Quantification of area at risk during coronary occlusion and reperfusion by means of MR perfusion imaging.

Abstract

Considerable clinical interest has focused on the size of ischemic myocardium. Fast MR imaging in conjunction with MR contrast media has the potential to identify hypoperfused and infarcted myocardium. This study used MR perfusion imaging to detect and quantify reperfused ischemic myocardium during a brief coronary occlusion and reperfusion, and to characterize the spatial extent of ischemic and reperfused ischemic myocardium relative to the "true" size of the area at risk as defined in histochemical morphometry at post mortem. The left circumflex (LCX) coronary artery in 8 dogs was occluded for 15 min followed by reperfusion in order to produce regional reversible myocardial ischemia. Perivascular Doppler probes were used to measure blood flow in the left anterior descending (LAD) and LCX coronary arteries. Fast inversion recovery-prepared gradient-recalled-echo images were acquired to delineate the ischemic area during occlusion, and the area of reversible ischemic injury at 1 and 30 min of reperfusion. The size of ischemic and reperfused ischemic myocardium were compared with the area at risk as determined by histochemical morphometry at post mortem. During LCX occlusion, LCX flow decreased from 16+/-1 to 0.2+/-0.1 ml/min. On contrast-enhanced images, ischemic myocardium was evident as a zone of relatively low signal intensity (SI) compared to normal myocardium. The size of the ischemic region was significantly smaller (30+/-2%) than at post mortem (36+/-3%; p<0.05). Immediately after reperfusion, LCX flow increased to 83+/-11 ml/min and the contrast medium caused greater enhancement in the reperfused ischemic region than in the normal myocardium (69+/-3 vs 42+/-3 arbitrary units; p<0.05). The increase in regional SI correlated closely with the increase in regional blood flow (r=0.73). At 1 min of reperfusion, the size of the reperfused ischemic myocardium was larger (48+/-3%, p<0.05) than the area at risk measured at post mortem. At 30 min of reperfusion, when the flow returned to baseline values (16+/-2 ml/min), contrast bolus produced no differential enhancement between the 2 myocardial territories. MR perfusion imaging has the potential to detect and quantify the size of ischemic myocardium and the region of post-occlusive hyperemia in the early reperfusion period. There is a significant direct linear relationship between the regional contrast enhancement of reperfused ischemic myocardium and the blood flow during post-occlusive hyperemia. The difference in the size of the area at risk at MR perfusion imaging and at histochemical morphometry may reflect an influence of coronary collateral circulation.