Magnetic resonance imaging of congenital heart disease: evaluation of morphology and function

Magnetic Resonance Imaging of Congenital Heart Disease: Evaluation of Morphology and Function Gautham P. Reddy and Charles B. Higgins

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HE PRIMARY GOALS of imaging in congenital heart disease (CHD) are the precise depiction of the morphology and appraisal of the function of the heart and great vessels. Because surgical repair and palliation of complex congenital cardiac anomalies are possible, accurate delineation of the lesions is imperative. Magnetic resonance imaging (MRI) has proven to be effective for anatomic evaluation of CHD1 and for quantitative functional assessment of the cardiovascular system.2,3 The advantages of MRI over angiography are its noninvasiveness, its ability to show extracardiac structures as well as the heart, its absence of ionizing radiation, and its capability of producing images without the use of iodinated contrast media. Echocardiography remains the primary noninvasive imaging modality used in the evaluation of CHD, in large part because it is widely available and because pediatric cardiologists are familiar with it. However, MRI has an important role as an alternative imaging technique and as an adjunct to echocardiography, especially in the assessment of supracardiac structures, delineation of complex CHD, quantification of function, and postoperative monitoring. INDICATIONS

Indications for cardiac MRI in CHD include morphological and functional evaluation before surgery and in the postsurgical period. The most important clinical indications are (1) demonstration of anatomy in patients with situs abnormalities; complex ventricular anomalies; or abnormalities of the pulmonary arteries, pulmonary veins, systemic veins, or thoracic aorta; (2) quantification of function, including calculation of right and left ventricular masses, measurement of collateral blood flow and pressure gradients in coarctation,

From the Department of Radiology, University of California, San Francisco, San Francisco, CA. Address reprint requests to Gautham P. Reddy, MD, Department of Radiology, Suite M396, 505 Parnassus Avenue, Box 0628, University of California, San Francisco, San Francisco, CA 94143-0628. © 2003 Elsevier Inc. All rights reserved. 0037-198X/03/3804-0009$30.00/0 doi:10.1053/S0037-198X(03)00055-5 342

and differentiation of blood flow between right and left pulmonary arteries; and (3) postsurgical imaging, including depiction of anatomy and postsurgical complications, and functional appraisal, such as measurement of conduit blood flow and pressure gradients. TECHNIQUES

The multislice spin-echo MRI technique with electrocardiographic gating and respiratory compensation is optimal for the precise depiction of anatomy in patients with CHD. Transverse images from the aortic arch to the cardiac apex are usually performed. Short- or long-axis magnetic resonance (MR) images of the heart are acquired, especially when quantifying ventricular volumes and function. The short-axis plane is prescribed from the long-axis view, which in turn is defined from the coronal or transverse planes. Coronal, sagittal, or oblique sequences can complement the transverse images. In general, 5-mm sections are used; however, thin (3 mm) slices are required for more detailed evaluation of the area of interest, especially in lesions such as aortic coarctation, septal defects, and complex anomalies. MR angiography, performed during the bolus intravenous administration of gadolinium chelate contrast agent, affords precise delineation of the thoracic aorta, pulmonary arteries and veins, and other vessels.4,5 Cine MRI is composed of multiple-gradient echo images obtained at different phases of the cardiac cycle. The cine technique can show abnormal blood flow in the heart and great vessels,6,7 permitting the diagnosis of valvular stenosis or regurgitation, ventricular outflow stenosis, or shunting across a septal defect. Cine MRI performed through the ventricles allows assessment of ventricular contraction and function. In addition, short-axis cine MRI acquired through the entire heart can be used to measure ventricular volumes and ejection fractions. Velocity-encoded cine MRI, a phase-contrast technique, permits the quantification of flow velocity and volume in vessels and across shunts. Because the most common reason for nondiagnostic examinations is patient motion, young children (under the age of 7 or 8 years) should be sedated or anesthetized for the study. Anesthesia

