks.
GASTROINTESTINAL ABNORMALITIES
Fetal scan of the gastrointestinal tract is based on
the presence of “natural” contrast media: swallowed
amniotic fluid passed to the intestines and meconium.
Swallowing of amniotic fluid begins by 9-10 gestational weeks (53); however, except for the stomach, it is not
until 25 gestational weeks that significant amount of
fluid enters the fetal intestines to serve as a natural
contrast medium. The full-term fetus swallows as
much as 750 ml/day. Meconium starts to accumulate
in the rectum by 18-20 gestational weeks, with retrograde filling of the colon and distal ileal loops.
Most abnormalities are diagnosed by ultrasound
and do not require a complementary diagnostic method.
The main indications for MRI in gastrointestinal
assessment are: identifying the site of bowel obstruction (bowel atresias, meconium ileus and bowel volvulus) and ruling out bowel ischemia (wall thickening,
meconium peritonitis) and malrotation.
Esophageal atresia
The most common is type A with proximal atresia
and distal tracheoesophageal fistula. Diagnosis is established by the presence of polyhydramnios, small sto-
mach, proximal esophageal pouch and IUGR in the
2nd or 3rd trimester (34) (Fig. 13). The tracheoesophageal fistula cannot be detected by MRI. It is associated
with trisomy 18 and, in the absence of fistula, it is more
common in trisomy 21.
Duodenal atresia or stenosis
Duodenal atresia or stenosis is diagnosed by
means of the double bubble sign of the stomach and
duodenum (hypointense on T1-weighted images and
hyperintense on T2-weighted sequences) (55), which
includes duodenal stenosis, annular pancreas, Ladd
bands and volvulus. Duodenal atresia is associated
with other abnormalities and trisomy 21 (56)-
Small bowel atresia or stenosis
Small bowel atresia is seen as dilated bowel loops
proximal to the obstructed segment. MRI provides
additional information since in the case of jejunal stenosis or atresia, dilated loops are hypointense on T1weighted images and hyperintense on T2-weighted
images (with triple bubble sign and polyhydramnios,
if proximal jejunum is affected) while in ileal atresia or
Página 13
Fig. 18: Retroperitoneal teratoma. (a) and (b) Axial SS FSE T2. (c) and (d) Coronal SS FSE T2. (e) Sagittal Fiesta. (f) Sagittal T1-weighted echo
gradient. Cystic mass on right adrenal location, with solid poles (white arrow), crossing the midline (arrowhead) anterior to the aorta. The mass
inferiorly displaces the right kidney with horizontalization of the kidney (curved arrow). GA: 35.5 weeks.
stenosis, dilated loops resemble a “string of sausages”
with meconium, being hyperintense on T1-weighted
images and iso-hypointense on T2-weighted images
(57)
. Distal atresias have a higher risk of perforation and
meconium peritonitis.
Meconium peritonitis
Meconium peritonitis is a chemical peritonitis
secondary to the perforation of a bowel loop, diagnosed on US with the finding of peritoneal calcification,
ascites and pseudocysts (54). MRI cannot identify calcifications but allows easy recognition of dilated loops,
ascites, polyhydramnios and cystic masses (meconium
pseudocysts). Pseudocysts may contain internal septations with a variable T1 signal and a high T2 signal (58),
hyperintense on T1-weighted images and intermediate on T2-weighted images (59) or hyperintense on T1and very low on T2-weighted images (60).
Megacystis-Microcolon–Intestinal Hypoperistalsis
Syndrome
Enlarged bladder (the most typical finding) is
easily diagnosed on ultrasound and MRI. On MRI, this
finding can be visualized as from 25 weeks of gestation
given the very small amounts of meconium present in
the rectum. At the end of gestation, more meconium
accumulates and the microcolon can be seen (55).
Large bowel atresia
Large bowel atresia mainly affects the rectum.
Atresia of the transverse colon produces dilation of the
proximal colon, small amount of distal meconium and
dilation of small bowel loops with absence of peristalsis. Cecal atresia causes dilation of the cecum and
ileum with abundant meconium (61).
Hirschsprung disease
Colon malrotations
Malrotations can be diagnosed on MRI as from 25
weeks of gestation. MRI shows the abnormal position
of the colon, using T1-weighted sequences, given
meconium content (55).
Página 14
In Hirschsprung's disease, there is rectal dilation
with hypointense inverted signal on T1-weighted images and intermediate signal on T2-weighted images (62).
Anal atresia
Bilateral renal agenesis
Small and isolated forms of anal atresia may go
undetected by MRI. V-shaped or U-shaped loops in
the pelvis suggest a diagnosis of anal atresia. They are
frequently associated with urinary tract fistula producing enteroliths (which are bettered identified on ultrasound) (63) or they may be part of the VACTERL syndrome ((vertebral anomalies, anal atresia, cardiac anomalies, tracheoesophageal fistula, renal and limb defect).
