Heart

Evan J. Zucker

Mark C. Liszewski

Bernard F. Laya

Ricardo Restrepo

Edward Y. Lee

INTRODUCTION

Imaging of the pediatric heart is one of the most complex topics in radiology. Expert interpretation requires intricate knowledge of embryology and normal anatomy, myriad relevant pathologies, and the expected posttreatment appearances of congenital heart disease (CHD). In addition, the heart is among the most technically challenging structures to image, mainly due to cardiac and respiratory motion and the need for nonstandard imaging planes. Echocardiography, supplemented by cardiac catheterization, is typically within the domain of pediatric cardiologists and remains the most commonly used modality for imaging pediatric cardiac disorders. Nevertheless, continuing advances in cross-sectional noninvasive imaging, such as computed tomography (CT) and magnetic resonance imaging (MRI), have solidified a role for radiologists’ imaging expertise and interdisciplinary collaboration in this complex specialty.

While a single chapter cannot cover the subject of pediatric cardiac disorders in its entirety, the aim of this chapter is to review the major modalities and entities with which general and pediatric radiologists and other clinicians caring for pediatric patients with cardiac disorders should be familiar. This chapter first discusses embryology, anatomy, and anatomic variants. Up-to-date imaging techniques are also reviewed, followed by discussion of the spectrum of pediatric cardiac disorders. In addition, life support and surgical devices used for pediatric cardiac patients are discussed. Furthermore, this chapter provides essential information related to surgical procedures commonly used in the treatment of CHD. Lastly, the current role of imaging in surgically corrected and adult CHD is reviewed.

ANATOMY

Embryology

The human heart begins to form during the gastrulation phase in the third week of development (Fig. 4.1). A subset of mesodermal tissue forms a crescent at the upper border of the embryonic disc. As the disc folds, the heart becomes repositioned and forms a tube made of endocardial cells with an internal lumen. The tube is in turn surrounded by myocardial cells, which become positioned within the developing pericardial cavity. Initially, the tube assumes the shape of an inverted “Y.” The “arms” give rise to the atria. A junctional component gives rises to the atrioventricular (AV) canal. Venous pathways drain to the atria via the left and right sinuses. At the same time, cells located distantly in the developing pericardial cavity migrate to the primary cardiac region and give rise to the primitive right ventricle and outflow tract. As the heart tube elongates with these new components, the developing left ventricle becomes unhinged from the mediastinum, which allows further development.¹,²,³,⁴

At this stage, looping of the heart tube occurs (Fig. 4.1). Through signaling pathways, the heart normally loops in a rightward direction and develops typical right- and left-sided structures. The ventricular loop contains inlet and outlet components. The outlet communicates with the outflow tract, which then communicates with the developing aorta and pharyngeal arches.¹,²,³,⁴

The next stage of cardiac development involves the formation of the cardiac chambers (Fig. 4.2). The heart tube divides into atrial and ventricular components that are separated by an AV canal and a common outflow tract. The systemic venous tributaries drain to the primary atrium. A constriction at the primary interventricular foramen develops between the inlet and outlet portions of the ventricular loop, giving rising to the left and right ventricles. As the lung buds form, a plexus of vessels connects to the heart tube atria via an initially solitary pulmonary vein. The atrial appendages are distinct, and their formation distinguishes the atria. Further ballooning of the heart tube leads to differentiation of the ventricles, with their characteristic differences in trabeculation. A common arterial trunk eventually separates into the aortic and pulmonary trunks.¹,²,³,⁴,⁵

FIGURE 4.1. Development of the human heart. This diagram shows the embryological development of the human heart during the first 5 weeks.

Normal Development and Anatomy

The key to understanding both normal and pathological cardiovascular anatomy lies in a rational and systematic approach to description. The most useful and universally adopted descriptive process is known as “segmental analysis.” The major approaches to segmental analysis were developed by the Van Praaghs, a married pair of pediatric cardiac pathologists at Boston Children’s Hospital, and Anderson, a London pediatric cardiologist. The Van Praagh notation is most commonly utilized in our practice and described herein.¹,²,³,⁴

FIGURE 4.2. Development of human heart. This diagram shows the formation of the four heart chambers.

Evaluation begins with assessment of visceral situs in the abdomen and chest. The normal situs configuration of the abdomen (situs solitus) is a single spleen in the left upper quadrant. The normal situs in the chest is best established by
atrial appendage morphology. The normal left atrial appendage (LAA) is an elongated, hooked, tubular structure with crenulations and a small junction with the venous portion of the atrium. In contrast, the typical right atrial appendage (RAA) is triangular and broad with a broad connection to the venous inflow tract. On the right, a groove or crest demarcates the appendage from the atrial body; on the left, this is not present. If atrial appendage identity cannot be reliably established, another methodology for establishing situs is the relationship of the airways to the pulmonary arteries. Normally, the right upper lobe bronchus is eparterial, with a takeoff superior to the origin of the right pulmonary artery. In contrast, the left upper lobe bronchus is normally hyparterial, taking off below the origin of the left pulmonary artery. Complete mirror-image situs anatomy is known as situs inversus. Other variations are known as situs ambiguous or heterotaxy and are most closely associated with underlying CHD.¹,²,³,⁴

Once situs is established, caval inflow should be determined. Normally, a single right superior vena cava (SVC) and a single left inferior vena cava (IVC) drain directly into the right atrium without any obstruction along the pathway. Next, pulmonary venous anatomy should be established. Most commonly, two right (upper and lower) and two left (upper and lower) pulmonary veins drain into the left atrium in an unobstructed fashion, without evidence of stenosis or anomalous venous return.¹,²,³,⁴,⁶,⁷

After caval and pulmonary venous drainage are determined, the cardiac anatomy is described. As detailed above, the right and left atria may be distinguished by differing appearances of their appendages and are normally separated by an intact interatrial septum. The AV connections are established next: the right atrium (RA) is normally contiguous with the right ventricle (RV) and the left atrium (LA) is contiguous with the left ventricle (LV). Normally, two distinct AV valves and an endocardial cushion are present.¹,²,³,⁴,⁵

Identification of the RV and LV is performed next. The RV and LV, separated by the interventricular septum, have several important morphologic landmarks that allow them to be differentiated from one another. The morphologic RV contains the moderator band and is heavily trabeculated, with papillary muscle attachments to both the interventricular septum and the free wall. In contrast, the LV is smooth walled and bullet shaped, with thin trabeculae and papillary muscle attachments only to the free wall.⁵

Looping of the ventricles should also be established. D(extro)-looping of the ventricles is the normal configuration. In most instances, if the RV is to the right of the LV, the ventricles are in a D-loop position. Otherwise, the ventricles are considered to be in an L-loop position (with “l” standing for “levo”). However, this method may fail in certain cases of complex CHD. It is more accurate to establish ventricular looping using the right-hand rule. One must imagine placing the palm of the hand along the septal surface of the morphologic RV, with the fingers pointed toward the outflow and thumb extended toward the inflow. This is only possible with either the right or left hand. The ventricles are D-looped if there is right-hand topology and L-looped if there is left-hand topology. Finally, the position of the ventricular apex of the heart, normally on the left (levocardia), should be established. Other configurations include a right-sided apex (dextrocardia) or central apex position (mesocardia). It is important to recognize that the position of the ventricular apex does not necessarily match ventricular looping, which is a separate parameter.¹,²,³,⁴

The next step in segmental analysis is to establish the ventriculoarterial connections. If the RV is contiguous with the main pulmonary artery (MPA) and the LV is contiguous with the aorta, the relationships are concordant. In addition, the relative position of the great arteries (MPA and aorta) should be classified. Normally, there is slight anterior-posterior offset of the great arteries, with the ascending aorta coursing posterior and to the right of the MPA (“D-position”). Reversal of this relationship is termed “transposition” (“L-position”). Malposition is characterized by a side-by-side configuration of the great arteries (no anterior-posterior offset), with “d-” or “L-” indicating whether the aorta is to the right or left of the MPA, respectively.¹,²,³,⁴

The final components of segmental analysis include assessment of the coronary arteries, aorta, and PAs. Normally, the right coronary artery (RCA) arises from the right sinus of Valsalva and the left coronary artery arises from the left sinus of Valsalva. The aortic arch is normally left-sided, with three vessels originating from the arch (the brachiocephalic or innominate, left common carotid, and left subclavian arteries). The presence of abnormal dilatation (aneurysm), coarctation (segmental narrowing), or aortopulmonary collaterals (aorta-PA connections) should be established. The MPA normally branches into right and left PAs and then lobar, segmental, and subsegmental branches paralleling the airways. Patency, dilatation, and stenosis of the PAs (if present) should be noted.¹,²,³,⁴

While a descriptive approach to segmental anatomy as above is encouraged, a three-letter notation is often used to summarize the major components of segmental analysis. The first letter denotes the situs (S-solitus, I-inversus, or A-ambiguous). The second letter denotes the ventricular looping pattern (D- or L-). The third letter denotes the great vessel position (S-solitus, I-inversus, d-TGV: D-transposition of the great vessels, l-TGV: L-transposition of the great vessels, d-MGV: D-malposition of the great vessels, or l-MGV: L-malposition of the great vessels). Thus, the normal segmental anatomy is summarized as {S, D, S}.¹,²,³,⁴

Anatomic Variants

While an extensive review of all cardiovascular anatomic variants is beyond the scope of this chapter, there are several commonly encountered variants with which the radiologist should be familiar. Many patients with complete situs inversus are asymptomatic and may be undiagnosed until medical imaging is performed for other purposes. A single left SVC draining to the coronary sinus or duplicated SVCs with or without a bridging innominate vein is not uncommon
and may be asymptomatic⁶ (Fig. 4.3). Pulmonary venous branching is highly variable. Three right pulmonary veins (upper, middle, and lower) or two left pulmonary veins with a common ostium are common variants. In addition, the ostium of the left lower pulmonary vein is often smaller than that of other pulmonary veins, as it drapes over the descending thoracic aorta.⁷

FIGURE 4.3. A 16-year-old boy with an incidentally detected left-sided superior vena cava on computed tomography (CT) obtained for evaluation of a right paratracheal mass which was found to be a right-sided aortic arch. A: Coronal enhanced CT image shows a left-sided superior vena cava (arrow) and a right-sided aortic arch (RA). B: 3D volume-rendered CT image in a sagittal oblique plane demonstrates a leftsided superior vena cava (arrow).

