Try wrapping the heart in cling-wrap once extracted from the mouse and put it in a container and put straight into dry ice. Use 1% TTC in PBS, incubate the sections of the heart on each side for 10 mins at 37C. After 10 mins flip the sections and incubate again for 10mins. Then put the sections in 10% buffered formation and leave overnight at room temperature. That way you get the best contrast. Without formalin, the infarct area is hard to see.