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Dr. W. Gregory Hundley, Cardiology Section, Wake Forest University Health Sciences, Medical Center Boulevard, Winston-Salem, North Carolina 27157-1045
Myocardial infarction (MI) remains one of the foremost causes of morbidity and mortality in both men and women and in developed and developing nations alike. In the evaluation of an MI, it is important to address the extent of the myocardial involvement, its impact on ventricular performance, the occurrence of mechanical complications (such as ventricular septal defect or papillary muscle dysfunction), and the adequacy of therapeutic intervention toward improving prognosis. The current management guidelines recommend a series of assessments including acquisition of history and physical examination, obtaining biomarkers, and performing noninvasive and/or invasive imaging investigations. The combination of all of these assessments also serves as a point of reference for a longitudinal follow-up for prevention of reinfarction and adverse cardiovascular events and preservation of ventricular function and exercise tolerance.
In the past 10 years, cardiac magnetic resonance (CMR) has evolved as an imaging modality, which as a single test can characterize myocardial tissue damage; assess systolic or diastolic, regional or global, and left or right ventricular function; identify inducible myocardial ischemia; identify structural complications; and intracavitary thrombi. Recent developments in both hardware and software allow the majority of this information to be acquired in most patients regardless of body habitus in a single setting without exposing individuals to ionizing radiation or ionic radio-opaque contrast materials.
CMR images can be acquired at spatial resolutions incorporating voxel sizes of 1 × 1 × 3 to 4 mm and temporal resolutions of 15 to 50 ms, and these high-resolution CMR images can characterize both large transmural and small subendocardial infarcts. This unique qualification of CMR is well illustrated in the accompanying iPIX by Mather et al. (1). As shown on the images, there is clear delineation of the wave front of myocardial necrosis that can be visualized in a fashion similar to what would be obtained histopathologically. Moreover, the microvascular obstruction and zones of no-reflow can be identified, as well as the extent of edema surrounding infarcts. CMR is well suited for describing global measures of ventricular function including chamber volumes and ejection fraction and regional measures of cardiac function, including regional wall motion, myocardial strain, myocardial relaxation, and twist. Employing gadolinium-enhanced myocardial perfusion studies, the regions of inducible myocardial ischemia can readily be identified. In addition, in several relatively small single-center studies involving hundreds of participants, the ability of CMR to combine tissue edema detection, delayed enhancement imaging, ischemia assessments with myocardial perfusion, and myocardial wall motion determinations has facilitated identification of MI and forecasting of cardiac prognosis in those emergency room settings. Another potentially unique opportunity for CMR includes guidance to both the selection and longitudinal follow-up of patients that may receive progenitor cells and the subsequent effect of these cells on myocardial function long term. Finally, because of the high fidelity and reproducibility of the function and perfusion measures, sample sizes for randomized trials of interventions for MI can be reduced. For this reason, it is likely CMR will remain an important clinical research tool for many years.
Although the CMR offers the most exquisite qualitative and quantitative estimates in patients with MI, there are some significant challenges. The foremost challenge is whether CMR is a clinically incremental test. For example, it still remains uncertain whether identification of relatively small infarcts or microvascular obstruction in a subacute setting would contribute to superior outcomes relative to currently available assessments obtained with other technologies. This question has particular interest since CMR can be used to identify scar tissue in patients with mildly reduced left ventricular ejection fraction, and further studies are necessary to determine if these patients should be treated as aggressively as those with a higher degree of reduction in their left ventricular ejection fraction. And, even if beneficial, would such an investigation be cost effective? At present, there are not many studies that examine the relative cost benefit of using CMR data for the assessment of post-MI outcomes. Another challenge involves training individuals to be proficient with the CMR techniques, as currently there are only a few cardiology fellowship training programs with access to the technology; the available opportunities are even fewer for practicing physicians and faculty.
In summary, CMR offers a unique opportunity to acquire tissue characterization, functional information, myocardial perfusion, and infarct extent in a single examination—without exposure to ionizing radiation—at a relatively high spatial and temporal resolution. However, the cost-effectiveness, lack of access and training avenues, and logistics of performing acute studies would need to be sorted out to decide whether only one shop is worth the stop?
Dr. Hundley is supported in part by National Institute of Health Grants RO1AG18915, R01HL076438, P30 AG21332, JHS MRI RC0006, and R33CA12196.
↵⁎ Editorials published in JACC: Cardiovascular Imaging reflect the views of the authors and do not necessarily represent the views of JACC: Cardiovascular Imaging or the American College of Cardiology.
- American College of Cardiology Foundation