is Chief Resident in the Department of Radiological Sciences at
University of California, Los Angeles (UCLA). He received his
medical degree from Boston University in 1998. His graduate
training included a Howard Hughes Research Fellowship, 19941996,
at Boston Children's Hospital. He is planning fellowship training
in Pediatric Radiology and both Adult and Pediatric
Drs. Ratib, Boechat, and Gomes
are senior faculty with expertise in MR imaging of pediatric
congenital heart disease in the Department of Radiological Sciences
Two of the most exciting advances in magnetic resonance
imaging (MRI) of pediatric congenital heart disease have been the
development of fast cine pulse sequences and the postprocessing
of gadolinium chelate (Gd)-enhanced MR angiographic data. Fast
cine pulse sequences can provide anatomic and physiologic
information to aid in the diagnosis of simple and complex
congenital heart lesions. Three-dimensional volume rendering of
Gd-enhanced MR angiographic data provides new and essential
information about both stenoses and the anatomic relationships of
blood vessels. This article provides an overview of these
technical advances in the use of MRI in pediatric congenital
Over the last decade there have been significant advances in the
role of magnetic resonance imaging (MRI) in pediatric congenital
heart disease. Advantages of MRI over echocardiography and
cineangiography--heretofore the standard techniques for
evaluation--include: 1) the ability to obtain both high-resolution
anatomic images in any orientation and dynamic (cine) images of the
heart to evaluate its mechanical and functional performance; 2) the
fact that the information represents a continuous three-dimensional
(3D) set obtained or reconstructed in any imaging plane; and 3)
superior evaluation of cardiac morphology, anatomy, and topography
of large thoracic blood vessels, including the aorta, central
pulmonary arteries, and both systemic and pulmonary veins.
Recent advances in MRI include the development of fast-pulse
sequences, that can provide anatomic and functional information,
and advanced postprocessing of MR gadolinium chelate (Gd)-enhanced
angiographic data. The radiologist must be familiar with the MRI
techniques applicable to the evaluation of pediatric congenital
heart disease lesions in order to properly tailor the examination
to the diagnostic problem. This paper will review the technical
aspects of MRI in respect to the more common types of pediatric
congenital heart disease, emphasizing recent technical
Getting started and patient preparation
Given the vast number of imaging techniques available, the
radiologist must choose the appropriate set of MRI protocols to
evaluate the specific clinical problem. If the patient has
undergone cardiac catheterization or echocardiography, this
information should be reviewed before determining the protocols for
the examination. It is also important that the radiologist be
familiar with the patient's clinical history because certain
abnormalities such as arrhythmias may affect the examination in
terms of proper electrocardiographic (ECG) gating. The radiologist
should discuss with the referring physician the type of sedation
(general anesthetic sedation, deep sedation, or conscious sedation)
needed for the study. Proper positioning of ECG leads is critical
for obtaining adequate ECG gating, which may be required to
overcome the artifact associated with cardiac contractions.
The MRI protocol for pediatric congenital heart disease is made
up of anatomic and physiologic components. ECG-gated spin-echo
imaging and MR angiography is needed for anatomic information,
while functional information may be obtained from dynamic imaging
protocols including cine imaging and velocity tagging.
Traditional pulse sequences
The traditional pulse sequences for the MRI of pediatric
congenital heart disease are spin-echo and gradient-echo imaging
sequences (figure 1).
For the spin-echo pulse sequence, the repetition time is set by the
R-R interval. For T1-wieghted images, the blood exposed to the 90š
pulse has moved out of the imaging section prior to receiving the
180š pulse, which leads to the blood being "black." However, if
blood is exposed to both the 90š and 180š pulse, as in diastole,
the blood will be of higher signal intensity, which should not be
mistaken for a thrombus.
A typical cardiac image consists of a 256 * 128 matrix and a
section thickness of 4 to 6 mm. The field of view will depend on
the size of the patient. The planes frequently used in spin-echo
imaging are the axial, coronal, and sagittal oblique views. The
axial plane is ideal for demonstrating ventricular and atrial
septal defects. Other planes may be tailored to the disease. For a
patient with a double aortic arch, the coronal view may be useful
in comparing the relationship between the right and left
Gradient-echo imaging employs a sequence of multiple images at
different phases of the cardiac cycle for a given imaging plane.
The individual images are viewed sequentially in a cine loop that
gives a visual effect of the beating heart. In the gradient-echo
sequence, flowing blood appears white, rather than black as it does
when using the spin-echo sequence. With gradient-echo sequence,
turbulent blood flow resulting from valvular or vascular stenosis
leads to a dark signal void.
