DOI:10.2214/AJR.04.1664
AJR 2006; 187:398-403
© American Roentgen Ray Society
Gadolinium-Enhanced 3D MR Angiography of Pulmonary Hypoplasia and Aplasia
Hakan Mutlu1,
Cinar Basekim1,
Emir Silit1,
Zekai Pekkafali1,
Ersin Ozturk1,
Bulent Karaman1,
Mecit Kantarci1,
Esref Kizilkaya1 and
Fevzi Karsli1
1 All authors: Department of Radiology, GATA Haydarpasa Teaching Hospital,
Tibbiye cad, 81327, Uskudar, Istanbul, Turkey.
Received October 26, 2004;
accepted after revision June 2, 2005.
Address correspondence to H. Mutlu.
Abstract
OBJECTIVE. The goal of this study was to determine the role of
gadolinium-enhanced 3D MR angiography (MRA) in patients with suspected
pulmonary hypoplasia and aplasia in a retrospective analysis of MRA and
digital subtraction angiography in 11 patients with clinical and/or radiologic
suspicion of pulmonary hypoplasia and aplasia.
CONCLUSION. Gadolinium-enhanced 3D MRA is capable of diagnosing
pulmonary hypoplasia and aplasia rapidly and accurately. Both pulmonary
hypoplasia and aplasia can be shown morphologically in a noninvasive manner,
obviating digital subtraction angiography.
Keywords: angiography • arteriography • developmental anomalies • digital subtraction • lung • MRI
Introduction
Pulmonary agenesis is the complete absence of the pulmonary parenchyma and
the vasculature and its bronchus. A whole lung, lobe, or segment can be
involved. Pulmonary agenesis and hypoplasia have been classified into three
types: pulmonary agenesis, the total absence of the bronchus, lung, and
vascular supply; pulmonary aplasia, the presence of a rudimentary bronchus and
the total absence of a lung and vascular supply; and pulmonary hypoplasia,
which is often a completely formed but congenitally small bronchus with
rudimentary parenchyma. In pulmonary agenesis and aplasia, the pulmonary
artery is absent, and both should, for practical reasons, be considered the
same entity. In pulmonary hypoplasia, the pulmonary artery is small
congenitally [1,
2].
The diagnosis of pulmonary hypoplasia or aplasia can be difficult. Chest
radiography, nuclear medicine studies, CT, and digital subtraction angiography
(DSA) are the imaging techniques for the diagnosis of pulmonary hypoplasia or
aplasia. These disorders are extremely difficult to diagnose with the
typically used imaging methods. DSA is the imaging technique of choice in
showing pulmonary vessels [3].
Few data have been published on the efficacy of MR angiography (MRA) in
evaluating pulmonary hypoplasia or aplasia
[4-6].
Recent advances in MR technology using fast gradients and contrast agents
have allowed MRA to make substantial advances
[4]. Gadolinium-enhanced 3D MRA
is a fast imaging technique that has been shown to accurately evaluate the
major arteries such as the carotid system, aorta, and coronary and renal
arteries [5,
6]. The use of pulmonary MRA
has been extended to the pulmonary arteries. It is possible to assess the
entire pulmonary tree in a single breath-hold during the injection of contrast
media. In this study, we aimed to describe the role of MRA in the diagnosis of
pulmonary hypoplasia or aplasia by comparing MRA with CT and DSA, which is
considered the gold standard.
Materials and Methods
Eleven patients (nine men, two women; age range, 20-50 years; mean age, 26
years) with a radiologic suspicion of pulmonary hypoplasia or aplasia on chest
radiographs were included in this study. There were one or more findings such
as poorly expanded lungs and overdistention of the contralateral lung, shift
of the mediastinum or diaphragm, or complete opacity of the hemithorax on
chest radiographs. The patients were asymptomatic or had symptoms of
exertional dyspnea and bronchitis episodes.
