++
Relatively recent advances in pediatric cardiovascular surgery, catheter-based
interventional therapies, intensive care, and medical management
have dramatically changed the landscape of the field of congenital
heart disease (CHD). The complexity of the anatomy and physiology
of patients surviving with CHD increases exponentially. The majority
will survive to adulthood, and the need for reintervention is common.
As such, the field is placing new demands on imaging to diagnose
and plan medical management, as well as to identify need for and
timing of reintervention. There are a number of imaging modalities
available to the clinician and radiologist when it comes to these
evaluations.
++
Echocardiography (ECG) has been and remains a mainstay of imaging
in congenital heart disease. Despite its importance in rapid diagnosis
and follow-up, it has limitations. The presence of postoperative
scar, chest wall deformities, overlying lung tissue, and large body
size as the patient ages often results in suboptimal transthoracic
echocardiographic windows. Transesophageal echocardiography provides
improved acoustic windows, but is limited by its small field of
view and more invasive nature.
++
Cardiac catheterization, employing x-ray fluoroscopy and contrast
angiography, has an expanding role in minimally invasive interventions,
but its role as a diagnostic procedure is rapidly diminishing. This
is in part due to its limitation as a 2-dimensional projection imaging
technique with poor soft tissue contrast and the substantial ionizing
radiation exposure involved and in part because both diagnostic
and functional analyses are often better performed with noninvasive
imaging techniques.
++
This chapter focuses on the evolving and expanding roles of other
imaging modalities in diagnosing and monitoring patients with congenital
heart disease, including cardiac magnetic resonance imaging (MRI),
cardiac computed tomography (CT), and radionucleotide scintigraphy.
++
Cardiac magnetic resonance imaging (MRI) has emerged over the
past few decades as an alternative, complementary, and frequently
superior imaging modality for investigating the anatomy and function
in the patient with congenital heart disease. It has many advantages over
other imaging modalities. It does not require the use of iodinated
contrast agents and does not involve exposure to ionizing radiation. This
is particularly important in a population of patients who have been,
and continue to be, exposed to large doses of contrast agent and
radiation during hemodynamic and interventional catheterization.
Additionally, many of these patients are children who are more susceptible
to the adverse effects of radiation. Major advances in MRI hardware
and software, including advanced coil design, faster gradients,
new pulse sequences, and faster image reconstruction techniques,
allow rapid, high-resolution imaging of complex anatomy and accurate,
quantitative assessment of physiology and function.
+++
Cardiac MRI Techniques
++
There are a number of MRI techniques useful in examining the anatomy
and physiology of the congenital heart disease patient.
++
ECG-gated gradient-echo sequences provide multiple images throughout
the cardiac cycle in prescribed anatomic locations. Display of these
images in a cine mode allows visualization of the dynamic motion
of the heart and vessels.1-3 Cine MRI techniques,
at a minimum, allow assessment of anatomy. More importantly, they
allow qualitative and quantitative assessment of physiology and
function. Specifically, cine MRI allows quantification of chamber
volumes, myocardial mass, and ventricular function. furthermore, cine
MRI allows qualitative assessment of focal and global wall motion
abnormalities; qualitative and quantitative assessment of valve
pathology, including the mechanism and severity of valve regurgitation
and the location and severity of valve stenoses; identification
and quantification of intra- and extracardiac shunts; and visualization
of other areas of flow turbulence.
++
The use of such cine MRI techniques for the quantification of chamber
volumes, myocardial mass, and ventricular function, both fast gradient-echo4-7 and
balanced steady-state precession (SSTP),1,2 have
been extensively evaluated and validated.8-10 Briefly,
evaluation of function begins with obtaining a series of contiguous
slices (cines) along the short axis of the ventricles, extending
from base to apex. These images are played back in a cine loop,
and the end-systolic and end-diastolic phases are chosen. The endocardial
borders are traced at both time points, and the epicardial borders
are traced at one of the 2 time points (Fig. 494-1).
