There is a new article by Laissy JP etal from France, in the Oct 2005 issue of Radiology that shows how the enhancement and first pass features of myocarditis and infarction are totally different. First pass imaging honestly does not matter in this situation and though the study was perfomed in the acute setting, it is valid in all clinical situations.
The differentiation is pretty straightforward. Infarcts show a defect on first-pass imaging, with a subendocardial-transmural enhancement corresponding to an infarct territory (Figs. 1, 2). Myocarditis shows no change on first pass imaging with mid-myocardial to epicardial, patchy areas of enhancement (Figs. 3, 4). Figs. 1 & 2 show a subendocardial infarct involving approximately 50-60% of the thickness of the myocardial in the anterior wall (arrows). Figs. 3 & 4 show mid-myocardial and epicardial septal enhancement (arrows).