Interstitial enhancement is related to alterations in the permeability of the blood-brain barrier, whereas intravascular enhancement is proportional to increases in blood flow or blood volume. If you “choose wisely,” enhancement will occur only in tissues without an intact blood-brain barrier. Extravascular or interstitial enhancement will also depend on the permeability of these capillaries to the chosen agent. The spinal cord, brain, and spinal nerves have specialized capillary vessels with a blood-brain barrier, giving them special properties of selective permeability. The higher the gradient, the greater the diffusion out of the vessel thus, giving a double or triple dose of a contrast agent will increase enhancement. If that capillary membrane is permeable to the contrast agent, it will rapidly leave the vessel and diffuse into the perivascular interstitial fluid space, driven by the concentration gradient. When a contrast agent is injected as a bolus, the blood level rapidly rises to a peak concentration that pushes the contrast agent against the capillary endothelial membrane. Parenteral contrast material is usually injected into a large peripheral vein, either slowly by a drip infusion or more rapidly by a short duration or bolus injection. In addition to neovascularity, which increases both blood volume and blood flow, vasodilatation of existing normal vessels (hyperemia) produces increased intravascular enhancement. Vascular enhancement is a combined product of blood volume, blood flow (delivery of contrast agent or “wash-in”), and “mean transit time” or time needed for “washout” of a contrast agent. Some “desirable” compounds, such as glucose, are facilitated to cross the vessel wall or are actively transported out of the vessel and into the tissue compartment. The normal intact blood-brain barrier is far more permeable to lipophilic compounds (as measured by octanol/water partition fraction), and the blood-brain barrier retards lipophobic compounds. The normal blood-brain barrier includes a continuous basement membrane, narrow intercellular gaps with junctional complexes, and only rare pinocytosis. The primary structure of the blood-brain barrier is from endothelial cell specialization produced by cooperation between these cells and the astrocyte foot processes. 1, 2 The brain, spinal cord, and nerves are supplied by capillaries that have a selectively permeable membrane that creates a “blood-brain barrier.” This selective barrier protects the nervous system from certain plasma proteins and limits inflammation by blocking inflammatory cells from entering the tissue. In the central nervous system (CNS) contrast enhancement is produced by two related, yet independent, processes: interstitial (extravascular) enhancement and vascular (intravascular) enhancement. Multiple gadolinium-based contrast agents have been developed, and six have been approved by the FDA for intravascular injection for contrast-enhanced MRI: Vasovist Injection Magnevist MultiHance Omniscan OptiMARK and ProHance. Modern iodine-based agents for CT are now usually low- and iso-osmolar compounds designed to lower the frequency of side effects and provide a higher safety margin. Food and Drug Administration (FDA)–approved urographic and angiographic iodine-based compounds for parenteral injection. The first intravascular contrast agents were U.S. MurphyĬontrast material has been essential to cross-sectional neuroimaging for almost 4 decades.
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