Intravascular contrast agents normally remain confined to the intravascular
space, compared to Gd-DTPA which distributes throughout the extracellular
fluid space. This is a result of intravascular agents having a molecular
weight of approximately 70,000 and above, compared to a molecular weight
of 590 for Gd-DTPA. There are several advantages of intravascular agents.
They can assess perfusion in areas of ischemia and provide information about
capillary permeability in areas of reperfusion. They can show the extent
of tumor neovascularity and associated permeability changes. Finally, they
are useful in studies requiring prolonged imaging. Three types of intravascular
contrast agents will be described: Gd-DTPA labeled albumin, Gd-DTPA labeled
dextran, and chromium-labeled red blood cells.