FIGURE 1: Transmural infarct area in right coronary artery territory. Breath-hold short axis ECG gated inversion recovery gradient echo image obtained about 10 minutes after the injection of gadolinium contrast agent, clearly indicates infarct enhancement in the right coronary artery territory (arrows). This area is nonviable.

What are the contraindications for use of cardiac MRI?
Contraindications for cardiac MRI are the same as for other MRI techniques. Certain device types, however, are much more frequently used for imaging patients with cardiac disease. All cardiac valves are currently considered to be safe at magnetic field strengths of up to 1.5 T. Implanted defibrillators remain an absolute contraindication for MRI. Although there have been reports of patients with pacemakers having MRI scans, this is considered experimental, and pacemakers are still considered an absolute contraindication. An increasing number of patients have coronary stents. While a number of coronary stents have been tested and reported to be MRI compatible, coronary stents must be assessed on an individual basis, with the medical team weighing the risks and benefits of the MRI procedure.

FIGURE 2: Long axis view showing transmural infarct in the inferior wall (arrow) and subendocardial infarct in the anterior wall (arrowhead) on the myocardial delayed enhancement image. High spatial resolution of the MR provides differentiation of the subendocardial and transmural infarct.

How are the patients prepared for cardiac MRI?
Up to 5% of patients may experience claustrophobia during the MRI examination. Anxiety may trigger cardiac symptoms, so the MRI examination should be carefully explained to patients. In particular, patients evaluated after a myocardial infarction may have pronounced symptoms. A mild sedative such as midazolam is administered to these patients to help relieve symptoms of anxiety.

Cardiac MRI images are acquired with electrocardiographic (ECG) gating. ECGs obtained within the MRI scanner, however, are degraded by the superimposed electrical potential of flowing blood in the magnetic field. Therefore, excellent contact between the skin and ECG leads is necessary. For male patients, the skin at the lead sites is shaved, and the skin of all patients is prepared using a mildly abrasive gel to improve lead contact. Breath holding at the end of expiration is practiced outside the MRI scanner to improve patient cooperation with the examination.

What are the advantages of cardiac MRI?

FIGURE 3: Breath hold double inversion recovery fast spin-echo (black blood) MR image of a patient with a life threatening coronary anomaly. The right coronary artery originates from the left aortic sinus and crossed to the left between the aorta and pulmonary trunk (arrows). This causes the coronary artery to be compressed and can cause myocardial infarction.

The primary advantage of MRI is extremely high contrast resolution between different soft-tissue types, including blood. There is precise demonstration of cardiac anatomy without the administration of contrast media. In certain disease states, such as myocardial infarction, the contrast resolution of MRI is further improved by the additional of extrinsic contrast agents. Direct visualization of the heart in oblique planes along the true cardiac axes is acquired.  Quantitative assessment of cardiac structure and cardiac function can be done without any geometric assumption. No other noninvasive imaging modality provides the same degree of contrast and temporal resolution for the assessment of cardiovascular anatomy and pathology.

How can MRI be used to evaluate myocardial viability?
Viable myocardium refers to the presence of myocardial cells that retain metabolism, but that may or may not retain contractile function. For example, hibernating myocardium results from chronic ischemia. Hibernating segments show diminished or absent contractility, but may recover function following surgical revascularization. Viable myocardium can be defined by the presence of myocardial metabolism, assessed using positron-emission tomography (PET)1; by changes between stress and rest perfusion, assessed using single-photonemission computed tomography (SPECT)2; and by evidence of preserved contractile reserve, assessed using dobutamine stress echocardiography or MRI.³ The newest (and, perhaps, simplest) test to assess myocardial viability is cardiac MRI using conventional gadolinium contrast agents.
In order to assess viable myocardium using MRI, the gadolinium contrast agent is injected at a dose of 0.15 to 0.2 mmol/kg. After about 10 minutes, short-axis and long-axis views of the heart are obtained using an inversion prepared ECG-gated gradient echo pulse sequence. The inversion pulse is adjusted to suppress normal myocardium. Areas of nonviable myocardium retain extremely high signal intensity using this pulse sequence (Figure 1).⁴ The total duration of the imaging protocol for viability is approximately 20 minutes, including scout images, first-pass images, cine images in two planes, and delayed myocardial enhancement images.
The major advantage of cardiac MRI, compared with PET, SPECT,  or echocardiography, is spatial resolution. These other diagnostic modalities classify viable myocardium as present or absent within a myocardial segment, but the higher spatial resolution of cardiac MRI makes it possible to assess transmural versus nontransmural myocardial infarction (Figure 2).⁵ A recent study⁶ of 31 patients showed that myocardial delayed enhancement not only agreed closely with findings on PET, but also identified additional areas of scar (chronic infarction) that PET did not detect. Cardiac MRI currently appears to be superior to PET in both sensitivity and specificity.

FIGURE 4: MR angiography of the right coronary artery in a 16-year-old male patient with Kawasaki disease. The artery is tortous, with areas of irregularity and small aneurysm formation.