MRI studies were performed with a 1.5-T scanner (Magnetom Vision, Siemens
Medical Solutions). Gadopentetate dimeglumine (0.2 mmol/kg) was injected via a
peripheral IV cannula by an automatic injector at the rate of 5 mL per second.
Patients were instructed to take several deep breaths before image
acquisition. The scan delay time (from injection start to scan start) was
calculated as estimated contrast travel time plus half the injection time
minus half the scanning time. The scan delay time ranged from 12 to 14
seconds. Two sequential breath-hold 3D MRA acquisitions were performed. The
MRA sequences consisted of a non-ECG-triggered 3D spoiled gradient-echo pulse
sequence. For the contrast-enhanced MRI, coronal T1-weighted 3D fast
multiplanar spoiled gradient-echo images were obtained with a 25° flip
angle, a 350- to 450-cm field of view, a 140 x 256 matrix, and
3-mm-thick sections. An axial test image was obtained with a 10° flip
angle, a 350- to 450-cm field of view, a 128 x 256 matrix, and
10-mm-thick sections. Acquisition times were 26 and 30 seconds,
respectively.
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Fig. 1A —20-year-old man with aplasia of left pulmonary artery. Scout
image from CT shows lung tissue in left upper hemithorax that mimics
hypoplasia of left lung and blind ending in left main bronchus
(arrowhead).
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Fig. 1B —20-year-old man with aplasia of left pulmonary artery. CT
scan reveals absence of left pulmonary artery (arrowhead) and
compensatory hypertrophy of right lung that extends to left hemithorax.
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Fig. 1C —20-year-old man with aplasia of left pulmonary artery.
Subvolume maximum-intensity-projection image (C) and 3D volume
reconstruction (D) of MR angiography show absence of left pulmonary
artery.
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Fig. 1D —20-year-old man with aplasia of left pulmonary artery.
Subvolume maximum-intensity-projection image (C) and 3D volume
reconstruction (D) of MR angiography show absence of left pulmonary
artery.
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Fig. 2B —21-year-old man with hypoplasia of left pulmonary artery.
Digital subtraction angiography (arrow, B) and 3D volume
reconstruction of MR angiography (arrowhead, C) show small
left pulmonary artery.
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View larger version (107K):
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Fig. 2C —21-year-old man with hypoplasia of left pulmonary artery.
Digital subtraction angiography (arrow, B) and 3D volume
reconstruction of MR angiography (arrowhead, C) show small
left pulmonary artery.
|
|
Interpretation of 3D image data was performed on a workstation (3D
Virtuoso, Siemens Medical Solutions) capable of multiplanar reformations
(MPRs) and maximum intensity projections (MIPs). The workstation allows
interrogation of each pulmonary artery down to the segmental level and beyond.
After inspection of the coronal source images, overlapping reformations and
MIPs encompassing several slices were obtained.
The right common femoral vein is the preferred venous access site in
pulmonary DSA because it provides a relatively straight course to the inferior
vena cava, right atrium, and right ventricle. If access to the right common
femoral vein was precluded, then the right basilic vein at the antecubital
fossa was used as an adequate access. Although many catheters are available
for pulmonary arteriography, we use the pigtail type. The amount of contrast
agent used is determined by the size of and flow in the vessel selected,
assessed during a test hand injection. In our practice, we find a total of 25
mL is usually sufficient.
Both angiographic examinations were reviewed separately, retrospectively,
and at random by three radiologists. After assessment of the examinations in
11 patients, a consensus review of the DSA by the three observers was
considered the reference standard for the statistical analysis. The
radiologists were in unanimous agreement for every study.