Ventricular volumes are then calculated as the sum of the traced volumes
(area times slice thickness). Myocardial mass is calculated as the
myocardial muscle volume times 1.05 g/mm3 (density
of myocardium). From these data, ventricular end-diastolic volume,
end-systolic volume, stroke volume, ejection fraction, myocardial
mass, and mass/volume ratio can be calculated for both
the right and left ventricles.
++
+++
Spin-Echo (Black
Blood) Imaging
++
ECG-gated spin-echo sequences, or black blood imaging represent
another important tool for imaging in the congenital heart disease
patient. Despite providing only static information, black blood
imaging has many benefits. allows assessment of anatomy with thin
slices, high spatial resolution, and excellent blood-myocardium
and blood-vessel wall contrast (Fig. 494-2).
Black blood techniques are superb for evaluating the spatial relationship
between cardiovascular and other intrathoracic structures such as
the chest wall and the tracheo-bronchial tree. These features hold
particular relevance when delineating complicated native and postsurgical
cardiac anatomy. Such techniques are also less susceptible to artifact
from metallic implanted devices such as stents, coils, occluder
devices, clips, and sternal wires.
++
++
Cardiac-gated gradient-echo sequences with flow-encoding gradients
are used to quantify the velocity and flow of blood (Fig.
494-3).11 These sequences are referred to
as velocity-encoded cine MRI or phase-contrast MRI. Two-dimensional
velocity-encoded cine MRI sequences are the most commonly used in
clinical practice. They can be used to quantify cardiac output, pulmonary
to systemic flow ratio (shunt), valvular regurgitation, differential
lung perfusion, coronary flow reserve, and to observe the location
and severity of flow obstruction.
++
+++
Gadolinium-Enhanced
3-Dimensional Angiography
++
Three-dimensional angiography (3D-MRA) sequences are typically
not cardiac gated and thus do not allow assessment of intracardiac structures.
Regardless, they provide excellent depiction of arterial and venous
vascular structures (Fig. 494-4). In the
congenital heart disease patient, 3D-MRA fills a significant diagnostic
role. It can be used to diagnose systemic arterial anomalies such
as aortopulmonary collaterals, shunts, vascular rings, and coarctation.
It is useful in diagnosing pulmonary arterial abnormalities such
as focal and diffuse stenoses and abnormal distal arborization patterns.
Using 3D-MRA methods is also useful for investigating systemic and
pulmonary venous abnormalities, both congenital and postoperative.
Also, 3D-MRA is useful for evaluating the relation between vascular and
other thoracic structures. With faster techniques and navigator pulses,
time-resolved 3D-MRA as well as noncontrast 3D-MRA are becoming
alternatives.12
++
+++
Coronary Artery
Imaging, Perfusion Imaging, and Myocardial Viability
++
Coronary artery abnormalities and ischemia are important issues
to be investigated in postoperative congenital heart disease patients. Not
only is this patient population aging sufficiently to develop atherosclerotic
coronary artery disease, they also commonly have congenitally abnormal
or postoperatively acquired coronary artery lesions. It is not uncommon
to find an anomalous origin or course of the left or right coronary
artery, postsurgical coronary obstruction (ie, after arterial switch
for transposition of the great arteries), coronary artery thrombus,
and abnormal fistulous connections (ie, pulmonary atresia with intact ventricular
septum and right ventricular-dependent coronary circulation). Identifying
such abnormalities is often critical to planning reintervention
and medical management. There is growing evidence
to support the concept that myocardial delayed enhancement in a
number of subsets of postoperative congenital heart disease patients
predicts poor outcome.13-16 In summary, although
still not as robust as routine coronary artery angiography with
x-ray fluoroscopy or cardiac-gated CT angiography at investigating
distal coronary artery lesions, MRI can image proximal coronary
arteries well,17-20 evaluate myocardial perfusion and
viability,21-24 and allow stress testing,25,26 all noninvasively
without exposure to contrast agents and ionizing radiation.