What are the most common current applications of coronary MR angiography?
Coronary MR angiography has been, and remains, the most challenging area of cardiac MRI. Coronary arteries have a small diameter, may be extremely tortuous, and move rapidly due to both respiratory and cardiac motion. Currently, radiographic angiography remains the gold standard for evaluating coronary artery stenosis. The most common current applications of coronary MR angiography include the evaluation of anomalous coronary arteries and the assessment of bypass-graft patency.
One of the earliest established indications for coronary MR angiography was the evaluation of anomalous coronary arteries.⁷ This condition has a prevalence of about 1.2%(Figure 3).⁷ Radiographic coronary angiography is limited in its ability to identify the anomalous vessels due to its projectional nature and is not used as screening tool in young adults. Coronary MR angiography has shown excellent results (93% to 100% of cases) in the identification and definition  of anomalous coronary arteries. In addition, coronary MR angiography may classify cases that could not be classified or were misclassified using radiographic angiography.^8,9 Coronary MR angiography has also been used for the noninvasive evaluation of children with Kawasaki disease,¹⁰ an acute vasculitis of unknown etiology that occurs predominantly in infants and young children. Coronary artery ectasia and aneurysm occur in 15% to 25% of children with Kawasaki disease (Figure 4).

FIGURE 5: MR tagging image, (cine-GRE short axis image with rectangular grid of the dark saturation lines). Post-processing software can measure the movement of the tag lines, to provide extremely precise measurement of cardiac contraction.

A coronary-artery bypass graft often has less motion, a larger lumen, and a straighter course than native coronary arteries. Therefore, MR angiography of bypass grafts achieves high sensitivity and specificity for significant stenosis. Two-dimensional black-blood and bright-blood techniques, breath-hold three-dimensional (3D) contrast enhanced, and navigator gated MR angiography techniques are used to demonstrate patency of coronary artery bypass grafts.^11,12 Sensitivities, specificities, and accuracy range from 86% to 100%, 56% to 90%, and 78% to 100%, respectively. Studies using contrast-enhanced 3D MR angiography report sensitivities of more than 90%.
What are the advantages of functional cardiac MRI, compared with echocardiography?

FIGURE 6: Arrhythmogenic right ventricular dysplasia (ARVD). There is enlargement of the right ventricle, and bulging (arrow) of the anterior wall of the right ventricle. ARVD can cause sudden death in young patients, typically of age 20-40 years.

Echocardiography and cardiac MRI are the two methods most commonly used to assess global and regional myocardial structure and function. Despite the ubiquitous use of echocardiography, there are known limitations. Global left and right ventricular measurements are made by using geometric assumptions that are inaccurate when the left ventricle is deformed by infarction or cardiomyopathy. Echocardiography evaluations of left and right atrial volumes and function are particularly limited. Cardiac MRI has no limitations for evaluation of any cardiac chamber. Direct (and more accurate) assessment of chamber size is performed by MRI, compared with echocardiography.^13-16

What is myocardial tagging?
MRI myocardial tagging is a well-developed method for evaluation of regional myocardial motion abnormalities. Regional wall-motion abnormalities are an excellent indicator of coronary stenosis. Regional wall-motion abnormalities actually precede both ECG abnormalities and chest pain as an indicator of myocardial ischemia.¹⁷

FIGURE 7: Hypertrophic cardiomyopathy. A) There is hypertrophy of the septum (arrow) and a turbulent jet in the outflow tract of the left ventricle (arrowhead). The jet is caused by abnormal systolic movement of the anterior leaflet of mitral valve. B) Post treatment of hypertrophic cardiomyopathy by alcohol ablation therapy of the septum. The first septal coronary artery perforator has been ablated, and the MR image clearly shows the area of septal thinning (arrowhead). This reduces the obstruction in the aortic outflow, with decreased abnormal anterior displacement of the mitral valve leaflet (arrow).

MRI tagging provides very precise quantitative estimates of muscle shortening and thickening. In this method, a thin plane of myocardial tissue is saturated using a sequence of radiofrequency pulses. Saturated myocardium does not give any MRI signal during myocardial contraction. Thus, myocardial tags deform with the underlying myocardium during systole and diastole (Figure 5). Because the cardiac MRI tags relax with the T1 of the heart, they are regenerated at the onset of each contraction. Postprocessing software can accurately estimate tag displacement to within 0.1 mm, and the temporal tag displacement can be mathematically processed to compute 3D myocardial strain maps.^18,19 Although tagging allows the full 3D displacement field to be calculated, two-dimensional analysis in the circumferential and radial directions is currently more practical, since the analysis is much more rapid.

