Right Ventricular Outflow Tract Imaging With CT and MRI

AJR:200, January 2013 W39

forming both the right ventricle (RV) and the outflow tract [4]. With looping of the heart tube, the ventricular trabeculations start to form at the outer curvature, permitting iden-tification of the cranial part of the tube as the outflow tract or conotruncus.

The initial outflow tract extends proximal-ly from the distal ventricular groove to the pericardial reflections and has a characteristic dog-leg bend, which divides it into two myo-cardial subsegments, a proximal subsegment (i.e., the conus) and a distal subsegment (i.e., the truncus) [5]. The truncus arteriosus is a short segment interposed between the conus and the aortic sac. The latter transforms into the ascending aorta and pulmonary trunks.

The initial outflow tract is mainly myocar-dial and increases almost sixfold in length between Hamburger-Hamilton stages 12 and 24 [6] (Fig. 1). Subsequently, the initial mus-culature of the walls of the truncus and dis-tal conus disappears by apoptosis, transdif-ferentiation, absorption into the developing RV, or a combination of these processes [7]. With further developments, these new por-tions will be remodeled into the intrapericar-dial portions of the aorta, pulmonary trunk, and arterial valves and their supporting si-nuses. In contrast, myocardial tissue is being added to the proximal portion of the conus.

Right Ventricular Outflow Tract Imaging With CT and MRI: Part 1, Morphology

Farhood Saremi1Siew Yen Ho2

José Angel Cabrera3

Damián Sánchez-Quintana4

Saremi F, Ho SY, Cabrera JA, Sánchez-Quintana D

1Department of Radiology, University of Southern California University Hospital, 1500 San Pablo St, Los Angeles, CA 90033. Address correspondence to F. Saremi ([email protected]).

2Cardiac Morphology Unit, Royal Brompton Hospital, London, United Kingdom.

3Department of Cardiology, Hospital Universitario Quirón Madrid, Universidad Europea de Madrid, Madrid, Spain.

4Facultad de Medicina de Badajoz, Departamento de Anatomía Humana, University of Extremadura, Badajoz, Spain.

Cardiopulmonar y Imaging • Review

CME/SAM This article is available for CME/SAM credit.

WEB This is a Web exclusive article.

AJR 2013; 200:W39–W50

0361–803X/13/2001–W39

© American Roentgen Ray Society

Anatomic knowledge of the right ventricular outflow tract (RVOT) can help one understand the spec-trum of complicated conotruncal

anomalies that arise from abnormal forma-tions of major vessels in this region. With rapid developments in imaging technology, cardiac CT and MRI have become the ideal methods for anatomic evaluation of the out-flow tracts and for diagnosis of abnormalities of this challenging region. The goal of the present review is to offer a general perspective on RVOT development, structure, and func-tion. In the first part, we mainly focus on the role of CT and MRI to evaluate morphology and developmental malformation of the RVOT. We also discuss postsurgical repair findings, because complications after surgery for con-genital heart disease are common, and many patients need follow-up imaging for diagno-sis and surgical planning.

Developmental ConsiderationsThe two fields of cardiac progenitors are

now recognized as the primary and second-ary, or anterior, heart fields [1–4]. It is the primary heart field that produces the straight heart tube. In mice, there is firm evidence that the primary heart field gives rise to the left ventricle (LV), with the secondary field

Keywords: cardiac CT, cardiac MRI, conotruncal anomaly, right ventricular outflow tract, tetralogy of Fallot

DOI:10.2214/AJR.12.9333

Received May 21, 2012; accepted after revision July 19, 2012.

Presented at RSNA 2011, in Chicago, IL, as exhibit LL-CAE1135-TUA (received the Magna Cum Laude award).

OBJECTIVE. MRI and CT have become the ideal methods for assessing the complex morphology of the conotruncal region, including the right ventricular outflow tract (RVOT). Detailed information about the embryology and anatomy of the RVOT provides a better un-derstanding of the spectrum of diseases of this region and helps to narrow the differential di-agnoses of abnormalities involving this important structure. In this review, we focus on the role of CT and MRI to evaluate morphology in relation to developmental malformation of the RVOT.

CONCLUSION. A spectrum of conotruncal anomalies with abnormally positioned great arteries may arise from a perturbation of RVOT formation. Complications after surgery are common, and many patients need follow-up imaging for diagnosis and surgical planning. In this regard, the spectrum of diseases, differential diagnoses, and postoperative findings are briefly described. With CT and MRI, the relationship of the RVOT to critical structures, such as the coronary arteries, can be revealed.

Saremi et al.Imaging of Right Ventricular Outflow Tract: Morphology

Cardiopulmonary ImagingReview

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W40 AJR:200, January 2013

Saremi et al.

As development proceeds, the single outflow tract undergoes remodeling into separate pul-monary and aortic arteries. The aorticopulmo-nary septation involves interactions between diverse cell types, including myocardium, en-docardium, and neural crest cells [8]. The dis-tal portion of aortic sac also is considered as an entirely neural crest derivative.