+++
General Clinical Applications
of Cardiac MRI
++
The indications for magnetic resonance imaging (MRI) in the congenital
heart disease (CHD) patient are evolving and expanding as MRI technology
advances and the medical issues of this population become more complex. In general,
MRI is indicated when transthoracic echocardiography does not provide
adequate diagnostic information (eg, postoperative patients; adult
with congenital heart disease), as an alternative to invasive and
costly diagnostic catheterization, and when the unique capabilities
of MRI can be exploited. The most common indications
for MRI in the CHD patient are detailed. Cardiac MRI is quite effective
in the segmental description of complex anatomy and it is unrivaled
in the quantitative evaluation biventricular function, particularly
the right ventricle, which is so often at risk in patients with
CHD. It is useful in evaluating arterial and venous anomalies and
detecting and quantifying shunts, valvular and vascular stenoses,
and regurgitant lesions. Cardiac MRI is also excellent for characterizing
coronary arterial anatomy and myocardial perfusion and viability.
++
Computed tomography (CT) has historically been quite useful in evaluating
vascular anatomy. The advent of high resolution CT and
cardiac gating has emerged as a useful tool for assessing intracardiac
anatomy, coronary anatomy, and myocardial function. Multi-slice
CT has advanced significantly over the past decade, particularly
with the emergence of 64-slice multidetector scanners in 2004. This
newest technology allows faster acquisition times, improved spatial
resolution, and thinner slices. Improved gating has decreased motion
artifacts. Although such technology is proving invaluable
in cardiovascular imaging in adults, its use in the pediatric population
in the assessment of congenital heart disease has just begun to
be explored.27-29
++
The strengths of CT imaging include excellent spatial resolution
(submillimeter isotropic resolution) and rapid acquisition times,
often precluding the need for anesthesia. It is unparalleled in
its ability to provide spatial relationships of intrathoracic structures,
including the vasculature and the airways. Some argue that CT provides
equal or even better image quality of the epicardial coronary vessels
than does coronary angiography. It can also be used in congenital
heart disease patients who have pacemakers or defibrillators.
++
Cardiac CT does have its weaknesses. There is less temporal resolution
than cardiac MRI or echocardiography and thus is limited in its
ability to characterize ventricular function. There are no CT techniques
to quantify flow. CT contrast, albeit improved and less toxic, still
has well-documented renal toxicity. Most importantly, there is significant
radiation exposure associated with CT. It is critical for the clinician
to consider this when ordering a CT, particularly in the young patient
and the patient who has been, and continues to be, exposed to large
doses of contrast agent and radiation during hemodynamic and interventional
catheterization. It must also be recognized that gated studies and higher-resolution
studies come at a cost of higher radiation doses. (See the as low
as reasonably achievable [ALARA] guidelines.30)
+++
General Clinical
Applications of Cardiac CT
++
The clinical indications for cardiac CT in congenital heart disease patients
are evolving and growing as technology advances. CT is particularly
indicated in patients with pacemakers and defibrillators in whom
cardiac MRI is precluded or in patients with metallic implants that
create unmanageable artifacts on cardiac MRI (ie, steel coils).
It also can take the place of cardiac MRI if the question being studied
is strictly vascular anatomy and not intracardiac anatomy or cardiac
function. CT can answer these questions with a rapid study, often
without sedation. CT is quite useful in evaluating the coronary arteries,
particularly if there is concern about distal coronary disease.
Cardiac CT has the added benefit of accurately delineating the relationship
of vascular structures and the airways.
++
Radionuclide imaging is used primarily for assessing cardiac
physiology. Although rarely used today for the assessment of right
and left ventricular function in pediatric patients, radionuclide
studies continue to provide valuable information for the evaluation
of intra- and extracardiac shunts and assessment of myocardial perfusion.
