Extracellular contrast agents such as gadolinium can also be used to characterize myocardial viability, as myocardial
perfusion correlates with function. When gadolinium is first injected into the systemic venous circulation, the contrast
agent will appear bright in the MR image as it perfuses throughout ventricles and into the myocardium. This enhancement
will occur within a few heartbeats. Contrast agents have altered T1 and T2 relaxation times in comparison to blood and
thus appear brighter in the image. The regional myocardial distribution in the contrast enhancement can be used to
determine the uniformity of perfusion, as well as the rate. Regions in the image which appear dim may suffer from an
obstruction in the microvasculature, or indicate an occluded coronary artery.
Delayed contrast enhancement using gadolinium is the preferred method to identify a scarred region caused by a chronic
myocardial infarction. Signal enhancement in scarred regions will occur approximately 5 minutes after the initial
contrast bolus. Scarred regions will retain the contrast agent as cell death and tissue edema (which occur after an
infarction and subsequent scarring) alter the wash-in and wash-out kinetics of the extracellular contrast agent.