Proton (hydrogen-1) magnetic resonance imaging techniques have potential for the detection and characterization of changes associated with myocardial ischemia. Since image contrast is dependent on TI and T2 relaxation times, we examined these parameters in a canine preparation of occlusion of the left anterior descending coronary artery. Of 16 dogs studied, seven underwent 3 hr of coronary artery occlusion and nine underwent 3 hr of occlusion followed by 1 hr of reperfusion. After the dogs were killed, the hearts of four from each group were imaged in a small bore, 1.4 tesla magnet. From all hearts myocardial segments were obtained from the normal zone and the central ischemic zone (CZ). These segments were divided into epicardial and endocardial sections and studied in a spectrometer (20 MHz). After 3 hr of occlusion, CZ endocardial Ti and T2 increased significantly (p < .01 and p < .05, respectively). Changes in CZ epicardial relaxation times were not as marked. Although Tl and T2 tended to be higher in the reperfused group compared with the nonreperfused group, the differences did not reach statistical significance. In keeping with the spectrometric findings, Ti-dependent inversion recovery images and T2-dependent spin-echo images of the excised hearts demonstrated excellent contrast between normal and ischemic myocardium. Modest correlations were noted for both groups between blood flows during occlusion measured by the microsphere technique and TI and T2 relaxation times. In summary, relaxation times Tl and T2 increase regionally after 3 hr of coronary artery occlusion and tend to be accentuated by reperfusion. These changes are associated with high-resolution magnetic resonance image contrast, suggesting that such imaging may be useful for the characterization of early myocardial ischemia.Circulation 71, No. 3, 595-601, 1985. MAGNETIC resonance imaging provides a potentially useful method for noninvasive detection and characterization of early myocardial ischemia. Since signal intensity is dependent in part on T1 and T2 relaxation times,' delineation of these parameters after an ischemic insult is of importance in predicting image con-