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

Abstract

Although ²³Na MRI has been shown to delineate acute myocardial infarction (MI), the time course of in vivo ²³Na MRI during infarct healing remains unknown. In this study ²³Na MRI was combined with contrast‐enhanced (CE) ¹H MRI to noninvasively characterize infarct healing in vivo. Serial in vivo 3D ²³Na MRI and ¹H 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 ²³Na MRI accurately delineated infarct size up to day 5 postinfarction in comparison with ¹H 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 ²³Na 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 ²³Na 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 ²³Na MRI accurately delineates infarct size up to day 5 postinfarction. In vivo ²³Na MRI signal intensity decreases during infarct healing as a result of the underlying infarct healing process. Magn Reson Med 53:843–850, 2005.