Echocardiography in Pediatric and Adult Congenital Heart Disease, 2nd Ed.

38. Cardiac Magnetic Resonance and Computed Tomographic Imaging in Congenital Heart Disease

INTRODUCTION

In the current era, multimodality imaging techniques are being used more frequently as their clinical utilities become better appreciated. There are several choices available when it comes to detailed imaging. Cardiac computed tomographic angiogram (CTA) and magnetic resonance imaging (MRI) provide excellent imaging alternatives in patients with limited acoustic windows, particularly in children who have undergone multiple surgeries as well as in adults with congenital heart disease. A general knowledge of the strengths and weaknesses of various imaging modalities can be helpful in choosing the ideal imaging modality in a given clinical scenario. Depending on the information sought, one or more imaging techniques may be utilized together to answer the clinical question. Table 38.1 gives a summary of the advantages and disadvantages of these imaging modalities. These parameters should be kept in mind when ordering an advanced imaging technique.

COMPUTED TOMOGRAPHY

General Principles

Rapidly rotating x-ray beams and detectors are utilized to create images in computed tomography (CT). The data from the detectors are then computer processed to generate a batch of images in parallel axial planes. Postprocessing of the acquired data set can then generate other fundamental (coronal and sagittal) or oblique planes to analyze anatomy and obtain precise measurements. The major strengths of CTA are a short scan time and very high spatial resolution. Because of its superior spatial resolution, CTA is the preferred noninvasive modality for imaging small vessels such as coronary arteries and aortopulmonary collaterals. However, the spatial resolution of CTA is not as good as an invasive angiogram performed during cardiac catheterization. With the advances in multidetector CT scanners, the scan time is becoming shorter along with continued improvement in image resolution. The temporal resolution of MRI has traditionally been better than that of CTA images. However, with the newer imaging techniques and postprocessing software, this difference is becoming less significant. The other advantage of CTA is its utility in patients with implanted devices (implanted defibrillators, pacemakers) and those where ferromagnetic foreign material such as stainless steel coils can cause significant artifacts with MRI. The limitations of CTA in imaging pediatric patients are exposure to ionizing radiation and the need for sedation. With the continued advances in the field, the radiation exposure during CTA is much less now but still remains significant. Owing to a short scan time, sedation is also becoming a less important issue. The use of an iodinated contrast agent can be another limiting factor, especially in patients with renal disease. For most patients, CTA will be sufficient to define the anatomy but flow analysis and tissue characterization cannot be performed with this modality.

Technical Aspects

Nonionic iodinated contrast is used for CTA to allow for better visualization of structures and to provide contrast between vascular and nonvascular structures. Preferably, the contrast is injected by a power injector via a large bore intravenous catheter to allow easy and uniform flow. However, this is not always possible, especially in small pediatric patients in whom only a small caliber intravenous catheter can be placed. In that situation, the contrast can be injected by hand. The CTA scan also has to be timed with the contrast injection. This scanning delay time is based on the structures of interest and the underlying cardiovascular anatomy. The goal is to acquire the images at a time when the maximum concentration of the contrast is in the area of interest. For example, to better visualize the pulmonary arteries in a patient who had undergone a cavopulmonary anastomosis, the scanning time delay will be shorter than in a patient with a normal four-chambered heart. The cardiac output and the circulation time are also factored into setting the scanning time delay. This delay can also be timed by using a marker in the area of interest which monitors the Hounsfield units (gray scale) in that particular area and triggers the scanner when this area changes in Hounsfield units due to the presence of the contrast. ECG-gating is used to reduce the blurring from cardiac motion and helps to provide better visualization of intracardiac anatomy and vascular structures close to the heart, such as coronary arteries and the aortic root. It is also needed to generate cine images for functional and volumetric assessments. For these reasons, a regular and slow heart rate is required for an optimal image acquisition. In infants with faster heart rates, esmolol infusion may be used to transiently decrease the heart rate. The vascular structures away from the heart such as the aortic arch, the descending aorta, and the branch pulmonary arteries can be imaged without ECG-gating. A brief breath-hold is also needed to abolish the movement artifact caused by breathing. Mild sedation with oral chloral hydrate or intranasal midazolam is usually sufficient in children less than 6 years of age. Rarely, infants may require deep sedation with propofol or ketamine, or general anesthesia. The anatomic coverage of the scan is decided based on the clinical question and the areas of interest; however, limiting the coverage area may lead to waste of matrix space and loss of resolution. A slice thickness is set based on the desired spatial resolution. For visualization of small structures, the thickness can be made smaller but only at the expense of increased radiation exposure.

Postacquisition Image Processing

The acquired data set has extensive possibilities of postacquisition image processing for both CTA and MRI. Various software are available and can be used to generate images in traditional and nontraditional planes and to perform volumetric analyses. These postprocessing techniques are fast and can be completed soon after the scan.

1. Multiplanar Reformatting: The fundamental imaging planes are axial, sagittal, and coronal which are at 90 degrees to each other. In multiplanar reformatting (MPR), oblique imaging planes are generated by altering three 2D imaging planes similar to the fundamental planes (Fig. 38.1). Since these three planes remain perpendicular to each other, the anatomic and spatial relationship of structures can be better understood. MPR can be used to generate a true short-axis plane of a vessel (e.g., ascending aorta) and is also referred to as double oblique technique. Two oblique planes are oriented along the long axis of the vessel which makes the third plane absolutely perpendicular to the direction of blood flow, generating a true short-axis image (Fig. 38.2). Curve-planar reformatting (CPR) is a variant of MPR where a curved or tortuous vessel is depicted in a single plane created by using multiple imaging planes (Fig. 38.3). CPR is usually used to assess caliber along the length of aorta, pulmonary arteries or coronary arteries. Only the vessel along the central line should be assessed in this format as the remainder of the anatomy in these images is distorted to straighten the vessel of interest.

Figure 38.1. Example of multiplanar reformatting. The three planes are perpendicular to each other, shown as bluered, and green lines on the top left and the bottom panels. A: A true apical four-chamber view is shown. The green line indicates the plane of a two-chamber view of the left ventricle shown in B. The blue lines represent a true short-axis of the left ventricle shown in DC: shows the volume-rendered 3D reformatted image. Image plane orientation is specified by the cube in right lower corner.

Figure 38.2. Measurement of aortic root using double oblique technique. The greenred, and blue lines indicate the planes of interrogation. A: A long-axis view of the left ventricle. The green line is directly parallel to the blood flow and indicates the plane which is shown in the bottom left panel. B: The red line is also parallel to the blood flow. Since these two planes are parallel to the blood flow, the third plane (depicted as the blue line) is perpendicular to the blood flow and creates an absolute short-axis view of the aortic root, shown in DC: shows the volume-rendered 3D reformatted image. Image plane orientation is specified by the cube in right lower corner.

2. Maximum Intensity Projection: Maximum intensity projection (MIP) is a technique in which a 2D image is given some depth to make it a slab rather than a slice. This gives a 3D effect to small structures and is primarily useful to better visualize longer lengths of small vessels such as coronary arteries or intrapulmonary vessels (Fig. 38.4).

3. Three-Dimensional Volume Rendering: Three-dimensional volume rendering allows reconstruction of 3D volumes based on 2D images (Fig. 38.5). This technique is of immense value in assessing the anatomic spatial relationship of various structures to each other such as airway and aortic branches.

MAGNETIC RESONANCE IMAGING

General Principles

MRI uses magnetic signals from hydrogen ions (protons) in various tissues to generate an image. These ions are also referred to as “spins” as they precess (or spin) around the axis of the external magnetic field of the MRI scanner. By use of a strong external magnetic field, hydrogen ions are stimulated to emit radiofrequency waves which are then computer processed to generate images. Water and fat yield the highest signals due to their higher concentration of hydrogen ions. Based on the variable concentration of water and fat content in adjacent tissues, signals of variable strengths are generated that allow delineation of structures and assessment of tissue characteristics. Owing to its property of creating a better tissue contrast, MRI has become the gold standard for ventricular volume and mass measurements. The additional major benefits of MRI are (a) freely selectable imaging planes without being limited by acoustic windows or overlapping structures, (b) avoidance of ionizing radiation, (c) accurate flow quantification by phase contrast velocity mapping, (d) strain analysis utilizing tagging of the myocardium, and (e) assessment of myocardial viability and perfusion. In contrast to CTA where the actual images obtained are a stack of tomograms in the axial plane, imaging planes in MRI can be oriented in any desired direction. This is especially useful in obtaining cine images in any desired plane comparable to echocardiography, but without being limited by acoustic windows.

The advantages of MRI come at the expense of prolonged acquisition time. A comprehensive assessment of cardiac anatomy and function in a patient with complex congenital heart disease can take up to several hours. Such a prolonged scanning time is not acceptable. Therefore, the imaging sequences to be employed during a cardiac MRI need to be carefully selected to obtain all of the required information while spending the least amount of time in the scanner. While also important for CTA imaging, it is of immense importance that the MRI operator have knowledge of the anatomic and functional details of the patient from other imaging modalities such as echocardiography and cardiac catheterization. In cases of postoperative assessment, reviewing the surgical notes is of utmost importance.

Figure 38.3. Curve-planar reformatted (CPR) images of the right coronary artery. Top right panel: The right coronary artery along its length is shown using volume-rendered 3D reconstruction with a slab thickness of 18 mm, represented by the yellow lines in the bottom right panel. The top left and the top middle panels are CPR images showing the long length of the right coronary artery. The bottom left and the bottom middle panels show short-axis views at the blue and red lines transecting the CPR images. Image plane orientation is specified by the cube in right lower corner.