+++
Assessment of
Left‐to-Right Shunts
++
Echocardiography with Doppler is the most widely used method
for determining the presence and magnitude of left-to-right shunts in
congenital heart disease, although its accuracy in quantification
of multiple level shunts or shunts in the presence of valve insufficiency
or multiple shunts is not optimal. The radionuclide technique
for left-to-right shunt quantification was established and validated
more than 30 years ago and is not affected by such considerations.31
++
The radionuclide method is most often used to assess the size
of left-to-right shunts in 4 major congenital lesions: atrial septal
defect, ventricular septal defect, patent ductus arteriosis, and
partial anomalous pulmonary venous return. In uncomplicated atrial
septal defect, the shunt is principally from the left atrium to
the right atrium, leading to an increased volume load in the right
ventricle, which becomes dilated (eFig. 494.1).
The left ventricle does not participate in the shunt. Anomalous connections
of pulmonary veins, generally to the right atrium, also result in
left-to-right shunts where the right ventricle handles the increased
volume. In the typical perimembranous ventricular septal defect,
the shunt is principally from the left ventricle, across the ventricular
septum, into the right ventricular outflow tract, and out the pulmonary
trunk (eFig. 494.2). The left ventricle handles
the increased volume load and dilates. Large left to right shunts
associated with ventricular septal defects can lead to pulmonary
hypertension and subsequent right ventricular hypertrophy. In patent
ductus arteriosus, the shunt occurs between the high-pressure aortic trunk
to the lower-pressure pulmonary trunk (eFig. 494.3).
The left ventricle handles the increased volume and dilates. As with
large ventricular septal defects, large patent ductus arteriosus
can lead to pulmonary hypertension and right ventricular hypertrophy.
In each lesion, knowledge of the size of the shunt, expressed as
pulmonary-to-systemic flow ratio (Qp/Qs), is essential
for decisions regarding corrective surgery.
++
++
++
++
The scintigraphic technique
involves the rapid injection of a bolus of radionuclide (usually
technetium-99m–labeled diethylenetriamine pentaacetate, or 99mTc-DTPA)
into the circulation while monitoring the transit through the heart
and lungs with the gamma camera. For small infants (ie, premature
newborn infants), a butterfly needle can be used in a temporal scalp
vein to deliver a compact bolus of activity to the central circulation.
In older children and adults, either a butterfly needle or a small
cannula can be inserted, preferably into an external jugular vein,
although an antecubital vein may also be used. The delivery of a
compact, nonfragmented bolus of activity is essential to allow accurate determination
of the size of the shunt. With good technique, the success rate
should be greater than 90%. It may be necessary to sedate infants
and some children, because crying simulates a Valsalva maneuver
that can impede bolus entry into the thorax and lead to fragmentation
of the bolus. As mentioned, 99mTc-DTPA is most commonly
used for shunt studies. Doses are 200 microcuries (µCi) per kilogram
of body weight, with a minimum dose of 2 mCi. The advantage of 99mTc-DTPA
over other technetium based agents is the fairly rapid renal excretion
and thus prompt clearance of background activity. This becomes important
if it is necessary to perform a second injection to improve the
quality of the bolus. Generally, no more than 2 sequential injections
are performed due to dosimetry limitations.
++
The study is done in the anterior projection using a converging
collimator (which provides magnification) in infants and ideally
a high-sensitivity parallel collimator in older children and adults.
A dynamic acquisition with a sampling rate of 2 to 4 frames per
second is adequate for evaluation of shunts. If ejection fraction
measurements are to be made by the first pass method, a rate of
at least 25 frames per second should be used. The sequential flow
study is reviewed in order to provide useful information regarding
chamber orientation and vascular connections. In the presence of
normal anatomic relationships, right heart structures will appear,
followed by the main pulmonary artery, lungs, and subsequently,
the left ventricle (levophase), and descending aorta. Persistent
pulmonary activity, resulting in the absence of a distinct levophase
is consistent with a moderate to large left-to-right shunt (eFig. 494.4). This appearance results from
recirculation of activity from heart back to lungs and vice versa, across
the shunt. This appearance has been called the “smudge
sign,” and generally indicates a pulmonary to systemic
flow of at least 1.6/1.
