Tetralogy of Fallot (TOF), the most common cyanotic congenital heart defect, accounts for about 5–10 % of all congenital heart diseases [61, 62]. Based on the main defects of this relatively common CHD (pulmonary stenosis, ventricular septal defect (VSD), overriding aorta, and hypertrophy of the right ventricle), these patients usually undergo corrective surgery in their first year of life. Advances in diagnosis, surgical techniques, and postoperative treatment have led to an increasing number of patients reaching adulthood [63] and a high survival rate of almost 90 % at 30 years [64].

Surgical treatment for TOF consists of the closure of the VSD and reconstruction of the RVOT [65]. Despite the great progress of surgical management, the majority of adult patients are burdened with functional abnormalities which can usually be tolerated well until adolescence [66]. Different surgical techniques have been used for RVOT reconstruction: homograft, transannular patch, or a valve-preserving technique. Despite the success of these interventional techniques, restenosis or aneurysm formation of the pulmonary outflow tract can complicate the patients’ mid- and long-term outcome [67, 68]. Pulmonary valve insufficiency with consecutive right ventricular volume overload is the most common and serious postoperative long-term complication in more than 70 % of TOF patients after repair [69, 70]. Progressive right ventricular dilatation or reduced ventricular function is an indicator of the need for valve replacement [71, 72].

MRI is superior to echocardiography for the evaluation of pulmonary artery stenosis, for detecting and quantifying pulmonary valve insufficiency, for assessing right ventricle volume, and for monitoring progression of pulmonary regurgitation and right ventricular dilatation [22, 24, 73]. The role of MRI as the preferred noninvasive imaging modality in patients with repaired TOF has gained international acceptance [66].

Standard MRI protocols based on ECG-gated SSFP sequences provide anatomic and volumetric information. Typically, the right ventricle is dilated and has a thickened trabecular wall. In the 4-chamber view, the interventricular septum is shifted backward to a horizontal position. Proximal stenosis of the left pulmonary artery (LPA) can often be observed on anatomic images, occasionally in association with poststenotic ectasia or in conjunction with hypoplastic LPA (Fig. 20.8). Time-resolved contrast-enhanced MRA can provide information on abnormal lung perfusion by depicting asymmetric contrast-medium distribution to both lungs (Fig. 20.8). MRA can additionally help to detect abnormalities of the aorta and supra-aortic arteries. In this context, post-processing of the MRA data to generate 3D surface-rendered or maximum intensity projection (MIP) images can often provide an improved 3D impression of the aortic and pulmonary vasculature (Fig. 20.9).

2D CINE MRI in oblique sagittal orientation to depict the RVOT should be acquired in addition to basic 4- and 2-chamber views. By viewing 2D CINE images dynamically over the cardiac cycle, jet flow phenomena or flow turbulences caused by valve stenosis or insufficiency can be observed in the pulmonary outflow tract and pulmonary arteries of repaired TOF patients (Fig. 20.10). To quantify aortic and pulmonary blood flow and the degree of valve regurgitation, 2D PC MRI is used. Postsurgical anatomy often complicates 2D PC MRI planning, and RVOT 2D CINE images can facilitate the exact placement of the 2D PC plane in the main pulmonary artery.

For the assessment of ventricular volumes and global cardiac function, 2D CINE SSFP planes in the ventricular short axis, which is preferred for biventricular volumetry, or in the axial plane should be performed. Both views allow for visual evaluation of regional wall motion abnormalities and ventricular asynchrony in contraction and relaxation.

This standard MRI protocol described above provides a broad overview over anatomy and function of the repaired heart after TOF but does not provide detailed information on potential right heart flow alterations associated with post-repair TOF. Since altered hemodynamics can result in right ventricular dilatation and thus at risk for sudden death and tachyarrhythmias [74], it is vital to obtain a better understanding of in vivo 3D flow patterns in TOF patients.