The outflow tract undergoes rotation dur-ing its remodeling. Rotation of the myocar-dium at the base of the outflow tract is prob-ably essential to achieve normal positioning of the great arteries with respect to each oth-er at the ventriculoarterial junction [9, 10]. In addition to abnormal outflow tract sep-tation caused by neural crest cell defects, a spectrum of conotruncal anomalies with abnormally positioned great arteries may arise from a perturbation of myocardial ro-tation, including tetralogy of Fallot (TOF), persistent truncus arteriosus, double-outlet RV (DORV), and transposition of great ar-teries (TGA) [10, 11]. A short outflow tract, as obtained experimentally through second-ary heart field ablation, may not allow nor-mal conotruncal rotation [4]. Although in conotruncal anomalies, including TOF, the infundibular septum is not always short, it is thought that, any time the RV shows a short outflow tract, a total or partial lack of conotruncal rotation and remodeling is inev-itably present [9].

Anatomic Evaluation of RVOTImaging Techniques

Cardiac CT and MRI allow comprehen-sive morphologic and functional assessment of the heart within a single examination. Higher spatial resolution and the availabil-ity of isotropic multiplanar data acquisition of cardiac CT angiography make it the pre-ferred technique over current routine MRI techniques for detailed anatomic study of the RVOT. With new CT scanners, an en-tire heart acquisition can be obtained in a short breath-hold combined with thin slices (0.5–0.75 mm). This greatly reduces motion artifacts, and the thin collimation improves the depiction of small structures. Anatomic analysis of the RV can be performed with a dedicated ECG-gated right heart study or as part of coronary CT angiography [12, 13]. In the latter, most of the time, sufficient attenu-ation for visualization of the right heart can be obtained by split-bolus injection, in which an initial bolus of contrast medium (50–75 mL) is followed by 50 mL of a 70%:30% saline-to-contrast-medium mixture and a

30-mL saline chaser at a rate of 4–5 mL/s [13]. A dedicated right heart examination with CT requires ECG-gating and -trigger-ing and homogeneous enhancement of the right atrium and ventricle to an optimum at-tenuation of 350–400 HU. The scan can be started early (i.e., main pulmonary artery triggering) to include the right heart only.

For certain patients with congenital heart disease, we have adopted a modified injec-tion protocol using dual-extremity contrast injections that provides artifact-free images of the right heart [14]. Using this protocol, a 20-gauge angiocatheter needle will be in-serted into the antecubital vein, and a second needle (similar in size but longer in length) will be placed in a femoral vein under ultra-sound guidance, local anesthesia, and ster-ile conditions. Nondiluted iodinated contrast medium (80 mL) will be injected, followed by 30 mL of saline using a standard dual-barrel injector in the femoral vein at a rate of 4 mL/s. Ten seconds after femoral injec-tion of contrast medium, 50–70 mL of 40% contrast medium diluted with saline (20 mL of contrast medium mixed with 50 mL of sa-line) will be injected manually in the ante-cubital vein at 3 mL/s. Standard ECG-gated acquisition is triggered at 150 HU in the de-scending aorta near the tracheal bifurcation.

Anatomic LandmarksThe RV in the normal heart is the most an-

teriorly situated cardiac chamber and marks the inferior border of the cardiac silhou-ette. In contrast to the nearly conical shape of the LV, the RV appears triangular when viewed from the side and crescent-shaped when viewed in cross section [15–18]. The curvature of the ventricular septum plac-es the RVOT anterocephalad to that of the LV, resulting in a characteristic crossover re-lationship between right and left ventricular outflows. This important spatial relationship can be lost in congenital heart malforma-tions such as TGA. The overlap between left ventricular inlets and outlets puts the aortic outflow tract immediately behind the septum that separates it from the RV inlet, giving the wedged position of the aortic root.

Traditionally, the RV is divided into the si-nus and conus parts, but in more recent de-cades, both the RV and the LV have been described as having three components: the inlet, apical trabecular, and outlet portions [15–17] (Figs. 2A and 2B). In the analysis of congenitally malformed hearts, this tri-partite concept is more useful than the tra-

ditional bipartite division. The inlet portion of the RV surrounds the leaflets of the tri-cuspid valve and supports its papillary mus-cles and tension apparatus. A distinguishing feature of the tricuspid valve is the direct at-tachment of its septal leaflet to the ventricu-lar septum. The apical portion of the RV is characterized typically by heavy trabecula-tions. Distinguishing features of the outlet portion of the RV are discussed in the fol-lowing subsections.

Pulmonary infundibulum—The pulmonary infundibulum (conus) is a tubular muscu-lar structure that supports the leaflets of the pulmonary valve. Its length, size, and angle vary. The size of the infundibulum is inde-pendent of the general size of the RV.

