Paaladinesh Thavendiranathan, MD, MSc, FRCPC and Scott D. Flamm, MD, MBA, FACC, FAHA

1. How does cardiac magnetic resonance imaging (CMR) produce images?

    CMR uses a strong magnet (1.5 to 3.0 Tesla; equivalent to 30,000 to 60,000 times the strength of the earth’s magnetic field), radiofrequency pulses, and gradient magnetic fields to obtain images of the heart. When placed in the bore of a magnet, positively charged protons, mainly from water, are aligned in the direction of the magnetic field, creating a net magnetization. Radiofrequency pulses are used to tilt these protons away from their alignment, shifting them to a higher energy state. These protons then return to their equilibrium state through the process of relaxation and emit a signal. The relaxation consists of two components: T1 and T2 relaxation. Magnetic gradients are applied across the tissue of interest to localize these signals. The signals are then collected using a receiver coil and placed in a data space referred to as k-space, which is then used to create an image. CMR uses differences in relaxation properties between different tissues, fluids, and blood, and changes that occur due to pathological processes to create contrast in the image.

2. What is unique about CMR?

    Similar to echocardiography, CMR allows the generation of images of the heart without exposure to ionizing radiation. Although the spatial resolution of CMR is comparable to echocardiography (approximately 1 mm), the contrast-to-noise and signal-to-noise ratios are far superior (Fig. 10-1). The latter allows easier delineation of borders between tissues and between blood pool and tissue. The contrast between blood pool and myocardium (see Fig. 10-1) is generated using differences in signal properties of the different tissues without the use of contrast agents. CMR is also not limited by “acoustic windows” that may hinder echocardiography, and images can be obtained in any tomographic plane. Finally, CMR can provide information about tissue characteristics using differences in T1 and T2 signals, and with addition of contrast agents.

3. What are the limitations of CMR?

    The major limitation of CMR is availability. Given the cost, the special construction necessary to host a CMR system, and the technical expertise and support necessary, CMR is not widely available at all centers. CMR is limited with respect to portability, unlike echocardiography where imaging can be performed at the patient’s bedside. There is also a set of contraindications (listed later) that limits the use of this technology in selected patient populations. Image acquisition can be challenging in patients with an irregular cardiac rhythm or difficulty holding their breath, though newer real-time techniques provide an opportunity to overcome these barriers (Fig. 10-2). Finally, because patients have to lie still in a long hollow tube for up to an hour during imaging, claustrophobia may become an important limiting factor.

4. What are the common imaging pulse sequences used in CMR?

    Pulse sequences are orchestrated actions of turning on and off various coils, gradients, and radiofrequency pulses to produce a CMR image. In very simple terms, the pulse sequences are based on either gradient echo or spin echo sequences. The most common sequences used are bright blood sequences (where the blood pool is bright); dark blood sequences (where the blood pool is dark); steady state free precession sequences (most commonly used for function or cine images); and inversion recovery sequences (e.g., delayed enhancement imaging used to assess myocardial scar).

5. What are the appropriate uses of CMR?

    Although echocardiography is usually the first-line imaging modality for questions of left ventricular (LV) function and assessment of valvular disease, CMR still has many important indications. The following questions and answers pertain to the appropriate use of CMR in the clinical setting, as recommended in the multisociety appropriateness criteria published in 2006.

6. What is delayed enhancement (DE) CMR imaging?

    One of the unique aspects of CMR is the ability to identify myocardial scar and/or fibrosis. This is most commonly performed using DE imaging. Gadolinium-based contrast agents are first administered, then after waiting approximately 10 minutes and allowing time for the contrast to distribute into areas of scar and fibrosis, an inversion recovery sequence is performed. This sequence nulls (makes it black) normal myocardium and anything that is bright within the myocardium is most likely myocardial scar or fibrosis (Fig. 10-3).

7. Does CMR have a role in the evaluation of chest pain?

    In patients with chest pain syndrome, CMR stress testing can be performed to assess flow-limiting coronary stenosis. This is most appropriate in patients with intermediate pretest probability of CAD with uninterpretable ECG or who are unable to exercise. The two stress methods used are vasodilator perfusion CMR or dobutamine stress function CMR. Vasodilator stress testing is performed identically to nuclear stress testing with the use of adenosine. With peak coronary vasodilation, gadolinium contrast agent is administered and perfusion of the myocardium during the first pass of the contrast agent is captured. Areas of perfusion abnormality can be detected as dark areas (Fig. 10-4). Resting perfusion is also commonly performed, mainly to differentiate imaging artifacts from a true perfusion defect. This is then followed by DE imaging to assess for myocardial scar. Alternatively, graded doses of dobutamine can be administered to provide a sympathetic stress with cine imaging to identify regions of wall motion abnormality, in a manner similar to echocardiography.

