²³Na MRI combined with contrast‐enhanced ¹H MRI provides in vivo characterization of infarct healing

Although (23)Na MRI has been shown to delineate acute myocardial infarction (MI), the time course of in vivo (23)Na MRI during infarct healing remains unknown. In this study (23)Na MRI was combined with contrast-enhanced (CE) (1)H MRI to noninvasively characterize infarct healing in vivo. Serial in vivo 3D (23)Na MRI and (1)H MRI were performed for up to 9 weeks postinfarction in 10 dogs. Radioactive microspheres were used to measure myocardial perfusion, and Hematoxylin-Eosin (H&E) and Masson's trichrome (MT) staining were used to assess interstitial cell infiltrate and collagen content. In vivo (23)Na MRI accurately delineated infarct size up to day 5 postinfarction in comparison with (1)H MRI (8.9% +/- 8.1% vs. 8.6% +/- 7.9% on day 1 postinfarction, P = NS; and 6.3% +/- 6.2% vs. 6.2% +/- 6.2% on days 4/5 postinfarction, P = NS). The in vivo (23)Na MRI signal intensity, expressed as the signal intensity ratio of infarcted tissue vs. noninfarcted tissue (MI/R) peaked on day 1 of infarction (2.04 +/- 0.23) but decreased significantly to 1.27 at 9 weeks postinfarction (P < 0.05) due to granulation tissue infiltrate and collagen deposition. To confirm the MI/R decrease during scar formation ex vivo, we performed (23)Na MRI in 12 rats on day 3 post-MI (N = 5) and after 6 weeks (N = 7). H&E and Picrosirius Red staining confirmed granulation tissue infiltrate on day 3 and scar formation after 6 weeks. MI/R decreased significantly from 1.91 +/- 0.45 on day 3 post-MI to 1.3 +/- 0.09 after 6 weeks. Thus, in vivo (23)Na MRI accurately delineates infarct size up to day 5 postinfarction. In vivo (23)Na MRI signal intensity decreases during infarct healing as a result of the underlying infarct healing process.