In contrast to CTA, images obtained by MRI are extremely operator dependent. This is particularly true in pediatric and adult congenital heart disease. Most of the MRI scanners are optimized for adult patients and thus require adjustments in its parameters for smaller pediatric patients. Similarly, many pediatric and adult patients with congenital heart disease may have foreign materials, such as coils or stents, in their chest which can create significant artifacts during an MRI scan. To optimize MRI images in the presence of such artifacts requires an in-depth knowledge of MRI physics and therefore requires extensive training. From the safety perspective, MRI is a great option to obtain detailed anatomic and functional information while avoiding ionizing radiation exposure. On the other hand, the presence of certain devices such as implantable defibrillators, pacemakers, and other implants may preclude the use of MRI due to safety concerns. Some of the newer devices are MRI compatible but require specially designed protocols to avoid harm. Other foreign materials like coils, clips, and wires, may be safe for MRI but can cause significant artifacts. Vascular occluding stainless steel coils are particularly problematic in creating significant artifacts. Most of the prosthetic valves, intravascular stents, and newer occluding devices create only a localized artifact which may hinder assessment of structures close to the foreign material. Specific sequences, which require more time but give less optimal tissue differentiation, can be employed to assess the structures lying within the artifact.

Figure 38.4. Maximum intensity projection image of the right coronary artery. Top right panel shows the MIP image of the right coronary artery showing the long length of the vessel. This image is in exactly the same plane as the bottom right panel with the addition of some depth to the image (slab). This slab is represented by the yellow lines in the multiplanar reformatted images on the left panels. Image plane orientation is specified by the cube in right lower corner.

Figure 38.5. Three-dimensional volume-rendered images of anomalous left coronary artery arising from the pulmonary artery (ALCAPA). Dilated right coronary artery with extensive collateral connections with the left coronary artery can be seen in a typical “adult-type” of ALCAPA.

Technical Aspects

The principles of MRI and its physics are far more complex than that of CTA. To familiarize readers with the MRI techniques, we will focus only on key concepts that are needed for cardiologists who generally do not perform these scans but review the images. Similar to CTA, MRI is also timed to the cardiac cycle. But in contrast to CTA, the images obtained by MRI are from averaged signals over several cardiac cycles. This timing can be achieved by the electrocardiogram or vectorcardiogram. This can also be achieved by using the intrinsic cardiac motion, referred to as self-gating. Self-gating is particularly useful in patients with arrhythmias or very abnormal ECGs (e.g., bizarre T waves or bundle branch block), which may not allow for appropriate triggering. Blurring of images resulting from breathing movement can be avoided by breath holding for 10–15 seconds. Unlike CTA which can be completed in a single breath-hold, MRI usually requires several breath-holds throughout the entirety of the exam. Alternatively, respiratory gating or navigator can be used to acquire data only at the end of expiration while the patient continues to breathe freely. This, however, can prolong the scan time significantly. Sedation may be needed especially in younger children. Infants, toddlers, and young school-age children may need general anesthesia. Adolescents can be scanned without sedation, but their cooperation to avoid movement and for breath-holding is needed. Claustrophobia may be another significant problem. Critically ill children are generally not suitable to undergo an MRI scan due to its prolonged scanning time.

Anatomic Assessment

To define anatomy, double inversion (DI) dark blood T1-weighted MRI yields high-resolution images of myocardial tissue and blood vessel walls where the blood yields low signal and appears dark (Fig. 38.6). Another technique called half-Fourier acquisition single-shot turbo spin echo (HASTE) can also be used to quickly acquire dark blood images, but at a lower resolution than the DI dark blood images. In the presence of ferromagnetic foreign material, the best images can be obtained by HASTE, fast spin echo (FSE) or turbo spin echo (TSE) sequences. Steady-state free precession (SSFP) imaging also yields high-resolution images of myocardial tissue and blood vessels. In contrast to DI images, in images acquired by SSFP the blood is bright and imaging is dependent on the contrast between the high signal of the blood and the low signal from tissue (Figs. 38.7 and 38.8, Videos 38.1 and 38.2). This type of imaging is currently the state of the art, but is very susceptible to artifacts in the presence of ferromagnetic material.

Gadolinium-based contrast agents can be used to enhance imaging. Gadolinium is an element with seven unpaired electrons in its outer shell and is paramagnetic. It is toxic in its native form and must be bound to a chelator such as diethylenetriamine penta-acetic acid (DTPA) to be used. This chelation increases renal excretion of gadolinium by a factor of approximately 500. The half-life is approximately 1.5 hours. Gadolinium works by increasing the relaxation of the surrounding protons in a dose-dependent fashion. Once gadolinium is injected into the cardiovascular system, the target structure (e.g., aorta) takes up the agent, whereas the background tissue does not. This method is principally useful in magnetic resonance angiography (MRA) to image vascular structures.

Figure 38.6. Examples of dark blood imaging. A: Patent ductus arteriosus (PDA) in a 3-week-old infant. B and C: A 3-month-old with tetralogy of Fallot with pulmonary atresia. A collateral (C) off the underside of the transverse aortic arch (TAo) going to the left (L) lung (B) and an additional collateral coming off the descending aorta (DAo) going to the right (R) lung (C) are clearly visualized. D: Diffuse DAo hypoplasia in a 2-year-old with Williams syndrome. E: A 1-year-old with tricuspid atresia who has undergone a hemi-Fontan procedure. F:Ventricular long-axis view of the atretic tricuspid valve and the left ventricle (LV). Superior vena cava (SVC)–to–right pulmonary artery (RPA) anastomosis. AAo, ascending aorta; MPA, main pulmonary artery.

Functional and Physiological Assessment

One of the more powerful applications of MRI is the assessment of cardiovascular physiology and function by the use of video images (see Figs. 38.7 and 38.8, Videos 38.1 and 38.2). In this technique, multiple lines of imaging data are obtained at multiple phases of the cardiac cycle in a given plane, allowing the visualization of both cardiac motion and blood flow. In this type of imaging, MRI produces a high-amplitude signal from the blood (white blood imaging) and a lower-amplitude signal from the tissue. Calculation of ventricular volume and mass, regional wall motion assessment, and blood flow visualization is performed in this manner. If turbulence is present, cine images will demonstrate a signal void (dark) in the region (see Fig. 38.8, Videos 38.2 and 38.3). This is used to detect valvar regurgitation, valvar stenosis, or blood vessel stenosis. Cine images can be obtained by two general sequences. The older form is termed spoiled gradient echo (SPGRE), where the signal of blood is higher than that of tissue. The more recent and more commonly used type is SSFP, where blood is exceptionally bright and there is a very high contrast between blood and tissue signals. SSFP is a much faster sequence and has elements of both T1- and T2-weighting. Because of its short acquisition time, it is used when performing “real-time” video or when using “interactive real-time” to allow “sweeps” to be performed interactively in real time. The SSFP sequences have susceptibility to artifacts from ferromagnetic materials. In the presence of such materials, alternative imaging techniques like fast or turbo gradient recalled echo (GRE) can create images with fewer artifacts at the expense of decreased image contrast and resolution.

Figure 38.7. Cine images of short-axis stack used to evaluate ventricular function. A: A true apical four-chamber view. Each imaging plane on B is represented by a yellow line on the left image (see Video 38.1).

Figure 38.8. Steady-state free precession sequence in a patient with nonischemic dilated cardiomyopathy. A: Two-chamber view of the left ventricle. B: Four-chamber view of the heart. The left ventricular geometry has a more spherical shape. The presence of dephasing across the mitral valve in the right panel is created by flow turbulence in the setting of mitral regurgitation (see Video 38.2).

Phase contrast velocity mapping is a special sequence in which the velocity of blood or any tissue can be determined. This sequence comes in two forms: (a) through-plane velocity mapping (Fig. 38.9), where velocity is encoded into and out of the plane of the image, and (b) in-plane velocity mapping (Fig. 38.10), where velocity is encoded in the plane of the image (similar to Doppler echocardiography). The advantage of through-plane velocity mapping is that if a blood vessel is imaged in cross section, all of the pixels that encode for velocity in the blood vessel can be summed over the entire cross-sectional area of the vessel and over the entire cardiac cycle. This can therefore be used to obtain flow (as in liters per minute, not just velocity). Directionality of flow is coded as a positive signal in one direction (bright on the image) and a negative signal in the other direction (dark on the image). By this method, for example, cardiac index can be obtained by performing phase contrast velocity mapping across the aortic valve. The pulmonary-to-systemic flow ratio (Qp/Qs) in a patient with an intracardiac shunt can be obtained by comparing this value with the phase contrast velocity mapping across the pulmonary valve. The regurgitant fraction of the aortic or pulmonary valves can be calculated similarly by dividing the backward flow by the forward flow and multiplying by 100.

Figure 38.9. Phase-encoded velocity mapping. Top: Patient with tetralogy of Fallot after a transannular patch repair with resultant pulmonary valve (PV) insufficiency. Right: Anatomic image of the pulmonary valve in cross section. Through-plane phase-encoded velocity mapping is used to quantify flow (see text) and direction is encoded as dark or bright. Left: Phase map at mid-systole. The signal is bright (arrow) across the pulmonary valve, demonstrating antegrade flow. Middle: Captured in early diastole and black across the pulmonary valve, demonstrating retrograde flow (arrow). Bottom left: Flow-time curve generated from this PV velocity map. Bottom middle and right: Phase-encoded velocity map and anatomic image, respectively, demonstrate a cross section of the trileaflet aortic valve (AoV) of the same patient with tetralogy of Fallot (arrow).

