| Abstract: | The blood, cerebrospinal fluid (CSF), and extracellular fluid of the parenchyma form the fluid compartments of the brain with three interfaces between, namely the blood–brain interface (BBB), the CSF–brain interface, and the blood–CSF interface. When either water‐soluble iodinated contrast media (CM) or water‐soluble paramagnetic CM are injected intravenously, they are rapidly brought into contact with both the BBB and the blood–CSF interface. It is the behaviour of the water‐soluble CM at these two interfaces that determines the normal and abnormal enhancement patterns demonstrated by either CT or MRI. Unlike lipophilic solutes, current iodinated and MRI contrast media all have high affinities for plasma water, low affinities for plasma proteins and, in particular, extremely low partition coefficients. Therefore they do not penetrate the normal BBB. On the other hand, radiopharmaceuticals used in positron emission tomography (PET) and single photon emission computed tomography (SPECT) to demonstrate regional cerebral blood flow are highly lipophilic and readily cross the intact BBB completely during the first pass through the cerebral vasculature. It is the inability of the current iodinated and MRI contrast media to cross the normal intact BBB that is the basis of their use in CT and MRI studies of the brain. |
