Quantitative MRI Measurements of Function, Perfusion and Viability in Microembolized, Moderately Ischemic Myocardium

Introduction: Multiple factors make assessment of microembolization clinically challenging. We aimed to 1) provide detailed analysis and comparison of infarct healing in ischemic myocardium with and without microemboli and 2) validate MRI measurements against histochemical/histological staining.Methods: Animals either served as controls (group I, n=8) or were subjected to one of two percutaneous coronary interventions (PCI) (n=8/group): 1) 45 min occlusion of the left anterior descending (LAD) coronary artery followed by reperfusion (group II) and 2) the same duration of occlusion followed by microemboli delivery prior to reperfusion (group III). Cine, perfusion and delayed contrast enhanced MRI (DE-MRI) were performed on a 1.5T scanner after 3 days and 5 weeks. The left ventricles (LV) were sliced and stained with TTC (triphenyltetrazolium chloride) and Masson trichrome to visualize, delineate and compute scar tissue planemetrically in groups II and III, respectively.Results: Both intervention groups showed on MRI an increase in end systolic volume and decrease in ejection fraction compared to controls and these changes were more pronounced in group III than II. Unlike group II, group III showed persistent perfusion deficits. Small but homogeneous MI on DE-MRI was visible in group II (1.3±0.9g) with substantially larger MI in group III (7.7±0.5g), however, the resorption was significantly smaller in group III (22%) than group II (60%, P<0.05). Postmortem TTC revealed similar extent of scar tissue to DE-MRI in groups II (1.6±1.0g) and III (group III 9.2±1.6g) at 5 weeks. Histology revealed larger infarct extent in group II (2.8±0.4g, P<0.05) and III (10.5±1.5, P<0.01) than DE-MRI.Conclusion: MRI detected structural and functional changes in moderately ischemic myocardium with and without microemboli. In ischemic microembolized myocardium, homogeneous infarct, patchy microinfarct and compensatory hypertrophy were visible at 5 weeks. Microvascular obstruction exacerbates the development of compensatory hypertrophy. DE-MRI has the potential to monitor infarct resorption after PCI, but has limited spatial resolution for estimating true infarct extent, which should be taken into account when used to testing the efficacy of newer cardioprotective therapies and distal protective devices in improving LV function