Cine Magnetic Resonance Imaging

ECG-gated gradient-echo sequences can be employed to provide multiple images throughout the cardiac cycle in prescribed anatomic locations. Display of these images in a cine mode permits visualization of the dynamic motion of the heart and vessels.^1–3 Cine MRI techniques, at a minimum, allow assessment of anatomy. More important, such techniques allow qualitative and quantitative assessment of function. Specifically, cine MRI permits quantification of chamber volumes, myocardial mass, and ventricular function. Further, cine MRI allows qualitative assessment of focal and global wall motion abnormalities, qualitative and quantitative assessment of valve disease (including the mechanism and severity of valve regurgitation and the location and severity of valve stenoses), identification and quantification of intracardiac and extracardiac shunts, and visualization of other areas of flow turbulence.

Cine MRI is the principal tool used to quantitatively assess ventricular function. Such techniques, both fast gradient-echo^4–7 and balanced steady-state free precession,^1,2 have been extensively evaluated and validated.^8,9 Briefly, evaluation of function begins with obtaining a series of contiguous cine slices along the short axis of the ventricles, extending from base to apex. The prescription of such slices should be performed from a true four-chamber view at end-diastole to ensure coverage of the entire ventricular mass (Fig. 50-1). These images are played back in a cine loop, and the end-systolic and end-diastolic phases are chosen. The endocardial borders are traced at both time points, and the epicardial borders are traced at one of the two time points (Fig. 50-2). Ventricular volumes are then calculated as the sum of the traced volumes (area × slice thickness). Myocardial mass is calculated as the myocardial muscle volume × 1.05 g/mm³ (density of myocardium). From these data, ventricular end-diastolic volume, end-systolic volume, stroke volume, ejection fraction, myocardial mass, and mass-to-volume ratio can be calculated for both the right and left ventricles. Most computer workstation software packages for cardiac MRI analysis provide semiautomated postprocessing tools to maximize efficiency.

FIGURE 50-1 To obtain a stack of short-axis cine images, a two-chamber (2-C) view is prescribed from an axial cine, then a four-chamber (4-C) view is prescribed from the two-chamber cine, then a short-axis stack is prescribed from the four-chamber cine at end-diastole.

FIGURE 50-2 Single image from a stack of short-axis cine images at end-diastole (A) and end-systole (B). Right and left ventricular endocardial contours are drawn at end-systole, and endocardial and epicardial contours are drawn at end-diastole. It is from these data that chamber volumes, myocardial mass, and ventricular function are derived.

Spin-Echo (Black Blood) Imaging

ECG-gated spin-echo sequences (black blood imaging) represent another important tool for imaging in the postoperative CHD patient. Despite providing only static information, black blood imaging has many benefits in this population. It allows assessment of anatomy with thin slices, high spatial resolution, and excellent blood-myocardium and blood–vessel wall contrast (Fig. 50-3). Black blood techniques are superb for evaluation of the spatial relationship between cardiovascular and other intrathoracic structures, such as the chest wall and the tracheobronchial tree. These features hold particular relevance in delineation of complicated postsurgical cardiac anatomy. Such techniques are also less susceptible to artifact from metallic implanted devices, such as stents, coils, occluder devices, clips, and sternal wires, which are commonly seen in the postoperative CHD patient.

FIGURE 50-3 Two axial spin-echo black blood images through the base of the heart. The initial surgery in this patient included a right ventricle–to–pulmonary artery conduit, which subsequently became stenotic. Unknown to this patient’s caregivers was the fact that the indication for a conduit rather than a transannular patch was an anomalous left coronary artery (LCA) from the right that traversed the right ventricular outflow tract (RVOT). This was important for the management of her conduit stenosis as a percutaneous stent placement may have caused coronary compression. The aorta (Ao), native pulmonary artery (native RVOT), right ventricle–to–pulmonary artery conduit (RV-PA conduit), and anomalous coronary artery are identified (A). CA, coronary artery. The connection of the conduit and native pulmonary artery is demonstrated (B).

