PET is one of the most accurate tests for assessing myocardial viability. PET does this by measuring myocardial glucose metabolism using FDG.
Normal myocardium can obtain energy from either fatty acids or from glucose. When a section of the myocardium becomes ischemic, that section switches over to anaerobic glucose metabolism as its sole source of energy. Because anaerobic glucose metabolism is so inefficient, the amount of glucose that is consumed can be almost as high as normal myocardium. However if that section of the myocardium is no longer viable, then it will not take up FDG at all.
When an area of hypo-perfusion continues to take up FDG, then it is viable (called a perfusion, metabolism mis-match). However, when an area of hypo-perfusion has a corresponding area of hypo-metabolism, that section of the myocardium is no longer viable.