1. Does cardiac magnetic resonance imaging (MRI) expose the patient to any radiation?
No. MRI uses an extremely strong static magnet 1.5–3.0 tesla (equivalent to 30,000–60,000 times the strength of the earth’s magnetic field), pulsed radiofrequency energy, and gradient magnetic fields to image the body. Most imaging uses positively charged hydrogen protons, most of which are in water molecules, as the means to image body structures. The pulse sequences actually used for the static and dynamic imaging of the heart are quite complex and far beyond the scope of this chapter. In very simple terms, many pulse sequences are based on either gradient echo (bright blood) or spin echo (black blood) sequences, as well as steady-state free precession imaging, which is used during cine cardiac MR imaging.
2. What are the primary indications for cardiac MRI?
Although echocardiography is usually the first-line imaging modality for questions of left ventricular function and assessment of valvular disease, cardiac MRI still has many important potential uses and advantages:
Assessment of left ventricular function, volume, and mass: Cardiac MRI allows for a reproducible and accurate assessment of left ventricular (LV) ejection fraction. Given its ability to image three-dimensional structures in any plane, it can produce accurate assessments of left ventricular end-systolic and end-diastolic volumes from which ejection fraction, as well as stroke volume and cardiac output, can be calculated. Unlike echocardiography, a patient’s body habitus does not affect image quality; therefore, issues like obesity or expanded lungs in patients with chronic obstructive pulmonary disease (COPD) are usually not an issue.
Cardiac MRI is considered by almost all as the gold standard for the measurement of ejection fraction (EF), LV volumes, and LV mass. It is generally felt to be more accurate and reproducible than echocardiography (at least two-dimensional echocardiography) and provides vastly more information than radionuclide imaging. Clinical studies, particularly those involving only limited numbers of patients, increasingly use cardiac MRI for the serial assessment of LVEF, volume, and mass.
Assessment of myocardial infarction and myocardial viability: Using a technique called late gadolinium enhancement, in which gadolinium is injected and then approximately 10 minutes later the heart is imaged using special sequences, infarcted myocardium can be imaged. This technique can very accurately detect and delineate infarcted myocardium (Fig. 9-1). Using this technique, infarcted myocardium appears bright; noninfarcted, viable myocardium appears dark.
This technique is very useful in assessing potentially hibernating myocardium in patients with depressed ejection fraction and multivessel disease who are being considered for bypass surgery. Kim and Judd have demonstrated that in a given area of the left ventricle in patients with multivessel disease, areas in which transmural infarction is demonstrated are unlikely to improve contractility after revascularization, whereas areas that show little or no demonstrated infarction are much more likely to improve contractility after revascularization. This modality is at least as good as if not better thannuclear or echocardiographic imaging in predicting functional recovery of contractile function afterrevascularization.
No. MRI uses an extremely strong static magnet 1.5–3.0 tesla (equivalent to 30,000–60,000 times the strength of the earth’s magnetic field), pulsed radiofrequency energy, and gradient magnetic fields to image the body. Most imaging uses positively charged hydrogen protons, most of which are in water molecules, as the means to image body structures. The pulse sequences actually used for the static and dynamic imaging of the heart are quite complex and far beyond the scope of this chapter. In very simple terms, many pulse sequences are based on either gradient echo (bright blood) or spin echo (black blood) sequences, as well as steady-state free precession imaging, which is used during cine cardiac MR imaging.
2. What are the primary indications for cardiac MRI?
Although echocardiography is usually the first-line imaging modality for questions of left ventricular function and assessment of valvular disease, cardiac MRI still has many important potential uses and advantages:
Assessment of left ventricular function, volume, and mass: Cardiac MRI allows for a reproducible and accurate assessment of left ventricular (LV) ejection fraction. Given its ability to image three-dimensional structures in any plane, it can produce accurate assessments of left ventricular end-systolic and end-diastolic volumes from which ejection fraction, as well as stroke volume and cardiac output, can be calculated. Unlike echocardiography, a patient’s body habitus does not affect image quality; therefore, issues like obesity or expanded lungs in patients with chronic obstructive pulmonary disease (COPD) are usually not an issue.