Flow Quantification

Electrocardiography-gated gradient-echo sequences with flow-encoding gradients are used to quantify the velocity and flow of blood (Fig. 50-4).¹⁰ These sequences are referred to as velocity-encoded cine MRI or phase contrast MRI. Two-dimensional velocity-encoded cine MRI sequences are commonly used in clinical practice. They can be used to quantify cardiac output, pulmonary-to-systemic flow ratio (shunt), valvular regurgitation, differential lung perfusion, and coronary flow reserve. They can be used to observe the location and severity of flow obstruction. In addition, velocity-encoded cine MRI assessment of flow is useful for corroboration of volumetric data obtained with cine imaging to ensure the interpreting physician that the data obtained are accurate.

FIGURE 50-4 From an image in the plane of the right ventricular outflow tract (RVOT), an imaging plane is chosen to capture the cross section of the main pulmonary artery (A). Velocity-encoded cine MRI is then performed in this plane to produce a magnitude (bottom left) and phase (bottom right) image (A). A region of interest encompassing the main pulmonary artery (MPA) is chosen for each image in the cardiac cycle. From these data, net flow is obtained. Flow is then displayed, allowing the calculation of peak velocity, net forward flow, and regurgitant fraction (B).

Newer velocity-encoded cine MRI sequences allow resolution of velocity vectors in three directions, with spatial coverage of a three-dimensional volume, temporally resolved throughout the cardiac cycle. Such techniques have been coined seven-dimensional flow encoding.^11,12 These techniques have the advantage of providing complete spatial and temporal resolution of velocity with a higher signal-to-noise ratio than in two-dimensional methods. Postprocessing tools permit the construction of vector field plots that highlight the intracardiac and intravascular nature of flow. Although they are currently limited by long scan durations, faster imaging techniques will likely allow such methods to reach clinical practice in the near future.

Gadolinium-Enhanced Three-Dimensional Angiography

Three-dimensional magnetic resonance angiography (MRA) sequences are typically not ECG-gated and thus do not allow optimal assessment of intracardiac structures. Regardless, such techniques provide excellent depiction of arterial and venous vascular structures (Fig. 50-5). In the population of postoperative CHD patients, three-dimensional MRA fills a significant diagnostic role. It can be used to diagnose systemic arterial anomalies, such as aortopulmonary collaterals, shunts, vascular rings, and coarctation. It is useful in the diagnosis of pulmonary arterial abnormalities, such as focal and diffuse stenoses and abnormal distal arborization patterns. Three-dimensional MRA methods are also useful for investigation of systemic and pulmonary venous abnormalities, both congenital anomalies and postoperative abnormalities. Finally, three-dimensional MRA is useful for evaluation of the relation between vascular and other thoracic structures. With the development of faster imaging techniques, ECG-gated three-dimensional MRA sequences are becoming more practical, allowing evaluation of intracardiac anatomy and acquisition of time-resolved three-dimensional MRA data sets.¹³

FIGURE 50-5 Three-dimensional gadolinium-enhanced MRA in a patient with single ventricle after the Fontan procedure. The image is displayed as a three-dimensional volume rendered reconstruction. A dilated Fontan pathway (F) is visualized.

Coronary Artery Imaging, Perfusion Imaging, and Myocardial Viability

Coronary artery abnormalities and ischemia are important issues to be investigated in postoperative CHD patients. Not only is this population of patients aging sufficiently to develop atherosclerotic coronary artery disease, they also commonly have congenitally abnormal or postoperatively acquired coronary artery lesions. It is not uncommon to find an anomalous origin or course of the left or right coronary artery, postsurgical coronary obstruction (i.e., after arterial switch for transposition of the great arteries), coronary artery thrombus (Fig. 50-6), or abnormal fistulous connections (i.e., pulmonary atresia with intact ventricular septum and right ventricle–dependent coronary circulation). Identification of such abnormalities is often critical to planning of reintervention or medical management. There is growing evidence to support that myocardial delayed hyperenhancement in a number of subsets of postoperative CHD patients is predictive of poor outcome, including patients with tetralogy of Fallot (Fig. 50-7).^14,15 Delayed hyperenhancement has been observed in other postoperative patients with CHD as well, the significance of which is being explored and elucidated.^16,17 In summary, although it is still not as robust as routine coronary artery angiography with x-ray fluoroscopy or ECG-gated CT angiography at investigating distal coronary artery lesions, MRI can image proximal coronary arteries well,^18–21 evaluate myocardial perfusion and viability,^22–26 and allow stress testing,^27–30 all noninvasively without exposure to contrast agents and ionizing radiation.