Figure 38.10. In-plane phase-encoded velocity mapping in a patient with tetralogy of Fallot after a transannular patch repair with pulmonary insufficiency. Top: Right ventricular outflow tract. Bottom: Pulmonary arteries in the bifurcation view. Left: Anatomic images. Top middle and right:Antegrade (bright signal encodes flow toward the head) flow in systole and retrograde (dark signal encodes flow toward the feet) flow in diastole through the pulmonary valve (arrows). Bottom middle (systole) and right (diastole) images demonstrate antegrade (bright signal encodes flow to the left) and retrograde (dark signal encodes flow to the right) in the branch pulmonary arteries.

Because of the quantitative nature of MRI, internal consistencies of the data can be easily evaluated. For example, cardiac index is measured from both the aorta and pulmonary artery and, in the absence of any intracardiac shunt and valve dysfunction, these values should be similar (within 10% of each other). The sum of flow in the right and left pulmonary arteries should be equal to that in the main pulmonary artery. Similarly, in the absence of atrioventricular valve insufficiency, the stroke volume of the left ventricle by volumetric analysis should be equal to the forward flow in the aorta by phase contrast velocity mapping. This approach ensures data integrity and is one of the unique features of MRI.

Myocardial tissue tagging (Fig. 38.11) is an MRI technique used to assess ventricular function. This methodology “magnetically labels” the walls of the myocardium and divides it into “cubes of magnetization.” This can be tracked over a cardiac cycle to demonstrate myocardial deformation. This technique is similar to speckle tracking in echocardiography and allows for the calculation of regional wall strain, wall motion, and torsion. A set of radiofrequency pulses are used to destroy the signals of the protons in parallel lines which results in dark bands on the image. This can be done in a “grid” pattern or as a series of one set of parallel lines using an MRI technique termed spatial modulation of magnetization (SPAMM). Similarly, blood tagging can also be performed, allowing for visualization of velocity profiles. This labeling can be done by bolus tagging, where only a thin stripe is laid down on the blood vessel to label it, or it can use a large stripe that is laid down on the blood to detect shunt flow.

Figure 38.11. Myocardial tissue tagging. This technique uses spatial modulation of magnetization (SPAMM) to lay a series of black parallel lines onto the myocardium dividing the myocardium into “cubes of magnetization.” As the cardiac cycle progresses from end-diastole (ED), the deformation of the regional myocardium can be tracked and strain/wall motion can be quantified. This is an example of a patient with hypoplastic left heart syndrome after the Fontan procedure in a short-axis orientation at ED (top left), end-systole (ES) (middle), and mid-diastole (MD) (bottom right). This video sequence can identify turbulent jets of blood by a loss of signal. In this example, the bottom right image demonstrates insufficiency from the neoaortic valve (Neo AoV). RV, right ventricle.

Tissue Characterization

Tissue characterization is a unique advantage of MRI. By suppressing signals from water or fat and by manipulating T1 and T2 weightings, composition of a tissue can be assessed (Fig. 38.12). These maneuvers are employed in evaluation of cardiac tumors, evaluation of myocardium (myocarditis, fatty infiltration, hemosiderosis, arrhythmogenic cardiomyopathy) and evaluation of pericardium (pericardial tumor, constrictive pericarditis).

The assessment of myocardial perfusion and viability has been used regularly in adult patients. For children, these assessments are primarily performed in patients with congenital coronary anomalies, Kawasaki disease, and postoperative assessment after arterial switch operation. These can also be useful in patients who have undergone cardiac transplantation as they are at risk of accelerated coronary artery disease and perfusion abnormalities. By using gadolinium enhancement, MRI assesses regional wall perfusion by a “first-pass” technique. Typically, short-axis views of the ventricle are obtained and the MRI sequence is set up such that the heart is imaged relatively motionless. Gadolinium is injected intravenously while the MRI scanner continuously images the ventricle (up to four or five short-axis slices may be imaged at once). The gadolinium bolus is followed from the right ventricular cavity to the left ventricular cavity and eventually to the ventricular myocardium. Defects in perfusion show up as dark portions of the myocardium while the remainder of the ventricle is signal intense. Image acquisition can be repeated after administration of a pharmacologic agent such as adenosine, a coronary vasodilator, to assess perfusion.

Figure 38.12. Tissue characterization by magnetic resonance imaging. The overall appearance of the myocardium is very dark and tissue characterization of the myocardium using T2* is consistent with iron deposition within the myocardium of this patient with hemochromatosis.

Infarcted or fibrosed myocardium can be identified by a contrast-enhanced technique called delayed enhancement (Fig. 38.13). Gadolinium is avidly taken up by scarred myocardium and remains in scarred tissue for an extended period of time, whereas it is “washed” out by coronary blood flow in normally perfused myocardium. Five to ten minutes after the injection, the infarcted myocardium continues to give signals of high intensity (appears bright), whereas normal myocardium does not. This technique has been demonstrated to accurately delineate the presence, extent, and location of both acute and chronic myocardial infarction. Foreign bodies such as patch materials can also become signal intense with this technique. In addition, various cardiac tumors can take up gadolinium, whereas others will not, and MRI uses this tissue property, along with T1-weighted images, T2-weighted images, and fat saturation to predict the type of tumor present.

Postacquisition Image Processing

For static images, the postacquisition image processing is very similar to CTA images. MPR, CPR, MIP, and 3D reconstructions can be done in a fashion similar to CTA. Additional postprocessing in MRI involves its inherent advantages which include volumetric measurements of the ventricles, and flow measurements on phase contrast imaging (Fig. 38.14).

CLINICAL APPLICATIONS AND CASE EXAMPLES

Shunt Lesions

Atrial and ventricular septal defects and patent ductus arteriosus are common congenital heart defects which may be isolated findings or present in association with other congenital cardiac defects. The importance of identification of these lesions is with respect to their hemodynamic influence which in turn is dependent upon their size, location, compliance of the receiving chamber, and resistance within the systemic and pulmonary circuits. In isolation, these lesions are traditionally interrogated with echocardiographic techniques. If echocardiography does not provide adequate assessment of these lesions either CTA or MRI may be performed as an ancillary noninvasive imaging test. Of these choices, MRI may be better suited because it allows a quantitative evaluation of shunts and determination of shunt fractions. As in any clinical evaluation, the clinical question to be answered is critical when deciding between CTA and MRI as an adjunctive test and when designing the appropriate imaging protocol when the patient presents for the imaging examination.

Figure 38.13. Delayed enhancement sequence after gadolinium injection. Abnormal myocardial delayed enhancement involving the anteroseptum and apex in a transmural pattern in a patient presenting with an anteroseptal myocardial infarction.

Figure 38.14. Volumetric and functional analysis of the ventricles. A stack of short-axis images is used to trace the blood-myocardial interface during systole and diastole to generate an area. A volume for each section is calculated based on the slice thickness. The sum of all these volumes gives the total end-diastolic and end-systolic volumes for each ventricle. Ventricular mass is also calculated in a similar manner.

Using CTA in the assessment of atrial septal defect (ASD), findings may include anatomic disruption of the interatrial septum (Fig. 38.15), however, smaller defects may be difficult to identify. Other adjunctive findings which are more easily appreciated include enlargement of the right-sided cardiac chambers and pulmonary arterial dilation. The superior spatial resolution allows complete interrogation of the pulmonary venous structures for the assessment of anomalous pulmonary venous return. This is especially important in sinus venosus ASDs which are mostly related to anomalous pulmonary venous connections. Similar to ASDs, ventricular septal defects (VSD) may be identified using ECG-gated cardiac CT, with large VSDs being relatively straightforward in their identification and localization. Smaller VSDs may be more challenging, and their identification is contingent upon careful study of the entire volume of images using multiplanar reformations. With a slice thickness <1 mm, cardiac CTA, with or without ECG gating, can readily identify the presence of a PDA.

Cardiac MRI can be used to complete the assessment of ASDs, VSDs, or a PDA when echocardiography is technically challenging. Imaging can be performed in any plane, which allows complete interrogation of the lesion of interest. The former, however, is predicated on appropriate operator experience. Ventricular volumes, mass, and contractile function can be assessed using SSFP sequences. Shunt volumes, Qp/Qs, valvular function, and pressure gradients may be estimated using velocity encoded phase contrast images. As with CT, vascular structures may be further evaluated using MR angiography when clinically indicated.

Figure 38.15. ECG-gated computed tomographic angiogram demonstrating unroofed coronary sinus. A is in an imaging plane axial to the body and demonstrates dilated right-sided heart chambers and coronary sinus. An imaging plane (yellow) is prescribed through the coronary sinus and results in the image B, which demonstrates the interatrial communication. A subsequent imaging plane (blue) prescribed through the left atrium and coronary sinus results in the image in C, which demonstrates the “unroofed” coronary sinus (arrow).

Aortic Anomalies

CT and MRI have important roles in the evaluation of patients with aortic disease. These include connective tissue disorders resulting in aneurysm formation, or structural abnormalities such as coarctation of the aorta and vascular rings and slings. Equally important is the role of these modalities in the serial pre- and postoperative surveillance of patients with aortic disease. Both MRI and CTA imaging modalities are well suited for visualization of the entire aorta and for this reason are attractive in the evaluation and surveillance of patients with a variety of aortic diseases. Despite the fact that MRI and CTA are fundamentally different in the way their images are generated, the approach to image interpretation is similar. We will begin with a discussion regarding the general approach to aortic measurements followed by issues specific to each imaging modality.

