Iron imaging in myocardial infarction reperfusion injury

Restoration of coronary blood flow after a heart attack can cause reperfusion injury potentially leading to impaired cardiac function, adverse tissue remodeling and heart failure. Iron is an essential biometal that may have a pathologic role in this process. There is a clinical need for a precise noninvasive method to detect iron for risk stratification of patients and therapy evaluation. Here, we report that magnetic susceptibility imaging in a large animal model shows an infarct paramagnetic shift associated with duration of coronary artery occlusion and the presence of iron. Iron validation techniques used include histology, immunohistochemistry, spectrometry and spectroscopy. Further mRNA analysis shows upregulation of ferritin and heme oxygenase. While conventional imaging corroborates the findings of iron deposition, magnetic susceptibility imaging has improved sensitivity to iron and mitigates confounding factors such as edema and fibrosis. Myocardial infarction patients receiving reperfusion therapy show magnetic susceptibility changes associated with hypokinetic myocardial wall motion and microvascular obstruction, demonstrating potential for clinical translation.

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Walter R. Witschey
Apr 2, 2020 Animal Cardiovascular Magnetic Resonance Protocol: MRI studies were performed on a 3 T whole-body system (Trio; VB17 scanner; Siemens Healthcare, Erlangen, Germany) with 40 mT/m gradients and 18 channel RF receiver arrays. Intraventricular pressure was interfaced to physiological monitoring software and filtered to facilitate dual respiratory and cardiac gating (LabView, National Instruments, Inc., Austin Texas, versio 2018). All 2D images were acquired in the short-axis during apnea and 3D with dual cardiac and respiratory gating. Apnea was achieved by temporarily disabling the animal ventilator. Human Cardiovascular Magnetic Resonance Protocol: MRI studies were performed on a 1.5 T whole-body system (Aera; VE11C scanner; Siemens Healthcare, Erlangen, Germany) with 40 mT/m gradients and 48 channel RF receiver arrays. Electrocardiograms were obtained during MRI and used for gating using a wireless ECG monitor. All 2D images were acquired in the short-axis during breath-holding and 3D with dual cardiac and respiratory gating.

October 2018
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All data is available upon reasonable request to the authors and approved through the University of Pennsylvania. The source data underlying Figures 1B-C Supplementary Figures 2, 8A, 10, 11A-E, 13 and Supplementary Tables 1, 2 are provided as a source data file. There may be restrictions on private health data. Source data from Supplementary Tables 3 and 4 were removed to protect patient confidentiality. MRI performed in the context of this research study did not influence the treatment of the patients enrolled in the study.
The animal sample size from our study demonstrated a significant difference in susceptibility and iron concentration at different reperfusion times and post-infarction timepoints, similarly to the myocardial infarction reperfusion study performed by Llaneras et al, Markovitz et al, Moainie, et al. Therefore, we did not increase the swine (N=38) sample size: reperfusion at 45 (n=3), 90 (n=10), 180 (n=5) minutes after infarct onset and permanent occlusion (n=4) at 1-week post-infarction. Three additional groups were studied at 3-day (n=3) and 8-week (n=8) after a 90-or 180-min occlusion followed by reperfusion and a separate set of controls without myocardial infarction (n=5). The clinical study was performed to test the feasibility of cardiac quantitative susceptibility mapping in a cohort of seven STEMI patients. Therefore, sample size power analysis was not performed prior to the clinical study. There were 38 animals, one animal in the 90 min reperfusion infarct group did not undergo LGE MRI, two animals in 180 min reperfusion infarct group did not undergo explant MRI due to unavailability of the MRI scanner, two animals in the 90 min reperfusion infarct group did not have ICP-OES analysis due to unavailability of the equipment. Seven animals in the 1-week timepoint group did not have ICP-OES analysis due to a subset of the animals being in a serial study and six animals in the 8-week timepoint group did not have ICP-OES due to unavailability of the equipment. The exclusion criteria were not pre-established, exclusion of animal data was due to unforeseen experiment circumstances.
Reproducibility analysis of magnetic susceptibility in patients (Fig. S7) performed by two raters (one was blinded). Bland-Altman and intra-class correlation coefficient show no significant bias and good correlation between raters.
Swine (n=17) were randomized by time-to-reperfusion into four groups: reperfusion at 45 (n=3), 90 (n=5), 180 (n=5) minutes after infarct onset and permanent occlusion (n=4). Histology and spectrometry tissue samples were allocated into time-to-reperfusion groups then into viable myocardium and infarct groups. Infarct regions were depicted based on visual inspection and viable myocardium was collected in remote regions that were not impacted by ischemia.
Cardiac structure, function and volumetric analysis were performed blinded for both animals and patients. Other MRI quantitative measurements and iron concentration measurements were unblinded.