FIGURE 4.4. A 4-year-old boy with an incidentally detected ductus bump. Sagittal enhanced maximum intensity projection CT image shows an outpouching (arrow) of the proximal descending aorta at the origin of the ligament arteriosum just distal to the origin of the left subclavian artery, which is a normal variant known as the “ductus bump” or “ductus diverticulum.”

Anomalies of coronary artery origin or course, even when asymptomatic and benign, may be of importance if surgery is planned. In addition, while the RCA is most often dominant (supplying the posterior descending [PDA] and posterior left ventricular [PLV] arteries), the left circumflex (LCx) may be dominant or there may be codominance (with the RCA supplying the PDA and the LCx supplying the PLV); these variations are not pathologic.⁸

Aortic arch branching is variable; common variants include a two-vessel arch with a common origin of the brachiocephalic and left common carotid arteries (bovine arch) and a left common carotid artery arising from the brachiocephalic artery. Another common arch variant is a four-vessel left-sided arch with the left vertebral artery arising directly from the arch between the left common carotid and left subclavian arteries. Slight outpouching of the proximal descending aorta at the origin of the ligament arteriosum just distal to the origin of the left subclavian artery is a normal variant known as the “ductus bump” or “ductus diverticulum” that should not be confused with posttraumatic pseudoaneurysm (Fig. 4.4). Finally, bronchial arteries arising from the descending aorta may hypertrophy in the setting of lung disease to provide collateral supply to the lungs.⁹,¹⁰,¹¹

IMAGING TECHNIQUES

Radiography

Although they are rarely able to render a precise diagnosis, chest radiographs (CXRs) nonetheless remain invaluable in the initial imaging assessment of CHD and may be the
first examination to suggest an underlying cardiovascular malformation. A systematic approach helps to avoid overlooking potential interpretive clues. Postsurgical material, support devices, and abnormal calcifications are usually plainly evident. Situs abnormalities can be readily detected by scrutinizing the stomach, spleen, and liver position and tracheobronchial branching pattern. While complete mirrorimage situs (situs inversus) is associated with no greater prevalence of CHD compared to normal situs (situs solitus), rates of CHD are significantly greater with intermediate situs variations (situs ambiguous, heterotaxy). Similarly, an anomalous position of the great vessels may be visible and should raise suspicion for underlying CHD. For example, a (single) right aortic arch, which manifests on radiographs as a right aortic knob deviating the trachea leftward of its expected position (normally slightly rightward of midline), is associated with CHD in 90% of cases (Fig. 4.5). Finally, an abnormal position of the cardiac apex in a location other than the left hemithorax (levocardia), that is, in the right hemithorax (dextrocardia) or in the midline (mesocardia), warrants attention. Of note, the apex position is not dependent on the anatomic arrangement of intracardiac and visceral structures.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸

FIGURE 4.5. A 14-year-old girl with a right aortic arch with an aberrant left subclavian artery. A: An axial enhanced CT image shows a right aortic arch (RA) and an aberrant left subclavian artery (arrow). T, trachea. B: Three-dimensional volume-rendered CT image in posterior view demonstrates a right aortic arch (RA) and an aberrant left subclavian artery (arrow) with compressed trachea (T).

The heart size and shape should also be assessed on every radiograph, although abnormalities are nonspecific. A cardiothoracic ratio (CTR; calculated as the maximal transverse heart diameter divided by the maximal transverse thoracic diameter from the inner edge of the ribs and pleura) of ≥0.5 is a useful indicator of cardiomegaly in older children undergoing posteroanterior (PA) radiography. However, the same tool is not reliable in neonates, in whom prior interpretative experience often ultimately proves most useful. Enlargement of the cardiac silhouette does not necessarily indicate intrinsic cardiac pathology. In fact, it may be due to extracardiac pathology such as anemia or an arteriovenous malformation or attributable to prominent normal thymic tissue. While certain cardiac shapes are classically associated with specific CHDs (as described later in this review), they are neither commonly encountered nor reliable for making a definitive diagnosis.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸

Although it can prove challenging in practice, an attempt should also be made to assess the pulmonary vasculature and pulmonary blood flow. Decreased pulmonary arterial blood flow, in which the hilum and vessels appear small and the lungs appear overly radiolucent, may be seen in cyanotic CHD. Increased pulmonary arterial blood flow, in which the vessels dilate and visible markings extend to the lateral third of the lungs, may be observed in left-to-right shunts and admixture lesions (Fig. 4.6). Pulmonary arterial abnormalities must be distinguished from pulmonary venous congestion and edema, in which the vessels appear hazy and ill-defined due
to pulmonary venous dilatation and migration of fluid into the perivascular tissues; such findings may be seen, for example, with impaired myocardial contractility. Concomitant lung disease may obscure the pulmonary vasculature, while enlargement of pulmonary vessels or cardiac chambers may secondarily cause focal atelectasis or air trapping.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸

FIGURE 4.6. A 2-month-old girl with increased pulmonary vascularity and cardiomegaly due to underlying atrial septal defect. Frontal chest radiograph shows increased pulmonary arterial blood flow, in which the vessels dilate and visible markings extend to the lateral third of the lungs due to underlying atrial septal defect. Cardiomegaly is also seen.

A variety of extracardiovascular findings appreciable on CXRs should raise suspicion for CHD or an associated syndrome. Osseous features associated with CHD include sternal hypoplasia, absence, or premature fusion; pectus excavatum; scoliosis; vertebral segmentation anomalies; abnormal number and morphology of ribs; and rib notching. In addition, a small thymus is typical of cyanotic infants subject to extreme stress and DiGeorge syndrome, which is associated with aortic arch abnormalities. Nonetheless, full characterization of any CHD requires additional imaging.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸

Echocardiography

Noninvasive, portable, widely available, and effectively riskfree, transthoracic echocardiography (TTE), or cardiac ultrasound, is generally regarded as the first-line and most important diagnostic test in the evaluation of CHD. In many cases, it can provide a precise anatomical roadmap, helping to reduce cardiac catheterization exam times and even sufficing for presurgical planning. In the same examination, cardiac function can be assessed. In addition, the use of Doppler techniques allows assessment of blood flow velocity and direction, facilitating estimation of shunt sizes and valvular gradients. Moreover, echocardiography provides excellent imaging guidance for certain interventions such as balloon atrial septostomy (Rashkind procedure). The advent of three-dimensional (3D) TTE, which can produce a dynamic 3D volume-rendered display of sonographic data, may also improve visualization of aortoseptal continuity and structures such as the interatrial septum and mitral valve.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰

TABLE 4.1 Sample CT Angiography Contrast Injection Protocols with Fixed Scan Delay

Weight (kg)	Injection Vol. (mL)	Contrast %	Saline Flush (mL)	Rate (mL/s)	*Delay (s)^,^†**
>2	15	30	5	1	18
>2.5	15	50	5	1	18
>3	15	60	5	1	18
>4	15	70	5	1	18
>5	15	80	5	1	18
>6	18	70	5	1	22
>7	18	80	10	1	22
>8	18	90	10	1	22
>9	18	100	10	1	22
>10	20	100	10	1	24
>11	30	70	10	1.5	24
>12	30	80	15	1.5	24
>13	30	90	15	1.5	24
>14	30	100	15	1.5	24
>15	2 mL/kg (max 150 mL)	100	20	Adjust rate for 30 s bolus duration	33
^*Add 3 s if lower extremity intravenous catheter is used. ^†For Fontan physiology, inject 2 mL/kg contrast at 1 mL/s and empirically scan at 2 min.