4D flow MRI pilot studies have shown that color-coded visualization by streamlines or particle traces can demonstrate increased peak velocity in the pulmonary trunk of TOF patients compared to normal controls. Higher flow velocity was frequently observed even in the absence of residual or restenosis of the RVOT (Fig. 20.11). Blood flow distribution to the right and left lung can easily be appreciated and measured by 4D MRI leading to an assessment of potential abnormal flow ratios (Fig. 20.12). In addition, 4D flow MRI can visualize diastolic retrograde flow in the pulmonary arteries and the RVOT (Figs. 20.11 and 20.12). A more precise assessment of pulmonary regurgitation is obtained by quantitative measurements. Accurate positioning of planes for regional flow quantification can be challenging in standard 2D phase-contrast imaging, particularly in TOF where the left pulmonary artery is often angulated or hypoplastic. Incorrect plane placement cannot be corrected after 2D data acquisition whereas the 4D data allow retrospectively for correct quantification of the regurgitation fraction. Feasibility studies depicted flow disturbances in the pulmonary trunk in terms of vortices or increased helical flow patterns which may be associated with pulmonary artery dilatation (Fig. 20.13). This observation has already been made in previous 4D flow MRI studies of the aorta showing that abnormal flow patterns and vortices contribute to ineffective vascular flow [56, 57, 75]. Similarly, pulmonary trunk abnormalities may influence the development of regurgitation and hemodynamic alterations. These observations in small patient cohorts are promising as 4D flow MRI may help to understand pathophysiological considerations of altered hemodynamics in TOF patients.

Transposition of the great arteries (TGA) is a cyanotic congenital heart defect characterized by ventriculoarterial discordance with the aorta originating from the right ventricle and the pulmonary artery from the left ventricle [76]. Initial surgical interventions by Senning and modification by Mustard (atrial switch procedures) left the right ventricle acting as the systemic ventricle [77, 78]. In this procedure, intra-atrial baffles were used to direct the blood to the corresponding ventricle, resulting in a physiological but not anatomic repair [79]. This condition gives rise to complications such as arrhythmia, progressive ventricular dysfunction, and heart failure [80, 81]. Subsequently, the treatment for TGA has been modified and is nowadays based on the arterial switch procedure during the neonatal period (Jatene principle) which involves a surgical correction of the arterial displacement [82]. The arterial switch operation consists of transection of the aorta and the main pulmonary artery at their roots to correct the ventriculoarterial discordance with transfer of the coronary arteries to the neoaortic root (Fig. 20.14).

In the typical postoperative Lecompte position, the pulmonary trunk is located anterior to the ascending aorta and the pulmonary arteries embrace the ascending aorta [83]. In some cases, e.g., in case of associate defects such as VSDs, the classical Lecompte position cannot be accomplished and the left pulmonary artery is routed posterior to the ascending aorta (Fig. 20.15).

Most common midterm and long-term complications include stenosis of the right ventricular outflow tract and the pulmonary arteries, coronary artery stenosis, aortic root dilatation, or valve insufficiency [84–88]. A recent study by Kempny and coworkers reported that coronary complications and aortic valve dysfunctions are rare. In contrast, RVOT obstructions or pulmonary valve insufficiencies were more frequently observed and lead to re-intervention [89]. Therefore, regular follow-up by echocardiography and MRI is mandatory to monitor ventricular size and function and to identify vessel stenosis or valve insufficiency needing intervention [90]. Echocardiography is challenging for older children due to the limited acoustic window by the postsurgically retrosternal location of the pulmonary trunk and the pulmonary arteries [91]. The complex postsurgical cardiovascular geometry requires multi-planar acquisitions as either dark-blood T2 HASTE or bright-blood SSFP MRI to completely capture the altered anatomy and detect a potential RVOT stenosis (Fig. 20.16) [92–95]. In analogy to the follow-up of Fallot patients, the use of 3D SSFP sequences with free-breathing and full volumetric coverage can facilitate the assessment of the complex cardiac anatomy and the measurement of vessel diameters [96].

Time-resolved CE MRA is most useful for the assessment of the extracardiac anatomy. It has the potential to demonstrate segmental pulmonary artery stenosis or morphologic alterations of the pulmonary trunk and the aortic root (Fig. 20.16). Because of the subsequent contrast filling of the different cardiac compartments, a detailed depiction of the vessels is warranted without interfering overlay by venous structures or suboptimal timing in contrast-medium arrival time using standard 3D MRA. In addition, information on the dynamics of the passage in the contrast agent through the cardiovascular system can be used to detect symmetric or delayed perfusion of the right and left pulmonary system (Fig. 20.17).

For the evaluation of ventricular function, a series of 2D CINE SSFP images is obtained in standard planes (4-chamber, 2-chamber, short-axis stack) and for the RVOT and LVOT. Similar to Fallot patients, an axial image stack may be used to assess the right ventricular volume. For flow quantification to evaluate valve insufficiency and residual shunts, 2D PC MRI is performed at the level of the ascending aorta and main pulmonary artery [97]. Blood flow in the pulmonary arteries can also be evaluated with 2D CINE PC MRI to investigate pulmonary stenosis arising from morphologic vessel lumen changes during the heart cycle [91].