Gradient-echo imaging usually requires less time than spin-echo
imaging. Typically the matrix is 250 * 128 with only two signals
acquired. In order to improve image quality for nonbreath-hold
acquisition, 16 to 32 phases of the cardiac cycle are obtained and
played in a cine loop. The number of phases of the cardiac cycle
obtained depends on the heart rate and the memory capability of the
MRI system. Other variations of cine imaging include
velocity-encoded cine MRI, myocardial tagging, and fast imaging
with steady-state precession (FISP) technique for acquisition of
high temporal and high spatial resolution images.
Velocity-encoded or phase-mapping cine MRI is a modified
gradient-echo technique that not only determines the direction of
blood flow, but also quantifies blood velocity and blood flow.
For myocardial tagging cine imaging, saturation bands are placed
across the myocardium at the end of diastole.
The tags are attached to tissue throughout the cardiac cycle.
Tagging techniques are used to quantitatively analyze regional
myocardial wall motion, especially for ischemic heart disease and
cardiomyopathy. In pediatric congenital heart disease, tagging
techniques have been used to evaluate right ventricular dysfunction
of patients with right ventricular outflow obstruction, and to
study the left ventricle of patients with transposition of the
Segmented k-space gradient-echo imaging
Breath-hold cardiac imaging makes use of segmented k-space
techniques and requires stronger gradient systems with faster
switching capabilities. With conventional spin-echo and cine
imaging, a single line of k space is acquired per R-R interval.
Segmented k-space techniques acquire multiple lines (eg, segments)
of k space per R-R interval, which decreases the imaging time
(figure 2). If the number of k space lines acquired per R-R
interval is high enough, the acquisition time can be reduced to a
single breath hold.
The segmented-k techniques use a very short repetition time,
approximately 4 to 10 msec. Lower repetition times require smaller
flip angles, which result in a lower signal-to-noise ratio compared
with conventional cine studies. The different types of segmented
k-space gradient-echo techniques include FAST-CARD, turbo-FLASH
(fast low angle shot), fast GRASS (gradient recalled acquisition in
the steady state) and true-FISP (figure 3). Pereles et al
showed that true-FISP imaging depicts morphologic and functional
abnormalities of congenital heart disease with great clarity and
more rapidly than FLASH images, allowing for greater diagnostic
confidence. The disadvantage of both the true-FISP and the FLASH
technique is that they are extremely susceptible to metal, due to
their sensitivity to magnetic field inhomogeneities, leading to
artifact (figure 4). A HASTE sequence is less susceptible to metal,
but cannot provide cine information (figure 4).
The three major types of MR angiography include time-of-flight
(TOF), phase contrast, and contrast-enhanced techniques.
Time-of-flight angiography can differentiate blood from stationary
tissue by means of flow-related enhancement. This technique
combines gradient-echo imaging and flow compensation. By using
gradient-echo imaging, the blood flow into the plane of the section
is fully magnetized, thus appearing bright in relation to the
surrounding stationary tissue. Images of the anatomic vascular
structures are created by combining contiguous sections in
two-dimensional (2D) or 3D TOF sequences. A maximum intensity
projection (MIP) algorithm is commonly used in the creation of TOF
The second type of MR angiography is phase-contrast MR
angiography that measures the phase shift of blood relative to
flow-related enhancement in TOF angiography. This technique is
similar to velocity-encoding cine imaging except that multiple
contiguous images are acquired, as opposed to multiple images of
the same section. The advantage of phase-contrast MR angiography is
that it permits calculation of blood velocity through determination
of the blood proton shift over a period of time.
The third type, contrast-enhanced MR angiography, is the method
of choice at our institution for the evaluation of pediatric
congenital heart disease. Because Gd-based contrast agents are
paramagnetic, they shorten the spin-lattice relaxation time of
blood, giving it a high signal intensity on T1-weighted images
compared with the surrounding tissue. Unlike TOF and phase-contrast
MR angiography, contrast-enhanced MR angiography is not dependent
on the inflow of saturated blood. It allows the repetition time to
be shortened without affecting signal-to-noise ratio.
Contrast-enhanced MR angiography is not cardiac gated, requires a
shorter time of acquisition, and allows 3D images to be obtained
during a breath hold. There are two possible strategies for
performing 3D contrast-enhanced MR angiography: the breath-hold
strategy and the nonbreath-hold strategy. Contrast-enhanced MR
angiography data may be postprocessed in multiple ways.
