Evaluation of myocardial viability with cardiac magnetic resonance imaging

Role of Cardiovascular Magnetic Resonance in Ischemic Cardiomyopathy

Ischemic heart disease is the most common cause of cardiovascular morbidity and mortality. Cardiac magnetic resonance (CMR) improves on other noninvasive modalities in detection, assessment, and prognostication of ischemic heart disease. The incorporation of CMR in clinical trials allows for smaller patient samples without the sacrifice of power needed to demonstrate clinical efficacy. CMR can accurately quantify infarct acuity, size, and complications; guide therapy; and prognosticate recovery. Timing of revascularization remains the holy grail of ischemic heart disease, and viability assessment using CMR may be the missing link needed to help reduce morbidity and mortality associated with the disease.

Accuracy of myocardial viability imaging by cardiac MRI and PET depending on left ventricular function

AIM

To compare myocardial viability assessment accuracy of cardiac magnetic resonance imaging (CMR) compared to [¹⁸F]-fluorodeoxyglucose (FDG)- positron emission tomography (PET) depending on left ventricular (LV) function.

METHODS

One-hundred-five patients with known obstructive coronary artery disease (CAD) and anticipated coronary revascularization were included in the study and examined by CMR on a 1.5T scanner. The CMR protocol consisted of cine-sequences for function analysis and late gadolinium enhancement (LGE) imaging for viability assessment in 8 mm long and contiguous short axis slices. All patients underwent PET using [¹⁸F]-FDG. Myocardial scars were rated in both CMR and PET on a segmental basis by a 4-point-scale: Score 1 = no LGE, normal FDG-uptake; score 2 = LGE enhancement < 50% of wall thickness, reduced FDG-uptake ( ≥ 50% of maximum); score 3 = LGE ≥ 50%, reduced FDG-uptake (< 50% of maximum); score 4 = transmural LGE, no FDG-uptake. Segments with score 1 and 2 were categorized “viable”, scores 3 and 4 were categorized as “non-viable”. Patients were divided into three groups based on LV function as determined by CMR: Ejection fraction (EF), < 30%: n = 45; EF: 30%-50%: n = 44; EF > 50%: n = 16). On a segmental basis, the accuracy of CMR in detecting myocardial scar was compared to PET in the total collective and in the three different patient groups.

RESULTS

CMR and PET data of all 105 patients were sufficient for evaluation and 5508 segments were compared in total. In all patients, CMR detected significantly more scars (score 2-4) than PET: 45% vs 40% of all segments (P < 0.0001). In the different LV function groups, CMR found more scar segments than PET in subjects with EF< 30% (55% vs 46%; P < 0.0001) and EF 30%-50% (44% vs 40%; P < 0.005). However, CMR revealed less scars than PET in patients with EF > 50% (15% vs 23%; P < 0.0001). In terms of functional improvement estimation, i.e., expected improvement after revascularization, CMR identified “viable” segments (score 1 and 2) in 72% of segments across all groups, PET in 80% (P < 0.0001). Also in all LV function subgroups, CMR judged less segments viable than PET: EF < 30%, 66% vs 75%; EF = 30%-50%, 72% vs 80%; EF > 50%, 91% vs 94%.

CONCLUSION

CMR and PET reveal different diagnostic accuracy in myocardial viability assessment depending on LV function state. CMR, in general, is less optimistic in functional recovery prediction.

Utility of CMR Markers of Myocardial Injury in Predicting LV Functional Recovery: Results from PROTECTION AMI CMR Sub-study

BACKGROUND

Adverse left ventricular (LV) remodelling following acute ST-segment elevation myocardial infarction (STEMI) has prognostic importance. We aimed to predict 90-day left ventricular (LV) function following acute STEMI using variables from clinical presentation, biomarkers, and cardiovascular magnetic resonance imaging (CMR).