Nevertheless, TTE has several limitations. It is highly operator dependent, and thus, findings and measurements may not be reliably replicated between examinations. For example, one study found a 53% rate of major or moderate diagnostic error in pediatric echocardiograms performed at adult community hospitals. In addition, acoustic windows may be suboptimal, limiting visualization; this problem is more likely in patients who have undergone surgery or have a large body habitus, severe emphysema, or narrow intercostal spaces. Even in the best hands, TTE generally underperforms in the assessment of structures including the thoracic aorta, PAs, pulmonary veins, and coronary arteries, particularly with complex malformations.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰

Transesophageal echocardiography (TEE) is a useful supplement to TTE and may offer new data, alter the underlying diagnosis, or provide intraoperative imaging guidance. However, TEE is invasive, with potential risks related to anesthesia, peri-procedural infection, and correct technical performance. Moreover, TEE is still operator-dependent with limited ability to adequately image structures such as the right ventricular outflow tract (RVOT), apicoposterior septum, pulmonary valve, proximal left PA, and distal right PA. Other limitations include restricted view planes and blind spots that may arise, for example, when implanted graft material is present. When echocardiography is insufficient, CT, MRI, or invasive angiography may be warranted.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰

Computed Tomography

Multidetector CT (MDCT) with angiographic technique allows rapid and exquisite anatomical assessment of the heart and great vessels in a single acquisition (Tables 4.1, 4.2 and 4.3).
Extracardiac structures such as the lungs and airways are also optimally evaluated. Postprocessing software allows multiplanar reformatting, maximum and minimum intensity projection reconstructions, and 3D volume rendering, facilitating visualization of complex CHD for diagnostic and presurgical planning purposes. Electrocardiographic (ECG) gating may be used not only to minimize image degradation from cardiac motion (e.g., as may be needed for coronary imaging) but also to quantify ventricular and regurgitant volumes, ventricular function, myocardial mass, cardiac output, shunt flow, and pulmonary to systemic flow ratios (Qp:Qs).¹²,¹⁴,¹⁵,²¹,²²,²³,²⁴,²⁵,²⁶

TABLE 4.2 CT Angiography Scan Considerations by Indication

Indication	Gated?	High-Pitch?	Breath-Hold?	Contrast Timing
Coronary artery anomaly	Yes	No	Yes	Bolus timing
Inflammatory coronary artery disease (Kawasaki)	Yes	No	Yes	Bolus timing
Newborn (intracardiac anatomy not critical)	No	Yes	No	Fixed or bolus timing
Newborn (intracardiac anatomy critical)	Yes	No	Yes	Fixed or bolus timing
Function (e.g., tetralogy of Fallot with pacemaker)	Yes (↓dose)	No	Yes	Fixed or bolus timing
Fontan	No	Yes	No	Fixed at 2 min
Pulmonary embolism	No	Yes	If possible	Fixed or bolus timing
Ventricular assist device	No	Yes	If possible	Fixed at 2 min

Exposure to ionizing radiation is a chief drawback of CT, with increased doses when cardiac gating is utilized. Prospective ECG triggering with axial sequential scanning can substantially reduce dose compared to retrospectively gated acquisitions (even with tube current modulation), while still providing reliable coronary artery evaluation. Yet, accurate functional assessment is not possible. Even lower doses may be achieved when high-pitch helical technique is combined with prospective triggering. For example, a recent study comparing prospectively ECG-gated 320-MDCT to ungated helical 64-MDCT in neonates and infants showed submillisevert (mSv) effective doses and subjectively better image quality in the 320-MDCT group, while effective doses were on average nearly 5 mSv in the 64-MDCT cohort.²⁴ Of course, without sufficient padding of the cardiac cycle, high-pitch helical acquisition may not ensure a reliably motion-free coronary phase, in which case retrospective or standard prospective scanning may be preferred.

In general, low²⁷,²⁸,²⁹,³⁰,³¹,³²,³³,³⁴,³⁵,³⁶,³⁷ kilovoltage peak (kVp) settings, with concomitant increase in tube current as needed, permit substantial dose reduction (e.g., 40% decrease when lowering kVp from 120 to 80). Iterative reconstruction algorithms also allow for reduced image noise at the same exposure, thus permitting dose decreases with maintained image quality. In addition, it should be remembered that a high-quality CT may decrease the number of runs needed in the cardiac catheterization laboratory or obviate the need for invasive angiography altogether, thus indirectly potentially lowering overall radiation dose. The theoretical risks of radiation exposure such as future cancer should also be weighed against competing risks such as surgical mortality.¹²,¹⁴,¹⁵,²¹,²²,²³,²⁴,²⁵,²⁶

TABLE 4.3 Representative Thoracic CT Angiography Scan Parameters

Weight (kg)	Pitch	Rotation time (sec)	Thickness (mm)	Ref. kVp	Ref. mAs
<55 kg	3.0^*	0.28	0.5-1 (50% overlap)	70-80	200
≥55 kg	3.0^*	0.28	1.5 (50% overlap)	100	160
^*Requires high-pitch scanning capability. If not, lower pitch (may require breath-hold).

Additional downsides to CT include the need for intravenous line (IV) placement, administration of iodinated contrast, and anesthesia (in some cases) to help reduce patient motion. Although the risks of an idiosyncratic allergic reaction and nephrotoxicity from iodinated contrast cannot be entirely removed, overall, nonionic low-osmolality agents such as iohexol (Omnipaque) and iopamidol (Isovue) have lower rates of adverse reactions. Iso-osmolar agents such as iodixanol (Visipaque) may be safer in premature infants and patients with known renal insufficiency. Achieving optimal contrast opacification in neonates can be technically challenging in view of 2 mL/kg contrast dose limits and 10 mL/kg total fluid limits. Injection protocols should be tailored to allow an injection duration for the entire scan length, including monitoring for peak enhancement of the area of interest, moving the CT table to the starting position while prompting a breath-hold (if one is needed), and actually performing the diagnostic scan. A foot injection may be preferred to an arm injection if dense contrast in the SVC may obscure the anatomy of concern. The potential risks of anesthesia, albeit uncommon, must be weighed against the potential diagnostic benefits of the scan. On the upside, anesthesia requirements for CT are typically less than those for MRI, which requires MR compatible devices that are less accessible to anesthesiologists and, usually, a longer depth and duration of sedation. Moreover, the rapid scan times of modern CT increasingly allow successful “awake” acquisition in patients who would have previously required anesthesia. Ultimately, if functional and flow information are
required, MRI is generally preferred to CT, unless patients are too ill to tolerate its longer scan times or it is contraindicated due to incompatible devices.¹²,²¹

Magnetic Resonance Imaging

In use for more than 20 years, MRI is among the most powerful imaging tools for evaluation of structural CHD (Tables 4.4 and 4.5). In a single examination, intracardiac and vascular anatomy as well ventricular function and blood flow can be accurately assessed. MRI images can be obtained in any plane, with capabilities for 3D visualization, without the “acoustic window” or operator-dependency limitations of echocardiography. Additionally, MRI does not expose the patient to ionizing radiation, and diagnostic imaging may be obtained even without IV contrast. Traditional MRI studies for CHD include some combination of spin-echo “black-blood” sequences, gradient-echo “bright-blood” cine sequences, MR angiography (MRA), and velocity-encoded phase-contrast (PC) sequences. While protocols must be tailored to the clinical indication, a typical CHD exam may include: 1) localizers; 2) bright-blood cine images in the four chamber, LV and RV two and three chamber, and RVOT planes; 3) twodimensional PC imaging at the level of the aortic and pulmonic valves; 4) contrast-enhanced MRA timed to the aorta or PA; and 5) possibly delayed enhancement images if assessment for viability or cardiomyopathy is needed. Additional sequences for myocardial tissue characterization, such as perfusion and T1 mapping, are less commonly needed for CHD evaluation.¹²,¹⁴,¹⁵,³⁸,³⁹

Traditionally, a major limitation of pediatric cardiovascular MRI has been the requirement for special technical expertise. As more physicians have been trained in this technique, it has become more widely available. Careful patient selection is required to determine whether MRI is the study of choice for a given indication, and attention must be paid to minimizing image artifacts. Prosthetic graft material and calcification are, in general, not optimally imaged by MRI due to their intrinsic signal characteristics. Motion and respiratory artifacts have traditionally been setbacks, although novel techniques such as motion compensation and compressed sensing have made free-breathing cardiac MRI a possibility. Despite the fact that it utilizes ionizing radiation, CT is preferred on account of its superior spatial resolution when anatomic imaging of the lungs, coronary arteries, or small pulmonary vessels is a primary goal of the exam, while MRI is more optimal for function and flow imaging.¹²,¹⁴,¹⁵,³⁸,³⁹

TABLE 4.4 Representative CMR Protocol for Structural Heart Disease

Goals: Evaluate biventricular function and delayed (late) gadolinium enhancement pattern
Sequence	Acquisition Planes
Cine “Bright-Blood” GRE	SAX stack, 4-Ch, LV 2/3 Ch Add AX stack for ARVD Add LV 3-Ch stack for HCM
T2 (DIR)	SAX stack if concern for myocarditis (edema)
T1	Select AX slices if ARVD for RV fat assessment (optional—not part of criteria)
2D Phase Contrast	Consider in LVOT plane if HCM (for peak velocity/gradient)
Delayed (Late) Gadolinium Enhancement	Prior to scanning, inject 0.2-0.3 mL/kg extracellular contrast agent, wait 8-10 min SAX stack, 4-Ch, LV 2/3 Ch
CMR, cardiac magnetic resonance; GRE, gradient echo; SAX, short axis; Ch, chamber; LV, left ventricle; AX, axial; ARVD, arrhythmogenic right ventricular dysplasia; HCM, hypertrophic cardiomyopathy; DIR, double inversion recovery; RV, right ventricle; LVOT, left ventricular outflow tract.