FIGURE 50-6 Short-axis (A) and four-chamber (B) cine images revealing delayed hyperenhancement (arrow) in the left circumflex distribution in this patient with {S,L,L} double-inlet left ventricle (LV).

FIGURE 50-7 In this short-axis view of the right ventricular outflow tract, delayed hyperenhancement is present at the location of the ventriculotomy and transannular patch (arrow). The right ventricle (RV) and left ventricle (LV) are labeled for orientation.

Tetralogy of Fallot

Tetralogy of Fallot (TOF), the most common form of cyanotic CHD, accounts for approximately 10% of all CHD and represents a large portion of postoperative CHD patients. This malformation consists of one embryologic abnormality, namely, anterior malalignment of the infundibular septum, leading to four constant features: malalignment ventricular septal defect, subvalvular (infundibular) and valvular pulmonary stenosis, overriding aorta, and right ventricular hypertrophy. Patients with TOF compose a varied group ranging from TOF with pulmonary atresia and multiple aortopulmonary collateral vessels to TOF with mild pulmonary stenosis.

Repair is also varied and has evolved during the past few decades. Early repair in the first few months of life is now possible and recommended as standard of care. Older patients referred for MRI will likely have undergone initial palliation to augment pulmonary blood flow (e.g., Blalock-Taussig, Waterston, or Potts shunt) with later definitive repair. The goals of definitive repair are to close the ventricular septal defect and to relieve right ventricular outflow tract obstruction (placement of a right ventricle–to–pulmonary artery conduit in patients with pulmonary atresia, placement of a transannular patch in those with severe pulmonary stenosis, or infundibular muscle resection in those with mild pulmonary stenosis).

Increasingly rare, imaging specialists will continue to see patients with only palliated TOF, pulmonary atresia, and multiple aortopulmonary collaterals. The goal of MRI in these patients is to delineate the anatomy of the pulmonary vascular bed and aortopulmonary collaterals. In this way, MRI can aid in determination of a patient’s candidacy for definitive repair.

Postoperative Management

Most patients with repaired TOF will have a certain degree of pulmonary regurgitation or stenosis, which has been shown to result in right ventricular dilation and dysfunction. In addition, progressive right ventricular dilation often leads to tricuspid valve annular stretch and ultimately regurgitation, which perpetuates continued right ventricular dilation. In addition, many patients with repaired TOF will have abnormal peripheral pulmonary vasculature and residual aortopulmonary collaterals, all of which contribute to their hemodynamic burden. Multiple studies in the recent literature have demonstrated that right ventricular dilation and dysfunction, as determined by MRI, predict adverse outcomes such as symptoms of right-sided heart failure, major arrhythmic events, and even death.³¹ Right ventricular dilation and dysfunction will often be detected by MRI before the onset of obvious clinical symptoms. In addition, delayed hyperenhancement has been shown to correlate with adverse outcomes in patients with repaired TOF.³²

For these reasons, MRI is useful in this population to delineate anatomy; to identify distal pulmonary vascular lesions; to identify the location and severity of pulmonary stenosis; to quantify pulmonary regurgitation; to quantify right and left ventricular mass, volumes, and function; and to identify regions of myocardial scar.

MRI for a patient with TOF may include the following:

• Four-chamber cine MRI for qualitative assessment of atrial and ventricular chamber size and identification of tricuspid valve regurgitation.

• Short-axis cine MRI to quantify biventricular mass, volume, and function (Fig. 50-8).

• Axial black blood MRI for assessment of chambers as well as branch pulmonary arteries and an overall look at coronary artery anatomy (Fig. 50-9; see also Fig. 50-3A).

• Three-dimensional MRA can be useful for identification of arch abnormalities, including right aortic arch and branch vessel abnormalities; for delineation of aortopulmonary collaterals; and for identification of postsurgical abnormalities, such as stenotic or absent subclavian arteries in the setting of previous palliative shunts.

