PHYSIOLOGICAL-BASIS OF MYOCARDIAL CONTRAST ENHANCEMENT IN FAST MAGNETIC-RESONANCE IMAGES OF 2-DAY-OLD REPERFUSED CANINE INFARCTS

Background Contrast-enhanced fast magnetic resonance (MR) images of ac ute, reperfused human infarcts demonstrate regions of hypoenhancement and hyperenhancement. The relations between the spatial extent and tim e course of these enhancement patterns to myocardial risk, infarct, an d no-reflow regions have not been well characterized. Methods and Resu lts The proximal left anterior descending coronary artery was occluded in 11 closed-chest dogs for 90 minutes followed by 2 days of reperfus ion. Regional blood flow was determined by use of radioactive microsph eres. The animals were studied at the 2-day time point with contrast-e nhanced fast MRI (Signa 1.5 T, General Electric). Thioflavin-S was adm inistered to demarcate no-reflow regions. The hearts were then excised , sectioned into five base-to-apex slices, stained with 2,3,5-tripheny ltetrazolium chloride (TTC), and photographed under room light (for TT C) and ultraviolet light (for thioflavin). The spatial extents of thio flavin-negative, TTC-negative, and risk regions were compared planimet rically with MRI hypoenhanced and hyperenhanced regions. The spatial l ocations of subendocardial hypoenhancement in MR images correlated clo sely with those of thioflavin-negative regions. Microsphere blood flow in these regions was significantly reduced compared with remote regio ns (0.37+/-0.09 versus 0.88+/-0.10 mL/min per gram, respectively, P<.0 01) and with baseline (0.37+/-0.09 versus 0.87+/-0.15 mL/min per gram, P<.01). The spatial extent of hyperenhancement was smaller than the r isk region (r=.64, slope=0.48, P<.001) but highly correlated with TTC- negative regions and were, on average, 12% larger (r=.93, slope=1.12, P=.035). Conclusions In contrast-enhanced MR images of 2-day-old reper fused canine infarcts, myocardial regions of hypoenhancement are relat ed to the no-reflow phenomenon. Approximately 90% of the myocardium wi thin hyperenhanced regions is nonviable.