++
++
For quantification, time-versus-radioactivity curves are generated
from regions of interest over the superior vena cava to assess the
quality of the bolus and the periphery of the right lung, for shunt
detection and magnitude (Fig. 494-5). A separate
curve may be generated from a region over the left lung if differential
shunting is expected (as may occur with a patent ductus arteriosus).
The normal pulmonary arterial curve has an ascending limb, reflecting
the arrival of tracer in the pulmonary circulation, and a symmetric
descending limb, reflecting the tracer exiting the lungs and entering the
left side of the heart. A late peak wsill appear, reflecting systemic
recirculation. In a left-to-right shunt, a shoulder will be present
on the downslope, indicating recirculation of activity back to the
lungs across the shunt. For shunt quantification, the shape of the
pulmonary portion of the curve is approximated by an algebraic expression
called a gamma variate function (Fig. 494-6).31 In
practice, the computer is given the coordinates of the upslope and initial
downslope of the pulmonary curve, and a curve is generated that
approximates the shape of the curve. The area under this pulmonary
curve is proportional to pulmonary flow, Qp. This fitted curve is
then subtracted from the initial time-versus-radioactivity curve,
and another gamma variate fit is done on the remaining curve. The
area under this second fitted curve is proportional to the shunt
flow, Qsh. The difference between the two fitted curves is a measure
of systemic flow, Qs. The resultant calculation of pulmonary to
systemic flow, Qp/Qs, is performed as:
++
++
++
++
Ratios less than 1.2:1 are consistent with the absence of left-to-right
shunts. The Qp/Qs calculation, by the gamma variate method, has
shown excellent correlation with shunt size determined at cardiac
catheterization over a clinically significant range of 1.2:1 to 3.0:1.31 This
relationship remains valid even in the presence of pulmonary hypertension,
tricuspid regurgitation, and heart failure.31,32 In
these conditions, extensive dilution and slow flow lead to a slow downslope
to the pulmonary curve. However, the upslope should be proportionately
slowed, and the curve fit method should generally apply, although
caution should be exercised. Since the method depends on the full
passage of the administered radionuclide through the lungs, left-to-right
shunts will be overestimated in the presence of right-to-left shunts.
Shunts greater than 3.0:1 are difficult to fit by the gamma variate
method due to distortions in curve shape as a result of the large
and torrential shunt flow. This is not a practical limitation, however,
as any shunt greater than 3.0:1 is very large. In general, a shunt
of 2.0 or greater is sufficient to warrant surgical correction.
++
With the anatomic detail provided by echocardiography, the hemodynamic
correlates from Doppler examination, and the precise quantitation
available from a radionuclide shunt study, it is sometimes possible
to proceed directly to surgery without preoperative cardiac catheterization.
This is particularly true with uncomplicated patent ductus and secundum
atrial septal defect. In anomalous pulmonary venous return, the
radionuclide determination of shunt size may be more accurate than
that determined at catheterization by oximetric methods due to the inability
to obtain a good mixed venous blood sample at catheterization.33 The
radionuclide method has also been used to measure changes in shunt
magnitude in response to oxygen therapy, and to assess the reactivity
of the pulmonary vascular bed in patients with pulmonary arterial
hypertension and large shunts.34 This is a very
important consideration in determining operability in patients with
moderate to large ventricular septal defects.
++
One of the leading indications for radionuclide shunt studies
is the postoperative assessment of residual shunt size in patients
with murmurs and echo Doppler evidence of persistent shunting after
surgical correction of septal defects. Doppler quantification of
shunt size is often not very reliable after patch closure of defects
due to the turbulence generated in the vicinity of the patch. In
this situation, the radionuclide technique has been helpful for
assessing the need for repeat catheterization and possibly reoperation.
