Cardiovascular magnetic resonance is an advanced noninvasive cardiovascular imaging technique that has become well established but continues to evolve. It has some advantages over SPECT myocardial perfusion imaging (MPI) in the evaluation of ischemic heart disease but also has limitations that restrict the potential patient population. Technical advances in hardware and software protocols are unlocking new diagnostic and prognostic possibilities. In many areas of cardiovascular disease, it is the reference standard. The versatility of CMR allows one scan to provide information on cardiovascular structure, function, fibrosis, perfusion, and tissue characterization. CMR is a robust technique for evaluating ischemic heart disease and cardiomyopathies and has a role to play in evaluating other conditions, such as valvular and pericardial disease. We will provide a brief overview of the methods of CMR imaging, discuss its advantages and limitations, and outline its use in specific disease states.

Magnetic resonance imaging is performed by having a patient lie in the bore of a large hollow magnet in which a magnetic field is generated, typically at 1.5 Tesla. The hydrogen atoms in the body, predominately in water and fat, behave like magnets and possess “spin.” When the hydrogen protons are exposed to the magnetic field they align their spins.^1,2 Radiofrequency pulses are generated by the magnet and excite the protons in specific planes of predetermined size and location so that their spins are aligned in a higher-energy state. Relaxation of the hydrogen nuclei to the lower-energy state gives off an electromagnetic signal that is detected by the scanner and processed into an image through a technique known as Fourier transformation.

There are many substantive advantages of CMR compared with other imaging modalities, including its high spatial resolution and high signal-to-noise ratio. CMR can also provide a 3D assessment of the heart, allowing the selection of any imaging plane, and is not susceptible to attenuation artifacts. Using different CMR sequences can also aid in tissue characterization, allowing multiple aspects of the myocardium to be assessed in one study (Table 29-1).

There are several limitations to the routine use of CMR. Despite recent improvements, there continues to be limited hardware availability, and cardiac-specific software is necessary. CMR remains an expensive imaging modality; this is challenging in the era of cost containment, although stress CMR and stress radionuclide imaging (RNI) are fairly similar in cost. Functional images are acquired over multiple beats gated to the electrocardiogram, which prolongs study times. Study times can be lengthy and are typically on the order of 40 to 60 minutes. Electrocardiographic gating can also result in artifacts in the setting of arrhythmias, particularly irregular rhythms such as atrial fibrillation and frequent premature ventricular contractions. Gating can be more difficult at higher field strengths. Patients must lie still, as all images are dependent on a fixed position relative to the magnetic field and RF coils. This can create compliance issues. Patients must hold their breath for most sequences, which can be challenging for symptomatic patients with limited respiratory reserve and also results in reduced compliance.¹ The confined nature of the magnet bore may be an issue for morbidly obese patients and can trigger claustrophobia, which can accentuate all of these issues. The magnetic field can interact with implanted devices. Some newer devices are potentially MRI compatible, but careful patient selection, well-defined safety protocols, and additional large-scale testing are needed prior to the widespread use of CMR in this population and in those without MRI-compatible devices. Performing protocols on the MRI machine and software requires highly trained staff. Adequate training is also required to perform the image processing that is required to quantify volumes and function. However, newer software is allowing these processes to be increasingly automated.³ The use of gadolinium-based contrast agents (GBCAs) are contraindicated in patients with stage 4 and 5 kidney disease due to the association with nephrogenic systemic fibrosis.⁴ Finally, in all but a few specialized laboratories, stress imaging can only be performed with pharmacologic stress, eliminating the most powerful prognostic marker in stress imaging, exercise capacity.⁵ Rapid advances in CMR hardware, software, and MR sequences have the potential to improve many of these limitations in the near future.

A CMR study is made up of a series of specific sequences that reveal different aspects of the heart and vascular system. The ordering physician supplies a clinical question and the performing laboratory determines the optimal CMR protocol to answer that question. Unique sequences can assess heart structure, myocardial function, tissue characterization, perfusion, valvular function, and angiography.

