MR imaging of experimental myocardial infarction

The signal enhancement in MR imaging of normal, infarcted and reperfused myocardium was investigated using different types of contrast agents. To investigate the organ distribution of a macromolecular contrast agent, normal rats were imaged before, and serially after injection of dextran-(Gd-DTPA)15. Dextran-(Gd-DTPA)15 had an essentially intravascular distribution, and significantly enhanced various organs in normal rats, including the heart. There was a linear relationship between the injected dose and observed enhancement. To investigate myocardial signal enhancement, acute myocardial infarction was induced in pigs by ligating a coronary artery. In two groups of pigs, dextran-(Gd-DTPA)15 or Gd-DPTA was administered i.v. after approximately 4 h of occlusion. Imaging was performed repeatedly in vivo. The animals were sacrificed about 2-2.5 h after injecting the contrast medium. The hearts were excised and re-imaged ex vivo. In three other groups of pigs, imaging was only carried out on excised hearts with 6-hour-old infarctions (a) without injecting contrast medium, (b) with injection of Dy-DTPA-BMA 3 min before sacrifice, and (c) with injection of Gd-DTPA-BMA 2 h before sacrifice followed by injection of Dy-DTPA-BMA 3 min before sacrifice. Without contrast medium, the infarctions were visualized as regions with a high signal intensity in the proton density- and T2-weighted images of excised hearts. Injection of dextran-(Gd-DTPA)15 resulted in infarct visualization also in the T1-weighted images ex vivo, due to a pronounced enhancement in parts of the infarct periphery and moderate enhancement in normal myocardium. Gd-DTPA accumulated in the infarctions, resulting in a better infarct visualization in the T1- and proton density-weighted images of excised hearts, compared with the control and dextran-(Gd-DTPA)15 groups. In vivo, however, the infarctions could not be visualized either before or after injection of dextran-(Gd-DTPA)15 or Gd-DTPA. Injection of Dy-DTPA-BMA improved infarct visualization mainly in the proton density- and T2-weighted images of excised hearts, due to susceptibility-induced reduction of signal intensity in the nonischemic myocardium. With T2-weighting, the infarct visualization was markedly better in the Dy-DTPA-BMA group than in the controls, or in the dextran-(Gd-DTPA)15 and Gd-DPTA groups. The double-contrast technique yielded an excellent infarct visualization ex vivo in all sequences, superior to that obtained in all other groups of pigs, due to Gd-DTPA-BMA-induced enhancement of the infarctions combined with Dy-DTPA-BMA-induced reduction of signal intensity in normal myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)