Cardiovascular Magnetic Resonance

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.

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.