Structure and Function

Steady-State Free Precession Cine Imaging

CMR is the gold standard for measuring left ventricular (LV) mass and LV and right ventricular (RV) chamber volumes and ejection fraction. Steady-state free precession (SSFP) imaging sequences are most commonly used to assess volumes and function, as they have excellent spatial and temporal resolution, a high signal-to-noise ratio, and short breath hold times.⁶ Stacking multiple short-axis slices together allows precise volume and function assessment without geometric assumptions.¹ The endocardial and epicardial borders are traced in end-diastole and end-systole, and the myocardial mass, stroke volume and ejection fraction can be calculated (Fig. 29-1).^6,7 Regional function can be quantified using a 17-segment model based on coronary distribution.⁸

In a comparison of LVEF between CMR, radionuclide ventriculography, and 2D echocardiography, the closest limits of agreement were with radionuclide ventriculography, which has also been considered a gold standard.⁹ There are wide variances with 2D and 3D echo, with systematic underestimation by 3D echo due to insufficient definition of endocardial trabeculae.¹⁰ CMR can orient the heart in any direction to assess function and precisely quantify RV and atrial function. CMR is particularly well suited for repeated studies for disease progression monitoring compared with echocardiography due to its accuracy and reproducibility, and RNI for the lack of radiation exposure.³

Several techniques are available to assess regional myocardial function and strain include myocardial tagging, harmonic phase imaging (HARP), and cine displacement encoded with stimulated echoes (DENSE).^3,6 Dobutamine CMR with tagging improves the identification of significant coronary lesions.¹¹ HARP is a low spatial resolution approach to tag analysis that has been used widely in population studies.¹² Cine DENSE has higher temporal and spatial resolution and will likely become dominant as the technology matures (Table 29-2).¹³

Tissue Characterization

T1-, T2-, and T2*-Weighted (W) Imaging

Adjustment of the electrical gradients, RF pulses, and timing of signal acquisition can emphasize different properties of the myocardium and aid in tissue characterization. The time it takes tissue to recover from excitation, the T1 relaxation time, is prolonged in fibrotic tissue and reduced in lipid-rich myocardium.¹⁴ T2 describes the time for the proton spins to lose their alignment in adjacent tissue (spin–spin relaxation time). T2-W accentuates tissue with a high water content, such as edema in early states of myocardial injury.¹⁵ T2* sequences can identify iron overload, such as in hemochromatosis or in conditions such as thalassemia after multiple transfusions.¹⁶ T1, T2, and T2* mapping are rapidly replacing static images due to their quantitative nature and robustness to artifacts.^17,18

Late Gadolinium Enhancement

Late gadolinium enhancement (LGE) is a method of scar assessment useful in assessing ischemic heart disease and cardiomyopathies. Patterns of gadolinium uptake during specific time periods following injection can help identify pathology. An absence of gadolinium uptake on delayed imaging indicates severe perfusion defects such as from microvascular obstruction after acute myocardial infarction (MI).¹⁹ Increased gadolinium uptake and delayed washout occur in infarcted myocardium due to intracellular accumulation in injured myocytes due to ruptured cell walls and in fibrotic scar due to its small intracellular fraction.¹⁹

Blood Flow

Blood flow quantification is performed in CMR through velocity-encoded imaging. Protons at motion develop a specific phase shift directly proportional to their velocity. Velocity-encoded imaging (also called phase-contrast) produces images with signal proportional to the velocity and direction of each individual pixel.⁷ Cumulative antero- or retrograde flows through specific vessels can be calculated by assessing blood flow through the region of interest over the entire cardiac cycle. Forward and regurgitant flow can be quantified and the regurgitant fraction calculated (Fig. 29-2).

Myocardial Perfusion

CMR perfusion imaging involves administering GBCAs and performing first-pass imaging, typically in several short-axis slices. Imaging is performed at rest and after stress (typically using a vasodilator agent) to assess for ischemia. Images are typically assessed qualitatively for visible defects. However, quantitative perfusion analysis can be performed and improves differentiation of moderate and severe stenoses and improves the identification of three-vessel CAD.²⁰ In combination with LGE imaging, ischemia and infarct can be differentiated with high accuracy (Fig. 29-3).²¹ The direct imaging of scar with LGE is an advantage over nuclear imaging.

CMR Coronary Angiography

SSFP imaging is used for coronary evaluation without contrast given the high blood T2/T1 ratio.²² SSFP is not susceptible to calcium, allowing imaging with heavy calcification. Images are limited by reduced spatial resolution (1–1.5 mm) and long imaging times requiring free-breathing imaging with respiratory gating.