Acute Myocardial Infarction: Cardiovascular Magnetic Resonance Detection and Characterization

Cardiovascular Magnetic Resonance for Detecting Acute Coronary Syndrome and Acute Myocardial Infarction

Balanced steady-state free precession (bSSFP) cine CMR has been accepted as a reference standard for assessing LV mass, volumes, and LVEF. Cine CMR is a powerful method for detecting regional wall motion abnormalities, as is evidenced by the diagnostic accuracy of dobutamine stress tests. This is an important set of validations because dobutamine stress testing poses some of the most extreme challenges for assessing regional wall motion (see Chapter 17 ). The diagnostic information must be acquired quickly (usually within about 2 minutes) at a time when the heart rate is 85% of age-predicted maximum and patients may be experiencing angina.

Cine CMR can assess regional wall motion with high image quality, allowing diagnosis of small wall-motion abnormalities with confidence that might be missed by other methods. For example, Fig. 19.1 illustrates a case in which a presumptive diagnosis of clinically unrecognized myocardial infarction could be made based on a focal anteroseptal wall-motion abnormality that was missed on a good-quality noncontrast enhanced transthoracic echocardiogram.

Cine CMR also performed well in detecting acute coronary syndrome (ACS) in patients presenting to an emergency department with at least 30 minutes of chest pain. Both qualitatively and quantitatively, regional wall-motion abnormalities by cine CMR had an accuracy of 82% for ACS, 89% for non–ST elevation myocardial infarction (NSTEMI), and 98% for ischemic heart disease. One important reason cine CMR performed so well may have related to how early patients were imaged after presentation to the emergency department. Specifically, 87% of subjects were scanned before the 4-hour troponin was available. By studying patients within 6 hours of presentation to the emergency department, stunned myocardium offers an additional mechanism that is capable of detecting infarction or unstable angina. When combined with LGE, the sensitivity for AMI increased to 100%. An example of a rest perfusion abnormality in a patient with a normal initial electrocardiogram (ECG) and normal initial troponin is shown in Fig. 19.2 .

However, a single CMR scan of regional wall motion does not differentiate acute from chronic conditions. In patients with no history of myocardial infarction, a definite regional wall-motion abnormality is reasonably specific for coronary artery disease (CAD) even though there are other conditions that can cause regional wall-motion abnormalities, such as myocarditis. However, using T2-weighted imaging to characterize the acuity of regional wall-motion abnormalities, Cury et al. replicated the primary findings of Kwong et al. but improved the specificity for diagnosing ACS.

Plein et al. formally studied the additive value of each available CMR method for diagnosing ACS. In that study, an LV wall motion by cine CMR had lower sensitivity (68%), perhaps as a result of the later timing of the CMR scan during the index hospitalization. In that setting, stress perfusion and CMR coronary angiography were the most sensitive components, and the comprehensive analysis of all methods used had 96% sensitivity. Ingkanisorn et al. also found that stress perfusion CMR had high diagnostic accuracy for detecting patients with possible ACS presenting to the emergency department. An example of a patient with a stress-induced perfusion defect that extends beyond the myocardial infarction is shown in Fig. 19.3 . In that study, CMR did not miss any patients with major adverse cardiac events over about 1 year of follow-up.

A series of studies from Wake Forest University examined the diagnostic accuracy and cost effectiveness of CMR to detect CAD in patients presenting to the emergency department with chest pain. Miller et al. found that use of adenosine stress CMR to assess patients with acute chest pain who were triaged to a chest pain observation unit had lower short-term and 1-year costs than those managed with standard inpatient care. In lower risk patients presenting to the emergency department with chest pain, provider-selected evaluation reduced costs compared with mandated stress CMR. That study was interesting because the primary stress test modality used was transthoracic echocardiography in 62% of patients. The primary provider used stress CMR in 32%, single proton emission computed tomography (SPECT) in only 2%, and computed tomography (CT) in 2%. In the setting of intermediate-risk patients with chest pain in an observation unit with a negative initial ECG and negative initial troponin, CMR was again a cost-effective modality.

Although CMR is not commonly used to evaluate patients with chest pain in the emergency department, these studies demonstrated several important concepts. Cine CMR is a powerful method of assessing global and regional LV systolic function and is useful in patients with acute presentations of possible CAD. However, the multiple ways that CMR can characterize the heart, including stress perfusion, LGE of myocardial infarction, and other methods create an even more powerful diagnostic tool. Despite the relatively high costs of CMR, the modality has high diagnostic accuracy and can be cost effective in the management of patients with possible ACS when applied to appropriate risk categories.

Late Gadolinium Enhancement of Acute Myocardial Infarction

LGE-CMR is accepted as a reference standard method for imaging myocardial scar. Before discussing applications of LGE, it is important to understand the different mechanisms leading to hyperenhancement of AMI versus the fibrotic or collagen scar associated with chronic myocardial infarction. As summarized in Fig. 19.4 , after intravenous injection, gadolinium (Gd)-based contrast agents rapidly enter the interstitial space, as represented by the yellow between cardiomyocytes. Thus contrast-enhanced normal or viable myocardium has a T1 that is shorter than native T1. The intact cell membranes of viable cells exclude Gd-based contrast agents from the intracellular space; hence the dark intracellular space in the diagram. Using inversion recovery methods, the inversion time is adjusted to null normal myocardium, making it appear dark on the LGE images.

Acutely infarcted myocardium enhances more strongly with Gd-based contrast agents than viable myocardium because damage to cell membranes allows Gd into what used to be the intracellular space. This is represented by yellow in both the intracellular and extracellular spaces in Fig. 19.4 . Finally, chronically infarcted myocardium also enhances strongly with Gd-based contrast agents because collagen scar has relatively little cellular volume and thus a large volume of distribution as represented by extensive yellow patches in Fig. 19.4 .