Demonstration of the area postrema with contrast-enhanced MR

Contrast media that distribute in the extracellular space, such as iodinated compounds used in cerebral CT and paramagnetic chelates used in MR imaging, do not enter areas of the CNS that have an intact blood-brain barrier. Studies of the barrier suggest that its location is the capillary endothelial cells, which differ from those outside the CNS because of the presence of zona occludens (tight junctions), narrow intercellular gaps, nonfenestrated endothelial basement membranes, and absence of pinocytosis [1]. Several brain sites do not possess these unique capillary endothelial cells, thus permitting intravascular contrast medium to cross into the brain (i.e., enhancement). These sites include the circumventricular organs, posterior pituitary gland, infundibulum, and choroid plexus [1, 2]. Increased density in both arteries and veins is seen on enhanced CT, whereas only areas of nonturbulent venous flow will show increased signal intensity after injection of paramagnetic contrast medium in MR imaging. This report describes visualization of enhancement in a circumventricular organ, the area postrema, with MR.