• Flow quantification. Velocity-encoded cine MRI is performed to quantify flow and degree of obstruction, if present. A two-dimensional plane is chosen across the main pulmonary artery for quantification of right ventricular stroke volume and pulmonary regurgitation (as a net volume as well as a percentage of the stroke volume; see Fig. 50-4). In addition, a two-dimensional plane can be prescribed in the plane of the right ventricular outflow tract to obtain a peak velocity and to quantify a peak pressure gradient if echocardiographic assessment proved insufficient.

• Delayed hyperenhancement to look for myocardial scar (see Fig. 50-7).

FIGURE 50-8 Cine images at one location in the short-axis plane of the ventricles in a patient with repaired tetralogy of Fallot at end-diastole (A) and end-systole (B). The right ventricle (RV) is severely enlarged. Analysis demonstrated severely depressed function. LV, left ventricle.

FIGURE 50-9 Black blood imaging at the level of the branch pulmonary arteries in a patient with tetralogy of Fallot. The right pulmonary artery (RPA) is mildly hypoplastic and the left pulmonary artery (LPA) is dilated. Ao, aorta.

At many institutions, MRI has become the standard of care for initial delineation of anatomy, monitoring for residual hemodynamic lesions, and surgical planning in patients with unrepaired and repaired TOF. A baseline study should be obtained in late childhood when sedation no longer is necessary (i.e., 6 to 8 years of age). Serial MRI evaluation should be performed every 1 to 4 years on the basis of initial findings and clinical symptoms (i.e., more frequently if there is pulmonary stenosis, moderate to severe pulmonary regurgitation, significant right ventricular dilation or dysfunction, left ventricular dysfunction, or clinical symptoms). If there is pulmonary stenosis, MRI should be paired with echocardiography.

Transposition of the Great Arteries

D-Transposition of the great arteries (D-TGA), in which the aorta arises anteriorly and rightward from the right ventricle and the pulmonary artery arises posterior and leftward from the left ventricle, is the most common form of TGA (accounting for 5% to 7% of all children born with CHD) and is a common form of cyanotic CHD (second only to TOF). This circulation is typically not compatible with life unless surgery is performed to redirect deoxygenated flow to the lungs and oxygenated flow to the body. Until the mid-1980s, this defect was surgically corrected with an atrial-level switch (Senning or Mustard procedure). The arterial switch (Jatene procedure) became popular in the mid-1980s and remains the standard of care in this population of patients.

Postoperative Management

After an atrial-level switch, patients suffer a number of residual hemodynamic burdens. The most important of these is systemic right ventricular failure as the right ventricle is not morphologically designed to handle a systemic afterload. Right ventricular dilation and failure are often accompanied by tricuspid (systemic atrioventricular valve) regurgitation, which leads to worsening right ventricular dilation and dysfunction. Patients who have undergone an atrial-level switch also suffer systemic or pulmonary venous baffle obstruction and baffle leaks with either right-to-left or left-to-right shunting, depending on the anatomy.

Patients who have undergone an arterial switch have the benefit of having a left ventricle as the systemic pumping chamber but still retain a number of potential hemodynamic burdens, such as supra-aortic and suprapulmonary stenoses at the suture lines, branch pulmonary artery stenoses secondary to stretch of the pulmonary arteries across the ascending aorta, and coronary ostial occlusions at the site of coronary artery reimplantation.

MRI for a patient with D-TGA who has undergone an atrial-level switch procedure may include the following:

• Four-chamber cine MRI to evaluate ventricular function, atrial pathways, and atrioventricular valve regurgitation (Fig. 50-10).

• Short-axis cine MRI to quantify biventricular mass, volume, and function. Images should extend coverage from the ventricles through the atrial pathways.

• Baffle cine MRI. Axial plane and coronal oblique planes through the superior vena cava and inferior vena cava pathways are useful for diagnosis of obstruction to flow. These images will often display baffle leaks (although the absence of a baffle leak on MRI does not rule out a baffle leak) (Fig. 50-11).