Four types of postprocessing methods are available, including
multiplanar reconstruction (MPR), shaded-surface rendering, maximal
intensity projection (MIP), and volume rendering (figure 3). In a
recent study, conducted by the authors of this paper, volume
rendering was found to provide additional information in 66% of
cases in which it was used, and 3D volume rendering provided
improved visualization of overlapping structures, which are not
separated by conventional MIP techniques.
Three-dimensional volume rendering was useful in the evaluation of
vessel stenoses and for demonstrating anatomic relationships in
complex congenital heart disease. We also found that 2D multiplanar
reformatting allowed improved accuracy in the measurement of vessel
diameter, particularly of tortuous vessels because of the ability
to properly align the measurement along the long axis of the
Other researchers have found that surface rendering does not add
more information in cases of pediatric congenital heart disease.
More research is needed comparing the different types of
postprocessing of Gd-enhanced MR angiographic data to
cross-sectional imaging (spin-echo imaging) for measuring vessel
diameter and stenoses.
MR studies of pediatric congenital heart disease provide both
morphological and functional information. Pediatric congenital
heart disease, because of its complexity, requires a systematic,
organized approach to achieve accurate interpretation. MR analysis
of this complex disease requires a rigorous approach that can be
separated into three parts: 1) localization of the three cardiac
segments (atria, ventricle, great arteries); 2) the determination
of the type of atrioventricular and ventriculoarterial connections;
and 3) the detection of associated anomalies.
The indications for MRI in the evaluation of patients with
congenital heart disease have been reported by the Task Force of
the European Society of Cardiology.
The major indications include: 1) segmental description of cardiac
anomalies; 2) evaluation of thoracic aortic anomalies; 3)
noninvasive detection and quantification of shunts, stenoses, and
regurgitations; 4) evaluation of cono-truncal malformations and
complex anomalies; 5) identification of pulmonary and systemic
venous anomalies; and 6) postoperative studies.
The remainder of this article will focus on the role of MR
imaging in the diagnosis of some of the more common types of
pediatric congenital heart lesions.
Atrial septal defects (ASD) are the most common congenital
cardiac defects. There are three main types: secundum (located in
the middle of the atrial septum), sinus venosus (located superiorly
at the junction of the superior vena cava and right atrium), and
primum (in the inferior portion of the atrial septum, near the
atrioventricular valves). Because of left-to-right shunting, right
atrial and right ventricular volume overload is common. With MRI,
the diagnosis of an ASD can be made by demonstrating the defect on
two adjacent axial images or during multiple phases of the cardiac
cycle during cine imaging at one anatomic level.
The sensitivity and specificity of MRI for the diagnosis of ASD are
both > 90%.
MRI also provides valuable information about ventricular septal
defects (VSD). The majority of VSDs occur in the perimembranous
septum, an anatomic area that is well evaluated with spin-echo or
gradient-echo MRI. MRI defines not only the extent of the defect,
but its relationship with other ventricular structures as well,
resulting in a more complete characterization.
Evaluation of the great arteries
MRI is useful in the evaluation of the pulmonary arteries with
respect to origin, presence, size, and confluence. Images in the
axial plane are usually adequate to demonstrate the main, right,
and left pulmonary arteries. If the left pulmonary artery cannot be
evaluated in the standard axial view, an oblique view paralleling
the long axis of the left pulmonary artery is useful.
Hypoplastic pulmonary arteries are found in the spectrum of
tetralogy of Fallot, the most common type of cyanotic congenital
heart disease. The main anatomic features of the tetralogy of
Fallot are infundibular pulmonary hypoplasia and stenosis, a large
malaligned VSD, an overriding aorta and secondary right ventricular
hypertrophy (figure 5). Determination of the morphology of the
pulmonary arteries in patients with right ventricular outflow
lesions is crucial when either corrective or palliative surgery is
Cineangiography has been considered the gold standard in the
imaging of the pulmonary arteries of patients with right
ventricular outflow lesions. Strouse et al
prospectively compared MRI to cineangiography in the evaluation of
the central pulmonary arteries of patients with right ventricular
outflow obstructive lesions. This study demonstrated that MRI was
equivalent to cineangiography in evaluating the size of central
pulmonary arteries. In addition, MRI was able to show the
appearance of central pulmonary arteries when cineangiography was
unable to demonstrate them.
MRI and echocardiography are similar in their ability to
demonstrate the abnormalities of the right ventricular outflow
tract and main pulmonary artery. However, MRI is superior to
echocardiography in showing abnormalities of the central right and
left pulmonary arteries. In general, measurements of the size and
status of the pulmonary arteries in MRI correlate well with those
obtained at conventional angiography.