Cardiac MRI is considered by almost all as the gold standard for the measurement of ejection fraction (EF), LV volumes, and LV mass. It is generally felt to be more accurate and reproducible than echocardiography (at least two-dimensional echocardiography) and provides vastly more information than radionuclide imaging. Clinical studies, particularly those involving only limited numbers of patients, increasingly use cardiac MRI for the serial assessment of LVEF, volume, and mass.
Assessment of myocardial infarction and myocardial viability: Using a technique called late gadolinium enhancement, in which gadolinium is injected and then approximately 10 minutes later the heart is imaged using special sequences, infarcted myocardium can be imaged. This technique can very accurately detect and delineate infarcted myocardium (Fig. 9-1). Using this technique, infarcted myocardium appears bright; noninfarcted, viable myocardium appears dark.
This technique is very useful in assessing potentially hibernating myocardium in patients with depressed ejection fraction and multivessel disease who are being considered for bypass surgery. Kim and Judd have demonstrated that in a given area of the left ventricle in patients with multivessel disease, areas in which transmural infarction is demonstrated are unlikely to improve contractility after revascularization, whereas areas that show little or no demonstrated infarction are much more likely to improve contractility after revascularization. This modality is at least as good as if not better thannuclear or echocardiographic imaging in predicting functional recovery of contractile function afterrevascularization.
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Fig. 9-1. Correlation between myocardial infarction as demonstrated by gross pathology and tissue staining and gadolinium-enhanced delayed MRI imaging. Left figure shows experimentally induced subendocardial infarction (arrow). Right figure demonstrates gadolinium-enhanced delayed hyperenhancement (arrow), corresponding almost exactly to the actual pathology. (Modified from Libby P, Bonow RO, Mann DL, et al: Braunwald’s heart disease: a textbook of cardiovascular medicine, ed 8, Philadelphia, 2008, Saunders. Courtesy Dr. R. J. Kim and Dr. R. M. Judd.)
Stress myocardial perfusion imaging: Just as adenosine is used to dilate the coronary arteries and increase blood flow in nonstenosed vessels during nuclear stress testing, adenosine can be used during MR stress imaging. Typically, the patient is treated with intravenous adenosine for 3 minutes. An intravenous bolus of gadolinium is then administered. The heart is imaged in real time. Gadolinium will cause normally perfused myocardium to appear bright, whereas nonperfused areas remain dark. Areas of relative or absolute perfusion defects can be easily detected (Fig. 9-2). Dobutamine stress testing can also be performed using cardiac MRI to assess for improvement in wall motion or decrease in wall motion (e.g., normally contracting wall during rest becomes hypokinetic during stress).
Valvular heart disease: Although echocardiography remains the first-line test in the evaluation of valvular disease, cardiac MRI can also be used for valvular assessment, especially
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Fig. 9-2. Stress perfusion imaging. The patient is first treated with adenosine for 3 minutes. A bolus of gadolinium is then administered. The gadolinium leads to perfused myocardium appearing bright, whereas nonperfused myocardium remains dark (arrow). (Courtesy Dr. Scott Flamm and Dr. Benjamin Chung.)
in patients with poor echocardiographic windows and those who are not candidates for or refuse to undergo transesophagael echocardiography (TEE).
Assessment of infiltrative diseases and inflammatory processes: Using gadolinium enhancement, infiltrative diseases (e.g., sarcoidosis; Fig. 9-3) and active inflammatory processes, such as myocarditis, can be visualized. Cardiac MRI is the procedure of choice in patients with suspected infiltrative diseases or myocarditis.
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Fig. 9-3. Gadolinium-enhanced delayed MRI of a 23–year-old patient with known sarcoidosis demonstrating cardiac sarcoidosis. The left ventricular myocardium should appear uniformly black, but patchy bright areas (arrows) are visible in both the horizontal long-axis view (A) and the short-axis view (B) as a result of the presence of sarcoid infiltration. (Modified from Hunold P, Schlosser T, Vogt FM, et al: Myocardial late enhancement in contrast-enhanced cardiac MRI: distinction between infarction scar and non–infarction-related disease, Am J Roentegenol 184:1420-1426, 2005.)
