Intravascular Contrast Agents

The main characteristic distinguishing intravascular contrast agents lies in their inability to pass across the vascular wall and leave the blood pool. Therefore, while being unsuitable for visualization and quantification of permeability and leakage into the interstitial space, these agents are especially suitable to analyze the vascular architecture and intravascular flow. The main characteristic distinguishing intravascular contrast agents lies in their inability to pass across the vascular wall and leave the blood pool. Therefore, while being unsuitable for visualization and quantification of permeability and leakage into the interstitial space, these agents are especially suitable to analyze the vascular architecture and intravascular flow. The size of the contrast-agent molecules is the distinguishing factor. While large size molecules cannot pass through the gaps between the vascular layers made of endothelial cells, smaller molecules can pass and leak into the extravascular space. In general, only molecules larger than few hundred nanometers can stay intravascular. Most contrast agents are therefore extravascular, unless specific binding to larger molecular complexes is realized. The only contrast agents which remain naturally intravascular are UCAs, as they are made of microbubbles with size larger than that of the endothelial gaps, even in the case of increased vascular permeability due to angiogenesis, i.e., the formation of a dense, irregular microvascular network in relation to cancer growth or other inflammatory processes (see Sect. 3.3.2). Therefore, although MR blood pool agents are also briefly treated, this chapter is mostly dedicated to UCAs, because of their intravascular nature.