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Microvascular injury in reperfused infarcted myocardium: noninvasive assessment with contrast-enhanced echoplanar magnetic resonance imaging ¹

^a Department of Radiology, University of California San Francisco, San Francisco, California, USA

View larger version (14K): [in a new window]   Figure 1 Graph showing the four study groups. GdDTPA-albumin (n = 8 in each group) or 123I-GdDTPA-albumin (n = 6 in each group) were injected before reperfusion (group 1); after h (group 2); after 1 h (group 3); after 24 h of reperfusion (group 4). Magnetic resonance imaging in each group was performed over a 1-h period, or hearts were excised 15 min after injection and autoradiography performed. Solid bar = time of coronary artery occlusion (1 h); hatched bar = time during reperfusion at which GdDTPA-albumin or 123I-GdDTPA-albumin were injected and MRI or autoradiography was performed.

View larger version (143K): [in a new window]   Figure 2 Axial IR-EPI of a rat heart acquired at the same anatomical level but with different TI settings. After 1 h coronary occlusion and 24 h reperfusion, GdDTPA-albumin was administered; IR-EPI were acquired 4 min later. A strong negative signal was observed for blood and the entire myocardium with TI = 20 ms (top left panel). Nil signal was obtained for blood at TI = 70 ms from left ventricular blood (LV in top right panel), for normal myocardium at TI = 320 ms (arrowheads in lower left panel) and for reperfused infarction at TI = 470 ms (arrows in lower right panel). Regions with faster T1 relaxation, that is, greater Gadolinium concentration, exhibit nil signal at shortest TI setting. Thus, Gadolinium concentration in blood > in normal myocardium > in reperfused infarction at 4 min after administration.

View larger version (21K): [in a new window]   Figure 3 R1 ratio over time postinjection of GdDTPA-albumin in normal and reperfused infarcted myocardium. GdDTPA-albumin was injected before reperfusion, at , 1 or 24 h of reperfusion. Note that the accumulation of GdDTPA-albumin was fastest when given before reperfusion. The accumulation was slower when administered at , 1 and 24 h after reperfusion.

View larger version (18K): [in a new window]   Figure 4 R1 and radioactivity ratios of reperfused infarct/blood (A) and normal/blood (B). When tracers were injected before reperfusion, at , 1 or 24 h of reperfusion, there were no significant differences between R1- and radioactivity-ratios. However, both ratios decreased when the tracers were administered after reperfusion as compared to administration before reperfusion (*p < 0.01), and decreased further at 24 h of reperfusion as compared to 1 h of reperfusion (p < 0.05). Solid bars = R1-ratio; white bars = radioactivity-ratio.

View larger version (149K): [in a new window]   Figure 5 Autoradiograms of midventricular slices of hearts subjected to 1 h of coronary occlusion and reperfusion. 123I-GdDTPA-albumin was injected either before reperfusion (A), at (B), at 1 (C) or at 24 h (D) after reperfusion. The accumulation of labeled albumin in the anterolateral wall of the left ventricle indicates vascular injury. Maximal and essentially homogeneous accumulation of labeled albumin occurs when administered before reperfusion (A). When 123I-GdDTPA-albumin was administered later during reperfusion, less indicator was observed in the infarct core (D, arrowheads).

View larger version (14K): [in a new window]   Figure 6 Blood volume of ischemically injured myocardium at various durations of reperfusion. Values are expressed as percentage of the blood volume of normal myocardium. The elevated blood volume at onset of reperfusion likely represents postischemic hyperemia; the decreased blood volume after 24 h of reperfusion may be the consequence of edema and external compression of the microvasculature, microvascular plugging and swelling of endothelial cells.

View larger version (45K): [in a new window]   Figure 7 Different imaging modalities of a heart subjected to 1 h of coronary artery occlusion and 24 h of reperfusion at the same anatomical level: (A) T1-weighted spin echo MRI, (B) IR-EPI, (C) postmortem TTC stain. Four minutes after injection of GdDTPA-albumin, the reperfused infarcted anterolateral wall (arrowheads) is heterogeneously enhanced (A), has a longer T1 value as compared to normal myocardium (B) and is TTC-negative (white) with hemorrhage (dark zone) (C).