Coronary artery imaging: Cardiac MRI can be used to visualize the coronary arteries. Due to issues such as cardiac motion and the size of the coronary arteries, coronary magnetic resonance angiography (MRA) is not currently as reliable and accurate as coronary computed tomography (CT) angiography, and images as good as Figure 9-4 are not always obtained. Coronary MRA is, however, an excellent technique to assess for aberrant origin of the coronary arteries and the course of such arteries (e.g., between the aorta and pulmonary artery). It is particularly useful in young patients, avoiding the radiation exposure of cardiac CT.
Assessment of tumors: Cardiac MRI is an excellent modality for the assessment of tumors (Fig. 9-5). Although initial hopes that it could definitely noninvasively characterize tumor tissue (e.g., a noninvasive biopsy) have not quite come to fruition, cardiac MRI is still helpful in determining whether a tumor is more likely to be benign or malignant and may be able to more specifically suggest the specific tumor type. It is also excellent in defining the borders of the tumor and can be used to further assess any mass visualized on echocardiography, many of which will turn out to be benign findings or pseudotumors.
Congenital heart disease: MRI is the test of choice in patients with suspected or known congenital heart disease. MRI allows both anatomic and physiologic assessment of lesions, defects, and shunts.
Arrhythmogenic right ventricular dysplasia/cardiomyopathy: Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is a rare condition in which fibrosis and fatty infiltration of the right ventricle are present, predisposing the patient to potentially lethal ventricular arrhythmias. MRI is the study of choice in assessing for this condition.
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Fig. 9-4. Coronary magnetic resonance angiography. Coronary cardiovascular magnetic resonance (CMR) at 3 Tesla. Left panel, Left coronary artery and branches (dotted arrows). Right panel, Right coronary artery (RCA). Ao, Aorta; LAD, left anterior descending artery; LCx, left circumflex artery; LMS, left main stem; LV, left ventricle; PA, pulmonary artery; RV, right ventricle. (From Stuber M, Botnar RM, Fischer SE, et al: Preliminary report on in vivo coronary MRA at 3 Tesla in humans, Magn Reson Med 48(3):425-429, 2002. Reprinted with permission of Wiley-Liss, Inc. a subsidiary of John Wiley & Sons, Inc.)
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Fig. 9-5. Cardiac MRI demonstrating a left atrial myxoma.
3. Can MRI be performed in patients with implanted cardiovascular devices?
Most implanted cardiovascular devices are nonferromagnetic or only weakly ferromagnetic. This includes most commonly used coronary stents, peripheral vascular stents, inferior vena cava (IVC) filters, prosthetic heart valves, cardiac closure devices, aortic stent grafts, and embolization coils. Pacemakers and implantable cardioverter defibrillators are strong relative contraindications to MRI scanning, and scanning of such patients should be done under specific delineated conditions, only at centers with expertise in MRI safety and electrophysiology, and only when MRI imaging in particular is clearly indicated.
A statement on the safety of scanning patients with cardiovascular devices was published by the American Heart Association, giving guidelines of the timing and safety of device scanning. (See the Websites box and Bibliography at the end of this chapter.) Before MRI scanning is performed, the safety of scanning that particular device should always be verified based on manufacturer device information and dedicated websites. (See the Websites box and Bibliography at the end of this chapter.) Particular care has to be taken in the case of patients with intracranial implants, in which an inappropriate MRI could have fatal consequences.
4. What is nephrogenic systemic fibrosis?
For many years, administration of gadolinium was considered to be completely safe in patients with chronic kidney disease. Over the last decade, it has become apparent that a rare condition called nephrogenic systemic fibrosis (NSF) can occur in patients with severe or end-stage renal insufficiency who are administered gadolinium (most commonly those on dialysis). The syndrome, which was previously called nephrogenic fibrosing dermopathy, involves fibrosis of the skin (hence the prior name), joints, skeletal muscles, and internal organs. Clinical involvement is usually noted in the first several months after exposure. Both the Food and Drug Administration (FDA) and radiology experts have issued recommendationsonthe issue ofgadolinium administration in patients with chronic kidney disease. The risks and benefits of gadolinium-enhanced MRI scans should be carefully weighted in patients with more severe renal disease or end-stage renal disease, and clinicians should consult with radiology services regarding the best imaging modality for a particular clinical circumstance. (Refer to the articles by Kanal et al and the FDA statement on the subject listed in the Websites box and Bibliography at the end of this chapter.)