• Flow quantification. Velocity-encoded cine MRI sequence is performed to assess flow. First, two-dimensional planes across the cross-sectional area of the ascending aorta and main pulmonary artery are obtained to quantify pulmonary-to-systemic flow ratio (if there is any concern for baffle leak). Velocity-encoded cine MRI sequences can be planned in the plane of the atrioventricular valves to assess the degree of tricuspid regurgitation.

• Delayed hyperenhancement to look for myocardial scar.

FIGURE 50-10 Four steady-state free precession images in a four-chamber plane demonstrating the pulmonary venous and systemic venous pathways (PV pathway, SV pathway, and IV pathway) in a patient with D-transposition of the great arteries after a Mustard (atrial switch) procedure. Note the right ventricular dilation and hypertrophy typical of the systemic right ventricle (RV). Also note the jet indicating tricuspid regurgitation (TR).

FIGURE 50-11 A, Coronal oblique steady-state free precession stack demonstrating the superior (SV pathway) and inferior (IV pathway) limbs of the systemic venous baffle draped like a pair of pants over the pulmonary venous pathway (PV pathway). B, Of note, there is evidence of a baffle leak in the inferior limb of the baffle with left-to-right flow.

Guidelines for MRI in patients with D-TGA who have undergone an atrial-level switch procedure include a baseline investigation and serial follow-up evaluation. The information uniquely provided by MRI includes quantitative systemic right ventricular function, visualization of the synchrony (or asynchrony) of ventricular contraction, atrioventricular valve regurgitation, systemic venous or pulmonary venous pathway obstruction, and baffle leaks (with cine imaging and quantification of systemic-to-pulmonary flow ratio). Depending on systemic right ventricular function, other anatomic and physiologic abnormalities, and clinical symptoms, MRI should be repeated every 2 to 4 years to monitor these.

MRI for a patient with D-TGA who has undergone an arterial switch procedure may include the following:

• Short-axis cine MRI to quantify biventricular mass, volume, and function.

• Axial cine MRI to assess for supra-aortic and suprapulmonary valve stenoses.

• Axial black blood imaging to provide better spatial resolution and more anatomic detail, complementing the axial cine MRI.

• Three-dimensional MRA if there is any concern about vascular abnormalities.

• Coronary artery, stress perfusion, and delayed hyperenhancement imaging can be used if there is any concern about coronary ostial obstruction.

MRI is useful for baseline and serial evaluation of patients with D-TGA who have undergone arterial switch. The information uniquely provided by MRI includes evaluation of the great vessels (supra-aortic and suprapulmonary stenosis and branch pulmonary artery obstruction). In addition, the coronary artery ostia and proximal coronary arteries can be evaluated. Stress MRI (dipyridamole, adenosine, or dobutamine) can additionally allow evaluation of perfusion defects and coronary flow reserve. Finally, MRI offers an assessment of ventricular function. Depending on baseline findings and clinical symptoms, MRI can be repeated serially.

Coarctation of the Aorta

MRI is useful for delineation of anatomy and physiology in patients after repair of coarctation of the aorta.^33,34 This can be monitored with serial MRI to identify the need for and to ensure appropriate timing of reintervention.

Coarctation of the aorta is a common form of CHD. Simple coarctation of the aorta accounts for 5% to 8% of all patients born with CHD and is a component of many more complex lesions. In coarctation of the aorta, ductal tissue surrounds the aortic isthmus, and with ductal closure, there is constriction leading to arch obstruction. If it is diagnosed in the newborn period, surgical repair with resection of the obstruction and end-to-end anastomosis is the preferred therapy. If it is diagnosed later in life, catheter-based intervention with balloon dilation or stenting of the obstruction can be performed as an alternative to surgery. Patients with native or repaired coarctation of the aorta often suffer aortic complications, such as restenosis, aneurysm formation of the ascending aorta or repair site, dissection, systemic hypertension with left ventricular hypertrophy, and early coronary artery disease. These complications are often asymptomatic and go unrecognized until it is too late.