TABLE 4.5 Representative CMR Protocol for Congenital Heart Disease (Repaired Tetralogy of Fallot)

Goals: Evaluate RV size/function, pulmonic regurgitation/stenosis, branch PA stenosis, Qp:Qs
Sequence	Acquisition Planes
Cine “Bright-Blood” GRE	SAX stack, 4-Ch, LV/RV 2/3 Ch, RVOT Ao/PA LAX localizers
2D Phase Contrast (2D PC)	Aortic and pulmonic valves Consider RPA/LPA for split flow calculation Other valves if regurgitant/stenotic
Contrastenhanced MRA	Sagittal or coronal acquisition, then reformat ECG-gated if available and breath-hold capacity tolerates
4D Flow	If available at institution. Choose VENC (typically 250-350 cm/s) based on 2D PC.
CMR, cardiac magnetic resonance; GRE, gradient echo; SAX, short axis; Ch, chamber; LV, left ventricle; RV, right ventricle; RVOT, right ventricular outflow tract; Ao, aorta; PA, main pulmonary artery; LAX, long axis; RPA, right pulmonary artery; LPA, left pulmonary artery; ECG, electrocardiogram; VENC, velocity-encoded gradient.

Unique challenges in performing cardiac MRI in children with CHD include planning appropriate imaging planes efficiently in real-time despite complex anatomy and achieving adequate visualization of small structures in the face of high heart rates. However, the landscape has changed with the advent and growing availability of 3D time-resolved (4D) flow MRI. This novel technique captures morphologic, function, and flow data from both intra- and extracardiac structures as a single 3D volumetric acquisition, performed with free breathing within 10 to 15 minutes. No special knowledge of often complex CHD anatomy is required before the scan is performed. Blood flow can be retrospectively quantified at any desired region of interest after the scan is performed. Moreover, anesthesia requirements can be lessened compared to traditionally longer exams requiring breath-holding. Although validation studies are ongoing, studies to date have shown good correlation between 4D flow and traditional sequences for quantifying ventricular volume, mass, and function, as well as blood flow. Workflow issues are a current drawback to 4D flow imaging, which amasses large datasets
requiring long reconstruction times and specialized software; however, these shortcomings are being addressed.¹²,²¹,⁴⁰,⁴¹,⁴²,⁴³,⁴⁴,⁴⁵,⁴⁶,⁴⁷,⁴⁸,⁴⁹,⁵⁰,⁵¹ The addition of ultrashort echo time (UTE) MR sequences can also allow simultaneous assessment of the airways and lungs, if indicated.⁵¹

In cardiac MRI applications, the use of IV contrast agents is more optimal than time-of-flight technique in 4D flow acquisition due to superior signal-to-noise characteristics in magnitude data and reduced noise in velocity data. Extended preservation of signal may be needed in cardiac imaging applications, necessitating use of contrast agents with relatively prolonged intravascular retention and long elimination half-times. At Stanford, ferumoxytol (Feraheme, AMAG Pharmaceuticals, Waltham, MA), an ultrasmall superparamagnetic iron oxide nanoparticle, has been preferred as an off-label IV contrast agent for more than 4 years. Ferumoxytol, which was originally developed for the treatment of iron-deficiency anemia, has been found to provide qualitatively greater vascular enhancement than the gadolinium-based blood pool agent gadofosveset (Ablavar, Lantheus Medical Imaging, Inc., North Billerica, MA) and has a circulating half-life of 14 to 15 hours. Thus, ferumoxytol permits longer acquisition times than gadolinium-based agents, which helps to improve the signal-to-noise ratio and allows for greater motion averaging. A notable drawback is increased risk of acute adverse allergic reactions compared to gadolinium-based agents; in fact, the United States (U.S.) Food and Drug Administration (FDA) strengthened warnings about the agent in March 2015. Nevertheless, ferumoxytol has a favorable safety profile when administered as a slow infusion of diluted agent and allows for reliably high image quality with lighter anesthesia requirements. Unlike gadolinium-based agents, ferumoxytol does not pose a known risk of nephrogenic systemic fibrosis (NSF) and may be administered in patients with renal failure.¹²,²¹,⁴⁰,⁴¹,⁴²,⁴³,⁴⁴,⁴⁵,⁴⁶,⁴⁷,⁴⁸,⁴⁹,⁵⁰,⁵¹

Because they quickly equilibrate between the intravascular and extravascular spaces and have a relatively short elimination half-time of 70 to 90 minutes, traditional extracellular gadolinium agents are not useful for many cardiac imaging applications. Gadolinium-based blood pool agents such as gadofosveset, which provides adequate vascular imaging for up to 45 to 60 minutes after administration and has an elimination half-time of up to 18 hours, are more suitable; however, gadofosveset is no longer readily available for purchase in U.S. markets.¹²,²¹,⁴⁰,⁴¹,⁴²,⁴³,⁴⁴,⁴⁵,⁴⁶,⁴⁷,⁴⁸,⁴⁹,⁵⁰,⁵¹

In addition to the above-described considerations related to contrast agents, general safety limitations and contraindications to MRI are important to consider, especially in patients with CHD. This patient population often has surgically implanted metallic materials that may move or heat during scanning and pacemakers that require calibration using electromagnetic devices. Pacemakers have traditionally not been MR compatible, although scanning may now be possible with some newer MR conditional models. Claustrophobia may be a concern in some patients but can usually be overcome with only mild sedation, especially with the aid of child life specialists.¹²,²¹

Nuclear Medicine

Radionuclide (nuclear medicine) imaging allows quantification of cardiac function and shunts. However, its use has declined with the growing adoption of competing modalities such as MRI that can provide equally accurate data with better anatomic definition and no ionizing radiation exposure. Nevertheless, selected applications for radionuclide imaging in CHD remain in use. These include ventilation-perfusion lung scintigraphy to identify and quantify abnormal patterns of pulmonary blood flow in patients with anomalous PAs, and stress-rest myocardial perfusion scans to evaluate for myocardial ischemia and infarction in individuals with coronary artery abnormalities. Radionuclide ventriculography (multiple-gated acquisition or MUGA scan) provides an accurate assessment of LV ejection fraction (LVEF), which may be followed serially, for instance, in oncology patients receiving cardiotoxic doxorubicin. In general, if MRI is contraindicated or not tolerated, nuclear medicine techniques provide a viable alternative. Hybrid radionuclide and anatomic imaging with combined cardiac positron emission tomography (PET) and MRI may present novel information in the assessment of CHD, although this modality is currently in early stages of development.¹²,¹⁴,¹⁵

Cardiac Catheterization

Cardiac catheterization has been considered the “gold standard” in CHD imaging for more half a century. Typical indications include coronary and hypoplastic PA assessment and presurgical planning. At the same time as providing anatomic information, invasive angiography allows measurement of pressure gradients and oxygen saturations, helping to define the direction and severity of shunts. Cardiac morphology and function are also delineated. Due to the dynamic nature of the exam, pulmonary vascular reactivity to vasoactive agents can also be determined. In addition, some interventions can be readily performed at the time of catheterization, such as balloon atrial septostomy, valvuloplasty, angioplasty, patent ductus arteriosus (PDA) and aortopulmonary collateral closure, and stent placement.¹²,²⁰

Nevertheless, with continued advances in noninvasive modalities such as cardiac CT and MRI, the routine use of cardiac catheterization has declined, particularly for less complex CHD. Even if catheterization cannot be avoided, supplementary noninvasive imaging may shorten procedure times. As an invasive procedure, catheter angiography carries risks of arterial and venous injury, bleeding, stroke, and even (rarely) death. Risks associated with IV contrast administration (allergic reaction, renal failure) and ionizing radiation exposure must also be considered. The radiation exposure from catheterization can be orders of magnitude higher than that of CTA; for example, in one retrospective cohort, the median effective dose for nongated chest CTA
was 0.76 mSv for children under one year of age compared to 13.4 mSv for catheterization.²³ Thus, it is not surprising that many institutions reserve catheterization for situations in which highly accurate hemodynamic data are crucial or interventional procedures are anticipated.

Advanced Visualization

The radiologist’s responsibility in pediatric cardiovascular imaging does not end with generating an accurate report. Indeed, his or her most important role is often to create a decipherable depiction of often complex anatomic and physiological data for the cardiologist preparing for catheterization or the surgeon preparing to operate. To that end, a variety of advanced visualization software packages allow direct importing of CT and MR data for production of multiplanar and 3D/4D representations. Such features may be directly accessible through the Picture Archiving and Communication System (PACS). A commonly used third-party software package known as AquariusNET (AQNET) is available from TeraRecon (Foster City, CA). The use of a thin-client model enables faster processing speeds; image data are uploaded to a secure host server, which then can be accessed and manipulated remotely from any number of remote clients that do not physically store the host data.