++
It is possible to calculate the extent of left-to-right shunts
using the equilibrium blood pool method. Stroke volume or amplitude
images can be used to measure the difference in stroke volume between
the ventricles, as is commonly performed for the evaluation of regurgitant
lesions (eFig. 494.5).35 With
a ventricular septal defect or a patent ductus arteriosus, the left ventricle
handles the excess volume of the shunt flow. The left ventricular
stroke volume is proportional to the pulmonary blood flow, and the
right ventricular stroke volume is proportional to the systemic
blood flow. The pulmonary-to-systemic flow ration can be calculated
as:
++
Qp/Qs = LV stroke volume/RV stroke volume
++
++
where, LV stroke volume equals LV end-diastolic volume minus
end-systolic volume, and RV stroke volume equals RV end diastolic volume
minus end-systolic volume.
++
For an atrial septal defect or anomalous pulmonary venous return, the
right ventricle carries the excess shunt flow. The Qp/Qs
can be calculated as:
++
Qp/Qs = RV stroke volume/LV stroke
volume
++
A good correlation (r = 0.79) has been
noted between the shunt Qp/Qs ratio calculated from stroke
volume ratios and oximetry.36 This approach may
be particularly useful in situations where attempts at a good bolus
injection were unsuccessful.
+++
Assessment of
Right‐to-Left Shunts
++
Right-to-left shunts can be detected by inspection of the first-pass radionuclide
angiogram, which reveals a premature appearance of radioactivity
in the left-sided chambers or aorta. Time-versus-radioactivity curves
generated from regions of interest over the carotid artery can be
analyzed by curve-fitting methods to quantify shunt size.37 Intravenous
injections of an inert radioactive gas, such as 133Xe or
krypton-81m, can also be used for detecting right-to-left shunts.38 Significant
systemic activity of these agents, which should be totally extracted
by the lungs and exhaled in the alveolar gas, indicates shunting.
++
The easiest and most commonly used method to assess right-to-left
shunts is the intravenous injection of 99mTc-labeled macroaggregated
albumin particles, similar to those used for the assessment of pulmonary perfusion.39 In
the absence of right-to-left shunting, all of the particles are trapped
in the lungs. When right-to-left shunting occurs at any level, particles
will enter the systemic circulation in proportion to the shunt flow,
lodging in the capillary and precapillary beds of systemic organs.
A series of whole body images are taken to determine the percentage
of right-to-left shunt as:
+
++
Pulmonary-to-systemic flow ratio can be calculated as:
++
Qp/Qs = lung counts/whole body counts
++
Radiolabeled macroaggregrated albumin particles can be used to assess
Qp/Qs in admixture lesions, where complete mixing of pulmonary
and systemic blood occurs in 1 cardiac chamber. Images in eFigure 494.6 were obtained from a 16-year-old cyanotic
patient who presented with a history of pulmonic atresia with intact
ventricular septum and hypoplastic tricuspid valve. He received
a Waterston aortic-to-right pulmonary artery shunt as a child. In
addition, a left subclavian-to-central pulmonary shunt (Blalock-Taussig
shunt) was done, with flow primarily to the left lung. The procedure
was done to assess the patency of the shunts.
++
++
Although technically there is a left-to-right shunt of blood
from the aorta to pulmonary artery, the major hemodynamic abnormality in
this cyanotic lesion is complete admixture of systemic venous and pulmonary
venous blood at the level of the left atrium. Both pulmonary and
systemic blood arise from the aorta. Note the presence of brain
and kidney uptake, consistent with right-to-left shunt. In this example,
differential pulmonary perfusion was measured by determining relative
counts in both lungs. Left lung activity represented 45% of
total lung activity, whereas right lung activity was 55% of
total. These findings suggested that both the Waterston and Blalock-Taussig
shunts were functioning. Additional important data were obtainable
from this macroaggregated albumin study. The pulmonary-to-systemic
flow ratio, Qp/Qs, was calculated as Qp = lung
counts, Qs = whole body counts – lung counts.
This formula is applicable since the patient has an admixture lesion.
There is complete mixing of the particles by the time the injected
dose reaches the aorta. The particles are subsequently distributed
in proportion to regional blood flow to the lungs and the body.