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using short-acting drugs is optimal to control patient motion for the duration of the examination. During anesthesia or deep sedation, the child’s electrocardiogram, heart rate, respiratory rate, oxygen saturation, expiratory pCO2, and blood pressure must be monitored. ANATOMIC EVALUATION

Situs Abnormalities MRI shows the characteristic features of the atria and ventricles, allowing the determination of the type of situs and the ventricular loop. The atria are distinguished by the configurations of their appendages: the left atrial appendage is tubular and has a narrow ostium, and the right atrial appendage is triangular, with a wider ostium. The inferior vena cava is almost always connected to the morphologic right atrium. The right ventricle can be identified most reliably by the presence of a muscular infundibulum separating the atrioventricular and semilunar valves. In the left ventricle, the atrioventricular and semilunar valves are in fibrous continuity. Other distinguishing features of the right ventricle are its moderator band at the apex and coarse trabeculations at the apical septum, as opposed to the smooth apical septum of the left ventricle. In situs solitus, the morphologic right atrium is located on the right, and the morphologic left atrium is on the left. There is a D-ventricular loop, with the anatomic right ventricle located to the right of the left ventricle. In situs inversus, the anatomic right (systemic venous) atrium is on the left, and the anatomic left (pulmonary venous) atrium is on the right. An L-ventricular loop is present, with the right ventricle positioned to the left side of the left ventricle. The diagnosis of isomerism can be made with MRI by showing the symmetric relationship of the bronchi and pulmonary arteries.8 CHD is usually present in patients with isomerism; right-sided isomerism is associated with asplenia syndrome, and left-sided isomerism with polysplenia syndrome. Abnormalities of Ventriculoarterial Connections This category of lesions includes transposition of the great arteries, double-outlet right and double-outlet left ventricle, and truncus arteriosus. MRI readily shows the morphology of these anomalies, including the connections of the ventricles to

Fig 1. Complete transposition of the great arteries. Axial spin-echo MR image showing the aorta (A) anterior to the main pulmonary artery (PA), a characteristic finding in this anomaly.

the great vessels and the relationship of the aorta and pulmonary artery to each other.1,9-12 Transposition of the great arteries. In complete transposition of the great arteries, transverse images reveal the aorta anterior and to the right of the main pulmonary artery (Fig 1). Transverse and coronal images show the aorta arising from the normally positioned right ventricle and the pulmonary artery arising from the normally situated left ventricle (D-ventricular loop). MRI depicts the thick walls and circular shape of the right ventricle compared with the thinner wall and elliptical configuration of the left ventricle, which are characteristic of d-transposition. Associated anomalies, such as a ventricular septal defect (VSD) or pulmonary stenosis, also can be diagnosed by MRI.10 In congenitally corrected transposition (l-transposition, L-ventricular loop), the aorta is anterior and to the left of the pulmonary artery. Transverse and coronal MR images depict the origin of the aorta from the morphologic right ventricle, which is connected to the left atrium and is positioned to the left of the left ventricle.13 MRI can be used in the postoperative evaluation of patients who have undergone arterial switch (Jatene) procedure12 or atrial baffle (Mustard or Senning) procedure.14 After arterial switch surgery, MRI can be used to show stenosis of the right

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the truncus with a Rastelli conduit, transverse and sagittal MRI can be used to identify stenoses at the anastomotic site or in the central pulmonary arteries. Tetralogy of Fallot and Pulmonary Atresia

Fig 2. Jatene arterial switch procedure for transposition. Axial spin echo MRI depicts a complication of this procedure: stenosis of the left pulmonary artery (arrow) secondary to compression by the ascending aorta (A).

