Cardiovascular magnetic resonance (CMR) gives unrestricted access to the heart and great vessels noninvasively and without ionizing radiation. It can provide biventricular functional assessment, flow measurement, myocardial viability assessment, angiography, and more, and is therefore recommended for long-term follow-up in adult congenital heart disease (ACHD). Transthoracic echocardiography remains the first-line approach to imaging the hearts of patients, and provides a relatively rapid and comprehensive evaluation of anatomy, function, and hemodynamic indices in most patients. However, the suboptimal penetration of ultrasound is a limitation, especially in adults after cardiovascular surgery. Moreover, echocardiography does not offer CMR’s repertoire of tissue contrast options, with or without contrast agent, and lacks its unrestricted fields of view and volumetric measurements of flow. For these reasons, a dedicated CMR service should be regarded as a required facility in a center specializing in the care of ACHD.
CMR is performed with a patient’s body located in a strong magnetic field, typically 1.5 T, where the patient will generally have to lie still for a period of 30 minutes or more. When 3.0 T is used, it enables a higher signal-to-noise ratio, which has the potential to improve image quality and acquisition speed particularly for contrast-enhanced angiography and perfusion imaging sequences. However, the higher field strength causes an increase in susceptibility-induced field variations. Local phase changes of the MR signal due to susceptibility differences lead to a signal loss in the image, more prominent in steady-state free precession (SSFP) sequences. Various approaches have been described to reduce these artifacts.
Claustrophobia can be problematic in about 5% of patients. Images are acquired by means of a radio signal that passes freely through the body and resonates with the nuclei of hydrogen in the body, whose spins are appropriately tuned and re-tuned by magnetic gradients superimposed on the main magnetic field. Images are computed by spectral analysis of re-emitted radio signals, interpreted in relation to the sequence of radio pulses and the magnetic gradients applied. Cardiac gated cardiovascular images are acquired using sequences applied at specific time delays after the R-wave of the electrocardiogram, usually through several successive heart cycles, so arrhythmias may degrade image quality.
Safety
Although CMR is noninvasive, nonionizing, and usually safe, the strong magnetic field with its gradient switches can present dangers under certain circumstances. CMR imaging of patients with implanted pacemakers or cardioverter defibrillators is no longer absolutely contraindicated. Magnetic resonance imaging (MRI) conditional devices are on the market and have been tested and approved for use in the MR environment. In patients with such devices, CMR can be performed safely under certain conditions according to the manufacturer’s recommendations ( Fig. 8.1 ). In patients with conventional pacemaker systems, CMR can be performed with low risk if procedure guidelines are followed. However, CMR should only be used if the benefit outweighs the risk, and alternative imaging techniques have to be considered. Common items of hospital equipment made of steel, such as scissors, wheelchairs, or gas cylinders, can become lethal missiles if inadvertently taken close to the magnet. However, most metallic devices and clips implanted in the chest are safe, as long as they do not incorporate electrical devices. Ferromagnetic implants cause local artifacts on images, but this does not usually negate the usefulness of the investigation. The severe complication of nephrogenic systemic fibrosis secondary to the use of gadolinium chelate contrast agents, which are widely used for CMR angiography or myocardial viability studies, was first described in 2000. This is rare and only in patients with preexisting renal failure. In cases where a contrast agent is indicated, renal function needs to be tested, and the potential risks weighed against the benefits of contrast-enhanced rather than noncontrast CMR imaging. Information regarding specific implants and CMR systems can be sought by logging on to www.MRIsafety.com .
General
Where a CMR service is available for investigation and follow-up of ACHD patients, it is soon found to be extremely valuable. Images and measurements obtained complement those by transthoracic and transesophageal echocardiography. They make diagnostic catheterization unnecessary in many cases, and expedite subsequent interventional catheterization. However, diagnostic catheterization may still be needed for measurement of pulmonary artery (PA) pressure and resistance. Alternatively, multislice ECG gated cardiac computed tomography (CT) may be preferable for detailed visualization of coronary arteries and in some patients with pacemakers.
CMR gives unrestricted access to the chest in multiple, freely chosen slices. It is noninvasive, free of ionizing radiation, and is usually well tolerated by patients who may need to return for repeated follow-up investigations. It provides clear images of anatomy throughout the chest. Cine imaging depicts movements of myocardium, valves, and flowing blood. Contrast-enhanced magnetic resonance angiography (CE-MRA) and three-dimensional (3D) SSFP noncontrast imaging can provide clear views of the pulmonary, systemic, and collateral arterial branches. CMR can answer functional as well as anatomic questions, including the location and severity of stenosis (eg, aortic coarctation or PA stenosis), severity of regurgitation (eg, pulmonary), the size and function of heart chambers (the right and the left ventricle [LV]), and measurement of shunt flow.
