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(1) To determine the accuracy of delayed enhancement multidetector computed tomography (MDCT) in measuring the extent of acute myocardial infarct and no-reflow areas using cardiac magnetic resonance imaging (MRI) as standard of reference and (2) to define the optimum timing between injection and MDCT image acquisition to characterize infarcted myocardium and no-reflow areas after reperfusion therapy.Nineteen patients were prospectively included after acute myocardial infarction and revascularization. Each patient had an MDCT acquisition before and 5 and 10 minutes after injection of 1.5 mL/kg iodinated contrast medium, and a contrast-enhanced MRI at 5 and 10 minutes after injection of 0.2 mmol/kg gadolinium chelate. We assessed image quality and infarct extent using MDCT and MRI, and we measured parameters related to iodinated contrast media kinetics (ΔHU and ΔHU ratio).The areas of hyperenhanced myocardium located on the MDCT corresponded to the occluded vessel located on the coronary angiogram (κ = 0.9). There were strong correlations between the extent of hyperenhanced infarcted myocardium on MDCT and MRI at 5 minutes (20.4% ± 2.7% of left ventricle (LV) and 20.9% ± 2.4%, respectively, R = 0.85; P < 0.0001) and 10 minutes after injection (21.0% ± 2.9% of LV and 19.4% ± 2.5%, respectively, R = 0.80; P < 0.0001). However, the correlation between the area of hypoenhanced myocardium measured using MDCT and CMR 5 minutes after injection (R = 0.86; P < 0.0001) was better than the measurement obtained 10 minutes after injection (R = 0.64; P = 0.002). On contrast-enhanced MDCT, 5 minutes after injection, the signal-to-noise ratio was significantly higher than at 10 minutes after injection in LV blood (28 ± 1 to 21 ± 1, respectively; P = 0.0007), normal myocardium (18 ± 1 to 15 ± 1; P = 0.0009), and hyperenhanced infarcted myocardium (24 ± 1 to 20 ± 1; P = 0.004). MDCT image quality was significantly better at 5 minutes (3.2 ± 0.1) than at 10 minutes (2.8 ± 0.2, P = 0.01, κ = 0.4). The ΔHU ratio increased slightly but significantly between 5 minutes (0.83 ± 0.01) and 10 minutes (0.93 ± 0.01; P = 0.01), suggesting a slow wash-in and wash-out of contrast medium in infarcted myocardium.In ST segment elevation myocardial infarction patients contrast-enhanced MDCT is an accurate method for characterizing and sizing myocardial infarct and no-reflow. Contrast-enhanced MDCT performed 5 minutes after injection yields a higher signal-to-noise ratio and image quality than the 10 minutes time point with no difference in the extent of infarct measurement.