Background Contrast medium–enhanced magnetic resonance images of acute, reperfused infarcts have shown hypoenhanced and hyperenhanced regions in areas of injured myocardium. The precise mechanisms that lead to these altered enhancement patterns are unknown. This study was designed to evaluate possible mechanisms and to relate altered enhancement patterns to myocardial perfusion and viability.

Methods and Results Thirteen rabbits underwent in situ coronary artery occlusion and reperfusion followed by isolated perfusion with cardioplegic solution. T1-weighted spin-echo images were acquired continuously during step changes in perfusate Gd-DTPA concentration. Regional blood flow was also measured by use of radioactive microspheres in all rabbits. There were marked differences in Gd-DTPA wash-in and washout time constants (wash-in, 0.8±0.1, 2.1±0.2, and 16.3±2.4 minutes, P<.001; washout, 1.6±0.1, 4.8±0.5, and 31.1±3.3 minutes, P<.001) in normal, infarct rim, and infarct core regions, respectively, resulting in differential enhancement of these regions. Microsphere flows in the infarct rim and core were 42.9±4.0% and 12.0±1.6% of normal myocardium and correlated well with washout time constants (r=.86, y=0.77x−0.002, P<.001), suggesting that these time constants index the severity of microvascular damage. In addition, spatial maps of washout time constants were produced. The extent of regions with abnormal time constants correlated well with triphenyltetrazolium chloride–determined infarct size (r=.94, y=0.95x+4.17, P<.001).

Conclusions In contrast-enhanced magnetic resonance images of acute, reperfused rabbit infarcts, differential image intensity is primarily due to regional differences in contrast agent wash-in and washout time constants. These regional differences in time constants also indicate the extent and severity of myocardial injury.