Contrast-Enhanced CMR Overestimates Early Myocardial Infarct Size Mechanistic Insights Using ECV Measurements on Day 1 and Day 7

OBJECTIVES This study aimed to investigate whether an overestimation of infarct size on cardiac magnetic resonance (CMR) versus triphenyltetrazolium chloride (TTC) exists acutely and whether it remains after 7 days in an experimental pig model and to elucidate possible mechanisms. BACKGROUND Overestimation of infarct size (IS) on Late gadolinium enhancement CMR early after acute myocardial infarction has been debated. METHODS Pigs were subjected to 40 min of Left anterior descending artery occlusion and 6 h (n = 9) or 7 days (n = 9) reperfusion. IS by in vivo and ex vivo CMR was compared with TTC staining. Extracellular volume (ECV) was obtained from biopsies using technetium 99m diethylenetriamine pentaacetic acid (99mTc-DTPA) and Light microscopy. TTC slices were rescanned on CMR enabling slice-by-slice comparison. RESULTS IS did not differ between in vivo and ex vivo CMR (p = 0.77). IS was overestimated by 27.3% with ex vivo CMR compared with TTC (p = 0.008) acutely with no significant difference at 7 days (p = 0.39). Slice-by-slice comparison showed similar results. A significant decrease in ECV was seen in biopsies of myocardium at risk (MaR) close to the infarct (sometimes referred to as the pen-infarction zone) over 7 days (48.3 +/- 4.4% vs. 29.2 +/- 2.4%; p = 0.0025). The ECV differed between biopsies of MaR close to the infarct and the rest of the salvaged MaR acutely (48.3 +/- 4.4% vs. 32.4 +/- 3.2%; p = 0.013) but not at 7 days (29.2 +/- 2.4% vs 25.7 +/- 1.4%; p = 0.23). CONCLUSIONS CMR overestimates IS compared with TTC acutely but not at 7 days. This difference may be explained by higher ECV in MaR closest to the infarct acutely that decreases during 7 days to the same level as the rest of the salvaged MaR. The increased ECV in the MaR closest to the infarct day 1 could be due to severe edema or an admixture of infarcted and salvaged myocardium (partial volume) or both. Nonetheless, this could not be reproduced at 7 days. These results have implications for timing of magnetic resonance infarct imaging early after acute myocardial infarction. (C) 2015 by the American College of Cardiology Foundation.