MRI allows diagnosis of bilateral renal agenesis, as
severe oligohydramnios or anhydramnios impairs
assessment by US. It is associated with congenital
heart disease and Potter's syndrome due to oligohydramnios (typical facies, joint contractures and pulmonary hypoplasia) (69).
Unilateral renal agenesis
GENITOURINARY ABNORMALITIES
Fetal urine is the main source of amniotic fluid. As
from 10 weeks’ gestation, fetal kidneys produce urine
and during the second half of pregnancy, they produce 90% of amniotic fluid.
MRI is indicated only when ultrasound findings are
inconclusive, especially in the cases of oligohydramnios or anhydramnios. 2D and 3D T2-weighted sequences with long repetition time (RT) are useful to obtain
urography images of the excretory system without
intravenous contrast (which is contraindicated).
Diffusion-weighted sequence would be useful in
cases of suspected renal infarction due to venous
thrombosis with a decrease in the apparent diffusion
coefficient (ADC) and in the twin-twin transfusion
syndrome, where the ADC of the donor twin is higher
than that of the recipient twin and is related to the
severity of the syndrome. Diffusion-weighted imaging helps in the diagnosis of (unilateral or bilateral)
renal agenesis because this technique is very sensitive
in detection of the renal parenchyma (64, 65).
Hydronephrosis
Hydronephrosis is clearly visualized on T2-weighted images. In the coronal plane, urography images
can be obtained, with visualization of the whole ureteral course (66).
Pyeloureteral junction stenosis
The pyeloureteral junction stenosis is the most frequent cause of hydronephrosis detected prenatally (67)
and it may progress to a displastic kidney with multiple cortical and medullary cysts.
Duplicated renal collecting system
A duplicated renal collecting system is the most
common anomaly (68). MRI can be helpful in the diagnosis of complete or incomplete duplex system, and
ectopic drain of an ureterocele associated with the
upper ureter.
Unilateral renal agenesis (Fig. 14) generally occurs
in isolation, and has a normal life expectancy,
although it may be occasionally associated with VACTERL syndrome. When a kidney is not located within
the renal fossa, MRI helps in identification of the ectopic kidney location.
Autosomal polycystic kidney disease
Autosomal recessive polycystic kidney disease is
characterized by large kidneys with high uniform
intensity on T2-weighted images and loss of corticomedullary differentiation; the pyelocaliceal system is
not identified (70).
Multicystic dysplastic kidney
Multicystic dysplastic kidneys (Fig. 15) are enlarged, with multiple cysts of different size peripherally
and centrally located, with little renal parenchyma (71).
These cysts demonstrate high signal intensity on T2weighted sequences and are separated from the pyelocaliceal system.
Bladder exstrophy
Bladder exstrophy is a rare malformation in which
the anterior wall of the bladder is absent, and the posterior wall is exposed externally. MRI identifies an
extruded solid mass below the umbilical cord insertion , with normal kidneys and amniotic fluid (72).
Cloacal malformation
Cloacal malformation is a rare cause of obstructive
uropathy secondary to failure of the division of the
primitive cloaca, with communication between the
gastrointestinal, urinary, and genital structures, resulting in a single perineal opening. Cloacal malformation is more common in women, with bilateral hydronephrosis, cystic lesion in the pelvis and difficulty in
visualizing the bladder because of its small size (Fig.
16). It is associated with meconium calcification in the
Página 15
Fig. 19: (a) Coronal FIESTA (b), (c), (d), (e) and (f) Axial FIESTA. (1) Supradiaphragmatic inferior vena cava; (2) Middle hepatic vein, (3) left
hepatic vein; (4) right atrium; (5) descending thoracic aorta; (6) azygos vein; (7)abdominal aorta; (8) hemiazygos vein; (9) right hepatic vein; (10)
left ascending lumbar vein. Agenesis of inferior vena cava secondarty to thrombosis, with collateral flow through paravertebral veins, azygos and
hemiazygos ascending lumbar veins. Bilateral adrenal hemorrhagic pseudocysts. Kidneys with cortical thickening (white arrows), mainly on the
left kidney. (g) Sagittal SS FSE T2. Left adrenal hemorrhagic pseudocyst and high signal intensity in the upper pole of kidney, suggesting renal
infarction (broken arrow). (h) Diffusion-weighted image. High intensity signal of left kidney suggesting probably venous infarction (white arrowhead). GA: 36 weeks.