Supraventricular crest—The posterior (para-septal) wall of the infundibulum is formed by a prominent muscular ridge, known as the supraventricular crest (crista supraventricu-laris or ventriculoinfundibular fold), which separates the inlet and outlet components of the RV (Fig. 2C). This is in contrast to the LV, where the aortic and mitral valves are in fibrous continuity. Although it looks like a ridge from the perspective of the RV cav-ity, the supraventricular crest is, in fact, an infolding of the ventricular wall (the ven-triculoinfundibular fold) inserting into the ventricular septum. It is separated from the aorta by the epicardial fat, and any incision through it will lead outside the heart into the vicinity of the right coronary artery (Figs. 2C and 3). Only the central portion of its infe-rior-most part between the limbs of the sep-tomarginal trabeculation contributes to the interventricular septum (muscular outlet sep-tum or conal septum) [17] (Fig. 2C). In the normal heart, however, this area is exceed-ingly small and can hardly be distinguished from the septomarginal trabeculation by CT.

Septomarginal trabeculation—The septo-marginal trabeculation is a prominent Y-shaped muscular strap reinforcing the septal surface. It bifurcates into anterosuperior and infero-posterior limbs (Fig. 3), which clasp the supra-ventricular crest. The anterosuperior limb ex-tends along the infundibulum to the leaflets of the pulmonary valve. The posterior limb runs onto and overlays the ventricular sep-tum, toward the RV inlet, giving rise to the medial papillary muscle complex. The body of the septomarginal trabeculation extends to the apex of the ventricle, where it gives rise to the moderator band and anterior papillary muscle before breaking up into the general apical trabeculations. The body of the septo-

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Imaging of Right Ventricular Outflow Tract: Morphology

marginal trabeculation is interventricular, rath-er than supraventricular, and when prominent can appear as a bump on the septum on cross-sectional imaging. When hypertrophied, the septomarginal band can divide the RV into two chambers (double-chambered RV) [19].

Moderator band—The moderator band is considered as part of the septomarginal trabec-ulation, supporting the anterior papillary mus-cle of the tricuspid valve and, from this point, crossing to the free wall of the ventricle. The moderator band incorporates the right atrio-ventricular bundle, as conduction tissue fi-bers move toward the apex of the ventricle be-fore entering the anterior papillary muscle. It is usually located equidistant from the tricuspid valve and the apex and can be identified in 90% of hearts. In 40% of hearts, the band is a short and thick trabeculation. The average thickness of the band is 4.5 mm, and its length is 16 mm, ranging from 11 to 24 mm [20]. The moderator band is supplied by branches of the left anteri-or descending (LAD) artery, named the artery of the moderator band. The LAD artery makes anastomotic connections at the base of the anterior papillary muscle with branches of the right coronary artery.

Medial papillary muscle of the conus—The medial (septal) papillary muscle of the conus is present in 82% of the hearts, whereas in the rest, it is replaced by tendinous chords [21] (Figs. 2C and 3). It is a single papilla in 50% and a double papilla in 30% of hearts. It con-nects with the septal and anterior leaflets of the tricuspid valve. It represents an important sur-gical landmark for the location of the right bundle branch to avoid injury to the bundle during surgical correction of certain types of ventricular septal defects (VSDs).

Pulmonary valve—Different nomenclature has been used to define the anatomic location

of the pulmonary valve sinuses on the basis of their spatial location in relation to the body of the heart itself [22] (Fig. 4). Because of the semilunar shape of the pulmonary leaf-lets (similar to the aortic valve), this valve does not have a ringlike annulus. The sinotu-bular junction of the pulmonary trunk marks the level of the commissures between the an-nuli (Fig. 4). A second junction exists at the ventriculoarterial junction. The bases of the sinuses within the ventricle cross the ana-tomic ventriculoarterial junction. The mus-culature of the subpulmonary infundibulum raises the pulmonary valve above the ventric-ular septum to position the pulmonary valve as the most superiorly situated of the cardiac valves. This anatomic feature makes possible the safe resection of the pulmonary valve, in-cluding its basal attachments within the in-fundibulum from the rest of the RVOT [22].

Arterial SupplyThe conotruncal structures are normally

vascularized by anterior and posterior arterial branches from the right and left coronary arter-ies [23]. On the right side, the branches arise from the conal branch of the right coronary ar-tery or directly from the aorta. On the left side, they arise from the LAD artery, the left main artery, or directly from the aorta. The right an-terior conal branch is the most constant and conspicuous branch participating in the prec-onal circulation, also known as Vieussens ar-terial ring [23]. This collateral intercoronary connection extends between the conus artery and the first right ventricular branch (left an-terior conus branch) of the LAD artery. The Vieussens arterial ring will become dilated when there is proximal LAD artery occlusion or, less frequently, right coronary artery oc-clusion [24]. Generally, three major collateral

pathways at the contruncal level provide circu-lation between the right and the left coronary systems in all congenital or acquired forms of one-sided coronary occlusion and are used as the basis for different classifications [24]. These three collateral circulation pathways are the preconal (precardiac), retroconal (interarte-rial), and retroaortic pathways.

Morphologic Changes of the RVOT in Adult Congenital Heart Disease

Major advances in cardiac surgery over the past 50 years have resulted in a marked increase in the number of patients with con-genital heart disease reaching adulthood [25]. In many cases, initial surgery is indi-cated on the basis of echo cardiography, with catheterization for physiologic assessment if required. CT and MRI have a prominent role in follow-up, either to monitor changes dur-ing staged surgical repair or to look for com-plications, which are common; thus, many patients need imaging for diagnosis and sur-gical planning. However, it is not unusual to discover an RVOT malformation for the first time and without a history of surgery.