In recent years, there has also been increasing interest in and applications for 3D printing, in which CT or MR datasets can be transmitted to a 3D printer, allowing creation of an individualized model of cardiovascular anatomy and even defining the effect of precise cuts along specific planes of the cardiac anatomy. Such models can be handled and manipulated by the surgeon prior to an operative case to provide a contextually improved surface roadmap. As with any new technology, studies are ongoing to determine the added value of 3D printing relative to cost. In order to achieve an accurate model, CT/MR data must be accurately segmented, which can be time consuming. At present, a “4D” printed model including cardiac motion is not yet available.⁵²,⁵³,⁵⁴,⁵⁵

SPECTRUM OF CARDIAC DISORDERS

Congenital Heart Disease

Septal Defects

Atrial Septal Defects

Atrial septal defects (ASDs) are defined by abnormal communication between the RA and LA. The subtypes of ASDs, defined by their anatomical locations, are detailed subsequently. Most pediatric patients with ASDs are asymptomatic and present with an incidentally detected murmur upon physical examination. However, sizable defects resulting in large shunts may lead to CHF, failure to thrive, or recurrent infection. In general, CXRs with large ASDs may show cardiomegaly and increased vascular markings (Fig. 4.7). Echocardiography can show the location of the ASDs and quantify the severity. CT and MRI can depict the ASDs but are not usually needed; however, they may be helpful in clarifying and characterizing ambiguous or complex conditions such as a sinus venosus defect associated with partial anomalous pulmonary venous return (PAPVR). In addition, MRI allows quantification of the shunt ratio (Qp:Qs), which is an indication for ASD closure if >1.5:1. Cardiac catheterization is not routinely performed for ASDs. However, for secundum ASDs below device size thresholds that have adequate adjoining tissue (rims), minimally invasive transcatheter ASD closure with devices such as an Amplatzer (AGA Medical Corporation, MN, USA) or CardioSEAL (NMT Medical, Inc., MA, USA) septal occluder can now be performed.⁵⁶,⁵⁷,⁵⁸ The cardiac septal occluder device has a characteristic radiographic appearance when properly positioned⁵⁹ (Fig. 4.8).

FIGURE 4.7. A 3-year-old boy with an atrial septal defect. Frontal chest radiograph shows increased pulmonary vascularity and cardiac enlargement.

Patent foramen ovale

A patent foramen ovale (PFO) results from incomplete fusion of the ostium primum and ostium secundum portions of the atrial septum (Fig. 4.9). It is commonly asymptomatic and, in fact, is found in nearly one-third of the population. However, when right atrial pressure exceeds left atrial pressure (as in the setting of pulmonary hypertension), the PFO may open in a manner similar to a cardiac valve. In turn, thrombi from the RA can pass to the LA and cause paradoxical embolism. Most pediatric patients with an asymptomatic PFO do not require treatment. Even for pediatric patients with a PFO and cryptogenic stroke, the need for treatment is currently controversial. In a multicenter, randomized controlled trial of adult patients, PFO closure was overall not superior to medical therapy (anticoagulation) alone, although it did appear to be superior in selective prespecified secondary analyses.⁵⁶,⁶⁰,⁶¹

FIGURE 4.8. A 2-year-old boy with atrial septal occluder (ASO) placement due to an underlying atrial septal defect. (Reprinted from Lee EY, et al. Amplatzer atrial septal defect occluder for pediatric patients: radiographic appearance. Radiology. 2004;233(2):471-476, with permission) A. Frontal chest radiograph shows the ASO as a flat disk (arrows). B. Lateral chest radiograph demonstrates two flat disks. The left atrial disk (arrow) is larger than the right atrial disk (arrowhead).

Ostium primum atrial septal defect

Ostium primum ASD is located in the lower part of the atrium close to the AV valves (Fig. 4.10). Overall, it comprises ˜20% of ASDs and is usually associated with an anomaly of the AV valves, most commonly a cleft in the anterior leaflet of the mitral valve that causes regurgitation.⁵⁶,⁵⁷,⁵⁸,⁵⁹,⁶⁰,⁶¹,⁶²,⁶³,⁶⁴,⁶⁵

Ostium secundum atrial septal defect

Ostium secundum ASD is located in the region of the foramen ovale and at the center of the atrial septum (Figs. 4.11 and 4.12). It is the most common type of ASD, comprising 70% of cases. Rarely, it is associated with PAPVR.⁵⁶,⁵⁷,⁵⁸,⁵⁹,⁶⁰,⁶¹,⁶²,⁶³,⁶⁴,⁶⁵

FIGURE 4.9. An 11-week-old girl with a patent foramen ovale (PFO) who underwent cardiac computed tomography angiogram for assessment of innominate artery compression. An axial enhanced CT image shows a defect (asterisk) in the atrial septum consistent with a PFO, which was confirmed by subsequently obtained echocardiography. RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.

Sinus venosus atrial septal defect

Sinus venosus ASD is located at the connection between the vena cava and the RA (Fig. 4.13A). Overall, it comprises 10% of ASDs. Superior sinus venosus defect is located at the connection of the SVC to the RA, whereas inferior sinus venosus defect occurs at the junction of the IVC, right inferior pulmonary vein, and RA. In the superior sinus venosus defect, the
atrial septum is actually intact, but the border between the SVC and some of the right pulmonary veins is not, allowing pulmonary venous blood to pass directly into the SVC and then the RA. The atria then communicate at the pulmonary vein confluence.⁵⁶,⁶⁶,⁶⁷ Sinus venosus ASD is associated with right upper lobe partial anomalous venous return and persistent left SVC (Fig. 4.13B).

FIGURE 4.10. A 17-year-old boy with trisomy 21 and an ostium primum atrial septal defect (ASD). Bright-blood magnetic resonance image in a four-chamber plane shows a large ostium primum ASD (asterisk). RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.

FIGURE 4.11. A 5-year-old boy with ostium secundum atrial septal defect (ASD). Frontal chest radiograph shows increased pulmonary vascularity and cardiomegaly.

FIGURE 4.12. A 16-year-old boy with ostium secundum atrial septal defect (ASD). Bright-blood magnetic resonance image in a fourchamber plane shows an ostium secundum atrial septal defect (asterisk). RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.

FIGURE 4.13. An 17-year-old asymptomatic boy with a sinus venosus defect incidentally discovered on a sport participation screening echocardiogram. A: Bright-blood magnetic resonance image in a four-chamber plane shows a sinus venosus atrial septal defect (asterisk). RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle. B: Coronal bright-blood magnetic resonance image demonstrates partial anomalous venous return (arrow) from the right upper lobe to the superior vena cava (SVC). Left-sided superior vena cava (arrowhead) is also seen.

Coronary sinus atrial septal defect

Coronary sinus ASD is also known as the “unroofed coronary sinus” (Fig. 4.14). This defect is the rarest of the ASDs, comprising only 1%. The “roof” of the coronary sinus in the region of the LA is absent, allowing blood to pass from
the LA to the coronary sinus and then the RA. In turn, the coronary sinus becomes widened. This defect is typically associated with a left SVC and, in some cases, total anomalous pulmonary venous return (TAPVR) or heterotaxy syndromes.⁵⁶,⁶⁸,⁶⁹

FIGURE 4.14. A 16-year-old boy with coronary sinus atrial septal defect (ASD) also known as “unroofed coronary sinus.” Bright-blood magnetic resonance image in a four-chamber view shows a dilated coronary sinus (C) with a defect (arrow) between the coronary sinus and left atrium (LA), consistent with coronary sinus ASD.

Ventricular Septal Defects

Ventricular septal defects (VSDs) are characterized by defects in the interventricular septum, arising during the first 7 weeks of gestation. They are the most common congenital heart defect, comprising 40% of all cases. They may also occur in conjunction with more complex defects or as a manifestation of chromosomal (e.g., trisomy 21) or genetic (e.g., Holt-Oram syndrome) disease. The significance of a VSD varies according to its size and underlying hemodynamics of the systemic and pulmonary circulations. VSDs may be classified as small, mid-size, or large, depending on whether they are <50%, 50% to 100%, or >100% of the diameter of the aortic root. A restrictive VSD is usually a small VSD that has a gradient or increased velocity of flow by Doppler, whereas a nonrestrictive VSD is usually large and has no gradient of increased velocity of flow by Doppler. While small VSDs may be asymptomatic, large, hemodynamically significant VSDs may cause CHF, dyspnea, failure to thrive, sweating, hepatomegaly, and pulmonary hypertension.⁵⁶,⁷⁰,⁷¹

CXRs may be normal in small VSDs; larger VSDs may result in cardiomegaly with LA and LV dilatation and increased vascular markings (Fig. 4.15). Echocardiography usually reliably demonstrates the cardiac septal defect and pressure gradient across the VSD. CT and MRI are not routinely indicated but may help clarify anatomical details or associated anomalies. Likewise, cardiac catheterization is not regularly needed, although may help clarify anatomy or hemodynamics or allow concomitant catheter-based closure. For infants with large, symptomatic shunts, VSDs should be closed during infancy. In older children, VSDs should be closed when the Qp/Qs >1.5 or there is LA or LV dilation. When there is concomitant aortic regurgitation (AR), surgery is indicated urgently to reduce the risk of needing aortic valve replacement surgery. Smaller VSDs may be closed surgically via suture, whereas larger VSDs require patch closure. The subtypes of VSD are detailed below.⁵⁶,⁷⁰,⁷¹

FIGURE 4.15. A 6-year-old girl with ventricular septal defect (VSD). Frontal chest radiograph shows increased pulmonary vascularity and cardiac enlargement.