What are the MRI findings in arrhythmogenic right ventricular dysplasia (ARVD)?
ARVD is a heritable cardiomyopathy characterized by partial or total thinning and fibropathy infiltration of the right ventricular myocardium. ARVD affects young adults 20 to 40 years old, with symptoms frequently occurring during exercise. Up to 10% of sudden death in patients less than 35 years old has been ascribed to ARVD.^20,21 Inheritance patterns suggest that ARVD is autosomal dominant, with variable expression and penetrance.

FIGURE 8: Gadolinium-enhanced MR angiogram of aortic coarctation. A focal stenotic segment (arrow) is seen at the proximal segment of the descending aorta.

A task force of the European Society of Cardiology and the International Society and Federation of Cardiology has established criteria for the diagnosis of ARVD. Cardiac MRI can identify both major and minor criteria for the diagnosis, including right ventricular aneurysm, dysfunction, and enlargement. Cardiac MRI is commonly used to diagnose ARVD due to its excellent soft-tissue contrast and the ability to depict morphology and function. To evaluate right-ventricle morphology, black-blood double inversion recovery fast spin echo sequences are used. Breath-hold cine sequences are used for evaluation of function. Steady-state free precession cine images are preferred.

FIGURE 9: Persistent left superior vena cava (arrow), dilated-tortuous left jugular vein and absence of the left subclavian vein are readily seen in the reformatted MR angiogram.

Particular emphasis is placed on the common sites of ARVD involvement, including the right ventricular inflow, apex, and outflow tract. Fat within the right ventricular wall is identified pathologically, but, in our experience, this is rarely prospectively identified by MRI.  A clear line of demarcation should exist between the epicardial fat and the right ventricular myocardium in normal subjects. Disruption of this line suggests fatty infiltration. Thinning of the right ventricular wall is difficult to detect due to artifacts and intrinsic limitations of spatial resolution.^22,23 Other characteristic morphological features are enlargement and dilatation of the right ventricle and right atrium, scalloping of the right ventricle free wall, and prominent trabeculations. On axial cine images, the right ventricle starts off as a large triangle in diastole and then becomes a smaller triangle in systole. Most of the contraction occurs in the long axis of the right ventricle from the movement of the tricuspid valve toward the right ventricular apex. Dyskinesia, free-wall systolic bulging, and aneurysms are detected in patients with ARVD (Figure 6). Localization of dyskinesia to the right ventricular outflow has been reported in right ventricular outflow-tract tachycardia and may be a feature distinguishing it from ARVD.²⁴

When is cardiac MRI used for the evaluation of cardiomyopathy?
Cardiac MRI is preferred for hypertrophic cardiomyopathy variants that involve the apex of the left ventricle. Patients treated with percutaneous transluminal septal ablation are assessed with delayed enhancement techniques and cine sequences to document the extent and successful location of the ablation injury (Figure 7).²⁷ An established indication for cardiac MRI is differentiation of restrictive cardiomyopathy from constrictive pericarditis. Pericardial thickening (of more than 4 mm) is present in patients with constrictive pericarditis, but is not seen in restrictive cardiomyopathy.^28,29 The presence of pericardial calcification, as seen in CT images, further supports the diagnosis of a constrictive pericarditis rather than restrictive cardiomyopathy.

What is the importance of cardiac MRI in congenital heart disease (CHD)? 
The evaluation of CHD was one of the first applications of cardiac MRI and continues to be one of its most important indications. Echocardiography provides efficient evaluation in infants and young children because the acoustic window of these patients usually allows clear visualization of the heart and great vessels. Substantial technological improvements, especially fast imaging, now make cardiac MRI competitive with echocardiography in the evaluation of infants and young children. Cardiac MRI techniques are useful in diagnosing and characterizing complex intracardiac anatomic details related to CHD, such as ventricular loop, atrioventricular connection, and ventriculoarterial connection. Major applications for CHD have been coarctation of aorta (Figure 8), branch stenosis of pulmonary arteries, and the depiction of 3D relationships in very complex CHD (Figure 9).^30,31

The trend over the past decade has been increasing acceptance for MRI as an important imaging tool for assessing adult CHD patients who have corrected or palliated CHD. The number of adult or adolescent CHD patient in the United States alone is now approximately 1 million,³² and their body size and the results of the surgery tend to limit echocardiographic access.³³ MRI provides quantitative measurement of pulmonary and systemic flow, valvular regurgitant fractions, and pulmonary-to-systemic flow ratios across shunts.

CONCLUSION
Cardiac MRI has become one of the most effective noninvasive imaging techniques for almost all groups of heart disease. Increased availability of dedicated cardiovascular MRI scanners with improved image quality will continue to increase the number of examinations performed with this modality.
Tuncay Hazirolan, MD, is research fellow at The Russell H. Morgan Department of Radiology and Radiological Sciences, The Johns Hopkins School of Medicine, Baltimore. He is supported by the Scientific and Technical Research Council of Turkey. David A. Bluemke, MD, PhD, is associate professor of radiology and clinical director of MRI at The Russell H. Morgan Department of Radiology and Radiological Sciences, The Johns Hopkins School of Medicine, Baltimore.

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