Differential MR Delayed Enhancement Patterns of Chronic Myocardial Infarction between Extracellular and Intravascular Contrast Media

Objectives Because the distribution volume and mechanism of extracellular and intravascular MR contrast media differ considerably, the enhancement pattern of chronic myocardial infarction with extracellular or intravascular media might also be different. This study aims to investigate the differences in MR enhancement patterns of chronic myocardial infarction between extracellular and intravascular contrast media. Materials and Methods Twenty pigs with myocardial infarction underwent cine MRI, first pass perfusion MRI and delayed enhancement MRI with extracellular or intravascular media at four weeks after coronary occlusion. Myocardial blood flow (MBF) was determined with microsphere measurement. The infarction histopathological changes were evaluated by hematoxylin and eosin staining and Masson's trichrome method. Results Cine MRI revealed the reduced wall thickening in chronic infarction compared with normal myocardium. Moreover, significant wall thinning in chronic infarction was observed in cine MRI. Peak first-pass signal intensity didn’t significantly differ between chronic infarction and normal myocardium no matter what kinds of contrast media. At the following delayed enhancement phase, extracellular media-enhanced signal intensity was significantly higher in chronic infarction than in normal myocardium. Conversely, intravascular media-enhanced signal intensity was almost equivalent among chronic infarction and normal myocardium. At four weeks after infarction, MBF in chronic infarction approached to that in normal myocardium. Large thick-walled vessels were detected at peri-infarction zones. The cardiomyocytes were replaced by scar tissue consisting of dilated blood vessels and discrete fibers of collagen. Conclusions Chronic infarction was characterized by the significantly reduced wall thickening and the definite wall thinning. First-pass myocardial perfusion defect was not detected in chronic infarction with two media due to the significantly recovered MBF and well-developed collateral vessels. Infarction remodeling enlarged the extracellular compartment, which was available for extracellular media but not accessible to intravascular media. Extracellular media identified chronic infarction as the hyper-enhancement; nonetheless, intravascular media didn’t provide delayed enhancement.研究目的 由于细胞外与血管内磁共振对比剂的分布容积及作用机制存在显著差异，采用细胞外或血管内对比剂的慢性心肌梗死强化模式可能也存在不同。本研究旨在探究细胞外与血管内对比剂在慢性心肌梗死磁共振强化模式上的差异。 材料与方法 本研究纳入20只冠状动脉闭塞4周后形成心肌梗死的小型猪，分别采用细胞外或血管内对比剂进行cine MRI（电影磁共振成像）、首过灌注磁共振成像及延迟强化磁共振成像检查。采用微球法测定心肌血流量（myocardial blood flow, MBF）。通过苏木精-伊红染色与马松三色染色法评估梗死区的组织病理学变化。 研究结果 电影磁共振成像显示，与正常心肌相比，慢性梗死区域的室壁增厚率降低。此外，电影磁共振成像还观察到慢性梗死区域存在明显的室壁变薄。无论使用何种类型的对比剂，慢性梗死区与正常心肌的首过信号强度峰值均无显著差异。在后续的延迟强化阶段，采用细胞外对比剂时，慢性梗死区的信号强度显著高于正常心肌；与之相反，采用血管内对比剂时，慢性梗死区与正常心肌的信号强度几乎无差异。梗死造模4周后，慢性梗死区的心肌血流量已接近正常心肌水平。梗死周边区域可见粗大的厚壁血管。心肌细胞被由扩张血管与散在胶原纤维构成的瘢痕组织所替代。 研究结论 慢性心肌梗死以室壁增厚率显著降低及明确的室壁变薄为特征。由于心肌血流量已显著恢复且侧支血管发育良好，采用两种对比剂均未在慢性梗死区检测到首过心肌灌注缺损。梗死重塑导致细胞外间隙扩大，该间隙可被细胞外对比剂渗透，但无法被血管内对比剂进入。细胞外对比剂可通过高强化效应识别慢性心肌梗死，而血管内对比剂则无法产生延迟强化效应。