The findings of MRA were confirmed by DSA in all patients. The other
radiologic studies used were CT or scintigraphy. CT images were reviewed by
two radiologists independently who were blinded to the DSA and MRA diagnoses,
the patient's name, clinical history, and symptoms. Helical CT and
conventional CT were performed in seven patients. The helical CT protocol
consisted of a 10 mm per second table feed during a 16.5-second breath-hold at
130 mA, with reconstruction of images at 10-mm intervals. Conventional CT was
performed with a 1-second scanning time, 145 mA, and contiguous 10-mm-thick
sections. CT enables distinguishing aplasia and hypoplasia from acquired
obstruction of the pulmonary artery. DSA and MRA only show pulmonary arteries,
but CT reveals bronchi and lung parenchyma. Acquired conditions may closely
mimic aplasia of a pulmonary artery, but the Swyer-James syndrome should have
demonstrably abnormal ventilation of the affected lung. Ventilation scans
showed a diffusely diminished uptake during the wash-in and equilibrium phases
on the involved side. No air trapping was detected during the washout phase
that excluded airway obstruction. MRA studies were obtained only for this
study. The study protocol was approved by the institutional review board, and
informed consent was obtained from all patients.
Results
The quality of DSA images was sufficiently diagnostic in all studies.
Angiography showed a small pulmonary artery in seven patients (hypoplasia) and
the absence of a pulmonary artery in four patients (aplasia). MRA showed
pulmonary hypoplasia in seven patients (in four patients on the left side and
in three on the right side) and absence of the pulmonary artery in the
remaining four patients (in three on the left side and in one on the right
side) (Figs. 1A,
1B,
1C,
1D,
2A,
2B, and
2C). The findings on DSA and
MRA were the same for each individual patient.
CT showed a small pulmonary artery in five patients (in two on the left
side and in three on the right side) and the absence of a pulmonary artery in
two patients (in one on the left side and in one on the right side)
(Table 1).
Scintigraphy showed diminished ventilation in the affected sides of five
patients, decreased perfusion in the affected sides of hypoplasia, and absence
of perfusion in the affected sides of aplasia
(Table 1).
The bronchus to the affected lung is either absent (agenesis) or
rudimentary (aplasia). The affected lung is small in pulmonary hypoplasia; the
mediastinum is displaced toward that side. The pulmonary vasculature and
bronchial tree may be deformed and stunted in pulmonary hypoplasia. The main
pulmonary artery was not present on the involved side, and MRA depicted this
pathology accurately in all patients with aplasia.
Table 1 summarizes the imaging
techniques, symptoms, and types used in the diagnosis of the abnormalities,
including the affected sides.
Discussion
Pulmonary hypoplasia or aplasia is part of the spectrum of malformations
characterized by the incomplete development of lung tissue
[1]. The severity of the lesion
depends on the timing of the insult in relation to the stage of lung
development and the presence of other anatomic anomalies. The hypoplastic lung
consists of a carina, a malformed bronchial stump, and absent or poorly
differentiated distal lung tissue
[1,
2]. To define pulmonary
hypoplasia, some investigators have devised specific criteria that are based
on reduced lung weight, volume, DNA content, and radial alveolar count
[7]. Although in more than 50%
of these patients coexisting cardiac, gastrointestinal, genitourinary, and
skeletal malformations are present along with variations in the
bronchopulmonary vasculature, we only evaluated main pulmonary arterial
anomalies.
Pulmonary MRA has been slow to develop into a clinically useful
application. It has overcome the problems of respiratory motion, cardiac
pulsation, and susceptibility artifact at air-tissue interfaces. Early
development of pulmonary MRA techniques focused on black blood and
time-of-flight (TOF) approaches. Neither of these has proven to be reliable.
Then 2D or 3D TOF techniques were used, the former with and the latter without
breath-holding
[8-10].
Three-dimensional contrast-enhanced MRA now offers several advantages that
make pulmonary MRA possible. The 3D spoiled gradient-echo technique has an
intrinsically short TE. This is sufficient to eliminate the susceptibility
artifact. Because of the enhancement with paramagnetic contrast, 3D
contrast-enhanced MRA is less affected by in-flow variations that create
pulsation artifact on TOF and phase-contrast imaging. Breath-holding
eliminates respiratory motion artifact
[8].
