SCMR 22nd Annual Scientific Sessions
Oxidative stress plays an important role in the pathogenesis of myocardial repair and remodeling after myocardial infarction (MI)1-3. Following MI, oxidative stress develops in the infarcted myocardium primarily due to neutrophil and macrophage infiltration, which peak at day 1 and day 4 post-MI respectively3-4. Nitroxide stable free radicals have been used as oxidative stress-sensitive MRI contrast agents in preclinical cancer imaging studies where they serve as T1-shortening contrast agents that lose their T1-shortening property as they undergo in vivo reduction reactions5-6. The rate at which nitroxide-induced T1-shortening decreases provides a measure of tissue oxidative stress. We tested the hypothesis that dynamic nitroxide-enhanced MRI can detect oxidative stress in infarcted myocardium at days 1 and 4 post-MI.
We employed a mouse model of MI. The nitroxide contrast agent 3-Carbamoyl-PROXYL (3CP) (Sigma-Aldrich, St. Louis, MO) was chosen because it is water soluble, commercially available, and well tolerated by mice. MI was induced by a 40 minute left coronary artery occlusion followed by reperfusion. Wild type male C57Bl/6 mice at day 1 (n=8), day 4 (n=9), and day 21 post-MI (n=6), and healthy control mice (n=10) underwent MRI studies using a 7T system (Clinscan, Bruker). DENSE MRI7 and proton-density (PD) weighted MRI were performed in a mid-ventricular short-axis slice before 3CP injection. Serial T1-weighted MRI (Table 1) was performed in the same slice before and consecutively after 3CP injection for 10 minutes. 3CP was administered as a 50mg/ml bolus solution through an indwelling tail vein catheter at 2 mmol/kg body weight. DENSE strain maps were used to identify infarct and remote noninfarcted regions of interest (ROIs) in the heart. In control mice, ROIs included all of the myocardium. ROI signal intensities were normalized by the PD signal intensities, and the 3CP decay rate from 3-10 minutes after injection was computed by least squares fitting of: ln(intensity) = constant – decay_rate*time.
Figure 1 shows example T1-weighted images before and serially after 3CP injection (A), an example DENSE strain map used to identify remote and infarcted ROIs (B), and example remote and infarct ln(intensity) vs time curves obtained from a mouse four days post-MI, along with linear fits for the period 3-10 minutes after injection (C). Figure 2 summarizes the time course of the 3CP signal decay rate in infarct and remote zone for all mice over 21 days post-MI.
We detected an elevated 3CP decay rate at days 1 and 4 post-MI in infarct regions compared to controls, remote regions, and the day 21 infarct region, indicating that nitroxide-enhanced MRI detects increased oxidative stress in the infarcted myocardium at time points when infiltrating leukocytes are at high concentration. These methods may be useful in preclinical studies that investigate the role of oxidative stress during infarct healing and post-MI remodeling.