CHAPTER 50 Magnetic Resonance Imaging in the Postoperative Evaluation of the Patient with Congenital Heart Disease

Alison Knauth Meadows, Karen G. Ordovas, Charles B. Higgins, Gautham P. Reddy

In recent years, advances in pediatric cardiovascular surgery, catheter-based interventional therapies, intensive care, and medical management have dramatically changed the landscape of the field of congenital heart disease (CHD). The complexity of the anatomy and physiology of patients surviving with CHD is increasing exponentially; the majority will survive to adulthood, and the need for reintervention is common. This changing field is placing new demands on imaging to plan medical management as well as to identify the need for and timing of reintervention. A number of imaging modalities are available to the clinician and imaging specialist when it comes to these evaluations. Given its ability to assess both anatomy and function, magnetic resonance imaging (MRI) holds a unique and growing position among these.

Echocardiography has been and remains a mainstay of imaging in CHD. Despite its importance in rapid diagnosis and follow-up, it has limitations in the evaluation of the postoperative patient with CHD. Postoperative scar, chest wall deformities, overlying lung tissue, and large body size as the patient ages often result in suboptimal transthoracic echocardiographic windows. Transesophageal echocardiography, although providing improved acoustic windows, is limited by its small field of view and more invasive nature, often requiring deep sedation or general anesthesia.

Cardiac catheterization, employing x-ray fluoroscopy and contrast angiography, has an expanding role in minimally invasive interventions, but its role as a diagnostic procedure is rapidly diminishing. This is in part due to its limitation as a two-dimensional projection imaging technique with poor soft tissue contrast and the substantial ionizing radiation exposure involved; also, both diagnostic analysis and functional analysis are often better performed with noninvasive imaging techniques.

Computed tomography (CT) has been useful in evaluating vascular anatomy, and with the advent of high-resolution CT and cardiac gating, it has emerged as a useful tool for assessment of intracardiac anatomy, coronary artery anatomy, and myocardial function. Nevertheless, the temporal resolution of cardiac CT remains limited, and advances in CT imaging technology have often come with increases in exposure to ionizing radiation.

MRI has emerged during the past few decades as an alternative, complementary, and frequently superior imaging modality for the investigation of anatomy and function in the postoperative CHD patient. It has many advantages over other imaging modalities. It does not require the use of iodinated contrast agents and does not involve exposure to ionizing radiation. This is particularly important in a population of patients who have been and continue to be exposed to large doses of contrast agent and radiation during hemodynamic and interventional catheterization. In addition, many of these patients are children, who are more susceptible to the adverse effects of radiation. Major advances in MRI hardware and software, including advanced coil design, faster gradients, new pulse sequences, and faster image reconstruction techniques, allow rapid, high-resolution imaging of complex anatomy and accurate, quantitative assessment of function.

This chapter highlights the MRI techniques frequently employed to evaluate the anatomy and physiology of the postoperative CHD patient. It provides information about the general application of MRI in this population of patients as well as sample protocols and guidelines for its use in the more commonly encountered lesions referred for MRI.

POSTOPERATIVE ASSESSMENT

A number of MRI techniques are useful to the examination of the anatomy and physiology of the postoperative CHD patient. These techniques are detailed in Chapters 13 to 17. Here, their importance to this population is highlighted.

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.