or left pulmonary arteries, an occasional complication of this operation (Fig 2). Double outlet right ventricle. Transverse and coronal MR images through the base of the heart show the characteristic parallel relationship of the aorta and the pulmonary artery, both of which arise from the right ventricle.15 On occasion, the aorta originates anterior to the pulmonary artery. Transverse images can depict a circle of myocardium (the right ventricular infundibulum) interposed between the atrioventricular valve from the aortic and pulmonary valves. Side-by-side rings of muscle may be identified in the right ventricular outflow tract. The location and size of the VSD can also be evaluated on transverse sections. When valvular or subvalvular pulmonary stenosis is present, double outlet right ventricle (DORV) may be clinically and angiographically similar to tetralogy of Fallot. Transverse MR images can distinguish these 2 anomalies, confirming the diagnosis of DORV by demonstrating a rim of muscle separating the aortic and semilunar valves. Truncus arteriosus. Transverse, coronal, and sagittal MRI planes depict a truncus located over both ventricles, straddling a VSD. Sagittal and coronal images can reveal the origins of the pulmonary arteries from the truncus. MRI can also provide other relevant information, such as the relative sizes of the ventricles. In both truncus arteriosus and pulmonary atresia, a single great artery takes rise from the heart. The diagnosis of truncus arteriosus can be excluded by showing a small infundibular chamber on transverse MR images, thereby establishing the diagnosis of pulmonary atresia. After operative repair of

The features of tetralogy of Fallot are a perimembranous VSD; an anteriorly located aorta overriding the VSD; infundibular pulmonary stenosis, and frequently valvular, supravalvular, or peripheral pulmonary stenoses; and right ventricular hypertrophy. Sagittal and transverse MRI demonstrate the stenotic right ventricular outflow region, the VSD, the overriding aorta, and the relative sizes of the aorta and the main pulmonary artery (Fig 3).1,16 MRI is the favored modality for imaging pulmonary arterial stenoses, which occur commonly in tetralogy of Fallot. These stenoses are optimally evaluated with oblique planes parallel to the pulmonary arteries. Pulmonary arteries can sometimes be more easily visualized on gadolinium-enhanced MR angiography than on a spin-echo sequence. Because it does not require opacification of the pulmonary arteries with contrast agent, MRI is the definitive method for determining the existence of central pulmonary arteries and of a central confluence of the right and left pulmonary arteries in patients who have pulmonary atresia with VSD. Transverse MR images in patients with this anomaly reveal a continuous band of muscle in the right ventricular outflow tract, indicating infundibular obstruction.17,18 Pulmonary atresia can be focal, involving only the valve, or it may involve a longer segment. Focal membranous pulmonary atresia may be difficult to differentiate from critical stenosis on spin-echo MR imaging. Cine MRI can be used to establish whether there is flow across the valve. Pulmonary atresia can be divided into 2 groups based on the presence or absence of a VSD. Pulmonary atresia with VSD is a severe variant of tetralogy of Fallot. The aorta overrides the VSD, and the central and peripheral pulmonary arteries are frequently stenotic, hypoplastic, or atretic; one of the principal advantages of MRI in these patients is the ability to appraise the central pulmonary arteries distal to the atresia. Transverse scans can be used to determine the presence and extent of atresia in the infundibulum and in the main pulmonary artery. Stenoses and atresia in the left and right pulmonary arteries are best defined with

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Fig 3. Tetralogy of Fallot. (A) Axial gradient echo cine image shows a VSD (arrow) and thickening of the right ventricular wall, indicating hypertrophy. (B) Coronal gradient echo cine image shows that the aorta (A) overrides the VSD (arrow). (C) Sagittal gradient echo cine image depicts a poststenotic flow jet (arrow) in the right ventricular outflow tract and pulmonary artery, indicating infundibular stenosis. (Courtesy of James Scatliff, MD.)