The pulmonary/systemic flow ratio, Qp/Qs, is a
very important determinant of systemic arterial oxygen saturation
in patients with admixture lesions.40 In the example
shown above, the Qp/Qs was 1.6. This modest ratio suggested
that pulmonary flow was not excessive, and increased systemic saturation
would likely result with a slightly larger degree of pulmonary blood
flow, that is, a larger Qp/Qs.
++
In spite of the general reluctance to administer particles to
patients with known right-to-left shunts, the method has proven
to be safe, accurate, and very easy to perform.39 The
particle number should be kept below 50,000 in pediatric patients.
++
The microparticle study is very useful for visualizing maldistribution
of pulmonary blood flow after surgery for congenital heart disease.
Pruckmayer et al41 studied 46 patients (mean age
8.2 years) with complex cardiac anomalies who had undergone either
a Glenn shunt or Fontan procedure. Glenn shunt and Fontan procedures
may be associated with abnormal pulmonary flow patterns and the
development of pulmonary atrioventricular fistula.41 Imaging
was done following sequential injections of ch494ref99mTc-microspheres
into upper and lower limb veins. In 31 of 46 patients, blood flow
from the superior vena cava (arm injection) drained preferentially
to the right lung, whereas blood flow from the inferior vena cava
(foot injection) drained equally to both lungs (eFig.
494.7). Lung perfusion scintigraphy after upper and lower extremity
injections detected more abnormal pulmonary flow patterns than did contrast
echo. In addition, the technique was able to quantify R-L shunt
volumes individually from the superior vena cava and inferior vena
cava, a unique attribute compared to any other imaging method. Quantitative
assessment of relative pulmonary perfusion before and after percutaneous
intervention for peripheral pulmonary artery stenosis42 has
become one of the most commonly ordered scintigraphic procedures
for cardiovascular assessment in pediatric patients in our practice
(eFig. 494.8).
++
++
+++
Assessment of Ventricular
Function
++
Although radionuclide methods are well suited for the assessment
of ventricular size and function in congenital heart lesions, these
methods have been largely replaced by echocardiography in current
clinical practice. Both first-pass and gated equilibrium methods
for the determination of ejection fraction have been validated in
the pediatric age group.43,44 Quantitative assessment
of absolute ventricular volumes45 and determination
of regurgitant fraction have been reported in children as well.46,47 For
infants, the imaging is optimized with the use of a converging collimator
to improve spatial resolution and increase the sensitivity. It is
feasible to measure ejection fraction even in tiny premature infants
with the pinhole collimator.48 Ventricular size
and function evaluation are useful at rest and with dynamic stress
in a variety of congenital lesions, both before and after surgical
correction.49,50 Residual structural and functional abnormalities
are very common, and careful, long-term follow-up is important.
+++
Assessment of Myocardial
Perfusion
++
Myocardial perfusion scintigraphy has been used for clinical
assessment in children for a number of years.51,52 In
the pediatric patient, perfusion imaging has been most widely used
for the noninvasive identification of anomalous left coronary artery.53-56To
evaluate patients for possible anomalous left coronary artery, a perfusion
tracer is injected intravenously, at rest, and images are acquired
in multiple planar projections, or single photon emission computed
tomography (SPECT) imaging is done. The usual anatomy in this rather
rare disease is for the left main coronary artery to arise from
the main pulmonary artery. This situation can lead to regional ischemia
and infarction of the left ventricle due to low perfusion pressure
from the pulmonary artery, which can create a coronary steal. Perfusion scintigraphy
typically reveals a segmental perfusion abnormality at rest (eFig. 494.9). This pattern is useful for identifying
anomalous left coronary, as opposed to myocarditis or cardiomyopathy,
as the etiology for poor ventricular function in infants. The condition
is often associated with Q-waves on the electrocardiogram. Echocardiography
is sometimes able to identify the aberrant origin of the left coronary,
but catheterization is required for confirmation. Multidetector
CT has shown promise for the noninvasive detection of anomalous
origins of coronary arteries.57,58 Direct aortic
implantation of the anomalous coronary artery is the preferred approach
to surgical correction.59,60
++
++
Another clinical condition for which perfusion scintigraphy may
be useful is Kawasaki disease.61 This syndrome
