Role of Cardiac MR Imaging in Cardiomyopathies Christopher M. Kramer Cardiovascular Imaging Center, University of Virginia Health System, Charlottesville, Virginia deformation than cine imaging and have been applied to studies Cardiac MR imaging has made major inroads in the new millennium of myocardial infarction (7) and hypertrophic cardiomyopathy in the diagnosis and assessment of prognosis for patients with (HCM) (8). cardiomyopathies. Imaging of left and right ventricular structure and Late gadolinium enhancement (LGE) refers to the discrimina- function and tissue characterization with late gadolinium enhancement tion of regions of scar, necrosis, or inflammation from normal (LGE) as well as T1 and T2 mapping enable accurate diagnosis of the tissue by the prolonged retention of gadolinium-based contrast underlying etiology. In the setting of coronary artery disease, either agents. Since the mid-1980s, investigators have appreciated T1 transmurality of LGE or contractile reserve in response to dobutamine shortening (increased enhancement) in regions of infarction after can assess the likelihood of recovery of function after revascularization. The presence of scar reduces the likelihood of a response to medical gadolinium administration (9–11). However, these early imaging therapy and to cardiac resynchronization therapy in heart failure. techniques were limited by long acquisition times, artifacts, and The presence and extent of LGE relate to overall cardiovascular insufficient contrast between normal and abnormal regions. With outcome in cardiomyopathies. A major role for cardiac MR imaging faster approaches and the implementation of an inversion recovery in cardiomyopathies is to identify myocardial scar for diagnostic and pulse sequence with the inversion time set to null normal myocar- prognostic purposes. dium, the signal difference between normal and infarcted seg- Key Words: cardiac MR imaging; coronary artery disease; late ments has been increased by 500%–1,000% (12). Using this gadolinium enhancement; ischemic cardiomyopathy; nonischemic approach, Kim et al. demonstrated that the spatial extent of cardiomyopathy; infiltrative cardiomyopathy LGE seen on CMR closely mirrored the distribution of myocyte J Nucl Med 2015; 56:39S–45S necrosis in the early period after infarction and that of collagenous DOI: 10.2967/jnumed.114.142729 scar at 8 wk (R 5 0.97, P , 0.001) (13), whereas in regions of the heart subjected to reversible injury, the retention of contrast did not occur (Fig. 1) (14). LGE accurately delineates infarction, as defined by histology, at various time points after injury (15). LGE is not unique to infarct scar and can demonstrate any cause of Cardiac MR imaging (CMR) is considered the gold standard fibrosis or infiltration in cardiomyopathies. for measurement of the structure and function of the left ventricle T2-weighted imaging can be used to demonstrate myocardial (LV) and the right ventricle (RV). Cine imaging is used to cover edema or injury. An understanding of the time course of edema is the LV in the short axis from the apex to the base, essentially creating key to the accurate interpretation of T2-weighted imaging. Using a 3-dimensional volumetric structure for analysis. In the 1990s, a canine model of myocardial infarction, Abdel-Aty et al. noted 6 gradient-echo cine imaging was the standard, but this technique that the onset of edema on CMR first became apparent 28 4 min was replaced at the turn of the millennium by steady-state free- after experimental coronary artery occlusion and well before the precession cine imaging, which offers a higher contrast-to-noise appearance of LGE or troponin elevation, signifying that CMR ratio between the dark myocardium and the bright blood pool (1). can visualize edema before the onset of irreversible myocardial Multiple studies have demonstrated the accuracy and reproducibility injury (Fig. 2) (16). Microsphere analysis in a canine model of of CMR for measuring LV volumes, ejection fraction (EF), and reperfusion after acute myocardial infarction suggested that T2- regional function (2). Measurements made with this 3-dimensional weighted imaging performed 2 d after myocardial infarction can dataset do not require geometric assumptions and are therefore less be used to delineate the area at risk (17). Increased T2 signal prone to error than 2-dimensional methods, such as 2-dimensional intensity can be seen in any cause of myocardial injury, such as echocardiography, in ventricles deformed by myocardial infarc- acute myocarditis (18). Older T2-weighted imaging approaches tion or cardiomyopathies. Interstudy and interscan reproducibility were prone to artifacts; more recently, T2 mapping has been used is high, allowing for reduced sample sizes in clinical