MRI of mice with acute (,24 hours) myocardial infarction injected with either (A,B) Gd-TO or (D, E) Gd-DTPA. MRI was performed 2-3 hours after injection. High levels of Gf-TO accumulate in acute infarcts producing signal hyperintensity on invertsion recovery images (A) and high R1 values (B). In contrast, within 2-3 hours of injection Gd-DTPA has completely washed out of the infarct and produces no change in signal intensity or R1 (D, E). The accumulation of Gd-TO peaks 9-18 hours after infacrtion, and within 72-96 hours has returned to baseline (C). Significant different (*p<0.01) in R1 in the infarct were seen between those mice imaged with Gd-TO within 48 hours of infarction versus more than 72 hours after infarction (F). Likewise, significant differences were seen between mice injected with Gd-TO versus GF-DTPA within 48 hours of infarction (F).