Fig. 20: Portosystemic shunt with aneurysm or venous lake in 30-gestational-week-old fetus. (a), (b), (c), (d) and (e) MRI. Axial FIESTA; (f) Sagittal
FIESTA. Umbilical cord (curved arrow). Absence of venous conduit, with umbilical vein (black arrow) draining into the portal vein (arrowhead) at
the level of the hepatic hilum. Venous lake or aneurysm (white arrow) in segment VIII. Anterior drainage vein (1). Posterior drainage vein (2). Vessel
collecting blood from (1) and (2) and draining into the inferior vena cava (3). Inferior vena cava (broken arrow). Left hepatic vein (black arrowhead).
Página 16
urinary tract and colon, which are better identified on
ultrasound (72, 74). There is a minor variant: the urogenital sinus, with communication between the vagina
and the urinary tract (which normally occurs during
embryological development). Such communication
may occur at any point from the urethral meatus to
the bladder, but the majority occur within the mid to
distal portion of the urethra. The two structures join
and exit on the perineum as a single common urogenital sinus channel (75).
MRI can better delimitate its organ-dependence. It
appears as a solid mass of uniform, slightly hyperintense signal on T2-weighted images.
Neuroblastoma is the most common malignant
tumor in the newborn (81). Most neuroblastomas arise
from the adrenal medulla and they can be solid, cystic or
solid and cystic adrenal masses(82). Liver metastases are
common (82). Differential diagnosis with adrenal hemorrhage should be established (MRI identifies hemorrhage
in different stages) (83), infradiaphragmatic pulmonary
sequestration (MRI can show systemic blood supply) (84)
and retroperitoneal teratomas (85) (Fig. 18).
Prune-Belly or Eagle-Barrett Syndrome
The Prune-Belly or Eagle-Barrett Syndrome is a
rare congenital malformation with severe hydronephrosis, abdominal muscle deficiency and chryptorchidism. MRI identifies hydronephrosis, bilateral ureteral dilatation and dilation of the entire urethra, unlike
posterior valves, which cause proximal urethral dilatation. Genitalia are absent and it is associated with
oligohydramnios (76).
ABDOMINAL MASSES
Most abdominal masses are benign cystic lesions:
Intestinal duplication cyst: unilocular cyst, with
double-layered wall of muscle and mucous membrane. The most common location is in the distal ileum.
Mesenteric cyst: considered as a lymphatic malformation. It may be unilocular or most commonly multilocular with multiple septa (Fig. 17). It may occasionally extend into the retroperitoneum and lower limbs.
Ovarian cyst: this is the most common female
abdominal mass in the third trimester. Ovarian ligaments are lax and the ovarian cyst may be found in
any location. It may be simple or complicated, with
torsion and bleeding.
Urachal cyst: unilocular cyst that occurs in the
midline between the bladder and the umbilical cord
insertion.
Choledocal cyst: unilocular cyst that occurs in the
right upper quadrant. Bile ducts may be seen entering
into the cyst.
TUMORS
MRI can detect, localize and characterize benign
cystic lesions, differentiating them from bowel loops (77, 78).
The experience with MRI in the diagnosis of congenital hepatic masses is very limited. Most are large and
solid masses (hemangioendothelioma, hepatoblastoma or metastatic neuroblastoma) , with cystic lesions
(mesenchymal hamartoma) being less common (79).
Mesoblastic nephroma is the most common renal
solid mass. It is a benign tumor with excellent prognosis associated in 70% of cases with polyhydramnios (80).
VASCULAR PATHOLOGY IMAGING
The use of intravenous contrast (gadolinium) is
not accepted. Gadolinium chelates cross the placenta
(86)
and there is potential risk of gadolinium-induced
nephrotoxicity (nephrogenic systemic fibrosis) (87).
Balanced sequences (FIESTA) are useful in assessing thoracic and abdominal vascular anatomy
without contrast (88), mainly in cases of intra- or extralobar sequestration, developmental abnormalities of
the inferior vena cava (Fig. 19) or vascular malformations such as hepatic portosystemic shunts (Fig. 20).
CONCLUSIONS
MRI plays an important role as complementary
method to ultrasound due to its multiplanar capability and tissue differentiation. Its main indication is
the assessment of diaphragmatic hernia, where MRI
provides diagnostic information, establishes a prognosis and helps in planning delivery and postnatal
surgery. In gastrointestinal pathology, MRI is useful in
the assessment of bowel obstruction, perforation and
malrotation. In genitourinary imaging, MRI does not
have the limitations of ultrasound (oligohydramnios
or maternal obesity) and it is used in the assessment of
renal agenesis, obstructive pathology and cystic kidney disease. Furthermore, it allows characterization of
congenital cystic lesions. With balanced sequences,
MRI angiographies can be performed without intravenous contrast; this is useful in the assessment of
congenital vascular pathology of large vessels and
pulmonary sequestration.
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Acknowledgements
We thank Sergio García Fauró, radiologic technologist.

Fetal MRI: thoracic, abdominal and pelvic pathology

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