8. Can CMR coronary angiography be used to assess chest pain?

    Coronary angiography for assessment of coronary stenosis is not commonly performed in clinical settings. However, it is appropriate to use coronary magnetic resonance angiography (MRA) to assess the origin, branching pattern, and proximal course of the coronary arteries to assess for coronary anomalies (Fig. 10-5). This can be performed without the use of contrast agents. However, the acquisition may require a substantial amount of time and is susceptible to artifacts. The use of this technique may be particularly important, however, in young patients, thereby avoiding radiation exposure from invasive coronary angiography or coronary CT angiography.

9. What is the role of CMR in the assessment of ventricular function?

    CMR is considered the reference standard for the assessment of ventricular volumes, ejection fraction, and ventricular mass of both the left and right ventricles, as a result of the excellent accuracy and reproducibility of this technique for these measurements. The most appropriate use of CMR is in the evaluation of LV function after myocardial infarction or in heart failure patients with technically limited echocardiographic images. In other patient populations, CMR can be used to assess LV function when there is discordant information from prior tests.

10. Does CMR have a role in the assessment of cardiomyopathies?

    CMR has an important role in the evaluation of specific cardiomyopathies, particularly in the infiltrative diseases (e.g., amyloidosis and sarcoidosis), hypertrophic cardiomyopathy, and cardiomyopathies due to cardiotoxic therapies. Although CMR-based ventricular morphology and functional images can provide certain clues to the etiology of the underlying cardiomyopathy, the most important CMR technique in the assessment of cardiomyopathies is DE imaging. Certain DE patterns have been associated with particular cardiomyopathies and can be easily identified by experienced readers (see Fig. 10-3). Another specific cardiomyopathy that can be assessed by CMR is arrhythmogenic right ventricular cardiomyopathy (ARVC). CMR may be specifically used in patients presenting with syncope or ventricular arrhythmias to provide excellent assessment of right ventricular (RV) function, RV dilation, focal aneurysms, and also myocardial scar using DE imaging both in the LV and RV.

11. What is the role of CMR in myocardial infarction?

    After an acute myocardial infarction, CMR can be used to assess the extent of myocardial necrosis (see Fig. 10-3, B) and areas of microvascular obstruction (“no reflow regions”) (Fig. 10-6) using DE imaging. Both the extent of necrosis and the presence of microvascular obstruction have been shown to have prognostic value. In addition, DE imaging can be used to determine the likelihood of recovery of function with revascularization (surgical or percutaneous) or medical therapy. Segments of myocardium with more than 50% transmural scar are less likely to recover with revascularization. Another important use of CMR is in patients who present with a positive troponin test but have normal coronary angiography. CMR can help identify myocarditis (see Fig. 10-3, E), Tako-tsubo syndrome, or even myocardial infarction with recanalized coronary artery.

12. Can valvular disease by assessed by CMR?

    Echocardiography remains the most commonly used method for assessment of stenotic and regurgitant lesions of both native and prosthetic valves. The role of CMR is mainly in patients with technically limited images from echocardiography. CMR can be used to assess the significance of the valve lesions using several techniques. First, an en face image of the valvular lesion can be obtained for visual assessment and planimetry of the stenotic or regurgitant orifices. However, the most useful CMR techniques are the quantitative assessment of valvular stenosis obtained by transvalvular gradients, or of regurgitant lesions obtained by volumes and flow measurements. The latter is often obtained using a combination of stroke volumes by planimetry of the ventricles and phase-contrast imaging (analogous to Doppler imaging in echocardiography) (Fig. 10-7).