thin-section (3 mm) oblique spin-echo images parallel to these vessels. Contrast-enhanced MR angiography can also be used but may not be effective in the delineation of the vessels distal to the site of atresia. MRI can show collateral blood supply to the lungs, most effectively with use of MR angiography and gradient echo cine images. MRI is ideal for the presurgical evaluation of the size of the central pulmonary arteries and the presence of a central confluence, factors that influence the decision to perform a Rastelli procedure. After operation, MRI can be used to demonstrate surgical shunts and their anastomoses. The postoperative condition of the pulmonary arteries is optimally assessed by MR images in the transverse and oblique (parallel to the left and right pulmonary arteries) planes. Pulmonary atresia with intact ventricular sep-

tum. In pulmonary atresia with intact ventricular septum, the pulmonary arteries are usually normal or almost normal in size, without stenoses. The size of the right ventricle is variable, from marked hypoplasia to dilatation; transverse and sagittal sections are ideal for the evaluation of right ventricular size. Pulmonary atresia with intact ventricular septum is not related to tetralogy of Fallot. Ventricular Septal Defect Spin-echo MRI can identify VSDs with an accuracy of greater than 90%.19 Transverse sections are especially useful because they demarcate the inflow and outflow regions of the ventricular septum (see Fig 3A).20 A perimembranous VSD, the most common type, is depicted on transverse images below the aortic root. MRI readily shows VSDs of the inlet septum, associated with atrio-

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ventricular septal defect (formerly known as endocardial cushion defect).21 The other components of the atrioventricular septal defect, an ostium primum atrial septal defect and a common atrioventricular valve, also can be seen on transverse MRI. Transverse, coronal, and sagittal MR scans delineate defects that involve the outlet septum, such as subpulmonic VSDs and malalignment VSDs, which are associated with tetralogy of Fallot, truncus arteriosus, and DORV. The size of the VSD is readily identified on transverse images. Infracristal and supracristal VSDs can be distinguished using short-axis or transverse sections through the right ventricular outflow tract. Identification of myocardium between the pulmonary valve and the septal defect indicates an infracristal VSD. Lack of myocardium is diagnostic of a supracristal VSD.22 Abnormalities of Atrioventricular Connections Atrioventricular (AV) connections can be classified as concordant or discordant. In normal (concordant) AV connections, the right atrium connects to the right ventricle, and the left atrium to the left ventricle. Congenitally corrected transposition (ltransposition, L-ventricular loop) is one type of discordance. The tricuspid and mitral valves always remain with the right and left ventricles, respectively. Patients with abnormalities of AV connections frequently have anomalies of ventriculoarterial connections as well. Transverse and coronal MRI readily depicts the complex morphological abnormalities. Specific anomalies of AV connections include complex lesions such as double-inlet ventricle (single ventricle), straddling AV valve (valve straddles the ventricles and drains into both ventricles), and atresia of the tricuspid valve or mitral valve. In patients with a single ventricle, both AV valves connect to 1 dominant ventricle, a morphologic left ventricle that connects to both the pulmonary artery and the aorta. In tricuspid atresia, MRI can identify a layer of muscle and fat separating the right atrium from the right ventricle. Associated anomalies in tricuspid atresia include atrial and ventricular septal defects, pulmonary stenosis or atresia, and transposition of the great arteries.

