The aim of this study was to test whether contrast-enhanced magnetic resonance (MR) imaging may assess in vivo the severity of the no-reflow phenomenon in a dog model of infarction with 2-hour coronary occlusion followed by reperfusion (6 hours). Subsecond MR imaging combined with intravenous bolus administration of superparamagnetic iron oxide particles (SPIO) was performed at the fifth hour of reperfusion. An MR index was calculated using the difference of signal-intensity enhancement between ischemic and nonischemic zones during the SPIO first pass. Dogs were separated into two groups according to the severity of ischemia: collateral blood flow in the central ischemic zone at 120 minutes of occlusion (radioactive microsphere technique) < 22.5% of the flow in the nonischemic zone (group I) or > 22.5% (group II). Mean collateral blood flow during occlusion was lower in group I (11.3% +/- 2.9%, n = 7) than in group II (66.8% +/- 19.8%, n = 6, p < 0.05). Mean infarct size was significantly larger in group I (58.6% +/- 4.9% of the area-at-risk, n = 7) than in group II (16.5% +/- 6.5%, n = 6, p < 0.05). For the entire population (n = 13), the infarct size was inversely correlated to the collateral blood flow (r = -0.64, p = 0.02, standard error of estimate = 0.24). The relative rate of enhancement in ischemic myocardium (MR index) was significantly lower in group I (38.1% +/- 10.9%) than in group II (142.8% +/- 32%; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)