RVOT Stenosis: Pre- and Postoperative FindingsRVOT stenosis is usually secondary to

pulmonary valve diseases, but stenotic le-sions at the subvalvular or supravalvular lev-els are not rare. Causes of RVOT stenosis are listed in Table 1.

Pulmonary valve stenosis—Isolated pulmo-nary stenosis (PS) is almost always congenital, and many patients can be asymptomatic when first diagnosed. Three morphologic types are described [18, 26–28] (Fig. 5 and Table 2). The most common type of PS is a dome-shaped pul-monary valve, which is characterized by a mo-bile valve and two to four raphes, with no

TABLE 1: Causes of Right Ventricular Outflow Tract Stenosis in Adults

Type of Stenosis Causes

Congenital

Truncal

Supravalvular Hour-glass deformity; main pulmonary artery membrane; main pulmonary artery stenosis; peripheral pulmonary artery stenosis (tetralogy of Fallot); associated with rubella and Alagille, Williams, and Keutel syndromes

Valvular Dome shaped; dysplastic; unicuspid or bicuspid

Conal

Intrinsic Tetralogy of Fallot

Extrinsic Aneurysm of sinus of Valsalva; aneurysm of membranous septum

Subconal Double-chambered right ventricle

Postoperative restenosis Native valve; prosthetic valve; conduit: Rastelli; peripheral stenosis: tetralogy of Fallot

Acquired Valvular; hypertrophy (pulmonary stenosis and pulmonary hypertension); masses; extrinsic (aneurysm and hematoma)

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Saremi et al.

complete separation into valve cusps (Fig. 5A). Dysplastic pulmonary valve is present in up to 20% of patients with PS and is asso-ciated with immobile thickened cusps and a hypoplastic ventriculoarterial junction [26] (Fig. 5B). Among patients with PS who re-quire active treatment, whether intervention-al or surgical (i.e., the more severe end of the spectrum of severity), dysplastic valves is far more common. A bicuspid or multicuspid valve is rare [27]. Different shapes can be equally distinguished with cardiac MRI and CT angiography.

Chronic PS results in RV hypertrophy, espe-cially at the RVOT. When prominent, RVOT hypertrophy can lead to secondary dynamic subvalvar stenosis. Distinguishing between valvar stenosis and subvalvular dynamic ste-nosis secondary to infundibular hypertrophy can be challenging. Subvalvular dynamic ob-struction (late systolic stenosis), in fact, often accompanies severe valvular PS and is charac-terized by a late-peaking jet in MRI similar to that of dynamic LV outflow obstruction. PS can also result in poststenotic dilatation of the pulmonary trunk.

Pulmonary valve interventions—For symp-tomatic patients with a dome-shaped pulmo-nary valve, balloon valvuloplasty is indicated when a peak instantaneous gradient greater than 50 mm Hg is present [28]. A success-ful procedure is defined by a final peak gra-dient of less than 30 mm Hg and is obtained in more than 90% of cases [29]. If the valve is dysplastic, surgery is more likely to be re-quired; if there is annular or pulmonary trunk hypoplasia, a transannular patch may become necessary. In patients with PS and significant pulmonary regurgitation, valve replacement is required. Mechanical valve replacement is used rarely because of thrombosis issues. Bioprosthetic valves and pulmonary homo-grafts are preferred [30].

TOF—TOF consists of a large nonrestric-tive subaortic VSD, the aorta riding up over the septal defect (> 50% overriding falls into the DORV subgroup), and RVOT obstruc-tion. Subpulmonary stenosis, which is an essential part of TOF, is mainly due to an-

terosuperior malalignment of the muscu-lar outlet septum relative to the limbs of the septomarginal trabeculation, coupled with thickened septoparietal trabeculations [17, 31, 32] (Fig. 6). Stenosis can also occur at the subpulmonic level by hypertrophy of the septomarginal trabeculation or the modera-tor band. This gives the arrangement often described as “two-chambered RV.” The sub-pulmonary infundibulum itself varies mark-edly in length and can sometimes be short. In most other instances, the narrowed infundib-ular chamber has considerable length (Fig. 7). Patients with TOF have remarkable in-trinsic histologic abnormalities and reduced elasticity in both the ascending aorta and the pulmonary artery, and it appears that TOF repair does not improve these abnormali-ties [33, 34]. Aortic root dilatation and sec-ondary aortic regurgitation are not rare [33]. Cardiac MRI can address these major clini-cal implications.