Aortic measurements are integral to the serial follow-up of individuals with connective tissue disorders affecting the aorta. The inherent variability within an imaging modality can cause significant confusion which is further compounded by additional variation imposed by differences between imaging modalities. Fundamentally there are three imaging planes that are principally identified relative to the body in the neutral position. These planes are sagittal, coronal, and axial respectively. It is important to recognize that an imaging plane that is axial to the body in the neutral position is not necessarily axial to an organ of interest residing within the body. By convention, the aorta is measured at different levels in a plane that is axial to the aorta at the level of interest. Diameter measurements are typically obtained by acquiring an imaging plane that is axial to the aorta at the level of measurement (double oblique measurement technique, see Figs. 38.2 and 38.16). This will provide a measurement that is perpendicular to the longitudinal or flow axis of the aorta to correct for the variable geometry of the aorta. The aorta is a long tubular structure which is anatomically divided into an ascending segment, an arch, and a descending segment. Measurements are made at several levels for the purpose of understanding the progression of disease, and more importantly to help in the preoperative planning for operative intervention (Fig. 38.17, Video 38.4). Typically, measurements are made at the (1) aortic valve annulus, which represents a plane that crosses the nadir of the aortic valve leaflets; (2) the sinus of Valsalva, which represents the largest diameter (sinus to sinus and sinus to commissure) of the aorta that resides between the aortic annulus and the sinotubular junction; (3) the sinotubular junction, which represents the continuation of the ascending aorta distal to the sinus of Valsalva; (4) the mid-ascending aorta at the level of the right pulmonary artery; (5) the distal ascending aorta, which is taken at a level just proximal to the take-off of the first branch of aortic arch; (6) the transverse or mid-aortic arch, which is taken at a point between the 2nd and 3rd branch of aortic arch; (7) the aortic isthmus, which is taken at a point just distal to the take-off of the 3rd branch of aortic arch; (8) the mid-descending thoracic aorta at the level of the pulmonary artery bifurcation; and (9) the distal descending thoracic aorta at the level of the diaphragmatic hiatus. In addition to these standard locations, the widest caliber of the aorta as demonstrated using the double oblique measurement technique is also reported.

Figure 38.16. Aortic root measurements. Top panels show the levels at which the measurements are made. (1) Aortic annulus. (2) Sinus of Valsalva. (3) Sinotubular junction. (4) Mid-ascending aorta. Bottom panels show measurements from sinus to commissure and from sinus to sinus.

With the advent of multidetector CTA and novel contrast injection protocols, CTA has evolved into one of the most accessible and time efficient modalities used to image the aorta. Specifically, the major advantage of CTA in the assessment of thoracic vascular structures is the presence of superior spatial resolution and the speed of image acquisition which enables evaluation of the arch to the femoral arteries in the setting of a single breath-hold. To evaluate the ascending aorta, the use of ECG-gating is recommended to avoid excessive motion related to the cardiac motion. This is particularly important in identifying the progression of aortic aneurysmal disease in which millimeter differences in measurements can be the difference between a surgical intervention versus continued expectant management. The ECG-gated image acquisition provides additional information that may be useful including an assessment of the coronary arteries as well as the left and right ventricular structure and function. The aortic valve can also be interrogated for morphology, the presence of calcification (both qualitatively and quantitatively), as well as planimetry of aortic valve area; although the major physiologic attributes of aortic valvular function are traditionally and remain within the purview of echocardiography.

Figure 38.17. Aortic arch measurements. The red arrows indicate the level of measurement. The actual measurements are made by using the double oblique technique as shown in Figure 38.16 (see Video 38.4).

MRI can provide much of the same information that CTA can while avoiding radiation. This makes MRI most suitable for elective surveillance imaging in order to avoid cumulative radiation exposure. The most significant disadvantage of MRI is the prolonged examination time, the inability to image calcific plaque as reliably as CT, and the inability to adequately image the full extent of the coronary arteries, which are best imaged noninvasively by CTA.

The goals of imaging aortic aneurysmal disease are primarily (1) to assess the location of the aortic aneurysm, (2) survey for the progression of aneurysmal disease using a standardized approach to aortic measurements, and (3) evaluate for complications of disease progression. With regard to patients with coarctation of the aorta, CT is particularly well suited to the interrogation of the location and severity of the coarctation (Figs. 38.18 and 38.19, Video 38.5). Additionally, it can identify the presence and extent of collateral vessels, which are an anatomic correlate of a hemodynamically significant lesion. In the setting of coarctation, MRI can provide similar information to CT, with additional information of flow quantification by using phase contrast imaging. In a hemodynamically significant coarctation, one major way blood can be directed distal to the site of coarctation is via systemic arterial collateral vessels. These are usually provided by the internal mammary artery, the intercostal arteries, or directly from the subclavian artery. These can all be delineated with the use of MR angiography. The presence of retrograde flow in these vessels, as demonstrated by phase contrast imaging, can allude to the presence of a hemodynamically significant coarctation.

Aortic arch anomalies are of particular relevance in the identification of patients with a vascular ring and resultant airway compromise. The importance of correct recognition of a vascular ring lies in the therapeutic approach—a complete ring can be treated with surgical intervention with hopeful resolution of symptoms of airway obstruction. For this reason, a complete delineation of aortic arch anatomy and its branching pattern is essential to the evaluation. Although the diagnosis is straightforward in the setting of a double aortic arch (Fig. 38.20, Video 38.6), it may be more challenging in other settings, in which case the key to diagnosis is the identification of the ligamentum arteriosum, which in some situations may be calcified. This finding allows the assessment of aortic arch anomalies better by CT imaging, given the superior spatial resolution and the ability of this modality to identify calcific lesions.

Pulmonary Artery Anomalies

Morphologic assessment of the pulmonary arteries can be performed by using either CTA or MRI. However, by using phase contrast imaging MRI can quantify total pulmonary blood flow (Qp) and differential pulmonary blood flow to both the lungs. Anomalous origin of the right pulmonary artery from the aorta and pulmonary artery hypoplasia can be assessed in MPR images followed by 3D reconstructions. Assessment of the branch pulmonary arteries in multiple planes is important especially for ribbon-like stenoses. Both branch pulmonary arteries can be assessed from their origin to their distal arborization. The assessment of the proximal segment of left pulmonary artery by CTA or MRI is particularly useful as it lies in a blind spot for transesophageal echocardiography. In the case of aortic origin of the right pulmonary artery, postoperative assessment by CTA or MRI can reveal stenosis at the reimplantation site of the right pulmonary artery.

Figure 38.18. Coarctation of aorta. Computed tomographic angiogram of aorta in a patient with coarctation (arrow). A: Maximum intensity projection in an oblique plane. B: Volume-rendered 3D reconstruction in the same orientation (see Video 38.5). Image plane orientation is specified by the cube in right lower corner.

Figure 38.19. Time-resolved three-dimensional gadolinium imaging. Top: Four phases of flow after a gadolinium injection in this sagittal view of a patient with an ascending-to-descending aortic conduit (C) for coarctation of the aorta. As the panels progress (left to right), the flow of gadolinium is seen to flow from the right superior vena cava (SVC) and right atrium (left) to the pulmonary arteries (PA) (second from left) to the aorta (Ao) and conduit (second from right) and then back to the superior vena cava (right). Bottom: Volume-rendered images obtained from this series of images. Left: Aorta, coarctation, and conduit in a sagittal view. If the pulmonary phase and the arterial phase are combined, a volume-rendered image of the entire heart is created as in the right three panels (third from right, anterior view; second from right, sagittal view looking from the left; right, off-axis sagittal view looking from the right). The course of the conduit and its relationship to various structures are clearly seen. Ao, aorta; C, conduit.

The usual MRI protocol used to assess the pulmonary arteries includes (1) HASTE sequence to obtain an axial stack, (2) SSFP at the pulmonary bifurcation and along the long axis of both branch pulmonary arteries, (3) phase contrast velocity mapping at a true short-axis of the main and the branch pulmonary arteries obtained by double oblique technique, and (4) Gadolinium-enhanced magnetic resonance angiography timed to get maximum contrast in the pulmonary arteries. In patients with prior stent placement in the pulmonary arteries for stenosis, CTA is preferable to assess anatomy. However, the presence of stents does not preclude an MRI examination, but it may cause susceptibility artifact on certain sequences, especially SSFP.

Figure 38.20. Vascular ring formed by double aortic arch. Volume-rendered 3D reconstruction of a computed tomographic angiogram (see Video 38.6).

In patients with absent pulmonary valve syndrome, the assessment of the pulmonary arteries and airways by CTA or MRI is considered state of the art. Both MRI and CTA can provide flawless information about the aneurysmal pulmonary arteries and their relationship to the tracheobronchial tree. A superior assessment of the airway can be achieved by CTA (Fig. 38.21). Preoperative information in absent pulmonary valve syndrome is acquired similar to tetralogy of Fallot with additional focus to potential airway obstruction due to the dilated pulmonary arteries. For postoperative assessment, attention is paid to diagnosing any residual atrial or ventricular septal defects, right ventricular outflow tract stenosis or valvar regurgitation, residual pulmonary artery dilatation, and airway compression. Three-dimensional reconstructions demonstrating the relationship of the dilated pulmonary arteries to the tracheobronchial tree are particularly useful for surgical planning.

Systemic and Pulmonary Venous Anomalies

Both MRI and CTA can be used to assess the patency, course, and connections of the systemic and pulmonary veins. The ability to capture a three-dimensional data set allows for multiplanar reconstruction of the anatomy, which is particularly helpful in the setting of anomalies of pulmonary or systemic veins. Delineation of the course and connections of systemic veins may be useful prior to procedures in the catheterization laboratory or device implantation. Visualization of anomalous pulmonary veins allows for preoperative planning (Fig. 38.22, Video 38.7), and either CTA or MRI can be used to assess the patency of baffles or conduits in the postoperative setting (Fig. 38.23). Intravenous contrast is necessary with CTA to assess patency and connections of systemic and pulmonary veins. SSFP or HASTE imaging obtained in the axial plane is usually done first with MRI to get an idea of the relevant anatomy, followed by a MRA obtained with injection of IV gadolinium. Conventional MRA or TRICKS (time-resolved imaging of contrast kinetics) can be used. ECG-gating is not required unless assessment of intracardiac structures is also needed.