trials of with CMR protocols to overcome these limitations (19). heart failure (3,4). Techniques such as myocardial tissue tagging T1 of the myocardium can be measured by mapping with a (5) or cine displacement–encoded stimulated-echo imaging (6) modified Look–Locker inversion recovery sequence (20) and allow more detailed analysis of regional myocardial motion and shorter versions of this approach (21). Native or precontrast T1 can be measured before contrast infusion, and then postcontrast T1 measures can be used to calculate the extracellular volume fraction in the myocardium (22). These techniques are being Received Nov. 13, 2014; revision accepted Dec. 26, 2014. applied to help in the differential diagnosis of cardiomyopathies, For correspondence or reprints contact: Christopher M. Kramer, Cardiovascular Imaging Center, University of Virginia Health System, P.O. as some have characteristically high native T1 values as well as Box 800170, Lee St., Charlottesville, VA 22908-0001. high extracellular volume fractions. Native T1 is especially E-mail: [email protected] COPYRIGHT © 2015 by the Society of Nuclear Medicine and Molecular useful in patients with concomitant renal disease, as patients Imaging, Inc. with stage 4 or 5 chronic kidney disease are not candidates for ROLE OF CMR IN CARDIOMYOPATHIES • Kramer 39S One study, however, demonstrated that in a minority of patients who had wall thinning without a substantial LGE burden (4% of 1,055 patients with CAD), contractile recovery could occur after revas- cularization (28). Nontransmural infarcts typically do not result in severe end-diastolic wall thinning. The extent of LGE is a useful marker of the likelihood of functional recovery with revascularization. In a landmark study, Kim et al. studied 50 patients who had LV dysfunction and were evaluated with contrast-enhanced CMR before surgical or percutaneous revascularization (29). LGE was noted in 80% of the patients. There was an inverse relationship between the transmural extent of LGE and functional recovery with revascu- larization (Fig. 4). Segments with greater than 50% transmural LGE were nonviable, whereas segments with no LGE had an approx- imately 80% likelihood of functional recovery. Similar findings were noted in a study of 52 patients undergoing coronary artery bypass surgery; in that study, 36% of segments developed new areas of LGE after surgery (30). FIGURE 1. Three-chamber long-axis phase-sensitive inversion recov- Cine imaging of long- and short-axis views of the LV after the ery LGE image in patient with heart failure and CAD. Transmural LGE administration of low-dose dobutamine (10 mg/kg/min) can be can be seen in inferolateral wall (arrow). used to assess contractile reserve and to predict areas of functional recovery with revascularization (27,31). Bove et al. studied patients receiving gadolinium because of concerns about nephrogenic sys- before and after coronary artery bypass surgery and divided segments temic fibrosis. into those with no LGE and those with 1%–25%, 26%–50%, or greater than 50% transmural LGE (31). Dysfunctional segments ESTABLISHING ETIOLOGY OF CARDIOMYOPATHY were defined as those having wall thickening of less than 27% (2 SDs less than normal myocardium). After revascularization with coronary The first step in the evaluation of the patient with new-onset artery bypass surgery, the improvement in wall thickening in seg- heart failure is to evaluate the underlying etiology and, importantly, to ments with 1%–50% transmural enhancement was 22% 6 4% for exclude ischemic heart disease as a potentially reversible cause. The dobutamine-responsive segments; this improvement was significantly presence of LGE in a coronary distribution can support the diagnosis higher than that for non–dobutamine-responsive segments (9% 6 4%). of underlying coronary artery disease (CAD), but its absence does not These findings suggested that contractile reserve was more pre- rule it out, as patients with extensive hibernating myocardium may dictive of functional recovery than infarct transmurality in infarcts have no LGE (Fig. 3) (23). In a study by Soriano et al. of 71 patients with 1%–50% transmural LGE. In a complementary study, 29 with new-onset heart failure and systolic dysfunction, the sensitivity patients with ischemic cardiomyopathy and a severely reduced of the infarct pattern of LGE for ischemic cardiomyopathy—as EF underwent both LGE and low-dose dobutamine cine imaging defined by the presence of obstructive CAD—was 81%, whereas (32). On the basis of the area under the receiver operating char- the specificity was 91% (23). Patients without obstructive CAD acteristic curve, contractile reserve with low-dose dobutamine was may have evidence of LGE in an infarct pattern due to thrombotic