Double-chambered RV—Double-chambered RV is characterized by subinfundibular ste-nosis due to aberrant hypertrophied septomar-ginal trabeculations or an abnormal moderator band that divides the RV cavity into a proxi-mal high-pressure chamber and a distal low-pressure chamber [19, 35, 36] (Fig. 8). The se-verity of double-chambered RV stenosis tends to increase with time [36]. Double-chambered RV is usually associated with a perimembra-nous VSD. MRI and CT are usually diagnos-tic, identifying the degree and location of the obstruction and the presence of a VSD. The degree of stenosis can be best quantified with MRI phase-contrast techniques. The indica-tions for surgery in double-chambered RV are similar to those for pulmonary valve stenosis (peak gradients > 50 mm Hg). Muscular resec-tion and correction of VSD have excellent long-term results and low rates of recurrence [37].

Changes after RVOT repair —Most patients with TOF in adult life have undergone either palliative or total repair early in life. Total re-pair involves a patch closure of the VSD and relief of the RVOT obstruction. In TOF, more than one third of patients receive a transan-nular RVOT patch using pericardium, poly-

ethylene terephthalate, or polytetrafluoroeth-ylene, and 10% of patients with TOF receive valved conduits (Hancock, homograft, or bo-vine jugular vein) [38]. An extracardiac con-duit interposition between the RVOT and the main pulmonary artery may be necessary in the presence of pulmonary atresia or an anom-alous left coronary artery crossing the RVOT [39]. Cryopreserved valved aortic homografts are more popular than pulmonary homografts, but accelerated aortic homograft fibrocalcifi-cations have been described [40]. The Conteg-ra valved bovine jugular vein has been used as a better alternative to homografts in RVOT reconstruction [41]. The diameter of the grafts is 12–22 mm, and their length is 10–12 cm. High pressure in the conduit may lead to an-eurysmal dilatation (one third of the conduits) and valve regurgitation. Polyethylene tereph-thalate conduits are least popular for exten-sive fibrous sheathing and calcification. Con-duit narrowing at the pulmonary anastomosis (distal suture line) is relatively common and may be associated with conduit dilatation. A complete assessment of the pulmonary arteri-al system with CT or MRI may be necessary before RVOT reoperation to find associated complications [42]. A finding of substantial branch pulmonary artery stenosis, especially in the setting of free pulmonary regurgitation, should be treated by balloon dilation with or without implantation of an endoluminal stent. Repeat sternotomy should be performed with special care in patients who have undergone outflow tract surgery because of the risk of conduit adherence to the sternum. CT of the chest is helpful in these complex patients.

Recently, percutaneous valve replacement has been deployed successfully in RV out-flow conduits and has now been extended to include patients with native PS [43, 44]. Documentation of severe pulmonary regur-gitation, exercise limitation, arrhythmia, and excessive RV dilatation, defined by an RV-to-LV ratio of 2:1 by MRI, have been used as criteria for valve replacement [42, 45]. The shape of the RVOT is a major determinant of suitability for percutaneous pulmonary valve replacement. This can easily be done by CT

TaBLE 2: Types of Congenital Pulmonary Valve Stenosis

Type of Stenosis Description

Dome shaped Very common (80–90% of all congenital right ventricular outflow tract lesions); low familial inheritance; 2–4 raphes but no separation into valve cusps; treated by balloon valvotomy

Dysplastic 10–20% of cases; trileaflet with markedly thickened leaflets; associated with hypoplastic ventriculoarterial junction and Noonan syndrome; treated with partial or total valvotomy or a transannular patch

Bicuspid or quadricuspid Rare; usually asymptomatic; common with other congenital heart diseases

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or MRI. Different RVOT shapes exist (Fig. 7). An aneurysmal (pyramidal) [45] shape is the most common (50%) and is related to the presence of a transannular patch. This shape is not suitable for percutaneous pulmonary valve implantation. In patients with conduits, other shapes are more common. The current device for pulmonary valve implantation, made of a platinum-iridium alloy, performs best in cylindric rigid (to avoid fracture) RV-OTs that measure 14–22 mm in diameter. These requirements make the device unsuit-able for most patients.

Other Conotruncal AnomaliesDORV—DORV is a type of abnormal ven-

triculoarterial connection in which both great vessels arise entirely or predominantly (> 50% circumference) from the RV [46]. A new clas-sification scheme defines four types of DORV according to the clinical presentation and sur-gical treatment approach: VSD type (24%), TOF type (36%), TGA type (Taussig-Bing) (18%), and DORV noncommitted VSD (22%) [46–48]. The VSD type of DORV is usually large and has four potential locations: subaor-tic, subpulmonic, doubly committed, or re-mote noncommitted [46]. MRI is accurate in pre- and postoperative assessment of patients with DORV [49]. The spatial relationship be-tween semilunar valves, great arteries, outlet septum, and VSD can be accurately assessed by MRI [49, 50]. Data for the role of CT in DORV are limited. In one study using electron beam CT, the range of diagnostic accuracy for all VSD types in DORV was 88–100% for 3D CT and 71–94% for echocardiography [51]. CT also provides clear delineation of the out-let septum, which defines the location of the VSD. The outlet septum attaches to the anteri-or or posterior limbs of septomarginal trabec-ulations in subaortic or subpulmonic VSDs, respectively. In the doubly committed VSD, the muscular outlet septum is absent [47]. The arterial trunks may vary in location, with the aorta generally to the right of the pulmonary trunk (Fig. 8). If the trunks spiral as they leave the base of the heart, the VSD is usually sub-aortic. If the trunks are parallel with the aor-ta anterior and rightward, the VSD is usually subpulmonic. When the VSD is only under the pulmonary trunk, the configuration is called the Taussig-Bing heart [18, 47]. Usually, there is no fibrous continuity between the semilunar and atrioventricular valves, with both great ar-teries arising predominantly from the RV.