Figure 38.21. Computer tomographic assessment of tetralogy of Fallot with absent pulmonary valve syndrome. A: Axial image at the level of pulmonary artery bifurcation. The left pulmonary artery (LPA) is aneurysmal and the right pulmonary artery (RPA) is anterior to the ascending aorta (Aao) after LeCompte maneuver. B: The arrow pointing at the anterior indentation in AAo from pulmonary arteries after LeCompte maneuver. C: 3D reconstruction of tracheobronchial tree with stenosis in right mainstem bronchus (arrow). D: 3D volume rendering showing relationship of pulmonary artery bifurcation (blue) anterior to aorta, aortic arch located on the right of trachea (green) and aberrant left subclavian artery posterior to the trachea. Image plane orientation is specified by the cube in right lower corner. DAo: descending aorta, RVOT: right ventricular outflow tract, T: trachea.

Figure 38.22. Anomalous pulmonary venous return. A: Axial image of computed tomographic angiogram. B: A corresponding image from 3D reconstruction with a 10 mm slab. AAo: ascending aorta, RMPV: right middle pulmonary vein, RUPV: right upper pulmonary vein, SVC: superior vena cava.

Tetralogy of Fallot

Tetralogy of Fallot (TOF) is typically diagnosed with echocardiography and complete repair is performed in infancy, consisting of relief of the right ventricular outflow tract (RVOT) obstruction and closing of the ventricular septal defect (VSD). In the past, the focus of the repair was on complete relief of the RVOT obstruction, commonly with a transannular patch. The ensuing chronic pulmonary valve regurgitation leads to right ventricular volume overload and subsequently right ventricular dysfunction. Cardiac MRI has become a powerful tool in the assessment of right ventricular volume to help determine optimal timing of pulmonary valve replacement (PVR). Several studies suggest that the right ventricular volume will not normalize postoperatively if PVR is performed once the right ventricular volume (indexed to BSA) is greater than 150–170 cc/m2. MRI offers an accurate and reproducible way to measure right ventricular volumes and function. A typical examination would consist of acquisition of SSFP cine images in the axial and short-axis planes for measurement of ventricular volumes and ejection fraction. Dedicated SSFP imaging of the right ventricular outflow tract may better demonstrate aneurysmal dilatation. Phase contrast imaging can be performed to quantitate the amount of pulmonary regurgitation or to measure the flow in each of the branch pulmonary arteries. MRA of the pulmonary arteries is useful for evaluation of branch pulmonary artery stenosis or aneurysmal dilation (Fig. 38.24). A right-sided aortic arch is commonly seen with TOF and anomalies of the aortic arch can be visualized with MRA. Dedicated sequences looking at the origin of the coronary arteries are important preoperatively, as anomalous origin of the left anterior descending artery from the right coronary artery crossing the RVOT is the most common coronary anomaly in patients with TOF. Although MRI is typically used for surveillance imaging in patients with repaired TOF, CTA can provide similar information including right ventricular volumes and function, pulmonary artery anatomy, and coronary artery anatomy. CTA is a less desirable surveillance tool due to concerns about radiation exposure, but the development of newer acquisition sequences with lower radiation doses makes this an acceptable alternative for patients unable to undergo MRI. CTA may be preferable in patients of TOF with pulmonary atresia, where it can help in defining the multiple aortopulmonary collaterals.

Figure 38.23. Computed tomographic angiogram after Warden procedure. A: Axial plane image at the level of pulmonary artery bifurcation. Arrow points at the superior vena cava (SVC) conduit. Asterisk (*) marks the stump of native SVC receiving blood from right upper pulmonary vein. Ao: aorta, RPA: right pulmonary artery. B: Axial plane image at the level of atria. Asterisk (*) marks the baffle in the right atrium (RA), directing the blood from the SVC stump and RUPV to the left atrium (LA) through the sinus venosus atrial septal defect. C:Sagittal plane image. Asterisk (*) marks the SVC stump. Arrow points at the SVC conduit.

Figure 38.24. Tetralogy of Fallot. Magnetic resonance angiogram demonstrating aneurysmal right pulmonary artery (RPA). The origin of left pulmonary artery (LPA) is being compressed by RPA. A: Axial plane image at the level of pulmonary artery bifurcation. B: Volume-rendered 3D reconstruction. Image plane orientation is specified by the cube in right lower corner. AAo: ascending aorta, DAo: descending aorta.

Complete Transposition of the Great Arteries

Complete transposition of the great arteries (TGA) is usually diagnosed shortly after birth, typically with TTE, and surgical repair is undertaken early in life. MRI and CTA do not play a significant role in the preoperative management of these patients. The subsequent issues in patients with surgically palliated TGA depend on the initial operation.

The first physiologic correction was the atrial switch (Mustard or Senning) operation. This consists of baffling systemic venous return to the morphologic left ventricle (Fig. 38.25), which pumps to the pulmonary arteries; and baffling the pulmonary venous return to morphologic right ventricle, which pumps to the aorta. This physiologic approach has several important long-term sequelae, the most important of which is that the right ventricle is the systemic ventricle. Right ventricular dilatation and failure is common in these patients, and MRI provides the opportunity for accurate quantification of right ventricular size and function. Baffle obstruction is another common complication, and can be difficult to visualize by transthoracic echocardiography. MRI and CTA both offer excellent visualization of these baffles to assess for stenosis. Large baffle leaks can also be identified but smaller leaks may not be detected.

Currently the operative approach to TGA is an anatomical correction, the arterial switch operation (ASO). The most common complications of the ASO include supravalvar or branch pulmonary artery stenosis (Fig. 38.26). Less common complications include neoaortic root dilation, neoaortic valve regurgitation, or stenosis of the reimplanted coronary arteries. Either MRI or CTA can be used to evaluate for all of these complications, although CTA may be better suited for evaluation of the coronary arteries.

A less common operative approach is the Rastelli procedure, which is performed when a large subaortic VSD and pulmonary stenosis are present. The patch is used to direct left ventricular outflow to the aorta through the VSD and the pulmonary valve is oversewn and a conduit is placed from the right ventricle to the pulmonary artery. Obstruction of the conduit is common and MRI or CTA can be used to evaluate this. Phase contrast imaging with MRI can also be used to assess the severity of the obstruction or regurgitation through the conduit.

Congenitally Corrected Transposition

Congenitally corrected transposition of the great arteries (CCTGA) is a defect in which atrioventricular and ventriculoarterial discordance results in the appropriate flow of deoxygenated blood into the lungs and oxygenated blood into the aorta, but through the “wrong” ventricle with the morphologic right ventricle supporting the systemic circulation (Fig. 38.27). Associated anomalies are common, including dextrocardia, situs inversus, VSD, ASD, pulmonary or subpulmonary stenosis, and the “Ebstein-like” abnormality of the systemic atrioventricular valve (morphologic tricuspid valve). Tricuspid valve regurgitation and systemic ventricular dysfunction are common. If associated lesions are not present, patients may not present until later in life and are often misdiagnosed. MRI may be a useful adjunct in the diagnosis as it offers the opportunity for complete morphologic assessment in patients with difficult echo images. SSFP images obtained in an axial plane may help with identification of cardiac chambers and also offers the ability to more accurately assess systemic ventricular function, which can be difficult by TTE due to the usual limitations in assessing right ventricular size and function.

Figure 38.25. Complete transposition status post Mustard procedure and left ventricle to pulmonary artery conduit insertion. ECG-gated computed tomographic angiogram demonstrating the drainage of pulmonary venous pathway (PVP) to the systemic right ventricle (RV). The systemic venous pathway (SVP) drains into the left ventricle (LV) which ejects the blood through the conduit (C) to the pulmonary artery (PA). A: Axial plane image showing the two venous baffles and the origin of the conduit. B: Maximum intensity projection in an oblique coronal plane showing inflow and outflow of the systemic RV. The superior vena cava (SVC) is heavily opacified by contrast. C: Volume-rendered 3D reconstruction showing the LV to PA conduit. Image plane orientation is specified by the cube in right lower corner.

Figure 38.26. Arterial switch operation and LeCompte maneuver. Computed tomographic angiogram. A: Axial plane image at the level of pulmonary artery bifurcation. The bifurcation is anterior to the ascending aorta (AA). B: Volume-rendered 3D reconstruction. Arrowpoints at the right pulmonary artery (RPA) stenosis. Ao: aorta, DA: descending aorta, LPA: left pulmonary artery, MPA: main pulmonary artery, RPA: right pulmonary artery.

Figure 38.27. Congenitally corrected transposition. ECG-gated computed tomographic angiogram. A: Axial plane image. B: Sagittal plane image. C: Coronal plane image. Ao: aorta, LA: left atrium, LV: left ventricle, PA: pulmonary artery, RA: right atrium, RV: right ventricle.

Heterotaxy Syndrome

Failure to generate left-right “asymmetry” in patients with heterotaxy syndrome poses a great challenge in understanding the cardiac segmental anatomy. The anatomic assessment of these patients extends beyond the heart and the chest. The lack of normal spatial orientation of abdominal and thoracic viscera is a diagnostic challenge which is a part of detailed anatomic assessment in these patients.