13. Can CMR be used in the assessment of congenital heart disease?

    Among imaging tests, CMR is likely one of the best modalities for the assessment of congenital heart disease. The ability to assess the heart and its relationship to the arterial and venous circulation in three-dimensional space and obtain hemodynamic data in the same study is unique to CMR. Images can be obtained in any preferred plane, often essential in patients with complex congenital heart disease or with complex repairs. Also, important parameters, such as LV and RV volumes and function, that are used to make important clinical decisions, can be measured in an accurate and reproducible manner. Furthermore, as these patients are young and typically require multiple imaging studies throughout their lives, CMR provides excellent assessment without ionizing radiation exposure. The major limitation of CMR in this population is the susceptibility to artifacts from previous surgeries involving surgical clips, valves, and coils.

14. Can CMR be used to assess cardiac masses?

    CMR is a good modality for the assessment of tumors (Fig. 10-8) as tissue characterization using various sequences may provide insights into the differential diagnosis of an identified mass. However, the initial hopes that CMR could definitively characterize tumor tissue (e.g., a noninvasive biopsy) have not quite come to fruition. Nevertheless, CMR can still help narrow the differential diagnosis and provide information about whether the tumor has characteristics suggestive of malignancy. However, small tumors and those with high frequency motion may not be appropriately assessed using CMR.

15. What are some of the other clinical uses of CMR?

    Some of the other appropriate uses of CMR includes the assessment of pericardial conditions such as pericardial mass and constrictive pericarditis. Specifically, in constrictive pericarditis, CMR can provide assessment of the thickness of the pericardium, the presence of pericardial enhancement suggestive of recent pericardial inflammation or neovascularization, and assessment of some of the physiological changes seen.

    CMR can also be used to assess the pulmonary vein anatomy prior to ablation for atrial fibrillation. The number, size, and orientation of the pulmonary veins can be assessed, and also information about the left atrium can be provided.

    CMR is also used for the assessment of aortic dissection. However, given the length of the examination and difficulty monitoring the patient closely during the examination, CMR is generally not considered the first modality for dissection assessment in the acute setting.

16. What are the contraindications to CMR?

    All devices should be checked for magnetic resonance imaging (MRI) compatibility prior to taking the patient into the MRI room. A good reference website to check the compatibility of a device is www.mrisafety.com. Alternatively, the manufacturer of the device should be contacted to check for compatibility. Some of the most common contraindications to CMR are provided in the list that follows.

17. Can CMR be performed in patients with implanted cardiovascular devices?

    Most implanted cardiovascular devices are not ferromagnetic 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. 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.

    Pacemakers and implantable cardioverter defibrillators are strong relative contraindications to MRI scanning, and scanning of such patients should be done only under specific conditions in centers with expertise in performing these studies. However, given the widespread use of pacemakers, concerns about not being able to perform MRI for even noncardiac indications have encouraged manufactures to make MRI-safe pacemakers. One such device is the Revo MRI SureScan system (Medtronic, Minneapolis), which was approved by the U.S. Food and Drug Administration (FDA) in 2011 for use as an MRI-conditional device. The conditional designation still necessitates specific patient and MRI protocols to be followed to ensure safety. Although this will make it possible to perform noncardiac MRI imaging, the artifacts that result from an implanted pacemaker and its leads will still make CMR challenging.

18. What is nephrogenic systemic fibrosis?

    Nephrogenic systemic fibrosis (NSF) is a fibrosing condition involving skin, joints, muscles (including cardiac myocytes), and other internal organs. Although first identified in 1997, the association of this entity with gadolinium-based contrast agents (GBCA) was only described in 2006. The most important risk factors for development of NSF after use of GBCA are advanced renal disease, recent vascular surgical procedures, dialysis, acute renal failure, and higher doses of contrast agent (more than 0.1 mmol/kg). On average, the time interval between administration of GBCA and NSF is 60 to 90 days. As a consequence of the concern of NSF, the FDA has issued a black-box warning on the use of GBCA, making its use in cardiac applications an “off label” use. However, since this black box warning was issued, centers have adopted methods to carefully screen patients for risk factors prior to GBCA administration. This has resulted in virtually no new cases of NSF reported over the past 3 to 4 years.

    In patients with advanced renal disease (glomerular filtration rate below 30 mL/min), the risks and benefits of gadolinium-enhanced MRI scans should be carefully weighed and alternate imaging modalities should first be considered. If a contrast-based CMR study is absolutely necessary, then the risks, benefits, and methods to avoid NSF should be discussed with imaging staff, nephrology service, and the patient prior to the study, and the patient should be carefully monitored after the CMR.

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