As an imaging modality, magnetic resonance has unrivaled versatility. The key to this versatility is control of the interaction between radio signals and nuclear spins in the tissues and blood, mainly by means of rapid, carefully designed sequences of applied magnetic gradients. The spins of protons are energized by pulses of radio energy and tuned and re-tuned by magnetic gradient switches. A repertoire of different sequences allows a variety of image appearances or flow measurements to be achieved, usually without a contrast agent ( Fig. 8.2 ).
The versatility of CMR is a great strength, but also a potential source of confusion. Different CMR systems, or different individuals using the same system, may use different approaches. Given so many choices, uniformity is not easy to maintain. CMR is also relatively expensive, but the cost of imaging should be weighed against potential costs of inappropriate management, which might entail complicated repeat surgery or longer hospitalization than necessary. Imaging specialists need not be deterred by anatomic variability found in congenital heart disease. The comprehensive anatomic coverage offered by CMR almost always allows useful diagnostic contributions to be made. Although it is recommended that CMR of more complex cases is undertaken by experts in specialist centers, this may not always be possible. If necessary, a relatively comprehensive and technically simple approach is to acquire one or more contiguous stacks of cine images covering the whole heart and mediastinum. Such cine stacks are easy to acquire and review. They reveal functional and anatomical information and allow the identification of any jet flow. This approach can be supplemented or replaced by patient-specific protocols as experience and confidence are gained.
Image Display and Analysis
Static films are not adequate for conveying all of the information available in multislice, cine, flow velocity, and 3D angiographic acquisitions. CMR acquisitions need to be replayed and analyzed interactively on a computer using appropriate software. The image display and analysis package should allow at least ventricular volume and flow measurements. For review of images in the setting of a multidisciplinary clinical meeting, images should be displayed via a computer linked to the image storage server and to a projector.
Techniques
Multislice Imaging
Transaxial, coronal, and sagittal stacks of multislice images should be acquired in ACHD patients. There are several methods of acquisition. Bright-blood images using SSFP acquisition have advantages in ACHD patients because they clearly show the pulmonary vessels, and each slice can be acquired rapidly. Adjacent slices can be acquired in consecutive heartbeats so that 20 or more static slices can usually be acquired in a single breathhold and then used for accurate alignment of subsequent breathhold cine acquisitions.
Cine Imaging
Cine imaging allows visualization of flow and the movements of the heart and vessel walls. Contiguous stacks of transaxial or coronal cine images covering the whole heart and mediastinum are recommended in ACHD, particularly in more complex cases. Such cine stacks are easy to acquire and review and reveal functional as well as anatomic information, showing the presence of any jet flow. However, because the images are composed of relatively long, thin voxels, the length being the slice thickness (typically 5 to 7 mm), thin structures such as valve leaflets or jet boundaries are seen clearly only where they are orientated perpendicular to the slice. SSFP cine images give good blood-tissue contrast, which is an advantage for imaging and measuring ventricular volumes and mass, and for visualizing heart valves. Sequences of this type can outline a coherent jet core clearly, if present, because of the localized loss of signal from the shear layers at the edges of a jet (see Fig. 8.2 ), and breathhold acquisition makes it possible to interrogate a jet area precisely and repeatedly. The approach of “cross-cutting,” locating an orthogonal slice through a partially visualized feature such as a valve orifice or jet, is an effective way of homing in on a particular jet. An alternative and more comprehensive approach is to acquire an oblique stack of relatively thin (5 mm) cines, without gaps, orientated to reveal all parts of a particular structure or region of interest such as a regurgitant mitral valve.
Phase Velocity Mapping
If correctly implemented, phase-contrast velocity mapping can provide accurate measurements of velocity and volume flow. However, understanding of the principles and pitfalls is needed for successful clinical application. Clinical uses include measurements of cardiac output, shunt flow, collateral flow, regurgitant flow, and where jets are of sufficient size and coherence, for measurements of jet velocities through stenoses. It is necessary to select a plane, echo time, velocity encoding direction, and sensitivity appropriate for a particular investigation.