Postoperative DORV—Depending on the anomaly, different surgical methods are used

in DORV. In the unrestrictive subaortic VSD type of DORV, the VSD is closed to include the aortic valve as part of the LV, creating a tunnel that excludes the RV from the sys-temic circulation. An intraventricular tunnel made of a polytetrafluoroethylene patch can baffle blood from the LV through the VSD to the aorta [52, 53]. In the TOF type of DORV, there is usually a subaortic VSD with PS. A Rastelli repair is performed, with the cre-ation of an intraventricular tunnel to baffle the LV to the aorta and the placement of an RV-to-pulmonary-artery conduit (valved ho-mograft) (Fig. 8). The TGA type of DORV usually has a subpulmonary VSD without PS. Complete repair with an arterial switch operation and a VSD-to-pulmonary-artery baffle is required in the neonatal period. Re-pair of DORV with a remote noncommitted VSD can be very complex [52]. In postoper-ative cases, MRI or CT can easily show the shape and patency of both outflow tracts. In postoperative patients, issues that should be assessed with imaging include the status of both ventricles, any evidence for subaortic or subpulmonary obstruction if a tunnel opera-tion has been performed, the presence of a residual VSD, and evidence for conduit ste-nosis or regurgitation.

Complete TGA—In complete TGA, ven-triculoarterial discordance exists, meaning that the aorta arises from the morphologic RV and the pulmonary artery arises from the mor-phologic LV [54, 55] (Figs. 9A and 9B). The most common outflow tract arrangement in TGA is with the aorta to the right and anterior to the pulmonary artery (56%) and the great arteries parallel, rather than crossing, as they do in the normal heart [56, 57] (Fig. 9A). The second most common arrangement is with the aorta just anterior to the pulmonary artery [57]; the rarest fetal variant is the side-by-side arrangement, with the aorta on the right and in front of the tricuspid valve [56]. RV dysfunc-tion and pulmonary hypertension are recog-nized late outcomes after the Mustard or Sen-ning procedure [58, 59]. In the arterial switch procedure, the pulmonary artery is brought forward anterior to the aorta, and the coronary buttons are sutured into the “neoaorta” [60]. Complications can be shown by CT or MRI. These include distortion of the RVOT and pul-monary arteries, neoaortic root dilatation with aortic regurgitation, and, rarely, coronary ar-tery stenosis [60].

Congenitally corrected TGA—In congeni-tally corrected TGA, blood flows in the nor-mal direction but through the “wrong” ven-

tricle (Figs. 9C and 9D). The morphologic LV and mitral valve supply the pulmonary circulation, and the morphologic RV and tricuspid valve supply the systemic circula-tion [61, 62]. The most common anatomic arrangement is situs solitus, with L-looping of the ventricles and the aorta anterior and leftward of the pulmonary artery [61]. At the earliest sign of deterioration in systemic ventricular function, systemic atrioventricu-lar valve regurgitation should be suspected [63, 64]. Most centers would not recommend a prophylactic double switch procedure for patients without associated abnormalities in whom RV and tricuspid valve function is normal. Regular assessment of ventricular function using cardiac MRI every 3 years is suggested [63].

Truncus arteriosus—Truncus arteriosus consists of a single arterial trunk giving ori-gin to the pulmonary arteries, coronary ar-teries, and the systemic circulation [65]. Sev-eral classifications of the common trunk have been proposed on the basis of the origins of the pulmonary arteries [65, 66]. Progressive dilatation of the common trunk as a result of cystic medial necrosis is common. The com-mon trunk usually overrides a large nonre-strictive VSD resulting from the absence of the infundibular septum [18]. It lies between the two limbs of the septomarginal trabecu-lation. The truncal valve is usually tricuspid but can vary between one and six cusps [18]. The basic repair involves closing the VSD and separating the pulmonary arteries and attaching them to a valved conduit arising from the RVOT [67] (Fig. 10). Most patients undergo reoperation within 10–12 years for conduit replacement (usually because of the small size of the original conduit) or trun-cal valve replacement because of valvular in-sufficiency. Truncus arteriosus should not be mistaken for hemitruncus [68]. Hemitrun-cus is best defined as a condition in which one branch of the pulmonary artery (usually the right) originates from the ascending aor-ta and the other branch has a normal course arising from a normal main pulmonary ar-tery (Fig. 10).