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Complex Ventricular Anomalies A number of reports have shown the utility of MRI in the assessment of complex cyanotic heart disease.8,23-26 Transverse MR images can effectively show situs, the position of the cardiac apex, the type of bulboventricular loop, and the relationship of the great arteries.23-25 In patients with splenic and heterotaxy syndromes, MRI of the upper abdomen displays absence of the spleen (asplenia) or the presence of multiple small spleens (polysplenia). MRI also depicts interruption of the inferior vena cava, seen in polysplenia, or bilateral superior venae cavae (more common in asplenia). MRI is capable of distinguishing complex ventricular anomalies, which are sometimes difficult to differentiate by cineangiography. Two reports have shown the advantages of MRI compared with angiography in the evaluation of complex ventricular lesions.23,25 These anomalies include single ventricle of the double-inlet type, atresia of an AV valve with hypoplasia of its respective ventricle, common AV valve, and common ventricle (absent ventricular septum). MRI delineates the size of the VSD, the septal thickness, the sizes of the ventricles, connections of the AV valves, and the ventriculoarterial connections. Atrial Anomalies MRI can reveal and precisely define the location and size of an atrial septal defect (ASD) (Fig 4A). Diagnosis of an ASD is considered definitive if the defect is visualized on 2 consecutive transverse MRI sections or if it is observed on cine MRI during several phases of the cardiac cycle at 1 anatomic level. The shunt across the ASD is shown as a signal void with cine MRI. One article has shown greater than 90% sensitivity and specificity for MRI in the diagnosis of ASD.27 MRI shows a sinus venosus ASD as the absence of the portion of the atrial septum between the superior vena cava and the left atrium. MR images can also display partial anomalous pulmonary venous connection (Fig 4B), which is associated with ASD, especially sinus venosus ASD. An advantage of MRI compared with echocardiography is the greater accuracy for demonstrating partial anomalous pulmonary venous connections in patients with ASD. Secundum ASDs are depicted as defects in the center of the atrial septum. The perimeter of the secundum defect is often thickened, a characteristic useful in differentiating

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Fig 4. Secundum atrial septal defect associated with partial anomalous pulmonary venous connection. (A) Axial gradient-echo cine image shows the atrial septal defect (arrow). (B) Coronal maximum intensity projection of a gadolinium-enhanced MR angiogram shows an anomalous right upper lobe pulmonary vein draining into the superior vena cava (S).

a true ASD from MRI signal loss secondary to septal thinning at the fossa ovalis. Transverse scans through the AV valve plane show primum ASDs as defects in the inferior portion of the septum. In patients AV septal defects, MRI shows a primum ASD, an inlet VSD, a common AV valve ring, and absence of a cardiac crux.21 Associated findings include shortening of the inlet ventricular septum and elongation of the left ventricular outflow tract. Spin-echo MR images are usually unable to delineate the cleft mitral valve. AV regurgitation associated with an AV septal defect can be diagnosed and quantified using velocity-encoded cine MRI.

venous connection involves the drainage of the right upper lobe pulmonary vein into the superior vena cava. Scimitar syndrome can be diagnosed with MRI by showing a common right pulmonary vein passing below the diaphragm, usually connecting to the inferior vena cava. Absence of pulmonary veins draining into the left atrium indicates total anomalous pulmonary venous connection. Depending on the site of anomalous drainage, transverse MRI may reveal a common pulmonary vein superior or posterior to the left atrium, an enlarged left superior vena cava, or a dilated coronary sinus. Thoracic Aortic Abnormalities

Pulmonary Venous Abnormalities MRI is highly effective for the appraisal of anomalous pulmonary venous connections. Partial anomalous pulmonary venous connection is associated with ASD, occurring in almost all individuals with sinus venosus ASDs. Therefore, pulmonary venous connections must be defined in all patients with ASD.27,28 Gadolinium-enhanced MRA is optimal for this purpose,29 but the anomalous veins can also be visualized on transverse and coronal MRI planes (Fig 4B). The most common type of partial anomalous pulmonary

MRI is the imaging modality of choice for evaluation of congenital anomalies of the thoracic aorta. Because MRI provides more information than cineangiography in most patients, the latter technique can be reserved for patients who have been diagnosed by MRI or another imaging modality and are candidates for percutaneous transluminal angioplasty and stent placement. Coarctation of the aorta. MRI is an ideal technique for delineation of coarctation, the most common congenital abnormality of the thoracic aorta.30 Contrast-enhanced MR angiography can