Velocity can be encoded in directions that lie in or through an image plane. Mapping of velocities through a plane transecting a vessel (velocity encoded in the direction of the slice selection gradient) allows measurement of flow volume. The cross-sectional area of the lumen and the mean axially directed velocity within that area are measured for each phase through the heart cycle. From this, a flow curve is plotted, and systolic forward flow and any diastolic reversed flow are computed by integration. Such flow measurements will only be accurate if phase shifts are caused by velocities and not by other factors such as eddy currents, concomitant gradients, motion artifacts, or background noise. Appropriate acquisition sequences must be used. On some systems, automated correction of phase offset errors, if available, or subsequent correction using corresponding phase maps acquired in a static phantom may be needed to remove errors.
Jet velocity mapping can be useful for assessment of certain stenoses where ultrasonic access is limited, for example in aortic coarctation, ventriculopulmonary conduits, PA branch stenoses, and obstructions at the atrial and atriopulmonary levels following Mustard, Senning, and Fontan operations. However, the limitations of the technique need to be recognized. The velocities of narrow, eccentric jets through mildly regurgitant tricuspid or pulmonary valves, which may be used in Doppler echocardiography for estimations of right ventricle (RV) or PA pressure, are unlikely to be measured accurately by CMR.
Four-Dimensional Flow-Sensitive Velocity Mapping
Visualization of the intra- and extracardiac structures and blood flow is an essential component of CMR in ACHD patients. Four-dimensional (4D) flow-sensitive velocity mapping CMR is a technique that can measure all three directional components of the blood flow velocities relative to the three spatial dimensions and the time course of the heart cycle. This allows quantification and visualization of even complex flow patterns throughout a 3D volume. Instead of using multiple planes to assess blood flow at valves or structures of interest, 4D flow-sensitive velocity mapping CMR permits assessing flow and anatomic data in a user-defined volume (eg, volume including the heart and the great thoracic vessels) with a single acquisition. Furthermore, 4D flow CMR has the advantage of retrospective placement of analysis planes at any location within the acquisition volume. This can be helpful in cases where several two-dimensional (2D) phase-contrast CMR scans are needed, such as assessment of systemic-to-pulmonary collateral flow in patients with palliated univentricular hearts.
The field of 4D flow CMR is rapidly evolving, and an increasing number of publications illustrate promising applications in congenital heart disease. However, there are several technical limitations, including the lack of sequence standardization across MR platforms and the often time-consuming pre- and postprocessing, which limits its routine clinical applicability.
Contrast-Enhanced Magnetic Resonance Angiography
To visualize vascular branches and collateral vessels, 3D angiographic acquisitions are used after venous injection of gadolinium chelate. This allows fast acquisition to be combined with good spatial resolution, allowing one or more 3D angiographic data sets to be acquired in a single breathhold. For optimal image quality, the timing of image acquisition needs to be adapted for contrast to be maximal in the anatomic region of interest. In time-resolved MR angiography, the timing of the acquisition is less important with the further advantage of obtaining dynamic information on the contrast agent distribution over time at the expense of spatial resolution.
MR angiography is useful for depiction of branches of the PA and aorta, and for assessment of aortic coarctation, recoarctation, or aortic aneurysm. It also allows assessment of obstructions in the venous channels after Mustard operation in transposition of the great arteries (TGA). The presence of metallic stents, sternal wires, or arterial clips can cause localized loss of signal in an angiogram, possibly leading to a false impression of stenosis.
Three-Dimensional Balanced Steady-State Free Precession
Bright blood SSFP sequences allow ECG-gated 3D imaging of cardiovascular cavities and structures, without the need for a contrast agent. This approach can be an alternative imaging modality to CE-MRA. This approach may be more suitable than contrast-enhanced angiography in patients after Fontan operation because it is not subject to the dilution of contrast from nonopacified caval inflow and is useful where 3D imaging of several heart chambers and arterial and venous vessels is required. It is also used in a single breathhold or when using diaphragm navigator respiratory gating, for MR coronary angiography. This allows the identification of anomalous coronary origins and proximal coronary course, although CT provides superior spatial resolution in shorter acquisition times for noninvasive coronary angiography, but at the cost of exposure to ionizing radiation.