Perimembranous ventricular septal defect

Also known as subaortic or membranous VSD, perimembranous VSD is characterized by a deficiency of the membranous septum (Fig. 4.16). However, the defect usually extends beyond the membranous septum and is thus “perimembranous.” These defects constitute 70% of all VSDs. The rare direct communication between the RA and LV is known as a Gerbode defect, a subtype of perimembranous VSD. A Gerbode defect permits shunting directly from the LV to the RA via a deficiency of the AV membranous septum or combined defects in the ventricular septum and septal leaflet of the tricuspid valve.⁵⁶,⁷⁰,⁷¹,⁷²,⁷³

Muscular ventricular septal defect

Muscular VSDs occur in the trabecular segment of the muscular septum (Fig. 4.17). They are often multiple, producing a “swiss cheese” appearance. They are the second most common VSD type, estimated to comprise 5% to 20% of all VSDs.⁷⁴ With advances in imaging, more muscular VSDs are now detected; however, many close spontaneously.⁵⁶,⁷⁴,⁷⁵,⁷⁶

Infundibular ventricular septal defect

In infundibular VSD, there is a defect in the outlet septum below the aortic and pulmonary valves. The aortic cusp may
bow into the VSD, leading to AR. This defect comprises only 5% to 8% of VSDs in Western countries but up to 30% in Asia.²⁷,²⁸,⁵⁶,⁷⁷

FIGURE 4.16. A 17-year-old boy with a membranous ventricular septal defect. Bright-blood magnetic resonance image in a fourchamber plane shows a deficiency (asterisk) of the membranous septum. RV, right ventricle; LV, left ventricle.

Inlet ventricular septal defect

Also known as an AV canal type VSD, this posterior defect occurs in the inlet septum, bounded superiorly by the tricuspid valve annulus. These anomalies constitute 5% to 8% of VSDs. Inlet VSD can occur in isolation but usually arises in conjunction with an ASD as an atrioventricular septal defect (AVSD), also known as an AV canal defect or endocardial cushion defect (described below).⁵⁶,⁷⁰

FIGURE 4.17. A 16-year-old boy with a muscular ventricular septal defect who presented with syncope. Bright-blood magnetic resonance image in a four-chamber plane shows a deficiency (arrow) in the trabecular segment of the muscular septum. RV, right ventricle; LV, left ventricle.

Malalignment ventricular septal defect

Malalignment VSD is characterized by abnormal displacement of the outflow tract septum, such that the semilunar valve overrides the VSD. These defects are associated with other abnormalities, most commonly in tetralogy of Fallot (TOF) (detailed later), which includes overriding of the VSD by the aorta.²⁸,²⁹,³⁰,⁵⁶

Atrioventricular Septal Defect

Atrioventricular septal defect (AVSD) is also known as an AV canal defect. Overall, AVSDs comprise 4% to 5% of CHD, with an incidence of 2 per 10,000 live births. These anomalies combine elements of both VSDs and ASDs. In particular, there is variable abnormal development of segments of the atrial and ventricular septa near the AV valve (a deficient endocardial cushion). Other features include a common AV valve for both ventricles, with a variable number of valve leaflets and openings, abnormal position and elongation of the ventricular outflow tract (“goose neck” deformity), and displacement of the AV node and excitation conduction system.³¹,³²,⁵⁶

Various subclassifications of the AVSD exist. Defects may be partial, intermediate, or complete, depending on whether there is one opening in the atrial septum, two separate atrial septal openings, or a single AV valve with a common opening, respectively. Further, defects may be classified as Rastelli A, B, or C depending on the location of the anterior bridging leaflet. An unbalanced AVSD is characterized by hypoplasia of one of the ventricles. Finally, an AV valve cleft refers to a single AV valve cleft in the absence of an ASD or VSD.³¹,³²,⁵⁶

The most common associated anomalies include patent ductus arteriosus (PDA; persistence of a prenatal shunt between the PA bifurcation and descending aorta) and TOF, each of which occurs in 10% of cases. Down syndrome is the most frequently associated syndrome. While complete AVSD may present with pulmonary hypertension and failure to thrive, partial forms mimicking a large ASD may initially be asymptomatic, later progressing to symptoms of CHF.³¹,³²,⁵⁶

As with other cardiac shunt lesions, CXRs may show cardiomegaly and increased vascular markings (Fig. 4.18). Echocardiography is usually diagnostic, delineating absence of the AV septum; AV valve morphology; “goose neck” left ventricular outflow tract (LVOT); bridging leaflets and attachments of valve leaflets; size, extent, and direction of shunting; ventricular sizes; and associated anomalies. CT and MRI are not routinely indicated but may help clarify complex associated anomalies (Fig. 4.19). MRI can allow shunt quantification. Cardiac catheterization also plays a limited role in selected cases in need of precise hemodynamic measurements. Temporizing conservative medical therapies for associated CHF include diuretics, beta-blockers, and angiotensin-converting enzyme (ACE) inhibitors. Surgery is
almost always necessary and is performed between the 3rd and 6th months of life for complete defects and between 2 and 4 years of age for partial and intermediate defects. Once irreversible pulmonary hypertension has occurred, however, surgery is contraindicated.³¹,³²,⁵⁶

FIGURE 4.18. A 4-month-old boy with an atrioventricular septal defect (AVSD). Frontal chest radiograph shows increased pulmonary vascularity and global cardiomegaly.

Eisenmenger Syndrome

The term “Eisenmenger syndrome” does not refer to a specific congenital heart defect. Rather, it names the pathologic state that arises when an unrepaired, high-pressure, high-flow shunt lesion leads to chronic and marked pulmonary arterial hypertension (PAH). Altered pressures in the state of left-to-right shunting eventually lead to shunt reversal (right-to-left), with resulting cyanosis. The most common underlying etiology of Eisenmenger syndrome is an unrepaired VSD (Fig. 4.20). AVSD or PDA may also be the underlying culprits; ASDs are rarely the cause. Eisenmenger syndrome is uncommon before age 2 and has an equal gender distribution. Affected pediatric patients typically present at a late stage with chest pain, dyspnea, fatigue, cyanosis, hemoptysis, and clubbing. No specific imaging features are present; however, findings mimic those of PAH with massive PA enlargement. Some emerging pharmacologic therapies, such as those used in PAH, are undergoing investigation.
However, therapy for Eisenmenger syndrome is generally palliative. The unfortunately poor prognosis of this syndrome highlights the importance of early detection, monitoring, and treatment of CHD for prevention of future complications.³³,³⁴,³⁵,⁵⁶

FIGURE 4.19. A 17-year-old boy with an atrioventricular septal defect (AVSD). Bright-blood magnetic resonance image in a fourchamber plane shows a large AVSD (asterisk). RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.

FIGURE 4.20. A 28-year-old male with Eisenmenger syndrome secondary to chronic patch leak at the site of prior ventricular septal defect (VSD) repair. A: Axial enhanced CT image shows marked enlargement of the main pulmonary artery (MPA) due to underlying pulmonary arterial hypertension. AA, ascending aorta; SVC, superior vena cava. B: Short-axis view demonstrates persistent communication (asterisk) between the ventricles despite prior VSD patch repair. RV, right ventricle; LV, left ventricle.

Conotruncal Anomalies

Conotruncal anomalies arise from anomalous development of the ventricular outflow tracts. The most common subtypes are TOF, transposition of the great arteries (TGA), double outlet left or right ventricle, and truncus arteriosus, as detailed herein. Overall, these conditions account for up to 20% of all CHD and comprise a leading cause of symptomatic cyanotic CHD.¹²,³⁶,³⁷,⁵⁶,⁷⁸

Tetralogy of Fallot

Tetralogy of Fallot (TOF) is characterized by four main features: a malaligned, subaortic, membranous VSD, an overriding aorta, right ventricular hypertrophy, and pulmonary stenosis (PS), which may be valvular, subvalvular, and/or supravalvular (Fig. 4.21). TOF is the most common cause of cyanotic CHD. In fact, although TOF arises in only ˜4 per 10,000 live births, it accounts for 7% to 10% of all CHD. The underlying pathogenesis of TOF is attributed to malposition of the conal septum in relationship to the ventricular endocardial cushion, resulting in lack of normal signaling for membranous septum formation.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

FIGURE 4.21. A 3-day-old boy with tetralogy of Fallot. Oblique coronal enhanced CT image shows a malaligned ventricular septal defect allowing communication (asterisk) between the right ventricle (RV) and left ventricle (LV) with an overriding aorta (OA). The right ventricle is hypertrophied.

FIGURE 4.22. A 9-year-old boy with tetralogy of Fallot with pulmonary atresia. Axial enhanced CT image shows pulmonary atresia (arrow). Note the mediastinal collaterals. RA, right aortic arch.