Echocardiographic assessment is often limited by poor acoustic windows. Although CT angiography provides anatomic detail, it is unable to characterize flow and the effect of the obstruction on the myocardium. MRI can noninvasively enhance our understanding of coarctation severity, and clinical assessment combined with MRI as a primary imaging modality can be more cost-effective than an approach using echocardiography as a primary imaging modality.^35,36

Postoperative Management

The initial goal of a cardiac MRI examination in a patient after repair of coarctation of the aorta is delineation of the anatomy. This includes anatomic characterization of recurrent or residual obstruction, relationship of the obstruction to other arch vessels (which is necessary for planning of surgical or catheter-based interventions), arch anatomy (e.g., aortic root or ascending aorta dilation, hypoplasia of the arch, post-stenotic dilation of the descending aorta, aneurysm at site of repair), and number of collateral vessels. The second goal is to evaluate the effect of the obstruction or resultant systemic hypertension on the myocardium, including left ventricular myocardial mass and left ventricular function. The final goal is to ascertain the physiologic severity of the obstruction by evaluating the nature of flow at the obstruction and in the descending aorta.

MRI for a patient with repaired coarctation of the aorta may include the following:

• Short-axis cine MRI for quantification of biventricular mass, volume, and function.

• Arch cine MRI. Sagittal oblique plane in the long axis of the aorta should be obtained, the so-called candy cane view. This allows visualization of the jet of turbulent flow at the coarctation of the aorta, thus providing a qualitative assessment of the severity of the obstruction (Fig. 50-12A).

• Black blood imaging in the sagittal oblique plane can be of additional value. This is particularly useful when there is a previously placed stent or if finer spatial resolution is required.

• Three-dimensional MRA to assess arch anatomy. It is from these images that cross-sectional areas can be measured, arch anatomy can be delineated, and collaterals can be defined. These findings are critical to planning of medical, catheter-based, or surgical interventions (Fig. 50-13).

• Flow quantification. Velocity-encoded cine MRI sequences are performed to assess flow. First, a two-dimensional plane across the cross-sectional area of the ascending aorta is obtained. If prescribed well, this often yields not only flow in the ascending aorta but also flow in the descending aorta just distal to the obstruction. It is also useful to obtain flow in the descending aorta at the level of the diaphragm. Delayed onset of systolic flow, damped rate of return to baseline, persistent flow during diastole, and augmentation of flow secondary to collaterals are suggestive of significant obstruction (Fig. 50-12B).

FIGURE 50-12 Sagittal oblique image in a patient with coarctation of the aorta (A). A jet of flow (arrow) at the isthmus is consistent with obstruction. The flow curves demonstrate the flow in the ascending aorta (B) and descending aorta (C). The delayed onset of systolic flow, damped rate of return to baseline, and persistent flow during diastole seen in the descending aorta are suggestive of significant obstruction.

FIGURE 50-13 Three-dimensional gadolinium-enhanced MRA in a patient with coarctation of the aorta (CoA). Images are displayed as a maximum intensity projection image. Both the region of coarctation of the aorta and the collaterals are identified (arrows).

In general, MRI should be used to evaluate anyone older than 6 years after repair of coarctation of the aorta with concern for recurrent or residual obstruction, aneurysm formation, or systemic hypertension. MRI should be used in younger children if questions remain after echocardiography. In patients who have undergone apparently successful repair, MRI should be obtained at baseline in late childhood when sedation is no longer necessary (i.e., 6 to 8 years of age). It should be repeated every 6 months to 5 years, depending on initial findings.

Single Ventricle After Fontan Palliation

Patients born with complex CHD and single-ventricle physiology (i.e., tricuspid atresia, hypoplastic left heart syndrome, pulmonary atresia with intact ventricular septum) represent a spectrum of disease severity, depending on their initial anatomy. In the current era, they typically undergo several surgical procedures during the first several years of life to provide a stable cardiopulmonary physiology. The first of these procedures is usually performed in the neonatal period and is directed at recruiting the single ventricle as the systemic pumping chamber and providing controlled pulmonary blood flow. The subsequent procedures involve sequential conversion to a physiology of separated systemic and pulmonary circulations with passive pulmonary blood flow (elimination of intracardiac mixing). The final circulation is named the Fontan circulation after the French surgeon who developed and first performed it in humans.³⁷ This palliation improves the patient’s cardiovascular efficiency and provides nearly normal systemic arterial oxygen saturations, but the physiology remains abnormal.