Right and Left Ventricular Function and Mass Measurement
CMR is well suited for volumetric measurements of the RV and the LV. The reproducibility of left ventricular measurements is excellent. Although published studies have shown good reproducibility, measurements of the RV are challenging and not easy to achieve reproducibly in ACHD patients in routine clinical practice. The myocardium of most of the free wall and the apical regions of the RV is highly trabeculated in most individuals. The trabeculations become more apparent when the RV is hypertrophied, but even if clearly visualized, they are not easy to outline individually. Furthermore, the base of the RV tends to be more mobile and difficult to delineate than the left. After repair of Fallot’s tetralogy, the right ventricular outflow tract can be dilated, akinetic, and may have no effective pulmonary valve. This can make it difficult to decide on distal limits of the outflow tract. Measurements of right ventricular volume and function require meticulous and clearly defined technique. An akinetic or aneurysmal region of the right ventricular outflow tract (RVOT) should be included as part of the RV, up to the (expected) level of the pulmonary valve. In the interests of time and reproducibility, the RV boundary may be traced immediately within the relatively thin compact myocardial layer of the free wall rather than by outlining the multiple trabeculations. However, semiautomated methods that identify blood-myocardial boundaries may become a practicable, even if not directly comparable, alternative. Regardless of the approach used, it is crucial that longitudinal comparisons are based on comparable methods of acquisition and analysis. Contour data for volumetric analysis should ideally be stored in a database and remain available for comparison at the time of a subsequent study.
Myocardial Infarction or Fibrosis Studied by Late Gadolinium Imaging
Late gadolinium enhancement inversion recovery imaging is well established for the visualization of previous myocardial infarction and for assessment of myocardial viability. The extent of right ventricular fibrosis identified late after surgery for tetralogy of Fallot (ToF) or TGA may be relevant to arrhythmic risk stratification. Right ventricular fibrosis is also strongly associated with adverse outcomes, especially arrhythmias in patients with TGA after atrial switch operation. However, localized enhancement in the regions of insertion of the right ventricular free wall into the LV is a frequent and nonspecific finding in ACHD, and is of doubtful clinical significance.
Myocardial Perfusion Imaging
The acquisition and interpretation of first-pass myocardial perfusion images by CMR at rest and during adenosine stress requires training and experience. However, because CMR perfusion imaging does not subject patients to the long-term hazards of ionizing radiation, it is likely to gain a clinical role in the assessment of ischemia in patients with congenital heart disease (CHD).
T1 Mapping for Quantitative Myocardial Tissue Characterization
Late gadolinium enhancement imaging was a key advancement in the development of myocardial tissue characterization techniques and is the reference standard for assessing focal myocardial fibrosis. However, it is unable to detect more diffuse/interstitial myocardial disease. Myocardial T1 mapping is a newer technique that allows a quantification of diffuse myocardial fibrosis. So far, data for ACHD patients are still limited, particularly with respect to application in the RV, but T1 imaging may be of future clinical usefulness in patients with CHD. Recently, T1 mapping quantification has been attempted in children and adults with ToF. One prospective study in adults with ToF demonstrated that left ventricular interstitial fibrosis quantified by CMR was higher than in controls, and higher interstitial left ventricular fibrosis was associated with other adverse clinical markers and with clinical outcomes (atrial arrhythmia and death). This and other descriptions of interstitial fibrosis in ACHD justify further work in this area, including extending the technique to the RV, where the thin-walled structure makes the applicability of the standard technique uncertain.
Combined Cardiovascular Magnetic Resonance and Cardiac Catheterization
Combined catheterization and CMR is also feasible. A promising application is for measurements of pulmonary vascular resistance based on simultaneous measurements of pulmonary flow by CMR and pressure by catheter transducer. Work is progressing in the use of CMR for catheter and device guidance, with the potential advantages of 3D localization, tissue characterization, and the avoidance or reduction of ionizing radiation, although this remains a field for research rather than for mainstream clinical use.
Although determination of mean PA and left atrium (LA) pressure (or pulmonary capillary wedge pressure, PCWP) is usually straightforward by right heart catheterization (RHC), the assessment of flow based on Fick or thermodilution is problematic (shunts/valvular regurgitation make dilution techniques inaccurate, whereas indirect Fick uses assumed oxygen consumption, introducing error). CMR is considered the gold standard for the assessment of great vessel flow measurement. One valued use of CMR is for MR-augmented cardiac catheterization, whereby patients undergo invasive RHC followed immediately by CMR with a balloon-tipped catheter (Swan-Ganz) remaining in the branch PAs (for mean PA and PCWP measurement) during simultaneous acquisition of CMR flow.
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