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Fig. 1—Developing outflow tract in embryonic chicken hearts at Hamburger-Hamilton stages 12, 24, 30, and 36. Area of outflow tract extends between distal ventricular groove (DVG) of right ventricle (RV) and junction with aortic sac at pericardial reflections and is divided into conus (proximal outflow tract, red) and truncus (distal outflow tract, light blue). Junction between conus and truncus is distal myocardial border (DMB). Images show that outflow tract is initially mainly myocardial (red) in its entirety, increasing in length up to Hamburger-Hamilton stage 24. Outflow tract myocardium subsequently shortens as result of ventricularization, contributing to trabeculated free wall, as well as infundibulum, of RV. Note absolute reduction in length of outflow tract between Hamburger-Hamilton stages 30 and 36, as well as relative reduction in relationship to ventricles, which have increased in size by cardiomyocyte proliferation. Outflow tract has also been divided by septation into pulmonic and systemic outflows, and aortic root has rotated to posterior position, where it connects with left ventricle (LV). Dotted line around heart indicates pericardium. A = primitive atrium, LA = left atrium, RA = right atrium, SHF = secondary heart field, SV = sinus venosus, V = primitive ventricle, VG = ventricular groove.

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a

Fig. 2—Right anterior oblique views (two chamber) of right heart. A, Cadaveric specimen. Inlet portion is outlined in green, and ventriculoinfundibular fold is outlined in red. AA = ascending aorta, CS = coronary sinus, IVC = inferior vena cava, MPA = main pulmonary artery, SVC = superior vena cava, TV = tricuspid valve.B, Volume-rendered CT shows that right ventricle (RV) comprises three components: inlet, apical trabecular, and outlet portions. Inlet portion of RV surrounds leaflets of TV and supports its papillary muscles and tension apparatus (green outline). Apical portion of RV is characterized typically by trabeculated muscle bundles. Outflow tract separates TV and pulmonary valve (PV). Axis of orifices of inlet and outlet roughly forms 60° angle. Inner heart curve between atrioventricular and arterial valves is called ventriculoinfundibular fold (red outline). RA = right atrium, RAA = right atrial appendage.C, Fold includes supraventricular crest at its septal margin, where it inserts between anterior and posterior limbs of septomarginal trabeculation. It is separated from right aortic sinus by epicardial fat (red line). Inferior central part is called outlet septum. This part is very small and cannot be resolved from septomarginal trabeculation by CT scan. Location of outlet septum (yellow arrow) above level of medial papillary muscle (MPM) is seen. IVS = interventricular septum, L = left aortic sinus, LA = left atrium, LVOT = left ventricular outflow tract, RVOT = right ventricular outflow tract.

B C

Fig. 3—Images show right ventricle (RV) infundibulum. Posterior wall of RV infundibulum (green arrows) is continuation of infundibulum into ventriculoinfundibular fold (supraventricular crest). It is free-standing wall except for its midinferior most part, which contributes to interventricular septum and also is known as outlet septum (yellow arrows). Defect in this portion can create ventricular septal defect. One slice above it (second axial cut) at orifice of right coronary artery (RCA) shows extension of fat plane between ascending aorta and right ventricular outflow tract (RVOT). RCA can be injured at time of pulmonary valve (PV) surgery. Ventriculoinfundibular fold is located between anterocephalad (septal band) (a) and posterocaudald (parietal band) (p) limbs of septomarginal trabeculations (SMTs), and its width is shown by double-headed purple arrows. Cradle between SMT limbs and ventriculoinfundibular fold form supraventricular crest. Note smooth surface of ventriculoinfundibular fold compared with rest of RV infundibulum, which is trabeculated. Anterior limb runs superiorly into infundibulum and supports leaflets of PV. Multiple muscular bundles that extend from cephalad margin of septomarginal trabeculation and run onto parietal wall of outflow tract are designated as septoparietal trabeculations (SPTs) (red arrows). Blue arrows denote medial papillary muscle. AA = ascending aorta, APM = anterior papillary muscle, AV = aortic valve, LVOT = left ventricular outflow tract, MB = moderator band, RA = right atrium, SAX = short axis.

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Fig. 4—Images show pulmonary valve sinuses. When heart is viewed in attitudinal anatomic position as sitting in thorax (i.e., axial views, left), pulmonary leaflets and sinuses are seen to be posterior (P), right anterolateral (Ra), and left anterolateral (La). However, in relation to heart (i.e., short axis [SAX] views, middle), pulmonary sinuses can be named anterior (A), left posterior (Lp), and right posterior (Rp). Relationship of pulmonary and aortic valve (green and red circles, top left and middle) as well as their orientation in relation to body (axial) and heart (SAX) are drawn. Black line shows location of interatrial septum. At top right, crossover arrangement between left (red arrow) and right (blue arrow) ventricular outflow tracts is also shown. Term “pulmonary annulus” is used to denote semilunar fibrous attachment of each of pulmonary leaflets. They are much less sturdy than aortic annuli. L = left aortic sinus, LA = left atrium, N = noncoronary sinus, R = right aortic sinus, STJ = sinutubular junction, VAJ = ventriculoarterial junction.