Transthoracic echocardiography is usually sufficient in infants to demonstrate the complex segmental anatomy of the heart. However, postoperative and especially adult patients with heterotaxy syndrome must be evaluated by using CTA or MRI. The information from these modalities can be invaluable in any surgical planning (Fig. 38.28). These evaluations are particularly useful in defining anomalous systemic, pulmonary, and hepatic venous connections, including presence or absence of right, left, or bilateral superior vena cavae, interrupted inferior vena cava with azygous continuation, course and drainage of azygous venous system, separate drainage of hepatic veins directly into the atrium, anomalous connection of pulmonary veins to a systemic vein, and ipsilateral pulmonary veins. Assessment of atrial morphology can be challenging by transthoracic echocardiography. The atrial appendages can be easily visualized by CTA and MRI imaging and can help in defining atrial situs. Juxtaposition of atrial appendages can also be ascertained. The intracardiac assessment of atrioventricular valves, ventricular morphology, atrial and ventricular septal defects, and ventriculoarterial connections are better assessed by MRI; however, CTA can also provide valuable information with the added advantage of much shorter scanning time. The postoperative assessment is predominantly the same as that in other patients who undergo single ventricle palliation.

Extracardiac anatomic assessment includes morphologic assessment of the lungs and mainstem bronchi, liver, spleen, stomach, and intestines. Tracheobronchial morphology can be better assessed by CTA imaging; however, a single coronal slice of HASTE sequence in MRI at the carina can clarify bronchial sidedness. Similarly, the location of the liver and the anatomy of its lobes can be delineated. Asplenia or polysplenia can be ascertained by either imaging modality.

Single Ventricle

The approach to the repair of single ventricle lesions is staged surgical reconstruction (two or three stages, depending on the anatomy and physiology), eventually culminating in the Fontan procedure. Understanding the various forms of functional single ventricles, their associated anomalies and physiologic/functional sequelae, as well as the various surgical reconstructive techniques, is important for optimal medical and surgical management of these complex patients.

MRI is typically used for imaging in young patients, unless there is a contraindication. As in all MRI cases, the exam must be individualized to the patient depending on the anatomy and the stage of surgical reconstruction. In all stages of surgical reconstruction, it is important to assess the following:

■Anomalous systemic and pulmonary venous structures

■Intracardiac systemic and pulmonary venous pathways

■Sites of intracardiac and extracardiac shunting

■Ventricular function, including regional wall motion abnormalities, ejection fraction, end-diastolic volume and mass, stroke volume, cardiac index, and atrioventricular valve regurgitant fraction

■Ventricular outflow tract obstruction, especially in patients with a bulboventricular foramen

■Pulmonary arterial anatomy including pulmonary artery stenosis, hypoplasia, and continuity

■Aortic arch anatomy, especially in patients with an aortic-to-pulmonary anastomosis to rule out aortic arch obstruction

■Aortopulmonary collaterals (typically by TRICKS or conventional MRA)

■Velocity mapping to assess cardiac index, Qp/Qs, relative flows to both lungs and regurgitant fraction of the semilunar and atrioventricular valves, and to assess ventricular outflow tract obstruction

Figure 38.28. Computed tomographic angiogram in a patient with asplenia syndrome. A: Axial plane. Ascending aorta is anterior and rightward to pulmonary artery (PA). Vertical vein (VV) is seen posterior to the right bronchus. Descending aorta (DAo) is on the left. B:Oblique sagittal plane. VV ascends and then drains into the superior vena cava (SVC). C: 3D reconstruction from posterior aspect showing pulmonary venous confluence (PVC) and VV. D: Oblique sagittal plane. AAo arising anteriorly from the right ventricle (RV). E: Double outlet right ventricle with malposed great arteries and severe subpulmonary stenosis. F: Classic Blalock-Taussig (BT) shunt with direct anastomosis of the right subclavian artery to the right pulmonary artery (RPA). Image plane orientation is specified by the cube in right lower corner. AoA: aortic arch.

In patients with hypoplastic left heart syndrome, Stage I palliation consists of either the Norwood procedure with a systemic to pulmonary shunt or a hybrid procedure. In the classic Norwood procedure a neoaorta is created from the main pulmonary artery and the aortic arch is reconstructed. Typically a modified Blalock-Taussig shunt is used to establish pulmonary blood flow and the atrial septum is resected. The Sano modification involves establishing pulmonary blood flow using a valveless conduit from the right ventricle to the pulmonary arteries instead of a BT shunt. The hybrid approach consists of stenting the PDA, surgically constricting the branch pulmonary arteries to limit pulmonary blood flow, and performing a catheter based atrial septostomy. Imaging of patients after the first stage of palliation should be focused on imaging of the neoaorta and arch to exclude obstruction and to ensure that the source of pulmonary blood flow is widely patent. The branch pulmonary arteries are assessed carefully for any stenosis. The retroaortic portion of the pulmonary artery and the anastomotic site of Sano conduit are common areas of stenosis. The neoaortic valve and atrioventricular valve regurgitation can be assessed quantitatively by MRI using phase contrast imaging. Similarly pulmonary and systemic blood flow can be quantified by phase contrast imaging.

Stage II palliation consists of the creation of a bidirectional cavopulmonary (Glenn) anastomosis. The superior vena cava is connected in an end-to-side fashion to the right pulmonary artery (Fig. 38.29). Imaging at this stage is focused on assessing patency of the bidirectional Glenn, assessing the distal pulmonary arteries, and assessing the aortic arch. This can be done with either MRI or CTA. SSFP sequences allow for assessment of patency of connections and assessment of ventricular size and function. MRA or CTA can both be used for assessment of the Glenn anastomosis and the pulmonary arteries, keeping in mind the importance of the timing of image acquisition as the superior vena cava is connected directly to the right pulmonary artery.

Figure 38.29. Hypoplastic left heart syndrome status post bidirectional cavopulmonary anastomosis. Magnetic resonance angiogram. A: Maximum intensity projection image in a coronal plane viewed from the front. B: 3D reconstruction viewed from the back. The anastomotic site is widely patent. There is significant stenosis (arrow) of the retroaortic portion of the pulmonary artery. Image plane orientation is specified by the cube in right lower corner. AA: aortic arch, LA: left atrium, LPA: left pulmonary artery, RPA: right pulmonary artery, SVC: superior vena cava.

The final stage is the Fontan procedure, which involves directing the IVC blood to the pulmonary arteries so that all systemic venous return is now directed to the pulmonary arteries. There are a number of anatomic possibilities, including (1) the classic atriopulmonary connection where the right atrial appendage is connected to the pulmonary artery, (2) the intraatrial conduit (lateral tunnel), or (3) the extracardiac connection (Figs. 38.30 and 38.31). When imaging any patient with the Fontan circulation, it is important to image the entire systemic venous pathway (Fontan circuit) for the presence of obstruction or thrombus. Assessment of the distal pulmonary arteries is also important as this will have a significant impact on filling pressures and cardiac output. It is important to remember that complete opacification of the pulmonary arteries will require injection into the upper and lower extremity simultaneously, as injection of the upper extremity alone will result in mixing of non–contrast-enhanced blood from the inferior vena cava which may lead to the inappropriate diagnosis of pulmonary emboli due to incomplete contrast opacification of the pulmonary arteries. Quantitative evaluation of ventricular function after the Fontan, including delayed enhancement to assess for myocardial fibrosis, is also important. Liver dysfunction with subsequent cirrhosis is now a recognized complication in patients with the Fontan circulation. MRI offers the ability to image the liver to look for evidence of cirrhosis and hepatocellular carcinoma. Magnetic resonance elastography (MRE) offers a noninvasive assessment of liver stiffness in these patients. The clinical utility of this type of imaging is yet to be determined, but may offer hope for earlier intervention prior to the development of cirrhosis.

Figure 38.30. Hypoplastic left heart syndrome status post total cavopulmonary anastomosis. Magnetic resonance angiogram. A: Coronal plane image showing the anastomotic sites of Fontan conduit (FC). The superior anastomotic site of FC is stenotic (white arrow). The retroaortic portion of the pulmonary artery (left pulmonary artery, LPA) is significantly stenosed (black arrow). The right pulmonary artery is not visible in this plane. B: Volume-rendered 3D reconstruction in a corresponding view. The two roots of neoaorta (NAo) can be seen. The neoaortic root appears dilated and the native aortic root is severely hypoplastic. The right pulmonary artery (RPA) can be seen in this image. The yellow arrows are pointing at the sites of stenosis. Fenestration between the FC and the right atrium is patent (blue arrow). Image plane orientation is specified by the cube in right lower corner. SVC: superior vena cava.

Figure 38.31. A variation of Fontan procedure. Volume-rendered 3D reconstruction of computed tomographic angiogram in a patient with hypoplastic left heart syndrome. A: Anterior view. Extracardiac Fontan conduit (FC) extends from inferior vena cava (IVC) to the innominate vein (asterisk). The left pulmonary artery (LPA) and the right pulmonary artery (RPA) are connected to the innominate vein using prosthetic material. The LPA passes anterior to neoaorta (AAo). B: Posterior view. The RPA is connected medial to the conduit and travels posterior to it. Mild coarctation of neoaorta (CoA) can be noted. C: Superior view from head. Image plane orientation is specified by the cube in right lower corner. DAo: descending aorta, IVC: inferior vena cava, LA: left atrium.

Cardiomyopathies

CTA and MRI have a developing role in the assessment of cardiomyopathic diseases. In the pediatric population the primary myocardial diseases of interest from the perspective of these imaging modalities include hypertrophic cardiomyopathy, noncompaction cardiomyopathy, and arrhythmogenic right ventricular dysplasia. Cardiac structure and function are integral to the assessment of these disorders, preferably by MRI. Cardiac CTA does have the ability to assess cardiac structure and function but is disadvantaged by the use of ionizing radiation and iodinated contrast, and lack of tissue characterization.