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be used to depict the coarctation as well as collateral channels (Fig 5).4 Thin (3 mm) sections in the transverse, sagittal, and oblique sagittal (parallel to the arch) planes are highly accurate in delineating the site of narrowing, the dimension of the distal aortic arch, poststenotic dilatation, and the relationship of arch vessels to the stenotic segment (Fig 6). Because of partial-volume averaging on transverse images, the site of coarctation and its diameter and length are best visualized on sagittal and oblique sagittal images. Velocity-encoded cine MRI can be used to estimate the pressure gradient across the coarctation site.31 Spin-echo MR images can show the presence of enlarged collateral vessels, and velocity-encoded cine MRI is useful for quantifying the volume of collateral blood flow and determining the hemodynamic significance of the lesion.32,33 After repair of coarctation of the aorta, MRI can be used to assess the diameter of the aorta.31 In patients with a long-segment stenosis, MRI can evaluate the patency of aortic bypass grafts. The outcome of balloon angioplasty and stent place-

Fig 5. Coarctation of the aorta. Gadolinium-enhanced MR angiogram depicts the long-segment coarctation (arrow) and profuse, enlarged collateral channels (arrowheads).

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Fig 6. Coarctation of the aorta. Oblique sagittal spin-echo MR image shows a discrete juxtaductal coarctation (arrow), the most common type. Velocity-encoded cine MR imaging was performed through the proximal descending aorta (P) and distal aorta (D) in the planes indicated by the white lines. In the normal individual, blood flow decreases slightly from the proximal to distal aorta. In this patient, flow increased by approximately 35% from the proximal to distal aorta, consistent with marked collateral circulation.

ment, used to treat teenagers and adults with newly diagnosed or recurrent coarctation, can also be shown with MRI. Aortic arch anomalies. MRI displays both the morphology of aortic arch abnormalities, known as vascular rings, as well as airway compression, if present.34,35 Airway compromise is best seen on transverse and sagittal MRI. The arch vessels are readily identified on MRI. Sagittal and transverse images can depict an aberrant retroesophageal subclavian artery. With a left aortic arch, an aberrant right subclavian artery usually has a normal caliber, and narrowing of the trachea or esophagus is unusual. In patients with a right aortic arch, transverse MRI shows the arch on the right side of the trachea and the proximal descending aorta to the right side of the spine. The descending thoracic aorta usually crosses over to the left side, and transverse or coronal images clearly display this transition. If

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there is an aberrant left subclavian artery, its proximal segment is usually dilated. This dilatation, known as a diverticulum of Kommerell, can compress the trachea or mainstem bronchi, resulting in symptoms, usually wheezing or dyspnea. Occasionally, the esophagus is compressed. Sagittal MRI can show this abnormal anatomy. Double aortic arch is diagnosed by identifying the trachea and esophagus between 2 vessels at the level of the arch. MRI is optimal for visualizing these structures. Sagittal and transverse MRI show the sizes of the 2 arches; the right arch is larger and higher than the left in approximately 80% of patients. During operative repair, the smaller arch is usually ligated. FUNCTIONAL ASSESSMENT OF CHD

Cardiovascular function in CHD can be evaluated with various MR imaging techniques, including cine MRI, breath hold (segmented K space) cine MRI, and velocity-encoded cine MRI. To measure ventricular size and function, cine MRI is acquired through the ventricles, and a volume of data is obtained for multiple phases of the cardiac cycle. The stacks of images at end systole and end diastole can be used to quantify ventricular masses and volumes. Stroke volume (SV) (SV ⫽ end systolic volume ⫺ end diastolic volume) and ejection fraction (SV/end diastolic volume), the most useful measure of ventricular function, can also be derived.7,36,37 Velocity-encoded cine MRI can be used to measure blood flow in the vessels.2,3,38 Ideally, the vessel of interest is scanned in a plane orthogonal to the flow direction using a phase contrast sequence. Flow velocity is calculated using a formula in which velocity is proportional to the change in phase angle of protons in motion. Mean blood flow is estimated by multiplying mean velocity and the cross-sectional area of the vessel. On cine MR images, a flow void can indicate valvular regurgitation or jet flow across a stenotic valve. Regurgitant volume in aortic or mitral insufficiency can be calculated by subtracting right ventricular SV from left ventricular SV; for pulmonary or tricuspid regurgitation, left ventricular SV is subtracted from right ventricular SV.39 Velocity-encoded cine MRI can be used to measure the pressure gradient across a stenotic valve or to quantify aortic or pulmonary regurgitation.40 Pulmonary regurgitation is a complication of surgical