is associated initially with persistent fevers, rash, adenopathy,
and mucous membrane abnormalities. Before the introduction of intravenous
gamma globulin therapy, about 20% to 25% of these
patients developed aneurysms of the coronary arteries. Treatment
with gamma globulin within 10 days of the onset of the illness reduces
the frequency of coronary aneurysms to about 4%. About
30% to 50% of such aneurysms will spontaneously
regress within the first 2 years of illness. The remaining aneurysms may
later thrombose and cause myocardial ischemia and infarction.62,63 Bypass
surgery has been advocated for some patients with objective evidence
of ischemia.64
++
Assessment of myocardial perfusion is important in the evaluation
and follow-up of patients with transposition of the great vessels
who have undergone the arterial switch procedure.65,66 D-transposition
of the great arteries is a lethal congenital malformation and represents
the most common cardiac cause of cyanosis in the neonate. Historically,
left untreated up to 90% of infants with the condition
died within the first year of life.67 The prognosis
for these infants, however, has been remarkably improved with the
introduction of palliative and corrective surgical techniques. Among
the earliest and most successful surgical procedures is a physiologic
correction of the circulation by redirecting the pulmonary and systemic venous
returns to the appropriate ventricles using intra-atrial baffles (Mustard
or Senning procedure).67 This atrial switch operation
has met with great success in the early operative period, and survival rates
at 10 and 20 years postoperatively of 90% and 80%,
respectively.67 Enthusiasm for this approach has been
dampened somewhat due to concerns that the right ventricle may be
unable to function successfully as the systemic ventricle for long
periods of time.
++
Anatomic correction of transposition is now commonly performed.68 The
procedure consists of switching the great vessels to the proper ventricles
and reimplantation of the coronary arteries to the newly formed
aorta. The morphologic left ventricle then becomes the systemic
pumping chamber for which it is better suited. Although technically
demanding, the procedure is best performed during the neonatal period,
prior to the regression of left ventricular mass as a result of
the progressive decrease in pulmonary vascular resistance that occurs
after birth. Alternatively, the left ventricle can be conditioned
to accept the high systemic vascular resistance by performing pulmonary
banding as an initial procedure. Concerns have been raised since
the inception of the arterial switch procedure about the possibility
that distortion or growth failure of the newly implanted coronary
arteries may result in myocardial ischemia and possibly infarction.
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Some studies suggest that coronary artery manipulation and reimplantation
do not result in late myocardial perfusion abnormalities, at least
in those infants with successful initial results.68 However,
large perfusion abnormalities have been seen in the territories
of stenosed coronary arteries following the switch procedure (eFig. 494.10).
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Given the remarkable advances over the past decade and the expected
advances over the next decade, it is an exciting time in cardiovascular
imaging. The future of magnetic resonance imaging (MRI) holds parallel processing
and navigator pulses, which will decrease scan times and decrease
need for sedation or anesthesia. Real-time imaging, MRI-guided interventions,
myocardial-tagging assessment of wall strain, and 7-dimension flow
techniques will reach the clinical armamentarium in the near future, making
MRI an even more useful tool in evaluating and managing this patient
population. The evolution of higher-field strength scanners (3 tesla)
may augment some MRI tools. The future of CT includes dual-source
scanners, which will decrease scan times and radiation dose, further
improve spatial resolution, and improve temporal resolution (necessary
for functional assessment). Radionuclide methods often lack sufficient
resolution to precisely characterize complex morphology in congenital
heart lesions. However, these methods provide accurate and reproducible quantitative
assessment of the physiological consequences of structural heart
disease.
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The field of congenital heart disease is advancing rapidly and,
as a result, patients with complex disease are surviving and patients
in general are living longer. Management of their complex residual cardiovascular
disease requires close monitoring. The growth of the field has challenged
and will continue to challenge the noninvasive imaging arena to
advance.