In the setting of hypertrophic cardiomyopathy, MRI provides excellent structural and functional imaging the using SSFP sequence (Fig. 38.32, Video 38.8). This initial approach provides accurate identification of the site and extent of ventricular hypertrophy and myocardial mass as well as identification of abnormal myocardial thickening, which may be an indicator of myocardial dysfunction in this cardiomyopathy. In particular, the ability to see the complete cardiac structure without limitations imposed by the need for specific acoustic windows and foreshortening of the ventricular apex, as seen in echocardiography, is one of the major advantages of cardiac MRI in this lesion. Obstruction is frequently visualized on SSFP sequence cine images as a signal void caused by dephasing of protons and represents a high velocity jet in the outflow tract. Using the same sequence, the mitral apparatus can be well delineated and the presence of systolic anterior motion of the mitral apparatus, as a signature of dynamic left ventricular outflow tract obstruction, can also be established. In the apical hypertrophic cardiomyopathy phenotype, MRI may also be better suited to assess for the presence of an apical pouch and thrombus. Perfusion abnormalities and abnormal myocardial delayed enhancement postintravenous gadolinium administration are also useful in assessing tissue characterization. The latter may be an indicator of abnormal myocardial architecture caused by fibrosis or necrosis. The cardiac MRI assessment of noncompaction cardiomyopathy is similar to that of hypertrophic cardiomyopathy with the emphasis on the delineation of areas of hypertrabeculation and left ventricular dysfunction.

Arrhythmogenic right ventricular dysplasia (ARVD) is an inherited cardiomyopathy whose imaging hallmark relates to fibrofatty replacement of the right ventricular myocardium, as well as right ventricular dilation and dysfunction. The left ventricle may also be involved in this cardiomyopathy in at least 15% of cases. Using the SSFP sequence in cardiac MRI, right ventricular function, microaneurysm formation, and focal areas of dyskinesis can be identified. Cardiac MRI is also uniquely suited to assess the right ventricular outflow tract, which can also be dilated and dysfunctional in this disorder. Black blood imaging may be utilized to assess for fatty infiltration. Assessment of abnormal myocardial delayed enhancement postintravenous gadolinium contrast administration is also relevant in the evaluation of these patients, as the presence of hyperenhancement correlates with the findings of inducible tachyarrhythmias. Compared to other cardiomyopathies, CTA may have a more prominent role in the assessment of patients with ARVD, since it is uniquely suited to demonstrate fatty replacement of the right ventricular myocardium. Similarly, the superior spatial resolution and the ability to obtain cine CTA images allows for delineation of right ventricular microaneurysms as well as right ventricular dilation and dysfunction. In particular, the ability to perform CTA imaging in patients who have already received an implantable device is an advantage in this group of patients.

Figure 38.32. Hypertrophic cardiomyopathy. Steady-state free precession sequence in magnetic resonance imaging showing a four- and five-chamber (long-axis) orientation. Note the asymmetric septal thickness consistent with diagnosis of hypertrophic cardiomyopathy. On video imaging there is dephasing in the left ventricular outflow tract consistent with dynamic left ventricular outflow tract obstruction at rest (see Video 38.8).

Myocarditis is an inflammatory myocardial disorder that has multiple etiologies. The clinical dilemma arises in the young adult presenting with symptoms, electrocardiographic changes and elevation in cardiac biomarkers. Here the evaluation is targeted to distinguish between ischemic and nonischemic etiologies of the presentation. Cardiac MRI is extremely useful in distinguishing ischemic heart disease from nonischemic causes of myocardial dysfunction. Several different MRI sequences have been used in the evaluation of the patient with suspected myocarditis with a focus on delayed enhancement imaging and T2 weighted sequences to assess for myocardial edema. The combination of these sequences may have incremental benefit in the clinical evaluation. The distinguishing feature lies in the pattern of myocardial delayed enhancement. In the setting of ischemic cardiomyopathy, the delayed enhancement is usually subendocardial to transmural in nature in the distribution of a vascular supply. Conversely, in the setting of myocarditis, the delayed enhancement pattern is more likely to be mid-wall or epicardially focused (Fig. 38.33). Cardiac CTA has a limited role in the evaluation of suspected myocarditis with its major strength being in the demonstration of normal coronary arteries. It is also important to emphasize that currently no noninvasive imaging methods can provide an etiologic diagnosis and should not be used to refute the need for tissue diagnosis; the latter should be determined based on clinical grounds.

Coronary Artery Anomalies

Clear identification of anomalies of the coronary arterial system is important in the management of patients with known congenital heart disease as well as in the young patient presenting with symptoms that are worrisome for an ischemic etiology. In older children and adolescents, anomalous coronary arteries are usually detected as an incidental finding on a transthoracic echocardiogram performed for some other reason. Anomalies of the coronary artery may represent anomalies of origin, course, termination, or a combination (Fig. 38.34). Their identification is important for two critical reasons. First, certain anomalies are considered high risk for sudden cardiac death, and in these settings therapeutic intervention may be some form of a surgical procedure. Indeed, coronary artery anomalies are the second most common cause of sudden cardiac death amongst young US athletes. Second, coronary anomalies may be found in combination with other congenital cardiac defects, in which case, their identification is integral to further surgical planning with regards to the primary congenital defect.

Traditionally, coronary anomalies are identified using echocardiographic and fluoroscopic techniques. With the development of both CTA and MRI, assessment of the coronary arteries has become fast and reliable. Cardiac CTA performed with ECG-gating defines, in great detail, the origin of the coronary arteries, definition of the presence or absence of an intramural course, the relationship of the coronary arteries to other cardiac and noncardiac structures, the presence of anomalies of course such as myocardial bridging, and the presence of anomalies of termination, including fistulous communication with a cardiac chamber or great vessel. Again, the major disadvantage of cardiac CTA is the compulsory radiation exposure. The need for iodinated contrast is dependent upon the clinical question, and if the identification of the origin and proximal course is all that is clinically indicated, then a straightforward coronary calcium study may be all that is required. This minimizes the radiation dose and allows for a study without the use of contrast. If, however, there is concern for myocardial bridging or fistulous communication with other vascular structures, or if there is a need to identify precise relationships of the coronary vasculature, iodinated contrast becomes an integral part of the examination. Cardiac MRI may also be useful in the assessment of the coronary vessels, especially in the identification of the origins and proximal course of the vessels. Cardiac MRI is limited by the long examination time and the current limited ability to reliably image the entire coronary anatomy; although it is the hope that, as the technology and science in the field advances, imaging time and quality will continue to improve.

Figure 38.33. Acute myocarditis. Steady-state free precession sequence in a short-axis imaging planes. A: Stack of short-axis images before Gadolinium injection shows nonspecific, heterogeneous enhancement of the left ventricular lateral wall. B: Stack of short-axis images 15 minutes after Gadolinium injection demonstrates a corresponding region of focal subepicardial hyperenhancement in the lateral wall of the left ventricle, characteristic of myocarditis.

Figure 38.34. Anomalous origin of coronary arteries. A: Coronal oblique plane demonstrating the origin of the left main coronary artery above the sinotubular junction (arrow). B,C: Anomalous origin of left anterior descending (LAD) coronary artery from the right coronary artery, travelling anterior to the right ventricular outflow tract (RVOT). This anomaly is important to recognize in patients with tetralogy of Fallot where a transannular incision in RVOT may result in transection of LAD. Ao: aorta, LV: left ventricle, MPA: main pulmonary artery, RV: right ventricle.

Figure 38.35. Ebstein anomaly. Steady-state free precession sequence using an imaging plane that is axial (A) to the body and in a short-axis (B) imaging plane in a patient with Ebstein anomaly of the tricuspid valve. Note the dilated right-sided chambers relative to the left sided cardiac chambers. Also note the challenge in identifying the annular plane in the short-axis imaging plane which can cause errors in volumetric quantitation (see Videos 38.9 and 38.10).

Ebstein Anomaly

Ebstein anomaly of the tricuspid valve, related to disorders of the normal delamination process of the tricuspid valve structure in utero, is a disorder that is traditionally imaged, and followed up with echocardiography. Both cardiac CTA and MRI are demonstrating a growing presence in the evaluation of patients with Ebstein anomaly. The current major indication for cardiac MRI in the assessment of Ebstein anomaly is in the quantitative delineation of right ventricular volume (Fig. 38.35, Videos 38.9 and 38.10). This includes measurement of end-diastolic and end-systolic volumes as well as right ventricular stroke volume and ejection fraction. Cardiac MRI assessment in this lesion is relatively straightforward, and for the most part can be done without the need for intravenous gadolinium contrast. Much of the cardiac MRI examination can be performed using an SSFP sequence that provides cine images of cardiac function. Right ventricular volumes are conventionally traced in a plane that is axial to the body. This is done because setting up such an imaging plane is straightforward, thereby reducing chances of significant interobserver variability. Additionally, identification of the tricuspid annulus is more reliable in the axial plane than in the short-axis plane where excursion of the tricuspid annulus into and out of the slice being traced can lead to significant error in measurement. In addition to volumetric assessment of the right ventricle, MRI offers one significant advantage over echocardiography, and that is the absence of limited imaging windows. The ability to image the cardiac structures in any desired imaging plane allows the examiner to assess the right ventricle in its entirety and this strength can help improve our current understanding of right ventricular structure and function. In the setting of coexisting congenital heart defects, additional measurements may be implemented at the time of the examination; for example, phase contrast imaging to derive Qp/Qs to quantify shunt fraction in the setting of an associated ASD.