Fig. 5—Dome-shaped pulmonary valve (PV) in two different patients. A, PV is shown from arterial aspect. It is characterized by narrow opening and incomplete separation of valve cusps. B, 46-year-old man with dysplastic pulmonary stenosis. CT shows mild supravalvular stenosis (arrow, bottom). Note trileaflet thickened valve with hypoplastic ventriculoarterial junction (top). In dysplastic PV, there are three distinct cusps and no commissural fusion. Obstructive mechanism is related to markedly thickened immobile cusps, characterized by presence of disorganized myxomatous tissue.

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Fig. 6— 36-year-old woman with tetralogy of Fallot (TOF) (left) and 55-year-old woman with normal heart (right). Left ventricle (LV) three-chamber (top) and right ventricle (RV) two-chamber (bottom) views are presented for comparison. In TOF, subaortic ventricular septal defect is closed with polyethylene terephthalate patch. In this example, infundibulum is short (blue arrows, bottom) and shows mildly thickened septoparietal trabeculations. No right ventricular outflow tract (RVOT) stenosis is seen. This is Eisenmenger defect. Malaligned superiorly displaced outlet septum is seen (yellow arrow, bottom left). Membranous septum (MS) is absent, and septal tricuspid (STV) leaflet is almost in fibrous continuity with aortic leaflet. In three-chamber view, left ventricle outflow tract (LVOT) appears longer in TOF, possibly because of dextroposition of aortic root. Note that interventricular communication (demarcated by patch) is cradled between limbs of septomarginal trabeculation. In normal heart, outlet septum is absent or involves small portion of supraventricular crest (red arrow, top right). AA = ascending aorta, RA = right atrium.

Fig. 7—Five different patients with repaired tetralogy of Fallot (TOF). Shape of right ventricular outflow tract (RVOT) is major determinant of suitability for percutaneous pulmonary valve replacement. Untreated TOF in 54-year-old woman is shown for comparison (top left). Type 1 (top middle) is aneurysmal (pyramidal) shape and most common (50%) and not good candidate for valve replacement. Type 2 (top right) is cylindric with constant diameter (14%). Type 3 (bottom left) has inverted pyramidal appearance (3%), type 4 (bottom middle) is fusiform (17%), and type 5 (bottom right) is narrow and tubular (13%). Note stenosis at distal end of homograft in fusiform RVOT (bottom middle). MRI or CT is necessary for 3D analysis and appropriate measurements. MRI assessment of right and left pulmonary arteries is necessary for hemodynamic analysis in each candidate.

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Fig. 8—Examples of double-chambered right ventricle (RV) and double-outlet RV (DORV) in two different patients. Images in top row show 48-year-old woman with double-chambered RV (DCRV). Short-axis MRI examinations are shown. Severe right ventricular outlet tract stenosis is seen as result of thickened septomarginal and septoparietal trabeculations (stars, top left). Jet flow is seen in systole (arrows, top right). Thickened wall RV inlet is shown. Images in bottom row show 22-year-old man after Rastelli repair for DORV. Axial (bottom left) and coronal (bottom right) MRI scans are shown. Note side-by-side position of aortic (a) and pulmonary (p) valves and outlet septum between them (stars, bottom). Intraventricular tunnel made of polytetrafluoroethylene patch (arrow, bottom right) was used to baffle left ventricular blood to aorta. LA = left atrium, LV = left ventricle, RA = right atrium.

Fig. 9—Outflow tract in transposition of great arteries (TGA) and congenitally corrected TGA in two different patients. A and B, 40-year-old man with TGA after atrial switch. In TGA, aortic valve (a) is located anterior to pulmonary valve (p) in most cases. There is fibrous continuity of mitral and pulmonary valves. Intraatrial baffle (yellow arrow, A) shifts deoxygenated blood of superior vena cava and inferior vena cava into left atrial (LA) vestibule, left ventricle (LV), and main pulmonary artery (MPA). Right ventricle is systemic ventricle and will be hypertrophied. RA = right atrium.C and D, 23-year-old woman with congenitally corrected TGA. In congenitally corrected TGA, ventricles are congenitally inverted with LV more anterior ventricle. Pulmonary (p) and aortic (a) valves are usually side by side with aorta on left. This anatomic arrangement of great arteries can be rarely seen in TGA (< 10%). Pulmonary artery in congenitally corrected TGA arises directly from LV with direct fibrous continuity between mitral and pulmonary valves. RV = right ventricle.

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Fig. 10—Examples of surgically repaired valves. Images in top row show 30-year-old woman with repaired truncus arteriosus. Prosthetic pulmonary valve (PV) is placed in right ventricle–pulmonary artery conduit. Conduit homograft is partially calcified. Images in bottom row show 25-year-old man with hemitruncus with anomalous origin of right pulmonary artery (RPA) from ascending aorta (AA). Patent ductus arteriosus (PDA) also exists. CT data are obtained at pulmonary phase, and only right heart is opacified. Left pulmonary artery (LPA) originates normally from main pulmonary artery (MPA). Delayed opacification of RPA as result of its origin from aorta has given double contrast to left atrium (LA) with incomplete filling of right pulmonary veins. AV = aortic valve, DA = descending aorta, LV = left ventricle, RV = right ventricle, SVC = superior vena cava.

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Right Ventricular Outflow Tract Imaging With CT and MRI

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