The anatomy in TOF is variable, and several discrete subtypes in addition to the conventional form are recognized. These include TOF with pulmonary atresia (Fig. 4.22), in which the central PAs are completely discontinuous from the RVOT; TOF with absent pulmonary valve, in which the pulmonic valve is rudimentary or nonexistent, leading to severe pulmonary regurgitation and PA dilation (Fig. 4.23); and TOF with major aortopulmonary collateral arteries (MAPCAs), in which the MPA is absent or diminutive and MAPCAs supply the lungs.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

TOF is often associated with a right aortic arch (25%); ASDs (15%; referred to as “pentalogy” of Fallot); and coronary artery anomalies, most commonly characterized by the left anterior descending (LAD) arising from the RCA. Presenting clinical features range from asymptomatic murmur or decreased exercise tolerance to hypercyanotic episodes (“tet spells”), typically evident by 2 to 6 months of age. In general, the degree of PS determines the onset and severity
of cyanosis; with only mild RVOT obstruction, neonates may be not cyanotic (“pink tetralogy”). The absent pulmonary valve form of TOF is unique in that the markedly enlarged PAs cause tracheobronchial compression and resultant respiratory distress.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

FIGURE 4.23. A 3-year-old with tetralogy of Fallot with absent pulmonary valve. Axial enhanced CT image shows dilated right (RPA) and left (LPA) pulmonary arteries.

FIGURE 4.24. A 4-month-old girl with tetralogy of Fallot. A: Frontal chest radiograph shows cardiomegaly with an elevated cardiac apex (arrow). Right-sided aortic arch (asterisk) is also seen. B: Three-dimensional volume-rendered CT image from a superior and posterior view demonstrates multiple major aortopulmonary collaterals (arrows) arising from the thoracic aorta (TA).

The classic (although not universal) radiographic findings in TOF include a “boot-shaped heart” with elevation of the cardiac apex due to right ventricular hypertrophy, a concave MPA silhouette, and a large, right-sided aortic arch (Fig. 4.24A). Pulmonary vascular markings are characteristically diminished but may be normal or engorged in “pink tetralogy” patients without significant PS. Echocardiography is the first-line imaging modality and provides an excellent assessment of the typical intracardiac anatomy in TOF, the magnitude and direction of shunting across the VSD, and the maximum velocity across the RVOT. The maximum velocity can be used to estimate the pressure gradient across the RVOT via the modified Bernoulli equation, ΔP = 4V², where ΔP is the pressure gradient in millimeters of mercury (mmHg) and V is the average maximum velocity is meters per second (m/s). Although the main and proximal branch PAs can be visualized, the distal PAs are usually not well evaluated.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

CT offers an excellent anatomical assessment of RVOT morphology, branch PA stenoses, and any MAPCAs (Fig. 4.24B). MAPCAs usually arise from the thoracic aorta but may originate from the abdominal aorta or the subclavian, internal mammary, intercostal, or even coronary arteries. When MAPCAs are present, for surgical planning purposes, it is important to delineate which segments of the lung are supplied by the native PAs, MAPCAs, or both. For TOF with absent pulmonary valve, CT provides superior assessment of tracheobronchial compression by the aneurysmal PAs. In addition, CT is preferred over MRI for anatomic assessment when metallic stents are present. Lung window CT images may reveal a “mosaic attenuation” pattern related to variations in regional lung perfusion and pulmonary hypertension.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

The major role of MRI in TOF is in postoperative assessment, when surgical material (such as conduits) limits visualization. A typical MRI for postoperative TOF includes evaluation of the following: (1) right ventricular volume and function; (2) RVOT anatomy, caliber, and peak velocity at the level of RVOT obstruction, if present; (3) severity of pulmonary regurgitation and differential branch pulmonary artery regurgitation (quantified using PC techniques); and (4) branch pulmonary artery stenosis (typically imaged with contrast-enhanced MRA). When performed, it is not uncommon for delayed enhancement imaging to demonstrate myocardial scarring in postoperative TOF patients (typically older patients with later repair), although its clinical significance is currently uncertain. MRI has a well-established role in assessing the appropriate timing for pulmonary valve replacement (PVR) in pediatric patients with repaired TOF based on quantitative parameters. In the presence of a pulmonary regurgitation fraction of ≥25%, indications for valve replacement include: (1) an RV end-diastolic volume index (EDVI) >150 mL/m² or Z-score >4, (2) an RV/LV EDV ratio >2, (3) a large RVOT aneurysm, (4) a RV ejection fraction (EF) <45% to 47%, or (5) LVEF <55%.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

Cardiac catheterization has limited additional diagnostic utility in TOF prior to initial surgery. Nevertheless, it is useful for quantifying hemodynamics, delineating the central and branch pulmonary arteries and MAPCAs, and demonstrating coronary anomalies. A major and increasing role for catheterization is percutaneous PVR and exchange, techniques that obviate more invasive surgery.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

Initial treatment for TOF depends on the underlying anatomy. The majority of affected pediatric patients are, at most, minimally cyanotic at initial presentation and undergo elective surgical repair between the ages of 6 months and 1 year of age. Definitive surgical correction consists of VSD closure and relief of RVOT obstruction, which may be performed with infundibulectomy, a transannular patch, or a pulmonary
artery valved conduit (usually a homograft). In pediatric patients with significant symptoms or cyanosis, a modified Blalock-Taussig (BT) shunt, typically consisting of a Gore-Tex graft from the right or left subclavian artery to the ipsilateral pulmonary artery, may be used to augment pulmonary blood flow. Other palliative procedures used as bridges to definitive surgery may include balloon pulmonary valvuloplasty or placement of an RVOT stent. In TOF with pulmonary atresia, initial maintenance of ductal flow is critical and may be accomplished via infusion of prostaglandin E1 (PGE1) and, sometimes, ductal stenting. Treatment in the presence of MAPCAs is complex but, in general, consists of multistage surgeries and catheterization to augment the PAs and allow forward pulmonary blood flow; the use of MAPCAs to supplement the native PAs is known as “unifocalization.” In TOF with absent pulmonary valve, management includes partial resection and repair of the aneurysmal PAs. After definitive repair, pediatric patients with TOF are at risk for complications ranging from RV dysfunction to arrhythmia to sudden death and require long-term monitoring, for instance, to determine the timing of PVR, as previously described.¹²,³⁶,³⁷,⁵⁶,⁷⁸,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴,⁸⁵,⁸⁶,⁸⁷

Pulmonary Atresia with Intact Ventricular Septum

Pulmonary atresia with intact ventricular septum (PA-IVS) is defined by total obstruction of the pulmonary valve with two separate ventricles (no VSD) and a patent (although usually hypoplastic) tricuspid valve. PA-IVS is rare, with a prevalence of 5 to 9 cases per 100,000 live births, accounting for 1% to 3% of CHD. The RV is usually hypoplastic to varying degrees. Associations include coexistent PFO (common) allowing right-to-left shunting; coronary arteriovenous fistulae (AVF) (10% to 50%); and tricuspid regurgitation (TR) (20%), associated with marked RV dilatation. Affected neonates develop severe cyanosis, hypoxemia, and tachypnea as the ductus closes after an initially uneventful course.¹²,³⁷,⁵⁶,⁷⁸,⁸⁸,⁸⁹,⁹⁰,⁹¹,⁹²,⁹³,⁹⁴,⁹⁵

The characteristic radiographic appearance of PA-IVS is known as the “wall-to-wall” heart, manifested by marked cardiomegaly. However, this is generally only seen in PA-IVS patients with severe TR. The heart size, pulmonary vascularity, and lungs may appear normal. Echocardiography shows a lack of forward flow from the RV to the PA. The RV is generally small in size. TR and right-to-left shunting across a PFO can also be visualized. While RV to coronary artery fistulae can be identified, they are often incompletely characterized. Typically, cardiac catheterization has been used for this purpose, with recent reports documenting the successful use of cardiac CT in identifying and characterizing these fistulae. MRI is generally reserved for postoperative patients and can be used to determine optimal timing of PVR, RV function, the presence of fibrosis, and the adequacy of repair (Fig. 4.25). Cardiac catheterization, in addition to assessing for coronary fistulae, can be used as guidance for transcatheter pulmonary valve perforation.¹²,³⁷,⁵⁶,⁷⁸,⁸⁸,⁸⁹,⁹⁰,⁹¹,⁹²,⁹³,⁹⁴,⁹⁵

FIGURE 4.25. A 2-year-old boy with pulmonary atresia with intact ventricular septum (PA-IVS), status post Glenn procedure. A: Axial ultrashort echo time (UTE) T1-weighted magnetic resonance image obtained after ferumoxytol administration shows a diminutive right ventricle (arrow) and dilated and hypertrabeculated left ventricle (LV), compatible with the diagnosis of PA-IVS. B: Oblique coronal UTE T1-weighted magnetic resonance image obtained after ferumoxytol administration demonstrates the bidirectional anastomosis (asterisk) between the superior vena cava (SVC) and right pulmonary artery (arrow).