Although cardiac CTA is not currently a mainstay of imaging in patients with Ebstein anomaly of the tricuspid valve, it has utility in those patients who require assessment of right ventricular function but have a cardiac device in place. In this instance an ECG-gated CTA with intravenous iodinated contrast will allow for measurement of right ventricular volumes as described above. Second, as patients with Ebstein anomaly may present at varying ages, assessment of the coronary arteries prior to surgical intervention may be afforded noninvasively using cardiac CTA.

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Questions

1.In cardiac CT imaging, ECG-gating of the study:

A.is useful for accurate determination of descending thoracic aortic measurements.

B.is not necessary for accurate determination of ascending thoracic aortic measurements.

C.is necessary for determination of cardiac volumes and ejection fraction.

D.can only be performed in a prospective mode.

E.is necessary for accurate assessment of branch pulmonary arteries.

2.When measuring right ventricular volumes using cardiac MRI the most acceptable technique is:

A.measurement of right ventricular volumes in a short axis imaging plane.

B.measurement of right ventricular volumes in a 2 chamber plane across the right ventricle.

C.measurement of right ventricular volumes in a plane that is axial to the body.

D.measurement of right ventricular volumes in a plane that is coronal to the body.

E.measurement of right ventricular volumes in a plane that is sagittal to the body.

3.Of the following imaging techniques, which one is associated with no radiation exposure?

A.Cardiac MRI

B.Retrospective ECG-gated cardiac CT

C.Prospective ECG-gated cardiac CT

D.Coronary angiography

E.Radionuclide imaging

4.Which of the following describes phase contrast imaging techniques?

A.Is used in echocardiography to determine shunt fraction.

B.Is used in cardiac CTA to create differential opacification of the cardiac chambers.

C.Is used in cardiac CTA to assess differential flow in pulmonary arteries.

D.Determines flow speed in cardiac MRI that can be used to determine shunt fraction.

E.Determines flow velocity in cardiac MRI that can be used to assess regurgitant lesions.

5.In the assessment of aortic aneurysms,

A.measurements of maximal aortic caliber are comparable across imaging modalities.

B.measurement of maximal aortic caliber is best made in a plane that is axial to the body.

C.measurement of maximal aortic caliber is best made in a plane that is axial to the aorta at the level of measurement.

D.intravenous contrast is always required for complete assessment of the aorta using MRI.

E.measurement of maximal aortic caliber is best made in a plane that is parallel to the aortic blood flow.

6.What is the role of cardiac MRI in the assessment of patients with Tetralogy of Fallot after repair?

A.Assessment of right ventricular volumes

B.Quantification of aortic regurgitation

C.Quantification of right ventricular mass

D.Assessment of left ventricular function

E.Quantification of Qp/Qs

7.What is the role of cardiac MRI in patients with a new diagnosis of coarctation of the aorta?

A.Assessment of the presence of left ventricular eccentric hypertrophy

B.Assessment of the presence of systemic venous collateral vessels using MRV

C.Assessment of the presence of aortopulmonary collateral vessels using MRA

D.Assessment of the presence of retrograde flow in the intercostal vessels proximal to the site of coarctation using phase contrast imaging

E.Assessment of pulmonary to systemic venous collaterals

8.With regard to radiation exposure in children undergoing cardiac CT assessment:

A.the radiation exposure is constant for a particular imaging protocol.

B.the radiation exposure is not dependent upon patient size.

C.dose reducing protocols should be used whenever feasible without compromise of diagnostic image quality.

D.ECG-gated cardiac CTA allows for the lowest radiation exposure.

E.contrast use reduces the radiation exposure.

9.Which of the following refers to coronary arterial anomalies?

A.Myocardial bridging is better assessed using cardiac MRI versus cardiac CT.

B.Anomalies of origin may be assessed adequately with echocardiography.

C.Intramural course of a coronary artery refers to a course between the great arteries.

D.Cardiac MRI is best suited to assess fistulous termination of coronary arteries.

E.Contrast is almost never needed to assess the coronary arteries in detail.

10.What is the role of cardiac MR imaging in patients who have undergone repair of a prior coarctation of the aorta?

A.Assessment for possible restenosis or aneurysmal dilation at the site of the initial coarctation.

B.Assessment of the regression of collateral vessels

C.Assessment of the presence or absence of right ventricular hypertrophy

D.To assess for the presence or absence of a bicuspid aortic valve

E.To assess the degree of aortic stenosis

Answers

1.Answer: C. ECG gating may be used in cardiac CTA imaging to assess the cardiac and vascular structures at different time points within the cardiac cycle. This is especially important given the fact that the heart and closely associated structures are constantly in motion. The descending thoracic aorta is not significantly influenced by cardiac motion and so its measurements are reliable in an ungated study. The ascending aorta, however, can have considerable motion during the cardiac cycle, which can be associated with significant motion artifact in an ungated study. An ECG gated study is a prerequisite to determine cardiac volumes. ECG gating can be performed both prospectively and retrospectively. Branch pulmonary arteries can be assessed without ECG-gating.

2.Answer: C. In determining right ventricular volumetric measurements the convention is to make tracings using the endocardial border in a plane that is axial to the body. This convention is practiced since the right ventricle has a complex geometry and using a short-axis imaging plane makes it challenging to determine the annular plane, which is crucial in such entities as Ebstein anomaly of the tricuspid valve. An imaging plane that is axial to the body is easily acquired and in such a plane, the annulus of the tricuspid valve is easily and conveniently identified.

3.Answer: A. Cardiac MRI is not associated with radiation exposure. It is an imaging technique that uses magnetization and its influence on proton precession and spin to ultimately produce images. All other responses are associated with radiation exposure.

4.Answer: E. Phase contrast imaging is a technique that can be used in cardiac MRI. It is a technique that can assess flow velocity perpendicular to the plane of interest. Using this technique cardiac output through the aorta and pulmonary artery can be determined to evaluate shunt fractions. Since velocity has an associated direction, this technique can also be used to determine severity of regurgitant valvular lesions. This technique can also be used in MRI to assess differential blood flow in the pulmonary arteries.

5.Answer: C. In assessment of aortic aneurysmal size it is important that an imaging plane is used that cuts across the axial plane of the aorta and is perpendicular to aortic blood flow. This ensures that the measurement is not over or underestimated by using a plane that is tangential to the aorta. Measurements across imaging modalities are not necessarily comparable, and differences in measurements must take into account the modality that is being used. Intravenous contrast is not always required for the complete assessment of the aorta using MRI. Intravenous gadolinium contrast is used as part of MR angiography. However, non-contrast sequences may be used, when reasonable, to assess the aorta with MRI.

6.Answer: A. One of the major indications for cardiac MRI in patients with Tetralogy of Fallot after repair is to assess right ventricular function. One of the postoperative issues that needs to be monitored in these patients is the presence and severity of pulmonary regurgitation. The right ventricle responds to volume overload with dilation, whereas the right ventricular response to pressure is hypertrophy. Since pulmonary regurgitation is predominantly a volume overload problem, right ventricular dilation will be the primary response. Therefore, one of the goals of surveillance imaging is to assess the degree of ventricular dilation so that optimal timing of pulmonary valve replacement can be planned.

7.Answer: D. The role of cardiac MRI in the assessment of patients with coarctation of the aorta is to locate the site of coarctation and then to assess its functional significance. The functional significance of a coarctation refers to its effects on the heart and vasculature. In a significant coarctation, there will be pressure overload of the left ventricle leading to concentric left ventricular hypertrophy. In a hemodynamically significant coarctation, the only way blood can be received distal to the site of coarctation is via systemic arterial collateral vessels. These are usually provided by the internal mammary artery, the intercostal arteries or directly from the subclavian artery, and these can all be delineated with the use of MR angiography. The presence of retrograde flow in these vessels, as demonstrated by phase contrast imaging, can allude to the presence of a hemodynamically significant coarctation. Formation of venous collaterals is not associated with the pathophysiology of this lesion.

8.Answer: C. When using cardiac CT, dose-reducing protocols should be used regularly such that the lowest possible radiation exposure is used that allows for the creation of diagnostic images. The radiation exposure depends upon both machine factors including the protocol being imaged, as well as patient factors such as body habitus. ECG-gating or contrast use by themselves do not reduce the radiation dose.

9.Answer: B. Myocardial bridging is better assessed using cardiac CTA versus MRI, specifically because the current spatial resolution using CTA affords better appreciation of small segments of bridging in the distal coronary vessels. Anomalies of origin can be assessed adequately using echocardiography. An intramural course refers to the initial course of an anomalous coronary artery within the wall of the vessel of origin. An interarterial course refers to a course between the great arteries. Although cardiac MRI continues to evolve, its ability to resolve the terminal portions of the coronary anatomy is challenging. It is hopeful that with faster imaging sequences the latter will improve, which will make imaging of the coronary arteries easier using MRI. The origin and proximal course of the coronary arteries can be seen in CTA without contrast, but for detailed assessment of distal coronary arteries contrast is usually required.

10.Answer: A. Surveillance imaging in patients who have undergone repair for coarctation of the aorta is focused on the evaluation of complications of the procedure that may require further surgical intervention. Such complications include restenosis with concomitant hemodynamic effects as demonstrated by persistence or development of concentric left ventricular hypertrophy or new systemic arterial collateral arteries. Aneurysmal dilation at the site of repair can be seen as a postoperative complication. The type of complication is predicated by the type of repair. As such, part of the comprehensive MRI approach for these patients includes a review of the exact surgical procedure performed. The assessment for the presence or absence of a bicuspid aortic valve should be achieved on the index imaging study (i.e., the first MR exam) and should not be performed as part of surveillance imaging. It is, however, important to assess aortic valve function in the setting of a bicuspid aortic valve as hemodynamically significant stenosis or regurgitation will influence clinical management of the patient. The preferred way to assess the degree of aortic stenosis is by echocardiography.