In our study, MRA successfully depicted the main pulmonary arterial
anomalies including hypoplasia and aplasia of the pulmonary artery. These
findings were confirmed with the other imaging techniques, such as DSA that is
the gold standard for these anomalies, in all of these patients. Bouros et al.
[3], studying six adult
patients with the pulmonary artery agenesis with DSA, found that this
technique was the procedure of choice for accurate anatomic depiction of
pulmonary artery abnormalities.
Bright blood imaging is one of the other MRA techniques. Bright blood
imaging is especially useful for visualizing the vessel lumen. Steady-state
free precession imaging (SSFP) is a new technique that produces
high-resolution bright blood imaging without the administration of an IV
contrast agent. This technique can be used to produce high temporal and
spatial resolution images of a vessel or of the heart. These images can be
viewed in a cine mode that allows for the visualization of cardiac
contraction. Bright blood SSFP also permits high-resolution cine imaging of
the vessel wall. These images are useful in the evaluation of aortic aneurysms
and pulmonary arteries
[10].
The advantages of MRA over angiography are noninvasiveness, lack of
ionizing radiation, and lack of a nephrotoxic contrast material
[11]. Sedation time is reduced
in MRA, it does not require expertise to be performed on the pediatric
population, and there is no need for an arterial puncture
[12].
Noninvasive contrast-enhanced CT of the chest produces an anatomic
definition of the pulmonary arteries with consistent quality
[3]. Nevertheless, for the
initial diagnosis, CT cannot substitute for invasive angiography, which
provides anatomic information in detail
[13]. An advantage of CT over
angiography is that the former allows concomitant evaluation of the bronchial
tree and lung parenchyma and the major vascular structures. CT shows the
pulmonary arteries in the affected lung. In bronchiectasis, both conventional
CT and high-resolution CT (HRCT) scans of the lung should be combined to
exclude the presence of bronchiectasis
[3].
Acquired conditions may closely mimic aplasia of a pulmonary artery, but
the Swyer-James syndrome should have demonstrably abnormal ventilation of the
affected lung. On HRCT, there was no air trapping in the lungs, which was
consistent with bronchiolitis obliterans in our study.
In small children, conventional MR image quality may be degraded because of
irregular heart rate, slow turbulent flow, and problems with a low
signal-to-noise ratio. Contrast-enhanced MRA increases the signal from blood
regardless of flow velocity. Because ultrafast MRA sequences have become
available, successful contrast-enhanced MRA is now possible in the pediatric
population [14]. The use of
MRA in pulmonary aplasia and hypoplasia would be much more relevant in a
pediatric population, in whom the use of ionizing radiation should be
minimized or avoided entirely when possible. In the pediatric population,
contrast-enhanced MRA has become an available and successful method in the
diagnosis of pulmonary aplasia and hypoplasia.
Helical CT or MDCT is the preferable method for studying pulmonary aplasia
and hypoplasia. In the adult population, one would probably want to perform CT
(as was done for seven of 11 patients in this study) to be able to look at the
lung parenchyma.
The results of this study indicate that gadolinium-enhanced 3D MRA is a
fast and accurate technique for delineation of the main pulmonary artery in
patients with pulmonary hypoplasia and aplasia and can be considered a
noninvasive alternative to DSA. Also, because of the advantages of lack of
radiation and risk of iodinated contrast media, pulmonary MRA may become
competitive with the other minimally invasive imaging techniques for
diagnosing pulmonary arterial anomalies. The main disadvantages of MRA are the
limited access to suitable MR technology, a patient's claustrophobia, and the
still relatively long measurement time. In MRA, both pulmonary arteries and
veins are enhanced with the contrast agent, and thus the reviewer must be
careful to distinguish them.
Gadolinium-enhanced 3D MRA is a rapidly evolving technique in the imaging
of the pulmonary vasculature. Both pulmonary hypoplasia and aplasia can be
shown morphologically in a noninvasive manner, obviating DSA.
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Abstract
Introduction
