You are probably better off doing it on the explanted tissue after euthanasia. Doing it prior would require either perfusing the animal IV with large amounts of TTC (depending on which animal you are using), which may not work if there is a technical issue with perfusion, or doing a direct injection into the cardiac coronaries after catheterization, which requires special instrumentation and expertise. Taking the heart out, slicing it and incubating into TTC is much quicker and simpler. The usual concentration is 1% TTC at pH of 7.8, in a bath prewarmed at 37C, for about 30 minutes. The TTC will only stain the tissue surface, so you must slice the heart first, then immerse slices in the bath. The slices must not overlap or be covered, because anything that prevents TTC contact with the tissue will interfere with the staining.

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.