Postoperative Management

These patients suffer many complications. First, as a result of the passive nature of pulmonary blood flow, they develop high right-sided filling pressures, dilated Fontan pathways, and atrial arrhythmias. As a result of high filling pressures, they often form systemic-to-pulmonary venous collaterals, which lead to cyanosis. As a result of sluggish pulmonary blood flow, they have poor left-sided preload and diminished output despite normal contractility. They often have single right ventricles that ultimately fail in the face of systemic afterload. They may have valvular disease that increases the pressure or volume load of the single ventricle. All of this inevitable pathophysiology must be monitored to ensure timely medical, catheter-based, and surgical interventions that may improve the patient’s clinical status.

The goal of a cardiac MRI evaluation of patients with single-ventricle physiology is first the delineation of anatomy. This postsurgical anatomy is often complex and often unexpected. The second goal is the characterization of physiology, including valvular disease, myocardial function, and flow assessment. A cardiac MRI protocol for such a patient may include the following:

• Axial cine MRI. Images should extend from the diaphragm to the arch vessels to begin to assess cardiac and vascular anatomy. These images help guide the remainder of the cardiac MRI examination.

• Short-axis cine MRI to quantify ventricular mass, volume, and function.

• Black blood imaging may be helpful if anatomy is in question or if there are metallic devices in place causing artifact on cine imaging.

• Flow quantification. Velocity-encoded cine MRI imaging is performed to assess flow. First, two-dimensional planes are prescribed across the aorta as well as the vessels supplying blood flow to the pulmonary vascular bed (i.e., right and left branch pulmonary arteries, main pulmonary artery in the setting of an old-style Fontan circulation in which the main pulmonary artery is connected to the right atrium). Two-dimensional flow assessment across the atrioventricular valves as well as the superior and inferior venae cavae is often helpful for assessment of atrioventricular valve regurgitation and for corroboration of other flow and volumetric data. A seven-dimensional flow technique may prove useful in this anatomy and physiology as it can obtain flow in all vessels in a single acquisition.

• Three-dimensional MRA is often useful for assessment of arterial and venous vascular anatomy, including identification of collateral vessels.

• Delayed hyperenhancement to look for myocardial scar.

Guidelines for the use of cardiac MRI in the setting of palliated single-ventricle physiology need to be tailored to the individual patient. If there are no concerns early, baseline cardiac MRI should be obtained in late childhood when sedation is no longer necessary (i.e., 6 to 8 years of age). Serial examinations should be performed, the frequency of which should be dictated by clinical status. Obviously, if there are concerns earlier (i.e., valvular disease, myocardial dysfunction, arterial or venous abnormalities, baffle leaks), MRI can be performed under sedation or general anesthesia.

KEY POINTS

The field of CHD is advancing rapidly, and as a result, patients with complex disease are surviving and patients in general are living longer.

Despite repair, postoperative CHD patients uniformly have residual hemodynamic burdens that need to be monitored carefully. Reintervention is often necessary. Choice of appropriate timing for reintervention is often challenging.

MRI is ideally suited as a primary noninvasive imaging modality, given its ability to provide accurate and reproducible anatomic and functional information. Multiple MRI techniques can be combined to provide a complete evaluation of anatomy and function, including cine (steady-state free precession) imaging, black blood (spin-echo) imaging, flow quantification (velocity-encoded cine) MRI, gadolinium-enhanced three-dimensional MRA, coronary artery imaging, perfusion imaging, and viability (delayed hyperenhancement) MRI.

The postoperative CHD patients presenting for MRI most commonly include those with tetralogy of Fallot, transposition of the great arteries, coarctation of the aorta, or single ventricles after Fontan repair.

MRI has advanced significantly during the past 3 decades and continues to advance at a rapid rate. Real-time imaging, MRI-guided interventions, myocardial tagging assessment of wall strain, and seven-dimensional flow techniques will reach the clinical armamentarium in the near future, making MRI an even more useful tool in evaluation and management of this population of patients.