MRI is also excellent for evaluating shunts between the systemic
and pulmonary circulations, including Blalock-Taussig, Glenn, and
Waterston anastomoses (figure 5), that are commonly used in
patients with right ventricular outflow tract lesions.
Coarctation of the aorta is defined as a congenital narrowing of
the aorta. The most common site of localized coarctation is just
distal to the left subclavian artery, near the level of the ductus
arteriosus (figure 6). MRI is useful in diagnosis of the lesion,
presurgical planning, and postsurgical follow-up.
Characteristics of the coarctation including its site, length,
severity, relationship to the left subclavian artery, and the
extent of collateral blood flow, should be evaluated
preoperatively. The coarctation should be evaluated postoperatively
to determine if restenosis or aneurismal dilatation has occurred.
MRI has been shown to be comparable to angiography in detecting
postoperative residual stenosis and aneurysms at the site of the
We have found that volume-rendering of Gd-enhanced MR angiographic
data is useful in demonstrating the anatomic distribution of
collateral flow in cases of coarctation (figure 6).
Vascular rings are also well evaluated by MRI.
The most common symptomatic vascular rings are the double aortic
arch and the right aortic arch with an aberrant left subclavian
artery. In patients with a double aortic arch, a right and left
arch arise from a single ascending aorta, with the right arch
usually higher and larger than the left arch. MR images in the
axial and coronal planes are the most useful for demonstrating the
vascular anatomy (figure 7).
Evaluation of systemic and pulmonary veins
Systemic venous return
MRI is useful in evaluating systemic venous anomalies associated
with congenital heart disease and heterotaxic syndromes.
Polysplenia and asplenia are heterotaxy syndromes in which there is
bilateral left (polysplenia) or right (asplenia) sideness. Asplenia
is associated with complex cyanotic disease, while polysplenia may
be associated with less complex acyanotic heart disease
(left-to-right shunts). Other types of systemic venous anomalies
that can be diagnosed by MRI include persistence of the left
superior vena cava, anomalous left innominate vein, left inferior
vena cava, and duplication of the inferior vena cava.
Pulmonary venous return
In total anomalous pulmonary venous return, the pulmonary venous
return is connected to venous structures such as the superior vena
cava, right atrium, or portal vein. The three types are
supracardiac, cardiac, and infracardiac, with the supracardiac type
being the most common type. MRI has been shown to be superior to
echocardiography and cineangiography for the diagnosis of anomalous
pulmonary venous connections.
MRI has also been shown to be excellent in showing obstruction or
stenosis of pulmonary venous structures after correction of total
anomalous pulmonary venous connections.
Conotruncal rotational abnormalities
A group of congenital heart lesions arise from abnormal rotation
of the conotruncus. Donnelly et al
demonstrated that axial MRI is an excellent method to show the
anatomic relationship between the aorta and pulmonary artery at the
level of the semilunar valves. These abnormalities include
D-transposition of the great arteries, L-transposition of the great
vessels, truncus arteriosus, double-outlet right ventricle, and
situs inversus (figure 8). In normal development, the primitive
truncus divides into the aorta and the pulmonary artery, which then
rotate 150š counterclockwise, so that the pulmonary artery lies
anterior and to the left of the aorta. The conotruncal
abnormalities represent different degrees of this rotation.
Patent ductus arteriosus
The ductus arteriosus is the result of the persistence of a
normal embryologic structure, the sixth aortic arch. The ductus
arteriosus extends from the origin of the left pulmonary artery to
the descending aorta just beyond the origin of the left subclavian
artery. Functional closure of the ductus arteriosus typically
occurs within 24 hours of birth. In some patients, the mechanism
for the closure of the ductus arteriosus is abnormal, resulting in
a persistent communication between the aorta and pulmonary artery.
MRI is a useful tool is demonstrating the presence of a patent
ductus arteriosus, both in the setting of other cardiac lesions
(figure 8D) and in isolation (figure 9).
Recent advances in MRI of pediatric congenital heart disease
complement echocardiography and cineangiography. Two of these
advances include the development of faster segmented k-space
gradient-echo imaging and postprocessing of Gd-enhanced MR
angiographic data including 3D volume rendering. MRI has a
definitive role in the evaluation of atria, ventricles, pulmonary
arteries, aorta, systemic veins, and pulmonary veins with respect
to congenital heart disease lesions. In order for MRI of pediatric
congenital heart disease to be effective and efficient, a basic
understanding of the technical aspects of the examination and of
the disease processes is essential.