Proton ('H) nuclear magnetic resonance (NMR) imaging has been used to define zones of myocardial infarction (MI), which appear as areas of relatively increased signal intensity (SI). However, zones of decreased SI have been observed within the areas of infarction and have been postulated to result from intramyocardial hemorrhage. To explore this phenomenon further, ex vivo spin-echo 'H NMR imaging at 1.5 Tesla was performed in 17 dogs after 24 hr (n = 9) and after 72 hr (n = 8) of coronary artery occlusion.
In all dogs, a zone of increased SI (1 18 5 9% compared with normal myocardium) was observed in the distribution of the occluded coronary artery. In 12 of the 17 dogs, zones of decreased SI (92 z 8% compared with normal) were seen within or around the central zone of increased SI. Gross inspection and histological assessment of sliced myocardium usually disclosed hemorrhage in the regions of decreased SI. In three of the five dogs with no apparent zones of decreased SI on NMR. the infarct was small, and only minor hemorrhage was observed by gross inspection, whereas in the remaining two dogs no hemorrhage was seen. Myocardial flow in the hemorrhagic regions was significantly higher than in the necrotic core (59 2 29% vs. 31 5 24% compared with control, P-0.05).
Image-derived calculation of T , relaxation times in the different infarcted regions revealed a significant shortening of T, in the infarcted hemorrhagic zones with decreased SI compared with the infarct zones with increased SI (49 2 8 msec vs. 6628 msec, Pi 0.05). This reduction in T , in the hemorrhagic zone is consistent with the paramagnetic effects of the deoxyhemoglobin associated with intramyocardial hemorrhage.
These observations improve our understanding of the changes in NMR SI within the infarcted regions and provide a noninvasive method for the detection of intramyocardial hemorrhage. This should help to characterize further the myocardium in patients with myocardial infarction and to assess the effect of myocardial hemorrhage following reperfusion therapy.