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repair of tetralogy of Fallot, and evaluation of this condition is an important indication for MRI. MRI provides 2 ways to quantify shunts. In the first method, right and left ventricular SVs are calculated using cine MRI.7 In the absence of a shunt, the ventricles have equal SVs. With a left-to-right shunt at the atrial level (ASD or partial anomalous pulmonary venous connection), right ventricular SV exceeds left ventricular SV by the volume of the shunt. If a left-to-right shunt is because of a patent ductus arteriosus, left ventricular SV will exceed right ventricular SV. The second technique uses velocity-encoded cine MRI to measure blood flow in the ascending aorta and in the main pulmonary artery.2,41 Normally, flow is equal in these 2 vessels. In the presence of a left-to-right shunt in patients with atrial septal defect, partial anomalous pulmonary venous connection, or ventricular septal defect, blood flow in the pulmonary artery exceeds flow in the aorta by the quantity of the shunt. The pulmonary-to-systemic flow ratio (Qp/Qs), equal to the pulmonary artery flow divided by the aortic flow, is an important indicator of the severity of the shunt.38 Collateral flow in coarctation of the aorta can be quantified with velocity-encoded cine MRI (Fig 6).32,33 Volume of blood flow is determined at 2 sites in the descending thoracic aorta, just beyond the coarctation and at the level of the diaphragm. In the normal individual, the volume of blood flow is slightly lower in the distal aorta than in the proximal aorta because intercostal arteries take some blood from the descending aorta. However, patients with coarctation may have higher flow distally, indicating retrograde collateral blood flow into the aorta via the intercostal arteries and the branches of the subclavian arteries. The volume of collateral circulation is equal to the difference between distal and proximal flow. Velocity-encoded cine MRI has the distinctive ability to quantify blood flow separately in the left and right pulmonary arteries.42 This technique can also be used to measure pressure gradients across stenoses of pulmonary arteries or surgical shunts43 and across coarctation of the aorta.31 EVALUATION OF THE POSTOPERATIVE PATIENT

As surgical repair of CHD becomes more sophisticated and successful even for complex anomalies, precise imaging is critical for monitoring

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postoperative patients. MRI is an ideal modality for assessing these patients (Fig 2).25 Because it can effectively define extracardiac anatomy, MRI is most valuable in the evaluation of patients who have undergone supracardiac surgery. These operations, involving shunts or anastomoses above the heart, include systemic-to-pulmonary artery conduits, and the Jatene, Damas, Rastelli, Norwood, and Fontan procedures.12,24,25,44 MRI is also useful for monitoring patients who have had intracardiac surgery. Intracardiac procedures that can be appraised by MRI include right ventricular outlet repair in tetralogy of Fallot, Mustard and Senning reconstructions, which used to be performed to repair complete transposition of the great arteries,15,45 and intraventricular baffles for correction of double-outlet right ventricle. MRI readily depicts postsurgical complications such as anastomotic stricture, closure, or pseudoaneurysm; pulmonary artery stenosis (such as after

Jatene arterial switch procedure for transposition of the great arteries)46; and hematoma, lymphocele, or seroma. Velocity-encoded cine MRI is used for functional assessment of the postoperative patient to show shunt patency and flow,43 to measure pressure gradients across anastomoses and stenotic vessels,31,43 to assess flow dynamics,47 to calculate blood flow in the pulmonary arteries,42 and to quantify the volume of pulmonary regurgitation after repair of tetralogy of Fallot.40 CONCLUSION

MRI has an important role in the evaluation of patients with CHD. Although echocardiography remains the primary imaging modality in these patients, MRI has value for a number of specific indications, especially the evaluation of supracardiac structures, complex CHD, quantitative function, and postoperative status.

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