METHODS

Consecutive patients undergoing primary percutaneous coronary intervention for anterior STEMI as part of the Selective Inhibition of Delta-protein Kinase C for the Reduction of Infarct Size in Acute Myocardial Infarction (PROTECTION-AMI) trial were enrolled into the CMR sub-study at selected sites. CMR was performed at baseline (days 3 to 5) and 90 days and used to evaluate infarct size, myocardial salvage index, infarct heterogeneity, microvascular obstruction and global LV function. Biochemical markers including creatinine kinase area under the curve (CK AUC), peak CK, peak CK-myocardial band (CK-MB) and AUC, and troponin I were collected at specific time-points.

RESULTS

Ninety-six patients were enrolled in the CMR sub study and 85 completed the 90-day follow-up, across 24 centres worldwide. LV ejection fraction (EF) was 56% (46-63%) at baseline and 60% (49-67%) at 90 days (p<0.001). Infarct size had moderate inverse correlation with 90-day EF (Spearman's rho=-0.7, p < 0.001) and had the strongest correlation when compared to myocardial salvage index (Spearman's rho=0.5, p=0.001), infarct heterogeneity (Spearman's rho=-0.4, p=0.02 or microvascular obstruction (Spearman's rho=-0.4, p<0.001). All biochemical markers had similar moderate relationship to LVEF at 90 days (Spearman's rho -0.6 to -0.8, p=0.001). In a multivariable model, only baseline LVEF, CMR infarct size and infarct heterogeneity independently predicted 90-day LVEF.

CONCLUSION

This study reports findings of a combined CMR protocol (including myocardial oedema imaging) in a multi-centre, multi-vendor setting. We found infarct size, infarct heterogeneity and myocardial salvage index correlated favourably with 90-day LVEF, however only the former two were independently predictive.

Myocardial viability and microvascular obstruction: role of cardiac magnetic resonance imaging

Established coronary artery disease has a prevalence of 7% in adult Americans, accounting for 16 million people. As morbidity and mortality rates have risen, research efforts to identify the pathophysiologic mechanisms of systolic dysfunction have risen in parallel. The current goal is to develop new therapeutic strategies with the potential to reverse systolic dysfunction in patients with established coronary artery disease. Cardiac magnetic resonance imaging has gained a key role in cardio vascular medicine. We will comment on the potential pivotal role of cardiac magnetic resonance imaging for the assessment of myocardial viability, including hibernating and stunned myocardium and microvascular obstruction.

Assessing the available techniques for testing myocardial viability: what does the future hold?

Left ventricular dysfunction in the setting of severe coronary artery disease poses a major diagnostic and therapeutic dilemma. While this clinical scenario is generally associated with poor outcomes, some but not all patients benefit from coronary revascularization. For example, patients with severe, transmural myocardial infarctions may derive little or no functional benefit from revascularization, as the underlying myocardium is irreversibly scarred. Furthermore, these patients may be exposed to high procedural risks with a low likelihood of deriving any perceivable benefit. Conversely, hibernating myocardium reflects a substrate whereby the nonfunctioning myocytes are chronically ischemic but may be viable. Existing data are somewhat inconclusive with regard to the benefits of performing viability testing in patients with ischemic cardiomyopathy. While this testing may predict regional and global functional myocardial recovery, the ability of viability studies to predict survival and prognosis remains unproven in prospective studies to date. Yet, viability testing may still be a valuable tool to guide therapeutic options in selected patients. A variety of noninvasive viability tests are available and newer technologies, such as PET and cardiac MRI, are likely to advance the scientific field in years to come.

Contrast-enhanced cardiac magnetic resonance imaging

Cardiac magnetic resonance (CMR) imaging has significantly evolved in the past decade and is well established in the evaluation of coronary artery disease (CAD). The evaluation of cardiac anatomy and contractility by high-resolution CMR can be improved by using intravenous administration of gadolinium-based contrast agents. Delayed enhancement CMR imaging has become the gold standard for quantification of myocardial viability in CAD. Contrast-enhanced CMR imaging may circumvent the need for endomyocardial biopsy or localize the involved regions, thereby improving the diagnostic yield of this invasive procedure. The application of contrast-enhanced CMR as an advanced imaging technique for ischemic and nonischemic diseases is reviewed.