However, in most cases of the rest scan, the image looks ok, with the exception of a dark area which would indicate a prior heart
attack that has killed some muscle. The stress
one will show if you have enough oxygen to the muscle when the heart is working harder. Obviously, the harder the heart works, the more oxygen is needed. So, if a blockage of say 80 % is present in a coronary artery, this will show the heart muscle as a dimmer colour in that vicinity.