Management of PA-IVS is determined by the severity of RV hypoplasia. Biventricular repair is the ideal goal but is generally only possible with an adequate-sized RV and absence of infundibular atresia and RV-dependent coronary circulation. In such cases, percutaneous radiofrequency (RF) pulmonary valvotomy alone may be sufficient. If a two-ventricle repair is not possible, a single-ventricle staged Fontan procedure may be performed, as later detailed. One alternative is the one and a half ventricle repair consisting of only a bidirectional Glenn (BDG); SVC blood communicates via the Glenn directly with the PA, while the RV actively pumps IVC blood to the PA. Management algorithms for PA-IVS will likely undergo continued modification as less invasive catheter-based therapies for this heterogeneous disorder continue to improve.¹²,³⁷,⁵⁶,⁷⁸,⁸⁸,⁸⁹,⁹⁰,⁹¹,⁹²,⁹³,⁹⁴,⁹⁵

Transposition of the Great Arteries

Although variable definitions exist in the literature, the term “transposition of the great arteries” (TGA) generally describes a state in which the aorta arises from the RV, while the MPA arises from the LV (ventriculoarterial discordance).
Overall, TGA is rare, with an incidence of 1 in 2,000 to 5,000 live births. Nonetheless, TGA comprises 5% to 7% of all CHD and 10% of all neonatal cyanotic CHD. TGA has a male predominance of 1.5-3:1 and arises most commonly in infants of diabetic mothers. Other risk factors include fetal exposure to antiepileptic drugs and herbicides.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

The most common form (d-TGA) is also known as complete transposition, which is characterized by the following additional findings: atrial situs solitus (normal atrial position), atrioventricular concordance (right atrium connected to RV and left atrium connected to LV), D-looping of the ventricles (typically RV to the right of the LV), and positioning of the aortic valve to the right of the pulmonary valve (Fig. 4.26). The d-TGA form is the second most common cause of cyanotic CHD diagnosed by one year of age (after TOF) and the most common cyanotic CHD that presents within the first day of life.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

TGA is usually isolated, but in 10% of cases may be associated with an extracardiac malformation or syndrome. Symptoms depend on the anatomic subtype. When the ventricular septum is intact, affected pediatric patients develop cyanosis by the first week of life due to lack of mixing between the in-parallel rather than normally in-series pulmonary and systemic circulations. If a VSD is present, cyanosis is typically minimal, but CHF ensues between 4 and 8 weeks of life. When a VSD and superimposed PS are present, the balance of cyanosis versus CHF is directly related to the severity of PS; marked PS essentially mimics an intact septum. Note that in the l-TGA subtype of TGA, in which there is also atrioventricular discordance (RA connected to LV and LA connected to RV), the native circulation is compatible with life; thus, affected pediatric patients may remain asymptomatic until the RV can no longer tolerate systemic pressures¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷ (Fig. 4.27).

FIGURE 4.26. Diagram of D-transposition of the great arteries (D-TGA). The aorta arises from the right ventricle and the pulmonary artery from the left ventricle. The position of the aorta is to the right and anterior relative to the position of the pulmonary artery.

FIGURE 4.27. Diagram of L-transposition of the great arteries (L-TGA). The aorta and pulmonary artery are transposed (switched), with the aorta anterior and to the left of the pulmonary artery. The right ventricle (RV) and left ventricle (LV) are also transposed. The aorta arises from the RV and the pulmonary artery from the LV. In L-TGA, circulation through the heart is physiologic or “congenitally corrected” because there is both ventriculoarterial and atrioventricular discordance.

The classic radiographic description of d-TGA is an “egg on a string” (Fig. 4.28). The “egg” refers to the heart, which characteristically has an enlarged and globular shape after the first few days of life (normal at birth), with an abnormally convex right atrial border and an enlarged left atrium. The “string” refers to the narrow mediastinum produced by the anteroposterior (AP) superimposition of the great vessel (aorta and MPA) shadows and thymic shadow, which is diminutive due to stress. Of course, the radiographic appearance is variable, depending on whether the great arteries are truly superimposed, as well as the size of the pulmonarysystemic communication and severity of pulmonary obstruction. Echocardiography is essential for preoperative diagnosis and allows detailed intracardiac assessment, including evaluation for atrioventricular and ventriculoarterial discordance, interatrial and interventricular communication, outlet valve morphology and function, and LVOT obstruction. Patency of the ductus arteriosus, the coronary artery origins, and determination of the presence of aortic coarctation can also be evaluated.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

As in other CHD, CT is highly useful in the presence of metallic stent material, providing an excellent anatomical assessment of such structures as the RVOT (Fig. 4.29), branch pulmonary arteries, coronary arteries, and postoperative
baffles, as well as optimal visualization of common sites of complications after surgery. MRI is also most valuable for postoperative assessment, both providing anatomic evaluation and allowing accurate quantification of biventricular function and assessment of maximum velocity at any sites of obstruction for pressure gradient estimation. The role of cardiac catheterization for TGA diagnosis has declined, except for coronary artery assessment, depending on local cardiac CT expertise. However, palliative procedures such as balloon atrial septostomy can be performed at the time of catheterization.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

FIGURE 4.28. A 6-day-old boy with D-transposition of great arteries (TGA) and ventricular septal defect. Frontal chest radiograph shows that the cardiac silhouette has a globular shape and the superior mediastinum is relatively narrow, producing an appearance referred to as an “egg on a string.”

FIGURE 4.29. A 17-year-old boy with D-transposition of the great arteries after arterial switch operation. Axial enhanced CT image shows the main pulmonary artery (asterisk) located anterior to the ascending aorta (AA) as the result of LeCompte maneuver. Also noted is stent (arrow) placement for treatment of stenosis of the left pulmonary artery. DA, descending aorta.

Initial survival in d-TGA depends on the presence of shunts, such as PFO, VSD, and/or PDA, that allow mixing between the systemic and pulmonary circulations. Infusion of PGE1 helps maintain ductus patency. With persistent cyanosis, balloon atrial septostomy is indicated. Definitive management is operative. The current surgery of choice is the arterial switch (Jatene) procedure, in which the positions of the aorta and MPA are reversed and the coronaries are reimplanted on the neoaortic root. This is often combined with pulmonary root translocation, known as the LeCompte maneuver. In affected pediatric patients who present early, complete repair is preferably performed between 1 and 2 weeks of age but can be delayed until 2 to 3 months of age if there is a VSD. However, in affected pediatric patients who present late, pulmonary artery banding is typically needed to “train” the deconditioned left ventricle prior to arterial switch.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

Venous/atrial switch operations, such as the Senning and Mustard procedures, are no longer favored, although many living individuals have had these surgeries. In these procedures, caval blood flow is redirected toward the mitral valve, while pulmonary venous flow is redirected toward the tricuspid valve via intra-atrial baffles after atrial septal tissue removal. However, as a result, the unequipped systemic right ventricle must pump against higher than expected pressures, leading to eventual failure; arrhythmia and baffle stenosis are also not uncommon. When PS is present, a modified BT shunt may be necessary for palliation. In addition, a Rastellitype surgical repair is typically pursued between the ages of 1 and 2; the VSD is repaired such that LV blood flow is rerouted to the aorta via an intraventricular patch and a conduit is inserted to connect the RV with the PA.¹²,³⁷,⁵⁶,⁷⁸,⁹⁶,⁹⁷

Truncus Arteriosus

Truncus arteriosus is characterized by a common arterial trunk (truncus) that arises from the base of the heart via a single semilunar valve and gives rise to the pulmonary, systemic, and coronary arteries, all of which carry mixed oxygenated and deoxygenated blood. The PAs arise distal to the coronary artery origins but proximal to the first aortic branch. The truncus arteriosus overrides a large conal septal VSD and usually straddles the ventricles, but may favor one ventricle. Aside from the VSD, the atria and ventricles are typically normally formed. Truncus arteriosus may be further subtyped according to the Collett and Edwards classification scheme (Fig. 4.30). In type I (50% to 70%), a single pulmonary trunk arises from the common trunk and divides into right and left PAs. In type II (30% to 50%), there are close but separate origins of the right and left PAs from the common trunk at its posterolateral aspect. In type III (6% to 10%), the branch pulmonary arteries arise at separate locations from the common trunk or aortic arch (Fig. 4.31). In type IV, neither pulmonary artery branch arises from the common trunk; this is now more appropriately characterized as a form of TOF with pulmonary atresia and VSD.¹²,³⁷,⁵⁶,⁷⁸,⁹⁸,⁹⁹,¹⁰⁰,¹⁰¹,¹⁰²

FIGURE 4.30. Diagram of Collett and Edwards classification of truncus arteriosus. There are four types of truncus arteriosus. In type I truncus arteriosus, a single pulmonary trunk arises from the common trunk and divides into right and left pulmonary arteries. In type II truncus arteriosus, the right and left pulmonary origins are in close proximity but arise separately from the posterolateral aspect of the common trunk. In type III truncus arteriosus, the branch pulmonary arteries arise at separate locations from the common trunk or aortic arch. In type IV truncus arteriosus, neither pulmonary artery branch arises from the common trunk.

Truncus arteriosus is rare, with an estimated incidence of 5 to 15 per 100,000 live births, and accounts for 1% to 2% of CHD. Other findings may include a right aortic arch (30% to 40%); interrupted aortic arch (IAA), with the interrupted segment usually located between the left common carotid and left subclavian arteries (type B); supernumerary and dysplastic truncal valve leaflets; truncal valve regurgitation; and coronary anomalies. In addition, there is an association with DiGeorge (22q11 deletion) syndrome. Affected pediatric patients are initially asymptomatic due to high pulmonary vascular resistance but develop stigmata of CHF within several weeks of life.¹⁰ Cyanosis is usually minimal and more pronounced with greater severity of PA stenosis.¹²,³⁷,⁵⁶,⁷⁸,⁹⁸,⁹⁹,¹⁰⁰,¹⁰¹,¹⁰²

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